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THE LATEST RESEARCH AND DEVELOPMENT NEWS IN MANUFACTURING AND TECHNOLOGYTECH FRONT New Method Creates Precise Graphene Nanoribbons t A


team of researchers from the Department of Energy’s (DOE) Lawrence Livermore National Laboratory (Berke- ley Lab) and the University of California, Berkeley, has


designed a new precision method of synthesizing graphene nanoribbons from molecular building blocks. The research, which created nanoribbons with enhanced properties, could be used in future electronic circuitry. These nanoribbons, which are narrow strips of graphene, exhibit properties that make them a prime choice for future nanoelectronic technologies, according to the researchers. The results of the research were published in a paper entitled “Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions,” in the journal Nature Nanotechnology. “This work represents progress towards the goal of controllably assembling mol- ecules into whatever shapes we want,” said Mike Crommie, senior scientist at Berkeley Lab, professor at UC Berkeley, affi liated with the Kavli Energy NanoScience Institute, and a leader of the study. “For the fi rst time we have created a molecular nanorib- bon where the width changes exactly how we designed it to.” In the past, scientists created nanoribbons that have a constant width, but this time the research team tried a new approach. “We wanted to see if we could change the width within a single nanoribbon, controlling the structure inside the nanoribbon at the atomic scale to give it new behavior that is potentially useful,” Crommie said. Felix Fischer, professor of chemistry at UC Berkeley, also


affi liated with the Kavli Energy NanoScience Institute, who jointly led the study, designed the molecular components to fi nd out whether this would be possible. Fischer and Crommie discovered that molecules of different widths can be made to chemically bond so that the width is modulated along the length of a single resulting nanoribbon. Nanoribbon synthesis has mostly involved etching ribbons out of larger 2D sheets of graphene. However, this lacks


precision, Fischer said, and each resulting nanoribbon has a unique, slightly random structure. Another method has been to unzip nanotubes to yield nanoribbons, which produces smoother edges than the “top-down” etching technique, but it is diffi cult to control because nanotubes have different widths and chiralities.


Bottom-up synthesis of graphene nanoribbons (left of image) from molecular building blocks 1 and 2. (a) The resulting ribbon, or heterojunction, has varied widths as a result of different width molecules 1 and 2. Scanning tunneling microscope image (right) of graphene nanoribbon heterojunction, with larger-scale inset of multiple ribbons.


“What we’ve done that is new is to show that it is possible


to create atomically-precise nanoribbons with non-uniform shape by changing the shapes of the molecular building blocks,” said Crommie. While the team has shown how to fabricate width-varying nanoribbons, it has not yet incor- porated them into actual electronic circuits. Crommie and Fischer hope to use this new type of nanoribbon to eventually create new device elements, including diodes, transistors, and LEDs that are smaller and more powerful than those in current use. Ultimately they hope to incorporate nanoribbons into complex circuits that yield better performance than today’s computer chips. In this effort, the researchers are collaborating with UC Berkeley electrical engineers. This research effort was supported by the Offi ce of Naval


Research BRC Program (molecular synthesis and character- ization); the DOE Offi ce of Science (instrumentation devel- opment, STM operation and simulations); and the National Science Foundation (image analysis, theory formalism).


April 2015 | AdvancedManufacturing.org 39


Image courtesy Berkeley Lab and University of California, Berkeley


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