This article presents developments in tip-enhanced Raman spectroscopy (TERS) that make possible nanoscale imaging of chemical and physical properties of graphene and other carbon species: Innovative integration of technologies brings high-throughput optics and high-resolution scanning for high-speed imaging without interferences between the techniques. Advances in near-field optical probes now provide reliable nanoscale spectroscopy solutions for academic and industrial researchers.
Raman spectroscopy is a well-known technique to study carbon species. It provides a very specific signature for the two crystal configurations: diamond (cubic or octahedral) and graphite (hexagonal).
Because Raman microspectroscopy is an optical technique, its spatial resolution is limited by the laws of diffraction: As a rule of thumb, the spatial resolution of the technique is about half the wavelength of the exciting laser wavelength. This value depends on several factors besides the excitation wavelength, including the numerical aperture of the objective lens used; however, this approximation is convenient and easy to understand for the purpose of this discussion. With a red laser excitation, this is on the order of 300 nm.Graphene flakes or coatings can be made quite large; however, other species of interest like single-wall carbon nanotubes (SWCNT) are typically in a diameter range of 1–3 nm, or two orders of magnitude smaller than the spatial resolution of the technique.
Raman spectroscopy brings chemical contrast for these species, and allows the determination of the diameter of a single wall carbon nanotube, or the number of layers in a multilayer graphene sample from the features present in the spectrum. Radial breathing mode frequency position is a good indicator of SWCNT diameter or the ratio and bandwidth of the G and two-dimensional (2D) bands in graphene are good indicators of the number of layers. However, the samples measured are generally not resolved spatially.
In contrast, scanning probe microscopy (SPM) and, in particular, the more popular technique atomic force microscopy (AFM) allow such materials to resolve spatially and provide even atomic-level resolution. Atomic resolution imaging of highly ordered pyrolytic graphite (HOPG) is a typical test for SPM calibration, using scanning tunneling mode (STM). The technique consists of scanning a very sharp probe over a sample and tracking the Z axis position via a feedback mechanism that depends on the mode used, to generate a topographic image. In STM, the feedback is done based on tunneling current intensity, and in AFM, a constant force is maintained between the probe and the sample by tracking the deflection of a laser on the back of a cantilever that holds the sharp probe. The cantilever bends according to its known mechanical properties that depend on size, shape, and material.
Beside topography, AFM can be used in a multitude of modes to obtain various physical properties. Looking at the way the probe interacts with the sample, it is possible to determine elasticity and plasticity of a material. Tracking the torsion of the level as it is scanned (lateral force) gives information about friction. Driving a current through a conductive probe, it is possible to obtain various electrical properties such as conductance, capacitance, and resistivity; surface potential is also determined by scanning above the surface. Using a magnetized probe, magnetic studies can be performed at various heights from the surface.
SPM and, in particular, AFM are very versatile techniques to image a multitude of physical properties, however, neither technique provides chemical specificity (except in the case of the use of functionalized probes designed for very specific chemical bonding).
The interest for combining the two techniques is obvious: Obtaining physical properties and chemical specificity of the same location, ideally simultaneously, without having to move the sample or find the location of interest each time is a huge time saver when studying nanomaterials.
The combination of the two techniques is in itself interesting, but furthermore reducing the spatial resolution gap between the two techniques is of the highest interest. This is where tip-enhanced Raman spectroscopy (TERS) comes into play.
TERS takes advantage of the enhancement of several orders of magnitude observed in techniques like surface-enhanced Raman spectroscopy (SERS), confining this enhancement at the tip of a specially crafted probe that acts as a plasmonic antenna. The result of this confined resonance is an extremely localized enhancement of the Raman signal in the very close vicinity of the tip of the probe, generally no further than 10 nm. A laser beam matching the probe's plasmonic excitation range is focused on the probe to produce the local enhancement. The focused laser beam is of course following the same diffraction rules mentioned above, and illuminates a field much larger than the probe tip radius, potentially exciting a far-field (diffraction limited) Raman signal, while the enhanced signal is localized just under the probe generating the near-field Raman signal. The ratio of the volumes involved in each case is also several orders of magnitude, so the ability to detect the near-field Raman signal is extremely dependent on the probe quality and its enhancement efficiency.
SWCNTs have been studied in the past with TERS, most often performing line scans (1) and a few produced images (2).
The novelty presented here and demonstrated in the experimental measurement of graphene oxide and single-wall carbon nanotube samples is in the combination of the techniques into an instrument in such an efficient manner that it is possible to obtain reliable enhancement and stable alignment to perform TERS imaging with very short acquisition times, which in turns frees the technique from the need to subtract far-field from near field and from potential drift issues inherent to the SPM technology.