Layer-By-Layer Assembly of Ag Nanowires into 3D Woodpile-like Structures to Achieve High Density “Hot Spots” for Surface-Enhanced Raman Scattering

Abstract

The surface-enhanced Raman scattering (SERS) "hot spots" are highly localized regions of enhanced electromagnetic field within a SERS substrate that dominate the overall SERS intensity. This results in inhomogeneous distribution of SERS intensity in a SERS substrate, thus limiting their application as reproducible and ultrasensitive sensing platforms. Here, we address this challenge by fabricating Ag nanowires into three-dimensional (3D) woodpile-like platforms via layer-by-layer Langmuir-Blodgett assembly. We focus on promoting strong electromagnetic coupling between parallel and vertically stacked Ag nanowire pairs within the woodpile structure to achieve a high density of "hot spots" across the entire 3D SERS substrates. Raman mapping (x-y plane) demonstrates that all of the 3D Ag nanowire arrays exhibit a homogeneous SERS Raman intensity over a large area, whereas their monolayer counterpart experiences >50% of zero and/or an undetectable SERS signal. The SERS enhancement factor increases from 3.1 × 10(3) to 2.6 × 10(4), as the assembled Ag nanowire layer increases from monolayer to three layers, respectively. We attribute the homogeneous SERS signal to the high density of "hot spots" arising from the vertical and lateral gaps within the woodpile layers. The SERS signals plateau off when the number of layers increase from three to five, which can be attributed to limited laser penetration depth. The assembled multilayered silver nanowires demonstrate a larger SERS depth cross section and angle-independent SERS intensity, making such woodpile 3D SERS substrate more reliable and versatile for future sensing applications.