Mechanistic Study On Ultrafast Dynamics Of Several Model Nanosystems

In recent decades,nanomaterials have received widespread attention as they have shown broad application prospects in many fields such as light-emitting diodes,solar cells,photodetectors,and photocatalysis,by virtue of their excellent and unique physical and chemical properties.Nevertheless,unlike bulk materials,due to the quantum confinement effect the strong Coulomb interaction in nanomaterials can bring about complicated excited-state relaxation and surface/interface charge transfer processes.To date,the understanding about the relevant microscopic dynamics remains rather lacking,thereby affecting material design and control as well as practical applications.Therefore,it has become one of the central challenges in the field to gain in-depth understanding about the excited-state microscopic mechanisms of nanomaterials.To this end,it is vital and necessary to rationally design model nanosystems and employ advanced spectroscopic methods(in combination of other characterization methods)to obtain mechanistic information about the excited-state dynamics.The motion of electrons/holes and energy transfer processes,which usually occur at a nano/picosecond and even shorter timescale,constitute one of the major contents in the nanomaterial dynamics studies.Upon photoexcitation,nanomaterials usually undergo a rapid electron-hole separation process forming hot carriers,followed by such excited-state relaxation processes as scattering,transfer,and recombination,and eventually reach an equilibrium state.During these relaxations the excited-state carriers may experience Auger,phonon bottleneck,multiple excitons,and surface/interface transfer processes,which are generally related to the intrinsic properties of nanomaterials as well as photoexcitation energy,wavelength,and polarization.Therefore,in order to explore and analyze the excited-state relaxation mechanisms,one needs to resort to the assistance from both the theoretical side and more importantly from the experimental side equipped with high-resolution spectroscopy and dynamics characterizations.In recent years,ultrafast spectroscopy has drawn substantial attention in the field of material physics and chemistry,as it provides a robust tool for solving various challenging problems pertinent to excited-state dynamics in nanosystems.Under this background,this dissertation is devoted to investigating several designed model nanosystems mainly by means of femtosecond time-resolved transient absorption(fs-TA)spectroscopy(in conjunction with other spectroscopic and property characterizations)to reveal a variety of excited-state dynamics involved in the nanosystems(e.g.,behaviors and effect related to plasmonic hot electrons,excited-state carriers,defect/trap states,Auger,carrier-phonon interactions).This dissertation includes the following four parts:1.Plasmonic hot-electron dynamics in gold nanorodsThe gold nanorods(Au NRs)are well known as a typical plasmonic material with two intrinsic bands originating from localized surface plasmon resonances.Despite the existence of abundant reports relating to Au NRs-based nanosystems,the two plasmonic modes are usually investigated or treated in an "isolated" manner and hence the elusive interplay between them remains far from being well explored.Here,we report a first observation of assisted reversible EnT(L→T)in such a system.We deliberately design and conduct control experiments,mainly using fs-TA spectroscopy,to identify and further manipulate the newly discovered EnT(L→T)process.The dielectric environment in which the Au NRs are dispersed turns out to play a key role in opening such a seemingly counter-intuitive EnT channel,whose efficiency is further found to be dependent of the aspect ratio of Au NRs.Moreover,an elusive dynamic inter-mode screening effect(i.e.,between L and T modes)is observed,which occurs normally in the binary heterostructured systems rather than the unary plasmonic one(i.e.,Au NRs)reported here.In addition to the mechanistic insights gained from this work,the implication associated with the manipulation of plasmonic hot electrons is also addressed.2.Hole-transfer dynamics in model nanosystemsTwo collaborative works have been carried out:(1)We have designed a model nanosystem composed of the metallic CoN porous atomic layers and the hole scavenger Na2S.The fs-TA spectroscopy disclosed that the IR-light excited electrons initially undergo fast intraband relaxation processes from the Fermi level to the trap states in the conduction band,and then decay on a nanosecond timescale through an interband recombination process;with the addition of Na2S solution,the intraband relaxation and interband recombination time was found to be greatly increased,confirming the key role of hole scavengers in prolonging the lifetime of photoexcited electrons.As a result,the IR-light driven CO2 reduction was improved significantly.(2)The classical photocatalytic oxidation system TiO2-CH3OH was selected as the research subject,in which CH3OH was used as a hole-sacrificial agent.The dependence of hole dynamics on the crystal planes of TiO2 nanocrystals was revealed,which confirmed the crystal plane effect of photocatalytic oxidation.3.The doping-induced defect-state dynamics in model nanosystemsSince the existence of defects has a non-negligible impact on the photoelectric properties of semiconductors,the impurities(or dopants)intentionally introduced into the host lattice of semiconductor nanocrystals usually bring about new phenomena or effects.Three pieces of work have been carried out:(1)Zn-doped CdTe quantum dots(QDs):We present a systematical scrutiny of the element doping-induced effects in a prototype system of CdTe QDs that is slightly doped with Zn.We reveal that the slight Zn-doping in CdTe QDs can greatly affect the involved carrier relaxation dynamics through modification of the density-of-state for both nearband-edge and localized surface trap states.Furthermore,such a slight doping is found to be quite significant in modulating the photoreduction efficiency(of particular relation to the localized surface trap states)as well as altering the involved relaxation/reaction activation energy and phonon effect in this QDs system.(2)Single-atom Cu-doped polymeric C3N4 system:Our key finding was that the simultaneous introduction of single-atom Cu into the interlayer and in-plane of C3N4 can open a new channel for electron transfer,greatly improving the separation efficiency of photogenerated carriers and hence improving the photocatalytic efficiency.(3)Dual cooperative defect sites in polymeric C3N4 system:We found that the cooperative effects of single-site copper and surface nitrogen defects can result in an optimal compromise of extended visible-light absorption and charge carrier dynamics.This work demonstrated the crucial role of dual defects in regulating photocatalytic activity,thereby offering useful guidance for the improvement of photocatalyst performance.4.Surface engineering inhibits Auger recombination in model nanosystemsAuger recombination is a nonradiative process,which usually leads to the loss of the carrier density and the reduction of quantum yield in nanomaterials,has become an important factor to impede the improvement of photoelectric performance.To suppress Auger process,surface engineering by replacement of certain ligands has shown great potential.In this part of the dissertation,we have chosen the CsPbBr3 QDs model system as the research subject.The main results are as follows:(1)by taking the surface-ligand modified inorganic perovskite QDs,i.e.,CsPbBr3-octanoic acid(OcA)and CsPbBr3-oleic acid(OA)QDs,as a pair of contrasting model,we look into the impact of surface-ligand modification on non-radiative Auger process.The detailed results obtained from steady-state/time-resolved PL spectroscopy and femtosecond time-resolved transient absorption(fs-TA)spectroscopy allow us to reveal that the subtle difference in ligand chain-length(i.e.,C8 and C18 alkyl chains for OcA and OA,respectively)can affect Auger recombination and hot-carrier cooling processes in a pronounced manner.Furthermore,we unravel that trapped and free carriers are formed when CsPbBr3 QDs are modified with short-and long-chain ligands,respectively,leading to distinctly different capability of suppressing the detrimental Auger process.Besides,the involved Auger suppression turns out to correlate with carrier population of a certain transition state in this model QD system.(2)We developed a hybrid surface passivation strategy to reduce the surface defects as well as improve carrier mobility in the fabricated CsPbBr3 QDs films for lowering efficiency roll-off in high-brightness light-emitting diodes.Using fs-TA and time-resolved photoluminescence spectroscopy,we demonstrated that the surface passivation can efficiently eliminate surface defects to enhance radiative recombination and suppress nonradiative Auger recombination,thereby providing a plausible explanation for the improvement of device efficiency from the ultrafast dynamics perspective.