Raman and AFM (Atomic Force Microscope) analysis can be combined on a single microscope system, opening interesting new capabilities and providing enhanced information on sample composition and structure by collecting physical and chemical information on the same sample area. Co-localized AFM/Raman measurement is the sequential or simultaneous acquisition of overlapped SPM (Scanning Probe Microscope) and Raman maps with pixel-to-pixel correspondence in the images.
On one hand, AFM and other SPM techniques like STM, Shear-Force or Normal-Force, provide topographic, mechanical, thermal, electrical, and magnetic properties down to the molecular resolution (~ nm, over μm2 area), on the other hand confocal Raman spectroscopy and imaging provides specific chemical information about the material, with diffraction limited spatial resolution (sub-micron).
TERS (Tip Enhanced Raman Spectroscopy) brings Raman spectroscopy into nanoscale resolution imaging1-6. TERS is a super-resolution chemical technique. Better yet, it is a label-free super-resolution imaging technique which has been extended by our novel technology into an important new imaging technology.
TERS imaging is performed with an AFM/Raman system, where a Scanning Probe microscope (SPM that can be used in atomic force, scanning tunneling, or normal/shear force mode) is integrated with a confocal Raman spectrometer through an opto-mechanical coupling. The scanning probe microscope allows for nanoscale imaging, the optical coupling brings the excitation laser to the functionalized tip (or probe), and the spectrometer analyzes the Raman (or otherwise scattered) light providing a hyperspectral image with nanometer scale chemical contrast.
A TERS system is based on a metallic tip (generally made of gold or silver) employed to concentrate the incident light field at the apex. The tip acts as a nano-source of light and local field enhancer, greatly improving the Raman sensitivity (by a factor of 103–107) and reducing the probed volume to the “nano” region immediately below the tip. The optical coupling that combines the two instruments uses a confocal scheme. Two different configurations exist for this coupling: one in transmission and one in reflection (Fig. 2), having their own advantages and drawbacks.
The transmission configuration allows the use of the highest numerical aperture (NA) objectives (including immersion objectives) giving high power density at the focus point and enabling the collection of high signal level, but it can only be used for transparent samples. The reflection configuration can be used for any kind of samples (opaque and transparent) but is limited to lower NA objectives. By combining point-by-point scanning with simultaneous spectrum acquisition, near-field Raman mappings can be performed with lateral resolution down to ten nanometers or less.
TERS provides similar chemical information to conventional far-field Raman spectroscopy. Typically, a Raman spectrum is a distinct chemical fingerprint based on vibrational characteristics of a particular molecule or material and can be used to quickly identify the material or differentiate it from others.
Thus, TERS provides information about:
While the availability of these contrast mechanisms on volumes of less than 10nm diameter is of itself most useful, the truly unique power of TERS is realised by combination with Scanning Probe Microscopy, which synchronises the motion of the active tip with respect to the surface with subnanometre precision enabling the generation of images, where each pixel in the image is represented both by the point physical property recorded but also by a complete spectrum representing the local chemical information. A single such “hyperspectral” image may contain tens of thousands of spectra, in pixel-to-pixel registration with the SPM image. These images show the distribution of individual chemical components, phases, variation in crystallinity or defects imaging down to the nanoscale.
The TERS effect comes from the strong local enhancement of the electromagnetic field occurring at the apex of a sharp noble-metal tip when illuminated with a focused laser light2. The phenomenon results from the combination of an electromagnetic ‘lightning rod effect’ and a localized surface plasmon (LSP) excitation.
This electromagnetic enhancement (EM) mechanism is associated with the excitation of surface plasmons and the strength of their EM fields near the surface. These fields can be significantly stronger than the incident fields. Theory has shown that if the tip is illuminated, a strong enhancement of the EM field can occur in the narrow space between the tip itself and the sample (consisting of an ideally metallic substrate on which are deposited adsorbates or nanomaterials). The metalized tip acts as an optical antenna that enhances both the incident and the emitted fields, in a region defined by the size of the tip apex (typically less than 30 nm).
Let us describe the field enhancement of the incident electromagnetic wave by a factor gi, and the enhancement of the scattered field by gsc. For g ≈ gsc ≈ gi (the so-called “g4 law”), the EM part of the enhancement is simplified to FEM = gi 2 gsc 2 ≈ g4. Then, a hundredfold increase of the EM field relative to the incident one thanks to the presence of the tip (i.e. gi = 100) would result in a local 10,000-fold intensity enhancement of the Raman signal (Iloc = gi 2 I0).
To summarize, in TERS, the Raman scattering process boosted by this local enhancement at the tip apex, since the scattering cross-section scales with the fourth power of the local electromagnetic field enhancement.