The importance of precursor molecules symmetry in the formation of self-assembled monolayers

We investigated the role of precursors molecular symmetry in the self-assembled formation of organosilane monolayer by applying atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD), Fourier transform infrared spectroscopy (FT-IR) and contact angle measurement. Octadencyltrichlorosilane (OTS), octadecylmethydichlorosilane (ODCS), and octadecyldimethylchlorosilane (ODMS) were used as raw materials to form organosilane monolayers. The organosilane monolayers were prepared by a liquid phase method at a reaction temperature of 5 degrees C and an humidity in reactor of 50%. AFM observations suggested that only OTS molecules grew two-dimensionally on the Si substrate surface. In GIXD results showed these OTS molecules to be arranged in a crystalline state on the Si surface. Thus "self-organization" occurred during OTS monolayer formation process. However, in the case of ODCS and ODMS, the monolayers formed were in an amorphous state. In addition, FT-IR and water contact angle measurement indicated that the packing density of the OTS monolayer was higher than that of the ODCS and ODMS monolayers. Only when the precursor molecules had an axis where the silanol groups were in three-fold did "self-organization" occurs in the formation of the organosiloxane monolayer.