Water enrichment/depletion of amorphous carbon coatings probed by temperature-dependent dc electrical conductivity and Raman scattering

Abstract

Amorphous carbon (a-C) is an appealing disordered multi-phase material widely used as a cost-effective barrier coating (10–100 nm thick) for silica optical fibers to protect them from harsh environments. Those coatings prevent strength degradation of the optical fibers by moisture and, also block hydrogen diffusion into the fiber. In this paper, we experimentally study the in situ temperature-dependent dc electrical conductivity and Raman scattering of dried/wetted a-C coatings under multiple heating/cooling cycles within the range of 40–120 °C. At temperatures of above 80 °C, we observe hysteresis in both the dc electrical resistance and the Raman scattering intensity. One of the mechanisms is related to the escape of intercalated (or spatially confined) water from the a-C coating, causing the resistance decrease due to a closing of the bandgap. The opposite is observed when some water molecules experience dissociative absorption at the edge and basal defects that may be decorated with CH, COOH/COH and COC/CO functional groups. The content of water within the a-C coating is quantified through relative contributions of the water and edge defect related Raman bands. This study expands our knowledge about a-C ultrathin films, originally designed as barrier coatings, towards promising ultracompact humidity sensors and next-generation adsorbents.