Reversible Exchange of Self-Assembled Monolayers of

Publication Date (Web): August 14, 2008 ... Zhao , Harsharn Auluck , Moonhee Kim , Wouter Vijselaar , Sander Kommeren , Andreas Terfort , and Paul S. ...
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J. Phys. Chem. C 2008, 112, 13997–14000

13997

Reversible Exchange of Self-Assembled Monolayers of Semifluorinated n-Alkanethiols and n-Alkanethiols on Au/Mica Surfaces S. N. Patole, C. J. Baddeley, D. O’Hagan, and N. V. Richardson* EaStCHEM School of Chemistry, UniVersity of St Andrews, St Andrews, Fife, KY16 9ST, United Kingdom ReceiVed: October 26, 2007

Scanning tunnelling microscopy (STM) and photoelastic modulation infrared reflection absorption spectroscopy (PM-IRRAS) have been used to investigate the exchange of self-assembled monolayers (SAMs) of n-octanethiolate by semifluorinated n-octanethiolate from ethanolic solution at 340 K. The process occurs via displacement of the thiols from domain boundaries. By contrast, the reverse exchange process occurs more homogeneously via the formation of mixed thiolate domains. The ability of semifluorinated octanethiolate to adopt a range of adsorption sites with approximately similar adsorption energies may lead to this contrasting behavior. Introduction In recent years, there has been enormous scientific and technological interest in self-assembled monolayers (SAMs) grown on metallic (in particular gold) surfaces from the point of view of corrosion inhibition,1,2 lubrication,3,4 sensing5-7 (both chemical and biological), nanodevice fabrication,8,9 and surface patterning.10-12 In addition, there is considerable interest in the use of SAMs to synthesize surfaces patterned on the nanometer scale13 which can subsequently be used for, e.g., electrochemical metal deposition.14 Patterning of this type often involves the exposure of a SAM (e.g., a thiolate monolayer on Au) to a solution of a second molecular species (e.g., frequently a second thiol). Clearly the stability of the first SAM in the presence of the second species is an important consideration. A number of investigations have been reported of thiol exchange at Au surfaces. These have included studies of SAMs created by solution deposition from mixtures of thiols,15-17 the influence of terminal and spacer groups on the exchange process from solution,18,19 exchange on micropatterned surfaces,20 investigations using electroactive thiols as probes of the exchange processes,21,22 comparisons between exchange processes by thiols from solution and from the gas phase,23 and more recently adetailedexaminationofdisplacementkineticsandmechanism.24-26 Generally, displacement is thought to be initiated at domain boundaries such that, during the exchange process, separate domains of each thiolate could be identified. We recently reported a detailed investigation of semifluorinated octanethiolate (FOT) SAMs on Au/mica using a combination of scanning tunneling microscopy (STM) experiment and theory.27 In agreement with previous studies,28 we found that FOT produced an overlayer with approximately (2×2) periodicity. However, we also found that the adsorption site of FOT was variable thereby substantially reducing its tendency to form domain boundaries. By contrast, the adsorption of octanethiol (OT) produces well-defined (3×3)R30° domains with clear domain boundaries. In this study we investigate, using STM and photoelastic modulation infrared reflection absorption spectroscopy (PM-IRRAS), the exchange reaction between FOT SAMs on Au/mica and OT in the forward and reverse directions. We show that the exchange of OT by FOT is initially faster than the reverse process and rationalize this observation in terms of the tendency of OT to form well-defined domains. * Corresponding author. E-mail: [email protected].

Experimental Section Substrates of mica with an epitaxial {111}-oriented gold layer 300 nm thick (Au/mica) were purchased from Georg Albert PVD, Heidelberg, Germany. The fluorinated n-octanethiol (FOT, CF3(CF2)5(CH2)2SH) and n-octanethiol (OT) were purchased from Aldrich and were used without further purification. Monolayers on Au/mica substrates were prepared by immersing the gold substrates into dilute ethanolic solutions (1 mM) of the respective thiol at 340 K. After refluxing for 18 h, the substrates were removed from the solution, rinsed with pure ethanol, and dried in a nitrogen stream. All STM measurements were carried out in air with a Molecular Imaging (PicoSPM) microscope. In all cases tips were prepared mechanically by cutting a 0.25 mm Pt/Ir alloy (8:2, Goodfellow) wire. STM images were recorded in constant current mode with tunneling currents between 10 and 20 pA and a sample bias between 1.2 and 1.6 V. Photoelastic modulation infrared reflection absorption spectroscopy (PM-IRRAS) data were acquired by using a Digilab FTS 7000 spectrometer with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. In the polarization modulation experiment, the infrared beam, which was incident at an angle of 80°, was modulated between s- and p-polarizations at a frequency of 37 kHz, using a Hinds Instruments PEM-90 photoelastic modulator. Signals generated from each polarization (Rs and Rp) were detected simultaneously by a lock-in amplifier and used to calculate the differential surface reflectivity (∆R/R): (Rp - Rs)/(Rp + Rs). The spectra were collected by using 256 scans with a spectral resolution of 4 cm-1. The spectrum of the interface is superimposed on a large sloping background, defined by a second-order Bessel function, which is caused by the wavelength dependence of the polarization modulation efficiency.29 The Bessel function is background subtracted revealing the spectra as presented in this paper. Exchange reactions were carried out in two steps. First, SAMs of FOT (or OT) on Au/mica substrates were prepared by immersing the gold substrates into dilute ethanolic solutions (1 mM) of the respective thiol at 340 K, as described above. After SAM formation, samples were immersed into ethanolic solutions of the second thiol also at 340 K for time intervals of 30, 60, and 240 min. After each time interval, substrates were removed from solution, rinsed with pure ethanol, dried in a nitrogen stream, and characterized with STM and PM-IRRAS.

10.1021/jp804888g CCC: $40.75  2008 American Chemical Society Published on Web 08/14/2008

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Figure 1. PM-IRRAS data of an FOT SAM on Au/mica as a function of exposure time in a 1 mM ethanolic solution of OT: (a) C-F region and (b) C-H region.

Results 1. Exchange of FOT SAM on Au/Mica by OT. Figure 1a shows the PM-IRRAS spectra of the initially FOT covered Au surface as a function of time in a 1 mM ethanolic solution of OT. The IR spectrum of FOT exhibits bands at 1079, 1123, 1145, 1248, 1294, 1323, and 1367 cm-1. After 30 and 60 min in the ethanolic OT solution, the bands are still visible in the spectra with slightly reduced intensities. After 240 min, no bands are observed in this region of the spectrum. Chidsey and Loiacono used similar IR data coupled with a consideration of the metal surface IR selection rule to determine an average tilt angle of ∼10° to the surface normal for semifluorinated alkanethiols on Au.4 In the liquid phase spectrum of FOT, the most intense bands are observed in the 1200-1250 cm-1 range. These modes each have dipole moments perpendicular to the carbon backbone. By contrast, the most intense modes in the adsorbed FOT SAM are at 1075, 1325, and 1375 cm-1. These modes are relatively complex in terms of atomic motion. However, importantly each of these modes has its associated dipole moment along the carbon backbone. In the first 60 min of the exchange process, there is a gradual and relatively uniform decrease in the intensity of the FOT related bands. In the same time interval, there is an increase in the C-H stretching bands characteristic of OT in the 2800-3000 cm-1 range (Figure 1b). There are some subtle differences in the relative intensities of these bands as a function of exchange time. Four distinct bands are observed at 2850, 2880, 2930, and 2970 cm-1. The 2850 cm-1 band is attributed to the symmetric methylene stretching mode (νs(CH2)) and the 2880 cm-1 mode is attributed to νs(CH3). The feature at 2930 cm-1 is broader than the other bands and contains contributions from the asymmetric methylene stretching mode at 2920 cm-1 (νa(CH2)) and a Fermi resonanceenhanced overtone of the symmetric methyl bending mode δs(CH3) at 2938 cm-1.4 The 2970 cm-1 band is assigned to the asymmetric methyl stretching mode (νa(CH3)). In comparison to the other IR spectra, after 60 min, the CH3 related modes at

Patole et al.

Figure 2. STM images of an FOT SAM on Au/mica as a function of exposure time in a 1 mM ethanolic solution of OT: (a) initial SAM (40 nm × 40 nm), V ) 1.2 V, It ) 10 pA, and (b) showing an area of panel a after Fourier filtering (30 nm × 30 nm), (c) after 30 min (40 nm × 40 nm), V ) 0.3 V, It ) 10 pA, (d) after 60 min (40 nm × 40 nm), V ) 0.5 V, It ) 10 pA, and (e) after 240 min (40 × 40 nm), V ) 0.5 V, It ) 10 pA.

2880, 2930 and 2970 cm-1 are more intense relative to the methylene related modes at 2850 and 2920 cm-1. The methylene related bands have dipole moments perpendicular to the carbon backbone. A decrease in their relative intensity implies that the alkanethiolates adopt a geometry that is closer to perpendicular to the surface than in the pure alkanethiolate layer where the molecular tilt angle is ∼30° from the surface normal. Figure 2a shows the STM image of the FOT covered layer prior to immersion in the OT solution. Figure 2b shows the Fourier filtered version of this image. Locally, molecules adopt a structure that is close to a p(2×2) periodicity. Over a length scale of a few nanometers, there is a substantial variation in contrast giving a characteristic mottled texture to the image.27 We have previously shown that there are a number of potential adsorption sites of similar adsorption energy for FOT on Au{111} and that the contrast in the STM image is strongly dependent on the adsorption site.27 Hence, FOT has a more flexible local structure on Au than, for example, OT reducing the tendency to observe well-defined domain boundaries. After 30 min of immersion in OT solution, ordered arrangements of molecular features appear in all areas of the STM images (Figure 2c). The separation of molecular features is consistent with a c(4×2) arrangement. [Note that this refers to the periodicity with respect to the underlying Au lattice not the c(4×2) superstructures commonly reported for thiolate adsorption on Au.30] However, the contrast of the molecular features within the c(4×2) “unit cell” is variable and there is a tendency for the contrast of rows of molecular features to alternate. The local coverage of molecules in these ordered domains is 0.25 ML, which is similar to the coverage of the p(2×2) structure observed for FOT on Au/ mica and lower than the 0.33 ML coverage associated with the (3×3)R30° structure of OT on Au/mica. It is interesting that the c(4×2) structure contains intermolecular spacings of 5 Å (i.e., typical of OT SAMs) and 5.7 Å (typical of FOT SAMs). After 60

Reversible Exchange of Self-Assembled Monolayers

Figure 3. PM-IRRAS data of an OT SAM on Au/mica as a function of exposure time in a 1 mM ethanolic solution of FOT: (a) C-F region and (b) C-H region.

min, the surface morphology consists of a series of parallel domains of average width ∼25 Å (Figure 2d). Figure 2e shows that by 240 min in OT solution, the surface is covered in well-defined domains with a molecular repeat consistent with a (3×3)R30° structure as is typical for OT on Au.27 2. Exchange of OT Layer by FOT. Figure 3 shows the PMIRRAS spectrum of the initially OT covered Au surface as a function of time in a 1 mM ethanolic solution of FOT. There is a gradual increase in the FOT related peaks in the 1000-1400 cm-1 range and a corresponding decrease in the C-H stretching peaks associated with OT in the 2800-3000 cm-1 range. In contrast to the behavior in the reverse system, there are no clear variations in relative intensity of the individual bands. Figure 4a shows the STM image of the FOT covered layer prior to immersion in the OT solution. The surface consists of large domains with a local periodicity consistent with a (3×3)R30° structure. At domain boundaries, a lower density structure is observed. After 30 min of exposure (Figure 4b) to the ethanolic solution of FOT, the (3×3)R30° domains appear relatively undisturbed while, by contrast, the lower density structure has largely disappeared to be replaced by features with no clear periodicity. After 60 min of exposure (Figure 4c), the area of the surface covered in (3×3)R30° domains has substantially decreased. After 240 min of exposure (Figure 4d), the surface is covered in molecular features of varying contrast. This is seen more clearly in the Fourier filtered image (Figure 4e) and is consistent with an FOT monolayer.27 Discussion The STM data suggest that there are clear differences between the mechanisms of exchange depending on which thiol is originally adsorbed on the surface. When the surface is initially covered in OT, the dominant exchange processes occur at domain boundaries as was reported, for example, for exchange between a dodecanethiolate SAM on Au and 11-mercapto-1undecanol from solution.23 In addition, Saavedra and coworkers24 have recently reported detailed kinetic studies of the

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Figure 4. STM images of an OT SAM on Au/mica as a function of exposure time in a 1 mM ethanolic solution of FOT: (a) initial SAM (40 nm × 40 nm), V ) 1.2 V, It ) 10 pA, and (b) after 30 min (40 nm × 40 nm), V ) 1.8 V, It ) 10 pA, (c) after 60 min (40 nm × 40 nm), V ) 1.5 V, It ) 10 pA, and (d) after 240 min (30 nm × 30 nm), V ) 1.5 V, It ) 10 pA (e) The image presented in panel d after Fourier filtering (30 nm × 30 nm).

displacement of a 1-adamantanethiolate monolayer on gold by n-dodecanethiol, which reveal that displacement is nucleated at defect sites in the 1-adamantanethiolate monolayer and that the displacement kinetics are inconsistent with Langmuir-like adsorption. Instead, the kinetics are consistent with a model of perimeter dependent island growth.24 In our study, we find that the lower density structures, seen as striped features in panels a and b of Figure 4, at boundaries between OT domains are quickly replaced while the tightly packed (3×3)R30° domains are still visible even after 60 min of exchange. By contrast, the replacement of FOT by OT appears to be more homogeneous. It is likely that this is caused by the lack of domain boundaries in the FOT monolayers. We have shown that there exist a number of discrete adsorption sites with similar adsorption energies for FOT.27 This allows for a much greater flexibility in the monolayer and a reduced tendency to form well-defined domain boundaries. A comparison of the relative rates of exchange at 340 K was carried out by measuring the rate of change in intensity of the various vibrational modes in the PM-IRRAS experiments. Figure 5 displays the change in the integrated intensity of all C-H and C-F related bands as a function of exchange time. [Note: There is some subtle variation in relative intensity of the various C-H (and C-F) related bands due to changes in molecular orientation etc. Therefore, the measurement of the integrated intensity of all peaks does not provide quantitative information on exchange kinetics. However, it does enable a comparison to be made as to which of the two exchange processes occurs more rapidly.] It is apparent that the exchange of OT by FOT is significantly more rapid than the reverse process. By 60 min, the exchange process is essentially almost complete when starting with an OT covered surface while, by

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Patole et al. in the carbon backbone being close to perpendicular to the surface. This is consistent with the behavior of the PM-IRRAS spectra in the mixed monolayer. It is not possible to determine whether a mixed domain has a 1:1 ratio of FOT and OT species. The contrast variation in these domains is perhaps an indication that the domains are random mixtures of FOT and OT. Conclusions

Figure 5. Integrated intensity of PM-IRRAS peaks during the exchange reactions.

The replacement of a FOT SAM by exposure to an ethanolic solution of OT at 340 K is significantly slower than the reverse process. Exchange occurs via the formation of an ordered array based on a c(4×2) molecular arrangementsa mixed domain of FOT and OT. In the reverse system, the dominant exchange process occurs at domain boundaries. We found no evidence for penetration of the OT domains by FOT. The greater vdW radius of FOT compared with OT hinders the incorporation of FOT species in the OT domains. The lack of domain boundaries in the FOT SAMs caused by the flexibility of the adsorption site of FOT species is likely to be the primary factor in attenuating the exchange process. Acknowledgment. We are grateful to EPSRC for the award of a research grant supporting this work (GR/T18585/01). References and Notes

Figure 6. Schematic diagram giving a possible explanation for the formation of the “c(4×2)” mixed phase following displacement of FOT species by OT. The sizes of FOT species (dark gray) and OT species (light gray) are represented in terms of their van der Waals’ radii.

contrast, when OT is replacing FOT, changes in band intensity are still evident beyond 60 min. Starting from an OT SAM, it seems likely that there is little or no mixing between the OT islands and the evolving FOT regions. In the reverse system, the facts that both FOT and OT are detectable by PM-IRRAS after 30 min of exchange and that the surface is essentially covered in stripes of molecular features suggest that mixed domains are being formed on the surface. A possible explanation for the “c(4×2)” periodicity is as follows (Figure 6). The local coverage in a pure FOT island is approximately 0.25 ML since FOT adopts a structure approximating to a p(2×2) array with one molecule per unit cell.27 The direct replacement of one FOT molecule by one OT molecule places the OT molecule in a low-density configuration compared to the 0.33 ML local coverage of the (3×3)R30°. It seems reasonable to assume that the formation of mixed domains in this case is more facile than in the reverse case where a single FOT molecule would be forced into a high-density environment in the middle of an OT island hindered by energetically unfavorable lateral repulsions due to its larger van der Waals (vdW) diameter (5.6 Å) compared with OT (4.2 Å).31 While it is energetically unfavorable for FOT molecules to align along the 〈-112〉 surface directions with the 3 (5.0 Å) periodicity, the mean vdW radius of an FOT and an OT molecule is ∼4.9 Å enabling FOT and OT species to align along such directions leading to the c(4×2) structure. Incorporation of an OT species into such an environment would likely result

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