A simple way to create very small patterns on a surface by using a mixture of top-down and bottom-up technology has been developed by American and Chinese scientists. They combine self-assembly with evaporation to make highly ordered nanoscale structures.
Top-down and bottom-up approaches are complementary routes to make very tiny structures. As the names suggest, structures can be made either by building up with very small building blocks or by making something large smaller, e.g. by spraying tiny droplets or by exploiting evaporation. Each approach has pros and cons, and there are few methods that combine both approaches.
Now a team led by Myunghwan Byun at Iowa State University has used a combination of top-down and bottom-up to create hierarchically ordered nanoscale structures made of a block copolymer on a silica surface. They took a solution of block copolymer which is designed to self-assemble easily, and distributed drops of it in solution onto a silica surface using a special wedge-shaped lens. They then allowed the solvent to evaporate from within the confines of the wedge and the surface, and took advantage of capillary forces that pulled the shrinking droplet across the surface to cover the surface with the droplet contents. Because of the balance of capillary and the polymer–surface interactions which caused a “stick–slip motion” across the surface, stripes formed as the polymer was distributed unevenly upon the surface.
The scientists then treated the initial stripe pattern with a different solvent that caused the polymer properties to change and parts of it to swell. This swelling turned the stripes into rows of vertically aligned nanocylinders. Such arrays of nanostructures could be used in electronics, optics, and studies of cell adhesion.
This combined top-down/bottom-up approach has a great advantage over current technology as it does not require lithography, which is a relatively expensive technique, or rely on any form of external field (electrical, magnetic, etc.). So the method should be cheaper and less complicated to use than current techniques for nanopatterning. The authors believe that their method is extendable to many different types of materials, or at least different copolymers, and could be used also to produce many different kinds of nanodevice.