Plasmonic color analysis of Ag-coated black-Si SERS substrate

Abstract

Red-Green-Blue (RGB) dark-field imaging can direct the choice of laser excitation for Raman enhancements on nanostructured plasmonic surfaces. Here we demonstrate that black silicon (b-Si) is a structured surface that has been shown to effectively absorb broad wavelengths of light, but also enables surface enhanced Raman scattering (SERS) when coated with silver (Ag). Coating b-Si with increasing amounts of Ag results in increased dark-field scattering at discrete frequencies associated with localized plasmon resonances. The dark-field scattering was monitored by collecting a far-field image with an inexpensive complementary metal oxide semiconductor (CMOS) camera, similar to what is available on most mobile phones. Color analysis of the RGB pixel intensities correlates with the observed SERS intensity obtained with either green (532 nm) or red (633 nm) laser excitation in SERS experiments. Of particular note, the SERS response at 633 nm showed low spectral variation and a lack of background scattering compared to SERS at 532 nm. The difference in background suggests sub-radiant (dark or Fano resonances) may be associated with the SERS response at 633 nm and a non-resonant character of SERS. These results indicate that b-Si serves a template where Ag nucleates during physical vapor deposition. Increased deposition causes the deposits to coalesce, and at larger Ag thicknesses, bulk scattering is observed. Comparison with a high enhancement Ag SERS substrate further illustrates that a high density of plasmonic junctions, or hotspots, is important for maximizing the SERS response. The randomness of the b-Si substrate and the corresponding Ag nano-features contributes to a broadband spectral response and enhancement in SERS. Metal-coated b-Si is a promising SERS substrate due to its performance and facile fabrication.