Atomistic Modeling of Charge Transport across a Carbon Nanotube-Polyethylene Junction

The conduction mechanism in carbon nanotube (CNT) polymer nanocomposites is complex, and there has been a considerable amount of work invested in understanding the role of the distribution of the CNTs in the composite and how it influences the conductivity. However, less interest has been devoted to the electron transport across a single CNT-polymer-CNT junction. We present a first atomistic study of the electron transmission through a CNT-polyethylene-CNT junction. The morphology of the junction is described using classical molecular dynamics simulations, and transport properties are calculated within density functional tight binding method. The electron transmission depends noticeably on the CNT-CNT separation and on the consequent polymer wrapping. At CNT-CNT distances shorter than 6 angstrom, the polyethylene molecules do not penetrate in the space between the CNTs. In this near contact regime, the electron transmission proceeds via direct tunneling between the two CNTs across a vacuum region without relevant contribution from the surrounding polymer. For distances larger than 6 angstrom, the PE molecules enter into the junction region. The frontier orbitals of the PE molecules in the junction provide localized states, which can couple to the CNT metallic states. This resonance tail increases the electron transmission probability between the CNTs across the junction by several orders of magnitude, thus lowering the effective barrier. The gradual interpenetration of the polymer is resembled in transmission fluctuations. An averaging of the transmission in energy and time along MD trajectories allows a quantitative estimation of the junction resistance and tunneling barrier.