Extending Lifetime of Plasmonic Silver Structures Designed for High-resolution Chemical Imaging or Chemical and Biological Sensing

摘要

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