What is tip enhanced Raman scattering, or TERS?

Tip enhanced Raman scattering (TERS) combines surface enhanced Raman spectroscopy (SERS) with Raman-AFM analysis.  The goal of this exciting research area is to offer true nanometer scale spatial resolution for Raman.

The principles of TERS are simple, although the practical use of TERS is complex, and requires considerable expertise in terms of spectroscopy and optics.

Surface enhanced Raman scattering (SERS) can provide orders of magnitude increases in Raman signal intensity.  If the tip of an atomic force microscope (AFM) is made SERS active (by coating with a SERS active metal or metal nanoparticles) then the SERS effect would be expected to occur only within the immediate vicinity of the tip.  Since the tip has dimensions typically <100 nm the spatial resolution of such a measurement would depend upon the tip itself, and would similarly be <100 nm.

A TERS experiment usually involves focusing the excitation laser beam through standard microscope objectives, to yield a diffraction limited ~0.5 - 1 µm spot size (depending on objective and laser wavelength).  The SERS active tip is then brought into contact with the sample within the laser spot. 

There will be two main Raman scattering processes

  1. ‘normal’ Raman scattering from the 0.5 – 1 µm diffraction limited laser spot
  2. surface enhanced Raman scattering (SERS) from the tip (e.g., tip enhanced Raman scattering)

Since SERS can give increases of intensity by factors up to 1014-15, the practical achievement of nanometer scale Raman analysis depends on the intensity of TERS being of a similar or greater magnitude than the ‘normal’ Raman signal.  This is not guaranteed, since there are concomitant orders of magnitude decreases in the number of molecules being sampled in TERS compared with ‘normal’ Raman.

Successful TERS measurements have been made on a range of sample types, but achieving such results is not trivial, and cannot be expected for every possible sample.

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