Single Particle Spectroscopy Study of Metal-Film-Induced Tuning of Silver Nanoparticle Plasmon Resonances

We present an experimental study of the tunability of the silver nanoparticle localized plasmon resonance in close proximity to a gold film. Broad-band tuning of the silver particle plasmon resonance from blue wavelengths into the near-IR region can be achieved due to strong electromagnetic coupling between the nanoparticle and the metal film. When the thickness of a thin silica spacer layer between the metal nanoparticle and the metal film is altered, the resonance frequency shifts. Single particle spectroscopy of over 250 isolated silver nanoparticles revealed evidence for the excitation of both horizontal and vertical plasmon modes. Distinct resonance features observed in the scattering spectra were assigned to specific modes based on a dipole-dipole interaction model. The experimental results suggest that low-loss silver nanoparticles can be used in surface-enhanced spectroscopy studies throughout the entire visible spectrum. The use of frequency-tuned spherical metal nanoparticles on solid substrates could lead to thermally stable substrates for plasmon-enhanced sensing applications, including surface-enhanced Raman scattering and refractive index based biodetection methods.