AVS2011 Session GR-TuP: Graphene and Related Materials Focus Topic Poster Session

Very recently, we demonstrated a fabrication of platinum catalysts supported on carbon nanoballs (CNBs), as well as characterization of their electrochemical activities (Saito et al., J. Vac. Sci. Technol. A, 27(4) pp.826-830). Originally-developed CNB is a highly crystallized nanosized carbon powder with spherical shape. The CNBs can be synthesized by thermal decomposition of ethylene and hydrogens under argon stream. Diameter of CNB can be tuned from 200 to 700nm by changing the mixing ratio of H₂. Platinum nanoparticles were densely loaded on the CNBs surface under modified solution plasma processing of CNBs and Pt ions with stabilizing agents. We obtained 1.6-fold incr ease in the electrochemical activity of Pt/CNBs system (0.26cm²/mg) compared with that of conventionally-used Pt/C system (0.16cm²/mg).

In this work, we propose newly developed CNBs, covered by carbon nanotubes (CNTs) with metallic conductivity in order to reduce contact resistance between neighboring CNBs in electrodes. We grew CNTs from various shaped Fe, Ni nanocrystals deposited onto an individual CNB, and subsequently decorated the surface of the CNBs with Pt nanoparticles as a catalyst by using solution plasma processing. The ratio of metallic to semiconducting CNTs of our all products was evaluated by Raman spectroscopy.

Graphene is currently produced typically through one of three routes: (1) some combination of mechanical and chemical exfoliation of graphite, (2) high temperature processing of SiC, or (3) through high-temperature catalytic methods on metal surfaces using simple carbon precursors such as methane. We are investigating and developing methods for synthesis of graphene through a molecular approach, in which the molecular precursors used already possess most of the C=C bonds necessary for graphene formation. A limitation of such approaches is that many of the precursors, e.g., pentacene, are only sparingly soluble and thus have difficulty being solution coated or have low volatility that prevent vapor phase deposition. Thus we have been investigating the use of soluble precursor molecules which form the graphene templates at higher temperatures or in the presence of catalysts. In particular, one approach toward solubilization that is under investigation is the use of reversible Diels-Alder chemistry to produce soluble grapheme precursor adducts. After solution coating of the materials onto appropriate substrates, the cycloaddition can be reversed at modest temperatures to produce the desired polyaromatic precursor and subsequently processed into grapheme through a further chemical or thermal consolidation. This presentation will review our progress to date in developing such techniques and will demonstrate the ability to form graphene from such approaches at relatively moderate temperatures. A discussion of the effect of choice of precursor on the ability to consolidate the material into graphene and its resulting quality will be presented.

Graphene is currently produced typically through one of four routes: (1) some combination of mechanical and chemical exfoliation of graphite, (2) oxidation of graphite to form grapheme oxide which is subsequently deposited onto surfaces and reduceed to grapheme, (3) high temperature processing of SiC, or (4) through high-temperature catalytic methods on metal surfaces using simple carbon precursors such as methane. Some applications for grapheme would benefit from low temperature methods (<400 ⁰C), and the first two methods mentioned above can inherently provide access to such low processing temperatures. However, simple exfoliation of graphite has only been demonstrated to produce relatively small, poorly controlled grapheme flakes that are not easily processed due to their limited solubility in solvents and lack of methods for further assembly into larger structures. Therefore, we have been investigating the ability to utilize grapheme oxide (GO) flakes as a soluble grapheme precursor that can be assembled onto surfaces to form both continuous grapheme films and directly form patterned grapheme microstructures. We will present a process in which we deposit and pattern aminosilane monolayers on a substrate, pattern them using lithographic techniques, and utilize such aminosilane patterns to control where GO assembles on the surface and subsequently is reduced to form grapheme. It will be demonstrated that by producing aminosilane monolayers that are dense and which possess a large fraction of non-hydrogen bonded amine terminal groups, continuous grapheme oxide and grapheme films and microstructures can be assembled on substrates. It will be shown that by subjecting the resulting assembled grapheme on such aminosilane layers to modest thermal treatments, that stable n-doped grapheme can be produced. The materials produced via such methods will be discussed in terms of their spectroscopic (e.g. Raman) and electrical properties (e.g. I-V curves for FET devices, carrier concentrations, mobilities, etc.)

We report layer-by-layer growth of macroscopically continuous and uniform graphene sheets on Co₃O₄(111) at 1000 K by carbon molecular beam epitaxy (MBE) from a graphite rod source. The direct growth of graphene on dielectric substrates is an essential step in the practical and scalable production of graphene-based devices. Co₃O₄(111) films 3 monolayers (ML) thick were formed from surface segregation of dissolved oxygen after deposition ~ 40 Å Co grown on Al₂O₃(0001) substrates at 750 K in UHV. Epitaxial Co₃O₄(111) films, as characterized by Auger spectroscopy and LEED, were formed by subsequent annealing to 1000 K in UHV, and exhibit th a O-O surface nearest neighbor distance of 2.8 Å, in good agreement with literature. The evolution of the Auger electron C(KVV) lineshape during carbon MBE indicates sp² hybridization, and layer-by-layer growth up to at least 3 ML average thickness. LEED spectra indicate that the sp² (111) graphene overlayer is incommensurate with the Co₃O₄(111) substrate. The graphene-related diffraction spots remain sharp from a coverage of 0.4 ML up to 3 ML, indicating that the graphene sheets are azimuthally in registry with each other. Exposure of the 3 ML graphene/Co₃O₄(111)/Co(111) sample to ambient results in no observable change in Auger or LEED spectra, indicating macroscopically continuous graphene sheets. Subsequent acquisition of XPS spectra in a separate chamber yields a graphite-characteristic asymmetric C(1s) peak at 284.9 eV binding energy, indicating graphene → oxide charge transfer, as observed for graphene/SiC and graphene/MgO. A π → π* satellite feature is also observed. Spectroscopic ellipsometry measurements carried out in a separate system confirm the presence of a π → π* resonance, and similarities with the optical absorption of graphene/SiC are observed. Raman spectra acquired at different, macroscopically separated sample areas indicate a uniform 3ML graphene film thickness. These results also strongly suggest that other non-polar (111) transition metal oxide surfaces with similar O-O nearest neighbor distances may act as suitable substrates for graphene growth at moderate temperatures, opening the way to controlled direct growth of high quality graphene on a variety of dielectric substrates, with materials and processing temperatures readily compatible with Si CMOS integration.