Spectres optiques de lames minces de carbone. effets de traitments thermiques

The optical study of turbostratic carbons was undertaken after having succeeded in making transparent carbon films of different thicknesses. The fundamental optical spectra ϵ 1 and σ have been plotted as a function of the energy hω of the incident photons in the infrared range. The X-ray study has shown that the sublimation of graphite in vacuum leads to a real ‘degraphitization’ if the vapour is collected on a cold surface; such films have optical properties quite different when compared with graphite. This type of carbon can be compared to a semiconductor with a gap of 0·2 eV but it contains some permanent free carriers. When these films are heated in vacuum up to 900°C, their structure changes into an oriented model of turbostratic carbon (1000°C). Their electronic properties can be compared with those of graphite: the optical conductivity σ comes close to that of graphite while ϵ 1 takes lower (and always negative) values than before the heat treatment. This fact shows that the free carriers are more numerous than in graphite, because of the presence of a great number of defects which act as electron traps. These results agree fairly well with the schema proposed by Mrozowski for graphitizable carbons.