臺灣博碩士論文加值系統

本論文主要是探討摻雜物對過渡性金屬第六族(鉬及鎢)和第七族(錸)的二硒單晶的非等向電學傳導與光學特性的影響程度。這項主題被探討的原因是由於此類材料有極佳的非等向電學，光學和力學特性。再加上此類材料的能帶正好匹配太陽光譜能帶，所以他們可以被應用在光電化學太陽能電池的電極材料。多方面的嘗試是在研究使用不同的摻雜物對這些非等向材料的影響效應。研究發現少量的摻雜物可以促進晶體成長進而產生大尺寸的材料。從這些大尺寸材料所提供的凡得瓦面和側面操作面積可以提昇非等向電學傳導和光學特性的研究。
在本項研究中所使用的晶體，主要以溴做為傳導劑，利用化學氣相傳導法來成長二硒化鉬摻雜鈮，二硒化鉬摻雜錸，二硒化鎢摻雜錸及二硒化錸摻雜鉬。X-ray繞射量測是用來確定所成長的晶體結構。光電壓是用來探討非等向晶體的光學特性，另外利用偏極化壓電及電解液電場調制光譜量測來研究摻雜物對於內部能帶直接躍遷的影響程度。霍爾及電阻率量測是用來探討非等向電學傳導特性。摻雜物對於無摻雜與有摻雜材料的非等向電學傳導與光學特性的影響程度將會有作一深入個比較和討論。

The purpose of this work is to characterize the effects of dopants on the electrical transport and optical anisotropy of transition metal diselenide single crystals in the groups VIB (Mo and W) and VIIB (Re). It is a subject of considerable interest because of their extremely anisotropic electrical, optical and mechanical properties. Since the band gaps of these materials are well matched to the solar spectrum, they can be used as an efficient photoconductive layer in photovoltaic devices and photoelectrochemical solar cells. Various efforts in studying the influence of their anisotropic properties of these materials have been produced with controllable different dopants. It appears that the addition of a small amount of dopants promotes the growth of material with extended crystal dimension. The larger surface areas of the van der Waals and edge planes facilitate sufficient working surfaces for the study of the electrical transport and optical anisotropy of the crystals.
In this work, single crystals of Nb-doped MoSe2, Re-doped MoSe2, Re-doped WSe2, W-doped ReSe2 and Mo-doped ReSe2 are grown using the chemical vapor transport method with Br2 as the transporting agent. XRD pattern is used to confirm the crystal structure of the as-grown material. The electrical transport anisotropy is being probed by conductivity and Hall measurements. Optical anisotropic properties have been studied with the aid of polarized photovoltage, electrolyte electroreflectance and piezoreflectance measurements for the effect of dopants on the interband transition energies of various features. The effect of dopants on the electrical transport and optical anisotropy of undoped and doped materials will be compared and discussed.

Abstract i
Acknowledgments iii
Table of contents iv
List of figures vi
List of tables xi
1 INTRODUCTION 1
1.1 Transition metal dichalcogenides 1
1.1.1 Crystal structure 1
1.1.2 Electronic structure 9
1.1.3 Synthesis 13
1.1.4 Electrical transport properties 14
1.1.5 Optical properties 15
1.2 The intercalation reaction and dopant effects 16
1.2.1 Intercalation definition 16
1.2.2 Dopant effects 17
1.3 Survey of recent literatures 19
1.3.1 The physical properties 20
1.3.2 The chemical properties 21
1.4 Statement of the present work 21
2 SINGLE CRYSTAL GROWTH AND STRUCTURAL INVESTIGATIONS 24
2.1 Single crystal growth 24
2.1.1 Compounds preparation 24
2.1.2 Crystal growth 25
2.2 Structural investigations of single crystals 27
2.2.1 Introduction to powder X-ray diffraction 28
2.2.2 Two-layered hexagonal crystal structure 30
2.2.3 Triclinic crystal structure 36
2.3 Energy dispersive analysis by X-ray 40
3 EXPERIMENTAL DETAILS 42
3.1 Conductivity and Hall effect measurements 42
3.1.1 Computer-controlled system for electrical characterization 43
3.1.2 Sample preparations in polarization electrical measurements 43
3.1.3 Definitions for conductivity and Hall calculations 49
3.2 Photovoltage measurements 51
3.2.1 Computer-controlled system for photovoltage measurements 52
3.2.2 Sample preparations in polarization photovoltage measurements 52
3.2.3 Definitions for band gap determinations 56
3.3 Modulation techniques 56
3.3.1 Computer-controlled system for modulation spectroscopy 59
3.3.2 Sample preparations in polarization piezoreflectance 59
3.3.3 Sample preparations in polarization electrolyte electroreflectance 61
3.3.4 Lineshape considerations for PzR and EER experiments 62
4 ELECTRICAL TRANSPORT ANISOTROPY STUDY OF DOPANT EFFECT 67
4.1 Electrical anisotropy of doped and undoped MoSe2 single crystals 67
4.2 Electrical anisotropy of doped and undoped WSe2 single crystals 72
4.3 Electrical anisotropy of doped and undoped ReSe2 single crystals 75
5 ROOM TEMPERATURE OPTICAL ANISOTROPY STUDY OF DOPANT EFFECT 80
5.1 Optical anisotropy of doped and undoped MoSe2 single crystals 80
5.2 Optical anisotropy of doped and undoped WSe2 single crystals 91
5.3 Optical anisotropy of doped and undoped ReSe2 single crystals 99
6 TEMPERATURE DEPENDENCE PIEZOREFLECTANCE STUDY OF DOPANT EFFECT 106
6.1 Excitons study of doped and undoped MoSe2 / WSe2 single crystals 106
6.2 Excitons study of doped and undoped ReSe2 single crystals 123
7 CONCLUSION 133
REFERENCES 137
PUBLICATIONS 147

LIST OF FIGURES

Fig. 1.1 Typical structure of layered TMDCs. Bonding is strong within each (triple-) layer, but much weaker across the separating “van der Waals gap”. The unit cell is defined with the c-axis perpendicular to the layers, and the a- and b-axes along the minimal chalcogen-chalcogen distance. 2
Fig. 1.2 Two possible types of MX2 layered TMDCs coordinations from ref. [3]: (a) Trigonal-prismatic coordination (space group ) and (b) Octahedral coordination (space group ). 4
Fig. 1.3 (a) Some common TMDCs polytypes represented by the projections on the (b) The crystal lattice of the 2Hb polytype of MX2 belongs to the non-symmorphic hexagonal space group ( ). 6
Fig. 1.4 (a) The representative scheme of the basal plane of ReSe2 with the chains of Re4 “diamond clusters” are shown. (b) The distorted unit cell (drawn by thick lines) which is derived from the perfect hexagonal lattice. 8
Fig. 1.5 Schematic diagrams of the density of states for TMDCs of groups IVB-VB-VIB after the Wilson and Yoffe model. From ref. [1] (a) ZrS2, octahedral (b) NbS2 and MoS2 trigonal-prismatic. 10
Fig. 1.6 Brillouin zone (BZ) for the hexagonal crystal structure. In the top of the BZ (A-L-H) there is generally a double degeneracy of each band. The interaction across the VdW plane is evident from the lift of degeneracy when going from A to Γ. 12
Fig. 2.1 (a) Temperature profile for the synthesis of ReSe2 polycrystalline compound. (b) Temperature profile for the growing ReSe2 single crystal. 26
Fig. 2.2 (a) The atoms, represented as full circles in the graph, can be viewed as forming different sets of lattice planes in the crystal. (b) Bragg’s reflection in a given set of lattice plane with an interplane distance of dhkl. 29
Fig. 2.3 Powder diffraction data can be collected using either (a) transmission or (b) reflection geometry. 31
Fig. 2.4 The surface photograph of as-grown (a) Nb-doped MoSe2, (b) Re-doped MoSe2 and (c) Re-doped WSe2 samples. 34
Fig. 2.5 The XRD patterns of as-grown (a) Nb-doped MoSe2, (b) Re-doped MoSe2 and (c) Re-doped WSe2 samples. 35
Fig. 2.6 The surface photograph of as-grown (a) W-doped ReSe2 and (b) Mo-doped ReSe2 samples. 37
Fig. 2.7 The XRD patterns of as-grown (a) W-doped ReSe2 and (b) Mo-doped ReSe2 samples. 38
Fig. 3.1 (a) A computer-controlled system for the temperature-dependent conductivity and Hall measurements. (b) The sample geometry and contacts placement used here in our measurements. 44
Fig. 3.2 Conductivity measurement of (a) I�姞 or I���駥 polarization configuration (b) I�姡, I���駬 and I���駥 polarization configuration. 46
Fig. 3.3 Hall measurements of (a) (I�姞, B���駥 ) polarization configuration (b) (I���駥, B�姞) polarization configuration (c) (I�姡, I���駬; B���駥) polarization configuration. 48
Fig. 3.4 (a) A computer-controlled system with PCI-8255 and IEEE-488 techniques for PV measurement (b) An electrochemical cell constructed from Teflon with a quartz window was designed to fit the sample. 53
Fig. 3.5 PV measurement of (a) (E�姞, E���駥; k�姞) and (E�姞, k���駥) polarization configurations (b) (E�姡, k���駥) and (E���駬, k�姞) polarization configurations. 55
Fig. 3.6 Computer-controlled setup with low temperature system used in modulation spectroscopy and modulation source for EER or PzR experiment. 58
Fig. 3.7 Sample preparations and arrangements for (a) PzR experiments with modulation source of AC sine-wave and (b) EER experiments with modulation source of 0.35 Vpp square-wave. 60
Fig. 4.1 (a) Temperature-dependent conductivity of Nb-doped, Re-doped and undoped MoSe2 parallel and perpendicular to the crystal c-axis. (b) Temperature-dependent mobilities of Nb-doped, Re-doped and undoped MoSe2 perpendicular to the crystal c-axis. 69
Fig. 4.2 (a) Temperature-dependent conductivity of Re-doped and undoped WSe2 parallel and perpendicular to the crystal c-axis. (b) Temperature-dependent mobilities of Re-doped WSe2 perpendicular to the crystal c-axis. 73
Fig. 4.3 (a) Temperature-dependent conductivity of W-doped and undoped ReSe2 parallel and perpendicular to the crystal b-axis. (b) Temperature-dependent mobilities of W-doped ReSe2 and undoped ReSe2 parallel and perpendicular to the crystal b-axis. 77
Fig. 4.4 (a) Temperature-dependent conductivity of Mo-doped and undoped ReSe2 parallel and perpendicular to the crystal b-axis. (b) Temperature-dependent mobilities of Mo-doped and undoped ReSe2 parallel and perpendicular to the crystal b-axis. 78
Fig. 5.1 (a) Relative photovoltage spectra for Nb-doped and undoped MoSe2 single crystals at 300 K. (b) Square root of the relative photovoltage intensity for Nb-doped and undoped MoSe2 single crystals for the determination of the indirect band gap. 85
Fig. 5.2 (a) Relative photovoltage spectra for Re-doped and undoped MoSe2 single crystals at 300 K. (b) Square root of the relative photovoltage intensity for Re-doped and undoped MoSe2 single crystals for the determination of the indirect band gap. 86
Fig. 5.3 (a) The EER spectra of Nb-doped and undoped MoSe2 single crystals over the energy range of 1.35 eV to 2.0 eV for the VdW (k���駥) plane. (b) The polarization-dependent EER spectra of Nb-doped MoSe2 single crystal over the energy range 1.4 eV to 2.0 eV for the edge (k�姞) plane. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies as indicated by the arrows. 87
Fig. 5.4 (a) The EER spectra of Re-doped and undoped MoSe2 single crystals over the energy range of 1.35 eV to 2.0 eV for the VdW (k���駥) plane. (b) The polarization-dependent EER spectra of Re-doped MoSe2 single crystal over the energy range 1.3 eV to 2.0 eV for the edge (k�姞) plane. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies as indicated by the arrows. 88
Fig. 5.5 (a) Relative photovoltage spectra for Re-doped and undoped WSe2 single crystals at 300 K. (b) Square root of the relative photovoltage intensity for Re-doped WSe2 and undoped WSe2 single crystals for the determination of the indirect band gap. 95
Fig. 5.6 (a) The EER spectra of undoped and Re-doped WSe2 single crystals over the energy range of 1.4 eV to 2.4 eV for the VdW (k���駥) plane. (b) The polarization dependent EER spectra of Re-doped WSe2 single crystal over the energy range 1.4 eV to 2.4 eV for the edge (k�姞) plane. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies as indicated by the arrows. 96
Fig. 5.7 (a) Relative photovoltage spectra for W-doped and undoped ReSe2 single crystals at 300 K. (b) Square root of the relative photovoltage intensity for W-doped and undoped ReSe2 single crystals for the determination of the indirect band gap. 101
Fig. 5.8 (a) Relative photovoltage spectra for Mo-doped and undoped ReSe2 single crystals at 300 K. (b) Square root of the relative photovoltage intensity for Mo-doped and undoped ReSe2 single crystals for the determination of the indirect band gap. 102
Fig. 5.9 Polarization-dependent EER spectra over the energy range 1.25 eV to 1.45 eV. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies. Arrows at the bottom of the figures show the peak positions of the two interband excitonic features, and , respectively. (a) W-doped and undoped ReSe2 single crystals. (b) Mo-doped and undoped ReSe2 single crystals. 103
Fig. 6.1 PzR spectra of the A and B excitonic transitions at 15, 77, 100, 150, 200, 250 and 300 K showing their temperature dependence for (a) Nb-doped MoSe2 (b) Re-doped MoSe2 and (c) undoped MoSe2. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies as indicated by the arrows. 109
Fig. 6.2 PzR spectra of the A and B excitonic transitions at 15, 77, 100, 150, 200, 250 and 300 K showing their temperature dependence for (a) Re-doped WSe2 and (b) undoped WSe2. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies as indicated by the arrows. 110
Fig. 6.3 Temperature variations of the A1, A2 and B excitonic transition energies for (a) Nb-doped MoSe2 (b) Re-doped MoSe2 and (c) undoped MoSe2. Representative error bars are shown. The dashed curves are the least-squares fits to Eq. (3.10) and the solid curves are the least-squares fits to Eq. (3.11). 114
Fig. 6.4 Temperature variations of the A1, A2 and B excitonic transition energies for (a) Re-doped WSe2 and (b) undoped WSe2. Representative error bars are shown. The dashed curves are the least-squares fits to Eq. (3.10) and the solid curves are the least-squares fits to Eq. (3.11). 115
Fig. 6.5 Temperature variations of the broadening parameters of the A1, A2 and B excitonic transition energies for (a) Nb-doped MoSe2 (b) Re-doped MoSe2 and (c) undoped MoSe2. Representative error bars are shown. The dashed curves are the least-squares fits to Eq. (3.12). 119
Fig. 6.6 Temperature variations of the broadening parameters of the A1, A2 and B excitonic transition energies for (a) Re-doped WSe2 and (b) undoped WSe2. Representative error bars are shown. The dashed curves are the least-squares fits to Eq. (3.12). 120
Fig. 6.7 Unpolarized PzR spectra of (a) W-doped ReSe2 (b) Mo-doped ReSe2, (c) undoped ReSe2 at several representative temperatures. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies ( and ) as indicated by the arrows. 124
Fig. 6.8 Polarized PzR spectra of (a) W-doped ReSe2 (b) Mo-doped ReSe2 and (c) undoped ReSe2 at 15 and 300 K. The solid curves are the least-squares fits to first-derivative Lorentzian lineshape expression which yield the excitonic transition energies ( and ) as indicated by the arrows. 125
Fig. 6.9 Temperature variations of the and excitonic transition energies for (a) W-doped ReSe2 and (b) undoped ReSe2. Representative error bars are shown. The dashed curves are the least-squares fits to Eq. (3.10) and the solid curves are the least-squares fits to Eq. (3.11). 130
Fig. 6.10 Temperature variations of the broadening parameters of the and excitonic transition energies for (a) W-doped ReSe2 and (b) undoped ReSe2. Representative error bars are shown. The dashed curves are the least-squares fits to Eq. (3.12). 131
LIST OF TABLES

Table 1.1 Most common polytypes found in TMDCs. 5
Table 1.2 Crystallographic data for 2H-WS2, 2H-WSe2, 2H-MoS2, 2H-MoSe2 [1]. 7
Table 1.3 Crystallographic data for Triclinic-ReSe2 [6]. 7
Table 1.4 Preparation conditions for the TMDCs single crystals used in this work. 13
Table 2.1 The calculated interplane distance dhkl (measured in Å units) of as-grown samples with 2H structure are deduced from the XRD patterns. 32
Table 2.2 Lattice parameters of as-grown samples with 2H structure and lattice constants of previous reports are listed for comparison. Some of the values quoted from previous reports for comparison purpose. 33
Table 2.3 The calculated dhkl (measured in Å units) of as-grown samples with triclinic structure are deduced from the XRD patterns. Some of the values quoted from previous reports for comparison purpose. 39
Table 2.4 Lattice parameters of as-grown samples with triclinic structure and lattice constants of previous reports are listed for comparison purpose. 40
Table 4.1 Electrical anisotropic transport properties of Nb-doped, Re-doped MoSe2 and undoped MoSe2 single crystals at 300 K. Some of the values quoted from previous reports for references and comparisons. 70
Table 4.2 Electrical anisotropic transport properties of Re-doped and undoped WSe2 single crystals at 300 K. Some of the values quoted from previous reports for references and comparisons. 74
Table 4.3 Electrical anisotropic transport properties of W-doped, Mo-doped and undoped ReSe2 single crystals at 300 K. Some of the values quoted from previous reports for references and comparisons. 79
Table 5.1 Optical anisotropic properties of Nb-doped, Re-doped and undoped MoSe2 single crystals at 300 K. Some of the values quoted from previous reports for comparison purpose. 90
Table 5.2 Optical anisotropic properties of Re-doped and undoped WSe2 single crystals at 300 K. Some of the values quoted from previous reports for comparison purpose. 98
Table 5.3 Optical anisotropic properties of W-doped, Mo-doped and undoped ReSe2 single crystals at 300 K. Some of the values quoted from previous reports for comparison purpose. 104
Table 6.1 Direct band gap Eg and exciton binding energy Re of the A exciton series for Nb-doped MoSe2, Re-doped MoSe2 / WSe2 and undoped MoSe2 / WSe2. Some of the values quoted from previous reports for comparison purpose. 107
Table 6.2 Direct transition energies and broadening parameters of the excitons A and B of Nb-doped, Re-doped and undoped MoSe2. If a Rydberg series is observed, the energy of the n = 1 feature is observed. Some of the values quoted from previous reports for comparison purpose. 112
Table 6.3 Direct transition energies and broadening parameters of the excitons A and B of Re-doped and undoped WSe2. If a Rydberg series is observed, the energy of the n = 1 feature is observed. Some of the values quoted from previous reports for comparison purpose. 113
Table 6.4 Values of Varshni and Bose-Einstein type fitting parameters describe the temperature dependence of the excitonic transition energies of Nb-doped MoSe2, Re-doped MoSe2 / WSe2 and undoped MoSe2 / WSe2. Some of the values quoted from previous reports for comparison purpose. 116
Table 6.5 Values of the parameters which describe the temperature dependence of broadening function ��(T) of the excitonic transition energies of Nb-doped MoSe2, Re-doped MoSe2 / WSe2 and undoped MoSe2 / WSe2. Some of the values quoted from previous reports for comparison purpose. 121
Table 6.6 Direct transition energies and broadening parameters of the excitonic features from PzR measurements for W-doped, Mo-doped and undoped ReSe2. Some of the values quoted from previous reports for comparison purpose. 126
Table 6.7 Values of Varshni and Bose-Einstein type fitting parameters describe the temperature dependence of the excitonic transition energies of W-doped and undoped ReSe2. Some of the values quoted from previous reports for comparison purpose. 127
Table 6.8 Values of the parameters which describe the temperature dependence of broadening function ��(T) of the excitonic transition energies of W-doped and undoped ReSe2. Some of the values quoted from previous reports for comparison purpose. 129

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