Fullerene nanotubes have been targeted for a number of applications, including nano-electronic device components, nano-actuators, and field emission sources for display applications. To better understand the mechanical and electronic properties of nanotubes for these and related applications, we have developed a self-consistent scheme that allows charge transfer and applied fields to be incorporated into semi-empirical tight-binding models of carbon systems. The method uses a self-consistent electron density as a perturbation to a reference electron density for diamond given by an existing environment-dependent tight binding scheme. Parameters within a valence Gaussian basis are fit to the electronic states and Mulliken populations for hydrocarbon clusters given by first principles density functional calculations. Analytic integrals produced by the Gaussian basis and a simplified exchange functional together with an efficient numerical method for achieving self-consistency yields a computationally efficient scheme capable of modeling the self-consistent electronic structure of systems composed hundreds of carbon and hydrogen atoms with modest computing resources.
Using this self-consistent, environment-dependent tight binding scheme, electronic state shifts, electrostatic potential, and dipole moment and energy as a function of field strength and field orientation have been calculated for finite-sized nanotubes in applied electric fields. The calculations predict complete screening of the applied field inside of the nanotubes for both metallic and semiconducting structures in agreement with prior first-principles calculations, as well as field enhancement at the tips of nanotubes in agreement with classical electrostatics. The model also predicts an enhancement in total energy for nanotubes as their axis is aligned with an applied field that is proportional to the tube length. Comparisons of the predicted polarization energy with the energy needed to bend and kink nanotubes is used to predict critical lengths for aligning the free ends of nanotubes with an applied field. These results are being related to the properties of nanotube actuators and field emitting nanotube mats.
This work is funded by the Office of Naval Research.