Packing spheres is easy; organizing non-spherical objects into ordered arrays is much harder, especially at the nanoscale. However, if the spheres can be modified after they have self-assembled into ordered structures, this problem could be overcome. Such a strategy has now been applied by X. S. Zhao and co-workers at the National University of Singapore to create two-dimensional arrays of ‘nano-vials’.

The process begins with the assembly of a close-packed monolayer of polystyrene nanospheres on a glass substrate coated with indium tin oxide. Electrochemical deposition of nickel then partly fills the spaces between the spheres to a height that is greater than their radius but less than their diameter. Dry etching with a plasma selectively removes the cores of the nanoparticles, with the nickel masking the rest of each sphere. Finally, wet-chemical etching with hydrochloric acid removes the nickel to leave an array of nano-vials.

The depth and diameter of the nanoholes can be easily tuned: longer exposure to the dry-etching step, for instance, makes the holes deeper. The nano-vials could be filled with other materials to make composite nanoparticles and may find applications in chemical or biochemical analysis.