Langmuir 2002, 18, 1561-1566
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Low-Coverage Decanethiolate Structure on Au(111): Substrate Effects W. P. Fitts† and J. M. White* Center for Materials Chemistry and Texas Materials Institute, Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712
G. E. Poirier‡ Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received May 23, 2001. In Final Form: November 19, 2001 The effects that a reconstructed (herringbone) and stepped Au(111) surface have on the structure of submonolayers of decanethiolate were studied by variable-temperature scanning tunneling microscopy (VT-STM) between 25 and 60 °C. At 25 °C, formation of lattice gas (R) species at low coverages alters the herringbone structure by shortening the periodicity of the elbows from 25 to 15 nm. In addition, small β-phase islands nucleate and grow anisotropically in regions of fcc stacking. These domains grow by incorporating nearby lattice gas species, consuming herringbone ridges and altering the remnant ridges that surround them. In a given domain, the β-phase rows take one of the three 〈121〉 directions of the Au(111) surface. No β-phase is found in regions of hcp stacking or in the bridge atom regions separating the fcc and hcp stacked regions. Increasing the coverage increases the β-phase island size at the expense of herringbone ridges that bound the domains. For a coverage that saturates the β-phase (∼0.25 of the closest packing achievable), raising the temperature to 30 or 40 οC increases the average size of the β-phase islands by condensation of neighboring islands with no evidence, at the selected coverage, for the presence of any other thiolate phase. At 60 °C, well above the thiolate melting point to form the -phase, small β-phase domains remain. These are stabilized by boundaries of two types remnant herringbone and step edges and for a given domain, fluctuations of the distribution between the - and β-phases were observed on a time scale of minutes.
Introduction Self-assembled monolayers (SAMs) form ordered twodimensional arrays of molecules upon exposure to a surface, with the degree of ordering being dependent on a number of parameters, including structure, substrate morphology, coverage, and temperature.1-3 Typically, SAMs are built of molecules containing alkyl chains of various lengths that attach to a solid surface. The alkyl chains provide interchain van der Waals interactions, while the headgroup anchors the chain to the surface via a chemical bond. SAMs are well-defined and can be “tailored” at the surface active headgroup, along the chain, and at the opposite end (“tail group”).4-10 Adsorbed on well-defined substrates, these systems serve as models for studying the fundamentals of two-dimensional phe* Corresponding author. E-mail:
[email protected]. † Present address: Intel Corp., 5200 N.E. Elam Young Parkway, MS AL3-66, Hillsboro, OR 97124. ‡ Deceased. (1) Ulman, A. An Introduction to Organic Films: from LangmuirBlodgett to self-assembly; Academic Press: San Diego, CA, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (3) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401. (4) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678-688. (5) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447-2450. (6) Jung, C.; Dannenberger, O.; Xu, Y.; Buck, M.; Grunze, M. Langmuir 1998, 14, 1103-1107. (7) Li, T.-W.; Chao, I.; Tao, Y.-T. J. Phys. Chem. B 1998, 102, 29352946. (8) Camillone, I. N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737-2746. (9) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Chem. Phys. 1996, 105, 2089-2092.
nomena.2,3 In applications such as biosensing,11,12 corrosion, and wetting,13-15 these films provide an effective method of controlling properties of the interfaces between different materials. Thiols on gold continue to be widely studied as model systems, and until recently the chemical state reached upon adsorption was not clear. There is now good evidence that, at 25 °C and the coverages we use here, that S-H dissociative adsorption occurs to form chemisorbed thiolates.16 Electron energy loss spectroscopy shows evidence for Au-S bonding, and thermal desorption spectroscopy exhibits H2 below 300 K. Thus, we present the discussion in terms of self-organization of decanethiolate, CH3(CH2)9S. The adsorption site continues to be investigated theoretically with varying conclusions regarding the relative stability of thiolate bound to 3-fold and bridged sites.3,17,18 Comparing the vibrational spectra with density functional (10) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, Ch.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102-7107. (11) Ferretti, S.; Paynter, S.; Russell, D. A.; Sapsford, K. E. Trends Anal. Chem. 2000, 19, 530-540. (12) Gano, K. W.; Myles, D. C. Tetrahedron Lett. 2000, 41, 42474250. (13) Zamborini, R. M. C. F. P. Langmuir 1998, 14, 3279-3286. (14) Emmons, H. Trans. Am. Inst. Chem. Eng. 1939, 35, 109. (15) Bigelow, W. C.; Pickett, D. L.; Ziseman, W. A. J. Colliod Interface Sci. 1946, 1, 513. (16) Kodama, C.; Hayashi, T.; Nozoye, H. Appl. Surface Sci. 2001, 264, 169. (17) Hayashi, T.; Morikawa, Y.; Nozoye, H. J. Chem. Phys. 2001, 114, 7615. (18) Groenbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839.
10.1021/la0107650 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/26/2002
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theory calculations indicates that methanethiolate (CH3S-) does not adsorb in a 3-fold site; rather, for monolayer coverages, it is most stable in a bridge site with its S-C bond tilted more than 45° away from the surface normal.17 As the coverage drops, the calculations indicate that the 3-fold site, while not the most stable, becomes more favorable. Another density functional theory calculation,18 at lower coverage, concludes that methanethiolate occupies a 3-fold site. While, for decanethiolate, the position, probably coverage-dependent, of the sulfur with respect to the surface gold atoms was not calculated, the bridge site is most likely. Previous results presented by our group culminated in the development of a 2D phase diagram of decanethiolate on Au(111) spanning coverages from submonolayer to saturation and temperatures from 0 to 65 °C.19,20 In developing this phase diagram, which comprises a lattice gas (R), striped phases (β and δ),21 a melt phase (), and a saturation phase (φ), we tacitly assumed that the Au(111) surface formed the ∼40 kcal mol-1 Au-S bond but was otherwise electronically and topologically flat. However, any Au(111) surface possesses an electronically corrugated lattice, a (23 × x3) “herringbone” reconstruction, surface defects, and single atomic steps, all of which may influence decanethiolate phase behavior. In this paper we present VT-STM images, mainly for coverages that, at 25 °C, are 25% of a full monolayer of the highest density φ-phase. This coverage was chosen because, at 25 °C, decanethiolate saturates the relatively easily imaged striped β-phase with no evidence for other ordered phases. The focus is on the interplay, as a function of temperature, between the organization of the decanethiolate and the steps and herringbone reconstruction of Au(111). STM, by nondestructively characterizing the real space positions of individual molecules,22-24 offers insight that is difficult using other tools.25-29 Experimental Section Experimental details have been addressed previously and will only be briefly presented here.19 All experiments were carried out in a commercially designed UHV VT-STM (Omicron Vacuumphysik) operating at a base pressure of 2 × 10-10 Torr. Sample cleaning was performed in the surface preparation section of the chamber while temperature-controlled dosing and imaging were done in the STM section. Substrates were composed of thin-film gold (200 nm) evaporated on mica. After in-situ Ar+ sputtering and thermal annealing, Auger electron spectroscopy (AES) confirmed surface purity and STM exhibited the expected herringbone reconstruction on terraces ∼50 nm wide. In a few instances, Au(111) atomic resolution was achieved. Decanethiol (19) Poirier, G. E.; Fitts, W. P.; White, J. M. Langmuir 2001, 17, 1176-1183. (20) Fitts, W. P.; Poirier, G. E.; White, J. M. Langmuir 2001, submitted for publication. (21) There is another phase, χ, probably metastable, that is observed but not discussed here. See refs 9 and 19. (22) Chen, C. J. Introduction to Scanning Tunneling Microscopy; Oxford Press: London, 1993. (23) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Appl. Phys. Lett. 1982, 40, 178-180. (24) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. Rev. Lett. 1983, 50, 120-123. (25) Schreiber, F.; Eberhardt, A.; Leung, T. Y. B.; Schwartz, P.; Wetterer, S. M.; Lavrich, D. J.; Berman, L.; Fenter, P.; Eisenberger, P.; Scoles, G. Phys. Rev. B 1998, 57, 12476-12481. (26) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456-3465. (27) Floriano, P. N.; Schlieben, O.; Doomes, E. E.; Klein, I.; Janssen, J.; Hormes, J.; Poliakoff, E. D.; McCarley, R. L. Chem. Phys. Lett. 2000, 321, 175-181. (28) Beardmore, K. M.; Kress, J. D.; Gronbech-Jensen, N.; Bishop, A. R. Chem. Phys. Lett. 1998, 286, 40-45. (29) Grunze, M.; Pertsin, A. J. J. Mol. Catal. A: Chem. 1997, 119, 113-123.
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Figure 1. STM images and schematic side view diagrams of the phases of decanethiol on Au(111). At the lowest coverage, the surface is comprised of a weakly interacting, mobile lattice gas, R. With increased coverage, interadsorbate interactions increase, and decanethiol first condenses into a series of lowdensity “striped” phases, β and δ, each with an increasing degree of out-of-plane interdigitation. A χ-phase is also observed for coverages (not shown) between β and δ. At saturation, the thiol axis lies up, 30° off normal, in a close-packed saturation phase, φ. The density of the melt phase, , lies between those of δ and φ. dosing times typically ranged from 1 to 5 min at pressures of 5 × 10-8 to 1 × 10-7 Torr. While doses were reproducible, we relied on STM images, calibrated on the basis of previous work, to determine the coverage.19,30 For STM at elevated temperatures, the sample was heated through a heated block to which the sample holder was mechanically fixed. The high-temperature limit using the heater block was 65 °C. STM experiments, with images taken using currents between 10 and 100 pA and voltages between -1.2 and +1.2 V, were repeated at least three times with reproducible results. Several experiments were also performed concurrently on a bulk Au(111) substrate using a separate STM at NIST. Both laboratories gave the same results; images taken at the University of Texas are shown here.
Results As discussed previously, decanethiolate sequentially forms a number of structural phases as the coverage increases to a full monolayer19,21,30 (Figure 1). These are, from top to bottom of Figure 1, a lattice gas, R, two striped phases, β and δ, a two-dimensional liquid phase, , and an upright (saturation) phase, φ. At 25 °C, the melt phase is not observed, but with increasing temperature it is observed as a stable phase while the striped phases sequentially become thermodynamically unstable on fcc terraces of Au(111). Phase entropy and molecular area trends account for the relative stabilities.19 For a thiolate coverage well below that required to saturate the striped β-phase, Figure 2A shows the resulting β-phase islands and herringbone ridges. As (30) (a) Poirier, G. E. Langmuir 1999, 15, 3018-3020. (b) Poirier, G. E. Langmuir 1997, 13, 2019-2026. (c) Poirier, G. E. Chem. Rev. 1997, 97, 1117-1127.
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Figure 2. (A) β-phase nucleation from lattice gas at fcc elbows of the Au herringbone reconstruction (oval). The herringbone distorts to accommodate the growth of these domains. The strain associated with thiol adsorption relaxes by consuming the herringbone structure. The three intersecting white lines show the 〈121〉 directions of Au(111). (B, C) Two views of the decanethiol-Au surface near β-phase saturation. Domains of β are separated by 4 nm wide bands, attributed to remnant herringbone bridging atom rows.
expected, the small β-phase domains nucleate and grow in fcc-stacked regions with rows that align with one of the three 〈121〉 directions (marked by three intersecting white lines). There is no β-phase growth in either hcp regions or across herringbone ridges. Compared to the starting pattern in which the elbows repeat every 25 nm, the herringbone is altered throughout the image. Along island edges where individual rows end, the elbows are longer and of variable length correlating with the length of the island edge. At edges where rows are added, the herringbone differs very little from that in regions where there are no β-phase rows. Away from the islands, the herringbone elbows are shorter than on clean Au(111); the repeat distance is 15 compared to 25 nm. There are several other noteworthy observations: (a) The longer elbows extend away from the island edges by at least three herringbone ridges. (b) Neighboring islands formed along the same fcc-stacked zone do not necessarily take the same row direction. (c) The herringbone behavior near the short row ends is one indication that longer elbows are formed at the expense of shorter ones by a process that shifts surface gold atoms to collapse two herringbone ridges into one (e.g., just below the white dotted line). This process expands the-fcc stacked area as the new β-phase row is added. At higher coverage, near β-phase saturation, and higher temperatures, the STM images, Figure 2B,C, evidence only β-phase islands separated by remnant herringbone ridges. The average island size is larger than in Figure 2A, the average number of rows/island is about the same as in Figure 2A, but the average row length is larger. Further, the average row length at 40 °C, Figure 2C, is longer than at 30 °C, Figure 2B. Apparently, small islands condense into larger ones by consumption of the herringbone ridges between them. We suppose, but cannot prove, that condensation occurs more readily for neighboring islands with rows aligned in the same direction. The β-phase domains in Figure 2B,C are separated by 3-4 nm wide dark bands that upon closer inspection comprise remaining bridging atom rows and hcp-stacked regions. Evidently, domain growth has removed or, less likely, covered much of the underlying herringbone. One interesting difference between Figure 2B and Figure 2C is the tendency in Figure 2C (40 °C) for the β-phase rows to run parallel to the herringbone ridges rather than toward them as in Figure 2B (30 °C). We do not observe the vacancy islands that accompany complete removal of the herringbone reconstruction.30 We conclude that the stress that accompanies partial removal of the herringbone by thiolate formation is, under our conditions of coverage
Figure 3. (A) β- two-phase (striped phase-melt phase) equilibrium at 60 °C. Domains are separated by remnant herringbone ridges (intermediate gray scale 4 nm wide rows). Approximately 10% of the herringbone persists at this exposure. No vacancy islands are observed. (B) Close-up of the domain marked by dashed lines in (A). The boundary between the striped phase and the melt fluctuates from image to image (not shown).
and temperature, insufficient to cause ejection of Au atoms. Consistent with this, other evidence suggests that complete removal requires higher coverage where some contribution of the more dense striped phases, χ and δ, is found and where images sometimes show β-phase domains overlaying residual herringbone ridges.19 Above 40 °C at the same coverage, the β-phase domains come into equilibrium with the -phase (melt).19 At 60 °C, i.e., above the critical point for appearance of β-phase decanthiolate on surfaces where the herringbone is fully removed, small domains of β-phase are imaged near steps and when surrounded by residual herringbone ridges, Figures 3-6. Only about 10% of the original herringbone remains at 60 °C, and most of the surface is covered with the expected melt phase (). As one example, Figure 3 shows a region supporting a number of small β-phase islands; the small island in the upper left-hand corner of panel A is expanded in panel B. It is bounded on the upper left by a barely visible step and around the rest of its periphery by herringbone ridges. Not shown here are other images of this region in which the boundary between the β and phases within this small domain oscillates reflecting the dynamic character of the thiolate structure. Importantly, we observed no variation of the herringbone ridges with these oscillations. We return to this point below. Regarding the stabilizing influence of atomic steps, Figure 4 shows a region with a number of single atom steps and intervening herringbone boundaries. In this image, there are β-phase rows adjacent and parallel to the steps but no islands on the terraces separated from the steps. It is, perhaps, not surprising that β-phase domains are stabilized on lower step edges at temperatures
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Figure 4. (A) Stepped area of the Au surface in β- equilibrium at 60 °C. Small domains of the striped phase are restricted to upper and lower step edges with rows parallel to step edges. β-phase stability falls rapidly away from the step edge. (B) Small, step-edge stabilized domain of the β-phase and a partially melted β-phase, termed “zigzag β”, on the terrace.
slightly above those where such domains would be stable on fcc-stacked 111 terraces of gold. However, it is unexpected to find stabilization of β-phase rows adjacent to upper step edges as observed in Figure 4A.31 We take this as another indication of an important role for residual herringbone ridges that define the outer boundary of the islands and to some extent appear to determine the number of rows that are stable. As the distance from the steps increases, β-phase domains are no longer stable with respect to the melt phase (). The β domains are no more than three stripes wide with a reduced length for stripes further from the step edges. One other interesting effect was occasionally found, namely, small domains of distorted β-phase that we term “zigzag β” (Figure 4B). These rows parallel the step edge and nearby herringbone ridges. We take this as evidence that forming the melt phase is more nearly a continuous, rather than abrupt, transition. And although we were unable to image them, oscillations of both the β-phase rows and the zigzag β regions with the melt phase () are likely, as for the oscillating boundary in Figure 3 and for domains discussed below. At 60 °C, the areal fraction occupied by the β- and -phases fluctuates in regions bounded by herringbone bridging atom ridges (Figures 5 and 6). Selected domains are outlined by narrow dark lines (that overlay remnant herringbone), and the region of interest is circled. Images, taken at 3-min intervals, clearly show fluctuating β-phase and -phase compositions. For example, between panel A and B of Figure 5, the β-phase fraction increases while 3 min later, panel C, it decreases and in panel D returns to the same fraction as in panel A. As another example, the (31) Kyuno, K.; Ehrlich, G. Surf. Sci. 1997, 394, L179-L187.
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Figure 5. Domain composition changes for a domain in β- two-phase equilibrium (circled). Images were taken 3 min apart. The domain (black line frame) contains ∼400 thiolates. The domain boundary is defined by remnant herringbone atom ridges and is outlined by narrow black lines. Between frames A and B, a portion of the domain condenses into the striped phase. The fractional increase of the β-phase is ∼12%. By frame D, the fractional coverage of β-phase returns to that found in (A).
Figure 6. Domain phase composition changes (circled) over a period of 9 min (3 min/frame). The domain boundary is defined by remnant herringbone atom ridges and is outlined by narrow black lines. There are ∼110 thiols in this ∼12 × 24 nm domain. Between frames A and B, the small domain completely melts, only to re-form the striped phase 3 min later (frame C). Similar fluctuations are clear elsewhere in these images.
5-stripe region below and to the right of the oval appears to be melting into the -phase but on a time scale longer than the region marked by the oval. In Figure 6, a small domain comprising ∼110 thiolates clearly converts fully from β- to - and back to β-phase in 6 min. Possible origins of these fluctuations are discussed below. Discussion Herringbone Structure of Au(111). Before discussing further the role of terraces, steps, and the herringbone, we review in more detail the nature of the reconstructed Au(111) surface. The outermost atomic layer of Au atoms is contracted laterally along the 〈121〉 direction of the bulk to form the (23 x x3) herringbone reconstruction.32 The positions of atoms in the contracted surface layer vary (32) Woll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B: Condens. Matter 1989, 39, 7988-91.
Low-Coverage Decanethiolate Structure on Au(111)
between face-centered cubic (fcc) and hexagonal closepacked (hcp) stacking order. Between the fcc and hcp areas are two partial surface dislocations33 that appear as 0.15-0.2 Å high ridges in STM images that take one of three 〈121〉 directions.34 The ridges periodically change direction by 60°. Compared to the unreconstructed Au(111) beneath it, there is one extra Au atom/unit cell of the reconstruction. A theoretical treatment of the gold substrate calculated a free energy difference of 5-6 kcal mol-1 favoring the fcc over the hcp sites.18 The energy difference helps to explain the preference of thiols to adsorb at unfaulted fcc lattice sites. Steps, Terraces, and Herringbone Effects on Thiolate Organization. Surface corrugation, steps, and the herringbone reconstruction play a major role in the structures formed upon decanethiol adsorption for the conditions employed in this papersrelatively low coverages (up to about 0.25 of the maximum thiolate packing) and high temperatures (60 °C). Steps clearly stabilize small β-phase domains on both upper and lower step edges (Figure 4), and as mentioned above, especially on the upper edges, remnant herringbone ridges appear to play an important role. On terraces, the clean Au(111) surface is electronically smooth in the sense that for constant current tunneling images atom spacings are seldom resolved; i.e., the valence electron density does not exhibit measurable variation as the tip is moved laterally across the surface. Nonetheless, there are specific sites to which thiol molecules preferentially adsorb and this subject continues to be investigated theoretically with varying conclusions regarding the relative stability of two favored sitessthiolate bound to 3-fold and bridged sites.3,17,18 Typically, cluster calculations have concluded that the 3-fold site is most stable,3 but recent solid-state slab calculations for methanethiolate, making use of density functional theory, conclude that the bridge site on a fcc-stacked (111) terrace is more stable than the 3-fold site.17 This assertion finds experimental support in measurements of vibrational spectra by electron energy loss spectrosocopy for a methanethiolate-saturated surface.17 Specifically, the unsplit Au-S stretch is consistent with binding at a bridge but not a 3-fold site. As the coverage drops, the calculations indicate that the 3-fold site, while not the most stable, becomes more favorable. Another density functional theory calculation,18 at very low coverage, concludes that methanethiolate occupies a 3-fold site. Overall, the following picture emerges for adsorption on terraces: thiolates at low coverage prefer to settle at bridge sites in fcc regions but the barrier to diffusion (2-3 kcal mol-1) between bridge sites via a 3-fold site is readily surmounted, even at 25 °C. All calculations indicate that top site Au-S bonding is significantly less stable (by 6 kcal mol-1 or more). Since the expected activation energy for surface diffusion (Ed,surf) is small (
