High Pressure Research On The Properties And Structures Of Transition Metal Dichalcogenides

Transition metal dichalcogenides（TMDs）show great potential in many fields such as electricity,optics,and optoelectronics due to their unique structures and excellent properties.Therefore,TMDs have attracted extensive attention in recent years.As an external regulation method,pressure can be used to explore the new structures and novel characteristics of materials under extreme conditions,and thus it widely used in the modulation of various materials.When pressure is applied to TMDs,their atomic spacing and electronic interaction can be effectively modulated,resulting in a series of interesting changes,such as pressure-induced phase transition,metallization and superconductivity,etc.In this thesis,we took 2H-Nb Se₂,1T-Hf Se₂ and 2H-WS₂ as the objects of our study.The understanding of the relationship between their structures and properties is enriched by an in-depth exploration of their evolution under high pressure.The main findings of this thesis are as follows:1.The evolution of the structure and superconducting property of 2H-Nb Se₂during compression has been systematically investigated through low temperature electrical transport measurements,synchrotron radiation X-ray diffraction experiments and related theoretical calculations.The experimental results show that the interlayer interaction of 2H-Nb Se₂ was gradually enhanced with increasing pressure,and covalent-like bonds were formed in the interlayer.Both the axial ratio and the axial compression ratio of 2H-Nb Se₂ showed discontinuous changes during compression,and its structure changed from quasi-two-dimensional to three-dimensional.In addition,the trend of superconducting transition temperature（T_c）changed along with the quasi-2D to 3D structural transition of 2H-Nb Se₂.The P-T_c diagram of 2H-Nb Se₂ is dome-shaped.With the increase of pressure,under the combined influence of impurity scattering enhancement and isostructural phase transition,its T_c gradually decreased after reaching a maximum value around 11 GPa.It can be seen that the change of superconductivity of 2H-Nb Se₂ during compression is related to its structural transition.2.The structure and properties of 1T-Hf Se₂ under pressure were explored by synchrotron radiation X-ray diffraction,Raman,infrared reflectivity,low temperature electrical transport measurements and combined with theoretical calculations.It was found that 1T-Hf Se₂ underwent two phase transitions under high pressure.When the pressure increased to 10.8 GPa,the initial phase of 1T-Hf Se₂ started to transform to the high pressure phase I and phase II,and coexisted with them,with further increase of pressure,the initial phase disappeared from the miscible structure at 31.7 GPa,leaving the two high pressure phases coexist;at 47.2 GPa,it completely transformed into the tetragonal high pressure phase with space group I4/mmm.The structure transition of1T-Hf Se₂ was also accompanied by a significant increase in carrier concentration.In the initial phase of Hf Se₂,with the increase of pressure,its valence and conduction bands gradually overlapped,and its band gap closed,this suggests that Hf Se₂ have changed from semiconductor to metal.These results further enrich our knowledge and understanding of TMDs high pressure behavior.3.By measuring the photoelectric properties,X-ray diffraction,Raman,and combined with relevant theoretical calculations,the changes of 2H-WS₂ photoelectrical properties during compression were analyzed.It was found that the photodetector based on 2H-WS₂ not only has an extremely fast response speed of 35/49μs,but also its response waveband can cover the wavelength up to 1650 nm.As pressure gradually increased,the band gap of 2H-WS₂ decreased with the decrease of interlayer spacing,its optical absorption coefficient and photoconductivity along the c-axis direction gradually increased.Benefit from the synergistic effect of the above pressure-induced effects,the photocurrent and responsivity of 2H-WS₂ increased significantly in the full spectral range under xenon lamp irradiation.In addition,in the near-infrared waveband of 980–1650 nm,the photoelectrical properties of 2H-WS₂ were enhanced more significantly,its responsivity and external quantum efficiency were both improved by two orders of magnitude,and its photocurrent showed an increase of three orders of magnitude.These conclusions deepen the understanding of the relationship between the photoelectrical properties and structures of TMDs,and further provide experimental and theoretical basis for the pressure could be used to design the high-performance broadband photodetectors.