Langmuir 1991, 7, 2013-2016
2013
Structure of CH3(CH2)17SH Self-Assembledon the Ag(111) Surface: An Incommensurate Monolayer P. Fenter'9t and P. Eisenberger Department of Physics, Princeton University, Princeton, New Jersey 08544
Jun Li, N. Camillone 111, S. Bernasek, and G. Scoles Department of Chemistry, Princeton University, Princeton, New Jersey 08544
T. A. Ramanarayanan and K. S. Liang Exxon Research and Engineering, Annandale, New Jersey 08801 Received June 14, 1991. I n Final Form: August 8, 1991 We describe the first experimental study of the molecular structure of the self-assembled monolayer of CH&H*)&H formed on the Ag(ll1) surface. Using both low-energy He diffraction and grazing incidence X-ray diffraction, we show that the monolayer formed on Ag is both incommensurate and rotated with respect to the Ag lattice, with a lattice spacing which is very similar to n-alkane chains in bulk hydrocarbon crystals. Lastly, by comparing the results of the X-ray and He diffraction experiments on the same sample, we observe that the outermost surface of the monolayer is less ordered than the interior.
Introduction In the past few years, there has been a great deal of interest in organic monolayers adsorbed on solid surfaces, generated by their unique properties as well as their technological Two classes of monolayers that have been very intensively studied are Langmuir-Blodgett (LB) films,Ce which are formed by transferring monolayers from liquid to solid substrates, and self-assembled (SA) monolayers,7-10 which form by chemisorption of molecules to a substrate from the liquid phase. Recent studies have indicated that LB films415form dense monolayers, with lattice spacings characteristic of the molecular size of the hydrocarbon chains, and in some cases very large domain In contrast, recent work on SA monolayers formed on Au(ll1) has shown that thesemonolayers have a lattice constant commensurate with the sub~trate,7*~JOeven though the spacing is 5 5% larger than the natural lattice constant of the bulk n-alkanes.l1 Since these SA monolayers are expected to interact strongly with the substrate through chemisorption,12it is important to understand how the structure of the monolayers is
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Also at Euon Research and Engineering, Annandale, NJ. (1)Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandroee, E. A.; Garoff, 5.;Ieraelachvili,J.; McCarthy, T. J.;Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932. (2)Whitesides, C. M.; Laibinis, P. E. Langmuir 1990,6,87. ( 3 ) Chidsey, C. E. D. Science 1991,251,919. (4)Sed,M.; Eieenberger,P.; McConnell, H. M. h o c . Natl. Acad. Sci. U S A . 1983,80,6796. (5) Caroff, S.;Deck", H. W.; Dunsmiur, J. H.; Alvarez, M. S.;Bloch, J. M. J. Phys. (Paris) 1986,47,701. (6)Meyer, E. Nature 1991,349,398. (7)Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989,91,4421. (8) Tidewell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Kosowsky, S. D.; Folkers, J. P.; Whitesides, G. M. J. Chem. Phys., in press. (9)Strong, L.; Whitesides, G. M. Langmuir 1988,4,546. (10)Samant, M. G.; Brown, C. A.; Gordon, J. G. I1 Langmuir 1991,7, 437. (11) Doucet, J.; Denicolo, I.; Craievich, A. J. Chem. Phys. 1981, 75, 1623. (12)Sellers, H.; Ulman,A.; Shnidman,Y.; Eilers,J. E. In Cluster Models /or Surface and Bulk Phenomena; Pacchioni, G., Bagus, P. S., Eds., Plenum Press: New York, 1991.
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influenced by the choice of the substrate. Recently, IR absorption ~ t u d i e s ' ~ -ofl ~SA monolayers have found that there are significant differences between films formed on Au(ll1) and Ag(lll), in spite of the similar lattice constants of these two metals. The most interesting difference is that the n-alkyl chains are found to be less tilted on Ag(ll1) than on Au(ll1). In this paper, we report the first experimental study of the intermolecular structure of CHs(CH2)17SH (referred to as CIS) adsorbed on Ag(ll1). We have used the complementary techniques of low-energyatom diffraction (LEAD)16 and grazing incidence X-ray diffraction (GIXD)17J8 to characterize the film structure. The combination of these two techniques provides a unique view of the film structure since He atoms scatter off only the topmost atoms in the film (the film surface), while the X-rays penetrate the film without any significant attenuation and consequently are sensitive to the average film properties. We show that the film structure of CIS absorbed on Ag(ll1) is incommensurate and rotatedwith respect to the substrate lattice and discuss possible reasons for this behavior.
Experimental Section Due to the reactivity of Ag to atmospheric gases, the preparation conditions of the film are important,and consequentlywe will detail them here. An Ag(ll1) single crystal substrate was used in this study. The surface was mechanically polished to ~ i t h i n 0 . 2ofthe ~ (111)crystallographicdirection,andchemically (13)Walczak, M. M.; Chung, C. S.; Stole,M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. SOC.1991,113,2370. (14)Laibinis, P. E.; Whitesides,G. M.; Allara,D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo,R. G. J. Am. Chem. SOC. 1991,113,OOO. (15)Nuzzo, R. G.; Korenic, E. M.; Dubom, L. H.J. Chem. Phys. ISSO, 93,767. (16)Rowntree, P.; Scolee, G.; Ruiz-Suarez, J. C. J. Chem. Phys. 1990, 94,8511. (17)Foues, P.; Liang, K. S.; Eieenberger,P. In Synchrotron Radiation Research Advances in Surface Science; Bachrach, R. Z.,Ed.; Plenum Publishing Co.: New York, 1991. (18)Feidenhans'l, R. Surf. Sci. Rep. 1989,10, 105.
0 1991 American Chemical Society
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2014 Langmuir, Vol. 7,No. 10, 1991 1 ,
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to the surface, Qll. The He atoms are scattered only from the topmost surface atoms and reflect the periodicity of the organic film's surface corrugation. In Figure l a , we show the intensity of the He diffraction signal as a function 11 = 0, correof Qll. These data show a large peak at 8 sponding to the specularly reflected He beam, and two diffraction peaks. These diffraction peaks are weak and broad, but it is clear that molecular order exists on the surface of the SA monolayer. In Figure lb, we plot the peak positions of the diffraction features (like those shown in Figure la) over a -200' range of azimuthal angles, 4, in the 811 plane. These data form wide arcs of concentric circles corresponding to the (-1,O) and (-1,-1) diffraction peaks of a hexagonal lattice. The width of these arcs, A 4 30-45", is much larger than that observed for similar films formed on A ~ ( l l land, )~ consequently, is due to the film structure (although some broadening is due to instrumental effects). The average reciprocal lattice spacing of these data is 811 = 1.55 f 0.08 A-l, where the error is the standard deviation of all of the measurements. The width, AQ11, of the diffraction peaks shown in Figure l a is relatively large and is found to be -0.5 A-l. These data correspond to a hexagonal structure in real space with a nearest neighbor spacing of a = 4.67 f 0.23 A. Since the lattice spacing of the expected (d3Xd3)R30° structure is 5.01 A,it is clear that the SA monolayer is incommensurate with the Ag(ll1) substrate, forming a denser monolayer as compared to the commensurate films formed on Au(ll1). The large azimuthal broadening reveals either orientational disorder or a more complex structure of the SA monolayer relative to the substrate. Grazing Incidence X-ray Diffraction. The X-ray diffraction was performed a t a constant grazing angle of (Y 1.2' with respect to the surface plane (see inset of Figure 2) corresponding to a momentum transfer perpendicular to the surface of 0.19 A-l. Since this is far above the critical angle for total external reflection from the organic film (ac= 0.15), the attenuation of the X-ray flux in the film is negligible.18 Consequently these measurements will reflect the average positions of the alkyl chains. In Figure 2a, we show the first order diffraction feature from the SA monolayer. The solid line is a fit to the data assuming a linear background and a Gaussian 11 = 1.52 peak shape. The peak position from this fit of 8 f 0.01 A-l corresponds to a film lattice spacing of 4.77 f 0.03 A, in reasonable agreement with the He diffraction results, but significantly different from the commensurate (v'3Xd3)R30° structure (811 = 1.452 A-l) found for Cla adsorbed on A ~ ( l l l ) ~ ,A~parallel J ~ . GIXD study of a CIS film adsorbed on Au(ll1) found the expected ( 4 3 x 4 3 ) R30' commensurate structure.20 The width of the diffraction peak in Figure 3a, AQII= 0.06 A-l, is broader than the instrumental resolution and corresponds to a domain size of 120 A. This peak is significantly narrower than the peak observed in the He diffraction data. The expected azimuthal orientation, 9, for a ( 4 3 x 4 3 ) R30' overlayer (as found for similar monolayers formed on Au( 111))is 9 = 30'. A measurement of the orientation of the film is shown in Figure 2b. Clearly, this monolayer behaves in a very different way, with two peaks occurring at 9 = 48.6' and 9 = 12.2'. These peaks are symmetrically displaced from the expected position of 4 = 30' by f18'. This indicates that there are two domains present to preserve the 6-fold symmetry of the (111) surface. In addition, the intensity distribution in the azimuthal direction of each of these peaks (at constant 811) is broad
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Figure 1. (a) Helium diffraction from CH&H2)17SH/Ag(lll) as a function of momentum transfer parallel to the surface, 811, at an arbitrary azimuthal angle. The incident energy is 15.1 meV (k = 5.38 A-I), 8i = 55.4" and the surface temperature is 30 K. (b) The helium diffraction peak positions plotted in the 2-d reciprocal space over a 180" range of azimuthal angles. etched by a solutionlg of 2% HzOz, 14% NHIOH, and 84% HzO to remove mechanical polishing damage. The crystal was sputtered (Ar, 500 eV) and annealed (400 "C) under ultrahigh vacuum (UHV) conditions until no impurities could be detected with Auger electron spectroscopy and the hexagonal low-energy electron diffraction pattern of the Ag(ll1) surface was observed. The crystal was then transferred from the UHV chamber directly into a dry NZ glovebox without exposure to the ambient. It was then immersed into a -1.3 mM solution of l-octadecanethiol in ethanol in order to form the SA monolayer. After -24 h, the crystal was removed from solution and rinsed alternately with ethanol and hexane before being placed under vacuum in the LEAD apparatus. The He diffraction measurements were carried out under the same conditions as those previously used to study alkanethiol . ~ measurements were carried out at monolayers on A ~ ( l l l ) The a crystal temperature of -30 K with a nearly monoenergetic (Au/u S 2%) incident helium beam energy of 15 meV (A = 1.16 A) with an incident angle of -60" with respect to the surface normal. Subsequently, the crystal was kept under vacuum for about 3 weeks until 5 days prior to the X-ray study. The GIXD measurements were performed at the Exxon XlOA beamline at the National Synchrotron Light Source on a 4-circle diffractometer, using an X-ray wavelength of X = 1.384 A. The measurements were taken with an in-plane longitudinalresolution of -0.03 A-1. In order to avoid sample contamination during exposure to the X-ray beam, the sample was contained in a cell with a slowly flowing He environment at atmospheric pressure.
Results Low-Energy He Diffraction. As shown in the inset of Figure la, the He diffraction scan was taken with the incident beam, detector, and the surface normal all in the same plane. The incident angle of the beam is kept fixed and the detector angle is varied in order to detect the diffraction signal at a different momentum transfer parallel (19) Holland, R. J. PhD. Thesis, Princeton University, 1986.
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(20)Fenter, P.; et al., unpublished material.
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Langmuir, VoZ. 7, No. 10,1991 2015
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Figure2. (a) X-ray diffractionthroughthe first-orderdiffraction peak of the SA monolayer as a function of Qn. The width of the peak is 0.06A-l. These data were taken in the scatteringgeometry shown in the inset, with a wavevector of k = 4.54 A-l, and an incident angle of a = 1.2". (b) An azimuthal scan through the peak shown in part a, showing clear peaks at 4 = 12.2" and 48.6". (4 is measured with respect to the observed Ag substrate diffraction features). The larger intensity in part b is due to a wider slit configuration. No peak was observed in the 4 = 30" azimuth.
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Figure 3. (a) A schematic of the observed reciprocal space structure, with small (large) circles representing the CISmonolayer (Ag substrate) diffraction features, and showing peak splitting from the expected 4 = 30" direction. The small filled (open) circles represent the +18" (-18") rotated domain. (b) A schematic of the real space structure of one of the two observed domains.
with a full width at half maximum of -3". A schematic of both the real and reciprocal space structures is shown in Figure 3.
Discussion and Conclusions Our results indicate that the structure of the SA monolayer of CH3(CH2)17SH/Ag(lll) is distinct from the structure of their known Au7v9J0and Si8 counterparts.
Although the SA film on Ag(ll1) is a dense and wellordered monolayer, the lattice constant of the film (d 4.7 A) is incommensurate, with a spacing that is - 5 % smaller than the substrate lattice spacing. In addition, this monolayer forms two domains rotated by f18" from the expected (d3Xd3)R30° orientation. It is interesting to look at the differences between the X-ray and He diffraction results. The X-ray diffraction results found a slightly larger lattice constant than was found with He diffraction (4.77 vs 4.67 A). Although this difference is small (considering the uncertainties), it may be due to an aging effect.21 More importantly, the widths of the diffraction peaks are found to be very different, with the surface of the film having a muchsmaller coherent domain size (- 12 A) than the interior of the film (- 120 A), even though the He diffraction measurements were done at a much lower temperature. This is then the first experimental confirmation of the prediction22 that the order in the alkyl chains decreases as one approaches the surface of the film. Lastly, the azimuthal disorder found in the He diffraction measurements (Figure 1) can be understood on the basis of the X-ray diffraction results. Since we find two domains separated by -36" from each other (Figure 3b), the minimum azimuthal peak separation becomes only 24" (compared to 60" for a (d3Xd3)R30° 3". Due to structure), each with a width of A 4 instrumental effects in the atom diffractometer, these peak doublets are observed as quasi-continuous arcs in Figure 2b. A further analysis of the intensity variation along these arcs has shown two peaks separated by -24" in full agreement with the X-ray data. These results are consistent with IR spectroscopy experiments which have found the alkyl chain to be less tilted on Ag substrates than on Au substrates (13' vs 260).13-15 It is thought that the alkyl chains tilt to acquire a closer packing of the molecule without changing the 2d periodicity. The difference in the measured tilt angle between monolayers formed on Ag and Au gives (in the simplest an 8% change in unit cell area, which is very similar to the 10%change that we have observed. It should be noted, however, that a recent X-ray diffraction studylo of an alkanethiol film on Au(ll1) found a much smaller tilt angle (12O) than found in the IR measurements.24 The differences in the film structure of alkanethiols adsorbed on the two different metals are probably due to the differences between the interaction of S with Ag and as well as Au. On Ag(lll), the adsorption of CH3S (ref 27) results in a (d7Xd7)R10.9" structure. In contrast, a recent calculation finds that, on Au(lll),CH3S prefers to bond in the 3-fold hollow site,12consistent with the (d3Xd3)R3Oo structure that is observed for the long chain monolayer^.^^^ Although the (d7Xd7)R10.9" structure is similar to what we have observed (that is, a hexagonal structure with two domains rotated by -12O), the nearest neighbor distance is far too small, corresponding to 4.4 A. Since we have observed that the molecular spacing is nearly the ideal bulk hydrocarbon spacing," it
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(21) We have observed some (-2%) variability in lattice constant for nominally similar films on Ag(lll), which is similar to the difference between the two measured values on the same sample. (22) Hauptman, J.; Klein, M. L. J. Chem. Phys. 1990,93,7483. (23) Bareman, J. P.; Klein, M. L. J. Phys. Chem. 1990,94, 5202. (24)Our recent measurements of an alkanethiol film on Au(ll1) indicate that the tilt angle is close to the value found by infrared spectroscopy. (25) Schwaha, K.; Spencer, N. D.; Lambert,R. M. Surf.Sci. 1979,81, 273. (26) Rovida, G.; and Prateei, F. Surf. Sci. 1981,104,609. (27) Harris, A. L.; Rothberg, L.; Dubois, L. H.; Levinos, N. J.; Dhar, L. Phys. Rev. Lett. 1990,64,2086.
2016 Langmuir, Vol. 7, No.10, 1991 appears that the monolayer has formed at the densest spacingconsistent with the molecular size. (In support of this, a study of a Langmuir monolayer of n-alkyl chains (CHs(CH2)mOH)on water found%(at the highest surface pressures) a nearest neighbor distance similar to the value we find for CU on Ag(lll).) But since the film picks out a particular azimuthal orientation, it is quite likely that the film/substrate interaction is not negligible. "his is very different from f i b formed on Au, where the substrate spacing determines the packing and orientation of the film. In contrast, for alkylsiloxane monolayers on Si (28) Barton, S. W.; Thomas,B. N.; Flom, E. B.; Rice, S. A.; Lin, B.; Peng, J. B.; Kettereon, J. B.; Dutta, P. J. Chem. Phys. 1988,89, 2257.
Letters substratese the substrate surface is amorphous and, consequently, has no effect on the orientation of the films, resulting in ringlike diffraction pattern. Acknowledgment. G.S.thanks Princeto.1 University for financial support and P.F. thanks Dr. G. Held and D. Keane for helpful discussions on technique prior to the experiment as well as H. H. Hung for experimental assistance. Part of this work was performed at the National Sychrotron Light Source which is supported by the Divisions of Material Science and Chemical Science under Department of Energy Contract No. DE-AC02-76CH00016.
