Llusar Jordi, du Fossé Indy, Hens Zeger, Houtepen Arjan, Infante Ivan
BCMaterials, Basque Center for Materials, Applications, and Nanostructures, UPV/EHU Science Park, Leioa 48940, Spain.
Department of Chemical Engineering, Optoelectronic Materials, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands.
ACS Nano. 2024 Jan 16;18(2):1563-1572. doi: 10.1021/acsnano.3c09265. Epub 2024 Jan 3.
Although density functional theory (DFT) calculations have been crucial in our understanding of colloidal quantum dots (QDs), simulations are commonly carried out on QD models that are significantly smaller than those generally found experimentally. While smaller models allow for efficient study of local surface configurations, increasing the size of the QD model will increase the size or number of facets, which can in turn influence the energetics and characteristics of trap formation. Moreover, core-shell structures can only be studied with QD models that are large enough to accommodate the different layers with the correct thickness. Here, we use DFT calculations to study the electronic properties of QDs as a function of size, up to a diameter of ∼4.5 nm. We show that increasing the size of QD models traditionally used in DFT studies leads to a disappearance of the band gap and localization of the HOMO and LUMO levels on facet-specific regions of the QD surface. We attribute this to the lateral coupling of surface orbitals and the formation of surface bands. The introduction of surface vacancies and their a posteriori refilling with Z-type ligands leads to surface reconstructions that widen the band gap and delocalize both the HOMO and LUMO. These results show that the surface geometry of the facets plays a pivotal role in defining the electronic properties of the QD.
尽管密度泛函理论（DFT）计算对于我们理解胶体量子点（QD）至关重要，但模拟通常是在比实验中普遍发现的量子点模型小得多的模型上进行的。虽然较小的模型有助于高效研究局部表面构型，但增加量子点模型的尺寸会增加晶面的尺寸或数量，这反过来又会影响陷阱形成的能量学和特性。此外，只有使用足够大以容纳具有正确厚度的不同层的量子点模型才能研究核壳结构。在这里，我们使用DFT计算来研究量子点的电子性质随尺寸的变化，直至直径约为4.5 nm。我们表明，增加DFT研究中传统使用的量子点模型的尺寸会导致带隙消失以及最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）能级在量子点表面特定晶面区域的局域化。我们将此归因于表面轨道的横向耦合和表面带的形成。引入表面空位并随后用Z型配体重新填充会导致表面重构，从而拓宽带隙并使HOMO和LUMO都离域。这些结果表明，晶面的表面几何形状在定义量子点的电子性质方面起着关键作用。
