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X-ray Studies of Self-Assembled Monolayers on Coinage Metals. 2. Surface Adsorption Structures in 1-Octanethiol on Cu(111) and Ag(111) and Their Determination by the Normal Incidence X-ray Standing Wave Technique Hugh Rieley,*,† Gary K. Kendall,†,‡ Robert G. Jones,§ and D. Phillip Woodruff| Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, U.K., Department of Chemistry, University of Nottingham, Nottingham NG7 2RD, U.K., and Department of Physics, University of Warwick, Coventry CV4 7AL, U.K. Received April 12, 1999. In Final Form: July 30, 1999 The adsorption structures of self-assembled monolayers (SAMs) formed at saturation coverages of 1-octanethiol SAMs, prepared under ultrahigh vacuum (UHV) conditions on Cu(111) and Ag(111) substrates, were measured using a normal-incidence X-ray standing wave (NIXSW) technique. A thorough and pedagogical exposition of the relationship between the structural parameters determined in NIXSW and a number of simple adsorption structures is presented. In the two SAMs studied, the NIXSW data indicated the formation of monolayers which induced a reconstruction of the outermost layer of the substrate. SAMs on Cu and Ag were indistinguishable using the NIXSW technique, but given the different steric requirements imposed by each surface it is proposed that a different adsorption geometry is adopted in each case, which is either incommensurate with the underlying substrate or commensurate on a large mesh. In the case of Ag(111) the NIXSW data are consistent with the formation of a distorted (x7×x7)R19.1° structure, proposed by other workers. In the case of Cu(111), it is proposed that the adsorbate structure is most likely a significant distortion of the (x3×x3)R30° structure observed in SAMs of alkanethiols on Au(111) and we speculate on the existence of an overlayer of CH3-(CH2)7-S-Cu3 clusters.
1. Introduction Despite intense research over the past decade into the properties of organosulfur self-assembled monolayers (SAMs), the precise nature of the sulfur/metal atom interaction remains unresolved. Addressing this problem is central to the development of many applications, since the macroscopic chemical and physical properties of SAMs are ultimately determined by forces acting at the molecular level. The proximity of adjacent headgroups, for example, will affect the degree of interaction between neighboring hydrocarbon chains. The interchain van der Waals and electrostatic forces will in turn play a part in defining the packing and ordering, and thus the surface chemistry, of the monolayer. Determining the adsorption geometry of the S head atoms is therefore crucial to the understanding of SAMs. The majority of the work to date has been concerned with SAMs of long-chain alkanethiols on gold, due in part to the inert nature of the substrate which facilitates straightforward sample preparation under ambient conditions. Sellers et al. performed ab initio calculations for the adsorption of SCH3 on Au and Ag surfaces, which indicated that sulfur energetically favors 3-fold hollow sites.1 The distance of the S atom from the surface plane was calculated to be 1.905 Å on Au(111) and 2.332 Å on Ag(111). An electron diffraction study of 1-docosanethiol on Au* Corresponding author. Present address: Unilever Research Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, U.K. † University of Liverpool. ‡ Present address: Mobil Oil Co. Ltd, Research and Technical Services Department, Stanford-le-Hope, Essex SS17 9LN, U.K. § University of Nottingham. | University of Warwick. (1) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.
(111) by Strong and Whitesides2 suggested that the S atoms were arranged in a hexagonal lattice, seemingly incommensurate with the substrate, in which the interchain spacing was 4.97 Å. Their results were reinterpreted as being indicative of a commensurate (x3×x3)R30° structure,3 placing S atoms either in all hexagonal close packed (hcp) or all face-centered cubic (fcc) 3-fold hollow sites. This model became widely accepted following supporting evidence from He diffraction,4-7 scanning tunneling microscopy (STM),8,9 atomic force microscopy (AFM),10 low energy electron diffraction (LEED),11 and X-ray diffraction (XRD)12 studies. The STM results, however, were called into question by Kim et al.13 when, in a study of ω-substituted thiols, it was found that a (x3×x3)R30° structure could be detected even when the size of the substituent group dictated otherwise. For example, the STM image of [2]staffane-3,3′-dithiol pentaamineruthenium(II), with a physical diameter of 7.0 Å imposed by the Ru(NH3)5 group, exhibited a nearest neighbor spacing of 4.8 ( 0.3 Å, consistent with the (2) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (3) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (4) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (5) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (6) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (7) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (8) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (9) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (10) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (11) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (12) Samant, M. G.; Brown, C. A.; Gordon, J. G., II Langmuir 1991, 7, 437. (13) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992, 96, 7416.
10.1021/la9904253 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/15/1999
X-ray Studies of SAMs on Coinage Metals
(x3×x3)R30° structure. It was speculated that the observed images were of the electronic distributions of Au atoms, perturbed by the adsorbed thiol, since STM only measures electron density and is insensitive to chemical identity. The (x3×x3)R30° structure has further been queried by a grazing-incidence X-ray diffraction (GIXD) study14 in which it appeared that thiols actually self-assemble as disulfide units. As a result, the S...S interatomic distance would necessarily be around 2.2 Å, in contrast to the ∼5 Å expected for a (x3×x3)R30° lattice. A further consequence of disulfide adsorption is that there must be (at least) two structurally and perhaps chemically distinct S species present in alkanethiolate monolayers on Au, since there are no equivalent high-symmetry sites on the (111) surface which lie 2.2 Å apart. In support of this conclusion, Zubra¨gel et al. performed an X-ray photoelectron spectroscopic (XPS) investigation of thiols on Au and Ag,15 in which the experimental data appeared to confirm the existence of two inequivalent S atoms. However, as discussed in part 1 of this series of work,16 the analysis of these results appears to be unsound. In addition to the work carried out on Au, thiol adsorption structures have been investigated on Ag and Cu surfaces. Given that the (111) nearest neighbor spacings for Au, Ag, and Cu are 2.88, 2.89, and 2.56 Å respectively,17 one might predict that SAM structures would appear similar on Au and Ag but differ significantly from those on Cu. However, a combined GIXD and He diffraction study of 1-octadecanethiol on Ag(111)18 revealed a SAM structure which was incommensurate and rotated with respect to the substrate. The hexagonal structure was found to have a lattice spacing of 4.67 ( 0.23 Å, similar to that of n-alkane chains in bulk hydrocarbon crystals. This suggests that the dominant force in the formation of a close-packed SAM on Ag is the chain-chain interaction. Two domains were formed, each rotated by (18° from the (x3×x3)R30° structure determined for 1-octadecanethiol on Au in a similar experiment.4 Dhirani et al. reported a similar structure in an STM study of 1-decanethiol on Ag(111), which they rationalized as a “distorted” (x7×x7)R10.9° monolayer lattice.19 It transpires that this structure is incorrectly labeled and we show later (Figure 10) that the correct definition of the intended adsorbate structure is (x7×x7)R19.1°swe shall continue to use the R19.1° label throughout this paper. This difference in SAM structure on Ag and Au was further evidenced by Laibinis et al., in which contact angle measurements and reflection-absorption infrared spectroscopy (RAIRS) indicated that SAMs on Ag were structurally more closely related to those on Cu.20 To examine the nature of SAM structures, it is useful to know precisely where the S atoms reside with respect to the substrate. The normal-incidence X-ray standing wave (NIXSW) technique has been used several times to identify adsorption geometries of sulfur-containing compounds on various substrates, e.g., mercaptide on Cu21 (14) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 226, 1216. (15) Zubra¨gel, C.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Wo¨ll, C. Chem. Phys. Lett. 1995, 238, 308. (16) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S.-H. Langmuir 1998, 14, 5147. (17) Kittel, C. Solid State Physics, 5th ed.; Wiley: New York, 1976. (18) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., III; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013. (19) Dhirani, A.; Hines, M. A.; Fisher, A. J.; Ismail, O.; GuyotSionnest, P. Langmuir 1995, 11, 2609. (20) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.
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and Ni,22 and atomic sulfur on Rh.23 Until now, this method has not been applied to investigate the geometry of alkanethiol SAM adsorption, although the aforementioned mercaptide (H3C-SH) is structurally related. NIXSW in conjunction with surface-extended X-ray absorption fine structure (SEXAFS) of mercaptide on Cu(111) indicated that the sulfur atom partially penetrated the outermost Cu atomic layer.21 To explain this observation, a model was proposed in which the S atom of mercaptide occupies the hcp 3-fold hollow sites, with a movement of neighboring Cu atoms parallel to the surface of 0.6 Å. The SEXAFS data suggested a layer spacing of S relative to the outer layer Cu of 1.2 Å. In this work the SAMs of 1-octanethiol on Cu(111) and Ag(111) have been examined using the NIXSW technique in order to assess the role of the substrate in determining the adsorption structures. A detailed account of the relationship between the structural parameters obtained in NIXSW and some likely common adsorbate structures is presented. The data obtained for the two systems studied are consistent with structures in which sulfur does not adsorb exclusively at 3-fold hollow sites. It is proposed that the geometry adopted in each case involves a reconstruction of the substrate. It will be shown that the monolayers formed on Cu(111) and Ag(111), although indistinguishable on the basis of the NIXSW data, are likely to adopt different detailed adsorption structures. 2. Normal Incidence X-ray Standing Wave (NIXSW) X-ray standing wave is a technique which can be used to deduce the adsorption geometry of adsorbate atoms on a single-crystal substrate.24-26 Its basis lies in the establishment of a standing wave on periodic lattice planes in the substrate. When an X-ray Bragg reflection is established in a crystal, the diffracted wavefield interferes with the incident wavefield to produce a standing wave, the periodicity of which is equal to the spacing of the scatterer planes. In the simplest case of a single diffracted beam and a nonabsorbing crystal, the result is a classical twobeam interference problem, in which the intensity varies spatially between zero and four times that of the incident beam [Io(1 ( 1)2, where Io is the incident X-ray intensity]. In reality, the incident X-ray wavefield is attenuated as it penetrates the substrate, and more flux is removed through backscattering from the surface. This is taken into account in the full dynamical analysis by Batterman,27 which shows that the finite penetration leads to a finite range in incidence angle or wavelength over which the standing wave is produced. Within this range, the phase of the standing wave shifts by half the bulk layer spacing, in a systematic fashion with respect to the scatterer planes. It is therefore possible, through the monitoring of X-ray absorption by atoms of interest, to deduce the location of these atoms relative to the (extended) bulk substrate planes. X-ray absorption may be measured in terms of photoemission, Auger electron emission, or X-ray fluo(21) Prince, N. P.; Seymour, D. L.; Woodruff, D. P.; Jones, R. G.; Walter, W. Surf. Sci. 1989, 215, 566. (22) Fernandez, A.; Espinos, J. P.; Gonzalez-Elipe, A. R.; Kerkar, M.; Thompson, P. B. J.; Lu¨decke, G.; de Carvalho, A. V.; Woodruff, D. P.; Fernandez-Garcia, M.; Conesa, J. C. J. Phys. Condens. Matter 1995, 7, 7781. (23) Mercer, J. R.; Scarel, G.; Santoni, A.; Cowie, B. C. C.; Lewis, D.; Robinson, A. W.; McGrath, R.; Dhanak, V. R. Surf. Sci. 1996, 369, 36. (24) Zegenhagen, J. Surf. Sci. Rep. 1993, 18, 200. (25) Materlik, G.; Zegenhagen, J. Phys. Lett. 1984, 104A, 47. (26) Malgrange, C.; Ferret, D. Nucl. Instrum. Methods Phys. Res., Sect. A 1992, 314, 285. (27) Batterman, B. W. Phys. Rev. 1964, 133, 759.
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rescence, as the incident energy is scanned through the Bragg energy. Since X-ray absorption is maximized when an antinodal plane of the standing wave coincides with an absorber atom, the intensity of a related emission as a function of the incident energy will determine the relative position of the absorber. Applying the X-ray standing wave to a second set of reflection planes which have a known spatial relationship to the first, it is possible to triangulate the position of the absorber precisely with respect to the atoms of the substrate surface.28 This has been successfully performed by Kadodwala et al. in the elucidation of the adsorption structure of Cu(111)-(x3×x3)R30°-Cl/Br.29 For each system, mixed site occupancy was observed: for Cl the fcc:hcp population ratio was 3:1, while for Br the ratio was 4:1. At normal incidence, the Bragg condition passes through a turning point in its dependence on incident angle (2DH sin θ ) λ at θ ) 90°, where DH is the periodicity of the reflecting planes) and is therefore relatively insensitive to small variations in incident angle due to crystal mosaicity and to limited incident-beam collimation.30 As such, the method is applicable to metal single-crystal substrates. In the NIXSW experiment it is essential to record, through the intensity of Auger emission or photoemission, the net X-ray absorption by the absorber atom. It is therefore necessary to measure the secondary electron background close in energy to the emission signal monitored. For example, when S adsorption structures on Cu(111) are studied, for which the Bragg energy is 2967 eV, the S 1s photoelectron feature at ∼505 eV (“on-peak”) is monitored together with the background signal at, say, 511 eV (“off-peak”). As such, the net X-ray absorption by S atoms may be distilled from fluctuations in the background intensity. It is here that problems arise when attempting to perform XSW measurements on S/Au systems. At 2631 eV, the Bragg energy for Au(111), there are no S Auger electron or photoelectron features which are energetically remote from substrate peaks. For example, the AuMNN signal at 2111 eV interferes with the monitoring of changes in the SKLL intensity at 2110 eV. In addition, the S 1s and SLMM features, at 155 and 146 eV, respectively, conflict with each other as the incident photon energy is scanned through the range (10 eV of the Bragg condition. It is for this reason that copper and silver were chosen as the substrates in this study. The NIXSW profiles obtained can be modeled, using a computerized fitting procedure, to yield two structural parameterssthe coherent position, dhkl, and the coherent fraction, fhkl, where hkl refers to the Miller index of the bulk scatterer planes employed. Equation 1 provides the theoretical basis of the fitting procedure and illustrates how dhkl and fhkl govern the shape of the XSW absorption profile.28
(
I ) 1 + R + 2fhkl xR cos φ -
)
2πdhkl DH
(1)
where R is the reflectivity of the substrate, φ describes the phase relationship between the incident and reflected waves, and DH is the layer spacing of the hkl scattering planes. (28) Woodruff, D. P.; Cowie, B. C. C.; Ettema, A. R. H. F. J. Phys. Condens. Matter 1994, 6, 10633. (29) Kadodwala, M. F.; Davis, A. A.; Scragg, G.; Cowie, B. C. C.; Kerkar, M.; Woodruff, D. P.; Jones, R. G. Surf. Sci. 1995, 324, 122. (30) Woodruff, D. P.; Seymour, D. L.; McConville, C. F.; Riley, C. E.; Crapper, M. D.; Prince, N. P.; Jones, R. G. Phys. Rev. Lett. 1987, 58, 1460.
Figure 1. Argand diagram representation of a single adsorption site z. The vector direction is defined by the phase angle 2πz/DH, while the magnitude f(z) is the probability of adsorption occurring at this position.
The values obtained for dhkl and fhkl are an average over all adsorbate positions existing within the area sampled by the X-ray beam. As such, these parameters are the convoluted result of a number of different effects, such as disorder due to thermal vibrations, areas of noncrystallinity, or perhaps the existence of any number of discrete adsorption sites. Using the Argand diagram representation devised by Woodruff et al.,28 it is possible to deconvolute these effects graphically, to elucidate the surface adsorbate structures. The basis of this analysis is found in eq 2, which shows the relationship between the measured quantities fhkl and dhkl, and the real spatial distribution of the adsorbate
(
fhkl exp
)
2πidhkl ) DH
∫0d
H
( )
f(z) exp
2πiz dz DH
(2)
where z is the position of an adsorption site with respect to the (extended) bulk scatterer planes. Effectively, each individual adsorption site can be represented by a vector, whose direction is defined by the phase angle 2πz/DH relative to the positive x-axis, while the modulus f(z) is the probability of the adsorbate occupying this position. The resultant vector sum is an integration over all adsorption sites, represented by a vector of length fhkl, and phase angle 2πdhkl/DH, such as that presented in Figure 1. In the discussion for the NIXSW data, extensive use will be made of the Argand diagram method, as results are analyzed and possible solutions are discussed. 3. Experimental Section 3.1. Experimental Apparatus. The experiments were conducted on Beamline 6.3 of the Daresbury Laboratory Synchrotron Radiation Source (SRS), Warrington, U.K. The SRS operated at an energy of 2 GeV and a ring current typically in the range 150-200 mA. With a twin Ge(111) crystal monochromator, the energy resolution was less than 1 eV in the energy region spanned by the Cu and Ag Bragg peaks of interest. The use of a chromatic premirror system comprising a plane and cylindrical mirror, adjusted to expose reflective surfaces of quartz and Cr, respectively, to the incident beam ensured suppression of higher order contributions to a negligible amount. The ultrahigh vacuum chamber in which experiments were conducted was equipped
X-ray Studies of SAMs on Coinage Metals
Figure 2. Section taken at beam level of UHV chamber used during NIXSW experiments. with liquid nitrogen cooling and resistive heating of the sample stage, argon ion gun, low-energy electron diffraction (LEED), and a cylindrical mirror analyzer (CMA) for Auger electron spectroscopy (AES). A hemispherical analyzer (HA) was employed for recording XSW data. In addition, a mass spectrometer was used both to check for leaks and to identify molecules entering the chamber during thiol dosing. A cross section through the ultrahigh vacuum (UHV) chamber, in the plane of the beam, is illustrated in Figure 2. 3.2. 1-Octanethiol on Cu(111). The copper substrate was prepared by a series of sputter and anneal cycles (2.6 kV sputter at a partial pressure PAr+ ) 3 × 10-5 mbar, followed by annealing to 450 °C for 10 min). Surface contaminants were monitored using AES, while LEED was employed to assess the crystallinity of the surface. Sputter/anneal cycles were repeated until a sharp centered hexagonal LEED pattern had been achieved, together with sufficient surface cleanliness (i.e., absence of oxygen, carbon, or sulfur Auger signals). The 1-octanethiol, stored in a glass finger on the gas-line, was connected to the main chamber via a leak valve. The liquid was degassed by several freeze-pump-thaw cycles, before dosing the Cu(111) crystal at a thiol partial pressure of 10-7 mbar. The required dosing period to achieve a monolayer coverage was determined, in a separate experiment, by observing the stabilization of the SLMM Auger signal relative to that of CKLL. The relative intensities of these two signals were dependent on the structure of the monolayer but remained constant after approximately 30 min. Energy distribution curves (EDCs) were recorded over the electron kinetic energy region 100-3000 eV prior to setting up each X-ray standing wave experiment, for two reasons: first, to check the surface for contaminants, and second, to locate an appropriate Auger or photoelectron signal for the XSW. Higher resolution EDCs were subsequently recorded over selected regions in order to determine the precise “on-peak” and “off-peak” energies to be studied. The X-ray standing wave was set up first at normal incidence to the (111) planes by tuning the monochromator to 2967 eV, the Bragg energy for copper at room temperature. The position of the reflected beam was monitored on a phosphorcoated screen, through which the incident beam had entered the chamber. The sample was rotated with the sample approximately 0.5° off-normal in order to avoid interference of the incident and reflected beams, which has an adverse effect on the beam intensity monitor. In setting up the (1 h 11) reflection, the sample was rotated about the vertical axis by 70.5°, before altering the azimuthal angle until the Bragg reflection was just off-normal, as before. Changes in the Cu 2p photoelectron yield were measured at energies (10 eV of the Bragg condition. The resultant data were least-squares fitted in order to assess the crystallinity of the substrate and to ascertain the energy broadening of the monochromator, parameters which were subsequently held constant. The intensity of the S 1s photoelectron emission was probed over the same incident energy range in order to determine the displacement of sulfur atoms with respect to each plane. Upon
Langmuir, Vol. 15, No. 26, 1999 8859 completion of the standing wave measurements the sample was cleaned, and a fresh monolayer prepared in an identical fashion in order to perform repeat experiments. A further XSW study of the 1-octanethiol/Cu(111) system involved cooling the sample, via the liquid nitrogen feed-through, to a temperature of approximately -140 °C, for the duration of the data collection. This had the effect of reducing the (111) plane layer spacing, due to crystal contraction, in turn increasing the Bragg energy by approximately 9 eV. The desired effect of this cooling process was to minimize contributions from thermal vibrations. 3.3. 1-Octanethiol on Ag(111). For the most part, the procedure adopted for experiments conducted on silver followed closely that of the copper study. Substrate cleaning proved slightly more difficult, and removal of surface sulfur was chief among the problems encountered. It was found that simply annealing the crystal did not sufficiently deplete the sputtered surface of sulfur atoms. A number of alternatives were explored, including holding the sample at 300 °C during Ar+ ion bombardment prior to annealing. It became apparent, however, that much of the sulfur contamination originated from the bulk of the Ag(111) crystal. By “flashing” the sample to 550 °C (i.e., heating very rapidly, by means of an electron-beam heater, and cutting power at the required temperature rather than maintaining for several minutes), this effect was minimized. A retractable stainless steel capillary allowed directional dosing of the thiol molecule. A further function of the capillary was to increase the effective dosage at the sample while minimizing the exposure of the main chamber components to organic molecules. It was found that saturation of the SLMM Auger signal occurred after 30 min at a thiol partial pressure of 1 × 10-8 mbar, an order of magnitude lower than that obtained in the previous copper experiments. NIXSW data were obtained by monitoring the intensities of the AgMNN and SKLL Auger electron emissions since both of these features remained free of interference from photoemission features over the range of incident photon energy.
4. Results The structural parameters fhkl and dhkl were determined via an automated nonlinear least-squares fitting procedure. Error limits were estimated by adopting the same fitting procedure for several data sets and then quoting the standard deviation about the mean over all data sets taken in each experiment. 4.1. 1-Octanethiol on Cu(111). Figure 3 shows X-ray standing wave profiles obtained for the Cu 2p and S 1s photoelectron peaks at room temperature in the (111) reflection. Best-fit curves are included, along with the structural parameters which describe them. The (nonstructural) energy broadening parameter was found, using the Cu 2p signal, to be 0.71 ( 0.07 eV. Since this parameter was inherent to the beamline, it was held constant throughout the subsequent fitting procedures. It is clear from the values obtained for the structural parameters that the substrate (Figure 3a) exhibited a high degree of crystalline order with respect to the (111) planes, with a best-fit mosaic width of 0.1°, a coherent fraction of f111 ) 0.97 ( 0.06, and a coherent position of d111 ) 2.12 ( 0.21 Å, i.e., within error equal to the Cu(111) bulk layer spacing of 2.08 Å. The adsorbate (Figure 3b) fitted to a considerably lower coherent fraction, f111 ) 0.67 ( 0.10, with a coherent position, d111 ) 1.21 ( 0.12 Å. The data recorded in the (1 h 11) reflection are illustrated in Figure 4. The Cu 2p profile (Figure 4a) agrees well with that in the (111) case, having a high coherent fraction, 0.90 ( 0.12, along with a coherent position of 2.12 ( 0.12 Å. It is immediately apparent that the S 1s (1 h 11) trace (Figure 4b) differs markedly from the (111) result and in fact exhibits an almost zero coherent fraction. It was possible to fit the data to the structural parameters of f-111 ) 0.10 ( 0.12, d-111 ) 1.73 ( 0.42 Å, as illustrated. The uncertainties quoted in each parameter were deter-
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Figure 3. NIXSW of Cu 2p (a) and S 1s (b) in (111) reflection, including best-fit curves and structural parameters.
Figure 4. NIXSW of Cu 2p (a) and S 1s (b) in (1h 11) reflection, including best-fit curves.
mined by fixing each parameter in turn in order to estimate the precision in the other, which assumes the errors are uncorrelated. However, when the coherent fraction is low,
Rieley et al.
Figure 5. NIXSW of 1-octanethiol on Ag(111) in (111) reflection: (a) AgMNN signal; (b) SKLL signal.
it should be noted that this is not the case and the shape of the profile is particularly insensitive to the value of the coherent position. It was found that cooling the sample made no noticeable difference to the form of the standing wave profile, with the exception of the Bragg energy shift remarked upon above. The obtained data were fitted to the same structural parameters (within error) as those recorded at room temperature. 4.2. 1-Octanethiol on Ag(111). Figure 5 shows the XSW data in the (111) reflection for 1-octanethiol on Ag(111). The substrate trace (Figure 5a) fitted to the structural parameters f111 ) 0.75 ( 0.12, d111 ) 2.38 ( 0.09 Å, but with the modification that the best-fit mosaic width of the crystal was 1.0°, indicating a specimen of poorer crystallinity than the Cu used in the previous experiment. It is important to note that despite the observed reduction in f111, the average atomic position, d111, was close to the Ag(111) bulk layer spacing of 2.36 Å. The adsorbate XSW profile, Figure 5b, exhibits a coherent fraction, f111, of 0.54 ( 0.12, with the coherent position, d111, equal to 1.39 ( 0.19 Å. The (1h 11) NIXSW data for the Ag substrate, illustrated in Figure 6a, is best described by a theoretical curve whose structural parameters are f-111 ) 0.72 ( 0.14 and d-111 ) 2.36 ( 0.09 Å, i.e., within the error of the curve describing the corresponding (111) data. In the case of the 1-octanethiol adsorbate, Figure 6b, the very low coherent fraction and poor signal-to-noise ratio render these data difficult to fit with accuracy. Illustrated is the theoretical trace described by the parameters f-111 ) 0.11 ( 0.12 and d-111 ) 1.94 ( 0.42 Å. The previous comments about the lack of significance of coherent position at low coherent fraction apply.
X-ray Studies of SAMs on Coinage Metals
Langmuir, Vol. 15, No. 26, 1999 8861 Table 1. Fractional Structural Parameters of 1-Octanethiol on Cu(111) and Ag(111) d111 f111 d-111 f-111
Figure 6. NIXSW of 1-octanethiol on Ag(111) in (1 h 11) reflection: (a) AgMNN signal; (b) SKLL signal.
5. Discussion 5.1. Integrity of the Adsorbate. It is important to state that in the experiments reported here there was no evidence of X-ray induced damage to the self-assembled monolayers formed by adsorption of 1-octanethiol on Cu(111) or Ag(111). Other workers have used X-ray techniques to support their evidence for the existence of two chemically and geometrically distinct adsorption sites for sulfur15 or for the existence of disulfide moieties at the metal surface.14 There has been some debate, however, on the validity of these conclusions since, in the former example, either beam damage or interdigitation of the adsorbate could provide an alternative explanation for the observations,16 while in the latter case, a later study revealed evidence for disulfide cleavage on adsorption.31 To reinforce the uncertainty in these conclusions, other workers have noted evidence for X-ray beam damage of SAMs on Ag,32 and there is some recent evidence that SAMs on Ag are much more sensitive to X-ray damage than are SAMs on Au. Such damage could lead to incorrect conclusions on both the geometric and chemical structure of SAMs. It is crucial, therefore, that the structural interpretation which follows is not the erroneous result of X-ray damage to the monolayer. Our evidence for the preparation and retention of a stable monolayer is as follows. In the preparation of the monolayers the ratio of the SLMM Auger signal relative to that of CKLL remained constant at saturation coverage and there was no evidence, in the mass spectrum of residual gases in the experimental (31) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261. (32) Rieke, P. C.; Baer, D. R.; Fryxell, G. E.; Engelhard, M. H.; Porter, M. S. J. Vac. Sci. Technol., A 1993, 2292.
S/Cu(111)
S/Ag(111)
0.58 ( 0.06 0.69 ( 0.10 0.83 ( 0.20 0.11 ( 0.13
0.59 ( 0.08 0.72 ( 0.19 0.82 ( 0.18 0.15 ( 0.17
chamber, for the desorption of adsorbate fragments from the surface. The NIXSW spectra of the substrates and sulfur of the adsorbate were also very reproducible over several hours of exposure to the synchrotron beam. These observations confirm that adsorbed species were retained at the surface and retained their structural integrity at the surface. The structural integrity of the pendant adsorbate was confirmed both by the position of the S-K edge (2742 eV), which was consistent with the thiolate, S(-II), species characteristic of SAMs on Au.33 Furthermore, the existence and persistence of an angularly dependent near edge X-ray absorption fine structure (NEXAFS) confirmed the integrity of the chemical linkage between the adsorbed sulfur atom and the pendant hydrocarbon chain. This is the subject of detailed study and discussion in part 3 of this work.34 5.2. Comparison of 1-Octanethiol on Cu(111) and Ag(111). Comparison of the results obtained for 1-octanethiol on copper and silver reveals that the structural parameters vary markedly between the two systems. The significant difference between the substrate coherent fractions is attributed to the fact that the Ag(111) crystal used was of poorer crystallinity (a higher mosaic width was necessary to achieve accurate fitting of these NIXSW spectra) than its Cu counterpart. Since the X-ray standing wave is defined by the periodicity of the substrate, any loss of coherence of the substrate will translate to a similar reduction in fhkl of the adsorbate. Therefore, when comparing the adsorbate data for each system, it proves to be instructive to calculate fhkl as a fraction of the corresponding substrate value. For example, the fractional value of f111 for the adsorbate on Ag(111) is 0.54/0.75 ) 0.72, while on Cu(111) it is 0.67/0.97 ) 0.69. A similar calculation of the fractional dhkl value for each substrate potentially allows comparison of relative S-metal spacings. The fractional structural parameters for the adsorbate, summarized in Table 1, reveal striking similarities between the two systems. The fractional values of fhkl and dhkl are the same, within error, on Cu and Ag. The former observation suggests that, in each case, the adsorbate S-atom layer structures exhibit the same degree of coherence with respect to the substrate. A possible explanation is that the two adsorbate structures are the same. However, the fractional coherent positions also appear to be the same within error, and this not only is surprising but is inconsistent with this conclusion. Since the values of d-111 are, individually, rather indeterminate, it is fortuitous (and probably misleading) that their values ratioed to the substrate are the same. The fractional values of d111 cannot be discounted so easily since the component adsorbate coherent positions are rather well determined. This would suggest that the S-metal layer spacing scales with the metal layer spacing in the 〈111〉 direction. If the adsorbate structures were the same, then this would dictate that the effective radius of the S atom scales with that of the metal; this does not tally with typical S-metal (33) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: London, 1991. (34) Rieley, H.; Kendall, G. K. Langmuir 1999, 15, 8867.
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bond lengths found by other methods.35 Although the NIXSW data are apparently unable to distinguish between the adsorbate structures of 1-octanethiol on Ag(111) and Cu(111), a possible scenario is that the adsorbate structures, although similar, are subtly different due to the different steric requirements imposed by each surface. In the remainder of the discussion we develop a model for the adsorbate structure of alkanethiols on Cu(111) and Ag(111) surfaces which satisfies these requirements. It will concentrate initially on the higher quality (better signal/noise) data obtained for 1-octanethiol on Cu before considering any modifications which may be required in the case of 1-octanethiol on Ag(111). 5.3. Ascertaining the Adsorbate Geometry. In the following, use will be made of the Argand diagram vector representation of the NIXSW structural parameters, as described above. Considering first the (111) reflection, for a sulfur atom distribution exhibiting a coherent fraction of ∼0.69, there are effectively two possible explanations in terms of the adsorption structure. The first assumes that there is a single position, with respect to the (111) planes, around which all S atoms exist. Phenomena such as thermal disorder, or small areas of imperfection, would account for the reduction in f111 from unity. A second model, which would also explain a loss of coherence of this magnitude, involves multiple adsorption sites; the S atoms may not all lie at identical heights above the (111) planes, i.e., there may be two or more discrete adsorption sites, in which sulfur occupies a different position depending on its substrate registry. For example, if there was an equal distribution of molecules in high-symmetry sites, it is clear that those in atop positions would sit higher above the surface plane than those in 3-fold hollows. Let us first consider the single position case, in which all sulfur atoms are positioned about an average fractional layer spacing d111 ) 0.58, with areas of disorder accounting for the value of f111 ) 0.69. Thermal effects may be ruled out on the basis that f111 was not increased by cooling. Given that the X-ray standing wave extends well beyond the metal surface, the actual adsorbate position may be an integer number of layer spacings greater than this value. The NIXSW technique cannot distinguish between atoms lying at, for example, d111 ) 0.5DH and d111 ) 1.5DH. Usually, however, all but a few of these ambiguities can be ruled out on the grounds that they are unphysical. The covalent radii of S and Cu are known;35 therefore it is possible to deduce the most likely scenario. In Figure 7, the top layer copper atoms are displayed in the section plane, together with sulfur atoms at the experimentally determined distance of 0.58 × 2.08 Å ) 1.21 Å. Given that the covalent radii of Cu and S are 1.17 and 1.04 Å, respectively,35 the anticipated Cu-S atomic distance is around 2.21 Å. It is therefore possible to immediately rule out three of the examples on the grounds that they result in bond lengths which are unrealistically high for a chemisorption interaction, namely, those in which the S sits at a vertical height of 3.29 Å above the Cu surface. Of those remaining, only 3-fold hollow adsorption, which gives a predicted Cu-S bond length of 1.91 Å, is close to the theoretical bond length of 2.21 Å. Even in this case a surface layer reconstruction would thus be necessary, in which Cu atoms move away from their preferred positions to accommodate a partially penetrative S thiolate group. What is clear, however, is that the lateral displacements required of the Cu atoms around a hollow site are so large that the outermost Cu (35) Emsley, J. The Elements, 3rd ed.; Oxford University Press: Oxford, 1997.
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Figure 7. Cross section of the Cu surface, in which the (111) experimental data are depicted in terms of single adsorption sites. The different possibilities for sulfur adsorption are displayed (all dimensions to scale).
layer must actually have a much lower Cu atom density than a close-packed Cu(111) bulklike layer. Once this conclusion has been reached, it is clear that the registry of the S relative to the underlying bulk could correspond to one or more, or a combination of numerous, configurations. It is possible that this outer layer could retain a 3-fold symmetry, implying that the S will be 3-fold coordinated relative to this lower density layer, but where this is relative to the underlying bulk is no longer defined. Adsorbate-induced reconstructions of this type are not uncommon. In the combined NIXSW and SEXAFS study of mercaptide on Cu(111),21 the authors concluded that substrate atoms must move away from the 3-fold hollow sites, parallel to the surface, by a distance 0.6 Å. This reconstruction, they argued, allows the mercaptide S atoms to attain a height above the (111) planes of 1.2 Å, in close agreement with the d111 value of 1.21 Å determined for 1-octanethiol on Cu(111). This assertion was supported by their SEXAFS data, which indicated a Cu-S bond length of 2.38 Å, at an angle 60 ( 5° to the surface plane. We take the view here that, in such cases where surface construction occurs (or is predicted to occur), that it is unwise to attempt a determination of the adsorbate site relative to the underlying bulk based on the coherent position alone. An alternative strategy is to attempt to rationalize the coherent fraction data taken in the (111) and (1 h 11) Bragg reflections. In particular, it is most instructive to rationalize how the low value of f-111 might arise and whether this could reveal information about the adsorbatesubstrate registry. Table 2 shows the measured values of the coherent fractions with respect to the (111) and (1 h 11) directions together with the values, calculated using the Argand diagram approach of Woodruff et al.,30 for some common adsorbate structures. The theoretical adsorbate structures are selected to exemplify the characteristics of the coherent fractions of simple high-symmetry adsorbate
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Table 2. Experimental and Theoretical Values of the Coherent Fractions with Respect to the (111) and (1 h 11) Directions coherent fractiona adsorbate system S/Cu(111) S/Ag(111)
f111 Experiment 0.67 ( 0.10 0.54 ( 0.12
Theory 100% atop f111 50% hcp: 50% fccb f111 100% bridge f111 33% atop: 33% hcp: 33% fccc f111 ideal coincidence net f111 incommensurate adsorbate f111
f-111 0.10 ( 0.12 0.11 ( 0.12 f111 f111/2 f111/3 0 0 0
a In the case of the theory, the value given for f -111 is the upper limit relative to f111. b As for example exists in the (x3×x3)R30° structure proposed for thiols on Au(111).3 c As for example exists in the (x7×x7)R10.1° structure proposed for thiols on Ag(111).19
structures versus those of multiple site adsorption structures. It can be seen that in the case of the simple highsymmetry adsorption structuresswith the exception of the equally occupied atop/hcp/fcc structuresthe value of f-111 is not close to zero (>f111/3). Conversely, the multiple site adsorption structures have a value of f-111 identical to zero. Since the small but nonzero value of f-111 could be explained by a modification in one of these ideal structures, it is worth considering both of these cases a little further. 5.2.1. Simple High-Symmetry Adsorbate Structures. As discussed above, the single site adsorption structure which most closely matches the (111) data is one in which S atoms adsorb in 3-fold hollow sites. Indeed this is the case for the (x3×x3)R30° structure proposed for thiols on Au(111).3 Predictably, given the likely reconstruction which occurs when this model is applied to the (1h 11) reflection, an apparent inconsistency is evident because adsorption in 3-fold hollow sites cannot, in isolation, account for the very low coherent fraction observed. Theoretically, the lowest value of f-111 for this adsorption geometry is f111/2, which would occur for an equal occupation of the geometrically inequivalent hcp and fcc 3-fold hollows present on the (111) surface. Figure 8 illustrates how these adsorption sites differ and how they combine to give a reduction in f-111. The upper part of Figure 8 shows that adsorbates situated in the hcp hollow are directly above substrate atoms in the second layer, while fcc-adsorbed atoms sit above third-layer substrate atoms. The consequence of this is that their respective perpendicular distances from the (1h 11) planes will differ by DH × cosine(70.5°), or DH/3. When translated to the Argand diagram picture, this difference imposes an angle of 120° between vectors, and the resulting f-111 observes a 50% reduction from f111, as illustrated in the lower part of Figure 8. This angle is independent of changes in d111, provided that both the hcp and fcc adsorbed atoms maintain the same value of d111. It is easy to see from the Argand diagram picture of Figure 8 that the magnitude of the resultant ()f-111 ) is reduced by increasing the angle included between its two component vectors. The effect of so doing is to move S atoms from hollow sites toward bridge sites. The limit to this reduction is when 100% of S atoms are in bridging sites and f-111 assumes the value f111/3. Allowing different vertical displacements of S atoms in different hollow sites also reduces the value of f-111 since this has the effect of increasing the included angle, although this leads to rather
Figure 8. The upper graphic is a 〈211〉 cross section of the Cu(111) crystal, illustrating the nonequivalence of the fcc and hcp 3-fold hollow sites. The lower part is the Argand diagram for (1 h 11) reflection, representing an equal occupation of fcc and hcp 3-fold hollow sites.
unphysical values for the coherent positions. A combination of such modifications could offer an explanation for the adsorption of SAMs exclusively in 3-fold hollow sites, yet still give a low f-111. Furthermore, work by Fenter et al.14 lends weight to the proposition that thiol adsorption occurs at more than one site. In a grazing incidence X-ray diffraction (GIXD) study, Fenter and co-workers suggested that thiols adsorb as disulfides on Au(111). The consequence of this is that S atoms cannot assume a single adsorption site, owing to geometrical constraints. A simple scenario which leads to a coherent fraction, f-111, equal identically to zero is one in which 3-fold hollow and atop sites are equally occupied at a uniform (111) layer spacing. This would be the case for the (x7×x7)R10.1° structure proposed previously for thiols on Ag(111).19 Since an atom bound in an atop site at d111 sits at a perpendicular distance of d111 × cosine(70.5°) from the (1h 11) planes, in comparison to that shown in Figure 8, the Argand diagram corresponding to this situation has a third vector of equal magnitude to, and at 120° to, the two existing vectors. The resultant magnitude is clearly equal to zero. In cases where surface reconstruction is not suspected, the value of the coherent position, d111, may militate against the existence of this structure. However, a modification of this simple scenario could form the basis for an explanation of the current data, since a minor
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distortion of this structure would almost certainly lead to a small but nonzero value of f-111. Such a distortion would, of course, constitute an example of a multiple site adsorption structure. 5.2.2. Multiple Site Adsorption Structures. The discussion of the scenarios above has shown that by modifying simple high-symmetry adsorption structures, f-111 may be reduced dramatically from unity. An alternative scenario, presented by multiple site adsorption structures, is one in which the starting point of the discussion is a coherent fraction of zero in the 〈1 h 11〉 direction. The (111) Bragg reflection data for Cu determined the value of d111 to be 1.21 Å, and taken in isolation, this suggests that there must be penetration of the Cu substrate and, as a consequence, reconstruction of the outermost layer. Such an outer layer reconstruction would in turn suggest, especially for the saturation coverages employed, the existence of multiple substrate coordinations. Turning now to the (1 h 11) data, it is clear that there must exist a range of S-Cu(1 h 11) layer spacings, since f-111 is very small, and it is not possible to state with confidence the value of d-111. An incommensurate monolayer structure, in which the adsorbate is randomly distributed parallel to the (111) surface, would produce a random set of projections onto the (1 h 11) reflection planes which are spread over the full range of values for d-111: the resultant coherent fraction being zero. Experimentally, it is not possible with NIXSW to distinguish between an adsorbate which adopts an incommensurate structure and an ideal coincidence net structure, i.e., a structure which contains a planar adsorbate layer with a regular submesh periodicity. In the latter, there is a fixed relationship between the periodicity of the adsorbate submesh and the substrate mesh which results in a finite set of equally spaced projections onto the (1h 11) planes; the coherent fraction sums exactly to zero. A theoretical example in which the adsorbate is equally distributed over seven layer spacings in the (1 h 11) direction, separated by 6/7DH to yield a resultant coherent fraction of zero, is described by Woodruff et al.28 The multiple site occupancy of the S atoms allowed by these two models would appear to offer a likely explanation of the adsorbate structure in 1-octanethiol on Ag(111) and Cu(111). 5.3. Proposed Adsorption Structures for 1-Octanethiol on Ag(111) and Cu(111). The preceding discussion has made it clear that for 1-octanethiol on Ag(111) and Cu(111) the most satisfactory explanation of the NIXSW data is that the adsorbate adopts either an incommensurate or large mesh commensurate overlayer. In both of these structures the adsorbate is free to occupy one of several positions parallel to the surface. Furthermore, since the d111 values are consistent with rather short Cu-S and Ag-S spacings, respectively, the S atoms must penetrate the underlying substrate and in so doing force a reconstruction of the surface. Determining the manner of this surface reconstruction is beyond the scope of the NIXSW experiment, since the coherent position d111 is defined as the S atom layer spacing with respect to the (extended) bulk scatterer planes. Consequently, provided the subsurface bulk substrate exhibits good crystallinity, the measurement of adsorbate position is well-defined relative to this extended lattice but is rather insensitive to the structure or positions of the outermost layer species. In other words the value of d111 may reveal that a reconstruction exists but can say nothing about the nature of the reconstruction.
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Figure 9. Argand diagram representation of (x7×x7)R19.1° structure in (1 h 11) reflection. Three vectors of equal length, corresponding to S atoms in atop sites, fcc hollows, and hcp hollows respectively, are separated by 120°. The resultant is zero.
It is, however, unlikely that the proposed adsorbateinduced reconstruction is completely random in nature, since the S atomic position is relatively tightly defined by f111 and d111. Two different modifications of the incommensurate or large-mesh commensurate overlayer model are now presented for 1-octanethiol on Ag(111) and 1-octanethiol on Cu(111), which satisfy the NIXSW data and the respective steric requirements of the two systems. 5.3.1. 1-Octanethiol on Ag(111). Using He diffraction and GIXD, Fenter et al. observed an incommensurate structure in SAMs of 1-octadecanethiol on Ag(111).18 A mechanism for alkanethiol adsorption on Ag was suggested by Dhirani et al., based on their STM study of 1-decanethiol on Ag(111).19 The proposed scheme involved the initial formation of the (x7×x7)R19.1° structure, in which adsorption occurs at both 3-fold hollow and atop sites. At saturation coverages the Ag surface reconstructs to allow the adsorbed alkanethiol molecules to achieve a common vertical distance from the surface plane, thus distorting the initial (x7×x7)R19.1° structure. Dhirani et al. argued that the driving force behind this reconstruction was the maximization of lateral cohesive van der Waals interactions between adjacent hydrocarbon chains.19 An Argand diagram for the (x7×x7)R19.1° monolayer lattice is presented in Figure 9. A scale drawing showing the plane view of this surface structure, is shown in Figure 10. This figure shows the relationship between the unit cells of the overlayer and the underlying substrate and illustrates the origin of the incorrect (x7×x7)R10.9° structural assignment reported elsewhere.1,19 It is noted that this equal occupation of fcc, hcp, and atop sites at a uniform (111) layer spacing results in a zero coherent fraction in the (1h 11) direction. It is likely, therefore, that a distortion of this structure would lead to a slight increase in f-111 from zero, as was observed in the experiments reported here. Furthermore, the relatively high value of f111 is consistent with some vertical displacement about the true atop and 3-fold hollow site positions which would serve to equalize the (111) layer spacings of sulfur. A distorted (x7×x7)R19.1° lattice is therefore presented as the most likely description of the adsorption structure for 1-octanethiol on Ag(111).
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Figure 12. The R-S-Cu3 cluster model. The ordered monolayer structure is based on a large, regular submesh which is commensurate with the substrate mesh. The outermost layer of the substrate is reconstructed to an unknown extent (H atoms are omitted).
Figure 10. Scale diagram showing the (x7×x7)R19.1° lattice structure. The distance between equivalent adsorption sites is given by x7 × 2.89 Å ) 7.65 Å. The relationship between the unit cells of the overlayer and the underlying substrate illustrates the origin of the incorrect (x7×x7)R10.9° structural assignment reported elsewhere.1,19
Figure 11. Scale diagram of the (x7×x7)R19.1° lattice on Cu(111).
5.3.2. 1-Octanethiol on Cu(111). SAMs on Cu(111) are, in general, less well characterized than those on Ag(111) and Au(111), and no detailed diffraction studies have been performed. However, an adsorbate structure for 1-octanethiol on Cu(111) cannot be based on the distorted (x7×x7)R19.1° model proposed for SAMs on Ag(111). A scale drawing showing the plane view of a (x7×x7)R19.1° surface structure for S on Cu(111) is shown in Figure 11. It is clear, by inspection of this structure, that the S‚‚‚S
nearest-neighbor distance of 3.91 Å dictated by the Cu(111) surface36 is insufficient to accommodate pendant hydrocarbon chains anchored in this arrangement because they have a preferred spacing of ∼4.5 Å (the separation in crystalline polyethylene1). Consequently, the adsorption structure on Cu(111) must be different from that on Ag(111). What is required is a model which satisfies the constraints of high adsorbate coherence perpendicular to the surface, close to zero coherence parallel to the surface, a small (111) layer spacing, and a reconstruction of the substrate. To sustain a S-Cu bonding distance of 1.9 Å, which is close to the theoretical 3-fold hollow separation of 2.21 Å, coupled with a coherent fraction of ∼0.69, it is highly likely that there exists a degree of local order around the sulfur headgroup. To illustrate how such a structure might appear, Figure 12 depicts several R-S-Cu3 clusters which preserve the expected sulfur adsorption geometry, while allowing for the adoption of multiple (1h 11) layer spacings. The figure portrays a regular large mesh commensurate structure, the dimensions of which depend not on the bulk lattice parameters but rather on the intermolecular forces between adjacent alkyl chains. An incommensurate structure may be described by arranging the proposed R-S-Cu3 clusters in a random fashion on the (111) surface. In each situation, the S atoms would attain d111 values consistent with partially penetrated 3-fold hollows, with areas of disorder accounting for the observed f111. With respect to the (1h 11) direction, adsorption might appear to be in hollow, atop, bridge, or sites of mixed character due to the reconstruction of the toplayer Cu atoms, thus leading to a near-zero f-111 and an indeterminate d-111. To confirm the precise nature of the reconstruction, it would be necessary to perform additional experiments. The SEXAFS technique, sensitive to local order, could be used to measure the S-Cu interatomic distance. This is not necessarily the same as the quantity which is inferred above, since NIXSW measures the perpendicular distance of S from the extended bulk substrate planes. The existence of long-range order might be probed using LEED, although it has been shown that SAMs are extremely sensitive to electron beam damage. Despite this, LEED has been (36) Kendall, G. K. Ph.D. Thesis, University of Liverpool, 1997.
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successfully applied to the investigation of SAM adsorption structures on Au surfaces.11,37 6. Summary The adsorption structures of UHV-prepared SAMs of 1-octanethiol on Cu(111) and Ag(111) have been probed with NIXSW. It is concluded that the respective monolayer structures cannot be distinguished, in terms of adsorption geometry, using this method. The observed similarity is in accordance with previous work, in which IR and ellipsometry data indicated the formation of similar SAM structures on Cu and Ag.20,38 In both cases, a reconstruction of the underlying substrate is necessary. The data obtained are not consistent with conventional 3-fold hollow adsorption. The results have been discussed (37) Balzer, F.; Gerlach, R.; Polanski, G.; Rubahn, H.-G. Chem. Phys. Lett. 1997, 274, 145. (38) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990.
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in terms of several scenarios, each of which involves a considerable modification of this simple model, leading to a reduction in the coherent fraction for the (1 h 11) reflection. The most likely model involves the formation of monolayers which are either incommensurate with the underlying substrate or commensurate on a large mesh, rationally related with the substrate submesh. On Ag(111), the most likely scenario is a distorted (x7×x7)R19.1° adsorbate structure. This scheme is not sterically feasible on the Cu(111) surface, and the formation of R-SCu3 clusters has been proposed as a possible structure for SAMs of 1-octanethiol on Cu(111). Acknowledgment. The authors thank the EPSRC for the allocation of SRS beamtime at Daresbury Laboratories, U.K., and the award of a studentship for G.K.K. LA9904253
