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 ${\mathit{sp}}^2$]/[${\mathit{sp}}^3$] ratio of about unity. Vibrational spectroscopy reveals that at the film surfaces H atoms are bound to C atoms in sp, aromatic ${\mathit{sp}}^2$, and ${\mathit{sp}}^3$ hybridization states. From the vibrational spectra, the presence of -C≡CH, (aromatic)=CH, and -${\mathrm{CH}}_{\mathit{x}}$, x=1,2,3 groups at the surface is inferred. Upon annealing, the films at higher temperatures, 500–1400 K, sp, ${\mathit{sp}}^3$, and ${\mathit{sp}}^2$ 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 ${\mathit{sp}}^2$ CH-related C-H stretch band and, accordingly, from EELS increased ${\mathit{sp}}^2$ bonding in the annealed films is obtained. This stability sequence of ${\mathrm{CH}}_{\mathit{x}}$ 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 ${\mathit{sp}}^3$ C-H bonding in accordance with a simple picture of the initial growth of the films.