CVD Growth Mechanism And Optoelectronic Properties Of Two-dimensional Transition-metal Dichalcogenides

Two-dimensional transition metal dichalcogenides(TMDs)have been regarded as one of the most important candidate materials for the development of nanoelectronic and optoelectronic devices due to their high stability,ultrathin atomic structure,strong optics-matter interactions,and tunable electronic and optical properties.The realization of large-area and high-quality fabrication of 2D TMDs is a key issue for the improvement of their optoelectronic properties and device performance.The chemical vapor deposition(CVD)is regarded as one of the most promising methods to reach this goal.However,the routine CVD-grown 2D TMDs often suffer from small grain size,higher defect density,and slower growth rate due to the uncontrolled nucleation density and source supply,which becomes one of main problems for limiting their applications in optoelectronic devices.In this thesis,we take Mo S₂ and Mo Se₂ as the main research targets and systematically study their growth mechanisms and optoelectronic properties by using the improved CVD growth processing,and exploring the feasible schemes for the improvement of crystal quality and optoelectronic properties.The main research contents include:1.We fabricate monolayer Mo Se₂ crystals via the molten-salt assisted CVD method and make a comparative study between the impact of vapor-liquid-solid(VLS)and vapor-solid(VS)growth mechanisms on the growth rate,structure and morphology,and photoluminescence properties of monolayer Mo Se₂ crystals by the combination of spectral analysis and theoretical calculations.The research results suggest that the addition of sodium salts makes that the growth rate of Mo Se₂ is 40times faster than that of Mo Se₂ via the routine CVD method.The ultrafast growth of Mo Se₂ benefits from the synergic effect of one-dimensional VLS growth and two-dimensional VS edge expansion.The transition of Mo Se₂ morphology from the regular triangles to fractal snow flowers can be realized by tuning the VLS and VS growth rates.Meanwhile,we find that the use of sodium salts significantly improves the crystal quality and exciton emission of Mo Se₂,which originates from that the intrinsic defects of Mo Se₂ can be passivated by the Br element of sodium salts.This work provides an important basis for the high-quality fabrication and optimization of optoelectronic properties of two-dimensional TMDs.2.We investigate the role of sodium halides in the CVD growth,structure and morphology,and optical properties of Mo S₂ by the combination of experimental spectra characterizations and first-principles(DFT)calculations.The research results show that the use of sodium halides(Na X,X=Cl,Br,I)can significantly promote the CVD growth rate of Mo Se₂ and lead to large crystal size and high crystal quality of synthesized monolayer Mo S₂ crystals.The PL characterizations suggest that the PL intensity of Mo S₂ can be greatly enhanced by 1?2 orders of magnitude.The XPS and DFT calculations indicate that the sulfur vacancies of Mo S₂ can be effectively passivated by the halide elements of sodium salts,which avoids the defect-induced nonradiative recombination.Hence,the crystal quality and exciton emission of Mo S₂can be improved.This work supplies a new route to tune the optoelectronic properties of two-dimensional TMDs.