Posted: May 31st, 2012 Tighter 'stitching' means better graphene, scientists say ( Nanowerk News ) Similar to how tighter stiches make for a better quality quilt,the "stitching" between individual crystals of graphene affects howwell these carbon monolayers conduct electricity and retain theirstrength, Cornell researchers report. The quality of this "stitching" -- the boundaries at which graphenecrystals grow together and form sheets -- is just as important asthe size of the crystals themselves, which scientists hadpreviously thought held the key to making better graphene. The researchers, led by Jiwoong Park, assistant professor ofchemistry and chemical biology and a member of the Kavli Instituteat Cornell for Nanoscale Science, used advanced measurement andimaging techniques to make these claims, detailed online in thejournal Science June 1 ( "Tailoring Electrical Transport Across Grain Boundaries inPolycrystalline Graphene" ). False-color microscopy images show examples of graphene grownslowly, resulting in large patches with poor stitching, andgraphene grown more quickly, resulting in smaller patches withtighter stitching and better performance. (Image: Muller lab) Graphene is a single layer of carbon atoms, and materialsscientists are engaged in a sort of arms race to manipulate andenhance its amazing properties -- tensile strength, high electricalconductance, and potential applications in photonics, photovoltaicsand electronics. |
Cartoons depict graphene like a perfect atomicchicken wire stretching ad infinitum. In reality, graphene is polycrystalline; it is grown via a processcalled chemical vapor deposition, in which small crystals, orgrains, at random orientations grow by themselves and eventuallyjoin together in carbon-carbon bonds. In earlier work published in Nature last January ( "Grains and grain boundaries in single-layer graphene atomicpatchwork quilts" ), the Cornell group had used electron microscopy to liken thesegraphene sheets to patchwork quilts -- each "patch" represented bythe orientation of the graphene grains (and false colored to makethem pretty). They, along with other scientists, wondered how graphene'selectrical properties would hold up based on its polycrystallinenature. Conventional wisdom and some prior indirect measurementshad led scientists to surmise that growing graphene with largercrystals -- fewer patches -- might improve its properties.
The new work questions that dogma. The group compared how grapheneperformed based on different rates of growth via chemical vapordeposition; some they grew more slowly, and others, very quickly.They found that the more reactive, quick-growth graphene, with morepatches, in certain ways performed better electronically than theslower growth graphene with larger patches. A scanning electron microscope (SEM) image of graphene crystalsgrowing on copper. The inset is a false-color SEM image of anelectrical device consisting of a single grain boundary ingraphene.
(Image: Wei Tsen/Park lab) As it turned out, faster growth led to tighter stitching betweengrains, which improved the graphene's performance, as opposed tolarger grains that were more loosely held together. "What's important here is that we need to promote the growthenvironment so that the grains stitch together well," Park said."What we are showing is that grain boundaries were a main concern,but it could be that it doesn't matter. We are finding that it'sprobably OK." Equal in importance to these observations were the complextechniques they used to make the measurements -- no easy task. Afour-step electron beam lithography process, developed by AdamTsen, an applied physics graduate student and the paper's firstauthor, allowed the researchers to place electrodes on graphene,directly on top of a 10 nanometer-thick membrane substrate tomeasure electrical properties of single grain boundaries.
"Our technique sets a tone for how we can measure atomically thinmaterials in the future," Park added. Collaborators led by David A. Muller, professor of applied andengineering physics and co-director of the Kavli Institute atCornell for Nanoscale Science, used advanced transmission electronmicroscopy techniques to help Park's group image their graphene toshow the differences in the grain sizes.
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