Making nanosized pieces of graphene is not an easy task and, in light of theoretical studies suggesting that the geometry of nanographene influences its electronic and magnetic properties, it remains an attractive synthetic goal. In the past, nanographene has been prepared by several methods including chemical etching of a graphene sheet, electrochemical etching of graphite, and unwrapping of carbon nanotubes by oxidation. These processes involve destructive conditions, however, and offer little control over the atomic structure of the edges. A bottom-up approach has also been demonstrated but only one type of edge structure — the ‘arm-chair’ structure — is produced.

Shintaro Fujii and Toshiaki Enoki at the Tokyo Institute of Technology in Japan1 have now shown how linear defects in an oxidized graphene sheet can act as a template to direct the cutting of the sheet using a scanning probe microscope (SPM). The regular spacing of the defects, thought to be lines of epoxide functional groups aligned in the zigzag direction of the hexagonal graphene lattice, causes the formation of nanosized graphene with well-defined edges, although the exact shape of the edge has yet to be determined.

Fig. 1: A predicted structural model showing the formation of periodic wrinkles in an oxidized graphene sheet (upper) from the linear arrangement of epoxide functions (lower, oxygen atoms shown as red) and the angular strain this arrangement causes.

Fujii and Enoki also investigated the surface morphology and electronic properties of the oxidized graphene sheet to establish the mechanism behind the directed cutting process. Although the graphene sheet is predominantly oxidized in a random manner, there are regions where ordered wrinkles are observed. These periodic wrinkles are thought to result from the preference of epoxide functions to exist in a linear arrangement and the subsequent angular strain caused by their presence (Fig. 1). Only samples containing the periodic wrinkles were successfully cut into nanosized pieces at the point contact of an atomic force microscope probe on the graphene sheet. Attempts to cut the randomly oxidized sections led to uncontrollable failure of the material.

“We believe our cutting method for producing well-defined edges and in situ SPM characterization in an ultra-high vacuum environment could provide opportunities to explore the relationship between geometry and the electronic structure and magnetic properties of nanographene,” says Fujii. “In particular, nanographene with zigzag edges and of different shapes, such as triangular or hexagonal islands, has been predicted to have unique magnetic properties.”