Vogel Yan B, Pham Le Nhan, Stam Maarten, Ubbink Reinout F, Coote Michelle L, Houtepen Arjan J
Department of Chemical Engineering, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands.
Institute for Nanoscale Science and Technology, College of Science and Engineering, Flinders University, Bedford Park, South Australia 5042, Australia.
J Am Chem Soc. 2024 Apr 10;146(14):9928-9938. doi: 10.1021/jacs.4c00402. Epub 2024 Mar 26.
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.
胶体量子点（cQDs）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表面的配体敏感，但它们如何依赖于其他实验条件（如溶剂化）则不太清楚。在这里，我们通过光谱电化学测量实验确定了PbS和ZnO cQDs薄膜的带边位置。为了实现这一点，我们首先仔细评估并优化了电子和空穴向PbS cQDs的电化学注入。这导致了电化学上完全可逆的电子注入，每个PbS cQDs有超过8个电子，从而能够定量测定不同直径和表面组成的PbS cQDs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，这一结果对ZnO cQDs也成立。我们认为溶剂与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这种巨大影响的原因。能量移动的趋势与明确溶剂中密度泛函理论（DFT）计算的结果相匹配，并与表面阳离子和溶剂之间的络合能量成比例。作为一阶近似，溶剂的路易斯碱性可以用作预测溶剂化cQDs导带和价带边缘移动的良好描述符。
