Stable green light-emitting quantum dots (QDs) are necessary for applications ranging from display technologies to bioimaging. Random, temporary fluctuations in emission known as “blinking,” and the complete lack of emission known as “photobleaching,” are some of the undesirable instabilities researchers looking to eliminate.

Following a comprehensive study of the relationship between a QD’s structure and its properties, McBride et al. used thick shells, with 6 to more than 14 monolayers of zinc sulfide, to suppress blinking and photobleaching in green QDs.

“Anytime a QD is non-emissive, its efficiency is sub-optimal and it is indicative of a non-radiative process hijacking photoluminescence – temporary in the case of blinking or permanent if photobleaching,” said author Jennifer Hollingsworth.

The researchers found adding a ZnS shell to the QDs via a successive ionic layer adsorption and reaction process provides stability against blinking and photobleaching under continuous laser excitation. However, this improvement comes at the cost of increased surface roughness and irregularity, which is detrimental to performance.

“Increased shell size alone does not doom these QDs to suboptimal efficiencies,” Hollingsworth said. “Rather, as we show, if the quality of the shell is closer to ideal, then the optical properties follow even for a very thick shell.”

Surprisingly, synthesizing the green QDs in a single-pot reaction with alternating additions of precursor layers does not yield the predicted chemical gradient nor the required smooth zinc sulfide shell, so a multi-step approach is necessary.

These findings point toward the complicated processes required for achieving thick, smooth and chemically pure shells required for bright and stable green QDs, and additional work is needed to enable defect-free growth.

Source: “Role of shell composition and morphology in achieving single-emitter photostability for green-emitting ‘giant’ quantum dots,” by James R. McBride, Nimai Mishra, Sophia M. Click, Noah J. Orfield, Feng Wang, Krishna Acharya, Matthew F. Chisholm, Han Htoon, Sandra J. Rosenthal, and Jennifer A. Hollingsworth, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0002772.