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 3 O 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 3 O 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 Co 3 O 4 (111) and 1.1 for CoO(111). Electron energy loss (EELS) spectra for Co 3 O 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 3 O 4 (111) indicate twinning during growth, in contrast to the LEED data for CoO(111). For Co 3 O 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 3 O 4 (111)/Ru(0001) indicate an average surface roughness below 1 nm. Electrical measurements indicate that Co 3 O 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 3 O 4 (111) films with topographical and electrical characteristics that are suitable for a variety of advanced device applications.

KEYWORDSphotoemission, electron energy loss, low energy electron diffraction, density functional theory ABSTRACT 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 5x5 unit cell of carbon, with one missing atom, forms on a 4x4 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 πàπ* loss feature. This 3 rd 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.2

Heterostructures consisting of 10 Å thick chromia films and 50 Å thick titania films display significant exchange bias at and above room temperature. Chromia films ∼10 Å thick were deposited by molecular beam epitaxy (MBE) of Cr at room temperature in ultrahigh vacuum on 50 Å thick TiO2–x (111) films (x < 0.3) also deposited epitaxially by MBE on Al2O3(0001). Cr deposition yields increased Ti­(III) formation in the titania substrate and the formation of a Cr2O3 overlayer, without Cr/Ti interfacial mixing, as determined by in situ photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). In situ low-energy electron diffraction (LEED) and XPS data indicate that the chromia overlayer is hexagonally ordered and ∼10 Å thick. Longitudinal and polar magneto-optic Kerr effect (MOKE) measurements at 285–315 K provide evidence of strong exchange bias between the boundary layer magnetization of chromia and the ferromagnetic substrate. These data demonstrate the robust room-temperature interaction of the boundary layer magnetization of a multiferroic antiferromagnet with a d0 ferromagnetic substrate.

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