Abstract:
© 2020 American Chemical Society. As-thermal-CVD-synthesized amorphous carbon (a-C) ultrathin coatings enable silica optical fibers to be mechanically robust and optically transparent in aggressive environments. This is achieved by mitigating both hydrogen diffusion and water ingression onto the fiber surface. Notwithstanding the advanced performance of CVD-deposited a-C films, their functionality may suffer from structural and topology defects, dopants, intrinsic stresses, and other imperfections. In this paper, we experimentally study the influence of water-anchored edge functional groups on dc electrical conductivity and Raman scattering of a-C films, ranging from 10 to 100 nm in thickness, exposed to thermal and electrical annealing. A series of heating/cooling cycles was found to affect the disorder D-family bands in the Raman spectra of a-C. In particular, two new Raman peaks at 1260 and 1400 cm-1, attributed to carboxyl (COOH) and hydroxyl (C-OH) groups, have been unraveled through polarization-controlled far-field Raman spectroscopy and tip-enhanced Raman scattering (TERS) spectroscopy. The TERS technique allows one to readily resolve the broad disorder D-family bands due to the excitation of coherent Raman scatters (nanosized graphitic clusters). A temperature-sensitive and thickness-dependent hysteresis in dc conductivity, observed at temperatures of above 80 °C, is explained by physical sorption/desorption of water at the surface of graphite-like crystals through hydrogen bonding and dissociative absorption of water with the formation of extra COOH/C-OH groups at the edge defects. The electroheating leads to a nonuniform distribution of the water-decorated edges owing to hot spots of a percolated carbon crystals network. This is directly evidenced by electroassisted TERS mapping of the a-C coating. We believe that our study will pave a way for the understanding of how water interacts with amorphous carbon and improve the performance of carbon coatings in harsh environments.
