Extending lifetime of plasmonic silver structures designed for high-resolution chemical imaging or chemical and biological sensing

High resolution chemical imaging of surfaces can be achieved using Tip Enhanced Raman Spectroscopy(TERS), an emerging technique that combines scanning probe microscopy with optical spectroscopy and takes advantage of apertureless near-field optics to obtain lateral resolution dramatically better than that provided by conventional optics. So far a 20 nm lateral resolution in chemical imaging of a surface has been achieved. The plasmonic structures on the tip used for imaging could also be used for novel, high sensitivity, local chemical and biological sensing. However, the silver plasmonic structures suffer from limited lifetimes due to morphological changes resulting from heating, wear during imaging, and tarnishing. The lifetimes of silver plasmonic structures on flat surfaces (as model systems) and on silicon nitride TERS tips have been extended by depositing over the silver an ultrathin (3nm) silicon oxide (SiO_x) coating. With this thickness protective coating, the contrast factor for the tip, which is the key parameter controlling one's ability to image with the tip, is decreased slightly (∼10%) initially, but the rate at which the signal enhancement degrades is sharply reduced. The silver layer on an unprotected tip was mechanically damaged after only three images of a polymer surface, while a silver layer protected by SiO_x remained intact after scanning three images.