AFM-Raman is now a well established technique offering a multi-technique platform for deeper understanding of materials at the nanoscale. Tip-enhanced Raman scattering (TERS) takes advantage of that same platform combined with surface plasmon resonance effects localized at the probetip to bring chemical information with nanometric spatial resolution. These techniques have the potential to transform spectroscopic research and sample characterization in many varied fields, including 1D and 2D nanomaterials, organic molecules, polymers and semiconductors. The examples given below illustrate the power of TERS analysis to provide new insights into the sample structure.
The synthesis of carbon nanotubes (CNTs) results in bundles that often contain a variety of tubes with different structural properties. Conventional Raman is widely used to characterize quasi-one dimensional structures such as CNTs, however the optical imaging of their nanoscale properties is severely restricted to the optical diffraction limit. A good illustration of how TERS can be used to gain information that would not have been accessible with conventional Raman can be seen in the following TERS image.
In this TERS map, Fig. 11, the intensity of the D band (blue pixels) is showing the imperfection in the structure of the latter; in contrast the areas in red correspond to the pure graphitic arrangement of the CNT through the intensity of the 2D band. Defects concentration37, local chirality changes from the different radial breathing modes38-39, pressure effect and strain distribution40 can be studied at the single carbon nanotube level through TERS. Not only carbon-based 1D nano-objects37-46 but Si47, Ge48-49 and GaN50 nanowires are also candidates for TERS.
The discovery51 of graphene in 2004 has given rise to a surge in research activities on 2D materials with novel properties52-54 and the need to characterize them at the nanoscale. As for 1D materials, TERS gives localized information down to 10 nm through the Raman signature and allows identification of defects (point defects, vacancies or dopants). More than identification, thanks to TERS imaging, the local distribution of the defects concentration in graphene and graphene oxide is possible Fig. 12.
Monitoring the defects of the graphene flakes and their concentration (via the distribution of the ratio of G to D band intensities) is of great interest to study the impact on the design of devices made with such materials.
Graphene56-82, but also any functionalized and decorated 2D materials, molybdenum disulfide MoS263-67, tungsten diselenide WSe268 and others 2D TMDCs (two-dimensional transition metal dichalcogenide materials) are good candidates for a TERS study.
In surface chemistry, catalysis or biology, the chemical characterization of molecules on a surface is crucial for understanding their reactivity and function. However, the small quantity of molecules present makes their spectroscopic characterization challenging and much of the time impossible with conventional far-field confocal Raman instrumentation.
As can be seen in Fig. 14, TERS, especially its gap-mode realization, proved to be an efficient tool for detection and nanoscale Raman mapping of molecular layers down to single molecule layers self-assembled on the surface (SAM)32. Imaging the chemical species at the nanoscale gives an insight into SAM grafting quality (in this example after a nano-stamping process) as well as information on the phase segregation that was impossible to observe in the pure AFM topographic image.
TERS is thus a great candidate to image organic molecules69-79 or polymer blends80-81. Not only does it allow nanoscopic domains with distinct chemical signature to be distinguished,but it also enables the study of the photocatalysis82-83 andothers plasmon-induced catalytic processes84.
Characterization of the strain present in semiconductor nanostructures such as strained Si (sSi) on insulator (sSOI) or epitaxially grown SixGe1-x on Si (SiGe/Si) is of special interest for the microelectronics industry. The presence of strain breaks the crystal symmetry and shifts the frequencies of the Raman modes with respect to the unstrained ones thus making possible the determination of the stress state. Far-field Raman spectroscopy only gives an average value of the strain, and the optical resolution is not suited to current problems of the microelectronics industry.
TERS allows the study the lattice mis-match strain in pseudoepitaxial growth down to the nanoscale in the present example of 150 nm lithographically defined SiGe nanostripes on Si(001) substrate. During the TERS experiment, the intensity and the frequency of the locally enhanced Si- Ge and Ge-Ge Raman modes across a single nano-stripe are monitored, giving the perpendicular strain profile with a lateral resolution of ~ 20 nm (Fig. 15). The strain is tensile and becomes maximum (~+1.4 %) at the centre of the nanostripe, decreasing close to zero at the edges. TERS studies on Si47,85-91, SiGe92-93 but also on GaN94-96, GaAs96 and CdS97 have been also published.