Carlos A. Barrios

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 (SiOx) coating. With this thickness protective coating, the contrast factor for the tip, which is the key parameter controlling ones 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.

Toward Robust High Resolution Chemical Imaging

Resonant plasmon excitations at the surface of noble metals can localize and amplify an electromagnetic field in a very small volume and are the enabling element of surface enhanced optical microscopies [1]. Tip enhanced Raman spectroscopy (TERS) combines scanning probe microscopy (SPM) with Raman spectroscopy, taking advantage of this enhancing mechanism [2]. So far a 20 nm lateral resolution in chemical imaging of a surface has been achieved. Unfortunately, pure noble metal nanostructures, the most active plasmon materials known, are fragile, and prone to mechanical, chemical, and morphological degradation (Fig 1). Means of protecting and extending the lifetime of these surfaces are key for making the plasmon-based high resolution chemical imaging a robust characterization technique.

Spectroscopic imaging at the nanoscale

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.