Chemical Imaging on the Nanoscale – Top-Illumination Tip-Enhanced Raman Spectroscopy

Abstract

A top illumination system for tip-enhanced Raman spectroscopy (TERS) in a gap-mode configuration is presented here, which allows chemical analysis of sample surfaces with a lateral resolution beyond the optical diffraction limit and optical detection of very small amounts of analyte molecules (down to single molecule sensitivity). The technique combines the high resolution of an STM with label-free, chemical-rich information of Raman spectra at ambient pressure. In this system, using a special geometry with illumination and detection perpendicular to the sample surface, a lateral resolution of <15 nm was achieved using low laser powers and split second acquisition times per spectrum. This was achieved due to a very high enhancement of the Raman signals in the order of 107 by etched metal tips, and allowed the acquisition of 64 × 64 to 200 × 200 pixels chemical Raman maps with full spectral information in every pixel within a reasonable time frame (<25 min for 64 × 64 pixels). The Raman maps give simultaneous information about localization and chemical nature of a sample with high sensitivity and high resolution.