Graphene helps study other 2D semiconductors

Molybdenum disulphide encapsulated between layers of insulating hexagonal boron nitride with graphene electrodes

Researchers have used graphene and 2D insulator boron nitride to make a clean environment in which to study other 2D semiconductors. Molybdenum disulphide has come out of the tests well.

At issue is the large surface area of 2D molecules which offeres huge possibilities for unwanted interaction.

In work at Columbia University in New York, two layers of boron nitride – an insulator, are used to keep the environment away from the material under test, while graphene provides electrical connections.

“These findings provide a demonstration of how to study all 2D materials. Our combination of BN and graphene electrodes is like a socket into which we can place many other materials and study them in an extremely clean environment to understand their true properties and potential,” said Columbia’s Professor James Hone.

MoS2 is a well-studied 2D semiconductor. Unlike graphene, it can form a transistor that can be switched fully ‘off’. However, tested on common insulating substrates such as SiO2, it shows varying mobility, well below its theoretical maximum.

Hone’s team built-up its heterostructure with MoS2 in the middle – from flakes of all three materials.

They found room-temperature mobility was improved by a factor of about two, approaching the intrinsic limit, according to Columbia, and at low temperature became faster by 5 to 50x depending on the number of atomic layers – all without the use of high vacuum.

“As a further sign of low disorder, these high-mobility samples also showed strong oscillations in resistance with magnetic field, which had not been previously seen in any 2D semiconductor,” said the university.

Analysis of low-temperature resistance and quantum oscillations indicates there remains contamination at the interfaces, indicating that further improvements are possible.

“This work motivates us to further improve our device assembly techniques, since we have not yet reached the intrinsic limit for this material,” said Hone.

Manchester University, where graphene was discovered, has been studying BN-graphene-BN sandwhiches since 2011.

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Alongside Columbia, collaborators came from: Harvard, Cornell, University of Minnesota, Yonsei University Korea, Danish Technical University, and the Japanese National Institute of Materials Science.

steve bush

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via Yuichun