Sample induced intensity variations of localized surface plasmon resonance in tip-enhanced Raman spectroscopy

Tip-enhanced spectroscopy techniques, in particular tip-enhanced Raman spectroscopy (TERS), rely on a localized surface plasmon resonance (LSPR). This LSPR depends on the near field antenna, its material and shape, and the surrounding medium with respect to its relative permittivity and the volume fraction of the optical near field occupied by the sample. Here, we investigate the effects of the surface composition and topography on the change of the LSPR intensity in tip-enhanced spectroscopy on SrTiO₃ nanoislands by monitoring the LSPR enhanced luminescence of gold tips. Our experimental results and analytical estimates indicate that by affecting the effective permittivity of the dielectric environment at the tip apex, the material composition as well as topography of the studied sample induce a change in LSPR intensity. This result significantly helps the understanding of the evolution or origin of the LSPR intensity during a typical TERS measurement, which in turn leads to a more accurate assessment of the relative intensity of different Raman modes in TERS.