Ultrafast Dynamics Study Of Few-layer WS₂ Nanosheets Under Hydrostatic Pressure

Two-dimensional transition metal dichalcogenides(TMDCs)are easily exfoliated to single or few layers due to their interlayer van der Waals interactions,and in addition the band gap of TMDCs is in the visible to near-infrared range of 1-2 e V,making the research on TMDCs semiconductors of high application value.The quantum efficiency of WS₂ as a material with a large band gap in TMDCs for optoelectronic devices is still very low,especially for few-layer or even bulk materials.Therefore,it is particularly important to study the carrier mobility,exciton binding energy and exciton relaxation dynamics of WS₂ materials to improve the quantum efficiency of optoelectronic devices.High pressure,as a means to modulate the interatomic distance to change the electronic energy band structure,is particularly effective in modulating the carrier dynamics processes in few-layer WS₂ materials.Therefore,we can analyze the effect of pressure on the electronic energy band structure and carrier dynamics processes of the material by studying the absorption spectroscopy,Raman spectroscopy and transient absorption spectroscopy of the few-layer WS₂nanosheets.(1)The variation of transient absorption spectroscopy at different pump fluence shows that with the increase of delay time,the A and B exciton peaks in the same spectroscopy show an obvious blue shift,which is thought to be caused by the band gap filling effect.The A exciton peak shows a red shift with the increase of pump fluence,and the intensity of each exciton peak shows an inflection point at the pump fluence of182?J/(88)~2,showing a trend of increasing and then decreasing this pump fluence induced red shift is caused by the quantum-limited Stark effect.The A exciton relaxation dynamics curves at all pump fluence were fitted with the exponential,and it was found that the thermalization and cooling processes of the carriers with a lifetime of about 1 ps did not change significantly with increasing pump fluence,and the second relaxation lifetime was around one hundred picoseconds,which was attributed to the defect-assisted Auger recombination,and more carriers filled the defect states at high pump fluence,which suppressed the defect-capture Auger recombination and prolonged the defect-assisted Auger recombination time.In addition,we also observe anomalous reinjection processes at high pump fluence,whose lifetimes are in the tens of picoseconds range and increase with increasing pump fluence,and such anomalous reinjection processes may be related to exciton-exciton annihilation and K-?intervalley transition.(2)The absorption spectroscopy,Raman spectra and transient absorption spectroscopy were measured at different pressures.The full width at half maxima increases in the low-pressure range at pressures less than 2 GPa because the pressure causes the energy band to change,which increases the inter-valley scattering rate;the full width at half maxima decreases in the high-pressure range at pressures greater than2 GPa,which may be related to the K-?crossover.Meanwhile,the Raman peaks2LA(M),E~1_2g(?)and A_1g(?)show a blue shift in each vibrational mode,and their widths also become larger with increasing pressure,indicating that the electron-phonon interaction is enhanced at high pressure.The transient absorption spectroscopy experiments at different pressures revealed that the peak intensity of each exciton peak decreased with increasing pressure and showed a significant blue shift.The exciton peaks at different delay time at the same pressure also show a blue shift,and the duration of the blue shift is similar to the fast relaxation time of carrier cooling of about1 ps obtained by fitting,while the carrier cooling process accelerates with increasing pressure.The trend of the relaxation process lifetime with pressure for the second 100picosecond scale also shows an inflection point at 2 GPa,where the lifetime of the relaxation process increases in the low-pressure range,and the lifetime suddenly decreases at the inflection point and decreases with further increase in pressure.Also,anomalous reinjection processes are observed at pressures greater than 2.4 GPa,and their relaxation lifetimes decrease with increasing pressure,which may be related to exciton-exciton annihilation and inter-valley scattering of carrier energy.