Quartz crystal microbalance (QCM) oscillators, or more generally, thickness shear mode (TSM) oscillators, have evolved from being used for thickness monitoring in vacuum deposition systems towards their use as electrochemical and biological sensors in liquid and gaseous environments. Because of that, studies of electrical and/or electrochemical properties, surface wettability and stability of biological functionalization of such oscillators have gained importance.
The use of diamond as a surface in such TSM or QCM oscillators for many biological and electrochemical applications would be interesting because diamond has a wide electrochemical window, tuneable wettability by simple stable surface terminations, low friction, extreme chemical stability and bio-inertness. The surface of diamond has been shown to be particularly stable when functionalized with bio-molecules.
In the Journal of the Electrochemical Society 149 (11), H179-H182 (2002), Zhang et al. describe the fabrication of a diamond-coated microbalance electrode by growing a freestanding diamond film and subsequently binding it to a QCM element.
In Diamond & Related Materials (2206), Volume 16, Issues 4-7, April-July 2007, Pages 966-969, Hakiki et al. describe the growth of diamond on langasite for surface acoustic wave (SAW) applications. As the adhesion of diamond on langasite is poor, the stress of the diamond layer should be low. This can be achieved by using a hydrogen-poor plasma during diamond growth, resulting in a diamond layer having a high sp2 bonding, i.e. graphite bonding. sp2 bonding is known to reduce the electrochemical window of diamond (Fausett et al, in Electroanalysis 12 (2000) 7. When applying voltages to electrodes in solution, sp2 or graphite bonding results in redox deterioration of the diamond layer and thus of the electrodes already at relatively low voltages, for example at 0.5V, 1V or higher. Direct growth of diamond on langasite, i.e. growth of diamond on langasite without interlayer, thus results in low quality diamond (i.e. high sp2 content or graphite bonding) with reduced stress to avoid delamination from the langasite. This low stress is achieved by using hydrogen poor (argon/methane) plasma to grow the diamond, which is known to produce material with more than 5% sp2 bonding. (See D. M. Gruen, in Annual Reviews of Materials Science, 29 (1999) 211.)