Roughening of Gold Atomic Steps Induced by Interaction with

Feb 8, 2008 - Exposure of a clean gold surface to tetrahydrofuran (THF) under ambient conditions was observed to cause roughening of atomic step edges...
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Roughening of Gold Atomic Steps Induced by Interaction with Tetrahydrofuran Warren R. T. Barden, Sherdeep Singh, and Peter Kruse* Department of Chemistry, McMaster UniVersity, 1280 Main St. W., Hamilton, Ontario, L8S 4M1, Canada ReceiVed June 13, 2007. In Final Form: December 6, 2007 Exposure of a clean gold surface to tetrahydrofuran (THF) under ambient conditions was observed to cause roughening of atomic step edges. This change was followed in situ using a scanning tunneling microscope during the exposure of a gold surface to a controlled stream of THF vapor. THF is a common solvent used in depositing molecules, self-assembled monolayers, and polymer films on surfaces, in electrochemistry, and in chemical reactions. Unlike other solvents, such as methanol, ethanol and diethyl ether, however, we found that THF itself has a profound effect on the surface morphology that needs to be taken into account when reporting on the interactions of solutes with a gold surface. At the same time, this finding may present new opportunities in catalysis or nanostructuring of surfaces.

Introduction Its electron configuration makes gold the least reactive of all metals.1 It is the only transition metal not to possess a thermodynamically stable oxide in ambient conditions. Yet in recent years its surface has been found to participate in quite a number of reactions,2 in particular at surface defects or in the form of gold nanoparticles. As the myth of gold’s inertness has been rebuked, the field of gold catalysis has flourished in recent years.2-4 Oxidation reactions in particular have attracted attention, for example, oxidation of CO5 or epoxidation of ethylene.6 Gold has since been found to catalyze a wide range of reactions, including, for example, de/hydrogenation or NH addition.7,8 Nevertheless, the gold surface is considered to be stable and therefore of interest to scientists and engineers working on organic/ inorganic interfaces, e.g., for sensor applications and molecular electronics. The interactions of gold surfaces with thiols have been subject to extensive scrutiny.9,10 Thiols can form wellordered self-assembled monolayers on gold, because their interactions with the surface are weak enough to allow for mobility and ordering within the monolayer, yet strong enough to form a robust film. All this has led to a rapidly growing literature about the surface chemistry of gold, even though many challenges remain. Over time, it was noticed that gold surfaces are not as static as they might initially appear.11 Contamination in ambient conditions can over several hours lead to a deterioration of the surface. Surface atoms are mobile at or near room temperature, * Corresponding author. Phone: (905) 525-9140, ext 23480. Fax: (905) 522-2509. E-mail: [email protected]. (1) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238-240. (2) Meyer, R.; Lemire, C.; Shaikhutdinov, Sh. K.; Freund, H.-J. Gold Bull. 2004, 37, 72-124. (3) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896-7936. (4) Hammer, B. Top. Catal. 2006, 37, 3-16. (5) Min, B. K.; Alemozafar, A. R.; Pinnaduwage, D.; Deng, X.; Friend, C. M. J. Phys. Chem. B 2006, 110, 19833-19838. (6) Deng, X. Y.; Min, B. K.; Liu, X. Y.; Friend, C. M. J. Phys. Chem. B 2006, 110, 15982-15987. (7) Mu¨llegger, S.; Winkler, A. Surf. Sci. 2006, 600, 3982-3986. (8) Deng, X. Y.; Baker, T. A.; Friend, C. M. Angew. Chem., Int. Ed. 2006, 45, 7075-7078. (9) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1169 and references herein. (10) Fendler, J. H. Chem. Mater. 2001, 13, 3196-3210 and references herein. (11) Emch, R.; Nogami, J.; Dovek, M. M.; Lang, C. A.; Quate, C. F. J. Appl. Phys. 1989, 65, 79-84.

leading to step mobility, which could be observed in real space and time as scanning tunneling microscopy (STM) became available as a tool for studying surfaces.11-13 Since the 1980s, researchers have been fascinated by the possibility to manipulate gold surface atoms with the STM tip in order to grow fingers extending from terraces11,14 and other features.15-17 These findings indicated that, in agreement with theory, gold atoms at surface defect sites such as adatoms, vacancies, step edges, or kinks sites are weakened in their interaction with bulk gold. Thus, it does not come as a surprise that these atoms interact more easily with adsorbed atoms or molecules, which can lead to enhanced mobility. In particular, in the case of self-assembled monolayers of thiols, the gold-sulfur interaction has been shown to lead to a weakening of the interaction of the surface atoms with the bulk.18-20 Recent studies have shown that sulfur,21,22 oxygen,23 and iodine,24 among other adsorbate atoms, can cause mobility of gold surface atoms. The resulting roughness of step edges has in some cases been attributed to excess atoms from the lifted herringbone reconstruction.22,23 Under electrochemical conditions, the effect of a large number of ions on the properties of gold electrodes has been studied.25-29 Both smoothening and roughening of the surface have been reported under different conditions. The most (12) Holland-Moritz, E.; Gordon, J.; Borges, G.; Sonnenfeld, R. Langmuir 1991, 7, 301-306. (13) Trevor, D. J.; Chidsey, C. E. D. J. Vac. Sci. Technol. B 1991, 9, 964-968. (14) Guo, Q.; Yin, F.; Palmer, R. E. Small 2005, 1, 76-79. (15) Wang, H.; Jing, J.; Chu, T. T.; Henriksen, P. N. J. Vac. Sci. Technol. B 1993, 11, 2000-2005. (16) Mendez, J.; Gomez-Herrero, J.; Pascual, J. I.; Saenz, J. J.; Soler, J. M.; Baro, A. M. J. Vac. Sci. Technol. B 1996, 14, 1145-1148. (17) Kim, J.; Uchida, H.; Yoshida, K.; Kim, H.; Nishimura, K.; Inoue, M. Jpn. J. Appl. Phys. 2003, 42, 3616-3618. (18) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775-2777. (19) Vericat, C.; Benitez, G. A.; Vela, M. E.; Salvarezza, R. C.; Tognalli, N. G.; Fainstein, A. Langmuir 2007, 23, 1152-1159. (20) Viana, A. S.; Abrantes, L. M.; Jin, G.; Floate, S.; Nichols, R. J.; Kalaji, M. Phys. Chem. Chem. Phys. 2001, 3, 3411-3419. (21) Biener, M. M.; Biener, J.; Friend, C. M. Langmuir 2005, 21, 1668-1671. (22) Biener, M. M.; Biener, J.; Friend, C. M. Surf. Sci. 2007, 601, 1659-1667. (23) Biener, J.; Biener, M. M.; Nowitzki, T.; Hamza, A. V.; Friend, C. M.; Zielasek, V.; Baumer, M. ChemPhysChem 2006, 7, 1906-1908. (24) McHardy, R.; Haiss, W. H.; Nichols, R. J. Phys. Chem. Chem. Phys. 2000, 2, 1439-1444. (25) Bunge, E.; Port, S. N.; Roelfs, B.; Meyer, H.; Baumga¨rtel, H.; Schiffrin, D. J.; Nichols, R. J. Langmuir 1997, 13, 85-90. (26) Zubimendi, J. L.; Vela, M. E.; Salvarezza, R. C.; Vazquez, L.; Vara, J. M.; Arvia, A. J. Langmuir 1996, 12, 12-18. (27) Giesen, M.; Kolb, D. M. Surf. Sci. 2000, 468, 149-164.

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significant effects of adsorbate molecules are observed under ambient or electrochemical conditions. An earlier study showed certain molecules to cause mobility in ambient conditions but could not find any measurable effect on the surface in vacuum.30 However, more recently the lifting of the herringbone reconstruction has been demonstrated due to exposure to molecules such as styrene and NO2 in ultrahigh vacuum conditions.31,32 In this paper, we will focus on the interaction of tetrahydrofuran (THF) with gold surfaces. THF is one of the most common organic solvents and as such is frequently used to deposit thin layers onto gold surfaces. It has been used, for example, to deposit metals33-35 or metal salts,36 nanotubes,37 and various polymers.38-42 A number of researchers have reported the deposition of selfassembled monolayers of thiols onto gold from THF.43-49 Some groups comment on solvent effects50-52 and a lack of order53 in cases where THF was used. In cases where the morphology was checked with STM or atomic force microscopy, it is noticeable that the surfaces appear fairly rough.54-58 An exception to that is one case where the thiol monolayer was deposited from a different solvent and THF was used to exchange some of the molecules within the layer.59 One instance has also been reported where exposure of a thiol-covered gold surface to THF led to (28) Berna, A.; Delgado, J. M.; Orts, J. M.; Rodes, A.; Feliu, J. M. Langmuir 2006, 22, 7192-7202. (29) Kang, J.; Rowntree, P. A. Langmuir 2007, 23, 509-516. (30) Peale, D. R.; Cooper, B. H. J. Vac. Sci. Technol. A 1992, 10, 2210-2215. (31) Baber, A. E.; Jensen, S. C.; Iski, E. V.; Sykes, E. C. H. J. Am. Chem. Soc. 2006, 128, 15384-15385. (32) Driver, S. M.; Zhang, T. F.; King, D. A. Angew. Chem., Int. Ed. 2007, 46, 700-703. (33) Aurbach, D.; Daroux, M.; Faguy, P.; Yeager, E. J. Electroanal. Chem. 1991, 297, 225-244. (34) Xing, X. K.; Bae, I. T.; Scherson, D. A. Electrochim. Acta 1995, 40, 29-36. (35) Lefebvre, M. C.; Conway, B. E. J. Electroanal. Chem. 2000, 480, 46-58. (36) Ntais, S.; Dracopoulos, V.; Siokou, A. J. Mol. Catal. A 2004, 220, 199205. (37) Gao, C. Macromol. Rapid Commun. 2006, 27, 841-847. (38) Schlenoff, J. B.; Dharia, J. R.; Xu, H.; Wen, L. Q.; Li, M. Macromolecules 1995, 28, 4290-4295. (39) Xia, S. J.; Birss, V. I. Electrochim. Acta 2000, 46, 463-474. (40) Xia, S. J.; Liu, G.; Birss, V. I. Electrochim. Acta 2000, 46, 475-485. (41) Bartlett, P. N.; Baumberg, J. J.; Coyle, S. Faraday Discuss. 2004, 125, 117-132. (42) Xue, C. H.; Chen, Z.; Wen, Y.; Luo, F. T.; Chen, J.; Liu, H. Y. Langmuir 2005, 21, 7860-7865. (43) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050-1053. (44) Checkik, V.; Stirling, J. M. Langmuir 1997, 13, 6354-6356. (45) Silin, V. I.; Wieder, H.; Woodward, J. T.; Valincius, G.; Offenhausser, A.; Plant, A. L. J. Am. Chem. Soc. 2002, 124, 14676-14683. (46) Cai, L. T.; Yao, Y. X.; Yang, J. P.; Price, D. W.; Tour, J. M. Chem. Mater. 2002, 14, 2905-2909. (47) Lee, M. T.; Hsueh, C.-C.; Freund, M. S.; Ferguson, G. S. Langmuir 2003, 19, 5246-5253. (48) Krapchetov, D. A.; Ma, H.; Jen, A. K. Y.; Fischer, D. A.; Loo, Y.-L. Langmuir 2005, 21, 5887-5893. (49) Colavita, P. E.; Miney, P. G.; Taylor, L.; Priore, R.; Pearson, D. L.; Ratliff, J.; Ma, S.; Ozturk, O.; Chen, D. A.; Myrick, M. L. Langmuir 2005, 21, 12268-12277. (50) Ravenscroft, M. S.; Finklea, H. O. J. Phys. Chem. 1994, 98, 3843-3850. (51) Tai, Y.; Shaporenko, A.; Rong, H. T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 16806-16810. (52) Berron, B.; Jennings, G. K. Langmuir 2006, 22, 7235-7240. (53) Vanderah, D. J.; Valincius, G.; Meuse, C. W. Langmuir 2002, 18, 46744680. (54) Cao, Z.; Xiao, Z.-L.; Gu, N.; Gong, F.-C.; Yang, D.-W.; Zhu, Z.-P. Anal. Lett. 2005, 38, 1289-1304. (55) Lin, S. Y.; Chen, I. W. P.; Chen, C. H.; Lee, C. F.; Chou, C. M.; Luh, T. Y. J. Phys. Chem. B 2005, 109, 7915-7922. (56) Kim, S. U.; Shin, H. K.; Kwon, Y. S. Colloids Surf. A 2005, 257-258, 211-214. (57) Kim, S.; Kim, B.; Park, J.; Shin, H.; Kwon, Y. Curr. Appl. Phys. 2006, 6, 608-611. (58) Lee, N. S.; Shin, H. K.; Kwon, Y. S. Jpn. J. Appl. Phys. 2006, 45, 426429. (59) Moore, A. M.; Mantooth, B. A.; Donhauser, Z. J.; Maya, F.; Price, D. W., Jr.; Yao, Y.; Tour, J. M.; Weiss, P. S. Nano Lett. 2005, 5, 2292-2297.

Figure 1. Changes to Au(111) due to exposure to pure liquid THF. (a) A characteristic clean Au(111) surface. (b) Sample after exposure to liquid THF. Imaging parameters: IT ) 1.0 nA, USB ) +400 mV, 200 ms/line, and scan size of 400 nm. The scale bar at the bottom right of the images is 50 nm long.

deterioration of the sample.60 The interactions of THF with a gold surface have been modeled;61 however, only a flat, defectfree surface was examined, in which case they were fairly insubstantial. In contrast, significant interactions of a copper surface with THF have been reported, as might be expected given the higher chemical reactivity of copper.62 To date, no detailed report of the interactions of THF with real gold surfaces is available. We studied the interaction of gold with THF and were able to follow the roughening process in real time. The results are compared to other solvents, such as methanol, ethanol, water, and diethyl ether. The latter is structurally closely related to THF, giving an indication of the underlying cause of the interaction. As hinted at by the significant amount of published studies that are relying on THF as a solvent alongside gold surfaces, our findings are of relevance to a large number of researchers. However, the roughening phenomenon may also be seen as an opportunity for catalysis research, where an increase in the number of active sites is desired. (60) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 1252812536. (61) Zhao, X.; Leng, Y.; Cummings, P. T. Langmuir 2006, 22, 4116-4124. (62) Mulligan, A.; Johnston, S. M.; Miller, G.; Dhanak, V.; Kadodwala, M. Surf. Sci. 2003, 541, 3-13.

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Figure 2. (a, b) Surface before exposure to THF and (c) Surface immediately after THF was introduced into the round-bottom flask. A distinct change in the morphology of the step edges can be seen. (d-f) The effects continue during exposure. (g) Immediately after THF ran out. (h) During nitrogen purge, the step edges remain stable in the roughened state. For all images, IT ) 1.0 nA, USB ) +400 mV, 200 ms/line, and scan size of 400 nm. The blue scale bar at the bottom right is 25 nm in all images.

Experimental Methods Before each experiment, a clean gold sample (250 nm Au/2.5 nm Cr/borosilicate glass; arrandee, Werther, Germany) was prepared by washing with methanol (HPLC grade, Fisher) and drying in air. After drying, the sample was heated using a butane torch with a flame temperature of approximately 1300 °C. The sample was in direct contact with the butane flame for 4 min and then was allowed to cool for 2 min. This process of heating and cooling was repeated four times in order to allow the surface to anneal. X-ray photoelectron spectroscopy (XPS) was used to ensure that no chromium from the adhesion layer made its way into the gold film or to the gold surface (see Supporting Information). The experimental setup consisted of a stoppered 50 mL roundbottom flask half-filled with approximately 22 mL of THF (freshly distilled solvent, but exposed to air for around 5 min prior to experiment) or diethyl ether and fitted with plastic tubing so that dry nitrogen could be bubbled through the liquid. Another plastic tube

leading out of the flask pointed at the sample being scanned by the microscope 3-4 mm away. This way the nitrogen flowing over the sample was saturated with THF (vapor pressure 165 Torr at 25 °C;63 typical temperatures in the lab were 18-20 °C) or diethyl ether (vapor pressure 532 Torr at 25 °C63). The PTFE tubing (Fisher) used was soaked in THF prior to the experiment to ensure cleanliness and to make sure there were no side reactions between the THF and the tubing. The result of the THF exposure was then monitored by continuously scanning the tip over the same sample area over a period of 2-3 h until all the THF in the round-bottom flask had evaporated. Data from only four experiments are discussed in this paper. The results, however, were reproducible many times over. Additional data are provided in the form of two STM movies as Supporting Information. (63) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press Inc., Boca Raton, FL, 1994.

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Scanning tunneling microscope images were taken under ambient conditions using a DI Nanoscope II STM with a type A head, a Pt/Ir (8:2) tip, and an RHK SPM 1000 controller. Typical scanning parameters were a tunneling current of IT ) 1.0 nA, a sample bias of VSB ) +400 mV, a scan speed of 200 ms/line, and a scan area of 400 nm × 400 nm with a resolution of 512 × 512 pixels. Any deviations from typical scanning parameters will be noted throughout the paper. Atomic resolution of the gold surfaces could not be achieved, since this work was performed under ambient conditions and a steady gas flow, as opposed to UHV conditions or electrochemical control. The images are nevertheless of sufficient clarity to support our conclusions. All images are unprocessed except for background subtraction.

Results Since the studies presented here were motivated by the observation of significant roughening of the gold substrate when we attempted to deposit molecules from THF, we started out by verifying the effect of pure THF on a flame-annealed gold surface. Figure 1a shows a typical image of one of our freshly annealed gold surfaces. Triangular terraces surrounded by atomic height steps as seen in this image are typical for Au(111) facets of gold grains in gold thin films as they are commonly obtained under our preparation conditions. The tip was then retracted and a droplet of liquid THF was applied to the region that had just been scanned. The THF was allowed to evaporate and a slight trace of a residue was left behind. The tip was reapproached to the surface and a new image (Figure 1b) was obtained. The area on the sample is likely not identical to Figure 1a, due to instrument drift as a result of THF deposition or tip changes. Nevertheless, the two images are typical of the surface before and after exposure, as seen in several repeat experiments. The flat terraces of the clean surface become globular and grainy, indicative of a change in the atomic structure of the surface from ordered Au(111) terraces to a highly disordered state. During repeated imaging, the surface gave the impression of being covered in some sort of a residue, which was initially thought to be poly-THF. However, we were unable to identify any trace of a polymeric adsorbate or even THF itself using Fourier transform infrared spectroscopy (FTIR) or secondary ion mass spectrometry (SIMS), despite a change in appearance from a clean gold surface (see Supporting Information). This question remains unresolved, also for lack of reproducibility under our conditions. Given that no vacuum-based method (SIMS, STM) turned up any evidence of adsorbed molecules, we suspect that the interaction between the THF molecules and the gold steps is too week to maintain coverage under vacuum conditions. XPS would have to be performed in vacuum and require at least 1% coverage of the relevant THF-related species for a sufficient signal. Given that our FTIR setup also did not detect any surface species, no spectroscopic study of the THF-gold interaction appears therefore possible at this moment. Multiple STM experiments were performed by placing a droplet of THF onto a gold surface and allowing time for the droplet to evaporate. We also attempted to continue scanning while the drop was being administered; however, we were unable to obtain “realtime” images of the surface modification process because of frequent tip crashes. A different method was devised in order to obtain images that showed the roughening process. THF vapor was blown over a gold surface while imaging with STM (Figure 2). The smooth step edges become gradually more and more distorted and elongated as a direct result of the applied THF. The step edge changes from smooth to very rough with a saw-tooth-like roughness appearing, reminiscent of fractal geometries that maximize the step length. The elongations appear

Figure 3. (a) Surface before diethyl ether exposure and (b) the same surface after prolonged diethyl ether exposure. Imaging parameters: IT ) 1.0 nA, USB ) +400 mV, 200 ms/line, and scan size of 400 nm.

to preferentially grow perpendicular to the step edges, but are overall random in direction. Very little change is noticed on the surface after the THF has run out and the surface continues to be purged with dry nitrogen. While the image in Figure 1b may have been taken for the possibility of a polymer, one needs to consider that (a) poly-THF is unlikely to be directly visible in STM under the given conditions since it is not a conductor and (b) we are not aware of any STM images of polymers in the literature that would be consistent with the data presented in Figure 2. Both advancing and retreating features are seen along the gold steps, not something that would be consistent with polymer growth. Poorly conducting polymer chains would also be expected to be much more mobile at room temperature than observed by us (e.g., compare Figure 2h,i) when imaged with 1 nA and 400mV. The oxygen-gold interaction must be sufficiently strong for the electronic structure to change enough to alter the diffusion energy barriers. The interaction with the surface likely takes place through the electron lone pairs at the oxygen atom. If this is the case, then other oxygen-containing solvents may give rise to similar phenomena. However, methanol was used to wash the gold samples in this study, and a clean, smooth surface was still obtained. Similarly, ethanol or water also have no observable effect on a freshly annealed gold surface. If methanol from the

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Figure 4. (a) Area of 400 × 400 nm2 with no THF applied to the surface, (b) zoom in at 150 × 150 nm2 again with no THF applied, (c) same region as in part b after exposure to THF vapor, and (d) the original 400 × 400 nm2 area after THF exposure. Scan parameters for all images: IT ) 1.0 nA, USB ) +400 mV, and 200 ms/line.

pretreatment or water from ambient background had an effect, this would impact our surface at the onset of each experiment (Figures 1a, 2a,b, 3a, 4a,b, Supporting Information). No such impact was evident in any of our experiments. Note that during our experiments (when surface changes were observed), water was excluded from the surface by the experimental design as described. Diethyl ether is a solvent with a very similar molecular weight and structure to THF (chain vs ring). We compared its effect on gold by blowing diethyl ether saturated nitrogen over a clean gold surface while scanning (Figure 3), using an identical experimental setup as with THF. Figure 3 shows that diethyl ether has no noticeable effect on the surface morphology. The vapor pressure of diethyl ether is 532 Torr at 25 °C compared to a vapor pressure of 165 Torr at 25 °C for THF.63 Therefore, the surface was exposed to higher concentrations of diethyl ether vapor as compared to THF. Still, its effect on the surface remained insignificant. We also checked the state of the STM tip after the experiment by using the same tip to image a clean gold surface. As an example, the Supporting Information contains an STM image of a clean gold surface taken shortly after the end of movie 2, on a different, freshly annealed sample. The ability of STM tips to move gold atoms along a surface has been demonstrated under a range of conditions.11,14,16,17 In order to exclude this effect in our case, a clean gold sample was

imaged at 400 × 400 nm2 for a couple of images (Figure 4a) and then the scan size was decreased to 150 × 150 nm2 (Figure 4b). The surface was then exposed to THF vapor during scanning as before and a corresponding change in surface morphology change was observed (Figure 4c). The image in Figure 4c was taken roughly 10 min after the image in Figure 4b. After the THF was used up, the scan size was increased to 400 × 400 nm, again in order to see if there was a region on the surface that differed from its surroundings (Figure 4d). We can see that the surface change is completely uniform and not restricted to only the square region that was being scanned at 150 × 150 nm2. These results demonstrate that there is indeed a direct interaction between THF and gold regardless of the presence or absence of the STM tip. Two additional observations should be noted. First, we unsuccessfully attempted to reproduce our observation with a variable temperature ultrahigh vacuum STM (base pressure 3 × 10-11 Torr, maximum dose of THF 1 × 10-5 Torr for 900 s) both at room temperature and at cryogenic temperatures (liquid nitrogen). Since we did not make any attempts to narrow the “dosage gap”, we are not reporting further details of those experiments as part of this paper. Second, we never observed a reversal of the roughening, even after samples were purged with

Roughening of Gold Atomic Steps

Figure 5. Model of a step edge on a Au(111) surface and its interaction with THF. (a) Smoluchowski smoothening leads to a dipole at a step edge that can interact with local charges at an adsorbate molecule. (b) Preferred (solid line) and alternative (dotted lines) ways of detaching a step atom to initiate migration on a clean fcc(111) surface. (c) Adsorbate-mediated change in the preferred migration direction of step atoms. (d) Possible structure of a roughened step as a result of the situation pictured in panel b.

dry nitrogen for several hours. No method other than flame annealing succeeded in removing the THF-induced surface roughness.

Discussion In order to understand these results, we have to take a closer look at the electronic structure of the surface. A likely explanation for the increased reactivity of step atoms is Smoluchowski smoothening, where there is a positive charge on atoms on the step edge and a negative charge on atoms below the step edge.64 This makes the step edge gold atom electrophilic and more willing to accept a lone pair of electrons from the oxygen atom of a THF molecule. This situation is illustrated in Figure 5a. Step edges naturally have undercoordinated kink sites as defects. The mobility of these kinks sites on clean Au(111) surfaces has been studied experimentally65 and is well-understood in theory.66-68 Figure 5b illustrates three possible events leading to step atom mobility: Expulsion of a step atom (least favored), detachment of a kink atom onto the “outer” step, or detachment of a kink atom along the ‘inner’ step. Normally the generation of mobile atoms diffusing along step edges and across terraces is in equilibrium with the reincorporation of those atoms into terraces. Mass flow at the surface can be generated by imposing a preferred direction of diffusion, e.g., with an STM tip. Typically, atoms at kink sites are the most mobile species on a surface and will detach easily. If they get redeposited at a kink site, they will retain their predisposition to mobility, whereas occasional annealing events will stabilize the surface and reduce roughness. In the case of THF-decorated steps, the situation appears to be different, as illustrated in Figure 5c,d. The strong interactions (64) Smoluchowski, R. Phys. ReV. 1941, 60, 661. (65) Verhoeven, G. S.; Frenken, J. W. M. Surf. Sci. 2007, 601, 13-23. (66) Giesen, M. Prog. Surf. Sci. 2001, 68, 1-153. (67) Agrawal, P. M.; Rice, B. M.; Thompson, D. L. Surf. Sci. 2002, 515, 21-35. (68) Boisvert, G.; Lewis, L. J.; Puska, M. J.; Nieminen, R. M. Phys. ReV. B 1995, 52, 9078-9085.

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between a gold step atom and a THF molecule permit it to be detached more easily for diffusion along steps. Annealing events are prevented by the THF molecules, because their interaction with terrace atoms is unfavorable. Vacancy and adatom formation and annihilation at a step edge are no longer in equilibrium with each other. On the basis of previous theoretical models of adsorbate-surface interaction and kink dynamics, we therefore propose that the observed phenomenon may be explained by the following two processes: (1) increase of step and kink atom mobility due to interaction with THF and (2) inhibition of step annealing by the adsorbed THF. The resulting roughening process is illustrated in Figure 5. Our observation is consistent with a recent computational study of noble and late transition metals using density functional theory, which found that surface steps are more favorable for adsorbate binding due to the electronic interaction between the bands of the metal and the molecular orbitals of the adsorbate.4 However, the observed effect is a subtle one with regard to electronic structure. A minute change in dipole moment and/or adsorption entropy (rigid ring vs floppy chain) between THF and diethyl ether is sufficient to turn off the effect. As stated above, alcohols and water also did not exhibit any noticeable roughening effect. Compounds involving heavier chalcogen analogues of oxygen (selenium and tellurium) are reported to form ordered monolayers in addition of course to the well-known thiols and thioethers,69 whereas roughened gold surfaces have been claimed to catalyze polymerization of pyrrole (the structure of which somewhat parallels that of furan).70 Other molecules have yet to be examined.

Conclusion Using scanning tunneling microscopy, we have shown that the commonly used solvent tetrahydrofuran strongly interacts with step edges on clean gold surfaces. As a result, steps are elongated and the surface is roughened. Even though the effect is very strong during exposure to liquid THF and could not be followed in-situ, we were able to follow the roughening process in the case of gold surfaces exposed to THF vapor. The exact mechanism leading to the roughening is not understood at this point. However, we propose that THF molecules can strongly interact with step atoms, thus encouraging their diffusion along the step and adjacent terraces, but at the same time preventing the incorporation of these atoms into a bulk terrace. Effectively, this leads to a fractal-style continued elongation of all steps on the surface. We have studied other solvents including water, lower alcohols, and diethyl ether, but we have not yet found any other molecules exhibiting this behavior. The roughening is permanent and of importance in cases where gold is used in molecular electronics or in a type of sensor where a smooth, well-ordered surface is required. Therefore, this report serves as a cautionary tale to researchers. On the other hand, it may be possible to utilize the roughening phenomenon in order to increase the number of catalytically active sites on a gold surface. Acknowledgment. We thank Holger Eichhorn at the University of Windsor for inspiring this study and Dave Emslie at McMaster University for providing us with some distilled THF. Thanks to Kevin Moonoosawmy for help with the in vacuo trial. Debabrata Pradham and Tong Leung from Watlabs assisted with XPS. Rana Sody and Peter Brodersen from Surface Interface Ontario recorded the SIMS data, and Steve Kornic at McMaster Chemistry assisted with the FTIR spectra. The work was (69) Nakamura, T.; Kimura, R.; Matsui, F.; Kondoh, H.; Ohta, T.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 2000, 16, 4213-4216. (70) Fujita, W.; Ishioka, T.; Teramae, N.; Haraguchi, H. Chem. Lett. 1994, 5, 933-936.

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financially supported by the Natural Science and Engineering Research Council of Canada and an Ontario Premier’s Research Excellence Award. W.R.T.B. was partially supported by the HRSDC of Canada. Supporting Information Available: Image sequences (STM movies) in avi format of two experiments documenting the gradual

Barden et al. modification of gold steps during exposure to THF; pdf file describing the context for both movies; XPS data of a gold surface as prepared for the experiments; SIMS data for clean and THF-exposed gold surfaces; FTIR data for THF, poly-THF, and a THF-exposed gold surface. This information is available free of charge via the Internet at http://pubs.acs.org. LA701757E