Water-repellent thin films from mixtures of fluoroalkylmethoxysilane and bis-(trialkoxysilyl)alkanes of various carbon-chain lengths using the sol–gel method and the fluoroalkylmethoxysilane dispersion mechanism

Water-repellent thin films were fabricated on silicon or germanium wafers by spin-coating sol mixtures of fluoroalkyalkoxysilane (FAS) and bis-(trialkoxysilyl)alkanes with various carbon-chain lengths (Cn: (C2H5O)3Si(CH2)nSi(OC2H5)3, and Cf: (CH3O)3SiC2H4(CF2)6C2H4Si(OCH3)3; n=1, 2, and 8. C0 is defined as Si(OC2H5)4). Before heat treatment, the contact angle of hexadecane on the films increased in the order Cf, C1, C0, C2 and C8 when the FAS concentration in the sol mixture was above 5 mmol/l. With heat treatment higher than 70 °C, the contact angles on the films decreased and the longer the carbon-chain lengths of Cn and Cf the greater the decrease. We show that during spin-coating of the Cn films, FAS accumulates on the outer surface of the film. In Cn films, with n=0, separation of FAS is governed by the repulsion between the non-polar and polar chemical groups while with n≧1, separation is governed by repulsion between the fluoroalkyl-chains and the alkyl-chains. It is speculated that heat treatment causes FAS evaporation, which increases as the carbon-chain length of Cn or Cf increases, owing to the differences in their readiness to undergo hydration.