The last two decades have brought a tremendous progress in the high resolution visualisation of single molecules. The molecules reside on an atomically flat surface and the imaging technique used is based on charting the tunneling current between the surface and a biased metallic tip, which is scanned a few Angstroms above the surface (scanning tunneling microscopy, STM). Here the electrons tunnel quantum-mechanically between the single molecule on the surface and the apex of the metallic tip, resulting into a tiny measurable current.
As seducing as the above presentation may sound, the interpretation of STM images is anything but trivial. Seeing and understanding the reality revealed by STM are two different pairs of shoes. Unfortunately, STM does not indicate the chemical nature of the structures it so finely shows. Often enough we end up with fresh unanswered questions after directly "seeing" the microscopic reality.
A very appealing technique designed to cure the chemical insensitivity of STM has been successfully introduced during the last decade. This technique is called "tip-enhanced Raman spectroscopy" (TERS) and is a subtle implementation of traditional Raman spectroscopy around the mtallic tip of an STM microscope.
Laser light is shone upon the atomically sharp STM tip in order to stimulate Raman transitions in the molecules located in its immediate vicinity (within about 100 Angstrom). Hereby the tiny Angstrom-sized gap between the surface and the scanning tip serves as an electromagnetic antenna for the laser light, the excitation of a very localized electric field ("plasmon") at the tip position being the result. The intensity of |E| in the tip region is 4-6 orders of magnitude enhanced in comparison with the value in the absence of the STM tip. This tip-induced enhancement is so substantial that the spectroscopy even of single molecules becomes technically feasible. As a next bonus (!) one even gets in TERS a spatial resolution of about 10 nm, way below the half-wavelength resolution limit of Classical Optics textbooks ("beating the diffraction limit").
Here comes my actual question:
in case you have already worked with an STM setup, have you ever tried to append it a commercially available Raman spectrometer? In case you have, what is your experience and what caveats have you identified during the construction of the STM-Raman coupling optics? Is a (knowledgeable) home-made optical setup fine enough to clearly put in evidence the tip-enhancement effect?
