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Perovskite chalcogenides: emerging semiconductors for visible to infrared optoelectronics
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Perovskite chalcogenides: emerging semiconductors for visible to infrared optoelectronics
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Perovskite Chalcogenides: Emerging Semiconductors for Visible to Infrared Optoelectronics by Shanyuan Niu A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Materials Science GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA Committee in charge: Professor Jayakanth Ravichandran, Chair Professor Andrea Armani Professor Rehan Kapadia Professor Han Wang May 2019 The dissertation of Shanyuan Niu is approved: Chair Date Date Date Date University of Southern California May 2019 i To my family, small and big ii Acknowledgments I would love to thank, first and foremost, my advisor, Prof. Jayakanth Ravichan- dran, for his mentorship and support throughout my doctoral study. I am blessed to be the first student supervised by Jayakanth and to witness the vision, passion, courage, and leadership he has demonstrated throughout the journey. He has struck a perfect balance between leaving me plenty freedom to explore things and leading me to focus on the central tracks. He has served not only as a great source of knowledge, vision, and inspiration in research, but also as a role model to help me grow as a researcher and as a person in general. I would also like to thank Prof. Anupam Mad- hukar for prior learning and research experience at USC and for instilling a clarity of vision in viewing nature and asking questions. My sincere thanks to my committee members, Prof. Andrea Armani, Prof. Han Wang, Prof. Rehan Kapadia, and my prior qualifying committee members, Prof. Andrea Armani, Prof. Jahan Dawlaty, Prof. Stephen Cronin, Prof. Paulo Branicio, for their support and mentorship in completing this work. I am grateful to all the professors who have taught me during my graduate and undergraduate materials science programs. I have had an incredible opportunity to develop a framework of fundamental understanding in materials science and to experience the energy and vision from many great mentors. I wish to inherit and practice such passion in my career. Special thanks to Prof. Yanqing Lu for his great lectures in optics, and more importantly, for the extraordinary inspiration. It was back then I decided to delve into scientific research and pursue an academic career. I have collaborated with an amazing set of researchers, whose contribution have been undoubtedly essential in the completion of my doctoral studies. These include Prof. Han Wang, Prof. Rehan Kapadia, Prof. Ralf Haiges, Dr. Matthew Meck- lenburg, Prof. Brent Melot, Prof. Stephen Cronin, Prof. Jahan Dawlaty, and Prof. Priya Vashishta at USC, and researchers from other universities, national labs and industry such as Prof. Rafael Jaramillo, Prof. Mikhail Kats, Prof. Austin Minnich, Prof. David Singh, Prof. William Tisdale, Prof. Anderson Janotti, Prof. Peter Khalifah, Dr. Tom Tiwald, Prof. Jeffery Snyder, Prof. Jan Seidel and Prof. Michael Manley. I am very grateful to work in such an interdisciplinary research field and to interactwithsomanyexpertsfromdifferentareas. Ithankallofthemandtheirgroup members for the access to various instruments and their expertise in completing this work. Special thanks to Phil at USC glass shop. He taught me how to make and seal quartz tubes. Huan and Debarghya deserve a special mention for collaborating on multiple projects and hanging in there with me during those overnight experiments. I want to give special thanks to link foundation and USC graduate program for fel- lowship support, which not only provided financial aid but also allowed extra courage and confidence in exploring new materials and experimenting adventurous ideas. I have truly enjoyed working with many students at USC. Within our group, Huaixun, Yucheng, Boyang, Kaihang, Kevin, Shiyang, and Shengyuan have closely worked with me during their time at USC. William, Yihong, Adrian, Malcolm, have iii worked with me on this project during their short visits to USC. Without their help, this work would not have been as complete or impactful. I am also thankful for the support and friendship from other group members, Tom, Yang, Mythili, Yuan, Jieyang. It has been a great pleasure working with all of you! I am glad to have had some amazing friends outside of the group. Boxiang, Shuran, Liang, Huan, Jie, Debarghya, Jiefei, Jian, Cai, Yunxiang, Jihan, Haotian. Thank you all for making my life at USC both entertaining and eventful. Thanks to my landlord, Betty, for putting up with me through these years. Thanks to my uncle and his family at LA, Mr. and Mrs. Su and Sarah, for all the wonderful trips and providing a home experience away from home. Last, and most importantly, I can never thank my family too much for their support and sacrifice. Thanks to my cat, tiger, for his endurance and ability to feed and clean himself. Thanks to my girlfriend, Xiaqing, for continued support through many years’ of overseas relationship and many lonely Valentine’s Days. I’m eternally grateful for my parents’ unconditional support in every way throughout my school years. The seed for science was placed the first time they let me into their chemistry lab and showed me the fireworks from the thermite reaction. i Abstract Perovskite Chalcogenides: Emerging Semiconductors for Visible to Infrared Optoelectronics by Shanyuan Niu Doctor of Philosophy in Materials Science University of Southern California Professor Jayakanth Ravichandran, Chair Large-scale deployment of electronic, photonic, and energy technologies rely on continuous discovery and invention of high performance electronic materials with earth abundant compositions. Carrier mobility and density of states are two criti- cal material parameters for electronic materials. These quantities are also crucial to enable efficient, high performance light-matter interaction. While the advantages of highcarriermobilityareevident, largedensityofstatescanleadtodesirableelectronic and optical properties such as enhanced light absorption and emission (efficient solar energy conversion and lighting), high carrier density (high current, power density), and large thermopower (thermoelectrics). To this end, transition metal perovskite chalcogenides (TMPCs) have been proposed as a class of semiconductor materials with rich tunability and functionality in the visible to infrared spectrum. Specifically, the coexistence of large density of states and high carrier mobility, along with tunable band gap, good thermal and aqueous stability, and benign composition could create opportunities for a broad range of photonic, optoelectronic, and energy applications, including solar cells, photodetectors, lighting devices, and photoelectrochemical de- vices. TMPCs have a general chemical formula of ABX 3 similar to the perovskite oxides and halides, where A is a metal such as Ba, Sr, B is a transition metal such as Ti, Zr, and X is S or Se. When the B-site is occupied by early transition metals, the valence bands and the conduction bands of TMPCs are primarily composed of chalcogen p orbitals and transition metal d orbitals, respectively. High density of states is ex- pected from the combination of highly symmetric structure and degenerate transition metald orbitals. TMPCs can be viewed as the inorganic alternatives to hybrid halide perovskites, with stable, benign, abundant composition, and high absorption coef- ficients. TMPCs can also be viewed as the chalcogenide counterparts of perovskite oxides, with much lower bandgap and improved responsivity to visible and infrared light. However, the physical properties of this class of materials remain underex- ii plored despite being structurally known for decades and the promise as predicted by theoretical studies. This dissertation focuses on the design, synthesis, and physical properties of TM- PCs. High quality synthesis of polycrystalline TMPCs were achieved with catalyzed solid state reactions in sealed ampoules. Single crystals up to several millimeters in size were obtained using vapor transport for hexagonal perovskite chalcogenide with quasi-one-dimensional network. Single crystals with lateral dimensions of sev- eral hundred microns were grown using salt flux method for perovskite chalcogenides with three-dimensional network and layered Ruddlesden-Popper chalcogenide crys- tals. Extensive structural and chemical characterizations for bulk, surface, or mi- crostructural studies were performed to test the quality of grown samples. In depth optical spectroscopy and transport studies were performed to extract the relevant optoelectronic properties of TMPCs. We employed static, quantitative, and tran- sient photoluminescence spectroscopy to probe the electronic structure and carrier dynamics in TMPCs with three-dimensional and quasi-two-dimensional structural networks. Desirable features including band gap tunability, strong luminescence, and long carrier lifetime were demonstrated. We also studied the anisotropic infrared op- tical properties as well as electrical and thermal transport properties in TMPCs with quasi-one-dimensional structures. Record high, broadband birefringence and linear dichroism were discovered. Thermal stability tests on TMPCs will also be briefly discussed to evaluate their potential for large scale deployment. iii Table of Contents List of Figures vi List of Tables xiii 1 Introduction 1 1.1 Background and motivation . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Perovskite oxides and chalcogenides . . . . . . . . . . . . . . . . . . . 3 1.3 Hybrid halide perovskites and perovskite chalcogenides . . . . . . . . 4 1.4 Revisit the “known” materials . . . . . . . . . . . . . . . . . . . . . . 6 2 Material synthesis and characterization 9 2.1 Polycrystalline synthesis with catalyzed solid state reaction . . . . . 9 2.2 Crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2.1 Vapor transport assisted growth . . . . . . . . . . . . . . . . . 10 2.2.2 Salt flux growth . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3 Structural characterizations . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.1 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.2 Rietveld refinement . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.3 Scanning electron microscopy . . . . . . . . . . . . . . . . . . 14 2.3.4 Transmission electron microscopy . . . . . . . . . . . . . . . . 15 2.4 Chemical characterizations . . . . . . . . . . . . . . . . . . . . . . . . 19 2.4.1 Energy dispersive X-ray spectroscopy . . . . . . . . . . . . . . 19 2.4.2 Wavelength dispersive X-ray fluorescence spectroscopy . . . . 19 2.5 Optical spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.5.1 UV-Vis-NIR transmission and reflection spectroscopy . . . . . 20 2.5.2 Fourier-transform infrared spectroscopy . . . . . . . . . . . . . 20 2.5.3 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . 22 2.5.4 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 22 3 Optical properties of three-dimensional TMPCs 24 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.2 Synthesis of polycrystalline BaZrS 3 , -SrZrS 3 and -SrZrS 3 . . . . . . 26 3.3 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 iv 3.3.1 Rietveld refinement . . . . . . . . . . . . . . . . . . . . . . . . 27 3.3.2 Grain size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.3.3 Octahedral tilting . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.4 Other characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.5 First principles calculations . . . . . . . . . . . . . . . . . . . . . . . 33 3.6 Bandgap determination . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.6.1 photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . 34 3.6.2 Diffuse reflectance spectroscopy . . . . . . . . . . . . . . . . . 35 3.7 Quantitative photoluminescence . . . . . . . . . . . . . . . . . . . . . 39 3.7.1 The "green gap" . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.7.2 External luminescence efficiency . . . . . . . . . . . . . . . . . 39 3.8 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4 Band gap evolution in quasi-two-dimensional TMPCs 43 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.2 Synthesis of Ba-Zr-S Ruddlesden-Popper phases . . . . . . . . . . . . 44 4.2.1 Polycrystalline synthesis . . . . . . . . . . . . . . . . . . . . . 44 4.2.2 Crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.3 Crystal structure of BaZrS 3 , Ba 3 Zr 2 S 7 , and Ba 2 ZrS 4 . . . . . . . . . . 46 4.4 First principles calculations . . . . . . . . . . . . . . . . . . . . . . . 52 4.5 Optoelectronic properties of Ba 3 Zr 2 S 7 . . . . . . . . . . . . . . . . . . 53 4.5.1 Optical spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 53 4.5.2 External luminescence efficiency . . . . . . . . . . . . . . . . . 54 4.5.3 Carrier recombination lifetime . . . . . . . . . . . . . . . . . . 57 4.6 Anomalous bandgap evolution in layered Ba-Zr-S Ruddlesden-Popper phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.6.1 Bandgap evolutions in Ruddlesden-Popper phases . . . . . . . 58 4.6.2 The role of octahedra tilting . . . . . . . . . . . . . . . . . . . 59 4.7 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 64 5 Building infrared optical anisotropy in quasi-one-dimensional TM- PCs 65 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 5.2 Quasi-1D structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 5.3 Polarizability engineering in hexagonal perovskite chalcogenides . . . 69 5.4 First principles calculations . . . . . . . . . . . . . . . . . . . . . . . 70 5.5 Crystal growth and characterizations of BaTiS 3 . . . . . . . . . . . . 73 5.5.1 Crystal growth of BaTiS 3 . . . . . . . . . . . . . . . . . . . . 73 5.5.2 Structural characterization . . . . . . . . . . . . . . . . . . . . 73 5.5.3 Chemical characterizations . . . . . . . . . . . . . . . . . . . . 77 5.5.4 Electrical transport . . . . . . . . . . . . . . . . . . . . . . . . 80 5.6 Optical anisotropy in BaTiS 3 . . . . . . . . . . . . . . . . . . . . . . 81 v 5.6.1 Polarization-resolved transmission and reflection . . . . . . . . 81 5.6.2 Generalized ellipsometry . . . . . . . . . . . . . . . . . . . . . 82 5.6.3 Giant birefringence . . . . . . . . . . . . . . . . . . . . . . . . 84 5.7 Optical anisotropy in other hexagonal perovskites . . . . . . . . . . . 87 5.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.7.2 Growth and characterizations of Sr 1+x TiS 3 . . . . . . . . . . . 88 5.7.3 Strong linear dichroism . . . . . . . . . . . . . . . . . . . . . . 91 5.8 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 92 6 Thermal stability of TMPCs 94 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 6.2 Thermal analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 6.3 Oxidation products . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 6.4 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 101 7 Conclusions and future directions 102 7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 7.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Bibliography 107 vi List of Figures 1.1 A qualitative comparison of carrier mobility and carrier density for several classes of semiconductors. . . . . . . . . . . . . . . . . . . . . 2 1.2 (a) An optical picture of the mineral perovskite (calcium titanate). Picture courtesy: Wikipedia. (b) A schematic for perovskite structure ABX 3 , the dark blue, green, and orange spheres represent A, B, and X sites, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 (a)A schematic showing the orbital contribution to conduction band and valence band in TMPCs. (b) A comparison of calculated density of states for BaZrO 3 and BaZrS 3 . Adopted from report[1]. (c) Calculated band gap of a series of TMPCs. Adopted from report[2]. . . . . . . . 4 1.4 Development of power conversion efficiency for various types of solar cells. ImageadoptedfromNationalRenewableEnergyLaboratorybest research cell efficiency chart.[3] . . . . . . . . . . . . . . . . . . . . . . 5 1.5 (a) Superior optoelectronic properties of hybrid halide perovskites in- clude tunable band gap, high external quantum efficiency (EQE), high absorption coefficient, and extremely long minority carrier lifetime. Figures adopted from reports[4–7]. (b) Additional desired features of TMPCs. The differential scanning calorimetry (DSC) measurement for a serious of TMPCs (top) showing stability at elevated temperatures and a comparison of absorption coefficients for representative TMPCs with Si and GaAs (bottom). . . . . . . . . . . . . . . . . . . . . . . . 6 1.6 Crystal structure schematics for structural variations of TMPCs with three-dimensional (3D) (a,b), quasi-one-dimensional (Quasi-1D) (c,d), and quasi-two-dimensional (Quasi-2D) (e) networks. . . . . . . . . . . 7 2.1 Optical pictures showing the synthetic process. (a) Sample loading in the glovebox. (b) Ampoule sealing with flame torch. (c) Heat treat- ment in furnaces. (d) As-obtained samples in sealed ampoules after heat treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2 Schematics for crystal growth with chemical vapor transport. . . . . . 11 2.3 Schematics for flux growth method. . . . . . . . . . . . . . . . . . . . 12 2.4 Representative optical pictures of grown crystals for several materials. 12 vii 2.5 (a) Schematic for XRD instrument configuration. (b) The Ewald sphere schematic illustrating the diffraction condition. Picture cour- tesy: Tara Marie Michels-Clark. . . . . . . . . . . . . . . . . . . . . . 13 2.6 A flow chart of main steps in Rietveld analysis. . . . . . . . . . . . . 15 2.7 Signals generated due to electron sample interaction in a SEM. Image courtesy: Thermo Fisher Scientific. . . . . . . . . . . . . . . . . . . . 16 2.8 Comparison of TEM mode (a) and STEM mode (b) in transmission electron microscopy. Figure taken from [8]. . . . . . . . . . . . . . . . 18 2.9 Schematics for fluorescence of X-ray from materials due to external radiation exciting core electrons. Image courtesy: Wikipedia. . . . . . 19 2.10 SchematicsfortheconfigurationoftheUV-Vis-NIRspectrophotometer with a 150 mm integrating sphere. Image courtesy: Perkin Elmer. . . 21 2.11 Schematics for configuration of polarization-resolved infrared transmis- sion and reflectance measurements. . . . . . . . . . . . . . . . . . . . 22 2.12 Schematics showing the process of photoluminescence. . . . . . . . . . 23 3.1 Optical pictures of the synthesized powder and cold-pressed pellet for -SrZrS 3 (a), -SrZrS 3 (b) and BaZrS 3 (c). . . . . . . . . . . . . . . . 26 3.2 Plots of Powder XRD patterns with Rietveld analysis for (a) -SrZrS 3 (red), (b) -SrZrS 3 (blue) and (c) BaZrS 3 (green). The black lines are simulated intensity profiles for three materials. . . . . . . . . . . . . . 28 3.3 A plot of synchrotron powder diffraction pattern with Rietveld refine- ment analysis for BaZrS 3 . The black, red and grey lines are exper- imental pattern, simulated pattern and their difference, respectively. Unmatched peaks in Rietveld analysis are marked with red star. The inset is the schematic for the refined BaZrS 3 structure. . . . . . . . . 30 3.4 (a) Schematic structure shows the out of phase tilt where the tilting of adjacent layers are in opposite direction. Ball-stick models show the projected view along the tilting axis (b) and the S-Zr-Zr-S torsion angle (blue-black-red) in perspective view (c). . . . . . . . . . . . . . 31 3.5 (a) Schematic structure shows the in phase tilt where the tilting of adjacent layers are overlapping. Ball-stick models show the projected view along the tilting axis (b) and the Zr-S-S-Zr torsion angle (pink- black-purple) in perspective view (c). . . . . . . . . . . . . . . . . . . 31 3.6 Raman spectra of BaZrS 3 , -SrZrS 3 , and -SrZrS 3 samples at room temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.7 EDS spectra of BaZrS 3 -SrZrS 3 -SrZrS 3 samples at 400 magnifica- tion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.8 WDXRF mapping on a BaZrS 3 pellet.(a) Ten locations for the map- ping.(b) Obtain Zr:Ba and S:Ba ratios at ten locations. . . . . . . . . 33 3.9 Calculated band structure for three materials. . . . . . . . . . . . . . 34 viii 3.10 (a) Calculated densities of states and (b) absorption spectra (direction averaged) for three materials. . . . . . . . . . . . . . . . . . . . . . . 35 3.11 Plot of isosurfaces of the calculated band structure 0.05 eV below the valencebandmaximum(VBM,blue) and0.05eV above theconduction band minimum (CBM, red) for three material. . . . . . . . . . . . . . 36 3.12 PL spectra for -SrZrS 3 (red), -SrZrS 3 (blue), and BaZrS 3 (green) show band gap values of 1.53 eV, 2.13 eV and 1.81 eV respectively. . . 37 3.13 Band gap determination with absorbance value obtained from diffuse reflectance and transmittance measurements on translucent powder layer using Kubelka-Munk theory. The deduced band gap values are 1.52 eV for -SrZrS 3 (red), 2.05 eV for -SrZrS 3 (blue), and 1.83 eV for BaZrS 3 (green). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.14 Band gap determination plot (a) with absorbance values (b) obtained from diffuse reflectance measurements of thick pellets using Kubelka- Munk theory. The deduced band gap values are 1.54 eV for -SrZrS 3 (red), 2.08 eV for -SrZrS 3 (blue), and 1.85 eV for BaZrS 3 (green). . . 38 3.15 A plot of nitride and phosphide LED efficiency as a function of wave- length. Adopted from report[9]. . . . . . . . . . . . . . . . . . . . . . 39 3.16 PL intensity comparison of-SrZrS 3 sample with InP and CdSe single crystal substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.17 External(black)andinternal(red)luminescenceefficienciesof-SrZrS 3 as a function of incident power densities. . . . . . . . . . . . . . . . . 41 3.18 Schematic structure, PL spectra and optical pictures for -SrZrS 3 , BaZrS 3 , and -SrZrS 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.1 Schematic crystal structures for RP series compounds withn=1 (per- ovskite), n=2, n=1 and n=0 (rock salt). . . . . . . . . . . . . . . . . 44 4.2 Optical pictures and SEM images of BaZrS 3 (a), Ba 3 Zr 2 S 7 (b), and Ba 2 ZrS 4 (c) crystals. . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4.3 Optical pictures and powder XRD of BaZrS 3 (a,b), Ba 3 Zr 2 S 7 (c,d), and Ba 2 ZrS 4 (e,f) polycrystalline samples. Expected peak positions are adopted from ICDD databases. . . . . . . . . . . . . . . . . . . . 47 4.4 EDS spectra of BaZrS 3 (a), Ba 2 ZrS 4 (b), and Ba 3 Zr 2 S 7 (c) samples. The obtained Ba:Zr:S ratios are 1.1:1.0:3.3, 2.3:1.0:4.1, and 1.6:1:1:3.5, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.5 Out-of-plane XRD of individual crystal for BaZrS 3 (a) and Ba 3 Zr 2 S 7 (b). The insets are rocking curve of the most intense peak. The cross- sectional STEM images of BaZrS 3 (c) and Ba 3 Zr 2 S 7 (d), the insets are corresponding SAED patterns. . . . . . . . . . . . . . . . . . . . . . . 49 4.6 Schematics for the P 4 2 =mnm phase and I4=mmm phase of Ba 3 Zr 2 S 7 . 50 ix 4.7 Rietveld refinement for polycrystalline BaZrS 3 samples. The difference between synchrotron XRD (red) and simulated curve (blue) is shown in the grey curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.8 The corresponding schematic crystal structure (a), calculated band structure (b), density of states (c), and absorption coefficients along out-of-planeandin-planedirectionsforBa 3 Zr 2 S 7 withP 4 2 =mnmstruc- ture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.9 DFT calculations for Ba 2 ZrS 4 and Ba 3 Zr 2 S 7 with I4mmm structure. Thecorrespondingperspectivecrystalstructure, calculatedbandstruc- ture, and absorption coefficients along different directions are shown for Ba 3 Zr 2 S 7 (top) and Ba 2 ZrS 4 (bottom), respectively. . . . . . . . . 54 4.10 Room temperature PL and Raman spectra for BaZrS 3 (a,b), Ba 3 Zr 2 S 7 (c,d), and Ba 2 ZrS 4 (e,f) crystal pieces. . . . . . . . . . . . . . . . . . 55 4.11 (a) PL intensity comparison of a Ba 3 Zr 2 S 7 crystal, a InP wafer and a GaAs wafer under the same measurement conditions. (b) Quantitative emission and corresponding V OC at different incident power density with 785 nm incidence. The error bar is included as multiple sets of data were obtained on several Ba 3 Zr 2 S 7 crystal pieces.The inset is external luminescence efficiency as a function of the incident power density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 4.12 (a) Spectral- and time-resolved emission map. The TCSPC intensity is indicated with the color bar. (d) Intensity decay profile of the emission peak as a function of time. Red dots are experimental data and the solid line is the bi-exponential fit convolved with the pump profile. A fast decaying time constant of 4.5 ns and a slow decaying time constant of 65 ns are extracted. . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.13 Normalized PL spectra from various compounds at room temperature. Spectra fro Ba 3 Zr 2 S 7 is taken with 785 nm excitation laser and the other materials are taken with 532 nm excitation laser. . . . . . . . . 59 4.14 (a) Measured band gap and (b) calculated band gap as a function of n for the Ba-Zr-S, Sr-Ti-O, and BA(MA)-Pb-I RP systems. The experimentalvaluesofSr-Ti-O,andBA(MA)-Pb-Isystemsareadopted fromreports[10,11]. ThecalculatedvaluesofBA(MA)-Pb-Isystemare adopted from report[12]. . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.15 SchematicrepresentationsofoctahedranetworkinABX 3 perovskitefor the orthorhombic phase with octahedra tilting (a) and the cubic phase with no octahedra tilting (b). Dark green and orange spheres represent Ba and S atoms respectively. (c) DFT density of states of BaZrS 3 in the cubic phase (top) and the orthorombic phase (bottom). (d) Band alignment of BaZrS 3 for the orthorhombic phase (left) compared to the cubic phase (right). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 x 4.16 Calculated band gaps for Ba-Zr-S RP compounds with different struc- tural variations. The dash line is the Ba-Zr-S RP phases band gap evolution trend without octahedra tilting. The solid line shows the decreasing band gap trend for the experimentally obtained structures with significant octahedral tilting for n = 2 and n =1. . . . . . . . . 63 5.1 Schematicsshowingthecontrolofphaseandamplitudeoflightthrough birefringence and dichroism. Image courtesy: Wikipedia. . . . . . . . 66 5.2 Optical pictures of some common birefringent crystals widely used in polarizing optics. Image courtesy: Union Optic. . . . . . . . . . . . . 66 5.3 The comparison of birefringence amplitude for common anisotropic crystals in their transmission window in the infrared. . . . . . . . . . 67 5.4 The illustration showing the easily accessible in-plane anisotropy in quasi-one-dimensional structures compared to two-dimensional layered structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 5.5 The contribution to the real part of polarizability as a function of electric field frequency. Figure taken from [13]. . . . . . . . . . . . . . 70 5.6 The schematic crystal structure of BaTiS 3 . Blue, green and orange spheres represent Ba, Ti and S atoms,respectively. TiS 6 octahedra are highlighted. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.7 The electronic polarizability of various ions. . . . . . . . . . . . . . . 71 5.8 Calculated absorption spectra for light polarized parallel and perpen- dicular to c axis. The inset is calculated electronic band structure of BaTiS 3 with Ti d z 2 band highlighted. . . . . . . . . . . . . . . . . . . 72 5.9 (a) The schematic diagram for vapor transport growth. (b) Optical image of the representative as grown BaTiS 3 crystal ribbon and plate. The crystal ribbon is clamped down on the glass substrate by two silver paint dots. SEM image of a BaTiS 3 plate (c) and a crystal ribbon (d). 74 5.10 (a) Powder XRD of ground BaTiS 3 crystallites. (b) Out-of-plane scan of a BaTiS 3 crystal plate. The inset is the rocking curve of the 200 reflection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.11 Measured neutron PDF (blue circles) and best fit (red line) at 300 K. The residual difference between the measurement and fit is also shown (green line). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5.12 A schematic showing the configuration of the rotational XRD map viewed along a axis (a) and c axis (b). (c) A XRD map of 2/! scans with varying in a thin film diffractometer. . . . . . . . . . . . . . . 78 5.13 (a) Schematic of crystal structure, (b) HAADF STEM images and (c) SAED pattern of BaTiS 3 viewed alonga axis. (d) Schematic of crystal structure, (e) HAADF STEM images and (f) SAED pattern of BaTiS 3 viewed along c axis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 xi 5.14 (a) EDS spectrum of a BaTiS 3 crystal. (b) EDS mapping of Ba (blue), Ti (red), and S (green) elements on a BaTiS 3 crystal needle. The scale bar is 50 m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5.15 (a) RBS measurement on a BaTiS 3 platelet. Black line shows the RBS data. Purple, blue and green lines show the fitting of S, Ti and Ba elements, respectively. Red line is the simulated curve assuming the ratio of Ba:Ti:S is 1:1:3. The nominal stoichiometry of BaTiS 3 shows a good fit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.16 (a) Measured electrical resistivity of a BaTiS 3 crystal as a function of temperature. The inset is an optical image of the sample used in the measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5.17 Infrared transmission (a) and reflection (b) spectra for incident light polarized perpendicular (dark green) and parallel (orange) to c axis. . 82 5.18 Ellipsometry measurement AnE data and fit for plane of incidence parallel to (a) and perpendicular to (b) optic axis. Reflection spectra and fitting for normal incidence with polarization parallel to (c) and perpendicular to (d) optic axis. . . . . . . . . . . . . . . . . . . . . . 83 5.19 Ellipsometry measurement and fitting for plane of incidence 14 to optic axis. AnE data (a), Aps data (b) and Asp (c) and their fitted results are shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.20 (a) Real (1) and imaginary (2) parts of the refractive indices for po- larization perpendicular and parallel to the c axis. (b) Birefringence (n), lineardichroism(), andnormalizeddichroismforwavelengths from 210nm to 16m. . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.21 A comparison of absolute birefringence value for various current bire- fringent materials and BaTiS 3 in infrared spectrum range. MWIR and LWIR spectra regions are highlighted. . . . . . . . . . . . . . . . . . . 86 5.22 (a) The schematic for crystal structure. (b) optical image of a crystal platelet. (c) Overlay of powder XRD, and the out-of-plane XRD scan of a crystal platelet. The inset shows the zoomed in platelet XRD near 4400 reflection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 5.23 (a) EDS spectrum of the crystal. (b) SEM image and EDS mapping of S, Ti, and Sr elements of a crystal wire and a platelet. The rotational XRD map of a crystal plate. Intensity of the reflections are indicated by the color bar contour. (c) The rotational XRD map of a Sr 1:145 TiS 3 crystal plate rotatied around the c axis . . . . . . . . . . . . . . . . . 89 5.24 (a) Normalized Raman spectra with incident laser linearly polarized parallel, 30 , 60 , and 90 to the c axis. (b) A polar plot of the three Raman peak intensities as a function of the angle between incident laser polarization and the c axis. . . . . . . . . . . . . . . . . . . . . . 90 xii 5.25 (a) The absorbance for normal incidence with linear polarizations par- allel, 30 , 60 , 80 , and 90 to the c axis in the crystal plate. (b) Transmission and absorbance value plotted in polar coordinate as a function of the incident polarization with respect to c axis. . . . . . . 91 6.1 Schematics of various ABX 3 crystal structures for (a) distorted per- ovskite phase, (b) needle-like phase, (c) hexagonal perovskite phase, and (d) Ruddlesden-Popper phase (n=2). The blue and orange spheres represent A site atoms and X site chalcogen atoms, respectively. The BX 6 octahedra are highlighted in green. . . . . . . . . . . . . . . . . . 95 6.2 Optical pictures of five samples before (a) and after (b) heat treat- ment. The materials in (a) from left to right are -SrZrS 3 , -SrZrS 3 , BaZrS 3 , Ba 3 Zr 2 S 7 , Ba 2 ZrS 4 , BaTiS 3 , and Sr 1:145 TiS 3 . The materials in (b) from left to right are -SrZrS 3 , -SrZrS 3 , BaZrS 3 , Ba 2 ZrS 4 , Ba 3 Zr 2 S 7 , BaTiS 3 , and Sr 1:145 TiS 3 respectively. . . . . . . . . . . . . . 96 6.3 (a) Thermogravimetric analysis mass change and (b) differential scan- ning calorimetry of seven samples as a function of the temperature. . 97 6.4 Powder XRD patterns overlaid with reference peak positions for Ba- Zr-S RP series before (a) and after (b) heat treatment. . . . . . . . . 98 6.5 Powder XRD patterns overlaid with reference peak positions for two SrZrS 3 polymorphs series before (a) and after (b) heat treatment. . . 99 6.6 Powder XRD patterns for BaTiS 3 and Sr 1:145 TiS 3 before (a) and after (b) heat treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 7.1 PL spectra overlay and schematic crystal structures of various TMPCs illustrating chemistry, structure, and dimensionality control. . . . . . 103 7.2 (a) Birefringence comparison of BaTiS 3 with other anisotropic crystals. (b) Schematic showing the anisotropic light interaction with hexago- nal chalcogenides. Picture courtesy: Talia Spencer. (c) Transmission and absorbance value plotted in polar coordinate as a function of the incident polarization with respect to c axis on a Sr 1:145 TiS 3 crystal. . 104 xiii List of Tables 3.1 Structural parameters for -SrZrS 3 from Rietveld refinement. . . . . . 27 3.2 Structural parameters for -SrZrS 3 from Rietveld refinement. . . . . . 29 3.3 Structural parameters for BaZrS 3 from Rietveld refinement. . . . . . . 29 4.1 Structural parameters from X-ray crystallographic analysis for BaZrS 3 . 50 4.2 Structural parameters from X-ray crystallographic analysis for Ba 3 Zr 2 S 7 . 51 5.1 Structural parameters from X-ray crystallographic analysis for BaTiS 3 . 77 5.2 Transmission of various linear polarizations, dichroic ratios, and ab- sorbance ratios of a Sr 1:145 TiS 3 crystal plate. . . . . . . . . . . . . . . 92 1 Chapter 1 Introduction 1.1 Background and motivation Rational design of new materials and discovery of novel functionalities in underex- plored materials, especially semiconductors, have been a key contributor in advancing electronic, photonic, and energy technologies. Carrier mobility and density of states (DOS) are two critical material parameters for semiconductors, which also quantify the performance and efficiency of light-matter interaction. While the advantages of high carrier mobility are evident, large density of states can lead to desirable elec- tronic and optical properties and corresponding applications such as enhanced light absorption and emission (solar energy conversion and lighting), high carrier density (high current, power density), and large thermopower (thermoelectrics). Most of the optoelectronic and photonic devices we encounter in daily life employ conventional semiconductors such as silicon, germanium, and compound semiconductors such as III-V and II-VI materials. These materials have been studied for decades. A deep understanding of how these materials interact with electromagnetic radiation has been established. The electronic bands in these materials often have s and p orbital character and large bandwidth. The highly dispersive conduction and valence bands in these semiconductors lead to high carrier mobility, and relatively small density of states at the band edges. Alternatively, material systems with flat bands would have large density of states, but also large effective mass and typically low mobil- ity. Among various material systems, the transition metal oxides (TMOs), especially transition metal perovskite oxides, have the highest achievable carrier density due to large density of states and large orbital degeneracy of the d-bands. However, carrier mobility in these materials are rather low, at least at room temperature. Such an in- verse correlation between carrier mobility and carrier density are depicted in Fig.1.1. Overall, DOS increases from s or p orbital bonded 3- or 4-fold coordination systems to 6-fold octahedral-bonded d orbital systems, while carrier mobility increases from ionic to more covalent semiconductors. To challenge such a contradiction between mobility and density of states, one would need to consider another important factor, 2 carrier scattering rates. In TMOs, apart from the large effective mass of small band- width d-bands, the mobility limitation arises mainly from large electron scattering rates, due to strong coupling of electrons with other quasi-particles and collective excitations such as other electrons, phonons, or spins.[14–17] Electron-phonon scat- tering or polaronic transport is a primary bottleneck for electron transport in TMOs, which is correlated with the large electronegativity difference between the oxygen and the transition metals.[16–18] Hence, increasing covalency in such a framework by replacing oxygen with lower electronegativity anions such as sulfur, selenium could lead to weaker electron-phonon interaction and consequently higher mobility limits, while retaining beneficial effects of large degeneracy and density of states of TMOs. To this end, transition metal perovskite chalcogenides (TMPCs), a class of materials withd 0 configuration and moderately covalent bonding, have been proposed for opto- electronic applications.[2, 19–27] TMPCs have a general chemical formula of ABX 3 , where A is an alkaline earth metal such as Ba, Sr, B is a early transition metal such as Ti, Zr, and X is S or Se. ������� ������ � ������� ������� � III-V Nitrides Graphene III-V Except Nitrides (GaAs, InP) Silicon Transition Metal Dichalcogenides Transition Metal Oxides (SrTiO 3 ) Organic and Amorphous Semiconductors Figure 1.1: A qualitative comparison of carrier mobility and carrier density for several classes of semiconductors. Perovskite is a crystal structure adopted from the mineral calcium titanate, as showninFig.1.2(a). Themineralisnamed”perovskite” aftertheRussianmineralogist, Lev Perovski. Unlike the diamond, zinc-blende, or wultzite structures adopted by Si andcompoundsemiconductors, perovskitestructurehasthreedistinctiveatomicsites, which can house many elements in the periodic table and offers rich design space in chemical composition. Another characteristic feature of perovskite structure is the octahedra formed by B atom in the center and six surrounding anions at the corners, as shown in Fig.1.2(b). 3 A-site B-site X-site (a) (b) Figure1.2: (a)Anopticalpictureofthemineralperovskite(calciumtitanate). Picture courtesy: Wikipedia. (b) A schematic for perovskite structure ABX 3 , the dark blue, green, and orange spheres represent A, B, and X sites, respectively. TMPCs can be viewed as the inorganic alternatives to hybrid organic-inorganic halide perovskites, with stable, benign, abundant composition, and ultrahigh absorp- tion coefficients. On the other hand, TMPCs can also be viewed as the chalcogenide counterparts of perovskite oxides, with much lower bandgap and improved response to visible and infrared light.[1, 2] The potential coexistence of large DOS and high mobility opens opportunities for a broad range of optoelectronic applications such as photovoltaics, photodetection and lighting.[27] 1.2 Perovskite oxides and chalcogenides TMPCs share a lot of desirable features with perovskite oxides. Similar to per- ovskite oxides such as SrTiO 3 , TMPCs are d 0 systems. The valence band and con- duction band are mainly from contribution of chalcogen p orbitals, and transition metal d orbitals, respectively, as shown in Fig.1.3(a). High DOS is expected from the combination of highly symmetric perovskite structure and degenerate d orbitals. Also shared with the oxides is the benign and abundant composition. Most elements involved in TMPCs are benign and earth abundant, which allows TMPCs to address the sustainability issues clouding various advanced materials. For example, in the case of BaZrS 3 and SrZrS 3 , Ba, Sr, Zr, and S have an earth abundance of 425, 370, 165, and 350 ppm, respectively, while Cd, Te, Sn, and In have an abundance of 0.15, 0.001, 2.3, and 0.25 ppm, respectively.[13, 28] The key differences between perovskite chalcogenides and oxides arise from the anions. Sulfur is larger, heavier, less electronegative than oxygen. S 2 is also much more polarizable than O 2 .[13] Apart from more covalent bonding, an important ef- fect is the reduced band gap in TMPCs. In typical oxides, large energy differences between transition metal d orbital and O 2p orbital lead to very large energy gaps (>3 eV), typically rendering them unsuitable for solar energy conversion or visible 4 BaZrO 3 3.9 eV BaZrS 3 1.7 eV (a) (b) (c) Figure 1.3: (a)A schematic showing the orbital contribution to conduction band and valence band in TMPCs. (b) A comparison of calculated density of states for BaZrO 3 and BaZrS 3 . Adopted from report[1]. (c) Calculated band gap of a series of TMPCs. Adopted from report[2]. optoelectronic applications. By replacing O with S or Se with much lower electroneg- ativity and closer p orbitals, the band gap is significantly reduced, allowing TMPCs to interact effectively with visible and infrared photons. A comparison of calculated DOS for BaZrS 3 and BaZrO 3 with very similar structures is shown in Fig.1.3(b). We can see that the band gap is reduced significantly from 3.9 eV to 1.7 eV due to the replacement of O with S.[1] As a result, TMPCs can take full advantage of the large DOS. Such a class of d band semiconductors with rich tunability and functionality in the visible to infrared spectral region,[2] as shown in Fig.1.3(c), open up many opportunities in optoelectronic applications. 1.3 Hybrid halide perovskites and perovskite chalco- genides Theorganic-inorganichybridleadhalideperovskitesareaclassofperovskitemate- rialsthathavereceivedalotofattentioninthepastdecade, duetotheirextraordinary 5 photovoltaic performance.[29–35] Hybrid halide perovskites are an exciting example of new materials achieving high efficiency in a short period of time in mature tech- nologies requiring long development lead times. The laboratory power conversion efficiency has reached 23.3%[3] within a relatively short period of time, surpassing CdTe, CIGS and other thin film solar cells, as shown in Fig.1.4. The extraordinary photovoltaic performance is a result of their superior optoelectronic properties such as tunable bandgap, high absorption coefficients, high external quantum efficiency (EQE), and long carrier lifetime.[4–7] However, their toxicity and instability issues pose serious challenges for large scale applications.[36] Figure 1.4: Development of power conversion efficiency for various types of solar cells. Image adopted from National Renewable Energy Laboratory best research cell efficiency chart.[3] TMPCs can be viewed as the inorganic alternatives to the hybrid halide per- ovskites with stable, benign, and abundant composition. As inorganic compounds with close packed structure, TMPCs generally have good thermal and aqueous sta- bility. We monitored the stability of these materials over the course of a year via samples stored in air, and observed no discernible degradation in physical properties. We also performed stability tests of these materials in air at elevated temperatures using differential scanning calorimetry, which indicate these materials are stable in air at elevated temperatures up to 600 C,[37] as shown in Fig.1.5(b). Another advan- tage of TMPCs over halide perovskites is the large density of states of the conduction band due to highly degenerate d orbitals. Large density of states leads to ultra-high 6 absorption coefficients near the band edge. Halide perovskite typically have absorp- tion coefficients around 10 4 cm 1 , while TMPCs can reach 10 5 cm 1 ,[22] which will enable very small absorption length in photovoltaic devices. Tunable Band gap High EQE High Absorption Long Carrier Lifetime Hybrid Halide Perovskites Inorganic TMPC (a) (b) Figure 1.5: (a) Superior optoelectronic properties of hybrid halide perovskites include tunable band gap, high external quantum efficiency (EQE), high absorption coeffi- cient, and extremely long minority carrier lifetime. Figures adopted from reports[4– 7]. (b) Additional desired features of TMPCs. The differential scanning calorimetry (DSC) measurement for a serious of TMPCs (top) showing stability at elevated tem- peratures and a comparison of absorption coefficients for representative TMPCs with Si and GaAs (bottom). 1.4 Revisit the “known” materials Strictly speaking, TMPCs are not new materials. These materials were known to exist and the synthetic efforts can date all the way back to over half a century ago.[38–42] Several TMPCs have been synthesized in ceramic form by either heating binary sulfide mixture for several weeks,[43] or sulfurization of corresponding oxides with CS 2 [39, 44] or H 2 S.[41] However, most reports were limited to the structural information, and despite these numerous attempts, there is very little experimental 7 data on their optical properties. Recently, these materials were rediscovered as an emerging class of semiconductors for optoelectronic applications. Many theoretical proposals have predicted their desirable optoelectronic properties, particularly for solar energy conversion.[2, 21, 23–25, 45–47] Experimental exploration are also well underway to reveal their potential,[19, 20, 22, 26, 37, 48–51] among which our group have contributed significantly.[22, 37, 49–51] Due to the significantly larger size of chalcogens compared to oxygen, TMPCs dis- play more structural distortions from the ideal cubic perovskite structure (Fig.1.6(a)). Four major types of structural variations in perovskite and related structures are of high interest, including distorted perovskite phase (Fig.1.6(b)), needle-like phase (Fig.1.6(c)), hexagonal perovskite phase (Fig.1.6(d)), and the layered Ruddlesden- Popper (RP) phases (Fig.1.6(e)). Cubic (SrTiO 3 type) Distorted (GdFeO 3 type) Needle like (NH 4 CdCl 3 type) Hexagonal (BaNiO 3 type) Corner-Sharing 3D network Quasi-1D Edge-Sharing Face-Sharing (a) Ruddlesden- Popper Quasi-2D Layered (e) (b) (d) (c) Figure 1.6: Crystal structure schematics for structural variations of TMPCs with three-dimensional (3D) (a,b), quasi-one-dimensional (Quasi-1D) (c,d), and quasi-two- dimensional (Quasi-2D) (e) networks. ThesestructuralvariationscanbeviewedasdifferentwaysinwhichBX 6 octahedra areconnectedinthethree-dimensional(3D)space. Thestructurethatisclosesttothe ideal cubic perovskite is the GdFeO 3 (GFO) structure with a typical space group of Pnma. Here, the BX 6 octahedra are connected through a corner sharing 3D network. Tilting of the octahedra breaks the cubic symmetry and leads to an orthorhombic structure for the distorted perovskite phase, as shown in Fig.1.6(b). When small A site ions are paired with large B site ions, the material tends to stabilize in the needle- like phase and adopts the NH 4 CdCl 3 structure with a space group of Pnma. The octahedra are connected by sharing edges, and two columns of edge-sharing octahedra form a chain. These parallel chains are extended along one direction and form a quasi-one-dimensional (Quasi-1D) structure, as shown in Fig.1.6(c). When smaller transition metals, such as Ti, sits in the B site, these compounds tend to stabilize in the hexagonal perovskite phase, which adopts the BaNiO 3 structure with a typical space group of P 6 3 =mmc. BX 6 octahedra share opposing faces and form parallel 8 single column chains. These chains are arranged in a hexagonal symmetry, as shown in Fig.1.6(d). The B-B bond length is much shorter along the chains than across the chains. These strongly bonded BX 6 chains are separated by A chains, forming a highly symmetric and yet highly anisotropic Quasi-1D structure. Notably, the strong bonding along the chain also ensures that the natural cleavage of the crystal is along the chain direction, leading to a prismatic plane termination with easily accessible anisotropy. Such easily accessible in-plane anisotropy provides a great platform to realized exotic anisotropic photonic and transport phenomena. Additionally, RP phases of perovskites are formed by alternating a set number (n) of perovskite layers with the chemical formula ABX 3 and a rock salt layer AX. One schematic crystal structure forn = 1 is shown in Fig.1.6(e). Such a 2D perovskite has a general formula of A n+1 B n X 3n+1 for the case of the same cations in perovskite and rock salt layer. The RP phases could host interesting, different sets of octahedra tilting and distortion. Several theoretical studies explored the possibility of achieving static polarization in such materials to demonstrate bulk photovoltaic effect.[21, 25] Although not all of these structures are perovskites using the rigorous definition of corner shared connectivity of octahedra, the characteristic BX 6 octahedra that controls the electronic property remains present in these variations and aforemen- tioned arguments regarding desirable optoelectronic properties mostly remain valid. We view all these perovskite related structures as an opportunity to introduce dimen- sionality control on top of structure and chemical control. This dissertation would use the structural network as a road map to explore physical properties of 3D, Quasi-2D, and Quasi-1D TMPCs. 9 Chapter 2 Material synthesis and characterization 2.1 Polycrystalline synthesis with catalyzed solid state reaction Polycrystalline TMPCs were synthesized with solid state reaction in sealed am- poules. The process involves mixing and loading the precursors in a glovebox with controlled environment (air-free and moisture-free) into a quartz tube. The quartz tube can then be evacuated and flame sealed for further high temperature treatment. Optical picture of such process are shown in Fig.2.1. The key challenge in the ce- ramic synthesis is the handling and reactivity of nonvolatile and air sensitive cationic precursors. Wehaveemployedanovelcatalyticgrowthmethodtosuccessfullysynthe- size this class of materials. The method is based on conventional solid-state reaction of alkaline earth metal sulfides (BaS and SrS) and elemental sources (Zr and S), with a critical amount of iodine as catalyst. Iodine has been used as a transporting agent in chemical vapor transport growth of single crystals. The catalytic addition of iodine could greatly enhance the reactivity by creating lower melting temperature volatile intermediate compounds of the precursors, such as metal iodides. For exam- ple, the melting temperature of BaS, Zr, and ZrS 2 are 2235 C, 1855 C, and 1480 C, respectively, while the melting temperature of BaI 2 , ZrI 4 are 711 C and 499 C, respectively.[13] As a result, the catalyzed reaction enables a one-shot synthesis with reduced reaction time of a few days compared to conventional solid-state reaction methods, which last for several weeks with laborious repeated grinding and annealing for homogenization. The highest achievable temperature in such synthesis is limited by the glass transi- tion temperature of the quartz tube. For high quality quartz tubes, extensive heating up to 1100 C can be routinely accessed while 1150 C can only be maintained in a short period of time. Another key point is to ensure minimal air exposure during 10 transfer of the tube to the sealing station. We built a custom "cap" for quartz tubes, which involves basically a ultra-torr fitting and a quarter turn plug needle valve. The valve remains closed to protect the air sensitive precursors until the extension tubing is connected to a vacuum pump and evacuated down to 5 mTorr. (a) (b) (c) (d) Figure 2.1: Optical pictures showing the synthetic process. (a) Sample loading in the glovebox. (b) Ampoule sealing with flame torch. (c) Heat treatment in furnaces. (d) As-obtained samples in sealed ampoules after heat treatment. 2.2 Crystal growth 2.2.1 Vapor transport assisted growth Chemicalvaportransport(CVT)isaprocesswheresolidprecursorsarevolatilized in the presence of a gaseous transport agent and deposited elsewhere in the form of crystals, as shown in Fig.2.2. Common transport agents include halogens and halogen compounds. The setup consists of a two zone furnace with the precursors in the source side and the grown crystals on the sink side. The precursors and transport agent are sealed in ampoules. Transport in such a mini-chamber is governed by two processes, convection and diffusion. Various parameters such as temperature, transport direction, mass transport rate, transport agent must be optimized for a successful CVT process. Though larger crystals can be obtained by increasing the transport rates favoring convection, the crystals are more likely to be inhomogeneous and prone to be defective. Thus optimization for each chemical system is vital. Depending on the free energy of the reaction between the species, the source and sink temperature can be altered. A reaction that is exothermic indicates transport from cold to hot end and the reverse is expected for an endothermic reaction. Also, if the 11 reaction between the species is highly exothermic or endothermic, minimal transport is expected to take place. Transport Agent Crystal T z Precursors Figure 2.2: Schematics for crystal growth with chemical vapor transport. 2.2.2 Salt flux growth Flux growth is a crystal growth technique that involves precipitation of solids from a supersaturated solution. The flux is a solvent used to dissolve the starting materials at a temperature lower than the melting point of the starting materials. Slowly lowering the temperature can gradually decrease the solubility of the target compound, thuscreatingasupersaturationtodrivethenucleationandgraingrowthof thethermodynamicallystablephase. Theflux, orsolvent, canbemetals, oxides, salts, hydroxides, and eutectic binaries. The desired flux for a crystal growth should have relatively low melting temperature and relatively high boiling temperature, should not react with the precursors or target compounds, and should be easy to separate from the grown crystals. In the case of Zr based compounds such as BaZrS 3 and its Ruddlesden-Popper Phases, BaCl 2 has been reported to be an effective flux.[52–54] WehavealsoexploredotherfluxoptionssuchasKI,andeutecticofBaCl 2 andMgCl 2 . They do not seem to significantly improve the size or quality of grown crystals. One advantage of using KI is that it is a lot easier to wash off compared to BaCl 2 . Optical pictures of representative crystals grown with flux growth method are shown in Fig.2.4(a,b,c), crystals grown with chemical vapor transport are shown in Fig.2.4(d,e). 12 Figure 2.3: Schematics for flux growth method. BaZrS 3 Ba 2 ZrS 4 Ba 3 Zr 2 S 7 BaTiS 3 Sr 1+x TiS 3 (a) (b) (c) (d) (e) Figure 2.4: Representative optical pictures of grown crystals for several materials. 2.3 Structural characterizations 2.3.1 X-ray diffraction X-ray diffraction (XRD) is a routine analysis technique extensively used to get structural information of crystalline materials. In a typical XRD setup, X-rays with known wavelength are generated and directed towards the sample. A detector will monitor the collected X-ray intensity as a function of angle between source and de- tector, 2, as shown in Fig.2.5(a). The primary effect of concern in XRD is the elastic scattering of X-ray from atoms within the target material. The interference of scattered X-rays generates the diffraction pattern. Similar to other diffraction based technique, the requirement for the XRD detector to collect signal from a constructive interference can be understood using an Ewald sphere diagram. In the case of XRD setup, the source and the detector are locked in a circle. The Ewald sphere is sim- 13 plified to a two-dimensional Ewald circle, as shown in Fig.2.5(b). When the change of wave vector between incident and scattered X-ray equals to a reciprocal lattice vector, a diffraction peak can be collected on the detector side. (a) (b) Figure 2.5: (a) Schematic for XRD instrument configuration. (b) The Ewald sphere schematicillustratingthediffractioncondition. Picturecourtesy: TaraMarieMichels- Clark. A primary use of the technique is the phase identification for polycrystalline sam- ples based on their diffraction pattern. The grain orientation in a polycrystalline sample is mostly random. Given the large of grains in typical XRD samples, We can see diffraction peaks from all the lattice planes without worrying about specific orientation of source or detector with respect to the target material. In such case, the requirement for XRD can be described by Bragg’s Law: n = 2dsin() (2.1) Each diffraction peak is related to a specific periodic spacing between one set of lattice planes within the target material. The diffraction pattern is then a map of all the lattice plane spacing available in a given material, which is a unique signature to the crystal structure with specific lattice constants. For samples with highly ordered grain orientation or very large grain size, such as single crystals or crystalline thin films, both the magnitude (2) and the direction (relative to sample orientation) of the X-ray wave vector change are critical for the diffraction condition: ~ S = ~ G hkl (2.2) IncontrasttothepowderXRDsetup, ahighresolutioninstrumentistypicallyusedto probe the well defined reciprocal lattice. In such thin film XRD systems, incident X- ray with better temporal and spatial coherency (monochromatic and parallel beam), a stage with accurate and versatile control of sample orientation (precise goniometer with rotation and tilting around several axes), and better detectors are typically used to achieve the desired resolution and control of sample orientation. These added degree of freedom allow probing the reciprocal space of target material along specific 14 paths, and constitute powerful techniques to extract different information about the crystallinity of the sample. Commonly used techniques include high resolution Bragg scan, rocking curve, pole figure, and reciprocal space mapping. 2.3.2 Rietveld refinement Routine powder XRD mostly utilizes the information regarding the position of diffraction peaks. However, there is also rich information contained in the diffraction pattern in terms of its peak shape, and relative intensity distribution, and back- ground. Rietveld refinement is a technique to extract detailed structural information from high quality powder diffraction patterns. This technique relies on the creation of a simulated diffraction pattern based on a input crystal structure convolved with instrumental configuration. One would then proceed to refine the difference between the simulated pattern and the experimentally generated pattern by tuning the in- put structure and/or instrumental settings in a least square approach. A flow chart for the Rietveld refinement process is shown in Fig.2.6. A reasonable fit to experi- mentally obtained pattern typically requires very high quality powder data to start with, and educated guess of the input structure. Good Rietveld refinement can be a very powerful tool to extract structural information including space group, lattice parameters, atomic positions, crystallite size, thermal expansion coefficients, etc. Ri- etveld refinement in this work was performed using FullProf software. The diffraction peaks were fit to a pseudo-Voigt profile, and the background was approximated using a 12-coefficient Fourier series. After initial structural parameters were refined, the background was switched to a linear interpolation between a set of background points with refinable heights for additional accuracy. To determine the goodness of fit for the simulated theoretical profiles, a 2 value was calculated using the weighted profile and expected R-factors as the following: 2 = ( R wp R exp ) 2 (2.3) 2.3.3 Scanning electron microscopy The scanning electron microscope (SEM) uses a focused beam of high-energy elec- trons (usually a few to tens of keV) to generate a variety of signals at the surface of solid specimens, as shown in Fig.2.7. The penetration depth of such high energy electrons in SEM depends on the specific composition and structure of sample, but typically falls in the range of a fewm. The signals that derive from electron-sample interactionsrevealinformationaboutthesampleincludingmorphology,chemicalcom- position, and crystal structure and orientation of materials making up the sample. In most cases, data are collected over a selected area on the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties by scanning the focused electron beam in this area. 15 Input Structure Model Calculated Pattern Compare with Experimental Pattern Optimize Input Structure and Iteration Agree Well Yes No Structural Information Figure 2.6: A flow chart of main steps in Rietveld analysis. The signals that are primarily used for imaging are secondary electrons and back- scatter electrons. Secondary electrons are generated as incident electron passing "near" sample atom and ionizing an electron (inelastic process) to escape from sample surface. Due to the small kinetic energy of secondary electrons (typically < 50 eV), only secondary electrons from the first a few nm of sample surface can be collected by the detector and used to generate an image that is very sensitive to the topology of the sample. Back-scattered electrons are generated as incident electrons collide with sample atom nucleus and scatter "backward" 180 in a mostly elastic process. The production of back-scattered electrons varies with the atomic number. Thus images generated by collecting back-scattered electrons are more sensitive to the Z contrast within the sample. Areas ranging from approximately 1 cm to 5 m in size can be imaged in a scanning mode using conventional SEM techniques with a spatial reso- lution of up to a few to a few tens of nm. The SEM is also capable of performing analysis of selected point locations on the sample; this approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions using energy dispersive X-ray spectroscopy (EDS), crystalline structure and crystal orientations using electron back-scattering diffraction (EBSD). 2.3.4 Transmission electron microscopy In a transmission electron microscope (TEM), the electron beam are generated from the electron gun, accelerated by a electric field, and pass through a thin spec- imen. Transmitting electrons are collected, focused, and projected onto the viewing device at the bottom of the column. The construction is somewhat similar to a light microscope. The light source of the light microscope is replaced by an electron gun. 16 Figure 2.7: Signals generated due to electron sample interaction in a SEM. Image courtesy: Thermo Fisher Scientific. The glass lenses are replaced by electromagnetic lenses. The eyepiece is replaced by a fluorescent screen or a digital camera. Unlike glass lenses, the power (focal length) of magnetic lenses can be changed by changing the current through the lens coil. And the imperfect electromagnetic lenses are the main limiting factors for spatial resolution in a TEM. In the conventional transmission electron microscopy (TEM) mode, the condenser lenses of the microscope are adjusted to illuminate the sample with a parallel beam of electrons, as shown in Fig.2.8(a). Selected area electron diffraction (SAED) pattern, and imaging due to amplitude contrast or diffraction contrast can be constructed. Atomic resolution imaging of lattice fringes can also be achieved with the phase contrast when combining more than one diffracted beam. Scanning transmission electron microscopy (STEM) combines the principles of transmission electron microscopy and scanning electron microscopy. Similar to SEM, the STEM technique scans a very finely focused beam of electrons across the sample in a raster pattern, as shown in Fig.2.8(b). Similar to TEM, STEM requires very thin samples and looks primarily at beam electrons transmitted through the sample. Al- 17 though the convergence of the beam results in the loss of spatial coherency, it enables the use of other signals that cannot be spatially correlated in TEM, including sec- ondary electrons, scattered beam electrons, characteristic X-rays, and electron energy loss spectra due to various electron-material interaction similar to what is shown in Fig.2.7. Similar to the back-scattered electron signals in SEM, Z-contrast images in STEM are formed by mapping the intensity of high-angle scattered electrons as the electron probe is scanned across the specimen using a high-angle annular dark field (HAADF) detector. High special resolution composition mapping may be obtained in STEM by collecting the EDS and electron energy loss spectroscopy (EELS) signals point-by-point as one scans the electron probe across the sample, to give composition and electronic structure information at atomic resolution. When it comes to the imperfection in the electromagnetic lenses, spherical aber- ration contributes primarily to the the blurring of image. The recent development of dedicated aberration-corrected TEM has enabled major advances in ultimate resolu- tion. State-of-art TEM platforms incorporate not only sophisticated spherical aber- ration corrections, but also more complex chromatic aberration corrections, which can enable spatial resolution well below 1Å. 18 (a) (b) Figure 2.8: Comparison of TEM mode (a) and STEM mode (b) in transmission electron microscopy. Figure taken from [8]. 19 2.4 Chemical characterizations 2.4.1 Energy dispersive X-ray spectroscopy In SEM or STEM, electrons bombarding the specimen can knock out the core electrons in the sample. The energy due to higher level electrons jumping into this core level can be released in the form of emitting a photon, usually a photon in the X-ray range, as shown in Fig.2.9. An energy dispersive X-ray spectrometer count and sort characteristic X-rays according to their energy. The energy difference between core levels is a unique characteristic of each element, and can be used to extract elemental composition within the sample. Such a technique is mostly used for qual- itative analysis of materials but is capable of providing semi-quantitative results as well. Note that due to relatively small penetration depth of electron beam into the sample, EDS is a relatively surface sensitive chemical characterization technique. Figure 2.9: Schematics for fluorescence of X-ray from materials due to external radi- ation exciting core electrons. Image courtesy: Wikipedia. 2.4.2 Wavelength dispersive X-ray fluorescence spectroscopy Wavelength dispersive X-ray fluorescence (WDXRF) is another type of X-ray flu- orescence instrumentation used for elemental analysis. In WDXRF setups, all of the elements in the sample are excited simultaneously. The different energies of the char- acteristic radiation emitted from the sample are diffracted into different directions by an analyzing crystal or monochromator (similar to the action of a prism dispersing different wavelengths of light into different directions). By placing the detector at a certain angle, the intensity of X-rays with a certain wavelength can be measured. Se- quential spectrometers use a moving detector on a goniometer to move it through an 20 angular range to measure the intensities of many different wavelengths. Simultaneous spectrometers are equipped with a set of fixed detection systems, where each system measures the radiation of a specific element. The principle advantages of WDXRF systems, as compared to energy dispersive X-ray fluorescence techniques, are high resolution (typically 5 - 20 eV) and minimal spectral overlaps. X-ray optics can be used to enhance WDXRF instrumentation. Polycapillary fo- cusingopticscollectX-raysfromthedivergentX-raysourceanddirectthemtoasmall focused beam at the sample surface with diameters as small as tens of micrometers. The resulting increased intensity delivered to the sample in a small focal spot, allows for enhanced spatial resolution for small feature analysis and enhanced performance for measurement of trace elements for Micro WDXRF applications. Additional colli- mating optics or the replacement of analyzing flat crystal with doubly curved crystal monochromator allow for efficient collection of the characteristic fluorescence X-rays from a small spot on the sample surface. 2.5 Optical spectroscopy 2.5.1 UV-Vis-NIR transmission and reflection spectroscopy In order to obtain a correct transmittance measurement for a solid, the possibility of the transmitted beam deviating in relation to the incident beam must be taken into account. The beam could also be diffused in all directions by the sample, producing measurement error. Overall transmittance, i.e. direct transmittance plus diffuse transmittance can be measured using an integrating sphere. There are also two kinds of reflectance, specular and diffuse. Specular reflectance refers to the part of the incident beam reflected at the same angle as the angle of incidence; Diffuse reflectance refers to the part of the incident beam reflected in all directions; powders produce diffuse reflectance when used as samples. Most samples produce a combination of specularanddiffusereflectance. UV-Vis-NIRspectroscopyinthisworkwasperformed in a Lambda 950 spectrophotometer with an integrating sphere of 150 mm. Such a setup allows the measurement of overall transmittance, and diffuse and specular reflectance over a range of 200 nm - 2500 nm. The sample is placed against the sphere and the beam transmitted or reflected by the sample is reflected onto the internal reflective surface of the sphere before reaching the detectors inside the sphere. The sample is placed in front of the sphere if transmittance is being measured and behind it if reflectance is being measured. The sphere’s internal surface is made of the polymer spectralon, which offers diffuse reflectance approaching 100%. 2.5.2 Fourier-transform infrared spectroscopy Fourier-transform infrared spectroscopy (FTIR) is another technique to measure how well a sample absorbs light at certain wavelengths. FTIR is commonly used to 21 Figure 2.10: Schematics for the configuration of the UV-Vis-NIR spectrophotometer with a 150 mm integrating sphere. Image courtesy: Perkin Elmer. obtain an infrared spectrum of absorption, emission, photoconductivity or Raman scattering of a solid, liquid or gas, widely used in physics, chemistry, and biology. It has the advantages of high spectral resolution, good signal-to-noise ratios, and the ability to measure a broad region of the spectrum in a short amount of time. This confers a significant advantage over a dispersive spectrometer that measures intensity over a narrow range of wavelengths at a time. FTIR uses an incandescent source of light to emit a bright ray in the IR wavelength range. This beam arrives at a half-silvered mirror, which divides it into 50% directed toward a fixed mirror and 50% to a movable mirror. The reflected beams return to the half-silvered mirror, which rejoins them and sends them to the detector. Even if the two beams arrive at the detector simultaneously, they have done so after covering a different optical path. Depending on the differences of optical paths of two rays, a constructive or destructiveinterferencehasthereforebeencreated. Thesignaldetectedinthismanner is proportional to the difference in the optical path of the two beams and to the position of movable mirror during analysis. A Fourier transform is required to convert the raw data into the actual spectrum at the detector, earning the name Fourier transform infrared spectroscopy. In this work, we have also performed polarization- resolved transmission and reflectance measurement, with the geometry shown in Fig. 2.11. 22 y z x Surface Normal Specific Crystal Axis B E Electric field direction (polarization) k incident light direction Figure 2.11: Schematics for configuration of polarization-resolved infrared transmis- sion and reflectance measurements. 2.5.3 Photoluminescence Photoluminescence (PL) is the light emission from any material after the absorp- tion of photons. In semiconductors, electrons are excited from valence band (VB) to conduction band (CB) upon absorption of photon with energy higher than the band gap. The excited carriers will relax to band edges and recombine. If the energy in such band-to-band recombination is released in the form of generating another photon, such luminescence process is referred to as photoluminescence. Static photo- luminescence can be used as a tool to extract the information about the band struc- ture. Time-resolved PL (TRPL) monitors such a luminescence process as a function of time, which allows the probe of relevant exciton dynamics and study of various recombination pathways in semiconductor materials. 2.5.4 Raman spectroscopy Raman spectroscopy is a spectroscopic technique used to observe vibrational, ro- tational, and other low-frequency modes in a system. Raman spectroscopy is an important tool in the field of vibrational spectroscopy and is complementary to in- frared absorption spectroscopy such as FTIR. Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identi- fied. It is worth emphasizing that these two spectroscopy techniques do not probe the same vibrational information. Raman spectroscopy is based on an inelastic scat- 23 VB CB Figure 2.12: Schematics showing the process of photoluminescence. tering process, whereas infrared spectroscopy is based on an absorption process. Ra- man spectroscopy detects vibrations involving a change in polarizability in molecules, whereasinfraredspectroscopydetectsvibrationsinvolvingachangeindipolemoment. As a result, the two spectroscopy techniques have different selection rules. In crys- talline solids, collective mode of lattice vibration can be quantized and captured in a quasi-particle phonon picture. Raman modes can then be viewed as the change in interacting photon energy by absorbing or creating a phonon. 24 Chapter 3 Optical properties of three-dimensional TMPCs 3.1 Introduction Rational design or discovery of new materials, especially semiconductors, has been a key contributor to several electronic, photonic, and energy technologies. Among current semiconductors, the dominant materials such as silicon, III-V, II-VI are typ- ically constructed by four-fold coordinated tetrahedral network of covalent bonds. Perovskites, a class of materials with highly symmetric close packed structure, have been researched heavily for their versatility in chemistry and physical properties. The organic-inorganic lead halide perovskites, which are shown to be efficient photovoltaic materials with current laboratory scale power conversion efficiency of over 23%,[3, 29– 35, 55] provide an exciting example. However, its toxicity and instability issues pose serious challenges for practical applications. On the other hand, the well known per- ovskiteoxideswithchemicalformulaABO 3 , whereAisanalkali, alkalineorrareearth metal and B is a transition metal, form the basis for our understanding of electronic properties of inorganic perovskites. Large energy differences between transition metal d-orbital conduction band and O 2p orbital valence band in these materials typically lead to very large energy gaps (> 3 eV) rendering them unsuitable for solar energy conversion or visible optoelectronic applications. A new class of semiconductors, transition metal perovskite chalcogenides (TMPCs), addresses this deficiency. By replacing O with S or Se, we move the valence band composed of mainly chalcogen (S, Se) 3p or 4p orbitals higher and decrease the band gaps to Visible âĂŞ IR range. The maximum achievable power conversion efficiency of a single junction solar cell is sensitively related to the semiconducting material’s band gap. Further, tunable band gap in a class of materials with similar structure and chemistry can also be of immense value for tandem solar cell architectures. TMPCs with a general formula, ABX 3 , where A is Ba, Sr or Ca, B is Ti, Zr or Hf, and X is S or Se, are predicted to provide a platform for band-gap engineering in the visible to infrared region.[2] 25 The ternary structure of perovskites opens up a larger design space compared to currently dominant single element (e.g. Silicon) and binary compound (e.g. GaAs, InP) systems. Theoretical calculations have shown that band gaps of individual TM- PCs span from the Far IR to Visible spectrum with more than 10 materials falling in between 1.0 eV to 2.5 eV.[2] Although alloying has been successfully employed to tune band gaps for other semiconductor materials, substitutional defects can lead to disorder and inferior physical properties. Hence, the chemical and structural flexi- bility of the ternary structure in TMPCs can lead to wide range of heterostructures with good interfacial quality, which is especially attractive for various optoelectronic applications. Additionally, all the elements involved in TMPCs are benign and earth abundant, which allows TMPCs to address the sustainability issues clouding various advanced materials. In the case of BaZrS 3 and SrZrS 3 , Ba, Sr, Zr and S has an earth abundance of 425, 370, 165 and 350 ppm, while Cd, Te, Sn and In has an abundance of 0.15, 0.001, 2.3 and 0.25 ppm respectively.[13] As inorganic compounds with close packed structure, these materials are also generally stable in ambient and elevated temperatures. Another key advantage of TMPCs over conventional semiconductors is the large density of states of the conduction band. Large density of states leads to large absorption coefficients near the band edge ( 10 5 cm 1 ), and small absorption length ( 100 nm absorbs >95% of incident solar photons above the band gap) in photovoltaic devices. Thin absorption layers increase the efficiency of collecting the excited carriers at the electrodes, and also lead to shorter synthesis times and lower materials costs. Recently several theoretical calculations were reported on the elec- tronic structure and optical properties of TMPCs. The electronic structure of BaZrS 3 has been theoretically calculated and direct band gap of 1.7eV,[1, 20] 1.72eV,[19] and 1.82eV[56] were reported. SrZrS 3 is expected to show a band gap of 1.2 eV for needle- like phase (-SrZrS 3 ) and 2 eV for distorted perovskite phase (-SrZrS 3 ).[2] On the other hand, several TMPCs have been synthesized in ceramic form over the last half century, by either heating binary sulfide mixture for several weeks,[41, 43, 57–60] or sulfur substitution of corresponding oxides with CS 2 [39, 44] or H 2 S.[41] Despite these numerous attempts, there is very little experimental data on their optical proper- ties, as most reports focused on structural characterization. Recently, two groups reported optical properties of BaZrS 3 . Meng et al.[19] reported a band gap of 1.85 eV for BaZrS 3 synthesized by conventional solid-state reaction of binary mixtures (BaS and ZrS 2 ) with repeated annealing. Perera et al.[20] reported a band gap value of 1.7 eV with photoluminescence and diffuse reflectance measurements on BaZrS 3 synthe- sized by high temperature sulfurization of oxides with CS 2 . With these preliminary reports in mind, it is imperative to study the physical properties, especially optical properties, of TMPCs in depth to evaluate their potential as a new class of versatile semiconductors for optoelectronic applications. 26 (a) (b) (c) Figure 3.1: Optical pictures of the synthesized powder and cold-pressed pellet for -SrZrS 3 (a), -SrZrS 3 (b) and BaZrS 3 (c). 3.2 Synthesis of polycrystalline BaZrS 3 , -SrZrS 3 and -SrZrS 3 The starting materials, Barium Sulfide powder (Alfa Aesar 99.7%), Strontium Sul- fide powder (Alfa Aesar, 99.9%), Zirconium powder (STREM, 99.5%), Sulfur pieces (Alfa Aesar 99.999%) and iodine pieces (Alfa Aesar 99.99%) were stored and handled in an Argon-filled glove box. Stoichiometric quantities of precursor powders with a total weight of 0.5 g was ground and loaded into a quartz tube with a diameter of 3 4 inch along with around 0.5 mg cm 3 iodine inside the glove box. The tube was then sealed using a blowtorch with oxygen and natural gas as the combustion mixture without exposing the contents of the ampoule to air. BaZrS 3 ,-SrZrS 3 and-SrZrS 3 samples were held at 600 C, 850 C, and 1100 C for 60 hours. All the samples were quenched to room temperature after the dwell time using a sliding furnace setup at approximately100Kmin 1 . Thentheobtainedsamplesweregroundandpressedinto 13 mm diameter pellets under uniaxial stress of around 600 MPa using a hydraulic cold press. The obtained pellets were mounted on acrylic epoxy base and polished with silicon carbide sand papers up to 3000 grit to yield a clean and flat surface. Obtained materials appeared in three different colors, as can be seen in the optical images in Fig.3.1, indicating distinct optical properties in the visible spectrum. 3.3 Crystal structure Despite short reaction times, our characterizations showed very high structural and chemical quality of the final products. The powder XRD characterization was carried out using a Bruker D8 Advance X-ray diffractometer with Co K radiation in Bragg-Brentano symmetric geometry with power setting of 35 kV and 40 mA, and a sample stage rotation rate of 15 rpm. The XRD patterns showed single-phase pu- rity for all three samples within instrument detection limit. The Rietveld analysis was performed to extract the structural information, as shown in Table 3.1, 3.2, 3.3. As expected, BaZrS 3 and high temperature SrZrS 3 phase adopt the distorted per- ovskite structure while lower temperature -SrZrS 3 adopts the needle like structure. 27 In BaZrS 3 and -SrZrS 3 the BX 6 octahedra are connected through a corner sharing 3D network. Tilting of the octahedra breaks the cubic symmetry and ends up with a distorted perovskite phase with an orthorhombic structure. In the powder patterns of BaZrS 3 and -SrZrS 3 , we can see most intense peaks qualitatively resemble peaks from the ideal perovskite structure, with weaker satellite peaks arising from the sym- metry breaking due to the octahedra tilting. -SrZrS 3 is the needle-like phase with the NH 4 CdCl 3 structure. In such a structure, the octahedra are connected by sharing edges, and two columns of edge-sharing octahedra form a chain. These parallel chains are extended along one direction. The two structurally distinct phases can both be stabilized at room temperature by quenching from different growth temperatures. 3.3.1 Rietveld refinement The Rietveld refinement was performed using Fullprof Suite software on scans from 15 to 90 with a 0.02 step and 2 second integration time for each data point. The initial guesses for the structural values were obtained from the Inorganic Crystal Structure Database (ICSD). Rietveld analysis showed lattice constants of a = 8.504 Å, b = 3.820 Å, and c = 13.917 Å for -SrZrS 3 ; a = 7.103 Å, b = 9.758 Å and c = 6.731 Å for -SrZrS 3 ; and a = 7.061 Å, b = 9.977 Å, and c = 7.014 Å for BaZrS 3 , with a space group of Pnma (62) for all three materials. The detailed structural parameters are available in Table 3.1, 3.2 and 3.3. Space Group Pnma Cell Parameters a(Å) b(Å) c(Å) 8.5037 3.8202 13.9174 Positional X Y Z B Parameters Sr 0.43337 0.25000 0.67921 0.00938 Zr 0.17149 0.25000 0.44242 0.00376 S 0.01670 0.25000 0.60627 0.00365 S 0.16148 0.25000 0.01394 0.00598 S 0.29478 0.25000 0.28133 0.00683 Agreement R p R w p R e 2 Factors 4.71 5.95 5.58 1.14 Table 3.1: Structural parameters for -SrZrS 3 from Rietveld refinement. 3.3.2 Grain size The instrument response function (IRF) of the Bruker D8 x-ray diffractometer, in terms of inherent peak broadening, was extracted by calibration measurements on a standard reference material, NIST 1976b (sintered alumina disc). The IRF file was 28 10 20 30 40 50 60 70 80 90 Intensity (arb. units) 2q (°) b-SZS Exp. b-SZS Cal. Peak Position Difference 20 30 40 50 60 70 80 90 BZS Exp. BZS Cal. Peak Position Difference Intensity (arb. units) 2q (°) 10 20 30 40 50 60 70 80 90 2q (°) a-SZS Exp. a-SZS Cal. Peak Position Difference Intensity (arb. units) (a) (b) (c) Figure 3.2: Plots of Powder XRD patterns with Rietveld analysis for (a) -SrZrS 3 (red), (b) -SrZrS 3 (blue) and (c) BaZrS 3 (green). The black lines are simulated intensity profiles for three materials. 29 Space Group Pnma Cell Parameters a(Å) b(Å) c(Å) 7.1030 9.7578 6.7312 Positional X Y Z B Parameters Sr 0.42330 0.25000 0.02155 0.60000 Zr 0.00000 0.00000 0.00000 0.76000 S 0.51739 0.25000 0.58608 0.43000 S 0.19099 0.04456 0.30872 0.76000 Agreement R p R w p R e 2 Factors 4.74 5.98 5.49 1.18 Table 3.2: Structural parameters for -SrZrS 3 from Rietveld refinement. Space Group Pnma Cell Parameters a(Å) b(Å) c(Å) 7.0605 9.9765 7.0139 Positional X Y Z B Parameters Ba 0.46331 0.25000 0.00293 1.12000 Zr 0.00000 0.00000 0.00000 0.76000 S 0.51047 0.25000 0.56230 0.69000 S 0.21761 0.02779 0.28127 0.98000 Agreement R p R w p R e 2 Factors 5.64 7.18 6.95 1.07 Table 3.3: Structural parameters for BaZrS 3 from Rietveld refinement. then used as an instrument resolution input in Rietveld analysis for the three samples. The obtained average crystallite sizes for BaZrS 3 ,-SrZrS 3 and-SrZrS 3 are 150 nm, 210 nm and 480 nm respectively. The crystallite size is typically an underestimation of the grain size. 3.3.3 Octahedral tilting Synchrotron powder XRD was also performed in collaboration with Prof. Peter Khalifah at Stony Brook University using synchrotron radiation with a wavelength of = 0.412735 Å. The structural refinement based on obtained pattern agrees well with that obtained in the laboratory setup, yielding a space groupPnma with lattice constants of a = 7.0654 Å, b = 9.985 Å, c = 7.0229 Å. Notably, there were a few unassigned peaks in the synchrotron XRD pattern, which were too weak to be picked up in the laboratory X-ray diffractometer. The rest of peaks were indexed well in the refinement, both in terms of positions and intensity distribution. The three unac- 30 * * * Figure 3.3: A plot of synchrotron powder diffraction pattern with Rietveld refinement analysisforBaZrS 3 . Theblack, redandgreylinesareexperimentalpattern, simulated pattern and their difference, respectively. Unmatched peaks in Rietveld analysis are marked with red star. The inset is the schematic for the refined BaZrS 3 structure. counted peaks were suspected to be signals from small amount of leftover reactants. A BaZrS 3 crystal structure was constructed based on the synchrotron refined model. Octahedra are sharing corners and form a 3D connected network. The structure is a distorted version of perfect cubic perovskite structure by incorporating certain ro- tations of all the ZrS 6 octahedra. The octahedra tilting can be either in phase or out of phase for the adjacent layers. For BaZrS 3 , the tilting along b axis is in phase, while the tilting along a and c axis are out of phase, following a Glazer notation of a b + c .[61] The tilting amplitude can be calculated by looking at the torsion angle of the Zr-S bonds. For out of phase tilt a , the torsion angle is half of the S-Zr-Zr-S torsion angle linking octahedra in two adjacent layers. For in phase tiltb + , the torsion angle half of 180 minus Zr-S-S-Zr torsion angle linking octahedra in the same layer. 3.4 Other characterizations Raman spectroscopy was performed at room temperature to gain further insight into the vibrational modes of the materials. Raman spectroscopy measurements were performed in a Renishaw inVia confocal Raman Microscope with a 532 nm laser and a 100X objective lens. To eliminate the possible signals from surface contaminations, measurements were done right before and after sanding of the sample surface with sand paper. The results are consistent in both cases. Obtained Raman spectra are shown in Fig.3.6. 31 (a) (b) (c) Figure 3.4: (a) Schematic structure shows the out of phase tilt where the tilting of adjacent layers are in opposite direction. Ball-stick models show the projected view along the tilting axis (b) and the S-Zr-Zr-S torsion angle (blue-black-red) in perspective view (c). (a) (b) (c) Figure 3.5: (a) Schematic structure shows the in phase tilt where the tilting of ad- jacent layers are overlapping. Ball-stick models show the projected view along the tiltingaxis(b)andtheZr-S-S-Zrtorsionangle(pink-black-purple)inperspectiveview (c). 32 (a) (b) (c) α- ��� ��� ��� ��� ��� ��� ��� � ��������� ( �� - � ) � �� ����� � ( ���� ���� ) β- ��� ��� ��� ��� ��� ��� ��� � ��������� ( �� - � ) � �� ����� � ( ���� ���� ) ��� ��� ��� ��� ��� ��� ��� � ��������� ( �� - � ) � �� ����� � ( ���� ���� ) Figure 3.6: Raman spectra of BaZrS 3 , -SrZrS 3 , and -SrZrS 3 samples at room temperature. Chemical composition analysis of these polycrystalline materials was carried out using EDS and WDXRF. BaS (Alfa Aesar 99.9%) was used as a standard input to obtain the Ba and S calibration factor for the quantitative EDS. The BaS powders were argon sealed in a glove box and gently pressed into a pellet right before entering the SEM exchange chamber. The calibration factor 1.36, was determined at four different magnifications with very minimal O signal. Similarly the calibration factor was obtained as 1.26 for Sr and S. The analytical data for both BaS and SrS from Alfa Aesarwasfoundtobe1:1justifyingtheneedforcalibration. Theobtainedchemical compositions for three samples in micron sized areas using EDS were deduced as Ba: Zr: S= 1: 0.93: 3.06 for BaZrS 3 , Sr: Zr: S= 1: 0.94: 2.90 for-SrZrS 3 and Sr: Zr: S= 1: 0.92: 2.99 for -SrZrS 3 (EDS spectra shown in Fig.3.7). The multi-spot analysis was performed using relatively more bulk sensitive WDXRF and showed consistent stoichiometrythrough10differentpointsonthepelletasshowninFig.3.8)Theatomic percent ratio variation for Zr: Sr and S: Sr were within 1% and 4% respectively for -SrZrS 3 and -SrZrS 3 ; The Zr: Ba and S: Ba variation are within 1% and 3% respectively for BaZrS 3 . These studies establish that the synthesized materials are crystalline and homogeneous both at the microscopic and macroscopic scales. (a) (b) (c) β- ��� � � � � � � ����� ( ��� ) � �� ����� � ( ��� � ���� � ) Zr Lα S Kα Sr Lα α- ��� � � � � � � ����� ( ��� ) � �� ����� � ( ��� � ���� � ) S Kα Sr Lα Zr Lα ��� � � � � � � ����� ( ��� ) � �� ����� � ( ��� � ���� � ) Zr Lα S Kα Ba Lα Figure 3.7: EDS spectra of BaZrS 3 -SrZrS 3 -SrZrS 3 samples at 400 magnification. 33 (a) (b) 0 1 2 3 4 1 2 3 4 5 6 7 8 9 10 Zr:Ba S:Ba Figure 3.8: WDXRF mapping on a BaZrS 3 pellet.(a) Ten locations for the map- ping.(b) Obtain Zr:Ba and S:Ba ratios at ten locations. 3.5 First principles calculations The first principles calculations were performed using the general potential lin- earized augmented planewave (LAPW) method[62] as implemented in the WIEN2K code.[63] We used highly converged basis sets corresponding to a cut-off of R min k max = 9.0, where R min is the minimum LAPW sphere radius and k max is the planewave sector cutoff. The LAPW sphere radii were 2.3 Bohr for Zr and S, and 2.5 Bohr for Ba and Sr. We did calculations with the modified Becke-Johnson potential (mBJ),[64] with experimental lattice parameters from literature and atomic positions determined by energy minimization with the Perdew-Burke-Ernzerhof (PBE)-generalized gradi- ent approximation (GGA) functional.[65] We also performed calculations with the PBE+U method, applying U to the Zr d states as a parameter to adjust the gaps. We find that it is possible to obtain the experimental gaps in this way, although it requires the use of different values of the parameter U for each of the three com- pounds. Results shown are from the mBJ functional, without band gap adjustment, unless noted otherwise. The theoretical band gap values from calculations with the mBJ potential are 1.12 eV, 1.73 eV and 1.55 eV for -SrZrS 3 , -SrZrS 3 and BaZrS 3 , respectively. The obtained band structures, DOS, absorption coefficients are shown in Fig.3.9 and 3.10. The shapes and positions of the valence and conduction band carrier pockets are illustrated in the isosurfaces Fig.3.11. It is worth noting that as seen in the isosurface plots, the conduction bands of BSZ and -SrZrS 3 are highly corrugated, showing a four armed star shape with arms along the [101] directions. From the point of view of transport, when doped this type of shape combines parts of the Fermi surface that have effectively high velocities favorable for conduction, with other parts (along the arms) that are heavy and lead to high density of states. In the context of solar applications, this type of band structure favors the otherwise rare combination of high absorption (due to high density of states) and efficient electron collection. It also favors the combination of high thermopower and high conductivity 34 BaZrS 3 β-SrZrS 3 α-SrZrS 3 (a) (b) (c) Figure 3.9: Calculated band structure for three materials. in the context of thermoelectric materials. 3.6 Bandgap determination 3.6.1 photoluminescence OpticalbandgapofthesematerialsweredeterminedbyPL.Theroomtemperature PL measurements were performed in a Renishaw inVia confocal Raman Microscope with a 532nm laser and a 100X objective lens. The PL spectra obtained at room temperature indicate band gap values of -SrZrS 3 as 1.53 eV, -SrZrS 3 as 2.13 eV and BaZrS 3 as 1.81 eV, as shown in Fig.3.12). These values agree well with the theoretical values from calculations, show the same trend as the experimental values, but are larger by0.3 eV - 0.4 eV. The intense PL peak of -SrZrS 3 was symmetric with FWHM of122 meV. -SrZrS 3 showed a symmetric but weaker peak with FWHM of 155meV, likely due to the lack of high symmetry corner sharing octahedral network. PL of BaZrS 3 is not very symmetric or as intense as other compounds, with a typical FWHM around 264 meV. The calculated optical absorption spectra of structurally similar -SrZrS 3 and BaZrS 3 both show a particularly strong onset of absorption at the band edges, reaching values above 2x105 cm 1 within a few tenths ofeVabovethegap. Thisisfavorableforuseasasolarabsorber, aswellasanemitter. It arises from the sharp onsets of the density of states (as shown in Fig.3.10)) at both the conduction and valence band edges, corresponding to weakly dispersive bands. Needle-like phase,-SrZrS 3 , shows a very different shape of the density of states, with broader bands at both the conduction and valence band edges and weaker onset in the absorption spectrum. Weak above gap absorption, despite a direct gap material, is a 35 α- ��� β- ��� ��� - � - � � � � � � � � � �� � ����� ( ��) ������ � �� � � �� �� ( ��� � � � � ) α- ��� β- ��� ��� �� � �� � �� � �� � �� � �� � �� � � �� �� �� �� �� �� �� ���� ��� ��� ��� ��� � ����� ( �� ) � ������ ��� ���� � ������ ( �� � �� - � ) � �������� � ( �� ) (a) (b) Figure 3.10: (a) Calculated densities of states and (b) absorption spectra (direction averaged) for three materials. consequence of matrix elements and band structure effects. It is interesting to note that-SrZrS 3 and-SrZrS 3 with same chemical composition show very different band gap values and absorption spectra, indicating structural control of optical properties. 3.6.2 Diffuse reflectance spectroscopy The band gaps were also estimated by the diffuse reflectance and transmittance measurements on powder samples using a spectrometer with an integrating sphere to account for both absorption and scattering. The measurements were carried out in a Lambda 950 UV-Vis-NIR spectrometer with an InGaAs integrating sphere detector accessory. The scans were from performed for a wavelength range of 400 nm to 1000 nm, with 1 nm step size. The detector switched from photo multiplier tube to InGaAs at 900 nm, which gave rise to experimental artifacts in the form of jitters. The sample was a thin layer of powders, sandwiched in between two cover glass pieces. The diffuse transmittance T was measured with samples clipped at the front entry port of the integrating sphere and the diffuse reflectance R 0 was measured at the exiting reflectance port with a black background. The transmittance and diffuse reflectance of two cover glass pieces were separately measured and accounted for in the final determination of the optical properties of the powder samples. The relationship between the measured diffusive reflectance and transmittance values to the absorption coefficient k, and a scattering coefficient s is given by the Kubelka- Munk theory.[66] The following equations are used for translucent powder layers to solve for both absorption coefficientk and scattering coefficients simultaneously from diffusive reflectance R 0 and transmittance T: s = ( 1 2bd ) tanh 1 ( bR 0 1aR 0 ) (3.1) k = (a 1)s (3.2) 36 BaZrS 3 β-SrZrS 3 α-SrZrS 3 (a) (b) (c) Figure 3.11: Plot of isosurfaces of the calculated band structure 0.05 eV below the va- lence band maximum (VBM, blue) and 0.05 eV above the conduction band minimum (CBM, red) for three material. a = (1 +R 2 0 T 2 )=2R 0 (3.3) b = (a 2 1) 1=2 (3.4) where a and b are quantities introduced to simplify expressions and carry no phys- ical purposes, and d is the thickness of the layer. The absorbance value = (1 R 1 ) 2 =2R 1 was used in (h) 2 versus h plot to find the band gaps of the materi- als. Obtained values were 1.52 eV, 2.05 eV and 1.83 eV for -SrZrS 3 , -SrZrS 3 and BaZrS 3 respectively, as shown in Fig. 3.13. The diffuse reflectance measurements on infinitely thick powder layers were also done to extrapolate the band gaps. We have T=0 and diffuse reflectance and . k changes significantly while s remains fairly invariant through the wavelength range. According to Kubelka-Munk model for an infinitely thick direct band gap materials, a plot of versus plot was used to find band gaps of the three materials, which turned out to agree with the above translucent powder method used, as shown in Fig. 3.14. 37 α- ��� β- ��� ��� �� � �� � �� � �� � �� � �� � �� � ��� ��� ��� ��� ��� � ����� ��� � ���������� � �� ����� � � �������� � ��� � Figure 3.12: PL spectra for-SrZrS 3 (red),-SrZrS 3 (blue), and BaZrS 3 (green) show band gap values of 1.53 eV, 2.13 eV and 1.81 eV respectively. α- ��� β- ��� ��� �� � �� � �� � �� � �� � �� � �� � � � � � � �� ��� ��� ��� ��� ��� � ����� ( �� ) ( α* � ν) � ( �� � ) � �������� � ( �� ) 1.52eV 1.83eV 2.05eV Figure 3.13: Band gap determination with absorbance value obtained from dif- fuse reflectance and transmittance measurements on translucent powder layer using Kubelka-Munk theory. The deduced band gap values are 1.52 eV for -SrZrS 3 (red), 2.05 eV for -SrZrS 3 (blue), and 1.83 eV for BaZrS 3 (green). 38 (a) (b) α- ��� β- ��� ��� �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � �� � ��� ��� ��� ��� ��� � ν ( �� ) ( α* � ν) ��( �� � � ) � �������� � ( �� ) α- ��� β- ��� ��� �� � �� � �� � �� � �� � �� � �� � �� � �� � ��� ��� ��� ��� ��� ��� � ν ( �� ) α � �������� � ( �� ) Figure 3.14: Band gap determination plot (a) with absorbance values (b) obtained from diffuse reflectance measurements of thick pellets using Kubelka-Munk theory. The deduced band gap values are 1.54 eV for -SrZrS 3 (red), 2.08 eV for -SrZrS 3 (blue), and 1.85 eV for BaZrS 3 (green). 39 3.7 Quantitative photoluminescence 3.7.1 The "green gap" Efficient light emitting devices (LEDs) have completely revolutionized the lighting industry. However, there is a discontinuity gap between highly efficient blue nitride LEDs and red phosphide LEDs,[9] as shown in Fig.3.15. One long-standing technical challenge is to increase the efficiency of deep green LEDs. Although most products today use phosphor conversion (PC) to produce white light from a blue LED, having a good green source could lead to color-mixed white sources that would avoid the losses associated with the PC approach. -SrZrS 3 with a strong luminescence right at the weakest green gap, along with its high stability, easily salable composition could be a good candidate for green light emission devices. Figure 3.15: A plot of nitride and phosphide LED efficiency as a function of wave- length. Adopted from report[9]. 3.7.2 External luminescence efficiency Acomparisonoftheradiativeemissionof-SrZrS 3 withreferencesinglecrystalline InP and CdSe wafers under the same illumination and measurement conditions is shown in Fig.3.16. Despite the non-ideal surface quality of the cold pressed pellets, integrated emis- sionintensityfrom-SrZrS 3 iscomparabletotheCdSe, withinoneorderofmagnitude compared to the atomically smooth state-of-art InP wafers. To directly quantify the strong emission of-SrZrS 3 , we also performed external luminescence efficiency ( ext ) measurements. These measurements were carried out by illuminating the sample with a known photon flux using a 532 nm laser, and then measuring the output photon 40 CdSe InP β-SrZrS 3 Figure 3.16: PL intensity comparison of -SrZrS 3 sample with InP and CdSe single crystal substrates. flux. Such quantitative photoluminescence measurements were performed in the same Renishaw inVia Microscope as the static PL measurements. This tool was calibrated using a transfer standard InP wafer. This was carried out by first measuring the sys- tem instrument response function (IRF) with a broadband calibrated reference light source. Next, a reference InP single crystal wafer with a known quantum efficiency from an independently calibrated photoluminescence measurement system was uti- lized to calibrate the absolute counts to photons conversion. The calibration was then cross checked across both tools. The reference InP and CdSe wafers used in Fig.3.16 were obtained from University Wafer and have a n-type carrier concentration around 10 17 cm 3 . Fig.3.17 shows both the measured external luminescence efficiency ext (black curve), and the internal luminescence efficiency (red curve), as a function of incident photon flux density. The error bar in Fig.3.17 is included as multiple sets of data were obtained on several samples. We see that the ext varies from0.06% to 0.15% as the power density is increased to 210 5 W m 2 , and then drops down upon further increases in photon flux. This corresponds to an internal luminescence efficiency between 3% - 6%, which is excellent for a polycrystalline material without any materials quality optimization. As a comparison, the ext for world record polycrystalline CIGS and CdTe cells are between 0.0001% - 0.19%,[67] after decades of research and development. The overall shape of the ext vs power density curve can be understood by considering the rel- ative rates of radiative recombination, and non-radiative Shockley-Read-Hall (SRH) and Auger recombination.[68, 69] Due to the carrier dependence of SRH, radiative, and Auger, which are proportional to the carrier density, the square, and the cube, 41 Figure 3.17: External (black) and internal (red) luminescence efficiencies of -SrZrS 3 as a function of incident power densities. respectively, it is expected that the measured efficiency will increase with incident optical density until it is limited by the non-radiative Auger recombination process. This behavior is observed here, with the Auger recombination appearing to come into play around 10 6 W m 2 , or 1000 suns. The peak of luminescence efficiency at such high injection carrier density is particularly desirable for light emitting applications. 42 3.8 Summary and outlook Insummary,TMPCsareproposedasanewclassofsemiconductorswithgreattun- abilityandsuperioroptoelectronicproperties. Threerepresentativeexamples, BaZrS 3 and SrZrS 3 in two room temperature stabilized phases, have been synthesized with an novel iodine catalyzed solid-state reaction process in sealed ampoules. Structural and chemical characterizations including XRD, Raman spectroscopy, EDS and WDXRF establish high quality of the bulk ceramic samples, and the optimized synthetic pro- cedure should be readily transferable to other TMPCs. Intense room temperature PL signals indicate band gaps of 1.53 eV for -SrZrS 3 , 1.81 eV for BaZrS 3 and 2.13 eV for -SrZrS 3 , and match well with the diffuse reflectance and transmittance mea- surements, and the theoretical calculations. Two structurally different SrZrS 3 phases show distinct optical properties indicating structural control of optical properties. - SrZrS 3 showsweakerabsorptionwiththesmallestbandgap, ascomparedtothesharp absorption onset and ultra-high absorption coefficients for the two higher symmetry phases, BaZrS 3 and -SrZrS 3 , with absorption coefficients approaching 210 5 cm 1 within only a few tenths of eV above the gap in the calculated absorption spectra. Additionally, the potential of these materials for light emitting applications was eval- uated by measurements of external luminescence efficiency. These results indicate the excellent optical properties and broad chemical and structural tunability of TMPCs as promising candidates for optoelectronic applications in general, and call for further in-depth experimental studies on TMPCs. Figure 3.18: Schematic structure, PL spectra and optical pictures for -SrZrS 3 , BaZrS 3 , and -SrZrS 3 . 43 Chapter 4 Band gap evolution in quasi-two-dimensional TMPCs 4.1 Introduction In the previous chapter, optical properties of perovskite chalcogenides such as BaZrS 3 , -SrZrS3, -SrZrS3 were explored. The luminescence studies showed the promise of such materials for photovoltaics, yet the demonstration of bandgap tun- ability down to the solar optimal single-junction value remains an outstanding chal- lenge. Several approaches such as alloying,[19, 70] and the exploration of quaternary chalcogenide[71] and other non-transition metal-based perovskite chalcogenides[72] havebeenproposedtoovercomethishurdle. Whileeffortstoachieveoptimalbandgap for single-junction solar cells and the demonstration of solar cells are underway, other approaches to add new functionalities to this family of materials are being consid- ered. For example, the introduction of ferroelectricity or any static polar order in a semiconductor can lead to interesting physical effects such as shift currents.[73] Several theoretical studies explored the possibility of achieving static polarization in the Ruddlesden-Popper type 2D layered TMPCs, to demonstrate bulk photovoltaic effect.[21, 25] Thus, studies of optical properties of 2D layered TMPCs are of broad interest beyond their applications as photovoltaic absorbers. Ruddlesden-Popper phases are two-dimensional homologues series of the perovskite structure. Such lay- ered structures can host interesting octahedral rotations and distortions that can lead to non-centrosymmetric structure, which is a prerequisite for both polar nature and ferroelectric properties. Quasi-2D perovskite chalcogenides are formed by alternating a set number (n) of perovskite layers with the chemical formula ABX 3 and a rock salt layer AX. The adjacent perovskite layers with corner-sharing BX 6 octahedra are intercalated by one AX layer, and offset by half a unit cell. Such a unique natural superlattice type Quasi-2D structure has a general formula of A’ 2 A n1 B n X 3n+1 , and boilsdowntoA n+1 B n X 3n+1 forthecaseofthesamecationsinperovskiteandrocksalt layers. For example, Ba 3 Zr 2 S 7 and Ba 2 ZrS 4 are n=2 and n=1 Ruddlesden-Popper 44 phases of the perovskite sulfide BaZrS 3 , respectively. 。。 。 n = ∞ n = 1 n = 2 n > 2 n = 0 Figure 4.1: Schematic crystal structures for RP series compounds with n=1 (per- ovskite), n=2, n=1 and n=0 (rock salt). 4.2 Synthesis of Ba-Zr-S Ruddlesden-Popper phases 4.2.1 Polycrystalline synthesis The polycrystalline materials were synthesized using solid state reaction method similar to that described in 3D perovskite chalcogenides. Barium Sulfide powder (Sigma-Aldrich 99.9%), Zirconium powder (STREM, 99.5%), Sulfur pieces (Alfa Ae- sar 99.999%) and iodine pieces (Alfa Aesar 99.99%) were stored and handled in an Argon-filled glove box. We have explored tuning various synthetic parameters to obtain high quality single phase Ba-Zr-S RP series ceramic samples, including solid state reaction with and without iodine addition, with raw elemental and binary sul- fide mixtures versus with pre-synthesized BaZrS 3 as precursors, and with different heating and cooling rates. The most successful synthetic procedures for the Ba-Zr-S RP compounds are the following. For BaZrS 3 , stoichiometric quantities of BaS, Zr, and S with a total weight of0.5 g was mixed with5 mg of I 2 . The powder mix- tures were heated to 400 C with a ramping rate of 100 C/min, dwell at 400 C for 4 hours, and then heated to 900 C with the same ramp rate, dwell at 900 C for 100 hours. The furnace was then turned off and allowed to naturally cool down after that. Note that the powder mixtures were sitting near the bottom of the tube during heat treatment, and the tube was placed in the tube furnace in such a way that the tip of tube was in the low-temperature air flow direction to allow iodine vapor to condense near the tip of the tube during cooling down. Ba 3 Zr 2 S 7 samples were obtained by 45 loading 0.5 g powder mixtures of BaS, Zr and S, or BaZrS 3 and BaS in a quartz tube together with5 mg of I 2 . The samples were heated to 400 C (ramping rate 100 C/h, dwell for 4 hours), then 650 C (ramping rate 100 C/h, dwell for 60 hours), and finally to 1100 C (ramping rate 100 C/h, dwell for 40 hours). At the end of the heat treatment, the tube was pushed out and quenched in water. The Ba 2 ZrS 4 samples were obtained by loading 0.5 g mixture of pre-synthesized BaZrS 3 powder and stoichiometric BaS, along with5 mg of I 2 . The mixtures were heated to 400 C (ramping rate 100 C/h, dwell for 4 hours), and then to 1100 C (ramping rate 100 C/h, dwell for 120 hours). At the end of the heat treatment, the tube was air quenched using a sliding furnace setup. Note that the tube was also placed in such a way that the iodine and sulfur vapor would preferably condense on the colder tip end during cooling down. 4.2.2 Crystal growth The single crystals were grown using salt flux method in sealed quartz ampoules with BaCl 2 flux. A schematic of the crystal growth is shown in Fig.2.3. The melting temperature of starting materials, BaS and Zr, is well above the highest growth temperature achievable in a quartz ampoule. The addition of low melting point BaCl 2 flux could dissolve the starting materials in a homogeneous solution at high temperature and allow much better mixing and reaction of the precursors. Then, one can lower the temperature slowly to gradually decrease the solubility of the target compound, thus creating a supersaturation to drive the nucleation and grain growth of the thermodynamically stable phase. Precursor powders with a total weight of 0.5 g was ground and loaded into a 3/4 inch diameter quartz tube along with0.5g BaCl 2 . The tubes were heated to 1050 C with a ramping rate of 0.3 C/min, held at 1050 C for 40 hours and then cooled to 400 C with a cooling rate of 1 C/min, and then allowed to naturally cool down after that by shutting off the furnace. The obtained samples were washed with deionized water repeatedly to remove the access flux, and dried with acetone and isopropyl alcohol. We have also tried growing these crystals with other salts, such as KI. The results were similar, the salt removing process was notably much easier, but the obtained crystals were not as large. In some cases, if there is still residue flux attached to the crystal surface after extensive washing, it can be removed by sonicating the crystals in isopropyl alcohol. In the case of BaZrS 3 , the predominant morphology of the obtained crystals was cube like with sharp edges and well-defined surfaces that presumably correspond to crystal facets. The obtained BaZrS 3 crystals were up to 500m in size, and showed shiny/reflective metallic luster when looked at with the right angle. In the case of RP phases, the attemptforsinglephasecrystalgrowthbyvaryingthestoichiometryofprecursorswas not successful. Salt flux growth is a thermodynamically slow process, and the energy barriers to get across RP phase boundaries are presumably shallow as they possess similar chemistry and structure. As a result, obtained crystallites were a mixture of 46 RPphase, withtherelativeyieldaffectedbythestartingstoichiometry. ForBa 3 Zr 2 S 7 , the predominant crystal morphology was cuboids and square platelets, with similar shiny metallic luster on the surface. However, when looked under microscope, the crystal facets are not as well defined, and show layered like terraces on the surfaces, presumably due to the stacking of perovskite slabs and rock-salt layer along the c axis. The isolated Ba 3 Zr 2 S 7 crystal pieces were up to 300 um in size. As for Ba 2 ZrS 4 , the obtained crystallites were much smaller. They typically adopt a tiny bar or irregular shape, with reflective luster on flat surfaces and dark brown colors. A small amount of Ba 4 Zr 3 S 10 crystallites were also found in certain cases. However, the effort to reliably grow Ba 4 Zr 3 S 10 was not successful. The predominant morphology of the obtained crystals was cubes and cuboids with well-defined surfaces that presumably correspond to crystal facets as shown in the SEM image in Fig.4.2). (a) (b) (c) Figure 4.2: Optical pictures and SEM images of BaZrS 3 (a), Ba 3 Zr 2 S 7 (b), and Ba 2 ZrS 4 (c) crystals. 4.3 Crystal structure of BaZrS 3 , Ba 3 Zr 2 S 7 , and Ba 2 ZrS 4 Fig.4.3 shows the optical pics of powder samples of the perovskite BaZrS 3 , and RP phase Ba 2 ZrS 4 and Ba 3 Zr 2 S 7 , synthesized with solid state reaction. We performed various characterizations to test the quality of the grown materials. Also shown in the Fig.4.3 is the powder X-ray diffraction overlaid with expected peaks from corresponding ICDD databases. These materials are phase pure within laboratory XRD detection limit. We can clearly see distinctive peaks at low angles. These 47 signature low angle peaks correspond to the large lattice constants of the layered structures alongc axis. Chemical composition analysis by EDS with varying locations andmagnificationsonthecrystalsshowedonlyexpectedelementsinaconsistentratio, as shown in Fig. 4.4. (a) (c) (e) (b) (d) (f) Figure 4.3: Optical pictures and powder XRD of BaZrS 3 (a,b), Ba 3 Zr 2 S 7 (c,d), and Ba 2 ZrS 4 (e,f) polycrystalline samples. Expected peak positions are adopted from ICDD databases. For the bigger crystals, BaZrS 3 and Ba 3 Zr 2 S 7 , we also did out-of-plane XRD scans on isolated crystal pieces. We see a set of narrow 00l type reflections corresponding to the termination plane of the crystals, as shown in Fig. 4.5. This confirmed that the crystal facets with layered-like features are along the (001) plane. The most intense peak has a full-width-at-half-maximum (FWHM) of less than 0.04 . The insets are rocking curves (RC) of the most intense peak. Notably, we were getting a RC FWHM of 0.011 for BaZrS 3 crystal, indicating high quality of the single crystal. However, for Ba 3 Zr 2 S 7 , the RC width is a lot larger, presumably arising from high mosaicity associated with the layered nature of the crystal structure. 48 (a) (b) BaZrS 3 Ba 2 ZrS 4 Ba 3 Zr 2 S 7 � � � � � � ����� ( ��� ) � �� ����� � ( ���� ) α-SrZrS 3 β-SrZrS 3 � � � � � � ����� ( ��� ) � �� ����� � ( ���� ) S Ka Zr La Ba La Sr La S Ka Zr La S Ka Zr La Sr La O Ka O Ka O Ka Ba La S Ka Zr La O Ka Ba La S Ka Zr La O Ka (a) (c) (b) Figure 4.4: EDS spectra of BaZrS 3 (a), Ba 2 ZrS 4 (b), and Ba 3 Zr 2 S 7 (c) samples. The obtained Ba:Zr:S ratios are 1.1:1.0:3.3, 2.3:1.0:4.1, and 1.6:1:1:3.5, respectively. We have performed single crystal X-ray diffraction analysis for the crystals. For BaZr 3 , we observed heavy twinning in the BaZrS 3 crystals and had to break the crystals into smaller pieces for a reasonable solution to the diffraction analysis. A lustrous dark red plate-like specimen of BaZrS 3 , approximate dimensions 0.005 mm 0.008 mm 0.008 mm, was used for the X-ray crystallographic analysis. The X-ray intensity data were measured on a Bruker APEX DUO system equipped with a TRI- UMPH curved-crystal monochromator and a Mo-K fine-focus tube ( = 0.71073 Å). The total exposure time was 20.80 hours. The frames were integrated with the Bruker SAINT software package using a SAINT V8.38A (Bruker AXS, 2013) algo- rithm. The integration of the data using an orthorhombic unit cell yielded a total of 4218 reflections to a maximum angle of 30.54 (0.70 Å resolution), of which 798 were independent (average redundancy 5.286, completeness = 94.8%, R int = 5.33%, R sig = 4.55%) and 623 (78.07%) were greater than 2(F 2 ). The final cell constants of a = 7.056(3) Å, b = 9.962(4) Å, c = 6.996(3) Å, volume = 491.8(3) Å 3 , are based upon the refinement of the XYZ-centroids of 1919 reflections above 20(I) with 9.169 < 2 < 60.88 . Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.764. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.7310 and 0.8970. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group Pnma, with Z = 4 for the for- mula unit, BaZrS 3 . The final anisotropic full-matrix least-squares refinement on F 2 49 020 040 060 080 FWHM 0.011° 18.48 18.50 18.52 18.54 Intensity a.u. BaZrS 3 Ba 3 Zr 2 S 7 BaZrS 3 Ba 3 Zr 2 S 7 (a) (b) (c) (d) Figure 4.5: Out-of-plane XRD of individual crystal for BaZrS 3 (a) and Ba 3 Zr 2 S 7 (b). The insets are rocking curve of the most intense peak. The cross-sectional STEM images of BaZrS 3 (c) and Ba 3 Zr 2 S 7 (d), the insets are corresponding SAED patterns. with 29 variables converged at R1 = 3.36%, for the observed data and wR2 = 7.57% for all data. The goodness-of-fit was 1.162. The largest peak in the final difference electron density synthesis was 1.804 e-/ÃĚ3 and the largest hole was -1.788 e-/ÃĚ3 with an RMS deviation of 0.356 e /Å 3 . On the basis of the final model, the calculated density was 4.386 g/cm 3 and F(000), 576 e . For Ba 3 Zr 2 S 7 , there were two possible structural variables from such high temper- ature synthesis, I4=mmm and P 4 2 =mnm. In the higher symmetry I4=mmm phase, there is no octahedra tilting. In the slightly distorted P 4 2 =mnm structure, the oc- tahedra tilting is only present along one of the in-plane axis, and the out-of-phase tilting axis keeps alternating from one block of double-layer to the next. The com- parison of these two structures are shown in Fig.4.6. It was challenging to resolve the difference between these two structural variation in powder XRD and out-of-plane 50 Space Group Pnma Temperature 100K Lattice Constants a(Å) b(Å) c(Å) 7.0563 9.9624 6.9963 Positional X Y Z U Parameters Ba1 0.5454 0.25 0.9903 0.00696 S1 0.4949 0.25 0.4382 0.00706 S2 0.2896 0.4678 0.7896 0.00604 Zr1 0.5 0.5 0.5 0.00312 Agreement R1 wR2 2 Factors 3.36% 7.57% 1.162 Table 4.1: Structural parameters from X-ray crystallographic analysis for BaZrS 3 . scan on crystal pieces. We performed single-crystal XRD studies and found that the crystals adopted the P 4 2 =mnm space group, as shown in Table 4.2. / / (a) (b) Figure 4.6: Schematics for the P 4 2 =mnm phase and I4=mmm phase of Ba 3 Zr 2 S 7 . A dark red prism-like piece of Ba 3 Zr 2 S 7 crystal, approximate dimensions 0.056 mm 0.086 mm 0.118 mm, was used for the X-ray crystallographic analysis in the same setup. A total of 2520 frames were collected. The integration of the data using a tetragonal unit cell yielded a total of 29151 reflections to a maximum angle of 30.50 (0.70 Å resolution), of which 1099 were independent (average redundancy 26.525, completeness = 99.9%, R int = 4.63%, R sig = 1.33%) and 890 (80.98%) were greater than 2(F 2 ). The final cell constants of a = 7.079(2) Å, b = 7.079(2) Å, c = 25.437(5) Å, = 90 , = 90 , = 90 , volume = 1274.7(8) Å 3 , are based upon the refinement of the XYZ- centroids of 386 reflections above 20(I) with 8.264 < 51 Space Group P 4 2 =mnm Temperature 100K Lattice Constants a(Å) b(Å) c(Å) 7.0792 7.0792 25.4375 Positional X Y Z U Parameters Ba1 0.25376 0.25376 0.5 0.01067 Ba2 0.26050 0.73950 0.18131 0.01025 S1 0.28552 0.71452 0.5 0.01504 S2 0.5 0.5 0.39138 0.01153 S3 0.22228 0.77772 0.30213 0.01433 S4 0.0 0.0 0.41587 0.01043 S5 0.5 0.0 0.40476 0.01242 Zr1 0.25035 0.74965 0.40026 0.00685 Agreement R1 wR2 2 Factors 2.45% 5.15% 1.127 Table 4.2: Structural parameters from X-ray crystallographic analysis for Ba 3 Zr 2 S 7 . 2 < 61.07 . Data were also corrected for absorption effects. The ratio of minimum to maximum apparent transmission was 0.768. The structure was solved and refined usingthespacegroupP 4 2 =mnm, withZ=4fortheformulaunit, Ba 3 Zr 2 S 7 . Thefinal anisotropic full-matrix least-squares refinement on F 2 with 40 variables converged at R1 = 2.45%, for the observed data and wR2 = 5.48% for all data. The goodness-of-fit was 1.127. The largest peak in the final difference electron density synthesis was 1.291 e /Å 3 and the largest hole was -0.748 e /Å 3 with an RMS deviation of 0.172 e /Å 3 . On the basis of the final model, the calculated density was 4.267 g/cm 3 and F(000), 1440 e . We further performed scanning transmission electron microscopy (STEM) studies on the crystals. The STEM image of a BaZrS 3 and a Ba 3 Zr 2 S 7 crystal and corre- sponding selected area electron diffraction (SAED) patterns are shown in Fig.4.5. In BaZrS 3 , we can see the extended 3D corner sharing network. In Ba 3 Zr 2 S 7 , as ex- pected, we observed the double-layer perovskite slabs sandwiching the extra rock-salt atomic layer offset by half a unit cell along the face diagonal of the in-plane square lattice. Much denser diffraction spots along c axis correspond to the much larger lattice constant. The highly symmetric SAED pattern also establishes high quality of the single crystal. For Ba 2 ZrS 4 , we performed synchrotron XRD and Rietveld refinement to extract the structural information,as shown in Fig. 4.7. Both the position and intensity of all the diffraction peaks match very well with the refined pattern, showing very high quality sample which are phase pure within synchrotron X-ray detection limit. The obtainedcrystalstructureisshownintheinset. Ba 2 ZrS 4 adoptsaveryhighsymmetry 52 tetragonal structure with a space group ofI4=mmm. The octahedra tilting present in BaZrS 3 and Ba 3 Zr 2 S 7 are completely suppressed, with all the Zr-S-Zr bonds straight. Ba 2 ZrS 4 Figure 4.7: Rietveld refinement for polycrystalline BaZrS 3 samples. The difference between synchrotron XRD (red) and simulated curve (blue) is shown in the grey curve. 4.4 First principles calculations The results of first principles calculations for BaZrS 3 were talked about in pre- vious chapters. For the RP phases, we also performed density functional calcula- tions using the same LAPW method[62] as implemented in the WIEN2K code.[63] The electronic structure and optical properties were also calculated with the mBJ potential.[64, 65] The contribution of spin-orbit coupling was included to improve the accuracy of the calculations. The calculated band structure, density of states, and absorption coefficients for Ba 3 Zr 2 S 7 in P 4 2 =mnm phase are shown in Fig. 4.8. The calculated electronic structure showed an indirect bandgap of 1.25 eV with a valence band maximum at M point, and a direct gap of 1.35 eV at point. Similar to the other early transition metal perovskite oxides and chalcogenides, the conduction and valence band of Ba 3 Zr 2 S 7 are primarily composed of Zr d orbitals and S p orbitals, respectively. The high density of states is also manifested in the flat valence band maximum and conduction band bottom in the band structure. These lead to a sharp absorption onset and large absorption coefficients greater than 10 4 cm 1 near the band edge. We also performed calculations for the Ba 3 Zr 2 S 7 and Ba 2 ZrS 4 in I4=mmm phase with no octahedral rotation. The corresponding indirect and direct band gap for 53 � � � Γ � � � Γ - � - � � � � � ����� ( ��) Total DOS Zr d Sp - � - � � � � � �� �� �� �� � ����� ( ��) �� � ( �� - � ��� � � � � ) (a) (b) (c) (d) a b c c a/b � � � � � � � �� - � �� � �� � �� � ���� ��� ��� ��� � ����� ( �� ) α ( �� � �� - � ) � �������� � ( �� ) Figure 4.8: The corresponding schematic crystal structure (a), calculated band struc- ture (b), density of states (c), and absorption coefficients along out-of-plane and in-plane directions for Ba 3 Zr 2 S 7 with P 4 2 =mnm structure. I4=mmm Ba 3 Zr 2 S 7 were 0.96 eV and 1.26 eV, respectively. The indirect and direct gap for Ba 2 ZrS 4 were 0.926 eV and 1.050 eV, respectively. 4.5 Optoelectronic properties of Ba 3 Zr 2 S 7 4.5.1 Optical spectroscopy PL spectra in 1.5 - 2.2 eV range were collected with 532 nm excitation laser while the lower energy range (1.2 eV - 1.5 eV) were collected in the same setup with a 785 nm excitation laser. Both measurements were done at room temperature in a back-reflecting geometry and under a microscope with 50 magnification, 0.7 NA objective lens. Raman signal were also collected along with luminescence signal at the same locations. The PL and Raman spectra for the crystals are shown in 54 Figure 4.9: DFT calculations for Ba 2 ZrS 4 and Ba 3 Zr 2 S 7 with I4mmm structure. The corresponding perspective crystal structure, calculated band structure, and absorp- tion coefficients along different directions are shown for Ba 3 Zr 2 S 7 (top) and Ba 2 ZrS 4 (bottom), respectively. Fig.4.10. BaZrS 3 showed a relatively broad peak at around 1.82 eV, similar to the polycrystalline samples. The two RP phases, Ba 3 Zr 2 S 7 and Ba 2 ZrS 4 , showed sharper peaks at 1.28 eV and 1.33 eV, respectively. Band gaps near 1.3 eV, on top of the high absorption coefficients, make the BaZrS 3 RP phases promising candidates for single junction solar cell devices. 4.5.2 External luminescence efficiency In addition to the optimal band gap, it is necessary to evaluate the material’s po- tential for achieving power conversion efficiency close to the Shockley-Queisser (SQ) limit.[67] The external luminescence efficiency ( ext ), defined as the ratio of output to incident photon numbers, is one of relevant material parameters for this evalua- tion. Inefficient external luminescence at open circuit is an indicator of non-radiative recombination and optical losses, and ext is thus a thermodynamic measure of the available open-circuit voltage (V OC ) in photovoltaic devices.[67, 74] A comparison of the radiative emission of Ba 3 Zr 2 S 7 with reference single crystalline InP and GaAs wafers under the same illumination and measurement conditions is shown in Fig.4.11. The reference InP and GaAs wafers used were as-received from University Wafer 55 (a) (d) (b) (e) (c) (f) Figure 4.10: Room temperature PL and Raman spectra for BaZrS 3 (a,b), Ba 3 Zr 2 S 7 (c,d), and Ba 2 ZrS 4 (e,f) crystal pieces. and have a n-type carrier concentration around 510 16 and 10 17 cm 3 , respectively. Despite the non-ideal surface quality of the flux-grown crystals, integrated emission intensity from Ba 3 Zr 2 S 7 is within one order of magnitude compared to the atomically smooth state-of-art III-V wafers. Measurement of Îůext was carried out using a 785 nm laser excitation with known incident photon flux, and the output photon flux was measured in a Renishaw inVia microscope system. This tool was calibrated using a transfer standard InP wafer. This was carried out by first measuring the system instrument response function (IRF) with a broadband calibrated reference light source. Next, a reference InP singlecrystalwaferwithaknownquantumefficiencyfromanindependentlycalibrated photoluminescence measurement system was utilized to calibrate the absolute counts to photons conversion. The calibration was then cross checked across both tools. External luminescence efficiency is defined as: ext = number of photons out number of photons incident (4.1) The VOC is calculated via:[67] V OC =kT ln( abs R 2 0 R =2 0 R 1 1 a(E;)b(E) cos()dEdd' ) +kT ln ext (4.2) abs is the absorbed photon flux calculated as abs =a(E exc ;) exc (4.3) 56 (a) (b) Figure 4.11: (a) PL intensity comparison of a Ba 3 Zr 2 S 7 crystal, a InP wafer and a GaAs wafer under the same measurement conditions. (b) Quantitative emission and corresponding V OC at different incident power density with 785 nm incidence. The error bar is included as multiple sets of data were obtained on several Ba 3 Zr 2 S 7 crystal pieces.The inset is external luminescence efficiency as a function of the incident power density. with E exc being the laser excitation energy, = 0 for normal incidence of the exci- tation. a(E;) is the effective absorbance of the incident flux by the material, given as: a(E;) =A(E)T () (4.4) The energy-dispersive absorptance A(E) is taken as 1 for energies greater than the bandgap, and 0 for the energies less than the band gap. This assumption was made due to the sample thickness being several times more than the absorption length. T () is the transmittance of the semiconductor-air interface calculated from Fresnel equations. b(E) is the blackbody spectrum: b(E) = 2n 2 h 3 c 2 E 2 ( 1 e E=kT 1 ) (4.5) The measured emitted photon flux at different incident photon flux and extracted V OC under different illumination powers are shown in Fig.4.11(b). The external luminescence efficiency remains 0.1% - 0.15% for incident power from around 10 3 W m 2 to 10 6 W m 2 , but quickly drops to below 0.1% as power further increases. This can be understood by considering the relative rates of SRH, radiative, and Auger recombination pathways, which are proportional to the first order, second order and thirdorderofcarrierdensity. Externalluminescenceefficiencywilldecreasewhennon- 57 radiative Auger recombination dominates as large excess carrier density is generated at high incident photon flux. 4.5.3 Carrier recombination lifetime (a) (b) Figure 4.12: (a) Spectral- and time-resolved emission map. The TCSPC intensity is indicated with the color bar. (d) Intensity decay profile of the emission peak as a function of time. Red dots are experimental data and the solid line is the bi- exponential fit convolved with the pump profile. A fast decaying time constant of 4.5 ns and a slow decaying time constant of 65 ns are extracted. We also performed time-resolved photoluminescence (TRPL) measurements at room temperature to study the non-equilibrium carrier dynamics in Ba 3 Zr 2 S 7 . PL was excited with a pulsed diode laser at a wavelength of 405 nm, and the PL transient was measured as a function of time and energy in the range of 900 nm - 1050 nm using spectrally-resolved, time-correlated single photon counting (TCSPC). Micro- scope mounted single crystals were photo-excited using a pulsed diode laser at 405 nm (PicoQuant, 2.5 MHz, 3.73 pJ/pulse, 590 ps FWHM pulse duration) focused onto the sample surface using a 40, 0.6 NA objective lens (Nikon) to a spot size of 2.52 m (FWHM). Emission was collected through the same objective lens and sent to a 0.5 m focal length spectrograph (Princeton Instruments, SP-2500) and spectrally dispersed with a 150 g/mm, 800 nm blaze grating. The spectrally dispersed light was projected through a slit onto an avalanche photodiode (PicoQuant, T-SPAD) so that the PL decay could be measured at each wavelength ( 2 nm). TRPL was recorded from 900 - 1050 nm at 2 nm increments. 58 Fig.4.12(a) shows the spectral- and time-resovled transient PL decay. Fig.4.12(b) shows the data integrated over the range 910 nm - 1010 nm to select band-to-band ra- diativerecombination, inordertoestimatethefreecarrierrecombinationlifetime. Us- ingtheknownpumpparametersandthecalculatedabsorptioncoefficientofBa 3 Zr 2 S 7 , we estimate the peak carrier concentration to be above 10 19 cm 3 within 30 nm of the illuminatedtopsurfaceofthecrystal. Therefore, Auger, surface, andbulkSRHareall likely relevant recombination mechanisms. Many material parameters of Ba 3 Zr 2 S 7 are as-yet unmeasured, including the electron and hole diffusivities, equilibrium carrier concentration and type, and the Auger coefficients. Absent such parameters, it is not possible to determine specific recombination rates from the data in Fig.4.12(b). An empirical, double-exponential fit convolved with the laser pulse width (0.59 FWHM) to the data yields time constants 1 around 4.5 ns and 2 around 65 ns. The longer time constant is an effective parameter that represents both bulk and surface recom- bination, and is typically a significant underestimate of the bulk SRH recombination lifetime.[75] Therefore, the data suggests that the room-temperature SRH lifetime in our Ba 3 Zr 2 S 7 crystals is well over 60 ns. This is a promising result, comparable al- ready to state-of-art CdTe, CIGS, and halide perovskite materials that support solar cells with one-sun power conversion efficiency over 20%.[76] 4.6 Anomalous bandgap evolution in layered Ba-Zr- S Ruddlesden-Popper phases 4.6.1 Bandgap evolutions in Ruddlesden-Popper phases Putting together the materials we have studied with photoluminescence so far, we would have Fig.4.13. This plot demonstrates the tunability in perovskite chalco- genides through the chemical composition control, structure diversity, and dimen- sionality control. If we look at the PL overlay. We find something very interesting. PL are photons emitted due to band-to-band recombinations, so position of emission peaks can serve as an estimate for the band gap, neglecting the exciton binding en- ergy. As we can see from Fig. 4.13. The band gap of of perovskite BaZrS 3 is larger than its layered phases. This is quite unusual. As typically in RP phases, the rock salt compounds with band gaps much larger than the perovskite materials will sever as a barrier and introduce a quantum confinement effect. Essentially these can be viewed as natural multiple quantum well structures. So, intuitively, the band gap of the layered phases should be larger than the bulk perovskite. This is indeed the case in oxides systems such as Sr-Ti-O RP series,[10] and halide perovskite systems such as BA(MA)-Pb-I RP series.[11] The experimentally obtained band gaps of Sr n+1 Ti n O 3n+1 RP series and MA n+1 Pb n I 3n+1 RP series are consistently higher and higher as n decreases, with the 3D perovskite SrTiO 3 and MAPbI 3 possesing the lowest band gap in the series.[10, 11] Both trends fall in line 59 α-SrZrS 3 β-SrZrS 3 BaZrS 3 Ba 3 Zr 2 S 7 Ba 2 ZrS 4 Figure 4.13: Normalized PL spectra from various compounds at room temperature. Spectra fro Ba 3 Zr 2 S 7 is taken with 785 nm excitation laser and the other materials are taken with 532 nm excitation laser. with the quantum confinement effect introduced by SrO and organic group barriers, as shown in Fig.4.14(a). Contrary to oxides and halide perovskites, Ba-Zr-S RP series show a opposite trend that is also manifested in first principles calculations. 4.6.2 The role of octahedra tilting One thing we think that is playing an important role is in the details of the crys- tal structure, specifically, the octahedral tilting. As talked about earlier, BaZrS 3 adopts a distorted orthorhombic phase rather than the ideal cubic phase with. We constructed a hypothetical cubic BaZrS 3 with no tilting, performed first principle calculations to compare the structures with and without octahedra tilting. The band structures were calculated with density functional theory and the screened hybrid functional HSE06[77] as implemented in the VASP code.[78, 79] Structural relaxation was performed under generalized gradient approximation and with PBEsol[80] for ex- change and correlation function. Both calculations were run with consistent settings. Fig.4.15 shows the structural schematic for orthorhombic phase (Pnma) with tilting and a hypothetical cubic phase (Pm3m) with no tilting, and the calculated density of states and band alignment for the two phases. The orthorhombic phase with tilting has a significant larger band gap than the cubic phase, due to a combined effect of higher conduction band minimum and lower valence band maximum. As mentioned earlier, these materials are d 0 systems, with valence band mainly from S p orbitals 60 0.0 0.5 1.0 1.2 1.6 2.0 2.4 3.2 3.6 Sr n+1 Ti n O 3n+1 Ba n+1 Zr n S 3n+1 Calculated Eg (eV) 1/n (BA) 2 (MA) n-1 Pb n I 3n+1 0.0 0.5 1.0 1.2 1.6 2.0 2.4 3.2 3.6 Sr n+1 Ti n O 3n+1 Ba n+1 Zr n S 3n+1 Experimental Eg (eV) 1/n (BA) 2 (MA) n-1 Pb n I 3n+1 (a) (b) Figure 4.14: (a) Measured band gap and (b) calculated band gap as a function of n for the Ba-Zr-S, Sr-Ti-O, and BA(MA)-Pb-I RP systems. The experimental values of Sr-Ti-O, and BA(MA)-Pb-I systems are adopted from reports[10, 11]. The calculated values of BA(MA)-Pb-I system are adopted from report[12]. and conduction band mainly from Zr d orbitals. Conceptually, the tilting will lead to bent Zr-S-Zr bonds and weaker overlap between the bonding orbitals. Such worse overlap would yield smaller hopping integral between neighboring wavefunctions and a smaller bandwidth. If we assume the correction to the Zr and S atomic orbitals do not change significantly, then the smaller bandwidth due to octahedra tilting will open up the band gap more, as shown in the calculated band alignment in Fig.4.15(d). We also performed calculations for the layered phases, Ba 3 Zr 2 S 7 in P 4 2 =mnm phase with tilting along one axis, Ba 2 ZrS 4 in I4=mmm phase with no tilting and the hypothetical Ba 3 Zr 2 S 7 I4=mmm phase with no tilting. If we just compare the Ba-Zr-S RP phases with no tilting, the band gap evolution trend actually follows the expected quantum confinement picture. The band gap increases as we go to smaller n andnarrowerquantumwell, asshowninFig.4.16. Webelievetherearetwocompeting factors in RP compounds that are influencing band gap evolution, octahedra tilting and quantum confinement. The significant suppression of octahedra tilting from 61 perovskite to layered RP phases is quite unique in the chalcogenide Ba-Zr-S series. Sr-Ti-O system and the hybrid halide perovskite system do not share such dramatic change in octahedra tilting. Larger bandgap in BaZrS 3 compared to layered phases can be viewed as the missing quantum confinement is over compensated by the band gap increase due to significant increase in octahedra tilting. 62 1.79 eV 1.17 eV 0.33 eV 0.28 eV -6 -4 -2 0 2 4 6 Zr_d 0 Zr_d 0 S_p Zr_d Density of States (a) (b) 3 3 (c) (d) Figure 4.15: Schematic representations of octahedra network in ABX 3 perovskite for the orthorhombic phase with octahedra tilting (a) and the cubic phase with no octahedra tilting (b). Dark green and orange spheres represent Ba and S atoms respectively. (c) DFT density of states of BaZrS 3 in the cubic phase (top) and the orthorombic phase (bottom). (d) Band alignment of BaZrS 3 for the orthorhombic phase (left) compared to the cubic phase (right). 63 No tilt Tilted 1.79 eV 1.17 eV 0.33 eV 0.28 eV Conduction band valence band -6 -4 -2 0 2 4 6 Zr_d 0 Zr_d 0 S_3p Zr_4d P42/mnm I4/mmm I4/mmm Pnma Pm3m _ Density of States Band gap (eV) 1.8 1.6 1.4 1.2 1.7 1.5 1.3 1.1 0.0 0.5 1.0 1/n Energy (eV) a b c d Figure 4.16: Calculated band gaps for Ba-Zr-S RP compounds with different struc- tural variations. The dash line is the Ba-Zr-S RP phases band gap evolution trend without octahedra tilting. The solid line shows the decreasing band gap trend for the experimentally obtained structures with significant octahedral tilting for n = 2 and n =1. 64 4.7 Summary and outlook In summary, we have explored the synthesis, characterization and optoelectronic properties of the RP phases of perovskite chalcogenides with Quasi-2D octahedra network. These layered compounds possess a natural superlattice-like structure of alternatingn layer perovskite blocks and single-layer rock salt structure. Specifically, we studied the polycrystalline synthesis and single crystal growth of Ba-Zr-S seris, BaZrS 3 (n =1), Ba 3 Zr 2 S 7 (n = 2) and Ba 2 ZrS 4 (n = 1). Ba 3 Zr 2 S 7 and Ba 2 ZrS 4 showedbrightphotoluminescencepeaksataround1.3eV.Specifically, alargeexternal luminescence efficiency of up to 0.15% was extracted via quantitative PL measure- ments. Wealsoperformedtime-resolvedphotoluminescencespectroscopyonBa 3 Zr 2 S 7 and obtained an effective recombination time well over 65 ns. The combination of optimal band gap, short absorption length, high external luminescence efficiency, and long carrier lifetime show that 2D Ruddlesden-Popper phases of perovskite chalco- genides are promising candidates for single-junction solar cell devices. These findings establish the growing potential of transition metal perovskite chalcogenides as semiconductors for broad optoelectronic applications. Urgent ef- forts are required to further study the carrier dynamics of these materials in-depth and determine of various critical carrier transport metrics such as carrier mobility, diffusivity, and specific recombination rates. Further, we envision the application and relevance of these materials to straddle beyond photovoltaics to other optoelectronic applications,particularly light emission and photodetection. Additional characterizations and spectroscopy are required to understandthedefectformationinthisclassofmaterials. Dopingstudyandsynthetic efforts towards film fabrication will also be of vital importance for device implemen- tation. 65 Chapter 5 Building infrared optical anisotropy in quasi-one-dimensional TMPCs 5.1 Introduction The interaction of light with anisotropic media is dependent on the geometric rela- tionship between the field and propagation direction of light and the crystallographic directions in the media. This interaction is widely used in optical components to manipulate the phase, amplitude and polarization of light. The nature of light prop- agation in an anisotropic system can be described by complex refractive indices (n + i) along principal axes of the system. The optical anisotropy in a material can be quantitatively described by the difference between the real parts of the indices as birefringence (n), and the difference between imaginary parts of the indices as dichroism (). When light pass through an optically thick media, different phase retardation and amplitude attenuation for different polarizations can be controlled via birefringence and dichroism, respectively. Birefringence and/or dichroism has been observed in natural crystals[81–87], liq- uid crystals,[88–90] metamaterial architectures such as plasmonic arrays[91] and multi-slotted nanophotonic structures,[92] strained films, two-dimensional materials with low-symmetry layers, and artificially aligned one-dimensional nanostructures. Artificial architectures with carefully patterned nanostructures can achieve optical anisotropy much larger than currently available in anisotropic crystals. However, the demanding size and geometric control severely limits large scale deployment of such technology.[93]. Liquid crystals, which are extensively used in electronic displays, typically have birefringence below 0.4[88], though careful designs such as connecting multiple aromatic rings and attaching terminal cyano- or iso-thiocyanate groups have achieved birefringence of up to 0.73[89]. However, these bulky molecules are difficult to synthesize and use[90]. Currently, inorganic solids widely used for high-performance polarizing optics include quartz (SiO 2 , n max = 0.013)[81], calcite (CaCO 3 , n max = 0.172),[81], rutile (TiO 2 , n max = 0.29)[82], yttrium vanadate 66 dichroismΔ amplitude attenuation birefringenceΔn phase retardation Figure 5.1: Schematics showing the control of phase and amplitude of light through birefringence and dichroism. Image courtesy: Wikipedia. YVO 4 Quartz α-BaB 2 O 4 MgF 2 LiNbO 3 Calcite (a) (b) (c) (d) (e) (f) Figure 5.2: Optical pictures of some common birefringent crystals widely used in polarizing optics. Image courtesy: Union Optic. 67 MWIR LWIR ▲ ▲ ▲ ▲ ▲ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ***************************** ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ● ● ● ● ● ● ▽ ▽ ▽ ▽ ▽ ▽ ■ ■ ■ ■ ■ ■ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ★ ★ ★ ★ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○○○ ○ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ ▼ ▼ ▼ ▼ ▼ ▼▼ ▼▼ ▼ ▼ ▼▼ ▼ ▼ ▼ ▼▼ ▼ ▼▼ ▼ ▼ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ � � � � �� �� �� �� �� ��� �� �� �� �� �� � �� � �� � �� � �� �� �� � �� �� � �������� � (μm) � ���� �������� | Δ �| � ����� (��) YVO 4 CdGeAs 2 AgGaS 2 Rutile Calcite a-BaB 2 O 4 LiNbO 3 AgGaSe 2 CdSe CdS MgF 2 Quartz ZnGeP 2 Figure 5.3: The comparison of birefringence amplitude for common anisotropic crys- tals in their transmission window in the infrared. (YVO 4 , n max = 0.222),[83], lithium niobate (LiNbO 3 , n max = -0.107),[84] and barium borate (BaB 2 O 4 , n max = -0.15)[85], with a maximum birefringence below 0.3. In addition, these materials are widely used in high performance polarizing optics in the visible, short-wave infrared (SWIR: 1.4 m - 3 m), and mid-wave infrared (MWIR: 3 m - 5 m) spectra. However, all these materials possess large losses in the long-wave infrared (LWIR: 8 - 14m) or the thermal imaging region, due to the prevalent vibrational resonances. Some materials such as CdS, CdSe do not possess vibrational resonances and are transparent in LWIR, but they typically possess very small optical anisotropy. Hence, it is of scientific and technological importance to design thermodynamically stable homogeneous materials with large anisotropy for high-performance compact optical components. 68 Optic axis Optic axis in-plane Quasi-1D 2D Figure 5.4: The illustration showing the easily accessible in-plane anisotropy in quasi- one-dimensional structures compared to two-dimensional layered structures. 5.2 Quasi-1D structures It needs to start with an anisotropic structure, obviously. Some of the most anisotropic examples one can think of would probably be two-dimensional layered materials such as graphite and transition metal dichalcogenides, due to differences between the inter-layer (van der Waals) and intra-layer (covalent) bonding.[94, 95] However, the high symmetry axis (optic axis) of these layered materials is typically the c axis. To access the a-c plane with large anisotropy, one would need to cleave large single crystals of these materials along this c axis direction with weak bonding. This leads to severe difficulties for experimental studies, and limits their practical applicationasanisotropicmaterials. Recently,blackphosphoruswithlower-symmetry individual layers and easily accessible in-plane anisotropy has attracted significant attention due to its anisotropy in vibrational, optical, and electrical properties.[96– 99] Nevertheless, the difference between different in-plane directions is much smaller, and as a result, the birefringence and dichroism in the a-b plane of black phosphorus remain modest compared to the other materials mentioned above.[98] Among various crystal systems, much larger and accessible in-plane anisotropy can be achieved in Quasi-1D materials, where atoms are arranged in parallel chain- like structures. These rigid chains running along a high-symmetry principal axis ensure that the optic axis and another principal axis are present in a cleavage plane, which naturally reveals large in-plane anisotropy between the intra-chain and inter- chaindirections. However, achievinglargeopticalanisotropyeveninsuchstructurally anisotropic materials has remained a challenge.[100, 101] 69 5.3 Polarizability engineering in hexagonal per- ovskite chalcogenides Apart from the structural aspect, chemical control can also play an important role in designing optically anisotropic materials. The refractive index of a homogeneous media can be related to the polarizability of constituent atoms, ions or molecules through the Clausius-Mossotti relation:[13] n 2 1 n 2 + 2 = 4 3 N (5.1) wheren is the refractive index, is the mean polarizability of the building block and N is the number density per volume. The polarizability in a solid mainly arises due to three mechanisms such as electronic polarization, ionic polarization, and orientational polarization. For the visible to infrared part of the electromagnetic spectrum, the characteristic frequencies of orientational and ionic polarization are too low to be a dominant contributing factor. Electronic polarization of the bound electrons and the nucleus of the constituent atoms, ions, or molecules is the dominant contribution to the index. Hence, a rational design of a birefringent material in the IR spectrum should consist of an anisotropic lattice with distinct electronic polarizabilities along different crystallographic directions. Although the Clausius-Mossotti relation does not readily apply for anisotropic crystals, it can provide a qualitative, and direct correlation between refractive index and electronic polarizability. Hence, one can achieve large optical anisotropy in Quasi-1D materials by tuning the anisotropy of the polarizability tensor, which requires controlling the nature and distribution of the constituent elements. One of the prominent Quasi-1D materials family is the hexagonal materials, which crystallize in the BaNiO 3 -type structure (shown in Fig.5.6). These materials possess a general chemical formula, ABX 3 , where A is typically an alkaline earth or alkali metal ion and B is a transition-metal ion sur- rounded by six anions (X). BX 6 octahedra share a common face and are connected one-dimensionally to form parallel chains along the c axis, which is the six-fold rota- tional axis of the hexagonal structure and optical axis of the uniaxial material. The tightly bonded BX 6 octahedral chains with bridging X 3 triangles alongc axis are not likely to be easily polarizable, while polarization parallel toa=b axis is large due to the weak inter-chain interaction with much longer spacing and ionic A-X bonds aligned along a=b axis. Therefore, one can expect it to be a negative uniaxial material with a smaller index for light polarized along c axis. On top of the structural anisotropy, optical anisotropy can be further enhanced by carefully tuning the chemistry in this structure. The electronic polarizability of some of the candidate ions for this struc- ture are shown in Fig.5.7. Notably, the polarizability of S 2 (10.2 Å 3 ) is much higher than O 2 (3.88 Å 3 ), and is comparable with Se 2 (10.5 Å 3 ). Ti 4+ with the lowest electronic polarizability among tetravalent transition-metal ions, and Ba 2+ with the highest value among common bivalent metal cations are a good combination to offer 70 Electronic Ionic Orientational Frequency Real part of polarizability Figure 5.5: The contribution to the real part of polarizability as a function of electric field frequency. Figure taken from [13]. large polarizability difference between the c axis and a=b axis. The larger Ba ion and the smaller Ti ion also works as a right combination to stabilize the hexagonal structure phase. We expect marginal improvement in birefringence in BaTiSe 3 due to slightly higher polarizability of Se 2 compared to S 2 , but exact value could be sensitive to the structural parameters. Thus, we chose BaTiS 3 as a model system among this class of Quasi-1D materials to demonstrate large optical anisotropy. 5.4 First principles calculations Hexagonal materials such as BaTiS 3 are uniaxial with a diagonal dielectric tensor, 0 B @ 1 0 0 0 1 0 0 0 2 1 C A We denote the two different components of the tensor with electric field perpendicular ( ? ) and parallel ( k ) to the c axis. To verify the heuristic selection process detailed above, we performed density functional calculations. First-principles calculations were done using the general potential linearized augmented planewave (LAPW) method[53], as implemented in the WIEN2k code[63]. We used the exper- imental lattice parameters. The non-symmetry-constrained atomic coordinates were determined by structure relaxation using the standard PBE GGA. The electronic structures and optical properties were then obtained using the standard PBE GGA, the GGA+U method, where correlation effects beyond GGA are treat at a semiempir- ical level within a rotationally invariant formalism, and the modified Becke-Johnson 71 a b c a b c a b c (a) (b) (c) Figure5.6: TheschematiccrystalstructureofBaTiS 3 . Blue, greenandorangespheres represent Ba, Ti and S atoms,respectively. TiS 6 octahedra are highlighted. ● ● ● ● ● ▲ ▲ ▲ ▲ ▲ ▼ ▼ ▼ ▼ ▼ ◆ ◆ ◆ ◆ ■ ■ ■ ■ ★ ★ ★ ★ ● + ▲ 2+ ▼ 3+ ◆ 4+ ■ - ★ 2- � �� �� �� �� �� �� �� �� �� � � �� � � ���� ������ � � ���� ����� � ������������ � α ( �� -�� �� � ) Ba 2+ S 2- O 2- Ti 4+ Se 2- Figure 5.7: The electronic polarizability of various ions. 72 0 5 10 15 20 25 30 35 40 45 0 0.2 0.4 0.6 0.8 1 1.2 α (10 6 /m) E (eV) xx zz ! " ! ∥ Absorption coefficient a (10 6 m -1 ) 40 30 20 10 0.8 0.4 1.2 0 Energy (eV) 0.2 0.6 1.0 1.0 0.5 0 -0.5 -1.0 E (eV) Γ M K Γ A L H A Energy (eV) 0.0 1.0 -1.0 G M K G A L H A Figure 5.8: Calculated absorption spectra for light polarized parallel and perpendic- ular toc axis. The inset is calculated electronic band structure of BaTiS 3 with Tid z 2 band highlighted. (mBJ) potential[64]. The representative data shown in Fig.5.8, which was obtained with a U value of 8 eV for Ti d orbitals. We used highly converged basis sets deter- mined by the criterion Rminkmax = 9 plus local orbitals and dense grids (up to 24 24 24) to sample the Brillouin zone. Interband optical absorption was calculated using the optical package of WIEN2k. The calculations yielded pronounced anisotropic optical properties and a large, broadband linear dichroism window. Calculated absorption coefficients for light po- larized parallel ( k ) and perpendicular ( ? ) to the c axis are shown in Fig.5.8. k shows a prominent absorption edge, while ? extends to substantially lower energies. Thecalculatedvaluesoftheabsorptionedgesinandaresensitivetotheapproximation used for correlation effect of Ti d orbitals. Irrespective of the parameters used, two distinct absorption edges in k and ? were observed. The origin of such anisotropic absorption edges can be understood by analyzing the band structure with dipole tran- sition selections rules, and has been discussed for similar hexagonal structures[102]. Note that the true fundamental band gap can be lower than the optical absorption edges for different crystallographic directions as seen here based on the nature of dipole selection rules. 73 5.5 Crystal growth and characterizations of BaTiS 3 5.5.1 Crystal growth of BaTiS 3 Large single-crystal platelets of BaTiS 3 with lateral dimensions of several millime- tres were grown by the vapour transport method with iodine as a transport agent. Starting materials, barium sulphide powder (Sigma-Aldrich, 99.9%), titanium powder (Alfa Aesar, 99.9%), sulphur pieces (Alfa Aesar, 99.999%), and iodine pieces (Alfa Aesar 99.99%) were stored and handled in an argon-filled glove box. Stoichiometric quantities of precursor powders with a total weight of 0.5 g were mixed and loaded into a 3/4 inch diameter quartz tube with 1.5 mm thickness along with around 0.5 mg cm 3 iodine inside the glove box. The tube was capped with ultra-torr fittings and a bonnet needle valve to avoid exposure to the air. The tube was then evacuated and sealed using a blowtorch, with oxygen and natural gas as the combustion mix- ture. The sealed tube was about 12 cm in length, and was heated to 1000 C with a 0.3 C/min ramp rate and held at 1000 C for 60 h. The samples were quenched to room temperature after the dwell time using a sliding furnace setup with a cooling rate of100 C/min. We encountered two predominant morphologies: needle-like and platelet-like crystals (Fig.5.9(b)). Scanning electron microscopy (SEM) images of these crystals (Fig.5.9(c,d)) show smooth crystal faces for both the platelets and needles. 5.5.2 Structural characterization We performed powder XRD on ground crystallites, the obtained spectra agree well with previous structural study, and show single phase purity of BaTiS 3 within instrument detection limit. We also performed the out-of-plane XRD scans of the crystal plate in a thin-film diffractometer. Only one set of sharp h00 type peaks is present. The rocking curve of the 200 peak has a FWHM of 0.024 . This proves that the crystal surface is along 100, with the c axis in this plane. We also performed neutron powder diffraction patterns at the Nanoscale Order MaterialsDiffractometer(NOMAD)instrumentattheSpallationNeutronSourceand performedapairdistributionfunction(PDF)analysisonpowdersofBaTiS 3 . Approx- imately 200 mg of powder was loaded in a 3 mm diameter quartz capillary and sealed with a nut and ferrule lid. The capillary was loaded in the sample shifter carousel NOMAD instrument. The NOMAD data reduction programs were used to normalize collected spectra against a vanadium rod, subtract background and container scatter- ing signals, and produce data appropriate for Rietveld and pair distribution function (PDF) analyses. PDF data G(r) were produced by Sine Fourier transformation of the total scattering structure factor S(Q) data with the Q range of 0.2 to 3.1 Å 1 . Fig.5.11 shows a representative fit to the PDF at 300 K using theP 6 3 mc space group from 1.5 to 30 Å 1 . 74 (a) 500 µm (c) (d) 100 µm (b) Figure 5.9: (a) The schematic diagram for vapor transport growth. (b) Optical image of the representative as grown BaTiS 3 crystal ribbon and plate. The crystal ribbon is clamped down on the glass substrate by two silver paint dots. SEM image of a BaTiS 3 plate (c) and a crystal ribbon (d). 75 16.10 16.15 16.20 16.25 Intensity a.u. FWHM 0.024˚ 200 300 400 100 101 110 200 002 201 102 112 121 202 300 220 400 100 (a) (b) Figure 5.10: (a) Powder XRD of ground BaTiS 3 crystallites. (b) Out-of-plane scan of a BaTiS 3 crystal plate. The inset is the rocking curve of the 200 reflection. 76 0.20 0.15 0.10 0.05 0.00 Atomic displ. param. (Å 2 ) 300 250 200 150 100 Temperature (K) Ti U 33 Ti U 22 S U 33 S U 22 Ba U 33 Ba U 22 4 2 0 -2 G(r) (Å -2 ) 30 25 20 15 10 5 r(Å) Measured (300 K) Calculated Difference 6 4 2 0 -2 G(r) (Å -2 ) 10 8 6 4 2 r(Å) Ti-S Ba-S, S-S 100 K 150 K 200 K 300 K Ba-Ti Ba-Ba, S-S Ti-S Ba-S, S-S, Ba-Ba Asymmetric nearest Ti-S a b c b c b a Ti S Ba 100 K 300 K d e Figure 5.11: Measured neutron PDF (blue circles) and best fit (red line) at 300 K. The residual difference between the measurement and fit is also shown (green line). We performed single-crystal X-ray diffraction (XRD) studies on the obtained crys- tals. The single crystal X-ray diffraction data were collected on a Bruker SMART APEX DUO 3-circle platform diffractometer and using Mo K radiation ( = 0.71073 Å) monochromatized by a TRIUMPH curved-crystal monochromator. The diffrac- tometer was equipped with an APEX II CCD detector and an Oxford Cryosystems Cryostream 700 apparatus for low-temperature data collection. A black plate-like specimen of BaTiS 3 , approximate dimensions 0.040 mm 0.100 mm 0.150 mm, was used for the X-ray crystallographic analysis. The crystal was mounted in a Cryo- Loop using Paratone oil. A complete hemisphere of data was scanned on omega (0.5 ) at a detector distance of 50 mm and a resolution of 512 512 pixels. A total of 2520 frames were collected. The frames were integrated using the SAINT algorithm to give the hkl files corrected for Lp/decay. The integration of the data using a hexagonal unit cell yielded a total of 2118 reflections to a maximum angle of 30.16 (0.71 Å resolution), of which 289 were independent (average redundancy 7.329, completeness = 100.0%, R int =2.02%, R sig =1.14%) and 268 (92.73%) were greater than 2(F 2 ). The final cell constants of a = 6.749(2) Å, b = 6.749(2) Å, c = 5.831(2) Å, = 90 , = 90 , = 120 , volume = 230.01(16) Å 3 , are based upon the refinement of the XYZ-centroids of reflections above 20(I). Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum appar- ent transmission was 0.467. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.2800 and 0.6590. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group P 6 3 mc, with Z = 2 for the formula unit, BaTiS 3 . The final anisotropic full-matrix least-squares refinement on F 2 with 20 variables converged at R1 = 1.76% for the observed data and wR2 = 4.51% for all data. The goodness-of-fit was 1.239. The largest peak in the final difference electron density synthesis was 1.214 e /Å 3 and the 77 Space Group P 6 3 mc Temperature 100K Lattice Constants a(Å) b(Å) c(Å) 6.7492 6.7492 5.8312 Positional X Y Z U Parameters Ba1 0.6667 0.3333 0.2982 0.01761 S1 0.8383 0.6765 0.7763 0.02018 Ti1 0.0 0.0 0.5223 0.02741 Ba1’ 0.6667 0.3333 0.3326 0.01761 S1’ 0.8302 0.6603 0.8503 0.02018 Ti1’ 0.0 0.0 0.6402 0.02741 Agreement R1 wR2 2 Factors 1.76% 4.36% 1.239 Table 5.1: Structural parameters from X-ray crystallographic analysis for BaTiS 3 . largest hole was -0.468 e /Å 3 with an RMS deviation of 0.115 e /Å 3 . On the basis of the final model, the calculated density was 4.064 g/cm 3 and F(000), 252 e . We extended the thin-film diffraction method to identify the c axis direction by confirming the six-fold rotational symmetry of the crystal, figure. A XRD map of consecutive 2/ scans as the sample is tilted at fixed angles (shown in Fig.5.12) in a thin film diffractometer clearly identifies the six-fold rotation axis of the crystal. The repetition of 100 type peaks as the crystal is tilted 60 away clearly indicates that the tilting axis of the stage is aligned with the c axis, the six-fold rotation axis of the crystal. Thec axis direction of the samples is readily identified by visible sharp step- like features on the surface as seen in Fig.5.9 and confirmed by these studies. The angle between (110) planes and (100) planes in a hexagonal structure is 30 , which agrees well with the position of 110 type peaks in the map. The absence of other peaks and the narrow dispersion of the peaks indicate the high crystalline quality of the plate. Highangleannulardarkfield(HAADF)scanningtransmissionelectronmicroscopy (STEM) images and selected area electron diffraction (SAED) patterns of the crystals along the a and c axis directions are shown in Fig.5.13. These images clearly reveal the parallel 1D chains along the c axis and the hexagonal arrangement of the chains in the (001) plane. 5.5.3 Chemical characterizations EDS mapping on the crystal pieces showed a uniform composition ratio of Ba:Ti:S around 1.06:1:2.95, with minimal O signal. EDS mapping of Ba, Ti, and S demon- strated uniform distribution of all components, as shown in Fig.5.14. We also used Rutherford Backscattering Spectrometry (RBS) for complementary 78 X-ray 2θ ω ψ c a b (110) (100) (010) ψ X-ray b a c (a) (b) (c) Figure5.12: AschematicshowingtheconfigurationoftherotationalXRDmapviewed along a axis (a) and c axis (b). (c) A XRD map of 2/! scans with varying in a thin film diffractometer. 79 4 nm 2 nm a b c a c b 5 1/nm (a) (b) (c) (d) (e) (f) Figure 5.13: (a) Schematic of crystal structure, (b) HAADF STEM images and (c) SAED pattern of BaTiS 3 viewed alonga axis. (d) Schematic of crystal structure, (e) HAADF STEM images and (f) SAED pattern of BaTiS 3 viewed along c axis. 50 µm Ba Ti S (a) (b) Figure 5.14: (a) EDS spectrum of a BaTiS 3 crystal. (b) EDS mapping of Ba (blue), Ti (red), and S (green) elements on a BaTiS 3 crystal needle. The scale bar is 50 m. 80 100 200 300 400 500 Channel 0 10 20 30 40 Normalized Yield 1.0 1.5 2.0 2.5 Energy (MeV) BTS crystal on C tape on glass Ba Ti S Simulation for BTS Figure 5.15: (a) RBS measurement on a BaTiS 3 platelet. Black line shows the RBS data. Purple, blue and green lines show the fitting of S, Ti and Ba elements, respec- tively. Red line is the simulated curve assuming the ratio of Ba:Ti:S is 1:1:3. The nominal stoichiometry of BaTiS 3 shows a good fit. quantitative determination of the composition of our BaTiS 3 samples. In RBS analy- sis, samples are bombarded with He 2+ ions at an energy in MeV range and the energy distribution and yield of the back-scattered ions at a given angle are detected. The energies of back-scattered ions depend both on the mass of atoms from which they are scattered as well as the the depth in the sample at which the collisions occur. The number of back-scattered ions is directly proportional to the concentration of a given element. For BaTiS 3 , we estimate that the uncertainty of RBS measurement can be as good as 2%. In Fig.5.15, we show the RBS determination of the stoichiometry of BaTiS 3 . We found that the nominal ratio (1:1:3) of BaTiS 3 gives a good fit to our RBS measurements, and thus we determined that the composition of our BaTiS 3 sample is BaTiS 3 to within the uncertainty of the RBS measurement (2-3%). 5.5.4 Electrical transport We measured the temperature dependence of electrical resistivity of a BaTiS 3 crystal. Four contacts were made by directly painting silver epoxy onto the crystal surface, as shown in the inset picture in Fig.5.16. Four-probe resistance was obtained by passing current through two end contacts and measuring voltage across the two in- 81 1mm Figure 5.16: (a) Measured electrical resistivity of a BaTiS 3 crystal as a function of temperature. The inset is an optical image of the sample used in the measurement. side contacts. Linear I-V curves were measured in the temperature range shown. The sample dimensions were measured in a microscope and electrical resistivity was cal- culated. The sample clearly shows semiconducting behavior with increased resistivity when cooled to lower temperatures, as shown in Fig.5.16. 5.6 Optical anisotropy in BaTiS 3 5.6.1 Polarization-resolved transmission and reflection Polarization-resolved infrared spectroscopy was performed with normal incidence on the (100) face of a BaTiS 3 plate mounted over a 200-m-diameter opening. The c axis direction on the surface was determined by XRD as discussed earlier. The transmission and reflection spectra of the incident light polarized parallel and per- pendicular to thec axis are shown in Fig.5.17. When the polarization is perpendicular to the c axis, the absorption edge was observed at 4.5 m (0.27 eV). However, when the polarization is parallel to the c axis, the material remains transparent until the absorption edge at 1.6m (0.76 eV). The thickness of the plate was estimated to be 13 m by fitting to the Fabry-Perot fringes in the spectra. Between 1.6 m and 4.5m, the samples appear transparent for extraordinary ray and remains completely opaque for ordinary ray. The reflection spectra are consistent with the transmission spectra as the fringes vanish at wavelengths corresponding to the two different absorption edges. We observe a large dichroic window at 1.5m - 4.5m for light with different linear polarizations. This experimental observation is in qualitative agreement with the theoretical calculations shown earlier. 82 ⊥ �- ���� ∥ �- ���� �� � � � � � �� �� �� �� �� �� �� �� �� � �� � �� � �� � �� � � �������� � ( μ �) ������ ����(% � ����� ( ��) ⊥ �- ���� ∥ �- ���� �� � � � � � �� � �� �� �� �� �� �� � �� � �� � �� � �� � � �������� � ( μ �) � ������� � ���� (% � ����� ( ��) (a) (b) Figure 5.17: Infrared transmission (a) and reflection (b) spectra for incident light polarized perpendicular (dark green) and parallel (orange) to c axis. 5.6.2 Generalized ellipsometry To fully quantify the degree of optical anisotropy, we performed generalized ellip- sometry measurements on the BaTiS 3 crystal plate for several sample orientations. Generalized ellipsometric data were acquired on a BaTiS 3 crystal plate mounted on glass substrate to determine the optical constants parallel and perpendicular to the c axis. The data were acquired using a focused-beam RC2 ellipsometer with Comple- teEASE software (J. A. Woollam Co.). The instrument spot size was 25m 60m, and the angle of incidence was 64.7 (from surface normal). The data were analyzed using WVASE softwareS2. The material was treated as infinitely thick since it is strongly absorbing in this spectral range. Data was used from three different sample orientations (optical axis parallel, perpendicular, and 14 to the plane of incidence) to back out the optical constants in this spectral range. The spectral range of the ellipsometry measurements was between 210 and 1500 nm, however, by incorporat- ing the polarization-resolved infrared spectroscopy data, the spectral range for fitting the optical parameters was extended up to 16.7 m. Generalized ellipsometric data types include: AnE, the ratio between the p-to-p-polarization reflection coefficient and the s-to-s-polarization reflection coefficient; Asp, the ratio between the s-to- p-polarization reflection coefficient and the s-to-s-polarization reflection coefficient; andAps, the ratio between the p-to-s-polarization reflection coefficient and the p-to- p-polarization reflection coefficient. A generic ratio of Fresnel reflection coefficients can be denoted by . Typically, spectroscopic ellipsometry data is given as and 83 where = tan( )e i (5.2) (a) (b) (c) (d) Figure 5.18: Ellipsometry measurement AnE data and fit for plane of incidence par- allel to (a) and perpendicular to (b) optic axis. Reflection spectra and fitting for normal incidence with polarization parallel to (c) and perpendicular to (d) optic axis. A two-step fitting process was performed. First, the measured generalized ellip- sometric data, corresponding to the ratios of Fresnel reflection coefficients between various polarization states, were fit with a single uniaxial layer model where the ma- terial’s optical constants were specified along axes parallel and perpendicular to the c axis. This allowed for Kramers-Kronig consistent oscillators to be fit to the features both parallel and perpendicular to the c axis. Afterobtainingagreeablefitsfortheopticalpropertieswithintheshortwavelength spectral range, further fitting of oscillator parameters was performed by incorporat- 84 (a) (b) (c) Figure 5.19: Ellipsometry measurement and fitting for plane of incidence 14 to optic axis. AnE data (a), Aps data (b) and Asp (c) and their fitted results are shown. ing the mid-infrared polarization-resolved transmission and reflection measurements in Fig.5.17. We extracted the optical properties of BaTiS 3 over the 210 nm to 16m wavelength range. The real (n) and imaginary () parts of the diagonalized refractive index tensor over the entire measured range are plotted in Fig.5.20. Notably, the peaks above the optical absorption edge in resemble transitions between molec- ular localized orbitals, and are indicative of the molecular nature of isolated TiS 6 chain-like structures. This behavior can be rationalized by analyzing the interband transitions along certain high-symmetry directions in the band structure, where the transitions between relatively flat higher-order bands yield strong absorption peaks. The extracted wavelength-dependent birefringence n, linear dichroism , and nor- malized dichroism ( ? k ? + k ) are shown in Fig.5.20. The normalized dichroism is near unity in the strong dichroic window of 1.5m-4.5m, and significant dichroism per- sists up to the visible range and changes sign at 300 nm. The fully transparent region for BaTiS 3 starts at 8 m and persists to the longest measured wavelength, 16.7 m. 5.6.3 Giant birefringence We emphasize that in this low-loss region, the material displays an unprecedented birefringence magnitude of up to 0.76, which is higher than the current largest bire- fringence in liquid crystals and more than twice as large as 0.29 in rutile (in their transparency regions)[82]. To the best of our knowledge, this is the highest reported birefringence among anisotropic crystals to our knowledge, and is 40 times larger than CdSe, the widely used long-wave infrared (LWIR) birefringent material (Fig.5.21). BaTiS 3 possesses broadband, giant birefringence (higher than 0.7 in magnitude) over the entire infrared spectrum, covering the short-wave infrared (SWIR), mid-wave infrared (MWIR), and LWIR atmospheric transmission windows. 85 Δn Δκ ( κ∥- κ⊥)/( κ∥+ κ⊥) �� � �� � � � � � �� - �� � - �� � �� � �� � � �� � � � �� � �� �� - �� � - �� � � �� � � �������� � ( μ �) � ���� �������� Δ � � ����� ( ��) ��������� Δκ � ⊥ � ∥ � ⊥ � ∥ �� � �� � � � � � �� � � � � � � �� � � � �� � �� �� � �������� � (μ � ) � / � � ����� (��) " " ∥ ∥ (a) (b) Figure 5.20: (a) Real (1) and imaginary (2) parts of the refractive indices for polariza- tion perpendicular and parallel to thec axis. (b) Birefringence (n), linear dichroism (), and normalized dichroism for wavelengths from 210nm to 16m. 86 MWIR LWIR BaTiS 3 YVO 4 CdGeAs 2 AgGaS 2 Rutile Calcite a-BaB 2 O 4 LiNbO 3 AgGaSe 2 CdSe CdS MgF 2 Quartz ZnGeP 2 ▲ ▲ ▲ ▲ ▲ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ♣ ***************************** ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ● ● ● ● ● ● ▽ ▽ ▽ ▽ ▽ ▽ ■ ■ ■ ■ ■ ■ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ★ ★ ★ ★ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○○○ ○ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ ▼ ▼ ▼ ▼ ▼ ▼▼ ▼▼ ▼ ▼ ▼▼ ▼ ▼ ▼ ▼▼ ▼ ▼▼ ▼ ▼ □ □ □ □ □ □ □ □□ □ □ □ □ □ □ □ □ □ □ □ □ □ � � � � �� �� �� �� �� ��� �� �� �� �� �� � �� � �� � �� � �� � �� � �� �� �� � �� �� � �������� � (μm) � ���� �������� | Δ �| � ����� (��) Figure 5.21: A comparison of absolute birefringence value for various current bire- fringent materials and BaTiS 3 in infrared spectrum range. MWIR and LWIR spectra regions are highlighted. 87 5.7 Optical anisotropy in other hexagonal per- ovskites 5.7.1 Introduction MWIR and LWIR spectral ranges cover the second ( 3 - 5m) and the third ( 8 - 14 m) atmospheric transmission windows in the infrared radiation.[103] Hence, MWIR and LWIR imaging or thermography systems have broad scientific, industrial, and military applications such as molecular fingerprint imaging, remote sensing, free space telecommunication, target discrimination, and surveillance.[104–108] Conven- tional bulk semiconductors such as Si and III-V alloys do not interact with the low energy photons effectively as one approaches the MWIR. The widely used materi- als in this regime include HgCdTe cadmium mercury telluride alloys,[109, 110] and quantum-well or quantum-dot nanostructures. [111–114] Several issues such as their environmentally hazardous composition and the need for sophisticated growth and fabrication processes post serious challenges to drive down the cost of the MWIR and LWIR detection devices. Besides the materials for detectors, very few materials pos- sess any appreciable anisotropy and/or non-linearity to act as linear or non-linear op- tical elements in this regime, which is another key to this puzzle. Hence, polarization- sensitive interaction with MWIR and LWIR light heavily relies on narrow bandwidth, extrinsic geometric patterns, which adds another layer of limitations to the perfor- mance of these systems. Thus, developing new broadband, MWIR and LWIR respon- sive materials and identifying intrinsic anisotropic functionalities of IR responsive ma- terials remains an important challenge. Linear dichroism is a property of a material, which represents the difference in the attenuation of light with polarizations parallel or perpendicular to a crystallographic axis. The spectroscopic technique that probes this property goes by the same name.[115] This intrinsic anisotropic response in ma- terials is key to enable novel photonic devices such as polarization rotation, polarizing filters, light modulators, and polarization sensitive photodetectors.[115–118] Previous attempts to demonstrate such an effect in technologically relevant MWIR and LWIR regime have mainly focused on the one-dimensional nanostructures, but the aspect ratios make the scalable, device fabrication and alignment of the nanoscale features challenging. Recent studies have shown linear dichroism in two-dimensional layered materials such as GeSe[115] in SWIR and black phosphorus (b-P)[119] in MWIR, which arises from their in-plane structural anisotropy.[96, 120] Within a few years of development, various optoelectronic devices based on b-P, such as high gain mid- IR photodetectors,[121] polarization-sensitive photodetectors[122] have been demon- strated. Given our successful attempt with BaTiS 3 , it is natural step to explore similar hexagonal perovskite materials for such infrared anisotropy as well. 88 5.7.2 Growth and characterizations of Sr 1+x TiS 3 We grew single crystals of Sr 1+x TiS 3 using chemical vapor transport method simi- lar to that used for BaTiS 3 .Niu:2018do, Niu:2017ce As-grown crystals revealed similar morphologies to BaTiS 3 , wires and platelets with smooth and clean naturally termi- nated facets, presumably corresponding to crystallographic planes. 200 µm (a) (b) (c) �� �� � θ (°) powder crystal plate �� �� �� �� �� �� �� � θ (°) � �� ����� � ( �� � � ) 1100 2200 3300 4400 Figure 5.22: (a) The schematic for crystal structure. (b) optical image of a crystal platelet. (c) Overlay of powder XRD, and the out-of-plane XRD scan of a crystal platelet. The inset shows the zoomed in platelet XRD near 4400 reflection. The structure of Sr 1+x TiS 3 falls in the broad category of BaNiO 3 type,[123] with face-sharingTiS 6 octahedralchainsandSrchainsextendingalongcaxisandalignedin a hexagonal setting of trigonal symmetry, as shown in Fig.5.22(a).[124, 125] However, unlike BaTiS 3 , the structure is distorted to a more complex incommensurate modu- lated structure that cannot be indexed in the three-dimensional formalism.[126, 127] BaNiO 3 structure has matched periodicities of Ba chains and NiO 6 octahedral chains, and thus equivalent number of alkaline metal and transition metal atom. In contrast, the periodicities of the Sr chains and TiS 6 chains in the distorted structure are not matched, resulting in an actual composition of Sr 1+x TiS 3 .[126, 127] It can be viewed as interpenetration of two sub-cells having common lattice constants perpendicular to thec axis and two different ones alongc axis. Previous studies on Sr 1+x TiS 3 resolved its structure through Rietveld refinement with a (3+1)-dimensional super space group formalism (hklm),[128] where the last two indices refer to the different c axis peri- 89 odicities for Sr and TiS 6 chains.[124, 127] We performed powder X-ray diffraction (XRD) studies on the ground crystals and high resolution out-of-plane scan on the crystal platelet. The out-of-plane scan on the platelet showed one set of sharp Bragg reflections with a d-spacing of 5.74, 2.87, 1.91, and 1.43 Å, respectively. Both the powder pattern and the extracted d-spacing agree with an Sr 1+x TiS 3 structure with x=0.145 reported earlier based on polycrystalline samples.[127] The powder pattern and Bragg reflections from the crystal platelet are overlaid in Fig.5.22. The platelet facet was indexed to be along 1100, following the aforementioned (3+1)-dimensional formalism. This again confirms that the naturally terminated surface is not the basal plane, but the prismatic plane instead, which enables the large anisotropy between intra- and inter- chain directions to be easily accessed by probing top surface of the platelet. Chemical composition characterization was carried out with EDS. EDS mea- Sr Ti S 100 µm � � � � � � � � ����� ( ��� ) � �� ����� � ( �� � � ) SrLa S Ka Ti Ka ' ' ' ' ' (a) (b) (c) Figure 5.23: (a) EDS spectrum of the crystal. (b) SEM image and EDS mapping of S, Ti, and Sr elements of a crystal wire and a platelet. The rotational XRD map of a crystal plate. Intensity of the reflections are indicated by the color bar contour. (c) The rotational XRD map of a Sr 1:145 TiS 3 crystal plate rotatied around the c axis surements on the crystal surface showed only expected elements with minimal signal from oxygen. A representative EDS spectrum at 400X magnification is shown in Fig.5.23(a). EDS mapping of Sr, Ti, and S elements on a wire and a platelet was 90 shown in Fig.5.23(b). A consistent ratio of Sr: Ti: S at varied locations and magni- fications was obtained to be around 1.1: 1: 2.9. This agrees well with the expected off-stoichiometry in this material. We used the rotational XRD map to identify the c axis. It is essentially a map of reciprocal space that is perpendicular to the tilting axis. The reciprocal lattice points are extended to long streaks along due to the rel- atively poor coherency of the probe in this direction. All the reflections are indexed with aforementioned four-dimensional formulism. All the reflections are (h k 0 0) type, confirming that the tilting axis is indeed c axis. We can see expected 5200, 4100, 3000, 500, 700, and 200 reflections, which follow the reflection conditions of h +k = 3n, when the crystal is tilted 13.9 , 19.1 , 30 , 40.9 , 46.1 , and 60 away from 1100 facet. This agrees well with the trigonal crystal symmetry and shows high quality of the crystal. ��� ��� ��� ��� ��� ��� � ��������� ( �� - � ) � �� ����� � ( ���� ) 90° 60° 30° 0° (c-axis) (a) (b) Figure 5.24: (a) Normalized Raman spectra with incident laser linearly polarized parallel, 30 , 60 , and 90 to the c axis. (b) A polar plot of the three Raman peak intensities as a function of the angle between incident laser polarization and the c axis. We performed the polarization-resolved Raman spectroscopy to study the vibra- tional response of such an anisotropic structure. Back-scattering geometry with a 532 nm laser excitation was used for the measurements. We recorded several sets of measurements with the polarization of the laser rotated in steps of 10 at room temperature. We have observed two Raman peaks at 204 cm 1 and 368 cm 1 . Peaks at 368 cm 1 show the highest intensity for the incident polarization parallel to c axis and the weakest for the perpendicular polarization, while peak at 204 cm 1 shows the opposite trend. Raman spectra with polarization parallel, 30 , 60 , and 90 to the c axis is overlaid in Fig.5.24(a). Intensity of the peaks in all 18 sets of measurements were plotted in a polar coordinate (Fig.5.24(b)). This large anisotropy in Raman peak intensities can also be used to identify the c axis of the crystal platelet. 91 � � � � �� �� �� � �� � �� � �� � � �������� � ( μ �) � ��������� 0° (c-axis) 80° 90° 60° 30° (a) (b) Figure 5.25: (a) The absorbance for normal incidence with linear polarizations par- allel, 30 , 60 , 80 , and 90 to the c axis in the crystal plate. (b) Transmission and absorbance value plotted in polar coordinate as a function of the incident polarization with respect to c axis. 5.7.3 Strong linear dichroism Polarization-resolved infrared spectroscopy was performed to study the optical response in the infrared region. Normal incidence transmission measurements were carried out for controlled polarization with 10 steps on the prismatic plane of a crystal platelet with a thickness of 30 m. High-resistivity silicon was used as the substrate. The c axis direction on the facet was predetermined with rotational XRD map and Raman measurements discussed earlier. When the incident polarization is parallel to the c axis, we observed a sharp optical absorption edge at2.5 m (0.5 eV). However, when the polarization is perpendicular to the c axis, the transmission shows a different sharp absorption feature at5 m (0.25 eV) instead. The crystal platelet appears highly absorptive for linear perpendicular polarization but remains transparent for the linear parallel polarization in the MWIR region. The strong, broadband dichroic window extends to LWIR region while the dichroic magnitude gradually decreases for longer wavelengths. Compared to BaTiS 3 , the strong and large dichroic window remains despite having a quite disordered incommensurate structure. However, the window is narrower. While the longer wavelength absorption edge for ordinary ray remains nearly unchanged, the shorter wavelength absorption edge, associated with extraordinary ray, moves to slightly longer wavelength than BaTiS 3 . It remains unclear whether such an effect is due to the slightly lighter and less polarizable Sr atom, or the structural disorder. The absorbance value of the transmission measurements with polarization parallel, 30 , 60 , 80 , and 90 to the c axis are plotted in Fig.5.25(a). The transmission and absorbance values at 5 m 92 for all measurements are shown in a polar plot in Fig.5.25(b), with a strong dichroic ratio (the ratio between transmission of parallel and perpendicular polarizations) of over 20. Note that at low wavelengths, the accurate determination of the nearly zero transmission of perpendicular polarization is limited by the instrument, thus leading to an underestimated dichroic ratio. The measured dichroic and absorbance ratios for various polarizations are listed in Table 5.2. wavelength 0 (c axis) 30 60 90 dichroic ratio ( I c I a ) a c 3m 17.8 14.6 6.5 1.8 9.9 2.3 4m 46.4 37.8 14.9 2.1 22.1 5.1 5m 63.4 51.7 20.7 3.1 20.5 7.5 6m 65.1 54.3 25.4 9.0 7.2 5.6 7m 70.2 60.1 32.5 16.8 4.2 5.1 8m 72.5 62.9 37.4 23.2 3.1 4.5 9m 73.2 63.9 39.1 25.3 2.9 4.6 10m 73.8 64.6 40.2 26.5 2.8 4.5 Table 5.2: Transmission of various linear polarizations, dichroic ratios, and ab- sorbance ratios of a Sr 1:145 TiS 3 crystal plate. 5.8 Summary and outlook Mid-waveinfrared(IR)andlong-waveIRspectralrangesareofgrowinginterestfor scientific, industrial, and military applications such as molecular fingerprint imaging, thermography-based remote sensing, free space telecommunication, target discrimi- nation, and surveillance. Developing new broadband, MWIR and LWIR responsive materials and identifying intrinsic anisotropic functionalities of IR responsive materi- alscoulddrivedownthecostofIRopticalsystemsandenablelargerscaledeployment. To this end, we have explored design principles for infrared anisotropic materials and experimentallydemonstratedunprecedenteddegreeofopticalanisotropyinhexagonal perovskite chalcogenides with Quasi-1D structures. We have shown strong, broad- band birefringence and dichroism in two model symtems, BaTiS 3 and Sr 1+x TiS 3 . This anisotropy is achieved in an easily accessible crystal plane, and is enabled by the Quasi-1D hexagonal perovskite structure of BaTiS 3 coupled with a judicious selec- tion of the constituent ions ("chemical polarizability engineering"). We synthesized single crystals of these materials with lateral dimensions up to several millimetres. Two distinct optical edges were observed for light with linear polarization parallel or perpendicular to the principle axis. Sr 1:145 TiS 3 possesses a linear dichroism ratio way 93 over 20, and BaTiS 3 possesses a broadband birefringence of up to 0.76, which more than doubles the value in any other transparent homogeneous solid. Our study introduces two materials with broadband dichroism and birefringence for MWIR/LWIR optics, and paves the way to finding other Quasi-1D materials with stronger anisotropy or broader bandwidth. We also expect further characterization of the non-linear optical properties of these materials will broaden their impact for integrated, polarization-sensitive IR optical systems. We anticipate that BaTiS 3 and other Quasi-1D materials will be broadly useful for next-generation imaging, com- munications, and sensing applications, especially for miniaturized photonic devices. We also expect these materials to possess large anisotropies in electrical, thermal and other physical properties. 94 Chapter 6 Thermal stability of TMPCs 6.1 Introduction In previous chapters, the exciting potential of TMPCs for optoelectronic and pho- tonic application are discussed. TMPCs share a lot of exciting features with the well-studied oxide counterparts, including rich, tunable chemistry, high stability, and environmentally friendly and earth abundant composition. Specifically, similar to the perovskite oxides, the valence band and the conduction band of TMPC are primarily composed of chalcogen p orbitals and transition metal d orbitals, respectively. High density of states is thus expected from the combination of highly symmetric structure and degenerate transition metald orbitals. These features are desirable attributes for high temperature thermoelectric candidates.[129] In fact, perovskite oxides have been extensively studied for thermoelectrics due to this advantage.[130–135] However, the high lattice thermal conductivity and large band gaps limit their thermoelectric per- formance. With the replacement of oxygen with larger, heavier, and less electronega- tive chalcogen elements, TMPCs are expected to possess lower thermal conductivity and lower band gaps spanning IR to visible spectrum, thereby mitigating those issues in the oxide counterparts for both thermoelectric and opto-electronic applications. Thus, it is important to study the high temperature thermal properties of TMPCs to evaluate their potential for thermoelectrics. Further exploration of the chemical and thermal stability for these materials, with respect to heating or exposure to moisture and air, is also one of the key remaining questions to extend their potential for cost- effective, large-scale deployment for those applications.[136] In this chapter, we will study the thermal stability of TMPCs in air, including 3D TMPCs-SrZrS 3 , BaZrS 3 , Quasi-2D TMPCs Ba 2 ZrS 4 and Ba 3 Zr 2 S 7 , and Quasi-1D TMPC-SrZrS 3 , Sr 1:145 TiS 3 and BaTiS 3 . 95 (a) (b) (c) (d) Figure 6.1: Schematics of various ABX 3 crystal structures for (a) distorted perovskite phase, (b) needle-like phase, (c) hexagonal perovskite phase, and (d) Ruddlesden- Popper phase (n=2). The blue and orange spheres represent A site atoms and X site chalcogen atoms, respectively. The BX 6 octahedra are highlighted in green. 6.2 Thermal analysis All the seven materials were synthesized in polycrystalline powder form. Ther- mogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were per- formed on the as synthesized samples to evaluate their thermal stability. DSC and TGAmeasurementswereperformedsimultaneouslyonaNetzschSTA449F3Jupiter. Prior to each sample run, a correction was performed using both the reference and sample crucibles to account for any variations within the crucibles themselves. Sam- ple powders obtained were weighed into alumina crucibles, equilibrated isothermally for 15 min, and heated to 1200 C at a heating and cooling rate of 8 C/min in air. All powders appear dark or dark brown colors before the heat treatment, as shown inFig.6.2(a). Thesepowdersarestableinambientconditions. Thereisnovisiblecolor change or measurable degradation over the course of a year. After the measurement, all samples turned into white powders, as shown in Fig.6.2(b). The weight change and DSC spectra as a function of the temperature are shown in Fig.6.3(a),(b). We can see that a loss of weight in most samples occurs around 96 (a) (b) Figure 6.2: Optical pictures of five samples before (a) and after (b) heat treatment. The materials in (a) from left to right are -SrZrS 3 , -SrZrS 3 , BaZrS 3 , Ba 3 Zr 2 S 7 , Ba 2 ZrS 4 , BaTiS 3 , and Sr 1:145 TiS 3 . The materials in (b) from left to right are - SrZrS 3 , -SrZrS 3 , BaZrS 3 , Ba 2 ZrS 4 , Ba 3 Zr 2 S 7 , BaTiS 3 , and Sr 1:145 TiS 3 respectively. 200 C and there are corresponding small peaks in DSC spectra. We attribute this to the evaporation of iodine, which was used as catalysis in the sample synthesis. This was most obvious in -SrZrS 3 samples, as those samples needed slightly higher iodine concentrations to stabilize the desired needle-like phase. Apart from the loss of iodine in the powder mixture, all samples remain fairly stable in air until being heated well beyond 500 C. The needle-like phase -SrZrS 3 is the first one subject to oxidation at550 C with a sudden weight loss and a subsequent gradual weight gain. The oxidation of the two distorted orthorhombic phases, BaZrS 3 and -SrZrS 3 happens at very close temperatures of just above 650 C. And the two RP phases of BaZrS 3 showed the highest stability with an oxidation onset a little below 800 C. All these oxidation reactions are signified by endothermic peaks in their corresponding DSC spectra. Another interesting feature in the TGA spectra is that -SrZrS 3 and 97 Ba 3 Zr 2 S 7 experienced sharp weight loss followed by weight gain, while other materials experienced no appreciable weight loss and only gradual weight gain. The weight loss can be understood by the replacement of S with lighter O atoms. And the competing weight gain at higher temperatures is attributed to the formation of metal sulfates, as indicated by the XRD measurement of the powders after heating. The TGA and DSC data indicate that the lower symmetry needle-like phase -SrZrS 3 is the most vulnerabletodegradationinelevatedtemperatures, andalltheotherhighersymmetry phases, including distorted perovskite phases (BaZrS 3 and-SrZrS 3 ), and RP phases (Ba 3 Zr 2 S 7 and Ba 2 ZrS 4 ) remain fairly stable in air up to well beyond 600 C. ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ▽ α-SrZrS 3 ○ β-SrZrS 3 △ BaZrS 3 □ Ba 2 ZrS 4 ◇ Ba 3 Zr 2 S 7 ▽ Sr 1+x TiS 3 ○ BaTiS 3 � ��� ��� ��� ��� ���� ���� - �� - �� - � - � - � - � � � ������� ��� (�°) �� � (μ �/ �� ) ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ △ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ◇ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ▽ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ▽ α-SrZrS 3 ○ β-SrZrS 3 △ BaZrS 3 □ Ba 2 ZrS 4 ◇ Ba 3 Zr 2 S 7 ▽ Sr 1+x TiS 3 ○ BaTiS 3 � ��� ��� ��� ��� ���� ���� - �� - �� - �� � �� �� �� � ������� ��� ( �°) ���� ������ (%) (a) (b) Figure 6.3: (a) Thermogravimetric analysis mass change and (b) differential scanning calorimetry of seven samples as a function of the temperature. 98 6.3 Oxidation products Tofurtherevaluatetheeffectofheattreatmentinairandunderstandtheoxidation end products, we performed XRD, Raman spectroscopy and EDS studies on the samples before and after heat treatment. The comparison of the results is shown in Fig.6.4,6.5,6.6. (a) (b) Figure 6.4: Powder XRD patterns overlaid with reference peak positions for Ba-Zr-S RP series before (a) and after (b) heat treatment. As mentioned earlier, in the pre-treatment XRD, one can clearly see distinctive patterns for the SrZrS 3 polymorphs and the BaZrS 3 RP phases. However, the post- treatment XRD of two SrZrS 3 polymorphs appear almost identical. The peaks can be attributed to a mixture of SrZrO 3 , SrSO 4 , and ZrO 2 . After the heat treatment, BaZrS 3 RP phases, with similar chemical compositions, showed very similar XRD patterns. The peaks can be attributed to a mixture of BaSO 4 and ZrO 2 . Such oxidation results for these ternary chalcogenides are surprising and interesting, as it confirms the formation of sulfates for the A site cations and binary oxides for the B site cations, instead of the dominant formation of corresponding ternary oxides. The formation of sulfates also explained the unexpected weight gain at high temperatures. Chemical composition study with EDS is in agreement with the structural study. Pre-treatment EDS spectra for all the materials showed expected composition with minimal O signal. The Ba: Zr: S elemental ratios obtained from EDS studies for BaZrS 3 , Ba 3 Zr 2 S 7 , and Ba 2 ZrS 4 are 1.06: 1: 2.73, 1.61: 1: 3.03, and 2.29: 1: 3.59, respectively. The Sr: Zr: S elemental ratios for -SrZrS 3 , -SrZrS 3 are 1.06: 1: 2.44, and 1.08: 1: 2.57, respectively. Post-treatment EDS spectra showed much 99 ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪▪ ▪ α-SrZrS 3 oxidation β-SrZrS 3 oxidation �� �� �� �� �� �� �� � θ (°) � �� ����� � ( ���� ) (a) (b) ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ● ● ● ● ♦ ♦ ♦ ♦ ♦ ♦ BaZrS 3 oxidation Ba 2 ZrS 4 oxidation Ba 3 Zr 2 S 7 oxidation �� �� �� �� �� �� �� � θ (°) � �� ����� � ( ���� ) ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪▪ ▪ ● ● ● BaSO4 m-ZrO2 t-ZrO2 ● ▪ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ SrSO4 m-ZrO2 t-ZrO2 SrZrO3 ● ▲ ▪ ♦ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ● ● ● ● ♦ ♦ ♦ ♦ ♦ ♦ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ● ● ● ● ● ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪▪ ▪ ▲ ▲ ▲ ▲ ▲ ▲ ● ● ● ● ● (a) (b) || | || | || || || || | || || || || | ||| | | | | || || || || || | | || |||| || ||| | || || || || | || |||| | | || || || ||| | | ||| | || |||| || || ||| | || | | || || || | ||| | || |||| ||| | || | | ||| | | | |||| |||| || || || || || | ||| | | ||| | ||| ||| | | | || || || | | || || | ||| |||| |||| ||| | || || | || || | | | | || || | | | | | | || | | | | | | | | | | | | | | || || | | | | | | | | | | | | || | | | | || | | | | | | | | | | | | | | | | | | | | | | || | | || || | | | | | || | | || | | | | | | | | | | | | | | | || | | | | | | | | | | | | | | | | | | || | | | | | | || | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | || | | | | | | || | | | | | | | | | | | | | ||| | || || | ||| | | ||| | | || || || ||| | | | || || || || | ||| ||| | ||| | || | | | | ||| ||| | | | || || || || || |||| ||| | || ||| | |||| || || |||| || || | | |||| ||| | ||| | || || || || || |||| | ||| ||| | || || | || || | | ||| || | | | | | | | || | | | | | | | | || | | | | | | | | | | | | || | | | | | | | | | | | | || ||| | || || || |||| ||| | | ||| ||| | |||| || |||| || || |||| || || || || || || |||| || || || ||| | | ||| |||| || || || || || ||| | || || || || || | ||| || || || || |||| || || |||| |||| |||| | ||| | ||| |||| | ||| |||| | | || | ||| ||| | || || | | |||| || | | |||| | ||| |||| ||| | | ||| || || || || | ||| ||| | α-SrZrS 3 | ICDD 04-011-9617 β-SrZrS 3 | ICDD 04-011-9618 �� �� �� �� �� �� �� � θ (°) � �� ����� � ( ���� ) Figure 6.5: Powder XRD patterns overlaid with reference peak positions for two SrZrS 3 polymorphs series before (a) and after (b) heat treatment. stronger O signal. Notably, S signal did not completely vanish, just significantly weaker, because of the transformation into sulfates. The Raman spectra for any given material changed dramatically before and after heat treatment. The Raman spectra comparison across the materials revealed a trend consistent with XRD. Pre-treatment samplesshoweddistinctiveRamanspectraduetodifferentstructures. BaZrS 3 showed two signature Ag peaks at130 cm 1 and 210 cm 1 , which agrees well with a previous extensive Raman study,10 while Ba 2 ZrS 4 and Ba 3 Zr 2 S 7 showed signature A1g Raman peak at305 cm 1 and 210 cm 1 , respectively. The post-treatment samples showed very similar Raman spectra, which agree well with the previously reported Raman spectra for the corresponding oxides and sulfates.36-38 The spectra are dominated by peaks from monoclinic and tetragonal phases of ZrO 2 , and the corresponding sulfates. Only two weak peaks at150 cm 1 and 410 cm 1 in the post-treatment SrZrS 3 polymorph Raman spectra can be attributed to the ternary oxide, SrZrO 3 . The Raman spectra again confirmed the dominant oxidation products to be the B site metal binary oxides and A site metal sulfates. 100 (a) (b) BaTiS 3 Sr 1+x TiS 3 �� �� �� �� �� �� �� � θ (°) � �� ����� � ( ���� ) BaTiS 3 oxidation Sr 1+x TiS 3 oxidation �� �� �� �� �� �� �� � θ (°) � �� ����� � ( ���� ) ● SrTiO3 TiO2 ● ♦ ▪ ♦ ♦ ● ● ● ● ● ● ♦ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ♦ BaSO4 TiO2 ♦ ▪ Figure 6.6: Powder XRD patterns for BaTiS 3 and Sr 1:145 TiS 3 before (a) and after (b) heat treatment. 101 6.4 Summary and outlook This chapter evaluated the thermal stability of several perovskite sulfides, includ- ingBaZrS 3 indistortedperovskitephase, twopolymorphsofSrZrS 3 (needle-likephase and distorted perovskite phase), as well as two Ruddlesen-Popper phases, Ba 2 ZrS 4 and Ba 3 Zr 2 S 7 . All samples used in this work were synthesized in high quality poly- crystalline form with solid state reaction in sealed quartz ampoules. Iodine was used to catalyze the reaction and reduce the synthesis time. TGA and DSC measurements were performed while these materials were heated to 1200 C in air. The needle-like phase of SrZrS 3 is the one subject to degradation at the lowest temperature, 550 C. All other phases appear more stable in air with an oxidation onset well beyond 600 C. XRD, EDS and Raman studies confirmed the dominant oxidation products as a mixture of A site metal sulfates (SrSO 4 , BaSO 4 ) and B site metal oxides, ZrO 2 . The desirable high thermal stability of this class of materials opens up possibilities for alternative thermoelectric materials with earth benign and abundant composi- tion. Furthermore, we believe that the rich tunability from structural diversity and vast chemical composition in these transition metal perovskite chalcogenides offers an exciting platform to realize innovative, desired functionalities for energy and op- toelectronic applications in general. 102 Chapter 7 Conclusions and future directions 7.1 Summary Conventional semiconductors dominate the electronic and photonic applications and have been the model systems to develop our understanding of the motion of electrons, phonons and photons in solid state systems. With emerging stringent constraints on the material properties to demonstrate novel devices and interesting phenomena, there is a push to discover new classes of semiconductors. Several energy conversion applications such as solar cells, thermoelectrics, and photo-electrochemical processes will also benefit from the discovery and use of such materials. TMPCs are composed of earth abundant and benign elements which are suitable for sustainable deployment in the proposed applications. Further, they are expected to conform to a new paradigm in semiconductor properties in which one can achieve large density of states semiconductors with high carrier mobility. This will challenge the conventional notionthatlargedensityofstatesandhighcarriermobilityarecontradictory. Thekey to achieving these contradictory properties lies in the understanding of how one can control scattering mechanisms while maintaining large density of states, specifically through the control of the structure and chemistry in these materials. Large orbital degeneracy in d-orbital based conduction bands leads to high effective mass, but other factors such as covalency of bonding play a key role in determining important scattering mechanisms, especially electron-phonon scattering. This work is centered on understanding the structure-chemistry-property correla- tions in TMPCs. A variety of perovskite sulfides are studied in this work. One major branch of the research is centered on materials with transition metal Zr in the B site. These materials were explored for visible to near infrared optoelectronic applications. We explored the impact of chemistry control by comparing BaZrS 3 and SrZrS 3 with very similar structure. We looked into the effect of structure variation by comparing two different phases of SrZrS 3 stabilized at room temperature. We further studied the dimensionality control by comparing 2D layered RP phases with 3D perovskite parent compound in Ba-Zr-S system. Synthetic effort was focused on ceramic synthe- 103 sis with solid state reaction and crystal growth using salt flux methods. High quality samples allowed the extraction of their relevant optoelectronic properties with exten- sive spectroscopic study. Clear and bright photoluminescence at room temperature were reported for the first time across these materials. Band gap tunability across the visible to near IR range was confirmed. We also performed quantitative and time- resolved spectroscopy studies. Large external luminescence efficiency up to 0.2% and long effective minority carrier lifetime well above 65 ns were also reported. β-SrZrS 3 BaZrS 3 3D α-SrZrS 3 Quasi-1D Ba 3 Zr 2 S 7 Ba 2 ZrS 4 Quasi-2D Figure 7.1: PL spectra overlay and schematic crystal structures of various TMPCs illustrating chemistry, structure, and dimensionality control. Another major direction in this work is centered on the perovskite sulfides with Ti as the B site metal. Due to the small size of Ti ion, these materials adopt a hexagonal perovskite structure with a Quasi-1D structural network. These parallel chains of face shared TiS 6 octahedra effectively resemble the behavior of ordered array of aligned, infinitely long nanowires. Such materials possess highly anisotropic light response and retain such exotic performance in the robust and scalable bulk crystals. We successfully grew high quality large single crystals of BaTiS 3 and demonstrated large broadbandlosslessbirefringence(>0.76)overthemid-waveinfrared(MWIR)tolong- wave infrared (LWIR) regime (Fig.5.21). It is worth noting that there are very few materials that are transparent and possess the useful anisotropy for the development of linear optical elements in this range (5 âĂŞ 17 m). This value more than double the highest achieved by Rutile (0.29), which is one of the highest birefringence values for any solid material for any wavelength, and 10 better than most birefringent materials in LWIR regime. 104 5 μm STS (a) (b) (c) Figure 7.2: (a) Birefringence comparison of BaTiS 3 with other anisotropic crystals. (b) Schematic showing the anisotropic light interaction with hexagonal chalcogenides. Picture courtesy: Talia Spencer. (c) Transmission and absorbance value plotted in polar coordinate as a function of the incident polarization with respect to c axis on a Sr 1:145 TiS 3 crystal. 7.2 Future work We are certainly still in the very early stage of the development of perovskite chalcogenides. Urgent efforts are required to study TMPCs in several directions: 1. Advanced Synthesis in either larger single crystals or high quality thin film growth. Although single crystals provide an avenue to study several intrinsic prop- erties of the materials, thin films are ideally better suited for device integration and for to measuremen of absorption coefficient, carrier mobility and density with a rea- sonable degree of quantitative accuracy. Sulfurization of corresponding oxide film has been tried, however, the quality of such film can be sub-optimal and detrimental to its electronic and optical properties. Sputtering could be a very promising technique, where one can deposit A site and B site metals on a substrate and anneal at high temperature in sulfur environment. Molecular beam epitaxy (MBE) can produce the highest quality of thin films. However, due to the very high melting temperature of involved metals, one might need to use electron beam cell rather than conven- tional Knudsen cells. Pulsed laser deposition (PLD) is probably the most promising approach for high quality thin film fabrication of perovskite chalcogenides. PLD en- ables stoichiometric transfer of cation elements from the targets and is commonly used to grow a wide range of oxides and chalcogenides. Main issue would be the control of anionic flux and their impact on the construction of the vacuum chamber. If solution based film growth technique could be developed for this class of materials, that would be big breakthrough and enable easy implementation of various device 105 structures. They main challenge in this direction is to identify non-oxidizing solvent for cationic metals, especially B site metals such as Zr and Ti. 2. Exploration of more materials. As mentioned earlier, there is rich design space in terms of chemical composition and structural diversity.It is a natural step to expand the synthesis efforts to cover more unknown territories. What’s more, various excellent databases and theoretical reports have already performed first principles calculations and laid out a framework for this class of materials. With such guided exploration towards desired properties, we expect more materials within TMPCs, or complex chalcogenides in general, to emerge as candidates for various optoelectronic applications. 3. Probing carrier dynamics with ultrafast spectroscopy. We have provided the first set of experimental data towards understanding of carrier transport in TMPCs using time-resolved PL. However, in general, dynamics of carrier transport at a time scale relevant for the proposed applications, which is crucial in understanding and tuning materials’ property, and ultimately determines the device performance, re- mains underexplored. For example, diffusivity and lifetime are the most important metrics of minority carrier transport that determine photovoltaic performance. Com- peting rates of surface and bulk, radiative and non-radiative recombination pathways are critical for quantum efficiency in photodetection and lighting. Carrier dynamics in the presence of dopants are important for device implementations. Therefore, it is imperative to study the carrier dynamics in TMPCs in depth to fully unveil their potential and provide guidance for device applications such as solar cells, light emit- ting devices, and photodetectors. Among various techniques, ultrafast spectroscopy is probably one of the easiest ways as it requires no electrical contact and minimal samples preparation. Most ultrafast spectroscopy use a pulsed light source to pump the material to an excited state, and then probe the excited state or monitor the relaxation process as a function of time. By varying injection level, excitation wave- length, temperature, or sample configuration, and conducting ancillary measurements in these techniques, ultrafast spectroscopy can be one of the most effective ways to deconvolute and determine the unknown optoelectronic parameters for new materials. 4. Transport measurement. Ultralow thermal conductivity in both single crystal form (0.4 W/mK) and polycrystalline form (0.6 W/mK) have been experimen- tally discovered. Further thermal and electrical transport are needed to explore the TMPC’s potential for thermoelectric applications. Also, low temperature resistiv- ity and hall measurements can provide valuable carrier tranport information about these materials such as carrier mobility, carrier density and carrier type. We have performed preliminary measurement on both polycrystalline pellet and single crystal pieces. The key challenge is to have good electrical contact on these materials. 5. Dopinganddefectstudy. Dopingisacriticalfactortocontrolthecarrierdensity and tune electronic properties of semiconductors. Successful experience in doping corresponding oxides include electron doping both A and B sites with trivalent and pentavalent metals, respectively. It would be great if detailed theoretical calculations 106 could be done to guide the experimental search for good dopants. Otherwise, the ones that work for oxides, such as La, Nb, can be a good starting point. Defect density is another critical factor that will ultimately determine the device performance. 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Abstract (if available)
Abstract
Large-scale deployment of electronic, photonic, and energy technologies rely on continuous discovery and invention of high performance electronic materials with earth abundant compositions. Carrier mobility and density of states are two critical material parameters for electronic materials. These quantities are also crucial to enable efficient, high performance light-matter interaction. While the advantages of high carrier mobility are evident, large density of states can lead to desirable electronic and optical properties such as enhanced light absorption and emission (efficient solar energy conversion and lighting), high carrier density (high current, power density), and large thermopower (thermoelectrics). To this end, transition metal perovskite chalcogenides (TMPCs) have been proposed as a class of semiconductor materials with rich tunability and functionality in the visible to infrared spectrum. Specifically, the coexistence of large density of states and high carrier mobility, along with tunable band gap, good thermal and aqueous stability, and benign composition could create opportunities for a broad range of photonic, optoelectronic, and energy applications, including solar cells, photodetectors, lighting devices, and photoelectrochemical devices. ❧ TMPCs have a general chemical formula of ABX₃ similar to the perovskite oxides and halides, where A is a metal such as Ba, Sr, B is a transition metal such as Ti, Zr, and X is S or Se. When the B-site is occupied by early transition metals, the valence bands and the conduction bands of TMPCs are primarily composed of chalcogen p orbitals and transition metal orbitals, respectively. High density of states is expected from the combination of highly symmetric structure and degenerate transition metal d orbitals. TMPCs can be viewed as the inorganic alternatives to hybrid halide perovskites, with stable, benign, abundant composition, and high absorption coefficients. TMPCs can also be viewed as the chalcogenide counterparts of perovskite oxides, with much lower bandgap and improved responsivity to visible and infrared light. However, the physical properties of this class of materials remain underexplored despite being structurally known for decades and the promise as predicted by theoretical studies. ❧ This dissertation focuses on the design, synthesis, and physical properties of TMPCs. High quality synthesis of polycrystalline TMPCs were achieved with catalyzed solid state reactions in sealed ampoules. Single crystals up to several millimeters in size were obtained using vapor transport for hexagonal perovskite chalcogenide with quasi-one-dimensional network. Single crystals with lateral dimensions of several hundred microns were grown using salt flux method for perovskite chalcogenides with three-dimensional network and layered Ruddlesden-Popper chalcogenide crystals. Extensive structural and chemical characterizations for bulk, surface, or microstructural studies were performed to test the quality of grown samples. In depth optical spectroscopy and transport studies were performed to extract the relevant optoelectronic properties of TMPCs. We employed static, quantitative, and transient photoluminescence spectroscopy to probe the electronic structure and carrier dynamics in TMPCs with three-dimensional and quasi-two-dimensional structural networks. Desirable features including band gap tunability, strong luminescence, and long carrier lifetime were demonstrated. We also studied the anisotropic infrared optical properties as well as electrical and thermal transport properties in TMPCs with quasi-one-dimensional structures. Record high, broadband birefringence and linear dichroism were discovered. Thermal stability tests on TMPCs will also be briefly discussed to evaluate their potential for large scale deployment.
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University of Southern California Dissertations and Theses
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Creator
Niu, Shanyuan
(author)
Core Title
Perovskite chalcogenides: emerging semiconductors for visible to infrared optoelectronics
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Materials Science
Publication Date
04/26/2019
Defense Date
02/01/2019
Publisher
University of Southern California
(original),
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Tag
infrared photonics,materials synthesis and characterization,novel semiconductors,OAI-PMH Harvest,perovskite chalcogenides,photovoltaics
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English
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Ravichandran, Jayakanth (
committee chair
), Armani, Andrea (
committee member
), Kapadia, Rehan (
committee member
), Wang, Han (
committee member
)
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nius@usc.edu,wzmjnwzmjn@gmail.com
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The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
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Tags
infrared photonics
materials synthesis and characterization
novel semiconductors
perovskite chalcogenides
photovoltaics