Optical And Electrical Properties Of Vanadium Selenide Nanofilms

With the reduction of the size of semiconductor devices,traditional semiconductor processes are facing huge challenges,and at the same time,heat dissipation and quantum effects of chips have become key factors affecting device performance.In order to solve this problem,researchers have been working to find new semiconductor materials.Vanadium diselenide(VSe₂),as a typical two dimensional semiconductor material,has properties such as high conductivity,high carrier concentration,room temperature ferromagnetism,and charge density wave phase transition behavior.These unique properties make VSe₂ in energy conversion.There is a lot of room for development in fields such as optoelectronics and microdevices.At present,the main way to prepare VSe₂ films is still achieved by mechanical peeling.Liquid phase reaction or physical vapor deposition were also reported to prepare VSe₂ films.To obtain high-quality,large-size,thickness less than 10 nm or even single-layer VSe₂ films remains a research hotspot.Therefore,this thesis takes VSe₂ as the research object,and uses CVD method to prepare VSe₂ films of different thicknesses on mica substrates to study the optical and electrical properties of VSe₂ films,and use variable temperature Raman and variable temperature resistance characterization methods to prove 1T-CDW phase transition temperature point of VSe₂ film.The main results are as follows:(1)1T-VSe₂ films were successfully prepared by chemical vapor deposition(CVD)on a mica substrate.The thickness of VSe₂ films can be regulated by changing the flow rate of Ar/H₂ mixture.The higher the flow rate,the thinner the VSe₂ films will be.The size of VSe₂films can be regulated by changing the temperature at which they grow.Generally speaking,the higher the temperature,the larger the size.In this study,the growth parameters of VSe2films grown by CVD were determined,and the van der Waals epitaxial growth model of VSe₂films was understood,which provided a method for batch preparation of VSe2 films of different thickness.(2)Optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM)were used to characterize the surface morphology of the VSe₂nanosheets.It can be observed that hexagonal and triangular VSe₂ films are randomly deposited on the mica substrate.The size of the VSe₂ nanosheet is about 20?m?50?m.Atomic force microscope(AFM)was used to characterize the thickness of the VSe₂ nanosheet.It can be found that the thickness of the VSe₂ film grown by CVD is about 5 nm to 40 nm.The crystal structure of the VSe₂ film was characterized by X-ray single crystal diffractometer(XRD).It can be found that only the(001)peak,(002)peak and(003)peak appeared in the VSe₂ film,and no other diffraction peaks appeared,which indicated VSe₂ nanosheets grow along the c-axis,and this preferential growth is perpendicular to the substrate.The element ratio of the VSe₂film was tested by energy dispersive x-ray spectrometer(EDS).It can be seen that the atomic ratio of V to Se is about 1:1.8,which is close to the ideal stoichiometric value of 1:2.The VSe₂film was tested by Raman spectroscopy,and an obvious Raman peak was found at 203.3 cm^-1,which corresponds to the A_1g mode of 1T-VSe₂.Through the theoretical calculation of the energy band,the band gap of the VSe₂ film is calculated to be zero,and the resistance of the VSe₂film is tested.It is found that the resistance value is between 100-300?,which is relatively small in two-dimensional materials,indicating the conductivity of the 1T-VSe₂ film The performance is better.(3)Through variable temperature Raman test,the CDW phase transition temperature points of 1T-VSe₂ films with different thicknesses in the temperature range of 90 K?350 K are analyzed.It is found that when the temperature drops from 350 K to 112 K,the A_1g mode shifts to a higher wave number,but when the temperature is further cooled to 110 K,the A_1g mode shifts to a lower wave number,but when the temperature drops further from 110 K to 90 K,the A_1g mode changes to a higher wavenumber,which indicates that the VSe₂ film undergoes a CDW phase transition near 112 K.Moreover,as the thickness of the VSe₂ film decreases,its CDW phase transition temperature will gradually decrease.The resistance changes of different samples in the temperature range of 90 K?350 K were tested by variable temperature resistance.It was found that the film resistance increases with the increase of temperature.But the resistance value of the 1T-VSe₂ film would drop sharply when the temperature point was 112K,indicating that the 1T-VSe₂ film was at 112 K.The CDW phase transition occurred,which is consistent with the variable temperature Raman test results.The research results are beneficial to the preparation of new electronic devices based on 1T-VSe₂ charge density waves.