Nondisturbing and Stable SERS-Active Substrates with Increased Contribution of Long-Range Component of Raman Enhancement Created by High-Temperature Annealing of Thick Metal Films

Abstract

High-temperature annealing of thick silver films (TSFs) deposited onto a smooth dielectric substrate leads to high-order self-organization of metal clusters on the film surface. A comparative atomic force microscopic (AFM) analysis of “as-deposited” and annealed TSFs (aTSFs) shows that uniform ellipsoidal roughness ∼41 × 25 nm in lateral cross section and ∼45 nm in height results after annealing. These metal clusters are mutually oriented so that the main lateral axes of the ellipsoids are nearly parallel. UV−visible data demonstrate a ∼300 nm hypsocromic shift of the bands corresponding to the collective surface plasmon modes. Additionally, a new (∼350 nm) band related to the normal component of the plasmon oscillations appears after annealing. This band was found to be strongly angle-dependent for p-polarized light. The aTSFs appeared extremely time- and organic solvent-stable versus as-deposited films. The aTSFs were found to be nondisturbing surface-enhanced Raman scattering (SERS)-active substrates in the application to studies of complexation of crown ether styryl dyes with metal ions. A pronounced SERS signal of the analyte rhodamine 6G was observed with aTSFs, even when the analyte was separated from the silver surface with five Langmuir−Blodgett monolayers of stearic acid. At the same time, depositing only a monolayer of stearic acid on the as-deposited film completely suppressed the SERS signal of the analyte. Finally, the self-assembling of Ag clusters on the surface of the aTSF, stimulated by the high-temperature annealing, results in the creation of a time- and organic solvent-stable SERS substrate with nanometer-scale quasi-periodical roughness, and this substrate exhibits an increased contribution of the electromagnetic component to the overall Raman enhancement.