Coupled synthesis and self-assembly of nanoparticles to give structures with controlled organization

Colloidal inorganic nanoparticles have size-dependent optical, optoelectronic and material properties that are expected to lead to superstructures with a range of practical applications(1,2). Discrete nanoparticles with controlled chemical composition and size distribution are readily synthesized using: reverse micelles and microemulsions as confined reaction media(3-5), but their assembly into well-defined superstructures amenable to practical use remains a difficult and demanding task This usually requires the initial synthesis of spherical nanoparticles, followed by further processing such as solvent evaporation(6-8), molecular cross-linking(9-14) or template-patterning(15-18). Here we report that the interfacial activity of reverse micelles and microemulsions can be exploited to couple nanoparticle synthesis and self-assembly over a range of length scales to produce materials with complex organization arising from the interdigitation of surfactant molecules attached to specific nanoparticle crystal faces. We demonstrate this principle by producing three different barium chromate nanostructures-linear chains, rectangular superlattices and long filaments-as a function of reactant molar ratio, which in turn is controlled by fusing reverse micelles and microemulsion droplets containing fixed concentrations of barium and chromate ions, respectively. If suitable soluble precursors and amphiphiles with headgroups complementary to the crystal surface of the nanoparticle target are available, it should be possible to extend our approach to the facile production of one-dimensional 'wires' and higher-order colloidal architectures made of metals and semiconductors.