Nanoscale chemical imaging using top-illumination tip-enhanced Raman spectroscopy

We present a new top-illumination scheme for tip-enhanced Raman spectroscopy (TERS) in a gap-mode configuration with illumination and detection in a straightforward fashion perpendicular to the sample surface. This illumination focuses the light tightly around the tip end, which effectively diminishes far-field background contributions during TERS measurements. The setup maintains the entire functionality range of both the scanning probe microscopy and the confocal optical microscopy of the setup. For the first time, we show large (64 × 64 up to 200 × 200 pixels), high-resolution TERS imaging with full spectral information at every pixel, which is necessary for the chemical identification of sample constituents. With a scanning tunneling microscope tip and feedback, these TERS maps can be recorded with a resolution better than 15 nm (most likely even less, as discussed with Figure 6). An excellent enhancement (∼10(7)×, sufficient for detection of few molecules) allows short acquisition times (<<1 s/pixel) and reasonably low laser power (in the microwatt regime) yielding spectroscopic images with high pixel numbers in reasonable time (128 × 128 pixels in <25 min). To the best of our knowledge, no Raman maps with similar pixel numbers and full spectral information have ever been published.