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Self-Assembled Binary Monolayers of n-Alkanes on Reconstructed Au(111) and HOPG Surfaces Z. X. Xie,*,† X. Xu,† B. W. Mao,† and K. Tanaka*,‡ State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China, and Advanced Research Laboratory, Saitama Institute of Technology, 1690 Fusaiji, Okabe, Saitama, Japan Received June 11, 2001. In Final Form: December 4, 2001 Self-assembly of n-heptadecane (n-C17H36) and n-hexatricontane (n-C36H74) was studied by means of scanning tunneling microscopy. A droplet of a solution of n-C17H36 containing 0.6 wt % of n-C36H74 formed an ordered binary alkane monolayer on the herringbone Au(111) surface, which was composed of 30% n-C36H74 and 70% n-C17H36. The alkane molecules were arrayed in parallel to the [011 h ] azimuth, and their intermolecular distance was 0.48 nm, which is equal to the distance of solid alkanes. In contrast, the alkane monolayer formed on a highly oriented pyrolytic graphite (HOPG) surface was predominantly composed of an n-C36H74 layer including only ca. 3% of n-C17H36 as an impurity. The n-C36H74 molecules on the HOPG surface were compressed along the molecular axis and in the perpendicular direction to the molecular axis. It is deduced that the intermolecular interaction is optimized when the surface structure is profitable for self-assembly of alkanes where no strong alkane-surface interaction is required.
Introduction Organic molecules arraying on the surface and their properties at the solid/liquid interface are crucially interesting, because the overlayer at the interface plays important roles in many technological phenomena, such as lubrication, friction, adhesion, molecular recognition, corrosion, and chemical reactions. The array of molecules on the surface depends on both the adsorbate-substrate and adsorbate-adsorbate interactions. The latter interaction is relatively weak in general, so the adsorbatesubstrate interaction has been dominantly considered to explain the adsorption of molecules as well as the growth of organic overlayers. In fact, the term “self-assembled monolayer” has been used for strong adsorption such as the adsorption of alkane thiol on the Au(111) surface. Real self-assembly, however, should sensitively depend on the adsorbate-adsorbate interaction. When the adsorbatesubstrate interaction becomes weak, we can expect that the adsorbate-adsorbate interaction will play more and more important roles in self-assembling of molecules on the surface. So far, the formation of an ordered organic monolayer was reported on various solid surfaces,1 but the mechanism was not well explored. The adsorption of normal alkanes (n-alkanes) was also studied on various single-crystal surfaces such as highly oriented pyrolytic graphite (HOPG),1-3 MoS2, and Au(111),4-10 but the * Corresponding authors. Prof. Ken-ichi Tanaka: Phone & Fax, (+81) 48-585-6874; E-mail,
[email protected]. Zhao-Xiong Xie: E-mail,
[email protected]. † Xiamen University. ‡ Saitama Institute of Technology. (1) Ikai, A. Surf. Sci. Rep. 1996, 26, 261. (2) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (3) Giancarlo, L. C.; Flynn, G. W. Annu. Rev. Phys. Chem. 1998, 49, 297. (4) Xia, T. K.; Landman, U. Science 1993, 261, 1310. (5) Cincotti, S.; Rabe, J. P. Appl. Phys. Lett. 1993, 62, 3531. (6) Giancarlo, L. C.; Fang, H.; Rubin, S. M.; Bront, A. A.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 10255. (7) Uosaki, K.; Yamada, R. J. Am. Chem. Soc. 1999, 121, 4090. (8) Yamada, R.; Uosaki, K. Langmuir 2000, 16, 4413. (9) Marchenko, O.; Cousty, J. Phys. Rev. Lett. 2000, 84, 5363. (10) Xie, Z. X.; Xu, X.; Tang, J.; Mao, B. W. Chem. Phys. Lett. 2000, 323, 209.
mechanism for the array of alkane molecules has not been well rationalized by the alkane-surface and alkanealkane interactions considering the surface structures. In this paper, we studied the formation of binary alkane monolayers on a reconstructed Au(111) surface and on a HOPG surface by the coadsorption of n-C36H74 and n-C17H36 from a liquid phase. Taking account of the fact that the adsorption of alkanes is weak on the herringbone Au(111) surface but relatively strong on the HOPG surface, we compared the effects of the surface structures and the alkane-surface interactions for the self-assembly of alkanes on these two surfaces. Experimental Section The Au(111) surface used in this experiment was prepared by epitaxial growth of gold on a fresh mica surface, and the reconstructed Au(111) surfaces were obtained by annealing it in a hydrogen flame. A droplet of heptadecane (C17H36) saturated with 0.6 wt % of n-hexatricontane (C36H74) at 298 K was made on a herringbone Au(111) surface or on a freshly cleaved HOPG surface, and the scanning tunneling microscopy (STM) was obtained by a Nano-Scope-III by putting a Pt/Ir tip in the alkane droplet.
Results and Discussion Figure 1a shows an STM feature of an ordered binary alkane layer of n-C36H74 and n-C17H36 formed on a reconstructed Au(111) surface. The long and short lamellar lines reflect the adsorbed molecules of n-C36H74 and n-C17H36. The dark troughs are the domain boundaries of adsorbed n-C36H74 and n-C17H36, but they are not perpendicular to the lamellar lines. Bright humps in Figure 1a indicated with arrows reflect the ridges of the herringbone structure of the Au(111) surface (see Figure 3). The length of the n-C36H74 molecule forming a long lamellar line is 4.8 ( 0.1 nm, and the short lamellar line of n-C17H36 takes a half length of it. These two lines agglomerate, and a plaidlike pattern is formed on the surface, where the ratio of the two alkanes is 30% n-C36H74 and 70% n-C17H36 in the monolayer. The dark trough in Figure 1a reflecting the domain boundary is perpendicular to the ridge of the herringbone
10.1021/la010869a CCC: $22.00 © 2002 American Chemical Society Published on Web 03/08/2002
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Figure 2. Self-assembled alkane monolayer formed on a HOPG surface by immersing in a saturated solution of C36H74 in C17H36. The image was obtained for 25 × 25 nm2 by a tunneling condition of Vb ) 1 V, It ) 0.1 nA.
Figure 1. (a) STM image of a self-assembled monolayer of binary alkanes on the reconstructed Au(111) surface. Coadsorption was performed in the saturated solution of C36H74 in C17H36. Big arrows indicate the ridges of the herringbone structure. (b) Dynamic replacement of alkane molecules proved by an STM image at room temperature. The STM feature shows an image taken 2 min after the image in (a), where the arrows indicate the replaced molecules. The image was obtained for 25 × 25 nm2 by a tunneling condition of Vb ) 0.1 V, It ) 1.2 nA.
structure. Therefore, the angle between the trough and the lamellar line is 62° ( 1°, that is, the molecular axis of n-C36H74 and n-C17H36 is inclined to the boundary. The domain boundaries are not always perpendicular to the ridges of the herringbone structure while the molecular axis is parallel to the [1h 01] or [011h ] direction. That is, the domain boundaries run to lower the total energy, because the surface energy of the herringbone Au(111) structure is heterogeneous. In contrast, the alkane molecules adsorbed on the HOPG surface make straight domain boundaries being perpendicular to the adsorbed alkane
molecules as will be discussed below. When n-C36H74 and n-C17H36 molecules are adsorbed on the herringbone Au(111) surface in parallel along the [1h 01] or [011h ] direction, they take an intermolecular spacing of 0.48 ( 0.02 nm. This spacing is almost equal to the distance of alkane chains in their solid crystals. It is known that the molecular axis of n-C36H74 and that of n-C17H36 are in a line through the two domains. If we look carefully, the n-C36H74 and n-C17H36 molecules in a line have a slight mismatch at the junction, which is caused by a different orientation of the terminal methyl (-CH3) groups of the odd and even carbon number alkanes. When the alkane molecules undergo adsorption along the [1 h 01] or [011 h ] direction on the reconstructed Au(111) surface, the alkane molecules can take an intermolecular distance of 0.48 nm, which is an optimized distance for making the self-assembled monolayer. On the other hand, the herringbone Au(111) surface such as that shown in Figure 3 has nonuniform surface energy so that the domain boundaries in the monolayer run to lower the total energy. Figure 1b shows an STM image for the same area acquired about 2 min after image a. When we compare these two STM images, it is known that the n-C36H74 replaces n-C17H36 molecules and vice versa as is marked with the arrows. From these experiments, we could conclude that the binary alkane monolayer observed in Figure 1a reflects an equilibrium array of the alkane molecules on the herringbone Au(111) surface. On the other hand, when a HOPG wafer was dipped in a n-C17H36 solution containing 0.6 wt % of n-C36H74, the surface was predominantly covered with a monolayer of n-C36H74 containing ca. 3% of n-C17H36 as shown in Figure 2, which is in remarkable contrast to the binary layer of 30% n-C36H74 and 70% n-C17H36 formed on the reconstructed Au(111) surface. The n-C36H74 layer containing 3% of n-C17H36 formed on the HOPG is also in equilibrium, because the adsorbed n-C36H74 molecules undergo replacement with n-C17H36 molecules and vice versa in keeping 3% at room temperature. The length of the n-C36H74 molecule is 4.7 ( 0.1 nm on the HOPG surface, which is undoubtedly shorter than the n-C36H74 (4.8 ( 0.1
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nm) on the reconstructed Au(111) surface. This fact suggests that the molecule is compressed on the HOPG surface along the molecular axis. On the other hand, the intermolecular distance of n-C36H74 in the monolayer was 0.425 ( 0.01 nm, which was almost equal to that of the other alkanes adsorbed on HOPG surfaces.1-3 These facts indicate that the zigzag (-CH2-CH2-CH2-) chain of the alkane molecule is forced to match with the hexagonal skeletal carbon rings on the graphite surface so that half of the hydrogens of the (-CH2-) are at the center of hexagonal carbon rings.11,12 As a result, the alkane monolayer on the HOPG surface is compressed along the molecular axis as well as perpendicular to the hydrocarbon chains. The structural fitting may increase the adsorption energy of alkane molecules so the longer the molecule, the stronger the adsorption on the HOPG surface. In other words, the strain of the monolayer is compensated with the adsorption energy gained by structural fitting on the graphite surface. Watel and Thibaudau13 reported that one n-C36H74 molecule adsorbed on the graphite surface shares 19 periodic hexagons; one of the 19 hexagons is used as a spacing of the domain boundary. If this is the case, the chain length of the n-C36H74 on the HOPG surface is estimated to be 0.246 nm × 19 ) 4.67 nm, which is in very good agreement with our observed value of 4.7 nm. On the other hand, a unit length of a bended (-CH2CH2-CH2-) is 0.252 nm for a free alkane molecule. Therefore, when a n-C36H74 molecule adsorbs with no compression, a periodic length of the molecule will be 4.79 nm (19 × 0.252 nm), which is in good agreement with 4.8 nm of the n-C36H74 molecule adsorbed on the Au(111) surface. Therefore, we can conclude that the alkyl molecules are not compressed on the Au(111) surface. So far, the thermodynamic argument for the formation of the ordered two-dimensional (2D) alkane layer was discussed in relation to the three-dimensional crystalline alkane,14 in which the effect of the surface structure was not seriously considered. However, the intermolecular distance of n-C36H74 is 0.425 nm on the HOPG surface, which is undoubtedly narrower than that of the crystal of n-C36H74 (0.48 nm).15 As we discussed above, the adsorbed n-C36H74 molecules on the HOPG surface are compressed along the molecular axis as well as in the 2D array of the molecules. Therefore, an apparent density of n-C36H74 molecules on the HOPG surface is higher than that of the crystalline n-C36H74. The better the fitting of the (-CH2-) groups to the hexagons of the HOPG surface, the stronger the adsorption of n-C36H74, which is the driving force for the formation of the compressed monolayer on the HOPG surface. This argument predicts that it will be difficult for shorter alkane molecules to form well-ordered monolayers on the HOPG surface. In fact, no STM image was reported for the alkane molecules shorter than n-C16H34 on the HOPG surface at room temperature. However, the normal paraffins shorter than n-hexadecane form monolayers and n-C7H16 to n-C14H30 alkanes form 2-3 monolayers on graphite at room temperature although these monolayers may not be well ordered.16
The results of this paper indicate that when the surface lattice spacing is profitable for the alkane-alkane interaction, the alkane molecules are self-assembled over the surface provided that the surface-alkane interaction is weak. In this case, the domain boundaries run to lower the total surface energy. The herringbone structure of (x3 × 22) Au(111) is formed by anisotropic contraction of surface atoms by about 4% as shown in Figure 3, where the lattice shortening occurs along the [11h 0] direction and 23 Au atoms are packed in a space of the 22 original Au h1 h 2] atoms.17-19 In this model, the ridges align along the [1 azimuth being perpendicular to the [11 h 0] direction. The alkane molecules on the herringbone Au(111) surface lie parallel to the [011h ] or [1 h 01] azimuth, so that the molecular axis is tilted 30° to the ridges. The [11h 0] and [011h ] azimuths are equivalent on the (1 × 1) Au(111) surface, but these two are distinctive on the herringbone surface. A periodic lattice distance along the [2 h 11] is 0.48 nm, while it is 0.50 nm in the [1h 1 h 2] azimuth (along the ridge) on the herringbone surface as shown in Figure 3. When the alkane molecules are on every two rows of the (x3 × 22) Au(111) surface in parallel to the [011 h ] azimuth, the alkane chains well fit with the lattice structure of the reconstructed Au(111) surface20 and have the intermolecular spacing of 0.48 nm. This intermolecular spacing is equal to the interchain distance of a solid alkane crystal so that the binary alkanes form a characteristic inlaid pattern on the reconstructed Au(111) surface. In fact, the alkane molecules from C12H26 to C17H36 take an intermolecular distance of 0.48 nm on the reconstructed Au(111) surface,20,21 and even such a short alkane as n-C12H26 (liquid at room temperature) can form a well-ordered 2D crystal layer on a reconstructed Au(111) surface.7,9,21,22 This is in remarkable contrast to the alkane molecules on the HOPG surface, where the alkanes shorter than n-C16H34 cannot form ordered monolayers at room temperature. Taking these facts into account, we can conclude that the alkanealkane interaction is enhanced by profitable structural fitting on the herringbone Au(111) surface. On the basis of these results, we could say that the values of 0.43 nm reported by Uosaki7,22 and 0.50 nm by Cousty9 are not correct experimentally and theoretically. The adsorption of normal alkanes on MoS2 or MoSe2 is relatively weak, and the heat of adsorption of n-dotriacontane on MoS2 is approximately 1/3 of the value obtained
(11) Groszek, A. J. Proc. R. Soc. London, Ser. A 1970, 314, 473. (12) Groszek, A. J. Nature 1964, 204, 680. (13) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. Lett. 1993, 281, L297. (14) Findenegg, G. H.; Liphard, M. Carbon 1987, 25, 119. (15) Small, D. M. The Physical Chemistry of Lipids: From Alkanes to the Phospholipids; Hanahan, D. J., Ed.; Plenum Press: New York, 1986; p 24. (16) Groszek, A. J. Adsorption at the Gas-Solid and Liquid-Solid Interface; Elsevier: New York, 1982; pp 55-67.
(17) Wo¨ll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 39, 7988. (18) Chambliss, D. D.; Wilson, R. J.; Chiang, S. Phys. Rev. Lett. 1991, 66, 1721. (19) Dakkouri, A. S.; Kolb, D. M. Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 151. (20) Xie, Z. X.; Xu, X.; Tang, J.; Mao, B. W. J. Phys. Chem. B 2000, 104, 11719. (21) Our unpublished results. (22) Yamada, R.; Uosaki, K. J. Phys. Chem. 2000, 104, 6021.
Figure 3. The schematic model of the reconstructed Au(111) surface.
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on graphite.12 Accordingly, the adsorbate-adsorbate interaction will promote the self-assembly of n-alkanes on MoS2 or MoSe2. In fact, it was confirmed that the alkane monolayer formed on MoS2 or MoSe2 takes the same intermolecular distance as that of the solid bulk alkane.5,23 In this case, however, the adsorbed alkane molecule is highly mobile so that it gives only poor-resolution STM images and only long-chain alkanes can make ordered monolayers.5,6 This is in contrast to the alkane molecules (23) Cincotti, S.; Burda, J.; Hentschke, R.; Rabe, J. P. Phys. Rev. E 1995, 51, 2090.
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adsorbed on the reconstructed Au(111) surface, where such a short alkane as n-C12H26 forms a well-ordered monolayer by the array of alkane molecules fitting with the reconstructed Au(111) surface. Acknowledgment. Z. X. Xie, X. Xu, and B. W. Mao appreciate the support of Grant Nos. 20023001 and 20021002 by the National Natural Science Foundation of China, and Z. X. Xie also appreciates Grant No. 20173046 and the Ministry of Education of China. LA010869A
