pubs.acs.org/Langmuir © 2010 American Chemical Society
Adsorption Site Determination for Au-Octanethiolate on Au(111) Fangsen Li,†,‡ Lin Tang,‡ Wancheng Zhou,† and Quanmin Guo*,‡ †
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China, and ‡School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, U.K. Received January 4, 2010. Revised Manuscript Received February 18, 2010
Self-assembled monolayers (SAMs) of Au-octanethiolate √ √on Au(111) have been studied using scanning tunneling microscopy 3)R30° layer at 353 K for 1 h leads to the formation of a √ √ (STM). Thermal annealing of the dense√( 3 √ (5 3 3)R30°√striped √ phase coexisting with the ( 3 3)R30° phase. High-resolution STM imaging shows that the unit cell of the (5 3 3)R30° phase consists of four adsorbed Au-thiolate species giving rise to an adsorbate coverage of 0.27 ML. The four Au-thiolate species take the standing-up orientation and occupy adsorption √ inequivalent √ √ sites: √ one on a bridge site and three on the hollow sites. By drawing connections between the (5 3 3)R30° and the ( 3 3)R30° √ √ phases, it is found that the adsorption site for Au-thiolate inside the ( 3 3)R30° phase must be either the fcc hollow or the hcp hollow site.
Introduction Self-assembled monolayers (SAMs) of molecules on solid substrates are well-studied systems because of their interesting physical properties and ease of preparation.1-3 Alkanethiol monolayers on metals surfaces, in particular, the Au(111) surface, are the most scientifically investigated SAMs because they are the ideal systems for probing molecule-substrate interactions, molecule-molecule interactions, and surface phase transitions.4 Many experimental techniques have been used to investigate this system, such as scanning tunnelling microscopy (STM),5-14 low-energy electron diffraction (LEED),15 helium atom scattering,16,17 X-ray photoelectron spectroscopy (XPS),12,18 high-resolution electron energy loss spectroscopy (HREELS),19 grazing incidence X-ray *Corresponding author. E-mail:
[email protected]. Fax: þ44 121 414 7327. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Castner, D. G.; Ratner, B. D. In Frontiers in Surfaces Science and Interface Science; Duke, C. B., Plummer, E. W., Eds.; North Holland: Amsterdam, 2002; p 28. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (4) Vericat, C.; Vela, M. E.; Benitez, G. A.; Martin Gago, J. A.; Torrellels, X.; Salvarezza, R. C. J. Phys: Condens. Matter. 2006, 18, R867. (5) Poirier, G. E. Langmuir 1999, 15, 1167. (6) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (7) Maksymovych, P.; Sorescu, D. C.; Yates, J. T., Jr. Phys. Rev. Lett. 2006, 97, 146103. (8) Toerker, M.; Staub, R.; Fritz, T.; Schmitz-Hubsch, T.; Sellam, F.; Leo, K. Surf. Sci. 2000, 445, 100. (9) Sharma, M.; Komiyama, M.; Engstrom, J. R. Langmuir 2008, 24, 9937. (10) Lssem, B.; Mller-Meskamp, L.; Karthuser, S.; Waser, R. Langmuir 2005, 21, 5256. (11) Teran, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. J. Chem. Phys. 1998, 109, 5703. (12) Xiao, X.; Wang, B.; Zhang, C.; Yang, Z.; Loy, M. M. T. Surf. Sci. 2001, 472, 41. (13) Guo, Q.; Sun, X.; Palmer, R. E. Phys. Rev. B 2005, 71, 035406. (14) Keel, J. M.; Yin, J.; Guo, Q.; Palmer, R. E. J. Chem. Phys. 2002, 116, 7151. (15) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. J. J. Chem. Phys. 1993, 98, 678. (16) Camillone, N., III; Eissenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (17) Pflaum, J.; Bracco, G.; Schreiber, F.; Colorado, R., Jr.; Shmakova, O. E.; Lee, T. R.; Scoles, G.; Kahn, A. Surf. Sci. 2002, 498, 89. (18) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092. (19) Noh, J.; Kato, H.; Kawai, M.; Hara, M. J. Phys. Chem. B 2006, 110, 2793. (20) Torrelles, X.; Barrena, E.; Unuera, C.; Rius, J.; Ferrer, S.; Ocal, C. Langmuir 2004, 20, 9396.
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diffraction (GIXD),20,21 normal incidence X-ray standing waves (NIXSW),22 infrared spectroscopy (IR),23,24 and temperatureprogrammed desorption (TPD).25 Previous studies have collected a wealth of information on surface phase transitions, molecular adsorption sites, and the desorption mechanism of SAMs, thus providing a sound foundation for the exploitation of SAMs in molecular devices, biosensing, lubrication, and corrosion inhibition.26-30 Although there have been a large number of investigations into the alkanethiol monolayers in the last 20 years, a full understanding of this system is nowhere near complete. Recent reports of the existence of gold adatoms at the molecule-substrate interface7,31-35 have cast some serious doubt on the previously established models where direct bonding of thiolate species on (1 1) Au(111) was assumed. Theoretical modeling, with a view to identifying the location of the sulfur atom within the surface unit cell, has so far been restricted to the very short chain alkanethiols.36-38 For the longer-chain alkanethiols, the van der Waals interaction between the hydrocarbon chains can become relatively more important in stabilizing a particular adsorbate (21) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (22) Roper, M. G.; Skegg, M. P.; Fisher, C. J.; Lee, J. J.; Dhanak, V. R.; Woodruff, D. P.; Jones, R. G. Chem. Phys. Lett. 2004, 389, 87. (23) Picraux, L. B.; Zangmeister, C. D.; Batteas, J. D. Langmuir 2006, 22, 174. (24) Chailapakul, O.; Sun, L.; Xu, C. J.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (25) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1997, 36, 2379. (26) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771. (27) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (28) Hatzor, A.; Weiss, P. S. Science 2001, 291, 1019. (29) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (30) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (31) Yu, M.; et al. Phys. Rev. Lett. 2006, 97, 166102. (32) Kautz, N. A.; Kandel, S. A. J. Am. Chem. Soc. 2008, 130, 6908. (33) Li, F.; Zhou, W. C.; Guo, Q. Phys. Rev. B 2009, 79, 113412. (34) Torres, E.; Blumenau, A. T.; Biedermann, P. U. Phys. Rev. B 2009, 79, 075440. (35) Voznyy, O.; Dubowski, J. J.; Yates, J. T., Jr.; Maksymovych, P. J. Am. Chem. Soc. 2009, 131, 12989. (36) Mazzarello, R.; Cossaro, A.; Verdini, A.; Rousseau, R.; Casalis, L.; Danisman, M. F.; Floreano, L.; Scandolo, S.; Morgante, A.; Scoles, G. Phys. Rev. Lett. 2007, 98, 016102. (37) Gronbeck, H.; Hakkinen, H.; Whetten, R. L. J. Phys. Chem. C 2008, 112, 15940. (38) Zhou, J. G.; Hagelberg, F. Phys. Rev. Lett. 2006, 97, 045505.
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structure. The effect of the chain-chain interaction on the structure of the SAMs was studied many years ago, and a clear difference between the structure of the shorter-chain thiols (e.g., methanethiol and ethanethiol) and that of the longer-chain Short-chain SAMs molecules (e.g., hexanethiol)15 was√observed. √ have only one ordered structure: ( 3 3)R30° and its c(4 2) variations. Longer-chain SAMs, however, can exist in several stable striped structures depending on the surface coverage, in √ √ addition to the √ ( 3√ 3)R30° structure at a full coverage. Because the ( 3 3)R30° structure is universal and independent of the chain length of the thiol molecule, its origin is assigned with much confidence to the ordering of the sulfur headgroups. The exact location of sulfur within the surface unit cell is still under some debate, with the hollow site, atop site, and bridge site all considered to be possible.22,39 Alkanethiol SAMs can exhibit rich phases at different molecular packing densities, and phase diagrams are available from previous investigations.40 In the early stages of adsorption, the alkanethiol molecules form a liquidlike phase on Au(111). The liquid phase gradually changes to a flat-lying striped phase as the coverage increases. Further increases in coverage lead to the √ formation of a series of (p 3)√striped √ phases before the formation of the final close-packed ( 3 3)R30° phase. Here, p denotes the period of the stripe in units of the lattice spacing √ of Au(111), although (p 3) does not usually represent an appropriate surface unit cell. During the transition between the various phases, a disordered phase is usually found to coexist with the ordered phases. √ The√phase transitions can also be studied by starting with the ( 3 3)R30° phase and reducing the coverage √ step by step using thermal desorption.8 The close-packed ( 3 √ 3)R30° structure and the low-coverage, lying-down striped phase are rather well understood, albeit there is still some debate about the exact of the gold-thiolate species.31-35 √ adsorption site 5,9,19,23,41 The (p 3) striped phases, however, are less well understood, and the consistency between various structural models is poor. STM imaging of the striped phases has so far been partially successful in identifying the period of the stripes but less successful in providing concrete evidence for a complete determination of the adsorbate sites of the Au-thiolate.9 We use the term gold-thiolate, rather than just thiolate, to indicate that Au-SR is regarded as the adsorbed unit, in view of recent developments in the field.31,33 √ Among the various striped (p √ 3) structures, with p = 4, 5, 6, 7.5, 9, 9.5, 11.5, and 13, the (7.5 3) phase is the most frequently observed structure5,9,19,23,41 and it has been found for every evennumbered-chain-length thiol from C4 to C10√using low-energy electron diffraction (LEED).15 The (7.5 3) √ structure √ was known, perhaps more appropriately, as the (5 3 3)R30° structure.15 This is because the latter is a proper representation of (39) (a) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (b) Yenganeh, M. S.; Dougal, S. M.; Polizzotti, R. S.; Rabinowitz, P. Phys. Rev. Lett. 1995, 74, 1811. (c) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokohama, T.; Ohta, T.; Shimomura, M.; Kono, S. Phys. Rev. Lett. 2003, 90, 66102. (d) Gr€onbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839. (e) Yourdshahyan, Y.; Zhang, H. K.; Rappe, A. M. Phys. Rev. B: Condens. Matter 2001, 63, 81405R. (f) Vargas, M. C.; Gianozzi, P.; Selloni, A.; Scoles, G. J. Phys. Chem. B 2001, 105, 9509. (g) Gottschalck, J.; Hammer, B. J. Chem. Phys. 2002, 116, 784. (h) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258. (40) Schwartz, D. K. Annu. Rev. Phys. Chem. 2001, 52, 107. (41) (a) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (b) Staub, R.; Toerker, M.; Fritz, T.; Schmitz-Hubsch, T.; Sellam, F.; Leo, K. Langmuir 1998, 14, 6693. (c) Kondon, H.; Kodama, C.; Sumda, H.; Nozoye, H. J. Chem. Phys. 1999, 111, 1175. (d) Noh, J.; Hara, M. Langmuir 2001, 17, 7280. Langmuir, 2002,18, 1953. (e) Fitts, W. P.; White, J. M.; Poirier, G. E. Langmuir 2002, 18, 1561. Langmuir, 2002, 18, 2096. (f) Qian, Y.; Yang, G.; Yu, J.; Jung, T. A.; Liu, G. Y. Langmuir 2003, 19, 6056. (g) Barrena, E.; Palacios-Lidn, E.; Munera, C.; Torrelles, X.; Ferrer, S.; Jonas, U.; Salmeron, M.; Ocal, C. J. Am. Chem. Soc. 2004, 126, 385.
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the surface unit cell but the former is not. In this article, we use both notations for the purpose of connecting √ to previous pub√ lications, but it must be emphasized√that only (5 3 3)R30° is the proper notation and the (7.5 3) notation has been used by various researchers √ as√a convenient substitute. Previous STM findings studies of the (5 3 3)R30°41 phase show consistent √ regarding the period of the stripes and also the 3 spacing along the close-packed direction of the adsorbed molecules. However, there is a great discrepancy in both the number of adsorbed molecules within each unit cell and the orientation of the alkane chains. One of the key reasons that √ there √ is not a good understanding of the structure of the (5 3 3)R30° phase is that the resolution in previous STM images was not high enough and hence it was not possible to determine if there were additional molecular rows in between the usual bright rows.41f We report in this article our √ recent √ findings with high-resolution STM imaging of the (5 3 3)R30° phase of an octanethiol SAM on Au(111). Our STM images allow us to identify, without any ambiguity, the exact number of molecular rows within one striped period. we are able to determine that √ Thus, √ the coverage of the (5 3 3)R30° √ phase √ is 0.27 ML, which is 80% of that associated with the ( 3 3)R30° phase. Moreover, using the high-resolution images and by making connections between the adsorbate √ √positions within the striped phase and those in the ( 3 √ 3)R30° phase, we can determine that √ Au-thiolate in the ( 3 3)R30° phase occupies the hollow site.
Experimental Section We conducted our experiments in ultrahigh vacuum (UHV) at room temperature (RT) with a base pressure of 5 10-10 mbar using an Omicron VT-STM. Tungsten tips, made by electropolishing, were used. The Au(111) sample was a thin film (∼400 nm) prepared by the thermal evaporation of gold onto a mica substrate in a BOC Edwards Auto 306 deposition system. The mica substrate was held at 633 K during deposition and annealed at 633 K for 30 min after Au film deposition in vacuum. The deposition rate was 0.5 ML/min as monitored with a quartz crystal microbalance. The freshly prepared Au(111) sample was then transferred into a 1 mM 1-octanethiol solution in ethanol (99.5%) and left for 24 h at room temperature for the completion of the 1-octanethiol monolayer. Octanethiol with a purity of 98.5% was purchased from Sigma-Aldrich and used without further purification. The sample with the SAM was then taken out of the solution, thoroughly rinsed with pure ethanol, and dried under N2 flux. After the drying process, the sample was transferred to our UHV system for STM scanning without further treatment. The initial√ sample√shows typical etch pits and adsorbed molecules with the ( 3 3)R30° structure. The sample was then annealed in the K for 1 h, which √ UHV √ chamber at√ 353 √ produces mixed (5 3 3)R30° and ( 3 3)R30° phases.
Results and Discussion The alkanethiol self-assembled monolayer structures are strongly dependent on the surface coverage. As mentioned earlier, several methods have been used to control the coverage: gas-phase adsorption,5 immersion into certain solutions after SAM formation,42 and thermal annealing of freshly prepared SAMs.41b,d,f In this work, we used the thermal annealing Figure 1a shows an STM image √ method. √ from a close-packed ( 3 √3)R30° √ SAM of 1-octanethiol with no surface treatment. Several ( 3 3)R30° domains are seen to be separated by domain boundaries. Etch pits found mostly at the intersections of √ domain √boundaries are also occupied by adsorbate with the same ( 3 3)R30° configuration. (42) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855.
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√
√ 3)R30° structure. Image size, 30 30 nm ; Vb = 1.5 V, and It = 0.05 nA. (b) STM image (40 40 nm ) showing the formation of a striped phase after the sample was annealed at 353 K for 1 h. Vb = 0.8 V and It = 0.05 nA.
Figure 1. STM images of an octanethiol SAM on Au(111). (a) As-prepared SAM showing a close-packed ( 3 2
After the sample is annealed at 353 K for 1 h, the surface structure has changed. The etch pits have disappeared as expected, and a striped structure with bright rows running along the the sample, the [112] direction is observed. In many places on √ √ striped structure is found to coexist with the ( 3 3)R30° phase as shown in Figure 1b. The transition from one phase to the other is smooth, especially in the direction perpendicular to the stripes. This suggests a close connection between the two phases. Some rows √ in the √ striped phase can be seen to extend naturally into the ( 3 3)R30° phase (e.g., lines A1-B1 and A2-B2). This gives a strong indication that adsorbed species along lines of A1-B1 and A2-B2 may occupy the same lattice site √ regardless √ whether they are in the striped phase or in the ( 3 3)R30° phase. As can be seen in Figure√1b, the periodicity of the bright spots along the rows √ is 0.5 √ nm ( 3 0.289 nm), which is the same as that in the ( 3 3)R30° phase. The measurement of distances using STM could involve relatively large uncertainties due to inaccurate piezo calibration as well as thermal drift. We√have eliminated those uncertainties by using the structure of ( 3 √ √3)R30° √ as a reference. Because the unit cell dimensions of the ( 3 3)R30° √ structure √ are well established and both the striped phase and the ( 3 3)R30° phase are measured within the same frame, uncertainties in our measurement are reduced to very small random errors. We point out that in Figure 1b there are two different types of rows, with every other row belonging to the same type. For instance, in Figure 1b the rows marked with white lines are the same type of rows. The distance between two adjacent rows of the same type is 2.16 ( 0.10 nm. This distance equals 7.5 times the lattice constant (7.5 0.289 nm) of the Au(111) surface, hence this striped phase was given a convenient notation of (7.5 √ √ 3) in previous studies. This (7.5 3) phase has been observed many times before for SAMs of various chain-length alka√ nethiols.5,8,15 The real structure of this (7.5 3) phase and the associated adsorbate coverage have not yet been determined, and there are several conflicting structural models reported in the literature. For the striped phase in Figure 1b, the dark band between two adjacent bright rows (rows of different types) appears not to be occupied by adsorbed species. If the number of visible spots per unit area is√counted √ and directly compared with that associated with the ( 3 3)R30° phase, then one gets a coverage of 9486 DOI: 10.1021/la1000254
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0.135 ML for the striped phase. However, this low value of coverage is incorrect. The true coverage is in fact 0.27 ML, which is supported by high-resolution imaging that reveals two “hidden” rows of adsorbate, thus there are a total of four rows within each stripe period (Figure 2a). Although the relative contrast of the rows depends on the imaging parameters, the existence of the four rows is observed without any ambiguity. For the convenience of discussion, we will name the four different rows p, q, r, and s. A surface √ √ unit cell is drawn in Figure 2a. This unit cell has a (5 3 3)R30° structure that was first observed using LEED √ many years ago.15 An equivalent unit cell, with c(15 3) notation, that is twice as large is also shown. Unfortunately, a √ number of later papers used less accurate notation, √ (7.5 3), for the same surface structure. Because the (7.5 3) notation, while emphasizing the stripe period, is not √ a correct √ surface unit cell notation, we decided to use the (5 3 3)R30° notation wherever The distance between two adjacent p rows in √ possible. √ the (5 3 3)R30° phase is 7.5a, and there √are four √ rows within this distance. For the same distance in the ( 3 3)R30° phase, one finds five rows. Because the dots are equally spaced along √ the direction for both phases, the coverage of the (5 3 [112] √ √ √ 3)R30° phase is 4/5 that of the ( 3 3)R30° phase: 0.27 ML. This coverage is too high for flat-lying molecules. Using the van der Waals size of the molecule, it can be estimated that the coverage for the flat-lying orientation cannot be higher than 0.1 ML.√ Because √ the 0.27 ML coverage is fairly close to the 0.33 ML of the ( 3 3)R30° phase that is known to consist of standing-up √ molecules, we conclude that the adsorbed species within (5 3 √ 3)R30° also takes a similar standing-up geometry. There may be a small change in the tilt angle that we cannot measure with STM. A standing-up structure with the same 0.27 ML coverage was proposed in an earlier LEED study.15 Although it was not possible to know the interior structure of the unit cell from LEED patterns, IR data provided strong evidence of the molecular orientation.15 The spacing between each pair of neighboring rows within one stripe period is marked in Figure 2a. A striking feature is that the spacing is not uniform and thus there is no mirror plane in the direction parallel to the rows. A quick glance at the image in Figure 2a also reveals that the spots along different types of rows occupy different sites. Before we attempt to analyze√the adsorption √ sites, we will draw a bit more connection between ( 3 3)R30° and the striped phases using the image shown in Figure 2b. Langmuir 2010, 26(12), 9484–9490
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Figure 2. (a) High-resolution STM image (11.7 11.7 nm2) showing four rows within each stripe period, with relevant distances between
rows (Imaging conditions: Vb = -0.93 V and It = 0.05 nA.) (b) STM image (14 14 nm2) showing the relation between rows in the √ labeled. √ ( 3 3)R30° phase and those in the striped phase. (Imaging conditions: Vb = 0.8 V and It = 0.05 nA.)
In Figure 2b, only the p and r rows are visible. The r row does not take the central location between two p rows. In fact, as shown in the Figure, an r row is 4a away from the p row to its left and 3.5a from the p row to its right. Also shown in Figure 2b is an interesting relationship the rows in the striped phase √ between √ and the rows√in the√( 3 3)R30° phase. Looking at the right edge of√ the ( √ 3 3)R30° island in Figure 2b, the very last row in the ( 3 3)R30° island is 3.5a away from the nearest row, which is a p row, in the √ striped √ phase. If, however, we move to the opposite site of the ( 3 3)R30° island, we find that the last row at the edge of the island is separated by 4a from the nearest row, an r row in this case, to its left in the striped phase. Therefore, the rows within the striped √ phase √ are correlated even when they are “interrupted” by a ( 3 3)R30° island. This demonstrates that the two phases are not two independent, noninteracting structures. The adsorbate at the phase boundary must occupy specific sites controlled by molecules from both sides of the boundary. In Figure 2b, if a random p row is selected, √ √then its distance to any vertically oriented row in the ( 3 3)R30° phase is (n þ1/2)a, where n is an integer. However, an r row √ if √ is selected, then its distance to a row in the ( 3 3)R30° phase is na. Now we know that there is perfect alignment √ √between the r rows and the close-packed rows within the ( 3 3)R30° phase. Next, we will determine the relationship between the spots along the rows. In Figure 2b, line √ M-N√is drawn over a row of spots. Part of this row falls in the ( √3 √3)R30° region, and the other part as an r row falls in the (5 3 3)R30° region. We randomly choose two spots along the row, one in each region, and then measure their distance. √ Without any exception, we always find that the distance is n 3a, where n is an integer. Therefore, we can conclude that the adsorbate √ along √the r rows occupies the same lattice site as those in the ( 3 3)R30° phase. This finding is valuable for the assignment √ of the √ so-far illusive adsorption sites for Au-thiolates in the ( 3 3)R30° phase. It is known that √ √ imaging the ( 3 3)R30° phase alone is not sufficient to determine the adsorption sites because the image shown in Figure 1a can arise from one of four possible adsorption √ √ sites: fcc hollow, hcp hollow, bridge, and atop. The (5 3 3)R30° phase has a much lower symmetry; therefore, for the highresolution STM image shown in Figure 2a, there may well be Langmuir 2010, 26(12), 9484–9490
just one possible real surface structure that can give rise to the observed √ √image. If we determine the adsorption sites for the (5 3 3)R30° phase, then we can use links to √ the established √ determine the adsorption sites for the ( 3 3)R30° phase. √ √ To determine the positions of all adsorbates inside one (5 3 3)R30° unit cell, we applied a filtering process to enhance the locations of the spots in the STM image. Figure 3a shows such a processed image. As a reference, the upper part of the image consists of raw data without any filtering applied to it.√In Figure 3a, two different √ √unit cells are highlighted: a c(15 3) unit cell and a (5 3 3)R30° unit cell. The 30° rotation in the latter is√relative to the unit cell of the Au(111) substrate. The c(15 3) unit cell is not a √ primitive √ unit cell, and it is twice as large as the cell defined by (5 3 3)R30°, which is an oblique unit cell. Four lines;OL, OM, ON, and OP;all starting from the same point O are drawn on the image in Figure 3a. These four lines are used to establish the relative positions of all spots in the image. The dashed line, OL, in Figure 3a, is perpendicular to the rows and parallel to the [110] direction. Figure 3b shows all of the possible adsorption sites. We now start constructing structural models on the basis of the information available from Figure 3a. To make the models useful, we first need to clarify what is exactly imaged in STM. Au-thiolate has a headgroup consisting of an adatom of gold bonded to sulfur and a tail group that is CH3. It is not likely that the bright features are from electrons tunnelling directly from the methyl group. This is because the HOMOLUMO gap for octane is greater than 5 eV, which is well beyond the accessibility of the STM with a bias voltage of less than 2.0 eV. Hence, the regular structure that is observed at room temperature in STM is most likely associated with the ordering of the headgroup that consists of a sulfur atom bonded to a Au adatom. We examine three cases: (a) spots along the p rows occupying hollow sites (fcc or hcp), (b) spots along the p rows occupying bridging sites, and (c) spots along the p rows occupying atop sites. In each case, we produce a ball model where we place the rest of the adsorbate in the unit cell into locations so that the structure is as close as possible to that suggested in Figure 3a. The only constraint that we apply is that the adsorbate must be on reasonably high symmetry sites: hollow, bridge, or atop. Lines O-L0 , O-M0 , O-N0 , and O-P0 , corresponding to the lines in Figure 3a, are drawn on the ball models for direct comparisons. DOI: 10.1021/la1000254
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Figure 3. (a) Filtered STM image to enhance the spatial locations of the spots for quantitative analysis. (b) Schematic diagram to show the possible adsorption sites on Au(111). (c-e) Structural models based on three different adsorption sites along the p rows: (c) hollow, (d) bridge, and (e) atop sites.
The first thing that we have noticed is that all three ball models predict that along the r rows, the adsorbate should√take the √ hollow sites. This means that the Au-thiolate within ( 3 3)R30° should take the hollow sites, in good agreement with positions of gold-methane thiolate determined using the normal incidence X-ray standing wave (NIXSW) method.31 Our analysis method, however, does not distinguish between the fcc hollow and hcp hollow sites. When we thoroughly examine the positions of adsorbed species in each model, we compare each “adsorbate spot” in the model with that in the STM image. By doing this, we find that the model in Figure 3d has an ∼100% match with the STM image whereas the other two models consist of several adsorption site mismatches. These mismatches are small but large enough to allow us to select the correct model with confidence. According to the model in Figure 3d, adsorbate along p rows occupy the bridge sites; along other rows they occupy hollow sites. Moreover, there are two different types of hollow sites. If the adsorption site along the r rows is the fcc hollow site, then along the q and s rows it is the hcp hollow site and vice versa. The extra brightness of the dots along the p rows in the STM images thus arises from bridge-bonded Au-thiolate. The darker appearance of the q and s rows in the STM images is probably due to a combined effect of the adsorption sites and the increased tilt angle of the molecular chains. The existence of a molecular layer between Au(111) and the STM tip can reduce the effective height of the tunnel barrier in comparison to a vacuum gap of the same width. If the tilt angle is increased, then the vertical height of the molecular layer is reduced, as is the tunnel current. STM imaging 9488 DOI: 10.1021/la1000254
unfortunately does not provide direct information on the molecular tilt angle. This makes the direct comparison of physical heights difficult and unreliable. Figure 4 shows a ball model illustrating the relative positions of √ √the Au-thiolate species in both the stripe phase and the ( 3 3)R30° phase, with all of √ the features observed in Figure 2b correctly reproduced. The (5 3 √ √ √ 3)R30° phase and the ( 3 3)R30° phase are closely related to each other. They have parallel unit cell vectors, thus the boundary along the [112] direction is free √ from √ defects. This explains why the long-range order of the (5 3 3)R30° √ phase is maintained despite the apparent “interruption” by a ( 3 √ 3)R30° island. One important outcome from the above analysis is that the Authiolate species occupy different sites within one unit cell: bridge, fcc hollow, and hcp hollow. This is not surprising in view of the fact that the STM image of Figure 2a clearly shows that more than one adsorption site is involved in Au-thiolate bonding. The reason behind the multiple bonding sites is that the adsorption energy on different sites is only marginally different39 and the interaction between the molecular chains plays a rather important role in the overall of the adsorbate. A final note is that the √ organization √ (5 3 3)R30° phase is a frequently observed structure for SAMs prepared from a number of alkanthiol molecules with very different chain lengths. Therefore, this particular structure must be due to a common adsorption geometry rather than different conformations of the alkane chains. Now that the adsorption sites have been determined, we will switch our discussion to the connection between the Langmuir 2010, 26(12), 9484–9490
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√
Figure 4. Ball model showing the relative locations of Au-thiloate species both in the striped phase and in the ( 3
√ 3)R30° phase.
√ √ √ Figure 5. Series of STM images showing the structural transitions along the phase boundaries between the ( 3 3)R30° and (5 3 √ 3)R30° phases. The image sizes and scanning conditions are also shown.
√ √ ( 3 3)R30° phase and the striped √ phase. √ The striped phase, obtained by thermally annealing the√( 3 √ 3)R30° phase at 353 K, has a lower coverage than the ( 3 3)R30° phase. Therefore, partial desorption has taken place during annealing. Previous studies43 have shown that at 353 K the SAM gets melted and becomes a liquid phase. Thermal desorption experiments identify the desorption of disulfide from the surface.44 If the melting SAM is then cooled to room temperature, then surface reordering must take place. Because of reduced surface coverage, it is not √ the √ possible to form the ( 3√ 3)R30° phase all over √ √ the√surface, thus a layer of mixed ( 3 3)R30° and (5 3 3)R30° phases is √ √ formed. In our study, after the 353 K annealing, the ( √3 √3)R30° phase is the minority phase embedded in the (5 3 3)R30° phase. What is interesting is that the island of (43) Schreiber, F.; Eberhardt, A.; Leung, T. Y. B.; Wetterer, S. M.; Lavrich, D. J.; Berman, L.; Fenter, P.; Eisenberger, P.; Scoles, G. Phys. Rev. B 1998, 57, 12476. (44) Hayashi, T.; Wakamatsu, K.; Ito, E.; Hara, M. J. Phys. Chem. C 2009, 113, 18795.
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√ √ the ( 3 3)R30° phase has a characteristic shape: long in the [112] direction and narrow in the [110] direction, which is not expected from a phase that has 3-fold symmetry. This √ rectangular √ shape strongly suggests that the formation of the ( 3 3)R30° phase during cooling is significantly influenced by the existence of the striped phase. √ √ Figure 5a shows an STM image where a long, thin ( 3 3)R30° island can be seen. Using a relatively low bias voltage √ of 0.5 V, we managed to disturb the molecular ordering of the ( 3 √ 3)R30° phase (Figure 5b). We then followed the surface reorganization in real time by recording STM images at constant intervals at room temperature, and the structural changes are shown in Figure 5c-f. It √ is clear√ from these images √ that the √ phase boundary between the ( 3 3)R30° and (5 3 3)R30° phases does not stay fixed. Instead, the boundary keeps moving as √ √ a result of the expansion or shrinkage of the ( 3 3)R30° island. Because the images are not all the same size, a triangular shape is drawn into each image as a marker so that a comparison between the images can be easily made. DOI: 10.1021/la1000254
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During the structural transformations, a narrow region of the disordered phase is found to bridge the two ordered phases. This disordered region seems to play the role of a buffer zone to store or to provide molecules to the two phases. The coverage inside the disordered zone is likely to be too high for the striped phase but too low for the dense phase. In Figure 5c, part of the striped structure has replaced the disordered phase near the top of the image, and this process releases Au-thiolate species that are found to have squeezed into the neighboring area, leading to the creation of a disordered region. After some changes, the final structure as shown in Figure 5f assumes the characteristics of that in Figure 5a. Although we could not directly monitor the structural changes taking place within the SAM at elevated temperatures, we can deduce some key processes on the basis of the sequence of images shown in Figure 5. At 353 K, we should have a melted SAM layer. Partial desorption takes place, resulting in a reduction in surface√coverage. √ Upon cooling, the SAM reforms but a uniform ( 3 3)R30° phase is not possible because of the reduced surface coverage. Thus, the striped phase formed. Because the overall √ √coverage is still higher than √ that√required for a√ single √ (5 3 3)R30° phase, a mixed ( 3 3)R30° and (5 3 3)R30° √ layer √ is formed. The proportion of the surface covered by the ( 3 3)R30° phase is determined √ √ by the overall surface coverage. The formation of the ( 3 3)R30° islands √ is strongly influenced by the presence of the surrounding (5 3 √ 3)R30° phase as demonstrated by the shape of the islands and the associated straight step-edges parallel to the [112] direction. Finally, we discuss our findings in relation to the recent models where gold adatoms are involved in chemisorbed thiol molecules. The formation of etch pits on Au(111) is very consistent with the process where surface gold atoms are released to form Authiolate. The electrochemical removal of SAM by the STM tip has also proven the existence of adatoms within the SAM.33 Our analysis is based on one Au atom per thiolate, although currently there are also models predicting one Au atom for every two thiolate species. If every two thiolates share one Au adatom and sulfur is imaged in STM, then we would have observed some correlated pairing features in high-resolution images. If only the Au adatoms are imaged in STM, then the number of bright
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features should never exceed 0.165 ML. Our data suggests that there is one Au adatom per thiolate, and the Au adatom is bright in the STM images. However, the existing models are based solely on adsorbed methyl thiolate, which does not have the relatively strong van der Waals interaction that is found to exist between the hydrocarbon chains in longer thiols. For thiol molecules with a relatively long hydrocarbon backbone such as octanethiol, the contribution of the van der Waals interaction to the overall stability of the adsorbate structure is rather significant. We need to wait until reliable theoretical modeling on longer-chain-length thiols becomes available before a definitive conclusion on this issue An important √ is reached. √ √ √finding from our study is that the (5 3 3)R30° and ( 3 3)R30° phases are closely related and one phase can readily change into the other at the phase boundaries. This can be used to test the accuracy of any model because a correct model must satisfactorily explain the structure of both √ phases. √ The current models have so far focused mainly on the ( 3 3)R30° phase, which has a high degree of symmetry. As already demonstrated √ √ in this article, the lower degree of symmetry of the (5 3 3)R30° phase provides a very stringent test criterion for selecting the correct structural model.
Conclusions High-resolution STM imaging gives direct evidence√that there are four adsorbed Au-thiolate species inside the (5 √3 √ √ √3)R30°/(7.5 3) unit cell. Thus, the coverage of the (5 3 3)R30° phase is determined to be 0.27 ML. The four Authiolate species within the unit cell occupy different surface sites: one bridging site and three √ hollow √ sites. The adsorption site determination for the (5 3 3)R30° √phase √also helped to identify that the Au-thiolate within the ( 3 √3)R30° √ phase occupies the hollow site. For SAMs with mixed ( 3 3)R30° √ √ √ √ and (5 3 3)R30° phases, the ( 3 3)R30° islands take a long, thin shape as a result of the direct influence of √ rectangular √ the (5 3 3)R30° phase. Acknowledgment. We thank the EPSRC for financial support, and F.L. thanks the Chinese Scholarship Council for providing a studentship.
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