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Scanning Tunneling Microscopy Observations of Butanethiol Self-Assembled Monolayers on Ag Underpotential Deposition Modified Au(111) Ming-Hsun Hsieh and Chun-hsien Chen* Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan 80424 Received April 26, 1999. In Final Form: July 26, 1999 The thermal stability of self-assembled monolayers (SAMs) of alkanethiols on gold substrates can be enhanced by underpotential deposition (upd) of a silver interlayer. In this study butanethiol SAMs prepared on Ag upd-modified Au(111) are examined in air by scanning tunneling microscopy (STM). When the upd layer is prepared at a potential of 460 mV (vs EAg+/o), the surface exhibits atomically smooth terraces, distinctly different from pit features of alkanethiol SAMs on bare Au. When the upd layer is prepared at potentials just prior to bulk deposition, butanethiol SAMs show protrusions with ca. 0.09 nm in height. Molecularly resolved STM images show that the nearest-neighbor spacing is 0.50 ( 0.02 nm, the same as long-chain alkanethiol SAMs on Au(111). However, the film exhibits a hexagonal lattice structure with a height modulation, distinctly different from that of butanethiol SAMs on bare Au(111) but similar to that of decanethiol monolayers on Ag(111). The similarity in structure demonstrates the effect of Au substrates and Ag interlayers on the formation of butanethiol SAMs.
Introduction Self-assembled monolayer (SAM)1,2 modified surfaces with desired functionalities can be easily prepared by taking advantage of strong sulfur-gold interactions and immersing gold substrates into organomercaptan-containing solutions. Research in this field has been intensive because of the ease in film preparation, the excellent properties as model systems, and potential applications to technologies integrated with surface engineering at the molecular level, such as microlithography, drug screening, and molecular shape-sensitive interfaces. The importance of these molecularly designed devices will be diminished if the thiol compounds desorb easily from the substrates. It is well-known that SAM surfaces exhibit pitlike features with monatomic depths arising from dissolution of Au atoms during preparation of SAMs by soaking the substrates in the deposition solutions.3-8 Dissolution of Au atoms, and thus thiol molecules, is attributed to the distribution equilibria of thiol molecules between the solution phase and the solid-liquid interface. The sulfurgold interactions are so strong that the gold atoms are etched away by the desorbed thiolates, suggesting that applications of SAMs in solutions are limited. Also, such monolayers are not stable at elevated temperatures (g 70 °C)9 and thus their applications are restricted. * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +886-7-5253909. Phone: +886-75252000 ext. 3917. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (3) Edinger, K.; Golzhauser, A.; Demota, K.; Woll, C.; Grunze, M. Langmuir 1993, 9, 4-8. (4) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853-2856. (5) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-3386. (6) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 1096610970. (7) Sondag-Huethorst, J. A. M.; Schonenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826-6834. (8) Schonenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (9) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.
Jennings and Laibinis10,11 recently developed an effective method to enhance the thermal stability of SAMs by underpotential deposition (upd)12-14 of Ag or Cu monolayers on the Au surface before generation of SAMs. Crooks examined this strategy and found that SAMs become more resistant to corrosion under harsh electrochemical conditions.15 Upd12-14 is an electrochemical phenomenon in which only a full monolayer or a submonolayer of foreign metal adatoms is deposited on the electrode surface at potentials positive to the reversible Nernst potential. Because of the work function difference between the adatom and the substrate, the formation of the adatomsubstrate bond occurs prior to that of the adatom-adatom bond. Upd-modified electrodes are often different from the unmodified ones in interfacial properties, such as electrocatalysis and anti-corrosion.12-14 The coverage of adatoms can be controlled by changing the electrode potential to manipulate the electrode properties. By varying the coverage of the Ag upd interlayer, Jennings and Laibinis10 found that the thermal stability of SAMs becomes better at higher coverages. A similar effect of enhancement on electrochemical stability was reported by Yoneyama et al.,16,17 who electrodeposited Cu or Ag monolayers after the alkanethiol SAMs were prepared. The purpose of the present scanning tunneling microscopy (STM) study is to enhance our understanding of the system of butanethiol SAMs modified with an Ag upd (10) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173-6175. (11) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208-5214. (12) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1978; Vol. 11, pp 125-271. (13) Juttner, K.; Lorenz, W. J. Z. Phys. Chem. (Wiesbaden) 1980, 122, 163-185. (14) Adzic, R. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1984; Vol. 11, pp 159-260. (15) Zamborini, F. P.; Campbell, J. K.; Crooks, R. M. Langmuir 1998, 14, 640-647. (16) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. Langmuir 1997, 13, 5215-5217. (17) Oyamatsu, D.; Nishizawa, M.; Kuwabata, S.; Yoneyama, H. Langmuir 1998, 14, 3298-3302.
10.1021/la990497u CCC: $19.00 © 2000 American Chemical Society Published on Web 12/07/1999
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Figure 1. Unfiltered STM images (Ebias ) 1.0 V, it ) 30 pA) of surfaces without Ag upd modification. (A) A 340 × 300 nm image of bare Au(111). The section profile shows that the height difference between terraces is equivalent to a single atomic step (0.24 nm). The inset (70 × 70 nm) shows the herringbone reconstruction. (B) Butanethiol monolayer on Au(111) (240 × 210 nm). The section profile shows that the pits are of monatomic depth. (C) Butanethiol monolayer on Au(111) (50 × 45 nm). (D) Molecularly resolved image of butanethiol monolayer on Au(111) (25 × 11 nm). Soaking time of the Au substrate in the butanethiol solution was 1 h for parts B-D.
interlayer. We employ Au(111) substrates and follow the method developed by Jennings and Laibinis10,11 to prepare butanethiol SAMs at two upd potentials. The images presented here reveal the significant effects of the upd interlayer on surface morphology and lattice structures of butanethiols. Experimental Section The Au(111) substrates (Metallhandel Schro¨er GmbH, Germany) were ∼200-300 nm thick gold films evaporated onto borosilicate glass that had a ∼1-4 nm thick Cr undercoating to improve adhesion of the gold film to the glass. Before each experiment the substrates were flameannealed with a butane microtorch until the sample glowed dark red and then were cooled under ambient conditions. The annealing procedure was repeated a few times, and finally the substrates were quenched in Milli-Q water (Millipore). Modification of Ag upd layers was carried on a PAR VersaStat potentiostat (EG&G Instruments Corp., Princeton, NJ) with a three-electrode configuration in which the reference and counter electrodes were a silver wire and a platinum wire, respectively. All potentials reported here are referenced to EAg+/0, i.e., ∼470 mV vs EAg/AgCl. The deposition solutions were 0.1 M H2SO4 (Fisher) containing 1 mM AgNO3 (Aldrich). The potential of the working electrode was cycled from 600 mV (vs EAg+/0) through the upd peaks and parked at 460 or 45 mV, where the coverages were half or approximately a full Ag monolayer, respectively. The electrodes were removed from the solutions under potential control, rinsed with copious ethanol (Ferak), blown dry in a stream of N2, and transferred rapidly through air to ethanol solutions of 1 mM butanethiol (TCI). This procedure followed that developed by Jennings and Laibinis.10,11 STM measurements were carried out with a NanoScope II (Digital Instruments, Santa Barbara, CA) using a lowcurrent scanning head (model HD-0.5I), which had an adjustable resistor to obtain a higher tunneling impedance. Commercial Pt/Ir tips were employed. Typical imaging conditions of tunneling current and bias voltage ranged from 50 to 15 pA and from 0.5 to 2 V, respectively. Although there was no discernible difference in structures between different imaging conditions, images with better quality were obtained at lower tunneling impedance.
Results and Discussion A typical image of a Au(111) surface after flame treatment is shown in Figure 1A. This 340 × 300 nm image exhibits large and smooth terraces. The height profile of the white line drawn in the upper part of the image shows that the step height between terraces is 0.24 nm, corresponding to one monatomic step of Au(111). The inset in Figure 1A is a 70 × 70 nm image obtained from a freshly prepared sample under ambient conditions and shows the herringbone feature of the (23 × x3) rectangular reconstruction, indicative of the high-quality Au(111) substrates employed in this study. Figure 1B is an STM image of butanethiol chemisorbed on Au(111). The surface shows numerous pits of monatomic depth which is manifested in the height profile. At a higher magnification, the pinstripe features such as those in Figure 1C can be observed. The interstripe distance is 1.02 ( 0.03 nm, corresponding to the p × 3 structures reported by Poirier5,6 and Kang and Rowntree.18 Presented in Figure 1D is a 25 × 11 nm image with resolution on the molecular scale. Figure 1 illustrates that, although our STM measurements are performed in air, the results are in good agreement with literature reports under UHV conditions, which ensures that the tunneling conditions employed in this study (g10 GΩ) are appropriate; viz., the structures of butanethiol SAMs are not perturbed by the scanning tip. For each sample, a parallel comparison was made between butanethiol SAMs with and without the Ag upd interlayer by applying Ag upd onto only a portion of the Au(111) substrate. Therefore, when a sample was immersed in a butanethiol solution, the surface contained both bare Au(111) and upd-modified regions to which the same assembly conditions were simultaneously applied. For all of the samples examined, the surface morphologies of butanethiol SAMs without upd modification always exhibited features of pits and stripes such as those shown in Figure 1B-D. On the other hand, when identical imaging conditions are employed on Ag upd-modified portions, the images show that assembly of butanethiol monolayers results in pit-free morphology. Details of butanethiol SAMs on Ag upd monolayers modified at 460 and 45 mV (vs EAg+/0) are reported as follows: (18) Kang, J.; Rowntree, P. A. Langmuir 1996, 12, 2813-2819.
Butanethiol Self-Assembled Monolayers
Figure 2. Unfiltered images (Ebias ) 0.5 V, it ) 20 pA) of butanethiol monolayers on Au(111) which is premodified with Ag upd at 460 mV (vs EAg+/0). (A) A 280 × 280 nm image obtained from a sample with a soaking time of 72 h in a 1 mM butanethiol solution. The inset (50 × 50 nm) is an image obtained from a sample with a soaking time of 120 h. (B) Image (12 × 12 nm) at the molecular scale.
Figure 2 shows typical images of butanethiol SAMs on a upd-modified surface at an Ag upd potential of 460 mV. At this potential, in situ atomic force microscopy (AFM) measurements19 show that the Ag adatoms adopt a 3 × 3 lattice structure and the coverage is approximately half of a full monolayer. Figure 2A is a 280 × 280 nm image obtained from a sample that was exposed to 1 mM butanethiol for 72 h. Shown in the inset is a 50 × 50 nm image obtained from another sample with a soaking time of 120 h. The significance of these two images is the revelation of the pit-free terraces, strikingly different from the morphological feature of butanethiol SAMs on bare Au(111) (Figure 1B). This observation is identical with that reported by Crooks et al., who employed a Cu upd interlayer.15 We have examined upd-modified samples with deposition periods as long as 7 days and found only a very few pits. Although the surface depicted in Figure 2A is covered with Ag atoms, this image is distinctly different from STM images of decanethiol SAMs on Ag(111) substrates where ca. 20% of the surface is covered by monatomic height islands with sizes ranging from 2 to (19) Chen, C.-h.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451-458.
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18 nm.20 Guyot-Sionnest et al. found dissolved Ag in the deposition solutions after the Ag(111) substrates were emmersed, suggesting that the Ag surface was etched by thiolates,20 similar to assembly of alkanethiols on Au. When Au surfaces are premodified with Ag upd monolayers, Jennings and Laibinis11 concluded from X-ray photoelectron spectroscopy (XPS) measurements that the Ag upd adatoms are not etched away by alkanethiolates because there is little (less than 10%) or no loss of Ag (3d5/2) signal after continued exposure to alkanethiol solutions for 5 days. The smoothness of terraces in Figure 2A provides a real-space evidence of the pronounced effect of the Ag upd layers in this regard. At higher magnifications, molecularly resolved images can be obtained, but no well-defined lattice structure has been observed. It has been proposed that the formation of the open 3 × 3 lattice of Ag upd on Au(111) is due to coadsorbed bisulfate (or sulfate) ions at this potential.19,21 In the current study, the lack of potential control and the removal of adsorbed sulfate ions during rinsing procedures11 may result in a disordered Ag upd layer, and thus the butanethiol overlayers do not exhibit lattice structures. Figure 2B is a 12 × 12 nm image where the spacing between the nearest molecules ranges from 0.46 to 0.51 nm. This span coincides with the lattice spacing of alkanethiols on Ag(111) (0.461-0.477 nm)20,22 and on Au(111) (ca. 0.50 nm). However, we have never found any local structures identical with those of thiolate SAMs on Au(111) or on Ag(111), which suggests that during exposure to the deposition solutions the surface has no bare Au(111) or bare Ag(111) islands due to aggregation of Ag upd adatoms. This observation is in a good agreement with the reflectance infrared (IR) results of Jennings and Laibinis11 that the spectra of such monolayers cannot be reconstructed using linear combinations of the spectra of alkanethiol SAMs on Au(111) and on Ag(111). In the case of butanethiol SAMs on Au(111), after the sample is removed from the deposition solution for a long time, the so-called liquidlike disordered domains turn into well-ordered (p × x3) structures as a consequence of evaporation of butanethiol from Au(111) substrates at room temperature.5 Such transformations, however, are not found when butanethiol SAMs are assembled on a predeposited Ag upd interlayer. The images shown in Figure 3 were obtained from butanethiol SAMs with an Ag upd layer premodified at 45 mV, just positive of bulk Ag deposition. Potentials more negative were not used in this study because of the concern of multilayer deposition. At a upd potential of 45 mV, the coverage of Ag adatoms is greater than 85% of a full monolayer.10,19 Both in situ STM23 and AFM19 experiments yield noisy images, which are attributed to a phase transition from an open upd structure to close-packed Ag(1×1). After the formation of butanethiol SAMs, the surface morphologies appear smooth under long-range scans, demonstrating that, at coverages close to a full monolayer, the Ag interlayer also inhibits butanethiol molecules from etching the surface. Therefore, we can conclude that the interaction between upd Ag adatoms and Au substrates is stronger than that between thiolate and Ag atoms. However, at this potential the surface is (20) Dhirani, A.; Hines, M. A.; Fisher, A. J.; Ismail, O.; GuyotSionnest, P. Langmuir 1995, 11, 2609-2614. (21) Mrozek, P.; Sung, Y. E.; Han, M.; Gamboa-Aldeco, M.; Wieckocoski, A.; Chen, C.-h.; Gewirth, A. A. Electrochim. Acta 1995, 40, 1728. (22) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., III; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013-2016. (23) Hachiya, T.; Itaya, K. Ultramicroscopy 1992, 42-44, 445-452.
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Figure 3. Unfiltered images (Ebias ) -0.3 V, it ) 30 pA) of butanethiol monolayers on Au(111) premodified with Ag upd at 45 mV (vs EAg+/0). (A) A 110 × 110 nm image. The section profile shows the heights of the steps and protrusions described in the text. (B) Molecularly resolved image (10 × 10 nm). The section profile shows the periodic height modulation of the white line drawn diagonally across the unit cell. The inset shows the spectrum of a two-dimensional fast Fourier transform. (C) A plausible model for butanethiol SAM with the Ag interlayer predeposited at 45 mV: right, open and gray circles representing the Au substrate and Ag interlayer with a coverage of 75%; left, the small circles showing the butanethiol molecules adopting a (x7 × x7)R19.1° structure on the reconstructed Ag layer.
distinctly different from those prepared at upd potential of 460 mV (Figure 2A). At higher resolutions such as the 110 × 110 nm image shown in Figure 3A, ∼15-20% of the surface area is covered by randomly distributed protrusions with diameters ranging from 1 to 4 nm. The section profile (Figure 3A) shows that the height of the protrusions is only 0.09 ( 0.02 nm, significantly lower than a monatomic step (0.24 nm). Therefore, the protrusions arise neither from butanethiol etching nor from redeposition of etched Ag atoms. Because the protrusions do not show a periodic pattern, the possibility of Moire´ patterns arising from the size mismatch of substrates and close-packed adsorbates must be ruled out. A plausible mechanism for the formation of these protrusions is as follows. For images with resolutions at the molecular level, a hexagonal lattice structure is generally observed. The lattice structure might improve the film’s intermolecular interactions and thus contribute to enhanced thermal stability for films prepared with higher upd coverages.10 The domain size is small, roughly 10 nm, and the continuity of the lattice generally terminates where the protrusions reside. A typical 10 × 10 nm image is shown in Figure 3B. The spacing between nearest-neighbor molecules is 0.50 ( 0.02 nm. The lattice spacing and hexagonal arrangement suggest a simple (x3 × x3)R30° structure. We note, however, that this does not explain the formation of the protrusions shown in Figure 3A. In the inset to Figure 3B the spectrum of Fourier transform shows two sets of concentric hexagonal spots which are rotated 30° relative to each other, indicating that the lattice exhibits a periodic modulation in height. By careful examination of the height profile diagonally across the unit cell (see the white line in Figure 3B), the modulation shows two lower molecules packed between two higher ones. At some distance from the domain boundaries the height difference between the highest and lowest molecules is only about 0.02 nm. Such modulation is absent in a simple (x3 × x3)R30° or a c(4×2) superlattice of long-chain alkanethiols on Au(111). It is known that, on Ag(111) surfaces, chemisorption of S atoms,24 SH,25 and (24) Schwaha, K.; Spencer, N. D.; Lambert, R. M. Surf. Sci. 1979, 81, 273.
SCH326,27 preferentially adopts a (x7 × x7)R19.1° lattice,28 which will have a height modulation similar to that shown in Figure 3B. However, such a structure would require a somewhat smaller nearest-neighbor spacing of 0.441 nm, rather than the 0.50 nm measured in this study. In a closely related case of decanethiol SAMs on Ag(111),20 STM measurements also showed that the lattice exhibits characteristics of a (x7 × x7)R19.1° structure, but the observed nearest-neighbor spacing and height difference are 0.461 nm and less than 0.01 nm, respectively. Guyot-Sionnest et al.20 proposed that the deviation from the simple (x7 × x7)R19.1° is a result of reconstruction of the outermost Ag layers induced by decanethiol molecules, which maximizes van der Waals interactions in the organic monolayers. This reconstruction mechanism explains the lattice as well as the protrusions in Figure 3 better than does the simple (x3 × x3)R30° structure. At a upd potential of 45 mV, where the coverage of Ag adatoms is less than a full monolayer, the nearest-neighbor spacing is larger than that of Ag(111). Assembly of butanethiol SAMs might further expand the spacing between Ag atoms because participation of thiolate adsorption can induce reconstructin of the Ag layer and the underlying Au lattice in order to force the molecules into better registry with each other. For a distorted (x7 × x7)R19.1° to have a nearest-neighbor spacing of 0.50 nm, the spacing of the underlayer Ag would have to be 0.33 nm, corresponding to 75% coverage of Ag on Au(111). Illustrated in Figure 3C is a plausible model for the proposed structure, where the open, gray, and small filled circles represent Au, Ag, and butanethiol, respectively. Consequently, some of the Ag adatoms will pack close together and appear as protrusions. Similar to the case of decanethiol SAMs on Ag(111),20 the effect of substrate reconstruction results in small height differences. (25) Rovida, G.; Pratesi, F. Surf. Sci. 1981, 104, 609-624. (26) Harris, A. L.; Rothberg, L.; Dhar, L.; Levinos, N. J.; Dubois, L. H. J. Chem. Phys. 1991, 94, 2438-2448. (27) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506-511. (28) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401.
Butanethiol Self-Assembled Monolayers
In summary, SAMs of butanethiol on Ag upd-modified Au(111) surfaces have been investigated down to the molecular level. The smooth morphology of such prepared surfaces demonstrates that the etching mechanism that occurs during the assembly of alkanethiols on Au and Ag can be inhibited by introduction of an Ag upd interlayer. When this upd layer is prepared at potentials just positive of that for bulk deposition, the surfaces of butanethiol SAMs exhibit 0.09-nm high protrusions and lattice structures. The periodic modulation in height, the lateral
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spacing of the lattices, and the protrusions on the terraces all suggest that the arrangements of underlying Ag and Au atoms are reconstructed by butanethiol molecules. Acknowledgment. Financial support of this work from NSC (National Science Council, ROC) is greatly appreciated. M.-H.H. thanks NSC for an undergraduate research fellowship. LA990497U
