Electric Transport Properties And Metallization Of Transition Metal Dichalcogenides Under High Pressure

High pressure investigation of transition metal dichalcogenides (TMDCs) plays an important role on both fundamental research and application research of condensed matter. Under high pressures several TMDCs undergo some changes, such as structural transition, semiconductor-metal transition and so on. The knowledge of their action under pressure is very useful for understanding the important progress in condensed materials, such as the bonding or dissciation between atoms. It's also useful for realizing the rule of interaction, finding out new rule, and presenting theoretical base for practical application. High pressure eleconic structure and electric transport properties investigation of TMDCs can provide correct guidance for optimization of optoelectronic and photovoltaic properties.Isostructural phase transition brings a new way for understanding the mechanics and the driving forces of high-pressure phase transitions. X-ray diffraction can be used to find the structure modification, but for pressure induced isostructural phase transition, the evidence of transition is not sensitive to X-ray diffraction, and in-situ physical property measurement has been used to determine this type of transition. Due to the limitation of high pressure experimental technique, isostructural phase transitions were mostly studied by theoretical calculations. Both in-situ electrical transport properties measurement and first-principles calculation were employed to inveatigate characteristic and rule about isostructural phase transition. In this paper, five kinds of TMDCs compounds:WS2, WSe2, MoS2, MoSe2 and TiS2 were chosen as research object. High pressure in-situ resistivity measurement, in-situ Hall effect measurement, in-situ X-ray diffraction measurement, and first-principles calculation were employed to conduct a synthetical analysis and research on high pressure phase transiton, mechanisms and trends of pressure induced metallization, and pressure behavior of electric transport properties.A pressure induced semiconductor-semimetal phase transition on WSe2 has been studied using in-situ electrical resistivity measurement and first-principles calculation under high pressure. The experimental results indicate that the phase transition takes place at 38.1 GPa. We have performed first-principles ultrasoft pseudopotential band structure calculations, which indicated that the zero-pressure structure is clearly a semiconductor with an indirect band gap of 1.2 eV. Pressure increases mixing of W 5dx2-y2, W 5dxy and Se 4p state near Fermi level, which indicates a decrease in the W-Se distance with increasing pressure, and WSe2 approaches the metallic phase. Therefore, we can conclude that the semimetallic phase of WSe2 under high pressure may be attributed to the W-Se covalent bonding in the interlayer rather than van der Waals bonding between layers. Pressure induced metallizaiton of MoSe2 occures at pressure of 35.7 GPa. We found that an abnormal change in the pressure dependence of resistivity of MoSe2 appears at 35.7 GPa, which means that an underlying phase transition exists herein. For confirming the phase transition of MoSe2 at 35.7 GPa, the temperature dependence of MoSe2 resistivity and in-situ X-ray diffraction were measured at different pressures. The result of temperature dependence of resistivity indicates that below 35.7 GPa the electrical resistivity of MoSe2 decreases with temperature increase and therefore MoSe2 shows a semiconductor behavior. Above 35.7 GPa the electrical resistivity shows a positive relationship with the temperature, which indicates MoSe2 has become metallic or semimetallic. High pressure X-ray differenction patterns indicate no change in structure of MoSe2 up to 36.4 GPa. So we can conclude that this semiconductor-semimetal transition in MoSe2 at 35.7 GPa is an isostructural phase transition. The first-principles band structure calculations indicate that the phase transition of MoSe2 caused by clousure of band gap.The continuous changes in pressure dependence of resistivity of WS2 and MoS2 indicated that there is no clear indication of any phase transition in our experimental pressure range. We have studied the transport active energy of WS2 through temperature dependence of resistivity under various pressures. Increase in pressure can shorten the interatomic distance, expend energy band, and narrow the band gap. So the electrons in impurity levels in band gap can be excited to conduction band to take part in electric conduction process. The pressure response of impurity levels is the root cause of continuous decreases in pressure dependence of resistivity of WS2 and MoS2. Through comparison the metallizaiton pressures of WS2, WSe2, and MoSe2, it has been perceived that smaller electronegativity difference between anion and cation can be helpful to charge transfer from chalcogenide to transiton metal and TMDCs would undergo metalized phase transiton at lower pressures. The d state of transition metal determines basic characters of TMDCs, but the p state of chalcogenide plays an important role in the process of metallization. Two totally conflicting opinions exist in the electronic structure of TiS2 and both of them have gained experimental and theoretical support of their own. The first-principles results show that DOS at Fermi energy (Ef) is controlled by the overlap between S 3p state (valence) and Ti 3d state (conduction). This indirect overlap results in a semimetallic state of TiS2-This theoretical result of the semimetallic nature of TiS2 can be verified by positive temperature dependence of resistivity. In view of metallic properties of TiS2 being attributed to impurities and defects, the compositions quantitative analysis was carried out by EDX and XRD, which indicate the sample used in this experiment is high-purity and good stoichiometric TiS2. So we can conclude that the metallic properties of TiS2 are intrinsic. The Hall effect measurements under various temperature illustrated that when the sample was cooled down to liquid nitrogen temperature, the electrical resistivity decreases by half of its original level, and the electronic concentration is not affected by temperature. Therefore, the decreasing resistivity may result from an increase in the Hall mobility of the carriers.The pressure behaviors of electric transport properties of TiS2 have been studied from both experimental standpoint and theoretical standpoint. The first-principles results show that the semimetallic state of TiS2 can be maintained under high pressure. We notice that slope change in resistivity of TiS2 variation with pressure is quite different before and after 15 GPa. To gain further insight into the electrical transport properties of TiS2, we performed in-situ Hall effect measurements under high pressure, which shows that the increase of electron concentration of TiS2 with increasing pressure up to 14.5 GPa should be caused by the pressure-induced ionization of impurity levels. Above 14.5 GPa, the saturated electron concentration suggests that the impurity levels are wholly ionized at higher pressure. The model of pressure induced ionization of impurity leves is used to interpret pressure response of semimetallic transport behavior reasonably.