STM Study on Nonionic Fluorosurfactant Zonyl FSN Self-Assembly on

Jan 8, 2010 - The mobility of surface atoms on the Au substrate is enhanced by the interaction between FSN molecules and the Au substrate; therefore, ...
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STM Study on Nonionic Fluorosurfactant Zonyl FSN Self-Assembly   on Au(100): 3 -1 Molecular Lattice, Corrugations, 1 1 and Adsorbate-Enhanced Mobility Jiawei Yan,* Yongan Tang, Chunfeng Sun, Yuzhuan Su, and Bingwei Mao State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, PR China Received August 31, 2009. Revised Manuscript Received December 13, 2009 Nonionic fluorosurfactant zonyl FSN self-assembly on Au(100) is investigated by using  scanning tunneling 3 -1 arrangement of the microscopy under ambient conditions. High-resolution STM images reveal that a 1 1 FSN SAMs is formed on Au(100). Different from the uniform structure of FSN SAMs on Au(111), the adsorption sites of FSN molecules on Au(100) change gradually and form a kind of corrugated structure. The change in the adsorption sites probably originates from the repulsive force among FSN molecules because the nearest-neighbor distance of FSN molecules is 0.41 nm, which is smaller than 0.50 nm on Au(111). The mobility of surface atoms on the Au substrate is enhanced by the interaction between FSN molecules and the Au substrate; therefore, no Au island is observed on the FSN-SAM-covered Au(100).

Introduction Zonyl FSN is a kind of nonionic fluorosurfactant, its molecular formula being F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH. A schematic illustration of the structure of the FSN molecule is shown in Figure 1, in which the hydrophilic part consists of a polyoxyethylene chain and the hydrophobic part consists of a fluorocarbon chain. Being a fluorosurfactant, FSN has high surface activity as well as good chemical and thermal stability for use in acidic or alkaline solution. Moreover, FSN has been used for electrogenerated chemiluminescence and HPLC assays.1-3 In our previous paper, scanning tunneling microscopy (STM) was employed to investigate FSN self-assembly on the Au(111) surface.4 It has been found that FSN SAMs on Au(111) have very large domain size with almost no defects, and a √ a√ ( 3  3)R30° adlayer structure was observed by STM. The as-prepared samples of FSN SAMs are very stable under atmospheric conditions for at least a month. An important characteristic of FSN SAMs on Au(111) is the absence of Au islands, which leads to the continuity of FSN SAMs. This phenomenon is in contrast to the thiolate SAMs on Au(111), which is characterized by Au adatom islands or Au vacancy islands because of the lifting of the surface construction.5 We inferred that the lifting of the reconstruction also occurs in the system of FSN SAMs on Au(111) but the generated Au islands are mobile enough to incorporate into the step edges of the Au surface during FSN assembly. However, no experimental evidence is obtained to support the enhanced mobility of Au atoms. Yamada et al. investigated structures of decanethiol SAMs on reconstructed and (1  1)-Au(100) surfaces by STM, and totally *Corresponding author. E-mail: [email protected].

(1) Li, F.; Zu, Y. B. Anal. Chem. 2004, 76, 1768. (2) Zu, Y. B.; Li, F. Anal. Chim. Acta 2005, 550, 47. (3) Lu, C.; Zu, Y. B.; Yam, V. W. W. Anal. Chem. 2007, 79, 666. (4) Tang, Y. A.; Yan, J. W.; Zhou, X. S.; Fu, Y. C.; Mao, B. W. Langmuir 2008, 24, 13245. (5) Yang, G. H.; Liu, G. Y. J. Phys. Chem. B 2003, 107, 8746.

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different structures were observed. They concluded that the formation mechanism of the decanethiol SAMs is strongly affected by surface reconstruction.6 The flame-annealed Au(100) surface is usually reconstructed; Au atoms of the first layer adopt a distorted-hexagonal packing arrangement on the second-layer Au atoms. About 25% extra Au atoms are released to form the monolayer islands because of lifting of the reconstruction. Here, the influence of surface reconstruction of Au(100) would be more obvious than that of Au(111) because the lifting of Au(111) reconstruction involves only the release of about 4% of the Au atoms. Therefore, the reconstructed Au(100) surface may be a good substrate candidate for observing the migration of Au atoms during the self-assembly process and the effect of reconstruction on the structure of the SAMs. Moreover, the different crystallography of the square atomic arrangement of the Au(100)-(1  1) surface from the hexagonal Au(111) surface would help us to understand how the structure of the substrate influences the structure of FSN SAMs. Such a molecular-level understanding of SAMs would also be beneficial to promoting wider applications of the FSN SAMs. In this article, we report an STM study of FSN self-assembly on the Au(100) surface. The structure of FSN SAMs on the square Au(100) surface is studied, and the enhanced mobility of Au atoms on the substrate surface is understood to be related to the proper interaction between FSN molecules and the Au substrate.

Experimental Section STM and AFM measurements were performed on a Nanoscope IIIa multimode SPM (Digital Instrument). All STM images were obtained in constant current mode in an ambient environment at room temperature. Tungsten tips were etched by an electrochemical method in 0.8 mol L-1 KOH solutions. Contact mode and commercial cantilevers with a spring constant of 0.58 N/m were used for AFM experiments. (6) Yamada, R.; Uosaki, K. Langmuir 2001, 17, 4148.

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Figure 1. Schematic illustration of the molecular structure of FSN.

Figure 2. STM images of an FSN-modified Au(100) surface. (a) Au(100) substrate cooled in nitrogen before self-assembly. (Inset) Au(100)-(1  1) structure (4 nm  4 nm). (b) Au(100) substrate exposed to air for 5 min before self-assembly. (Inset) Section analysis of Au islands on the Au(100) substrate. The Au(100) substrate was prepared following the Clavilier method.7 Briefly, one end of a Au wire with a diameter of 0.5 mm was melted in a hydrogen-oxygen flame to form a single-crystal bead with a diameter of about 3 mm. The bead was then fixed on a Au foil with one of the (100) facets facing upward and served as the substrate for STM measurements. Prior to each experiment, the Au surface was subjected to electrochemical polishing followed by flame annealing and cooling in nitrogen to obtain a clean, high-quality surface. Contact angle measurements were performed on an SL200B contact angle meter (Solon, China) using a massive Au(100) single crystal. Zonyl FSN-100 (F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH) was purchased from Aldrich and used as received. All solutions were prepared with Milli-Q water (18.2 MΩ cm, Millipore). FSN aqueous solutions were freshly prepared prior to self-assembly experiments. The FSN SAMs were prepared by immersing an electrochemically polished and flame-annealed Au(100) substrate into a 0.25% FSN aqueous solution for 3 h, which was then thoroughly rinsed with Milli-Q water and dried in air.

Results and Discussion Figure 2a shows the STM image of FSN SAMs on Au(100). The Au(100) surface has a very large terrace size with only one monatomic height step on the lower left side over an area of 400 nm  400 nm. On this scale, the FSN SAMs on Au(100) appear to be uniform without Au islands, Au vacancy islands, and domain boundaries. Such a characteristic is the same as that of FSN SAMs on Au(111) but very different from alkanethiol SAMs on a Au(100) surface and even the bare Au(100) surface. It is well known that flame-annealed Au(111) and Au(100) √ surfaces reconstruct into Au(111)-( 3  22) and Au(100)-(hex), respectively. The reconstructed Au(100)-(hex) structure has a packing density that is about 25% higher than that of the Au(100)-(1  1) structure, but the extra Au atoms may be expelled from the surface upon lifting of the construction and coalesce to (7) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (8) Gao, X.; Edens, G. J.; Hamelin, A.; Weaver, M. J. Surf. Sci. 1993, 296, 333.

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form monatomic high Au islands, resulting in a Au-island-rich surface.8-10 To examine how the FSN assembly influences the stability of the as-formed Au islands, flame-annealed Au(100) substrates are first exposed to air for 5 min to lift the reconstruction and then immersed into the FSN-containing solutions for 3 h. An STM image of the above sample is shown in Figure 2b. Different from samples that have not been exposed to air, some islands ranging from 20 to 100 nm are observed. The height of these islands is about 0.21 nm and the coverage is about 25%, indicating that these islands are made of Au. Because only larger Au islands are observed, the emergence of some small islands must have already taken place during the process of FSN assembly. The mobility of Au islands is inspected by monitoring the changes in island shape and density as a function of time. Figure 3 shows sequentially recorded STM images of an FSN-modified Au(100) surface with a time interval of 128 s. The arrows in the Figure are placed to aid the eye in tracking the time-dependent change in the Au islands. Ostwald ripening of Au islands is observed: The smallest Au island (island 1) disappeared first, followed by the disappearance of the two next-smallest Au islands (islands 2 and 3). Meanwhile, two islands merge into a larger island as highlighted by the blue arrow. We performed a control experiment on a Au-island-covered Au(100) surface without FSN SAMs as shown in Figure 2s; such a substantial change in Au island shape was not observed, which confirms that only in the presence of FSN molecules is the mobility of Au atoms enhanced. We point out that on the bare surfaces some Au islands, which are smaller than the changing islands on the FSNcovered surface in Figure 2, are still stable. Molecule-surface interaction is responsible for the surface mobility of both molecules and surface atoms of the substrate. Interactions that are too strong (e.g., thiolate SAMs) will decrease the surface mobility of Au atoms in comparison with that of the FSN/Au system because the Au atoms prefer to move along with the adsorbed thiol molecules. However, interactions that are too weak (e.g., alkane SAMs) cannot even lift the reconstruction of the Au substrate; therefore, they cannot enhance the mobility. As for FSN SAMs on the Au substrate, the interaction is strong enough to lift the reconstruction of the Au substrate and weaken the Au-Au interaction but it is weaker than the interaction of the thiol/Au system. It is the moderate molecule-surface interaction that enhances the surface mobility of Au atoms. The question remains as to why the Au islands are completely absent when island-free reconstructed Au(100) and Au(111) surfaces are used for FSN assembly as shown in Figure 2a and in our previous paper,4 respectively. Plausibly, the enhanced surface mobility of Au atoms encourages all of the expelled Au atoms to incorporate into the lattice at the step edges of existing terraces before the FSN form a large SAM domain, leaving no islands on the surface. However, the enhanced surface mobility of the Au atoms should also increase the probability that the expelled Au atoms themselves will form Au islands, which is in violation of the experimental observation of the absence of the Au islands. This leads us to infer that the lifting of the reconstruction is initiated only at the step edges of terraces and proceeds toward the center of the terrace. Under this circumstance, once expelled from the surface because of the lifting of reconstruction, the Au atoms can be immediately incorporated into the lattice of the terrace. Kolb’s group investigated ethanethiol and butanethiol SAMs on Au(100).11,12 STM images recorded in air reveal a disordered (9) Magnussen, O. M.; Hotlos, J. R.; Behm, J.; Batina, N.; Kolb, D. M. Surf. Sci. 1993, 296, 310. (10) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109. (11) Schweizer, M.; Hagenstrom, H.; Kolb, D. M. Surf. Sci. 2001, 490, L627. (12) Loglio, F.; Schweizer, M.; Kolb, D. M. Langmuir 2003, 19, 830.

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Figure 3. Sequential STM images of an FSN-modified Au(100) surface, where the Au(100) substrate was exposed to air for 5 min before assembly. The arrows are placed to aid the eye in tracking the time-dependent changes in Au-island shape and density. Scan rate: 2 Hz. Scan size: 400 nm  400 nm.

Figure 4. Schematic diagrams of FSN self-assembly processes (a) on a reconstructed Au(100) surface and (b) on an island-covered Au(100) surface via lifting of the reconstruction.

ethanethiol adlayer on a surface that has a 25% coverage of monatomic high Au islands that originate from the lifting of the hex reconstruction during thiol adsorption. In contrast, the elevated network of Au islands and the striped structure of the butanethiol SAM are observed in air. Poirier studied the molecular packing structure of butanethiol SAMs on Au(100) by using an ultrahigh-vacuum STM. Site-selective Au atom ejection results in highly anisotropic Au/SAM islands. The thiolate molecules pack in a distorted hexagonal arrangement with a c(2  8) unit cell.13 However, the present work has shown no islands of FSN SAM-covered Au(100) unless Au(100) is exposed to air prior to the self-assembly process. Moreover, the shape of Au islands under FSN SAMs appears to be random, which is different from the stable, rectangularly shaped Au islands on Au(100) surfaces covered with alkanethiolate SAMs. Here, the strength of FSN adsorption is crucial to the observed absence of the Au island upon the lifting of reconstruction. Schematic diagrams of the FSN self-assembly processes on reconstructed Au(100) and island-covered Au(100) are shown in Figure 4. As mentioned above, in the STM image with a scan area of 400 nm  400 nm, the FSN SAMs on Au(100) present the same characteristic as that of FSN SAMs on Au(111). However, a close examination of the high-resolution STM image in Figure 5a shows that the FSN SAMs on Au(100) form parallel stripes of corrugation. The kind of corrugation could be observed in two (13) Poirier, G. E. J. Vac. Sci. Technol., B 1996, 14, 1453.

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perpendicular directions, which is consistent with the 4-fold symmetry of the Au(100) substrate. The height of the corrugations is 0.05 nm. These corrugations remarkably degrade the uniformity of FSN SAMs. It should be pointed out that the above corrugations are not observed on the Au(111) surface, revealing the influence of the surface crystallography of the substrates on the SAMs structures. Figure 5b is a molecular-resolution image of FSN SAMs on Au(100), which shows that FSN molecules are present on both sides of a corrugation and an ordered structure is formed between two corrugation stripes. The corrugation stripes are formed because of the deviation of FSN adsorption from the ordered adsorption structure as shown by the black lines in Figure 5b. Domains with ordered structure are usually very large and extend to the whole STM imaging area in the direction parallel to the corrugations. The structural transition of two normal regions is a kind of domain boundary. Different from the disordered structure of domain boundaries of thiolate SAMs, the domain boundaries of FSN SAMs on Au(100) show special structure (i.e., the adsorption sites of FSN molecules change gradually from bridge sites to near-top sites in the corrugation region). To indicate the difference, we called the transitional region a corrugation. To reveal the structural registration of FSN SAMs with the Au(100) substrate, we tried to acquire atomic-resolution images of the Au(100) substrate in the presence of FSN by adjusting the tunneling conditions. Under the condition of a bias voltage DOI: 10.1021/la903250m

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Figure 5. High-resolution STM images of FSN SAMs on a Au(100) surface on different scales: (a) 50 nm  50 nm and (b) 10 nm  10 nm. The inset of panel a is a sectional profile showing the height of the corrugations.

Figure 6. High-resolution STM image of an FSN SAM on Au(100). The lower part shows the atomically resolved Au(100) surface acquired at a constant tunneling current of 11 nA and a bias voltage of 4 mV; the upper part shows the ordered structure of the FSN SAM acquired at a constant tunneling current of 0.8 nA and a bias voltage of 200 mV. Scan size: 10 nm  10 nm.

of 4 mV and a tunneling current of 11 nA, the atomic structure of the Au(100) substrate was imaged as shown in the lower part of Figure 6. Because of the existence of FSN SAMs, the image quality of the Au(100) substrate is not very good but the [011] direction of the Au(100) surface can be clearly identified as is indicated by the short arrow in the lower part of the image. The closest distance of two bright lines is 0.29 ( 0.02 nm, which is consistent with the atomic distance of the Au(100) substrate. By raising the bias to 200 mV and decreasing the tunneling current to 0.8 nA, an image of a different structure is obtained with the closest distance of two bright dots of 0.41 ( 0.03 nm, which should originate from the adsorbed molecules; see the upper part of Figure 6. There is a rotation of 72 ( 3° of the close-packed direction of the adsorbed molecules (indicated by the long arrow) from the [011] direction of the Au(100) substrate. On the basis of ! the above experimental results, a

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proposed as shown in Figure 7. In this√model, the distance between two nearest FSN molecules is ( 2)a (a = 0.288 nm, 3832 DOI: 10.1021/la903250m



3 -1 1 1 ture and corrugation on the Au(100) surface.

Figure 7. Schematic illustration of the

 adlayer struc-

√ Au(100) lattice constant) in the 2 direction √ and the distance between two next-nearest FSN molecules is ( 10)/2a. The direction of the next-nearest FSN molecules rotates by 71.6° relative to the [011] direction of the Au(100) substrate. The parameters of the proposed model are in good agreement with our STM data. The x in the FSN formula means that the hydrophilic part of FSN has a variable chain length. However, the SAMs still look flat. It is likely that the STM tip penetrates into the SAMs and scans inside the SAMs in order to maintain the preset feedback current. In this case, the variable chain length of the FSN molecules will not produce different contrast in the STM images. To obtain high-quality STM images, the tunneling current and bias voltage need to be adjusted to appropriate values. Similar high-quality STM images of the FSN SAMs could be acquired under a tunneling current of 0.4-1 nA and a bias voltage of 50-400 mV. Lower current and higher bias (i.e., higher tunneling gap impedance) would significantly degrade the quality of the STM images. If the FSN molecule is bound to the substrate via its Langmuir 2010, 26(6), 3829–3834

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oxygen atom, then we prefer to believe that it is the oxygen atom that is imaged by STM, which would give a relatively high tunneling current. The hydrophilic part of the FSN molecule consists of a hydroxyl group, and the structure and adsorption behavior of FSN may be similar to those of an alkanethiolate to a certain extent. It is necessary to compare the adsorption sites of FSN with those of thiolate. STM has been used to examine the structures of benzenethiol adlayers on Au(100). All benzenethiol admolecules are assigned to adsorb at the most favorable 4-fold hollow sites.14 The self-assembly of bis(2-anthraquinyl)disulfide15 and the adsorption of 4-pyridinethiolate16 on the Au(100) surface were studied by Yoshimoto et al. using STM. It is proposed that bis(2-anthraquinyl)disulfide is adsorbed via the sulfur atoms at 4-fold hollow sites but 4-pyridinethiolate is adsorbed via the sulfur atom at the bridging site. Yoshimoto et al. also immobilized p-tert-butylcalix[4]arene-1,3-dithiol (BCAD) on a Au(100) surface. The two sulfurs of BCAD are assumed to be located on the near 2-fold bridging site on the basis of the distances between the molecules and the molecular orientation observed in the STM images.17 There was also a controversy about the adsorption site of alkanethiolate on Au(111).18-21 Recently, Cossaro et al. investigated hexanethiol and methylthiol on Au(111) by density functional theory-based molecular dynamics simulations and grazing incidence X-ray diffraction. They found that the sulfur atoms of the molecules bind at two distinct surface sites; some on the ontop site form the RS-Au-SR motif and others adsorb on bridging sites.22 As for FSN SAMs on the Au(100) surface, bright spots in the STM images of Figure 5b, except in the corrugation area, appear to be similar in contrast, which indicates that these FSN molecules bind at the same sites. As shown in the schematic illustration of FSN SAMs in Figure 7, FSN molecules are proposed to sit on the 2-fold bridging sites along the [011] and [011] directions, which explains the similar contrast in STM images. In the proposed model, the distance between two adjacent FSN molecules is in good agreement with our STM data. If some FSN molecules are proposed to sit on 4-fold hollow sites, then the others must sit on top sites in order to match the distance and the angle of the FSN arrangement observed in STM images; a different contrast would have been observed in the STM image. As shown in Figure 7, on both sides of a corrugation, the same unit cell lattice √ in which one of the unit cell lattice directions is along the 2 direction (i.e., [010] direction) can be assigned to describe the structure of FSN molecules. However, there exists a parallel shift of the adsorptive sites for FSN molecules adsorbed on different sides of the corrugation, as indicated by solid curves. (14) Liu, G. Z.; Ou Yang, L. Y.; Shue, C. H.; Ma, H. I.; Yau, S. L.; Chen, S. H. Surf. Sci. 2007, 601, 247. (15) Yoshimoto, S.; Hirakawa, N.; Nishiyama, K.; Taniguchi, I. Langmuir 2000, 16, 4399. (16) Yoshimoto, S.; Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Electrochem. Commun. 2000, 2, 39. (17) Yoshimoto, S.; Abe, M.; Itaya, K.; Narumi, F.; Sashikata, K.; Nishiyama, K.; Taniguchi, I. Langmuir 2003, 19, 8130. (18) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258. (19) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokohama, T.; Ohta, T.; Shimomura, M.; Kono, S. Phys. Rev. Lett. 2003, 90, 66102. (20) 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. (21) Torrelles, X.; Vericat, C.; Vela, M. E.; Fonticelli, M. H.; Millone, M. A. D.; Felici, R.; Lee, T. L.; Zegenhagen, J.; Munoz, G.; Martin-Gago, J. A.; Salvarezza, R. C. J. Phys. Chem. B 2006, 110, 5586. (22) Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M. L.; Scoles, G. Science 2008, 321, 943.

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It is the parallel shift that results in the corrugations and discontinuity of FSN SAMs. An interesting phenomenon is that in the [010] direction the distance between two corrugations is only about 2.5-10 nm (i.e., the width of a uniform film is small) but the length of the uniform film is usually hundreds of nanometers, running throughout the whole STM image. This feature should originate from different arrangements of FSN molecules in different directions. In the√[010] direction, the distance between two FSN molecules is ( 2)a. We reported in our previous paper that FSN SAMs on Au(111) are free of defects √ and the smallest distance between two FSN molecules is ( 3)a. Therefore, we infer that because of the smaller distance there exists a repulsive interaction between FSN molecules in the [010] direction. The accumulation of such repulsive interactions causes the FSN molecules to deviate from the bridge site in the corrugation region, (i.e., FSN molecules move gradually to near-top sites). Liu √ √ et al. reported that benzenethiol molecules adapt a ( 2  2)R45° adlayer structure on the Pt(100) surface because the Pt-S bond is strong enough to compensate for the repulsive interaction between the admolecules.14 However, the interaction between FSN molecules and the Au substrate is weaker than the Pt-S bond; therefore, FSN molecules prefer to adjust their adsorptive site to relieve strain and stabilize the SAMs. The corrugations look brighter in STM images, and the deviation of adsorptive sites may account for the change in contrast in STM images. For the next-closest packed direction, the √distance √ between two FSN molecules is ( 10)/2a, larger than ( 2)a, by which the repulsive interaction is expected to be lessened to allow a longer extension of the uniform structure in this direction. We also acquired molecular-resolution images of FSN SAMs on an island-covered Au(100) surface; the same structure has been observed in Figure 5. We performed AFM measurement to estimate the thickness of the FSN SAMs by the following procedures. First, STM was used to confirm that the ordered FSN SAM was formed on the Au(100) substrate. Then, the AFM tip scanned across an area of 100 nm  100 nm to remove the FSN molecules by using a larger contact force (about 435 nN). As a result, a square pit was generated as shown in Figure S1 of Supporting Information. Note that no pits can be formed on the bare Au(111) under the above experimental conditions. Finally, an area of 2 μm  2 μm that enclosed the pit was imaged. Sectional analysis gives the depth of the pit to be about 1.1 nm, which indicates that the molecules in the SAMs are not lying flat on the substrate surface but are in a standing-up configuration with a tilt angle. It is probable that the FSN molecules are somehow bent and do not have an alkanethiol-like standing-up geometry. To estimate the possible orientation of the FSN molecules, contact angle measurements, which are sensitive to the hydrophobicity of FSN SAMs, were conducted on Au(100). The water contact angle of FSN SAMs on Au(100) is about 90°. This value indicates that the hydrophobic part of the FSN SAMs is exposed as the outer surface. If it were the hydroxyl groups of the FSN molecules that were exposed, then the contact angle of FSN SAMs would have been about 20°, given that the contact angle of a hydrophilic HO(CH2)11S-Au surface is 22°.23 The observed characteristics of the FSN SAMs on Au(100) are the result of surface crystallography and the mobility of Au atoms and adsorbed molecules during self-assembly. The interaction between the FSN molecules and the Au substrate is strong enough to lift the reconstruction of the Au substrate and weaken the Au-Au interaction. Therefore, the enhanced mobility of Au (23) Banks, J. T.; Yu, T. T.; Yu, H. Z. J. Phys. Chem. B 2002, 106, 3538.

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atoms and adsorbed molecules makes some kinds of defects such as Au islands absent. The absence of defects remarkably increases the continuity of SAMs. However, the adsorption structure of FSN molecules onto the quadratic atomic arrangement of Au(100) creates a repulsive interaction among FSN molecules because of the limited distance. The accumulation of such repulsive interactions causes FSN molecules to deviate from the bridging site. However, the corrugations thus formed decrease the uniformity of the SAMs to some extent.

sites except for corrugation regions. √ The distance √ between the two nearest FSN molecules is ( 2)a in the 2 direction; the repulsive interaction makes FSN molecules deviate from the bridging site in the corrugation region and gradually move to near-top sites. It is the repulsive interaction that results in the corrugations. The characteristics of the FSN SAMs on Au(100) are the results of surface crystallography and the mobility of Au atoms and adsorbed molecules during the self-assembly process.

Conclusions

Acknowledgment. This work was supported by the Natural Science Foundation of China (NSFC no. 20973144), the SRF for ROCS, SEM, China (2008-890), and the Special Funds for Major State Basic Research Project of China (973 Project nos. 2007CB935600 and 2009CB220102). We sincerely thank Dr. Yanbing Zu for his suggestion to perform STM studies on FSN self-assembly. We also thank Dr. Deyin Wu and Mr. Yifan Huang for their helpful discussions.

We have shown that high mobility plays an important role in forming uniform FSN SAMs on Au substrates. No Au islands form; even extra 25% monolayer Au atoms exist for the reconstructed Au(100) substrate. The expelled Au atoms originated from lifting of the reconstruction could emerge into step edges because of the FSN adsorbate-enhanced mobility of ! Au atoms. High-resolution STM images reveal a

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tration with the Au(100) surface and some parallel corrugations. FSN molecules should reasonably sit on the 2-fold bridging

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Supporting Information Available: Additional AFM and STM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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