Lending support to graphene: By design

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  • Published: Mar 15, 2015
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Lending support to graphene: By design

Structural criterion

Researchers have developed a set of criteria that could allow materials scientists to find suitable substrate materials for graphene in a targeted way. X-ray and photoelectron spectroscopic studies lend support.

Researchers have developed a set of criteria that could allow materials scientists to find suitable substrate materials for graphene in a targeted way. X-ray and photoelectron spectroscopic studies lend support.

Scientists at Forschungszentrum Jülich (the Jülich research centre) and colleagues at other centres have worked out a set of criteria that should allow them and others to target substrates for the two-dimensional all-carbon material graphene for which the intriguing material's optical and electronic properties are preserved.. A simple structural parameter emerged as a useful tool estimated what influence a given substrate will have on graphene's electronic properties and is reported in the journal Physical Review Letters. Graphene can be fabricated on a silicon carbide substrate, for instance, provide that material has been treated with hydrogen in order to electrically decouple the graphene. The distance between the two layers, minus the respective van der Waals radii, gives an approximate value for the interaction strength.

Pining down underpinnings

Graphene is harder than diamond, tougher than steel and many times more conductive than silicon. An interesting new property seems to be discovered on a regular basis for what many scientists have dubbed the latest "wonder material". Such notable properties have made it the focus of countless research teams around the world hoping to find ways to exploit such properties in their laboratory experiments and in many cases to extrapolate the science to technological applications. Graphene is yet to emerge from the laboratory in technological form though despite its promise. One major obstacle on the road to innovation is finding the necessary support, after all a single atomic layer is an impractical substance to handle commercially in many regards.

"We simply wanted to find an accessible parameter which can be used to compare different substrates directly," explains post-doctoral researcher François Bocquet of Jülich's Peter Grünberg Institute. "The decisive criterion turned out to be the atomic distance between the graphene layer and the underlying substrate," he adds. The van der Waals radius, which is known for a given free atom size, can be used to determine the interaction strength. Computer simulations carried out by scientists from the Berlin Fritz Haber Institute of the Max Planck Society provide a mathematical underpinning to this point. Bocquet and colleagues have now used the Diamond synchrotron radiation source in Didcot, Oxfordshire, UK, to pin down the concept experimentally.

They used X-ray diffraction to produce a standing wave in the vicinity of the interface. Combined with the chemically sensitive photoelecton electron spectroscopy technique, they could measure the distance between graphene and its substrate with a precision close to the picometre range with hydrogen-modified silicon carbide as substrate for their graphene sample. This substrate was originally developed by the team at the Max Planck Institute for Solid State Research in Stuttgart, Germany, as a semiconductor substrate. By contrast to the more conventional metallic substrate, there are practically no interactions between the substrate layer and the graphene, thus the graphene effectively behaves independently of the substrate.

Weak interactions

"With the emergence of this new class of substrates, it was time for a new criterion with which even very weak interactions can be detected precisely," explains Stefan Tautz, who heads the sub-institute Functional Nanostructures at Surfaces (PGI-3). "With the techniques available so far, for example photoelectron spectroscopy, the degree of interaction with the substrate could only be deduced indirectly. Bonds as weak as these could hardly be detected."

Related Links

Phys Rev Lett, 2015, 114, 106804: "Approaching Truly Freestanding Graphene: The Structure of Hydrogen-Intercalated Graphene on 6H-SiC(0001)"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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