Showing papers by "Mark S. Hybertsen published in 2006"

Dependence of single-molecule junction conductance on molecular conformation

Abstract: Since it was first suggested1 that a single molecule might function as an active electronic component, a number of techniques have been developed to measure the charge transport properties of single molecules2,3,4,5,6,7,8,9,10,11,12. Although scanning tunnelling microscopy observations under high vacuum conditions can allow stable measurements of electron transport, most measurements of a single molecule bonded in a metal–molecule–metal junction exhibit relatively large variations in conductance. As a result, even simple predictions about how molecules behave in such junctions have still not been rigorously tested. For instance, it is well known13,14 that the tunnelling current passing through a molecule depends on its conformation; but although some experiments have verified this effect15,16,17,18, a comprehensive mapping of how junction conductance changes with molecular conformation is not yet available. In the simple case of a biphenyl—a molecule with two phenyl rings linked by a single C–C bond—conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. Here we use amine link groups to form single-molecule junctions with more reproducible current–voltage characteristics19. This allows us to extract average conductance values from thousands of individual measurements on a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through π-conjugated biphenyl systems13.

Single-Molecule Circuits with Well-Defined Molecular Conductance

Abstract: We measure the conductance of amine-terminated molecules by breaking Au point contacts in a molecular solution at room temperature. We find that the variability of the observed conductance for the diamine molecule−Au junctions is much less than the variability for diisonitrile− and dithiol−Au junctions. This narrow distribution enables unambiguous conductance measurements of single molecules. For an alkane diamine series with 2−8 carbon atoms in the hydrocarbon chain, our results show a systematic trend in the conductance from which we extract a tunneling decay constant of 0.91 ± 0.03 per methylene group. We hypothesize that the diamine link binds preferentially to undercoordinated Au atoms in the junction. This is supported by density functional theory-based calculations that show the amine binding to a gold adatom with sufficient angular flexibility for easy junction formation but well-defined electronic coupling of the N lone pair to the Au. Therefore, the amine linkage leads to well-defined conductanc...

Renormalization of Molecular Electronic Levels at Metal-Molecule Interfaces

Abstract: The electronic structure of benzene on graphite (0001) is computed using the $GW$ approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.

Renormalization of Molecular Electronic Levels at Metal-Molecule Interfaces

Abstract: The electronic structure of benzene on graphite (0001) is computed using the $GW$ approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.

Auger recombination of excitons in one-dimensional systems

Abstract: In tightly confined one-dimensional (1D) systems, the effective Coulomb interaction is greatly enhanced and optical transitions generally lead to the formation of strongly bound excitons. When more than one exciton is present, the Coulomb interaction also leads to rapid exciton-exciton annihilation through an Auger recombination process. This effect, which may be significant even at low exciton densities, can be described by a rate law governing two-body interactions. The Auger recombination rate for excitons in a strongly confined 1D system is analyzed. The rate increases sharply with exciton binding energy, but varies only weakly with temperature. An explicit expression for the Auger recombination rate in terms of the exciton binding energy, optical matrix element and reduced carrier mass is derived for a two-band model in which the Coulomb interaction is approximated by a point-contact potential. Results for the prototypical 1D system of single-walled carbon nanotubes are obtained and compared with experiment.

Performance of carbon nanotube-dispersed thin-film transistors

Abstract: A numerical technique that relies on modifying the organic semiconducting host with metallic carbon nanotubes (CNTs) to increase the transconductance or, equivalently, reduce effective channel length (Leff) has recently been proposed. The authors use an extensive set of experimental data to analyze the performance of these transistors using the theory of heterogeneous two-dimensional percolating networks of metal-semiconducting CNTs embedded in the organic host. Their analysis (i) reproduces experimental characteristics, (ii) shows that Leff scales as a power law of CNT-doping density (ρ), (iii) illustrates the importance of an active subpercolating network of semiconducting CNTs in an organic host, and (iv) establishes the upper limit of transistor count for an integrated circuit based on this technology as a function of ρ, on current (Ion), and circuit-failure probability (F).

Frustrated Ostwald Ripening in Self-Assembled Monolayers of Cruciform π-Systems

Abstract: This study details a scanning tunneling microscopy investigation into the mechanism of chiral grain growth in highly ordered, self-assembled monolayer films composed of cruciform π-systems. Although the molecules themselves are achiral, when they adsorb from solution onto graphite, they adopt a gear-like conformation that, by virtue of the surface, is chiral. These handed subunits arrange themselves into enantiomeric two-dimensional domains. The unique finding from this study is that Ostwald ripening is frustrated between domain boundaries that are of opposite chirality because direct interconversion between the chiral units on the surface is energetically inhibited.

GRAPHITE, TUBULAR PAHs, AND THE DIFFUSE INTERSTELLAR BANDS

Abstract: Planar polycyclic aromatic hydrocarbons (PAHs) are candidates for the diffuse interstellar band (DIB) carriers. Nanometer-size Fe metallic particles, expected to be present during astrophysical graphite growth, are catalytic for formation of a related species—large tubular PAH molecules. We propose that tubular PAH molecules are a component of the interstellar medium. Electronic structure calculations, on a specific family of tubular PAH molecules derived from elongated C60, reveal intense electronic transitions in the visible and near-IR, which vary systematically with length. We analyze these molecules as DIB carriers within the known constraints. Subject headings: astrochemistry — dust, extinction — ISM: molecules — stars: carbon

First-principles studies of the electronic structure of cyclopentene on Si(001) : density functional theory and GW calculations

Abstract: The electronic structure and ground-state geometry of cyclopentene monolayers on Si(001) are studied using ab initio pseudopotential density-functional theory (DFT). Quasiparticle excitation spectra are calculated within the GW approximation. Both cis and trans cyclopentene monolayers are considered. In both geometries, a strong electronic coupling between the monolayer and Si substrate is found; substantial inter-molecular interactions are present, as indicated by broadening in molecular levels that are decoupled from the substrate. It is argued that the cis structure is kinetically favored, and we evaluate self-energy corrections to the eigenstates of this configuration near the band edges within the GW approximation. The calculated self-energy corrections are large, exceeding those of bulk Si, and increase the energy gap between occupied and unoccupied frontier adsorbate states by 1.3 eV.

Ion scattering simulations of the Si(100)-SiO2 interface

Abstract: We carry out ion scattering simulations to investigate the nature of the transition region at the Si(100)-SiO2 interface. Ion scattering experiments performed in the channeling geometry provide us with a genuine interfacial property, the excess Si yield, resulting from distortions in the Si substrate and from Si atoms in intermediate oxidation states. To interpret the ion scattering data, we first generate a series of model structures for the interface by applying sequentially classical molecular dynamics and density-functional relaxation methods. These models reproduce atomic-scale features consistent with a variety of available experimental data. Then, we design a classical scheme to perform ion scattering simulations on these model interfaces. In our study, we separate the excess Si yield obtained from experiments in two distinct contributions. First, Si atoms in intermediate oxidation states account for similar to 25% of the excess Si yield, a contribution that is fully determined by the population of suboxide determined from photoemission data. The remaining similar to 75% of the excess Si yield characterizes the amount of lateral distortion of the substrate Si layers in the vicinity of the Si(100)-SiO2 interface. The comparison between calculated and experimental excess Si yields indicates that the distortions propagating from the interface into the Si substrate are consistent with interfacial transition structures extending over more than two Si layers, eventually including a disordered bonding pattern. Nearly abrupt interfaces induce distortions in the upper layers of the Si substrate which are insufficient for reproducing the experimental excess Si yields.