Fluoroalkylsilane monolayers formed by chemical vapor surface modification on hydroxylated oxide surfaces

Water-repellent surfaces have been prepared by exposing Si substrates with a hydroxylated surface oxide to fluoroalkyl silane (FAS) vapor. Since this chemical vapor surface modification (CVSM) is based on the chemical reaction between organosilane molecules and hydroxyl groups at the oxide surface, prior to CVSM, the substrate surface was completely hydroxylated by irradiating in air with a 172-nm ultraviolet light until the water contact angle of the surface became almost 0 degrees. Under atmospheric pressure, the substrate was then exposed to vapor of an FAS precursor, that is, one of three types of FAS having different perfluoroalkyl chain lengths [CF3(CF2)(n)CH2CH2Si(OCH3)(3), where n = 0, 5, or 7, referred to as FAS-3, FAS-13, and FAS-17, respectively]. The FAS molecules chemically reacted with the hydroxyl groups on the substrate surface and adsorbed onto it, forming a thin layer of less than 2 nm in thickness, The water repellency of the substrate surface increased with an increase in perfluoroalkyl chain length. The maximum water-contact angles of the substrates treated with FAS-3, -13, and -17 were ca. 86 degrees, 106 degrees, and 112 degrees, respectively.