Materials

Graphene + Boron Nitride = Semiconductor

MIT researchers have taken a major step towards making wonder material graphene with a property called a band gap, which is required to utilize graphene in the manufacturing process for electronic devices.

Graphene hBN Heterostructure
Graphene and boron nitride hexagons, almost aligned.

By placing a sheet of graphene, the now almost legendary one atom thick carbon-based material, on top of hexagonal boron nitride, another one-atom-thick material with similar properties, the new material adds the band gap, whilst sharing graphene’s ability to conduct electrons. Graphene is an extremely good conductor of electrons, while boron nitride is a good insulator, blocking the passage of electrons.

This was no easy task, as to ensure that the hybrid material’s conductor/ insulator properties work, the hexagonal atomic lattices required alignment with phenomenal accuracy, particularly as the boron nitride lattice is 1.8 percent larger. Currently, this task is difficult enough that it results in a 1 in 15 attempt success rate.

The hybrid also creates some advanced phenomena in itself, as the differential between the lattice size brings forth the opportunity to ‘tune’ the material via alignment rotation, whilst, against theoretical prediction, the band gap is maintained at a constant level.

This, frankly, is weird.

Graphene Hbn
Insulating states and superlattice minibands in a graphene/hBN heterostructure

It’s kind of like being annoyed that the sun-heated swimming pool is full around the area where the sun has been on recently, only to be pleasantly surprised that not only is the bit that you slide into also warm, but the whole pool is of equal temperature. Kind of.

Put another way, regions of local quasi-epitaxial alignment lead to opposite signs of the sub-lattice asymmetry in different regions with

the heterostructure, whilst, against prediction, the area between the conduction band and valance band is maintained with ubiquity, regardless of proximity to moire unit cells.

The MIT researchers state that the band gap created so far in the material is smaller than that needed for practical electronic devices; finding ways of increasing it will require further work.

An alternative semiconductor approach has been made by etching graphene sheets into narrow ribbons, but this degrades graphene’s electrical properties: this new method produces no such degradation.

And with heavy emphasis on 3D printing R&D at MIT too, one can only speculate on if/when the teams will hook up?

Read more at MIT

REFERENCES: B. HUNT ET AL., MASSIVE DIRAC FERMIONS AND HOFSTADTER BUTTERFLY IN A VAN DER WAALS HETEROSTRUCTURE, SCIENCE, 2013, DOI: 10.1126/SCIENCE.1237240

[Image Credits: B. Hunt et al./Science]
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