The Study Of Modulating Optoelectronic Properties Of Monolayer WS₂

Monolayer tungsten disulfide(WS2)is an important two-dimensional transition metal sulfide(TMDs)with semiconductor properties,which has become a research hotspot because of its width forbidden band and highly optoelectronic conversion efficiency.In this paper,the monolayer WS2,prepared by molten-salt-assisted vapor transportation method,is taken as the research object to explore the modulation effects of defects and alloying factors on its optoelectronic properties.In the process of vapor transportation method,the microstructure and physical properties of WS2 are easily affected by defects.The energy band gap is strongly modulated and defect levels are generated.The non-uniform distribution of defects leads to strong non-uniformity in photoluminescence transmission and optoelectronic properties.In this paper,tip enhance photoluminescence(TEPL)combined with photosensitive Kelvin potential force microscopy(KPFM)was used to characterize the exciton distribution and surface photo-sensitivie potential variation of high defective monolayer WS2 in nanoscale.The contact potential(CPD)of defect region presents weak variation without light,but strong non-uniformity on the different excitation wavelength and laser power.TEPL measurement results show that,the strength of trion excitons and neutral excitons is strongly modulated by defects,and changes rapidly on the scale of several hundred nanometers,and the strength distribution is closely related to CPD.TEPL measurement reveals the trion and neutral exciton intensity are strongly mediated by the defect and vary quickly in scale of few hundreds of nanometers,and the intensity distributions of trion and neutral excitons strongly depend on the CPD.In addition,the alloying effect of WS2 can prepare two-dimensional alloy samples with tunable bandgap,exciton photoluminescence energy and optoelectronic properties.In this paper,monolayer alloy samples with different selenide components(x=0,0.1,0.2,0.5,0.7 and 1)were prepared by molten-salt-assisted vapor transportation method.With the increase of selenium content,the A exciton luminescence energy of the monolayer alloy sample changs significantly from 1.95 eV to 1.6 eV.The Raman resonance characteristics of these single-layer WS2(1-x)Se2x alloys are strongly modulated by stoichiometry,and the intensity and width of the main peak of Raman are closely related to the stoichiometric and excitation laser wavelength.The Raman intensity resonances of WS2-like mode(A'w-s)and WSe2-like mode(E'w-se)modes of the alloy samples is similar to A' and E' of the single-layer WS2 and WSe2 modes,respectively,but the resonance curve is redshifted by modulation of exciton energy.