Ultrafast Dynamics And High-pressure Studies Of Transition Metal Dichalcogenide WSe₂

Due to the weak van der Waals forces between layers of two-dimensional transition metal dichalcogenides（TMDCs）,these materials can be mechanically or liquid-phase exfoliated into few-layer or single-layer nanosheets,which have important application values in electronics,optoelectronic catalysis,sensors,and other fields.As a typical representative of two-dimensional transition metal dichalcogenides（TMDCs）,tungsten diselenide（WSe₂）can be used to make photovoltaic cells and photodetectors due to its excellent optoelectronic properties.By studying the dynamic parameters of carrier lifetime and mobility of WSe₂,it is possible to improve the efficiency and response speed of photovoltaic devices,and optimize the performance of photodetectors and phototransistors.We have systematically studied the dynamic changes of WSe₂ carrier under normal and high pressure using time-resolved spectroscopy and steady-state absorption spectroscopy.This provides theoretical support for the preparation of high-sensitivity,high-stability,and repeatable pressure sensors,and reveals the changes of its electrical properties under different pressures.In this paper,we use liquid phase exfoliation to peel off WSe₂ bulk materials into few-layer WSe₂nanosheets.By changing the centrifugation speed of the centrifuge,we successfully control the number of layers peeled off,with an average of 2-4 layers.For steady-state absorption spectra of WSe₂with different numbers of layers,two fitting peaks A and B are obtained.The peak energies of A and B peaks increase with decreasing centrifugation speed（number of layers）.In the transient absorption spectra with fixed number of layers and pump flux,as the delay time increases,both A and B excitonic peaks experience a blue shift,which is considered to be caused by the Bernstein-Moss effect.At a fixed delay time of 3 ps,with the increase of pump flux,the A excitonic peak exhibits a red shift,which is considered to be caused by the Stark effect.The A exciton relaxation dynamics curves for WSe₂ with different numbers of layers under the same pump flux are fitted with a double exponential,and it is found that the rapid relaxation process at around 1 ps is due to the cooling of the hot excitons,and the slow relaxation process at around 100 ps is the charge recombination process of surface defects.The entire relaxation process is linearly dependent on the number of layers,and the relaxation process gradually accelerates as the number of layers decreases.The A exciton relaxation dynamics curves for the same number of layers under different pump fluxes are also fitted with a double exponential,and it is found that the rapid relaxation process is due to the cooling of hot carriers,while the slow relaxation process is due to the defect-assisted Auger recombination.As the pump flux increases,the generated photogenerated carriers gradually fill the defect states in the sample,forcing the Auger capture of carriers to become inter-band recombination,thereby slowing down the Auger capture rate.The entire relaxation process is linearly dependent on the pump flux,and the relaxation process gradually slows down as the pump flux increases.From the transient absorption spectra of few-layer WSe₂ under different pressures,it was found that high pressure causes the A and B exciton bleaching peaks to blue-shift and the exciton lifetime to significantly shorten.At a fixed delay time of 3 ps,the A exciton peak gradually blue-shifts with increasing pressure,attributed to the upward shift of the K point of the conduction band of few-layer WSe₂ under pressure.The relaxation dynamics curve of the A exciton under different pressures was fitted,with the first lifetime around 1 ps,which is the process of thermal carriers cooling.The second lifetime is around several tens of picoseconds,which is the result of exciton-exciton annihilation and defect-assisted Auger recombination.When the pressure is below 1.25 GPa,the main process is exciton-exciton annihilation,and as the pressure increases,the positive relaxation process is accelerated by exciton re-injection.When the pressure is higher than 1.25 GPa,the carrier transfers from the K valley to theΛvalley through phonon scattering due to the phenomenon of energy band valley crossing,making the exciton re-injection process completely apparent and accelerating the exciton recombination rate.