Cutting graphene into nanostructures alters its electronic properties due to the creation of edges. Subtle differences in edge structures, e.g. zigzag, by cutting along the outer atoms of the hexagons, or armchair by a 30° offset cut, have been predicted to produce measurably different physical properties. For example, edge states in zigzag ribbons are spin polarized, whose interactions across the ribbon open a band gap in graphene’s linear dispersion, making it a semiconductor. These predictions have never been experimentally tested due to challenges in fabricating well-ordered ribbons just a few nanometers in width.
In this work, we have achieved this feat by Fe nano-particle assisted hydrogen etching of epitaxial graphene on the Si-face SiC(0001) in ultrahigh vacuum, and studied these ribbons’ electronic properties using an STM. Our investigations reveal that single layer ribbons as small as 1 nm wide are supported on single or bi-layer graphene. The weak van der Waals interaction minimizes the impact from the supporting layer, allowing the determination of a ribbons’ intrinsic properties for the the first time.
Using tunneling spectroscopy, we have measured systematically the electronic properties of ribbons 1-14 nm in width, and discovered width-dependent energy gaps in their density of states. Above a threshold of 3 nm, a constant gap of 0.4 eV is found, and below, the gap strongly depends on width, up to 1.6 eV for a 1 nm ribbon. The origin of this behavior is elucidated by first-principles calculations.
Our findings not only confirm the theoretical predictions of spin-polarized edge states and the effect of their interactions on the energy gap, but also open the prospect of building energy-efficient nanoscale devices from graphene based on their charge and/or spin.
Published in Nature Communications:
“Direct experimental determination of onset of electron-electron interactions in gap opening of zigzag graphene nanoribbons“, Y. Y. Li, M. X. Chen, M. Weinert, L. Li, Nature communications 5, 4311 (2014).
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