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: H 3 C(CH 2 ) 17 Si(OCH 3 ) 3 ), n ‐(6‐aminohexyl)aminopropyltrimethoxysilane (AHAPS: H 2 N(CH 2 ) 6 NH(CH 2 ) 3 Si(OCH 3 ) 3 ) and fluoroalkylsilane (FAS: heptadecafluoro‐1,1,2,2‐tetrahydro‐decyl‐1‐trimethoxysilane, F 3 C(CF 2 ) 7 (CH 2 ) 2 Si(OCH 3 ) 3 ). Characteristics of these self‐assembled monolayers (SAMs) formed at a temperature of 100–150°C were studied based on ellipsometry and x‐ray photoelectron spectroscopy. Chemical properties of the monolayers were characterized further by water contact angle and ζ‐potential measurements. The SAMs formed from ODS and FAS were hydrophobic so as to show water contact angles of >100°, whereas that of the SAM formed from AHAPS was ∼60°. ζ‐potentials and isoelectric points (IEPs) of SiO 2 substrates covered with each of the SAMs were measured and compared with a naked SiO 2 substrate. The IEPs of the SiO 2 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 SiO 2 substrate was pH ∼2.0, 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.