Communications of the ACM
Nanoribbons Enable 'on-Off' Switch For Graphene
This graphene nanoribbon was formed from a molecular precursor.
Credit: Oak Ridge National Laboratory
Scientists have discovered a new way to synthesize semiconducting graphene ribbons. They used polymer precursors and injected charges at certain spots on a surface to form molecules. The new method yields graphene ribbons a few-molecules wide that are wide band gap semiconductors. The ribbons may be useful for a broad range of electronics applications.
The work is described in "Controllable Conversion of Quasi-Freestanding Polymer Chains to Graphene Nanoribbons," published in the journal Nature Communications.
Scientists say durable, lightweight graphene could improve electronics if it had an "on-off" switch for electricity. Graphene sheets, made from a one-atom-thick layer of carbon atoms, are always "on." They conduct electricity. Narrow ribbons of graphene can control electricity, but they're hard to produce.
Previous efforts to create narrow graphene ribbons used a metal surface. The surface interfered with the ribbon's electronic properties. In their recent work, scientists used precursors and injected charges to grow ultra-narrow graphene ribbons without a metal substrate.
The new method may address a shortcoming that has prevented the material from achieving its full potential in electronic applications. Graphene nanoribbons exhibit different electronic properties than two-dimensional sheets of the material. Graphene in sheets conducts electricity like a common metal, but narrowing graphene can turn the material into a semiconductor if the ribbons are made with a specific edge shape.
Previous efforts to make graphene nanoribbons employed a catalytic metal substrate that hindered the ribbons' useful electronic properties. Now, scientists at Oak Ridge National Laboratory and North Carolina State University have grown graphene nanoribbons without involving the catalyst effect from a metal substrate. Instead, they injected charge carriers that promote a chemical reaction that converts a polymer intermediate into a graphene nanoribbon.
At selected sites, this new technique can create interfaces between materials with different electronic properties. Such interfaces are the basis of semiconductor electronic devices from integrated circuits and transistors to light-emitting diodes and solar cells.
In wide sheets, graphene doesn't have an energy gap — an energy range in a solid where no electronic states can exist — so it acts like a metal and the current flow cannot be turned on or off. This severely limits graphene's application in digital electronics. Only when graphene is made in the form of a narrow ribbon can an energy gap be created.
Making the researchers' finding even more attractive for future applications is the way such ribbons' electronic properties can be tuned: narrower ribbons have a wider energy gap, and in very narrow nanoribbons, how structures terminate at the edge of the ribbon is important, too. Cutting graphene along the side of a hexagon creates an edge that resembles an armchair and a material that can act like a semiconductor; in contrast, excising triangles from graphene creates a zigzag edge — and a material with metallic behavior.
The Nature Communications article is authored by Chuanxu Ma, Honghai Zhang, Liangbo Liang, Jingsong Huang, Bobby G. Sumpter, Kunlun Hong, and An-Ping Li of Oak Ridge National Laboratory, Zhongcan Xiao of North Carolina State Univeristy, and Wenchang Lu and J. Bernholc, who are affiliated with both ORNL and NC State.
This work was supported by the Center for Nanophase Materials Sciences, a U.S. Department of Energy Office of Science user facility at ORNL. Simulations at NC State were supported by the DOE Office of Science. Supercomputing time was provided by the U.S. National Science Foundation at the National Center for Supercomputing Applications and by DOE through the Oak Ridge Leadership Computing Facility and the National Energy Research Scientific Computing Center, DOE Office of Science user facilities at ORNL and Lawrence Berkeley National Laboratory, respectively. The Office of Naval Research funded electronic characterization of samples at CNMS and the corresponding calculations at NC State. Oak Ridge Associated Universities supported Zhongcan Xiao's work at CNMS. ORNL supported Liangbo Liang's Eugene P. Wigner Fellowship.
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