Study on Photoinduced Ultrafast Electron Dynamics of Several Thin Film Materials

Author:Zhang Ping Li

Supervisor:ding da jun


Degree Year:2019





Femtosecond time-resolved pump-probe technique has been widely used in studying ultrafast photophysical processes of semiconductors and their interfacial composite structures.The key factor restricting the photoelectric conversion efficiency of perovskite solar cell is the carrier separation and transmission performance.However,there are few studies on carrier separation and transport between the interfaces of all-inorganic perovskite materials.In addition,femtosecond time-resolved pump-probe technique is also an effective method for studying the ultrafast energy relaxation dynamics in materials.The microcosmic mechanism of energy relaxation dynamics of different magnetic materials is significantly different.In this paper,a femtosecond transient absorption technique is used to study all-inorganic perovskite materials,ferromagnetic Co thin film,CoFeB alloy thin film,and half-metal Heusler alloy Co2FeAl0.5Si0.5.The specific research content is as follows:(1)CsPbX3(X=Cl,Br,I)was synthesized by sintering and assembled with mesoporous TiO2 layer to form CsPbX3/TiO2 composite structure.The excited state dynamics and electronic properties of CsPbX3/TiO2 composite systems were investigated by the femtosecond transient absorption spectroscopy combined with the calculation based on the first principle density functional theory.The singular value decomposition and global fitting of the femtosecond transient absorption spectrum data were carried out to obtain four kinetic components,which were respectively attributed to the processes of hot carrier cooling,free exciton formation,electron transfer and charge recombination,and thus,a charge transfer mechanism model of CsPbX3/TiO2 system was proposed.It is found that the introduction of iodine reduced the band gap value of the mixed perovskite system,resulting in a longer charge recombination time in the system.The theoretical results are in agreement with the experimental results.Our ultrafast dynamics data provide a basis for the control of interfacial electron transfer and charge recombination processes of this new type of all-inorganic perovskite materials.(2)Co,as a typical ferromagnetic material,was selected as the sample to study the ultrafast energy relaxation dynamics related to spin feature of materials.Firstly,Co/MgO thin film sample was prepared by magnetron sputtering method,and the energy relaxation dynamics of Co/MgO sample was measured by femtosecond transient absorption.The theoretical description is based on a three-temperature model including electron,spin,and lattice.The results show that,after a femtosecond laser pulse excitation,the transient absorption curve reaches the extreme value in the time scale of 220 fs,and then gradually decays until it maintains a constant value.The electron temperature of the transient absorption curve with different delay times was calibrated by the three-temperature model,and the effect of different temperature electrons on energy relaxation dynamics was detected by changing the energy density of the pump pulse.Three lifetimes were obtained by analyzing the femtosecond transient absorption dynamics data.According to this model,they were assigned as electron-spin interaction,electron-lattice interaction,and spin-lattice interaction,respectively.Electron-spin relaxation time accelerates when increasing the pump pulse energy.This phenomenon can be explained as the temperature effect under high excitation energy accelerates the response process of spin system.The time of electron-lattice interaction is affected by lattice thermal diffusion.In addition,the time of spin-lattice interaction increases with the increase of the pump pulse energy,as the higher the peak of the electron temperature generated by the pump pulse,the longer the time of spin-lattice interaction is.Furthermore,the femtosecond transient absorption technology was conducted on the CoFeB alloy thin film,comparing with Co electronic relaxation.The result shows that the electron-spin interaction of CoFeB is significantly slower than that of Co.It can be explained by the low Curie temperature of CoFeB,which prolong the response time of the spin system.Our results provide experimental basis for controlling the ultrafast energy relaxation dynamics of Co-based ferromagnetic materials.(3)Finally,the ultrafast energy relaxation dynamics of two materials,half-metal Co2FeAl0.5Si0.5(CFAS)and ferromagnetic metal Co,were studied by femtosecond transient absorption technique.The Density of States(DOS)of the two materials were calculated and the DOS effect on the ultrafast energy relaxation times was discussed for these materials.The ultrafast energy relaxation of half-metal CFAS occurred at about 587 fs,larger than that of ferromagnetic transition metal Co(about 285 fs).For half-metal CFAS materials,the slow energy relaxation time may be due to the existence the band gap of minority electron states,which inhibits the spin-flip process.In order to further understand the relationship between electron band and electron energy relaxation dynamics,we studied the ultrafast dynamics of CFAS alloy film with different thickness.With increasing the film thickness,the decay process in the short time scale gradually slows down,while the decay process in the long time scale is accelerated.The measurement gives that the ultrafast energy relaxation time for 40 nm thin film sample is lager than those of the15 nm and 30 nm thin films,indicating that the state density distribution of minority spin band in the thicker thin film inhibited the electron-spin energy relaxation channel through spin-flip scattering.With the increase of film thickness,the half-metal properties of CFSA alloy films become more prominent.