| Abstract
| - Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to measure the molecularorientation in semifluorinated self-assembled monolayers (SAMs) prepared by vapor deposition of mono-(F3C(CF2)8(CH2)2Si(CH3)2Cl, m-F8H2), di- (F3C(CF2)8(CH2)2Si(CH3)Cl2, d-F8H2), and trichloroorganosilanes(F3C(CF2)8(CH2)2SiCl3, t-F8H2) on flat silica-covered substrates. The average tilt angles (from the samplenormal) of the fluorocarbon part, F(CF2)8-, of t-F8H2, d-F8H2, and m-F8H2 measured by carbon K-edgeNEXAFS are 10 ± 2°, 35 ± 2°, and 45 ± 3°, respectively. We show that the increase of the tilt angle isassociated 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 toestimate the grafting densities of the F8H2 molecules on the substrates. We present a simple one-dimensionalgeometric model to show that the grafting density of m-F8H2 is approximately one-half of that correspondingto the t-F8H2 SAM. Finally, we show that the results of this simple model are in accord with estimatesof the two-dimensional fluorine areal density obtained from the fluorine K-edge NEXAFS spectra.
|
