Optical absorption in ZrO2–x-C nanocomposite films on polymer substrates: application of an effective medium theory
Résumé
RF sputtering from separate zirconia and graphite targets enables room-temperature co-deposition of ZrO2–x and carbon onto polycarbonate substrates. Films consist of amorphous carbon dispersed in tetragonal nanocrystalline zirconia; their optical properties have been determined by reflectance measurements at normal incidence and over the visible range. The optical constants of each component of the composite have been extracted; the experimental reflectance spectra have been fitted to Maxwell-Garnett effective medium theory, leading to the effective dielectric constants of the film. Absorption is characterized by the band gap, the energy and lifetime of the optical transition of the absorbing carbon phase. These parameters are deduced from the imaginary part of the dielectric function using Tauc-Lorentz model (TL) and from the extinction coefficient using Forouhi-Bloomer model (FB) in which an energy dependence of the matrix element of the transition has been introduced. Furthermore, the energies of the peak transition and the maximum of the extinction coefficient are so similar that they prove the self-consistency of the modified-FB model applied to the amorphous carbon phase of the composite films.
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