François Triozon

Anomalous Doping Effects on Charge Transport in Graphene Nanoribbons

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.

Charge transport in disordered graphene-based low dimensional materials

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 confinement 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.

Chemically Induced Mobility Gaps in Graphene Nanoribbons: A Route for Upscaling Device Performances

We report a first-principles based study of mesoscopic quantum transport in chemically doped graphene nanoribbons with a width up to 10 nm. The occurrence of quasi-bound states related to boron impurities results in mobility gaps as large as 1 eV, driven by strong electron-hole asymmetrical backscattering phenomena. This phenomenon opens new ways to overcome current limitations of graphene-based devices through the fabrication of chemically doped graphene nanoribbons with sizes within the reach of conventional lithography.

Effect of the chemical functionalization on charge transport in carbon nanotubes at the mesoscopic scale.

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.

Orientational Dependence of Charge Transport in Disordered Silicon Nanowires

We report on a theoretical study of surface roughness effects on charge transport in silicon nanowires with three different crystalline orientations, [100], [110] and [111]. Using an atomistic tight-binding model, key transport features such as mean-free paths, charge mobilities, and conductance scaling are investigated with the complementary Kubo-Greenwood and Landauer-Büttiker approaches. The anisotropy of the band structure of bulk silicon results in a strong orientation dependence of the transport properties of the nanowires. The best orientations for electron and hole transport are found to be the [110] and [111] directions, respectively.

Quantum dynamics in two- and three-dimensional quasiperiodic tilings

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 wave packets, using an algorithm that allows us to treat systems of about 10 6 sites. The present approach to quantum dynamics, which gives also access to the low-frequency conductivity, opens interesting perspectives in the analyzis of two- and three-dimensional models.

Atomistic Boron-Doped Graphene Field-Effect Transistors: A Route toward Unipolar Characteristics

We report fully quantum simulations of realistic models of boron-doped graphene-based field-effect transistors, including atomistic details based on DFT calculations. We show that the self-consistent solution of the three-dimensional (3D) Poisson and Schrödinger equations with a representation in terms of a tight-binding Hamiltonian manages to accurately reproduce the DFT results for an isolated boron-doped graphene nanoribbon. Using a 3D Poisson/Schrödinger solver within the non-equilibrium Greens function (NEGF) formalism, self-consistent calculations of the gate-screened scattering potentials induced by the boron impurities have been performed, allowing the theoretical exploration of the tunability of transistor characteristics. The boron-doped graphene transistors are found to approach unipolar behavior as the boron concentration is increased and, by tuning the density of chemical dopants, the electron-hole transport asymmetry can be finely adjusted. Correspondingly, the onset of a mobility gap in the device is observed. Although the computed asymmetries are not sufficient to warrant proper device operation, our results represent an initial step in the direction of improved transfer characteristics and, in particular, the developed simulation strategy is a powerful new tool for modeling doped graphene nanostructures.

Electronic conduction in multi-walled carbon nanotubes: role of intershell coupling and incommensurability

Abstract Geometry incommensurability between weakly coupled shells in multi-walled carbon nanotubes is shown to be the origin of unconventional electronic conduction mechanism, power-law scaling of the conductance, and remarkable magnetotransport and low temperature dependent conductivity when the dephasing mechanism is dominated by weak electron–electron coupling.

Electronic transport properties of carbon nanotube based metal/semiconductor/metal intramolecular junctions

The electronic structure and the conductance of a carbon nanotube based metal/semiconductor/metal intramolecular junction is investigated numerically. The nature of electronic states at the interfaces and in the semiconductor section is analysed. The quantum conductance of the system is calculated in the coherent regime and its variations with energy and length are shown to be related to contributions from different kinds of electronic state.

Quantum calculations of the carrier mobility: Methodology, Matthiessen's rule, and comparison with semi-classical approaches

We discuss carrier mobilities in the quantum Non-Equilibrium Greens Functions (NEGF) framework. We introduce a method for the extraction of the mobility that is free from contact resistance contamination and with minimal needs for ensemble averages. We focus on silicon thin films as an illustration, although the method can be applied to various materials such as semiconductor nanowires or carbon nanostructures. We then introduce a new paradigm for the definition of the partial mobility μM associated with a given elastic scattering mechanism “M,” taking phonons (PH) as a reference (μM−1=μPH+M−1−μPH−1). We argue that this definition makes better sense in a quantum transport framework as it is free from long range interference effects that can appear in purely ballistic calculations. As a matter of fact, these mobilities satisfy Matthiessens rule for three mechanisms [e.g., surface roughness (SR), remote Coulomb scattering (RCS) and phonons] much better than the usual, single mechanism calculations. We als...