Y. Li, E. Fahrenthold
University of Texas at Austin,
Keywords: quantum modeling, materials design, CNT, nanocomposites, iodine doping
Summary:The development of new high strength, high reliability, high ampacity conductors can benefit a wide range of commercial and military systems. Improved conductors are needed to perform a variety of power and data transmission functions, and their strategic research importance is highlighted in numerous publications. The most promising opportunities for fundamental improvements in conductors appear to be offered by carbon nanotube (CNT) based composites. Material architectures of interest include packed nanotube arrays, gel spun nanotube based fibers, doped nanotube wires and cables, and nanotube-copper composites. Although the electrical conductivity of current CNT fibers lag copper by an order of magnitude, a mass specific comparison shows that CNT composites are promising candidates for disruptive advances in conductor technology. The majority of published research on carbon nanocomposite conductors has taken an experimental approach. Although experimental research has been productive, the complexity of the materials design problem motivates complementary efforts on simulation. Simulation can serve as a valuable adjunct to experiment, in particular when published experimental studies speculate on physics which may not be amenable to direct experimental measurement. In recent research the authors have performed conductance analyses of doped and undoped carbon nanotubes, and doped and undoped carbon nanotube junctions, using the Density Functional Theory code Siesta (TransSiesta). The analysis considers iodine doping, studied experimentally in published work, and applies modeling techniques previously applied to study transition metal doping of CNTs. The models assume ballistic transport: the mean free path of an electron is assumed to be greater than length of the conductor. All calculations were performed for metallic single-walled carbon nanotubes of chirality (5,5). The modeled dopant was iodine. The analysis considered six CNT configurations: (1) single CNT, undoped, (2) single CNT, doped, (3) junction of two CNTs, aligned and undoped, (4) junction of two CNTs, misaligned and undoped, (5) junction of two CNTs, aligned and doped, and (6) junction of two CNTs, misaligned and doped. Here the term `aligned' refers to the positioning of the dopant atom with respect to the two nanotubes which form the junction. The results of the quantum modeling of the doped CNT junctions may be summarized as follows: (1) in the undoped case, conductance increases monotonically with overlap, (2) in the doped case, conductance shows a relative maximum at an overlap of approximately two nanotube diameters, (3) the junction conductance is sensitive to alignment effects, in particular for doped junctions, (4) the beneficial effects of doping do not extend beyond an overlap of approximately five nanotube diameters, and (5) at the best modeled combination of doping, alignment, and overlap, junction conductance is approximately eighty percent of that for a single ideal nanotube. The following general conclusions are suggested: (1) the experimentally observed benefits of doping appear to be due primarily to effects at the nanotube junctions, (2) an `optimal' overlap was observed only for doped configurations. Additional modeling work is in progress, including refined versions of the quantum models analyzed to date and studies of additional nanotube and dopant configurations.