Scientists discover edge states of graphene nanoribbons

"Until now, no one has been able to test theoretical predictions regarding nanoribbon edge states, because no one could figure out how to see the atomic scale structure at the edge of a well ordered graphene nanoribbon," claimed researcher Michael Crommie, who led the team. "By studying specially made nanoribbons with a scanning tunneling microscope, we were able to measure their electronic properties within nanometers of the edge." Crommie believes the team's research could lead to the building of quick acting, energy efficient nanoscale devices from graphene nanoribbon switches, spin valves and detectors based on either electron charge or electron spin. He maintained that it could also lead to devices with tunable giant magnetoresistance and other magnetic and optical effects. "Making flakes and sheets of graphene has become commonplace," Crommie said, "but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity - typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well ordered on the atomic scale." Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, solved this problem with a novel method of 'unzipping' the carbon nanotubes chemically. Using this method, the graphene could be wrapped at almost any angle to make a nanotube. According to Dai , a cut made straight along the outer atoms of a row of hexagons produced a zigzag edge, and a cut made at a 30 degree angle from a zigzag edge went through the middle of the hexagons and yielded scalloped edges, known as 'armchair' edges. The team looked at more than 150 high quality nanoribbons with different chiralities, all of which showed an unexpected feature - a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature and not the artifact of a folded ribbon or a flattened nanotube, the chirality and electronic properties of well ordered nanoribbon edges could be measured and the edge regions theoretically modelled Crommie says achieving reproducibility on the atomic scale is the next big step for the physicists. "Meeting this challenge is a big reason for why we do research," he said. "Nanoribbons have the potential to form exciting new electronic, magnetic and optical devices at the nanoscale." Although he says getting there won't be simple, Crommie maintained that what the research team has shown is that it is possible to make nanoribbons with good edges and that they do have characteristic edge states similar to what theorists had expected. "This opens a whole new area of future research involving the control and characterisation of graphene edges in different nanoscale geometries," he concluded.