Molecular packing of semifluorinated alkanethiol self-assembled monolayers on gold: Influence of alkyl spacer length

The structures of semifluorinated alkanethiol self-assembled monolayers (SAMs) generated by the adsorption of CF₃(CF₂)₉(CH₂)_nSH (F10HnSH, n = 2, 6, 11, 17, 33) onto gold were investigated with atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and surface plasmon resonance spectroscopy. Images obtained by AFM of the F10H2SH SAM showed a remarkably ordered 2D hexagonal lattice with a lattice constant, a = 5.9 ± 0.1 Å, on Au(111)/mica. As the total number of carbon atoms in the alkyl spacer groups (n) was increased, the flourocarbon adlayer structure appeared more disordered in the AFM images; however, the thicknesses of the fluorocarbon layers estimated from the C 1s (CF₃), C 1s (CF₂), and F 1s XPS signal intensities were indistinguishable in all of the SAMs. In contrast, the C 1s (CH₂) signals revealed that the tilt angles of the alkyl spacer groups depended strongly on n. We utilized a contrast variation SPR technique with various contacting media for an independent determination of the refractive indices and the film thicknesses of the semifluorinated SAMs. The obtained data were consistent with our AFM and XPS results, which show that the longer alkyl chains pack more densely than the shorter ones in these SAMs.