Photocarrier Dynamics In Transition Metal Dichalcogenides And Their Heterostructures Studied By Ultrafast Laser Spectroscopy

Layered materials,which are represented by the transition metal dichalcogenides,are very convenient to be fabricated due to their unique interlayer van der Waals force.They have been widely used in photodetector,photovoltaic and light-emit devices due to their outstanding optical absorption.Photocarriers in transition metal dichalcogenides play a impotanat role in the properties of optoelectronic devices.Therefore,the study of their dynamic is of great significance.In this dissertation,the transient absorption measurement is performed on molybdenum disulfide,molybdenum selenide and graphene heterostructure by the using ultrafast laser pump-probe technique.The transfer process between layers is detected,which provide important theoretical basis on photoelectric device base on this heterostructure.In addition,The mechanism for interband absorption has its inherent limitations.Here,a new probing mechanism named exciton intraband absorption is provieded,which is used to perform ultrafast laser measurement on MoS2 and WS2,the absorption cross section of these materials are calculated,and the problems in interband absorption mechanism are avioded.The contents of as follows:1.Dynamic of carriers in monolayer molybdenum disulfide,molybdenum diselenide,bulk material molybdenum disulfide and their heterojunction with graphene have been studied by using time-resolved transient absorption spectroscopy.The sample are fabricatied by exfoliation and a dry transfer process.The high efficiency electron transfer process from monolayer molybdenum disulfide and molybdenum diselenide to graphene layer was detected.However,in the heterojunction formed between graphene and molybdenum disulfide,the interlaminar charge transfer efficiency is low.It is found that the carrier of graphene can regulate the absorption coefficient of molybdenum disulfide and molybdenum selenide layer.This method can be used to realize the coupling and control between heterojunction layers.These results provide valuable information for the development of a new generation of high-performance optoelectronic devices based on graphene and molybdenum disulfide or molybdenum diselenide heterojunction.2.The exciton dynamics of molybdenum disulfide and tungsten disulfide are studied by ultrafast laser measurement based on exciton intraband absorption.It is found that the absorption coefficient of exciton is proportional to the exciton density.unlike the interband absorption mechanism,the saturated absorption is not present in the intraband absorption.However,exciton-exciton annihilation will occur when the pump fluence goes too high.In addition,The exciton intraband absorption mechanism reduces the limitation on the probe wavelength and then widens the application of ultrafast laser measurement method.Finally,the absorption cross sections of the two materials are calculated by the result of the differential reflectance signal.