Temperature and chemical sensitivity of carbon films on quartz

Insulating to conductive carbon films were obtained by controlled vacuum annealing of the disordered carbon films deposited on quartz by sputtering of carbon. The change of conductance of the films has been studied on exposure to the vapors of NH3 and NO2, chosen for their opposite donor-acceptor properties. The chemical sensitivity of conductance along with temperature sensitivity is discussed in terms of atomic ordering of the as-deposited films induced by heating at high temperatures and revealed by Raman scattering measurements. The observed dependence of conductance on temperature is explained by a model based on the charge tunneling through thermal vibrational barriers, the average height and width of which change with atomic ordering. Phenomenological description of chemical sensing by the films is given in terms of the time constant τ and the maximum relative change in conductance β, related to the surface and material properties of the films respectively. The expression σ = β/τ is considered a measure of the chemical sensitivity of the films and changes with modifications in the material structure of the films on annealing.