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How surface periodicities can open a gap on graphene

Semiconductors are the basis of electronics, because their band gap is the key for controlling the current flow on devices, whose presence or absence defines the logical states. One graphene cousin exhibits precisely this intensively searched property in graphene, a band gap. This graphene cousin consists of an atomic layer of C atoms with the same honeycomb structure as graphene, but periodically attached to the underlying SiC substrate. In a collaboration between LPS, SOLEIL and GeorgiaTech, we have observed demonstrated that the gap opening seems to be triggered by the periodical bonding of the buffer layer to the substrate.

When Si on SiC sublimates upon a high-temperature thermal treatment, the excess of carbon at the surface can reorganize into graphene. However, before a first graphene layer grows, a structurally similar layer with the honeycomb structure but without its electronic properties, appears. This layer is called the buffer layer and exhibits a band gap of more than 500 meV [1, 2 and references]. The reason of this band gap opening remained to be elucidated. It was usually accepted that this gap is opened by a global covalent bonding of the buffer layer to the substrate. However, such a strong covalent bonding was incompatible with the previously observed lifting of the buffer layer by hydrogen intercalation to give rise to standard graphene. Motivated by this controversy, we performed complementary photoemission, scanning tunnelling microscopy (STM), high-resolution transmission electron microscopy and ab-initio calculations to understand the system. We have demonstrated that the surface periodicity observed in the STM images is associated only to a local, and not global, attachment of the buffer layer to the substrate. Together with a precise analysis of symmetries and band foldings of the experimental and theoretical band structures, we have concluded that the band gap is opened due to a nanoscale periodicity.

[1] M. S. Nevius, M. Conrad, F. Wang, A. Celis, M. N. Nair, A. Taleb-Ibrahimi, A. Tejeda, and E. H. Conrad. Physical Review Letters 115, 136802 (2015) [2] A. Lanzara, Physics 8, 91 (2015).


Wide Band Gap Semiconductor from a Hidden 2D Incommensurate Graphene Phase
M. Conrad, F. Wang, M. Nevius, K. Jinkins, A. Celis, M. Narayanan Nair, A. Taleb-Ibrahimi, A. Tejeda, Y. Garreau, A. Vlad, A. Coati, P. F. Miceli, and E. H. Conrad
Nano Letters 17, 341 (2017).

Band Gap Opening Induced by the Structural Periodicity in Epitaxial Graphene Buffer Layer
M. N. Nair, I. Palacio, A. Celis, A. Zobelli, A. Gloter, S. Kubsky, J.-P. Turmaud, M. Conrad, C. Berger, W. de Heer, E. H. Conrad, A. Taleb-Ibrahimi, and A. Tejeda
Nano Letters 17, 2681 (2017).


Antonio Tejeda