Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences 978-953-51-2681-2

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences Edited by Mark T. Stauffer

Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences Edited by Mark T. Stauffer

Published by ExLi4EvA Copyright © 2016 All chapters are Open Access distributed under the Creative Commons Attribution 3.0 license, which allows users to download, copy and build upon published articles even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. After this work has been published, authors have the right to republish it, in whole or part, in any publication of which they are the author, and to make other personal use of the work. Any republication, referencing or personal use of the work must explicitly identify the original source. As for readers, this license allows users to download, copy and build upon published chapters even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. Notice Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published chapters. The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book.

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Contents

Preface

Chapter 1 Fourier Transform Infrared and Raman Characterization of Silica-Based Materials by Larissa Brentano Capeletti and João Henrique Zimnoch Chapter 2 Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy by Lin Sun, Lingcong Shi and Chunrui Wang Chapter 3 Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and Scanning Electron Microscopy by Pedro R.S. Prezas and Manuel P.F. Graça Chapter 4 Raman Spectroscopy, a Useful Tool to Study Nuclear Materials by Laura J. Bonales, Jone M. Elorrieta, Álvaro Lobato and Joaquin Cobos Chapter 5 Infrared Spectra and Density Functional Theoretical Calculation of Transition Metal Oxide Reaction with Monochloromethane by Yanying Zhao, Xin Liu and Shuang Meng Chapter 6 Vibrational and Electronic Structure, Electron-Electron and Electron-Phonon Interactions in Organic Conductors Investigated by Optical Spectroscopy by Andrzej Łapiński Chapter 7 Infrared and Raman Spectroscopic Characterization of Porphyrin and its Derivatives by Metin Aydin and Daniel L. Akins

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Chapter 8 Novel Pressure-Induced Molecular Transformations Probed by In Situ Vibrational Spectroscopy by Yang Song Chapter 9 Conformational Analysis of Molecules: Combined Vibrational Spectroscopy and Density Functional Theory Study by Partha P. Kundu and Chandrabhas Narayana Chapter 10 Geometric and Electronic Properties of Porphyrin and its Derivatives by Metin Aydin and Daniel L. Akins Chapter 11 Applications of Molecular Spectroscopic Methods to the Elucidation of Lignin Structure by Tingting You and Feng Xu Chapter 12 Using Fluorescence Spectroscopy to Diagnose Breast Cancer by Tatjana Dramićanin and Miroslav Dramićanin Chapter 13 Applications of 1H Nuclear Magnetic Resonance Spectroscopy in Clinical Microbiology by Lara García‐Álvarez, Jesús H. Busto, Jesús M. Peregrina, Alberto Avenoza and José Antonio Oteo Chapter 14 Improving Food Safety by Using One- and Two-PhotonInduced Fluorescence Spectroscopy for the Detection of Mycotoxins by Lien Smeesters, Wendy Meulebroeck and Hugo Thienpont Chapter 15 Microprobing Structural Architecture Using Mid-Infrared Vibrational Molecular Spectroscopy by Yuguang Ying and Peiqiang Yu Chapter 16 Fluorescence Spectroscopy for the Analysis of Spirit Drinks by Jana Sádecká, Veronika Uríčková and Michaela Jakubíková

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Chapter 17 Laser Spectroscopy in Hollow‐Core Fibers: Principles and Applications by Philip G. Westergaard Chapter 18 Enhanced Molecular Spectroscopy via Localized Surface Plasmon Resonance by Lu Sun, Ping Chen and Lie Lin Chapter 19 Using Raman Spectroscopy for Characterization of Aqueous Media and Quantification of Species in Aqueous Solution by Ivana Durickovic

Preface The goal of this book is to present an overview of applications of molecular spectroscopy to investigations in organic and inorganic materials, foodstuffs, biosamples and biomedicine, and novel characterization and quantitation methods. This text is a compilation of selected research articles and reviews covering current efforts in various applications of molecular spectroscopy. Sections 1 and 2 deal, respectively, with spectroscopic studies of inorganic and organic materials. Section 3 provides applications of molecular spectroscopy to biosamples and biomedicine. Section 4 explores spectroscopic characterization and quantitation of foods and beverages. Lastly, Section 5 presents research on novel spectroscopic methodologies. Overall, this book should be a great source of scientific information for anyone involved in characterization, quantitation, and method development.

Chapter Provisional chapter1

Fourier Transform Fourier Transform Infrared Infrared and and Raman RamanCharacterization of Silica-Based Materials Characterization of Silica-Based Materials Larissa Brentano Capeletti and Larissa Brentano Capeletti and João Henrique Zimnoch João Henrique Zimnoch Additional information is available at the end of the chapter

Additional information is available at the end of the chapter http://dx.doi.org/10.5772/64477

Abstract Fourier Transλorm Inλrared and Raman are powerλul techniques to evaluate silica and hybrid silica structure. It is possible to evaluate the silica network λormation alonμ the hydrolysis and condensation reactions in terms oλ siloxane rinμs λormation and Si–O(– Si) anμle deλormation due to the introduction oλ orμanic μroups, the employed synthetic route or encapsulated species interaction. The siloxane λour- or six-membered rinμs imply in a more riμid or lexible network, respectively, in order to accommodate the orμanic μroups. “ structural analysis oλ the materials is oλ hiμh importance, since interactions between the encapsulated molecules and the matrix are critical λor the device perλormance, such as sensors. This type oλ device needs the permeation oλ an analyte to activate the encapsulated receptor molecules inside the silica structure. Fourier transλorm inλrared spectrometry can be also used to determine parameters oλ the silica network as a λunction oλ the hydrophilicity/hydrophobicity deμree and the siloxane rinμ structure with respect to thin ilm porosity. This silica structural analysis is reviewed alonμ the text in a tentative oλ beter explorinμ the data resultinμ λrom these powerλul techniques. In addition, the λunctionalization oλ silica structures by the use oλ orμanoalkoxysilanes, which is important to the creation oλ hiμh-speciic materials, can be well described by these two complementary techniques. The Si–C bonds and the maintenance oλ the orμanic substituents such as methyl, octyl, octadecyl, vinyl, phenyl, aminopropyl, mercaptopropyl, isocyanatopropyl, iodopropyl, chloropropyl and μlicydoxypropyl could be evaluated aλter the sol-μel synthesis process. The literature reμardinμ silica vibrational spectroscopy is also explored creatinμ a data bank oλ wave numbers λor the most important bonds λor diferent types oλ silica and hybrid silica materials obtained by diferent synthetic routes. Keywords: hybrid silica, molecular imprintinμ, silica-based materials, FTIR, Raman

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. Introduction on silica-based materials Silica-based materials have a wide ield oλ applications nowadays, since it is very lexible in terms oλ material characteristics and λabrication methods [1]. Diferent types oλ devices such as catalysts, chromatoμraphic phases, sorbents, sensors, coatinμs, etc. can be produced with tuned properties to enhance activity and/or robustness. In terms oλ catalysts, several diferent reactions can take advantaμe oλ the silica surλace usaμe, resultinμ in heteroμeneous processes. The hydroμen production, λor example, needs hiμh surλace area supports, open porosity, nanostructure with uniλorm morpholoμy, hiμhly and relatively uniλorm dispersed active phase, which can be achieved by a silica matrix [2]. In addition, very complex structures can be desiμned as ratle-type maμnetic silica composite with nonporous silica-coatinμ maμnetic iron oxide encapsulated in mesoporous silica hollow sphere which can also contain active metallic nanoparticles (Figure A), resultinμ in a Pt-based catalyst λor hydroμenation that exhibits hiμh activity, selectivity and excellent reusability [3]. Complex hierarchical structures can be also obtained with independent λunctionalization oλ macropore and mesopore networks on the basis oλ chemical and/or size speciicity afords control over the reaction sequence in catalytic cascades [4]. The catalyst preparation strateμies also include the addition oλ other metal oxides to the silica network: in desulλurization process, the prepared mixed oxide would take advantaμe oλ both titania, probably as the main active component, and silica, λor its hiμh thermal stability, excellent mechanical strenμth and hiμh surλace area [5] and the same approach can be employed λor photocatalytic practices [6]. The mixed oxides can also be used as polymerization catalysts, where the presence oλ oxides such as WO3, CrO3 and MoO3 in the silica network decreased the necessary cocatalyst amount, suμμestinμ that the support nature has a considerable inluence on the process [7].

Figure . “) Ratle-type maμnetic silica composite with nonporous silica-coatinμ maμnetic iron oxide encapsulated in mesoporous silica hollow sphere which can also contain active metallic nanoparticles [3]. ”) Molecular imprintinμ process adapted with permission λrom Zhao et al. [9].

For chromatoμraphic phases and sorbents, the main maneuvers are the so-called molecular imprintinμ or the silica λunctionalization with μroups that retain the analytes. Takinμ into consideration the second approach, it is possible to desiμn hybrid silica monoliths λunctionalized with, λor example, aminopropyl or cyanopropyl μroups and utilize them as selective stationary phase λor microextraction by packed sorbent (MEPS). This method could determine druμs, such as antipsychotics in combination with antidepressants, anticonvulsants and

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anxiolytics in plasma samples λrom schizophrenic patients throuμh liquid chromatoμraphytandem mass spectrometry (LC-MS/MS) in the multiple reactions monitorinμ (MRM) mode [8]. On the other hand, the molecular imprintinμ is a methodoloμy which creates cavities λor the analytes encapsulatinμ the analyte itselλ or a template within the silica network that is λollowed by an extraction process (Figure B) [9]. The resultinμ material is oλ hiμh speciicity, increasinμ the selectivity and perλormance oλ the sorbent/phase. This methodoloμy has been employed to extract important compounds such as the β-N-methylamino-L-alanine amino acid λrom cyanobacteria which is hypothesized to be linked to amyotrophic lateral sclerosis and Parkinson dementia complex λrom people livinμ in Guam island [10] and to pretreat, detect and analyze trace levels oλ toxic pyrethroid insecticides in soils [9]. Diferent types oλ sensors can also be prepared takinμ advantaμes oλ silica materials’ lexibility. Optical sensors use to employ the methodoloμy oλ a receptor element encapsulation within the silica network and the structural properties and addition oλ orμanic μroups can be correlated with the device perλormance [11]. They λrequently include the encapsulation oλ an orμanic dye which can chanμe color when in contact with the analyte [12]. The introduction oλ mixed oxides and orμanic μroups can be oλ hiμh importance in this ield to avoid leachinμ oλ these dyes durinμ usaμe [13]. It is also possible to manuλacture diferent devices’ coniμuration such as nanosensors λor pH measurement [13], optical ibers λor volatile orμanic compounds’ detection [14], electrochemical [15], electrochemiluminescence [16] and biosensors [17]. Furthermore, silica is commonly used to protect or μive special λeatures to surλaces. In terms oλ protection, orμanosilanes are well known as corrosion protectors λor metallic surλaces such as steel and aluminum alloys [18], where other metal such as cerium [19], metallic nanoparticles such as NiFe2O4 [20] and other compounds such as phosphonic acid [21] can also be added to the coatinμ to enhance the protection. The surλace characteristics are another λeature that are able to be tuned by the silica coatinμs. In this ield, it is possible to mention the superhydrophobicity that is widely explored with the possibility oλ selλ-cleaninμ surλace creation [22, 23] and also the addition oλ antimicrobial properties is oλ hiμh importance [24, 25]. Thus, the chapter shall be structured accordinμ to the λollowinμ subitems: .

Introduction on silica-based materials. “ panorama on the diferent applications oλ such devices (catalysts, chromatoμraphic phases, sorbents, coatinμs…) should be provided, illustratinμ recent examples λrom the literature (2014–2015). Some comments on the μeneral aspects oλ their production (synthetic routes based on sol-μel routes, μraλtinμ reactions, encapsulation via nonhydrolytic sol-μel processes, molecular imprintinμ).

.

Organic groups on silica surface. Recent examples oλ the use oλ FTIR and FT-Raman spectroscopies in the monitorinμ oλ surλace reactions between silanol μroups and liμands λor orμanic and orμanometallic compounds. The possibility oλ distinμuishinμ liquid-like and crystalline coniμuration λor lonμ-chain alkyl μroups λrom the C–H stretchinμ vibrations position.

.

Molecules within bulk silica. The use oλ FTIR in monitorinμ the encapsulation oλ molecules within silica network by deconvolution oλ Si–O stretchinμ reμion. The correlation between siloxane λour- or six-membered rinμs and device characteristics/properties.

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.

Silica molecular imprinting. The use oλ FTIR and Raman in the monitorinμ oλ cavity interaction between templates/tarμet molecules with λunctional μroups λrom silica pores.

.

FTIR and Raman modes. Discussion on the complementary inλormation provided by samplinμ accessories and detection modes in the characterization/evaluation oλ hybrid silica materials, namely: atenuated total relectance (“TR), DRIFTS, photoacoustic spectroscopy (P“S), inλrared emission spectroscopy (IRES), micro-FTIR and Raman.

. Organic groups on silica surface Recent examples oλ the use oλ FTIR and FT-Raman spectroscopies in the monitorinμ oλ surλace reactions between silanol μroups and liμands λor orμanic and orμanometallic compounds. The possibility oλ distinμuishinμ liquid-like and crystalline coniμuration λor lonμ-chain alkyl μroups λrom the C–H stretchinμ vibrations position. The orμanic μroups’ presence and interactions are monitored in diferent types oλ materials, as the ones discussed above. In terms oλ corrosion protection coatinμs, the orμanosilanes are widely known that due to their eicient properties as couplinμ aμents, representinμ an interestinμ and environmentally λriendly alternative in the ield oλ surλace treatments [26]. One oλ the employed orμanosilanes is the μlycidyloxypropyltrimethoxysilane (GPS) that when mixed with methyltriethoxysilane can improve coatinμ resistance, charμe transλer resistance and present low-λrequency impedance parameters. FTIR was employed by Foroozan et al. [18] to investiμate the reaction between μlycidyl μroups oλ GPS molecules with silanol μroups. It was possible to identiλy bands relectinμ the epoxy rinμ breathinμ around ~910 and 840 cm−1 and also a new band appeared near 1730 cm−1 λor C=O stretchinμ that could be associated with the oxidation oλ the epoxide rinμ. The spectra conirmed a stronμ network reticulation as a result oλ the reaction between μlycidyl μroups oλ GPS molecules with hydrolyzed silanes. However, the complimentary results oλ water contact anμles decreased probably due to hiμher amount oλ –CH–OH, produced in the reaction between μlycidyl and silanol μroups, so they reached an optimum point at which a more reticulated structure overcomes the silane layer hydrophilicity. Silanol μroups are also employed to investiμate the μraλtinμ oλ catalytic compounds at the silica surλace. Ochędzan-Siodłak et al. describes a catalytic system where metallocenes and postmetallocene compounds are immobilized in an ionic liquid modiied silica surλace. The modiication process could be λollowed by the Si–OH stretchinμ vibration, at 3700 cm−1, disappearinμ aλter the ionic liquid modiied silane reaction with the silanol μroups (Figure A) [27]. This strateμy is becominμ popular nowadays, where diferent orμanosilanes are irst used to modiλy the surλace with speciic chemical μroups with which is possible to μraλt the catalysts species or precursors itselλ [28, 29]. Similarly, the Si–OH vibration can be used to λollow direct μraλtinμ oλ catalyst in the silica surλace, where the silanol μroups can react with a metallic center producinμ chemical bond between the catalyst compound and the silica surλace. Li et al. [30] describes the μraλtinμ oλ a nickel complex in silica and alumina surλaces by λollowinμ a band at 3745 cm−1, which is assiμned to isolated surλace hydroxyl μroups drops μradually with

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials http://dx.doi.org/10.5772/64477

reaction time and almost completely vanishes aλter 24 h and correspondinμly the absorption bands at 3070–2877 cm−1 related to C–H stretchinμ vibrations oλ the catalyst allyl μroups steadily raise in intensity (Figure B). Capel-Sanchez et al. [31] also investiμated the μraλtinμ by silanol μroups and in the development oλ a sinμle site titanium on an amorphous silica surλace, they report that the titanium precursor is preλerentially anchored over the silica surλace by the bridμinμ hydroxyl μroups (broad band around 3500 and 3700 cm−1) over the isolated ones (~3700 cm−1).

Figure . Monitorinμ oλ Si–OH stretchinμ vibration, at 3700 cm−1, disappearinμ aλter (“) the μraλtinμ oλ a nickel complex in silica and alumina surλaces [27] and (”) the ionic liquid modiied silane reaction with the silanol μroups [30].

”y usinμ Raman spectroscopy, there is also the possibility oλ inal conλormation studies in the case oλ hybrid silica with lonμ alkyl chains at the silica surλace. Structure and order inλormation about alkane-based systems can be obtained λrom multiple indicators in their Raman spectra, especially in the ν(C–H) reμion between 2750 and 3050 cm−1. “lso, siμniicant conλormational order inλormation exists λor alkane systems in the ν(C–C) and δ(C–H) reμions between 900 and 1500 cm−1. However, it is necessary some care about luorescence phenomena interλerences in Raman around this reμion [32]. Usinμ this methodoloμy, ”rambilla et al. evaluated the μauche and trans conλormation oλ hybrid silica with diferent octadecyl μroups (octadecylsilane [ODS]) content by Raman spectroscopy. The two bands centered at 1080 and 1062 cm−1 are assiμned, respectively, to ν(C–C) λor gauche e trans conλormation oλ the ODS alkyl chains. The ratio in intensity oλ these two bands was evaluated in order to monitor the inluence oλ the TEOS/ODS molar ratio in the orμanic μroups’ behavior. For all ratios, the intensity between the two bands laid above 1, meaninμ there is a predominance oλ trans conλormation in comparison to gauche one, indicatinμ thereλore an intense molecular orμanization in the hybrid silica prepared by the sol-μel method. In addition, it was λound that the orμanization deμree oλ alkyl chains decreases with the ODS content increase, with data λrom 29Si-NMR and FTIR detected in atenuated total relectance (“TR) mode, corroboratinμ to the indinμs [33].

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. Molecules within bulk silica The FTIR technique can also be employed to evaluate silica network characteristics such as hydrophilicity/hydrophobicity deμree and siloxane rinμ structure reμardinμ thin ilm porosity [34, 35]. In terms oλ sensors, these λeatures may impact the analyte interaction and access to the encapsulated molecules, known as receptor elements, within the silica matrix. Iλ the encapsulated molecules’ interaction with the silica network occurs throuμh their active sites, it is possible that these active sites are not available to λurther interact with the analyte. Reduced sensor perλormance can also occur iλ the silica network is nonpermeable. In this case, the silica network itselλ limits the pathways the analyte can travel to reach the encapsulated receptor molecules hinderinμ the reaction. Consequently, both cases could reduce the sensor perλormance or completely disable it [36]. Silica materials have a prominent band correspondinμ to the Si–O–Si bond asymmetric stretchinμ in the reμion λrom 1300 to 1000 cm−1. Literature reports that the maximum centers and relative intensities oλ the lonμitudinal optic (LO) and transversal optic (TO) modes oλ this bond are shiλted with the introduction oλ chemical μroups or orμanic molecules in the silica network [35]. So, a complete analysis oλ its components can be conducted, includinμ the deconvolution oλ the LO and TO modes in their relative main contributions: the λour-membered (SiO)4 and six-membered (SiO)6 siloxane rinμs (Figure A), resultinμ in a total oλ λour components (LO6, LO4, TO6 and TO4) (Figure B). Usually, materials with hiμher content oλ chemical μroups or orμanic molecules use to present hiμher λormation oλ less stressed sixmembered rinμ, thereby allowinμ a beter accommodation oλ the nonreactive orμanic μroups [37]. Furthermore, there is a correlation between the λormation oλ six-membered rinμs and an increase in the relative deμree oλ crystallinity, as well as with the lonμ-ranμe orμanization in hybrid silica materials that is normally observed with an increase in the deμree oλ matrix alkylation [38].

Figure . “) Two oλ the most common cyclical arranμements oλ SiO4 structural units in xeroμels: λour-membered siloxane rinμ (SiO)4 above and six-membered siloxane rinμ (SiO)6 below. ”) ”and deconvolution to asymmetric stretchinμ ν(Si–O(–Si)) bond [39].

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials http://dx.doi.org/10.5772/64477

Usinμ this approach, a series oλ silica-based acid-base optical sensors prepared by encapsulatinμ pH indicators usinμ three diferent sol-μel routes was investiμated [36]. The employed routes were: nonhydrolytic, acid-catalyzed and base-catalyzed and the pH indicators were alizarin red, brilliant yellow and acridine. The FTIR spectra were perλormed λor all the materials and the peak correspondinμ to the Si–O–Si asymmetric stretchinμ was deconvoluted and their respective components analyzed and Figure assembles the results. For the acidic and nonhydrolytic routes, a positive correlation between the pH indicator content and the increase in (SiO)6 percentaμe was established, thereby indicatinμ the silica network structure rearranμement in order to accommodate the indicator molecules. Usinμ a basic route, the reached indicator contents were notably low and so this relationship was not observed. In addition, no relationship between the (SiO)6 percentaμe and the response time could be established in spite oλ less dense networks, with biμμer rinμs, miμht render easier the analyte permeation. This behavior may indicate that the analyte probably accessed the receptor elements throuμh the passaμes between the siloxane rinμs and not throuμh the siloxane rinμs themselves [36].

Figure . Comparison oλ (SiO)6 percentaμe () and encapsulated indicator content () in each sample [39].

It is remarkable that dependinμ on the sol-μel route, the rate between hydrolysis and condensation reactions will chanμe with the pH medium and can explain the behavior oλ the basiccatalyzed sensor material. Under basic conditions, the condensation reactions oλ silanol μroups are stronμly accelerated, and the particles are rapidly λormed. Thus, the probability oλ rearranμement as a λunction oλ the presence oλ other molecules, as pH indicators λor example, durinμ the synthesis decreases, since the tridimensional network is quickly λormed. When the

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condensation reactions are slower, such occurs in acid pH, the network can be more inluenced by the addition oλ molecules to be encapsulated [39].

. Silica molecular imprinting The molecular imprintinμ methodoloμy involves a molecular recoμnition process where the analyte can recoμnize and preλerentially bind to speciic sites built by usinμ a template oλ the tarμet molecule durinμ the matrix network λormation. “λter an extraction process in order to remove the template, the resultinμ material is bulk silica with cavities that are morpholoμically and stereochemically compatible with the analyte. Thereλore, an option oλ analytical technique to λollow this procedure is FTIR. It is possible to track the template interaction with the silica network by the template bands appearance and/or SiO2 vibrations chanμe. With this approach, Morais et al. [40] describe the interactions oλ diferent druμs such as luoxetine, μentamicin, lidocaine, morphine, niλedipine, paracetamol and tetracycline with silica matrix durinμ the preparation oλ molecular imprintinμ materials. “ll these druμs present nitroμen atoms as primary, secondary or tertiary amines that can interact with silanol μroups. The investiμation was perλormed comparinμ the vibrations oλ bare druμ with the encapsulated druμ, beλore removal by extraction. The silica spectrum presents intense bands in the reμion oλ ~3500 cm−1 assiμned to the O–H vibrations oλ silanol μroups and adsorbed water; and in the reμion oλ 1200–800 cm−1, where the Si–O stretchinμs are observed. “s a result oλ these stronμ bands and the λrequently low concentrations oλ the encapsulated compounds, it is common to observe an overlappinμ oλ their siμnals by the silica ones avoidinμ this type oλ evaluation [36]. “s an example, Figure A illustrates the spectra oλ lidocaine with its main bands at 1675 cm−1 is assiμned to the ν(C=O) oλ the amide chemical μroup, 1655 and 1546 cm−1 atributed to δ(C– N–H) oλ the amine μroup and the band at 1476 cm−1 to the δ(C–CH3) oλ the methyl μroup; and Figure B shows the spectra oλ the lidocaine-silica composite where the main bands oλ the druμ can still be observed. However, the bands assiμned to the carbonyl stretchinμ and amino bendinμ modes were shiλted to 1683 and 1639 cm−1, respectively, suμμestinμ a potential medicine-silica network interaction throuμh these chemical μroups. Similar behavior was observed λor the other druμs, considerinμ the machine resolution oλ 4 cm−1. Most oλ the pharmaceutical presented inλrared band shiλts toward hiμher wave numbers (bathochromic shiλt) when encapsulated, indicatinμ they are interactinμ with the silica structure, resultinμ in a rearranμement oλ the chemical μroups, which was conirmed by the rotational isomerism oλ the molecule. In addition, some oλ the bands were shiλted λor lower wave numbers (hypsochromic shiλt) relectinμ a tension increase in the molecule rotational conλormation, since the encapsulation process may incur in diiculty oλ the λunctional μroups vibrational movements demandinμ more enerμy. Most oλ the nitroμen-related bands, λor all samples, had their wave numbers shiλted. These results denote possibility oλ a hydroμen bondinμ interaction throuμh electron donation between these μroups and silica network, as illustrated by Figure . “monμ the studied druμs, the exception was tetracycline which presented a shiλt in the OH μroup deλormation band and so indicatinμ an interaction by this μroup [40].

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials http://dx.doi.org/10.5772/64477

Figure . FTIR spectra oλ bare lidocaine (“) and the respective encapsulated system (”) [40].

Figure . Proposed interactions oλ the druμs with silica network [40].

In other approaches, hybrid silica networks have been used to improve both the process oλ molecular imprintinμ as the λollowinμ usaμe oλ the material as an extraction matrix. Han et al. reports the λunctionalization oλ silica with amino μroups provided by aminopropyltriethoxy-

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

silane to help interaction with the toxic herbicide pentachlorophenol [41]. ”y usinμ FTIR to monitor this process, they were able to identiλy the N–H bond around 1560 cm−1 and C–H bond around 2935 cm−1, suμμestinμ the –NH2 μraλtinμ onto the activated silica μel surλace. In this case, imprinted and nonimprinted sorbents showed similar location and appearance oλ the major bands, relectinμ the already mentioned problem oλ overlappinμ bands with the major bands oλ silica network. Similar behavior was observed by Chrzanowska et al., Ren et al. and Li et al. [42–44]. The irst one employed the λunctionalization oλ silica nanoparticles surλace with aminopropyl μroups to promote the encapsulation oλ biochanin “, producinμ a selective solid-phase extraction oλ biochanin “, daidzein and μenistein λrom urine samples [42]. “naloμously, Ren et al. employed the same procedure with aminopropyl μroups, althouμh the tarμet analyte was bisphenol “ [44]. Finally, the later one made use oλ propylthiocyanate μroups to modiλy the silica surλace, creatinμ a selective phase λor selective removal oλ cadmium(II) competinμ with copper, zinc and lead in aqueous solution [43]. In all the cases, the assistinμ orμanic μroups’ bands were detected; however, the molecular imprinted and nonimprinted spectra were really similar.

. FTIR and Raman modes When analyzinμ hybrid silica materials, sometimes it is necessary to use complementary techniques to beter evaluate the materials’ characteristics. The same occurs λor vibrational spectroscopy methods. “ wide investiμation was perλormed with a series oλ diferent hybrid silica prepared with tetraethoxysilane (C0), methyltriethoxysilane (C1), octyltriethoxysilane (C8), octadecyltrimethoxysilane (C18), vinyltrimethoxysilane (Vy), phenyltrimethoxysilane (Ph), mercaptopropyltrimethoxysilane (SHp), isocyanatepropyltriethoxysilane (NCOp), chloropropyltrimethoxysilane (Clp) and μlycidoxypropyltrimethoxysilane (Gp) [45]. Usinμ FTIR, the main bands oλ silica were well determined λor all the hybrid silicas and they showed shiλts dependinμ on the orμanic μroup presentinμ at the network, althouμh the orμanic μroups’ bands were barely seen. Usinμ Raman spectroscopy, the orμanic μroups were well described and some oλ the silica network bands were also observed. The reμion around 3600–3000 cm−1 is atributed to hydroxyl μroups ν(O–H) stretchinμ modes. The shoulder at ~3600 cm−1 matches the O“H vibrations associated with alcohols that are a subproduct oλ sol-μel reaction, while the maximum at ~3425 cm−1 is related to surλace –OH participatinμ oλ hydroμen bonds. Water is also described here as a shoulder at ~3230 cm−1, which is also observed at ~1630 cm−1. “s mentioned beλore, silica presents a characteristic reμion oλ peaks λrom 1250 to 700 cm−1 that can provide structural characteristics oλ the network. Specially, when related to the main bands between 1250 and 1000 cm−1 correspondinμ to the asymmetric ν(Si–O–H) and their deconvolution on LO at ~1130 cm−1 and TO at 1047 cm−1 modes. The Si–O(H) bond stretchinμ appears at sliμhtly diferent positions in the FTIR (~950 cm−1) and Raman (~980 cm−1) spectra. The symmetric mode oλ ν(Si–O–Si) band is λound at ~791 (FTIR) and ~799 cm−1 (Raman), while the Si–O− rockinμ mode was observed at ~540 cm−1 (IR). Finally, the Raman spectra also show the siloxane rinμ breathinμ mode (with 3 or 4 SiO units) located at ~490 cm−1 [45].

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials http://dx.doi.org/10.5772/64477

The silica-related bands presented wave number shiλts dependinμ on the employed orμanic μroup in the diferent hybrid silicas. For Si–O(–Si) LO mode, the larμest shiλt occurs λrom the nonhybrid to the hybrid samples. In this case, shiλts to lower wave numbers use to be related to the network deλormation in order to accommodate the orμanic μroups within the inorμanic silica matrix resultinμ in larμer siloxane rinμs and μreater Si–O–Si anμles and lonμer Si–O bond lenμths. LO and TO mode shiλts occur mainly near the surλace oλ the material, which can be beter detected employinμ atenuated total relectance (“TR) mode oλ FTIR spectroscopy. In addition, the orμanic μroups’ introduction can oriμinate λrom heteroμeneous reμions that may introduce local deλormations in the network resultinμ in the diferences observed λor Si–O bond lenμths and Si–O–Si anμles in the diferent hybrid materials. “nother interestinμ behavior is reported to Si–O(H) band, considerinμ that, in a μeneral way λor this case, the wave number shiλts result λrom hydroμen bond λormation with the silanol μroups. “ siμniicantly hiμher wave number occurred λor C18 while the lower ones occurred λor Clp and Gp [45]. The very hydrophobic C18 orμanic chain can hinder hydroμen bond λormation by comparison with the other samples, thus, the Si–O bond lenμth is decreased and the wave number is shiλted to hiμher values. However, the μroups Clp and Gp can λacilitate hydroμen bond λormation with the silanol μroups and, as a consequence, the Si–O bond lenμth is increased and the vibrational wave number is decreased [46]. “s mentioned beλore, the characterization oλ the orμanic μroups oλ hybrid network is beter done usinμ Raman spectroscopy than FTIR. The last one can observe only some bands, while Raman presents a series oλ them showinμ the complementarity oλ both techniques λor hybrid materials’ characterization. Table compiles some important assiμnments reμardinμ the orμanic μroups and their respective wave numbers detected by both techniques, when applicable [45]. The complementary use oλ FTIR and Raman spectroscopies can be also employed to deeply investiμate the processes takinμ place durinμ sol-μel process and there are other types oλ detection modes λor these vibrational techniques as DRIFTS, P“S, IRES, micro-FTIR and Raman which can help. DRIFT spectroscopy is primarily used on samples where most oλ the relected radiation is difused. It is important that the specular relectance is reduced to a minimum because it distorts the DRIFT spectrum and lowers the band intensities [47]. Durinμ the last years, it has become the most efective technique λor studyinμ the processes takinμ place at the μas-solid interλace [48]. Ivanovski et al. investiμated the reμion oλ the OH stretchinμ vibrations oλ silica μel activated at diferent temperatures λor the purpose oλ checkinμ the availability oλ the OH μroups λor λurther reaction with 3-aminopropyltrimethoxysilane (“PTMS) molecules and whether chemisorption was successλul, indinμ evidence whether chemisorption oλ “PTMS involves all available methoxy μroups. It is also possible to investiμate the conλormation oλ the aminopropyl μroups (“PS) backbone on the silica μel surλace and the possible proton transλer between the NH2 μroups oλ “PS and OH λrom silica μel detected by the λormation oλ NH3+ and SiO− which is spectroscopically detectable throuμh the appearance oλ the δ(NH3+) vibrations at 1668 cm−1 [49]. In addition, ”ukleski et al. developed a direct quantitative method oλ quantiication oλ maximal chemisorption oλ 3-aminopropylsilyl μroups on silica μel usinμ

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

DRIFT spectroscopy, as (“PS) modiied silica μel plays an important role as a precursor λor λurther modiications, where “PS acts as a spacer or bridμinμ molecule. ”y inteμratinμ the spectra in the λrequency ranμe oλ the ν(CH2)/ν(CH3) vibrations between 3014 and 2808 cm−1, the mass λraction oλ “PTMS oλ 19.04% was λound to correspond to a maximal concentration oλ “PS on silica μel oλ 2.23 µmol m−2, which was conirmed by elemental analysis λor carbon [47]. Assignment

C

C

C

C–H(3) asym

2979 R

2955 I

2957 I

C–H(3) sym

2916 R

C–H(2) asym

C–H(2) sym

2884 R

2883 R

2932 R

2930 R

Vy

SHp

NCOp

Clp

Gp

2928 R

2939 R

2962 R

2926 R

2926 I

2918 I

2942 I

2945 I

2960 I

2935 I

2861 R

2850 R

2894 R

2897 R

2901 R

2894 R

2856 I

2848 I

2880 I

C–H arom Si–C

Ph

3058 R 1412 R

1122 R

1280 I CH2 bend

1463 R

1460 R

1457 I

1468 I

C–C

1064 R

1062 R

1013 R

C=C

1603 R

=C–H term

3072 R

=C–H term bend

1412 R

999 R

1278 R (Si)C–H C–H bend

2991 R 1432 i

Rinμ breath

737 I

Rinμ deλ

698 I

1260 R

S–H

2574 R

C–S

652 R

C=N

1553 I

N=C=O

1449 R

H–C–Cl deλ

1412 R

C–Cl(H)trans

645 R

OC–H

1456 R

*asym: asymmetric; sym: symmetric; bend: bendinμ; term: terminal; deλ: deλormation; R: Raman; and I: inλrared. Table . Orμanic μroup bands detected by the complementary techniques oλ FTIR and Raman [45].

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials http://dx.doi.org/10.5772/64477

The photoacoustic spectroscopy (P“S) FTIR is a broad-applicable mid-inλrared solution when samples present opacity problems [50]. It is a unique extension oλ IR spectroscopy which combines the utility oλ interλerometry with the standard sample-μas microphone oλ the photothermal technique λor depth-proile analysis oλ materials. Its siμnal μeneration processes automatically and reproducibly isolates a layer extendinμ beneath the sample surλace which has suitable optical density λor analysis without physically alterinμ the sample. P“S involves measurement oλ acoustic wave (pressure oscillations) in a hermetically sealed cell ited with a very sensitive microphone. The microphone siμnal, when ploted as a λunction oλ wavelenμth, contains a spectrum proportional to the absorption spectrum oλ the sample. The wave μeneration λollows absorption oλ liμht, which is modulated at a λrequency in the acoustic ranμe, by the sample. Most FTIR instruments provide modulation λrequencies between 50 and 500 Hz in the 400–4000 cm−1 wave number span [51]. Thereλore, this approach can be employed to evaluate silica samples as described by Gao et al. to widely explore a polyethylene-co-Znacrylic acid hybrid material prepared by the sol-μel process. They could identiλy both vibrations λor silica and the orμanic μroup. ”y investiμatinμ the silica bands it is noted that not only silica but also other λorms oλ silicon μroups are λormed via the sol-μel reaction, while the existence oλ the Si–OH μroup indicates that the condensation reaction was not completely inished. However, analyzinμ the peaks reλerrinμ to the orμanic μroups, it was reported that –CH2– and –CH3 positions in the reμion oλ 2800–3000 cm−1 remain the same aλter hybrid material λabrication, indicatinμ no chemical bond between silicon and these two μroups λorms aλter the sol-μel reaction. The band at 1700 cm−1 is the C=O stretchinμ mode λrom the –COOH μroup was also noticed λor both materials. This is due to unneutralized acrylic acid. While the transparency oλ the hybrid is a result oλ stronμ orμanic-inorμanic interaction, and a hiμh deμree oλ mixinμ, no evidence oλ hydroμen bondinμ is observed λrom the FTIR peak position [52]. Inλrared emission spectroscopy (IRES) is a method in which a sample is enerμized by heatinμ and so on, and the inλrared liμht emited λrom the sample is measured to obtain a spectrum. It utilizes the contrast between the sample and the base material with possibility oλ an improved siμnal-to-noise ratio compared to absorption spectroscopy. Ideally only photons emited by the sample are detected (“zero backμround”), λree λrom the noise produced by the continuum lamp in an absorption experiment. This improvement in sensitivity is particularly useλul λor the spectroscopy oλ transient molecules because oλ their intrinsically low concentrations [53]. In the case oλ hybrid silicas, this methodoloμy can be successλully employed to evaluate the thermal stability oλ materials. ”rambilla et al., λor example, describe the behavior oλ octadecylsilane (ODS) hybrid silicas prepared by μraλtinμ and sol-μel methods with a spectra series λrom 100 to 900°C between 4000 and 500 cm−1. The decrease oλ physically adsorbed water band and the surλace silanol interactions, leadinμ to an increase in the band at 3747 cm−1, atributed to isolated silanol μroups was observed. For temperatures hiμher than 450°C up to 900°C, the band assiμned to isolated silanol μroups (3747 cm−1) is reduced due to condensation reactions and structural reorμanization with μeneration oλ siloxane μroups. Concerninμ the ν(C–H) stretchinμ reμion, the bands at 2963, 2929 and 2855 cm−1 are reduced in the ranμe oλ 250–550°C, as a result oλ the thermal deμradation oλ ODS chains. Figure presents the relative band area λor this vibration. It was possible to compare the thermal stability oλ hybrid materials

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

prepared by μraλtinμ (GR100) and sol-μel method (SG10“), conirm the hiμher thermal stability oλ the irst one [54].

Figure . Relative area oλ ν(C–H) bands versus temperature λor μraλtinμ prepared (GR100) and sol-μel prepared (SG10) hybrid silicas [54].

Finally, micro-Raman and micro-FTIR modes have less sensibility, however, they are key methods employed when spatial resolution oλ a λew micrometers is necessary. It can be employed, λor example, to evaluate radial distribution oλ the ictive temperature in pure silica optical ibers [55], porous silica supports λor individual livinμ cells [56] and phenyl-bridμed polysilsesquioxane positive and neμative resist λor electron beam lithoμraphy where the technique helped to propose a description oλ the tone switchinμ mechanisms [57].

. Final remarks Observinμ all the procedures and results described above, it is noticeable that a vibrational spectrum is not collected only to simply evaluate peak positions anymore. Nowadays, it is possible to obtain deep inλormation about materials’ λormation, evolution and structure, as well as to acquire spatial resolution spectra or in hiμh-resolution modes with low siμnal/noise ratios. The reaction chemical processes λollowinμ methods are μetinμ more and more speciic, as collected data are more and more exploited in order to μive the maximum results inλormation, brinμinμ λast advance in the materials characterization ield.

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials http://dx.doi.org/10.5772/64477

Author details Larissa ”rentano Capeleti and João Henrique Zimnoch* *“ddress all correspondence to: [email protected]λrμs.br Chemistry Institute, Federal University oλ Rio Grande do Sul, Porto “leμre, ”razil

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Graλted onto Silica or “lumina: “ Molecularly Dispersed Nickel Precursor λor Syntheses oλ Supported Small Size Nickel Nanoparticles. Chemical Communications, 50 (2014) 7716–7719. DOI: 10.1039/c4cc02962c. [31] M.C. Capel-Sanchez, G. ”lanco-”rieva, J.M. Campos-Martin, M.P. de Frutos, W. Wen, J.“. Rodriμuez, J.L.G. Fierro, Graλtinμ Strateμy to Develop Sinμle Site Titanium on an “morphous Silica Surλace. Lanμmuir, 25 (2009) 7148–7155. DOI: 10.1021/ la900578u. [32] C.J. Orendorf, J.E. Pemberton, Raman Spectroscopic Study oλ the Conλormational Order oλ Octadecylsilane Stationary Phases: Efects oλ Electrolyte and pH. “nalytical and ”ioanalytical Chemistry, 382 (2005) 691–697. DOI: 10.1007/s00216-005-3133-4. [33] R. ”rambilla, G.P. Pires, N.P. da Silveira, J.H.Z. dos Santos, M.S.L. Miranda, R.L. Frost, Spherical and Lamellar Octadecylsilane Hybrid Silicas. Journal oλ Non-Crystalline Solids, 354 (2008) 5033–5040. DOI: 10.1016/j.jnoncrysol.2008.07.031. [34] R.M. “lmeida, T.“. Guiton, C.G. Pantano, Detection oλ LO Mode in V-SiO2 by Inλrared Difuse Relectance Spectroscopy. Journal oλ Non-Crystalline Solids, 119 (1990) 238–241 DOI: 10.1016/0022-3093(90)90847-λ. [35] “. Fidalμo, L.M. Ilharco, Chemical Tailorinμ oλ Porous Silica Xeroμels: Local Structure by Vibrational Spectroscopy. Chemistry – “ European Journal, 10 (2004) 392–398. DOI: 10.1002/chem.200305079. [36] L.”. Cappeleti, E. Moncada, J. Poisson, I.S. ”utler, J.H.Z. Dos Santos, Determination oλ the Network Structure oλ Sensor Materials Prepared by Three Diferent Sol–μel Routes Usinμ Fourier Transλorm Inλrared Spectroscopy (FT-IR). “pplied Spectroscopy, 67 (2013) 441–447. DOI: 10.1366/12-06748. [37] “. Fidalμo, R. Ciriminna, L.M. Ilharco, M. Paμliaro, Role oλ the “lkyl-“lkoxide Precursor on the Structure and Catalytic Properties oλ Hybrid Sol–μel Catalysts. Chemistry oλ Materials, 17 (2005) 6686–6694. DOI: 10.1021/cm051954x. [38] S.L.”. Lana, “.”. Seddon, X-Ray Difraction Studies oλ Sol–Gel Derived Ormosils ”ased on Combinations oλ Tetramethoxysilane and Trimethoxysilane. Journal oλ Sol-Gel Science and Technoloμy, 13 (1998) 461–466. DOI: 10.1023/a:1008685614559. [39] L.”. Capeleti, J.H.Z. Dos Santos, E. Moncada, Dual-Tarμet Sensors: The Efect oλ the Encapsulation Route on pH Measurements and “mmonia Monitorinμ. Journal oλ SolGel Science and Technoloμy, 64 (2012) 209–218. DOI: 10.1007/s10971-012-2849-9. [40] E.C. Morais, G.G. Correa, R. ”rambilla, C. Radtke, I.M. ”aibich, J.H.Z. dos Santos, The Interaction oλ Encapsulated Pharmaceutical Druμs with a Silica Matrix. Colloids and Surλaces ” – ”iointerλaces, 103 (2013) 422–429. DOI: 10.1016/ j.colsurb.2012.10.059. [41] D.M. Han, G.Z. Fanμ, X.P. Yan, Preparation and Evaluation oλ a Molecularly Imprinted Sol–μel Material λor On-Line Solid-Phase Extraction Coupled with Hiμh Perλormance

Fourier Transform Infrared and Raman Characterization of Silica-Based Materials http://dx.doi.org/10.5772/64477

Liquid Chromatoμraphy λor the Determination oλ Trace Pentachlorophenol in Water Samples. Journal oλ Chromatoμraphy “, 1100 (2005) 131–136. DOI: 10.1016/j.chroma. 2005.09.035. [42] “.M. Chrzanowska, “. Poliwoda, P.P. Wieczorek, Surλace Molecularly Imprinted Silica λor Selective Solid-Phase Extraction oλ ”iochanin “, Daidzein and Genistein λrom Urine Samples. Journal oλ Chromatoμraphy “, 1392 (2015) 1–9. DOI: 10.1016/j.chroma. 2015.03.015. [43] Z.C. Li, H.T. Fan, Y. Zhanμ, M.X. Chen, Z.Y. Yu, X.Q. Cao, T. Sun, Cd(II)-Imprinted Polymer Sorbents Prepared by Combination oλ Surλace Imprintinμ Technique with Hydrothermal “ssisted Sol–Gel Process λor Selective Removal oλ Cadmium(II) λrom “queous Solution. Chemical Enμineerinμ Journal, 171 (2011) 703–710. DOI: 10.1016/ j.cej.2011.05.023. [44] Y.M. Ren, W.Q. Ma, J. Ma, Q. Wen, J. Wanμ, F.”. Zhao, Synthesis and Properties oλ ”isphenol “ Molecular Imprinted Particle λor Selective Recoμnition oλ ”P“ λrom Water. Journal oλ Colloid and Interλace Science, 367 (2012) 355–361. DOI: 10.1016/j.jcis. 2011.10.009. [45] L.”. Capeleti, I.M. ”aibich, I.S. ”utler, J.H.Z. dos Santos, Inλrared and Raman Spectroscopic Characterization oλ Some Orμanic Substituted Hybrid Silicas. Spectrochimica “cta Part “ – Molecular and ”iomolecular Spectroscopy, 133 (2014) 619–625. DOI: 10.1016/j.saa.2014.05.072. [46] “. Fidalμo, L.M. Ilharco, The Deλect Structure oλ Sol–μel-Derived Silica/Polytetrahydroλuran Hybrid Films by FTIR. Journal oλ Non-Crystalline Solids, 283 (2001) 144–154. DOI: 10.1016/s0022-3093(01)00418-5. [47] M. ”ukleski, V. Ivanovski, E. Hey-Hawkins, “ Direct Method oλ Quantiication oλ Maximal Chemisorption oλ 3-“minopropylsilyl Groups on Silica Gel Usinμ Driλt Spectroscopy. Spectrochimica “cta Part “ – Molecular and ”iomolecular Spectroscopy, 149 (2015) 69–74. DOI: 10.1016/j.saa.2015.04.026. [48] N. Tasinato, D. Moro, P. Stoppa, “.P. Charmet, P. Toninello, S. Giorμianni, “dsorption oλ F2C=CFCL on TiO2 Nano-Powder: Structures, Enerμetics and Vibrational Properties λrom Driλt Spectroscopy and Periodic Quantum Chemical Calculations. “pplied Surλace Science, 353 (2015) 986–994. DOI: 10.1016/j.apsusc.2015.07.006. [49] V. Ivanovski, M. ”ukleski, M. Madalska, E. Hey-Hawkins, Vibrational Spectra oλ a Ferrocenyl Phosphine Derivative Chemisorbed on 3-“minopropylsilyl Modiied Silica Gel. Vibrational Spectroscopy, 69 (2013) 57–64. DOI: 10.1016/j.vibspec. 2013.09.009. [50] J.F. McClelland, R.W. Jones, S. Luo, L.M. Seaverson, “ Practical Guide to FTIR Photoacoustic Spectroscopy, In: P.”. Coleman (Ed.) Practical Samplinμ Techniques λor Inλrared “nalysis, CRC Press, ”oca Raton, 1993, pp. 320.

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[51] R. Kizil, J. Irudayaraj, Fourier Transλorm Inλrared Photoacoustic Spectroscopy (FTIRP“S), In: G.C.K. Roberts (Ed.) Encyclopedia oλ ”iophysics, Sprinμer ”erlin Heidelberμ, ”erlin, Heidelberμ, 2013, pp. 840–844. [52] Y. Gao, N.R. Choudhury, N. Duta, J. Matisons, M. Readinμ, L. Delmote, Orμanic– inorμanic Hybrid λrom Ionomer Via Sol–μel Reaction. Chemistry oλ Materials, 13 (2001) 3644–3652. DOI: 10.1021/cm010179s. [53] P.F. ”ernath, 6 Inλrared Emission Spectroscopy. “nnual Reports Section "C" (Physical Chemistry), 96 (2000) 177–224. DOI: 10.1039/”001200I. [54] R. ”rambilla, J.H.Z. dos Santos, M.S.L. Miranda, R.L. Frost, Thermal Stability oλ Octadecylsilane Hybrid Silicas Prepared by Graλtinμ and Sol–μel Methods. Thermochimica “cta, 469 (2008) 91–97. DOI: 10.1016/j.tca.2008.01.010. [55] C. Martinet, V. Martinez, C. Coussa, ”. Champaμnon, M. Tomozawa, Radial Distribution oλ the Fictive Temperature in Pure Silica Optical Fibers by Micro-Raman Spectroscopy. Journal oλ “pplied Physics, 103 (2008) 4. DOI: 10.1063/1.2905321. [56] O. Cristini-Robbe, K. Raulin, F. Dubart, R. ”ernard, C. Kinowski, N. Damene, I. El Yazidi, “. ”oed, S. Turrell, Porous Silica Supports λor Micro-Raman Spectroscopic Studies oλ Individual Livinμ Cells. Journal oλ Molecular Structure, 1050 (2013) 232–237. DOI: 10.1016/j.molstruc.2013.06.063. [57] L. ”riμo, V. “uzelyte, K.“. Lister, J. ”ruμμer, G. ”rusatin, Phenyl-”ridμed Polysilsesquioxane Positive and Neμative Resist λor Electron ”eam Lithoμraphy. Nanotechnoloμy, 23 (2012) 7. DOI: 10.1088/0957-4484/23/32/325302.

Chapter Provisional chapter2

Investigations ofof Phonons in Zinc Blende and Wurtzite by Investigations Phonons in Zinc Blende and Wurtzite Raman Spectroscopy by Raman Spectroscopy Lin Sun, Lingcong Shi and Chunrui Wang Lin Sun, Lingcong Shi and Chunrui Wang Additional information is available at the end of the chapter

Additional information is available at the end of the chapter http://dx.doi.org/10.5772/64194

Abstract The importance oλ phonons and their interactions in bulk materials is well known to those workinμ in the ields oλ solid-state physics, solid-state electronics, optoelectronics, heat transport, quantum electronic, and superconductivity. Phonons in nanostructures may act as a μuide to research on dimensionally conined phonons and lead to phonon efects in nanostructures and phonon enμineerinμ. In this chapter, we introduce phonons in zinc blende and wurzite nanocrystals. First, the basic structure oλ zinc blende and wurzite is described. Then, phase transλormation between zinc blende and wurzite is presented. The linear chain model oλ a one-dimensional diatomic crystal and macroscopic models are also discussed. ”asic properties oλ phonons in wurzite structure will be considered as well as Raman mode in zinc blende and wurzite structure. Finally, phonons in ZnSe, Ge, SnS2, MoS2, and Cu2ZnSnS4 nanocrystals are discussed on the basis oλ the above theory. Keywords: phonons, zinc blende, wurzite, Raman spectroscopy, molecular vibration

. Zinc blende and wurzite structure Crystals with cubic/hexaμonal structure are oλ major importance in the ields oλ electronics and optoelectronics. Zinc blende is typical λace-centered cubic structure, such as Si, Ge, Ga“s, and ZnSe. Wurzite is typical hexaμonal close packed structure, such as GaN and ZnSe. In particular, II–VI or III–V μroup semiconductor nanowires always coexist two structures, one cubic λorm with zinc blend (Z”) and another hexaμonal λorm with wurzite (WZ) structure. Sometimes, this coexistence between zinc blende and wurzite structure leads to λorm twinninμ crystal durinμ the phase transλormation between zinc blende and wurzite [1, 2].

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

. . Basic structure of zinc blende and wurzite The crystal structure oλ zinc selenide in the zinc blende structures is shown in Figure , which is reμarded as two λace-centered cubic (λcc) latices displaced relative to each other by a vector a

� +

a

�+

a

� , where a is latice constant. Close-packed planes oλ zinc blende are {111} alonμ

, and the stackinμ is …“”C“”C“…; the adjacent plane separation is 3/3a. “lonμ , the sackinμ is …“”“”“”…; the adjacent plane separation is a/ . “lonμ , the sackinμ is …“”“”“”“…; the adjacent plane separation is / a. Zinc blende structures have eiμht atoms per unit cell.

Figure . Zinc blende crystal structure.

Figure is wurzite structure oλ zinc selenium. Close-packed planes oλ wurzite are {0001} alonμ , and the stackinμ is …“”“”“…. “djacent plane spacinμ is c/2. Wurzite structures have λour atoms per unit cell. In zinc blende, the bondinμ is tetrahedral. The wurzite structure may be μenerated λrom zinc blende by rotatinμ adjacent tetrahedra about their common bondinμ axis by an anμle oλ 60° with respect to each other.

Figure . Wurzite crystal structure.

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

. . Phase transformation between zinc blende and wurzite Research into controllinμ nanowire crystal structure has intensiied. Several reports address the diameter dependency oλ nanowire crystal structure, with smaller diameter nanowires tendinμ toward a WZ phase and larμer diameter nanowires tendinμ toward a Z” phase. “llowinμ λor ZnSe, two phases, zinc blende (Z”) and wurzite (WZ), exist, and the (111) λaces oλ Z” phase are indistinμuishable λrom and match up with the (001) λaces oλ WZ phase, the subtle structural diferences oλ which lead to the atendant small diference in the internal enerμies (∼5.3 meV/atom λor ZnSe). The WZ-Z” phase transλormation is considered to be caused by the crystal plane slip. Take the λormation oλ ZnSe lonμitudinal twinninμ nanowires, λor example [3]. Structurally, the (001) planes oλ WZ and the (111) planes oλ Z” are their correspondinμ close packinμ planes. “”“” stackinμ λor WZ and “”C“”C stackinμ λor Z” are shown in Figure a and b, respectively. It was noteworthy that the arranμement oλ atoms in “/” packinμ planes was diferent in WZ phase. So the phase transition could not be realized until the smaller Zn atoms moved to the interspaces provided by three neiμhborinμ biμμer Se atoms, within the plane ”. In this case, the new layers ”' were obtained, and then, the slip occurs between neiμhborinμ planes “ and ”’ by Figure a.

3

� +

3

b , that is direction, indicated in

Figure . Phase transλormation between zinc blende and wurzite. (a) The arranμement oλ atoms in WZ phase; (b) The arranμement oλ atoms in Z” phase; (Se is shown with the biμμer sphere and Zn is shown in litle one.) (c) The stackinμ sequence schematic model showinμ the phase transλormation process λrom WZ phase to Z” phase.

Generally, there are three equivalent directions to realized the slip, which are , < 0>, and < 0>. Such a displacement could be indicated in Figure c, and the Z” structure could be obtained throuμh the slip between every second close-packed layer in the WZ sequence to λorm the “”C stackinμ.

. Linear-chain model and macroscopic models To the simple double latice, latice vibration can be described by the one-dimensional diatomic model. The linear-chain model oλ a diatomic crystal is based upon a system oλ two atoms with masses, m and M, placed alonμ a one-dimensional chain as depicted in Figure . The separation between the atomics is “a”, and the vibration in the vicinity oλ their equilibrium position is treated as the simple harmonic vibration. The properties oλ optical phonon can be described

25

26

Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

based on the macroscopic ields. It is the model based on the Huanμ and Maxwell equations, which has μreat utility in describinμ the phonons in the uniaxial crystals such as wurzite crystals.

Figure . One-dimensional diatomic linear-chain model.

. . Polar semiconductors Polar semiconductor is the crystal that consists oλ diferent ions. In polar semiconductor, the latice vibration is associated with the electric dipole moment and electric ield μeneration. “ssume that the vibration λrequency is ω, wave vector is q , then the intensity oλ polarization can be writen as λollows, r r rr P = P0 e i (wt - q ×r )

(2-1)

Solve the simultaneous λormula (2-1) and Maxwell equations can obtain, r r 2 r r r w 2 P - qc (q × P) E= e 0 (q 2c 2 - w 2 )

(2-2)

To lonμitudinal polarity latice mode, p / / q , λormula (2-2) can be simpliied as λollows, r r P EL = -

e0

(2-3)

To transverse polarity latice mode, p ⊥ q , λormula (2-2) can be simpliied as λollows, r ET =

r w2 P 2 e 0 (q c - w ) 2 2

(2-4)

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

“s was apparent above, polar optical phonon vibrations produce electric ields and electric polarization ields that may be described in terms oλ Maxwell's equations and the drivenoscillator equations. “ssume that the mass oλ the ions are M+, M−, the charμes are ±Ze, displacements are u±, the λorce constant is k,

r &&r = - kur + ZeE M +u + e

(2-5)

r &&r = kur - ZeE M -u e

(2-6)

where Ee is the efect electric ield, u = u + − u −, then r &&r = - kur + ZeE Mu e

where M =

(2-7)

M+M− is reduced mass. M + + M−

The latice vibration is associated with the electric dipole moment μeneration, which can be described as λollows, r 1 r r P = (Zeu + a Ee ) W

(2-8)

where Ω is the volume oλ the primitive cell, and � is the electron polarization. Under the efective ield approximation, the efective ield can be described as λollows, r r r P Ee = E + 3e 0

(2-9)

Replace the value oλ p in λormula (2-9) with (2-8),

r r r 3e 0 WE + Zeu Ee = 3e 0 W - a

(2-10)

Then, take λormula (2-7) and (2-9) into (2-10), r &&r = Aur + BE u

(2-11)

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28

Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

r r r P = CE + Du

(2-12)

k Z2e 2 + M M(3e 0 W - a )

(2-13)

where A=-

B=

3e 0 WZe M(3e 0 W - a )

C=

3e 0a 3e 0 W - a

D=

3e 0 Ze 3e 0 W - a

(2-14)

(2-15)

(2-16)

λormula (2-11) and (2-12) are the Huanμ equations, which are the basic equations oλ describinμ the vibrations oλ lonμ wave in the polar crystals. From the λormula (2-14) and (2-16), one can ind that, B=

W D M

(2-17)

When the system is under the hiμh-λrequency electric ield, λormula (2-12) reduces to r r P = CE

For ∞ = +

�

�0�

(2-18)

, λormula (2-18) can be writen as λollows, C = e 0 [e (¥) - 1]

(2-19)

Compute the curl oλ λormula (2-11) and solve the simultaneous equations oλ (2-12) and electrostatic equations ∇ × E = 0,

A = -w02

(2-20)

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

When the system is under the static electric ield, u¨ = 0, and λormula (2-11) reduces to r B r u=- E A

(2-21)

r BD r P = (C )E A

(2-22)

r r P = [e (0) - 1]e 0 E

(2-23)

Take λormula (2-21) into (2-12),

Replace the electrostatic equation,

“nd take λormula (2-23) and (2-20) into (2-22), BD = [e (0) - e (¥)]e 0w02

(2-24)

Solve the simultaneous equations oλ (2-17) and (2-24) can obtain B=(

D=(

1 W 21 ) {[e (0) - e (¥)]e 0 } 2 w0 M

1 M 21 ) {[e (0) - e (¥)]e 0 } 2 w0 W

(2-25)

(2-26)

Solve two simultaneous Maxwell and Huanμ equations, (2-27a) (2-27b) r Ñ×D = 0

(2-27c)

r Ñ×H = 0

(2-27d)

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30

Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

“ssume the solution λorms are (2-28a) (2-28b) (2-28c) (2-28d) Take (2-28) into the Huanμ and Maxwell equations, r r BD P0 = [+ C ]E0 2 A+w r r (q × E0 )[e 0 + C -

BD ]=0 A + w2

(2-29)

(2-30)

To the lonμitudinal wave, q · E 0 ≠ 0, (2-30) reduces to

e0 + C -

BD =0 A + w2

(2-31)

Take (2-19) (2-20) (2-25) (2-26) into (2-31) 2 wLO =

e (0) 2 w e (¥) 0

(2-32)

Equation (2-23) is the dispersion relations oλ lonμitudinal wave, which is commonly called Lyddane-Sachs-Teller (LST) relationship. LST relation indicates that the λrequency oλ lonμitudinal wave is a constant and independent on the wave vector. Similarly, to the transverse wave, q · E 0 = 0, solve the simultaneous equations oλ Maxwell and

Huanμ equations,

q2

m0w

æ BD ö = w çe0 + C ÷ + w2 ø A è

(2-33)

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

Replace the values oλ A, B, C, and D into (2-33),

c2

w

2

q 2 = e (¥) +

e (0) - e (¥) 2 w0 w02 - w 2

(2-34)

Equation (2-34) is the dispersion relations oλ transverse wave. One can ind that the λrequency oλ transverse is dependent on the value oλ wave vector q , but independent on its direction [4, 5]. . . Dispersion relations One-dimensional diatomic model can be reμarded as the simple double latice. In the simple linear chain model, it is assumed that only nearest neiμhbors are coupled, and that the interaction between these atoms is described by Hooke's law; the sprinμ constant α is taken to be that oλ a harmonic oscillator. Thus, the kinematical equations are established, &&r = - b (2u && 2n - u && 2n + 1 - u && 2n -1 ) mu 2n

(2-35a)

r r r &&r Mu 2n +1 = - b (2u 2n +1 - u 2n + 2 - u 2n )

(2-35b)

where m and M are the mass oλ the adjacent atoms ���   � � − , u n, u n + ,  ���  , u n + are the displacements oλ the atoms at the position oλ 2n-1, 2n, 2n + 1, and 2n + 2, respectively. � is the λorce constant. The solution λorms oλ (2-35) can be writen as λollows (2-36a) (2-36b) where q is the phonon wave vector and ω is its λrequency. Take λormulas (2-36a) and (2-36b) into λormulas (2-35a) and (2-35b), - mw 2 A1 = b ( e - ia ×q + e ia ×q ) A2 - 2 b A1

(2-37)

- Mw 2 A2 = b ( e - ia ×q + e ia ×q ) A1 - 2 b A2

(2-38)

r r

r r

Eliminatinμ A1 and A2,

r r

r r

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

w2 = b

r r 1 ü M+mì 4 Mm sin 2 ( a × q )] 2 ý í1 ± [1 2 Mm î ( M + m) þ

(2-39)

The relationship between λrequency and wave vector is commonly called dispersion relation [5].

. Basic properties of phonons in wurzite structure In this section, we discuss the phonon efects in wurzite structure. The crystalline structure oλ a wurzite material is depicted in Figure . There are λour atoms in the unit cell. Thus, the total number oλ optical modes in the lonμ-wavelenμth limit is nine: three lonμitudinal optic (LO) and six transverse optic (TO). In these optical modes, there are only three polar optical vibration modes. “ccordinμ to the μroup theory, the wurzite crystal structure belonμs to the space μroup C6v, and the phonon modes at Γ point oλ the ”rillouin zone are represented by the λollowinμ

irreducible representations:

G = 2 A1 + 2 B + 2 E1 + 2 E2 Due to the anisotropy oλ wurzite structure, the vibrational λrequency oλ oscillates parallel and perpendicular to the optical axis is denoted by ωeT and ωoT, and the correspondinμ dielectric

constants are denoted by εes, εe∞ and εos, εo∞. The correspondinμ components can be writen

as the λorm oλ Huanμ equations, and the dispersion relation can be obtained by solvinμ two simultaneous equations oλ Maxwell and Huanμ equations. q 2c 2

w2

= e0 =

2 woT e os - w 2e o¥ 2 woT - w2

2 weT2 e es - w 2e e¥ woT e os - w 2e o¥ )( ) 2 2 - w2 - w2 woT woT q 2c 2 = = e q w2 w 2 e - w 2e e ¥ w 2 e - w 2 e o¥ ( eT es2 )cos 2 q + ( oT os2 )sin 2 q 2 weT - w woT - w 2

(3-1)

(

(3-2)

where εo and �� is the dielectric constants oλ ordinary and extraordinary wave, is the included anμle between wave vector and optical axis.

When the wave vector is parallel to the optical axis, θ = 0, λormula (3-2) reduce to

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

eq =

2 woT e os - w 2e o¥ 2 woT - w2

(3-3)

which is the same as λormula (3-1). When the wave vector is perpendicular to the optical axis, θ = 90 , λormula (3-2) reduces to

eq =

weT2 e es - w 2e o¥ weT2 - w 2

(3-4)

Formula (3-4) indicates that the extraordinary wave is transverse wave when the wave vector is perpendicular to the optical axis. When q ≫ ω/c, λormulas (3-1) and (3-2) can be rewriten as λollows,

w = woT

(3-5)

and

(

2 weT2 e es - w 2e e¥ woT e os - w 2e o¥ 2 )cos ( )sin 2 q = 0 + q 2 - w2 weT2 - w 2 woT

(3-6)

Formula (3-5) indicates that λrequency oλ ordinary phonon is independent on the wave vector q. Formula (3-6) indicates that the λrequency oλ extraordinary phonon is dependent on the orientation oλ the wave vector, but independent on its value. It is most convenient to divide uniaxial crystals into two cateμories: (a) the electrostatic λorces dominate over the anisotropy oλ the interatomic λorces and (b) the short-ranμe interatomic λorces are much μreater than the electrostatic λorces. It has been turned out that crystals with the wurzite symmetry λall into the irst cateμory. In this case, ωeT − ωoT ≪ ωeL − ωoT and ωoL − ωoT , εe∞ ≈ εo∞ = ε∞, λormula (3-5) reduces to

(

weL2 - w 2 w2 - w2 )cos 2 q + ( oL )sin 2 q = 0 2 2 2 - w2 weT - w woT

(3-7)

thus, 2 w 2 » weT2 sin 2 q + woT cos 2 q

(3-8)

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34

Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

and

w 2 » woL2 sin 2 q + weT2 cos 2 q

(3-9)

. Raman mode in zinc blende and wurzite structure Raman spectroscopy is a non-destructive technical tool used to μain inλormation about the phonon behavior oλ the crystal latice throuμh the λrequency shiλt oλ the inelastically scatered liμht λrom the near surλace oλ the sample. It is well known that diferent crystal phases have diferent vibrational behaviors, so the measured Raman shiλts oλ diferent phases are mostly unique and can be seen as inμerprints λor the respective phases. This provides the possibility oλ detectinμ diferent phases in a sample. It has been developed to be a versatile tool λor the characterization oλ semiconductors leadinμ to detailed inλormation on crystal structure, phonon dispersion, electronic states, composition, strain, and so on oλ semiconductor nanostructures. In a zinc blende structure, the space μroup oλ the cubic unit cell is F43m(Td) containinμ λour

λormula units. The primitive unit cell contains only one λormula per unit cell, and hence, there are three optical branches to the phonon dispersion curves. “s there is no center oλ inversion in the unit cell, the zone-center transverse optic (TO) and lonμitudinal optic (LO) optic modes are Raman active. The optic mode is polar so that the macroscopic ield liλts the deμeneracy, producinμ a non-deμenerate lonμitudinal mode that is at a hiμher λrequency than the two transverse modes. The wurzite crystal structure belonμs to the space μroup C6v and μroup theory predicts zone-

center optical modes are A1, 2B1, E1, and 2E2. The A1 and E1 modes and the two E2 modes are Raman active, whereas the B modes are silent. The A and E modes are polar, resultinμ in a splitinμ oλ the LO and the TO modes [6].

. Phonons in ZnSe, Ge, SnS , MoS , and Cu ZnSnS nanocrystals In addition to the atached reλerences, this chapter is primarily writen on the basis oλ our research works. Here, we select ZnSe, Ge nanowires and CdSe/Ge-based nanowire heterostructures, two-dimensional semiconductors SnS2 and MoS2, and candidate absorber materials oλ thin-ilm solar cells Cu2ZnSnS4. These examples will help us to understand the phonons behaviors in nanostructures. It is well known that ZnSe has two structures: cubic zinc blende (Z”) and hexaμonal wurzite (WZ) due to the diference oλ the stackinμ sequence oλ successive layers, whereas Ge has diamond structure. SnS2 and MoS2 belonμ to the wide λamily oλ compounds with layered

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

structures. SnS2 crystal is isostructural to the hexaμonal CdI2-type structure. MoS2 usually consists oλ a mixture oλ two major polytypes oλ similar structure, 2H (hexaμonal) and 3R (rhombohedral), with the λormer beinμ more abundant. “s λor quaternary Cu2ZnSnS4 (CZTS), the parent binary II-VI semiconductors adopt the cubic zinc blende structure, and the ternary I-III-VI2 compounds can be μenerated by mutatinμ the μroup II atoms into pairs oλ μroup I and III atoms. The quaternary CZTS materials are λormed by replacinμ the two In (III) atoms with Zn (II) and Sn (IV), respectively (see Figure ).

Figure . Evolution oλ multinary compounds.

We use Raman spectroscopy to identiλy crystal structure oλ ZnSe one-dimensional material (Figure ). In sample S3, the Raman peaks at 204 and 251 cm-1 are atributed to the scaterinμs oλ the transverse optic (TO) and lonμitudinal optic (LO) phonon modes oλ ZnSe, respectively. “ stronμ peak at 232 cm-1, between the TO and LO phonons, is thouμht to be surλace mode. The Raman peak at ∼176 cm-1 is atributed to the hexaμonal phase E1(TO) mode oλ ZnSe, which is inhibited in Raman spectrum (RS) oλ Z” ZnSe. Compared with S3, Raman peaks at 205.6 (TO mode) and 252 cm-1 (LO mode) oλ S1 show tiny blue-shiλt. However, in S1, there is no Raman peak correspondinμ to the surλace mode, as well as El (TO) mode, which is suppressed in the

Figure . Room temperature Raman spectra oλ ZnSe. S1, S2, and S3 stand λor Z”, coexist oλ Z” and WZ, WZ ZnSe nanostructure.

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

Z” phase. This indicates the existence oλ Z” phase in S1. Thus, structure oλ the sample can be shown throuμh RS, and we μot S1-Z” phase, S3-WZ, S2 the coexist oλ Z” and WZ [7] (cm-1). Figure shows the room temperature RS oλ CdSe/Ge-based nanowires. The LO mode oλ Ge in CdSe-Ge (or CdSe-Ge-CdSe), -CdSe-Ge core/polycrystalline Ge sheath, and -Ge-GeSe heterostructural nanowires has a downshiλt by 8, 5, and 2 cm-1 in comparison with that oλ the bulk counterpart Ge (299 cm-1), respectively. With reμard to the microstructure oλ heterostructural nanowires, the downshiλt oλ the LO mode may be caused by tensile stress, which afects the Raman line by a downshiλt. “nd the diferent shiλt scales are atracted by the diferent sizes oλ the Ge subnanowires and Ge nanocrystalline [8].

Figure . Raman spectrum oλ (a) CdSe-Ge biaxial nanowires and CdSe-Ge-CdSe triaxial nanowires. (b) CdSe-Ge biaxial nanowire core/polycrystalline Ge sheath heterostructures. (c) Ge-GeSe biaxial heterostructure nanowires.

The individual layer in SnS2 is known as an S-Sn-S sandwich bonded unit. Each Sn atom is octahedrally coordinated with six nearest neiμhbor sulλur atoms, while each S atom is nested at the top oλ a trianμle oλ Sn atoms. The sandwich layers in the elementary cell occur alonμ the c axis and bonded toμether by Vander Waals λorces. The normal modes oλ vibration in SnS2 are μiven by the irreducible representations oλ the D3d point μroup at the center oλ the ”rillouin zone: Γ = Alμ + Eμ + 2A2u + 2E2u. Two Raman-active modes (A1μ and Eμ) and two IR-active modes (A2u and Eu) are λound. In view oλ the existence oλ an inversion center, the IR- and Raman-active modes are mutually exclusive. On the other hand, six atoms in the unit cell oλ SnS2 extend over two sandwich layers. Eiμhteen normal vibration modes can be represented by the λollowinμ irreducible λorm: Γ = 3A1 + 3B1 + 3E1 + 3E2. ”ased on the analysis above, there are six modes, which are both IR- and Raman-active, belonμinμ to A1 and El, and three Raman-active modes belonμinμ to E2. The B1 modes are silent, while the three acoustic modes belonμ to A1 and E1 [9]. The RS oλ β-SnS2 nanocrystal is illustrated in our λormer work [10]. The spectra show one irstorder peak at 312 cm-1 that correspondinμ to A1μ mode. The RS oλ as-prepared SnS2 shows a sliμht redshiλt in comparison with that oλ bulk materials (peak at 317 cm-1). The redshiλt oλ phonon peaks is due to spatial coninement oλ phonon modes. The irst-order Eμ mode (peak

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

at 208 cm-1) cannot be observed, which likely results λrom a nanosize efect. “ wide peak between 450 and 750 cm-1, which only observed in the bulk materials at lower temperature, may be atributed to second-order efects. The phonon dispersion oλ sinμle-layer MoS2 has three acoustic and six optical branches derivatized λrom the nine vibrational modes at the Γ point. The three acoustic branches are the in-plane lonμitudinal acoustic (L“), the transverse acoustic (T“), and the out-oλ-plane acoustic (Z“) modes. The six optical branches are two in-plane lonμitudinal optical (LO1 and LO2), two in-plane transverse optical (TO1 and TO2), and two out-oλ-plane optical (ZO1 and ZO2) branches. For 2L and bulk MoS2, there are 18 phonon branches, which are split λrom nine phonon branches in 1LMoS2. The phonon dispersions oλ 1L and bulk MoS2 are very similar, except λor the three new branches below 100 cm-1 in bulk because oλ interlayer vibrations. There are similar optical phonon dispersion curves λor 1L, 2L, and bulk MoS2 because oλ the weak Vander Waals interlayer interactions in 2L and bulk MoS2 [11]. Raman spectroscopy is also used to accurately identiλy the layer number oλ MoS2. The λrequency diference between out-oλ-plane A1μ and in-plane E2μ1 mode oλ MoS2 is denoted as . ��. From monolayer to bulk MoS2, �� monotonically increases λrom 19.57 cm-1 to 25.5 cm-1. In our work [12], two stronμ peak at ∼379 cm-1 and ∼402 cm-1 can be assiμned as in-plane E2μ1 mode and out-oλ-plane A1μ mode oλ MoS2, respectively, which has a redshiλt in comparison with that oλ the bulk MoS2. The �� is about 23 cm-1, indicatinμ that the as-μrown MoS2 contains tri-layer MoS2.

The phonon dispersion and density-oλ-states curves alonμ the principal symmetry directions oλ kesterite CZTS were calculated usinμ a density λunctional theory by Khare et al. [13]. The phonon states around 50–160 cm-1 are mainly composed oλ vibrations oλ the three metal cations with some contribution λrom the sulλur anions. The phonon states around 250–300 cm-1 are mainly composed oλ vibrations oλ the Zn cations and S anions with some contribution λrom the Cu cations. The phonon states λrom 310 to 340 cm-1 are mainly a result oλ vibrations oλ S anions, whereas those λrom 340 to 370 cm-1 are composed oλ the vibrations oλ Sn cations and S anions.

To more exactly conirm secondary phases in Cu2-II-IV-VI4 semiconductors, Raman scaterinμ studies have been extensively perλormed. From the vibrational point oλ view, the zone-center phonon representation oλ the kesterite structure space μroup I is constituted oλ 21 optical modes: Γ = 3A + 6B + 6E1 + 6E2, where 12B, E1, and E2 modes are inλrared active, whereas 15A, B, E1, and E2 modes are Raman active. “ccordinμ to our work [14], the sinμle peak at about 328 cm-1 oλ Raman spectrum oλ the as-prepared CZTS nanocrystals can be assiμned to breathinμ mode oλ sulλur atoms around metal ions in CZTS. Moreover, Raman spectrum oλ CZTS has about 8 cm-1 redshiλts compared with that oλ the respondinμ bulk counterpart which may be due to a smaller size efect. In our work oλ λabrication oλ Cu2ZnSnSxSe4-x solid solution nanocrystallines [15], RS revealed that vibratinμ modes were modulated by x-values. The peak position oλ 170, 189, and 229 cm-1 shiλted to hiμher λrequency with increasinμ x-value in CZTSSe, respectively. Those peaks

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

completely disappeared when x = 4. Moreover, a wide peak located at about 330 cm-1 appeared when x > 0 and the relative intensity increased with increasinμ x-value. Such results indicate that Se elements were μradually replaced by S elements in CZTSSe solid solution system.

Acknowledgements This work was supported by the National Natural Science Foundation oλ China under Grant Nos. 11174049 and 61376017.

Author details Lin Sun, Linμconμ Shi and Chunrui Wanμ* *“ddress all correspondence to: crwanμ@dhu.edu.cn Department oλ “pplied Physics, Donμhua University, Shanμhai, China

References [1] Xu, J., Wanμ, C., Wu, ”., Xu, X., Chen, X., Oh, H., ”aek, H., & Yi, G. C. Twinninμ efect on photoluminescence spectra oλ ZnSe nanowires. Journal oλ “pplied Physics. 2014;116(17):174303. doi:10.1063/1.4900850. [2] Xu, J., Wanμ, C., Lu, “., Wu, ”., Chen, X., Oh, H., ”aek, H., Yi, G., & Ouyanμ, L. Photoluminescence oλ excitons and deλects in ZnSe-based lonμitudinal twinninμ nanowires. Journal oλ Physics D: “pplied Physics. 2014;47(48):485302. doi: 10.1088/0022-3727/47/48/485302. [3] Xu, J., Lu, “., Wanμ, C., Zou, R., Liu, X., Wu, X., Wanμ, Y., Li, S., Sun, L., Chen, X., Oh, H., ”aek, H., Yi, G., & Chu, J. ZnSe-based lonμitudinal twinninμ nanowires. “dvanced Enμineerinμ Materials. 2014;16(4):459–465. doi:10.1002/adem.201300405. [4] Zhanμ, G. et al. Latice Vibration Spectroscopy. ”eijinμ: Hiμher Education Press; 2001 [5] Huanμ, K., & Han, R. Solid-State Physics. ”eijinμ: Hiμher Education Press; 1988. [6] Stroscio, M. “., & Duta, M. Phonons in Nanostructures. Cambridμe: Cambridμe University Press; 2005. [7] Wanμ, H. Luminescence and vibratinμ properties oλ Zn-based μroup II-VI nanostructures. Master's thesis. Donμhua University. 2012.

Investigations of Phonons in Zinc Blende and Wurtzite by Raman Spectroscopy http://dx.doi.org/10.5772/64194

[8] Cai, J. Controllable synthesis and vibratinμ properties oλ CdSe based heterostructure nanowires. Master's thesis. Donμhua University. 2011. [9] Smith, “. J., Meek, P. E., & Lianμ, W. Y. Raman scaterinμ studies oλ SnS2 and SnSe2. Journal oλ Physics C: Solid State Physics. 1977;10(8):1321. doi:10.1088/00223719/10/8/035. [10] Wanμ, C. Synthesis and properties oλ iodine and sulide nanomaterials. PhD thesis. University oλ Science and Technoloμy oλ China. 2002. [11] Zhanμ, X., Qiao, X. F., Shi, W., Wu, J. ”., Jianμ, D. S., & Tan, P.H. Phonon and Raman scaterinμ oλ two-dimensional transition metal dichalcoμenides λrom monolayer, multilayer to bulk material. Chemical Society Reviews. 2015;44(9):2757–2785. doi: 10.1039/C4CS00282”. [12] Fu, Y. Fabrication and properties oλ ZnO/CdS/MoS2 heterostructure nanorod arrays. Master's thesis. Donμhua University. 2016. [13] Khare, “., Himmetoμlu, ”., Johnson, M., Norris, D. J., Cococcioni, M., & “ydil, E. S. Calculation oλ the latice dynamics and Raman spectra oλ copper zinc tin chalcoμenides and comparison to experiments. Journal oλ “pplied Physics. 2012;111(8):083707. doi: 10.1063/1.4704191. [14] Wanμ, C., Chenμ, C., Cao, Y., Fanμ, W., Zhao, L., & Xu, X. Synthesis oλ Cu2ZnSnS4 nanocrystallines by a hydrothermal route. Japanese Journal oλ “pplied Physics. 2011;50(6R):065003. doi:10.1143/JJ“P.50.065003. [15] Cao, Y.. Fabrication and characterization oλ Cu2ZnSnSxSe4-x thin ilm solar cell absorber layer material. Master's thesis, Donμhua University. 2012

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Provisional chapter Chapter 3

Structural Characterization Characterization of of Lithium Lithium Niobate Niobate Structural Nanoparticles Prepared by the Sol-Gel Process, Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and Using X-Ray and Raman Spectroscopy and Scanning Electron Microscopy Scanning Electron Microscopy Pedro R.S. Prezas and Manuel P.F. Graça Pedro R.S. Prezas and Manuel P.F. Graça

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter http://dx.doi.org/10.5772/64395

Abstract The widespread use oλ lithium niobate (LN) in several technoloμical applications, notably in optical and electrooptical systems, is a consequence oλ its remarkable piezoelectric, electrooptical, photoelastic, acousto-optic, and nonlinear optical coeicients. In this chapter, the structural and electrical characterization oλ LN nanosized particles synthesized by the Pechini route is discussed. Compared to solidstate reaction processes, wet chemistry processes can be advantaμeous alternatives λor the synthesis oλ polycrystalline LN, because they require lower processinμ temperatures, and thus the loss oλ stoichiometry and λormation oλ secondary phases can be minimized. The powders obtained by dryinμ the μel (base powder) were heat-treated λor 4 h at temperatures between 400 and 1000°C, accordinμ to the diferential thermal analysis (DT“) results. It was λound that the powders sintered at 450°C contain only the LN phase, while those heat-treated at 500°C already contain the secondary LiNb3O8 phase. The structural and electrical characterization oλ the samples sintered at 450°C, λor diferent times, was perλormed usinμ X-ray difraction (XRD) in conjunction with Rietveld reinement, Raman spectroscopy, scanninμ electron microscopy (SEM), and impedance spectroscopy in the temperature ranμe between 200 and 360 K and in the λrequency ranμe between 100 Hz and 1 MHz and by measurinμ the ac and dc conductivities.

Keywords: lithium niobate, structural properties, X-ray spectroscopy, Raman spectroscopy, electrical properties, lithium triniobate, sol-μel process

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. Introduction Lithium niobate (LiNbO3, LN) is a well-known artiicially synthesized λerroelectric material with considerable technoloμical importance, beinμ in competition with barium titanate (”aTiO3, ”TO) in several hiμh-tech applications. In λact, an inspection oλ the number oλ publications related with LN and ”TO will show that since the mid-1990s, the number oλ reports on both materials has been λollowinμ the same increasinμ trend, with similar number oλ publications, reinλorcinμ the importance oλ LN amonμ the scientiic communities. Table displays some oλ the main physical properties oλ sinμle-crystalline LN [1]. Meltinμ temperature (°C)

1260

Curie temperature (°C)

1210

Density at RT (μ/cm3)

4.64

Reλractive index (ordinary), n0

2.296

Electrooptical coeicient, r33 (m/V)

30 × 10−12

Transparency window (µm)

0.4–5

Resistivity, ρ (c-axis) (Ω cm)

loμ ρ = (7150/T) − 2.823 (at RT) = 1021

Dielectric constant at RT (ε′)—c-axis

80 (100 kHz) >1000 (1 kHz)

Dielectric loss at RT (tan δ)—c-axis

≈0 (100 kHz)

Coercive ield (at 1210°C) (V/m)

20

Spontaneous polarization at RT (Ps [×10−2 C m−2])

70

Piezoelectric coeicient (d33 [pC/N])

6

Thermal conductivity at RT (W m−1 K−1)

3.92

Table . Main physical properties oλ sinμle-crystalline stoichiometric LN. RT stands λor room temperature (μenerally 300 K) [1–3].

LN sinμle crystals display several excellent properties, such as hiμh piezoelectric, electrooptical, photoelastic, acousto-optic, and nonlinear optical coeicients. They are known to have very low acoustic losses, oferinμ a μreat versatility as a substrate λor inteμrated optic systems: a considerable number oλ optical devices have been developed based on LN, such as wave μuides, surλace acoustic wave (S“W) devices, electrooptical wavelenμth ilters and polarization modulators, nonlinear λrequency converters (λrequency doublinμ and second harmonic μeneration), nonvolatile memories, and ultraλast optical processinμ systems. Its combination oλ electrooptical and photoμalvanic efects makes it photoreλractive without the need oλ applyinμ an external electrical ield, thus beinμ able to be applied in holoμraphic data storaμe. It ofers also the possibility oλ beinμ easily doped, in a controllable way, with optical-active ionic species, usinμ standard techniques such as ion implantation or thermal difusion.

Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and and Scanning Scanning Electron Electron Microscopy Microscopy Raman Spectroscopy http://dx.doi.org/10.5772/64395

The physical properties aλorementioned are optimized λor the case where LN is μrown as a sinμle crystal and with stoichiometric composition. Reμardinμ the stoichiometry, LN has a relatively broad composition ranμe, and thereλore, it can be labeled as conμruent lithium niobate (cLN, 48.35–48.6 mol% Li2O) and stoichiometric (sLN, ~50 mol% Li2O). Figure displays the phase diaμram oλ the Li2O-Nb2O5 binary system, showinμ the possibility oλ μrowinμ pure LN crystals by usinμ 50% up to ~52% oλ Nb2O5. It also shows the transition oλ λerroelectric phase to paraelectric by increasinμ the synthesis temperature and Nb2O5 content. The larμe majority oλ LN sinμle crystals are μrown by the conventional Czochralski method, which yields cLN sinμle crystals. Some competitor methods have been developed λor μrowinμ stoichiometric crystals, includinμ the vapor transport equilibration (VTE) method, which is a post-μrown procedure [4]. The λormer is more suitable λor thin and small samples, because λor larμer and thicker crystals, very larμe solid-state difusion times are required λor the Li/Nb ratio equilibration. To atain larμer stoichiometric sinμle crystals, more direct μrowth methods can be applied, such as the double crucible Czochralski method with an automatic power supply [5, 6] or the hiμh-temperature top-seeded solution μrowth (HTTSSG) method λrom the K2O-Li2O-Nb2O5 ternary mixture, which is the one capable oλ yieldinμ compositions closest to 50 mol% Li2O [5, 7]. The nonlinear and photoreλractive properties will μenerally deμrade with the loss oλ stoichiometry and consequent increase oλ deλects and impurity density. “s a mater oλ λact, the conμruent composition ranμe is reμarded as havinμ an intrinsic deλect structure which is dominated by lithium vacancies, known as the lithium vacancy model, which translates in Eq. (1) [6]: (1) In this model, as depicted in Eq. (1), λor every λour lithium vacancies created, a niobium ion occupies a lithium network site, assurinμ electrical charμe neutrality. The cLN is not suitable λor hiμh-temperature applications, because deμradation processes can start to occur λor temperatures startinμ λrom 300°C [6, 8]. On the other hand, studies show that sLN can be stable

Figure . Phase diaμram oλ the Li2O-Nb2O5 binary system [3].

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

up to temperatures oλ at least 900°C [6, 9], because some properties like the electrical conductivity do not chanμe under thermal cyclinμ up to such temperatures. “s it was aλorementioned, the dominant process in the μrowth oλ LN sinμle crystals is the Czochralski method. However, this method is known λor its technical and economic drawbacks, as well as beinμ time-consuminμ. Thus, alternative preparation processes have been researched and explored. Solid-state reaction processes μenerally require hiμh processinμ temperatures (>1000°C), which lead to the loss oλ lithium by evaporation [10]. “s a consequence, secondary crystalline phases such as Li3NbO4 and LiNb3O8 can be λormed, chanμinμ the stoichiometry and deterioratinμ the properties. Wet chemistry methods, such as sol-μel methodoloμies and hydrothermal methods, can be μood alternatives because they require lower processinμ temperatures, such as calcination and thermal-sinterinμ treatments, and thus the λormation oλ secondary phases can be minimized [10, 11]. Sinμle crystals in the nanometer and micrometer size ranμes can be synthesized at low temperatures, such as 240°C, by these methods [11]. When such methods are applied, typically polycrystalline LN samples are produced, i.e., a material composed by small sinμle crystals in the micrometric or nanometric ranμe randomly distributed with no evident preλerential orientation. Polycrystalline LN inds a lot oλ applications, especially as thin ilms λor inteμrated optic applications, althouμh μenerally the properties oλ polycrystalline materials are not as μood as their sinμle-crystalline counterpart. For example, the piezoelectric properties oλ polycrystalline LN are inλerior to the sinμle crystal, and in the best case, they miμht approach them iλ all oλ its domains are perλectly orientated. Further, relative to sinμle-crystalline thin ilms, the μrain boundaries in polycrystalline ilms may lead to increased liμht scaterinμ and larμer optical losses in wave μuides, which may reduce their utility and potentiality in some applications [12]. However, their production is cheaper and easier compared with the μrowth processes λor sinμle crystals, and all these cons and drawbacks have to be considered and well balanced λor potential applications. “morphous LN is also important λor some applications. In an amorphous material, there is no lonμ-ranμe order, and the network can be described as distorted unitary cells randomly oriented. LN sinμle crystals and polycrystals have low electrical conductivity (~10−12 S/cm at 500 K λor sinμle crystals [13]), and ionic difusion or mobility is reduced in these materials. However, the amorphous structure is always a more open structure compared with the crystalline composition or, in other words, has a smaller density, which promotes and λacilitates the ionic difusivity, makinμ them suitable λor technical application such as solid-state electrolytes λor Li-ion bateries. In λact, the reported activation enerμies λor the ionic difusivity are considerably smaller (halλ in the 25–150°C temperature ranμe [13]) in amorphous LN. In this chapter, the structural and electrical characterization oλ LiNbO3 nanosized particles prepared by the Pechini route, also known in the sol-μel methodoloμies as the polymeric precursor route, is discussed. The powders obtained by dryinμ the μel (base powder) were heat-treated λor 4 h at temperatures between 400 and 1000°C, accordinμ to the diferential thermal analysis (DT“) results. The sinterinμ temperature revealed to be, as expected, an important parameter in controllinμ the development oλ secondary crystalline phases, and it was λound that the powders sintered at 450°C contain only the LN phase, while those heat-

Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and and Scanning Scanning Electron Microscopy Raman Spectroscopy http://dx.doi.org/10.5772/64395

treated at 500°C already contain the secondary LiNb3O8 phase. Their structural characterization was perλormed usinμ X-ray difraction (XRD) in conjunction with Rietveld reinement, Raman spectroscopy, and scanninμ electron microscopy (SEM). The μrains observed have sizes lower than 100 nm and an approximately spherical μeometry. The electrical characterization oλ pellets made λrom the base powder heat-treated at 450°C was made by measurinμ the dc and ac conductivities and measurinμ the complex impedance (Z*) in the temperature ranμe between 200 and 360 K and in the λrequency ranμe between 100 Hz and 1 MHz. From the measured complex impedance values, the complex permitivity (ε*) was calculated, since the μeometrical characteristics oλ the pellets leμitimate the use oλ the parallel plate capacitor model. The correlation between the structure and morpholoμy with the electrical and dielectric properties is one oλ the main topics oλ the present chapter.

. Structural, morphologic, and electrical properties . . Structural properties LN belonμs to a μroup oλ materials whose crystalline structure is a perovskite. This structure has the typical chemical λormula “”O3, where the cation “ usually is too larμe λor an efective complete packaμinμ, causinμ a distortion in the unit cell and leadinμ to a displacement oλ the O2− anions λrom their expected sites. However, in the case oλ LN, the distortion is related with the small radius oλ the lithium ion. For temperatures lower than 1415 K, which is the Curie temperature (Tc) oλ sLN, this material is in its λerroelectric state, and consequently, it exhibits a spontaneous polarization, due to a nonuniλorm charμe distribution oλ the lithium and niobium ions. In this λerroelectric phase, this material has a triμonal crystalline structure, with threeλold rotation symmetry around its c-axis. Figure a and c shows the atomic model oλ LN in its λerroelectric crystalline phase. In this coniμuration, the crystalline structure consists in layers oλ oxyμen atoms parallel to each other with the Li+ and Nb5+ cations lyinμ alonμ the

Figure . (a) and (c) “tomic model oλ the LN λerroelectric phase. (b) “tomic model oλ the LN paraelectric phase. ∆Li indicates the displacement alonμ the c-axis oλ the Li+ cations, represented as black spheres, while ∆Nb indicates the displacement alonμ the c-axis oλ the Nb5+ cations. ”oth displacements are represented relatively to the center oλ the oxyμen (red spheres) planes [14].

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

c-axis, surrounded by oxyμen octahedra: in the unitary cell, one-third oλ the octahedral interstices are occupied by Li+ cations; another one-third by Nb5+ cations and the remaininμ (one-third) interstices are structural voids [2, 14]. “s depicted in Figure , in the λerroelectric state, the displacement oλ the Li+ and Nb5+ cations relatively to the center oλ the oxyμen planes oriμinates a spontaneous polarization alonμ the c-axis with a maμnitude oλ 0.7 C/m2 at 300 K (see Table ). The displacement can be up or down with respect to the oxyμen sublatice, and both cations are displaced in the same direction because oλ the Coulomb repulsion. “bove the Curie temperature, due to the thermal expansion oλ the crystalline latice axes, the structure is no lonμer distorted, because the Li+ and Nb5+ cations move to latice sites lyinμ in the planes oλ the oxyμen layers, as Figure b displays. Thus, the transition to the paraelectric state occurs, and LN ceases to exhibit a permanent spontaneous polarization [2, 14]. “s it was stated in Section 1, polycrystalline LN is also oλ μreat technoloμical importance. In this λorm, the structure can be described as composed by small sinμle crystals in the micrometric or nanometric ranμe randomly distributed with no evident preλerential orientation. Relatively to the λerroelectricity, it is composed by several λerroelectric domains, which are reμions with diferent orientations oλ the spontaneous polarization Ps. In this case, the thermodynamic potential λor describinμ the λerroelectric phase transition has to account with a nonuniλorm Ps distribution, orμanized in domains, and thereλore the domain depolarization enerμy WE and the enerμy oλ the domain walls WW are introduced in the potential [3]. The thermodynamic model λor the sinμle-crystal case, an “ideal” λerroelectric, does not need to include the λormer enerμy terms WE and WW and can be described by Eq. (2) [3]: (2) This model is based on the second-order phase transition as described by Landau-Ginsburμ; with at least a λourth-order polynomial in P, the polarization. G(P,T) is the Gibbs λunction, and α and are second- and λourth-order expansion temperature-dependent terms. “s it was said, this rather simple λorm applies λor the case when Ps is uniλorm λor all the material, as in the case oλ a sinμle crystal. In the λerroelectric/paraelectric phase transition, the behavior oλ the plots oλ the type G(P,T) versus P, λor diferent temperatures, is shown in Figure [3]. “s it can be seen, λor T2 and T0,, there is only one minimum, while λor T1, in the λerroelectric phase, two minimum values exist, which correspond to the values oλ the spontaneous polarization Ps (can be positive or neμative, accordinμ to the direction—Figure ). These values can be determined by solvinμ the diferential (∂G/∂P)Ps = 0, resultinμ in �� = ±

−� λor temperatures lower than �

Tc and Ps = 0 λor temperatures hiμher than Tc [3]. The parameters α and are related with the dielectric constants oλ LN, and more speciically, α can be expressed above Tc, in the paraelectric phase, accordinμ to the Curie-Weiss law shown in Eq. (3) [3, 15]:

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(3)

Figure . The Gibbs λree enerμy in λunction oλ the polarization P. T0 is equal to Tc, and the quantitative relation between the temperatures is T1 < T0 < T2 [3].

where χ is the dielectric susceptibility and C is a material-dependent constant. The above-considered model λails when predictinμ quantities such as the coercive ield, both λor conμruent and stoichiometric LN, because the inversion mechanisms oλ Ps occur throuμh the λormation oλ λerroelectric domains and the model does not account with Ps discontinuities. However, this thermodynamic model can be improved to beter characterize a λerroelectric material containinμ domains, accordinμ to Eq. (4) [3]: (4) “s it was stated, the terms depolarization enerμy WE and the enerμy oλ the domain walls WW are here introduced. The inteμration volume, WE and WW depend on the domain structure and μeometry. In [3] the WE and WW expressions are described λor a simple periodic domain structure model. The manipulation oλ the structure and μeometry oλ the domain walls in a λerroelectric such as LN was, and still is, an important subject oλ study, because dependinμ on the technoloμical application, some μeometries/shapes may be preλerred over others: λor example, acoustic and optical λrequency conversion devices will beneit with periodic μratinμs oλ antiparallel domains [15]. The domain shape will depend on the temperature at which they are created, throuμh the application oλ external electric ields and also on the crystal stoichiometry/composition. When created at room temperature, they can show diferent shapes due to small variation oλ stoichiometric composition [15]. Figure shows the preλerred shapes oλ domains created at 25 and 125°C, λor a conμruent LN. It is visible that the domains have a polyμonal shape with six sides, known as y walls. Curiously, the domain shape as depicted in Figure a is the same λor stoichiometric LN, while in other λerroelectric materials such as LiTaO3, the same does not happen [15].

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Figure . Piezoelectric λorce microscopy phase contrast imaμes obtained in a conμruent LN. The domains were created at 25 and 125°C [15].

Figure . XRD spectra revealinμ the efect oλ heat treatments on amorphous LN prepared by complete hydrolysis oλ the LN double alkoxide [16].

“s λor the structure oλ amorphous LN, it was said that the network can be described as distorted unitary cells randomly oriented. Figure reveals the efect oλ heat treatments (HTs) on amorphous LN prepared by complete hydrolysis oλ the LN double alkoxide [16]. The XRD spectra oλ amorphous LN have the typical λorm oλ the spectrum λor 473 K displayed in Figure , with two broad bands around 30 and 50–60°. These broad bands are a trademark oλ amorphous materials, and they are typically visible λor difraction anμles where the crystalline phase has the most intense difraction peaks, revealinμ at least a short-ranμe-order preservation. The heat treatments promote the reconiμuration oλ the amorphous phase to a more thermodynamically stable crystalline phase; althouμh λor low treatment temperatures and times, the material may be composed by a heteroμeneous mixture oλ an amorphous and a crystalline phase: λor the heat treatment at 573 K λor 0.5 h, it is still noticeable the coexistence

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oλ a broad band with the difraction peaks oλ the LN crystalline phase [16]. The structure oλ amorphous LN was also described by Kitabatake et al. to be constructed λrom the network oλ NbO6 octahedra which contains a micronetwork similar to crystalline LN [17]. The hiμh dielectric constant and a relaxation mechanism were atributed to the hiμh mobility oλ the Li+ ion in the LN structure [17]. Goinμ λurther on the structural properties, the Raman spectroscopy is a useλul nondestructive technique to access about the structure and composition oλ materials. The Raman spectrum oλ LN will μenerally depend on its stoichiometry, i.e., the shape, width, and position oλ some Raman shiλts may chanμe accordinμ to the Li/Nb ratio [18]. Furthermore, the Li/Nb ratio oλ a μiven LN sample may be determined by analyzinμ the width oλ some Raman lines, λor a μiven temperature [18, 19]. Figure exhibits experimental λull width at halλ maximum (FWHM) values oλ the Raman lines detected at about 153 and 876 cm−1, λor samples with diferent lithium contents (mol%) [19]. The measurements were carried out at room temperature (note that the Raman lines’ width also depends on the temperature). The FWHM dependency with the Li content is approximately linear, and hence a calibration line can be obtained. The uncertainty related with the Li content determination by this method was calculated to be 0.05 mol%, with an estimated uncertainty oλ 0.2 cm−1 in the 876 cm−1 line FWHM and 0.1 cm−1 in the 153 cm−1 line [19]. Thereλore, this technique can be a simple nondestructive method to estimate the Li/Nb ratio in LN crystals, with an excellent accuracy.

Figure . Full width at halλ maximum oλ the Raman lines at the wavenumbers 153 and 876 cm−1. The dots are experimental values obtained λor samples with diferent lithium contents (mol%), at room temperature, and the lines are the linear least-squares its [19].

Figure

displays the Raman spectra oλ a nearly stoichiometric LN crystal, with xc = 49.7 %,

and a conμruent LN crystal with xc = 48.5 %, where xc is μiven by �� =

�� × 00 % [18]. �� + ��

“ccordinμ to the μroup theory, when belonμinμ to the R c spatial μroup, eiμhteen vibrational

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modes are to be expected, which can be reduced in the representation 4“1 + 9E + 5“2 [18]. The “2 vibrational modes are not active in Raman and FTIR (silent modes), while both “1 and E modes are active in Raman and FTIR. The “1 modes are polarized alonμ the Z-axis, while the E modes represent vibrations alonμ the X- and Y-axes (see Figure ). Thereλore, in the XYZ coordinate system, as indicated in Figure , Z-axis lies in the c-axis direction while the X-axis in the a-axis crystalloμraphic direction. The Y-axis is perpendicular to Z and X. The notation represented in the same iμure is a universally used notation irst described by Damen et al. For example, in X(YZ)Y, the symbols inside the parenthesis are, λrom leλt to riμht, the polarization oλ the incident and scatered liμht, while the ones outside the parenthesis, λrom leλt to riμht, represent the directions oλ the incident and scatered liμht, respectively [20]. “s depicted in Figure , the E(TO) transversal modes can be detected in the X(ZY)Z coniμuration, while E(TO) and E(LO) can be detected in both X(ZY)Z and X(YZ)Y coniμurations [18]. The “1(TO) phonons, represented in Figure , at riμht, can be detected in the X(ZZ)Y coniμuration. The spectra clearly show that there is a broadeninμ oλ the lines in the conμruent composition, relatively to the nearly stoichiometric, i.e., the nearly stoichiometric spectrum lines are more resolved. Furthermore, there are lines that are only clearly visible in the nearly stoichiometric LN, and thus vibrational mode atribution in conμruent LN can be an incomplete task [18]. “s a inal remark, when dealinμ with polycrystalline LN, the discussion about the diferent possible coniμurations to detect diferent vibrational modes is not applicable, since in the polycrystalline sample, we have nano- or micrometric sinμle crystals randomly oriented, and thus interaction volume oλ the laser beam with the sample will include all these diferent orientations. In Section 2.4, the case study, we include the Raman spectra oλ polycrystalline LN, where a typical overlappinμ oλ vibrational modes is visible. The overlappinμ is due to the λact that, as Figure shows, some oλ the E(TO + LO) and “1(TO) vibration modes are in the same wavenumber ranμe, and consequently, they will overlap in the polycrystalline LN samples.

Figure . “t leλt: Raman spectra oλ two LN crystals with composition xc = 49.7 % (nearly stoichiometric) and xc = 48.5 % (conμruent), exhibitinμ the E(TO) and E(LO) vibrational modes. The arrows hiμhliμht lines that are more clearly visible in the nearly stoichiometric composition. “t riμht: the coniμuration X(ZZ)Y allows the detection oλ the “1(TO) phonons [18].

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. . Morphological characteristics Morpholoμy means “the study oλ λorm or patern,” and morpholoμical characterization techniques, such as scanninμ electron microscopy (SEM) or transmission electron microscopy (TEM), allow to characterize the morpholoμy oλ a μiven material. The morpholoμical characteristics oλ LN will obviously rely upon the synthesis process, stoichiometry, and concentration oλ intrinsic deλects. Dependinμ on the synthesis process and crystal μrowth conditions, the μrains observed λor polycrystalline LN can display well-deined symmetries and a diferent ranμe oλ sizes.

Figure . “t leλt: SEM microμraph oλ polycrystalline LN prepared by a low-temperature hydrothermal route [21]. “t riμht: SEM microμraph oλ polycrystalline LN prepared by the reactive molten salt synthesis (RMSS) process [10].

Zhan et al. [21] report in their work a low-temperature hydrothermal route to prepare polycrystalline LN. The XRD characterization revealed the λormation oλ a pure hexaμonal sinμle phase oλ stoichiometric LN. The SEM microμraph oλ the LN powder, presented in Figure , shows a reμular rhombohedral μrain morpholoμy, in aμreement with the XRD results, althouμh some imperλections, such as bended surλaces, are visible. The μrain size ranμes λrom 300 nm to approximately 1 µm. Kamali et al. [10] prepared polycrystalline LN usinμ a modiication oλ the molten salt synthesis (MSS): in conventional MSS, the mixed powders are heated above the liquids’ temperature oλ the salt mixture, and this molten salt acts as the reaction medium, remaininμ inert durinμ the synthesis. Salt mixtures such as KCl-NaCl are typically used. In the MSS modiication approached by [10], the salt can react with other reaμents durinμ the synthesis process, beinμ labeled as reactive molten salt synthesis (RMSS). They heat-treated at 973 K Nb2Cl5 powder in a LiCl molten salt, in a water-containinμ atmosphere, whereby the molten salt is one oλ the precursors λor LN synthesis [10]. Usinμ this RMSS approach, they produced sinμle-phased LN, i.e., the loss by evaporation oλ Li2O was avoided. “ SEM microμraph oλ the obtained LN particles is shown in Figure , on the riμht side [10]. The revealed morpholoμy shows μrains with dimensions ranμinμ λrom several hundreds oλ nanometers to a λew micrometers. The rhombohedral symmetry is also visible, althouμh cleavaμes and bends are visible. In Figure , it is visible a TEM briμht-ield microμraph oλ the polycrystalline LN prepared by the RMSS process. The inset on the riμht top shows a selected area electron difraction patern, beinμ the area marked by the black arrow. The difraction patern is consistent with sinμle-

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crystalline rhombohedral LN. The inset on the leλt botom shows a hiμh-resolution microμraph oλ the area marked by the white arrow. The rhombohedral LN (104) atomic plane paterns are visible, with an interplanar spacinμ oλ 0.27 nm [10].

Figure . TEM briμht-ield microμraph oλ the LN particles prepared by the reactive molten salt synthesis (RMSS). Inset on the top riμht: selected area electron difraction patern oλ the area marked by the black arrow. Inset on the leλt bottom: hiμh-resolution microμraph oλ the area marked by the white arrow [10].

The μrain morpholoμy will be determined by the μrowth conditions in such a way that the inal morpholoμy relects the coniμuration with the minimum surλace enerμy. In crystalline solid materials, the surλace tension will depend on the crystalloμraphic planes and direction, because to create a new surλace, it is necessary to break bonds. “t a constant pressure and temperature, the work required to create a new portion oλ surλace dAs in a one-component system is μiven by Eq. (5) [22]: (5) where γ is the surλace enerμy (J/m2). This represents an excess oλ enerμy relatively to the bulk and will depend on the number oλ bonds oλ the surλace (crystalloμraphic plane) and their bond enerμy. The chanμe oλ the Gibbs λree enerμy can be writen accordinμ to Eq. (6) [22]: (6) where γ is deined as in Eq. (7) [22]: (7) The λormation oλ new surλaces leads to a positive Gibbs enerμy contribution, whereby smaller particles will be unstable when compared with larμer particles. The equilibrium morpholoμy

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oλ the crystal will be determined by the surλaces with lower Gibbs enerμy, while surλaces oλ hiμher enerμy are sacriiced. Diferent crystalloμraphic planes will have a diferent number oλ bonds per unit oλ area, and the bond strenμths can also chanμe accordinμ to the composition. “ctually, very oλten when a surλace enerμy value oλ a μiven crystalline material is indicated, it is in λact an averaμe oλ the surλace enerμy oλ the diferent crystalline λaces. Takinμ as example a λace-centered cubic latice, when increasinμ the Miller indices, typically the atomic density oλ the planes decreases. The exception is that in the plane λamily [1 1 1], which contains six nearest neiμhbors, three bonds λor each surλace atom have to be broken when cutinμ the crystal alonμ such direction, while λor [1 0 0] and [1 1 0] planes, with lower atomic density, λour and six bonds have to be broken, respectively [22]. Thereλore, the surλace enerμy oλ the λormer planes is larμer relatively to the [1 1 1] plane λamily. Planes with the hiμhest density have a lower surλace enerμy and rate oλ μrowth, and thereλore the inal morpholoμy oλ the crystal μrowth will be deined by the hiμh-density atomic planes [21, 22]. However, several studies have indicated that a spherical morpholoμy, which is the coniμuration that minimizes the surλace area, is enerμetically more λavorable λor solids at hiμh temperatures, and the diference oλ surλace enerμy between diferent crystalloμraphic planes becomes a less important λactor [22]. . . Electrical and dielectric properties In this section, we will start to address the electrical properties oλ LN sinμle crystalline and polycrystalline. “λterward, we will address polycrystalline LN. However, λor both cases, it is important to analyze the properties λor diferent temperature ranμes and diferent stoichiometries, since both parameters have inluence on the mechanisms oλ electrical conduction.

Figure . Dependence oλ a LN stoichiometric sample (sample 3 oλ Figure ) total electrical conductivity with the oxyμen partial pressure p0, at 1173 K (900°C). The solid line is the linear it oλ the experimental values, while the doted line is the extrapolation [6].

It is well known that the electrical conductivity oλ LN sinμle crystals, as well as some optical properties, is stronμly dependent on the surroundinμ environmental characteristics, in

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particular the partial oxyμen pressure p0, as well as the Li/Nb ratio (stoichiometry). It was demonstrated that the electrical conductivity in hiμh temperature ranμes, between 600 and 1300 K, has a dependency oλ the type p0−1/4 λor low p0 ≲1 torr (1 torr ≈ 1/760 oλ a standard atmosphere). Figure presents the dependence oλ a sample with [Li]/[Nb] = 1 (sample 3; see inset table oλ Figure ) total electrical conductivity with p0, at 1173 K (900°C) [6]. The solid line, which represents the linear it oλ the experimental values, has a slope oλ approximately ¼ [6].

Figure . Dependence oλ σdc and E“ at room temperature with reduction temperature λor diferent reduced conμruent LN sinμle crystals [23].

Figure . “rrhenius representation oλ the electrical conductivity oλ sinμle-crystalline LN samples with diferent lithium contents, in the temperature ranμe between 500 and 900°C. [VLi] represents the mol% oλ lithium vacancies, calculated throuμh the lithium vacancy model (Eq. (1)). The activation enerμies (E“) λor the diferent samples are also indicated (adapted with permission λrom [6]).

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The electrical properties oλ LN are conditioned by the oxidation or reduction atmosphere, durinμ thermal annealinμ: in a low p0 (≲1 torr) environment, it will consist oλ a reducinμ atmosphere. The efects oλ a reducinμ atmosphere on LN crystals are typically reλerred in the literature to oriμinate the λollowinμ modiications: the irst is loss oλ oxyμens λrom the structure, which leads to the release oλ electrons which are trapped by Nb5+ cations, consequently oriμinatinμ Nb4+ cations; the second is that the reducinμ atmosphere leads to the difusion and loss oλ lithium cations, creatinμ more lithium vacancies and an excess oλ niobium cations relatively to lithium, thus leadinμ to the occupation oλ Li+ latice sites by the Nb5+ species (the so-called anti-site niobium deλects NbLi), accordinμ to the lithium vacancy model, presented in Eq. (1) [6, 23]. Dhar et al. studied the low temperature (77–373 K) dependency oλ dc electrical conduction in reduced conμruent LN sinμle crystals [23]. The samples had diferent levels oλ oxyμen reduction accordinμ to the temperature oλ reduction, in a vacuum oλ approximately 10−5 mbar. Figure shows the dependence oλ the dc conductivity (σdc) and activation enerμy (E“) at room temperature with the reduction temperature λor diferent samples [23]. The presence oλ a maximum in σdc and a minimum in E“ can be explained by the Mot’s variable ranμe hoppinμ (VRH) mechanism: the oxyμens released durinμ reduction can produce λree electrons accordinμ to Eqs. (8) and (9) [23]: (8) and (9) and thereλore the release oλ oxyμen durinμ reduction oriμinates λree electrons that can μet trapped in niobium ions, and the conduction mechanism is assiμned to polaronic hoppinμ between Nb5+ and Nb4+ cations [23]. The maximum and minimum observed in Figure can be explained accordinμ to the ratio oλ Nb4+/Nb5+ states: λor low reduction temperatures, λew Nb4+ states will be created, while λor hiμh reduction temperatures, Nb4+ will predominate. However, λor intermediate reduction temperature, there will be a case where we will μet Nb4+/Nb5+ = 0.5. In that case, a maximum in the conductivity and a minimum in E“ are to be expected, because the diferent oxidation states throuμh which the polaronic hoppinμ occurs are closer to each other [23]. They also concluded that actually the reduction annealinμ in low p0 does not lead to a siμniicant loss oλ lithium ions, as it was aλorementioned (and is λrequently mentioned in the literature), because they have shown that when reheatinμ the reduced samples in an oxyμen-rich atmosphere, without any content oλ lithium vapor, the conductivity decreases and reμains practically the same value as unreduced samples, which imply that annealinμ at low p0 does not have an important role in lithium loss [23]. For a polaronic VRH, the conductivity has a dependency as expressed in Eq. (10): (10)

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where σ0 is the pre-exponential λactor, T0 is Mot’s characteristic temperature, and s is the exponent λor the VRH model. For reduced LN sinμle crystals, the exponent s = ¼ is the one which beter describes the temperature dependency oλ σdc (check on [23] to see a loμ(σdc) vs T  − 1/4 plot). For hiμher temperature ranμes, in reduced LN sinμle crystals, the VRH oλ polarons continues to be one oλ the mechanisms λor the electrical conduction. However, and especially λor conμruent samples, lithium difusion starts to be thermally activated, and when increasinμ the temperature, the main contribution λor the total electrical conductivity may become ionic, assiμned to lithium difusion throuμh lithium vacancies in the network (once aμain, we recall the lithium vacancy model) [6]. Figure shows the “rrhenius representation oλ the total electrical conductivity oλ LN sinμle-crystalline samples with diferent lithium oxide molar percentaμes, in the temperature ranμe between 773 and 1173 K [6]. The conductivity increases with the decrease oλ the Li2O content, indicatinμ the inluence oλ lithium difusion throuμh lithium vacancies. The ionic conductivity will be larμer λor conμruent LN, because the density oλ lithium vacancies is larμer. With respect to the λrequency dependency oλ the electrical properties, typically studied by means oλ impedance spectroscopy (IS), some results oλ the work done by Mansinμh and Dhar will be addressed, namely, those related with the ac electrical conductivity (σac) and the dielectric constant (ε′) oλ conμruent LN sinμle crystals [24]. This work was published in 1985; however, more recent papers reportinμ dielectric studies as a λunction oλ λrequency and temperature λor LN sinμle crystals are surprisinμly not that easy to ind, because most oλ them deal with polycrystalline LN or with LN doped with other elements.

Figure . Dependence oλ σac and σdc with the temperature (77–700 K) λor diferent ixed λrequencies (kHz): (•) DC, (○) 0.1, (x) 1, (Δ) 10, (□) 100 [24].

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Figure shows λor a conμruent LN sinμle crystal the dependence oλ σac and also σdc (althouμh in a more limited temperature ranμe relatively to σac) with the temperature (77–700 K) λor diferent λrequencies, between 100 Hz and 100 kHz. It is visible that λor lower temperatures σac presents hiμh-λrequency dispersion, and it is considerably hiμher than σdc, while λor hiμher temperatures it becomes practically λrequency-independent and stronμly temperaturedependent. Moreover, the temperature at which σac starts to have the same value as σdc increases with the increase oλ the λrequency. The mechanism λor lower temperatures was λound to be well described by a hoppinμ-over-the-barrier (HO”) mechanism, and it was correlated with electron hoppinμ between diferent valence states oλ the niobium, because oλ the reduction oλ Nb5+ due to oxyμen deiciencies (as it was reλerred beλore) [24].

Figure . Frequency dependence oλ σac λor some ixed temperatures (K): (◑) 77, (▼) 220, (▽) 320, (■) 415, (□) 475, (▲) 530, (∆) 580, (●) 625, (○) 650, (x) 680 [24].

Figure shows the λrequency dependence oλ σac λor some ixed temperatures. For lower temperatures, up to 530 K, the dependency oλ σac with the λrequency can be expressed by the relation presented in Eq. (11) [24]:

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(11)

Figure . Frequency dependency oλ the dielectric constant (ε’) λor diferent ixed temperatures (K): (◑) 77, (▲) 530, (∆) 580, (●) 625, (○) 650, (x) 680 [24].

This is the well-known relation λound by Mot and Davis which describes the λrequency dependency oλ σac λor many amorphous and crystalline materials. The HO” mechanism presents a λrequency dependency which can be described by Eq. (11). Furthermore, the HO” mechanism predicts a decrease oλ the λrequency exponent s with the increase oλ temperature, and the values oλ s calculated by Mansinμh and Dhar (~1 λor 77 K and ~0.6 λor 530 K) aμree satisλactorily with the HO” model [24]. For hiμher temperatures, as it was aλorementioned, σac becomes practically λrequency-independent and with a maμnitude close to σdc. “t the same time, λor the same hiμh temperature ranμe (relatively to σac, see Figure ), ε′ is characterized by a stronμ λrequency dispersion, as shown in Figure . The temperature dependence oλ ε′ λor diferent ixed λrequencies, between 100 Hz and 100 kHz, is also shown in Figure . It can be noted in Figure that the temperature λrom which ε′ shows a sharp increase increases with the increase oλ λrequency. So, λrom the presented plots oλ σac and ε′, it is evident that the temperature dependences show evidence oλ two distinct mechanisms λor the conductivity. “t low temperatures, the mechanism was already identiied, the HO” mechanism with a hiμh distribution oλ relaxation times. For hiμher temperatures, the stronμ ε′ dispersion is probably associated with the dc conduction mechanisms, while both ac and dc conductivities are determined by the same mechanism, lonμ-ranμe hoppinμ oλ charμe carriers [24]. “ sample with thickness reduced to halλ, keepinμ the electrode surλace area constant, was included in Figure to demonstrate that the sharp increase oλ ε′ is not related with the electrode barriers and spatial charμe accumulation at the electrode/sample interλace [24].

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Figure . Temperature dependency oλ the dielectric constant (ε’) λor diferent ixed λrequencies (kHz): (○) 0.1, (x) 1, (Δ) 10, (■) 100, (●) Sample with halλ oλ the thickness, keepinμ the same area [24].

We will end this section by briely addressinμ the electrical and dielectric properties oλ polycrystalline LN. In such case, the behavior oλ the reλerred properties can relect the presence oλ μrain boundaries in the material. This efect oλ μrain boundaries can be more clearly seen in IS measurements, because the characteristic λrequencies (or times) at which μrain boundary processes occur are diferent λrom those that occur in the bulk oλ the μrains, and thereλore Nyquist diaμrams [plot oλ the neμative oλ the imaμinary part oλ the impedance, −Im(Z), in the y-axis over the real part oλ the impedance, Re(Z), in the x-axis] oriμinate successive semicircles, where each point oλ the semicircle corresponds to a diferent λrequency value which increases counterclockwise. Lanλredi and Rodriμues report in their work IS studies oλ the electrical conductivity and dielectric constant oλ polycrystalline LN [25]. Figure presents Nyquist diaμrams λor diferent temperatures oλ two polycrystalline LN samples a and b. Sample b has approximately halλ oλ the thickness to electrode surλace area ratio (l/“) relatively to sample a.

Figure . “t leλt: Nyquist diaμram λor diferent temperatures λor a polycrystalline LN sample with a thickness to electrode surλace area ratio l/“ = 0.200 cm−1 (sample a). “t riμht: Nyquist diaμram λor diferent temperatures λor sample a polycrystalline LN sample with a thickness to surλace area ratio l/“ = 0.105 cm−1 (sample b), approximately halλ oλ sample a. For both samples, the smallest semicircle corresponds to a measurement perλormed at 700°C [25].

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Comparinμ both diaμrams, it is visible that λor the same temperature, the real part oλ the complex impedance Re(Z) oλ sample b is approximately halλ oλ sample a, thus conirminμ that the hiμh-λrequency semicircle is the bulk response oλ the samples, since the resistance is directly proportional to the path lenμth [25]. Furthermore, the λrequency value distribution in the bulkresponse semicircle is the same λor both samples, indicatinμ the homoμeneity oλ the bulk response, and that the relaxation λrequency, μiven by the peak oλ the bulk semicircle (in the peak, the relation 2πf0RbCb = 1 is λulilled, where Rb and Cb are bulk resistance and capacitance, respectively), is an intrinsic property oλ the material and does not depend on μeometrical λactors [25]. “s expected, the bulk resistance decreases with the increase oλ the temperature. The low-λrequency semicircle is assiμned to the response oλ μrain boundaries, and its depressed shape is an indicator oλ a nonhomoμeneous electrical behavior oλ μrain boundaries [25]. This nonhomoμeneous behavior can be related with an existence oλ a distribution oλ relaxation times. The complex impedance semicircle λor the bulk response can be well ited by simple RbCb equivalent circuit. “ bulk electrical conductivity σb can be deined accordinμ to Eq. (12): (12)

Figure . “rrhenius representation oλ the bulk electrical conductivity σb oλ the polycrystalline LN samples a and b, in the temperature ranμe between 450 and 800°C [25].

Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Raman Spectroscopy and Scanning Electron Microscopy Spectroscopy and Scanning Electron Microscopy http://dx.doi.org/10.5772/64395

Figure

. Frequency dependency, in the ranμe oλ 5–107 Hz, oλ ε′ λor some ixed hiμh temperatures, λor sample b [25].

R b can be determined throuμh the Nyquist plots by the second interception (just beλore the μrain boundary response) oλ the bulk semicircle with the real axis. Figure displays λor both samples a and b the “rrhenius representation oλ σb in the temperature ranμe between 450 and 800°C [25]. The activation enerμies are very similar. To conclude this section, it is included in Figure the λrequency dependency, between 5 and 107 Hz, oλ ε′ λor some ixed hiμh temperatures, λor sample b. For lower λrequencies, a stronμ dispersion oλ ε′ is observed. This is due to spatial charμe accumulation at the μrain boundaries, and the charμe accumulation at interλace electrode/ sample may also contribute to the sharp increase oλ ε′ λor lower λrequencies. This behavior is oλten observed λor polycrystalline materials. . . Case study: preparation and characterization of polycrystalline LN by the Pechini method 2.4. . Preparation process: the Pechini route The sol-μel process is a well-known route λor the synthesis oλ diferent types oλ materials, and in its basic description, it can be reλerred as a technique that synthesizes a solid compound throuμh a chemical reaction in solution at low temperatures. There are diferent sol-μel methodoloμies accordinμ to the type oλ precursors used and the chemical reactions leadinμ to the λormation oλ the μel: there is the sol-μel methodoloμy based on the hydrolysis-condensation oλ metal alkoxides, the “chelate-μel” route, involvinμ aqueous solutions containinμ metal chelates, and the Pechini route. The sol-μel methodoloμies have μeneral advantaμes such as the very μood control oλ the stoichiometry and purity oλ the inal material, low processinμ temperatures, possibility, and μood lexibility in developinμ thin ilms as well as the possibility

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to have control over some important characteristics, such as the size and shape oλ the particles and homoμeneity. The Pechini route takes its name on its developer, Maμμio Pechini in 1967 [26]. In particular, it was developed to include metals which are not suitable λor traditional sol-μel reactions due to their unλavorable hydrolysis equilibria, and thus this method has the advantaμe oλ not requirinμ that the metallic species involved λorm stable hydroxo-complexes. This method is known λor its use on the synthesis oλ multicomponent metal oxide materials [26], and basically this method uses an R-hydroxycarboxylic acid, such as citric acid (C“), to lead the λormation oλ stable metal complex, i.e., the metallic cations oλ interest λorm stable complexes known as chelates. “λter this step, a polyalcohol, such as ethylene μlycol (EG), is used to promote the polyesteriication oλ the chelates, leadinμ to the λormation oλ a polymeric resin, where the metallic cations are trapped in the orμanic polymeric network. In other words, the polyalcohol is able to create links between the chelates by polyesteriication reactions. The λormation oλ the polymeric resin results in the λormation oλ the μel. The subsequent dryinμ process leads to the pyrolysis oλ the orμanic compounds, resultinμ in the λormation oλ multicomponent metal oxide [26]. In this case study, the precursors lithium nitrate (LiNO3) and niobium chloride (NbCl5) (purity > 99%) were chosen. “ molar ratio oλ 1:1 between LiNO3 and NbCl5 was established in order to enhance the λormation oλ the LiNbO3 stoichiometric phase. Firstly, the LiNO3 and NbCl5 were dissolved in deionized water and in a hydroμen peroxide solution (H2O2, 3%, V/V), respectively. For each μram oλ NbCl5, 3.2 ml oλ H2O2 was used, oriμinatinμ a yellow transparent and clear solution. ”oth precursor solutions were mixed with citric acid (C“), ixinμ a molar ratio oλ 1:1 between the C“ and the metallic cations, in order to λorm the metal complexes (chelates). The mixinμ was perλormed usinμ a maμnetic stirrer λor 30 min at room temperature. “λter the mixinμ process, ethylene μlycol (EG) was added to promote the polyesteriication oλ the chelates. “ mass ratio oλ 2:3 was established between the C“ and EG to determine the quantity oλ EG to use. The inal solution was mixed aμain with a maμnetic stirrer λor about 3 h, and the inal μel was yellow and transparent, maintaininμ its macroscopic appearance λor a lonμ time period (>1 month). Figure outlines the entire preparation process. The base powder was obtained aλter dryinμ the μel at 300°C λor 1 h, with a heatinμ ramp oλ 5°C/min, yieldinμ a black/μrayish powder. 2.4.2. Thermal, structural, and morphological properties The base powder was subjected to several heat treatments (HTs) at temperatures between 400 and 1000°C. These temperatures were chosen accordinμ to the diferential thermal analysis (DT“) results, presented in Figure . This thermal technique was perλormed between room temperature and 1200 °C usinμ a Linseis 63“ apparatus. The heatinμ rate was 20 °C/min and “l2O3 powder as used as reλerence. In Figure , the thermoμram shows the presence oλ three exothermic thermal processes at the temperatures oλ 490, 790, and 1050°C, approximately. Consequently, HTs were perλormed at 450, 500, 800, and 1000°C, λor 4 h.

Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and and Scanning Scanning Electron Microscopy Raman Spectroscopy http://dx.doi.org/10.5772/64395

Figure μlycol.

. Diaμram oλ the Pechini method used λor the synthesis oλ the LN base powder. C“, citric acid; EG, ethylene

Figure . DT“ thermoμram oλ the base powder synthesized by the Pechini method. This thermal analysis was perλormed between room temperature and 1200 °C, with a heatinμ rate oλ 20 °C/min.

The XRD measurements perλormed on these powders (not shown here) revealed that λor the HT at 500°C, the LN and LiNb3O8 crystalline phases are present and that the increase oλ the HT temperature promotes the development oλ the LiNb3O8 phase. However, the powder HT

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at 450°C only contained the LiNbO3 phase, whereby this powder was used to prepare pellets which were then sintered at 450°C λor 4, 12, 24, 48, and 96 h. Thirty milliμrams oλ powder was used λor the preparation oλ the 10-mm diameter pellets, resultinμ in a thickness oλ about 1 mm when applyinμ a uniaxial pressure oλ 1.5 tons. Hereaλter, we will reλer to these pellets as samples.

Figure . XRD paterns oλ the base powder HT at 450°C and oλ the samples sintered λor 4, 12, 24, 48, and 96 h (× LiNbO3; O LiNb3O8).

Figure depicts the XRD paterns oλ the samples sintered durinμ the aλorementioned time intervals. The paterns show that the sinterinμ process activated the λormation oλ the LiNb3O8 phase. The XRD technique was perλormed on a Philips X’Pert MPD (CuKα radiation, λ = 1.54056 Å), with a step 0.02° in 1 s, in the 2θ anμle ranμe oλ 10–60°. The identiication oλ the crystalline phases was made usinμ the database oλ the Joint Commitee on Powder Difraction Standards–International Center λor Difraction Data. To μet a λurther insiμht about the contents oλ each phase in the samples as well as to calculate the crystallite sizes associated with each phase, a Rietveld reinement was perλormed λor all the difraction paterns shown in Figure , usinμ the PowderCell soλtware. In Figure , the Rietveld its oλ the samples 4 h and 4 h + 96 h are presented.

Figure . The Rietveld its oλ the XRD paterns λor the powder HT at 450°C, containinμ only the LN crystalline phase and the sample sintered λor 96 h, containinμ both LN and LiNb3O8.

Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and and Scanning Scanning Electron Electron Microscopy Microscopy Raman Spectroscopy http://dx.doi.org/10.5772/64395

The Rietveld it parameters indicate a μood it oλ the structural models to the experimental data (Table ). “lthouμh the XRD paterns shown in Figure may suμμest that the LiNb3O8 phase is present in small amounts, the mass percentaμes shown in Table show that the presence oλ this phase is relevant, reachinμ the maximum value λor the sample 4 h + 96 h. In λact, both LN and LiNb3O8 have relections lyinμ in close difraction anμles, and some oλ the observed peaks contain a contribution oλ the LiNb3O8 phase, besides the LN phase, explaininμ the relatively hiμh mass percentaμes. The crystallite size oλ both phases stands approximately constant λor the diferent samples, especially λor the LiNb3O8 phase, which has larμer sizes relatively to the LN phase. Sample 4h

R wp

R exp

χ

Crystallite size nm

Mass%

LiNbO

LiNb O

LiNbO

LiNb O

′

tan

8.78

6.20

2.01

43.90

–

100

–

–

–

4h+4h

10.40

8.72

1.42

45.52

61.94

72.48

27.52

8.64

0.06

4 h + 12 h

6.26

4.77

1.72

48.30

61.32

66.48

33.52

9.37

0.05

4 h + 24 h

6.31

4.49

1.97

50.23

61.03

66.94

33.06

16.85

0.09

4 h + 48 h

9.72

8.23

1.39

44.84

62.28

75.90

24.10

16.36

0.13

4 h + 96 h

9.37

7.70

1.48

46.65

59.50

61.76

38.24

13.06

0.18

Table . The initial three parameters are the weiμhted proile R-λactor (Rwp), the expected R-λactor (Rexp), and the “chi squared” χ2. The crystallite size and mass percentaμe oλ each crystalline phase in all the samples are also indicated. The last two columns show the dielectric constant (ε′) and loss tanμent (tan δ) at 10 kHz and room temperature (300 K).

Figure . The Raman spectra oλ the samples sintered λor diferent times between the time ranμe 4 and 96 h, perλormed at room temperature.

In Figure , the Raman spectra oλ the sintered samples are presented. The spectra show the presence oλ vibrational bands which are the result oλ an overlappinμ oλ vibrational models oλ LN and also oλ the LiNb3O8 crystalline phase, as a consequence oλ the polycrystalline structure

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oλ the prepared samples. For a λurther analysis oλ the LN and LiNb3O8 vibrational modes, the authors suμμest the readinμ oλ ”artasyte et al. report [27]. The room-temperature Raman spectroscopy was perλormed in backscaterinμ μeometry usinμ a T64000 Jobin-Yvon spectrometer. “ microscope objective (50×) λocused the excitinμ liμht (“r+ laser, λ = 532 nm) onto the sample (spot diameter 95%) doped with ission products and transuranium elements [10], which are λound as bubbles (Xe, Kr), metallic precipitates (Mo, Tc, Ru, Rh, and Pd), oxide precipitates (Rb, Cs, ”a, and Zr), solid solutions, and transuranium elements dissolved by U substitution in the UO2 matrix [11]. These elements are not distributed homoμeneously as a consequence oλ the thermal μradient within the UO2 pellet (temperature as hiμh as 1700°C at the center oλ the pellet and decreasinμ to 400°C outwards) [12, 13]. ”esides, the spent nuclear λuel sufers substantial microstructural modiications λrom the initial λresh λuel such as coarseninμ oλ the μrains and extensive microcrackinμ. Thus, SNF can be described as a complex, hot, and radioactive waste and thereλore extremely danμerous. ”runo et al. [14] provide a particularly suitable example to demonstrate how hazardous the SNF is, that is, …”One year after discharge from a reactor…a person exposed to this level of radioactivity at a distance of one meter would receive a lethal dose in less than one minute….” “λter several thousands oλ years, the total radioactivity oλ the SNF equals the radioactivity oλ natural uranium [15]. Thereλore, within the manaμement oλ spent nuclear λuel, the saλe storaμe oλ this radioactive waste λrom the discharμe oλ the reactor until the decay reaches natural uranium radioactivity is considered. Countries adopt diferent staμes oλ these nuclear storaμes accordinμ to their internal policies [16], but usually the λollowinμ steps are suitable: (i) spent λuel pools [17], (ii) intermediate storaμe or reprocessinμ [18], and (iii) inal storaμe [19]. “λter SNF is removed λrom the reactor, it is stored λor the irst λew years on-side in water containers or pools, located close to the reactor in order to allow the spent λuel to decay, both radioactively and thermally. Then, it can be transported to a reprocessinμ λacility or to a deinitive storaμe λacility. However, since inal repositories λor spent λuel do not exist λor the moment, interim storaμe is required. NPPs use the spent λuel dry-cask storaμes, which are steel and concrete containers illed with an inert μas as a irst step λor interim storaμe. “lthouμh no ultimate storaμe in operation exists, the deep μeoloμical repository is internationally accepted as the best solution [20]. The perλormance oλ the mentioned repositories requires knowledμe oλ the SNF stability at diferent storaμe conditions. Thereby, the studies oλ the spent λuel behavior can be mainly divided into dry and wet conditions, μiven the diferent evolution observed in each case. The studies oλ spent nuclear λuel under dry conditions are mainly λocused on the oxidation oλ both the UO2 matrix and the “nO2 [21] present in the SNF.1 In case oλ shieldinμ λailure, the oxidation oλ “nO2 and UO2 takes place owinμ to its contact with the atmospheric oxyμen and the hiμh

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temperatures present (up to 400°C) [15]. This oxidation occurs via oxyμen incorporation into the luorite structure (λcc) oλ the stoichiometric oxide, λor example, some actinides as plutonium and uranium can oxidize to “nO2+x (x < 0.25) maintaininμ the λcc structure [22]. Further oxidation to hiμher-oxidation states (V and VI) leads to diferent structures. For example, the transλormation oλ UO2 into U3O8 via the two-step reaction [23] UO2 → U4O9/U3O7 → U3O8 entails an increase in the volume oλ around 36% and, consequently, it miμht cause the loss oλ the UO2 matrix inteμrity. On the other hand, the studies oλ spent nuclear λuel under wet conditions are λocused on the corrosion process oλ this waste. This miμht happen in case the SNF shieldinμ λails while stored in pools or in the deep μeoloμical repository at timescales oλ the order oλ some thousands oλ years [24] when it is assumed that the barriers that protect the waste will be breached and SNF will be in contact with water [25]. The UO2 matrix oλ the spent nuclear λuel miμht dissolve with water and then the release to the biosphere oλ the SNF radioactive contents miμht occur [26–28]. This corrosion process is primarily described by the oxidation oλ uranium, U(IV) → U(VI), and then the alteration products λormation, usually containinμ UO22+ in their crystal structures [29] U(VI) → UO22+ (s). Great efort has been perλormed to analyze the reaction mechanism and to establish the key parameter that controls the corrosion oλ the SNF such as leachinμ/dissolution experiments [30–32] and studies oλ the uraninite, a natural analoμ oλ the spent nuclear λuel matrix [33, 34]. These stability studies require the characterization oλ the SNF and its reaction products, with O2 and/or water, which is a μreat challenμe not only because these materials are very complicated (containinμ almost the entire periodic table) [11] but also because intense radiation ield inherently associated to these materials makes it diicult to examine them in saλe conditions. In order to minimize radiation doses and the release oλ radioactive material, the workinμ procedure employed to study these materials must λulill the “L“R“ principle (acronym λor “as low as reasonably achievable”) [35]. Such a reliable procedure must, hence, minimize the time that radioactive materials are handled and maximize the distance to them. Raman spectroscopy is an analyzinμ technique that has been established in recent years as a useλul tool since it λulills the mentioned saλe principles, as shown by the increase in the number oλ publications dealinμ with the characterization oλ nuclear materials by this technique [36–44]. This is due to some oλ its λeatures as (1) that it does not require any special preparation oλ the sample, (2) it allows the analysis oλ a very small amount oλ sample, and (3) it is a nondestructive technique. ”esides these saλety principles, the coninement oλ the whole apparatus in a μlove box or a hot cell is also very common, which obviously complicates the measurements [45]. Despite the advantaμes mentioned above, the characterization oλ these SNF and related nuclear materials is λar λrom beinμ well established. Existinμ databases must be improved and new methods 1 “ meaninμ minor actinides such as Np, Pu, and “m.

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must be developed. Due to the hazardous λeature oλ nuclear materials, both the development oλ new protocols and Raman spectra acquisition (λor the purpose oλ extendinμ the databases) are usually perλormed irst by analyzinμ the behavior oλ diferent SNF analoμs and, once the method is λeasible, by applyinμ it to the real SNF. Such analoμs can be divided into two kinds: synthetic analoμs such as uranium dioxide (UO2) [46, 47] or SIMFUEL (simulated λuel) [48, 49], and natural analoμs such as uraninite. In this context, this chapter is structured as λollows: In the irst part, Raman spectroscopy is described. First, the theoretical aspects on an introductory level are explained. Second, the main components oλ the Raman spectrometers are presented and, as an example, the LabRam HR Evolution spectrometer is described in more detail. The “Results” section has been divided into two, correspondinμ to dry and wet conditions; in each part, the developed method and the results λound λor analoμs oλ the SNF are shown. Namely, the materials studied in this section are the diferent uranium oxides, UO2+x (0 < x < 0.25), U4O9/U3O7, and U3O8, and several secondary phases such as rutherλordine, soddyite, uranophane alpha, or kasolite.

. Raman spectroscopy technique . . Description of the Raman phenomena Raman efect owes its name to the Indian physicist Chandrasekhara Venkata Raman [50] who won the Nobel Prize λor its discovery. In his Nobel lecture, μiven on December 11, 1930, Sir C.V. Raman said…“The frequency diferences determined from the spectra, the width and character of the lines appearing in them, and the intensity and state of polarization of the scatered radiations enable us to obtain an insight into the ultimate structure of the scatering substance […]. It follows that the new ield of spectroscopy has practically unrestricted scope in the study of problems related to the structure of mater” [51]. “s other molecular spectroscopy techniques, Raman scaterinμ is based on the analysis oλ liμhtmater interaction [52], that is, absorption, emission, or scaterinμ oλ a photon. Two interpretations oλ this phenomenon can be considered: the quantum mechanical method and the classical interpretation. In the purely classical interpretation, the radiation is considered as an electromaμnetic wave, and the mater as an assembly oλ independent classical rotors and vibrators. This model can explain satisλactorily the main λeatures oλ the liμht scaterinμ such as the λrequency dependence and some key aspect related to their selection rules. Raman efect is described as an inelastical scaterinμ oλ liμht. From a macroscopic point oλ view, liμht scaterinμ consists in a deviation oλ liμht λrom its straiμht trajectory (oriμinal direction oλ incident liμht). Molecules scater liμht because the electric ield oλ the incident liμht wave λorces the electrons within the molecule to oscillate (see Figure ), producinμ oscillatinμ electric moments leadinμ to the reemission oλ radiation in all directions [53].

Raman Spectroscopy, a Useful Tool to Study Nuclear Materials http://dx.doi.org/10.5772/64436

Figure . Liμht scaterinμ produced by the interaction oλ the incident liμht’s electric ield and the molecule electrons.

Such process produces two types oλ radiation, Rayleiμh radiation, which has the same λrequency that the incident liμht (ν0), and Raman radiation, which consists in a new set oλ λrequencies with more or less enerμy than the incident radiation (ν0 ± ν1), where ν1 is typically related to the rotational, vibrational, and electronic levels oλ the molecule. In Figure , a μeneral scheme oλ the scaterinμ process and its diference with the absorption process λrom the point oλ view oλ the photons and the enerμy levels oλ the molecule are represented.

Figure . Enerμy level diaμrams describinμ the physical phenomenon oλ (1) IR absorption, (2) Rayleiμh scaterinμ, and (3) Raman scaterinμ.

”eλore the interaction oλ the radiation with the system, there are N photons oλ enerμy hcν0. In the case oλ the absorption process, the interaction oλ the radiation with the system leads to the excitation oλ the molecule to a hiμher enerμy state resultinμ in a radiation which consists in N − 1 photons oλ enerμy hcν0. This process can occur, iλ and only iλ the incominμ photon has the same enerμy as the diference between the initial and inal state oλ the molecule, Eλ − Ei = hcνi = hcν0, λulillinμ the condition oλ enerμy conservation. Let us now consider the scaterinμ process, the interaction between the incident radiation and the system produces the annihilation oλ a photon oλ enerμy hcν0, and simultaneously the creation oλ a new photon with enerμy Es. Now the radiation consists in N − 1 photons oλ enerμy hcν0, a new photon oλ enerμy hcνs, and the transition to the molecule to a inal state with enerμy Eλ. In the overall process, the enerμy must be conserved so, hcν0 = hcνs + Eλ. This two-photon process can be visualized as two simultaneous staμes. First, the annihilation staμe which leads the molecule to a virtual hiμh-enerμy state. Virtual states are created when

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the laser interacts with the molecule and causes polarization; hence, their enerμy is determined by the λrequency oλ the incident liμht source used (ν0). “t this staμe, there is no enerμy conservation implyinμ that the role oλ the incident radiation in the scaterinμ is to perturb the molecule μiven the possibility to allow diferent spectroscopic transitions rather than the absorption process. Second, the creation staμe, where the molecule reaches its inal state with enerμy Eλ producinμ the new photon. “t this point, we can consider two types oλ scatered liμht: (1) the Rayleiμh scaterinμ, or elastic scaterinμ, where the inal state oλ the molecule is its own initial state, Eλ = Ei, and, correspondinμly, the enerμy oλ the scatered liμht corresponds to the initial λrequency value, νs = ν0. (2) The Raman scaterinμ, or inelastic scaterinμ, in which the molecule reaches a inal state diferent λrom its initial state; hence, the enerμy oλ the scatered liμht has a diferent λrequency value λrom the incident radiation, νs = ν0 ± νi. This process is much less probable than Rayleiμh scaterinμ (only 10−5 − 10−8 oλ the incident beam intensity). Iλ the inal state has a hiμher enerμy than the initial state, Eλ > Ei, the scatered photon loses enerμy, νs = ν0 − νi. This radiation is known as Stokes Raman scaterinμ. ”y contrast, iλ the inal state has a lower enerμy than the initial state, Eλ < Ei, the scatered photon increases its enerμy, νs = ν0 + νi, μivinμ the anti-Stokes Raman scaterinμ. Relative probabilities oλ Stokes and anti-Stokes radiation depend on the population oλ the molecule states, f and i, and thereλore on temperature accordinμ to the Maxwell-”olzmann distribution. “s both μive the same inλormation, it is customary to measure only the “Stokes” side oλ the spectrum. Even thouμh this μeneral scheme describes scaterinμ phenomena in a qualitative way, it hiμhliμhts some key aspect oλ the Raman spectroscopy and its diferences with the absorption process. Nevertheless, it is worth to describe the classical treatment oλ the Raman scaterinμ in order to provide a deeper insiμht in the λrequency dependence and the microscopic oriμin oλ the scatered liμht. Classical wave interpretation [54, 55] oλ the Raman efect is based on the time-dependent polarizability oλ the molecules. Consider one oλ the simplest scaterinμ systems, a vibratinμ diatomic “” molecule. Such a system can be modeled, at irst approximation, as two balls atached by a sprinμ (Figure ). “ccordinμ to Hook´s law, its relative movement can be described by the second Newton law as λollows:

Figure . Simpliied model oλ a diatomic “” molecule.

æ d 2x d 2x ö m ç 21 + 22 ÷ = K ( x1 + x2 ) dt ø è dt

(1)

Raman Spectroscopy, a Useful Tool to Study Nuclear Materials http://dx.doi.org/10.5772/64436

where µ represents the reduced mass oλ the molecule, x represents the displacement, and K represents the bond strenμth. For small vibrations, the harmonic approximation holds, and then the normal coordinates q(t) oλ the vibratinμ molecule can be expressed as q = q0 cos ( 2pn mt )

(2)

where q is the amplitude and νm is the natural vibration λrequency which is deined in terms oλ its bond strenμth as

nm =

1 2p

K

m

(3)

When incident liμht interacts with a molecule, induces a dipole moment, P, equal to the product oλ the polarizability oλ the molecule, α, and the electric ield oλ the incident liμht source E P = a E0 cos ( 2pn 0t ) ,

(4)

where E0 and νo are the electric ield amplitude and λrequency, respectively. “s λar as the molecule is vibratinμ, its polarizability varies accordinμ to the relative displacement oλ these atoms and thereλore we can express α as a power series q æ ¶a ö a = a0 + q ç ÷ +¼ è ¶q ø0

(5)

which when combined with Eqs. (3) and (5) results in, æ ¶a ö P = a 0 E0 cos ( 2p v0t ) + q0 cos ( 2p vmt ) E0 cos ( 2p v0t ) ç ÷ è ¶q ø0

(6)

æ ¶a ö P = a 0 E0 cos ( 2p v0t ) + ç ÷ q0 E0 éëcos ( 2p {v0 - vm } t ) + cos ( 2p {v0 + vm } t ) ùû è ¶q ø0

(7)

From Eq. (7), it is evident that the induced electric dipole is λormed by three diferent terms. The irst one μives rise to an oscillatinμ moment at the same λrequency oλ the incident liμht, the Rayleiμh scaterinμ, and two additional terms which accounts λor the Stokes and anti-stokes Raman scaterinμ. Thereλore, Rayleiμh scaterinμ arises λrom an electric dipole which oscillates at the same λrequency induced in the molecule by the electric ield oλ the incident radiation,

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whereas Raman scaterinμ arises λrom the modulation oλ the electric dipole with the natural λrequency oλ the vibratinμ molecule. This modulation is produced by the electrons oλ the molecule, whose rearranμement produces a couplinμ between the nuclear motion and the electric ield oλ the radiation. . . Dispersive Raman spectrometer From the basis oλ the Raman efect described above, it is easy to deduce that in a conventional (or dispersive) Raman spectrometer,2 the main diiculty lies in separatinμ the intense stray liμht oλ the Rayleiμh scaterinμ λrom the much weaker Raman-scatered liμht. ”esides, as Raman scaterinμ has low eiciency, the optimization oλ each oλ the instrumental components becomes critically important. The main components oλ a Raman setup are as λollows: (1) Excitation source (2) Sample illumination systems and collection optics (3) Wavelenμth selectors and separators (4) Detector (5) Recordinμ device . Excitation source: Traditionally, mercury arc lamps were used as liμht sources until beinμ replaced by laser sources. Laser beams are hiμhly monochromatic, present small diameter and, with the help oλ diferent optic devices, can be λocused on small samples. Diferent lasers can be used as the liμht source in Raman spectrometry, as the ones shown in Table [56]. Laser

Wavelength nm

“r ion

530.9/647.1

He-Ne

632.8

Near IR diode

785/830

Nd-Y“G

1064

Frequency-doubled Nd:Y“G

532

Nd:YVO4 diode

532

Table . Lasers used as liμht source in Raman spectroscopy.

In addition, in order to enhance the laser quality it is possible to employ a pass-band ilter, desiμned to pass only a certain band oλ λrequencies while atenuatinμ all siμnals outside this band. This component is commonly known as interλerometric ilter. 2. Sample illumination system and collection optics: The collimation and λocusinμ optics oλ the excitinμ radiation onto the sample depends on the experimental setup. In principle, excitation 2 Raman systems are subdivided into two principals accordinμ to the spectral analysis oλ the Raman liμht, namely Fouriertransλorm (FT) systems usinμ an interλerometer, and dispersive systems.

Raman Spectroscopy, a Useful Tool to Study Nuclear Materials http://dx.doi.org/10.5772/64436

and collection λrom the sample can be accomplished in any μeometry, althouμh 90 and 180°C (backscaterinμ) are more λrequently employed. The use oλ iber optics helps to make the spectrometers more versatile. . Wavelength selectors and/or separators: The separation or removal oλ the intense Rayleiμh scaterinμ can be achieved by usinμ two diferent types oλ ilters: notch and edμe ilters. Notch ilters allow the acquisition oλ the anti-Stokes and Stokes Raman spectra down to ~30 cm−1, but their use is expensive since they must be replaced very λrequently (~2 years). For this reason, the use oλ edμe ilters is widespread. These are wide pass-band ilters, which imply that the anti- Stokes Raman spectrum cannot be obtained and typical minimum wavenumbers are ~50 cm−1. “λter the removal or suppression oλ the Rayleiμh radiation, the separation oλ the diferent Raman radiations scatered by the sample should be perλormed. The irst Raman spectrometers used prisms, but nowadays these are replaced by μratinμs that are typically holoμraphically produced. It is worth notinμ that ilters can be neμlected iλ couplinμ oλ two or three monochromators is set in a series. This coniμuration allows not only to separate the Raman lines but also to remove the Rayleiμh scater. 4. Detectors: Just like in other spectrometers, the λormer detectors, that is, photoμraphic ilms, were substituted irst by photodiode array detectors and then by charμe transλer devices (CTDs) such as charμe-coupled devices (CCDs). CCDs are silicon-based semiconductors arranμed as an array oλ photosensitive elements, each one μeneratinμ photoelectrons and storinμ them as an electrical charμe. Charμes are stored on each individual pixel as a λunction oλ the number oλ photons strikinμ that pixel and then read by an analoμ-to-diμital converter [54]. “ schematic representation oλ a modern micro-Raman spectrometer is shown in Figure .

Figure . Descriptive scheme oλ the main components oλ a Raman microspectrometer.

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Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences

In the micro-Raman technique, a microscope is inteμrated in a conventional Raman spectrometer, enablinμ both visual and spectroscopic measurements. “s can be seen in Figure , in these types oλ equipment the λocusinμ and collection optics oλ the scatered radiation are identical. In addition to the analysis oλ a sinμle point, these spectrometers allow mappinμ and imaμinμ measurements.

. Results “s explained beλore, this section has been divided into two parts correspondinμ to dry and wet conditions; into the correspondinμ part the developed methods and the results obtained λor analoμs oλ the spent nuclear λuel matrix and its alteration products are shown. Namely, the materials studied in this section are the diferent uranium oxides, UO2+x (0 < x < 0.25), U4O9/ U3O7, and U3O8, and several secondary phases such as rutherλordine, soddyite, uranophane alpha, and kasolite. The results shown in this section have been obtained by usinμ the LabRaman HR Evolution (Horiba Jobin Yvon Technoloμy, i.e., a dispersive spectrometer equipped with a microscope that enables the uniication oλ both λocusinμ and collection optics. It is possible to couple any laser to the spectrometer optical system as the excitation source. We speciically use the internal HeNe laser oλ 20-mW nominal power and an excitation wavelenμth oλ 632.8 nm (red). The laser beam is λocused on the sample throuμh a conλocal microscope with diferent maμniications (5×, 20×, 50×, and 100×). Scatered radiation is then collected by the microscope on its way back (180° scaterinμ) and the Rayleiμh contribution removed by an edμe ilter that cuts at