Ultrafast Dynamics Of Molecular System And Semiconductor Nanostructures

This dissertation includes two parts:(1) Mapping out the ultrafast dynamics in the dark triplet space of molecular systems;(2) Ultrafast carrier dynamics of semiconductor nanostructures:(a) Ultrafast carrier dynamics of a unique semiconductor-metal-graphene ternary stack;(b) Ultrafast carrier dynamics of the two-dimensional nanostructure of bismuth oxychloride.(1) Ultrafast dynamics of the dark triplet space for molecule systemsA molecule or a molecular system always consists of excited states of different spin multiplicities. With conventional optical excitations, only the (bright) states with the same spin multiplicity of the ground state could be directly reached. How to reveal the dynamics of excited (dark) states remains the grand challenge in the topical fields of photochemistry, photophysics, and photobiology. For a singlet-triplet coupled molecular system, the (bright) singlet dynamics can be routinely examined by conventional femtosecond pump-probe spectroscopy. However, owing to the involvement of intrinsically fast decay channels such as intramolecular vibrational redistribution and internal conversion, it is very difficult to single out the (dark) triplet dynamics. Herein, we develop a novel strategy that uses an ultrafast broadband white-light continuum as a excitation light source to enhance the probability of intersystem crossing, thus facilitating the population flow from the singlet space to the triplet space. With a set of femtosecond time-reversed pump-probe experiments, we demonstrate on a proof-of-concept molecular system (i.e., the malachite green molecule) that the pure triplet dynamics can be mapped out in real time through monitoring the modulated emission that occurs solely in the triplet space. This newly developed approach may provide a useful tool for examining the mechanisms underlying molecular luminescence/photonics.(2) Ultrafast carrier dynamics of semiconductor nanostructures(a) Ultrafast carrier dynamics of a unique semiconductor-metal-graphene ternary stackIt has a long-standing challenge how to suppress the electron-hole recombination in the photo-excited semiconductor via rational materials design. Together with our collaborators at USTC, we designed a ternary stack structure of semiconductor-metal-graphene. This unique hybrid nanostructure was proved to have higher photocatalytic efficiency by using ultrafast spectroscopy and steady-state photoluminescence, with the assistance of photocurrent measurements and theoretical simulations. In this designed ternary stack, the Pd nanocrystals with perfect work function serve as the interface medium between{100} facets of Cu2O nanocubes and reduced graphene oxide (rGO). The Pd nanocrystals are introduced to effectively trap charged carriers from Cu2O owing to Schottky junction, followed by the carrier extraction by a two-dimensional graphene nanoplate. As a result, the tremendous interfacial defects which can be recombination centers in the hybrid of Pd-decorated Cu2O are reduced and the difficulty of the charge carriers created in the cores being transferred beyond the shells in the Cu2O-Pd core-shell structure is overcome perfectly; as such, the detrimental electron-hole recombination is efficiently suppressed. The developed hybrid system exhibits superior performance in both photoelectrochemical and photocatalytic water splitting to any of its Cu2O-based counterparts.(b) Ultrafast carrier dynamics of the two-dimensional nanostructure of bismuth oxychloride As novel and typical bismuth-based compound with special electronic structure, layered oxyhalides have stimulated surge of interest in the field of photocatalysis. BiOCl nanomaterials exposed with{100} facets exhibit excellent photoactivity, but the mechanistic understanding remains lacking. We track the real-time carrier dynamics of relevance to the surface states in BiOCl by means of ultrafast transient absorption spectroscopy. We have obtained some preliminary results and analyzed the carrier dynamics of interest in this nanosystem.