Vapor-deposited Au thin films modified by plasma etching for surface-enhanced Raman scattering active substrates

Abstract

Surface-enhanced Raman scattering (SERS) has been used in many fields as a high sensitivity spectroscopic technique. SERS involves local amplification of the electromagnetic field in close proximity to metal nanostructures. Therefore, the morphology of nanoscale structures greatly affects SERS spectroscopy. We carried out modification of the morphology of vapor-deposited Au thin films by plasma etching to form islandlike structures for the development of SERS-active substrates. The modified Au films were evaluated by atomic force microscopy and finite-difference time-domain simulation. The islands on plasma-etched Au films had finer structures because of the collisions of high-energy ions. Furthermore, the island size on the plasma-etched films was almost independent of the film thickness from 5 to 30 nm. When the film thickness decreased below ∼5 nm, many small islands were generated. Due to small islands, active sites where the electric field was enhanced significantly increased. From the results of SERS spectroscopy, it was found that SERS intensities by the film, which had a large number of small islands, are strongly enhanced. We concluded that the thinner Au film with thickness below ∼5 nm was the most suitable for SERS spectroscopy.