Chemical vapor deposition of fluoroalkylsilane self-assembled monolayers.

Abstract

Surface chemistry manipulation by grafting molecular chains of various chemistries has garnered increasing attention in recent decades. By designing molecular-scale surface mono-/multi-layers, many essential surface properties can be achieved, such as anti-stiction, slipperiness, and liquid-repellency. Organosilane-based self-assembled monolayers can be considered more durable alternatives for air-mediated super-liquid-repellent surfaces, as the latter relies on fragile surface structures. Nevertheless, molecular-scale defects can significantly affect the performance of the deposited layers. Contact angle hysteresis, as a measure of surface homogeneity, has been widely utilized as a measure of homogeneity for modified surfaces. Although alkylsilane and silicone-based layers have often exhibited low hysteresis values, the use of more rigid variants, such as fluoroalkylsilanes, results in higher hysteresis. This is because of the lack of self-smoothing behavior observed in more flexible structures, leading local defects to be spontaneously repaired after deposition. Accordingly, fluoroalkylsilanes are much more sensitive to deposition defects. The present thesis investigates the effect of moisture content on the homogeneity of fluoroalkylated surfaces fabricated by chemical vapor deposition. It is shown that high-quality and highly repeatable fluoroalkylated surfaces with hysteresis values comparable to alkylsilanes can be fabricated using a simple, custom-built reactor. For this purpose, trichloro(1H,1H,2H,2H-perfluorooctyl)silane is used as a precursor. Substrates are exposed to various levels of humidity inside the reactor prior to deposition to control the vapor-phase and surface-bound water content. The procedure suggested in this thesis can guide the fabrication of low-defect fluoroalkylated layers showing liquid-like properties regardless of their high molecular rigidity.