  The conductivity of carbon nanotube (CNT) networks can be improved markedly
by doping with nitric acid. In the present work, CNTs and junctions of CNTs
functionalized with NO$_3$ molecules are investigated to understand the
microscopic mechanism of nitric acid doping. According to our density
functional theory band structure calculations, there is charge transfer from
the CNT to adsorbed molecules indicating p-type doping. The average doping
efficiency of the NO$_3$ molecules is higher if the NO$_3$ molecules form
complexes with water molecules. In addition to electron transport along
individual CNTs, we have also studied electron transport between different
types (metallic, semiconducting) of CNTs. Reflecting the differences in the
electronic structures of semiconducting and metallic CNTs, we have found that
besides turning semiconducting CNTs metallic, doping further increases electron
transport most efficiently along semiconducting CNTs as well as through a
junction between them.
