Graphene nanoribbons have different properties than the normal "two-dimensional" sheets of the single-atom thick carbon allotrope. Researchers at Oak Ridge National Laboratory and North Carolina State University have even shown that the difference can be as critical as graphene being an excellent electrical conductor but their nanoribbons being semiconductors provide the edge is tuned.
An-Ping Li and colleagues have grown graphene nanoribbons without a metal substrate, something that stymied the material's electronic properties in earlier work. The team's approach involved injecting charge carriers that promote the conversion of polymer precursor into graphene. They explain that if used selectively the technique can form interfaces between materials with different electronic properties and so be used to build semiconductor devices. [Li et al. (2017) Nature Commun; DOI: 10.1038/ncomms14815]
"Graphene is wonderful, but it has limits," suggests Li. "In wide sheets, it doesn't have an energy gap. That means you cannot turn it on or off." Apply a voltage to graphene and electrons flow as freely as they do in a metal. However, ribbons of the materials do have a band gap and the narrower the ribbon the bigger that gap. For nanoribbons, the edge structure becomes important. Where the hexagonal "chair" conformation is present (as opposed to the cyclohexane "boat"), the material will behave as a semiconductor. Conversely, excising triangles from the edge of the graphene ribbon creates a zigzag edge which endows the ribbon with metallic properties. The use of metal substrates in previous attempts to form the ribbon led to a smaller band gap with the same numbers of chairs and zigzag.
Li and colleagues used the tip of a scanning tunneling microscope (STM) to inject charge carriers (electrons or holes) to see which would initiate the right reaction. The holes have it and they were able to make graphene ribbons just seven carbon atoms wide with edges in the chair conformation.
"We figured out the fundamental mechanism, that is, how charge injection can lower the reaction barrier to promote this chemical reaction," Li explains. Moving the tip along the polymer precursor chain, the researchers could select where they triggered the reaction and convert one graphene hexagon at a time. The next step will be to construct heterojunctions from different precursors that might direct electrical energy flow or facilitate solar energy conversion. "It's a way to tailor physical properties for energy applications," Li adds. "This is an excellent example of direct writing. You can direct the transformation process at the molecular or atomic level."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".