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

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Abstract
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.