The authors report experimental studies of surface-enhanced Raman scattering (SERS) of wurtzite-type GaN and ZnO crystalline samples covered with Ag-nanoparticles. The longitudinal optical phonons consistently exhibit unusually intense Raman enhancement in comparison with other phonons. The anomaly is interpreted by a proposed model based on a resonant Raman scattering process assisted by metal-induced gap states at the Ag/GaN and Ag/ZnO interfaces. This study suggests that SERS of lattice vibrations in inorganic semiconductors is sensitive to their propagation nature, providing a progressive perspective view on electron-mediated enhanced Raman scattering.

Uniform and highly surface-enhanced Raman spectroscopic (SERS)-active substrates have been fabricated using Ag nanoparticle arrays with unprecedented small tunable gaps. The dependence of the enhancing capability of the substrate on the gap size provides quantitative evidence for the collective SERS effect and confirms predictions of interparticle-coupling-induced Raman enhancement.