Silicon Nanocrystal Superlattice Nucleation and Growth.

Colloidal dodecene-passivated silicon (Si) nanocrystals were dispersed in hexane or chloroform and deposited onto substrates as face-centered cubic superlattices by slowly evaporating the solvent. The uniformity of the nanocrystals enables extended order; however, the solvent and the evaporation protocol significantly influence the self-assembly process, determining the morphology of the films, the extent of order, and the superlattice orientation on the substrate. Chloroform yielded superlattices with step-flow growth morphologies and (111)SL, (100)SL, and (110)SL orientations. Hexane led to mostly island morphologies when evaporated at room temperature with exclusively (111)SL orientations. Higher evaporation temperatures led to more extensive step-flow deposition. A model for the surface diffusion of nanocrystals adsorbed on the superlattice surface is developed.

Orientationally Ordered Silicon Nanocrystal Cuboctahedra in Superlattices.

Uniform silicon nanocrystals were synthesized with cuboctahedral shape and passivated with 1-dodecene capping ligands. Transmission electron microscopy, electron diffraction, and grazing incidence wide-angle and small-angle X-ray scattering show that these soft cuboctahedra assemble into face-centered cubic superlattices with orientational order. The preferred nanocrystal orientation was found to depend on the orientation of the superlattices on the substrate, indicating that the interactions with the substrate and assembly kinetics can influence the orientation of faceted nanocrystals in superlattices.

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.

Nanocrystal superlattices that exhibit improved order on heating: an example of inverse melting?

Grazing incidence small angle X-ray scattering (GISAXS) measurements reveal that superlattices of 1.7 nm diameter, gold (Au) nanocrystals capped with octadecanethiol become significantly more ordered when heated to moderate temperatures (50-60 °C). This enhancement in order is reversible and the superlattice returns to its initially disordered structure when cooled back to room temperature. Disorder-order transition temperatures were estimated from the GISAXS data using the Hansen-Verlet criterion. Differential scanning calorimetry (DSC) measurements of the superlattices exhibited exotherms (associated with disordering during cooling) and endotherms (associated with ordering during heating) near the transition temperatures. The superlattice transition temperatures also correspond approximately to the melting and solidification points of octadecanethiol. Therefore, it appears that a change in capping ligand packing that occurs upon ligand melting underlies the structural transition of the superlattices. We liken the heat-induced ordering of the superlattices to an inverse melting transition.