Synthesis and characterization of nanomaterials: graphene, silicene and carbon nano- onions

Abstract

The electronic structure of the graphene/Ni(111) system was investigated by means of electron energy-loss spectroscopy (EELS). A single layer of graphene has been obtained on Ni(111) by dissociation of ethylene. Angle-resolved EEL spectra show a low energy plasmon dispersing up to about 2 eV, resulting from fluctuation of a charge density located around the Fermi energy, due to hybridization between Ni and graphene states. The dispersion is typical of a two-dimensional charge layer, and the calculated Fermi velocity is a factor of ~0.5 lower than in isolated graphene. The interface-π plasmon, related to interband transitions involving hybridized states at the K point of the hexagonal Brillouin zone, has been measured at different scattering geometries. The resulting dispersion curve exhibits a square root behavior, indicating also in this case a two-dimensional character of the interface charge density. As well, it has been shown that it is possible to use EELS in the reflection mode to measure the fine structure of the carbon K-edge in monolayer graphene on Ni(111), thus demonstrating that reflection EELS is a very sensitive tool, particularly useful in cases where the TEM-based ELNES cannot be applied. Clean Ag(111) surface and the two phases of silicene on Ag(111), mixed (4×4, √13×√13R19°, 2√3×2√3R30°) and 2√3×2√3R30, have been studied by XPS, LEED and EELS. EEL spectra of the Ag(111) surface covered by silicene in the (4×4, √13×√13R19°, 2√3×2√3R30°) mixed phase shows a well-defined plasmon peak whose center is located at about 1.75 eV. The 2√3×2√3R30° phase shows EEL spectra that exhibit a peak located at about 0.75 eV loss, which moves clearly towards higher energies with increasing momentum transfer. The typical parabolic dispersion relation obtained from such spectra confirms that the peak is due to a collective excitation which is evidently associated to the silicene layer. These plasmons associated to silicene have never been observed in the past. Our results show that the plasmonic properties of silicene on Ag(111) are strongly dependent on the geometrical arrangement of Si atoms with respect to the substrate. Carbonaceous nanomaterials have been obtained by underwater arc discharge between graphite electrodes. TEM images showed that the resulting particles suspended in water consist of CNOs with other carbonaceous materials such as CNTs and graphene. We observed for the first time the formation of a solid agglomerate on the cathode surface. Raman and TEM studies revealed that the agglomerate is made exclusively of CNOs. The defragmentation of such agglomerate allows to obtain CNOs free of other carbonaceous materials without the complex purification procedures needed for floating nanomaterials