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Structure of Heptanethiolate Monolayers on Au(111): Adsorption from Solution vs Vapor Deposition H.-J. Himmel,† Ch. Wo¨ll,*,†,‡ R. Gerlach,‡ G. Polanski,‡ and H.-G. Rubahn‡ Max-Planck-Institut fu¨ r Stro¨ mungsforschung, Bunsenstrasse 10, 37073 Go¨ ttingen, Germany, and Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Received July 17, 1996. In Final Form: December 16, 1996X A combined low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) study on the structural properties of heptanethiol (C7H15SH) monolayers prepared by either adsorption from the liquid or chemical vapor deposition (CVD) on Au(111) single crystal surfaces has been performed. Deposition from the vapor produces a commensurate c(17×x3) superstructure which exhibits a well-defined LEED pattern. XPS data recorded for these films reveal that the hydrocarbon molecules are orientated with their axes parallel to the substrate surface. Compared to the more commonly used fabrication of alkanethiolate self-assembled films by adsorption from solution the c(17×x3) CVD film exhibits a coverage of less than 40%.
1. Introduction The demonstration that ultrathin (nanometer thick) films can be prepared by spontaneous self-assembly of organic molecules on the surface of a solid which is immersed in a solution1 has opened up a variety of possible technical applications in different areas, including lubrication, sensor devices, and lithography.2 Whereas the preparation of such self-assembled molecules (SAMs) from alkylsilanes on, e.g., hydroxylated Si substrates1 still requires clean-room conditions if a high structural quality of the films is desired,3 it was observed several years ago4 that alkanethiols on Au substrates produce monolayers in a significantly more reliable and robust way. Additional interest in these films was generated when it could be demonstrated that the structure of the films adopts the crystalline structure of the substrate. Welldefined diffraction patterns for thiolate films on Au(111) substrates were observed in high-energy electron scattering,5 low-energy electron diffraction (LEED),6 He atom scattering,7 and X-ray diffraction.8 These results indicate that at temperatures below 100 K7 the hydrocarbon chains form a highly ordered crystalline phase and are thus well suited also for fundamental studies on intermolecular interactions and adhesion. In addition these films, in particular when using functionalized alkanethiols, can be employed to build up well-defined, ordered organic surfaces.6 Although for technical applications the standard preparation of SAMs by immersion in solution is the most interesting, the formation of these organic monolayers by * To whom correspondence may be addressed at Universita¨t Heidelberg. ‡ Universita ¨ t Heidelberg. † Max-Planck-Institut fu ¨ r Stro¨mungsforschung. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self Assembly; Academic: Boston, MA, 1991. (3) Bierbaum, K.; Ha¨hner, G.; Heid, S.; Kinzler, M.; Wo¨ll, Ch.; Effenberger, F.; Grunze, M. Langmuir 1995, 11, 512. (4) Bain, C. D.; Troughton, E. B.; Yao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (5) Strong, L.; Whiteside, G. M. Langmuir 1988, 4, 546. (6) Dubois, L. H.; Zegarski, R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (7) Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. Camillone, N., III; Leung, T. Y. B.; Scoles, G. SPIE Proc. 1994, 2125, 174. (8) Fenter, P.; Eisenberger, P. Phys. Rev. Lett. 1993, 70, 2447.
S0743-7463(96)00704-4 CCC: $14.00
Figure 1. Molecular arrangement in alkanethiolate films grown by adsorption from solution (a) and by chemical vapor deposition (b).
chemical vapor deposition (CVD), i.e., adsorption from the gas phase in an ultrahigh vacuum (UHV) chamber, on well-defined surfaces of single crystals offers several advantages. In addition to the prospect of obtaining films with even higher structural quality, the growth by deposition from the vapor phase6 allows the application of UHV-based analytical methods (in particular electron diffraction and electron spectroscopies) in situ. In recent studies it has been found that CVD alkanethiolate films exhibit structural properties which are different from those of the solution-grown films.6,9,10 Whereas general consensus has been achieved that the solution-grown alkanethiolate films are characterized by the alkyl chains being tilted away from the surface (Figure 1a), it has been proposed that in these new structural phases the alkyl chains are oriented parallel to the surface9,10 (see Figure 1b). Direct experimental evidence for such a parallel orientation, i.e., from IR spectroscopy or X-ray diffraction, could, however, not be provided. For higher coverages different structures without the presence of a high amount of order were observed and were attributed to the onset of the growth of islands with a structure similar to that of the solution-grown thiolate films,9 i.e., molecules tilted away from the surface. We will in the following present data obtained by LEED and X-ray photoelectron spectroscopy (XPS) which demonstrate directly that chemical vapor deposition of heptanethiol (HS-C7H15) on an Au(111) single crystal indeed (9) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (10) Camillone, N., III; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737.
© 1997 American Chemical Society
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Figure 2. (a) LEED pattern obtained at an electron energy of 49 eV from an Au(111) crystal exposed to 20 langmuirs of heptanethiol. (b) Positions of LEED spots for a c(17×x3) structure.13
leads to the formation of well-ordered overlayers with the hydrocarbon axis parallel to the surface. 2. Experimental Section The LEED measurements have been performed in the third chamber of a three-chamber UHV apparatus11 with a base pressure of 10-10 mbar. After a mechanical polish, the Au(111) substrate was installed in the UHV chamber and then subjected to several Ar+ sputtering and annealing cycles. After this procedure the widths of the (0,0) and (1,0) LEED spots were found to be limited by the instrumental transfer width revealing the presence of ordered domains with a coherence length in excess of 120 Å. The heptanethiol was deposited from the gas phase in a transfer chamber (base pressure of 10-8 mbar) at pressures in the 10-5 mbar regime with the sample at room temperature. A dual micro-channelplate LEED (Omikron MCP/ESDIAD) was used for obtaining LEED patterns at incident electron fluxes of typically 100 pA/mm2. This low electron flux is necessary in order to avoid electron beam damage.6 A CCD camera with a frame grabber system in front of the fluorescence screen allowed the storage and processing of the LEED pictures. The XPS data were taken in a different UHV apparatus, which consists of several chambers connected by a transfer system. Dosing of heptanethiol was carried out in a deposition chamber as described above. One of the chambers is equipped with a load-lock system which allows the introduction of samples from the ambient into the UHV analysis chamber. The X-ray photoelectron spectra reported here were recorded using a 100 mm hemispherical electron energy analyzer and a (non-monochromatized) Al KR X-ray source.
3. Results Figure 2a shows a LEED pattern obtained with an electron energy of 49 eV from a Au(111)-substrate exposed to 20 langmuirs (1 langmuir = 1 s × 10-6 Torr) of heptanethiol. For this low electron energy the (1×1) spots of the clean Au surface lie outside the part of the pattern visible on the screen; the hexagonal superstructure in Figure 2a is thus due to the presence of the adsorbed molecules.12 The diffraction pattern allows to deduce the presence of a well-ordered, rectangular c(17×x3) structure.13 A possible real-space unit cell is shown in Figure (11) Polanski, G.; Rubahn, H.-G. J. Vac. Sci. Technol., A 1996, 14, 110. (12) The (22×x3) reconstruction of the clean Au(111) surface (Harten, U.; Lahee, A. M.; Toennies, J. P.; Wo¨ll, Ch. Phys. Rev. Lett. 1985, 54, 2619. Wo¨ll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. 1989, B39, 7988.) could not be resolved in the present experiments. (13) A c(17×x3) overlayer can be described in matrix notation by
(
7 2 -
17 x3 3 3 2
-
x3 3 2 17 2
)
with respect to the Au(111) substrate base vectors.
4, the corresponding positions of the diffraction peaks are displayed in Figure 2b. The c(17×x3) structure could be observed also for larger exposures. Beyond 40 000 langmuirs, however, the pattern started to become rather diffuse. This observation is consistent with the expected growth of a thiolate film with the same structure as in the solution-grown films. The intensities of the various spots in the c(17×x3) LEED pattern were found to strongly vary with the incident energy of the electron beam. A detailed analysis of such I(V) curves would in principle allow for more precise conclusions on the structure of the adsorbate layer. Recent work has shown that also in the case of hydrocarbons chemisorbed on metal surfaces detailed structural parameters can be derived from such an analysis.14 In the present case, however, because of the large unit cell a huge and presently unfeasable computational effort would be necessary for such a calculation. We are not aware of any previous LEED studies of heptanethiol molecules adsorbed on Au(111). However, Dubois, Zegarski, and Nuzzo6 have performed a LEED study of the structure of other vacuum-deposited alkanethiols such as hexanethiol. From their data they have characterized the observed structures as (nx3×x3), with integer n. In some cases n was found to coincide with the number of C atoms in the respective alkyl chain, but in most cases it was found that n ) 5.6 Note, however, that these authors proposed an ordered array of alkanethiols with their axes tilted away from the surface (same orientation as observed for films grown in solution, Figure 1a) to explain their LEED patterns.6 Actually, the observation of a 5-fold periodicity for a liquid grown alkanethiolate film is rather surprising given the fact that both He atom diffraction7 and grazing incidence X-ray diffraction8 have shown the presence of a c(4×2) (short-hand notation for c(4x3×2x3)R30°) structure. Several molecular dynamics simulations have also yielded a molecular arrangement with 4-fold periodicity.15,16 A way to elucidate this apparent discrepancy is to employ XPS in order to determine the relative thiolate coverages of the c(17×x3) CVD and a solution deposited film. In Figure 3a a C1s XPS spectrum recorded for a solution-grown film is shown. It consists of a sharp line at 284.5 eV, typical for condensed alkanes.17 In Figure 3c a spectrum recorded for a thiolate film grown by CVD of heptanethiol on Au using an exposure of 20 langmuirs is displayed, the same exposure which was used to prepare the film, the LEED pattern of which is shown in Figure 2. The CVD XPS spectrum exhibits a significantly broader C1s line which can be decomposed into a large component centered at 283.5 eV and a smaller one at 284.3 eV. The 1.0 eV shift of the larger component toward lower binding energies with regard to the value of 284.5 eV typical for alkane multilayers17 is caused by core-hole final state relaxation effects and is a typical signature of alkane monolayers adsorbed with their axes oriented parallel to a metal surface.17 The C1s peak areas of the spectra shown in Figure 3 were determined and reveal a C1s signal of the CVD-deposited film which is 29% ( 10% smaller than that of the film grown in the liquid. In agreement with the reduced coverage in case of the CVD films, the Au 4f signal was found to be significantly less intense for the solution-grown films, a quantitative analysis considering the finite escape depths of photoelectrons revealed a (14) Stellweg, C.; Held, G.; Menzel, D. Surf. Sci. 1995, 325, L379384. (15) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994. Hautman, J.; Klein, M. L. J. Chem. Phys. 1990, 93, 7483. (16) Pertsin, A. J.; Grunze, M. Langmuir 1994, 10, 3668. (17) Witte, G.; Wo¨ll, Ch. J. Chem. Phys. 1995, 103, 5860.
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Figure 4. Two possible unit cells for heptanethiol molecules on the Au(111) surface obtained by chemical vapor deposition. The thick circles denote the thiolate molecules, the thin circles the substrate atoms. The small parallelogram depicts the primitive unit cell of the Au(111) substrate.
Figure 3. C1s XPS data recorded for heptanethiolate adlayers on Au(111). The points correspond to the raw data; the solid line is the result of a fit (see text). Individual peaks are shown as dotted lines. (a) Preparation by immersing the crystal for 6 h in liquid heptanethiol. (b) Preparation by exposure of a clean Au(111) surface to 90 000 langmuirs and (c) to 20 langmuirs of heptanethiol at room temperature.
decrease in thickness from 9 Å for the solution-grown film to 2.1 ( 0.5 Å for the CVD films.18 X-ray photoelectron spectra comparable to those for the solution-grown films as regards peak area and position (fitted value 284.3 eV, see Figure 3b) could only be obtained for prolonged exposures of more than 40 000 L (Figure 3b). Spectra were also recorded for the S2p-region (not shown) and reveal for all systems studied a S2p3/2 peak position of around 162 eV, typical for S-atoms bonded to Au20 or Cu21 surfaces. The C1s/S2p ratio was found to be 8 ( 1, indicating the presence of intact heptanethiolate moieties on the surface. In the case of the solution-grown film the XPS data are fully consistent with the presence of a well-ordered alkanethiolate film with its alkyl chains tilted away from the surface as expected from the structure shown in Figure 1a. For the CVD films, in contrast, the shift of the C1s peak to lower binding energies and the C1s/Au4f ratio reveal that the molecular axes of the alkyl chains are oriented parallel to the surface. Both, the XPS data and the LEED patterns observed here are consistent with a molecular arrangement in the CVD deposited films as shown in Figure 4, where we have (18) In this anaylsis an escape depth of 45 Å has been used for the Au(4f) photoelectrons (kinetic energy 1403 eV in case of an Al KR source) transmitted through an alkane thiolate film.19 (19) Hansen, S.; Tougaard, S.; Biebuyck, H. J. Electr. Spectr. Rel. Phen. 1992, 58, 159. (20) Zubra¨gel, Ch.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Wo¨ll, Ch. Chem. Phys. Lett. 1995, 283, 308. (21) Go¨lzha¨user, A.; Panov, S.; Wo¨ll, Ch. Surf. Sci. 1994, 314, L849.
Figure 5. Model for the centered c(17×x3) structure of heptanethiol on Au(111) formed by chemical vapor depositon.
plotted two possible unit cells, namely, a centered one and a primitive one. The dimensions of the primitive unit cell (24.5 × 5 Å) are similar to that found via STM measurements for vacuum deposited decanethiol, namely 22 × 5 Å.9 Actually, if we use decanethiol instead of heptanethiol we find from our measurements within error bars the same size of the unit cell, namely, 21.6 × 5 Å.22 Figure 5 shows a possible packing of the heptanethiolate moieties on the Au surface, assuming that the thiolates chemisorb with their axes parallel to the surface and that they form disulfides. The length of the centered unit cell is then twice as long as the length of the disulfides, which is 24.7 Å if one assumes bond angles of 112° and the bond lengths 1.541 Å (C-C), 1.81 Å (C-S), 1.073 Å (C-H), and 2.2 Å (S-S), respectively. The van der Waals radius of the bonded hydrogen is 1.2 Å. The coverage resulting from such a structure with two thiolate molecules per unit cell amounts to 35% of the accepted structure for the films grown in liquid (4 molecules per c(4x3×2x3))R30° unit cell7). A relative coverage of 35% is within error bars (22) Gerlach, R.; Polanski, G.; Rubahn, H.-G. Chem. Phys. Lett. Submitted for publication
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consistent with the ratio of the integrated XPS C 1s peaks recorded for the CVD and liquid grown films. The differences between this c(17×x3) unit cell present in monolayers of heptanethiolate and the (5x3×x3) structure (which corresponds to a rectangular c(15×x3)) observed by Dubois et al.6 for hexanethiolate are rather subtle and will be discussed in a more comprehensive report on diffraction patterns observed for different alkane thiolates.22 Note, that the structure shown in Figure 5 is strictly valid only for low temperatures. In contrast to the strong S-Au bond the interaction between the alkyl chain and the surface is very weak. For example in the case of octane adsorbed on a Cu surface, the desorption temperature amounts to 180 K,25 and little differences are expected for a Au substrate. Therefore at room temperature a significant number of alkyl chains is expected to be tilted away from the surface as a result of thermally induced disorder, i.e., conformational defects (gauche conformations).23 In fact the strong shoulder at energies of 284.3 eV observed in the C1s X-ray photoelectron spectra for the CVD films is perfectly consistent with a small amount of molecules with alkyl chains dynamically tilted away from the surface. (23) Such a temperature-induced phase-transition accompanied by alkyl segments bending away from the surface has been observed for alkanes adsorbed on Pt(111).24 (24) Hostetler, M. J.; Manner, W. L.; Nuzzo, R. G.; Girolami, G. S. J. Phys. Chem. 1995, 99, 15269. (25) Fuhrmann, D.; Wo¨ll, Ch. Submitted for publication.
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In conclusion, we have demonstrated that exposure of a clean Au(111) surface to heptanethiol leads to the formation of an ordered c(17×x3) structure, where the molecular axes are oriented parallel to the substrate surface. Only prolonged exposure to the alkanethiol (>40000 langmuirs) does lead to films which are comparable in coverage to those obtained from solution. This observation can be rationalized by considering that the well-ordered, rather closely packed layers which are formed when the alkanethiol molecules adsorb with their molecular axes parallel to the surface are so dense that the sticking coefficient for molecules hitting this monolayer drops to very small values. Note that for sticking a rather close contact between the SH group and the Au surface is required in order to dissociate the H atom and to form a S-Au bond. Acknowledgment. Partial financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged by H.-G.R. (grants RU 425/9-1 and RU 425/12-1,2) and Ch.W. (grants SFB 247/E2). We are indebted to Professor J. P. Toennies, Go¨ttingen, for his continuous interest and support. We have also profited from discussions with and the support of Professor M. Grunze, Heidelberg. In addition we thank Professor G. Scoles, Princeton, for very valuable comments on the manuscript. LA960704F
