Nanostructure and sp¹/sp² clustering in tetrahedral amorphous carbon thin films grown by femtosecond laser deposition

Diamondlike amorphous carbon films have been deposited on silicon and quartz substrates by laser ablation of graphite using 120 fs pulses from an amplified Ti:sapphire laser operating at 800 nm. Ultraviolet/visible (UV/VIS) and micro-Raman spectra of these materials have shown that the sp³-bonded carbon fraction in these films is ≈27%, 55%, and 20% when deposition occurs at substrate temperatures of 77, 300, and 573 K, respectively. The presence of sp¹ chains in these films is indicated by the appearance of an excitation band at 2000-2100 cm^-1 in UV-Raman spectra. We also find a remarkable increase in the Tauc energy gap calculated from in situ UV/VIS optical spectra immediately after exposure to air together with a 1 eV redshift of the C 1s core-level energy in x-ray photoelectron spectra of these samples. The properties of sp¹-, sp²- and sp³-bonded components of these materials have also been studied using UV/VIS Raman spectroscopy. The enhanced stability of sp¹ chains in tetrahedral carbon matrix is discussed. The present study sheds light on novel tetrahedral carbon materials embedded with both sp¹ chains and sp² clusters.