Most folks who follow the graphene scene are familiar with the honeycomb structure (hexagonal network) shown in diagram after diagram but I imagine there’s more than one of us who didn’t realize that defects can occur at the boundaries, from the Jan. 15, 2012 news release on EurekAlert,
When graphene is grown, lattices of the carbon grains are formed randomly, linked together at different angles of orientation in a hexagonal network. However, when those orientations become misaligned during the growth process, defects called grain boundaries (GBs) form. These boundaries scatter the flow of electrons in graphene, a fact that is detrimental to its successful electronic performance.
The Jan. 14, 2013 University of Illinois Beckman Institute news release written by Steve McGoughey, which originated the item on EurekAlert, provides insight into the problem and its solution,
Beckman Institute researchers Joe Lyding and Eric Pop and their research groups have now given new insight into the electronics behavior of graphene with grain boundaries that could guide fabrication methods toward lessening their effect. The researchers grew polycrystalline graphene by chemical vapor deposition (CVD), using scanning tunneling microscopy (STM) and spectroscopy for analysis, to examine at the atomic scale grain boundaries on a silicon wafer. They reported their results in the journal ACS Nano.
“We obtained information about electron scattering at the boundaries that shows it significantly limits the electronic performance compared to grain boundary free graphene,” Lyding said. “Grain boundaries form during graphene growth by CVD, and, while there is much worldwide effort to minimize the occurrence of grain boundaries, they are a fact of life for now.
“For electronics you would want to be able to make it on a wafer scale. Boundary free graphene is a key goal. In the interim we have to live with the grain boundaries, so understanding them is what we’re trying to do.”
Lyding compared graphene lattices made with the CVD method to pieces of a cyclone fence.
“If you had two pieces of fence, and you laid them on the ground next to each other but they weren’t perfectly aligned, then they wouldn’t match,” he said. “That’s a grain boundary, where the lattice doesn’t match.”
Their analysis showed that when the electrons’ itinerary takes them to a grain boundary, it is like, Lyding said, hitting a hill.
“The electrons hit this hill, they bounce off, they interfere with themselves and you actually see a standing wave pattern,” he said. “It’s a barrier so they have to go up and over that hill. Like anything else, that is going to slow them down. That’s what Justin was able to measure with these spectroscopy measurements.
“Basically a grain boundary is a resistor in series with a conductor. That’s always bad. It means it’s going to take longer for an electron to get from point A to point B with some voltage applied.”
In the paper, the researchers were able to report on their analysis of the orientation angles between pieces of graphene as they grew together, and found “no preferential orientation angle between grains, and the GBs are continuous across graphene wrinkles and Si02 topography.” They reported that analysis of those patterns “indicates that backscattering and intervalley scattering are the dominant mechanisms responsible for the mobility reduction in the presence of GBs in CVD-grown graphene.”
The researchers work is aimed not just at understanding, but also at controlling grain boundaries. One of their findings – that GBs are aperiodic – replicated other work and could have implications for controlling them, as they wrote in the paper: “Combining the spectroscopic and scattering results suggest that GBs that are more periodic and well-ordered lead to reduced scattering from the GBs.”
“I think if you have to live with grain boundaries you would like to be able to control exactly what their orientation is and choose an angle that minimizes the scattering,” Lyding said.
Here’s a citation and link for the article,
Atomic-Scale Evidence for Potential Barriers and Strong Carrier Scattering at Graphene Grain Boundaries: A Scanning Tunneling Microscopy Study by Justin C. Koepke, Joshua D. Wood, David Estrada, Zhun-Yong Ong, Kevin T. He, Eric Pop, and Joseph W. Lyding in ACS Nano, Article ASAP DOI: 10.1021/nn302064p Publication Date (Web): December 13, 2012
Copyright © 2012 American Chemical Society
The article has not been published in print and it is behind a paywall.