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

Published

November 1, 2002

Author(s)

Jan Genzer, K Efimenko, Daniel A. Fischer

Abstract

We study the molecular orientation in semifluorinated (F₃C(CF₂)₈(CH₂)_2-, F8H2) self-assembled monolayers (SAMs) prepared by vapor deposition of mono- (m-F8H2), di- (d-F8H2) and trichloro (t-F8H2) organosilanes on flat silica-covered substrates. Near-edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to measure the molecular orientation in m-F8H2, d-F8H2, and t-F8H2 on silica substrates. The average tilt angles of the fluorocarbon part, F(CF₂)_8-, of t-F8H2, d-H8H2, and m-F8H2 measured by 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 F-edge NEXAFS spectra.

Citation

Langmuir

Volume

18

Issue

No. 24

Pub Type

Journals

Keywords

monolayers, NEXAFS, orientation, silanes, soft x-ray

Citation

Genzer, J. , Efimenko, K. and Fischer, D. (2002), Molecular Orientation and Grafting Density in Semifluorinated Self-Assembled Monolayers of Mono-, Di- and Trichloro Silanes on Silica Substrates, Langmuir (Accessed April 24, 2024)