Research On Defect Control Of Two-dimensional MoS₂ Photoelectric Performance

Two-dimensional(2D)transition-metal dichalcogenides(TMDs)represented by layered molybdenum disulfide(MoS2).are 2D semiconductor materials with wide interest.They have ultra-thin bulk thickness,atomically flat interface,suitable bandgap width and considerable room temperature carrier mobility,which make them show great potential in future electronic and optoelectronic devices.However,unlike traditional semiconductor materials,lattice defects play a more significant role in the electronic structure of 2D materials(such as TMDs.black phosphorus,InSe).Furthermore,their photoelectric properties have been plagued by lattice defects,and the mechanism of the defects is still unclear.In this paper,the influence mechanism of sulfur vacancies on optical and electrical properties of monolayer MoS2 is studied.The influence mechanism of sulfur vacancy concentration on electronic transport of monolayer MoS2 is systematically elucidated,and an effective way to enhance monolayer MoS2 electronic devices is provided.On this basis,we designed and constructed high-performance lateral monolayer MoS2 homojunction and vertical stacking bilayer MoS2 homojunction by utilizing the ability of sulfur vacancies to regulate the electronic structure of monolayer MoS2.Monolayer MoS2 has been prepared by oxygen-assisted chemical vapor deposition.The effects of growth parameters on the morphology and size of monolayer MoS2 were investigated by optical microscopy.The preparation of high-density large-size monolayer MoS2 was achieved by adjusting the growth parameters.The assisting effect of oxygen is systematically clarified,which limits the vertical growth of MoS2 and promotes lateral epitaxial growth along the edge of monolayer MoS2 to grown large-scale monolayer MoS2.Monolayer MoS2,graphene and few-layer boron nitride have also been prepared by micromechanical exfoliation.On this basis,some high-quality van der Waals heterostructures have been constructed by precise transfer at micro-nano scale.We establish a powerful poly(4-styrenesulfonate)(PSS)-treated strategy for sulfur vacancy healing in monolayer MoS2 to precisely and steadily tune its electronic state.The self-healing mechanism,in which the sulfur vacancies are healed spontaneously by the sulfur adatom clusters on the MoS2 surface through a PSS-induced hydrogenation process,is proposed and demonstrated systematically.The electron concentration of the self-healed MoS2 dramatically decreased by 643 times,leading to a work function enhancement of～150 meV.This strategy is employed to fabricate a high performance lateral monolayer MoS2 homojunction which presents a perfect rectifying behavior,excellent photoresponsivity of～308 mA/W and outstanding air-stability after two months.Unlike previous chemical doping,the lattice defect-induced local fields are eliminated during the process of the sulfur vacancy self-healing to largely improve the homojunction performance.The strong interlayer coupling vdW homojunctions are stacked by the two monolayer MoS2 with different electronic structures via defects self-healing.Compared with vdW heterostructures,this homojunction exhibits stronger interlayer coupling effect.Meanwhile,the strong interlayer coupling effect at the lattice-matched interface can greatly enhance the interlayer charge transfer efficiency and promote the emergence of the photovoltaic effect.The ultrafast interlayer charge transfer takes place within～447 fs,which is faster than those of most vdW heterostructures.Furthermore,the homojunction photodiode manifests outstanding rectifying behavior with an ideal factor of～1.6,perfect air-stability over 12 months,and high responsivity of～54.6 mA/W that greatly exceed those obtained for previous stacked or epitaxial heterobilayers.The observations compare the interlayer coupling differences between vdW heterostructures and vdW homojunctions,and provide the interlayer coupling damage mechanism about lattice mismatch for the preparation of high-performance 2D vdW structures.We report an opposite strategy to enhance the electronic properties of 2D materials by intentionally manipulating the defect concentration in monolayer 2D-TMDs.By systematically tailoring the sulfur vacancies in monolayer MoS2,we find an unusual mobility enhancement can be achieved when sulfur vacancies are increased the right amount.Temperature dependent studies reveal the transport in monolayer MoS2 follow a Mott variable range hopping model and a proper concentration of sulfur vacancies is critical for ensuring sufficient localized states,maximized the charge hopping probability,and thus optimized charge transport.We show,by precisely controlling the sulfur vacancies to 3.5%,a record high carrier mobility exceeding 110 cm2 V-1 s-1 is achieved in monolayer MoS2,which has further allowed us to construct high performance logic inverter with a record high voltage gain exceeding 100.Lastly,we further show transport enhancement by defect engineering is general for monolayer 2D semiconductors and can be readily applied to other 2D-TMDs,including WS2,MoSe2,WSe2.