Scanning probe microscopies & Raman
Scanning Tunneling Microscopy: STM
Invented in 1981 by Binnig and Rohrer (who won the Nobel Prize for this invention in 1986), tunnel microscopy consists of approaching a metal tip of a conductive or semiconductor surface. Before touching the surface but when the distance is less than 1 nanometre, a very small current (typically less than 1 nA) passes between these two elements. By moving the sample (or tip) while controlling the system to maintain a quasi-constant current, a very accurate topography of the sample can be obtained. Under the best conditions, subatomic spatial resolution is possible. In addition, this technique can be implanted in an electrochemical environment if a 4-electrode system is used, and the tip is isolated (except the tip!) to avoid leakage currents.
The figure above shows an image of a graphite surface on only 5x5 nm2. The apparent patterns are carbon atoms.
Atomic Force Microscopy: AFM
Invented very soon after tunneling microscopy, atomic force microscopy is based on a measure of the interaction force between the tip and the surface. Its great advantage on the STM is that it can work on insulating substrates. It can also operate in an electrochemical environment. The measurement of the interaction between the tip and the surface is made by means of a laser that is reflected on the lever supporting the tip. A variant consists in fixing a tip on a quartz tuning fork (typically used in watchmaking) and measuring the perturbations on resonance induced during interaction with the surface. In addition, atomic force microscopy provides many other information in addition to topography: electrical conductivity, elasticity, friction, etc.
The figure above shows an AFM image of a 3D self-organization of gold nanoparticles. The individual nanoparticles are visible in the image on the right. Collaboration with Alexa Courty (MONARIS Laboratory, UPMC)
Co-localized SPM/Raman measurements
As explained here, Raman spectroscopy is complementary to local probe microscopy because it provides direct chemical analysis. Measurements using local probe microscopy and Raman spectroscopy therefore allow a much better characterization of the sample.
The figure shows Raman measurements taken on a gold surface modified by a molecular monolayer on which gold nanoparticles have been deposited. Only areas with nanoparticles provide a chemical signature of the molecular layer because they amplify the electromagnetic field locally (SERS effect).
However, here, Raman measurements remain limited by light diffraction (about 300 nm). To descend below, it is necessary to use the TERS.
To obtain a spectral signature on a scale of a few nanometers, the source of the Raman excitation must itself be nanometric. This recent methodology uses, like the SERS, the amplification of the electromagnetic field and therefore of the Raman signal observed at nananoantennas (by peak effect): this is the TERS effect. This new methodology was first proposed in the 2000s. Aligning the laser beam on the tip to find the "hot spot" for which the exaltation is maximum is sometimes difficult to implement on some samples. The design of effective STM-TERS or AFM-TERS probes is a major challenge in popularizing the technique.
The figure opposite shows the spectra obtained when a laser illuminates a gold STM tip in tunnel contact with a surface modified by an azobenzene derivative (hot spot, blue) or just the surface (pink), showing an enormous gain in sensitivity in TERS.
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 or metal evaporation).