Effects of manifold gaps on SERS enhancement in coupled plasmonic nanoantennas

Since a couple of decades, surface-enhanced Raman scattering (SERS) is playing a crucial role for high-sensitive label-free detection in several research fields, from biophysics and molecular biology up to materials science... [ view full abstract ]

Since a couple of decades, surface-enhanced Raman scattering (SERS) is playing a crucial role for high-sensitive label-free detection in several research fields, from biophysics and molecular biology up to materials science [1-2]. Even single molecule detection regimes have been reported [3-4], thanks to very high enhancing factors accomplished with specific experimental conditions. In order to make SERS results as reliable and reproducible as possible, large efforts have been also dedicated to the study of plasmonic resonances conditions in nanostructures. Coupled plasmonic elements, like circular dimers and rod-shaped nanoantennas where plasmonic resonances are tuned by geometrical features [5], have been widely investigated and proposed as SERS nanosensors.

In this work we investigate three different kinds of nanostructures in the shape of trimers (3-arms), pentamers (5-arms) and eptamers (7-arms). All the arrays of nanostructures are fabricated over Si substrates by electron beam lithography in combination with thermal metal deposition and electroless Ag deposition. The geometrical parameters relevant for plasmonic resonances are kept constant through all kinds of nanostructures, more in details all the arms are 160nm×60nm while all the gaps between arms are 20nm wide. Rhodamine (R6G) is deposited over the samples before micro-Raman measurements, in order to probe their SERS capabilities. Keeping constant all the experimental conditions, the largest Raman signals are achieved on the 5-arms nanostructures (pentamers), even if the 7-arms ones have a larger number of SERS active spots.

This result suggests that plasmonic resonances occurring in these nanogaps is not relying on the single gap behavior only, but that a potential coupling effect between neighboring gaps could lead to a further enhancing factor. Obviously the inter-gap enhancement would strongly rely on spacing between gaps, and further improvements in SERS sensitivity could be achieved by proper design of nanostructures with manifold gaps.

1. K. Kneipp et al., J. Phys.: Condens. Matter (2002), 14:R597–R624

2. J. Chao et al., Journal of Materials Chemistry B (20 16), 4:1757-1769

3. K. Kneipp et al., Phys. Rev. Lett. (1997), 78:1667-1670

4. S. Nie et al., Science (1997), 275:1102-1106.

5. O.L. Muskens et al., Optics Express (2007) 15:17736-46