| In recent decades,nanomaterials have attracted a lot of attention in the area of modern sciences owing to their cornerstone roles in nanoscience and nanotechnology.When the size of particles reaches the nanoscale,their acoustic,optical,electrical,magnetic,and thermal properties will exhibit novel characteristics.As a bridge connecting atoms,molecules,and macroscopic systems,the study of nanosystems deepens the understanding of nature.In the study of nanomaterials,changes of structure order and non-equilibrium properties of states during the evolution of nanomaterials into macroscopic systems will significantly affect the nature of the systems.By studying these changes,the intrinsic evolution processes from the micro-to macroscopic regimes can be revealed and elucidated.Static spectroscopy gives basic information about the structures,which is indispensable for studying the optical properties of nanomaterials.However,to determine the structure-function interactions and the related physical mechanisms for nanomaterials,the dynamic information about the lifetime of carriers or excitons and their related relaxation pathways is necessary.Therefore,we start to study the femtosecond time-resolved spectroscopy and dynamics in the time domain.Specifically,femtosecond transient absorption spectroscopy was employed to investigate the microscopic mechanisms of two-dimensional heterojunction materials,photocathode devices,photocatalytic micro/nano materials,and perovskite luminescent materials.On the basis of the information of rate characteristics and transfer/transport pathways of carriers and energy combined with theoretical modeling,the physical mechanisms of electron and energy transfers for the studied materials can be revealed,which provides useful guidance for related practical researches.Therefore,the researches in the time domain is rather important.Meanwhile,we show that the researches with local-field spatial-resolved spectroscopy is also very important.Through confocal Raman detection and atomic force microscopy,we carry out studies of optical properties and microscopic imaging.Finally,combining the spatial resolution with the ultrafast temporal resolution,it is expected to promote the development of ultrafast nonlinear spectroscopy and dynamics techniques that integrally possess multiple functions of ultrafast detection,nanoscale spatial resolution,and energy resolution.This thesis mainly includes two parts:(i)Femtosecond time-resolved ultrafast spectroscopy and dynamics studies(a)Ultrafast spectroscopy and dynamics of interface-state carriers in micro/nano systemsFirstly,by introducing a heterogeneous material with a suitable band structure and high carrier mobility,a semiconductor heterojunction constructed by black phosphorus nanosheets and conductive polymer P3HT is used to improve the exciton dissociation efficiency of two-dimensional materials.As a result,efficient carrier-related photoresponse performance is achieved.The transient absorption spectroscopy shows that the interface formed in the constructed heterojunction makes the free charge carrier relaxation process significantly accelerated,which is responsible for the performance improvement of the deviceSecondly,a scheme of directly coating the MOF material Cu3(BTC)2 on the surface of Cu2O photocathode was designed.Moreover,the ultrafast dynamics study shows that the integration of Cu3(BTC)2 in contact with Cu2O forms an interface state,which allows photo-generated electrons generated on Cu2O to preferentially transfer to the interface state after photoexcitation.As a result,more catalytically active sites are activated in the interface state,which increases the reduction rate of CO2.This work provides a new and simple means for designing functional devices that can protect the photocathode from photocorrosion and improve the catalytic efficiency(b)Ultrafast spectroscopy and dynamics of defective carriers in micro/nano systemsFirstly,benefited from advantages of simple synthesis method,high stability,and high tolerance to structural adjustment,the MOF system of UiO-66-NH2 has become an ideal system for studying effects of defects on the photocatalytic performance.The ultrafast transient absorption spectroscopy reveals that the sample has the shortest relaxation lifetime and the highest charge separation efficiency when a moderate amount of structural defects is introduced,while too many defects will hinder the relaxation processes and reduce the charge separation efficiency.Our study unravels the microscopic mechanism of the photocatalytic activity trend and hence provides valuable information for improving device performance by regulating structural defectsSecondly,by using transient absorption measurement combined with theoretical simulations,we show that the introduction of low-content palladium(0.1 wt%)causes the band structure change of C3N4.The presence of near-band edge defect states accelerates the carrier separation and transfer,which is responsible for the effective charge separation and the more hydrogen adsorption sites.This study provides instructive guidance for the development of monoatomic catalysts with enhanced atomic efficiency and activity.(c)Photoluminescence spectroscopy and kinetics of lead-free double perovskitesIn order to explore the luminous characteristics of lead-free metal halide perovskite nanocrystals with stable optical properties and environmental protection,the ion-doping method was used.Compared with the original Cs2NaBiCl6 NCs with low dark-blue light-emitting,the photoluminescence(PL)performance of Cs2NaBiCl6 NCs doped by Ag+,Mn2+ or Eu3+ions is significantly modulated and enhanced.Femtosecond time-resolved transient absorption spectroscopy was used to reveal the mechanisms responsible for the ion-doping induced PL enhancement in Cs2NaBiCl6 NCs,including the self-trap exciton emission mechanism due to Ag+doping and the energy transfer-related PL mechanism in the Mn2+and Eu3+doped Cs2NaBiCl6 NCs.These results show the great potential of double perovskite materials in the field of photovoltaics.(ii)Local-field spatial-resolved spectroscopy and imaging studies(a)Spatial distribution of low-frequency Raman modes of twisted bilayer grapheneAs a typical van der Waals interlayer coupled double-layer system,the twisted bilayer graphene(tBLG)has novel physical properties and tunable electronic structure.By folding a single layer of graphene,such tBLG can be obtained.In this case,the specific folded configuration of tBLG has a significant effect on its physical properties.However,for the folded tBLG,the physical properties of internal different material regions are non-uniform.In order to study the heterogeneous physical properties inside tBLG,we used the micro-area low-frequency Raman spectroscopy imaging technique to investigate the spatial characteristics of the shear mode(C)and the breathing mode(ZO'),as well as the C+ZO' combination mode.The spatial distribution of different low-frequency modes is well suited to spatially reflect internal physical properties of different regions in tBLG,thereby deepening the understanding of graphene-based materials.(b)Mechanistic study on low-frequency modes of twisted bilayer grapheneWe futher studied the internal mechanism of the spatial distribution characteristics of low-frequency Raman modes for the folded tBLG.For C and ZO'modes,the spatial distribution is throughout the entire folded tBLG.The specific twist angle of 14 degree in tBLG is the key point to enable the extensive distribution of ZO'and C modes.In contrast,the Raman image of C+ZO' combination mode has local distribution in two different areas inside the folded tBLG.Studies show that unlike the other regions in tBLG,specific strain with a tensile character exists in these two regions,which makes C+ZO' meet the activation condition of momentum matching.As a result,the C+ZO' combination mode is activated in these specific two regions.The spatial distribution characteristics of C+ZO' mode are expected to be a new indicator for describing the mechanical strain distributions in tBLG. |
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