Phase-selective Synthesis And Properties Of Two-dimensional MoS₂

MoS₂ is a typical polyphase layered-material and its physical and chemical properties highly depend on the phases.Thermodynamically stable 2H phase MoS₂monolayer has a direct bandgap of 1.9 e V and holds potential applications as ultrathin channels in semiconductor industry.While thermodynamically metastable 1T or distorted 1T?1T'?phase MoS₂ exhibits distinctive physical properties,such as superconductivity,providing a unique platform for the investigation of new fundamentals in low-dimensional systems.Controlled synthesis of high quality 2D MoS₂ is crucial for exploring both fundamental properties and practical applications.However,there are still many challenges in phase-controlled synthesis of two-dimensional?2D?MoS₂,including the phase-selective synthesis of metastable phases;the correlations between properties,phases and structures;the chemical integration of hetero-phase 2D MoS₂ into new functional structures.Here,we focus on the phase-selective synthesis of 2D MoS₂ and this dissertation can be divided into the following four parts:1.Developed a CVD approach on the nucleation-controlled synthesis of 2D MoS₂ in thermodynamically stable 2H phase.Due to low-efficiency of nucleation in the CVD growth,we designed a metal-organic complex as the precursor to facilitate the nucleation process.This precursor supplies Mo sources and seeding promoters at the same time.The seeding promoters are absorbed onto the substrate prior to the growth of MoS₂ and decrease the surface energy of growth substrate,which greatly facilitates the nucleation process.This strategy shed new lights on the control of nucleation in the CVD process.2.Developed a new approach on the phase-selective synthesis of MoS₂monolayers.Because the formation energy of 1T'phase is dramatically higher than that of 2H phase,by combing with theoretical research,we proposed a potassium-assisted CVD method to adjust the formation energy of the two phases.Using K₂MoS₄ as the precursor,we realized the phase-selective synthesis of MoS₂ monolayers by simply switching the growth atmospheres and obtained a phase-diagram of MoS₂ growth.The obtained 1T'phase monolayers exhibit high crystallinity and?100%phase purity.This approach opens up new possibilities on the controllable synthesis of other 2D TMDCs with metastable phases.3.Investigated the anisotropic properties and catalytic performance of 2D 1T'MoS₂.Using angle-resolved characterizations,we revealed the intrinsic in-plane anisotropic spectroscopic and electrical properties of 2D 1T'MoS₂.The direction of maximum Raman intensities is parallel to zigzag directions,as well the direction of maximum conductance.The Tafel slope of hydrogen evolution reaction?HER?for 1T'MoS₂ is 51 m V/decade,indicating efficient charge transfer in the HER process,which gives great potentials on the design and preparation of high-performance catalyst based on MoS₂.4.Explored the controlled synthesis of hetero-structures and hetero-phase structures and constructed 1T'-contacted devices based on hetero-phase structures.Using the method of phase-selective synthesis,we obtained 1T'/2H in-plane monolayers and out-of-plane bilayers of MoS₂ with the sequential growth of 2H and1T'phase at different temperatures.Meanwhile,various in-plane hetero-structures and hetero-phase structures were also obtained by epitaxial growth of 2H or 1T'MoS₂ on other TMDCs,such as Re S₂,WSe₂,and so on.We fabricated field effect transistors?FETs?with the 1T'/2H out-of-plane structures.The FETs with 1T'contact show higher mobility in comparison with conventional metal-contacted devices and make the Schottky barrier height?SBH?decrease from 200 me V to 110 me V,showing potentials in the design and fabrication of high-performance devices based on MoS₂.