Ultrafast Dynamics And Carriers Distribution Modulation Of Two-Dimensional Materials And Their Heterostructures

Two-dimensional materials,such as transition metal dichalcogenides（TMD）,have attracted extensive attention in recent years.The unique excitonic effect and high optical quantum yield bring them great potential in optical and optoelectronic applications.However,the performance of optical and electronic devices based on two-dimensional materials is generally inferior to theoretical prediction.First of all,the quantum yield of individual two-dimensional material decreases rapidly under high power.Moreover,lots of defects would induce non-radiative recombination channels.This non-radiative decay would reduce the photoluminescence（PL）and affect the carrier transport.In addition,heterostructures based on two-dimensional materials are also beneficial to functional devices such as photovoltaic devices.Charge transfer process determines the optical and optoelectronic properties of devices based on heterostructures.But until now,the understanding of the charge transfer mechanism and the modulation of carrier distribution are still lacking.Thus,the optimized properties of those devices based on TMD heterostructures have not been obtained.Here,we study the recombination and transfer behavior of carriers in two-dimensional materials and their heterostructures by ultrafast dynamics.Monolayer MoS₂is modified by oxygen plasma and Janus is composited to modify the exciton recombination in TMD.Moreover,TMD layers,laser and Janus build-in electric field are introduced to modify the charge transfer process in heterostructures.The main research contents are summarized as follows:1.Monolayer MoS₂ is modified by oxygen plasma to enhance quantum yield under a high incident power.It is found that the fundamental reason for the rapid decline of quantum yield at high exciton generation rate（G）is that the non-radiative recombination rate is much faster than the radiative recombination of MoS₂.Therefore,an effective way to increase quantum yield is to accelerate the radiation recombination.With oxygen plasma,besides the defects passivation in the MoS₂monolayer,a flat molecular state at the top of the valence band is introduced.The（PL）enhancement of MoS₂is more than 35 times at G～10¹⁹cm^-2s-1.Transient PL（TRPL）analysis and first-principles calculation（DFT）analysis confirm that oxygen doping introduces the molecular state.This molecular state can accelerate exciton radiative recombination,decrease exciton-annihilation and improve the quantum yield of MoS₂.2.Janus is composited and its exciton dynamics is studied.The Janus structure is formed by replacing the top S atoms with Se atoms in MoS₂or WS₂monolayer.Due to the different electronegativity of Se and S,there is a build-in dipole moment in Janus.The pump-probe technique is used to study the ultrafast carrier dynamics in Janus.Comparing with the pristine TMD,exciton-phonon interaction is stronger in Janus due to the presence of the built-in electric field.And this enhanced exciton-phonon interaction would accelerated the exciton formation process in Janus.Moreover,the built-in electric field can also separate the wave functions of electrons and holes in Janus,and thus prolong its radiation recombination lifetime.This extension of radiation lifetime is beneficial to improve the performance of photoelectric and photoelectric devices.3.The charge transfer process in the heterostructures can be modified with different layers of TMD.The 1L-MoSe₂/NL-MoS₂heterostructures are constructed and studied by Pump-Probe.It is found that charge transfer time can be tuned from 0.2 ps to several ps by changing the number of TMD layers.And the lifetime of interlayer excitons can also be changed.Because the exciton-phonon interaction decreases with the number of TMD layers,the charge transfer and interlayer exciton recombination time are affected by the number of TMD layers.4.Charge transfer process in heterostructures is modified by pre-adding laser.The pre-added laser separates electrons and holes at the heterojunction interface,which forms an electric field and affects the subsequent charge transfer process.Pump-Probe results show that the charge transfer time is reduced by two times and the signal peak intensity is reduced by about 20%with various power of the pre-added laser.5.The charge transfer process in Janus/TMD is modified by Janus built-in electric field.For carrier transfer from Janus to regular TMD,the Janus field allows（blocks）the transfer with the current direction along the same（opposite）direction of the field.For carrier transfer from regular to Janus TMD,Janus field allows charge transfer in both directions.The results allow Janus to be a charge blocking part in optoelectronic devices.