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.
References
Yan, Q. et al. J. Mater. Chem. 16 2132–2134 (2006).
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Cantrill, S. Arrays made easy. Nature Nanotech (2006). https://doi.org/10.1038/nnano.2006.8
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DOI: https://doi.org/10.1038/nnano.2006.8