Quantum Dot Supercrystals: Assembly and properties*

The assembly of semiconductor nanoparticles, quantum dots (QD), into dense crystalline nanostructures holds great promise for future optoelectronic devices. Emulsion-templated assembly has recently been shown to provide three-dimensional quantum-dot supercrystals of excellent crystal quality. We combine emulsion-templated assembly with in situ small-angle X-ray scattering to obtain direct insight into the nanoscale interactions underlying the nucleation, growth and densification of quantum dot supercrystals. We show that while hard-sphere behaviour governs their initial nucleation, at later stages the ligands play a crucial role in balancing steric repulsion against attractive van der Waals attraction. We also elucidate the optoelectronic properties of these supercrystals. We show that the supercrystals exhibit Mie resonances that can lead to absorption efficiencies larger than one, beneficial for photovoltaic applications. Ultrafast spectroscopy reveals energy transfer processes due to quantum-dot coupling in these supercrystals.