Institute of Microwaves and Photonics, School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom.
ACS Nano. 2011 May 24;5(5):3614-21. doi: 10.1021/nn200723g. Epub 2011 Apr 27.
Photocharging has been suggested recently as the explanation for the spread of carrier multiplication yields reported by different groups. If this hypothesis can be plausible in the case of PbSe, it is inconsistent with the reported experimental data relative to CdSe nanocrystals and cannot therefore explain the large discrepancies found in that material system between static and stirred samples. An alternative explanation, photoinduced surface trapping, is suggested here, based on the results of atomistic semiempirical pseudopotential calculations of the Auger recombination rates in a number of excitonic configurations including a variety of surface traps, which show that the photoinduced surface trapping of the hole, which leaves the core negatively charged (but the nanocrystal neutral overall), can lead to recombination rates that are indistinguishable from those of a conventional biexciton with four core-delocalized carriers and therefore result in exaggerated CM yields in static samples. In contrast, the recombination rate of a charged exciton is found to be at least a factor of 2.3 smaller than that of the biexciton and therefore easily distinguishable from it experimentally. Although increased trapping at surface states was dismissed as unlikely for PbSe nanocrystals, in the case of CdSe, this hypothesis is further supported by much experimental evidence including recent spectroscopic measurements on CdSe nanostructures, single-nanocrystal photoionization studies on CdSe core/shell nanocrystals, and state-resolved transient absorption studies of biexcitonic states, all showing increased probability of surface trapping for highly excited states. These results suggest that multicarrier processes could be mediated by different mechanisms in CdSe and PbSe nanocrystals.
最近有人提出，光电荷可以解释不同研究小组报告的载流子倍增产率的扩展。如果这一假设在 PbSe 中是合理的，那么它与相对 CdSe 纳米晶体的实验数据不一致，因此不能解释在该材料体系中静态和搅拌样品之间发现的巨大差异。本文提出了一种替代解释，即光诱导表面俘获，这是基于对一些激子构型中的俄歇复合速率的原子半经验赝势计算结果，包括各种表面陷阱，结果表明，空穴的光诱导表面俘获会导致复合速率与具有四个核心离域载流子的传统双激子几乎相同，从而导致静态样品中 CM 产率过高。相比之下，带电激子的复合速率至少比双激子小 2.3 倍，因此在实验中很容易与它区分开来。尽管 PbSe 纳米晶体中表面态的俘获增加不太可能，但在 CdSe 的情况下，这一假设得到了更多实验证据的支持，包括对 CdSe 纳米结构的最近光谱测量、CdSe 核/壳纳米晶体的单纳米晶体光电离研究以及双激子态的态分辨瞬态吸收研究，所有这些都表明表面俘获的概率对于高激发态有所增加。这些结果表明，多载流子过程可能在 CdSe 和 PbSe 纳米晶体中通过不同的机制介导。
