Percolation threshold gold films on columnar coatings: characterisation for SERS applications

TL;DR

This study investigates the controlled percolation of ~15 nm gold films on nanostructured porous and columnar coatings to optimize SERS performance, revealing significant enhancement mechanisms and phase retardation effects.

Contribution

It introduces a method to control gold film percolation on specific nanostructured substrates for improved SERS applications, highlighting the effects of deposition parameters.

Findings

Gold percolation films achieve up to 10^3 SERS counts/(mW.s)

Phase retardation of up to 5% in visible range

Additional SERS enhancement via Fresnel near-field mechanism

Abstract

Percolation of gold films of ~15 nm thickness was controlled to achieve the largest openings during Au deposition. Gold was evaporated on 300-nm-thick films of nanostructured porous and columnar SiO2, TiO2 and MgF2 which were deposited by controlling the angle, rotation speed during film formation and ambient pressure. The gold films were tested for SERS performance using thiophenol reporter molecules which form a stable self-assembled monolayer on gold. The phase retardation of these SERS substrates was up to 5% for wavelengths in the visible spectral range, as measured by Stokes polarimetry. The SERS intensity on gold percolation films can reach 10^3 counts/(mW.s) for tight focusing in air, while back-side excitation through the substrate has shown the presence of an additional SERS enhancement via the Fresnel near-field mechanism.