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 p repared by hydrocarbon-ion-beam deposition at Pt(100) and Pt(111) sing le-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 el ectronic and vibronic characteristics. The hydrogen content of the fil ms was determined at [H]/[C]=0.05-0.5, depending on deposition paramet ers such as feed gas and target temperature. AES and EELS identify the films deposited at 350 K as being of a distorted graphitic nature wit h 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 spect ra, the presence of -C = CH, (aromatic) = CH, and -CH(x), x = 1,2,3 gr oups at the surface is inferred. Upon annealing, the films at higher t emperatures, 500-1400 K, sp, sp3, and sp2 groups get destroyed sequent ially, paralleled by evolution of hydrogen (major product) and hydroca rbon species (minor product) from the films. After annealing at 1040 K , vibrational spectra exhibit only a sp2 CH-related C-H stretch band a nd, accordingly, from EELS increased sp2 bonding in the annealed films is obtained. This stability sequence of CH(x) groups at the carbon ne twork is in accordance with the expectations drawn from kinetic and th ermodynamic data of hydrocarbons. The chemical structure of the deposi ted C:H films is insensitive to the nature of the deposition ion, e.g. , methane, ethane, ethylene, or benzene, from which it is suggested th at specific C-H bonding in the deposition ions does not act as a precu rsor 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.