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

The structures of semifluorinated alkanethiol self-assembled monolayers (SA Ms) generated by the adsorption of CF3(CF2)(9)(CH2)(n)SH (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 sp ectroscopy. Images obtained by AFM of the F10H2SH SAM showed a remarkably o rdered 2D hexagonal lattice with a lattice constant, alpha = 5.9 +/- 0.1 An gstrom, on Au(111)/mica. As the total number of carbon atoms in the alkyl s pacer groups (n) was increased, the fluorocarbon adlayer structure appeared more disordered in the AFM images; however, the thicknesses of the fluoroc arbon 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 ls (CH2) signals revealed that the tilt angles of the alkyl spacer groups d epended 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 lon ger alkyl chains pack more densely than the shorter ones in these SAMs.