Stacking graphene sheets

An electric field can change the structure of a tri-layer of graphene, converting it from a metallic form to a semiconductor form, according to physicists at the University of Arizona. The discovery might open up the applications of the all-carbon material being heralded as the natural successor to silicon in microelectronics. [LeRoy et al, Nature Mater, 2014, online; DOI: 10.1038/nmat3965]

Brian LeRoy and his collaborators point out that changing the crystal structure of most materials requires the application of a high temperature, high pressure or both, but an electric field is all that is required to alter the stacking of the three layers between their two stable forms. The first layer is the familiar graphene monolayer. In the second layer, half the atoms sit over the center of the hexagon in the bottom layer and the other half sit over an atom in the bottom layer.  For the third layer, half the atoms again sit over the hexagon in the second layer.  Now, there is a choice for the other half of the atoms, they can either sit directly over atoms in the second layer or over atoms in the bottom layer. The second layer sits over half the holes in the bottom layer.  The third layer either sits directly above the first layer (ABA) or over the other half of the holes in the bottom layer (ABC).

The team explains that both stacking patterns can thus exist in the same trilayer graphene flake. Nevertheless, there is a sharp boundary between the arrangements involving strain among the hexagons of carbon atoms in each graphene layer.

"Due to the different stacking configurations on either side of the domain wall, one side of the material behaves as a metal, while the other side behaves as a semiconductor," explains LeRoy. The team was probing these trilayer materials with the metal tip of a scanning tunneling microscope and discovered that they could move the position of the domain wall within a flake of graphene, as they moved the domain wall, the crystal structure of the trilayer graphene changed in its wake.

"We had the idea that there would be interesting electronic effects at the boundary, and the boundary kept moving around on us," LeRoy adds. "At first it was frustrating, but once we realized what was going on, it turned out to be a most interesting effect." By applying an electric field to move the boundary, it is now possible for the first time to change the crystal structure of graphene in a controlled fashion. "This basically gives us an on-off switch, which had not been realized yet in graphene," he suggests. While silicon's crown is unlikely to be usurped in the very near future given its vast industrial legacy and prominence, developments such as this nudge graphene another step forward in its accession to the throne.

"The next step in the work is to show that we can move the domain walls using electrostatic gates instead of needing the STM tip.  This ability would open the way to using the motion of the domain walls in devices," LeRoy told Materials Today.

David Bradley blogs at http://www.sciencebase.com and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".