Ordered three-fold symmetric graphene oxide/buckled graphene/graphene heterostructures on MgO(111) by carbon molecular beam epitaxy

Theory and experiment demonstrate the direct growth of a graphene oxide/buckled graphene/graphene heterostructure on an incommensurate MgO(111) substrate. X-ray photoelectron spectroscopy, electron energy loss, Auger electron spectroscopy, low energy electron diffraction, Raman spectroscopy and first-principles density functional theory (DFT) calculations all demonstrate that carbon molecular beam epitaxy on either a hydroxylated MgO(111) single crystal or a heavily twinned thin film surface at 850 K yields an initial C layer of highly ordered graphene oxide with C_(3v) symmetry. A 5 × 5 unit cell of carbon, with one missing atom, forms on a 4 × 4 unit cell of MgO, with the three C atoms surrounding the C vacancy surface forming covalent C–O bonds to substrate oxide sites. This leads to a bowed graphene-oxide with slightly modified D and G Raman lines and a calculated band gap of 0.36 eV. Continued C growth results in the second layer of graphene that is stacked AB with respect to the first layer and buckled conformably with the first layer while maintaining C_(3v) symmetry, lattice spacing and azimuthal orientation with the first layer. Carbon growth beyond the second layer yields graphene in azimuthal registry with the first two C layers, but with graphene-characteristic lattice spacing and π → π* loss feature. This 3rd layer is also p-type, as indicated by the 5.6 eV energy loss feature. The significant sp^3 character and C_(3v) symmetry of such heterostructures suggest that spin–orbit coupling is enabled, with implications for spintronics and other device applications

Epitaxial growth of cobalt oxide phases on Ru(0001) for spintronic device applications

Cobalt oxide films are of technological interest as magnetic substrates that may support the direct growth of graphene, for use in various spintronic applications. In this work, we demonstrate the controlled growth of both Co_3O_4(111) and CoO(111) on Ru(0001) substrates. The growth is performed by Co molecular beam epitaxy, at a temperature of 500 K and in an O_2 partial pressure of 10^(−4) Torr for Co_3O_4(111), and 7.5 × 10^(−7) Torr for CoO(111). The films are distinguished by their dissimilar Co 2p x-ray photoemission (XPS) spectra, while XPS-derived O/Co stoichiometric ratios are 1.33 for Co3O4(111) and 1.1 for CoO(111). Electron energy loss (EELS) spectra for Co_3O_4(111) indicate interband transitions at ~2.1 and 3.0 eV, while only a single interband transition near 2.0 eV is observed for CoO(111). Low energy electron diffraction (LEED) data for Co_3O_4(111) indicate twinning during growth, in contrast to the LEED data for CoO(111). For Co_3O_4(111) films of less than 20 Å average thickness, however, XPS, LEED and EELS data are similar to those of CoO(111). XPS data indicate that both Co oxide phases are hydroxylated at all thicknesses. The two phases are moreover found to be thermally stable to at least 900 K in UHV, while ex situ atomic force microscopy measurements of Co_3O_4(111)/Ru(0001) indicate an average surface roughness below 1 nm. Electrical measurements indicate that Co_3O_4(111)/Ru(0001) films exhibit dielectric breakdown at threshold voltages of ~1 MV cm^(−1). Collectively, these data show that the growth procedures yield Co_3O_4(111) films with topographical and electrical characteristics that are suitable for a variety of advanced device applications