All-optical Reversible Manipulation Of Exciton And Trion Emissions In Monolayer WS₂

Two-dimensional transition metal sulfides（TMDs,such as Mo S2,WS₂,etc.）because of their novel physical and chemical characteristics,such as significant quantum-confinement effect,stable intralayer excitons at room temperature,and tunable interlayer interaction,high current ON/OFF ratio,excellent valley optical characteristics,etc.,not only provides a new platform for the study of basic physical effects,but also shows extremely attractive potential application of new optoelectronic devices.In terms of basic physics,the novel Moire superlattice and exciton Hall effect have been observed in single-layer TMDs,and the regulation of multibody interactions（charged excitons,double excitons,etc.）has been achieved.In terms of applications,FET based on TMDs,flexible electronic devices,micro lasers,light detectors,etc.have been constructed;These devices exhibit comparable or better performance than traditional devices.In order to further improve the performance of these devices,it is particularly important to manipulate their photoelectric characteristics,especially exciton radiation characteristics.Currently reported manipulate means include chemical/defect doping,electric field,and molecular adsorption.Among them,chemical and defect doping are generally irreversible and will permanently damage the sample;and electric field manipulation and molecular adsorption can reversibly manipulate TMDs,which has more important practical significance.However,electric field manipulation requires the preparation of micro-nano electrodes,and molecular adsorption requires vacuum equipment,which brings certain limitations to practical applications.In particular,both of these methods are global manipulation of the whole sample（at least the electrode region）and lack of spatial resolution.In order to simplify the manipulation equipment and improve the spatial resolution of manipulation,this paper uses the different laser power to promote the adsorption and desorption process according to the dependence of gas adsorption on the surface temperature of the sample.and realizes the all-optical reversible manipulation of single-layer WS₂ exciton radiation.The manipulation process only needs to change the laser power,which is convenient to manipulate and the spatial resolution reaches micron level.The main contents of paper are as follows:1.According to the thermal motion theory of gas molecules,it is assumed that the desorption activation energy of molecules on the single-layer TMDs surface is lower than the adsorption activation energy;It is proposed that the laser-induced thermal effect is used to change the adsorption effect of gas molecules on the monolayer TMDs.When the activation energy provided by the thermal effect is lower than the desorption activation energy,the adsorption effect is dominant on the monolayer TMDs surface.At this time,free electrons on the surface of the single-layer TMDs are transferred to the adsorption molecule,which changes its exciton radiation characteristics.When the activation energy provided by the thermal effect is higher than the desorption activation energy,the surface of the single-layer TMDs is dominated by the desorption effect,and the electrons return to the surface of the single-layer TMDs.2.In the experiment,we prepared a single layer WS₂ with a certain surface charge density by chemical vapor deposition method,so that the photoluminescence spectrum of WS₂ has significant exciton and charged exciton characteristics.When the laser power is more than 500 k W/cm2,the photoluminescence intensity is gradually quenched,the exciton ratio decreases,and the charged exciton ratio increases,indicating that the surface charge density of WS₂ increases,and molecules are gradually desorbed from WS₂ surface.The laser power was reduced to less than 30 k W/cm2,and the quenched photoluminescence gradually recovered.At this time,the exciton ratio increased and the charged exciton ratio decreased,indicating that the surface charge density of WS₂ decreased and the molecules gradually adsorbed on the WS₂ surface.Further research shows that continuous switching of laser power can achieve reversible manipulation of the photoluminescence intensity of single layer WS₂.This experimental phenomenon verifies our theoretical model.Based on this all-optical reversible manipulation,we propose a reversible optical storage scheme with spatial resolution up to the order of micron.3.Our theoretical model provides a simple and convenient all-optical reversible manipulation method,which provides a new method for optimizing the performance of WS₂-based photoelectric devices.In order to improve the manipulation speed,we studied the effect of femtosecond laser irradiation on single layer WS₂,and found that the photoluminescence intensity increased by more than two orders of magnitude,and the manipulation speed was only in the order of second.