Absolute surface density of the amine group of the aminosilylated thin layers: Ultraviolet-visible spectroscopy, second harmonic generation, and synchrotron-radiation photoelectron spectroscopy study

The surface of a fused silica and oxidized silicon wafer (SiO2/Si(100)) was treated with (3-aminopropyl)triethoxysilane (1), (3-aminopropyl)diethoxymethylsilane (2), and (3-aminopropyl)ethoxydimethylsilane (3) for 72 h in solution. Thickness of the aminosilylated film out of 1 increases rapidly, and it turns out to be around 100 A in 72 h. Rather slow increase of the thickness is observed for 2, which produces the layers of 8 and 14 Angstrom in 10 and 72 h, respectively. The reagent 3 produces the monolayers of constant thickness 7-8 Angstrom during the whale span of the reaction. The aminosilane layer was allowed to react with 4-nitrobenzaldehyde to form an imine, and absorbance (A(surf)) of the imine was recorded by UV-vis spectroscopy. The imine was hydrolyzed in a known volume of water to produce 4-nitrobenzaldehyde, and subsequently its absorbance was measured. Thus observed number of the aldehyde molecules is equivalent to the number of the imine molecules on the surface. With the known surface area of the substrates, the absolute surface density of the imine, that is, the reactive amine group on the surface, was calculated. At an early stage of the aminosilylation with 2, a surface density of 3.9 amine groups per 100 Angstrom(2) was measured. A relatively low value was observed for 3. It is also observed that 1 produces a multilayer with rough surface morphology, of which surface density increases sharply by the reaction time. Tilt angle of the imine was estimated from the A(surf) and the absolute surface density. The angles vary among 23-47 degrees depending on the reaction condition. Second harmonic generation of a laser light of 1064 nm was observed for the imine-formed substrates. X-ray photoelectron spectroscopy utilizing a synchrotron radiation source, in particular for the N(1s) binding energy region, confirmed the chemical transformation. Also, it is observed that the nitro group of the imine is cleaved by X-rays.