Carbon nanotubes, long, thin cylinders of carbon, are large macromolecules that are unique for their size, shape, and remarkable physical properties. They can be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a cylinder. These intriguing structures have sparked much excitement in recent years and a large amount of research has been dedicated to their understanding.
We are interested in the optical and electrical properties of this fascinating material, as well as how to be able to produce them in a more controlled way. We synthesize our nanotubes with the chemical vapor deposition method. At the Harrison Spectroscopy Lab, we are building a Raman & Photoluminescence setup with tunable excitation to characterize our nanotube materials.
(1) Combining Optical Experiments and Transport Measurements
In carbon nanotubes the optical and electronic properties are strongly correlated due to the presence of the van Hove Singularities in the density of electronic states. The energies of the electronic transitions for many different nanotubes lie within the range of visible light, and therefore it is clear that the electronic properties of semiconducting nanotubes will be greatly affected by emission or absorption of photons with specific energies.
ELECTRON-PHONON INTERACTION: Phonons are well known to be the greatest contributors for the resistivity of materials. In the case of one-dimensional systems such as carbon nanotubes, the interaction with low energy acoustic phonons is known to be very weak, which results in a quasi-ballistic electron transport at low-biases. In a high-bias condition, the optical phonons, which interact very strongly with electrons, play an important role in the resistivity. The correlation between the density of optical phonons and the transport properties of nanotubes can be explored by combining Raman spectroscopy and transport measurements.
(3) Chirality assignment of carbon nanotubes
During the past few years Raman spectroscopy has become one of the most commonly used methods to characterize carbon nanotubes. It was demonstrated that under energy resonant conditions it is possible to obtain a Raman signal from a single nanotube and from its Raman spectra this nanotube's chirality can be derived . However, this usually involves an elaborative analysis with uncertainties and only limited number of nanotubes can be probed among many others in a sample. With our tunable Raman setup, we will be able to perform more accurate and straightforward chirality assignment. In addition, we plan to investigate on the relatively un-explored Raman features, such as the intermediate frequency modes (IFM) , both to study the scattering mechanisms that give rise to these features but also to use them as a supplemental identification for the chirality assignment. An example is in Figure 3(b) and (c), where the two nanotubes are both under resonant conditions with excitation energy E laser =1.58eV, and both give a radial breathing mode (RBM) frequency of 237cm -1 . However, their IFM features are completely different, indicating that they are two nanotubes of different chiralities.