Colloidal semiconductor nanocrystals are promising materials for a number of energy transfer and photocatalytic systems due to their large absorption cross sections, significant material tunability, and relatively long excited state lifetimes. To understand the effectiveness of CdS nanocrystals in these systems, it is also necessary to understand competing intrinsic properties within the nanocrystal, as well as factors which drive both electron and hole transfer processes. We have discovered that many CdS nanocrystal systems undergo a photocharging process wherein photogenerated holes transfer to the native surface capping ligand, which subsequently generates a long lived, negatively charged nanocrystal. This charged state persists for many minutes after illumination, and the sample eventually fully recovers to the original absorptive state without the use of external redox agents. We explore the dependence of this photohcarging process on a variety of surface capping ligands, solvents, and nanocrystal morphologies. We further note that CdSe nanocrystals do not exhibit photocharging, even with similar nanocrystal sizes and capping ligands. These results have significant impacts on a broad range of photophysical optical studies, which operate under conditions which can readily photocharge nanocrystals. Furthermore, understanding and controlling this ligand mediated charging process has significant implications for other photo-redox systems, as the degree and rate of photocharging is highly sensitive to the exact system properties.

Controlling and understanding ligand mediated photocharging of CdS nanocrystals

ACS Spring 2022

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