Quantitative evaluation of perfluorinated alkanethiol molecular order on gold surfaces

Perfluorinated or fluorocarbon surfaces are used in many industrial applications ranging from biomedical vascular prostheses to lubricating, sensing, non-adhesive, and protective films in many different technologies.^1–5 The type, surface concentration, and orientation of the fluorinated chemistries (e.g., CF₃, CF₂, CF) can all affect, for example, surface wetting, barrier quality, tribology, and tenacity of protein binding to a fluorinated biomaterial surface.^6–12 Design and innovation for developing fluorinated surfaces and materials would benefit from knowledge of the orientation and concentration of the different fluorinated surface groups to determine optimized surface properties that confer the desired interfacial properties.^13,14

Different techniques have been employed to fabricate fluorinated surfaces, including RF glow discharge deposition of fluorinated films,^6,15,16 Langmuir–Blodgett techniques,^17,18 surface blooming of trace fluorinated components from bulk liquids,^19–22 and self-assembly of perfluorinated alkanethiol molecules as self-assembled monolayers on metal substrates (SAMs)^23–34 and perfluoroalkylated silanes on oxide supports.^35–38 While many methods produce perfluorinated films, the need persists to quantitatively determine degrees of ordering and orientation at the surface of these films to obtain and control the desired interfacial properties of perfluorinated coatings.^1,39

Chain lengths of the molecules comprising alkyl thiol SAMs are critical determinants for the ordering and orientation of chains in the monolayer.^40–45 Thus, quantifying the effects of chain length on the tilt of the molecules in perfluorinated SAMs is critical to determining the orientation of the CF₃ and CF₂ groups at the surface.³⁹ While it is known that alkanethiol molecules shorter than C10 generally yield poor quality SAMs,⁴⁵ the analogous situation in perfluoroalkyl systems has not been as extensively studied. The effects of chain length for hydrocarbon thiols on gold SAMs have been investigated.⁴⁶ However, the substitution of a predominantly fluorocarbon chain for the hydrocarbon chain has been found to affect the SAM packing density as well as chain tilt.^{23,25–32,39,47–49} This difference in the packing density is not unexpected since fluorine has a much larger isosteric radius than hydrogen, forcing distortion of the carbon chain conformation from all-trans (hydrocarbon) toward helical (fluorocarbon). Due to this steric hindrance of the fluorine groups, perfluoroalkanethiol chains are believed to be helical in conformation,⁴⁷ as observed in bulk poly(tetrafluoroethylene) (PTFE).⁵⁰ These helical fluorocarbon chains differ in structure and aggregate properties from hydrocarbon chains in a predominantly planar all-trans configuration. Short (<12 CF₂ groups) fluorocarbon chains should maintain their helical structure in the monolayer due to intramolecular stabilization^47,49,50 and be densely packed in SAMs (Refs. 47, 49, and 51) by the terminal thiolate epitaxial anchoring on commensurate gold lattice surface binding sites.

AFM and grazing incidence x-ray diffraction data showed that (CF₃(CF₂)₁₁CH₂CH₂SH) monolayers have a tilt angle of 12 ± 2° with respect to the surface normal.⁵¹ Based on AFM results, the shorter CF₃(CF₂)₅CH₂CH₂SH appeared to form a close-packed monolayer similar to the (CF₃(CF₂)₁₁CH₂CH₂SH) monolayer. However, the tilt angle differences between shorter and longer fluorocarbon chains were not investigated. NEXAFS studies of perfluoralkyl silane monolayers on silicon oxide showed how the orientation and grafting density depended on the number of chlorine atoms bonded to the Si atom in the perfluoralkyl silane molecules.³⁵ NEXAFS studies of semi-perfluorinated thiol monolayers were used to determine the orientation of the perfluoro and alkyl portions of their corresponding SAMs.^25,52,53

In this study, self-assembled perfluorinated molecules based on the structure CF₃(CF₂)_nCH₂CH₂SH (n = 3, 5, 7, and 9) were analyzed with angle-dependent x-ray photoelectron spectroscopy (XPS), static time-of-flight secondary ion mass spectrometry (ToF-SIMS), and near edge x-ray absorption fine structure (NEXAFS). XPS is used to determine the type and amount of elements present and the bonding environment of those elements in the outermost 9 nm of the film.^54,55 Static ToF-SIMS generates a mass spectrum from the outermost 1.5 nm of the film, and the fragments detected provide detailed information about the molecular structure and chemistry of the surface.^54,56 XPS and ToF-SIMS data are complementary methods for determining organic film composition and structure.⁵⁷ Partial electron yield NEXAFS provides information about the bonding environment of surface atoms and is used to determine the molecular orientation and ordering of surface species.^54,58 These complementary analytical techniques were combined to uniquely determine how chain length affects ordering and the orientation of fluorocarbon thiol SAMs on polycrystalline gold.^13,54,59

1-iodo-1H, 1H, 2H, 2H-perfluorohexane, 1-iodo-1H, 1H, 2H, 2H-perfluorooctane, 1-iodo-1H, 1H, 2H, 2H-perfluorodecane, and 1-iodo-1H, 1H, 2H, 2H-perfluorododecane were purchased from PCR (Gainesville, FL) and used without any further purification. Thiol acetic acid (Aldrich) was freshly distilled before use. Tetrahydrofuran (THF) was distilled from CaH2. Lithium aluminum hydride (LAH) was purchased from Aldrich and used without further purification. Methanol (MeOH), sodium hydroxide (NaOH), and sodium hydride (NaH) were reagent grades (Fisher) and used as received. Perfluoroalkanethiols of perfluoroalkyl (R_f) chains lengths of 4, 6, 8, or 10 R_f carbons were all synthesized by identical methods. For brevity, one complete synthetic iteration of one chain length (R_f = 6) is described below and outlined in Fig. 1.^{36,52,60–62}

To a flame-dried 250 ml round bottom flask, dry THF (40 ml) was added and combined with NaH (18.99 mmol, 0.76 g in 60% dispersion) under a flow of nitrogen. The slurry was cooled to 0 °C when thiol acetic acid (18.99 mmol, 1.44 g) was added drop-wise to the slurry over a 15 min period. The reaction was then stirred at 0 °C for 45 min and cooled to −78 °C. 1-iodo-1H, 1H, 2H, 2H-perfluorooctane (6.32 mmol, 3 g) was then added drop-wise to the slurry. After 3 h, the cold bath was removed and the reaction mixture was warmed to room temperature. Methanol (20 ml) was then combined and stirred for 15 min. The volatile organics were evaporated under vacuum followed by the addition of de-ionized H2O (40 ml) and extraction with methylene chloride (3 × 20 ml). Flash chromatography (hexanes, R_f ∼ 0.30) provided 1H, 1H, 2H, 2H-perfluorooctanethiol acetate (1.96 g, 73%) as a yellow oil after vacuum evaporation. ¹H NMR (CDCl₃ Brueker 300 MHz): 2.34 (m, 5 H CF₃(CF₂)₅–CH ₂–CH₂–S–CO–CH ₃), 3.07 (t, 2 H CF₃(CF₂)₅–CH₂–CH ₂–S–CO–CH₃).

To a flame-dried 250 ml round bottom flask fitted with an addition funnel, LAH (1.18 mmol, 0.095 g) was added and combined with dry tetrahydrofuran (30 ml) under a flow of nitrogen. The reaction slurry was cooled to −78 °C and 1H, 1H, 2H, 2H-perfluorooctanethiol acetate (0.236M, 1.18 mmol, 0.50 g) was added drop-wise over 20 min. The reaction was stirred at −78 °C for 45 min and then methanol (20 ml) was combined and stirred again for an hour while allowing the reaction mixture to warm to room temperature. The volatile organics were evaporated under vacuum and then de-ionized H₂O (40 ml) was added and extracted with methylene chloride (4 × 20 ml). Flash chromatography (hexanes, R_f ∼ 0.45) provided the 1H, 1H, 2H, 2H-perfluorooctane-1-thiol product (0.30 g, 67%) as a clear oil. ¹H NMR (CDCl₃ Brueker 300 MHz): 1.58 (t, 1H SH), 2.50 (m, 2H CF₃(CF₂)₅–CH ₂–), 2.80 (dt, 2H SH–CH ₂–).

Gold substrates for perfluorinated alkyl thiol self-assembly were prepared by thermal evaporation of a fresh layer of 1500–2000 Å gold (99.999%, Johnson-Mathey) onto Pd-coated (200 Å) silicon wafer surfaces as described previously.⁶³ Gold-coated substrates (cut into 1 cm square pieces) were immersed into ethanolic solutions of each purified perfluorinated alkyl thiol (0.5–1.0 mM) for 6–18 h at ambient temperature, followed by thorough sequential rinsing with pure EtOH and Millipore water and drying under pure N₂. Solution adsorption time made no detectable difference in the resulting SAM properties. Freshly prepared SAMs were stored under N₂ in Fluoroware containers prior to analysis.

The XPS experiments used a Surface Science Instruments S-probe spectrometer (SSI, Mountain View, CA) using a monochromatic Al K_α x-ray source (hν = 1486.6 eV). The binding energy (BE) scales for the monolayers on gold were referenced by setting the Au4f_7/2 BE to 84.0 eV. The high-resolution C1s and S2p spectra were acquired with an analyzer pass energy of 50 eV. XPS elemental composition scans were acquired with an analyzer pass energy of 150 eV. At this pass energy, the transmission function of the spectrometer was assumed to be constant.⁶³ The peak areas were normalized by the number of scans, points per electron volt, Scofield's photoionization cross sections,⁶⁴ and sampling depth. The sampling depth was assumed to vary as KE^0.7, where KE is the kinetic energy of the photoelectrons.⁶³ 

To assess the SAM compositional variation with depth, angle-dependent XPS data were collected at nominal photoelectron takeoff angles of 0°, 55°, and 80°.⁶⁵ The takeoff angle was defined as the angle between the surface normal and the axis of the analyzer lens system. Using mean free paths calculated from the equations given by Seah and Dench,⁶⁶ the sampling depth (three times the mean free path) should decrease from 9 to 1.5 nm as the takeoff angle increases from 0° to 80°.

The NEXAFS experiments were performed at the National Synchrotron Light Source U7A beamline located at Brookhaven National Laboratory, utilizing a ∼85% polarized, high-intensity beam. This beamline uses a dual grating (600 and 1200 l/mm) monochromator. The 600 l/mm grating used to acquire the carbon K-edge spectra provided a full-width at half-maximum energy resolution of ∼0.15 eV. The 1200 l/mm grating used to acquire the fluorine K-edge spectra provided a full-width at half-maximum energy resolution of ∼0.25 eV. The monochromator energy scale was calibrated by setting the C1s → π* transition in the graphite carbon K-edge NEXAFS spectrum to 285.35 eV.⁶⁷ All NEXAFS spectra were normalized by the partial electron yield (I₀) from an in situ gold coated, 90% transmission grid placed in the incident x-ray beam. Partial electron yield (PEY) was monitored by a channeltron with the cone negatively biased (−100 to −150 eV). Samples were mounted to allow rotation about the vertical axis, thus changing the polar angle between the sample surface and the incident x-ray beam. The polar angle is defined as the angle between the incident x-ray beam and the sample surface. Hence, normal incidence of the x-ray beam on the surface corresponds to a polar angle of 90°, while glancing incidence x-ray beam corresponds to a polar angle of 20°. This definition of the polar angle for NEXAFS should not be confused with the definition of XPS photoelectron takeoff angles. The electric field vector (E) is perpendicular to the x-ray beam, so for normal incidence, the E vector of the x-ray beam lies parallel to the surface.

The ToF-SIMS data were acquired using a Model 7200 Physical Electronics instrument (PHI, Eden Prairie, MN). The 8 keV Cs⁺ ion source was operated at a spot size of 50 μm and a pulse width of ∼1 ns. Data were acquired over a mass range from m/z = 0 to 1500 for both positive and negative secondary ions. The Cs⁺ ion beam was moved to a new spot on the sample and rastered over a 100 × 100 μm² area for each spectrum. The total ion dose used to acquire each spectrum was less than 3 × 10¹² ions/cm².

The mass scale for the negative secondary ions was calibrated using the CF (30.9984), CF₃ (68.9952), C₂F₅ (118.9920), and C₃F₇ (168.9888). The positive secondary ion mass scales were calibrated using CF, CF₂(49.9968), C₂F₅, and C₃F₇. The fit between the expected and observed masses was less than 15 ppm for both the positive and negative calibration ions.

Perfluoroalkyl thiol synthesis using hydride reduction for transforming commercially available perfluoroalkyliodides^{36,52,60–62} to corresponding perfluoroalkanethiols significantly improves the yields of these products over former routes based on hydrolysis from the common thioacetyl perfluoroalkyl intermediate.^{23–25,40,48,68} Hydride reduction eliminates the product-reactant equilibrium in hydrolysis methods.^{36,52,60–62} Figure 1 displays proposed mechanistic differences that lead to a variety of undesired side reactions in hydrolysis approaches (Path A), particularly oxidation of thiol under equilibrium base-catalyzed hydrolysis conditions. Hydride reduction (Path B) removes several of these side reaction possibilities, improving the overall yield and purity. Table I displays the improved synthetic yields for both reaction steps in Path B for all R_f chain lengths.

The angular dependence of the elemental compositions and carbon functional groups for the four different chain length perfluorocarbon thiol monolayers assembled onto gold was analyzed by XPS. Table II compiles a summary of the average compositions and standard deviations (n = 4) for the four monolayers at a 55° photoelectron takeoff angle (see the supplementary material⁷⁵ for representative survey spectra for each monolayer). The CF₃(CF₂)₉CH₂CH₂SH (F10), CF₃(CF₂)₇CH₂CH₂SH (F8), and CF₃(CF₂)₅CH₂CH₂SH (F6) SAMs showed reasonable surface coverage and relative uniformity, as seen by the low amount of compositional variance. The CF₃(CF₂)₃CH₂CH₂SH (F4) monolayer, however, exhibited a larger amount of variance, indicating the monolayer quality produced by this thiol was not as high as the longer chain fluorocarbon SAMs. This observation has also been reported for an analogous short-chain perfluoroalkyl thiol monolayer terminated with a –SF₅ group.³⁰ Data in Table II show that concentrations of gold, hydrocarbon species, and sulfur all increased as the length of the fluorocarbon tail decreased, while the concentration of fluorine and fluorocarbon species decreased as the length of the fluorocarbon tail decreased. These trends are in the directions expected from the stoichiometry of the fluorocarbon thiol molecules. Oxygen, a ubiquitous contaminant that should not be present in the fluorinated SAMs, was detected at low levels (<1 at. %) in the F10 SAM. The oxygen concentration generally increased as the length of the fluorocarbon tail decreased, indicating that the shorter chain SAMs were more susceptible to oxygen contamination that the longer chain SAMs.

Monolayer thicknesses were calculated for each fluorocarbon SAM using the XPS compositional data taken at the 55° takeoff angle. Equations from Ertl and Küppers⁶⁹ were used for these calculations and we assumed that a complete monolayer of uniform thickness was formed in each case. Mean free paths were calculated using an equation given by Seah and Dench.⁶⁶ From this analysis, the F10 monolayer was calculated to be 1.5 nm thick. This thickness is comparable to the length of 1.6 nm that was calculated for the F10 molecule using crystallographic parameters of PTFE for the fluorocarbon portion of the F10 molecule⁷⁰ and consistent with the molecules in the F10 SAM standing upright on the gold surface. It is also consistent with numerous reports for similar molecule monolayers on gold measured with optical reflectometry methods (e.g., ellipsometry).^{23–32,35,37} Monolayer thicknesses for the F8, F6, and F4 SAMs calculated from these XPS data were found to be 1.2, 0.96, and 0.2 nm, respectively, decreasing as expected with decreasing length of the fluorocarbon chain. The F4 monolayer experimental thickness of 0.2 nm is much less than the calculated F4 molecular chain length of 0.9 nm, indicating that the F4 molecules are not upright on the surface and likely do not form a complete monolayer.³⁰ 

Angle-dependent XPS was used to determine the compositional depth dependence of these monolayers. Angle-dependent XPS composition measurements for each of the SAMs are summarized in Table III. Calculated atomic percentages have been renormalized to reflect only the atoms present in the fluorocarbon thiol monolayers by removing the contributions from the gold substrates. The expected (theoretical) values shown in Table III are based on stoichiometric atomic percentages of the molecules (excluding hydrogen, which is not detected by XPS). Experimental fluorine atomic percentages for the F10, F8, and F6 SAMs were higher at all angles than the corresponding theoretical values. This indicates the perfluorinated tails of the F10, F8, and F6 thiol molecules locate at the outer SAM surface. Atomic sulfur percentages for the F10, F8, and F6 SAMs were lower at all angles (depths) than the corresponding theoretical values. Also, experimental sulfur values for each of the three SAMs decreased as the photoelectron takeoff angle increased from 0 to 80° (sampling depth decreased from ∼9 to ∼1.5 nm). These two observations indicated that sulfur atoms in the F10, F8, and F6 SAMs were located near the gold surface. In contrast, XPS results for the F4 monolayer showed lower fluorine concentrations and higher carbon concentrations compared to theoretical values (Table III). The sulfur concentration of the F4 monolayer showed little, if any, change with takeoff angle, indicating no overall preferential positioning of sulfur in the monolayer. The high percentage of carbon is probably due to the presence of significant hydrocarbon contamination on this sample.

The high-resolution XPS S2p spectra for the F10, F8, and F6 monolayers (see the supplementary material⁷⁵ for S2p spectra for each monolayer, which are similar to those previously published in Refs. 28 and 63) showed that nearly all (>90%) of the thiols in these monolayers were bound as a thiolate to the Au substrate, as seen by the S2p_3/2 thiolate peak at 162 eV.⁶³ The S2p spectrum of the F4 monolayer showed a significant (∼20%) contribution of a peak at 163.5 eV due to unbound thiols.⁶³ The high-resolution S2p results are consistent with the XPS compositional results for the four fluorinated SAMs.

High-resolution scans of the C1s regions at 0° (normal) and 80° (glancing) takeoff angles (representing 9 nm and 1.5 nm sampling depths, respectively) for F10 and F4 are shown in Figs. 2(a) and 2(b). The F10 SAM [Fig. 2(a)] clearly shows three distinct carbon peaks due to hydrocarbon (CH, 285 eV), perfluoromethylene (CF₂, 291 eV), and perfluoromethyl (CF₃, 293 eV) species.²⁸ The relative intensities of these peaks change between glancing and normal takeoff angles, with the angular dependence clearly supporting monolayer organization. The hydrocarbon peak is more intense at 0° takeoff angle (9 nm sampling depth) while the CF₃ peak is enhanced at the glancing angle (∼1.5 nm sampling depth). At both photoelectron takeoff angles, the experimental CH₂ concentration is less than theory and the experimental CF₃ concentration is more than theory. Thus, the CF₃ groups are enriched at the surface of the F10 SAM and the CH₂ groups are located near the gold-SAM interface, consistent with a well-organized SAM. The high-resolution C1s spectra for the F4 monolayers [Fig. 2(b)] show large hydrocarbon CH₂ peaks (∼285 eV) compared to the small perfluorocarbon CF₂ (291 eV) and CF₃ (293 eV) peaks. The relatively small amounts of CF₃ and CF₂ groups and their lack of angle-dependence indicate that the F4 thiol does not form an ordered, complete monolayer.

The F10 and F4 C1s angle-dependent results along with those for the F8 and F6 monolayers are summarized in Table IV. The C1s percent species for the CH₂, CF₂, and CF₃ peaks are reported at normal and glancing angles as well as the theoretical atomic percentage of these groups in the different fluorinated molecules. The C1s results for the F8 and F6 SAMs were similar to the F10 SAM results. The CF₃ peak signal in for all three monolayers was stronger at the glancing takeoff angle than at the normal angle while the CH₂ peak signal was stronger at the normal angle, reflecting the anticipated structural organization. Consistently, the experimental percentage of CF₃ groups for F10, F8, and F6 monolayers was greater than the theoretical value for both angles, strongly supporting CF₃ group positioning at the outermost surface of the F10, F8, and F6 SAMs.

The angle-dependent elemental composition and XPS C1s results for the F10, F8, and F6 SAMs are consistent with film structural organization and anisotropy: the hydrocarbon thiol end of the molecules attached to the gold surface and the fluorocarbon chain located at the outermost surface of the SAM. The close agreement between the XPS determined thickness of the F10, F8, and F6 SAMs and the molecular chain length of the corresponding fluorinated thiols (discussed above) along with the angle-dependent XPS results both indicate that the fluorinated molecules are oriented relatively normal to the surface in the F10, F8, and F6 SAMs. This is consistent with previous studies of fluorinated SAMs^{23–32,35–37,49,51} and is further supported by the NEXAFS results described below.

Near edge x-ray absorption fine structure (NEXAFS) was used to further investigate the ordering and orientation of the F10, F8, F6, and F4 monolayers on gold. Polarized x rays were incident on the sample at various angles and scanned through the carbon and fluorine absorption edges. Absorption of x rays as a function of energy was detected by monitoring the PEY. As mentioned above, rotation of the sample changed the polar angle (θ) (see inset in Fig. 3 and description in Sec. II). Also, note that the NEXAFS angular definition is different from XPS photoelectron takeoff angular definition. In NEXAFS, incident x rays are absorbed by the sample when the electric field vector of the incident x rays overlaps (i.e., is parallel to) an unoccupied antibonding sigma orbital in the sample, resulting in the transition of a core 1 s electron to that antibonding orbital. This orientation-dependent absorption is observed as a peak in the x-ray spectrum specific for each antibonding orbital.

The carbon K-edge NEXAFS results for the F10 monolayer are shown in Fig. 3 for x-ray beam incident angles of both θ = 90° and 20°. Peaks at 292.6 and 299.0 eV are assigned to transitions from the C1s → C–F* orbitals.^6,71,72 As shown in Fig. 3, these peaks are most intense for 90° incidence x rays (x-ray E vector parallel to the SAM surface). By rotating the sample to glancing x-ray incidence (θ = 20°) the observed C–F* peaks diminish and the intensity of the 295.8 eV peak, assigned to the C1s → C–C* transition,^6,71,72 increases. At this glancing incidence angle (θ = 20°), the x-ray E vector is nearly perpendicular to the SAM surface. Enhancement of the C–C* peak at θ = 20° indicates that the F10 molecules are standing upright on the Au surface (SAM C–C bonds nominally perpendicular and C–F bonds nominally parallel to the Au surface). Similar NEXAFS results for upright, highly ordered perfluorinated chains have been previously observed in studies of downstream RF glow discharge polymerized tetrafluoroethylene and adsorbed perfluorotetracosane.^6,71,72

The small valleys observed at 285 and 290 eV are due to improper normalization caused by absorption of the x rays by carbon contamination on the reference grid (I₀). Dividing the PEY of each sample by its reference spectrum subsequently produces small negative peaks in the resulting net spectrum. However, since these artifact peaks occur at lower energies than the fluorocarbon peaks, they do not interfere with the intensity or interpretation of the fluorinated SAM peaks reported in this study.

The bottom spectrum in Fig. 3 shows the difference spectrum from C1s NEXAFS data collected at 90° and 20° angles for the F10 monolayer. This difference spectrum emphasizes relative changes in the monolayer x-ray absorption and, hence, orientation, as the incident x-ray angle is varied. The two peaks at 292.6 and 299.2 eV correspond to the C–F* transitions that are more prominent at 90°, while the negative valley at 296.2 eV is the C–C* transition that is reduced at 90°.

The 90°–20° difference peak at ∼292.6 eV (C–F* transition) decreases in intensity with decreasing fluorocarbon chain length (see the supplementary material⁷⁵ for the difference peaks for each monolayer). Figure 4 is a plot of this difference peak area as a function of fluorocarbon chain length. In general, the larger the differences between the peak intensities at 90° and 20° incident angles, the greater the structural anisotropy and ordering in the monolayer. Because differences between the 90° and 20° angle spectra seen in the C–F* peaks decrease as the SAM perfluorocarbon molecular chain length decreases (Fig. 4), the degree of ordering of the monolayer chains must also decrease as perfluorocarbon chain length decreases.

The upper portion of Fig. 5 shows the F K-edge spectra for the F10 monolayer at 90° and 20° incident x-ray angles. The lower portion of Fig. 5 shows the 90°–20° F1s difference spectrum for the F10 monolayer. The peak at 693 eV is due to the F1s → C–F* transition and is enhanced when the incident x-ray angle is at 90° (where the x-ray E vector is parallel to the SAM surface). This dataset is consistent with the C K-edge results, also showing that the C–F bond in the F10 monolayer lies relatively parallel to the surface.

The perfluorocarbon portions of the molecules examined in this study are expected to be in a helical conformation (e.g., PTFE is a 15/7 helix),^26,27,29,50 so in this model all the C–F bonds extend outward at the same angle from the carbon–carbon backbone molecular axis (see Fig. 6). While the C–F bonds are expected to have no preferential azimuthal orientation, they will have polar orientation with respect to the surface normal. Thus, under this assumption, the average tilt angle of the molecular axis of the perfluoroalkyl thiolates can be determined from the change in intensity of the C–F* peak with incidence x-ray angle. To do this, the C K-edge spectra from the F10 monolayer were acquired at 10° increments for incident x-ray angles between 20° and 90°. The first C–F* peak at 292.6 eV (see Fig. 3) was fit with a Gaussian peak and the area under the Gaussian peak was calculated for each spectrum. In Fig. 7, the C K-edge C–F* peak area as a function of incident x-ray angle was fit according to equations by Stöhr and Outka^58,73 with a few modifications. Since both the orientation angle and degree of ordering can affect the measured peak intensities, a tilt angle of 12° from the surface normal was assumed for the orientation of the F10 SAM.²⁵ Then, the degree of disorder in the F10 monolayer was calculated from the fit of the measured C–F* peak intensities as a function of the x-ray angle. This provided a disorder value of 17%, similar to the percentage of the CF₃ C–F bonds in F10. This implies, as expected, that the three C–F bonds in the F10 terminal group are not oriented at the same angle from the molecular axis as the C–F bonds are in the CF₂ groups, or, alternatively, that the CF₃ groups can freely rotate in F10 monolayers.

Similar calculations for the F8, F6, and F4 SAMs showed that the degree of disorder increased significantly with decreasing perfluorocarbon chain length (see Table V). This increased amount of monolayer disorder with decreasing chain length was also observed in the difference spectra C–F* peak areas in Fig. 4. For F8 and F6 monolayers, it is reasonable to attribute the decrease in the amount of polarization observed in the NEXAFS peaks solely due to an increase in the disorder of the F8 and F6 molecules since the XPS-measured thicknesses for these monolayers are close to the expected values for upright orientation of the helical fluorocarbon molecules on the Au surface. However, this is not the case for F4, so the decreased polarization observed for this monolayer is probably due to both an increase in the disordering and average orientation from the surface normal. In fact, the F4 monolayer shows little difference between the 90° and 20 °C K-edge spectra.

Due to its surface sensitivity and molecular specificity, static ToF-SIMS is useful to interrogate surface properties of fluorocarbon thiol monolayers. Static ToF-SIMS in tandem with XPS and NEXAFS provide a complementary, more complete characterization of the fluorinated monolayers.⁵⁴ Static ToF-SIMS spectra from each different monolayer show characteristic peaks (parent ions) that are representative of each particular fluorocarbon thiol monolayer. The high mass negative ion region from the F10 SAM in Fig. 8 has peaks at m/z = 973 and 1355 due to the Au₂(M-H) and Au(M-H)₂ ions, where M = CF₃(CF₂)₉CH₂CH₂SH. A peak from the parent ion (M-H) is also present at m/z = 579 in the high-mass region (data not shown). The negative ion SIMS spectrum from the F8 monolayer shows molecular peaks at m/z = 873 and 1155 for the Au₂(M-H) and Au(M-H)₂, where M represents the F8 molecule (data not shown). The masses of these molecular ion peaks are consistent with the difference between the F10 and F8 molecules, namely, a loss of two CF₂ groups (100 amu) for each parent species, M. Similar results were seen for the F6 and F4 monolayers, supporting ejection of intact molecules from the SAM surface by ToF-SIMS (data not shown). In all cases, parent ion-gold ion species are detected, consistent with anchored fluorocarbon thiolate species.

The low-mass region of the F10 negative and positive ion SIMS spectra contain the C_xF_y fragments typically observed from fluorocarbon surfaces (e.g., see Figs. 8 and 9).⁷⁴ In addition, species such as SO₃⁻ and (M–H + 3O)⁻ are also detected, indicating some oxidation of the perfluoroalkyl thiols has occurred. No oxidized sulfur species were detected by XPS, which means the concentration of the oxidized sulfur species is less than 0.1 at. %. However, the detection limit of static ToF-SIMS is several orders of magnitude lower than XPS, so it is straightforward to detect trace small amounts of thiol oxidation with static ToF-SIMS. The percentage of oxidized sulfur in each monolayer increases as the length of the fluorocarbon chain decreases. It should be noted that SO₃⁻ has a significantly higher secondary ion yield than S, so the fact that these two ions have similar intensities does not imply that the concentration of oxidized sulfur is similar to the concentration of unoxidized sulfur.

While fragments in static ToF-SIMS spectra from most organic surfaces that have the strongest intensities can typically be related directly to the molecular structure of the surface being examined, this is not the case for perfluoroalkyl thiolate monolayers. For these SAMs, recombination peaks (i.e., fragments that contain a set of atoms that are not directly bonded to each other in the monolayer) with strong intensity are detected. An example is shown in Fig. 9 for the positive secondary ions detected in the m/z = 200–300 range from the F10 SAM. The intensity of recombination peaks at m/z = 211 (AuCH₂), 225 (AuC₂H₄), 228 (AuCF), 247 (AuCF₂), and 278 (AuC₂F₃) is similar, and in some cases even higher, than peaks consistent with the F10 structure at m/z = 230 (AuSH), 243 (AuSCH₂), and 256 (AuSC₂H₃).

The perfluoroalkyl thiol synthetic approach based on hydride reduction for transforming commercially available perfluoroalkyliodides to corresponding perfluoroalkanethiols has been used to prepare a series of perfluoroalkanethiols [CF₃(CF₂)_xCH₂CH₂SH (x = 3, 5, 7, and 9)] with improved yields compared to other published routes. Monolayers prepared by self-assembly of these perfluoroalkanethiols onto gold were characterized XPS, NEXAFS, and static ToF-SIMS. F10, the longest molecule, produced a well-ordered SAM (∼17% disorder) with the F10 tilted ∼12° from the surface normal. As the chain length of molecules decreased, the amount of disorder in the monolayer increased. For the F10, F8, and F6 SAMs, the molecules were bonded in an upright configuration with the CF₃ group located at the outer surface of the monolayer and the thiol group located at the monolayer-Au interface. The F4 monolayer was highly disordered, with the majority of the F4 molecules probably lying down on the Au surface and containing significant amounts of hydrocarbon contamination.

L.J.G. and D.G.C. gratefully acknowledge support from UWEB (No. NSF EEC-9529161) and NESAC/BIO (NIH Grant No. EB-002027). The NEXAFS studies were performed at the NSLS, Brookhaven National Laboratory, which is supported by the DOE, Division of Materials Science and Division of Chemical Sciences. Daniel Fischer is thanked for the technical expertise and contributions he provided to the NEXAFS experiments. D.W.G. gratefully acknowledges support from NSF (Grant No. DMR-9596023) and faculty research fellowships from 3M and DuPont. Discussions with N. R. Holcomb and Gary Gard (Portland State University, USA) were very helpful to this study.

The authors have no conflicts to disclose.

Ethics approval is not required.

Lara J. Gamble: Data curation (equal); Formal analysis (equal); Investigation (lead); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). David Radford: Investigation (equal); Methodology (equal). David W. Grainger: Conceptualization (equal); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). David G. Castner: Conceptualization (equal); Funding acquisition (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.