In the face of a slowing Moore’s law for silicon-based CMOS technology, researchers are on the hunt for a successor to silicon. One of the more promising candidates is graphene, a one-atom thick layer of carbon prized for its strength, flexibilty, lightness and conductivity.
Despite graphene’s potential, it is not without challenges. Its biggest shortcoming: it lacks the energy band gap necessary to produce switching devices, like transistors.
The big question is how to best imbue graphene with this critical semiconductor functionality. Researchers with the Department of Energy’s Oak Ridge National Laboratory (ORNL) and North Carolina State University have developed a new nanoribbon growing technique that does just this.
Previous efforts had created nanoribbons using a metal substrate that impeded the desired electronic properties. In the journal Nature Communications, the ORNL-North Carolina State research team report that they are the first to grow graphene nanoribbons without a metal substrate.
The ORNL writeup of the research explains: “Instead, they injected charge carriers that promote a chemical reaction that converts a polymer precursor 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.”
“Confinement changes graphene’s behavior,” said An-Ping Li, a physicist at ORNL.
“When graphene becomes very narrow, it creates an energy gap,” Li said. “The narrower the ribbon is, the wider is the energy gap.”
It’s not just the narrowing into nano-sized strips that enables the nanoribbons’ semiconducting ability; they must be cut using a specific edge shape.
“Cutting graphene along the side of a hexagon creates an edge that resembles an armchair; this material can act like a semiconductor,” said ORNL. “Excising triangles from graphene creates a zigzag edge–and a material with metallic behavior.”
The effort to unlock the potential of this silicon alternative for electronic applications was aided by today’s highest-performance silicon-based number crunchers. Simulation work for the project was conducted by the Center for Nanophase Materials Sciences (CNMS), a DOE Office of Science User Facility. 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 (at ORNL) and the National Energy Research Scientific Computing Center (at Berkeley National Lab).
For more information, there’s a great writeup of the research by ORNL’s Dawn Levy.
Also see the researchers’ March 2017 Nature paper: “Controllable conversion of quasi-freestanding polymer chains to graphene nanoribbons.”