Emerging Materials: Design, Characterization and Applications 9811913110, 9789811913112

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Laxman Raju Thoutam Shubham Tayal J. Ajayan   Editors

Emerging Materials Design, Characterization and Applications

Emerging Materials

Laxman Raju Thoutam · Shubham Tayal · J. Ajayan Editors

Emerging Materials Design, Characterization and Applications

Editors Laxman Raju Thoutam SR University Warangal, India

Shubham Tayal SR University Warangal, India

J. Ajayan SR University Warangal, India

ISBN 978-981-19-1311-2 ISBN 978-981-19-1312-9 (eBook) https://doi.org/10.1007/978-981-19-1312-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Contents

Design and Characterization Novel Emerging Materials: Introduction and Evolution . . . . . . . . . . . . . . . Laxman Raju Thoutam, Malleswararao Tangi, and S. M. Shivaprasad

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Synthesis and Characterization of Emerging Nanomaterials . . . . . . . . . . . Chidurala Shilpa Chakra, Velpula Divya, Konda Shireesha, Sakaray Madhuri, Thida Rakesh Kumar, Adapa Uday Krishna, and Deshmukh Rakesh

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Seeded Crystal Growth of Cd-Zn-Te (CZT) Assisted via Numerical Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Saketh Kakkireni, Magesh Murugesan, Benjamin Montag, and John McCloy Design Techniques for High Reliability FET by Incorporating New Materials and Electrical/thermal Co-optimization . . . . . . . . . . . . . . . . . . . . 133 Young Suh Song, Shiromani Balmukund Rahi, Shubham Tayal, Abhishek Upadhyay, and Jang Hyun Kim Recent Advances in Energy Harvesting from Waste Heat Using Emergent Thermoelectric Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Saurabh Singh, Keisuke Hirata, Sudhir K. Pandey, and Tsunehiro Takeuchi Challenges and Opportunities for Emerging Material Systems . . . . . . . . . 185 Ribu Mathew, Avirup Das, and Harihara Padhy Applications of Emerging Materials Emerging Materials for Biosensor Applications in Healthcare . . . . . . . . . 213 P. P. Muhammed Shafeeque Rahman, Merin Joseph, Lakshmi V. Nair, and T. Hanas

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Emerging Nanostructures in Dental Applications . . . . . . . . . . . . . . . . . . . . . 265 Debarati Ghose and Dhiraj Kumar Emergent Catalytic Materials Towards CO2 Reduction . . . . . . . . . . . . . . . 315 S. S. Sreejith, Nithya Mohan, and M. R. P. Kurup A Brief on Emerging Materials and Its Photovoltaic Application . . . . . . . 361 Deboraj Muchahary, Sagar Bhattarai, Ajay Kumar Mahato, and Santanu Maity Applications of Emerging Materials: High Power Devices . . . . . . . . . . . . . 407 J. Ajayan, Shubham Tayal, and Laxman Raju Thoutam An Insights into Non-RE Doped Materials for Opto-Electronic Display Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 Satya Kamal Chirauri, M. Rakshita, and D. Haranath

Design and Characterization

Novel Emerging Materials: Introduction and Evolution Laxman Raju Thoutam, Malleswararao Tangi, and S. M. Shivaprasad

Abstract Material discovery and research is at the forefront of all fundamental scientific and technological revolutions. The pursuit of synthesis for high quality materials with novel physical properties mandates precise control of growth process(es) and profound knowledge of structure–property relationships. The book chapter introduces the fundamentals of energy bandgap and energy– momentum dispersion relationships and its implications on the electronic properties of recent emerging material systems including, graphene, topological insulators, Weyl semimetals and III-V nitride semiconductors. The book chapter briefs on the process(es) related issues in synthesizing high quality two-dimensional material systems and introduces twistronics, a new branch of device engineering wherein the relative (twist) angle between the vertically stacked two-dimensional layers dictates the physical properties. The intricate effect of intentional and unintentional doping, electrostatic field effect gating technique on various topological insulator materials are discussed. The importance of high charge carrier mobility in Dirac materials and its significance for functional device and sensor applications are highlighted. The latter part of the chapter discusses the fundamental properties of group-III nitrides and their advantages in optoelectronics and electronics applications. The significance of Group-III nitrides and their alloys emerged as technologically outstanding materials with direct bandgap spanning deep UV to near infrared wavelengths in addition to their excellent electronic properties are discussed. The high-performance optoelectronics and electronic devices of III–V nitrides provided mankind with several benefits with state-of-the-art display and power-saving technologies. All authors have contributed equally to this work. L. R. Thoutam (B) Department of Electronics and Communications Engineering, SR University, Warangal, Telangana 506371, India e-mail: [email protected] M. Tangi · S. M. Shivaprasad Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India e-mail: [email protected] S. M. Shivaprasad e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_1

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Keywords Graphene · Dirac materials · Topological insulators · Weyl semimetals · Semiconductors · Group-III nitrides · Direct bandgap · Optoelectronics

1 Introduction Material discovery is the driving force for all the recent technological innovations and created new scientific avenues to explore fundamental structural, chemical, magnetic, optical, mechanical, and physical properties for functional device applications. The advancement of electronic devices and emerging technologies is continuously decreasing the physical transistor feature size and is reaching its fundamental limit in conventional two-dimensional chip manufacturing process. It is expected that any further decrease in transistor size will result in inevitable increase of tunneling and leakage currents that deteriorates the device performance. Material discovery and development provides unique opportunities to overcome these shortcomings and offer a feasible solution to match the current state-of-art technological demands. The availability of high-quality materials at different length scales (centimeter to nanometer) and dimensions (three dimensional-3D, two dimensional-2D and one dimensional-1D) enabled the researchers to discover elusive quantum phenomena in the last few decades. However, new materials systems offer us with many unknown challenges like reliability and economic viability that needs to be accounted for the design and development of future technologies. The current chapter gives you a basic overview of recent emerging materials systems with interesting topological features. The chapter discusses the fundamental principles of different classes of topological materials and their novel electronic properties. Lastly, the chapter introduces the fundamental properties and evolution of group III-nitride semiconductors over the past few decades, which make them technologically important for electronics and optoelectronics applications.

2 Dirac Materials Materials are classified into metals, insulators and semiconductors based on their electronic properties. These electronic properties primarily depend on the electronic band structure, type of bonding and the density of free charge carriers. Traditionally, metals have no forbidden energy gap and conduct electricity; insulators have large forbidden gap and do not conduct electricity; and semiconductors have small forbidden gap that enable the flow of current with external perturbation. The band theory of solids is very successful in explaining the electronic band structure and type of bonding for most materials which helps to predict and understand the electronic properties under appropriate conditions [1]. It is to be noted that electronic band structure, bonding type, and density of free charge carriers inside a material can

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be altered by subjecting the material to different external stimuli such as structural deformation (stress and strain), temperature, pressure, magnetic field, and doping. An electronic band structure is a two-dimensional representation of possible allowable electronic energy levels inside a solid material. The energy “E” of the band structure is usually calculated in “momentum space” or “k-space” and requires some deeper understanding of the band theory and is not trivial. The curvature of the electronic band structure determines the mobility of the charge carriers through different energy bands. For instance, in most traditional materials, an electron in a free space with zero external potential has its energy proportional to square of the momentum E∝ k 2 , related by free electron mass m0 . However, the free electron mass is replaced by effective mass m* under the presence of periodic potential inside a material, which plays a pivotal role in defining charge carrier dynamics. The effective mass of the charged particle describes the myriad external forces acting on it when interacting with other identical charged particles in its vicinity for a given thermal distribution of energy. The effective mass can be isotropic, anisotropic, and even be negative [1, 2]. The parabolic E-k diagram consists of many discrete points with each point corresponding to a discrete possible state that is allowed to exist inside the material, as shown in Fig. 1a. The forbidden energy gap, which is the difference between top of the valence band and bottom of the conduction band determines the type of material. The E-k diagram and its curvature dictates the physical properties of a material. The charged particles inside a material which follow parabolic dependence on energy usually obey Schrodinger’s Hamiltonian wave equations and the physics of which is well understood over the years. However, recent advancements in the field of condensed matter physics and materials science opened new area of material research, where in the parabolic dependence of energy is replaced by linear dependence at lower energy excitation spectrum, as shown in Fig. 1b. The linear E ∝ k dependence appear at some specific points (called Dirac points) on the Fermi surface, where charged particles behave as massless particles [3, 4]. The materials which show the linear dispersion relation are termed as Dirac materials. The valence band and conduction band states of Dirac materials

Fig. 1 Energy–momentum diagram for materials with a with parabolic dispersion relation and b linear dispersion relationship. Open circles correspond to all occupied states in valence band and closed circles correspond to all available empty states in conduction band

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meet at Dirac point yielding zero density of states. Dirac materials usually have zero or narrow band gap where two or more bands strongly coupled at level crossing [3–5]. The quasi-charged particles occupy all possible states below the Dirac point (open circles in Fig. 1b) and while the states above the Dirac point are empty (closed circles in Fig. 1b). The linear dispersion relation near the Dirac points produces mass-less charged quasi particles that obey relativistic Dirac equation rather than usual Schrodinger Hamiltonian (as in the case of parabolic energy–momentum relationship) [3–5]. The increased velocity of the charged particles due to vanishingly small effective mass at Dirac points potentially lead to high mobilities and currents that can be optimized for faster device performance in semiconductor and electronics industries. The unique cone like structure near the Dirac points leads to some novel and interesting physical properties [6]. Intrigued by the discovery of recent theoretical and experimental physical properties at Dirac points, material researchers are striving to understand the implications of linear dispersion in different material systems. The use of latest nano fabrication and characterization tools are best put in practice to control the charged particles at the Dirac nodes to reveal new and novel physical phenomena. The static and dynamic in-situ control of the charged particles at the Dirac points can be achieved by fine tuning the inversion symmetry, sublattice symmetry and the time-reversal symmetry [3–5]. However, the effect of scattering and the presence of external perturbations like stress, strain, magnetic field, pressure, and temperature at Dirac points vividly alter the nature of the charged particles to show different physical properties in different material systems. The linear dispersion relation in the materials can be achieved by synthesizing high quality materials using myriad growth techniques that have precise control over stoichiometry and defect density or through reduced dimensionality, as in the case of two-dimensional materials. The book chapter focuses on recent Dirac material systems that are of significant interest due to their interesting electronic properties in the recent past.

3 Graphene The discovery of graphene, a monolayer of carbon atoms has revolutionized the approach of tinkering materials at nanoscale for functional device applications. The experimental proof of isolating graphene onto insulating substrates using scotch-tape was successfully demonstrated by K.S. Novoselov and A.K. Geim at University of Manchester UK in the year 2004 [7], for which they are awarded with Nobel Prize in physics for the year 2010 [8]. This simple yet ground-breaking discovery of synthesizing nanoscale material using mechanical exfoliation technique paved way for the discovery of many other new two-dimensional (2D) materials that span most of the elemental periodic table. The ultra-thin material graphene offers a perfect Dirac cone with vanishing zero carrier mass in its band structure that leads to interesting physical properties including ballistic electronic transport [9] which renders high electron mobility [10], berry phase curvature [11, 12], quantum hall effect [13] and fractional

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Fig. 2 Change in resistance of a graphene device as a function of externally applied gate voltage at room temperature. The inset shows the optical microscope image of the graphene device, where the dark outlined region is the monolayer graphene, and four Ti/Au contacts are used for four-probe measurement of resistance. Note that the inset also shows a thicker graphite layer (gray color) adjacent to the actual device, which is exfoliated along with the monolayer graphene

quantum hall effect [14] in mechanically exfoliated samples. The simplicity and the non-trivial nature of producing mechanically exfoliated graphene offers electric field tunability of the chemical potential inside the band structure, that invariably controls the charge carrier density of electrons and holes, as seen in Fig. 2. The existence of the sharp maximum peak around −5 V is consistent with the reduced carrier density as the fermi level approaches the Dirac point, consistent with Fig. 1b, at which the density of the states vanishes. The decrease in resistance on either side of the Dirac point is due to population of charge carries i.e., electrons for positive gate voltages and holes for negative gate voltages. Ideally, the Dirac point should exist at zero applied gate voltage. However, the position of the peak resistance and its corresponding value at zero applied gate voltage is variable and is usually dependent on sample cleanliness. The presence of any native defects on the insulating substrate or the dangling bonds; and the photo/e-beam resist residues related disorder induced during photo/electron lithography process can alter the position of the Dirac point considerably [15, 16]. The quantum hall effect, a quantum phenomenon is usually observed at low temperatures in two-dimensional materials and usually requires high charge carrier mobility and vanishing carrier mass. The mono layer carbon, graphene being a perfect example of two-dimensional material is expected to show quantum hall effect revealing rich quantum physics that elucidates the interaction of charge carrier at nanoscale. Graphene due to its unique structure, vanishing carrier mass and high charge carrier mobilities (~10,000 cm2 /V-s) under ambient conditions showed quantum hall effect [17]. This result fosters the researchers to access the quantum properties of the materials at elevated temperature and helps to design emerging materials with novel properties. With the advancement in new techniques like sandwiching the graphene between insulating hexa boron nitride thin layers (to avoid substrate and surface contaminations), ultimate room-temperature electron mobility

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around ~105 cm2 /V-s [17] was reported and establishes graphene as a powerful Dirac material for making of next generation electronic devices. With the success of manipulating electronics states in monolayer graphene, researchers have experimented with topological superlattices of bi-layer and tri-layer graphene with different stacking orientations [18]. The ability to tune properties by varying the twist angle between the stacked graphene layers facilitates the novel material graphene to achieve new and novel electronic states with interesting physical properties that include superconductivity [19, 20], mott metal–insulator transition [21, 22]. and ferromagnetism [23, 24]. The twisted bi-layer graphene material system has opened a new research area “Twistronics” in the field of condensed matter physics, where in the twist angle between the stacked layers controls the electronic properties of the materials, provides a new tuning knob for device engineering [18, 25]. This new platform enables switching between different electronics states in a single device. Twistronics is triggering a flurry of research on the twisting angles (and the so-called magic angle around ~1.1° in graphene) between various other two-dimensional quantum material systems to explore unknown material [3] properties with unconventional electronic properties. It is be noted that any influence of defects, misalignments and sample preparation procedures can drastically affect the electronic properties in twistronics research and careful analysis is needed to disentangle pristine and altered electronic properties in the presence of any defects. Dirac materials with versatile electronics properties are not just limited to twodimensional materials but can also be found in higher three-dimensional bulk materials which exhibit similar linear dispersion relation at low- energy fermionic excitations, where charged particles behave as massless Dirac particles [3–6]. A threedimensional Dirac material can be considered as three-dimensional analogue of graphene with linear dispersion relation around Fermi points. The 3D Dirac materials are usually narrow (or zero) gap semiconductors where two or more bands cross the Fermi level as seen in Fig. 3 [6].

Fig. 3 Illustrations of schematic energy–momentum diagrams of different representative material systems. a Direct band gap semiconductor b two-dimensional graphene c topological insulator d semi-metal with valence band and conduction band touching at different momentum points and e topological semi-metal with multiple band crossings at Fermi level (dashed line). Adapted from [6]

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4 Topological Insulators Topological insulators (TI) are a special case of Dirac materials where in it has an energy gap in the bulk interior and localized conducting states on the surface, which are protected by time reversal symmetry as depicted in Fig. 3c [26, 27]. The charged particles moving on the surface states are characterized by Bloch states possessing quasi-momentum whose corresponding energy levels are periodic over entire Brillouin zone. It is to be noted that Dirac equation and the corresponding linear dispersion relation is a local phenomenon and is not observed in the entire Brillouin zone. In general, the bulk of a TI acts as an insulator with finite band gap between conduction and valence bands, while the surface behaves as gapless conductor with Dirac cones in 3D and (as edge states in 2D) in which spin and momentum are locked-up and perpendicular to each other [26, 27]. The electrons on the surface state have zero effective carrier mass with interesting spin properties that can potentially result in new magnetic and spintronic devices [28]. The first known 3D TI material is a strong spin–orbit coupled alloy of bismuth and antimony Bix Sb1-x whose unusual surface stated are mapped by angle-resolved photo emission spectroscopy (ARPES) characterization technique [26]. ARPES technique is proven to be a very strong experimental technique to directly probe the distinct band structure of surface and bulk states of the TI materials [29]. The recent interest in the novel surface states has resulted in evolution of many material systems like Bi2 Se3 , Bi2 Te3 and Sb2 Te3 that are theoretically and experimentally characterized using various structural characterization techniques [26, 30], scanning probe techniques like scanning tunneling microscope [31–33]; and signature electronic transport measurements like Shubnikov-de Haas oscillations for decoupling the contribution of bulk and surface states [34–36]. The alloy Bix Sb1-x is unique among the 3D TI family as it can host high 2D carrier mobility around 104 cm2 /V-s, which makes it an ideal platform to study novel quantum transport properties. Unfortunately, the complex surface states and small bulk gap of 0.3 eV in Bix Sb1-x alloy makes it hard to disentangle the contribution of individual bulk versus surface states electronic transport states [26, 27, 37]. The sensitivity of surface states at ambient conditions, and the contribution of interior bulk states caused by alloy and native disorder, thermal activations due to small bulk band gap make this material system not an ideal system for functional device applications. The suppression on bulk contribution can be achieved by improving material quality and controlling the defect density through optimized growth approaches. The next generation of TI materials including Bi2 Se3, Bi2 Te3 and Sb2 Te3 are stacked in A-B-C-A-B-C manner and are bonded through weak Van-der- Walls forces, facilitating mechanical exfoliation much like graphene and host a single Dirac cone [38–40]. Additionally, these materials can be grown in thin films using a myriad thin film growth technique [27, 41–43]. These ultra-thin TI materials improves the surface-to-volume ratio and yields a higher surface contribution. The single Dirac cone of the gapless surface state in a 3-quintuple layer Bi2 Se3 thin film grown using molecular beam epitaxy technique is shown in Fig. 4 [44].

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Fig. 4 Band-structure of a 3-quintuple layer Bi2 Se3 thin film on AlN substrate imaged by angle resolved photo emission spectroscopy around the center of the Brillouin zone (Adapted from [44])

The Dirac point is located at −0.5 eV from the Fermi level indicating electron type conduction possibly due to Se vacancies. The bulk like bandgap between interior conduction and valence band is around 0.47 eV [44]. The intrinsic bulk conduction can be avoided by fine tuning the chemical composition or selective doping of the as-grown TI films to compensate donors/acceptors. However, the selective doping introduces dopant related disorder and can affect the surface state electronic properties. It is of great interest to manipulate the Dirac cone in TI materials and its associated characteristics in as-grown films to observe new and distinct electronic properties. However, the manipulation of Dirac cone arising from metallic spin-helical surface state of a TI invariably changes the interior bulk contribution. Indigenous techniques like electrostatic field effect gating technique on TI thin films or using artificial heterostructures can mitigate this issue by decreasing the interior bulk contribution and enhance the surface state contributions. The use of electrostatic gating technique using high K-dielectrics like HfO2 , Al2 O3 on TI devices have resulted in ambi-polar conduction (consistent with gapless surface states), modulation of surface-conduction states, and dynamic surface-to bulk coupling of low temperatures magneto transport properties including weak localization and weak antilocalization [34, 36, 45]. The use of high K-dielectrics over conventional SiO2 yields high surface charge carrier density modulation on electrostatic gated TI-devices, which resulted in a 500% resistance modulation on surface-states as demonstrated on a 20 nm Bi2 Se3 using 16nm HfO2 gate dielectric [45]. Alternatively, use of heterostructure engineering uses interfacial coupling to effectively modulate the surface state Diraccone and its associated properties. For example, Bi2 Te3 grown on Bi2 Se3 shows a shift of Dirac cone towards the Fermi level (electron-doping and hole-doping) due to interface charge transfer between the two prototypical TI’s. The interface charge transfer is primarily due to the difference in work functions of Bi2 Te3 (5.25 eV) and Bi2 Se3 (5.60 eV) respectively [46] Heterostructure engineering using magnetic materials with proximity to a TI material is an elegant way to induce magnetism

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via TI’s surface state coupling to ferromagnetic materials using Ruderman–Kittel– Kasuya–Yosida (RKKY) interaction [47–49]. Prokes et al., interfaced TI Bi2 Se3 with a low temperature ferrimagnetic materials EuSe and retained bulk EuSe magnetic properties in the bilayer system, showcasing the true potential of TI based magnetic devices [47]. A giant enhancement of ferromagnetic ordering that persists near the room temperature (300 K) was reported in Bi2 Se3 /EuS heterostructure grown using molecular beam epitaxy technique [50]. It is to be noted that bulk ferromagnetic insulator EuS shows a ferromagnetic ordering near 17 K. The increase in curie temperature is argued to be due to large spin–orbit interaction and spin-momentum locking of TI surface states [50]. Additionally, the acquired magnetism in proximity coupled TI/magnetic material bilayers can also be modulated by electrostatic gating technique as demonstrated by Mathimlar et. al., in Bi2 Se3 /EuS system [51]. The acquired magnetism due to proximity effect of magnetic materials in TI-based heterostructures create new opportunities in spin-based electronics that are dominated by gapless surface states.

5 Topological Semi-metals A semi-metal is defined as a material with small overlap between bottom of the conduction band and top of the valence band as depicted in Fig. 3d. With renewed increased interest in TI, research on semi-metals possessing a stable Fermi surface with energy band crossings evolved and are termed as topological semi-metals (TSM) [3–6, 52]. The band crossings in the crystal band structure are not new and has been studied during the development of band theory by Herring in 1937 [53]. The renewed interest in these band crossings is attributed to its topological behavior, where the quasiparticle excitations near the band crossings can behave both like elementary particles and exotic fermions connecting the high energy physics with condensed matter physics theories [54, 55]. TSMs are characterized by gapless band structure usually with charge compensation i.e., equal number of electrons and holes contribute to the conductivity of the material. TSM’s can be further categorized into different types based on the (i) degeneracy of band crossings; (ii) occurrence of the band degeneracy (point degeneracy vs line degeneracy) and the (iii) dispersion in the vicinity of the band crossings [54, 55]. Additionally, the origin of the band crossing i.e., whether it is symmetry enforced or due to band inversion can also result in different TSM behaviors [55, 56]. Based on above attributes coupled with their topological characteristics, TSM are classified into Dirac Weyl semimetals, nodal line semi-metals, type I and type II semimetals, multifold fermion semimetals and triple-point semimetals [55, 56]. The 2D material graphene is a perfect example of Dirac semimetal as discussed in Sect. 3. In 3D, the Dirac semimetal’s conduction band and valence bond touch at a discrete point in the Brillouin zone and disperse linearly in all directions around the discrete contact point. The bands that cross in a Dirac material are two-fold spin degenerate and the corresponding Dirac point is four-fold degenerate [55, 56]. The Fermi surface

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of a 3D Dirac material consists of many such degeneracies of Dirac points [57]. The band crossings in a Dirac material are protected by both inversion symmetry and time-reversal symmetry [54–60]. Cd3 As2 and Na3 Bi are the most researched 3D Dirac materials for their novel quantum electronic transport properties [60, 61]. The charge carriers in 3D Dirac material obey linear energy dispersion, much like 3D analogue of graphene to yield high charge carrier mobility that has true potential to show quantum linear magnetoresistance behavior at the quantum limit, where all the carriers occupy the lowest Landau level [32, 60, 62]. Berry phase curvature is another characteristic feature of Dirac fermions in 3D Dirac materials which is due to cyclotron motion of charge carriers around the Dirac points and can be experimentally accessed by Shubnikov-de Hass (SdH) oscillations [54]. The magnetoresistance measurements by Zhao et. al., on Cd3 As2 single crystals [63] reached quantum limit (n = 1 Landau level) at an applied magnetic field of 43 T, showing berry curvature and SdH oscillations consistent with theoretical predictions, as shown in Fig. 5. The SdH oscillations of Cd3 As2 single crystal can be clearly seen in Fig. 5a at low magnetic fields (B < 15 T). With increase in applied magnetic field (15 T < B < 43 T) SdH oscillations with Zeeman splitting appear, followed by high field quantum linear magnetoresistance. The inset in Fig. 5a shows inverse of magnetic field vs resistivity curve clearly revealing the SdH oscillations with Zeeman splitting after subtracting polynomial background. The Landau index plot based on the maxima and minima of observed resistivity oscillations as a function of inverse of the applied magnetic field reveals the lowest Landau level as shown in Fig. 5b. The linear dispersion relation around the Dirac points located at the Fermi level of Cd3 As2 single crystal reveals low temperature ballistic transport regime with carrier mobilities in the range of 9.2 × 106 to 4.6 × 107 cm2 /Vs and a mean free path around ~1 mm at T = 6 K [63]. The ultra-high carrier mobility coupled with long mean free path can be used to design faster electronic

Fig. 5 a Low temperature high field perpendicular magnetotransport measurement of Cd3 As2 single crystal. Zeeman splitting can be observed by periodicity in resistivity curve at high magnetic fields. Inset shows the SdH oscillations of the same after subtracting a polynomial background. b Landau level index plot showing lowest Landau level (n = 1) reaching around 43 T suggesting the ultimate quantum limit. (Creative Commons Attribution 3.0 License.63.)

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devices for optimum performance. With increase in defect density and quality of the single crystal growth mechanisms of 3D Dirac materials, the quantum linear magnetoresistance in Cd3 As2 even persists near the room temperature [60, 64]. The high room temperature magnetoresistance can be used to design magnetic sensors for useful practical applications. With renewed interest in novel electronic transport properties of 3D Dirac materials, researchers explored the evidence of 3D Dirac semi-metallic nature in Bi1-x Sbx [65], TlBiSSe [66], YbMnBi2 [67], α-Sn on InSb [68], and ZrTe5 [69] material systems using myriad structural and electronic transport characterizations.

6 Weyl Semimetal A Weyl semimetal is observed if Dirac point in three-dimensional band structure is split into a pair of Weyl points in the band structure by breaking either inversion vs time-reversal symmetry. Weyl semimetals are characterized by topologically protected surface states, which have an arced Fermi surface with two singly degenerate band crossings at discrete points namely Weyl nodes. The fermions near the Weyl node disperse linearly in all three momentum space directions away from each Weyl node. The fermions in Weyl semimetal possess distinct chiralities, either left-handed or right-handed leading to chiral charges, which can be understood as monopoles and antimonopoles of Berry flux in momentum space [3–6, 52, 57]. The presence of opposite chiral charges in momentum space of a Weyl semimetal result in the formation of Fermi surface arcs. The Fermi arc’s constant energy contours are open arc that connect the Weyl nodes of opposite chiralities on the surface [55]. In Weyl semimetals, the band crossings are spin split and thus making each band singly degenerate [57]. A Weyl monopole is formed when a spin up band and spin down band crosses near the Fermi level with a specific chirality associated to the crossing. The Weyl cones always comes in pair and the resulting Weyl Fermions are chiral in nature [57]. Weyl semimetals of the transition metal monophosphides TaAs, TaP, NbAs and NbP gained a lot of interest in recent times for their novel quantum electronic properties in the field of condensed matter physics [70–74]. Angle resolved photo emission spectroscopy technique was quite successful in revealing their bulk and surface band structures consistent with Weyl picture in transition metal monophosphides [75, 76]. However, the band structure of NbP differs from the conventional TaAs Weyl semimetal due to low atomic mass of Nb resulting in weak spin–orbit coupling in its band structure. This results in the existence of conventional parabolic semi metallic bands apart from the linear Dirac bands as shown schematically in Fig. 3(e) [77]. NbP peculiar band structure hosts the co-existence of normal quadratic bands from hole pockets and linear Weyl bands from electron pockets [78]. The use of advanced growth techniques like molecular bean epitaxy method enables the precise control of surface terminations and doping density that enable the suppression of trivial parabolic states and increase the topological nature of NbP to observe

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elusive quantum transport properties [79]. Electronic transport measurements on transition metal monophosphides revealed ultra-high mobility due to the presence of Weyl nodes [78], extreme large magnetoresistance [78, 80, 81], and chiral-anomaly induced negative magnetoresistance [82–84]. In general, topological semimetals including Dirac and Weyl semimetals exhibit high low-temperature carrier mobility accompanied by large magnetoresistance due to the existence of Dirac cones/Weyl points with vanishing density of electron effective mass. Please note that the low temperature charge carrier mobilities of same family of the material(s) reported by different research group vary and it invariably depends on the sample quality. Residual resistivity, defined as the ratio of room temperature resistivity to experimentally accessible low temperature value seems to be a good parameter to gauge the material quality. Typically, the lower the residual resistivity, the higher is the mobility and this can be achieved maintaining perfect stoichiometry in as-grown materials, lowering the defect density using myriad growth techniques using optimum growth conditions to produce ultra clean samples. The mobility versus magnetoresistance of various class of Dirac semimetals and Weyl semimetals is shown in Fig. 6. The MR shows increasing trend with increasing mobility values until ~105 cm2 /V-s, and saturates at mobility values beyond 106 cm2 /V-s. Please note that the mechanism of MR is not of same origin across various topological semimetals systems and the observed high MR can be due to simple Lorentz force, mobility fluctuation due to disorder or sample inhomogeneity [85], electron–hole charge compensation [86, 87], or of quantum origin [62]. These new emerging material systems with extremely large magnetoresistances and high carrier mobilities pave way for design of the next generation of the electronic devices. Fig. 6 Comparison of mobility versus magnetoresistance (MR) values of various well-known Dirac and Weyl semimetals at 2 K. The compounds with dark circles show linear MR behavior (Sukriti Singh et al. 2020 J. Phys. Mater. 3 024,003 @Creative Commons Attribution 3.0 license[158])

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7 Group-III Nitride Semiconductors The technological importance of III-V compound semiconductors is well reviewed in numerous books [88, 89] and articles [90–92]. The group III-nitrides AlN, GaN, and InN being part of III–V semiconductors are direct bandgap semiconducting materials exhibiting the band gaps of 6.2 eV, 3.4 eV and 0.63 eV for AlN, GaN and InN, respectively. Energy gap for various semiconductors with respective lattice constants are shown in Fig. 7. The major advantage of group-III nitrides is exhibiting similar wurtzite structure and valance states which facilitate them for bandgap engineering. The ternary alloys AlGaN and InGaN formed out of these binary compounds cover the energy spectrum 6.2 eV (200 nm) to 0.63 eV (1965 nm). This makes these materials to be well suitable for high performance electronic and optoelectronic devices such as high electron mobility transistors (HEMTs), light emitting diodes (LEDs), lasers, detectors and, solar cells. The commercialization of LEDs and laser diodes is an ultimate development that being accomplished by III-V nitride research community. Especially the invention of bright blue GaN-based LEDs by S. Nakamura [93, 94] introduced an approach for achieving white light sources. The LEDs substituted the incandescent light bulbs as they provide white bright illumination at less power consumption with longer lifetime. Therefore, III-nitride LEDs turned in to current-generation illuminating sources. The applications of nitride-based LEDs are versatile. For instance, the standard green to violet LEDs are utilized in traffic lights, automotive panel instruments photovoltaic technology of solar cell and, full color displays whereas UV LEDs plays a crucial role as disinfectants, in water purification. On the other hand, though a significant progress has been achieved in the area of solar cells [95], achieving high conversion efficiency remains still challenging. There are several approaches were proposed among one is to build a solar cell such a way that it absorbs as wide energy range of solar

Fig. 7 Relation between lattice constant and band gap energy for the III-nitrides and other various semiconductors at room temperature [96] (Adapted from Ref. [96, 157])

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spectrum which is possible by using multi-layers of InGaN alloys covering the most of the solar spectrum, from 0.63 to 3.42 eV, from bandgap engineering as shown in Fig. 7. Thus, there is considerable interest in evolving full-spectrum-response highefficiency solar cells, by using tandem stacks of III-nitride-based materials [90]. Like other III-V GaAs and InP semiconductors, it is impossible to accomplish largearea single-crystalline nitride bulk substrates for homo-epitaxy of these materials. Hence, the growth of group-III nitrides remained as hetero-epitaxy on usual substrates like Sapphire, Si and, silicon carbide (SiC). Thus, the heteroepitaxial growth hinder the preparation of device quality materials and thereby performance which is also limited by various other reasons. The group-III arsenide and phosphide LEDs could span infrared to yellow wavelengths. The (Ga,Al)As based LEDs spans from 1.5 to 2.2 eV and (Al,Ga,In)P based LEDs spans from 1.3 to 2.5 eV, (shown in Fig. 7) whereas group-III nitrides cover wide range of the solar spectrum from 0.63 to 6.2 eV. On the other hand, in comparison to silicon (Si), germanium (Ge), III arsenides, phosphides (GaAs, GaP and InP) or zinc selenide (ZnSe) material systems, group III- nitrides have a higher physical and chemical stability. This allows them to be used in harsh environments and for high electric currents, for intense light illumination, high thermal conductivities, high radiation hardness, larger avalanche breakdown fields, larger piezoelectric constants, and larger room temperature electron mobility. These discerned properties make them suitable for high-frequency and high-power optical and electrical devices. Group III-nitride semiconductors have been studied for more than half a century. Among them, GaN is the most investigated materials and InN is least studied in the literature. In 1928, Tiede et al. [97] first reported AlN growth, later in 1938 Juza et al. [98] reported the synthesis of GaN which was made by flowing NH3 above gallium and InN was prepared by reducing (NH4 )3 InF6 . These initial studies were intended to analyze the fundamental band gap measured from optical properties and relate it to the lattice parameters obtained from XRD measurements for these materials. Later, Maruska et al. [99] first prepared GaN epitaxial thin films having band gap at 3.39 eV by vapor phase deposition on sapphire substrates, followed by the deposition of GaN epitaxial layers were deposited using hydride vapor phase epitaxy (HVPE) in the 1960s. After this report, in 1971 blue LEDs from GaN:Zn/n-GaN heterostructures were prepared by Pankove et al. [100] on vapour phase epitaxial GaN. This structure was used for the injection of hot carriers resulted from avalanche breakdown to create holes. Regardless of the consequent improvement in growth, preparation of p-GaN remained difficult owing to the high defect concentration of available heteroepitaxial films. In 1989, by employing a two-step growth method, Amano et al. accomplished growing good quality Mg:GaN epi-layer on c-sapphire with a low temperature AlN buffer using metal organic vapour phase epitaxy (MOVPE) [101]. At Japanese company Nichia Chemical Industries, Nakamura et al. used Mg doping to achieve p-type material which helped in preparing the brightest blue LEDs [93, 94]. As a consequence, InGaN high-power LEDs for white, blue and green light emission having a lifetime of 105 h were announced and marketed by the same company in 2002. Since then, several remarkable attempts have been made to investigate laser diodes and even high electron mobility transistors (HEMT) using the group-III

Novel Emerging Materials: Introduction and Evolution

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nitride compounds and their alloys. During 1990s, it was demonstrated that HEMTs are possible due to the accumulation of 2D electron gas at Alx Ga1−x N/GaN interface resulting from the interfacial strain and spontaneous piezoelectric polarization effects [102]. In 2001, Kuzmik suggested to use the ternary InAlN in place of AlGaN in order to avoid the degradation of devices which is a consequence of the strain relaxation, by varying in composition in InAlN the strain can be modified while preserving the high sheet carrier concentration [103]. Later on, strain has been completely removed by using InAlN/GaN heterostructure with reduced lattice mismatch, which is explained in detail by Jain et al. [104, 105] and Bhuiyan et al. [106]. The superior optical and electrical properties of group-III nitride compound semiconductors lead to the fabrication of advanced device structures. The tunable III nitride alloys having variable Indium composition, and thus band-gaps, made them potential candidates for stateof-the-art electronic and optoelectronic devices, that operate over a wide range of the electromagnetic spectrum. A higher band gap is chosen for microwave transistors, while smaller band gap is also required for a full spectrum solar cell from AlGaN and InGaN ternary alloys.

7.1 Crystal Structure The group III nitrides exist in three crystal structures, namely wurtzite, zincblende, and rocksalt. Among wurtzite structure is energetically favorable as compared to the others. The cohesive energy per bond for the bulk AlN, GaN, and InN, are 2:88 eV (63.5 kcal/mol), 2.20 eV (48.5 kcal/mol), and 0.67 eV (15.5 kcal/mol) respectively [107], whereas the calculated energy difference values between wurtzite and zincblende phases are as small as −18:41 meV/atom for AlN, −9:88 meV/atom for GaN, and −11:44 meV = atom for InN. The wurtzite crystal structure with hexagonal unit cell pertains to the P63mc space group and c/a ratio is 1:63. The wurtzite structure is constituted two interpenetrating hexagonal close-packed sublattices, which displaced with respect to each other along the threefold out of plane c-axis by 0.375. Each sublattice having four atoms per unit cell and every group III atom is accompanied tetrahedrally by four nitrogen atoms, or vice versa, which are being positioned at the edges of a tetrahedron. The actual nitrides deviate from the above-mentioned ideal structure, which is signified by the c/a ratio or u value. However, rocksalt phase in Fm3m space group symmetry is not stable, a phase transition towards rocksalt structure of group-III nitrides can be possible at extremely high external pressure owing to decreased lattice parameters that results in inter-ionic Coulomb interaction that facilitates ionicity over covalent behavior. On the other hand, metastable zincblende structure can be attained by growing III-nitrides on cubic substrates such as Si, MgO and GaAs. The equivalent bond length of zincblende structure is around 1.62 Å. The zincblende crystal structure is composition of two interpenetrated facecentered cubic (fcc) sublattices having displacement of one quarter of its body diagonal exhibiting the space group of F 4 3 m. Figure 8 shows the crystal structure

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Fig. 8 Atomic structures of wurtzite and zincblende group-III nitrides in their hexagonal and cubic conventional cells, respectively. Larger and smaller balls stand for III-metal and N atoms [109]

for zincblende and wurtzite type group-III nitrides. The unit cell consists of four atoms with tetrahedral coordination. The zincblende structure can be distinguishable from wurtzite structure by stacking sequence of the diatomic planes however both show similar tetrahedral coordination. Owing to the non-centrosymmetric nature, the wurtzite crystal structure exhibits crystallographic polarity which is useful for field effect transistors on the other hand it is deleterious effect for LED and LASER applications by means of quantum confined stark effect [108]. The non-polar and semipolar oriented III-nitrides need to be explored to suppress polarization effects on the efficiency of optoelectronics devices.

7.2 Elastic and Thermal Properties: The elastic properties of materials are usually associated with the elastic theory of solids. The parameters such as Young’s and bulk modulus, hardness, stiffness constant, and yield strength are utilized to understand the respective mechanical properties of semiconducting materials like group-III nitrides. The elastic theory for hexagonal symmetry reveals that there are independent elastic constants C11, C33, C12, C13 and C44. Among, C11 and C33 represent longitudinal modes along the [1000] and [0001] directions respectively, whereas C44 and C66 can be derived from the speed of sound of the orthogonal modes propagating in opposite directions. The other coefficients are related to the velocity of the modes moving in low-symmetry directions. The bulk modulus can be deduced from the elastic coefficients [148]. For the case of isotropic materials, the Young’s modulus and shear modulus can be assessed from Poisson’s ratio which can be determined from high resolution x-ray diffraction measurements that demonstrated by Moram et al. [110]. Several efforts have been made in determining the mechanical properties of compound semiconductors by utilizing ultrasound measurements which require bulk and single crystalline materials. The other technique used for the same is Brillouin scattering while other

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forms of x-ray techniques energy dispersive x-rays, angular dispersive x-rays, xray absorption spectroscopy in addition to aforementioned HRXRD measurements. From such techniques the Poission’s ratio was observed to be about 0.28, 0.3 and 0.2 for AlN, GaN and InN respectively. The hardness of group-III nitrides can be found from nano and micro-indentation. On the other hand, Infrared Reflection and Raman spectroscopies are promising techniques to understand phonon modes and their linear relationship with the applied pressure. Thermal properties of group-III nitrides can be understood by exploring the thermal expansion coefficients (TEC) in terms of temperature dependent variation in the lattice parameters along in-plane and out-of-plane orientations. The thermal expansion accommodates strain during material growth and consequently controls the electronic properties at the interfaces formed by the heterostructures. For instance, AlGaN/GaN high electron mobility transistors and InGaN multiple quantum wells in LEDs. The TEC of AlN was reported as 4.03 × 10–6 in the temperature range of 298–473 K. Along a and c lattice direction, thermal expansion was not found at low temperatures whereas it was found to be linear expansion at higher than 750 K. Thermal expansion coefficients were stated to be 3.38 × 10–6 and 2.68 × 10–6 K−1 along a and c directions for AlN. The thermal expansion coefficient for wurtzite GaN is 5.59 × 10–6 K−1 for a/a while the data for InN reported in the range of 100–673 K and the respective values are 3.6 × 10–6 and 2.6 × 10−6 K−1 . Thermal conductivity plays an important role in group III nitride based high-power and high temperature electronic and optoelectronic devices. Heat transport in III-nitrides is mainly resulted from phonon–phonon Umklapp scattering as well as phonon scattering which associated with point, line defects and grain boundaries. Thermal conductivity values for AlN, GaN and InN were reported to be respectively 3.3, 4.1 and 0.45 W cm−1 K−1 [149].

7.3 Electrical Properties Advantages of group-III nitride semiconducting materials include high break down voltages, sustaining large electric fields, low noise generation, and high temperature and high-power operation. Low field electron mobility, large satellite energy separation, and high phonon frequency are among the other qualities. The electron transport in group III nitride semiconductors treated at low and high electric fields. At low fields, the energy acquired by the electrons from the applied external electric field is small as compared to the thermal energy of electrons. Hence energy of electrons is unperturbed by such a low electric field. But at high electric fields the energy gained by electrons is no longer ignored and thereby the electron distribution function changes enormously from its equilibrium value. The Hall effect is widely explored way of understanding the transport properties of epitaxial thin films. For any semiconductor thin films, it gives the information about the carrier density, type of carriers, mobility. The temperature dependent Hall measurements infer about the impurities, uniformity, scattering mechanism and other defects. In order to understand various

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scattering mechanisms, one would explore the relaxation time which determines the rate of change in electron momentum as it passes through the semiconductor crystal. And various scattering mechanisms can be understood by Matthiessen’s rule. The resulted mobility of carriers is associated with several scattering mechanisms such as ionized impurity scattering, phonon scattering, piezoelectric scattering and scattering at dislocations. Among III-nitrides, InN is an enormously potential material for electronic and optoelectronic devices because of its outstanding material properties such as lowest effective mass (0.04mo ), narrowest band gap (0.63 eV), highest predicted electron mobility (4400 cm2 /Vs) and high peak and saturation velocities (107 cm/sec). In spite of these fascinating properties, InN has remained the least studied and understood material among III-nitrides, since the preparation of high-quality epitaxial InN material is challenging due to its low dissociation temperature (≈550 °C), high equilibrium vapor pressure of nitrogen over the InN films during growth and lack of suitable (native) substrate material. Thus, the complexity in preparing high quality InN films using different experimental techniques, has led to a wide spectrum of reported fundamental properties like band gap values that range from 0.6 to 2.2 eV in the literature [111–115]. The high band gap values observed earlier, have been largely attributed to be resulting from the incorporation of indium oxide, hydroxide, or oxynitride and related complexes as these films were grown using low vacuum growth techniques like rf-sputtering [116]. Recently, due to the emergence advanced growth techniques, high quality single crystalline InN films have been prepared. Over the last two decades, a major advance in InN research was made, when the growth of good quality single crystalline films of InN by MBE on sapphire substrates was reported, whose the band gap energy was claimed to be ≈0.67 ± 0.05 eV [117–119]. Since then, the number of publications in the area of InN research have been significantly increased. As a consequence of its lowest effective mass for electrons, InN exhibits the highest mobility and high saturation velocity among III-nitrides. The theoretically estimated maximum mobilities of InN and GaN at 300 K are 4400 and 1000 cm2 /Vs, respectively, while at 77 K the limits are beyond 30,000 and 6000 cm2 /Vs, respectively [120]. The electron and hole mobility values are listed in the Table 1 for zincblende and wurtzite binary group-III nitride semiconductors. The electron transport in wurtzite InN studied by an ensemble Monte Carlo method [121–123], showed that InN exhibits an extremely high peak drift velocity at room temperature, with saturation velocity much larger than that of gallium arsenide (GaAs) and gallium nitride (GaN). Figure 9 describes the velocity-field characteristics associated with wurtzite GaN, InN, AlN, and zincblende GaAs, which shows that the respective critical field at which the peak drift velocity was achieved. InN achieves the highest steady-state peak drift velocity of 4.2 × 107 cm/s at 65 kV/cm which contrasts that of GaN, 2.9 × 107 cm/s, AlN, 1.7 × 107 cm/s, and of GaAs, 1.6 × 107 cm/s. It is also reported [123] that the electrical transport properties and electron mobility of InN higher than that of other binary III–V semiconductors, in a temperature ranging from 150 to 500 K. The doping concentration is up to 1019 cm−3 for such InN films which highly depend on the variation of temperature and doping levels, in contrast to GaAs. This indicates that there could be unique benefits provided by using InN in high-frequency centimetre and millimetre wave devices.

3.24

6.0865

6.02

6.813

6.903

β-AlN (ZB)

α-GaN

β-GaN (ZB)

α-InN

β-InN (ZB)

4.986

4.52

4.38

3.548

3.1896

3.112

a (Å)

5.76

5.1855

4.982

c (Å)

a (Å)

(g/cc)

3.258

α-AlN

Zincblende

Density

Binary

Wurtzite

Structure and Lattice parameters

III-nitrides

0.56

0.7–1.1

3.231

3.42

5.2

6.2

Eg (eV)

Bandgap

0.03

0.044

0.15

0.21

0.26

0.29 − 0.45

(me /mo )

Electron

1.959

1.27

1.77

(mHH */mo )

Heavy hole

Effective mass

0.098

0.21

0.35

(mLH */mo )

Light hole

3100

760

1245

39

350

370

14

12.2

no

9.4

8.07

(cm2 /Vs)

(cm2 /Vs) 125

εs

μh

μe

7.92

5.35

4.25

ε∞

Dielectric constant

Mobility

Table 1 Various properties of group-III nitride semiconductors with zincblende (β) and wurtzite (α) crystal structures are listed [132, 157]

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Fig. 9 Variation of drift velocity for III–V compound semiconductors as increasing field (Adapted from [121])

The transient electron transport was also considered for InN which is the main electrical transport in nanometer to micrometer devices [121]. It was demonstrated that InN shows the highest peak and overshoot velocity which sustains for longer times as compared to other nitrides GaN and AlN. Moreover, group-III nitride field-effect transistors (FET) have tremendously high speed with a frequency range over 1 THz for 0.1 mm gates. Thus, the properties of group-III nitrides show that it is a remarkable material for the device fabrication of high-speed, high-performance hetero junction FETs, with several applications. Moreover, group-III nitride based heterostructure field-effect transistors (HFETs) have been evolved as promising electronic devices for high-voltage, high-power operation at microwave frequencies [124, 125]. The electron effective mass of GaN is higher than that of GaAs, consequently GaN exhibited low-field mobility which is lower than the mobility of GaAs. Wurtzite group-III nitrides exhibited spontaneous and piezoelectric polarization which are much higher than other conventional III–V and II–VI semiconductor compounds and are similar to the values of ZnO [126]. The spontaneous polarization in group-III-nitrides can result in the fields up to 3 MV/cm whereas lattice mismatch induced strain in pseudo morphically grown AlGaN/GaN or lnGaN/GaN hetero-films can produce an extra piezoelectric field of about 2 MV/cm. Such high polarizations resulted in electric fields which provides interface charge densities at the interfaces. This also helps in the separation of the hole and electron wave functions in quantum wells (QWs). GaN exhibits a high peak and saturation velocities in addition to it’s higher thermal stability, and a larger band gap, which allow them to be adaptable for channel materials in microwave devices. Additionally, attaining the exceptional performance of AlGaN/GaN HFETs is the capable of accomplishing two-dimensional electron gas (2DEG) through sheet carrier density of 1013 cm−2 or even higher in proximation with the hetero-interface without purposefully obtained doping levels. This 2DEG is remarkably higher than those acquired in other III–V semiconducting materials. Especially AlGaN/GaN FETs exhibiting piezoelectric polarization induced by strain is higher than that of AlGaAs/GaAs structures, causing a considerable increase in the sheet density [127]. The 2DEG was found in Ga-face and N-face wurtzite Alx Ga1-x N/GaN/Alx Ga1-x N

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heterostructures employed for preparing FETs. The interface charges that induced by piezoelectric and spontaneous polarizations were attributed to the remarkable high sheet density and substantial confinement effect at pseudo-morphically grown hetero-interfaces with either Ga polar or N-polar faces [128]. The heterostructures with Ga and N-face are shown in Fig. 10b. A typical heterostructure, similar to the Fig. 10a, having the thickness of 2000 nm which is GaN buffer film without doping subsequently 25 nm thick n-type GaN was deposited over a 25 nm Al0.1 G0.9 N which is not shown in Fig. 10. This structure was grown on c-Al2 O3 substrate by metal organic chemical vapor deposition (MOCVD). Figure 10c shows Hall carrier mobilities plotted versus temperature from 80 K to RT where the open circles accompanied by those for a 4000 nm n-doped GaN thin film deposited on Al2 O3 substrates with similar growth condisitons which were employed for GaN thin film in this system. Figure 10d a typical photoluminescence obtained for an AlGaN (30 nm) grown over GaN (1000 nm) system excited with an excimer laser (at 193 nm) [129]. The optical properties of group-III nitride semiconductors are further discussed in the next section. The mobility of GaN raises from 450 cm2 /V s at RT to 1200 cm2 /V s at 150 K which diminishes at lower temperatures owing to

Fig. 10 a Shows schematic representation of AlGaN/GaN polar hetero-structures prepared by MOCVD and MBE on c-plane Sapphire substrates. b Depicts a wurtzite GaN having Ga and Nfaces [128]. c Shows electron mobility with temperature for a typical GaN/AlGaN layers. The open circles depict experimental observations for the sheet and bulk densities. The line represents the 2DEG exhibiting electron density of 0.5 × 1018 /cm3 , and ionized impurity concentration N T = 6.5 × 1016 cm−3 . d A typical photoluminescence obtained for an AlGaN (30 nm) grown over GaN (1000 nm) system excited with an excimer laser at wavelength of 193 nm [129] (Adapted from Ref [129])

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scattering associated with the impurity states. Instead, the carrier mobility raises up to 5000 cm2 /V s at 150 K and stays more or less same up to 80 K. Thus, the enhancement of carrier mobility resulted from the 2DEG formation at the heterointerfaces. This improvement in the 2DEG electron properties is due to increased carrier density in contrast to the bulk material, consequently showed an effective carrier screening and higher Fermi energy [129].

7.4 Optical and Optoelectronic Properties The optical properties of a semiconducting materials exhibiting direct bandgap play crucial role in the contemporary optoelectronic devices. The optical transitions in high quality single crystalline defect-free intrinsic semiconductors, happen at conduction band and valence band, involving excitonic states from the Coulomb effect. The excitons involved in such interactions can be free and bound type excitons. In the case of high-quality semiconducting thin films having low defect densities, the free excitons can also include the transitions to the excited states as well as their ground states, whereas bound excitons are associated with the dopants or unintentional defect-levels, resulting in a discontinuous level under the gap states, thereby affecting the optical character of the excitons in the photon emission and absorption. The excitons can be associated with either neutral or charged donors and acceptors. Several characterization techniques have been employed to examine the optical properties of group III nitrides. Optical absorption, transmission, photo-reflection, photoluminescence, time-resolved photoluminescence, cathodeluminescence, calorimetric, pump-probe spectroscopy techniques have been widely utilized to study the optical properties of group-III nitride semiconductors. The photoluminescence properties of binary III-nitrides are discussed below. Neuschl et al. reported optical properties of MOVPE grown undoped AlN epitaxial layer with dislocation density less than 104 cm−2 . The spectrum exhibits several bound exciton lines (Do X) in addition to the free exciton emission (XA) and an excited state. Perhaps the emission resulted from the excitons that bound to Si (Sio X) is found to be 28.5 meV lower than the XA emission[150]. Kuokstis et al. reported the PL spectrum of C-plane single crystalline AlN at different temperatures exhibited a free-exciton line, and bound exciton where the authors also reported temperature dependence of the exciton line [151]. Viswanth et al. reported the photoluminescence spectrum of a GaN epitaxial layer grown on c-plane sapphire substrate at 12 K. FX (A): free-exciton A; FX (B): free- exciton B; DX: donor bound exciton; AX: acceptor bound exciton; FX (A)–LO: phonon-assisted free-exciton transitions; and DX–LO: phonon assisted donor bound exciton transition. The discontinuous lines represent a Lorentzian fit to the peaks. A He–Cd laser is used for excitation[152]. Figure 11a shows Low-temperature cathodoluminescence (CL), optical reflectivity (OR) and transmission (trans.) spectra of AlN bulk crystals in the near-band-edge energy range. FXA indicates the position of the free exciton-A. The CL spectrum was measured using 5 keV e-beam energy and 5 mA current. This spectrum showed five

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Fig. 11 a Low-temperature cathodoluminescence (CL), optical reflectivity (OR) and transmission data for AlN bulk materials close to the near-band-edge. FXA indicates the position of the free exciton-A [153]. b Photoluminescence spectrum of a GaN layer grown by MBE [154]. c Experimental PL spectrum of InN with lowest carrier density and the inset shows the recombination ways of A1 and A2 by respectively a shallow and deeper acceptor states [131]. Adapted from Ref [153, 154]

different transitions, which have been tentatively assigned to free and bound exciton recombination. The A-exciton showed at approximately 6.03 eV as a shoulder of 6.010 eV attributed to a neutral donor- bound-exciton recombination. Reflectance measurements well corroborated with A-exciton, as depicted in the OR spectrum in Fig. 11a [153]. Figure 11b shows a PL spectrum of an undoped 1.5 mm-thick GaN layer was grown by molecular beam epitaxy (MBE) on a thick GaN. The 10 × 10 mm template was grown by hydride vapor-phase epitaxy (HVPE) on a c-Al2 O3 subsequently by a lift-off process. The spectrum from area A is very identical to a typical spectrum from high-quality GaN. The full-width at half-maximum of the strongest peak at 3.472 eV, classified as a neutral donor bound exciton (DBE), is 1.5 meV. At higher photon energy, the free exciton (FE) peak is visible. At lower energies the authors identified two-electron satellite peaks (DBE2e), LO phonon replicas of the major exciton lines, and the shallow donor–acceptor pair (DAP) band. In the PL spectrum from the area B, the same lines are present which are broadened, and two characteristic PL lines: the Y4 line at 3.35 eV and the Y7 line at 3.21 eV followed by two LO phonon replicas, whereas the PL intensity in the area B is much weaker as compared to area A. Monemar reviewed electronic structure of bound excitons in GaN. Figure 11b describes the reported photoluminescence spectrum of HVPE-grown undoped 80 μm thick GaN layer, having the two dominant donor bound excitons, and the most common acceptor bound exciton at lower energy. The spectra are shifted up in energy by about 6 meV compared to those of unstrained GaN, due to a residual biaxial strain field in the layer [130]. The band gap of InN was initially believed to be around 1.8 eV for sputter grown films, later due to advancement in the growth techniques such as MBE, MOVPE and MOCVD, researchers achieved good quality InN thin films with low bandgap of around 0.7 eV [116]. Figure 11c is an experimental PL spectrum of InN with lowest carrier density of 7.7 × 1017 cm−3 . The inset depicts the recombination of A1 and A2 through a shallow and deeper acceptor states, respectively, which

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were employed to describe the results. The position of Eg, along with EA1 and EA2 band edge states were also described [131]. The superior optical and electrical properties of group III-nitride compound semiconductors lead to the fabrication of advanced device structures. The tunable III nitride alloys having variable Indium composition, and thus band-gaps, made them potential candidates for state-of-the-art electronic and optoelectronic devices, that operate over a wide range of the electromagnetic spectrum. A higher energy-gap is favored for microwave transistor devices, while smaller energy gap is also required for InGaN full-spectrum solar cell. In 1962, the importance of Electronic Lighting Technology and communication was realized with the discovery of first light emitting diode (LED) [133], and consequently, light amplification by stimulated emission (LASER) using semiconductors [134, 135]. This led to drastic revolution in semiconductor technology, by the fabrication of bright light emitters employed in fiber based optical networks, data storage by CDs and in optical printers. Nevertheless, GaAs LEDs emits solely in the red to yellow wavelength range of the solar spectrum. Though, SiC or II-VI semiconductors were used in making LEDs, its indirect energy gap nature they do not emit with enough intensity. First time invented blue LEDs based on the group-III nitride semiconductors were commercialized by Nichia company in early 1994 and paved the path for high brightness optical devices. Since then, tremendous research efforts have been accomplished on group-III nitrides covering the ternary and quaternary alloys which are promising for the current and future generation LEDs and Lasers.

8 Light Emitting Diodes A Light Emitting Diode (LED) is PN diode which generates photons of a single wavelength by combining holes and electrons in a semiconductor. Typical LED structure is illustrated in Fig. 12. Irrespective of the many dislocations and other deects, IIInitrides endure as a remarkable material with several implications in high-efficiency optoelectronics devices, like LEDs, and laser diodes [90]. Group-III nitride semionductors prone to have defects that resulted from lattice mismatch with Sapphire, SiC and Si substrates. Ternary and quaternary alloys can provide direct energy gaps ranging from 0.63 eV to 6.2 eV that match with near infrared to deep ultraviolet wavelengths. The firstly inverted InGaN blue and green light emitting diode structure has a 30 Å layer Inx Ga1-x N (where x = 0.2) grown between p-AlGaN and n-GaN. This entire structure was grown over Al2 O3 substrate [136]. White LEDs of 30–40 lumens/watt efficacy were established by adding yellow phosphorus on LED structure that makes white light [137], which are not capable as the fluorescent bulbs which give 70–80 lumens/watt. Joining red, blue and green LEDs with same characteristics able to use in the full color displays and white light lamps. The red wavelength can be tuned by choosing an appropriate Indium composition. The International Business Machines Corporation showed that the nanowire LED, releasing

Novel Emerging Materials: Introduction and Evolution

27

Fig. 12 The device structure of GaN based light emitting diode (LED) (Adapted from [155])

infrared light, can be the future possibility of resource for optical-communication between devices fabricated on submicron chips, which can be useful in making fast computers. The achievement of tuning wavelengths in nanowires LEDs on the same substrate, would allow to fabricate LEDs at cheaper rates and possess greatly improved performance[138]. Nakamura et al. grew LED structure using two flow MOCVD method on c-plane sapphire. Fig a shows schematic of grown green LED structure at Nichia Chemical Industries. This LED consists of 30 nm GaN buffer layer, a 4000 nm thick GaN:Si as n-type material, a 100 nm thick n-AlGaN, 50 nm thick n-type InGaN, a 100 nm thick layer of AlGaN:Mg as p-type material and a 500 nm p-GaN. Ni/Au and Ti/Al metallization contacts were deposited on p- and n-type GaN layers, respectively. The active region of device is formed by InGaN sandwiched between n-InGaN and a pAlGaN layers. The indium composition of InGaN in the active region was tuned from 20 to 70% to obtain blue to yellow wavelengths. The electroluminescence spectra of such LEDs showed emission for blue (450 nm), green (525 nm) and yellow (590 nm) single quantum well LEDs with different Indium composition. The output power reduces from blue to yellow LEDs due to the reduction of crystal quality with increasing Indium content in the active layer. It saturates at higher currents because of generation of heat[139].

9 Laser Diodes A laser diode (LD) is a similar device as a light-emitting diode discussed previous section. In contrast to the LED, the laser diode is pumped with electrical current thereby creating lasing conditions at the active region of device junction. A typical

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Fig. 13 Schematic view of a blue laser diode (LD) structure grown a Si substrate. Adapted from [156]

laser diode structure is shown in Fig. 13 which is adapted from an article reported by Sun Y et al. [156]. Haase et al. demonstrated first Laser Diode (LD) by utilizing ZnSe based II-VI Zn-Se based materials [140]. Due to their short life time up to few hours prevented its commercialization. Later, Shuji Nakamura et al. achieved the shortest emission wavelength at 417 nm on their first time demonstrated InGaN multi quantum well LDs made from group III-V nitride compound materials [141]. The typical device structure of Nakamura’s InGaN MQW LD where GaN buffer layer grown on c-Sapphire then followed by /GaN/n-AlGaN/n-GaN/26 periods of InGaN MQW/p-AlGaN/p-GaN/p-AlGaN/p-GaN structure. Here n-AlGaN and pAlGaN with Al composition 0.15 were cladding layers for optical confinement. The area of LD stripe is 30 μm × 1500 μm. High reflection facets were served for the reduction of threshold current. The evaporation of Ni/Au and Ti/Al metal contacts were made on p- and n-GaN, respectively. The threshold current density was about 4000 A/cm2 . The optical emission spectra of such MQW LD. Above a threshold value of the current, a strong emission at 417 nm having FWHM of 1.6 nm was observed. The long-life time and high efficiency of group-III nitride high power LEDs and LDs made them commercially available at present. The development of LEDs allowed later to fabricate high efficient diode lasers that spans UV to green wavelengths. The main benefit of blue LDs having a stimulated emission at 405 nm over a red laser with emission at 605 nm is that it facilitated fivefold storage capacity. Subsequently Blue-ray disc technology permitted over traditional DVDs, by several consumers which altered high density recording and rewriting [142]. The commercialization of LEDs by the world leading markets such as blue-ray was the initial effort towards the advantages that possible by this technology. However, it is evident that the incorporation of group III elements in appropriate portions to prepare ternary and quaternary alloys enabling the diode lasers that stimulates photons ranging from deep UV to infrared wavelengths, stimulated emission was experimentally obtained in the range of wavelengths spanning from near ultraviolet to the green. Later further development of the devices was limited by the preparation of single-0crystalline defect free semiconducting compounds. The 1D structures happen to be an interesting approach owing to the easier growth and

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avoids defect formation, having better lasing performance. Hu et al. published an observation of stimulated emission from InN nanostructures prepared by MOCVD [143], which was treated as a promising achievement in the area of nanophotonics that can have huge impact on understanding basic sciences, and optical network communications.

10 Photodiodes A photodetector is an optoelectronic device permitting the detection of the photons by producing a current when light having energy higher than bandgap of the material are absorbed. Like other optoelectronic devices, the Photodetectors (PDs) from group-III nitrides utilize unique benefit of inbuilt spectral selectiveness for acquiring the detection edge from 6.2 eV to 0.63 eV. The other promising benefits like radiation hardness low noise and dark current as compared to narrow bandgap semiconducting materials for instance Si and Ge. Photodetection has several applications such as solar-blind detection, optical communications, photolithography, water purification, flame detection and missile plume detection [144, 145]. Photodetectors consists of three mechanisms, the creation of free carriers arising from the absorption of incident light, the propagation of photogenerated electrons and holes and thereby processing current. External quantum efficiency and responsivity are important parameters in quantifying photodiodes. There are several types of photodiodes like photoconductors, Schottky like metal–semiconductor junctions, p–i–n and metal–semiconductor– metal (MSM) photodiodes. A photodiode can be a PN diode or it can have mainly of three constituent layers: an intrinsic layer that can be grown between n and p type films, both can be doped to form a PIN diode. When the diode operates in the reverse bias conditions, the depletion regime forms in the undoped film and turns into an absorption unit. When the electrons and holes move across the depletion regime, the carriers will have the transit time and the capacity at the formed hetero junction. These parameters play a role in deciding the device performance and rely on the relative permittivity, the surface of over grown film and the thickness of the absorption layer. In order to overcome avoid the filters and achieve visible-blind operation, UV detectors associated with wide energy gap group-III nitride semiconductors were investigated. The advancement in the (In, Al, Ga) N semiconductors allow to study visible-blind UV detectors. The detection of UV, VIS and IR regimes can be achieved using AlGaN and (In, Ga) N alloys [146]. For instance, Carlos Rivera et al. reviewed and reported that the bandpass photo response of UV boundary and visible (In,Ga)N/GaN multi-quantum-well PIN photodetectors which were tuned by varying In composition of 10 to 25%. Higher In content of 25% exhibited wider responsivity [145]. Figure 14 is the spectral response of AlGaN MSM photodiodes operating at various bias voltages. The spectral responsivity remained constant above energy gap, having a cutoff wavelength moved to lower wavelengths as raising Al content whereas Photocurrent increases almost linearly as increasing the optical power [146, 147].

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Fig. 14 Spectral response of Al0.25 Ga0.75 N and GaN Metal–Semiconductor-Metal (MSM) photodiodes, measured at different bias [146, 147]. Adapted from Ref. [147]

11 Summary The rise of graphene, a perfect two-dimensional material with interesting linear energy–momentum dispersion relationship, obeying Dirac equation yielding novel physical and chemical properties have transpired the material research community to explore many other two-dimensional materials across the periodic table. The book chapter discussed the fundamentals of linear energy–momentum dispersion relationship and its implications on the electronic transport properties of the various Dirac material systems. The book chapter presented a unified description on various topological insulators with emphasis on surface vs bulk energy states. The effect of doping and electrostatic gating on topological insulators to disentangle the surface vs bulk states is discussed. The book chapter briefed the basics of zero-gap and narrowgap semiconductors band crossings and their implications on electronic properties including high charge carrier mobility and unusually high magnetoresistance that can be used for functional device and sensor applications. The other part of the chapter briefly outlines the fundamental properties of III-V nitride semiconducting materials. Remarkable advancement has been evolved in the last few decades in the synthesis, doping and device fabrication of this materials system. The most significant property is engineering its direct bandgap of alloys which spans over wide range of solar spectrum from deep UV to near infrared wavelengths. This allowed researchers to achieve high performance light emitters such as LEDs and Laser diodes. The other property is the polar nature of wurtzite crystal structure which makes them to be used in high electron mobility transistors due to the 2DEG formation at the AlGaN/GaN interfaces. Moreover, the mechanical and thermal properties of III–V Nitrides motivated the III-nitride research community to use their electronic and optoelectronics devices in high temperature and high harsh environments. This chapter provides the importance of III-V nitride semiconductors for the state-of-the-art electronics and optoelectronics applications.

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Synthesis and Characterization of Emerging Nanomaterials Chidurala Shilpa Chakra, Velpula Divya, Konda Shireesha, Sakaray Madhuri, Thida Rakesh Kumar, Adapa Uday Krishna, and Deshmukh Rakesh

Abstract In recent scientific technologies, the study of Emerging materials has been given a major priority due to its novelty and potentials in various sectors including Energy storage and conversion, Opto-electronics, Nano-devices, Biosensors, Aerospace, Biomedical and Agriculture. Among different Emerging materials, nanomaterials have been studied extensively due to their extraordinary properties and enhance potential abilities in various applications. However, with the advance of research towards traditional methods in materials design and characterization are lagging behind. Fortunately, some new and advanced technologies viz. Electrodeposition, Electrospinning, Emulsion, Hydrothermal, Liquid/Liquid interfacial, Microwave, Polyol and Sonochemical etc. are now emerging in the design or fabrication of nanomaterials to produce unique properties such as controlled shape and size, less agglomeration, high active surface area and stability. Hence these advanced properties of nanomaterials provide enhanced efficiency in wide range of applications such as supercapacitors, lithium-ion batteries, biosensors, preventive medicine, electrocatalysis and fuel cell. Enormous efforts have been made by various scientific groups in the worldwide to provide advanced technologies in the synthesis of nanomaterials and high precision material characterization techniques like X-Ray Diffraction, Chronoamperometry, Cyclic Voltammetry, Scanning Electron Microscopy, Transmission Electron Microscopy etc. that reveals various insights C. Shilpa Chakra (B) · V. Divya (B) · K. Shireesha · S. Madhuri · T. Rakesh Kumar · A. Uday Krishna · D. Rakesh Center for Nano Science and Technology, JNTUH Institute of Science and Technology, Hyderabad 500085, India e-mail: [email protected] V. Divya e-mail: [email protected] K. Shireesha e-mail: [email protected] S. Madhuri e-mail: [email protected] T. Rakesh Kumar e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_2

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and provide emerging applications in diverse fields. The purpose of this book chapter is to address high quality, latest scientific insights of state-of-the-art advances on the design, characterizations and applications of various emerging nanomaterials. Keywords Emerging nanomaterials · Liquid/Liquid interfaces · Electrodeposition · Cyclic voltammetry · Chronoamperometry · Scanning electron microscopy · Transmission electron microscopy · Nano-opto electronics · Energy applications · Environmental applications

1 Introduction The nanotechnology branch has experienced tremendous growth during the previous century. Moreover, nanotechnology is involved in various research studies, either directly or indirectly. Nanotechnology is defined as the process of synthesizing, characterizing, and applying materials and devices by altering their size and shape on a nanoscale. The prefix “nano” is used as a keyword in every stream, including commercial advertising. The term “nano” comes from the Greek word “Nanos” or the Latin word “Nanus”, which means “dwarf.” It combines physics, chemistry, materials science, solid-state physics, and biosciences. As a result, knowledge of a single field will not be sufficient; the combined knowledge of material science, physics, solid-state, chemistry, and biosciences will be necessary. Nanotechnology applications are now evident in almost every field of science and technology [1]. The British Standards Institution has recently recommended the following definitions for commonly used scientific terms: • Nanoscale: A size range of around 1–1000 nm. • Nanomaterial: A material that has any internal or exterior nanoscale structures. • Nanoparticle: A nano-object with three tiny dimensions on the outside. When the longest and shortest axis lengths of a nano-object differ, the words nanorod or nanoplate are used instead of nanoparticle (NP). • Nano-object: A material with one or more nanoscale dimensions on the periphery. • Nanofiber: A nanomaterial is called nanofiber if it has two comparable external nanoscale dimensions and a third bigger dimension. • The aspect ratio of a nanosphere is 1.21. (Aspect ratio: It is defined as the ratio of the length of the major axis to the width of the minor axis). • Nanorod: When the lengths of the shortest and longest axes vary. Nanorods have a width of 1–100 nm and a larger than 1:1 aspect ratio. • Nanowire: Nanorods with a greater aspect ratio than nanorods. • Nanocomposite: A multiphase structure containing at least one nanoscale phase. • Nanostructure: In the nanoscale realm, a structure is made up of linked constituent pieces. • Nanostructured materials: Materials with nanostructures on the inside or the surface.

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• Nanotechnology: Using scientific knowledge from a variety of industrial and healthcare applications, manipulate and control matter on the nanoscale dimension. • Nanoscience: The science and study of matter at the nanoscale that focuses on understanding size and structure-dependent characteristics, as well as comparing the emergence of individual atoms or molecules or bulk material-related changes [2]. • Engineered nanomaterials: Engineered nanomaterials are intentionally created materials with one or more dimensions of less than 100 nm.

1.1 Introduction to Nanomaterial Nanomaterials are cornerstones to both nanoscience and nanotechnology. Nanotechnology is a diverse subject of study and development that has attracted a lot of attention in recent years all around the world. It has the potential to change the way materials and products are made, as well as the sorts of functionality available. It already has a large commercial impact, which will undoubtedly increase in the future. Nanomaterials are intriguing because, at such a tiny size, they display unique electrical, optical, magnetic, and other characteristics. These new characteristics might have far-reaching implications in fields such as electronics, health, and others [3].

1.2 Classification of Nanomaterial Nanoparticles are defined as a cluster of atoms and molecules with a size of 1– 100 nm. They can be made up of one or more atoms or molecules, and they can have a diversity of size-dependent features. Nanoparticles bridge the gap between small molecules and bulk materials within this size range in energy states [4]. Nanoparticles are mainly classified based on their Dimensionality.

1.2.1

Classification of Nanomaterials on the Basis of Dimension

Nanomaterials come in various morphologies, such as nanorods, nanoparticles, and nanosheets, and can be classified according to their dimensionality. Nanoparticles are zero-dimensional nanomaterials, nanorods or nanotubes are one-dimensional nanomaterials, and films and layers of type one are two-dimensional nanomaterials. These are mainly used to classify single isolated nanomaterials. The physical properties of two or more particles will alter due to their interaction. These particles of different constituents are called bulk or three-dimensional nanomaterials [1]. They are majorly classified as follows based on nanoscale dimensions and materials.

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Table 1 Type of nanomaterials based on dimension Dimensionality

Type of nanomaterials

0-D

Nanoparticles, quantum dots

1-D

Nanorods, nanotubes, nanowires

2-D

Nanofilms, nanolayers

3-D or bulk nanomaterials

Dispersions of nanoparticles, bundles of nanowires and nanotubes as well as polycrystals in which the 0D, 1D and 2D structural elements are in close contact with each other and form interfaces

(1)

(2)

(3)

(4)

Zero-dimensional nanomaterials (0-D): These nanomaterials have all three dimensions in the nanoscale range. Nanoparticles will be included in this category. One-dimensional nanomaterial (1-D): In this case, any one dimension will be in the nanoscale range, and the remaining two dimensions will be beyond the nanoscale range. This class includes nanorods, nanotubes, and nanowires. Two-dimensional nanomaterials (2-D): Any two dimensions are in the nanoscale region, while the remaining one is not. Nanocoatings, Nanofilms, Nanolayers are examples among these. Three-dimensional or bulk nanomaterials (3-D): These nanomaterials are not in the nanoscale range in any dimension (Table 1).

They are >100 nm scale in three randomly chosen dimensions. Nanocomposites, core shells, multi nanolayers, nanowire bundles, and bundles of nanowires are among them.

1.2.2

Classification of Nanomaterials Based on Materials

Nanomaterials can be categorized as: (1) Carbon-Based Nanomaterials; (2) Inorganic-Based Nanomaterials; (3) Organic-Based Nanomaterials; and (4) Composite-Based Nanomaterials [5]. Carbon-Based Nanomaterials: These nanomaterials are carbon-based and come in various shapes and sizes. Hollow tubes or spheres, carbon nanofibers, Fullerenes, and graphene are examples of carbon-based nanomaterials. Chemical vapor deposition (CVD), arc discharge, and laser ablation are some of the processes utilized to make carbon-based nanomaterials. Inorganic-Based Nanomaterials: Metals and metal oxides are made up of these nanoparticles. Metals like Ag, Au, and Fe can be used to make them, while metal oxides like TiO2 , ZnO, and MnO2 can be used to make them. Silicon and ceramic materials are also used to make semiconductor nanoparticles. Organic-Based Nanomaterials: Organic material other than carbon and inorganic material makes up organic-based nanomaterials. Self-assembly or transformation of organic material into the desired structure is used to make these nanomaterials. In these materials, the noncovalent (weak) interaction is observed.

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Composite-Based Nanomaterials: Another layer of nanoparticles makes up composite nanomaterials. These nanomaterials are used with other nanoparticles, bulk materials, and more complicated materials such as metal frameworks. Metal, ceramic, organic, inorganic, carbon-based, or bulk polymers are some materials that can be used to make composites. Depending on the synthesis and desired qualities for the uses, these materials have a variety of morphologies.

1.3 Introduction to Characterization Techniques The surface-area-to-volume ratio of nanoparticles differs in orders of magnitude larger than that of macroscopic materials. Surfactant additives, reactant quantities, temperature at the time of synthesis will influence the size and structure of nanomaterials. The characterization of materials is proven to be significant in developing repeatable nanomaterial fabrication that provides information about the composition, structure, chemical/electrical/physical/magnetic attributes. Many techniques exist to characterize nanomaterials; however, each methodology has some level of uncertainty. Here are some characterization techniques which are extremely useful to characterize novel emerging materials. (a)

X-ray diffraction Spectroscopy

X-ray diffraction (XRD) is a well-known technique for creating high-resolution atomic pictures. The constraints have been discovered, including the single confirmation condition when the material is accessible and the low intensity compared to electron diffraction. The data includes things like crystal structure variations, phase quantification and identification, crystallite shape and size, lattice distortion, size, and periodicity of noncrystalline and orientation, and so on [6]. The crystal to be measured is placed on the goniometer and bombarded with slow-rotating X-rays. When crystals are too small, they may have poor resolution or precision. Electromagnetic energy produces X-rays, and crystals are assumed to be atom arrays. A regular array of caterers generates spherical waves of regular arrays. Bragg’s law states that the waves are constructive. Debye-Scherer’s equation was used to calculate the average crystallite sizes of as-synthesised composites (1). D=

0.9λ β cos θ

(1)

where D (nm) represents the crystallite size, K (0.9), λ (0.15406 nm) denotes the Kα component of wavelength of Cu radiation source, β reflects the full width at half maximum of individual observed peak and θ is Bragg’s angle.

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Transmission Electron Microscopy (TEM)

A microscopy method takes advantage of the interaction between a thin sample and a uniform current density electron beam (i.e., the energies are usually between 60 and 150 keV). When the electron beam stricks the sample, some electrons are transferred, while the rest are scattered elastically or in-elastically. The size, density, and elemental content of the sample all influence the amplitude of the interaction. The information gathered from the sent electrons creates the final image. The unique range of physical properties of Nanoparticles, such as optical, magnetic, electrical, and catalytic, and their interaction with biological systems, are defined by size and morphology. TEM provides direct images of the nanomaterial that also give information about nanoparticle homogeneity [7]. (c)

Mass spectrometry analysis

The use of mass spectrometry (MS) as a powerful technique for the accurate analytical characterization of Nanoparticles has piqued interest. MS provides essential molecular and elemental information on the composition and structure of Nanoparticles, as well as their bio-conjugation to target bio-molecules. It can also be used to determine the amount of bio-conjugation. Apart from being a susceptible method, MS is suitable for any sample. Furthermore, it is simple to combine with separation techniques to obtain real-time data. In this way, new and varied insights into the nature of Nanoparticles and their ultimate uses and applications may be gained. Furthermore, with the mass spectrometry technique, the sample must be ionized and then sorted in magnetic and electric fields based on the mass to charge ratio. Ablation with a high-intensity laser can aid the desorption and ionization process [7]. (d)

Thermogravimetric Analysis

The mass and composition of the stabilizers can be determined using TGA. A nanomaterial sample is heated in this approach, and components with varying temperatures disintegrate and vaporize, leaving a mass change. The TGA instrument records the temperature and mass loss, and the type and quantity of Nanoparticle organic ligands are determined based on the initial sample mass [7]. Apart from having the sample in a dry state, the advantages of TGA include that it is a straightforward approach that requires no special sample preparation. A disadvantage of traditional TGA is that it requires only a few milligrams of nanomaterial sample, increasing the cost or complicating lab-scale production. (e)

Brunauer–Emmett–Teller (BET) technique

Characterization of nanoscale materials is also done using the Brunauer–Emmett– Teller (BET) technique. It was named after the developers’ surnames, Brunauer, Emmett, and Teller, and is based on the physical adsorption on a solid surface. It is extensively used to determine the surface area of nanoparticles since it is a relatively accurate, rapid, and simple method [7]. The particular surface area of a material or product is one of the first indicators of whether it contains Nano and

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qualifies as a nanomaterial. Because nanoparticles are so small, their surface-tovolume ratio is extremely high. As a result, nanoparticles have a much higher surface area per unit mass than micrometer-sized particles. The BET analysis, which uses physical gas adsorption of nitrogen gas at an analysis temperature of 77 K, can accurately measure and quantify the specific surface area of porous, non-porous, and nanoparticle materials. The BET surface area is typically referred to as the specific surface area obtained by this analysis, and the BET specific surface area is represented in m2 /g. (f)

Raman spectroscopy

It is a technique for observing low-frequency vibrational, rotational, and other modes. It is a method for obtaining information on the crystalline structure of macromolecules and changes in polymeric structure in membranes that are used in conjunction with infrared spectroscopy. In a nutshell, this method relies on the inelastic Raman scattering of monochromatic laser light in the visible to near-ultraviolet region. When laser light interacts with molecular vibrations, phonons, or other excitations, the energy of the laser photons changes, and the energy shift provides information on the vibrational modes. Raman spectroscopy has long been utilized in material sciences and chemistry to produce a fingerprint that can directly identify and interpret distinct compounds [8]. The light source, monochromator, sample holder, and detector are essential components of a Raman spectrometer. The Raman scattered light is split into Raman spectra according to wavelength. High signal-to-noise ratio, instrument stability, and appropriate resolution are the factors that influence the quality of Raman spectra [8]. (g)

Fluorescence spectroscopy

Fluorescence spectroscopy is electromagnetic spectroscopy that uses fluorescence to identify components. It is an essential analytical tool for both quantitative and qualitative research. Two-photon emission processes like fluorescence and phosphorescence occur during molecular relaxation from an electronically excited state. Polyatomic fluorescent molecules move between vibrational and electronic states during the photonic process. The molecules are energized and subsequently relaxed to the excited state’s vibrational level. It’s employed in DNA sequencing, forensics, genetic analysis, environmental, industrial, and medical diagnostics, and it happens in femtoseconds to picoseconds [9, 10]. (h)

Scanning electron microscopy (SEM)

SEM is a flexible technology for analyzing micro-and nanostructures with a wide range of applications. The SEM approach offers information on surface composition, topography, and crystallographic data. Adjustable magnifications from reading glass magnification and the imposition of structure viewing are two of the benefits of SEM. It extracts data from signals generated by specific interactions with material compositions. SEM is a surface-imaging technique in which an electron beam examines the sample surface. It reflects topographic detail and atomic composition by using

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sample contacts to generate signals. Before a sample is put, an SEM is utilized to inspect the surface. A FESEM is a device that can visualize very minute topography characteristics on the surface of whole or fractioned objects. This approach is used by biologists, chemists, and physicists to detect structures as small as 1 nm (billionth of a millimeter). The FESEM can be used to investigate organelles and DNA material in cells and synthetic polymers and microchip coatings [11, 12]. (i)

Energy Dispersive X-Ray Spectroscopy

Energy-dispersive X-ray spectroscopy (also known as EDS, EDX, or EDXA) is a helpful tool for determining the elemental composition. The key operational principle that permits EDS to work is the ability of high-energy electromagnetic radiation (X-rays) to eject ‘core’ electrons (electrons not in the outermost shell) from an atom. Moseley’s Law established a direct relationship between the frequency of light released and the atom’s atomic number. EDS comprises three main components: an emitter, a collector, and an analyzer. These components are also commonly seen on an electron microscope, such as an SEM or TEM. Combining these three elements allows for the study of the number of X-rays radiated and their energy (in comparison to the power of the initial X-rays emitted). The EDS data is plotted as a graph, with the x-axis representing KeV and the y-axis representing peak intensity. The peak location on the x-axis are converted into the atoms that the energy changes represent by a computer program [13, 14]. (j)

UV–Visible spectroscopy

UV–Vis spectroscopy is a method of determining how many unique wavelengths of UV or visible light are absorbed or transmitted by comparing a sample to a reference or blank sample. The sample composition influences this property, giving information about the sample’s contents and concentration. The molecule absorbs light from the UV range. The electrons in the ground state are excited to higher energy states by UV radiation. The UV light that is absorbed is equal to the difference between the ground state and the higher state. The visible zone of electromagnetic radiation is between 400 and 700 nm, while the UV region is between 200 and 400 nm. UV spectroscopy uses the Beer-Lambert law. The Beer-Lambert law states that the more molecules capable of absorbing light of a specific wavelength, the higher the degree of light absorption. UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds, and certain biological macromolecules. (k)

FT-IR spectroscopy

The vibrations of atoms in a molecule or crystal lattice are excited when infrared radiation is absorbed, resulting in spectra bands generally expressed in terms of the unit wave number in cm–1 . The FTIR spectra are combined with KBr, an IR transparent material, and crushed into a clear pellet for analysis. Thermal treatment is often used because it eliminates weakly physisorbed and chemisorbed components from the sample’s surface and is temperature dependant. FTIR spectroscopy is the

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most effective instrument for analyzing the bonding of generated nanocomposite materials. FT-IR spectroscopy is used to analyze the existence of functional groups by determining the vibrational frequencies of chemical bonds. The vibrational excitation energy of molecules in the range of 1013 –1014 Hz corresponds to IR radiation. IR spectroscopy can detect qualitative and quantitative vibrational changes of functional groups on the nanoparticle surface. It is a noninvasive and nondestructive approach, but it has a high signal-to-noise ratio, making it challenging to work with samples with low transmission and weak spectra [15, 16]. (l)

Cyclic Voltammetry (CV)

In many areas of chemistry, CV has become a well-known and widely used electroanalytical technique. It is commonly used to investigate redox processes, analyze electrochemical reactions between ions and electrode surface atoms, that give information about stability of reaction products, and qualitative properties of charge transfer reactions between the ions (electrolyte) and electrons (electrode surface). As a result, it is a potent tool for studying the redox reaction, which is essential for understanding the charge storage process in supercapacitors. CV is a three-electrode electrochemical cell that measures current by varying the applied voltage at a working electrode in both directions, i.e., forward and reverse (at a specific scan rate). The current that passes between the working and counter electrodes can be monitored with respect to the potential (measured between work and reference electrode). CV is a potentiodynamic electrochemical measurement in which the working electrode’s potential is ramped in the opposite direction to return to its initial value. These cycles can be performed over and over again. The depiction of current versus applied voltage is known as a cyclic voltammogram [17]. (m)

Chronoamperometry

Chronoamperometry is a time-dependent technique that employs a square-wave potential on the working electrode. The current of the electrode measured as a function with time and is changes according to the diffusion of an analyte toward the surface of the working electrode. The following processes govern the detection or occurrence of current change in electrochemical experiments: mass transfer (the transfer of analyte from solution to a solution-electrode interface) and charge transfer (electron transfer in the working electrode surface). The occurrence of reactions at the surface such as protonation, adsorption, desorption, polymerization, and Nucleation, is studied using this technique which makes the understanding of the synthesis, characterization and applications of emerging nanomaterials. (n)

Pump probe spectroscopy

Many reactions such as atomic motion, molecular vibrations, photon absorption and emission, and numerous scattering events occur briefly. These processes can occur in as little as a few picoseconds or femtoseconds. Observing these fast events is crucial to properly comprehend the dynamics of various excitations in the matter, which has fueled significant advances in the efficiency of time-resolved measurements over the

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last few years. To quantify ultrafast processes accurately, timing uncertainty must be less than the process’s time scale, requiring a temporal resolution of 10–15 s. The approach, known as pump-probe spectroscopy, involves indirectly detecting the status of a process by observing a “probe” laser pulse. Pump-probe spectroscopy has been a popular tool for time-resolved research, resulting in breakthroughs in various domains. In Pump-probe spectroscopy, an ultrashort laser pulse is separated into two sections: pump (a stronger beam used to excite the material and create a nonequilibrium state) and probe (a weak beam used to measure the pump-induced changes (reflectivity or transmission) in the sample. Each of the two pulses reaches the sample on a distinct path, which the investigator determines through mirrors. It is important to note that pump-probe spectroscopy is a method for studying dynamical processes, which necessitates perturbing the physical system from its equilibrium state. This is accomplished by the pump pulse hitting the analyte quickly as it travels a shorter path. The pump pulse interacts with the analytical sample and can be modified in various ways (including energy, intensity, polarization) to excite the sample in a preferred manner. Consider the electron energy levels in an atom to illustrate this process. Suppose the energy difference between two levels equals the pump pulse energy. In that case, the particles in the sample will absorb photons from the pump pulse, causing an increase in the population of electrons in the higher energy state. The probe pulse reaches the sample after the pump pulse has disrupted the sample. The path length difference between the pump and probe pulses can be adjusted to control the delay between the pump and probe pulses and can be possible by optical mirrors. After the probe pulse interacts with the sample, it is monitored with a detector. The population dynamics of the electron energy levels can thus be described by monitoring the intensity modulation of the probe pulse as a function of delay [18]. By using this method, temporal resolution is no longer restricted by the detector. Figure 1 describes the working principle of Pump-probe spectroscopy, while the SEM image depicts the presence of Au Np which is fabricated using electron beam lithography onto the SiO2 substrates which is coated with ITO thin films. While the Au Np

Probe beam

Ext.

Sample

800

Wave length (nm)

Fig. 1 Working principle of pump-probe spectroscopy

1300

Intensity (a.u.)

Pump beam

Out put

Laser

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spectra denotes the extinction spectra for the Au Np along with spectrum for the pump probe (red curve) [19]. (o)

Angle-Resolved Photo Emission Spectroscopy

Angle-Resolved Photo Emission Spectroscopy (ARPES) enables the direct measurement of a material’s electronic band structure, yielding instrumental perspectives into its electronic properties. This technique necessitates using an instrument configuration that allows the investigation of photoemission intensity as a function of emission angle and can differentiate the kinetic energy of photoelectrons. The first ARPES experimental studies samples seemed to be a few millimeters wide, and photoelectrons were identified with a hemispherical analysis tool with a narrow acceptance angle and a simplified electron counting detector. The electron analyzer plays a critical role as it is the component that allows measuring photoelectron kinetic energy and generating an intensity versus kinetic energy spectrum using the simplest 1D detector (Fig. 2). Electron analyzers were used in the experimental investigations to select a slight solid angle of the entire photoelectron emission cloud, which correlates to a small fraction of the k-space. Figure 2a shows the illumination of the sample with a light source or discharge lamps over a relatively large and homogeneous surface. The photoelectrons are emitted from the center of the Brillouin zone as it is located at the sample average (Fig. 2b, e). To operate ARPES, the specimen had rotated in both polar and azimuthal orientations such that the electron analyzer could procure photoemission spectra from various angles (Fig. 2c). The material’s band structure will be obtained by combining different angles of photoemission signals. It was

Fig. 2 a Schematic representation of conventional ARPES, ϕ and θ represent the azimuthal and polar angles respectively; b 1D detector spectrum c series of 1D spectra from points  to M that represents in (e). d 2D detector acquisition. e The 1st Brillouin zone of TiSe2 with acquisition plane (yellow) and high symmetry points (, M and K) (Got copy right permission from [20])

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time-consuming to acquire a complete band structure of a sample with 1D detector mapping as it required complex sample movements. The 2D electron detector is introduced that promotes the decrease in complexity of sample rotations (Fig. 2d). Electron analyzers with multichannel plate 2D detectors can discriminate electron distribution in the analyzer plane, permitting the acquisition of a slice of the kspace (Fig. 2d, e), which is identical to a polar scan with a 1D detector (Fig. 2c). This technology advance significantly improved the quality of spectra and reduced acquisition times for conventional ARPES. The importance of this advancement can be seen by comparing Fig. 2c, d. A recent upgrade in ARPES is the modification of analyzer systems (using suitable photon source energy) that enables angular scans in two dimensions in k-space to be performed analysis without rotating or tilting the sample. The scanning of the k-space is done electronically with the help of dedicated deflectors. The shaded area in Fig. 2e represents an example of a k-space region that can be tested with these detectors [20].

2 Synthesis of Emerging Nanomaterials Synthesis protocol of nanomaterial is crucial for various emerging applications as the efficiency towards various activities mainly depends on the types of synthesis and reagents conditions used during the formation. Nanomaterials fabrication can be divided into two main categories, namely top-down and bottom-up techniques [21].

2.1 Top-Down and Bottom-Up (i)

Top-down Approaches:

To generate the requisite nano-structural designs, fabrication via etching away or pulverizing of bulk material is a key to the top-down method. This approach can be accomplished by lithographic, Ball milling, Etching, sputtering, laser ablation, Molecular beam epitaxy, Arc Discharge, Flam synthesis, Electro Explosion, Electrospinning procedures (Fig. 3). The Advantages and disadvantages of Top-down Approaches are as follows [22]. Advantages of Top-down Approaches: (a) (b) (c)

Techniques that are well-established and well-understood Provides control and accuracy if lithography is used to shape the materials Techniques produces repeatable results.

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Fig. 3 Schematic representation of nanomaterials synthesis by top-down and bottom-up approaches

Disadvantages of Top-down Approaches: (a) (b) (c)

Extremely sensitive to defects as features get tiny Narrower compliance as features grow smaller Costlier than self-assembly methods.

(ii)

Bottom-up Approaches:

The bottom-up technique includes building structures from atom scale or molecular levels to the desired size. Sono-chemical routes, chemical reduction method, photochemical process, electrochemical method, co-precipitation, emulsion synthesis, microwave-assisted synthesis, template-based synthesis, spray pyrolysis, and solvated metal atom dispersion, polymer-based synthesis techniques, solvothermal and hydrothermal routes, solution combustion synthesis can considered as bottom-up methods. Further, Sol–Gel, Electrodeposition, Reverse micelle, Self-assembly techniques, Biological methods—such as bacteria fungi are also used for nano-particle synthesis (Fig. 3). The use of magnetostatic bacteria is also well established for the natural synthesis of the magnetic nanoparticle. These methods help form covalent or supramolecular interactions between the molecules to obtain adequate structure [21]. Advantages of Top-down Approaches: (a) (b) (c)

Self-assembly is a low-cost, simple, and quick process for creating emerging nanomaterials. Offers supreme limits of miniaturization Choices available to the produce a wide range of nanostructured materials by these synthetic approaches

50

C. Shilpa Chakra et al.

Disadvantages of Top-down Approaches: (a)

At this moment, excellence of self-assembly is confined to fairly simple nanostructured materials, being not able to produce integrated devices.

The semiconductor industry has used top-down nanofabrication with apparent progress for several decades, with engineers and scientists exploiting successively tiny pieces of material using photolithography and other related techniques; however, the top-down approach is rapidly attaining its economic and physical boundaries due to power consumption and more maintenance charges. On the other hand, Bottom-up nanofabrication provides an exceptional limit of miniaturization, opens up practically infinite possibilities for the construction and design of nanomaterials, and can be much more cost-effective than top-down nanofabrication. This book chapter provided various synthetic approaches in bottom-up synthesis and a newly emerging technique that produces uniform nanofibers, i.e., electrospinning, in a detailed manner.

2.2 Emerging Nanomaterials Metal nanoparticles (gold, silver, platinum, palladium, Nickle, Iron, Zinc, Tallium, Titanium, etc.), metal oxides (NiO, ZnO, Mn2 O3 , TiO2 , CuO, Ag2 O, and RuO), Bimetals (Pt–Au, Pt–Ni, Pd–Ag, Pd–Au, Au–Ag, Au–Cu, etc.), Bimetallic oxides (NiMn@O, ZnMn@O, NiAn@O, NiMg@O, and ZnAg@O, etc.), Trimetals (Pt– Au–Ag, Pd–Pt–Ag, Pd–Ni–Ag, and Ag–Au–Pd, etc.), hydroxides (NaOH, MgOH, LiOH, CoOH, MnOH), Bimetallic hydroxides (NiMgOH, NiMnOH, NiCoOH, ZnMnOH), Carbon materials (CNT, Graphene, MXenes, Carbon nanofiber, Graphene oxide, and Reduced Graphene oxide, etc.), polymers (Polyaniline, Polypyrrole, polyindole, polythiophene, polyazulene, and polypyrene, etc.), polymer composites (Metal-polymer, Metal oxide-Polymer, Carbon-Polymer, and Carbon– metal oxide-polymer, etc.) have emerged as remarkable nanomaterials for various applications. The exceptional characteristics of these nanoparticles, i.e., high active surface area, conductivity, ease of fabrication, functionalization ability, thermal stability, etc., make them the best aspirant for applications in environmental, medicinal, optical, energy and manufacturing sectors. Table 2 provides examples of synthetic method, reagents, parameters, properties, and possible applications of emerging nanomaterials.

Microwave

Indole, HAuCl4 .3H2 O, dichloromethane, triple distilled water

Polyindole/Au nanocomposite

Ni(NO3 )2 , rGO

4-Methylbenzaldehyde, NaHCO3 , Na2 SO4

Metallopolymer films

Nickel oxide carbon nanocomposite

Aniline, tetrabutyl ammonium perchlorate, lithium perchlorate, tetradecylsulfate sodium salt dichloromethane

AgNO3 , chitosan, HNO3

Ag nanoparticles

Polyaniline

SnCl2

Tin-nanorods

Liquid/Liquid interfacial

NiSO4 .6H2 O, pyrrole monomer, H2 SO4

Ni-polypyrrole

Electrodeposition

Precursor/Source material/Oxidizing agent material

Target material

Method

NaOH

Indole

Dodecylaldehyde

Aniline, applied potential

Potential sweep

Constant potential

Constant potential

Reducing agent/ Fuel/Capping

H2 O

Dichloromethane, water

Dichloromethane, water

Dichloromethane, water

Water

HCl, HClO4 , CH3 COOH, HCOOH, H3 BO3

Water

Solvent

10 min

30, 60, 90 s. 24, 48 h

24 h

1h

–

30, 60, 120 s

60 s

Reaction time

Operating frequency of 2.45 GHz, power of 900 W, pH = 7

25 °C

25 °C

350 °C

–

–

–

−1.0 to 1.5 V

–

−0.55, − 0.60, −0.65 and −0.70 V –

–

−1.3 V

0.18, 0.14, 0.0 V

Calcination temperature

pH, temperature/ potential/ distance

17 nm

5 nm to 10 μm

–

50 nm

–

23 nm

–

Particle/ Crystallite size

16.7 nm

–

–

–

–

–

–

Porosity

Table 2 Emerging nanomaterials, synthetic components and conditions, characteristic properties and possible applications

Nano circular plate-like structures

Perennial flowers, Porous, fibres,

Sheet (film)

Nano fibers, nano spheres

Hexagonal, leaf, dendrites

Rods, rectangular, polygonal, grain-like,

Spherical

Morphology

Supercapacitors electrocatalysis sensors

Sensors

Drug delivery, water filtration membranes

Supercapacitors

Catalyst

Bio sensors

Electromagnetic shielding

Possible applications

(continued)

[29]

[28]

[27]

[26]

[25]

[24]

[23]

References

Synthesis and Characterization of Emerging Nanomaterials 51

Electrospinning

Sonochemical

Method

Nickel acetate and chromium acetate hydroxide

Polystyrene multi-walled carbon nanotube

Tin chloride anhydrous, formic acid and acetic acid

LiCoO powders and artificial graphite powers, LiPF6

N-doped metal-free Li–O2

Carbon nanofibers @SnO2

LiCoO2 @g

Tungsten disulfide (WS2 ), H2 O2

WO3 flower-like spheres

NiCr-CNT@C

Thallium(I) nitrate, lithium iodide, cadmium nitrate

Tl4 CdI6 nanostructures

Cobalt acetate tetrahydrate, sodium selenite

Cobalt selenide nanosheets

KMnO4 , MnCl2 .4H2 O and Sr(NO3 )2

Ammonium meta vanadate, nitric acid

Vanadium pentoxide nanostructures

SrMnO3 nanoparticles

Precursor/Source material/Oxidizing agent material

Target material

Table 2 (continued)

PAN

KOH

PAN

(Polyacrylonitrile) PAN

–

CTAB, SDS, sodium salicylate, EDTA, and (PVP-40000)

KOH

Hydrazinehydrate and ammonia

Moringaoleifera leaves

Reducing agent/ Fuel/Capping

LiCoO2 @g

Carbon nanofibers @SnO2

N-doped metal-free Li–O2

NiCr-CNT@C

HCl Aqueous solution

H2 O

H2 O

H2 O, ethanol

H2 O

Solvent

6–24 h

5–48 h

24 h

24 h

3.5 h

20 min

30 min

10 min

30 min

Reaction time

5h

12 h

6h

26 h

–

–

–

100 °C at high mode power of 700 W

Operating frequency of 2.45 GHz, power of 900 W, pH = 2&3

pH, temperature/ potential/ distance

120 °C

60 °C

280 °C

85 °C

350–500 °C

–

400 °C

80 °C

400 °C

Calcination temperature

–

2.1 nm

> 50 nm

200 nm

–

25–33 nm

41–56 nm

–

25 nm

Particle/ Crystallite size

67 nm

200–125 nm

–

5.3 nm

8.3 nm

190.55 nm

19.28 nm

6.4 nm

–

Porosity

Nanofibers

Core shell nanofibers

Nanofibers

Nanofibers

Spherical

–

–

Nano sheets

–

Morphology

Li-ion batteries

Bio mass based energy storage

Carbon based carriers

Electrochemical

Supercapacitors electrocatalysis sensors

Supercapacitors electrocatalysis sensors, fuel cells

Supercapacitors electrocatalysis sensors, water splitting

Supercapacitors electrocatalysis sensors

Supercapacitors electrocatalysis sensors, fuel cells

Possible applications

(continued)

[38]

[37]

[36]

[35]

[34]

[33]

[32]

[31]

[30]

References

52 C. Shilpa Chakra et al.

Polyol

C6 H8 O7

PVP, NaOH

TiOSO4

Ni(CH3 COO)2 , Mn(CH3 COO)2 , Co(CH3 COO)2 , LiOH

Cu(NO3 )2 •3H2 O

TiO2

LiNi0.4 Mn0.4 Co0.2 O2

Cu2 O

NaOH

NaOH

PVP

IrCl3 , CuCl2

BiCl3

Bismuth

NH4 OH

PVP

HNO3 , NH4 OH, (NH4 )2 SO4 , C2 H5 NS,

C6 H5 (NH4 )3 O7

C2 H3 NaO2 , CTAB

CTAB, docusate sodium salt, toluene, lithium iodide

CTAB, NaOH

CTAB, 1-Butanol 1-Hexanol,

Sodium hydroxide

Reducing agent/ Fuel/Capping

IrCu

AgNO3

FeCl3 , CoCl2

Magnetic cobalt ferrite

Ammonium heptamolybdate

MoO3

Silver

Al(NO3 )3

Al2 O3

Zinc nitrate

Zinc oxide

FeCl3

Neodymium III nitrate hexahydrate strontium nitrate, Cobalt(II) nitrate

Nd0.67 Sr0.33 CoO3 -δ (NSC)

Fe3 O4

Iron oxide

Superparamagnetic iron oxide nanoparticles

Hydrothermal

Native sago (Metroxylon sagu) starch powder

Starch nanoparticles

Microemulsion

Precursor/Source material/Oxidizing agent material

Target material

Method

Table 2 (continued)

EG

TrEG

TEG

EG

EG

EG, DEG, TrEG

EG

Water

Water

Ethylene glycol

Ethanol, acetone

DI water

Water

cyclohexane, acetone, ethanol and n-butanol and ultrapure water

Solvent

15 min

3h

3h

60 min

5h

3–24 h

30 min

6–24 h

5–48 h

24 h

1h

2h

–

1h

Reaction time

172 °C

230 °C

165 °C

190 °C

200 °C

pH: 8–12, 197 °C

100–150 °C

180 °C, pH-1

170–230 °C

200 °C

110 °C

70 °C

RT

80 °C

pH, temperature/ potential/ distance

–

850 °C

400 °C

–

250 °C

–

–

500 °C

550 °C

–

450 °C

800 °C

–

–

Calcination temperature

–

∼40 nm

12 nm

180 nm

–

–

–

–

1.4 μm

2.6 nm

–

–

13.7 nm

5.5 nm

40–100 nm

–

7.8 nm

18-42 nm

25 nm

100–600 nm

23 nm

20–50 nm

5.5 nm

–

0.3 μm 20–50 nm

–

Porosity

83 nm

Particle/ Crystallite size

–

–

–

Wire

Spherical

Spherical

Spherical

Nanorods

Spherical

Microsphere

Hexagonal plate

–

–

Distinct

Morphology

Eletcronic, magnetic and structural

Energy

Self-cleaning

Optical and electro-optical device

–

Biomedical

Biomedical

Gas sensing

Catalyst

Dye degradation

Semiconductors

Catalyst

Bio applications

Drug formulation, Packaging materials

Possible applications

[52]

[51]

[50]

[49]

[48]

[47]

[46]

[45]

[44]

[43]

[42]

[41]

[40]

[39]

References

Synthesis and Characterization of Emerging Nanomaterials 53

54

C. Shilpa Chakra et al.

3 Fabrication of Emerging Materials and Their Applications 3.1 Electrodeposition Techniques Synthesis of nanomaterials by electrodeposition is a embolden technique in recent years to attain grain size nanoparticles with right morphology and to improve surface characteristics of a wide varieties with simple controlled over potential, current, time, electrolyte concentration and other electrochemical parameters using inexpensive equipment i.e. electrochemical work station. The electrodeposition process customarily referred as three-electrode cell system that consists of (i) working electrode (metals, alloys, semiconductors, chemically modified electrodes, diverse forms of carbon etc.) on which the reaction (deposition) occurs; (ii) reference electrode (Saturated Calomel Electrode (SCE), Ag/AgCl/Cl− (aqueous), Hg/HgO, Ag/Ag+ (non aqueous) etc.) for ensuring a potential difference and (iii) counter or auxiliary electrode (Pt wire or Pt coil) for maintaining the current. The three electrodes are immersed into a conducting solution known as electrolyte (aqueous (KCl, KNO3 , K2 SO4 , H2 SO4 , KClO4 , LiClO4 ), Organic (Propylene Carbonate (PC), Ethylene Carbonate (EC), Acetronitrile (ACM), Tetrahydrofuran (THF)) in which the targeted analyte or precursor will be dissolved. By controlling the potential or current, deposition occurs from the monomer/metal ion in the electrolyte solution to polymer/metallic state at the conducting substrate. Most importantly, the nanomaterials are directly immobilized at the conducting substrate, providing further applications in manufacturing sectors (conducting cloths), Health care and bio technology (synthesis of nanomaterials, biochips, biosensors), Aerospace (Thin film coatings), Opto electronics (Surface enhanced Raman spectroscopy), Nano electronics (electrocatalysis, sensors, supercapacitors) etc. A variety of electrodeposition methods can be employed for the synthesis of metals or polymers based nanomaterials viz potentiodynamic method, galvanostatic method and potentiostatic methods. Potentiodynamic method: The deposition of nanoparticles is carried out using optimizing potential windows (forward and backward scan), number of cycles and scan rates. Galvanostatic method: The particles are deposited by applying constant current for optimum time durations. Potentiostatic methods: The deposition occurs by applying a single potential pulse or sequence of pulses to the solution containing monomer or metal precursor by optimizing time duration. Irrespective of the Electrodeposition method, the deposition of the material will occur only by the movement of molecules or ions from electrolyte to working electrode. The three essential modes of transports pertaining to any electrochemical studies are (i) diffusion–governed by Fick’s laws of diffusion, the driving force being the chemical potential gradient; (ii) migration described by Ohm’s law, the driving

Synthesis and Characterization of Emerging Nanomaterials

55

force being the potential gradient and (iii) convection represented by hydrodynamic equations, the driving force being the velocity.

3.1.1

Importance of Electrodeposition Method

In the synthesis of nanomaterials, wet-chemical methods are effective routes for controlling the morphology; however, the use of templates/surfactants introduces heterogeneous impurities and hence these methods are complicated in various applications due to the difficulty in the removal of the template/precursor. Electrodeposition is a simple facile strategy for the synthesis of various metals, polymers with well-defined nanostructures viz nanowires and nanorings, nanofibers, nanobelts, nanourchins, nanotriangles and nanoflowers etc., with high purity and uniform morphology. The electrodeposition method is energy efficient, cost effective and the materials can be achieved at room temperature. If stable electrolytes with long lives are used, materials utilization in electrodeposition procedures can be close to 100%. Electroplating allows for conformal coverage, grain size control, crystallinity (or lack thereof), and alloy mixing. This technique’s applicability is well-suited to the large-area photovoltaic sector, and it can help with the successful mass production of copper indium gallium selenide solar modules. The obvious advantages of Electrodeposition method is as follows: [53] a. b. c. d. e. f. g. h.

Rapid reaction rate Cost effectiveness Free from impurities Industrial transferability Production of shape controlled, free-standing structures with sizes ranging from nm to μm Binder free, stable nanostructures over the electrode Produce low-cost multilayer coatings on wide range of substrates High ability to produce and control compositions that are unattainable by other techniques.

3.1.2

Various Materials That Can Be Synthesized by Electrodeposition Method

Electrodeposition is a versatile technique for deposition of various materials such as metals metal oxides, bimetallic nano composites, Trimetallic composites, carbon composites, polymers and polymer composites etc. Some of the materials and their synthetic protocols are provided in the below Table 3. For clear understanding of method, characterizations, and application we provide one emerging material with detailed analysis i.e. Shape-Controlled Synthesis of Palladium Nanostructures from Flowers to Thorns: Electrocatalytic Oxidation of Ethanol.

Aniline

Na2 SeO3 , H2 SO4 3CdSO4 ·8H2 O

ZnCl2 , eosin Y KCl

Silver plating solution

Polyaniline

CdSe

ZnO and ZnO/EY hybrid films

Ag Film

Silver plating solution with an Ag concentration of 28.7 g/l

SDS

HCl/H2 SO4 , HNO3 /LiClO4 / KClO4 /HClO4

Pyrrole

Polypyrrole

Electrolyte

Precursors/ Source material

Target material

WE: 316L stainless steel plates CE: Pt mesh

WE: RDE, RE: Ag/AgCl CE: zinc wire

WE: n-Si 100 RE: Ag/AgCl CE: Pt wire

WE: GC RE: SCE CE: Pt wire

–

–

300 rpm

10 mV/s

−0.2 to −0.9 V –

–

–

–

Potential Scan rate/ window frequency

Working electrode – (WE): pencil graphite electrode (Faber Castell, 0.5 HB) Reference electrode (RE): saturated calomel electrode (SCE) Counter electrode (CE): graphite rod

Electrodes

SEM, XRD, TGA, photoluminescence analysis, XPS I-V Characteristics, Electrochemical Impedance, SEM, BET SEM, photoluminescence analysis, XPS

−0.91 V, ZnO (10 min) ZnO/EY (45 min) −1.7 V, 1 min

UV–Vis, FTIR, SEM, CV, impedance, rotating disk electrode voltammograms

FTIR, SEM, TEM, CV, impedance

–

0.8 V/100 s

(0.70, 0.75 and 0.80 V/ ranging from 5 to 180 s

Potential/ Characterizations current/time

[55]

[54]

References

Manufacturing sector

Manufacturing sector

(continued)

[58]

[57]

Optoelectronics [56]

Nano electronics

High power devices

Applications

Table 3 Synthesis of emerging nanomaterials by electrodeposition process, components and conditions, characteristic properties and possible applications

56 C. Shilpa Chakra et al.

NiSO4 .6H2 O, NiCl2 . 6H2 O,

CNT, NiCl2 ˙6H2 O, SeO2

K2 PdCl4

Ni coating

CNT@NiSe

Pd

WE: CNT/SS substrate RE: Ag/AgCl CE: Pt wire

WE: 316L stainless steel RE: Ag/AgCl CE: Pt mesh

WE: Cu plate RE: Ag/AgCl CE: Pt plate

WE: GC RE:SCE CE: Pt wire

WE: GC

Electrodes

KOH/KCl/KClO4 / WE: ITO KNO3 RE: Ag/AgCl CE: Pt coil

KCl

H3 BO3

H3 PO3 , H3 BO3 , H3 PO4

NiSO4 ·6H2 O, NiCl2 ·6H2 O,

NiO flms

Na2 SO4

PBS

CuCl2 CoCl

CuCo bimetallic dendrite

Electrolyte

PPy-RGO-AuNPs/GCE Pyrrole, GO, HAuCl4

Precursors/ Source material

Target material

Table 3 (continued)

20 mV s − −1 V/ 1 1200 s

0 to − 1.5 V

50 mV s − 0.482 V/ 1 0.382 V/ 0.300 V/ 0.282 V/ 0.182 V 180 s

−0.1 to 0.6 V

–

0.1 Hz

−0.7 and 0.2 V

3 A/60 min

–

–

–

25 mV/s

0 to − 1.5 V

High power devices

Agriculture

Aerospace

CV, High power chronoamperometry, devices, nano XRD, EDS, TEM, electronics SEM, impedance

XRD, SEM, TEM, EDS, XPS

Potentiodynamic polarization, XRD, SEM, EDS, XPS, impedance

XRD, FESEM, EDX, water contact angle measurements potentiodynamic polarization, Impedance

CV, SEM and amperometry

Biotechnology

Healthcare

CV, DPV

−0.8 V/ 600 s

–

Applications

Potential/ Characterizations current/time

–

Potential Scan rate/ window frequency

[64]

[63]

[62]

[61]

[60]

[59]

References

Synthesis and Characterization of Emerging Nanomaterials 57

58

3.1.3

C. Shilpa Chakra et al.

Synthesis of Shape-Controlled Palladium Nanostructures

The shape controlled syntheses of Pd nanostructures were carried out by chronoamperometry (Potentiostatic methods) as it is superior to other electrodeposition methods in production of diverse morphological patterns with least efforts. However, the nature of the supporting electrolyte, concentration of the precursor, choice of potential, and time of deposition play crucial roles in shape-controlled synthesis. The author studied the impact of various electrolytes (KOH, KCl, KClO4 , and KNO3 ) on the formation of Pd nano structures over the working electrode (Indium Tin Oxide). The KOH electrolyte produces irregular ice-cube-like morphologies, while KCl gives spherical Pd microparticles. Conversely, cotton-wool-like structures were notice with the use of KClO4 and the thorn-like morphology of pd were obtained with KNO3 electrolyte. Among various morphologies thorn-like structures are interesting as the growth and uniformity are higher. The concentration of the precursor was varied as 1, 10, 100 mM, while 10 mM is chosen as it is producing good particle distribution. The impact of potential on shape controlled synthesis was carried out by varying the potentials as 0.482, 0.382, 0.300, 0.282, and 0.182 V. The roseflower-like (PdRFL) structures are obtained at 0.482 V, whereas the deposition at 0.382 V yield splintery flower (Pd-SFL) morphology, 0.282 V produces chrysanthemum flowers (Pd-CFL) and 0.182 V gives thorns (Pd-THO) of Pd nano structures. The application of 0.30 V, an intermediate (Pd-INT) structure comprising both Pd-SFL and Pd-CFL is formed. The aforementioned observations reflect the key role of the applied potential in dictating the shape controlled Synthesis. The particular potentials were selected from cyclic voltammetry as peak potentials and before and after peak potentials (Fig. 4). (b )

a

(b)

(a)

b

-0.5

c

-1.0

-0.4 -0.4

-1.5 d

0.482 0.482 Pd-FL Pd-SFL 0.382 0.382 Pd-INT 0.182 Pd-CFL 0.182 0.300 0.3 0.282 0.282 Pd-THO

-0.8 -0.8

-2.0

-1.2 -1.2

-2.5

-1.6 -1.6 0.0 0.0

-3.0 e 0

1 µm

1 µm

0.0 0.0

Current Current / /mm AA

C u rre n t / m A

(a ) 0.0

0.3 0.3

(c) 1 µm

0.6 0.6

(e)

(d)

Potential //VV Potential

30

60

90

Time /sec

120

150

180

1 µm

1 µm

Fig. 4 A Chronoamperometric curves for the deposition of Pd on ITO at various potentials (a) 0.482 V; (b) 0.382 V; (c) 0.300 V; (d) 0.282 V and (e) 0.182 V while the inset depicts the cyclic voltammogram and the corresponding potentials (a) to (e) are marked with the colored arrows. B SEM images of (a) Pd-RFL, (b) Pd-SFL, (c) Pd-INT, (d) Pd-CFL and (e) Pd-THO obtained at different potentials, the arrows denoting the occurrence of both features of Pd-SFL and Pd-CFL in Pd-INT

Synthesis and Characterization of Emerging Nanomaterials

59

3.2 Liquid/Liquid Interfacial Method The interface between two immiscible liquids makes a special environment with dielectric, morphological, and chemical characteristics that differ significantly from the bulk liquids. Furthermore, in biology and chemistry, the reactions that occur at all these interfaces are critical. Mass transport at the interface between lipid bilayer membranes and aqueous bodily fluids, for example, is essential in a variety of biological activities. Because of the unknown contact architecture and electrostatic force across the contact, progress in this sector has been modest; yet, there is significant potential for the synthesis of a wide range of materials and technologies. L/L interfaces are used in chemical, biomedical, medicinal, and manufacturing uses to offer defect-free supports for nanomaterials and their assemblies.

3.2.1

Importance of Liquid/Liquid (L/L) Interfacial Method

L/L interfacial methods are used in various chemical, biomedical, medicinal, and manufacturing industries to offer defect-free supports for nanomaterials and their self-assemblies. The L/L interface is commonly used in the research of membranes, since it plays an important role in all organisms’ cells. L/L interfaces can also be used to create basic models of biological membranes. The fundamental processes like biocatalysis, charge transport, ion pumping, membrane fusion, and numerous photosynthetic processes take place at these interfaces. L/L interfaces offer enormous opportunity for theoretical research of various genres because to their anisotropy atmosphere, coupling of aqueous and organic solutions, physical partitioning of charges at the junction, and the consequences for reactions between species of differing solubility. These nanometer-thick surfaces of immiscible interfaces are beneficial for self-assembly processes and have applications in catalysis, sensors, microchannels, gene delivery, and other fields (Fig. 5). Drug activity, complexation catalysis, biphasic, and charged particle transfer processes can all be studied using L/L interfaces. Many industrial applications, such as solvent extraction, drug delivery, and electrical energy storage, rely on L/L interfaces. Theoretical and experimental scientists equally can benefit greatly from research on the L/L interface.

3.2.2

Various Materials That Can Be Synthesized by Liquid/Liquid (L/L) Interfacial Method

To synthesise nanomaterials at L/L interfaces, a variety of chemical and electrochemical approaches can be used, and each strategy has its own merits in terms of objectives. The features of interfaces in chemical synthesis vary depending on the species, their concentrations, the polarity of immiscible solvents, and the temperature range. Here the reaction can take place on a flat surface that separates immiscible liquids [65]. While, in electrochemical synthetic strategy, the properties of

60

C. Shilpa Chakra et al.

Fig. 5 Importance of liquid/liquid interfaces

the L/L interface are depended with the potential, current, time, method of deposition [66]. Here the reactions occurred at immiscible interface and at the working electrode. The Emerging materials such as metal nanoparticles, hydroxides, metal oxides, sulfides, chalcogenides, alloys, carbon nanotube-nanomaterial composites, polymer, graphene-polymer composite, metal-polymer composites have been synthesized using L/L interfacial methodology. Hence, the interface between two immiscible liquids offers enormous potential for the creation of diverse nanostructures, which can then lead to novel applications. Some of the Emerging materials and their characterizations along with applications are provided in Table 4. The detailed synthesis of one material from the above table is provided below.

3.2.3

Synthesis of Siver-Polyindole Composite

The L/L interface is formed by dissolving indole (0.029 g) in dichloromethane (5 mL) and then adding 0.3M Ag2 O (5 mL) containing 3.0 M HNO3 . Different colour changes at different time intervals can be detected as the reaction time increases (Fig. 6A) indicate the amount of the indole monomer’s interactions with the oxidising agent (Ag2 O). Initially, the oxidising agent produces oligomers, which cause a pink colour in the aqueous phase. As time passes, green compound formation at the interface denotes the increase of polymeric chain and is due to the formation of emulsions, which reduce interfacial tension. A Pt loop is used to collect composites at the L/L interface, which is then transported to Whatman filter paper. To eliminate soluble oxidising agents as well as unreacted components from the aqueous and organic phases, the composites is washed

HAuCl4 .3H2 O, n-hexane, HCl, thiophene, ultrapure Milli-Q water

Tetrahydrofuran, N4,N4,N4  ,N4  -tetraphenyl-[1,1 biphenyl]-4,4 -diamine, N-bromosuccinimide, Bis(pinacolato)diboron, Ir(cod)(OMe)]2,3,4ethylenedioxythiophene, dtbpy, tris(4-bromophenyl)amine, Pd(dppf)Cl2, n-BuLi, 2,7-dibromo9H-fluorene, tetrabutylammonium iodide

Indole, HAuCl4 . 3H2 O, Dichloromethane, Triple distilled water

Polythiophene/gold nanoparticles

Triphenylamine-based conjugated polymer films

Gold/polyindole nanocomposite

Indole

NaOH

Thiophen

Precursor/Source Reducing agent/ material/Oxidizing agent Fuel/Capping material

Target material

24 h

4.5, 8.5, 12.5, 16.5 or 20.0 h and pH 7.0

Reaction time/pH

Dichloromethane, water 24 h

Acetonitrile, dichloromethane, chloroform, hexane, ethyl acetate, xylene, ethanol, methylbenzene, water

Water, n-hexane

Solvents

Opto-electronics (organic photovoltaics cells)

Applications

SEM, XRD, TGA, FTIR, TEM, in situ optical, fluorescence and laser scanning microscopic studies, four-probe impedance spectroscopy

Interfacial behaviour study

Raman, FTIR, UV–Vis, Electrochromic CV, electrochromic applications stability, open-circuit memory,

XRD, FTIR, SEM, TEM, Raman spectra, UV–vis, thermogravimetric analyses (TGA), SEM, EDS, CV

Characterizations

(continued)

[69]

[68]

[67]

References

Table 4 Synthesis of emerging nanomaterials by liquid/liquid interfacial methodology, components and conditions, characteristic properties and possible applications

Synthesis and Characterization of Emerging Nanomaterials 61

Piperazine, trimesoyl chloride, polyethersulfone membrane

Ammonium persulfate, indole monomer, silver nitrate

Ferric chloride, pyrrole monomer, multi walled carbon nanotubes

Aniline, pyrrole, thiophene, indole, tetrabutyl ammonium perchlorate, lithium perchlorate, tetradecylsulfate sodium salt dichloromethane

1,3,5-triethynylbenzene (1) and 1,3,5- tris(4azidophenyl)benzene, copper(II) sulfate and sodium L-ascorbate, alkyne, azide

Catechol, dodecylaldehyde, 4-methylbenzaldehyde, heptaldehyde, iron(II) acetate, and copper(II) acetat, Nile red and brilliant blue,

Polyamide nanofiltration membrane

PIn, Ag-PIn

Polypyrrole-multi walled carbon nanotube composites

Polyaniline, polypyrrole, polyindole, polythiophene

Covalent two-dimensional polymer nanosheet

Functional catechol−metal polymers

Catechol, dodecylaldehyde

Sodium L-ascorbate

Aniline/pyrrole/ thiophene/indole

Pyrrole monomer

Indole monomer

Sodium dodecyl sulfate

Precursor/Source Reducing agent/ material/Oxidizing agent Fuel/Capping material

Target material

Table 4 (continued)

12 min

Reaction time/pH

24 h

Dichloromethane, chloroform, methanol, hexane, dimethylformamide, ethanol, toluene, acetonitrile, water

~24 h

Dichloromethane, water 48 h

Dichloromethane, water 1 h

Diethyl ether, water

Dichloromethane, water 72 h

Hexane, water

Solvents

Manufacturing sector

High power devices, nano electronics

Opto-electronics

High power devices, nano electronics

Manufacturing sector

Applications

1 H and 13 C NMR Heath and spectra, Mass biotechnology spectrometry, UV−vis–NIR Spectrophotometry, FE-SEM, FTIR, contact angle measurements

XPS, FTIR, optical microscopy, SEM, EDS, AFM, TGA

FESEM, CV, UV–Vis

XRD, FESEM, FTIR, UV–Vis, TEM, Z-scan, four-probe technique

XRD, XRF, Raman, UV–Vis, FTIR, SEM EDS, CV, Impedance spectroscopy

SEM, TEM, XPS, scanning probe microscope

Characterizations

(continued)

[27]

[73]

[26]

[72]

[71]

[70]

References

62 C. Shilpa Chakra et al.

Polystyrene-bpoly(butadiene)-bpoly(methylmethacrylate), PS-PBPMMA, triblock terpolymers,

HAuCl4 .3H2 O, Thiophene, CNT

Janus nanoparticles

PT/Au/CNT

Thiophene

Precursor/Source Reducing agent/ material/Oxidizing agent Fuel/Capping material

Target material

Table 4 (continued)

Water, n-hexane

Toluene, water

Solvents

20 h

–

Reaction time/pH

Applications

XRD, FTIR, SEM, TEM, Raman spectra, UV–vis, TGA, EDS, CV

Opto-electronics (organic photovoltaics cells)

FTIR, Inductively Manufacturing sector coupled plasma- optical emission spectrometry, TEM

Characterizations

[75]

[74]

References

Synthesis and Characterization of Emerging Nanomaterials 63

64

C. Shilpa Chakra et al.

a Water DCM

b

Ag2O + 2 H +

2 Ag0 + H 2O

Fig. 6 A the time-dependent growth and B a feasible mechanism of Ag–PIn composite formation [9]

with water and then with DCM. The hypothesised mechanism for the creation of nanocomposite is shown in Fig. 6B.

3.3 Microwave (MW) Assisted Synthesis Nanomaterials serves as the foundation for studying unique properties and applications of nanoscale materials, nanoparticle synthesis is one of the most important research topics in nanoscience and nanotechnology. As a result, this chapter examines some of the methods for producing nanoparticles using microwave radiation. The study of employing microwave irradiation to heat materials is known as microwave chemistry. It is based on a substance’s ability to absorb microwave radiation and convert it to heat (e.g., solvents and/or reagents). When using a mass-production process that produces nanoparticles with a tiny particle size distribution, improving nanoparticle functions is not expensive. Microwave heating has been employed in nanoparticle synthesis in recent years. The benefits of employing microwaves, as well as possible mechanistic insights and the benefits of doing syntheses are • Using different frequencies of microwaves, • The ability to control the shapes of nanoparticles, and microwave local heating effects. 3.3.1

Importance of Microwave Method

The interaction of electromagnetic waves with polar solvent molecules in solution provides the basis for microwave synthesis. Excellent heating rates and homogeneous heating with high energy efficiency are created when electromagnetic waves interact directly with the reactants present in the solution [76]. When adopting a microwaveassisted synthesis procedure, the reaction time can be drastically decreased, and

Synthesis and Characterization of Emerging Nanomaterials

65

irradiation of molecules can result in smaller crystals. Because of its homogenous heating, rapid kinetics, outstanding phase purity, and high yield rate of products in a very short time, the microwave synthesis technique has piqued attention. Quick crystallisation, homogenous nucleation, easy morphological control, phase selectivity, particle size reduction, and rapid warming are all advantages of MW method. Furthermore, one of the benefits of MW is that it allows you to adjust the particle size distribution.

3.3.2

Various Materials That Can Be Synthesized by MW Method

Using MW method various materials such as metals, metal oxides, metal sulfides, bimetallic hydroxides, bimetallic oxides, carbon composites etc. Table 5 describes the possible synthetic materials and their properties with applications. For a thorough knowledge of the synthesis technique, characterizations, and application, we give one developing material with a complete analysis: Reducing Agents’ Effect on NiMg@OH/Reduced Graphene Oxide Nanocomposites Synthesis: Supercapacitor.

3.3.3

Synthesis Protocol of Microwave with Example

Microwave aided approach was used to investigate NiMgOH@rGO nanocomposites because it creates homogeneous crystalline powders with varied morphologies. The concentration of the precursor, various reducing agents, pH value, and kind of solvent all play important roles in microwave synthesis. The author investigated the influence of reducing agents, such as NaOH and NH4 OH, on the production of NiMgOH and labled as NMS, NMA respectively, [91]. Bimetallic hydroxides are made using 0.1 M Ni(NO3 )2 , 0.1 M Mg(NO3 )2 , 2 M NaOH, and Ammonia as precursors. Different weight percentages of rGO(0.5, 1, 1.5) were added to each of these precursors individually and agitated for 2 h at 80 °C to obtain a homogeneous mixture. The creation of NiMgOH is suggested by the development of apple green precipitate with 0 wt% rGO (NMS, NMA). Whereas, after adding 0.5% rGO, ash colour precipitates (NMS-1, NMA-1), and the intensity of the colour grows as the wt% of rGO increases (NMS-2, NMA-2, NMS-3, NMA-3). The production of NiMgOH-rGO composites is reflected in this. Finally, these precipitates were microwaved for 10 min before being dried in a 60 °C oven for 12 h (Fig. 7).

3.4 Sonochemical Synthesis The sonochemical method is a more unique, cost-effective, simple, efficient, and easy approach to nanomaterial synthesis, in which the particles are broken down into tiny sizes using bubbles. Ultrasound is favoured for nanoparticle production not

Cr(acac)3

Lead nitrate, sodium sulfide, and silver nitrate

Cupric acetate, sodium hydroxide

Mercury (II) chloride, ammonium metavanadate

Ammonium vanadate and ZnCl2

Graphite powder, nickel chloride hexahydrate, cobalt(II) nitrate hexahydrate, potassium hexachloroplatinate

Zinc nitrate hexahy Drate

Auric trichloride

Chromium (VI) oxide

Ag-doped PbS nanoparticles

CuO nano particles

Mercury vanadate (Hg2 VO4 ) (MV) nanoparticles

Zinc vanadate nano particles

PtNiCo/rGO nanocomposites

ZnO nanoparticles

Gold nano particles

Myristica fragrans leaf extract

Indian bael juice

Sodium nitrate

Ammonium hydroxide

–

Cordia africana Lam leaves

Cetyltrimethylammonium bromide

CTAB

Precursor/Source Reducing agent/Fuel/ material/Oxidizing agent Capping material

Target material

H2 O

H2 O

Ethylene glycol and H2 O

Milli-Q Water

Diethylene glycol and H2 O

H2 O

Milli Q Distilled water

Ethylene glycol

Solvent

1 min

10 min

140 min

60 min

30 min

–

60 min

Reaction time

Frequency of 2450 MHz and power of 800 W

–

–

500 °C

–

500 °C

pH = 8.8 200 °C, pH = 10

400 °C

–

–

300–700 °C

Calcination temperature

Microwaves at a power of 500 W

–

–

microwave irradiation (220 C, 200 psi, 300 W)

pH, temperature/ Microwave Parameters

XRD, FTIR, UV–Vis, HR-TEM

XRD, FTIR, UV–Vis, DLS, SEM, TEM, BET, Raman

XRD, TEM, PSA, EDX, Raman

XRD, SEM, EDX

XRD, SEM, EDX, FTIR, UV–Vis

XRD, FTIR, SEM_EDX

FESEM, EDX, TEM, Zetasizer Nano-Z, UV, FTIR

XRD, UV–Vis, HR-TEMFTIR, PSA

Characterizations

[84]

[83]

[82]

[81]

[80]

[79]

[78]

[77]

Ref

(continued)

Nano electronics

Manufacturing sector, healthcare and biotechnology, agriculture

Manufacturing sector, nano electronics

Nano and optoelectronics

Optoelectronics

Nanoelectronics

Optoelectronics

Manufacturing sector

Applications

Table 5 Synthesis of emerging nanomaterials by MW process, components and conditions, characteristic properties and possible applications

66 C. Shilpa Chakra et al.

Ni(acac)2 Fe(acac)3

Pure Tin

Zinc nitrate hexahydrate, Dodecylamine manganese(II) nitrate tetrahydrate

Chitosan flakes, glutamic and aspartic acid, Chloroauric acid

The squeezed camellia oleifera residue, HCl, KOH

Graphite oxide powder, MnSO4 ·H2 O and CoCl2 ·6H2 O

Ni(NO3 )2 and Mg(NO3 )2

NiFe2 O4 nanoparticles

Porous g-C3 N4 /SnO2 nanocomposite

ZnMn2 O4 nanoparticles

Chitosan aerogels

Carbon nanosheet

Reduced graphene oxide-wrapped manganese cobaltite ternary hybrids

NiMg@OH/Reduced graphene oxide nanocomposites

NaOH, ammonia

Ammonia

–

Orange peel extract

Urea, ammonia

–

Precursor/Source Reducing agent/Fuel/ material/Oxidizing agent Capping material

Target material

Table 5 (continued)

4h

30 min

60 min

Reaction time

H2 O

H2 O

H2 O

10 min

60 min

120 min

H2 O, 1,2-propanediol 25 min

Ethanol

HCl

Rac-1-phenylethanol

Solvent

Operating frequency of 2.45Ghz, power of 900 W, pH = 7

Microwave power of 900 W

Microwave treatment in 2450 MHz, 400 °C

Microwave power 300 W, 140 °C

Operating at low power (~300 W)

Microwave oven operated at 2.45 GHz and 900 W, pH = 12

–

pH, temperature/ Microwave Parameters

–

–

–

–

300–600 °C

550 °C

300–500 °C

Calcination temperature

Nano electronics

Manufacturing unit

Nano electronics

Opto electronics

Nano electronics

Applications

XRD, UV–Vis, PSA, FTIR, FESEM, XPS, Raman

Nano electronics

TGA, XRD, SEM, EDX, Nano electronics BET, XPS

XRD, FESEM, TEM, BET, XPS, Raman, FTIR

DLS, UV–Vis, FTIR, SEM

TGA, XRD, Raman, TEM, UV–Vis

XRD, FESEM, HR-TEM, EDX, XPS, FTIR,

XRD, TEM, Raman, EDX, DRIFTS, BET, TGA, XPS, Mössbauerspectroscopy

Characterizations

[91]

[90]

[89]

[88]

[87]

[86]

[85]

Ref

Synthesis and Characterization of Emerging Nanomaterials 67

68

C. Shilpa Chakra et al.

Fig. 7 Synthetic illustration for NiMgOH and NiMgOH-rGO with NaOH and NH4 OH reducing agents

only because of its ease of use and wide range of applications, but also because it allows atomic-level mixing of constituent ions. The creation of an amorphous phase of nanoparticles occurs as a result of atomic-level mixing, which can later be converted into a crystalline phase by simple annealing or calcination at a lower temperature. The sonic cavitation principle is used in this sonochemical approach [92]. The formation and collapse of liquid bubbles create acoustic cavitation. A sudden reduction in pressure causes small vapour bubbles to appear in the solvent solution. These tiny bubbles combine to form larger bubbles, which subsequently collapse, releasing massive amounts of energy and generating pressure shocks of up to 100 MPa. According to the hot spot theory, bubble collapse can result in extremely high temperatures (>5000 K), which can drive a variety of chemical processes.

3.4.1

Importance of Sonochemical Method

The advantage of using the sonochemical approach is that the size and crystallinity of the materials generated may be precisely controlled. Sonochemical is the favoured method for producing amorphous nanoparticles with excellent morphological control. This is due to the fact that reaction parameters like temperature, sonication time, and ultrasonication power all have an impact on the final result’s morphology. Management of aqueous and non-aqueous solutions, and suspensions, are considered as ultrasonic applications. Sonication was thought to affect the microstructure of solid state reaction products as reaction rates increased, resulting in lower reaction temperatures [93]. Some of the advantages of this method was as follows • Ultrasound aided synthesis allows for the manufacture of consistently distributed and equally sized nanocomposites in less time and with less energy.

Synthesis and Characterization of Emerging Nanomaterials

69

• Sonochemistry allows for high reaction rates, leading in time-saving synthesis. • Kinetics, selectivity, extraction, dissolution, filtering, and crystallinity all showed improved properties. • It is eco-friendly. • Increased yields and average particle size as a result of faster reactivity due to rapid micromixing. • Various nanostructures materials can be generated with minor changes to the reaction conditions. 3.4.2

Various Materials That Can Be Synthesized by Sonochemical Method

Various metal oxides, bimetal oxides, metal nanoparticles, hydroxides can be synthesized by this method and most of time we will get porous materials. Table 6 describes the parameters and properties of various emerging material synthesized by sonochemical method.

3.4.3

Synthesis Protocal of Sonochemical Method with Example

The NiMnO@pr-GO nanocomposite is prepared by using sonochemical route are shown in Fig. 8. The precursors nickel chloride and manganese chloride were dissolved in double distilled water and mixed using a magnetic stirrer to create the NiMnO nanocomposite sonochemically. The mixture was then treated with dilute NaOH until the pH reached 10. After that, the solution was sonicated in an ultrasonic bath for 2 h at a frequency of 37 kHz with a power of 150 W. The brown precipitate was washed with water/ethanol and dried in the air. The NiMnO nanocomposite was made by calcining the resultant powder at 550 °C in an air atmosphere for 3 h. NiMnO@pr-GO nanocomposite was made by mixing GO powder and NiMnO (1:1) in a beaker with 50 mL of ethanol and stirred. The obtained solution was ultrasonically treated for 15 min to make the NiMnO@pr-GO nanocomposite. The solution was dried for 12 h at 60 °C [97].

3.5 Electrospinning Technique The budding technology plying its pivotal scientific role as well as commercial venture with universal economic benefits is Nanotechnology. J. F. Cooley patented the electrospinning process in May 1900. This techniques has been widely used in the late 20th and early twenty-first centuries. Polymers, composite and ceramic materials are included for the production of fibers. Nanofiber technology involves synthesis, processing, manufacturing and application with nanodimension. Low cost,

Precursor/Source material/ Oxidizing agent material

Zinc chloride (ZnCl2 ), sodium hydroxide (NaOH)

Manganese sulfate and iron sulfate

Iron chloride, Iron sulfate heptahydrate

FeCl3 .6H2 O and NiCl2 .6H2 O

Nickel chloride and manganese chloride

Zinc chloride, carbon black, polyvinylidenedifluoride (PVDF) and Sodium tungstate dihydrate

Praseodymium nitrate, ammonium meta vanadate

Target material

ZnO Nano particles

MnFe2 O4 Nano particles

Iron oxide nanoparticles

NiFe2 O4 nano particles

Nickel-manganous oxide nanocrumbs

ZnWO4 nanoarchitectures

PrVO4 nanostructures

H2 acacpn ligand and ammonia

Cetyl-trimethylammonium bromide (CTAB), and analar potassium hydroxide

NaOH

NaOH

NaOH

NaOH

–

Reducing agent/Fuel/ Capping

H2 O

H2 O

H2 O

H2 O

H2 O

H2 O

H2 O

Solvent

20 min

20 min

120 min

60 min

60 min

120 min

70 min

Reaction time

550 °C

pH = 10

600 °C

800 °C

pH = 12

pH = 6–7

–

pH = 11–13

600 °C

–

pH = 14

–

–

Calcination temperature

–

pH, temperature/ potential/ distance

Manufacturing sector, high power devices

Opto electronics

Opto electronics

Applications of emerging materials

[96]

[95]

[94]

References

XRD, EDS, FTIR, SEM, TEM, UV–Vis,

XRD, XPS, FTIR, Raman, HRTEM

FESEM, EDX, FTIR, XRD, Raman

Opto electronics

Nano electronics

Nano electronics

(continued)

[99]

[98]

[97]

XRD, UV–Vis, Nano and Opto [92] FTIR, SEM, EDS electronics

FTIR, Raman, PSA, XRD, FESEM, EDS, HRTEM

XRD, UV–Vis, SEM, EDX TEM, Raman

XRD, UV–Vis, SEM, BET, Raman

Characterizations

Table 6 Synthesis of emerging nanomaterials by sonochemical synthesis, components and conditions, characteristic properties and possible applications

70 C. Shilpa Chakra et al.

Fe(NO3 )2 ·9H2 O, NH4 VO3

Chloroauric acid or gold (III) chloride hydrate

Iron vanadate (FeVO4 ) nanoparticles

Gold nanoparticles

Sodium citrate

Ammonia, l-alanine, valine, and leucine

Copper acetate monohydrate Ammonia

CuO/Cu2 O/Cu nanoparticles

Reducing agent/Fuel/ Capping

Precursor/Source material/ Oxidizing agent material

Target material

Table 6 (continued)

H2 O

H2 O

H2 O

Solvent

10 min

30 min

3h

Reaction time

600 °C

pH = 5

–

500 °C

pH = 11

–

Calcination temperature

pH, temperature/ potential/ distance

UV-is, PSA, Zeta potential, XRD

XRD, FTIR, EDS, SEM, PSA, TEM, UV–Vis

SEM, DRS, XRD, UV–Vis

Characterizations

Nano electronics

Opto electronics

Opto electronics, healthcare and biotechnology, agriculture

Applications of emerging materials

[102]

[101]

[100]

References

Synthesis and Characterization of Emerging Nanomaterials 71

72

C. Shilpa Chakra et al.

Fig. 8 Sonochemical synthesis of NiMnO@pr-GO nanocomposite

high productivity, reproducibility and simplicity makes electrospinning with potentialities being used at industrial level. Electrospinning depends on solution properties and processing parameters with which specific fibers can be produced for various applications. Nanofibers are the fibers which falls under the nanometer range diameter with large surface area to volume ratio, flexibility, low density and porosity due to which it takes center stage. Modern challenges were refined by the development of nanofibers having exiting applications to tissue engineering, encapsulation of bioactive molecules, wound dressing, adsorbent, protein separation, immobilization, energy conversion and storage, air and water filtration, medical, membrane and composite materials. Electrospinning is a plain, worldly mechanism used for producing nanofibers. Electrospinning or electrostatic fiber spinning is a red-hot Voltage driven process restrained by electrodynamic phenomena that uses electric force to pull charged threads in the order of hundred nanometers which does not require high temperatures to yield solid threads without carrying solvent into the final resultant from the polymer solution. Variety of materials mainly polymers shapes into controlled nanofibers. Systems to synthesize nanofibers Nanofibers are produced using two systems: needle based electrospinning and needle less electrospinning.

Synthesis and Characterization of Emerging Nanomaterials

73

Fig. 9 Schematic representation for electrospinning instrument

Needle based electrospinning techniques such as multi-axial, coaxial, tri-axial, bicomponent, multi-needle, gas jet, magnetic field assisted, conjugate and centrifugal electrospinning with starting polymeric fluid stream solution tightly sealed in a reservoir minimizing solvent evaporation. Advantages: flexibility, controlled flow rate, minimized solution waste. Needleless electrospinning techniques such as bubble, two layer fluid, splashing, melt differential, gas assisted melt differential, rotary cone, rotating roller, edge, blown bubble electrospinning with starting polymer solution. Advantages: Mass production. Disadvantages: Morphology and quality precisely not controlled, limited raw materials, limits fiber production, uncontrollable flow rate. Set-up The electrospinning Set up consists of three major parts high voltage power source, syringe holds up the polymer solution and a conductive grounded collector (Fig. 9). These solutions will be pushed to the spinneret at a constant speed out of the tank. 20–40 kV of high voltage will be applied to spinneret and the polymer solution will be ejected when electrical attraction exceeds surface tension of polymer solution that is volatilized to reduce to nanometer when they reach collector. The diameter of Electrospun fibers ranges from tens of nanometers to few micrometers. As the charge liquid moves from the tip of the syringe to the collector, the circulating current shifts from ohmic to convective as the charge moves to the fibres.

3.5.1

Importance of Electrospinning

Large surface area, light weight with small diameters, ease of functionalization, ease of process and superior mechanical properties of nanofibers makes if ideal in the applications such as sensors, filtration, drug delivery, tissue engineering, energy storage and functional materials.

74

C. Shilpa Chakra et al.

Advantages: versatile process to produce fibers with numerous arrangements and morphological structures. Disadvantages: Inhomogeneoues cell distribution, use of toxic solvents.

3.5.2

Various Materials That Can Be Synthesized by Electrospinning Technique

Different composites such as carbon, metal, bimetallic, polymer, metal oxides can be synthesized by employing electrospinning technique (Table 7).

3.5.3

Synthesis Protocol of Electrospinning Method with Example

For clear understanding, one of the Ref. [108] briefly described. Polyvinylideneflouride-Hexaflouropropylenewere added to a beakerand polyethylene glycol was added to the above contents. N,N-dimethylformamide and acetone in the ratio of 4:1 was added. This mixture was stirred for 8 h and poured into spinneret. The parameters such as speed was set at 0.08 mm per minute controlled at voltage of 18 kV with 20 cm of distance between needle and collector. Electrospun nanofibers were obtained after 6 h and membrane was evaporated at 60°c. These nanofiber membranes were cut into disks with 19 mm in diameter for further application.

3.6 Microemulsion Method Microemulsion/transparent emulsion/swollen micelle/micellar solution and solubilized oil is a thermodynamically stable isotropic fluid mixtures of oil, water and surfactant that differs from kinetically stable emulsion. Microemulsion was first used by T. P. Hoar and J. H. Shulman at Cambridge University in 1959. Microemulsions are of direct (oil in water), reversed (water in oil) and bicontinous where the immiscible phases are bestowed with a surfactant forming a monolayer at the oilhydrophobic tails and water interface and Oil being a mixture of hydrocarbons and olefins. Microemulsions have Ultralow interfacial tension between water and oil phase. Particle size of the microemulsion derived synthesis is about 10–300 nm due to which they appear translucent solutions. It is a Single isotropic phase with mixture of two immiscible liquids water, oil and a surfactant. The influence of Volume fractions of these ternary composites can be represented by Gibbs phase diagram. The structure of microemulsion differ from the orientation of amphiphiles. In W/O microemulsion, the hydrophilic groups reside in the dispersed oil droplets with hydrophobic portion protruding in the continuous phase. Even though microemulsions are stable they undergo coalescence, deformation breakdown and

Precursor/Source material/Oxidizing agent material

Nickel acetateandchromium acetate hydroxide

Polystyrene multi-walled carbon nanotube

Tin chloride anhydrous, formic acid andacetic acid

LiCoO powders and artificial graphite powers, LiPF6

Ammonium metavanadate, lithium nitrate

Graphene nanosheets

Nickel(II) chloride hexahydrate, cobalt(II) chloride,

Target material

NiCr-CNT@C

N-doped metal-free Li–O2

Carbon nanofibers @SnO2

LiCoO2 @g

Li3 VO4 /N

HN-CNFs/GNs

C-NiCoO NFs//activated carbon

PAN, KOH

PAN, PMMA

PAN

PAN

KOH

PAN

PAN

Reducing agent/Fuel/ Capping

DMF, deionized water

Acetone and DMF

DMF

DMF acetone

Ethanol

DMF

N, N-dimethylformamide,

Solvent

4

24

12

5

12

6

26

Reaction time (h)

110 °C

250 °C

250 °C

120 °C

60 °C

280 °C

85 °C

Temperature

700

850

600

90

800

700

1000

Calcination temperature (°C)

Applications

Batteries

HRSEM, FETEM, STEM-EDX, XRD, XPS, RAMAN, TGA, FTIR, BET, ICP-MS, CV, GCD, EIS

FE-SEM, TEM, Raman, XRD, TG, CV, GCD, EIS, XPS

SEM, TEM, SAED, Raman, XRD, TG, CV

SEM, BET, DSC, TG, GCD

Supercapcitors

Supercapacitors

Batteries

Batteries

SEM, XRD, XPS, Supercapacitors BET, TGA

FE-SEM, XRD, LABRAM HR, XPS, BET, TGA

TEM, SEM XRD, Batteries EIS

Characterizations

Table 7 Synthesis of emerging nanomaterials by electrospinning, components and conditions, characteristic properties and possible applications

(continued)

[105]

[104]

[103]

[38]

[37]

[36]

[35]

References

Synthesis and Characterization of Emerging Nanomaterials 75

Precursor/Source material/Oxidizing agent material

Si, TiO2 and PAN

Acrylic yarn, tin powder

Urea

Vanadium (III) acetylacetonate, argon, H2 O2

Target material

Si/TiO2 /Ti2 O3

N-doped mesoporous carbon nanofibers

PVP based carbon nanofibers via pyrolysis of g-C3 N4

VO x @CNFs

Table 7 (continued)

PAN, PVP

PVP

–

H2 gas

Reducing agent/Fuel/ Capping

DMF

Ethanol, Deionized water

DMF, deionized water

DMF

Solvent

2

12

2

48

Reaction time (h)

60 °C

25 °C

280 °C

RT

Temperature

1000

800

650

800

Calcination temperature (°C)

SEM, TEM, SAED, TGA, RAMAN, CV, GCD, EIS

SEM, XRD, FTIR, TGA, RAMAN, CV, GCD, EIS

XRD, XPS, BET, SEM, TEM, HRTEM, Raman, FTIR, CV, GCD, EIS

FE-SEM, TEM, XRD, RAMAN, BET, XPS, FTIR, CV, GCD

Characterizations

Alkaline ion batteries

Batteries

Battery anodes

Lithium-ion batteries

Applications

[109]

[108]

[107]

[106]

References

76 C. Shilpa Chakra et al.

Synthesis and Characterization of Emerging Nanomaterials

77

shape fluctuations of the droplets. Surfactants must lower the interfacial tension to assist the dispersion process and possess hydrophilic and lipophilic character [110]. Due to low surfactant concentration winsor phases (W I, W II, W III, W IV) exists in equilibrium. They can be classified from normal emulsions due to transparency, viscosity ability to transform spontaneously. Under normal conditions theoretically they have infinite shelf life. Interface is stabilized by the balanced combinations of surfactants and co-surfactants. Microemulsions are characterized using Differential scanning calorimetry (DSC), polarization microscopy, photon correlation spectroscopy (PCS) total intensity light scattering (TILS), dynamic light scattering (DLS), static light scattering (SLS), small angle neutron scattering (SANS), self-diffusion nuclear magnetic resonance (SD NMR), small angle X-ray scattering (SAXS).

3.6.1

Importance of Microemulsion

Microemulsion discovery has attained increasing drift both in lab scale as well as industry scale. Their peculiar properties such as large interfacial area, thermodynamic stability, ultralow interfacial tension. By using microemulsion technique different metals, metal oxides, polymers and various composites can be synthesized.

3.6.2

Different Materials Synthesized by Microemulsion Technique

Materials such as metal nanoparticles, polymers, carbon based materials, metalpolymer based composites, bimetal oxide composites etc. can be synthesized by microemulsion technique (Table 8).

3.6.3

Synthesis Protocol of Electrospinning Method with Example

Aniline was dissolved in methanol in a beaker and to this continuously stirred solution, copper nitrate solution was added drop wise. During the addition, the solution became green indicating the formation of polyaniline. After complete addition, the solution was stirred for another 30 min and a green precipitate was formed in the solution (Fig. 10). The entire reaction was carried out at room temperature under continuous stirring. The ppt was filtered under vacuum using a cellulose acetate filter paper. For removing the soluble oligomers, the precipitate was washed with water followed by methanol and dried at 30 °C.

Precursor/Source material/ Oxidizing agent material

Native sago (metroxylon sagu) starch powder

Iron oxide

Neodymium III nitrate hexahydrate strontium nitrate, cobalt(II) nitrate

Zinc nitrate

Vulcan carbon nafifion solution

Copper foil

Target material

Starch nanoparticles

Superparamagnetic iron oxide nanoparticles

Nd0.67 Sr0.33 CoO3 -δ (NSC)

Zinc oxide

Carbon supported Pt-WO3

Porous CuO nanosphere films

CTAB, Sodium bicarbonate

Triton X-100, H2 So4

CTAB, docusate sodium salt, toluene, Lithium iodide

CTAB NaOH

CTAB, 1-Butanol 1-Hexanol

Sodium hydroxide

Reducing agent/Fuel/ Capping

Reaction time

Isopropanol

Butan-1-ol cyclohexane, methanol, Deionized water

Ethanol, acetone

DI water

Water

24 h

30 min

1h

2h

–

Cyclohexane, 1h acetone, ethanol and n-butanol and ultrapure water

Solvent

25 °C

RT

110 °C

70 °C

RT

80 °C

Temperature

–

–

450 °C

800 °C

–

Calcination temperature Nano drug delivery

Applications

XRD, FESEM, TEM, TG

TEM, HRTEM, EDX, TGA, CV

CTAB, XRD, SEM, HR–TEM, UV

FE-SEM, XRD, TEM, BET, XPS, CV, GCD, EIS

Lithium ion batteries

Electrocatalyst for methanol oxidation

Dye-sensitized solar cells

Lithium-air batteries

TEM, XRD, DLS Bioapplications

SEM

Characterizations

(continued)

[112]

[111]

[42]

[41]

[40]

[39]

References

Table 8 Synthesis of emerging nanomaterials by microemulsion technique, components and conditions, characteristic properties and possible applications

78 C. Shilpa Chakra et al.

Precursor/Source material/ Oxidizing agent material

Lithium nitrateiron nitrate nonahydrate ammonium dihydrogen phosphate

Lithium nitrate, manganese nitrate tetrahydrate, nickel nitrate hexahydrate, ferric nitrate nonohydrate, Chameleon

CNTs, titianium oxide, cyclohexane

Cobalt acetate, cetyl-trimethyl-ammonium bromide mnn-mixture

Copper nitrate trihydrate, aniline

Target material

LiFePO4 /C

Li1.2 Mn0.54 Ni0.22 Fe0.04 O2

Li4 Ti5 O12

Water/cetyl-trimethyl-ammonium bromide/n-hexanol

Cu-PANI

Table 8 (continued)

Methanol

Hexanol

Nitric acid, sulfuric acid,

HNO3 , cyclohexane

Cyclohexane

Reducing agent/Fuel/ Capping

Triply-distilled water

Tetrabutyl titanate acetylacetone, deionized water

–

De-ionized water, n-butanol, kerosene

Solvent

30 min

45 min

24 h

12 h

12 h

Reaction time

RT

– 5 °C

350 °C

110 °C

600 °C

Temperature

–

–

800 °C

900 °C

–

Calcination temperature

UV–VIS, FTIR, HR-SEM, EDAX, TGA, BET, XRD

XRD, TEM, DS C, XPS, CV, GCD

XRD, SEM, TGA

XRD, BET, SEM, TGA

XRD, SEM, TEM, TG, BET

Characterizations

Battereis

Alkaline secondary batteries

Lithium ion battery

Batteries

Lithium ion batteries

Applications

[116]

[115]

[114]

[113]

[110]

References

Synthesis and Characterization of Emerging Nanomaterials 79

80

C. Shilpa Chakra et al.

Fig. 10 Chemical synthesis of mesoporous cu-PANI composite

3.7 Hydrothermal Synthetic Protocol As technology advances, we are getting closer to being more innovative. According to advanced sciences, nanotechnology is one of the driving forces that have the potential to change material science in the modern era. The hydrothermal approach is widely utilized to create nanomaterials around the world. The term “hydrothermal/solvothermal method” refers to the process of crystallizing chemical compounds straight from the aqueous phase by adjusting thermodynamic variables such as pressure, temperature, and chemical composition. When water is employed as the solvent, the process is called “hydrothermal.“ In a few circumstances, this phrase refers to a process in natural conditions. When organic compounds are utilized as solvents, the term “solvothermal process” is used. When discussing the hydrothermal/solvothermal process, the following phrases are frequently used: • Mineralizers: inorganic or organic additives with high concentrations (e.g., 10 M) to control the pH of solutions Precursors: reactants in the form of solutions, gels, and suspensions Mineralizers: inorganic or organic additives with high concentrations (e.g., 10 M) to control the pH of solutions • Additives: modest amounts of organics or inorganics are needed to promote regulated crystal shape or excellent particle dispersion [117]. 3.7.1

Importance of Hydrothermal Method

Today, the hydrothermal method has found a place in a variety of research and technological fields, spawning a slew of associated approaches with deep roots

Synthesis and Characterization of Emerging Nanomaterials

81

in the hydrothermal method. So there is hydrothermal synthesis, hydrothermal growth, hydrothermal alteration, hydrothermal treatment, hydrothermal metamorphism, hydrothermal decomposition, hydrothermal dehydration, hydrothermal extraction, hydrothermal phase equilibria, hydrothermal sintering, hydrothermal reaction sintering, hydrothermal electrochemical reaction, and so on; these all require materials scientists, earth scientists, physicists, chemists, biologists etc. [118]. In most cases, hydrothermal reactions are carried out in a sealed pressure reactor vessel, an autoclave, or a high-pressure bomb. These hydrothermal reactors are metal autoclaves with alloy linings or Teflon or containing an extra can/beaker/tube made of platinum, gold, Teflon, or silver to protect the body of the autoclave from the highly corrosive solvents, which is carried at high pressure and temperature [117] (Fig. 11). Figure 11 shows the hydrothermal setup used for preparation of materials in laboratory scale. Hydrothermal synthesis has several advantages over traditional and non-traditional ceramic synthesis techniques. Powders, fibers, single crystals, and monolithic ceramics can be made via hydrothermal synthesis—coatings on metals, polymers, and ceramics, as well as ceramic bodies.

Stainless steel autoclave Teflon vessel

nuts and bolts

Stainless steel cap Teflon led

Spanner

Fig. 11 Autoclave used in laboratories for hydrothermal method

82

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C. Shilpa Chakra et al.

Various Materials That Can Be Synthesized by Hydrothermal Method

Microporous crystals, superionic conductors, chemical sensors, electronically conducting solids, complex oxide ceramics and fluorides, magnetic materials, and luminescence phosphors have all been successfully created using hydrothermal synthesis. It is also a way to make unusual condensed materials, including nanoscale particles, gels, thin films, equilibrium defect solids, helical and chiral structures, and materials with a stacking sequence. Hydrothermal synthesis has been essential in biology and environmental science, for example, in the beginning of life and the supercritical water oxidation process for degrading organic wastes and the synthesis of new materials [119] (Table 9).

3.7.3

Synthesis Protocol of Hydrothermal with Example

The hydrothermal method of material preparation is a crystallization process that involves two steps: crystal nucleation and subsequent growth. The final products could be manufactured with desired particle sizes and morphologies by adjusting processing variables, such as temperature, pH, reactant concentrations, and additives. The total nucleation and growth rates, dependent on supersaturation, underpin the size and morphological control through manipulating the process factors. The schematic representation of mechanisms of crystal growth by hydrothermal methods is shown in Fig. 12a, b shows the synthesis process of nanomaterials. The ratio of actual concentration to the saturation concentration of the species in the solution is known as supersaturation. Although the existence of a large number of species in the hydrothermal/solvothermal solution makes determining the exact reaction equilibria difficult, numerous thermodynamic models have been developed to calculate the species solubility [117]. For MoS2 Nanostructures, 0.0575 g of MoO3 and 0.1332 g of thiourea were mixed in 40 mL of water. The reaction mixture was agitated for 30 min before being transferred to a 50 mL Teflon-lined autoclave and stored at 200 °C for 24 h. The autoclave was allowed to cool naturally once the reaction was completed, and the black residue was obtained by filtration and rinsed multiple times with water and ethanol. The washed precipitate was dried for 12 h at 60 °C [45].

3.8 Polyol Method The liquid-phase synthesis of high-boiling multivalent alcohols is known as polyol synthesis, and it is mainly used to make nanoparticles. The polyol family begins with ethylene glycol (EG), the essential member. The polyols are divided into two groups based on EG: (i) Diethylene glycol (DEG), Triethylene glycol (TrEG), Tetraethylene glycol (TEG), and up to polyethylene glycol (PEG), usually contains over 2000

Precursor/Source material/ Oxidizing agent material

Graphite powder

Biomass (microcrystalline cellulose, furfural, 5-hydroxymethylfurfural)

Sucrose

Multi-walled carbon nanotubes (MWCNTs)

Stannous chloride

Ammonium heptamolybdate

Target material

Reduced graphene oxide (rGO)

Carbon quantum dots

Carbon microspheres

Spherical carbon nanoparticles

SnO

MoO3

HNO3 , NH4 OH, (NH4 )2 SO4 , C2 H5 NS,

CH4 N2 O, HCl,

HNO3 , C3 H7 NO, NaOH

C6 H8 O7

–

H2 SO4 , KMnO4 , NaNO3

Reducing agent/Fuel/ Capping

H2 O

H2 O

H2 O

H2 O

H2 O

H2 O

Solvent

6–24 h

4h

12 h

6h

30 min

24 h

Reaction time

180 °C, pH-1

120–500 °C

120 °C

190 °C

220 °C

~35 °C

pH, temperature/ potential/ distance

500 °C

500 °C

–

–

–

105 °C

Calcination temperature

Applications

Healthcare and biotechnology

[121]

[120]

References

XRD, FESEM, BET

XRD, XRF, Raman, UV–Vis, TEM

XRD, Raman, UV–Vis, FTIR, SEM, EDX, XPS, TEM

(continued)

High power [45] devices, nano and opto-electronics

High power [124] devices, nano and opto-electronics

High power [123] devices, nano and opto-electronics

SEM, Particle size Healthcare and [122] distribution, XRD, biotechnology, TGA, FTIR high power devices, nano and opto-electronics

UV–Vis, FTIR, TEM

XRD, FTIR, SEM, Healthcare and TEM biotechnology, aerospace, agriculture, high power devices, nano and opto-electronics

Characterizations

Table 9 Synthesis of emerging nanomaterials by hydrothermal method, components and conditions, characteristic properties and possible applications

Synthesis and Characterization of Emerging Nanomaterials 83

MAX phase (Ti3 AlC2 )

Gallium nitrate

Ammonium heptamolybate CH4 N2 S, tetrahydrate C3 H8 O, H2 SO4

Nickel(II) chloride and red phosphor

MXene

γ-Ga2 O3

MoS2

Ni2 P

KOH

CH3 COO

NaBF4 , HCl

N2 H4 , NaNO3 , KMnO4 , NaOH, C2 H5 OH

Samarium nitrate hexahydrate

Sm2 O3

Reducing agent/Fuel/ Capping

Precursor/Source material/ Oxidizing agent material

Target material

Table 9 (continued)

H2 O

H2 O

H2 O

H2 O

H2 O

Solvent

10 h

24 h

24 h

8–32 h

24 h

Reaction time

200 °C

200 °C

180 °C

180 °C

180 °C, pH > 7

pH, temperature/ potential/ distance

–

–

–

–

800 °C

Calcination temperature

XRD, EDS, TEM, SEM

XRD, SEM, TEM

HRTEM, EDS, XPS, XRD

XRD, FESEM, EDX, TEM, BET

XRD, FTIR, FESEM,

Characterizations

References

Nano and opto-electronics

[129]

High power [128] devices, nano and opto-electronics

High power [127] devices, nano and opto-electronics

Healthcare and [126] high power devices, nano and opto-electronics

High power [125] devices, nano and Opto-electronics

Applications

84 C. Shilpa Chakra et al.

Synthesis and Characterization of Emerging Nanomaterials

85

a Precipitation

Growth

Nucleation

Solute molecules

Nuclei

Growth units

Crystal

b

Hydrothermal 250 oC 15h

Precursors

Autoclave

Filtration

Dry & Calcined

Morphology

Nano powder

Fig. 12 Schematic representation of a crystal growth mechanism b synthesis of materials under the hydrothermal method

ethylene groups and possessing a molecular weight up to 100,000 g mol–1 ; and (ii) Propanediol (PDO), Butanediol (BD), Pentanediol (PD), and more [130]. The polyol method is an extensively used synthesis method for many inorganic compounds, including metal nanoparticles, alloys, oxides, fluorides, sulfides, tellurides, and other inorganic compounds. This approach for nanomaterials synthesis is widely applicable due to the improved solubility of widely accessible starting materials. Precursors, relative quantities of polyols, ligands, and other factors must be considered [131].

3.8.1

Importance of Polyol Method

Polyols provide a number of advantages. Due to the polyols’ water-comparable polarity and solubility, essential metal salts are employed as precursor materials. In terms of economics and practical handling, this is far more easy than volatile and reactive carbonyl metals or complicated organometallic complexes. The polyols become reductive at high temperatures, which is a big benefit. As a result, the polyol may quickly decrease dissolved metal cations, resulting in metal nanoparticles with sufficient surface functionalization and stability in the excess polyol solvent. In this approach, polyol synthesis may be thought of as a one-pot process that combines numerous characteristics at the same time. Besides its benefits, polyol synthesis of metal nanoparticles has two major drawbacks: (i) the polyol’s reducing power is restricted, and (ii) the polar polyol commonly fails to stabilize non-polar metal surfaces [130].

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Various Materials That Can Be Synthesized by Polyol Method

Fe, Co, Ni, Cu, Pd, Ag, Sn, Ru, Rh, Pd, W, Pt, Fe–Cu, and Ni–Cu nanostructured powders have all been developed. On Pyrex TM, Kapton TM, Teflon TM, aluminum nitride, carbon fibers, and alumina fibers, nanocrystalline metallic coatings of Co, Ni, Rh, Re, W, Pt, and Au were deposited. Fine metallic powders of Co, Cu, Ni, Pb, Ag, and other elements are made using the polyol process. The metal precursor(s) is suspended or dissolved in a polyol like ethylene glycol throughout this procedure. The glycol-metal precursor combination is then heated to reflux, causing the metallic moieties to precipitate out of the solution. Single elements were used to make these particles, which ranged in size from micron to submicron. Additional reducing agents were frequently used in the reaction, Depending on the metal precursors that were used [52] (Table 10). The range of crystallite size is given when it is concentration dependent [52].

3.8.3

Synthesis Protocol of Polyol with Example

The typical technique for the synthesis of various metallic powders and films consisted of suspending the appropriate metal precursors in ethylene glycol or tetraethylene glycol and then heating the derived mixture to refluxing temperature (often between 120 and 200 °C) for 1 to 3 h, as shown in Fig. 13. The metallic moieties precipitated out of the mixture during this reaction time. The metal-glycol solution was cooled to room temperature, filtered, and the precipitate recovered was dried in the air [52]. Silver nitrate, a precursor of Ag, was dissolved in ethylene glycol with polyvinylpyrrolidone, preventing agglomeration of the produced silver nanoparticles. This solution was aggressively agitated in a reactor equipped with a reflux condenser, then heated to temperatures between 100 and 150 °C at a steady rate of 1–7.5 °C min–1 . At each reaction temperature, the reaction was kept for 30 min. The reaction was brought to room temperature, and the silver particles were removed from the liquid by centrifugation and rinsed with ethanol several times. At room temperature, the resultant particles were dried [46].

4 Characterizations: Detailed with Example The synthesized emerging materials were characterized by XRD (Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation (wave length = 1.5405 Å)) and TEM (JEM-2100F). Further LDI MS studies were carried out by Voyager DEPRO Biospectrometry Workstation (Applied Biosystems), and BET, BJH, N2 adsorption– desorption isotherm were obtained by (ASAP 2020 physisorption instrument). While FT-Raman spectra were obtained from Bruker RFS 27: Stand alone FT-Raman Spectrometer. The SEM and EDX were acquired using (FEI) HR-SEM Quanta FEG 200. The fluorescence spectra were acquired using Jasco FP-6300 spectrofluorometer

Synthesis and Characterization of Emerging Nanomaterials

87

Table 10 Synthesis parameters and products of different polyol reactions Material

Precursors

Concentration range used (mol/L)

Average crystalline size (nm) of powder

Average crystalline size (nm) of coating

Reaction time (h)

Fe

Fe(C2 H3 O2 )2

0.01–0.20

20

Co

Co(CH3 CO2 )2 ·4H2 O; CoCl2 ·6H2 O

0.05–0.20

12.1 14

15 (K) 14 (P) 23 (T)

2

Ni

Ni(CH3 CO2 )2 ·4 H2 O

0.02–0.20

20

9 (K) 30 (K) 15 (P)

2

Cu

Cu(CO2 CH3 )2 ·4H2 O

0.02–0.025

10–80

12 (AIN) 43 (K)

2

Ru

RuCl3

0.021

5

Rh

RhCl3

0.01

9 (P)

1

Pd

PdCl2

0.02–0.15

10

18 (K) 22 (P)

1

Ag

AgNO3

0.05–0.2

40

34 (T) 43 (K) 50 (P)

1

Sn

SnO

0.01–0.03

36

Re

ReCl3

0.02

14 (P)

1

W

H2 WO4 Na2 WO4

0.012–0.20 0.03

8 10

12 (P) 12 (P)

3 3

Pt

K2 PtCl6

0.01–0.20

2

10 (K) 12 (T) 14 (GF) 15 (AF) (SF)

1

Au

AuCl3

0.01–0.20

28

32 (P)

2

Fe–Cu

Fe(C2 H3 O2 )2 ; Cu(CO2 CH3 )2 ·4H2 O

0.016–0.16 0.018–0.14

27–47

–

2

Co–Cu

Co(CH3 CO2 )2 ·4H2 O; Cu(CO2 CH3 )2 ·4H2 O

0.01–0.20

17–35

–

2

Ni–Cu

Ni(CH3 CO2 )2 ·4 H2 O Cu(CO2 CH3 )2 ·4H2 O

0.0321 0.0963

8

–

1

2

1

2

K-kapton; P-pyrex; T-teflon; G-graphite; A-alumina; S-sapphire; F-fiber

and the FTIR spectra from JASCO FT/IR-4100 spectrometer. UV–Vis spectra were obtained by systronics 2202 double beam spectroscopy, while the FT-IR spectra from Bruker Alpha II spectrometer. (a)

X-ray diffraction Spectroscopy

88

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Reducing agents Calcined

Polyol 120oC 1h

Precursors

Magnetic stirrer

Centrifuge

Nano powder

Final Nanostructures

Fig. 13 Synthesis of nanostructures by polyol method

(a) (111) (111)

(b)

Pd-RFL Pd-FL

(200) (200)

(a)

Pd-SFL Pd-SFL

(220)

Intensity (a. u.)

(111)

(b)

(200) (111)

(111)

Pd-INT Pd-INT

(220)

Pd-CFL Pd-CFL

(220)

P

B 55 º A 55 º

(c)

(200)

(200)

(220)

Pd-THO Pd-THO

(d) (e)

ITO 40

50

60

70

2θ (degree)

Fig. 14 A XRD patterns of (a) Pd-RFL; (b) Pd-SFL; (c) Pd-INT; (d) Pd-CFL and (e) Pd-THO on Indium Tin Oxide working electrodes. B TEM image of the nanospike of Pd-THO illustrating the growth directions. The left inset depicts the SAED pattern and right inset shows the magnified view of the spike [64]

Pd-RFL, Pd-SFL, Pd-INT, Pd-CFL, and Pd-THO X-ray diffraction patterns show the development of a face centred cubic (FCC) structure, with three peaks at 2θ values of 40.3°, 46.7°, and 68.3° denoting reflection from the (111), (200), and (220) planes, respectively (Fig. 14). The high intensity peak at 40.3° indicates that PdTHO, Pd-CFL, Pd-INT, and Pd-SFLs grow by stacking (111) facets, but Pd-RFLs grow by stacking (200) facets, with the (111) plane being less intense. Although the actual origin of the higher intensity associated with the (200) plane is unknown at this time, it suggests that Pd-RFL is essentially growing from it; additionally, the lower intensity in comparison to ITO indicates reduced Pd particle coverage. The diffraction peaks 51.2° and 60.1° refer to (440) and (622) planes of the Indium tin oxide [64]. (b)

Transmission electron microscopy

The growth pattern of Pd-THO crystals can be seen in TEM (Fig. 14). The nanospikes of Pd-THO have well-defined lattice fringes, and the interplanar distance of 0.23 nm corresponds to planes (111), while 0.19 nm refers to planes (200). Furthermore, the nanospikes grow along the (111) direction with the main branch (labelled as OP in Fig. 14) of the (200) plane and the side branches (OA, OB). The angle of 55 degrees between the main and side branches corresponds to the theoretical estimate of the

Synthesis and Characterization of Emerging Nanomaterials

Intensity

50000

N

N n*

Series1 Series 3

40000

734 659

Series 2

30000

914 839

1094

720

20000

900

1019

0 500

1199

1379 1260

1000

b

90 80 70

0.15

100

0.12

1454 14401559

a c

0.09 0.06 0.03 0.00 0

60

200

400

600

800

Temperature (°C)

90 80

0.5 0.4 0.3 0.2 0.1 0.0 0

200

400

600

800

Temperature (°C)

70

50

1274

1080

540

b

Deriv. Weight (%/°C)

*

100

Weight (%)

∆M=180

∆M=180

Deriv. Weight (%/°C)

a

554

60000

10000

110

110

70000

Weight (%)

A

89

40

60

30

1500

m/z

50 0

200

400

600

Temperature (°C)

800

1000

0

200

400

600

800

1000

Temperature (°C)

B

Fig. 15 A (a) mass spectrometry of pernigraniline, (b) TGA of Cu-PANI composite and (c) TGA of pernigraniline. The insets depict DTGA of the corresponding compounds. B (a) BET, (b) BJH and (c) N2 adsorption–desorption isotherms of the Cu-PANI composite [116]

preferred development path between (111) and (200). The facets (111) and (200) of Pd-THO demonstrate its fcc structure, whereas the SAED pattern validates its crystalline nature [64]. (c)

Mass spectrometry analysis

The molecular weight of polymer is calculated from mass spectrometry and is recorded in positive-mode with m/z ranging from 500 to 2000. It is of interest to note that the appearance of various series of peaks with m/z difference of 180 indi). Three of them is shown cates the formation of pernigraniline ( in Fig. 15A(a) for clarity and the formation of eight repeated units is noticed [64]. (d)

Thermogravimetric Analysis

TGA emphasizes the thermal stability of Cu-PANI composite and has to carried in nitrogen atmosphere with uniform heating rate (here 20 ◯ C per minute). The Cu-PANI composite shows four-stages of degradation: i.e. at (a) 60–70 °C (loss of methanol); (b) at 110–300 °C (removal of oligomers); at (c) 300–600 °C (degradation of PANI chains) and (d) at 610–1160 °C (decomposition of polymer back bone and benzene ring opening). The derivative weight loss data is shown as an inset of Fig. 15A(b). Unlike coating of PANI on Cu nanoparticles, here no weight gain was observed at 350 °C indicates the importance of synthetic protocol in production of more stable CuPANI composite. Figure 15A(c) depicts the TGA of polymer (pernigraniline) and its derivative weight loss is depicted in the inset. Similar to composite the polymer is also showing four types of degradation i.e. (i) 121 °C (elimination of water molecules), (ii) at 230 °C (decomposition of oligomers), (iii) at 408 °C (decomposition of PANI backbone) and (iv) at 556 °C (opening of benzene and quinonoid rings) [116].

90

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(e)

BET, BJH, N 2 adsorption-desorption isotherm

The BET isotherm and the BJH plot are commonly used to measure surface area and pore dimension. The Cu-Polyanilne composite has a surface area of 63.193 m2 g−1 and a pore volume of 0.153 cm3 g−1 (Fig. 15B(a)). Figure 15B(b) shows the pore size distribution estimated using the BJH method, with an average pore radius of 1.773 nm. The type of nitrogen adsorption–desorption isotherm for this composite was investigated in order to define the pore structure and determine the type of isotherm. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, it has a hysteresis loop of type H4 that is displayed in Fig. 15B(c) and is connected with slit-like pores [116]. (f)

Raman spectroscopy

The comparison of Raman spectra of Ag-Polyindole nanocomposite with polymer (polyindole) reflects the synergic effect of silver nanoparticles in composite in enhancing signals in Raman spectroscopy (Fig. 16(i)). The stretching vibration of a pyrrole ring polyindole is reflected as bands at 1329 cm−1 . The νC7 C8 + νC5 C6

Ag-PIn Polyindole

(ii) A Ag-PIn

Polyindole

p Polyindole

Ag-PIn Ag-Pin

0.020 0.015

C Intensity

Intensity / a.u.

(i) 0.025

0.010 0.005

Polyindole

80

Ag-PIn

70 60 50 40 30

0.000 0

1000

2000

3000 -1

Raman shift / cm

(iii)

B

20 375

400

425

450

475

500

Wavelength / nm

C 20μm

4000

(iv) E D 1 μm

Element CK NK OK AgL Matrix

Wt% 22.56 02.66 06.41 68.37 Correction

At% 60.54 06.12 12.92 20.43 ZAF

A

1 μm

Fig. 16 (i) Raman spectra of Ag-PIn and PIn (ii) Photographs of Ag-PIn and PIn upon visible light and UV radiation, and the corresponding emission spectra. (iii) SEM of Ag-PIn composites (A, B and C) and the corresponding images (iv) EDAX spectra [9]

Synthesis and Characterization of Emerging Nanomaterials

91

+ νC8 C9 ; νC8 C9 + δC4 H + δC7 H; δC5 H + νC8 N + νC4 C5 ; δC6 H + δC7 H + δC5 H and νC4 C5 + νC5 C6 + νC6 C7 are reflects at 1615 cm−1 , 1497 cm−1 , 1455 cm−1 , 1150 cm−1 , 1112 cm−1 , 1048 cm−1 respectively. The νC8 C9 + γNC2 C3 + γNC2 C3 C9 ; πC6 H + πC5 H + πC3 H and πC2 H + πC6 H + πNH are denoted as bands at 894 cm−1 , 740 cm−1 and 638 cm−1 respectively [9]. (g)

Fluorescence spectroscopy

The polyindole (PIn) and Ag-PIn composite exhibits fluorescence under UV light in DMF solvent. In General PIn, its derivatives show blue fluorescence, but here the material emits Violet-Blue light (430 nm). Here, the wide peak distribution attributes to the distributions of Ag-PIn and PIn molar mass. As Ag nanoparticles exhibit the strongest Plasmon resonance of all the noble metals tested, composites incorporating Ag nanoparticles have received a lot of attention. The fluorescence intensity of the Ag-PIn composite is double to that of PIn, which is interesting (Fig. 16(ii)). This increase is attributable to the migration of the energy of the plasmon resonance from Ag to PIn due to the surface plasmons of Ag nanoparticles during light stimulation of the Ag-PIn composite. This means that the spacing between the polyindole and the Ag nanoparticles is ideal for energy transfer [9]. (h)

Scanning electron microscopy (SEM)

The SEM is an important tool for prediction of morphological texture. The long polyindole fibres with quasi spherical silver nanoparticles are noticed for Ag-PIn composite (Fig. 16(iii)). The non-uniformity of Ag nanoparticles over polymer fibers depicts the fastness of polymerization at the immiscible interface. This type of morphology is expected due to interaction of polymer formed at the interface with reduced Ag cluster. The EDAX spectra show the elemental composition and formation of Ag-PIn composite (Fig. 16(iv)) [9]. (i)

V–Visible spectroscopy

The presence of Mg(OH)2 is shown by the absorption peak at 236 nm in UV–vis spectra (Fig. 16A). The red shift was seen at 232 nm, 233 nm, and 236 nm for NMS-1, NMS-2, and NMS-3, respectively, when the rGO weight percent increased. Because of the overlap of the Mg(OH)2 absorption peak, the absorption peak about 232 nm was missed. The development of shoulder peaks between 270 and 330 nm shows—* transitions of aromatic –C=C bonds and n- * transitions of –C=O, while the absorption peak around 232 nm was obscured by the Mg(OH)2 absorption peak. The broad peak between 330 and 385 nm was dominated by Ni(OH)2 d-d transitions. The transitions 3 A2 → 3 T1 of Ni are reflected by the broad absorption peak in the range of 385–450 nm Ni(OH)2 [91]. (j)

FT-IR spectroscopy

The broad peak at 3492 cm−1 is attributed to the starching of the –OH bond and is made up of water molecules in the interlayer as well as Ni and Mg-hydroxyl groups and is depicted from Fig. 17. The bending vibrations of water molecules

92

C. Shilpa Chakra et al.

Fig. 17 A UV–Vis spectra and B FTIR spectra of (a) NMS, NMS-1, NMS-2, NMS-3 and (b) NMA, NMA-1, NMA-2, NMA-3.[91]

caused the band at1634 cm−1 , while the bending vibrations of the –OH group in Mg(OH)2 caused the band at 1380 cm−1 . The Ni/Mg-O, O–Ni/Mg–O, and Ni/Mg– O–Ni/Mg vibrations are responsible for the broad peak centred at 613 cm −1 . C–O groups of rGO are described by the band centred at 1118 cm−1 . It is worth noting that the width of the peak near 3490 cm−1 increases as the weight percent of rGO increases, indicating intramolecular hydrogen bonding between the corresponding rGO molecules and NiMgOH [91].

5 Applications Nanomaterials have always sparked the interest of scientists due to their unique and novel characteristics. Engineered nanomaterials offer great promise for catalysis, sensor development, corrosion control, medicine, electronics, environmental remediation, and other applications.

Synthesis and Characterization of Emerging Nanomaterials

(a)

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Nanoparticles in Energy sectors:

Emerging nanomaterials have a wide range of energy-related applications, including catalysis, solar cells, light-emitting diodes, lithium ion batteries, fuel cells, and supercapacitors. Over the last decade, significant progress has been made in energy conversion, energy storage, and environmental decontamination using nanomaterials such as semiconductor nanocrystal quantum dots, carbon materials, two- and mixeddimensional nanomaterials, and perovskites. Because of the inherent properties of these materials, as well as their engineering strategies, they are capable of harvesting energy and converting it into other forms that are convenient for storage [64]. (b)

Nanoparticles in Food sectors:

The rapid development of nanotechnology, in particular, has created numerous opportunities for food sample pretreatment. Because of their ultra-small size, large surface area, unique structure, and functional properties, nanomaterials are considered excellent adsorbents, allowing for the efficient isolation of contaminants from food matrices. The use of nanomaterials as adsorbents has emerged as a promising trend in the field of food safety screening in recent years. Metal–organic frameworks, covalent-organic frameworks, ordered mesoporous silicas, polydopamine-derived materials, carbon-based materials, molecularly imprinted polymers, and other novel nanomaterials have all been evaluated in food sample pretreatment [132]. When compared to conventional adsorbents, emerging functional nanomaterials exhibit superior performance for the extraction and pre-concentration of food contaminants, significantly improving detection sensitivity, selectivity, precision, and accuracy. (c)

Nanoparticles in Environmental/waste water treatment sector:

Since nanomaterials have novel and substantially changed physical, chemical, and biological properties, owing to their structure, higher surface area-to-volume ratio, and prevention of pollution, they can be used for treatment and remediation, sensing and detection, and pollution prevention. Based on their functions in unit operations, these unique properties of nanomaterials, such as high reactivity and strong sorption, are being investigated for application in wastewater treatment. Nanoparticles can dive deeper and thus treat water in ways that conventional technologies cannot. The potential applications of some nanomaterials in water/wastewater treatment is explained as follows: CNTs or Nanometal oxide or nanofibers’ large specific surface area and ascertainable adsorption sites, selective and more adsorption sites, short intra—particle diffusion distance, tunable surface characteristics, easy reusability, and other properties led to successful in waste water treatment. The disinfection application was driven by the powerful antimicrobial properties, low toxicity and cost, and high stability of Ag or TiO2 nanoparticles or CNTs. TiO2 nanoparticles, Fullerene derivative products used in photocatalysis have photocatalytic activity in the solar spectrum, relatively low toxicity, excellent stability and sensitivity, low cost, and so on. In addition, Ag/TiO2 nanoparticles, as well as CNTs, Zeolites, and Magnetite, are used as Membranes in water purification [133]. Adsorbent materials have recently received a lot of attention in the field of food safety screening.

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Nanoparticles in Manufacturing Sector:

The unique nature regarding structure, strength, conductivity and reactivity makes nanomaterial emerging in manufacturing sector. For example the break strength, wear resistance and light weight property of CNTs makes them to use widely in manufacturing sectors as bulletproof vests, spaceship components, bike/car frames, industrial blades/robot arms, air craft accessories and boat hulls etc. [134]. While the ring structure of CNTs makes them efficient in removing of physical, chemical biological pollutants in water treatment industry. Further the shape of CNTs promotes shielding and wrapping of drug molecules in drug delivery application. The antimicrobial and satin resistance property of carbon nanofiber helps them in production of safety wear in biotextile industry. The composites of heavy metal nanoparticles with carbon nanoparticles are prominent in production of high strength, lighter weight, cost effective components in material industry especially steel industry. The interacting ability of nanoparticles with biomolecules brings revolution medical industry in device fabrication of nanoscale robots/machines/naites [135]. The electronic property of nanomaterials leads emerging applications in fabrication of flexible devices and circuit boards. (e)

Nanoparticles in Health sector:

The manipulation of particles size and surface characteristics of emerging materials to nanoscale leads development in imaging systems, therapeutic actions or in other smart systems. The drug that was manufactured using nanomaterials is promising in managing, targeting and treating of various diseases. The effective treatment with nanomaterials makes positive impact on several illness including Cancer and AIDS [136].

6 Conclusions In this book chapter, the introduction of emerging nanomaterials, classifications, introduction to various characterization techniques have been reported. In specific, the importance of various synthetic methodology viz. Electrodeposition, Liquid/ Liquid Interfacial Method, Microwave Assisted Synthesis, Sonochemical, Electrospinning Technique, Emulsion Method, Hydrothermal Synthesis and Polyol Method in production of emerging nanomaterials is described with diverse examples. The key role of characterizations in analysing the nanomaterial is described with specific examples. In addition, the applications of emerging nanomaterials in various sectors such as energy, food, Environmental, Manufacturing, and Health etc. are also described with examples. This book chapter provides complete knowledge on emerging nanomaterials with their synthetic protocols, characterizations and applications. Acknowledgements This work has been supported in part by “Department of Science and Technology (DST), Ministry of Science and Technology”, “Govt. of India”, for its generous financial

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support for the Women Scientist (WOS-A) project (sanction no. SR/WOS-A/CS-13/2019, dated 28.12.2020) through the DST Kiran Division at New Delhi and DST SYST division with Research Funds (no. SP/YO/2019/1599(G)).

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Seeded Crystal Growth of Cd-Zn-Te (CZT) Assisted via Numerical Modelling Saketh Kakkireni, Magesh Murugesan, Benjamin Montag, and John McCloy

Abstract Cd-Zn-Te/CdTe is material of choice for room temperature X-ray and gamma-ray detector applications, spanning security, medical and astronomical imaging. However, potential applications of this material are limited by unfavorable thermo-physical properties that make the crystal growth of large volume material challenging (particularly at faster growth rates). This chapter provides an overview of an attempt to develop a potential alternate growth method capable of growing large volume material at faster growth rates (2 mm/h), without the need for post processing. The growth technique discussed involves seeding in combination with the accelerated crucible rotation technique (ACRT). Successful seeding is accomplished by modifying the heat sink setup in an electro-dynamic gradient (EDG) furnace, and the effects of different heat sink configurations are discussed. Repeatability of seeding is also discussed along with the effect of initializing ACRT on tellurium inclusion distribution and grain structure. The propagation of grain structure along the crystal is demonstrated by consecutive crystal growth experiments. Keywords CZT crystal · CGSim simulation · Numerical modeling · Seeded crystal growth · Bridgman Technique

1 Importance of Numerical Modelling in Crystal Growth Heat and mass transport forms the basis of crystal growth, and the optimization of these transport phenomena is critical for obtaining high quality single crystalline materials [1–3]. Crystal growth proceeds with solidification of the mother phase (liquid, vapor, or solution) with subsequent change in enthalpy, and the rate of this process is often limited by the transport of constituents to the growth interface and/or by the rate of heat removal from the interface, and the limiting factor depends on S. Kakkireni · M. Murugesan · J. McCloy (B) Institute of Material Research, Washington State University, Pullman 99613, USA e-mail: [email protected] B. Montag Radiation Detection Technologies, Inc, Manhattan, KS 66506, USA © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_3

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Fig. 1 Different transport phenomena involved in crystallization, redrawn from [2]. Here V is the crystallization velocity and n is the normal direction

the growth method [2]. Briefly, the phenomena involved during crystallization can be seen in Fig. 1 [2]. For crystal growth from the melt (not solution growth), heat transfer is the primary rate-limiting factor, and the ability to drive the latent heat of fusion released during solidification away from the growth interface will determine the maximum possible stable growth rates for that particular material. Hence, the ability to control precisely the process parameters that govern these transport phenomena during the crystal growth is critical in determining the end quality of the crystal. For example, control of the heat flux at the growth interface is determined by the temperature gradient imposed during crystal growth, and the mass flow rate to the growth interface during solution growth depends on the crucible rotation rates imposed [4]. However, each process parameter generally affects multiple crystalline properties of the grown crystal, and the optimum parameter values are chosen based on the desired quality required for an application. Even a basic crystal growth parameter, such as temperature gradient at the solid/liquid interface, influences the permissible growth rate, constitutional undercooling, melt convection, dislocation density, and the growth interface shape. However, the ability to impose a desired thermal gradient is itself a limitation of the crystal growth method and system that has been emerging, continuously depending on the physical properties of the growing material. The classic Bridgman method [5] to grow single crystal metals was modified by Stockbarger [6] by adding an additional heating zone to achieve higher thermal gradients required for the growth of lithium fluoride crystals, which eventually led to the current electro-dynamic gradient (EDG) growth method used at Washington State University (WSU) with multiple heating zones [7]. The Accelerated Crucible Rotation Technique (ACRT) was introduced by Scheel [8] in the 1970’s to improve

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crystal sizes and reduce striations for the solution growth of GdAlO3 , and this modification has been implemented at WSU for the growth of Cd-Zn-Te (CZT) using EDG [9] and Nd:YAG using Czochralski [10], to achieve faster stable growth rates and improved dopant uniformity. For the specific case of CdTe/Cd-Zn-Te, an extensive review of challenges associated with the bulk-crystal growth are outlined by Rudolph [2, 17] with experimental data supporting the observations. Another valuable source of information is still the early report by Route [18]. Briefly, the challenges with crystal growth of CdTe/CdZn-Te can be classified broadly into two categories: (a) presence of grain boundaries (single crystal yield), and (b) presence of second phase particles (inclusions and precipitates). The material properties that make obtaining a high single crystalline yield a challenge include: (1) (2)

(3) (4)

(5)

(6)

Low thermal conductivity of the solid (~1 W/(m–K)), (leading to problems with control of growth interface shapes) Melt/solid thermal conductivity ratio greater then unity, (leading to unstable heat transfer at the growth interface, and problems with control of growth interface shape) High ionicity (~70%) resulting in an associated nature of the melt, (affecting the initial nucleation and formation of secondary grains) High symmetry of the zincblende (ZnS, CdTe) crystal structure, resulting in the absence of a strong growth direction and the tendency for any growth fluctuations to cause secondary nucleation Low Critically Resolved Shear Stress (CRSS) values, resulting in the tendency to dislocate under high temperature gradients, promoting cracking and presenting challenges with processing Accumulation of Te inclusions at the crystal growth interface, limiting the growth rates and affecting detector performance [11].

These above-mentioned factors limit the usable parameter range of crystal growth variables to optimize CdTe/Cd-Zn-Te the growth process, as each of these challenges requires different growth conditions to overcome them. For example, low thermal conductivity of the solid requires high temperature gradients to be able to drive latent heat of crystallization away from the growth interface and also control the growth interface shape, but at the same time a low CRSS value causes higher dislocation density under increased thermal gradients. This is just an example to illustrate the challenge of optimizing various crystal growth parameters, but in practice, imposing a desired thermal gradient is itself a challenge as mentioned earlier. As crystal growth experimentation is expensive and extensive in terms of both time and money, numerical simulations prove to be a valuable tool to understand the impact of changing growth parameters. Various authors have previously performed numerical modelling for the specific case of CZT to study the influence of temperature gradients [12], crucible materials [13], heat sinks [14], and crucible rotation effects [15] on the interface shape, segregation, and crystal quality, and some of the results have been experimentally validated [16]. In this work, numerical modelling is used as

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a tool to optimize thermal environments to achieve successful seeding by controlling the growth rate, temperature gradient, and the shape of the seed/melt interface.

2 Seeded Crystal Growth of CdZnTe Seeding with Cd-Zn-Te is a technological challenge due to its peculiar melt structure and solid properties (thermal conductivity), and very few successful reports exist in literature for the seeded melt growth of CZT [17]. Seeding is a traditional strategy to reproducibly achieve high single crystalline yield during the crystal growth process and is successfully implemented commercially for the growth of different semiconductors. However, for CZT this is not an easy task, and the challenges with successful seeding procedure are: (a) ability to successfully replicate the grain structure of the seed crystal (control melt back, have slow growth rate near the seed/melt interface), and (b) ability to successfully propagate the seeded grain structure throughout the boule. Both components present independent challenges but are also correlated. An oriented single crystalline seed is extremely essential for successful propagation of the grains along the length of the crystal. Although CZT does not have a strong preferential growth direction, the major crystal orientations in un-seeded growth are found to be < 110 > and < 111 > [18]. In the following sections, the task of successful replication of the grain structure of the seed is discussed in detail with the aid of numerical modelling and modifications to the growth system.

3 Numerical Modelling During this work, two commercial crystal growth software packages were used, crysMAS and CGSim, to simulate heat transfer and to assist with structural optimization in WSU’s EDG furnace setup, with practical implementation being of primary importance. This has proven to be a valuable tool to be able to predict the thermal influence of design changes during the crystal growth and achieve successful seeding of CZT in our growth system. Most of the work presented here on the EDG setup is performed using CGSim. CGSim is a versatile modelling package designed by STR group for the optimization crystal growth process [19]. It is worthwhile to mention that in this effort, the modelling capability of the CGSim has been limited to only the thermal influences of design changes and subsequent optimization, while the package is much more capable of addressing complex problems of the crystal growth, including species transport, stress distribution, and turbulent convection (gas and melt). CGSim uses a finite volume method to solve the energy, momentum, and mass conservation equations on each grid cell to arrive at a converged solution. The basic steps consist of

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constructing geometry, assigning materials, defining material properties (conductivity, emissivity, viscosity, specific heat capacity, and latent heat), generating grids, and allocating boundary conditions. Grid sizes are optimized in order for the solver to reach a convergence criterion of a relative residual value of 0.0001. The required temperature profile at the control point(s) (position(s) of control thermocouple) is given as input, and power required for each set of heaters are backcalculated using a proportional–integral–derivative (PID) algorithm in an iterative manner. Control points for the input temperature profile are basically the locations of control thermocouples in the furnace assigned to a set of heaters in the software module. The location of the control points and the specific heater that majorly influences the temperature of the particular thermocouple (TC) can be mentioned in the software module. The maximum heater power should be initially mentioned during the parameter specification module, and the PID algorithm adjusts the heater power according to the temperature specified for the particular control point. Boundary conditions are mentioned along the outer layer of furnace insulation and play a major influence on calculation of the furnace power, but these have negligible effect on the actual thermal profile in the furnace.

4 EDG Furnace The main body of the actual experimental furnace consists of 45 insulation disks stacked on each other, with grooves provided for heating coils. The outer diameter (OD) of these disks is 13.95 and the inner diameter (ID) is 3 or 4 based on the furnace used. Spiral Kanthal® advanced powder metallurgical (APM) or Pt heating elements with an OD of 0.5 and a wire thickness of 1 mm are inserted into the grooves between the disks. The furnace is divided into 43 independently controlled heating zones, out of which 32 are quadrant zones (1 zone is divided into 4 radial zones) to provide radial uniformity in temperature. Axially, the growth charge is mainly located within the quadrant zones (Fig. 2). The furnace liner consists of an alumina tube, 46.5 in length with a 74 mm OD and 72 mm ID for the 3 furnace and ~90 mm ID and ~98 mm OD for the 4 furnace. Control thermocouples (S-type) which provide feedback for the controller are placed 1–2 mm from the liner for each zone. The furnace liner shields the heating elements in case of failure. Two 1 thick refractory boards at the bottom and top of the furnace cut to the OD of the liner are provided to center the liner. On the bottom of the furnace, the liner rests on a steel plate attached to fiberboard with a hole cut to its ID. The Crystal growth assembly (Fig. 3) consists of the ampoule, crucible, pedestal support, and a set of monitor thermocouples (four shoulder TCs and one tip TC). The ampoule holds the charge-containing crucible; it is GE214® quartz for high purity. The ampoule is 15 in length with the closed end tapered to 45◦ . The ampoule OD is 69 mm, and the ID is 65 mm. The crucible along with the charge is sealed using a rotary torch under vacuum in the ampoule, with an end cap inserted into it. Refractory fiber is stuffed into the top of the end cap to reduce radiative heat loss during growth.

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Fig. 2 Basic construction and zone position of the 3-inch diameter furnace

Fig. 3 a Crystal growth assembly containing ampoule, crucible, ampoule support, etc., b positions of auxiliary TC’s

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The crucible is 7 long with an OD of 61 mm and wall thickness of 0.7 mm. The end is tapered to 45◦ to match the ampoule. The pedestal support consists of two tubes, one outer and one inner. Both of these are made of silicon carbide (SiC) ceramic, which has a thermal conductivity of ~60 W/(m·K) at 1000 °C [20]. The inner SiC tube is 14 in length and 1 in diameter with a 0.5 diameter through hole, providing space for argon flow and the center monitor thermocouple. The outer SiC tube has an OD of 61 mm and an ID of 28 mm, with grooves provided for monitor thermocouples to be placed. The sealed ampoule containing the crucible rests on the pedestal support. A sample temperature profile at IMR is given in Fig. 4a indicating different segments of the growth experiment. The ramp consists of reaching the melting temperature of the Cd-Zn-Te at a specific heating rate; this heating rate is varied during the ramp, slowed down from ~85 °C/h (up to 700 °C) to 45 °C/h before the melting point has reached. Soak refers to the system being at a constant temperature, typically superheated to ~50 °C above the melting point, but the degree of superheating depends highly on the charge composition. Growth segment refers to cool down under a specific temperature gradient to achieve solidification starting from the tip of the ampoule. The imposed gradient is about ~50 °C/inch, but again is particular to the conditions and the desired outcome of the experiment. Recently, IMR’s CZT is mostly grown from highly off-stoichiometric melts and the growth is generally extended up to ~700 °C (gradient zone) to ensure stable freezing towards the heel region of the crystal and to reduce the extent of the fast frozen section formed during quenching after the growth. After the growth is completed, the crystal is cooled down at a specific rate to room temperature, ~30 °C/hr.; the rate of cool down can affect the inclusion formation in Cd-Zn-Te [21, 22]. The temperature control of the 4 furnace is designed to control the temperatures precisely with a root mean squared (RMS) deviation from set points of ~(0.06–0.09) °C [23]. The rate of cool down during the growth segment depends on the imposed growth rates assuming a constant melting point. However, the melting point keeps changing as the growth progresses, as the excess component (Te) is continuously rejected into the melt. This effect becomes prominent as the off-stoichiometry of the melt keeps increasing. A sample plot for change in melting point during the growth is shown in Fig. 4b and also the difference in actual versus imposed growth rates in Fig. 4c. The set of auxiliary thermocouples represented in Fig. 5 provide a valuable source of information during the crystal growth, including providing a signature in case of a failure and an indication of charge material melting and nucleation. The observation of some of these phenomena during crystal growth are shown in Fig. 5 during typical experiment.

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Fig. 4 a Temperature profile showing different segments of crystal growth; change in b melting point, c growth rate during growth for different initial melt off-stoichiometry (weight% Te)

Seeded Crystal Growth of Cd-Zn-Te (CZT) Assisted via Numerical … b

Shoulder TC

1150 1100

Melting, Endothermic

1050 1000

Temperature ( °C)

Temperature ( °C)

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Average (Shoulder's) Tip

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Nucleation ~ 1088 °C

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Time (hrs)

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Time (hrs)

Fig. 5 Auxilary TC data during the crystal growth showing signatures of a melting and b nucleation during crytsal growth

Fig. 6 Typical grain structure of CdTe for superheating a 27 K, b superheating of 3 K-redrawn from ref [17]. Here 1 is the polycrystalline tip region, 2 show twin lamellae, and 3 are boundaries around large grains

5 Initial Work with Heat Sink Systems in Un-Seeded Growth Initial work with numerical modelling was focused on improving the single crystalline yield during the static, un-seeded growth in the standard ampoule (conical, 45°) configuration at WSU’s Institute of Material Research (IMR). To improve the yield in an un-seeded growth, it is extremely critical to be able to control the initial nucleation (by controlling the extent of undercooled region) and growth interface shape during the initial part of the growth region (conical section of the crucible) to ensure maximum yield and proper grain selection. The associated nature of the CdTe melt, as a result of high degree of ionicity, results in lower formation energy required for nucleation during the growth, and any perturbations during the growth can cause grain formation if the melt is not superheated enough. So, CdTe is often superheated >10 K [17] to be able to break down these complexes and suppress grain formation after initial nucleation; this favorable break down has been observed by

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several authors via thermal superheating [24] and also with mechanical vibration including a rocking furnace mechanism [25]. The use of thermal superheating to disrupt the associated nature of CdTe will result, somewhat counterintuitively, in an undercooling ~50 K due to increase in the formation energy required for initial solidification. This in turn leads to non-negligible initial ‘fast frozen’ region with multiple small nuclei in the absence of a high-thermal gradient, which is generally the case for the melt growth. A typical grain structure of CdTe grown from two melts with various degrees of superheating can be seen in Fig. 6. After initial spurious random nucleation, to ensure high single crystalline yield, it is critical to maintain the convex growth interface throughout the growth and suppress the propagation of any secondary nucleation occurring at the crucible walls. Additionally, the undesirable concave interface also provides a favorable radial temperature profile for nucleation to occur at the walls, as can be seen in Fig. 7. However, the control of the interface shape is extremely challenging with CZT and CdTe due to its low thermal conductivity (solid, ~1 W/(m·K)) and >1 melt-to-solid thermal conductivity ratio. Control of growth interface shape also becomes complex with an increase in crystal diameter and length of the crystal grown. The influence of the heat sink configuration in controlling the above-mentioned parameters is given in the following sections with both numerical and experimental results. A focus on practical challenges and considerations for the particular case of WSU’s EDG furnace are described. In Fig. 7, the conditions to achieve a convex growth interface clearly shows that the walls should be hotter than the central region of the crystal. This is achieved by modifying the ampoule support system that the melt-containing ampoule rests on, which is shown in Fig. 8. This ampoule support consists of an outer tube and inner tube. This idea was proposed theoretically [14] and also was validated experimentally with different heat sink configurations [26]. Below, the practical limitations and challenges in un-seeded growth are mentioned that led

Fig. 7 Thermal conditions to promote convex interface in crystal growth—redrawn from ref [17]

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Fig. 8 Standard heat sink setup with auxillary thermocouple at WSU

to the development a novel heat sink setup and its eventual importance in seeded growth.

5.1 Different Heat Sink Configurations, 1st Iteration As mentioned in the previous section, to favorably alter the shape of the crystallization front in the initial region of the growth, modifications are made to the heat sink setup in simulation the model to assess the thermal impact. However, the changes made to the setup are based on the best available resource for practical applicability. As the first change, the highly thermally conductive layer of SiC, k ~65 W/(m·K) @ 1000 °C, which forms the outer layer of ampoule support in the standard heat sink setup at WSU (Figs. 8 and 9), has been replaced by Zircar (Alumina composites RS-101) k ~0.7 W/(m·K) @ 1000 °C, in the model [27]. A snapshot of the growth interface for these two different heat sink systems during a point of time in growth is given in Fig. 10. Significant points to be noted between the two setups are the height of the solidified crystal (isotherm axial position) and the shape of the isotherms in the solid. These two snapshots are compared during the same time in the growth, and the temperature profile along with furnace line (imposed profile) remains the same for these two different configurations. Clearly, the height of the solidified crystal in the SiC/SiC setup is larger compared to the Zircar/SiC setup due to the enhanced thermal conduction of the larger area SiC ampoule support of the SiC/SiC setup. This difference is also very evident from the actual run data collected during the crystal growth experiment with these two different ampoule support systems. The shoulder thermocouple (TC) always reads a higher temperature for the Zircar/SiC setup than the SiC/SiC setup; however, the tip thermocouples read almost the same temperature during the growth, as can be seen in Fig. 11. Observing the isotherm shapes during the initial part of the growth (conical region, Fig. 10), a clearly convex growth interface is seen with the Zircar/SiC, resulting from the insulation effect of the Zircar shielding on the outer side. Interestingly, as the growth progresses to the cylindrical region of the crucible, the SiC/SiC support seems

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Fig. 9 Structural configuration of the heat sink support, pyrolytic boron nitride (pBN), and quartz ampoule at WSU

Fig. 10 CGSim comparison of isotherms/interface shape in two different heat sink configurations, a SiC/SiC and b Zircar/SiC, under the same temperature profile at a similar time during the growth

favorable; this is again due to the increased axial thermal gradient in the solid region of the growth after the progression into the cylindrical part, due to the enhanced thermal conduction from SiC/SiC. This inversion of the interface shape with progression from the conical to cylindrical part of the growth ampoule is also seen experimentally with room temperature

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photoluminescence (PL) mapping. Figure 12 shows the PL profiles across the radial section of a Cd-Zn-Te crystal growth with nominal 10% zinc (i.e., Cd0.9 Zn0.1 Te) using the SiC/SiC heat sink support; higher zinc in the plot corresponds to the firstto-freeze section during the growth owing to its segregation coefficient >1[28]. Since the melting point and band gap [29] of CdTe changes with Zn concentration, PL mapping can be used to measure the Zn concentration, and as the freezing isotherm during the growth will have same Zn concentration, axial profiles of Zn can be used to estimate the interface shape during the growth. This also proves to be the qualitative validation of the model, and agrees well with the previous numerical results published on a different ampoule support systems [14]. Since it is critical to ensure proper grain selection in the first-to-freeze region of the growth, the Zircar/SiC heat sink system was chosen to be experimentally implemented in an un-seeded growth to study the effects on the single crystal yield, as the numerical results support favorable modification of growth interface in the conical region. The primary practical challenge that this heat sink system posed was to be able to have a mechanical contact between the crystal growth ampoule and the central SiC holder. Considering the shape of the growth ampoule, the tolerances in the manufacturing have to be flush (close to ‘0’) to be able to achieve perfect mechanical contact between the central SiC tube and the ampoule. Not having the SiC tube touching the ampoule will negatively impact the interface shape during the growth, resulting in a less convex shape. This can be seen from the modelling results presented in Fig. 13, comparing the two cases where the SiC is either in ‘contact’ or ‘non-contact’ configuration with the growth ampoule. The temperature at the tip of the ampoule can be clearly seen to be ~10 K lower for the ‘contact’ case, indicating better heat removal effect caused by enhanced thermal conduction. However, the interface shape remains convex in the ‘non-contact’ case in comparison to the standard SiC/SiC configuration shown in Fig. 14, and hence the crystal growth experiment has been performed with this non-contact configuration. Interestingly, the result of the crystal growth, numbered CG 184 (Zircar/SiC), grown under a very similar configuration to that modelled (Fig. 13b), showed a very polycrystalline tip in Fig. 14 (right)), especially when the Zircar/SiC setup should have had a favorable interface condition for achieving high single crystalline yield. Careful analysis of the real-time run data of the monitor TCs revealed that undercooling of ~50 °C occurred in the Zircar/SiC ampoule support, leading to the polycrystalline structure of the tip as observed. This undercooling effect has not been seen in the crystal growth performed with the SiC/SiC ampoule support system. The comparison of the actual run-data of the monitor TCs between these two crystal growths reveal a higher degree of superheating as a result of changing the ampoule support system to the Zircar/SiC system (Fig. 11) and thus resulting in the observed undercooling. However, one experiment does not provide enough data to conclude, as undercooling could be dependent on multiple growth parameters. This has been further confirmed by analyzing the crystal growth data from about ~70 crystal growths performed at IMR. Clearly seen in Fig. 15 is the increased average degree of the undercooling for Zircar/SiC heat sink system compared to that of the SiC/SiC system. The randomness in the data corresponds to the influence of other

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crystal growth parameters on the undercooling (e.g., growth rate, crucible material, charge composition, etc.), but a trend is evident. This trend can also be seen in the temperature data of the monitor TCs during the different crystal growth runs, which are on average higher for the Zircar/SiC system. It is important to note that other important factors such as change in the TC position during each growth can also cause changes in the observed nucleation temperatures between each growth, which is extremely difficult to control. For example, a 3 mm change in the axial position of the TC can cause ~6 °C of variation in the recorded temperature for an imposed thermal gradient of ~50 °C/inch. Here, the data is used to describe a trend with no intention of determining the exact superheating and undercooling.

Fig. 12 Radial Zn profiles provided by analysis of PL data along the length of the crystal grown using SiC/SiC heat sink setup

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Fig. 13 Comparison of isotherms in two different cases where a central SiC tube is in contact with the quartz ampoule, b central SiC tube is not in contact with the quartz ampoule

Fig. 14 Auxiliary TC data of Zircar/SiC setup showing undercooling (left), SiC/SiC with no sign of nucleation (middle) and picture of undercooled tip grown with Zircar/SiC configuration (right)

5.2 Different Heat Sink Configurations, 2nd Iteration A new compound heat sink setup was necessary to address the problem of thermal undercooling caused from superheating of the melt for the Zircar/SiC configuration. Comparing the monitor TC temperatures during the actual growth run between the heat sink systems in the previous section, Fig. 11, it is evident that the increased radial gradient/convexity in Zircar/SiC system is caused by the increase in the temperature at the shoulders while the tip temperature remains almost the same. Therefore, a new compound heat sink setup was necessary that could cool the central part of the growth ampoule preferentially, to be able to achieve the desired convex interface shape without causing much increase in superheating. However, for that purpose, a better heat removal mechanism at the central part of the growth ampoule was necessary. The limited availability of highly thermally conductive materials that are stable at high temperatures posed a critical challenge. The Fig. 16 shows a plot of

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the thermal conductivities of different materials at ~1000 °C. SiC is the best material that is relatively inexpensive and stable to use. It was already employed in one of the heat sink configurations that resulted in thermal undercooling in combination with Zircar. The adjustments to the temperature profile could have been made to reduce the undercooling, but the quest for a better heat sink at the center was also propelled due to its need in a seeded crystal growth application. In seeded growth, higher thermal gradients are essential to protect the seed from melting and to control the growth rate near the seed/melt interface. As can be seen in the model in Fig. 17, when the SiC is not in contact with the growth ampoule, the primary mode of heat transfer is the radiative loss between the ampoule and the SiC tube. Thermal conduction towards the cooler regions of the furnace through the SiC tube determines the temperature of

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Fig. 16 Thermal conductivites of different comercially available materials at 1000 °C

Fig. 17 Major modes of heat transfer near the tip region of the ampoule in the non-contact SiC configuration

the surroundings in the region close to the tip of the ampoule. The higher the degree of thermal conduction through the central tube, the cooler the temperature near the tip of the ampoule, and hence the effectiveness of radiative cooling. The only other class of highly thermally conductive materials, alternative for SiC, which can operate safely at higher temperatures, were Pt and Ir. The cost of making a Pt tube of these dimensions would be very expensive; however, reports exist for the use of Pt for the crystal growth of Cd-Zn-Te [25], and this configuration proved to produce effective results. Considering the cost limitations, Mo was chosen to be used as a heat sink to effectively cool the central region of the ampoule. Enhanced thermal conductivity of Mo under the same imposed temperature profile can effectively cool the central region of the ampoule. However, the oxidation of molybdenum (Mo), which forms

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Fig. 18 image showing Zircar/Mo heat sink setup, Mo rods sealed in quartz are shown.

a volatile oxide, is a concern for implementation in the furnace at ~1150 °C. Hence, it was proposed that the Mo rods be sealed under vacuum in a quartz tube to prevent oxidation and to allow safe use in the furnace. Fou r molybdenum rods, 0.5 in diameter and 12 in length, were sealed in quartz ampoules of 1 mm thick under a vacuum ~10–2 torr. The design is shown in Fig. 18. Using this heat sink configuration, the effect of lowering the temperature surrounding the tip is seen both numerically and experimentally. Partial transparency of quartz to infrared (IR) also helps effective radiative heat transfer between the ampoule and the Mo. Furthermore, quartz does not pose any challenges regarding purity and is an inherent part of the CZT growth system. Note that even if Pt is used in the center, as mentioned earlier, it is extremely difficult to have a mechanical contact at the center, and radiative heat transfer will be the primary mode of heat transfer cooling the center of the ampoule. Hence, Pt would act very similar to Mo sealed in quartz. Numerical results of the growth interface at a similar time during the growth are shown in Fig. 19 for all the three different heat sink systems. A higher degree of convexity can clearly be seen in the initial part of the growth for the Zircar/Mo heat

Fig. 19 Comparison of isotherms/interface shape in different heat sink configurations, a SiC/SiC; b Zircar/SiC; and c Zircar/Mo under same temperature profile at a similar time during the growth. The lowest tip temperatures are seen with Zircar/Mo setup

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sink, along with a much lower temperature near the tip region of the ampoule. The model has been validated experimentally with the PL mapping of an axial slice grown with the Zircar/Mo heat sink setup, and a clearly convex interface shape is seen in the cone part of the growth region in the Fig. 20. Also, the actual run time data from the monitor TCs clearly recorded much colder tip temperatures, and the shoulder TC temperature is in between the temperatures recorded for SiC/SiC configuration and Zircar/SiC configuration seen in Fig. 21. The Zircar/Mo heat sink configuration was extremely essential to achieve a thermal environment favorable for seeded crystal

Fig. 20 a Grain structure along the axial slice of the crystal grown wih Zircar/Mo configuration, b Room temperature photoluminescence (RTPL) map showing the interface shape of the same crystal showing a convex growth interface in the cone region

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growth, and ultimately lead to achieving successful seeding, as described in the following sections.

6 Seeded Growths of CZT: Initial Work with Furnace It is essential to have a precise knowledge and control of the temperature in the furnace to be able to control the seeding process and to be able successfully replicate the grain structure of the seed crystal. Interface temperature and the temperature gradient at the seed-melt interface are the most critical for controlling the melt back and growth rate at the interface. However, to precisely control the temperature at the seed-melt interface, it is critical to know the exact position of the monitor TC’s and how the imposed temperature profile and the growth environments affects the temperature distribution. Therefore, before the implementation of seeding, a large amount of effort went into improving the control system of the furnace and reducing the fluctuations in temperature that could adversely affect the growth process. The furnace control system showed degradation over time (over years) and there was random noise in the control TC readings in the furnace. Prior to attempting seeded growth these fluctuations were ignored, as ACRT tends to reduce these effects by homogenizing the melt, but for seeding it was essential to have a precise control. After multiple trials with about three weeks of effort to determine the source of noise in the control system, it was found that the 5B signal conditioning modules, which are the amplifiers of the TC voltage with subsequent transfer to the data acquisition system, were the source of noise. All 43 amplifiers were replaced, and the control was improved significantly. Before and after replacing the amplifiers, the difference in temperature fluctuations is shown in Fig. 22. In addition, it was found that there was ~20 °C offset in the

Fig. 22 Temperature fluctuations in the control TCs a after and b before replacing the TC amplifiers

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temperature reading of the wireless TCs used to monitor the temperature during ACRT, which had to be corrected to impose the required temperature profile. Since the seeding was planned in combination with ACRT, this was critical information, as the monitor TCs are valuable resources in estimating the melt back and growth rate in the furnace during the growth. Thus, after considerable work in improving the control of the furnace, a temperature profile for the seeded crystal growth was designed and implemented. The implications of heat sink design and numerical modelling in enabling the desired temperature profile is described in the next section.

7 Design of the Seeded Crystal Growth The details of the challenges associated with seeding in CZT are detailed in the literature [17, 18, 30], but briefly, the major challenges include the absence of a strongly preferred growth direction, achievement of the convex growth interface, and controlling the melt back of the seed crystal. Seeding is an extremely challenging task with very few successful reports on melt-grown CZT to date, which sometimes require a special kind of inversion furnaces to simultaneously superheat the melt and protect the seed crystal [31, 32]. The initial work with the seeding at WSU has been described in detail in [33], and the main conclusions from that work include the challenges involved in being able to control the interface shape and growth rate near the seed-melt interface with regions of large fast-frozen section (i.e., polycrystalline section). In addition, the limitations of the furnace for imposing a desired thermal profile are discussed in the previous work. Learning from the previous work, and with assistance from numerical modelling, a temperature profile was designed to favor successful seeding. For imposing a desired temperature profile that requires a high thermal gradient in the furnace to protect the seed crystal from melting, the heat sink Zircar/Mo design was used. Fig. 23 shows the actual thermal gradients in the furnace, compared between two different heat sink systems from different crystal growth experiments under identical conditions except the heat sink setup. Clearly, a ~50 °C higher gradient is registered for the Zircar/Mo heat sink system. This is critical to be able to protect the seed crystal and control the melt back and growth rate near the seed-melt interface. This effect of controlled growth rate is also shown in Fig. 23 where the growth rate is estimated based on the temperature recorded by monitor TCs (tip and shoulder TCs). Clearly, a slower growth rate is seen in the Zircar/Mo heat sink configuration. Another advantage of this heat sink system is the convex growth interface shape in the conical region as seen in Fig. 20. All these details play major roles in the seeding process. Due to the abovementioned advantages, the Zircar/Mo heat sink was utilized for seeded growth trials, along with careful design of the temperature profile to control the melt back and protect the seed crystal. Different thermal profiles were first simulated, before experimentation, to arrive at the correct temperature near the seed-melt interface.

124 Fig. 23 Comparison of actual growth rate in a Zircar/Mo heat sink system and b SiC/SiC system; c thermal gradient comparison between the different heat sink systems (SiC/SiC bottom line and Zircar/Moly top line)

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Fig. 24 Images showing the initial seed structure cut from un-seeded growth and subsequent replication of grain structure in two seeded crystal growths

The seed crystal used was a conical seed, cut from an un-seeded growth. This contained twins, as can be seen from Fig. 24; these are also replicated in the crystals grown in the subsequent seeded growth trials. The crystal from which the seed crystal was cut was grown with a nominal Zn concentration of ~10% under ACRT conditions. Based on our past experience with ACRT, the Zn concentration at the surface of the used seed-crystal was estimated to be 10% Zn. The raw material loaded above the seed crystal consisted of a pre-compounded Cd-Zn-Te charge with nominal Zn concentration of ~10%. The seed was loaded into the same crucible in which it was grown for precise mechanical fit to prevent melt seeping around the seed crystal. The temperature profile was designed to impose an initial slow growth rate (~0.5 mm/h) for up to 30 mm of growth, during which the ampoule remained in static position. ACRT was then switched on for the remaining (~45 mm) of the growth, and the growth rate was subsequently increased to 2 mm/h (Fig. 25). Replication of the grain structure has been demonstrated twice, ensuring process reproducibility.

Fig. 25 Temperature profile for seeded growth showing the regions of static growth and onset of ACRT

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However, there are still problems with secondary nucleation and seed propagation which are discussed in the subsequent sections.

8 Seed Propagation and Sidewall Nucleation It can be seen from Fig. 24 that the grain structure of the seed is replicated in two consecutive crystal growths; however, the propagation of the seed grain structure is still a challenge and the factors responsible are analyzed here. Comparison of the grain structure very close to the seed crystal can be seed in Fig. 26. The grain structure of the seeded crystal is very similar to the seed-crystal, with small random grains close to the periphery of the crystal. These are mainly due to the structural imperfection (chipping and cracking) of the seed crystal before the experiment, which are formed due to the sharp edges of the conical seed crystal being damaged due to handling, including etching and loading into the ampoule (Fig. 27). These structural imperfections can cause melt to seep around the seed crystal into the regions of lower temperature and cause super cooled grains. This effect is magnified during the second seeded growth, as the number of smaller grains increase from the first growth (Fig. 26). To overcome this problem, there is a need for an extremely smooth crystal surface, free of chips and cracks. Considering the machining difficulties associated with CZT, due to its tendency to dislocation and cracking, this is difficult, but often processes such as diamond turning are used [31]. In this work, no particular care has been taken to use an oriented seed, which would also have a significant effect on seed propagation through the entirety of the boule. The goal of these experiments was to demonstrate a continuous crystal growth (including seed) without the presence of cracks or a fast-frozen section. As mentioned previously, in these crystal growth experiments, the temperature profile and onset of ACRT are chosen such that ACRT is started after ~30 mm of growth (starting from

Fig. 26 Grain structure of in the initial regions of seed crystal for two consecutive crystal growth experiments

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Fig. 27 Images of the seed-crystal (etched) before loading in to the crucible; chips around the periphery cause secondary nucleations

the seed crystal, which is ~20 mm). However, due to uncertainty of exact location of the isotherm, it was found that the ACRT had been started actually after ~12 mm of growth on the seed crystal. The initiation of ACRT and sudden transition to faster growth rates are the reasons for the reduced grain size, as the growth interface still is desirably convex near the region of onset of ACRT. The PL mapping was carried out for axial slice of crystal and the Zn concentration was calculated according to the relation:  −0.54 + 1.7728 − 0.92E g (x) ((1)) x= 0.46 which is obtained by solving the band gap bowing equation for the concentration x: E g (x) = (1 − x)E gCdT e + x E gZ nT e − bx(1 − x)(2)

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Here x is the fraction of Zn, the endmember band gaps are taken as E gCdT e = 1.61 eV, E gZ nT e = 2.38 eV, and the bowing parameter b = 0.23 eV and E g (x) is obtained by spectral fitting [34]. The Zn distribution on the axial slice, and also from the inclusion distribution along the length of the crystal growth region are given in Figs. 28 and 29. The tellurium inclusions are more prevalent in the static region than in the ACRT region, which indicates that Te inclusion is reduced drastically by ACRT. The Zn distribution at the bottom or close to the conical shape is 8.2% and then varied 9.3% then 8.2% at the center of crystal and finally 6% at the heel portion of the crystal. The Zircar/Mo heat sink simulated result showed that the crystal growth interface shape is convex, and the same convex interface shape is experimentally verified by PL mapping for axial crystal. A successful seeded grain propagation will be achieved only with optimization of the transition region into ACRT after the static region, including the growth rate and rotation profile.

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Fig. 28 a Grain structure along the axial slice of seeded growth (CG 230); b PL map of the slice showing variation in zinc and onset of ACRT

Fig. 29 IR microscope image showing difference in Te inclusions before and after onset of ACRT during the seeded growth

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9 Conclusion We have successfully developed a new compound heat sink system to implement seeding in the melt growth of Cd-Zn-Te. The experimental challenges with different heat sink systems were analyzed and their effect of undercooling in an un-seeded growth were described. The reasons for unsuccessful seed propagation through the entirety of the crystal were described with supporting evidence from Zn and Te distributions. Results from numerical modelling were qualitatively validated by experimental results, and it proved to be valuable as a tool for crystal growth optimization. Further improvement in seeding CZT can be made through use of an oriented single crystalline seed. Acknowledgements Crystal growth efforts performed as part of this work are dedicated to late Professor Kelvin G. Lynn (1948–2020), who initiated and supervised Cd-Zn-Te research at WSU. The authors acknowledge former PhD students Dr. Jedidiah McCoy and Dr. Amlan Datta for helping in many way during the Ph.D. work of SK. We thank the members of IMR, WSU, Jasdeep Singh, and Becky Griswold for their assistance. We acknowledge the input provided by Dr. Mia Divecha, and Prof. Jeff Derby of the University of Minnesota towards implementing ACRT at WSU. Authors would also like to acknowledge the feedback provided by Dr. Aleksey Bolotnikov of Brookhaven National Laboratory. This work was supported by US Department of Energy—DE-SC0020023 STTR project.

References 1. R. A. Brown, Theory of transport processes in single crystal growth from the melt. AIChE. J. 34(6), 881–911. John Wiley & Sons, Ltd (1988). https://doi.org/10.1002/aic.690340602 2. P. Rudolph, Transport phenomena of crystal growth—heat and mass transfer. AIP Conf. Proc. 1270(1), 107–132 (2010). https://doi.org/10.1063/1.3476222 3. W. R. Wilcox, Transport phenomena in crystal growth from solution. Prog. Cryst. Growth Charact. Mater. 26(C), 153–194 (1993). https://doi.org/10.1016/0960-8974(93)90014-U 4. J. Zhou, M. Larrousse, W.R. Wilcox, L.L. Regel, Directional solidification with ACRT. J. Cryst. Growth 128(1–4), 173–177 (1993). https://doi.org/10.1016/0022-0248(93)90314-M 5. P.W. Bridgman, Some properties of single metal crystals. Proc. Natl. Acad. Sci. 10(10), 411–415 (1924). https://doi.org/10.1073/pnas.10.10.411 6. D.C. Stockbarger, The production of large single crystals of lithium fluoride. Rev. Sci. Instrum. 7, 133 (1936). https://doi.org/10.1063/1.1752094 7. A. Datta, K. A. Jones, S. Swain, K. G. Lynn, Modified vertical bridgman growth of Cd1−x Znx Te detector grade crystal in a 4 inch EDG furnace, in 2009 IEEE Nuclear Science Symposium Conference Record (NSS/MIC), pp 1771–1776 (2009). https://doi.org/10.1109/NSSMIC.2009. 5402202 8. H.J. Scheel, E.O. Schulz-Dubois, Flux growth of large crystals by accelerated crucible-rotation technique. J. Cryst. Growth 8(3), 304–306 (1971) 9. J.J. McCoy, S. Kakkireni, Z.H. Gilvey, S.K. Swain, A.E. Bolotnikov, K.G. Lynn, Overcoming mobility lifetime product limitations in vertical bridgman production of cadmium zinc telluride detectors. J. Electron. Mater. 48(7), 4226–4234 (2019). https://doi.org/10.1007/s11664-01907196-5

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Design Techniques for High Reliability FET by Incorporating New Materials and Electrical/thermal Co-optimization Young Suh Song , Shiromani Balmukund Rahi , Shubham Tayal , Abhishek Upadhyay , and Jang Hyun Kim

Abstract This chapter addresses overall CMOS design techniques for the future technology nodes (especially, sub 7-nm node technology). In the introduction, the emerging new materials and novel structures for the next-generation CMOS device are covered, and their advantages and disadvantages over conventional CMOS devices are addressed. Then, three main factors for next-generation CMOS device design—better electrical characteristics/improved thermal characteristics/high semiconductor reliability—are explained, and three optimization techniques are specifically suggested as a method to accomplish these three factors. For the first optimization method, gate dielectric material engineering technique is proposed by utilizing the structure of hetero-gate-dielectric. Then, CMOS design by utilizing high thermal conductivity of aluminium oxide (Al2 O3 , alumina) is suggested as a second optimization method, for improved thermal performance and better device reliability. Thereafter, various doping techniques are introduced for CMOS design. Finally, this chapter is concluded by specifically addressing three expected challenges that could arise when these optimization techniques are adopted.

Y. S. Song (B) Korea Military Academy, Seoul, Republic of Korea e-mail: [email protected] S. B. Rahi Mahamaya College of Agriculture Engineering and Technology, Ambedkar Nagar, Akabarpur, Uttar Pradesh 224122, India Narendra Dev University of Agriculture and Technology, Kumargang, Faizabad, Uttar Pradesh 224229, India S. Tayal SR University, Warangal, India A. Upadhyay Fundamentals of Electrical Engineering and Electronics, Technische Universit¨at Dresden, Barkhausen building room 263, Helmholtzstraße 18, 01069 Dresden, Saxony, Germany J. H. Kim Pukyong National University, Busan, Republic of Korea e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_4

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Keywords CMOS design · Design technology co-optimization (DTCO) · Electrical/thermal co-design · Thermo-electric co-design · Semiconductor reliability · Next-generation semiconductor device · International Technology Roadmap for Semiconductor (ITRS) · IRDS

1 Introduction Recently, the information technology (IT) market has been on a very important turning point. With the development of cloud technology and fifth generation (5G) technology, the amount of information needed to be processed has been increasing exponentially [1]. According to this demand, lots of semiconductor manufacturing companies (Intel, AMD, TSMC) have been continuously increasing the number of transistors per unit area of central processing unit (CPU) through the introduction of 3/5/7-nm node technology [2]. Fortunately, with the help of advancement in extreme ultraviolet lithography (EUV) lithography, many CPU manufacturers have successfully increased the transistor density so far [3]. Specifically, the transistor density has increased from 8 millions/mm2 (in 2010) to 105 millions/mm2 (in 2020). However, at the same time, as the density of the transistor increases, the amount of generated heat by the CPU also concomitantly increases and ‘heat’ issue emerges [4]. The problem is that this heat not only heats up the computer, laptop, or mobile phone, but also reduces the CPU’s 1) lifespan (reliability degradation) and 2) speed (performance degradation) [5]. In fact, this heat problem has been conventionally solved by using thermoelectric (considering thermal and electrical characteristics simultaneously) circuit design [6]. However, as the technology node approaches 7-nm node or less, CPU generates heat too much to solve it by simply applying conventional methods [7–10]. Therefore, when designing a transistor in a recent CPU, it is really critical to consider this heat issue. In this framework, this chapter is going to address the metal–oxide–semiconductor field-effect transistor (MOSFET) design methodology with design technology cooptimization (DTCO) which aims to simultaneously improve device performance and device reliability. In addition, trade-off issues will be also precisely discussed for future semiconductor design. After that, the risks and expected challenges for future MOSFET design will be covered.

1.1 Integration of MOSFET and Moore’s Law In 1965, Gordon E. Moore (chairman emeritus and co-founder of Intel Corporation) published the ground-breaking paper addressing the evolution of transistor density in integrated circuits (IC) [11]. He predicted that the density of transistor in IC

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Fig. 1 Report of future semiconductor roadmap predicted by the IRDS 2017 [15]

technology would quadruple every three years [11]. This prediction has been called ‘Moore’s law’ and noticeably well followed by the semiconductor industry for the last fifty years [12, 13]. Since lots of semiconductor companies and academy have been eager to confirm this Moore’s law and precisely predict the future demand of semiconductor density, the organization named ‘International Technology Roadmap for Semiconductor (ITRS)’ was founded in 1998 [14]. ITRS organization regularly issues reports predicting the future demand of semiconductor specification. These reports usually explain the type of technology, design methodology, emerging material utilization, metrology tools that would be needed to be incorporated in order to match supply and demand of future semiconductor [13]. These reports have served as a major benchmark for the semiconductor industry, and the name of ITRS was changed to ‘International Roadmap for Devices and Systems (IRDS)’ in 2016 [13]. As shown in the Fig. 1, the demand for integration will steadily increase in the future, and therefore MOSFET also needs to be reduced in terms of size [15]. In fact, as shown in the future semiconductor roadmap by IRDS (Fig. 1), MOSFET will need more scaling for higher integration until 2033 and the gate length (L gate ) of the future MOSFET needs to be reduced [15]. However, reducing L gate for integration is not that easy. This is because various undesirable short channel effects (SCEs) and self-heating effects (SHEs) appear as L gate decreases. This will be specifically discussed in the next Sect. 1.2.

1.2 Short-Channel Effects and Emerging New Materials Basically, the MOSFET is operated by change of gate voltage (V gate ). For example, when V gate increases, many free electrons are generated in channel (inversion layer is formed in channel), and these generated electrons move from source to drain. On the other hand, when V gate decreases, there are less free electron in channel, less electrons

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move from source to drain. Therefore, more current flows as V gate increases, and less current flows as V gate decreases. In this perspective, the control ability of gate is really important in the operation of MOSFET. However, when the dimension of MOSFET becomes smaller and L gate decreases, the control ability of gate becomes smaller. This is because ‘the effect of potential difference between source and drain’ becomes greater as L gate decreases, which decreases the control ability of gate. Namely, as the dimension of MOSFET is decreased for more integration, the source and drain becomes closer, and therefore the gate controllability (ability of gate electrode to control the operation of channel) eventually decreases. As a result, several undesirable results, called ‘short channel effect (SCE)’ start to deteriorate the electrical performance of MOSFET. For example, due to SCE, the off current (I off ) increases and the power consumption during steady state also increases. Considering all practical purposes, it seems nearly impossible to shrink L gate below 14 nm [13]. If MOSFET is scaled with the simple conventional method of decreasing L gate , Moore’s law might reach an end. Therefore, it is critical to mitigate this SCE as much as possible and most semiconductor designers aim to solve this SCE for MOSFET design. In order to exactly quantify the amount of SCE, ‘Drain-Induced Barrier Lowering (DIBL)’ is usually used. In specific, when L gate decreases and channel becomes shorter, energy barrier between source and drain decreases. As a result, significant amount of current flows during off-state, which deteriorates the MOSFET performance. The specific formula of DIBL will be described in the following paragraph. For more numerical analysis on SCE, there is a sophisticated tool called the Voltage-Doping Transformation model (VDT) [13], which has been widely utilized to numerically analyze the effect of scaling device parameters such as L gate or drain voltage (VDS ) into electrical performances. Two important parameter SCE and DIBL could be interpreted with the following expressions from this VDT model [13].   x 2j tox tdep εsi εsi Vbi = 0.64 E I Vbi SCE = 0.64 1+ 2 εox εox L el L el L el   x 2j tox tdep εsi εsi DIBL = 0.80 VDS = 0.80 E I VDS 1+ 2 εox εox L el L el L el

(1)

(2)

where L el /Vbi /t ox /x j are the effective channel length/built-in potential of source or drain/gate oxide thickness/source and drain junction depth, respectively [13]. t dep is corresponding to the penetration depth of the gate field in the channel area, which is the same as the depth of the depletion region [13]. The parameter EI is the abbreviated form of ‘electrostatic integrity (EI)’ factor, which is strongly affected by the device geometry. From the above two Eqs. 1 and 2, the threshold voltage (VTH ) of a MOSFET could be obtained by the following Eq. 3 with the long-channel device threshold voltage (VTHL ) [13].

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(3)

The VTH is the parameter where the transistor changes its state between on-state and off-state. When Vgate >VTH , the transistor approaches to on-state, and when Vgate < VTH , the transistor approaches to off-state. This VTH is affected by L gate . If MOSFET has long channel, VTH is equal to VTHL , and I off and power consumption are simultaneously suppressed. However, if MOSFET has short channel, VTH decreases by ‘SCE + DIBL’ from VTHL , and I off and power consumption are concomitantly increases. Namely, the VTH strongly depends on SCE and DIBL, and therefore VTH is strongly affected by L gate and device geometry. Usually, the shortened L gate causes the decrease of VTH , which is called ‘threshold voltage roll-off’.

1.3 Roadmap for Future MOSFET Technology with Emerging New Materials Figure 1 shows the change of L gate and geometric structure in the past and future, suggested by IRDS up to 2033 [15, 16]. When we look at the most important parameter L gate in Fig. 1, we can find the L gate has been continuously reduced and needs to be further reduced in the future. In addition, it can be also found that the geometric structure of MOSFET has also constantly changed to alleviate the SCE caused by scaling. Among critical geometric changes in MOSFET, the shape of the gate has especially changed a lot. This is because the loss of gate controllability could be mitigated by changing the gate shape [17]. As shown in Fig. 2, early MOSFET is designed

Fig. 2 Geometric evolution of MOSFET from planar structure to gate-all-around (GAA) MOSFET structure

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with planar structure that covers only one side of the channel. This planar MOSFET evolved into a double gate structure that covers two sides of the channel. After that, the shape of the gate changes to fin structure for covering approximately three sides of the channel. This structure is usually called the ‘FinFET’ structure. Thereafter, the gate shape is changed to an omega () structure, and then gate-all-around (GAA) structure that completely wraps all around channels. Namely, the structure of MOSFET has been continuously changed in order to increase the gate controllability (how efficiently the gate controls the channel). Even though GAA structure shows excellent gate controllability and electrical performance, a big problem also concomitantly arises. In GAA MOSFET, since the gate dielectric material (which usually has low thermal conductivity) completely covers the channel, the heat generated from the channel is hard to escape to the outside. More importantly, since the most commonly used gate dielectric material hafnium oxide (HfO2 ) has significantly lower thermal conductivity than silicon dioxide (SiO2 ), this heat issue becomes worse in GAA MOSFET [5, 18–21]. This heat problem in GAA MOSFET increases the temperature of the channel, which causes various self-heating effect (SHE) problems. These SHE issues could be classified into two types (Fig. 3). Firstly, SHE causes reliability problems. If heat is not easily dissipated during operation, significant heat will be eventually accumulated in the MOSFET and the lifespan of the field-effect transistor (FET) will be reduced. The accumulated heat in MOSFET also affects the temperature of the metal line connecting MOSFETs, which eventually leads to a decrease in the lifespan of the CPU.

Fig. 3 Step by step illustration for understanding the SHE. The problems from SHE could be classified by 2 main types: reliability problem and performance problem

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Fig. 4 Illustration describing workflow of the chapter “Synthesis and Characterization of Emerging Nanomaterials”

Secondly, SHE causes performance problems. If the temperature of the channel is high, the mobility of charge carriers (electrons and holes) in the channel also undesirably decreases. Consequently, the on-current (I on ) of MOSFET concomitantly decreases, which could further worsen the resistance–capacitance (RC) delay [23–25]. Figure 4 shows the workflow of this chapter for the better and intuitive understanding of readers. In Sect. 2, the recent demand for SHE improvement will be discussed with recent CPU issues and various perspectives. Then, in Sects. 3 and 4, the optimization methodology will be carefully explained in terms of structural approach and material approach. Finally, in Sect. 5, the design of the future MOSFET with DTCO will be concluded with expected challenges.

2 Recent Demand for SHE Improvement 2.1 Recent CPU Issue As previously mentioned in Sect. 1, as the technology node advances, the density of the transistor increases and gate dielectric completely covers all channels, and therefore heat issue in CPU becomes more and more serious. On YouTube, there is a fun video that clearly shows this. Figure 5 summarizes a video of ‘BBQ on CPU (Meatsink)’ which is uploaded on YouTube ‘たれみみ (Taremimi) Channel’ in March 2020.

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Fig. 5 YouTube video showing new method of grilling meat (uploaded by Taremimi channel, https:// www.youtube.com/watch?v = zAEXuONMJCQ). This video points out important heat issues in recent CPU

In this video (source: https://www.youtube.com/watch?v=zAEXuONMJCQ), engineer Taremimi shows the process of cooking the meat. Firstly, he applies cooking oil on the CPU, and then places a piece of beef on it. After some minutes, the beef is finally well done. This video had over 3.5 million views in a year and a half, and has become a hit. Even though this video interestingly introduces a new method to grill meat just for fun, it actually addresses a very important issue among recently released CPUs. Namely, the recent CPU generates tremendous heat which is difficult to handle. This large amount of heat not only degrades the lifespan of the CPU, but also degrades the performance of the CPU.

2.2 Demand for SHE Improvement—Performance Aspect Figure 6 explains the mobility of electrons according to the temperature [26]. Electron mobility usually increases as the temperature increases, and after a certain turning

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Fig. 6 Relationship between electron mobility and temperature, in silicon channel. Before certain turning point, the impurity scattering becomes dominant, and after certain turning point, the lattice scattering becomes dominant. (This figure was re-edited from the https:// nptel.ac.in/content/storage2/ courses/113,106,065/Week% 203/Lesson8.pdf source figure in) [22]

point, it decreases as temperature increases. This is because impurity scattering dominates until temperature reaches a certain turning point, and then lattice scattering becomes dominant after a certain turning point [26]. Specifically, charge carriers experience increased lattice scattering at high temperatures due to increase in phonon vibrations. At low temperatures, this phonon vibrations decrease and other impurities like impurity scattering, electron to electron (e–e) interaction and localization phenomenon evolve at low temperatures. More importantly, above 300 K where MOSFET usually operates, the mobility of electrons generally decreases as the temperature increases. Therefore, in a typical MOSFET design, an increased amount of heat causes a decrease in mobility, which consequently leads to decrease of Ion. As a result, the CPU becomes hard to perform the high performance which the CPU was originally designed for.

2.3 Demand for SHE Improvement—Reliability Aspect As shown in Fig. 7, as device temperature increases, the failure time (Tfail ) of MOSFET decreases. That is to say, if device temperature increases, MOSFET shows breakdown faster. This SHE also causes lots of reliability issues as well. For example, regarding transistor perspective, SHE (1) brings negative-bias temperature instability (NBTI) [27], (2) decrease of threshold voltage [28], and (3) hot-carrier induced degradation [29]. In addition, regarding circuit perspective, SHE brings (4) reliability issues such as metallization lifetimes of circuit [30]. Regarding (1) NBTI, as device temperature increases, silicon-hole (Si–H) bonds at Si/SiO2 interface become broken, and significant diffusion of hydrogen (H) is formed at Si/SiO2 interface, which makes severe performance instability. These phenomenon

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Fig. 7 Failure time of MOSFET depending on device temperature. In general, the failure time of MOSFET becomes shorten as temperature increases

also results in (2) decrease of threshold voltage, which concomitantly increase I off and undesirable power consumption. Regarding (3) hot-carrier induced degradation, ‘the increased energy of electron due to increased temperature’ could increase formation of electron–hole pair (hotcarrier), and this newly added hot-carrier could penetrate into gate oxide or Si/SiO2 interface. Eventually, the quality of SiO2 degraded, and the performance of MOSFET consequently becomes deteriorated as well. In addition, the increased temperature of MOSFET causes the position change of metal line (metal line that connects several MOSFET). Due to this phenomenon, 4) the metallization lifetimes of circuit could be significantly reduced, which make circuit-reliability worse at the same time. In terms of CPU perspective, this SHE could eventually cause melt-down issues [31], and even worse, CPU could explode at some case [32].

2.4 Main Design Goal ‘3H’—High Heat Dissipation, High Performance, High Reliability In previous sections, we learned how SHE affects device performances and device reliability. Then, when designing a MOSFET, what kind of goals should be aimed? First of all, the performance of the device should be prioritized. For example, high performance (HP) devices should have high I on and low performance devices should have low I off in order to obtain better device performance and reduce power consumption. However, unfortunately, when semiconductor engineers design MOSFET for HP, device reliability usually becomes worse. For example, in recent CPUs, HfO2 is the most commonly used gate dielectric material among various high-k materials, and this HfO2 brings excellent results in performance but poor results in reliability. For example, the increased gate leakage current due to HfO2 makes the degradation of HfO2 quality, and MOSFET might shows breakdown earlier.

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Specifically, as mentioned in the previous sections, the SHE problem arises from the low thermal conductivity of HfO2 . This SHE degrades the overall lifetime of device, circuit, and CPU. In addition, HfO2 has a disadvantage in gate leakage current. Since HfO2 has a low bandgap (5.3 eV ) compared to SiO2 (8.9 eV ), more current usually flows through the gate dielectric materials. In this process, a significant number of electrons are trapped in the HfO2 layer, resulting in a threshold shift, which leads to a reduction in the lifetime of the device. Therefore, co-optimization method that simultaneously considers the performance and reliability should be essential when designing a MOSFET. In general, the project team for device performance and the project team for device reliability are separated in many cases. If engineers in various fields work together to adopt the design technology co-optimization (DTCO) method that considers various factors at the same time and design the device considering SHE, the performance and reliability of the device could be simultaneously improved, despite the trade-off issues. In the following Sects. 3 and 4, techniques for designing next-generation MOSFETs using these DTCO methods will be addressed. First, Sect. 3 will discuss how to optimize the device by tuning the geometric structure of the MOSFET. This is a well-known traditional and classic approach, which can effectively improve device performance and reliability at the same time. However, there are lots of limits in changing the structure of MOSFET, and additional methods are needed. Therefore, Sect. 4 will continue to introduce additional techniques for optimizing MOSFET by utilizing emerging new materials.

3 Optimization Method with Structure Adjustment In this section, we will introduce optimization methods for GAA MOSFET which has been widely researched in recent sub 7-nm node technology. Figure 8a illustrates the cross-sectional view of GAA MOSFET [33]. In the case of GAA MOSFET, the gate dielectric and metal completely cover the channel. Figure 8b shows another crosssectional view of the GAA MOSFET [33]. In this structure, a hetero-gate-dielectric (HGD) structure is adopted as the gate dielectric structure. Specifically, SiO2 is used as the gate dielectric at the drain side and HfO2 is used as the gate dielectric at the source side (Fig. 8b) [33]. Song et al. team proposed this structure in 2020, and this structure was designed considering the well-known fact that most heat is generated in the channel of the drain side [33]. As electrons are gradually accelerated when they move from the source to the drain, the electrons in the drain side channel usually have high energy, and as a result, the drain side generates the most heat. As a result, it has been demonstrated that the maximum temperature is significantly reduced from 498 to 431 K by adopting HGD GAA MOSFET structure (Figs. 4.9 and 4.10] [33]. This is because SiO2 in the HGD acts as an effective heat sink, as shown in Fig. 4.10 [33].

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Fig. 8 a Cross-sectional view illustrating GAA MOSFET, and b another cross-sectional view describing GAA MOSFET utilizing hetero-gate-dielectric (HGD). In HGD GAA MOSFET, SiO2 is partially adopted as gate dielectric of source side. SiO2 in HGD acts as effective heat sink and suppressor of gate leakage current

For this design optimization technique, there are additional advantages in addition to SHE [33]. As mentioned in the previous sections, SiO2 has a high bandgap of 8.9 eV, and the gate leakage current could be significantly lowered by utilizing the HGD structure (Figs. 9, 12]. Specifically, gate leakage current could be lowered by more than 100 times by adopting this HGD structure (Fig. 9) [33]. Also, although not covered in this research paper [33], it is expected that this HGD GAA MOSFET is also possible to reduce DIBL. For most MOSFET devices, Fig. 9 Maximum temperature and gate leakage current with regards to L SiO2 of HGD

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Fig. 2.10 Distribution of lattice temperature in HGD GAA MOSFET with regards to various L SiO2 . As HGD is utilized, the thermal characteristic is effectively improved, since SiO2 in HGD acts as efficient heat sink. Specifically, as L SiO2 increases, the maximum lattice temperature decreases

due to the potential difference between gate and drain in off-state (Vgate = 0 V), a significant DIBL current flows between source and drain. However, in HGD GAA MOSFET [33], the DIBL current might be desirably reduced, since the capacitance between the gate and the drain is reduced by SiO2 in HGD. Perhaps, some readers might wonder ‘If HGD is adopted, won’t Ion decrease due to SiO2 in HGD?’. However, fortunately, in HGD GAA MOSFET, the maximum temperature is lowered, and thus the mobility is also improved. Therefore, even if HGD is adopted, I on might not significantly decrease (Fig. 11) [33]. In other words, there might be no big difference in Ion, whether the conventional GAA MOSFET structure is used or HGD GAA MOSFET structure is used [33]. Instead, the Fig. 11 Actual on-current (Ion) when SHE is considered. Even though the effective permittivity of HGD might be lower than pure HfO2 film, the actual I on will not significantly due to improved thermal characteristics

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Fig. 12 Energy band diagram in HGD GAA MOSFET (right), compared to the conventional GAA MOSFET (left)

three factors, SHE, gate leakage current, DIBl could be simultaneously improved by utilizing HGD GAA MOSFET structure. Furthermore, since SHE issue is improved in the transistor, the increase in the temperature of the metal line connecting the MOSFET is also suppressed, and therefore the performance of the circuit is also expected to be improved. In this Sect. 3, some design techniques are covered to simultaneously improve the performance and reliability of MOSFET by optimizing the device structure. Even though much improved MOSFET could be designed by adopting this structure optimization method, this method is not perfect and has some challenges to overcome. Namely, this sophisticated HGD GAA MOSFET structure is hard to fabricate. In other words, the method of optimizing MOSFET by changing the geometric structure often complicates the device process, so it is necessary to apply not only the structure optimization method but also another optimization method by material engineering. This material optimization method will be discussed in the next Sect. 4.

4 Optimization Method with Emerging New Materials Figure 13a shows the vertically stacked GAA pMOSFET structure designed for high I on [34]. Song et al. has proposed the novel structure for vertically stacked GAA MOSFET by utilizing the emerging new materials [34]. In the proposed structure, it is suggested to use Al2 O3 as gate dielectric instead of HfO2 for reliability improvement. Al2 O3 has been widely researched as high-k material especially in the pMOSFET field, however it was hard to be adopted in the real industry due to its low permittivity compared to HfO2 [35–41]. Nevertheless, Al2 O3 has hidden advantages in terms of thermal conductivity and bandgap. The thermal conductivity of Al2 O3 is 29 W /(K*m), which is 59 times that of HfO2 (under bulk condition), and this superiority on thermal conductivity is maintained even in a thin film condition [34]. In addition, the bandgap of Al2 O3 is 7.0 which is relatively higher than that of HfO2 (5.3 eV), which can be really advantageous in terms of gate leakage current. Figure 13b illustrates vertically stacked GAA pMOSFET when HfO2 is used (left) and Al2 O3 is used (right) under same equivalent oxide thickness (EOT) condition [34]. Of course, since the dielectric constant of Al2 O3 is lower than that of HfO2 ,

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Fig. 13 a Cross sectional view illustrating vertically stacked GAA pMOSFET, and b another cross sectional view illustrating stacked GAA pMOSFET with HfO2 (left) and Al2 O3 (right). Due to the lower permittivity of Al2 O3 , Al2 O3 should be designed with thinner thickness, compared to HfO2

Al2 O3 has to be designed with thinner thickness for the same EOT. Therefore, it is nearly inevitable to use a thinner thickness of Al2 O3 , which may act as a disadvantage in gate leakage current, however this disadvantage is compensated by the high bandgap of Al2 O3 . On the other hand, regarding SHE, the thinner thickness of Al2 O3 could give remarkable advantages in terms of heat dissipation. This is because the heat dissipates better as gate dielectric becomes thinner. Figure 14 demonstrates that the SHE is considerably improved when Al2 O3 is utilized for vertically stacked GAA pMOSFET [34]. Specifically, it has been demonstrated that the maximum temperature is remarkably improved from 564 to 408 K (Fig. 14), because Al2 O3 acts as an effective heat sink due to its high thermal conductivity [34]. This advantage becomes stronger as the L gate decreases (Fig. 15a) [34]. This is because VTH roll-off occurs as the L gate decreases, and I on consequently increases (Fig. 16), and the temperature of the channel consequently increases (Fig. 15a) [34]. This proposed technique could be widely adopted for various EOTs (Fig. 2.15b) [34].

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Fig. 14 Illustration describing SHE improvement achieved by Al2 O3 . Because of high thermal conductivity of Al2 O3 , the thermal characteristic could be remarkably improved by utilizing Al2 O3 as gate dielectric in vertically stacked GAA pMOSFET. In Al2 O3 -based structure, maximum lattice temperature is 428 K . In contrast, in HfO2 -based structure, maximum lattice temperature is 564 K

Because of this improvement of SHE, hole mobility could be also improved (Fig. 17), and I on could be simultaneously improved under the same EOT condition (Fig. 18) [34]. These improvements could be found not only under direct current (DC) conditions, but also under alternating current (AC) condition. Specifically, under AC bias condition (Fig. 19), SHE could be also significantly improved under 100 MHz/1 GHz/10 GHz condition (Fig. 20). As demonstrated in these results [34], even if we use the same device structure, the performance and reliability could be remarkably improved by material engineering. This material methodology could be applied not only to the gate dielectric material, but also to other materials. For example, there is another research demonstrating that the operating temperature of the device could be improved from 572 to 410 K when using a silicon-on-sapphire (SOS, Al2 O3 -based) substrate instead of siliconon-insulator (SOI, SiO2 -based) substrate [42].

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Fig. 15 SHE improvement by Al2 O3 with regards to various a L gate , and b EOT

Fig. 16 VTH roll-off caused by shorten L gate . Consequently, I on increases as L gate decreases

Namely, not only structural optimization but also material optimization is very critical in designing next-generation MOSFET. In particular, while there are relatively numerous restrictions on changing the structure of MOSFET, there are relatively few restrictions on changing the materials used in MOSFET. In other words, it is relatively easy to change the type of material by considering compatibility

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Fig. 17 Hole mobility improvement by Al2 O3 with regards to various L gate

Fig. 18 I on improvement by Al2 O3 under same EOT condition

Fig. 19 Additional bias condition for AC analysis

between materials, compared to changing the shape of the device. Therefore, in designing next-generation semiconductor devices, the structural optimization and material optimization with emerging new materials should be simultaneously carried out.

Design Techniques for High Reliability FET … Fig. 20 SHE improvement under various AC conditions. (In all conditions, Al2 O3 shows lower maximum temperature, compared to HfO2 )

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5 Limitation and Challenges of Future MOSFET Design with DTCO So far, the advanced design technique with DTCO has been widely discussed for the future MOSFET design. The core of DTCO is considering the trade-off issues between different research areas. However, most semiconductor industries separately operate the project team for device performance and the project team for device reliability. There is often various conflict between these two teams, since clear trade-off makes them hard to collaborate. Therefore, it is paramount to foster future engineers who simultaneously understand the device performance and reliability. If so, the future MOSFET design could be more accelerated, and hence it could be more possible to advance technology for humanity. Acknowledgements The chapter “Synthesis and Characterization of Emerging Nanomaterials” has been written by utilizing some information obtained from various previous studies, and the sources of the information are indicated in a reference format. The author would like to express heartfelt thanks to previous researchers who have conducted various researches and advanced technology for humanity. Especially, some figures in chapter “Synthesis and Characterization of Emerging Nanomaterials” were written by re-editing and utilizing the figures from the open access journal, after obtaining permission from all authors of the corresponding manuscripts (Refs. 34 and 35). In addition, several plagiarism checks (Turnitin) have been conducted in writing this chapter. As a result, under 4% similarity has been confirmed in all sentences of this chapter (except ‘reference list part’). I would like to express my gratitude to the previous studies and plagiarism screening program (Turnitin) for providing a variety of information.

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Recent Advances in Energy Harvesting from Waste Heat Using Emergent Thermoelectric Materials Saurabh Singh, Keisuke Hirata, Sudhir K. Pandey, and Tsunehiro Takeuchi

Abstract A powerful economy and a sustainable society are based upon the available energy resources. A nonrenewable energy source is limited in its availability as a result of high consumption rates, output efficiency, and when used at large scale generates a large amount of waste heat and toxic gases. The eco-friendly and long lasting human society has always worked hard to find an alternate source of renewable and clean energy. Material scientists have found possible solutions with the discovery of thermoelectric materials that utilize the waste heat and convert it into useful electricity. This chapter focuses on thermoelectric (TE) materials. The content of this chapter is presented with the assumption that readers have a basic understanding of material’s structural, chemical, and transport properties, as well as the mechanisms responsible for displaying these properties based on computational results. We begin the chapter with an overview of thermoelectric materials and their applications in waste heat energy harvesting. The physical parameters and selection criteria for screening high-performance TE materials are also discussed. To gain a better understanding of TE properties, a brief description of the computational and experimental methods for investigating a material’s crystal structure, S. Singh (B) · K. Hirata · T. Takeuchi Research Center for Smart Energy Technology, Toyota Technological Institute, Nagoya 468-8511, Japan e-mail: [email protected] K. Hirata e-mail: [email protected] T. Takeuchi e-mail: [email protected] S. Singh · T. Takeuchi CREST, Japan Science and Technology Agency, Tokyo 102-0076, Japan S. K. Pandey School of Engineering, Indian Institute of Technology Mandi, Kamand 175075, Himachal Pradesh, India e-mail: [email protected] T. Takeuchi MIRAI, Japan Science and Technology Agency, Tokyo 102-0076, Japan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_5

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electronic and heat transport properties is provided before classifying the category of different materials. With a better understanding of materials properties, a wide range of emergent materials, discovered in recent past years, with high TE performance over a wide temperature range, are covered. We concentrated on the TE properties of chalcogenides, Si–Ge-based alloys, and oxides, in this chapter. For the realization of practical applications, everything from materials to device fabrication and output performance is elaborated. The limitations of conventional approaches and commonly available tools used in TE research, as well as potential solutions, are discussed, paving the way for further investigation of the exceptional properties governed by unusual crystal and electronic structure of materials. Finally, we present a summary of thermoelectric materials as a potential candidate for renewable energy sources in next-generation technological applications. Keywords Thermoelectricity · Thermoelectric generators · Transport properties · Thermal diodes · Energy materials

1 An Overview of Thermoelectric Materials and Its Applications In modern society, energy in the form of electricity is one of the most important things for a better life, and it has now become a basic survival need after food. This is mainly due to the involvement of a large number of equipment and electronic gadgets in human life style which mainly work by consuming the energy in the form of electricity in either direct or indirect form. In order for everything to run smoothly, the demand for electrical energy production rate has increased exponentially over time. The demand for a high consumption rate stems from both our daily needs and the industrial level. Currently, the primary energy resources used to generate electricity are nonrenewable natural resources such as petroleum, natural gas, and coal. Of course these natural resources are a gift from nature, and they can be used for as long as they are plentiful and do not damage environment. However, when such nonrenewable energy sources are consumed in excess, a large amount of heat and toxic pollutant gases are produced. Unbalanced waste heat and pollution in the form of greenhouse gases are extremely dangerous to all living things and are continuously destroying the environment. The time is not far away (approximately fifty years) when the situation will be worse and out of human control both economically and environmentally because of the crossover of the demanding rate of energy due to increasing population and the limited available energy resources. The use of non-renewable resources for better human society has advantages such as being naturally available, but also has disadvantages such as destroying the natural environment, which takes decades to recover from. The excessive use of fossil fuels has resulted in the most serious environmental issue, the global climate crisis. The National Aeronautics and Space Administration’s (NASA) database of scientific reports alerted global researchers to the danger situation on the current global

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mean temperature of the Earth. According to scientific evidence [1], the global mean temperature has risen by 1 °C in the last 60 years since the industrial revolution as shown in Fig. 1. With an increase of 1 °C, the variation in our body temperature in daily life appears to be very small. However, due to its strong influence on the number of hot and cold days across the globe, such a small increase in the global mean temperature can result in a very severe disaster [2]. According to a climate-based research report, the World Meteorological Organization, WMO (Switzerland), 2 °C may cause an irreversible disaster all over the world. Therefore, not only specific and responsible organizations, but also nearly all countries, have come up with a common agreement to control the faced climate problem, and have proposed a plan and effective policy to lower the global mean temperature. The rise in global mean temperature is primarily determined by the amount of CO2 emitted. As a result, all countries are making concerted efforts to reduce CO2 emissions. This is practically accomplished by encouraging the use of public transportation facilities rather than the adoption of private transportation in a number of countries, including the European Union, the United States of America, and Japan. Although such processes contribute to a net reduction in CO2 emissions, finding alternative clean energy sources that reduce reliance on fossil resources would be more effective [3]. To find a better and more sustainable solution, scientists and researchers all over the world are striving to find an alternate source of clean energy that will reduce our dependence on natural resources as much as possible. In the traditional process of converting electrical energy using coal, natural gases and oils, nearly 67% of the energy is lost to the environment in the form of heat. Therefore, the technology that converts a large amount of waste heat energy into useful electrical energy will be widely used in a variety of applications [4, 5]. In this regard, thermoelectric generators (TEGs) made of high performance thermoelectric materials have been found to be one of the possible solutions that provide Fig. 1 Schematic diagram of increasing trend in global mean temperature with time

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Fig. 2 Schematic diagram of Seebeck effect shown for a typical n-type of solid metal

the best sources of clean energy by converting waste heat into useful electricity [5]. TEG devices are very effective for energy harvesting due to their non-moving parts and direct conversion of heat energy into electrical energy without the emission of toxic gas. Thermoelectric materials are used not only to convert waste heat to electricity, but also to create fluid-free solid-state devices for refrigeration applications. In addition to these, several types of sensors with varying shapes and sizes can be fabricated in the form of thermocouples to detect the temperature. Before delving into the high performance thermoelectric materials that have emerged as some emerging materials in recent years, it is necessary to consider the mechanism by which heat management applications can be realized [6–9]. The thermoelectric effect is the fundamental mechanism by which a material can be used in the energy conversion from heat to electricity [10]. In a broader sense, this effect can be explained by three thermodynamically reversible processes known as the Seebeck effect, Peltier effect, and Thomson effect. The energy harvesting from waste heat and refrigeration process are mainly based upon the Seebeck and Peltier effect, therefore, some brief introductions of these two effect are mentioned prior to the detailed discussions of thermoelectric materials and TEG device efficiency. The schematic diagram by which Seebeck effect can be describe is shown in Fig. 2. About 200-year age (i.e. 1821), experimental observation of thermoelectric voltage was noticed by creating the temperature gradient across a typical metal and semiconductor materials [11, 12]. This mechanism of electric voltage formation by heat gradient is well classified as thermoelectricity. Further, this phenomenon was given a name called as Seebeck effect or thermoelectric effect. In a specific way, when a typical metal of rectangular bar (e.g. nickel) heated at one end, shown in Fig. 2, the number of free electrons at hot end experience the high temperature and thus gain high thermal energy. These electrons, within the approximation of Drude theory of solids, move towards the lower temperature end (cold end) through the diffusion mechanism [13]. The accumulation of electrons at cold end continues until the electric field created (in the direction of electron motion) prevent any further accumulation near cold end, and attain the equilibrium state condition for electrons motion. In a such steady state, the resultant thermoelectric voltage (ΔV ) between hot and cold ends is proportional to the temperature gradient (ΔT ) across the material. The proportionality constant (i.e. the ratio of ΔV and ΔT ) is called as Seebeck coefficient (S) or thermopower. The Seebeck coefficients can be positive or negative

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for a given material, and the sign of S can also possible to change in some material goes through the structural and electronic phase transition when subject to different range of temperature. The sign of the Seebeck coefficients (positive for p-type and negative for n-type) provide the information about dominant charge carriers which contribute to transport properties of the material. The approximation used in the simple Drude theory of solid have the limitation to understand temperature dependent behavior in many systems which possess the complex electronic band structure such as strongly correlated electron systems, topological materials, etc. To understand the mechanism of thermpower behavior in such materials Bloch-Boltzmann transport equation based on quantum theory of solids have been found effective in explaining the temperature dependent behavior. Further, utilizing the both n and p-type thermoelectric materials, a new phenomenon was observed in 1834 by a French physicist Jean Charles Athanase Peltier, which later known as Peltier effect [14]. In the Peltier effect, when n and p-type materials are connected electrically in series, the heat absorption or generation can occur at junction point by passing an electric current in the circuit. The direction of current decides the heat absorption or liberation at junction points, i.e. changing the direction of current change hot or cold condition of the junction point. Each material has different electronic structure and the charge carriers have different electronic energy distribution in the available energy states (chemical potential). At the junction point, when current applied in the circuit, charge carriers move from one materials to another by either gain of energy or loss of energy depends upon the chemical potential of second materials have higher or lower energy states. In the practical applications, the strength of cooling and heating at the junction points are mainly determined by the magnitude of passing current and Peltier coef  ficients of each material. Consider that a and b are the Peltier coeffcients of two materials a and b, and I is the applied current through the junction, then an expression ˙ can be defined as of the amount of heat generated or removed per unit time, ( Q), follows: Q˙ = (Πa − Πb )I,

(1)

The amount of heat carried out per unit charge for a given material is represented by its Peltier coefficient. As shown in Fig. 3, the negative charge carriers move opposite direction to the current flow, whereas holes move along the current direction. In order to maintain the continuity of current across the junction, carriers exchange their thermal energy and charge. At a given temperature, charge carriers have different thermal energy in distinct materials, which lead to the deficiency or excess of the heat energy resulting in the change of temperature at the junction [15, 16]. At this point, with basic ideas of Seebeck and Peltier effects discussed above, a further more key information related to the requirement of thermoelectric properties of materials, which helps for fabrication of an industrial thermoelectric generator is essential. The typical thermoelectric generator having two legs each of n, and p-type

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Fig. 3 Schematic diagram of device which demonstrate the Peltier effect

thermoelectric materials connected electrically in series with external load is shown in Fig. 4. For a robust and large applications of commercial TEG device, the energy conversion capability of waste heat to electricity need to be efficient enough so that it can be satisfied with the device fabrication cost charged on its output performance. Therefore, the heat-energy conversion efficiency is very key parameter which decides the device utilization for the customer. For a typical TEG as shown in Fig. 4, the conversion efficiency of a device can be estimated by the hot and cold end temperature difference, and thermoelectric figure of merit, ZT [4, 5], of the materials used for the n and p-legs. The mathematical expression for the efficiency can be written as:  1 + Z Tavg − 1 TH − TC  η= TH 1 + Z Tavg + TC /TH Fig. 4 Schematic diagram of the thermoelectric generator made of one pair of n-p thermoelectric material

(2)

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where η, TH , TC , and Z Tavg are the maximum energy conversion efficiency, temperature of the hot end, the temperature of the cold end, and the average value of ZT for each thermoelectric materials used as legs, respectively [5]. The expression 2 of energy conversion efficiency consist of two part (i) first part is Carnot efficiency, (T H − T C )/T H , for the case of heat engine this term is ideally maximized (ii) second term is material dependent factor, can be called as percentage of Carnot efficiency. The dependency of percentage Carnot efficiency on figure of merit, ZT, is shown in Fig. 5. The increasing trend of % Carnot efficiency obtainable with materials figure of merit ZT (i.e. efficiency 20%: ZT = 1, 50%: ZT = 6, 85%: ZT = 100, and 90%: ZT = 400) suggests that one can achieve higher efficiency by increasing the magnitude of ZT. Thus, for efficient energy generation from waste heat using thermoelectric technology, materials having the large value of ZT is very useful. The selection of the appropriate materials for the thermoelectric applications are mainly determined by the dimensionless quantity, ZT [5]. The value of ZT can be estimated by using the expression: ZT =

S2σ T κ

(3)

where S, σ, κ, and T are the measured physical quantity known as Seebeck coefficients, electrical conductivity and thermal conductivity and absolute temperature, respectively. The total thermal conductivity for a given material is the sum of the contributions come from electronic part and lattice part i.e. κ = κ e + κ l . These two term are known as electronic thermal conductivity (κ e ) and lattice or phonon thermal conductivity (κ l ), respectively. Fig. 5 The predicted trend shown for percentage of Carnot efficiency as a function ZT

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For large ZT, material should possess large Seebeck coefficients and electrical conductivity with lower thermal conductivity. Due to the interdependence of S, σ and κ e to each other via common physical quantity known as carrier concentration (n), it is experimentally a challenging issue to optimize the condition for finding high ZT in a material. The mathematical expression (3) suggests that by decreasing the denominator part i.e. thermal conductivity (in which mainly κ l ), overall magnitude of ZT can be increased. Experimental and computational methods have their own limitations in terms of accuracy, the number of steps and parameters involved in material processing, cost and time it takes in finding the high performance material with large value of ZT. With trial and error method, many semiconductor materials have been investigated in past several decades from the thermoelectric aspects [17]. The basic knowledge of solid state physics provides us good direction in search of material which can show the good thermoelectric properties. Some of the ideas which is proposed and found to be successful are “the low-dimensional nano-structures” [18, 19], “the strongly correlated materials” [20, 21], “the weakly bonded rigid heavy clusters” [22], “the electron crystal and phonon glass” [23, 24], “the Kondo-semiconductors” [25], “metastable crystal structure with pseudo gap”, “the strong an-harmonic scattering”. Using these ideas, the values of ZT in wide class of materials have been increased. However, still state-of-the-art materials which are commercially being used have ZT about unity. Considering the formalism based on linear response theory, which is used for the understanding of electron transport properties, a constructive guiding principle to obtain the large ZT by electronic structure modification is proposed by Takeuchi et al. in 2009 [26–28]. The approach of electronic structure modification work very well to increase ZT by enhancement of power factor (PF = S 2 σ ) in the case of semiconductor materials with low thermal conductivity. Let us briefly summarize the conditions of electronic structure modification for obtaining the high ZT.

2 Selection Criteria and Strategy for Screening the High Performance Materials The linear response theory is applied both in Kubo-Greenwood formula and Boltzmann transport equations used for understanding the electron transport properties of the solids. The expressions of electronic transport coefficients (σ, S, κ el ), as a function of temperature, which describes the thermoelectric properties of the solid material are given as: ∞ σ (T ) = −∞

  ∂ f F D (ε, T ) dε σ (ε, T ) − ∂ε

(4)

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1 S(T ) = − |e|T

κel (T ) =

1 e2 T

−

∞ −∞

1 e2 T

∞

 − μ)σ (ε, T ) − ∂ f F D∂ε(ε,T ) dε  ∞ ∂ f F D (ε,T ) − dε σ T (ε, ) −∞ ∂ε

−∞ (ε

163

(5)

  ∂ f F D (ε, T ) dε (ε − μ)2 σ (ε, T ) − ∂ε 2  ∂ f F D (ε,T ) dε − μ)σ T − (ε (ε, ) −∞ ∂ε  ∞ ∂ f F D (ε,T ) dε −∞ σ (ε, T ) − ∂ε

 ∞

(6)

where σ (ε, T) is spectral conductivity, f F D (ε, T ) is Fermi–Dirac distribution function, and μ is the chemical potential (Fermi level εF at 0 K), which changes with variation in temperature [29]. From the expression 4, 5, and 6, it can be clearly understood that transport coefficients are mainly dependent on the energy derivative of the Fermi–Dirac distribution function, spectral conductivity, and temperature dependent of the chemical potential near band edge. With help of density functional theory based computational codes, the electronic structures can be calculated within the approximations. Further, using the information of the density of states, calculated information of the group velocity as a function of energy, the required electronic transport coefficients at desired temperature is obtainable. In the strong scattering limit, spectral conductivity is mainly determined by the energy dependence of density of states. Thus, constructive modifications in density of states in the form of impurity states near chemical potential lead to the large Seebeck coefficients. With suitable electronic structure which can provide the large power factor, further, ZT of the material can be enhanced by reducing the thermal conductivity. Several approaches have been taken into considerations such as nano-structuring, complex structure, heavy mass substitutions, dimensional effects i.e. making thin films or nano wires [16–19]. These methods have shown an effective impact in suppression of the thermal conductivity covering wide class of materials and temperature region [30–32]. In a brief way to summarize the strategy, for a material which possesses low thermal conductivity in its nanostructure form, it is rather easy to modify the electronic structure for tuning the Seebeck coefficients and electrical conductivity. Such strategies are always a preferable choice from the experimentalist point of view. Let us now discuss the applications and high performance TE materials which have been extensively investigated in past few years.

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2.1 Applications and Emergent Thermoelectric Materials Historically, the Seebeck effect has been widely used in the temperature detector which is called by a thermocouple. The well-determined temperature-dependent Seebeck coefficient between the two metal wires which are electrically- and thermally-connected gives us the temperature information. On the other hand, the TEG has been used limitedly because of the poor energy-conversion efficiency. In a specific situation where only limited energy sources can be used especially in space, the high-output power is more important than energy conversion efficiency. So NASA developed the Radioisotope Thermoelectric Generator (RTG) which is using radioisotopes to generate the required temperature difference for their space missions [33, 34]. The thermoelectric materials research was started in a broad materials science field. However, a remarkable achievement had not been presented for a long time because of the lack of quantum mechanical insight which can improve the performance of thermoelectric materials [35]. From the end of the twentieth century, the research of the high-performance thermoelectric material gradually became popular because of the remarkable experimental and theoretical report as follows. M. S. Dresselhaus et al. suggested the effect of the quantum-well structure can improve the performance of thermoelectric materials [18]. Y.I. Ravich et al. reported the possibility of increasing the thermoelectric performance through the resonance charge carrier scattering [36]. G.J. Snyder published notable papers which suggest that the complex structure enhances the thermoelectric performance [30, 37]. Including these three innovations, a number of papers have been published with remarkable achievements as shown in Fig. 6 [38–43]. Fig. 6 Figure-of-merit for different class of thermoelectric materials

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The field of thermoelectric research is very old from the point of view to understanding of transport properties and their implementation into fabrication of device. As shown in Fig. 6, some of the selected high performance of materials mimic the consistent increasing trend in figure of merit with time. In search of good TE materials with the consideration of criteria discussed above, the various type of materials has been explored, and some of them have been found suitable for the fabrication of TE devices [30]. Experimental and theoretical investigations of thermoelectric properties have been carried out in various categories of materials with the different structure, and chemical composition. The different class of materials which show the thermoelectric properties are bismuth chalcogenides, tellurides, inorganic clathrates, magnesium group IV compounds, silicides, skutterudite thermoelectrics, oxide thermoelectrics, half and full Heusler alloys, electrically conducting organic materials, silicon–germanium, sodium cobaltate, amorphous materials, functionally graded materials, nanomaterials and superlattices [43– 65]. The research is continuously being carried out to improve the ZT values of these materials with the appropriate combination of elements, suitable doping, lowering the dimension, creating defects mechanism, nano structuring and band engineering, etc. [66]. Among these, nano structuring method has been one of the novel and effective approach for getting the higher ZT values; as decreasing the grain size to the nanoscale region increases the phonon scattering in the intra granular region [67]. Due to the scattering effect, phonon mean free path reduces and results in the decrement in the value of κ. In many nanocrystalline size materials, overall values of κ were found to be much lower than that of corresponding bulk or single crystal material [68]. The state-of-the-art thermoelectric materials used in thermoelectric applications are BeTe-based alloys (ZT ≈ 0.8 to 1.4) for both n and p-type TE systems; which are mostly useful for refrigeration and waste heat recovery up to 450 K temperature [51]. For the intermediate temperature region, i.e. 500 K to 900 K, Pb–Te-based alloys (maximum ZT > 1.5 at 800 K) are more suitable [52]. For higher temperature (>900 K), silicon and germanium based alloys are used in making the TE generators. Although Be-Te, Pb–Te based alloys, and silicon–germanium alloys remain high demanding materials for making the commercial TE generator and refrigerator, the environmental issues of hazardous Bi and Pb alloys restrict them from several applications. Further, these materials are not suitable for high-temperature due to their easy decomposition and oxidation in the air. Moreover, use of these materials should be avoided as far as possible, as they are toxic, and not environmentfriendly. In Comparison to the conventional TE alloys, Si–Ge based alloys and TE oxides are the two different group of materials which is found to be more suitable for high-temperature applications because of their low manufacturing cost, environmental friendliness, nontoxic character, availability in nature, structural and chemical stability and oxidation resistance property [69, 70]. Although numerous categories of materials are studied but few of them, chalcogenides and silicon–germanium based alloys, are found to be commercially usable due to their high performance and cost effectiveness. With modern technology development trend, it is expected that wide class of materials showing high performance

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near room temperature will be highly demandable for the industrial scale applications. In the next part of this chapter, the emergent materials among chalcogenides, Si–Ge based alloys, and oxides are discussed.

3 Chalcogenides Among several materials investigated as good materials for thermoelectric applications, the chalcogenides based materials have found a special importance due to their high thermoelectric performance near room temperature. Many chalcogenides material possess narrow band gaps, suitable electronic structures, and strong anharmonics lattice vibrations. These unique and favorable characteristics coexisting altogether in chalcogenides based materials result into the high thermoelectric properties. The recent materials which have been recently investigated with high ZT is tabulated in the Table 1, and same is plotted in the Fig. 7, in order to get a better idea with a quick visualization. From the values of ZT shown in Table 1, it can be clearly realizing that, chalcogenides materials are the most prominent among inorganic based thermoelectric materials where the high ZT is obtainable in wide temperature range 300–900 K. This is mainly due to the fact of these materials possesses low thermal conductivity, and large power factor. This is also one of the most important reason behind their priority for the commercialization as compared to the other class of materials. As it is obvious that Pb, Sb, Se, Sn, Te based composition shows high ZT, beside their high thermoelectric performance these compounds contain the all the constituent elements with high toxicity [71–89]. Among these materials, the best performance (ZT = 3.1 at 782 K) is found in case of Na0.03 Sn0.965 Se due to ultralow lattice thermal conductivity (0.07 Wm−1 K−1 ) [89]. Comparatively to these compounds, silver chalcogenides Ag2 (S, Se, Te) based alloy shows high performance close to the room temperature as they possess inherent low thermal conductivity (κ < 1Wm−1 K−1 at 300 K), moderate Seebeck coefficients and low electrical resistivity [77, 78]. Jood et al. investigated the role of chalcogenide site off-stoichiometry on thermoelectric properties and results shows that with slight variation of composition in Ag2 SeChy (y ≤ 0.01; Ch = Se, S) the value of ZT ~1.0 at 300 K is obtainable with metastable phase [79]. Several chalcogenide materials show the ionic characteristics at high temperature due to highly mobile nature of transition metal elements within lattice space. For the thermoelectric applications, high performance near room temperature possesses by these materials would be best with minimum effect of contributions from the ionic conduction. By substitution of S at Se site the operating temperature region is drastically lower down up to 50 K with maintaining the high thermoelectric performance in Ag2 S0.4 Se0.6 compositions [78]. Utilizing the coexistence of two phase region (insulating and metallic phase) near phase transition by applying the temperature gradient across the material, ZT about 20 (390–430 K) and ~471 (348 K) are observed in Ag2 S and Cu2 Se material, respectively [90, 91]. Discovery of such materials open a new path to understand

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Table 1 Chalcogenides thermoelectric materials Sr. No.

Alloys

ZT

Temperature (K)

Year

Refs.

1

Pb0.935 Cd0.04 Na0.025 Se0.5 S0.25 Te0.25

2.0

900

2021

[71]

2

Pb0.99 Ag0.01 Se-1.5%SrSe

1.2

873

2021

[72]

3

Pb0.99 Cu0.01 S-0.01 Cu

1.2

773

2021

[73]

4

TlCuSe

1.9

643

2021

[74]

5

Pb0.96 Na0.04 Te

1.9

860

2020

[75]

6

Pb0.89 Sb0.012 Sn0.1 Se0.5 Te0.25 S0.25

1.8

900

2021

[76]

7

Ag2 Se

1.2

390

2017

[77]

8

Ag2 Se

1.14

390

2019

[78]

9

Ag2 S0.2 Se0.8

0.97

360

2019

[78]

10

Ag2 S0.4 Se0.6

1.08

350

2019

[78]

11

Ag2 SeChy (y ≤ 0.01; Ch = Se, S)

1.0

300–375

2020

[79]

12

Ag2 Sb0.02 Te0.98

1.4

410

2020

[80]

13

Ag20 S7 Te3

0.8

600

2021

[81]

14

Cd doped AgSbTe2

2.6

573

2021

[82]

15

Cu3 Sb1-x-y Bix Sny S4 (x = 0.06, y = 0.05)

0.76

623

2020

[83]

16

(Ag0.2 Cu0.785 )2 S0.7 Se0.3

0.95

800

2021

[84]

17

α-Cs(Cu0.96 Sb0.04 )5 Se3

1.3

980

2020

[85]

18

Cu3 DyTe3

0.9

900

2020

[86]

19

Sn0.88 Ti0.03 Mn0.09 Te

0.7

723

2021

[87]

20

Ge0.9 Mg0.04 Bi0.06 Te

2.5

700

2021

[88]

21

Na0.03 Sn0.965 Se

3.1

782

2021

[89]

22

Ag2 S

20

390–430

2021

[90]

23

Cu2 Se

471

348

2019

[91]

the thermoelectric properties near phase transition region for best utilization of their performance in the thermoelectric applications. In past several decades, many different classes of thermoelectric materials are discovered for making the thermoelectric module. Near room temperature applications, classic material used in commercial devices for electricity generation are based upon the bismuth-telluride and lead-telluride materials with ZT about unity. These commercial devices are best suitable when use with flat and static heat sources. Also, the use of toxic constituents is very harmful when these devices come in direct contact of body. The key, scientific questions are to find the non-toxic, cheap, earth abundant, high performance thermoelectric materials with flexibility near room temperature. The major challenges arise when they used to the curve and flexible heat sources, where they become non-compatible due to mechanical bending constraint. Due to brittle nature of commercial materials, they need to be further processed by making

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Fig. 7 Figure of merit of Chalcogenide thermoelectric materials

them flexible using thin films on flexible substrate or combine them with organic materials such as polymers. Adding the flexibility by external materials or substrate support make another practical challenges for the device such as performance, their compatibility with attached flexible materials, temperature they used, and high cost in materials processing. In this scenario high performance thermoelectric materials with good flexibility have advantageous as the number of applications would be significantly enlarged and device will also be cost effective. The most frequent and best region of temperature, where human can use thermoelectric material is near room temperature. The direct applications of the room temperature thermoelectric devices are to supply the input power in wearable gadgets from body heat. Several high-performance inorganic chalcogenides which is brittle in nature attain the flexibility when fabricated in the form of thin film on flexible substrates including polymer based materials [92–101]. The power factor for different flexible inorganic based thermoelectric materials is shown in Fig. 8. Here, the ZT of these materials are not shown due to the challenges in the measurement of temperature dependent thermal conductivity of thin film samples. As shown in Fig. 8 the possible flexible substrate which is compatible in providing the flexible nature to the inorganic materials are copy paper, PVPDF, cellulose fiber, gass substrate, plastic substrate, epoxy substrate, and epoxy membrane. Although, the flexible nature is obtainable, but, the thermoelectric performance decreases, and even degrade when used several cycle in actual device due to the interface barrier and non-compatibility of mechanical property between thermoelectric material and flexible substrates. To overcome such problem, materials with high thermoelectric performance and good ductility is being focuses of investigation for making the wearable thermoelectric devices [102].

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Fig. 8 Power factor (S 2 σ ) of flexible chalcogenides materials at room temperature

4 Si–Ge Based Alloys TE Materials The required input power of electricity for running the electronic devices in space craft mission, NASA organization used Si–Ge based alloys [33, 34]. Due to the instability of normal fuels, working and operational challenges of battery based power supply in space atmosphere, limitation of available solar light for the functioning the solar cell based devices, and the tremendous cost require to carry the excess weight in a sophisticated space mission such as Voyager-1, Voyager-2, New Horizons, Curiosity, the energy generation from thermoelectric materials become one of the best and cost effective choice. The performance of the composition of Si–Ge alloys which have been used since long time by NASA is relatively low (ZT ~0.5–0.9) in comparison to those called as the state of the art thermoelectric materials (Bi–Te based alloy: ZT ≥ 1.0 near room temperature). Notably, Si–Ge based alloys are found to be the only options with robust chemical and thermal stability, and also have shown the consistent performance as a solid state device in providing useful electricity through radio-isotopes thermoelectric generators (RTGs). Device utilize the waste heat available from the radioactive material used as a main fuel in operation of space craft. In RTGs, material used are the bulk structure (crystallite size or grain size in μm) range, the performance is somehow lower due to the large thermal conductivity. By knowing the electronic structure of individual component i.e. Si and Ge, which is the typical semiconductor, several researchers try to make the nano-structure of Si– Ge based compositions, mixing the external dopants, and also make the composite

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with other compatible alloys to achieve the high ZT. The common strategy which most of the researcher follows are the enhancement of power factor through optimized carrier concentrations, and reduction of thermal conductivity via scattering of phonons covering the low-mid-high frequencies in nano-structure material. Here, we should know the importance of mean free path of charge carriers (electrons or holes) and phonons for understanding the thermoelectric performance of the Si–Ge based materials. Generally, in the case of Si–Ge, the electron mean free path is typically ~5 nm, while the phonon mean free path are 200–300 nm [103]. The large difference between the mean free path of charge carriers, responsible for electronic transport properties, and phonons which mainly determine the lattice thermal conductivity (κl ) suggests that even one reduce the particle size in range of 10–50 nm for effective reduction of κl , it would not affect the electronic transport behavior. The values of ZT obtained in different compositions of Si–Ge based alloys is shown in figure and tabulated in the Table 2 [104–116]. By most of the researchers, the base Si–Ge composition in which ZT is improved by Nano structuring is Si80 Ge20. The reason of selection for this composition is mainly due to the high-cost of Ge as compared to the Si, and observation of decrease in the thermal conductivity when Ge is mixed with main matrix of Si. In the samples having particle size in nm range (smaller than 50 nm), total thermal conductivity was found to be reduced due to the scattering of phonons at grain boundaries. The synthesis conditions and dimension effects play very crucial role in the thermoelectric performance of Si–Ge based materials (Fig. 9). As we can see from the figure, the magnitude of ZT, mostly in n-type material, have been increased more than unity, ZT max = 1.84 at 1073 K, between year 2008 to 2019 via only considering the nano-structuring effect [104–116]. The operating temperature remains high i.e. above 900 K. Such high temperature in the form of waste heat Table 2 Figure-of-merit for Si–Ge based alloys Sr. No.

ZT max

Temperature (K)

Year

Refs.

1

0.95

1073

2008

[104]

2

1.7

900

2009

[105]

3

1.84

1073

2014

[106]

4

1.2

1173

2015

[107]

5

1.81

1100

2016

[108]

6

1.7

1173

2017

[109]

7

0.60

873

2019

[110]

8

0.91

1000

2019

[111]

9

1.63

973

2019

[112]

10

1.88

873

2019

[113]

11

1.38

1100

2020

[114]

12

1.63

973

2020

[115]

13

2.61

300

2021

[116]

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Fig. 9 Figure of merit of Si–Ge based alloys

is generally available in industrial places. Lowering the operating temperature while maintaining the high ZT enhance the applications in wide scale. Since Si–Ge based alloys contain the elements which possess the semiconductor electronic structure. As discussed earlier, based on the expression of spectral conductivity obtain from the linear response theory, any modifications in electronic structure near chemical potential would result into the increase in the Seebeck coefficients. By maintaining the low thermal conductivity with nano structuring, and optimizing the carrier concentrations through external dopant elements give the large power factor. The implementations of these two effects in same materials lead to simultaneous improvement of power factor and low thermal conductivity resulting in large ZT. Another benefit of the electronic structure modification is the finding of p-type Si– Ge alloy for making the TEGs, as it requires both n, and p-type material. In addition to this, if the both type of materials with same thermal expansion coefficients are available, it will be very good for stable and consistent practical operations, especially at high temperature where mismatch of dimension degrade the performance. By considering the synergic approach of electronic structure modifications, nanostructuring, and optimization of carrier concentrations, the large values of ZT in Si–Ge based alloys have been successfully achieved in both n-type (1.88 at 873 K), and p-type (1.63 at 973 K) in bulk shape of materials [113–115]. Such strategy works very well in case of the thin films, where optimized carrier concentrations, increasing the scattering probability of phonons by controlling the phonon mean free path through thickness in nm range, and the electronic structure modification lead to the large ZT. Applications become large using thin-films as on-chip devices can be made to achieve the large output power density. The important point which one have to always consider for high efficiency from the device is the average ZT possessed by the material. The operating temperature of Si–Ge alloys in bulk shape is still so high, thus its application is also very limited. Recently, in the thin-film form with

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embedded nanocrystal in Si–Ge based alloy a large ZT = 2.61 is reported at room temperature [116]. Such high ZT is breakthrough results so far and can lead to the huge applications beside the performance is reproducible at device scale. By taking the look on the several approach adopted in the improvements of ZT in Si–Ge based material, the consistent results strongly let us believe that electronic structure modifications may lead to even much higher ZT in near future.

5 Oxide Thermoelectric The unique properties of oxide materials as described earlier makes them suitable candidate for high-temperature thermoelectric applications in air atmosphere with long durability. In spite of their various advantages, oxide materials were discarded by the industry until the early 1990s, as the values of ZT were insufficient to consider it for the efficient thermoelectric device. Generally, in the metal oxide, oxygen anion forms the ionic bond with metal cation and they have the strong Coulombic interaction between them. Due to the high polarizable nature of the metal–oxygen bonds, there will be more tendency of localization of conduction electrons on the metal cations which are positively charged [117]. Also, in ionic compounds, the extent of overlapping of atomic orbitals is smaller compared to that of the covalent compounds. This is one of the reasons the carrier mobility value in oxides is generally 2 to 3 orders lesser than those of silicon and other covalent compounds [118]. Moreover, the oxide materials generally possess a high value of lattice thermal conductivity (κ) [119]. This nature of the oxides is because, the ionic bonds have large bonding energies and the oxygen atom in the compound has the small atomic mass which, in turn lead to the high velocity of phonons propagation in the crystal lattice of oxides. From the well-known criteria for the high figure of merit of a thermoelectric, one can find that these inherent properties of the metal oxide materials are opposite to the high-efficiency yields [120, 121]. In 1997, the discovery of large and positive thermopower in NaxCoO2 compound [20] gave a new breakthrough for oxide materials as a potential TE candidate for the hightemperature range applications. After this, high ZT value (∼0.8 at 800 °C) was observed in Ca3 Co4 O9+δ compound and further research on oxide materials got accelerated to search the suitable material for high-temperature thermoelectric applications [122]. The interesting results were found in the oxide thermoelectrics and nearly ten times increase in the value of ZT has been achieved in the past twenty years. In the last few decades, transition-metal oxides with strongly correlated electron systems have also attracted much attention due to spin, charge, orbital and lattice degrees of freedom of electrons [120, 121, 123]. Among the discovered transition-metal oxides, the small band gap semiconductor oxides are suitable for TE applications as they show large thermopower and electrical conductivity. Figure 10 shows the trend of figure of merit achieved in past two decades. In comparison to other materials, improvement in case of oxide is difficult due to the high thermal conductivity and electrical resistivity even though most of them show

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Fig. 10 Figure of merit for oxide thermoelectric materials

the large Seebeck coefficients. The most studied compositions are ZnO, SrTiO3 , CaMnO3 based material [124–135]. The doping of small amount of aluminum with combination of reduced graphene oxides in ZnO, an effective enhancement in thermoelectric performance are experimentally demonstrated [134]. Among the polycrystalline oxide materials, the best thermoelectric performance is experimentally obtained for the Nb doped SrTiO3 + 0.5% graphene (ZT = 1.42 at 1050 K) [136]. Recently, based on the computational results reported, figure of merit more than 2.0 can be obtained in CaBi2 O and Na0.74 CoO2 materials [137]. The experimental verification on CaSb2 O and CaBi2 O materials will provide the high performance material and open a new path to fabricate the thermoelectric device for industrial applications (Table 3). So far we have discussed the thermoelectric performance of chalcogenides, Si–Ge based alloys, and Oxides materials suitable for the making thermoelectric devices for energy harvesting from waste heat. In addition to this, heat management using thermal diodes is also an emergent field of applications using thermoelectric materials. In the next section we will discuss in more detail about the phenomenon, materials and emergent materials usable for the making thermal diodes and thermal rectifiers.

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Table 3 High temperature performance of Oxide thermoelectric alloys Sr. No.

Alloys

ZT

Temperature (K)

Year

Refs.

1

CaMn0.98 Nb0.02 O3

0.32

1060

2008

[124]

2

Zn0.96 Al0.02 Ga0.02 O

0.65

1247

2009

[125]

3

La-doped SrTiO3

0.37

973

2014

[126]

4

0.6wt% G + La doped SrTiO3

0.42

300

2015

[127]

5

ZnV2 O4

0.30

900–1400

2016

[128]

6

LaCoO3

0.35

600–1100

2017

[129]

7

Nb-La-doped SrTiO3

0.60

1000–1100

2017

[130]

8

Sr0.9 Dy0.1 TiO3

0.2

900

2017

[131]

9

2% ZnS-coated Zn0.98 Al0.02 O

0.20

1032

2021

[132]

10

Ba1/3 CoO2

0.11

300

2021

[133]

11

rGO-Al-ZnO

0.52

1100

2021

[134]

12

Nb doped SrTiO3 + 0.5% graphene

1.42

1050

2021

[135]

13

Ca4 Sb2 O

1.58

1000

2021

[136]

14

Ca4 Bi2 O

2.14

1000

2021

[136]

6 Thermal Rectifier and Thermal Diode The materials possessing different temperature dependence of thermal conductivity can be employed in making device for effective control of heat flow, known as thermal diode or thermal rectifier [138–140]. Such devices are very useful for the constructive thermal management in the wide range of applications e.g. cooling, thermal detection, thermal isolation, etc. [141, 142]. In a wide time, span researches of heat management devices, using both inorganic and organic based materials, have been carried out for the development of solid and liquid based thermal rectifier [143– 146]. Current chapter is mainly focused on the performance of devices made up with solid materials, as they are very convenient in both understanding the mechanism of heat flow phenomenon from basic physics view point and wide practical applications. Thus, the scientific information related with solid state materials are described throughout. In the solid materials, based on diffusion phenomenon, heat current density (J Q ) can be simply described by classical Fourier’s law known as J Q = − κ∇T, where κ and T are the thermal conductivity and temperature, respectively. For a typical solid state thermal rectifier, the heat-flow in one direction is much larger than that of opposite direction. To achieve this condition, two solid materials (A and B), with different temperature dependence of thermal conductivity, are required to mechanically coupled together, and placed between two heat reservoirs kept at high (T H )

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Fig. 11 Schematic representation of principle of thermal diode fabricated using two material having different temperature dependent behavior of thermal conductivity. Shown are a setup for large heat current flow (J large ) and b low heat current flow (J small )

and low temperature (T L ), with a minimum thermal contact resistance. The typical arrangement of a device under test is schematically illustrated in the Fig. 11a and b. Consider that the thermal conductivity of material A increases with temperature, while it decreases for the material B. When the end of A in contact to the reservoir is placed at high temperature and B is kept at low temperature, a large heat current (J large ) flow in a direction from A to B because both materials stay in the temperature range of large thermal conductivity. In the reverse situation, the smaller thermal conductivity of constituent materials makes the magnitude of heat flow (J small ) smaller than J large . The potential capability of the device for thermal rectification is evaluated by the thermal rectification ratio (TRR) defined as TRR = (J large /J small ), this suggests the magnitude more than unity of TRR is obtainable with a condition of J large > J small. In this process, the magnitude of TRR is mainly decided by the temperature dependence of thermal conductivity of the individual material. Therefore, to achieve the large TRR in efficient thermal rectifier, employing the materials with large variation in thermal conductivity with temperature is an essential requirement. The conceptual idea based on one dimensional anharmonic-lattice model was theoretically proposed for the solid stated thermal rectifier [138, 139, 147, 148]. Based on the inorganic materials, solid state devices and their TRR values with best performance conditions is tabulated in Table 4 [149–157]. By taking the two oxide materials, LaCoO3 : well-ordered insulator and La0.7 Sr0.3 CoO3 :disordered metal, showing significant temperature dependent behavior in thermal conductivity, a bulk thermal rectifier with TRR = ~1.5 was developed, but, the high performance of the device was achievable only at cryogenic temperatures [149]. Few other oxides based thermal rectifier developed with improved TRR, where high performance require to set one side of the device at low temperature below the 300 K [150, 152]. Utilizing the insulator metal transition characteristics of VO2 material above 300 K, Ito et al. find the condition where a significant improvement in TRR ~2.0 was realize in thin film of W doped VO2 deposited on silicon substrate [151]. Considering the wide range of practical applications, the low cost and high-performance thermal rectifier should be

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Table 4 Thermal rectification ratio (TRR) of the different alloys. IQC: icosahedral quasicrystals, T H : Hot end temperature, T L : Cold end temperature, ΔT: temperature gradient, TRR: Thermal rectification ratio Alloys A/B

T H (K)

T L (K)

ΔT (K)

TRR

Refs.

Year

LaCoO3 /La0.7 Sr0.3 CoO3

98.9

40

58.9

1.43

[149]

2009

La1.98 Nd0.02 CuO4 /MnV2 O4

57

55

2

1.14

[150]

2012

W doped VO2

350

300

50

2.0

[151]

2014

Ni0.85 Fe0.15 S/Al2 O3

347

250

97

1.51

[152]

2021

Ag2 Ch (Ch = S, Se, Te)

473

405

68

2.1

[153]

2020

Ag2 (S,Se,Te)

413

300

113

2.7

[154]

2020

Al72.6 Re17.4 Si10 /Al71.6 Mn17.4 Si11

500

300

200

1.12

[155]

2012

Al61.5 Cu26.5 Fe12 IQC/Si

900

300

600

1.81

[156]

2014

Al61.5 Cu26.5 Fe12 IQC/Al2 O3

900

300

600

2.01

[156]

2014

Al61.5 Cu26.5 Fe12 IQC/CuGaTe2

900

300

600

2.20

[156]

2014

Al61.5 Cu26.5 Fe12 /Ag2 Te

543

300

243

1.63

[157]

2015

developed. Such thermal rectifier can be obtainable with materials possessing drastic variation in thermal conductivity above 300 K. The cryogen free operating TRR devices, having 1/1-cubic approximant of icosahedral quasicrystal (IQC) with narrow pseudogap, were developed by utilizing the unusual electronic contribution in the thermal conductivity above room temperature. Using the simple method, Takeuchi et al. succeeded in fabrication of IQC based thermal rectifier which shows the high TRR = ~1.12 (see the definition used for TRR in this chapter), where one end of the heat reservoirs was kept at 300 K [155]. By changing the chemical compositions of IQCs as the material A and the counter supporting material B with effective variation in thermal conductivity, Takeuchi et al. observed a net increment in TRR up to 2.2 [156]. The large TRR > 2.0 obtained with cryogen free heat reservoirs by keeping one side temperature at 900 K was a definite prerequisite for achieving the high performance. Again, it creates another difficulty in maintaining high temperature difference of ~600 K. In order to reduce the upper limit of high temperature end, chalcogenides become one of the best choice of component material in practical thermal rectifiers due to their significant temperature dependence of thermal conductivity in 300–600 K. Combining IQC with Ag2 Te the TRR ~1.63 was obtainable with T H and T L set at 543 K and 300 K, respectively. As stated earlier, large TRR is always in demand for excellent performance of the thermal rectifier in low-cost, earth-abundant materials with minimal effort in controlling and maintaining heat reservoir temperature. Hirata et al. developed the best thermal rectifier device withTRR ~2.1 (T H = 473 K, T L = 405 K) and ~2.7 (T H = 413 K, T L = 300 K) using Ag2 (S, Se, Te) materials [153, 154]. He used the temperature dependence behaviour of thermal conductivity and the applicability of vast freedom in tuning the operating temperature and magnitude of thermal conductivity.

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Fig. 12 Thermal rectification ratio, TRR = J large /J small , of several materials developed in past several years

These reported value of TRR = ~2.7 in cryogen free operating temperature using both A and B materials from chalcogenide family are much higher than the recently reported value of TRR ~1.51 in Ni0.85 Fe0.15 S/Al2 O3 alloys (T H = 347 K, T L 250 K) [152, 154]. The progress of improvement in the value of TRR using the different materials is shown in Fig. 12. It can be clearly seen that, by effective knowledge of heat propagation in the solid, a record improvement of ~170% is achieved in the time span of about one decade. Further, by controlling the phonon structure of materials with large variation in thermal conductivity even much large TRR can be obtainable in future.

7 Summary In summary, we have discussed about the basic phenomenon related with the thermoelectric properties of solid state materials. With basic understanding of the physical parameters involved in deciding the thermoelectric performance, the strategy to find high performance thermoelectric materials are elaborated. The three different class of materials namely chalcogenides, Si–Ge based alloys, and oxides are discussed. These material are robust, cheap, and their performance are capable to fabricate the thermoelectric device for conversion of heat energy to the useful electric energy as an alternate source for power application. The emergent materials discovered in the recent past years with help of computational and experimental tools are summarized in the form of table and bar diagram. This information will help further to investigate the new materials with better performance. At the end we discussed another applications of thermoelectric materials in heat management using thermal diodes

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and thermal rectifiers. The large variation in thermal conductivity near room temperature can be utilized for making high performance thermal diodes for wide range of applications by controlling the heat flow.

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Challenges and Opportunities for Emerging Material Systems Ribu Mathew, Avirup Das, and Harihara Padhy

Abstract In recent times, two dimensional (2D) materials have been widely explored due their extensive applications in realizing electronics, optoelectronics and energy generation/storage devices. Such materials depict improved performance with modifications in their structural orientation, with selective doping, and by formation of heterojunctions. These monolayer structures encompass transition metals and metal oxide (MO) based system. The 2D materials depict high surface to volume ratio resulting in highly sensitive electronic property making them sensitive with respect to changes in their surroundings. In the present chapter, we elucidate three important applications of these 2D materials: (i) opto-electric applications, (ii) energy storage, and (iii) electronic sensing. In this chapter, we will elucidate different novel and high performance 2D materials which are used for the aforementioned applications which includes: (i) graphene and graphene heterojunction as charge transport layer and ternary layer in solar cell and optoelectronic devices, (ii) mxene as a novel material for Li and Na ion batteries, (iii) MoS2 as sensing material for biosensors. Here, a thorough discussion on these 2D materials and the specified applications. In addition, vital advantages of these 2D materials over other conventional materials have also been discussed. This chapter will serve as a guideline for researchers and scholars to obtain an insight into 2D materials and their applications in thrust areas of sensing, energy generation and storage.

R. Mathew (B) School of Electrical & Electronics Engineering (SEEE), VIT Bhopal University, Bhopal, India e-mail: [email protected] A. Das School of Advance Sciences & Languages (SASL), VIT Bhopal University, Bhopal, India H. Padhy Department of Chemistry, GITAM Institute of Science, GITAM (Deemed To Be University), Visakhapatnam, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_6

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Energy

2D-Materials

Electronics sensing

Optoelectronics

• Metal nitrides and carbides (Mxene)

• MoS2

• Graphene

1 Introduction In recent times, nanomaterials have been extensively researched due to their unique electrical, mechanical, thermal, magnetic and chemical properties that can be tailored by controlled addition of impurities compliant with modern integrated circuit (IC) fabrication techniques. Nanomaterials depict unique properties due to its large surface to volume ratio. Treatise encompasses examples of nanomaterial applications in various domains such as high performance commercial IC design, defence, space, healthcare, energy, storage to cite a few. Among the aforementioned domain much focus has been on healthcare, energy generation, and energy storage. With the surge in biological warfare, advances clinical diagnosis techniques, telemedicine, smart healthcare, and bioengineering, much focus has been on developing biosensors/biochemical sensors for detection of biological/biochemical entities. Biosensors are an integral module of micro-total analysis system or lab on chip devices which provide real time, economic and point of care diagnostics. For instance, biosensors have been reported for the detection of virus [1], protein [2], DNA [3], RNA [4], biochemical like myoglobin [5], to mention a few. 2D materials have been extensively explored for developing such biosensors due to enhanced sensing properties exhibited by 2D materials. Another critical domain is energy generation and storage due to tremendous increase in energy consumption in the last decade [6, 7]. With increasing demand of portable electronics and off grid electricity, relevance of a high energy density, long cycle life, chemically and structurally stable secondary energy storage device has increased. So a monstrous effort has been done on the search of an ideal electrode material for these energy storage devices mainly Li ion battery. The inherent problems of existing electrode materials like volume expansion, low electronic conductivity, reactivity towards electrolyte, slow ion migration, shifts the attention towards 2D transition metal carbides/nitrides popularly known as Mxenes. It has opened a great possibility of using them as electrode material in Li ion battery. Naquib et al. first

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proposed these materials as electrodes of Li ion battery [8]. After that extensive research has been followed with different transition metals based Mxenes. Further, their composites with CNT, SnO2 and other materials have also been explored. A good cyclability with improved battery characteristics has showed a feasibility of using them in practical scenario. In the recent decade, 2D materials has evolved as an important component of solar cells [9, 10]. Graphene has sparked interest in a variety of solar cell architecture, due to its (a) improved electrical property, (b) tuneable band gap, (c) high mechanical strength, and (d)great flexibility. It can be used as electrodes, electrolytes, and light harvesting materials for dye sensitised solar cells (DSSCs), organic/inorganic heterojunction, and perovskite solar cells. In the subsequent sections, we will elucidate the role of emerging materials in various the thrust areas of healthcare in developing biosensors, energy storage and energy generation.

2 Sensing Application Traditionally, semiconductors were the preferred material to realize sensors due to their advantages like controllability over resistivity, compatibility with IC fabrication technique, availability, matured process/technology, to cite a few. However, with advancement in nano-electro-mechanical-systems (NEMS) technology [11– 15], limitation of semiconductors in sub-nm regime, and expansion in material set for nanotechnology, new materials were explored for realizing high performance to cost index sensors [16]. Among new materials, nano materials such as metal oxides [17–19], conductive nano material doped polymers [20, 21], carbon nano tubes (single wall/multi-wall) [22, 23], silver/gold nano particles [24, 25], etc., have been reported to realize high sensitivity sensors. Polymers such as PDMS [26], polyethylene terephthalate (PFT) [27], SU-8 [28], Parylene [29], polyamide [30], etc., have been extensively explored for realizing sensors especially flexible electronics [31]. Such polymers are converted into electrical conductive material by doping with the carbon black (CB), Au nano particles, Ag nano particles, graphene, carbon nano tube (CNT), metal nanowires, etc. However, in recent times much focus has been 2D materials due to their capability to gauge the changes external environment. 2D materials are crystalline compound with a few atoms of thickness. The capability of 2D materials in sensing the change in external parameters by change their properties is due to their high surface to volume ratio. Among 2D materials, graphene was the first to be explored due to its unique mechanical, electrical, optical, and thermal properties. Subsequent to the discovery of Graphene, other 2D materials were explored for unique properties. One of the 2D materials that have been extensively investigated in scientific community is Molybdenum disulphide (MoS2 ). MoS2 has unique features such as large band gap (~1.8 eV) and direct bandgap material characteristics in thin films. Premise enables

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Fig. 1 Atomic structure of 2H, 1 T, and 1 T’ phases of MoS2 monolayer [32] (copyright permission obtained from Elsevier)

better downscaling to realize electronic devices unlike graphene that has nearly zero bandgap. MoS2 exhibits high mobility and its processing steps are compatible with IC fabrication techniques, thus becoming a possible candidate for realizing high speed electronic devices. Unlike silicon that depicts quantum and tunnelling effects in nano scale regime, MoS2 shows favourable optical and quantum effects when scaled down from bulk to nanoscale.

2.1 MoS2 Properties Molybdenum disulphide (MoS2 ) is one of the compounds in transition metal dichalcogenides (TMD) series. A TMD compound constitutes a transition metal (M) and a Chalcogen (element of group 16). MoS2 consists of Molybdenum (Mo) metal atom and two atoms of Sulfur (S). In 2D MoS2 material, a layer of Mo atoms is sandwiched between two layers of S atoms bonded together with covalent bonds resulting in a typical monolayer thickness of MoS2 around 6.5 Å (shown in Fig. 1). In bulk form, MoS2 constitutes monolayers bound to each other with Van-der-Waals forces. Features of MoS2 such as direct bandgap, no inversion centre (with odd monolayers), when combined with other materials results in hetero-structures with enhanced performance.

2.2 MoS2 Synthesis The properties of MoS2 thin layer depends strongly on the synthesis technique, parameters and environment. Thus, the choice of MoS2 thin layer synthesis technique and controllability of synthesis process parameters become critical in terms of maximizing the performance to cost index for specific application. 2D-MoS2 thin layers have been synthesised by using both top down and bottom up approaches.

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MoS2 synthesis techniques along with their features and limitations are summarized in Table 1.

2.3 MoS2 Sensors Like graphene, silicon and its derivatives, MoS2 material based sensors have been reported for wide application such as detection of prostate specific antigen (PSA) [33], cancer [34], coronavirus [35], to cite a few. Unlike graphene, silicon and its derivatives which are not biocompatible, MoS2 is non-reactive to human body cells thus making it suitable to realize sensors that can be embedded inside human body. MoS2 based sensors based on the reported applications can be broadly classified as the following: • • • •

MoS2 MoS2 MoS2 MoS2

biosensor gas sensor refractive index sensor photo sensor (photodetector).

The aforementioned sensor are realized with various sensing techniques such as electrochemical, calorimetric, fluorescence, field effect transistor (FET), optical, electro-chemo-luminescence, surface plasmon resonance (SPR), to mention a few. In sensors, the active layer of MoS2 is synthesised as nano rods, nano sheets, quantum dots, nano pores to obtaining high sensitivity and maximizing performance to cost index. FET sensors have been reported to detect various chemical/biological molecules such as ethanol [36], NO2 gas [37], prostate cancer biomarker [38], to cite a few. FET sensors have been extensively explored primarily due to the advantages of high sensitivity, better scalability, real time detection capability, wide detection range and low detection limits, to cite a few. FET biosensors have been reported with CNT, Si-nano wires, graphene, 2D materials, etc., as the channel material. However, 2D materials have been preferred over other materials for realizing channel due to its various advantages over 1D and graphene discussed earlier. A FET sensor converts change in concentration of molecules into an equivalent electrical signal (drain current). The FET structure is made specific to detect a particular species of biomolecules (target molecules) by immobilizing the surface with receptor molecules specific to targets. There are various immobilization protocols that has been reported in the literature for realizing FET biosensors that includes Van der Waals force [39, 40], 3-aminopropyltriethoxysilane (APTES) in combination with glutaraldehyde (GA) [38, 41], DNA tetrahedron and biotin-streptavidin (BSA) [42], to mention a few. The typical detection range of Van der Waals force, 3-aminopropyltriethoxysilane (APTES) in combination with glutaraldehyde (GA), DNA tetrahedron and biotin-streptavidin (BSA) protocols based FET biosensors are 1 pg/mL–1 ng/mL, 100 fg/mL–1 ng/mL, and 1 fg/mL–100 ng/mL respectively.

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Table 1 Synthesis techniques of MoS2 , their features, applications and limitations Sr. no

Technique

Features and application

Limitations

01

Mechanical (mechanical cleavage) and chemical exfoliation

– Process involves exfoliation of thin films of MoS2 from bulk MoS2 using low surface tension tape – Simple process – High crystalline quality mono and a few layer MoS2 – Sensing application

– Controllability issues to produce uniform 2D layer thickness and size – Dependence on user expertise – Not suitable for high throughput and large area application – Chemical exfoliation based MoS2 monolayer loose semiconducting properties

02

Chemical vapour deposition (CVD)

– – – –

03

Atomic layer deposition (ALD)

– High quality thin films at – Thin film surface is rough low temperature (150–350 – Low throughput °C) – Relatively low scalability – Thin film growth by sequential surface reactions – High quality crystalline thin films—Excellent step coverage – Deposition of film on different substrate is possible

04

Pulsed laser deposition (PLD)

– Excellent process controllability – Thin film thickness of a few nm to a few micron – High deposition rate – Enables deposition of metastable phases at RT – Better process controllability

Economic and high quality – Large process dependence Large volume production of the techniques i.e. TVS, Bottom up approach TVD and TD – The quality and properties Better controllability of MoS2 varies resulting in good significantly various as a morphology and function of process crystallinity – Methods: Thermal vapour parameters sulfurization (TVS), thermal vapour deposition (TVD), thermal decomposition (TD)

– Costly setup

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Fig. 2 FET biosensor: a device structure, b functionalization process flow. Source [42] (copyright permission obtained from Elsevier)

Fig. 3 FET biosensor characteristics: a real time detection, b response time, and c sensitivity. Source [42] (copyright permission needs to be obtained)

A typical FET biosensor and the functionalization process flow is shown in Fig. 2. Upon exposure to specific target molecules, target-receptor interaction takes place at the FET surface resulting in a change in drain current. This change in drain current is equivalent to the change in concentration of molecule. Typical FET biosensor response i.e. real time continuous response as a function of target concentration, and sensitivity are shown in Fig. 3. Similar process is used to detection gases, chemicals apart from biomolecules. Another important sensor category is gas sensors that finds numerous applications in detecting toxic gases like CO, NO2 , sulphur based gases that are not only hazardous to buildings but also human health. A summary of various applications of MoS2 sensors reported in the literature is provided in Table 2.

3 Energy Storage Application Secondary rechargeable batteries have become an essential component in every household or industry level electronics equipment’s. A growing interest in internet

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Table 2 Summary of MoS2 sensors reported in the literature References

Category

[43]

Environmental gas FET device with MoS2 sensor functional layer

Sensor type

Application – Gas sensing

[37]

Environmental gas FET device with MoS2 sensor functional layer

– NO2 gas sensing

[44]

Environmental gas FET device with thin EOT sensor MoS2 layer

– Gas sensing

[36]

Environmental gas FET device with multilayer sensor CVD-graphene and MoS2 layers

– Ethanol gas sensing

[45]

Environmental gas MoS2 optoelectronic Gas sensor Sensor

– NO2 gas detection

[46]

Opto-electronics photodetector

MoS2 based amorphous silicon heterojunction

– Light sensing

[47]

Opto-electronics photodetector

MoWO3 /VO2 /MoS2 /Si UV Schottky device

– UV sensing

[48]

Medical biosensor

Electro-chemical microfluidic Immunosensor with functionalized MoS2 nano sheet

– Pathogen detection

[49]

Medical biosensor

MoS2 nano sheet based microfluidics biosensor

– DNA detection

[34]

Medical biosensor

Epitaxial monolayer MoS2 flakes based photoluminescence device

– Breast cancer biomarker detection

[33]

Medical biosensor

MoS2 Nano sheet based fluorescent device with aptamer-functionalization protocol

– Prostate specific antigen detection

[50]

Medical biosensor

MoS2 nano pores devices based on optical detection

– DNA sequence detection

[38]

Medical biosensor

MoS2 FET biosensor with label free detection

– Prostate cancer biomarker detection

of things, electric vehicles, and smart electronics has made rechargeable batteries as an essential part of human life style. Secondary rechargeable batteries like Li ion or beyond Li ion batteries are manufactured as a micro battery and large scale rechargeable batteries. However, the present Li ion battery system has some inherent obstacles like (i) Low power density, (ii) low cycle life, (iii) slow charging process, and (iv) electrode degradation process. One of the reasons for this poor performance is sluggish ion dynamics in electrode material. The restricted lattice space restricts the ion migration pathways in the electrode structure. It will make the charge discharge cycle sluggish in the li ion battery system. High energy density is another important requirement of li ion battery. For this purpose electrode needs an optimum no of

Challenges and Opportunities for Emerging Material Systems Fig. 4 Application of Mxenes in energy domain

Li/Na baery electrode

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Supercapaciotr

Mxenes in energy field

Water spling

triboelectric nano generator

available sites for intercalation mechanism along with a high surface area. A perfect electrode should handle the volume expansion problems during the charge discharge cycle and provide a long cycle life of batteries without any negligible chemical and thermal degradation. It is significantly limited by phase transitions and mechanical deterioration of electrode materials. So these important factors need to look after with utmost care. 2D electrode material can be a good solution for the problems inherited by the li ion batteries. The high surface area of 2D material helps with better ion diffusivity and storage capacity [51–54]. Typically Graphene/Graphene oxides, Chalcogenides, Mxenes, Metal oxides, and Mono elemental 2D materials are popular choice for Li ion battery electrode material [55]. Along with this the band gap tenability and possibility to have different architecture helps these 2D structures to have good ion transport channel, along with excellent electrical and mechanical stability [56–58]. Various applications of Mxenes in the field of energy are shown in Fig. 4. Different 2D structures like transition metal oxides, graphene, carbides, nitrides, chalogenosides has already been explored as possible 2D electrodes in li ion battery. In the present study we will review transition metal nitrides and carbides (Mxene) based electrode materials. It has previously explored as an electrode material for (i) Li/Na battery, (ii) water splitting, (iii) tribo electric nano generator, (iv) super capacitor [59–72]. A detail study on different performance parameters like, energy density, power density, cyclability, and other electrochemical property of these materials as 2D electrode in Li ion battery will be discussed here [73]. Ti3 C2 Tx can be a probable candidate for Li ion battery electrode. The intercalation of Li ion can be facilitated by the multi-layered structure of Mxenes. During the charging process Li atom can intercalate inside the Mxene as per the reaction given below. T i 3 C2 + 2Li = T i 3 C2 Li 2 Transition metal nitrides and carbides have high ionic conductivity and significant mechanical stability to be used as electrode material. Also the psudocapacitance property of the Mxanes helps them to store charges effectively.

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3.1 Synthesis of Mxenes MAX phase is the primary precursor of Mxenes, and they are prepared via selective etching process. MAX phase has general formulae as Mn+1 AXn . Here M represents transition metals like Ti, Sr, V, Cr, Ta, Nb, etc. Typical structure of MAX phase and Mxenes, and their SEM images are shown in Figs. 5 and 6 respectively. A represents group III and IV elements and X represents nitride and carbides. In a classical MAX system, MX layers are hexagonally staked in between ‘A’ layers. The bond between M-X atoms is ionic or covalent bond makes them stronger then the metallic bond between M-A layers. So it’s possible to selectively etching of the MAX system. However, this etching process is not easily performed by using mechanical strength or ultra-sonication, a chemical root need to be followed for that. After the etching process, Mn+1 Xn Tx structures forms where T represents different functional groups like –O, –OH, –F. These lamellar structures possess good electrical conductivity along with favourable structure for intercalation [74].

Fig. 5 Structure of MAX phase and Mxenes. Source (Ref: [74]) (copyright permission obtained from Elsevier)

Fig. 6 a SEM image of MAX phase, and b Mxene phase. Source [75] (copyright permission: open access)

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Fig. 7 Mxene structure

T i 3 AlC2 + H F = Al F3 +

3H2 + T i 3 C2 2

T i 3 C2 + 2H2 O = T i 3 C2 (O H )2 + H2 T i 3 C2 + 2H F = T i 3 C2 F2 + H2 Generally, HF, LiF/HCL, NaOH, is used to extract layer “A” from the MAX phase to obtained final Mxene phase. However different method of removal of “A” significantly changes the surface property along with functional groups attached on them. This significantly affects the properties of the electrode. Tang et al. have summarized different etching agent applied on MAX phase to obtain Mxenes, along with their change in lattice constant [73]. It evidently proves the enhanced lattice parameter due to wet chemical etching of MAX phase. Recently a rigorous study is going based on Mxene/metal oxide nano composite preparation and its application in battery electrode material. For this purpose different processing route has been taken, they are mainly as follows: i. ii. iii. iv.

Direct reduction of metal oxide in the presence of Mxene (Mxene/Ag composite by Zou et al.) Atomic layer deposition (SnO2 /Ti3 C2 Tx by Ahmed et al.) Solution phase approach (Sb2 O3 / Ti3 C2 Tx by Guo et al.) Hydrothermal route (MoS2 / Ti3 C2 Tx by Wu et al.)

Mxenes has extensively used as electrode material for Li ion battery, Na ion battery, Super capacitor, Hydrogen storage, etc. However in the present manuscript we will provide a detail description on application of Mxenes in Li ion battery electrode. Typical Mxene structure is shown in Fig. 7.

3.2 Performance of Mxenes as Li Ion Battery Electrode Material From Mxene family of material mostly titanium based mxenes has been extensively investigated for electrode material in Li ion battery. Initially, Ti3 C2 Tx system was used in Li-ion battery and has reported a very high capacity of 225mAh/g. Further

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Table 3 Materials along with battery capacities Mxanes

Battery characteristic

References

Ti3 C2 Tx /carbon nanotube composite “paper”

100 mAh/g @ 0.1 °C and 50 mAh/g @ 10 °C

[76]

MoS2 /Ti3 C2 -Mxene@C

1200 mA/g

[77]

Mxene /Ag composite

550 mA/g

[78]

Hfo2 coated SnO2 /Mxene

843 mAh/g

[79]

Ti3 C2 Tx /Co3 O4

645 mAh/g

[80]

Ti3 C2 Tx /NiCoO4

650 mAh/g

[80]

Ti2 C2 Tx

225 mAh/g

[81]

Nb2 C2 Tx

170 mAh/g

[82]

Nb4 C3 Tx

380 mAh/g

[83]

(Nb0.8 ,Ti0.2 )4 C3 Tx

158 mAh/g

[84]

(Nb0.8 ,Zr0.2 )4 C3 Tx

132 mAh/g

[84]

different composite system has been prepared by integrating Ti3 C2 Tx with other materials like carbon nano tube, MoS2 , Ag, SnO2 , Co3 O4 , etc. and obtained good reversible capacity. The improved performance of Mxene/metal oxide composite system has been proved to be an interesting choice for battery materials. Further the free standing Mxene/metal composite prepared by Zhao et al. can be readily used as anode material in Li ion battery with good reversible capacity of 645 and 650 mAh/g. Some of these materials and their capacities have been enlisted in the following Table 3. The 2D structure of Mxenes offers adequate sites for electrochemical interactions due to their high surface area and weak interlayer forces. So, it is possible to realize much high capacity battery system using them. Main disadvantage of using Mxenes as Li ion electrode tends to lie its processing. Mxenes are an excellent electrode material for their good intercalation property. But restacking of these layers during the electrode processing is the main constrain. It will reduce the conducting property along with effective surface area of these samples.

4 Energy Generation Application The wonder material Graphene is one of the carbon allotropes having one-atomthickness sheets of sp2 -hybridized carbon atoms. In a honeycomb crystal lattice, the atoms are packed closely together. [85]. Graphene is the thinnest known material in the universe and the strongest ever material measured. Basic property of the graphene has been tabulated in Table 4.

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Parameter

Value

Specific surface area

2630 m2 g−1

Intrinsic electron mobility

200,000 cm2 v−1 s−1

Young’s modulus

~1.0 TPa

Thermal conductivity

~5000 Wm−1 K−1

Pptical transmittance

~97.7%

Along with Graphene its other derivatives like (i) graphene oxide (GO), (ii) graphene nano sheets (GNS), and (iii) graphene nanoribbons, etc. have extensively been explored [86, 87]. Graphene-based materials due to its unique properties has deep impact on photonic, electronic, biomedical and energy storage, and polymer nano composite [88, 89]. Graphene has been used as an electrode and/or active layer in solar cells for both electron–hole separation and hole transport [90]. The 3D hexagonal honeycomb lattice of bulk graphite consists of stacked single graphitic lattices connected by the weak van der Waals force. Furthermore, lattices have the basic building blocks of sp2 bonded carbon atoms. These single graphite layers can arrange in different forms to form various carbon-based materials such as fullerene, carbon nanotube or graphene when they form a sphere or cylindrical structure or 2D structure, respectively Fig. 8. These graphitic layers can be stacked in

Fig. 8 2D hexagonal nano sheets of graphene [91] (copyright permission obtained from Wiley)

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many layers to form multilayer graphene [91]. Monolayer, single atomic, and bilayer graphene are formed by one or two layers of graphitic. Multilayer graphene, thick graphene, or nano crystalline thin graphite are all terms used to describe graphene with 20–30 layers stacked together. Because of its high electrical conductivity and high optical transmittance, as well as its chemical inertness, graphene has shown to be a durable material for use in flexible optoelectronics, energy harvesting, storage, and a variety of other optical devices. It’s also becoming a contender for replacing transparent conductive metal oxides like indium tin oxide (ITO). Graphene-based solar cells and storage devices have recently piqued the interest of numerous academics due to their increased efficiency.

4.1 Graphene Synthesis In order to apply in the optoelectronics industries, it is of great interest to develop a large-scale synthesis process of graphene in a cost effective and reliable way with the high yield and quality. Generally, graphene synthesis methods for large scale production are classified into two categories: the top-down (destruction) and bottomup (construction) methods. In the top-down process, individual graphene sheets are able to separate from the graphite by overcoming the Van der Waals forces in a destructive way [92]. The top-down methods include (i) mechanical exfoliation, (ii) arc discharge, (iii) oxidative exfoliation-reduction, (iii) liquid-phase exfoliation and (iv) unzipping of CNT have been reported to delaminate the graphene sheets [93]. Some of these methods are very scalable and creates high-quality goods but challenges like surface defects and re-agglomerating of separated sheets associated in this approach. Furthermore, these methods follow tedious synthetic procedure offering low yields and rely heavily on the finite graphite precursor. In the bottomup constructive approach, carbon molecules obtained by various sources apart from graphite considered as building blocks to form a graphene sheet. From atomic-sized precursors, these processes produce graphene materials. Some of the bottom-up methods include the following- (i) chemical vapour deposition (CVD), (ii) epitaxial growth, (iii) substrate-free gas-phase synthesis, (iv) template route and (v) total organic synthesis produces the graphene sheets defect-free. Though the graphene sheets prepared from this approaches suffers from low surface area, this method revealed the possible production graphene nano-ribbons and graphene dots in a large scale [94]. During initial stage of discovery of graphene, micromechanical cleavage of graphite is the most common method employed to synthesize high quality, defectfree graphene. In this method, a longitudinal or transverse stress is generated on the material surface with a simple scotch tape or atomic force microscopy (AFM) tip. Premise is performed to strip off a single layer or few layers from the material [95]. In chemical exfoliation process, alkali metal ions such as Li, K, Na, Cs, are used by dispersing in the solution to react and intercalate with the bulk graphite to produce

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graphene layers. Sonication process can be introduced in this process to achieve un-oxidized monolayer graphene [96]. In chemical reduction process of graphite oxide, nano meter to a few micron sized Graphene nano flakes could be obtained by reducing the oxides through chemical or thermal treatment [97]. Reduced Graphene is found suitable as conductive inks and paints, polymer fillers, organic solar cells, electrochemical electrodes, sensors, etc. Advantages of chemical synthesis process include its low temperature operation and the fact that it’s a solution-based process, whereby allowing tuning in scalability and direct synthesis of graphene film on various substrates at different thickness. However, graphene production with chemical synthesis process produces the products of higher surface defects and lower purity than thermal methods. Later stage, thermal chemical vapour deposition (CVD) was used as a feasible alternative. Thermal CVD requires hydrocarbon gases at elevated temperature (∼1000 °C) under reduced atmospheric condition to grow the on substrates [98]. It can produce large-scale graphene but suffers in obtaining homogeneous layers. Additionally, large number of defects is formed due to grain boundaries and ripples which limit its electrical, thermal, and optical properties. Graphene also can be produced through the epitaxial growth process by fabricating graphene onto SiC substrate at 1100 °C [99]. As thin graphene films are obtained by this method (>50 µm), it can be applied to use in the transistors and circuits. The surface and size of substrate influences the thickness, mobility and carrier density of graphene produced. Compared to Graphene obtained from exfoliated process, Graphene synthesised from this procedure tends to have weak antilocalization. However, similar to Graphene obtained by drawing or peel off method, epitaxial graphene displays extremely large, temperature independent mobility. Graphene transfer is one of the important processes for realizing the graphene (dispersion)-based photovoltaic devices. Other processes apart from dispersion include (i) drop-casting, (ii) spin coating, (iii) dip casting, (iv) vacuum filtration spraying, (v) hot press lamination process stamping process, and (vi) roll-to-roll transfer process. Langmuir–Blodgett assembly and electrophoretic deposition have been demonstrated are used for transferring graphene on various substrates. A review of popular Graphene synthesis techniques is summarized in Table 5.

4.2 Graphene for Solar Cells Application Recently, Graphene became one of the best-chosen materials for various solar cells that includes (i) solid-state solar cells, (ii) quantum dot solar cells (QDSCs), (iii) perovskite solar cells, (iv) Dye-sensitized solar cell (DSSC) and (v) organic solar cells. Graphene improves the quantum efficiency of solar cells when used as a transparent electrode, electron/hole transport layers, catalytic counter electrodes, etc. Graphene’s broad solar spectrum; exhibits higher charge transfer, manufactures a flexible device with greater heat dissipation. It’s employed in a variety of material

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Table 5 Popular graphene synthesis techniques and their potential applications Method

Typical dimension

Application

References

Micromechanical exfoliation

Flakes (5–100 µm)

Research purpose

[100]

CVD

Thin films: very large (cm)

– Touch screens – Smart windows – Flexible optoelectronic devices

[98]

Liquid phase exfoliation Nano sheets: (nm~µm

– Polymer fillers – Transparent electrodes and sensors

[96]

Epitaxial growth on SiC Thin films: (>50 µm)

– – – –

[99]

Carbon nanotube unzipping

Nano ribbons: (~µm)

– FETs – Composites

[95]

Chemical reduction of graphene oxide

Nano flakes, powders: (nm~µm)

– Supercapacitors – Sensors – Electrochemical electrodes – Polymer fillers

[97]

Transistors Circuits Interconnects Semiconductors

development procedures to help devices function better. Various Graphene based solar cells is summarized in Fig. 9.

4.2.1

Graphene in Dye-Sensitized Solar Cells

The basic design of a DSSC is shown in Fig. 10, which include the following: (i) transparent conductive electrode, (ii) a semiconducting layer, (iii) an electrolyte, and (iv) counter electrodes. In these kind of solar cell light is absorb by the dye, leading to excitation. Subsequently, electron migrates to the current collector via conduction band. Recently, search for new materials (photo anodes, electrolytes, sensitizers, and conductive electrodes) has been a challenging task for researchers to improve the efficiency and stability of DSSCs to bring into commercialization where Graphene in each sections could be part. Owing to the transparency, flexibility and electron mobility, graphene is considered to act as effective conductive electrode alternative to the ITO substrate. A graphene single-layer sheet possesses a sheet resistance (Rsh ) of 6.45 /sq. Thereby, the material with layers can attain a Rsh of ~1 k/sq for T = 90%, which is comparable to ITO material [100]. Typically, a semiconducting layer of DSSC is deposited on a transparent conducting oxide. Usually loss of electron is the main challenge in the performance of DSSC due to recombination of electrons. It has been found that, introducing a barrier

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Fig. 9 Application of graphene nanostructures in different solar cells

Fig. 10 Basic design architecture of a dye-sensitized solar cells

layer at the interface of current collector/electrolytes can reduce electron losses. 20– 100 nm TiO2 coatings are used normally as a blocking layer sandwiched between ITO and the electrolyte. Further, the introduction of graphene-TiO2 composites increases device performance by increasing charge collection efficiency [101].

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Higher charge collecting efficiency in DSSCs is accomplished by lowering TiO2 charge recombination, which lengthens the electron diffusion length. As a result, incorporating graphene into TiO2 nanomaterials increases electron transport and lowers charge recombination, resulting in increased device efficiency. Furthermore, higher absorption of wide range of solar spectrum and compatible energy level states with both electron injection and dye regeneration, graphene quantum dots (GQDs) with a diameter ranging from 1 to 30 nm has been used as a photosensitizer in DSSCs. Under optimal conditions, GQDs depicts a maximum power conversion efficiency of approximately 6.1%. Therefore, it is predicted that, by the synergetic approach which has integrated components of (i) hot electron harvesting and (ii) minimizing recombination at TiO2 /GQD interfaces, GQDs are superficial sensitizers in mesoscopic solar cells [102]. Graphene is also used as additives in electrolytes to enhance device performance in DSSCs due to its outstanding electron accepting capacity. It has been reported that cell performance efficiency increases from 5.8% to 7% in a DSSC when 0.005% of graphene nanoribbons as an additive in an electrolyte is incorporated [103]. To reduce the electrolyte oxidized redox couple, counter electrode plays an important role in the DSCC. Generally, for the iodide/triiodide mediators, Platinum is considered to the best counter electrode because of their high activity, mainly for the iodide/triiodide mediators. Platinum being a costly, to reduce the cost, investigating platinum free counter electrode became the huge interest of researchers. Graphene, being transparent and conductive, it has been investigated as DSSC counter electrodes. In addition, combination of graphene and poly (3,4-Ethylenedioxythiophene) (PEDOT) as carbonaceous electro catalysts has attracted considerable attention in DSSC [104].

4.2.2

Graphene in Perovskite Solar Cells

Perovskite solar cells have been extensively explored in recent years, because of their cost effectiveness, and high conversion efficiency than the DSSCs. The structural design of these solar cells is inspired by DSSCs architectures. It consist of the following: (i) electron-transport layer, (ii) perovskites, (iii) hole transport layer, and (iv) an electrode. In Perovskite solar cells, graphene-based materials are used as an electron/hole transport layer. HRG-TiO2 nanocomposites and graphene quantum dots have been integrated between perovskite and TiO2 to enhance the cell performance [105]. Furthermore, graphene oxide sheets and highly reduced graphene oxide are used as a hole conductor and hole transport material with CH3 NH3 PbI3 as an absorber, respectively for improving efficiency [106].

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Fig. 11 Simple architecture of organic solar cells

4.2.3

Graphene in Organic Solar Cells

In recent days, owing to low weight, flexibility, and low cost, organic solar cells extensive research and development for commercialization [107]. It consist of the following: (i) a transparent anode, (ii) an electron transport layer, (iii) a photo active layer, (iv) a hole transport layer, and (v) a cathode (Fig. 11). These cells customarily contains TiO2 , ZnO, or Graphene oxides and PEDOT:PSS as the as the electron collection layer and hole-collecting layer respectively. Because of their high absorption, high electron mobility, and flexibility, they are used to improve the performance of organic solar cells. Several researchers have looked at using graphene-based materials for OPVs as transparent electrodes, electron transport layers, hole transport layers, and cathode materials to improve solar cell power efficiency [108–110]. Further, for making efficient charge separation inside the photoactive layer i.e. at Donor–Acceptor interfaces, owing to the larger electron affinity of graphene, it is used as a promising electron acceptor [109]. Due to high charge mobility, surface area and flexible band gap, graphene quantum dots are also incorporated as an electron acceptor and as a potential cathode layer material in photovoltaic devices [111]. Not only in organic solar cells, but also in graphene is also assimilated in Schottky junction solar cells. Usually, Schottky junctions are made from the metal and semiconductor materials. However, efficiency of 4.35% achieved from graphene-based heterojunction solar cell using graphene and an n-type silicon semiconductor [112].

5 Challenges and Future Prospects In the near future, 2D material will play critical role in the development of electronic sensors, energy harvesting and storage. In the case of sensor development, 2D materials due to their high sensitivity to external environmental changes have been integrated as the active sensing material. Although, 2D materials especially MoS2 based sensors depict high performance to cost index, development and real time application of such sensors remains a challenges due to the following factors: (i) controllability issues related to 2D material synthesis as the 2D properties largely depend on synthesis parameters, (ii) selectivity of specific species (target molecules)

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in a mixture, (iii) selection and controllability of fabrication process and process parameters, (iv) electrical sensitivity of sensors, (v) sensor output drift due to the impact of external parameters like temperature and humidity, and (vi) scalability and compactness of system. Similarly, Mxenes has already proven to be a good candidate for Li ion battery electrode. However, new insights are needed in the following sectors to properly use Mxenes at their full capacity: (i) a thorough investigation is needed on processing method from MAX phase to Mxene phase, also effects of various functional groups and surface property. During this process the homogeneity of the functional groups need to take care of, (ii) a rigorous study is needed to understand the possibilities if composite formation with Mxenes, and (iii) different architecture of Mxenes needs to be investigated to overcome the “restacking” problem. Since last decade, significant progresses in graphene-based solar cells are established and found to be the possible material to substitute the conventional solar materials due to its enhanced or comparable performances to those of conventional devices. However, following challenges need to be addressed for adopting this material fully in the future solar research: (i) for the excellent properties of the graphene, controlled synthesis i.e. a reliable growth method for the mass production of Graphene with high yield is necessary, (ii) tuning the defect concentration, conductivity and transparency of the graphene for better activity, (iii) adjusting the sheet resistance and fine-tuning the graphene–semiconductor interface for lowering the recombination probability, and (iv) controlling graphene morphology by using several functionalisation’s for improving the interfacial charge transfer kinetics at the electrode–electrolyte interfaces.

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Applications of Emerging Materials

Emerging Materials for Biosensor Applications in Healthcare P. P. Muhammed Shafeeque Rahman, Merin Joseph, Lakshmi V. Nair, and T. Hanas

Abstract Recent advances in the field of materials science add a lot to the development of biosensors. The various synthesis and processing techniques that can be used for developing microparticles to nanoparticles allow their applications in biosensors. The development of new biosensors was the major focus of application in the past few decades due to its wide range of applications in the health care industry, environmental monitoring, drug delivery, and disease diagnosis. The selectivity and sensitivity are the major challenges of biosensors which can be addressed by finetuning their performance. This chapter focus on emerging materials that can be used to develop biosensors for affordable healthcare. The section of the book will provide an insight into the different classes of materials that can be used for the evolution, design, principle, and classification of biosensors. Keywords Biosensors · Emerging materials · Nanomaterial

1 Sensors In the present scientific and technological era, we enjoy the efficacy of numerous electronic gadgets like computers, fridges, air conditioners, mobile phones, smoke detectors, IR thermometers, cameras, and remotes, etc. Most of them work with the help of sensors. Sensors have a great impact on our day-to-day lives. They are known as the heart of any measurement system. Sensors are defined as those devices which could be able to detect changes in physical parameters like current, heat, pressure, movement, humidity, force, etc., [1]. On monitoring any of these parameters, the sensor converts the signals in a detectable and measurable form and analyzes these signals with respect to the surroundings. An important term to be considered here is P. P. M. S. Rahman · M. Joseph · L. V. Nair (B) · T. Hanas School of Materials Science and Engineering, National Institute of Technology Calicut, Kozhikode, Kerala, India e-mail: [email protected] T. Hanas Department of Mechanical Engineering, National Institute of Technology Calicut, Kozhikode, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_7

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‘transducers’ or energy converters, which are devices that convert a form of energy to an electrical form. The output signal is the main distinction between a sensor and a transducer. Both a sensor and a transducer are used to detect changes in the environment or an object they are attached to; however, a sensor will produce the same output format while a transducer will turn the measurement into an electrical signal. The main characteristics of an ideal sensor are selectivity, linearity, sensitivity, resolution, repeatability and reproducibility, calibration, drift, and response time [2]. Due to the advancement of sensor technology, sensors are widely used in health care, environmental monitoring, food quality industry, automobile industry, etc. This portion of the book chapter mainly focuses on the use of sensors in the field of health care industry/academia.

1.1 Classification of Sensors Sensors are broadly classified as (a) (b) (c) (d) (e)

Active and Passive sensor Relative and Absolute sensors Contact and Non-Contact sensors Physical, Chemical, Thermal, and Biological sensors Analog and Digital sensors.

Active sensors operate with the aid of an external energy source such as microphones, strain gauges, thermistors, inductive and capacitive sensors. The operation of active sensors necessitates the use of an external power source and hence is also known as parametric sensors. For example, a capacitive biosensor with an integrated microfluidic chip was developed and used for the detection cardiac biomarker such as C-reactive proteins (CRP). The chip having capacitor biosensor was fabricated using the surface modification of gold thiol interaction and the conjugation of antibodies. The so developed sensor shows greater selectivity and sensitivity. It showed a limit of detection of 0.001 ng/mL [3]. In passive sensors (self-generating sensors) no external energy is needed to generate signals. Photodiodes, thermocouples, piezoelectric sensors, etc. are examples of passive sensors. As an example a passive sensor was developed for the detection of waterborne pathogens using an optical microfluidic device by incorporating organic photodiodes. Gold nanoparticles was incorporated to this photodiode for enhancing chemiluminescent immunoassay. This sensor provided a higher resolution of 40,000 cells/mL (25 times higher) for Legionella pneumophila without the incorporation of gold nanoparticle [4]. Relative sensors detect the signals relative to a reference signal. For example, the thermocouple that measures the pressure and temperature difference concerning atmospheric pressure. It is shown that a microbiosensor having a thermopile integrated onto a quartz chip were used for the detection of glucose from 2 to 25 mM. It shows a relative standard deviation of 5% for 100 number of glucose samples. Here

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the thermopile was developed by doping boron and aluminium in polysilicon. The microchannels were sealed using silicon rubber and catalase and glucose oxidase were coimmobilized on a controlled pore glass beads [5]. Absolute sensors respond to the signals on an absolute scale. A thermistor is an example of an absolute sensor. A calorimetric thermal micro-biosensor was fabricated as a versatile, portable and sensitive sensors having a linearity response of 100 mM. The efficacy of the sensor was analysed using penicillinase, catalase and enzymes. This sensor shows greater sensitivity than conventional enzyme based thermistor [6]. In contact sensors as the name implies it needs direct (physical contact) with the stimulus. Temperature sensors are examples of contact sensors. Graphite pencil traces patterned on cellulose paper are used to make single material thermocouples by R Mulla et al. [7]. Whereas non-contact sensors don’t need direct physical contact. Examples are IR thermometers, magnetic and optical sensors. MM Safaee et al. wirelessly monitored oxidative stress with optical fibrous biomaterial surrounded with nanosensors [8]. In analog sensors signal generated is continuous in time that is signal is in the analog form. Examples of analog sensors include strain gauges, resistance temperature detectors, thermocouples, etc. SH Min et al. developed an analogue Nanoparticle Sensor for the measurement of strain and vibration Monitoring [9]. A digital sensor transforms measured physical quantity as pulses. Encoders are examples of digital sensors. JH Low et al. demonstrated a stretchable insole fluidic model for the gait analysis of patients which can be digitally transmitted to smart phone [10]. Signals are generated in a physical sensor by monitoring the physical quantity and converting those quantities in the form of signals. Environmental changes like pressure, volume, mass, density, rate of flow, acceleration, and force, etc. can be detected using physical sensors. These sensors are extensively used in the field of disease diagnosis and monitoring. Compact and more accurate sensors were fabricated using MEMS (microelectromechanical system) technology using different measuring technology. Chemical sensors are those sensor devices that convert chemical information to a useful analytical signal. One can monitor the amount of the analyte molecule in a given sample using chemical sensors. These sensors can monitor the required species from liquid or gaseous form. Clinical diagnosis, drug and food analysis, pollution monitoring can also be done using chemical sensors. Thermal sensors can measure the input temperature and convert the input information into an electronic signal so as to monitor or record the signal of changes in temperature. Thermistors, thermocouples, etc. are examples of temperature sensors. Biomolecular processes like enzymatic interaction, antibody/antigen interactions, or cellular signaling processes, etc. were monitored using biological sensors. Biological sensors are also known as biosensors. This section of the book chapter mainly focuses on biosensors in the field of health care industry.

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1.2 Biosensors Biosensors are devices capable to undergo conformational changes upon interaction with an analyte of interest. According to International Union of Pure and Applied Chemistry (IUPAC), biosensors are devices which detects analyte of interest such as chemical compounds by utilizing optical, electrical, or thermal signals by making use of biochemical reactions of tissues, whole-cell, isolated enzymes, or immune systems [11]. The main components of biosensors are receptor, transducer, and detection system. Receptors are those components in a biosensor capable to bind with the analyte molecule and upon interaction, the transducer generates signals and which are quantified and converted into useful information by the detector. The generally employed detection systems were classified into electrochemical, optical, or piezoelectric detection systems. In comparison with the traditional techniques employed for the detection of bio-analytes such as enzyme-linked immunosorbent assay (ELISA), one can design fully automated rapid and real-time analysis having enhanced reproducibility and performance. Biosensors were shown to have an inevitable part in the field of diagnostics of diseases, defense, environmental monitoring, contaminants in food, etc., [12–15]. As per 2019 World Health Organization (WHO)’s global health estimates, the top ten global causes of death are ischaemic heart disease, kidney diseases, diabetes mellitus, stroke, diarrhoeal diseases, lower respiratory infections, chronic obstructive pulmonary disease, trachea, bronchus, neonatal conditions, Alzheimer’s disease and other forms of dementia, and lung cancer. All these conditions became fatal if it is not identified at the early stage. Using properly designed biosensors one can monitor these diseases at their early stage and that will ideally help in the management of these diseases.

1.3 Types of Biosensors The commonly used biosensors are classified as tissue-based, enzyme biosensors, Deoxyribonucleic acid (DNA) based biosensors, optical and thermal, immunosensors, and piezoelectric biosensors [12]. In tissue-based sensors, the tissue sources are from animal and plant sources. During the sensing process, the analyte acts as an inhibitor or substrate. Arginine sensor was the first tissue-based sensor [16]. Organelle-based biosensors were fabricated using membranes, microsomes, chloroplasts, and mitochondria. These kinds of sensors have high stability. The major limitations are low specificity and longer response time [17]. Enzyme biosensors are the class of biosensors where the enzymatic adsorption occurs due to enzyme adsorption through different van der Waals forces, ionic bonding, or covalent bonding. The commonly employed enzymes are oxidoreductases amino oxidases, peroxidases, and polyphenol oxidases [18].

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Antigen–antibody affinity was utilized in immunosensors. The mechanism is that the antibodies can selectively bind with pathogens or other toxins and thereby interact with the immune system of the host. The DNA biosensors work based on the efficacy of the single-stranded nucleic acid molecule to recognize and bind with its complementary strands in the analyte sample. This is happened mainly due to the hydrogen bond formed between the two nucleic acid strands [19]. Magnetic biosensors are the miniaturized biosensors to detect micro and nanoparticle of magnetic nature by utilizing the magneto resistant effect of microfluidic channels. These sensors offer high sensitivity [13]. Piezoelectric biosensors work based on the change in the resonance frequency of the piezoelectric crystal. This change in resonance frequency is due to the change in mass on the crystal structure. There are two types of piezoelectric biosensors such as surface acoustic wave devices and quartz crystal microbalance [20]. Another common class of biosensors is optical biosensors. It consists of a light source associated with different optical components. These optical components help to generate desired wavelength or characteristics of light. This light will pass through a modulating agent before reaching the analyte or sample and then to the photodetector [21]. Thermal biosensors or colorimetric biosensors measure the absorbed or released heat during a biochemical reaction. This sensing mechanism is based on the absorption of the analyte material into a physical transducer [22]. The details of each type of sensor will be discussed in the classification of biosensors part of this chapter.

2 Evolution of Biosensors Biosensor evolution can be categorized into three generations [23] according to the integration of bioanalyte detection entity to the transducers. Mediator-less amperometric biosensors are first-generation biosensors. The mode of sensing here is the reaction with bioreceptors. Upon interaction with the bioreceptors, it diffuses towards the transducer. This will produce an electric response upon interaction with the bioanalyte. The components of biosensors were first explained by Leland Charles Clark Jr 1956 [24]. By using a specific electrode, the oxygen concentration in blood was measured. Experimentally demonstrated an amperometric glucose monitoring using enzyme electrode in the year 1962 [25]. Later in 1967, Updike and Hicks modified Clark’s work using glucose oxidase on oxygen sensors [26]. The first potentiometric sensor based on enzyme electrodes was reported by Guilbault and Montalvo in 1969 for the detection of urea [27]. Four years later, Lubrano and Guilbault developed a hydrogen peroxide sensor that was reported by using lactate and glucose enzyme on a platinum electrode [28]. Thermistors (enzyme sensors susceptible to heat) were developed in 1974 by Klaus Mosbach [29]. In the next year, the same concept was utilized in the fabrication of an optical biosensor for the monitoring of alcohol by Opitz and Lubbers [30].

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Fig. 1 Illustration of all three generation biosensors. Here Mox MRed represents oxidized mediators and reduced mediators respectively (Adapted from [23])

The second-generation biosensor consists of a layer of individual components over the biological component layer so as to enhance the sensing efficacy. The layer of individual components includes nanomaterials or mediators and auxiliary enzymes. Such kinds of sensors are also known as mediator amperometric biosensors [23]. In 1976 developed an electrochemical biosensor for glucose detection by Clemens et al., [31]. Catheter-based continuous monitoring of blood glucose was introduced by VIA Medical [32]. Bioreceptor molecule is an important part of third-generation biosensors which acts as a base for element sensing. In this generation of biosensors, the mediators and the enzymes are on the same electrode. The interaction between the electrode and enzyme was achieved via direct electron transfer. The main advantage of these third-generation biosensors is the efficacy of having repeated measurement and low cost [33]. Surface plasmon resonance (SPR) phenomena were used for the real time dependency reaction by Liedberg in 1983 [34]. In 1987 pen-sized detector was developed in the USA for the detection of blood glucose levels. Figure 1 shows a schematic representation of all the three generation biosensors [23].

3 Principle and Design Biosensors 3.1 Basic Principle An analytical device capable to analyse the changes occurred during a biological process and convert them into useful signal can be termed as biosensor, in a broader

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Fig. 2 Block diagram showing an important component of a biosensor

perspective. Here biological processes can include any biological element or material such as cells, tissue, DNA, RNA, enzymes, microorganisms, etc. The basic principle of sensing may vary based on different approaches and techniques. When the biomolecule present in the analyte needs to be tested the biological response upon interaction will be converted into a useful signal by the transducer. As an example, if the biological material is in the form of an enzyme; the Electroenzymatic process converts the enzyme into an electrical signal using a transducer. The common biological response to an enzyme is oxidation. This oxidation acts as a catalyst to change the pH of the enzyme which in turn affects its current carrying efficacy. Figure 2 represents the block diagram showing different components of biosensors.

3.2 Components of a Biosensor A basic biosensor has different components such as analyte, bioreceptor, transducer, electronics, and display [35]. 1.

2.

3.

Analyte: Analytes are defined as those substances whose components needed to be identified or detected. Any substance or biomolecule (e.g., lactose, glucose, alcohol, ammonia, bacteria, virus, etc.) that needs to be detected is known as a bioanalyte. Bioreceptor: Bioreceptors are those molecules or biomolecules or biological substrates or elements capable detect the analyte or the target substrate. Examples of bioreceptor include antibodies, deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), aptamers, cells, and enzymes. The signal generation or transduction is achieved in the form of plant or animal tissue, charge or mass change, microbial products, pH, heat, and light. Biorecognition is occurred by the interaction of analyte and bioreceptor. Transducer: Transducers are those devices that transform one form of energy into other forms. Transducers play a vital element or role in biosensors which helps to convert biorecognition events into a detectable signal upon interaction with biological target or analyte. The electrical or optical signal generation depends on the number of the interaction of bioreceptor molecule. This process of changing of energy is called as signalization. Based on the principle of operation, transducers are broadly classified as electronic, thermal, optical, electrochemical, and gravimetric transducers.

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4 Characteristics of Biosensors The field of biosensors has a promising impact on society by enabling the diagnosis of the disease at the early stage. In other words, the advancement of science and technology opens a new door in the biosensor area to achieve its widespread goal. The overall performance of a biosensor will depend on certain parameters or characteristics. The performance of the biosensor was optimized for commercial purposes based on certain specifications [36]. In order to achieve better performance one could tune the following properties such as. 1.

2.

3. 4. 5.

6.

Selectivity is the prime requirement for designing a biosensor by suitably selecting a bioreceptor. An efficient bioreceptor can particularly target analyte molecules from a mixture of samples that contains other contaminants. Lowest detection limit (LOD) is another major factor that specifies the performance of the biosensor. It is the minimum concentration of the analyte that a sensor can be detected without interacting with another component in the target mixture. Response time is the time needed for a sensor to get 95% of the results. Linearity represents how accurately the measured result is. The higher linearity means the sensor can detect a higher concentration of substrate. Stability is one of the important aspects of biosensors. A commercial biosensor should possess high stability. Depending on environmental parameters stability of the biosensor will vary. So it is important to continuously monitor the stability of biosensor molecules even outside the device. If the sensor is not stable, it will affect the binding affinity of the system and finally lead to degradation of the biosensor device. Reproducibility is the most important characteristic of a biosensor. The results obtained from a biosensor each time should be the same ie., reproducible at the same conditions. The technical term used is precision which means the same output signal when the same sample is measured several times. It is important to produce a mean value closer to the actual value for every time. It is important for a sensor to be precise and accurate then only it will lead to an application.

5 Classification of Biosensors The biosensors are mainly classified based on different factors. This portion of the book chapter explains the different classes of biosensors.

5.1 Biosensors Based on Receptor Biosensors are classified into affinity and catalytic biosensors or non-catalytic biosensors based on the principle of biorecognition [37]. A catalytic biosensor utilized the

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analyte and bioreceptor interaction resulted in the formation of new biochemical molecules. Such biosensors include enzymes, whole cells, tissue, and microorganisms. For a non-catalytic biosensor, the analyte molecule interacts irreversibly with the receptors. No new reaction product is produced in a non-catalytic biosensor. Nucleic acids, cell receptors, antibodies, etc. are the common target for non-catalytic biosensors [38].

5.1.1

Biosensors Based on Enzyme

Protein molecules that catalysis biological reactions are known as enzymes. Usually, the rate of biological reactions is enhanced in presence of enzymes. A biosensor in which the enzyme acts as a bioreceptor in association with a transducer to create the output signal. The signal generated solely depends on the amount of analyte molecules. These kinds of biosensors are called enzymatic biosensors. Possible mechanisms for the enzyme based biosensors are in which analyte molecules are metabolized by the enzyme followed by its measuring, i.e., the analyte is metabolized by enzyme and measuring the change in concentration of analyte and the third step is to study the change in enzyme characteristics due to analyte metabolism [39]. The history of enzymatic biosensors begins with the development of glucose sensors by Professor Leland C. Clark in 1956. He developed the biosensor by trapping glucose oxidase enzyme in a dialysis membrane and monitoring the amount of glucose by measuring a decrease in oxygen concentration. In continuation with Dr. Clark, many scientists developed various biosensors based on enzyme activity, especially glucose and urea sensors. A film of polyaniline based electrochemical sensor platforms was fabricated by incorporating lactate dehydrogenase and glucose oxidase enzymes. These biosensors were used as a third-generation biosensors for glucose monitoring. The developed electrode shows a 90 s response time with two weeks of shelf life. An amperometric glucose biosensor was developed in the brain by Cordeiro et al., to monitor glucose in real-time in vitro. The sensor is based on W-Au with enzyme and it monitors the metabolic changes in the brain [40]. Lakshmi et al. experimentally demonstrated a urea sensor based on urease activity. This sensor is based on fluorescence of gold cluster present in it. On urease activity aggregation of gold cluster increases and the detection of this signal makes the sensor [41]. E. Morallón and coworkers reported a stretchable and screen-printed glucose biosensor immobilizing glucose oxidase enzyme on Pt-decorated graphite and was able to show performance comparable to a commercially available glucose meter [42].

5.1.2

Biosensors Based on Antibody Activity

Antibodies are also known as immunoglobulins (Ig); they are large “Y” shaped proteins present in the immune system to identify foreign objects (viruses, bacteria, etc.). These biorecognition elements are widely used for biosensing applications by

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making use of their antigen–antibody interactions. There are five classes of antibodies present based on the difference in its chain—IgG, IgM, IgA, IgD, and IgE [43]. Immunosensors are biosensors in which antibodies are immobilized and work in the principle of antigen–antibody interaction. There are two types of immunosensors present—labeled and non-labeled. In labeled immunosensors, a particular label molecule is incorporated into the system and the changes that occurred in this label are recoded with antigen–antibody interaction. Whereas in non-labeled immunosensors, the complex formed by antigen–antibody interaction is directly measured by recording its physical changes. In both types, various biosensors are available. A label-free immunosensor was constructed by Madurro et al., for the diagnosis of ovarian cancer by making use of CA125 biomarker concentration. The anti-CA125CA125 (antigen–antibody) interaction showed a linear relationship to a particular range and possesses a detection limit of 1.45 UmL−1 [44]. Three-dimensional biosensors are made of G protein-coated particles for the immobilization of IgG (immunoglobulin) and IgG crystallizable fragment domain (Fc)-tagged protein. Also, these sensors were optimized for the detection of antigen–antibody interaction. The antibody-based biosensor is effective in mimicking the cell surface and evaluating the binding of immune therapeutics with their analyte molecule. Aflatoxin B1 (AFB1) was detected using an electrical and optical method which is a labeled method using gold (Au) nanobipyramids (NBPs). This detection was based on surface plasmon resonance of gold nanoparticles and showed a 0.4 nM detection limit and the electrical method using impedimetry showed a 0.1 nM limit of detection [45]. Shimayali Kaushal et al. put forward a hybrid antibody biosensor for detecting bacteria responsible for foodborne diseases. Specific antibodies are covalently attached by EDC-NHS chemistry to Polyethylene glycol (PEG) grafted graphene oxide (GO) coated AuNPs for sensing Escherichia coli and Salmonella typhimurium followed by antibacterial activity by photoablation after infra-red (NIR) irradiation [46].

5.1.3

Biosensors Based on Aptamer

The term aptamer was formed from the Latin terms “aptus,” meaning to fit, and “meros,” meaning part. These are single-stranded short nucleic acid sequences (DNA or RNA) that can selectively bind with molecules like proteins, peptides, small molecules, toxins, etc. These aptamers will form helices and loop structures rather than sequences. The target binding of the aptamer usually involves its 2D or 3D structure. In this, the target has greater surface density and less spatial blocking [47]. Three dimensional shape, hydrophobic interactions, base-stacking, and intercalation are responsible for target recognition and binding of aptamers. Aptamers can be chemically synthesized, and due to nucleic acid characteristics, they are stable to a range of temperature and pH (2–12). They have the capacity for thermal refolding. These aptamers can be further modified according to the target specificity. Systematic Evolution of Ligands by exponential enrichment (SELEX) is a method of in vitro selection of aptamers. It can also be isolated from oligonucleotides libraries. Different

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types of SELEX are present for the recognition of aptamers. Various types of biosensors (optical, electrochemical, and piezoelectric) are made by taking the advantage of the aptamers. These types of biosensors are further classified as label or label-free; SPR is the most common technique used in label-free detection. In label biosensors, fluorescence is one of the techniques in which the fluorescent molecule acts as the label in the aptamer system. Since fluorescent nanoparticles and quantum dots provide many advantages over conventional dyes they are used as label molecules. Aptamer-Qd conjugate was used to identify cancer cells, proteins, etc., [48]. Since gold nanoparticles are more biocompatible, show easy formation and conjugation, and have less toxicity; they have been commonly used in aptamer based sensors than Qds. A 0.01 pM sensitive L-Arg detection was performed using electrochemical impedance spectroscopy by Feng et al. [49]. Nilofar Ahmadi et al. designed a nanoaptasensor based on graphene oxide (GO)-aptamer- nanocrystals (NCs) ensemble utilizing fluorescence property of nanocrystals for the detection of lysozyme. Fluorescence resonance of transferring energy (FRET) happening between aptamer and nanocrystals led to quenching of fluorescence which on lysozyme aptamer binding increases with the concentration of lysozyme [50].

5.1.4

Whole-Cell-Based Biosensors

Biosensors that take the advantage of microbes like bacteria, fungi algae, etc. for the detection of specific analytes are termed whole-cell based biosensors. These types of cells are self-replicable and produce specific elements like antibodies which will act as recognition elements. This method does not need any further purification. Compared to larger animal or plant cells these types of cells are easy to handle and rapid [51]. These cells can interact with a variety of analyte molecules and produce corresponding electrochemical signals which are recorded in the transducer system. This is called the whole-cell based biosensor principle [52]. These types of sensors have good sensitivity, selectivity and have a high capability of detection of molecules. Due to these peculiarities, these types of biosensors are extensively used for environmental monitoring, detection of heavy metals and pesticides, detection of organic contaminants, and drug screening. Escherichia coli biosensors were fabricated for the detection of the pyrethroid insecticide. It is having a detection limit of 3 ng mL−1 .

5.1.5

Nanoparticle-Based Biosensors

Nanoparticles are widely used in different types of biosensor devices. Nowadays the biosensors which use nanoparticles are considered a new class of biosensors. Nanoparticles act as bioreceptors for different molecule detection [53]. Nanoparticles can act as both bioreceptor as well as transducers. Cancer biomarkers for breast cancer sensors were fabricated using a nanoparticle array of graphene with high selectivity and sensitivity. 1 pM detection limit is achieved due to the enhancement in the surface area of the nanoparticles [54]. A low concentration of the cancer biomarker can be

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Fig. 3 Illustration of nanoparticle based biosensors for virus detection. Adapted from the permission of [55] (Copyright © 2017 with the permission from Elsevier)

detected using the nanoparticle. Cerium oxide nanoparticles showed biomimetic catalytic activity which will be good in bioreceptor property. Noble metal nanoparticles, Quantum dots (Qds) graphene, CNTs, etc. possess transducer capabilities due to their peculiar optical and electronic properties. Figure 3 shows the schematic representation of nanoparticle based sensors for the detection of viruses. Benediktas Brasiunas et al. fabricated a colorimetric biosensor for detecting lactose, glucose, mannose, and fructose. AuNPs formation as a result of redox reaction between reducing sugars and AuCl4—ions were monitored using a spectrophotometer [56].

5.2 Biosensors Based on Transducer Based on the transducer principle, biosensors are widely classified as electrochemical, optical, thermal, electronic, and gravimetric.

5.2.1

Electrochemical Biosensors

Biosensors in which the detection was achieved with the help of the electrochemical property of the analyte or transducers are termed as electrochemical biosensors.

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These possess high selectivity and sensitivity. In the case of electrochemical sensors, the electrochemical reaction occurs on the surface of the transducer by the bioreceptor and analyte. It will result in the production of electrochemical signals which can be detected in the form of voltage, current, impedance, and capacitance [57]. Electrochemical biosensors are again classified based on transduction principles- Potentiometric, impedimetric, voltammetric, amperometric, and conductometric biosensors [58]. Potentiometric biosensors: It measures the accumulated charge in the working electrode due to the interaction of analyte and bioreceptor relative to the reference electrode under zero current. For potentiometric biosensors, ion-selective electrode and ion-sensitive field-effect transistors are used to transform biochemical reactions into potential signals [59]. The detection of the BRCA1 gene related to breast cancer in low concentration was developed using a graphene-based electrochemical DNA sensor was developed by Rasheed et al. [60]. Impedimetric biosensors: The change in charge conductance and capacitance of sensor surface was measured in impedimetric biosensor. A lower amplitude AC voltage is applied at the sensor electrode. With the help of an impedance analyzer, the in/out-of-phase current response is measured as a function of frequency. Voltammetric biosensors: It measures the current produced during the controlled variation of the applied potential. This method is highly sensitive and was used for the detection of different analytes [57]. Amperometric biosensors: It can be operated in two or three-electrode systems. When the working electrode is kept at a constant potential, the current is produced due to oxidation or reduction of electroactive species present. This current produced on the working electrode with respect to the reference electrode was measured and it is proportional to the concentration of analyte present in the solution. The advantages of these types of sensors are their fast, precise and linear response with an analyte and these are highly sensitive. Interferences from other electro-active substances and poor selectivity are the major drawback of such biosensors [61]. Conductometric biosensors: It measures the change in conductance between pair of electrodes during an electrochemical reaction. This change is due to the conductive properties of the analyte. These types of sensors are commonly used to detect the metabolic processes in living systems [62].

5.2.2

Optical Biosensors

Optical biosensors are those sensor devices in which the biorecognition element is combined with an optical transducer system. The working of optical sensors is based on the generation of the optical signal depending on the analyte concentration present in it. It utilizes nanoparticles, enzymes, antibodies, aptamers, etc. as a biorecognition element. The transduction process involves optical property changes such as absorption, reflection, phase, refraction, transmittance, frequency, etc. corresponding to the change in biorecognition element due to binding of analyte molecule. Optical biosensors are classified into label and label-free based on the principle of

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working. The signal produced due to the interaction between the transducer and analyte molecule is termed as label-free sensing. On the other hand, in the label method, the optical signal is produced by luminescence or fluorescence method. Different optical biosensors are produced with the help of different optical principles like fluorescence, chemiluminescence, Surface Plasmon Resonance (SPR), refractive index, Surface-enhanced Raman scattering (SERS), etc. The most widely employed optical biosensors are SPR based sensors, optical fiber-based sensors, fluorescencebased sensors, and chemiluminescence-based sensors.An optical sensor was developed by Bilal Ahmed et al. for L-cysteine detection by plasmonic amplification in the presence of Cr3+ using a surface Plasmon Resonance based gold nano particle as a probe [63]. Surface Plasmon Resonance-based sensors: Molecular interaction on the metal surface with surface plasmon wave produces a change in the indices of refraction. The linear dependence of the angle of SPR, θSPR, on the refractive index of the media at the interface is monitored in SPR-based biosensors. Such kinds of biosensors are coming under the class of label-free biosensing technology. When a polarized light illuminates onto the surface of the metal at the interference between two media having different diffractive indices, at a certain angle it produces a wave of electron charge density called a plasmon. With a layer thickness of the metal surface, the plasmon resonance shows a decrease in the reflected light intensity compared to the incident light at the resonance angle. This decrease in reflected light intensity depends on the mass on the surface [64]. SPR method measures the refractive indices of the analyte molecule to the biorecognition element. SPR biosensors are widely employed in disease diagnosis, food adulteration, and environmental monitoring. When it comes to consideration with nanoparticle SPR is replaced with localized surface plasmon resonance (LSPR). LSPR is the total internal reflection at the surface of the nanomaterial rather than the surface of the metal. Different types of biosensors are developed using metal nanoparticles based on LSPR technology [65]. Optical fiber-based biosensors: Optical fiber-derived sensors quantify biological species serving application of the optical field. The optical fiber technique is an evanescent field sensing, which is similar to that of tapered optical fibers. When light passes through an optical fiber due to total internal reflection results in the formation of an evanescent wave at the sample interface. This evanescent wave produced decays exponentially as the distance from the interface changes. The evanescent wave produced in this manner will excite the molecule present in the close proximity of the sensing surface. The optical fibers are widely used with different transduction processes like refractive index, absorption, fluorescence, etc. These types of sensors are promising and serve as an alternative method in comparison with the traditional method of sensing. A three-dimensional fiber optics-based based sensor was fabricated using gold nanoparticles and zinc oxide nanowires for the diagnosis of prostate cancer [66]. This sensor shows a linear response towards the change of the refractive index. It shows enhancement in the localized surface plasmon resonance response. The developed sensor has a detection limit of 0.5 pg ml−1 [66]. Huang et al.

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demonstrated cholesterol and glucose sensor using poly(N-isopropylacrylamide)-coacrylamide)(P(NIPAAm-co-AAm))-magnetic immobilized glucose oxidase(GOD) complex (PMIGC) and glucose oxidase (GOD) [67]. Biosensors based on fluorescence property: Fluorescence is a form of luminescence, in which electromagnetic radiation, usually visible light, is reemitted immediately (≈10−8 s) after exciting the material and can be used for the labeling of analyte molecules. It is widely investigated and used in medical diagnosis, food adulteration, and environmental pollution. It showed high selectivity, fast response, and greater sensitivity. Fluorescent molecules like dyes, quantum dots (Qd), metal clusters, and proteins show fluorescence property, and this property is utilized for effective analyte detection. Different techniques such as quenching, enhancement, and foster resonance energy transfer (FRET) were used to detect the changes in the analyte molecules. FRET-based biosensors were developed for cancer diagnosis, sensing, and aptamer analysis. Another FRET-based system was developed by Liu et al., by making use of carbon dots and gold nanorod for the detection of lead ions with a limit of detection of 50 nM [68]. Chemiluminescence-based optical biosensors: The process of production of light energy via a chemical reaction is known as chemiluminescence. This type of sensing got wide acceptance due to wide calibration limit, affordable instrumentation, low detection limit, etc. Nowadays chemiluminescence-based biosensors were extended to use nanomaterials so as to improve intrinsic sensitivity and detection capacity. A DNA chemiluminescence sensor was developed by He et al. using graphene oxide. It exhibit high sensitivity and showed a detection limit of 34 pM [69]. A chemiluminescence sensor was developed using luminol conjugated AuNPs and AgNO3 for the ultrasensitive manganese (Mn2+ ) detection in water samples. In presence of manganese ions, the nanoparticle shows enhancement in the chemiluminescence property due to the production of hydrogen peroxide. This simple and facile sensor shows detection from 1 nM to 0.600 μM having a 0.3 nM detection limit [70].

5.2.3

Gravimetric Biosensors

Mass-based biosensors show effective response when there is a small difference in the mass of the analyte molecules on the surface. Gravimetric biosensors are developed using a piezoelectric quartz crystal. These crystals will vibrate at a certain frequency and the frequency depends on the mass and current of the analyte molecule [71]. Magnetoelastic biosensors, piezoelectric biosensors, and QCM (quartz crystal microbalance) biosensors are employed in the gravimetric transduction method. Gravimetric transducers help to monitor pathogens and antigens. A piezoelectric biosensor along with a crystal shows elastic deformation upon application of current or potential. The application of current at a specific wavelength creates waves in the crystal. The sensor molecule detects the change in the resonant frequency and hence yields the result. Yang et al. developed a lead titanate zirconate based piezoelectric biosensor for the diagnosis of cancer biomarkers. This sensor shows a high sensitivity of 0.25 ng/ mL [72]. QCM biosensors work on the principle of the piezoelectric

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effect. For this, a thin quartz crystal is placed between two conducting electrodes. As the mass of the analyte molecule changes the frequency of resonance of quartz will also change. A selective sensor for miR-21 molecules using a QCM biosensor was developed by Lim and Lee. The quartz crystal microbalance sensor was developed for the detection of miR-21 with the help of a TiO2 photocatalytic silver enhancement reaction. This method showed high sensitivity with a lower detection limit of 0.87 pM [73]. The authors demonstrated sensing from serum samples as well. A magnetoelastic sensor ( MES) is a method to detect stress, force, pressure, and strain. It is working in the principle of magnetostriction; ie., mechanical deformation is developed by an applied magnetic field. When a time varying magnetic field is applied, magnetoelastic vibrations are produced. It results in a field-generated strain which in turn produces longitudinal elastic waves. These will produce detectable magnetic flux within the magnetoelastic material. Due to its passive nature, small size, costeffective and wireless characteristics MES got wide attention even in the biomedical field [74]. A detector for Fe3 O4 magnetic nanoparticles was developed using MES without any coating by Atalay et al. An amorphous ribbon was used in the fabrication of MES with calcification. Using this sensor, the authors were able to measure 0.025 mg of the magnetic nanoparticle [75]. M. Andac et al. demonstrated a gravimetric nanosensor for the perception of Cu (II) ions with the application of nanotechnology with high selectivity properties of ion-imprinted polymers [76].

5.2.4

Thermal Biosensors

The amount of heat energy released or absorbed during a reaction is utilized for the designing of a thermal biosensor. In other words, a thermal biosensor utilized the endothermic or exothermic property of a biological reaction. The total temperature change or heat energy absorbed or released by this sensor is proportional to the enthalpy and number of molecules produced and inversely proportional to the heat capacity [77]. T = −(npH )/Cp where T —change in temperature, np—number of molecules, H enthalpy, and Cp—heat capacity. Calorimetric biosensors measure the heat change and are used to calculate the structural dynamics of biomolecules in the dissolved state or the extent of reaction. Poor specificity and long experimental procedures limited the use of calorimetric devices. Also, it is difficult to identify the specific and non-specific heat change using calorimetric devices. But the development of thermistor based enzyme sensors having heat sensing elements and immobilized catalysts helps to overcome the abovementioned disadvantages. Thermal biosensors consist of an enzyme reactor that shows a differential measurement of temperature across the reactor. It also consists

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of thermal transducers (thermopiles or thermistors) which helps to the formation of the product by the conversion of the substrate molecule. The temperature generated during the catalytic reaction is proportional to the concentration of the analyte while detecting using a thermistor. The simultaneous reactions occurring in the thermometric detection are useful because the sensitivity of the sensing assay depends on the sum of all enthalpies. Thermistors or thermopiles are commonly employed temperature sensors. Among which thermistors possess sensitive temperature transducers. Thermistors detect the variation in electrical resistance with respect to the temperature. From this one can determine the absolute temperature with low sensitivity. Whereas thermopiles detects the temperature difference between the two regions. Thermopiles consist of a thermocouple junction developed using metals, semiconductors, and other semiconductor substrates. The thermopile is capable of detecting temperature change in two regions. Apart from these liquid gas expansion, bimetallic strips, metal resistance, and microelectromechanical systems (MEMS) are also used as temperature transducers in thermal biosensors. Among which thermocouples having excellent sensitivity is the better candidate in detecting the change in temperature. Monitoring of metabolic activity was done using microelectromechanical (MEMS) thermal sensors on the basis of changes in temperature. The advantages of MEMS are low fabrication cost and can be easily incorporated into a smaller electronic device with low cost. MEMS thermal sensors show low thermal mass, enhanced thermal isolation, high sensitivity with a linear range of response, low power consumption. Apart from these, it requires less time for the temperature measurement, and also multiple samples can be monitored simultaneously [58]. Estelle Glais et al. demonstrated a thermal probe of high local temperature measurement in the photoexcited thermalized aggregate of gold nanorods with ZnGa2O4 : Cr3+ , Bi3+ nanothermometers with the strong temperature dependence of chromium (III) luminescence lifetime and high sensitivity over a wide temperature range [78].

5.2.5

Electronic Biosensors

The electronic biosensor working is based on the principle of field-effect transistors (FETs). The field-effect transistor is a three-terminal device that regulates the flow of current using an electric field, which operates between the source and drain terminal on a semiconductor [1]. The main advantage of FET-based transducers is their efficacy to translate the interaction between the surface (FET) and analyte [79]. The potential on the surface of the sensor is changed upon binding of the analyte molecule to the bioreceptor surface. Due to this, there will be a change in the current flow between the source and drain terminals [80]. Chemical changes during bioreceptor interaction with analyte molecules are obtained due to the large input impedance of the semiconductor-based transducers. Compared to other biosensing method FET based method have certain advantages like high spatial resolution, and high sensitivity. At the same time, it possesses some limitations when employed in in vitro applications [81, 82]. Based on the technique of design, application of

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the gate voltage, and material of the gate and the channel region, transistor-based sensors in biological applications are commonly classified into metal–oxide–semiconductor field-effect transistors and ion-sensitive field-effect transistors. FET-based glucose sensor made of ZnO nanorod using AC frequency mixing possesses stability of 38 h with a sensitivity of 1.6 mA [83]. The lowest detection limit of the developed ZnO nanorod towards glucose is 1.0 μM. S. Rani et al. investigated the behavior of graphene (Gr) and hexagonal boron nitride (BN) and their heterostructure-based single electron transistors (SET) to detect / detect nucleobases such as DNA and RNA [84].

5.2.6

Acoustic Biosensors

Acoustic biosensors work based on the properties of change in an acoustic wave with respect to the amount of analyte interacted with the sensor [85]. Commonly used transducers are piezoelectric materials due to their efficacy to transmit and produce frequency-dependent acoustic. Here in the case of acoustic biosensors, it depends on the property of a piezoelectric crystal and its physical dimension. Change in the mass of the material on the crystal surface can induce variation in the natural resonant frequency of the crystal [86, 87]. Surface-acoustic waves and bulk-acoustic waves are the main acoustic transducers based on mass balance. A surface-acoustic wave transmits the acoustic wave from one place to another through a single crystal. In a bulk-acoustic wave device, the acoustic wave transmits from one crystal surface to another [88, 89]. The gas-phase interaction of these devices is well studied but in liquid media, the interaction was barely understood. Piezoelectric material will be an alternative for the transduction process in affinity biosensors. Still, this material lacks sensitivity and specificity towards analyte molecules [90]. A quartz crystal resonator (QCR) covered with a homogeneous layer of reduced graphene oxide was developed to create a highly accurate carbon dioxide acoustic wave sensor without moisture interference (RGO). The designed sensor could measure the levels of water molecules and CO2 at the same time, which can greatly reduce moisture disturbances [91].

6 Emerging Materials in Biosensors Some important materials that are used in designing transducers in biosensors are discussed here. This section also discusses the general principle behind using these materials as transducers.

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7 Metals 7.1 Optical Biosensors (Surface Plasmon Resonance-Based) The variations in optical properties of metals are exploited by metal-based biosensors, namely Surface Plasmon Resonance (SPR), light absorption, reflectance, surfaceenhanced Raman scattering (SERS), chemiluminescence, and, metal enhanced fluorescence (MEF) as a medium for sensing action. One or more of these optical properties are influenced by the presence or some kind of interactions of the analyte with receptor molecules and are in turn converted to any kind of signal and detected. Among which SPR-based biosensors are a commonly used method in the biosensor field. Figure 4 depicts a scheme for surface plasmon resonance. Surface plasmons (SPs) can be described as the surface phenomena of coherent electron oscillations occurring on the interface of a metal-dielectric medium. A sign change occurs for these oscillations at the interface for the real part of the dielectric function. In more simple terms, SPR is the resonance interaction of electromagnetic radiation, light, with the surface free electrons on the bulk or nanosized metals which decays exponentially. The most interesting factor is that the action of SPR defines or differentiates the bulk from nanosized metallic materials (in bulk it is called bulk SPR or simply, SPR and in nanosized materials as localized SPR or LSPR). The dependency of these oscillations on the properties of surrounding media is utilized by researchers for developing various sensors.

Fig. 4 Schematic representation of surface plasmons from (a) metallic thin-film generating propagating SPs (b) LSPR in nanoparticles. Adapted from [92]

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The working principle of bulk and localized SPR (LSPR) based biosensors are different since they apply to materials in two extreme dimensions. The changes in interparticle distance, shape, size, and the dielectric properties of the nanoclusters or nanoparticles can influence the absorption of SPR. Furthermore, SPR absorption depends on the dielectric properties of the surrounding medium [93]. The sensing principle behind the bulk SPR based biosensors is the linear dependence of the angle of SPR, θSPR, on the refractive index of the media at the interface. The components of an SPR based biosensor include a metal surface with biorecognition elements, a solution of an analyte, and the set-up for capturing and converting the signal. Analyte molecules are easily recognized and captured by biorecognition elements embedded on the metal surface. The analyte-bioreceptor interaction induces a local alteration in the refractive index. These variations are depicted as an increase in the propagation constant of SPs propagating on the metal surface. Figure 5 shows a sharp dip obtained when normalized reflection intensity is quantified on plotting with the angle or wavelength of the incident light with and without the analyte, while all other parameters are kept unaffected [93]. This high sensitivity of the resonance angle towards differences in the refractive index of the surrounding medium on the metal surface is manipulated as sensing action. SPR biosensors are always been a subject of interest in a vast spectrum of practical applications by researchers employing a wide variety of biorecognition elements like antigens, antibodies, receptors, proteins, growth factors, enzymes, etc. This resulted in large advances in food safety testing, disease detection and monitoring, biotechnology, drug screening, and environmental protection [95–97]. The phenomena of SPR were first extended to biosensors by Liedberg et al. in 1983. They developed a simple prism coupler SPR based biosensor for immunology and demonstrated the coupling of anti-IgG with Human IgG adsorbed on a silver metal surface with a shift of resonance angle of 0.9º [34]. The recent researches in SPR biosensors are up to the development of a sensor that detects antibody of SARS-CoV-19. Masson’s research group has developed a portable SPR instrument for the determination of

Fig. 5 Pictorial representation of the preparation of MOF and resultant structure demonstrating the crosslinking through polymerization between organic linkers and the potential void formation. Adapted from [94]

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even nanomolar levels of SARS-CoV-2 antibodies [83]. This is achieved by SARSCoV-2 nucleocapsid recombinant (rN) protein (bioreceptor) bound on a gold metal surface (transducer). This allows the detection of antibodies present in patients and thus, helps in developing vaccines [98]. With surface-enhanced Raman scattering (SERS) of 4-mercaptobenzoic acid (pMBA) in gold nano aggregates, J Kneipp et al. achieved spatially resolved probing and mapping of the pH value in living cells using mobile and biocompatible nanosensors [99]. HM Kim et al. developed a highly sensitive and selective plasmon sensor approach based on a surface plasmon-enhancing resonance nanosensor with polymer micro-tip and composite gold nanoparticles for the detection of AFB1 (extremely toxic) [76]. The use of an enzymatic reaction in a highly selective SPR-based fiber optic hydrogen peroxide sensor was described by Vivek et al. The sensor probe was manufactured by depositing layers of gold and graphene oxide (GO) on the uncoated core of glass fiber and then immobilizing the catalase enzyme through EDCNHS coupling [100].

7.2 Metal–Organic Framework (MOF) Based Biosensors Another class of metal-based biosensors is metal–organic framework-based biosensors (MOFs). MOFs are defined as a hybrid material of metal ions or metal clusters and organic molecules having a large percent of void space coordinated in a unique fashion. The widespread interest in MOFs is that can be tailored according to the needs by functionalizing the linkers, modifying metal–ligand coordination, or pore size and hence can be used for catalysis, sensing, drug delivery, etc. Moreover, numerous combinations are possible varying metal atoms, organic linkers, and pore size [94, 101]. The major advantages of MOFs include the higher stability, chance to tune the properties, tolerance with harsh and extreme temperature and pH conditions, however, binding kinetics of synthetic materials are not appreciable (Fig. 5) [94]. The synthesis methods involve normal solvothermal methods using autoclaves or under microwave-assisted conditions with suitable reagents to facilitate coordination polymerization [102]. Most of the reported works of MOF biosensors utilize the potential of the MOF as both receptor and transducer so that quantification and biorecognition by electrochemical, mechanical, or optical means are done by polymerized hybrid material [103, 104]. Biorecognition of analyte is made possible through the capability of H-bonding, π-π stacking interactions, dative covalent bonding, or electron donor/ acceptor interactions of linker molecules or the electrostatic interaction capability of metal ions or clusters with the analyte. Liu et al. developed MOFs for DNA sensing for the first time based on fluorescence quenching property. Zimidazolate framework-8 nanoparticles (ZIFNPs) were manipulated to detect HIV-single strand DNA. On binding ZIFNPs to FAMlabeled ssDNA probe that is specific to HIV-ssDNA, quenching happens which is otherwise highly fluorescent. The binding happens through electrostatic and π–π

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interactions. However, the sensitivity was lower as the fluorescence quenching efficiency exhibited was only 53% [105]. A water dispersible MOF framework, NH2MIL-53(Fe), was manipulated by Neha Bhardwaj et al. for Staphylococcus aureus biosensing. NH2-MIL-53(Fe) were conjugated with specific bacteriophages of S. aureus through glutaraldehyde covalently and thus facilitated sensing via detection of photoluminescence quenching with a LOD of 31 CFU/mL [106].

8 Polymers Widely used polymer-based biosensors are conducting polymers (CPs) based electrochemical biosensors. This is due to its easiness of fabrication, cost-effectiveness, and high sensitivity that provides a direct electrical readout in presence of the analyte. CPs can act as either transducer that produces some kind of electrochemical signal or both transducer and biorecognition elements which makes the sensor much simpler. The important factor that leads the sensor development is the stability of the transducer matrix and its abundance of functional groups present in it or the ability to get modified. The progression happening in the field of polymer-based biosensors has contributed appreciably to the improvement in healthcare initiatives [107, 108]. Carbon-containing conjugated polymeric systems with electrical conductivity are known as conducting polymers (CPs) or intrinsically conducting polymers (ICPs) [109]. They show high electrical conductivity with good redox activity and electron affinity. The most popular CPs are given in Fig. 4. Their vast popularity among biosensor researchers is due to their highly organized π conjugated system, i.e., any changes in its confirmation can easily be sensed. Perturbations caused to the CP films by target hybridization or attachment to the biorecognition element are converted to potential signals. The immobilization of the biorecognition elements like enzymes, proteins, antibodies, aptamers, etc. on the transducer CP is a very important part while considering biosensors. Immobilization can be done by covalent bonding, adsorption, electrochemical entrapment, and affinity-based attachment [107]. The functional groups on biorecognition elements like −NH2 , −COOH, and −SH, are employed for binding with transducer covalently. The Van der Waals’ forces, electrostatic interactions due to cationic and anionic charges on CPs and biomolecules, etc. play a significant role in getting biomolecules physically adsorbed on transducer CP. Dicks et al. first introduced the adsorption based biosensors demonstrating the adsorption of glucose oxidase on polypyrrole (PPy). This method is efficient for enzymes, oligonucleotides, but not much effective for DNA due to the relatively weak interactions involved and often leads to leaching out [110]. PPy is the most appreciable material for electropolymerization embedding since it can be polymerized at neutral pH with low potentials in aqueous solutions, widely known, after reported electrochemical entrapment of glucose oxidase into PPy films [111]. Eventhough, the affinity based attachment immobilization method is similar to covalent binding in terms of strong binding, unlike the latter, the former does not involve chemical reagents for binding. For

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example, the avidin–biotin system, in which biotin species is entrapped by electrodeposition on the monomers and copolymerized to form polymer matrix to which avidin is introduced [112]. Among the polymeric materials used for designing biosensors polyaniline (PANI) and its derivatives got peculiar properties. Their specialty is its strong bimolecular interactions, good environmental and electrochemical stability, high conductivity, good electrical and optical properties [113]. Piezoelectric immunosensors with PANI immobilization were developed by V. V. R. Sai et al. which could differentiate the target analyte concentrations between appreciable nanogram to milligram per mL with nonspecific binding of ∼10%. Human IgG was immobilized on PANI covalently utilizing a crosslinker, glutaraldehyde [114]. Incorporating metals, metal oxides, carbon-based materials into CPs enhanced their performance, especially as electrochemical biosensors. Fabrication of these composite materials employs various synthetic methods such as vapor polymerization, template-oriented synthesis, in situ generations, electrochemical techniques, and chemical functionalization. An electrochemical biosensor of low biofouling was realized for the detection of hepatocellular carcinoma biomarker, Alpha-fetoprotein (AFP) in which AuNPs and polyethylene glycols (PEG) loaded PANI acted as substrate. The conducting property of both AuNPs and polyethylene glycols (PEG) and the anti-biofouling of PEG facilitated an efficient immobilizing unit of the AFP immunosensor [115]. Nanocomposites of PANI/Graphene oxide (PANI-GO) were recognized in the simultaneous sensing of dopamine, ascorbic acid, and uric acid [116].

9 Ceramics Brittle and hard inorganic materials containing both metal and non-metals are ceramics that comprises many popular items like potteries, tiles, etc. Ceramics are realized in fabricating microneedle type in vivo neuro sensors based on amperometric sensing [61]. Gerhardt’s group studied extensively and developed microelectrodes based on ceramics as parallel materials for silicon. Unlike silicon, ceramics are more durable and nonconducting, so became more acceptable for in vivo measurements. Gerhardt’s group prepared a microneedle of 0.127 mm thick ceramic substrate having 1 cm length that gradually narrows to a ∼2–5 μm end portion or tip on which recording sites, as well as connecting lines, have a platinum coating [117]. Utilizing these microelectrodes different amperometric biosensors are designed for choline, acetylcholine, glutamate, and lactate. Furthermore, using cyclic voltammetry, a carbon-ceramic electrode was used for folic acid detection. Multisite microelectrodes made of ceramic are demonstrated as a biosensor for detecting L- Lactate measures of brain tissue. The microelectrode consists of three functional layers. The response time and selectivity of these electrodes offer better performance for the measurements in brain tissue in vivo [118]. Karol Malecha and coworkers designed ceramic-based biosensor making use of low temperature

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co-fired ceramics (LTCC) technology for glucose detection. Glucose is oxidised in presence of glucose oxidase enzyme and byproduct hydrogen peroxide is sensed amperometrically [119].

10 Alloys Metallic materials composed of two or more metallic elements are termed alloys. A variety of non-enzymatic biosensors for glucose detection are developed using alloys. Bimetallic combinations of elements like Pt, Pd, Au, Ag, Ni, Cu, Ru, Ir, Pd, and Pb are shown to be promising materials in this field. Two theories are explaining the possible action of transition metals in glucose electrooxidation. According to the first theory proposed by Pletcher [120], glucose gets adsorbed to the surface of the catalyst containing transition metals, and the hemiacetalic hydrogen atom is abstracted which can be due to interaction between d-electrons and the absorbate. The second theory was put forward by Burke [121], which focuses on the role of OH radicals in the process of electrocatalysis, known as ‘Incipient Hydrous Oxide Adatom Mediator’ (IHOAM). The excellent catalytic activity shown by Pt-based alloys made them popular for electrooxidation of glucose and other organic molecules among others. Sun et al., concluded Pt2 Pb is the best combination for glucose oxidation. However, surface poisoning by chloride ions remained the greatest challenge. Three-dimensional heterostructures were demonstrated for the nonenzymatic recognition of glucose using Co3 O4 /Ni on the porous Ni substrate via the hydrothermal method. They showed a good sensing capability for glucose, numerically, 13,855 μA mM−1 cm−2 [122]. Joseph Wang and coworkers developed Ru-Pt alloy dispersed carbon-paste enzyme electrodes for glucose biosensing with an appreciable selectivity and higher sensitivity, unlike pure metal dispersions [123]. Similarly, PtAg and PtCu alloys which are nanoporous were immobilized with glucose oxidase (GOx) after modification with enzymes were reported by Caixia Xu et al. for glucose sensing [124].

11 Metal Oxides Crystalline solids made of a metal cation and oxide anions are known as metal oxides. Metal oxide has shown to be a better emerging material in the field of sensing application. They are considered as ideal immobilization matrices or transduction platforms for a biosensor owing to their biocompatibility, fast electron transfer capability, chemical stability, as well as, increased surface area. Among the metal oxides, zinc oxide (ZnO) has contributed appreciably to sensing applications. It is an ntype semiconducting transition metal oxide having a broad bandgap of 3.37 eV, excellent film forming capability, good immobilization capacity, high isoelectric

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point, excellent stability and conductivity, good corrosion resistance, and appreciable biocompatibility on reducing to nanostructures [125]. Semiconducting metal oxide-based biosensors, metal oxide composite materialsbased biosensors are advancements to this area. Semiconducting metal oxides are used in electrochemical biosensors which are affixed on the surface of the electrode by electropolymerization, electrodeposition, covalent bonding, or physisorption [126]. Glucose sensing was demonstrated by semiconducting TiO2 (tetragonal columnarshaped) nanorods as immobilizing platform for glucose oxidase [127]. Xiaoli Zhang et al. designed ZnO nanostars for signaling the presence of miRNA-21 in tumor cells with a LOD value of 18.6 aM based on electrochemiluminescence [128].

12 Nanomaterial Based Biosensors or Nano Biosensors Nanoscience and nanotechnology are the study of the creation of ultra-small material in the nanometer length scale and their application. Using this technology one can create a varied property by manipulating the material in the nanoscale. Nanomaterials consist of ultra-small materials comprising the material length of scale less than 100 nm in any of the three-dimension, typically 1 to 100 nm. The high reactivity and high surface-to-volume ratio provide better performance in all areas of science technology from the energy, environment, and biomedical field. Nanoscience and technology is a highly interdisciplinary area that clubs all the branches of science and engineering together so as to create materials for diverse applications. These materials showed better performance in the biosensing area due to the enhanced surface area offered by this material allowing greater sensing efficacy. Mainly two approaches are used in manufacturing nanomaterial-based sensors. They are top-down and bottom-up approaches. In the top-down, the breakdown of bigger entities without atomic-level control is done to form a nanoscale device or sensor. In the bottom-up approach, sensors or devices are manufactured with the selfassembly of molecules by chemical or physical forces to form a large structure. The nano biosensors are formed by the principle of science extended to the atomic level for the application of life science and biological parameters as sensing templates. Bioanalysis with nanomaterials provides better optical and electrochemical sensing properties with immobilizing nonmaterial as a sensor. The amplification of sensitive biosignals in a controlled manner can be utilized for further application in a novel way for biosensing applications.

12.1 Nanoparticle Based Sensors The characteristic physical and chemical properties of nanoparticles are due to the size, shape, and chemical compositions that enabled them to create novel sensing platforms for the manufacturing of sensor devices. Because of the unique properties

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offered by the nanoparticles, ie., the size-dependent optical, electronic, magnetic properties, and presence of more binding sites, due to larger surface area offered opens up to a unique candidate in the area of biosensors with better sensitivity, accuracy, and selectivity [129]. Multiple recognition ability, accuracy in producing results, sensitivity and precision, fewer sample requirements, and fast performance are the major highlights of nanoparticle based sensors. A variety of nanoparticles are made of different elements mainly silver, gold, copper, platinum, and palladium, and their different chemical compositions helped to detect trace elements with high precision and accuracy by utilizing the changes optical, electronic, magnetic properties. As a result, nanoparticle based sensors were widely demonstrated as a potential biosensor during recent years [130]. Nanoparticles in colloidal form and quantum dot nanoparticles in association with ligand functionalization and coupling make them a special tool for the recognition and enhancement of biosignals [131]. Nanoparticle assembling labels and biomolecule interaction can increase the sensitivity like polymerase chain reaction detection. DNA and protein detection using biomolecule functionalized nanoparticles provides better sensitivity in bio diagnostics and bioanalysis. These nanoparticles are either used for bioanalytic applications as transducers or for biosensing on which biomolecule conjugates. Among the nanoparticles, metal-based nanoparticles are widely used as sensors because of their size-dependent surface plasmon property. When the size of the nanoparticle is less on compared to the wavelength of light there exists a coherent oscillation of conduction electron on the metal surface resulting in the emergence of surface plasmon resonance (SPR). Biocompatability, non-toxicity, high colloidal stability and ease of functionalization of metal nanoparticle allows them to use in the area of biosensors. When the metal nanoparticles interacts with specific analyte there exist changes in the SPR peak which is used for quantifying the analyte molecules of the developed sensor. Another uniqueness of this metal nanoparticle is that most of them shows visible color change upon interaction with analyte, this is not true for all cases. Silver nanoparticle synthesized via greener route was utilized for the detection of agro-fungicide mancozeb. The sensor shows a linear response towards the fungicide and sensitivity is 39.1 nm/mM [132]. Silver nanoparticles deposited in and out of a TiO2 nanotube were used as a non-enzymatic biosensor for glucose. The linear pattern obtained as a response of detection of glucose from 20 to 190 mM having a limit of detection of 24 μM [133] is shown in Fig. 6.

12.2 Metal Oxide Nanoparticle Metal oxide nanoparticle is one of the most promising candidates in the field of biosensors. This is mainly because of the controllable size/shape, stability, ease of fabrication, optical catalytic properties, electron-transfer kinetics, biocompatibility, and, strong adsorption ability [134]. Increased electron mobility has attracted a variety of biological and therapeutic applications. The advantage of these metal oxide nanoparticles in the area of biosensors is their high adhesion capability, film forming

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Fig. 6 Non-enzymatic biosensor for glucose using AgNps. Adapted from [132] https://pubs. acs.org/doi/10.1021/acsomega.0c01136 (Copyright @ 2020American Chemical Society). Further permissions related to the material excerpted should be directed to the ACS

capacity, small grain size, enhanced oxygen storage efficacy, high surface area, resistance to oxidation, and corrosion efficacy. Apart from these it also possesses a high isoelectric point that owes to its strong adsorption properties. It is highly useful in biomolecular sensing applications due to its fine grain size, better chemical stability, recommendable catalytic activity due to strong adsorption of oxygen, large surface area, strong reluctance for oxidation and corrosion, appreciable adhesion capability, and excellent film forming ability. The good electrical conductivity of these metal oxide nanoparticles helps to fabricate stable, fast, and accurate sensor devices [135]. The combination of the nanotextured surface and metal oxide leads to the production of nanohybrid materials. Because of the improved optical and electronic properties, is anticipated to open up novel avenues for diagnosis and treatment [136]. Sol–gel techniques, soft templating, hydrothermal deposition, radiofrequency sputtering, etc. are the underlying synthetic techniques involved in fabricating metal oxide nanoparticles [137, 138]. Zinc, cerium, titanium, zirconium, tin, magnesium, and iron are commonly studied metal oxide nanoparticles as biosensors. The advantages of these nano oxides of metals are their non-toxic nature, biocompatibility, unique morphology, and enhanced catalytic activity. Metal oxide nanoparticle shows excellent optical and catalytic and electrical property due to interaction of phonons and electrons compared to different substrate materials. Enhancement in the surface property such as surface area, reactivity at the surface, modified working function over the surface, strong absorption resulted in the enhancement in the unique properties offered by the metal oxide nanoparticles. Because of the nanometer size of these metal clusters allows them to use for quick and simple in vivo examination. Different novel signal transduction technologies can be developed using metal oxide nanoparticles [139]. A biosensor is a

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miniaturized device that uses biological elements as analytes (antibodies, enzymes, proteins, nucleic acids) and sensors to detect signals. Biosensors utilize the selectivity of biomolecules and the capability of contemporary optoelectronics and microelectronics to create an efficient analytical instrument that can be utilized in health care, medical diagnosis, and many other areas. Due to specificity, low cost, mobility, and, a fast response time of metal oxide nanoparticle based biosensors are anticipated to play an important role in the medical and health care field. Miniaturization of metal oxide nanoparticles allows the fabrication of many miniature electrodes and the sensors to be packaged as a biochip device resulted in the high-density bio-array design [140]. For the use of biosensors of metal oxide nanoparticles, the major focus is on solid–liquid interfaces and nano bio-interfaces. It is critical to choose suitable metal oxide nanoparticles for immobilization of the relevant biomolecule when fabricating an efficient biosensor. Surface charge, roughness, surface area, physical states, hygroscopic nature, functional groups, and porosity plays a vital role in the creation and characteristics of nano-bio interface creation. Apart from these the creation of nano-bio interface depends on the short-range force originated from solvent interaction, steric depletion, etc., The functional groups created using suitable chemical reaction helps the covalent binding of the biomolecule to the metal oxide nanoparticles. By creating a biocompatible microenvironment, one can create a suitable metal oxide bio nano interface so as to maintain sensing efficacy having high stability. The interactions of the metal oxide nanoparticles to biomolecule influence the rate of electron transport [141]. Through the application of metal oxide nanoparticles, it is possible to fabricate the biosensor having higher efficacy, limit of detection, stability, and sensitivity with reduced cost and longer shelf life. Metal oxide nanoparticles can immobilize a variety of biomolecules, including enzymes, nucleic acids, and antibodies. Chitosan treated iron oxide (Fe3 O4 ) nanoparticles were deposited on an ITO plate immobilized with glucose oxidize (GOX) via physical deposition were used for the glucose determination (Fig. 7). This sensor exhibits linear behaviour towards glucose having a sensing capability of 9.3 μA /(mg dL cm2 ) from 10–400 mg/dL [142]. The change in capacitance in accordance with a change in glucose was monitored by a metal oxide nanoparticle sensor for the application of a highly selective capacitive nanosensor for non-enzymatic glucose detection. ARDUINO UNO was used to test the ability of polyvinyl alcohol (PVACuO) coated copper oxide thin films on indium tin oxide (ITO) coated glass [143]. To identify gases, different ultrasound effects are used in the continuous detection reactions of an ultrasound irradiated metal oxide semiconductor gas sensor to different target gases [144].

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Fig. 7 Schematic representation of the mechanism of sensing glucose using iron oxide nanoparticles. Adapted from [142]. (Copyright © 2008 with permission from Elsevier)

12.3 Quantum Dot Quantum dots are nanometer or subnanometer-sized semiconducting core shell structured materials. Their size-dependent optical and photoluminescence make them fluorescent or electrochemical probes in biosensing applications. Zero dimensional nano semiconductor materials are known as the quantum dot [145]. It possesses unique physical and optical characteristics makes them a unique candidate in molecular biology, material science, chemical analysis, and other fields. When semiconductor nanomaterials like CdS, CdTe, PbS, and ZnS are stimulated at different wavelengths, they can glow due to their discrete energy states. Semiconductor nanoparticles also known as quantum dots are one of the most common nanoparticle biomarkers employed in the electrochemical analysis. Quantum dots based electrochemical biosensors helps to detect biological molecules. Firstly the specific interactions like antigen and antibody interaction and the interaction of biotin and avidin are some of the commonly performed interactions for the detection in the majority of the quantum dot based sensors. Secondly, the concentration of targets is determined quantitatively using the voltammetric response of the sensor molecule. Controllable light emission, good light intensity and stability, larger Stokes shift, longer fluorescence lifetime, functionalized active surface and good biocompatibility are the major properties of the quantum dots. Based on the detection mechanisms employed in biosensors based on quantum dots can be divided into three categories.

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They are target-ligand interactions, quantum dot-target interactions, and fluorescent resonance energy transfer. The primary issue in the fabrication or designing of quantum dot based biosensors is the precise monitoring of target proteins from complicated analyte samples. When it comes to the in vivo studies due to the slight toxicity of Cd or Se limits its usage. But there exist different methods, such as passivating the surface or reducing the concentration of quantum dots, etc., to reduce the toxicity. For example, the quantum dot synthesized by Nair et al. is shown to be slightly toxic but when it is functionalized with a single-wall carbon nanotube doesn’t show any toxicity [146]. Cadmium selenium (CdSe) based quantum dots were designed for the selective recognition of streptavidin using an assay; steric hindrance hybridization. The developed sensor can detect streptavidin from 1.96 pg/mL to 1.96 μg/mL having 0.65 pg/mL is the LOD value [147]. Pooja et al. investigated a functionalized CdTe fluorescence nanosensor for the sensitive detection of environmentally harmful metal ions. The use of COOHfunctionalized CdTe quantum dots (QD) as fluorescent nanosensors for the detection of various environmentally hazardous metal ions like (Cr3+ , Pb2+ , Cu2+ , Zn2+ , and Co2+ ) was analyzed in the aqueous phase [148]. Jin wang et al. developed Capillary Sensors with CdTe Quantum Dots for the detection of Cu2+ in Real-Time. Aqueous synthesis has been used to produce colloidal quantum dots (CQD) of CdTe coated with glutathione (GSH). CdTe-CQDs have been used due to their selective quenching of fluorescence in the presence of Cu2+ [149].

12.4 Anisotropic Nanoparticle Based Biosensors As the size of the materials is diminished in size to the nano regime, they start to show new and peculiar properties and functionalities. Metal nanostructures are exceptionally compelling in light of the fact that such qualities are constrained by altering shape, structure, and size. Anisotropic nanoparticles are non-round structures that have shape-subordinate synthetic and actual properties and can be employed in a significantly broad range of applications spanning from biosensing, imaging, catalysis, and so on. When considering gold nanoparticles, AuNPs are classified into two groups depending on their shape and properties: isotropic and anisotropic. By solving the Maxwells equation Mie described the interaction of light with nanomaterials materials [150] and thereby explained the phenomena of scattering by gold nanoparticles upon interacting with suitable electromagnetic radiation. In nutshell, Mie’s theory explains when the size of the particle increases the plasmon absorption shifts towards the longer wavelength because of the change in the density of charge at the surface. Furthermore, a slight change in particle size or shape resulted in a drastic change in plasmon absorption energy. The anisotropic shape of gold nanomaterial shows a difference in plasmon absorption due to their anisotropic shape. Changing the morphology of gold nanomaterial switches their surface plasmon absorption from visible to the near-infrared region (NIR) of the electromagnetic spectrum. The NIR

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absorption allows deep tissue penetration make them a promising candidate in the medical field for disease detection and therapy [151]. Also, owing to their enhanced optical response, anisotropic gold nanoparticles have proven to be a notable candidate in the field of biosensing with high selectivity and accuracy [152]. In addition to that, the absorbed energy can be converted to heat in a smaller time scale of the order of picoseconds because of the electron–electron interaction or electron–phonon interaction [153]. Gold nanorod is shown to be an inevitable candidate in the field of biomedical imaging, sensing of biomolecules, and plasmonic photothermal therapy [53, 154]. The anisotropic gold nanoparticle has great potential in many substitution domains due to its size-dependent optical properties. Nanorods, nanotriangles, nanoprisms, nanocubes, nanoarrows, nanostars, pentagons, icosahedrons, are the commonly synthesized anisotropic nanoparticle due to the improvements in synthetic procedures. For the chemical synthesis of anisotropic AuNPs, there are two major methods: (1) seed-based methodology [155] and (2) seedless-based approaches [156]. The reduction of Au3+ ions is used in both methods. Small spherical AuNPs are employed as seeds in seed-based methods. Those seeds act as nucleation sites for Au3+ reduction, allowing for anisotropic development. Seedless-based methods, on the other hand, rely on the reaction kinetics and essential chemical variables (such as stabilizing, reducing, and capping agents) to regulate growth. Furthermore, Green chemistry methods are accessible for the synthetic methods of both types of AuNPs because both synthetic methodologies have significant costs and negative environmental consequences. An affinity sensor based glucose biosensor was developed using gold nanorod by using concanavalin-A/dextran/glucose. The longitudinal plasmon resonance peak depicted a hypsochromic shift to 714 nm of dextran conjugated gold nanorods on the addition of concanavalin on account of dissociation of dextran coated gold nanorods. The degree of the shift in wavelength is correlated with concanavalin-A. The detection limit is from 1 to 30 mM of glucose [157]. Dizajiet al. used 3D anisotropic gold nanorod (GNR) arrays to create a highly active Surface Enhanced Infrared Absorption Spectroscopy (SEIRA) platform for bacterial identification [158]. The use of laser-written Femtosecond Fiber Bragg Grating (FBG) sensors for real-time monitoring of tumor temperature during Nano Gold Rod (GNR) mediated photothermal therapy has been described by L Bianji et al. Temperature detection in subcutaneous breast tumors in mice was precise when used the FBG sensor [159].

12.5 Quantum Cluster Based Sensors Metal Nanoclusters are considered as the smallest of nanoparticles of 25 nm) based on the particle size [274]. In general, antibacterial, or antimicrobial activity is triggered by penetration of bacterial cell membrane followed by generation of reactive oxygen species (ROS). The ROS is known to cause protein denaturation, interruption of electron transport mechanism, and DNA damage leading to bacterial cell death. The colloidal Pt nanoparticles’ proand anti- oxidative properties can affect the activity of molecules like antibiotics as well as normal cellular functions. The Pt NPs have been reported to mitigate oxidative and mitochondrial stress in THP-1 macrophage cells by Hanako Kachi et al. [164], and in a similar study by Sai Ma et. al., the incubation of quaternary ammonium based antibacterial monomers in the presence of colloidal platinum (2.5 mM) reduced the antibacterial property of monomers against S. mutans bacterial cells [213]. On the other hand, the antibacterial property of divalent Pt NPs with zero-valent Pt core has

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been explored to activate with near-infrared (NIR) exposure on both gram-negative E. coli and gram-positive S. aureus (methicillin-resistant) suggested by significant reduction in colony forming units (CFU) and cell viability. The also reported NIR activated Pt NPs exposure resulted in bacterial cell membrane and DNA damage as well as variation to ROS and ATP metabolism [74]. The colloidal Pt nanoparticles conjugated to latex beads have also been known to have anticancer effect against oral squamous cell carcinoma (HSC-3-M3) in a nude mice with xenograft model suggested by aggregation of NPs at site followed by destruction of cell membrane and globular structure [327]. In the last decade copper (Cu) and bismuth (Bi) nanoparticles have also drawn attention for prevention and treatment of peri-implantitis and peri-implant mucositis because of antibacterial properties. Bismuth nanoparticles in colloidal or coating form are known to improve the biocompatibility of medical devices and inhibit the bacterial growth as well as biofilm formation of various gram-positive and gramnegative bacterial species [26, 43, 108, 134, 286]. In the case of copper nanoparticles, concentration dependent studies suggested improved the mechanical property, resindentin bond strength, and adhesive property investigated with caries affected dentin and resin-adhesive/orthodontic-adhesive system as well as maintained antibacterial property under in vitro conditions [19, 115–117, 340] and also in a clinical study [221]. The copper nanoparticles on a TiO2 nanotubes array based dental implant [285], embedded into silicone and polyurethane based resin composite [298], or hybridized with chitosan nanoparticles [64] have shown promising potential as antibacterial agent against cariogenic bacterial species as well as against antibiotic resistance developed bacteria. Under similar in vitro conditions Cu NPs deposited surface (ZrNb alloy) showed improved osteoblast adhesion and antibacterial property against S. aureus [184] and colloidal Cu NPs are even known to be a potential disinfectant for abutment under in vitro conditions [345]. Biosynthesized nano-copper is also known to have similar antibacterial property against gram-positive and gram-negative bacteria [280] and has been employed as antibacterial agent against periodontitis and periodontal disease-causing bacterial species [79].

2.4 Titanium Dioxide Nanoparticles Apart from pure titanium-based devices in dental application, the titanium dioxide (TiO2 ) nanoparticles (NPs) have also been explored extensively for various photocatalytic and photovoltaic devices as well as biomedical and biological applications like drug delivery, photoablation therapy, biomedical imaging. Also, TiO2 NPs are widely being used in paints, cosmetics, toothpaste, and food colorings [222]. The white color of TiO2 nanoparticles can be particularly favorable for dental application. The synthesis of TiO2 is typically done by sol-gel techniques followed by calcination that changes the crystal structure of TiO2 from anatase to brookite to

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rutile with increasing temperature [257]. Other techniques namely hydrothermal, coprecipitation, micro-emulsion, and laser ablation etc. have been reported in literature for the synthesis of TiO2 nanostructures [257]. The addition of TiO2 NPs (0.1%) mixed with methacrylate based macromers has been known to increase the degree of conversion and reduce the polymerization time via photo-polymerization reaction as well as improved the adhesive strength of composite [58, 318–321]. In addition to that mixing up to 5 wt% of TiO2 NPs with glass-ionomer in a commercially available formulation KavitanTM Plus or Bis-GMA/TEGDMA/UDMA resin-based composite significantly improved the fracture toughness, comprehensive strength and flexural strength but further increase in concentration resulted in loss of mechanical properties without compromising bond strength with dentine [80, 124, 282]. Apart from positively affecting the physical and mechanical properties of resin composites in orthodontic and dental applications [160, 161, 243], TiO2 nanostructures have also shown promising results as antibacterial [84, 85, 373], antimicrobial [46], bactericidal [147] and biofilm inhibiting [88] agent when used ‘as prepared’ or modified [58, 162, 163] against both gram-positive and gram-negative bacterial species. Even though the TiO2 nanoparticles are known to improve physical mechanical properties as well as antibacterial/antimicrobial but when amalgamated with resin or coated on device surface, these nanostructures encounter the healthy cells in oral environment. In a study with animal model, dip coated as well as laser deposited TiO2 nanoparticles on dental implant, provided a rough biocompatible implant fixture that supported the peri-implant bone healing process as well as osseointegration [27]. Unlike previous examples, the exposure of TiO2 NPs to human gingival fibroblast cells resulted in significant increase in IL-1β induced prostaglandin E2 production, COX-1, and COX-2 protein expression, and IL-1β induced metabolic changes suggesting gingival inflammation and a monitored application of TiO2 NPs containing materials in the case of patients suffering from gingivitis or periodontitis [102]. While in another study with human epithelial cells, the exposure of nitrogen doped TiO2 NPs resulted in oxidative stress but had no effect on cell viability [373]. Also, Mohamed Ibrahim et al., have reported disruption of cytoskeletal networks and cell adhesion properties when human osteoblast cells (SaOS-2) were incubated with an increasing concentration of TiO2 NPs of different sizes (5 and 40 nm) [148]. So, these biological changes in the presence of TiO2 nanostructures could be NPs size, concentration and or NPs surface chemistry dependent or cell type could be another factor that can certainly affect the cell-nanoparticle interaction.

2.5 Zinc Oxide (ZnO) Nanoparticles Among the various oxide nanoparticles, ZnO nanoparticles have been also synthesized using both chemical [129] and bio-synthesis route [122, 125, 310, 355]; and have been ingredient of many commercial enzymes, ointments, and sunscreens because of UVA and UVB light absorption. The factors like particle shape, size,

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concentration, and exposure time are known to have impact on physical–mechanical (hardness, thermal stability, glass transition temperature, and the hydrophilicity), biological, and antimicrobial properties of nanoparticles including ZnO [57]. Increasing the ZnO size from 21.11 to 2012 nm mixed with eugenol bonding resin resulted in loss of compression strength, hardness and interlocking bond [28]. While in a similar study, the amalgam of spherical or flower like nano-ZnO structures in a resin-modified glass ionomer cement resulted in the loss of surface hardness compared to control while mixing nanorods had no effect on mechanical properties of cements [254]. The ZnO NPs have been mixed with other additives like drug molecules and other nanoparticulate systems in resin based adhesive, sealant, or filler composites for the prevention of dental prosthesis failure, biofilm formation, peri-implantitis, tooth decay, and dental caries (composite filled in bovine teeth) [2, 3, 117, 123, 171, 251, 254, 330]. Either it is the light activated ZnO/curcumin or just ZnO nanostructures, the nanoparticles are known to work as an orthodontic adhesive additive against multispecies cariogenic biofilm and without the loss of physicochemical, mechanical, and biological properties when mixed with methacrylatebased resins [267, 288, 331]. The antibacterial or antimicrobial activity of ZnO nanoparticles incorporated in different resin composites formulations have been investigated against bacterial or fungus species like Lactobacillus, Bacillus cereus, Bacillus licheniformis, Streptococcus mutans, Streptococcus sobrinus, Staphylococcus aureus, Staphylococcus epidermidis, Rothia dentocariosa, Rothia mucilaginosa, Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa, Candida albicans (yeast/fungus), and Clostridioides (Clostridium) difficile (highly resistant endospores) for dental and or orthopedic applications [25, 111, 117, 122, 123, 168, 171, 180, 224, 228, 251, 297, 310, 356]. The working mechanism for ZnO NPs is like any other nanoparticulate system previously reported. It starts with the cell membrane damage and increase in permeability due to physical update of nanoparticles leading to ionic imbalance, and generation of reactive oxygen species (ROS) like H2 O2 , (OH)− , (O2 )2− , leading to oxidative stress, mitochondrial stress and DNA/RNA damage followed by bacterial cell death [312]. In a study by Jingyu Wang et al., the ZnO nanoparticles showed antibacterial activity against Porphyromonas gingivalis and Actinomyces naeslundii (causing root canal infection); and were nontoxic to mouse NIH3T3 and OCCM-30 cells suggested by MTT assay [352]. Under similar in vitro conditions, ZnO NPs mixed in Portland cements enhanced the odontogenic differentiation for human dental pulp stem cells [277] while in another study green-synthesized ZnO NPs improved the osteogenic differentiation and mineralization potential in the case of human osteoblast (MG63) cells [329]. The PMMA-ZnO nanocomposite formulation for denture bases has been reported to be cyto-compatible against HeLa cell line [59]. In addition to these studies’ anticancer activity of ZnO nanoparticles has also been reported [120]. The biosynthesized ZnO nanoparticles induced the cell death by apoptosis and inhibited the cell proliferation in laryngeal cancer cells [355].

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2.6 Zirconia (Zirconium Oxide) Nanostructures In literature the zirconia has been reported to exist in three crystalline states, namely monoclinic that is stable at room temperature and two metastable polymorphs— tetragonal and cubic: known for their stoichiometry, thermal stability and crystal lattice structure. The tetragonal phase has been actively investigated for its physical and mechanical properties making it ideal ceramic material for engineering application [287]. The synthesis of Zirconia (Zirconium oxide—ZrO2 ) nanostructures have been reported by different routes, like sol/gel [139], vapor phase, hydrolysis [323], hydrothermal [83], laser vaporization [206], laser ablation [34], microwave [308] and thermal treatment [175, 269, 271, 293]. The ZnO2 nanoparticles either during synthesis or after synthesis are capped by either metal oxides like yttrium, calcium, cerium, magnesium, niobium, or molecules like polyvinylpyrrolidone (PVP) to further improve the phase stability, and biocompatibility [118, 149, 175, 287, 293, 346]. The ZrO2 nanostructures have drawn significant attention because of natural color, high strength, toughness, chemical stability, enhanced mechanical properties and microbial resistance when mixed with resin-based composites for dental and prosthodontic applications [98, 206, 367]. These nanostructures have found application as additive for adhesive, denture base, filler, primers, cements, coating for dental abutment, as provide reduced impact strength, improved flexural, fracture toughness, transverse strength, and increased tensile strength when incorporated with methacrylate based macromers [98, 99, 119, 166, 206, 248, 306]. The antibacterial or antimicrobial property of pure zirconia nanostructures is under investigation considering the extensive use of zirconia as additive and or coating to improve the physical and mechanical properties. The zirconia nanoparticles have been reported to show strong antibacterial property against both grampositive and gram-negative bacterial species like Bacillus subtilis, Escherichia coli and Salmonella typhi as result of large surface-volume ratio [192, 281]. Under similar in vitro condition, the human teeth dip coated with zirconia and exposed to different beverages showed negligible weight loss and hardness compared to uncoated and were found to be antibacterial against E. coli, Streptococcus and Bacillus bacterial species [291]. On the other hand, the antibacterial, antimicrobial, antifungal, and or antibiofilm properties of zirconia when stabilized/modified with other metal, metal oxides have been reported against (both gram-positive and gram-negative) Bacillus subtilis, Streptococcus mutans, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Klebsiella oxytoca, (fungus) Candida albicans, Candida glabrata, Aspergillus niger, and Aspergillus flavus [24, 50, 75, 97, 138, 255, 270, 275]. The zirconia-based composites have shown cytocompatibility with human bone marrow mesenchymal stem cells, human gingival fibroblast cells, human osteoblast cells and mouse fibroblast [50, 105, 138, 166, 247]. The exposure of these nanoparticles via dental implants, coating, or composites in the oral environment have been detected in rat lungs in an in vivo study as a result of inhaling while polishing and grinding of filler composite [305].

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2.7 Cerium Oxide Nanoparticles Cerium oxide (CeO2 ) nanoparticles or nano-ceria are known for optical, and catalytic property and for biomedical application the properties like radical scavenging, antiinflammatory, antioxidant properties as a result targeting oxidative stress [140, 145, 196]. The several synthesis methods have been developed for ceria including hydrothermal, co-precipitation, flow method, and microwave assisted heating with or without the presence of capping agent [170, 196, 309]. The green and bio synthesis of cerium oxide nanoparticles has been reported [169, 179, 185]. CeO2 NPs are known to improve the physico-chemical and mechanical properties of formulation in the dental field [328]. The nano-ceria composite is known to improve the biaxial flexure strength and mechanical durability of dental restoration [31]. Under laboratory conditions, the nano-ceria with acrylate-based resin mixture is known to improve the degree of conversion, flexural strength, and elastic modulus of the resin when exposed to chemical and mechanical stress [214]. The antibacterial or antimicrobial properties of cerium oxide nanoparticles for dental application have also been investigated. In a recent study, nanoceria with high Ce (III)/Ce (IV) ration has been known to produce reactive oxygen species (ROS) under acidic conditions, more over it has the ability to cause ATP deprivation leading to bacterial cell starvation, acting as antibacterial agent but biocompatible with osteoblastic MC3T3-E1 cells [375]. The antibacterial and antibiofilm property of cerium oxide nanoparticles has been reported against various gram-negative and gram-positive bacterial species like Streptococcus mutans, Staphylococcus aureus, Enterococcus faecalis, Klebsiella pneumonia, Staphylococcus epidermidis, Bacillus subtilis, Salmonella typhimurium, Listeria monocytogenes, Bacillus cereus, Escherichia coli, and Pseudomonas aeruginosa suggested by biofilm and growth inhibition [5, 37, 150, 186, 264, 272, 333, 375]. The cerium oxide nanoparticles synthesized by green method are known to have antifungal properties too, tested against Aspergillus fumigatus, Fusarium solani, Mucor sp. (FCBP 0041), Aspergillus niger, and Aspergillus flavus [219, 227]. The nanoceria surface modified with silk, pectin, polyvinyl pyrrolidone (PVP) and dextran have also been reported to have antibacterial properties [212, 260, 283, 300]. Contrary to these findings, the cerium oxide nanoparticles 25–50 nm in size failed to inhibit the growth and biofilm formation by various gram-negative and gram-positive bacterial species compared to antibiotic, ciprofloxacin [220]. The study also concluded that incubating nanoceria in the presence of ciprofloxacin resulted in loss of antibiotic property of the drug. Apart from antibacterial or antimicrobial properties, cerium oxide nanoparticles are known to have other biological properties. As reported by Chinmaya Mahapatra et al., the different shaped cerium oxide nanoparticles were able to rescue the oxidative stress in human dental stem cells by scavenging the reactive oxygen species (ROS) and controlling the DNA, cell membrane and protein damage [216]. A concentration-based study of 3, 10–20 and 30 nm nanoceria has reported minor effect on metabolic activity of human oral cells with increasing concentration [37] while

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in another study the nanoceria has been known to be biocompatible and promote the osteoblastic differentiation of human osteoblast cells used in dental implant [272]. Under similar in vitro conditions nanoceria have been reported to inhibit the production of inflammatory mediator species in macrophage cells and in the similar study the particles were non-toxic, showed no oxidative stress and inflammatory marker expression in mouse model [140]. In another 3 weeklong in vivo study, the 19.5 nm nanoceria has been reported to promote revascularization of limb tissue and muscle function restoration [258]. Under similar in vivo, PEGylated and lipid coated nanoceria (3–4 nm) were able to cross the blood brain barrier suggested by therapeutic effect on ischemic brain tissue [182].

2.8 Magnesium Oxide Nanoparticles In the last decade, magnesium oxide (MgO) nanoparticles (NPs) have also been investigated for dental application. The MgO NPs are known to improve the mechanical properties of ceramics [353] as well as are known to stimulate the bone regeneration under in vitro and in vivo conditions when mixed with polyethylene or poly (lactic-co-glycolic acid) (PLGA) based composites [202, 266]. Such MgO NPs based nanocomposites have been used as dental cements and are known to have antibacterial, antibiofilm, and antimicrobial properties against cariogenic and endodontic pathogens like Staphylococcus aureus, Streptococcus mutans, Enterococcus faecalis, Streptococcus sobrinus, and Candida albicans [231, 237, 238, 246, 370].

2.9 Silica Nanoparticles With/Without Phosphate (Bio-Glass) 2.9.1

Silica Nanoparticles

In an age when antibiotic resistance is on an alarming rise, novel ways to combat bacterial infections is the need of the hour. To that effect, we now discuss an increasingly popular kind of nanoparticles, the silica nanoparticles. Its otherwise non-toxic nature makes it a popular choice to be studied for potential applications in countering bacterial infections and more. Like most types of nanoparticles, there are several methods for the synthesis of silica nanoparticles, including but not limited to the Stöber process [317], which owing to its recent modifications can be used to synthesize nanoparticles as small as 2 nm [299, 363], and through the use of nanoreactors in the form of micelles in the microemulsion technique [20, 210, 299] which can lead to the synthesis of nanoparticles of myriad sizes. Further details on the synthesis of silica nanoparticles are beyond the scope of this chapter. In this section, we will focus on an important application of silica nanoparticles in the field of medicine. For some time now, silica nanoparticles have been extensively

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studied to be used for the successful delivery of several synthetic macromolecules like peptides and antibodies that make up for a significant percentage of modern-day drugs. Despite the steady increase in the number of macromolecules approved to be used as therapeutic medicine, an effective and safe way of administering the same has mostly remained a challenge. Several characteristics of silica nanoparticles make them a preferred candidate for one of the most convenient methods of drug delivery, the oral route [4]. The ability to control parameters like shape, size, surface modifications, as well as biocompatibility makes silica nanoparticles a good candidate for potential drug delivery. Silica nanoparticles can be synthesized in a wide range of sizes and controlling various synthesis parameters enable the size to be controlled anywhere between 2 and 500 nm, thus making it useful in the delivery of macromolecules of various sizes. In addition to size, the shape of the silica nanoparticles also contributes to its ability to succeed as a drug delivery system. In vivo studies with one of the most common types of silica nanoparticles, called the mesoporous silica nanoparticles (MSN) showed that decreasing the aspect ratio, defined as the length of the particle divided by the diameter, increased the systemic absorption of the particles by several organs including the small intestine, increased liver distribution, and a decrease in urinary excretion [199]. Additionally, further in vivo studies also showed how shape affects the pharmacokinetic behavior of silica nanoparticles. Their observations included that longer rod-shaped nanoparticles evaded clearance by the reticuloendothelial system and circulated in blood much longer than shorter rods and spherical nanoparticles, and rod-shaped nanoparticles were eliminated slower from the gastrointestinal tract than spherical ones [381]. The improved understanding on the best ways to design the synthesis of silica nanoparticles have led to better use of these particles for specialized drug delivery, especially through the mouth. Among the drugs that must be taken continuously over a very long period, one of the most frequently used is insulin. And for any longterm drugs, oral intake is the safest, quickest, cheapest and the most preferred and painless route of administration. Hence it is not surprising that the ability of silica nanoparticles to serve as an efficient mechanism for oral insulin uptake is one of the most researched topics in nanomedicine. Among the different types of silica nanoparticles, the mesoporous nanoparticles are one of the best suited for oral delivery of insulin, owing to its size, adjustable pore volume, ability to be modified, extremely low cytotoxicity and other characteristics [55, 153, 325]. The presence of silanol groups on the surface of these silica nanoparticles have proved to aid in the modification of the surfaces of these particles [15, 361] that further improve their use as drug delivery systems. Modified silica nanoparticles have also been studied with considerable success in the oral delivery of insulin. Andreani et al. [16] worked on coating silica nanoparticles with chitosan and other mucoadhesive polymers. This modification led to a better availability of insulin when delivered into the gut and showed little to no toxicity against human cancer cell lines (like HepG2 and Caco-2). In another study, it was observed that spherical silica nanoparticles when complexed with polymethacrylate based polymers are capable of a controlled release of insulin in the gastrointestinal tract, that is sensitive to pH changes [55]. Additionally, it was studied that when mesoporous

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silica nanoparticles are first coated with dextran-maleic acid and then complexed with 3-amidophenylboronic acid, the resulting particles are sensitive to both pH and glucose levels along with an increased capacity to load and release insulin in vivo [322]. Among other modifications, it has also been seen that coating silica nanoparticles with polyethylene glycol led to a faster release of insulin into the intestine [15]. The versatility of the use of silica nanoparticles against bacterial infections extends well into the world of dentistry. Streptococcus mutans is historically the most studied cariogenic bacterium when it comes to oral microbiology. A study showed that tooth surfaces polished with silica nanoparticles made it much easier to eliminate this “notorious” oral bacterium compared to unpolished teeth and teeth polished with commercial light polishing toothpastes [100]. Whether it is biological teeth, or dental implants, infection caused by bacteria continues to be the top concern for dentists. Among the oral bacteria capable of causing infection, those that colonize forming a biofilm are particularly difficult to eliminate. Continuing the quest to find ways to control the colonization by oral pathogens and biofilm formation, a group found that when silica nanoparticles (containing disulfide bridges) are complexed with silver nanoparticles, the resulting particles can be used as an effective vehicle to deliver chlorhexidine (as well as silver ions) in acidic environments like those created in a biofilm of S. mutans [211]. In addition to this, mesoporous silica nanoparticles when coupled with poly-L-glycolic acid, also showed improved release of chlorhexidine in low pH environments, over a long period of time, thus showing considerable antimicrobial properties [9]. A study by Kallas et al. [165] also showed that when dental implants are functionalized using silica nanoparticles, the attachment of bacteria like Staphylococcus aureus and Streptococcus mitis can be significantly reduced. S. aureus has long been associated with oral infections like periodontitis [65, 93, 183] and peri-implantitis [261]. On the other hand, although S. mitis is traditionally defined as a commensal in the oral microbiome, it is often associated with oral and other diseases like endocarditis and septicemia [60, 226, 302]. Thus, a reduction in the ability of such bacteria to inhabit dental implants through use of silica nanoparticles is of immense clinical significance. Even outside dentistry silica nanoparticles have shown promising results in the fight against bacterial infections. To this point, Letchmanan et al., [198] showed that mesoporous silica nanoparticles in conjunction with poly(methyl-methacrylate) bone cements is a promising choice for drug delivery in patients recovering from orthopedic surgeries. A study also showed that these mesoporous silica nanoparticles when coated in a liposome worked as an excellent delivery mechanism for the drug, ciprofloxacin which helped in the clearing of a food-borne pathogen Salmonella typhimurium from the vacuoles [235]. These robust mesoporous nanoparticles have been used in another study [380] on how they can improve delivery of molecules like the human defensin peptide that has the promise to be a future antibiotic against multi-drug resistant bacteria including Escherichia coli. Their results showed that these silica nanoparticles considerably aided the peptide to permeate bacteria including the outer membrane in E. coli. Further modification of these particles using succinylated casein helped in the delivery of the modified

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peptide at a controlled rate, thus proving to be a potential oral drug delivery option for treating inflammation caused by bacterial infections. Thus, this section helps us understand that silica nanoparticles have several distinct advantages that make them a promising candidate in the field of nanomedicine. To name a few, characteristics like the ability to be synthesized in a wide range of sizes and shapes, pore sizes, ease of functionalization, stability, biocompatibility, negligible to very low cytotoxicity, antimicrobial potential, explain why silica nanoparticles are so extensively studied in myriad fields of research, from drug delivery to combat clinically important diseases like diabetes, to oral infections, to post-operative care for orthopedics and many more.

2.9.2

Bioglass®

In this section we will focus on yet another brilliant invention that has significantly contributed to healthcare, including oral health. Bio active glass or bioceramics are known to play a prominent role in various implants used in different branches of medicine. The first and most important among them was invented by Professor Larry Hench over five decades ago at the University of Florida in 1969, after being asked to make a material that will survive the human body by a US Army Colonel [130]. The composition he used was 45% SiO2 , 24.5% Na2 O, 24.5% CaO and 6% P2 O5 , and the material was termed 45S5. 45S5 is however famously known as Bioglass® which is a trademark registered by the University of Florida. The ability of this material to bond with bones and help in bone growth make it one of the most researched biomaterials for bone regeneration in response to trauma, accidents, chronic problems among others. The dissolution of the glass leading to the deposition of a hydroxycarbonate apatite (HCA) layer is considered to form the basis of bone bonding. The similarity of HCA to bone mineral is expected to help its interaction with collagen fibrils ultimately leading to bone bonding [130, 132, 158, 159]. Further discussion on the mechanism of action of Bioglass 45S5 is beyond the scope for this chapter. An important application of Bioglass® lies in its potential for use in synthetic bone grafting. In bone damage cases arising from accidents, chronic illnesses, birth defects and from other sources of trauma, autografting is mostly performed where the bone is taken from another location in the patient’s body. An ideal alternative would be using materials like Bioglass® that can bond with bone and serve as a template for bone growth and repair. The birth of the concept of osteostimulation, which meant that Bioglass could help osteoblasts produce bone tissue faster than other materials, led to the development and approval of using materials like Perioglass® (1993–1995) to be used in grafting at tooth extraction sites and NovaBone® (1999) to be used in general orthopedic bone grafting [131]. NovaBone® has been used to date in hundreds of thousands of bone grafting cases, an exemplary use of this was in lumbar spinal fusions [334]. For the remainder of this section, we will focus on the roles played by bioactive glass in the field of dentistry and oral healthcare. The most notable application of Bioglass lies in its potential to treat teeth hypersensitivity. Hypersensitivity results

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from dentinal tubules, otherwise covered by enamel, being exposed to hot or cold foods and liquids. A Bioglass based formulation, called calcium sodium phosphosilicate was developed over two decades ago, and is popularly known to date under the trade name of NovaMin® . The use of this technology is currently (since 2010) owned by GlaxoSmithKline Consumer Healthcare (GSK) [195]. When this Bioglass is added as very fine particulate matter into a toothpaste, the material quickly forms a HCA layer that attaches to the dentin, thus occluding it from hot and cold foods and drinks, thus helping hypersensitivity by relieving pain. The release of sodium ions from this formulation upon contact with saliva in the mouth, raises the pH that ultimately aids the formation of the HCA on the dentin [195]. The use of this material is currently commercially popular in the United Kingdom in the form of a toothpaste, Sensodyne® Repair and Protect, GSK Consumer Healthcare, Weybridge, Surrey, UK, that contains about 5% NovaMin® [195]. This genius formulation based on Prof. Hench’s Bioglass® counts as one of the most significant contributions of this particular material in human healthcare. However, even though NovaMin® containing toothpastes have been popular in several countries for a while now, and much research has been done to prove the efficacy of NovaMin® in helping with tooth hypersensitivity and dental remineralization, it continues to be unavailable in the United States to date. But thankfully, even though NovaMin® is not available in the US, the applications of Bioglass goes well beyond toothpastes. Bioactive glass displays significant antibacterial properties, which adds to why it is advantageous to use Bioglass in dentistry. A study showed that the viability of several supra and sub-gingival bacteria like Streptococcus sanguis, Streptococcus mutans, Porphyromonas gingivalis, Fusobacterium nucleatum, Actinomyces viscosus among others, were severely reduced in the presence of particulate Bioglass 45S5. This group also reported that direct contact between Bioglass and the bacterial cells were not necessary for optimal antibacterial effect and that the effect significantly improved at higher pH [11]. They also observed that a strong antibacterial effect is seen against biofilms formed by Streptococcus sanguis and other anaerobic bacteria that can form mixedspecies biofilms subgingivally when grown on Bioglass compared to other inert glass controls, for up to two days, thus proving their potential to be used to help wounds heal after oral surgeries [12]. Additionally, it has also been studied that when silver, another material well-known for its antimicrobial properties, is complexed with Bioglass in a sol–gel based system, the resulting SiO2 –CaO–P2 O5 –Ag2 O (AgBG) displayed bacteriostatic properties when added at 0.02–0.2 mg/ml of the culture medium. A complete bactericidal effect was also observed when AgBG was added at a final concentration of 20 mg/ml [29]. Apart from oral bacteria, it has been reported that Bioglass is also effective against bacteria like Staphylococcus aureus, Staphylococcus epidermidis and Escherichia coli [144]. This study found over 98% bactericidal effects by Bioglass at a concentration of 50 mg/ml or higher. Also, it was very recently shown that Bioglass when complexed with lithium (BGLi) proved toxic to bacteria like S. mutans and P. gingivalis, when used at a concentration of 50– 200 mg/ml. However, it was effective against an oral pathogen involved in aggressive

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periodontitis, Aggregatibacter actinomycetemcomitans at a much lower concentration of 20 mg/ml [63]. In addition to an elevated pH, the formation of Bioglass debris that mimic sharp needle-like glass can also cause cell wall damage and thus help in bacterial cell death [144]. However, Begum et al. [35] observed that even at an alkaline pH, Bioglass was only effective against E. coli NCTC 10538 and not Staphylococcus aureus ATCO 6538. There could be several reasons explaining the susceptibility of Escherichia coli over S. aureus but the presence of a thinner peptidoglycan layer and the absence of teichoic and lipoteichoic acid could help in the killing of E. coli cells. Additionally, the presence of porin proteins on the outer membrane could also interact with the products produced from the breakdown of Bioglass, thus allowing them to enter the cells and increase intracellular pH [35]. Another study [70] showed that when triazine and niobium phosphate bioglass were incorporated into orthodontic adhesives at a concentration of 20%, led to a decreased count of the oral pathogen S. mutans along with other Streptococci compared to controls, in patients, over at least a two-week period. The antibacterial capacity of bioactive glass coupled to the remineralizing potential of niobium pentoxide upon contact with saliva can thus be a useful option for orthodontic treatments. In addition to its antibacterial potential, Bioglass has also been studied for its ability to help against enamel demineralization during teeth bleaching, using abrasive agents like hydrogen peroxide [73]. This study showed that incorporating Bioglass during teeth bleaching helped control mineral loss and demineralization, along with a potential for possible remineralization, without any impeding effect on the whitening process. They further concluded that including Bioglass within the bleaching regimen showed better results compared to treating with Bioglass before or after whitening. Bioglass is also steadily gaining prominence in the field of nanomedicine. Researchers have prepared mesoporous Bioglass nanoparticles whose adjustable hollow structure allowed for the efficient packing of an effective quantity of drugs [354]. This coupled with Bioglass’ ability to bond with bones make up for an efficient new vehicle for drug delivery and therapeutic healing in orthopedic surgeries and other diseases. A common application would be the use of these nanoparticles for targeted delivery of anti-inflammatory drugs like ibuprofen that can deem essential for treatments like bone repair which can otherwise be excruciatingly painful [354]. In another study [344], a classic metal with antimicrobial properties, silver, was incorporated into Bioglass forming a new class of bioactive nanoparticles, silver bioglass nanoparticles that showed antibacterial effect against E. coli and S. aureus. This opens another potential for Bioglass based nanomedicine to be used to fight infections, in both bone-related treatments as well as dental surgeries. Also, zinc, which has often been associated with bone health, when incorporated into nanoparticles along with Bioglass, has shown to help in cell differentiation involved in the formation of new teeth as well as the generation of new blood cells [376]. Thus, zinc based Bioglass nanoparticles could prove useful in regenerative medicine, for both bones and teeth. In addition to complexing Bioglass nanoparticles with metals, research by Kwon et al., [193] reported that Bioglass nanoparticles, when added to methacrylated gelatin based cryogels at a concentration of 2.5% w/w, displayed high degree of mesenchymal cell differentiation and improved bone regeneration.

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In conclusion, we think it is safe to say that even though Bioglass® has been around for over five decades, the true potential of this material in human healthcare, especially in the field of nanomedicine, is only beginning to be harvested. Studies involving Bioglass® based nanoparticles, though very much in its nascent stages, are already proving that this material is a perfect candidate for several fields of regenerative medicine including but not limited to, orthopedics and dentistry.

2.9.3

Hydroxyapatite (Ca10 (PO4 )6 (OH)2 ) Nanoparticles

An object of increasing research interest in dentistry is a principal component of teeth and an exceptionally stable salt containing calcium and phosphate, hydroxyapatite (Ca10 (PO4 )6 (OH)2 ) (HA). In recent years much work has been done to synthesize HA nanoparticles which have been characterized for its potential in various biomedical and biochemical applications. Their stability over a wide range of temperature and pH makes them ideal candidates for therapeutic as well as industrial applications. Several approaches are used to synthesize HA nanoparticles, like the use of high temperature and pressure in controlled tubes during hydrothermal synthesis [42, 208, 209], and the wet chemical precipitation methods [7, 191, 205, 207, 208, 256, 360]. The second technique allows multiple parameters to be varied like temperature, calcium concentration, precipitation time etc. and is a cost-effective way of synthesizing HA nanoparticles with controllable phenotypic properties [208]. A third technique that is often used to yield HA nanoparticles that are well-separated and not clustered is the microemulsion technique [40, 208, 263]. Detailed discussions on the synthetic techniques are beyond the scope of this chapter, but a good summary of these techniques can be found in the article by Loo et al. [208] as well as in the review by Balhuc et al. [30]. Other methods like the solid-state and the mechanomechanical method (together termed as dry methods) as well as high temperature-based methods like combustion and pyrolysis can also be used to generate more nontraditional shaped HA nanoparticles [30]. The ability to control shape and other physical characteristics of these particles make them ideal candidates to be used in modern medicine. As HA makes up for a major mineral component of teeth, it is widely applied in the field of dentistry. In a recent study it was seen that when complexed with two other important components of teeth and bone regenerations, fluoride and strontium, the resulting nanoparticles were bioactive and biocompatible. They were also shown to promote dental remineralization when studied on synthetic samples [279]. Oral bacteria, especially the biofilm forming ones are the single most common reason for degradation of oral implants. Since most of these bacteria are resistant to almost every available antibiotic, Abdulkareem et al. [2, 3] wanted to study if coating titanium discs with zinc oxide (ZnO) nanoparticles or HA nanoparticles or a combination of both (ZnO + HA) provided a protection against bacterial biofilms. They reported a significantly reduced population of facultatively anaerobic bacteria and the classical oral bacterial genus Streptococcus sp. after a 4-day study period on the discs coated with zinc oxide nanoparticles and the ones coated

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with the combination nanoparticles (ZnO + HA), and a slightly reduced protection was observed on the discs coated with only HA nanoparticles [2, 3]. A recent study [154] showed that when part of a dental composite made up of the traditional 2,2-bis [p-(2 hydroxyl-3 -methacryloxypropoxy) phenyl] propane (bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) was replaced by HA nanoparticles, the resulting new composite showed a release of calcium and orthophosphate ions which increased with the increase in the percentage of HA nanoparticles. This release was pH sensitive, and the highest release was at an acidic pH of 4 compared to pH between 5.5 and 7. The released ions were also seen to contribute to the remineralization of dental lesions caused by S. mutans containing biofilms, thus proving that HA nanoparticles can be an ideal candidate for novel dental composites. Additionally, when used in an experimental fluoride containing oral paste on subjects, as well as exposed to bacteria capable of causing caries, it was reported that compared to placebo and commercial products, the experimental paste showed exceptional capacity to control demineralization of the dentin samples and contribute to enamel remineralization [316]. However, it was not capable of inhibiting demineralization of the enamel. Other in situ studies have also shown that HA nanoparticles of myriad shapes and sizes can bind to both enamel as well as artificial dental surfaces made of materials like titanium, ceramic and polymethyl-methacrylate (PMMA) [245]. These interactions were shown to be aided by the salivary pellicle. These studies pave the way for further studies on the role of HA nanoparticles in oral applications both in toothpastes as well as composite materials. HA nanoparticles are also proving to be a valuable material in the research for improved dental fillers, research shows that when cubic-shaped and to some extent when whiskers-shaped HA nanoparticles were included in dental fillers, the resulting materials showed improved resistance to friction and slowed down wear and tear compared to controls [8]. The use of HA also extends into cosmetic oral practices. Hydrogen peroxide comes with its challenges when used in tooth bleaching, but when complexed with HA, the resulting mix can be used as an effective dental bleaching agent while resisting enamel demineralization and tooth weakening [157]. As with any novel material, questions also arise on the cytotoxicity and overall safety of using HA nanoparticles in products intended for use on humans. To address such concerns, a study by Coelho et al. [62] showed that a commercially available nano-HA displayed no toxicity towards human gingival fibroblasts and tested using an in vitro hen egg test on the chorioallantoic membrane (HET-CAM) assay, the test material displayed no irritation potential. All their tests incorporated mimicking the normal brushing procedure and their results deemed the use of nano-HA commercially on oral products to be safe [62]. In addition to dentistry, HA nanoparticles can also be used in other potential therapeutic applications like the use of these particles to encapsulate fluorescent dyes like Cascade Blue, rhodamine WT, Cy3 amidite etc., and similar smaller molecules, thus serving as a vehicle in bio-imaging applications [53, 208, 234]. Like dyes, HA nanoparticles can also be used to package radioisotopes, like the use of HA complexed with 153 Sm-Ethylene-diamine-tetramethylene-phosphonate (EDTMP) in patients with chronic knee synovitis [61]. The versatility of HA nanoparticles extends into its use as a drug delivery vehicle. Plate- and needle-shaped HA nanoparticles

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(25–100 nm) were shown to successfully bind anticancer drugs like cisplatin and di (ethylenediamineplatinum) medronate (DPM) respectively, and when studied over a two-week time period, they showed extremely slow and controlled release [33, 208, 253]. When it comes to drug delivery-based research, insulin remains the most studied drug, yet the best ways to deliver insulin still remain a challenge. Zhang et al. [379] attempted to address that by taking advantage of the bioactivity, biocompatibility and the porous structure of HA nanoparticles. They complexed the nanoparticles with a non-toxic and extremely popular ingredient in drug delivery, polyethylene glycol (PEG). Insulin was then conjugated to the PEG. Additionally, gallic acid was also added along with insulin, to be used as a free-radical scavenger to address the oxidative stress issues related to diabetes. The resulting stable, non-toxic quadruple nanoparticles when administered to rats improved their type 1 diabetes and in Caco-2 cells showed absorption by the small intestine epithelium [379]. In another study, it was reported that plasmid DNA can also be packaged into rod-shaped HA nanoparticles and be used to successfully transfect gastric cancer cells [386]. Much care needs to be exercised to optimize the techniques and parameters used in packaging molecules and other particles into these nanoparticles, for example high temperatures are used instead of ambient temperatures, it can lead to DNA and protein denaturation. Several in-situ and ex-situ techniques are currently used to load HA nanoparticles like co-precipitation, surface adsorption, electrostatic interactions etc. [208]. Research to date, thus indicates that HA nanoparticles of myriad shapes and other physical properties carry immense potential in several biomedical applications like controlled drug delivery, bio-imaging, gene therapy among others.

3 Organic Nanoparticle-Based Formulations 3.1 Nanogels Based Formulations Now leaving inorganic nanoparticles behind, it would be interesting to focus on organic nanoparticles and their application in dentistry. The composite resins (organic polymer) for restorative application have been exploited in the last 50 years and advancements have been made by using different types of filler materials to improve the longevity and mechanical properties. A dental resin composite formulation is composed of hydrophobic and hydrophilic macromers or monomers along with photo/co-initiators. The hydrophobic monomer on photo-polymerization has been known to provide the strength and reduce the water interaction. On another hand, the hydrophilic macromers in dental formulations have been known to enhance the bonding of esthetic restoration to the dentin surface on photo-polymerization. That being said, there have been reports suggesting the phase separation of these water miscible monomers on interaction with water molecules during or prior to photopolymerization. A well polymerized dental adhesive polymer matrix is known to be permeable and water sorption has been reported as one of the main causes of

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adhesive/restoration failure because of degradation/biodegradation of ester group in resin composite or dentin-collagen matrix [36, 187, 188]. Apart from filler glass isomers, metal and or silica nanoparticles are used as stabilizers in dental composite formulations to enhance the physical–mechanical properties. More recently, nanogels have been the prime focus to improve the physical and mechanical properties by incorporating the nanogels in prepolymer adhesive formulation [232]. In a typical synthesis for nanogel, macromer like Hexamethylene diisocyanate (HMDI), m-Xylene diisocyanate (XDI), ethoxylated bisphenolA dimethacrylate (Bis-EMA), bisphenol-A glycerolate dimethacrylate (Bis-GMA), 1,3-bis (isocyanatomethyl) cyclohexane (BIC), isobornyl methacrylate (IBMA), urethane dimethacrylate, 2-hydroxyethyl methacrylate (HEMA), Polycaprolactone tetra(3-mercaptopropionate) (PCL4MP), ethoxylated-trimethylolpropane tri (3mercaptopropionate) (ETTMP), tetraethylene glycol dimethacrylate (TEGDMA), and or isocyanatoethyl methacrylate (IEM) are used in different combinations along with the thermal inhibitor/s [69, 94, 101, 110, 203, 204, 232, 276]. The macromers in different combination and concentration are selected and mixed, based on the user application to amend the physical, mechanical and surface chemistry properties, in excessive volume of a solvent like toluene, hexane, dichloromethane (DCM) or ethyl acetate along with photocuring agents [69, 94, 110, 203, 204, 232, 276, 292]. During certain synthesis procedures, the addition of a chain transfer agents like 2-mercapto ethanol (ME), methyl ethyl ketone (MEK), or α-methylstyrene dimer (AMSD) has been known to control the macro-gelation, nanogel size, and nanogel molecular weight [94, 110, 203, 204, 232]. These 10–100 nm nanogels in prepolymer or monomer mixture of cement, adhesive or composite restorative formulation are known to reduce polymer shrinkage, stress, improve material resistance towards degradation, and bond interfaces on polymerization by increasing cross-linking density or cross-polymerizing between nanogel and surrounding monomer molecules within the polymer matrix [68, 95, 233, 276]. The Fig. 3 represents the schematic representation of nanogel synthesis (a) [233] and mechanism behind lower shrinkage stress (b) in the presence of nanogels

Fig. 3 Methacrylate based monomer for nanogel synthesis (a). when incorporated in resin formulation on polymerization reducing shrinkage stress because of matrix crosslinking. The images reused from the publications [203, 233]

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[203]. Such nanogel based formulations have been investigated for various dental applications. Another such formulation loaded with ciprofloxacin (0.0032%) in a 3D co-culture environment of human keratinocytes with S. aureus and P. aeruginosa resulted in elimination of biofilm [349]. The nanogels have found application as antimicrobial, anti-bacterial, local anesthetic and molecular delivery vectors for bone regeneration when loaded with molecules like chlorhexidine, turmeric extract, lidocaine hydrochloride, prilocaine hydrochloride and biomolecules like human fibroblast growth factor 18 (hFGE18) and human bone morphogenetic protein 2 (hBMP2) under in vitro and in vivo conditions [47, 114, 178, 181].

3.2 Polymeric Nanoparticle-Based Formulations Sometime instead of nanogels, the term “polymer/ic nanoparticles” is widely used when the natural polymers like chitosan, starch, alginate, and or synthetic polymers like polycaprolactone (PCL), polylactic acid (PLA), poly(lactide-co-glycolide) (PLGA), polyethyleneimine (PEI—linear or branched), polyethylene glycol (PEG), Pluronic and polymethacrylate (PMA) are used for the synthesis of polymeric nanoparticles [225]. These polymeric nanoparticles have been extensively investigated for various dental or oral applications namely, dental caries/hypersensitivity, periodontal (gum) disease, bacterial growth/biofilm inhibition, varnish, local anesthesia, drug/molecular delivery, endodontic, and oral cancer for therapeutic purpose. One such example of polymeric nanoparticle that have drawn attention in the last decade is chitosan based, known for its properties like biocompatibility, biodegradability, bioadhesive, antifungal, wound healing and immune system stimulation [236]. Chitosan nanoparticles have also been employed for anti-caries application when loaded with chimeric lysin ClyR, or sodium fluoride [244, 357, 388]. Such NPs formulation with or without the active molecules like with Mentha piperita natural oil, ceftriaxone, rutin have been known to inhibit the biofilm formation by S. mutans, Candida, Bacillus pumilus and Enterococcus faecalis biofilm inhibition when loaded or applied on dental or denture surface [22, 77, 107, 259]. The application of chitosan nanoparticles have been exploited for the formation of an aprismatic enamel like layer for demineralized enamel recovery, ion release, remineralization of tooth surface, molecular/protein delivery under in situ or in vitro conditions, as well as act like multifunctional formulation for antimicrobial and anti-inflammatory for periodontitis treatment with the delivery of triclosan flurbiprofen under in vivo condition [14, 314, 350, 358, 387]. Contrary to these beneficial properties, the chitosan nanoparticles with size 50–67 and 318–350 nm have also shown some cytotoxicity against human dental pulp cells [10]. Apart from Chitosan nanoparticles, there have been limited reports on starch and alginate nanoparticles, and NPs have been used for molecular delivery of active molecule for the recovery of dental caries and periodontitis by controlling the biofilm and bacterial growth of cariogenic bacteria [284, 311]. The polymeric nanoparticles

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composed of dextran-curcumin loaded have been used for photodynamic therapy against S. mutans, C. albicans, and S. aureus in vitro [342] while under in vivo conditions carbamide peroxide nanoparticles are known for tooth whitening application [201]. The synthetic polymer like poly(lactide-co-glycolide) (PLGA) have also found application in dental field for dental caries, dental plaque, periodontitis, and root canal infection management by releasing the active molecule payload or with the help of photodynamic therapy at the site by controlling the biofilm and bacterial growth [1, 18, 252, 296, 351]. In a clinical pilot study with human patients with chronic periodontitis condition when treated with methylene blue (MB) loaded PLGA nanoparticles and exposed to 660 nm red light for photodynamic therapy inhibited the biofilm formation and bleeding up to 1 months [92]. While in a comparative in vitro study, moxifloxacin loaded PLGA nanoparticles are known to have strong antibacterial effect against E. faecalis up to 336 h compared to chitosan NPs [217]. Endodontic and periodontic fields have also drawn the interest of polymeric nanoparticles. The polymeric nanoparticles synthesized by methacrylate-based monomer (2-hydroxyethyl methacrylate, PolymP-n) and loaded with zinc or antibiotic have shown potential to prevent the resin-dentin interface biodegradation by inhibiting the cariogenic bacteria/biofilm growth and attachment to the interface [250, 337, 338]. Again, substantial work has been done by Manuel Toledano group in the field of periodontitis. The PolymP polymeric nanoparticles loaded with doxycycline has been reported to show concentration and time dependent anti-bacterial/microbial properties against P. gingivalis, S. mutans, L. lactis, S. gordonii, and S. sobrinus [336], and such nanoparticles are reported to have bioactivity which means the potential to support bone growth when encapsulated with calcium or zinc metals [249]. The methacrylate-based monomers (2-(dimethylamino ethyl methacrylate (DMAEMA, butyl methacrylate (BMA, 2-propylacrylic acid (PAA are known to form pH-response polymeric nanoparticles and the NPs have been used for biofilm disruption in dental caries and drug delivery application to oral biofilm [141, 307, 385].

4 Conclusion In this chapter, we have tried summarizing some of the many advances in the development and use of nanoparticles in healthcare, with a particular focus on dentistry. With every new nanoparticle based biomedical application arises the questions of bioavailability, biocompatibility, cytotoxicity among others. But thanks to successful research, several nanoparticles like those of gold, silver, calcium, titanium and many more are already finding use in aiding several treatment related issues like targeted drug or gene delivery, bone and teeth protection and regeneration, as well as antibacterial properties, the need for which is severely exacerbated with the alarming increase in antibiotic resistance. But this field is truly emerging, with research advancing by the hour. Much remains to be done in this growing field, but every progress is helping us answer some of the pressing needs in the field of dentistry and in medicine at large.

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150. N. Iqbal, A. Anastasiou, Z. Aslam, E.M. Raif, T. Do, P.V. Giannoudis, A. Jha, Interrelationships between the structural, spectroscopic, and antibacterial properties of nanoscale (−0.73 V vs. RHE), 12 electron reduced ethanol is formed. Highest faradaic efficiency of 48% for electrochemical reduction of CO2 to ethanol was obtained at a relatively low potential (−0.8 V vs RHE). DFT (Density Functional Theory) studies and other reduction studies using the proposed intermediates show that the intermediates involved in the reduction process like carboxyhydroxyl radical intermediates, formyl radicals etc. are stabilised at the Co(III) metal centre. On changing the active metal centre from Co(III) to Mn(III) in the above catalyst changes the dynamics of the CO2 reduction completely which again reinforces the importance of the redox centre in the catalyst [164]. Mn(III) containing catalyst now reduces CO2 to acetate (8 electron reduced product) at −1.25 V versus Ag/AgCl with a Faradaic Efficiency of 63%. The reduction process occurs with the formation of a Mn(II) species with which CO2 is adsorbed and also stabilises the reduced products. This then lead to the formation of a Mn(III) bound oxalate intermediate which is subsequently reduced to acetate. The results were further substantiated by reduction studies of oxalic acid and DFT calculation and offers an easy one step solution for CO2 reduction to acetic acid which currently is a multistep process.

3.3 Metal Organic Layers/Frameworks Metal Organic Layers and Frameworks have been widely used for electrochemical CO2 reduction. Owing to their highly ordered crystalline nature, Metal Organic Frameworks (MOFs) offer unique blend of activity and selectivity. On the other hand, Metal Organic Layers (MOLs) due to their more accessible active sites and ultrathin nature results in enhancement of electrocatalytic CO2 reduction. MOFs and metal organic layers offer the opportunity to incorporate well-defined and highly active

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sites into a rigid scaffold which results in excellent catalytic efficiency and selectivity. Moreover, these species owing to their porous nature allow for the controlled mass transfer between the active sites and the well-defined architecture permits the proper calculated modification and tuning of the catalytic activity. Achieving a high Faradaic efficiency for CO2 reduction reaction (CO2 RR) is a real challenge in any electrochemical CO2 RR. One way to attain this is to minimize the competing hydrogen evolution reaction (HER) at the requisite reduction potential. Metal Organic Layers (MOLs) have been employed to achieve this feat [178]. MOLs act as 2D scaffold and tune the microenvironment of Co-protoporphyrin (CoPP) electrocatalyst in aqueous media. The microenvironment of CoPP from phenyl rings in BTB (benzenetribenzoate) to py moieties in TPY (4 -(4-benzoate)(2,2 ,2 -terpyridine)-5,5 -dicarboxylate) enhances the efficiency of CO2 RR considerably. This was achieved by the post synthetic modification of MOLs leading to the change of the microenvironment resulting in the increase of CO/H2 current density ratio from 2.7 (BTB-MOL-CoPP) to 11.8 (TPY-MOL-CoPP) at a reduction potential of −0.86 V vs reversible hydrogen electrode (Fig. 8). The use of MOFs for electrocatalyst modification has also been used to improve CO2 reduction selectivity to CO. Modification of a Ag electrocatalyst is done by growing thin films of MOFs (UiO-66) on the catalyst’s flat surface [179]. The thin films grown are continuous, homogenous and porous in nature and the thickness of the film can be tuned by changing the volume of precursor solution drop-casted (DMF solution of ZrOCl2 , acetic acid and 1,4-benzene-di-carboxylic acid (BDC)). SEM (Scanning Electron Microscope) and PXRD (Powder X-ray Diffraction) analyses confirmed the modification and XPS (X-ray photoelectron spectroscopy) analysis confirmed that the core electronic nature of Ag remains unchanged after modification. This again reveals an interesting fact that there are some other factors which is tuned by MOF modification resulting in the increase in the CO2 to CO selectivity. Specifically, the concentration of dissolved H+ ions is drastically reduced when compared

Fig. 8 Schematic showing the structure of TPY-MOL-CoPP and the cooperative activation of CO2 by CoPP and pyH+ (Reprinted (adapted) with permission from Ref. [178]. Copyright 2019 American Chemical Society.)

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to that of CO2 which shifts the reduction process towards CO production. Also, the missing cluster (MC) in MOF layer results in the efficient transfer of protons to stabilize *COO- intermediates which again increases the rate of CO production. The selectivity of CO2 reduction to CO was increased from 43% for the bare Ag to 89% (at −0.8 v vs RHE) in the case of the modified electrocatalyst. MOFs and related modified electrocatalysts have also been reported to be used for CO2 reduction towards higher reduced products (C2 or higher). A Cu based MOF (HKUST-1) was used for the conversion of CO2 to ethylene with a faradaic efficiency of 51% [180]. CO2 reduction to ethylene is a 12 electron process and copper catalysts have been reported to show a high selectivity [181]. But the process is often restricted by the limitation in the surface area of the catalyst and MOFs offer a solution since they have a large surface area with abundance of catalytic sites. The Cu-based MOF was prepared using calcination process and the use of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) analysis show that the under a negative applied bias the catalyst has a Cu@Cux O core@shell structure. The calcination process was employed for the simultaneous modification of the morphology and oxidization state of catalysts. The pristine catalyst has an octahedral MOF structure in which copper dimers are separated by adjacent benzene tricarboxylate and at the calcination temperature of 265 °C the dissociation of organic ligand occurs. MOF containing the mixed phase Cu+ , Cu2+ oxides cause distorted grains with high tensile strain, which might promote the activation of CO2 molecules. The stabilization of Cu+ species by the catalyst promotes the *CO stabilization at Cu0 /Cu+ interface which suppresses the methane production while promoting ethylene conversion. By now, it is a proven fact that the stability and degree of binding of intermediate is a key factor in a CO2 RR. The use of reticular chemistry employed on a metal complex encapsulated MOFs has shown to control the binding of intermediates which in turn enhances CO2 reduction efficiency [182]. Critical factors of CO2 RR like CO2 adsorption, pore size, Lewis acidity of MOFs is tuned by systematic variation of metal node and linker in MOFs. Initially the starting material Zr-fcu-MOF-BDC which is prepared from zirconium chloride (ZrCl4 ) and 1,4-benzenedicarboxyllic acid (BDC) is found to consist of Zr based hexanuclear clusters (Secondary building units, SBU). The interaction with CO2 was enhanced by modifying the linker with functionalization of amine group (Zr-fcu-MOF-NH2 BDC) (Fig. 9). Other tuning like pore size enhancement is done by changing benzene in BDU with naphthalene (Zr-fcu-MOF-NDC). Owing to its reputation to produce CO on high selectivity, Ag nanoparticles was selected as the active material and incorporated into the above two types of MOFs (Ag/Zr-fcu-MOF-NH2 BDC and Ag/Zr-fcu-MOF-NDC). This is further confirmed by CO2 RR using Ag free MOF where it returned only H2 . Using the method of solution impregnation, the Ag nanoparticles (uniformly sized of about 5 nm) were grown inside the pores of MOF. Through TEM and FE-SEM analyses it was found that the Ag nanoparticles in addition to the pores, were incorporated onto the exterior surfaces of the MOFs. This is a positive outcome as the presence of Ag on the exterior enhances the electrical conductivity of MOFs.

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Fig. 9 Tuning of Zr-fcu-MOF-BDC for enhanced CO2 RR activity: a addition of amine functional group (−NH2 ) to organic linker, b changing the organic linker to naphthalene dicarboxylic acid, and c changing the element in metal node site from Zr to Y. (Reprinted (adapted) with permission from Ref. [182] Copyright 2020 American Chemical Society)

In situ Raman spectroscopy was used to monitor the binding of intermediates onto the catalyst surface during CO2 RR. Both Ag/Zr-fcu-MOF-NH2 BDC and Ag/Zr-fcuMOF-NDC showed *CO Raman peaks which is due to the increased CO concentration in these species near the Ag nanoparticles and is a result of tuning done on these MOFs. In the case of Ag/Zr-fcu-MOF-NH2 BDC, the CO FE (Faradaic efficiency) increased to over 80% (it was less than 74% in bare MOF (Ag/Zr-fcu-MOF-BDC)) and H2 FE decreased to less than 8% (it was 29% in bare MOF). The mechanism includes the involvement of carbamate intermediate which enhances CO2 RR activity along with increased CO concentration [183]. Similarly in Ag/Zr-fcu-MOF-NDC at a slightly higher potential, CO FE showed an increment with a total of 94% and H2 FE was suppressed to 1% which shows that the selectivity is even better than amine functionalized MOF. Please do note that pH also has an influence in the reaction and here 0.5 M KHCO3 was used and Lewis acidity of the overall MOF remained close to neutral due to the use of Zr. On the aspect of the importance of the organic linker, it was shown that there are 2 different CO binding modes which depends upon the regulation of pore size which is dictated by the organic linker used (Fig. 10). The two organic linkers used here are −1,4-benzenedicarboxylic acid (BDC) to −1,4naphthalenedicarboxylic acid (NDC) which directs CO to adopt a COatop and CObridge binding modes respectively. The difference in the binding mode affects the CO selectivity of Ag nanoparticles located at the pore site of the MOF where the COatop mode exhibits higher selectivity. The dramatic increase in both FE and selectivity for CO product formation indicates that the same method can be used for improving the activity of similar catalysts for a higher reduced or even higher carbon containing products for CO2 RR.

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Fig. 10 The different binding modes of CO; COatop and CObridge regulated by the change in organic linker from −1,4-benzenedicarboxylic acid (BDC) to −1,4-naphthalenedicarboxylic acid (NDC). (Reprinted (adapted) with permission from Ref. [182] Copyright 2020 American Chemical Society.)

Another effective way to improve the performance of MOF based electrodes is to tune the synthetic aspect via the use of rapid electro-synthesis template in an ionic liquid [184]. The results are impressive with a HCOOH FE of 99.1% and a current density of 46.1 mA cm−2 which is the best that a MOF based electrocatalyst can offer till date in an organic electrolyte. The increased selectivity/performance is due to the incorporation of In3+ active defect sites in the prepared MFM-300(In)-e/In electrode. The mechanism was delineated via DFT studies which enabled the quantification of Gibbs free energy for the electroreduction of CO2 to HCOOH. In3+ defective sites present in the defective catalyst promotes the *COOH intermediate formation when compared to that of pristine catalyst and the associated energy barrier is also low. This easy formation of *COOH intermediate over *CO intermediate in the defective catalyst explains the increased production of HCOOH. The activity of the electrode is not only due to the presence of defective sites but also due to the strong interaction that it has with the Indium foil support. This strong interaction results in the reduction of interfacial resistance enhancing selectivity, activity and stability for CO2 RR which brings an exceptional HCOOH FE of 99.1%. A bimetallic synergistic interaction between Cu and Zn centers in a 2D MOFs have shown promising results for electrochemical CO2 RR in terms of selectivity and durablity [185]. The easy tunability stems from the fact that the CO2 RR product ratio i.e. H2 /CO can be varied by the adjustment of metal centers and the associated catalytic potentials which opens up a myriad of possibilities. The catalyst material (PcCu-O8 -Zn) was prepared by using phthalocyaninato copper as the ligand (CuN4 ) and zinc-bis(dihydroxy) complex (ZnO4 ) as the linkage and this was made into a composite with carbon nanotube (CNT) and was loaded onto a carbon paper to use as working electrode. At a potential of −0.7 V versus RHE this 2D MOF system exhibits excellent selectivity for CO2 to CO conversion (88%) with a turn over frequency

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(TOF) of 0.39 s−1 . On probing the catalytic sites and the reaction process using both experimental and theoretical analyses it was revealed that the linkage which consists of Zn complexes exhibit high activity for the CO production and the protonation process. Hydrogen formation was also seen along with CO2 RR and the whole process is performed by the synergistic action of CuN4 in the phthalocyanine macrocycles. In a similar way using the freedom of reticular chemistry, another 2D MOF based on phthalocyanine ligand exhibits an increased CO FE of 85% with high current density of about −17.3 mA cm−2 [186]. Analysis of CO2 RR show that the presence of Co metal center of the phthalocyanine over Cu nodes is required in addition to the presence of oxygen based linkages (CoPc−Cu−O). Here the catalyst material is supported on carbon black with a mass ration of 1:1 and is deposited on carbon fiber electrode with the conversion happening at a relatively low overpotential of − 0.63 V. The material exhibits excellent stability with a stable current density for more than 10 h. Computational analysis shows the involvement of a carboxyl intermediate (*COOH, active site bound (rate determining step)) which is then transformed into an active-site bound CO species and finally desorbed as CO. Ligand doping extends another option to boost the electrochemical CO2 reduction using MOFs [187]. An efficient electron donor ligand 1,10-phenanthroline was doped onto Zn-based MOFs in order to enhance its activity. Both the experimental and theoretical analysis point out that the electron donating nature of the doped ligand results in charge transfer which activates the neighboring sp2 carbon sites of imidazole ligand (as obtained from DFT studies). This facilitates the generation of *COOH resulting in enhancement of CO2 RR activity and FE for CO generation. The *COOH formation is facilitated by the transfer of electron from the active sites imidazolate to the antibonding orbitals of CO2 . The MOF used is that of a zeolitic imidazolate framework-8 (ZIF-8) which has been modified by ligand doping and the modification imparts the electron density onto MOF and increases the percentage of unprotonated N. The high catalytic activity of nanoparticles is explored widely nowadays and a recent report on structural rearrangement of bismuth-based metal–organic framework Bi(1,3,5-tris(4-carboxyphenyl)benzene) denoted as Bi(btb) at reducing potentials into catalytic Bi-based nanoparticles dispersed on a porous organic matrix draws much attention [188]. This structural transformation is important since the nanoparticles formed, display high selectivity and activity towards CO2 R to formate. In order to probe the electrochemical CO2 R activity Bi(btb) was deposited on a glassy carbon electrode and works as a precatalyst for the formation of Bi nanoparticles with highly exposed surface area at a potential of −0.97 V versus RHE. The electrocatalyst formed from MOF contains active metal sites (primarily due to the reduction of Bi3+ to Bi0 ) which are distributed in a continuous mesh format with a highly porous network. The experimental results reveal that this material is able to selectively reduce to formate with a FE of 95% at an overpotential of 770 mV which denotes a superior performance as compared to other Bi based catalysts.

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3.4 Metal Oxide-Hybrid Nanoparticles Metal oxides were used together with other materials as an active material towards the electrochemical CO2 reduction harnessing the power of mixed active sites [10]. Use of bulk metals-based electrodes for CO2 RR is a well-established technique but suffers from serious drawbacks like poor product selectivity, high overpotentials etc [189]. In this context, the use of nanoparticles which offer high surface area combined with judicious tailoring of active sites and the use of defects for the benefit of CO2 RR is a viable alternative. No wonder that there are increasingly large number of these materials for CO2 RR which leverages on the use of fine structural tuning by well controlled exposure of certain surface facets. Metal oxides are reported to possess the ability to act as active component for CO2 RR and at the same time as a platform, thus providing a high degree of tunability [11]. The fascinating catalytic activity of metal oxide interface is well established by now, especially in the case of heterogenous catalytic processes. This may be due to the strong interfacial interaction which has been reported to stabilize highly active site towards molecular activation [190]. By the construction of the metal-CeOx interfaces, it was reported that the catalytic properties appeared to be much enhanced than that of the pristine metal or metal oxides [191]. Here both Au and Ag metals were used for the metal-CeOx interfaces. It has been found that the CO2 RR activity and selectivity was increased dramatically and in the case of Au-CeOx /C, CO geometric current density was 1.6 times compared to that of Au/C with a Faradaic efficiency of 89%. Also, a similar result was exhibited by Ag-CeOx /C interface. This enhancement can be credited to the increased CO2 adsorption and activation of subsequent adsorption of CO2 on ceria terraces. The chemical nature of the adsorbed species was probed using synchrotron-radiation photoemission spectroscopy (SRPES) which revealed that it is the formation of COδ− 2 on CeOx -Au(111) and on ceria [192]. In contrast to the above findings, CO2 is not at all adsorbed onto pure Au(111) surface while it only undergoes a weak physisorption on the CeO2 (111) film that too at the surface defects. It has been also found that the CO2 RR is critically influenced by the presence of hydroxyl group which is formed by the spontaneous dissociation of water. This facile dissociation of water at interfacial Ce3+ sites resulting in the enhancement of hydroxyl formation which on the other hand facilitates the removal of lattice oxygen upon annealing. Along with the promotion of ceria reduction to form high concentration Ce3+ sites, water presence also stabilizes COδ− 2 species which subsequently enables the hydrogenation process. The reaction mechanism of the CO2 RR was studied by DFT methods and the following route was proposed (Eqs. 15–17). C O2 (g) + ∗ + H + (aq) + e− → ∗C O O H

(15)

∗C O O H + H + (aq) + e− → ∗C O + H2 O(l)

(16)

∗C O + ∗+ → C O(g)

(17)

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A representative free energy diagram of CO2 RR formation was constructed for Ce3 O7 H7 /Au(111) and it was found that the formation of carboxyl species via protonation (Eq. 15) was the potential limiting step. It is the same even for Au(111) but the energy cost is lower by 0.33 eV on Ce3 O7 H7 /Au(111). Subsequently the protonated decomposition of *COOH to *CO and then the following desorption to CO readily occurs on Au-CeOx interface (Eqs. 16 and 17). Bader analysis reveal that the exceptional stability of *COOH intermediates on Ce3+ site which drives the whole process is due the direct interaction with one of the terminal oxygen atoms. Enhancement of electrochemical CO2 RR by the creation of oxygen vacancy is an emerging trend and amorphous MnOx catalysts have been reported to perform the same by using plasma treatments [193]. Electrochemical CO2 RR activity of a series of these compounds have been checked with a varying degree of oxygen vacancies ranging from low (a-MnOx -L), pristine (a-MnOx -P) and high (a-MnOx -H). On comparison, it is seen that the one with the largest oxygen vacancy (a-MnOx -H) exhibits the highest CO FE (94.8%) with a partial current density of 10.4 mA cm−2 at a low overpotential of 510 mV. In order to probe the oxygen vacancy O 1 s X-ray photoelectron spectroscopy (XPS) was used which showed two peaks at 531.5 eV and 529 eV. These peaks correspond to O atoms near the oxygen vacancies and that of Mn–O–Mn. Further the binding energy calculations of Mn 2p3/2 show that there is a charge transfer occurring from oxygen vacancies to the Mn species. This is seen the greatest for the species with the highest oxygen vacancy which also unsurprisingly translated into better CO2 to CO conversion. DFT calculations also corroborate the experimental results and it can be seen that as the concentration of oxygen vacancies increases, it leads to an increase in the electronic concentration of the surface catalyst resulting in a decrease of the band gap. The induction of abundant electrons into the Fermi level is the reason for the decrease of band gap and furthermore this also augments the adsorption of CO2 onto MnOx surfaces. The proposed mechanism includes the formation of a CO.− 2 species followed by the hydrogenation leading to *COOH species stabilised at the active site. *COOH then undergoes proton coupled electron transfer reaction leading to an adsorbed *CO species which is finally desorbed to give the product. The take away from this result is that by the engineering of oxygen vacancies on a suitable catalyst with a different adsorption property towards *CO intermediate (here MnOx is used), we can stabilize *CHO intermediates or even higher carbon containing intermediates which can yield C2 or C3 products at an appreciable yield. Another area in which there has been excellent progress in terms of electrocatalytic CO2 reduction is in hybrid materials. They can be designed to integrate various properties such as selectivity and activity particularly the ones synthesized using the combination of organic–inorganic components. Generally, these components can be combined using the array of noncovalent interactions like hydrogen bonds, Vander Waals bonds etc. The goal is to combine facets of organic materials like flexibility, variation etc. with that of the inorganic materials like redox activity, conductivity, large surface area etc. It also aims to enhance upon CO2 adsorption and reduction ability by the increase in the number and efficiency of active sites thereby

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increasing the stability of intermediates adsorbed at these active sites and reducing overpotentials. It is well accepted fact that the electrochemical CO2 RR is favored by the initial adsorption of CO2 onto the electrocatalytic surface which is at the same time quite challenging too. A comparative study on the CO2 adsorption capabilities of pure Sn and Sn/rGO composites show that the later has an adsorption capability of about 4 times than that of the former [194]. The onsite linear sweep voltammograms were used to probe position dependence of onset potential. The current vs potential graphs reveal that a major source of catalytic reaction of CO2 reduction occurs on the interface between Sn and rGO when compared to that of pure Sn and pure rGO. This enhanced CO2 RR is directly linked to the increased CO2 adsorption which is called as the sequential adsorption of migration of CO2 provided by rGO onto the Sn surface. Here apart from the direct adsorption of CO2 onto Sn surface, CO2 is also adsorbed onto the oxidized functional groups of rGO which are near Sn surface. These adsorbed CO2 , then migrates to Sn surface which occurs even at room temperature. DFT calculations revealed that the increased CO2 reduction is due to the lowering of free energy of *COOH intermediate and suppression of *CO intermediate formation on the CO2 rich Sn surface. A more significant outcome of this study is the fact that this mechanism can be applied to previously reported metal catalyst/carbon composites [195–197]. Mixed metal oxides are also used for electrochemical CO2 reduction and one such example include the use of Cu/Ni oxides nano porous composites [198]. The synergistic effect of the two oxides combined with the porous structure and low charge transfer resistance contribute to efficient electrochemical CO2 reduction. The porous structure of Cu/Ni mixed oxide (Cu3 NiO Composites where 3 and 1 indicate the atomic ratio of Cu and Ni determined using XPS (X-ray photoelectron spectroscopy)) was monitored using SEM (scanning electron microscopy) and TEM (tunnelling electron microscopy) analyses and the results reveal that the pores generated are as a result of stacking of the materials. The catalytic performance was analyzed in 0.5 M KHCO3 electrolyte for 5 h. At a potential of −0.37 V versus RHE, HCOO– formation was initiated on the catalytic surface which is indicated by an onset overpotential of relative low potential of about 0.147 V (E0 for the CO2 /HCOO– reaction is −0.2 V versus RHE in aqueous electrolyte) [199, 200]. Optimum conversion of CO2 to formate was observed at a potential of about −0.57 V versus RHE with a partial HCOO– current density of about 10.5 mA cm−2 and Faradaic efficiency of 76.19%. The result indicates that this particular combination Cu/Ni oxides exhibits high activity and selectivity for CO2 reduction to formate over all the other possible combinations and over pure oxides. The surface area and pore volume of the Cu3 NiOCs was found to be 117.7 m2 g−1 and 0.118 cm3 g −1 , respectively and this is much larger than that of the corresponding complex since the organic ligands are removed in the composites. This porous structure of the composites results in increase of active sites which is translated into large electrochemical surface area and low charge transfer resistance. The mechanism includes the initial CO2 adsorption followed by the reduction and stabilization of

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CO.− 2 intermediate on Cu. Finally, this Cu bound intermediate is transformed into HCOO–. In the pursuit to achieve high stability and selectivity for CO2 RR, a key element for any catalytic reaction i.e., the catalytic stability is often overlooked. Stability of a model catalytic system (Cu/TiO2 nanoparticles) synthesized by the selective deposition of Cu nanoparticles over TiO2 nanoparticles on a highly oriented pyrolytic graphite (HOPG) substrate was carried out under electrochemical conditions [201]. Following the electrochemical CO2 reduction, the morphological changes on the catalytic surface was analyzed using atomic force microscopy (AFM) and Scanning electron microscopy (SEM) was used to monitor the structure and movement of specific particles. Further the chemical composition changes were tracked using X-ray photoelectron spectroscopy (XPS) and the amount of nanoparticles after electrochemical reduction. There are enough defects (edge defect, defect at the Cu and HOPG interfaces and Ti3+ sites) possessed by TiO2 nanoparticles (average size of 15 nm) which are conductive and permit the electrochemical reaction. The model catalyst is stable under CO2 electrochemical conditions but there is an observance of mobility of about 30% of TiO2 nanoparticles which eventually leads to agglomeration of these particles after 30 CV cycles. Even though the amount of copper on the TiO2 nanoparticles decreases with electrochemical CO2 reduction, it is seen that the catalyst remains viable for electrochemical CO2 RR. The result shows that judicious combination of nanoparticles and metal oxide can in fact increase the catalytic stability and has the potential for upscaling. A remarkable achievement of reducing CO2 to methanol with high current density of 41.5 mAcm−2 and a Faradaic efficiency of 77.6% was obtained using copper selenide nanoparticles [202]. This feat was achieved at a relatively low overpotential of 285 mV using a Cu1.63 Se(1/3) nano catalyst system yielding an outstanding current density of 41.5 mA cm−2 with FE of 77.6% at −2.1 V versus Ag/Ag+ . These catalysts were prepared using cupric chloride salts, Na2 SeO3 with the addition of hydroxylamide to a mixed solvent consisting of diethylenetetramine (DETA) and deionized water with varied volume ratio (eg: VDETA /VH2 O = 1/3). The nanoparticles thus produced were deposited onto the carbon paper to use as the electrode. A probable reaction pathway is proposed as follows; in Cu2−x Se(y) nano catalysts; the use of electrolyte containing ionic liquids enhances the concentration of CO2 in the electrolyte. This in turn increases the possibility of adsorption of CO2 onto the .− catalyst surface as CO.− 2 species. This adsorption of CO2 species on the active sites of the catalyst triggers formation of adsorbed CO species. The Cu active sites on the catalytic surface can further enhance the reduction of these adsorbed CO species to form adsorbed CHO species which is subsequently reduced to give methanol. The proposed mechanism for the CO2 RR to methanol is given in Fig. 11. In the case of C2 products obtained from electrochemical CO2 RR using composites, report of Graphene oxide (GO) supported Cu/Cux O nanocomposites exhibiting reduction of CO2 to ethylene stands as a significant milestone worth studying [203]. Care should be taken right from the fabrication method employed for the preparation of electrode for achieving such higher reduced products. By comparison it was established that the electrochemically co-deposited composites show enhanced

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Fig. 11 a Proposed mechanism on Cu2−x Se(y) electrode and b the free energy calculation on Cu1.63 Se(1/3) electrode. (Reprinted (adapted) from Ref. [202] which is an open access article distributed under the terms of the Creative Commons CC BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.)

activity compared to the other methods like layer-by-layer coating of electrocatalysts. Using these nanocomposites at a potential of −0.985 V versus RHE with an overall Faradaic efficiency (FE) of 34%, a conversion rate of 194 g−1 h−1 was achieved. The catalytic activity is dictated by the degree of interactions between GO and copper oxide nanostructures. The electrodes were prepared in two ways in order to compare the activities i.e., (1) electrodeposited thin films (fCu/CuxO, fCu/CuxO-GO and fCu/CuxO@GO) on CFP cathode were used as such without any modifications, (2) for samples obtained as powder, 2.5 mg of each was dispersed in 3:1 iso-propanol and water mixture, with 5 mL Nafion for 30 min and the dispersion was fed to an air spray gun and coated as a film on the carbon fiber paper (CFP) under the flow of nitrogen as carrier gas and also (3) CFP was modified with GO by pressure spray and further air sprayed with Cu/Cux O (pCu/Cux O@GO). On carrying out the analysis of electrochemical reduction reactions, it was found that when compared to the electrodes fabricated using layering of catalyst samples pCu/Cux O@GO is most active and even gives C2 product like ethylene. The underlying mechanism indicates the involvement of *CO coupling [204] as noticed from the lowered production of CO in pCu/Cux O@GO. The loose and porous nature of the deposited which is a result of the combined effect of the presence of GO and the nature of the cathode material. One of the major hurdles in the advancement of electrochemical CO2 reduction is the competing reactions like H2 evolution at high overpotential which lowers the efficiency. This problem is tackled successfully by the use of a Fluorine doped carbon nanocage material [196]. The nanocage has a porous nature (hierarchically porous achieved by derived polymer method [205]) and is tuned by controlled fluorine doping which enables it to have high electrical conductivity and large surface area. This Fluorine doped cage like porous carbon (F-CPC) shows excellent catalytic activity for CO2 RR owing to its cage-like structure and the presence of optimum mesopores/micropores ratio. This amazing feat was achieved by synthesizing these catalytic materials using SiO2 particles as templates and these templates were later removed by etching with HF.

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The synthesis process involves the aldol reaction (between resorcinol and formaldehyde precursors) on the surface of SiO2 with the simultaneous addition of polytetrafluoroethylene as the source of fluorine. The sample was then calcined at 900 °C and further open pores were created on the carbon shell by CO2 activation (Fig. 12a). The F-CPC was selected after optimization process done using F-doped carbon spheres (F-CS), F-doped hollow carbon spheres (F-HCS), and Fdoped commercial carbon (F-PC). The superior catalytic ability of F-CPC is due to the uniform cage like morphology and hollow structure which were confirmed from SEM and TEM images (Fig. 12b, c). The enlarged TEM image shows the the presence of mesopores at the surface (Fig. 12d). The CO2 activation at high temperature process has a shrinking effect which resulted in the thinner carbon shell and smaller diameter and the addition of semi-ionic state fluorine into the carbon skeleton enhances the CO2 RR activity. In a nutshell, the morphology and tuning of the structure of the catalyst facilitates the capture and diffusion of CO2 adsorbate which was confirmed from various analytical techniques. But the deciding factor which equips the F-CPC catalyst for enhanced CO2 RR activity even at high overpotentials is the accumulation of the electron around the edge sites of circular opening pores at the surface of the catalyst. This provides high electric field for concentration electrolyte cations which significantly lowers the thermodynamic energy barrier for CO2 RR.

Fig. 12 a Schematic representation of formation of fluorine-doped cagelike porous carbon (F-CPC) along with its b SEM and c TEM d magnified TEM images. (Reprinted (adapted) with permission from Ref. [196]. Copyright 2020 American Chemical Society.)

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4 Conclusion It is evident that the advancements in both photochemical and electrochemical CO2 RR are plenty and the area is vast but major ones include the tuning of catalytic species for increased activity and selectivity, efforts to produce high electron/carbon containing products at a respectable selectivity and efficiency. Obviously, what is discussed till now only forms a drop in the ocean and several other efforts including the ones which uses the principle of green chemistry and those which couple the CO2 reduction with the reaction like water oxidation are there worth mentioning. It is imperative for us to have a deep understanding of the mechanistic aspects of these reactions so that it can catapult our journey towards a more sustainable and greener environment. Looking at the future, exploration of novel material compositions and structural tuning will be spearheading the research in this area with a serious leverage on upscaling and commercialization of these projects.

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A Brief on Emerging Materials and Its Photovoltaic Application Deboraj Muchahary, Sagar Bhattarai, Ajay Kumar Mahato, and Santanu Maity

Abstract In this chapter, an overview of different novel materials used in solar cell is presented. The inside physics and most common materials used in a variety of solar cells are described. Material is the heart of a solar cell device and selection and development of proper material are a crucial task for device development. Amongst different type hybrid, perovskite and organic solar cells are cheap and provide comparable power conversion efficiency to the conventional silicon based solar cell. Superior element for a hybrid solar cell is PEDOT:PSS blend and Si heterojunction. Different additives to improve the efficiency in such solar cell are discussed. Moreover, different organo-metallic perovskite materials with ferroelectric characteristics are discussed in this chapter. The ferroelectricity uplifts the open circuit voltage of the solar cell beyond its energy band gap. The analytical description of device performance in such under different charge carrier transport materials are presented as well. Both organic and inorganic materials are suitable for electron and hole transporting layer. The perovskite materials are leading its way to lower dimensional versions which are suitable for both solar cell and other optoelectronic devices. The stability of lower dimensional perovskite leads to Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) phases and are discussed in this chapter. Moreover, working physics and materials used in organic solar cell are also a part of this chapter.

D. Muchahary (B) · A. K. Mahato Department of Electronics and Communication Engineering, National Institute of Technology Raipur, Raipur, India e-mail: [email protected] A. K. Mahato e-mail: [email protected] S. Bhattarai Department of Basic and Applied Science, National Institute of Technology Arunachal Pradesh, Arunachal Pradesh, India S. Maity School of Advanced Materials, Green Energy and Sensor Systems, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_10

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Keywords Solar cell · PEDOT:PSS · DSSC · Modelling · Ferroelectric · Perovskite · Organic

1 Introduction Material is the platform of almost all the electronics and mechanical devices. It is customary to choose suitable material based on certain properties while designing a device. In a solar cell design the material selection for different layers depends on its type. In conventional semiconductor p–n junction based solar cell, the absorber and emitter layer are crucial materials that decide the power conversion efficiency (PCE) of the device. Crystalline silicon begs the highest priority in such a case due to wide range absorption of solar spectrum. In addition to that the low charge carrier recombination inside the bulk crystalline silicon enhances the solar to power conversion efficiency. Nevertheless, the surface and interface trap states due to dangling bond is a bottleneck for such a solar cell design but advanced surface passivation and interface layer engineering has reduced the impact. The solar to electric power conversion efficiency depends on the mobility of charge carrier as well. Crystalline silicon with its electron and hole mobility approximately 1441 cm2 /V s and 470 cm2 /V s respectively serves for this purpose [1]. Moreover, well sophisticated material processing and device fabrication methodology for silicon is abundant. All those reasons have drawn strong attention of researchers and industrialists to the crystalline silicon solar cell technology. On the dark side, the processing cost of such a pure crystalline material is quite high. As a result, the mass production is limited from economic point of view. Such a scenario has diverted the roadway of solar cell material to structurally degraded version of silicon such as polycrystalline and amorphous due to relatively low cost. Almost all the silicon based solar cell available in the market is made of polycrystalline material. The p–n junction embedded inside such a solar device is homojunction in nature. The cost of production relies on the quantity of Si used to manufacture the solar device as well. Typical silicon based solar cell available in the market is designed with a thickness of thousands of microns. The absorption coefficient of silicon is of the order 106 cm−1 in the ultra-violet range and monotonically reduces to 10–5 cm−1 in infra-red region of the solar spectrum [2]. It indicates thin layer of silicon can absorb the lower wavelength photons but is not sufficient for higher wavelength photons. This mandatorily demands for reasonably thick absorber silicon for optimal solar to electric power conversion. In addition, requirement of thick substrate to support the device and high temperature processing steps contribute to the production cost. The required surface area for solar panel installation is quite high and often this causes problem in some terrains. In this context, transparent organic–inorganic materials are used to replace Si in solar cell. The above discussions lead us to a new paradigm of methodologies and technologies that are used to design solar cell bearing low cost and uncompromised conversion efficiency. Amongst those thin films, dye-sensitized, quantum dot (QD) sensitized,

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QD, transparent, organo-metallic perovskite, and ferroelectric solar cell technologies are quite popular.

2 Diversity of Solar Cell Solar cell is a diverse field of study consisting of several technologies. These technologies are categorised as shown in Fig. 1. A brief on those technologies are discussed in this section. The main objective of those is reduction in cost without compromising the PCE of the device.

2.1 Thin Film Technology As mentioned above the cost of solar cell depends on the quantity of material used. Thin film solar cell is an approach to reduce the material cost. This type of solar cell encompasses materials that have good photon absorption capacity for a specific range of solar spectrum. These materials belongs to I-III-VI2 chalcopyrite family. The general formula for these materials is CuMN2 where M can be Ga or In and N is Se or S. Copper Indium Gallium Selenide (CIGS) belongs to that family and has gain vast application in solar cell. CIGS grown with different methodologies are used as absorber material in various thin film solar cells [3–8]. The thickness of CIGS layer is only a few microns in those devices and so is named as thin film solar cell. The CIGS has absorption coefficient as high as 104 cm−1 up to ~620 nm of solar spectrum and it reduces beyond [9]. As a result the photons passing through CIGS get absorbed partly. However, a metallic back reflector layer solves such ramification. The reflected photons are allowed to pass repeatedly through the absorber layer so that reasonably good number of photons are absorbed. The thin absorber layer in such solar cell

Fig. 1 Diversity of solar cell

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reduces the material requirement and thus the cost of production. In addition, CIGS can be deposited on both rigid and flexible substrates and so thin technology extends its usefulness towards flexible devices as well. Other most commonly used thin film materials are Cadmium Telluride (CdTe) [10–12], amorphous (a-Si) [13–16] etc. In addition to thin absorber layer the thin film solar cell consists of electron transport layers (ETL) and transparent conducting oxides (TCO) for contact formation.

2.2 Hybrid Technology Another approach to reduce the quantity of Si and the cost is organic–inorganic hybrid solar cells. It appears that Sailor and co-workers have spin casted poly(CH3 )3 Si which is an organic polymer on an n-type silicon for its application in solar cell for the first time back in 1990 [17]. The organic polymer showcases a good contact to the n-type silicon and suitable for charge extraction. At present varieties of organic material such as Poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) which is commonly known as PEDOT:PSS, graphene, poly(3-hexylthiophene-2,5diyl) also known as P3HT, carbon nanotube etc. are used for this purpose [18]. Among these PEDOT:PSS is popular and used in commercial purpose as well. Its wide optical band gap, mechanical flexibility and low temperature fabrication process have attracted the research community. In addition, its electrical conductivity and optical transparency are appraisable. Such reasons made PEDOT:PSS a widely applicable transparent polymer in solar cell. Yoon et al. deposited PEDOT:PSS on a textured Si and reported efficiency of more than 17% [19]. This is a champion efficiency provider (power conversion efficiency more than 17%) under hybrid heterojunction solar cell category at present. Noteworthy, siloxane oligomer compound is sandwiched between PEDOT:PSS and Si in their work which reduces defect states and makes a better contact at the hybrid junction. Similarly, a PEDOT:PSS doped with 3-glycidoxypropyltrimethoxydsilane was deposited on Si which can provide PCE of 15.01% in the work of Lu et al. [20]. Power conversion efficiency of 15.10% was reported for a Ag/PEDOT:PSS/Si/TiOx /LiFx /Ag solar cell by He et al. [21]. The performance parameter of similar type of solar cell are listed in Table 1.

2.3 Transparent Technology Transparent solar cell implementable on any surface is another hot topic in the PV energy research community. These devices are different from conventional one in terms of photon absorption range. Normally, these are designed to absorb and convert invisible range of solar spectrum. Infrared and ultraviolet photons are absorbed by the active material whereas visible light are transmitted through such device. The major advantage of such device is suitability to install on surfaces like window, rooftop, roadway etc. Most commonly used absorber materials in such

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Table 1 Performance parameters of some hybrid solar cells Heterojunction

PCE (%)

FF (%)

Ag/PEDOT:PSS/Siloxane/Si/Siloxane/Ti/Ag 17.34 74

JSC (mA cm−2 ) VOC (V) References 38.41

0.610

[19]

Ag/PEDOT:PSS:GOPS/Si/TiN/Al

15.01 69.03 33.00

0.660

[20]

Ag/PEDOT:PSS/Si/TiOx /LiFx /Ag

15.10 75.6

31.90

0.626

[21]

Ag/PEDOT:PSS/Si/Cs2 CO3 /Al

12.74 63.80 34.22

0.583

[22]

Ag/PEDOT:PSS/Si/Al

12.60 69

29.60

0.619

[23]

Ag/PEDOT:PSS/Si/F-N2200/Al

14.50 73.30 31.10

0.635

[24]

Ag/PEDOT:PSS/Si/SnO2 /Al

14.16 72

33.16

0.593

[25]

Ag/PEDOT:PSS/Si/SiOx /EDTA—SnO2 /Ag

11.52 71.20 28.80

0.562

[26]

Note GOPS—3-glycidoxypropyltrimethoxydsilane; PBDB-T:ITIC—a non-fullerene blend bulk heterojunction where PBDB-T—Poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b ]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5 c ] dithio phene4,8-dione)] and ITIC—3,9-bis (2- methylene (3–(1,1-dicyanomethylene)-indanone))-5,5,11,11tetrakis(4-hexylphenyl)dithieno-[2,3-d:2 ,3 -d ]-s-in daceno[1,2-b:5,6-b ]dithiophene; PM6:Y6— organic bulk heterojunction where PM6—(poly[(2,6-(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2yl)benzo[1,2-b:4,5-b ]dithiophene))-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2c:4,5-c ]dithiophene-4,8-dione))] and Y6—(2, 2 -((2Z, 2Z)–((12, 13-bis (2-ethylhexyl)-3,9diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,"3 :4 ,50]thieno[2 ,3 :4,5]pyrrolo[3,2g]thieno[2 ,3 :4,5] thi eno [3,2-b] indole–2,10-diyl) bis (methanylylidene)) bis (5,6-difluoro-3-oxo2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile); PEDOT:PSS:α-In2 Se3 —composite film; PDINO–N,N-bis(3,3-dimethyl-3-amino N-oxide-n-propyl)perylene diimide; PFN-Br—Poly(9,9bis(3 –(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) dibro mide; F-N2200—fluorinated naphthalene diimide based conjugated polymer; EDTA—Ethylene diamine tetraacetic acid

a device are organic. For example, Lunt and co-workers fabricated a heterojunction between an organic donor molecule known as chloroaluminium phthalocyanine (ClAlPc) and acceptor molecule C60 on an ITO coated glass substrate to form a transparent solar cell [27]. Apparently such an organic junction is the first transparent solar cell. Due to the application of organic materials in these type of solar cell the flexibility of the device is increase and low production cost is maintained. However, absorption peak of ClAlPc demands a reflector at near infrared wavelength range. Another pair of organic compounds that has gain reasonable attention of the researchers on transparent solar cell is poly(3-hexylthiophene) (P3HT):[6,6]phenyl-C61- butyric acid methyl ester (PCBM). The structure of such solar cell is depicted in Fig. 2. The front electrode in such solar cell must be transparent to visible light and good conductor of charge carriers. Lee et al. used graphene oxide as front transparent electrode in P3HT:PCBM based solar cell [28]. Some of the most commonly used transparent front oxides are PEDOT:PSS, nanowires of silver, carbon nanotubes etc. [29, 30]. However, the transparency of P3HT:PCBM pair to the visible range of light is not up to the mark. Consequently, this organic pair provides a semi-transparent solar cell instead of the pure transparent one. To improve that,

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Fig. 2 Schematic diagram of transparent solar cell

Table 2 Performance parameters of some transparent solar cells Materials

Voc (V)

Jsc (mA/cm2 )

ClAlPc

0.8

4.7

PBDTT-DPP:PCBM

0.77

12.60

FF (%)

Efficiency (%)

References

55

2.4

[27]

54.40

5.28

[30]

Chen et al. used heterojunction of poly(2,6/ -4,8-bis(5-ethylhexylthienyl)benzo-[1,2b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiop hen-2-yl)pyrrolo[3,4c]pyrrole-1,4-dione) in short PBDTT-DPP with PCBM (PBDTT-DPP:PCBM) as a transparent solar cell [30]. Silver nanowire was used as front electrode in their device. Table 2 exhibits device parameter reported for different transparent solar cells.

2.4 Sensitized Solar Cell Technology Dye sensitized solar cell (DSSC) is a low cost device. The heart of a DSSC is a thin film consisting of small nanoparticles which is sintered with a material bearing highenergy band gap and charge carrier mobility such as TiO2 , SnO2 , Nb2 O5 and ZnO. These oxides are known as photoanode. The photoanode is deposited on a transparent conducting oxide (TCO) like FTO on glass. Double-layered photoanode structures such as TiO2 /SiO2 , TiO2 /SnO2 , ZnO/TiO2 , ZnO/SnO2 are also used in DSSCs [31, 32]. The photoanodes are sensitized on its surface with organic dye molecules like Zinc Porphylin (C20 H14 N4 Zn) [33], N719 (C58 H86 N8 O8 RuS2 ) [34], collaborative dye between ADEKA-1 and LEG4 [35] etc. Concurrently, the photoanode is kept immersed in an electrolyte consisting of redox couple such as Iodide and tri-iodide couple [36], Cobalt(II, III) [33] etc. Such couples bear a redox potential measured with respect to hydrogen electrode. Iodide and tri-iodide couple has redox potential of 0.35 V which is ~0.2 V more than that of Cobalt (II, III) [35]. On the other hand, the back side of the device is fitted with a counter electrode such as FTO along with

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a catalyst like Platinum (Pt) [35], carbon [37] and poly(3,4-ethylenedioxythiophene) (PEDOT) [38]. In modern era the Pt is tried to replace by graphene and transition metal di-chalcogenides such as MoS2 , TiS2 , CoS2 NbSe2 , NiS2 , Bi2 Se3 etc. [39]. The reason being expensive and scarcity of Pt metal on earth. Unlike in conventional solar cell devices, the carrier generation and transportation takes place in two different materials in DSSC. The generation takes place in dye molecule and transportation via photoanode. The photon of wavelength in the range of UV to near IR gets absorbed in the dye molecule and excitons are generated. Those electrons resides in the lowest unoccupied molecular orbital (LUMO) of dye molecule are then injected to the conduction band of nanostructured photoanode (for example TiO2 ). Inside the photoanode the electrons get transported via hopping from one nanoparticle to the other. These are finally collected at the back side FTO contact. The dye molecule upon loss of electron oxidises and become positively charged dye ion (Dye+ ). However, Dye+ reduces back due to the possession of electron generated in the redox couple. The redox couple present in the electrolyte of DSSC reduces and such electron are generated. For example Iodide (I− ) is oxidized to tri-iodide (I3 − ) to reduce back the oxidized dye in Iodide and tri-iodide couple system. The electron transported through photoanode, travels through external circuitry and reaches the counter electrode, the FTO. Such electrons are used by a catalyst such as Pt present on the FTO to reduce back the redox couple such as I3 − back to I− at the counter electrode. The open circuit voltage (VOC ) of such a device is equal to the difference between Fermi level of photoanode and redox potential of the electrolyte [40]. The complete mechanism of a DSSC is presented in the Fig. 3 based on the report of Lee et al. [36]. In the mechanism, following sequence of reactions takes place [41]. The photoexcited dye molecule oxidizes to Dye+ : Fig. 3 Carrier conduction mechanism in DSSC

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Dye −→ Dye+ + e− The electrons enter into the TiO2 nanostructure and reaches to FTO by hopping between the nanoparticles (shown by dotted arrowhead lines). Oxidized Dye+ then reduces back to normal Dye molecule with electron (e− ) originated due to oxidation of I− by following a number of intermediate steps. Oxidized Dye+ in contact of an I− forms a complex (Dye….I). Dye+ + I− −→(Dye . . . .I) Further availability of another I− dissociates the complex to reduce to normal state Dye. A non-stable di-iodide radical (I2 −* ) is also formed during this dissociation. (Dye . . . .I) + I− −→ Dye + I−∗ 2 When two such I2 −* meet and react, it produces single of both tri-iodide and iodide. ∗ − − 2I− 2 −→ I3 + I

Electrons collected at the front electrode travels through external circuit, meet oxidized tri-iodide at the counter electrode, and reduce it to iodide. − − I− 3 + 2e −→ 3I

The open circuit voltage of such device mainly depends on the rate of I3 − reduction reaction to I− and rate of electrons entering at the counter electrode. Generally VOC of a DSSC is given by the following equation [41, 42]. Voc =

  Je_ce kT ln q kr eco n 0

(1)

In the equation J e_ce , k reco and n0 stands for electron flux entering at the counter electrode, reduction rate of I3 − to I− and dark electron concentration in the conduction band of I3 − respectively. An alternative to DSSC is quantum dot sensitized solar cell (QDSSC). A typical quantum dot sensitized solar cell consist of a QD sensitized nanorods of high band gap metal oxides like TiO2 and ZnO as photoanode, an electrolyte consisting of redox couple such as polysulfide and a counter electrode like Cu2 S [43]. The photoanode is deposited on a glass substrate coated with thin TCO layer and is immersed in an electrolyte consisting of redox couple. The counter electrode is deposited on the other side of the device. The basic structure of a QDSSC is depicted in Fig. 4. Upon illumination photons get absorbed in the QDs and electron hole paired in the form of exciton are generated. Heterojunction between QD and metal oxide separates the electrons and holes towards two opposite sides of it. Noteworthy, the band offset

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Fig. 4 Schematic diagram of QDSSC

can be easily tuned with the help of size and shape of QD. The large band gap of metal oxide and near infrared equivalent energy gap of QD forms a minimal band offset for CB at the interface. On the other side, band offset of VB is several orders of magnitude larger than that of CB. As a result, free electrons that are generated can easily pass through the heterojunction while holes experiences large barrier on its passage. The electrons generated in p-type QD thus get injected into the n-type metal oxide (provided electron affinity of metal oxide is more than that of QD). In general, metal oxide is shaped in the form of nanorod or other nanostructures that provide a direct pathway for electron transport to the collecting electrode. The electrons thus are transported through the nanorod, TCO and passes through the external circuit with infinitesimal resistance in its path. The oxidized QD is then reduced with reduction of redox electrolyte leaving behind oxidized component. These residual components in the electrolyte are then reduced with the electrons collected at the counter electrode after passing through external circuit. This illustrates complete photon energy to electricity conversion mechanism undergoing in a QDSSC. In addition, since the electrons and hole get separated and transported to different materials as soon as they are generated the chance of carrier recombination is negligible and is a major advantage of this kind of device over conventional thin film heterojunction. The complete process of electric current generation in QDSSC is depicted in the Fig. 5. Upon exposure of solar light, QD excites its VB electron to CB. The heterostructure between QD and ZnO develops a CB offset that act as a driving force for excited

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Fig. 5 Complete process flow of QDSSC

electron to inject into the ZnO nanoparticles. On the other side due to electron injection QD is oxidized to QD+ . A redox electrolyte consisting of polysulfide (S) oxidizes S2− to S. These electrons reduce QD+ to its ground state QD. S2− −→ S + 2 e− QD+ + e− −→ QD Subsequently, the oxidized S reduces back to S2− at the counter electrode with two electrons that arrives after passing through an external circuit. S + 2 e− −→ S2− One of the drawbacks in such solar cell is that the QD used for sensitization of nanoparticle photoanode is either not passivated or not well passivated. So a secondary QD layer, the shell layer, is deposited on the QD in order to form core/shell QD structure. The type of core/shell QD heterostructure depends mainly on the electronic properties of the underused materials. For example PbS (core)/CdS (shell) QDs form a type I heterojunction because the later has more energy bandgap. In such QD heterojunction, the charge carrier is confined to the core layer only. However, the shell layer provides a good passivation to the core. On the other side, junction between CdS/CdSe QDs is a type II heterojunction. In this type of core/shell structure the two types of charge carriers are separated spatially and their recombination probability is nullified. Other type II core/shell QDs extensively used in QDSSC is CdTe/CdS, ZnSe/CdS, CdTe/CdSe etc.

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2.5 Quantum Dot Solar Cell Another type of solar cell is quantum dot solar cell (QDSC). Quantum dot in the form of colloidal state (CQD) is a promising candidate for low cost high efficient solar cell. Solution processing and successive ionic layer adsorption and reaction (SILAR) methods are generally used to synthesis CQD. The earlier version of QDSC consists of a transparent conductive oxide with front electrode deposited on it followed by a doped CQD layer making Schottky contact with a high work function metal like Al, Ca, Mg and Ag on the other end [44]. Under illumination, light passes through TCO and gets absorbed in the QD. The generated electrons in the CB of QD spill over to the metal because of its low work function. These electrons circulate through external circuitry and reach the TOC front contact and recombine there with hole injected from QD. The major advantages of Schottky junction CQD solar cell are its easy fabrication methodology and less junction defect density due to limited heterojunction. In addition, it provides a platform for testing of CQD film electrical and optical properties. However, the problem associated with a Schottky contact of CQD based solar cell lies on the Fermi level pinning at the CQD-metal contact that imposes a limitation to the open circuit voltage of the device. However the evolved QDSC, keeping in view to eradicate the limitations of earlier version of the device, consist of a TCO front electrode followed by high energy band gap semiconductor (metal oxides like TiO2 and ZnO) making heterojunction with infrared absorber CQDs of semiconductor. The heterojunction between doped CQDs and nanoparticle metal oxides are arranged in various form such as n-p and p-i-n junction like CQD(ntype)/CQD(p-type) or (n-type) CQD/(p-type) metal oxide (doping may be in opposite order as well) and (p-type) CQD/(intrinsic)CQD/(n-type) metal oxide. The end of the stack is done with deposition of an oxide layer such as MoO3 or deep work function metal like Au followed by deposition of metallic layers of Al or Ag. In general, metal oxide is prepared n-type and p-type QD is deposited on it. The n-type material is highly doped so that maximum of depletion region lies in the QD side of the junction. Moreover, the width of depletion region can be controlled with the size and shape of the QD used. Illumination is onset on the front TCO and light passes through it and is absorbed with maximum rate near the heterojunction. Photon of wavelength in the range of visible to infrared light get absorbed in the CQD and excitons are generated. Photoexcited electrons from VB to CB of QD are drifted towards high band gap energy metal oxide under the driving force of electric field inside the junction depletion region. The holes in the VB of QD move in opposite direction and enter in the back electrode. The alignment of CB of CQD and metal oxide is noteworthy because it allows electrons to enter into the metal oxide side whereas VB offset of the two materials are high and thus provides hindrance to holes. As a result, excitons get separated and electron passing through external circuit provides electricity. Carrier transport in both Schottky contact and heterojunction type QDSC is reported by Abraham et al. and is depicted in the Fig. 6a, b [45]. The Schottky contact of Al on PbS CQD possessing ITO front electrode is used as solar cell device in their work. Electron from its pool in Al jumps to the CQD and a thin depletion

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Fig. 6 Carrier transport mechanism in two major types of QDSC [45]: a Schottky junction QDSC and b heterojunction QDSC

layer is formed in the CQD side of the contact. The 1S excited state of electron in CQD is well above the Fermi energy level of Al. Thus, photoexcited electrons in CQD get injected into the Al. However, holes generated in the CQD can easily inject into the electron collecting contact. This leads to adverse effect on the efficiency, FF of the device. In another structure of CQD based solar cell, heterojunction between TiO2 , and PbS CQD is presented in their work. Electron and hole diffusion at the heterojunction produces a depletion region. Transparent TiO2 in the structure acts as an electron collecting contact. The 1S excited state of electron in CQD is ~0.3 eV above the CB minima of TiO2 . On the other side 1p excited state of hole in CQD is ~1.5 eV above the VB maxima of electron collecting contact. Such structure allows injection of photogenerated electrons into TiO2 whereas blocks the hole from entering the electron collecting contact and thus bounces it back as shown in the figure. In the figure E fn and E fp are quasi-Fermi level for electrons and holes respectively. Transportation direction of photo generated electrons and holes are denoted by jn_ill and jp_ill respectively. However unintentional hole injection in a direction is denoted by jp_for . The CQD are decorated with surfactants or ligands on its surface in order to make a proper passivation and improve stability of the colloidal solution. However, these ligands are long organic compounds and insulating in nature that degrades device performance when used in solar cell. Therefore, a short ligand possessing high conductivity is desirable in this scenario. As a result, a ligand exchange process is applied to solution processed QD prior to its deposition in solar cell. In recent type of QDSC, a hole transport layer of organic materials like PdS-EDT [46] and SpiroOMeTAD [47] are used between QD and back electrode for better hole collection. Figure 7 shows a representative QDSC reported in [46] based on ZnO nanorods and PbI2 CQD. The conventional solar cell is costlier as it requires crystalline and purest form of expensive substrate materials like Si. Evolved version of the devices attempted cost reduction by choosing alternate materials that has the ability to provide efficiency close to that of Si based device. Emitter layer in such device plays a crucial role

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Fig. 7 Schematic diagram of a QDSC

in development of open circuit voltage and short circuit current by band gap alignment and being transparent to visible light highly absorbed in the Si wafer. Wide energy band gap materials like ZnO, SiC and TiO2 etc. are some of the prominent candidates for the purpose. New generation solar cell includes DSSC and QDSSC as family members implementing a new approach of solar to electric power conversion technology. In addition, exploring ability to control both electrical and optical properties of QD by its shape and size has drawn a new paradigm for solar cell industry. Dye molecules in DSSC are replaced with QDs for better solar cell performance. Heterojunction between a high energy band gap nanostructured semiconductor and IR absorber QDs are implemented for solar cell included in this version. Among the nanostructured semiconductors, ZnO is an important candidate due to its high transparency and low sheet resistance. All the three types of new generation solar cells namely DSSC, QDSSC and QDSC provides high efficiency but is not as efficient as Si based conventional device. However, the cost of production is comparatively much less than the earlier version.

2.6 Perovskite Technology The technologies discussed above are in a stage of evolution. Recently, most of the attention of the research community has been focused on organo-metallic perovskite materials and its ferroelectric phenomenon. The detailed description of organometallic perovskite is given in Sect. 2.7.3. Consequently, main attention is given to perovskite material based solar cell and its ferroelectric effect in this chapter. Organo-metallic compound with perovskite structure are on the verge of replacing polysilicon solar cells. Worldwide, research is going on over this solar cell technology. Several attempts have previously been done that have resulted in greater PCE at a lower manufacturing cost [48–51]. Apart from that, researchers are interested in perovskite-based solar cells because of its intrinsic features, which include

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better optical properties, superior carrier mobility, longer carrier diffusion lengths, and a simpler but precise structure with effective production method [52]. It appears that Kozima et al. first used perovskite material for photovoltaic based device in 2006 [53]. They used methyl ammonium lead tri-iodide (CH3 NH3 PbI3 ), a perovskite material as a sensitizer in dye sensitized solar cell device. Gradually, the growing research has developed this concept up to completely solid and planar perovskite solar cell [54]. This has become possible due to ambipolar charge carrier transport property possessed by the perovskite material. Subsequently, Kojima et al. in 2009 reported a PCE of 3.81% and 3.13% utilizing iodine (I) and bromine (Br) as halide materials, respectively in a planar type solar cell [55]. It is also a very interesting fact to mention that in less than one decade, much-improved efficiency of up to 22.1% is achieved with MAPbI3 as the light-absorbing material in the year 2017 [56]. Concurrently, several earlier studies also reported that a much-improved efficiency can be achieved with the similar outcomes having the structural modification in the perovskite solar materials respectively. In addition, perovskite solar cell demands for suitable ETL and hole transport layers (HTL). Different organic and inorganic materials are used for these purposes. For example ZnO, TiO2 , SnO2 , [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) etc. are used as ETL in perovskite solar cell [57]. Similarly, Spiro-MeOTED, P3HT, CuI, CuSCN etc. are used as HTL. Perovskite materials are consists of both organic and inorganic species. The general format used to indicate perovskite material is ABX3 where both A and B are cations with larger and smaller ionic radii respectively. In general A consists of methyl ammonium, Caesium or ethyl-ammonium with ionic radii of 0.216 nm, 0.253 nm and 0.167 nm respectively [58]. Metals like Pb, Ge and Sn are used as B cation in the perovskite material. However, Sn is not suitable from stability perspective because it can oxidise easily. The X represents halogens mainly Cl, Br or I. Composite halides such as I1-x Clx and I1-x Brx are incorporated in perovskite as well. The latest reports of Bhattarai et al., Raoui et al. and Abdelkader et al. showcases successfully designed and highly efficient PSC device [59–61]. A distinctive perovskite layer having a smaller bandgaps such as CsSnI3 , (1.3 eV), FASnI3 (1.41 eV) can also be proposed for the different device modelling of the perovskite solar cells [60–62]. The insertion of Caesium (Cs) in place of Methyl-ammonium (MA) can be very much efficient for the future work of the perovskite solar cells.

2.6.1

Modelling Approach of Perovskite Solar Cell

The fundamental structure of a perovskite solar cell consist of a light absorbing material also known as active material sandwiched between ETL and HTL. The light generated excitons spilt up and move towards the respective electrodes causing electric current. Such a solar cell can be modelled using one dimensional drift diffusion and charge carrier transportation equations in semiconductor [51, 57]. These equations are implemented in various 1D simulators such as SCAPS 1D, AFORSHET etc. The potential inside the perovskite material is deduced using the 1D Poisson equation as given below.

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 d 2 V (x) q Nd (x) − Na (x) + p(x) − n(x) ± Nde f (x) =− dx2 εa

(2)

where V, q, x, εa , Nd , Na , p, n and Nde f represent electric potential, electronic charge, thickness of layer, relative permittivity, ionised donor doping concentration, ionised acceptor doping concentration, hole, electron and defect densities respectively. Under steady state condition the transportation of electrons and holes across the device is approximated by the semiconductor transport equation as given below. d Jp = G − R(n, p) dx −

(3)

d Jn = G − R(n, p) dx

(4)

where J p and Jn are current densities due to transportation of hole and electron respectively. These currents are contributed by drift and diffusion mechanisms. In addition, G and R(n, p) represent the optical generation rate and recombination rate of charge carriers respectively. Some of the generation profile is inbuilt in the simulator whereas analytical profile can also be incorporated in the simulation. On the other hand, R(n, p) is contributed from band-to-band, SRH and Auger recombination. Some of the perovskite materials used to implement solar cell are MAPbI3 , MAGeI3 , MASnI3 , FASnI3 and CsSnI3 . The electrical parameters of those materials are presented in Table 3. The energy band gap, electron affinity, relative permittivity, density of state of conduction band and valence band, charge carrier mobility and doping density are required in the simulation. In addition, mid-band gap and interface defects can be included in the model as well. The recombination models such as Shockley–Read–Hall, Auger and band-to-band are incorporated in those simulator. Moreover, both standard and analytical generation profile can be included in Table 3 Material properties of the perovskite materials used in solar cells [60, 61, 63] Parameters

MAPbI3

MAGeI3

MASnI3

FASnI3

CsSnI3

Bandgap (eV)

1.5

1.9

1.3

1.41

1.3

Electron affinity (eV)

3.93

3.98

4.17

3.52

3.6

Relative permittivity

30

10

8.2

8.2

9.93

Effective DoS (m−3 )

at CB

2.5 × 1020

1 × 1016

2 × 1018

1 × 1018

1 × 1019

at VB

2.5 ×

1×

2×

1×

1 × 1018

Mobility (cm2 /Vs)

1020

1015

1018

1018

Electron

50

1.62 ×

Hole

50

1.01 × 101

1.6

22

5.85 × 102

Variable

1×

0

0

Variable

Variable

1 × 109

3.25 × 1025

7 × 1016

Variable

Donor concentration

(cm−3 )

Acceptor concentration (cm−3 )

101

1.6

22

1.5 × 103

109

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the simulation environment. The flexibility in input process of absorption coefficient profile to the simulation is an additive feature as well. The absorbance coefficient in the perovskite solar cell plays significant role in the overall selecting the better efficient perovskite materials. As in the Fig. 8, it can be depicted that the material of perovskite i.e. MAGeI3 has shown a very high absorption coefficient up to 1.6 × 105 cm−1 . Furthermore, a wide wavelength range (from 200 to 680 nm) can be obtained for the high absorption of the photons [63]. On the contrary, the Fig. 9a shows the schematic diagram of the perovskite solar cell where electron and holes are carried by the ETL material (ETM) and HTL material (HTM) respectively [63]. The ETL is chosen from those materials with lower LUMO than that of the active material. On the other hand, HTL has higher highest occupied molecular orbital (HOMO) than that of the perovskite. The electrons and

Fig. 8 The absorption coefficient of MAGeI3 perovskite materials [63]

Fig. 9 a Schematic diagram of a typical perovskite solar cell and b Energy band diagram of perovskite solar cell without transport layer [63]

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holes generated inside the active material move across ETL and HTL respectively. These are driven by the electric field present at the two sides of the active material. Moreover, in Fig. 9b, represents the energy band diagram of the Perovskite Solar Cell without carrier transport layer. The electrons and holes are diffused in the layers for having a steeper slope as depicted in the figure.

2.6.2

Electron Transport Layers

The primary function of electron transport layer in a perovskite solar cell is to extract electrons from the main absorber perovskite layer in order to improve the collection efficiency of photo-generated electrons. In addition, it prevents the hole from migrating to the counter electrode and reduces charge carrier recombination. One of the suitable materials for electron transport layer is large band gap TiO2 . Moreover, ZnO is another material which is frequently utilized as electron transport layer in flexible perovskite solar cells. Many research on electron transport materials concentrate on the effects of various n-type semiconductor materials and architectures on the efficiency of power conversion [62]. Following is the foundation for selecting electron transport materials: • High carrier mobility n-type semiconductor material. • As located at the front of the device, the material should transmit light through it as much as possible. • The material may be produced at low temperatures so that underlying perovskite material does not degrade in its crystal structure. • The band structure of the material must be compatible with that of the perovskite materials. Arzi et al. modelled CH3 NH3 PbI3 perovskite solar cell and analysed the effect of different ETLs and HTLs on the device performance parameters specially the short circuit current, open circuit voltage, power conversion efficiency and filling factor [57]. Similarly, Bhattarai et al. analysed the effect of ETLs and HTLs on the performance of CH3 NH3 GeI3 based solar cell [63]. Some of the most commonly used ETLs with organo-metallic perovskites and its energy level diagram is depicted in Fig. 10. As can be seen the ITO is the most suitable ETL for solar cell application. However, high cost and low abundancy of Sn material demands for alternate ETLs. Inorganic transparent oxides such as Zinc oxide (ZnO), In and Ga co-doped ZnO (IGZO), Titanium di-oxide (TiO2 ), are used as ETLs. In addition, organic materials are also used as ETL and those can be deposited on low temperature condition and are cheap as well. PCBM is a commonly used organic ETL in perovskite solar cells.

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Fig. 10 Energy level diagram of some commonly used ETLs and CH3 NH3 PbI3

2.6.3

Hole Transport Layers

The hole transport layer is responsible for extraction and transportation of hole generated inside the light absorber perovskite layer. In addition, it blocks the electrons to pass through it and aids in reduction of recombination. For hole transport layer, HOMO in hole transport materials must match the valence band of perovskite materials. The energy level diagram of some commonly used HTLs is depicted in Fig. 11. Both organic and inorganic materials are used as hole transport layers in the perovskite solar cells. Spiro-OMeTAD is the most widely utilized organic hole transport material. The energy band alignment of this organic material is well matched with HOMO of some most commonly used perovskite materials. However, the hole mobility is less in this material and electrical conductivity is less. To increase hole mobility, several researchers doped a p-type composite such as cobalt compounds or

Fig. 11 Energy level diagram of some commonly used HTLs and CH3 NH3 PbI3

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certain additives like bis(trifluoromethane) sulfonimide lithium (LiTFSI), and 4-tertbutyl pyridine (TBP) into organic materials [62]. The Polymer hole carrying materials have gotten a lot of interest lately because of their greater film-forming ability and increased hole mobility when compared to organic small-molecule compounds. One of the ammines known as PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine) is widely employed as the hole transport material in perovskite solar cells. This material has hole mobility approximately two order of magnitude higher than conventional counterparts. Using PTAA as the hole transport material for MAPbI3 based perovskite solar cells with the greatest PCE of 16.2% and a short-circuit current of 21.3 mA cm−2 [62]. P3HT, PCDTBT, PCPDTBT and PEDOT:PSS are examples of other polymer hole transport materials. Recently, Bhattrai et al. also described the carrier transport less perovskite solar cell which effectively increases the efficiency with having less cost for unavailability of the carrier transport layer materials. The efficiency of model was reported up to 12.35% respectively, which can be very nice for future prospective [51].

2.6.4

Advances in Modelled Perovskite Solar Cells

Perovskite Solar cells have made significant progress in the recent period of time. The Perovskite Light-Absorbing Layer is a layer of perovskite material that absorbs incident photons. The perovskite materials have a key role in light absorption and photoelectric conversion in the perovskite solar cells, out of all the constituent components. One of the keys to increasing photoelectric conversion efficiency is to optimize materials and architectures i.e. the structure of the device. Table 4 depicts a few typical devices its material and performance analysis with parameters. The comparison of various perovskite materials in terms of performance exhibit simulated efficiency as high as 28.97%. The optimization of the PCE and other parameters by selection for both absorber and charge carrier transport layer is a trend in the current scenario. Table 4 Performance of some of the simulated perovskite solar cells Perovskite Materials

Jsc (mA/cm2 )

Voc (Volt)

FF (%)

PCE (%)

References

MAPbI3

25.59

1.13

81.54

23.55

[62]

MASnI3

31.85

0.921

77.81

22.82

[62]

MAGeI3

16.27

1.74

64.5

18.28

[63]

FASnI3

22.40

1.277

86.36

24.70

[62]

CsSnCl3

25.92

0.959

74.31

18.47

[62]

a CsSnI 3

31.53

1.048

87.66

28.97

[62]

a

Optimized

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2.7 Ferroelectric Photovoltaic Technology Conventional silicon based solar cell has a limitation of open circuit voltage. Ideally such a voltage can be raised up to the energy band gap of the absorber material and the theoretical efficiency limit is impractical to overcome. Crystalline silicon has a band gap of ~1.1 eV and thus cannot provide large VOC . Contrary to these photovoltaic devices where ferroelectric materials are used as absorber layer are high potential VOC provider [64, 65]. Such devices are named as ferroelectric photovoltaic (FPV). The mechanism manifesting the FPV is generation of high open circuit voltage across the two terminals of a bulk homogeneous ferroelectric due to additional voltage contribution by the ferroelectric phenomenon [66]. In addition, those materials provide steady photovoltaic current under short circuit condition and uniform illumination.

2.7.1

Ferroelectricity

In a ferroelectric material asymmetric crystal structure generates electric polarization. These polarizations correspond to electric dipole moments due to ionic displacement inside the crystal. Such dipole moments have random directions inside the crystal. However, the polarization direction can be changed and made unidirectional using some external electric field across the material. The ferroelectricity is a property of material which ensures existence of remnant electric dipoles even if the external electric field is removed. Under no electric field, the electric dipoles are randomly oriented in a macroscopic scale but there exist some microscopic regions inside the crystal where polarization is unidirectional and are known as ferroelectric domains. In addition, the polarization directions of ferroelectric domains in such material are modulated with the electric field applied to it. This leads to the existence of hysteresis loop in the plot of electric field (E) against the induced polarization (P) for ferroelectric materials likewise observed in ferromagnetic materials. This is known as P-E hysteresis loop. For a typical ferroelectric material the P-E hysteresis loop is depicted in Fig. 12. In absence of E, P is zero and it is due to random orientation of ferroelectric domain which cancels out one another. This is represented by point A in the figure and the corresponding polarization direction is shown as well. Application of E re-orients polarization directions of all the domains and the moment all the domains are directed same the P finally saturate as shown in the figure by the point B. The polarization direction can be flipped by applying an opposite E larger than coercive field (Eco ) and saturation of P is obtained at the opposite quadrant as denoted by point C. The trace of P for a range of E follows the path A-B-C-B and completes a hysteresis loop. It is noteworthy the P remains at a non-zero value even at the moment E is completely removed. This value of P is known as remnant polarization (Pre ). The ferroelectric materials have more than nine space groups and crystal structure varies from cubic to monoclinic. These are classified as nonoxide, double oxide and hydrogen bonded ferroelectrics in most of the literature. The most commonly used ferroelectric materials in photovoltaic are perovskite oxides which belong to

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Fig. 12 Polarization-Filed hysteresis loop of a typical ferroelectric material

double oxide type and organo-metallic perovskites. The polarization induced inside the crystal of ferroelectric is a reason for voltage generation higher than the energy band gap of absorber material and high photocurrent. These voltage and currents are known as photovoltage and photovoltage current respectively. Notably such voltage and current are different from normal photocurrent arises in the semiconductor under illumination. One of the unique properties of photovoltaic current is its direction which is always along the polarization. Such current is linearly dependent on the thickness of the device as well [67]. However, the photovoltage current generated inside FPV is small and thus the efficiency is small as well. Boosting of the efficiency of such solar cell is a concern for the research community at present.

2.7.2

Physics of Ferroelectric Photovoltaic

The physics behind the generation of ferroelectric photovoltaic inside a crystal is supported by various theoretical concepts such as bulk phenomenon, schottky barrier modulation, ferroelectric domain concept etc. According to the former concept, the ferroelectric phenomenon is a bulk phenomenon which has again different origins. One of the origin is randomly located but unidirectional wedges (scattering centre) present in the material causing drift and diffusion of charge carriers at those locations [68]. This establishes a short lived and locally existed electric currents due to increased entropy but together constitutes a net electric current across the material. Another type of origin is anisotropic in potential well at the generation centre as depicted in Fig. 13 [69]. Assuming E0 (ground energy state), E1 and E2 are the three energy levels and V1 and V2 represent potential in the right (B site) and left (A site)

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Fig. 13 Anisotropic potential well causing ferroelectric in a bulk material [69]

respectively of the well. Obviously, V1 >V2 , qV1 > E2 > qV2 and qV2 > (E1 and E0 ). Any photon absorption that causes the charge carrier to excite from E0 to E1 remains bounded inside the potential well and constitute no electric current. But if the excitation of the charge carrier takes place up to E2 or any energy level between V1 and V2 , it can move either towards A or B. Some of the excited carriers moving towards B tunnels through and rest are scattered from the barrier and thus a net carriers move towards A. This kind of carrier movement produces a net current of charge carriers towards A and contributes to photocurrent. However the movement of charge carriers decays along its path. Noteworthy, the optimized thickness of the crystal where the contribution to photocurrent in maximum is equal to the decay length of the carriers moving towards A. The fundamental structure of FPV is ferroelectric material sandwiched between two electrodes as depicted in Fig. 14. The ferroelectric effect causes electric dipole inside the crystal of absorber layer which can be altered with biasing condition as mentioned in Sect. 2.7.1. In fact, to do alteration in polarization an electric field more than coercive field needs to be Fig. 14 Schematic of a typical ferroelectric photovoltaic device

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applied through the contacts. In the above figure direction of dipole moment or polarization of all the ferroelectric domains are towards anode. Even after the removal of external electric field these polarization direction remain the same especially in ferroelectric materials. This contributes an additional component to the intrinsic electric field present inside the absorber material. Due to such electric field (E) a number of excitons that are generated under illumination inside the absorber get separated. The identification of contributors of additional electric field is a controversial topic. But the net electric field is assumed to be contributed by three sources: non-uniform defect states inside the crystal, the schottky barrier junctions at anode and cathode interfaces. The former builds a fixed directed electric field whereas the other two sources provide alterable fields. The schottky barrier height at the two contact interfaces of ferroelectric material is modulated by the polarization [70]. The FPV device ITO/BiFeO3 /SrRuO3 /SrTiO3 is fabricated using RF magnetron sputtering and analysed its photovoltaic performance in [71]. The schematic diagram and energy band alignment of this device is depicted in Fig. 15a and b respectively. Schottky barriers are formed at the ITO/BiFeO3 and BiFeO3 /SrRuO3 interfaces respectively and the corresponding electric fields are denoted as El and Er respectively. These can be altered using and external electric field. In addition, another electric filed Eb exists due to defect QSC such as oxygen vacancy inside the absorber material which is BiFeO3 in this case. Noteworthy, the field Eb is fixed and cannot be altered even applying an external electric field. Chen et al. described schottky barrier modulation with polarization in a typical FPV based on ferroelectric halide perovskite which have an energy band diagram as depicted in Fig. 16 [72]. In their work a thin CH3 NH3 PbI3-x Clx is sandwiched between Au and an N-type ETL to analyse its photovoltaic performance. Obviously, schottky barriers are formed at the two ends of the perovskite due to work function mismatch of both Au and ETL to that of the perovskite. Correspondingly, depletion width of Wa and Wb are formed near ETL and Au contacts respectively. In other words electric fields Eia and Eib are

Fig. 15 a Schematic of ITO/BiFeO3 /SrRuO3 /SrTiO3 PV device and b Energy band alignment of ITO/BiFeO3 /SrRuO3 /SrTiO3 PV device. Redrawn based on [71]

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Fig. 16 Energy band diagram of FTO/TiO2 /CH3 NH3 PbI3-x Clx /Au, a under zero net polarization, b under positive bias condition and c under negative bias condition [72]

developed and both of these are directed towards Au contact as depicted in Fig. 16a. The net electric field for this condition is E = Eia + Eib . The macroscopic polarization (P) is zero under this condition due to randomly directed domains (not shown in the diagram). Application of an external electric field (positive biasing) ensures unidirectional polarization of domains and a net P exists as depicted in Fig. 16b. As a result an electric field EP directed same as Eia and Eib is developed. The depletion width under this condition is enhanced to Wa+ and Wb+ and E = Eia+ + Eib+ where Eia+ >.Eia and Eib+ > Eib . Contrarily, reversal of the external electric field (negative biasing) reduce the depletion widths to Wa- and Wb- as depicted in Fig. 16c. Under such condition, E = Eia- + Eib- where Eia- < Eia and Eib- < Eib . The EP plays an important role in open circuit voltage of the FPV device. Larger the magnitude of EP more is the open circuit voltage provided its direction is same as that of the schottky barrier electric field.

2.7.3

Hybrid Halide Perovskite Materials

Materials bearing perovskite structure are mostly used in different photovoltaic applications. The intrinsic properties of these materials include low temperature crystalline structure, minimal defect, high absorption coefficient, tuneable band gap and efficient charge transfer. The fundamental representation of organo-metallic halide perovskite is ABX3 . Most of the materials belonging to this category have organic cation as A and inorganic cation as B. As a result it is named as hybrid halide perovskite in the research community. These are classified as type-1, type-2, type-3 and type-4 based on the structure. Table 5 showcases the chemical formula and composition of all those types. Type—I perovskites are extensively used in photovoltaic application. These consist of CH3 NH3 + and Pb2+ at A and B site respectively and have chemical formula of CH3 NH3 PbI3 . This material exhibits distinct phase of symmetry under different temperature. These phases are cubic, tetragonal and orthorhombic at the temperature below 162.2 K, intermediate and more than 327.4 K respectively [77]. Moreover, Pb

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Table 5 Types of hybrid halide perovskite and its elements Category

Molecular formula

Elements/compounds

Layers

Type—1 [73]

ABX3 and A2 A/ n-1 Bn X3n+1

A = CH3 NH3 + , CH(NH2 )2 + , Cs+ etc. B = Ge2+ , Sn2+ , Pb2+ etc. X = halide anions A/ = inorganic cations

Single

Type—2 [74]

A3 B2 X9

A = CH3 NH3 + , CH(NH2 )2 + , Cs+ , Rb+ etc. B = Bi3+ , Sb3+ etc. X = halide anions

Type—3 [75]

A2 BX6

A = Cs+ , CH3 NH3 + , CH(NH2 )2 + etc. B = Tetravalent cations X = halide anions

Type—4 [76]

A2 BB/ X6

A = Rb+ , Cs+ , K+ etc. B = In+ , Ag+ etc. B/ = Bi3+ , Sb3+ etc. X = halide anions

Double

is a toxic element which demands its replacement with bivalent cations such as Sn2+ , Ge2+ etc. The fundamental structure of perovskite with the cation A as inorganic is depicted in Fig. 17a whereas the crystal structure for CH3 NH3 PbI3 is depicted in figure (b). The inorganic cation B is coordinated with six X anions in an octahedral cage. The octahedral cages are edge connected and form a cubic lattice unit cell. The cation A is coordinated with 12 X anions and caged inside cuboctahedral geometry surrounded by eight octahedrals. In case of organometallic perovskite like CH3 NH3 PbI3 the

Fig. 17 Fundamental structure of a ABX3 perovskite in cubic phase and b CH3 NH3 PbI3 in cubic phase which occurs at high temperature

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inorganic cation i.e. Pb2+ and anionic halide (I− ) form octahedral cage. However the organic cation (CH3 NH3 + ) is caged inside the cuboctahedral. Our discussion so far indicates the site A and B are organic and inorganic cations. But stability of such material is less and is a serious concern for solar cell application. The reasoning are moisture, ion migration, thermal degradation etc. [78]. in 2014 a group of researchers used organic cations in both A and B sites and formed (CH3 NH3 )x (CH(NH2 )2 )1-x PbI3 and reported power conversion efficiency of fabricated solar cell using the mentioned perovskite material to be 14.9% [79]. The tuning of energy band gap using different values of x is also reported in their work. Similar, tuning is done by using two inorganic cations at the B site. For example, in (CH3 NH3 )Pbx Sn1-x Br3 both Pb2+ and Sn2+ cations are incorporated along with CH3 NH3 + cation to tune the energy band gap and optical absorption wavelength [80]. Moreover, two dimensional and quasi two dimensional perovskite materials are quite popular in the research community at present. The lower dimensional perovskite can provide better stability of perovskite crystal lattice structure [81–83]. This is due to hydrophobic nature of the organic cations present in these type of crystal structure. The two dimensional perovskite materials with (100) crystal orientation are represented by A/ 2 An-1 Bn X3n+1 where A/ is a monovalent cation with larger ionic radii than the other two cations A and B [84]. In general, A/ and A are organic and inorganic cations respectively but the possibility of both of these being organic is acceptable. The parameter n decides the dimension of the perovskite lattice structure. For example, n = 1, 2 < n < 5 and n ~ ∞ represent two, quasi two and three dimensional structures respectively. The two dimensional perovskite materials are classified further, based on the phase of stability, in to Dion-Jacobson (DJ) and RuddlesdenPopper (RP) [85]. The former and the later are represented by A// An-1 Bn X3n+1 and A/ 2 An-1 Bn X3n+1 respectively. A// is an organic divalent cation with large ionic radii. For example, 3-(aminomethyl)piperidine shortly known as (3-AMP) is used in place of A// to form a two dimensional perovskite of DJ phase which is represented as (3-AMP)(CH3 NH3 )2 Pb3 I10 . In DJ phase an interlayer of divalent organic compound is formed between the two consecutive sheets made of organic–inorganic perovskite structure. Most commonly used organic cations for A// site are diamine compounds which has two NH3 groups at its edges and 3-AMP [86, 87]. The two NH3 groups of diamine form bonding to the nearest organic– inorganic perovskite sheets. For example, 1, 3–propanediamine (PDA) occurs as intermediate layer in (PDA)(CH3 NH3 )3 Pb4 I13 perovskite crystal structure forming DJ phase as depicted in Fig. 18a [85]. The amines at the two ends of a PDA are bonded to two (CH3 NH3 )PbI3 layers and is denoted by dotted lines in the corresponding figure. On the other hand, use of organic materials such as iodoethylammonium, butylammonium, iso-butylammonium and phenethylammonium leads to RP phase. In this phase two interlayer organic sheets are formed between the consecutive organic–inorganic perovskite structures. These interlayers share a common boundary formed by van der Waals interaction. For example, Ahmad and co-workers incorporated propylamine (PA) in place of A/ to give rise to (PA)2 (CH3 NH3 )3 Pb4 I13 and its crystal structure is depicted in Fig. 18b [85]. The amine of each PA is bonded to only one organic–inorganic perovskite sheet

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Fig. 18 a Dion-Jacobson phase of (PDA)(CH3 NH3 )3 Pb4 I13 quasi-two dimensional perovskite. Redrawn based on [85]. b Ruddlesden-Popper phase of (PA)2 (CH3 NH3 )3 Pb4 I13 quasi-two dimensional perovskite. Redrawn based on [85]

and a Van der Waals interaction exists between the two nearest amines. In both DJ and RP phases the interlayer organic cation protects the perovskite sheets of the material from external factors such as moisture. However the structural stability is more in case of DJ as compared to RP phase due to single organic cationic interlayer. In addition, the interlayer thickness of the RP is larger than that in DJ phase.

2.7.4

Ferroelectricity in Hybrid Halide Perovskites

The existence of ferroelectric phenomena inside CH3 NH3 PbI3 crystal is determined by various methods. The most suitable methods focuses on the measurement of PE hysteresis, ferroelectric domain and centre-symmetric of ions inside the crystal structure. In general, presence of P-E hysteresis, high resolution microscopy image of ferroelectric domain or displacement of cations from its centre of symmetry inside the crystal structure indicates that the material is ferroelectric. The observation of

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high resolution microscopy image that elucidate ferroelectric domain can be found using piezoelectric force microscopy (PFM) [88], heterodyne megasonic piezoresponse force microscopy (HM-PFM) [89], interferometric displacement sensor PFM (IDS-PFM) [90], Kelvin probe force microscopy (KPFM) [91], transmission electron microscopy (TEM) [92], band excitation PFM [93], conductive atomic force microscopy (c-AFM) [91] and dual AC resonance tracking PFM [93]. To detect the displacement of cation inside the crystal structure or presence of non-centresymmetric time resolved photoluminescence (Tr-PL) method is used. Presence of second harmonic generation (SHG) in the corresponding Tr-PL indicates the existence of ferroelectric phenomenon inside the crystal of the material under treatment. Almost all the polar materials show SHG [94]. However, the ferroelectric phenomenon in CH3 NH3 PbI3 is a controversial topic at present. The source of P-E hysteresis exhibited by this perovskite is not vivid and is leading to a debate. In addition, the plot of current density versus voltage applied across this perovskite (J-V) exhibits hysteresis loop. This means J-V plot for V varying from 0 V to open circuit voltage (VOC ) is different from that for V sweet from VOC to 0 V. There are two different groups of experimental and theoretical results, one of which indicates that CH3 NH3 PbI3 is not ferroelectric in nature especially at room temperature. All those phenomena are due to charge trapping, ion migration and leakage current [89, 95– 98]. In 2014, Fan et al. reported no evidencing outcomes supporting ferroelectricity while testing experimentally the P-E hysteresis and domain using piezoresponse force microscopy at room temperature [95]. Similarly, in 2015, Sharada and co-workers reported existence of no non-centre-symmetric of ions based on the results of second harmonic generation method and concluded that no ferroelectric phenomenon exists inside CH3 NH3 PbI3 [96]. In the same year, Yamada and co-workers performed TrPL measurement on single crystal CH3 NH3 PbI3 and reported existence of no SHG profile [97]. Recently, Zeng et al. reported no noticeable ferroelectricity behaviour in CH3 NH3 PbI3 using a well sophisticated and advanced method [89]. Some of the related works suggests that the observation of P-E hysteresis and. The other group tells that this perovskite has ferroelectric properties. For example, In 2014, Kutes et al. found ferroelectric domains of size approximately 100 nm in CH3 NH3 PbI3 and concluded the presence of ferroelectric phenomenon in the material [88]. Similarly, In 2016, Sewvandi et al. mentioned existence of anti-ferroic behaviour in CH3 NH3 PbI3-x Clx which can be made ferroic applying a polarization field [83]. The piezoresponse force microscopy was used to get microscopic image of ferroelectric domain in their work. In the same year, Hermes et al. reported an unique PFM image of CH3 NH3 PbI3 highlighting twin domain [99]. According to their PFM image approximately uniformly spaced stripes of high contrast regions are present in each grain. These stripes are aligned in two forms which are orthogonal to each other. In addition, it is predicted that the spacing of stripes are correlated to the grain size. It is worth noting the change in orientation of domain ensure the materials ferroelectric property. In 2017, Rakita et al. presented experimental results that showcases presence of SHG, ferroelectric domains and spontaneous polarization in a sample of CH3 NH3 PbI3 and concluded that perovskite as ferroic [94]. To get nearly artefact free P-E hysteresis the authors used imaginary response of dielectric

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instead of energy storage part. In 2018, Ahmadi et al. reported existence of SHG in CH(NH2 )2 Pbx Sn1-x I3 (for x = 0 and 0.85) and (CH(NH2 )2 )y (CH3 NH3 )1-y PbI3 for y = 0.85 and summarized these materials as ferroic [100]. Recently, a group of researchers mentioned the existence of ferroelectric behaviour in CH3 NH3 PbI3 using PFM image [101].

2.7.5

Photovoltaic in Ferroelectric Perovskites

In general two common structures are used to implement ferroelectric solar cell. These are depicted in the Fig. 19a, b. The simplest structure of ferroelectric solar cell consist of an active layer sandwiched between anode and cathode. The arrowhead lines and charge pairs indicate polarization direction and dipole present inside the active layer. Another most commonly used structure can be built by inserting an electron transport interlayer (ETIL) and hole transport interlayer (HTIL) adjacent to anode and cathode respectively. These layers are used to improve the charge carrier collection at the respective electrodes. The physical mechanism inside the ferroelectric solar cell is believed as the same as conventional one. The performance of solar cell is analysed using current voltage characteristics in solar cell. The open circuit voltage, short circuit current, fill factor and power conversion efficiency are calculated based on that. In ferroelectric solar cell the same analysis is done. However, both the current and voltage components of the device have an additional contribution from the ferroelectric phenomenon. The total charge carrier transportation current and voltage can be written as below [102]. Jtot = Ji ± J f

(5)

Vtot = Vi ± V f

(6)

Fig. 19 Schematic of ferroelectric solar cell a without carrier transport layer and b with ETIL and HTIL layers

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Table 6 Mixed halide perovskite solar cells with more than 20% power conversion efficiency Active material

PCE (%)

FF (%)

JSC (mA cm−2 )

VOC (V)

References

MAPI:TMX

20.25

76.9

22.77

1.134

[103]

MAPI:IMS

20.84

82.82

22.08

1.140

[104]

PTAA/MAPI/PCBB-3 N-3I

21.10

81

23.46

1.110

[105]

MAPI:AAT

20.03

78

23.39

1.100

[106]

FA0.15 MA0.85 PbI3

21.88

81

24.23

1.120

[107]

MAPI:P(VDF-TrFE)/P(VDF-TrFE)

21.38

78

24.10

1.140

[108]

Note MAPI—CH3 NH3 PbI3 ; TMX—1,3,7-trimethylxanthine; IMS—imidazole sulfonate zwitterion, 4-(1H-imidazol-3-ium-3-yl)butane-1-sulfonate; PCBB-3N-3I—Lewis acid based fullerene; PTAA—poly(triarylamine); AAT—1-alkyl-4-amino-1,2,4-triazolium; FA—formamidinium; MA—methyl ammonium; P(VDF-TrFE)—polyinylidene fluoride-trifluoroethylene polymer

where Ji and Vi are intrinsic current and voltage of the device and J f and V f are contributions from ferroelectric phenomenon. Noteworthy, the former pair has a fixed direction inside the device whereas the later is switchable in direction. The application of CH3 NH3 PbI3 in solar cell has uplifted the PCE more than 20%. This is attributed to high open circuit voltage of the device. Some of the CH3 NH3 PbI3 based solar cell with PCE more than 20% and its performance parameters are listed in the Table 6. These solar cells are made using MAPI as active layer but some additives are used to improve the PCE. Moreover, oxode ferroelectric materials are also used for solar cell application. The most commonly used oxide ferroelectric perovskite are BiMnO3 (BMO), BiFeO3 (BFO) and BiFeCrO6 (BFCO) [102]. These have energy band gap less than 3 eV and suitable for visible light absorption and charge carrier generation process. Nevertheless, the VOC in those solar cells is quite high the power conversion efficiency is poor due to limited charge conductivity through these oxides. The discussion on oxide ferroelectric based solar cell is not within the scope of this chapter.

2.8 Organic Solar Cell Technology Organic solar cells could be considered as a branch of low-cost emerging photovoltaic technology, which deals with various organic semiconductor and conducting materials as light absorbing layer for photovoltaic applications [109–112]. Conventionally, Silicon (Si) in various polymeric form such as crystalline [113, 114], monocrystalline [115], multi-crystalline [115], and poly-crystalline [116–118] forms have been widely explored for solar cell application; exhibiting superior efficiency. However, the major constraint for Si-technology for solar cell applications is high

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Fig. 20 Molecular structure of various organic semiconductor materials used in solar cell

manufacturing/fabrication cost, requirement of sophisticated machinery for thinfilm deposition and many more. In last one decade, inorganic semiconductor materials such as hydrogenated amorphous Silicon (a-Si: H) [119, 120], CdTe [121– 123], and CIGS) [124, 125] have been explored significantly for the solar cell application; which requires lower manufacturing cost compared to conventional Sitechnology. Recent research on solar cell are focused on discovering new/novel materials and device structures which offer superior device performance at lower fabrication/manufacturing cost. This objective can be achieved by using organic technology for solar cell applications. Figure 20 shows the molecular structure of various organic semiconductor materials, which have been explored for solar cell application over the past few years. The use of organic technology to fabricate solar cell endow several key benefits including cost effectiveness, which are summarized as follows: (a).

(b).

Ease of Device Fabrication/Cost Effectiveness: Organic Semiconductor materials are solution processable, and can be processed using various solution processable deposition scheme such as spin-coating [126], spray-coating [127, 128], blade-coating [129, 130], roll coating [126], inkjet printing [131, 132], and screen printing [133] for thin film deposition; which eventually resulting in lower fabrication/manufacturing cost over large area applications. Molecular Engineering: The most attractive feature of organic technology is molecular engineering i.e. the molecular structure of the organic molecule can be modulated very easily which eventually resulting in modulation in the

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(c).

(d).

(e).

(f).

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semiconducting properties such as band-gap, charge carrier mobility, energy levels; allowing for electronics tunability [134–136]. Wide Absorption Spectrum: The organic molecule exhibit wide absorption spectra in UV–Vis absorption spectrum; which eventually resulting in better photon absorption during light/or photo-illumination condition. Lower Active Semiconductor Layer Thickness: The thickness of the active layer in organic solar cells is much thinner than the conventional silicon solar cells (few hundred of nm thickness of active semiconductor layer in organic solar cell [130] compared to few tens of μm in conventional silicon solar cells [137]); resulting in requirement of lesser active semiconductor material in device fabrication. The requirement of lesser material for active layer deposition with relatively simpler cell processing in organic solar cells makes the organic technology based solar cells as the potential candidate for future low-cost solar cell technology for large area application. Light Weight, Mechanical Flexibility, and Environmental Friendly: In comparison with the inorganic counterparts, organic electronic based solar cells are light weight [138], mechanical flexible [139, 140] and environmental friendly [130]. Miscellaneous: Moreover, in the existing literature, it is found that the organic technology based solar cells are capable of exhibiting high power generation efficiency under low light intensity also [141, 142].

In spite of inferior device performance in organic devices compared to their inorganic counterparts; various advantages with organic semiconductor materials favoured organic technology as a potential candidate for future low-cost solar cell applications. In recent studies, efficiency in the range of 10–17% has been achieved in organic electronics based solar cell [143, 144]. The major constraints of organic technology based solar cells include sensitivity towards environmental conditions, life-time, poor generation and separation of photo-generated charge carriers, and inferior efficiency compared to their inorganic counterparts. Historically, the first and second generation solar cells were focused upon single crystal silicon solar cell and amorphous thin film solar cell technology. DSSC and organic solar cell technology based solar cells are classified as third generation solar cell technology. In DSSC technology, use of liquid electrolyte causes temperature stability problem, which limits it for future solar cell technology.

2.8.1

Device Structure of Organic Solar Cells

A typical organic solar cell structure is depicted in Fig. 21a. It consists of an anode terminal: where the photo-generated holes have been collected, a cathode terminal: where the photo-generated electrons have been collected and an active semiconductor layer: where charge carriers are generated during photo-illumination conditions, and it is sandwiched between anode and cathode terminals.

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Fig. 21 Device structure of a simplified and b improved model of an organic solar cell

Examples of various commonly used organic semiconductor materials for organic solar cell application includes Magnesium Phthalocyanine (MgPc) [145], poly(3nethyl-thiophene) [146], and poly(phenylene-vinylene) (PPV) [147]. Under photoillumination condition, photo-generation of excitons takes place in organic semiconductor materials. The difference in the workfunction of the electrode terminals develops an electric field in the organic solar cell; which helps in splitting of photogenerated excitons into photo-generated electrons and holes. Additionally, it assists these photo-generated electrons and holes to be collected at the cathode and anode terminal respectively. Finally, if any resistance/load is connected between the cathode and anode terminals, electric current starts to flow between the electrodes. The major constraint in this organic solar cell is the recombination of photo-generated electrons and holes. During photo-illumination conditions, there could be a probability that some of the photo-generated holes are moving towards the cathode terminal, recombine with the photo-generated electrons and get lost. Similarly, the photo-generated electrons may move towards anode terminal, recombine with the photo-generated hole and get lost. These kinds of recombination process of photo-generated charge carriers diminish the overall efficiency of the organic solar cells. Thus, the efficiency of the organic solar cell shown in Fig. 21a could be further improved by using hole-blocking layer (HBL) and electron-blocking layer (EBL) at the cathode and the anode terminals respectively. The resultant improved version of the organic solar cell is shown in Fig. 21b. The hole blocking layer (HBL) is acts as a filter layer, which blocks the flow of hole and allows the flow of electrons towards the cathode terminal; similarly an electron blocking layer (EBL) blocks the flow of electron and allows the flow of hole towards the anode terminal. Moreover, different structures in the light absorbing layer of organic solar cell allows its classification in to single layer, bi-layer and bulk heterojunction type as depicted in Fig. 22. In bi-layer structure a planar heterojunction between two different organic materials are used as light absorber layer. In the bulk junction type heterojunction between two different organic materials is non-planar and spread all over the bulk.

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Fig. 22 Device structure of a single layer, b Bilayer, and c bulk heterojunction

(a)

Single Layer Solar Cells

The device of single layer based solar cell is shown in Fig. 22a. The working principle of this type of solar cells has been discussed earlier. The major constraints in this kind of organic solar cells include inferior efficiency, insufficient electric field to drive the transportation of charge carriers, poor fill factor resulting due to large series resistance associated with the insulating nature of the organic semiconductor layer, recombination of photo-generated electrons and holes, and field-dependent generation of charge carrier in organic semiconductor layer under photo-illumination conditions. Table 7 represents the literature review on organic solar cell with single layer of active semiconductor materials. (b)

Bilayer/Two-layer Solar Cells

Figure 22b represents the device structure of bilayer/two-layer organic solar cell. Organic solar cells of this kind consist of bilayer/two layer of organic semiconductor materials with different electron affinity and ionization energies as active layer. Generally, the values of electron affinity and ionization energy in organic and inorganic semiconductor materials range from 2–4 eV and 4.5–6.5 eV respectively. Typically, organic semiconductor materials with high values of electron affinity (>5.0 eV) are classified as strong electron acceptors or p-dopant [148]; whereas, the organic semiconductor materials with low ionization energy ( 17%) Si-organic hybrid solar cells by simultaneous structural, electrical, and interfacial engineering via low-temperature processes. Adv. Energy Mater. 8(9), 1702655 (2018) 20. Z. Lu et al., Achieving a record open-circuit voltage for Organic/Si hybrid solar cells by improving junction quality. Solar RRL 5(8), 2100255 (2021) 21. J. He et al., 15% Efficiency ultrathin silicon solar cells with fluorine-doped titanium oxide and chemically tailored poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate) as asymmetric heterocontact. ACS Nano 13(6), 6356–6362 (2019) 22. R. Devkota et al., Solution-processed crystalline silicon double-heterojunction solar cells. Appl. Phys. Express 9(2), 022301 (2016) 23. M. Pietsch, S. Jäckle, S. Christiansen, Interface investigation of planar hybrid n-Si/PEDOT: PSS solar cells with open circuit voltages up to 645 mV and efficiencies of 12.6%. Appl. Phys. A 115(4), 1109–1113 (2014) 24. Y. Han et al., Naphthalene Diimide-based n-type polymers: efficient rear interlayers for highperformance silicon–organic heterojunction solar cells. ACS Nano 11(7), 7215–7222 (2017) 25. L. Chen et al., 14.1% efficiency hybrid planar-Si/organic heterojunction solar cells with SnO2 insertion layer. Solar Energy 174, 549–555 (2018) 26. Y. Zheng et al., Optimization of SnO2 -based electron-selective contacts for Si/PEDOT: PSS heterojunction solar cells. Solar Energy 193, 502–506 (2019) 27. R.R. Lunt, V. Bulovic, Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications. Appl. Phys. Lett. 98(11), 61 (2011) 28. Y.-Y. Lee et al., Top laminated graphene electrode in a semitransparent polymer solar cell by simultaneous thermal annealing/releasing method. ACS Nano 5(8), 6564–6570 (2011) 29. A.A.F. Husain et al., A review of transparent solar photovoltaic technologies. Renew. Sustain. Energy Rev. 94, 779–791 (2018) 30. C.-C. Chen et al., Visibly transparent polymer solar cells produced by solution processing. ACS Nano 6(8), 7185–7190 (2012) 31. Y.R. Bak et al., Fabrication and performance of nanoporous TiO2 /SnO2 electrodes with a half hollow sphere structure for dye sensitized solar cells. J. Sol-Gel Sci. Techn. 58, 518–523 (2011) 32. Y.J. Shin, J.H. Lee, J.H. Park, N.G. Park, Enhanced photovoltaic properties of SiO2 -treated ZnO nanocrystalline electrode for dye-sensitized solar cell. Chem. Lett. 36, 1506–1507 (2007) 33. S. Mathew et al., Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers. Nat. Chem. 6(3), 242–247 (2014) 34. X. Mao et al., High efficiency dye-sensitized solar cells constructed with composites of TiO2 and the hot-bubbling synthesized ultra-small SnO2 nanocrystals. Sci. Rep. 6, Article number: 19390 (2016) 35. K. Kakiage et al., Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes. Chem. Commun. 51(88), 15894–15897 (2015)

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Applications of Emerging Materials: High Power Devices J. Ajayan, Shubham Tayal, and Laxman Raju Thoutam

Abstract SiC power MOSFETs, AlGaN/GaN-HEMTs (High Electron Mobility Transistors), AlGaN/GaN-MOSHEMTs and β-Ga2 O3 MOSFETs have become the most attractive transistors for future high power electronic applications due to their unique characteristics like very low RON (ON Resistance), excellent mobility of electrons in the channel, outstanding breakdown performance and high temperature operation. This chapter highlights the various architectures and RF & power performance of SiC power MOSFETs, AlGaN/GaN-HEMTs (High Electron Mobility Transistors), AlGaN/GaN-MOSHEMTs and β-Ga2 O3 MOSFETs. Moreover, it also describes the emergence of new materials for the development of above mentioned emerging transistors. This chapter also throw lights on the use of AlGaN as a channel layer material in AlGaN/GaN-HEMTs, ITO (Indium tin oxide) as a transparent gate electrode material in p-GaN/AlGaN-HEMTs, Gd2 O3 as a high-k gate oxide material in AlGaN/GaN-HEMTs and also ScAlN as a barrier layer material in AlGaN/GaN-HEMTs. Keywords SiC power MOSFETs · AlGaN/GaN-HEMTs · AlGaN/GaN-MOSHEMTs · β-Ga2 O3 MOSFETs · High-k dielectrics

1 Emergence of AlGaN as a Channel Material in HEMTs Wide band gap semiconductors such as GaN and SiC have outstanding material properties over Si & GaAs for use in RF & high power applications including pulsed power for electric ships and avionics, advanced control electronics and power management and solid state drivers for heavy electric motors. GaN and SiC based power devices offer significant savings in cost and energy due to their outstanding power handling capability and efficiency. However, further improvement in environmental operation region and breakdown field in GaN HEMTs requires the replacement of traditional GaN channel with AlGaN channel. Al0.7 Ga0.3 N is a wider band gap material (Eg = J. Ajayan (B) · S. Tayal · L. R. Thoutam Department of Electronics and Communication Engineering, SR University, Warangal, Telangana, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_11

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Table 1 The electrical properties of some of the materials which can be used for developing high power devices [1] Properties

GaN

GaAs

4H-SiC

Ga2 O3

Al0.7 Ga0.3 N

AlN

Si

Eg (Band gap in eV)

3.4

1.42

3.25

4.85

5.7

6.2

1.12

Dielectric permittivity (ε)

9

12.9

9.7

10

8.86

8.5

11.8

Breakdown field (EC)

3.9

0.4

2.5

8

12.7

12

0.3

Mobility of electrons (cm2 /Vs)

1250

8500

1000

300

310

1090

1500

Maximum Velocity (× 3 107 cm/s)

1

2

2

1.6

2.2

1

Thermal conductivity, λ (W/cm K)

0.5

4.9

0.23

0.07

2.85

1.5

2.53 2.221532

5.7 eV) has a higher critical breakdown field of 12.7 MV/cm compared to GaN which has a critical breakdown field of 3.9 MV/cm. The electrical properties of loping high power devices are shown in Table 1. Obtaining a low contact resistance is one of the primary issues in the realization of AlGaN channel HEMTs. AlGaN channel with high aluminium (Al) content have emerged as a promising solution for future switching power electronic applications. AlGaN channel HEMTs grown on AlN substrates offer extremely low leakage current, low sub-threshold swing and higher ION /IOFF . The cross-sectional heterostructure of traditional and schottky drain AlGaN channel HEMTs are shown in Fig. 1. The device consists of Silicon substrate, AlN nucleation layer, AlGaN buffer, C-doped AlGaN buffer, UID (Unintentionally doped) Al0.1 Ga0.9N buffer, AlN insertion layer, layer, Al0.1 Ga0.9N channel, Al0.4 Ga0.6N barrier layer, GaN cap layer and Al2 O3 dielectric layer. Au-free technology and silicon wafer need to use to promote the commercialization of AlGaN channel based HEMTs by efficiently minimizing the cost. AlGaN HEMTs can also be grown on sapphire substrate. The use of Au-free technology in the fabrication of AlGaN channel HEMTs is beneficial for industry to eliminate Au diffusion issues. AlGaN material has a lower electron affinity than GaN. Therefore, the barrier-height of AlGaN channel HEMTs is higher compared to GaN channel HEMTs for the same schottky gate metal work function. This makes it very difficult to obtain a low ohmic contact resistance and threshold voltage. AlGaN material with higher Al content is very difficult to grow. The epitaxial material quality play a very important role in the breakdown performance of power devices. Usually MOCVD systems are used to grow AlGaN channel HEMTs on silicon wafer. Al on top of silicon wafer acts as nucleation layer, a thick (3 μm) UID graded AlGaN and 1 μm C-doped Al0.1 Ga0.9 N can be used as buffer layers to improve the material quality. 400 nm UID Al0.1 Ga0.9 N material acts as the channel, 1 nm AlN acts as insertion layer, 24 nm Al0.4 Ga0.6 N layer acts as barrier layer and a 2 nm GaN material acts as

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Fig. 1 The cross-sectional heterostructure of a traditional and b schottky drain AlGaN channel HEMTs [3]

cap layer. Ti/Al metal combination can be used to create Au-free ohmic contacts in AlGaN channel HEMTs. For efficiently reducing the turn on voltage, schottky drain structure can be used in AlGaN channel HEMTs by W/Al metal work function at the drain. Polarization engineered InAlGaN barrier layer can be used for significantly enhancing the RF and power performance of AlGaN channel HEMTs [2]. AlGaN channel HEMTs on AlN substrate with Al content of 0.51 exhibits a higher breakdown voltage (VBR ) of 1800 V [4]. A large LGD (gate-drain spacing) is desirable for achieving high VBR in AlGaN channel HEMTs. The deposition of GaN on AlGaN layer will induce holes due to spontaneous polarization and piezoelectricity effects, which can decrease the concentration of electrons in the AlGaN channel. The impact of temperature on the IDS -VDS curves for AlGaN channel HEMT is depicted in Fig. 2. When temperature raises IDS decreases and breakdown voltage (VBR ) increases due to the reduction in carrier mobility in the AlGaN channel due to charg.e scattering effects. The relationship between IDS and VDS can be expressed as [1] Ld VDS = RS + R D + I DS μW ε(VG S − VO F F ) VDS IDS RS RD VGS

Drain to source voltage. Drain to source current. Source resistance. Drain resistance. Gate to source voltage.

(1)

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Fig. 2 a–c The impact of temperature on the IDS -VDS curves for AlGaN channel HEMT [1] d VBR versus LGD characteristics [5]

L W d ε VOFF

Gate length. Gate width. Barrier layer thickness. Dielectric constant. Pinch off voltage.

The sub-threshold swing of AlGaN channel HEMT can be expressed as [1]   Dit q KT ln10 1 + SS = q C AlGa N SS CAlGaN Dit

(2)

Sub-threshold swing. Capacitance of the AlGaN layer. Interface trap density.

2 The Emergence of Gd2 O3 as High-K Gate Dielectric in AlGaN/GaN MOS-HEMTs Gd2 O3 (Gadolinium Oxide) have recently emerged as a promising high-k gate dielectric material to replace HfO2 and other traditional high-k dielectric materials [6].

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HfO2 offer a higher band gap of ≈5.5 eV and a higher dielectric constant of above 20. However, it has the disadvantage of lower crystalline temperature (below 500 °C). Another disadvantage of HfO2 is that it can’t efficiently avoid the diffusion of O2 , leading O2 from gate dielectric to react with wafer material to form a low-k layer. Gd2 O3 is gaining huge attention because of its large band gap of above 5 eV, outstanding thermal and chemical stability and extremely high crystalline temperature (below 1000 °C) compared to HfO2 . But Gd2 O3 offer a dielectric constant of less than 18 [7, 8]. The structure of Gd2 O3 /AlGaN/GaN MOSHEMT on silicon wafer is depicted in Fig. 3. The use of Gd2 O3 as gate dielectric helps to significantly minimize the gate leakage current in AlGaN/GaN MOSHEMTs [10]. Moreover, Gd2 O3 helps to improve the ION/IOFF of AlGaN/GaN MOSHEMT on silicon wafer to approximately 108 . A large mismatch in lattice constants between silicon and GaN induces a large amount of threading dislocations in the GaN/Si interface. For efficiently minimizing this threading dislocations at the GaN/Si interace, graded AlGaN layers with varying Al content (80–25%) can be grown on AlN nucleation layer using MOVPE technique. An epitaxial Gd2 O3 layer can be grown on GaN/AlGaN heterostructure with the help of solid-state MBE technique. There is no significant variation in gate leakage current density with rise in temperature (Fig. 4a). This is mainly due to the outstanding thermal stability of Gd2 O3 material at higher temperature. The threshold voltage (Vth ) of the MOSHEMT can be estimated as [9] Fig. 3 The structure of Gd2 O3 /AlGaN/GaN MOSHEMT on silicon wafer [9]

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Fig. 4 Influence of temperature on the DC performance of MOSHEMT with Gd2 O3 [9]

 Vth = ϕ2 − E C −

QPI



C AlGa N

 −

QI



C Gd2 O3

(3)

The electric field across Gd2 O3 can be computed as [9] E Gd2 O3 = Q D −q Nit −qn2D E G εGd O ε0 2 3

φ2

height or the barrier between Gd2 O3 and Ti

QI = QD − qNit QD Nit = Gd2 O3 /AlGaN EC CAlGaN CGd2O3 QPI

(4)

positive donor charges at AlGaN surface. interface trap density. Conduction band offset between AlGaN and Gd2 O3 . Capacitance at the AlGaN layer. Capacitance at the Gd2 O3 layer.

total polarization charge at AlGaN/GaN interface.

(5)

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Fig. 5 ION /IOFF versus temperature for HEMT and MOSHEMT [9]

At higher temperature the drain current and transconductance decreases due to the enhancement of optical phonon scattering (Fig. 4b–d). ION /IOFF remains constant with rise of operating temperature (Fig. 5) [9, 11].

3 The Emergence of Indium Tin Oxide (ITO) as Gate Electrode Material in P-GaN/AlGaN/GaN HEMTs In recent years transparent devices have emerged as an attractive solution for future sensing devices, display technologies and optoelectronic applications. Due to low resistance and excellent optical transparency, ITO is used as a transparent conducting material in many optoelectronic applications. Materials like GaN and InGaN are used as transparent channel materials in transparent field effect transistors (TFTs). AlGaN/GaN HEMTs are widely used in applications like power electronics, optoelectronics and wireless communications [12–26]. The performance comparison of some of the transparent device technologies are given in Table 2. In 2015, T. H. Chang et al. [29] reported the successful fabrication of AlGaN/GaN HEMT on flexible & transparent wafer. Similarly, ITO can be used as a transparent electrode in AlGaN/GaN HEMTs. The schematic heterostructure of AlGaN/GaN HEMT using ITO gate is demonstrated in Fig. 6. It consists of a sapphire or silicon wafer, a thick buffer layer, UID GaN channel layer, Alx Ga1-x N barrier layer, a p-type GaN layer, Al2 O3 passivation layer and a ITO gate. E-beam evaporation technique can be used to deposit ITO gate. The IDS VDS curves of fully transparent and conventional AlGaN/GaN HEMTs are plotted in Fig. 7. The traditional AlGaN/GaN HEMT exhibits higher ION and ION /IOFF compared to fully transparent AlGaN/GaN HEMT [27, 40]. Gate first process is suitable for p-GaN/AlGaN/GaN HEMTs. Sputtering can also be used for making ITO gate electrodes. Optical lithography is commonly used to define gate region,

Transparent structure

HEMT

FT-HEMT

HEMT

HEMT

TFT

TFT

TFT

TFET

NW-TFT

TFT

TFT

TFT

TFT

TFT

Ref

[27]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

IGZO

IZO

ZnO

ZnO

a-Si:H

SnO2

InGaO3 (ZnO)5

ZITO

IGZO

InGaN

GaN

GaN

GaN

GaN

Channel material

50

100

1500

50

32

3.8

50

200

80

–

0.4

0.7

3

3

LG (μm)

0.1

0.15

0.005

0.006

0.07

1.46

14.5

0.45

0.75

30

600

700

179

192

Peak current (Ma/mm)

80

1.6

8.3

15

−5

0.97 0.3

−1 10

1

145

3

3

3

19

−4

–

−2 22

–

−1

9.1

1004

−2.3

−5

1012

−2.3

−5.4

μFE (cm2 /Vs)

Vth (V)

Table 2 The performance comparison of some of the transparent device technologies [27–39] Transmittence (%)

–

–

75

80

–

70

80

–

80

–

–

–

70–80

–

SS (mV/dec)

1225

1200

1177

500

500

270

385

797

858

376

–

269

250

95

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Fig. 6 Heterostructure of AlGaN/GaN HEMT using ITO gate [40]

Fig. 7 The IDS -VDS curves of fully transparent and conventional AlGaN/GaN HEMTs [27]

then ITO gate can be defined using wet etching process. In order to selectively remove the p-GaN layer in the access region, selective etching is preferable. Finally, for 30 s, a rapid thermal annealing can be done at N2 ambience for ITO gate.

4 The Emergence of Scaln and Scalgan Barrier Layers in Algan/Gan HEMTs In recent years, Scx Al1-x N has emerged as a potential barrier layer material in AlGaN/GaN HEMTs due to its large spontaneous polarization charge. Usually, Alx Ga1-x N has been used as barrier layer in high power AlGaN/GaN HEMTs. The key

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Fig. 8 The structures of ScAlN barrier layer based GaN HEMTs on SiC wafer

benefit of using Scx Al1-x N as a barrier layer in GaN HEMT is that it has the capability of providing superior RF performance with aggressive barrier layer thickness and gate length scaling. The structure of ScAlN barrier layer based GaN HEMT on SiC wafer is illustrated in Fig. 8. It consists of SiC wafer, Fe-doped buffer, GaN channel, AlN back barrier, AlGaN spacer, ScAlN barrier, n+GaN source/drain regions SiN passivation layer. The influence of ScAlN and ScAlGaN on the GaN HEMT performance is demonstrated in Fig. 9. ScAlGaN based HEMT provides better DC performance compared to ScAlN barrier layer based HEMTs. However, ScAlN based HEMTs offer relatively better RF performance compared to ScAlGaN based HEMTs. GaN HEMTs with ScAlGaN offers low RON (ON resistance) due to higher sheet charge density [41–43].

5 SiC Power MOSFETs Silicon based IGBTs (insulated-gate bipolar transistors) are widely used for motor drives in electric vehicles and other industrial applications. IGBTs have a large short—circuit time (tSC ) of above 10 μS which helps to detect the short circuits and followed by turning off the gate voltage to avoid device failure. However, these IGBTs can be replaced with SiC-power MOSFETs for efficiently minimizing the switching power loss. SiC-MOSFETs also have the capability to increase the operating frequency of inverters. Unfortunately, the SiC power MOSFETs have a relatively low tSC of 5 μS which makes it difficult to detect the short circuits which may leads to the failure of SiC power MOSFETs. The failure of SiC power MOSFETs due to short circuit will happen when the temperature increases above 700 °C. When

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Fig. 9 The influence of ScAlN and ScAlGaN on the GaN HEMT performance [41]

temperature rises over 700 °C, the source metal usually Al melts that results in the destruction of SiC power MOSFETs. One method to avoid the failure of SiC power MOSFET due to rise in temperature is to minimize the short circuit current as low as possible. Short circuit current can be efficiently reduced by minimizing the gate drive voltage. Another method is to increase the gate length of the SiC MOSFETs. However, these two methods significantly increase the RON of SiC power MOSFETs [44–48]. The schematic of SiC power MOSFETs are shown in Fig. 10. The SiC vertical planar power MOSFET consists of an n+ substrate, N-drift region, poly-Si-gate, JFET region and channel region. SiC MOSFETs are capable of operating at more than 3300 V [49]. The specific channel resistance (Rch,sp ) of the SiC power MOSFETs can be estimated as [50] Rch,sp = Lch Wch W S μch COX

L ch (W S) μch Wch Cox (VG S − Vth )

Channel length. Channel width. Width of the MOSFET unit cell. Pitch of the MOSFET unit cell. Inversion layer mobility. Oxide capacitance.

(6)

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Fig. 10 The schematic of SiC power MOSFETs [46, 52]

Vth

Threshold voltage.

Saturation drain current density of SiC power MOSFET can be estimated as [51] Jd,sat =

VG S − Vth μch Wch C O X (VG S − Vth )2 = 2L ch (W S) 2Rch,sp

Short circuit time (tSC ) of SiC MOSFET can be estimated as [51]

(7)

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t SC ≈ ρ VB Cp EC WJ Tmax

ρC p Jd,sat E C



Wj S



3Tmax 2VDS /VB + 1

419

(8)

Material density. Blocking voltage. Specific heat. Critical field. Width of the JFET region. Maximum acceptable temperature increase.

In the off state blocking mode, a thin highly doped drift layer, owing to the large critical field of SiC, results in a large electric field at the oxide region. In conventional SiC power MOSFETs, the gate oxide layer at the bottom end corner of trench is not fully shielded by a traditional single p-base structure. This is because the trench is deeper compared to p-base/N-drift interface. double trench in both source and gate regions, bottom thick oxide and bottom protection p-well are the various techniques that can be used for protecting the gate oxide layer in SiC power MOSFETs. The bottom protection p-well is more effective among these techniques because the trench is directly shielded by heavily doped p+ regions. In order to effectively suppress the crowding of electric field at the gate oxide regions, a double p-base structure can be used [52]. Suppression of electric field crowding will effectively improve the breakdown performance of SiC power MOSFETs [53]. The influence of DPG (Distance between the trench side wall and deep p-base edge) on the drain current density of SiC-power MOSFET is illustrated in Fig. 11a. For obtaining large drain current density a higher trench depth is required. Similarly, the effect of DPG on the breakdown voltage of SiC power MOSFET is illustrated in Fig. 11b. increasing the depth of trench results in the degradation of breakdown voltage of SiC power MOSFETs. The reliability of gate oxide is the major concern in the development of SiC MOSFETs [54–58]. TDDB (Time dependent dielectric breakdown) and BTI (Bias temperature instability) are the two primary reliability issues in SiC MOSFETs

Fig. 11 Impact of DPG of on a drain current density and b breakdown voltage, BV (V) [52]

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[59]. The interface quality of SiC/SiO2 is poor than Si/SiO2 interface. Different types of charges like mobile charges, oxide trapped charges, fixed charges, near interface oxide trapped charges (NIOTs) and interface trapped charges affect the interface quality of SiC/SiO2 (Fig. 12). Among these charges, both NIOTs and interface traps limit the field effect mobility (μfe ) in 4H-SiC power MOSFETs. In 2019, Maria Cabello et al. [60] experimentally proved that boron treatment in gate oxide (in other words boron doped gate oxide) of SiC power MOSFET significantly improve the interface quality of SiC/SiO2 interface (Fig. 13). Boron treatment introduction of boron atoms in gate oxide helps to efficiently reduce the NIOTs, which may leads to the enhancement of μfe . Another concern in SiC power MOSFET is the shifting of threshold voltage (Vth ) due to the BTI. The influence of positive and negative HTGB (High Temperature Gate Bias) stress on the transfer and output curves of SiC power MOSFET is demonstrated in Figs. 14, 15. From Fig. 14, it is clear that Vth decreases with negative bias and increases with positive bias. Similarly, Fig. 15 points out that RON decreases due

Fig. 12 Different charges available in a SiC-MOS structure (Reprinted from Ref. [60] with permission from Elsevier)

Fig. 13 Boron treatment effects in SiC MOSFETs [60]

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Fig. 14 The influence of positive and negative HTGB stress on the transfer curves of SiC power MOSFET (Reprinted from Ref. [59] with permission from Elsevier)

Fig. 15 The influence of positive and negative HTGB stress on the output curves of SiC power MOSFET (Reprinted from Ref. [59] with permission from Elsevier)

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Fig. 16 a SEM image of failure in SiC MOSFET (Top view) (Reprinted from Ref. [62] with permission from Elsevier), b SEM image of damaged cell (cross sectional view) (Reprinted from Ref. [62] with permission from Elsevier)

to NBTI and increases due to PBTI. The channel resistance (Rch ) of SiC power MOSFET can be calculated as [59] Rch =

L ch W μn Cox (VG − Vth )

(9)

Equation (9) indicates that a shift in Vth due to HTGB significantly affect the Rch and RON of SiC power MOSFETs. The primary mechanism that leads to the short circuit (SC) failure of SiC power MOSFET is the thermal runaway failure which occurs due to the high power dissipation [61–73]. Short circuit failure (short circuit between source and gate terminals by the SC stress) is the socondary mechanism that may also leads to the failure of SiC power MOSFETs. Figure 16 shows the SEM image of the failure in SiC MOSFETs. The formation of carbon clusters at SiC/SiO2 interface severely degrades the μfe of SiC power MOSFETs. Figure 17 compares the C6 rings in bulk SiC and interface of SiC side. C6 ring is represented by blue colour atoms in the figure. Figure 17b, d respectively illustrate the PDOs (Partial density of states) in C6 in SiC and C6 at SiC interface. Similarly Fig. 18 shows the vertical C6 ring at interface SiO2 side and in bulk SiO2 . Corresponding PDOs are given in Fig. 18b, d respectively. The formation of C9 and C14 carbon clusters and their PDSos at SiC/SiO2 interface are illustrated in Fig. 19.

6 β-Ga2 O3 MOSFETs Recently, β-Ga2 O3 has emerged as a potential power semiconductor due to its wide band gap of 4.8 eV and critical field of 8 MV/cm [75]. In 2018, Yuanjie Lv et al. [76]

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Fig. 17 C6 rings in bulk SiC and interface of SiC side and their PDOs [74] (Reprinted from Ref. [74] with permission from Elsevier)

reported a β-Ga2 O3 MOSFET (Fig. 20) that consists of Fe-doped β-Ga2 O3 substrate, UID-Ga2 O3 buffer, and a Si-doped n-type Ga2 O3 channel, HfO2 gate dielectric, Ti/Au source/drain ohmic contacts and Ni/Au gate electrode. The 2 μm gate length device had a LGS (gate to source spacing) of 2.5 μm and LGD (gate to drain spacing) of 7.5 μm. This device exhibited a peak drain current density of 20.7 mA/mm @VGS = 6 V. A low gate recess depth is required to achieve higher drain current density in β-Ga2 O3 MOSFETs (Fig. 21). For enhancing the breakdown voltage of β-Ga2 O3 MOSFETs, source field plate technology can be used [77]. The use of source field plate efficiently reduces the peak electric field and improves the breakdown performance of β-Ga2 O3 MOSFETs. The Vth of β-Ga2 O3 MOSFET can be estimated as [77]. Vth = −Vbi − Vbi

Built in potential.

q Nd W d Al2O3 q Nd W 2 − ε0 ε Al2O3 2ε0 εGa2O3

(10)

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Fig. 18 Vertical C6 ring at interface SiO2 side and in bulk SiO2 and their PDOs (Reprinted from Ref. [74] with permission from Elsevier)

W Nd doxide εGa2O3

Channel thickness. Donor doping density. Oxide thickness. Permittivity of Ga2 O3 .

β-Ga2 O3 MOSFETs have a breakdown voltage of over 1850 V [77–85]. An Al2 O3 surface passivation in β-Ga2 O3 MOSFET efficiently improves drain current, reduces off current and minimizes hysteresis effects [86]. A gate connected field plate in combination with a large LGD can provide high breakdown voltage in β-Ga2 O3 MOSFETs [87]. A large LGD helps to efficiently suppress the critical electric field and improves the breakdown voltage of β-Ga2 O3 MOSFETs with gate connected field plate. In 2021, Arkka Bhattacharyya et al. [88] demonstrated a β-Ga2 O3 MOSFET with gate connected field plate that exhibited a breakdown voltage of 2462 V. Flourine based plasma treatment can be done in β-Ga2 O3 MOSFETs for effectively improving its electrical characteristics. CF4 treatment helps to achieve good ohmic behaviour. The surface morphology of CF4 plasma treated β-Ga2 O3 MOSFET is shown in

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Fig. 19 Formation of C9 and C14 carbon clusters and their PDOs at SiC/SiO2 interface (Reprinted from Ref. [74] with permission from Elsevier)

Fig. 20 a Ga2 O3 MOSFET structure [76] b SEM picture of the fabricated Ga2 O3 MOSFET [76] (Reprinted from Ref. [76] with permission from Elsevier)

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Fig. 21 Output curves of β-Ga2 O3 MOSFET with a recess depth of a 110 nm and b 220 nm (Reprinted from Ref. [76] with permission from Elsevier)

Fig. 22. However, the effect of different dopants (n type and p type) and unintentional doping has to be critically analysed and understood for the realization of β-Ga2 O3 based devices in high power device applications[90].

Fig. 22 The surface morphology of CF4 plasma treated β-Ga2 O3 MOSFET (Reprinted from Ref. [89] with permission from Elsevier)

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7 Summary Outstanding power handling capability and speed of operation, good reliability, good thermal and chemical stability and low cost are the key requirements for the developments of future high power devices. Si-power MOSFETs have been widely used for high power applications. But, now Si-power MOSFETs have reached their physical and fundamental limits. Therefore, its high time to think beyond Si-based power devices. Ultra wide band gap and high critical field are the two most important characteristics of materials that can be used for fabricating high power RF devices. SiCpower MOSFETs, GaN HEMTs and β-Ga2 O3 -MOSFETs have emerged as attractive solutions for future high power RF applications. Researchers are shifting their focus from GaN HEMTs towards AlGaN channel HEMTs due to the outstanding material properties of AlGaN over GaN for high power RF applications. This chapter covered the role of emerging materials and device architectures for the development of next generation high power RF devices.

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An Insights into Non-RE Doped Materials for Opto-Electronic Display Applications Satya Kamal Chirauri, M. Rakshita, and D. Haranath

Abstract For Opto-Electronic Display technologies, non-rare earth activated phosphor materials play an significant role. As a result, incorporating of non-rare earth ions in phosphors systems must be carefully chosen and described based on the material characteristics. Many researchers have made significant contributions in this regard, but the urgent challenges that arise during the development process have largely unsolved. In this chapter, we first provide an introduction to display device technologies followed by its emission mechanism and requirements of materials for display devices. Secondly, the state-of-the-art and developing innovative display devices were shown. Despite the fact that various phosphors systems have been described over the years, there are only a limited number of applications for phosphor based displays. In the end, this chapter closes with concluding remarks and suggestions that need to be addressed for usage of phosphor in display technology. Keywords Non-rare earth · Phosphor · Field emission display · Laser-powered phosphor display · Inorganic electroluminescent display

1 Introduction Phosphor based materials have recently dominated the display market through their energy efficiency, stability, and ease of integration into smart lighting technologies [1–3]. Canonical phosphors for illumination consists of micron powders Y3-x Cex Al5 O12 (YAG: Ce) emitting about 550 nm, sometimes combined with sulfide or nitride doped Eu2+ to obtain emissions of warm white. The high efficiency of YAG: Ce is due to its unique optical properties of Ce3+ ions—i.e., the fast-optical transition

S. K. Chirauri (B) Department of Sciences, IIITDM, Kurnool, Andhra Pradesh, India e-mail: [email protected] M. Rakshita · D. Haranath National Institute of Technology, Warangal, Telangana, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 L. R. Thoutam et al. (eds.), Emerging Materials, https://doi.org/10.1007/978-981-19-1312-9_12

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between 5d- and 4f-electronic levels—and also YAG’s excellent hosting characteristics [4, 5]. However, the discovery of GaN based blue light emitting diode (blueLED) by Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for which Noble prize awarded in 2014, a new horizon of display technology arises that are energy efficient, cooler and possess improved color quality which lasts longer. In global scenario, the penetration of white light emitting device (LED) technology enabled 1.1 quadrillion British thermal units (quads) in energy savings [6]. This triggered extensive efforts to design light emitting materials/devices by mixing various colors emitted by rareearth ions and an excitation source [7–12]. However, due to adoption of efficient LED technologies, utilization of several critical RE raw materials also increased, which are very expensive and geopolitical concern [13, 14]. Thus, development of strategic RE-free phosphors excitable in the blue/NUV range and exhibiting a broad emission band extended in the entire visible range are remains a major scientific challenge [15, 16]. Many researchers extensively investigated non-rare earth doped phosphors for display applications, among them defect-related luminescent materials synthesised via chemical methods are promising candidates because they typically combine high PL efficiency, good stability, ease of fabrication, and a wide PL emission [17, 18]. Pioneering work on these components was done on silicate-based mixtures, [15, 16] which now include phosphates, [19, 20] metal oxides, [21, 22] boron, carbon oxynitride (BCNO), [23–25], and carbon-based nano-materials. [26] Although these phosphors generally emit blue or bluish-white light, their quantum efficiencies are not systematically indicated, and their PL mechanisms are not completely understood. Moreover, metals which are toxic and hazardous to both human health and the environment, are present in some RE salts. In order to address this pressing issue of RE shortage supply all over the world extensive research is required by developing self-illuminated and rare-earth-free phosphors with high colour rendering index and outstanding luminous efficiency. Besides LED technology, new-generation flat panel display (FPD) devices include Field emission display (FEDs), Laser-powered phosphor display (LPD) and Inorganic electroluminescent (IEL) display devices had numerous features, such as self-emission, thin panel, broad viewing angle, fast reaction time, high brightness, high contrast ratio, and low energy consumption, drew a lot of attention. Until now, efficient sulfide-based phosphors have been employed for FPD. Despite the fact that the majority of them have been commercialized, there are still numerous issues that have severely limited their widespread use as FPDs. As it is well understood that decomposition of the sulphur gases from the phosphor, on irradiation with high-energy electron beams leads to reduction of the performance of the device. Furthermore, the incorporation of RE ions also increases the production costs. Therefore, it is important to develop phosphors by doping with non-rare activators such as, Mn4+ , Cr3+ , Bi3+ , Ge4+ , In3+ etc., and self-illuminating phosphors coupled with laser driven, UV- and electrical excitation. Up to date, a large number of nonrare metal ion activated phosphor systems have been investigated, such as oxides (aluminates, vanadates, molybdates, tungstates, gallates), sulphides, semiconductors, carbon dots materials for LED applications.

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In this chapter, we will discuss on non-rare activated and self-illuminating phosphor materials for development of opto-electronic display devices. It begins with a brief introduction, phosphors and its luminescence, overview of LED and Flat Panel Display device configurations, and followed with wide range of materials such as binary oxides, vanadates, tungstates, gallates, perovskites, semiconductors, carbon and silicon-based materials etc., Finally, we will also discuss the recent developments and concludes with challenges to be accomplished for further development of LEDs and FPD devices.

2 Phosphor and Luminescence Light is a type of electromagnetic energy that allows things to be visible. Light from the sun is abundantly available and can be captured and converted into electrical energy. However, all new technologies cannot depend on a renewable source of light, i.e., sunlight. As a direct consequence, there is a need to artificially generate light. There are two types of artificial light that emit light: incandescence and luminescence. Luminescence occurs when a material emits electromagnetic radiation (preferably in the visible range) after being exposed to X-rays, ultraviolet (UV), and infrared (IR) radiations without being heated. The phenomenon of incandescence occurs when a material is heated to such a high enough temperature that the atoms vibrate and emit light. Therefore, luminescence is classified as a cold process when compared to incandescence [27, 28]. In the case of an incandescent bulb, the emission is caused by heating of the filament, which results in a large amount of energy loss in the form of heat [29]. Furthermore, the incandescent bulb has a life span of approximately 1000 h, which is very short in comparison to compact fluorescent lamps (CFL) and light emitting diodes (LEDs). Because the luminescence phenomenon is a cold process, materials that exhibit it have a higher efficiency [30]. A phosphor is a light-emitting solid inorganic substance. Luminescence occurs when a phosphor absorbs energy from an external source and emits light in the visible to near-infrared (NIR) spectrum. Fluorescence is the spontaneous emission of light from solid phosphor material during the excitation process. Phosphorescence, also referred as visible emission, could indeed, on the other hand, be observed even after the material has been stimulated. Luminescence phenomena categorized based on the type of excitation source: Photoluminescence, Thermoluminescence, Chemiluminescence, Radioluminescence, Bioluminescence, Electroluminescence, Cathodoluminescence, Sonoluminescence. Photoluminescence When light energy, or photons, induce the emission of a photon, photoluminescence occurs. Fluorescence and phosphorescence are the two types of photoluminescence.

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Fluorescence

When a material absorbs light or other electromagnetic radiation, it emits fluorescence [31]. It’s one of the kinds of photoluminescence. The substance absorbs energy first, then emits light. The fluorescence stops when the light source is turned off. It’s a type of phosphorescence. The emitted light has a longer wavelength (or lower energy) than the absorbed radiation in most situations. The most noticeable features of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, namely the absorption is invisible to the human eye, but the emitted light is visible. The decay time of fluorescence is ~10–8 s. (ii)

Phosphorescence

Phosphorescence, emission of light from a substance exposed to radiation and persisting as an afterglow after the exciting radiation has been removed. Phosphorescence is a distinct type of luminescence. Slower time scales of re-emission are associated with “forbidden” energy state transitions in quantum mechanics. Because some materials take so long to transition, absorbed radiation may be re-emitted at a lower intensity for several hours after the initial excitation. [32]. Toys that glow in the dark, paint, and clock dials are all examples of phosphorescent materials that glow for an extended period of time after being exposed to bright room light. In dark conditions, the afterglow typically fades out within a few minutes (or up to a few hours for certain materials). The decay time of phosphorescence is ~10–5 s. Thermoluminescence Thermoluminescence is a type of luminescence that occurs in certain crystalline materials, such as minerals, when previously absorbed energy from electromagnetic or other ionizing radiation is re-emitted as light when the substance is heated. The phenomenon must not be confused with the radiation from the dark body. Chemiluminescence The emission of light (luminescence) as a result of a chemical reaction is known as chemiluminescence (also chemo-luminescence). There would also be a small amount of heat emitted. Radioluminescence Radioluminescence is the process by which light is produced in a substance after it has been bombarded with ionising radiation such as alpha particles, beta particles, or gamma rays. Radioluminescence is a low-level light source that is used for nighttime illumination of instruments or traffic signs. When an incoming ionising radiation particle collides with an atom or molecule, it excites an orbital electron to a higher energy level, resulting in radioluminescence. The particle is typically produced by the radioactive decay of an atom of a radioisotope, which is a radioactive isotope of an element. The electron then emits the extra energy as a photon of light, returning to its ground energy level. A phosphor is a chemical that emits light of a specific colour when exposed to ionizing radiation.

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Bioluminescence The production and emission of light by a living organism is referred to as bioluminescence. It is a type of chemiluminescence. Bioluminescence is found in many marine vertebrates and invertebrates, as well as some fungi, microorganisms (including some bioluminescent bacteria), and terrestrial insects like fireflies. A light-emitting pigment, luciferin, and an enzyme component, luciferase, are involved in this reaction. For example, the firefly luciferin/luciferase reaction requires magnesium and adenosine triphosphate (ATP) and generates CO2 , adenosine monophosphate (AMP) and pyrophosphate (PP) as waste products. Electroluminescence Electroluminescence (EL) is an optoelectronic phenomenon in which a material emits light through the application of an electric current or even a strong electric field passing through it. Electroluminescence is caused by the radiative recombination of electrons and holes in a material, which is typically a semiconductor. Photons— light—are produced when excited electrons release their energy. Even before to recombination, electron—hole pairs can be separated by either doping the material to form a p–n junction (as in semiconductor electroluminescent devices such as lightemitting diodes) or excited by the impact of high-energy electrons accelerated by a strong electric field (as with the phosphors in electroluminescent displays). Cathodoluminescence Cathodoluminescence is an optoelectromagnetic (optical and electromagnetic) phenomenon in which electrons collide with a luminescent material (phosphor), causing the emission of photons with visible wavelengths. A well-known example is the generation of light by a beam of electrons scanning the phosphor-coated inner surface of a cathode ray tube-based television screen. Cathodoluminescence is the inverse of the photoelectric effect, in which photon irradiation induces electron emission. Sonoluminescence The emission of light from imploding bubbles in a liquid when stimulated by sound is known as sonoluminescence. The luminescence of a chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state. The photon energy of the emitted photon is equal to the energy difference between the two states. There will be many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element’s emission spectrum is unique. Thus, understanding the light emission mechanism and spectroscopic properties of non-RE ions is critical when studying phosphor and its luminescence. The emission phenomena and spectroscopic characteristics non-RE activator ions are discussed in the next section.

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3 Emission Mechanism and Spectroscopic Properties of Non-RE Metal Ions During the synthesis of phosphors, firing is made at >1000 °C with the addition of halides (such as NaCl) with low melting point as chemical fluxes. It was found that if the firing is made without the addition of dopants but with a halide chemical flux, some luminescence (emission) is produced. This type of luminescence is called self-activated emission. Till date many researchers have reported the origin of selfactivated emission mechanism from many phosphors due to oxygen defects, size dependent emission properties, charge transfer phenomena, size dependent and due to activator doping etc. which can be controlled by different synthesis conditions and annealing atmospheres [33–38]. A collection of possible mechanisms has been discussed and summarized as follows:

3.1 Oxygen Related Defect Emissions Oxygen vacancies can be considered as point defects, which are pervasive in metal oxides. They may be created during manufacturing or field cycling when bonds are weakened by the electric field, as it is utilized in resistive random-access memories (RRAMs) [39]. Emissions related to oxygen related defects are very well-known defects in oxygen dominant luminescent materials. Further tunable of emissions from the host lattice by generating ionized oxygen vacancies with typical synthesis conditions acts as luminescence centers in the process. Procedures such as wet chemical reactions, pyrolysis, etching, annealing etc. are performed for causing defects which may be stoichiometric or non-stoichiometric. Some means by which oxygen vacancies may be formed include: (i) created during preparation of the material, (ii) created during preparation of a doped material and ionically compensate the charge introduced by the dopant (ionic compensation), (iii) preexisting oxygen vacancies diffuse to and gather at the interfaces, causing an interlayer of a different crystal phase due to the their influence of the phase stability. The oxygen defects generally have impact on both lattice symmetry and local potential that might trap the holes in the lattice of the crystalline solids, depending on the system energy levels. A succession of mid-gap states, such as surface defect states or deep levels in the energy gap of the system that might be connected to wide emission phenomena, can help explain the hole trapping phenomenon. For example, Oxygen vacancies play an important role for the performance of ceria as an oxygen storage material and as a redox catalyst. The oxygen vacancy formation in ceria results in partial reduction of the material where the two electrons, left when removing a neutral O atom, reduces Ce4+ to Ce3+ and in this context an interesting finding by [40] Shah et al. (2007) for humidity sensing of magnesium ferrite has been reported.

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3.2 Carbon Related Defects Emission Carbon related defect emissions depends on the synthesis procedure and annealing treatment, many reports revealed the presence of carbon based material (polymerizable silane pre-functionalized carbon dot (SiCD)) [41] and by optimizing the annealing temperature generates broad emissions covering entire visible region. When phosphors synthesized using polymer-based network shows broadband emissions extending in the whole visible range. Another benefit of optimizing heat treatment of phosphors is the prevention of the development of contaminants such as pyrolytic carbon or carbonate. Defect-related phosphor materials that have high internal quantum yields can be made even more efficient by heating them. Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials with unpaired electrons from which the intensities observed during EPR measurements are directly related to internal quantum yields, suggesting that carbonyl radicals emit strong white light when they are non-bridging defects.

3.3 Size Dependent and Self-activated Defect Emission The white light emission from nano sized phosphor materials is due to disordered lattice structure, increase of surface–volume ratio with size reduction and low dimensionality. When compared with bulk counter parts, reduction in size of particle exhibits a broad emission spreading all over the entire visible light region. Moreover, at nano regime the electronic states get excited when compared with bulk material, which is due to the particle size engineering. For example, White light (WL) emission for Eu2 O3 nanocrystals due to interaction between Eu, S and oxygen atoms in the solution archives enhancement from 7 to 10% on cooling the sample solution [42]. This study reveals the size dependent effect on the tuning the luminescence properties. Further, self-activated defect-induced emission can be observed in inorganic phosphors which includes vanadates, tungstates, gallates and molybates [43, 44]. They exhibit range of emission wavelengths from 400 to 700 nm. For example, Ca2 NaZn2 V3 O12 phosphor shows emission maximum around 500 nm when excited with a wavelength of 389 nm [45]. The broadband emissions from Ca2 NaZn2 V3 O12 , is assigned to charge transfer processes of the [VO4 ]3− units and transitions that happen among the energy levels are positioned between the valence and conduction bands. Herein, the charge transfers directly originated between 1 A1 → 1 T1 and 1 A1 → 1 T2 transition inside the (VO4)3− tetrahedral structure, and as a consequence of the cubical structure of CKZVO phosphor predicted to be the direct energy gap [46]

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3.4 Emissions Due to Doping The spectroscopic properties of the various non-rare activators such as Mn2+ , Mn4+ , Cr3+ , Bi3+ , Ge4+ , In3+ etc., due to the absorption and emission band structures of metal ions. On the wide absorption and emission bands, there may be some relatively thin line characteristics overlaid on them. Therefore, in the current chapter, the configurational characteristics due to doping have been discussed in detail. As a result of the simultaneous absorption or emission of both photons and phonons, these transitions are known as the vibronic transitions. Some particular ions (groups) will be explored in detail in this chapter to better understand luminescence phenomena in luminous materials. The d shell orbitals of non-RE metal ions are partly filled. For this arrangement, Tanabe and Sugano [47, 48] took into consideration both the electron–electron interactions as well as the crystal field to calculate energy levels. In general, it can be said that there are two basic factors that influence the emission radiation of a phosphor. The first one is the strength of the crystal field at the sites of the luminescing ions, and the second is the degree of covalence (or coordination number) of these ions with the surrounding oxygen ions. The role of the crystal field is to split the dopant energy level into sublevels; the luminescence always takes place from the lowest sublevel [49]. In the case of d1 and d3 and d9 , for example: d1 configuration is the simplest of the three. In octahedral symmetry, free ion features fivefold orbital degeneracy two dimensional (2D) which separates into two levels (2 E and 2 T2 ). As a consequence, the only feasible absorption transition is from 2 T2 to 2 E which is shown in Fig. 1a. The crystal field strength s 2 E − 2 T2 . In conclusion, this explains how non-RE metal ions are colored. The 2 T2 → 2 E transition is a clear example of a transition whose energy is dictated by crystal field strength. Figure 1b shows the Tanabe–Sugano (T-S) diagram for d3 configuration, for example Cr3+ (3d3 ), the ground state level is 4 A2 . There will be only three possible transitions

Fig. 1 a The absorption spectrum of Ti3+ (d1 ) in aqueous solution. b T-S diagram for d3 configuration (Reprinted with permission from Michael D. Seltzer et al. J. Chem. Educ. 1995, 72, 10, 886 Copyright (2015) American Chemical Society)

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i.e. 4 A2 → 4 T2 , 4 T1 (4 F). From Fig. 1b it is clearly seen from the spectra the Cr3+ ion is made up of three low-intensity absorption bands. The last configuration is d9 , for instance Cu2+ is is a well-known probe that is utilized in various luminous materials. The optical absorption spectra phenomena of Cu2+ ion is explained as follows: Ground state electronic distribution of Cu2+ in an octahedral crystal field is t2g 6 eg 3 , which gives rise to the 2 Eg term in the crystal field. A 2 T2g term corresponds to t2g 5 eg 4 , which is the excited electronic state of the system. In an octahedral crystal field, just one electron transition, 2 Eg → 2 T2g is predicted and the separation between these energies is 10Dq, which is crystal field energy. As an octahedron is stretched or compressed, the coordination is deformed, resulting in tetragonal geometry. Due to the Jahn–Teller effect, the 2 Eg ground state is normally divided, and a decrease in symmetry is predicted for the Cu2+ ion. This state splits into 2 B1g (dx2 -y2 ) and 2 A1g (dz2 ) states in tetragonal symmetry and the excited term 2 T2g also splits into 2 B2g (dxy ) and 2 Eg (dxz , dyz ) levels. In rhombic field, 2 Eg ground state is split into 2 A1g (dx 2 -y 2 ) and 2 A2g (dz 2 ) whereas 2 T2g splits into 2 B1g (dxy ), 2 B2g (dxz ) and 2 B3g (dyz ) states. Thus, three bands are expected for tetragonal (C4v ) symmetry and four bands are expected for rhombic (D2h ) symmetry. A tetragonal elongated environment d-orbital energy level diagram is illustrated in Fig. 2a. These are the transitions in the tetragonal field. It is 2 B1g → 2 A1g , 2 B1g → 2 B2g , 2 B1g → 2 Eg . The optical absorption bands observed for Cu2+ in octahedral coordination with rhombic (D2h ) symmetry are: 2 A1g (dx 2 -y 2 ) → 2 A2g (dz 2 ), 2 A1g (dx 2 2 →2 B1g (dxy ), 2 A1g (dx 2 -y 2 ) → 2 B2g (dxz ), 2A1g(dx 2 -y 2 ) → 2B3g(dyz ) states respecy ) tively. This is shown in Fig. 2b. In rhombic (D2h ) field, i.e., Assuming C2V symmetry, the strong band 2 A1g (dx 2 -y 2 ) → 2 B1g (dxy ) has a 10Dq value that depends on the compound’s nature. s2 configuration ions exhibits optical absorption in UV-region due to s2 → sp transition. In s2 configuration the ground state is represented by 1 S0 . The sp configuration produces the increment of energy in the order of 3 P0 , 3 P1 , 3 P2 and 1 P1 levels. From the spin selection rule, 1 S0 → 1 P1 is the only transition is expected here. However, the transition 1 S0 → 3 P1 can also be observed from the spectra, this is caused by spin–orbit coupling and combines with spin triplets and singlets. In addition to this, but in ions like As3+ , Sb3+ , Bi3+ have high intensity for

Fig. 2 a Energy level diagram of Jahn–Teller distortion in d-orbital in octahedral and tetragonal elongation. b Energy level diagram of d-orbitals in rhombic distortion

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the spin-forbidden transition 1 S0 → 3 P0 (J = 0). The 1 S0 → 3 P2 transition also remains forbidden (J = 0), in spite of the fact that it may be detected by coupling with vibrations. Therefore, there are differences between the bonding with p electrons and the one with s electrons lead to strong absorption bands are and s2 ions bands depends greatly on the host lattice Fig. 3. Ions with d10 configurations like In3+ , Sb5+ , Ge4+ etc. shows that the shorter UVregion has a strong and wide absorption band. Many researchers investigated and observed that such ions show good luminescence [50]. The charge transfer transition from the ligands’ 2p orbital of oxygen to an anti-bonding orbital, which is located partially on the d10 ion and partly on the ligands, explains the optical absorption. Hence, due to large band width and high intensity absorption bands are observed in these types of ions. Fig. 3 Absorption spectrum of KI-Tl+ . The Tl+ ion has 6s2 configuration (Reprinted from Philip H. Yuster and Charles J. Delbecq Optical Properties of Potassium Iodide-Thallium Phosphors J. Chem. Phys. 21, 892, AIP Publishing Copyright (1953))

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4 Material Characteristics Required for Display Device Applications Among many luminescent materials, a few only matches well with the properties considered as potential materials for practical applications. A framework of requirements has been described for improving the efficiency of the device system described as following (Fig. 4).

4.1 Spectral Efficiency Spectral efficiency has been a central focus for improving the lighting performance which measured in terms of lumens, Correlated color temperature (CCT) and Color Rendering Index (CRI). Correlated color temperature (CCT) is a measure of light source color appearance defined by the proximity of the light source’s chromaticity coordinates to the blackbody locus, as a single number rather than the two required to specify a chromaticity. Color temperature is a description of the warmth or coolness of a light source. When a piece of metal (often mysteriously referred to as a black body radiator) is heated, the color of light it emits will change. This color begins as red in appearance and then slowly turns to orange, yellow, white, and then blue-white to deeper colors of blue. The temperature of this metal is measured in degrees Kelvin. What’s confusing is that higher Kelvin temperatures are cool and lower temperatures are warm; directly opposite to the temperature in an oven. Color temperature is not an indicator of physical heat. Cool light is preferred for visual tasks because it produces Fig. 4 Material characteristics for efficient display devices

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higher contrast than warm light. Warm light is preferred for living spaces because it is more flattering to skin tones. CRI is a measurement of a light source’s accuracy in rendering different colors when compared to a reference light source with the same correlated color temperature. The closer a light source is to a score of 100, the better its color rendering. The higher the CRI, the better the visual perception of colors. Therefore, spectral efficiency defines the ability of emission spectrum to generate the desired spectral response for display device applications. This concept of spectral efficiency is peculiar for general lighting applications, since fine control of spectral response has only been made relatively measurable for display technology. Therefore, having better spectral efficiency of the luminescent material will exhibit possible benefits for advanced lighting technology.

4.2 Emission Spectrum The peak position, emission wavelength and full width at half-maximum (FWHM) are crucial as they control the luminescence properties of materials. Phosphors with wide emission spectra covering the entire visible region with high color rendering indices and CCT are the major requirement. Hence, emphasis should be taken while selecting the phosphor with suitable broad emission spectra.

4.3 Excitation Spectrum In general, the wavelength range of UV and blue LED chip is 355–400 and 440– 460 nm, respectively. The excitation spectral overlap with the emission of the LED source is very important for fabrication of White Light Emitting Diode (WLED). Consequently, broad excitation spectrum of the phosphor material maintains the color stability in FPD device.

4.4 Higher Quantum Efficiency The Photoluminescence quantum yield or PLQY of a molecule or material is defined as the number of photons emitted as a fraction of the number of photons absorbed. This characteristic property of a fluorophore or fluorescent molecule is important for understanding molecular behaviour and interactions for many key materials. Similarly, the electroluminescence quantum yield, or ELQY, is the number of photons emitted divided by the electron current of a device. This is important for lighting, display devices, and photovoltaic materials. Further quantum efficiency is classified into Intrinsic quantum efficiency and Extrinsic quantum efficiency. In the case of

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phosphors, the intrinsic QE is the ratio of photons emitted and absorbed. The extrinsic QE of phosphors mainly considered with the conversion of the excited photons to out-put power. The quantum efficiency of phosphor materials is highly dependent on the phosphor’s elemental makeup, crystal structure, and surface imperfections.

4.5 Phosphor Degradation Temperature-dependent luminescence performance of phosphors is needed in order to adjust the device’s intensity and hue. By studying the deterioration of phosphor, we can see how crystal structure, chemical compositions, and defect impurities change over time.

4.6 Maintaining Color Stability Maintaining the chromaticity refers to the stability in spectral power distribution for long time. The phosphor materials employed in WLEDs system can deform and its color stability based on the selection of the suitable phosphor system. Hence, phosphors must maintain their luminance under high power irradiations for longer lifetimes.

4.7 Cost Effective Other main requirement is usage of cost-effective phosphors for fabrication of display which is increasingly important for general illumination purpose. Designing and development of such low-cost phosphors with desired color specifications can fulfill by decreasing the cost of material processing.

4.8 Environment Friendly Employing the eco-friendly phosphors for fabrication of displays are to be devised. Limiting the elements such as cadmium, sulphur which are incompatible under moisture and humid conditions, in which luminescence from the such phosphors leads to degradation.

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5 Non-RE Activated Materials for Opto-Electronic Display Devices During the last decade, several researchers have indicated about the importance of non-rare-earth activated materials for solid state display applications. Especially when compared to organic based light emitting materials, inorganic phosphors have more potential due to essential qualities of activator ion emissions such as wavelength, intensity, and brightness. Many families of self- illuminating phosphor materials have been studied for building efficient devices, including gallates tungstates, vandates, carbon-based materials. As a conclusion, there is also an essential need to design and develop new and novel RE free and self-activated phosphor to fulfil the display device firm’s requirements.

5.1 Light-Emitting Diode Display Although main stream of research is devoted towards RE activators for phosphor converted LEDs (pc-WLEDs) with flexibility in design, high efficiency, and cost effective for mass production. pc-LEDs mostly used for general illumination which had become as integral part in our daily life. Firstly, pc-WLEDs were fabricated by integrating GaN LED coated with YAG:Ce3+ yellow phosphor layer Fig. 5a, which exhibits a low color rendering index (CRI) < 75 it isn’t appropriate for normal lighting (CRI > 85) [34, 51–57]. In type two, GaN LED to pump a phosphor combination of

Fig. 5 Types of p-LEDs a blue LED + yellow phosphor, b blue LED + red/green phosphors, c UV LED + blue/green/red phosphors and d UV-LED + self-activated (single compound) or RE free Phosphor

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green/red or green/yellow/red light producing phosphors [58, 59] Fig. 5b. In another approach, the tri-color (RGB) phosphors are coupled with a near UV diode. While the color temperature is not constant due to various driving currents for various color diodes, complicating the fabrication process, the blue emission efficiency is low due to the significant reabsorption of blue light by the red and green-emitting phosphors in such a system Fig. 5c. Using self-activated phosphors, coupled with a UV LED, the color rendering properties are improved. (Fig. 5d) [60]. On one hand, a tremendous effort has been devoted in identifying suitable non-rare activator ion doped phosphors for general illumination purpose, as a result decreases the overall usage of RE content in phosphors. Besides, self-activated phosphors have extensively studied. Among them, defect-related materials are potential candidates, since they are able to emit complete range from 400 to 700 nm covering entire visible region. When they are coupled with UV LEDs their efficiency, good stability, as well as ease of fabrication also can be a breakthrough in lightning industry. Many spearhead works have been under went on many materials such as binary oxides, vanadates, tungstates, and zinc gallates, perovskites etc. [61–65].

5.2 Light-Emitting Device Working Mechanism Figure 6 shows how an LED works. In principle, LEDs work on the same principle as a p–n junction diode, which generates light when applied with a voltage. Electric charges reunite at the depletion zone when an external voltage is supplied, as electrons move from the n-region to merge with holes in the p-region. While moving from the valence to conduction bands, electrons leave a hole in the process. Thus, electrons’ energies rise in contrast to those of holes’ energies as a result.

Fig. 6 Schematic of mechanism of LED

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5.3 Non-RE Based Phosphors for LED Display Recently, there emerges great consideration in developing non-RE activator doped phosphors and self-illuminating materials for Solid State Lighting (SSL) applications, by employing the inorganic oxide phosphors. Binary oxides have been extensively used in luminescent devices applications because of their optoelectronic and chemical characteristics resulting from the 4f electronic shells [66–69]. Recently G. Bilir and coworkers [70] developed single compound nanosized WL emitting undoped Y2 O3 host material, using continuous monochromatic IR light, emission is detected in the 400–900 nm region (803.5 and 975 nm). The observed emissions at nanoscale regime replicates the emission from incandescent lamp with efficiency (864 lum/W) and with color rendering index (99%). Recently C. S. Biju et al. reported [71] ZnO nanopetals prepared using selective self-etching method. From PL studies, on varying sintering temperature, creation of polychromatic defect emissions quench the intensities remarkably. Hence, low temperature prepared ZnO nanopetals exhibited enhanced broad defect emissions and higher strain in ZnO lattice leads to improvement when compared with annealed samples at higher temperatures. Hence, low temperature prepared ZnO nanopetals exhibited enhanced broad defect emissions and higher strain in ZnO lattice leads to improvement when compared with annealed samples at higher temperatures. Further, R. Tomala et al. [72] reported that on irradiation of laser (808 and 975 nm) on Y2 Si2 O7 nanocrystals exhibits of white emission ranges from 450 to 900 nm, with a peak at 680 nm are demonstrated in Fig. 7a, b. When excitation with 1.00 to 1.67 W the band peak maximum changed by 13.5 nm and corresponding emission intensity exhibits an enhancement by 5.6 times. Hence, due to the multiphoton ionization

Fig. 7 The power dependence of Y2 Si2 O7 white emission intensity under laser excitation: a evolution of intensity with 975 nm excitation b emission spectra under 445, 655 and 808 nm LD (Reprinted from Robert Tomala et al., Laser induced broadband white emission of Y2 Si2 O7 nanocrystals, Journal of REs, 37, 11, Copyright (2019), with permission from Elsevier)

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process leads to broad band white light emission. Taking these results into consideration that LEDs are going to replace by Laser diode as sources for generation of direct white light emission which replicates sunlight. Very recently, M. Stefanski et al. and coworkers [73] observed broad emissions observed from Sr2 CeO4 due charge transfer from to metal to ligand i.e., Ce4+ and O2− . The excitation power density and power range for two infrared wavelengths was equal to 0.04–2.14 W and 0.04–2.02 W. The slow accumulation of electrons in conduction band facilitates an avalanche process of photons which leads to remarkable increment in emission intensity. In addition to the function of power range, Sr2 CeO4 phosphor also obtained emission spectra as a function pressure for 975 nm excitation lines. The pressure generated inside the chamber dissipates the heat by the air gas molecules on laser excitation. Hence, this pressure provides transfer of heat energy to the sample and causes to increase its WL emission intensity. Recently, Spinel structured zinc gallate (ZnGa2 O4 ) phosphor material shown excellent broad emission properties. [74, 75]. The emission spectra of ZnGa2 O4 originates due to self-activation of the octahedral GaO6 sites [76]. C. R. Garcia et al. prepared crystalline ZnGa2 O4 phosphors with two different kind of precursors: Gallium acetylacetonate and Gallium nitrate. Gallium acetylacetonate precursor shows oxygen defect vacancies and intense emission bands at 450 and 515 nm [77]. GaO6 levels in octahedral configuration exhibits blue emission whereas oxygen defects emits green-yellow emission. The recorded lifetime ≈212 s was assigned due to the transfer of energy among the gallium and oxygen vacancies. The ZnGa2 O4 (Precursor Gallium acetylacetonate) sample shows (x = 0.2812, y = 0.3738) (Fig. 14b), in terms of the CIE coordinate for pure white light. Hence in most cases, luminescent host materials obtain a well-defined emission property [78–80] for obtaining white light emissions. In case of vanadium-based materials broad-emission spectra, reported by Aditya Sharma et al. Ca2 V2 O7 , Sr2 V2 O7 , and Ba2 V2 O7 pyrovanadate’s prepared by chemical precipitation method [81]. Charge-transfer transitions in VO4 tetrahedra have established broad photoluminescence characteristics in pyro-vanadates. Yellow-orange (CEI colour coordinates; x = 0.5308, y = 0.4151), green–blue (CEI colour coordinates; x = 0.2445, y = 0.3357) and blue-indigo (CEI colour coordinates; x = 0.2053, y = 0.2666) emissions have been observed from the Ca2 V2 O7 , Sr2 V2 O7 and Ba2 V2 O7 samples, respectively. The emitted colour coordinates could be tailored by mixing of the compounds and has resulted in advanced photoluminescence properties which are comprehensively suitable for the rare-earth element free white-light-emitting devices. The observed emission spectra from 400 to 700 nm , due to charge-transfer (CT) transitions of VO4 tetrahedra. The 3 T1 –1 A1 and 3 T2 –1 A1 transitions, from the VO4 , consisting of compounds that emit two different emission peaks, which are affected by alkali-earth metal ions differing in atomic radius. When the color coordinates are tweaked, photoluminescence characteristics improve, making them more appropriate for white-light producing devices that do not require rare-earth elements. Further, among various luminescence materials, metavanadates AVO3 (A:K, Rb, and Cs) and M3 V2 O8 (M = Mg and Zn) are very luminous, emitting light from 400 to 700 nm. [82]. T. Nakajima et al. [83] reported

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Fig. 8 CIE chromaticity diagram of the KCa2Mg2V3O12 phosphor (λex = 346 nm) (Reprinted from X. Huang et al. KCa2 Mg2 V3 O12 : A novel efficient rare-earth-free self-activated yellow-emitting phosphor, Journal of Photochemistry and Photobiology A: Chemistry, 401, 112765, Copyright (2020), with permission from Elsevier)

emission spectra from 380 to 800 nm derived from the CT transition in the VO4 tetrahedra. According to X. Huang et al., the KCa2 Mg2 V3 O12 phosphor has a wavelength range of 400–800 nm. Self-activated broadband yellow emission is assigned to 3 T2 → 1 A1 and 3 T1 → 1 A1 transitions of [VO4 ]3− groups [84] (Fig. 8). However, researchers have tried to replace nano-semiconductors with incandescent and fluorescent lights as a novel component for the production of solid, white light generating devices. ZnSe nanocrystals with direct white light emission [85]. Their WLEDs have a CIE chromaticity coordinate of using this white-light ZnSe material (0.38, 0.41). In addition, Rosenthal’s group has observed wide surface-state and band-edge emission from CdSe nanocrystals with a magic size of 1.5 nm [74]. As a result of the connection of uncoordinated surface selenium sites, wide band emissions have been recorded. When electrons and holes recombine, band edge emission occurs, whereas deep trap emission occurs when a photo-generated hole stuck in midgap state [75]. It was shown that post-preparative treatments with a formic acid solution increased the quantum yield of CdSe by up to 45% [76]. Carbon quantum dots (CQDs) have emerged as ideal component for fabrication WL emitting devices due to its characteristic wide band emission, excellent stability and low cost. Further, white light emissive CQDs have been reported for their induced red-shifting emission band and [76–78] offer a promising prospect for high rendering properties. On tuning the CQD emissions, shift in peak maxima towards shorter wavelength and due to aggregation phenomena increase in particle size of CQDs peak maxima tends to move towards long wavelengths, [83] which broadens the emission spectra covering the entire visible region helps to realize the CQDs as LED material. Therefore, to mimic the sunlight and to achieve pure white light emissions by avoiding the drawbacks which includes decrement in color life time and complicated device configurations, researchers show interest in direct white light emitting CQDs materials.

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Recently, many investigated the incorporation of non-RE ions in different materials, in particular fluorides [86, 87]. As Mn4+ activator ion is cheap and having potential luminescence properties owing to their absorption in the UV region and blue region and red emission band in the spectrum [88–90]. But unfortunately, fluoride phosphor materials are reportedly very unstable and moisture sensitive. As a result, many trials have been underway to find the potential host for the Mn4+ . There are limited number of Mn4+ doped oxide phosphors reported up-to date which exhibits high red emissions (Mg14 Ge5 O24 , (K,Rb)2 Ge4 O9 , CaAl12 O19 , Sr2 Al6 O11 ,) [91–97]. Efforts have been undertaken to understand how oxygen vacancies affect the phosphor emission characteristics [98]. Oxygen vacancies, which modify electron interaction and raises donor state valence state by breaking octahedron’s symmetry, introduce local chemical pressure disturbance [99]. These effects indeed exhibits changes in the opto-electronic properties in particular for transition metal phosphors [100, 101]. In addition, the oxygen vacancies created by non-rare-earth ion activation, persistent luminescent phosphors also create electron traps for modifying the emission characteristics [102]. Just like Mn4+ ions, Cr3+ also exhibits d3 configurations. However, for Cr3+ doped aluminate and gallate phosphors, there is no problem regarding the imbalance of charge, because Cr3+ ion is having a similar valence state as Al3+ , hence Cr3+ ions create the considerable environment for improving the luminescent performance effectively. Further, it is very known that, Cr activated Al2 O3 material is commercially used as efficient laser material [103, 104]. In Mg2 SiO4 :Cr3+ , a narrow red emission peak was detected at 674 nm, which was attributed to the Cr3+ ions 2 E − 4 A2 transition Fig. 9 [90]. There have also been several reports on Cr3+ ion doped [105, 106], which is well known for its promising optoelectronic applications. However, Ga2 O3 :Cr3+ studies would be one of interest. The red emission due to Cr3+ doping in Ga2 O3 is attributed to electronic transitions of $of the red luminescence can be observed from the excitation spectrum in the visible and UV regions which consists of mainly three peaks one centered around Fig. 9 Emission spectrum of Mg2 SiO4 : Cr3+ excited at 354 nm (Reprinted from R. Naik et al., Effect of fuel on auto ignition route, photoluminescence and photometric studies of tunable red emitting Mg2 SiO4 : Cr3+ nanophosphors for solid state lighting applications, Journal of Alloys and Compounds, 682, 815–824 Copyright (2016), with permission from Elsevier)

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4.9–5.0 eV , corresponding to the band gap energy of β-Ga2 O3 , and others at 2.88 and 2.08 eV. The luminescence mechanism is explained using the life time decay graph, where a three-level model is illustrated in this study between 4 A2 , 2 E, and 4 T2 of Cr3+ in this Ga2 O3 [106]. Hence, doping with Mn4+ , Cr3+ will be a promising way for improving the red or NIR emissions for LED applications. In addition, T. Li et al. [107] demonstrated luminescence and photocatalytic properties of Ga2 O3 :Cr3+ activated by In3+ doping shows improved persistent luminescence and photocatalytic activity, respectively. The emission and excitation spectra of In3+ doped Ga2 O3 :Cr3+ phosphors show no was emission peak due to In3+ ions. Similarly, Kuang et al. [108] observed the trapping effects in In3+ doped CdSiO3 phosphor from PL and thermoluminescence (TL) spectrum. From the results it is concluded that activation of In3+ into CdSiO3 lattice produces a dense cadmium vacancy trap level, which leads to the origin of long afterglow luminescence. Maximenko et al. reported the blue to red light emission on doping Sn4+ in Ga2 O3 NWs [109]. Farvid et al. prepared alloyed gallium indium oxide nano materials at 250 °C, where In3+ ions occupied octahedral sites in in the γ-Ga2 O3 lattice [110] which are responsible for emission in blue region. The blue photoluminescence from In3+ doped γ-Ga2 O3 is due induced interactions among defect sites such as electron donors and acceptors, which creates the are broad emission tunning in the visible spectral range. Similarly, Esub et al. doped In3+ β-Ga2 O3 phosphors and confirms red-shift in emission peak maximum, for improved blue light intensity. The red-shift emission peaks are due to decrease in the band-gap doped β-Ga2 O3 phosphors Fig. 10. This behavior is due to incorporation of In3+ ions which is assigned to strong Donor– Acceptor Pair transition which favored the increase of Bhor radii and leads to faster

Fig. 10 Emission and excitation of a pure β-Ga2 O3 and b In3 + : β-Ga2 O3 nanoparticles (Reprinted by permission from Springer, Journal of Materials Science: Materials in Electronics Optical insights of indium-doped β–Ga2 O3 nanoparticles and its luminescence mechanism Esub Basha Shaik, et al. 31, 6185–6191 Copyright (2020))

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transition. Hence, the authors also performed DFT studies to confirm the shift in the emissions peaks and these results are promising to breakthrough in cost-effective LEDs and display devices [111]. A Considerable works have undergone on luminescent properties of Bi3+ in various host lattice. The luminescence properties of the Bi3+ ions come under s2 electronic configuration are attributed to corresponding 1 S0 –3P0,1 transition which arises from the S–P inter-configurational transition [112–114]. In addition, the Bi3+ luminescence properties greatly depend on its local environment conditions in host lattice because it has s2 outermost electron configuration. From the photoluminescence (PL) spectra of the Bi3+ doped Zn2 SiO4 phosphor shows an interesting photoluminescence property where the emission can be tuned as a function of excitation wavelength. The blue-yellow light of Bi3+ ions are quite diverse in Zn2 SiO4 phosphor. On exciting with 336 nm CaSb2 O6 :Bi3+ shows a high blue intensity emission at peak at 437 nm. Further, a great role is observed due to the charge compensation in performance of luminescence properties for CaSb2 O6 :0.75Bi3+ ,0.75 Na+ . The results indicates that the emission properties of oxide-based phosphors materials can be modified their structures accordingly by co-activation with aliovalent ions for desired luminescence properties (Fig. 11). Further, Bi3+ activated phosphors such as Ca12 Al14 O32 Cl2 :Bi3+ , Ca3 Al2 O6 :Bi3+ , ScVO4 :Bi3+ were as also showing the potential results for lighting and display applications [115–119] Since the sensitivity behavior of Bi3+ ions in its local environment, luminescence phenomena will be varying with different crystallographic structures, which makes it convenient or producing the high intensity emissions [119–122]. Besides activating with cations will also control the environment around Bi3+ ion. Recently, H. Zhuwe et al. prepared K2 MgGeO4 :Bi3+ phosphor by incorporating alkali metal A+ (A = Li, Na, Rb) to replace the K+ in host lattice. The modifications in emission and excited patterns at different wavelengths is due to creation of luminescent centers and occupancy of Bi3+ ions in K2 MgGeO4 :Bi3+ under the effect of Rb+ , Li+ and Na+ activation. Fig. 11 Emission spectra of CaSb2 O6 :0.75Bi and CaSb2 O6 :0.75 Bi, 0.75 M (M¼Li, Na, K) (Reprinted from S. Yao et al., Enhanced luminescence of CaSb2 O6 :Bi3+ blue phosphors by efficient charge compensation, Journal of Materials Science in Semiconductor Processing, 41, 265–269 Copyright (2016), with permission from Elsevier)

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Cation incorporation shows an effective strategy in controlling the local environment around Bi-site and leads tuning of the spectrum. In this study, inclusion of cation in K2 MgGeO4 :Bi3+ metal A+ (A = Li, Na, Rb) to replace K+ exhibits entirely different emission on exciting with various wavelengths which leads to the extra luminescent centers creation and alteration Bi3+ ion occupancy K2 MgGeO4 :Bi3+ under the incorporation of Li+ and Na+ , which also produces the expansion of full widths at half-maximum (FWHM) to 204 nm emitting bright white emission. Development of such kind of phosphors will emphasis the research on alkali metal incorporation with great significance in controlling the emission properties and enhancement in thermal stability of luminescent materials [123]. Table: Some emerging phosphor materials for display device applications Dopant

Compound

Luminous efficiency (lm W−1 ) References

Mn4+

K2 GeF6

125

[124]

K2 SiF6

116

[125]

K2 (Si,Ge)F6

145

[126]

Mg14 Ge5 O24

109.42

[91]

(K,Rb)2 Ge4 O9

–

[92]

CaAl12 O19

–

[93]

Sr2 Al6 O11

–

[94]

Li3 Mg2 SbO6

87

[127]

Zn2 SiO4

70

[128]

SrAl2 Si2 O8

45

[129]

LiGd9 (SiO4)6 O2

65

[130]

CaZnOS

–

[131]

Single compound CdSe

41

[132]

Single compound Sr2 CeO4

–

[73]

Single compound InP/ZnS

16.7

[133]

(Gd2.97 Ce0.03 )Mg2 Ge2 AlO12 71.6

[134]

ZnGa2 O4

–

[222]

ZnGa2 O4

–

[135]

Ga2 O3

–

[111]

K2 MgGeO4

–

[123]

CaSb2 O6

–

[115]

ScVO4

–

[119]

BaGa2 O4

–

[136]

Mn2+

Ge4 + In3+ Bi3+

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6 Field Emission Display Device (FED) The device configuration of FED is comparable to a cathode ray tube (CRT). There is no need for components such as a colour filter or polarizer in this sort of device construction. Further they feature a faster reaction time and a wider viewing angle than LCDs as well as a superior temperature stability [137, 138]. In such type of display device electron beam is used to activate the phosphor for generating the light in Fig. 12. There is a distinctive aspect to a FED’s construction, it is composed of subpixels (the phosphor coating on the array) and edge-emitters. The intensity of each array’s emission is operated by voltage application and in that array the phosphor is utilized. Since the emitters generate electrons when exposed to a high electric field, this produces a visible picture on the faceplate’s phosphor coating. FED Device can classify into two categories based on voltage application: low voltage (1 kV) and high voltage (3–8 kV). For safety reasons, low-voltage applications are preferred. There’s also a conductive layer coating necessary for low-voltage operation since the device has high current densities, which leads to rapid detoration via outgassing. While phosphor deterioration is slower at high voltage than at low voltage, breakdown difficulties and poor resolution are unavoidable. For improving the FED technology important components like electron emitter, vacuum packing, phosphor material and driver circuits is very much mandatory. Among these components, it is very critical and need of the hour for developing a novel type of phosphor material which have great brightness, high luminous efficiency, high color purity, superior thermal stability, and endurance at low voltage operation (1 kV) with higher current densities. Therefore, apart from RE doped phosphors the need for design and development of a suitable non-RE activated phosphor is also vital [139, 140].

Fig. 12 Cross-section of a diagram of a field emission display device [15]

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6.1 Non-RE Activated Phosphors for Field Emission Display Device Generally, at low voltage operation, sulfide-based phosphor materials were utilized like CuS, ZnS: Cu, Al, SrGa2 S4 [141–143]. Sulfide-based phosphors, on the other hand, have a high brightness and efficiency; nevertheless, they have drawbacks such as decomposition and the release of toxic sulphur fumes when impinged with an electron beam, which reduces their life lifetime. Because of these drawbacks, researchers and industry professionals have concentrated their efforts on oxygen-dominant phosphors for efficient FEDs. ZnO nanostructures are used in various optoelectronic devices, including as ultraviolet UV and FED devices, because of their outstanding optical and electrical characteristics [144, 145]. Zinc oxide (ZnO) has great characteristics that also make it useful in photonic devices, and it is easy to prepare nanostructures. Since its inherent and extrinsic radiation defect levels emit visible light, it has caught the interest of numerous researchers. As a result of ZnO’s nano-scale structure, the material’s field enhancement factor and emission characteristics were increased. The current density increases as a result of the increased number of field emitters. Materials with low voltages and high current densities are crucial for display devices. As a result, the number of pixels on a display device increases while the power usage decreases. As a result, researchers made many efforts by varying the morphology and structural characteristics of ZnO which is excellent for display device applications such as FEDs [146–148]. Recently, Anshika Nagar [149] developed Zinc Oxide nanoflowers and observed ZnO exhibited high current density with low turn on field and which is suitable for FED device applications. Furthermore, rare-earth sources, which have recently been in short supply, are in increasingly scarce. As a result, employing bismuth as the activator in host lattice has gained popularity in developing phosphors for FED devices [150, 151]. The outmost 6s and 6p electrons of the bismuth ion allow for regulated emission, and the parity-allowed transitions of the bismuth ion result in excellent light output efficiency [152, 153]. Thus, more Bi3+ incorporated phosphors have been explored such as Ba2 ZnGe2 O7 : Bi3+ , Ca2 MgWO6 : Bi3+ , Ca5 (BO3 )3 F: Bi3+ , and for field emission display devices [154–157]. Nevertheless recently, Shanshan Ye et al. developed Bi3+ -activated Ca3 Lu2 Ge3 O12 show green emission, comparable to commercial green phosphor (Fig. 13). β-sialon: Eu2+ , to imply Ca3 Lu2 Ge3 O12 : Bi3+ can be well suited for cathodoluminescence properties [158]. The emission mechanism of Ca3 Lu2 Ge3 O12 : Bi3+ were studied systematically, where the ground-state electrons can be excited to the 1 P1 and 3 P1 energy levels by NUV light, and the majority electrons positioned at in the 1 P1 level will jump into the 3 P1 level via the spin–lattice relaxation, which confirms from weak excitation band centered at 300 nm. Whereas on comparing with commercial Eu2+ or Ce3+ activators, the PL properties of Bi3+ -doping provides a highly compact and symmetrical site occupation. This kind of local environment changes the electron − phonon coupling and constraint the expansion of excited level, which leads to the narrowband emissions. Due low work function and affinity (2.7–3.3 eV) and high chemical

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Fig. 13 Emission and excitation spectra of Ca3 Lu2 Ge3 O12 : Bi3+ along with the cyan-emitting sample of Ca3 Lu2 Ge3 O12 : Bi3+ (Reprinted with permission from Shanshan Ye et al., Design of a Bismuth-Activated Narrow-Band Cyan Phosphor for Use in White Light Emitting Diodes and Field Emission Displays, ACS Sustainable Chem. Eng., 8, 49, 18187–18195 Copyright (2020) American Chemical Society)

and physical stability, GaN is a promising material for optoelectronic devices, such as LEDs, laser diodes, and FEDs. The morphologies of prepared GaN material in recent years have shown excellent field emission properties. On activating with In, Ge, Si, Mg, Al, Zn, Mn and Fe ions also enhancing the electrical, and optical, of GaN [159–162] which made this material to be promising candidate. Recently Enling Li reported [163] Ge-doped GaN nanorods which demonstrates excellent emission property at low electric field 2.93 V/μm, which suggests Ge incorporation in GaN could be well possible for FED applications. The PL spectrum of the Ge-doped GaN nanorods with excitation wavelength of 325 nm exhibits an Fig. 14 FE J–E curve of the large-area patterned defect-rich WO3 NWs film (Reprinted with permission from Zufang Lin et al.., Defect-Enhanced Field Emission from WO3 Nanowires for Flat-Panel X-ray Sources, ACS Appl. Nano Mater. 2, 8, 5206–5213 Copyright (2019) American Chemical Society)

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emission peak at 379 nm which assigned to band edge emission of the wide band gap of GaN. Further a red shift of 14 nm when compared with the Bohr radius of GaN excitons (11 nm) is larger, which confirms no blue shift occurs as due to effect quantum confinement effect. Two emission peaks at 417 and 437 nm are due to the Ge presence energy gap of the Ge doped GaN nanorods. So, mechanism of the Ge-doped GaN nanorods confirms that it can used for photoelectric devices. Zinc germanium nitride (ZnGeN2 ) material is also having similar structural properties when compared with GaN. Many experimental studies have undergone on its preparation and characterization for its luminescence properties [164–167]. However, the PL and CL properties of ZnGeN2 and ZnGeN2 :Mn2+ phosphors exhibit high intensity yellow-orange and orange-red emissions. Especially, orange-red light emitting phosphor ZnGeN2 :Mn2+ shows a color purity of 93% which confirms ZnGeN2 :Mn2+ as a promising nitride-based phosphor, FED device with brilliant CRI properties and a high color gamut. Comparing germanates to sulphur dioxides for FEDs, germanates are more thermally stable and eco-friendly materials. Aside from that, germanates have prospective uses in photodetectors, electroluminescence and LED fields, among others. A major role has been played in current general illumination and display device applications by the transition metal Mn2+. Mn2+ doped Germanates such as Li2 ZnGeO4, (Zn, Mg)2 GeO4 exhibits a green emission which is much higher than its commercial counter-part green phosphor ZnO: Zn. Further, the CL properties of germanates based phosphor greatly depends on applied accelerating voltage, filament current, and life time decay pattern with CL emission intensity on electron collision. The germanates phosphor material CIE coordinate also demonstrates the color purity and stability of green intensity. Therefore, germanates are used for in full-color field-emission displays. Similarly, cost effective ZnAl2 O4 :Mn2+ green phosphor developed by X. Wang et al. Under the low-voltage excitation Mn2+ -doped ZnAl2 O4 showed both 508 and 517 nm bands with high voltage brightness saturation and great colour purity, due to 4 T1 (4 G) → 6 A1 (6 S) transitions of Mn2+ on octahedral and tetrahedral sites [168]. Zufang Lin et al. [169] achieved high field emission current from defect-rich WO3 NWs. The high conductivity of the NWs can be explained from the observed high emission current. The maximum emission current is determined by the thermal equilibrium process. Because electron emission causes less Joule heating, more conductive emitters may provide larger maximum emission currents. However, imperfections in semiconductor NWs will affect the electron transport process, affecting the temperature change in conductivity. The transport mechanism then influences the supply of emission electrons and the emission current. The authors explained high emission of current (Fig. 14) was caused by more defect concentration in the NWs, which also further confirmed from the theoretical studies. As a promising large area field emitter for flat panel display device the performance of the device was also examined.

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7 Laser-Powered Phosphor Display The worldwide Laser Powered Phosphor Display (LPD) Technology market was valued at 2586.21 million USD in 2020, and it is anticipated to grow at a 2.48% CAGR from 2020 to 2027. The primary excitation source for the Laser Powered Phosphor Display is a blue laser diode. Its components include laser diodes, mirrors, and a phosphor screen. Diodes provide a laser signal to mirrors, which project lasers onto an RGB phosphor screen to produce an image sequence that may be viewed on a monitor or television. Furthermore, the device makes use out of movable mirrors to guide laser light onto a screen coated in RGB phosphor stripes. As a result, when the laser is projected onto the screen, it undergoes optical modulation, and the phosphor stripes produce a colour image on the screen. LPD was invented and patented by Prysm, Inc. [170] Fig. 15. LPD works in the same way as a laser printer, except it refreshes much quicker. LPD use lasers rather than electron guns, and laser phosphor displays are equivalent to FEDs. Unlike other display technologies, an LPD does not suffer from screen burns. This is mostly due to the fact that laser phosphor display technology provides great image quality as well as wide viewing angles. Furthermore, phosphor pixels in LPD do not fail mechanically or electrically.A laser diode is used as a source to activate the phosphor coating, which produces colour pictures in LPD technology. The laser display system is made up of a phosphor-coated layer on a substrate. The phosphor layers are placed parallely as strips, resulting in RGB phosphor stripes: each phosphor absorbs light at a certain wavelength and produces its associated colour.

7.1 Non-RE Activated Phosphors for Laser-Powered Display Devices To obtain full colour LPDs, it is necessary to develop phosphors with higher colour purity than currently passing RGB phosphors, as well as to design and develop

Fig. 15 Simplified diagram of an LPD screen’s workings

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novel phosphors with CIE chromaticity coordinates that are not contained within the triangle region encircled by RGB chromaticity coordinate points. Sulphide based ZnS, CaS and CdS phosphors have been emplyoed as phosphor screens in many display devices during the past decade as they exhibit unique physical properties and luminescence properties [171, 172]. Sulphides based phosphors as host materials doped with Ag+ , Cu2+ , Mn2+ displays higher efficiency and light output. Since, for better resolution images the phosphor particles morphology should be smaller sizes and which greatly depends on the synthesis procedure [173, 174]. Recently, Toshihiro Nakamura, demonstrated a procedure for developing K2 SiF6 :Mn4+ phosphor particles by laser irradiation in aqueaous solution. After pulse irradiation, the distribution of particle sizes reduced to micro regime approximately 2 m, this shift in the size is due to his difference in absorbance. The micronized particles shows red emissions with quantum efficiency Therefore, such type of microsized articles exhibits uniform emission and expected to show more uniform angular emissions than conventional sulphide based phosphors which are suitable for Laser- powered phosphor displays [175]. T. P. Tang et al. [176] developed pure ZnSe phosphor emitting green, to orange, and red light as a function of temperature. The reason behind this tunning of emission color is due to annealing the samples in presence of temperature where the oxygen atoms will occupy the defects inside the increasing ZnSe lattice, which develops the lattice distortion, and improves in luminescence properties. I. M. Yirtici reported, PL intensity of Cu doped ZnS0.94 Se0.06 nanoalloys. The emission spectra demonstrated red shift as shown in Fig. 16. The excitonic emission observed at 388 nm in the spectra was reduced due to incorporation of Cu impurity, due to, photon created excitons are can be trapped by Cu ions in the lattice. Further on doping with Mn2+ in the lattice it also exhibits excellent green color which is also promising material for LPD and FEDs [177]. In generally the sharp emission bands of Mn4+ ions are attributed due to d-d transition with d5 configuration. The doping of Mn4+ in Mg4 GeO5.5 : Mn4+ shows red emission due to 2 E − 4 A2 transition which is relatively very sharp bands when Fig. 16 Emission spectra of KTF phosphors modified with silane (0.5%) (Reprinted with permission from Y. Y. Zhou et al., Waterproof Narrow-Band Fluoride Red Phosphor K2 TiF6 :Mn4+ via Facile Superhydrophobic Surface Modification, ACS Appl. Mater. Interfaces, 10, 1, 880–889 Copyright (2018) American Chemical Society)

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compared with other non-RE ions. This phosphor is used as mainly in phosphor strips LPD device. Currently, phosphors like β-SiAlON and K2 SiF6 :Mn4+ are been commercially using for LPD and LED devices, especially, K2 SiF6 :Mn4+ red phosphor exhibiting color gamut’s333 to 85–127% as per NTSC. As a result, researchers are looking for new narrow-band red-emitting phosphors with rapid LPD responses. Now, recently novel Rb2 MoO2 F4 :Mn4+ (RMOF) phosphor developed by Y. Zhou et al. with no phonon emission line, via incorporation of [MnF6 ]2− into [MoO2 F4 ]2− lattice, which also further confirmed by energy to balance between Mn4+ and Mo6+ ions. Such [MnF6 ]2− incorporation plays a vital role and the significant effect of Mn4+ in oxyfluoride based phosphors for fast-response in displays [178]. As Mn4+ ions activated fluoride based red light phosphors having unique blue light absorption property shows significant interest by many researchers for LPD and back lit applications. But it is inevitable that such kind of phosphors having moisture sensitivity and hygroscopic issues. Similarly, Yayun Zhou group reported K2 TiF6 :Mn4+ red emitting phosphor (Fig. 16) by modifying surfaces with Octadecyltrimethoxysilane (ODTMS) on K2 TiF6 :Mn4+ surfaces. It is demonstrated that luminous efficiency (LE) improved from 83.9 to 84.3%. Further, the prepared phosphor also demonstrates excellent low color temperature, higher color rendering index and high luminous efficiency indicate improving moisture resistance, thereby acting as suitable phosphor for potential display devices [179]. Similarly for phosphors emitting green color, Mn2+ -doped germanates systems such as Mg2 SnO4 , Zn2 GeO4 , Mg2 GeO4 and Li2 ZnGeO4 lattice are suitable for luminescence properties for use of laser driven and electron excitation based display devices [180–182]. Since, Mn2+ ion having green emission corresponding to 4 T1 − 6 A1 transition [183]. Because of weak crystal field surroundings, Mn2+ doped materials exhibits excellent green light from 500 to 530 nm with a small band width and also suitable in color purity when compared with the commercial green ZnO:Zn phosphors. By determining new preparatory procedures for increasing efficiency of phosphors and more basic research on luminescence properties are need of the hour, so there’s still plenty of room for future development.

8 Inorganic Electroluminescent (IEL) Display For many years, researchers have been studying inorganic electroluminescent (IEL) devices because of their remarkable properties (such as maximum luminance, maximum efficiency, and luminescence color) and market importance for display technology. When phosphors are exposed to alternating current (AC) voltage, they produce light. Electrodes, phosphor (emissive layer), and one or more dielectric layers make up an inorganic electroluminescent (IEL) device. Here, IEL are classified mainly into two types: AC-driven thin-film electroluminescent (AC-TFEL), alternating current thick dielectric electroluminescents (AC-TDEL). A IEL display device light output is totally dependent on high energy electron collisions with luminescent centres in the emitting layer or on electron–hole recombination depending on

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Fig. 17 Schematic diagrams of example IEL structures: a TFEL; b TDEL

the device’s configuration when AC voltage is applied. There are greater advantages to AC-driven IEL devices (Fig. 17) than DC-driven EL devices. We can increase efficiency and longevity by changing the applied electric field polarity often. Short circuits in the device can be prevented by using dielectric as an insulating material [184]. IEL devices may be easily integrated with AC power sources of 100/400 V at 50-10 kHz because to their unique characteristics (depends on type of material). In addition to being energy efficient and cost-effective, IEL devices offer a quick response time and are sustainable in harsh weather situations. There are two dielectric layers on the device, one on top and one on bottom, and they will evenly distribute the electric field among the phosphor particles in the emission layer. EL devices with thick and thin phosphor layers are technologically different. These changes in the thickness of the phosphor layer i.e. lower electric fields is required for thick EL devices when compared with thin-film EL devices. IEL device intensity–voltage characteristics will be affected by changes on electric field magnitudes during device operation. However, an increase in the voltage of a thin-film device will result in an increase in intensity, whereas the intensity–voltage curve for thick-film devices displays an exponential increase in intensity with respect to voltage. Due to the following processes, these differences can be explained as follows: on AC excitation electrons will pass through to phosphor layer from electrodes via dielectric layers. This will cause transitions between the excited energy levels in these centres, which will result in EL emission. The excited energy levels of the luminescent centre will relax to the ground level and EL emission will result. Figure 18 illustrates the hot electron impact excitation process responsible for EL emissions [184]. In addition to impact excitation, field-induced ionisation is responsible for EL emission. This phenomenon is well understood for alkaline-earth sulfides, where the incorporated elements get ionised on application of field. Due to the accelerated hot electrons, the dopants (cathode) are ionised and pushed towards the anode when the electric field is activated. Diffusion occurs when the applied field is relaxed. This

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Fig. 18 Scheme of the hot-electron impact excitation principle

allows electrons to combine with the ionised dopant activators to produce emission. Along with impact excitation, energy will be transfer from the host lattice to activator ions is expected for thick electroluminescent devices; that is to say that initially the host lattice is excited, and then the energy will be transfering to host lattice to activator ions occurs (Fig. 18).

8.1 Inorganic Electroluminescent Non-RE Activated Phosphor Materials In recent years, usage for AC Inorganic electroluminescent devices in recent reduced due to the advent of other light sources with greater brightness, power consumption, and lifespan. When compared to other light sources ACIEL are easy to fabricate, low cost, flexible, and produce cool light. Many applications have shown their potentiality such as usage on badges on t-shirts as promotional items, backlights panels for instruments and for military purposes such as mobile landing lights or low-light cockpit lighting. The materials which satisfy the previously mentioned properties are mainly large bandgap compounds such as, ZnS, CuS CdSe, CaS, SrS and some oxides ZnGa2 O4 , Zn2 SiO4 . Zinc sulfide (ZnS) belongs o II–VI group semi-conductors which had been explored for electro-luminescence (EL) applications [185]. Multicolor emission from ZnS phosphors is been developed by modifying the size of the ZnS particles and incorporating suitable dopant ions. A number of ions such as Mn2+ , Ag+ , Cu2+ or Cu+ , were used for doping in ZnS host [186–190]. Furthers many researchers also explored the incorporation in the ZnS phosphor in various kinds of polymer matrices for developing EL devices [191, 192]. Sunghoon Lee et al. [193] prepared Through

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powder processing, a tri-functional device based on a ZnS phosphor layer and PVDF matrix as a dielectric. On application of external voltage, the gadget produced light from the phosphor and sound from the piezoelectric sheet, and the mechanical energy is transformed into EL light. Similarly, Li Wen et al. [194] demonstrated an electroluminescence emission by employing tetrapod-like ZnO nano structures into bottom electrodes instead of phosphor layer in the device. This process results in emitting color like blue, green and orange with high brightness which is due the combine effect of T-ZnOw and ZnS phosphor which blended in polydimethylsiloxane (PDMS) layer. A. Yakoh group [195] developed a first of its kind device using ZnS based electroluminescent display by introducing nanocomposite between graphene oxide and nafion deposited on phosphor layer of EL display for sensing applications. Besides, Banseok You et al. [196] fabricated water proof stretchable electroluminescence device by using ZnS phosphor coupled with PUU is printed onto the AgNWs/hydroxylated PDMS substrate and combined with suitable electrode. Due to excellent adherence between PUU and PDMS materices in water is the reason for waterproof properties of the EL devices (Fig. 19). In another work, Chen et al. demonstrated of a large-area EL device with a using based ZnS phosphor [197] which exhibits both white and blue emission from both sides the phosphors utilized. Jeong et al. developed EL device by introducing quantum dots with ZnS based phosphor/PDMS for achieving cool white light. Jeong et al. and Satya Kamal et al. fabricated AC-driven EL device using screen printing technology of using ZnS: Cu phosphors whose emission intensity is due to the recombination emission theory [198, 199]. Further, Yellow phosphor ZnS:Mn is

Fig. 19 Luminance of a fabricated EL device measured during immersion in water (Reprinted with permission from B. You et al., Highly Stretchable and Waterproof Electroluminescence Device Based on Superstable Stretchable Transparent Electrode, ACS Appl. Mater. Interfaces 2017, 9, 6, 5486–5494)

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Fig. 20 An AC voltage of 600 V was applied at a fixed frequency of 10 kHz across the terminals for recording electro-luminescence spectra (Reprinted from Ch. Satya Kamal et al. Influence of Ge4+ doping on photo- and electroluminescence properties of ZnGa2 O4 , Journal of Alloys and Compounds, 852, 156967 Copyright (2021), with permission from Elsevier)

also widely investigated and employed phosphor in EL devices back panels. However, SrS:Ag, Cu phosphors also demonstrated blue emission for potential EL applications [200–203]. The cadmium selenidebased semiconductor is also significant material for fabrication of opto-electronics devices like light emitting devices [204–208], flat panel displays etc. because of is properties like less weight, cost effective in processing, low voltage application, high intensity and efficiency, fast reaction time, and compatibility to flexible substrates [209–214]. Sarita Kumari et al. [215] prepared CdSe nanocrystals with hexagonal phase and demonstrated PL and EL emission peak at 498 nm which is due to surface/defect states of the lattice. Michael A. Schreuder et al. fabricated WLEDS with ultrasmall CdSe nanocrystals, which exhibits electroluminescence from CdSe nanocrystals which exhibits excellent color properties (A, B). Generally, the sulfide-based phosphors are commercialized for EL devices which have moisture issues and low lifetime when compared with fluorescent lamp with long lifetime [216, 217]. Thus, oxide based phosphors are introduced to address these drawbacks. Phosphors like ZnGa2 O4 , Zn2 SiO4 activated with Mn2+ ions emit green color with the highest efficiency of 0.8 lm/W [218]. In addition, emission color was modified by replacing Ge ions for Si ions in Zn2 (Si,Ge)O4 :Mn2+ EL device [219]. Further, it is known that on excitation with low voltage electrons, ZnGa2 O4 emits blue light, this due to GaO6 by the lattice [220, 221]. By incorporation Ge4+ in ZnGa2 O4 and excitation by electrical method are resulted in tuning of blue emission from Ge4+ activated ZnGa2 O4 as shown in Fig. 20 [222].

9 Concluding Remarks RE free and self-activated light emitting materials are the challenging sources for opto-electronic devices will dominate the actual market of light emitting for indoor and outdoor applications. In the past decade RE-based phosphors were widely used

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for generation of light, including individual color mixing, color conversion, and direct white-light generation. Display devices RE free and self-illuminating phosphor materials have the advantage of cost effective and easy fabrication, but more concrete efforts are required to resolve the problems in achieving high rendering properties compared with RE-based phosphors. Therefore, despite of great developments for quality light emitting materials, the primary focus for the future development of RE free and self-illuminating phosphor materials will be new out comers with increase in efficiency and reducing the cost for large-area lighting production. To begin we have mentioned and discussed: the present scenario and material requirements. Then, we have explained about the emission mechanism and spectroscopic properties of non-RE metal ions followed by the recent developments of emission from RE free materials covering red, green, blue regions which are reported in the literature upon excitation with UV or laser light. Therefore, developments of RE free materials should be a primary goal due to their enhanced performance features in display devices.

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