Langmuir 1997, 13, 3055-3058
Desorption of Butanethiol from Au(111) during Storage in Ultrahigh Vacuum: Effects on Surface Coverage and Stability toward Displacement by Solution-Phase Thiols David A. Hutt and Graham J. Leggett* Department of Materials Engineering and Materials Design, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. Received October 21, 1996. In Final Form: March 11, 1997
In recent years, significant advances have been realized in our understanding of the structures of self-assembled monolayers (SAMs) through the application of scanning tunneling microscopy (STM).1-6 For SAMs of long chain methyl-terminated molecules (CH3(CH2)n-1SH, where n g 8), the model predicted by diffraction studies, based on a (x3×x3)R30° lattice relative to the Au(111) substrate, has been confirmed and subsequently refined to include a c(4×2) superlattice.1-3 Poirier and co-workers7-9 have performed a number of elegant studies in which they have examined the behavior of butanethiol monolayers on Au. These have shown a very different series of structures to those formed by long chain SAMs. Upon deposition from solution, a 2D liquid-like phase is first observed over the whole surface, with no resolvable structure. On storage in vacuum, however, this converts, over a period of several days, to an ordered phase ascribed to a p×x3 (p ) 8-10) unit mesh.7 This appears to consist of rows of thiol molecules aligned along the next-nearest neighbor direction (i.e., 〈112h 〉) of the Au(111) substrate, separated by several (determined by p) exposed Au rows. Similar structures have been observed by other groups10,11 using He atom diffraction for long chain molecules deposited under ultrahigh vacuum (UHV) conditions. In addition, p×x3 structures have also been noted in STM studies of long chain molecules deposited by very rapid exposure to dilute solution.6 It therefore appears that this arrangement is a stable low coverage structure formed before completion of the (x3×x3)R30° layer. Support for this comes from studies7,10 of monolayers originally possessing the (x3×x3)R30° structure which, upon annealing in vacuum, convert after sufficient time and elevation of temperature to the p×x3 layer, indicating desorption of some of the monolayer. For longer chain molecules, the value of p has been found to correlate closely with the length of the methylene chain, leading to the suggestion that the p×x3 structure may consist of a disulfide molecule attached to the surface with the methylene chains lying down parallel to the gold substrate.11 There is some debate as to the actual part of the thiol molecule that is imaged during STM measurements, * Corresponding author. (1) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (2) Anselmetti, D.; Baratoff, A.; Guntherodt, H.-J.; Delamarche, E.; Michel, B.; Gerber, Ch.; Kang, H.; Wolf, H.; Ringsdorf, H. Europhys. Lett. 1994, 27, 365. (3) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (4) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Chem. Phys. 1996, 105, 2089. (5) Chiang, S. Science 1996, 272, 1123. (6) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (7) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (8) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (9) Poirier, G. E. J. Vac. Sci. Technol., B 1996, 14, 1453. (10) Camillone, N.; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (11) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737.
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although it is generally thought that it is the electronic states of the S atom adsorbed at the gold surface that are probed instead of the methyl tail groups. Support for this has come from atomic resolution STM studies by Cyr et al.,12 in which thiol molecules lying flat on graphite show much brighter contrast for the thiol head group compared to the methylene chain. This difficulty in interpretation of the STM images has resulted in some speculation as to the actual structure of the p×x3 layer. In contrast to the open structures described above, a recent STM study by Kang and Rowntree13 has suggested some alternatives in which there are no uncovered areas of gold. There is clearly a need to establish the surface coverage of thiol species in the p×x3 arrangement to distinguish between the many suggested structures. To date, we are not aware of any spectroscopic investigations of the rearrangements of self-assembled monolayers in vacuum or of the p×x3 structure of butanethiol. In the present study, we have carried out an X-ray photoelectron spectroscopy (XPS) study of butanethiol monolayers prepared from solution and stored in vacuum for several days. Our objectives were first, to establish the extent of desorption from the butanethiol layers during the reorganization to form the p×x3 structure; second, to examine whether spectroscopic data supported one or other of the alternative p×x3 structures suggested by STM and diffraction data; and third, to examine the effect of the formation of the p×x3 structure on monolayer stability, by comparison of its resistance to solution-phase displacement, by another thiol, with that of the initially formed, 2-D liquid phase monolayer. Studies of the change in monolayer stability during reorganization may well illuminate the driving forces that determine SAM stability more generally, and the kinetics of self-assembly, which still remain the subject of much debate. The monolayers used in this work were prepared on evaporated gold films (∼500 Å, 99.99+%, Goodfellow Metals) supported on Cr primed (∼20 Å, 99.99+%, Goodfellow Metals) glass coverslips (Chance, no. 2 thickness). The SAMs were formed by immersion of the freshly deposited gold film into a 1-2 mM solution of butanethiol (>97%, Fluka) in degassed ethanol (99.9%) for approximately 18 h. Following this, the samples were removed from solution, rinsed with ethanol, and dried with a stream of N2 gas. Samples were then cut up into smaller pieces for XPS analysis using a diamond-tipped scribe. All glassware used in the preparation was cleaned by soaking in hot (∼90 °C) “Piranha” solution14 for 30 min before rinsing with reverse osmosis water and drying in an oven at ∼70 °C. XPS measurements were made using a VG ESCALAB instrument equipped with an unmonochromated twin anode X-ray source and 100 mm radius hemispherical electron energy analyzer. Samples, stored in vacuum, were left in the preparation chamber of the instrument, at a base pressure of 1 × 10-9 Torr, for the specified period of time, before being moved through vacuum to the analysis chamber. Al KR radiation was used at all times to record the XPS spectra. Several spectra were recorded for each sample, with the analyzer operated in fixed transmission mode, using different pass energies to obtain the best resolution/sensitivity for the region being scanned. A (12) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747. (13) Kang, J.; Rowntree, P. A. Langmuir 1996, 12, 2813. (14) “Piranha” solution is a mixture of concentrated (95%) sulfuric acid and 30% hydrogen peroxide solution in the ratio 7:3. Caution: this solution may detonate spontaneously on contact with organic materials.
© 1997 American Chemical Society
3056 Langmuir, Vol. 13, No. 11, 1997
Notes
Figure 1. XPS spectra obtained from butanethiol monolayers on gold (a) immediately after preparation and (b) after storage in vacuum for 67 h. Spectra are offset in y direction for clarity. C 1s spectra have been fitted with a single symmetrical Lorentzian (30%)/Gaussian (70%) peak.
survey scan was taken first, followed by detailed scans of C 1s (10 eV pass energy, high resolution), O 1s/Au 4p3/2 (20 eV pass energy), and S 2p (50 eV pass energy, high sensitivity) regions, together with separate Au 4f7/2,5/2 spectra recorded using all three pass energies for comparison. Peak area ratios were corrected using empirically determined sensitivity factors, to account for elemental variations in photoelectron yields. Figure 1 shows XPS spectra of butanethiol SAM samples obtained immediately upon loading into the vacuum system and after storage in vacuum for 67 h. The S 2p region of the spectrum of the as prepared sample shows a broad peak centered at a binding energy of 162.5 eV. This peak is composed of two components, the S 2p3/2 and 2p1/2 peaks. However, the high analyzer pass energy, used to obtain maximum sensitivity, has prevented them from being resolved. The C 1s region for this sample shows a single symmetrical peak centered at 284.9 eV, indicating a single environment for the C atoms in the molecules, in agreement with other studies15 (the S atom does not produce a measurable chemical shift to the adjacent CH2 group16). The studies of Poirier et al.,7,8 indicated that storage of a monolayer of butanethiol in vacuum for 67 h results in complete conversion of the 2D liquid phase to the p×x3 structure. The XPS spectra obtained from the sample left in vacuum for this period of time (Figure 1b) show significant differences from those of the as prepared monolayer. The most important change is the large reduction in the size of the S 2p3/2,1/2 peak. In addition, there has been a small increase in the size of the C 1s peak, and both the S and C peaks have shifted slightly to higher binding energy. The increase in C peak area is thought to be due to the adsorption of small quantities of carbonaceous contaminants from the background vacuum, which has caused the slight increase in binding energy of the peaks due to additional charging in the monolayer. Attenuation of the S 2p photoelectrons by the extra C is (15) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (16) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; J. Wiley & Sons: Chichester, 1992.
not significant enough to explain the large decrease in S 2p area, however, especially as the Au 4f peaks (not shown) are only slightly reduced in intensity. By taking the ratio of the S 2p peak area to the Au 4f peak area, the effect of the contaminating layer can be excluded as S and Au have similar escape depths, because of the close binding energies of the peaks,17 and because both signals are attenuated by the same thickness of material. For the as prepared monolayer the S 2p(3/2+1/2)/Au 4f(7/2+5/2) XPS peak area ratio is found to be 0.084 ( 0.003, and for the sample left in vacuum for 67 h the ratio is reduced to 0.036 ( 0.001 implying a reduction in S coverage to 42 ( 2% of the original value. In order to demonstrate further the openness of the p×x3 structure, displacement experiments were conducted using mercaptopropanol. Monolayers of butanethiol freshly prepared and following storage in vacuum for 63 h, were each dipped into a 2 mM solution of 3-mercaptopropanol in ethanol for 7 min, before removal, rinsing with ethanol, and drying with N2 gas. The period of immersion was selected on the basis of extensive studies in this laboratory on the stabilities of SAMs and photopatterned monolayers toward solution-phase thiols; these studies indicate that for such short time periods, a solutionphase thiol will adsorb to a region of bare Au but will not be able to displace adsorbates from a complete monolayer of thiols.18 Figure 2 shows the XPS spectra obtained from these samples. The S 2p regions of the spectra show no difference between the two samples, indicating a saturation coverage of S on the surface. However, examination of the C 1s region of the spectrum shows significant differences. For the as prepared sample dipped in mercaptopropanol, there appears to be little change from (17) On the basis of the work of Whitesides et al. (Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017), the escape depth, λ, of photoelectrons passing through a self-assembled monolayer can be calculated using the equation λ(Å) ) 9 + 0.022K.E. (eV), where K.E. is the kinetic energy of the photoelectrons in electronvolts. This gives values of 38.1 Å for S 2p electrons and 39.9 Å for Au 4f photoelectrons. (18) Cooper, E.; Hutt, D. A.; Parker, L.; Wiggs, R.; Leggett, G. J.; Parker, T. L. J. Mater. Chem. 1997, 7, 435. Cooper, E., Leggett, G. J. Unpublished data.
Notes
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Figure 2. XPS spectra of butanethiol monolayers following exposure to mercaptopropanol solution for 7 min. In (a) the butanethiol SAM was exposed immediately after preparation, while in (b) the SAM was stored in vacuum for 63 h before exposure to mercaptopropanol. Spectra have been fitted with symmetrical Lorentzian (30%)/Gaussian (70%) peaks to highlight features. Spectra are offset in y direction for clarity.
the spectrum of an undipped sample (cf. Figure 1a). However, the vacuum-stored and dipped sample has a very different C 1s peak shape and can be fitted with two components, separated by 1.6 eV in the area ratio of ∼2:1. This spectrum is indistinguishable from that expected for a mercaptopropanol monolayer on Au19 and indicates the complete replacement of the butanethiol layer. Further evidence of the adsorption of mercaptopropanol onto the surface can be seen in the O 1s/Au 4p3/2 region of the XPS spectra. The spectrum of the freshly prepared butanethiol monolayer dipped in mercaptopropanol shows only two features: a large peak at 546.9 eV due to Au 4p3/2 and a broad hump centered at approximately 536.8 eV attributed to satellites from the Au 4p peak, generated by the unmonochromated X-ray source. There is no contribution to the spectrum from O 1s around 532 eV; however, the sample stored in vacuum and then dipped in mercaptopropanol shows a very large O 1s contribution at 532.6 eV. The C:O ratio for this layer is found to be ∼3:1, again identical to the value that would be expected for a monolayer of mercaptopropanol,19 supporting the conclusion that the butanethiol has been completely replaced on the surface by it.20 The monolayer that has been stored in vacuum therefore has a very different stability toward displacement by solution-phase thiols than the as prepared monolayer. It appears from this work that the formation of the p×x3 structure of butanethiol on Au(111) involves extensive loss (∼60%) of S-containing species from the surface of the original 2D liquid monolayer. If the bright regions in the “pinstripe” structure observed in the STM images of Poirier et al.7,8 are indeed S head groups adsorbed onto the gold surface, then the coverage would be 33% of that of a densely packed (x3×x3)R30° structure (assuming a (19) Hutt, D. A.; Leggett, G. J. Langmuir, in press. (20) For comparison, the same experiments were conducted on long chain SAMs of dodecanethiol. Following storage in vacuum for 3 days, there was no apparent change in the size of the S 2p XPS peak indicating no loss of S, in stark contrast to the butanethiol data. In addition, the C 1s spectra also remained unchanged, with only a single symmetrical peak for both samples. Replacement experiments with mercaptopropanol were also performed on these samples, but no indication of any O in the spectra could be seen for either the freshly prepared or vacuum stored samples. These observations are in good agreement with previous STM studies7 of these materials which show the immediate formation of a (x3×x3)R30° structure which does not reorder over many days in vacuum.
value for p of 9). However, if the gray areas between alternate pinstripes contain close packed thiol molecules in a (x3×x3)R30° arrangement,13 then the coverage should be 66% of that of a complete monolayer. The 2D liquid initially formed by butanethiol is likely to have a density similar to, but less than, that of a (x3×x3)R30° SAM. This assumption is supported by the observation that the 2D liquid phase is able to resist replacement by mercaptopropanol. The coverage of 42 ( 2% relative to the 2D liquid phase obtained in our work is therefore likely to be a slight overestimate of the actual surface coverage of the p×x3 phase and best supports the pinstripe structure consisting of single rows of butanethiol molecules separated by bare regions of gold with a coverage of 33% of a (x3×x3)R30° layer. The spacing between the remaining thiol rows is approximately twice the length of the methylene chain of butanethiol, and based on the assignment of the coverage, it is possible for us to speculate that the gray areas between the alternate pinstripes observed by Poirier et al.7 are caused by the methylene chains lying down tail to tail on the gold surface. This structure is supported by the He diffraction studies of Camillone et al.11 which show an inter-row spacing equivalent to twice the methylene chain length for a range of long chain molecules deposited from the gas phase onto gold surfaces. Subsequently, interactions between the alkyl chains and the Au surface are able to stabilize the p×x3 structure against further desorption of adsorbates. The mechanism for the formation of the p×x3 structures is uncertain but clearly involves the desorption of thiol species from the surface. This seems in disagreement with the basic understanding of alkanethiols on gold which are thought to possess a very strong S-Au interaction, making room temperature desorption unlikely. In addition, the open structure has far fewer stabilizing van der Waals interactions than the close packed (x3×x3)R30° arrangement. A significant driving force must therefore be at work for the restructuring to occur. There are several possible routes by which the thiol species may desorb from the surface, assuming that in the 2D liquid phase the molecules are chemisorbed at the surface rather than physisorbed. The combination of the thiolate head group with hydrogen from the background vacuum is one possible route which would lead to a species that may desorb readily; however, the most likely mechanism for
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the desorption process is the combination of two thiolate head groups to form a stable disulfide species which can leave the surface directly. Support for this second mechanism can be found in the recent X-ray diffraction study of Fenter et al.21 who postulated the dimerization of the S head groups, in a (x3×x3)R30° SAM of long chain molecules, to create the c(4×2) superlattice. After the thiols have desorbed from the surface, the formation of the p×x3 structure must involve the further reorganization of the remaining molecules. This process has been noted in the work of Camillone et al.10 for structures formed on annealing of preformed (x3×x3)R30° SAMs of decanethiol. They found that annealing the SAM to 348 K resulted in the appearance of p×x3 islands within the (x3×x3)R30° structure; however, p was not single valued, but varied across the surface. Annealing to 438 K resulted in the formation of structures with a value of p of 7.5 which subsequently increased to 11 after storage at room temperature in vacuum for several days. This p value was the same as that observed for vapor-deposited monolayers of decanethiol and indicated that the spacing of the thiol rows had increased, implying the desorption of even more of this long chain molecule at room temperature. It therefore appears that once the density of the monolayer is reduced from that of the (x3×x3)R30° layer, the desorption of molecules from the surface becomes very facile as the layer tries to form the next most stable structure. The observations described above lead us to speculate that it is the interchain interactions which determine the desorption process. In the butanethiol 2D liquid phase the van der Waals interactions between chains are clearly weaker than the thermal energy at room temperature preventing the formation of a (x3×x3)R30° structure. These weak interchain forces also cannot prevent the desorption of the disulfide species from the surface. For the dense (x3×x3)R30° arrangement formed by longer molecules, the stronger interchain forces are significant enough to prevent desorption of the disulfide moieties. However, once the structure is broken up by annealing in vacuum, either by a melting or desorption process, the van der Waals interactions are reduced sufficiently in some areas of the surface for desorption to occur and the molecules arrange themselves into the next most stable structure which is the p×x3, maintaining the (x3×x3)R30° in the remaining regions. While the initially formed 2D liquid phase monolayer protected the Au substrate against adsorption of mercaptopropanol, the p×x3 structure clearly provided no such protection and was readily displaced by a second, (21) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216.
Notes
solution-phase thiol. This confirms that the structure of the adsorbate layer becomes much more open following storage in vacuum; however, it may also provide some indications as to the nature of the bonding between S and Au in the p×x3 arrangement. If the S-Au bonding was strong, it would be expected that mercaptopropanol might adsorb into the missing rows in the p×x3 structure, but not that all of the butanethiol molecules would be displaced. The complete displacement of the butanethiolate and the rapidity of the displacement would therefore only be explicable if some significant driving force were in operation. It may be that exposure of the p×x3 structure to the mercaptopropanol solution results in its destabilization and ready replacement at the surface. However, the formation of hydrogen bonds between mercaptopropanol tail groups leading to an accompanying stabilization with respect to butanethiol may explain this behavior. Such a suggestion would be consistent with the recent observations of Poirier et al.,4 who reported a change in the structure of mercaptohexanol monolayers on exposure to water vapor and attributed this to the dependence of SAM structure on tail group intermolecular interactions. In summary, storage of a self-assembled monolayer of butanethiol on gold in vacuum leads to the desorption of a significant fraction of the adsorbed molecules. It is estimated that the coverage of the structure that results is less than 42% of the coverage expected for a monolayer of close-packed adsorbates in a (x3×x3)R30° arrangement. When this low coverage structure was exposed to a solution of mercaptopropanol, butanethiol was found to be completely displaced, confirming the openness of the reorganized adsorbates. These findings are consistent with the formation of the p×x3 structure proposed by Poirier et al. on the basis of STM data.7 We speculate that monolayers of butanethiol on gold are only weakly adsorbed, with desorption of a large fraction of the adsorbates occurring before ultimate stabilization of a lowcoverage structure via interactions between adsorbate alkyl chains and the gold surface. It may be that interchain interactions are of much greater significance in stabilizing self-assembled monolayers than has previously been thought, with the sulfur-gold interaction being just strong enough to guide assembly of the adsorbates, rather than playing a determining role in influencing their well-attested stability. Acknowledgment. The authors are grateful to the Engineering and Physical Sciences Research Council (Grant GR/K28671) for financial support. LA961014E
