Ultrafast Spectroscopy Study And Pressure Manipulation Of Carrier Dynamics In Transition Metal Dichalcogenides

The discovery of graphene at the beginning of 21^st century makes people deeply realize that the properties of material can be remarkably modified by only changing its dimension.With the on-going extensive research of graphene,the fabrication,characterization and property control of atomically thin layered nano-materials become established techniques,which also facilitate the fast development of a variety of two-dimensional materials like two-dimensional transition metal dichalcogenides(2D-TMDCs)beyond graphene.Together with graphene,2D-TMDCs belongs to the family of van der Waals(vd W)materials,in which the adjacent layers are combined with weak vd W forces,while the atoms in the same layer are connected with strong chemical bonds.With the benefit of the strength difference between the intra-layer and inter-layer interactions,top-down technique like mechanical and liquid-phase exfoliation can be applied to obtain mono or few-layer 2D-TMDCs from their bulk counterparts.The property differences of TMDCs with transition metal elements of different groups are quite huge.The chemical formula of group-VI TMDCs can be written as MX₂,where M stands for the transition metal atom(Mo,W)and X stands for chalcogenide atoms(S,Se,Te).When the chalcogenide atom is S or Se,the four corresponding TMDCs(MoS₂,MoSe₂,WS₂,WSe₂)show semiconductor characteristics.It has been proved that these four TMDC semiconductors share a band gap in the energy range from 1 to 2 e V,which correspond to the visible-NIR spectral range,and this feature makes these materials promising candidates in the field of photovoltaic technology.Based on theoretical and experimental studies,all these four TMDC semiconductors are indirect-gap semiconductors in their bulk phase.However,when the number of layers or the thinckness of these materials decreases,their electronic band structures will change greatly with the enhancement of the quantum confinement effect.When the monolayer limit is reached,a striking crossover from indirect to direct-gap semiconductor together with the emergence of photoluminescence has been discovered.Compared with the bulk or few-layered counterparts,these mentioned monolayer TMDCs possess greatly enhanced luminescence quantum efficiency and sub-nanometer thinckness.With these unique structures and optical properties,group-VI TMDC semiconductores play a key role in the development of optoelectronic devices,and they also provide solutions to the next generation of chip manufacturing extend the Moore's law.In optoelectronic devices consisting of TMDC semiconductors and their heterostructures,all the phenomena like light absorption,photoluminescence,charge transfer and energy transfer are directly connected with many-body interactions and ultrafast dynamics of carriers inside these layered materials.Thus,it is quite insufficient to merely study the band structure of these TMDC semiconductors in their steady or equilibrium state,and in-depth experimental and theoretical studies are urgently needed on the many-body interactions and ultrafast dynamics of free carriers and excitons inside these TMDC semiconductors based on the researches of their band structures.Only with these in-depth studies,deeper understandings of the optoelectronic properties can be obtained,which is of great importance for the manipulation and performance optimization of electronic and optoelectronic devices with the variation of external conditions.There are plenty of experimental methods to tune the optical and electronic properties of 2D-TMDC semiconductors,such as charge carrier doping,gating,changing the dielectronic environment or the substrates,interlayer twist,or even chemically changing the constituent elements and their proportion.Meanwhile,because of their unique structure and weak vd W interlayer interactions,it's easy to induce great changes to the electronic band structure of these 2D-TMDC semiconductors by external tensile or compression strain.In this thesis,few-layer MoS₂ samples were prepared firstly through liquid-phase exfoliation from bulk MoS₂,after which statistical Raman analyses were employed and resulted in the dominance of four-layered MoS₂in the sample.The sample was then put inside a diamond anvil cell to perform compression strain onto it,after which the home-built steady-state absorption spectroscopy and the femtosecond time-resolved transient absorption spectroscopy under high hydrostatic pressure were conducted to experimentally study the pressure-induced evolution of electronic band structure,many-body interactions and ultrafast carrier dynamics.The specific contents are as follows:(1)Utilizing the diamond anvil cell,we created a hydrostatic pressure environment in the range from 0.08 GPa to 3.20 GPa for the few-layer MoS₂.By measuring steady state absorption spectra of the few-layer MoS₂under different pressures,we found that both of the A and B exciton absorption peak positions underwent a monotonical blue-shift as the pressure increased,and the energy difference between these two exciton absorption peaks were enlarged accordingly.Meanwhile,the linewidth of both A and B exciton absorption peaks were broadened.It has been proved that the K valley in the conduction band,where A and B exciton are located,will move upwards under pressure,which leads to the blue-shift of both exciton absorption peaks.Besides,as the interlayer distance discreases under pressure,the interlayer interactions will be enhanced,with results in a larger valence band maixmum splitting(VBM splitting)at the K point in the first Brillouin zone and manifests itself as an increase of the energy difference between A and B exciton.Meanwhile,it's a common fact that different valleys near the band edge will experience relative energy shifts under external strain,and when the pressure increases,the relative energy shifts among nearby valleys will result in stronger intervalley scattering between carriers and phonons,which is the main reason for the linewidth broadening of both exciton absorption peaks.Based on these results from steady state absoprtion spectroscopy,we further utilized femtosecond time-resolved transient absorption sepctroscopy to systemically study the ultrafast carrier relaxation dynamics of few-layer MoS₂ under pressure.We found that as the pressure increased,the ground state bleaching peaks of both A and B resonances decayed faster.With the help of global fitting analysis,the decay process was divided into two transient kinetics,which were attribuated to the hot carrier cooling process followed by the exciton dissociation process.As the carrier-phonon scattering is enhanced under pressure,the hot carrier cooling process will accelerate accordingly.Along with the cooling process,the carriers will relax to the band edge and bound into excitons by Coulomb interactions.As the interlayer distance decreases,the dielectric screening between adjacent layers will become stronger,which will reduce the exciton binding energy,and together with the enhanced scattering between excitons and phonons,the exciton dissociation will then get faster.(2)Under ambient pressure,we first systemically studied the spectral signature changes of A exciton absorption spectra of few-layer MoS₂ under nonresonant pump with increasing power density at fixed time delay utilizing the femtosecond time-resolved transient absorption spectroscopy.We determined experimentally that the critical density for Mott transition,i.e.,the Mott density,of the tested few-layer MoS₂ under ambient pressure is aboutN_Mott=4.2×10¹³cm^-2.When the Mott transition happens,the free carrier density will increase,which will lead to stronger positive absorption and result in an overall uplift of the A exciton bleaching peak in the transient absorption spectra.Meanwhile,as the elastic and inelastic scattering between excitons and free carriers become more frequent,exciton dissociation will get faster,and the linewidth of A exciton bleaching peak will be broadened together with a decrease of its intensity.Based on these findings,we further utilized the diamond anvil cell to create hydrostatic pressure enviornment around the few-layer MoS₂ samples,and we fixed the photocarrier density n₀ less than the measured Mott density N_Mott under ambient pressure to study the spectral signature changes of A exciton bleaching peaks in the transient absorption spectra as the pressure increased.We found that when the pressure increased to 1.81 GPa,the linewidth of A exciton bleaching peak was obviously broadened.As the pressure further increased,the linewidth broadening continued with a reduced intensity of the bleaching peak,and the same spectral feature change appeared as an overall uplift of the A exciton bleaching peak,which made it above the zero and exhibited as a sunk positive peak.According to these spectral feature changes,we can conclude that when the pressure is above 1.81 GPa,the Mott transition happens to the tested few-layer MoS₂ samples with a fixed photocarrier density of n₀=3.29×10¹²cm^-2,and the occurrence of this Mott transition is the result of the pressure-induced reduction of the Mott density.