Abstract
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 ring-like 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.
Highlights
Controlled chemical vapour deposition (CVD) of well-defined, highly ordered large-area graphene layers is mandatory for a variety of potential technological applications, e.g. nanoscaled ultrafast field effect transistors, novel energy-storage materials, batteries, transparent conducting electrodes, etc
The technique of surface X-ray diffraction (SXRD) is sensitive to atomic surface roughness25, which can be modelled by ‘missing’ electron density and the value obtained for the occupancy here would correspond to a root-mean-square roughness less than 0.5 Å, i.e. to a very smooth surface
The Auger Electron Spectroscopy (AES) spectrum of the quasi-clean state does not show a clear C signal, which means that its concentration is below the detection limit of roughly 5%
Summary
Controlled chemical vapour deposition (CVD) of well-defined, highly ordered large-area graphene layers is mandatory for a variety of potential technological applications, e.g. nanoscaled ultrafast field effect transistors, novel energy-storage materials, batteries, transparent conducting electrodes, etc.. CVD of graphene has, been studied extensively on a variety of metal substrates4 Some of these metals exhibit a rather poor solubility of carbon in the bulk, whereas others – such as the Ni[111] substrate used in this work – dissolve carbon quite well in the surface near bulk regions already at temperatures above 673 K. The temperature-dependent growth mechanisms of graphene by chemical vapor deposition under UHV conditions by exposure to ethylene (10−8 to 10−6 mbar) are reviewed and the thermal stability of graphene and the surface-confined Ni2C nickel carbide are compared. As a consequence put forward in our present study, nucleation of graphene is likely to become easier once additional carbon supersaturation of the surface-near regions below the 2D Ni2C layer is provided. In LEEM studies of the same group, a carbon-denuded transition zone between the initially formed metastable Ni2C domains and the advancing graphene front was observed, highlighting the complex function of subsurface carbon during the transition from Ni2C toward the final product monolayer graphene
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