STM Characterization And Optical Properties Of Two-dimensional Transition Metal Dichalcogenides

During the past years,two-dimensional（2D）transition metal dichalcogenides（TMDs）have attracted considerable attention owning to their unique optical,electronic,and optoelectronic properties.The exciton binding energy of 2D TMDs has been significantly enhanced due to their atomically thin structure,which strongly influences their optical and optoelectronic properties.On the other hand,the defects and grain boundaries of the TMDs play an important role in their optical,electronic,and optoelectronic properties.Thus,a comprehensive study of the atomic and electronic structures at the defects and grain boundaries of the TMDs,as well as the relation between the electronic properties and the defects and grain boundaries are significant for promoting their applications.Scanning tunneling microscope and spectroscopy（STM/S）provides a powerful tool for studying atomic and electronic structures of solids at atomic scale,while the pump-probe technique can monitor carrier dynamics at femtosecond scale.Hence,the combination of STM/S and pump-probe spectra can provide an ultra-high temporal and spatial resolutions method to character the TMDs.In this paper,we give a comprehensive study of atomic and electronic structures and carrier dynamics of TMDs using the STM/S and pump-probe technique.The main content is listed as follows:（1）The formation process of the trion of the physical vapor deposition（PVD）grown layered WS₂ nanosheets has been explored by the combination of STM/S and ultra-fast pump-probe spectra.The ultra-fast pump-probe measurements reveal that the formation of trion of the WS₂ nanosheets leads to the photo-induced absorption signal in the transient reflection spectra:the electrons in the valance band maximum（VBM）in WS₂ nanosheets are excited into the conduction band minimum（CBM）by the pump pulse to form excitons with holes in the VBM,and then another electron or hole excited by the probe pulse join into the exciton to form a trion.Atomic-resolution STM images and STS spectra reveal that three types of defects exist in the different layers of the WS₂ nanosheet:O_S top（an O atom substitutes a top S atom）,O_S bottom（an O atom substitutes a bottom S atom）,and Mo _W（a Mo atom substitutes a W atom）.Moreover,the defect states related to both the O_S top and O_S bottom are close to the VBM while that related to the Mo _W are close to CBM,which can increase the formation efficiency of the trion due to the coupling between the defect and trion states.The difference between the trion absorption spectrum with pump-on and the exciton adsorption spectrum with pump-off gives the transient reflection spectrum,showing a strong minus peak.By the combination of the STM/S and pump-probe spectroscopy,the puzzled minus peak in the transient reflection spectra was interpreted at nanoscale.（2）The interfacial electronic structure and band alignment of the neighboring layers of the PVD-grown spiral WS₂ nanosheets and the moirésuperlattice of the spiral WS₂ nanosheets are studied by STM/S.STS measurements demonstrate that the interface of the neighboring layers of the spiral WS₂ nanosheets possesses abundant edge states.Moreover,the type I band alignment of the neighboring layers of the spiral WS₂ nanosheets is directly revealed by the STS contour plot map.Atomic-resolution STM and STS measurements reveal that a moirésuperlattice was existent on the spiral WS₂ nanosheet,which can regulate their electronic structures.（3）The nonlinear optical and electronic properties,as well as the carrier dynamics at the interfaces between the WS and WSe domains of the PVD grown spiral WS_2xSe_2-2x alloy nanosheets are systematically studied.Second-harmonic generation tests demonstrate that these nanosheets exhibit a very strong layer-dependent nonlinear optical effect.Atomic-resolution STM/S measurements reveal that S and Se atoms are non-uniformly distributed,forming WS domains,WSe domains,and defect-related areas.Atomic STM images and STS maps demonstrate enhanced local density of states by electron scattering at the WS/WSe interfaces,providing a detailed nanoscale interpretation of the S/Se ratio-dependent lifetimes observed in pump-probe spectroscopy measurements.