| Valleytronics can encode,process and store information by manipulating valley degrees of freedom of the Bloch electron,which has the advantages of high integration,fast processing speed,low information loss and low energy consumption,and can meet the high demands of sustainable device development.Therefore,valleytronics has received much attention in recent years,making valleytronic materials,valley degree of freedom manipulation,and valley transistor devices become current research hotspots.Monolayer transition metal dichalcogenide(TMD)possesses the unique spin-valley locking effect that allow its valley degree of freedom to be excited and probed by circularly polarized light,making it the optimal material system for studying valleytronics.However,due to the strong Coulomb interaction in monolayer TMD,the valley depolarization time is on the picosecond scale,which limits the application of monolayer TMD in valleytronics to ultrafast time scales.In contrast,interlayer excitons in TMD van der Waals heterostructures are spatially indirectly separated and thus have long valley polarization lifetimes,making them good candidates for exploring valleytronic devices.However,the emission intensity of interlayer excitons in TMD heterostructures is highly dependent on the twist angle,and it usually requires the resonant circularly polarized light excitation or external electric and magnetic fields to achieve effective valley polarization control,which limits the practical development of valley-polarized exciton devices.Based on this,this thesis proposes to construct chiral two-dimensional(2D)perovskite/monolayer TMD van der Waals heterostructures to realize strong interlayer coupling without considering the twist angle.Chiral 2D perovskites with high spin polarization were used to effectively manipulate the valley degrees of freedom of intralayer excitons and interlayer excitons under non-resonant linearly polarized light excitation.In addition,the exciton emission type of 2D perovskite/monolayer TMD heterostructures can be controlled by electrical means to enable highly tunable valley polarization switching and exciton diffusion modulation.The specific research content includes the following parts:(1)The valley polarization manipulation of intralayer excitons in monolayer TMD by chiral 2D perovskite.High-quality chiral 2D perovskite single crystals were synthesized by solution methods and their circularly polarized emission was characterized.Type II chiral 2D perovskite/monolayer TMD heterostructures were prepared by mechanical exfoliation and dry transfer method,and the selective spin injection from chiral 2D perovskite to monolayer TMD was analyzed by low-temperature,temperature-and power-dependent circularly polarized photoluminescence(PL)spectra.The valley polarization of intralayer excitons in monolayer TMD was effectively manipulated under non-resonant linearly polarized excitation without external field,where the average degree of valley polarization was about 10% at 78 K,and the average spin injection efficiency can reach 78%.(2)The valley polarization control of interlayer excitons in chiral 2D perovskite/monolayer TMD heterostructures.The optical properties of interlayer excitons in different types of heterostructures composed of different 2D perovskites and TMD monolayers were systematically analyzed by temperature-,power-,and electric-field-dependent PL spectra and time-resolved PL spectra.Based on this,the valley polarization of interlayer excitons in chiral2 D perovskite/monolayer TMD heterostructures was manipulated by highly spin-polarized chiral 2D perovskites.The circularly polarized emission was only related to the chirality of chiral 2D perovskites and was independent of the polarization of the excitation light.(3)Electrical control of exciton states and valley polarization in 2D perovskite/monolayer TMD heterostructures.The transition from type II to type I heterostructure was achieved by electrically tunning the band alignment of the heterostructure,where the PL spectra changed from charged interlayer excitons emission to charged intralayer excitons emission.By effectively electrically switching between the two exciton states,on the one hand,the degree of valley polarization of the device was manipulated to enhance the valley contrast and achieve a highly stable and reversible valley polarization switch device with a maximum on/off ratio of 15.8;on the other hand,the exciton diffusion and lifetime were also effectively controlled,where the diffusion length can be adjusted in the range of 1.75～2.89 μm and the exciton lifetime was adjustable in the range of 0.10～3.46 ns. |
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