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 (SiOx) 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 SiOx remained intact after scanning three images.
Several technologies have attempted to deliver the analytical capabilities of Raman and fluorescence spectroscopies to developing nanotechnologies. They have, however, two limitations when applied to nanoscale structures: (i) diffraction limit and (ii) weak signal due to a small sampling volume. To overcome the first obstacle, researchers traditionally use aperture-limited near-field optics based on optical fibers with extremely small apertures (down to ~50 nm). Low transmission through the apertures exacerbates the second limitation by strongly decreasing the measured optical signal. An alternative method based on plasmon optics, strong and very local enhancement of the electric field of light in the vicinity of plasmon nanoparticles (usually Ag or Au), helps to overcome both problems. We overview developments in apertureless near-field optics that are based on a combination of optical spectroscopy and scanning probe microscopy (SPM), with SPM tips modified to have plasmon resonance at the apex. Apertureless near-field microscopy enables traditional confocal optical imaging, scanning probe microscopy (SPM), and a combination of optical and SPM imaging with spatial resolution ~10-20nm, unprecedented for optical techniques. We demonstrate simultaneous Raman and SPM imaging of semiconductor structures and also discuss the challenges facing widespread applicability of this emerging technology, for areas as far ranging as biomedical, semiconductor, and composite materials research.