Chemical and thermal stability of carbyne-like structures in cluster-assembled carbon films

Nanostructured carbon films consisting of sp chains ~polyynes and polycumulenes! embedded in an sp2 matrix are grown using supersonic carbon cluster beam deposition in ultrahigh vacuum at room temperature. All the specimens have been analyzed by in situ Raman spectroscopy. The use of different excitation wavelengths ~532 and 632.8 nm! confirms the presence of distinct carbynoid species. Chemical stability of the sp species has been studied by exposing the as-deposited films to 500 mbar of H2, He, N2, and dry air. Gas exposure produces an exponential decay of the carbynoid fraction slightly affecting the sp2 component. Helium, hydrogen, and nitrogen do not chemically interact with the sp chains whereas oxygen reacts with the carbynoids species causing their fast and almost complete destruction. The films have been also thermally annealed at 20°, 100°, 150°, and 200 °C. The amount of carbynoid species is rapidly and strongly reduced at temperature larger than room temperature. The relevance for material science and interstellar chemistry of the production of a bulk form of carbon where sp and sp2 hybridizations coexist is addressed.