We present single molecule tip-enhanced resonance Raman spectra from brilliant cresyl blue (BCB) submonolayers adsorbed on a planar Au surface with Ag tips. A gap of 1 nm between a Ag tip and the Au substrate was employed to create a highly enhanced electric field and to generate Raman scattering from an area of ∼100 nm 2 . Three lines of evidence are presented to prove the single molecule sensitivity of our experiments: (1) Extremely diluted samples were used. Estimations show that at most a few molecules were excited by the Ag tip. (2) Spectroscopic fluctuations, including intensity fluctuations, frequency shifts, and line shape changes were observed. A histogram analysis of the intensity fluctuations of two different BCB coverages was carried out. The results clearly show the features of single molecule behavior. (3) Discrete signal losses also were observed. This is because of photochemical processes involving single molecules. Besides BCB, which shows a strong resonant absorption at 633 nm (the wavelength of the excitation laser), a self-assembled monolayer of benzenethiol, which does not strongly absorb at 633 nm, was studied. Good quality spectra were recorded with a short exposure time (10 s) and time-dependent spectral changes were also observed.

Methods for chemical analysis at the nanometer scale are crucial for understanding and characterizing nanostructures of modern materials and biological systems. Tip-enhanced Raman spectroscopy (TERS) combines the chemical information provided by Raman spectroscopy with the signal enhancement known from surface-enhanced Raman scattering (SERS) and the high spatial resolution of atomic force microscopy (AFM) or scanning tunneling microscopy (STM). A metallic or metallized tip is illuminated by a focused laser beam and the resulting strongly enhanced electromagnetic field at the tip apex acts as a highly confined light source for Raman spectroscopic measurements. This Review focuses on the prerequisites for the efficient coupling of light to the tip as well as the shortcomings and pitfalls that have to be considered for TERS imaging, a fascinating but still challenging way to look at the nanoworld. Finally, examples from recent publications have been selected to demonstrate the potential of this technique for chemical imaging with a spatial resolution of approximately 10 nm and sensitivity down to the single-molecule level for applications ranging from materials sciences to life sciences.

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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.