Research On The Relaxation Process Of Conduction Electron Of TiO₂ In Au-TiO₂ Nanoparticle System

In this paper,the relaxation process of conduction electron of TiO₂nanoparticle in Au-TiO₂ nanoparticle system excited by femtosecond laser is investigated by time of flight photoemission electron microscopy.Firstly,the power dependence of photoemission yield of TiO₂nanoparticles and Au nanoparticles are measured by photoemission electron microscopy;the corresponding multiphoton orders of photoemission electron from nanoparticles are obtained in this thesis..According to the incident photon energy,work function of Au nanoparticle is in the regime between4.65eV to 4.71eV,meanwhile the minimum energy for electron to emission of TiO₂ nanoparticle is in the regime of 5.77eV to 5.84eV This lays the fundamental for further investigation on electronic relaxation process of Au-TiO₂ nanoparticle system with the combination of femtosecond laser pump-probe technology.The characters of photoemission electron from nanoparticles are investigated by analyzing photoemission yield of TiO₂nanoparticles and Au nanoparticles to the parameters of the incident laser.Secondly,utilizing the ability of spatial resolved and energy resolved of photoemission electron by time of flight photoemission electron microscope,photoemission spectra from Au-TiO₂ nanoparticle are investigated under the illumination of fundamental frequency laser,double frequency laser and both the two laser pulses;the origin of photoemission electron from the mixed nanoparticle is investigated.Under the illumination of fundamental frequency laser,only a few electrons with small kinetic energy can be emitted from Au and TiO₂ nanoparticle,however,there are two spectral peaks in the photoelectron spectra from Au-TiO₂ nanoparticle where the spectral peak with high kinetic energy originated from transfer of hot electron from Au to TiO₂nanoparticle.Under the illumination of double frequency laser,only a few electrons with small kinetic energy appear in photoelectron spectrum from Au nanoparticle,meanwhile,photoemission from conduction electron after the absorption of photon of valence electron from TiO₂ nanoparticle in Au-TiO₂nanoparticle leads to the high spectral peak in the photoelectron spectra.Finally,relaxation process from conduction electron in TiO₂ nanoparticle originated from transferred electron of Au nanoparticle is investigated by adjusting the consequence of fundamental and double frequency laser pulses.With the combination of photoemission electron microscopy and femtosecond laser pump-probe technology,relaxation processes of photoemission electron from TiO₂ and Au-TiO₂ nanoparticle are investigated according to electron excitation and emission process.With pump of fundamental frequency laser and probe of double frequency laser,the relaxation time of photoelectron from Au-TiO₂ nanoparticle is around 728fs and independent of the wavelength of the incident laser pulse.When the pump and probe laser is reversed,the relaxation time of photoemission electron from Au-TiO₂ nanoparticle is ranging from 215fs?740nm?to 718fs?840nm?determined by the wavelength of the incident laser.Furthermore,relaxation time of conduction electron in Au-TiO₂ nanoparticle is increased when only a small fraction of hot electron transfer from Au nanoparticle to TiO₂ nanoparticle.This research is of critical importance for deeply understanding the transfer process of hot electron in Au-TiO₂ nanoparticle system and improving photon to electron conversion efficiency.