Tip Enhanced Raman Spectroscopy
Tip Enhanced Raman Spectroscopy (TERS) is a vibrational spectroscopy that utilizes an STM tip to enhance Raman scattering phenomena on a local (nanometer) scale, allowing to perform Raman spectroscopy with a high spatial selectivity.
Before discussing TERS further, we will briefly discuss the more general method: Raman spectroscopy. This method analyzes the energy loss or gain of photons upon a Raman scattering event between a photon and molecule. (If the photon-molecule interaction is due to the photon-induced polarization of the initially unpolarized molecule, one refers to Raman scattering.) During these processes, photons interact with molecules and can either transfer some of their energy to molecular vibrations (“Stokes” process) or gain some energy from a molecular vibration (“Anti-Stokes” process). By analyzing the photon energies after those interactions, one can draw conclusions about the energy levels in the exposed molecules.
Raman scattering is, in general, a weak effect, with approximately only one out of 1011 photons participating. There are, however, experimental ways to amplify the effect in a given configuration. One way is to place a sharp metallic structure in close proximity to the molecules of interest. In Surface Enhanced Raman Spectroscopy (SERS) this is achieved by adsorbing molecules on rough, disordered metallic adsorbates. In Tip-Enhanced Raman experiments, a metallic STM tip is brought in close proximity to the adsorbed molecules, substituting, in a manner of speaking, the rough surface structures of SERS on a very local scale. The enhancement effect in TERS is oftentimes discussed in context with plasmon excitations in the tip and the metallic structures; however, some of the aspects of the enhancement effects in SERS and TERS are still debated.