Preparation And Ultrafast Carrier Dynamics Research Of Two-dimensional WS₂/WSe₂ Heterostructure

A two-dimensional layered material with atomic thickness is a rich material system that includes insulators,semiconductors,semi-metals,and superconductors.Among them,transition metal sulfides（TMDCs）are layered under the action of Van der Waals forces and have suitable band gaps.Single-layer materials have a direct band gap,high carrier mobility,and high fluorescence quantum yield;ultra-thin photodetectors,light-emitting diodes and other optoelectronic and light collection devices have excellent performance in applications.The very thin longitudinal dimension makes the layered material have a strong confined effect and a large exciton binding energy（hundreds of meV）,so many TMDCs single-layer materials have very strong light-matter interactions.In addition,due to the weak van der Waals force combination,different TMDCs materials can be stacked to form heterostructures,thereby obtaining novel optoelectronic properties that do not exist in a single material.It is of great significance to study the carrier dynamics such as charge separation,transfer direction,time scale and transfer efficiency in heterostructures to understand the photoelectric properties of layered materials heterostructures and to guide the development of TMDCs devices.In this paper,high-quality,large-area single-layer transition metal sulfur compounds WS₂ and WSe₂ were first prepared,and then a heterostructure was constructed using the two materials.Characterization of the heterostructure structure indicates that the two materials have good coupling strength.Then,using the femtosecond ultrafast time-resolved spectrum,steady-state fluorescence and absorption spectrum,the charge separation and carrier dynamics of the layered material on the ultrafast time scale were systematically studied.The main contents are as follows:（1）Large-area,high-quality single-layer TMDCs have excellent optoelectronic properties,and the significance of successfully preparing such materials is extremely important.At present,the preparation of some large-area TMDCs materials is based on silicon oxide or glass,however,the layered materials on silicon oxide or glass are inconvenient to transfer to other substrates,which increases the difficulty of use.Researchers have found that TMDCs grown on sapphire are easy to transfer.Based on this,the author prepared a large area of WS₂ and WSe₂ materials using chemical vapor deposition using C-plane sapphire as the substrate.Raman spectrum and PL characterization proved that the prepared materials were single-layer and highly photo-induced luminescence emission intensity.Then the author used surface energy assisted transfer to prepare the heterojunction structure of the two materials.After annealing,the same excitation power was used to perform low-wavenumber Raman characterization of the heterojunction.A new peak 25cm^-11 appeared here,and the PL of both materials has quenched,indicating that the two materials are well coupled,the high-quality TMDCs heterostructures structure was synthesized,which provided an excellent system for the study of carrier dynamics.（2）The author uses femtosecond transient absorption spectroscopy to study the ultrafast carrier dynamics at the WS₂/WSe₂ heterostructure interface.Exciting samples with 400 nm pump light above the band gap of the two materials to detect the bleaching peak of the A exciton of WSe₂,then changes the pump power to obtain a linear region where the intensity of the bleaching peak changes linearly with the power.Subsequent resonance excitation of the A exciton peak of WSe₂resulted in an electron transfer time of 250 fs and a hole transfer time of 360 fs from WSe₂ to WS₂.Subsequently,the author used a variable wavelength pump.When the pump wavelength is 400 nm,the pump light excites the electron holes to a relatively high energy state.They interact with the phonons,release excess energy and then relax to the ground state.It takes longer.As the wavelength increases,the energy state of the excited photo-generated carriers is lower,and the thermal exciton can relax to the band edge quickly and be detected,that is,the rising edge becomes faster.Under the excitation of high pump intensity,the increase of hot carrier density leads to the accumulation of non-equilibrium phonons,and the excess number of hot phonons increases the reabsorption of phonons,as a result,the relaxation rate of the net carrier decreases,resulting in a slower rising edge.Next,the authors used high-quality single-layer TMDCs to prepare high-quality heterostructures after transfer.Using transient absorption methods,the process and time of charge transfer in the heterostructures were obtained,and the understanding of future heterostructures was provided.The idea provides a theoretical basis for the preparation of devices based on heterostructures.