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In Situ Scanning Tunneling Microscopy Observation of the Self-Assembly Process of Alkanethiols on Gold(111) in Solution Ryo Yamada and Kohei Uosaki* Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Received August 28, 1997. In Final Form: December 11, 1997 The self-assembly process of alkanethiols of various chain lengths [CH3(CH2)n-1SH: n ) 10, 15, and 18] on a Au(111) surface in very dilute (0.5 µM, Figure 1f), the gold terrace was almost entirely covered with the island structure. The time required for the islands to cover the whole surface was highly dependent on the surface conditions and concentration. It took typically ca. 30 min to 1 h in ca. 0.3 µM solution.42 At this
stage, the (x3 × x3)R30° structure with defects of the dimension of one or several molecules was observed on the islands, as shown in Figure 2. Although the c(4 × 2) structure was not observed here, this does not mean that the c(4 × 2) structure did not exist, as molecular structures observed by STM are highly dependent on the sharpness and symmetry of the tip and imaging conditions.54,55 Multilayer formation was not observed even in solutions of higher concentration (∼5 µM). Although Poirier and Tarlov reported the Ostwald (54) Touzov, I.; Gorman, C. B. J. Phys. Chem. B 1997, 101, 5263. (55) Lio, A.; Morant, C.; Ogletree, D. F.; Salmeron, M. J. Phys. Chem. B 1997, 101, 4767.
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Figure 2. Closed up STM image (15 × 15 nm2) of the island seen in Figure 1f.
Figure 4. (a) Typical closed up STM image (50 × 50 nm2) of the region that shows the coexistence of different stripe structures. (b) Sectional views along lines A and B in Figure 4a.
Figure 3. STM image (200 × 200 nm2) obtained in ca. 0.2 µM solution of C10SH in heptane: (A) the wide stripe structure; (B) the narrow stripe structure; (C) the mesh structure; (D) the island structure.
ripening of the pits,56 such a phenomenon was not observed in the present study. The discrepancy is probably due to the differences in the observation period and the mobility of the gold adatom (their observation was carried out with butanethiol [CH3(CH2)3SH]). When the SA process was monitored in solutions of higher concentration, e.g., 3 µM, the line structures were hardly observed and the gold surface was covered with the islands very quickly, e.g., within 5 min. The lineshaped structures were not observed either because the rate of the phase transition from the line-shaped structures to the islands was too fast to be observed by STM or the coverage became high enough to form the island structure as soon as the gold was dipped into the modifying solution. To investigate the initially observed structures more carefully, the STM investigation was carried out in heptane containing ca. 0.2 µM C10SH so that the island growth was suppressed. Figure 3 shows a typical STM image of the gold surface. At least four kinds of structures, i.e., two types of stripe structure (A and B), a meshlike structure (C), and islands (D), were observed. A higher resolution image (Figure 4) provides clearer evidence for (56) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966.
the coexistence of the different stripe structures. Molecular rows of each structure run in the same direction, but their periods are different (see Figure 4b). Thus, these two different structures are not due to convolution with an asymmetric tip but are real. These structures were observed for more than 1 h without significant change, indicating that they were stable under this condition. In many cases, the narrower stripe and the mesh structures were dominantly observed and the wide stripe was rarely observed. We must mention here that these structures except the island structure were hardly observed in air40,41 and we observed the transition from the stripe and mesh structure to the island structure.57 Figure 5a shows a typical STM image of structure A. We refer to this structure as a wide stripe structure. It has a 3.2 nm corrugation period perpendicular to the stripe (Figure 5b). This corrugation period is close to the one of the 11 × x3 structure observed for the SAMs at low coverage in UHV conditions.19d,39 This stripe structure consists of a paired bright row. Considering the space between the stripes, the wide stripe structure seems to consist of the molecules that lie on the surface; i.e., the molecular axis is aligned with the surface plane. This structure is not expected if the Au-S chemical bond was formed because the sp or sp3 orbital of the S atom is known to be used for a Au-S-C bond and the Au-S-C angle is 104° and 180° for the former and the latter orbitals, respectively.60 Thus, the molecules in this wide stripe (57) Yamada, R.; Uosaki, K. Unpublished result. (58) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747. (59) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. Part 2 1996, 35, L799.
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Figure 5. (a) Closed up STM image (25 × 25 nm2) of a wide stripe structure. (b) Sectional view along line A.
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Figure 7. (a) Closed up STM image (30 × 30 nm2) of a mesh structure. (b) and (c) Sectional views along lines A and B, respectively.
Figure 8. Closed up STM image (30 × 30 nm2) that shows a transitional state from the mesh structure to the stripe structure.
Figure 6. (a) Closed up STM image (25 × 25 nm2) of a narrow stripe structure. (b) Sectional view along line A.
structure should be physisorbed. A similar structure was reported by Cyr et al. for self-assembled monolayers of alkanethiols on graphite.58 Their observation showed that alkanethiols were preferentially oriented head to head, i.e., thiol to thiol, but occasionally oriented head to tail. In the present case, however, no head to tail orientation was observed. This may be because molecules were in dimerized form, i.e., disulfide.20b,26,59 (60) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.
Figure 6a shows a typical STM image of structure B in Figure 3. We refer to this structure as a narrow stripe structure. It consists of a paired bright row and has a 2.3 nm corrugation period perpendicular to the stripe (Figure 6b). The paired structure was reproducibly observed in different experiments, and thus it is not the artifact due to a double tip. The corrugation period is close to the one expected for a (5x3 × x3)R30° (or 7.5 × x3) structure, which was reported to appear after thermal treatment of SAMs of alkanethiols.17d,25,40 This value is larger than the single molecular length but less than 2 × (molecular length). Troung and Rowntree carried out ex situ STM and IRRAS measurement of the self-assembly process of butanethiol in dilute solution and showed that when the gold substrate was soaked in dilute solution for a long time, the stripe structure consisting of upright molecules
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Figure 9. Real time monitoring of the self-assembly process of C15SH in a 0.5 µM solution of C15SH in heptane. Each image was obtained at (a) just upon addition of the modifying solution droplets (150 × 150 nm2) and (b) 1 min and (c) 4 min after the addition of the droplets of the modifying solution (150 × 150 nm2).
was formed.24,61 Their results suggest that the narrow stripe structure of the present study may also consist of upright molecules. IRRAS measurement should be carried out to confirm the formation of these structures. In addition to the stripe structures, which are very similar to those reported in UHV condition,19d,38,39,43,56 a meshlike structure shown in Figure 7a, which has never been reported, was observed. The unit structure has a shape of a parallelogram and consists of paired rows with the separation distance of 1.3 nm. The corrugation period is 2.5 and 4.5 nm in the direction of short side (A) and the long side (B), respectively (see Figure 7b,c). Note that Figure 7b shows an averaged height in the direction of the paired rows within a rectangle drawn in Figure 7a. This structure was dominantly observed at the initial stage of the SA process of C10SH. There were cases when only the mesh structure covered the gold surface. Although it is difficult to discuss the molecular arrangement of the mesh structure only from the STM image, this structure seems to be a precursor of the narrow stripe structure, as a structure that seems to show a transition from the mesh to the narrow stripe structure was observed at a certain portion of the surface, as shown in Figure 8. At the center of the figure, the unit structures of the mesh begin to be connected to each other. Stripes are also seen in the figure. (b) Self-Assembly of Alkanethiols of Longer Alkyl Chain Length (C15SH and C18SH). Figure 9 shows STM images of the gold surface obtained in a heptane solution containing ca. 0.5 µM C15SH. The narrow stripes and pits were observed even just after the addition of the modifying solution. Although the STM images of the same region obtained 1 min (Figure 9b) and 4 min (Figure 9c) after the addition of the modifying solution clearly showed that the pits and the stripe structure grew with time, the growth of the latter was more significant. The observed SA process was similar to that of C10SH except the fact that the wide stripe and the mesh structure were rarely observed. The corrugation period perpendicular to the stripes was about 2.7 nm. This value was larger than that of the narrow stripe of C10SH. Figure 10 shows an STM image of the gold surface observed in a heptane solution containing ca. 0.5 µM C18SH. Although the growth process of C18SH was very similar to that of C15SH, the corrugation period perpendicular to the stripe lines was larger than those of C10SH and C15SH, i.e., ca. 3.3 nm. After further addition of the modifying solution, the STM image became obscure and we were not able to observe the island structure. This may be because a tunneling gap impedance is not high enough to resolve the long alkanethiol tail and the tip destroyed the structure. (61) Kang, J.; Rowntree, P. A. Langmuir 1996, 12, 2813.
Figure 10. Closed up STM image (50 × 50 nm2) of the stripe structure of C18SH. (b) Sectional view along line A.
Conclusion The present results have shown that there are at least three steps in the SA process of alkanethiols of various chain lengths on Au(111). At the initial stage, adsorbed molecules do not arrange in ordered structure. The VIs of the gold surface are created at this stage. Further adsorption of the molecules leads to the initial ordered self-assembled structures such as mesh and stripe, i.e., the p × x3 structure (p ) 7.5 and 11 for C10SH) when the concentration of the solution is low. The size of VIs does not change significantly after this stage, suggesting the etching is a minor effect on VIs formation. Finally, the islands on which molecules arrange in the (x3 × x3)R30° structure, i.e., solid phase, grow and the monolayer formation is completed. The dynamic equilibrium state exists at the early stage of the island growth. The p × x3 structures, whose values of p are not consistent with the surface-aligned molecular model, also exist at the early stage of self-assembly of C15SH and C18SH. These results seems to be consistent with the model based on the results from macroscopic measurements that have shown that there are at least two steps in the SA process, i.e., the initial faster step and following slower step.3,26,27,31,34,36,37 It seems that the formation of the preformed structures and the islands corresponds to the initial faster and
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following slower step, respectively. To confirm the consistency of the present and macroscopic studies, the SA process should be simultaneously monitored by other in situ analytical methods such as IR-QCM,62 which is sensitive to the changes in both structure and mass on the surface.
Acknowledgment. This work was partially supported by Grants-in-Aid for Scientific Research on Priority Area Research of “Electrochemistry of Ordered Interfaces” (09237101) from the Ministry of Education, Science, Sports, and Culture, Japan. R.Y. acknowledges the Japan Society for the Promotion of Science for a fellowship.
(62) Shimazu, K.; Ye, S.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 375, 409.
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