Glossary of Key Terminology
- AFM Raman
- Co-located confocal Raman microscope with an AFM tip enabling simultaneous acquisition of AFM and Raman spectroscopy images from the same location on the surface.
- AFM Raman TERS
- Collective name for high resolution Raman measurements including Raman confocal microscope integrated with SPM microscope and metalized TERS probe.
- Graphene TERS
- TERS has been successful in measuring the spectral characteristics of graphene with very high spatial resolution.
- Surface enhanced Raman scattering (SERS). A well-known Raman enhancing effect where a roughened metal surface can provide orders of magnitude increase in Raman signal intensity.
- Conventional TERS probes
- Typically Au or Ag metalized AFM probes or STM probes.
- Strained Silicon TERS
- ERS measurements on strained silicon substrate for TERS probes characterization in terms of the enhancement efficiency.
- TERS AFM
- Same as AFM Raman TERS
- Tip enhanced Raman scattering (see TERS effect)
- TERS Effect
- enhancement of the Raman signal using the metalized AFM tip as the source for enhancement. By using such a small dia meter tip, enhancement occurs only in the immediate vicinity of the tip providing a high spatial resolution for the Raman measurement. TERS provides significant improvement in resolution over conventional AFM-Raman methods.
- TERS Microscope
- A microscope fitted with TERS equipment, including a lens (optical microscope), AFM head, TERS probes, Raman spectrometer, and CCD camera.
- TERS Probes
- Specialized probes suitable for AFM/TERS measurements. A gold or silver ball at a variety of diameters is embedded at the end of a glass cantilevered probe to generate the enhancement of the Raman signal near the probe
Nanonics probes are extended and transparent allowing for all modes of TERS operation: Reflection, transmission and side illumination.
- TERS Raman
- As TERS stands for “Tip-Enhanced Raman Spectroscopy”, this term is redundant on its own, but used by searchers to specify this particular meaning of the term “TERS”.- It is the same as “TERS” or “TERS effect”
- TERS Tips
- same as TERS probes.
- Reflection TERS
- TERS measurements on opaque sample, when the SPM integrated with upright confocal Raman microscope.
- Transmission TERS
- TERS measurements on transparent or half transparent samples, when the SPM integrated with inverted confocal Raman microscope.
- Side illumination TERS
- TERS measurements on opaque sample, when the SPM integrated with upright confocal Raman microscope, when the laser for Raman excitation illuminates the sample by the 45˚-60˚ relatively to the axis of the TERS probe.
Raman measurements are also possible in liquids, but they require the specialized liquid immersion objectives available on our systems. Below is a Raman image of a Si/SiO2 grid immersed in liquid collected at 532nm. The image on the right was collected using a 50x objective with an NA of 0.45, a typical optical objective used in an air environment. The periodic grid features are poorly resolved. On the left, the same grid was imaged with the water immersion optical objective clearly showing the grid features with excellent resolution.
Below is a short 15 minute video explaining the basics of AFM technology and feedback mechanisms:
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.