11-04 :: April/May 2011
nanotimes News in Brief
Graphene // Berkeley Lab Scientists Discover the Edge States of Graphene Nanoribbons
P
hysicists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Ber-
keley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise mea- surements of the “edge states” of well-ordered nanoribbons.
A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide.
“Until now no one has been able to test theoreti- cal 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 and how, at the same time, to measure its electronic properties within nanometers of the edge,” says Michael Crommie of Berkeley Lab’s Materials Sciences Division (MSD) and UC Berkeley’s Physics Division, who led the research. “We were able to achieve this by studying specially made nanoribbons with a scanning tunneling micro- scope.”
The team’s research not only confirms theoreti- cal predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and de-
tectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tuna- ble giant magnetoresistance and other magnetic and optical effects.
“Making flakes and sheets of graphene has become commonplace,” Crommie says, “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 nano- ribbon 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, a member of the research team, solved this problem with a novel method of “unzipping” carbon nano- tubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are un- zipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.
Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrap-
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