The features of the characteristics of metallic carbon nanotubes (-CNTs) in different experimental setups are studied using semiclassical Boltzmann transport equation together with the heat dissipation equation to account for significant thermal effects at high electric bias. The model predicts that the shape of the -CNT characteristics is basically controlled by heat removal mechanisms. In particular, the authors show that the onset of negative differential resistance in freestanding nanotubes finds its origins in strong transport nonlinearities associated with poor heat removal unlike in substrate-supported nanotubes.
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Here we neglect acoustic phonon effects in a first approximation, and as a result, nonequilibrium phenomena between optical and acoustic phonon populations. By identifying the temperature profile obtained from Eq. (3) with the optical phonon temperature in the transport equation [Eq. (2)], our model overestimates the nanotube temperature. However, this approximation has minor effects (at most a slight renormalization of the scattering rate values) on the onset of the nonlinear transport (saturation and NDR) as seen in Figs. 1 and 3 while it sets an upper limit on the maximum temperature reached in the nanotubes. Inclusion of nonequilibrium phonon population in our self-consistent scheme will be the subject of a forthcoming analysis.
