Few chemical structures are as instantly recognisable as graphene. The orderly honeycomb construction of this two-dimensional carbon nanosheet has become very familiar to physicists and chemists alike.
Yet two decades after its discovery, graphene continues to surprise. When two sheets of graphene are layered together, then twisted out of atomic alignment with the other, the familiar honeycomb appearance dissolves.
As the twist angle changes, the complex patterns the two offset sheets create move from intricate mosaic-like arrangements to arrays of giant hexagons. In each case, the repeating nature of each sheet of graphene’s structure remains, but translated into periodic pattern of far grander scale.
The visual effect of this twist, known as a moiré pattern, is not unique to two-dimensional nanomaterials. ‘Moiré patterns also appear at macroscale,’ says materials chemist Yury Gogotsi from Drexel University in Philadelphia, US. When 2D nanomaterials are twisted into a moiré superlattice structure, however, their properties also change, sometimes inverting completely.
A tiny twist can turn a non-magnet into a magnet, or an insulator into a conductor. In the case of graphene, it can turn a conductor into a superconductor. ‘When people discovered that you could take two layers of non-superconducting material, twist it by roughly one degree and it becomes superconductor, that captured the imagination,’ says Gogotsi.
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