Site selective surface enhanced Raman scattering on nanostructured cavity arrays

Raman spectroscopy is an extremely powerful analytical tool. Surface enhanced Raman scattering (SERs) enables sample sensitivity to extend down to the single molecule level. There is presently great interest in using uniform nanostructured surfaces to give reproducible and strong surface enhanced Raman (SER) signal. The nanocavities studied here have spherical cap architecture and are arranged uniformly in an Au array. These structures support both localised and delocalised plasmons. Localised surface plasmon polaritons exist inside the nanocavities and delocalised or propagating surface plasmon polaritons exist on the flat surface of the sample (Bragg plasmons). The angle dependence property of surface enhanced Raman is used in the present work to enable comparison between SERs caused by localised plasmons and SERs caused by delocalised plasmons. The samples used here were modified to enable separate investigations of the two plasmon types. The externally modified array had dye placed only on the flat top surface of the array. The internally modified array had dye placed only on the internal walls of the cavities. Results show that the changes in Raman intensities with respect to the incident angle depend on the location of dye on the array.