Charge Carrier Decay Dynamics Of Two-dimensional Materials Studied By Transient Absorption Microscopy

Due to their unique structure and superior properties,two-dimensional materials have promised broad application prospects in many fields.The dynamics of photo-excited carriers in two-dimensional materials play a decisive role in the performance of optoelectronic devices.Therefore,it is central to fully investigate the dynamics of photo-excited carriers in two-dimensional materials.It helps us to better understand the complex photophysical processes occurring in materials,then maximizes the application potential of two-dimensional materials in the field of optoelectronic devices.Transient absorption microscopy is utilized as a useful experimental tool to study the dynamics of photo-excited carriers in materials.Transient absorption images and charge carrier decay dynamics traces of two-dimensional materials were measured by home-build transient absorption microscopy,which can reveal the relaxation processes,including interactions between carriers-carriers,carriers-phonon and other information of photoexcited carriers in materials.However,the ultrafast photophysical processes and their influence mechanism occurring in materials are not fully interrogated and understood.In this thesis,aiming at the shortcomings and problems of current research,home-built transient absorption microscopy was used to fully investigate the ultrafast charge carrier decay dynamics of monolayer graphene,black phosphorus（BP）and indium selenide（In₂Se₃）.Firstly,a home-built transient absorption microscopy with ultra-high temporal and spatial resolution was constructed based on the pump-probe technique.Then,heterostructure construction and vapor deposition method were used to prepare different substrates,we transferred graphene from copper to hexagonal boron nitride（h BN）,Octadecyltrichlorosilane self-assembled monolayers（OTS SAMs）and glass substrates using the polymethyl methacrylate（PMMA）-mediated method.We investigated the charge carrier decay dynamics and transient absorption images of monolayer graphene supported by h BN,OTS SAMs and glass by transient absorption microscopy.Fitting the charge carrier decay dynamics traces,we found that the carrier relaxation was tuned by those three substrates.The charge carrier of graphene decayed fastest on h BN substrate,slowest on glass substrate and the decay rate was somewhere in between that on h BN and glass on OTS SAMs substrate,which is due to the hot phonons lifetime.Smaller charge fluctuations and less p-doping level of monolayer graphene on h BN and larger thermal conductance of h BN help cool the hot phonons and hence accelerate the charge carrier decay in graphene on h BN.Smaller charge fluctuations and less p-doping level of monolayer graphene on OTS SAMs substrate help cool the hot phonons and hence accelerate the charge carrier decay in monolayer graphene on OTS SAMs.Then,the polarization-resolved transient absorption microscopy was conducted on thin black phosphorus（BP）flakes to study the decay dynamics of charge carrier and coherent acoustic phonon.Charge carrier decay dynamics and coherent acoustic phonon oscillation show obvious anisotropic.Fitting the charge carrier decay dynamics traces,we obtained that the damping time of the coherent acoustic phonon oscillation was increased when the polarization direction of the probe pulse was changed from armchair direction to zigzag direction.Furthermore,the initial amplitude of coherent acoustic phonon oscillation in BP was larger for the polarization direction of the probe pulse along the armchair direction than along the zigzag direction.The anisotropic coherent acoustic phonon oscillation in BP was induced by the polarization-dependent absorption length derived from the intrinsic anisotropic refractive index of BP.Based on the initial oscillation amplitude and further oscillation phase analysis,we considered that the direct deformation potential mechanism has a leading role in the generation of coherent acoustic phonons oscillation in our experiment.In addition,the sound velocity of coherent acoustic phonon in BP was calculated from the frequency of acoustic phonon oscillation and acoustic echo effect.Finally,we explored the charge carrier decay dynamics in thinα-In₂Se₃ andβ-In₂Se₃ via transient absorption microscopy.Forα-In₂Se₃,the band gap of 1.43 e V is less than the photon energy of 800 nm.From the charge carrier decay dynamics traces,we found that the band filling effect and bandgap renormalization effect jointly governed the time evolution of the transient absorption signal.Then,we measured the charge carrier decay dynamics traces ofα-In₂Se₃with different carrier densities.Further fitting and analysis indicated that the magnitude and sign at different delays in charge carrier decay dynamics were determined by the weights between the band filling effect and bandgap renormalization effect,depending on the carrier density.When the carrier density increased,the absolute value of the photoinduced absorption became smaller.Forβ-In₂Se₃,the band gap of 1.55 e V is close to the probe photon energy,so we probed the electron transition from the top of the valence band to the bottom of the conduction band,producing a stronger band filling effect.Therefore,only photoinduced bleach signal was detected,which was attributed to the strong band filling effect.In addition,we studied the influence of carrier diffusion on the charge carrier decay dynamics ofα-In₂Se₃.Then,we obtain that the different charge carrier decay dynamics were induced by the change of carrier density,which was attributed to carrier diffusion.