Ultrafast Electron-phonon Dynamics Of Several Low-dimensional Functional Nanomaterials

In recent decades,with the development of femtosecond laser technology,femtosecond time-resolved spectroscopy technology is more and more used in the basic research of materials science.By detecting the transient absorption spectrum,we can not only observe the carrier motion in the ultrafast physical or chemical process in"real time",but also study the ultrafast response of material structure or state by detecting the periodic coherent phonon oscillation signal.With the development of nanomaterials preparation technology and its attractive prospect in the fields of electronics,optoelectronics,biosensors and other applications,exploring its internal micro quantum(carrier and phonon,etc.)dynamics processes,such as charge transfer,energy transfer,electric phonon coupling,etc.,is of great significance for understanding the macro performance of nanomaterials and devices and developing their applications in more fields.Femtosecond transient absorption spectroscopy study on the electron-phonon dynamics of low-dimensional nanomaterials has become an important focus in the study of ultrafast dynamics in recent decades.In this thesis,the femtosecond transient absorption spectroscopy technology is used to study the electron-phonon dynamics of nanomaterials from zero-dimensional gold(Au)-silver(Ag)dimer nanoparticles to one-dimensional single-walled carbon nanotubes,and then to two-dimensional monolayers MoS₂.The main research results are summarized as follows:(1)Two kinds of coherent phonon oscillation modes,radial breathing mode and axial stretching-like mode,of the zero-dimensional Au/Ag nanosphere dimer(from Ag-Ag dimer to Au-Ag core-shell dimer,and then to Au-Ag core-shell eccentric dimer)were studied by femtosecond transient absorption spectroscopy technology.The period of the mode oscillation is simulated by finite element method,and the influence of particle morphology and component on the mode oscillation is further analyzed.The theoretical simulation results show that the oscillation period is consistent with the experimental results.The results show that the influence of the center distance between Au/Ag dimer particles on the oscillation period of the breathing mode is negligible,while the oscillation period of the stretching mode is obviously dependent on the increase of the distance.The effect of Ag coated Au spheres on the oscillation period of breathing mode of core-shell dimer is small,and it has a strong mass-like effect on the oscillation period of stretching mode.These studies are of great significance to the application of the vibration characteristics of Au/Ag bimetal nanostructures,and provide a method to predict the vibration characteristics of bimetal nanostructures.(2)By adopting sub-5 femtosecond transient absorption spectroscopy technology,the coherent electron superposition states between the lowest single excited state(E₁₁)and the lowest higher excited state(E₁₁-E₂₂)of one-dimensional(6,4)carbon nanotubes are selectively pumped by two kinds of excitation spectra,narrow band and broad band.By observing the parameter changes of two kinds of coherent phonon oscillation modes(radial breathing mode and axial G-stretching mode)in carbon nanotubes under different excitation conditions,the influence of electronic coherent motion on phonon(or nuclear structure oscillation)mode is explored.The results show that,compared with the narrow band pump,the broad band pump makes two kinds of phonon modes show the obvious characteristics of electronic motion regulating the phonon field.The amplitude ratio of radial breathing mode and axial G-stretching mode,as well as the initial phase of axial G-stretching mode have been significantly changed.This study provides a possibility to control the Phonon oscillation by exciting the electron motion.(3)It is crucial for the application of optoelectronic materials and devices based on two-dimensional materials to explore the regulation of band gap and exciton binding energy by the strong multi-body interaction between carriers after photoexcitation.Studied with sub 10 femtosecond transient absorption spectroscopy technology,in the process of thermal equilibrium and"cooling"relaxation of MoS₂ following the ultrashort pulse excitation,the coulomb shielding effect between photoexcited carriers and between excitons changes with excitation energy density and sample temperature,as well as the effect on band gap renormalization.In the process of"cooling"relaxation,many mechanisms such as coulomb shielding,band filling and band gap renormalization compete with each other,leading to the evolution of band gap width with time.Then,by detecting the coherent phonon dynamics of c-plane sapphire crystal substrate,the coherent phonon motion of sapphire crystal and its possible influence on the hot carrier dynamics of single-layer MoS₂ are discussed.