Abstract
Tip-enhanced Raman spectroscopy (TERS), an emerging technique combining scanning probe microscopy (SPM) and Raman spectroscopy, provides the sensitivity and selectivity necessary for chemical imaging with nanoscale resolution. We demonstrate that coating the fragile plasmonic structures used to enhance the electric field with a protective layer improves significantly its mechanical, chemical,
and thermal stability without loss of the extreme optical enhancement that appears as a “blinking” phenomenon. This observation provides important information on the mechanism of blinking. The fact that TERS blinking from a polymer layer is preserved when the analyte is separated from the plasmonic nanostructure by a 3 nm thick coating proves that chemical enhancement and charge transfer do not play a significant role in the blinking. Rather, thermally activated diffusion appears to be the dominant mechanism. While extreme enhancement persists only for minutes for an unprotected silver-coated tip, it persists for tens of hours with plasmonic structures coated by an ultrathin layer of alumina (Al2O3), opening broader opportunities for the study of blinking and for chemical imaging and sensing based on plasmonic structures.
