We report on a numerical study of electronic transport in chemically doped 2D graphene materials. By using ab initio calculations, a self-consistent scattering potential is derived for boron and nitrogen substitutions, and a fully quantum-mechanical Kubo-Greenwood approach is used to evaluate the resulting charge mobilities and conductivities of systems with impurity concentration ranging within [0.5, 4.0]%. Even for a doping concentration as large as 4.0%, the conduction is marginally affected by quantum interference effects, preserving therefore remarkable transport properties, even down to the zero temperature limit. As a result of the chemical doping, electron-hole mobilities and conductivities are shown to become asymmetric with respect to the Dirac point.

Two-dimensional graphene, carbon nanotubes, and graphene nanoribbons represent a novel class of low dimensional materials that could serve as building blocks for future carbon-based nanoelectronics. Although these systems share a similar underlying electronic structure, whose exact details depend on confi nement effects, crucial differences emerge when disorder comes into play. In this review, we consider the transport properties of these materials, with particular emphasis on the case of graphene nanoribbons. After summarizing the electronic and transport properties of defect-free systems, we focus on the effects of a model disorder potential (Anderson-type), and illustrate how transport properties are sensitive to the underlying symmetry. We provide analytical expressions for the elastic mean free path of carbon nanotubes and graphene nanoribbons, and discuss the onset of weak and strong localization regimes, which are genuinely dependent on the transport dimensionality. We also consider the effects of edge disorder and roughness for graphene nanoribbons in relation to their armchair or zigzag orientation.

We present first-principles calculations of quantum transport in chemically doped graphene nanoribbons with a width of up to 4 nm. The presence of boron and nitrogen impurities is shown to yield resonant backscattering, whose features are strongly dependent on the symmetry and the width of the ribbon, as well as the position of the dopants. Full suppression of backscattering is obtained on the pi-pi* plateau when the impurity preserves the mirror symmetry of armchair ribbons. Further, an unusual acceptor-donor transition is observed in zigzag ribbons. These unconventional doping effects could be used to design novel types of switching devices.

For carbon nanotubes, we determine the role of disorder and helicity in the transport length scales and intrinsic conductance. Our results highlight different physical phenomena originating from defect scattering and multishell conduction. Those effects are sensitive to the position of the Fermi level, and allow for a consistent interpretation of recent transport experiments in doped nanotubes. DOI: 10.1103/PhysRevB.69.121410 PACS number͑s͒: 73.63.Fg, 71.15.Ϫm Helical symmetries in carbon nanotubes are at the spectacular origin of a metallic versus semiconducting electronic status.1 Metallic single-wall nanotubes ͑SWNT's͒, with their Fermi level at the charge neutrality point ͑CNP͒, have been further demonstrated to behave as exceptionally long ballistic conductors, due to a vanishing contribution of elastic backscattering ͑pseudospin symmetry conservation͒, 2,3 and upscaling of the mean free path with nanotube diameter was derived in the framework of the Fermi golden rule. 4 Multiwall nanotubes ͑MWNT's͒ are made from a few to several tens of weakly coupled concentric shells, separated by few angstroms and with random helicities. Although ballistic conduction has been reported in several experiments in SWNT's and MWNT's, 3,5,6 the bias dependence of the conductance in MWNT's highlights the possible role of disorder and/or intershell conduction.7-11 At large bias, the contribution of several subbands is expected, with an enhanced backscattering for the so-called massive subbands, as compared to the two massless subbands crossing at the CNP.3 On the other hand, it has been argued theoretically that incommensurability between coupled shells can drive conduction to either diffusive, ballistic, or intermediate transport regime.12-15 Such scenario has been debated in subsequent works, restricted, however, to qualitative arguments or numerical computation on small systems. 14,15 As it is now experimentally possible to have a simultaneous access to the helicity and transport phenomena in MWNT's, 16 it is important to better clarify the fundamental length scales and transport regimes. Besides, the issue of electronic mobilities in carbon nanotube based field effect transistors is also of great concern. 17In this work, the relative effect of elastic scattering on massless and massive subbands is quantified, in disordered nanotubes as long as 10 m. This is performed by computing the energy dependence of diffusion coefficients and length scaling of the Kubo conductance, using a real-space numerical approach. Two sources of elastic scattering are considered separately. First, the effect of static ͑Anderson-like͒ disorder is studied in SWNT's and commensurate MWNT's. Second, multishell conduction is shown to be sensitive to incommensurability, which introduces scattering centers in otherwise structurally perfect MWNT's.Transport is controlled by low-energy excitations close to the Fermi level E F , which turn out to be well described by a simple tight-binding Hamiltonian treating only the coupling between electrons: ͑1͒Th...

We investigate the properties of electronic states in two and three-dimensional quasiperiodic structures : the generalized Rauzy tilings. Exact diagonalizations, limited to clusters with a few thousands sites, suggest that eigenstates are critical and more extended at the band edges than at the band center. These trends are clearly confirmed when we compute the spreading of energy-filtered wavepackets, using a new algorithm which allows to treat systems of about one million sites. The present approach to quantum dynamics, which gives also access to the low frequency conductivity, opens new perspectives in the analyzis of two and three-dimensional models.

We report on theoretical results about contact-dependent effects and tunneling currents through DNA molecules. A tetranucleotide PolyGACT chain, connected in between metallic contacts, is studied as a generic case, and compared to other periodic sequences such as PolyAT or PolyGC. Remarkable resonance conditions are analytically derived, indicating that a strong coupling does not always result in a larger conductance. This result is properly illustrated by considering intrinsic features of bias-dependent tunneling currents in the coherent regime. DOI: 10.1103/PhysRevB.71.113106 PACS number͑s͒: 72.80.Le, 72.20.Ee, 87.14.Gg In recent years, many experimental measurements have directly probed the electrical current as a function of the applied potential across DNA molecules. 1-5 These experiments are performed in a variety of conditions where important factors, including the substrate surface, contacts to the electrodes, counterions, and DNA structure are not kept constant.3 This state of affairs considerably makes difficult a proper comparison among different experimental reports, which range from completely insulating to semiconducting, and even superconducting, behaviors. 2 In turn, such scatter makes it difficult to set the basis for a meaningful theoretical approach to the intrinsic DNA electrical transport properties.To this end, the role of contacts deserves particular attention. In many measurements, contact with metal electrodes was achieved by laying down the molecules directly on the electrodes. In this case, it is rather difficult to prove that the DNA molecule is in direct contact with the electrodes. Even so, the weak physical adhesion between DNA and metal may produce an insulating contact. Recent transport experiments have shown that deliberate chemical bonding between DNA and metal electrodes is a prerequisite for achieving reproducible conductivity results. [3][4][5] Generally, any current measured through a DNA molecule results from the carrier injection onto the stack of bases, combined with the intrinsic conduction along the DNA sequence. At low voltage, the main contribution to the resistance comes from the metal-DNA junction potential mismatch ͑barrier͒, whereas for high enough voltage, new conduction channels are provided by the molecular states. The I͑V͒ characteristics are thus somehow inferred from the energy difference between the metallic work function and the lowest ionization energy levels of the DNA ͑in case of hole transport͒.6 Besides, charge transfer in DNA has been proven to be mainly conveyed by intrastrand -coupling, 7 through sequential incoherent hopping or coherent tunneling ͑superexchange͒. 7 The latter mechanism might be expected to dominate the conduction in the very low-temperature regime. Despite great experimental efforts, 8,9 few theoretical works have so far precisely addressed the nature of measured currents and its relation with device characteristics.In this work, we present a theoretical study on coherent charge tunneling in DNA molecules connected in between...

We report on a numerical study of quantum diffusion over m lengths in defect-free multiwalled nanotubes. The intershell coupling allows the wave packet spreading over several shells, and when their periodicities along the nanotube axis are incommensurate, electronic propagation is shown to follow a nonballistic law if a sufficient number of shells are involved in conduction. This results in magnetotransport properties which are exceptional for a disorder free system.

We present first-principles calculations of quantum transport in chemically functionalized metallic carbon nanotubes with lengths reaching the micrometer scale and random distributions of functional groups. Two typical cases are investigated, namely, a sp2-type bonding between carbene groups (CH2) and the nanotube sidewalls and a sp3-type bonding of nanotubes with paired phenyl groups. For similar molecular coverage density, charge transport is found to range from a quasi-ballistic-like to a strongly diffusive regime, with corresponding mean free paths changing by orders of magnitude depending on the nature of the chemical bonding.