Molecular Orientation and Grafting Density in Semifluorinated Self-Assembled Monolayers of Mono-, Di-, and Trichloro Silanes on Silica Substrates

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to measure the molecular orientation in semifluorinated self-assembled monolayers (SAMs) prepared by vapor deposition of mono- (F 3 C(CF 2 ) 8 (CH 2 ) 2 Si(CH 3 ) 2 Cl, m-F8H2), di- (F 3 C(CF 2 ) 8 (CH 2 ) 2 Si(CH 3 )Cl 2, d-F8H2), and trichloroorganosilanes (F 3 C(CF 2 ) 8 (CH 2 ) 2 SiCl 3, t-F8H2) on flat silica-covered substrates. The average tilt angles (from the sample normal) of the fluorocarbon part, F(CF 2 ) 8 -, of t-F8H2, d-F8H2, and m-F8H2 measured by carbon K-edge NEXAFS are 10 ± 2°, 35 ± 2°, and 45 ± 3°, respectively. We show that the increase of the tilt angle is associated with the steric hindrance of the methyl groups attached to silicon close to the bonding substrate. We also show that the molecular orientation obtained from the NEXAFS measurements can be used to estimate the grafting densities of the F8H2 molecules on the substrates. We present a simple one-dimensional geometric model to show that the grafting density of m-F8H2 is approximately one-half of that corresponding to the t-F8H2 SAM. Finally, we show that the results of this simple model are in accord with estimates of the two-dimensional fluorine areal density obtained from the fluorine K-edge NEXAFS spectra.