Organosilane self-assembled monolayers formed at the vapour/solid interface

Self-assembly of organosilane molecules at the vapour/solid interface under atmospheric pressure conditions was explored in order to form a monolayer on oxide-covered silicon substrates. Three types of precursors were employed: n-octadecyltrimethoxysilane (ODS: H3C(CH2)(17)Si(OCH3)(3)), n-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPS: H2N(CH2)(6)NH(CH2)(3)Si(OCH3)(3)) and fluoroalkyl-silane (FAS: heptadecafluoro-1,1,2,2-tetrahydro-decyl-l-trimethoxysilane, F3C(CF2)(7)(CH2)(2)Si(OCH3)(3))Characteristics of these self-assembled monolayers (SAMs) formed at a temperature of 100-150degreesC were studied based on ellipsometry and x-ray photoelectron spectroscopy. Chemical properties of the monolayers were characterized further by water contact angle and zeta-potential measurements. The SAMs formed from ODS and FAS were hydrophobic so as to show water contact angles of >100degrees, whereas that of the SAM formed from AHAPS was similar to60degrees. zeta-potentials and isoelectric points (IEPs) Of SiO2 substrates covered with each of the SAMs were measured and compared with a naked SiO2 substrate. The IEPs of the SiO2 substrate covered with the ODS- or FAS-SAM was pH 3.5-4.0, which was almost the same as those of polyethylene and polytetrafluoroethylene plates, whereas that of the naked SiO2 substrate was pH similar to2.0, as predicted from results on silica particles. In the case of the AHAPS-SAM, its surface was positively charged under acidic conditions due to protonation of surface amino groups. Consequently, the AHAPS-SAM-covered substrate showed an IEP of pH 7.5-8.0. Copyright (C) 2002 John Wiley Sons, Ltd.