What is TERS?



TERS, or tip enhanced Raman spectroscopy, is a technique that was developed to increase the lateral and axial resolution of Raman spectroscopy and thus to obtain chemical composition on the nanoscale.


The typical resolution of conventional confocal Raman spectroscpy is approximately 250nm, while TERS resolution is <50nm. TERS was derived from SERS (surface enhanced Raman spectroscopy) where the Raman signal was enhanced by several orders of magnitude near stationary gold or silver nanoparticles distributed on the sample. TERS was then developed as an alternative to the destructive SERS method.

In TERS, a very sharp tip is coated by a noble metal such as either gold or silver. The electrical field near the tip apex is strongly enhanced as the result of excitation of the localized surface plasmons at the noble metal tip by the illumination laser. In order to excite the surface plasmons, the wavelength of the illumination laser should match the resonance of the surface plasmons. The tip of the TERS probe with the now strongly enhanced electrical field becomes a hotspot.

The principle of TERS operation is shown in the schematic on the right, where a sharp tip now functions as an antenna to localize the Raman laser right underneath the tip. Once the laser is aligned onto the tip at the correct hotspot location, the sample stage then scans the sample underneath the tip, without disturbing the laser alignment onto the tip. Furthermore, instruments with laser-free feedback modes to keep the tip-sample interaction constant (such as tunneling current or tuning fork) are advantageous so that there is no interference with the Raman laser.

An excellent example of the power of TERS and its improved Raman lateral resolution is shown here. In this image, TERS and far-field measurements were performed at the same time on a strained silicon/silicon grating. The strained silicon line cannot be deteted with conventional confocal Raman imaging (right) but can be detected with TERS (image on left)

The power of TERS unfolds in samples and applications where high resolution chemical (Raman) information is required along with topography/morphology.