Semiconductor nanocrystals emit light from excitons confined to their core, as well as from their surfaces. Time-resolving the emission from the core yields information on the band edge exciton, which is now well understood. In contrast, the emission from the surface is ill-characterized and remains poorly understood, especially on long time scales. In order to understand the kinetics of charge trapping to the surface and electronic relaxation within the surface, we perform time-resolved emission spectroscopy on CdSe nanocrystals with strong surface emission. The time-resolved spectra reveal a time scale of electron transfer from core to surface much slower than previously thought. These spectra also unveil electron dynamics in the surface band, which gives rise to an average lifetime spectrum. These dynamics are explained by invoking two surface states. This simple model further rationalizes the role of ligands in tuning the surface emission of nanocrystals. These experimental results provide a critical test of our understanding of the electronic structure of the surface.

中文翻译：

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半导体纳米晶体表面的电子动力学

半导体纳米晶体从局限于其核心的激子以及从其表面发射光。对铁心的发射进行时间分辨会产生关于带边缘激子的信息，这已为人们所熟知。相反，来自表面的发射的特征不充分，尤其是在长时间范围内，了解仍然很少。为了了解电荷捕获到表面和表面内电子弛豫的动力学，我们对具有强表面发射的CdSe纳米晶体进行了时间分辨发射光谱。时间分辨光谱揭示了从核到表面的电子转移的时间尺度，比以前认为的要慢得多。这些光谱还揭示了表面带中的电子动力学，从而产生了平均寿命光谱。通过调用两个表面状态来解释这些动力学。这个简单的模型进一步合理化了配体在调节纳米晶体表面发射中的作用。这些实验结果为我们对表面电子结构的理解提供了重要的测试。