High-Throughput Fabrication of Bimetallic Au-Ag Annular Nanogap Arrays Featuring 5 nm Gaps for Surface-Enhanced Raman Spectroscopy.

The confinement of light into nanometer-sized (typically <10 nm) metallic nanogaps can generate extremely high near-field enhancement, dramatically boosting absorption, emission, and Raman scattering of target molecules situated within the gaps. However, the scarcity of cost-effective, reliable, and high-throughput nanofabrication techniques capable of fabricating sub-10 nm gaps, particularly in binary nanostructures, has significantly hindered their practical applications. Here, a high-throughput nanofabrication strategy that combines anodic aluminum oxide (AAO) templates, molecular self-assembly, and glancing-angle ion-beam etching (IBE) is proposed to controllably produce large-scale bimetallic gold-silver (Au-Ag) arrays with approximately 5 nm gaps. These arrays serve as exceptional surface-enhanced Raman spectroscopy (SERS) substrates, allowing for the ultrasensitive detection of target molecules with limits of detection (LODs) as low as 10-11 M for rhodamine 6G (R6G) and 4-mercaptobenzoic acid (4-MBA) and 10-8 M for bilirubin. The bimetallic Au-Ag nanogap arrays leverage the synergistic effects of gold's strong absorption and silver's superior plasmonic properties, achieving significantly enhanced electromagnetic fields compared to monolithic Au-Au nanogap arrays. This cost-effective and efficient nanofabrication approach overcomes the limitations of conventional methods for fabricating binary nanostructures, providing a viable solution for developing high-sensitive and reliable SERS-based detection platforms toward chemical-/bio-sensing.