Scientists say they solved the problems of one-dimensional conductivity and contamination in graphene similtaneously

Breakthrough could help graphene reach 'true potential'

Graphene scientists have managed to make electrical contact with the wonder material only along its one-dimensional edge for the first time.

At the same time, the researchers from Columbia University’s engineering department have developed a new assembly technique for layering the material that prevents contamination at the interfaces resulting in the cleanest graphene yet realised.

In previous experiments graphene researchers have made electrical contact with graphene from the top of its 2D structure, which exposes the material to contaminants.

According to Cory Dean, who led the research as a postdoc at Columbia and is now an assistant professor at The City College of New York, the team solved both the contact and contamination problems at once.

He said: "One of the greatest assets of 2D materials such as graphene is that being only one atom thick, we have direct access to its electronic properties. At the same time, this can be one of its worst features since this makes the material extremely sensitive to its environment.

“Any external contamination quickly degrades performance. The need to protect graphene from unwanted disorder, while still allowing electrical access, has been the most significant roadblock preventing development of graphene-based technologies.

“By making contact only to the 1D edge of graphene, we have developed a fundamentally new way to bridge our 3D world to this fascinating 2D world, without disturbing its inherent properties. This virtually eliminates external contamination and finally allows graphene to show its true potential in electronic devices."

After first encapsulating graphene in boron nitride, the multilayer stack is etched to expose only the very edge of the two-dimensional graphene layer The researchers, whose findings were published in journal Science today, fully encapsulated the 2D graphene layer in a sandwich of thin insulating boron nitride crystals, employing a new technique in which crystal layers are stacked one-by-one.

Once they created the stack, they etched it to expose the edge of the graphene layer, and then evaporated metal onto the edge to create the electrical contact. By making contact along the edge, the team realized a 1D interface between the 2D active layer and 3D metal electrode.

And, even though electrons entered only at the 1D atomic edge of the graphene sheet, the contact resistance was remarkably low, reaching 100 Ohms per micron of contact width – a value smaller than what can be achieved for contacts at the graphene top surface.

At room temperature, these devices exhibit previously unachievable performance, including electron mobility at least twice as large as any conventional 2D electron system, and sheet resistivity less than 40 Ohms when sufficient charges are added to the sheet by electrostatic "gating”. Amazingly, this 2D sheet resistance corresponds to a "bulk" 3D resistivity smaller than that of any metal at room temperature.

The team is now working on applying these techniques to develop new hybrid materials by mechanical assembly and edge contact of hybrid materials drawing from the full suite of available 2D layered materials, including graphene, boron nitride, transition metal dichlcogenides, transition metal oxides and topological insulators.

An illustration of an encapsulated two-dimensional graphene sheet that is electrically contacted only along its one-dimensional edge "This is an exciting new paradigm in materials engineering where instead of the conventional approach of layer by layer growth, hybrid materials can now be fabricated by mechanical assembly of constituent 2D crystals," said Electrical Engineering Professor Ken Shepard, co-author of the paper. "No other group has been able to successfully achieve a pure edge-contact geometry to 2D materials such as graphene."

He added that earlier efforts have looked at how to improve 'top contacts' by additional engineering such as adding dopants: "Our novel edge-contact geometry provides more efficient contact than the conventional geometry without the need for further complex processing. There are now many more possibilities in the pursuit of both device applications and fundamental physics explorations."

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