Interlayer Charge Transfer Excitons Dynamics In Few-layer Van Der Waals Heterostructures Based On Tungsten Disulfide

Two-dimensional transition metal dichalcogenides（TMDs）,with their unique intralayer sandwich lattice structure,thickness-dependent band gap structure,excellent optoelectronic properties and strong spin-orbit coupling,have attracted great attention in basic research and optoelectronics,and are considered to be the most promising two-dimensional semiconductor materials.Van der Waals heterostructures（vd W）constructed of two-dimensional materials can achieve performance beyond that of a single material and have broad application prospects in the field of optoelectronic devices,driving the vigorous development of next-generation optoelectronic devices.Although research on monolayer TMDs and their van der Waals heterostructure has made impressive progress in frontier areas such as transistors,photodetection,photovoltaics and sensing,studies have shown that few-layer TMDs have the advantages of high light absorption efficiency,good environmental stability and easy preparation on large areas,which are more practical than monolayer TMDs devices.However,there are few in-depth studies on few-layer TMDs heterostructure,especially the lack of sufficient understanding of the interlayer ultrafast photocarrier carrier processes,which are crucial for the design and application of optoelectronic devices.In order to deepen the understanding of the photophysical processes at the interface of few-layer van der Waals heterostructure and essentially explore the application prospects of few-layer TMDs heterostructure in optoelectronic devices,this paper designs several TMDs heterostructure for the core problems of interlayer exciton transport,layer thickness effect and bandgap hybridization,and reveals the dynamics of interlayer charge transfer excitons in few-layer heterostructure using transient spectroscopy:I.The trilayer heterostructure（Mo S₂/WS₂/Mo Se₂）with a ladder band aligments was systematically investigated using a high spatial and temporal resolution pump-probe technique.Based on the understanding of the ultrafast charge separation between layers,the formation of cross-layer excitons and the dynamics of long composite lifetime interlayer excitons on the time scale,the transverse nonlinear diffusion behavior of thermal excitons in this heterostructure is observed for the first time.Based on the classical diffusion theory and Einstein’s relation,the time-containing diffusion model of thermal excitons is deduced and agrees well with the experimental results.The reason for the nonlinear variation of the diffusion coefficient is revealed to be the presence of the intermediate layer WS₂ restoring the shielding effect of the dielectric,which in turn reduces carrier-carrier and carrier-phonon scattering.The initial exciton diffusion coefficient of up to 24.7 cm² s^-1,the relaxation time of about 97 ps of hot excitons and the exciton complex lifetime of up to 200 ps in the heterostructure are significantly better than that of the monolayer structure.The additional energy relaxation process in the layer less heterostructure system makes it possible to exploit the thermal exciton effect to improve the response time of optoelectronic devices and enhance the overall device efficiency.II.Charge transfer and separation are the main features of type-II heterostructure.The ultra-fast interlayer charge transfer between monolayer heterostructure has been widely demonstrated,while the charge transfer between few-layer heterostructure and its transfer rate as affected by the layer thickness are still lacking in understanding.To address this issue,we explored the variation of charge transfer rate with WSe₂ with layer thickness in WSe₂/WS₂heterostructure under type II band alignment,and demonstrated that compared with monolayer WSe₂,few-layer WSe₂ heterostructures reduce the interlayer charge transfer rate and extend the interlayer exciton lifetime to more than twice.As the layer thickness increases,the interlayer coupling changes,which leads to a change in the position of the bottom of the conduction band and the top of the valence band,which in turn affects the path of charge transfer and exciton complexation,so the charge transfer becomes slower and the exciton lifetime is prolonged.These results help to understand the modulation of charge separation and lifetime in heterostructures by layer thickness.Few-layer heterostructures reduce the charge transfer rate and prolong the exciton lifetime,allowing better conversion and utilization of excitons and charges,which is of importance in optoelectronic devices.III.The band hybridization is a special band alignment,especially the conduction band minimum hybridization,which is of special significance for optoelectronic devices.For typical TMDs materials,the band gap decreases as the number of layers increases,while the bottom of the conduction band gradually approaches.In this case,the band hybridization becomes the main band alignment structure of the few-layer TMDs heterostructures.For this purpose,trilayer Mo Se₂ and trilayer WS₂ heterostructures were constructed,and a complete physical picture of post-excitation charge transfer and photocarrier dynamics was comprehensively given using three different pump-probe configurations.The simultaneous bidirectional transfer process of electrons between heterostructure interfaces and the formation of quasi-equilibrium state electron distribution at the bottom of the conduction band are demonstrated.Further analysis shows that the band hybridization can be realized in the T valley in the few-layer indirect band gap semiconductor heterostructure,and this distribution can mediate the rapid compounding of electrons on both sides of the heterostructure.The special band structure formed by the hybridization of the conduction band bottom in the few-layer heterostructure can be used as a band engineering for bridging type-I and type-II heterostructures,which is important for the application fields such as excitonic devices and optoelectronic devices.