Towards Crystals of Crystals of NanoCrystals: a Self-Assembly Study

Abstract

In this thesis several methods to synthesise monodisperse nanoparticles and how to self-assembled them within emulsion droplets are presented. The self-assembly behaviour of nanoparticles within the spherical confinement of emulsion droplets resulted in highly ordered crystalline supraparticles that were readily redispersable and thus in turn can be self-assembled again, opening the way to hierarchical self-assembly. By examining the highly ordered internal and external structures of the supraparticles a transition region in self-assembly behaviour as a function of the number of nanoparticles in the supraparticle was observed. When supraparticles consisted of more than ~65 thousand nanoparticles bulk-like self-assembly behaviour was observed. Supraparticles with lower numbers of constituents resulted in twenty-fold icosahedral ordering, which is incompatible with long range order. Simulations of hard-spheres self-assembled in spherical confinements showed the same icosahedral ordering. This showed that this twenty-fold ordering of spherical constituents is entropically favourable over bulk crystallization for assemblies consisting of less than ~50.000 spheres. Supraparticles consisting of photoluminescent core-multishell semiconductor nanoparticles ( CdSe-ZnS quantumdots) showed bright photoluminescence at equal quantum yields as the individual constituents. Suspending these quantumdot supraparticles on a bed of zinc oxide nanowires showed strong modulation in the photoluminescence spectra which is indicative of laser action and likely the result of optical resonances or Mie modes over the surface of the supraparticles. This suggests that low threshold lasing is possible using these self-assembled systems.