Formation of porosity in sputtered amorphous hydrogenated carbon films

Results are reported of the porosity and the microstructure of amorphous hydrogenated carbon films produced by direct current (d.c.) sputtering. The as-deposited films were initially slightly mesoporous. Heating the material in a vacuum, however, produced a highly porous material. The amount of porosity and the pore size distribution were directly related to the quantity of material which was outgassed during heating. The dependence on the baking temperature of the degree of sp² hybridization in the films had the same form as the dependence of the porosity and of the quantity of outgassing. The plasmon-energy data for the films was related to the process of morphology and composition reconstruction during the heating. The outgassing quantity was strongly dependent on the oxygen content of the as-prepared material. The morphology changes in the material can be regarded as a carbon-activation process involving the incorporation of oxygen in the film during deposition, followed by the outgassing of carbon-oxygen compounds during heating in vacuum. Many of the properties of the hydrogenated amorphous carbon (a-CH) film were very similar to those exhibited by conventional activated carbons, and it is therefore concluded that the porous baked a-CH film is a form of activated carbon.