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Coverage-Dependent Phases and Phase Stability of Decanethiol on Au(111) G. E. Poirier* National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received September 29, 1998. In Final Form: November 23, 1998 Using ultrahigh vacuum scanning tunneling microscopy, we have characterized the structural phases of decanethiol on Au(111) at coverages below saturation. As coverage increases, the monolayer sequentially adopts five discrete structural phases. At low surface coverage, decanethiol exists as a lattice gas. Above a critical surface coverage, the molecules condense into islands of a commensurate crystalline lattice. These islands grow in equilibrium with the lattice gas until saturation. As coverage increases, the surface layer sequentially undergoes two first-order phase transitions, first to a metastable phase then to a stable phase. The first three condensed phases are characterized by alignment of the molecular axes with the surface plane but with discretely increasing degrees of out-of-plane interdigitation. Above saturation coverage of the densest surface-aligned phase, the monolayer undergoes an edge-mediated melting transition. The evidence suggests that the resulting fluid is a supercooled, two-dimensional liquid. The highest-density phase, characterized by alignment of the molecular axes close to the surface normal, grows by homogeneous nucleation from this supercooled liquid. These data provide a fundamental understanding of the mechanistic pathway of molecular monolayer self-assembly.
Introduction Molecular self-assembly is a phenomenon ubiquitous in nature: it governs the building of cell walls and the folding of proteins. Alkanethiols mimic biological molecular self-assembly by spontaneously forming organized monolayer films on metal surfaces.1-5 These films provide an effective and parsimonious method to control the chemical, physical, and electron-transfer properties of electrode surfaces, and they have potential applications in biosensing,6 biomimetics,7 corrosion inhibition,8 and wetting inhibition.9 Various techniques have been used to study aspects of the growth of alkanethiols on Au including ellipsometry and contact angle measurements,10 surface acoustic wave (SAW) devices,11 second harmonic generation,12 near-edge X-ray absorption fine-structure,13 quartz crystal microbalance,14 X-ray and helium diffraction,15,16 atomic force microscopy,17 and scanning tunneling microscopy (STM).18-21 These studies found Langmuir * Telephone: (301)975-2603. Fax: (301)869-5924. E-mail: gregp@ nist.gov. (1) Blackman, L. C. F.; Dewar, M. J. S. J. Chem. Soc. 1957, 162. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (4) Ulman, A. Chem. Rev. 1996, 96, 1533. (5) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (6) Haussling, L. et al. Makromol. Chem., Macromol. Symp. 1991, 46, 145. (7) DiMilla, P. A. et al. J. Am. Chem. Soc. 1994, 116, 2225. (8) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (9) Emmons, H. Trans. Am. Inst. Chem. Eng. 1939, 35, 109. (10) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (11) Thomas, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1991, 7, 620. (12) Buck, M. et al. J. Vac. Sci. Technol., A 1992, 10, 926. (13) Hahner, G.; Woll, C.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955. (14) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (15) Schreiber, F. et al. Phys. Rev. B 1997, 57, 12476. (16) Eberhardt, A.; Fenter, P.; Eisenberger, P. Surf. Sci. 1998, 397, L285. (17) Xu, S. et al. J. Chem. Phys. 1998, 108, 5002. (18) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (19) Poirier, G. E. Langmuir 1997, 13, 2019. (20) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (21) Kondoh, H.; Kodama, C.; Nozoye, H. J. Phys. Chem. 1998, 102, 2310.
10.1021/la981374x
growth kinetics11,12,14,22 and found that monolayer assembly proceeds by nucleation and growth of a phase characterized by alignment of the molecular axes in the surface plane and, subsequently, a phase characterized by alignment of the molecular axes close to the surface normal.18,20 These prior studies provide a picture of the kinetics and mechanism of alkanethiol monolayer assembly. However, questions regarding aspects of the surface phases remain unanswered; for example, How many phases precede the saturation coverage phase and what are their packing structures? What is the nature of the transitions between structural phases? Do the coverage-dependent phases represent the thermodynamic minimum energy structures or are they metastable? The goal of this paper is to provide a detailed molecularscale picture of the coverage-dependent phases and phase transitions occurring during assembly of decanethiol monolayers onto Au(111). This system is one of the most widely studied and is prototypic of a wide class of amphiphile monolayers on metals. We begin with a discussion of the experimental apparatus and methodology. This is followed by a structural interpretation of molecular-resolution topographs of the phases that exist prior to nucleation and growth of the saturation coverage phase. Next, we develop a model for the mechanistic pathway of monolayer assembly by inferences from a sequence of STM topographs acquired during monolayer growth. Finally, we address the issue of the thermodynamic stability of each phase by exploring the temporal evolution of the molecular packing. These data provide a fundamental understanding of alkanethiol monolayer selfassembly; and this knowledge can be used to gain insight into the forces that govern molecular self-assembly. Experiment The studies were accomplished by using gas-phase deposition of decanethiol onto clean Au(111) single crystals in an ultrahigh vacuum (UHV) scanning tunneling microscope. Our system has a base pressure of 3 × 10-8 Pa (2 × 10-10 Torr) and is equipped with a rapid-entry load-lock. Single crystals of Au(111) were (22) Eberhardt, A. S. Ph.D. Thesis, Princeton University, Princeton, NJ, 1997.
This article not subject to U.S. Copyright. Published 1999 by the American Chemical Society Published on Web 01/26/1999
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Poirier Figure 1. Molecular-resolution constant-current STM topographs of phases of decanethiol on Au(111). Schematized molecular models are overlaid on topographs to show hypothetical registry. Molecules drawn in bold represent displacement out of the surface plane. The scale bar and crystallographic indicator in A also applies to parts B-E. (A) Au(111) exposed to 10-6 Torr of decanethiol for 240 s and imaged at 400 pA and 1.2 V sample bias. The surface region shows two commensurate domains of β-phase decanethiol separated by a region of R-phase lattice gas. (B) Au(111) exposed to 10-6 Torr of decanethiol for 380 s, stored in ultrahigh vacuum for 5 days, and imaged at 350 pA and 1.0 V sample bias. The left half of image is characteristic of the decay product of the χ phase, and the right half is characteristic of the β phase. The line segment in the right half shows lateral displacement of β-phase rows by influence of the Au reconstruction that underlies the monolayer at the region indicated by the crossed lines. (C) Au(111) exposed to 10-6 Torr decanethiol for 300 s and imaged at 400 pA and 1.2 V sample bias. Parallel line segments indicate antiphase boundaries. (D) Au(111) exposed to 10-6 Torr for 480 s and imaged at 150 pA and 1.2 V sample bias. The surface region shows coexistence of β, δ, and phases. Azimuthal orientation of the alkyl chains is retained across the β-δ phase boundary. Parallel line segments indicate antiphase boundaries. (E) The same surface preparation and tunneling conditions as those in D but with a different geometry of tip-apex atoms. We assume that the tip used in D highlights alkyl chain corrugation whereas the tip used in E highlights thiolate-bonding electrons. spectrometry. Dosing pressures were typically 1 × 10-4 Pa (1 × 10-6 Torr). Measurements were also made on monolayers prepared by solution deposition. For solution deposition, 4.5 µM solutions of decanethiol in ethanol were prepared, and the coverage was controlled by varying the incubation time. STM tips were prepared from single-crystal tungsten wire using a DC etch. All STM imaging was done at room temperature in constantcurrent mode. The tunneling current set-point was fixed between 5 and 500 pA, and the bias voltage was set between ((200 to 1200) mV.
cleaned by sputtering and annealing to 500-600 °C for 10 min. Following this preparation, X-ray photoelectron spectroscopy revealed a contamination-free surface, and STM topographs showed the herringbone reconstruction characteristic of clean Au(111).23 These topographs were used to establish the sample’s crystallographic orientation with respect to the STM scan direction. For gas-phase deposition, decanethiol was stored in an ambient-temperature, blackened-glass vial attached to the UHV chamber via a variable-aperture leak-valve. The neat decanethiol was purified using freeze-pump-thaw cycles, and the purity was confirmed using in situ quadrupole mass
The Subsaturation Coverage Structural Phases Because of the break in bonding symmetry, Au(111) surfaces adopt a different packing structure than that of the bulk (111) planes. The atomic packing of the equilibrium bare Au(111) surface is characterized by a 4.4% uniaxial lateral contraction relative to the bulk layers.23-25 This contraction causes variations in registry between the surface and subsurface atomic layers such that the stacking arrangement alternates between normal ABC stacking and faulted ABA stacking with faulted and unfaulted regions delineated by rows of bridging Au atoms.25 These bridging rows are manifest in STM topographs as paired elevated ridges aligned with substrate 〈121〉 directions.23 To further reduce surface energy, the paired ridges form hyperdomains characterized by alternating 60° turns.23 Figure 1A shows the reconstructed Au(111) surface exposed to ∼1 × 10-6 Torr of decanethiol vapor for 240 s. The displayed region comprises two commensurate islands (labeled β) of a crystalline surface phase of decanethiol. The right half of Figure 1B shows a domain of this same crystalline phase. This is the first condensed phase that nucleates on Au(111) during deposition of decanethiol and has been observed in prior diffraction studies.26,27 This phase exhibits corrugated rows aligned with substrate 〈121〉 directions with a 5 Å corrugation periodicity and an inter-row spacing of 32 Å. There is no evidence of incommensuration in the row direction, neither in this STM data, nor in prior diffraction studies.26,27 The data therefore indicate that molecular rows bind in registry with Au(111) next-nearest-neighbor sites (spaced at 4.995 Å). Conflicting models have been proposed for the packing structure perpendicular to the molecular rows. The
Decanethiol on Au(111)
authors of an electron diffraction study proposed a centered rectangular (23×x3) unit mesh containing four molecules.26 The authors of a helium diffraction study proposed a primitive rectangular (11×x3) unit mesh containing two molecules.27 Nonlinearities in the piezo actuator displacement typically result in 5-10% length uncertainties in STM measurements; therefore, it is difficult for us to discriminate which of the two proposed models is correct. The differences between the two unit cells are subtle, and it is possible that both coexist or that the c(23×x3) (molecular area ) 82.8 Å2/molecule) converts to the slightly higher density p(11×x3) (molecular area ) 79.2 Å2/ molecule) with increasing coverage. Because the evidence for both models is strong, we will assume this later case as a working hypothesis: that the nascent β-phase islands have a c(23×x3) packing arrangement and that they convert to a p(11×x3) at higher coverage.25 The c(23×x3) is equivalent to an oblique (x133×x3) primitive unit mesh and has a row spacing of 11.5 a (≈33 Å); where a ) 2.884 Å, the Au(111) lattice constant.26 A proposed model of this packing structure is shown in Figure 2β. The overlays in parts A and B of Figure 1 show the expected registry between the β phase and the STM topographs, assuming that the brightest features are due to Au-thiolate electrons and the less bright features are associated with the alkyl chains. The edges of the β-phase islands in Figure 1A are fragmented. If decanethiol molecules can diffuse around a β-phase island perimeter on a time scale that is rapid compared to the ∼1 min STM image acquisition time, then this would lead to a fragmented appearance of island edges. Likewise, if frequent evaporation-condensation events occur between the condensed β phase and a twodimensional lattice gas phase, this would also result in fragmented island edges. Apart from the fragmented island edges in Figure 1A, other observations suggest the presence of a lattice gas of decanethiol. Specifically, distributions of β-phase islands undergo Ostwald ripening (see parts A and B of Figure 5), and low exposures to decanethiol vapor result in alteration of the herringbone gold reconstruction prior to nucleation of β-phase islands. We conclude from these observations that the surface region labeled R in Figure 1A is characterized by a twodimensional lattice gas phase of decanethiol on Au(111). A two-dimensional lattice gas is a dilute ensemble of mobile, weakly interacting particles that are confined to a corrugated surface and are quantified by a lateral pressure, π. Figure 1C shows a unit-cell-resolved image of a phase that we term χ and that occurs at higher coverage than that for β. Parts D and E of Figure 1 show moleculeresolved images of a phase that we term δ and that occurs at higher coverage than that for χ. Phase χ precedes δ during growth; however, δ will be discussed first because the χ-phase structure appears to be an admix of β and δ. The δ phase exhibits row segments that have a corrugation periodicity of 5 Å parallel to substrate 〈121〉 directions and an inter-row periodicity of 22 Å (see Figures 1D, 1E, 3A, and 3C). As was the case for the β phase, we attribute the corrugated row segments to rows of molecules in registry with the Au(111) next-nearest-neighbor spacing. If it is assumed that the molecular lattice is commensurate, (23) Chambliss, D. D.; Wilson, R. J.; Chiang, S. J. Vac. Sci. Technol., B 1991, 9, 933. (24) Sandy, A. R. et al. Phys. Rev. B 1991, 43, 4667. (25) Woll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 39, 7988. (26) Gerlach, R.; Polanski, G.; Rubahn, H.-G. Appl. Phys. A 1997, 65, 375. (27) Camillone, N. et al. Langmuir 1996, 12, 2737.
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Figure 2. Proposed packing structures for the three striped phases of decanethiol on Au(111). Open circles represent Au, shaded white circles represent H, shaded gray circles represent C, and filled circles represent S. Details of the molecular packing, such as registry of molecular overlayer with Au surface, S-S spacing, and orientation of the hydrocarbon chain about its symmetry axis, were chosen arbitrarily. (β) Top view of the β-phase model. Primitive unit cell is oblique (x133×x3) and is equivalent to a centered rectangular (23×x3). (χ) Top and edge views of the χ-phase model. Primitive unit cell is oblique (x364×x3), comprises four molecules, and results from alternate stacking of oblique (x133×x3) and oblique (x57×x3) unit cells. Counter-oriented chains in the (x57×x3) rows are lifted out of the surface plane by incorporation of a gauche defect near the sulfur terminus, resulting in out-of-plane interdigitation. The oblique (x364×x3) unit cell is equivalent to a centered rectangular (19×x3). (δ) Top and edge views of packing structure proposed for the δ-phase decanethiol on Au(111). The primitive unit cell is oblique (x57×x3), comprises two molecules, and is identical to the model proposed for alternate rows of the χ phase. The oblique (x57×x3) is equivalent to the hexagonal (5x3×x3)R30°.
then the spacing between rows must be an integer number of substrate lattice constants with a rectangular primitive unit cell, or it must be a half integer number of substrate lattice constants with an oblique primitive unit cell. The allowed row spacing that is closest to our measured value is 7.5a; therefore, we assign an oblique primitive unit cell with dimensions of (x57×x3). The (x57×x3) is identical to a centered rectangular (15×x3) and to a hexagonal (5x3×x3)R30°. The (5x3×x3)R30° was reported previously in the literature, and we will retain this nomenclature.20,26-28 The (5x3×x3)R30° was first seen in a low-energy electron diffraction study in which the authors proposed (28) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678.
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a packing arrangement characterized by an expanded variant of a (x3×x3)R30° with 80% of saturation density.28 The proposed model should give rise to STM topographs exhibiting a furrowed hexagonal packing with a x3a lattice constant; however, the STM topographs show only linear corrugations. The (5x3×x3)R30° was also seen in an STM study conducted in heptane solution20 and in a helium diffraction study conducted in UHV;27 both authors propose a model whereby the molecules tilt out of the plane, but at an angle less than the final 60° inclination. This model calls for a two-molecule unit cell with the chain tilt azimuths aligned either parallel or antiparallel. The former case should give rise to an interrow periodicity of 7.5a/2 ) 10.8 Å; the later case should also give rise to a row feature at half of the 7.5a periodicity. In contrast, the STM topographs parts D and E of Figure 1 do not show any feature at half of the 7.5a periodicity. Therefore, both of these previously proposed structures are inconsistent with the STM results reported here and previously.20 We will consider five other possible molecular packing arrangements that could explain the δ-phase unit cell. An arrangement with 1 molecule/unit cell and with the molecules arranged head-to-tail would have a molecular area of 108 Å2/molecule, larger than the 82.8 Å2/molecule for the β phase. This model is therefore not consistent with our observation that the δ phase occurs at higher coverage than the β phase. A two-molecule unit cell with molecules arranged head-to-head and the molecular axis aligned perpendicular to the row direction and strictly confined in the surface plane would result in overlap of the chains; consequently, this model is forbidden by steric constraints. A higher packing density could be achieved by swiveling the molecular axes in the surface plane away from the perpendicular of the row direction. Efficient methylene chain packing is predicated on satisfaction of the ratcheting condition29
sin λn )
nd l
(1)
where λn is the angle by which the chains tilt from the perpendicular of the lamella, n is a nonnegative integer, d is the second-neighbor methylene distance (2.54 Å),29 and l is the distance between chains in the plane of the lamella (4.995 Å for next-neighbor Au commensuration). Swiveling the chains of a two-molecule unit cell strictly in the surface plane to the first ratcheting condition, λ1 ) 30.6°, would result in overlap of the chains; therefore, this model is prohibited by steric constraints. The second ratcheting condition is prohibited because eq 1 has no solution for n)2, d ) 2.54 Å, and l ) 4.995 Å. Finally, we consider a model of a two-molecule unit cell with chains packed head-to-head, oriented perpendicular to the row direction, and aligned with the surface plane but with one or two gauche defects near the sulfur terminus of chains in alternate rows allowing them to lie atop the counteroriented rows. We refer to this model, pictured in Figure 2δ, as out-of-plane interdigitation. This model structure has a packing density greater than that of β and less than that of the saturation coverage structure,30 it does not violate steric constraints, and it is consistent with the corrugations in the STM topographs shown in parts D and E of Figure 1. If one assumes that the aforementioned seven models comprise a complete list of reasonable (29) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultants Bureau Enterprises, Inc.: New York, 1961. (30) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503.
Figure 3. Constant-current STM topographs and selected cross-sectional profiles from β-δ and β-χ phase boundaries. Schematized molecular models are overlaid on topographs to show hypothetical registry. Models with angled chains represent displacement out of the surface plane. The scale bar and crystallographic indicator in A applies also to B. (A) Constantcurrent STM topograph of β-δ phase boundary. (B) STM topograph of β-χ phase boundary. Dashed line indicates twin boundary in molecular lattice. (C) and (D) Cross-sectional height profiles from corresponding regions in A and B. For C and D, the distance axis was calibrated from the β-phase domain in B assuming a β-phase periodicity of 11a.
structures, then the out-of-plane interdigitation model is the best candidate. The overlays in parts D and E of Figure 1 show the expected registry between the δ phase and the STM topographs if one assumes that the brightest features are due to Au-thiolate electrons and that the less bright features are associated with the alkyl chains. Figure 1C shows an STM topograph of a phase that we term χ and that has not been previously reported in the literature. We assume that χ belongs to the same family of surface-aligned phases that precede it, β, and succeed it, δ. A cross-sectional profile (Figure 3D) aligned close to the β-phase chain axis direction indicates a periodicity of 19a with a pattern of protrusions that suggests alternating
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Table 1. Molecular Areas and Relative Packing Densities of the Commensurate Solid Phases of Decanethiol on Au(111)a phase
unit cell
molecular area (Å2/molecule)
θ/θsat.
β χ δ φ
c(23×x3) c(19×x3) h(5x3×x3)R30° c(3×2x3)
82.8 68.4 54.0 21.6
0.261 0.316 0.400 1.000
a
c ) centered rectangular, and h ) hexagonal.
segments of 11.5a and 7.5a. The β-χ phase boundaries shown in Figure 3B suggest that phase χ arises by lateral shifting of molecular row segments perpendicular to the row direction and alternate bunching of the molecular rows. Figure 2χ shows a model that is consistent with the data shown in Figures 1C, 3B, and 3D. It comprises alternating row segments of β and δ phases, has a centered rectangular (19×x3) unit cell, and contains four molecules. The centered rectangular (19 × x3) is identical to an oblique (x364×x3). Figure 3B comprises two domains labeled D and D′ that are related by mirror reflection symmetry. The angle measured between low index directions in D and D′ is 9°. Measurements on five other such boundaries yield an average included angle of 9.5° with a standard deviation of 1.3°. In close agreement, a boundary between two domains of the model unit cell Figure 2χ that are related by a mirror reflection (a twin boundary) would cast an included angle of 10.4°. Figure 2 shows the packing structures predicted based on the molecule-resolved STM data and on existing literature data. Figures 1, 3, and 4 show that the χ- and δ-phase molecular rows exhibit regular deviations from alignment with substrate 〈121〉. In the next sections, we provide evidence that these deviations are antiphase boundaries that arise from the mechanism of the phase transitions and are metastable. Table 1 summarizes the unit cells and packing densities of the three striped phase models shown in Figure 2 and the saturation coverage centered rectangular (3×2x3) phase. No attempt was made to adjust the reported packing densities for the antiphase defects that exist in the nascent χ- and δ-phase domains. The Assembly Mechanism Figure 4 shows the evolution of the apparent surface structure during vapor-phase deposition of decanethiol, commencing from below saturation of the β phase and continuing to just after nucleation of the highest density phase, φ. Figure 4A shows coexistence of β-phase islands with the lattice gas, R. Increasing the coverage of decanethiol results in lateral growth of β-phase islands and coalescence of neighboring islands with elimination of the lattice gas, R. Increasing the coverage above saturation of the β phase results in heterogeneous nucleation of χ-phase domains at the β-phase domain boundaries (see Figure 4B). At the molecular level, we speculate that phase β transforms into phase χ when the lateral pressure exceeds a threshold and induces the methyl termini of a β-phase row subset of, for example, three chains, to slide between the counter-oriented chains, up out of the surface plane, and with concerted formation of gauche conformations near the sulfur terminus. The end result is out-of-plane interdigitation. This model is confirmed by Figures 3B and 4B which show that the azimuthal orientation of the alkyl chains is retained across the β-χ phase boundary. This bunching of molecular row segments results in a local compaction perpendicular to the row direction of
roughly 11.5a - 7.5a ) 4a. If this bunching occurs at a β-phase domain edge the tangent of which is parallel to the β-phase rows, then the compaction propagates into the β-phase domain by creation of a paired antiphase boundary defect (white arrow in Figure 4B). If this bunching occurs at a β-phase domain edge the tangent of which is perpendicular to the β-phase rows, then the edge propagates into the β-phase domain by conversion of β into χ (black arrows in Figure 4B). The alternating conversion of β-phase row segments to δ-phase segments is an interesting outcome. We speculate that conversion of a row segment to the out-of-plane interdigitated configuration induces a distortion of the sulfur-paired neighbors and that this distortion increases the barrier to formation of out-of-plane to the next row. This would shift the conversion down one row and result in the observed alternation. The confinement of this local compaction to molecular-row subsets is what gives rise to the regular antiphase boundaries in large domains of the χ phase (see black line segments in Figure 1C). The remaining β-phase domains in the topograph Figure 4B exhibit a height modulation that follows a sinuous path through the domain center (black pointing fingers). These modulations are similar to the Au reconstruction in height and shape;23 therefore, we ascribe them to remnant Au herringbone ridges below the β-phase decanethiol. Remnant herringbone ridges also exist below the crossed line segment in the right half of Figure 1B and result in a lateral shifting of the β-phase molecular row perpendicular to the row direction. Increasing the coverage of decanethiol results in lateral growth of the χ phase at the expense of the β phase. Figure 4C shows a surface dominated by the χ phase and with residual β-phase domains that trace a sinuous path by influence of the underlying Au reconstruction. Increasing the coverage above saturation of the χ phase results in heterogeneous nucleation of δ-phase domains at the χ-phase domain boundary network (see Figure 4D). We speculate that the δ phase forms from the χ phase when the lateral pressure induces the methyl termini of the alternating β-like χ-phase row segments to counterpropagate, thereby converting the phase to pure out-of-plane interdigitated segments. This model is confirmed by the phase boundaries in Figures 1D, 3A, and 4D which show that the azimuthal orientation of the alkyl chains is retained across the β-δ and χ-δ phase boundaries. This evidence suggest that the regular antiphase boundaries in large domains of the nascent δ phase (see black line segments in Figure 1D) result from the mechanistic pathway of the phase transition from the χ phase. Increasing the coverage above saturation of the δ phase results in formation of a new phase, labeled in Figure 4E, that is nonperiodic and dynamic. The phase nucleates heterogeneously at the δ-phase domain boundary network and grows laterally. Similar topographies were observed for hexanethiol on Au(111) (see Figure 1C in ref 21) and for Ge(111)c(2×8) below its melting point (see Figure 3 in ref 31). For the Ge(111)c(2×8) studies, the authors ascribe the disordered regions to edge premelting. Similarly, we attribute the phase to a fluid of decanethiol formed by edge-mediated melting. The molecular density of the phase should be higher than that of the δ phase, making it more characteristic of a liquid than of a gas. The phase is a one-molecule-thick layer sandwiched between crystalline Au and ultrahigh vacuum; therefore, it is a twodimensional liquid. Allowing this surface to relax several minutes results (31) Feenstra, R. M.; Slavin, A. J.; Held, G. A.; Lutz, M. A. Ultramicroscopy 1992, 42-44, 33.
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Figure 4. Constant-current STM topographs showing the evolution of the apparent surface structure after gas-phase exposure of Au(111) to ≈10-6 Torr decanethiol. The scale bar and crystallographic indicator in A applies also to B-F. (A) Au(111) exposed for 300 s shows striped molecular monolayer islands (β) coexisting with lattice gas (R). The dark feature in the upper right is a preexisting vacancy island surface defect. (B) Au(111) exposed for 420 s shows χ and β phases in coexistence. Black arrows indicate the β-χ phase boundary, white arrow indicate the paired antiphase boundary defect in the β-phase domain, black pointing fingers indicate the Au herringbone reconstruction residing beneath β-phase domains, and white pointing fingers indicate monatomic Au step edges, the contrast of which was artificially lowered. (C) Au(111) exposed for 480 s shows a surface dominated by χ-phase domains with residual sinuous β-phase domains. Pointing fingers indicate nascent, assembly-induced Au vacancy islands. (D) Au(111) exposed for 480 s shows δ and χ phases in coexistence. Black arrows indicate the χ-δ phase boundary, and white arrows indicate χ-phase defects. β-phase domains persist in the upper right. (E) Au(111) exposed for 510 s shows nucleation and growth of the fluid phase () at the δ-phase domain boundary network. (F) The same surface region and exposure as that in part E. The dense phase (φ) nucleates homogeneously in the -phase domains and grows laterally. Residual β- and -phase domains persist.
Decanethiol on Au(111)
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Scheme 1. Sequence of Monolayer Phases with Increasing Coverage of Decanethiol on Au(111)
in formation of a new phase, labeled φ in Figure 4F. Because of the absence of heterogeneous nucleation sites in a liquid phase, φ-phase islands nucleate homogeneously, rather than heterogeneously, in the -phase domains. High-resolution topographs of the φ-phase domains show that they exhibit a centered rectangular (3×2x3) lattice.32 The c(3×2x3) is synonymous with a hexagonal (4x3×2x3)R30°. Historically, this phase has been referred to as a c(4×2) superlattice of a hexagonal (x3×x3)R30°. It is somewhat unconventional to refer to a hexagonal phase as centered; therefore, we prefer the nomenclature c(3×2x3). This is the densest known phase of decanethiol monolayers on Au(111) and is characterized by alignment of the hydrocarbon chains upright but canted 30° away from the surface normal.30,32-35 On the basis of inferences from the coverage-dependent STM topographs, we propose a growth model that is shown in Scheme 1. The molecules initially adopt a lattice gas phase (Scheme 1A), and then sequentially adopt three discrete striped phases characterized by alignment of the molecular axes with the surface plane (Scheme 1B-D). Interestingly, in these first three solid phases, a close pairing of sulfur headgroups is maintained. Above saturation of the δ phase, the monolayer melts, and then, the saturation coverage phase, φ, characterized by alignment of the molecular axes close to the surface normal, nucleates homogeneously from this melt (Scheme 1E). The φ phase grows laterally until saturation (Scheme 1F). The monolayer undergoes five phase transitions during the approach to saturation coverage. In each case, the boundaries between coexisting phases are molecularly abrupt, suggesting that all observed phase transitions are first-order. The Thermodynamic Stability of the Phases During self-assembly by vapor-phase transport, the monolayer sequentially adopts six discrete structural phases: R, β, χ, δ, , and φ. Of fundamental importance is the question of whether each of these phases is the thermodynamic minimum energy configuration (stable) or is some higher energy configuration (metastable). There are several methods to prove that a phase is metastable at some T, π, and A (T, P, and V, in three dimensions). The time test measures the sample structure over time and, if the phase is metastable, eventually the stable phase (32) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (33) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (34) Delamarche, E. et al. Langmuir 1994, 10, 2869. (35) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216.
Figure 5. Constant-current STM topographs of decanethiol monolayers prepared in R-β and β-χ coexistence on Au(111) and then stored in ultrahigh vacuum. (A) Decanethiol grown to R-β coexistence and stored for 15 h. (B) The same surface region as that in A but after storage for an additional 10 h. The large central island of β-phase decanethiol exhibits shape fluctuations, and the small island in the upper middle reverts to vapor. Data suggests that the redistribution of β-phase island material is mediated by evaporation and condensation of a lattice gas of decanethiol (R phase). Stability of the β phase is indicated by persistence of the β-phase structure. Curvilinear bright bands are due to Au reconstruction. (C) Decanethiol grown to β-χ coexistence and stored for 5 days. The χ-phase domains convert to a mixture of β′ and δ′, and the β-phase domains exhibit unaltered packing structure. Topograph suggests that the β phase is stable, and the χ phase is metastable. White arrows indicate monatomic Au step edges, the contrast of which was artificially lowered.
1174 Langmuir, Vol. 15, No. 4, 1999
Poirier Figure 6. Constant-current STM topographs of decanethiol monolayers prepared to coverage above φ-phase nucleation and then stored in ultrahigh vacuum. Phase boundaries are indicated by white arrows, domain boundaries are indicated by black arrows. (A) Decanethiol prepared in β, δ, , and φ coexistence by gas-phase deposition and stored in ultrahigh vacuum overnight. The β and -phase domains have disappeared, and phase and domain boundaries approach molecular width. Dark triangles are Au vacancy islands, and the irregular feature close to the center is attributed to contamination. The contrast was enhanced by adding to the topograph, z(x,y), the derivative of the topograph, dz(x,y)/dx. The topograph indicates that, at this coverage, the δ and φ phases exist in thermodynamic equilibrium. (B) A decanethiol monolayer prepared in δ-φ coexistence by incubation of Au(111)(x3×23) in 4.5 µM solution for 4.3 min. Phase and domain boundaries approach molecular width. The topograph indicates that at this coverage the δ and φ phases exist in thermodynamic equilibrium and that the assembly pathway for solution deposition mirrors that for gasphase deposition. (C) A decanethiol monolayer in δ-φ coexistence prepared by annealing a saturation φ-phase monolayer to 75 °C for ∼10 min in ultrahigh vacuum. Phase and domain boundaries are molecular width. In contrast to the δ phase accessed by growth from solution or gas phase, the δ phase accessed by partial desorption from the φ phase exhibits linear molecular rows.
will nucleate and grow. The annealing method is often employed in the spirit of an accelerated time test, in which one attempts to coax the sample into a stable phase using thermal energy; however, one always runs the risk of converting to another phase that is only stable near the annealing temperature. Another method is to slowly approach from both above and below, in either temperature or pressure. If the phase appears in only one direction of approach, then it is likely metastable. Proving stability is more difficult than proving metastability. For example, a negative result on the time test could mean that the phase is stable, or it could equally mean that the experimenter lacked the requisite patience. In this section, we will address the thermodynamic stability of the phases in the order in which they appear.
Consider that the sample is a constant-area (≈1 cm2) two-dimensional container into which we are placing decanethiol molecules. As the number of molecules increases, the lattice gas vapor pressure, πR, increases linearly and achieves a degree of supersaturation that depends on the flux, the critical island size, and the presence of heterogeneous nucleation sites.36,37 The β-phase islands then nucleate from this supersaturated lattice gas, causing π to recover to the two-dimensional β-phase vapor pressure, πβ. Adding decanethiol molecules to the system results in lateral growth of the β-phase islands, islands that are growing in equilibrium with the lattice gas, R.37 The time evolution of a surface in R-β coexistence shows redispersion of the island distribution (see parts A and B of Figure 5), indicating that the islands are in thermodynamic equilibrium with the lattice gas on a phase boundary. The chain of events that we observe is therefore consistent with conventional two-dimensional nucleation and growth.36 The only time that the system deviates from the thermodynamic minimum energy configuration (neglecting edge-energy contributions) is in the supersaturated regime; therefore, the R phase at pressures equal to or below πβ is a stable phase. To ascertain the thermodynamic stability of the β phase, monolayer surfaces were prepared in R-β coexistence and in β-χ coexistence and then stored in UHV. The results, shown in Figures 1B and 5A-C, show that the β-phase islands persisted for more than 5 days in UHV. Electron diffraction studies also showed that the β phase was obtained for both the case of increasing from zero coverage (by gas-phase deposition) and the case of decreasing from saturation coverage (by thermal desorption).26 These observations all indicate that the β phase is stable. A similar strategy was employed to test the thermodynamic stability of the χ phase. The results, shown in Figures 1B and 5C, indicate that the χ-phase domains do not persist; rather, they decay into a mixture of β and δ like regions. The χ phase was not seen in prior diffraction experiments,26,27 possibly because it is transitory or resides in a shallow potential well. The evidence therefore indicates that the χ phase is metastable. Molecular resolution scans of the β′ regions (labeled in Figure 1B) suggest that they are β-phase but with a regular shifting of row segments perpendicular to the row direction
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and by an amount equal to roughly half of the row spacing. The packing structures of the β and β′ phases are similar, and therefore, it is reasonable to assume that they are close to isoenergetic. Molecular resolution scans of the δ like regions, labeled δ′ in Figure 1B, fail to provide convincing evidence for any specific structural assignment. The features are similar to δ but with a ∼10 Å row spacing rather than 22 Å. The δ′ regions typically appear as isolated short segments of paired rows; they were never seen to encompass large areas. This suggests that δ′ is a metastable packing arrangement that can be accessed when the monolayer escapes from the metastable χ phase. The thermodynamic stabilities of the and the δ phases were characterized by preparing a sample in β, δ, , and φ coexistence, such as that shown in Figure 4F, and then storing it in UHV overnight. The observation of four-phase coexistence (Figure 4F) is curious because phase rules demand coexistence of at most two phases for a singlecomponent system at equilibrium and away from a triple point.38,39 The results, shown in Figure 6A, show that the β-phase domains disappear. This suggests that the β-phase domains are not in thermodynamic equilibrium at this coverage; we speculate that they persist by virtue of their interaction with the residual herringbone reconstruction ridges. Figure 6A also shows that the monolayer evolves by lateral growth of δ- and φ-phase domains at the expense of the phase. This suggests that is a metastable phase, a supercooled liquid. Monolayers in δ-φ coexistence were also prepared by controlled growth from micromolar solutions (see Figure 6B). The results show φ-phase domains in equilibrium with δ-phase domains that have 7.5a row periodicity. The phase boundaries and domain boundaries are both of molecular width and the δ-phase domains exhibit deviations from linearity that are similar to those of monolayers prepared by gas-phase deposition. If it is assumed that these deviations arise from the mechanistic pathway of the transformation from the χ phase, then these data suggest that solution- and gas-phase assemblies can follow roughly the same mechanistic pathway. Monolayers in δ-φ coexistence were also prepared by thermal desorption in UHV from saturation coverage of the φ phase (see Figure 6C). The results show φ-phase domains in equilibrium with δ-phase domains, and the phase boundaries and domain boundaries are both of molecular width. The rows exhibit a 7.5a row periodicity, but the rows do not exhibit the regular deviations from linearity; rather, they are linear and aligned with substrate 〈121〉. We speculate, therefore, that the deviations from linearity in nascent δ-phase domains are antiphase defects and that these defects may be removed by annealing. An electron diffraction study also observed the 7.5a row periodicity for monolayers prepared by thermal desorption from saturation of the φ phase.26 The results presented here and in the literature therefore suggest that δ is a stable phase but that the regular deviations from alignment with 〈121〉, in the nascent δ phase, are a metastable property of the phase. The transition from the δ to the φ phase proceeds by an intermediate metastable melted phase whereas the transitions from β to χ and from χ to δ do not. We speculate
that this is due to a critical island-size effect. For nonfractal islands, the critical island size scales as the ratio of the island edge energy to the free energy of two-dimensional condensation.36 It is reasonable to assume that the edge energy between the striped phases, which are closely related, is less than the edge energy between δ and φ. Other things being equal, this would result in a larger critical island size for the φ phase than than those for the χ and δ phases. Because formation of critical islands relies on stochastic molecular motion, the delay time to nucleation increases with increasing critical island size.36 Thus, with increasing coverage, the monolayer transiently melts because the lateral pressure is too high to support the δ phase; yet, sufficient time has not elapsed for nucleation of stable φ-phase islands. The data shown in Figure 6 indicate persistence of the φ phase under storage in a vacuum for 1 day and under exposure to ethanol and air. STM measurements in this lab (data not shown) indicate that the φ phase is stable under storage in a vacuum for over 30 days. There is no indication in prior literature reports that the φ phase is metastable;30,32-35 therefore, the weight of the evidence indicates that the φ phase is stable.
(36) Zinke-Allmang, M.; Feldman, L. C.; Grabow, M. H. Surf. Sci. Rep. 1992, 16, 378. (37) Fowler, R.; Guggenheim, E. A. Statistical Thermodynamics; Syndics of the Cambridge University Press: New York, 1960. (38) On the Equilibrium of Heterogeneous Substances; Gibbs, J. W., Ed.; Longmans and Green: New York, 1906; Vol. Scientific Papers, p 315. (39) Zernike, J. Chemical Phase Theory; Kluwer: Antwerp, 1955.
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Conclusion These studies provided a detailed, real-space picture of the phases formed and the mechanistic pathways followed during self-assembly of decanethiol molecular monolayers on Au(111). We observe a total of six discrete structural phases: two are fluids (R and ), and four are commensurate solids (β, χ, δ, and φ); two are metastable (χ and ), and four are stable (R, β, δ, and φ). On the basis of molecular resolution topographs, we assigned a structure that is characterized by out-of-plane interdigitation to the previously observed δ phase. We also observed a new phase, the χ phase, that we believe is metastable and has a structure and a packing density that are intermediate between the β and δ phases. We also assigned a structure to the phase, which transiently forms an intermediate between the highest density stripe phase and the upright phase. The data suggest that it is a twodimensional, supercooled liquid. We speculate that the complex phase behavior observed here is a reflection of the complex interplay of forces that influence monolayer self-assembly including spreading pressure,37,40 edge energy, heat of condensation, and molecular conformation energy. The system studied here is similar to insoluble monolayers at the liquid/air interface41-43 and to phospholipid bilayer membranes in living systems;44 therefore, the results reported here may have some bearing on the complex phase behaviors observed for these related systems. Finally, we hope that these studies will facilitate synthesis of useful monolayerbased biosensitive interfaces by providing developers with a fundamental understanding of the structural phases adopted and the mechanistic pathways followed during monolayer self-assembly. Acknowledgment. The author gratefully acknowledges helpful discussions with J. W. Cahn, R. E. Cavicchi, A. S. Eberhardt, T. M. Herne, J. M. H. Levelt-Sengers, R. D. Mountain, G. Scoles, and M. J. Tarlov.
(40) Dervichian, D. G. J. Chem. Phys. 1939, 7, 931. (41) Harkins, W. D.; Boyd, E. J. Phys. Chem. 1941, 45, 20. (42) Rasing, T.; Shen, Y. R.; Kim, M. W.; Grubb, S. Phys. Rev. Lett. 1985, 55, 2903. (43) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509. (44) Lee, A. G. Biochim. Biophys. Acta 1977, 472, 237.
