Probing the Anisotropy of SERS Enhancement with Spatially Separated Plasmonic Modes in Strongly Coupled Silver Nanocubes on a Dielectric Substrate

The utilization of substrateparticle interactions provides a route to tuning the optical properties of strongly coupled supported plasmonic nanoparticles. In this work fine control over interparticle and particlesubstrate interactions is demonstrated using Langmuir-Blodgett monolayers of silver nanocubes deposited onto titanium oxide (TiOx) thin films of varying thickness. By using two Raman reporters, a Rhodamine-B (RhB) and benzenethiol (BT), surface-enhanced Raman spectroscopy (SERS) independently examines electromagnetic (EM) enhancement at the substrate/nanocube interface (RhB) and at the surface of the cubes, where the label is predominately 20-40 nm away from the dielectric substrate (BT). For RhB the SERS enhancement factor (EF) drops as much as an order of magnitude on 20 nm TiOx with respect to glass. However, for BT, a maximum SERS EF of (2.5 +/- 0.4) x 10(5) was observed on TiOx compared to (1.5 +/- 0.4) x 10(5) on glass, an increase of 60%. Control over the organization of the nanocube monolayer reveals that maximum enhancement occurs in small, discrete clusters of nanocubes as opposed to large aggregates. Fine control over the optical properties and near-field EM distribution of coupled nanostructures can be accomplished through tuning of the dielectric properties of the substrate yielding a route to optimizing properties for field-enhanced plasmonic applications.