15 August 2012

Graphene’s conductivity depends on the underlying material

Researchers at MIT have shown that graphene behaves differently depending on the material it's wrapped around.

When the team placed sheets of graphene on substrates made of different materials, properties such as how the graphene conducts electricity and how it interacts chemically with other materials were found to be drastically different, depending on the nature of the underlying material.

When the material underneath was silicon dioxide, a standard material used in electronics, the graphene became 'functionalised' when exposed to certain chemicals. But when graphene sat on boron nitride, it hardly reacted to the same chemicals.

"It's very counterintuitive," commented Michael Strano, the Charles and Hilda Roddey professor of chemical engineering at MIT. "You can turn off and turn on graphene's ability to form chemical bonds, based on what's underneath."

The team found that this is because the material is so thin that how it reacts is strongly affected by the electrical fields of atoms in the material beneath it. Therefore it is possible to create devices with a micropatterned substrate covered with a layer of graphene whose chemical behaviour will then vary according to the hidden patterning. This could enable the production of microarrays of sensors to detect trace biological or chemical materials.

"You could get different molecules of a delicate biological marker to interact [with these regions on the graphene surface] without disrupting the biomolecules themselves," said MIT postdoc Qing Hua Wang. Most current fabrication techniques for such patterned surfaces involve heat and reactive solvents that can destroy these sensitive biological molecules.

The next step for the MIT team is to investigate whether bi layer graphene reacts differently to the single layer material.

The image shows a graphene layer on top of a patterned substrate and highlights the difference in chemical reactivity of the side opposite the substrate. The wide red stripe is an area over a silicon dioxide substrate, making the top surface of the graphene highly reactive. The narrow blue stripe is graphene over a layer of hydrocarbon (called OTS), and there is almost no reactivity on the side not in contact with the substrate.

Simon Fogg

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