Tuesday, February 24, 2009

The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons

Kyle A. Ritter, Joseph W. Lyding (2009). The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons Nature Materials, 8 (3), 235-242 DOI: 10.1038/nmat2378

Characterization/electronic properties testing paper. These authors did mechanical exfoliation of graphite, but used a silicon wafer (Si(100)-2X1:H) instead of SiO2 and made sure to pulverize their sample as much as possible. They were able to find both graphene quantum dots (GQD, basically small square or round pieces of graphene) and graphene nanoribbons (GNR's). They then used Scanning Tunneling Microscopy (STM) and it's cousin Scanning Tunneling Spectroscopy (STS) to determine different pieces morphology, edge state (zigzag vs. armchair), and band gap (HOMO-LUMO gap, for us chemists; a band gap of 0 means the material is metallic). Their results suggest that zigzag GQD's are metallic, while armchair GQD's are semiconducting. The nanoribbons, it turned out, were semiconducting no matter what the edge state, but zigzag ribbons had a lower band gap (0.14eV, meaning they were more conductive) than armchair ribbons (0.38eV), as expected.

The large disparity in conductivity between edge states in carbon nanotubes is a big reason why they didn't pan out to be the wonder materials everyone thought they would, since most synthetic procedures gave a mixture of zigzag and armchair (metallic and semiconducting) nanotubes. This paper shows us that GQD's would have exactly the same problems. Although the difference is not as big in graphene nanoribbons, it would still be very difficult to build a device where some of your connecting elements would be twice as conductive as others.

Moral of the story: Zigzag-edged graphene structures are more metallic than armchair structures, with zigzag GQD's being metallic, zigzag GNR's semiconducting, and armchair GQD's and GNR's semiconducting.

This article also got some press, which (in my humble opinion) gets the quantum dot and nanoribbon results a little bit mixed up:
EE Times
Science Daily