Research On Modulation Of Carrier Behaviors And Optoelectronic Performance Of Van Der Waals Heterostructures

Two-dimensional semiconducting transition metal dichalcogenides(TMDCs),represented by MoS₂ and WSe2,display suitable band gaps,high carrier mobility,and strong light-matter interactions.Besides,TMDCs with dangling-bond-free surfaces can be flexibly assembled into heterostructures utilizing interfacial van der Waals(vdWs)force,truly realizing "design by demand",which are considered as ideal candidate materials for next-generation optoelectronic devices.Effective modulation of carrier behaviors of van der Waals hetero structures is crucial for designing and constructing high-performance optoelectronic devices.This thesis focuses on TMDCs vdWs heterostructures.High-quality monolayer MoS₂(1L-MoS₂)was synthesized and the fabrication process of vdWs hetero structures was optimized.The effect of electrode contact and layer number on carrier behaviors of MoS₂/WSe2 heterostructures was clarified.The high-performance MoS₂/WSe2 heterojunction photodetector was developed.A new method for constructing ZnO/MoS₂ hetero structure arrays with periodical strain distribution was proposed.The mechanism of strain modulation on carrier behaviors of ZnO/MoS₂ hetero structures was revealed.The ZnO/WSe2 hetero structure arrays with gradient-strain distribution were designed.The effective gradient-strain modulation on the optoelectronic performance of ZnO/WSe2 hetero structure arrays was realized.Firstly,IL-MoS₂ was prepared by oxygen-assisted chemical vapor deposition.It was found that as the sulfur temperature increased,the shape of 1L-MoS₂ transited from hexagon to triangle.The inhibitory effect of oxygen on the vertical growth of MoS₂ was clarified.A new strategy that increases the number of substrates in the constant-temperature zone to control the number of nucleation sites was proposed.The transfer techniques for chemical-grown MoS₂ and exfoliated two-dimensional materials were explored.An optimized layer-by-layer annealing strategy for enhancing the interlayer coupling of vdWs hetero structures was proposed.Secondly,the MoS₂/WSe2 vdWs hetero structures were designed and constructed.Three different electrode contacts were integrated into one device for comparing performance in situ.The carrier transport and collection of this heterostructure was optimized by the vertical graphene vdWs contact that can shorten the carrier transport path and avoid interface Fermi-pining.The influence of WSe2 layer number on the optoelectronic performance was explored.It was confirmed that WSe2 with 11 layers(7 nm)can well balance the layer-number-dependent competition between carrier generation,separation,transport,and collection.Based on the synergistic optimization of electrode contact and layer number selection,a MoS₂/WSe2 photovoltaic photodetector with ultra-high external quantum efficiency(61%)and ultra-fast response speed(4.1 ?s)was developed.Thirdly,ZnO/MoS₂ heterostructure arrays were designed and constructed.A new technique that utilizing ZnO nano indentation force to apply biaxial tensile strain to 1L-MoS₂ during the wet transfer process was proposed.The periodical strain distribution of 1L-MoS₂ was demonstrated by Raman spectroscopy characterization.The maximum tensile strain(0.6%)was obtained at the contact interface.In the ZnO/Al2O3/MoS₂ heterostructure,the strain-induced exciton funneling effect was found,and the photo luminescence intensity of the 1L-MoS₂ increased by 50%.The strain-enhanced carrier separation efficiency of ZnO/MoS₂ was verified.Through theoretical calculations,the mechanism of strain-enhanced carrier separation of vdWs heterostructures was revealed:strain raises the Fermi level of 1L-MoS₂ and reduces the peak barrier at the ZnO/MoS₂ interface,promoting the separation of interfacial carriers.Finally,ZnO/WSe2 hetero structure arrays were designed and constructed.The ZnO/WSe2 photovoltaic photodetector was constructed.By precisely controlling the height of the ZnO nanorod arrays,the ZnO/WSe2 hetero structure array with gradient-strain(0.6%,1%,1.6%,and 2%)was developed.It was found that as the strain increased from 0.6%to 2%,the responsivity of the hetero structure increased from 75 mA/W to 140 mA/W.Through spectroscopic characterization with theoretical calculations,the mechanism of the gradient-strain modulation on the optoelectronic performance of the ZnO/WSe2 hetero structure was clarified:strain-induced WSe2 exciton funneling and strain-optimized band structure of ZnO/WSe2 synergistically improve the interfacial carrier separation efficiency.