Anisotropic surface enhanced Raman scattering in nanoparticle and nanowire arrays

Silver nanoparticles and nanowires self-aligned on pre-patterned rippled substrate are presented as active surface enhanced Raman scattering (SERS) substrates. The reported inter-particle gap of 5 nm and array periodicity of 35 nm are much lower than current lithographic limits. The observed anisotropy in SERS and surface plasmon resonance in such arrays is attributed to different plasmonic field enhancement along and across the chains of nanoparticles not due to shape anisotropy. For nanoparticle arrays higher SERS intensity is found along the particle chain, but for nanowire arrays higher SERS intensity is found for excitation across the wires. Higher intensity across nanowire arrays supports the argument that the SERS phenomenon is due to electromagnetic field enhancement (hot-junctions) caused by localized surface plasmon resonance across the nanowires having a 35 nm gap. The effect of inter-particle gap, ordering, and aspect ratio on field enhancement is demonstrated. Higher SERS intensity is observed in aligned elongated nanoparticles compared to aligned spherical, non-ordered nanoparticles, or aligned nanowires. Aligned silver nanowires enhance Raman scattering more strongly than aligned gold nanowires.