Transport in Carbon Nanotubes

M. Ahlskog, P. Hakonen, M. Paalanen, L. Roschier, R. Tarkiainen, and A. Zyuzin

The electrical properties of carbon nanotubes depend on several factors, e.g. the number of concentric layers, number of conducting channels, disorder strength, and carrier concentrations (the level of doping), which can all vary over a wide range and which all are hard to control experimentally. We have studied very disordered, catalytically grown CVD multiwalled carbon nanotubes (MWCNT). Resistance vs. temperature measurements on CVD tubes with good-quality contacts (Rc ~ 1 kW) and resistance of ~ 30 kW/Bm displayed rather large conductance corrections which we have analyzed in terms of the interaction effects. As a function of voltage, heating effects tend to dominate, and the dependence can be best modeled by using the equation for diffusive heat transport. The density of states of these tubes has been studied using high impedance Al-AlOx-NT contacts (Rc ~ 100 kW). We have compared our results with the theoretical calculation on tunneling into 1-dimensional disordered system, and obtained good agreement with the results beyond the first order corrections.

There are several classes of experiments that call for good quality MWCNTs for their successful implementation.  For this purpose, we have tested tubes made using plasma-enhanced CVD (PECVD) method by S. Iijima in Japan. These MWCNTs have a diameter in the range of 3-10 nm, and they are of better quality and uniformity than the tubes made using previous methods of synthesis. These tubes have been employed in rf-SET work where they gave a charge resolution of 1.10-5 e/. This value is an order of magnitude worse than predicted, indicating that there are either inherent fluctuations (disorder) on the nanotubes or the electron-phonon coupling is extremely weak.