Advanced Materials for Solid State Lighting (Progress in Optical Science and Photonics, 25) [1st ed. 2023] 981994144X, 9789819941445

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Progress in Optical Science and Photonics

Vijay Kumar Vishal Sharma Hendrik C. Swart   Editors

Advanced Materials for Solid State Lighting

Progress in Optical Science and Photonics Volume 25

Series Editors Javid Atai, Sydney, NSW, Australia Rongguang Liang, College of Optical Sciences, University of Arizona, Tucson, AZ, USA U. S. Dinish, Institute of Materials Research and Engineering (IMRE), A*STAR, Singapore, Singapore

Indexed by Scopus The purpose of the series Progress in Optical Science and Photonics is to provide a forum to disseminate the latest research findings in various areas of Optics and its applications. The intended audience are physicists, electrical and electronic engineers, applied mathematicians, biomedical engineers, and advanced graduate students.

Vijay Kumar · Vishal Sharma · Hendrik C. Swart Editors

Advanced Materials for Solid State Lighting

Editors Vijay Kumar Department of Physics National Institue of Technology Srinagar Hazratbal, Jammu and Kashmir, India

Vishal Sharma Institute of Forensic Science and Criminology Panjab University Chandigarh, Chandigarh, India

Hendrik C. Swart Department of Physics University of the Free State Bloemfontein, South Africa

ISSN 2363-5096 ISSN 2363-510X (electronic) Progress in Optical Science and Photonics ISBN 978-981-99-4144-5 ISBN 978-981-99-4145-2 (eBook) https://doi.org/10.1007/978-981-99-4145-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 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

Rare-Earth Doped Inorganic Materials for Light-Emitting Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irfan Ayoub, Rishabh Sehgal, Vishal Sharma, Rakesh Sehgal, Hendrik C. Swart, and Vijay Kumar

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Charge Transfer in Rare-Earth-Doped Inorganic Materials . . . . . . . . . . . Amol Nande, Swati Raut, and S. J. Dhoble

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ZnO-Based Phosphors Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. J. Mofokeng, T. P. Mokoena, L. L. Noto, T. A. Nhlapo, M. J. Sithole, D. E. Motaung, and M. R. Mhlongo

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Dynamics of Perovskite Titanite Luminescent Materials . . . . . . . . . . . . . . . S. J. Mofokeng, L. L. Noto, T. P. Mokoena, T. A. Nhlapo, M. J. Sithole, M. W. Maswanganye, and M. S. Dhlamini

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Rare-Earth-Doped Ternary Oxide Materials for Down-Conversion and Upconversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Irfan Ayoub, Rishabh Sehgal, Vishal Sharma, Rakesh Sehgal, Hendrik C. Swart, and Vijay Kumar Luminescence Properties of Rare-Earth-Doped CaO Phosphors . . . . . . . 149 Umer Mushtaq, Irfan Ayoub, Nisar Hussain, Vishal Sharma, Hendrik C. Swart, and Vijay Kumar SnO2 Based Phosphors Materials: Synthesis, Characterization, and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Radha Verma, Sahil Goel, Krishan Kant, Rajesh Kumar, and Rashi Gupta Red Emitting Phosphors for Display and Lighting Applications . . . . . . . . 199 Athira K. V. Raj and P. Prabhakar Rao Spectroscopic Studies of Rare-Earth-Doped Glasses for LED Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 Asmahani Awang, S. K. Ghoshal, and Alireza Samavati v

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Quantum Dots and Nanoparticles in Light-Emitting Diodes and Displays Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Vishnu Chauhan, Yogendra Kumar, Deepika Gupta, Anita Sharma, Deepika, Sonica Upadhyay, and Rajesh Kumar Organic Material-Based Phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Umer Mushtaq, Rishabh Sehgal, Vishal Sharma, Rakesh Sehgal, Hendrik C. Swart, and Vijay Kumar Recent Advances in Long-Persistent Luminescence in Rare-Earth-Doped Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Meram S. Abdelrahman, Hend Ahmed, and Tawfik A. Khattab Optical and Luminescent Properties of Lanthanide-Doped Strontium Aluminates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Meram S. Abdelrahman, Hend Ahmed, and Tawfik A. Khattab Potential Use of Photo-Excited Phosphors in Energy-Efficient Plant Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 T. Krishnapriya, Adon Jose, and P. R. Biju Luminescence Characteristics and Energy Transfer Dynamics of Rare-Earth Ion Co-activated Borosilicate Glasses for Solid-State Lighting Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Adon Jose, T. Krishnapriya, and P. R. Biju

Rare-Earth Doped Inorganic Materials for Light-Emitting Applications Irfan Ayoub, Rishabh Sehgal, Vishal Sharma, Rakesh Sehgal, Hendrik C. Swart, and Vijay Kumar

1 Introduction Incandescent and fluorescent sources of illumination, which rely on the phenomena of incandescence or gas discharge, were some of the earlier traditional sources used for lighting purposes. But these sources are found to have significant energy losses as a result of the involvement of different processes such as high temperatures and large Stokes shifts. Keeping in mind significant energy losses, the scientific community provided an alternative in the form of a light-emitting diode (LED) [1]. LED technology has advanced to the point where it clearly outperforms previous technology in every way. The LEDs possess several advantages over the previous ones, not only in terms of brightness but also with respect to their compact size, lifetime, mechanical stability, and extremely high luminescence efficiency that approaches approximately I. Ayoub · V. Kumar (B) Department of Physics, National Institute of Technology Srinagar, Hazratbal, Jammu and Kashmir 190006, India e-mail: [email protected] R. Sehgal Department of Electrical and Computer Engineering, University of Texas at Austin, Austin, TX 78751, US V. Sharma Institute of Forensic Science and Criminology, Panjab University, Chandigarh 160014, India e-mail: [email protected] R. Sehgal Department of Mechanical Engineering, National Institute of Technology, Hamirpur, Himachal Pradesh 177005, India H. C. Swart · V. Kumar Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein ZA9300, South Africa e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_1

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theoretical limits. It is well known that lighting accounts for about 20% of the worldwide energy use; in this regard, the LED technology proves to be very efficient and effective [2]. The replaced technology, “LED”, also helps in decreasing environmental pollution by decreasing the emission of CO2 and other toxic gases in the environment. They also do not possess any mercury content and thereby have negligible health effects due to their low UV radiation [3]. All the mentioned benefits have diverted the attention of the scientific community towards this evolving technology, which results in accelerating the development of LEDs for illumination. The evolution of LEDs can be broadly categorized into three generations, each marked by remarkable breakthroughs in manufacturing technique and equipment, the emergence of new phosphor materials, and heat-dissipation packaging solutions. With the growing progress in solid-state lighting technology, LEDs have become brighter over time. In addition to this, the color variation has also become more adaptable along with the improvement in the efficiency and effectiveness of the light. The first commercially available LED was fabricated in the 1960s. This first generation of LEDs remained in use for 20 years up until the 1980s, during which they mainly remained applicable for machinery indicators and for alpha-numeric displays. In the 1980s, Fairchild produced the first commercially effective LED with high brightness. These LEDs became very popular as second-generation illumination sources, which lasted from the 1990s until 2014. This second generation of LEDs found applications in motion displays, LED flashers, mobile phones, automotive LED lighting, and architectural illumination. The research continued for further achievements and finally led to the arrival of the third generation of LEDs, for which the modern technological world had been waiting for a long time. These devices have been developed with the aim of saving the world from energy catastrophes and reducing pollution in the environment. In addition to solving the purpose of general illumination, the third generation of LED devices has marked its presence in different fields of science and technology, such as lighting communication, medical and environmental fields, sensor technology, purifiers, etc. [4, 5]. From the theoretical point of view, Moore’s law estimates that the count of the “Si” transistors on a chip gets doubled within every 18–24 months. In a similar fashion, the luminous flux of the LEDs probably follows Haitz’s law. This law asserts that it has been observed for nearly 30 years that the luminous flux for each package doubles within 18–24 months. This paradigm in the technological improvement of LEDs relies on industry-driven R&D activities aimed at high efficiency along with cost-effective solutions that substantially provide an energy-saving competitor for the current LED applications [6]. These solid-state lighting sources (LEDs) arise in different multidimensional arrays, starting from the zero-dimensional (0D) to the three-dimensional (3D). LEDs have a high energy efficiency, resulting in lower power consumption with low voltage and current (700 mA) for operation. When compared to traditional available lighting sources, these devices have a life span of approximately 50,000 h and superior thermal control. The LEDs offer excellent characteristics such as an ultra-fast reaction time, a wider range of programmable color temperatures between 4500 and 12,000 K, a variable working temperature range from 20 to 85 °C, and the fact that they do not possess any type of low temperature starting issues. The emission ranges of the LEDs remain limited

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to a single color of the spectral band, such as red, yellow, green, or blue. However, due to the varied distribution of luminous intensity, spectrums, and shades, various efforts for enlarging the emission spectral band by generating white light rely on color mixing and architectural design. The use of blue LEDs with phosphor is the modern trend in LEDs for producing white light. White light is a synthesis of all visible wavelengths. Besides the presence of the blue color, it also contains green, yellow, and red components [4]. By studying the different properties and the related effects, it has been observed that the optical, electrical, and mechanical characteristics of the different materials at the nanoregime altogether depart from their comparable bulk counterparts. The optical absorption spectra of nanocrystalline materials provide a simple method for determining the quantum size impacts. The process of excitation of the electron from the valance band to the conduction band because of photon absorption is strongly correlated to their bandgap energy. The spectroscopic research is intimately tied to the different luminescence pathways and can modify the adjustable changes in the size, content, and surface of these nanomaterials. Since the luminescence centers that possess broadband emissions always remain sensitive to their coordinating environment, the different and varied emissions caused by the different activator sites within the host crystal might substantially alter the type of emission [7]. The different changes observed in the energy bandgap strongly depend on the essence and concentration of the transition metal ions. Improvements in internal quantum efficiency can be achieved in a variety of ways, including sensitization due to host lattice and co-dopant, reduction in surface defects, electrical defects, and so on [8]. The surface modification of the nanoparticles (NPs) plays a vital role in the inhibition of the defect sites, which results in increasing the photoluminescence quantum yield (PLQY). The phenomenon of the luminescence revealed by the different nanomaterials is explained by the radiative recombination of the delocalized conduction band electrons with a localized hole, which widens the bandwidth, causes a significant Stokes shift, and extends the exciton lifetimes [9]. But the spectral range spanned by these semiconductor devices is restricted. Therefore, in order to overcome the drawback associated with these LED chips and thereby extend the spectral range, phosphor materials came into the picture. The phosphor-converted light-emitting diodes (pc-LEDs) provide significant benefits over other available sources, such as in terms of luminous effectiveness, power efficiency, working life span, etc. [10, 11]. The pc-LEDs use phosphors, which are usually mounted on LED chips and stimulated by the UV or blue light generated by the chip. The mainstay of these pc-LEDs is the inorganic phosphors doped with the inner transition metal ions. Aside from the inner transition, the transition metal ions also act as possible dopants for revealing the phenomenon of luminescence. The most commonly used ones among them are Cu, Mn, Ni, and Cr, with oxidation states from +1 to +4. These dopants usually act as activators in different inert hosts. The inert host matrix may exist in enormous forms such as oxide, nitride, phosphate, fluoride, borate, sulfate, aluminate, vanadate, molybdate, etc. [12–16]. This book chapter aims to provide an overview of the different aspects of the phosphor LED, the role played by the transition metal ions, the properties that an efficient host should possess, etc.

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2 Electroluminescent Device The LED devices are typically composed of either single- or multi-layer semiconductors holding the p–n junction wherein under the biased condition the electrons and holes stream from the respective electrodes to the junction areas resulting in the generation of the incoherent light. The energy bandgap of the semiconductor material is determined by the wavelength of the light emitted by that material. This fundamental principle serves as a cornerstone for the widespread and growing usage of LED systems as lighting devices [17]. Also, the 2014 Noble Prize in physics was conferred for the creation of the blue LEDs with excellent efficiency and extended lifetime. The subsequent year was celebrated as the international year of light [18]. In the direct bandgap semiconductor materials such as gallium nitride, the recombination of the electrons and holes took place without having any shift in momentum due to interaction with the phonon. The energy–momentum graph illustrates that for an electron to reintegrate in an indirect semiconductor, more momentum in the form of a phonon is necessitated, with the phonon’s participation being compensated by lower optical efficiency. Because of these factors, the indirect semiconductors are underutilized in the LEDs. Furthermore, the important consideration in selecting an electroluminescent material is the ability to easily create a p–n homojunction or to acquire a heterojunction in conjunction with the other compounds. Further, the compound stability needs to be ensured and eventually at a cheap cost. Also, an appropriate substrate having lattice parameters comparable to that of the electroluminescent compound is needed. The combination of different characteristics has substantially stimulated the development of InGaN-based white LEDs with emission wavelengths ranging in the blue region (375–450 nm). This ranging nature in the wavelength emission is attained by varying the indium concentration in the GaN matrix. The different alternatives that are efficient and useful are still in the research and developmental phase but possess significant limits in terms of efficiency and stability. So far, an enormous number of LEDs have been studied and different techniques have been employed for increasing their efficiencies [17]. In this regard, zinc oxide (ZnO) is gaining popularity because of its exceptional characteristic features such as large direct bandgap of 3.3 eV and binding energy of the order of 60 meV [19]. These characteristic features of the ZnO LEDs make them an efficient candidate for solid-state lighting as compared to the GaN-based LEDs. Nevertheless, only few luminescent gadgets which do hold too much stability have been manufactured so far [20, 21]. The significant drawbacks of the ZnO which hinder its usage for the LEDs are its poor mobility in comparison to the GaN, its greater electron–phonon coupling (nearly four times), and its low thermal conductivity. The fundamental issue for the ZnO is realizing its homojunction, especially for the p-type ZnO. It is very challenging to generate the homojunction in these devices in a reliable and cost-effective manner [22]. On the other hand, the heterojunctions, have been accomplished by utilizing a variety of p-type materials such as Si [23], GaN [24], AlGaN [25], SrCu2 O2 [26], etc. However, the fundamental cause that has impeded the advancements of these gadgets is the challenge of attaining stability.

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In addition to the II–VI range compounds, there do exist a lot of other options such as ZnSe, the emission range of these compounds varies from blueish green while covering the near UV range also [27]. Nevertheless, these compounds possess the direct bandgap, but the difficulty in obtaining the strongly doped sample with acceptable repeatability has resulted in very poor efficiency and short lifespan. Although the devise that relies on the SiC (indirect bandgap) have been marketed, they too face the problem of achieving efficient brightness [28]. Recently, significant developments in solid-state light-emitting devices have been revealed, exemplified by the utilization of metal halide perovskite structures. Diverse light-emitting LEDs relying on the perovskites have demonstrated excellent quantum efficiencies and stable operational parameters. The standard structural formula of these structures is ABX3 , where A, B, and X generally represent the cation, metal, and halogen atom, respectively. The developed pure inorganic perovskite structures have revealed improved characteristics in comparison to the organic–inorganic halide perovskites. They also have revealed greater photoluminescence quantum efficiencies along with efficient thermal stabilities. Therefore, there is a need to investigate the different characteristic properties such as efficiency, mass manufacturing, toxicity, and most importantly operational stability before performing any sort of commercialization step [29].

3 Properties of Phosphor The credibility of study of any light source is determined by the quality and the quantity of the output obtained from that light source. While examining any source of light for lighting, it is important to comprehend how the human eye perceives it. For evaluating and optimizing the lighting systems in terms of the apparent qualities, a variety of parameters need to be satisfied [30]. A brief description of the different parameters is presented in the next section.

3.1 Emission Spectrum Phosphor’s output spectra are inextricably associated to the pumping LEDs. The span of the phosphor is decided by the characteristic features of the pumping LEDs. It has been observed that if the pumping LED had a near UV spectral range, then that phosphor will span the whole visible region of the electromagnetic spectrum. On employing the pumping LED for the pumping purpose, it has been observed that the emission output of the phosphor gets combined slightly with the transmitted light from the pumping source. Under such circumstances, the peak emission wavelength of the LED light source is typically fixed at around 450 nm, but the pumping energy and the sensitivity remain high. Usually, for the single phosphor, peak emission wavelengths should lie around the 500–600 nm range of the electromagnetic

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spectrum, but it has been observed that the peak emission of the multiple phosphors often spans up to 700 nm, thereby partially overlying each other. As a consequence of the spin–orbit coupling observed in the ground state of the Ce3+ , the Ce: YAG exhibits extremely wide emission spectra with FWHM of 100 nm. The yellow emission of the Ce: YAG when gets combined, the blue emission generated from the pumping LED yields the white light with an appropriate color rendering index (CRI) value of around 70 [17]. The purity of the white light is governed by its few typical parameters, i.e., radiation emission efficiency and great color quality. The first one is characterized by calculating the luminous efficiency of radiation (LER), which is defined as the brightness of the radiation averaged by the human eye reaction [31]. Mathematically, it is defined as  830 830 LER(lm W ) = 683 I (λ)V (λ)d λ I (λ)d λ 

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In the above equation, I (λ) represents the intensity of the emission spectrum, V (λ) represents the response record of the eye. So far, the peak value achieved is 683 lm/W from the monochromator radiation of around 555 nm which corresponds to the peak for receiving the respond from the eye. Since white light is achieved by the combination of colors among which the two important ones correspond to the two extremes of the visible spectrum thus the value of the LER remains always less than 683 lm/W, typically around 350 lm/W. The Commission Internationale de I’Eclaiage (CIE) coordinates are frequently used to characterize the color of a wide emission. The CIE had been established in 1913 as an international autonomous agency for developing the global standards for light and sharing ideas about light science, color, vision, and lighting. So far, three different models of the CIE came into existence which are CIE 1931, CIE 1960, and CIE 1976, and are presented in Fig. 1. The versions of 1960 and 1976 are just the new representations of the old version with minor alterations to the original 1931 CIE graphic. These new versions have

Fig. 1 Suggested models for the CIE in 1931, 1960, and 1976, respectively. Reproduced with permission from ref. [30], Copyright 2020, Elsevier

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been introduced only to ensure the non-uniformity exhibited in 1931 and therefore failed to outperform the prominence of the original version. To depict the composition of colors of the color space, tristimulus values X, Y, and Z are being used. The isolated ratio of these tristimulus values with that of the sum of the total tristimulus values leads to the generation of the new set of coordinates (x, y, z) known as CIE coordinates [30]. There do exist a lot of other factors that must be taken into consideration while considering the light source for some specific application. This is because the behavior of the light depends on a number of factors such as absorption and the reflectivity spectra, reaction of an object to the optical radiations as it varies from source to source and depends on whether both (object and the source) may be optically perceived as white or not. Another parameter that is commonly acknowledged as a metric for the light source is referred to as color rendering index (CRI). The CRI is determined by performing the relative color test of some samples. The color attained by illumination of the particular light source and the reference source is being analyzed properly on behalf of which the CRI value is being determined. The CRI value can also be determined as a function of the distance between different standard colors reflected coordinates and is given in the equation [32]. 1 Ei 8 1 8

CRI = 100 − 4.6

where E i denotes the gap between the colors of test items when illuminated by the test and the reference source. Color-correlated temperature (CCT) is often used to describe the color appearances of light sources by associating it with the temperature of an ideal blackbody radiator. The value of the CCT is expressed in terms of Kelvin (K). This phrase is largely confined to those luminaries that become operational by the initiation of heat radiation such as incandescent bulbs. However, there do exist a variety of alternative light sources which release the light by the process of luminescence instead of incandescence. In such instances, the color temperature of the light radiated by the light source is not a genuine color temperature therefore the term CCT comes into play [33]. Also, warm-colored light is more like infrared radiation, whereas a cool-colored light is more similar to blue radiation. The temperature connected with these phrases appears to be a little bit perplexing. Warm color temperature ranges from 2500 to 3500 K, while the cold colors possess temperature above 5000 K [34, 35] as depicted in Fig. 2.

3.2 Excitation Spectrum The spectral receptivity of the phosphor is considered to be the second important characteristic feature of the phosphor materials. In this regard, the phosphors with possess high receptivity in the near-UV to the blue range of the electromagnetic

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Fig. 2 Values of the color-correlated temperature for different light sources

spectrum are desired. However, the increase in the temperature close to the junction of the LED may tend to modify the optical characteristics of the materials and thereby a reasonable broad excitation spectrum is desired to mitigate such fluctuations. In order to reduce the phenomenon of auto-absorption, it is necessary to ensure that there does occur any overlap among the absorption and the emission spectrums of the phosphor. The blue LED that has been used as an illuminating source for the excitation of the Ce: YAG possesses suitably broad excitation at 450 nm that overlaps well with the LED emission spectrum. The observation has revealed that the spinallowed and parity-allowed 4f-5d absorption results in strong absorption strengths and enhanced threshold before the occurrence of the concentration of the quenching [36].

3.3 Thermal Behavior As there occurs a strong correlation between the phosphor and the LEDs, thus the heat dependency of the optical characteristics is significant. For the general sources of illumination, only 10–20% of the incoming electrical power is transformed into optical power and the remaining power results in the generation of heat which is dissipated by the system. Also, in the LED devices, it has been observed that a tiny amount of the electrical power (approximately 20%) gets wasted in the form of thermal energy [17]. Further, almost each and every point of the LED chip attains temperature ranging from 400 to 450 K, and therefore, it is necessary for the phosphor to hold its optical characteristics in the mentioned temperature range. Performing any change in the emission spectrum, emission color of the whole device gets changed

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which results in reducing the quantum efficiency of device. Moreover, this variable range of the temperature is also responsible for fluctuating the excitation spectrum which results in altering the absorbed portion of the pumping LEDs output thereby also causing a change in the observed light. In this regard, the reported characteristic features of the Ce: YAG has revealed that it exhibits excellent thermal quenching performance by retaining almost 50% of the room temperature emission energy at 700 K while revealing only minor changes in the emission spectrum [36].

3.4 Quantum Efficiency The quantum efficiency (QE) is an important parameter while dealing with illumination technology. The QE in general is of two types, i.e., internal quantum efficiency (IQE) defined as the ratio of photons emitted to the absorbed ones and external quantum efficiency (EQE) defined as the ratio of the number of photons emitted to the incident ones. Both these defined characteristics are critical for the selection of the acceptable phosphor and the value is considered to be efficient when nearest to unity. While keeping the excitation wavelength fixed, it has been observed that boosting of the dopant concentration results in enhancing the absorption capability of the phosphor. But there exists an inverse correlation between the absorption and the IQE, thus enhancing the absorption results in decreasing of the IQE due to quenching phenomenon. Among the two, i.e., IQE and EQE the first one is material property and totally relies on the amount of incoming light, while the later one also relies on the morphology and the architecture of the fabricated LED device. Also, in multilayer phosphor materials, the scattering of the light causes enhancement in the absorption losses there by decreasing the process of retrieving the photon from the phosphor. The inefficiencies that occur due to the scattering rely on the particle size as well their spatial and size distribution. The wide particle size distribution often results in intermitted emission of different colors. For this reason, achieving the nanosized Ce: YAG particles which possess efficient QE is still a concern [37].

3.5 Long-Term Stability The lifespan of an LED is referred to as the amount of time during which its lumen output drops to 70% of its initial output and is represented by L70. Currently, the lifespan of the available devices ranges from 15,000 to 100,000 h. The enhanced lifespan as compared to the older version of the illumination sources such as incandescent lamps, fluorescent lamps, CFLs, etc., is due to the presence of the significant electrical accessories which do not cause any heat dissipation and the device architecture. The phosphor materials also reveal comparable efficiencies over a long period of time but they face complications with regard to their chemical and photostability [38, 39].

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3.6 Saturation The parameter of the saturation reveals the inter-correlation between the emitted light and the excitation intensity. The phenomenon of saturation is inextricably linked with the temporal behavior of the radiative decay route as well as with the decay rate of the emission. This phenomenon becomes effective when a large flow of the excitation photons keeps on interacting with the phosphor and it becomes important to assure that the absorption and the re-emission of light remain efficient and effective. Under such circumstances, quick decay is one among the different desirable options. For Ce: YAG, the decays constant considerably remains below 100 ns, whereas much quicker responses have been observed in various other matrices [40–43].

4 Relevance of Transition Metal Ions in LED Phosphor The activator ions present in the LED phosphor devices play a very crucial role as they function as the luminescence center in the phosphor. Activator ion which is doped into host lattices remains exposed to the local crystalline surroundings that significantly possess a great impact on the luminescence characteristics of that activator ion. This activator ion can either be the rear-earth ion or the transition metal ion. Here, the focus is to discuss the relevance of the transition metals ions as an activator. These activator ions in general remain controlled by the ligands in the host. The phenomenon of the crystal field interaction in-between the positively charged activator ion and the negatively charged unpaired electrons of the ligands gets developed due to the mutual interaction of the opposite charges. The development of the crystal field causes the splitting of d-orbitals of the luminous centers, i.e., transition metal or rear earth. However, it has been observed that the crystal environment tends to impact the spectroscopically active d-orbitals of transition metals more as compared to that of the rear-earth ions. Due to which the transition metal ions are subjected to significantly higher crystal effects [30]. Among the large number of transition metal ions, Mn4+ ion has received greater attention in the field of LED phosphor. Due to the unfilled d-orbital (contains only three electrons) configuration, Mn4+ is well known for generating the effective bright red luminescence in the 600–750 nm range of the electromagnetic spectrum [44, 45]. Because of the simple accessibility of raw manganese ore and the low manufacturing costs, the Mn4+ -doped phosphor has received a lot of interest in the scientific community. Mn4+ possesses various distinguished characteristic features that allow the Mn4+ -doped phosphors to match the parameters of an excellent red-emitting phosphor for the LEDs. Since the Mn4+ possesses a large effective positive charge, as a result of which they exhibit significant crystal field effects. As a result of the strong crystal field, the emission spectra of the Mn4+ act as a tool for differentiating the weak and strong crystal field instances [46]. It has been observed that in the presence of the weak crystal field, the spin-allowed 2 T2g –4 A2g transition results to produce a wide emission band. In

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contrary to this, the spin-forbidden 2 Eg –4 A2g transitions result in the generation of the sharp emission peak under the influence of the strong crystal field. The emission spectrum of Mn4+ has the dominance of the sharp line generated due to the spin-forbidden 2 Eg –4 A2g transitions. The 4 A2g state is among the emerging spinquartet orbital singlet which forms the ground state of the Mn4+ ion. The excitation spectrum of Mn4+ has been found to consist of two sharp bands which have been analyzed to occur due to the spin-allowed transitions 4 A2g –4 T2g and 4 A2g –4 T1g from the 4 F state. The third spin-allowed transition 4 A2g –4 T1g from the 4 P state remains frequently obscured due to the phenomenons of the host lattice absorption or the charge transfer transitions [47]. As the excitation bands of the Mn4+ lie in the UV to the blue regions of the electromagnetic spectrum, it has been observed that Mn4+ doped phosphor efficiently absorbs in that range and generates sharp emission lines in the red region of the spectrum. This feature clearly verifies its potential usage for the LED phosphors excited with the NUV or blue LEDs, which also reveals that they can be readily mixed with the yellow or the green phosphors with negligible chance of visualizing any kind of reabsorption effect. It has been observed that energy of the spin-allowed transition 2 Eg –4 A2g of Mn4+ varies from one host to another such as for the Na2 SiF6 :Mn4+ the 2 Eg –4 A2g of Mn4+ has been observed at 16,181 cm−1 (618 nm) [48], for Sr4 Al14 O25 :Mn4+ at 15,360 cm−1 (657 nm) [49], for Mg2 TiO4 :Mn4+ , Bi3+ at 15,220 cm−1 (725 nm) [50], for SrTiO3 : Mn4+ at 13,792 cm−1 (725 nm) [51], etc. This variation in energy observed in the 2 Eg –4 A2g transition is not due to any crystal field as the energy associated with these transitions of the d3 electronic configuration is competent for any influence of crystal field [47]. In this regard, it is the covalency or the iconicity of the Mn4+ -ligand bonding present in hosts that play an important role in this energy variation. The energy acts as a parameter for deciding the covalent or the ionic nature of the host. In case the transition energy (2 Eg –4 A2g ) is greater than 15,000 cm−1 , the host is regarded as ionic and if the value of the energy is less than 15,000 cm−1 then the covalent nature of the host comes into play. This property of the Mn4+ permits it to be used as a reliable indication for the detection of ionic or covalent nature of the host. The Mn4+ is widely doped in the oxide and fluoride hosts for the fabrication of the red-emitting LED phosphor. Among them, the oxides prove to be the most efficient host for the Mn4+ due to their strong thermal and chemical stability. It has been revealed that the Mn4+ stably dwells in the octahedral position of the host matrix [52]. Also, in most of the circumstances, it has been observed that the atoms around Mn4+ create a deformed octahedron around it. The parameters of some of the hosts used for the fabrication of LED phosphor doped with Mn4+ are given in Table 1. In addition to the +4 oxidation state, manganese also exhibits the phenomenon of photoluminescence in the +2-oxidation state. In this configuration, the d3 shell of the Mn2+ consists of five electrons only with the 6 A1 orbital singlet forming the ground state. However, under the influence of the strong crystal field, this spin-sextet is unable to retain its position and gets replaced by the spin-doublet 2 T2 state. The Mn2+ ion when gets doped in the host either coordinates in octahedral or tetrahedral site, but, in both the situations, it exhibits the spin-forbidden transition, i.e., 4 T1 –6 A1 . In contingent with the intensity of the crystal field, the emission color of the d–d transition keeps on fluctuating. Under the strong intensity of the crystal

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Table 1 Characteristics parameters of Mn4+- doped phosphor hosts Host

Current (mA)

CIE (X, Y)

CCT (K)

Ra

R9

Refs.

Na2 SiF6

350

0.3126, 0.2951

6875

LE (lm/W) 78

86

61

Nguyen et al. [48]

Cs2 SiF6

300

0.4248, 0.4029

3205

111

85

78

Deng et al. [53]

Na2 TiF6

20

0.3960, 0.4090

3841

34

91

–

Wang et al. [54]

Cs2 TiF6

20

0.4150, 0.3890

3272

125

80

–

Zhou et al. [55]

Rb2 TiF6

20

0.4310, 0.4060

3123

188

92

–

Wang et al. [56]

K3 GaF6

–

0.3979, 0.3939

3691

92

87

50

Deng et al. [57]

Cs2 HfF6

20

0.4020, 0.3690

3377

107

93

–

Yang et al. [58]

field, Mn2+ shows the emission spectrum in the orange-red area, while under the weak intensity of the crystal field, the Mn2+ reveals the green emission spectrum. Also, the strong crystal field favors the octahedral coordinated site and the weak crystal favors the tetrahedral coordinated site as a result of which it has been revealed that Mn2+ exhibits red and green emission for the octahedral and the tetrahedral sites, respectively. However, in the 400–520 range of the electromagnetic spectrum, Mn2+ possesses the weak and narrow excitation band due to which it is unable to exhibit effective green or red emission until sensitized by either Eu2+ or Ce3+ ion [46, 59–62]. The excitation and the emission characteristics of a few known hosts co-incorporated with Eu2+ and Mn2+ are given in Table 2. Like manganese, Cr3+ ions too reveal the property of luminescence in the red and NIR regions of the electromagnetic spectrum [63, 64]. These ions are especially used for the fabrication of solid-state lasers; however, they are occasionally explored as a possible luminous ion for developing LED phosphors in the NIR range of the electromagnetic spectrum [65, 66]. The electronic configuration of the Cr3+ ions is identical to that of the Mn4+ ions wherein the 4 A2 forms its ground state. Under the influence of the crystal field, it has been observed that the 4 F levels of the Cr3+ ions undergo the splitting at the octahedral site into 4 A2 , 4 T2, and 4 T1 states, respectively. However, the crystal field does not cause any splitting effect on the 4 P state of Cr3+ ion; instead, it directly transforms it into 4 T1 state with the result Cr3+ possesses dual 4 T1 state. In the visible region, the spin-permitted transitions 4 A2 –4 T2 and 4 A2 –4 T1 of Cr3+ result in the generation of the two large absorption bands. The doping of the Cr3+ ion in any phosphor host generally occupies the octahedral coordination site and shows dualistic transition modes, that is, 4 T2 –4 A2 and 2 E–4 A2 in weak and strong crystal fields, respectively [67]. In addition to these, there do exist a lot of other non-rear earth-doped phosphors

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Table 2 Emission and excitation characteristics of Mn2+- doped phosphor hosts Host

λex (nm) λem (nm) Refs.

Host

λex (nm) λem (nm) Refs.

Na2 CaMg(PO4 )2 365

407, 627

Zhou and Xia [68]

KCaY(PO4 )2 365

480, 652

Liu et al. [69]

Ca3 SiO4 Cl2

395

509, 568

Ding et al. [70]

Ca9 Y(PO4 )7

486, 638

Huang et al. [34]

BaMgAl10 O17

370

514

Ke et al. [71]

Ba3 MgSi2 O8 375

SrAl2 Si2 O8

365

406, 565

Zheng Sr2 B2 P2 O10 et al. [73]

370

430, 600

Li et al. [74]

Sr3 Y(PO4 )3

355

490, 605

Guo et al. [75]

370

420, 670

Yang et al. [76]

SrZnP2 O7

365

437, 504, Ma 623 et al. [72]

which are gaining popularity in this field. Among the post-transition metal ions, the Bi3+ when combined into the different host matrix has been observed to achieve. Diverse luminescence outputs in the visible region of the electromagnetic spectrum [77]. This variability in the emission is attained due to the surrounding crystal field. Although the presence of the large number of unshielded states in the bismuth may jeopardize the phosphor, it also tends to enhance the luminescence characteristic of bismuth-activated phosphor materials in the UV to IR range. Thus, for the bismuth-activated phosphor materials it has been observed that their luminescence is altogether dependent on the host parameters [78, 79].

5 Some Phosphor Hosts The research is continuously going on for further advancements of the illumination setups. From the past century, efforts are continuously being made for the development of phosphor systems with optimal characteristic features as discussed above. In this regard, the development of phosphor-converted white LED (pc-w-LED) still needs a large number of advancements. The pc-w-LEDs can be obtained by mixing either multiple phosphors or by using single phosphor that possesses a broad emission range as shown in Fig. 3. However, these approaches face the drawback in terms of developing homogeneous and standardized phosphor mixtures. Furthermore, such types of phosphors have low thermal stability along with variable nature [80]. The general technique for producing the warm white light is to clump the yellow or the green-emitting phosphor with the red one; however, it has been observed that the white light obtained by following this technique possesses low efficiency. Also, the

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Fig. 3 Different methods for the generation of white light for the development of pc-w-LEDs. Reproduced from ref. [17], MDPI

red-emitting phosphor that plays a significant role in the generation of white light possesses the fundamental drawback of holding its major portion of the emission spectrum in the infrared region which contributes to a significantly small lumen property. Thus, the development of efficient red-emitting phosphors with suitable phosphor parameters is still a demand. The red-emitting phosphor hosts also faces the drawback of chemical instabilities along with the phenomenon of heat quenching. In this regard, it has been observed that the fundamental prerequisite for discovering the new red-emitting phosphor is to search for a simple synthesis technique which does not rely on high pressure and high temperature and should also be cost-effective. Each system possesses specific characteristic parameters that govern the efficiency of the system. Similarly, the perquisites for the fabrication of acceptable phosphor for LED applications are selection of the efficient host lattice, tweaking of the chemical composition, host–activator ion interaction, and engineering of the energy transfer pathways [81]. Till date, phosphors are considered to be the most important for the fabrication of the w-LED devices and are broadly classified on the basis of their structural design in various hosts as will be discussed in this section.

5.1 Oxide Phosphors Among the large number of oxide hosts, the crystal structure of BaSc2 O4 has been observed to be monoclinic with the basic building block as pseudo-orthorhombic unit cell. In its crystal structure, different atoms hold multiple sites with multiple coordination numbers that is two Ba2+ sites with coordination number 12 and three Sc3+ sites with coordination of either 5 or 6. Visualizing different crystal sites, it is evident

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that it can be doped with Bi3+ or with some rear earth ion. Dang et al. [82] investigated the luminescence characteristics of Bi3+- , Li+- , and Eu3+ -doped BaScO4; they observed that there occurs a significant electron transfer among the dopants which eventually helps in adjusting the different parameters. They observed that the modifications among the concentration ratios of the Bi3+ and Eu3+ help in attaining the control of the color tuneability across blue to red regions of the spectrum. They have also analyzed that adjusting the concentration of Eu3+ between 0 and 30 mol % results in a significant change in the CIE coordinates of the host from (0.155, 0.141) to (0.439, 0.268) which demonstrates significant electron transfer from Bi3+ to Eu3+ ions. The observed results of the host suggest that this host might be a good choice for the fabrication of the phosphor-based w-LEDs. The gadolinium aluminum garnet (GaAG) has been observed to hold a better crystal structure as compared to the YAG because the Gd ions sensitize and tend to increase the luminescence characteristics of the activator. But it is very difficult to attain the stable structure of the Tb3+ /Ce3+ doped GdAG due to the fact that ionic size of the Ce3+ is very large as compared to the Gd. However, Wang et al. [83] attained success in fabricating Gd3 Al5 O12 :Tb3+ / Ce3+ phosphor by stabilizing it with the coding of Y3+ ions, although its ionic radii are too small as compared to Ce3+ . While performing the analysis, they observed that the CIE coordinates of the (Gd0.9 Y0.1 )0.9-X Tb0.1 CeX AG compound lies at (0.49, 0.49) which lies in the yellow region of the electromagnetic spectrum, whereas the emission spectrum of the (Gd, Y)AG: Tb3+ /Ce3+ lies in the red region of the electromagnetic spectrum. The analysis revealed that this host will be highly productive for the fabrication of the LED devices as compared to the existing YAG: Tb3+ / Ce3+ and LuAG: Tb3+ /Ce3+ phosphor. Ln2 Ti2 O7, where Ln stands for the lanthanide, possesses either pyrochlore or the layered perovskite structural characteristic features and are supposed to act as efficient hosts for the fabrication of phosphor. The criteria for attaining efficacious results by doping are generally determined by the ratio of ionic radii of Ln3+ and Ti4+ . Recently, researchers have reported the fabrication of Sm-doped Gd2 Ti2 O7 and have studied its structural along with the luminescent characteristic features. It has been observed that the emission spectrum of the phosphor lies in the orange-red region of the electromagnetic spectrum. The observed strong emission bands have been attributed to the transitions arising due to the Sm3+ that is 4 G5/2 -6 H5/2 , 4 G5/2 -6 H7/2, and 4 G5/2 -6 H9/2 in both Gd1.20 Sm0.80 Ti2 O7 and LaSmTi2 O7 phosphors. Similarly, the Ca14 Al10 Zn6 O35 (CAZO) host matrix has been found to possess high thermal stability having the simple manufacturing procedure and is costeffective. The CAZO host co-doped with the Mn4+ which possesses blue absorption around 470 nm has also been used for the manufacturing of the w-LEDs. The thermal stability of the said phosphor has also been confirmed and has been observed that its emission intensity persists up to 88% even working under the heating temperature of 425 K [84]. The crystal structure and the luminous characteristics of the CAZO: Bi3+ , Sm3+ phosphor have been explored by Zhang et al. [85]. The excitation and the emission spectra of the Ca13.9-Y Al10 Zn6 O35 :0.1Bi3+ , ySm3+ have revealed that there occurs a significant energy transfer among the dopants, that is, from Bi3+ -Sm3+ , due to the phenomenon of dipole–quadruple interaction. It has been observed that when the Sm doping concentration reaches 0.3 mol% the efficiency of energy transfer

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was found to be 46.3%. Feng et al. [86] investigated the luminous characteristics of Ce3+ and Mn2+ co-activated CaO phosphors. As compared with other red available phosphors such as ZnGeN2 :Mn2+ , Ca9 Gd (PO4 )7 : Eu2+ , and Mn2+ which possess the quenching temperature of the order of 120 °C and 160 °C, the quenching temperature of CaO:0.007Ce3+ , 0.014Mn2+ has been observed to be of the order of 175 °C. In addition to the fact this phosphor holds suitable phosphor parameters such as low CCT value of the order of 3973 K, high Ra value of the order of 83.1, and luminous effectiveness of the order of 22.9 lm W−1 , this phosphor still faces the drawback of poor absorption and low Ce3+ concentration values. Furthermore, being chemically active, there are significant chances that it may react with the CO2 along with the ambient moisture and will either form CaCo3 or Ca(OH)2 . Thus, for enhancing the stability of the mentioned phosphor, there is a need to concentrate on the aspect of developing some protective layer which will ensure stability. Among the different traditionally available red-phosphors, ZnMgAl10 O17 :Eu2+ is the most significant and it would be fascinating to extend its luminous capabilities up to the nano-level. Verma et al. [87] investigated the optical properties of these nanophosphors. While studying the PL spectra of the nanophosphors they visualized a shift in the spectra which has not been reported earlier. The sharp shifted blue peak with its maxima around 440 nm as shown in Fig. 4 has been attributed to small particle size. Li et al. [88] studied the Ca2.5 Sr0.5 Al2 O6 host co-doped Ce3+ , Mn2+ and have observed that it can act as an efficient candidate for the development of near-UV-excited w-LEDs. However, it has been observed that when the concentration of the Mn2+ is increased in the said phosphor there occurs a significant decrease in the blue and cyan emissions of the phosphor. The observed phenomenon implies that the energy transfer efficiency significantly differs between blue- and cyan-emitting Ce3+ and Mn2+ . Also, the coion substation technique has been employed by Chen et al. [89] for the fabrication of red-emitting Lu3 Al5 O12 :Mn4+ , Mg2+ garnet phosphor for the development of the efficient w-LED which possesses efficient characteristic parameters such as Ra = 81.4, CCT = 4766 K. The emission spectrum of this phosphor has shown narrow red emission bands around 670 nm which has been attributed to the spin-forbidden (2 E – 4 A2 ) transition of Mn4+ . To conclude, it has been observed that the significant hybridization effects among the oxide hosts tend to induce a red shift in the emission spectrum of the dopants. Comprehensive research on the lattice characteristics of the oxide hosts indicated that the preferred occupancy of the dopants at their respective sites gets substantially influenced by the crystal field strengths. Furthermore, different possibilities have revealed that the thermal stability of these phosphor hosts may also depend on the lattice components and on the binding character between the activator and the ligand ion along with coordination number.

5.2 Phosphate-Based Phosphor This class of phosphor materials has got its recognition since very long ago due to their characteristic properties such as significant chemical stability, pragmatic

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Fig. 4 a The PL spectra of Eu2+ -doped ZnMgAl10 O17 with λex = 340 nm and λem = 440 nm b Influence of doping concentration on PL intensity c CIE coordinates of different doping concentrations d Illustration of a typical degradation curve for the ZnMgAl10 O17 :0.01Eu2+ phosphor. Reproduced with permission from ref. [87], Copyright 2018, Elsevier

phonon energy, high threshold, etc. In general, the lattices of the phosphate are made up of PO4 tetrahedron, where the four oxygen atoms remain in coordination with the dopant ions (transition or rear earth) and result in the formation of an advanced crystal structure. The researchers have studied general lattice parameters and the crystal structure of the phosphate host that is AI BII PO4 (where AI : Li, K…, BII : Sr, Ba, …) by employing the general structure analysis refinement system and XRD technique. The effect of the different doping on the structure and the coordination environment has been studied at different temperatures for revealing the thermal quenching behavior. The different observations attained from these phosphor materials have revealed that they possess high thermal stabilities which are necessary for luminous materials used for w-LEDs [90]. Li et al. [91] emphasized the electron transfer mechanisms that occur between the dopants and the activator ions in different phosphor materials. They have also analyzed the concept of the color tuneability for the different colors present in the visible region of the electromagnetic spectrum and have summarized different methods that will help in the fabrication of LEDs holding the electron transfer mechanisms. So far, an enormous number of the hosts phosphate-based phosphor hosts such as KMg4 (PO4 )3 :Eu2+ , Li3 Sc(PO4 )3 :Eu2+ , Sr8 MgGd(PO4 )7 :Eu2+, etc., have been examined for studying their luminous properties by numerous groups [92]. Zheng et al. [92] employed

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the density functional simulation for investigating the lattice characteristics and the electronic structure of Sr5 Cl(PO4 )3 :Eu2+ phosphor. They observed that the color purity of this phosphor at 0.01% doping gets enhanced up to 99% which is very significant as compared to the readily available Eu-doped BaMgAl10 O7 phosphors which possess the color purity of only 88%. Furthermore, the thermal quenching studies performed at a temperature of 150 °C have demonstrated that the emission intensity of the phosphor retains at 87.61% of the starting value which signifies its thermal stability also. The properties have suggested that it might be a significant alternative for the development of the blue-emitting phosphor for w-LEDs. By the amalgamation of this phosphor with the readily available (Ba, Sr)2 SiO4 :Eu2+ and CaAlSiN3 :Eu2+ using InGaN as substrate researchers have successfully fabricated the near-UV w-LED. The attained w-LED was found to possess high CRI and low CCT values of the order of 94.65 and 3567.84 K, respectively, with CIE coordinated as 0.3952 and 0.3709. Guo et al. [93] studied the luminous characteristics of doping effect of Ce3+ and Tb3+ and their mutual effect on the Na3 Sc2 (PO4 )3 phosphor host. They also studied the electron transfer mechanism by employing the concentration-changing approach and analyzed that this phenomenon takes place due to the exchange and multi-pole–multi-pole interactions. At the doping percentage of 0.03 for Ce3+ and 0.1 for Tb3+ , the phosphor has revealed excellent thermal stability and internal quantum efficiency of the order of 65% and thus might be considered as a viable green emitter for the fabrication of w-LEDs. Because of the different drawbacks found in integrating the phosphors, it is required to discover single-phase w-LED phosphor that will possess the simple energy transfer mechanism. Huang et al. [94] studied the effect on the emission spectra of the Eu-doped rhombohedral Ca9 Y(PO4 )7 phosphor host due to the crystal field splitting of Mg2+ , Ca2+, and Sr2+ ions. By performing the concentration change of the divalent metal ion, proper tuning of the emission color in the visible region of the electromagnetic can be achieved. Also because of the high spectrum overlap, the investigations have also indicated an effective resonance energy transfer from Eu2+ to Mn2+ . Liang et al. [95] studied the luminous characteristics of (Ca, Sr)9X Sc(PO4 )7 :Eu2+ , Mn2+ phosphor. For analyzing the emission spectrum, they utilized UV light of wavelength 368 nm as an excitation source and observed a progressive red shift in the emission spectrum of host as Sr2+ concentration keeps on changing. This red shift has been attributed to the development of the solid solution in the scenery of the Eu2+ ions of the lattice. It has been observed that as soon as Sr2+ completely replaces the Ca2+ ion, a broad shift is observed in the emission wavelength that from 483 to 525 nm. The occurrence of shift is the consequence of solid solution which eventually changes the coordination framework of the Eu2+ ion. It is very exciting to observe how the tuning of twodimensional emission colors can be obtained on the CIE diagram by integrating the phenomenons of the crystal field and energy transfer. By varying the concentration and monitoring the changes under 20 mA current, a significant enhancement has been observed in the CRI value that is it changes from 66 to 85 with a CCT value approximately of the order of 3000 K. The emission spectrum has been observed to cover the whole visible range of the electromagnetic spectrum extending from 400 to 780 nm. The general conclusion drawn from the various observations shown that by

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choosing the phosphate host and dopant, efficient emission characteristics and better CIE coordinates can be ensured. Also, their PL excitation spectra have revealed that the doping of the rear earth significantly enhances the optical absorption spectra of these phosphor hosts in the UV region of the electromagnetic spectrum. These phosphors hold a wide variety of CCT values and thus can be used for making effective w-LEDs by choosing appropriate pairs of phosphors. Further, the observation has revealed that attaining the white light by intermixing of different phosphor possess the drawback of disrupting the color balance, the stabilization of which raised the cost of production. Thus, a single-component phosphate-based host which will emit white light can become an efficient alternative option [92].

5.3 Silicate-Based Phosphor In view of the compositional changes, the apatite-based compounds are highly adaptable as they are crystalline in nature, thus possess excellent luminescent features. The apatite structures possess a large number of cation and anionic sites which can be readily filled by a variety of dopant ions. The researchers have studied the effect of crystal field on the La6 Ba4 (SiO4 )6 F2 (LBSF) host and have observed that it is very complex and dynamic in nature [96, 97]. Ye et al. [98] have fabricated Sm-doped LBSF host which possesses two cationic sites of lanthanum and barium and their associated crystal field. For analyzing the emission spectrum, they excited the host at an excitation wavelength of 404 nm and observed the emission of the reddish-orange light in the emission spectrum peaking at 603 nm. Significant thermal stability of the host has been attained at 0.12% doping of the dopant (Sm). At this doping percentage, the said host has revealed high intensity of the order of 72% when observed at a temperature of the order of 150 °C. The characteristic features revealed by the oxyapatite structures suggests that they can be ideal host for luminescent devices and thus can prove to be effective phosphor that will meet the demands. Among the enormous number of silicate hosts, the lanthanide-based silicate hosts, that is, M2 Ln8 (SiO4 )6 O2 (where M = Mg, Ca, Ba: Ln: Y, Gd, La) are of considerable importance. Singh et al. [99] reported the fabrication of green-emitting Tb-doped Sr2 Ln8 (SiO4 )6 O2 phosphor host. According to the CIE coordinates observed at an excitation wavelength of 233 nm, the emission color can be varied from sky blue to green by changing the concentration of the dopant. The crystal field and the symmetries of the host play a critical role in the phenomenon of energy transfer of the lattice thus resulting in the generation of many emission bands. Kim et al. [100] studied the optical characteristics of the Sr2.85 Eu0.15 MgSi2 O8 phosphor host. While observing the absorption spectra, they found that there exist two wide excitation bands at 200 and 450 nm from which it is ensured that the host can be stimulated by UV, near-UV, or by some blue LED. Utilizing the near UV source for the excitation, the said phosphor has shown enhanced internal and external quantum efficiencies, that is, of the order of 76% and 70%, respectively. An effective and efficient phosphor powder has been attained by intermixing of Eu-doped Ba2 SiO, (Sr, Ba)2 SiO4 , Sr3 SiO5 , CaAlSiN3, and CaAlSiBN

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phosphor hosts, respectively. In order to replicate the natural light, the tuning has been performed by using “Minitab” software. The light obtained from w-LED was in close approximation with the natural as its spectrum matches up to 92% with the spectral distribution of the standard illuminant A, while the spectrum of the already available YAG: Ce3+ matches just up to 73.3%. This phosphor came near to emulate the natural white daylight and possesses superb characteristics parameters such as CRI value of the order of 97 and CCT value of the order of 2874 K as shown in Fig. 5 [100]. The readily available red phosphor materials which are thermally less stable as compared to commonly available YAG: Ce3+ yellow phosphor are challenging to integrate into the traditional silicate glasses. The phosphor in glass (PiG) is obtained by the intermixing of the phosphor and glass powder followed by proper sintering of the powder. These materials act as an excellent and stable matrix for the phosphors to work with. PiGs are much more realistic inorganic converters that can be tweaked to emit red light. Lee et al. [101] employed a Pb-free glass matrix for scattering the light from YAG: Ce3+ phosphor which was latterly employed for the fabrication of the blue LEDs. It was observed that white light production gets substantially influenced by the variation of the glass-to-phosphor ratio (GTP). The variation in the GTP enables to change the CRI value from 82 to 91. The PiG thickness plays a significant role in shifting the color coordinates of the phosphor and is substantiated by the observation of the yellow transition as the thickness gets changed from 250 to 300 nm. It has been observed that increasing the thickness of the PiG causes an enhancement between the yellow and the blue phosphor thereby resulting in greater scattering. The development of such devices demands complex technologies and can be fruitful for only massive and multichip-based LEDs. Xu et al. [102] studied the effect of doping and coding of Ce3+ and Tb3+ /Eu2+ on Ca2 (Mg0.75 Al0.25 ) (Si1.75 Al0.25 )O7 (CMAS) phosphor host. The results have shown that introducing Ce3+ /Tb3+ /Eu2+ ions did not cause any noticeable change in the crystal structure and thus can be considered that they get entirely dissolved in the host. The PL investigations have revealed that significant energy transfer takes place from Ce3+ to Tb3+ and Eu2+ due to the fact that the emission spectra of the Ce3+ ions and the excitation spectrum of Tb3+ /Eu2+ are comparable. Among the large number of silicate-based phosphors, the fluorosilicatebased silicate phosphor is being found to hold weak physical, chemical, and thermal stabilities. Same has been reported by Maggay et al. [103] when they analyzed the NaCaBeSi2 O6 F:Eu2+ phosphor and discovered that its emission intensity drops to 40% of its initial value at 150 °C. Furthermore, the observed quantum efficiency of the host was also found to be very low as compared to other phosphors. But the significant CRI value that is 82 and efficient CCT value that is 6016 K as shown in Fig. 6 suggests that the further development of this phosphor might lead to a viable contender for w-LED systems. Silicates are stiff, solid crystal structures in which the silicon and oxygen are joined by a covalent bond, thus potentially offer a disordered habitat to the activator ions. It has been observed that the insertion of the halide ions into the silicate host lattices causes a red shift in the excitation and the emission bands of the activator Eu2+ ion. This shift is attributed to the tendency of halide ions that they may form strong bonds with activator ions and thus result in enhancing the crystal field

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Fig. 5 a Spectral overlap of sunlight and electroluminescence spectra of an as-fabricated LED by using YAG: Ce3+ blue light for excitation. b–d Spectral overlap of sunlight and electroluminescence spectra of Sr2.8 5Eu0.15 MgSi2 O8 phosphor revealing the features of warm light, natural light, and cool light spectra, respectively. Reproduced with permission from ref. [100], Copyright 2015, Elsevier

splitting of Eu2+ ion. The researchers have also reported the fabrication of the Eu2+ doped Ba5 Si2 O6 Cl6 green-emitting phosphor. While performing the luminescence investigations it has been observed that raising the concentration of the Eu2+ ions tends to change the lattice characteristics of the host which results in enhancing the crystal field splitting. Increasing the concentration also causes a significant increase in stokes shift which in turn results in red shifting of the emission band. In order to develop an efficient w-LED, the researchers have paired the Eu2+ -doped Ba5 Si2 O6 Cl6

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Fig. 6 Electroluminescence spectra of the device under the UV excitation wavelength of 380 nm and a proper blend of NaCaBeSi2 O6 F:Eu2+ (blue), (Ba, Sr)2 SiO4 :Eu2+ (green), and CaAlSiN3 : Eu2+ (red) phosphors. In the inset, a digital image of the w-LED is also presented. Reproduced with permission from ref. [103], Copyright 2018, Elsevier

with the red- and blue-emitting Eu2+ -doped Sr2 Si5 N8 and BaAl10 MgO17 phosphor hosts on InGaN blue chip. On comparative analysis of the phosphor, the CIE finding revealed that this phosphor possesses great stability and an efficient CCT value [104]. Soon after Nakano et al. [105] reported the luminescence qualities of Eu2+ -doped Ba9 Sc2 Si6 O24 , different researchers investigated various alkaline earth silicates with Sc3+ ions substituted by earth ions for prospective use in pc-converted w-LEDs. According to the analysis performed by Liu et al. [106], modulating the dopant (Ce3+ ) concentration in Ba9 Sc2 Si6 O24 results in enhancing the internal quantum efficiency up to 82% which is equivalent to other existing nitride phosphors. However, these phosphors have revealed better thermal stabilities as compared to others since they are able to retain their internal quantum efficiency up to 94% when operated under the temperature of 160 °C. Khan et al. [107] reported the development of Ce3+- , Mn2+-, and Tb3+ -doped Ba9 Sc2 Si6 O24 as a single-component full-color-emitting phosphor. It has been observed that in this host the trivalent dopants replace the Ba2+ sites, where the charge compensation is attained by the presence of Li+ ion. Due to significant energy transfer among the dopant ions, that is, Ce3+ -Mn2+ and Ce3+ -Tb3+ , there occurs three color full bands (blue, green, and red) in the emission spectrum of the phosphor. Thus, from the discussion, it can be concluded that the CIE coordinates of these phosphors can be efficiently modified for the attainment of white light. Also, as per the investigations, the occurrence of the concentration quenching in silicates/orthosilicates hosts is the consequence of electric multiple contacts rather than exchange interactions. In addition to the above-mentioned host, there do exist a lot of other phosphor hosts such as borate, fluoride, sulfide, nitride, and many other phosphor hosts. The phosphors materials devised from these also have revealed many significant properties and applications for the solid-state lighting technology. There is a need for further progress in the different aspects of these devices and for enhancing the characteristic parametric features of these phosphor hosts.

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6 Conclusions The demand for energy increases daily. The surge in energy visualization has encouraged researchers to search for new efficient light sources compared to traditionally available sources such as incandescence, tungsten lamps, and xenon arc lamps. The newly developed sources need to have efficient characteristic parameters, such as high luminous efficiency, enhanced brightness, low energy usage, and longer lifetime. Considering the above circumstances, LED-based phosphor technology has proven to be very efficient. According to the present situation, it has been analyzed that the w-LED technology has the potential to conserve at least 20% of yearly energy consumption. Valence, local coordination, and the ionic distribution are unique factors responsible for the outstanding luminous features of the oxide-based phosphor hosts. Considering the synergetic and optical parameters, solid-state lighting devices necessitate careful selection and analysis of the phosphor hosts. The pursuit of a novel red-emitting phosphor is necessary to ensure a high photoluminescence quantum yield, which can be attained by carefully visualizing the synthesis, chemicals, and other thermodynamic entities. Furthermore, a significant LER can be attained by reducing the waste–phonon ratio, narrowing the bandwidth, and maximizing the blue shift in the spectrum. Different initiatives should be taken to improve the crystal structure of the host lattices and the crystal field symmetry of the activator ions. The attainment of optimal color coordinates necessary for warm w-LEDs that is CRI, Ra = 90, R9 = 90, color quality scale in the order of 90, etc., continues to be a significant problem for the lighting industry without affecting the lighting energy efficiency of readily available sources. To achieve a wide color gamut in display systems, it is necessary to ensure the creation of the narrowband red and green phosphors. There are different aspects such as suitable host and the activator, tuning crystal symmetry and spectra, and improving the optical performances of phosphors that play a critical role in regulating the efficiency and color rendition of the fabricated phosphor. These typical motivations are based on obtaining the inherent features that determine the efficiency of phosphors. Furthermore, an enhanced CRI does not guarantee that a light source will produce vivid colors. Instead, researchers need to design new red-emitting phosphors that are stable, and inexpensive, should have improved luminescence features and should attain the capability of dual activation modes that are both by near-UV light and blue LEDs. From the above discussion, it is clear that next-generation display technology will rely on the line-type or the narrowband red-emitting phosphors. The focus of the future LED phosphor developments must be diverted towards natural mineral-inspired prototype layouts. In addition to articulating the design, it is necessary to encourage the easy synthesis of new components/ phases. The employment of genetic algorithms such as combinatorial chemistries might act as an important tool in optimizing the chemical variation of novel phosphor materials. Also, the manganese-doped phosphors have revealed that they can be co-doped with Ce3+ , Eu2+, etc., within a single host matrix thereby enabling the emission of tuning from near-UV to white light. The incorporation of the various rear-earth co-dopants in the nitride phosphors to achieve persistent red luminescence

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tends to enhance the luminescence by introducing more defects. The incorporation of co-dopants also enhances the thermal stability of the phosphor systems. Different manufacturing conditions such as high annealing temperatures, limiting atmospheric conditions, and extended time may result in the generation of inhomogeneous materials, necessitating the development of new modified and less critical procedures that will prove to be beneficial for enhancing the optical characteristics of the phosphor. Lastly, it is worth mentioning that the ratio among the different activators within the host can also act as an option for the fabrication of single-phase hosts. In this context, different phosphor systems such as phosphates, orthophosphates, molybdates, and tungsten hosts may be among the most efficient choices.

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102. C. Xu, H. Guan, Y. Song, Z. An, X. Zhang, X. Zhou, Z. Shi, Y. Sheng, H. Zou, Novel highly efficient single-component multi-peak emitting aluminosilicate phosphors co-activated with Ce3+ , Tb3+ and Eu2+ : Luminescence properties, tunable color, and thermal properties. Phys. Chem. Chem. Phys. 20, 1591–1607 (2018). https://doi.org/10.1039/c7cp07108f 103. I.V.B. Maggay, K.Y. Yeh, B. Lei, M.G. Brik, M. Piasecki, W.R. Liu, Luminescence properties of Eu2+ -activated NaCaBeSi2 O6 F for white light-emitting diode applications. Mater. Res. Bull. 100, 26–31 (2018). https://doi.org/10.1016/j.materresbull.2017.11.059 104. S. Dutta, T.M. Chen, An efficient green-emitting Ba5 Si2 O6 Cl6 :Eu2+ phosphor for white-light LED application. RSC Adv. 7, 40914–40921 (2017). https://doi.org/10.1039/c7ra07221j 105. T. Nakano, Y. Kawakami, K. Uematsu, T. Ishigaki, K. Toda, M. Sato, Novel BaScSi-oxide and oxynitride phosphors for white LED. J. Lumin. 129, 1654–1657 (2009). https://doi.org/ 10.1016/j.jlumin.2009.04.028 106. Y. Liu, J. Zhang, C. Zhang, J. Xu, G. Liu, J. Jiang, H. Jiang, Ba9 Lu2 Si6 O24 :Ce3+ An efficient green phosphor with high thermal and radiation stability for solid-state lighting. Adv. Opt. Mater. 3, 1096–1101 (2015). https://doi.org/10.1002/adom.201500078 107. S.A. Khan, Z. Hao, H. Wei-Wei, L.Y. Hao, X. Xu, N.Z. Khan, S. Agathopoulos, Novel single-phase full-color emitting Ba9 Lu2 Si6 O24 :Ce3+ /Mn2+ /Tb3+ phosphors for white LED applications. J. Mater. Sci. 52, 10927–10937 (2017). https://doi.org/10.1007/s10853-0171162-y

Charge Transfer in Rare-Earth-Doped Inorganic Materials Amol Nande, Swati Raut, and S. J. Dhoble

1 Introduction Wiedemann coined the term “luminescence”, which is derived from the Latin word for light [1, 2]. Luminescence employs cold radiations from certain materials when these materials are excited by an external source at room temperature [3]. Luminescence materials and complexes emit light in a variety of energy ranges, posing interesting challenges for fundamental and applied research. Luminescence materials have opportunities in the field of optical devices and fundamental studies of material physics. The applications of luminescence materials involve the design and tailoring of several science needs. The applications vary from health and safety to lamps, displays, thermometers, lasers, photocatalysts, sensors, imaging, biomolecular probes, solar cells, and many more [4–15]. Luminescent materials, or phosphors, are commonly solid inorganic and organic materials [3]. However, the study will focus on rare-earth-doped inorganic phosphors and their photoluminescence applications. The inorganic phosphors consist of a host lattice and a rare-earth metal ion, which act as activators. Thus, the rare-earth ions show transitions from the UV to the infrared energy spectrum [16, 17]. Transitions in rare-earth-doped inorganic phosphors are caused by a parity-restricted electronic transition within the 4F shell [2, 18]. In photoluminescence, the phosphors are excited using a light source, and two types of spectra are depicted by the phosphors: excitation or absorption and emission spectra [7]. The excitation spectra contain energy absorption peaks due to host materials as well as from the dopant ions. In general, the absorption transitions suggest the excitation of the active ions from their ground energy state to a higher, or excited, A. Nande (B) Guru Nanak College of Science, Ballarpur, Maharashtra, India e-mail: [email protected] S. Raut · S. J. Dhoble Department of Physics, R. T. M, Nagpur University, Nagpur, Maharashtra, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_2

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energy state. The most probable transition is obtained by exciting electrons from the lowest vibrational energy level with an R0 value (the maximum value of vibrational wave functions) to the edge of the excited parabola (the highest amplitude of the vibrational energy levels) [19]. For the other transitions, the optical transition probability between ground levels and the excited state energy levels is proportional < state energy > to 𝚿v|𝚿v ' . Here, 𝚿e, 𝚿g, 𝚿v, 𝚿v ' , and r are excited state wave function, ground state wave function, ground state vibrational wave function, excited state vibrational wave function, and the electric dipole operator. If R0 ' is the equilibrium distance at the excited state, then the parabola offset is R0 ' − R0 and if the offset is zero, then the absorption consists of only one line which refers to as no phonon or zero-vibrational transition as shown in (Fig. 1). When the offset is large, the absorption band will be broader [20–22]. Although the spin selection and parity selection rules do not allow for transitions with different spins and the same parity (such as d–d and f–f), spin–orbit coupling and electron–vibration interactions provide some relaxation and allow for transitions [22–24]. These transitions are the most important aspect of luminescence characteristic spectra. This chapter will discuss the analysis of these transitions using charge transfer mechanisms. However, in emission spectra, the observed peaks are basically due to 5d → 4f transitions (band emission) and 4f → 4f transitions (due to rare-earth metal ions) [25]. The emission peaks signify the area of interest of prepared samples. The luminescence mechanism, its operations, and applications have already been covered in the book. Hence, the chapter is focused on recent activities and properties of host materials, especially oxides, phosphates, aluminates, and sulfates. Later in the chapter, using recently published work, the charge transfer mechanism in rare-earth-doped inorganic phosphors is discussed. As previously stated, inorganic phosphors are primarily composed of activators doped in host matrix. The host material should be chemically and thermodynamically stable, as well as stable in the environment around it. Further, these materials should Fig. 1 A schematic configurational coordinate diagram of an absorption transition (R0 and R0 ' are the ground state and excited state equilibrium distances, respectively)

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have low phonon energy, as low phonon energy causes less non-radiative losses due to a lower vibrational frequency. The host materials are generally transparent to the excitation source used for studying luminescence phenomena, but this is not true for every aspect. Further, the host matrix influences the optical properties of activators; the host matrix absorbs the excitation radiation and transfers energy to the activators. As the activators, like rare-earth metal ions, have f–f transitions that are latitudinally forbidden, they have poor absorptions and low quantum yields. When such activators are doped in the host matrix, the induced perturbation causes the odd parity of rareearth metal ions to mix with the even parity of the host matrix’s d- or p-orbitals. If the host is not symmetric around the activators, it relaxes the section rule and has a significant impact on the emission and absorption spectra. Further, the host materials should have better stability (thermal as well as chemical) and the ability to take in large amounts of dopant concentration in order to have a good quantum yield. Thus, the rare-earth-doped host material either directly or indirectly excited the rare-earth metal ion, and the host material later transferred energy to the rare-earth metal ion. The host matrix ions that absorb energy from the excitation source are referred to as sensitizers. The properties of the host matrix modify the emission or absorption spectra of luminescence materials, which results in the covalency, crystal field, and inhomogeneous broadening of measured spectra. Thus, the luminescence properties of inorganic phosphors are affected by the host and activator combination. Hence, the selection of the host materials is also important for a better luminescence output. Aluminates, arsenates, borates, bromides, carbonates, halides, chromates, cuprates, ferrites, gallates, magnates, nitrides, oxides, phosphates, sulfides, titanates, tungstates, urinates, vanadates, zirconates, etc., are the host materials for inorganic phosphors. Out of which, we discussed oxides, phosphates, aluminates, and sulfates. In this section, we summarize important properties and recent work for the abovementioned host materials.

2 Oxides-Based Phosphors for Solid-State Lighting Applications These are materials that contain one or more oxygen atoms as well as other elements. They are in the form of binaries, ternaries, and complex oxide materials, which are used as hosts for luminescence phosphors. Oxides are mostly chemically and thermodynamically stable. These are also insensitive to moisture or humidity in the atmosphere. Further, oxides have low phonon energy, which makes them less susceptible to non-radiative losses. These can be handled in the open air and can be used for a long time without changing their chemical composition. Furthermore, oxide structures are adaptable and stable, and additions of extra metal ions (activators) within the cations grid do not significantly alter the host grid for oxides. Furthermore, the synthesis processes of oxides do not involve any reactive or corrosive chemicals or gasses and are easy to prepare compared to other materials. However, these materials

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require really high temperatures for annealing and sintering. Moreover, oxides have a wide band gap and are considered in designing the best-performing inorganic phosphors. These materials involve metal oxides like ZnO, Al2 O3 , CuO, etc., borates, phosphates, vanadates, aluminates, tungstates, titanates, tantalates, sulfates, niobates, gallates, germinates, molybdates, perovskites, and silicates. A few of them will be discussed as a separate topic in this chapter. Rare-earth-doped oxides are well known for their excellent luminescence properties and are used in a variety of applications. Zikriya et al. [26] created crystalline Dy3+ -doped TiO2 inorganic phosphors that could be used to emit white light. The inorganic phosphors were synthesized using the co-precipitation method followed by thermal annealing, and their structural, optical, and luminescence properties were studied. The results confirmed that the samples had solar cell applications. Yadav et al. [27] synthesized La2 O3 : Er, Yb, and La2 O3 : Er, Yb, Bi co-doped samples using the solid-state reaction method. The EDS measurements confirmed the presence of Er, Yb, and Bi elements in the samples. Also, due to the presence of Er, Yb, and Bi elements in the samples, a large number of absorption bands were observed in the UV–Vis–NIR spectra. A luminescence study showed that the synthesized samples showed intense emission in the green region and weak emission in the red region. Further investigation revealed that the emission for these samples was upconverting emission caused by the absorption of two photons. These samples could be suitable for photonic and optical bistable devices. Wang et al. [28] synthesized and studied the Ba2 In2 O5 : Ho, Yb, and Ba2 In2 O5 : Er, Yb series of samples. These samples were synthesized using the solid-state reaction method and exhibit efficient visible upconversion and near-infrared downshifted emissions. These series of samples were used as two pairs of two thermally coupled levels with sensitivities of 0.0065 K−1 at 498 K (for Ba2 In2 O5 : Er, Yb) and 0.0025 K−1 at 273 K (for Ba2 In2 O5 : Ho, Yb). The study suggested these inorganic phosphors could be promising thermometers and have an application as optical temperature sensors with high sensitivity. Pyngrope et al. [29] synthesized Eu-doped Gd2 O3 nanomaterials using the polyol route method and compared the properties of undoped Gd2 O3 and Eu-doped Gd2 O3 annealed at different temperatures. The annealed samples showed strong red emission at 613 nm (5 D0 → 7 F2 ) and other emission peaks centered at 580 nm, 594 nm, 613 nm, 623 nm, and 648 nm, corresponding to 5 D0 → 7 Fj (j = 0, 1, 2, 3, 4) transitions. The sharp peaks confirmed the complete incorporation of Eu dopant in the Gd2 O3 matrix. Further, excitation spectra confirmed that the energy transfer took place from Gd2 O3 to Eu2 O3 . Along with rare-earth-doped oxides, transition metal-doped ions demonstrated excellent emission and charge transfer mechanisms. Wang et al. [30] synthesized red phosphors (Li3 Mg2 NbO6 :Mn4+ ), which can be used for warm white light using the solid-state reaction method. The study showed that the inorganic phosphor was crystalline in nature, and all metal ions—Li+ , Mg2+ , and Nb5+ ions—formed four octahedrons with neighboring O2− ions. These isolated octahedrons shared edges with the adjacent structures, forming a rock salt structure, and Mn4+ replaced multiple sites in the structure. Further, the band gap was estimated using the experimental and DFT calculations, which came out consistent and equal to 3.8 eV. Moreover, a

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Table 1 Oxide-based inorganic phosphors with observed emission transitions Inorganic phosphors

Observed emission/transitions

Refs.

TiO2 :Dy

Blue emission (4 F9/2 → 6 H5/2 ) and yellow emission (4 F9/2 → 6H 13/2 )

Zikriya et al. [26]

La2 O3 : Er, Yb, Bi

4G 4 2 4 4 4 2 4 11/2 → I15/2 , P3/2 → I13/2 , F5/2 → I15/2 , H11/2 → I15/2 , 4 S3/2 → 4 I15/2 , 4 F9/2 → 4 I15/2 , and 4 I9/2 → 4 I15/2 Red emission (2 Eg → 4 A2g )

Yadav et al. [27]

Li3 Mg2 NbO6 : Mn4+

Wang et al. [30]

Li3 Mg2 SbO6 : Mn4+

Red emission at 666 nm (2 Eg → 4 A2g )

Zhong et al. [31]

Li4 AlSbO6 : Mn4+

Red emission at 673 nm (2 Eg → 4 A2g )

Li et al. [32]

Li5 La3 Ta2 O12 : Mn4+

Red emission at 714 nm (2 Eg → 4 A2g )

Cao et al. [33]

Ba2 In2 O5 : Ho, Yb Ba2 In2 O5 : Er, Yb

Ba2 In2 O5 : Er,Yb—526 nm (2 H11/2 → 4 I15/2 ), 549 nm (2 S3/2 → Wang et al. 4I 2 4 [28] 15/2 ), and 658 nm ( F11/2 → I15/2 ) 5 Ba2 In2 O5 : Ho, Yb—550 nm ( H4 (5 S2 ) → 4 I8 ), and 653 nm (5 F5 → 4 I8 )

Gd2 O3 : Eu

580 nm (5 D0 → 7 F0 ), 594 nm (5 D0 → 7 F1 ), 613 nm (5 D0 → 7 F ), 623 nm (5 D → 7 F ), and 648 nm (5 D → 7 F ) 2 0 3 0 4

InMO4 : Tb For Tb3+ doped oxides—green emission (5 D0 → 7 FJ ) InMO4 : Yb For Yb3+ doped oxides—near-infra-red emission (2 F5/2 → (M = V, Nb, and 7 F7/2 ) Ta)

Pyngrope et al. [29] Botella et al. [34]

SiO2 @Yb/Tm/ ZnO

Tm (3 F2,3 , 3 H4 → 3 H6 )

Li et al. [35]

NiO: Eu

468 nm (5 D2 → 7 F0 ), 571 nm (5 D1 → 7 F3 ), 611 nm (5 D0 → 7 F ), and 665 nm (5 D → 7 F ) 2 0 4

Mokoena et al. [36]

Crystals— LaAlO3 : Eu LaAlO3 : Tb

Tb3+ − 5 D3 → 7 F6 , 5 D3 → 7 F5 , 5 D3 → 7 F4 , 5 D3 → 7 F2 , 5 D4 → 7 F6 , and 5 D4 → 7 F5 Eu3+ − 5 D1 → 7 F2 , 5 D0 → 7 F1 , 5 D0 → 7 F2 , 5 D0 → 7 F3 , and 5D → 7F 0 4

Pejchal et al. [37]

photoluminescence study showed that the studied phosphor was excited using UV to the blue range and exhibited red emission at 668 nm due to the 2 Eg → 4 A2g transition. The other examples of oxide-based inorganic phosphors with emission transitions are given in Table 1.

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3 Phosphate-Based Phosphors for Solid-State Lighting Applications Phosphates are compounds that possess good thermal, hydrolytic, charge, and chemical stability [38, 39]. Also, these compounds are easily prepared at low temperatures, and the particle analysis can be controlled using different synthesis processes. The phosphates have various structures, such as ortho-, pyro-, meta-, and polyphosphates, depending on their structures and sizes. The phosphate compounds can involve ABPO4 , A3 P4 O13 , A3 (PO4 )2 , ABP2 O7 , A2 B3 P4 O15 , A2 BC(PO4 )3 , A2 BP2 O7 , etc., where A, B, and C are transition and rare-earth metal ions [40]. The luminescence and charge transfer phenomena are structure-dependent. The current research showed that the phosphate-based inorganic phosphors activated with rare-earth metal ions were utilized to create some novel phosphors with outstanding photoluminescence properties and numerous applications [41–45]. In this section, we discuss recent publications on phosphate-based inorganic phosphors and their luminescence properties. Xiang et al. [46] synthesized Eu3+ -doped NaAlP2 O7 using the solid-state reaction method. The synthesized samples had a monoclinic structure with space group p21 /c. Further, XRD analysis confirmed that Eu3+ did not change the crystal structure of the host material, NaAlP2 O7 . Further, FT-IR analysis of Eu3+ -doped NaAlP2 O7 showed that six vibrations (symmetric and antisymmetric stretching vibrations of PO3 , symmetric and antisymmetric stretching vibrations of P–O–P, and symmetric and antisymmetric bending vibrations of PO3 ) are similar to [P2 O7 ]4− . Further, the estimated band gap (4.07 eV) of NaAlP2 O7 from the absorption spectra was smaller than the calculated band gap (5.28 eV), which was due to the fact that the transparent light wavelength region of the crystal was wider compared to the powder. Furthermore, the measurement’s absorption band confirmed the combined electronic transition from O to Na, Al, and Eu, as well as a small blue shift observed between the doped and undoped absorption peaks of NaAlP2 O7 . Further, the emission and excitation spectra showed that the inorganic phosphor had potential application for red solid-state LEDs. The most noticeable transitions were 5 D0 → 7 F2 (red emission) and 5 D0 → 7 F1 (orange emission), which were caused by magnetic dipole and forced electric dipole transitions, respectively. Also, the red emission transition was highly dependent on the variance of the inversion symmetry sites of Eu3+ ions. Furthermore, as concentration increased, the intensities of the phosphor decreased, resulting in the concentration quenching phenomenon caused by the multipole–multipole interaction between different Eu3+ ions. Zhen et al. [47] synthesized Sr3 (PO4 )2 :Er/Yb upconverting phosphors using the solid-state reaction method. The synthesized host showed a rhombohedral phase with an R3m space group. The luminescence study confirmed exhibition of deep red upconverting emission due to the intra-4f transition of Er metal ions. Further, temperature dependence characterization, as well as the estimated fluorescence intensity ratio and stark component parameters, demonstrated that the inorganic phosphor could be used in luminescence thermometry. According to Saidi and Dammak [48], Er/Yb co-doped Na3 Gd(PO4 )2 phosphor can be used as

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a luminescence temperature sensor and optical heater. The series of phosphors were synthesized using the solid-state reaction method. The XRD analysis confirmed the formation of a monoclinic structure with a C2 /c space group and proved that Er3+ and Yb3+ sites replaced or incorporated Gd3+ sites. Further, the upconverting luminescence emission spectra study showed the peaks in green and red regions corresponding to 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4 F9/2 → 4 I15/2 transitions. The concentration of doping had an effect on signification but not on emission intensity or peak positions. Beyond 3% dopant concentration, concentration quenching was observed due to the effect of non-radiative electronic transitions and cross-relaxation between Er3+ ions. Further, they investigated thermally coupled levels (2 H11 , 4 S3/2 ) and non-thermally coupled levels (2 H11/2 , 4 F9/2 ) by analyzing temperature-dependent upconverting emission analysis in the 300–440 K temperature range, suggesting the phosphor had a potential application in optical thermometry. Also, the thermal effect analysis showed that it could also be used in thermal therapy applications. In another interesting paper, Zhao et al. [49], synthesized polycrystalline samples of K3 BiTaP3 O13 : Eu inorganic phosphors using the solid-state reaction method. The structural analysis confirmed that the compound was formed in the hexagonal system space group P63 /mcm . The framework was made up of [TaO(PO4 )2 ]∞ , three types of K chains, and [Bi(PO4)]∞ chains that were almost parallel to the c-axis, suggesting the formation of an organic oxysalt compound. Each individual unit cell contained one bismuth atom, one tantalum atom, two phosphorous atoms, and six oxygen atoms. In the structure, the bismuth and tantalum atoms formed distorted octahedra with the oxygen atoms and phosphorous tetra-coordinated to form PO4 groups. Later, each octahedron of tantalum atoms shares an oxygen atom to form a pseudo-onedimensional [TaO(PO4 )2]∞ chain along the c-axis of the hexagonal structure. Alternatively, the BiO6 -PO4 chain was formed by sharing two O5 atoms, resulting in an almost one-dimensional [Bi(PO4 )]∞ chain along the (001) direction. K2 atoms were arranged to form a one-dimensional chain overlapping with the 3-axis of the crystal lattice, and each K2 atom was hexa-coordinated to O2 atoms. On the other hand, the K1 atom was hepta-coordinated by O atoms and located at the generated sites between [TaO(PO4 )2 ]∞ and [Bi(PO4 )]∞ chains. Further, the K3 atoms located in pseudo-octagonal rings delimit voluminous tunnels formed due to BiO6 rings. Moreover, the K3 BiTaP3 O13 doped with Eu and Tb for different concentrations showed the same hexagonal crystal structure in which Bi3+ ions were replaced by Eu3+ or Tb3+ ions without changing the arrangement of the groups. The photoluminescence study confirmed that Bi3+ ions provided the ideal doping environment due to the comparable ionic radii of Eu3+ and Tb3+ ions. Further, for Eu3+ -doped phosphor, a broad transfer transition peak was observed at 280 nm, which was due to a charge transfer transition between Eu3+ and O2 . Also, nearly seven peaks in photoluminescence excitation spectra and five emission peaks were observed, suggesting a number of different luminescence centers were present in the host. Out of five emission peaks, the most prominent peak at 610 nm was attributed to the 5 D0 → 7 F2 electric dipole transition, while the second most prominent peak at 592 nm was attributed to the 5 D0 → 7 F2 magnetic dipole transition. This suggested the Eu3+ activators took up a noncentrosymmetric site in the host crystal structure. The Tb3+ -doped host showed green

38

A. Nande et al.

emission but no concentration quenching. These sets of inorganic phosphors were used for white light-emitting diodes. Table 2 summarizes a few recent works to gain a better understanding of phosphate-based rare-earth-doped inorganic phosphors. Table 2 The crystal structure and applications of phosphate-based rare-earth-doped inorganic phosphors Inorganic phosphor

Structure of the host

Possible application

Refs.

NaAlP2 O7 : Eu

Monoclinic phase

Solid-state LEDs

Xiang et al. [46]

Sr3 (PO4 )2 : Er/Yb Rhombohedral phase

Upconverting phosphors and thermometer with tunable sensitivity

Zheng et al. [47]

Na3 Gd(PO4 )2 : Er/Yb

Monoclinic phase

Temperature sensing, display device and optical heater

Saidi and Dammak [48]

Na2 ZnP2 O7 : Eu, Dy

Tetragonal phase White luminescence—white light-emitting diodes

Na3 Gd(PO4 )2 : Tetragonal phase White light-emitting diodes Dy Monoclinic K3 Gd(PO4 )2 : Dy phase

Fhoula et al. [50] Bedyal et al. [51]

LaPO4 : Yb/Tm/ Ln (Ln = Eu,Tb)

Monoclinic phase (Monazite-La type)

Luminescent ink and luminescent fibers

Tymi´nski et al. [52]

Mg3 In4 P6 O24 : Eu/Tb

NA

Fluorescent lamps

Zhang et al. [53]

MgIn2 P4 O14 : Tm/Dy

Monoclinic phase

White light-emitting diodes

Zhang et al. [54]

Sr3Y(PO4)3: Yb/ Cubic structured Ln eulytite-type (Ln = Ho, Er, Tm)

Optical temperature sensors

Liu et al. [55]

AgLaP2 O7 : Tb

–

Hami et al. [56]

Orthorhombic phase

NaBaPO4 : Gd

Hexagonal phase UV phototherapy lamps

Singh et al. [57]

Ba3 Bi2 (PO4 )4 : Sm

Eulytite-type

Warm white light-emitting diodes

Jayachandiran and Kennedy [58]

Ca10 M(PO4 )7 (M = Li, Na, K)

Trigonal phase (Ca10 K(PO4 )7 )

–

Zhao et al. [59]

KNa4 B2 P3 O13 : Sm

Orthorhombic phase

Warm white light-emitting diodes

Fang et al. [60]

NaCaPO4 : Dy

Orthorhombic phase

Cool white light-emitting diodes

Nair et al. [61]

Charge Transfer in Rare-Earth-Doped Inorganic Materials

39

4 Aluminates-Based Phosphors for Solid-State Lighting Applications Aluminates attracted researchers due to their chemical stability, wide band gap, and luminescence behavior. Also, these materials are studied for valence states and spectroscopic properties. Rare-earth-doped aluminates have shown promise as phosphors. However, over the past several years, there has been huge development for aluminates as host materials, due to their potential applications in the fields of lamp phosphors, solid-state laser materials, luminescent paints, scintillations, panel display materials, dosimeter materials, long-lasting phosphors, etc. Aluminates can be classified based on their chemical composition as well as their crystal structure. On the basis of chemical composition and arrangement, aluminates are classified as ortho-aluminate (MO: Al2 O3 ), di-aluminate (MO: 2Al2 O3 ), and hexa-aluminate (MO:6Al2 O3 ); here, M stands for metal ion. In an ortho-aluminate structure, AlO4 is occupied at the corners of the framework tetrahedron. In this structure, each oxygen atom shares space with two aluminum ions, giving a net negative charge to each tetrahedron. The negative charge is balanced by the divalent cation present in the interstitial sites of the tetrahedral framework. However, hexaaluminates like CaAl12 O19 and SrAl12 O19 have a magnetoplumbite structure, while compounds like BaAl12 O19 have a beta-alumina structure. The magnetoplumbite has 12-coordinated cations, while the beta-alumina structure has 9-coordinated cations. Table 3 shows a selection of recent aluminates compounds with structures studied for luminescence properties. Rare-earth-doped alkaline earth aluminates demonstrated persistent luminescence with a significantly long duration of emission when exposed to sunlight. The first application of Eu-doped SrAl2 O4 was in rare-earth-doped strontium aluminates. These materials have high luminescence efficiency and good chemical and physical stability [70]. For improved afterglow intensities, stability, and emission lifetimes, aluminates such as Eu and Dy co-doped SrAl2 O4 phosphors are used instead of traditional ZnS-based phosphors [70, 71]. These phosphors can be further broadly categorized as (a) binary alkaline earth aluminates, (b) alkaline earth hexa-aluminates Table 3 Compound name of aluminates with structures and respective references are mentioned Compound name

Structure

Refs.

CaAl12 O19

Magnetoplumbite structure

Medina et al. [62], Li et al. [63]

SrAl12 O19

Magnetoplumbite structure

Zhu et al. [64]

BaAl12 O19

Beta-alumina structure

Zhu et al. [64]

BaMgAl10 O17

Beta-alumina structure

Wang et al. [65]

BaMg3 Al14 O25

Beta-alumina and spinal structure

Verstegen and Stevels [66], Xabanova et al. [67], Villars et al. [68]

CaMg2 Al16 O27

Magentoplumbite and spinal structure

Li et al. [63, 69]

40

A. Nande et al.

analogous to magnetoplumbite and beta-alumina with their superstructures, and (c) rare-earth hexa-aluminates with the magnetoplumbite structures. We summarize photoluminescence studies for rare-earth-doped aluminates and inorganic phosphors in this section. Weber et al. [72] synthesized and studied Er-doped rare-earth aluminate glass. Chaware et al. [73] synthesized Eu3+ -doped SrAl12 O19 nanophosphors using the urea-assisted combustion synthesis method. The study showed that the emission spectra had peaks at 590 and 614 nm referred to 5 D0 → 7 F1 and 5 D0 → 7 F2, respectively. Further, Judd–Ofelt’s analysis confirmed that high covalency in Eu3+ and ligands. Further, Eu3+ is located at the site with distorted symmetry. When it excited by 980 nm, fluorescence was observed at 1450–1700 nm, 2650– 3000 nm, 650–700 nm (red), and 530–570 nm (green) referring to transitions from 4 I13/2 , 4 I11/2 , 4 F9/2 , and 4 S3/2 /2 H11/2 to electronic states, respectively. Neema et al. [74] synthesized Ce3+ and Dy3+ co-doped SrAl2 O4 inorganic phosphors using solid-state reaction method. The photoluminescence study showed that emission spectra were dominated by Dy3+ activator ions and emission was observed in blue region but for higher concentration Ce3+ ions the emission shifted to red region. Muresan et al. [75] synthesized a series of samples of yttrium aluminum doped with Gd, Ce, Eu, and Tb rare-earth using homogenous precipitation method. The study showed that the presence of Tb3+ ions increased the enhancement of Ce3+ emission to 23% but for Eu3+ ions intensity decreased by 23%. When yttrium was replaced by gadolinium, the emission Ce3+ shifted from 527 to 554 nm and intensity of emission decreased but Eu3+ emission increased due to radiative processes between Gd3+ to Eu3+ . Further, analysis showed that variable luminescence was obtained from green to white to red for the synthesized samples under 254 nm excitations. Aluminate-based phosphors are famous for persistent luminescence; for example, Holsa et al. [76] synthesized and studied a series of samples of Eu2+ -doped CaAl2 O4 and SrAl2 O4 inorganic phosphors. The samples were synthesized using the solid-state reaction method. Both series of samples had very similar luminescence spectra, confirming that the luminescent center was Eu2+ ions in both cases. The effect of doping on the size and shape of the luminescence spectra was negligible, but the afterglow Thermoluminescence was enhanced. As a result, the rare-earth-doped aluminates demonstrated excellent photoluminescence and persistent luminescence. The rare earth is the ideal host for rare-earth metal doping and provides an excellent host matrix for the rare-earth metal ions.

5 Sulfate-Based Phosphors for Solid-State Lighting Applications To completely fill their valence shell, sulfur anion can bond to a metallic or semimetallic material. To form the tetrahedral sulfate (SO4 )2− anion, sulfur cations form a strong bond with four oxygen atoms [77]. The sulfates can be anhydrous or hydrous sulfates. Anhydrous sulfates are sulfates without water molecules or without the

Charge Transfer in Rare-Earth-Doped Inorganic Materials

41

hydroxyl anion attached to the lattice structure of the sulfates. On the other hand, the hydrous sulfates are the ones that have a hydroxyl or water molecule attached to them, for example, gypsum (CaSO4 .2H2 O), chalcanthite (CuSO4 .5H2 O), etc. [78]. Hydrous sulfate has a sheet-like structure in which metal ions and sulfate ions are separated by water molecules. Further, if the water has been removed from the anhydrous sulfate, then the anhydrous sulfate structure will collapse into the respective hydrous sulfate structure. Sulfates are the most attractive candidates for their luminescence properties. These materials have a very simple chemical composition and have various applications such as glasses, paints, thermal energy storage, light-emitting devices, etc. It is interesting to know that sulfates are the first artificial phosphor that was made using the calcination method [79, 80]. Sulfate-based inorganic phosphors have been extensively studied, but many groups are still working on interesting projects due to their high luminescence intensity, significant applications in thermoluminescence (but not so much in photoluminescence), and prominent energy transfer between ions [81–84]. The chapter is focused on the charge transfer in luminescence phosphors, which is observed in emission and excitation spectra. Due to this, the photoluminescence from the previously published literature is mentioned here. Liang et al. [85] synthesized rareearth hydroxides rigidly pillared by sulfate ions using the homogeneous precipitation method, which was driven by the hydrolysis of hexamethylenetetramine. The samples were doped with Eu and Tb, which showed red and green emissions, respectively. The prominent red emission in Eu-doped samples was corresponding to 5 D0 → 7 F2 sharp transition ascribed to 4f-4f transitions in the Eu3+ metal ions. Along with this, a broad band at lower wavelength was due to the charge transfer between O2− and Eu3+ ions, the charge transfer phenomena are discussed later in the chapter. Kher et al. [86] synthesized Dy- and Eu-doped BaSO4 and MgSO4 phosphors were synthesized using solid-state reaction method. Dy-doped BaSO4 and MgSO4 showed peaks at 470 and 580 nm which was due to 4 F9/2 → 6 H5/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+ ions. Further, for Eu-doped BaSO4 sample, only one peak was observed at 380 nm but, for MgSO4 samples, three peaks were observed at 360, 589, and 620 nm. Out of which, peaks between 360 and 440 nm was referring to 4f 6 5d → 4f 7 transitions and peaks at 589 and 620 nm were referring to 5 D0 → 7 F1 and 5 D0 → 7 F2, respectively, transition of Eu3+ ions. Low energy emission (near at 612 nm) was due to 4f–4f transitions, while the high energy emission was from dipole allowed 5d–4f transitions. The high energy peaks were also depending on crystal structure of host materials. Durgakar et al. [87] synthesized Eu, Dy, and Sm doped KNa(SO4 )1-x (MO4 )x (where, M = PO4 , VO4 , and WO4 ) phosphors using solid-state diffusion technique. Eu-doped KNa(SO4 )1-x (MO4 )x phosphor showed orange-red emission. Dy-activated doped KNa(SO4 ) showed intense radiation in the white region and intensity increased when Dy-doped in KNa(SO4 )1-x (PO4 )x . Smdoped KNa(SO4 ) and KNa(SO4 )1-x (PO4 )x showed emission in orange-red region (the latter one showed higher intensity compared to the former one), while Sm-doped in KNa(SO4 )1-x (VO4 )x showed emission in the green region. All these emissions were suitable for white light-emitting diodes. The preceding discussion confirmed

42

A. Nande et al.

that rare-earth-doped sulfate-based phosphors have potential applications as well as fundamental research potential.

6 Charge Transfer Mechanism in Rare-Earth-Doped Phosphors Charge transfer (CT) is associated with electron transfer between two or more molecules or different parts of a large molecule. Generally, it occurs in inorganic ligands involving metals. CT in luminescence involves a transition from host materials as well as from the central ions. In general, CT suggests a transfer of electrons from one orbital to the other; however, in rare-earth-doped phosphors, it is affected by the spatial expansion of the charge distribution around the luminescent center and not by the actual transfer. In rare-earth inorganic phosphors, doping is due to the transfer of electrons from the highest occupied energy level of the valance band in the host lattice to the rare-earth metal ions [88–90]. The expected absorption peak of the CT transition is observed in the UV–visible region, and it is a strong and broad absorption peak. It also does not follow any selection rule (except for intervalance CT transitions, for which the Laporte selection rule is valid), so the absorption bond is broad and intense [89]. The width of the CT transition may vary from 5000 to 10,000 cm−1 , and in some cases, a huge stroke shift is observed. The intensity of the CT transition peak depends on the displacement of the charge across the interatomic distance. This is also responsible for a large transition dipole moment and large oscillator strength. Generally, the energy of the CT transition is dependent on the ligand ion’s electronegativity, i.e., covalency. For low electronegative values or higher covalency, the CT transition between the ions shifts to a lower energy. For example, the CT transition absorption band for YF3 for Eu3+ ions is toward high energy when compared to a higher covalent material like Y2 O3 environment [91–93]. Similarly, the observed CT transition band for Eu3+ ions was 45 cm−1 in YPO4 compared to La2 O3 or LaOCl, suggesting the same thing. Further, it is observed that, if the Eu3+ ion state is not stable in some of the host matrix, it also causes the low energy of CT transitions. For example, sulfides favor the divalent ion of a rare-earth ion (like Eu2+ in Y2 O2 S), causing a low-energy CT transition that suggests the trivalent nature is not stable [92– 94]. To have a better understanding of CT mechanisms, first, the rare-earth metal ion transitions are discussed. Later, in the section, 4f–5d and charge-transfer transitions are discussed, as well as recent work on CT transitions. The rare-earth metals are also known as lanthanides, which contain 15 elements from La (atomic number 57) to Lu (atomic number 71). From Ce3+ to Lu3+ , 1– 14 electrons are added to 4f orbital along with the inner shell configuration (the inner shell configuration for these elements is equivalent to Xenon). Out of the 15 rare-earth elements, La and Lu are, respectively, no-filled (zero-electrons in the 4F shell) and completely filled (14 electrons in the 4F shell); thus, they do not possess

Charge Transfer in Rare-Earth-Doped Inorganic Materials

43

electronic energy levels that lead to the excitation or emission of luminescence in the near visible region. In contrast to the 3dn transition metal ions, the other rare-earth metal ions (Ce3+ to Yb3+ ) have a partially filled 4f shell in which electrons in the 4f orbital are shielded from their surroundings by fully filled 5s2 and 5p6 orbitals. The electronic levels are classified using the classical 2 S + 1 LJ Russell–Saunders notation for spherical symmetry. This suggests the influence of host lattice on the luminescence or optical transitions within the 4F energy level is very small (but cannot be neglected). As a result, these ions have distinct electronic energy levels that emit and excite luminescence in or near visible regions. Also, the presence of crystals or host lattices affects the position of transition energy levels, which is termed the naphaelauxetic (cloud expanding) effect. Further, the width of emission and excitation is small in comparison with the transition metal ion transitions. All the rare-earth ions have trivalent forms; along with trivalent forms, Ce, Tb, and Pr have tetravalent forms, and Eu, Sm, Yb, and Tm have divalent forms as well. The detailed ground state, number of transitions, excitation, and emission transitions with references are provided in Table 4. The Stark effect of the crystal field causes the splitting of energy levels J into sublevels. The most possible splitting is estimated using (2 J + 1) for integer J and (J + 1/2) for the half-integer J. The number of sub-energy levels for rare-earth metal ions is different for all the ions; the lowest non-zero energy levels are 2 for Ce3+ , while the highest was observed for Tb3+ , i.e., 3106. The other rare-earth ions La3+ , Pr3+ , Nd3+ , Pm3+ , Sm3+ , Eu3+ , Gd3+ , Dy3+ , Ho3+ , Er3+ , Tm3+ , Yb3+ , and Lu3+ have 0, 20, 107, 386, 977, 1878, 2725, 2725, 1878, 977, 386, 107, and 20 energy levels, respectively. The characteristic energy levels of trivalent lanthanide ions of 4f electrons were investigated in detail [95]. However, too many energy levels are observed in rare-earth metal ions and are shown in the figure of ref [95]. Thus, the extension of Dieke diagram and experimentally observed strongest lines were studied by Wegh et al. [96] and is shown in Fig. 2. These Dieke diagrams and the extension of Dieke diagram are very useful to assign the peaks observed in absorption emission spectra. The rare-earth ion transitions can be of two types: intra-configurational 4f n → n 4f transitions and interconfigurational 4f n−1 5d → 4f n transitions. Electric dipole and magnetic dipole interactions in 4f levels are the main cause of electronic transitions between 4f levels and originates luminescence phenomenon. Generally, intraconfigurational 4f n → 4f n transitions give sharp lines in infrared, visible, and UV regions in the emission and excitation spectra. The obtained transitions are comparatively weak and also the lifetime of the transitions is in milliseconds. The electric dipole oscillator strength of the f–f transitions is of the order of 10–6 and has weak transition due to weak ion lattice coupling. These transitions are forbidden and follow Laporte and spin selection rules. The Laporte selection rule can be relaxed due to electron–phonon interaction and interaction with higher orbitals. Further, the rule suggests the initial and final states must have opposite parity and follows ∆l = ±1, where, l is the orbital quantum number. On the other hand, spin selection rule suggests the conservation of spin (∆S = 0) in electric dipole transitions. The rule

9

3H 4

4I

Pr3+

Nd3+

9/2

5

2F 5/2

Ce3+

2

0

1S

La3+

Expected transitions

Ground state

Rare-earth metal ions

366 nm 4I 2 2 4 9/2 → I13/2 + D5/2 + D11/2 438 nm 4I 2 2 9/2 → P11/2 + D5/2 480 nm 4I 4 2 2 9/2 → G11/2 + D3/2 + G9/2 523 nm 4 I9/2 → 4 G9/2 538 nm 4I 4 3 9/2 → G7/2 + K13/2 598 nm 4I 4 2 9/2 → G5/2 + G7/2 649 nm 4 I9/2 → 2 H11/2 693 nm4 I9/2 → 4 F9/2 758 nm 4I 7 4 9/2 → F7/2 + S3/2 818 nm 4I 4 2 9/2 → F5/2 + H9/2

3H → 3P ; 3H → 3P 4 2 4 1 3H → 3P ; 3H → 1I 4 0 4 6 3H → 1D 4 2

–

–

Excitations

Most favorable transitions

(NIR region) 4 F3/2 → 4 I9/2 4F 4 3/2 → I11/2

(Blue region) 3 P0 → 3 H4 (Red region) 3 P → 3 H ,3 F 1 D → 3 H 0 6 2 2 4

~420 nm (2 D → 2 F5/2 ) ~500 nm (2 D → 2 F7/2 )

–

Emissions

Table 4 Ground state, number of transitions, excitation, and emission transitions in rare-earth metal ions

(continued)

Lee et al. [99], Kamimura et al. [100]

Jamalaiah et al. [98]

Gavhane et al. [97]

Refs.

44 A. Nande et al.

20

3

30

6H 5/2

7F 0

8S 7/2

7F 6

Sm3+

Eu3+

Gd3+

Tb3+

7

14

4

5I

Pm3+

Expected transitions

Ground state

Rare-earth metal ions

Table 4 (continued)

–

→ 6 D7/2 ) → 6 D9/2 ) → 6 I15/2 , 6 I13/2 ) → 6 I9/2 ) → 6 I7/2 )

→ 5 D0 ) → 5 L7 ) → 5 L6 ) → 5 D3 )

246 nm (8 S7/2 253 nm (8 S7/2 273 nm (8 S7/2 276 nm (8 S7/2 279 nm (8 S7/2

365 nm (7 F0 384 nm (7 F0 400 nm (7 F0 420 nm (7 F0

6H 6 6 4 5/2 → P3/2 ; H5/2 → D3/2 ; 6H 6 6 6 5/2 → P5/2 ; H5/2 → P7/2 ; 6H 4F ; 6H 6H → → 7/2 13/2 5/2 5/2

Excitations

Most favorable transitions

→ 7 F0 ) → 7 F1 ) → 7 F2 ) → 7 F3 ) → 7 F4 )

380 nm (5 D3 → 7 FJ ) (Green region) 490–550 nm (5 D4 → 7 FJ )

307 nm (6 P5/2 → 8 S7/2 ) 313 nm (6 P7/2 → 8 S7/2 )

580 nm (5 D0 594 nm (5 D0 613 nm (5 D0 623 nm (5 D0 648 nm (5 D0

(Yellow–red region) 4G 6 4 6 5/2 → G5/2 ; G5/2 → H5/2 ; 4G 6H ; 4G 6H ; → → 7/2 9/2 5/2 5/2 4G 6 5/2 → H11/2

820 nm 5 F1 → 5 I4 930 nm 5 F1 → 5 I5 1098 nm 5 F1 → 5 I6 790 nm 5 F2 → 5 I4 890 nm 5 F2 → 5 I5 1042 nm 5 F2 → 5 I6

Emissions

(continued)

Garcia et al. [110], Jain et al. [111]

Singh et al. [109]

Kadam and Dhoble [107], Li et al. [108]

Khan et al. [104], Xiang et al. [105], Halappa et al. [106]

Beach et al. [101], Shinn et al. [102], Carnall et al. [103]

Refs.

Charge Transfer in Rare-Earth-Doped Inorganic Materials 45

5I

4I

3H 6

2F 7/2 1S 0

Ho3+

Er3+

Tm3+

Yb3+

Lu3+

6H 15/2

Dy3+

15/2

8

Ground state

Rare-earth metal ions

Table 4 (continued)

1

25

62

79

79

58

Expected transitions

–

363 nm 6 H6 → 1 D2

–

365 nm 5 I8 → 3 H4 382 nm 5 I8 → 4 G4 415 nm 5 I8 → 5 G5 ~ 450 nm 5 I8 → 5 G6

–

Excitations

Most favorable transitions → 6 H15/2 ) → 6 H13/2 ) → 6 H11/2 ) → 6 H9/2 ,6 F11/2 ) → 6 H7/2 ,6 F9/2 )

–

4H → 4I 12 15/2 4H → 4S 12 3/2 4F 4 9/2 → I15/2 455 nm 1 D2 → 3 F4 650 nm 1 G4 → 3 F4 780 nm 1 G4 → 3 H5 980 n2 F5/2 → 2 F7/2

490 nm 5 F3 → 5 I8 532 nm (5 F4 + 5 S2 ) → 5 I8 635 nm 5 F5 → 5 I8

485 nm (4 F9/2 575 nm (4 F9/2 660 nm (4 F9/2 755 nm (4 F9/2 840 nm (4 F9/2

Emissions

Wang et al. [118]

Li et al. [117]

Li et al. [116]

Öztürk and Karacaoglu [114], Singh et al. [115]

Bedyal et al. [112], Gupta et al. [113]

Refs.

46 A. Nande et al.

Charge Transfer in Rare-Earth-Doped Inorganic Materials

47

Fig. 2 Strongest experimental lines observed for middle rare-earth ion which is also called the extension of the Dieke diagram in the visible–ultraviolet region. Reused with permission from [96]. (copyright taken—Copyright © 2000 Elsevier Science B.V.)

relaxes in the presence of spin–orbit coupling; thus, transitions also involve spinforbidden features. Also, spin–orbit coupling increases the intensity of rare-earth ion transitions compared to transition metal ions. This suggests the optical transitions in rare-doped inorganic phosphors are due to the electric dipole transition induced by odd crystal field components, electric dipole transitions induced by odd vibrations, magnetic dipole transitions, and electric quadrupole transitions. The other transitions in rare-earth ions are interconfigurational 4f n−1 5d → 4f n transitions. These transitions are usually broad bands without any structure and are observed in the above ultraviolet spectral region. The emission transitions are due to d → f transitions originating from the lowest onset of 5d states, while excitation spectra involve non-radiative relaxation via d states and spin-allowed or spin-forbidden transition to lower f levels. For rare-earth metal ions, as the energy levels are spanned over 4f energy levels, two additional electronic differences from these energy levels are observed. These energy levels are 4f n−1 5d1 energy states and charge transfer states [119]. In the 4f n−1 5d1 energy states, the electron or electrons transfer to the 5d orbital, while in the CT state, the electron or electrons in the neighboring anions transfer to the

48

A. Nande et al.

Fig. 3 Types of CT process (i) fundamental host transition, (ii) intervalance CT transition and CT from ligand to dopant. Reused with permission from [120]. (Copyright © 2010 published by Elsevier B.V.)

4f orbitals. The 4f n−1 5d1 energy states and CT transitions depend more on the ion environment than the 4f energy states. The CT transitions involve a rare-earth ion and its surroundings, which are either ligand-to-metal charge transfer or metal-to-ligand charge transfer, or there is another possibility in which charge is transferred from metal-to-metal ions. The absorption or emission bands due to all possible charge transfers are broad. However, the positions of emission and excitation states are found to be almost equal for the same series of rare-earth ions in different host materials. The schematic of the charge transition is shown in Fig. 3. CT is classified into three types: (i) host transitions—charge transfer from the oxygen valance band to the conduction band; (ii) charge transfer from a trivalent rare-earth (RE3+ ) ion to the conduction band, producing a rare-earth 4+ ion; and (iii) charge transfer from an O2– ion to an incompletely fielded 4f shell of rare-earth 3+ ions, producing a rare. CT transitions or bands are observed in many rare earth metal oxides and raredoped inorganic phosphors. The trivalent rare-metal ions which can have divalent (like Sm3+ , Eu3+ , and Yb3+ ) depict charge-transfer absorption in the ultraviolet region. In contrast, the rare-earth metal ions which tend to have tetravalent state (like Ce3+ , Pr3+ , and Tb3+ ) show 4f → 5d absorption bands in the ultraviolet region. The CT transitions are due to the energy transfer of the highest occupied valance band orbital to the rare-earth metal ions. These transitions have predominant effect on emission spectra and their applications. Such as CT transitions in Sr2 SiO4 : Eu2+ and Sr3 SiO5 : Eu2+ inorganic phosphors had a vital role in photoluminescence emission process which gave a wide emission in visible region. This made these phosphors suitable for white light-emitting diodes [121–123]. Sometimes, the broad emission is associated with ligand distribution around the metal ions like in Eu2+ -doped Sr2 SiO4 due to the arrangement of distribution of oxygen ion around two Sr metal ions: nine oxygen

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ions around co-ordination sites of Sr(I) and ten oxygen ion around co-ordination site Sr(II) site [124]. Similarly, Yb- and Y-doped Y2 O3 inorganic phosphors also showed wide or broad CT transition band [125, 126]. Here, we discuss the CT transition observed in excitation and emission band observed in inorganic phosphors. In recent work, Ran et al. [127] studied photoluminescence properties of KLaMgWO6 : Eu phosphors along with their crystal structure and electronic structure. The photoluminescence study suggests that the excitation spectra were located in near-ultraviolet and blue regions. Along with excitation peaks at 394 nm (7 F0 → 5 L6 ) and 465 nm (7 F0 → 5 D2 ), a wide asymmetric excitation band from 250 to 420 nm was observed which was CT band. The CT band is assigned to the absorption from p orbital of oxygen atom to the 4f orbital of Eu atom and 5d orbital of W atom. Further, analysis confirmed that the CT band locations of O2− → Eu3+ and O2− → W6+ were centered at 294 nm and 345 nm. Moreover, the efficient energy transferred from WO6 to the 5 D0 energy level of Eu3+ ions, later a set of 5 D0 → 7 FJ characteristics transitions were exhibited by the phosphors. Shang et al. [128] studied the photoluminescence properties of CaCe(PO4 )14 : Eu/Tb/Mn phosphors. The excitation spectra of CaCe(PO4 )14 : Eu showed a broad peak in 200–300 nm range which was centered at 250 nm resulting from the absorption of the Eu3+ –O2− CT transition. The other transitions observed in the excitation spectra were due to the 4f–4f characteristics of Eu3+ ions. Further, the emissions of the samples were measured under the excitation of 254 nm which showed a broad transition of Ce3+ ions at 366 nm and a sharp peak at 617 nm due to Eu3+ ions. The energy transfers from Ce3+ to Eu3+ ions were due to metal-to-metal CT. This suggested that it was difficult for Ce3+ and Eu3+ ions to coexist in the same host. Further, with increasing Eu3+ molar concentration, the emission peak of Ce3+ decreased. A similar result was observed for Tb doped CaCe(PO4 )14 ; an excitation broad peak was observed ranging from 200 to 350 nm which was due to f–d transition of Ce3+ and some weak peaks at 350–500 nm due to f–f transition of Tb3+ . It was observed that with the excitation of the phosphor by 365 nm, the Ce3+ emission from the host lattice decreased gradually, while the concentration of Tb3+ ion increased. Pan et al. [129] studied the CT energy between F– and Eu3+ ions. The photoluminescence excitation spectra for CaF2 : Eu phosphors showed a broad peak at 277 nm and other sharp peaks at 363 nm, 412 nm, 465 nm, and 531 nm. The broad peak at 277 nm was due to the CT from F– to Eu3+ ions; it was further shown that the CT broad band was dependent on the excited wavelength. The road shifted with the excited wavelength suggesting Eu3+ ions in CaF2 were present in three different lattice environments. Here, the CT energy was due to the excited electrons on the F– ions transferred to the 4f orbits of Eu3+ to form excitation state. This suggested after the excitation, the electrons of F– 2p orbits could transfer to the 4f orbits of Eu3+ . Barandiaran et al. [130] reported the color control of Pr3+ -doped CaTiO3 and CaZrO3 phosphors by electron–hole recombination energy transfer process. The excitation spectra showed two broad bands at 330 and 370 nm, which attributed to the O2− to Ti4+ ligand-to-metal charge transfer. Further, the reflectance and excitation spectra were due to the host to Pr (1 D2 ) energy transfer. Also, ab initio calculation showed that the sole presence of the 1 D2 red emission and quenching of greenish

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blue emission from the 3 P0 level was due to the electron–hole recombination energy transfer Pr-doped LuNbO4 had two broad bands at 265 and 305 nm which were due to NbO4 3− from the host to 1 D2 energy level of Pr3+ ions. Wu et al. [131] studied Pr/Tb co-doped LuNbO4 phosphors which could use as optical thermometer. The photoluminescence excitation spectra of absorption and Pr3+ –Nb5+ intervalance CT absorption. Similar behavior was observed for Tb-doped LuNbO4 phosphor which showed the charge transfer from NbO4 3− group and Tb3+ –Nb5+ intervalance charge transfer. For Pr/Tb co-doped LuNbO4 phosphor two excitation bands were attributed to NbO4 3− absorption and Pr3+ Tb3+ –Nb5+ intervalance transfer. Guo et al. [132] studied the effect of Ta-O charge transfer band on the emission properties of Dydoped SrLaMgTaO6 phosphors. In excitation spectra along with sharp excitation peaks centered at 354, 390, and 452 nm, a broad host absorption peak centered at 250 nm was observed. Further, excitation spectra monitored at 460 nm showed two absorption bands ranging from 200 to 280 nm and 300 to 400 nm. When the phosphor was excited using 250 and 378 nm the phosphor showed a broad emission band from 420 to 600 nm originating from TaO6 group. The study confirmed the energy transfer from the host to Dy3+ metal ion which suggested that the host acted as the activator to enhance luminescent properties of the Dy3+ ions. When the phosphor was excited by 250 nm, the prominent emission of Dy3+ increased with increasing the concentration, in contrast to the emission intensity from TaO6 group. This confirmed the energy transfer from the host to the Dy3+ ions. Table 5 provides the summary of CT transitions from the recent times of selected papers. Table 5 Summary of CT-transitions of selected rare-earth-doped inorganic phosphors Inorganic phosphors

CT transitions

La2 LiSbO6 : Ln (Ln = Eu, Tb, Tm, Sm Ho)

The CT-band was located in the excitation spectra at Zhang et al. [133] 240–360 nm, which was attributed to the host absorption of O2-(2p) → Sb5+ (4d) CT of the [SbO6 ]7− group and CT between O2− and Ln3 . Phosphor was excited by the CT band along with the characteristic transition of Ln3+ ions, resulting in a strong emission band observed at 393 nm, which was attributed to the internal CT of [SbO6]7−

Lu2 MoO6 : Sm

A CT band was observed from 250 to 400 nm, which Li et al. [134] was attributed to the CT band of O2 —Mo6+ and O2 —Sm6+ . O2 —Mo6+ CT band in the excitation spectra of Sm3+ ions showed that the excitation energy was transferred from MoO6 6− groups to Sm3+

Refs.

(continued)

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Table 5 (continued) Inorganic phosphors

CT transitions

Refs.

Y4 Zr3 O12 : Eu

CT from a completely filled 2p orbital of O2− to a partially filled 4f orbital of Eu3+ ions was observed from 220 to 320 nm. With increasing Eu3+ ion concentrations, the CT band position shifted to a higher wavelength or lower energy

Park and Yang [135]

Ba2 CaWO6 : Pr

CT band observed in the ultraviolet region, ranging Sreeja et al. [136] from 230 to 350 nm, centered at 314 nm, corresponding to CT absorption from oxygen’s 2p orbitals to tungsten’s 5d orbitals. Under 314 nm excitation, the CT state is due to an electron getting excited from the 2p orbitals of oxygen to the 5d orbitals of tungsten. Along with this transition, the excitation energy was transferred to the Pr3+ ion’s 3 P2 level, followed by rapid non-radiative relaxation. Through energy transfer from WO6 6− to Pr3+ , efficient Pr3+ emission was achieved using tungstate as a host

Ba2 Y5 B5 O17 : Bi/ Eu

Excitation showed a broad band with a peak at Annadurai et al. 289 nm (ranging from 250 to 310 nm), which was due [137] to the O2− to Eu3+ CT band, and the other peaks were characteristic transitions of Eu3+ ions

SrLaLiTeO6: Eu

Excitation showed a broad band with a peak at Lal et al. [138] 293 nm, which was due to the O2− to Eu3+ CT band; as well as the other characteristic peaks of Eu3+ ions. When the phosphors were excited at 293 nm, energy was transferred from the ligand to the metal charge transitions—the oxygen 2p orbital to the empty 4f orbit of Eu3+ ions

SrAl3 BO7 : Eu3+ , Eu2+

In the excitation spectra, the CT band was observed due to O2− to Eu3+ charge transfer in the range of 200–350 nm. Under UV excitation, two broad bands due to Eu: 5d-4f were observed at 400 nm and 470 nm, with highly intense characteristics in orange luminescence

Liu et al. [139]

MgAl2 O4 : Dy3+

A broad peak of CT was observed in the range of 250–450 nm, centered at 354 nm, which was attributed to O2− –Dy3+ . It could be due to electron transfer from the 2p orbit of O2− ions to the 4f shells of Dy3+

Pratapkumar et al. [140]

7 Conclusions The goal of this chapter is to go over CT transitions in rare-earth-doped inorganic phosphors as well as explain the CT band seen in emission and excitation spectra. To have a better understanding, the oxide-based host is discussed, and the chapter also

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provides a collection of research papers on the respective host materials. The chapter also has an explanation of energy levels and the most probable energy transitions of rare-earth metal ions. Further, in the chapter, fundamentals and types of CT transitions have been discussed. Also, the CT transitions of trivalent and divalent rare-earth ions in inorganic phosphors and the study of local structure for the rare-earth ions have been discussed using recent research papers about the CT transitions. The chapter also provides information about how the host matrix around the rare-earth metal ions affects the CT transition. In the last section, we have provided a summary of the recent works carried out by different groups, which is summarized in the table.

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ZnO-Based Phosphors Materials S. J. Mofokeng, T. P. Mokoena, L. L. Noto, T. A. Nhlapo, M. J. Sithole, D. E. Motaung, and M. R. Mhlongo

1 Introduction Phosphors are regarded as an inorganic material in the form of powders or thin films that can release light. Most phosphors are nanostructured materials that consist of a host matrix and an activator, i.e. a relatively low content of impurities like rare earths, transition metals, and noble metals that are deliberately introduced into the host lattice. The major applications of nanophosphors exist in light display technologies like lamps, screens or light-emitting diodes; optical storage; biochemical probes; and medical diagnostics [1]. The phosphor nanomaterials are more attractive because of their fascinating advantages, such as extraordinary optical efficiency, a relatively long lifetime, low energy consumption, durability and high brightness [2]. Phosphor materials tend to create micron-sized particles when they are synthesized through solid-state reactions, and such particles display robust scattering features due to their S. J. Mofokeng (B) · L. L. Noto · M. J. Sithole Department of Physics, School of Science, Science Campus, CSET, University of South Africa, Private Bag X6 Christiaan de Wet and Pioneer Avenue, Florida Park, Johannesburg 1710, South Africa e-mail: [email protected] T. P. Mokoena Department of Physics, University of the Free State (QwaQwa Campus), Private bag X13, Phuthaditjhaba 9866, South Africa T. A. Nhlapo Department of Medical Physics, Sefako Makgatho Health Sciences University, P.O. Box 146, Medunsa 0204, South Africa D. E. Motaung Department of Physics, University of the Free State, Bloemfontein 9300, South Africa M. R. Mhlongo Department of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94, Medunsa 0204, South Africa © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_3

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relatively large particle size. Thus, for phosphor materials to be used in light-emitting displays, nanoparticles that are 50 nm ought to be synthesized to prevent scattering [3]. Therefore, this proved that the selection of synthesis methods for the fabrication of nanophosphors is a stepping stone towards the desired applications. Broad transparency range, relatively high refractivity, tuneable crystal phase, small cut-off phonon frequency, low photon energy and superior thermal stability are the benchmarks for selecting an exceptional host lattice in luminescence applications [4]. Numerous lattice-based phosphors have been used, and most of them are unique in their own way. However, oxide lattice phosphors have proved to be the best candidates compared to their counterparts, and this is due to their relatively high stability and colour purity, which enable light harvesting through the up- and downconversion processes [5, 6]. Zinc oxide (ZnO) is one of the oxide lattice phosphors, and it is a versatile semiconducting metal oxide (SMO) with a wide energy bandgap ranging from 3.2 to 3.4 eV. This material links semiconducting, piezoelectric and optical features together. Thus, it may be utilized in a broad spectrum of applications, from sensor technology to optoelectronic devices [7]. In general, ZnO semiconductors in various crystals, ceramics and powders emit a narrow near-ultraviolet band and a broad emission band in the ultraviolet and visible regions of the electromagnetic spectrum, respectively. The UV emission band is typically obtained from the ZnO absorption edge, which is also referred to as nearband-edge (NBE) luminescence. As a result of free exciton recombination, NBE luminescence has an excitonic nature. The intense luminescence of ZnO in the visible area makes this material to be a preferable candidate for phosphor applications. Dissimilar and/or various native defects display luminescence in various regions such as green and blue emissions assigned to Vo , VZn and Zni , respectively (refer to Fig. 1a–c). The imperfections in the host lattice are known as intrinsic defects, when constituent atoms diffuse, migrate or are eliminated from the native lattice sites. These intrinsic defects are zinc vacancy, zinc interstitials and zinc antisite. In the case of zinc vacancy, the atom is misplaced or eliminated from its native or host lattice and it forms vacancies like oxygen vacancy and zinc vacancy. Furthermore, for zinc interstitials, an additional atom is in an interstitial position, and these are called interstitial defects like oxygen interstitial and zinc interstitial. For zinc antisite, an atom is positioned on the native lattice site of another atom where these are referred to as antisite defect like, the oxygen atom located at zinc site is denoted as O-antisite and zinc atom located at the incorrect lattice or in place of O atoms is considered as Zn-antisite. For the development of highly efficient optoelectronics, it is very critical to study the origin of these emissions thoroughly, and on most occasions, it was witnessed that the origination of defect-related deep-level emission greatly relies on the synthesis methods and growth conditions [8, 9]. However, the incorporation of transition metal, rare-earth (RE) ions, non-metal, alkali metal and alkali earth metal dopants into the lattice structure of ZnO are described to be beneficial to its wider applications. ZnO is a multipurpose semiconductor with a high phonon energy threshold, which is combined with luminescent centres and a large number of intrinsic defects. The combination of these features makes ZnO a suitable host luminescent material for RE ions due to their special

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Fig. 1 a Literature survey on the origin of defects associated with blue, green and yellow luminescence. b Illustration of PL emission spectra for blue, green, yellow and red emission from intrinsic defect of ZnO [8]. Reproduced with permission from Rai H, Prashat, Kondal N. A review on defect-related emissions in undoped ZnO nanostructures. Mater Today: Proc. 2022; 48:1320–1324. Copyright Elsevier (2022). c A proposed PL mechanism of ZnO under the UV excitation of 325 nm

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thermal quenching effect on the efficiency of their luminescence. The advancements in the modification of the optical response of ZnO nanostructured through RE doping are reported to exhibit remarkable optical properties such as photoluminescence, magnetic behaviour and transmittance, just to mention a few [10]. Additionally, ZnO nanostructured semiconductor modified by transition metals usually possesses excellent visible light photocatalytic activity [11]. The RE ions are well known as lighting elements which proved to be the centre of attention for luminescence applications due to their exceptional features, such as multifunctional properties to realize a wealth of luminescent characteristics by behaving as wavelength converting system (up-conversion (UC) and down-conversion (DC) or down-shifting (DS)). The RE ions refer to a series of consecutive elements in the periodic table, which are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). Some of these elements are classified as down-converting or down-shifting elements and some as up-converting elements [10, 12]. UC process includes elements, which are emitting UV or visible light via the sequential absorption of numerous near-infrared photons, while the DC or DS process includes elements that can emit visible light via the consecutive absorption of numerous ultraviolet light [13, 14]. An interesting characteristic feature of these ions is arising from their distinct electronic transitions within 4f orbitals configuration as a result of strong emission in ultraviolet, visible or near-infrared spectral regions that can introduce new spectral features, such as high purity of the colour of emissions [12]. However, late transition metals doped suitable phosphor materials are also suitable candidates for the DC process because their hybrid form tends to create the unique electronic structural changes and local atomic environment in the doped phosphor systems [15–17]. The general objective of this chapter is mainly focused on the new trends of ZnO nanostructures. This also includes the RE ions and transition metals that are used to improve the quality of ZnO nanostructures for various applications; we focused on enhanced photoluminescence properties with doping of down- and up-converting ions and metals.

2 Crystal Structure of ZnO ZnO is a versatile II–VI compound SMO whose ionic behaviour is located at an intermediate position amid the covalent and ionic semiconductors. The crystal structures that are common in ZnO are wurtzite (B4), zinc blende (B3) and rocksalt (or Rochelle salt) (B1) as presented by diagrams in Fig. 2. Amongst the mentioned structures, the wurtzite symmetry structure is utmost thermodynamically stable under ambient pressures and temperatures [18, 19]. It has been shown using theoretical calculations that a fourth phase, cubic cesium chloride, could be at extreme temperatures. Nevertheless, this phase has not been detected experimentally [20]. In contrast, the zincblende symmetry structure is thermodynamically metastable and may be steadied only by

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epitaxial growth on cubic substrates. The rock-salt phase or Rochelle salt (NaCl) has commonly been observed as thin layers, grown on cubic substrates at higher pressures [18, 19]. The normal crystal structure of ZnO semiconductor is generally determined by tetrahedrally coordinated bonding geometry which consists of oxygen (O2− ) and zinc (Zn2+ ) layers. The traditional view of an arrangement of the bonding geometry of the structure of this semiconductor depends on the stick-andball stacking sequence of the bi-layers. This phenomenon can result in hexagonal wurtzite structure, cubic zincblende structure or cubic rock-salt structure [8, 19]. Figure 2 illustrates the polymorphs of ZnO semiconductor: rock-salt, zinc-blende and hexagonal wurtzite structures. These structures consist of alternating stacking arrangements of tetrahedrally coordinated zinc (Zn2+ ) ions and oxygen (O2− ) ions along the c-axis direction [18, 19], see Fig. 2. The ideal wurtzite crystal structure possesses a hexagonal unit cell with two lattice parameters a = b = 3.2495 Å and c = 5.2069 Å, primitive unit cell volume per formula unit V = 47.625 Å3 , Z = 2 and the density of 5.605 g.cm−3 [18, 21]. It also belongs 4 in the Schoenflies notation and P6 3 mc in the Hermann– to the space group C6ν Mauguin notation. The lattice parameters a and c are commonly recognized as the basal plane lattice parameter and axial lattice parameter, respectively. Besides, these lattice parameters can be changed by external factors such as doping and lattice strain which are commonly caused by stress field induced by the excess volume in grain

Fig. 2 Three schematic representations of ZnO crystal structures: a cubic rock-salt (B1), b cubic zinc-blende (B3), and c hexagonal wurtzite (B4). Yellow and blue spheres denote Zn and O atoms, respectively [19]. Reproduced with permission from Onga CB, Ng LY, Mohammad AW. A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications. Renew Sust Energ Rev. 2018;81:536–551. Copyright Elsevier (2018)

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boundaries [19]. The axial ratio c/a and u parameter (that is used to quantity the amount by which each atom is displaced concerning the next along the c-axis) are associated by the association u(c/a) = (3/8)1/2, where c/a = 1/2 and u = 3/8 for a superlative crystal. ZnO crystals diverge from this idyllic arrangement by altering both values. This nonconformity happens in a way that the tetrahedral distances are maintained almost the same in the lattice. The real values for the u and c/a, i.e. for wurtzite ZnO, were experimentally observed in the range u = 0.3817–0.3856 and c/ a = 1.593–1.6035 [19, 22–24]. Apart from inducing the inherent polarity in the crystal of ZnO, the tetrahedral coordination of this compound is as well a mutual pointer of sp3 covalent bonding. Besides, the Zn–O bond further has a robust ionic character, and hence, ZnO lies on the marginal between being categorized as a covalent and ionic compound, with an ionicity of f i = 0.616 on the Phillips ionicity scale [25]. The schematic representation of the wurtzite structure is categorized by two interrelating sublattices of Zn2+ and O2− , representing a structure. This may be specified as numerous alternating planes of Zn ion, which is enclosed by tetrahedra of O ions, and vice versa stacked layer-bylayer along the c-axis as shown in Fig. 2. This tetrahedral coordination gives rise to polar symmetry along the hexagonal axis and this polarity is accountable for several characteristics of ZnO, as well as its piezoelectricity and spontaneous polarization, and is correspondingly an important feature in crystal growth, etching and defect creation [26]. The crystal structure of zinc-blende (Fig. 2b) is characterized as a cubic closet packing (ccp) array, which comprises of dual face-centred cubic sublattices displayed by ¼ of the body diagonal axis. As mentioned earlier, it is regarded as the structure that is commonly stabilized by epitaxial grown on the substrates and its symmetry of the structure is arranged in such a way that the zinc ions occupy half of the tetrahedral sites to accomplish charge neutrality [27, 28]. Figure 2a shows the schematic representation of the high-pressure phase of the rock-salt polymorph. It has been reported that the wurtzite ZnO structure is transformed to the ZnO rock-salt polymorph at relatively modest external hydrostatic pressures (~9GPa) and cannot be epitaxially stabilized as in the case of GaN. This high-pressure polymorph has been proven to be metastable for a long period of time above 100 °C and even at ambient pressure. The structural phase transformation from wurtzite to rack-salt with a transition pressure was reported to take place due to the reduction of the lattice dimensions which causes the interionic Coulomb interaction to favour the ionicity more over the covalent nature. The cubic symmetry of the rock-salt structure is six-fold coordinated and belongs to the space group Fm3m [19, 29, 30]. Different researchers have calculated the ZnO electronic band structure using various methods [31–37]. Figure 3 shows the findings of the band structure calculated utilizing the Local Density Approximation (LDA) and including atomic selfinteraction corrected pseudopotentials (SIC-PP) to precisely justify for the Zn 3d electrons [37]. The band structure and high symmetry lines in the hexagonal Brillouin zone are shown in the Figure. Using both the valence band (EV ) maxima and the lowest conduction band (EC ) minima, which happen at the  point k = 0, it is possible to conclude that ZnO is indeed a direct band gap semiconductor. The

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Fig. 3 The LDA band structure of bulk wurtzite ZnO estimated by utilizing the dominant atomic SIC-PP. This approach is effective at treating the d–bands in comparison to the standard LDA method [26]. Reproduced with permission from Vogel D, Kruger P, Pollmann J. Self-interaction and relaxation-corrected pseudopotentials for II–VI semiconductors. Phys Rev B54 (1996) 5495–5505. Copyright APS (1996)

bottom 10 bands, which occur roughly −9 eV are associated with Zn-3d levels. The next 6 bands between −5 eV and 0 eV are linked to O 2p bonding states. The first two EC states are strongly Zn localized and associated to unfilled Zn-3 s levels. The higher EC (not shown here) are free-electron-like. The O-2 s bands (results not shown here) related to core-like energy states happen at roughly −20 eV. The bandgap as extracted from this calculation is 3.77 eV. This relates well with the experimental value of 3.4 eV, which is nearer than the value attained from standard LDA calculations that tend to underrate the bandgap by ~3 eV because of its failure in precisely modelling the Zn-3d electrons [37].

3 Experimental Work 3.1 Synthesis Methods Several synthesis approaches have been previously applied to prepare ZnO nanostructures [8, 38]. These miscellaneous synthesis methods can be divided into different physical-based and chemical-based methods. The physical-based synthesis includes pulsed laser aberration/deposition, ultrasonication, radiofrequency, magnetron sputtering, arc discharge and molecular beam epitaxy, just to mention a few, and the chemical-based approaches involve chemical solution combustion method, sol–gel

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method, co-precipitation, hydrothermal synthesis, spray pyrolysis, electrochemical deposition and chemical vapour deposition [39, 40]. It is worth noting that the improvement of the structural and optical properties of materials, especially crystallite size, surface morphology, lattice strain, orientation, charge carriers, defects and luminescence characteristics of ZnO nanostructures, are strongly depending on the selection of the synthesis method and synthesis conditions. Among these parameters, most of the researchers have focused on the optical and luminescence characteristics of ZnO nanostructures, which enable it as a crucial phosphor material in various areas of nanotechnology, to name a few, renewable and sustainable energy, transparent conductive contacts, gas sensors, solar cells, etc. [38, 40–42]. Among the abovementioned synthesis approaches, it was found that the chemical synthesis method is noteworthy as a fast technique used to fabricate ZnO nanostructures for luminescence applications because this technique is effective, simple cost and reproducible [43]. With this method, the ZnO nanostructured materials are commonly produced through the reaction between precursor reactants and reactive solvents. The precursor reactants, in this case, are zinc salts such as zinc acetate (Zn(C2 H3 O2 )2 , zinc nitrate (Zn(NO3 )2 ), zinc sulfate (Zn(SO4 )2 , zinc chloride (ZnCl2 ), sodium hydroxide (NaOH), while the reactive solvents include deionized water (H2 O) and ethanol (C2 H5 OH). The main benefit of this approach is that the final particle size of the material could be regulated either by choosing chemicals comprising of stable particles when the optimal size is achieved or by ending the growth at a precise size. In addition, researchers have successfully proven that in the chemical synthesis, the bandgap energies and charge carrier separation of oxides materials are crystal phases, crystal size and crystallinity dependent. In chemical synthesis techniques, several parameters can be adjusted and controlled to produce the required crystal structure and optical properties of ZnO nanoparticles. Such parameters are reactants concentrations, temperature, pressure, reaction time, pH, etc., [40, 44, 45]. Thus, by governing the above-mentioned parameters, this method can yield high purity and crystalline ZnO product. Generally, when the reaction in the chemical synthesis technique occurs, the nucleation process is first forms and then followed by crystal growth of the product of the nanosized regime. The nucleation system is caused by the molecules, ions or atoms in the chemical reaction solution. Recently, the chemical synthesis method is considered the most widely used method of fabricating nanomaterials in the industry [46].

3.2 Luminescence of ZnO Nanostructure-Related Phosphors As indicated earlier, the photoluminescence of ZnO at room temperature displays violet, green and blue emission lines assigned to various defects. The PL emission spectrum of ZnO prepared by a chemical-based approach (co-precipitation) that was recorded at room temperature is shown in Fig. 4a. Two emission bands were observed in the UV and visible regions. The minor UV emission around 384 nm is attributed to the excitonic recombination and broad emission in the visible region around 600 nm

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is attributed to the defects present in the ZnO phosphor material. The inset shows a magnified view of the excitonic recombination emission of ZnO. The observed wide emission in the visible region of the spectrum was de-convoluted using Gaussian fit. The de-convolution proved the presence of the three emissions because of native point defects in the crystal structure of ZnO; green (~530 nm), yellow (~590 nm) and red (~680 nm) which originate from singly ionized oxygen vacancy (Vo + ), doubly ionized oxygen vacancy (Vo ++ ) and oxygen interstitials (Oi ), respectively [40]. Yet another PL emission spectra of ZnO nanoparticles synthesized using the coprecipitation method by varying the molar ratio of sodium hydroxide during the

Fig. 4 a De-convolution of PL spectrum [40] (Reproduced with permission from Ntwaeaborwa OM, Mofokeng SJ, Kumar V, Kroon RE. Structural, optical and photoluminescence properties of Eu3+ -doped ZnO nanoparticles. Spectrochim Acta A Mol Biomol Spectrosc. 2017;182:42– 49. Copyright Elsevier (2017)), b PL spectra [47] (Reproduced with permission from Banerjee D, Kar AK. Effect of hydroxide ion concentration on the evolution of nanostructures and structure correlated luminescence of ZnO nanopowders. Opt. 2019;89:430–440. Copyright Elsevier (2019)) and energy-level illustration of the luminescence mechanism [48] of ZnO nanoparticles. Zni , Vzn , Vo and V+ 0 represent zinc interstitial, zinc vacancy, oxygen vacancy and singly ionized oxygen vacancy, respectively [48]. Reproduced with permission from Rajkumar C, Srivastava RK. UV–visible photoresponse properties of self-seeded and polymer-mediated ZnO flower-like and biconical nanostructures. Results Phys. 2019;15:102,647. Copyright Elsevier (2019)

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synthesis is shown in Fig. 4b. According to Banerjee et al. [47], the observed broad emission consists of several emissions at around 400, 415, 440, 452, 470, 485, 495 and 539 nm. It was highlighted that the broadness of observed emission is because of the presence and overlapping of emission from the deep trap levels. However, variation of the molar ratio of the precursors altered the intensity of the emission bands, while their overall shape was not altered, see Fig. 4b, signifying that the concentration of the hydroxide ions tuned the structure and improved the luminescence profile of ZnO nanostructures. That is, when hydroxide changes the shapes of nanostructured ZnO, there exists the chance for the surface area of each nanostructure to change. This phenomenon is normally resulting in the development of more defect states in the crystal structure which can result in wealth luminescence [47]. Figure 4c depicts the energy level diagram of nanostructured ZnO. The figure shows different transitions responsible for luminescence emissions (violet, blue and green) due to the point defects and levels in ZnO [48]. Sharma et al. [49] synthesized and grew ZnO nanowires (NWs) on a quartz substrate using chemical method. The excitation wavelength of 350 nm was used, and the samples emitted intense UV emission (400 nm) peak compared to visible emission peak. This UV emission (Fig. 5a) of ZnO NWs was governed by a Zni defect-related level, with the defects increasing as the diameter of the NWs increased. The increasing of the diameter of the NWs induced the red shift in the NBE emission peak indicating that those particles can be confined in the infinite potential well at the surface of ZnO NWs. As a result, the concentration of defects at the surface increased, resulting in the suppression of the NBE emission peak. However, the emission spectra of the sample grown at 24 h growth time could be deconvoluted to six peaks (see Fig. 5b). These peaks were a contribution from various defect centres with the band gap of the ZnO and emission of those defects. Furthermore, Fig. 5b shows the energy level diagram indicating the possible defects present in ZnO NWs. As a result, the two mechanisms are accountable for the blue emissions: transitions between Zni and Vo result in broad blue emission in the range of ~425 nm to ~492 nm. Green emission is formed by the Vo transitions at ~535 nm [49]. As mentioned earlier, transition metal doped ZnO nanostructures are considered versatile phosphor nanomaterials not only for the UC process but also for the DC process. However, it is also vital to understand the defects induced DC multicolour emission process in DC transition elements/metals activated ZnO for wider luminescence applications. There are several synthesis techniques like hydrogen plasmametal reaction, mechano-thermal method, chemical vapour deposition, electrospinning, arc discharge, microemulsion method, combustion method, vapour phase laser pyrolysis, sol–gel method, hydrothermal method, liquid phase microbial processing, sonochemical method, solid phase and ball milling that can be used to synthesize nanostructured materials for optoelectronic and biomedical applications [15, 50]. For example, Rao et al. [15] prepared selenium (Se)-doped ZnO nanostructure through mechano-thermal for optoelectronic and biomedical applications. However, according to Rao et al. [15], selenium metal is used in this case because it induces luminescence emission due to the creation of different zinc and oxygen defect centres in the ZnO crystalline. The main objective of their research was to investigate the

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Fig. 5 a Luminescence spectra of the ZnO NWs grown at different growth times, b De-convolution of spectrum at 24 h growth times and c schematic diagram showing defect state of the emission spectra of ZnO NWs [49]. Reproduced with permission from Sharma P, Tiwari SK, Barman PB. Abnormal red shift in photoluminescence emission of ZnO nanowires. J Lumin. 2022;251:119,231. Copyright Elsevier (2022)

influence of defects in ZnO:Se NRs to achieve large Stokes shift and strong wide emission [15]. In their results, a large Stokes shift (~250 nm) was achieved between the absorption maxima (λabs ~ 368 nm) and emission maxima (λem ~ 625 nm), see Fig. 6a. However, the photoluminescence spectra (refer to Fig. 6b) revealed ultraviolet emission (~388 nm) and a solid broad visible emission centred at ~625 nm [15]. So far, it is adopted that UV emission of ZnO is associated with excitonic recombination. Additionally, the broad emission of ZnO in the visible region corresponds to defects within the structure [9, 15, 40]. The broad deep-level emission in Fig. 6b was deconvoluted and there are three peaks according to Rao et al. [15], these emissions are associated with the presence of different energy states (shallow and deep-level defects) and extrinsic defects because of Se doping. This is justified based on the energy transfer between ZnO defect levels and the overlapping of the different trapped energy states which resulted in a large Stokes shift and strong broad emission [9, 51]. This showed a single photon down-conversion which played a vital role to produce a single colour emission due to the increase of the limited defects in ZnO [15]. Figure 6c shows

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Fig. 6 a UV–vis and PL spectra of ZnO NRs, b Fitted PL spectrum and (c) Schematic energy-level diagram of defect states in Se-doped ZnO NRs [15]. Reproduced with permission from Rao AVRK, Chelvam V. Defects induced multicolour down- and up-conversion fluorescence in Se-doped ZnO nanorods by single wavelength excitation. Opt. 2020;107:110,122. Copyright Elsevier (2020)

the mechanism showing electronic transitions from different defects existing in the Se-doped ZnO nanostructures [15]. Several transition metals like copper (Cu), aluminium (Al), silver (Ag) and gold (Au) are versatile and have been successfully utilized to regulate and alter the photoconductivity and optoelectronic properties for different applications. Among these, Ag has been investigated for possible enhancement of optical emission and UV photodetection as well as efficient and consistent prototype UV sensors. Furthermore, ZnO semiconductors in various nanostructure morphologies are frequently used in conjunction with catalytic nanoparticles and have excellent electron transfer ability to improve electron yield and transfer rate [52]. In general, transition metals are used to improve the luminescence properties of ZnO for sensors applications [53]. For example, Mishra et al. [52], for example, conducted a study on the effect of Ag on the defects within the emissions of a ZnO nanostructure-based UV sensor. Figure 7 shows an NBE peak at 397 nm and a broad blue emission band in the visible region. The deconvolution (shown inset) confirmed the presence of several emission bands associated with defects in the ZnO lattice structure. These four emission bands

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Fig. 7 Emission spectra of undoped and Ag-doped ZnO under the excitation of 325 nm [52]. Reproduced with permission from Mishra SK, Tripathi UK, Kumar R, Shukla RK. Defects mediated optical emissions and efficient photodetection characteristics of sol–gel derived Ag-doped ZnO nanostructures for UV sensor. Mater Lett. 2022;308:131,242. Copyright Elsevier (2022)

are located at 459 nm, 484 nm and 506 nm, respectively, and correspond to blue emission, blue-green emission and green emission. The observed emissions, according to the authors, are caused by defects such as zinc interstitials (Zni ), zinc vacancies (VZn ), oxygen interstitials (Oi ), oxygen vacancies (VO + ) and oxygen antisites (OZn ) [52].

3.3 Luminescence of ZnO Nanostructured Doped with RE Ions Transition metal oxides (TMO)-based ZnO has been investigated for various photocatalytic applications. This is due to TMOs’ incredible ability to introduce charge trap sites into the ZnO lattice. Furthermore, RE ion-doped ZnO have been studied to improve the luminescence of ZnO due to their unique properties such as easy synthesis conditions, strong conductivity, large surface area and high stability [54]. To enhance the photoluminescence and colour purity of ZnO semiconductor, Zhao et al. [55] reported the synthesis and luminescent properties of P2 O5 –SrO–BaO– B2 O3 –ZnO (PSBBZ) glasses co-activated with Tm3+ –Dy3+ yo improve the photoluminescence and colour purity of ZnO semiconductor. Figure 8a depicts the excitation

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spectra of Tm3+ –Dy3+ co-activated PSBBZ glasses measured in the 300–500 nm wavelength range. The spectra were recorded for different molar contents of Tm2 O3 and a fixed molar content of Dy2 O3 . Furthermore, Fig. 8a shows eight excitation bands, which are assigned to 4f transitions of Dy3+ ions with varying intensities [55, 56]. In this case, the trend of increasing and then decreasing excitation intensities are referred to as cross-relaxation of RE ions. The band at 350 nm is the strongest of these excitation bands. As a result, the prepared PSBBZ: Tm3+ and Dy3+ glasses were shown to absorb UV light with a wavelength of 350 nm. Following that, as shown in Fig. 8b, the 350 nm wavelength was used to record emission spectra for all PSBBZ: Tmx , Dy0.1 glasses. Four peaks were visible in the emission spectra: 454 nm (blue), 484 nm (greenish blue), 575 nm (yellow) and 665 nm (red). As shown in Fig. 8b, the observed emission peaks correspond to Tm3+ and Dy3+ transitions [55, 57, 58]. The intensities of the emission peaks increased as the Tm3+ molar concentration increased up to 0.4 mol%. This enhancement could be attributed to the transfer of energy from sensitizer (Tm3+ ) to activator (Dy3+ ). Furthermore, higher Tm3+ ion concentrations (>0.4 mol.%) decreased emission intensities of Dy3+ . Under the excitation of 350 nm UV light, electrons move from the valence states, 3 H6 (Tm3+ ) and 6 H15/2 (Dy3+ ), into the conduction states, 1 D2 (Tm3+ ) and 4 M15/2 + 6 P7/2 (Dy3+ ), from which they deexcite 4f states of Tm3+ and Dy3+ ions. As shown in Fig. 8c, Tm3+ electrons in the 1 D2 state deexcite to the 3 F4 state via the radiative relaxation process and emit blue light. Electrons in the 4 M15/2 + 6 P7/2 level of Dy3+ deexcite to the 3 F4 state through a non-radiative relaxation process. Following that, there is efficient resonance energy transfer (ET) between Tm3+ and Dy3+ ions through three energy transfer channels; ET1: Dy3+ → Tm3+ : 4 F9/2 (Dy3+ ) + 3 H6 (Tm3+ ) → 6 H11/2 (Dy3+ ) + 3 F2 (Tm3+ ), ET2: Dy3+ → Tm3+ : 4 F9/2 (Dy3+ ) + 3 H6 (Tm3+ ) → 6 F5/2 (Dy3+ ) + 3 H5 (Tm3+ ) and ET3: Tm3+ → Dy3+ : 1 D2 (Tm3+ ) + 6 H15/2 (Dy3+ ) → 3 F4 (Tm3+ ) + 4 I15/2 (Dy3+ ). In this context, electrons deexcite radiatively to Dy3+ ion transitions 6 H15/2 , 6 H13/2 , and 6 H11/2 . These transitions produce blue (484 nm), yellow (575 nm) and red light (665 nm). PSBBZ: Tm3+ , Dy3+ emission colour gradually approached standard white light, manifesting in a rectifying manner, implying that these glasses could have applications in W-LED devices [55]. Shi et al. [58] used a facile low-temperature method to create single-phased emission-tunable ZnO: Mg, Ce quantum dots for white light-emitting diode applications. X-ray photoelectron spectroscopy (XPD) was used to investigate the surface chemical state of ZnO: Mg, Ce QDs using a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer. In this chapter, only the XPS results corresponding to the oxygen peaks (O-1 s) and Ce-3d spectra are discussed, as shown in Fig. 9a–c. The O-1 s XPS peak obtained for the ZnO: Mg, Ce QDs could only be deconvoluted into two signals. These peaks were caused by lattice oxygen (OL) at 530 eV BE and O2− in oxygen vacancies (OH) in ZnO. The Ce-3d spectra (Fig. 9:a) revealed six binding energy signals at approximately 882.1, 887.7, 897.7, 901.2, 908.2 and 916.5 eV, which were assigned to the final +4 oxidation states of Ce. However, the increased molar concentration of Ce is found to increase the oxygen vacancy concentration in the ZnO: Mg, Ce QDs, which is also responsible for the conversion of Ce from +4 oxidation states to +3 oxidation states [58].

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Fig. 8 a Excitation spectra, b PL spectra and c Chromaticity CIE diagram of Dy3+ –Tm3+ codoped P2 O5 –SrO–BaO–B2 O3 –ZnO glass [55]. Reproduced with permission from Zhao Y, Zhong Y, Chang H, Liu W, Xiao Z, Zhong Y et al. Luminescent properties of Tm3+ Dy3+ co-doped P2 O5 SrOBaOB2 O3 ZnO glasses for white LED applications. J Non-Cryst Solids. 2021;573:121,121. Copyright Elsevier (2022)

Figure 10a depicted the EL spectra of the fabricated white LED device at 3 V and 200 mA driven current. On the inset of Fig. 10a, a photograph of the fabricated white LED device was shown. The EL spectra include the emission peak from the NUV chip as well as an emission peak with a maximum at 545 nm from Ce0.008 Mg0.1 Zn0.892 O QDs. The observed PL emission is consistent with the EL emission peak at approximately 545 nm (see Fig. 10c). Figure 10b shows the excitation spectra of CexMg0.1 Zn0.9-x O at various Ce concentrations: (i) = 0, (ii) x = 0, (iii) x = 0.004, (iv) x = 0.008 and (v) x = 0.01. The electron transfer from the exciton energy level to the valence band of the ZnO semiconductor resulted in a weak peak at 374 nm in the spectra [58, 59]. The excitation peaks revealed a blue shift as the concentration of Ce increased. This blue shift was caused by a high concentration of dopant, which causes Ce ion incorporation into the ZnO lattice and, as a result, band gap changes. The PL emission spectra of all samples showed a broad visible emission peak. As mentioned earlier, the presence of defects such as oxygen vacancies (Vo), zinc vacancies (Vzn ), zinc interstitials (Zni ) and zinc vacancies (Vzn )

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Fig. 9 XPS core-level scan of O-1 s peak with a variation of Ce content; a x = 0.004 b x = 0.008, c x = 0.01 and d Ce-3d spectra of Cex Mg0.1 Zn0.9−x O [58]. Reproduced with permission from Shi Q, Ling K, Duan S, Wang X, Xu S, Zhang D et al. Single-phased emission-tunable Mg and Ce co-doped ZnO quantum dots for white LEDs. Spectrochim. Acta A Mol. 2020;231:118,096. Copyright Elsevier (2020)

Fig. 10 a EL spectra, b excitation spectra and c emission spectra of Cex Mg0.1 Zn0.9−x O [58]. Reproduced with permission from Shi Q, Ling K, Duan S, Wang X, Xu S, Zhang D et al. Singlephased emission-tunable Mg- and Ce co-doped ZnO quantum dots for white LEDs. Spectrochim. Acta A Mol. 2020;231:118,096. Copyright Elsevier (2020)

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causes broad visible emission of ZnO [58–60]. As a result, the authors concluded that the broad visible emission of Cex Mg0.1 Zn1-x O QDs was directly related to oxygen vacancies (Vo ). In general, these findings show that the incorporation of Mg and Ce improves the luminescence properties of ZnO for white LED performance, with the Ce concentration tuning the band gap of ZnO [58]. Er3+ and Yb3+ are widely used as luminescent centers in various phosphor materials for efficient UC applications. They are therefore the most popular and important UC transition metals dopants for phosphor materials because phosphors co-doped with these elements are well known to be promising candidates for optical thermometry, advanced anticounterfeiting, dye-sensitized solar cells, non-contact temperature sensors and colour display [61–63]. In Fig. 11a, PL spectra verified the UC luminescence emissions of Er3+ /Yb3+ co-doped ZnO nanostructured (optimized sample). The figure displayed blue (400–425 nm), green (515–480 nm) and red (640–700 nm) emission bands, which were assigned to the 2 H9/2 → 4 I15/2 , 2 H11/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 intra-configurational 4f transitions of Er3+ [61–63]. According to Kumar et al. [61], the Er3+ ions act as an activator and Yb3+ ions as a sensitizer. It is known that for the co-doping system, the sensitizer absorbs more photon energy and transfers it to the activator through the energy transfer process. Studies have shown a superior red emission band, which was approximately 145 times and 64 times larger in comparison to the blue and green emission bands, respectively. Additionally, it was found that the prepared phosphor material exhibits red colour, which slightly varied and approach the ideal red with the variation of the sensitizer (Yb3+ ) as shown in Fig. 11b [61].

Fig. 11 a UC luminescence mission spectrum and b CIE diagram of ZnO co-doped with Er3+ and Yb3+ ions [61]. Reproduced with permission from Kumar V, Pandey A, Swami SK, Ntwaeaborwa OM, Swart HC, Dutta V. Synthesis and characterization of Er3+ -Yb3+ -doped ZnO upconversion nanoparticles for solar cell application. J Alloys Compd. 2018;766:429–435. Copyright Elsevier (2018)

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4 Potential Applications Nanotechnology in recent years has become a key to innovative technology for designing and engineering different nanoscale materials, like metals, SMO and polymers. ZnO phosphors have attracted a lot of attention in optical industries due to their low cost and environmental friendliness [64]. It has semiconducting, piezoelectric and excellent optical properties. ZnO is a semiconductor material and comprises of a bandgap of about 3.37 eV and an exaction-binding energy that is roughly 60 meV [7, 64, 65]. Tuning the bandgap by choosing a suitable synthesis method, synthesis temperatures and type of dopants can make ZnO suitable for different applications, such as sensing, luminescent devices (i.e. screen display), photoelectrochemical cells, optoelectronics, Li-ion battery photodetectors, photoelectrons, catalysis and biomedicine [66–69]. In addition, its availability in different nanostructured morphologies, cost-effective synthesis methods and chemical stability make it an easy-to-use compound for different applications [66, 67, 69, 70]. Among semiconducting materials, ZnO has been widely investigated for its physicochemical properties [64, 71, 72–74]. Thus, Sect. 3.4 provides a brief discussion on various applications of ZnO in industries.

4.1 Sensing The advances in modern technology led to the development of sensors, which are important in warning us about physical quantities in our environment. They can pick up quantities that can’t be picked up by our body sensors, which include radiation, gases, pressure, temperature, light, very low sounds and magnetism just to mention a few. In addition, they now assist us to operate machinery in the workplace in our absence [75]. ZnO is a probable material for sensor applications due to its excellent sensing response, non-toxicity and chemical stability. It has been used as a refractive index sensor in optical fibre materials, a chemical sensor and a gas sensor [75, 76].

4.2 Optical Fibre Sensing Metal oxide materials play a vital role in science due to their effective applications in chemical, gas, and biosensing due to their electrical, electronic, antibacterial and optical properties. ZnO is a promising semiconductor material because of its wide bandgap (3.37 eV), high excitation energy (60 meV) and excellent thermal and chemical stability [77]. Optical fibre is a cable fabricated using a plastic or glass fibre that can transmit signals in the form of photons. These are created by forming nanofilms or nanostructures into cylindrical waveguides. In addition, they can perform authentic and remote measurements, are resistant to electromagnetic

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interference and are small in size. There are numerous advantages to using ZnO as an optical fibres sensor coating. In comparison to a sensor without a coating, it allows for a wider measuring range and improved sensitivity of the measured parameter. It also allows measurements to be taken in a medium with a refractive index close to that of an optical fibre core (n = 1.4) [78]. Moreover, ZnO deposition techniques have advanced significantly, particularly the Atomic Layer Deposition (ALD) method, which ensures coating uniformity. However, ZnO can have a negative impact on the surrounding medium in a number of situations, so a CVD nanocrystalline diamond sheet attached over the ZnO is required to protect both the sensor head and the measured medium from damage in the event of an unwanted reaction. Optical fibre is a cable fabricated using a plastic or glass fibre that can transmit signals in the form of photons. The telecommunications industry recently prefers optical fibres over copper-based type of communication cables, because of the faster and larger bandwidth signals that can be transmitted using optical fibre technology. It is made up of different fibre glasses, with the fibre core and cladding fibre glass being the most important (Fig. 12). The fibre core is a medium in which the photon signal is transmitted [79]. The signal can be affected by the external environment parameters, which include the magnetic field. Such effects are observed from the refractive index variations. ZnO incorporated in the optical fibre cladding layer has proven to be an important refractive index variation sensor, because of its semiconducting properties and its transparent thin films. Furthermore, doping ZnO with different materials can improve its sensitivity and selectivity [76].

Fig. 12 An optical fibre cable schematic [80]

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4.3 Gas Sensing ZnO is one of the most utilized nanomaterials in gas sensing. This semiconductor has high density at temperatures lower than 800 °C which makes it vital for multifunctional material with applications for various, gas sensors, transparent electrodes and catalysts. Due to its specific chemical, surface and microstructural properties, ZnO has a wide range of applications. It is widely used as a commercial material and has shown performance characteristics [65]. Its electrical resistance can be drastically influenced during adsorption and desorption of gas molecules on its surface due to electron capture or release. Therefore, electrical properties together with electronic structures are crucial components that affect the ability of ZnO to be gas sensitive. Surface defects, like Vo and Vzn were found to enhance the sensing characteristics [66]. Yang et al. [81] investigated the NO2 using three-dimensional crumpled MXene Ti3 C2 Tx /ZnO spheres for suitable room temperature applications. They observed that ZnO-based sensor displayed a reversible NO2 sensing behaviour. The sensor further demonstrated improved response and superior selectivity towards NO2 . The addition of impurities can significantly improve the gas sensing of ZnO. Agarwal et al. [82] prepared ZnO hollow tubes-like microstructures modified with Ag nanoparticles and investigated their sensing properties towards H2 gas. They observed a significant improvement in the ZnO gas sensing properties after incorporating the Ag nanoparticles in the ZnO sensor. Such improvement could be justified by a spillover mechanism induced by Ag nanoparticles, leading to a significant change in sensor resistance. In addition, they discovered that these composites exhibited rapid response and recovery properties, excellent repeatability and stability. Therefore, the gas-sensing ability of ZnO can be improved by introducing either defects or impurities or introduction of SMO–SMO heterostructure [82].

4.4 Photo Detector The room temperature bandgap of 3.37 eV, the excitation energy is 60 meV, which is similar to that of GaN, strong excitation binding energy and the thermal energy at room temperature (26 meV) of ZnO may guarantee an effective excitation-emission at room temperature under relatively low excitation energy [83]. These properties make ZnO materials to be attractive to be used for optoelectronic applications, in addition to its wideband ZnO materials can also be considered as a capable photonic material in the blue–UV region [83, 84]. Ibraheam et al. [85] studied various properties of Al: ZnO materials as UV photodetector and witnessed that the optical bandgap of ZnO improved when the concentration of dopant (Al) was increased to reach 3.23 eV [85]. This makes ZnO materials suitable for UV photodetector.

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4.5 Display Screens The luminescent materials have attracted a strong attention in the display technology, by enabling colour TV and smartphones with images that closely resemble real images [86]. Through advances in television display technology, we are now able to see other parts of the world in colour images. The gadgets support internet browsing and gaming in colour and they also enable us to capture images in colour photography. All these technologies are supported by luminescent materials that emit green, red and blue emissions, which are coated into thin films that are stacked into a range of pixel matrices [87]. Then these materials are excited using liquid crystals, plasma or an electron beam, and produce a colour image by mixing the emission from the pixels [86], as shown in Fig. 13. ZnO is an important luminescent material with a tunable bandgap, which enables it to have a wide range of luminescent frequencies, like blue, green and red emissions [69, 70]. Though not currently used at a commercial level, the material has been tested at the laboratory level, and seems to be a promising candidate for display technology [88]. The compound can easily be synthesized to have nanoscale spherical particles using wet chemistry and optically transparent [89]. This is an added benefit for its application in the display industry because spherical particles have better packing density.

Fig. 13 A schematic showing colour pixel matrix [87]

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4.6 Optoelectronic Technology Low-dimensional nanodevices such as field effect transistor photodetectors and lasers have recently received a lot of attention due to their distinct properties and large surface-to-volume ratios. ZnO has emerged as the most important element due to its ease of fabrication and excellent optical response, which can be tuned by adjusting its energy band gap [66, 90]. Because of its high electron mobility and high exciton binding energy [76], it has superior semiconducting properties [83]. Furthermore, it has an excellent optical response that may be tuned by tuning its energy band gap [66], superior semiconducting properties owing to its high electron mobility [76] and high exciton binding energy [90]. These unique characteristics make it an excellent candidate for devices like solar cells [76, 91]. A solar cell is a device that absorbs energy from the photons and converts it to electrical energy. Hence, they are referred to as optoelectronic devices. In the modern days, they are used to convert photon energy from the sun to electricity, which is used to power home appliances [92]. The materials used to fabricate these devices determine which energy range and percentage of sun rays will be absorbed. This leads to photovoltaic solar cells having different power conversion efficiencies [93]. In dye-sensitized solar cells (Fig. 14), the ZnO has been used because of its high electron mobility, and in most solar cells as an antireflective p–n heterojunction. In both instances, it played a pivotal role in enhancing the power conversion efficiency of these devices [76, 93, 94].

Fig. 14 Solar cell schematic showing ZnO nanorod inside the device [94]. Reproduced with permission from Lai MH, Tubtimtae A, Lee MW, Wang GJ. ZnO-nanorod dye-sensitized solar cells: New structure without a transparent conducting oxide layer. Int J Photoenergy. 2010;2010:1–5. Copyright Elsevier (2010)

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4.7 Li-Ion Battery Li-ion is widely used in portable devices, like smartphones, laptops and cordless tools and appliances. Vehicle batteries are also using Li-ion-based batteries. Recently, ZnO has been considered and raised attraction as a probable anode because of its low cost, natural abundance and sufficient capacity of 978 mAhg-1 [65, 95]. Compared to graphite, which has a theoretical capacity of approximately 372 mAhg-1 and is the most commonly used material as an anode in lithium batteries. Furthermore, due to its ease of synthesis, it theoretically outperforms other transition metals such as Cu, Ni and Fe. ZnO is unique in that it can be synthesized in a wide range of processes, morphologies and structures making it easily tunable for applications. However, its practical utilization is obstructed due to volume change during lithiation or de-lithiation. This possess is a big problem that affects electrical conductivity due to the poor electrochemical cell. Therefore, modification is needed to improve the electric structure. The intrinsic defects, such as oxygen vacancies in ZnO materials control the charge transfer and prevent charge accumulation. Oxygen vacancy also plays a role in Li-ion transportation through the electrode, thus improving the battery performance [95].

4.8 Catalysis Applications ZnO materials are widely used for catalysis due to their high electrochemical coupling coefficient, wider radiation absorption, high chemical stability and high photostability [96]. The relatively large ZnO bandgap limits the recombination of charge carrier by creating a compatible band structure. This material can also be used as the carrier to increase the stability of less stable photocatalyst in aqueous solution system [97]. The ZnO materials have been used to synthesize alcohols, methanol, in particular, gasoline, and other higher hydrocarbons through CO2 hydrogenation. Methanol steam reforming (MSR) for hydrogen gas production, biomass-related catalytic reactions, carbon monoxide oxidation and thermocatalytic reactions [96]. Omir et al. [97] investigated the influence of ZnO/Mn concentration on the microstructure and optical characteristics of ZnO/Mn–TiO2 nanocomposite for photocatalysis applications. They observed that the photocatalytic activity is dependent on the Zn–M weight ratio loaded into TiO2 composite. They also observed the highest photocatalytic activity on Mn50%–Ti composite [97].

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4.9 Medical Applications The applications of novel ZnO nanoparticles have received vast interest in research due to many potential applications in biomedicine. These include bio-imaging, biosensing and therapeutic applications [98]. Properties, such as high surface area, structure, active sites, thermal stability, electron properties (band gap), greater response towards the photoelectric reaction and the biocompatible nature, make ZnO nanomaterials to be among candidates for the fabrication of useful chemical and biological sensors for a diverse range of moieties [97]. The characteristics of more surface atoms has created the sensing layer for detection of materials the surrounding environment. In addition, the small grain size and amount of adsorbed oxygen species, in the ZnO-based nanostructure, greatly control the detection response to different types of poisons. For bio-sensing, ZnO nanoparticles are used for improved images during diagnosis in magnetic resonance imaging (MRI), positron emission tomography (PET), computed tomography (CT), etc., [98, 99]. For ZnO nanoparticles to be considered in bio-sensing as drug carriers, the nanoparticles should have abilities to contain the drug, have stability in blood serum, and can release the drug at write time at the correct sport. In addition, they must undergo biodegradation or be biologically inert. Studies were done about ZnO-based materials and revealed that these materials are bio-degradable thus making them suitable for applications in drug, gene and vaccine delivery [99]. Embedding ZnO composites with ZnO nanoparticles in polymers brings stability and flexibility [100]. Besides greater antibacterial, antimicrobial and outstanding UV-blocking properties, ZnO materials also have strong UV absorption properties, which makes them mostly utilized in personal care products, like cosmetics and sunscreens [101].

5 Conclusions Tuning the properties of ZnO with flexibility and wide scope of luminescence applications remains as the main objective in the family of semiconductors. To achieve this, we have comprehensively highlighted and discussed some recent breakthrough research works for enhancing the efficiency of the ZnO semiconductor phosphors. In this context, several strategies such as impurity doping and suitable synthesis methods result in various defects, improved electronic structures and performances of ZnO semiconductors towards respective photocatalytic, renewable energy, optoelectronic, storages, biomedical applications and so on. In general, the effect of impurity doping over the bandgap absorption of ZnO nanoparticles depends on their molar concentration and morphological features in the prepared ZnO phosphor material. Furthermore, the performance of impurity doping ZnO nanostructures has been advanced remarkably by varying various reaction parameters (reaction time, concentrations, solution pH, molar ratio, ignition temperature, etc.). This has been achieved using numerous synthesis methods to prepare nanostructured ZnO-based phosphor.

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Despite the progress achieved in the development of impurity-doped ZnO from the past decades, researchers are still facing some obstacles because the luminescence dynamics (emission and wide wavelength responses) of the down- and up-conversion dopants for ZnO nanoparticles have not been explored thoroughly to advance the nanotechnology in the industry. However, room still exists for developing efficient ZnO for diverse down and up-conversion applications. This includes the synthesis of down- and up-conversion-doped and co-doped ZnO followed by the integration with various cations with small ionic radii and co-catalysts. This will make semiconductors like ZnO nanostructures a suitable forefront in terms of developing efficient phosphor material.

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Dynamics of Perovskite Titanite Luminescent Materials S. J. Mofokeng, L. L. Noto, T. P. Mokoena, T. A. Nhlapo, M. J. Sithole, M. W. Maswanganye, and M. S. Dhlamini

1 Introduction Perovskite-based nanostructured materials are normally utilized in various applications like a tunable laser, infrared (IR) and visible light-emitting diodes, solar cells and piezo and ferroelectricity. Generally, perovskite titanates are widely utilized as phosphor hosts, due to their relatively high stability and they can accommodate the RE elements within the lattice sites [1]. It is a well-known fact that inorganic host-based phosphors activated with RE ions are the prime components of phosphorconverted white (pc-w) light-emitting diodes (LEDs). Most of the host materials like ternary or binary compounds of oxides, nitrides, sulphates, titanates, molybdates, etc., have been explored before. Phosphors based on titanate have gained so much attention compared to their counterparts, owing to the titanate group’s high refractive index and wide bandgap as emission intensity increases. The chromaticity of titanatebased materials can be regulated by fine-tuning some of the synthesis factors and by incorporating 3d or 4f system through doping process [2].

S. J. Mofokeng (B) · L. L. Noto · M. J. Sithole · M. W. Maswanganye · M. S. Dhlamini Department of Physics, School of Science, Science Campus, CSET, University of South Africa, Private Bag X6 Christiaan de Wet and Pioneer Avenue, Florida Park, Johannesburg 1710, South Africa e-mail: [email protected] T. A. Nhlapo Department of Medical Physics, Sefako Makgatho Health Sciences University, P.O. Box 146, Medunsa 0204, South Africa T. P. Mokoena Department of Physics, University of the Free State (QwaQwa Campus), Private bag X13, Phuthaditjhaba, 9866, South Africa M. W. Maswanganye Department of Physics, University of Limpopo, Private Bag X1106, Sovenga, 0727, South Africa © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_4

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Among titanate-based phosphors materials, calcium titanate (CaTiO3 ) proved to be a versatile inorganic material with numerous applications in luminescence display devices, sensors, colour-tuneable phosphors, biomedical applications like fluorescence resonance energy transfer assays and optical imaging. These outstanding applications from CaTiO3 are fuelled by its low phonon energy, prominent physical properties and non-toxicity [3]. Hence, RE ions such as europium (Eu), samarium (Sm) or praseodymium (Pr) have been incorporated into CaTiO3 through the doping process, simply because of their environmental friendship and chemical stability. Out of all other phosphors, CaTiO3 : Eu3+ is widely used in red LEDs applications, due to its pure red emission. What is more fascinating, and very unique from the development of broad band red-emitting oxynitride/nitride-based phosphors, Eu3+ -doped CaTiO3 has a narrow band emission that is beneficial to the light purity of LEDs used in displays but has poor thermal stability [4]. However, in other similar studies, the appropriate content of silver (Ag) has been coated onto Eu3+ -doped CaTiO3 phosphors resulting in an enhancement in heat dissipation and photoluminescence (PL) intensity, as shown in Fig. 1a–d. Therefore, the pc-LED lamp derived and/or packaged from Ag-coated Eu3+ -doped CaTiO3 phosphor also displays a greater efficiency and more steady operating conditions compared to the uncoated ones [5]. In some events, the insertion of alkali ions was utilized into Eu3+ -doped CaTiO3 phosphors for enhancement of photoluminescence characteristics, particularly for white LEDs manufacturing (refer to Fig. 1e–f). Generally, the incorporation of alkali ions is used to improve the luminescence intensity through the charge compensation process and created asymmetry around RE ions in the host lattices [6]. It is well known that samarium (Sm3+ ) displays a robust emission in red–orange proximity for the application of full-colour display devices. Ha et al. [7], studied the luminescence features of Sm3+ -doped CaTiO3 phosphors, and the results revealed a relatively robust red–orange emission assigned to 4 G5/2 → 6 H7/2 transition of Sm3+ upon the near ultra-violet excitation. On the other side, praseodymium (Pr3+ ) is normally doped into CaTiO3 phosphors, it was detected that the CIE coordinates of Pr3+ -doped CaTiO3 are located at x = 0.680 and y = 0.311 nearby the red emission. CaTiO3 : Pr3+ displays a strong sharp emission located at 612 nm and is an appropriate phosphor material for field emission display (FED) applications due to its high electron irradiation resistance and sustained luminescence efficiency. Lately, it was recently discovered that this material emits an afterglow in red with a persistence time of minutes [8]. In addition, CaTiO3 is not restricted to certain applications only, several reports revealed that the CaTiO3 phosphors-based material can be used in up-conversion (UC) applications [9–11]. Our special interest was in zinc titanate (ZnTiO3 ) material as well, where this material is considered as a dual-metal oxide semiconductor, simply because it can accommodate RE ions and TMs to achieve magnificent luminescence due to its wide bandgap, unique physical and chemical characteristics. A relatively high thermal and chemical stability of ZnTiO3 , allows this material to be utilized in optical thermometers, particularly in harsh conditions. Generally, ZnTiO3 has a cubic defect spinel structure and/or rhombohedral perovskite arrangement, and when the crystallization temperature is too high, it will transform into the high-temperature phase of

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Fig. 1 Excitation and emission of the CaTiO3 :Er3+ before and after coating with Ag (a and c); the opposites attract the coating process of CaTiO3 :Er3+ and Ag nanoparticles (b); Ag nanodisks and nanotriangular plates resonate with the absorption of exciting and emission light (d) [5]. Reproduced with permission from Chen Z, Qin X, Zhang Q, Li Y, Wang H. Enhanced fluorescence and heat dissipation of calcium titanate red phosphor based on silver coating. J. Colloid Interface Sci. 2015;459:44–52. Copyright Elsevier (2015). PL emission spectra of CaTiO3 : Eu3+ co-doped with various alkali ions under different excitation wavelengths (e) 398 and (f) 466 nm [6]. Reproduced with permission from Singh P, Yadar RS, Rai SB. Enhanced photoluminescence in a Eu3+ -doped CaTiO3 perovskite phosphor via incorporation of alkali ions for white LEDs. J Phys Chem Solids. 2021;151:109,916. Copyright Elsevier (2021)

Zn2 TiO4 [12]. By the way, zinc titanates arose from ZnO/TiO2 composites; however, it was revealed that several parameters such as ZnO/TiO2 molar ratios and annealing conditions have significant effects on the phase formation [13]. Recently, the research on the UC properties of ZnTiO3 has gained momentum and is growing rapidly, moreover, especially in solar cell applications. This is due to the fact that the restricted absorption can be enhanced by employing materials with a broader absorption range, which allows the absorption and transform it to radiation that photovoltaic (PV) cells can absorb. Thus, those materials consist of up-converting and down-converting layers-based nanophosphors [14, 15]. Based on the literature review, there are scarce reports on the ZnTiO3 as the host material for UC phosphors and this might be due to the complexity of the synthesis techniques that have been used. However, Mofokeng et al. [16], unpacked that the solid-state reaction

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method is one of the best and simplest synthesis techniques for the preparation of up-conversion RE ion-doped ZnTiO3 phosphors, with the results attained, we can attest that this material can be endorsed and used as the up-converting layer on the photovoltaic applications. Perovskites are titanate materials with a crystalline structure in the form of ABO3 [17]. These materials also have incredible and diverse properties such as piezoelectricity, semi-conductivity, superconductivity, ferromagnetism, thermoelectricity and conductivity [18]. Perovskite oxides have been investigated for ferroelectric films, which are crucial in many high-technology applications such as optical waveguides, catalysis, integrated optics and high-capacity memory cells [19]. Perovskite oxide ferroelectric films on low-dimension and mini-sized substrates have received significant attention because of their potential applications in microelectronics, sensors, energy storage systems and harvesting, piezoelectric actuators, memories, ultrasonic energy converters, mechanical processing and optical devices [19, 20]. According to Gao et al. [20], electronic devices must be more flexible, transparent and even compact in order to develop artificial intelligence and the internet of things, as well as meet the burgeoning diversified demand in human daily life. With the most recent research for the advancement of current technology, the development of these flexible electronic devices has imposed new demands on the bendability of perovskite oxide materials. Furthermore, the developed high-performance perovskite oxide films can undoubtedly meet the applied demand for flexible electronic devices [20]. Perovskites have been characterized as a class of materials having a crystalline structure in the form of ABO3 , where A represents a monovalent organic cation (FA+ = CH(NH2 )2 + and MA+ = CH3 NH3 + ) or inorganic metal cation (Rb+ and Cs+ ), B represents the divalent metal cation (Sn2+ , Pb2+ , Ca2+ , Ge2+ and Mg2+ ) and O represents the halide ion (Br− , I− and Cl− ) [18–21]. The octahedral crystal lattice of these perovskite structures is characterized by the B cation and O anions structure, which forms a 3D network of BO6 octahedra as shown in Fig. 2, with the A cations occupying the halogen ion’s octahedral voids. The ideal ABO3 perovskite structure has a space group of Pm3m and a cubic symmetry, while cation B has a 6-fold coordination with the neighbouring O ions, and cation A has a 12-fold coordination with them [22–24]. The perovskite material’s structural stability can be predicted using the Goldschmidt tolerance factor (t) from the below equation: rA + rX t=√ 2(r B + r X )

(1)

where the ionic radii of the A, B and X sites are r A , r B and r X , respectively. A cubic perovskite phase is typically stable when the t value is between 0.9 and 1. For t values 1, the perovskite structure is distorted due to the tilt of the BO6 octahedra. The non-perovskite phase occurs when t values are greater than 1 and less than 0.8 [25]. The octahedral factor (μ), where μ = r B /r X , was later added to provide additional information regarding the requirements for perovskite formation. The stability of the perovskite can be predicted by combining tolerance and octahedral factor. Consequently, element radius is an essential component of the structure

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Fig. 2 Perovskite lattice structure of ABO3 . Yellow, cyan and red colours represent A, BO6 and O, respectively [18]. Reproduced with permission from Mamba G, Mafa P.J, Muthuraj V, Mashayekh-Salehi A, Royer S, Nkambule TIT, Rtimi S. Heterogeneous advanced oxidation processes over stoichiometric ABO3 perovskite nanostructures. Mater Today Nano.2022;18: 100,184. Copyright Elsevier (2022)

because it alters band alignments that have the potential to influence the redox reaction throughout the PC process. Perovskites have a number of physiochemical properties that are easily influenced by their various compositions and constituent elements. Among these are, to name a few, high mobility, ferroelectricity, superconductivity, colossal (CMR), ion diffusivity and high catalytic performance [26]. Due to their improved charge carrier separation and quantum yield, inorganic perovskite oxides (IPOs) have recently received a lot of attention for photochemical water splitting. IPOs typically have the structure ABO3 , in which B is a transition metal cation and A is an alkali, RE, or alkaline earth metal cation. In the ABO3 structure, each A-cation is surrounded by 12 oxygen (O) anions, resulting in a cubic close-packing structure. Figure 2 depicts the coordination of A and B cations as well as O anion. In Fig. 2, the B-cation coordinates with O anions to form a 3D-octahedron at the crystal structure’s corner. These perovskite oxides are not used in industrial applications due to issues such as large particle size and surface area, low-temperature treatment resulting in weak crystallinity, low O2 adsorption and rapid charge recombination [27]. Hence, in recent years, an increasing number of researchers have paid attention to the advantages of perovskites itself, such as excellent water stability, abundant oxygen vacancies, tunable electronic structure, electron mobility, photoinduced lifetimes and shape that makes these perovskite oxides to be unique in order to improve them [22]. As mentioned earlier, perovskite oxide materials of our interest in this chapter are CaTiO3 and ZnTiO3 . These are two materials with perovskite crystal structures and are classified as inorganic materials. As previously stated, the crystal structure of these compounds has a general stoichiometry given by the formulae ABO3 , where A and B are typically metal cations and O is frequently an oxygen anion.

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In the case of CaTiO3 and ZnTiO3 perovskite structures, A represents a site occupied by Ca and Zn ions, respectively, and B represents a site occupied by Ti ions in both compounds [3, 28]. The symmetry of CaTiO3 perovskite material changes with temperature where it is divided into four space groups according to changes in temperature: cubic (Pm3m), orthorhombic (Pbnm), tetragonal (I4/mcm) and orthorhombic (Cmcm). The orthorhombic phase (Pbnm) is the most stable of these phases [29]. It has an orthorhombic symmetry with space group Pbnm at temperatures below 1380 K. It undergoes a phase transition to a tetragonal structure between 1380 and 1500 K, and once it reaches 1500 K, it assumes a tetragonal symmetry with a space group of 14/mcm. Lastly, CaTiO3 exhibits a cubic structure with a space group Pm3m at temperatures above 1580 K [3]. ZnTiO3 is a difficult material to synthesize because the direct synthesis of ZnO–TiO2 compounds usually results in the formation of three known phases: ZnTiO3 , Zn2 TiO4 and Zn2 Ti3 O8 . ZnTiO3 exists in cubic or hexagonal structures, whereas Zn2 TiO4 exists in cubic or tetragonal structures, and Zn2 Ti3 O8 exists only in cubic structures [15]. The cubic phase of ZnTiO3 is observed at ~600 °C, while the hexagonal phase appears at ~700 °C but completely crystallizes at ~800 °C [15, 16]. There have been numerous reports on the synthesis of CaTiO3 and ZnTiO3 nanoparticles using various synthesis methods [3, 15, 16, 29].

2 Synthesis Methods Perovskite titanites materials can be synthesized by several methods to achieve the desired structural and optical properties. These methods include sonochemical method [30], solution combustion synthesis [31, 32], polymeric precursor method [33], solution casting method [34], the sol–gel method [35], solid-state reaction method [15, 36], colloidal process [36], molten salt reaction method [36, 37] and many more. Recently, the application of perovskite titanate materials in renewable energy, material science, catalytic process and geophysics has gained tremendous interest [36]. We mainly focus on the luminescence dynamics of perovskite titanate compounds for various applications. Given that when working with these compounds for different applications, there are various processes of attaining the desired properties and expanding their possible applications. This involves nanoparticle sizes, exposed lattice planes, uniform surface morphology, lattice defects and high specific surface with mesoporosity [36]. Of all preparation methods for perovskite titanates, solid-state reaction method is the most common synthesis protocol for the synthesis of perovskite titanate compounds for advanced applications [15, 36]. In general, solid-state reaction synthesis takes place through appropriate physical or chemical interactions between precursors such as sulphates, oxides, hydroxides, nitrates or carbonates. Furthermore, this method is preferred for the synthesis of titanate compounds due to its numerous advantages, which include energy efficiency, a short firing time, controllable crystal growth and the ease with which RE dopant ions can be incorporated [15]. As a

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result, the solid-state reaction can take place either through a mixture of precursors and solvent, or the ball milling of the precursors. The homogeneous mixture is then post-heated at elevated temperatures to attain ABO3 or ABX3 perovskitetype compounds. In this case, A, B and X represent organic cations, metal cations and halogen anions, respectively. However, the end-product of perovskite titanate compounds prepared through this method consist of morphological and structural properties with non-uniform and submicron particles [3, 15, 36, 37]. In this work, we discuss the recent advances in perovskite CaTiO3 and ZnTiO3 compounds prepared by versatile chemical solid-state reaction and other methods. Research shows that the synthesis of CaTiO3 through chemical solid-state reaction entails a combination of calcium carbonate (CaCO3 ) or oxocalcium (CaO) and titanium dioxide (TiO2 ) powders as starting reagents. The homogeneous mixture of the oxides is normally calcined at higher temperatures in the range of approximately 700–1200 °C where the CaTiO3 perovskite is formed [3, 38]. The conventional solid-state reaction mechanisms of CaTiO3 compound formation are illustrated by two balanced chemical reactions with stoichiometric molar ratios of 1CaCO3 :1TiO2 or 1CaO:1TiO2 and are shown in Eqs. (2) and (3) [3, 38]: CaCO3 + TiO2 t o C CaTiO3 + CO2

(2)

CaO + TiO2 · t o C · CaTiO3

(3)

Various researchers have been interested in CaTiO3 synthesized via a chemical solid-state reaction strategy. Noto et al. [39], employed this method to synthesize CaTiO3 :Pr3+ . A sintering temperature of 1200 °C led to the synthesis of high crystallinity of pure perovskite phase and the particle size was uniform. However, doping and co-doping improved the luminescence properties of this material and increased the phosphorescence lifetime [39]. ZnTiO3 perovskite material is also synthesized through chemical solid-state reaction method which utilizes zinc oxide (ZnO) and titanium dioxide (TiO2 ) as starting materials [15, 40, 41]. The mechanism of the conventional solid-state reaction for the formation of ZnTiO3 is presented in Eq. 4: ZnO + TiO2 · t o C · ZnTiO3

(4)

Mofokeng et al. [15] synthesized ZnTiO3 perovskite by combining stoichiometric amounts of ZnO and TiO2 in a ball mill at room temperature. The resulting homogeneous mixture of powder was annealed at 800 °C and formed high crystallinity of pure perovskite phase of ZnTiO3 [15]. This compound is known to have high thermal and chemical stability and resembles an ABO3 perovskite structure. Pure ZnTiO3 is commonly studied for the application of microwave dielectric resonators, photocatalysts and gas sensors for the detection of nitric oxide (NO) and carbon monoxide (CO) [15, 42, 43]. Mofokeng et al. [15], on the other hand, reported that

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when ZnTiO3 is doped with a suitable activator ion and synthesized using a solidstate reaction method, its electronic and catalytic properties can be tuned to broaden its potential applications to luminescence materials. In this case, the ZnTiO3 was doped with RE ions such as erbium (Er3+ ) and ytterbium (Yb3+ ) ions where the materials possessed luminescence properties. As a luminescent compound, ZnTiO3 : Er3+ ,Yb3+ showed to be a suitable material for improving the power conversion of solar cell [37]. Therefore, it is noteworthy that the sintering temperature is the most important factor in the chemical solid-state reaction methods because it is controlling the crystalline product [3, 15, 36–41]. Chen and colleagues [44] used solid-state reaction to create a series of CaTiO3 : Pr3+ and CaTi0.5 Si0.5 O3 : Pr3+ phosphors. Praseodymium (III) oxide, calcium nitrate tetrahydrate, silicon dioxide, boric acid and nitric acid were used as the starting materials for the synthesis. During the fabrication of these perovskite materials, it was discovered that the variation of molar concentrations of Pr3+ and SiO2 which were used as precursors in CaTiO3 had an effect on the luminescence dynamics of CaTiO3 : Pr3+ and CaTi0.5 Si0.5 O3 : Pr3+ phosphors, respectively, as shown in Fig. 3. Upon varying the molar concentrations, the luminescence intensities of CaTiO3 : Pr3+ and CaTi0.5 Si0.5 O3 : Pr3+ phosphors were increased and decreased [44]. Furthermore, it was discovered that the energy transfer of CaTiO3 to Pr3+ plays an important role in increasing the intensity and brightness of the prepared compound’s emission [44, 45]. The decrease in intensities is due to concentration quenching effect [44–46]. When compared to those synthesized without SiO2 , those synthesized with SiO2 have a remarkable luminescent property and some intensities of the crystal directions [44]. Noto et al. [47], used a solid-state reaction to synthesize CaTiO3 /CaGa2 O4 : Pr3+ rods-like nanostructures by conventional solid-state reaction method. For the synthesis of the composite, gallium(III) trioxide was used as one of the precursor materials. This composite was created by combining all of the reagents into a slurry

Fig. 3 Photoluminescence spectra of a CaTiO3 :Pr3+ phosphor with different content of Pr3+ at 900 °C for 5 h and b CaTi1-x Six O3 : Pr3+ with x value from 0.1 to 0.9 [44]. Reproduced with permission from Chen R, Chen D. Enhanced luminescence properties of CaTiO3 : Pr3+ phosphor with the addition of SiO2 by solid-state reaction. Spectrochim Acta A Mol. 2014;127:256–260. Copyright Elsevier (2014)

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with ethanol and preheating it at 100 °C for 10 h before sintering at 1200 °C in the crucible for 4 h in air. In order to make CaTiO3 /CaGa2 O4 composite for various luminescence applications, foreign elements such as RE ions must be added. As a result, the concentration of Pr3+ was optimized to determine the optimum amount of RE ion concentration required for high luminescence properties. Based on the observations, 0.4 mol% Pr3+ was determined to be the appropriate concentration for the synthesis of CaTiO3 /CaGa2 O4 : Pr3+ with a surface morphology made up of irregularly shaped particles and rods of varying sizes that are agglomerated (Fig. 4a). The nature of the irregularly shaped particles and rods of various sizes that are agglomerated in this case is attributed to high temperatures during the synthesis. The UV–vis diffuse reflection spectrum of CaTiO3 /CaGa2 O4 : Pr3+ compound is shown in Fig. 4b. CaTiO3 : Pr3+ has two band-to-band absorptions at 330 and 360 nm, while CaGa2 O4 material has a band-to-band absorption around 260 nm, according to the authors. In comparison to UV–vis diffuse reflectance spectra of CaTiO3 : Pr3+ and CaGa2 O4 compounds, which both exhibit strong UV absorptions, the CaTiO3 /CaGa2 O4 : Pr3+ composite exhibits redshift of the absorption edges, i.e. band-to-band absorptions at 262, 329 and 388 nm, which was attributed to the two compounds being synthesized in one pot. The UV–vis diffuse reflection spectrum in this case, on the other hand, revealed and demonstrated that one pot synthesis tuned and reduced the band gap energy, thereby expanding the absorption region of CaTiO3 : Pr3+ and CaGa2 O4 when they were combined. Furthermore, the one-pot synthesis, in this case, was critical for charge transfer, which is known as inter-valence charge transfer (IVCT) in CaTiO3 : Pr3+ compound. The change in IVCT was discovered to narrow the band gap energy in CaTiO3 /CaGa2 O4 : Pr3+ rods composite, resulting in blue emission due to the 3 P0 → 3 H4 transition [47]. Cathodoluminescence (CL) was used in this study to investigate one of the optical phenomena for CaTiO3 /CaGa2 O4 : Pr3+ composite phosphor. The cathodoluminescence (CL) maps of CaTiO3 /CaGa2 O4 : Pr3+ composite phosphor collected in different areas are shown in Fig. 4c, d. Squares 1 and 2 in the figure represent the area where the corresponding CL spectra were collected. When comparing the two scanned areas, the CL results successfully revealed the type of emission achieved by scanning the composite in different regions. Blue and red emissions were discovered in the square 2 region, while only red emission was discovered in the square 1 region. The observed blue and red emissions are attributed to 3 P0 → 3 H4 and 1 D2 → 3 H4 transitions of Pr3+ , respectively. The blue emission was discovered due to the presence of CaGa2 O4 phosphor in the prepared CaTiO3 /CaGa2 O4 : Pr3+ composite phosphor [47, 48]. These findings indicated that CaTiO3 /CaGa2O4: Pr3+ rod phosphors with narrowed band gap are promising candidates for improving visible light photocatalysis activities such as solar-assisted water splitting reactions [47, 49]. Different dopants have been used to dope CaTiO3 . Taikar and Dhoble [50] reported on the photoluminescence studies of Pb2+ and Dy3+ co-doped CaTiO3 compounds. Figure 5 depicts CaTiO3 materials doped and co-doped with different concentrations of Pb2+ and Dy3+ ions. The intensity of the emission from Pb2+ -doped CaTiO3 phosphor (Fig. 5a) increases as the doping concentration increases up to 1.2 mol%, and then decreases. Because of the excitation energy that is lost through non-radiative

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Fig. 4 a FE-SEM micrograph, b diffuse reflectance spectrum, c cathodoluminescence map with a scale bar indicating total intensity over the scanned area and d photoluminescence spectrum of the composite phosphor [47]. Reproduced with permission from Noto LL, Shaat SKK, Poelman D, Dhlamini MS, Mothudi B.M, Swart HC. Cathodoluminescence mapping and thermoluminescence of Pr3+ doped in a CaTiO3 /CaGa2 O4 composite phosphor. Ceram Int. 2016;42:9779–9784. Copyright Elsevier (2016)

Fig. 5 PL emission spectra of a CaTiO3 : Pb2+ under 345 nm excitation and b CaTiO3 : Pb2+ : Dy3+ under 348 nm excitation [50]. Reproduced with permission from Taikar DR, Dhoble SJ. White light emission via Pb2+ to Dy3+ energy transfer mechanism in CaTiO3 phosphor. Optik. 2022;261:169,215. Copyright Elsevier (2022)

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decay, this phenomenon is known as concentration quenching [44, 46, 50]. The authors used Blasse’s expression [51] to figure out what was causing the concentration quenching:  Rc = 2

3V 4π X C N

1/ 3

(5)

where RC is the critical distance between Pr2+ ions, V represents the volume of the unit cell of CaTiO3 (V = 223.45 Å3 ), X c represents the optimum concentration of Pb2+ ions (X c = 0.012) and N represents the number of available sites for the Pr2+ in the unit cell of CaTiO3 (N = 4). Using Eq. 2, the critical distance (Rc ) for CaTiO3 :Pr2+ was determined to be 20.71 Å. This critical distance was found to be greater than 5 Å, indicating that concentration quenching occurs in the prepared phosphor through an electric multipole–multipole interaction mechanism [48, 50]. According to their findings, CaTiO3 : Pb2+ , Dy3+ material has a potential to be used in applications such as display devices, white light-emitting devices and other related phosphor applications [50]. Figure 5b shows the emission spectra of CaTiO3 : Pb2+ co-doped with various concentrations of Dy3+ ions. The spectra displayed blue emission (~483 nm and ~545 nm) and yellow emission (~587 nm) which are attributed to electronic transitions of Dy3+ ions. The blue emission band is attributed to the magnetic dipole allowed 4 F9/2 → 6 H15/2 transition of Dy3+ ions, whose intensity is not easily affected by the crystal field. The yellow emission band is assigned to the forced electric dipole 4 F9/2 → 6 H9/2 transition of Dy3+ ions which is hypersensitive to the local environment [50]. Most importantly, in this study, the Dy3+ was used to increase the optimum luminescence intensity of the Pb2+ , and an emission peak corresponding to Pb2+ ion electronic transitions was observed at 365 nm. Furthermore, while the intensity of Dy3+ ions increased with increasing Dy3+ content, the intensity of Pb2+ ions decreased. The increased luminescence intensity in the co-doped system can be attributed to an efficient energy transfer process from Pb2+ to Dy3+ ions [50]. The variation in emission intensities as the content of Dy3+ ions in CaTiO3 : Pb2+ : Dy3+ phosphor is shown in the inset of Fig. 5b. This shows that as the content of Dy3+ ions is increased in CaTiO3 : Pb2+ : Dy3+ phosphor the intensity of Pr2+ decreases. Clearly, the significant effect of the blue and yellow emission intensities indicates that the efficient energy transfer occurs from Pb2+ to Dy3+ and Pb2+ ion acts as a sensitizer for Dy3+ emission. According to the findings of this study, CaTiO3 doped and co-doped with Pb2+ and Dy3+ has the potential to be used in applications such as display devices, white light-emitting devices and other related phosphor applications [50]. Z. Jieqiang et al. [52] used a solid-state reaction route to synthesize a spherical particle-like shaped Mgx Ca1–x TiO3 : Eu3+ nanophosphor from Ti(OC4 H9 )4 , NH2 CONH2 , NH4 OH and oxidizer Ca(NO3 )2 . The synthesis procedure continued by using ethanol as a solvent, and the mixture was sintered at 1200 °C for 4 h. Mg2+ was discovered to be important in the particle size growth rate of CaTiO3 :

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Fig. 6 FE-SEM micrographs of a CaTiO3 :0.03Eu3+ and Mgx Ca1-x TiO3 :0.03Eu3+ where b x = 0.1, c x = 0.2, d x = 0.3, e x = 0.4 and f x = 0.5 [52]. Reproduced with permission from Jieqiang Z, Yanwei F, Zhaoyang C, Junhua W, Pengjun Z, Bin H. Enhancing the photoluminescence intensity of CaTiO3 : Eu3+ red phosphors with magnesium. J Rare Earths. 2015;33(10):1036–1040. Copyright Elsevier (2015)

Eu3+ samples. The effect of various Mg2+ contents (0.1, 0.2, 0.3, 0.4 and 0.5 mol%) on the morphology of nanophosphor compounds was studied. Figure 6 shows the SEM results that successfully revealed the type of morphology achieved by varying these Mg2+ contents. When the Mg2+ concentration was around 40%, SEM analysis revealed that the phosphor particles were uniformly distributed in the 600–800 nm range [52]. Figure 7 shows the excitation and emission spectra of CaTiO3 :0.03Eu3+ and Mgx Ca1-x TiO3 :0.03Eu3+ . Here, the excitation spectra of the compounds (Fig. 7a) were obtained in the range from 350 to 500 nm monitoring emission wavelength at 617 nm. The observed excitation peaks in this range are related to the f –f transitions of Eu3+ in the CaTiO3 host lattices, as shown in the figure. Under 398 nm excitation with a 150 W xenon lamp, Mgx Ca1-x TiO3 :0.03Eu3+ nanophosphor exhibits two luminescence bands at ~594 and ~617 nm, which are attributed to the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of the Eu3+ ions, respectively [52, 53]. However, the emission spectra show relative increases in the intensity of PL emission in Mgx Ca1-x TiO3 :0.03Eu3+ . These findings point to improved luminescence properties in the CaTiO3 compound as a result of the lattice distortion produced by Mg2+ within the host lattices of CaTiO3 . Furthermore, increasing the molar concentration of Mg2+ to 40% resulted in a reduction in the symmetry of the host lattices, resulting in a significant improvement in the PL intensity of the prepared nanophosphors. As a result, it was discovered that the optimal concentration of Mg2+ was required for the development of CaTiO3 nanophosphor for potential applications in w-LEDs [52]. (Ca, Sr, Ba)TiO3 are suitable perovskite host materials for RE ions for red-emitting phosphors due to their high charge storage capacity, high dielectric constant, good

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Fig. 7 Luminescence spectra of a CaTiO3 :0.03Eu3+ and Mg0.4 Ca0.6 TiO3 :0.03Eu3+ , and b Mgx Ca1-x TiO3 :0.03Eu3+ for different values of x (0.1, 0.2, 0.3, 0.4 and 0.5) [52]. Reproduced with permission from Jieqiang Z, Yanwei F, Zhaoyang C, Junhua W, Pengjun Z, Bin H. Enhancing the photoluminescence intensity of CaTiO3 : Eu3+ red phosphors with magnesium. J Rare Earths. 2015;33(10):1036–1040. Copyright Elsevier (2015)

insulating property, excellent optical transparency and excellent chemical and physical stabilities [54, 55]. Yang et al. [52] used hydrothermal synthesis to create CaTiO3 : Pr3+ , Ga3+ with cubic morphology with similar sizes. When the surfactant is not present, the samples appear to have only irregular particles at first. However, alcohol was used as a surfactant during sample synthesis, and it was critical in forming the cubic morphology predicted by the TEM images in Fig. 8a, b. The authors explained the formation of the cubic morphology in terms of dissolution–recrystallization mechanism. The luminescence studies were conducted as part of the further investigation of the properties of the CaTiO3 :Pr3+ ,Ga3+ phosphor. Figure 8c depicts the emission spectra of doped and co-doped samples. The emission spectra clearly shows that the two emission peaks are due to 1 D2 → 3 H4 and 1 D2 → 3 H5 transitions of Pr3+ ions, and that the addition of Ga3+ significantly enhanced these emissions. It was also discovered that Ga3+ is responsible for reducing the number of non-radiative centres in the CaTiO3 : Pr3+ compound, where the red fluorescence lifetimes and intensities are clearly increased. This is because Ga3+ is likely to replace Ti4+ sites, reducing the number of defects that could act as nonradiative centres for the 1 D2 state of Pr3+ [52]. Numerous studies have been focusing on perovskite materials and RE ionactivated perovskite compound for the development of nanotechnology. Mainly, the luminescence properties are key thought for incorporating RE ions in the lattice structures of perovskite semiconductor materials; particularly if the semiconductor exhibit moderate phonon energy. The motive for this is that the compound with moderate phonon energy results in high luminescence intensity, especially up-conversion luminescence (UCL) intensities. Hence, oxide materials that exhibit low phonon energy such as ZnO and TiO2 are acting as host materials for RE ions, and these materials tend to be paired to form the effective perovskite ZnTiO3 host for RE dopant ions [15,

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Fig. 8 TEM images (a) and (b), luminescence spectra (c) and decay curves (d) of CaTiO3 : Pr3+ and CaTiO3 : Pr3+ , Ga3+ [55]. Reproduced with permission from Yang L, Cai Z, Yang L, Hu J, Zhao Z, Liu Z. Solid-state synthesis, luminescence and afterglow enhancements of CaTiO3 : Pr3+ by Ga3+ codoping. J Lumin. 2018;197:339–342. Copyright Elsevier (2018)

56–58]. Mingze et al. [59] investigated the photocatalytic performance of a threedimensional ordered hollow spherical ZnTiO3 –ZnO–TiO2 (x) composite modified with CdS quantum dots (QDs). The non-emulsification polymerization technique was used to create the composites. Figure 9 shows the elemental composition and chemical valence states of the prepared composite as analysed using X-ray photoelectron spectroscopy (XPS). Figure 9a depicts a complete survey scan with characteristic peaks of Zn-2p, Ti-sp, Cd-3d, O-1 s, C-1 s and S-2p, confirming the presence of Zn, Ti, Cd, O, C and S elements in the composite. High-resolution spectra of Ti, Zn, Cd and S elements are shown in Fig. 9b–e. Figure 9b shows that the Ti element in the composite is in the form of Ti4+ , with binding energies of 459.0 eV and 464.8 eV corresponding to Ti 2p3/2 and Ti 2p1/2 , respectively [59, 60]. Figure 9c confirmed that the Zn element appears in the composite as Zn2+ , with binding energies of 1022.3 eV and 1045.4 eV corresponding to Zn-2p3/2 and Zn-2p1/2 , respectively [59, 61]. In addition, the Cd element is in the form Cd2+ , with peaks at binding energies of 404.9 eV,

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Fig. 9 a Survey scan spectrum; b Ti-2p; c Zn-2p; d Cd-2p; e S-2p; f O-1 s of CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 composite [59]. Reproduced with permission from An M, Li L, Wu Q, Yu H, Gao X, Zu W, Guan J, Yu Y. CdS QDs modified three-dimensional ordered hollow spherical ZnTiO3 –ZnO–TiO2 composite with improved photocatalytic performance. J Alloys Compd. 2022;895:162,638. Copyright Elsevier (2022)

405.8 eV and 411.6 eV, 412.6 eV corresponding to Cd-3d5/2 and Cd-3d3/2 , [59, 62] as shown in Fig. 9d. Furthermore, two distinct peaks at 169.0 and 162.7 eV correspond to S-2p3/2 and S-2p1/2 , respectively, confirming that S is in the form of S2− in the CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 (x) composite [59, 60]. Figure 10 depicts the optical properties of the various prepared samples as well as the CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 (x) composite. Figure 10a shows that the spectra of ZnO and TiO2 samples showed a significant absorption between 200 and 400 nm, which corresponds to the absorptions of ZnO and TiO2 semiconductors. The 3DOH ZnTiO3 –ZnO–TiO2(550) composite sample, on the other hand, showed a red shift in the absorption bands. This phenomenon can be explained by the synergistic effect of ZnO, TiO2 and ZnTiO3 , which increases light absorption in the visible region by the 3DOH spherical structure. Furthermore, the addition of CdS QDs to the 3DOH ZnTiO3 –ZnO–TiO2 (550) composite resulted in a red shift of the strong absorption from 440 to 520 nm, which is attributed to the synergistic effect in the 3DOH ZnTiO3 – ZnO–TiO2 (550) composite, giving it a high photon transport ability and absorption performance in the visible light region [59]. Figure 10b depicts the Kubelka–Munk energy plot for each synthesized sample, and Table 1 displays the calculated band gap energies. The results show that the band gap values are significantly reduced due to the size effect of the addition of CdS QDs, which synergistically enhanced the composite’s absorption of visible light [59]. Photoluminescence is a process that describes materials that absorb external energy and emit it to their surroundings. This can be seen with a variety of excitation

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Fig. 10 a Absorption spectra, b Kubelka–Munk plots and c emission spectra of CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 composite [59]. Reproduced with permission from An M, Li L, Wu Q, Yu H, Gao X, Zu W, Guan J, Yu Y. CdS QDs modified three-dimensional ordered hollow spherical ZnTiO3 –ZnO–TiO2 composite with improved photocatalytic performance. J Alloys Compd. 2022;895:162,638. Copyright Elsevier (2022)

Table 1 Band gap energies of the different samples [59]

Samples

Band gap (eV)

TiO2

3.20

ZnO

3.14

3DOH ZnTiO3 –ZnO–TiO2 (550)

2.94

CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 (500)

2.81

CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 (550)

2.73

CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 (600)

2.91

sources. The wavelength of light emitted can be used to investigate the properties of luminescent materials [63]. Mingze et al. [59] investigated the luminescence properties of the prepared different samples and composites, as shown in Fig. 10c. The samples were taken with an excitation wavelength of 325 nm. The samples exhibited characteristic peak emissions at 435–500 nm of varying intensities, due to the addition of CdS QDs, which attributed to the enhancement of the composite’s absorption

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in the visible light range. Furthermore, among the synthesized samples, the CdS QDs 3DOH ZnTiO3 –ZnO–TiO2 (550) composite demonstrated good photocatalytic performance [59]. Mofokeng et al. [15], investigated the efficacy of ZnTiO3 as a host perovskite material doped with Er3+ and co-doped with Yb3+ ions for UC luminescence properties in order to improve the efficiency of dye-sensitized solar cells (DSSCs). These samples were synthesized using the conventional solid-state reaction method. The photoluminescence spectra of ZnTiO3 :Er3+ displayed green emission (~527 nm and ~545 nm) and red emission (~665 nm) as shown in Fig. 11a. These green and red emissions are assigned to electronic transitions of Er3+ ions. However, the Yb3+ (sensitizer) was utilized to enhance the optimum luminescence intensity of the Er3+ and the new violet (~410 nm) and blue (~480 nm) up-converted emissions corresponding to electronic transitions of Er3+ ions were observed (Fig. 11a). The remarkable enhancement of luminescence intensity in the co-doped system is normally attributed to effective energy transfer process from the sensitizer to activator ions [15, 54]. In Fig. 11c–d, the ratio of the emission intensity of the samples was observed. Clearly, lowering the green/red ratio has a significant effect on both single and co-doped compounds [15]. The decay curve shown in Fig. 12a proved that the incorporation of the sensitizer Yb3+ in the Er3+ -activated ZnTiO3 remarkably increased the lifetime of the prepared samples. Figure 12b shows the proposed energy-level diagram of Er3+ –Yb3+ for the possible energy transfer mechanism in the ZnTiO3 : Er3+ , Yb3+ compounds which is illustrated by the excitation pathways populating different UC emission states through possible energy transfer processes from Yb3+ to Er3+ ions. In this case, electrons need to be excited and populate higher energy level of the sensitizer by the energy from the external source (980 nm), as shown in Fig. 12b. Furthermore, electrons from the excited state of the sensitizer are transferred to metastable levels of Er3+ ions, and then relax to the ground state of Er3+ ions in the form of up-converted violet, blue, green and red emission due to the transition of Er3+ ions. Furthermore, the obtained results from the developed materials confirmed that it can be applied for highly efficient NIR to visible up-converters applications [15]. Dutta et al. [58] also investigated the luminescence properties of ZnTiO3 nanophosphors co-doped with Er3+ and Yb3+ elements with enhanced UC emission for temperature sensing applications. The nanophosphor samples were synthesized by conventional solid-state reaction method using a stoichiometric amount of the commercial powders. Different concentrations of Yb3+ ions were incorporated into a ZnTiO3 :1%Er3+ host lattice. The absorbance spectra of ZnTiO3 :1%Er (Fig. 13) demonstrated the red-shifted absorption wavelengths for Yb co-doped ZnTiO3 : Er samples upon system excitation. Furthermore, absorption peaks corresponding to Er3+ ion transitions were observed at 490, 525 and 980 nm. However, the absorption peak observed at 980 nm corresponds to the superposition of the absorption transitions 2 F7/2 → 2 F5/2 (Yb3+ ) and 4 I15/2 → 4 I11/2 (Er3+ ). As a result, the intensity of the absorption edge confirmed that it improved photocatalytic efficiency and absorbance capacity in visible light [58]. Figure 14a, b depicts SEM micrographs of Er3+ -doped and Er3+ –Yb3+ co-doped ZnTiO3 compounds, respectively. In these images, it is reported that the particles are

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Fig. 11 UC luminescence spectra of a ZnTiO3 : Er3+ (different molar concentrations of Er3+ ) and b ZnTiO3 : Er3+ , Yb3+ (fixed molar concentrations of Er3+ and variation of molar concentrations of Yb3+ ). Comparison of integrated intensities of green and red emission emissions for c ZnTiO3 : Er3+ and d ZnTiO3 : Er3+ , Yb3+ compounds [15]. Reproduced with permission from Mofokeng SJ, Noto LL, Kroon RE, Ntwaeaborwa OM, Dhlamini MS. Up-conversion luminescence and energy transfer mechanism in ZnTiO3 : Er3+ , Yb3+ phosphor. J Lumin. 2020;223:117,192. Copyright Elsevier (2020)

formed on a nanometre scale with an average particle size of ~156 nm. Figure 14c, d shows the emission spectra of the samples excited at wavelengths of 426 nm and 980 nm at room temperatures, respectively. Under the excitation wavelength of 426 nm (refer to Fig. 14c), green emission (~525 nm) and red emission (~656 nm) due to the transitions of Er3+ ions were observed. However, Fig. 14d shows the blue emission bands (~490 nm), green emission bands (~525 nm and ~548 nm) and red emission bands (~656 nm) when the samples were excited at 980 nm. These emission bands are attributed to the transitions of Er3+ ions. The optimum concentrations of the developed samples were found to be 1 mol% Er3+ and 10 mol% Yb3+ . As a result, the increased up-conversion luminescence intensity caused by Yb3+ co-doping is due to an efficient energy transfer from the sensitizer to the activator. This result shows that the prepared ZnTiO3 : Er3+ , Yb3+ may be suitable luminescence material in NIR to visible up-converters and temperature sensing applications with maximum sensitivity ~4.7 × 10− 3 K−1 at 300 K [58]. Zhu et al. [12] used a hydrothermal method to successfully synthesize ZnTiO3 hexagonal prisms. The stoichiometric amounts of Zn(CH3 CHOO)2 and Ti(OC4 H9 )4

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Fig. 12 a Decay curves of the 4 F9/2 → 4 I15/2 transition and b schematic energy-level diagram of Er3+ –Yb3+ showing the energy transfer mechanism in ZnTiO3 : Er3+ , Yb3+ phosphor [15]. Reproduced with permission from Mofokeng SJ, Noto LL, Kroon RE, Ntwaeaborwa OM, Dhlamini MS. Up-conversion luminescence and energy transfer mechanism in ZnTiO3 : Er3+ , Yb3+ phosphor. J Lumin. 2020;223:117,192. Copyright Elsevier (2020)

Fig. 13 Absorbance spectra ZnTiO3 :1%Er3+ , xYb3+ compound for different molar concentrations of Yb3+ ; (i) x = 0%, (ii) x = 5%, (iii) x = 10%, (iv) x = 15% [58]. Reproduced with permission from Dutta J, Chakraborty M, Ra VK. Investigation on ZnTiO3 co-doped Er3+ / Yb3+ nanophosphors with enhanced up-conversion emission and in temperature sensing application. Optik. 2021;233:166,558. Copyright Elsevier (2021)

were used during the synthesis process. The authors investigated the effect of foreign elements (Eu3+ and Al3+ ) on the structural and photoluminescence properties of ZnTiO3 perovskite. Figure 15 depicts SEM images as well as photoluminescence excitation and emission spectra of ZnTiO3 : Eu3+ co-doped with varying concentrations of Al3+ charge compensator. Figure 15a, b demonstrates and confirms that a single-doped sample (ZnTiO3 : Eu3+ ) and co-doped samples (ZnTiO3 : Eu3+ , Al3+ ) are composed of regular hexagonal prisms containing secondary nanoparticles [12].

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Fig. 14 FE-SEM images of the ZnTiO3 a doped Er3+ (1% Er3+ ) and b co-doped Er3+ –Yb3+ (1% Er3+ –5% Yb3+ ) compounds. c, d UC luminescence emission spectra of ZnTiO3 : Er3+ , Yb3+ compounds [58]. Reproduced with permission from Dutta J, Chakraborty M, Ra VK. Investigation on ZnTiO3 co-doped Er3+ /Yb3+ nanophosphors with enhanced up-conversion emission and in temperature sensing application. Optik. 2021;233:166,558. Copyright Elsevier (2021)

The excitation spectra were recorded while the emission wavelength at 614 nm was monitored (Fig. 15c). The spectra contain a charge transfer band (CTB) between 275 and 350 nm, as well as several sharp lines at 395, 416 and 465 nm. The CTB is attributed to the electron migration from the ligand 2p orbit O2− to the f–f transitions of Eu3+ ions. The sharp peaks at 395, 416 and 465 nm are attributed to the 7 F0 → 5 L6 , 7 F0 → 5 D3 and 7 F0 → 5 D2 of Eu3+ , respectively [12, 64]. The emission spectra of ZnTiO3 : Eu3+ co-doped with a varied molar concentration of Al3+ charge compensation under the excitation of a 465 nm laser are shown in Fig. 15d. The emission exhibited five distinct emission lines in the 550–750 nm range, which corresponded to the usual emissions of Eu3+ ions. The 4f–4f transitions, 5 D2 , 5 D0 → 7 FJ (J = 0–6) are widely accepted as the primary causes of Eu3+ ion glow. The five emission bands, with centres at 579, 593, 614, 654 and 703 nm, were assigned to the transitions 5 D0 → 7 F0 , 5 D0 → 7 F1 , 5 D0 → 7 F2 , 5 D0 → 7 F3 and 5 D0 → 7 F4 of Eu3+ ions, respectively [65]. According to Judd–Offlet theory [66], the environment around Eu3+ has an effect on its intra-level transitions. Figure 15d shows that the magnetic dipole (MD) 5 D0 → 7 F2 transition of the Eu3+ ion is stronger than the electric dipole

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Fig. 15 a, b SEM micrographs, c excitation spectra and d emission spectra of Eu3+ , Al3+ (x mol%) co-doped ZnTiO3 hexagonal prisms [12]. Reproduced with permission from Zhu B, Yang Q, Zhang W, Cui S, Yang B, Wang Q, Li S, et al. A high sensitivity dual-mode optical thermometry based on charge compensation in ZnTiO3 : M (M = Eu3+ , Mn4+ ) hexagonal prisms. Spectrochim. Acta A Mol. 2022; 274:121,101. Copyright Elsevier (2022)

(ED) 5 D0 → 7 F1 transition in the emission spectrum. This indicates that the Eu3+ ions are located in a non-inversion symmetric site in the prepared ZnTiO3 lattice [57, 66]. The results, however, revealed that varying the molar concentration of Al3+ charge compensation slightly increased the emission intensity of Eu3+ ions and began to decrease at 0.4 mol% Al3+ concentration and above. The charge balance in the Eu3+ doped ZnTiO3 crystal can describe this. In this case, the Al3+ charge compensation tends to promote charge balance by forming electron-negativity during the synthesis of the ZnTiO3 : Eu3+ , Al3+ compound [12, 57].

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3 Potential Applications 3.1 Applications of Undoped and Rare-Earth-Doped ZnTiO3 Due to the fast development wireless communication in industries, nanomaterials have been widely studied due to their distinctive features such as magnetic, electronic, optical and chemical properties that differ from bulk counterpart materials [67]. In recent years, nanotechnology has become an innovation when designing and tuning nanomaterials such as metal oxides. ZnTiO3 materials are one of the suitable materials to prepare quantum-dot-sensitized solar cells (QDSSCs) due to their high electron mobility and bandgap structure. The suitable bandgap structures allow light to enter the QD layer with minimum energy loss [42]. The ZnTiO3 materials also have photoanode characteristics due to the increased electron transport rate and reduced recombination probability during electron transfer. In addition, ZnTiO3 has received attention recently and has been widely used in building electron transport channels because of their appropriate bandgaps, efficient electron mobility and simple synthesis methods [42]. Due to these properties, ZnTiO3 has been reported to have properties suitable for application in microwave dielectrics, photocatalysis, wireless communication products like telephones, wireless local area networks (WLAN) and global positioning systems (GPS). ZnTiO3 perovskite material has also received consideration as nitric oxide (NO), carbon monoxide (CO) gas sensors, LEDs, pigment and solar cells [68]. It is known that the crystal structure of host lattice, synthesis methods, particle sizes and the type of dopant ion affect the luminescent dynamics of phosphors. Recently, inspiring progress has been made in the studies on the luminescence of RE-doped perovskite materials due to their abundant energy levels and excellent luminescence [15, 56, 69–71]. ZnTiO3 doped with RE can be used in LEDs as the lighting source. This has made it to receive attention, due to the advantages of high luminous efficiency, low energy consumption, reliability, long lifetime, antibacterial properties and environmental protection [56]. Due to rich energy-level structures, high photostability, tunable wavelengths, long emission lifetimes, sharp bandwidths and relatively low toxicity, the RE-doped ZnTiO3 have been used in numerous applications, such as UC of near-infrared (NIR) photons into visible or ultraviolet photons [15].

3.2 Applications of Undoped and Rare-Earth-Doped CaTiO3 CaTiO3 has a perovskite structure with a bandgap of 3.68 eV and possesses optoelectronic properties widely used in many applications such as photocatalysis, LEDs, solar cells, capacitors, biodiesel production, drug chemical/biological sensors and catalysts [72]. TiO2 group in these environments has a large refractive index and a wide bandgap of 3.2 eV, which results in powerful luminescence [73]. Surface

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morphology and structural properties of the CaTiO3 influence its physicochemical properties [74]. The increasing production of CO2 from fossil fuels has raised concerns over the past years due to its high contribution to global warming resulting in a large global climate change. CaTiO3 can be used as a CO2 absorbent due to the presence of oxygen vacancies in TiO2 acting as both, CO2 and H2 O adsorbent and excited electron attractor. This causes all of the excited electrons from irradiation to be transferred to the oxygen vacancy sites in TiO2 , producing CO and CH4 from CO2 [3]. CaTiO3 can also be used in the degradation of organic pollutants. Semiconductor photocatalysis is a non-toxic and eco-friendly method for degrading hazardous pollutants. Numerous photocatalysts have been investigated in recent years. Among them, CaTiO3 is a well-known titanium-based perovskite material that has been extensively researched for environmental remediation applications. Pure and modified CaTiO3 loaded metal and metal oxide can also be used for the improvement of photodegradation of water. Incoporating RE elements into CaTiO3 has always been an excellent way of improving its photoluminescence properties and making them suitable for application in field emission displays, monitors, photonic devices, phosphors, w-LED, and optical material [3]. The type of RE dopant determines the luminescent properties and therefore the suitable applications. Attention has been paid to the luminescence of RE ions in different hosts materials making them suitable for various applications [3, 75–79]. The chemical and thermal stability as well as the low phonon energy (470 eV) of CaTiO3 makes it a good host in a matrix for UC of phosphors. Thus, doping CaTiO3 with up-converting RE ions improves the light absorption edge of the photocatalysts which make it suitable for application in the production of pollutant degradation in water and DSSCs [3].

4 Conclusions The ongoing enhancement of the luminescence properties and stability of perovskite titanate compounds in the family of titanates makes them suitable for a broader range of future practical applications. Incorporating RE elements into CaTiO3 and ZnTiO3 perovskites has always been a great way to improve the photoluminescence dynamics of these materials. In this context, various strategies such as appropriate synthesis methods, single and co-doping with foreign elements/RE have resulted in improved photocatalyst light absorption edge, charge carrier separation, quantum yield and high performances of CaTiO3 and ZnTiO3 towards respective photocatalytic, renewable energy, gas sensors, LEDs, pigment and solar cells, among others. In general, the luminescent properties of ZnTiO3 and CaTiO3 phosphors are affected by their molar ratios and concentrations, synthesis methods, morphological features, particle sizes and the type of dopant ions used, as well as the type of perovskite phosphor materials used. Furthermore, the high performance of doped ZnTiO3 and CaTiO3 phosphors has been remarkably achieved by varying various reaction parameters such as reaction time, molar ratios, particle size, crystal surface morphology, ignition temperature,

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mixing or blending hard metals and so on. Despite the extensive work done by various researchers on perovskite ZnTiO3 and CaTiO3 materials, the utilization and effect of RE for both UC and DC processes to cover the wider emission wavelength responses to develop efficient ZnTiO3 and CaTiO3 materials remain a challenge. This opens the possibility of future research into the luminescence dynamics of ZnTiO3 and CaTiO3 with varying particle molar ratios and concentrations, morphological features and particle sizes. A precise synthesis method, such as the conventional solid-state method, may be a better way to prepare such materials. Furthermore, this will allow for further research into the precise mechanism that may be causing the enhanced luminescence properties and brightness of perovskite CaTiO3 and ZnTiO3 phosphors for a broader range of practical applications.

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Rare-Earth-Doped Ternary Oxide Materials for Down-Conversion and Upconversion Irfan Ayoub, Rishabh Sehgal, Vishal Sharma, Rakesh Sehgal, Hendrik C. Swart, and Vijay Kumar

1 Introduction In view of the persistent rise in the demand for energy and the impacts of utilizing current energy sources on the environment, the dwindling of oil resources and geopolitical situations have prompted researchers to search for an efficient and economical alternative. Thus, without facing a catastrophic energy crisis, the contemporary world is progressively reducing its reliance on fossil fuels and is shifting towards different sustainable energy choices such as solar, wind, hydrogen, nuclear power, and many more for the future [1–4]. Among the different forms of sustainable energy sources, solar energy is one of the most plentiful and environmentally friendly sources of energy. Similarly, nuclear energy has net-zero CO2 emissions, a small environmental impact, and generates little waste [5, 6]. However, the widespread use of solar energy I. Ayoub (B) · V. Kumar Department of Physics, National Institute of Technology Srinagar, Hazratbal, Jammu and Kashmir 190006, India e-mail: [email protected] R. Sehgal Department of Electrical and Computer Engineering, University of Texas at Austin, Austin, TX 78751, US V. Sharma Institute of Forensic Science and Criminology, Panjab University Chandigarh, Chandigarh 160014, India e-mail: [email protected] R. Sehgal Department of Mechanical Engineering, National Institute of Technology, Hamirpur, Himachal Pradesh 177005, India H. C. Swart · V. Kumar Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein ZA9300, South Africa e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_5

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(photovoltaic cells) has been hindered by its low conversion efficiency, spectral dissonance, and strong absorption spectrum [7]. In addition, regarding the nuclear form of energy, it is believed that the radiation from nuclear power plants could be harmful to the ecosystem of the area in which the plant is located [8]. Therefore, different researchers are searching for the best materials to increase the solar energy absorption in traditional photovoltaic (PV) cells and provide protection in the nuclear environment by imposing the concomitant restrictions. To achieve this goal, researchers have attempted different methods, however, the incorporation of REOs has remained an active study area owing to their efficient characteristics [9]. The incorporation of the rare earth element Ytterbium with Bi3+ in the Y2 O3 host material, which functions as a down converter, has revealed a substantial enhancement in the efficiency of solar cells. The increase in efficiency occurs because of the availability of higher energy orbitals 3 P1 –1 S0 for the Bi3+ [10]. Similarly, the synthesis of ZnO/Yb2 O3 nanocomposite, Er3+- , Li3+- , and Yb3+ -doped Y2 O3 luminescent host, LiYb(MoO4 ), Eu3+ -doped LiYb1-x Eux (MoO4 )4 and other different efforts made by researchers have shown significant enhancements in the efficiency of PV cells [11–14]. Upconversion polycrystalline powders, such as Y2 O3 , were used to broaden the emission energy bands [13]. The observation has also revealed that organic and inorganic dyes deteriorate very fast under the influence of UV radiation, which tends to decrease the dyesensitized solar cell (DSC) endurance [15]. Studies have revealed that the problem of dye degradation can be eradicated by the incorporation of rare earths relying on the DC and UC mechanisms in the photoanode layers [16–20]. The incorporation of the DC/UC rare earths in the photoanode layers increases the parameters like surface area, scattering of light, durability, and recombination capability [21]. Additionally, the REOs were employed in thermophotovoltaic (TPV) systems as targeted radiation emitters for the achievement of the right energy bands and to improve the carrier mobility in perovskite solar cells [22–24]. Due to excellent shielding capability against harmful radiations, mechanical strengths, and temperature resistance, the DC/UC-based REOs are regularly taken into consideration for preserving the living entities from dangerous radiations in nuclear reactors, medicine, and aerospace technology [25]. The rare-earth ions are nowadays frequently in all newly developed applications because of their unique energy-level structure, strong intra-4f–4f transitions, and efficient protection of the outer orbitals in the host matrices by their 5 s and 5p orbitals [26–28]. Due to the unique energy-level structure of the solid host materials, they are also able to absorb and emit photons in UV to IR regions of the spectrum. Because of the things we’ve talked about, rare-earth-based DC and UC materials are good at tuning UV and NIR light into visible light. This makes them good candidates for solar-based technology [29]. Another problem faced by the DSCs utilizing the liquid electrolyte is that of their stability, as the solvents utilized are thermodynamically unstable. Long-term storage and natural environment exposure necessitate the prevention of solvent exudations and the progressive degradation of dye molecules from anode materials. In order to address these problems, dye was replaced by perovskite and the liquid electrolyte by solid-state hole transporting material as they proved to be very beneficial, particularly in large-scale perovskite solar cell (PSC) applications [30, 31]. As compared to DSCs, perovskite materials have

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garnered much more attention because of their rapid, exponential rise in efficiency, which is approximately 22.1%. Furthermore, compared to the dyes, the perovskite materials offer outstanding electric and optical characteristics. However, the primary study in this area is optimizing the crystallization for creating the perovskite layers with a desirable shape. But the prominently utilized perovskite material, MAPBI3 , ingests the part of the light incident on it falling within a wavelength range of 350 and 800 nms due to its exclusive optical bandgap of the order of 1.55 eV, which leads to wastage of a large amount of solar energy over visible to NIR range [31]. As a potential substitute, it has been observed that the amalgamation of the DC and UC materials has proved to be a successful way of increasing the photocurrent which in turn tends to enhance the conversion efficiency limit of the solar cells. Particular rareearth-doped DC and UC nanomaterials are now the subject of intense study due to the possibility of using them in solar cells [32, 33]. It is anticipated that the incorporation of DC or UC materials, which possess efficient charge transport ability, can minimize the electron–hole recombination and will increase the utilization of visible and NIR light in PSCs [33]. Incorporation of these also tends to enhance the light scattering ability along with the perovskite filtration, which further improves the efficiency of PV cells. Additionally, it significantly improves the capacity of the perovskite films to resist moisture, thus increasing the stability of PSCs [34–36]. Over recent years, remarkable advancements in this field of study have been achieved at a very rapid pace by eminent research organizations, leading to an enormous number of publications every year. This publication includes review articles and chapters citing the application of DC/UC REOs almost in every domain of science and technology, such as biomedical, optical, sensors, detectors, electronic devices, and many more [37–39]. The overview of different prospects, challenges, and applications of REOs is presented in Fig. 1.

Fig. 1 Prospects, applications, and challenges of rare-earth oxides

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Keeping in view the importance of these materials, this chapter aims to provide an insight into the DC and UC phenomena. The different mechanisms governing the DC and UC phenomena will also be discussed. Then the main focus will be to highlight the applicative domain of these DC/UC-based materials. In the applicative section, the discussion will focus on the present research state, significant issues, and prospective research gaps, along with the future research orientation of DC/ UC REOs. Before concluding the chapter, challenges of the rare-earth oxides in the different fields will also be discussed which will be followed by the future perspective.

2 Down-Conversion The phenomenon of DC is a non-linear optical process. In this process, high-energy photon gets split into two low-energy photons as depicted in Fig. 2a. This marvels phenomenon was first proposed in 1957 by Dexter. For achieving this phenomenon in different DSCs and PSCs, a layer of DC materials is placed on the glass-side of the substrate which helps in the transformation of photons from shorter to longer wavelengths by absorbing the visible light. As a result, the devices produce more electron–hole pairs, which helps in enhancing the efficiency of the device. At the very beginning the phenomenon of DC was observed in the fluoride-based materials doped with rare-earth ions such as Pr3+ , Tm3+ , and Er3+. that possess broader emission

Fig. 2 Pictorial representation of down conversion and upconversion phenomenons

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spectra [40]. The DC materials are generally classified on the presence of the rareearth/lanthanide ions in the host that is lanthanide and nonlanthanide DC materials. Both of these subclasses are briefly discussed below.

2.1 Lanthanide DC Materials Lanthanides consist of a group of fifteen different elements present in the f-block of the periodic table. The atomic number of these elements ranges from 57 to 71. In addition to these, they also include two different elements, that are scandium and yttrium, having the atomic numbers 21 and 39, respectively. These lanthanides are a group of elements that start after lanthanum in the periodic table and constitute the subsequent filling of electrons in 4f-subshells. Despite the fact that lanthanum, scandium, and yttrium don’t completely occupy their 4f subshells, their characteristics are quite similar to those of lanthanides; thus, they are counted among the members of this group. Lanthanides, which are often commonly referred to as “rare-earth elements,” are becoming more and more significant from a research point of view because of their distinctive optical, magnetic, and catalytic characteristic features. The property of luminescence lies at the core of lanthanides, and as a result, these materials are being used for multiple purposes such as lighting, electronic displays, sensors, LEDs, and security [41, 42]. Photoluminescence is one of the most desirable characteristic features of rare earths as they possess distinct emission peaks with efficient color purity. On the other hand, these materials possess poor luminescence intensity because of their poor light-absorbing capacity. The amount of light absorbed is proportional to the luminescence produced; thus, even a small amount of lanthanide absorption affects the amount of photoluminescence produced [43]. In order to circumvent the problem of weak absorption and adjust the luminescence characteristics in an appropriate manner, the sensation process, also known as the antenna effect, is a well-known phenomenon opted by the lanthanide ions. Furthermore, the lanthanide ions and the environment around them are the primary factors responsible for determining the color of emission. The phenomenon of luminescence is the consequence of the radiative and non-radiative transitions that occur in an electrically stimulated ion. All of the lanthanide ions possess the completely filled outermost shell, but the number of electrons progressively keeps on increasing in their 4f orbitals with the increases in the atomic number. The distinctive chemical and physical properties possed by them are governed by these unfilled 4f orbitals. These shells are shielded by their outermost 4 s and 5p shells. This shielding of the inner 4f shells is responsible for enhancing the efficiency of numerous valuable features of lanthanide ions, that is narrowband emission, luminescence, and the lifespan of the excited state [44]. In the process of synthesizing luminous materials to be used in the visible region of the electromagnetic spectrum, Eu3+ , Gd3+ , and Tb3+ are some of the most commonly used lanthanide ions. However, in cases when Eu3+ is excited by UV light, emission of two red photons occurs, thereby resulting in the occurrence of the DC process [44]. The main phenomenon governing the phenomenon of DC is

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the transfer of energy from the rare-earth ion to others. There exist different modes through which energy transfer can take place, namely via migration of electrons and holes, via overlapping of resonance energy levels of sensitizer and activator ions, or by resorbing the photons released by the respective activator and sensitizer ions. However, the energy absorbed by the lanthanide ions possesses the ability to reach out up to the defects of the host lattice, where they can recombine non-radiatively, thereby affecting the effectiveness of the DC process [40]. Thus, it is necessary that all the luminous DC materials be extremely crystalline with minimal lattice imperfections.

2.2 Non-lanthanide DC Materials Among the enormous number of materials, the fabrication of effective DC materials for spectral conversion especially in PV cells is of utmost importance in order to improve their power conversion efficiency without changing the fundamental design. As was previously mentioned, the majority of the luminous materials used in DC devices are rare earths and their compounds. However, the majority of the rareearth materials possess similar characteristics that are comparable to one another, but their process of separating, refining, and purifying them is very laborious and costly [45]. Apart from this, there has been a tremendous increase in the overall usage of lanthanides over the past several years [46]. Keeping all these aspects in view, along with the fact that mining and refining processes are extremely costly and harmful to the environment, the focus of research has shifted towards the growth of luminescent materials without using the rare-earth elements [47]. During the past few years, the observations have revealed that fabrication of rare-earth-free DC materials is possible through the use of four primary methods: That is, (I) Transition metals used as luminescent centers: Aside from lanthanides, several transition metal-based compounds and complexes exhibit very bright luminescence. As a luminous material, the compounds of elements such as vanadium, copper, manganese, iron, cobalt, and nickel, are very common. Among these d-metals, manganese and vanadium display luminescence even at room temperature and are available in bulk quantities in nature [48]. (II) Luminescence arising from defects: During recent years, attention of researchers has significantly diverted towards the study of materials such as silica-based metal oxides, and boron–carbon oxynitride materials, that reveal the luminescence property due to the presence of defects. The essence of luminescence arising via defects is still a matter of discussion; however, most of the time, it is due to the presence of different vacancies, impurities, radicals, and many more [44]. (III) Utilization of self-luminescent materials: Self-activated luminous materials that display the property of luminescence without the presence of an activator ion is another alternative for the generation of luminescence without rare-earth ions. In this group, the most prevalent element is considered to be vanadate, which exhibits the charge transfer phenomenon due to the existence of (VO4)3− clusters. Self-luminous

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materials made from vanadate have a wide range of light emission, are inexpensive, and only need a mild heating process to work. In addition, tungstate- and molybdate-based compounds belong to the same class of self-luminescent materials. The luminous activity is mostly due to charge transfer transitions, defect recombination, exciton emission, and so on [44]. (IV) Development of luminescent quantum dots (QDs): These are another type of DC material with adjustable luminescence. They outperform the conventional rare-earth-based luminescence materials because of their excellent efficiency, color tuneability, and purity. Furthermore, their applicative domain is significantly larger as compared to other luminescent materials due to their nanosized dimensions. To date, the most commonly used QDs are made from InP, CdSe, carbon, graphene, ZnO, and other materials [44].

3 Upconversion and Mechanisms This phenomenon was first discovered by Auzel in 1966. The characteristic of this process is the sequential absorption of two low-energy photons, which ultimately results in the emission of single high-energy photon. Hosts which show this phenomenon are usually doped with two optically active ions called sensitizer and an activator. In this process, the absorption of photons that falls below the bandgap results in the creation of electron–hole pairs. This process occurs via two different transitions that took place in sequential order that is from the valance band to the intermediate level followed by the conduction band as depicted in Fig. 2b. In this process, the main phenomenon is the transfer of energy among the different energy levels. The energy-transfer process associated with the UC process occurs through various processes that is ground-state absorption (GSA), excited-state absorption (ESA), energy-transfer UC (ETU), photon avalanche (PA), and cooperative UC (CUC). Among the different mentioned processes GSA/ESA is considered to be a fundamental UC mechanism, in which the electron absorbs a photon for reaching the first excited state from the ground state and then absorbs another photon for reaching the higher excited state as depicted in Fig. 3a. Because of the concentration quenching processes caused by the occurrence of non-radiative relaxations at higher concentration, ESA process is typically manifested in the single rare-earth ions at lower concentrations (30,000 h under the strong excitation fluences in solid state lighting applications.

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1.9 Color Purity and Color Coordinates The red phosphors should show good chromaticity coordinates close to the standard red phosphor values (0.67, 0.33) from National Television Standard Committee (NTSC). Also they should exhibit high color purity better than 90% and the color purity was determined from the equation [12, 13]. / Color purity =

(x − xi )2 + (y − yi )2 × 100 (xd − xi )2 + (yd − yi )2

where (x, y) are the CIE chromaticity coordinates of the phosphors, (x i , yi ) are the chromaticity coordinates of white light (0.333, 0.333), and (x d , yd ) are the coordinates of the dominant wavelength in the emission.

1.10 Low Light Scattering The red phosphors powder particles should be uniform in size ≤1 μm for easy dispersion in the matrix and also spherical in shape to reduce the scattering of light.

1.11 High CRI and Low CCT Phosphors should have CRI 90–95 and CCT ~2500–4000 K for warm white emissions.

2 Applications of Red Phosphors The red phosphors have many important technological applications such as white light emitting diodes (WLEDs), agriculture, displays, thermometry, security, biomedical and solar energy (Fig. 2).

2.1 White LED Applications The down converting phosphors play a key role in the phosphor converted white light emitting diodes (pc-WLED). Currently, the commercialized WLED is realized by the combination of broad yellow emitting phosphor (YAG:Ce3+ ) along with blue LED chip. However, this method of producing white light faces some severe problems such

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Fig. 2 Red phosphor applications in various fields

as low CRI, high CCT due to the absence of red component. To overcome these issues, the mixing of red phosphors is adapted along with other phosphors to produce warm light with high CRI and low CCT. In this regard, numerous efforts have been made to discover novel red phosphors for WLED applications. The red phosphors should have broad emission, good emission intensities, low thermal quenching and good photo stability to qualify for the LED applications. Among the reported large number of red phosphors, some of the potential phosphors are CaAlSiN3 :Eu, Ba3 MgSi2 O8 :Eu, Mn and K2 SiF6 :Mn [14–16].

2.2 Agricultural Applications The agricultural product yield mainly depends on the photosynthetic efficiency. The plants perform the photosynthesis process by absorption of light through the pigments: chlorophyll A and B. These two chlorophyll pigments have strongest absorption of light in the blue region 400–500 nm and orange-red region 600–680 nm wavelength regions. Several studies suggest that by continuous irradiation of blue and red light on the plants, the crop yield will be significantly increased by enhanced physiological process in plants. In the adverse conditions of sunlight and also for indoor cultivation, the LED lamps serve the purpose of red light to enhance the photosynthetic activity of the plants. The red phosphors under the excitation of the unwanted UV light helps in filtering out the sunlight. Many Mn4+ doped germanates

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red phosphors show their promising application in optical agriculture and indoor plant cultivation. The emitted wavelength by these phosphors matches with the absorption range of chlorophylls responsible for the photosynthesis. Some of the potential Mn4+ doped systems for plant growth applications are Ba2 TiGe2 O8 :0.006Mn4+ , Mg3.5 Ge1.25 O6 :0.0125Bi3+ , 0.01Mn4+ and BaAl2 Ge2 O8 :0.001Mn4+ [17–19]. Thus the crop yield can be increased significantly through the use of red phosphors in plant growth LEDs.

2.3 Display Applications Apart from solid state lighting applications, deep red emitting phosphors find potential applications in various displays such as television, large display panels and cathode ray tubes (CRTs). Mostly Mn4+ doped fluoride phosphors are preferred choice with deep red emission in the wavelength region around 650 nm for enhancing the color gamut of ultra-high definition (UHD) TVs. Mn4+ activated red phosphor Li3 RbGe8 O18 showing deep red emission peaking at 667 nm is one of the promising phosphor for display application that increases the color gamut in UHD TV [20]. Also other promising red phosphors for solid-state displays and lasers are Ca2 Ge7 O16 :Eu3+ , Mg14 Ge4.5 Ti0.5 O16 :Mn4+ , MgAl2 O4 :0.05Cr3+ due to their good CCT and CIE values [21, 22].

2.4 Thermometry Applications Red phosphors are also drawing more attention for temperature sensors due to their fast response, good spatial resolution and relatively low perturbation of sample temperature during measurement. The present phosphors for optical thermometry have some drawbacks such as small temperature range, energy loss due to reabsorption in multiple activator system and low sensitivity. In this direction, a single activated red phosphors with high sensitivity and large temperature range are required for thermometry applications. The change in emission intensity or the decay time are used for sensing the temperature. The best examples of red phosphors are Er3+ , Yb3+ doped Bi7 Ti4 NbO21 and LiLaP4 O12 :Cr [23, 24].

2.5 Security Applications Red phosphors play a key role in security and forensic applications including fingerprint detection. The detection of fingerprint is an easy way to identify the individual’s information in the area of forensic science, since each one has a unique finger print image. Luminescent materials are more effective in terms of good contrast, high

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sensitivity and visibility and low interference with the background on comparison with the traditional tactics of regular and metallic powder methods. In addition, the issues of counterfeiting of currency notes and forgery of sensitive documents can be solved very effectively by the impregnation of the fluorescent materials in these items. For these applications, the emission of the phosphors should be in the higher wavelength for reducing the interference of the background and texture within the eye sensitivity region. The red phosphors based Eu3+ , Mn4+ and Cr3+ are more apt for these security and forensic applications. The best examples are BaY2 ZnO5 :Eu3+ , Ba2 LaNbO6 :0.25Mn4+ , Zn3 Ga2 Ge2 O10 :0.5% Cr3+ etc. [25–27].

2.6 Biomedical Applications The long after glow phosphors more typically the red phosphors have unique biomedical applications in the areas of bio imaging, tumor therapy, image storage etc. Deep red emitting phosphors in the wavelength of region 650 nm are preferred choice due to the least scattering of light enhancing the high contrast of the images. Also the deep red region is transparent to the biological tissues which makes them suitable for the bio imaging applications. A few Mn4+ , Cr3+ , Pr3+ , Eu2+ , Dy3+ doped red phosphors have shown potential for these applications. Some of the good examples are Zn2.94 Ga1.96 Ge2 O10 :Cr3+ , Pr3+ , SrAl2 O4 :Eu2+ , Dy3+ , Mg2 GeO4 :Mn4+ etc. [28–30].

2.7 Solar Energy Applications The mismatch between the band gap of the Si solar cell and the incident photon energy from the sunlight reduces the solar cell efficiency in harvesting the total solar energy into electricity. The band gap of Si solar cell is more tuned to the absorptions of red and deep red regions of the light which contains all parts of the spectral region from UV to NIR region. To enhance the efficiency of the solar cells, luminescence solar concentrators are employed to match the band gap energy of Si cells. The luminescent material coated on the solar cells converts the high energy UV photons to a red rich wavelength region enhancing the efficiency of the solar cells. Many Sm3+ , Cr3+ and Eu2+ activated red phosphors are used extensively for down converting the UV energy to red and deep red regions by harvesting the total solar energy in the solar cell applications. Best examples are La3 GaGe5 O16 :Cr3+ , Ba2 ZnS3 :Eu2+ , CaSc2 O4 :Eu3+ , Sm3+ [31–33].

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3 Common Red Phosphor Activators The applications of red phosphors in different areas have been highlighted in the previous sect. 2. Some of these applications can be achieved if these phosphors show strong red emission intensities with good quantum efficiency and yield. Among the phosphors, red phosphors have been widely studied due to its technological importance. The common activators whose electronic transitions lie in the red and deep red region are Eu3+ , Eu2+ , Pr3+ , Sm3+ , Mn4+ and Cr3+ .

3.1 Eu3+ Activator Ion Among the rare earth activator ions Eu3+ has been extensively studied in different hosts that gives strong emissions in the orange red region. Its luminescence originates from the intra configurational of 5 D0 –7 FJ (J = 0–4) transitions that are highly hypersensitive and strongly dependent on the surrounding of Eu3+ . The probability of these transitions is also dependent on the site symmetry of Eu3+ , when Eu3+ occupies a non centro symmetric site the hypersensitive forced electric dipole (ED) transition dominates over the allowed magnetic dipole (MD) transition. Generally, both these transitions exist simultaneously with different degrees of intensity depending on the prevailing site symmetry. The ratio of the intensities of ED to MD is called as an asymmetric ratio (R) that provides the degree of distortion from the inversion symmetry of its local environment in the host. In addition, the splitting of these transitions gives further knowledge of the crystal field effect in the Eu3+ environment. Also, the appearance of 5 D0 –7 F0 in the emission spectra of Eu3+ indicates the low symmetry environment as it is allowed only for low symmetry elements like C s , C n , C nv . The presence of intense 5 D0 –7 F4 transition reveals the highly polarizable chemical environment with low symmetry [34, 35]. Thus, Eu3+ ion is also considered as structural probe to investigate these symmetry coordination and environment around the cations in the crystalline structure. Eu3+ can be easily accommodated in most of the hosts with coordination ≥VI.

3.2 Eu2+ Activator Ion Eu2+ ions show broad band emissions due to parity allowed electronic transitions from the 4f 6 5d 1 excited states into the 4f 7 ground states. The emission wavelength is strongly influenced by the host lattice parameters such as coordination of the cations, Eu–O bond length, the symmetry and the covalency and it varies from the UV to the red spectral range. For example, Eu2+ doped fluorides exhibit violet emission whereas in chlorides and bromides violet-blue colors and in oxides blue, green and red emissions [36]. To achieve the red emission with Eu2+ , the proper host lattice

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is to be chosen to create high covalency and good crystal field strength in the Eu2+ surroundings.

3.3 Pr3+ Activator Ion The emission color from Pr3+ mainly depends on the host crystal system and gives emissions; red, blue, green and ultra violet. The emission color is mainly determined by the excited state of Pr3+ . The red luminescence of Pr3+ is predominantly observed in perovskite systems due to 1 D2 –3 H4 transition. The intensity of the Pr3+ red luminescence in these perovskite systems increases in the order of cubic SrTiO3 , tetragonal BaTiO3 and orthorhombic CaTiO3 . The increasing f-f transitions is caused by the point symmetry lowering that is governed by either by crystallographic site symmetry or the solid solution effect at the alkaline earth site. In the case of some rare earth fluorites the lowest crystal field component of the 4f 5d state of Pr3+ is located above the 1 S0 level. Exciting the Pr3+ ions from the 3 H4 ground state into the 4f 5d level decays non radiatively to the 1 S0 level. From here the Pr3+ ions return to ground state by two photon luminescence that means the photoluminescence shows a group of transitions in the blue and another group in the green and in the red. Here the red luminescence is ascribed to the transition from 3 P0 level to 3 H4 . If the excited state of 4f 5d is below the 1 S0 level, the two photon luminescence process is no longer observed. The luminescence from the 4f 5d state has been observed in many host lattices like YPO4 , LiYF4 etc. If the excited states of 4f 5d are located at still lower energy, no 5d–4f emission is observable. In this case the emission from the 3 P0 level takes place. This shows the emission characteristics of Pr3+ is entirely dependent on the excited state levels of the 4f 5d [37].

3.4 Sm3+ Activator Ion Among the trivalent rare earth ions Sm3+ ion shows interesting luminescence properties with relatively high quantum efficiency. The reddish orange emission of Sm3+ ion is caused by the transitions from 4 G5/2 to 6 HJ where J = 5/2, 7/9, 9/2 and 11/ 2. Sm3+ ion has a sharp excitation with strong excitation at 400 nm and exhibits narrow orange red emissions due to the characteristics intra configurational 4f –4f transitions of Sm3+ ions in the host. Sm3+ doped materials have been extensively investigated for their red luminescence in various fluorescent devices, color displays and temperature sensors [38].

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3.5 Mn4+ Activator Ion Mn4+ -activated phosphors are an important family of rare earth free red phosphors and a possible substitute to the commercial Eu2+ activated nitride/oxynitride phosphors which can give white light with better CRI and CCT for the applications in the solid state lighting. Manganese (Mn) is the 25th element in the periodic table whose 3d electron shell is unfilled. It has two stable oxidation states, namely, +2 and +4. Mn4+ ions have 3 electrons in the unfilled 3d shell, hence all the energy levels in a crystal field (if the spin–orbit coupling is considered) remains at least as doubly degenerate. In the crystal field, 2S + 1L terms of the Mn4+ ions split into a number of sublevels, whose number depends on the symmetry of the nearest environment. It is to be noted that Mn4+ ions enter the octahedral sites in crystals, and their six nearest neighbouring ions form an octahedron, often deviating from the ideal shape. In the case of Mn4+ ions, the ground state is always the spin quartet orbital singlet 4 A2 arising from the 4F term. For small crystal field, the first excited state is the spin-quartet orbital triplet 4 T2 , whereas the spin-doublet orbital 2 E becomes the first excited state for strong crystal field case. The point where the 4 T2 and 2 E levels cross each other distinguishes between the weak and strong crystal cases. Mn4+ activated phosphors exhibits a series of relatively sharp emission lines or several broad emission bands in the red to deep red spectral region (at ∼600 − 750 nm) [39].

3.6 Cr3+ Activator Ion Generally, the Cr3+ ions in the octahedral coordination exhibits sharp emission lines from the 2 Eg states. However, broad band NIR emission from the 4 T2g is also observed either alone or in combination with 2 Eg lines in weak crystal field environments provided by ligands. The lowest excited state is 2 E and the excitation is intense from the charge transfer transition from O2− to Cr3+ . The Cr3+ ions show mostly persistent luminescence in the long wavelength regions 700 nm wavelength due to 2 E to 4 A2 transition under ultra violet excitation. The co doping of Sm3+ ions with Cr3+ enhances significantly the persistent luminescence intensity by more than thirty-five times. The typical example is Cr3+ –Sm3+ co doped LaAlO3 , that shows very good luminescence at 694 nm wavelength and this is considered as a potential phosphor for in-vivo imaging applications [40].

4 Common Host Lattices for Red Phosphors Host lattices play an important role to influence the electronic transitions of the activator ions. To give an example, the crystal field produced by the host lattice may relax the parity selection rule and change the f –f transition probability. The host

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lattice should facilitate to accommodate the activator ions into the system without altering its crystalline structure. The host lattice should also have low lattice phonon energies to minimize the non-radiative losses and improve the radiative emission. Generally, the halide based materials such as chlorides, fluorides, bromides, iodides exhibit very low phonon energies and high chemical stability. Some common host lattices with different structural families are described here to gain knowledge for their red luminescence characteristics with different activator ions.

4.1 Orthosilicates Orthosilicates are also known as neosilicates and these materials mainly constitute the SiO4 tetrahedra that are not linked to each other through the bridging oxygens. The garnet based silicates: Ca3 Al2 (SiO4 )3 is also included in the orthosilicates. Among the silicates, some of the largely studied compositions M2 SiO4 , M3 SiO5 , M2 MgSiO7 and M3 MgSiO8 where M = Ca, Sr or Ba [34, 35]. Eu2+ doped silicates are extensively studied for red emitting phosphors. Among the silicates, Eu2+ doped M2 SiO4 are widely investigated. These silicates crystallize into different structures depending on the metal cation (M). In the barium system, orthorhombic structure with a space group Pmcn exists whereas strontium compound forms β type monoclinic with a space group P21 /n and above 85 °C, it polymorphs similar to the barium compound. The calcium silicate at room temperature crystallizes into γ type orthorhombic olivine structure with a space group Pbnm and also it possesses metastable β phase isostructural to β-Sr2 SiO4 . Eu2+ activated M2 SiO4 silicates exhibit broad red emission in the spectral region 500–600 nm wavelength under UV excitation radiation from 300 to 400 nm wavelength. In the case of barium and calcium compound, the emission wavelength is shifted to the blue side and giving emission around 500 nm. In the case of strontium, the emission shifts towards the red side around 575 nm. The solid solutions of these metal silicates offers tunable emission colors. These silicates also possess high quantum efficiency at room temperature. The Eu3+ doped silicates exhibit intense red emission around 600–620 nm under the near UV excitation. In all the silicates, the forced electric diploe transitions are dominantly observed due to non-inversion symmetry existing in the Eu3+ site. The studies of Sm3+ and Pr3+ doped M2 SiO4 silicate phosphors are limited, but they also exhibit red emission around 600–620 nm under the excitations of near UV and blue regions. Incorporating another metal ion into these silicates gives formula M3 SiO5 and the symmetry of these materials changes depending on the ionic size of the metal ion and it increases in the symmetry with the ionic size. They crystallize from a low symmetry structure like monoclinic (Cm) to high symmetry structures tetragonal (P4/ncc) to tetragonal (I4/ mcm) from calcium, strontium and barium respectively. Blasse et al. investigated the luminescence characteristics of these silicates with Eu2+ . They exhibit a broad emission from 500 to 600 nm wavelength under charge transfer excitation and these silicate systems offer color tunability to higher wavelength by substituting the metal

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ions from smaller to bigger (Ca to Ba). Another magnesium incorporated silicate system M2 MgSi2 O7 is also investigated for their red luminescence with Eu3+ activator. This particular system is different from the other two silicates M2 SiO4 and M3 SiO5 structures because the Si tetrahedra are connected in layers by Mg tetrahedra. Irrespective of the metal cation, all the silicates crystallize into tetragonal structure with a space group P421m. On activation of Eu3+ into these systems they exhibit strong red luminescence due to forced electric dipole transition (5 D0 –7 F2 ) under near UV excitations. Apart from the above silicates, many other silicates are studied as a host lattice with different activators such as Eu3+ , Eu2+ , Sm3+ and Pr3+ for their red luminescence. The crystalline structure, excitation and emission wavelengths of these phosphors are listed in Table 2. Table 2 Some of the common silicate based red phosphors with different activators Host

Activator

Structure

Excitation wavelength (nm)

Emission wavelength (nm)

References No.

M2 SiO4 M = Ba, Sr, Ca

Eu3+

Orthorhombic

395

613

[34]

Ca2 SiO4

Eu3+

Monoclinic

400

612

[35]

Na2 CaSiO4

Eu3+

Cubic

393

613

[41]

Ba2 SiO4

Eu3+

Orthorhombic

393

614

[42]

K4 CaSi3 O9

Eu3+

Cubic

396

616

[43]

Gd9.33 (SiO4 )6 O2

Eu3+

Hexagonal apatite

532

614

[44]

Ca2 MgSi2 O7

Eu3+

Tetragonal

393

613

[45]

K3 YSi2 O7

Eu2+

Hexagonal

394

620

[36]

Ca3 ZrSi2 O9

Eu2+

Monoclinic

365

635

[46]

CaSrSiO4

Eu2+

Orthorhombic

370

580

[47]

Ca2 SiO4

Sm3+

Monoclinic

405

601

[38]

M2 SiO4 M = Ba, Sr, Ca

Sm3+

Orthorhombic

405

599

[34]

Ca2 MgSi2 O7

Sm3+

Tetragonal

401

600

[45]

Bi4 Si3 O12

Sm3+

Cubic eulytite

405

609

[48]

Ca2 SiO4

Pr3+

Tridymite

447

606

[37]

Li2 SrSiO4

Pr3+

Hexagonal

452

610

[49]

Ba2 La8 (SiO4 )6 O2

Pr3+

Apatite

475

611

[50]

Sr2 Al2 SiO7

Pr3+

Tetragonal

443

602

[51]

MgAl2 Si2 O8

Mn4+

Triclinic

258

662

[52]

Mg2 Al4 Si5 O18

Mn4+

Orthorhombic

323

680

[39]

CaY2 M2 Al2 SiO12 (M = Al, Ga, Sc)

Mn4+

Garnet

481

674

[53]

BaSiF6

Mn4+

Hexagonal

467

636

[54]

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Many Mn4+ activated silicates have been investigated extensively for their deep red luminescence. These red phosphors are of current interest in the area of Plant emitting lighting systems for enhanced plant yield. The Mn4+ activated phosphors show broad emission in the red region 620–680 nm under the near UV and blue excitations. They also find applications in the phosphor converted white LEDs for better CRI and low CCT values.

4.2 Nitrides and Oxynitrides Nitrides and oxynitrides have been largely studied due to their interesting properties of high covalency of the crystal lattices and strong crystal field. Among the activators, Eu2+ and Ce3+ doped oxynitrides exhibit longer emission wavelengths than the silicon based oxides due to the lower electron negativity of the nitrogen over oxygen [55, 66]. The lower electronegativity nitrogen induces strong nephelauxetic (covalency) effect and the higher negative charge of N3− also influences larger crystal field will give splitting of the d orbital of Eu2+ /Ce3+ . However, the emission of Eu2+ doped oxynitrides mostly shows blue to yellow wavelength region as observed in Ba3 Si6 O12 N2 :Eu2+ , SrSiAl2 O3 N2 :Eu2+ etc. Whereas, the Eu2+ doped nitrides show yellow to deep red emissions as observed MSiN2 :Eu2+ (M = Ca, Sr, Ba), SrAlSi4 N7 :Eu2+ , CaAlSiN3 :Eu2+ etc. Also Eu2+ doped M2 Si5 N8 shows broad emission in the wavelength region 570–680 nm and a broad excitation spectrum from 300 to 500 nm. This broad excitation of these nitrides makes them suitable to be excited with either UV or blue LED chip. In addition, these nitrides based red phosphors exhibit high thermal stability with good quantum yield. Further, the incorporation of Si–N in place of Al–O in other oxide compounds like YAG:Ce and BaAl2 O4 :Eu2+ shifts the emission towards the longer wavelength. Though these nitrides based phosphors exhibit good luminescence characteristics, but their synthesis is very cumbersome due to high temperature calcinations around 1700–2000 °C. Apart from the Eu2+ activator red phosphors, some reports are seen for red luminescence with these nitrides doped with other activators such as Eu3+ , Yb3+ , Sm3+ , Mn2+ etc. (Table 3). These phosphors also show broad red emission in the wavelength region 600–680 nm under broad excitations 270–500 nm except Eu3+ based phosphors.

4.3 Sulfides Like N, the electronegativity of S is also smaller than that of O and also S polarizes easily in the host. The nephelauxetic effect is largely observed due to the smaller electron negativity of S and consequently introduces large centroid shift of Ce3+ or Eu2+ in the host lattices [73, 78]. The Ce3+ and Eu2+ doped metal sulfides exhibit strong emissions from green to red region under the excitation of blue light. These phosphors are highly suitable for excitations from the blue LED chip in pc-White

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Table 3 Some of the common nitride based red phosphors with different activators Host

Activator Structure

Excitation Emission References wavelength wavelength No. (nm) (nm)

M3 B2 N4 (M = Ca, Sr)

Eu3+

Cubic

370

613

[55]

M2 Si5 N8 (M = Ca, Sr, Ba)

Eu2+

Ca: monoclinic Sr, Ba: orthorhombic

395

600–680

[56]

CaAlSiN3

Eu2+

Orthorhombic

393

613

[57]

Sr2 Si5 N8

Eu2+

Orthorhombic

460

642

[58]

Sr0.8 Ca0.2 AlSiN3

Eu2+

Orthorhombic

393

618

[59]

Sr3 B2 O6−3/2x Nx

Eu2+

Cubic

447

620

[60]

Sr2 CaSi(O1−x Nx )4

Eu2+

Orthorhombic Monoclinic

466

605–630

[61]

CaAl2 Si4 N8

Eu2+

Trigonal

410

590

[62]

Sr2 BeAl3 N5

Eu2+

Triclinic

440

622

[63]

Ca1−x Srx CN2

Mn2+

Rhombohedral 270

680

[64]

AlN

Mn2+

Hexagonal wurtzite

640

[65]

CaSiN2

Ce3+

Orthorhombic

450

610

[66]

La3 (Si, Al)6 (O, N)11

Ce3+

Tetragonal

530

640

[67]

SrLiAl3 N4

Ce3+

Triclinic

470

615

[68]

Li38.7 RE3.3 Ca5.7 [Li2 Si30 N59 ]O2 F

Ce3+

Trigonal

440

638–651

[69]

CaAlSiN3

Yb2+

Orthorhombic

410

629

[70]

Li2 CaSi2 N4

Sm3+

Cubic

UV

Red

[71]

SiAlON

Pr3+

Hexagonal

460

624

[72]

254

LEDs. The co-doping of Ce3+ with Eu2+ doped metal sulfides improves the emission due to the energy transfer from Ce3+ to Eu2+ . Further, the introduction of Eu2+ in solid solutions of CaS and SrS increases the emissions linearly with the bond distance of MS. apart from the pure metal sulfides, some Eu2+ –Ce3+ doped thiosilicates: Ba2 SiS4 , BaSi2 S5 and Ca2 SiS4 show emissions in the deep blue to red wavelength region that makes them suitable for applications in pc-WLEDs (Table 4). For example, the combination of Ca2 SiS4 :Eu2+ phosphor with blue LED chip produces white light with low CCT and CRI ~ 67. These red phosphors are commercially employed in the fabrication of white LEDs Eg : Luxeon III star LED. The main drawback of these sulfide phosphors is lack of thermal and chemical stability. These phosphors are sensitive to moisture and also disintegrate under high temperatures and degrade the performance of the LED.

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Table 4 Some of the common sulfide based red phosphors with different activators Host

Activator

BaLa2 Si2 S8

Eu2+

Trigonal

405

645

[73]

CaS

Eu2+

Trigonal

539

642

[74]

SrS

Eu2+

Cubic

460

620

[75]

BaY2 S4

Eu2+ , Er3+

Orthorhombic

455

663

[76]

Y2 O2 S

Eu3+

260

612

[77]

(Y, Gd)FS

Ce3+

Tetragonal

450

660

[78]

Y2 (Ca, Sr)F4 S2

Ce3+

Tetragonal

470

590

[79]

Structure

Excitation wavelength (nm)

Emission wavelength (nm)

References No.

4.4 Other Oxides Numerous number of oxide based red phosphors are developed in many common structural families such as perovskites, pyrochlores, molybdates, tungstates and fluorites with different activators: Eu3+ , Eu2+ , Pr3+ , Sm3+ , Mn4+ etc. The oxide based phosphors are thermally and chemically stable compared to the sulfides. Also, they can be easily synthesized by different wet chemical and solid state routes. They exhibit proper red emissions in the wavelength region 590–630 nm with the most of the activators. Among the activators in these oxide based phosphors, Eu3+ activated systems are widely studied because of their specific emissions in the orange red region due to magnetic dipole 5 D0 –7 F1 and electric dipole 5 D0 –7 F2 transitions respectively. It also provides information related to the structural symmetry of the cations and hence considered as a structural probe. Some of the widely studied systems are briefly described below and their red luminescence data is presented in Table 5. (a) Perovskites based red phosphors Perovskites having the formula ABO3 form an important class of materials in many technological applications including phosphors. Perovskites, exist naturally as CaTiO3 and also can be synthesized many derived oxides due to their tolerance for many cation substitutions. Because of their robust crystalline structure, chemical and thermal stability, they form good candidates as a host for many red activators. Also they offer to accommodate many activators for their incorporation without altering the crystalline structure. In this regard, Eu3+ doped simple perovskites of the formula ABO3 and their derived systems like double perovskites: A2 BB' O6 , AA' BB' O6 , etc. and triple perovskites: A3 B' B2 '' O9 , etc. show orange red luminescence depending on the site symmetry of the Eu3+ ion. The Eu3+ absorptions around 394 and 464 nm are prominently strong matching with the near UV and blue LED chips making them potential for LED applications. The Pr3+ doped simple perovskites ATiO3 (A = Ca, Sr and Ba) shows red luminescence that is achieved through the

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Table 5 Some of the common oxide based red phosphors with different activators Host

Activator

Phase structure Excitation wavelength (nm)

Emission wavelength (nm)

References No.

Ca2 LaMO6

Eu3+

Perovskite monoclinic

465

615

[80]

Ba2 YZrO6

Eu3+

Perovskite cubic

464

612

[81]

Ca2 Y3 Nb3 O14

Eu3+

Pyrochlore cubic

393

613

[82]

CaGdSnNbO7

Eu3+

Pyrochlore cubic

395

612

[83]

CaGdNbMoO8

Eu3+

Powellite tetragonal

395

614

[84]

Y2 Ce2 O7

Eu3+

Fluorite cubic

467

613

[85]

Sr2 ScAlO5

Eu2+

Perovskite cubic

427

615

[86]

Ca2 LaNbO6

Sm3+

Perovskite monoclinic

407

651

[87]

CaY0.5 Ta0.5 O3

Sm3+

Perovskite monoclinic

407

603

[88]

RE2 Ti2 O7 (RE = Gd, La)

Sm3+

Pyrochlore cubic

292

608

[89]

Gd2 Zr2 O7

Sm3+

Fluorite cubic

230

609

[90]

BaMoO4

Sm3+

Scheelite tetragonal

404

646

[91]

NaNbO3

Pr3+

Perovskite orthorhombic

310

612

[92]

La2 Hf2 O7

Pr3+

Pyrochlore cubic

255

607

[93]

CeO2

Pr3+

Fluorite cubic

463

608

[94]

LiY(MoO4 )2

Pr3+

Scheelite tetragonal

456

657

[95]

Sr2 LuTaO6

Mn4+

Perovskite monoclinic

346

686

[96]

Gd2 ZnTiO6

Mn4+

Perovskite monoclinic

365

705

[97]

RE2 Sn2 O7 RE = Y, Lu, Gd

Mn4+

Pyrochlore cubic

275

670

[98]

Sr9 Gd2 W4 O24

Mn4+

Perovskite cubic

355

694

[99]

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conduction band states to the emitting Pr3+ levels. The intensity of red emission of Pr3+ ions increases in the order of cubic SrTiO3 , tetragonal BaTiO3 and orthorhombic CaTiO3 . The probability increase of f –f transitions is mainly caused by lowering the symmetry of the cations with the substitutions. Unlike the titanates, the Pr3+ doped zirconates exhibit greenish-blue luminescence due to stable tetravalent zirconium. Pr3+ doped many perovskites exhibit persistent red luminescence for example Ca3 TiO7 :Pr3+ . The reddish orange emission of Sm3+ in the perovskites is mainly due to transitions from 5 G5/2 to 6 HJ (J = 5/2, 7/2, 9/2, 11/2). Sm3+ doped stannate perovskite such as Srn+1 Snn O3n+1 (n = 1, 2, …) is a good example to demonstrate the effects of crystal structure on the luminescence properties. The red emission intensity for Sr3 Sn2 O7 :Sm3+ is three folds better than that for the simple perovskites due to effective confinement of transfer energy in the two dimensional layer. The red luminescence of Mn4+ activated perovskites have been studied in a wide variety of oxides including germanates, silicates and aluminates. They exhibit deep red luminescence in the wavelength region 680–710 nm under near UV excitations. Mn4+ doped double perovskites Gd2 MgTiO6 shows deep red luminescence located at 681 nm under 315 nm excitation. In another example Mn4+ activated double perovskites La2 LiSbO6 and La2 MgTiO6 shows tunable red luminescence with an increase of octahedral site distortion. The germanate type double perovskite La2 MgGeO6 :Mn4+ exhibit deep red luminescence peaking at 708 nm under UV excitation. This system also shows persistent red luminescence in the wavelength range from 670 to 720 nm for 60 min. The other double perovskite Ba2 GdSbO6 phosphor shows strong red luminescence due to spin forbidden transition of Mn4+ in the region 620–750 nm. These Mn4+ based red phosphors are potential for plant growth applications. (b) Pyrochlore based red phosphors Pyrochlore based phosphors plays a significant role among the luminescent materials mainly due to their thermal and stability and allowing wide variety of cation substitutions into it. The simple pyrochlore oxides having the formula A2 B2 O7 (A = Rare earth ions, B = Tetravalent ions) are most commonly studied hosts with different activator ions for the red luminescence. The Eu3+ doped RE2 M2 O7 (RE = Y, La, Gd, Lu; M = Ti, Zr, Hf, Sn, Ce) exhibits the characteristic Eu3+ emissions both in the orange and red regions depending on the site symmetry. The extent of distortion of the coordination of the polyhedra is more related to the ionic size of the cation. By losing the D3d symmetry due to induced distortion, these phosphors show red emission through forced electric dipole transitions. The splitting of these transitions increased with the increased size of the A cation suggesting that larger cations create more highly distorted surroundings of the Eu3+ environment. The probability of emission transitions also can be tuned by substituting different ionic sizes at the A and B sites in order to drive the structure from disorder to order cations in the structure. The example is Eu3+ doped rare earth zirconate RE2 Zr2 O7 where RE = La and Y, that undergoes pyrochlore to fluorite phase. The disordered phase of Y2 Zr2 O7 :Eu3+ exhibits multiband emission whereas La2 Zr2 O7 :Eu3+ shows dominant orange emission due to allowed transition. The distortion of the D3d pyrochlore symmetry can be broken in the Eu3+ doped quaternary pyrochlore oxides like CaREMNbO7 ; RE = Y,

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La, Gd; M = Sn, Ti. Consequent to this, these phosphors show strong red luminescence under near UV and blue excitations. Apart from Eu3+ , the other activators Sm3+ and Pr3+ doped pyrochlore systems are scantly reported for their red luminescence under UV excitations. These phosphors show strong excitations at near UV and blue light matching with the emission wavelengths of the near UV and blue LED chips. Only the drawback is that the excitations and emissions are narrow, however they are good with high color purity and also improves the efficacy of the LED using these phosphors. (c) Molybdates and tungstates based red phosphors The alkaline earth and rare earth metal tungstates and molybdates offer as potential hosts for many activator ions for the red phosphors. These compounds exhibit broad charge transfer band in the wavelength region 250–350 nm due to charge transfer of O2− –M6+ where M = Mo, W. The presence of these intense charge transfer band in the near UV region helps in exciting the Eu3+ ions efficiently and give strong red luminescence due to the f –f transitions of the Eu3+ ion. Further, the charge transfer (CT) band is shifted to the blue region by co doping with Bi3+ ions in order to further strengthen the Eu3+ absorptions in the blue region. Due to the presence of broad. CT band many Eu3+ doped tungstates and molybdates have been widely studied for their red luminescence. The typical examples are CaMoO4 :Eu3+ , AREMo2 O8 :Eu3+ , Y2 MoO6 :Eu3+ , Y2 (MoO4 )3 :Eu3+ and CaLaNbMoO8 :Eu3+ for their strong red luminescence. The phosphor Gd1.2 Eu0.8 (MoO4 )3 shows a higher luminescent intensity than that of phosphor Y2 O2 S:0.05Eu3+ under the excitation of near UV and blue light. It has also been observed that the quenching concentration in Gd2 (MoO4 )3 :Eu3+ is much higher than that in CaMoO4 :Eu3+ due to the large distance between rare earth in the latter case. Also, Sm3+ and Pr3+ doped molybdates and tungstates are reported for their red luminescence. These red phosphors are considered for potential applications in the pc-converted white LEDs. The main drawback of these phosphors is low quantum efficiency and thermal quenching. (d) Fluorite based red phosphors Fluorite based oxides are also potential hosts due to high coordination and structural maneuverability. Eu3+ doped CeO2 and RE2 Ce2 O7 exhibit dominant red luminescence due to forced electric dipole transitions under near UV and blue excitations. The control of Ce3+ defect concentration and oxygen vacancy ordering greatly influence the absorbance and luminescence properties. Further the Eu3+ doping in Y2 Ce1.9 O7 enhances significantly the red luminescence properties than that of the pure CeO2 . Further the cation ordering with oxygen vacancy ordering facilitated uniform distribution of Eu3+ ions in these fluorite host lattices, thus increasing higher doping activator concentrations without causing quenching in the system and consequently increasing the radiation lifetimes. These fluorites type red phosphors are find potential for strong blue excitations along with a blue LED chip in pc-WLEDs.

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5 Challenges in Developing Red Phosphors for Display and Lighting Applications The major challenges for the development of red phosphors in the display and lighting applications are as follows i. Weak absorption of most of the activators in the near UV and blue wavelength region. ii. Large stokes shift causes reduction in efficiency of the phosphors. iii. Broad emission from the activators like Eu2+ /Ce3+ /Mn4+ reduces the efficacy of the LED light and the emissions spill over to the longer wavelength beyond the eye sensitivity. iv. Narrow emissions from the activators like Eu3+ /Sm3+ /Pr3+ improves the efficacy but doesn’t cover the whole spectral red region. v. Trade-off between efficacy and spectral distribution is to be made depending on the desired applications. vi. Though nitride based red phosphors meet the requirement for the display and lighting applications but their synthesis is a tedious process and energy intensive for commercial scale production. vii. Some of the commercially employed sulfide based red phosphors lack chemical stability and their processing is not environment friendly. viii. The emission intensities of red phosphors are relatively lower than the green and blue phosphors. As can be seen there are many challenges in developing efficient red phosphors for display and lighting applications. The discovery of novel hosts with proper red activators is inevitable for improving the color rendering index and lowering the correlated color temperature for general illumination lighting sources.

6 Conclusions The development of efficient red phosphors has become a prime focus of interest for the lighting and display applications. The red phosphors in combinations with the blue and green phosphors in pc-WLEDs can mimic the sunlight with good CRI and low CCT for indoor lighting applications. Though there are efficient green and blue phosphors but the lack of efficient red phosphor hinders the development of solid state lighting with good efficacy and full visible spectral region. In this scenario several series of red phosphors in different host lattices have been developed to meet the above requirements. Among these, the notable developed red phosphors are CaAlSiN3 :Eu2+ , Y2 O2 S:Eu3+ and (Sr, Ba)3 SiO5 :Eu2+ which have shown commercial potentiality for display and lighting applications. However, they have their own advantages such as both broad and narrow red emissions, good thermal and chemical stability (except sulfides), high quantum yield and efficiency, high

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temperature quenching and disadvantages such as weak absorptions mainly for the trivalent rare earth activators, lack of thermal and chemical stability and hazardous processing conditions for the sulfide host based phosphors, tedious and energy intensive processing for the nitrides and lack of good crystallinity for the silicate based hosts. Thus there are great challenges in discovering novel red phosphor materials that can surpass the above bottlenecks for the display and lighting applications. The future development of red phosphors will emphasize on the following aspects (i) The discovery of novel red phosphors with excellent luminescence properties under efficient excitations of both near UV and blue light. (ii) Optimization of the existing red phosphors for their improvements in terms of quantum efficiency, thermal quenching, easy synthesis routes, environment friendly materials and reducing the cost factor. (iii) A deep theoretical and experimental understanding of the luminescence properties of the compounds in relation to the crystal structure and chemical composition.

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Spectroscopic Studies of Rare-Earth-Doped Glasses for LED Applications Asmahani Awang, S. K. Ghoshal, and Alireza Samavati

1 Introduction Glass is a material of high importance in both ancient times and the present day. Dating back to the Bronze and Iron ages, the earliest methods of introducing the lattice defect have involved the use of the forging process to alter the atomic structures of naturally created crystalline materials [1, 2]. Since the early 1900s, salt mixtures, simple salts, and combinations of ionic minerals have been the most renowned materials for exploring the mechanisms of glass creation [3]. These days, common solidstate materials investigated widely for distinct applications are in a non-crystalline state due to immense industrial and technological growth [1]. The advantages of glass, such as its ease of formation, in addition to its capacity to be designed in any preferred shapes such as fibers, glass wools, sheets, thin films, and rods, contribute to the emergence of glass as a prevalent material in diverse applications. Glass is formed by the fusion process of an inorganic product cooled to a rigid condition to prevent crystallization [4]. Glass, with its highly transparent dielectric features, beautiful color, and the ability to accommodate the impurities required to generate the desired structural, optical, mechanical, conducting, and magnetic properties, is a favorable material that can be practically adapted to various applications [2]. A. Awang (B) Industrial Physics Programme, Faculty of Science and Natural Resources, Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia e-mail: [email protected] S. K. Ghoshal Advanced Optical Materials Research Group, Physics Department and Laser Centre, Faculty of Science, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia A. Samavati Physics Department, Faculty of Science, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_9

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Glass is known to have a highly attractive group of resources amid the disordered systems of nature. Volcanic obsidian glass, which is a type of natural glass, has been utilized by people since the most primitive ages. Archaeological discoveries such as knives, carving arrowheads, and other objects required for diurnal endurance support this idea. Dating back to about 7000 BC, the first glass objects produced by man were found in Egypt and Mesopotamia. Egyptians around 1500 BC were probably the first people to recognize the benefits of heating glass; the first guide to glass fabrication appeared millennia later [5]. Practically, glass was revealed by coincidence; the vibrant molten color is achieved by using natron slabs containing a blend of sodium bicarbonate and sodium carbonate. During the process, the Egyptians kept the heating pots liquefied and added additional ingredients such as sand that could be obtained from the coast. Indeed, natron was applied as a household material for body application and home usage. The combination of natron and oil was used as an initial method to produce soap. Glass fabricators most likely applied an ingredient with the same composition as the one discovered in the cuneiform library of the Assyrian king Assurbanipal, dating back around 650 BC. According to this record, the formation of glass required a specific composition containing sand in a range of 60 portions, ash from sea plants in a range of 180 portions, and lime in a range of 5 portions. Apart from something lacking, such as a furnace working at a temperature of 1000–1200 °C, most recent flat windows adhere to this initial raw ingredient arrangement consisting of sand, which is silicon oxide, calcium, potassium carbonates, and sodium [5]. According to the literature, the term ‘glass’ originates from the Teutonic term ‘glaza’, which implies amber. Though the origins of glass are unknown, the Mesopotamians in the fifth century BC discovered ash by accident while heating clay pots to glaze ceramics or dissolving copper. In Egypt, the excavation of Pharaohs’ funeral places dating from the early fourth century BC discovered greenish glass beads, which are described as premeditation in glass fabrication. From the second century BC, the core-wound technique began to emerge to produce rings and small figures. Around 200 BC, Syrian craftsmen discovered the Syrian blowing iron, which led to the manufacturing of thin-walled hollow pots in a broad range of forms. Glass was used as a part of the structure cover in public baths in Herculaneum and Pompeii during the early Roman era. This fact was disclosed during the excavation process. These panes were installed using a wood or bronze frame, or even without any frame. This method spread to the Alpine in the northern region during the Middle Ages, resulting in the early production of utensils such as claw beakers, drinking horns, and mastos vessels. Furthermore, glass was applied to the construction of buildings such as churches and monasteries [4]. Glass is used as a promising host for encapsulating both metal nanocrystals and rare-earth ions due to its remarkable features such as the simplicity of production in different sizes and shapes, good mechanical strength, and the non-appearance of metal–ligand interaction [6]. Moreover, glass exhibits special features such as high hardness, favorable transparency at room temperature, adequate strength, and high corrosion resistance. Glass is also associated with additional benefits such as low cost, large-volume production possibility, chemical durability, thermal stability,

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physical isotropy, non-existence of grain boundaries, and tunable composition to fit practical use. The exploration of the properties of glass is a prerequisite to its usage in the fields of engineering and technology [7]. The glass type affects its physical characteristics. The dynamic viscosity of glass possesses a value of 1020 dPas at ambient temperature. In comparison, the viscosity values of water and honey are 1 dPas and 105 dPas, respectively. It is highly difficult to observe the flow effect by using the naked eye due to the high viscosity of glass at room temperature [4]. Glass can be produced in different forms with varying chemical compositions. It can be used in the construction industry, for example, for the manufacturing of windows and/or doors for buildings. Further, glass can be utilized in kitchen utensils and vehicles. Glass properties, particularly its refractive index, are affected by the composition and type of treatments used on it. Different elements such as Si, Mg, Ba, and Na can be utilized to develop typical glass [8]. The fabrication of glass is feasible, and the composition of glass can be designed according to specific applications. Glasses that contain rare-earth ions have broad-ranging functions due to their promising applications as luminescent devices and lasers. In comparison with halide crystals, the vibrational frequencies of glass are greater. This causes the glass to produce a lower quantum yield for some of the fluorescing levels due to a higher non-radiative relaxation rate. Amplification in the electric field intensity close to the rare-earth ions or favorable energy transfer to the luminescent centers stimulates the rare-earth ion luminescence [9].

2 Types of Host Glass The selection of an appropriate host matrix is vital to attaining optimum optical properties [10]. Silicate, borate, and phosphate, among the oxide types of glass, possess high phonon energies. This characteristic causes multiphonon relaxation to become a prevalent method for transitioning small energy gaps in these types of glass [11]. Further, soda-lime glass has displayed novel properties in terms of low phonon energy, higher values of refractive index, and high third-order non-linear powerlessness in the close vicinity of other altered lattices that were developed to improve the conductivity of electricity and the relaxation quality [12]. Borate glass has attracted researchers’ attention because of its favorable features, such as affordability, a small melting point, and feasible glass formation [2]. In general, germanium-based types of glass show higher phonon energy, elevated thermal stability, and good mechanical strength in comparison with tellurium-based ones [13]. Tellurite (glass with TeO2 elements), chalcogenide (glass with S, Se, and Te elements), and fluoride (glass with ZrF4 or AlF3 elements) are non-silica types of glass, which display superior optical transparency in the wavelength region of 0.4– 7 µm, 1–16 µm, and 0.3–8 µm, respectively. These features facilitate the use of these types of glass as fiber materials for mid-infrared non-linear optical usages [14]. Tellurite glass is nominated as a laser host owing to its outstanding features [11]. TeO2 -based glass possesses the following characteristics: the smaller value of

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maximum phonon energies in contrast to the available oxide glasses, good transmission in the range of visible to near IR up to 4.5 mm, higher value of the refractive index, optimal mechanical strength, acceptable chemical durability, small melting temperatures, small working temperatures, non-hygroscopic, slow crystallization rate, possibility to be incorporated with a high concentration of rare-earth ions, high dielectric constant, large third-order non-linear susceptibility, wide transparency ranging from ultraviolet to middle infrared region, and good infrared transmissivity [15].

2.1 Tellurite as Host Glass In the outermost shell, the arrangement of the valence electrons for all the species in lanthanide atoms is similar, while the 4f orbitals are gradually occupied with rising atomic numbers. The similar chemical and physical properties of the elements were developed from the screening of the 4f orbitals [16]. Extensive experiments and indepth studies have been performed by various researchers to find glass hosts with small phonon energy and outstanding stability to be doped with lanthanide ions. Glass with low phonon energy is important for hosting lasing ions because it can minimize non-radiative losses [17]. The incorporation of lanthanide ions decreases the multiphonon de-excitation between the Ln ion energy levels and facilitates the shifts between levels, which are positioned in close space to improve the luminescent transitions and quantum efficiency [18]. There is a need to fabricate a new type of functioning material with an advanced glass-based design in which tellurite-based glass fulfills these criteria by possessing unique properties [19]. Various tellurite-based glasses with distinct main elements have been chosen as hosts to achieve rare-earth luminescence with optimum luminescence quantum efficiencies due to its remarkable features of small cut-off phonon energy in the range of ~750 cm−1 among oxide materials and also it shows outstanding optical properties, good thermal stability, as well as large homogeneity range [20– 22]. Some tellurite glass can be incorporated with 25% of rare-earth oxides [23]. Therefore, tellurite glass emerges as an ideal host for rare-earth ions and can sustain the incorporation of a large amount of rare-earth ions without clustering [24]. Therefore, tellurite glass has emerged as a suitable contender for the expansion of various types of active photonic instruments due to its potential to accommodate a great number of rare-earth ions [25]. Pure TeO2 comprises TeO4 trigonal bipyramids (tbps). In this structure, a lone pair of electrons dominates one equatorial site of the sp3 d hybrid orbitals. Meanwhile, oxygen atoms dominate the other two equatorial and axial sites [26]. Four oxygen atoms surround the trigonal bipyramidal tellurium and form a bridging network [27]. The incorporation of alkali or alkaline-earth modifiers into a pure TeO2 system alters the coordination polyhedron of Te from TeO4 tbps to TeO3 trigonal pyramid (tp). In this structure, a lone pair of electrons occupy one of the Te sp3 hybrid orbitals [26]. Adding transition metal oxides to tellurite glass changes the structure of glass by forming new ionic bonds and generating non-bridging oxygens. This process

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changes the electrical conductivity, optical properties, and thermal stability of glasses induced by the strong polarizability of the tellurium lone pair electrons [28, 29]. The elongation of one oxygen-tellurium linkage in TeO4 and the generation of TeO3+1 and TeO3 structural units are attained with the incorporation of metal ions [29]. The formation of glass for the tellurite glass system requires at least 50 mol% TeO2 . A variation on the structural motives of network-forming TeOx species is assisted by the wide range of glass-forming configurations. The TeOx species consists of bipyramidal TeO4 groups at high TeO2 components and isolated pyramidal TeO3 groups at small TeO2 concentrations. The variation of elements necessitates the selection of structural sites for the addition of dopant elements such as erbium ions. Therefore, ion agglomeration, which causes attenuation in fluorescent emission and inefficient amplification, could be avoided. In addition, broadened emission spectrum can be obtained by varying the dopant sites [30]. Glass exhibits an amorphous nature and is not limited to specific stoichiometries compared to crystalline materials; therefore, it has a broader range of potential compositions [31]. The production of new materials with distinctive properties by mixing two or more different components has been an approach adopted for many years [32]. Multicomponent tellurites (TeO2 -based glasses) with the addition of one or more chemical modifiers have been introduced because TeO2 is unable to form glass structures easily under normal conditions [33]. The improvement of the glassforming ability and optical properties for diverse applications could be achieved by using alkali oxides, alkaline-earth oxides, or transition metal oxides [33, 34]. Rareearth and heavy metal elements appear as good glass dopants due to their tendency to form glass easily [35]. Modifiers that fill empty spaces in the crystal lattice space are flexible and do not have a specific arrangement in the case of multicomponent glasses. Alkali metal, alkaline-earth metal oxides, and several transition metal oxides are types of modifiers in oxide glasses [36]. Improvement in the glass former in the form of a homogeneous mixture of dopant ions could be attained by incorporating additional elements, i.e., a network modifier. The incorporation of several network modifiers such as aluminum oxide (Al2 O3 ), sodium oxide (Na2 O), zinc oxide (ZnO), lead oxide (PbO), lithium oxide (Li2 O), and bismuth oxide (Bi2 O3 ) into the glass former might alter the fundamental network of glass. The incorporation of Li2 O into glass leads to the creation of ionic bonds with oxygen (non-bridging oxygen), which strengthens the structural bonding, improves the moisture resistance, reduces the generation of bubbles, and enhances thermal and ionic conductivity in tellurite-based glass [19]. Meanwhile, Na2 O facilitates homogenization, reduces the melting point, enhances rare-earth solubility, and enables the incorporation of a high amount of dopants [37]. In the meantime, the reduction in the melting point of glass and enhancement in the glass-forming ability can be achieved by incorporating zinc oxide in glass fabrication. Furthermore, zinc oxide can be used to lessen the optical energy gap and rise the refractive index [10]. Therefore, to achieve optimum properties of glass, it is crucial to use network modifiers with a proper composition [38].

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3 Glass Containing Rare-Earth Ions Rare-earth and heavy metal elements are preferred as glass dopants due to their tendency to form glass easily [35]. The doping process of rare-earth in glass as network formers, modifiers, or luminescent ions helps explore further their optical, electrical, and magnetic properties. Substantial luminescence quantum efficiency in the longer wavelength transitions can be attained for host glass containing rare-earth ions [39]. Trivalent rare-earth ions produce prominently noticeable emissions aroused from the transitions of 4f → 4f, which are susceptible to the neighboring crystalline field of ions caused by the screening effect of outer 5s and 5p shell electrons. These transitions have been explored in-depth for diverse utilization particularly scintillators, phosphors, optical amplifiers, lasers, and up-conversion and down-conversion devices [40]. By the excitation of high-energy radiation such as ultraviolet or X-ray, glasses containing rare-earth ions can produce prominent luminescence in the spectral regime extended from violet to near-infrared. In glasses containing rare-earth ions, quantum cutting is associated with the absorption of photons of elevated energy and transformed into two or more photons with smaller energies. Rare-earth ions appear as potential candidates for photon down-conversion and photon up-conversion due to their distinctive ladder-like energy states. In addition, the availability of many energy levels with extended lifetimes and optimum thermal stability has nominated the rareearth ions-doped glass as an important material for exploring photon up-conversion. The available energy level plays a vital role as an intermediate state for consecutive transitions in the multiphoton-induced absorption (MPA), which stimulates the up-conversion. These intermediate states are essentially inhabited at substantial excitation intensities, which, in turn, causes the phenomenon of supersaturation. These features facilitate the up-conversion process due to the reduction of the number of simultaneously absorbed photons and the excitation intensity [41]. The International Union of Pure and Applied Chemistry (IUPAC) recognized rare-earth as a class of 17 elements that exhibit comparable physicochemical characteristics. Fifteen elements are listed in the group of lanthanides with atomic numbers ranging between Z = 57 and Z = 71: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu); the list is added with two other elements: scandium (Sc, Z = 21) and yttrium (Y, Z = 39). There are two different groups of rare-earth elements: light rare-earth elements and heavy rare-earth elements. The former comprises La, Ce, Pr, Nd, Pm, and Sm, which are abundant in nature and possess lower atomic masses, greater solubility, and acceptable alkalinity. On the other hand, the latter comprises Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, which exhibit greater atomic masses, lower solubility, and lesser alkalinity [42]. Complexities in distinguishing one rare-earth element from another because of their close resemblance in chemical and physical properties make them known as ‘rare’. Meanwhile, ‘earth’ manifests an old chemical name for oxides because

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they were first discovered as rare-earth oxides. Rare-earth elements are frequently specified as the ‘treasury’ of new-found materials vital in accelerating innovation in the manufacturing industry, which support traditional and high-technology products [43]. Rare-earth elements have been developed as particularly essential materials in the world of technology due to their distinctive optical, magnetic, electronic, phosphorescent, anticorrosive, and catalytic properties [16, 44]. Based on their electronic configuration, the rare-earth elements possess comparable chemical and physical features due to the particularly stable oxidation state of ‘+3’, and a minor but continuous decrease in the ionic radius, with a rising value in the atomic number, known as lanthanide contraction. The optically active rare-earth ions are generated frequently in the form of trivalent due to the withdrawal of one 4f electron and both 6s electrons. The intra-4f or 5d–4f transitions contribute to the occurrence of luminescence from these ions. The 4f states are isolated from outside interaction by 5d states; therefore, the intra-4f transitions are comparatively impartial to the host material. It should be noted that when rare-earth ions are integrated into a host material, the intra-4f transitions for free ions are parity forbidden; however, they are moderately permitted via the combination of opposite parity wave functions. This characteristic leads to low oscillator strengths and extended luminescence lifetimes. In contrast, 5d states are directly uncovered to the local environment, which makes the 5d–4f transitions very sensitive to the neighboring ligands. The modification of the matrix structure alters the peak positions, spectral shapes, and intensities [45]. It has been acknowledged that the energy level structure and neighboring ligand environment produced by the host matrix spectral characteristics of rare-earth ions affect the spectra characteristics of these ions [46]. In this section, the physical appearance of glass will be explained further by using our unpublished data for tellurite glass containing zinc oxide, sodium oxide, and erbium content. According to Baynton [47], the fabrication of tellurite glass using commercial tellurium dioxide usually yields glass with a yellow-green color. Glass coloring can be achieved by using various processes such as varying the chemical composition, colorant, varying the number of non-bridging oxygens, oxidation state, and oxygen/sulphur coordination [48]. However, the option of using trivalent rareearth elements in the glass matrix has become popular due to the nature of color centers inside the glass network [49]. In a heterogeneous solid, the propagation of light involves the absorption and scattering process. The perceived color is due to the light which is not absorbed and is diffusely reflected during the process. The selective absorption of light and adequate backscattering of light determine the visual appearance of a material [50]. The physical appearance of tellurite glass without and with erbium content is shown in Fig. 1a, b, respectively. As can be seen, the glass without erbium content appears pale yellow in color, whereas the one with erbium content appears orangish. The formation of rare-earth sites facilitates the absorption of different wavelengths in the visible spectrum in the host glass. This causes noticeable alterations in glass color, which convert the glass color from pale yellow to orange [51]. The complete shielding of 4f electrons by the outer shells of particular host media such as insulators with wide bandgap has a restricted effect in rare-earth [40]. Tellurite

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Fig. 1 Physical appearance of tellurite glass a without the incorporation of erbium content and b with the incorporation of erbium content

glasses containing erbium content exhibit chemical and optical properties, which support the optical applications due to their beneficial features of elevated linear and non-linear indices of refraction, smaller phonon energy spectra, various valence states of tellurium, low-bonding strength of Te–O, low glass transition, and acceptable chemical durability. The incorporation of alkaline dopants (alkali, Zn) alters the glass structure, units, and network, which is useful for optical applications [52]. Furthermore, the addition of chemical modifiers such as alkali oxides, alkaline-earth oxides, or transition metal oxides may enhance the ability to form glass by developing multicomponent oxide-based tellurite glasses with good glass stability [33]. Though, modifying a glass matrix to fulfill particular requirements depends on the selection of appropriate parent glass-forming and glass-modifier systems [53]. Latest trends have pictured a remarkable surge in research activities due to the addition of rare-earth ions to the glass matrices in different forms, for instance, modifiers, network formers, or luminescent ions. Compared to oxide glasses; fluoride, tellurite, and chalcogenide glass systems have attracted more attention in different research fields due to their lower phonon energies [54]. Tellurite glass has been known to exhibit outstanding features such as high dielectric constants, high refractive index, large third-order non-linear optical susceptibility, and broadband infrared transmittance. Moreover, tellurite glass possesses non-hygroscopic properties, high densities, and low transformation temperatures, which make them unique compared to its counterparts such as borate and phosphate glass. These features make tellurite glass appear as a potential host for active element doping [55]. The benefits of constituent glass formers are merged in multicomponent tellurite glass. Despite the significantly improved properties of these glass systems, there is an obstacle in the data analysis and understanding of the experimental results due to difficulty in the structure of the glass matrix by the inclusion of various glass formers. This is due to the possible creation of different structural sites by using multicomponent tellurite glasses. Though, using multicomponent tellurite glass probably prevents the undesirable agglomeration of dopant elements and creates a path for sufficient fluorescence emission [8]. The chemical composition of glass impacts the refractive index and the value of the energy break. It can be concluded that the properties of glass are altered by the presence of distinct oxides in the matrix. For instance, the substitution of PbO or TeO2 by WO3 adds one or two more oxygen atoms to the glass network, which

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leads to the presence of a large number of oxygen atoms, increases the polarizability, and decreases the value of the energy break. In the tellurite glass structure, a varying order is created in the form of coordination polyhedrons, establishing a glass network and possessing their crystalline matching part in addition to voids due to intramural spaces. In these spaces, the occurrence of materials modifier ions is probable [36]. The non-bridging oxygen is likely to occur due to the high polarity of the trivalent erbium ions, Er3+ . The formation of non-bridging oxygen is stimulated by the disruption of the bridging oxygen of erbium ions, which, in turn, elevates the polarizability of the materials. It has been acknowledged that non-bridging oxygen exhibits larger polarizability compared to bridging oxygen. Polarizability is directly proportional to the refractive index. Therefore, higher polarizability causes the glass system to exhibit a higher refractive index. Strong linear polarizability of glass can be achieved by using highly polarizable Er3+ ions that possess empty unfilled d-orbital on the outer electronic configuration of 5d0 6s2 [10]. The choice of glass constituents is vital in the exploration of high-performance tellurite glass because the mechanical, optical, and chemical properties of the host are affected by its composition [37]. The rare-earth in a glassy matrix is greatly reliant on several factors such as crystal field effects, phonon energies that extend into the bandgap, and the local environment [56]. In terms of optical gain, this feature can be enhanced by varying the amount of rare-earth content [30]. Under certain circumstances, the void between adjacent Er3+ ions is reduced at elevated erbium doping levels, which consequently improves the possibility of Er–Er ions interaction. This, in turn, stimulates the deprivation of the performances of the amplifiers, particularly the power transformation efficiency due to the transfer of energy occurring between them. There are two possible mechanisms of energy transfer with varying distances between the neighboring Er3+ . The first mechanism is homogeneous up-conversion, which occurs due to doping the ions being positioned at a nanometer scale apart from each other. The second mechanism is the pair-induced quenching that occurs when the distance between the Er3+ ions is reduced to an ion diameter scale. In this second mechanism, the two ions are impossible to be concurrently excited to their first excited level (4 I13/2 energy level) due to an effective energy transfer. The ion-ion interactions taking place at the beginning of a non-saturated absorption significantly vary depending on the manufacturing process [57]. The crystallization kinetics of glass can be examined using Differential Thermal Analyzer (DTA) [58]. The resistance of glass to the devitrification at the heating process manifests the glass stability. Glass stability can be estimated using several parameters established by the characteristic temperature in the glass heating process, including glass transition temperature (T g ), onset temperature (T x ), crystallization temperature (T c ), and melting temperature (T m ) [59]. The number and strength of coordinate links produced between the cations and oxygen atoms in the glass network and the density of covalent cross-linking, which reflect the rigidity of the network former, are represented by the transition temperature, T g [60]. Meanwhile, a typical measurement of the glass’s thermal stability against crystallization can be estimated using the difference ∆T = (T x − T g ) [46]. The other method to assess the glass stability includes the determination of the variance between T g and T m or the ratio of

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these two temperatures. Moreover, the stability parameter can be determined using the ratio of T x and T m as well as the variance between T x and T g . The common parameters used in this process, which consist of three characteristic temperatures, are the Hruby parameter K H = (T x − T g )/(T m − T x ), parameter introduced by Weinberg K W = (T c − T g )/T m , and the parameter suggested by Lu and Liu, K LL = T x /(T g + T m ) [59]. The transformation from one glassy system to another causes alteration in the glass resistance due to the devitrification process. Therefore, there is a modification to the glass stability parameters. Higher values of the K H , K W , and K LL parameters indicate glass possesses high stability toward devitrification [61]. Higher thermally stable glass can be made by incorporating transition metal oxides and alkaline earth into the tellurite glass [62]. In other studies, the incorporation of two, or extra component oxide systems by the integration of TeO2 with various oxides for example Na2 O, Li2 O, Tl2 O, K2 O, MgO, BeO, BaO, SrO, PbO, ZnO, Al2 O3 , B2 O3 , GeO2 , SeO2 , In2 O3 , ThO2 , TiO2 , V2 O5 , Nb2 O5 , P2 O5 , and WO3 has produced stable glasses based on tellurium dioxide. The thermal stability of several categories of soft glass such as silicate, phosphate, and borate can be enhanced by demonstrating the method of shifting from two-component systems to multicomponent systems. This method also can be applied to tellurite glasses [59]. To investigate the characteristic temperature of the glass, this study refers to the TG-DTA spectra of tellurite glass, which were reported by Yuhari et al. [63]. Figure 2 illustrates the TG-DTA curve of glass with T g , T x , T C1 , and T C2 , which are observed at 300 °C, 415 °C, 435 °C, and 696 °C, respectively. Several crystallization processes contribute to the segregation of phase in the glass indicated by the appearance of two crystallization peaks [64, 65]. The emergence of crystallization peaks at lower temperatures exemplifies the TeO2 crystallizing phase. Further, the occurrence of crystallization peaks at higher temperatures manifests the Zn3 TeO6 crystallization phase [66]. The thermal stability, ∆T, of glass is found to be 115 °C. In another study [13], the glass composition and individual components in the glass were found effective on the thermal properties of glass. According to Plewa et al. [67], higher values of ∆T indicate higher stability of the glass. Though, the characteristic temperatures and thermal stability of tellurite glass can be enhanced by increasing the concentration of erbium content. For example, increasing the erbium content raises the transition temperature T g , which is associated with the dominant bonding of erbium ions with non-bridging oxygens and elevates the rigidity of the glass network [13]. Glass has a unique structure and intensity distribution, which makes it compatible with optical properties [10]. Nevertheless, dopant environments become an integral element and rise very significant interest in glasses containing rare-earth ions. In the case of all rare-earth ions possessing over one f-electron, the hypersensitive transitions are likely to be observed in the spectra. Anomalous sensitivity of line strength to the nature of the dopant surroundings facilitates the occurrence of hypersensitive transitions of rare-earth ions. There are distinct mechanisms associated with optical absorption in solids. In this process, the absorption of photon energy by either the electrons or the lattice causes the transfer of energy. The optical spectra can be

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recorded using Ultraviolet-Visible (UV-Vis) spectrophotometer. Substantial information regarding the absorption of radiation and atomic vibrations that take place in the infrared region can be obtained from the lattice (or phonon) absorption. In addition, details concerning band structure, energy gap, and optically induced transitions of non-crystalline materials can be extracted from optical absorption methods. In this process, a photon with energy larger than the band gap energy will be absorbed [68]. To investigate the absorption spectra of glass, this study refers to the previous work conducted by Ferodolin et al. [69]. The absorption spectra of glass in Fig. 3 show the appearance of eight absorption bands located at 409, 450, 491, 523, 544, 653, 799, and 975 nm. The absorption bands are associated with the transition from the ground state of Er3+ to the excited states, which originate from 4 I15/2 → 2 H9/2 , 4 I15/2 → 4 F3/2 , 4 I15/2 → 4 F7/2 , 4 I15/2 → 2 H1/2 , 4 I15/2 → 4 S3/2 , 4 I15/2 → 4 F9/2 , 4 I15/2 → 4 I9/2 , and 4 I15/2 → 4 I11/2 , respectively. The hypersensitive transition is evidenced in the 4 I15/2 → 2 H11/2 transition with the appearance of a prominent peak. By following the selection rules, |∆J| ≤ 2, |∆L| ≤ 2 and ∆S = 0, the hypersensitive transition becomes very sensitive to slight modifications to the surrounding environment of lanthanide ions [70, 71]. The following equation can be used to estimate the optical band gaps of the materials [72, 73]: α(ω) =

β

(

ω − E opt ω

)n (1)

where at an angular frequency of ω = 2π v, α(ω) represents the absorption coefficient, β is a constant, è is the Planck constant divided by 2π, E opt represents the optical band gap, and n possesses values of 0.5, 1.5, 2, and 3 following the characteristics of the electronic transitions accountable for the absorption. In the Urbach energy, the extent of the exponential tail of the absorption edge can be determined using Eq. (2) [74]:

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Fig. 3 Absorption spectra of glass give information about the transition of rare-earth ions from the ground state to the excited state. Adapted with permission from [69]. Copyright 2022 Elsevier

( α(ω) = β exp

ω E tail

) (2)

where E tail is the width of band tails of electron states. The Tauc plot to determine direct and indirect band gap is displayed in Fig. 4a, b. Then, the plot of lnα versus photon energy to determine the Urbach energy is illustrated in Fig. 4c. The optical energy band gap can be obtained by extrapolating the straight-line area to the energy axis, in which the interception occurs on the x-axis [75]. Further details regarding the electronic states can be extracted from the higher energy region of the spectra predominantly those related to the integrand electronic transition. The absorption coefficient α(ω) arises from the excitation of electrons from the valence band to the conduction band, meanwhile, the corresponding energy represents the energy gap [10]. The Urbach energy can be estimated by determining the reciprocal of the slopes of the linear region from the graph of lnα versus èω [72]. The tailing of the density of states is linked with the presence of defects or the deficiency of crystalline long-range order in the amorphous materials [76]. The glass exhibits direct band gap, indirect band gap, and Urbach energy with values of 3.161 eV, 3.063 eV, and 0.232 eV, respectively. In general, the optical band gap of pure TeO2 glass is in the range of 3.79 eV. The materials can be categorized into two types, i.e., semiconductors and insulators. Semiconductors possess lower band gaps in the range of 0–3 or 4 eV, whereas insulators exhibit higher band gaps in the range of 4–12 eV [77]. Several researchers have reported a decrease in the values of direct and indirect band gaps of glass with the incorporation of higher concentrations of rare-earth [53, 78, 79]. According to Oo et al. [80], the reduction in the optical band gap is due to the movement of the absorption band to lower energy and is associated with the generation of non-bridging oxygen (NBO), which holds exited electrons of non-bridging oxygen less strongly as compared to bridging oxygen. Therefore,

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Fig. 4 a Tauc plot to determine the direct band gap, b Tauc plot to determine the indirect band gap, c plot of lnα versus photon energy to determine the Urbach energy. Adapted with permission from [69]. Copyright 2022 Elsevier

there is a reduction in the rigidity of glass. Increasing the number of non-bridging oxygen permits less tight oxygen anions due to an upsurge in the Lewis basicity of oxide ions, which leads to the formation of stronger covalent Te–O bonds in TeO3 units. This process generates fewer tight oxygen anions in the glass network. Thus, further addition of rare-earth ions causes a significant decrease in the optical band gap energies due to the formation of less tightly bound oxygen anions (valence electrons). On the opposite side, Urbach energy manifests the degree of disorder in the crystalline and amorphous materials. According to the Urbach rule, an exponential function of photon energy is observed for the optical absorption coefficient close to the absorption edge. Urbach proposes an empirical rule that emerged worldwide and is relevant to various types of disorders materials including glass, semiconductors, and insulators [81]. The higher structural stability of the glass system is reflected by a smaller value of Urbach energy [76]. An increase in Urbach energy with the addition of rare-earth content is reported by various researchers, in which the increase in Urbach energy manifests the rise in the structural disorder of the glasses [82–84].

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The most popular studies are related to those types of glass that govern their luminescent properties [85]. Though, several factors affect the luminescence properties of glass, such as the host composition, active centers, and crystallinity [86]. Recently, activator-doped glass has drawn considerable attention among the numerous kinds of glass because it facilitates the exploration of fundamental scientific concepts and practical applications. Activators such as rare-earth cations are frequently incorporated for phosphor materials because traditional glass or glass containing defects possesses low luminescent intensity [85]. High demand for optical sources leads the luminescence area stimulated by the rare-earth ions to be gradually expanding during the past decade. Rare-earth ion luminescence has been investigated for many years, in which the prominent luminescence bands originating from the lanthanides have been established since the early twentieth century. Comprehensive studies have been performed involving the energy levels of the lanthanide ions, particularly in crystals and for several decades, the rare-earth ions have been commonly utilized as the active ions in phosphors [13]. The noticeable transitions of the triply charged ions of erbium (Er3+ ), terbium (Tb3+ ), europium (Eu3+ ), and cerium (Ce3+ ) to generate the saturated green, red, and blue emissions required for full-color display have been used in Cathode-Ray Tubes (CRT) phosphors [87]. Photoluminescence is affected by active centers, adjacent host composition, and their interactions. Therefore, photoluminescence has emerged as a very sensitive analytical method [88]. Under optical excitation, photoluminescence can be portrayed as the emission of light from a material. Photoluminescence is one of the common phenomena of luminescence, which occurs due to the emission of optical radiation originating from different types of excitations such as electrical energy, biochemical changes, reactions in crystals, stimulation of an atomic system, or subatomic motions. Photon absorption and electronic excitation are induced when the light of adequate energy incident to a material. In the last stage, the electrons return to the ground state and produce emanated light in the form of a photoluminescence signal if this relaxation is radiative [89]. A feasible interplay in the glass matrix between the incident light and component atoms is shown in Fig. 5. Photoluminescence spectroscopy or emission spectroscopy can be used to illustrate the mechanism of radiation emanating from an atom or a molecule. An explicit emission spectrum is generated by each atom or molecule during the electronic transition that takes place from a high energy level to a lower energy level [90]. Heavy metal oxide (HMO) glass containing trivalent rare-earth ions has emerged as an important material owing to its strong luminescence features due to small cutoff phonon frequencies [78]. To investigate the luminescence features of glass, the present study referred to the previous work conducted by Ferodolin et al. [69]. The emission spectra of glass in Fig. 6 revealed six typical bands of Er3+ centered at 415 nm, 447 nm, 459 nm, 486 nm, 547 nm, and 668 nm due to the (2 G, 2 F, 2 H)9/2 → 4 I15/2 , 4 F3/2 → 4 I15/2 , 4 F6/2 → 4 I15/2 , 4 F7/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4 F9/2 → 4 I15/2 transitions, respectively. However, the appearance of four bands in the blue region positioned at 415, 447, 459, and 486 nm were omitted because of adverse vivid blue emissions concerning the three-step energy transfer pumping process [91]. Bands

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Fig. 5 Graphical image represents the optical interaction of light with constituent atoms in the glass matrix

located at 547 nm and 668 nm represent the green and red bands, respectively. In another study conducted by Rajagukguk et al. [38], the appearance of distinct peak profiles and glass intensity is linked to the local ligand fields around Er3+ sites. The incorporation of different concentrations of rare-earth ions into host glass leads to the alteration of local ligand fields near the rare-earth ions sites. According to Pisarska et al. [92], the crystal field effect plays a significant role as the luminescence intensity is reliant on the glass host lattice. The force of the electric-dipole transition affects the optical radiation of rare-earth ions. Thus, the symmetry and crystal field strength alters the luminescence spectra and relative band intensities of rare-earth. Boetti et al. [93] reported that rising dopant concentration amplifies the peak emission intensity stimulated by the enlarged absorption of the pump source at the initial stage. However, Elkhoshkhany et al. [94] reported that further incorporation of rare-earth content deteriorates the photoluminescence of the system which causes attenuation in the photoluminescence intensity. The attenuation occurs due to the effect of the concentration quenching governed by two mechanisms: (i) a rise in the electric dipole–dipole interactions between the Er3+ ions, and (ii) restriction in the interaction with the energy or transfer of energy due to declining the ions spacing between the neighboring Er3+ ions. Therefore, to produce devices with improved characteristics, the rare-earth ions concentration needs to be small enough to minimize the luminescence quenching [78].

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Fig. 6 Emission spectra of glass containing rare-earth ions with an excitation wavelength of 527 nm. Adapted with permission from [69]. Copyright 2022 Elsevier

The partial energy level diagram of Er3+ in Fig. 7 illustrates significant stages that generated the emission spectra. At the first stage, the Er3+ ion in the ground state 4 I15/2 is elevated to the excited level 2 H11/2 due to the ground state absorption (GSA) process with an excitation of 527 nm. Then, at the second stage, a fast non-radiative (NR) decay via the multiphonon relaxation process facilitates the population of 4 I11/2 and 4 I13/2 excited states. At the third stage, the generation of a green emission is achieved through the NR decays from 2 H11/2 to 4 S3/2 populating this level. Finally, the generation of red emission is assisted by the NR decays from 4 S3/2 to 4 F9/2 populating the later level [94–96]. Fig. 7 Partial energy level diagram of erbium ions indicates feasible stages to generate emission spectra. Adapted with permission from [69]. Copyright 2022 Elsevier

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4 Judd–Ofelt Parameters In this section, all the data which was used to calculate the Judd–Ofelt parameters were derived from our unpublished data for tellurite glass containing sodium oxide and erbium content. The oscillator strength provides indirect data regarding the symmetry and bonding of rare-earth ions within the glass matrix [97]. The intensities of the transitions for the rare-earth ions can be estimated by using the Judd– Ofelt theory [39]. In this method, the absorption spectrum is utilized to calculate the Judd–Ofelt intensity parameters and assess the radiative properties [46]. Three intensity parameters of Ω 2 , Ω 4 , and Ω 6 are explained in detail in this theory. The local environment of the rare-earth ions affects these three parameters and makes them highly sensitive. The three above-mentioned parameters can provide vital information governing the optical features such as radiative transition probability for spontaneous emission and a radiative lifetime of the excited states. In addition, the prediction of the fluorescence intensity of laser transitions can be done by using the branching ratio. The branching ratio can be utilized further to investigate the reliance of spectroscopic parameters on the variety of glass constituents [39]. In another study, Damak et al. [98] investigated the Judd–Ofelt parameters of glass, which are associated with the local structures around the rare-earth ion sites. They found out that the Judd–Ofelt parameters give beneficial information to evaluate the emission properties of glass containing rare-earth. According to Tanabe [99], the isomer changes and quadrupole splitting under certain circumstances provide clear details about the rare-earth environment in glass, particularly the bond covalency and symmetry. The combination of the lifetime measurement, which incorporates the effect of multiphonon decay, energy transfer such as cross-relaxation, and cooperative up-conversion, the non-radiative decay rate can be estimated by experimenting. By performing the integration process for each absorption band, the following equation is used to estimate the experimental oscillator strength (Pexp ) of the transitions [100, 101]: Pexp =

2303mc2 π e2 N0

( ε(v)dv

(3)

where c is the velocity of light, e is the electron charge, and m is the mass of the electron. The molar extinction coefficient ε(v) at the wavenumber v (cm−1 ) by following the Beer–Lambert law is determined as follows [100, 101]: ε(v) =

log0 (I0 /I ) Cd

(4)

where log0 (I 0 /I) represents the measured absorbance at v (cm−1 ), d is the thickness of the sample in cm, and C is the concentration of the rare-earth ions (mol/1000 cm3 ). In the 4f configuration, for a transition from the ground state (AJ) to an excited state (bJ ' ) of Er3+ ions, the total probability of the dipole oscillator strength is expressed as follows [100, 101]:

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Pcal = Ped + Pmd =

8π 2 mcv (χed Sed + χmd Smd ) 3h(2J + 1)e2 n 2

(5)

At a well-localized center in a medium of isotropic refractive index n for electric and magnetic-dipole transitions, the effective field is expressed as follows [100–102]: )2 ( n n2 + 2 χed = 9

(6)

χmd = n 3

(7)

The line strength for electric (S ed ) and magnetic-dipole (S md ) transitions are as follow [100–102]: ∑ | | >|2 ( ) Sed a J, b J ' = e2 λ ||2 e2 || 10 h), resistance to chemical agents, non-radioactivity, nontoxicity, and recyclability [37].

2 Anticounterfeiting Photochromic Films The forgery of banknote and recognizable brand items is a significant problem for governments, citizens, and economy. The counterfeiting of products has contributed billions to the growth of the black market [38]. Because of this, the development of cutting-edge anticounterfeiting technologies and materials is an exciting area of study. Since optical materials are both difficult to copy and easy to validate, they are often used in anticounterfeiting technology [39]. Quantum dots and Plasmonic nanomaterials are only two examples of the very effective anticounterfeiting optical technologies now under development. The high fluorescence effectiveness and very small, uniform particles of these materials can make them ideal for use in anticounterfeiting applications [40]. Because of their reliance on short emission, printed substrates like banknotes can cause significant interference with anticounterfeiting optical signals. Incorporating long-lasting phosphorescent (i.e., phosphorescence) materials allows for the production of anticounterfeiting goods that are not affected by the background fluorescence interference. Certificates, banknotes, passports, and ID cards are just some of the documents that have recently had trouble with security

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authentication [41]. Thus, several common authentication techniques, such as digital, ink, and paper methods, have been reported. Encoding methods such as holographic labels and identity cards are among them. However, the time and money required for such verification procedures are prohibitive for many situations [42]. Photochromic materials, such as polymer-based quantum dots, have been used to create anticounterfeit composites to print secure patterns. These photochromic chemicals can be printed onto cellulosic surface in either solvent or aqueous formulations. Anticounterfeiting polymer composites have attracted considerable interest because of their remarkable durability and optical stability [43]. Polymer nanoparticles have been employed in the preparation of anticounterfeiting nanocomposites because of their enormous surface area and negligible light reflection. It has been widely noted that the accumulation of these polymeric nanoparticles in printer nozzles is a significant challenge. As a result, new techniques using luminescent nanoparticles dispersed in a polymer matrix have been developed to improve authentication patterns [44, 45]. Alfi et al. [46] prepared photochromic PVA/CMC composite label embedded with lanthanide-doped aluminate nanoparticles (35–115 nm). The developed photochromic film exhibited transparency, antimicrobial performance, superhydrophobicity, UV protection, and photostability, which is promising for a variety of applications, including smart packaging and anticounterfeiting of merchandise. El-Newehy et al. [47] developed transparent photochromic polyacrylonitrile films that change color in response to ultraviolet light by electrospinning (Fig. 1). The nanofibrous films were differentiated by a preparation procedure that was easy, effective, quick, and cheap. In terms of photochromic performance, the electrospun film with a 0.4% ratio of lanthanide-aluminate nanoparticles was deemed the best. The average particle size of strontium aluminate was measured at 6–19 nm, whereas the average fiber size of polyacrylonitrile was measured at 100–250 nm. Transparency, adaptability, and elasticity were all present in the processed films. Fibrous film contacting angles improved from 141.3 to 158.5° when strontium aluminate was added in increasing amounts, indicating that the films became more superhydrophobic. With increased tensile elongation, the stretch-rubbery performance remained stable. Absorbance and emission maxima were seen to be at 354 and 424 nm, respectively. There are a number of sensing gadgets that have been made using smart nanofibers, including ones used for anticounterfeiting. However, the electrospinning technology has a limited yield, high cost, and high voltage, which are all drawbacks [48–50]. The solution blowing spinning process, in which significant investment has lately been made, manufactures nanofibers at high yields and cheap charge without the requirement of high voltage [51, 52]. Cellulose nanowhiskers (CNW) can improve and reinforce nanofibrous coatings efficacy. Thus, Khattab and coworkers [43] developed dual-mode security encoding authentication films using the intriguing fluorescent photochromism technology. This film is made up of a cellulose nanowhiskers-supported polyacrylonitrile composite embedded with alkaline earth aluminate. Polyacrylonitrile was used as the binder matrix, the strontium aluminate phosphor was used as the photochromic agent, and cellulose nanocrystals (CNWs) served as the drying and crosslinking material to create a coating for photochromic

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Fig. 1 Schematic illustration of electrospinning setup [47]. “Reprinted with permission from {ElNewehy et al. Luminescence 2022, 37, 40–50}. Copyright {2022} Wiley”

authentication nanofibers. The polyacrylonitrile composite must have a high dispersion of strontium aluminate and CNW in order to produce a transparent film. Luminosity spectra revealed a greenish emission at 527 nm from the paper covered with nanofibers as shown in Fig. 2. The paper sheets coated with photochromic nanofibers showed high reversibility in both ultraviolet and visible lights. This method can be reported as an competent method to create genuine goods efficiently. To obtain a micropowder (12–35 µm) of the lanthanide-doped strontium aluminate, the solid-state high-temperature synthesis was applied. The top-down technique was used to introduce a nanopowder of the lanthanide-aluminate [54, 55]. TEM analysis of the phosphor particles revealed diameters in the range of 23–47 nm. Hydrolysis of rice straw waste with sulfuric acid was used to produce CNW. The polyacrylonitrile-based film was reinforced using carbon nanotubes. TEM analysis of CNW shape revealed that their crystals had lengths of 133–156 nm and diameters of 3–7 nm (Fig. 3). As expected, the luminous CNWsupported polyacrylonitrile coats exhibited varied optical characteristics depending on the phosphor ratio. To guarantee that the AEAN was evenly distributed throughout

Fig. 2 Photochromism of solution blow spun nanofibers displaying color change from off-white below daylight to greenish below ultraviolet and greenish-yellow in darkness [53]. “Reprinted with permission from {Khattab et al. Cellulose 2022, 29, 6181–6192}. Copyright {2022} Springer”

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Fig. 3 TEM images of cellulose nanowhiskers [53]. “Reprinted with permission from {Khattab et al. Cellulose 2022, 29, 6181–6192}. Copyright {2022} Springer”

the composite matrix, the solutions were subjected to stirring and ultrasonic vibrations, which rendered the solutions colorless. As part of the solution blowing spinning process, the solutions were spread over a Wahttman sheet to create a fibrous film. The fibrous materials displayed photochromism, providing anticounterfeiting secure patterns with greenish color emissions that disappears in daylight. Figure 4 depicts the XRD spectrum of the lanthanide-doped strontium aluminate phosphor along with Quartez standard blank pattern. The diffraction results were identical to those obtained from the monoclinic crystal structure of SrAl2 O4 . The SrAl2 O4 crystals were analyzed, and no further signals were found, suggesting that the dopants (Eu2+ or Dy3+ ) were fully incorporated into the crystal lattice [18]. Also, this demonstrated that the phase composition is a monoclinic low-temperature phase.

Fig. 4 XRD analysis of SrAl2 O4 :Eu2+ /Dy3+ (a), and Quartez standard blank sample (b) [53]. “Reprinted with permission from {Khattab et al. Cellulose 2022, 29, 6181–6192}. Copyright {2022} Springer”

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3 Photochromic and Afterglow Textiles Muscle vibration sensor, foodstuff quality screening, chemical detection, body temperature monitoring, infections detection, medicine delivery, and heat and radiation monitoring are just some of the intriguing uses for smart fabric sensors. Superhydrophobic, antimicrobial, photoluminescent, and flame-retardant fabrics are only a few examples of the functional textiles touted as high-performance clothes designed for non-aesthetic objectives [56–58]. Smart textile is a term used to describe functional clothing that responds to environmental cues like light and heat by altering its color or emission activity. For instance, after being exposed to a light source, textile fibers that glow in the dark need sustained emission [59]. Crystals and traps are the two primary elements in most cases of long-lasting phosphorescence. Crystals are distinguished by their light-absorbing properties, whereas traps gather the absorbed energy. The light photons that were previously bound are freed. Smart packaging and photochromic inks are only two examples of the various applications identified for lanthanide-activated aluminates [60]. Methods including lithography, nanofibers, chemical etching, and sol–gel can all be used to create superhydrophobic materials. But these approaches have shown flaws, such tough and delayed processing, as well as the need for experienced personnel and specialized instrumentations [61–63]. It has been said that the pad-dry-cure method is an easy and inexpensive way to coat fabrics. Since hydrophobic materials tend to be less stable, they are less useful in transportation and packing. To be considered superhydrophobic, a material has to have a static contact angle of more than 150° and a sliding angle of less than 10° [64]. The use of hydrophobic compounds has benefited antifouling, marine sector, oil–water separation, and corrosion prevention. Fabricating hydrophobic materials requires the creation of micro/nanohierarchical structures. These superhydrophobic surfaces have been created with the use of fluorinated chemical agents. However, they are costly and harmful to human and environmental health [65]. Khattab et al. [66] also reported the development of photochromic textile framework by the screenprinting technology. The printed cottons had three excitation wavelengths at 272, 325, and 365 nm associated with three emissive wavelengths at 418, 495, and 520 nm. As shown in Fig. 5, the cotton textiles were dyed with reddish reactive dyestuff, which improves the visualization of the greenish-yellow emission under UV. The photochromism and emission spectra were explored as shown in Figs. 6 and 7. The absorption intensity faded rapidly with time after turning the excitation source off. The excitation and emission intensities were found to increase with increasing the UV-illumination time (0–70 s). The cotton fabrics showed greenishyellow fluorescence as reported by florescent optical microscopic images under UV illumination. Hameed and coworkers [67] provided a unique and easy method for creating multifunctional cellulose fabric by means of pad-dry-curing. The luminescent composites are made up of a combination of environmentally benign Exolet-AP422 as a flameretardant, lanthanide-doped aluminum strontium oxide nanoparticle, and environmentally benign silicone rubber as a water-repellent agent. It was determined that

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Fig. 5 Photographs of cotton (0.08% w/w) under visible and UV lights [66]. “Reprinted with permission from {Khattab et al. Carbohydrate polymers 2018, 195, 143–152}. Copyright {2018} Elsevier”

the coated textiles significantly outperformed the blank cotton sample in terms of flame-retardant performance, as measured by the char length. Under the flammability test, the blank cotton cloth was found to be fully destroyed, while the coated samples recorded char lengths between 45 and 39 mm. Coated cotton textiles maintained their flame-retardant properties even after 35 washings. Both X-ray diffraction and transmission electron microscopy were used to investigate the produced phosphor nanoparticles (15–24 nm). When raising the quantity of lanthanide-aluminate, the contact angle was observed to rise from 153.6 to 164.5°, while the sliding angle was observed to fall from 12° to 8°. When additional pigment nanoparticles were introduced, the wettability time increased from 45 min to over 60 min. Luminescence was observed in the treated cotton samples, which continued to emit a strong greenyellow glow for prolonged periods of time. The treated cotton samples showed great promise as a material for the potential large-scale production of smart textiles due to their ease of preparation, cheapness, effective hydrophobic and flame-retardant characteristics, and afterglow activity with fatigue resistance to reversibility [66] (Fig. 8). Smart textiles, such as photochromic and photoluminescent clothing, change color and emission spectra in response to light stimuli. In order to include photochromic and photoluminescent features, Ahmed et al. [68] prepared recycled nonwoven polyester textiles printed with lanthanide-aluminate pigment. From recovered polyester waste, an industrial spinning process and nonwoven fibrous mat production were carried out. Direct applications of aqueous phosphor-binder composites comprising various ratios of lanthanide-aluminate with outstanding thermal and photostability were made onto polyester textiles. Even at lower contents of lanthanide-aluminate (0.5 wt%) in the printing paste, the screen-printing approach created a consistent photochromic and photoluminescent layer onto polyester surface, with a bright greenish emission color (440 nm) bellow UV. The printed nonwoven polyester samples had their excitation wavelength tracked at 382 nm. High quantities of phosphor produced a greenish-yellow glow that persisted for longer periods of time in the dark. The phosphor nanoparticles were measured to have diameters between 4

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Wavelength (nm) Fig. 6 Excitation (a) and fluorescence (b) spectra of cotton fibers (0.08% w/w) [66]. “Reprinted with permission from {Khattab et al. Carbohydrate polymers 2018, 195, 143–152}. Copyright {2018} Elsevier”

and 11 nm as reported by transmission electron microscopy (TEM), while the crystal size was determined to be 9 nm as reported by X-ray diffraction (XRD). The XRD spectra of phosphor nanoparticles were studied. Signals from the phosphor diffraction experiments were indistinguishable from those obtained from crystals of pure monoclinic SrAl2 O4 . When illuminated with UV light, the printed fabric displayed instant and reversible photochromic emission. As the pigment ratio in the printing

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paste increased, the UV protection, antibacterial, and superhydrophobic qualities all improved. There was an increase of 108.6–132.6° in the static contact angle and 12–7° in the sliding angle. The printing process did not alter the fabric natural fiber structure or its physical qualities. The printed polyester; however, showed clusters

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Fig. 8 Fluorescence optical microscopic image of cotton (0.08% w/w) [66]. “Reprinted with permission from {Khattab et al. Carbohydrate polymers 2018, 195, 143–152}. Copyright {2018} Elsevier”

of lanthanide-aluminate particles in scanning electron microscopic (SEM) images (Fig. 9) [68]. As described by Al-Qahtani and coworkers [69], spray coating was used as a straightforward method for producing linen textiles with photoluminescent, ultraviolet (UV) blocking, and antibacterial properties. An organic fire-retardant, Exolit AP422 was used because of its low environmental impact. Silicone rubber, which is safe for the environment, was used as a water-repellent agent and a film bulk to hold the fire-resistant and lanthanide-doped aluminate particles (25–55 nm) on linen surface. The spray-coated textiles were excited at 366 nm, and the emission wavelength was measured to be 519 nm. Treating linen with 1.5% or more of phosphor nanoparticles was sufficient to produce long-lasting phosphorescence. The linen treated with the lowest phosphor concentration displayed the greenest fluorescence emission, which can be described as the best candidate for anticounterfeiting measures. During the flammability test, the char length was in the range of 52–40 mm, but the blank linen was fully consumed by flames. When the amount of lanthanide-aluminate particles was increased, the contact angle rose from 139.8° to 152.4° and the sliding angle dropped from 15° to 8°.

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Fig. 9 SEM images of phosphor-printed nonwoven fibrous mat of recycled polyester [68]. “Reprinted with permission from {Ahmed et al. Journal of Polymers and the Environment 2022, 30, 5239–5251}. Copyright {2022} Springer”

4 Anticounterfeiting Inks In materials, long-lasting photoluminescence is a desirable property because it allows for a “glow-in-the-dark” effect even after the excitation source is removed. After the excitation source is switched off, the stored energy is slowly released by the long-lived luminous material by gradual de-trapping and emissive re-combination of carriers at a certain wavelength, which might take several minutes or even hours [70]. Emergency signs in blackout situations, safety clothing, and decorative uses are just some of the many places you may find long-lasting luminous materials like dysprosium and europium doped strontium aluminate. To cover paper quickly, with little agglomeration, and using less coating paste aerosol, a novel, simple, and inexpensive non-contact technology called spray coating has been described [15]. Khattab and coworkers [54] reported that spray-coating sheets of cellulose paper with an aqueous solution of binder/phosphor were regarded as a straightforward method of producing functional paper. Long-lasting phosphorescence was achieved

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by immobilizing strontium aluminate on sheets while preserving the paper mechanical characteristics. This lighting-in-the-dark sheet absorbs light and then continues to shine brightly in the dark, although at a lower intensity, for a while after the original source of lighting has been removed. Coated cellulose paper sheets are promising for many applications due to their fatigue resistance, thermal and photostability, and other characteristics. The treated cellulose sheets showed a range of colors, including greenish-yellow, dazzling white, and turquoise as shown in Fig. 10. Abdelhameed et al. [71] presented recently the development of secure prints on documents with excellent photostability and durability to anticounterfeiting valuable items, along with an easy and cheap preparation procedure. In order to create photochromic ink, both resin and strontium aluminate was combined. Under regular visible light, the printed film looked transparent; however, below UV light, the photoluminescent layer produced green color, which immediately faded when the lamp was turned off. The mechanical characteristics showed only minor variations from blank sheets. By grinding the nanoscale pigment phosphor and mixing it into the sticky ink formulation, we were able to readily construct a transparent photochromic and fluorescent layer. Due to its fatigue resistance, quick reversibility, photo- and thermal stability, and ability to be placed at room temperature on a wide range of commercial hard surfaces, this transparent film is ideal for anticounterfeiting applications including brand protection and security printing. Scanning electron microscopy

Fig. 10 Photographs of sprayed paper sheet: before (a) and after (b) ultraviolet (365 nm) illumination, and some seconds (c) to 30 min (d) following to removing the ultraviolet illumination supply [54]. “Reprinted with permission from {Khattab et al. Carbohydrate polymers 2018, 200, 154–161}. Copyright {2018} Elsevier”

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Fig. 11 SEM micrographs (top) and EDX spectra (bottom) of printed sheets using 0.1% (w/w) of the pigment phosphor [71]. “Reprinted with permission from {Abdelhameed et al. Luminescence 2021, 36, 865–874}. Copyright {2021} Wiley”

(SEM), energy-disperse X-ray (EDX), and elemental mapping were utilized to investigate the morphological characteristics of blank and luminous films, as shown in Figs. 11 and 12. SEM images confirmed that the pigment phosphor had been successfully immobilized onto the paper surface, revealing a smart film of nanoscale sized strontium aluminate. Using the screen-printing deposition method, pigment was successfully distributed homogeneously onto the surface of the cellulose paper. The lone-pair of electrons on the hydroxyl substituents bearing from the cellulose could be responsible for the coordinated binding of aluminum with phosphor. The pigment size was about 10–35 nm. As a result of the nanoparticle nature of the pigment, it was evenly distributed throughout the cellulose surface. NH4 OH produced an alkaline medium for the hydroxyl groups of the cellulosic polymer chains, causing their dissociation and, in turn, producing negative charges. This means that the negatively charged reactive sites of cellulose polymer chains may readily attract the phosphor reactive cationic sites [71]. As described by Snari et al. [72], a fingerprint on off-white paper was spraycoated with a novel photochromic ink that becomes green when exposed to UV light, making them easy to detect. Ink preparation using lanthanide-doped aluminate nanoparticles for dual-mode detection of fingerprints at a crime scene was monitored

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Fig. 12 Elemental mapping of printed sheets using 0.1% (w/w) of the pigment phosphor [71]. “Reprinted with permission from {Abdelhameed et al. Luminescence 2021, 36, 865–874}. Copyright {2021} Wiley”

to be possible. This approach can be used as a low-cost, straightforward smart ink with outstanding photostability and durability. Nanocomposite ink, made by mixing phosphor nanoparticles with a polyacrylic acid binding agent, was used to adhere the nanoparticles to the sheet surface. There was the most noticeable fluorescence photochromism and greenest fluorescence in the fluorescent fingerprinted sheet with a 1% ratio of the lanthanide-aluminate particles. The fingerprinted sheet surface exhibited absorbance at 366 nm and emission at 517 nm. Because the nanoscale pigment particles were dispersed throughout the material bulk, the fingerprinted layer appeared transparent. The particle size of the pigment was measured to be between 27 and 49 nm. When placed on the off-white sheets and exposed to the visible spectrum, the films gave off a transparent appearance. The composite film was found to have dual-mode fluorescent photochromism, with green emission seen under UV illumination.

5 Photoluminescent Hard Surfaces Elsawy et al. [73] reported recently the preparation of smart windows with optical transmittance, photostability, UV blocking, durability, toughness, and hydrophobicity. Lanthanide-aluminate nanoparticles (6–13 nm) were embedded into recycled polyvinyl chloride. The contacting angles increased from 127.6 to 140.6° upon increasing the lanthanide-aluminate concentration from 0 to 11%. The lanthanidealuminate ratio of 1% was ideal to accomplish a transparent appearance with the optimal greener fluorescence. The film with a lanthanide-aluminate ratio of 11% generated the strongest green phosphorescence. The films displayed emission bands of 429 nm (weak/minor blue emission) and 513 nm (strong/major green emission) upon excitation at 367 nm. Al-Qahtani et al. [74] immobilized the rare-earth aluminate phosphor (3–8 nm) onto the sol–gel glass host to provide transparent glasses, changing color to green under UV. The contact angles improved from 138.5° to 152.6°

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Fig. 13 Colorimetric change of luminescent colorless wood between white (a) below UV, and greenish (b) in darkness [75]. “Reprinted with permission from {Aldalbahi et al. Luminescence 2021, 36, 1922–1932}. Copyright {2021} Wiley”

with raising the phosphor ratio. Recently, there is a rise in interest in transparent wood with multifunctional features as a source for effective construction materials. Aldalbahi et al. [75] reported recently the production of translucent wood with afterglow, robust surface, photostability, high durability, protection against UV light, hydrophobicity, and fire resistance. Smart wood with long-lasting phosphorescence has been shown to maintain its luminescence for many hours. A mixture of lanthanidealuminate nanoparticles, ammonium polyphosphate, and methylmethacrylate were immobilized into lignin-modulated hardwood bulk, making the photoluminescent transparent wooden substrate. The photoluminescent transparent wood changed colors from colorless under UV light to dazzling white, and then greenish-yellow in the dark (Fig. 13). The phosphorescent wood substrates that were synthesized showed absorption at 365 nm and emission at 516 nm. In addition to enhance fire resistance, UV blocking, and superhydrophobicity, the phosphorescent colorless wood also exhibited a reversible and long-lasting phosphorescence in response to UV radiation without showing signs of fatigue. Those transparent hardwood substrates with several uses, such as smart window, soft interior and outdoor lighting, and directing marks, were shown to be a viable option for large-scale manufacturing. Al-Qahtani and coworkers [76] prepared photoluminescent, superhydrophobic, and UV protective glasses coated with a composite of phosphor (15– 21 nm)-embedded polystyrene matrix. The coated samples showed fluorescent photochromism at low phosphor ratios as high as 0.5% (w/w). Afterglow emission was monitored for the phosphor ratios as low as 0.75% (w/w). When raising the phosphor content, the hydrophobic activity changed in both of contact and sliding angles in the ranges of 91.4–123.1° and 7–15°, respectively. The use of strontium aluminate that had been doped with Eu(II) and Dy(III) was useful in creating afterglow cobbles without sacrificing the material origin look, toughness, or smoothness. In order to prepare afterglow cobbles, an easy and cheap admixing process was recently devised by Khattab et al. [77]. Such a pigment-epoxy combination is suitable for commercial use at room temperature, making it simple to apply to flagstone surfaces. Lanthanidealuminate and epoxy resin were combined in the presence of a hardening agent. The photoluminescent cobble showed no discernible visual variations from the blank epoxy cobble devoid of phosphor, and only minute alterations in hardness qualities were found. The created photoluminescent cobble is promising for several applications toward electricity-free lighting-in-the-dark because of their fatigue resistance to reversibility, and photostability. The solid-state high-temperature method [54] was

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Fig. 14 Color change of epoxy cobble (3% w/w) from transparent below visible spectrum (a), to green below ultraviolet (b), white after 30 s in darkness (c), and greenish-yellow after 75 min in darkness (d) [77]. “Reprinted with permission from {Khattab et al. Luminescence 2020, 35, 478–485}. Copyright {2020} Wiley”

applied to produce the lanthanide-doped aluminate. Molar ratios of Sr:Al:Dy:Eu = 1:2:0.02:0.01 were applied to disperse a combination of aluminum(III) oxide, europium(III) oxide, boric acid, dysprosium(III) oxide, and strontium(II) carbonate in absolute ethanol. Sonication curing at 25 kHz for 30 min was applied to allow for a consistent admixing. After being dried at 90 °C for three hours, the combination was grinded for two hours before being sintered at 1300 °C for three hours at a heating rate of 10 °C/min in a carbon atmosphere. The phosphor with a small particle size (about 10–30 µm) was made by grinding and sieving the sintered phosphor. As shown in Fig. 14, he transparent epoxy cobbles displayed a color change from transparent below daylight (a), to intense green below ultraviolet (b), white after 30 s in darkness (c), and greenish-yellow color after 75 min in darkness (d). Aragonite polygonal tablets make up the building blocks of nacre; these tablets are tessellated to create individual layers, and the nacre structure reveals that neighboring layers and tablets are bound together by a biological polymer. Snari et al. [78] reported the preparation of a nacre-like coating with light-emitting properties in the dark by combining epoxy, graphene oxide, and lanthanide-aluminate. This coating maintained the transparency and hardness of a phosphor-free nacre-like coating. Before adding the isophorone diamine hardener, the lanthanide-aluminate nanoparticles were efficiently disseminated in the thick epoxy/graphene oxide mixture. The nanoparticle dispersion in epoxy/graphene oxide allowed for a completely colorless coating. Using TEM, the pigment nanoparticles were measured to have sizes ranging from 37 to 171 nm. It was found that 367 and 518 nm were the peak excitation and emission wavelengths, respectively. Only nacre-like coatings made with phosphor emitted a bright green light when exposed to UV light. The contact angle was analyzed to show that raising the phosphor concentration makes the nanocomposite nacre-like coatings more water resistant. Reports put the coating contact angle at 136.2°. Increasing the phosphor percentage from 0.5 to 14% resulted in contacting angles of 139.7° to 151.6°, respectively. The nacre-like coat with a phosphor concentration of 10% was reported to have the typical roughness, and its contact angle was the maximum at 152.8°. The current method is simple and inexpensive, and it can be used to produce afterglow emission of nacre-like transparent coats that can be applied to various surfaces, including glasses, ceramics, metals, and woods, to provide lighting at night and to prevent counterfeiting.

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Fig. 15 Photographs of phosphorescent plant (globe artichoke root) before (a) and after (b) illumination with ultraviolet; and few seconds in the dark (c) [79]. “Reprinted with permission from {Khattab et al. Journal of Molecular Structure 2019, 1176, 249–253}. Copyright {2019} Elsevier”

Divalent europium doped strontium aluminate phosphor is a potential tool for providing long-lasting phosphorescence to plants, especially those grown for aesthetic purposes [79]. The phosphorescence effect was triggered when the phosphor was introduced to the plant feeding system partly dissolved in a resin (Fig. 15). Therefore, this method is novel and might pave the way for improved light processing facilities in the future. We have now achieved phosphorescence, which by increasing pigment absorption, can be prolonged to produce phosphorescence signature afterglow. We expect that by making certain adjustments to the phosphor pigment, it will be easier for plants to take it up. These light-up plants may be grown all over the globe with the help of phosphor added to their fertilizers, allowing for the elimination of the need for artificial lighting or the burning of fossil fuels at night.

6 Afterglow and Photochromic Coatings One of the most widespread forms of solid pollution in the world is sugarcane bagasse, an agricultural byproduct [80–82]. As a result, the introduction of a straightforward process to transform sugarcane bagasse into high-value products has been a challenge. Al-nami et al. [83] presented four components-epoxy coating bulk, including strontium aluminate nanoscale particles serving as a phosphorescent agent, a crosslinking agent in the form of graphene oxide, and a curing agent in the form of isophorone diamine, which were used to create nanocomposite paints with a variety of useful properties. The oxidation of sugarcane bagasse, an agricultural byproduct, with ferrocene has resulted in a one-step method for producing graphene oxide. For epoxy coatings, the oxidized sugarcane bagasse nanostructures were used as a drying agent, corrosion inhibitor, and crosslinking agent. Minor shifts in hardness and impact performance were detected with raising the phosphor ratio, although no visual shifts were detected. To ensure the development of a colorless paint, phosphor in nanoparticle form was integrated into epoxy. Transmission electron microscopy images were used to analyze the morphological features of the generated phosphor particles, which indicated diameters of 3–7 nm. Measurements of luminescence spectra revealed that nanocomposite paints exhibited both color-changing and afterglow capabilities. After being excited at 372 nm, the emission shifted to 520 nm. According to reports, the

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Fig. 16 Color changes of polyester film (6% w/w) from transparent under the visible spectrum, green phosphorescence under ultraviolet, white in darkness directly after removing the ultraviolet source, and greenish-yellow after 100 min in darkness [84]. “Reprinted with permission from {Abumelha. Luminescence 2021, 36, 1024–1031}. Copyright {2021} Wiley”

water-repellency of the manufactured paints improved as the phosphor ratio rose. The contacting angle was 141.3° for the blank coating before being improved to 152.1° with increasing the phosphor content. Substrates with corrosion-resistant coatings were subjected to immersion in NaCl(aq) for up to thirty days. Including lanthanidealuminate and graphene oxide in epoxy dramatically improved its corrosion resistance. However, increasing the phosphor concentration did nothing to improve the anticorrosion properties. With the maximum afterglow, translucent look, superhydrophobicity, and anticorrosion with the lowest phosphor content, the paint with 11% of the phosphor nanoparticles has shown to be the optimal option. Because of this, the current technique may be characterized as a simple method for the mass production of phosphorescent, superhydrophobic, and anticorrosive coats. The created smart coating has excellent photostability and fatigue resistance, opening the door to a diversity of applications such as anticounterfeiting. Abumelha [84] mixed polyester resin with lanthanide-doped aluminate particles (5–10 nm) to introduce lighting-inthe-dark transparent coating with an emission of 525 nm upon excitation at 360 nm. The highest contact angle was reported at 175.6°. The transparent coating displayed green phosphorescence under ultraviolet, white in darkness directly after removing the ultraviolet source, and greenish-yellow after 100 min in darkness (Fig. 16).

7 Conclusions There has been a lot of research and commercial interest in smart materials that can change color in response to one or more stimuli. Many compounds based on photochromic have been created and put to use in manufacturing. New photochromic anticounterfeiting materials have been created, and their use in smart materials has been proven. However, due to technological restrictions during application and relatively high cost, photochromic anticounterfeiting materials have seen very limited use in both the scientific sector and in commercial settings. As a result of these technological limitations, photochromic materials form the foundation of most commercially available chromic fibers. For example, photochromic materials may find practical use in high-performance sensing textile-based systems that are programmed to alter their

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color in response to external light inputs. Advantages of photochromic fibrous-based sensors over other sensing devices include a lack of complex apparatus, specialized employees, or electrical circuitry, all of which might hinder accuracy. There have been advancements in knowledge on the best circumstances for applying chromic colorants to fabrics, which has led to improved performance. But to increase the variety of such compounds and improve their capabilities on fibrous substrates, continuous study of lanthanide-doped colorants created for photochromic materials is required. The ability to do this will be crucial to realizing future lofty goals.

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52. T.A. Khattab, M. Rehan, S.A. Aly, T. Hamouda, K.M. Haggag, T.M. Klapötke, Fabrication of PAN-TCF-hydrazone nanofibers by solution blowing spinning technique: Naked-eye colorimetric sensor. J. Environ. Chem. Eng. 5(3), 2515–2523 (2017) 53. T.A. Khattab, M.E. El-Naggar, A.G. Al-Sehemi, M. Pannipara, M.A. Abu-Saied, M.F. Abou Taleb, Facile preparation strategy of photochromic dual-mode authentication nanofibers by solution blowing spinning of cellulose nanowhiskers-supported polyacrylonitrile. Cellulose 29(11), 6181–6192 (2022) 54. T.A. Khattab, H. Abou-Yousef, S. Kamel, Photoluminescent spray-coated paper sheet: Writein-the-dark. Carbohyd. Polym. 200, 154–161 (2018) 55. L. Krishnia, P. Thakur, A. Thakur, Synthesis of nanoparticles by physical route, in Synthesis and Applications of Nanoparticles (Springer, Singapore, 2022), pp. 45–59 56. M.S. Abdelrahman, S.H. Nassar, H. Mashaly, S. Mahmoud, D. Maamoun, M. El-Sakhawy, T.A. Khattab, S. Kamel, Studies of polylactic acid and metal oxide nanoparticles-based composites for multifunctional textile prints. Coatings 10(1), 58 (2020) 57. K. Zhang, Z. Yang, X. Mao, X.-L. Chen, H.-H. Li, Y.-Y. Wang, Multifunctional textiles/ metal−organic frameworks composites for efficient ultraviolet radiation blocking and noise reduction. ACS Appl. Mater. Interfaces. 12(49), 55316–55323 (2020) 58. M.E. El-Naggar, O.A. Abu Ali, D.I. Saleh, K.M. Abu-alnja, A.M. Mnsour, M.A. Abu-Saied, T.A. Khattab, Production of Smart Cotton-nickel Blend Fibers Using Functional Polymers Comprising Ammonium Polyphosphate and Silicone Rubber. Fibers Polym. 1–12 (2022) 59. M.S. Abdelrahman, S.S.M. Elhadad, M.E. El-Naggar, H.E. Gaffer, T.A. Khattab, Ultravioletsensitive photoluminescent spray-coated textile. Coatings 12(11), 1686 (2022) 60. S. Seipel, J. Yu, V.A. Nierstrasz,Effect of physical parameters and temperature on the piezoelectric jetting behaviour of UV-curable photochromic inks. Sci. Rep. 10(1), 1–10 (2020) 61. Q. Zeng, H. Zhou, J. Huang, Z. Guo, Review on the recent development of durable superhydrophobic materials for practical applications. Nanoscale 13(27), 11734–11764 (2021) 62. M. Ghasemlou, F. Daver, E.P. Ivanova, B. Adhikari, Bio-inspired sustainable and durable superhydrophobic materials: from nature to market. J. Mater. Chem. A 7(28), 16643–16670 (2019) 63. Y.A. Mehanna,S. Emma, R.L. Upton, A.G. Kempchinsky, Y. Lu, C.R. Crick, The challenges, achievements and applications of submersible superhydrophobic materials. Chem. Soc. Rev. 50(11), 6569–6612 (2021) 64. A.H.A Kader, S. Dacrory, T.A. Khattab, S. Kamel, H. Abou-Yousef, Hydrophobic and flameretardant foam based on cellulose. J. Polym. Environ. 30(6), 2366–2377 (2022) 65. H. Wei, F. Wang, X. Qian, S. Li, Z. Hu, H. Sun, Z. Zhu, W. Liang, C. Ma, A. Li, Superhydrophobic fluorine-rich conjugated microporous polymers monolithic nanofoam with excellent heat insulation property. Chem. Eng. J. 351, 856–866 (2018) 66. T.A. Khattab, M. Rehan, T. Hamouda, Smart textile framework: Photochromic and fluorescent cellulosic fabric printed by strontium aluminate pigment. Carbohyd. Polym. 195, 143–152 (2018) 67. A. Hameed, E. Aljuhani, T.M. Bawazeer, S.J. Almehmadi, A.A. Alfi, H.M. Abumelha, G.A.M. Mersal, N. El-Metwaly. Preparation of multifunctional long-persistent photoluminescence cellulose fibres. Luminescence 36(7), 1781–1792 (2021) 68. H. Ahmed, M.S. Abdelrahman, N.G. Al-Balakocy, Z. Wen, T.A. Khattab, Preparation of photochromic and photoluminescent nonwoven fibrous mat from recycled polyester waste. J. Polym. Environ. 30(12), 5239–5251 (2022) 69. S.D. Al-Qahtani, K. Alkhamis, A.A. Alfi, M. Alhasani, M.H.E. El-Morsy, A.A. Sedayo, N.M. El-Metwaly, Simple Preparation of Multifunctional Luminescent Textile for Smart Packaging. ACS Omega 7(23), 19454–19464 (2022) 70. S. Gültekin, S. Yıldırım, O. Yılmaz, ˙I.Ç Keskin, M.˙I. Katı, E. Çelik, Structural and optical properties of SrAl2O4: Eu2+/Dy3+ phosphors synthesized by flame spray pyrolysis technique. J. Lumin. 206, 59–69 (2019) 71. M.M. Abdelhameed, Y.A. Attia, M.S. Abdelrahman, T.A. Khattab, Photochromic and fluorescent ink using photoluminescent strontium aluminate pigment and screen printing towards anticounterfeiting documents. Luminescence 36(4), 865–874 (2021)

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Optical and Luminescent Properties of Lanthanide-Doped Strontium Aluminates Meram S. Abdelrahman, Hend Ahmed, and Tawfik A. Khattab

1 Introduction Materials that change their behavior in response to environmental cues including heat, cold, light, humidity, pressure, chemicals, magnetism, electricity, and solvent polarity are considered “smart goods” [1–3]. Some smart medical films can gently release medication onto the skin, while others can dampen muscle vibrations during exercise or even release ingredients that can regulate the human body temperature [4]. Smart materials that respond to external stimuli can display different colors, different images or videos, or light up in different patterns [5]. Materials that are photochromic can change their color in response to light stimulus. Luminescence, which includes fluorescence, is the process through which a substance gives off light. Photochromism, on the other hand, is the reversible transition between two optical states in a chemical substance in response to light. Therefore, photochromic liquids or solid-state materials change color in response to a light stimulus and return to their original color when the stimulus is removed [6]. There are two kinds of photochromic substances: those that shift color in the visible spectrum, and those that shift color in the UV spectrum. Scientists have given this color-changing technology serious thought because of the many practical applications it offers in industry, including optical data storage, optical switches, displays, memory, and ophthalmic lenses [7– 9]. Such high visibility has been used in the creation of colorful ads and road signs made from photochromic materials. By incorporating photochromism into composite films, manufacturers may find new ways to create multipurpose and high-tech goods with features like electronic displays, sensors, brand protection, security barcode, UV blocking, security printing, visually appealing decorations, and the ability to

M. S. Abdelrahman · H. Ahmed · T. A. Khattab (B) Dyeing, Printing and Auxiliaries Department, National Research Centre, El-Buhouth Street, Dokki 12622, Cairo, Egypt e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_13

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track environmental conditions [10–16]. Additionally, photochromic materials can be used for military reasons to provide light-responsive camouflage [17–24].

2 Optical Films and Nanofibers Two-dimensional thin layers, or films, have both great mechanical flexibility and low weight due to their lack of thickness. They are significant because they provide the possibility of cheap processing using the least amount of raw materials necessary to fulfill the needs of the application [25, 26]. They have been used in light-emitting diodes, air/water filters, antireflective coats, medicine delivery, and photovoltaics [27–31]. Optical, catalytic, sensory, water-repellent, antibacterial, UV protection, and electrical conductivity nanocomposite films immobilized with nanoparticles have garnered considerable attention owing to their desired features [32, 33]. When it comes to the construction of various nanocomposite films, cellulose acetate has shown to be a crucial engineering biopolymer. It was one of the first cellulose derivatives used for things like packaging sheets and membranes for separating gases and purifying water [34–36]. It is distinguished by its low complexity and high-quality features, such as its wide availability from a variety of sources, high transparency, biodegradability, favorable physical performance, straightforward processing, and unique mechanical performance [37]. The large surface area and high porosity of nanocomposites based on cellulose acetate contribute to their heightened sensitivity and increased responsiveness [38]. The incorporation of nanomaterials into cellulose acetate has resulted in the material acquiring novel characteristics such as magnetic and catalytic activity, flame-retardant, bioactivity, blocking of ultraviolet rays, antimicrobial activity, and gas permeability. One of the most important features of nanomaterials is their ability to preserve the transparent nature of cellulose acetate films [39–41]. Fluorescent and photochromic materials can alter their color and emission behavior in response to light [42]. Nanoparticles of inorganic lanthanide-aluminate were encased in an organic cellulose acetate hosting material, creating photochromic/ fluorescent nanocomposite films, as described recently by Khattab et al. [43]. The produced films showed important properties for packaging, including transparency, photostability, UV protection, superhydrophobicity, and antibacterial activity. Measurements of photophysical characteristics and the CIE Lab color space revealed that, when exposed to ultraviolet, the emissive color of the photochromic material changed from colorless to green (Fig. 1). Without altering its original physico-mechanical properties, the produced films showed enhanced ultraviolet protection, antibacterial activity, and superhydrophobic activity. The mechanical behavior, handling, photostability, aesthetics, and comfortability were all preserved throughout the preparation process, which was also quick, cheap, and straightforward. The physical integration of lanthanide-aluminate into cellulose acetate yielded the photochromic/fluorescent film. Compared to films containing organic dyes, these ones showed superior color photostability. The provided photochromic films are

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Fig. 1 Photographs of phosphor (0.5% w/w)-immobilized cellulose acetate film before and under illumination with ultraviolet (365 nm) [43]. “Reprinted with permission from {Khattab et al. Luminescence 2021, 36, 543–555}. Copyright {2021} Wiley”

extremely promising for a variety of typical applications, including security prints, brand protection, and military camouflage due to their photostability and strong reversibility. These multipurpose films are important for a wide range of potential applications, including optical electronics, antireflective coats, and protective materials to enhance man safety at work such as traffic warning and anticounterfeiting products [43]. The emission spectra of rare-earth aluminate can be assigned to the Eu+2 transitions from 4f6 5D1 ↔4f7 [44]. In general, no distinctive emission peaks are monitored for Eu+3 or Dy+3 to prove a complete switch of Eu+3 to Eu+2 , and confirm that the light absorbed by the Dy+3 transfers to Eu+2 . The main role of Dy+3 is to motivate the formation of hole traps, which discharges after switching off the ultraviolet light supply. The liberated traps are reassigned to Eu+2 , which returns to its ground state leading to the emission of light [45, 46]. El-Newehy et al. [47] also prepared dual-mode UV-induced photochromic films by electrospinning polystyrene nanofibers (200–300 nm) encapsulating lanthanidealuminate particles (5–12 nm). The optimal photochromic properties were assigned to nanofibers with a lanthanide-aluminate ratio of 0.5%. The highest contact angle was reported at 138.1°. The films exhibited an excitation band at 363 nm and an emission band at 520 nm. This is a competent technique owing to the characteristics of the prepared fibrous films such as flexibility, high surface area, elasticity, and transparency. To combat fakes, nanoparticles of lanthanide-doped aluminate were inserted into polypropylene electrospun nanofibers as reported by the Al-Qahtani research group [48]. This created a photochromic film that changed color when exposed to ultraviolet light. The method used to manufacture the nanofibrous film was straightforward, productive, rapid, and low-priced. It was determined that the film with a phosphor ratio of 0.8% exhibited the most desirable photochromic features, including transparency, and the highest fluorescent greener hue. The morphological investigations confirmed the presence of the phosphor nanoparticles across a uniform distribution inside the polypropylene nanofibers. Nanoparticles of rare-earth-doped strontium aluminate ranged in size from 40 to 300 nm, whereas the diameter of phosphor-infused polypropylene fibers was measured at 450 to 550 nm (Fig. 2). The films showed signs of transparency, adaptability, and elasticity. The addition of the

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Fig. 2 SEM images of polypropylene electrospun nanofibers (pigment concentration of 0.8%) at various magnifications [48]. “Reprinted with permission from {Al-Qahtani et al. Microscopy Research and Technique 2022, 85, 2607–2617}. Copyright {2022} Wiley”

rare-earth-doped strontium aluminate to the fibrous films improved their superhydrophobic properties, as measured by an increase in contact angle from 132° to 152.4. Stretch-rubbery performance was shown to be steady even while tensile elongation was increased. The absorbance and emission maxima of the fibrous films were found to be 365 nm and 517 nm, respectively. Those films are perfect for a variety of uses, such as anti-reflective coatings and security printing. As reported by Abumelha et al. [49], the solution-blowing spinning method was used to create a UV-induced lanthanide-doped aluminate/thermoplastic polyurethane anti-counterfeiting film. In terms of time, effort, and cost, the procedure of making the photochromic films was ideal. At a phosphor ratio of 0.4 wt%, the most desirable photochromic effects were observed. The resulting films were transparent, elastic, and flexible. The diameter of the phosphor nanoparticles ranged from 3 to 17 nm, whereas that of the lanthanide-doped aluminate/thermoplastic polyurethane fibers was between 200 and 250 nm. Absorbance was strongest at 367 nm and emission peaked at 431 and 517 nm. The hydrophobic activity of the manufactured nanofibrous films rose as the contact angle improved, increasing between 147.1° and 163.7°. The photoluminescence spectra show that the films are suitable for anticounterfeit markings to operate as green patterns under UV illumination with no traces below the visible spectrum. El-Newehy et al. [50] developed anticounterfeiting nanofibers by the solution blow spinning. The lanthanide-aluminate particles (4–9 nm) were embedded in polyester nanofibers (180–220 nm). The emission maxima were reported at 519 nm. The highest contact angle was reported at 157.8°.

3 Smart Inks Dual-mode secure encoding has been shown on substrates that use fluorescent photochromism, which has proven to be an alluring method for the generation of efficient authenticating substrates. To combat counterfeiting, Abou-Melha [51] has created a unique photochromic ink to be used as a stamped layer that emits strongly in both visible and ultraviolet wavelengths. Lanthanide-aluminate particles

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suspended in a polyacrylic acid binding agent were used to create an inorganic/ organic nanocomposite ink. The cellulose papers were successfully stamped with the photochromic composite ink, and then thermally fixed. For a colorless appearance and, as shown by CIE Lab, a green color change when exposed to the ultraviolet spectrum, a uniform photochromic film was printed onto the paper surface. Photoluminescence spectra of the papers with stamps on them revealed absorption peaked at 364 nm and emission peaked at 438 nm. Under normal lighting conditions, the induced security encoding was invisible, but when exposed to UV light, its color changed from clear to a bright greenish yellow. Lanthanide-doped aluminate nanopowder was produced. Under both visible and ultraviolet light, the printed sheets exhibited a reversible photochromism without any signs of fatigue. Mokhtar et al. [52] reported the preparation of transparent nanocomposite films with a green emission photochromic activity (Fig. 3) for counterfeiting purposes such as ATM cards, banknotes, and checks. The created photochromic documents showed a simple preparation technique, photostability, efficiency, excellent mechanical qualities, and cheapness. TEM analysis of the phosphor nanoparticles gave us typical sizes of 7–12 nm (Fig. 4).

Fig. 3 Photochromism of coated paper displaying a change in color from off-white below visible spectrum to green below ultraviolet [52]. “Reprinted with permission from {Mokhtar et al. Luminescence 2021, 36, 1933–1944}. Copyright {2021} Wiley”

Fig. 4 TEM images of rare-earth strontium aluminum oxide particles [52]. “Reprinted with permission from {Mokhtar et al. Luminescence 2021, 36, 1933–1944}. Copyright {2021} Wiley”

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Fig. 5 Photochromism of self-healing photochromic composite ink printed onto paper sheet [53]. “Reprinted with permission from {Alkhamis et al. Industrial and Engineering Chemistry Research 2022, 61, 16,962–16,971}. Copyright {2022} American Chemical Society”

By using unique self-healing photochromic inks with dual-mode optical properties (fluorescence and photochromism), sheets of paper were produced that changed color from black to green in the presence of UV light (Fig. 5), as reported by Alkhamis and coworkers [53]. As a practical phosphor, lanthanide-doped strontium aluminate was used in the production of nanocomposite ink, where it showed remarkable photostability and durability. The composite ink was created using a combination of lanthanide-doped strontium aluminate (photochromic agent) and poly(2-acrylamido2-methyl-1-propanesulfonic acid) (self-healing agent). There was the most noticeable photochromic green emission from the prints with a phosphor ratio of 0.8%. The excitation peaked at 365 nm and fluorescence peaked at 520 nm were visible in the prints. TEM measurements indicated that the diameter of the phosphor particles was between 4 and 9 nm. This colorless smart ink can be easily applied to a wide range of objects for low-cost, self-healing, and effective authenticating reasons, such as trademark protection and banknotes.

4 Photoluminescent Coatings Smart paints that use long-lasting phosphorescent technology can glow in the dark for an extended length of time, reducing the need for artificial lighting. Al-Qahtani et al. [54] reported the development of a novel class of epoxy/silica nanocomposite paints that include lanthanide-doped aluminate nanoparticles at varying concentrations. The phosphor-encapsulated between SiO2 nanoparticles was originally created by coating the lanthanide-doped aluminate nanoparticles with SiO2 using the heterogeneous precipitation method. Nanocomposite paints made from lanthanide-aluminate particles, silica, and epoxy were applied to the steel. The transparent films showed green emission peaked at 518 nm when excited at 368 nm, demonstrating ultravioletinduced luminescence characteristics. The nanocomposite coating films containing 25% of the phosphor@SiO2 displayed the optimal long-lasting luminous qualities for 90 min, demonstrating good anticorrosion and hydrophobic properties. El-Naggar et al. [55] successfully developed afterglow and hydrophobic woods coated with nanoparticles (14–23 nm) of lanthanide-aluminate integrated into a polystyrene composite. The colorless (374 nm) coated wood changed color to green (518 nm) below UV.

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5 Smart Windows Energy-efficient materials with higher light transmission have been developed by scientists in an effort to lower residential building energy consumption [56]. Lighttransmitting windows have long been made using glass. However, there are safety and cost concerns associated with using glass windows [57]. The non-crystalline vitreous sheets made from the thermoplastic polymer polymethyl methacrylate are used as a lightweight and shatter-resistance substitute for glass. Even though polymethyl methacrylate is not a silica-based glass, it is nonetheless considered a kind of glass due to its similar physical properties. Because it lacks the potentially dangerous bisphenol-A units found in polycarbonate, it has been used as a low-cost substitute for polycarbonate [58]. Polymethyl methacrylate is known for its inexpensive price and ease of manufacturing and handling. To add to its superiority over inorganic glass, modified polymethyl methacrylate also exhibits increased resistance to scratching and impact. Polymethyl methacrylate has been used for several purposes, including car taillights, eyeglass lenses, and appliance lenses, because of its transparency and durability [59]. Road signs and shatter-resistant panels for windows are just some of the many uses for polymethyl methacrylate sheets. They have also found use in medical and dental settings, as well as in the production of polymer coatings, lenses, and inks. Polymethyl methacrylate has gained a lot of popularity as of late because of its many desirable features, including its optical transmittance, shatterproof nature, mechanical toughness, and low thermal conductivity, which make it an ideal energyefficient material. It proved useful in real-world scenarios, such as the manufacturing of smart windows [60]. Photochromic and afterglow photoluminescent polymethyl methacrylate smart window has been claimed to have energy-saving properties [61]. El-Newehy and coworkers [62] reported recently the development of transparent polymethyl methacrylate plastic windows with photochromic activity using a simple technique for the manufacturing of possible smart windows. Free-radical polymerization of methyl methacrylate combined with lanthanide-aluminate particles resulted in the development of the photoluminescent transparent polymethyl methacrylate plastic. Through this straightforward method, photochromic polymethyl methacrylate sheets with good transparency, resistance to UV light, and superhydrophobic qualities were presented to the market. Nanoparticles of phosphor were created and measured by TEM to have sizes of 4–19 nm. Photoluminescence spectra confirmed that the luminescent polymethyl methacrylate sheets displayed photochromism, changing from colorless (370 nm) during daytime light to green (513 nm) under ultraviolet illumination. By measuring the contact angle of the polymethyl methacrylate sheet, we were able to quantify an increase in the contact angle from 147.6° to 162.6° when the phosphor nanoparticles-to-PMMA ratio was increased. The hardness was measured to confirm that raising the phosphor particle-to-matrix ratio did not result in any appreciable changes. Different concentrations of alkaline earth aluminate phosphor were tested, and the results showed that a 0.75% concentration produced a colorless sheet that fluoresced greenest when exposed to UV light, indicating that this concentration was optimal for photochromism. However, an 8% ratio showed that

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long-persistent phosphorescence was tracked, revealing a colorless sheet that radiated greener phosphorescence when exposed to ultraviolet. Strong photostability was also observed in the luminescent polymethyl methacrylate sheets [62]. Figure 6 shows the morphology of the finished polymethyl methacrylate sheets. The surface shape of the polymethyl methacrylate sheets changed little with increase in the phosphor ratio. As no particles were found on the surface of the examined polymethyl methacrylate samples, the smoothed surface of the sheets generated indicates that the phosphor particles were completely incorporated into the polymethyl methacrylate bulk. El-Newehy et al. [63] presented the preparation of photochromic and afterglow recycled polyester plastic incorporated with lanthanide-doped aluminate nanoparticles (11–23 nm). The samples were colorless (365 nm) below the visible spectrum and green (512 nm) below the ultraviolet. The contacting angle was detected between 148.5° and 161.6°. Afterglow phosphorescence, a robust surface, great photostability, blocking from UV rays, hydrophobicity, optical transmittance, and durability are only some of the features of the transparent hardwood substrate that was recently presented by El-Naggar et al. [64]. This phosphorescent hardwood was capable to maintain its light emission for extended periods of time. Phosphor nanoparticles made of rare-earth-doped aluminate were immobilized in a solution of epoxy resin on lignin-modified wood. Transmission electron microscopy (TEM) analysis of the

Fig. 6 SEM images of the polymethyl methacrylate sheets using various concentrations of the phosphor nanoparticles, including 0% a–c, 0.75% d–f, and 8% g–i [62]. “Reprinted with permission from {El-Newehy et al. Luminescence 2022, 37, 97–107}. Copyright {2022} Wiley”

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manufactured lanthanide-aluminate nanoparticles, which were designed to enhance pigment dispersion, revealed that the particles had a size range of 8–14 nm. The lanthanide-aluminate has to be evenly diffused in the methyl methacrylate solution without clumping in order to maximize the formation of long-lasting phosphorescent colorless woods. Colorimetric parameters from CIE Lab showed that phosphorescent woods became green below UV and greenish-yellow in darkness. Absorbance peaked at 365 nm and phosphorescence peaked at 431 and 520 nm were observed in the afterglow wood samples. The static contact angle increased from 147.2° to 163.6° and the sliding angle from 9° to 14° when the rare-earth-doped aluminate ratio went up. To achieve maximum long-term photoluminescence, the rare-earth-doped aluminate ratio of 8% was found to be ideal. With the current technique, many functional types of wood for several uses, such as smart windows, can be mass-produced [64]. Figures 7 and 8 show the results of scanning electron microscopy (SEM) analysis of phosphorescent wood samples. Due to the absorption of visible light by lignin, the unprocessed lignin-modified wood appears brown. As a result of the enhanced light reflection at the border amid the micro(nano)-scaffolds and canals, the treated wood gradually changed color from brown to white. The development of translucent woods followed the combination of the lignin-modified wooden substrate with a mixture of phosphor@epoxy, which reduced the mismatch of refraction index while suppressing light reflection. As a result of the increased regularity of canals, the phosphor@epoxy was able to diffuse straight through lignin-modified wood without damaging the wood microstructures throughout the lignin-modification process. The wood’s mechanical strength was increased because of the role phosphor@epoxy played as a glue agent connecting the cellulose fibers in the wood [64]. Al-Qahtani and coworkers [65] developed smart windows that use photochromic transparent timber materials. The prepared photochromic and fluorescent transparent

Fig. 7 SEM images of wood embedded with epoxy and different concentrations of strontium aluminate particles; 0 wt% a–c; 10 wt% d–f [64]. “Reprinted with permission from {El-Naggar et al. Journal of Rare Earths 2022}. Copyright {2022} Elsevier”

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Fig. 8 SEM images of lignin-modified wooden substrate a–c, and cross-section of photoluminescent wood embedded with epoxy and phosphor ratio of 10 wt% d–f [64]. “Reprinted with permission from {El-Naggar et al. Journal of Rare Earths 2022}. Copyright {2022} Elsevier”

wood can change color between the ultraviolet and visible sections of the spectrum. By the permeation of methyl methacrylate and lanthanide-aluminate through the lignin-modified wood, they were able to create a fluorescent and photochromic translucent wood with high photo- and thermal stability. According to the colorimetric findings of the CIE Lab, this transparent hardwood substrate went from being colorless in the visible spectrum to green when exposed to UV rays. An absorption peaked at 365 nm and two emissions peaked at 433 and 517 nm were detected for the generated photoluminescence transparent wood. The results showed that the manufactured translucent luminous wood provided superior resistance to ultraviolet rays and displayed superhydrophobic properties.

6 Textile Materials Khattab research group [66] reported the development of smart warning textiles with adjustable photoluminescent performance using a straightforward spray-coating method. A spray coating method was carried out employing an aqueous binder and inorganic phosphor. The coated wool materials were not significantly different in their stiffness or air permeability as compared to blank wool, demonstrating that their pliability and breathability were preserved. The coated wool substrates show promise for general applications in specific domains like warning textiles due to their very good photostability and thermal stability. As such, this method can be seen as novel, easy, and significant, ushering in new possibilities for the manufacture of warning smart textile, in particular for safety purposes, which can be adapted for mass production on a wide range of textile materials. The strontium aluminate

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phosphor was applied to wool fibers. The morphology and elemental compositions were determined as shown in Fig. 9. SEM images show that spray-coating was effective in depositing clusters of irregularly shaped microstructures of lanthanidealuminate onto the wool surface. The generated micro-sized pigment had a size variation of 10–40 µm. Nonetheless, this nano/micro-sized lanthanide-aluminate tends to cluster and spreads out quite uniformly throughout the wool surface. Technical protective gear is designed to make workers less vulnerable to harm while they’re on the job. This clothing might save a life or prevent serious illness in certain cases. Khattab et al. [67] also designed a photoluminescent warning cotton fabric that keeps generating light for a considerable period after the excitation tool has been turned off, improving traffic safety. In order to create a cross-linkable formula for application onto cotton through the spray-coating method, lanthanide-aluminate was disseminated in an aqueous solution of polyacrylic acid. In order to implement a transparent luminescent film, the lanthanide-aluminate particles must be uniformly disseminated to avoid clumping. The spray-coated film on cotton fabric had its highest band of excitation wavelength at 365 nm, whereas its emission was measured at 515 nm. Alsharief et al. [68] reported the use of lanthanide-doped aluminate to develop smart textiles with UV-activated photochromic prints. The shape of phosphor particles was analyzed by using TEM images to indicate sizes of 4–9 nm. By directly printing lanthanide-aluminate particles onto a polypropylene textile surface, a smart composite film was produced with superior water repellency, antibacterial activity, and UV shielding. After 371 nm of excitation, a green emission was detected at 515 nm. Lower phosphor ratios (0.1–0.5%) showed bright green fluorescence when exposed to UV light, whereas larger phosphor ratios (>1%) emitted a greenishyellow afterglow that persisted for a considerable amount of time in the dark. It was discovered that the multifunctional properties of printed textiles might be enhanced by increasing the phosphor ratio. When the lanthanide-doped aluminate ratio was increased, the contact angle showed an increase from 98.7 (pigment content of 0%) to 126.8° (pigment content of 10%). Fabrics with a phosphor ratio of 10% were found to have the best hydrophobic characteristics. The fabric aesthetic and comfort qualities were not affected by the phosphor ratio. At the highest lanthanide-doped aluminate ratio (14%), the cytotoxicity analysis of the present phosphor-printed polypropylene was exceptionally low (1.1%) on man skin. Printing using strontium aluminate on fabric resulted in far more vibrant and long-lasting colors than using organic dyes. The present technique is appealing for use in a range of niche sectors, such as security encoding, because of its high photostability, durability, remarkable reversibility, and fatigue resistance. The lanthanide-aluminate was synthesized using the solid-state high-temperature technique [62], and the phosphor nanoparticles were obtained using the top-down method [69]. Figure 10 depicts the morphology of lanthanide-aluminate particles as monitored by transmission electron microscopy, which were found to be spherical and with diameters of 4–9 nm. Low-temperature plasma pretreatment was used first on the nonwoven polypropylene fabrics, and then screen printing was applied.

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Fig. 9 Mapping of elements a and b, energy-dispersive X-ray spectra c, and SEM images d of wool fabric with a pigment ratio of 3% w/w [66]. “Reprinted with permission from {Khattab et al. Industrial and Engineering Chemistry Research 2018, 57, 11,483–11,492}. Copyright {2018} American Chemical Society”

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Fig. 10 TEM micrographs of lanthanide-aluminate nanopowder [68]. “Reprinted with permission from {Alsharief et al. Journal of Materials Research and Technology 2022, 20, 3146–3157}. Copyright {2022} Elsevier”

The polypropylene textile morphology was analyzed using scanning electron microscopy as shown in Fig. 11. The printed fabrics (with or without plasma treatment) showed identical morphologies. Surface morphologies of printed polypropylene were drastically different from those of blank fabric smooth surfaces. The phosphor-printed polypropylene surface is more uneven than that of blank polypropylene. It was discovered that the printing paste was entrapped in the spaces between the fibers. Phosphor particles seemed to be compatible with the printing paste, since the printing paste was deposited onto the cloth surface. Phosphor ink on nonwoven polypropylene fabric resulted in a rougher surface morphology, proving that the polypropylene chains could form bonds with the lanthanide-aluminate molecules [68]. Polyester fibers have found use in a wide variety of manufacturing contexts, including plastic furniture, vehicle components, medical equipment, and LCD

Fig. 11 SEM micrographs of plasma-pretreated screen-printed nonwoven polypropylene at phosphor ratio of 0.1% a–c, and 14% d–f [68]. “Reprinted with permission from {Alsharief et al. Journal of Materials Research and Technology 2022, 20, 3146–3157}. Copyright {2022} Elsevier”

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screens [70–72]. Polyester, on the other hand, has never taken color well because it has no functional staining sites. Attar et al. [73] reported that the lanthanide-aluminate was dyed onto the polyester surface by means of the plasma technique, which was utilized to construct reactive dyeing sites. To make polyester fibers, scientists first used recycled polyester trash. TEM analysis of phosphor nanoparticles revealed that their average size was between 3 and 8 nm. Polyester was given antibacterial, photoluminescent, and UV protective properties due to the use of lanthanide-doped aluminate. The dyeing activity was significantly improved in comparison to polyester that had not been treated with plasma. The contact angle rose from 149.3° to 155.8° when the lanthanide-aluminate ratio was increased, but sliding angles dropped from 13° to 8°. The softness, air permeability, and stiffness of treated polyester fibers were not altered by the immobilization of phosphor nanoparticles. In vitro testing of the antibacterial properties of the blank polyester fibers yielded negative results. When compared to plasma-free samples, the fibers that were subjected to pretreatment with plasma showed significantly enhanced antibacterial activity. Samples treated with plasma exhibited between good and very strong antibacterial activity against C. albicans, E. coli, and S. aureus. Plasma-pretreated fibers provided high to outstanding colorfastness. The luminous polyester showed green phosphorescence at 439 nm after being excited at 382 nm. When treated polyesters were maintained in the dark for a long time, a significant amount of greenish-yellow light emission was observed. The best findings from the colorfastness and superhydrophobic tests were assigned to the optimal sample with a phosphor concentration of 10%. Multifunctional textile fibers can benefit from the development of photostable polyester fibers with UV blocking, antibacterial activity, hydrophobicity, and sustained luminescence. As a result, the present approach is crucial for a wide range of smart applications, including smart packaging and trademark protection [73].

7 Other Applications Many scientists interested in biomimetic materials chemistry have been motivated by the intriguing possibilities presented by the combination of organic and inorganic materials in hierarchical nacreous architectures. The synergistic combination of these materials is on the rise as a way to produce a single inexpensive lightweight object with all the qualities of its constituent parts [74, 75]. Graphene-based polymer composites are frequently employed as hybrid biomimetic nanobuilding blocks because of their large surface area and acceptable aspect ratio in addition to their remarkable optical, electrical, thermal, and mechanical capabilities. Graphenes have been used as nanosized fillers in a wide variety of nanocomposite manufacturing such as tissue engineering [76]. Multi-scale alignment is made possible by combining graphene nanosheets with a polymer of interest. In addition, composites with new characteristics and multifunctionality can be produced by the hybridization of inorganic nanostructures with graphene-based nanomaterials [77, 78]. These composites may exhibit unusual magnetic, electrical, catalytic, optical, or other capabilities.

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Casting techniques were used to create nacre-like structures from two-dimensional graphene-based nanomaterials. The organic layers of natural nacre are separated by inorganic aragonite, which accounts for the vast majority of its weight (~96% w/w). Recently, synthetic nacre was created with an inorganic-to-organic ratio that is the opposite of natural nacre, consisting mostly of an organic substance with layers of graphene between them [79, 80]. As previously reported by Hameed et al. [81], glow-in-the-dark nacres were created using graphene oxide (GO), low molecular weight unsaturated polyester, and lanthanide-aluminate while retaining the hardness and transparency of phosphorfree nacre. The phosphor particles were dispersed in the thick polyester/GO mixture before the methylethylketoneperoxide catalyst was added to stop them from clumping together. This transparent nacre was achieved by spreading phosphor nanoparticles uniformly across the polyester/GO matrix. TEM analysis of milled phosphor nanoparticles revealed that their sizes ranged from 36 to 137 nm. The maximum wavelengths for excitation (365 nm) and emission (524 nm) were found. Only photoluminescent nacres glowed bright green with phosphorescence for around 90 min when exposed to UV light. The water-repellency of the nanocomposite nacres was determined by measuring the contact angles between the nacre and the water. The nacre-like blank sample (0 wt% of phosphor) was measured to have a static contact angle of 144.1°. It was observed that the static contact angles for the nacre-like substrate (0.5% of phosphor) and nacre-like substrate (13 wt% of phosphor) varied between 145.0° and 152.9°, respectively. With a static contact angle of 153.7°, the nacre-like substrate (9 wt% of phosphor) was found to be the roughest and most desirable surface. It was shown that the hardness performance was enhanced with an increased pigment ratio. This method can be applied to create afterglow nacrelike materials that can be used for electricity-free illumination in the dark. Glasses, ceramics, metals, and wooden surfaces are only some of the substrates that can benefit from the use of these transparent long-persistent phosphorescence nacres. Alzahrani et al. [82] developed smart concretes with excellent stiffness, optical transmittance, superhydrophobicity, UV blocking, photostability, and durability via a simple immobilization of lanthanide-aluminate into polyester. Lightweight photoluminescent plastic concrete was made by combining silica nanoparticles (filler), polyester (bulk material), methylethylketoneperoxide (hardening agent), and rareearth-doped aluminate (photoluminescent agent). After curing for a few minutes under ambient conditions, the operation was complete. After being measured by TEM, it was determined that the average size of phosphor nanoparticles is between 6 and 11 nm. In response to an excitation wavelength of 372 nm, the plastic concretes showed an emission peak at 515 nm. Only little differences were detected between phosphor-containing and phosphor-free concretes for the majority of the studied hardness metrics. The photoluminescent plastic concrete appeared the same under normal lighting conditions as the blank plastic concrete. Hydrophobicity was measured to have increased from 139.2 to 150.7 when phosphor concentration was raised. The produced photoluminescent concrete substrates have excellent reversibility, fatigue resistance, and photostability, suggesting a broad variety of potential applications for glow-in-the-dark materials.

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In order to substitute electric power in medical endoscopes, a novel and longlasting photoluminescent electrospun nanofibrous-walled tube was created. Khattab et al. [83] encapsulated lanthanide-aluminate nano-scaled particles into nanofibers of polycaprolactone-cellulose acetate to facilitate the development of a simple method for creating an organic–inorganic luminescent nanocomposite. The nanofibrous luminescent tubes showed a 355 nm excitation band and a 517 nm phosphorescence band. The photochromic transition from white in the visible spectrum to green in the ultraviolet spectrum was shown by the photoluminescence spectra. The electrospun nanofibers were examined for their mechanical and superhydrophobic capabilities. Increased hydrophobic performance was observed in the nanofibrous-walled tubes without any compromise to their other desirable properties. Figure 12 shows SEM images examining the morphologies of the manufactured nanofibers encapsulated by lanthanide-aluminate nanoparticles. There were no notable surface differences upon increasing the pigment ratio. No pigment particles were found on the manufactured tube surface, proving that the phosphor particles were thoroughly lodged inside the nanofibers. By directly encapsulating the luminous lanthanide-aluminate into nanofibers, the presented luminescent nanofibers were constructed. As shown in Fig. 13, the created luminescent nanofibers had a white backdrop in natural daylight, greenish-yellow under UV, and a vivid green hue after being exposed to darkness.

Fig. 12 SEM micrographs of electrospun tubes embedded with different ratios of the phosphor nanoparticles; 1% a–c, and 10% d–f [83]. “Reprinted with permission from {Khattab et al. Industrial and Engineering Chemistry Research 2021, 60, 10,044–10,055}. Copyright {2021} American Chemical Society”

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Fig. 13 Photos of phosphor-embedded nanofibrous tubes (phosphor ratio of 5%) under daylight a, under UV b, and in darkness c [83]. “Reprinted with permission from {Khattab et al. Industrial and Engineering Chemistry Research 2021, 60, 10,044–10,055}. Copyright {2021} American Chemical Society”

8 Conclusions Scientists are very interested in materials that can alter their color in response to external stimuli, both for academic and commercial purposes. Many different types of businesses have benefited from manufacturing and using chromic chemicals. The feasibility of using photochromic-based materials in fibrous and dense materials has been shown. Photochromic materials have been limited in both research and commercial usage due to their high price and application challenges. For this reason, there are only a few photochromic fibrous and dense materials on the market. Possible applications for photochromatic materials include functional fibers that respond to light stimuli with apparent color changes. Photochromatic counterfeiting sensors provide various advantages over conventional sensing devices since there is no need for expensive instruments, trained personnel, or electrical infrastructure. Performance and ideal circumstances for applying photochromic colorants to fabrics have both seen substantial increases in recent years. Thus, research on photochromic colorants must be ongoing. For future lofty targets, this will be of crucial relevance. There is potential for further development of photochromism-based applications, such as smart windows, anticounterfeiting, smart packaging, and medical diagnostics.

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Potential Use of Photo-Excited Phosphors in Energy-Efficient Plant Lighting T. Krishnapriya, Adon Jose, and P. R. Biju

1 Introduction The recent impacts made by global climate change are actually a clarion call for us to wake from the dreams of technological and economic growth to the very different dreams of sustainable use of soil, water, air, and biomass with a great concern for the environment. Actually, in a world with an already stretched energy infrastructure where the major and growing use of energy is the generation and use of electricity for lighting, the idea of designing new, efficient, and cost-effective light-emitting materials is an enduring recipe to take us on a safer trajectory to a carbon-neutral earth. Lighting may be counted as the first amenity rendered and still remains one of the ultimate specific uses of electrical utilities. On a global basis, a lion’s share of the primary energy consumption points toward lighting, and the resulting carbon emissions amount to three times the emissions due to aerial navigation and 70% of the same from global passenger automobiles. With escalating climate risks, there is an urgency to adopt innovative climate services and solutions in the fields of agriculture, infrastructure, research and development, industries, etc. As a part of these developments, we are witnessing a major shift from traditional farming to “controlled environment agriculture.” Traditional farming, which was developed in open fields, is now facing many challenges due to variations in the natural lighting features, inadequate daylight during periods of winter, and unexpected climate changes that are leading to substandard yields and crop failures around the globe [1]. This situation made us think of controlled environment agriculture (CEA), also called protected horticulture, with some major manifestations such as indoor farming, vertical farming, urban farming, greenhouses, and plant factories [2]. The ever-increasing population and the resulting demand for food and infrastructure have led to a boom in indoor

T. Krishnapriya (B) · A. Jose · P. R. Biju School of Pure and Applied Physics, Mahatma Gandhi University, Kerala 686560, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_14

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plant cultivation as well as building construction. So, designing and retrofitting buildings with vegetation in the built-in environment in the form of living walls and green roofs is much welcomed [3]. Another leading area is planetary exploration, with a view to establishing and extending permanent settlements into various other parts of the solar system. Making these missions cost-effective by recovering water and oxygen and generating food with an adequate supply of irradiation for bioregenerative life support facilities is one of the other challenges [4]. Another interesting field of research is the realization of light-emitting plants, specifically “glow in the dark” plants engineered by direct incorporation of luminescent material so as to replace streetlights in the future. Some of the major applications of phosphor in the agricultural field consist of LED plant lights and light conversion films for their use in greenhouses. Solar radiation, being the primary source of energy, is the driving force that initiates the food chain, which is an integral part of the ecosystem. The quality, intensity, and duration of light exposure are some of the key factors that influence the overall physiological development of plants by regulating their photosynthesis, photoperiodism, metabolism, phototropism, gene expression, photomorphogenesis, and so on. The solar spectrum extends over a wide waveband ranging from ultraviolet (UV), visible, to infrared (IR) light, which is crucial for plant growth [5]. There are four major pigments in plants: chlorophyll a, b, phytochrome PR, and PFR, which absorb in the blue, red, and far-red regions, respectively [6–9]. At the same time, light having wavelengths shorter than 400 nm is also relevant to plants, not for photosynthesis but for photomorphogenesis, which is the adaptation of plant growth according to the changes in light quality. Say, for example, that the photoreceptors UVR8, phototropin, and cryptochrome respond to UV-B, UV-A, and blue radiations [10]. In the early days of plant lighting, incandescent, fluorescent, and high-intensity discharge lamps were in use as artificial light sources in crop production [1]. Contrary to indoor plant lighting, plant factory artificial lighting, etc., and high-definition displays, the spectral requirement for plant growth necessitates specific design and attention. Photosynthesis and photomorphogenesis are two important factors to consider when designing [11]. In the former, the photosynthetic pigments chlorophyll a and b absorb light in the wavelength regions 662 and 642 nm, respectively, while in the latter, light-mediated plant development such as gene expression and differentiation of cells, tissues, and organs happen under the control of the photoreceptors called phytochromes PR and PFR, which have absorption in the 550 to 700 nm range [1]. In addition, plants also sense variations in the relative length of day and night (photoperiodism) and the direction of the incoming light source (phototropism) [12]. The right spectral composition of light radiation is one of the determining factors for maximal plant growth, and it has been proven that blue emission positioned at 450 nm and red emission positioned at 660 nm are optimum. As a result, the most desired characteristics of light-emitting diodes as growth lights are those that combine emission bands in blue and red, in accordance with the photosynthesis action spectrum [12, 13]. In fact, the current uptrend in climate extremes, such as haze weather, droughts, and floods, has led to crop failures and suboptimal

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yields. These facts prompt us to adopt indoor farming, where plants are grown in a controlled environment including light, water, and temperature. During the night, lighting is performed by phosphor-converted LEDs. Recent advancements in plantgrowth lighting, particularly the development of LED technology, enable a grower or researcher to tailor the light spectrum to their specific needs. LEDs also have a flexible spectrum, monochromaticity, a longer lifespan, a high luminous flux with low radiant heat output, cost-effectiveness, and so on. Apart from these ordinary WLEDs that satisfy human color perception, there are some special-purpose phosphors like plant habitat-conscious phosphors. Here, the thing is that extensive use of the abovementioned sources can collapse the natural habitats of indigenous plants. Usually, plants adjust their flowering time depending on the photoperiod at a particular time of the year in a geographical area with the help of certain genes that control the circadian clocks. But when such plants in their natural terrain, especially in dark sites such as woods and caverns, are subjected to ordinary white light from WLEDs, it hampers the flowering and fruiting of short-day plants while boosting the same in long-day plants. The results are an obvious imbalance in the natural vegetation of an area [14]. With this fear in mind, scientists are working hard to develop plant habitat-conscious white LEDs (PHC-WLEDs), whose emission spectrum is designed to contain nighttime photosynthesis, thereby preserving endemic species in a geographical area. In this regard, pc-WLEDs made of Dy3+ –doped phosphors have attracted attention and have proven to be an excellent choice for reducing nighttime photosynthesis due to their very low red or NIR emissions [15]. Phosphors are luminescent materials that emit light when sufficiently excited. The emitted light may lie in the UV, visible, or IR regions. On receiving suitable stimuli, the electronic system of the phosphor material undergoes transitions from the ground state to excited states and decays from the higher energy excited states to the lower lying levels with the emission of light. The emission transition will be the unique characteristic of the material and have no explicit dependence on the exciting source. As we know, most phosphors consist of a host and an activator. The host lattice can be silicates, oxides, tungstates, aluminates, sulfides, nitrides, phosphates, etc., whereas the activator can be any of the transition metals or rare earth (RE) ions (Fig. 1).

Fig. 1 A plant factory with artificial lighting. Reused with permission from [57]

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Converting light from less efficient spectral bands to more efficient bands with the help of luminescent coatings is assumed to stimulate photosynthesis in greenhouses and is verified via numerous experimental studies. The studies have proven the increased growth of plants under fluorescently coated greenhouses when compared to bare ones [16, 17]. The prime focus of this chapter is to familiarize the reader with the role of various phosphors in man-made lighting systems, especially those used in the growth and development of plants, along with the analysis of the major lighting parameters for growing vegetables efficiently and also to briefly describe the influence of specific light emissions on photoreceptors, plant pigments, and phytochemical profiles. The relationship between plants and light is very complex, and a comparative assessment of the effects of various artificial lighting systems on plants is also detailed. It is not hard to imagine the futuristic use of plants as natural powerhouses that could transform sunlight during the daytime into energy as well as bioregenerative life-support systems.

2 Spectral Effects on Plant Growth Light is the major energy source required for all the physiological processes associated with plant growth. Biologically active photons having sufficient energy could excite various plant pigments and photoreceptors, which in turn enhance growth rate, plant quality, and productivity [18]. Among these, light intensity, spectral distribution, and light duration are three key light parameters that are pivotal in the overall growth and development of plants.

2.1 Light Intensity The quantity of light or its intensity plays a pivotal role in regulating biosynthesis in plants by influencing the photochemical reactions that convert atmospheric carbon dioxide to carbohydrates and many other physiological processes. In other words, during photosynthesis and transpiration processes, the transport of carbon dioxide and water through stomata is affected by the intensity of light. Sufficient light quantity or irradiance within limits is followed by a higher growth rate as additional photons could drive photosynthesis, causing an improved accumulation of biomass. This can be justified through the observations made in the experiments done in spinach, where a linear increase in biomass has been reported when irradiance was raised from 125 to 620 µ mol m−2 s−1 whereas a saturation of phytonutrients occurs at 200 mol m−2 s−1 [19].

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2.2 Spectral Distribution Spectral distribution is another determining factor, which refers to the selection of certain areas of the spectrum with red, blue, green, or other visible UV or IR wavelengths to which various plant pigments are sensitive. It is possible for plants to sense and respond to various radiations in the UV (280–400 nm) and far-red (700–800 nm) regions of the spectrum. The two major plant pigments, namely phytochrome and blue-absorbing pigments, are sensitive to red or far-red light and blue or UV-A light, respectively [20]. At present, the spectral distribution of various phosphor-converted LEDs is possible as demanded by various plant species, thus enhancing the nutritional quality and total biomass accumulation of many plants [21].

2.3 Photoperiod Photoperiod is actually a relatively unexplored parameter and environmental signal that controls flowering, which when modulated could promote crop breeding [22]. The optimal photoperiod has a great effect on plants throughout their growth process, from germination to flowering. It also helps in promoting the crop productivity of plants in controlled environments with artificial lighting. But when compared to the other two parameters, light intensity and spectral distribution, photoperiod has less impact on the growth process and accumulation of phytonutrients, and there is no impetus to use prolonged photoperiod in a closed production system [23]. This could lead to an economically viable and sustainable system.

3 Applications 3.1 Artificial Light Sources and Plants The sun’s light, which is the natural source of light, is certainly the only source that is continuously supplied to and endlessly accumulated to maintain the existence of all forms of life on earth, specifically via photosynthesis. Even though sunlight is the key player behind the process of photosynthesis, it suffers from certain limitations because it used to attenuate a lot before it hit the earth, specifically the leaves. When sunlight travels through the atmosphere, a part of its energy is absorbed, and a part is redistributed by scattering. An appreciable amount of light is absorbed by the atmosphere itself. Specifically speaking, the absorption bands at the UV, NIR, and mid- or far-IR regions occur due to absorption by the ozone, oxygen, water, and CO2 molecules, respectively [24]. Depending on the sun’s position, the light radiation has to pass through different layers of the atmosphere before hitting the earth’s surface. The longer the path of light through the atmosphere, the lower the sun is. Therefore,

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the spectral distributions of sunlight in the morning, noon, and dusk are not the same, which will certainly influence the growth and development of solar plants. In such situations, it is essential to have some sort of artificial means. Usually, a major share of solar energy is wasted since plants reflect green light and less efficiently absorb orange light. Nowadays, with the advent of nano-phosphor technology, the plant lighting environment has been optimized to a great extent. Two important ways to increase the light absorption by plants are illustrated as follows: The first is a specially fabricated lamp consisting of red, blue, and UV emissive nanoparticles. The second is utilizing the light-converting film that is incorporated with luminescent nanoparticles for converting the full-color solar spectrum into the responsible spectral wavelength [25]. A brief discussion is made regarding the various conventional lighting sources along with the next-generation lamps for plant lighting. Even though insolation, which is the natural source of light for plants, extends over a broad band within the wavelength range of 300–1000 nm, only half of the radiant energy is made available to plants in the form of photosynthetically active radiation (PAR), which encompasses the 400–700 nm range. During photosynthesis, this radiant energy, fetched by specific photoreceptors in plants, is converted to chemical energy. Plants also transduce different wavelengths of solar radiation into specific chemical signals for various growth and developmental processes [26]. The prospect of growing food crops without being affected by dry spells, flooding, haze weather, or weak or intense light motivates us to pursue controlled environment agriculture. But, minimizing the total cost of energy generally and for plant lighting specifically, which has evolved to be the first priority for optimizing artificial lighting for plants, may have the greatest potential for CEA energy savings in the long run. In greenhouses, artificial lighting is used to augment insufficient solar radiation, whereas electrical lighting is the only light source for crop production in a closed environment since there is no sunlight available or it is quite expensive to guide sunlight into it. In the early days, light with a spectrum similar to that of sunlight was considered to be the best for plant growth. However, subsequent research has shown that not all bands in the spectrum are equally important for plant growth, but rather specific regions. Conventional plant lighting sources consist of incandescent, fluorescent, high-intensity discharge lamps, high-pressure sodium, plasma grow lights, induction lights, and metal-halide lamps [27]. In incandescent lamps, the filament starts emitting electromagnetic radiation in the visible range with a reddish-yellowish tone and a low color temperature close to 2700 upon being heated by electric current. However, when compared to the energy required to illuminate the bulb, an extremely small amount of energy is spent productively for plant growth, and the lion’s share (90%) of energy is dissipated as heat, so these incandescent lamps must be kept at a minimum distance of 24 inches from the plant foliage to avoid thermal scorching. Incandescent lamps are more often used for flowering plants since their red component could promote the growth of buds and flowers. Also, the lack of light intensity can be managed by increasing the duration of exposure to light or by incorporating some sort of reflective surface. Incandescent lamps are also used to prolong the short days during autumn, winter, and spring. But the problem is that even though incandescent bulbs are cheap in terms

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of their capital cost, they are expensive from an operational point of view. Moreover, the average usable hours (750 h) and energy efficiency of these lamps are also very poor [28]. In the late 1940s, fluorescent lamps were the most popular light sources for photobiological applications, and they encompassed more than 10% of the blue wavelength of the overall light emission within the photosynthetically active radiation spectrum (PAR, 400–700 nm). Fluorescent tubes use low-pressure mercury vapor within the tube, which is not actually environment-friendly when disposed of carelessly. The color temperature of these lamps may vary from 2700 to 10,000 K, and they are available as cool, warm, or full spectrum. These were quite popular, not only because they could give twice the output of incandescent lamps but also because they put out very little heat, which allowed them to be kept near the plants. However, their fluorescent output intensity decreases gradually over time, limiting the growth rate and biomass accumulation in plants [5]. A high-intensity discharge lamp, also called a high-pressure discharge lamp, works on the principle of electric discharge through a gas, and ballasts are required for creating a striking voltage and maintaining the arc [5]. They mainly include metal halide lamps and high-pressure sodium vapor lamps. The former uses an electric arc in the quartz tube as a gas discharge through a mixture of vaporized mercury and metal halide gas and has a high luminous efficacy of around 75–150 lm/w and a life expectancy close to 15,000 years. Slightly different from the former, high-pressure sodium vapor lamps use a small amount of neon and argon gas to start the gas discharge, and once the discharge is started, a dim red or pink light is emitted to warm up the sodium metal, after which the emission sends out bright visible light. A fixed-spectral-output source of light to grow plants productively was made possible with the invention of HIDLs. But the huge thermal radiation output from these lamps necessitates some kind of thermal management system to diffuse the heat generated, thus imposing an extra burden on plant growers [5, 28]. LED, a solid-state form of light production that was invented in the 1960s, was not powerful enough to be used as a full-fledged grow light until the 1980s [29, 30]. It consists of a p–n junction that emits light when electrical current is applied to recombine the electrons, and the holes within the junction are where the energy is released in the form of photons, and the color of the light is determined by the energy band gap in the junction [29]. Nowadays, in order to attain different broadspectrum lights as well as narrow-band lights, we use a phosphor material coated on the top of a monochromatic short-wavelength LED, usually a blue or UV LED. LEDs have a number of advantages over other artificial light sources, including the ability to operate at low voltage direct current, durability, robustness, compactness, and waveband selectivity. Furthermore, it is possible to keep this source closer to plants as LEDs employ heat sinks remote from photon-emitting surfaces. They have a remarkably huge life expectancy of about 100,000 years. Despite the fact that the first generation of LEDs was somewhat expensive, LED fixtures have become the most popular choice for greenhouses and controlled environment agriculture [30]. Table 1 gives a brief idea regarding the various electrical lamps used for plant lighting.

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Table 1 Various electrical lamps used for plant lighting [30] Type of lamp

Incandescent

Fluorescent

HIDL

LED

Nature of spectrum

Broad

Broad

Broad

Narrow

Power efficiency (%)

5

40

30

60

Spectrum tunability

Nil

Nil

Nil

Yes

Heat production

High

Low

High

Low

Lifetime (h)

1000

1000−30,000

10,000−20,000

100,000

Cost

Low

Medium

Medium

Cheap

As previously stated, light quality is critical in determining crop productivity, and agriculture is becoming smarter with the introduction of new engineering and technology. The applications of these phosphors in the agricultural field include pcLEDs for indoor plant lighting, plant habitat-conscious LEDs (PHC-LEDs), lightconverting films for greenhouses, luminescent plants, etc. Figure 2 illustrates a schematic of various phosphors under daylight and suitable excitation. Among them, pc-LEDs are very common in various walks of life owing to their cost-effectiveness, better electricity-to-light energy conversion efficiency, spectral control, relatively low surface temperature, long lifespan, solid-state construction without gas, etc. Horticultural LED fixtures are made up of red (660 nm), blue (450 nm), white, and/or far-red (730 nm) emitting LED patterns. On the contrary, other wavelengths are less common as they have lower efficiencies and efficacies. LED lighting has extended the day and increased the quality and quantity of yield by means of top lighting, inter-lighting, etc. The monochromaticity as well as the controllability of its spectrum are beneficial for the advanced control of plant physiology and morphology. They also impose less thermal stress on plants, even if kept closer, since the amount of radiant heat is relatively lower. In addition, LED-integrated digital control systems can brighten and dim the light output or control the spectral pattern so as to mimic the natural daylight cycle [31]. A comparison of the utilization of light by various photosynthetic pigments with the spectral output of various light sources is shown in Fig. 3. Regarding specific applications such as PHC-LEDs, which are used in dark sites, specific phosphors that lack red or far-red emission are to be developed to inhibit nighttime photosynthesis. This could conserve the indigenous habitat of a natural terrain. Another interesting application is the fluorescent-coated light-converting film that can be used in greenhouses, and this technology is actually in its budding stage. Next is a really fascinating and futuristic technology called luminescent plants or trees, which can illuminate streets and pavements in the near future.

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Suitable Excitations

Fig. 2 A schematic of various phosphors under daylight and under suitable excitation

Fig. 3 A comparison of the utilization of various wavebands of light by photosynthetic pigments with spectral output of various light sources. Reused with permission from [1]

3.2 Agricultural Fields As we know, there are a number of phosphor matrices available on the market that satisfy human color perception. But some specific phosphors are to be developed for specific applications, like plant lighting. Eu3+ ions as activators are very popular for their intense red emissions at around 616 nm, which, being much farther from the deep red region (650–740 nm), are not good for indoor farming [24]. On the other hand, Mn2+ /Mn4+ -activated phosphors such as Ca3 La2 W2 O12 : Mn4+ [32], (Ca, Sr)14 Zn6 Ga10 O35 :Mn4+ [33], (Ca1-x Yx )(Al12-x Mgx ) O19 :Mn4+ [34], SrLaAlO4 :

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Mn4+ [35], Li2 MgZrO4 :Mn4+ [36], Sr9 Gd2 W4 O24 : Mn4+ [37], NaLaMgWO6 : Mn4+ [38], SrMg2 La2 W2 O12 : Mn4+ [39], LiLaMgTeO6 :Mn4+ [40], Ba2 GdTaO6 :Mn4+ [41], Ba2 LaSbO6 :Mn4+ [42], Sr3 LiSbO6 :Mn4+ [43], Li5 La3 Nb2 O12 :Mn4+ [44], Ca2 LaNbO6 :Mn4+ [45], BaAl2 Ge2 O8 :Mn4+ [46] have already been reported to be potential for their use in LEDs meant for controlled environment plant cultivation. This confirms the potential use of Mn4+ -doped phosphors in LEDs for indoor plant lighting. It is also observed that the wide emission band of Mn4+ ions extends in the red or far-red region, which matches well with the absorption bands of the phytochromes. Later, researchers attempted to improve the luminescent properties of these phosphors by incorporating metal ions into them, where some of the metal ions contribute to sensitizing, charge compensation, flux, and so on, thereby increasing the intensity manifolds. One example is the co-doped Mg2+ , Zn2+ , and Ca2+ ions in SrAl3 BO7 :Mn4+ [46]. Here, the metal ions act as charge compensators, and the co-doped matrices seem to have better stability, quantum efficiency, and emission intensity when compared to the singly doped matrices. Some other such phosphors include Li+ , Na+ , K+ , and Mg2+ co-doped Ca2 GdTaO6 : Mn4+ [47], Mg2+ ion codoped CaYAlO4 :Mn4+ , Li+ [48], Mg2+ , and Ge4+ ion-incorporated GdAlO3 :Mn [49], trivalent aluminum-doped Li6 CaLa2 Sb2 O12 :Mn4+ [50], Yb3+ ion-coactivated Gd2 ZnTiO6 :Mn4+ [51], Yb3+ co-doped in Ba2 LaNbO6 :Mn4+ [52], Ca3 (PO4 )2 :Eu2+ , Mn2+ [53], Lu2 GeO5 :Bi3+ , Eu3+ [54], La2 MgGeO6 :Mn4+ , Dy3+ [55], etc. Because of the electron transfer mechanism, the literature survey revealed better photometric results for various metal ion co-doped Mn-activated oxide phosphors. In addition, there seems to be a blue or red shift to the emission peak of Mn ions due to codoping. A specific interest is given to ytterbium-coactivated upconversion phosphors to improve the emission in the NIR region. The electromagnetic spectrum consists of the entire range of frequencies of electromagnetic radiation. It is clear from Fig. 4 that the visible region of the spectrum is just a small part of the whole electromagnetic spectrum, which lies within the wavelength range 380–750 nm, and each color is not a single wavelength but possesses a range of the spectrum. In the case of plant lighting, not only the visible light but also the other parts of the spectrum, such as UV and IR, play important roles. The visible region is the most effective wavelength range, and, within the range of a particular color, the impact of each single or combination of wavelengths on plant growth can be different. The contributions of some of the most important regions of the electromagnetic spectrum are discussed in detail in the forthcoming sections. Red emission is an inevitable wavelength in most steps of the cropping cycle and is the major waveband required for indoor plant cultivation. Also, even if red photons have the lowest intrinsic energy, they are essential for the development of the photosynthetic system as well as for the preparation of various chemical compounds. Among the various photosynthetically active radiations (PAR), red light has the highest quantum yield, and its comparatively low intrinsic energy content makes red light-emitting diodes attain a large emission efficacy (mmol photon output per joule electrical energy input). The typical paradigm has been that red and far-red act oppositely to hamper or actuate shade avoidance symptoms, such as lengthening of stems, leaf orientation, and decreased branching. [56]. However, the larger absorbance of

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Fig. 4 An illustration of the electromagnetic spectrum. Reused with permission from [5]

red light by chlorophyll means that the effect of red emission on shade responses may be overestimated. However, because early-generation red-light arrays used for plant lighting only covered the lower waveband red region (620–630 nm), they played a smaller role in triggering the various photomorphogenic responses. On the contrary, the next generation of LED patterns having top-range red emission (660–680 nm) can be more related to the selective enhancement of vegetative growth. Moreover, the red light-based treatment conditions are reported to generate the highest leaf carbohydrate conditions, induce fresh and dry weight gain, shoot elongation, and leaf dimension [56]. In terms of quantum efficiency for promoting photosynthetic processes, broadband blue emission in the short-wavelength/end of the PAR spectrum is about 25% less efficient than red light [1]. Previous research has shown that plants grown in sufficient red light alone perform much better when a small amount of blue component is added. Relatively compact blue fluorescent light sources were used earlier to recognize the efficacy of blue light in plant development, which were later substituted by blue LEDs as they became more energy-saving and efficient. In some plant species, exposure to blue radiation was critical for initiating chlorophyll synthesis and promoting leaf thickness. The blue waveband is known to comprise approximately one-third of the photosynthetically active radiation from sunlight during midday, whereas the share of blue light required for plant growth under solid-state lighting conditions is much lower than that under normal sunlight. Blue light sources play a variety of important roles in stem elongation, leaf expansion, phototropism, and stomatal aperture, introducing purple pigmentation related to phenolic and anthocyanin production, and increasing the amount of carotenoids, lutein, glucosinolates, minerals, and other phytonutrients in tissues of leafy greens, microgreens, and herbs grown solely with LED lighting [57–59]. Still, achieving dark-green, thick leaves under intense blue light may come at the expense of leaf dimension and plant size. However, as the blue/red ratio of LED lighting increases, plant growth in terms of size decreases, necessitating research on dimmable settings for all regions of light

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wavelengths in general, and blue in particular, in the case of next-generation LED arrays [60]. The broad green emission band within the 500–600 nm range covers the mid-third of the 400–700 nm photosynthetically active radiation spectrum. Green LEDs were overlooked to a great extent due to the misconception that green is not important for photosynthesis in entire leaflets. In fact, its photon efficacy lies between that of blue and red, and certain wavelengths in the green region have a higher quantum yield for photosynthetic action relative to some other wavelengths in the blue regime [61]. Above all, it is interesting to know that one of the first roles of the green band was to make the visual inspection of crops grown under red and blue LEDs easier since it is really tough to perceivably detect stress or disease disorders under such narrow-band red-blue lighting [62]. Furthermore, in multilayered leafy species, the upper leaf layers strongly absorb red and blue light, whereas green penetrates deeply into leaf covers before being absorbed and eventually assisting photosynthesis [61]. Even though the green light regime comes in third on the PAR scale of relative quantum yield, on a whole plant, a green band can enhance overall photosynthetic productivity. Still, more research is needed to improve the properties of green LEDs, as their photon efficacy lags far behind that of blue, red, and white LEDs. Far-red light is very important for the production of higher value vegetables since it prevents aggravation of eating quality and the bitterness caused by the excessive growth of vegetables. Photo-morphogenetic responses that are mediated by phytochromes are usually related to the sensing of the light quality through the red (R) to far-red (FR) ratio [29]. In the case of phytochromes, the ground state is Pr (“P” for phytochromes and “r” for the red-light-absorbing material) with a maximum in the 650–670 nm range. Once a red photon is absorbed, the Pr undergoes a rapid change to form the Pfr state (where “fr” indicates far-red), in which wavelengths in the range of 705–740 nm are preferentially absorbed. The recent availability of far-red LEDs (700–800 nm) with improved photon efficacy offers novel opportunities for plant photocontrol under plant factory artificial lighting. Furthermore, by combining FR light with shorter wavelengths in the PAR band, shorter wavelength photosynthetic efficiency can be increased, resulting in the “Emerson enhancement effect” [63]. As mentioned in the previous section, just like green light, FR penetrates into deeper foliar canopies, where lower leaves are mutually shaded by upper leaves and where the main effects occur. Many flowering plants use the sensitivity of phytochromes to light in the red and far-red regions to regulate the time of flowering and to set circadian rhythms. It also regulates other responses, including the germination of seeds, the elongation of seedlings, the size, shape, and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings [9]. In multiple dimensions, the utilization of narrow-spectrum lighting fixtures from LEDs for crop production made us rediscover and better understand the value of broad-band white light. Because of the complicated array of available choices and interactions, it has long been asked if broad-band white light should be the default LED of choice for growing crops in a plant factory with sole-source lighting. After all, it is a fact that plants evolved their complex pigmentation and photoresponse

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systems under solar radiation, which is a broad-band emission. But the thing is that the intensity, spectral composition, and duration vary diurnally and seasonally, and today’s crop species are genetically and physiologically keyed to respond to such dynamic changes. Moreover, several studies have revealed that the availability of additional photomorphogenic and photosynthetic wavelengths in white light has benefited net photosynthetic productivity. The emissions from white LEDs themselves affect plant physiology and crop production differently depending on whether they are cool, neutral, or warm in tint or color temperature. To be more specific, plants grown under cool-white LEDs generally have more compact and shorter stems and smaller leaves, whereas plants developed under warm-white LEDs are significantly larger, and those under neutral-white LEDs are intermediate in size [4, 60]. Plants are adapted to ultraviolet (UV)-B (280–315 nm) and UV-A (320–400 nm) radiations of the solar spectrum over time [64]. The crop appearance and quality may be different when grown indoors under SSL in the absence of a UV component than when grown outdoors, especially due to the reduction or lack of secondary metabolites and phytonutrients in the former case. But providing UV radiation is a little bit complicated due to the radiation-safety concerns as well as the high cost and inefficiency of the then-current UV LEDs [65]. The future prospect of UV LED being included in the plant factory, artificial lighting is determined by upcoming breakthroughs in UV LED in terms of electrical efficiency and photon efficacy, a database on secondary metabolism studies conducted with UV and other wavebands, and the development of technologies that could meet the safety concerns of workers from UV exposure in plant-growth environments.

3.3 Light Converting Film (LCF) Agriculture facilities based in greenhouses are becoming more popular globally as a result of alarming food shortages and rising food prices. According to the covering materials, there are two kinds of greenhouse facilities: glass and plastic film greenhouses. Moreover, some previous works have proposed that dual-excited and dualemission phosphors could be incorporated into agriculture films for improving solar light harvesting [66]. The sun’s light that reaches the surface of the earth ranges from the ultraviolet (UV) to the infrared (IR). However, only the visible radiation that is between 400 and 700 nm is photosynthetically active in the absence of other wavebands. Fluorescence coatings that can convert less photosynthetically efficient photons to more efficient photons were proposed as part of research to improve the properties of photovoltaic devices. Enhancing the sunlight harvest by doping the agriculture films with luminescent nanoparticles could improve the photosynthesis in greenhouses. When compared to bare agricultural films, fluorescent coatings consisting of nanoparticles with downconversion properties that transfer UV light to visible light are more preferred since UV light is the main problem that leads to agricultural film aging. Because of their high electronic energy levels, rare earth ionbased nanophosphors are ideal for conversion film applications. Another complex

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issue is the high indoor temperature of plastic greenhouses due to IR radiation. The use of light-converting films coated with rare earth ion-doped upconversion nanoparticles that efficiently transfer long-wavelength light to short-wavelength light is one possible solution [67]. The luminescent intensity of upconversion phosphors can be enhanced by using the core/shell construction strategy. The perceived enhancement in plant growth from using luminescent coatings could be further improved if the diffusely reflected light could be directed back toward the plants. This would be possible if the fluorescent coating were used as a reflector rather than a filter. A solar-light-illuminated vertical farm, capturing solar light using parabolic collectors and a light distribution system comprised of splitters, optical fibers, and fluorescent reflectors, is also being proposed as a promising strategy for greenhouses. Sunlight is collected by a parabolic trough collector and transmitted into greenhouse shelves by splitters and optical fibers. Lastly, the output of optical fibers is focused on a reflector having a coating comprised of fluorescent-doped glass to modify the solar spectrum and increase photosynthetic efficiency [68]. In the first stage of the development of light-converting films for greenhouses, organic phosphors were widely used and are still in use. Sm1-x Tbx (TTA)3 phen as surfaces modified by SiO2 , 2-pentylene-4,6-di(1-phenyl))-1, 3butadiene)-1, 3, 5-s-triazine are among them. Red photoluminescent PMMA nanohybrid films for controlling the solar spectrum distribution within greenhouses, fluorescent Eu2+ (M-acryloyt-Oxymethyl-15-crown-5-oligoether) polymer complexes, and so on [68]. However, these organic matrices have limitations such as fast decay time, conversion of only UV light to red light, fast diminishing intensity, and poor matching of their emission spectra and plant absorption wavelengths, which reduces their commercial value even further. To improve the properties of these greenhouse films, more emphasis is being placed on inorganic phosphors, with a particular emphasis on rare earth and other metal ion-doped matrices. They were found to convert ultraviolet and green colors to red and were really economical. But the currently available light-converting agents are relatively hygroscopic and oxidize under sunlight. This leads to an active investigation into the synthesis of new, efficient, and stable inorganic materials. Ce3+ /Eu3+ in CaS phosphor, CaS:Eu2+ thin film, CaS:Eu3+ thin film, CaS:Eu, Sm, CaS:Ce, Sm films, and others are examples of widely used fluorescence coatings [69].

3.4 Plant Imaging Imaging methods in plant tissue culture systems, as well as connecting engineeringplant tissue culture to a new dimension of understanding, are critical to plant research and development. The role of luminescent nanoparticles in imaging plant phenotype and improving photosynthetic efficiency is very crucial due to their tunable luminescence, biocompatibility, low cytotoxicity, and stability [70]. Due to these advantages, they are promising for their application in bioimaging, biosensors, drug delivery, genetic engineering, etc. Luminescent nanoparticles include upconversion

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nanoparticles, carbon dots, silicon nanoparticles, and nanophosphors and are applied for plant imaging in vivo and in vitro. Plant imaging could assist in analyzing the plant cell construction and revealing the process of dynamic metabolic transmission, describing the signal-transmitting process on the membrane, as well as revealing the whole plant tissue formation and nutrition transportation. However, when compared to animal cell imaging, plant cell imaging by luminescent nanoparticles is a less traveled path due to the difficulties encountered due to auto-fluorescence from chlorophyll, carotenoid, and other secondary metabolites [71]. Moreover, the luminescent nanoparticles used for plant imaging should be hydrophilic and have good fluorescent properties to achieve a high signal-to-noise ratio. Another technical bottleneck is that the overlay tissues impede the excitation and emission of light. Using rare earthdoped nanoparticles to create imaging materials like upconversion nanoparticles and photo-stimulated phosphor (PSP) that can be excited by long-wavelength light after being irradiated with UV light is a better way to achieve high signal-to-noise ratio imaging [72]. Parthiban Ramasamy et al. [67] studied the improved upconversion luminescence in Fe3+ co-doped NaGdF4:Yb, Er for bioimaging applications.

3.5 Space Agriculture Light-emitting diodes are a futuristic auxiliary light source for plant growth facilities in space shuttles to evaluate the growth and development of plants for ensuing space bioregenerative life-support systems [73]. Space agriculture is of great importance for research purposes, the astronauts’ diet improvement, and air regeneration inside biological life support systems. LEDs have a near-optimal spectral output for photosynthesis and are available with electrical conversion efficiencies close to the most efficient lamp sources used for plant lighting. Enhancing the properties of blue light-emitting LEDs would enhance the development of an LED plant irradiation system for applications in a space environment [74]. In order to develop and sustain self-sufficient space colonies, plants are the only reliable source to regenerate enough oxygen, recycle water, and produce food. But for plants to be grown in space travel, new, efficient, and compact light sources are to be developed instead of the standard sources that are being used in homes and greenhouses [75]. LEDs were tested for the first time in the Astroculture plant chamber in space shuttles with the aid of NASA funding for the University of Wisconsin’s Center for Space Automation and Robotics (WCSAR) [21]. Following this, patents were taken for using LEDs for plant growth, leading to years of expeditions at Kennedy Space Center and other NASA-funded groups. The past few decades have witnessed a virtual revolution in the use of LEDs as light sources in indoor farming, and this stands as the best ever example of how research for space and extraterrestrials has benefited terrestrial farming.

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3.6 Luminescent Plants Plants are the pristine form of food resource that sustains life on Earth. They are the ultimate provider of resources that make up the basis of fossil fuels that meet the ever-increasing power requirements of the global community. Nowadays, active research is being made to further make use of the power of these living power stations by enabling them to collect sunlight during the day and transduce it into visible emission during the night, thereby materializing those imaginary luminescent trees in public green-glowing parks depicted in the “Avatar” movie. This could, in the future, lead to the replacement of streetlights by luminescent trees. Rare earthbased photoluminescent materials having long afterglow properties are preferred for this application and are in the budding stage of their research and development. A photoluminescent substance consists of crystals of elemental aggregations and deep traps and is distinguished for its photoluminescence due to the emission it gives when excited by light, while its capability to store light photons is characterized by the presence of traps. The above-mentioned merits make them long-lasting phosphors [76]. These phosphors are initially excited with photons, which could be from sunlight, and even after the exciting light is removed, the material will remain excited and continue to emit light originating from the energy stored in the traps. Conventional electric lights devour a huge number of resources and are highly expensive in terms of maintenance. Even though a lot of attempts are being made to generate such glowing living plants via genetic engineering, the very thought of disturbing the delicate balance of biodiversity prevents them from taking that path. Some researchers have reported the development of glowing roots based on photoluminescence as an initial step toward the development of luminescent trees and public green-glowing parks [76]. SrAl2 O4 :Eu2+ , Dy3+ is really promising as a long-lasting phosphor in luminescent plants.

3.7 Fluorescent Carbon Dots for Light Harvesting and Enhanced Photosynthesis Luminescent quantum dots are gaining popularity in a variety of fields as light conversion agents to improve light harvesting from the sun and photosynthetic efficiency [77]. The light conversion potentials of quantum dots of various semiconductors, rare earth-doped upconversion nanoparticles, and metal nanoclusters were well explored for the purpose of plant growth and development. For the past few years, luminescent carbon dots have gained greater interest in horticultural applications due to their lower toxicity and biocompatibility. There are several reports on these nanoparticles, especially far-red or near-infrared quantum dots that harvest light and enhance photosynthesis, thereby increasing crop yield. The thing is that only the visible part of the solar spectrum is utilized by chloroplasts. Several strategies, like genetic engineering, are applied to stimulating the photosynthetic performance in plants, and one

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of the most exciting advancements happens with the very idea of developing a manmade hybrid photosynthetic system like the chloroplast-CDS photosystem that can efficiently exploit the entire solar spectrum [77]. Wang et al. [78] have studied the effect of NIR-CD treatment on the photosynthetic action in Nicotiana benthamiana. They confirmed the absorption of ultraviolet radiation followed by far-red emission, as well as an accelerated electron transfer from photosystem II (PS II) to photosystem I (PS I) and overexpression of specific genes that stimulate photosynthesis further. This would be a great leap in the quest for sustainable plant cultivation practices [78].

4 Conclusions The setting up of a desirable lighting condition for agricultural purposes has gained much interest in the last few decades. Most of the money is spent on lighting equipment when building and running a plant factory. So, making plans, deciding on, arranging, and regulating the system should be properly performed to significantly reduce the cost of lighting, increase its efficiency, and create better lighting effects. To attain these features, the research community has to conduct adequate experiments and trials for different crops under various cultivating environments to arrive at the required optimization of the system. Currently, several leading research teams all over the world are working on improving the intensity of light and spectra for specific applications such as improving photosynthetic and photomorphogenic processes. This made the researchers think of various luminescent materials that could serve to meet the lighting requirements of plant lighting. Large-scale crop production in controlled environments could be aided by the use of LEDs, which use significantly less electricity while providing high-quality spectral parameters. This study is quite relevant at a time when the global community faces a shortfall in open cultivable lands due to the ongoing climate extremes and consequent environmental issues. In this chapter, we have provided a comprehensive treatise on the effects of various lighting sources, various lighting parameters for plant growth, the role of various emission regions on photosynthesis and photomorphogenesis, etc. We have also discussed the importance of developing a multitude of phosphor materials for the purpose of improving the properties of plant-growth LEDs and light-converting films used in greenhouses. The light-converting films play a more significant role by promoting the early ripening of fruits, improving production, and enhancing quality. This is because they emit the blue and red light that is required for photosynthesis when excited by ultraviolet and/or green light, and they can enhance the rate of photosynthetic activity and the judicious use of light. A brief discussion is given regarding the various currently available phosphors for various plant lighting applications. Luminescent carbon dots with multiple benefits have given immense hope for enhanced crop growth. A detailed literature survey has shown that the NIRCDs exhibit immense potential in their applications to increase crop productivity via UV light harvesting and greater photosynthetic efficiency. In the near future, all

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these approaches with phosphor materials will lead to a greater understanding of the spectral quality effects on plant growth and morphology, further pinpointing the importance of phosphor-converted LEDs in plants grown in restricted environments such as growth chambers, greenhouses, vertical farming, and living walls. Future research will be mainly focused on improving the emission intensity, increasing the light conversion efficiency, and reducing the degradation of these phosphors over time. The chapter is concluded with the hope that the future will be illuminated with long-lasting photoluminescent trees and plants that could replace the indoor and outdoor electric lighting systems that devour huge amounts of resources.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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Luminescence Characteristics and Energy Transfer Dynamics of Rare-Earth Ion Co-activated Borosilicate Glasses for Solid-State Lighting Applications Adon Jose, T. Krishnapriya, and P. R. Biju

1 Introduction “It is only when one tries to imagine a world without glass that one realizes how many ways it is used and the extent of our unthinking acceptance of it.” Jo Marshall’s well-known quote captures mankind’s long and intimate association with glasses and glassy materials [1]. The world would be unthinkable without glass. It’s in the spectacles on our faces, the light bulbs in our home, the windows that let us see outside, and many other innumerable objects. Glass is neither a crystalline solid nor a liquid, but rather an amorphous solid material that is prepared through rapid quenching of molten materials. It has transformed the globe more than any other material, and, in many sneaky ways, it’s been the defining material of the human era since primitive times. The pace of innovation in glass research has accelerated dramatically in recent decades because of breakthroughs in our basic knowledge of glass physics and chemistry [2, 3]. Further focus on fundamentals will be crucial to setting the foundations for the next wave of glass innovation as a technologically advanced material to meet some of the world’s biggest hurdles today. From glass windows to lenses to optical waveguides, glasses have renovated how humans interact with the rest of the world, and such innovations in glass science and technology have been the vital instigators of modern civilization throughout history [4]. Furthermore, glassy materials have received much attention for the fabrication of environment-friendly, engineer-safe, and fully recyclable smart materials. Luminescent glass contributes these special properties to photonic applications like lighting and display devices and A. Jose (B) · T. Krishnapriya · P. R. Biju School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala, India e-mail: [email protected] A. Jose St. Stephen’s College, Uzhavoor, Kottayam 686634, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 V. Kumar et al. (eds.), Advanced Materials for Solid State Lighting, Progress in Optical Science and Photonics 25, https://doi.org/10.1007/978-981-99-4145-2_15

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photovoltaics by converting UV light to visible or NIR light, which can be used in cameras, smart windows, and LED-based lighting devices, among many other applications [5]. Steady growth in the field of luminescent materials shown by rare-earth embedded matrices has taken significant strides because of their astonishing applications in up- and down-converting lasers, solar cells, optical fiber amplifiers, flat-panel displays, and optical waveguides [6–11]. In the field of photonics, researchers are looking for novel materials that have better properties than other materials, such as low fabrication costs, a combination of hardness and transparency at room temperature, sufficient strength, high performance, excellent corrosion resistance, and main resistance to thermal and water. As a result of these significant features of luminescent glasses, their number of applications is continuously growing and is progressively being manipulated in combination with other substances for high-tech applications [12–19]. Recently, tunable or multicolor luminescence achieved through energy transfer (ET) between the RE3+ ions doped in a single glassy host has become an interesting topic of research. Other than the characteristic luminescent features, the codoping approach extends the luminescence over a wide range. The fascinating optical features attained through the co-doping of rare-earth ions can be significantly tuned via the occurrence of ET processes and are extensively studied in diverse systems not only for their technological relevance but also for identifying the fundamental mechanisms that underlie these optical phenomena [20–25]. This chapter focuses on the luminescent properties of certain RE3+ ion-activated fluoroborosilicate glasses and the role of the ET mechanism when they are co-doped (doubly and triply) in a single glassy matrix for various photonic applications. Tailoring the luminescence of light in the UV, Vis, and IR ranges has always been a challenge of materials research because of its number of applications in optoelectronic devices [26–30]. Among the diverse routes explored for such applications and research, trivalent rare-earth ion-integrated glasses can be considered an active contender since their applicability includes merits such as optical transparency in the range of interest, chemical solubility, ease of synthesis correlated with a moderately low cost of production, as well as ease of shaping in the desired form depending on the targeted application. In a glassy system, the luminescence of rare-earth ions is homogeneously broadened because of the characteristic distribution of ions within a broad ensemble of local crystal-field sites. The crystal-field effect offered by the host matrix on the rare-earth ion has a notable influence on its optical characteristics, and thus a worthy glass host is very significant for the efficient fluorescence of rare-earth ions [31, 31–33]. Because of their unique and distinct energy-level features, studies based on the optical properties of rare-earth-integrated matrices have sparked a lot of interest. Thus, the present chapter is mainly focused on the concentration-dependent spectroscopic characteristics of various RE3+ ions doped in multicomponent borosilicate glasses. The significant features of rare-earth-activated glassy matrices, such as the possibility of altering compositions in comparison with other fluorescent materials, more homogeneous luminescence, low cost, and greater ease of production, attract more attention to them in the scientific community. Consequently, now a day’s rare-earth incorporated glassy systems became an integral part of photonic

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devices due to their potential application in developing compact solid-state lasers, sensors, high-density optical memory devices, optical broadband amplifiers, and visible display devices [15, 34–37]. Therefore, in this chapter, we have discussed the luminescent characteristics of europium, neodymium, erbium, dysprosium, and samarium ions incorporated with multicomponent borosilicate glasses for photonic applications. The proper glass host is critical in the fabrication of optically active glass matrices for efficient optoelectronic devices. Studies have been reported on the optical traits of oxide-based glass matrices because of their chemical resilience and thermal steadiness. In the present chapter, we have preferred fluoroborosilicate glasses as the glassy matrix due to their better corrosion and heat resistance, greater glass formation capability, high optical transparency, and especially because they are eco-friendly in nature. Moreover, it offers a better luminescent lifetime, large luminescent efficiency, and better durability with the incorporation of suitable activators (rare-earth ions), since it unites the merits of fluorides, borates, and silicates with alkali–alkaline earth metals. Among various glass formers, B2 O3 is regarded as an important glass former and can form a glassy network along with BO3 triangles by creating three-member (boroxol) rings coupled by B–O–B linkages. The incorporation of alkali metal or alkaline-earth metal oxides into the borate glass leads to the formation of complex cyclic groups such as di-, tri-, tetra-, and penta-borate groups. These modifiers can raise the glass transition temperature (Tg ) and decrease the thermal expansion coefficient, while silicate glasses have some peculiarities, such as good mechanical and chemical steadiness, which makes them a universal matrix material. Thus, the grouping of borate and silicate glasses (borosilicate glasses) receives much attention due to its higher transparency in an optical window, better rare-earth ion solubility, chemical durability, refractive index with low dispersion, softening temperature, and moisture resistance [21, 38, 39]. However, these borosilicate glass matrices have an environment with greater phonon energy, which leads to the possibility of nonradiative transitions and a reduction in the luminescence efficiency. This disadvantage of borosilicate glass has been mentioned in several publications. However, it has been reported that heavy metal fluoride (HMF, ZnF2 ) embedded glasses outperform conventional borate, phosphate, and silicate glass matrices. They consist of low phonon energy, wide transparency from near-ultraviolet to mid-infrared, and the capability to inculcate a large amount of rare-earth ions. The occurrence of ZnF2 (HMF) in the glassy matrix enhances the quality of radiative emissions due to its ability to lower phonon energies, and it possibly increases the lifetime of the excited states of the rare-earth ions and the luminescence efficiency without reducing the optical transparency or chemical durability. Also, it is reported that OH absorption in the host matrix strongly decreases due to the reaction of fluorine with the OH group. It is well understood that hydroxyl groups coordinated with the host matrix can have a significant impact on PL traits [28, 40, 41]. Conversely, fluoride glasses have poor chemical and mechanical stability compared with oxide, which restricts their practical applications. Consequently, it is important to combine the advantages of borosilicate and fluoride glass matrices. A glassy host with an alkali–alkaline earth base has

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the possibility of possessing a comparatively longer PL lifetime, large luminescent efficiency, and better durability in addition to its various chemical and physical properties. Hence, the features of alkali–alkaline earth metals, fluorides, and borosilicate glasses are really interesting to integrate since these systems offer benefits, and the limits of each system are minimized. The multicomponent borosilicate glass matrices with fluoride (ZnF2 ) and alkali–alkaline earth metal (K2 O, BaO) content thus seem to be a better glassy system because of their promising characteristics [33, 42].

2 Methods Employed for the Fabrication of the Fluoroborosilicate Glasses The well-known technique for the fabrication of glass is by quenching a melt achieved through the fusion of one or more raw materials. When the rate of cooling is sufficiently fast to bypass crystallization, it is possible to retain the disordered state of the liquid in the solid state. The majority of glasses used for practical applications are synthesized by this method. As in the case of multicomponent borosilicate glasses doped with various rare-earth ions, about 6 g of the batch composition was thoroughly crushed in an agate mortar and the homogeneous mixture was heated in a porcelain crucible in an electric furnace for 1 h at 950 °C until a bubble-free melt was formed. The melt was quickly moved onto a preheated brass mold with spherical channels and annealed at 400 °C for 10 h. Table 1 lists the glass compositions of rare-earth-activated multicomponent borosilicate glasses that will be discussed in this chapter.

3 Energy Transfer Between Lanthanide Ions The majority of the lanthanide ions exhibit sharp luminescence bands owing to intra-configurational f transitions within the f-f manifold. The enhancement of RE luminescence is usually ascribed to the energy exchange between the RE ions and their surroundings. The fundamental energy transfer processes (radiative and nonradiative) that exist in lanthanide-embedded systems are shown in Fig. 1. As in the case of an inductive resonant mechanism or dipole–dipole interaction, the variance of the transfer probability W(R) with changing distance R between the donor and acceptor pair is proportional to R−6 , R−8 , and R−10 . The overlap integral of the donor and acceptor electron clouds is the dependent factor for transfer probability. Also, energy transfer is observed to be effective at a relatively short distance between the ions, so it usually occurs in high activator concentrations. As per Forster–Dexter theory, ET efficiency relies on the area of overlap between the luminescence spectra of the donor atom and the excitation (absorption) spectra of the acceptor [11, 43, 44].

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Table 1 Multicomponent borosilicate glass compositions with glass codes Glass

Glass composition

FBSEU

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (50–x) B2 O3 + xEu2 O3 (x = 0.1, 0.5, 1, 1.5, and 2 mol%)

Synthesis method

Jose et al. [33]

References

FBSND

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (50–x) B2 O3 + xNd2 O3 (x = 0.1, 0.5, 1, 1.5, and 2 mol%)

Jose et al. [38]

E10

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (50–x) B2 O3 + xEr2 O3 (x = 0.1, 0.5, 1, 1.5, and 2 mol%)

Jose et al. [32]

D10

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + 49B2 O3 + 1Dy2 O3

Jose et al. [25]

S10

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + 49B2 O3 + 1Sm2 O3

Jose et al. [25]

ND1E

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (49–x) B2 O3 + 1Nd2 O3 + xEu2 O3 (x = 0.1, 0.25, 0.5, 0.75, and 1)

Jose et al. [21]

DS

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (49–x) B2 O3 + 1Dy2 O3 + xSm2 O3 (x = 0.1, 0.25, 0.5, 0.75, and 1)

Jose et al. [22]

ED

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (49–x) B2 O3 + 1Er2 O3 + xDy2 O3 (x = 0.1, 0.25, 0.5, 0.75, and 1)

Jose et al. [20]

ES

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (49–x) B2 O3 + 1Er2 O3 + xSm2 O3 (x = 0.1, 0.25, 0.5, 0.75, and 1)

Jose et al. [23]

DES

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (48–x) B2 O3 + 1Er2 O3 + 1Dy2 O3 + xSm2 O3 (x = 0.1, 0.25, 0.5, 0.75, and 1)

Jose et al. [25]

ESD

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (48–x) B2 O3 + 1Er2 O3 + 1Sm2 O3 + xDy2 O3 (x = 0.1, 0.25, 0.5, 0.75, and 1)

Jose et al. [24]

DSE

10BaO + 10ZnF2 + 10K2 O + 20SiO2 Melt-quench technique + (48–x) B2 O3 + 1Sm2 O3 + 1Dy2 O3 + xEr2 O3 (x = 0.1, 0.25, 0.5, 0.75, and 1)

Jose et al. [39]

3.1 Non-radiative Relaxation Process Non-radiative transitions emerge through the interaction of RE ions with the vibrations of the host material and are also known as phonon-assisted transitions. The possibility of these kinds of transitions usually depends on the phonon energy and the number of phonons needed to bridge the energy gap. When the electronic states

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Fig. 1 ET processes amid a donor (D) and an acceptor (A). The initial and final levels are represented by green and blue lines, respectively. PD and PDA are the radiative and non-radiative decay rates of the donor, respectively (Reused with permission from Ref. [43] Copyright © ROYAL SOCIETY OF CHEMISTRY)

of the rare-earth ion lie close enough so that the gap can be covered by the absorption or emission of phonons, the process will be fast and effective. The non-radiative transitions can be categorized as follows [45]: (i) multiphonon relaxations; (ii) crossrelaxation; (iii) cooperative up-conversion. Among these, multiphonon relaxations occur when a single phonon is not enough to cross over the variation in energy levels, and thus it can only be bridged via multiple phonons. The cross-relaxation process is simply the energy transfer from one excited ion (donor) to another adjacent non-excited ion (acceptor) in the ground state and results in the simultaneous change of the initial states of both ions to an intermediate energy state. Cooperative up-conversion, on the other hand, occurs when two adjacent ions, say X and Y, are excited. The ion X drives the ion Y to an excited level, and, subsequently, X relaxes to the ground state. The ion Y in its new state can relax radiatively or non-radiatively.

3.2 Fluorescence Resonance Energy Transfer (FRET) The principle of Fluorescence Resonance ET (FRET) depends on the transfer of excitation energy from a donor fluorophore to a nearby acceptor fluorophore in a NR fashion via intermolecular long-range dipole–dipole coupling [46]. In contrast, the following factors play a significant role in FRET: (i) Donor and acceptor molecules must be close to one another (typically 1–10 nm). (ii) The emission spectrum of the donor molecule must overlap with the absorption or excitation spectrum of the acceptor, and the diagram that includes both of these together is referred to as a spectral overlap diagram. (iii) The donor and acceptor transition dipole orientations must be approximately parallel to each other. (iv) The luminescence lifetime of the donor molecule must be of ample duration to allow the FRET to happen. The coupled transitions involved among the donor fluorescence emission and acceptor absorbance in FRET are referred to as the Jablonski diagram [47]. Jablonski diagrams of no-FRET and FRET cases are epitomized in Fig. 2.

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Fig. 2 Jablonski diagram of a no-FRET, and b FRET (Reused with permission from Ref. [47] Copyright © Elsevier)

4 Rare-Earth-Doped Glasses for Solid-State Lighting Applications The luminescent and ET characteristics of RE3+ co-activated glasses primarily depend on the optical absorbance, photoluminescence excitation (PLE), and emission (PL) of the singly embedded glasses. Thus, the absorbance, PLE, and PL spectra of the RE3+ ions (Eu3+ , Nd3+ , Er3+ , Dy3+ , and Sm3+ ) in activated fluoroborosilicate glasses are elaborated in the subsequent section to study the optical characteristics of singly doped glasses. Figure 3a [33] optimizes the absorbance spectrum of trivalent Europium ionintegrated fluoroborosilicate glass (10K2 O + 10BaO + 10ZnF2 + 48B2 O3 + 20SiO2 + 2Eu2 O3 ). Figure 3a depicts nine transitions, five from the 7 F0 level and four from the 7 F1 level, and is labeled with their respective transitions. The observed excitation peaks from the same glass under the λemi of 612 nm are epitomized in Fig. 3b. The spectra illustrate that 7 F1 → 5 L6 transition holds the highest intensity and is thus preferred as the λexc for recording the PL spectra of FBSEU glass samples. The photoemission spectra of FBSEU glasses recorded with 392 nm excitation are shown in Fig. 3c. The spectra display five emission transitions with an intense emission peak at 612 nm (5 D0 → 7 F2 ) [48–50]. From the PL spectra of prepared samples, it can be identified that the 2 mol% of Eu3+ -doped multicomponent borosilicate glass showed higher emission intensity. The optical absorbance spectrum of 1 mol% Nd3+ -integrated multicomponent borosilicate glass (FBSND10) is depicted in Fig. 4a, and the observed bands are

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Fig. 3 a Absorbance spectrum of 2 mol% of Eu3+ -activated glass. b Excitation spectrum. c PL spectra of Eu3+ -doped fluoroborosilicate glasses (Reused with permission from Ref. [33] Copyright © Springer)

assigned to the absorption transitions of Nd3+ ions [38]. The 4 I9/2 → 4 G5/2 transition is found to be the dominant transition in Fig. 4a, which depicts the transition at 584 nm. Figure 4b depicts the PLE spectrum of the FBSND10 sample recorded with an λemi of 1060 nm. The intense PLE bands shown in Fig. 4b correspond to Neodymium ion f-f transitions, and the peaks correspond to 4 I9/2 → 4 F3/2 , 4 F5/2 + 2 H9/2 , 4 F7/2 , 4 F9/2 , 4 G5/2 , 4 K13/2 + 2 G7/2 , 2 K15/2 , 2 P1/2 , and 4 D3/2 + 4 D5/2 transitions, respectively [38, 40, 51–53]. Figure 4c displays the near-infrared PL spectra of Nd3+ −activated multicomponent borosilicate glassy matrices under 584 nm excitation, and it exhibits intense PL peaks owing to 4 F3/2 to 4 I9/2 , 4 I11/2 , and 4 I13/2 transitions. The observed spectra indicate that 1 mol% of Nd3+ integrated glass is the optimal concentration. It is reported that the occurrence of cross-relaxation channels within the glassy system is the main factor for the observed concentrations in the FBSND glasses. The room-temperature absorbance spectrum of an Er3+ –activated fluoroborosilicate glassy sample is epitomized in Fig. 5a and b. The spectrum shows 13 absorption peaks in the Vis–NIR region [25, 32]. The PLE and Vis–NIR photoemission spectra of the 1 mol% Er3+ –activated glassy sample are depicted in Fig. 5c–e. Figure 5d and

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Fig. 4 a Absorbance spectrum of 1 mol% of Nd3+ –doped multicomponent borosilicate glassy sample. b Excitation spectrum. c NIR photoemission spectra of Nd3+ –activated multicomponent borosilicate glasses under λexc = 584 nm (Reused with permission from Ref. [38] Copyright © Elsevier)

e depicts the photoluminescence spectrum examined with an λexc of 379 nm in the visible and near-infrared ranges. Figure 5 shows four emission bands at 410, 526, 546, and 1536 nm, which correspond to (2 G, 4 F, 2 H)9/2 → 4 I15/2 , 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4 I13/2 → 4 I15/2 transitions, respectively [25, 32, 54–57]. The absorbance spectrum of 1 mol% trivalent Dysprosium embedded in fluoroborosilicate glass in the 300–1800 range is shown in Fig. 6a and b. The figure illustrates twelve absorption bands owing to the transitions from the lower state to diverse upper energy states, and among these 6 H15/2 → 6 P7/2 transitions, one was found to be the prominent [25, 58]. Figure 6c and d shows the PLE and PL spectra of the same glassy sample. The PLE spectrum was recorded using a 574 nm λemi , and the intense excitation band was observed at 348 nm (6 H15/2 → 6 P7/2 ) and corresponds to the 6 H15/2 → 6 P7/2 transition [59–63]. The photoemission spectra of the 1 mol% Dy3+ –activated multicomponent borosilicate glassy matrix recorded with λexc of 348 nm depicted in Fig. 6d give three PL peaks at 481, 574, and 665 nm [25, 58].

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Fig. 5 a Ultraviolet–Visible and b Near-infrared absorbance spectrum. c PLE spectrum of 1 mol% of Er3+ –activated multicomponent borosilicate glassy sample. d Visible and e NIR photoemission spectrum of E10 glass sample (Reused with permission from Refs. [25, 32] Copyright © Elsevier)

The optical absorption spectrum of the 1 mol% Sm3+ -integrated fluoroborosilicate glass in the UV–visible and near-infrared ranges is shown in Fig. 7a and b. The spectrum reveals that the 6 H5/2 → 6 P3/2 and 6 H5/2 → 6 F7/2 absorption bands are more intense than those of other bands [25, 64–71]. The PLE spectrum of the same glassy sample recorded with an emission wavelength of 598 nm is shown in Fig. 7c. It is clearly evident from the spectrum that 6 H5/2 → 6 P3/2 transition possesses the greater PLE intensity, and hence it is selected as the λexc for recording the photoemission spectra. The fluorescence spectrum of a 1 mol% Sm3+ activated fluoroborosilicate glassy sample recorded in the region of 450–750 nm under λexc = 400 nm is shown in Fig. 7d. Figure 7 shows PL bands centered at 562, 600, 645, and 707 nm [25, 64]. The overall excitation and luminescence properties of the reported rare-earth-activated fluoroborosilicate glass samples are summarized in Table 2. It was pointed out earlier that the enhanced optical features observed from the codoped glasses achieved via the ET mechanism are mainly dependent on the spectral overlap (SO) of the donor ion’s PL spectrum and the absorbance/PLE spectrum of the acceptor ion [6, 72–78]. This section illustrates the spectral overlap diagrams and the probability of ET between the various RE3+ ions integrated into multicomponent borosilicate glasses. The probability of ET between the RE3+ ions for the synthesis of co-activated glassy matrices can be verified with the help of the SO diagram. In accordance with Forster–Dexter theory (FDT), relevant criteria for the ET mechanism amid the RE3+ ions in a matrix can be confirmed with the SO of the donor ion’s emission and the PLE spectrum of the acceptor ion [79–82]. The SO diagram of Nd3+ excitation and Eu3+ emission in fluoroborosilicate glassy samples is portrayed

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Fig. 6 a UV–Vis. b NIR absorbance spectrum. c PLE and d Photoemission spectrum of 1 mol% Dysprosium-activated fluoroborosilicate glassy system (Reused with permission from Refs. [25, 58] Copyright © Elsevier)

in Fig. 8 [21]. Figure 8a shows the SO of PLE band 4 I9/2 → 4 G5/2 of Nd3 + ion and the PL bands 5 D0 → 7 F1 and 5 D0 → 7 F0 of Europium ion. The energy difference between the 5 D0 state of Eu3+ and the 4 G5/2 state of Nd3+ is 142 cm−1 . These findings validated the energy transfer possibility when these RE3+ ions are embedded in the present glassy system [21]. The spectral overlying diagram (Fig. 8b) reveals that the fluorescence (4 F9/2 → 6 H15/2 ) transition of Dysprosium shows overlap with the PLE (6 H5/2 → 4 I11/2 ) band of Samarium [22]. This SO envisages the ET existence between the 4 F9/2 (Dy3+ ) and 4 I11/2 (Sm3+ ) states, since the energy disparity amid the 4 F9/2 (Dy3+ ) and 4 I11/2 (Sm3+ ) is of the order of 48 cm−1 . The possibility of ET between Erbium and Dysprosium ions when co-activated in multicomponent borosilicate glassy matrices is predicted using spectral overlying diagrams depicted in Fig. 8c and d [20]. Figure 8 depicts two possible ET pathways between the Erbium and Dysprosium ions in a fluoroborosilicate glass system. The SO examined between the PLE transition 6 H15/2 → 4 I13/2 (Dysprosium) and the Erbium emission band (2 G, 4 F, 2 H)9/2 → 4 I15/2 demonstrates the first ET route

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Fig. 7 a UV–Visible and b Near infrared absorbance spectrum. c Excitation spectrum and d Photoemission spectrum of 1 mol% Samarium-activated fluoroborosilicate glassy sample (Reused with permission from Refs. [25, 64] Copyright © Elsevier) Table 2 Excitation wavelengths and luminescence bands of various rare-earth ions embedded in multicomponent borosilicate glass samples Rare-earth ion

Excitation wavelength (nm)

Emission transitions

Eu3+

392

Nd3+

584

Er3+

379

Dy3+

348

Sm3+

400

5D → 7F 7F 7F 7F 7F 0 4, 3, 2, 1, 0 4F 4 4 4 3/2 → I9/2 , I11/2 , I13/2 (2 G, 4 F, 2 H)9/2 → 4 I15/2 , 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 4F 6 6 6 9/2 → H15/2 , H13/2, H11/2 4G 6 6 6 6 5/2 → H5/2 , H7/2 , H9/2 , H11/2

→ 4 I15/2 , 4 I13/2

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Fig. 8 Spectral overlap diagrams of a Eu3+ PL and Nd3+ PLE. b Samarium and dysprosium. c Dy3+ : 6 H15/2 → 4 I13/2 ; Er3+ : (2 G, 4 F, 2 H)9/2 → 4 I15/2 . d Er3+ :4 I15/2 → 4 F7/2 ; Dy3+ : 4 F9/2 → 6H 3+ 3+ 15/2 excitation e Er and Sm singly activated fluoroborosilicate glassy samples (Reused with permission from Ref. [21] Copyright © Elsevier)

(Fig. 8c). The significant overlap shown between Erbium’s 4 I15/2 → 4 F7/2 excitation band and Dysprosium’s 4 F9/2 → 6 H15/2 emission transition constitutes the other ET route and has a higher probability of occurring within the multicomponent borosilicate glassy matrix due to the small energy difference (722 cm−1 ) between the 4 F7/2 and 4 F9/2 states (Fig. 8d) [20]. The SO amid the normalized PL and PLE spectra of Erbium and Samarium singly activated multicomponent borosilicate glasses envisages the ET between these ions and is depicted in Fig. 8e [23]. The emission transition of Er3+ ((2 G, 4 F, 2 H)9/2 → 4 I15/2 ) exhibits an appreciable overlap with 6 H5/2 → (6 P, 4 P)5/2 PLE band of Sm3+ ion, proposing an efficient energy transfer from the (2 G, 4 F, 2 H)9/2 of Er3+ to (6 P, 4 P)5/2 state of Sm3+ in the fluoroborosilicate glassy system [23]. ET is a radiative or nonradiative (NR) process amid two or more diverse rare-earth ions in a host matrix so as to improve lasing or luminescence performance via co-doping. This mechanism is significant in the advancement of novel laser substances and can minimize the threshold energy of laser oscillations in some solid-state lasers. Moreover, the codoping approach also leads to the generation of extended luminescence from a single glassy system that can find significant applications in the lighting sector [79, 83–85]. The reported PL spectra of different co-activated fluoroborosilicate glasses along with partial energy-level diagrams are illustrated in the upcoming section. The visible and near-infrared PL spectra of Nd–Eu co-doped multicomponent borosilicate glasses under λexc = 394 nm are displayed in Fig. 9a. The spectra display five emission transitions in the visible region that correspond to the transitions from the europium ion’s metastable state 5 D0 . Among these, the 5 D0 → 7 F2 band shows

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Fig. 9 a Visible and NIR luminescence (inset) spectra. b Partial energy-level diagram. c NIR luminescence spectra of Nd–Eu co-activated fluoroborosilicate glasses (Reused with permission from Ref. [21] Copyright © Elsevier)

strong, intense red luminescence and has a higher intensity than that of other electricdipole bands. Three intense luminescence transitions from the Nd3+ metastable state 4 F3/2 under 394 nm excitation were also observed in the co-activated glasses [21]. The peaks are assigned as 4 F3/2 → 4 I9/2 , 4 I11/2 , and 4 I13/2 bands, and among these, the 4 F3/2 → 4 I11/2 band is observed as the intense one. Usually, Nd3+ singly activated glass does not show any PL band in the visible region under this excitation wavelength, and it started to display near-infrared PL bands only after the integration of europium ions in the glassy system. Consequentially, through a co-doping approach, the occurrence of both visible and near-infrared fluorescence was realized under λexc = 394 nm. The main reason for this extended luminescence range from the co-activated glasses is attributed to the ET and is portrayed in the partial energy-level diagram (PELD) illustrated in Fig. 9b [21]. Figure 9c depicts the NIR emission spectra of Nd–Eu co-activated fluoroborosilicate glasses irradiated with the characteristic excitation wavelength Nd3+ ions (584 nm). The spectra display a decrement in PL intensity with the rise in the content of europium ions. The observed quenching mechanism

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is mainly attributable to the existence of Nd3+ and Eu3+ ions in the present glassy matrix. The decrement in the PL is ascribed to the cross-relaxation (CR) mechanism from Nd3+ to Eu3+ ions and is also depicted in Fig. 9b. The fluorescence spectra of Dy3+ –Sm3+ co-activated fluoroborosilicate glasses excited at 348 and 400 nm with varying Samarium ion concentrations are shown in Fig. 10a and b. Figure 10a shows bands caused by both dysprosium and samarium ions. The spectra show a decrease in emission intensity corresponding to Dy3+ ion peaks and an increase in Sm3 + ion content. Consequently, the intensity of the Sm3+ PL band is found to have increased, which confirms the presence of NR ET from Dysprosium to Samarium in the glass matrix, whereas the luminescence spectra recorded at λexc = 400 nm show an increase in Sm3+ luminescence transitions (4 G5/2 → 6 H5/2 , 7/2 , and 9/2 ) with increasing Sm3+ ion content [22]. Furthermore, the decrease in Dy3+ luminescence bands (4 F9/2 , 6 H15/2 , and 13/2 ) is examined in Fig. 10b in relation to the increase in Sm3 + PL bands, which is also related to the presence of ET among these ions. The whole PLE and PL transitions, along with the possible ET routes, are illustrated in the partial energy-level diagram depicted in Fig. 10c. The reported work evidences both forward and backward ETs and certain cross-relaxation channels in the Dy3+ –Sm3+ co-activated fluoroborosilicate glassy matrix [22]. Figure 11a and b shows the luminescence spectra of Er–Dy co-activated fluoroborosilicate glasses with varying levels of Dysprosium ion under 348 and 379 nm excitation. Figure 11a illustrates the emission bands of dysprosium ions only and is found to be enhanced with the increasing content of Dy3+ ions in the co-activated glassy system [20]. Figure 11b shows the PL spectra of co-activated glassy samples irradiated at λexc = 379 nm. Interestingly, Fig epitomizes the co-occurrence of both Dy3+ and Er3+ emission bands, and it is significant to notice that under 379 nm excitation only Er3+ ions get excited since Dy3+ ions do not have intense PLE transitions at that particular wavelength. The spectra also describe that, with increasing content of Dysprosium, the intensity of Erbium ion fluorescence decays gradually, and, as a result, the PL intensity of Dy3+ ions tediously improves [20]. The luminescence observed from co-doped glasses under the characteristic excitation of Er3+ ions validated the ET from Er3+ to Dy3+ in the co-doped multicomponent borosilicate glassy matrix and is attributed as the cause of these interesting PL features, as shown in Fig. 11c. The luminescence properties of Er–Sm co-activated fluoroborosilicate glassy samples with varying Samarium ion content were investigated by exciting the codoped glasses at 379 and 400 nm, as shown in Fig. 12a and b. Under 379 m excitation, co-doped glasses show luminescent bands owing to both erbium and samarium ions. From Fig. 12a, it can be concluded that the samarium ions are displaying emission in Er–Sm co-doped multicomponent borosilicate glasses even without the direct excitation of samarium ions. The shift in emission intensity peaks demonstrates the existence of ET from Erbium to Samarium ions. The PL spectra of co-activated glasses under 400 nm excitation are depicted in Fig. 12b, and they only show the characteristic emission peaks of the Sm3 + ion with an absence of emission bands owing to Er3+ . It eliminates the probability of backward ET in the glass matrix, whereas the PL intensity corresponding to Sm3+ emission bands increases with the

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Fig. 10 a PL emission spectra of Dy–Sm co-activated fluoroborosilicate glassy samples under the excitation wavelength of 348 nm. Inset epitomizes CIE diagram and digital image of DS100 glassy sample under 348 nm excitation. b Photoemission spectra of DS glass samples under 400 nm excitation. Inset displays the CIE diagram and digital image of DS100 glass under 400 nm excitation. c PELD of Dy–Sm co-activated fluoroborosilicate glasses (Reused with permission from Ref. [22]. Copyright © Elsevier)

rise in Sm3+ content up to 0.5 mol%, and afterwards a decrement is noticed because of concentration quenching [23]. The overall PLE and PL, along with ET routes, are detailed in the partial energy-level diagram (Fig. 12c). The search for an ideal day like light, has a long history since human civilization depends to a great extent on lighting. Later on, the technology advanced the design and development of new lighting sources. Researchers paid much attention to developing new materials and methods for designing new lighting and other photonic devices. As a result, the development of phosphor-integrated light-emitting diodes was a significant step toward achieving this goal. But it has several disadvantages

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Fig. 11 a PL spectra of Er–Dy co-activated multicomponent borosilicate glassy samples under λexc = 348 nm. Inset portrays the PL spectra of Dy3+ singly activated multicomponent borosilicate glass. b PL spectra of Er–Dy co-activated multicomponent borosilicate glasses at an excitation of 379 nm. Inset portrays the PL spectra of Er3+ singly activated multicomponent borosilicate glass. c Partial energy-level diagram of Er3+ /Dy3+ co-activated multicomponent borosilicate glass (Reused with permission from Ref [20]. Copyright © Elsevier)

[39, 86, 87]. Therefore, an Ultraviolet Light Emitting Diode chip embedded with white light-emitting glass is favorable for preparing WLED bulbs because of their diverse features of better thermal stability, longevity, and higher emission intensity. A mixture of red, green, and blue (RGB) colors emanating from a single material can generate white light. As a result, researchers who have proposed various RE3+ ion single and co-activated materials for this framework frequently use the technique of tailoring the emission by changing rare-earth ions in diverse matrices. The quest for suitable light-emitting materials has focused on triply doped luminescent glassy substances. The advancement of white light luminescence works via co-doping was dependent on ET processes connecting rare-earth ions [25, 37, 88, 89]. Better emission of yellow and blue, reddish-orange, and green from Dysprosium, Samarium, and Erbium ions in diverse works predicts a grouping of these ions in a single host as

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Fig. 12 a Emission spectra of Er3+ –Sm3+ co-embedded fluoroborosilicate glasses under 379 nm excitation and CIE color chart and digital image of ES100 glass under 379 nm excitation. b Photoemission spectra of Erbium–Samarium co-activated multicomponent borosilicate glass samples under 400 nm excitation and CIE color chart and digital image of ES100 glass under 400 nm excitation. c PELD of Er–Sm co-doped glass (Reused with permission from Ref. [23]. Copyright © Elsevier)

an active contender for white light luminescence from luminescent glassy samples. This section elucidates the luminescence investigation on the dependence of each rare-earth ion in this triply doped composition by varying the concentration from 0.1 to 1 mol%, whereas the remaining rare-earth ions are kept fixed at a concentration of 1 mol%. The PL spectra for the DES glasses under ultraviolet and visible light excitations (348 nm (Dy3+ ), 379 nm (Er3+ ), 400 nm (Sm3+ ), and 374 nm (Sm3+ and Er3+ )) are depicted in Fig. 13 (a–d) [25]. Figure 13a shows the luminescence spectra of DES glass samples (1 mol%:Dy3+ and 1 mol%:Er3+ with varying Sm3+ :0.1–1 mol%) at λexc = 348 nm. The spectra exhibit the characteristic peaks of Dy3+ ions, and no other emission peaks were observed. The occurrence of the NR ET process, which occurs from Dysprosium to Samarium ions, was also evidenced by a decrease in emission intensity peaks in the spectra as Samarium ion content increased [25]. The PL spectra at 379 nm are depicted in Fig. 13b. It epitomizes the PL transitions of all three rare-earth ions, and the observed extra PL transition peaks owing to Dy3+ and Sm3+ ions under Er3+ excitation highlight the existence of ET mechanisms amid these rare-earth ions. Figure 13 illustrates that, with a rise in samarium ion content from 0.1 to 1 mol%, the PL intensity of Sm3+ bands is enhanced tediously, whereas the PL intensity of Dy3+ and Er3+ bands portrays a notable fall with the rise in Sm3+ content. Moreover, the dysprosium and samarium emissions are noticeable even without their characteristic excitation wavelengths. The significant decay in PL intensities of erbium and dysprosium is witnessed because of the increased availability of Sm3+ ions to receive the excitation energy. Figure 13c and d also displays

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Fig. 13 Photoemission spectra of DES glass samples under a λexc = 348 nm, b λexc = 379 nm, c λexc = 400 nm, d λexc = 374 nm, and e PELD (Reused with permission from Ref. [25]. Copyright © Elsevier)

the PL spectra of DES glasses irradiated with the excitation wavelengths of 400 and 374 nm, respectively, and they follow the same trend as that of λexc = 379 epitomized in Fig. 13b. The energy transfer pathways from Erbium to Dysprosium, Dysprosium to Samarium, and Erbium to Samarium are attributable to this enhancement in the intensity of Sm3 + luminescent bands. The remarkable decrement in PL intensities of Erbium and Dysprosium is also ascribed to the increased availability of Samarium ions to collect the excitation energy [25]. At lesser rare-earth ion content, interactions among the rare-earth ions are moderately low, whereas along with the increment in rare-earth ion content, the distance among these ions decays, leading to an increase in ET between active rare-earth ions (Er3+ to Dy3+ , Er3+ to Sm3+ , and Dy3+ to Sm3+ ). The presence of characteristic PL transitions of Er3+ , Dy3+ , and Sm3+ ions in the PL spectra points out the existence of ET mechanisms in the glassy matrix and is detailed in the PELD (Fig. 13e) with five possible ET routes [25]. From the luminescence spectra (Fig. 13a–d), it can be seen that with lower Samarium ion content, PL peaks owing to Dy3+ (at 480 and 574 nm) ions appear with maximum intensity when compared with the photoemission bands owing to Erbium and Samarium ions (λexc = 374, 379, and 400 nm). Intense photoemission bands owing to Dysprosium in the PL spectra are related to the existence of strong ET amid Er3+ and Dy3+ in the triply activated glass matrix [25]. The ET ways ETM 1 and ETM 4 are the probable ET processes accountable for this interesting PL effect and are dominant than other ET ways (ETM 2, 3, and 5) in the case of lower Sm3+ concentration. The triply doped glasses exhibited a luminescence enhancement owing to the photoemission peaks of Sm3+ ions with respect to the rise in Sm3 + ion content, whereas the PL bands owing to Er3+ and Dy3+ are found to be diminished simultaneously. These results conclude that ETM 2, 3, and 5 ET pathways get strengthened with respect to the rise in Sm3+ content [25].

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Fig. 14 PL spectra of ESD glass samples under a λexc = 348 nm. b λexc = 374 nm. c λexc = 379 nm. d λexc = 400 nm (Reused with permission from Ref. [24]. Copyright © Elsevier)

The photoemission spectra of Dy3+ /Er3+ /Sm3+ triply integrated fluoroborosilicate glass samples (ESD glasses) with varying concentrations of Dy3+ ion (0.1–1 mol%) were recorded with excitation wavelengths of 348, 374, 379, and 400 nm and are epitomized in Fig. 14a–d, respectively. The glasses show luminescent bands of Dy3+ and Sm3+ ions together at λexc = 348 nm (Fig. 14a). The PL bands owing to the dysprosium ion are observed to be raised, whereas those of the Sm3 + ion are decreased with the rise in content of Dy3+ ions. There are two probable pathways for the examined decay in samarium luminescence intensity. The former describes how, due to the closeness of their energy levels, a redistribution of excitation energy within the samarium and dysprosium ions may occur when these two rare-earth ions are co-activated in a matrix, i.e., there is only a minimum disparity in the energy states of these ions [24]. The photoluminescence (PL) spectral profiles (Fig. 14a–d) of ESD glasses under 374, 379, and 400 nm exhibit similar luminescence characteristics, and they show various fluorescent crests in the range of 400–700 nm that correspond to Dy3+ , Er3+ , and Sm3+ ions. From the figures, it can be seen that the photoluminescence arising because of Dysprosium ion enhances with the raise in Dy3+ concentration, and this

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rise in PL intensity of Dy3+ ion also affects the luminescence intensity of Erbium and Samarium ions. The considerable decay in Er3+ and Sm3+ PL intensity with the rise of Dy3+ content is related to ET lies in the ESD glass matrix [24]. The ET mechanisms have been also illustrated in the PELDs (Fig. 13e). The PL profiles (Fig. 14a–d) of ESD glass samples under four different excitations (348, 374, 379, and 400 nm) with constant fixed Erbium and Samarium ion content and altered Dy3+ ion content evidence the presence of possible ET amid these ions and figure out that via triple-doping tunable luminescence (Green, Blue, Yellow, Reddish-orange) from a single matrix could be attainable and is advantageous for white light generation. PELD illustrating the ET pathways are epitomized in Fig. 13e. The decrement in the intensity of Sm3+ and Er3+ PL bands with increasing Dy3+ content may be assigned due to the supremacy of ETM 1, ETM 4, and ETM 6 ET pathways and is also related to the redistribution of excitation energy amid these ions that we described earlier. Along with the ETM 1 and ETM 6 ET pathways, a backward (ETM 7: Dy3+ (4 F9/2 ) → Er3+ (4 F7/2 )) and forward energy transfer (ETM 5: Dy3+ (4 F9/2 ) → Sm3+ (4 I11/2 )) is also present in this triply activated glass matrix. The noticed rise owing to Dy3+ luminescence bands in every recorded PL spectrum corroborates that the contribution of these ETs (ETM 5 and 7) is relatively lesser than the other proposed ET ways (ETM 1, 2, 3, 4, and 6) [24]. From the results, we can say that, when the content of Dy3+ increases from 0.1–1 mol%, the energy transfer routes enhance the emission from Dy3+ ions. The backward ET (ETM 6) from Sm3+ to Dy3+ contributes a significant role in this luminescence enhancement of Dy3+ in this triply activated glass matrix [24]. Photoemission spectra of the Dy3+ /Er3+ /Sm3+ triple-activated fluoroborosilicate (DSE) glassy samples with varying concentrations of Er3+ ions recorded with different λexc (348, 374, 379, and 400 nm) were portrayed in Fig. 15a–d. Figure 15a displays the emission profile of DSE glasses with λexc of 348 nm (Dy3+ ), and it only possesses the emission bands of Dy3+ ions [39]. The luminescence spectra (Fig. 15b) of DSE glasses irradiated with a λexc = 374 nm show a wide range of Dy3+ , Sm3+ , and Er3+ ion PL transitions from 400 to 700 nm. It is also shown in Fig. 15b that the PL intensity of the dysprosium and samarium bands drops off with the increase in erbium concentration, i.e., the PL peak appears because the Er3+ ion enhances with an increase in Er3+ content, while that of the Dy3+ and Sm3+ ions decreases simultaneously [39]. DSE glassy matrices irradiated with 379 nm show the PL bands of Dy3+ , Sm3+ , and Er3+ ions as depicted in Fig. 15c. Under 379 nm excitation, we have examined the co-existence of Er3+ , Dy3+ , and Sm3+ PL bands in single fluorescence spectra. The 480, 574, and 665 nm transitions of Dy3+ and 600 and 646 nm transitions of Sm3+ are found to decrease with increasing Er3+ ion content, while the Er3+ ion band at 546 nm is continuously increased [39]. Figure 15d depicts the photoluminescent emission profile of DSE glass samples irradiated with a λexc of 400 nm. It was discovered that the Er3+ ion peak at 546 nm increased tediously with the increase in Er3+ content, while all other peaks of Dy3+ and Sm3+ decreased in intensity. The 400 nm excitation is also regarded as a possible excitation wavelength for Er3+ ions. Thus, the increase in Er3+

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Fig. 15 Photoemission spectra of DSE glass samples under a λexc = 348 nm, b λexc = 374 nm, c λexc = 379 nm, and d λexc = 400 nm (Reused with permission from Ref. [39]. Copyright © Wiley)

PL intensity is attributable to this rise in Er3+ concentration [39]. The photoluminescence spectra (Fig. 15a–d) of the DSE glass matrix show that the PL bands caused by Er3+ ions only show an increase in PL intensity with increasing Er3+ concentration, whereas those caused by Dy3+ and Sm3+ ions show a significant decrease. The overall excitation and emission in the DSE glasses are illustrated in Fig. 13e. This substantial decay with a rise in Er3+ content highlights the existence of Sm to Dy and Dy to Er ET pathways followed by ETM 1, 2, 3, 4, and 5 in the DSE glass matrix. Thus, a back transfer from Sm3+ :4 I11/2 to Dy3+ :4 F9/2 state (energy disparity = 48 cm−1 ) may occur in the DSE glass matrix and is portrayed as ETM 6 in Fig. 13e. Furthermore, an ET from Dy3+ : 4 F9/2 to Er3+ : 4 F7/2 (ETM7) may occur in the glass samples (energy disparity = 722 cm−1 ) as reported by Zhang et al. in Er/Dy co-activated glass matrices. The phonon energy of this fluoroborosilicate glass is 990 cm−1 . In both cases (ETM7 and 6), the energy difference is less than the host’s phonon energy [39]. This highlights the possibility of ET existence in the DSE glass matrix from Sm to Dy and Dy to Er, i.e., upon irradiation, Er to Dy, Dy to Sm, and Er to Sm ETs may happen initially in the DSE glass matrix, followed by Sm to Dy and Dy to Er ETs. A considerable increase in the intensity of Er3+ , PL transitions is not observed from

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the recorded spectra and is attributed to the forward and backward energy transfers from the Erbium ions, i.e., at lower erbium ion concentrations, Er to Dy, Dy to Sm, and Er to Sm ET pathways (ETM 1, ETM 2, ETM 3, ETM 4, and ETM 5) dominate in the glassy system, whereas a rise in erbium content improves backward ET pathways (ETM6 and 7) and is regarded as the main cause of the fall in Dy and Sm PL intensities in the recorded PL spectra [39]. The detailed investigation reliant on the concentration and excitation-dependent fluorescence of Dy3+ /Er3+ /Sm3+ triply activated fluoroborosilicate glasses by varying the concentrations of Dy, Sm, and Er ions showed interesting luminescence features in the visible region. The Sm3 + ion variation in the triply doped composition results in tunable emissions of orange and white light via the dominance of ETM 2, 3, and 5 energy transfer pathways from the as-prepared glasses. The Dy3+ ion variation of 0.1–0.75 mol% in the triply doped system has increased due to Dy3+ luminescent bands in all recorded emission spectra, while Er3+ and Sm3+ ion peaks have decreased. The remarkable luminescence enhancement (rise in the PL intensity owing to Dy3+ ion peaks and a fall in that of Er3+ and Sm3+ ions) with respect to the rise in Dy3+ ion content explored the dominance of ETM 1, 4, and 6 ET routes and is also helping to achieve the visible luminescence, which corresponds to reddishorange, orange, and different shades of white light, whereas the visible luminescence of greenish-white, orange, and white light is observed from the triply doped glasses with varying concentrations of Er3+ ions (0.1–0.75 mol%) and is mainly attributable to the dominance of ET routes ETM 6 and 7 in the prepared glasses. Moreover, it is significant to notice that the luminescence characteristics shown by the triply doped glass with 1 mol% of Dy, Er, and Sm ions (DES100, ESD100, or DSE100) illustrate the existence of all these 7 energy transfer routes. As in the case of DES100 (ESD100 or DSE100) glass, due to the heavier dopant ion concentration, a quenching in luminescence characteristics may happen. This will result in the enhancement or diminution of the luminescence corresponding to each rare-earth ion with respect to the dominance or strength of certain energy transfer pathways. It is observed that for DES100 (ESD100 or DSE100) glass, under 348, 374, and 379 nm excitations, the luminescence peaks correspond to the Dy3+ ion and are more intense than those of the Er3 + and Sm3 + ions, which describes the fact that the energy transfer routes ETM 1, 4, and 6 play a significant role in the luminescence shown by the DES100 (ESD100 or DSE100) glass, whereas under 400 nm excitation of the same glass, the luminescence owing to Sm3 + ions becomes more prominent. It is the characteristic excitation wavelength of Sm3 + ions and has the tendency to excite more Sm3+ ions. The systematic investigation based on luminescence characteristics clearly aided in understanding the role of each rare-earth ion in the triply doped composition for the observed extended visible luminescence. The concentration and excitation-dependent tunable luminescence from the Dy3+ /Er3+ /Sm3+ triply activated fluoroborosilicate glasses result in the visible luminescence of cool/warm white light, reddish-orange, orange, and greenish-white light that can be found in both indoor and outdoor lighting applications.

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Recent works related to the optical characteristics of doped and undoped borosilicate glasses give some promising results that may potentially emphasize their suitability in various optoelectronic applications [49, 59, 90–96]. Interestingly, Cerro et al. [97] reported persistent luminescent borosilicate glasses using direct particle doping for the very first time. The glasses were synthesized via a direct doping method. Commercial PeL SrAl2 O4 :Eu2+ and Dy3+ microparticles (MPs) were added to the borosilicate glass after melting [97]. The recent work of Al-Hazmi et al. investigates the impact of sunlight on the optical features and light field parameters of nickel borosilicate glasses. The work evidently illustrated the impact of sunlight (solarization) on the stability, optical features, and ligand field parameters of glasses used for solar cells and smart windows [98]. The influence of CaO content on the structural, chemical, and optical properties of Sm3+ -doped calcium sodium borosilicate glasses was investigated by Ceniceros-Orozco et al., who concluded that the photoluminescence intensity of Sm3+ -doped borosilicate glasses is increased with the addition of CaO [99]. Singla et al. investigated the incorporation of gold nanoparticles to improve the optical response of rare-earth-doped borosilicate glasses. The work reported an increase in the Y/B ratio observed in the Dy3+ /Au co-doped bismuth borosilicate glass from 4.3:1 to 6.2:1 after gold nanoparticle addition [100]. Yin-Po designed the Ce3+ , Tb3+ , and Sm3+ doubly and/or triply activated borosilicate glass to reveal the influence of rare-earth (RE) dopants on their controllable and adjustable structure, luminescence, and color [101].

5 Conclusions Modern science and technology and its development with innovative concepts mostly depend on the invention of novel materials that can easily replace conventional materials and devices with better characteristics and efficiency. This thought imparts greater momentum for developing new materials and devices useful to society. Materials emitting in the broad spectral range from UV–Vis to IR, in particular, have enormous potential and have established some practical applications. Research has been carried out to fabricate high-power laser sources in the NIR wavelength for diverse practical applications. Moreover, rare-earth-incorporated glasses have received significant research interest in the scientific community due to their enchanting luminescence features. Among these, multicomponent borosilicate glasses are worthy due to their unique characteristics like a lower melting point, high transparency, better chemical durability, thermal stability, and good rareearth solubility compared to the other glasses. Through the successful incorporation of various rare-earth ions and their co-doping, it was possible to extend the luminescence range of the rare-earth-activated multicomponent borosilicate glasses and, in that way, generate tuneable luminescence from a single host material. This chapter evidently illustrated the photoluminescence characteristics of Eu3+ , Nd3+ , Er3+ , Dy3+ , and Sm3+ ions doped and co-activated with fluoroborosilicate glasses.

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The energy transfer routes are sufficiently validated and illustrated on the basis of partial energy-level diagrams. The work presented in this chapter paves the way for various methods and choices for luminescent materials and applications. Following the results of white light generation, it is planned to fabricate W-LEDs based on glasses by improving the color characteristics with the appropriate amount of doping. Exploration of up-conversion luminescence in RE3+ -doped glass can lead to the development of biomedical sensors. An intriguing area of research is the effect of double and triple doping in up-conversion processes. Moreover, future work is ongoing in the direction of the fabrication of channel waveguides via femtosecond laser writing in these glasses and the study of amplification and propagation characteristics via the waveguides. Plasmonic sensors can also be made by integrating metal nanoparticles into the glassy host. The fabrication of optical planar waveguides by ion exchange is also a topic of interest. The dielectric characteristics of the glass may also be studied for storage applications. It is observed that the rare-earth ions embedded in oxide glasses are used in many functional applications. As a result, systematic investigation into the fundamental properties of these rare-earth-embedded glasses will greatly expand the capabilities of solid-state lighting devices.

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