"Our technique sets a tone for how we can measure atomically thin materials in the future," Park added.Ĭollaborators led by David A. A four-step electron beam lithography process, developed by Adam Tsen, an applied physics graduate student and the paper's first author, allowed the researchers to place electrodes on graphene, directly on top of a 10 nanometer-thick membrane substrate to measure electrical properties of single grain boundaries. We are finding that it's probably OK."Įqual in importance to these observations were the complex techniques they used to make the measurements - no easy task. "What we are showing is that grain boundaries were a main concern, but it could be that it doesn't matter. "What's important here is that we need to promote the growth environment so that the grains stitch together well," Park said. They found that the more reactive, quick-growth graphene, with more patches, in certain ways performed better electronically than the slower growth graphene with larger patches.Īs it turned out, faster growth led to tighter stitching between grains, which improved the graphene's performance, as opposed to larger grains that were more loosely held together. The group compared how graphene performed based on different rates of growth via chemical vapor deposition some they grew more slowly, and others, very quickly. Conventional wisdom and some prior indirect measurements had led scientists to surmise that growing graphene with larger crystals - fewer patches - might improve its properties. They, along with other scientists, wondered how graphene's electrical properties would hold up based on its polycrystalline nature. In earlier work published in Nature last January, the Cornell group had used electron microscopy to liken these graphene sheets to patchwork quilts - each "patch" represented by the orientation of the graphene grains (and false colored to make them pretty). In reality, graphene is polycrystalline it is grown via a process called chemical vapor deposition, in which small crystals, or grains, at random orientations grow by themselves and eventually join together in carbon-carbon bonds. Cartoons depict graphene like a perfect atomic chicken wire stretching ad infinitum. Graphene is a single layer of carbon atoms, and materials scientists are engaged in a sort of arms race to manipulate and enhance its amazing properties - tensile strength, high electrical conductance, and potential applications in photonics, photovoltaics and electronics. The researchers, led by Jiwoong Park, assistant professor of chemistry and chemical biology and a member of the Kavli Institute at Cornell for Nanoscale Science, used advanced measurement and imaging techniques to make these claims, detailed online in the journal Science June 1. The quality of this "stitching" - the boundaries at which graphene crystals grow together and form sheets - is just as important as the size of the crystals themselves, which scientists had previously thought held the key to making better graphene. Similar to how tighter stiches make for a better quality quilt, the "stitching" between individual crystals of graphene affects how well these carbon monolayers conduct electricity and retain their strength, Cornell researchers report. The inset is a false-color SEM image of an electrical device consisting of a single grain boundary in graphene. Each ETP has a unique risk profile, detailed in its prospectus, offering circular, or similar material, which should be considered carefully when making investment decisions.A scanning electron microscope (SEM) image of graphene crystals growing on copper. The degree of liquidity can vary significantly from one ETP to another and losses may be magnified if no liquid market exists for the ETP's shares when attempting to sell them. An ETP may trade at a premium or discount to its net asset value (NAV) (or indicative value in the case of exchange-traded notes). The return of an index ETP is usually different from that of the index it tracks because of fees, expenses, and tracking error. ETPs that use derivatives, leverage, or complex investment strategies are subject to additional risks. ETPs that target a small universe of securities, such as a specific region or market sector, are generally subject to greater market volatility, as well as to the specific risks associated with that sector, region, or other focus. Foreign securities are subject to interest rate, currency exchange rate, economic, and political risks, all of which are magnified in emerging markets. Exchange-traded products (ETPs) are subject to market volatility and the risks of their underlying securities, which may include the risks associated with investing in smaller companies, foreign securities, commodities, and fixed income investments.
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