Controlled Synthesis And Surface Enhanced Raman Scattering Of WS₂ Flakes

In recent years,two-dimensional graphene-like materials have attracted a great deal of attentions owing to their excellent optical,electronic and mechanical properties.Unlike graphene,as direct band gap semiconductors,single-layered two-dimensional transition metal dichalcogenides?TMDs?have shown great potential applications in many fields.Compared with other two-dimensional TMDs materials,the higher density and molecular weight,strong spin-orbit coupling and bipolar electronic transport properties makes WS₂ apply in many fields.However,the controllable synthesis of WS₂ still faces a huge challenge.In our report WS₂ films were prepared by chemical vapor deposition?CVD?.Not only the effects of different growth parameters on the morphology of WS₂ sheets but also the growth mechanism,Raman enhancement effects of R6G molecules on WS₂ sheets were studied.The main contents of this paper are as follows:Firstly,WS₂ thin films were successfully grown on SiO₂?300nm?/Si substrate by traditional low.pressure CVD method.Three controllable growth parameters?the time of S-precursor introduction,the temperature of WO₃ precursor and the growth temperature?were considered.It was found that the morphology of the WS₂ thin film and the products are highly dependent on the concentration of S and W precursor,and the ratio of W and S atoms?W/S?.If the W/S?1:2,triangular and hexagonal WS₂ sheets will be obtained.Whereas,if the W/S>1:2,apart from the WS₂ sheet,W nanowires will be formed due to an excess of W atoms.Secondly,the WS₂ thin film was successfully prepared on SiO₂?300nm?/Si substrate by CVD method.The size of WS₂ thin film obtained by atmospheric pressure CVD method was larger than that by low pressure CVD,and the average length of the triangular WS₂ film boundary is about200?m.The effects of three growth parameters:the temperature of WO₃ precursor,the distance between substrate and WO₃ precursor,and the flow rate on the morphologies of WS₂ are further analyzed.The results show that it is easier to be controlled and obtain WS₂ film,but not W nanowires in low pressure CVD.Finally,SERS of R6G molecule was investigated by using WS₂ film as the subtrates.It is found that the single-layer WS₂ film has the most prominent Raman enhancement effect on R6G molecules.Besides,the Raman enhancement effect of R6G molecules gradually decreases with the increase of layer number of WS₂ film.Our further analysis indicates that the Raman enhancement effect of WS₂ film on the R6G molecule is mainly due to the charge transfer between the WS₂ film and the R6G molecule.