Marino Emanuele, Sciortino Alice, Berkhout Annemarie, MacArthur Katherine E, Heggen Marc, Gregorkiewicz Tom, Kodger Thomas E, Capretti Antonio, Murray Christopher B, Koenderink A Femius, Messina Fabrizio, Schall Peter
Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands.
Department of Chemistry, University of Pennsylvania, 231 S. 34th St., Philadelphia, Pennsylvania 19104, United States.
ACS Nano. 2020 Oct 27;14(10):13806-13815. doi: 10.1021/acsnano.0c06188. Epub 2020 Sep 18.
Semiconductor nanocrystals, or quantum dots (QDs), simultaneously benefit from inexpensive low-temperature solution processing and exciting photophysics, making them the ideal candidates for next-generation solar cells and photodetectors. While the working principles of these devices rely on light absorption, QDs intrinsically belong to the Rayleigh regime and display optical behavior limited to electric dipole resonances, resulting in low absorption efficiencies. Increasing the absorption efficiency of QDs, together with their electronic and excitonic coupling to enhance charge carrier mobility, is therefore of critical importance to enable practical applications. Here, we demonstrate a general and scalable approach to increase both light absorption and excitonic coupling of QDs by fabricating hierarchical metamaterials. We assemble QDs into crystalline supraparticles using an emulsion template and demonstrate that these colloidal supercrystals (SCs) exhibit extended resonant optical behavior resulting in an enhancement in absorption efficiency in the visible range of more than 2 orders of magnitude with respect to the case of dispersed QDs. This successful light trapping strategy is complemented by the enhanced excitonic coupling observed in ligand-exchanged SCs, experimentally demonstrated through ultrafast transient absorption spectroscopy and leading to the formation of a free biexciton system on sub-picosecond time scales. These results introduce a colloidal metamaterial designed by self-assembly from the bottom up, simultaneously featuring a combination of nanoscale and mesoscale properties leading to simultaneous photonic and excitonic coupling, therefore presenting the nanocrystal analogue of supramolecular structures.
半导体纳米晶体，即量子点（QDs），兼具廉价的低温溶液处理工艺和令人兴奋的光物理特性，使其成为下一代太阳能电池和光电探测器的理想候选材料。虽然这些器件的工作原理依赖于光吸收，但量子点本质上属于瑞利区域，并且表现出仅限于电偶极共振的光学行为，导致吸收效率较低。因此，提高量子点的吸收效率以及它们的电子和激子耦合以增强电荷载流子迁移率，对于实现实际应用至关重要。在这里，我们展示了一种通用且可扩展的方法，通过制造分级超材料来提高量子点的光吸收和激子耦合。我们使用乳液模板将量子点组装成晶体超粒子，并证明这些胶体超晶体（SCs）表现出扩展的共振光学行为，相对于分散量子点的情况，在可见光范围内吸收效率提高了两个多数量级。这种成功的光捕获策略通过在配体交换的超晶体中观察到的增强激子耦合得到补充，通过超快瞬态吸收光谱实验证明，并且在亚皮秒时间尺度上导致形成自由双激子系统。这些结果引入了一种通过自下而上自组装设计的胶体超材料，同时具有纳米级和中尺度特性的组合，导致光子和激子同时耦合，因此呈现出超分子结构的纳米晶体类似物。
