A defining characteristic of semiconducting nanocrystals, or quantum dots (QDs), is that their geometry dictates their optical properties.Computational models have been developed that can accurately predict the electronic structure and thus optical properties of a variety of QD materials. However, these computational models applied to core/shell and other heterostructure QD materials have received less experimental corroboration owing to the difficulty in systematically synthesizing large ranges of geometries. Notably, most models predict a significant effect of core size on the radiative rate in core/shell II-VI QDs. In the current work, we synthesized a library of core/shell CdSe/CdS QDs with varying core sizes and shell thicknesses, and have characterized their radiative recombination rates. We find that the core size had only a modest effect on the radiative recombination rates. To explore this finding in more depth, we implemented an effective mass computational model that solves wavefunctions for excited electron/hole pairs in any spherical geometry. Our model uses the finite difference method, which simplifies boundary conditions that would otherwise be difficult to solve analytically and leaves us with a method that can be applied to any arbitrary core/shell/shell or core/alloy/shell spherical QD structure. We use the aforementioned experimental data to empirically modify this model. We find that the conduction band offset between CdSe and CdS must be empirically altered based on QD core size in order to fit our experimental data. This is hypothesized to be a result of reduced interfacial strain in core/shell QDs with small cores. The derived relationship between conduction band offset and core size is used to create a predictive map of radiative lifetime as a function of core size and shell thickness. This map may be useful to researchers implementing CdSe/CdS core/shell QDs for a variety of applications since it can provide geometry specific excited state lifetimes.

Core size has a relatively small effect on the radiative rate of CdSe/CdS core/shell quantum dots

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.