Investigation of sub-monolayer, monolayer, and multilayer self-assembled semifluorinated alkylsilane films

Semifluorinated self-assembled (FAS SA) films fabricated from trifunctional precursors are frequently used in myriad applications, yet an understanding of the effects of fabrication conditions, including deposition time, on adsorption mechanisms and molecular architectures is still being developed. In this work we prepared SA films based on the F(CF2)8(CH2)2SiCl3 (FAS-17) precursor and characterized these films using a suite of surface analytical techniques. Contact angle, sum frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and ellipsometry results are consistent with the formation of disordered sub-monolayer structures at short deposition times, well-ordered monolayers at intermediate deposition times, and inhomogeneous multilayers at long deposition times. Correlation of SFG and XPS results demonstrates a change in FAS-17 chain orientation as the deposition time increases from 2 s to 5 min. Group theory-based calculations, SFG studies, and Fourier-transform infrared (FTIR) results also afford additional evidence in support of the assignment of the SFG signals at ∼1345 and ∼1370 cm−1 to the asymmetric stretching mode of the semifluorinated silane chain’s terminal CF3 group rather than to its axial CF2 stretches. To our knowledge, this is the first report of SFG studies on semifluoroalkyl silane self-assembled films in the C–F stretching frequency region.

Graphical abstractSemifluorinated self-assembled films fabricated from trifunctional precursors have disordered sub-monolayer structure at short deposition times, well-ordered monolayer structure at intermediate deposition times, and inhomogeneous multilayer structure at long deposition times.Figure optionsDownload full-size imageDownload high-quality image (42 K)Download as PowerPoint slideResearch highlights► Ordered semifluorinated silane monolayers formed at 5-min deposition time. ► Inhomogeneous semifluorinated silane multilayers formed at 20-min deposition time. ► Signals at 1345 and 1370 cm−1 from the asymmetric stretching mode of the CF3 group. ► Orientation changed for semifluorinated films with different deposition times.