Study On Extremely Fast Time Resolution And Local Field Spatial Resolution Spectroscopy And Dynamics

In recent years, our group has been working on the development of highly sensitive nonlinear spectroscopy and dynamics techniques with ultrafast femtosecond time, nano-space, and energy resolutions. First, our efforts are devoted to femtosecond time-resolved spectroscopy and dynamics studies. By using femtosecond transient absorption spectroscopy and time-resolved fluorescence spectroscopy, we have conducted research on deciphering mechanisms underlying complex molecular systems, solar energy devices, and photocatalytic micro-nano systems. We aim at gathering information regarding molecular interactions, electron and energy transfer channel, velocity and coupling characteristics and so on, in conjunction with theoretical simulation, to provide new fundamental sense of inspiration or guidance to the relevant systems. While temporal resolution in the time domain is important, spatial resolution in the local space domain is also important. To this end, part of our attention has been paid to the space-based local field spectroscopy and dynamics studies. By using confocal Raman detection, atomic force microscopy, and near-field optics, we can obtain some useful knowledge of surface plasmon. The final goal of these studies is to unite femtosecond time-resolved and local field space-resolved spectroscopic techniques to develop useful tools with multi-domain resolution features.This dissertation includes two parts:(1) Ultrafast spectroscopy and dynamics:(a) The molecular mechanism of fluorescence switch effect; (b) The carrier dynamics of the ZnO/CdS/CdTe interface layer; (c) The electron dynamics of BiOBr in the process of photocatalytic dyes degradation; (d) The graphene-like C3N4 excitons dynamics. (2) Local field spectroscopy and dynamics:(a) The directional scattering of surface plasmon polariton; (b) The gold nanoparticle surface plasmon emission; (c) The imaging of gold nanoparticle plasmon.(1) Ultrafast spectroscopy and dynamics studies(a) The molecular mechanism of fluorescence switch effectBy means of ultrafast transient absorption spectroscopy, we reveal the dynamics-based mechanism responsible for the fluorescence switch effect in a class of chemical sensors-fluorescent molecule chelated with two valent mercury ions, which holds promise in living cells imaging.(b) The carrier dynamics of ZnO/CdS/CdTe interface layerThe properties of the electron donor-acceptor interface play an important role in the photoelectric conversion of the nano-core-shell rod array solar cells. By using ultrafast transient absorption spectroscopy to characterize a simple low-cost ZnO/CdS/CdTe structure, we reveal the interface layer thickness effect on its carrier separation-collection and charge recombination efficiency. These results provide a useful guidance to the performance improvement of all-inorganic core-shell photovoltaic devices.(c) The electron dynamics of BiOBr in the process of photocatalytic dyes degradationWe have successfully fabricated ultra-thin BiOBr nanosheets by a simple solvothermal method to ensure the exposure of its (001) crystal surface. In the relevant ultrafast spectroscopy studies on the photocatalytic degradation of Rhodamine B process, we propose a parameter, proportion of electron photocatalytic effect, to evaluate the photocatalytic efficiency, which can be expected to offer a useful measure for quantitative photocatalyst design in industry.(d) The graphene-like C3N4 excitons dynamicsFor decades enormous efforts have been devoted to understanding the behavior of photogenerated carriers in polymer semiconductor-based photocatalysts, while the exciton effect was usually ignored. Given that the strong multi-body effect of metal-free polymer materials, the exciton kinetics should be taken into account and the results are much more complex than those of inorganic materials. In the case study of graphene-like C3N4, we for the first time provide insights into the involved exciton processes and mechanisms by means of ultrafast transient absorption spectroscopy and steady-state/time-resolved fluorescence spectroscopy. We reveal that the observed triplet enrichment originates from the enhanced spin-orbit coupling effect due to the narrowing of the singlet-triplet energy gap. We also find that its fluorescence is P-type delayed rather than thermally activated E-type delayed fluorescence, which results from the TTA (triplet-triplet annihilation) process. This work not only sheds new light on the excitonic processes in polymer matrix of graphitic carbon nitride, but also provides valuable guidance from a fundamental perspective for other relevant material design based on excitonic engineering.(2) Local field spectroscopy and dynamics(a) The directional scattering of surface plasmon polaritonThe mechanism of the directional scattering of surface plasmon has long been in question. Through a series of optical design experiments, we confirm that the directional scattering comes from the scattered light whose electric field is perpendicular to while propagation is parallel to the surface, which couples with the surface plasmon to emit light at a specific angle (local field enhancement-100 folds). We also find the dependence of emitted light intensity on laser polarization.(b) The gold nanoparticle surface plasmon emissionWe integrate ultralow-wavenumber Stokes and anti-Stokes Raman modules into a confocal imaging system, on which we obtain the full spectra of surface plasmon emission of gold nanoparticles and perform imaging observations based on peak area (a significant local field enhancement is observed). We also discuss the influence of laser polarization on surface plasmon emission,(c) The imaging of gold nanoparticle plasmonOn an atomic force detection system, we carry out surface plasmon-based imaging experiments on gold nanoparticles using a set of self-designed optical layout. In the study of excitation power dependence, we find that the excitation of surface plasmon bears a power "transition point". This finding may be of instructive value for other surface plasmon-related investigations.