Samia A C S, Lou Y, Burda C, Senter R A, Coffer J L
Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, USA.
J Chem Phys. 2004 May 8;120(18):8716-23. doi: 10.1063/1.1695318.
Femtosecond pump-probe absorption spectroscopy is used to investigate the role of Er(3+) dopants in the early relaxation pathways of photoexcited Si nanocrystals. The fate of photoexcited electrons in three different Si nanostructures was studied and correlated with the effect of Er-doping and the nature of the dopant architecture. In Si nanocrystals without Er(3+) dopant, a trapping component was identified to be a major electron relaxation mechanism. Addition of Er(3+) ions into the core or surface shell of the nanocrystals was found to open up additional nonradiative relaxation pathways, which is attributed to Er-induced trap states in the Si host. Analysis of the photodynamics of the Si nanocrystal samples reveals an electron trapping mechanism involving trap-to-trap hopping in the doped nanocrystals, whereby the density of deep traps seem to increase with the presence of erbium. To gain additional insights on the relative depths of the trapping sites on the investigated nanostructures, benzoquinone was used as a surface adsorbed electron acceptor to facilitate photoinduced electron transfer across the nanocrystal surface and subsequently assist in back electron transfer. The established reduction potential (-0.45 V versus SCE) of the electron acceptor helped reveal that the erbium-doped nanocrystal samples have deeper trapping sites than the undoped Si. Furthermore, the measurements indicate that internally Er-doped Si have relatively deeper trapping sites than the erbium surface-enriched nanocrystals. The electron-shuttling experiment also reveals that the back electron transfer seems not to recover completely to the ground state in the doped Si nanocrystals, which is explained by a mechanism whereby the electrons are captured by deep trapping sites induced by erbium addition in the Si lattice.
飞秒泵浦-探测吸收光谱法用于研究铒（Er(3+)）掺杂剂在光激发硅纳米晶体早期弛豫途径中的作用。研究了三种不同硅纳米结构中光激发电子的命运，并将其与铒掺杂的影响和掺杂剂结构的性质相关联。在没有Er(3+)掺杂剂的硅纳米晶体中，一种俘获成分被确定为主要的电子弛豫机制。发现将Er(3+)离子添加到纳米晶体的核心或表面壳层会开辟额外的非辐射弛豫途径，这归因于硅基质中铒诱导的陷阱态。对硅纳米晶体样品光动力学的分析揭示了一种电子俘获机制，该机制涉及掺杂纳米晶体中陷阱到陷阱的跳跃，由此深陷阱的密度似乎随着铒的存在而增加。为了深入了解所研究纳米结构上俘获位点的相对深度，使用苯醌作为表面吸附的电子受体，以促进光诱导电子在纳米晶体表面的转移，并随后协助反向电子转移。电子受体确定的还原电位（相对于饱和甘汞电极（SCE）为-0.45 V）有助于揭示掺铒纳米晶体样品比未掺杂的硅具有更深的俘获位点。此外，测量表明内部掺铒的硅比铒表面富集的纳米晶体具有相对更深的俘获位点。电子穿梭实验还表明，在掺杂的硅纳米晶体中，反向电子转移似乎没有完全恢复到基态，这可以通过一种机制来解释，即电子被硅晶格中铒添加诱导的深俘获位点捕获。
