Credit: © 2007 ACS

Building functional devices at the nanoscale requires that very small objects be manipulated and organized into ordered structures. Carbon nanotubes, with their unique mechanical and electronic properties, are appealing nanoconstruction materials and their rational assembly is widely studied.

Now, Chad Mirkin, George Schatz and co-workers1 from Northwestern University in the USA have shown how carbon nanotubes can be bent into circular structures using a patterned surface as a template. Bending nanotubes is of particular interest because it allows their unusual curvature-dependent properties to be studied.

Using dip-pen nanolithography, a gold surface was coated with a monolayer in which circular islands comprising hydrophilic molecules were surrounded by a sea of hydrophobic ones. When placed on this surface, nanotubes assembled along the boundaries between the hydrophobic and hydrophilic regions, forming circles with diameters as small as 100 nm. By performing simulations, it was shown that ring formation could be predicted by comparing two opposing forces — the strain energy involved in bending a nanotube and van der Waals forces between the nanotube and the surface groups.

Although nanotube rings have been made previously, this approach can be used to pattern circles of defined diameters on a surface. In combination with the modelling, this work provides a systematic framework that can explain the surface assembly and organization of carbon nanotubes.