There is considerable interest in using nano-structured surfaces instead of metal nanoparticles as SERS substrates.  With the advent of more accessible and reliable micro-fabrication and nano-lithographic techniques these types of surfaces can be made increasingly easily.

The need to aggregate metals nanoparticles in a controlled fashion to produce high Raman enhancements can limit the reproducibility of enhancements from these types of substrates. The aggregation step is difficult to control and can result in the formation of different shapes of metal clusters that can produce variable levels of enhancement. For this reason there is now considerable interest in using nano-structured surfaces instead of metal nanoparticles, as SERS substrates.  With the advent of more accessible and reliable micro-fabrication and nano-lithographic techniques these types of surfaces can be made increasingly easily.  Additionally substrates made from nanoparticles held together in regular arrays, or bound to surfaces, for instance with thiol bridges, are of considerable interest as they also offer improved reproducibility of enhancement.  These structured substrates have the advantage of being more reproducible than metal nanoparticles and being able to be tailored to balance the enhancement levels and reproducibility required for the desired application.  Current research at NPL is focused on evaluating the performance of such substrates and comparing these with metal nanoparticle-based systems.

This research includes the electromagnetic modelling of predicted Raman enhancements from different SERS substrates.  This allows us to design and optimise structures at the nanoscale to produce high enhancement. We are seeking the optimum balance between metallic structures that can reproducibly achieve high enhancement SERS enhancements, but yet can be manufactured simply and cost-effectively using methods such as e-beam lithography.

The figure below is an example illustrating the predicted electromagnetic field distribution when a laser is incident upon a pair of silver nanoparticles. The red colours represents higher field enhancement and therefore Raman enhancement, while the blue colours represent lower field enhancement and therefore lower Raman enhancements.

Modelled Raman enhancements around two silver nanoparticles of radius 50nm, separated by 1nm and illuminated with radiation of wavelength 514nm. The inset shows an enlarged portion of the main picture, around the area between the nanoparticles. The colours in red are the areas of highest Raman enhancement and those in blue are the areas of lowest Raman enhancement.  A maximum enhancement of approximately 10⁷ is predicted.