Layered MoS₂ And WS₂:Synthesis And High Pressure Modulation

Layered Transition Metal Dichalcogenides?TMDs?are a bunch of new graphenealike quasi-2D materials.The 2H phase structure of TMDs attracts tremendous attentions.In the 2H monolayer,each Transition Metal atom is centered and covalently connected with the chalcogenide atoms prismatic.Their electronic bandgap range from 1.5eV to 2.7 eV,rendering themselves promising for the applications of versatile devices.This dissertation takes MoS₂ and WS₂ as examples to explore the larger-scale synthesis of thin layers by Chemical Vapor Deposition?CVD?and tune the crystal and electronic structure by Diamond Anvil Cell?DAC?.Main results are as followed.Firstly,relative vapor concentration of transition metal trioxide to sulfur determines whether the epitaxy occurs as in-plane or out-of-plane growth.Higher temperature undermines the nucleation.The interface of ?-sapphire facilitates epitaxy in 2D plane due to the crystallographic resemblance of c-axis-cut sapphire and the in-plane hexagonal TMDs structure.Hydrogen assist in reducing oxygen in transition metal trioxide and entices larger scale epitaxy.Secondly,the fact that in-plane Raman mode E_2g² splits under pressure indicates the relative slippage of monolayers in bulk MoS₂,corresponding to previously reported 1st order phase transition from 2Hc to 2Ha.Monolayer MoS₂ on Si/SiO₂ substrate behaves splitting of in-plane E' and out-of-plane A1'mode,which indicates the structural distortion in monolayer.Similarly,the monolayer WS₂ also has activated out-of-plane B mode and splitting of B and A1',while no splitting is observed on monolayer WS₂ on diamond anvil,verifying the Raman splitting comes from extra strain of Si/SiO₂ distortion.Finally,the photoluminescence of 1s A exciton blueshifts under pressure both in monolayer MoS₂ and monolayer WS₂ and their intensity continuously decrease.The absorption spectra of exciton confirm the enlargement of optical bandgap and constant spin-orbital coupling on the top of the valance band.Combined analysis of photoluminescence and absorption of exciton under pressure confirms the enhanced Coulomb interaction that accounts for the non-radiative transition from exciton to trion.