Enhancement of SERS background through charge transfer resonances on single crystal gold surfaces of various orientations.

Surface-enhanced Raman scattering (SERS) spectra are accompanied by broad background emission, which limits improvements in the signal-to-noise ratio. Despite the close correlation between the background generation and the SERS enhancement, the chemical origin of the background emission has remained somewhat mysterious. In this work, SERS spectra of organic monolayers are systematically measured on an atomically defined single crystalline gold surface of various orientations, which specifically define metal-molecule chemical interactions. The use of sphere-plane type plasmonic nanogap structures on a well-defined surface enables us to evaluate the contribution of charge transfer resonances to SERS enhancement. The present results not only reveal that charge transfer resonance at metal-molecule interfaces increases the intensity of plasmon-mediated broadband emission but also provide us a consistent view about electronic structures of metal-molecule interfaces.