Interlayer Bandgap Modulation And Valley Polarization Research Of 2D-Layered Transition Metal Dichalcogenides

Research on graphene has opened the door to new two-dimensional layered semiconductor materials.Among them,transition metal dichalcogenides has layered crystal structure,ultra-thin physical size,continuously tunable band gap,novel electronic structure and valley spin,showing great scientific research value and application prospect.Meanwhile,different two-dimensional layer semiconductor materials by van der Waals force can constitute a vertical stack heterostructure,because of the strong interaction with each layer,van der Waals heterostructures based on these layered TMDCs have been developed into many novel functions and offer an excellent platform to explore basic physics such as the interfacial charge transfer.Two-dimensional layered semiconductor materials are important components in the next generation of optoelectronic integrated devices.Therefore,it is very important to explore the controllable preparation and basic physical properties of two-dimensional layered semiconductor materials.With these ideas in mind,chemical vapor deposition technology was used to synthesize transition metal dichalcogenides and van der Waals heterostructures constructed by transition metal dichalcogenides.The valley degree of freedom in monolayer transition metal dichalcogenides was operated and detected by alloy method.By means of alloy method,the band arrangement of the heterostructure constructed by transition metal dichalcogenides,and the continuous control of interlayer band gap is realized in the heterostructures.To experimentally probe the valley-dependent Second-harmonic generation（SHG）in AA stacking multilayer MoS₂,and the polarization degree of SHG is nearly uniform.The main achievements are summarized as follows;（1）The generation and manipulation of valley-spin polarization are essential for two-dimensional（2D）layered transition-metal dichalcogenides（TMDCs）for spin-/valley-tronic applications.Here,high crystal quality WS_2xSe_2（1-x）monolayers with sulfur composition tuning from 0 to 1 were prepared through a controlled chemical vapor deposition method.The crystal structure retains perfect C3-rotation symmetry,with the circular polarization degree of second harmonic generation（SHG）achieving near unit.Both steady-state and time-resolved circular polarization-resolved PL characterizations demonstrate that the valley polarization degree of WS_2xSe_2（1-x）monolayers can be monotonically improved as gradually increasing the sulfur concentration.A phenomenological model and the corresponding rate equations were established to describe the valley polarization dynamics of the bandgap engineered monolayer WS_2xSe_2（1-x）,and a real band-edge intervalley scattering lifetime can be determined by fitting the circularly polarized PL decay curves using this model.The physical origin of the phenomenon that increasing degree of valley polarization with the decreasing of the hot electron energy has been revealed due to the continuous tuning of the initially injected polarization as varying the composition ratio.Our work gives insight into the underlying valley depolarization mechanism in 2D alloyed monolayers and provides a potential pathway for controllable synthesis of high-quality atomically thin alloys with tunable valley physics.（2）The wavelength-tunable interlayer exciton（IE）from layeredsemiconductor materials has not been achieved.van der Waals heterobilayers constructed using single-layer transition metal dichalcogenides can produce continuously changed interlayer band gaps,which is a feasible approach to achieve tunable IEs.In this work,we design a series of van der Waals heterostructures composed of a WSe2 layer with a fixed band gap and another WS_2（1-x）Se_2x alloy layer with continuously changed band gaps.The existence of IEs and tunable interlayer band gaps in these heterobilayers is verified by steady-state photoluminescence experiments.By tuning the composition of the WS_2（1-x）Se_2x alloy layers,we realized a very wide tunable band gap range of 1.97-1.40e V with a wavelength-tunable IE emission range of 1.52-1.40 e V from the heterobilayers.The time-resolved photoluminescence experiments show the IE emission lifetimes over nanoseconds.（3）With unique valley-dependent optical and optoelectronic properties,2D transition metal dichalcogenides（2D TMDCs）are promising materials for valleytronics.Second-harmonic generation（SHG）in 2D TMDCs monolayers has shown valley-dependent optical selection rules.However,SHG in monolayer TMDCs is generally weak;it is important to obtain materials with both strong SHG signals and a large degree of polarization.In the work,MoS₂ samples with differently stacked were synthesized by CVD onto Si O₂/Si substrates.Through SHG experiment,it is demonstrated that the AB-stacked multilayer MoS₂ samples with 2H phase.Meanwhile,AA-stacked multilayer MoS₂ samples with 3R phase（breaking inversion symmetry）.Through circular-polarization-resolved SHG experiments,it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room tem-perature.By studying multilayer MoS₂samples（3R phase）with different twist angles,it is found that there is no significant effect of multilayer interlayer interaction on valley-dependent SHG.