© Copyright 2002 American Chemical Society
MARCH 19, 2002 VOLUME 18, NUMBER 6
Letters Final Phase of Alkanethiol Self-Assembled Monolayers on Au(111) Jaegeun Noh and Masahiko Hara* Local Spatio-Temporal Functions Laboratory, Frontier Research System, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Received May 31, 2001. In Final Form: September 6, 2001 Time-dependent phase transitions of closely packed alkanethiol self-assembled monolayers (SAMs) on Au(111) were investigated by scanning tunneling microscopy (STM). We report a new phase, the 6 × x3 superlattice, that exhibits deviations from the hexagonal packing arrangement formed through long-term rearrangement of the usual c(4 × 2) superlattice of alkanethiol SAMs. Our STM images show clearly systematic phase transitions with molecular-scale images from the c(4 × 2) phase to the 6 × x3 phase. This result implies that the monolayers in the c(4 × 2) phase are still in the nonequilibrium state, although it has been regarded to be in the equilibrium state in a number of previous studies. The striking implication in this study is to provide new features for closely packed alkanethiol SAMs for the first time.
Alkanethiol self-assembled monolayers (SAMs) on a gold surface have been extensively studied for more than a decade because of fundamental interest in surface science as well as the possibility of a variety of technological applications such as wetting, molecular recognition, and molecular patterning.1-4 In particular, a number of scanning tunneling microscopy (STM) studies revealed at the molecular level that monolayers at saturation surface coverage have a c(4 × 2) superlattice5-7 in addition to the simple hexagonal (x3 × x3)R30° overlayer6-8 on Au(111). The consensus from both experimental studies * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +81-48-467-9600. Fax: +81-48-462-4630. (1) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (2) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1998, 120, 7328. (3) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (4) Kumar, A.; Biebuyck, H. A.; Whiteside, G. M. Langmuir 1994, 10, 1498. (5) Poirier, G.; Tarlov, M. J. Langmuir 1994, 10, 2853. (6) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (7) Noh, J.; Hara, M. Langmuir 2001, 17, 7280. (8) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805.
including electron,9 X-ray,10 and helium diffraction11 measurements together with STM results, and theoretical studies12-14 of the structure of alkanethiol SAMs on gold is that a commensurate (x3 × x3)R30° overlayer exists. It has become a conventional belief that the c(4 × 2) structure is observed when the monolayers reach the equilibrium state. However, despite numerous studies on alkanethiol SAMs, there are no further fundamental and practical questions such as: “Is the c(4 × 2) phase really the final equilibrium state of the monolayers?” and “Can this phase remain the same without any structural changes for a long time?” In fact, the long-term stability of SAMs should be considered essential for technical applications of thiol-based molecular devices. In this Letter, we provide the answers to those questions with molecular-scale STM images. (9) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (10) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (11) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (12) Mar, W.; Klein, M. L. Langmuir 1994, 10, 188. (13) Bhatia, R.; Garrison, B. J. Langmuir 1997, 13, 4038. (14) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.
10.1021/la010803f CCC: $22.00 © 2002 American Chemical Society Published on Web 02/13/2002
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Figure 1. STM images of octanethiol self-assembled monolayers adsorbed on Au(111) surface: (a) STM image showing the c(4 × 2) superlattice, domain boundaries, and depressions from a SAM sample after a 1-day deposition (75 nm × 75 nm, Vb ) 0.83 V (sample positive), and It ) 0.12 nA); (b, c) STM images exhibiting mixed phases of the c(4 × 2) superlattice (region A) and newly appeared linear features (region B) from SAM samples that were prepared by a 1-day deposition, and then kept under a vacuum condition of ∼400 Torr for 3 months at room temperature ((b): 80 nm × 80 nm, Vb ) 0.83 V (sample positive), and It ) 0.12 nA; (c): 40 nm × 40 nm, Vb ) 0.83 V (sample positive), and It ) 0.12 nA).
Au(111)/mica substrates used in this study were prepared by vacuum evaporation as described elsewhere.15 Octanethiol (OT, n-CH3(CH2)7SH) SAMs were obtained by immersing clean gold substrates into a freshly prepared 1 mM ethanol solution of octanethiol for 1 day. After the SAM samples were taken out from the solutions, the resulting monolayers were rinsed thoroughly with pure ethanol to remove physisorbed molecules from the surface. The SAM samples were dried under a stream of nitrogen for 15 min. To examine time-dependent surface structures, the OT SAMs were immediately put in a Petri dish, and then it was tightly sealed under a vacuum of ∼400 Torr with a vinyl bag using an automatic vacuum-seal machine to prevent further oxidation and contamination of the monolayer at room temperature, for 3 months and 6 months. We would like to note that, using this simple storage method, we could observe an ordered molecular structure from various SAM systems without any troubles in STM imaging, even after storage for 1 year. This strongly suggests that there is no a significant effect related to the contamination of monolayers by a trace of water or oxygen or other contaminants that may remain under this vacuum condition, because it is hard to imagine molecular structure from the contaminated monolayers. This result shows that this simple method can be used in a study of material systems with spatiotemporal functions. We checked the chemical composition of the monolayers by X-ray photoelectron spectroscopy (XPS) to confirm that no oxidation of the monolayers took place during the storage. XPS spectra in the S(2p) region are identical to those obtained from as-deposited SAM samples.16 We did not observe any peaks corresponding to oxidized sulfur at around 167 eV, reflecting the absence of oxidation of the SAM samples we used. All STM images for these SAM samples were acquired in air at the constant current mode. The STM image of as-deposited OT SAMs after a 1-day deposition on Au(111) in Figure 1a shows general features of the monolayer such as a closely packed c(4 × 2) phase, a variety of domain boundaries, and depressions with a monatomic depth of 2.5 Å, as can be observed in a variety of alkanethiol SAMs.5-7 On the other hand, STM images in Figure 1b and Figure 1c clearly exhibit two mixed phases consisting of the c(4 × 2) phase (region A) and a linear (15) Noh, J.; Hara, M. Langmuir 2000, 16, 2045. (16) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092.
feature phase (region B) acquired from SAM samples after keeping OT SAMs at room temperature for 3 months. Interestingly, after this time period, a new phase in the monolayer was formed as shown in region b. This result implies phase transitions from the c(4 × 2) phase to the linear feature phase. This new phase shows three domain orientations reflecting the symmetry of Au(111), and linear features have a row spacing of 4.3 Å, and they look like a kind of striped phase where the molecular axes are parallel to the surface in the less-dense phase of SAMs.1,15,17-19 However, while the striped phase depends strongly on the molecular length of alkanethiols adsorbed on the gold surface, this linear feature phase is not such a striped phase because the row spacing is completely different from that found in the striped phase. The detailed molecular structure of this phase will be described later with a molecularly resolved STM image (Figure 2b). For samples after the 3-month storage period, most linear features were observed at domain boundaries and near depressions (white arrows in Figure 1c). Dislocation rows were also observed in closely packed domains (dark arrows in Figure 1c). From the STM observation, we infer that phase transitions primarily start from domain boundaries and near depressions, and subsequently propagate slowly into closely packed domains. This sequence for phase transitions can be attributed to the fact that molecules at domain boundaries and depressions have larger degrees of freedom for conformational changes than those in closely packed domains. STM images shown in Figure 2 were obtained from a SAM sample prepared after keeping OT SAMs at room temperature for 6 months. A single crystalline domain showing alternating bright and dark linear features resulting from the complete termination of the c(4 × 2) phase was clearly observed, as shown in Figure 2a. Our STM results demonstrate for the first time that the c(4 × 2) phase found at saturation surface coverage is definitely not a monolayer structure of the thermodynamic equilibrium state that we have believed so far, even if the phase is relatively stable. We assumed that such phase transitions from the c(4 × 2) phase to the linear feature (17) Poirier, G. E. Langmuir 1999, 15, 1167. (18) Noh, J.; Murase, T.; Nakajima, K.; Lee, H.; Hara, M. J. Phys. Chem. B 2000, 104, 7411. (19) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737.
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Figure 2. (a) STM images exhibiting single-crystalline linear phase and (b) molecularly resolved structure formed after annihilation of the c(4 × 2) phase in octanethiol SAM samples that were prepared by a 1-day deposition, and then kept under a vacuum condition of ∼400 Torr for 6 months at room temperature ((a): 60 nm × 60 nm, Vb ) 0.60 V (sample positive), and It ) 0.19 nA, (b): 10 nm × 10 nm, Vb ) 0.60 V (sample positive), and It ) 0.19 nA). The inset is 2-D fast Fourier transform. (c) A schematic model describing a new 6 × x3 superlattice observed after phase transitions from the c(4 × 2) phase. The rectangular unit mesh is composed of four alkanethiol molecules, and the lattice constants of the unit cell are a ) x3ah ) 4.9 Å, b ) 6ah ) 17.5 Å, where ah ) 2.89 Å and denotes the interatomic spacing of the Au(111) lattice. White circles represent Au atoms, and colored circles represent alkanethiol molecules adsorbed on Au(111).
phase is due to the rearrangement of alkanethiol molecules adsorbed on Au(111) to reach the thermodynamically favorable monolayers. The molecularly resolved image of a linear feature phase shown in Figure 2b exhibits a rectangular primitive unit mesh with cell dimensions of 4.9 Å × 17.5 Å containing of four alkanethiols, which can be described as a 6 × x3 superlattice. On the basis of the STM image in Figure 2b, we propose a schematic model explaining the 6 × x3 superlattice, as shown in Figure 2c. The molecular arrangements of this phase show clear deviations from the hexagonal packing observed at saturation coverage of alkanethiol SAMs. On the other hand, we should note that the 6 × x3 superlattice we observed is entirely different from the p × x3 phase having a low molecular packing density as reported elsewhere19,20 because the new phase shows an average areal density of 21.5 Å2/ molecule, which is nearly the same as the expected areal density of 21.6 Å2/molecule required for closely packed alkanethiol SAMs.21 From this result, we confirm that phase transitions originated from rearrangements of adsorbed molecules and not from the change in surface coverage that might be caused by the desorption of molecules in the course of sample preparation. Furthermore, we should mention that under our imaging conditions, there were no indications of structural deformation even when the STM tip was scanned repeatedly over the same area of the monolayer for longer than 1 h. Meanwhile, although structural fluctuations between (x3 × x3)R30° and c(4 × 2) superlattice were observed by a recent STM study,22 we never found such structural transitions for the new phase during the scanning of the tip. Considering phase transitions via the long-term rearrangement process of adsorbed molecules as well as the high structural stability during the scanning of the tip, the 6 × x3 phase may be energetically more favorable than the (x3 × x3)R30° phase or the c(4 × 2) phase. The molecular features in Figure 2b show three different variations in brightness of the four molecules comprising (20) Larsen, N. B.; Biebuyck, H.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017. (21) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (22) Arce, F. T.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1998, 14, 7203.
the unit mesh together with deviations from the hexagonal packing arrangements. Variation in contrast of the packing arrangements may be influenced chiefly by the difference in twist about the chain axis of the adsorbed molecule11 and/or the change in the local density of states near the Fermi level of the surface caused by moleculesubstrate interaction.23-25 A diffraction experiment11 revealed that the variation in contrast that appeared in the c(4 × 2) superlattice5-7 might have originated primarily from the difference in twist about the chain axis rather than from the electronic effects between adsorbate and substrate. This is because sulfur atoms in a simple hexagonal (x3 × x3)R30° structure occupy identical 3-fold hollow sites of the Au(111) lattice, resulting in similar electronic effects between adsorbate and substrate. However, the difference in adsorption sites of sulfur headgroups on the Au(111) surface can give rise to changes of the twist of alkyl chain and electronic effects between adsorbate and substrate, resulting in the difference in contrast in the STM images.22,26 Interestingly, based on the STM image in Figure 2b, when alkanethiol molecules comprising molecular rows showing three different contrasts were placed on the Au(111) lattice, such molecular rows are located exactly at the center of three adsorption sites, namely, on-top, bridge, and 3-fold hollow sites (see Figure 2c). From this result, we assume that the variation in contrast of molecules is mainly caused by changes in the location of sulfur binding sites, leading to the change in interaction between molecule and substrate. As a result, the STM tip detects a tunneling current difference between itself and the molecules. Therefore, we consider that phase transitions are mainly due to the movement of sulfur headgroups from 3-fold hollow sites to bridge sites or ontop sites in alternating rows along the nearest-neighbor (NN) direction, resulting in the deformation of the hexagonal packing arrangement. It has been pointed out that the flexibility of the sulfur atom helps to relieve the strain in packing arrangements.27 Such rearrangement (23) Nejoh, H. Appl. Phys. Lett. 1990, 57, 2907. (24) Smith, D. P. E.; Ho¨rber, J. K. H.; Binnig, G.; Nejoh, H. Nature 1990, 344, 641. (25) Fisher, A. J.; Blo¨chl, P. E. Phys. Rev. Lett. 1993, 70, 3263. (26) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (27) Li, T.-W.; Chao, I.; Tao, Y.-T. J. Phys. Chem. B 1998, 102, 2935.
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of sulfur headgroups may be driven by the rearrangement of the first gold layer by Ostwald ripening.28,29 As evidence of surface migration of gold atoms in this present work, we clearly observed the coalescence of depressions from a number of small depressions (Figure 1a) to a few large ones (Figures 1b and 2a) as a function of the storage period of SAMs at room temperature. From this result, even though it is generally accepted that sulfur headgroups are bound to identical 3-fold hollow sites of the Au(111) lattice,14,30 it can be also considered that the sulfur headgroups bind to the different adsorption sites such as bridge and on-top sites.31,32 On the other hand, the difference in alkyl chain tilt within each row may be another possibility for the variation in contrast and packing structure. Such different chain tilts would produce a lateral shift in the positions of the terminal methyl groups. Recently, our high-resolution energy loss spectroscopy (HREELS) measurement shows that the time-dependent phase transition is manly due to the conversion of adsorption site of the sulfur headgroups adopting multiple adsorption sites as well as change in orientation of alkyl chains as mentioned above (data not shown here). However it is still difficult to understand clearly the imaging mechanism in the observed structure because of a monolayer structure formed by a complex interplay of molecules, which lead to intricate electronic effects between adsorbate and substrate.5,11,22-26 In addition, it is very interesting to discuss the relation between this phase transition and dimerization of sulfur headgroups that was proposed by Fenter et al.26 The HREELS study by Kluth et al.
elucidated that dimerization of sulfur headgroups on gold occurs only when the SAM sample was annealed to an elevated temperature, suggesting that there is an activation energy for dimerization.33 Also, this result strongly implies that the sulfur headgroups in alkanethiol SAMs exist as a monomer at room temperature, as revealed by many earlier works. Our HREELS result shows that there is no S-S stretching peak for the dimer at around 540 cm-1 from time-dependent OT SAM samples, reflecting no relation between this phase transition and dimerization. This result will be reported elsewhere separately. Finally, we would like to mention that the interconversion of (x3 × x3)R30° and c(4 × 2) structures only requires changes in the tilt structure (i.e., the tilt angle, twist angle, and tilt direction) of alkyl chains,11,13 whereas the interconvesion of these to the 6 × x3 structure requires a concerted motion of the sulfur headgroups and the tilt structure. In this study, molecular-scale STM images revealed phase transitions from the c(4 × 2) phase to the new 6 × x3 phase after long-term storage of alkanethiol SAMs at room temperature. One striking finding is that the c(4 × 2) phase is still in the nonequilibrium state, although it has been regarded to be in the equilibrium state in a number of earlier studies. Our new finding provides new insights into alkanethiol SAMs after the formation of closely packed monolayers. We hope that our results will provide a basis for the design of new thiol-based molecular devices and a better understanding of organic molecules adsorbed on metal surfaces.
(28) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (29) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (30) Ulman, A. Chem. Rev. 1996, 96, 1533. (31) Hayashi T.; Morikawa, Y.; Nozoye, H. J. Chem. Phys. 2001, 114, 7615. (32) Yeganeh, M. S.; Dougal, S. M.; Polizzotti, R. S.; Rabinowitz, Phys. Rev. Lett. 1995, 74, 1811.
Acknowledgment. We would like to thank K. Nakajima for many helpful discussions. LA010803F (33) Kluth, G. J.; Carraro, C.; Maboudian, R. Phys. Rev. B 1999, 59, 10449.
