Electrochemical cathodic treatment: A non-destructive way to hydrogenate conductive ultrananocrystalline diamond films

Abstract

The excellent conductivity of nitrogen-doped ultrananocrystalline diamond (UNCD) films mainly originates from sp2-bonded carbon located in the grain boundaries. But conventional hydrogen plasma treatment to achieve surface hydrogenation would preferentially etch sp2-bonded carbon and deteriorate the conductivity. In this article, an electrochemical cathodic treatment (ECT) was developed as an alternative method to hydrogenate the surface of nitrogen-doped conductive UNCD films. The treatment was done in acidic solution at −30 V for 10 min. X-ray photoelectron spectroscopy revealed that, after the ECT, the photoelectron subpeaks related to carbon–oxygen bonding significantly decreased and there was no obvious variation in the content of sp2-bonded carbon. Compared to pristine UNCD (P-UNCD) films, the hydrogenated UNCD (H-UNCD) films exhibited a marked change in electrochemical responses such as cyclic voltammetry, capacitance–voltage and electrochemical impedance spectroscopy, suggesting the different surface termination between two UNCD films. More importantly, Raman spectra, scanning electron microscopy and Hall-effect measurement, revealed that no obvious variations in atomic bonds and surface morphology as well as electrical properties existed before and after the ECT. All of these results demonstrated that ECT can be a non-destructive surface hydrogenation method for nitrogen-doped conductive UNCD films with a high content of sp2-bonded carbon atoms.