The Study Of Defect And Band Structure Engineering In Optoelectronic Devices Based On Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides(2D TMDCs)with have been considered as promising building blocks for next generation electronic and optoelectronic devices,including transistor,photodetector,light-emitting diode(LED)and so forth,owing to their relative high mobilities,on-off ratios and strong light-matter interaction.However,the properties of 2D materials and performances of devices built on TMDCs are generally inferior to theoretical prediction,e.g.low photoluminescence quantum yield and mobility in MoS₂.The discrepancies between theory and reality mainly arise from the inevitable intrinsic defects in 2D materials.The defects can act as undesirable carrier traps,or scattering center,or recombination centers and lead to a significant impact on the optical,electronic and optoelectronic properties of TMDCs.On the other hand,the properties of TMDCs can be easily modulated by defect engineering.The rational design of defects is able to induce effective doping,tailor the band structures and carrier transport,based on which high performance devices can be constructed.Here,defect engineering by chemical,ozone and plasma treatment are studied,which not only improve the performance of devices based on TMDCs,but also reinforce the understanding of defects.The main contents are summarized as follows:1.ReS₂ photoconductors with high sensitivity and fast response were achieved by molecule decoration.The existence of sulfur vacancies(SVs)in as-prepared ReS₂ was confirmed by high-resolution transmission electron microscopy(HR-TEM).Density functional theory(DFT)calculations showed that SV would introduce several localized states within the bandgap of ReS₂.These localized states contain large amount of deep traps,which greatly prolong the response time of ReS₂.Protoporphyrin(H₂PP)molecules were utilized to passive the SVs.After the removal of most SVs,the shallow traps started to dominate the photoconductivity decay of the device.As a result,decay time was improved by 3-4 orders of magnitude and photoconductivity gain as high as 10⁴ was achieved at ultralow laser power(5 pW).Meanwhile,the specific detectivity of the phototransitor was greatly enhanced(as high as?1.89×10¹³ Jones)due to reduction of dark current through charge transfer between ReS₂ and molecules.2.Near-infrared(NIR)photodetectors based on bilayer WSe₂ were prepared by ultraviolet(UV)ozone exposure.HR-TEM results confirmed the existence of oxygen substitutions.The upper layer of WSe₂ turned into WSeO,WO₂ and amorphous WO_x after UV ozone exposure,and dominated by WSeO.The electronic band of WSe₂-WSeO was calculated by DFT and the bandgap is reduced to0.89 eV.Photodetector based on oxidized bilayer WSe₂ showed photoresponse up to 1500 nm.Meanwhile,the photoresponsivity of device in visible region was improved by 2-3 orders of magnitudes with slight reduction in response speed.3.Phase transition in 2H MoTe₂ was achieved by soft hydrogen plasma treatment,as revealed by Raman spectra and mapping studies,X-ray photoelectron spectroscopy(XPS)analysis and FET device measurements at varying temperatures.The phase transition can be attributed to the warping of Te-Mo bonds and the lateral sliding of the top Te-layer induced by the soft hydrogen ion bombardment.In addition,the hydrogen ions can also induce phase transition to underlying layers via penetrating the surface layer while oxygen and argon plasma can't induce phase transition.Meanwhile,such phase transition can be readily reversed by post annealing at 300?.