Entropy driven and reaction driven self-assembly of colloidal semiconductor nanocrystals

Abstract

Formation of binary superlattices (BNSLs) from colloidal nanocrystals (NCs) by self-assembly is a promising pathway towards novel materials with unique optoelectronic properties. While a host of different superlattice structures and material combinations have been reported, the driving forces behind BNSL formation are not fully understood. This information, if available, would be helpful in the rational design of nanostructured materials. In my research project we investigated the driving forces involved in the formation of binary (and ternary) superlattices. The main focus has been on superlattices built from semiconductor nanocrystals and combinations of semiconductor and metal nanocrystals. By comparing of results with calculations of crystal formation (among others performed at the Soft Condensed Matter group at Utrecht University) I found that the binary self-assembly process of semiconductor nanocrystals is entropy driven, while for metallic nanocrystals enthalpic (attracting) forces have to be taken into account. While the binary self-assembly is mainly entropy driven, single component assembly can also be achieved by ‘activation’ of crystal facets. In this way I have achieved oriented linear attachment of PbSe NCs into large rods, and 2-D square and honeycomb super lattices that are also atomically coherent. It is vital to understand the optical properties of the separate building blocks. While in literature PbSe and CdSe are widely investigated, the core|shell particles PbSe|CdSe are less characterized. I investigated the optical properties of PbSe|CdSe NCs, and found a striking result: the light-emission of the lowest exciton consist of two peaks that are not in thermal equilibrium.