SPECTROSCOPIC INVESTIGATION OF ELECTRONIC AND VIBRONIC PROPERTIES OF ION-BEAM-DEPOSITED AND THERMALLY TREATED ULTRATHIN C-H FILMS

C:H films in the thickness range <1 to about 20 monolayers have been prepared by hydrocarbon-ion-beam deposition at Pt(100) and Pt(111) single-crystal surfaces covered with a graphite monolayer. The films were investigated by Auger electron, electron-energy-loss (EELS), and high-resolution electron-energy-loss spectroscopies to investigate their electronic and vibronic characteristics. The hydrogen content of the films was determined at [H]/[C]=0.05-0.5, depending on deposition parameters such as feed gas and target temperature. AES and EELS identify the films deposited at 350 K as being of a distorted graphitic nature with a carbon [sp2]/[sp3] ratio of about unity. Vibrational spectroscopy reveals that at the film surfaces H atoms are bound to C atoms in sp, aromatic sp2, and sp3 hybridization states. From the vibrational spectra, the presence of -C = CH, (aromatic) = CH, and -CH(x), x = 1,2,3 groups at the surface is inferred. Upon annealing, the films at higher temperatures, 500-1400 K, sp, sp3, and sp2 groups get destroyed sequentially, paralleled by evolution of hydrogen (major product) and hydrocarbon species (minor product) from the films. After annealing at 1040 K, vibrational spectra exhibit only a sp2 CH-related C-H stretch band and, accordingly, from EELS increased sp2 bonding in the annealed films is obtained. This stability sequence of CH(x) groups at the carbon network is in accordance with the expectations drawn from kinetic and thermodynamic data of hydrocarbons. The chemical structure of the deposited C:H films is insensitive to the nature of the deposition ion, e.g., methane, ethane, ethylene, or benzene, from which it is suggested that specific C-H bonding in the deposition ions does not act as a precursor for the film structure. Films deposited in the submonolayer range exhibit primarily sp3 C-H bonding in accordance with a simple picture of the initial growth of the films.