Optimization of mica surface hydroxylation in water vapor plasma monitored by optical emission spectroscopy

Abstract

Mica, a mineral consisting of alumino-silicate layers, can be cleaved to obtain atomically smooth surfaces, and it is often used as a model substrate for surface force measurements. Due to mica's chemical inertness, covalent surface modifications are not straightforward. By applying a water vapor plasma treatment, reactive silanol groups can be generated on the surface. Up to now, the optimization of the plasma process was a time-consuming trial-and-error process. Furthermore, no clear correlation between the plasma parameters and the hydroxyl surface density was elucidated. To overcome such limitations, we invoke in-situ optical emission spectroscopy (OES). A correlation between the pressure-normalized OES hydroxyl intensity and the surface density of hydroxyl groups was found, which allows real-time optimization of the plasma conditions. The density of silanol groups was quantified via chemical derivatization X-ray photoelectron spectroscopy (XPS) using a fluorinated monochlorosilane. An apparent upper limit for the hydroxyl surface density of 2.7 SiOH/nm2 was found—the limitation being imposed by packing restraints of the derivatizing reagent. While real surface density may be underestimated by derivatization, the obtained maximum value is an order of magnitude higher than obtained from wet-chemical hydroxylation, and, it is above the hitherto highest reported values for water vapor plasma treatment.