Elettra | Nanospectroscopy, NanoESCA The peculiar case of graphene growth on Co(100) The widespread use of graphene in everyday technologies still faces a major hurdle: how to produce it on a large scale in a cost-effective and reliable way. Among the different approaches explored so far, chemical vapor deposition (CVD) has emerged as a particularly promising method, allowing the growth of high-quality graphene on metal surfaces. Transition metals such as iridium, platinum, copper and cobalt serve as catalytic platforms where graphene layers can assemble upon CVD. Up to now, graphene has been synthesized via CVD mainly on top of Co films with hexagonal symmetry, where the relatively similar lattice constant allows for the formation of epitaxially aligned graphene. While the presence of a lattice mismatch may already lead to the appearance of moiré superstructures, the stabilization of graphene on symmetry mismatched substrates leads to different moiré motifs, which may influence the electronic properties of the interface. At the same time, the symmetry mismatch can influence the graphene growth in a profoundly different way from the symmetry-matched hexagonal surfaces. In this work, using low-energy electron microscopy (LEEM), we follow in realtime the CVD growth of graphene on top of thin Co(100) films deposited on carbon-reconstructed W(110) crystal and unravel the electronic and magnetic properties of the interface using a combination of surface-sensitive spectro-microscopy methods. The selection of the carbon-reconstructed tungsten is motivated by the fact that such a surface acts as a template for Co(100) growth, and it has a very low tendency towards Co-W intermixing. Cobalt is deposited on top of the carbon-reconstructed W(110) surface up to a thickness of 6 nm. On top of this surface, Co adopts an fcc(100) crystalline arrangement and has its lattice dilated by 2.8% as compared to its bulk spacing. As visible from the low-energy electron diffraction (LEED) pattern in the inset of Fig. 1 (a), after the room-temperature deposition and subsequent mild annealing to 570 K, the Co film possesses an fcc(100) structure with two mirror domains, along with a surface c(2×2) reconstruction due to surface carbon segregation. Graphene is grown on top of the c(2x2) surface by ethylene CVD while keeping the sample at 800 K. As visible from the LEEM image sequence (Figs. 1 a-c), the surface nucleation of graphene occurs at several surface points with nanometer-sized islands. The islands expand along the terraces of the Co substrate (following the crystalline directions - see the LEED pattern), as can be appreciated by their elongated shape upon reaching a dimension of about 1 µm. Within each island, graphene has several rotational domains, as visible from the μ-LEED pattern in the inset of Fig. 1 (c) acquired from a single island. The CVD synthesis was stopped upon reaching a graphene surface coverage of about 40%, allowing for laterallyresolved chemical characterization of the interface using X-ray Photoemission Electron Microscopy (XPEEM) at the C 1s core level. The surface regions not covered by graphene (Fig. 1 d bottom) are characterized by a sharp peak centered at 282.7 eV and a smaller and broader component at 283.3 eV, which can be identified as the presence of surface and bulk-dissolved surface carbon, respectively. In the graphenecovered regions, instead, the C 1s spectrum is deconvoluted into four components, with two prominent peaks found at binding energies of 284.3 eV and 284.9 eV in addition to the same two components observed in the uncovered regions. The peak at higher binding energy is assigned to strongly interacting carbon atoms with the cobalt substrate. Instead, the contribution at lower binding energy is tentatively assigned to carbon atoms, which weakly interact with the underlying support. This behavior can be justified either by considering that there are strongly and weakly interacting parts within the surface unit cell or by considering that graphene is fully decoupled and only strongly bound at the boundaries of the nano-sized islands below the resolution limit of the microscope. The electronic properties of the interface were analyzed using k-PEEM. The energy vs. momentum map acquired on Co(100) before graphene growth is displayed in Fig. 1 (h). Sharp states can be identified in the proximity of the Fermi level and are ascribed to the emission from the Co 3d bands. The energy vs. momentum map after the synthesis of monolayer graphene is visible in Fig. 1 (i). The sharp parabolic feature with the vertex in Γ (indicated by the blue arrow) is the characteristic π band of graphene. Its apex is found at 8.3 eV binding energy and is compatible with the presence of a weakly interacting graphene layer on top of cobalt. In addition, a faint intensity (indicated by the red arrow) could be observed outside the main parabolic band, with a shape similar to the main π band, which is labeled as π’. This band is downshifted in energy by about 2.4 eV and agrees well with the emission from strongly interacting graphene on top of cobalt. This observation aligns well with the XPS data reported in Fig. 1 (d), where the double-peak structure, deriving from strongly and weakly interacting carbon atoms in graphene, was observed. Finally, we provided a comprehensive investigation of the CVD growth of graphene on thin ferromagnetic carbonrich Co(100) films, offering detailed insights into the growth mechanism and interfacial chemistry. The presence of a carbidic interlayer effectively decouples graphene from the cobalt substrate, rendering it almost freestanding. The ability to control the decoupling and binding of graphene to the substrate has significant implications for developing graphene-based devices and designing functional interfaces in graphene-based systems. Elettra Highlights 2024–2025 126
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