Interfacial Charge/Energy Transfer Dynamics In 2D Van Der Waals Heterostructures

Two-dimensional（2D）materials,represented by graphene,transition metal dichalcogenides（TMDs）,have unique and interesting physical,chemical and mechanical properties.2D van der Waals（vd W）heterostructures,which can integrate the advantages of various 2D materials,have been found of great potential in advanced optoelectronic,photonic,valleytronic and photo/electrocatalytic applications.As a fundamental step,the dynamics of charge/energy transfer at the van der Waals heterojunction interface determines the performance of 2D optoelectronic devices,valleytronic and photo/electrocatalytic devices,and has attracted extensive research interest.This paper achieved a comprehensive understanding of charge/energy transfer mechanisms in 2D vd W heterostructures and developed methods to modulate the dynamics of excited states by barrier layers.These greatly make up for the deficiencies in previous studies and paint a clear physical picture for the excited state dynamics of 2D vd W heterostructures,and are of great guiding significance for optimizing the performance of optoelectronic,valleytronic and optoelectronic devices based on 2D van der Waals heterostructures and the promoting of their practical applications.This paper is mainly divided into three parts:the first part is the second chapter,which introduces the experimental methods and test principles;the second part includes the third and fourth chapters,mainly focusing on the charge transfer dynamics in the type II van der Waals heterostructures;The third part includes the fifth and sixth chapter,which discuss the energy transfer mechanism.The third chapter explores the charge transfer mechanism at the interface of WSe₂/WS₂type-II vd W heterostructures.The experimental results of transient spectroscopy show that the electron transfer process between the two layers is robust,on an ultrafast time scale（30femtoseconds）,and is not affected by the external dielectric environment.Combined with the temperature-independent ultrafast charge transfer process,we suggest that there is a sufficiently strong electronic coupling phenomenon in the heterostructures that the electron transfer process occurs adiabatically on ultrafast timescales.Through the research in the third chapter,we found that external factors cannot affect the charge transfer process of type-II vd W heterostructures.In the fourth chapter,we insert boron nitride（BN）in the middle of WSe₂/WS₂ to precisely control the spacing between WSe₂and WS₂.By transient absorption spectroscopy,we found that the interfacial electron and hole transfer rates（k（d））exhibit an exponential decay trend with increasing BN thickness（d）,which is consistent with the quantum tunneling model.Not only that,we also found that the BN barrier layer can effectively prolong the lifetime of the interlayer excitons of the vd W heterostructures.This study provides a general approach to modulate the charge transfer dynamics and recombination lifetime of 2D vd W heterostructures with atomic precision.Finally,we investigate the energy transfer mechanism in 2D vd W heterostructures in the fifth and sixth chapters.The fifth chapter explores the TMD/Gr heterostructures,which has great potential for optoelectronic applications.After photoexcitation of the TMD layer,both electron/hole and exciton transfer from TMD to graphene can occur.The traditional transient absorption spectroscopy can only measure the population but cannot distinguish the species of the carriers,so the accurate energy transfer mechanism cannot be obtained.The unique spin-valley coupling of monolayer TMD endows the carriers with a new spectrally detectable spin-valley degree of freedom.Helicity-resolved transient spectroscopy,combining population and polarization of photoexcited carriers,offers a new opportunity to probe the dynamics of specific carriers（electrons or holes or excitons）in real time.We found that in TMD/Gr heterostructures,electrons and holes are transferred into graphene by asymmetric charge transfer mechanism（electron and hole transfer with different rates）,rather than the F?rster and Dexter energy transfer mechanism.In the fifth chapter,we described the great role and prospect of helicity-resolved transient absorption spectroscopy in the study of interfacial charge/energy transfer mechanisms,which could be introducing to type-I vd W heterostructures（WS₂/Mo Te₂）.We found the transfer mechanism is also asymmetric charge transfer rather than F?rster and Dexter energy transfer.Then,we introduced a WSe₂ layer to control the position of holes and found transfer of electrons from WS₂ to Mo Te₂ closely follows the dynamics of holes,which are coupled together by the dragging effect of the Coulomb force.The position of holes determines the fate of electrons,which explains the two-step charge transfer mechanism in WS₂/Mo Te₂.This study reveals the influence of Coulomb interaction on excited state dynamics,demonstrating the spatiotemporally coupled electron-hole dynamics.