Enhanced Raman Scattering from Nanoparticle-Decorated Nanocone Substrates: A Practical Approach to Harness In-Plane Excitation

摘要

We investigate surface-enhanced Raman scattering (SERS) from gold-coated silicon-germaniumudnanocone substrates that are decorated with 30-nm spherical gold nanoparticles (AuNPs). Finite-elementudsimulations suggest that individual nanocones generate stronger electromagnetic enhancement with axialudpolarization (i.e., polarization parallel to the vertical axis of the nanocones) than with transverse polarizationud(i.e., polarization in the plane of the nanocone substrate), whereas the excitation in a typical Raman microscopeudis mainly polarized in the transverse plane. We introduce a practical approach to improve the SERS performance ofudthe substrate by filling the valleys between nanocones with AuNPs. Simulations reveal an enhanced electric fieldudat the nanoscale junctions formed between AuNPs and nanocones, and we explain this lateral coupling with audhybridization model for a particle-film system. We further experimentally verify the added enhancement byudmeasuring SERS from trans-1,2-bi-(4-pyridyl) ethylene molecules absorbed onto the substrates. We report overudone order-of-magnitude increase in SERS activities with the AuNP decoration (compared to the nanocone substrateudwithout AuNPs) and achieve a spatially averaged enhancement factor of 1.78 x 10^8 at 785-nm excitation.udUnderstanding and implementing the enhancing mechanism of structured metallic surfaces decorated withudplasmonic nanoparticles open possibilities to substantially improve the SERS performance of the existing processengineered substrates.