Although graphene (a sheet of carbon just one atom thick) has numerous unique mechanical and electronic properties (including high electron mobility and extremely high strength), it does suffer from an important drawback. This is that it has no gap between its valence and conduction bands. Such a bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off. However, one way of introducing a bandgap into graphene is to make extremely narrow ribbons of the material.

Like graphene, graphene nanoribbons (GNRs) also have unique electronic and optical properties. They are unique materials whose bandgaps vary with their width and they go from being semimetal to semiconducting as they become narrower. They could be used in high-performance nanoelectronics devices, such as high-frequency transistors and sensors, and could also be ideal as interconnects in nanoelectronics circuits. Although a number of "planar" techniques exist to make GNRs (such as graphene patterning and shadowing using copolymer masks), most of these methods are quite complicated and not easily scalable, so researchers prefer bottom-up methods such as nanotemplated growth.

GNRs with widths of less than 1 nm

Researchers led by Hisanori Shinohara of Nagoya University say they have now exploited the hollow interior of single-walled carbon nanotubes (rolled up sheets of graphite) to make GNRs with widths of less than 1 nm. They did this by annealing coronene molecules inside the nanotubes at temperatures as high as 700 °C for 48 hours. “The high temperature induces fusion reactions in coronene and leads to the synthesis of ultrathin GNRs inside the CNTs,” explains Shinohara. “The resulting structures are known as (GNRs)@CNTs for short and the CNTs act as templates for the reactions.”

The researchers, reporting their work in ACS Nano DOI: 10.1021/nn507408m, then functionalized the surface of the (GNRs)@CNTs with 4-bromobenzene diazonium tetrafluoroborate and found that the bandgap of the GNRs significantly increased so that the structures became transparent to light.

"Clever and creative"

The team, which includes scientists from Tokyo Metropolitan University, the University of Tsukuba and AIST, also in Tsukuba, says that they may be able to apply its experimental technique to other novel, ultrathin 2D materials – like the transitional metal dichalcogenides (TMDCs), which are promising for electronics and optoelectronics applications too. And since the technique is basically a simple combination of vacuum sublimation and high-temperature annealing, it could easily be scaled up.

“To this end, we are now trying to separate and isolate the ultrathin GNRs we made,” Shinohara tells nanotechweb.org.

James Tour of Rice University in the US, who was not involved in this work, comments that using carbon nanotubes as the confined environments to control GNR growth is “extremely clever and creative”.

For recent developments in terahertz applications of carbon nanomaterials visit the Nanotechnology topical review by Hartmann, Kono and Portnoi.