Scanning Tunneling Microscopy Investigation of l-Lysine Adsorbed on

the molecules lie down on the surface and connect to their neighboring molecules by hydrogen bonds so to form two slightly different superstructures, ...
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Scanning Tunneling Microscopy Investigation of L-Lysine Adsorbed on Cu(001) Xueying Zhao, R. G. Zhao, and W. S. Yang* Mesoscopic Physics Laboratory and Department of Physics, Peking University, Beijing 100871, China Received March 13, 2000. In Final Form: June 29, 2000 Adsorption of L-lysine (or, in absolute nomenclature, S-lysine) on Cu(001) has been studied with a UHV-STM. The binding energy is estimated to be not lower than 1.6 eV, and thus lysine is expected to be in its anionic form on the surface. The adsorbates have two phases, that is, the two-dimensional (2D) gas and solid phases. We speculate that in the 2D gas phase the molecules stand upright on the surface with their two oxygen atoms at the atop sites and can diffuse frequently, and that in the 2D solid phase the molecules lie down on the surface and connect to their neighboring molecules by hydrogen bonds so 4 1 4 1 to form two slightly different superstructures, Cu(001)( -2 4)- and Cu(001)( -3 4)-L-lysine, for which models have been proposed for further investigation. Surprisingly, adsorption of L-lysine can make all steps restructuring into {3 1 17} facets with the same chirality. This phenomenon shows that chiral metal surfaces have potential in discrimination of molecular chirality.

1. Introduction Scanning tunneling microscopy (STM) has proved to be a powerful tool for investigation of organic films or adsorbates on various substrates. Well-ordered organic and biological films on solid surfaces have important technological applications in biocatalysis and biomaterials.1 The investigation on the protein-surface interaction is of great importance in certain aspects of biocompatibility of artificial biomaterials, medical implants, and biosensors.2 Because of the complexity of protein-surface interactions, model systems have also been frequently introduced to help. Adsorption of amino acids, which are the building blocks of proteins and many other biologically important systems and thus are much simpler compared to proteins, on various surfaces have long been used as model systems to study protein-surface interactions in aqueous solutions.3-7 Toward the same goal, fundamental studies of the adsorption of amino acids on metal surfaces in UHV environments have also been carrying out especially in recent years.8-18 In this paper, as a logical continuation of our recent investigations of glycine15-17 * To whom all correspondence should be addressed. E-mail: [email protected]. (1) Turner, A. F. Sensors Actuators 1989, 17, 433. (2) Elwing, H.; Askenda, A.; Ivarsson, B.; Nilsson, U.; Welin, S.; Lundstro¨m, J. In ACS Symposium Series 343, Proteins at Interfaces; Brash, J. L., Horbett, T. A., Eds.; American Chemical Society: Washington, DC, 1987; p 468. (3) Liedberg, B.; Carlsson, C.; Lundstro¨m, J. J. Colloid Interface Sci. 1987, 120, 64. (4) Salaneck, W. R.; Lundstro¨m, I.; Liedberg, B. Prog. Colloid Polym. Sci. 1985, 70, 83. (5) Liedberg, B.; Lundstro¨m, I.; Wu, C. R.; Salaneck, W. R. J. Colloid Interface Sci. 1985, 108, 123. (6) Ihs, A.; Liedberg, B.; Uvdal, K.; To¨rnkvist, C.; Bodo¨, P.; Lundstro¨m, I. J. Colloid Interface Sci. 1990, 140, 192. (7) Roddick-Lanzilotta, A. D.; Connor, P. A.; McQuillan, A. J. Langmuir 1998, 14, 6479. (8) Atanasoska, L. L.; Buchholz, J. C.; Somorjai, G. A. Surf. Sci. 1978, 72, 189. (9) Lange, W.; Jirikowsky, M.; Benninghoven, A. Surf. Sci. 1984, 136, 419. (10) Ernst, K. H.; Christmann, K. Surf. Sci. 1989, 224, 277. (11) Williams, J.; Haq, S.; Raval, R. Surf. Sci. 1996, 368, 303. (12) Booth, N. A.; Woodruff, D. P.; Schaff, O.; Giessel, T.; Lindsay, R.; Baumga¨rtel, P.; Bradshaw, A. M. Surf. Sci. 1998, 397, 258. (13) Barlow, S. M.; Kitching, K. J.; Haq, S.; Richardson, N. V. Surf. Sci. 1998, 401, 322.

and alanine18 adsorbed on Cu surfaces, we study the adsorption of L-lysine on a well-defined single-crystal Cu(001) surface in UHV environments. L-Lysine [(2S)-2,6-diaminocaproic acid, NH2(CH2)4CH(NH2)COOH] is an essential amino acid and is characterized by having an amine group at -C. At room temperature, lysine is a white crystalline solid with the vapor pressure on the order of 10-10 Torr,19 which exists as an inner salt or in the zwitterionic form NH3+(CH2)4CH(NH2)COO-.20 The decomposition point of lysine is 497 K.21 When warmed, solid lysine vaporizes and converts to the gas phase, but the conformation in the gas phase is not clear yet, probably in its neutral form (NH2(CH2)4CH(NH2)COOH), same as in the cases of glycine and alanine.22-25 In aqueous solution, its form depends on the pH value, varying from divalent cationic (NH3+(CH2)4CH(NH3+)COOH through monovalent cationic (NH3+(CH2)4CH(NH3+)COO-) and zwitterionic (NH3+(CH2)4CH(NH2)COO-) to anionic (NH2(CH2)4CH(NH2)COO-) as the pH increases.7 Ogura et al. conducted electrochemical and in situ Fourier transform infrared spectroscopy (FTIR) studies on adsorption and oxidation of lysine on Pt electrode in alkaline medium and found that adsorption of lysine on the Pt electrode, in certain potential range, was achieved through the COO- group of fully unprotonated anions.26 However, as far as we know, (14) Hasselstro¨m, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Petersson, L. G. M.; Samant, M. G.; Sto¨hr, J. Surf. Sci. 1998, 407, 221. (15) Zhao, X.; Zhao, R. G.; Gai, Z.; Yang, W. S. Acta Phys. Sinica 1998, 47, 1304 (Chinese). (16) Zhao, X.; Gai, Z.; Zhao, R. G.; Yang, W. S. Acta Phys. Sinica 1999, 48, 94 (Chinese). (17) Zhao, X.; Gai, Z.; Zhao, R. G.; Yang, W. S. Surf. Sci. 1999, 424, L347. (18) Zhao, X.; Zhao, R. G.; Yang, W. S. Surf. Sci. 1999, 442, L995. (19) Svec, H. J.; Clyde, D. C. J. Chem. Eng. Data 1965, 10, 151. (20) Bozack, M. J.; Zhou, Y.; Worley, S. D. J. Chem. Phys. 1994, 100, 8392. (21) Barrett, G. C. Chemistry and Biochemistry of the Amino Acids; Chapman and Hall: New York, 1985. (22) Godfrey, P. D.; Brown, R. D. J. Am. Chem. Soc. 1995, 117, 2019. (23) Ding, Y.; Krogh-Jespersen, K. Chem. Phys. Lett. 1992, 199, 261. (24) Csaszar, A. G. J. Phys. Chem. 1996, 100, 3541. (25) Gronert, S.; O’Hair, R. A. J. J. Am. Chem. Soc. 1995, 117, 2071. (26) Ogura, K.; Kobayashi, M.; Nakayama, M.; Miho, Y. J. Electroanal. Chem. 1998, 449, 101.

10.1021/la000378a CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/2000

STM of L-Lysine Adsorbed on Cu(001)

adsorption of lysine on metal surfaces in UHV conditions has never been studied before. In addition, as model catalysts, single-crystal transitionmetal surfaces have been intensively investigated about their structure, adsorption, and catalysis characteristics,27 whereas studies about their asymmetric adsorption and enantioselective heterogeneous catalysis properties almost have not been involved.28,29 The basic reason apparently is that transition-metal crystals are achiral. Whether or not asymmetric adsorption as well as related phenomena on achiral crystals, such as on transition metals, may also take place is the question. Recent researches28,29 have shown that, although transition-metal crystals are achiral themselves, some high Miller index surfaces may be chiral. In principle, similar to the definition of molecular chirality,30 a crystal surface can be thought of as chiral if it lacks reflection symmetry and center symmetry,28 because such a surface is not superimposable on its mirror image. Since chirality in some high Miller index surfaces of transition metals exists, it should be possible to observe asymmetric adsorption of chiral molecules on such surfaces. Nevertheless, until now, no definite experimental evidence has been provided.28,29 Here we report an intriguing discovery of the chiral restructuring of steps on Cu(001) surfaces driven by adsorption of L-lysine. This phenomenon evidently indicates that it is possible for chiral surfaces of transition metals to exhibit enantiospecific properties and will have important consequences for the investigations of chiral separations and asymmetric heterogeneous catalysis. 2. Experimental Section The experiment was conducted with the home-built UHVSTM system that was used in our recent amino acid/Cu works.15-18 In the STM experiment the bias voltage is applied to the sample and the tip is grounded. The tip is made of pure W wires with a diameter of 0.5 mm with electrochemical etching. The constant current mode of the STM was used throughout to work, and the scanning rate was from 200 to 2000 Å /s. Images given here were acquired at room temperature with the ac mode (same as the ac input mode of oscilloscopes, sometimes also called differential or local-contrast enhanced mode), unless otherwise mentioned. The Cu(001) sample was cleaned by repeated cycles of argon ion sputtering followed by annealing at 670 K. The deposition source was a Ta oven containing the L-lysine commercial powder with purity better than 99.0% and used without further purification. The source was degassed in situ for about 10 min at about 370 K before the first deposition. During depositions the oven was kept at about 410 K while the Cu(001) sample kept at room temperature facing the opening of the oven. The dose was about 10 L or lower (1 L ) 1.33 × 10-4 Pa s), and the rate was about 2 L/min. Since we do not know the sticking coefficient of lysine molecules on the surface, we do not know the real coverage either, even though we know the dose. However, we have noticed that once the surface is entirely covered by lysine further increasing the dose by a factor of 2 or even more does not change the surface structure. We thus believe that for the lysine/ Cu(001) system the sticking coefficient of second layer molecules must be very, very low, if not zero.

3. Results and Discussion 3.1. Early Stage of Adsorption and the 2D Gas Phase. At low exposures up to about 2 L of lysine vapor, (27) Davis, S. M.; Somojai, G. A. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1982; p 217. (28) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483. (29) Sholl, D. S. Langmuir 1998, 14, 862. (30) Atta-ur-Rahman; Shah, Z. Stereoselective Synthesis in Organic Chemistry; Springer-Verlag: New York, 1993.

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no adsorbed structures can be identified on Cu(001) terraces, except adsorbates at step edges. However, as we shall show below, the adsorbed lysine molecules form a two-dimensional (2D) gas on the terraces at this stage, though we could not directly image individual molecules of this 2D gas, likely because of the disparity between the time scales of STM data acquisition and diffusion as well as conformation change of the molecules. Further exposing the surface to about 4 L results in the nucleation of superstructure islands, which mostly exhibit a strip pattern as shown in Figure 1a. At this point, the coverage of the 2D gas phase reaches the saturated value and thus the excess adsorbed molecules segregate to form superstructure islands or 2D solid islands. Interestingly, right after exposing to 4 L of the lysine vapor, the surface only had many very small and narrow 2D solid islands randomly distributed on its Cu(001) terraces (see Figure 1a), but several hours afterward only fewer but larger such islands were found on the surface (see Figure 1b). This phenomenon may be indicative of the homogeneous nucleation mechanism of superstructure islands and the subsequent “Ostwald ripening”, which is distinctly different from the adsorption behavior of some other amino acids such as glycine and alanine on Cu.17,18 In the latter case, the ordered domains preferentially form at step edges. Nevertheless, even in the present case inhomogeneous nucleation may still play some role because domains connected to steps were occasionally observed (see Figure 1b). To show that the vast areas in Figure 1b other than the 2D solid islands contain indeed the 2D gas, a dc mode STM image of the surface is given in Figure 2a, which shows clearly that the areas imaged without any structures actually are 3.4 Å higher than the 2D solid islands (see Figure 2b), rather than lower as one would expect if no molecules adsorbed in these areas. Moreover, such large a height difference is an indication of that in the 2D gas phase the molecules orient upright with respect to the Cu surface, whereas in the 2D solid phase they orient horizontally. Actually, it has been known that the glycine molecule also tend to orient upright on Cu surfaces at low coverages.13,31 Interestingly, we observed once fluctuation of the boundaries between the two phases. Figure 3 shows three sequential STM images of nearly the same area where the 2D gas phase and the 2D solid phase coexisted. In the arrow pointing region, the width of the 2D gas region first became narrower (see Figure 3b) and then wider again (see Figure 3c). Obviously, this feature indicates that the 2D solid islands grows in equilibrium with the 2D gas phase. We also made an interesting tip-disturbing attempt, as shown in Figure 4. Scanning the surface with a small bias voltage of 500 mV and a large tunneling current of 10.0 nA can disturb the molecules so severely that the islands were almost completely destroyed. However, new islands in a different orientation but with almost the same total area appeared within 3 min (see Figure 4b). This indicates not only that the ordered phase is in equilibrium with the 2D gas phase but also that lysine molecules can diffuse on Cu surfaces very quickly even at room temperature. 3.2. Superstructures on (001) Terraces. At an even higher coverage all (001) terraces of the as-deposited surface can be covered only by the 2D solid phase, which may have two different superstructures, that is, Cu(001)4 1 4 1 ( -3 4)-L-lysine and Cu(001)( -2 4)-L-lysine. Annealing at (31) Wilson, W. D.; Bisson, C. L.; Schaldach, C. M. J. Colloid Interface Sci. 1997, 187, 201.

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Figure 1. (a) STM image of a typical area of the as-deposited lysine/Cu(001) surface right after the exposure of about 4 L of lysine at room temperature, showing the formation of randomly distributed narrow islands of the ordered superstructure (270 Å × 270 Å, 1.0 V, 1.0 nA). (b) STM image of a typical area of the same surface but 14 h later, showing the growth of ordered domains (270 Å × 270 Å, 1.0 V, 1.0 nA). The [3 5 0] and [-3 5 0] directions are marked on the basis of the atom resolved images of the clean Cu(001) surface (not shown here).

Figure 2. STM images (265 Å × 265 Å, 1.0 V, 0.6 nA) obtained from the lysine/Cu(001) surface that had both the 2D gas and ordered phases on. (a) Obtained with the dc mode (same as the dc mode of oscilloscopes). (b) From the same area as in a, but with the ac mode.

430 K, however, can make the former transforming to the latter and thus make the entire surface covered only by the latter. Given in Figure 5 are typical high-resolution STM images of the two superstructures obtained from the surface before and after annealing, respectively. Since in these images there are only two protrusions in each unit cell of the superstructures, we suppose that the unit cell consists of two lysine molecules, which is also considered reasonable from the van der Waals dimensions of the L-lysine molecule. This means that, same as in the

case of glycine/ and alanine/Cu(001)17,18 and probably not accidentally, each lysine molecule was imaged only as one protrusion. Of course, from such STM images alone one should not even try to guess the adsorption geometry of the molecules in these superstructures. Fortunately, some useful clues to the adsorption characters of lysine molecules on metal or Cu surfaces can be found from the relevant published papers. As it has been demonstrated that different forms of glycine may take completely different adsorption geometries and thus may have very

STM of L-Lysine Adsorbed on Cu(001)

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Figure 3. Sequential STM images showing the fluctuation of the domain sizes of the ordered structure and the 2D gas (160 Å × 160 Å, 1.0 V, 1.0 nA). Note that the width of the 2D gas domain the arrow points to varied from wider ((a) t ) 0) to narrower ((b) t ) 7 min) and to wider again ((c) t ) 14 min).

Figure 4. Sequential STM images showing the influence of an intentional tip disturbing (200 Å × 200 Å, 1.0 V, 0.6 nA). (a) Before the tip disturbance, there are two parallel striped domains. (b) After the tip disturbance, i.e., scanning at bias voltage 0.5 V and tunneling current 10 nA, the vertical ordered domains have become two horizontal domains.

different binding energies even on the same metal surface,31 we need, at least, to have some estimation on the binding energy of the lysine/Cu(001) system. Since within 20 min no detectable desorption of lysine occurred at 497 K (the decomposition point of lysine) it is reasonable to take 20 min as the lower limit of the mean lifetime τ of adsorbed molecules at this temperature. Then according to EB ) kT ln(τν), where EB, k, T, and ν are binding energy, Boltzmann constant, temperature, and attempt frequency (1013/s, as usually taken for simple activated processes), respectively, the binding energy has to be g1.6 eV. Such large a binding energy hints strongly that lysine adsorbs on the Cu(001) surface also in its anionic form,18,31 same as its adsorption form on Pt26 or TiO27 from solution. Actually, many amino acids adsorb on metal surfaces in their anionic form.9,11-14,32 The main chain of lysine, (32) Benninghoven, A.; Lange, W.; Jirikowsky, M.; Holtkamp, D. Surf. Sci. 1982, 123, L721.

excluding the side chain (-(CH2)4NH2), obviously is comparable with glycine, and hence the two oxygen atoms and the nitrogen atom are expected also to stay at or near the atop site as in the case of glycine/Cu(110).12-14 As for the conformation of the side chain -(CH2)4NH2, we refer to the conformations of lysine molecule in various crystals.33-35 Similarly, the two oxygen atoms of the formate14,36 and acetate14,37 on Cu(100) and (110) and the nitrogen atom of ammonia NH3 on Cu(100) also take atop (33) Koetzle, T. F.; Lehmann, M. S.; Verbist J. J.; Hamilton, W. C. Acta Crystallogr. B 1972, 28, 3207. (34) L’Haridon, P. P.; Lang, J.; Pastuszak, R.; Dobrowolski, J. Acta Crystallogr. B 1978, 34, 2436. (35) Prasad, G. S.; Vijayan, M. Acta Crystallogr. B 1991, 47, 927. (36) Woodruff, D. P.; McConville, C. F.; Kilcoyne, A. L. D.; Lindner, Th.; Somers, J.; Surman, M.; Paolucci, G.; Bradshaw, A. M. Surf. Sci. 1988, 201, 228. (37) Weiss, K.-U.; Dippel, R.; Schindler, K.-M.; Gardner, P.; Fritzsche, V.; Bradshaw, A. M.; Kilcoyne, A. L. D.; Woodruff, D. P. Phys. Rev. Lett. 1992, 69, 3196.

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4 1 Figure 5. High-resolution STM images (133 Å × 133 Å, 1.0 V, 0.6 nA). (a) The Cu(001)( -3 4)-L-lysine structure on the as-deposited 4 1 surface, with a unit cell outlined. (b) The Cu(001)( -2 4)-L-lysine structure on the annealed (430 K) surface, with a unit cell outlined.

sites.38,39 As the most favorable conformation, the side chain is all trans and thus fully extended.35 On the basis of above considerations, a unified model has been proposed 4 1 4 1 for both the ( -3 4) and the ( -2 4) superstructures for further investigation (see Figure 6). In the model the Cu(001) substrate is assumed to be bulklike (or unreconstructed) except for some relaxation, because we are not aware of any cases where the metal substrate is reconstructed by adsorption of amino acids. In the model, the two oxygen atoms of the carboxylate group and the two nitrogen atoms of the two amine groups locate approximately at the atop site while the side chain stays in the favorable trans conformation. As Figure 6 shows, the two superstructures are qualitatively similar. Specifically, all lysine molecules in both superstructures presumably are connected by O2-H-N hydrogen bonds to form headto-tail molecule chains either in the [3 5 0] direction or in the [-3 -5 0] direction; each [3 5 0] chain presumably is bound to a [-3 -5 0] chain by O2-H-N hydrogen bonds forming a molecule-chain pair; and both of the superstructures consist of only such molecule-chain pairs. The difference between the two superstructures is in the arrangement of the molecule-chain pairs and the relevant 4 1 coverage of lysine. In the ( -3 4) structure, as one can see from the model, no hydrogen bonds could form between 4 1 neighboring molecule-chain pairs, whereas in the ( -2 4) 1-H-N hydrogen bonds may form. This structure many O seems to indicate that the latter is energetically more favorable than the former and thereby may explain why after annealing only the latter exists on the surface. The 4 1 4 1 coverage for the ( -2 4) and ( -3 4) model is 2/18 and 2/19 ML, respectively, because for both there are two lysine molecules in a unit cell while their unit cell is 18 and 19 times larger than the (1 × 1) unit cell, respectively. We assume that, apart from formation of the hydrogen bonds, (38) Hussla, I.; Seki, H.; Chuang, T. J.; Gortel, Z. W.; Kreuzer, H. J.; Piercy, P. Phys. Rev. B 1985, 32, 3489. (39) Booth, N. A.; Davis, R.; Toomes, R.; Woodruff, D. P.; Hirschmugl, C.; Schindler, K. M.; Schaff, O.; Fernandez, V.; Theobald, A.; Hofmann, Ph.; Lindsay, R.; Giessel, T.; Baumga¨rtel, P.; Bradshaw, A. M. Surf. Sci. 1997, 387, 152.

Figure 6. Schematic drawing of the model proposed for the 4 1 two superstructures of the L-lysine/Cu(001) system. A ( -3 4) 4 1 unit cell (upper) and a ( -2 4) unit cell (lower) are outlined.

bonding of the N atoms in both the amine group and side chain to the Cu substrate may also have some contribution to stabilization of the superstructures,6,14 although in the gas phase the N atoms are not directly bonded to the substrate, as mentioned above. 3.3. Transition from the Gas Phase to the Solid Phase. To give a more comprehensive view of the 2D gas and solid phases of lysine adsorbed on the Cu(001) surface, as well as their relationships, we discuss further about

STM of L-Lysine Adsorbed on Cu(001)

their relationship. As addressed above, from the desorption temperature the binding energy of a lysine molecule on the surface is estimated to be about 1.6 eV or larger. This binding energy implies that lysine adsorbs on the surface in its anionic form because this value is very close to that of anionic glycine on Cu surfaces.31 As it has been known that at low coverages anionic glycine molecules “stand” on Cu(001)31 and (110)13 we thus expect that in the 2D gas-phase anionic lysine molecules also stand upright on the surface at the atop sites.13 Obviously, there must be some electrostatic repulsive forces between two such molecules, especially at small separations, because an anionic lysine molecule has not only a net negative charge but also a dipole momentum. We suppose that it is these repulsive forces that prevent the molecules from forming clusters or islands but a 2D gas homogeneously covering the entire surface. After the number density of lysine molecules of the 2D gas reaches a critical value any attempt to further increase it would fail because otherwise the intermolecular repulsive forces would be too strong and thus the 2D gas would become energetically too costly. Consequently, further depositions of lysine onto the 2D gas that has the maximum number density will result in nucleation of the 2D solid phase or superstructure islands, because the number density of lysine molecules can be higher in the solid phase where the molecules lie down on the surface. The major diving force for that presumably is, as mentioned above, formation of the hydrogen bonds between the neighboring lysine molecules, especially in 4 1 the ( -2 4) superstructure where each molecule forms 6 hydrogen bonds with 4 neighboring molecules, or 3 hydrogen bonds per molecule. This corresponds to an interaction energy of about 0.6 eV per molecule because a medium strength hydrogen bond has an interaction energy of about 0.2 eV.40 Compared with the binding energy of a single lysine molecule on the Cu(001) surface, that is, about 1.6 eV, this is almost 40% and thus is indeed a big gain in energy. Moreover, by lying down of the molecules the repulsive forces between dipoles can also be reduced. Therefore, although individual lysine molecules take the upright position on the Cu(001) surface, when there are more lysine molecules than what the 2D gas phase can take it is energetically favorable if the excessive molecules lie down on the surface, and the more there are the excessive molecules the smaller will be the area ratio of gas-to-solid until the surface is entirely covered by the 2D solid phase. As the transition proceeds through nucleation and growth of the solid phase it, by definition, is first order. Sometimes, we see regions such as what imaged in Figure 7, where different parallel narrow domains coexist. Clearly, the superstructure in the ordered domains is 4 1 normal ( -2 4) and its molecule chains are parallel to the 〈3 5 0〉 direction. In some other domains, especially the very narrow ones, however, some striped features with a width of one molecule chain or even some molecule-like features similar to those imaged from the ordered domains can be seen, though a bit vaguely. On the other hand, such domains are still higher than the 2D solid domains. Combining these features, we would guess that in such regions the status of the molecules is different from but similar to what in the 2D gas and solid phases. Note that the imaged features of the transition areas are real, rather than any artifact induced by the tip, because in Figures 1a, 2, and 4 no such features are seen near the edges of (40) Rigby, M.; Smith, E. B.; Wakeham, W. A.; Maitland, G. C. The Forces Between Molecules; Clarendon Press: London, 1986; p 18.

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Figure 7. STM image showing the coexistence of alternating striped domains of the solid phase and transition phase (270 Å × 400 Å, 1.0 V, 0.6 nA). The latter consists of parallel molecular rows running along the [3 5 0] or [-5 3 0] directions with row width of about 5.3 Å, which evolved from the gas phase.

narrow 2D solid domains. At this stage we do not precisely know when the transition areas appear. 3.4. Chiral Restructuring of Steps into {3 1 17} Facets. Although in the case of glycine/Cu(001)17 and L-alanine/Cu(001)18 the adsorption of the molecules may cause steps faceting to 〈3 1 0〉 directions even at room temperature, our observation shows that L-lysine molecules only randomly adsorb along steps and do not cause any step faceting at room temperature. However, annealing at 430 K for some 20 min can make all steps faceting and bunching to form very regular {3 1 17} facets (see Figure 8). The adsorption structure on the Cu{3 1 17} facets is (4 × 1)-L-lysine. After a careful inspection of many images such as Figure 8, it has been found that always only four, i.e., (-1 -3 17), (3 -1 17), (1 3 17), and (-3 1 17), rather than eight {3 1 17} facets, as expected from the C4V symmetry of the Cu(001) surface, appear on the well-annealed surface. In contrast, in the case of glycine/Cu(001)17 and L-alanine/ Cu(001)18 all eight {3 1 17} facets can be found on the surface. It is even more interesting to point out that in the present case the four {3 1 17} facets that appear on the surface all have the R chirality (according to the nomenclature proposed by McFadden et al.)28 and thus are rotationally equivalent relative to the [0 0 1] direction, while the rest four {3 1 17} facets that have never been seen on the surface, that is, (3 1 17), (-1 3 17), (-3 -1 17), and (1 -3 17), all have the S chirality and, of course, are also rotationally equivalent relative to [0 0 1]. Obviously, this could not be a result of any kinetic limitation as it happens on the well-annealed surface. In other words, on the R-{3 1 17} facets the adsorption energy of an L-lysine

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Figure 8. (a) STM image (550 Å × 550 Å, 1.0 V, 0.6 nA) acquired from the L-lysine/Cu(001) surface annealed at 430 K for 20 min, showing coexistence of (001) terraces and {3 1 17} facets formed through bunching of 〈3 1 0〉 faceted steps. Note that two neighboring intersection lines of (0 0 1) and {3 1 17} planes are always in 〈3 1 0〉 directions and thus are perpendicular to each other. As discussed in the text, this is a result of the homochirality of the {3 1 17} facets. (b) High-resolution STM image (125 Å × 125 Å, 1.0 V, 0.6 nA) of the Cu{3 1 17}4 × 1-L-lysine superstructure (upper right), with a (4 × 1) unit cell outlined.

molecule must be higher than that on the S-{3 1 17} facets. At this stage we do not know the precise difference but as the surface was annealed at 430 K, the difference must be larger than kT of this temperature, that is, 37 meV. Nevertheless, the difference ought to be high enough to provide some enantioselectivity if, say, a Cu(3 1 17) surface is used in adsorption of DL-lysine. A natural question then is why this does not happen for the glycine/ and alanine/Cu(001) systems. Actually, this is not really a question for the glycine case, because the molecules are achiral, although in the case of L-alanine this seems to be indeed a question. However, it can be well understood on the basis of the “three-point interaction model”41 for the mechanism of chiral recognition on an independent chiral stationary phase (CSP), which says for chiral recognition to happen, the CSP must have a minimum of three simultaneous interactions with one of the enantiomers.41 When using a clean Cu{3 1 17} surface, which is chiral,28 as the CSP, the amino group and the carboxylate group of an amino acid molecule can form strong chemical adsorption bonds with the surface, in other words, providing two interactions with the CSP.6,14 For an alanine molecule, only these two active functional groups are connected to its asymmetric R-carbon. However, the side chain of a L-lysine molecule, i.e., the -(CH2)4NH2 group, provides the third simultaneous interaction and thus makes chiral recognition possible. 4. Summary In summary, L-lysine adsorbs on the Cu(001) surface with a binding energy estimated to be not lower than 1.6 (41) Pirkle, W. H.; Finn J. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1993; p 87.

eV and thus is expected to be in its anionic form. The adsorbates may form a 2D gas phase and a 2D solid phase, and they may exist alone or coexist on the surface, depending on the coverage and deposition rate. We speculate that in the 2D gas phase the molecules stand upright on the surface with their two oxygen atoms at the atop sites and can diffuse frequently as they are far apart from each, and that in the 2D solid phase the molecules lie down on the surface and are connected to their neighboring molecules by hydrogen bonds to form two 4 1 slightly different superstructures, that is, Cu(001)( -2 4)4 1 and Cu(001)( -3 4)-L-lysine. The former has a slightly higher coverage and probably also a higher density of hydrogen bonds, can survive annealing up to 497 K, and thus is more stable. Depending on the deposition rate, some transition features may or may not appear between a gas domain and a solid domain. L-lysine adsorbates on the Cu(001) surface can make all steps restructuring into four rotationally equivalent Cu{3 1 17}4 × 1-L-lysine facets with a R chirality, indicating that L-lysine has a higher adsorption energy on Cu{3 1 17} facets with a R chirality than on those with a S chirality. This fact demonstrates that chiral metal surfaces do have potential in discrimination of molecular chirality.42 Acknowledgment. This work was supported by the National Natural Science Foundation of China (grant no. 19634010) and by the Doctoral Program Foundation of Institution of Higher Education of China. LA000378A (42) Ahmadi, A.; Attard, G.; Feliu, J.; Rodes, A. Langmuir 1999, 15, 2420.