The experimental conditions and the apparatus used for the diamond film growth have been described in detail elsewhere [40]. The films were prepared by deposition of the BDD thin films on highly conductive n-Si (1 1 1) substrates by microwave plasmaassisted
chemical vapor deposition. Deposition was usually carried out for 10 h to achieve a film thickness of approximately 30m.The nominal B/C atomic ratio in the gas phase was 1:100, and the typical boron-doping level in the film was ca. 1021 cm−3. The BDD electrodes were rinsed with ultra-pure water prior to use.