A step towards graphene-based electronic devices
Nanostructures such as carbon nanotubes have extraordinary properties but are extremely challenging to pick up, handle and assemble them into for example electronic devices.
Now Ju Li, associate professor in the Department of Materials Science and Engineering in Penn’s School of Engineering and Applied Science and Jianyu Huang from Sandia National Laboratories, have demonstrated the formation of interconnected curvy carbon nanostructures directly integrated on graphene substrate in a simple assembly process that involves heating few-layer graphene sheets to sublimation using electric current. They took advantage of the fact that graphene, an atomically thin two-dimensional sheet, bends easily after open edges have been cut on it, which can then fuse with other open edges permanently. This discovery of self-assembled carbon nanostructures may lead to a new approach of graphene-based electronic devices.
In their experiment the researchers used electrical current from a Nanofactory scanning probe that generated heat up to 2000°C. This method of Joule-heating the multilayered graphene caused the carbon atoms at the edge of the material to escape into the gas phase (sublimation), leaving freshly exposed edges on the solid graphene. The remaining graphene edges curl and often welded together to form graphene bi-layer edges or BLEs. According to the researchers this kind of behavior is due to nature’s driving force to reduce capillary energy, dangling bonds on the open edges of monolayer graphene, at the cost of increased bending energy. Because of this reason when the researchers applied the electric current to the multilayered graphene they observed the on site creation of many interconnected, curved carbon nanostructures, such as “fractional nanotube”-like BLEs; BLE rings on graphene equivalent to “anti quantum-dots”; and nanotube-BLE assembly connecting multiple layers of graphene.
Another interesting result from this experimental work was the experimental confirmation of the theoretical predications (that were based on symmetry considerations and energy calculations) that BLEs are the stable edges in graphene because the researchers observed that more than 99 percent of the graphene edges formed during sublimation were curved BLEs rather than flat monolayer edges.
Li said, “This study demonstrates it is possible to make and integrate curved nanostructures directly on flat graphene, which is extended and electrically conducting. Furthermore, it demonstrates that multiple graphene sheets can be intentionally interconnected. And the quality of the plumbing is exceptionally high, better than anything people have used for electrical contacts with carbon nanotubes so far. We are currently investigating the fundamental properties of graphene bi-layer edges, BLE rings and nanotube-BLE junctions.”
According to theoretical predications the BLEs, or “fractional nanotubes,” possess novel properties of their own and may find applications in devices.
June 11, 2009