Raman And Photoluminescence Spectra Of Monolayer MSe₂?M:Mo,W? Under Pressure And Tensile Strain

MoSe₂ and WSe₂,belong to Transition metal dichalcogenides,have special layered structure.The inner-layer has strong covalent bond,while between the layers the combination is just Van der Waals forces so that the MSe₂ material can be mechanically exfoliated.When the bulk material is exfoliated to monolayer,a special physical property will be presented that the MSe₂ materials change from the indirect semiconductor to the direct semiconductor.The thickness-dependent physical property enables them to show prospect in the field of photonics,which becoming a heated research spot in the near future.Provided that MSe₂ show unique electronic properties,researchers pay more and more attention to the manipulation of their electronic structure by external fields.Among various external fields,the strain can effectively change the crystal structure and electronic structure and it is an important tool for the generation of phase transition.It's really challenging for the two-dimensional monolayer materials like MoSe₂ and WSe₂.Because they are too thin and the transfer is so difficult,it is still a new research field.Therefore,studying the behavior of single layer MoSe₂ and WSe₂ under the strain has important significance for the development of Nano scale strain components?switches,sensors,etc.?.In this thesis,monolayer MoSe₂ and WSe₂ were prepared by the method of micro mechanical exfoliation and the sample was successfully placed on DAC and self-made Micro precision uniaxial tension equipment.With the application of compressive strain and tensile strain respectively,we obtained and analyzed the in-situ Raman and photoluminescence spectra to study the structure and electronic structure of MSe₂ caused by strain.1.Under the effect of pressure,the Raman-active phonon modes of monolayer MoSe₂ and monolayer WSe₂ all moved to the high wave number and the splitting occurred when the pressure was up to a certain pressure.Both split sub-peak and the original peak blue shifted at different speeds with the increasing of pressure.With the increasing of pressure,the photoluminescence peak position of monolayer MoSe₂ and monolayer WSe₂ moved,which indicated the pressure leading to the gradual increase of the band gap.The reason is that at the K point the minimum of the conduction band and the maximum of the valence-band were kept away from Fermi level under the pressure effect so that the gap increased.At the same time,the photoluminescence???intensity decreased rapidly with the increased pressure.When the pressure reached to a certain pressure,photoluminescence almost disappeared.It means the transition from direct band gap to indirect band gap,deriving from the band maxima hybridization between p orbit and d-orbit of metal atoms for the se atomic with r point valence-band maximum formed the Brillouin.2.With the increasing of tensile strain,Raman characteristic peak of monolayer MSe₂?M:Mo,W?was broadened,and spitting can also be observed from the single layer WSe₂,which indicated that the tensile strain leaded to the change of crystal structure.For the photoluminescence spectra,with the increasing of the strain,the photoluminescence peak position of monolayer MSe₂?M:Mo,W?shifted to the direction with low energy,which showed the band gap decreased.The intensity for monolayer MoSe₂ of photoluminescence spectra decreased with the increase of strain,while the photoluminescence intensity of WSe₂ single layer was obviously enhanced as result of the monolayer WSe₂ always kept the direct band gap in the process of monolayer WSe₂.In this thesis,the research of Raman and photoluminescence spectra under high pressure and tensile strain of MSe₂?M:Mo,W?shows strain can significantly change the structure and electronic structure of the two-dimensional material and it is an effective method to manipulate two-dimensional electronic materials and optical properties,which plays an important role in the two-dimensional optoelectronics field.