Two types of fabrication method have been proposed in the past: top-down techniques, such as electron beam lithography or focused ion beam milling, and bottom-up approaches based on natural lithography. The former offers good control over particle size, shape and spacing, but has drawbacks due to the serial nature of the process – low throughput, low speed, high cost of equipment – and can often result in nanostructures of poor chemical and crystalline quality. On the other hand, natural lithography using two-dimensional colloidal crystals is massively parallel and allows the fast synthesis of large arrays of nanostructures at low cost.

Optimal geometry

Researchers at the Swiss Federal Laboratories for Materials Testing and Research (EMPA) have developed a simple and controlled synthetic procedure for making well defined silver nanostructures. The technique, which involves electroless plating through a tailored mask of nanospheres, also allows for the morphology to be changed at will.

To study the SERS activity of its designs, the team probed the Raman response of an organic dye covering the substrate. The scientists found that silver nanograins arranged in nanorings represent an optimal geometry by maximizing the number of SERS-active hotspots.

The arrangement of nanostructures delivered strong and easy-to-measure Raman bands (see blue spectrum in image) from a quantity of crystal violet that gave no Raman signal at all when deposited on bare silicon (see green spectrum in image).

Further nanostructures with different, sophisticated morphologies made from other SERS-active metals, such as gold, are currently being investigated at EMPA.

The researchers presented their work in Nanotechnology.