Self-assembly of nanoparticles into structured spherical and network aggregates

Multi-scale ordering of materials is central for the application of molecular systems in macroscopic devices. Self-assembly based on selective control of non-covalent interactions provides a powerful tool for the creation of structured systems at a molecular level, and application of this methodology to macromolecular systems provides a means for extending such structures to macroscopic length scale. Monolayer-functionalized nanoparticles can be made with a wide variety of metallic and non-metallic cores, providing a versatile building block for such approaches. Here we present a polymer-mediated 'bricks and mortar' strategy for the ordering of nanoparticles into structured assemblies. This methodology allows monolayer-protected gold particles to self-assemble into structured aggregates while thermally controlling their size and morphology. Using 2-nm gold particles as building blocks, we show that spherical aggregates of size 97 +/- 17 nm can be produced at 23 degrees C, and that 0.5-1 microm spherical assemblies with (5-40) x 10(5) individual subunits form at -20 degrees C. Intriguingly, extended networks of approximately 50-nm subunits are formed at 10 degrees C, illustrating the potential of our approach for the formation of diverse structural motifs such as wires and rods. These findings demonstrate that the assembly process provides control over the resulting aggregates, while the modularity of the 'bricks and mortar' approach allows combinatorial control over the constituents, providing a versatile route to new materials systems.     

Electrically induced structure formation and pattern transfer

The wavelength of light represents a fundamental technological barrier to the production of increasingly smaller features on integrated circuits. New technologies that allow the replication of patterns on scales less than 100 nm need to be developed if increases in computing power are to continue at the present rate. Here we report a simple electrostatic technique that creates and replicates lateral structures in polymer films on a submicrometre length scale. Our method is based on the fact that dielectric media experience a force in an electric field gradient. Strong field gradients can produce forces that overcome the surface tension in thin liquid films, inducing an instability that features a characteristic hexagonal order. In our experiments, pattern formation takes place in polymer films at elevated temperatures, and is fixed by cooling the sample to room temperature. The application of a laterally varying electric field causes the instability to be focused in the direction of the highest electric field. This results in the replication of a topographically structured electrode. We report patterns with lateral dimensions of 140 nm, but the extension of the technique to pattern replication on scales smaller than 100 nm seems feasible.