Structural analysis of hydrogenated diamond-like carbon films from electron energy loss spectroscopy

Electron energy loss spectral analysis of the π and (π + σ) electron plasma resonances are used to analyze the structure of hydrogenated diamond-like carbon (a-C:H) films. Energy loss peaks associated with the resonances of the π and (π + σ) plasmons in a-C:H are identified by comparison with reference spectra taken on natural diamond and on highly oriented pyrolytic graphite. The decrease in energy of the π plasmon with increasing hydrogen atom fraction provides direct experimental evidence that addition of hydrogen serves to reduce the density of π bonds in a-C:H. Under several important assumptions, the mass density, the sp³/sp² site ratio, and the average coordination number are related to the resonance energies of the π and (π + σ) plasmons. The mass density of a-C:H samples inferred from the energy of (π + σ) electron plasma resonance is in the range from 1.46 to 1.69 g/cm³, which is in general agreement with an independent sink-float measurement. The ratio of sp³ (tetrahedral) to sp² (trigonal) carbon sites increases from 0.29 to 0.75 and the average coordination number of each atomic site decreases from 2.6 to 2.3 as the hydrogen increases from 28 to 44 at.%. The fully constrained covalent network model is used to discuss the experimental results. The measured ratio of sp³/sp² carbon sites and the average coordination numbers are in agreement with the predictions of the model, particularly at high hydrogen concentration.