D. Häfliger, J. Plitzko, and R. Hillenbrand
Applied Physical Letters 85, 4466 (2004)
We show that the scattering efficiency and image contrast of commercial Si tips used in “apertureless” scattering-type near-field optical microscopy can compare well with PtIr- or Au-covered tips as predicted by a simple dipole model. However, high scattering efficiency of Si tips is only achieved after etching an unexpected up to 20-nm-thick amorphous SiO2 layer generally found at the tip apex. The oxide was revealed by transmission electron microscopy. The unverified presence of SiO2 may explain contradicting reports published on the optical performance of Si probes. Bare Si tips offer advantages over metal probes due to reduced fluorescence quenching, low nonlinear optical coefficient, and less wear.