Nanoscale Raman and TERS Research

Investigate chemical bonds, changes, and vibrational signatures

Like infrared spectroscopy, Raman spectroscopy complements atomic force microscopy as a non-destructive, label-free method to identify the presence of particular chemical bonds through their vibrational state signature. With its different selection and favoring of moieties with high polarizability, such as aromatic carbon compounds and heavy elements, Raman spectroscopy is an ideal counterpart to infrared spectroscopy. In addition, its use of typically visible light wavelengths enables standard confocal Raman microscopy to provide chemical information with a spatial resolution down to 500nm or below. Combining such a confocal Raman map with a correlated AFM image of the same sample region is particularly powerful when the AFM utilizes PeakForce Tapping. As seen in the below figure, chemical changes are often closely associated with changes in nanomechanical properties. With the correlation information from the Raman map, a PeakForce QNM property map can thus effectively become a nanoscale resolved chemical map.

Colocated Modulus and Raman maps of polystyrene polypropylene
Colocated PeakForce QNM Modulus (a) and Raman (b) maps of polystyrene-polypropylene blend.

In addition to correlated information, Raman spectroscopy can provide a direct means for nanoscale spectroscopy in the form of tip-enhanced Raman spectroscopy (TERS). TERS relies on the scanning probe microscopy tip to serve as a plasmonic structure with very strong local field enhancement, very efficiently coupling far field radiation into the near field under the tip and coupling Raman scattered radiation back out into the far field. With a strongly enhancing tip such as an IRIS TERS tip, TERS can capture the weak Raman scattering from small ensembles, with use in a range of applications, from the measurement of strain at the nanoscale to the detection of thin molecular films that are not otherwise detectable.

TERS of thin Nile Blue film on gold
TERS spectrum of a thin Nile Blue film on gold. While undetectable in the far field (i.e. in conventional confocal Raman), strong signal enhancement enables identification of the Nile Blue Raman spectrum in the near optical near field (i.e. with TERS).