Serendipitous tuning: Well-stacked graphene

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  • Published: Oct 1, 2011
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Serendipitous tuning: Well-stacked graphene

Graphene chancer

An accidental discovery at the University of California, Riverside, might allow materials scientists to control the way sheets of the carbon allotrope stack together and so allow them to tune the electrical properties of the material. Raman spectroscopy was used to reveal the stacking order and so may have played a crucial role in the development of graphene nanoelectronics devices of the future.

The serendipitous fabrication of tri-layers of graphene by Jeanie Lau and colleagues revealed that when the material is piled up like so many pancakes, its electrical properties change markedly. The conductance value, for instance, depended on how the layers stack together leading to one form being conducting and another insulating, the research suggests.

"What we stumbled upon is a simple and convenient ?knob? for tuning graphene sheets? electrical properties," explains Lau. Graphene can be thought of as a monolayer of graphite, a sheet of carbon atoms a single atom thick in which the carbons are hooked together in the familiar hexagonal chicken wire arrangement. Even before it earned Andre Geim and Konstantin Novoselov at Manchester University, UK, the 2010 Nobel Prize for Physics, researchers were keenly studying its material properties, its strengths, its weaknesses, and the way it behaves electronically and optically. Some pundits have suggested that graphene might one day displace silicon as the material of choice for future, not micro, but nano-electronics devices. However, there are many experiments to conduct before such fanciful notions become hard-wired into computer and device circuitry.

Stacked chicken wire

The fact that graphene sheets are flat and have this chicken wire type structure means that researchers expect them to stack together in the "Bernal stacking" arrangement. A Bernal-stacked bilayer has one corner of the hexagons of the second sheet located above the centre of the hexagons of the lower sheet. This would, one might assume, be the lowest energy, well-stacked arrangement. In a Bernal-stacked trilayer (ABA), the upper, third, would have the same relationship to the middle layer but its atoms would appear to be coincident with the positions of the carbons in the lower sheet looking directly down on the stack. There is also a third arrangement, a rhombohedral-stacked (ABC) trilayer, in which the upper sheet is shifted by the distance of an atom, so that the upper sheet is related to the lower sheet in the Bernal arrangement.

Mind the gap

Lau explains that metallic graphene in which the tri-layer graphene is arranged ABA is the most stable formation. "Amazingly, if we simply shift the entire topmost layer by the distance of a single atom, the trilayer - now with the ABC or rhombohedral stacking - becomes insulating," she says. Why this should be is not clear to the experimentalists who are now hoping that the theoreticians might devise a viable mechanism by which this shift from the ABA to the ABC arrangement would switch the graphene stack from a metallic conducting state to a non-metallic insulating state.

Lau's team used Raman spectroscopy to examine the stacking order of the graphene and is now planning experiments to investigate the insulating state, ABC-stacked, form. They also hope to reveal more about the insulating material's band gap, the energy range that excludes electrons and is mission critical to the development of electronic and optical devices based on stacked graphene.

"The presence of the gap in ABC-stacked graphene that arises, we believe, from enhanced electronic interactions is interesting since it is not expected from theoretical calculations," Lau explains, "Understanding this gap is particularly important for the major challenge of band gap engineering in graphene electronics."


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

An accidental discovery at the University of California, Riverside, might allow materials scientists to control the way sheets of the carbon allotrope stack together and so allow them to tune the electrical properties of the material. Raman spectroscopy was used to reveal the stacking order and so may have played a crucial role in the development of graphene nanoelectronics devices of the future.
Tri-layered graphene

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