The shaped nanosphere lithography process starts with the formation of a self-assembled monolayer of polystyrene spheres. Then the nanosphere array is exposed to a reactive ion etching process (RIE) where, under the appropriate conditions, the spheres shrink, collapse and become mushroom-shaped (see 1a). Gold deposited through either sputtering or evaporation onto these structures fills in the gaps between the spheres and also builds up under the lips of the shaped spheres.

A key innovation in this work is the use of non-conductive edge strips placed on top of the samples prior to metal removal (see 1b). Such elements help direct the flow of reactive ions during plasma etching, and lead to highly uniform and reproducible production of either crescents or rings (see 1c and 1d) over areas greater than 1 mm2. Fabricating such an array using conventional techniques – for example, by electron beam lithography – would be a time-consuming process. For the Rice team, it took just a couple of minutes to make a surface that contained billions of nanostructures.

Furthermore, the scientists can easily manipulate feature diameter, height and, where relevant, the crescent gap size. In the study, they fabricated features with outer diameters in the range 200–300 nm, which had strong infrared extinction peaks.

Currently, the team is developing nanoring- and nanocrescent-based substrates for surface enhanced infrared absorption spectroscopy (SEIRA), which allow more sensitive and, in some cases, more selective chemical sensing. The researchers are also working on the fabrication of metamaterial components with diameters under 100 nm, which have potential for generating resonances in the visible range.

More details can be found in the journal Nanotechnology.