Manchester explains unusual electronic and optical behaviour in graphene

Researchers at the University of Manchester found a mechanism that could explain unusual electronic and optical behaviour in graphene, and another route to give it a bandgap, writes our technology editor Steve Bush.

Manchester Graphene

Graphene in detail

The work involved spreading mono-layer flakes of graphene on a thin (20nm) layer of boron nitride (BN).

Both materials have a hexagonal two-dimensional structure – there is increasing interest in the interaction of various 2-d materials, phosphorene and molybdenum di-sulphide are others. LINK

Some language is required here: when two layers of graphene are aligned with one another exactly, atoms from the layers attract each other and lock the alignment, and they are said to be ‘commensurate’.

Boron nitride’s lattice constant is 1.8% larger than graphene.

“There has been work done to see if graphene and boron nitride would become commensurate. Everyone had assumed they would stay incommensurate – there would be no way to click together,” project scientist Colin Woods told Electronics Weekly.

Monolayer graphene flakes were known to stick to boron nitride, by the same van der Waals’ forces that hold two aligned graphene layers together, but the lattice mis-match was thought to be enough that they would always remain incommensurate.

Woods’ examined graphene flakes flat on the boron nitride in various rotational orientations, using atomic force microscopy, scanning tunnelling microscopy and Raman spectroscopy.

He discovered that when the lattices were clearly misaligned, they were incommensurate.

However when the axes of the hexagons were aligned exactly, the graphene locally stretched to match the boron nitride and became commensurate, and also developed a bandgap.

“The badgap is small. We don’t know if it exists in un-aligned state, where there is a very small bandgap or no bandgap,” said Woods, “but the commensurate/incommensurate transition definitely opens a bandgap. This is a new mechanism for creating a bandgap in graphene.”

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Steve Bush

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