Research On The Excited Carrier Dynamics Of Several Two-dimensional Materials

With unique structural features and the excellent properties of various aspects,two-dimensional materials have involved many fields of practical application,especially in photoelectric applications such as battery electrodes,light detector,super capacitor,show a good development prospect and attract more and more attentions.On the other hand,as the formal coming of information age,the rise of optoelectronic devices for the progress of the society plays a vital role,and interdisciplinary combination of materials science and optoelectronic technology becomes a popular research area in recent years.Although the performance of many photoelectric devices has been improved by two-dimensional materials,the understanding of the photophysical processes occurring in these two-dimensional materials is not very clear.Generally speaking,studying the photophysical properties of materials is mainly to understand the transition rules of excited carriers and energy relaxation processes in materials excited by light.The exploration of these excited state processes helps to grasp the physical origins of superior photoelectric properties in various systems.Therefore,we chose several typical two-dimensional materials as the research objects,aimingat the problems in their respective systems,we carried out a detailed study of the excited state dynamics in the target system with ultrafast time-resolved spectroscopy as the detection means.Based on the principle of pump detection,a transient absorption spectrum measurement system has been successfully constructed by using femtosecond laser.The system realizes the time resolution by converting the delay interval between the pump pulse and the detection pulse.We summed up the adjustment methods of the optical path and the main equipment,meanwhile summarized the fitting and analysis of the data and the mathematical treatment methods.The ultrafast process in the mixture system of methyl blue and graphene oxide was studied by transient absorption spectroscopy.A carrier dynamic model in the excited state is proposed.The experimental results show that the relaxation rate of the photo-excited carriers in graphene oxide and methyl blue dimer is slower than that in the methyl blue dimer under the same conditions,which is caused by the energy transmission between materials in the aggregation system.However,for the aggregation system of graphene oxide and methyl blue monomer,the relaxation rate of the photo-excited carriers in them is faster than that in methyl blue monomer under the same conditions.This is due to the emergence of new intermediate states between the highest occupied orbital and the lowest unoccupied orbital in the mixture system.Theyprovide a new photochemical relaxation pathway for the photo-excited carriers,thereby enhancing the relaxation rate.The kinetic process of carrier relaxation in the excited state of molybdenum disulfide nanodots was studied by transient absorption spectroscopy.We find the relaxation process of the photo-excited carriers is not simple direct band-to-band recombination of electrons and holes.Defects play an important role in the process of the photo-excited carrier recombination.At least two or more defect states with different carrier capture rates are involved in the recombination process of the photo-excited carriers.Meanwhile,the main mechanism of the process is auger scattering,basing on the dependence of the pump intensity on the recovery rate in the dynamics curves.In the model of the carrier being captured by defects,the relaxation process of the photo-excited carrier includes mainly four steps: thermalization and cooling of carriers,the capture of carriers in the defect states with high capture rate,the capture of carriers in the other defect states with low capture rate,and the direct interband recombination between photo-excited electrons and holes.The kinetic process of carrier relaxation in the excited state of multilayer molybdenum disulfide films at different temperatures was detected by transient absorption experiments.The experimental results show that there are two main photo-excited carrier transition paths in the sample: the photo-excited electrons in the conduction band and thephoto-excited holes in the valence band are captured by the defect states,and have a direct interband noradiation recombination.The photo-excited electrons and holes have a indirect interband noradiation recombination by phonon scattering.We find that the increase in temperature will enhance the interaction of the phonon and the photo-excited carrier,and the ratio of indirect interband noradiation recombination is enhanced in the two transition modes.Thus,the overall recombination rate of the photo-excited carrier is slowed down.In addition,the variation of the carrier dynamics under the influence of temperature does not change with the increase of the carrier density in a certain range.