Dielectric transition of nanostructured diamond films

The dielectric behavior of nanostructured diamond films has been investigated by using an impedance analyzer up to 500°C. Impedance data are presented in the form of the Cole-Cole plot. It is found that: (i) the resistivity contributed both from bulk grain interior and grain boundary decreases with increasing temperature; (ii) above 250°C, the impurities at grain boundaries are thermally activated, and thus contribute to the dielectric relaxation; and (iii) the electrical conductivity of diamond films follows an Arrhenius law with an activation energy transition from 0.13 to 0.67 eV at 250°C. Similar activation energy is found for the Arrhenius plot of relaxation frequencies from 0.14 to 0.73 eV. The dielectric transition is explained as the change of crystal field caused by the thermal expansion or by surface bond contraction of nanosized particles.