Femtosecond pulsed laser deposition and optical properties of diamond-like amorphous carbon films embedded with sp-bonded carbon chains

Diamond-like amorphous carbon films have been deposited on silicon and quartz substrates by ablation of graphite using 120 fs pulses from an amplified Ti:sapphire laser operating at 800 nm. The results clearly show that fs-DLC films are composed of sp, in addition to sp(2) and sp(3) bonded species. We find that the sp concentration can be somewhat increased by deposition in vacuum at liquid nitrogen temperature. Vibrational modes involving sp, sp(2) and sp(3) bonds in putative molecular species in these materials have also been studied using surface enhanced Raman spectroscopy (SERS) and these results are compared to DFT simulations of Raman spectra of several linear carbon chains. It is found that the chain length of sp-bonded carbon species increases when deposition occurs in a He buffer gas indicating that sp-bonded chains present in the gas phase also become incorporated in the film during condensation. SERS spectra together with spectral simulations using DFT show that fs-DLC contains a limited number of well-defined structural components formed as the result of mixed sp, sp(2) and sp(3) bonding. (C) 2008 Elsevier B.V. All rights reserved.