Search

To realise the technological potential of subnanometer graphene junctions, there is a need to understand how their electronic and spintronic properties are controlled by the carbon chains bridging their gaps. Motivated by the recent experimental efforts to form in-situ all-carbon junctions using graphene electrodes connected to carbon chains, here we systematically study a wide variety of such structures. We find that although a wide range of transport properties are possible, the junctions can be divided into a small number of categories, according to their qualitative transport properties. For example, we find that junctions bridged by even-numbered chains of carbon atoms tend to have a lower conductance than those bridged by odd-numbered atomic chains. We also find that junctions with ferromagnetically aligned electrode surface edges have a higher transmission than anti-ferromagnetically aligned ones, because ferromagnetic alignment tends to increase the transmission of one of the spin carriers. We also examine the effect of terminal rings, electrode terminal edges and the effect of saturation of edges with hydrogen on transport properties of all-carbon junctions. Just like genomics, our findings provide a complete set of information to construct junctions formed by carbon chains with desired properties for spintronic applications. [Display omitted] [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Understanding and controlling quantum interference (QI) in single molecules is fundamental to the development of QI‐based single‐molecule electronics. Simple rules such as counting rules, curly arrow rules (CARs), circuit rules, and more recently magic ratio rules have been developed to predict QI patterns in polycyclic aromatic hydrocarbons. CARs are widely used to predict destructive QI. Here we examine the validity of CARs in fully conjugated anthracene and dihydroxyanthracene, cross‐conjugated anthraquinone, and broken conjugated dihydroanthracene attached to either graphene or gold electrodes through π‐π stacking or thiol and Au−C anchors. For the first time, we demonstrate that CARs break down in molecular junctions formed by cross‐conjugated anthraquinone. In contrast with the destructive QI predicted by CARs for a meta‐connected anthraquinone core, we demonstrate that QI is constructive. This behavior is independent of the choice of electrode material or anchor groups. This is significant, because by changing the redox state of meta‐connected dihydroxyanthracene to form meta‐connected anthraquinone, the conductance of the junction increases by a couple of orders of magnitude due to the crossover from constructive to destructive QI. This opens new avenues for realization of QI‐based single‐molecule switches. Short and curly: Curly arrow rules break down in molecular junctions formed by cross‐conjugated anthraquinone. In contrast with the destructive quantum interference (QI) predicted by curly arrow rules for a meta‐connected anthraquinone core, it is demonstrated here that QI is constructive. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We propose an implementation of a valley selective electronic Veselago lens in bilayer graphene. We demonstrate that in the presence of an appropriately oriented potential step, low-energy electrons radiating from a point source can be re-focused coherently within the same band. The phenomenon is due to the trigonal warping of the band structure that leads to a negative refraction index. We show that the interference pattern can be controlled by an external mechanical strain. [ABSTRACT FROM AUTHOR]

The effectiveness of five different anchor groups for non‐covalent interfacing to graphene electrodes are compared. A family of six molecules is tested in single‐molecule junctions: five consist of the same porphyrin core with different anchor groups, and the sixth is a reference molecule without anchor groups. The junction formation probability (JFP) has a strong dependence on the anchor group. Larger anchors give higher binding energies to the graphene surface, correlating with higher JFPs. The best anchor groups tested are 1,3,8‐tridodecyloxypyrene and 2,5,8,11,14‐pentadodecylhexa‐peri‐hexabenzocoronene, with JFPs of 36% and 38%, respectively. Many junctions are tested at 77 K for each molecule by measuring source‐drain current as a function of bias and gate voltages. For each compound, there is wide variation in the strength of the electronic coupling to the electrodes and in the location of Coulomb peaks. In most cases, this device‐to‐device variability makes it impossible to observe trends between the anchor and the charge‐transport characteristics. Tetrabenzofluorene anchors, which are not π‐conjugated with the porphyrin, exhibit different charge transport behavior to the other anchors tested, and they show multiple Coulomb peaks with characteristically small molecular electron‐addition energies of 0.3–0.7 eV, whereas the other compounds give single Coulomb peaks. Molecules can be interfaced with graphene electrodes using non‐covalent interactions to generate single‐molecule transistors. A family of π‐stacking anchor groups is synthesized and the yield of transistors depends strongly on the nature of the anchor group. Whilst coupling to electrodes varies between devices, it is possible to identify junctions functionalized with single molecules if they exhibit characteristic addition energies. [ABSTRACT FROM AUTHOR]

Devices formed from single molecules attached to noble-metal electrodes exhibit large conductance fluctuations, which inhibit their development as reproducible functional units. We demonstrate that single molecules with planar anchor groups attached to carbon-based electrodes are more resilient to atomic-scale variation in the contacts and exhibit significantly lower conductance fluctuations. We examine the conductance of a 2,6-dibenzylamino core-substituted naphthalenediimide chromophore attached to carbon electrodes by either phenanthrene anchors or more extended anchor groups, which include oligophenylene ethynylene spacers. We demonstrate that for the more spatially extended anchor groups conductance fluctuations are significantly reduced. The current-voltage characteristic arising from long-range tunneling is found to be strongly nonlinear with pronounced conductance suppression below a threshold voltage of approximately 2.5 V. [ABSTRACT FROM AUTHOR]

We report quantum interference effects in the electrical conductance of chemical vapor deposited graphene nanoconstrictions fabricated using feedback controlled electroburning. The observed multimode Fabry-Pérot interferences can be attributed to reflections at potential steps inside the channel. Sharp antiresonance features with a Fano line shape are observed. Theoretical modeling reveals that these Fano resonances are due to localized states inside the constriction, which couple to the delocalized states that also give rise to the Fabry-Pérot interference patterns. This study provides new insight into the interplay between two fundamental forms of quantum interference in graphene nanoconstrictions. [ABSTRACT FROM AUTHOR]

We report transport measurements on a graphene-fullerene single-molecule transistor. The device architecture where a functionalized C60 binds to graphene nanoelectrodes results in strong electron-vibron coupling and weak vibron relaxation. Using a combined approach of transport spectroscopy, Raman spectroscopy, and DFT calculations, we demonstrate center-of-mass oscillations, redox-dependent Franck-Condon blockade, and a transport regime characterized by avalanche tunnelling in a single-molecule transistor. [ABSTRACT FROM AUTHOR]

If quantum interference patterns in the hearts of polycyclic aromatic hydrocarbons could be isolated and manipulated, then a significant step toward realizing the potential of single-molecule elec tronics would be achieved. H e re we demonstrate experimentally and theoretically that a simple, parameter-free, analytic theory of interference patterns evaluated at the mid-point of the HOMO--LUMO gap (referred to as Mfunctions) correctly predicts conductance ratios of molecules with pyrene, naphthalene, anthracene, anthanthrene, or azulene hearts. M-functions provide new design strategies for identifying molecules with phase-coherent logic functions and enhancing the sensitivity of molecular-scale interferometers. [ABSTRACT FROM AUTHOR]

Experiments using a mechanically controlled break junction and calculations based on density functional theory demonstrate a new magic ratio rule (MRR) that captures the contribution of connectivity to the electrical conductance of graphene-like aromatic molecules. When one electrode is connected to a site i and the other is connected to a site i' of a particular molecule, we assign the molecule a "magic integer" Mii'. Two molecules with the same aromatic core but different pairs of electrode connection sites (i,i' and j,j', respectively) possess different magic integers Mii' and Mjj'. On the basis of connectivity alone, we predict that when the coupling to electrodes is weak and the Fermi energy of the electrodes lies close to the center of the HOMO--LUMO gap, the ratio of their conductances is equal to (Mii'/Mjj')². The MRR is exact for a tight-binding representation of a molecule and a qualitative guide for real molecules. [ABSTRACT FROM AUTHOR]

Todemonstrate the potential of nanopores in bilayer graphene forDNA sequencing, we computed the current–voltage characteristicsof a bilayer graphene junction containing a nanopore and found thatthey change significantly when nucleobases are transported throughthe pore. To demonstrate the sensitivity and selectivity of exampledevices, we computed the probability distribution PX(β) of the quantity β representing the changein the logarithmic current through the pore due to the presence ofa nucleobase X (X = adenine, thymine, guanine, or cytosine). We quantifiedthe selectivity of the bilayer-graphene nanopores by showing that PX(β) exhibits distinct peaks for eachbase X. To demonstrate that such discriminating sensing is a generalfeature of bilayer nanopores, the well-separated positions of thesepeaks were shown to be present for different pores, with alternativeexamples of electrical contacts. [ABSTRACT FROM AUTHOR]

We demonstrate a technique for creating unique forms of pure sp²-bonded carbon and unprecedented heteromolecules. These new structures, which we refer to as sculpturenes, are formed by sculpting selected shapes from bilayer graphene, heterobilayers, or multilayered materials and allowing the shapes to spontaneously reconstruct. The simplest sculpturene is topologically equivalent to a torus, with dimensions comparable to those of fullerenes. The topology of these new molecular structures is stable against atomic-scale defects. We demonstrate that sculpturenes can form the basic building blocks of hollow, multiconnected structures, with potential applications to nanofluidics and nanoelectronics. [ABSTRACT FROM AUTHOR]