‘On-off switch’ controls crystal structure of graphene

American researchers have found an ‘on-off switch’ for graphene which enables them to change its crystal structure in a controlled fashion. Most materials require high temperature or pressure to change their structure, but researchers from the University of Arizona discovered that applying an electric field to trilayer graphene allows them to change the material from metallic to semiconducting, and control the flow of electrons. “It is extremely rare for a material to change its crystal structure just by applying an electric field,” said Brian LeRoy, Associate Professor of Physics. “Making trilayer graphene is an exceptionally unique system that could be utilised to create novel devices.” Trilayer graphene can be stacked in two ways: Bernal and rhombohedral. The two configurations can naturally exist in the same graphene flake and are separated by a sharp boundary where carbon hexagons are strained to accommodate the transition from one stacking pattern to the other. LeRoy said that the different stacking configurations on either side of the boundary means one side of the material behaves as a metal, while the other behaves as a semiconductor. While probing the boundary with an electric field applied an extremely sharp metal scanning tunnelling microscope tip, the team discovered they could move the boundary’s position within the graphene flake, with the crystal structure of the trilayer graphene changing 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 said. "At first it was frustrating, but once we realised what was going on, it turned out to be the 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. “Now we have a knob that we can turn to change the material from metallic into semiconducting and vice versa to control the flow of electrons,” LeRoy said. “It basically gives us an on-off switch, which had not been realised yet in graphene.” While more research is needed before it can be applied in technological applications on an industrial scale, it might be possible to create transistors from graphene. The work has been published in Nature Materials. Electric field control of soliton motion and stacking in trilayer graphene