SUM-FREQUENCY VIBRATIONAL SPECTROSCOPIC STUDY OF MIXED ALKYLSILOXANE MONOLAYERS SELF-ASSEMBLED ON FUSED-SILICA SURFACE

Self-assembled monolayers of fluorinated and non-fluorinated alkylsilo xanes were prepared on fused silica substrates and studied by both con ventional IR and nonlinear sum-frequency vibrational spectroscopy comp lemented with XPS. Simultaneous self-assembly of the two alkylsiloxane s results in preferential adsorption of the fluorinated compound and f ormation of the mixed monolayer. Different species are found to be clo sely coadsorbed in the mixed monolayers which distinguishes self-assem bly from Langmuir-Blodgett deposition where the similar species tend t o separate into different phases. Nonlinear sum-frequency spectroscopy of C-H stretching region detects symmetry perturbations of the alkyl chain induced by trans-cis defects and partial fluorination and clearl y distinguishes between them. All three characterization methods consi stently indicate the presence of a hydrocarbon residue in all deposite d monolayers, which is ascribed to the hexadecane used as a solvent du ring the self-assembly process. (C) 1997 Elsevier Science S.A.