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June 10, 2009

Interconnected Carbon Nanostructures Made With Graphene and An Engineered Tunable Bandgap Made With Graphene

Two separate research developments with graphene advance applications with graphene for electronics. One is the engineering of tunable bandgaps and the other is formation of nanostructures that connect graphene layers.

1. Wang, who is also an assistant professor in the Department of Physics at the University of California at Berkeley, have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts (250 meV, or .25 eV).


On the left, a microscope image looking down through the bilayer-graphene field-effect transistor. The diagram on the right identifies the elements. (Image: Feng Wang and colleagues, Lawrence Berkeley National Laboratory)

Moreover, their experiment was conducted at room temperature, requiring no refrigeration of the device. Among the applications made possible by this breakthrough are new kinds of nanotransistors and – because of its narrow bandgap – nano-LEDs and other nanoscale optical devices in the infrared range. Researchers can precisely tune a bandgap in bilayer graphene from zero to the infrared.




2. Engineers from the University of Pennsylvania, Sandia National Laboratories and Rice University have demonstrated the formation of interconnected carbon nanostructures on graphene substrate in a simple assembly process that involves heating few-layer graphene sheets to sublimation using electric current that may eventually lead to a new paradigm for building integrated carbon-based devices.










The "knife" and "welding torch" used in the experiments, which were performed inside an electron microscope, was electrical current from a Nanofactory scanning probe, generating up to 2000°C of heat. Upon applying the electrical current to few-layer graphene, they observed the in situ creation of many interconnected, curved carbon nanostructures, such as "fractional nanotube"-like graphene bi-layer edges, or BLEs; BLE rings on graphene equivalent to "anti quantum-dots"; and nanotube-BLE assembly connecting multiple layers of graphene.

Remarkably, researchers observed that more than 99 percent of the graphene edges formed during sublimation were curved BLEs rather than flat monolayer edges, indicating that BLEs are the stable edges in graphene, in agreement with predictions based on symmetry considerations and energetic calculations. Theory also predicts these BLEs, or "fractional nanotubes," possess novel properties of their own and may find applications in devices


Researchers induced the sublimation of multilayer graphene by Joule-heating, making it thermodynamically favorable for the carbon atoms at the edge of the material to escape into the gas phase, leaving freshly exposed edges on the solid graphene. The remaining graphene edges curl and often welded together to form BLEs. Researchers attribute this behavior 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.

"This study demonstrates it is possible to make and integrate curved nanostructures directly on flat graphene, which is extended and electrically conducting," said Li, associate professor in the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science. "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."


Abstract: In situ observation of graphene sublimation and multi-layer edge reconstructions.

We induced sublimation of suspended few-layer graphene by in situ Joule-heating inside a transmission electron microscope. The graphene sublimation fronts consisted of mostly {1100} zigzag edges. Under appropriate conditions, a fractal-like “coastline” morphology was observed. Extensive multiple-layer reconstructions at the graphene edges led to the formation of unique carbon nanostructures, such as sp2-bonded bilayer edges (BLEs) and nanotubes connected to BLEs. Flat fullerenes/nanopods and nanotubes tunneling multiple layers of graphene sheets were also observed. Remarkably, >99% of the graphene edges observed during sublimation are BLEs rather than monolayer edges (MLEs), indicating that BLEs are the stable edges in graphene at high temperatures. We reproduced the “coastline” sublimation morphologies by kinetic Monte Carlo (kMC) simulations. The simulation revealed geometrical and topological features unique to quasi-2-dimensional (2D) graphene sublimation and reconstructions. These reconstructions were enabled by bending, which cannot occur in first-order phase transformations of 3D bulk materials. These results indicate that substrate of multiple-layer graphene can offer unique opportunities for tailoring carbon-based nanostructures and engineering novel nano-devices with complex topologies.


8 pages of supporting information in a pdf.



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