All-graphene edge contacts: Electrical resistance of graphene T-junctions

Using ab-initio methods we investigate the possibility of three-terminal graphene "T-junction" devices and show that these all-graphene edge contacts are energetically feasible when the 1D interface itself is free from foreign atoms. We examine the energetics of various junction structures as a function of the atomic scale geometry. Three-terminal equilibrium Green's functions are used to determine the transmission spectrum and contact resistance of the system. We find that the most symmetric structures have a significant binding energy, and we determine the contact resistances in the junction to be in the range of 1-10 kOhm which is comparable to the best contact resistance reported for edge-contacted graphene-metal contacts. We conclude that conducting all-carbon T-junctions should be feasible

Failure of multi-layer graphene coatings in acidic media

Being impermeable to all gases, graphene has been proposed as an effective ultrathin barrier film and protective coating. However, here it is shown how the gastight property of graphene-based coatings may indirectly lead to their catastrophic failure under certain conditions. When nickel coated with thick, high-quality chemical vapor deposited multilayered graphene is exposed to acidic solutions, a dramatic evolution of gas is observed at the coating-substrate interface. The gas bubbles grow and merge, eventually rupturing and delaminating the coating. This behavior, attributed to cathodic hydrogen evolution, can also occur spontaneously on a range of other technologically important metals and alloys based on iron, zinc, aluminum and manganese; this makes these findings relevant for practical applications of graphene-based coatings

Large-scale tight-binding simulations of quantum transport in ballistic graphene

Graphene has proven to host outstanding mesoscopic effects involving massless Dirac quasiparticles travelling ballistically resulting in the current flow exhibiting light-like behaviour. A new branch of 2D electronics inspired by the standard principles of optics is rapidly evolving, calling for a deeper understanding of transport in large-scale devices at a quantum level. Here we perform large-scale quantum transport calculations based on a tight-binding model of graphene and the non-equilibrium Green's function method and include the effects of $p-n$ junctions of different shape, magnetic field, and absorptive regions acting as drains for current. We stress the importance of choosing absorbing boundary conditions in the calculations to correctly capture how current flows in the limit of infinite devices. As a specific application we present a fully quantum-mechanical framework for the "2D Dirac fermion microscope" recently proposed by B{\o}ggild $et\, al.$ [Nat. Comm. 8, 10.1038 (2017)], tackling several key electron-optical effects therein predicted via semiclassical trajectory simulations, such as electron beam collimation, deflection and scattering off Veselago dots. Our results confirm that a semiclassical approach to a large extend is sufficient to capture the main transport features in the mesoscopic limit and the optical regime, but also that a richer electron-optical landscape is to be expected when coherence or other purely quantum effects are accounted for in the simulations.Comment: 12 pages, 10 figure