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.