Formation of uniform aminosilane thin layers: An imine formation to measure relative surface density of the amine group

The surface of a fused silica and oxidized silicon wafer (SiO2/Si(100)) was treated with (4-aminophenyl)-trimethoxysilane (1), (3-aminopropyl)triethoxysilane (2), (3-aminopropyl)diethoxymethylsilane (3), and (3-aminopropyl)ethoxydimethylsilane (4) in solution. The thickness of thus formed aminosilane layers was determined with ellipsometry. In most cases silane coupling agents produce monolayers of 6-10 Angstrom thickness, but reagent 2 gives multilayers with variable thickness (6-100 Angstrom) depending upon the dipping time. The aminosilane layers were allowed to react with 4-nitrobenzaldehyde, and formation of the corresponding imines was confirmed by UV-vis spectroscopy. Relative surface density of the amines was calculated from the observed absorbance; In aqueous medium the imines were easily hydrolyzed to regenerate the amine group. The process, the formation, and the subsequent hydrolysis of the imines, can be repeated several times without any noticeable degradation of the absorption characteristics. The ellipsometric data and the measured absorbance show that 3 gives the most uniform molecular layer with the highest surface density of the amine functionality. Meanwhile, 2 provides multilayers lacking uniformity, and the other reagents produce uniform thin layers but with lower surface density of the amine.