Precise QD placement begins with spinning a thin resist on a substrate. Then an electron-beam lithography process defines an array of holes. Next a QD solution (6 nm diameter CdSe or 5 nm diameter CdSe/CdZnS) is drop cast. The dots fill the holes and coat the resist. Finally, the remaining resist is removed by dissolution in acetone, leaving only the QDs that were touching the substrate.

To minimize the number of QDs at a desired position, the team has optimized the QD solution, QD adhesion to the substrate, resist thickness and the electron-beam lithography process. This QD-placement technique resulted in an average of three QDs per location. Furthermore, QDs could be placed in close proximity to one another, with a minimum separation of 12 nm.

In the study, the researchers performed photoluminescence measurements on the QD clusters, demonstrating that the QDs remain optically active after the fabrication process, presenting intermittent photoluminescence.

This optimized top-down lithographic process is a step towards the integration of individual QDs in excitonic and plasmonic circuits.

A full description of this work can be found in the journal Nanotechnology.

This material is based on work supported as part of the MIT Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088.