Electronic And Optical Properties Control Of Two Dimensional Transition Metal Dichalcogenides And Heterojunction

Transition metal dichalcogenides(TMDs)possess superior optical and electrical properties,which attract intense research interest currently.In order to exploit TMDs for novel optical and electrical applications,it is critically important to be able to regulate and control the electronic structure of TMDs.In this work,we achieve this goal with different methods,including chemical molecule adsorption,alloy doping,and electrostatic gate control of TMDs hetero-bilayer.These researches provide new platforms for novel optical and electrical applications in two dimension.In the first work(chapter ?),we focus on chemical molecule adsorption of TMDs.Atomic defects can be easily created in the TMDs monolayers devices and cause even more severe influence than in the bulk since the quantum conductance paths are obviously suppressed in the two-dimensional transport.Here we find a drop of chemical solution can repair the defects in the single-layer MoSe₂ field-effect transistors(FETs)through vacancy adsorption.Both the room temperature electron mobility and hole mobility of the FET device increase by two to three orders of magnitude,with the electron mobility increases from 0.1 cm²/Vs to around 30 cm²/Vs,and hole mobility is increased to over 10 cm²/Vs after the solution processing.The defect dynamics is interpreted by the combined study of the first-principles calculations,high-resolution transmission electron microscopy(HRTEM),and Raman spectroscopy.Rich single/double Selenium vacancies are identified by the high-resolution microscopy,which cause some mid-gap impurity states and localize the device carriers.They are found to be repaired by the processing with the result of extended electronic states.Such a picture is confirmed by a 1.5 cm^-1 red shift in the Raman spectra.In the second work(chapter ?),we focus on the monolayer TMDs alloy.Monolayer TMDs possess superior optical properties,including the valley degree of freedom that can be accessed through the excitation light of certain helicity.Although WS₂ and WSe₂ are known for their excellent valley polarization due to the strong spin-orbit coupling,the optical bandgap is limited by the ability to choose from only these two materials.This limitation can be overcome through the monolayer alloy semiconductor,WS_2xSe_2(1-x),which promises an atomically thin semiconductor with tunable bandgap.In chapter IV,we show that the high-quality h-BN encapsulated monolayer WS_0.6Se_1.4 inherits the superior optical properties of tungsten-based TMDs,including a Trion splitting of⁶ meV and valley polarization as high as⁶0%.In particular,we demonstrate for the first time the emerging and gate-tunable interlayer electron-phonon coupling in the hBN/WS_0.6Se_1.4/h BN van der Waals heterostructure,which renders the otherwise optically silent Raman modes visible.In addition,the emerging Raman signals can be drastically enhanced by the resonant coupling to the 2s state of the monolayer WS_0.6Se_1.4 A exciton.The hBN/WS_2xSe_2(1-x)/h BN van der Waals heterostructure with a tunable bandgap thus provides an exciting platform for exploring the valley degree of freedom and emerging excitonic physics in two-dimensionIn the third work(chapter ?),we focus on the TMDs hetero-bilayer.Heterostructure of two different monolayer TMDs provides a unique platform to manipulate exciton dynamics.The ultrafast carrier transfer across the van der Waals interface of the TMDs hetero-bilayer can efficiently separate electrons and holes in the intralayer excitons with a type II alignment,but it will funnel excitons into one layer with a type I alignment.In this,we demonstrate the reversible switch from exciton dissociation to exciton funneling in a MoSe₂/WS₂ heterostructure,which manifests itself as the photoluminescence(PL)quenching to PL enhancement transition.This transition was realized through effectively controlling the quantum capacitance of both MoSe₂ and WS₂ layers with gating.PL excitation(PLE)spectroscopy study unveils that the PL enhancement arises from the blockage of the optically excited electron transfer from MoSe₂ to WS₂.Our work demonstrates electrical control of photoexcited carrier transfer across the van der Waals interface,and the understanding of which promises novel applications in quantum optoelectronics.