J. Phys. Chem. C 2007, 111, 16909-16919
16909
Influence of Molecular Structure on Phase Transitions: A Study of Self-Assembled Monolayers of 2-(Aryl)-ethane Thiols Piotr Cyganik,*,†,‡ Manfred Buck,*,† Thomas Strunskus,‡ Andrey Shaporenko,§ Gregor Witte,‡ Michael Zharnikov,§ and Christof Wo1 ll‡ EaStChem School of Chemistry, St. Andrews UniVersity, North Haugh, St. Andrews, KY16 9ST, United Kingdom, Lehrstuhl fu¨r Physikalische Chemie I, UniVersita¨tsstrasse 150, 44801 Bochum, Germany, and Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ReceiVed: May 23, 2007; In Final Form: August 11, 2007
Self-assembled monolayers (SAMs) prepared on Au(111) substrates from solutions of ω-(4′-methylbiphenyl4-yl)ethane thiol (CH3(C6H4)2(CH2)nSH, n ) 2, BP2), at room temperature and subsequently annealed at temperatures of up to 423 K were studied using scanning tunneling microscopy, low-energy electron diffraction, high-resolution X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy. Upon annealing a phase transition occurs from the low-temperature (5x3 × 3) structure common to all SAMs prepared from the series of BPn homologues with n ) even studied so far, to a new structure which is markedly different from the high-temperature phases of the higher BPn homologues. Although its basic structure can be approximated by a (2x3 × 2) unit cell, the regular occurrence of line defects running exclusively along the 〈112h〉 direction is the most characteristic feature of this new phase. Irrespective of these defects the phase transition dramatically improves the stability of the BP2 monolayer as demonstrated by exchange experiments. In contrast to BP2, SAMs made from the closely related 2-phenylethane thiol (C6H5(CH2)2SH, P2) do not show any phase transition. The differences between BP2, its higher homologues, and P2 highlight the subtleties of the interplay of different factors determining the structure of a SAM.
I. Introduction A key challenge for molecular electronics is the controlled arrangement of molecular entities exhibiting electronic functionality on a length scale ranging from externally addressable down to molecular dimensions. This requires materials which combine the desired electronic properties with the possibility of patterning them at very different length scales. Aromatic selfassembled monolayers (SAMs) is one class of systems which are currently explored along this route.1,2 Experiments performed in recent years have demonstrated that aromatic SAMs allow control of charge transfer and tailoring of electronic functionality3-12 but also offer new opportunities in lithography.13-19 However, to address the nanometer scale in aromatic SAMs, a high level of control over the structure and properties of these systems is of key importance. At present, unfortunately, it is not possible to precisely control the structure of a SAM in a rational fashion (i.e., by deliberately adding functional groups to the organothiol) since the properties of a SAM are determined by a complex interplay of several factors including intermolecular interactions, molecule-substrate bonding, and lattice mismatch between the molecular lattice and the substrate, to just name a few. The latter factor is presumably a major limiting factor for the structural quality of thiol-derived SAMs with a purely aromatic backbone, in which the mismatch between the packing preferred by the rigid aromatic layer20 and the substrate lattice results in significant stress. This stress is released predominantly by the formation of defects (i.e., domain * Corresponding authors. E-mail:
[email protected] (P.C.); mb45@ st-and.ac.uk (M.B.) † St. Andrews University. ‡ Lehrstuhl fu ¨ r Physikalische Chemie I. § Universita ¨ t Heidelberg.
boundaries, dislocation faults, point defects)21 which results in the observed lack of structural quality in purely aromatic thiolderived SAMs.21-26 As demonstrated in our previous studies on biphenyl-substituted alkanethiols (CH3-(C6H4)2-(CH2)nSH, BPn, n > 1)27 this problem can be partly overcome by introducing an alkane spacer chain between the thiol head group and the aromatic moiety. The individual thiolates forming these SAMs have additional degrees of freedom (e.g., conformational ones) through which stress is reduced without breaking up the structure.27 However, beyond the improvement of the quality of the film the insertion of the “flexible” alkane spacer also exerts a crucial influence on the film structure, i.e., the molecular orientation and packing. Our previous studies28-31 have revealed that BPn SAMs exhibit a pronounced alternation in molecular orientation and packing density with the change between n ) odd and n ) even. This odd-even variation is also reflected in a number of properties of the BPn system such as stability against exchange by other thiols,32 electrochemical stability,19,33 and electron-induced modification.34 However, the most striking manifestation of the odd-even behavior is probably, as demonstrated recently, the molecule-dependent occurrence of polymorphism35-37 and irreversible phase transitions35,36 which occurs only for n ) even. Such a transition is accompanied by a dramatic increase in the level of perfection, resulting in domains with the area exceeding 105 nm2. Furthermore, this phase transition causes a surprising switch in stability against exchange by other thiols when immersed into the corresponding solutions. The phase transition seen in these biphenyl-based organothiolate adlayers is fundamentally different from temperature-induced structural transitions of alkanethiolate adlayers into low-density “flat lying” phases38-43 which are not stable when immersed in solutions under conditions where a high-
10.1021/jp073979k CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007
16910 J. Phys. Chem. C, Vol. 111, No. 45, 2007 density phase forms. Detailed spectroscopic and microscopic investigations of the structures of BPn SAMs28-31 and the phase transitions observed in SAMs of BP4 and BP6 provided substantial insight into the basic factors behind these phenomena.36 The model which emerged from these studies explains the occurrence of phase transitions and their dependence on the molecular structure by the either cooperative or competitive way the different factors determining the energetics of a SAM enter into the energy balance. This is illustrated by a simple qualitative model (see Figure 11 in ref 36) where the Au-S-C bending potential and the density of S-Au bonds along with intermolecular interactions are key factors determining the energy of the system. Whereas for n ) odd all factors act cooperatively, the Au-S-C bending potential opposes the other factors in even-numbered BPn SAMS. As a consequence, several structures exist for BPn SAMs with n ) even which are similar in energy and thus can undergo thermally induced structural transitions. Building on the studies of BP4 and BP6, the present work explores the idea of competitive design further. Choosing molecules which are closely related to the ones investigated previously but which exhibit systematic differences this work aims at an improved understanding of the relative importance of structure-determining factors. II. Experimental Section Sample Preparation. The synthesis of the ω-(4′-methylbiphenyl-4-yl)ethane thiol (CH3(C6H4)2(CH2)nSH, BPn, n ) 2) molecule has been described elsewhere.28 2-Phenylethane thiol (C6H5(CH2)2SH, P2) has been obtained from Aldrich. Commercially available Au/mica substrates from Georg Albert PVD, consisting of 150 nm Au evaporated onto mica (rate 2 nm/s, temperature 340 °C), were flame-annealed in a butane/oxygen flame and subsequently immersed into a 0.1 mM solution of BP2 or P2 in ethanol at room temperature for 24 h. After immersion, samples were rinsed with pure ethanol and blown dry with nitrogen. Annealing of the SAMs was done in a sealed container which was purged with nitrogen prior to temperature treatment. Scanning Tunneling Microscopy (STM) Measurements. All STM measurements were carried out in air at room temperature using a Molecular Imaging Picoscan STM instrument. In all cases tips were prepared mechanically by cutting a 0.25 mm Pt/Ir alloy (8:2, Goodfellow) wire. The data were collected in constant current mode using tunneling currents between 400 and 700 pA and a sample bias between 600 mV and 1.0 V (tip positive). No tip-induced changes were observed. Low-Energy Diffraction (LEED) Measurements. LEED data were obtained in a multichamber ultrahigh vacuum (UHV) system using a microchannel plate LEED system (OCI) and a charge-coupled device (CCD) camera connected to a frame grabber card for image acquisition. This system is capable of recording LEED data with very low electron fluxes (5-15 nA/ cm2)44 and thus allows us to reduce total electron dose during the entire LEED experiment (up to 5 min) to 1.5-4.5 µC/cm2, which is far below the doses leading to electron-induced damage of BP2 and P2 SAMs.34,45,46 Even though a diffraction pattern of the SAM was observed at room temperature, the Au single crystal was heated to 340 K for a few minutes to improve the quality of the data. For the LEED measurements the sample was cooled down to 110-120 K to reduce the inelastic background and to increase the intensity of the diffraction spots which are determined by the Debye-Waller factor. High-ResolutionX-rayPhotoelectronSpectroscopy(HRXPS) Measurements. The HRXPS experiments were performed at
Cyganik et al. the bending magnet beamline D1011 at the MAX II storage ring of the MAX-lab synchrotron radiation facility in Lund, Sweden. The HRXPS spectra were acquired in normal emission geometry at photon energies of 350 and 580 eV for the C 1s range and 350 eV for the S 2p region, respectively. In parallel, the Au 4f spectra were acquired and the O 1s range was monitored. The binding energy (BE) scale of every spectrum was individually calibrated using the Au 4f7/2 emission line of AT-covered Au substrate at 83.95 eV. The latter value is the latest ISO standard.47 It is very close to a value of 83.93 eV, which has been obtained by us for Au 4f7/2 using a separate calibration to the Fermi edge of a clean Pt foil.29 The energy resolution was better than 100 meV (typically ∼60 meV), which is noticeably smaller than the full width at half-maximum (fwhm) of the photoemission peaks addressed in this study. Thus, these fwhms are representative for the natural widths of the respective lines. HRXPS spectra were fitted by symmetric Voigt functions and a Shirley-type background. To fit the S 2p3/2,1/2 doublet we used two peaks with the same fwhm, the standard48 spin-orbit splitting of ≈1.18 eV (verified by a fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The fits were performed self-consistently: the same fit parameters were used for identical spectral regions. Near-Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy Measurements. The NEXAFS spectroscopy measurements were performed at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. The spectra acquisition was carried out at the C K edge in the partial electron yield mode with a retarding voltage of -150 V. Linear polarized synchrotron light with a polarization factor P of ≈82% was used. The energy resolution was ≈0.40 eV. The incidence angle of the light was varied from 90° (E-vector in surface plane) to 30° (E-vector near surface normal). The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The energy scale was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV. Contact Angle Measurements. Advancing contact angles of distilled water were measured with a Kru¨ss goniometer, model G10. The experiments were performed under ambient conditions with the needle tip in contact with the drop. Averaged values of at least 10 measurements at different locations on each sample are reported here. Deviations from the average were less than (1°. III. Results ω-(4′-Methylbiphenyl-4-yl)ethane Thiol (CH3(C6H4)2(CH2)2SH, BP2). STM Measurements. Figures 1 and 2 summarize STM data obtained for the BP2 system prepared at room temperature from the respective solution and then annealed at different temperatures. Images collected at large scale (Figure 1a-d) reveal two important features. First, in comparison to BP2 samples prepared at room temperature (see Figure 5 in ref 31 for direct comparison) the density of substrate depressions49-51 (black areas in the STM images) is strongly reduced due to Ostwald ripening. Although this is commonly observed upon annealing of SAMs,52,53 the extent to which this occurs for BP2 is more pronounced compared to other types of thiol SAMs and is comparable to its higher homologues BP4 and BP6.36 Second, proper annealing conditions yield a perfectly uniform SAM (Figure 1c), whereas lower (Figure 1, parts a and b) or higher (Figure 1d) temperatures result in heterogeneous SAMs. Closer inspection of Figure 1, parts a and b, reveals the coexistence of two ordered phases one of which is the R-phase
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J. Phys. Chem. C, Vol. 111, No. 45, 2007 16911
Figure 1. BP2 SAMs on Au(111). STM images with different resolutions showing samples prepared at room temperature and subsequently annealed in N2 atmosphere for 15 h at 373 (a), 393 (b and e), 417 (c and f), and 423 K (d). R and δ indicate the areas covered by different phases as described in the text. White dotted loops in (d) indicate the destroyed areas of the δ structure. White dashed lines in (e) indicate the translational domains with the unit cell shifts marked by white solid lines.
known from our previous studies of BPn SAMs. The R-phase is a rectangular (5x3 × 3) structure with eight molecules per unit cell and an area per molecule of 27.05 Å2. This structure is characteristic for BPn systems with an even number of CH2 units prepared from solution at room (or slightly elevated) temperature.27,30,31 Similar to BP4 and BP6,36 annealing of BP2 SAMs results in a modification of the R-phase. As shown by the high-resolution image reproduced in Figure 1e narrowly spaced translational domain boundaries are formed which, as indicated by the white dashed lines, are about 5-7 nm apart. We interpret the formation of translational domain boundaries as relief of stress. As discussed previously27 stress in SAMs originates from the misfit between the molecular lattice and the substrate lattice. Upon increase of temperature this stress is then released by forming domain boundaries. Whereas for purely aromatic thiol SAMs, e.g., anthracene thiol, a high density of translational domains is already observed at room temperature,21 for BPn films prepared under corresponding conditions the density of such defects is rather low. This can be rationalized by taking into account that the conformational degrees of freedom of the alkane spacer (in combination with a possible restructuring of the Au-S interface) make the BPn systems more tolerant to lattice mismatch.27 However, there seems to be a limit to this “soft” way of stress reduction and continuous growth of domains at higher annealing temperatures demands more effective ways to relieve stress resulting in the break up of the regular molecular pattern as seen in Figure 1e and observed previously for BP4 and BP6.36 The other phase labeled δ seen in Figure 1, parts a and b, is not present in the native BP2 SAM but emerges during annealing. In contrast to the annealed R-phase with its high density of translational domain boundaries, the new δ-phase is of pronounced uniformity as illustrated by Figure 1f. Increasing the annealing temperature (keeping the same annealing time of 15 h) from 373 to 393 K (Figure 1, parts a and b) results in a continuous increase of the area occupied by the δ-phase and yields a complete transition at 408 K (Figure 1c). Even higher temperatures cause a gradual damage of the SAM resulting in loss of molecular order in the dark areas marked by dotted loops in Figure 1d. This process starts mainly at steps for which we hold responsible possible defects in the SAMs related to (1) the distortion of the local SAM structure by the substrate step and/or (2) possible accumulation of contaminations at substrate steps prior to SAM formation. For a detailed analysis of the structure of the δ-phase we turn to the high-resolution STM images presented in Figure 2.
The first characteristic feature of this structure is the appearance of densely packed rows of molecules which alternate in the STM contrast. These rows running along the 〈11h0〉 directions are common for the structures reported previously for the odd and even BPn systems30,31 and can be assigned to the herringbone arrangement of the biphenyl moieties. In the case of the δ-phase, however, the orientation of the alternating rows is along the 〈11h0〉 instead of the 〈112h〉 directions observed for the R-phase. This is also different from the β-phase of SAMs of BP4/BP6 where the rows also run along the 〈112h〉 direction. This indicates a significant rearrangement of the molecules during the R f δ phase transition. Another characteristic feature of the δ-phase are dark lines running perpendicular to the alternating molecular rows. From the images taken on a larger scale (Figure 1f) it follows that these dark lines define a large unit cell of the δ structure. However, a closer inspection of the structure (see Figure 2a) reveals that there is not just a single value for the distance between these lines but discrete values are measured for the long axis of the unit cell which jump between about 48 and 62 Å. This suggests that these lines correspond to highly ordered line defects. The high-resolution data presented in Figure 2b together with the corresponding cross sections (Figure 2c) reveal the arrangement of molecules outside and inside the region of the line defects. Although the intermolecular distances appear regular along the 〈112h〉 direction, one can clearly discern two types of variation of the intermolecular distances along the 〈11h0〉 directions. In the line scan labeled A one molecule is slightly displaced toward its neighbor, whereas line B reveals a missing molecule. The result of the STM analysis is summarized in a schematic model of the δ-phase presented in Figure 3. Outside the regions of the line defects, the average distance between molecules along the 〈11h0〉 and 〈112h〉 directions amounts to 5.9 ( 0.4 and 10.0 ( 0.6 Å, respectively, where the averaged values and errors have been obtained from a set of five different images. The basis of the δ-phase can, thus, be described as a rectangular (2x3 × 2) lattice with an area per molecule of 28.7 Å2. The area of the line defect is characterized by (i) a variation of intermolecular distances (a ) 5 Å, b ) 6.6 Å in Figure 3) in every second row but with an average identical to the 5.8 Å of the unit cell and (ii) by a missing molecule in the adjacent row resulting in increased intermolecular distances of c ) 7.9 Å and d ) 9.4 Å. Two things are important to note at this point. First, even though the proposed commensurate structure describes the dimensions rather accurately one should bear in mind that it is the mismatch between molecular lattice and substrate
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Figure 3. Proposed model for the molecular arrangement in the δ-phase of BP2 SAMs on Au(111). Au atoms are marked by open circles. Adsorption sites of the BP2 molecules (threefold hollow sites are chosen for the modelssee text) are marked by gray circles. The unit cell of the rectangular (2x3 × 2) lattice corresponding to an area per molecule of 28.7 Å2 is marked by the black rectangle. The orientation of the phenyl rings in the herringbone pattern is also indicated (for clarity the tilt angle of molecules is omitted). The commensurate rectangular (2x3 × 2) lattice on the right and left sides is divided by the line defect arising due to the missing molecules. The positions of the molecules in the defect area have been obtained from STM images (see the corresponding distances marked in Figure 2b) and assuming that the molecules adjacent to the vacancy have changed their adsorption sites from face-centered cubic (fcc) to hexagonal close-packed (hcp) (a ) 5.00 Å, b ) 6.52 Å, c ) 7.88 Å, d ) 9.40 Å).
Figure 2. BP2 SAMs on Au(111). (a and b) High-resolution STM images recorded for samples prepared at room temperature and subsequently annealed in N2 atmosphere for 15 h at 417 K. (c) Height profiles along the lines depicted in (b).
lattice which gives rise to the line defects, and therefore, the suggested dimension along the 〈11h0〉 directions should be considered an approximation. Second, the thiols have been arbitrarily positioned in threefold hollow sites for referencing purposes in order to facilitate illustration of the various intermolecular distances. The true position is not known, and various recent experiments suggest other adsorption sites of the sulfur.54,55 Furthermore, as discussed in our previous publication on phase transitions in BP4 and BP636 and evidenced by studies of short-chain alkanethiols56,57 a precise model has to include a restructured S-Au interface. LEED Measurements. Independent information on the structure of the δ-phase of the BP2 system has been obtained from the LEED patterns. In order to suppress the inelastic background signal and to minimize the Debye-Waller attenuation of the diffraction peaks all LEED data were recorded at a sample
Figure 4. (a and c) LEED data recorded for the δ-phase of BP2 SAMs on Au(111). The diffraction patterns have been obtained for electron energies of 80 (a) and 30 (c) eV at 120 K. (b and d) Schematic presentation of the diffraction patterns for 80 (b) and 30 (d) eV. The dashed hexagon in (b) indicates the position of the first-order diffraction spots of the bare Au(111) substrate recorded at the electron energy of 80 eV. (e) A comparison of the observed diffraction spots with the calculated diffraction pattern of a rectangular (2x3 × 2) structure including (rotational and mirror) domains. The color of the diffraction spots marked in (e) directly corresponds to the color of the spots marked in (b) and (d).
temperature of 120 K. In Figure 4, parts a and c, two LEED patterns are displayed which were acquired at electron energies of 80 and 30 eV, respectively, together with a schematic
SAMs of 2-(Aryl)-ethane Thiols
Figure 5. Normalized C 1s (left panels) and S 2p (right panels) HRXPS data recorded for the R (upper panels) and δ (bottom panels) phases of BP2 SAMs on Au(111). The spectra were acquired at a photon energy of 350 eV. The spectra are fitted assuming the presence of two different components in the case of the C 1s data and a doublet in the case of the S 2p data (see text for details).
representation of these diffraction patterns (Figure 4, parts b and d) showing the most intense diffraction spots as colored balls. The dashed hexagon and gray balls in Figure 4b indicate the position of the first-order diffraction spots of the bare Au(111) substrate recorded at the same electron energy of 80 eV. A comparison of the observed diffraction spots with the calculated diffraction pattern of a rectangular (2x3 × 2) structure including (rotational and mirror) domains which is displayed in Figure 4e reveals a close correspondence and thus confirms the commensurability of the δ-phase. We note that, due to a complex phase dependence, only few diffraction spots become visible for a given incident electron energy. Despite several attempts, no ordered diffraction pattern could be resolved for the R-phase of the BP2 system. As in our previous LEED study30 we attribute this effect to the large rectangular (5x3 × 3) unit cell of the R-phase which leads to a large number of rather closely spaced diffraction spots which apparently cannot be resolved (already for one domain of the rectangular (5x3 × 3) structure about 100 superlattice diffraction spots are expected to appear within the hexagon of the gold substrate first-order spots). Moreover, due to the large number of domain boundaries and lateral stacking faults, this phase reveals only a small coherence length which results in an additional broadening of the diffraction peaks. HRXPS Measurements. The S 2p and C 1s HRXPS data of the R- and δ-phases of BP2/Au are presented in Figure 5. The spectra are normalized to the intensity of the incident X-ray beam and the number of scans so that a direct comparison between the individual spectra in the same spectral range is possible. The S 2p HRXPS data of both R- and δ-phases exhibit a single S 2p3/2,1/2 doublet at a BE of ≈162.05 eV commonly assigned to the thiolate species,58,59 with no evidence for disulfides, alkylsulfides, or oxidative products. There is, however, a weak signal at 161.0 eV (S 2p3/2) for the δ-phase, whose origin is unclear. Molecular species in SAMs not attached by a thiolate bond have been proposed in previous work,60-63 but we cannot exclude an assignment to atomic sulfur.64 Since slight changes in the annealing conditions substantially affect the quality of the layers (see Figure 1) an onset of thermal decomposition in a small fraction of the substrate area could
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16913 escape the rather local STM imaging but would be seen in the data recorded with a spatially averaging spectroscopy like XPS. The presence of such a heterogeneity is supported by the fact that the intensity of the 161.0 eV doublet exhibits slight variations from sample to sample due to inevitably small variations in the preparation procedure such as slight differences in annealing temperature.33 The intensities of the thiolate-related doublet in the R- and δ-phases are essentially identical. This suggests very similar packing densities of both phases in accordance with the STM data. The fwhm of the S 2p3/2,1/2 components is quite small for both R- and δ-phases: it amounts to about 0.50 eV for both films. Such a small value suggests a small heterogeneity of the adsorption sites of the BP2 molecules in these films.33 Note that, since the instrumental broadening is below 100 meV, the value of the fwhm is essentially determined by the natural line width of the S 2p3/2,1/2 lines and the inhomogeneity of bonding configurations of the thiolate headgroups (e.g., occurrence of several different adsorption sites). The C 1s HRXPS data of the R- and δ-phases of BP2/Au are quite similar. The spectra exhibit a main emission peak at a BE of 284.23 and 284.30 eV, respectively, which is assigned to the aromatic backbone and a shoulder at ≈0.7 eV higher BE. Similar shoulders have been observed previously for different aromatic SAMs and have been assigned to the carbon atom bonded to the sulfur headgroup or to shakeup processes.63,65,66 Since the probing depth of HRXPS is rather small at the photon energy used in the present experiments, the former assignment seems to be rather questionable and an assignment to a shakeup process is more likely. The fwhm of the main C 1s emission peak for the δ-phase is slightly larger than that for the R-phase (0.79 vs 0.73 eV) which could be indicative of a somewhat larger inhomogeneous broadening in the δ-phase due to, e.g., changes in the crystalline packing. Like the sulfur signals, the total C 1s intensities of the Rand δ-phases do not differ within the experimental error supporting that the average packing densities of the BP2 molecules are very similar in both phases, see below. The thickness of these films was evaluated on the basis of the Au 4f spectra and C 1s relative intensities seen for a photon energy of 580 eV (data not shown). A standard expression for an exponential attenuation of the photoemission signal was assumed. The attenuation lengths of the C 1s and Au 4f photoelectrons were taken in accordance with the data of Lamont and Wilkes.67 The derived film thickness amounts to ≈14.2 Å for both R- and δ-phases of BP2/Au. At first sight the same average thickness as determined by XPS seems to be at variance with the structural models proposed for the two phases. Assuming an average spacing between the line defects in the δ-phase of 51.84 Å (i.e., equivalent to nine unit cells of the δ-phase), then, from the model presented in Figure 3, one can obtain an average area per molecule of 30.5 Å2, which is about 12.5% less dense compared to the R-phase (27.05 Å2). However, considering that the rotational domains in the R-phase prepared at room temperature are much smaller compared to the δ-phase, i.e., about 10 and 100-400 nm in diameter, respectively, the effective difference in density will be certainly reduced to only a few percent due to the larger concentration of defects in the R-phase. NEXAFS Measurements. NEXAFS spectroscopy is a synchrotron-based spectroscopic technique to probe electric dipole transitions from core levels to unoccupied molecular orbitals close to the continuum.68 Absorption resonances in NEXAFS spectra give a signature for a characteristic bond, a chemical
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Figure 6. C 1s NEXAFS spectra for R (a) and δ (b) phases in the BP2 SAMs on Au(111) samples acquired at X-ray incident angles of 90° and 30° (shadowed).
group, or a molecule. In addition, from the linear dichroism in X-ray absorption data68 information on the molecular orientation of adsorbed species can be obtained. In Figure 6, parts a and b, we present the C 1s edge NEXAFS spectra obtained for the R- and δ-phase of the BP2 SAMs, respectively. The spectra exhibit characteristic absorption resonances (marked in the top part of Figure 6a) related to the phenyl rings.69-72 The most prominent is the π1* resonance at ≈285.0 eV, which is accompanied by a weaker π2* resonance at ≈288.9 eV, and several broad σ* resonances at higher photon energies. The difference in the NEXAFS spectra taken at X-ray incidence angles of 90° and 30° as seen in Figure 6 for both phases of the BP2 system reveals a pronounced linear dichroism and thus directly demonstrates the presence of a high degree of orientational order. To determine the average tilt angles of the biphenyl backbones in the R- and δ-phases, the entire sets of spectra acquired at different incidence angles θ were used. For simplicity, only the intensity I of the most pronounced π1* absorption resonance was monitored. The angular dependence was evaluated according to the following theoretical expression (for vector-type orbital and substrate with threefold symmetry)68
1 1 I(F, θ) ∝ P 1 + (3 cos2 θ - 1)(3 cos2 F - 1) + 3 2 1 (1 - P) sin2 F (1) 2
[
]
where P denotes the polarization factor (P ≈ 0.82), F corresponds to the angle of the transition dipole moment (TDM) for the transition in question relative to the surface normal, and the incidence angle θ of the X-ray beam is defined with respect to the surface plane. To avoid normalization problems, not the absolute intensity but intensity ratios I(F, θ)/I(F, 30°) were analyzed. The experimental results together with best fits based on eq 1 are presented in Figure 7. The average values of the angle F determined for the R- and δ-phases are very close, i.e., 64° and 65°, respectively. Since the TDM of the π1* resonance is oriented perpendicular to the plane of the phenyl rings (see Figure 1), the value of the angle φ corresponding to the tilt angle of the biphenyl moiety is given by the following equation:28
sin φ )
cos F cos ϑ
(2)
where ϑ is the twist angle of the coplanar biphenyl moiety around the 4,4′ axis. Assuming a herringbone arrangement of the biphenyl moieties, which is typical for aromatic systems, and a twist angle ϑ as in bulk biphenyl20 (32°),36,69,70,73,74 φ values of about 31° and 30° were obtained for the R- and δ-phases, respectively. However, by a combination of NEXAFS and infrared reflection absorption spectroscopy (IRRAS) data in our previous study of the BPn (n ) 1-6) homologues,28 a value of ϑ ≈ 60° was estimated which yields φ values of about 61° and 58° for the R- and δ-phases, respectively. The NEXAFS data, thus, suggest very similar orientation of the BP2 molecules in both phases with an indication of a smaller tilt angle φ for the δ-phase. This is quite unexpected, since the lower density of the δ-phase would indicate, if any, an appositive change in the tilt angle, i.e., a higher value for the lower density δ-phase. A similar, but even more pronounced, effect was observed in our previous study of the R f β phase transition in the BP4 and BP6 system.36 Calculating the above value of the tilt angle φ (using eq 2), it was assumed that the twist angle ϑ remains unchanged during the phase transition, and therefore, we take the discrepancy described above as an indication that the simplifying assumption of a phase-independent constant value of ϑ is not valid. As discussed in more detail previously36 one can reasonably assume that the change in packing density by about 6% upon the R f δ phase transition is accompanied by a change in the twist angle ϑ of the phenyl rings. In addition (and in contrast to the previously reported R f β transition), the R f δ phase transition reported here leads to a reorientation of the herringbone structure relative to the Au(111) substrate (see the model in Figure 3), and thus, apart from change in the density this is another factor which may be associated with the change in the twist angle ϑ upon the phase transition. Contact Angle Measurements. To compare the relative stability of the R- and δ-phases of the BP2/Au(111) system against exchange by other thiols, a series of contact angle measurements was performed. For this purpose respective samples were incubated at room temperature in 1 mM ethanolic solution of ω-mercaptohexadecanoic acid (HS-(CH2)15COOH) for a given time. The drop in the contact angle value (toward the value of about 50° obtained for ω-mercaptohexadecanoic acid SAMs formed by adsorption on clean Au(111) substrate) was used to monitor the exchange process. The results obtained by this procedure are summarized in Figure 8. Comparison of the exchange process for the R- and δ-phases shows that the R f δ phase transition results in a pronounced improvement of the film stability against exchange by other thiols. 2-Phenylethane Thiol (C6H5(CH2)2SH, P2). Figures 9 and 10 summarize STM data obtained for the P2 SAMs prepared at room temperature from the respective solution and then investigated either as prepared or annealed at different temperatures. Images collected at larger scale are presented in Figure 9. For the samples prepared at room temperature (Figure 9ac), formation of an ordered structure has been observed with, however, some disordered regions located at the borders between the rotational domains (see Figure 9c). As documented by Figure 9, parts d and e, additional annealing at 373 K for 15 h leads to a significant increase in the size of the well-ordered domains, i.e., from 5-10 to 20-50 nm. The high-resolution STM data did not indicate any change in the packing of the molecules. At the same time, the density of the substrate depressions (black islands in the STM images) decreased due to Ostwald ripening as already mentioned above. The STM data shown in Figure 9, parts f and g, reveal that annealing at an even higher temperature
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Figure 7. Angular dependence of the π1* resonance intensity ratio I(θ)/I(30°) for the R (a) and δ (b) phase of BP2 SAMs on Au(111) samples (θ denotes the X-ray incidence angle). The experimental data are presented together with the best fits (solid line) according to eq 1. The values of the derived average TDM tilt angle (F) are given together with the respective fits.
Figure 8. Static water contact angles recorded for R (filled circles) and δ (open circles) phases of BP2 SAMs on Au(111) as a function of the incubation time in 1 mM ethanolic solution of ω-mercaptohexadecanoic acid at room temperature.
of 393 K (keeping the same annealing time of 15 h) leads to a partial destruction (starting from defects located at the boundaries of the rotational domains) of the film with, however, no structural changes in the remaining ordered areas. The highresolution image presented in Figure 10a reveals details of the P2 film structure. Similar to BP2 and other BPn systems, also in this case one can observe the appearance of densely packed rows of molecules (along the 〈112h〉 directions) exhibiting an alternating contrast in STM. As discussed above, this effect can be attributed to a herringbone-type arrangement of the phenyl rings. The orientation of the rows along the 〈112h〉 directions is the same as for BP2 and other BPn SAMs prepared at room temperature. The cross sections taken from this image (Figure 10b) reveal periodicity of about 11.7 ( 0.6 and 4.8 ( 0.4 Å in the 〈11h0〉 and 〈112h〉 directions, respectively (the averaged values and errors have been obtained from a set of five different images). These distances are very close to the corresponding values of 11.52 and 4.98 Å for the commensurate rectangular (4 × x3) lattice with an area per molecule of 28.7 Å2. A schematic molecular arrangement for the P2 SAM on Au(111) is shown in Figure 10c, with, as previously commented for the BP2 system, arbitrarily assigned adsorption sites. IV. Discussion The annealing behavior of BP2 and P2 SAMs exhibits remarkable differences. On one hand in BP2 SAMs a phase transition is observed which results in a structure distinctly different from the ones observed in SAMs made from the higher homologues BP4 and BP6. In P2 SAMs, on the other hand, no temperature-induced phase transition is observed. The substantial difference in the behavior of these closely related molecules
which differ only in the number of methylene spacer units or phenyl rings, respectively, illustrates the subtle balance of the factors which determine the energy and the structure of thiol SAMs. Microscopic and diffraction data show that thermal annealing of the BP2 system leads to an irreversible phase transition from the R-phase described by a rectangular (5x3 × 3) unit cell into the δ-phase which is based on a rectangular (2x3 × 2) lattice but which has very characteristic regular line defects. On the one hand, BP2 represents another manifestation of the pronounced odd-even difference in the annealing behavior of BPn SAMs on Au(111) with only n ) even exhibiting polymorphism as a consequence of the competitive design mentioned in the Introduction and discussed in detail before.35,36 On the other hand, the R-phase which is common to all three even-numbered BPn SAMs (n ) 2, 4, 6) studied so far evolves into a structure very different from those seen for BP4/BP6 (see the summary of even BPn/Au(111) structures shown in Figure 11). The latter two form very large domains with a perfectly homogeneous arrangement of molecules described by an oblique (6x3 × 2x3) unit cell,35,36 whereas BP2 realizes a defect-rich structure. It is, however, quite remarkable that the defects are very regular, that is, they occur as parallel line defects which run exclusively along the 〈112h〉 directions at a rather close distance of 8-11 unit cells (4.8-6.2 nm). Since the only difference between the molecules is the length of the alkane spacer this must account for the different structures of the δ- and β-phases. Thus, the thermally induced transformation to different structures for BP2 and BP4/BP6 highlights the vital importance of the spacer for the structure of SAMs beyond the odd-even effect. As revealed by studies on the homologue series of BPn SAMs with n ranging from 0 to 6 the spacer is crucial for coping with stress arising from the mismatch between the molecular lattice and the underlying substrate lattice.27 Whereas BPn SAMs with n < 2 yield poorly ordered SAMs, i.e., the stress is released in a rather random way by forming small, irregular domains, one obtains well-ordered layers for n g 2. We have suggested that the conformational degrees of freedom (in combination with relaxation processes at the S-Au interface27) are responsible for accommodating the mismatch of molecular and substrate lattices and, thus, reducing stress. For the phases generated by annealing this still holds for BP4 and BP6, whereas for BP2 it seems that there are limits to the extent to which the alkane spacer can cope with stress. Therefore, BP2 seems to represent a system located at the boundary between SAMs which cannot cope with stress other
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Figure 9. P2 SAMs on Au(111): STM images with different resolutions recorded for samples prepared at room temperature (a-c) and subsequently annealed in N2 atmosphere for 15 h at 373 (d and e) and 393 K (f and g). In (b) the white rectangle marks the area shown in (c). In (c) black lines mark the areas of defects in arrangement of molecules in the rectangular (4 × x3) lattice (marked by black rectangles) identified outside these defect regions.
than by forming defects/domain boundaries and SAMs which have “soft” ways of stress relief without breaking the molecular order. A most interesting aspect is that defect formation is highly ordered, i.e., the stress is highly anisotropic. This does not only result in well-oriented defect lines but induces a long-range order as illustrated by Figure 1, parts c and f. Unfortunately, since numerous factors determine the structure of a SAMs a reliable modeling of these systems (which at present does not exist) would be required to elucidate the relative importance of the different factors involved such as intermolecular interactions, conformational states, and the structure and energetics of the S-Au interface. At this state we can only say that the stress anisotropy might not be too surprising keeping in mind that the C-S-Au plane has preferred direction/orientation and that the packing of the biphenyl moieties in SAMs on Au(111) substrate requires an anisotropic deformation with respect to the preferred packing of biphenyl moieties in biphenyl crystal. We note that a related anisotropy effect has been observed in BP6 SAMs where annealing at temperatures low enough to avoid transition to the β-phase results in highly elongated domains.31 Concluding the comparison of BP2 SAM with its higher homologues, the R f δ phase transition in the BP2 system leads to a phase with a lower density similar to what has been observed for the R f β phase transition in the BP4 and BP6 systems. Despite the decreased density, which is about 30% lower compared to a densely packed (x3 × x3)R30°-type structure of alkanethiols, both the β- and the δ-phase are surprisingly stable against exchange by other thiols, in contrast to the R-phases of BP2 (Figure 8) and BP4/BP6.36 However, even though BP2 and BP4/BP6 SAMs are of similar stability, there is a remarkable difference in their molecular packing. The β-phase of the latter forms very large domains of a uniform structure, whereas BP2 is characterized by missing molecules along lines parallel to the 〈112h〉 direction. It is interesting that despite these defects the BP2 SAMs in the δ-phase are perfectly stable against exchange by other thiols or back-conversion to the R-phase upon extended immersion in BP2 solution. Also
thermal destruction of δ-phase at higher temperatures is not initiated from these defects but only from the step edges indicating high stability of the δ-phase. Although for BP4/BP6 kinetic stabilization cannot be completely ruled out it fits well to the interpretation that the phase transition is thermodynamically driven by lowering the energy related to stress and possibly restructuring of the interface.36 In this context it is noteworthy that, like the β-phase of BP4/BP6 SAMs, the δ-phase of BP2 SAM lacks any Moire´-type contrast variation in the STM images, in contrast to the R-phase. We now turn to an STM analysis of the temperature-induced changes in morphology as seen for the P2 SAMs. In this case, preparation at room temperature results in the formation of a rectangular (4 × x3) structure with an area per molecule of 28.7 Å2. This structure is not consistent with the (7 × x3) lattice reported in an earlier STM study of this system26 which corresponds to a structure with a much lower molecular density (50.3 Å2/molecule). The reason for this difference is not clear at present. It should be noted, however, that the preparation conditions were different and that the resolution of the previously reported micrographs was somewhat inferior to the data presented here. We would like to note that the long side of the rectangular (7 × x3) unit cell (20.16 Å) proposed in the previous study26 is very close to the distance between the black lines in Figure 9c, which corresponds to defect lines embedded in the rectangular (4 × x3) lattice. Such defects have been commonly observed by us for P2 SAMs prepared at room temperature (but not for the film annealed at 373 K, see Figure 9, parts d and e). Their presence is evidenced by a change in the STM contrast (dark lines in Figure 9a) and by a change in the arrangement of molecules (Figure 9c) visible in images taken at lower and higher magnification, respectively. Interestingly and independent of these differences in the observed structures [(7 × x3) vs (4 × x3)], the difference between P1/P3 SAMs26 exhibiting a (2x3 × x3)R30° structure and P2 SAMs is another manifestation of the odd-even behavior28 and, thus, the crucial influence of the SAM/substrate interface. Although the structures of Pn SAMs parallel the odd-even effect of BPn SAMs, the
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Figure 11. Summary of structures observed for even BPn SAMs on Au(111) substrate: (a) room-temperature R-phase observed for BP2, BP4, and BP6 characterized by a rectangular (5x3 × 3) and an area per molecule of 27.05 Å2; (b) β-phase observed for BP4 and BP6 after annealing (15 h at 423 K for BP4 and 24 h at 423 K for BP6) characterized by an oblique (6x3 × 2x3) unit cell and an area per molecule of 32.4 Å2; (c) δ-phase observed for BP2 after annealing (15 h at 417 K) characterized by a rectangular (2x3 × 2) unit cell and an area per molecule of 28.7 Å2; (d) schematic presentation of the relative size and orientation of R-, β-, and δ-phase.
Figure 10. P2 SAMs on Au(111). In (a) a high-resolution STM image of a sample prepared at room temperature and subsequently annealed in N2 atmosphere for 15 h at 373 K is shown. Unit cell and topographic cross sections A and B are marked in red. In (b) topographic cross sections A and B taken in (a) are presented. In (c) an illustration of the molecular arrangements of P2 SAMs on Au(111) is shown. Au atoms are marked by open circles. Adsorption sites of the P2 molecules (threefold hollow sites were arbitrary chosen) are marked by gray circles. The unit cell of the rectangular (4 × x3) lattice with an area per molecule of 28.7 Å2 is marked by a black rectangle. The schematic arrangement of the phenyl rings in the herringbone pattern is also marked (for clarity the tilt angle of molecules is not included).
most important finding in the present STM study of P2 SAMs is that, in contrast to the even-numbered BPn systems, no thermally induced phase transitions could be observed. As for odd-numbered BPn SAMs, only an increase in the size of domains was seen before a gradual destruction of the film structure upon further increasing the annealing temperature. The comparison of BP2 SAMs with P2 SAMs clearly reveals the crucial importance of an additional phenyl ring for the properties of the SAMs, in particular with respect to its annealing properties. This conclusion is consistent with a recent study of terphenyl-substituted alkanethiols [C6H5(C6H4)2(CH2)nSH, TPn, n ) 1-6] on Au(111) substrates. Preparation of a TP6 SAM at elevated temperature75 shows the formation of the β-phase already at 333 K, whereas in the corresponding experiment with BP6 the onset of the transition is seen only at a significantly higher temperature of 393 K.
Besides the odd-even effect, which is seen not only for Pn and BPn SAMs but also persists in TPn SAMs,73,75 the phenyl ring dependent polymorphism is another manifestation of how sensitively the structure of a SAM depends on the various factors entering into its energy balance. As outlined in the Introduction, this balance is determined by partially competing factors31,35,36 and is affected significantly by adding another aromatic moiety. As a consequence of the increased contribution of phenylphenyl intermolecular interactions the energy landscape is substantially altered, and new pathways for energy optimization reflected by phase transitions open up. In general, (small) changes of the molecular structure in the organothiol used to fabricate a SAM can result in structural transitions either by lowering the activation barrier for the transition from the initial to the final structure or by changing the energy difference between two different structures. The former implies that the transition is kinetically controlled. If the activation energy for the phase transition becomes too high, the competing channel of thermal desorption will open upon raising the temperature and the phase transition may cease to be a viable route. This may be the case for the P2 SAMs. The latter is a thermodynamic argument, and the detailed contributions to the change in total energy, which result from a complex interplay between intermolecular interactions, molecular degrees of freedom, and structural effects at the SAM-substrate interface, are particularly difficult to predict. Most likely both types of mechanism play a role, and the difference in the transition temperature between BP6 and TP6 suggests a change in the transition state. However, as we have pointed out previously,35,36 for a full understanding of the polymorphism in BPn SAMs it is necessary to understand the extent to which the energetics of the interface plays a role and how it is affected by the molecular structure. The need to understand these issues
16918 J. Phys. Chem. C, Vol. 111, No. 45, 2007 in detail is, first, highlighted by the crucial influence of the molecular structure on the energetics of the interface as revealed by ion- and electron-induced desorption experiments.46 In this study a higher stability of the Au-S and S-C bonds was found for P2 SAMs compared to BP2 SAMs. Second, the change of step directions and the disappearance of stress upon phase transitions is another sign that restructuring of the interface is one of the key factors toward the understanding of the phase transitions.35,36 Third, two recent studies of alkanethiols which found a significant deviation of the thiol-Au interface from the bulk-terminated Au(111) structure is another piece of evidence illustrating the importance to understand the SAMAu interface.56,57 V. Conclusions The structure of BP2 SAMs obtained after annealing is remarkable since, to the best of our knowledge, it is the first example of a thiol SAM where defects form in such a welldefined way. In the series of homologue BPn SAMs, BP2 appears as a boundary case between n < 2 which do not form well-ordered layers and n ) 4, 6 where defect-free structures are formed. A comparison of the new high-temperature phase of the BP2 system with the corresponding ones of BP4 and BP635,36 demonstrates that the details of such phase transitions with respect to transition temperature, time, and resulting structure are the result of a subtle balance of factors. Nevertheless, despite the differences between the structures of BP2, BP4, and BP6, the occurrence of phase transitions across the range of CH2 units and results for TP675 show that this is a general phenomenon which occurs for a large number of related compounds. However, the lack of such a transition in P2 SAMs demonstrates that the relative magnitude of the factors entering the energy balance is crucial. This can be expected to be of particular importance for the competitive design. A too unbalanced contribution of the various factors causes the SAM structure to be dominated by one (or more cooperatively acting) factor(s) with the consequence that one structure is energetically strongly favored and phase transition impeded. It will be interesting to see how far the concept of competitive design can be carried with respect to structural variations and controlled introduction of defects. Acknowledgment. This work was supported by The Leverhulme Trust, German Science Foundation, EPSRC, and SHEFC. P.C. was a Postdoctoral fellow of the Alexander von Humboldt Foundation. This work has been supported by the German BMBF (05KS4VHA/4) and European Community (Access to Research Infrastructure action of the Improving Human Potential Program). References and Notes (1) Ulman, A. Acc. Chem. Res. 2001, 34, 855-863. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1170. (3) Fan, F. R. F.; Lai, R. Y.; Cornil, J.; Karzazi, Y.; Bredas, J.; Cai, L. T.; Cheng, L.; Yao, Y.; Price, D. W.; Dirk, S. M.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2004, 126, 2568-2573. (4) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904-907. (5) Tran, E.; Duati, M.; Ferri, V.; Mullen, K.; Zharnikov, M.; Whitesides, G. M.; Rampi, M. A. AdV. Mater. 2006, 18, 1323-1328. (6) Joachim, C.; Ratner, M. A. Nanotechnology 2004, 15, 1065-1075. (7) Katsonis, N.; Kudernac, T.; Walko, M.; Molen, S. J.; Wees, B. J.; Feringa, B. L. AdV. Mater. 2006, 18, 1397-1400.
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