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Séminaire QUEST

Jeudi 13 octobre à 9h30,
Salle Rémy Lemaire, K223

Orateur : A. Artaud (CEA and Institut Néel)
"Fourier transform microscopy-based classification of moiré superlattices"


Graphene’s crystal symmetry and bi-partite lattice, with two carbon atoms by unit cell, impose electronic excitations reminiscent of massless Dirac fermions. Breaking the equivalence of the two atoms in the unit cell – like in hexagonal BN – has proven a strategy to modify the properties of graphene [1]. Maintaining its properties in the pristine form while enriching it with others requires milder strategies, for instance involving slowly varying superpotentials such as generated by moiré superlattices.

However, two challenges emerge from the countless examples of moirés. First, for most substrates, many structural models have been interpreted, and identifying the physical one is still debated. Secondly, in spite of many geometrical models [2,3], no unified picture grasps the diversity of all observed moirés.

Using a consistent direct and Fourier analysis of scanning tunneling microscopy (STM) images, and assuming commensurate graphene-substrate relations, we achieve 0.001Å lateral precision, so the exact atomic structure underlying a moiré superlattice can be deciphered. This also reveals a so far overlooked fact : graphene is usually strained, twisted and sheared with respect to its substrate. Based on this analysis, a geometrical model can be formulated, so as to classify every possible structureand rationalize them [4].

[1] K. Novoselov, Graphene : Mind the gap, Nature Mater.6, 720-721 (2007)
[2] K. Hermann, Periodic overlayers and moiré patterns : theoretical studies of geometric properties, J. Phys. : Condens. Matter 24, 314210 (2012)
[3] P. Zeller, and S. Günther, What are the possible moiré patterns of graphene on hexagonally packed surfaces ?, New J. of Phys.16, 083028 (2014)
[4] A. Artaud, L. Magaud, T. Le Quang, V. Guisset, P. David, C. Chapelier, J. Coraux, Universal classification of twisted, strained and sheared graphene moirésuperlattices, Sci. Rep.6, 25670 (2016)

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