PORTLAND, Ore. Graphene, sheets of pure carbon atoms akin to unrolled nanotubes laid flat, could herald a new era of ultra-high speed semiconductors. But the precise topology required to make them has yet to be verified experimentally.
Now, the gap between theory and experimental observations has been bridged, according to an electrical engineering professor at the University of Illinois.
The theory stated that nanoribbons of graphene measuring less than 10-nanometers wide could serve as ultra-fast channels for transistors, but they needed to be cut across the grain of the hexagonal lattice of carbon atoms. Nanoribbons cut with the grain, resulting in a zig-zag pattern at their edges, could serve as good metallic interconnections, but theorists predicted that only so-called "armchair" nanoribbons would make good semiconductors.
Professor Joseph Lyding and co-researcher Kyle Ritter at Micron Technology Inc. (Boise, Idaho) claim to have validated the theory.
"If you want graphene to be semiconducting, its edges must be armchair," said Lyding, a member of the university's Micro and Nanotechnology Laboratory. "If its edges are zigzag, then the metallic state of that edge decays into the interior completely converting it into the metallic state."
The mechanism by which graphene exhibits semiconducting properties is not completely understood, but electrical measurements show that large sheets of the material are naturally metallic. However, when ribbons measuring just a few nanometers are fabricated with armchair edges, quantum effects define a band-gap of forbidden energy levels for electrons, thereby converting the material into a semiconductor.
In their experiment, the researchers did not fabricate nanoribbons with a particular edge topology, but instead pulverized graphene sheets into a random mixture of zigzag and armchair topologies. The researchers were able to measure their energy states and confirm that only the pieces with armchair edges were semiconducting.