Radiation Laboratory, University of Notre Dame , Notre Dame, Indiana 46556, United States.
Department of Chemistry and Biochemistry, University of Notre Dame , Notre Dame, Indiana 46556, United States.
ACS Nano. 2016 Jul 26;10(7):7208-15. doi: 10.1021/acsnano.6b03700. Epub 2016 Jul 12.
Long-range charge and exciton transport in quantum dot (QD) solids is a crucial challenge in utilizing QDs for optoelectronic applications. Here, we present a direct visualization of exciton diffusion in highly ordered CdSe QDs superlattices by mapping exciton population using ultrafast transient absorption microscopy. A temporal resolution of ∼200 fs and a spatial precision of ∼50 nm of this technique provide a direct assessment of the upper limit for exciton transport in QD solids. An exciton diffusion length of ∼125 nm has been visualized in the 3 ns experimental time window and an exciton diffusion coefficient of (2.5 ± 0.2) × 10(-2) cm(2) s(-1) has been measured for superlattices constructed from 3.6 nm CdSe QDs with center-to-center distance of 6.7 nm. The measured exciton diffusion constant is in good agreement with Förster resonance energy transfer theory. We have found that exciton diffusion is greatly enhanced in the superlattices over the disordered films with an order of magnitude higher diffusion coefficient, pointing toward the role of disorder in limiting transport. This study provides important understandings on energy transport mechanisms in both the spatial and temporal domains in QD solids.
量子点（QD）固体中的长程电荷和激子输运是利用 QD 实现光电应用的关键挑战。在这里，我们通过超快瞬态吸收显微镜映射激子种群，直接观察高度有序的 CdSeQD 超晶格中的激子扩散。该技术的时间分辨率约为 200fs，空间精度约为 50nm，可直接评估 QD 固体中激子输运的上限。在 3ns 的实验时间窗口中观察到激子扩散长度约为 125nm，并且对于由中心到中心距离为 6.7nm 的 3.6nmCdSeQD 构建的超晶格，测量到的激子扩散系数为(2.5±0.2)×10(-2)cm(2)s(-1)。测量的激子扩散常数与Förster共振能量转移理论吻合良好。我们发现，激子在超晶格中的扩散比无序薄膜大大增强，扩散系数高一个数量级，这表明无序在限制输运方面起着重要作用。这项研究为 QD 固体中的能量输运机制提供了在空间和时间域的重要理解。
