Chemical Production of Thin Protective Coatings on Optical Nanotips for Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) provides topographic and chemical information simultaneously with a spatial resolution well below the optical diffraction limit; however, the typically used noble-metal nanotips are prone to unwanted chemisorption, and when silver is used, the enhancement quickly fades due to chemical degradation during storage and use. In the present work, we demonstrate a method to produce strongly enhancing tips for TERS, which are protected against undesired chemisorption and (in the case of silver) have a significantly longer lifetime than unprotected tips. This was achieved by chemical coating of an ultrathin (few nanometers) and pinhole-free silica shell on top of the enhancing noble-metal nanostructures. The enhancement achieved with the protected silver tips was comparable with the typical tip enhancement for TERS; that is, no major constraints for TERS experiments resulted from the protective coatings. These shell-isolated tips could widen the scope of tip-enhanced Raman spectroscopy by potentially allowing for in situ studies of, for example, surface catalysis and biological systems.