Heat capacity of hydrogenated diamond-like carbon films

We have used differential scanning calorimetry to measure the heat capacity of diamond-like carbon (DLC) film prepared by a plasma immersion ion processing method. The same calorimeter was used to measure the heat capacity of single crystal natural diamond and of high purity graphite. The amount of atomic hydrogen trapped in the DLC films was determined by elastic-recoil-detection spectrometry. The present data and literature values were used to deduce an expression for the specific heat that factors out the contribution from the specific bonding and from the atomic hydrogen trapped in the carbon. The data shows that the hydrogen contribution to the specific heat of carbon is independent of the sp³/sp₂ bonding and amounts to about 0.63k_B per hydrogen atom. We propose a simple method to determine the sp³/sp₂ bonding ratio in hydrogenated DLC films based on measuring the specific heat and the hydrogen content of the sample.