Bubble Assemblies of Nanocrystals: Superlattices without a Substrate.

A method was developed to create free-standing nanocrystal films in the form of solidified bubbles. Bubbles of octadecanethiol-capped gold nanocrystals were studied by in situ grazing incidence small-angle X-ray scattering (GISAXS) to determine how the absence of an underlying substrate influences a disorder-order transition of a nanocrystal superlattice. We find that the presence of the substrate does not significantly change the nature of the disorder-order transition but does lead to reduced interparticle separation and reduced thermal expansion. Bubble assemblies of silicon and copper selenide nanocrystals are also demonstrated.