The optoelectronic properties of colloidal quantum dots (cQDs) depend critically on the absolute energy of the conduction and valence band edges. It is well known these band-edge energies are sensitive to the ligands on the cQD surface, but it is much less clear how they depend on other experimental conditions, like solvation. Here, we experimentally determine the band-edge positions of thin films of PbS and ZnO cQDs via spectroelectrochemical measurements. To achieve this, we first carefully evaluate and optimize the electrochemical injection of electrons and holes into PbS cQDs. This results in electrochemically fully reversible electron injection with >8 electrons per PbS cQDs, allowing the quantitative determination of the conduction band energy for PbS cQDs with various diameters and surface compositions. Surprisingly, we find that the band-edge energies shift by nearly 1 eV in the presence of different solvents, a result that also holds true for ZnO cQDs. We argue that complexation and partial charge transfer between solvent and surface ions are responsible for this large effect of the solvent on the band-edge energy. The trend in the energy shift matches the results of density functional theory (DFT) calculations in explicit solvents and scales with the energy of complexation between surface cations and solvents. As a first approximation, the solvent Lewis basicity can be used as a good descriptor to predict the shift of the conduction and valence band edges of solvated cQDs.

中文翻译：

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溶剂化使胶体量子点的带边位置移动近 1 eV

胶体量子点（cQD）的光电特性主要取决于导带和价带边缘的绝对能量。众所周知，这些带边能量对 cQD 表面上的配体敏感，但尚不清楚它们如何依赖于其他实验条件（例如溶剂化）。在这里，我们通过光谱电化学测量实验确定了 PbS 和 ZnO cQD 薄膜的带边缘位置。为了实现这一目标，我们首先仔细评估和优化电子和空穴向 PbS cQD 的电化学注入。这导致电化学上完全可逆的电子注入，每个 PbS cQD 具有 >8 个电子，从而可以定量测定具有不同直径和表面组成的 PbS cQD 的导带能量。令人惊讶的是，我们发现在不同溶剂存在下，带边能量移动了近 1 eV，这一结果也适用于 ZnO cQD。我们认为溶剂和表面离子之间的络合和部分电荷转移是溶剂对带边能量产生如此大影响的原因。能量转移的趋势与显式溶剂中的密度泛函理论 (DFT) 计算结果以及表面阳离子和溶剂之间的络合能量相匹配。作为第一个近似，溶剂路易斯碱度可以作为一个很好的描述符来预测溶剂化 cQD 的导带和价带边缘的移动。