Semiconducting quantum dots (QDs) exhibit tunable size dependent properties, leading some to designate these materials as “artificial atoms”. Synthetic methods of CdSe QDs have been highly refined and typically involve one-pot seeding and growth, resulting in the generation of colloidal crystallites ranging in diameter from ~1nm to ~6nm and with size distributions as low as 10%. Several reports describe the observation of “magic size” QDs during the early stages of growth, characterized by a stepwise increase in QD size—as opposed to the typically-observed continuous growth of QD sizes. While postulated to result from QD structures of energetic stability, evidence of the nature of stable structures is limited in experiment by microscopy and in computational simulation by the sheer number of electrons present within even the smallest of QDs. This work highlights a new computational approach toward treatment of QD behavior that forgoes quantum mechanical (electronic state) information in lieu of computational speeds many orders of magnitude faster than ab initio methods. The modification of traditional charge equilibration (qEQ) methods to also account for the unique dielectric environment experienced by a QD during its synthesis (QD-qEQ) yields an approach that is fast and atomistic, providing rich information about the partial charge of each atom in the system at energetic equilibrium. When applied to a structure-set of 35,000+ unique wurtzite CdSe QDs with diameters ≤ 2.50 nm, energetic depressions are apparent at certain QD sizes, which agree generally with the sizes at which CdSe magic size QDs have been reported experimentally. Further, analysis of QD surface characteristics associated with such magic sizes yield surfaces made up of highly coordinated and relatively low energy atoms, suggesting that the magic size phenomenon is due to a thermodynamic stability afforded by certain QD surface atom arrangements. The accuracy and versatility of QD-qEQ positions this method as a potential candidate for simulation of QD properties at much larger scales, an intractable problem with traditional computational methods.

Applying charge equilibration methods to CdSe quantum dots to gain atomistic insight into the magic-size phenomenon

2019 Spring National Meeting

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