Characterization of a Homogeneously Mixed Octyltriethoxyilane/Octadecyltrichlorosilane Self-Assembled Monolayer and Analysis of Its Formation Mechanism

The wettability, surface topography, surface potential, and degree of order of a homogeneously mixed octyltriethoxysilane (C8TES)/octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) were characterized by means of contact angle analysis, atomic force microscopy (AFM), electrostatic force microscopy (EFM), and Fourier transform infrared (FTIR) spectroscopy. The formation mechanism of the SAM was studied as the monolayer was constructed using a stepwise approach, taking advantage of the steric hindrance of C8TES. The SAM was found to have a contact angle of 105 and the formation process of the mixed SAM was obviously different from the SAM formation mechanisms of both pure C8TES and OTS. The AFM and EFM characterizations indicated that the mixed SAM had a smooth surface and a homogeneous surface potential distribution with a typical, normal surface potential frequency distribution. The internal structure was highly homogeneous over regions ranging from 500 nm x 500 nm to 20 mu m x 20 pm in size. The FTIR analysis indicated that the mixed SAM had a double-layer film structure, and that the molecular densities were different in the two layers, with the layer closer to the substrate being denser. This work shows that steric hindrance effects can be used to allow the stepwise formation of homogeneously mixed SAMs, and that this method is especially applicable to the construction of the homogeneously mixed SAMs composed of two types of molecules having different head groups.