Tip-Enhanced Raman Spectrosocpy (TERS)
To obtain a spectral signature at a finer scale than the micrometer (currently in the order of a few nanometers), the source of the Raman excitation must itself be nanometric. This recent methodology exploits the amplification of the electromagnetic field and therefore of the Raman signal observed at nano-antennas (by peak effect), it is the TERS effect. This new methodology, first proposed in the 2000s, is difficult to implement and remains the preserve of a few specialized laboratories.
Near and far field: Unlike SERS, TERS measurements use a tapered tip (gold, silver) illuminated by a laser source and brought into contact with the sample to be analyzed. Thanks to the confinement of the electric field at the tip end (hot spot), the Raman signal of the compounds at the tip/substrate junction (optical near-field signal) is considerably amplified. The under-tip signal largely dominates the microRaman signal from the illuminated surface of the sample (far-field signal), allowing the extraction of chemical signatures at the nanoscale.
TERS Gap-mode: Increased sensitivity and spatial resolution can be achieved in the TERS gap-mode configuration where the analyte is sandwiched between the tip and a gold or silver surface.
Hot spot: While the SERS signal can come from several hot spots simultaneously, the single hot spot developed at the tip end of the tip, which is very spatially limited, must be carefully identified and located. The precise focusing of the excitation laser at the tip end and the search for the hot spot are achieved by using high magnification and high numerical aperture lenses mounted on piezoelectric plates (displacement 30 x 30 µm).
Polarized laser source: To induce LSPR at TERS probes, polarized laser sources are used and the main component of the electric field is aligned along the tip axis. For opaque samples, a linearly polarized light source is focused at the end of the tip through the lens positioned either on the side of the tip or on top (elephant trunk tip). For transparent samples deposited on microscope slides, the tip end can be illuminated from below through the sample using a radially polarized laser source.
TERS probes - While instruments for robust optical coupling required for TERS measurements have appeared on the microscopy market, the main obstacle to the development of TERS has been and remains the difficulty in designing TERS probes with controlled properties. The opening angle, the radius of curvature, the chemical (oxidation of silver) and mechanical stability are all parameters that determine the appearance and maintenance over time of an effective plasmon resonance at the tip end for a given excitation wavelength, conditions necessary for a strong and reproducible excitation of the Raman signal. LISE has the know-how to develop TERS probes (electrochemical dissolution of gold and silver wire).
TERS principle: a) a laser excitation laser is focused at the end of a metal tip (gold/silver for visible excitation) creating a considerable amplification of the local electromagnetic field at the tip/sample junction and thus of the Raman scattered signal under the tip that dominates the signal from the surface illuminated by the laser, b) the energy difference between the scattered Raman photons (inelastic scattering) and the incident photons corresponds to the energy difference between the vibrational levels of the molecule (u0 - u1), similar to IR spectroscopy, c) the Raman signature of a molecular layer can be obtained if the laser is very precisely positioned on the tip of the tip, at the exact location of the hot spot, using a lens mounted on a piezo plate.