Steven W. D. Bailey

Functionalization mediates heat transport in graphene nanoflakes.

The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for in situ monitoring we demonstrate that the hotspot temperature was lowered by ∼28 °C for a chip operating at 1,300 W cm−2. Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.

A study of planar anchor groups for graphene-based single-molecule electronics.

To identify families of stable planar anchor groups for use in single molecule electronics, we report detailed results for the binding energies of two families of anthracene and pyrene derivatives adsorbed onto graphene. We find that all the selected derivatives functionalized with either electron donating or electron accepting substituents bind more strongly to graphene than the parent non-functionalized anthracene or pyrene. The binding energy is sensitive to the detailed atomic alignment of substituent groups over the graphene substrate leading to larger than expected binding energies for -OH and -CN derivatives. Furthermore, the ordering of the binding energies within the anthracene and pyrene series does not simply follow the electron affinities of the substituents. Energy barriers to rotation or displacement on the graphene surface are much lower than binding energies for adsorption and therefore at room temperature, although the molecules are bound to the graphene, they are almost free to move along the graphene surface. Binding energies can be increased by incorporating electrically inert side chains and are sensitive to the conformation of such chains.

Conformation dependence of molecular conductance: chemistry versus geometry

Recent experiments by Venkataraman et al (2006 Nature 442 904) on a series of molecular wires with varying chemical compositions revealed a linear dependence of the conductance on cos2 θ, where θ is the angle of twist between neighbouring aromatic rings. To investigate whether or not this dependence has a more general applicability, we present a first-principles theoretical study of the transport properties of this family of molecules as a function of the chemical composition, conformation and the contact atom and geometry. If the Fermi energy EF lies within the HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) gap, then we reproduce the above experimental results. More generally, however, if EF is located within either the LUMO or the HOMO states, the presence of resonances destroys the linear dependence of the conductance on cos2 θ and gives rise to non-monotonic behaviour associated with the level structure of the different molecules. Our results suggest that the above experiments provide a novel method for extracting spectroscopic information about molecules contacted to electrodes.

Giant magnetoresistance of nickel-contacted carbon nanotubes

We present ab initio theoretical results for the giant magnetoresistance of single-wall carbon nanotubes (CNT) in contact with nickel electrodes. These show that Ni atoms located on the surface or axis of CNT contacts can induce a significant magnetic moment on the carbon atoms. For non-gated, undoped CNTs, this produces room-temperature GMR ratios of between 45% and 100% of the anti-aligned conductance.

Giant magneto-conductance in twisted carbon nanotubes

Using the Landauer-Buttiker formalism, we calculate the effect of structural twist on electron transport in conducting carbon nanotubes. We demonstrate that even a localized region of twist scatters the propagating π electrons and induces the opening of a (pseudo-) gap near the Fermi level. The subsequent conductance reduction may be compensated by an applied axial magnetic field, leading to a twist-induced, giant positive magneto-conductance in clean armchair nanotubes.

Tuning the electrical conductance of metalloporphyrin supramolecular wires.

In contrast with conventional single-molecule junctions, in which the current flows parallel to the long axis or plane of a molecule, we investigate the transport properties of M(II)-5,15-diphenylporphyrin (M-DPP) single-molecule junctions (M=Co, Ni, Cu, or Zn divalent metal ions), in which the current flows perpendicular to the plane of the porphyrin. Novel STM-based conductance measurements combined with quantum transport calculations demonstrate that current-perpendicular-to-the-plane (CPP) junctions have three-orders-of-magnitude higher electrical conductances than their current-in-plane (CIP) counterparts, ranging from 2.10−2 G0 for Ni-DPP up to 8.10−2 G0 for Zn-DPP. The metal ion in the center of the DPP skeletons is strongly coordinated with the nitrogens of the pyridyl coated electrodes, with a binding energy that is sensitive to the choice of metal ion. We find that the binding energies of Zn-DPP and Co-DPP are significantly higher than those of Ni-DPP and Cu-DPP. Therefore when combined with its higher conductance, we identify Zn-DPP as the favoured candidate for high-conductance CPP single-molecule devices.

Electronic properties of sculpturenes

We investigate the electronic properties of sculpturenes, formed by sculpting selected shapes from bilayer graphene, boron nitride or graphene–boron nitride hetero-bilayers and allowing the shapes to spontaneously reconstruct. The simplest sculpturenes are periodic nanotubes, containing lines of non-hexagonal rings. More complex sculpturenes formed from shapes with non-trivial topologies, connectivities and materials combinations may also be constructed. Results are presented for the reconstructed geometries, electronic densities of states and current–voltage relations of these new structures.

Negative differential electrical resistance of a rotational organic nanomotor

Summary A robust, nanoelectromechanical switch is proposed based upon an asymmetric pendant moiety anchored to an organic backbone between two C60 fullerenes, which in turn are connected to gold electrodes. Ab initio density functional calculations are used to demonstrate that an electric field induces rotation of the pendant group, leading to a nonlinear current–voltage relation. The nonlinearity is strong enough to lead to negative differential resistance at modest source–drain voltages.