The optical properties of two-dimensional (2D) materials are accurately described by many-body methods including specifically pronounced electron−electron and electron−hole effects. Such methods are, however, computationally demanding and applicable on small computational cells only. We provide approximate optical gaps for 2D materials from time-dependent (TD) density functional theory based on a set of specific screened hybrid functionals and show that this approach effectively accounts for all important physical effects including excitons. Optical gap values obtained from the TD-HSE06 approach for a broad gap range 1−6 eV of eight 2D materials are in agreement with both experimental optical gaps and accurate GW+BSE calculations. Further, we show that such an approach is eligible and practicable for van der Waals heterostructures containing incommensurate cells of different monolayers and enables detailed analysis of intra-and interlayer excitonic wave functions. TD-HSE06 is therefore a suitable method for a reliable description of the optical properties of extended periodic 2D systems.

We use time-dependent density functional theory together with the HSE06 hybrid functional to investigate the optical and excitonic properties of two-dimensional transition metal carbides, MXenes. We determine reliable optical gaps,...

The conductivity of carbon nanotube thin films is mainly determined by carbon nanotube junctions, the resistance of which can be reduced by several different methods. We investigate electronic transport through carbon nanotube junctions in a four-terminal configuration, where two metallic single-wall carbon nanotubes are linked by a group 6 transition metal atom. The transport calculations are based on the Green’s function method combined with the density-functional theory. The transition metal atom is found to enhance the transport through the junction near the Fermi level. However, the size of the nanotube affects the improvement in the conductivity. The enhancement is related to the hybridization of chromium and carbon atom orbitals, which is clearly reflected in the character of eigenstates near the Fermi level. The effects of chromium atoms and precursor molecules remaining adsorbed on the nanotubes outside the junctions are also examined.

We have studied the effect of AuCl4 functionalization on the conductivity of carbon nanotube networks by first-principles electronic structure calculations. The functionalization results from treating carbon nanotube networks by dissolved AuCl3. Band structures and electronic transmission functions for single-walled semiconducting carbon nanotubes with physisorbed AuCl4 anions are computed using the density functional theory. The resulting p-type doping of nanotubes accompanied by a downshift of the Fermi level and balanced by negatively charged AuCl4 anions make the nanotubes metallic. Moreover, the influence of AuCl4 functionalization on the conductance of the junction between two semiconducting carbon nanotubes is considered. Increasing the AuCl4 coverage lowers the Fermi level rapidly until it is pinned by a van Hove singularity of the nanotube electronic structure. For these doping levels our calculations based on the density-functional nonequilibrium Green’s function method show a significant increase in the intratube electron transmission. Moreover, our electron transport calculations for crossed nanotubes indicate a simultaneous increase in the intertube conductance. These factors explain the experimentally observed robust conductivity improvement of carbon nanotube networks treated by dissolved AuCl3.

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 also study electron transport between different types (metallic, semiconducting) of CNTs. Reflecting the differences in the electronic structures of semiconducting and metallic CNTs, we find that in addition to turning semiconducting CNTs metallic, doping further increases electron transport most efficiently along semiconducting CNTs as well as through the junctions between them.