Credit: © 2006 IOP

The self-assembly of colloidal nanoparticles is being increasingly used to build ordered three-dimensional structures. These ‘artificial opals’, as they are widely known, are of interest as photonic crystals. However, research to-date has centred on the synthesis and applications of spherical particles. The availability of methods for producing surfaces with other textures would open up the possibility of novel applications in photolithography, chemical sensing and optical cavities.

Now, Léon Woldering and colleagues1 from the University of Twente, and the FOM Institute for Atomic and Molecular Physics in The Netherlands have developed a milling process that uses a focused beam of gallium ions to selectively etch 40-nm cavities in individual nanospheres on the surface of artificial opals made from silica. It is possible, with this technique, to control both the size and position of the cavities with nanometre precision. Futhermore, the results were independent of the size of colloidal particles, the method used to grow the opal films, and whether chemicals were introduced to enhance the etch rates. Nanoparticles could also be etched all the way through to produce doughnut-like ‘nanobeads’.

This method allows for the formation of much smaller cavities in opal films than other techniques based on electron or laser beams, and such arrays could be used as building blocks for lithographic masks.