Charge Transfer And Energy Transfer In Two-dimensional Material Heterojunction

The adjustment of material properties is directly related to the optimization of applied devices,and is very important for the study of physical properties and the development of nanotechnology.In optoelectronic devices,the rate of charge transfer between layers directly determines the basic response speed and photoelectric conversion efficiency of the device.Therefore,a deep understanding of charge transfer has important guiding significance for device design and optimization.Transition metal disulfide heterojunction has wide band gap,strong light and material interaction,and ultra-fast interlayer charge transfer.Low wavenumber Raman and ultrafast pump detection can be used to study the charge transfer time between layers in a two-dimensional heterojunction.First,we mainly introduce the special preparation of sub-200nm thick quantum dot film and the nonlinear optical control results of quantum dot molybdenum disulfide heterojunction.At the same time,how the prepared quantum dot film greatly enhances the nonlinear optical response of the two-dimensional atomic monolayer material.We found that the nonlinear signal of the enhanced two-dimensional material exceeds the nonlinear signal of the two-dimensional material itself by three orders of magnitude,and this huge nonlinear enhancement is universal.Different orders of harmonic emission can be enhanced,such as optical second harmonic emission(SHG),optical third harmonic emission(THG),optical fourth harmonic emission(FHG).Secondly,we simultaneously attenuated and strengthend the SHG of single-layer MoS₂ by graphene.In the MoS₂/graphene heterojunction,it is found that the SHG of MoS₂ is attenuated when excited by resonance through the experiments.Besides,the SHG of MoS₂ is enhanced when excited by non-resonance in the MoS₂/graphene heterojunction.We explained the SHG attenuation mechanism and enhancement mechanism of MoS₂.The interface energy transfer between MoS₂ and graphene causes the SHG change of MoS₂.Thirdly,we report the realization of Van der Waals interface engineering technology for easy control of nonlinear optical response in the MoS₂ momolayer.I found that the interface of the single-layer graphene will weaken the intensity of the exciton oscillator in the MoS₂ monolayer and suppress the second harmonic generation(SHG)intensity to30%under the excitation of the band gap resonance.When using non-resonant excitation,the SHG intensity will increase by up to 130%,which is presumably caused by the interlayer excitation between MoS₂ and graphene.My research provides an effective method for controlling the nonlinear optical properties of two-dimensional materials,thus contributing to their future applications in optoelectronic and photonic devices.Finally,we studied the adjustment of uniaxial tensile strain on the carrier dynamics of MoS₂ monolayer.We found that the carrier dynamic process in MoS₂has four channels with different time scales(ie Auger scattering,trap state scattering,carrier-phonon scattering and radiation recombination process).Strain can significantly extend the carrier lifetime related to defect trapping by 440%/1%strain,while the other three carrier dynamic channels remain largely unchanged.