Evaluation of probes for tip-enhanced Raman scattering by darkfield microspectroscopy and calculation

Tip-enhanced Raman scattering (TERS) can be observed highly sensitive spectral image with high spatial resolution. However, it shows low reproducibility due to difference and change in optical properties of the metallic tips. For surfaceenhanced Raman scattering (SERS), the spectra can be reproduced by the scattering spectra due to localized surface plasmon resonance (LSPR) of the individual metallic nanostructures, which observed with a dark field illumination, and the calculated electromagnetic field around the nanostructures. In the present study, we tried to relate TERS spectra with the LSPR spectra and the calculation, in a similar way of SERS. By conventional dark field illumination, LSPR scattering spectra at the apex of the tip were measured and were compared with the corresponding TERS spectra. By excitation using polarization parallel to the tip, the polarized LSPR peak was stronger than that by perpendicular polarization. Also in the case of TERS, the similar trend was observed. It was confirmed whether the vertical polarization to the sample plane (Z-polarization) is effective or not by the polarized LSPR and TERS spectra. By excitation at different wavelengths, moreover, TERS enhancement factors were compared. In the calculation for TERS, the nanostructure like a monopole antenna was adopted, because the EM field is enhanced not at both sides, but at only apex. The dependence on taper and curvature of the tip were compared with the calculated results for the nanostructure like a conventional dipole antenna.