domingo, 25 de julio de 2010

New technique opens a gap in graphene

Researchers in Germany and Switzerland have developed a new way to make extremely narrow graphene ribbons with specific widths and electronic bandgaps. The ribbons also have smooth edges, something that is crucial for making electronic devices out of graphene.
Graphene is a flat sheet of carbon just one atom thick – with the carbon atoms arranged in a honeycomb lattice. Since the material was first created in 2004, its unique electronic and mechanical properties have amazed researchers who say that it could be used in a host of device applications. Indeed, graphene might even replace silicon as the electronic material of choice in the future.
However, unlike the semiconductor silicon, graphene 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. One way of introducing a bandgap into graphene is to make extremely narrow ribbons of the material.

Cutting or unzipping
Until now, these graphene nanoribbons were made using top-down approaches, such as "cutting" the ribbons from larger graphene sheets or "unzipping" carbon nanotubes. Such methods produce ribbons that are relatively wide (more than 10 nm across) with rough edges. For high-efficiency electronics devices, the ribbons need to be much smaller than 10 nm wide and, importantly, their edges need to be smooth because even minute deviations from the ideal edge shapes, "armchair" and "zigzag", seriously degrade graphene's electronic properties.
The new technique, developed by a team led by Roman Fasel of the Swiss Federal Laboratories for Materials Science and Technology (Empa) and Klaus Müllen from the Max Planck Institute for Polymer Research in Germany, is a simple, surface-based bottom-up chemical process. It involves first spreading specially designed halogen-substituted bianthryl monomers onto gold and silver surfaces under a high vacuum. Next, the monomers are made to link up to form polyphenylene chains.

'Important first step'
Fasel and colleagues then remove hydrogen atoms from the polymers by heating up the ensemble. This leads to the polymer chains interconnecting to form planar, aromatic graphene ribbons that are just one atom thick, 1 nm wide and up to 50 nm long. The ribbons are narrow enough to have an electronic bandgap and thus switching properties. "Such switching is an important first step for the shift from silicon microelectronics to graphene nanoelectronics," say the researchers.
And that is not all: the edges of the graphene ribbons are smooth and armchair-shaped, and the ribbons themselves are either straight or zigzagged, depending on the monomers used to make them. The smooth edges will be important for studying fundamental experimental physics too, says the team – for example, observing how magnetic properties of the ribbons change with different edge structures. Until now, previous methods to make graphene nanoribbons always produced rough edges that were difficult to study
The new technique could also be used to dope the graphene ribbons by using monomers containing nitrogen or boron atoms. And monomers with additional functionalities should allow the researchers to create positively and negatively doped ribbons – for making p–n junctions in transistors, for instance.

Solar cells
Going further still, a combination of various monomers might even allow heterojunctions (interfaces between different types of graphene nanoribbon, such as those with large or small bandgaps) to be created. Such structures could be used in applications like solar cells or high-frequency devices. Fasel and colleagues have already shown that this technique is viable by connecting three separate graphene ribbons together using two suitable monomers.
The team, which includes scientists from ETH Zürich and the Universities of Zürich and Bern, is now working on creating the nanoribbons on semiconductor surfaces, rather than on just metallic substrates as in this work. This will be critical for making real-world electronic devices.