Carbon Nanotubes
Carbon nanotubes have attracted the fancy of many scientists worldwide. The small dimensions, strength and the remarkable physical properties of these structures make them a very unique material with a whole range of promising applications. As the increase of carbon nanotubes in commercial productions, a quick analytical tool for quality verification of the nanotubes becomes more and more important. Diameter, chirality and phonon structure of carbon nanotubes, are related to the mechanical and electrical properties. They can be either metallic or semi conducting, depending on their chirality.
Raman spectroscopy
There are a few microscopy methods to detect and qualify CNT selectively. Transmission Electron Microscopy, Scanning Probe Microscopy, Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) are widely used to determine the morphology of single CNT particles. Raman spectroscopy has been established as a powerful technique to characterize the structure and electronic properties of carbon nanotubes minimal sample preparation. Raman spectroscopy provides a powerful tool to differentiate between two different sp2 carbon nanostructures (carbon nanotubes and graphene) which have many properties in common and others that differ.
Raman spectroscopy has good spatial resolution (~0.5 micrometers) and sensitivity (single nanotubes) and is rather informative. Consequently, Raman spectroscopy is probably the most popular technique of carbon nanotube characterization. Raman scattering in SWCNTs is resonant.
Similar to photoluminescence mapping, the energy of the excitation light can be scanned in Raman measurements, thus producing Raman maps which contain oval-shaped features uniquely identifying (n, m) indices.
The main features in the Raman spectra of carbon nanotubes are the radial breathing mode (RBM); the disorder-induced D-band, and its corresponding second-order G'-band; and the tangential G-band. The information revealed in Raman spectra provide the important information about the diameter, chirality and phonon structure of carbon nanotubes, which are related to the mechanical and electrical properties.
Analysis modes
Radial Breathing Mode (RBM) is specific to SWNT and usually observed in the region from 150 cm-1 to 300 cm-1. Raman peak position, which is inversely proportional to the tube diameter, of this mode is used to classify the diameter distribution in carbon nanotubes. The G band, a tangential shear mode, corresponds to the stretching mode of the carbon-carbon bond in the graphite plane. The fine structure seen in the G-band depends on tube diameter and chirality. The line shape of the band can be used to help identify metallic and semi conducting nanotubes. The D band is often referred as the disorder or defect band. The D band/ G band ratio is usually used for evaluating the quality of carbon nanotubes.
Visible to NIR laser excited Raman spectroscopy of CNTs are resonance process, which is excitation wavelength dependence of the spectra resulting from the electronic band structure. During the measurement it is important to keep the low laser power to decrease heating effect since Raman shift/shape is dependent on temperature.