Effects Of Different Preparation Condiyions On Growth Mechanism And Optical Properties Of Vertical WSe₂ Thin Films

In recent years,two-dimensional materials have attracted extensive attention of researchers due to their excellent properties,such as significant size effects,surface effects,and quantum effects.Especially transition metal dichalcogenides(TMDs).It is a quasi-two-dimensional layered material with high carrier mobility,unique and excellent nonlinear optical properties,and good electrical conductivity in terms of electrical properties,making it suitable for optical limiting,optical Switches and other fields have shown great application potential.Tungsten diselenide(WSe₂)is a typical representative of the TMDs family.Pure phase WSe₂shows good stability in air and excellent optoelectronic properties.It has a wide range of applications in photocatalysis,laser processing,photodetectors and other fields.However,the realization of large-area controllable WSe₂thin films has become a bottleneck for its wide application in electrical devices.In this paper,radio frequency magnetron sputtering technology was used to successfully obtain WSe₂nano-films on double-throw p-type Si substrates by controlling the sputtering time,film temperature,target power,gas pressure and other preparation parameters.Morphology,crystal structure,linear optical properties and the effects of nonlinear optical properties.The main work of the paper is divided into the following three aspects:(1)By adjusting the preparation conditions,a"worm-like"special morphology and a"standing"growth structure were successfully obtained on the surface of the Si substrate.The growth mechanism of standing WSe₂films was analyzed by morphology changes and side structures.The study found that with the extension of time,the"worm-like"changed from dispersion to cluster,and the film thickness gradually increased from 290 nm to 1020 nm.Raising the film temperature can further increase the crystallinity of the film and make the particles more tightly bound.The increase of sputtering power accelerates the deposition rate of particles and changes the surface morphology of the film.As the power increases,the grains become larger,the"worm-like"gradually appears,the particles gradually densify,and the clustering phenomenon appears.(2)The crystal structure and chemical composition of the WSe₂film were analyzed,and the XRD results showed that the WSe₂film preferentially grew along the(008)crystal plane during the growth process.Interestingly,the(002)peak gradually blue-shifted with the increase of time.It is caused by the presence of W ions and the influence of pressure.The XPS results just prove that Se^2-and W⁴⁺coexist in the WSe₂film.In order to know the energy level vibration of the film,the Raman spectrum of WSe₂was analyzed to determine the peak position of the characteristic peak and its vibration mode,which proved the existence of functional groups in the film.The TEM confirmed that the film is multi-layered,and functional groups exist in it,which is consistent with the conclusion of Raman spectroscopy.(3)Linear and nonlinear optical properties of WSe₂thin films:The study of linear UV-Vis absorption spectra of WSe₂thin films found that increasing the sputtering time and substrate temperature can increase the absorption of WSe₂thin films in the visible region,and the absorption band edge gradually redshifts.The increase of pressure and power will significantly increase the absorption strength,making it closer to the direct bandgap semiconductor,and the bandgap absorption will be stronger.The characteristic exciton absorption peaks appearing at 300-500nm are caused by the direct transition from the valence band to the conduction band,involving the spin-orbit valence band of the K-point in the Brillouin zone.The Z-scan images of WSe₂thin films were studied on the nonlinear absorption properties.The effects of preparation conditions and laser intensity on the nonlinear optical absorption properties of WSe₂thin films were discussed,the physical mechanism was explained,and the nonlinear absorption coefficients were calculated.It was found that the nonlinear absorption mechanism of WSe₂films changed from saturable absorption(SA)to reverse saturable absorption(RSA)with the sputtering time,substrate temperature,target power,gas pressure and other preparation conditions.The RSA of the WSe₂film is gradually enhanced when the laser intensity is gradually increased.In addition,the nonlinear absorption coefficient of WSe₂thin film is relatively large,which can be calculated to be on the order of 10^-7m/W,so it has a very bright prospect in optoelectronic device applications.