Spectral Measurement And Control Of Interlayer Coupling In Van Der Waals Heterojunction

The atomic-level thickness and layered structure of van der Waals heterostructures make it exhibit very different characteristics from other systems,such as fabrication without considering lattice matching,indirect excitons between layers,ultrafast interlayer charge transfer,etc.,which not only provides a platform for exploring novel physical characteristics,but also shows great advantages in the field of on-chip integrated micro-nano devices,which is expected to make up for the shortcomings of the silicon-based integrated circuit industry in the"post-Moore era".The key to designing and constructing devices based on van der Waals heterostructures is to analyze and regulate the coupling between layers.The recent studies have shown that band structure alignment and space between layers changing can effectively affect interlayer coupling,and then produce different photoelectric responses.In this thesis,interlayer coupling is studied when energy barrier and interlayer expansion existing,and the content is as follows:（1）Research on charge transfer effect in MoSe₂/WS₂/MoSe₂/Graphene heterostructure where energy barrier existing in band alignment through ultrafast spectral techniques:We designed this energy-barrier band alignment heterostructure where Graphene could be used as the ultrafast channel for carrier collection and thus to realize the just accumulation of holes in the top MoSe₂.The middle layer WS₂ could be the energy barrier for holes because of band alignment.Then,photoluminescence spectroscopy and ultrafast transient absorption spectroscopy were used to analyze the dynamical process of interlayer tunneling of holes.The results show that holes of the top layer MoSe₂ can tunnel through the WS₂ barrier layer,transfer to the bottom MoSe₂ layer and finally recombine in Graphene layer.The time scale of the whole dynamical process was analyzed and goes about 20 ps,and the interlayer tunneling time shows slightly increasing with the more injected carriers.Results of this study can provide theoretical and experimental support for the design of complex functional optoelectronic devices based on van der Waals heterostructures.（2）All optically generating and monitoring interlayer expansion of h-BN/Ga Se through second harmonic generation:A method for all-optical generating and monitoring of interlayer expansion is proposed where single-photon absorption is used to induce interlayer expansion through thermal effect,and the level of interlayer expansion is monitored by intensity of the second harmonic signal.Several h-BN/Ga Se samples were successfully prepared by mechanical exfoliation and dry-transfer technique.Interlayer expansion in h-BN/Ga Se was firstly studied in detail by power-dependent and layer-dependent second harmonic spectra.The results show that thermal effect induced by single-photon absorption will lead to interlayer space increasing and thus weaken the second harmonic intensity,which can be supported by time-resolved and in-situ temperature dependent second harmonic results.Finally,a superposition model of electric field was proposed to confirm this expansion effect.Our all-optically generating and monitoring of interlayer expansion results can provide a new method to manipulate interlayer coupling in van der Waals heterostructure devices.