Controllable Synthesis And Formation Mechanism On Morphology Of Two-dimensional WS₂ Flakes

As a representative of two-dimensional layered materials,graphene has received a lot of attention because of its excellent thermal conductivity and high carrier mobility.It has a wide range of applications in electrical,optical,electrochemical and biomedical fields.Besides graphene,transition metal chalcogenide compounds?TMDs?as graphene-like materials are also the focus of research.Compared with other TMDs,the two-dimensional WS₂ film has a lower effective electron quality and exhibits a little higher carrier mobility or output current.At present,the formation mechanism of WS₂films and the controllable preparation of WS₂ films with desired morphologies still face challenges.In this paper,we use chemical vapor deposition to prepare WS₂ films on different substrates and explore their growth mechanism with different substrates,different morphologies.The main contents of this paper are as follows:Studying the effect of rough?unpolished?Si O₂?300 nm?/Si substrate and single crystal Al₂O₃substrate on the growth of two-dimensional WS₂ films by using chemical vapor deposition?CVD?.Firstly,we successfully prepared a large-area single-layer WS₂ film on the surface of rough Si O₂?300nm?/Si substrate and found that the rough Si O₂/Si substrate has two advantages for the growth of single WS₂ films:first,the"hills and valleys"structure of rough substrate reduces the nucleation density of the WS₂ film,which is conducive to providing a large growth space.Second,the"hills and valleys"structure makes the growth rate of WS₂ film higher.Secondly,for the single crystal Al₂O₃substrate,the WS₂ films are orientation control and affected by the Sapphire substrate for its layer-over-layer mode.A two-dimensional WS₂ film was synthesized on a Si O₂/Si substrate under atmospheric pressure chemical vapor deposition?APCVD?system.In our work,AFM,Raman,optical microscope?OM?and scanning electron microscope?SEM?were used to characterize the thickness and morphology of the WS₂ film.The results show that under the same experimental conditions,different shapes of WS₂films can be obtained at the same time.Further analysis shows that the competition of edge free energy and the local ratio of W:S atoms are the fundamental reasons for the formation of two-dimensional WS₂ films of different shapes:the edge free energy determines the growth rate of each edge,and the ratio of W:S atoms affects the growth process and morphology of two-dimensional WS₂film.These results may have new implications for the synthesis of a variety of different two-dimensional nanomaterials.By observing the change of the film after the second growth,it was found that the film will accelerate oxidation under the condition of controlled heating.The oxidized film can be observed through an optical microscope To the existence of grain boundaries.At the same time,after exposing the WS₂ film grown in the CVD experiment to room temperature for a period of time,it was found that the film will also oxidize,accompanied by aging effects,including a large number of cracks and morphological changes.We use AFM and Raman to characterize the changes of the corresponding parameters of the film before and after oxidation.