The Investigation Of Photocarrier Dynamics Of Heterostructures Based On Bismuth Oxyselenide

Recently,atomically thin bismuth oxyselenide（Bi₂O₂Se）exhibits attractive properties for electronic and optoelectronic applications,such as high charge-carrier mobility,good air stability and narrow bandgap.The development of Bi₂O₂Se-based heterostructures have attracted enormous interests with prospects in compensating the shortcomings and extending the properties of logic devices.Although the electrical properties of Bi₂O₂Se and Bi₂O₂Se-based heterostructures have been widely studies,the photocarrier dynamics on an ultrafast time scale in these systems remain elusive,despite its importance in exploring the limitation of device performance from the intrinsic properties of the material.In this thesis,Bi₂O₂Se,as a typical non-van der Waal layered material,is selected as the research object,a comprehensive experimental study on optical properties and photocarrier dynamics in its monolayer and nanoplates is presented.On this basis,the main transport forms and interlayer charge transfer properties of non-van der Waals interlayer coupling formed by Bi₂O₂Se and representative materials in transition metal dichalcogenides（namely,WS₂/Bi₂O₂Se and MoS₂/Bi₂O₂Se）are revealed.The main innovative results are as follows:1.In 21 layers Bi₂O₂Se nanoplates,a direct optical transition near 1.72 eV is identified by optical transmission and transient absorption spectroscopic measurements for the first time,which is attributed to the transition between the valence and conduction bands in theΓvalley.Time-resolved differential reflection measurements reveal ultrafast carrier thermalization and energy relaxation processes and a photocarrier recombination lifetime of about 200 ps.Furthermore,by spatially resolving the differential reflection signal,a photocarrier diffusion coefficient of about 4.8 cm² s^-1 is obtained,corresponding to a mobility of about 180 cm² V^-1 s^-1.A similar direct transition is also observed in monolayer Bi₂O₂Se,suggesting that the states in theΓvalley do not change significantly with the thickness.The temporal dynamics of the excitons in the monolayer is quite different from the nanoplates,with a strong saturation effect and fast exciton-exciton annihilation at high densities.The differences in the carrier dynamics in monolayer and nanoplate samples could be attributed to their different electronic structures and the reduced dielectric screening in monolayers.Spatially and temporally resolved measurements yield an exciton diffusion coefficient of about 20 cm² s^-1.These results provide basic information that is useful for understanding,designing,and optimizing electronic and optoelectronic devices based on ultrathin Bi₂O₂Se.2.A comprehensive experimental investigation on the interlayer charge transfer properties of the heterostructure formed by Bi₂O₂Se and WS₂ monolayers is for the first time reported.Kelvin probe force microscopy（KPFM）is used to measure the work functions of the samples,which are further employed to establish type-II band alignment of WS₂/Bi₂O₂Se heterostructures.Photoluminescence（PL）quenching of about 20 times of WS₂ at 620 nm is observed in the heterostructure,suggesting high charge transfer efficiency.Time-resolved and layer-selective pump-probe measurements further prove the ultrashort charge transfer time（0.45 ps）and nearly 100%of the holes excited in the Bi₂O₂Se layer that participate in the transfer process,resulting in the formation of long-lived interlayer excitons.These results establish the feasibility of integrating non-van-der-Waals materials with van der Waals ones to fabricate heterostructures with novel charge transfer properties and provide insight for understanding the performance of optoelectronic devices based on such 2D heterostructures.3.The high dark current caused by the high carrier concentration has become an obstacle for further development of Bi₂O₂Se-based devices,toward this end,the MoS₂/Bi₂O₂Se with typeⅡband alignment is designed and constructed to regulate carrier concentration in the Bi₂O₂Se layer.The MoS₂/Bi₂O₂Se forms the type-Ⅱband alignment heterojunction is verified by KPFM,with the conduction band minimum and valence band maximum located in the MoS₂and Bi₂O₂Se layers,respectively.The A excitonic emission peak of MoS₂in the heterostructure at 659 nm exhibits significant PL quenching with 25 times quenching factor,indicating high charge transfer efficiency.The near-unity of the electrons excited in the Bi₂O₂Se layer of the MoS₂/Bi₂O₂Se heterojunctions can be transferred to the MoS₂ layer in the ultrafast time scale of 0.51 ps,companying with the formation of long-lived interlayer excitons,is further revealed by the time-resolved and layer-selective pump-probe measurements.The results suggest that the MoS₂/Bi₂O₂Se heterostructure shows promising potential to realize low power consumption and fast optical response devices through reducing the high carrier concentration of the Bi₂O₂Se.