Optimizing the Quality of Monoreactive Perfluoroalkylsilane-Based Self-Assembled Monolayers

Abstract

Self-assembled monolayers (or SAMs) created from monoreactive perfluoroalkylsilanes by deposition from a toluene solution are investigated for the dependence of their quality on processing conditions. Surface-sensitive spectroscopic techniques are used to provide feedback on the processing conditions in which solution temperature, silane concentration, and reaction time are optimized to improve the quality of these SAMs. For these analyses, monolayers are formed at 20, 40, 60, or 80 °C from solutions containing between 0.5 and 5 mM perfluoroalkylsilane over a period of up to 5 h. Physically adsorbed molecules are removed from these surfaces by extraction to determine the quality of the covalently bound monolayer. Water contact angle measurements, spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), respectively, are used in combination to assess the uniformity of the surface hydrophobicity, monolayer thickness, composition of the assembled perfluoroalkylsilane molecules, and topography of these monolayers. A comparison is also presented for two approaches to fill defects within these solvent extracted monolayers with more perfluoroalkylsilane molecules, aiming to improve the quality of these SAMs. A detailed XPS analysis is used to assess both the relative changes in density and average tilt of molecules within the monolayers as the process temperature is increased in increments from 20 to 80 °C. The observed differences in quality of the SAMs are attributed to temperature- and time-dependent organization and reactivity of the silane molecules. Although the assembly of these monoreactive perfluoroalkylsilanes is driven by thermodynamics, the quality of the monolayer is ultimately limited by the kinetics and mass transport during this assembly process. Lessons from these studies can be exploited for improving the quality of monolayers composed of other alkylsilane molecules that are covalently bound to the surfaces of oxides.

Microwave assisted formation of monoreactive perfluoroalkylsilane-based self-assembled monolayers

“…16 This value is comparable to previous literature on FDDCS based SAMs prepared from immersion in 1 mM FDDCS in toluene for 1 h at 80°C followed by Soxhlet extraction. 16 To gain further insight into the density of the SAMs formed by microwave processing, high resolution XPS analysis was performed on the C 1s region to assess the relative signals for CF 2 -CF 2 and CF 3 (Figure 3). 16 A comparison of the ratio of the integrated areas for CF 2 -CF 2 and CF 3 , as determined by high resolution XPS analysis, was used to assess the This journal is © The Royal Society of Chemistry 2012 packing density of the perfluoroalkylsilane molecules.…”

“…However, such a process can take over 10 days to grow densely packed, uniform SAMs. 9 In order to accelerate the formation of these SAMs, heat can be applied to the solution during their formation, 2,10,16 but can still require a relatively long processing time. Silane molecules can also form SAMs on oxide surfaces through multiple iterations of the deposition process to improve their quality, 19,21 but this approach can be cumbersome and induce unwanted intermolecular polymerization.…”

“…A perfluorinated backbone was chosen in part because it is more easily characterized by XPS than a hydrocarbon backbone. 16 To observe the impact of microwave radiation on the formation of FDDCS SAMs, samples were treated with microwave radiation for up to 10 min at a constant power of 300 W. Time increments of 1 min were investigated to evaluate the progressive change in surface coverage of SAMs with an increased duration of exposure to microwave radiation. The substrates were each left in a solvent containing the silane molecules for a total immersion time of 18 min.…”

“…After the microwave treatment and reaching 18 min of immersion in the solvent, Soxhlet extraction was performed to remove adsorbed molecules and contaminants from the surfaces as per previously described procedures. 16 Extraction was performed for 1 h to effectively rinse the surfaces and remove physisorbed molecules. The impact of microwave radiation on quality of these SAMs was explored by observing the relative change in hydrophobicity and hysteresis for water contact angle (WCA) measurements, as well as through atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analysis of the surface topography and composition, respectively, of the monolayer modified surfaces.…”

“…16 To gain further insight into the density of the SAMs formed by microwave processing, high resolution XPS analysis was performed on the C 1s region to assess the relative signals for CF 2 -CF 2 and CF 3 (Figure 3). 16 A comparison of the ratio of the integrated areas for CF 2 -CF 2 and CF 3 , as determined by high resolution XPS analysis, was used to assess the This journal is © The Royal Society of Chemistry 2012 packing density of the perfluoroalkylsilane molecules. 16,31 Increased density of perfluoroalkylsilane molecules results in an increased CF 3 to CF 2 signal.…”