Conformation dependence of charge transfer and level alignment in nitrobenzene junctions with pyridyl anchor groups

The alignment of molecular levels with the Fermi energy in single molecule junctions is a crucial factor in determining their conductance or the observability of quantum interference effects. In the present study which is based on density functional theory calculations, we explore the zero-bias charge transfer and level alignment for nitro-bipyridyl-phenyl adsorbed between two gold surfaces which we find to vary significantly with the molecular conformation. The net charge transfer is the result of two opposing effects, namely Pauli repulsion at the interface between the molecule and the leads, and the electron accepting nature of the NO$_2$ group, where only the latter which we analyze in terms of the electronegativity of the isolated molecules depends on the two intra-molecular torsion angles. We provide evidence that the conformation dependence of the alignment of molecular levels and peaks in the transmission function can indeed be understood in terms of charge transfer for this system, and that other properties such as molecular dipoles do not play a significant role. Our study is relevant for device design in molecular electronics where nitrobenzene appears as a component in proposals for rectification, quantum interference or chemical gating.Comment: 10 pages, 6 figure

Effective models for charge transport in DNA nanowires

The rapid progress in the field of molecular electronics has led to an increasing interest on DNA oligomers as possible components of electronic circuits at the nanoscale. For this, however, an understanding of charge transfer and transport mechanisms in this molecule is required. Experiments show that a large number of factors may influence the electronic properties of DNA. Though full first principle approaches are the ideal tool for a theoretical characterization of the structural and electronic properties of DNA, the structural complexity of this molecule make these methods of limited use. Consequently, model Hamiltonian approaches, which filter out single factors influencing charge propagation in the double helix are highly valuable. In this chapter, we give a review of different DNA models which are thought to capture the influence of some of these factors. We will specifically focus on static and dynamic disorder.Comment: to appear in "NanoBioTechnology: BioInspired device and materials of the future". Edited by O. Shoseyov and I. Levy. Humana Press (2006

Modeling molecular conduction in DNA wires: Charge transfer theories and dissipative quantum transport

Measurements of electron transfer rates as well as of charge transport characteristics in DNA produced a number of seemingly contradictory results, ranging from insulating behaviour to the suggestion that DNA is an efficient medium for charge transport. Among other factors, environmental effects appear to play a crucial role in determining the effectivity of charge propagation along the double helix. This chapter gives an overview over charge transfer theories and their implication for addressing the interaction of a molecular conductor with a dissipative environment. Further, we focus on possible applications of these approaches for charge transport through DNA-based molecular wires

The role of contacts in molecular electronics

Molecular electronic devices are the upmost destiny of the miniaturization trend of electronic components. Although not yet reproducible on large scale, molecular devices are since recently subject of intense studies both experimentally and theoretically, which agree in pointing out the extreme sensitivity of such devices on the nature and quality of the contacts. This chapter intends to provide a general theoretical framework for modelling electronic transport at the molecular scale by describing the implementation of a hybrid method based on Green function theory and density functional algorithms. In order to show the presence of contact-dependent features in the molecular conductance, we discuss three archetypal molecular devices, which are intended to focus on the importance of the different sub-parts of a molecular two-terminal setup.Comment: 17 pages, 8 figure

Sum rule for transport in a Luttinger liquid with long range interaction in the presence of an impurity

We show that the non-linear dc transport in a Luttinger liquid with interaction of finite range in the presence of an impurity is governed by a sum rule which causes the charging energy to vanish.Comment: 5 pages, RevTeX, 1 figure, to be published in Europhysics Letter

Hofstadter butterflies of bilayer graphene

We calculate the electronic spectrum of bilayer graphene in perpendicular magnetic fields nonperturbatively. To accommodate arbitrary displacements between the two layers, we apply a periodic gauge based on singular flux vortices of phase $2\pi$. The resulting Hofstadter-like butterfly plots show a reduced symmetry, depending on the relative position of the two layers against each other. The split of the zero-energy relativistic Landau level differs by one order of magnitude between Bernal and non-Bernal stacking.Comment: updated to refereed and edited versio

Electron transport in carbon nanotube-metal systems: contact effects

Carbon nanotubes (CNT) have a very large application potential in the rapid developing field of molecular electronics. Infinite single-wall metallic CNTs have theoretically a conductance of 4e2/h because of the two electronic bands crossing the Fermi level. For finite size CNTs experiments have shown that other values are also possible, indicating a very strong influence of the contacts. We study electron transport in single- and double-wall CNTs contacted to metallic electrodes within the Landauer formalism combined with Green function techniques. We show that the symmetry of the contact region may lead to blocking of a transport channel. In the case of double-wall CNTs with both inner and outer shells being metallic, non-diagonal self energy contributions from the electrodes may induce channel mixing, precluding a simple addition of the individual shell conductances

Vibrational Instabilities in Resonant Electron Transport through Single-Molecule Junctions

We analyze various limits of vibrationally coupled resonant electron transport in single-molecule junctions. Based on a master equation approach, we discuss analytic and numerical results for junctions under a high bias voltage or weak electronic-vibrational coupling. It is shown that in these limits the vibrational excitation of the molecular bridge increases indefinitely, i.e. the junction exhibits a vibrational instability. Moreover, our analysis provides analytic results for the vibrational distribution function and reveals that these vibrational instabilities are related to electron-hole pair creation processes.Comment: 19 pages, 3 figure