Preparation And Physical Properties Of Transition Metal Dichalcogenides VSe₂

Transition metal dichalcogenides（TMDs）are two-dimensional layered materials with strong in-plane bonding forces and weak out-of-plane van der Waals interactions.Due to their atomic level thickness,direct band gap,strong spin orbit coupling,and excellent electronic and mechanical flexibility,TMDs have broad application prospects in electronics,optoelectronics,spintronics,valley electronics,energy storage,and flexible electronics.In recent years,Vanadium diselenide（VSe₂）,as a member of the TMDs family,has attracted great interest from researchers in spintronic devices and memory devices due to its unique physical properties.However,the preparation of VSe₂ thin layers and large-area continuous films remains a challenge.The difficulty of material preparation not only influences the research of the basic physical properties of VSe₂,but also limits further exploration of its potential application value.Aiming at the above existing problems,this thesis mainly focuses on the preparation of thin layers and large-area continuous films of VSe₂,exploring different preparation processes for VSe₂,and conducting in-depth research on its microstructure and physical properties.It is mainly divided into the following parts:1.Thin layer nanosheets of 1T-VSe₂ were prepared by mechanical exfoliated method to study the influencing factors of phonon vibration of 1T-VSe₂.It was found that the dependence of phonon vibration on thickness is that the thinner the sample,the more obvious the phenomenon of redshift in the Raman characteristic peak of 1T-VSe₂.On the other hand,the influence of charge density wave phase transitions on phonon vibrations is discussed.In two-dimensional material systems,charge density wave phase transition originates from lattice distortion caused by strong electron-phonon coupling,which leads to the emergence of new Raman peaks,accompanied with phonon softening.On this basis,further research was conducted on the phase transition mechanism and implementation pathway of 1T-VSe₂.Using 1T-VSe₂ nanosheets prepared by mechanical exfoliated method as precursors,2H-VSe₂ was successfully prepared by annealing at 400℃ in a vacuum,providing an important material foundation for the study of the physical properties of 2H-VSe₂ in fields such as ferromagnetism and valleytronics.2.The growth of VSe₂ thin films was systematically studied via pulsed laser deposition.By optimizing the growth process,continuous 1T-VSe₂ single crystal epitaxial thin films were successfully grown on mica by van der Waals epitaxy.The van der Waals epitaxial properties of VSe₂/mica heterostructures were confirmed using high-resolution X-ray diffraction（XRD）and transmission electron microscopy.1T-VSe₂/mica thin films exhibit obvious metallic properties,and their electrical transport properties are comparable to those of single crystal nanosheets.The test results of mechanical flexibility indicate that the resistance stability of VSe₂/mica is excellent,and it has good bending resistance.In addition,by utilizing the weak van der Waals forces of heterojunction interface between VSe₂ and mica,the VSe₂ thin film can be separated from the mica,resulting in a free-standing VSe₂ thin film that can be transferred to other flexible or silicon substrates,demonstrating the important application prospects of VSe₂ thin film in the fields of flexible electronic devices and traditional silicon-based integrated circuits.3.At 550 ℃ and 1 Pa oxygen pressure,the pure M1 phase VO₂ single crystal epitaxial thin films were prepared on Al₂O₃ substrate by pulsed laser deposition,which showed obvious metal-insulator transition characteristics,and the resistance mutation before and after the phase transition reached more than three orders of magnitude.Further investigate the selenization of VO₂ single crystal thin films to explore a new process for the preparation of VSe₂ large-area continuous thin films.The selenidation products of VO₂ thin films under different process conditions were studied in detail by high-resolution XRD and Raman spectroscopy.At lower temperatures,a mixture of vanadium oxide and M1 phase VO₂ was formed;as the temperature increases,a mixed phase of VO₂ and V₂O₃ is formed;at 500 ℃,almost all are V₂O₃.Based on the analysis of the selenization products of VO₂ thin films,a follow-up experimental scheme for preparing VSe₂ continuous thin films by selenization of VO₂ thin films was proposed.