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Formation of a Self-Assembled Monolayer by Adsorption of Thiophene on Au(111) and Its Photooxidation Mark H. Dishner, John C. Hemminger,* and Frank J. Feher* Department of Chemistry, University of California, Irvine, California 92697 Received August 26, 1996. In Final Form: November 1, 1996X In direct conflict with theoretical predictions, thiophene monolayers were adsorbed onto Au(111) by immersing an evaporated thin film of mica supported gold in an ethanolic solution of thiophene. The resulting self-assembled Monolayer (SAM) was shown to be annealable and to be comprised of large domains adopting the unit cell (2x19×x3)R30°. The SAMs can be removed by photooxidation in air and rinsing with ethanol to afford clean reconstructed Au(111).
Introduction Self-assembled monolayers (SAMs) prepared by adsorbing alkanethiols or dialkyl disulfides onto Au(111) have attracted widespread interest because of their ease of preparation and potential for practical applications.1-7 Numerous surface analytical techniques have been used to study Au(111)-supported alkanethiol SAMs,8-16 but the application of the scanning tunneling microscope (STM) has allowed unprecedented molecular-level examination of these important sulfur-based monolayers.17,18 Many properties of Au(111)-supported alkanethiols and dialkyl disulfides are well-established. Monolayers prepared with both classes of compounds appear to adopt identical surface structures, but the identity of this structure and the details of the self-assembly process are still controversial. For example, recent work by Fenter, Eberhart, and Eisenberger10 suggests that alkanethiols react on Au(111) to produce adsorbed disulfides, whereas the conventional belief is that both alkanethiols8,19,20 and dialkyl disulfides8,14,19,20 are reduced to thiolates upon chemisorption on gold. Similarly, our recent observation21 that photochemical oxidation of dodecanethiol SAMs X Abstract published in Advance ACS Abstracts, December 15, 1996.
(1) Horvath, I. T.; Rabai, J. Science 1994, 266, 72-75. (2) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103-4108. (3) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (4) Behm, J. M.; Lykke, K. R.; Pellin, M. J.; Hemminger, J. C. Langmuir 1996, 12, 2121-2124. (5) Mrksich, M.; Whitesides, G. M. Trends Biotechnol. 1995, 13, 228235. (6) Kumar, A.; Biobuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (7) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (8) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825-1831. (9) Dubois, L. H.; Zegarki, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678-688. (10) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218. (11) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493-8502. (12) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-3386. (13) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (14) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (15) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 437463. (16) Chidsey, C. E. D.; Lui, G.-L.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421-4423. (17) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 1096610970. (18) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853-2856. (19) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 17661770.
S0743-7463(96)00840-2 CCC: $12.00
induces rapid healing of the “pits” formed upon chemisorption of the thiol is inconsistent with traditional etching and annealing mechanisms.2,17,22-25 As part of an effort to elucidate the details of SAM formation, we have been using a STM to examine Au(111) surfaces which have been exposed to a variety of organosulfur compounds. In this paper we describe the formation and characterization of a well-ordered monolayer prepared by adsorbing thiophene (C4H4S) onto Au(111). Our results, which contradict recent theoretical work predicting that thiophene would not interact with gold,26 have important and interesting implications concerning the chemisorption of alkanethiols, dialkyl disulfides, and related compounds on Au(111). Experimental Section Thin films of gold (100 nm) were prepared by thermally evaporating gold (1 Å/s) onto freshly cleaved mica (AshevilleSchoomaker AV-STM grade) at 2.5 mbar and 380 °C. The films can be used as is or annealed with a small butane torch to remove any carbon contaminants. This procedure regularly produces highly crystalline films with terraces in excess of 1500 Å × 1500 Å of clean, defect-free [111] oriented gold. SAMs of thiophene are prepared by immersing Au(111) in an ethanol solution of thiophene (4 mM, 2.5 h), followed by rinsing with ethanol and drying in a stream of nitrogen. Thiophene solutions were prepared with Aldrich gold label (99+% purity) thiophene. In order to insure the integrity of our experiment, thiophene solutions were treated with gold powder (1-3 µm spheres, 0.30 m2/g) to adsorb any impurities before the gold films were immersed. Thiophene monolayers were photochemically oxidized in air with a Stromart HBO 200 W high-pressure Hg vapor lamp. The light was passed though an IR filter and focused onto the sample (1.5 cm2) at a distance of 25 cm. Typically the photolysis time was 15 min. The oxidized thiophene monolayer could be rinsed off with ethanol to afford clean Au(111), which, in turn, could be reimmersed in thiophene solution to produce another well ordered SAM. All STM images were collected with a Besocke type STM in constant current mode with bias and current conditions (typically 0.30 V and 1.0 nA) that yielded images with the highest possible (20) Ulman, A. In Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (21) Dishner, M. H.; Feher, F. J.; Hemminger, J. C. Chem. Commun. 1996, 1971 and 1972. (22) Buchhloz, S.; Fuchs, H.; Rabe, J. P. J. Vac. Sci. Technol., B 1991, B9, 857-861. (23) Cavalleri, O.; Hirstein, A.; Kern, K. Surf. Sci. Lett. 1995, 340, L960-L964. (24) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (25) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826-6834. (26) Elfeninat, F.; Fredricksson, C.; Sacher, E.; Selmani, A. J. J. Chem. Phys. 1995, 102, 6153-6158.
© 1996 American Chemical Society
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a
Figure 1. 1500 Å × 1500 Å STM image (differentiated) of an unannealed thiophene monolayer on Au(111) recorded at 0.30 V and 1.0 nA.
b
Figure 2. 1500 Å × 1500 Å STM image (differentiated) of an annealed thiophene monolayer on Au(111) recorded at 0.30 V and 1.0 nA. signal to noise ratio. Atomic resolution and atomic steps of clean Au(111) were used to calibrate the spatial dimensions of the images.
Results and Discussion Exposure of Au(111) surfaces to an ethanol solution of thiophene produces a stable SAM which is strikingly similar to SAMs obtained by reacting Au(111) with alkanethiols or dialkyl disulfides. A typical large scale STM image (1500 Å × 1500 Å), shown in Figure 1, exhibits well-defined step edges and uniformly distributed vacancy islands or “pits” which are one gold atom deep. In most thiol-based systems vacancy islands are observed by STM,2,9,12,17,18,21,24,25,27,28 and these defects are healed by annealing the monolayer.2,17,23 In the thiophene SAM, the vacancy islands in the monolayer are stable at room temperature but quickly anneal upon warming to 40 °C (30 min). Figure 2 shows a large scale STM image (1500 Å × 1500 Å) of a typical sample after annealing and cooling to room temperature. Vacancy islands are no longer present, and large, well-ordered domains (striped pattern) are clearly visible. Figure 3a shows a higher resolution (280 Å × 280 Å), true height, STM image of a thiophene SAM which was (27) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096-1102. (28) Camillone, N.; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 1103111036.
Figure 3. (a) Nondifferentiated STM image of unannealed thiophene on Au(111) recorded at 0.30 V and 1.0 nA. The image is 280 Å × 280 Å. The dark bar is one typical line scan contributing to the 150 line scans averaged and shown in part b. (b) Line scan resulting from averaging 150 line scans from part a. The minima between bright rows clearly alternate in depth between gray scale value of ∼150 for the shallow valleys and ∼140 for the deeper valleys.
not annealed. Inspection of Figure 3a reveals 3-4 Å diameter features (spaced by 5 Å) that exist in lines which are separated by approximately 12 Å. These lines correspond to the stripes which are visible in Figure 2. As expected for adsorption on the (111) surface, we observe three symmetry equivalent sets of domains which are rotated by 120° with respect to each other. At a place where a SAM domain intersects a monatomic step in the gold substrate, the angle between the bright lines and the straight step edge is observed to be either 30° or 90°, depending on the domain orientation. Since the straight step edges of the {111} facets will be predominantly along the gold closest packed directions, this means that the lines of bright spots are rotated 30° away from the rows of close-packed gold atoms. Once again, this geometry is similar to that observed for SAMs prepared from alkanethiols. As domains like those shown in Figure 3a are monitored with a STM, we sometimes observe the growth or shrinkage of a domain by an even number of bright lines. Moreover, the domains always contain an even number of bright stripes; thus, the bright lines appear to exist in pairs. Figure 3b provides an additional indication that the fundamental unit of the structure which we observe consists of pairs of stripes. Figure 3b shows a plot of the average of 150 line scans which were taken perpendicular to the bright stripes in the structure. The minima between bright stripes clearly alternate in depth. Taking into account the spacing of the molecules, the pairing of the bright stripes, and the alternating depth
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Figure 4. (2x19×x3)R30° unit cell consistent with the 24 Å and 5 Å intermolecular spacing measured in Figure 3. Only molecules that are easily identifiable in STM images are shown; however, it is quite likely that additional thiophene molecules exist between the features used to construct the unit cell.
of the valleys between stripes, the unit cell which is consistent with our data is (2x19×x3)R30°. This unit cell is shown schematically in Figure 4. At present, we do not know what leads to the pairing of rows of thiophene molecules. It is also not possible on the basis of our STM images to determine the orientation of the thiophene molecules relative to one another because the width of the thiophene molecule measured in the plane of the aromatic ring or perpendicular to it is almost the same. However, all of our observations are consistent with a model in which the bright stripes are due to π-stacked thiophene molecules bonded to the Au(111) surface via the sulfur atoms and spaced by 5 Å (i.e., x3 times the Au-Au nearest neighbor distance). The 12 Å spacing between rows of thiophene molecules is quite large, and it is tempting to propose that additional thiophene molecules reside within this region, but we do not observe features attributable to inter-row thiophene. Like SAMs prepared by adsorbing alkanethiols onto Au(111).3,21,29 we have observed that thiophene monolayers can be removed by UV photolysis in air followed by rinsing with ethanol. When unannealed SAMs are photooxidized, the vacancy islands in the thiophene monolayer disappear. The oxidation product in this case is presumably 1,1thiophene dioxide (C4H4SO2),30 but this has yet to be confirmed. In light of the generally poor ligand properties of thiophene,31 the prediction that thiophene should not react with Au(111),26 and the many similarities between thiophene SAMs and alkanethiol SAMs, we were concerned that the monolayer obtained after immersing Au(111) in thiophene was due to a trace impurity in thiophene rather than the thiophene itself. In order to address this (29) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (30) Miyahara, Y.; Inazu, T. Tetrahedron Lett. 1990, 31, 5955-5958. (31) Angelici, R. J. Coord. Chem. Revs. 1990, 105, 61-76. (32) 20 mg of Au powder (1-3 µm, 0.30 m2/g) removes approximately 1% of the sulfur-containing species from 1 mL of a 4 mM solution of thiophene. (33) March, J. Advanced Organic Chemistry Reactions, Mechanisms, and Structure; John Wiley and Sons: New York, 1992; pp 1224 and references therein. (34) Both water and molecular hydrogen have been proposed as sinks for the hydrogen atoms on the thiol functionality.
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possibility, SAMs were prepared using thiophene solutions treated with fine (1-3 µm) gold powder, which is so effective at scavenging sulfur-containing compounds that it can remove all malodorous substances from ethanol solutions of thiophenesincluding all of the thiophene.32 When SAMs were prepared by immersing Au(111) into solutions of high-purity (i.e., Aldrich “gold label”) thiophene containing enough gold powder to adsorb 1% of all sulfurcontaining species, monolayers identical to those in Figures 1 and 3 were observed. The SAMs in Figures 1-3 are clearly due to adsorbed thiophene. Thiophene adsorption onto Au(111) raises some interesting questions about sulfur-based monolayers because the sulfur atom in thiophene is part of a stable aromatic ring. Unlike alkanethiols and dialkyl disulfides, which can in principle react with a gold surface in a number of ways, thiophene is relatively inert; it is difficult to reduce, and its ring structure is difficult to cleave. In the case of dialkyl disulfides, which are prone to reductive S-S cleavage,33 the formation of surface thiolates upon adsorption is chemically reasonable.8,19,20 Similarly reasonable surface-mediated reactions can be proposed for the formation of surface thiolates upon chemisorption of alkanethiols,8,15,19,20 but the fate of the hydrogen atoms from the thiol functional group has yet to be convincingly established.34 Unlike disulfides which might be reductively cleaved, and alkanethiols which might reduced to thiolates8,15,19,20 or oxidized to disulfides,10 the likelihood of thiophene participating in chemical reactions beyond simple molecular adsorption is quite small. Our results with thiophene clearly demonstrate that sulfur-containing molecules can adsorb on Au(111) to form well-ordered monolayers without reducing the adsorbate and oxidizing the gold substrate. Although it is still possible that alkanethiols and dialkyl disulfides undergo surface reactions subsequent to adsorption on Au(111), both species possess lone pairs of electrons that should in principle be capable of coordinating to Au(111) in the same manner as we observe for thiophene. In fact, our observations present the intriguing possibility that the formation of well-ordered SAMs from alkanethiols and disulfides does not require the cleavage of S-H bonds and S-S bonds, respectively. Our results also have important implications for the mechanism by which vacancy islands or “pits” are created during chemisorption. In particular, our observation that vacancy islands can be induced by adsorption of molecules which cannot formally oxidize gold strongly suggests that these features are not the result of chemical etching. Instead, it seems more likely that the “etch pits” observed when alkanethiols, dialkyl disulfides, and thiophene are adsorbed on Au(111) are the consequence of surface reconstruction. Finally, the observation that thiophene can form wellordered SAMs on Au(111) suggests that a wide variety of other sulfur-containing molecules can be used to create Au(111)-supported surface structures. Our efforts to prepare and study the chemistry of new sulfur-based monolayers with a STM and by laser-induced desorptionFTMS will be reported in a future article. Acknowledgment. We thank the U.S. Department of Energy and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. LA960840K
