Ultrafast Dynamics Study Of Transition Metal Dichalcogenides Under Hydrostatic Pressure

Two-dimensional transition metal dichalcogenides（TMDCs）are layered materials,with adjacent layers connected by weak van der Waals forces,and thus can be exfoliated into single or fewer layers,yielding many new physical properties that are different from those of bulk materials.There are many types of Group-VI TMDCs,the most common of which is semiconductor material represented by MoS₂.With its layer-dependent band gap and high carrier mobility,it has good prospects for applications in field effect transistors,photodetectors and molecular sensors.In photoelectronic devices consisting of TMDC semiconductors,carrier interaction and ultrafast dynamics processes within these materials profoundly affect the photoluminescence,light absorption,and energy transfer processes of the device.Therefore,using ultrafast spectroscopy to study the carrier dynamics process inside the material can deepen the understanding of the photoelectric properties of the material and promote the development and application of semiconductor materials.At present,the most common modulation methods include applying external electric field,changing pressure and controlling temperature.Due to the special layered structure of two-dimensional TMDCs,the lattice structure and band structure of the materials can be changed by applying tensile or compressive strain.Therefore,the internal relationship between structure and property of TMDC semiconductors is deeply explored by means of high voltage regulation,which provides direction for the regulation and optimization of photoelectric device performance.In this paper,we prepared MoS₂ samples by mechanical exfoliation method,and determined that the samples were trilayer MoS₂ by Raman spectroscopy.On this basis,we systematically studied the carrier dynamics of MoS₂ under different pressures by diamond anvil cell,steady-state absorption spectroscopy measurement system and femtosecond transient absorption system.（1）From the steady-state absorption spectra of trilayer MoS₂ in the pressure range of 0-3.55 GPa,it is found that the A and B exciton peaks undergo a continuous blue shift process with the increase of pressure,and the absorption peaks continue to widen,while the C exciton peak does not change significantly.This is because under the action of pressure,the conduction band K valleys of A and B exciton move up,resulting in an increase in the direct band gap width and a blue shift of exciton.In addition,the high pressure also leads to the decrease of the energy difference between K andΛvalley in the conduction band energy,and the increase of the scattering probability between the valleys,which increase the width of the absorption peak.（2）According to the relaxation dynamics curves of A,B and C exciton drawn from the transient absorption spectra of MoS₂ under different pressures,it is found that the decay of the ground state bleach peak of the exciton gradually accelerates with the increase of pressure.Four relaxation dynamics processes are obtained by fitting the fourth-order e-exponential function,which correspond to the carrier cooling process,exciton dissociation process,Auger recombination process and radiative recombination process,respectively.As the pressure increases,the carrier-phonon scattering is enhanced,leading to a faster carrier cooling process.At the same time,the carriers relax to the band edge and form excitons.The pressure enhances the interlayer dielectric screening,leading to a decrease in exciton binding energy and a faster exciton dissociation process.Then it enters the Auger recombination process,which is related to interband carrier-phonon scattering.Finally,the carriers enter the electron-hole recombination process,and the high pressure makes the recombination rate faster.（3）In the transient absorption spectra of MoS₂ at room temperature and at atmospheric pressure,it is found that the oscillation phenomenon occurs near the wavelength corresponding of the C exciton peak position.The spectra are fitted globally,and then the experimental data are differentiated from the fitted data to obtain the coherent phonon oscillation curve.The frequency spectrum obtained by Fourier transform shows that the oscillation frequency is 10.75 cm^-1,which is speculated to be caused by coherent optical phonons.The coherent phonon dynamics curve and frequency spectrum under different pressures show that the oscillation frequency increases with the increase of pressure.Under pressure,the atomic spacing decreases,the electron-phonon coupling interaction increases,and then the lattice temperature increases,this causes the increase of phonon oscillation frequency.