FDTD simulation study of size/gap and substrate-dependent SERS activity study of Au@SiO_2 nanoparticles

We use Au@SiO2 nanoparticles（NPs） to systematically and comprehensively study the relationship between nanostructure and activity for surface-enhanced Raman scattering. Calculation simulation using the finite different time domain method verifies the experiment results and further reveals that the particle size and the distance between the NPs play vital roles in the surface-enhanced Raman scattering（SERS）. Furthermore, in order to better simulate the real experiment, a Au@SiO2 nanosphere dimer is placed on the silicon substrate and Au substrate, separately. The simulation results show that the large EM field coupling is due to the ＂hot spots＂ transferred from the NP–NP gaps to NP–surface of metal gaps,meanwhile, more ＂hot spots＂ occur. We also find that the signal intensity strongly depends on the position of the probe molecule. This work provides a better understanding of EM field enhancement.