Ultra-smooth and hydrophobic nitrogen-incorporated ultranano-crystalline diamond film growth in C-H-O-N gas phase system via microwave plasma CVD

The ultra-smooth surface and hydrophobic nitrogen-incorporated ultra-nano-crystalline diamond (N-UNCD) was directly synthesized by equilibrating the etching effect of OH radical and growth promotion of CN and CH in the 5% CH4 added H2 plasma environment with additional feeding of constant 0.16% O2 and from 0.3% to 3.3% N2 contents. The initially decreasing and then increasing trend of surface roughness, i.e., from as-grown appearance with pits to smooth and then to worm-like surface, was resulted from the offset and even suppressive effect on OH etched holes by the faster growth rate that under the help of more N2 promoted chemical groups of CH and CN. In addition, chemical composition, i.e., the large amount of sp2 phases (sp2/sp3 ratio up to 1.399) and H termination of N-UNCD surface (proportion was 75.54 ± 3.8%), played an another enhancing function for repelling water (contact angle from 81.3° to 93.8°) although the surface roughness decreased to smoothest of 1.36 nm in Root-Mean-Square (RMS) in the presence of N2 addition from 0.3% to 1%. However, continuing adding N2 to 3.3%, the formed surface (RMS 8.98 nm) with worm-like ultra-nano diamond crystallites together with higher H reconstructed sp2 contents (sp2/sp3 ratio rose to 1.478) further boosted the hydrophobicity, at which the contact angle was finally increased to 110.2°. Therefore, in C-H-O-N gas system, ultra-smooth and uniform N-UNCD surface with excellent hydrophobicity can be obtained by a one-step method without any post-treatment.