Investigation on Initial Process of Photoexcitation at Semiconductor/Solution Interface by Ultrafast Spectroscopy

本研究利用泵浦探针时间分辨漫反射光谱分析了光激发钛粒子和电极的初始过程，旨在阐明半导体材料中光激发电子与正空穴复合的决定性因素。利用310nm泵浦（激发）脉冲和620nm探针脉冲的超快飞秒激光系统，我们证明了钛粒子在带隙激发下会产生非常快的吸收上升，随后会产生相对快速和缓慢的吸收衰减。快速衰减服从二级动力学，表明吸收是由于捕获电子引起的，它们与正空穴的复合导致了快速衰减。在多晶二氧化钛的电极中也观察到类似的行为，表明电极中的每个二氧化钛颗粒都是独立的。研究了锐钛矿和金红石型二氧化钛粒子的衰减与泵浦脉冲波长的关系。结果表明，当粒子在晶体带隙附近被能量泵浦脉冲光激发时，会出现非常快的衰变成分。我们得出结论，这是由于激子的形成。

This study aimed at clarification of decisive factor of recombination between photoexcited electron and positive hole in the semiconducting materials by analysis of initial process in the photoirradiated titania particles and electrodes with pump-probe time-resolved diffuse reflectance spectroscopy. By using ultrafast femtosecond laser system with 310 nm pump (excitation) and 620 nm probe pulses, we have proved that the bandgap excitation of titania particles induces very fast rise followed by relatively rapid and slow decay of absorption. The rapid decay obeyed second order kinetics suggesting that the absorption is due to trapped electrons and their recombination with positive holes give the rapid decay. Similar behavior was also observed in the electrode of polycrystalline titania showing that each titania particle in the electrode behaves independently. Dependence of these decays on the pump pulse wavelength was investigated for anatase and rutile titania particles. It was clarified that when the particles were photoexcited by the pump pulse of energy close to the bandgaps of crystallites very fast decay component appeared. We concluded that this is due to the formation of excitons.