Abstract

Colloidal nanocrystals are thought to be viable candidates for the fabrication of solar cells, photo-detectors, etc. …. In such a device context, the nanocrystals will always be deposited in thin layers. However, device – relevant properties of these colloids (such as absorption cross section, exciton lifetime, …) are typically evaluated in solution, where uncoupled optical dipoles are used as modeling framework. The latter assumption breaks down when nanocrystals are put in close packed layers since the depolarization fields of neighboring dots can influence e.g. the polarizability of a nanocrystal through (electrostatic) near-field coupling. More in particular, the absorption cross section of colloidal quantum dots in close-packed monolayers shows a 4 (CdSe) to 5-fold (PbS) enhancement compared to quantum dots in a dilute dispersion. Quantitative agreement is demonstrated between the value and the size dependence of the experimentally determined enhancement and theoretical model predictions based on dipolar coupling between neighboring quantum dots. Having verified the validity of this model, we show that absorption anisotropy and modified spontaneous emission could occur in these super-lattice structures. Current research focuses on demonstrating these exciting new phenomena, in particular the absorption anisotropy. The collective optical behavior in nanocrystal super-lattices, demonstrated for the first time by our work, offers a new degree of freedom in the design of custom optical properties for electro-optical devices using wet-processable colloidal nanocrystals.