In order to understand better the electronic properties of π-bonded materials, some optical and photoluminescence properties of amorphous carbon films have been investigated as a function of film density (0.9 to 1.7 g·cm-3). This study gives an overview of the radiative recombination properties in relation with local structure characterizations (in situ infrared ellipsometry and Raman spectroscopies) for a series of dual-plasma deposited polymer-like a-C:H films. Medium range topology has consequences in the hyperconjugation effects seen on infrared bands, as well as in optical and resonant Raman characteristics. Photoluminescence (PL) excitation spectroscopy reveals resonance features which are attributed to exciton-like electron-hole pairs in close Coulomb interaction. The PL efficiency shows a sharp quenching for densities above 1.3 g·cm-3 where a clear transition also occurs in the Raman "fingerprint". In addition, quantitative analysis of IR ellipsometry and Elastic Recoil Detection give evidence of a strong decrease of both the effective dynamical charge e*(C-H) and the bandwidth of sp3 C-H vibrations; this is interpreted as being a result of the increase of local strains in the carbon skeleton, meaning that matrix distortions already appear at H content values as high as 46 H at.% due to film densification. An expected consequence is the mixing between π and σ molecular orbitals and the enhancement of the dissociation rate of confined electron-hole pairs. PL quenching would thus result from both a decrease of exciton confinement and an increase of the density of accessible nonradiative centers.
Infrared Ellipsometry Analysis of Heritage Photographic Prints
