Study Of The Preparation,Layer Number Identification And Optical Properties Of Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional?2D?nanomaterials are receiving much attention since the graphene was successfully prepared in 2004.Up to now,more than two thousand kinds of 2D materials have been reported.Transition metal dichalcogenides?TMDs?,which were recognized as the typically 2D graphene-like materials,have unique electronic energy band structure that is strongly dependent on their layer number.The conventional TMDs?MoS₂,WS₂,MoSe2,WSe2,etc.?can be transformed from indirect band gap materials to direct band gap materials when their thicknesses decrease to monolayer.As a result,the optoelectronic properties of 2D TMDs are strongly correlated to their layer number.Therefore,the non-destructively rapid and accurate determination of layer number of TMDs materials is the first priority in their fundamental research and practical applications.In this thesis,MoS₂,WS₂ and WSe2nanofilms with different thicknesses were prepared by mechanical exfoliation and chemical vapor deposition?CVD?.On this basis,the correspondence between optical contrast values,Raman vibration frequency,photoluminescence?PL?peak-position and few-layer number of samples were confirmed.Moreover,the optical properties of monolayer MoS₂ were controlled by modifying differently electronegative dopant on the surface of MoS₂.Also,the many-body bound state in monolayer TMDs was discussed in detail.The specific content is as follows:?1?The layered Mo S₂,WS₂ and WSe₂ nanosheets with different thicknesses were prepared by mechanical exfoliation and CVD methods.The optical microscope,scanning electron microscope?SEM?,Atomic Force Microscope?AFM?,Raman and PL spectra were used to characterize the morphology and optical properties of samples.And the CVD-prepared MoS₂ samples with different temperature and weight of sulfur were compared to investigate the influence of the growth conditions on layered MoS₂ samples.?2?The thicknesses of samples were measured by AFM to identify their layer number.Then,based on the analysis of contrast values in optical microscope images,frequency difference between E₂¹ _g and A_1gg vibration modes in Raman spectra,peak-position differences between indirect transition and direct transition??IT-DT?in PL sapectra of samples,the quantitative correspondences with few-layer number were confirmed.The potential application and inadequacies of these methods to the layer number identification of 2D TMDs materials were pointed out.?3?The ? IT-DT in PL sapectra has significant variation when the few-layer number of samples changed.So it is propitious to few-layer number identification of conventional TMDs.To gain further insight into the band gap structures of WS₂ and WSe2,first-principles calculations based on the density functional theory?DFT?were carried out.Theoretical calculation confirms that the stronger thickness-sensitivity of indirect transition than direct transition derives from the diverse distribution of electron density of state?E-DOS?of W atom d-orbitals and chalcogen atom p-orbitals to different high-symmetry points in the Brillouin zone.?4?The surface of monolayer MoS₂ was modified with dopants that have different electronegativity in order to investigate the influence of many-body bound state on the optical properties of monolayer TMDs.The carrier transfer between MoS₂ and dopant changed electron density of MoS₂ materials.This realized effective doping to monolayer MoS₂ and converting between exciton and negative trion.So the optical properties of monolayer MoS₂were controlled.By means of Three-level model and Mass action model,the variation of electron density of monolayer MoS₂ was calculated when it was modified with different dopant.