Studies On The Atomic Structures Of Defects And Related Optical And Electronic Properties In Two-Dimensional Transition Metal Sulfide

Two-dimensional（2D）transition metal dichalcogenides（TMDs）have been demonstrated to possess great application potential in the fields of optical,electronic,optoelectronic,catalyze,and energy storage owing to their striking physical and chemical properties.Due to their atomic thickness and saturated chemical bonds at the surfaces,the defect engineering can effectively modulate optical,electronic,and optoelectronic properties of 2D TMDs,and extend their applications.Among the methods for defect engineering,substitutional doping can controllably modulate optical,electronic,and optoelectronic properties by precisely adjusting the substitutional doping concentration with accurate dopant doses.Therefore,studying the atomic structure of the substitutional defects and their influence on the optical,electronic,and optoelectronic properties of 2D TMDs is important for extending their applications.In this thesis,the influence of substitutional dopants on the atomic and electronic structures,photoluminescence,and transient spectra properties of the monolayer WS₂and Mo S₂ are well studied with the combinations of scanning tunneling microscopy/spectroscopy（STM/STS）,spherical aberration corrected transmission electron microscope（STEM）,micro confocal microscopy,and ultrafast pump-probe spectroscopy.The defect-assisted valley depolarization mechanism is proposed,and the interactions between dopants and 2D TMDs hosts are interpreted in this thesis,which is important for the development of defect engineering.This main work is listed as below:1.Studies on the influence of O substitutional S defects on atomic structures,and optical and electronic properties of WS₂ monolayers.The combination of STM/STS characterizations with theory calculations reveals that the popular defects in the monolayer physical vapor deposition（PVD）grown WS₂ is O substitutional S defect.The insertion of O in the lattice shifts the energy level atΓpoint of the valence band maximum of the monolayer WS₂ toward the Fermi level.Both atomic STM images and STS maps demonstrate enhanced local density of states from electron scattering at the O substitutional S sites.By further comparing the valley polarization dynamics of the defective monolayer PVD-grown WS₂ with that of the mechanical exfoliation monolayer WS₂,the defect-assisted valley depolarization mechanism is proposed:the carrier scattering at O substitutional S defects could reduce the valley polarization lifetime of A excitons.2.Studies of the influence of Ce substitutional W defect on atomic structures,and optical and electronic properties of WS₂ monolayers.The Ce-doped monolayer WS₂（Ce-WS₂）was synthesized by PVD method.The STEM images reveal that doped Ce atoms enter into the WS₂ lattice by replacing W atoms.The combination of STS spectra with Density Functional Theory（DFT）calculations demonstrates that the monolayer WS₂ crystal field can effectively split the electronic bands of Ce-f orbitals,producing split electronic bands in the bandgap near the conduction band minimum of the monolayer WS₂.Both photoluminescence and transient spectra reveal that the electrons in Ce-f electronic bands can bind the holes in the valence band maximum of the monolayer WS₂,forming bound excitons.The bound excitons collide with the free A excitons when increasing the pump fluences,reducing the A exciton’s lifetime.Such highly temporal and spatial resolutions provide a mechanism at nanoscale for the modulations of optical and electronic properties of TMDs with lanthanides.3.Studies of the influence of In substitutional Mo defects on atomic and electronic structures of Mo S₂ monolayers.The In-doped Mo S₂ monolayers were synthesized using the chemical vapor transport method.STEM images demonstrates that doped In atoms enter into the Mo S₂ lattice by replacing Mo atoms.The STS spectra and DFT calculations reveal that the embedding of In atoms into the Mo S₂ distorts the lattice,broadens the quasiparticle bandgap,and introduces six defect states in the band gap.This provides fundamental understandings for the studies about the optical properties of In-doped TMDs.