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研究生： 黃松勳
Huang, Song-Hsun
論文名稱： 陰離子空缺對於層狀多晶與單晶1T-TiSe2-d的能帶與侷域結構之影響
The impact of anion vacancy defects on band picture and local structure of layered polycrystalline and single crystal 1T-TiSe2-d
指導教授： 劉祥麟
Liu, Hsiang-Lin
周方正
Chou, Fang-Cheng
學位類別： 博士
Doctor
系所名稱： 物理學系
Department of Physics
論文出版年： 2017
畢業學年度： 105
語文別： 英文
論文頁數： 89
中文關鍵詞： Transition metal dichalcogenides (TMDCs) 、Charge density waves (CDW) 、Excitonic insulator 、Semiconductor
英文關鍵詞： Transition metal dichalcogenides (TMDCs), Charge density waves (CDW), Excitonic insulator, Semiconductor
DOI URL： https://doi.org/10.6345/NTNU202201994
論文種類： 學術論文
相關次數： 點閱：136  下載：4 
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    A systematic study of 1T-TiSe2 polycrystalline and single crystalline with controlled Se deficiency level indicates that a significant Se loss could be responsible for the controversial charge density wave (CDW) phase and on whether the nominal 1T-TiSe2 should be categorized as a semiconductor or a semimetal at room temperature. In the polycrystalline form, the second order CDW phase transition near ~200 K is found to be most pronounced in samples with δ ~0.12, corresponding to about one Se atom missing per eight formula units in average, which is incommensurate to the hexagonal symmetry and naturally leads to the charge ordering of 2a × 2a × 2c superlattice via exciton-phonon coupling. The anomalous resistivity ρ(T) peak between 100 and 200 K indicates not only resistivity increase due to charge ordering, but also a concomitant p- to n- carrier type change. An interpretation using band model for an extrinsic p-type semiconductor with an impurity band (IB) in proximity to the valence band (VB) is proposed to explain the evolution of Se vacancy level and electronic structure change for 1T-TiSe2-δ, from the low doping bound (δ ~0.08) of semiconducting behavior to the heavily doped (δ ~0.17) dirty semiconductor showing metallic-like n-type conduction. Supporting experimental evidences for the Se vacancy existence are provided by the integrated chemical and physical property analyses, including electron probe microanalysis (EPMA), Hall coefficient, and magnetic susceptibility. In single crystal form, the Se vacancy and Ti-intercalation are dominant near the crystal surface as explored by the scanning tunneling microscopy (STM). The Se vacancy level is found reduced on the crystal surface after prolonged annealing at high temperature, but the intercalated Ti level grows, which implies the occurrence of local re-structuring near the Se vacancy sites. Room temperature Raman scattering spectrum shows a red shift of A1g phonon mode and a blue shift of Eg phonon mode after the long time high temperature post-annealing. The high temperature post-annealing procedure has different impact on polycrystalline and single crystal samples, while samples of small grain size (~10-30 μm) have dominant Se deficiency in equilibrium, samples of large grain size (≳ 1 mm) shows significant amount of Ti-intercalation. This is most likely due to the different level of local re-structuring near the Se vacancy sites.

    Keywords: Transition metal dichalcogenides (TMDCs), Charge density wave (CDW), Excitonic insulator, Semiconductor.

    Contents Acknowledgements i Abstract ii Contents iv List of Figures vi List of Tables xi Chapter 1 Introduction 1 Chapter 2 Brief review of transition metal dichalcogenides 6 2.1 Transition metal dichalcogenides (TMDCs) 6 2.2 Principles of the CDW: the Peierls transition 9 2.3 CDW and other collective phenomena in the TMDC family 11 2.4 CDW state in 1T-TiSe2 16 2.4.1 Crystal structure 16 2.4.2 Transport properties 20 2.4.3 Excitonic condensate and Jahn-Teller effect 21 2.4.4 The intrinsic behavior of 1T-TiSe2 without Se vacancy defects 22 Chapter 3 Sample preparation and experimental methods 24 3.1 Polycrystalline sample synthesis and single crystal growth 24 3.2 Powder X-ray diffraction 26 3.3 Rietveld refinement method 30 3.4 Electron probe micro-analyzer (EPMA) 32 3.5 Magnetic property measurement system (MPMS) 34 3.6 Physical Property Measurement system (PPMS) 35 3.7 Scanning Tunneling Microscopy (STM) 38 3.8 Raman scattering spectroscopy 41 Chapter 4 Tunable Se vacancy defects and the unconventional charge density wave in 1T-TiSe2−δ 44 4.1 Experimental details 44 4.2 Synthesis and defects 45 4.3 Crystal structure and defects 48 4.4 Transport properties 51 4.5 Semiconductor or semimetal 54 4.6 Band picture description 55 4.7 Raman scattering study 58 4.8 Summary 60 Chapter 5 STM study of single crystal 1T-TiSe2 surface after the post-annealing 61 5.1 Experimental details 62 5.2 Structure and transport property 62 5.3 STM images on the crystal surface 67 5.4 Raman scattering study 74 Chapter 6 Summary and proposed further works 79 References 81 List of Figures Fig. 2.1: The basic forms of layered compounds for TMDCs. Fig. 2.2: The different stacking types of MX2 for M = group V and VI. Fig. 2.3: The polytypic transformation in TaSe2 system via different temperature heating process. Fig. 2.4: Peierls transition in a one-dimensional chain. The atomic arrangement and the corresponding electronic band structure (a) before and (b) after the transition. Fig. 2.5: Fermi surface nesting in 1D, 2D, and 3D electron gas. Fig. 2.6: The synchrotron X-ray transmission Laue pattern for 1T-TaSe2 crystal which exhibits a commensurate CDW √13 × √13 superlattice at room temperature. Fig. 2.7: The temperature dependence of resistivity in 1T- TaS2. A metallic state is above 550 K; an incommensurate CDW (ICCDW) phase is above 350 K; an near commensurate CDW (NCCDW) phase is from 190 to 350 K; a commensurate CDW (CCDW) phase is below 190 K. Fig. 2.8: The temperature–pressure phase diagram of 1T-TaS2. Fig. 2.9: The electronic phase diagram for CuxTaS2. Cu is proposed being intercalated into the vdW gap of the TaS2 planes. Fig. 2.10:The crystal structure of 1T-TiSe2. Fig. 2.11: In-plane electrical resistivity perpendicular for crystals of 1T-TiSe2 grown by iodine vapor transport at different growth temperatures (Tg). Fig. 2.12: (a) Lattice parameters of CuxTiSe2. (b) The CuxTiSe2 T–x electronic phase diagram. Fig. 2.13: The phase diagram of 1T-CuxTiSe2, where the horizontal axes stand for pressure and the content x of the intercalated Cu. Fig. 2.14: Pressure–temperature phase diagram of TiSe2. Fig. 2.15: The densities of states for octahedral and trigonal prismatic layered materials that the ‘d_(z^2 )’ band is slightly lower for trigonal prismatic than for octahedral coordination in TiSe6. Fig. 2.16: There are two kinds of Ti environments for TiSe6 octahedron: (a) top and bottom face of Se atoms rotate around Ti, and (b) Ti moves closer to Se-Se edge of the octahedron. Fig. 3.1: The single crystal growth of 1T-TiSe2 by the CVT method using iodine as the transport agent. The temperature gradient is TH = 650 ℃ and TL = 550 ℃ in 35 cm. Fig. 3.2: The as-grown 1T-TiSe2 single crystal deposited at the 550 ℃ cold zone by the CVT method. Fig. 3.3: A sketch of X-ray diffractometer. Fig. 3.4: The Bruker D2 PHASER X-ray diffractometer. Fig. 3.5 The Bruker D8 ADVANCE X-ray diffractometer. Fig. 3.6: A sketch of synchrotron radiation and the beamline distribution in NSRRC, Taiwan. The powder X-ray diffraction is operated at BL01C beamline. The photon energy of 20 keV (~0.61993 Å) incident beam passing through the sample and scattered to a detector Mar345 to form a diffraction pattern. Fig. 3.7: The XRD pattern for 1T-TiSe2 polycrystalline which obtained by synchrotron X-ray. Fig. 3.8: (a) A 2D diffraction patterns forming a reciprocal lattice. (b) The random orientation powder scattering rays forming bright cycles. Fig. 3.9: Different types of interaction: (1) ionization and (2) generation of Bremsstrahlung. Fig. 3.10: QD-SQUID VSM instrument with the helium cryo-cycle compressor in the Center for Condensed Matter Sciences (CCMS), National Taiwan University. Fig. 3.11: The QD-PPMS instrument with the helium cryo-cycle compressor in the Center for Condensed Matter Sciences (CCMS), National Taiwan University. Fig. 3.12: A sketch of four probe method for resistivity measurement. Fig. 3.13: A sketch of Hall effect measurement. Fig. 3.14: A Schematic diagram of STM. Fig. 3.15: Schematic representation of operation of a scanning tunneling microscope in (a) constant current mode and (b) constant height mode. Fig. 3.16: A sketch of the setup of the micro-Raman scattering. Fig. 4.1: X-ray diffraction (XRD) patterns for 1T-TiSe2-δ which were obtained by using synchrotron radiation with wave length λ = 0.61993 Å and indexed with space group P3 ̅m1 (No.164). Fig. 4.2: The Se vapor condensed on the sealed quartz tube inner wall after post-annealing. Fig. 4.3: An Arrhenius plot of Se deficiency level δ vs. inverse annealing temperature for 1T-TiSe2-δ powder samples. Fig. 4.4: The magnetic susceptibility for different Se deficiency level of 1T-TiSe2-δ. The CDW transition temperature is shown in inset of figure and predicted by red dash arrow. Fig. 4.5: The Se deficiency level δ dependence of the lattice constant a and c in 1T-TiSe2-δ. Fig. 4.6: The Se deficiency level δ dependence of the bond length and bond angle in 1T-TiSe2-δ. Fig. 4.7: The coordination number (CN) from TiSe6 of CN = 6 to TiSe5 of CN = 5 due to the Se vacancy. Fig. 4.8: The temperature dependence of resistivity for 1T-TiSe2-δ. Fig. 4.9: The temperature dependence of Seebeck coefficient for 1T-TiSe2-δ. Fig. 4.10: The temperature dependence of Hall coefficient for 1T-TiSe2-δ where δ = 0.14. Fig. 4.11: Band pictures of 1T-TiSe2-δ. Fig. 4.12: Temperature dependence of electrical resistivity which has a semiconducting background subtraction for 1T-TiSe2-δ, δ = 0.12. Fig. 4.13: The room temperature Raman scattering spectra for 1T-TiSe2-δ polycrystalline (powder pellets with polished surface). The laser power is 2 mW. Fig. 4.14: The room temperature Raman scattering spectra for (a) no grinded and (b) grinded of 1T-TiSe2-δ polycrystalline. The laser power is 0.2 mW. Fig. 4.15: The room temperature Raman spectra for 1T-TiSe2-δ polycrystalline (no grinding powder), δ = 0.17. The laser powers are 0.2 mW for black line and 2 mW for red line. Fig. 5.1: The synchrotron X-ray diffraction for different annealing temperature of 1T-TiSe2 crystals which are grinded to powders. Fig. 5.2: The lattice constants of 1T-TiSe2 crystals as function of annealing temperature. Fig. 5.3: The Se deficiency level of 1T-TiSe2 polycrystalline and crystals with different annealing temperature which obtained by EPMA. Fig. 5.4: The temperature dependence of resistivity for the ab-plane of 1T-TiSe2 single crystals which annealing at different temperature. Fig. 5.5: The temperature dependence of magnetic susceptibility for the ab-plane and c-direction of 1T-TiSe2 single crystals which annealing at different temperature. Fig. 5.6: STM image of 1T-TiSe2 by 550 ℃ post-annealing. 20 × 20 nm2, V = 0.15 V, I = 0.8 nA at 77 K. Fig. 5.7: STM image of 1T-TiSe2 by 750 ℃ post-annealing. 20 × 20 nm2, V = 0.15 V, I = 0.8 nA at 77 K. Fig. 5.8: STM image of 1T-TiSe2 by 950 ℃ post-annealing. 15 × 15 nm2, V = 0.6 V, I = 0.8 nA at 77 K. Fig. 5.9: Bias dependence of STM images for 1T-TiSe2 after 950 ℃ post-annealing. Sample bias V = (a) -0.1, (b) -0.2, (c) -0,3, (d) -0.4, (e) -0,5, (f) -0.6, (g) -0.7, (h) -0.8, and (i) -0.9 V (filled State), I = 0.8 nA, 15 x 15 nm2 at 77 K. Fig. 5.10: Bias dependence of STM images for 1T-TiSe2 after 950 ℃ post-annealing. Sample bias V = (a) 0.1, (b) 0.2, (c) 0,3, (d) 0.4, (e) 0,5, (f) 0.6, (g) 0.7, (h) 0.8, and (i)-0.9 V (empty State), I = 0.8 nA, 15 x 15 nm2 at 77 K. Fig. 5.11: Number density of Se vacancy in 1T-TiSe2 crystals for different annealing temperature. Fig. 5.12: Number density of Ti-intercalation in 1T-TiSe2 crystals for different post-annealing temperature. Fig. 5.13: The defect distributions (Ti-intercalation) on 750 ℃ post-annealing 1T-TiSe2 surface. 30 × 30 nm2, V = -0.5 V, I = 0.8 nA at 77 K. Fig. 5.14: Displacement vectors for the infrared- and Raman-active modes in the 2H and 1T polytypes. Fig. 5.15: Room temperature Raman scattering spectra of 1T-TiSe2 crystals for different post-annealing temperature excited by 532 nm laser line. Fig. 5.16: (a) The red shit of A1g and (b) the blue shift of Eg Raman phonon modes. Fig. 5.17: The Ti atom was introduced into the van der Waals gap and bonded with top and bottom layer of TiSe2. List of Tables Table 1.1: Single crystal growth of 1T-TiSe2 in the literature, including growth temperature, transport agent used in CVT, and physical property measurements. Table 2.1: The common groups of transition metals for TMDCs, MX2, X = S, or Se, or Te. Table 2.2: The CDW and superconductivity transition temperature of TMDCs. Table 4.1: The lattice parameters, selected bond length and angle, and goodness of fit for 1T-TiSe2-δ polycrystalline varied with different Se deficiency level δ. Table 4.2: Carrier concentrations and mobilities derived from resistivity and Hall effect measurement results for δ ∼0.14. Table 4.3: The activation energy Ea and CDW onset of 1T-TiSe2−δ. Table 5.1: The lattice parameters, selected bond length and angle, and goodness of fit for 1T-TiSe2 single crystals varied with different annealing temperature. Table 5.2: The atomic ratio for different annealing temperature of 1T-TiSe2 polycrystalline and crystals by EPMA. Table 5.3: Symmetries and selection rules for the long-wavelength acoustic and optic phonons in the 1T and 2H polytype geometries.

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