Surface modification of nanocrystalline diamond/amorphous carbon composite films

The surfaces of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films deposited from a 17% CH₄/N₂ mixture have been subjected to a variety of plasma and chemical treatments, namely H₂ and O₂ microwave plasmas, a CHF₃ 13.56 MHz plasma, and a chemical treatment with aqua regia (HCl:HNO₃ 3:1). The resulting surfaces have been studied with respect to their chemical nature by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS), concerning their morphology with atomic force microscopy, and by contact angle measurements to study their hydrophobicity and their stability. As-grown surfaces are hydrogen terminated, but the number of C–H bonds can slightly be increased by a H₂ microwave plasma, while treatment with aqua regia considerably lowers the number of C–H bonds at the surface. O₂ and CHF₃ plasmas, on the other hand, lead to a replacement of the terminating C–H bonds by C–O or C–OH and C–F_x groups, respectively. Finally, by contact angle measurements over a period of 150 days it could be shown that the H-terminated surface is very stable whereas the contact angle of the O-treated surface changed considerably with time, probably due to the adsorption of contaminants.