Ultrafast Spectroscopic Study On Photophysical Properties Of Two-dimensional Semiconductor Heterostructures

As we enter a new era of big data and artificial intelligence,there is an increasing demand for increased computing power.Traditional silicon-based materials have become increasingly difficult to support the further development of integrated circuits.Two-dimensional(2D)materials are expected to break through the limitations of traditional chips and become a new generation of optoelectronic materials.The van der Waals heterostructure formed by different 2D materials can combine the excellent properties of each 2D material component,design and create more functional 2D structures,and provide more possibilities for the preparation of new nano-optoelectronic devices.In recent years,studies based on 2D heterostructures are flourishing,such as gate-tuned Mott insulators,superconducting effects,Moiréinterlayer excitons and other physical phenomena,as well as near-infrared photoelectric detectors,interlayer exciton lasers,new spintronic devices based on van der Waals heterostructures,etc.Although significant progress has been made in the research of2D materials and their heterostructures,there are still many unresolved problems in charge transfer and valley spin dynamics.The first problem is the factors that affect the charge transfer of the 2D heterostructure.Studies have shown that the charge transfer rate of a 2D heterostructure is robust.The only way to modulate the charge transfer rate of a heterostructure is to change their spatial distance so far.However,it is very difficult to control the atomic-scale spatial distance with the current technologies.Therefore,it is crucial to find an alternative method.Secondly,the charge transfer mechanism of the2D heterostructure is still controversial,and the classical Marcus charge transfer model is no longer applicable to the 2D heterostructure system.A complete explanation of charge transfer mechanism in 2D heterostructure is needed for optimizing the design and development of devices based on the heterostructure.In addition,the spin-valley dynamics in heterostructures need to be further studied,and the factors that affect the depolarization of the spin-valley of the heterostructure and the process of spin-valley depolarization are still unclear.Solving the above problems has a very important scientific significance for us to better understand 2D materials and related heterostructures,and it is also the basis for the development of various optoelectronic devices based on 2D materials and related heterostructures.In this paper,we designed heterostructures with different band structures around the two themes of charge transfer and spin-valley depolarization in van der Waals heterostructures based on TMDs.We explored the effects of strain and the number of layers on the charge transfer rate,the process of charge transfer and interlayer exciton transfer are studied in detail,and the spin-valley depolarization process of different heterostructures are well-studied by broadband ultrafast spectroscopy.(1)Study on the dynamics of charge transfer in 2D heterostructures under strainWe systematically investigate the transient absorption spectra and dynamics of exciton in monolayer MoS₂,monolayer WSe₂ and MoS₂-WSe₂ heterostructure under strain.When the mechanical tensile strain applied to the monolayer MoS₂ and WSe₂increases from 0%to 1.2%,the bandgap at K point shrinks 41 me V and 16 me V for monolayers of MoS₂ and WSe₂,respectively,according to the redshift of A-exciton bleaching peak measured by broadband transient absorption spectroscopy.Furthermore,the bandgaps at K point for MoS₂ and WSe₂ in the MoS₂-WSe₂ heterostructure shrink about 26 me V and 24 me V due to the mismatched strain response for the excitons in monolayer MoS₂ and WSe₂,respectively.It is found that the band offset change for MoS₂-WSe₂ heterostructures under tensile strain have little effect on the band edge charge transfer processes.From the viewpoint of optoelectronic applications,robust rate of charge transfer under strain engineering in TMD heterostructures is very beneficial for the performance of flexible devices based on monolayer TMDs and their composites.(2)Study on the dynamics of layer-dependent charge transfer in 2D heterostructuresThe charge transfer and charge recombination processes in nLMoS₂/mLWSe₂heterostructures have been systematically investigated by broadband femtosecond pump-probe experiments.This study shows that in addition to controlling the charge transfer rate in TMD heterostructures by separating the donor and acceptor layers,there is another way to change the electron transfer and recombination rates by fabricating nLMoS₂/mLWSe₂ multilayer heterostructures.The rate of electron transfer has an asymmetric layer-dependent characteristic,especially the electron transfer and charge recombination rates of 1L MoS₂/2L WSe₂ heterostructure are 2.3 and 12 times that of1L MoS₂/1L WSe₂ heterostructure,respectively,which have been competitive with that in the 1L MoS₂/h BN/1L WSe₂ heterostructure.This could be due to the changes in interlayer interactions caused by changes in dielectric environment.From the perspective of device applications,well-designed 2D multilayer heterostructures provide an alternative way to modulate the electron transfer and recombination rates,which could be more beneficial for the charge extraction in 2D optoelectronic devices and the resulting improvement of device performance.(3)Study on exciton effect and charge transfer mechanism in 2D heterostructuresWe observed the band-edge exciton formation in monolayers MoS₂(WS₂)and MoS₂-WS₂,ultrafast dynamics of charge transfer process and interlayer exciton transfer in trilayer-WS₂-MoS₂-WSe₂ and trilayer-MoS₂-WSe₂-WS₂ heterostructure under band-edge excitation,respectively,by broadband pump-probe spectroscopy.We found that intra-band relaxation of carriers may occur in the form of exciton state.Besides,a two-step process of charge transfer was directly observed after exciting WSe₂ in trilayer-WS₂-MoS₂-WSe₂.The electrons in WSe₂ are initially transferred to the high lying electronic state of MoS₂-WS₂ on a time scale of tens femtoseconds,and then electrons eventually relax into the conduction band minimum of MoS₂ within 1 picosecond.In addition,the transfer of interlayer excitons was observed for the first time in trilayer-MoS₂-WSe₂-WS₂ heterostructure.Both transfer processes can be better understood by Dexter charge exchange model,while the exchange interaction is caused by the orbital overlap between the high energy levels of interlayer exciton and the low energy level of exciton of the charge donor in TMD heterostructures.Due to the nature of Dexter type transfer that the exchange rate exponentially depends on the donor-acceptor distance,the interlayer exciton transfer rate is nearly hundred times slower than that of exciton transition in bilayer HSs.Our results deepen understanding of charge transfer in 2D vd W HSs and also indicate that the exciton effect and orbital hybridization make HS as a strong coupling system.(4)Study on spin-valley depolarization of 2D heterostructuresWe measured the spin-valley depolarization process of electrons and holes in type-II MoS₂-WSe₂ 2MoS₂-WSe₂ and MoS₂-WS₂ heterostructures through valley-resolved broadband femtosecond pump probe experiments.The different depolarization paths between electrons and holes make them have different spin-valley polarization lifetimes.The valley polarization lifetimes of electrons and holes in the MoS₂-WSe₂ system at room temperature are 6 ps and 78 ps,respectively.The spin-valley depolarization path of the hole is mainly dominated by the phonon-assisted intervalley scattering process,while intravalley and intervalley coupling can trigger additional electron depolarization paths.In the trilayer 2MoS₂-WSe₂ heterostructure,the hole polarization lifetime can be further extended to more than 3 times.In comparison,for MoS₂-WS₂ with strong orbital hybridization of M and W atoms,both electrons and holes lose the spin-valley polarization extremely soon after charge separation,behaving similarly to intra-excitons in monolayer.Understanding this kind of short-range coupling mechanism holds potential significance for facilitating the effort towards longer lifetime valleytronic devices for information transfer and storage applications.