Surface Topography And Photoluminescence Properties Of WSe₂ Monolayers

Transition metal dichalcogenides（TMDCs）have a layered structure similar to graphite and belong to layered material,relying on weak van der Waals binding force between layers,and the inner layer is bonded by a covalent bond.Thus,TMDC monolayer can be obtained by mechanical exfoliation.For a typical TMDC material（such as WSe₂）,when its layer number decreases from the bulk material to the monolayer,its band structure will change from the original indirect band gap to direct band gap,and there will be some properties,such as valley selectivity and spin-valley coupling,which makes TMD material become a hot material for the study of optoelectronics,optical polarization and valley electronics.On the other hand,due to the novel physical properties of WSe₂ film,it is also a hot research direction to regulate the optical properties of WSe₂ by shaping the surface topography characteristics of few or single layers.However,there are few reports on the intersecting directions of the two.It is a significant and challenging new research field to study the influence of surface topography on the properties of exciton states and photoluminescence polarization of single layer or few layer thin films by correlating film shaping with the properties of valley electronics and optical polarizationIn view of the above problems,the surface topography and photoluminescence properties of WSe₂ monolayers are studied in this paper.The WSe₂ monolayer is prepared by mechanical exfoliation and the WSe₂ monolayer is covered on the Bi2Se3 bulk and the WS2 bulk by the dry transfer here.Through the surface topography measured by atomic force microscope（AFM）and polarized PL spectra,we study the change of the topography,optical polarization and the influence of temperature on optical characters,which are induced by surface effects of the substrate.The obtained results are following:1.The spikes or micro-nano wrinkles is introduced into the monolayer films.Because of the inevitable tearing and kinking during the transfer process,many sharp spikes may appear in the monolayer.Each spike is about 0.1-1 μm lateral size and 5-40 nm height.The shape and size of the spikes are not uniform.These spikes are sometimes aligned into wrinkles with a clear direction.2.The correlation between the nano-scale characteristic topography and the PL signal and optical polarization of monolayers.The small spikes and wrinkles induce strain into the monolayer,so that the PL peak of the WSe₂ monolayer has obvious redshift at room temperature.Due to the structural difference of the spikes in different excitation regions,the amount of redshift is different,and the center wavelength of the peak can red shift from 745 nm to maximal 785 nm.However,the WSe₂ monolayer PL at low temperature（10K）generates new characteristic peaks,and the new characteristic peaks have a significant energy redshift with the redshift up to 150 me V,compared with the intrinsic exciton peak.The new peaks can only be observed obviously in samples with a substrate of Bi2Se3.The topological surface states of Bi2Se3 provide quenching channels for the intrinsic excitons and trions in the monolayers,eliminating the strong interference of these intrinsic emissions,thus highlighting the weak PL signals related to the micro-nano topography.Besides,the new characteristic peaks also have high linear-polarization degree,and the average linear-polarization degree is > 50%.The orientation of linear-polarization is related to the direction of surface topography.For a uniformly spiky region,the PL spectra from different spots within the region are almost the same,and the orientations of linearpolarization are nearly identical.But the peak positon and orientations of linearpolarization of the peaks from the differently spiky region are totally different.The orientations of linear-polarization of the closed energy peaks are related to the alignment of the wrinkles in the corresponding sample region.Moreover,we discover that the PL intensity of the new peaks decreases during the temperature rising process and quenches up to zero at 40 K.Based on the distorted WSe₂ monolayers,this paper experimentally studies the relevance between the orientation of linear-polarization of PL and surface topography for the first time,and apply the topological surface state of Bi2Se3 the to quench intrinsic exciton of WSe₂ monolayer.This paper lays a foundation for the research on the studies of shaping the topography of 2D layers and the development of ideal surface devices of superior optical polarization performances.