Protecting TERS Probes While Keeping Extreme Enhancement

Protecting the probes used for tip enhanced Raman spectroscopy (TERS) with alumina coatings reduces chemical, mechanical and thermal degradation that otherwise limit the applicability of this emerging technique. Protected plasmonic structures are of interest for surface enhanced Raman spectroscopy (SERS) generally, especially for SERS-based sensors, but we focus particularly on the special case of TERS. Most recently we have focused on the detailed effect of the protective coating for cases in which the enhancement is particularly strong. ``Blinking,'' which is characteristic of extreme enhancement, has been observed with TERS on polymer films for the first time. An Al$_{2}$O$_{3}$ coating prolongs the duration of blinking from 20 minutes to 2 days without significant detriment to the extreme enhancement. The fact that blinking has been observed in the presence of the alumina coating allows us to eliminate chemical enhancement as a major mechanism of the extreme enhancement evidenced by blinking.