This paper addresses the assembly of body centered-cubic (bcc) superlattices of organic ligand-coated nanocrystals. First, examples of bcc superlattices of dodecanethiol-capped Au nanocrystals and oleic acid-capped PbS and PbSe nanocrystals are presented and examined by transmission electron microscopy (TEM) and grazing incidence small-angle X-ray scattering (GISAXS). These superlattices tend to orient on their densest (110) superlattice planes and exhibit a significant amount of {112} twinning. The same nanocrystals deposit as monolayers with hexagonal packing, and these thin films can coexist with thicker bcc superlattice layers, even though there is no hexagonal plane in a bcc lattice. Both the preference of bcc in bulk films over the denser face-centered cubic (fcc) superlattice structure and the transition to hexagonal monolayers can be rationalized in terms of packing frustration of the ligands. A model is presented to calculate the difference in entropy associated with capping ligand packing frustration in bcc and fcc superlattices.

Vesicles of dioleoylphosphatidylcholine (DOPC) formed by extrusion (liposomes) with hydrophobic alkanethiol-capped gold (Au) nanocrystals were studied. Dodecanethiol-capped 1.8 nm diameter Au nanocrystals accumulate in the lipid bilayer, but only when dried lipid/nanocrystal films were annealed with chloroform prior to hydration. Without chloroform annealing, the Au nanocrystals phase separate from DOPC and do not load into the liposomes. Gold nanocrystals with slightly longer capping ligands of hexadecanethiol, or with larger diameter of 4.1 nm, disrupted vesicle formation and created lipid assemblies with many internal lamellar attachments.

Colloidal silicon (Si) nanocrystal superlattices Nanocrystal superlattices have finally been made of the most commercially relevant semiconductor, silicon. These sterically-stabilized nanocrystals self-assemble into face-centered cubic (fcc) superlattices. Their thermal stability is much more robust than other types of nanocrystal superlattices because of the strong covalent bonding of the hydrocarbon capping layer.

Simple hexagonal (sh) AB2 binary superlattices (BSLs) of organic ligand-capped silicon (A; 5.40(±9.8%) nm diameter) and gold (B; 1.88(±10.1%) nm diameter) nanocrystals were assembled by evaporation of colloidal dispersions and characterized using transmission electron microscopy (TEM) and grazing incidence small-angle X-ray scattering (GISAXS). When deposited on tilted substrates by slow evaporation, the sh-AB2 superlattice contracts slightly towards the substrate with centered orthorhombic structure. Heating the BSL to 200°C in air led to gold coalescence and segregation to the surface of the assembly without disrupting the Si nanocrystal sublattice, thus creating a simple hexagonal superlattice of Si nanocrystals.

Dodecanethiol-capped gold (Au) nanocrystal superlattices can undergo a surprisingly diverse series of ordered structure transitions when heated (Goodfellow, B. W.; Rasch, M. R.; Hessel, C. M.; Patel, R. N.; Smilgies, D.-M.; Korgel, B. A. Nano Lett.2013, 13, 5710–5714). These are the result of highly uniform changes in nanocrystal size, which subsequently force a spontaneous rearrangement of superlattice structure. Here, we show that halide-containing surfactants play an essential role in these transitions. In the absence of any halide-containing surfactant, superlattices of dodecanethiol-capped (1.9-nm-diameter) Au nanocrystals do not change size until reaching about 190–205 °C, at which point the gold cores coalesce. In the presence of halide-containing surfactant, such as tetraoctylphosphonium bromide (TOPB) or tetraoctylammounium bromide (TOAB), the nanocrystals ripen at much lower temperature and superlattices undergo various ordered structure transitions upon heating. Chloride- and iodide-containing surfactants induce similar behavior, destabilizing the Au–thiol bond and reducing the thermal stability of the nanocrystals.