Epitaxial graphene grown on SiC is currently the best candidate for use as a metrological resistance standard based on the quantum Hall effect (QHE) in which the Hall resistance is quantized in 2D electron systems,” explains team member Cassandra Chua of the Cavendish Laboratory at Cambridge University. “This type of graphene is also ideal for making RF transistors for wireless devices. Studying the behaviour of bilayer patches – which naturally form as monolayer epitaxial grows on SiC – will thus allow us to better understand how to use this material for these applications.”

The main difficulty here, however, is to produce single-phase epitaxial graphene that does not contain multilayer graphene inclusions, which have a very different resistance quantization from the monolayer material. Bilayer inclusions in otherwise monolayer graphene samples usually begin to grow along the step edges on the surface of the SiC substrate and form stripes or islands.

Metallic shortcuts or insulating islands

Now, a team of researchers from Cambridge University, NPL, Lancaster University, Chalmers University of Technology and Linköping University is saying that the bilayer patches can act as either metallic shortcuts or insulating islands depending on how the monolayer-bilayer composite is doped and gated. When the SiC substrate highly dopes the composite and provides it with a large number of charge carriers (electrons and holes), it behaves like a metal. And, when it is feebly doped by the SiC, it acts like an insulator.

In the insulating case, pairs of closely placed insulating bilayer inclusions can even create naturally defined constrictions and point contacts in monolayer graphene. And the researchers say that they have managed to control electrical transport through such constrictions using local electrostatic fields applied using a conducting atomic force microscope (AFM) tip.

Edge currents

“We measured the magnetotransport properties of the sample by electrically connecting it via a probe and placing it in the centre of liquid helium cooled superconducting coils that allow us to make electrical measurements while applying a magnetic field at the same time,” explained Chua. “We then performed scanning gate measurements using the same set up but using an AFM attached to the probe too. Finally, we measured the sample’s electrical transport characteristics as a function of the tip’s position above the sample – in brief, we measured changes in electronic transport as we attached a gate electrode to different parts of the sample.”

The researchers found that in the quantum Hall state, only the edges of the 2D material contribute to electronic transport. These are called edge currents. Any metallic material, like an electrode, that bridges between the two edges can then short them and thus modify the voltages measured across the sample.

“We also found that closely spaced insulating bilayers guide the flow of electrons,” Chua told nanotechweb.org. “Since electrons are unable to flow through insulators, they are thus ‘forced’ to take any available remaining conductive path.”

The team says that it is now looking into possibly utilizing templated growth of graphene bilayer patches to engineer new device structures.

The research is detailed in Nano Lett DOI: 10.1021/nl5008757.