Quasi-3D gold nanoring cavity arrays with high-density hot-spots for SERS applications via nanosphere lithography.

Large-scale ordered arrays with dense hot spots are highly desirable substrates for practical applications such as surface-enhanced Raman scattering (SERS). In the past decade, most work has focused on using lateral gaps between two metal structures. However, the strength and density of the generated hot spots are limited to a 2D arrangement of nanostructures. In this work, we present a novel quasi-3D nanoring cavity structure, which contains a nanoring and a nanopillar in a nanohole. The fabrication is based on nanosphere lithography incorporated with dry etching and gold coating. Gold nanostructures with one layer (nanohole), 2 layers (nanohole + nanodisc), and 3 layers (nanohole + nanoring + nanopillar) were successfully fabricated and compared. The SERS performance of the three-layered nanostructures is about two orders of magnitude higher than the others. Finite-difference time-domain (FDTD) simulations show that incorporating nanopillars and nanorings into a nanohole array not only significantly increases the density of the hot spots but also achieves stronger electromagnetic field enhancements compared to a nanohole array. The simple fabrication of multilayered quasi-3D nanostructures provides a large-area and highly efficient SERS substrates for biological and chemical applications.