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Chain Length Dependence of the Striped Phases of Alkanethiol Monolayers Self-Assembled on Au(111): An Atomic Beam Diffraction Study N. Camillone III,†,‡ T. Y. B. Leung,† P. Schwartz,§ P. Eisenberger,| and G. Scoles*,† Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544 Received December 1, 1995. In Final Form: February 29, 1996X Low-energy helium atom diffraction measurements of the surface structure of n-alkanethiol films deposited from a molecular beam on to the (111) face of gold single crystals (at an impingement rate on the order of 1011(1 molecules cm-2 s-1) show that the thiols form “striped” overlayers. These structures are similar to those previously seen by Dubois et al. in a recent low energy electron diffraction (LEED) study of vapor-deposited overlayers (J. Chem. Phys. 1993, 98, 678), by Poirier et al. in a scanning tunneling microscope (STM) study of low coverage solution-grown short-chain thiols monolayers (Langmuir 1994, 10, 3383), and more recently by us, Poirier, and Tarlov by thermal treatment of the full-coverage c(4x3×2x3)R30° phase formed in the standard way by self-assembly from solution (J. Chem. Phys. 1994, 101, 11031). The surface periodicity of the monolayer structures observed in the present study can be described (with respect to the Au(111) surface lattice) in terms of a rectangular p×x3 unit mesh where p, the periodicity of the stripes, scales linearly with the length of the adsorbed thiol. The absolute value of the stripes’ period is, with a maximum deviation of 3%, 1.9 times the length of the corresponding fully stretched thiolate fragment which coincides with the length of the corresponding fully stretched dialkyl disulfide. The present results, analyzed in the context of the others, confirm the presence of coverage-dependent and chain-lengthdependent phase behavior in these systems and suggest that, at the lowest “full” coverages, the molecules may assume a near-flat configuration on the gold substrate.
I. Introduction “Self-assembly” methods provide a wet-chemical approach to the fabrication of highly ordered organic monolayers.1 Simple immersion of a clean substrate into a dilute solution of surfactant molecules can, for certain systems, result in the spontaneous assembly of a monolayer film possessing long-range (∼102-103 Å) molecular order.2-7 During the past few years, n-alkanethiols (CH3(CH2)n-1SH, referred to as Cn) adsorbed on gold have served as the archetypical examples of such systems and are arguably the most intensively studied and best characterized class of organic thin films.8 The relative simplicity of self-assembly methods conceals the complexity of intermolecular and interfacial interactions involved in the formation of the monolayers. †
Also Chemistry Department, Princeton University. Present address: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599. § Also Department of Astrophysical Sciences, Princeton University. | Also Physics Department, Princeton University. X Abstract published in Advance ACS Abstracts, May 1, 1996. ‡
(1) See, for example: (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (d) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (e) Bain, C. D.; Evall, J.; Whitesides, G. J. Am. Chem. Soc. 1989, 111, 7155. (f) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (g) Ulman, A. Adv. Mater. 1990, 2, 573. (h) Whitesides, G. M.; Laibinis, P. Langmuir 1990, 6, 87. (2) Strong, L.; Whitesides, G. Langmuir 1988, 4, 546. (3) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (4) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (5) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (6) Camillone, N., III; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 99, 744. (7) (a) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (b) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (8) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437.
S0743-7463(95)01097-3 CCC: $12.00
This complexity is reflected in both the structure of the monolayers and the kinetics of their formation. For example, it is now well documented that the packing of molecules in the monolayer differs from that expected based on simple analogy with the structure of bulk n-alkane crystals.4,7,9,10 The periodicity along one of the primary unit cell directions of the monolayer is twice that found in analogous bulk crystal phases.4 The root cause of this doubling is almost certainly due to interactions at the organic/inorganic interface and has been attributed to the formation of a disulfide-like moiety.7b In this paper we report the results of helium atom diffraction studies of monolayers, mostly, but not exclusively, prepared in vacuo via molecular beam deposition. In the context of the complexity of the interactions and kinetics involved in monolayer formation, in vacuo molecular beam deposition of n-alkanethiols onto gold surfaces is of interest for a variety of reasons. First, molecular beam deposition provides a means to prepare monolayers of short-chain thiols under very well-controlled conditions. We have recently focused our attention on these short-chain monolayers (n e 10), expecting the ordering to be more strongly influenced by interfacial forces when compared with that of the longer-chain homologues. We found that, for short-chain thiols, the conventional self-assembly methodology results in monolayers of irreproducible quality, possibly due to the inferior ability of the shorter chains to provide protection against diffusion of atmospheric and solution-borne contaminants to the interface. Since in vacuo preparation of monolayers avoids contact with atmosphere and solution, it provides the means to study short-chain monolayer formation free from any external contamination. Recently Chailapakul et al.11 have compared, using nondiffractive techniques, (9) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (10) (a) Anselmetti, D.; Baratoff, A.; Gu¨ntherodt, H.-J.; Delamarche, E.; Michel, B.; Gerber, Ch.; Kang, H.; Wolf, H.; Ringsdorf, H. Europhys. Lett. 1994, 27, 365. (b) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869.
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the properties of monolayers self-assembled from solution to those prepared by equilibrium vapor phase deposition. While some differences were found, no clear-cut pattern emerged from that study. Second, thermal treatment of solution-prepared monolayers has recently been observed by us12,13 and others15 to result in the formation of “striped” phases: i.e., monolayer structures characterized by rows spaced at intervals in the range of 7.5-13 times the gold-gold nearest neighbor separation, dAu-Au, which appear as a pinstripe pattern in STM images.13,15 Some of the structures observed in our experiments are similar to the structures observed by LEED in recent vapor dosing experiments which have been proposed to be lower in density relative to the familiar solution-grown c(4x3×2x3)R30° monolayer.16 The two sets of observation do overlap in general, but disagree in some details. Furthermore, n-alkanethiol monolayers are known to be damaged by electron beam exposure.2 Since the structure of the striped monolayers may depend on subtle electronic effects associated with adsorbate-induced reconstruction of the gold surface, they may be even more sensitive to electron beam exposure than their solutiongrown counterparts. Nonperturbative helium atom diffraction studies of molecular beam deposited striped films is clearly free from these problems and provides a route for probing the coverage dependence of the overlayer structure. Finally, the preparation of monolayers in vacuo allows easy access to nonequilibrium routes of monolayer growth. It is known, for example, that the molecules in a monolayer in solution are in dynamic equilibrium with the molecules in the supernatant solution.17,18 This equilibrium is thought to play a role in the dissolution of the gold surface.19-22 This process is not present under normal operating conditions (near room temperature and below) in the vacuum apparatus, where desorption of chemisorbed thiols is expected to be negligible at temperatures less than ∼50 °C.13 The present work shows that the nonequilibrium molecular beam deposition of short-chain (Cn, 6 e n e 10) n-alkanethiols on Au(111) single crystal substrates, at relatively low exposures, results in the formation of striped phases. Our observation that the period of the stripes is, within 3%, the same as the length of the corresponding dialkyl disulfide (see below) and that the same phases can be obtained by thermal treatment (i.e., evaporation) of these monolayers indicates that these striped phases may have a surface density substantially lower than that corresponding to a fully occupied c(4x3×2x3)R30° overlayer lattice. Since the observed periodicities are related (11) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (12) Camillone, N., III; Leung, T. Y. Becky; Scoles, Giacinto SPIE, OE/LASE 1994 Proceedings 1994, 2125. (13) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (14) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (15) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (16) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (17) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (18) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (19) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (20) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775. (21) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (22) Bucher, J.-P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979.
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to the periodicities observed for thermally-treated, solution-grown monolayers, we complement the results obtained by molecular beam deposition with a few measurements obtained by thermal treatment of solutiongrown phases so to arrive at a rudimentary phase diagram that relates chain length with monolayer structure and surface coverage. II. Experimental Section (a) Substrate Preparation. Clean gold surfaces were prepared in the following manner: a gold single crystal cut to expose the (111) face is cleaned as outlined in our previous work by repeated cycles of Ar+ sputtering and annealing to ∼800 °C.3-6 Once the surface is confirmed to be clean and well-ordered by Auger spectroscopy and LEED, the crystal is removed from the UHV chamber and immersed into a dilute solution of C10, C11, or C12. The crystal is then installed into the helium atom diffractometer. (The diffraction apparatus is described in detail in previous publications.23 ) Upon completing our study of the solution-grown monolayer, we cleaned the gold surface by desorbing the monolayer by heating the crystal to ∼190 °C for ∼48 h. During and subsequent to the desorption of the monolayer, the crystal is maintained within two, concentric, liquid-nitrogencooled enclosures. The cleanliness and order of the gold surface are confirmed by helium atom diffraction studies. Surfaces are judged clean when (1) the intensity of the specular reflection is ∼20% of that of the incident beam, (2) a well-resolved first-order diffraction peak is observed, and (3) evidence of the well-known 23×x3 surface reconstruction is observed. As confirmed by these criteria, the crystal can be kept clean in our apparatus for at least 1 month, provided that the crystal enclosures are maintained at the temperature of boiling nitrogen. (b) Procedures for in Vacuo Monolayer Growth. To guarantee the purity of the thiols impinging on the crystal surface, the beam deposition source (nozzle diameter = 0.5 mm) was constructed out of Teflon. As early attempts to dose thiols using a gas manifold of copper and stainless steel construction were frustrated by nozzle clogs and the accumulation of yellow and white powders inside the manifold, it became obvious that the manifold needed to be constructed of less reactive materials. The manifold used in the present experiments was constructed of Teflon tubing, nylon connectors, and glass valves with Teflon plungers. Prior to dosing, the manifold is evacuated by a mechanical pump through a liquid nitrogen trap. During dosing, the thiol pressure in the deposition source is typically 200-800 mTorr. All n-alkanethiols (96-99% purity) were obtained from the Aldrich Chemical Company and used without further purification. Monolayer growth was accomplished by exposing the crystal to a molecular beam extracted from the vapor of liquid thiols (hexane-, octane-, and decanethiol). From the attenuation rate of the intensity of the specular reflection during dosing at low temperatures (where the sticking coefficient can be assumed to be unity), the thiol flux at the surface during all of the in vacuo growth experiments is estimated to be of the order of 1011(1 molecules cm-2 s-1. We have experimented with two methodologies: low-temperature growth and near-room-temperature growth. At crystal temperatures near 100 K, the sticking probability is much higher, and it becomes possible to completely cover the gold surface after 2 min, depending on the intensity of the thiol beam. When the surface is at 100 K, we were able to monitor the intensity of the specularly-scattered helium beam during the deposition. Upon exposure to the thiol flux, the specular drops dramatically in the first 30 s and reaches saturation at I(0,0)/I(0,0),t)0 ∼ 10-2, within 2 min. No surface order is observed subsequent to such lowtemperature depositions. We expect that the molecules simply stick where they land, and, at 100 K, there are not enough thermal fluctuations to enable diffusion and reorganization producing a very rough, disordered surface that has very weak specular reflectivity. Upon heating, however, the molecules reorganize, and diffraction patterns are observed. Normally, the crystal is heated incrementally: the crystal is brought to an elevated temperature, held there for 1-5 min, and then cooled back down (23) Danielson, L.; Ruiz, J. C.; Schwartz, C.; Scoles, G.; Hutson, J. M. Faraday Discuss. Chem. Soc. 1985, 80, 47.
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to 40 or 50 K prior to being probed by helium diffraction. In this way, the temperature at which reorganization occurs can be determined, and the dependence of the structure upon annealing temperature can be monitored. In general we find that a single, well-defined diffraction pattern develops after annealing to temperatures between 260 and 350 K. Heating to lower temperatures leaves the films somewhat disordered, and heating to higher temperatures (∼400 K) results in desorption. Near-room-temperature growth involves exposing the crystal to the thiol beam at elevated temperatures, usually between 280 and 330 K. Since helium diffraction is very sensitive to vertical (thermal) displacements of the molecules at the surface, observation of diffraction from these soft organic materials requires lowering the surface temperature to ∼50 K. Thus, we performed interrupted growth studies: the crystal is held at temperatures near 300 K and exposed to the thiol beam for ∼5 min intervals. The crystal is cooled after each deposition and the surface is probed by helium diffraction. Since the sticking probability for the thiols is expected to be quite low at room temperature, at least one-half hour total exposure time is required to grow a monolayer in this manner. In the course of this work we have found that desorption of self-assembled monolayers results in gold surfaces of varying degrees of cleanliness. In general, C10 and C12 self-assembled monolayers which give high-quality helium atom diffraction patterns desorb leaving behind high-quality gold surfaces (as evidenced by a >20% specular reflectivity, a well-resolved firstorder diffraction peak, and observation of the well-known 23×x3 surface reconstruction). Most poorly-diffracting self-assembled monolayers desorb leaving behind only a somewhat contaminated metal surface (∼5-10% specular reflectivity). (c) Data Collection. All measurements are done in the inplane scattering configuration, i.e., the primary helium beam, the surface normal, and the detector are all in the same plane. Diffraction patterns are taken by measuring the intensity of the scattered helium as a function of polar angle θ as the detector is rotated in the scattering plane. In order to obtain a detailed picture of the reciprocal space, polar angle scans are taken at 5° increments of the azimuthal angle φ spanning a range of 120°. These data in θ space can be transformed into the momentum space by the use of the equation
∆K// ) ki(sin θf - sin θi) where ∆K// is the momentum transfer parallel to the surface, θf is the detector angle, ki and θi are the incident wavevector and angle. The θ resolution is estimated to be 0.5° which corresponds to 1% at ∆K// ) 6 Å-1 and 15% at ∆K// ) 0.2 Å-1. The φ resolution depends on θ and ranges from 7° to 40° respectively at 90° and 10° from the specular reflection when the latter is located at 60° from the normal to the surface. The uncertainty in the angular position of a peak is dependent on the quality of the diffraction pattern. For the organic surfaces described here, the uncertainty is typically 0.05°. A worst case analysis shows that this translates into an uncertainty of less than (1% in ∆K// for any individual readings. For weak or asymmetrical peaks, the uncertainty in θ may be as large as 0.2°, which translates into an uncertainty of (4% in ∆K// in the worst cases. Also, because of the poor quality of the diffraction patterns obtained for some of the solution-grown monolayers, the calibration of the sample alignment could not be carried out exactly. This may cause an uncertainty of 4.5% in the observed periodicities. For clarity, all diffraction peaks described here are indexed according to the hexagonal unit mesh of either the overlayer or the substrate as indicated.
III. Results (a) n-Hexanethiol. Early experiments involving molecular-beam deposition of n-hexanethiol on Au(111) produced diffraction patterns consistent with the formation of a (5x3×x3)R30° overlayer lattice.12 The real space unit cell and expected reciprocal space (diffraction pattern) are illustrated in Figure 1a,b. The (5x3×x3)R30° mesh is the simplest unit mesh capable of explaining the observed diffraction pattern. However, the resolution
function of the instrument and the quality of the early experimental data made it all but impossible to distinguish unequivocally between a (5x3×x3)R30° pattern and the rectangular 7.5×x3 pattern shown in Figure 1c,d, or any of a family of structures characterized by rows of crystallographically identical molecules spaced (along the rows) at intervals of x3×dAu-Au with the repeat distance between the rows of 7.5 ×dAu-Au. We shall refer to structures belonging to this family as 7.5-fold, striped structures. Since Dubois et al.16 clearly observed a (5x3×x3)R30° LEED pattern for C6/Au(111), it is reasonable to assume that, when the repeat distance between equivalent rows is 7.5×dAu-Au, the molecules arrange themselves according to this unit mesh. With the present higher-quality data, we will show below that it is in fact a (5x3×x3)R30° unit mesh. A more gradual molecular-beam deposition of C6 results initially in a diffraction pattern along the nearest-neighbor direction of the gold substrate (hereafter referred to as the 〈11 h 0〉 direction) comprised of intensity near the (x3×x3)R30° positions and peaks quite close to the specular (see Figure 2a), and a pattern along the next nearest-neighbor direction of the gold substrate (hereafter referred to as the 〈11 h 2〉 direction) characterized by a doubling of the bare gold surface periodicity (see Figure 2b). A closer look at the pattern in Figure 2a reveals that the peaks are not consistent with a (5x3×x3)R30° overlayer (i.e., a 7.5-fold periodicity relative to dAu-Au). Rather, as indicated in Figure 2a, they are in good agreement with an 8-fold periodicity, strongly suggesting a rectangular 8×x3 lattice. The average over six samples provides the values of (7.9 ( 0.2)×x3 or 22.8 ( 0.6 Å. Examination of the 〈11h 2〉 spectrum strongly supports this conclusion (Figure 2b). The deviation of these results from both those of our earlier work, as well as the results of Dubois et al. raises at least two questions. First, the patterns shown in Figure 2a,b were obtained by dosing at a surface temperature of 100 K, followed by annealing to room temperature, while the results of Dubois et al. were obtained by growing the adlayer at 300-350 K (and may be affected by beam damage which is virtually unavoidable with electrons). Therefore, we should pose the question whether or not the 7.5-fold (5x3×x3)R30° pattern can be obtained by dosing or annealing to higher temperatures. Second, a larger unit cell (8-fold periodicity instead of 7.5-fold) is suggestive of a lower-coverage structure. Is it possible that repeated dosing will eventually lead to production of a 7.5-fold pattern? To answer the first question, we carried out interrupted growth experiments, as described in section II.b, at a crystal temperature of ∼316 K. (The small sticking coefficient for C6 on Au(111) at high temperatures made 316 K the highest temperature at which we could carry out the experiment over reasonable time scales.) Selected scans taken subsequent to cooling the crystal to 40 K are presented in Figure 3. We discover two new features of the growth of C6/Au(111): (1) with repeated exposure the pattern shifts from agreeing with an 8-fold periodicity to agreeing with a 7.5-fold periodicity and (2) extra peaks half-way between the “1/5-order” spacing (see Figure 3) can be discerned, indicating the possible existence of a doubling of the unit mesh, i.e., a 15-fold periodicity (possibly a (10x3×x3)R30° lattice). Due to the relatively weak intensity of these extra peaks and lack of azimuthal resolution of the present experimental setup, we cannot provide unambiguous confirmation of this (10x3×x3)R30° lattice: the apparent half-order peaks may actually be diffraction belonging to nearby azimuths (see Figure 1). We have also observed that annealing a fully-formed,
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Figure 1. (a, b) Illustrations of reciprocal-space (a) and real-space (b) for the (5x3×x3)R30° lattice. In (a), the reciprocal-space unit mesh is outlined by a solid line. The total pattern is the superposition of three identical lattices rotated by 120° with respect to each other. Each lattice is marked with a different symbol (9, (, and b). The positions coinciding with the reciprocal-space lattice of the Au(111) surface are also marked (O). The dashed-line box illustrates how, in this case, for a scan along the 〈11 h 0〉 azimuth, the resolution function of the apparatus can cause off-azimuth diffraction to appear as half-order diffraction. The real space unit mesh is outlined in (b) (shaded circles) atop the Au(111) surface lattice (open circles). The real space diagram shows that the distance between equivalent rows in the (5x3×x3)R30° lattice is equal to 7.5 times the gold nearest-neighbor spacing. Coverage should not be inferred from this diagram. (c, d) Illustrations of reciprocal-space (c) and real-space (d) for a rectangular p×x3 lattice with p ) 7.5. In (c), the reciprocal-space unit mesh is outlined by a solid line. The total pattern is the superposition of three identical lattices rotated by 120° with respect to each other. Each lattice is marked with a different symbol (9, (, and b). The positions coinciding with the reciprocal-space lattice of the Au(111) surface are also marked (O). The dashed-line box is the same as in (a). Note that the distribution of overlayer spots in the vicinity of the (1,0) substrate spot is markedly different from that in (a). This region in (a) and (c) forms the basis for Figure 5. Again, coverage should not be inferred from the real-space lattice diagram.
100 K deposited, 7.5-fold, striped structure to ∼400 K results in a decrease in diffracted intensity and a shift in peak positions toward the 8-fold periodicity (data not shown). This observation, as well as those made in the high-temperature dosing experiment, are consistent with the picture that the periodicity depends on the coverage and not simply on the temperature to which the monolayer has been annealed or heated during growth. Figure 4 shows that repeated dosing at low temperatures results in the same shift from an 8-fold to a 7.5-fold periodicity with increasing coverage as was observed in the high-temperature dosing experiment. The transition requires several 100 K dosings followed by room temperature annealings (not all data are shown). The 7.5fold structure (Figure 4, trace c) appears to be (at least) a local minimum in free energy, as further dosing does not result in continued contraction of the lattice. These observations are also consistent with the picture that the periodicity depends on the coverage. We further note that the improved quality of the data relative to our earlier measurements12 provides now strong evidence for the (5x3×x3)R30° structure (Figure 1a,b) and against the 7.5×x3 rectangular lattice (Figure 1c,d). This evidence is presented in Figure 5. Detection of offazimuth diffraction along the 〈11h 2〉 azimuth near the position corresponding to the (-1,0) substrate diffraction (heavy solid vertical line) is consistent with that expected for a (5x3×x3)R30° overlayer lattice (solid vertical lines). No evidence for peaks expected for a 7.5×x3 rectangular lattice (dashed vertical lines) is observed.
There is another interesting observation involving lowtemperature deposition. When, after the initial 100 K deposition, instead of raising the temperature directly to 300 K, the annealing is done incrementally, we find that at temperatures of ∼180 K, diffraction consistent with a 7.5-fold periodicity first appears, eventually giving way to the 8-fold periodic pattern at ∼260 K (see Figure 6). This low-temperature-annealed structure has the same periodicity as the “saturation-coverage” structure; however, it has an entirely different intensity distribution: the “2/5-order” peak dominates in the 180 K-annealed case, while it is the weakest of the first 5 orders of diffraction in the 300 K-annealed case. While a quantitative understanding of the diffraction intensity distribution is presently beyond our capabilities, we can safely state that the two structures clearly have different corrugations. There are at least two possible explanations for the lowtemperature diffraction pattern: (1) it may correspond to some sort of bilayer or multilayer structure or (2) it may correspond to a monolayer similar in coverage and form to the saturation-coverage structure, only in this case the molecules are adsorbed physically to the surface, as the needed activation energy has not yet been supplied to induce chemisorption.16 Annealing to higher temperature results in both chemisorption and desorption and, thus, produces the lower-coverage 8-fold, striped structure. More careful coverage-dependence studies, or combined helium diffraction/X-ray photoelectron spectroscopy measurements may yield additional insight into these issues.
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Figure 2. (a) Helium atom diffraction from C6/Au(111) along 〈11 h 0〉 azimuth. The C6 was deposited onto the surface at 100 K and annealed to 290 K. The solid vertical line on the left marks the expected position for first order diffraction from a simple hexagonal (x3×x3)R30° overlayer. The other solid vertical line marks the position expected for a peak at ∆K// equal to 3/5 times that for the (x3×x3)R30° first order peak, i.e., a peak location consistent with diffraction expected from a (5x3×x3)R30° adlayer. Instead, the data agree well with peak positions expected for an 8×x3 lattice, which are marked by dashed vertical lines. The incident wavevector, ki, was 5.24 Å-1, the crystal temperature, Tc, was 40 K, and the incident angle, θi, was 61.26°. (b) Diffraction from C6/Au(111) along the 〈112h 〉 azimuth. The C6 was deposited onto the surface at 100 K and annealed to 290 K. The solid vertical lines mark the expected positions for integer- and half-integer-order diffraction with respect to the bare Au(111) surface lattice (the first order is specifically labeled). The dashed vertical lines mark locations of off-azimuth diffraction expected for a 8×x3 overlayer. The excellent agreement of the data with the latter strongly supports our assignment of the unit mesh. ki ) 4.84 Å-1; Tc ) 45 K, and θi ) 61.26°.
(b) n-Decanethiol. High-quality diffraction patterns have been obtained from n-decanethiol overlayers deposited either by exposure of a room temperature crystal to the thiol beam or by exposure of a cold (100 K) surface followed by annealing to room temperature. The diffraction patterns obtained in these two cases are nearly identical to each other. While we have not made a detailed study of the response of these overlayers to annealing at higher temperatures, we have observed that heating to 350 K results in an increase in the intensity of the diffraction peaks. Figure 7a is an example of diffraction from molecularh 0〉 direcbeam-deposited C10/Au(111) taken along the 〈11
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Figure 3. Selected scans taken from the surface of C6/Au(111) grown by molecular beam deposition at 316 K. With repeated exposure, the pattern shifts from agreement with an 8×x3 lattice (marked by dashed lines) to agreement with a 7.5-fold periodicity (actually a (5x3×x3)R30° lattice, marked by solid lines). Since there are 5 orders of diffraction up to the peak corresponding to first-order diffraction from a simple (x3×x3)R30° overlayer lattice, we refer to the diffraction peaks in trace d in terms of “1/5-order” diffraction. The predominant peaks are the 1/5-, 3/5-, and 5/5-orders. Extra peaks half-way between 1/5-order spacing are observed but may not indicate a doubling of the unit mesh (see text and Figure 1a). 〈11h 0〉 azimuth, ki ) 4.84 Å-1; Tc ) 40 K, and θi ) 61.5°.
tion. Upon inspection of this diffraction pattern, our attention is immediately drawn to three important features: (1) while there is intensity near the positions expected for a (x3×x3)R30° overlayer, the peaks in the diffracted intensity are not in good agreement with those positions, especially at the first order position, (2) the diffraction features near the (x3×x3)R30° positions are comprised of a series of diffraction peaks, and (3) there is a progression of peaks very close to the specular, corresponding to a relatively large real-space periodicity. Figure 7b is a diffraction scan taken parallel to the 〈112h 〉 direction. Here we find diffraction peaks at spacings 1/2, 3/ , and 5/ times the spacing observed for a clean gold 2 2 substrate, indicating a doubling of the periodicity in the 〈112 h 〉 direction, i.e., a periodicity equal to x3 times the gold-gold nearest neighbor spacing, dAu-Au. Figure 7c provides an expanded look at the near-specular diffraction along the 〈11h 0〉 azimuth from which we extract a periodicity equal to (11 ( 0.5)×dAu-Au, or 31.7 ( 1.4 Å (average over six samples). Therefore, the data are consistent with a rectangular unit mesh with dimensions 11×x3 times dAu-Au. This unit mesh is illustrated in both real and reciprocal space in Figure 8. The structure shown in Figure 8 is quite distinct from the c(4x3×2x3)R30° structure obtained by conventional self-assembly in solution. The observed lattice is identical to the so-called “striped” phases found subsequent to thermal treatment of solution-grown C10 monolayers (see Figure 9). We also remind the reader that, as shown in ref 13, that thermal treatment of C10 produces first a (5x3×x3)R30° lattice (which corresponds to a p value of
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Figure 4. Selected scans taken from the surface of C6/Au(111) grown by molecular beam deposition at 100 K followed by annealing to 290 K. As in Figure 3, with repeated exposure the pattern shifts from agreement with an 8×x3 lattice (marked by dashed lines) to agreement with a 7.5-fold periodicity (actually a (5x3×x3)R30° lattice, marked by solid lines). 〈11h 0〉 azimuth, ki ) 4.84 Å-1; Tc ) 45 K, and θi ) 61.37°.
Figure 5. Diffraction from the same monolayer as that in Figure 4. These data were collected along the 〈112 h 〉 azimuth. Close inspection of the region in the vicinity of the position corresponding to first-order diffraction from bare Au(111) (marked by the heavy vertical line) reveals agreement with a (5x3×x3)R30° lattice (positions marked by solid lines) and no evidence for a rectangular 7.5×x3 lattice (marked by dashed lines). Also compare parts a and c of Figure 1. ki ) 4.84 Å-1; Tc ) 45 K, and θi ) 61.08°.
7.5 as shown in Figure 1) followed later by the striped phase. (c) Other Chain Lengths. Finally, we report the most recent results of measurements made on adlayers of C8, C11 and C12. n-Octanethiol was deposited onto Au(111) in the same manner as C6 and C10, dosing at a surface temperature of 100 K followed by annealing to room temperature to
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Figure 6. Systematic annealing of a 100 K-deposited C6 overlayer. The top traces were collected subsequent to annealing to 180 K, while the bottom trace was collected subsequent to annealing to 260 K. With repeated annealing the pattern shifts from agreement with a (5x3×x3)R30° lattice (marked by solid lines) to an 8×x3 lattice (marked by dashed lines). Note the predominance of the 2/5-order peak in the former case. 〈11h 0〉 azimuth, ki ) 4.84 Å-1; Tc ) 40 K, and θi ) 61.5°.
produce the pattern shown in Figure 10. This pattern is well-described by a (9.4 ( 0.2)×x3 rectangular lattice (average over five samples). Alternatively, the pattern is consistent with an oblique unit mesh described by the vectors a ) (0, x3×dAu-Au) and b ) (9.5×dAu-Au, x3×dAu-Au/2), which is the simplest commensurate overlayer lattice belonging to the family of structures comprised of stripes spaced at 9.5×dAu-Au. Similarly to the case of C6/Au(111), we do not expect that we would be able to easily distinguish between the two lattices. The pattern for the C12 overlayer shown in Figure 10 was produced by in vacuo thermal treatment of a solutiongrown monolayer which initially displayed a welldeveloped c(4x3×2x3)R30° diffraction pattern. Thermal treatment consisted of repetitive cycling of the crystal to increasingly higher temperatures in the vicinity of 100 °C. The pattern shown in the upper trace of Figure 10 was actually obtained subsequent to thermal treatment followed by approximately 1 week at room temperature. Similarly long reorganization times have been observed previously for C10.13 The pattern in Figure 10 is in excellent agreement with a periodicity described by a (13 ( 0.3)×x3 unit mesh. Finally, very recently, the thermal behavior of two solution-grown C11 monolayers which initially showed a c(4x3×2x3)R30° diffraction pattern was studied. Subsequent to thermal cycling the monolayers to ∼180 °C followed by about 2 days for one sample and 5 days for the other sample at room temperature, a new diffraction pattern was observed which can be described by an (11.8 ( 0.8)×x3 unit mesh (data not shown). Since a certain amount of “aging” at room temperature has been found useful to observe the striped phase, we feel it useful to summarize here the amount of times during
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Figure 8. (a) Reciprocal space diagram of diffraction from the bare Au(111) surface (O) and from C10/Au(111) (b). Prior to deposition, only the six equivalent first-order peaks (one is marked (1,0)) are observed. Subsequent to deposition, halforder peaks appear along the 〈112 h 〉 azimuth (vertical in the figure), and a progression of closely spaced peaks appear along the 〈11h 0〉 azimuth (horizontal). The reciprocal-space unit mesh is outlined. This unit mesh corresponds to the real space unit mesh outlined in (b).
Figure 7. (a) Diffraction from the surface of C10/Au(111) grown by molecular beam deposition. The vertical dashed lines mark the expected peak locations for a simple hexagonal (x3×x3)R30° overlayer, i.e., a periodicity equal to 1.5 × 2.885 Å. 〈11h 0〉 azimuth, ki ) 5.239 Å-1; Tc ) 45 K, and θi ) 61.36°. (b) Diffraction from the surface of C10/Au(111) grown by molecular beam deposition. The dashed lines mark expected peak locations for a periodicity of x3 times the gold next-nearestneighbor spacing, i.e., x3 × 2.885 Å. Only the peak labeled (-1,0) is observed in scans taken of the clean gold surface. 〈112 h 〉 azimuth, ki ) 5.24 Å-1; Tc ) 45 K, and θi ) 61.71°. (c) An expanded view of the near-specular region of the 〈11 h 0〉 diffraction scan shown in (a). The dashed lines mark expected peak locations for a periodicity equal to 11 gold nearest neighbor spacings, i.e., 11 × 2.885 Å.
Figure 9. A comparison of diffraction from the surface of C10/ Au(111) along the 〈11h 0〉 azimuth: (a) deposited from a molecular beam and (b) deposited from solution and thermally treated in vacuo. Both show close agreement with a periodicity that is 11 times the gold nearest-neighbor spacing which is marked by the dashed lines. ki ) 5.24 Å-1; Tc ) (a) 40 K, (b) 45 K and θi ) (a) 61.47°, (b) 60.70°.
which the different monolayers have been observed since it will provide the reader with a feeling about the reaching of equilibrium and about the stability of these systems.
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Figure 10. Helium atom diffraction from Cn/Au(111) along the 〈11h 0〉 azimuth. The C6, C8, and C10 layers were prepared by molecular beam deposition and the C12 by partial desorption of a solution grown monolayer. The intervals δ mark the characteristic reciprocal lattice spacings, 2π/δ gives the realspace distance between stripes. C6: ki ) 4.84 Å-1; Tc ) 40 K, and θi ) 61.51°. C8: ki ) 4.84 Å-1; Tc ) 45 K, and θi ) 61.22°. C10: ki ) 5.24 Å-1; Tc ) 45 K, and θi ) 60.72°. C12: ki ) 4.84 Å-1; Tc ) 45 K, and θi ) 61.36°.
Several films of C10, C11, and C12 have been found stable in vacuum at room temperature for times of the order of 5 days. However on this time scale some increase in specular intensity was observed which may indicate either a small amount of evaporation or molecular diffusion on the surface producing larger domains of bare gold. While a study on the stability of the C6 films has not been carried out in details, we have observed almost complete desorption in one of the samples at room temperature on the same time scale. For C8, no stability observations were made. IV. Discussion Taking the results for the five molecules, C6, C8, C10, C11, and C12, together, and including both the present results as well as those of the earlier thermal treatment experiments, we find a linear relationship between the periodicity of the diffraction pattern and the number of carbons in the thiol (see Figure 11). The slope (0.83 ( 0.04) is very nearly equal to twice the distance between carbon atoms parallel to the molecular backbone (2.55 Å) expressed in terms of gold lattice constants (2.55/2.885 ) 0.884). Furthermore, for these so-called “striped” structures, the periodicity of the stripes is very nearly equal to twice the overall length of the molecules forming the overlayer which is consistent with the possibility that the molecules are adsorbed as disulfides7b (see Table 1). The coincidence of these numbers suggests that the striped structures consist of thiols lying flat, i.e., with their axes parallel to the gold surface, in a fashion similar to that found by LEED for normal paraffin vapors deposited on Pt(111)24 as well as to that found by STM for cyclic alkanes (24) Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 2901.
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Figure 11. A plot of observed periodicity (given in terms of gold lattice spacings, p ) 2π/2.885δ) of the striped structures vs the number of carbons in the molecules comprising the layer (solid circles). The data represent measurements made for layers prepared as follows: C6 and C8 - molecular beam deposition only; C10 - molecular beam deposition as well as thermal treatment; C11 and C12 - thermal treatment only. For the (5x3×x3)R30° structure observed for C6 and C10, the distance between equivalent rows is equal to 7.5 times the gold nearestneighbor spacing (open circles). Table 1. Chain Length Dependence of the Lattice Parameter p of the Striped Phase of SAMs n
p
6 7.9 ( 0.2 8 9.4 ( 0.2 10 11 ( 0.5 11 11.8 ( 0.8 12 13 ( 0.3
“disulfide” 2π/δ (Å) fragment (experiment) length (Å)a R1b length (Å)c R2d 22.8 ( 0.6 27.1 ( 0.6 31.7 ( 1.4 34 ( 2.3 37.5 ( 0.9
11.8 14.4 16.95 18.25 19.5
1.93 1.88 1.87 1.86 1.92
22.1 27.3 32.4 35.0 37.5
1.03 0.99 0.98 0.97 1.00
a The length of the thiol fragment, assuming bond angles of 112°, bond lengths of 1.541 Å for C-C, 1.81 Å for C-S, and 1.073 Å for C-H, and van der Waals radii of 1.85 Å for sulfur and 1.2 Å for hydrogen. b The ratio of the observed periodicity and the fragment length. c The length of disulfide-like moiety at which the sulfur head groups of the two thiolate fragments dimerized with S-S length of 2.2 Å. d The ratio of the observed periodicity and the length of the disulfide moiety.
on graphite.25 (The latter work includes images which are very similar in appearance to those of Poirier et al.9,13,15) Each unit mesh would therefore contain two thiols. This would suggest that the coverage for these structures is from 0.4 to 0.23 times that of the full-coverage c(4x3×2x3)R30° phase (as the periodicity of the stripes ranges from 7.5 to 13, respectively). It would be, of course, interesting to explore the bonding environment of the sulfur atoms in the striped phases using the available methods of electron and X-ray spectroscopy to confirm or contradict the presence of the disulfide bonds. Since we have no direct evidence that the molecules indeed lie prone on the surface, we must consider other alternatives. One possibility is that the periodicity of the striped structures be a consequence of the variation of adsorbate-adsorbate interaction strength as a function of chain length. For the case of the thiols, the strength of the adsorbate-adsorbate interaction varies linearly with the number of carbons in the chains, so we could expect, in principle, a linear relationship between the chain (25) Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N. Langmuir 1993, 9, 2778.
SAM Striped Phases Scheme 1. A Rudimentary Phase Diagram Showing the Relationship among the Observed Phases of Cn/Au(111) for n ) 6, 8, 10, 11, and 12a
a
The arrows indicate observed transitions.
length and the length scale of a superlattice modulation responsible for the stripes. In this context, we note that for all the structures measured the value of p is, within experimental error, an integer or half-integer number. Another possible explanation for the structure of the low coverage phases is related to a quantization of the magnitude of the tilt of the molecular axis from the surface normal. It is known that as a consequence of the interdigitation of methylene groups, hydrocarbon chains prefer to pack with certain tilt angles and two-dimensional packing densities,26 explaining why certain structures (in particular the (5x3×x3)R30° phase) seem to be common to a number of chain lengths. Interdigitation of closelypacked methylene groups and tilt quantization have been observed to result in displacement of chains along their molecular axes in Langmuir-Blodgett films.27 Such a displacement would have a marked effect on surface corrugation and could explain the observed helium diffraction patterns for the thiols on gold system. After examining these alternative we feel, however, that the flat-on-the-surface arrangement of the molecules is the simplest and most likely explanation of our data. An overall look at our results has led us to the formulation of a graphic representation for the chainlength-dependent and coverage-dependent phase behavior presented in Scheme 1. The (5x3×x3)R30° structure has so far been observed by us for both C6 and C10. Dubois et al. observed this same structure for every evennumbered chain length from C4 to C10.16 One could then make the hypothesis that at very low coverages the molecules lie flat and, as the coverage increases, that the molecules pack more densely giving rise to the (5x3×x3)R30° structure. This would happen to a critical coverage where the molecules switch from a prone position to an orientation directed more parallel to the surface normal. Thus this particular structure would be common to several chain lengths. Neither Dubois et al. nor Poirier et al. observe the linear relationship between periodicity and chain length described here; rather Dubois et al. predominantly observe the (5x3×x3)R30° structure16 and Poirier and Tarlov observe a wide variety of periodicities.9,13,15 There are several possible explanations for this: (1) the periodicity varies with local coverage, to which the STM measurements may be more sensitive than our technique which averages over the whole surface, (2) some or all of the measurements involve nonequilibrium structures, thus the observations are strongly dependent on the exact method of preparation of the adlayer and its thermal (26) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D.; Rotermund, H. H. Phys. Rev. Lett. 1987, 59, 1321. (27) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. Langmuir 1993, 9, 1384.
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history, or (3) the structures observed by us at 40 K differ from those observed at near room temperature by Dubois et al. and Poirier et al. In any case all indications are that the phase vs coverage behavior of these systems is quite complicated, and, for a more complete understanding, may require further study. For example, a grazing incidence X-ray diffraction measurement of the inclination of the molecules in the (5x3×x3)R30° structure would serve not only to specify the structure more completely but also to shed additional light on the issues raised in this discussion. While the (5x3×x3)R30° phase has never been seen with grazing incidence X-rays, by exposing a clean gold substrate to decanethiol vapor at a background pressure 10-6 Torr, c(4x3×2x3)R30° diffraction patterns have recently been observed using this technique.28 The difference between the present results and those obtained by X-rays may be due to the substantially lower dosing rate used here. Efforts are underway in both laboratories to prepare monolayers under comparable conditions. The achievement of much larger deposition fluxes is hindered, in our setup, by the unavoidable presence of the cryopumping action of the liquid helium cryostat which supports the bolometer helium beam detector. While background gas deposition cannot be achieved under any circumstances, we are presently building a shielded doser located much nearer to the gold surface, which will be capable of producing much larger impingement rates for the thiol deposition. V. Conclusions We have shown that molecular beam deposition of n-decane-, n-octane-, and n-hexanethiol on gold, at the thiol flux on the order of 1011(1 molecules cm-2 s-1, does not produce the c(4x3×2x3)R30° structure usually observed for n-decane-7b,9,13 and n-octane-9 and n-hexanethiol9 saturation-coverage monolayers prepared by self-assembly in solution. Rather, we observe the formation of striped structures characterized by rows of molecules. These rows run parallel to the 〈112h 〉 direction of the Au(111) substrate and the periodicity along the rows is x3 times the gold nearest-neighbor distance. So far our observations show that, at surface temperatures of 40 K, the perpendicular distance between the rows scales linearly with the length of the molecules which comprise the adlayer and is, in fact, nearly equal to twice the overall length of those molecules. We have seen that molecular beam deposition of C10 results in an 11×x3 structure that is identical to that ultimately produced by thermal treatment of a c(4x3×2x3)R30° solution-grown monolayer. C6 has been seen to form two structures: (1) 7.9×x3 at low coverage and (2) (5x3×x3)R30° at higher coverage. The latter has the same periodicity as an intermediate structure observed for C10 in the thermal treatment experiments13 as well as the structures of a variety of chain lengths prepared by background gas dosing by Dubois et al.16 The recurrence of the (5x3×x3)R30° mesh in the different chain length suggests that this structure may correspond to a preferred packing density and (non-surface-parallel) tilt angle at some intermediate coverage. The (5x3×x3)R30° is bracketed on the one side by the fullcoverage c(4x3×2x3)R30° structure in which the molecules are tilted about 30° from the surface normal and on the other side by the lowest coverage monolayers in which the molecules lie parallel to the surface (as suggested by the trend shown in Scheme 1). We expect, therefore, (28) Eberhardt, A.; Fenter, P. Private communication.
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that the “striped” phases are the result of increasing molecular tilt with decreasing coverage. The wide variety of periodicities observed by us and others suggests that the periodicity of the stripes (and, probably, therefore, the tilt angle) depends on coverage, chain-length, and thermal history. Once they have been fully explored, the rich phase diagrams of these intriguing systems will provide an excellent testing ground for theory. If intermolecular force field and molecular-dynamics code can be developed to a level where they are capable of reproducing this rich variety of structures, it is reasonable to expect that they will also be able to accurately describe the properties of cell membranes and other “solution”
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structures which, by their own nature, cannot provide us with structural data of equivalent precision. Acknowledgments. We acknowledge the fruitful discussions with A. Eberhardt, P. Fenter, and G. Poirier and Terri Cummings for her participation in the recent measurements. We also thank Professor C. Chidsey for giving us the undecanethiol sample used in the measurements. This work has been supported by the Materials Science Program of the Office of Basic Energy Sciences of DOE under Grant DE-FG02-93ER45503. LA951097J
