Self-assembled perfluoroalkylsilane films on silicon substrates for hydrophobic coatings

Abstract

In this work, four novel perfluoroalkylsilanes were included to fabricate hydrophobic coatings on silicon substrates by the method of self-assembled films. The surface chemical compositions and structures of self-assembled perfluoroalkylsilane films were analyzed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectrometer, and results showed that perfluoroalkylsilane molecules have uniformly assembled onto the surface of hydroxylated silicon substrate successfully. Water contact angles and surface free energy were tested to characterize the surface wettability of self-assembled perfluoroalkylsilane films. Results showed that the self-assembled bisfluorooctyltrichlorosilane film with annealing process (BPFOTS-Si-heat) had the highest water contact angle as 117.6 ± 0.9° and hexadecane contact angle as 76.7 ± 1.3°. This may be attributed to the incorporation of two longer perfluoroalkyl groups into the silane molecule. On the other hand, annealing process also contributed much to improve the hydrophobicity of self-assembled films. The adhesive force of annealed self-assembled perfluoroalkylsilane films was lower than that of untreated ones. Dynamic contact angle measurement showed that the BPFOTS-Si-heat had the smallest water contact angle hysteresis (14 ± 2°) and hexadecane contact angle hysteresis (10 ± 2°). Surface morphology of self-assembled films with different assembling time was obtained through atomic force microscopy, and the surface was found to be gradually covered by dense sharp peaks with the extension of assembling time.