Thz spectroscopy study of metal insulator phase transition on vanadium dioxide

THZ SPECTROSCOPY STUDY OF METAL-INSULATOR PHASE TRANSITION ON VANADIUM DIOXIDE LIU HONGWEI (B. Sc, SHANDONG UNIVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2013 i ii ACKNOWLEDGEMENT In past four years, many people helped me a lot. It would never have been possible for me to write this thesis without their generous support. It is a great pleasure for me to acknowledge all the people who have helped and encouraged me during my PhD project. First and foremost I owe sincere and earnest thankfulness to my supervisor, Prof. Tang Sing Hai, for his guidance, patience, support and encouragement. For me, he is not only a wonderful supervisor, but also an erudite scholar, a wise elder and an energetic artist. From him, I learn not only physics, but also life, choice, duty, attitude and so on. I would never be able to have such an enjoyable PhD experience without him. I also would like to express my sincere gratitude to my supervisor, Prof. Sow Chorng Haur. He is a passionate scientist and great leader. Thanks for the friendly and joyful lab environment he has created for us, which gave me many unforgettable memories. Besides, I am grateful for the high standards he sets for the research group, and for the support I have had in exploring different research directions. A great thank to my co-supervisor Dr. Zhang Xinhai. Dr. Zhang always gives me valuable and in-depth suggestions on experiments and data analysis. He led me into the fantastic world of THz ultrafast spectroscopy with his expertise, enthusiasm and great patience, and always offered me his advice and iii encouragement when I had difficulties in my research. I am also grateful to Dr. Guo Hongchen. He taught me quite a lot, such as the basic operation of laser system, the experience on ultrafast measurement. He enjoyed to share his life experience and gave valuable advice to me. He is so nice both as a good friend and as a senior colleague. I also want to express my thanks to Mr. Wu Tong Meng and Ms. Yong Anna Marie. As the lab officers for nanophotonics lab and THz lab, they have managed and organized the labs so well that we can concentrate completely on research. I am also grateful Dr. Wang Shijie and Ms. Wong Lai Mun. As experts on pulsed laser deposition, they gave me support in sample preparation. I also would like to thank Mr. Wang Yinghui. He is an expert on lithography and etching, who helped me a lot in etching VO2 thin films. Life out of lab was also delightful, especially as a member of Shandong University alumni in department of physics. I want to thank Dr. Chu Xinjun, Dr. Wang Yuzhan, Dr. Diao Yingying, Dr. Ma Fusheng, Dr. Xie Lanfei, Dr. Yao Guanggeng, Ms. Wang Qian, Dr. Chen Xiao, Dr. Wang Xiao, Ms. Li Yanan, Ms. Pan Huihui, Mr. Xu Wentao, Mr. Wang Yinghui and so many other friends. I have spent many wonderful weekends with you when we had big meals and played card games. Personally, I would like to thank my family. I am grateful to my parents for raising me up and for the continuous support, encouragement and love. Actually, any word cannot express my deepest love to them. The love always iv gives me power and pushes me to work harder when I was depressed in difficulties. Finally, I would like to thank my husband, Junpeng, who has been with me over the last seven years. As a husband, he gives me consistent understanding, care, support and love. As a research partner, he provides so many different samples for me. Without his help, I could not study ultrafast spectroscopies on II-VI 1D materials and topological insulator nanowires and could not have so many published papers and ongoing manuscripts. Thanks for pointing out all my shortcomings in both research and life. I wish all our dreams come true. v TABLE OF CONTENTS ACKNOWLEDGEMENT . iii TABLE OF CONTENTS vi SUMMARY ix LIST OF TABLES xi LIST OF FIGURES .xii Chapter Introduction . 1.1 Basic mechanisms of metal-insulator transitions . 1.2 Metal-Insulator transition and vanadium dioxide 1.2.1 background 1.2.2 Principle mechanism of the metal-insulator transition of VO2 1.2.3 Solid-state-device concepts . 12 1.3 Outline of the thesis 15 Chapter Experimental Techniques and Data Analysis 17 2.1 Why THz? 17 2.2 Scientific applications of THz spectroscopy 19 2.2.1 Static THz-TDS of solid-state materials . 19 2.2.2 THz-TDS of water and aqueous solutions 20 2.2.3 OPTP of semiconductor nanostructures . 21 2.2.4 OPTP of strongly correlated systems . 23 2.3 THz generation and detection 23 2.3.1 Photoconductive Antennas 24 2.3.2 nonlinear-optical crystal 27 2.3.3 Air-plasma THz generation and detection . 31 2.4 THz time-domain spectroscopy technique . 34 2.4.1 THz-TDS methodology . 36 2.4.2 TDS analysis 38 2.5 Optical-Pump THz-Probe (OPTP) Spectroscopy . 41 vi 2.5.1 OPTP methodology . 42 2.5.2 OPTP analysis 45 2.5.3 Conductivity model . 46 Chapter Ultrafast photoinduced MIT in VO2 thin films . 53 3.1 Introduction 53 3.2 Experiments 55 3.3 Results 57 3.3.1 Raman and X-ray diffraction characterization 57 3.3.2 OPTP measurements at room temperature 59 3.3.3 OPTP measurements at low temperatures . 61 3.3.4 Transient complex conductivity spectra 63 3.4 Discussions . 65 3.5 Conclusions 68 Chapter Effect of oxygen stoichiometry on MIT in VO2 thin films . 69 4.1 Introduction 69 4.2 Methodology 70 4.2.1 Sample preparation 70 4.2.2 Characterization technique 71 4.3 Results and discussions 72 4.3.1 Characterization of vanadium oxides thin films 72 4.3.2 Fluence-dependent OPTP spectroscopy 73 4.3.3 Transient photoconductivity of different vanadium oxides thin films 79 4.4 Conclusions 84 Chapter Size effects on MIT in individual VO2 nanobelts . 86 5.1 Introduction 86 5.2 Methods and experiments . 88 5.3 Results and discussions 90 5.4 Conclusion 101 Chapter Fabrication and Characterization of VO2 Metamaterials 103 vii 6.1 Introduction 103 6.2 Experimental section 108 6.2.1 Process flow . 108 6.2.2 Design of split-ring resonator 109 6.2.3 Fabrication . 110 6.2.4 Experimental characterization . 112 6.2.5 Simulation software: CST Microwave Studio . 113 6.3 Characterization of the VO2 metamaterials 115 6.4 Conclusions 119 Chapter Conclusions . 120 7.1 Summary 120 7.1.1 Ultrafast photoinduced phase transition behaviors in VO2 . 120 7.1.2 Effect of oxygen stoichiometry on phase transition in VO2 121 7.1.3 Size effects on phase transition behaviors in single VO2 nanobelts 123 7.1.4 Fabrication and characterization of VO2 split-ring resonators 124 7.2 Limitations and future works . 124 BIBLIOGRAPHY 126 APPENDIX 138 viii Chapter Conclusions 7.1.4 Fabrication and characterization of VO2 split-ring resonators We fabricated structured VO2 split ring resonators, which have overall structure size 40 m and structural details down to m. Experimental characterization of the sample have produced spectral responses in the range 0.4-3.5 THz. Numerical predictions using the commercial software CST Microwave Studio were found to be in agreement with experimental values. We have demonstrated a THz modulator based on VO2 metamaterials by tuning the angle between electric field and SRR gap. 7.2 Limitations and future works Being a preliminary investigation, this thesis still has some limitations and the following aspects are recommended for future research.  Besides the oxygen stoichiometry and dimensionality, external strain may affect MIT properties by deposition vanadium dioxide thin films on different substrates. As VO2 thin films employed in this work grow on the same substrate (c-sapphire), effects of external strain could not be investigated. VO2 thin films grown on different substrates having significantly different external strain will be an appealing direction for future work. 124 Chapter Conclusions  Furthermore, doping with other transition metal, e.g. Cr, W and Mo, can also affect the MIT process and/or phase transition temperature. Therefore, another possible area of future work is to study the doping effects in VO2 by utilizing OPTP spectroscopy.  It should be noted that VO2 with different morphologies show great potential to fabricate devices utilizing its properties related to phase transition, such as thermochromic devices, thermoelectrical devices, polarizers and metamaterials. Thus, a possible avenue of future research is to further develop the VO2-based devices. 125 Bibliography BIBLIOGRAPHY (1) Sommerfeld, A. Z. Physik 1928, 47, 1. (2) Bloch, F. Z. Physik 1929, 57, 545. (3) Wilson, A. H. Proc. R. Soc. Lond. A 1931, 133, 458. (4) Wilson, A. H. Proc. R. Soc. Lond. A 1931, 134, 277. (5) Fowler, R. H. Proc. R. 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(2) Liu Hongwei, Wong Lai Mun, Wang Shijie, Tang Sing Hai, Zhang Xinhai, Effect of oxygen stoichiometry on the insulator-metal phase transition in vanadium oxide thin films studied using optical pump-terahertz probe spectroscopy. Appl. Phys. Lett. 2013, 103, 151908. (3) Lu Junpeng, Liu Hongwei, Deng Suzi, Zheng Minrui, Wang Yinghui, van Kan, Jeroen, Tang Sing Hai, Zhang Xinhai, and Sow Chorng Haur, High-sensitive and multispectral responsive phototransistor using tungsten-doped VO2 nanowires. Prepared. (Co-first author) (4) Liu Hongwei, Lu Junpeng, Fan Haiming, Sow Chorng Haur, Tang Sing Hai, Zhang Xinhai, Temperature and composition dependence of photoluminescence dynamics in CdSxSe1-x (0 ≤ x ≤ 1) nanobelts. J. Appl. Phys. 2012, 111, 073112. (5) Liu Hongwei, Lu Junpeng, Teoh Hao Fatt, Li Dechun, Feng Yuan Ping, Tang Sing Hai, Sow Chorng Haur, Zhang Xinhai, Defect Engineering in CdSxSe1–x Nanobelts: An Insight into Carrier Relaxation Dynamics via Optical Pump–Terahertz Probe Spectroscopy. J. Phys. Chem. C 2012, 116, 26036-26042. (6) Liu Hongwei, Sun Cheng, Lu Junpeng, Zheng Minrui, Lim Kim Yong, Mathews Nripan, Mhaisalkar G. Subodh, Tang Sing Hai, Zhang Xinhai, Sow Chorng Haur, Improved electrical property of Sb-doped SnO2 nanonets as measured by contact and non-contact approaches. RSC Advances 2012, 2, 9590-9595. (7) Lu Junpeng, Liu Hongwei, Sun Cheng, Zheng Minrui, Nripan Mathews, Chen Gin Seng, Subodh G. Mhaisalkar, Zhang Xinhai, Sow Chorng Haur, Optical and electrical applications of ZnSxSe1-x nanowires-network with uniform and controllable stoichiometry. Nanoscale 2012, 4, 976-981. (8) Liu Hongwei, Lu Junpeng, Zheng Minrui, Tang Sing Hai, Zhang Xinhai, Sow, Chorng Haur, Composition-Dependent Ultra-high Photoconductivity in Ternary CdSxSe1-x Nanobelts as Measured by Optical Pump-Terahertz Probe Spectroscopy. 138 Appendix Nano Res. 2013, 6, 808-821. (9) Lu Junpeng, Sun Cheng , Zheng Minrui , Mathews Nripan, Liu Hongwei, Chen Gin Seng, Zhang Xinhai, Subodh G. Mhaisalkar, Sow Chorng Haur, Facile One-Step Synthesis of CdSxSe1–x Nanobelts with Uniform and Controllable Stoichiometry. J. Phys. Chem. C 2011, 115, 19538-19545. (10) Li Kai, Qin Wei, Ding Dan, Tomczak Nikodem, Geng Junlong, Liu Rongrong, Liu Jianzhao, Zhang Xinhai, Liu Hongwei, Liu Bin, Tang Ben Zhong, Photostable fluorescent organic dots with aggregation-induced emission (AIE dots) for noninvasive long-term cell tracing. Sci. Rep. 2013, 3, 1150. (11) Deng Liyuan, Teng Jinghua, Liu Hongwei, Wu Qing Yang, Tang Jie, Zhang Xinhai, Maier A. Stefan, Lim Kim Peng, Ngo Chun Yong, Yoon Soon Fatt, Chua Soo Jin, Direct Optical Tuning of the Terahertz Plasmonic Response of InSb Subwavelength Gratings. Adv. Optical Mater. 2013, 1, 128-132. (12) Liu Zhen, Huang Chia-Yi, Liu Hongwei, Zhang Xinhai, Lee Chengkuo, Resonance enhancement of terahertz metamaterials by liquid crystals/indium tin oxide interfaces. Opt. Express 2013, 21, 6519-6525. (13) Ma Fusheng, Qian You, Lin Yu-Sheng, Liu Hongwei, Zhang Xinhai, Liu Zhen, Tsai Ming-Lin Julius, Lee Chengkuo, Polarization-sensitive microelectromechanical systems based tunable terahertz metamaterials using three dimensional electric split-ring resonator arrays. Appl. Phys. Lett. 2013, 102, 161912-5. (14) Lu Junpeng, Liu Hongwei, Lim Sharon Xiaodai, Tang Sing Hai, Sow Chorng Haur, Zhang Xinhai, Transient Photoconductivity of Ternary CdSSe Nanobelts As Measured by Time-Resolved Terahertz Spectroscopy. J. Phys. Chem. C 2013, 117, 12379-12384. (Co-fisrt author) (15) Liu Hongwei, Lu Junpeng, Tang Sing Hai, Sow Chorng Haur, Zhang Xinhai, Composition dependence electron transport in CdSxSe1-x nanobelts: A study using THz time-domain spectroscopy. Submitted to Optics Letters. (16) Liu Hongwei, Lu Junpeng, Tang Sing Hai, Sow Chorng Haur, Zhang Xinhai, Terahertz Spectroscopic Study of Topological Insulator Bi2Se3 Microcrystals and Nanowires. Prepared. (17) Liu Hongwei, Lu Junpeng, Tang Sing Hai, Sow Chorng Haur, Zhang Xinhai, Unambiguous identification of recombination lines in ZnSSe nanowires. Prepared. 139 [...]... the framework of Common undoped as Bloch-Wilson insulator) conventional band theory semiconductors Electron-electron Mott-Hubbard insulator V2O3 interactions (correlations) Electron-phonon (lattice) Peierls insulator K0.3MoO3 interactions Disorder-induced Anderson insulator Si:P localization Control mechanism of MIT Control Type Representative approach Example materials Temperature control Heating/cooling... is another kind of insulator, namely band insulator or Bloch-Wilson insulator, which is under the frame of conventional band theory, i.e., without considering the electron-electron interactions Table 1.1 summarizes the four types of insulators briefly Table 1.1 Classification of insulators and control mechanisms of MIT Classifications of insulators Type Origin Example materials Band insulator (also... subject.11,12,23-28 Great interest in MIT in transition metal oxides started from Morin‟s29 paper on phase transition behaviors in binary transition metal oxides in 1959 In this study, the conductivities of some transition metal oxides, such as titanium sesquioxide (Ti2O3) and vanadium oxides (monoxide (VO), dioxide (VO2), and sesquioxide (V2O3)), increased by several orders of magnitude when the temperature... characterization of the thermal-emission lines of simple molecules and the vibrational and rotational resonances With photon energy in the meV range, the THz radiation strongly interacts with systems that have picosecond-range characteristic lifetime and energetic transitions in meV range Examples of such systems comprise excitons,94-97 bound electrical charges,98 phonons in crystalline solids,99 strongly confined... deeper insight on understanding the mechanism of MIT The results about depressed phase transition temperature show an intriguing possibility of designing different room-temperature phase transition device x LIST OF TABLES Table 1.1 Classification of insulators and control mechanisms of MIT 5 Table 2.1 Advantages and disadvantages of employing air plasma for THz generation and detection 34... bandwidth 1.2 Metal- Insulator transition and vanadium dioxide 1.2.1 background Metal- insulator transition (MIT) in transition oxides has attracted long-standing interests in condensed matter sciences Theoretical and experimental studies to find out more about the mechanism of MIT have been ongoing for almost half a century A great number of reviews on MIT materials and mechanisms indicate the consistent... close to that of the partially 3d band near Fermi level for some heavier transition -metal elements such as copper and nickel Thus the charge gap of the Mott insulator cannot be only accounted with d electrons, but p electrons have also be considered The MIT can also occur because of reasons other than electron correlation effect For instance, MIT can be induced by electron-phonon interaction, which is... external perturbation, e.g thermal, optical, electrical or magnetic In some cases, combination of different excitations can also efficiently result in the phase transition Depending on the employed external perturbation, the time constant is different between the OFF and ON transitions As we known, the time constants will determine the speed of the switch The energy needed for MIT onset corresponds to the... condensed matter physics Theoretical description of metals, insulators and transitions between them is related to noninteracting or weakly interacting electron systems The theory distinguish metals and insulators based on the filling of the electronic bands at zero temperature: For metals, the highest filled band is partially filled; for insulators, it is completely filled In noninteracting electron... the metal- insulator transition 1.1 Basic mechanisms of metal- insulator transitions In the past century, much progress has been made from both experimental and theoretical aspects in understanding strongly correlated electrons and MITs In theoretical sides, Mott made a significant contribution to understand how electron-electron correlations could explain the insulating state, which is called Mott insulator. 8-11 . THZ SPECTROSCOPY STUDY OF METAL- INSULATOR PHASE TRANSITION ON VANADIUM DIOXIDE LIU HONGWEI (B. Sc, SHANDONG UNIVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF. transitions 2 1.2 Metal- Insulator transition and vanadium dioxide 6 1.2.1 background 6 1.2.2 Principle mechanism of the metal- insulator transition of VO 2 9 1.2.3 Solid-state-device concepts 12. TABLE OF CONTENTS ACKNOWLEDGEMENT iii TABLE OF CONTENTS vi SUMMARY ix LIST OF TABLES xi LIST OF FIGURES xii Chapter 1 Introduction 1 1.1 Basic mechanisms of metal- insulator transitions