Effect of Nanogap Curvature on SERS: A Finite-Difference Time-Domain Study

Surface-enhanced Raman scattering (SERS) is one of the most important plasmonic effects, offering a wide range of applications. Strong SERS arises from narrow nanogaps between nanoparticles. The SERS enhancement factor (EF) depends on many parameters that define the nanogap, such as the gap distance, gap geometry, and size and material of the constituent nanoparticles. In this study, we focus on the effect of the curvature of the nanogap on SERS. We perform finite-difference time-domain (FDTD) simulations for Au nanocube-nanosphere dimers, where nanocubes are attached to nanospheres on their vertices with various radii of curvature. The calculations reveal that the induced electric field becomes more localized around the vertex of the nanocube in the nanogap with the decrease in the curvature radius. The EF also drastically increases when the corner of the cube in the dimer sharpens. The EF of the nanocube nanosphere dimer at,similar to 10(13) is far greater than that of nanosphere dimers or nanorod-nanosphere dimers. Through systematic changes of the variables that may affect the SERS EF, we find that in addition to the sharp local structure, a sizable volume is required to obtain the maximum EF. The curvature effect is the dominant contributor to the highest SERS EF for the nanocube nanosphere dimers, overwhelming radiation damping or plasmon damping by the interband transition. This study identifies the governing factors for SERS and provides a design principle for the best SERS substrates.