Plasma-enhanced chemical vapor deposition of ortho-carborane: structural insights and interaction with Cu overlayers.

X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) are used to investigate the chemical and electronic structure of boron carbide films deposited from ortho-carborane precursors using plasma-enhanced chemical vapor deposition (PECVD), and the reactivity of PECVD films toward sputter-deposited Cu overlayers. The XPS data provide clear evidence of enhanced ortho-carborane reactivity with the substrate, and of extra-icosahedral boron and carbon species; these results differ from results for films formed by condensation and electron beam induced cross-linking of ortho-carborane (EBIC films). The UPS data show that the valence band maximum for PECVD films is ∼1.5 eV closer to the Fermi level than for EBIC films. The XPS data also indicate that PECVD films are resistant to thermally-stimulated diffusion of Cu at temperatures up to 1000 K in UHV, in direct contrast to recently reported results, but important for applications in neutron detection and in microelectronics.

Novel alloy polymers formed from ortho-carborane and benzene or pyridine.

Polymer films have been formed by electron-induced cross-linking of condensed ortho-carborane and benzene (B(10)C(2)H(X):BNZ) or pyridine (B(10)C(2)H(X):py) at 110 K, followed by warming up to room temperature. High resolution core-level photoemission and molecular orbital calculations demonstrate that the reaction of the icosahedra with the aromatic group is site-specific: bonding occurs between a C atom on the aromatic group and a B site bound to other boron atoms on the icosahedron. This site specificity determines a systematic variation in the valence band maximum relative to the Fermi level from -4.3 eV for cross-linked ortho-carborane to -2.6 eV for B(10)C(2)H(X):BNZ and -2.2 eV for B(10)C(2)H(X):py. The results indicate the ability to form a new class of materials that are a cross between a molecular solid and a network polymer. Further, the electronic properties of these materials can be systematically tuned for a broad variety of applications in neutron detection, nano-electronics and spintronics.

Graphene mediated domain formation in exchange coupled graphene/Co3O4(111)/Co(0001) trilayers.

Graphene grown directly on Co3O4(111)/Co(0001) by molecular beam epitaxy exhibits extrinsic p-type doping, as demonstrated by photoemission and conductivity measurements. Trilayer heterostructures of graphene/Co3O4(111)/Co(0001) reveal an unconventional magneto-optical Kerr hysteresis with vanishing remanence for temperatures up to 400 K. Magnetic force microscopy measurements demonstrate that the vanishing remanence is due to a complex domain state, indicating substrate-induced graphene spin polarization. The domain formation of the Co magnetization is in strong contrast to the magnetic behavior of Co in Co/Co3O4 bilayers. This suggests that the Co3O4 interlayer mediates the variable Co magnetization and induced graphene spin polarization, with possible retroaction of graphene on the Co film.

Direct graphene growth on Co3O4(111) by molecular beam epitaxy.

Direct growth of graphene on Co(3)O(4)(111) at 1000 K was achieved by molecular beam epitaxy from a graphite source. Auger spectroscopy shows a characteristic sp(2) carbon lineshape, at average carbon coverages from 0.4 to 3 ML. Low energy electron diffraction (LEED) indicates (111) ordering of the sp(2) carbon film with a lattice constant of 2.5(±0.1) Å characteristic of graphene. Sixfold symmetry of the graphene diffraction spots is observed at 0.4, 1 and 3 ML. The LEED data also indicate an average domain size of ~1800 Å, and show an incommensurate interface with the Co(3)O(4)(111) substrate, where the latter exhibits a lattice constant of 2.8(±0.1) Å. Core level photoemission shows a characteristically asymmetric C(1s) feature, with the expected π to π* satellite feature, but with a binding energy for the 3 ML film of 284.9(±0.1) eV, indicative of substantial graphene-to-oxide charge transfer. Spectroscopic ellipsometry data demonstrate broad similarity with graphene samples physically transferred to SiO(2) or grown on SiC substrates, but with the π to π* absorption blue-shifted, consistent with charge transfer to the substrate. The ability to grow graphene directly on magnetically and electrically polarizable substrates opens new opportunities for industrial scale development of charge- and spin-based devices.