A comparative study of the growth of octadecyltrichlorosilane and 3-mercaptopropyltrimethoxysilane self-assembled monolayers on hydrophilic silicon surfaces

Abstract

Self-assembled monolayers of two different organosilane precursors, methyl-terminated nonpolar n-octadecyltrichlorosilane (OTS, Cl 3Si(CH 2) 17CH 3) and thiol-terminated polar 3-mercaptopropyltrimethoxysilane (MPTMS, (CH 3O) 3SiCH 2CH 2CH 2SH), were prepared separately on hydrophilic silicon surfaces by immersion in millimolar solutions of the respective precursors in toluene at room temperature. Ex situ atomic force microscopy (AFM), lateral force microscopy (LFM) and X-ray photoelectron spectroscopy (XPS) were used to study the growth and the properties of OTS and MPTMS SAMs. For OTS SAMs, generally speaking, small islands surrounded large dendrite-shaped islands. But for MPTMS SAMs, sporadic small round islands appeared, but no dendrites. The impact of the solution age was more significant on the growth of OTS SAMs than MPTMS SAMs. At the same precursor concentration and solution age, the growth of OTS SAMs was much faster than MPTMS SAMs due to the greater hydrolysis ability of Si–Cl bonds in OTS as compared with that of Si–OCH 3 bonds in MPTMS. The difference in hydrolysis ability was confirmed by the absence of a Cl signal in the XP spectrum of OTS SAMs and the existence of a C 1s peak corresponding to unhydrolyzed Si–OCH 3 bonds in the XP spectrum of MPTMS SAMs. This trend together with the difference in alkyl chain length had a strong influence on the surface morphology and coverage of these two SAMs. According to the individual adsorption behavior of the components, the predictable kinetic difficulty of preparing OTS/MPTMS mixed SAMs by co-adsorption is pointed out. Furthermore, a potential reaction condition for stepwise adsorption is suggested.