Structural and optical properties of brominated plasma polymers

Novel brominated amorphous hydrogenated carbon (a-C:H:Br) films were produced by the plasma polymerization of acetylene-bromoform mixtures. The main parameter of interest was the degree of bromination, which depends on the partial pressure of bromoform in the plasma feed, expressed as a percentage of the total pressure, RB. When bromoform is present in the feed, deposition rates of up to about 110 nm min− 1 may be obtained. The structure and composition of the films were characterized by Transmission Infrared Reflection Absorption Spectroscopy (IRRAS) and X-ray Photo-electron Spectroscopy (XPS). The latter revealed that films with atomic ratios Br:C of up to 0.58 may be produced. Surface contact angles, measured using goniometry, could be increased from ~ 63° (for an unbrominated film) to ~ 90° for RB of 60 to 80%. Film surface roughness, measured using a profilometer, does not depend strongly on RB. Optical properties - the refractive index, n, absorption coefficient, α(E), where E is the photon energy, and the optical gap, Eg, were determined from film thicknesses and data obtained by Transmission Ultraviolet-Visible Near Infrared Spectroscopy (UVS). Control of n was possible via selection of RB. The measured optical gap increases with increasing FBC, the atomic ratio of Br to C in the film, and semi-empirical modeling accounts for this tendency. A typical hardness of the brominated films, determined via nano-indentation, was ~ 0.5 GPa.