Control of Vertical Gap and Statistical Distribution in Hole-Sphere Gold Nanogaps for Sensitive and Quantitative SERS

Surface-enhanced Raman scattering (SERS) is a promising, sensitive, and label-free molecule detection scheme. However, uniformity and reproducibility of signal enhancement have remained elusive, making quantitative evaluation difficult. In this work, we propose a simple fabrication approach to quantitative SERS sensors that satisfies all the sought-after characteristics: a gold hole-sphere nanogap SERS substrate that is uniform, reproducible, sensitive, large, and cost-effective. Here, we achieve a sensing uniformity of 4.2% averaged over 4 points throughout the entire 6-in. substrate and a SERS enhancement of 4.6 × 108. Our approach provides for gap control in the vertical direction, thus granting very precise control with subnanometer accuracy and the statistical distribution of nanospheres in plane. This combination enables a remarkably uniform and reproducible SERS sensitivity over the entire substrate. The SERS spectra from DNA bases are also measured and their corresponding peaks are well defined down to 10 pM concentration. The proposed approach should be a key to quantitative SERS.