Electron transport through dangling-bond silicon wires on H-passivated Si(100).

We compute the electron transmission through different types of dangling-bond wire on Si(100)-H (2 × 1). Recent progress in the construction of atomic-size interconnects (Weber et al 2012 Science 335 64) shows the possibility to achieve atomic-size circuits via atomic-size wires using silicon surfaces. Hence, electron transport through quasi-1D Si-based structures is a compelling reality. Prior to these achievements, wires formed by controlled desorption of passivating H atoms off the monohydride Si(100) surface have been shown to be subject to 1D correlations and instabilities (Hitosugi et al 1999 Phys. Rev. Lett. 82 4034). The present calculations are based on density functional theory and evaluate the electron transmission though the minimum-energy 1D structures that can be formed when creating dangling-bonds on Si(100)-(2 × 1)-H. The purpose of this study is twofold: (i) to assess the transport properties of these atomic-size wires in the presence of 1D instabilities; (ii) to provide a fingerprint for experimental identification of the instability through the transport characteristics of the wires. To these aims, we evaluate the electron transport through the wires in the absence of instabilities, in the presence of distortions (Jahn-Teller instabilities) and in the presence of magnetic instabilities (ferro- and antiferro-ordering). We find that instabilities substantially reduce the transport capabilities of dangling-bond wires leading to transmissions that vary so differently with electron energy that an unambiguous identification of the wire type should be accessible in transport experiments.

Electroresistance effect in ferroelectric tunnel junctions with symmetric electrodes.

Understanding the effects that govern electronic transport in ferroelectric tunnel junctions (FTJs) is of vital importance to improve the efficiency of devices such as ferroelectric memories with nondestructive readout. However, our current knowledge (typically based on simple semiempirical models or first-principles calculations restricted to the limit of zero bias) remains partial, which may hinder the development of more efficient systems. For example, nowadays it is commonly believed that the tunnel electroresistance (TER) effect exploited in such devices mandatorily requires, to be sizable, the use of two different electrodes, with related potential drawbacks concerning retention time, switching, and polarization imprint. In contrast, here we demonstrate at the first-principles level that large TER values of about 200% can be achieved under finite bias in a prototypical FTJ with symmetric electrodes. Our atomistic approach allows us to quantify the contribution of different microscopic mechanisms to the electroresistance, revealing the dominant role of the inverse piezoelectric response of the ferroelectric. On the basis of our analysis, we provide a critical discussion of the semiempirical models traditionally used to describe FTJs.

Electronic transport between graphene layers covalently connected by carbon nanotubes.

We present a first-principles study of the electronic transport properties of metallic and semiconducting carbon nanotube (CNT) junctions connecting two graphene layers, for different CNT lengths and link structures. Transport is analyzed in terms of the scattering states originated from the π and π* states of the finite-length CNTs, which couple to the graphene states producing resonances in the transmission curves. We find that, for metallic CNTs, the conductance is nearly independent of the tube length, but changes strongly with the link structure, while the opposite occurs for semiconducting CNTs, where the conductance in the tunneling regime is mainly controlled by the tube length and independent of the link structure. The sizable band offset between graphene and the CNTs yields to considerable effects on the transport properties, which cannot be captured using simple empirical models and highlights the need for a first-principles description.

Divacancies in graphene and carbon nanotubes.

Divacancies are among the most important defects that alter the charge transport properties of single-walled carbon nanotubes (SWNT), and we here study, using ab initio calculations, their properties. Two structures were investigated, one that has two pentagons side by side with an octagon (585) and another composed of three pentagons and three heptagons (555777). We investigate their stability as a function of tube diameter, and calculate their charge transport properties. The 585 defect is less stable in graphene due to two broken bonds in the pentagons. We estimate that the 555777 becomes more stable than the 585 for a diameter of about 40 A (53 A) for an armchair (zigzag) SWNTs, indicating that they will prevail in large diameter multiwalled carbon nanotubes and graphene ribbons.

Oxygen clamps in gold nanowires.

We investigate how the insertion of an oxygen atom in an atomically thin gold nanowire can affect its rupture. We find, using ab initio total energy density functional theory calculations, that O atoms when inserted in gold nanowires form not only stable but also very strong bonds, in such a way that they can extract atoms from a stable tip, serving in this way as a clamp that could be used to pull a string of gold atoms.

Adsorption of benzene-1,4-dithiol on the Au(111) surface and its possible role in molecular conductance.

It is a consensus in the field of molecular electronics that the transport of charge across a single molecule depends sensitively on the details of the interaction between the molecule and the metallic leads, such as the molecular orientation. To advance the design of complex molecular devices, it is crucial to have a detailed understanding of these many aspects that influence the electron transport. A simple system that has been used as a paradigm of the class of conjugated aryl molecules is the benzene-1,4-dithiol (BDT). However, we still do not have a full understanding of the BDT transport experiments. Usually the geometries considered in transport calculations assumed that the BDT was connected to the two Au leads via the S atoms, and that the molecule was either perpendicular or close to a perpendicular configuration relative to the Au surfaces. Using ab initio calculations, we show that, for an isolated molecule, the configuration with largest adsorption energy has the BDT phenyl ring closer to being parallel to the surface, and we then argue, based on nonequilibrium Green's function-density functional theory calculations, that, depending on the experimental procedure, this may be the relevant configuration to be used in the transport calculations.

Effect of impurities in the large Au-Au distances in gold nanowires.

Experimentally obtained atomically thin gold nanowires have presented exceedingly large Au-Au interatomic distances before they break. Since no theoretical calculations of pure gold nanowires have been able to produce such large distances, we have investigated, through ab initio calculations, how impurities could affect them. We have studied the effect of H, B, C, N, O, and S impurities on the nanowire electronic and structural properties, in particular how they affect the maximum Au-Au bond length. We find that the most likely candidates to explain the distances in the range of 3.6 A and 4.8 A are H and S impurity atoms, respectively.