Nanoribbons Enable “On–Off” Switch for Graphene

Built from the bottom up, nanoribbons can be semiconducting, enabling broad electronic applications.

Image courtesy of Oak Ridge National Laboratory; scanning tunneling microscopy by Chuanxu Ma and An-Ping Li

The Science

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. Scientists 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 Impact

Previous efforts to create narrow graphene ribbons used a metal surface. The surface interfered with the ribbon’s electronic properties. For the first time, scientists used precursors and injected charges to grow ultra-narrow graphene ribbons without a metal substrate. The new method yields semiconducting graphene ribbons; that is, the few-molecule wide ribbons are wide band gap semiconductors. The ribbons may be useful for a broad range of electronics applications.

Summary

A new way to grow narrow ribbons of graphene, a lightweight and strong structure made of a layer — just a single atom thick — of carbon atoms linked into hexagons, 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 become the first to grow 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 this 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.

Contact

An-Ping Li
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
[email protected]

Funding

The Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy (DOE) Office of Science user facility at Oak Ridge National Laboratory (ORNL), supported the work. Simulations at North Carolina State University (NCSU) were supported by the DOE Office of Science. Supercomputing time was provided by the 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 NCSU. Oak Ridge Associated Universities supported Zhongcan Xiao’s work at CNMS. ORNL supported Liangbo Liang’s Eugene P. Wigner Fellowship.

Publications

C. Ma, Z. Xiao, H. Zhang, L. Liang, J. Huang, W. Lu, B.G. Sumpter, K. Hong, J. Bernholc, and A.P. Li, “Controllable conversion of quasi-freestanding polymer chain to graphene nanoribbons.” Nature Communications 8, 14815 (2017). [DOI: 10.1038/ncomms14815]