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R. Rameshan, V. Vonk., D. Franz, J. Drnec, S. Penner, A. Garhofer, F. Mittendorfer, A. Stierle, B. Klötzer:
"Role of Precursor Carbides of Graphene Growth on Ni(111)";
Scientific Reports, 8 (2018), 266201 - 266213.

Surface X-ray Diffraction was used to study the transformation of a carbon-supersaturated carbidic
precursor toward a complete single layer of graphene in the temperature region below 703 K without
carbon supply from the gas phase. The excess carbon beyond the 0.45 monolayers of C atoms within a
single Ni2C layer is accompanied by sharpened reflections of the |4772| superstructure, along with ringlike
diffraction features resulting from non-coincidence rotated Ni2C-type domains. A dynamic Ni2C
reordering process, accompanied by slow carbon loss to subsurface regions, is proposed to increase
the Ni2C 2D carbide long-range order via ripening toward coherent domains, and to increase the local
supersaturation of near-surface dissolved carbon required for spontaneous graphene nucleation
and growth. Upon transformation, the intensities of the surface carbide reflections and of specific
powder-like diffraction rings vanish. The associated change of the specular X-ray reflectivity allows to
quantify a single, fully surface-covering layer of graphene (2 ML C) without diffraction contributions
of rotated domains. The simultaneous presence of top-fcc and bridge-top configurations of graphene
explains the crystal truncation rod data of the graphene-covered surface. Structure determination of
the |4772| precursor surface-carbide using density functional theory is in perfect agreement with the
experimentally derived X-ray structure factors.