Mechanism of enhanced wettability of nanocrystalline diamond films by plasma treatment

The mechanism of wetting behavior of nanocrystalline diamond films is examined in terms of surface free energy, morphology, and bonding characteristics. The films are prepared by microwave plasma-enhanced chemical vapor deposition using Ar-rich/N ₂/CH ₄ and Ar-rich/H ₂/CH ₄ mixtures, followed by microwave hydrogen and oxygen plasma exposures separately. Contact angle measurement with water, ethylene glycol, and formamide reveals that both the as-deposited and hydrogen plasma treated films are hydrophobic, while the oxygen plasma treated film is extremely hydrophilic such that the contact angle is reduced down to almost zero degree. Fourier transform infrared spectroscopy reveals that the hydrogen atoms are dominantly bonded to diamond and amorphous sp ³-bonded carbon, and they are removed by the oxygen plasma treatment. For the oxygen plasma treated film, the mean value of oxygen concentration for the top surface to bulk (∼ 1 μm) measured by energy-dispersive X-ray spectroscopy is ∼ 10 at.%, while that for the top several monolayers surface measured by X-ray photoelectron spectroscopy is much higher at ∼ 37 at.%, indicating a higher degree of oxidation toward the surface. The carbon bonding state in the oxidized layer is disordered by incorporation of a large amount of oxygen in form of polar CO bonds, which is accountable for a greater polar component of the apparent surface free energy and stronger dipole-dipole interactions.