Probing Optical Properties Of CVD Grown Monolayer WS₂ And MoS₂-WS₂ Heterojunction

Since the discovery of graphene and transition metal dichalcogenides?TMDCs?,their unique structure has provided with excellent optical,electrical,and thermal properties,which stimulated the research and received extensive attention.Heterojunctions are the cornerstones of the modern electronics and optoelectronics industry.Due to the rich physical properties of TMDs,the performance of heterojunctions can be controlled by tuning the stacking materials.However,there was no detailed reports about the physical mechanism of exciton transfer process at the interface of heterojunctions.Therefore,studying the optical properties of heterojunctions is vital for research and application.In order to study the exciton transfer in heterojunctions,optical properties of the monolayer WS₂ and atomic layer thickness MoS₂-WS₂lateral heterojunctions grown by chemical vapor deposition?CVD?were studied by optical methods such as Raman,photoluminescence?PL?and absorption spectroscopy.The main research contents are as follows:1.Probing the optical properties of the monolayer WS₂ by optical methods.Monolayer WS₂ with a size of 30?m was synthesized by CVD.We found that the monolayer WS₂ grown by CVD has an extraordinary PL phenomenon in PL mapping at the room temperature.In details,the edge area shows higher PL intensity with peak energy red-shift,and the center area shows lower PL intensity with peak energy blue-shift.Furthermore,it could be found from Raman spectroscopy that the local strain and the electron concentration are the reasons of the extraordinary PL phenomenon.In addition,the relationship between the Stokes shift and surface electron concentration was studied systematically by absorption spectroscopy.2.Probing the optical properties of the MoS₂-WS₂ lateral heterojunctions.In order to decrease the cost of inexpensive metallic molybdenum sheets were used as precursors to prepare MoS₂-WS₂ heterojunctions.According to the analysis of Raman spectroscopy,the E¹_12g2g vibration of MoS₂ at the heterojunction interface shows slight red-shift which indicates the strain in the interface of the heterojunction.In addition,there is no prominent change in the other MoS₂ region,indicating the other region of MoS₂ is strain-free.In PL mapping,the intensity of MoS₂ quenches near the heterojunction interface,and the fluorescence intensity of MoS₂ gradually increases when moving away from the heterojunction interface until arriving a certain distance.The PL intensity quenching is attributed to the charge separation effect of the heterojunction.The heterojunction was further characterized by spatially resolved absorption spectroscopy.It could be found that the linewidth of the A exciton absorption peak of MoS₂ is broadened near the heterojunction.3.Probing the electrical properties of lateral MoS₂-WS₂ heterojunctions.The built-in potential of the lateral MoS₂-WS₂ heterojunction grown by CVD was studied by Kelvin potential microscope.