Physisorption-Induced Surface Reconstruction and Morphology

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Physisorption-Induced Surface Reconstruction and Morphology Changes: Adsorption of Glycine on the Au(110) 1 × 2 Surface Xueying Zhao, Hao Yan, R. G. Zhao, and W. S. Yang* Mesoscopic Physics Laboratory and Department of Physics, Peking University, Beijing 100871, China Received September 4, 2001. In Final Form: February 14, 2002 Although it is well-known that chemisorption may change the reconstruction or even the morphology of metal surfaces, one would not expect to see similar effects induced by physisorption. In this paper, we report, however, exactly such effects induced by physisorption of glycine on the Au(110) 1 × 2 surface. Specifically, at low coverage the adsorbates can change the 1 × 2 reconstructed surface to a well-ordered 1 × 3, whereas further adsorption can even dramatically modify the surface morphology until becoming completely faceted. As byproducts, two new reconstructions of clean Au(110), i.e., 1 × 1 and 1 × 3, have been found, with the former being metastable at or below room temperature while the latter at higher temperatures not beyond 170 °C. The fast and reversible room-temperature local transition between the two along with its possible mechanism will also be discussed.

Introduction There has been a growing interest in the interaction between organic molecules and metal surfaces, and the driving force behind this, apparently, is manifold. For instance, molecule/metal interfaces exist inevitably in all molecular electronic devices,1-3 while it has been shown that organic molecules, such as proteins, can alter inorganic microstructures, offering a very powerful tool for the design of novel materials,4 and that structural order over many length scales can be created by sophisticated use of self-organizing materials, and this may hold the key to developing new structures and devices.5 In addition, a fundamental understanding of the biocompatibility of proteins with artificial implant materials was also the motivation behind many recent studies of amino acid adsorption on surfaces.6-10 As far as adsorption of amino acids on single-crystal metal surfaces is concerned, the most studied so far are copper,6-19 in contrast to gold or silver substrates. * Corresponding author. E-mail: [email protected]. (1) Collier, C. P.; Wong, E. G.; Belohradsk’y, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (2) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (3) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (4) Stupp, S. I.; Braun, P. V. Science 1997, 277, 1242. (5) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225. (6) Wilson, W. D.; Bisson, C. L.; Schaldach, C. M. J. Colloid Interface Sci. 1997, 187, 201. (7) Barlow, S. M.; Kitching, K. J.; Haq, S.; Richardson, N. V. Surf. Sci. 1998, 401, 322. (8) Hasselstro¨m, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Samant, M. G.; Sto¨hr, J. Surf. Sci. 1998, 407, 221. (9) Booth, N. A.; Woodruff, P. D.; Schaff, O.; Giessel, T.; Lindsay, R.; Baumga¨rtel, P.; Bradshow, A. M. Surf. Sci. 1998, 397, 258. (10) Nyberg, M.; Hasselstro¨m, J.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem Phys. 2000, 112, 5420. (11) Atanasoska, L. L.; Buchholz, J. C.; Somorjai, G. A. Surf. Sci. 1978, 72, 189. (12) Colton, R. J.; Murday, J. S.; Wyatt, J. R.; Decorpo, J. J. Surf. Sci. 1979, 84, 235. (13) Lange, W.; Jirikowsky, M.; Benninghoven, A. Surf. Sci. 1984, 136, 419.

Considering how much effort has been spent on investigation of adsorption of organosulfur compounds on gold,20 this fact of course does not mean that gold substrates are less important in application or basic research but, obviously, because adsorption of amino acids on gold substrates is physisorption in character.21 Physisorption systems have been studied mainly either in basic research of phase transitions of quasi-2D systems22 or for other purposes in applications such as physisorbed-precursormediated chemisorption,23 because one would not expect any effect induced by the strong adsorbate/substrate interactions in chemisorption systems to happen also in physisorption systems. In this paper, however, we studied adsorption of glycine on the Au(110) surface and have found that the adsorption, albeit indeed being physisorption in character, can, surprisingly, induce significant changes in surface reconstruction and surface morphology or even can bring the clean Au(110) 1 × 2 surface into some metastable status, which otherwise would be impossible. Experimental Section The experiment was conducted with the home-built roomtemperature UHV-STM system that was used in our recent amino acid/Cu works.14-18 Briefly, in the STM experiment the bias voltage is applied to the sample and the tip is grounded. The tip was made of pure W wires with a diameter of 0.5 mm with electrochemical etching. The constant current mode of the STM (14) Zhao, X.; Gai, Z.; Zhao, R. G.; Yang, W. S. Surf. Sci. 1999, 424, L347. (15) Zhao, X.; Zhao, R. G.; Yang, W. S. Surf. Sci. 1999, 442, L995. (16) Zhao, X.; Zhao, R. G.; Yang, W. S. Langmuir 2000, 16, 9812. (17) Wang, H.; Zhao, X.; Zhao, R. G.; Yang, W. S. Chin. Phys. Lett. 2001, 18, 445. (18) Zhao, X.; Zhao, R. G.; Yang, W. S. Langmuir 2002, 18, 433. (19) Zhao, X.; Wang, H.; Zhao, R. G.; Yang, W. S. Mater. Sci. Eng. C 2001, 16, 41. (20) See, for example: Ulman, A. Chem. Rev. 1996, 96, 1533. (21) Lange, W.; Jirikowsky, M.; Benninghoven, A. Surf. Sci. 1984, 136, 419. (22) See, for example: Stranburg, K. J. Rev. Modern Phys. 1988, 60, 161. (23) See, for example: Leisenberger, F. P.; Surnev, S.; Koller, G.; Ramsey, M. G.; Netzer, F. P. Surf. Sci. 2000, 444, 211. Loepp, G.; Vollmer, S.; Witte, G.; Woell, Ch. Langmuir 1999, 15, 3767.

10.1021/la011390l CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002

Adsorption of Glycine on the Au(110) 1 × 2 Surface

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Figure 1. STM images (500 Å × 500 Å) obtained from the Au(110) surface in the course of the low-rate (see text) glycine deposition: (a) the clean Au(110) 1 × 2 surface before deposition, obtained with a bias voltage of -2.0 V and a tunnel current of 0.6 nA; (b) image (1.0 V, 0.6 nA) of the 1 × 3-reconstructed surface obtained at the end of the low-rate deposition process; (c) (top half) top view (upper) and side view (lower) of the missing-row model of the Au(110) 1 × 2 surface, (bottom half) top view (top) and side view (bottom) of the 1 × 3-reconstructed Au(110) surface; (d) dc mode image (0.1 V, 0.6 nA) obtained at the early stage of the low-rate deposition process (note that on the two major terraces 1 × 3-reconstructed islands and long stripes separated by the deeper and wider troughs start to develop on the surface, although the major portion of the surface is still 1 × 2-reconstructed); (e) ac mode image (1.0 V, 0.6 nA) of the same surface as the one imaged in (d). 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, unless otherwise mentioned. Note that ac and dc modes are the same as the correspondent input modes of oscilloscopes. In the case of STM, the ac mode sometimes is also called as differential or local-contrast enhanced mode. 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 glycine commercial powder with the purity better than 99.0% and was used without further purification. The source was degassed in situ for about 10 min at about 370 K before the first deposition. During deposition the source was kept at about 400 K and the Cu(001) sample was either facing the opening of the source or staying at the STM stage, depending on the required deposition rate, but was always at room temperature. The sample and source temperature were monitored with a PtRh-Pt and copper-constantan thermocouple, respectively. It has been known that in the case of chemisorption of amino acids on the Cu(001) surface the desorption rate at room temperature is negligibly small14-19 and thus in principle with any none-zero deposition rate one can achieve any coverage up to saturation, provided that time is not limited. It should be pointed out that this is no longer the case for physisorption systems such as the present one where, as we shall see, significant desorption exists even at room temperature as long as there are some molecules adsorbed on the surface. Therefore, it is not easy to keep the coverage constant for a long time, because only when the deposition rate equals the desorption rate or, in other words, only when the surface is in dynamic equilibrium can the coverage be stable. Of course, it is even more difficult to reach a specific coverage and then keep it unchanged, because the desorption rate itself must be, apparently, coverage-dependent. Actually, the coverage of the surface must be always changing more or less, either increasing or decreasing. In the experiment we used, roughly speaking, three typical deposition rates, that is, the low, medium, and high rates. To get the high deposition rate, the

source temperature was set to about 400 K so to make the vapor pressure to be about 2 × 10-5 Pa (with the ion pump off) and the sample was put in front of the source about 5 cm away. The medium deposition rate was obtained at the same vapor pressure but with the sample staying at the STM stage, where the sample does not face the source. We call the rate resulted from the residual vapor (with the ion pump on) the low deposition rate, although this rate inevitably became gradually higher after many times of high- and medium-rate depositions. Note that under these deposition rates the coverage may either be increasing or decreasing, depending on the coverage itself. Nevertheless, with the three deposition rates, which apparently are defined a bit vaguely, we were able to explore a quite large coverage range and thus have touched most or even, hopefully, all typical phenomena possible for the system.

Observation and Discussion For the reasons mentioned above, we address first the phenomena observed during net deposition processes and then describe what we have seen during net desorption processes. 1. Phenomena Observed with Increasing Coverage. A. Low-Rate Deposition. The clean and well-annealed Au(110) surface is 1 × 2 reconstructed (see Figure 1a), and the “missing-row” model (the upper half of Figure 1c) describes its structure.24,25 The low-rate deposition of glycine onto the surface did not result in any visible features on the surface but only makes imaging of the 1 × 2 reconstruction more difficult, indicating that the adsorbates exist in the 2D gas phase, which has been seen to exist in the case of adsorption of amino acids on the (24) Ho, K. M.; Bohnen, K. P. Phys. Rev. Lett. 1987, 59, 1833. (25) Yamagishi, T.; Takahashi, K.; Onzawa, T. Surf. Sci. 2000, 445, 18.

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Cu(001) surface.18,19 The only difference is that in the present case individual molecules are not visible by means of STM because the diffusion barrier for physisorption systems must be, apparently, smaller than that for chemisorption systems, and thus, the molecules diffuse faster than their counterparts in chemisorption systems do.19 Despite this, the low-rate deposition, surprisingly, can make the 1 × 2 reconstructed surface a well-developed 1 × 3 (see Figure 1b). Chemisorption-induced change in surface reconstruction has been well-known. For instance, CO chemisorption on the Pt(110) 1 × 2 surface can lift the 1 × 2 reconstruction, making the surface consisted of small 1 × 1 patches,26 while oxygen can change the reconstruction of this surface to 1 × 3.27 Physisorption-induced change in surface reconstruction, however, is rare, if not never having been reported at all. As one can see from Figure 1a,b, the atomic corrugation of the 1 × 3 reconstructed surface is significantly larger than that of the 1 × 2 reconstructed surface. This fact leads us to the straightforward model of the 1 × 3 reconstructed surface (see the lower half of Figure 1c), which is the same as the faceting model proposed for the “clean” 1 × 3 reconstructed surface obtained by heating the clean Au(110) 1 × 2 surface in an oxygen atmosphere.27 The width of the long (111) nanofacets of this reconstruction is greater than that of the missing-row model and is about just right for adsorption of a glycine molecule. This presumably is the driving force of the 1 × 3 reconstruction. Presented in Figure 1d,e are two images acquired at the early stage of the low-rate deposition process when some narrow 1 × 3 reconstructed islands just started to grow on the clean 1 × 2 reconstructed surface. From Figure 1d one can see that narrow 1 × 3 reconstructed islands started to grow on top of the 1 × 2 reconstructed terraces, meanwhile 1 × 3 reconstructed strips started to develop in the terraces at the cost of the 1 × 2 reconstructed areas. This indicates that in the transition process from 1 × 2 to 1 × 3 reconstruction the excessive atoms “dug out” from a terrace (see Figure 1c) may aggregate nearby to form 1 × 3 islands on the terrace rather than necessarily diffusing away from it. From Figure 1e one can see that 1 × 3 reconstructed rows in a terrace tend to appear in pairs. As the width of a pair of 1 × 3 rows is the same as that of three 1 × 2 rows (see Figure 1c), this fact reflects that the surface tends to avoid 1 × 1 rows from appearing at 1 × 2 terraces, on one hand, and not to disturb the remaining 1 × 2 area of the terrace, on the other. B. Medium-Rate Deposition. Deposition of glycine onto the clean 1 × 2 surface by means of the medium rate, however, resulted in many different visible features as well as significant surface morphology changes. The intermediate phase18,19 (or the chain phase28) appeared first, where the molecules align into chains with a length of about 45 Å, and the chains further aggregate into domains separated by domain walls (see Figure 2a). Further deposition resulted in at least three different superstructures coexisting simultaneously with the chain phase: Au(110) c(4 × 4)-glycine (Figure 2b); Au(110)Xglycine (Figure 2c,d); Au(110) c(2 × 4)-glycine (Figure 3b). Of particular interest is that, as Figure 3 shows, the c(2 × 4)-glycine domains tend to develop at a level below the surface and thus give rise to many troughs at the originally flat terraces. Considering what we are dealing (26) Jackmann, T. E.; Davies, J. A.; Jackson, D. P.; Unertl, W. N.; Norton, P. R. Surf. Sci. 1982, 120, 389. Gritsch, T.; Coulman, D.; Behm, R. J.; Ertl, G. Phys. Rev. Lett. 1989, 63, 1086. (27) Frey, P.; Moritz, W.; Wolf, D. Phys. Rev. B 1988, 38, 7275. (28) Zhao, X.; et al. To be published.

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Figure 2. STM images of the intermediate phase (or chain phase) and superstructures appeared in the course of mediumrate (see text) deposition process: (a) the intermediate phase (240 Å × 240 Å, 1.0 V, 0.6 nA), where the molecules align into chains with a length of about 45 Å and the chains aggregate into domains separated by domain walls (running from lower left to upper right); (b) an island of the Au(110) c(4 × 4)-glycine superstructure surrounded by the intermediate phase (250 Å × 250 Å, 1.0 V, 0.6 nA); (c) the intermediate phase (lower-right portion) and another superstructure Au(110)X-glycine, which has not been identified yet (260 Å × 260 Å, 1.0 V, 0.6 nA); (d) an enlarged image of the Au(110)X-glycine superstructure (100 Å × 100 Å, 1.0 V, 0.6 nA).

with is a physisorption system, this significant morphology change is really astonishing. C. High-Rate Deposition. Deposition with the high rate for only several minutes resulted in a dramatic change in the surface morphology, as imaged in Figure 4a,b, where larger and smaller pits as well as facets exist everywhere. Figure 4c,d presents high resolution images obtained from the surface, showing that the molecules are actually well ordered, especially in the [11 h 0] direction, very similar to the images of the 2D solid phase of amino acid adsorbates on the Cu(001) surface.14-19 We thus believe that the molecules are also flat-lying on the surface and the structures are also stabilized by H bonds.14-19 The only difference seems to be that in the present case the 2D molecular net is incommensurate with the surface, at least in the direction perpendicular to the [11 h 0] direction. Considering the physisorption character of the present system, this is quite understandable. However, also considering the physisorption character of the present system, the dramatic surface morphology change was really unexpected, because physisorption-induced small change (deformation) of metal surfaces has been reported only very recently.29 2. Desorption and the Resultant Metastable Clean Au(110) Surfaces. A. Desorption of the Au(110) 1× 3-Glycine Surface. The Au(110) 1 × 3-glycine superstructure obtained from the low-rate deposition is not stable even at room temperature as it gradually loses the adsorbed molecules and becomes a (110) 1 × 1 surface. Imaged in Figure 5a is the Au(110) 1 × 3-glycine surface in the course of room-temperature desorption, while in Figure 5b is the final clean Au(110) 1 × 1 surface with (29) Levi, A. C. Surf. Sci. 1999, 426, 308.

Adsorption of Glycine on the Au(110) 1 × 2 Surface

Figure 3. STM images the Au(110) c(2 × 4)-glycine superstructure, which coexisted with the structures imaged in Figure 2, and the accompanying surface morphology change: (a) a large-scale image (550 Å × 550 Å, 1.0 V, 0.6 nA), showing some troughs and ridges start to develop (note that the terraces are covered by the chain phase, instead of the clean 1 × 2 reconstruction); (b) an enlarged image (150 Å × 150 Å, 1.0 V, 0.6 nA), showing that at the bottom of the troughs are the Au(110) c(2 × 4)-glycine superstructures; (c) a large-scale dcmode image (580 Å × 580 Å, 1.0 V, 0.6 nA), showing that adsorption of the molecules can make the originally flat surface quite rough; (d) another dc-mode image (225 Å × 225 Å, 1.0 V, 0.6 nA), indicating the formation process of the Au(110) c(2 × 4)-glycine superstructure. Note: A large amount of gold atoms was “dug” out from the troughs and left aside forming the ridges. In this image the depth of the trough is one atomic layer and the height of the ridge is also one atomic layer.

some 1 × 3 reconstructed rows remaining. Surprisingly, this Au(110) 1 × 1 surface is very stable at room temperature. B. Reversible Transition between Clean Au(110) 1 × 1 and 1 × 3. However, after being annealed at a temperature not higher than 170 °C and then cooled to room temperature, this clean Au(110) 1 × 1 surface becomes 1 × 3 reconstructed again. Although this 1 × 3 reconstructed Au(110) surface looks indistinguishable with the Au(110) 1 × 3-glycine surface imaged in Figure 1b, it is, obviously, clean because physisorbed molecules are not able to survive that annealing. Interestingly, this clean Au(110) 1 × 3 surface, albeit stable at higher temperatures, gradually transfers back to 1 × 1 again if it is left at room temperature for several hours. However, as Figure 6 shows, even at room temperature there are always some 1 × 3 reconstructed rows coexisting with the 1 × 1 surface, indicating that the transition temperature is slightly below room temperature and thus partial transition to 1 × 3 already happens at room temperature. Unfortunately, with our STM we can image the surface only at room temperature. Thus the transition between 1 × 1 and 1 × 3 is reversible, provided that the annealing temperature is not higher than 170 °C. If the Au(110) 1 × 1 surface is annealed at a temperature higher than 170 °C then it changes to 1 × 2 reconstructed and remains to be that at room temperature, indicating that the 1 × 1 and 1 × 3 reconstructed surfaces are only metastable (one at room temperature and the other at higher temperatures below 170 °C). This is understandable because the calculated surface energies of the 1 × 1 and 1 × 3 reconstructed

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Figure 4. STM images obtained during and after the highrate (see text) deposition process: (a) large-scale image (820 Å × 820 Å, 1.2 V, 0.6 nA), showing the significant surface faceting induced by fast adsorption; (b) another large-scale image (820 Å × 820 Å, 1.2 V, 0.6 nA), showing the surface morphology in the room-temperature desorption process, where the faceted areas were still covered by the well-ordered adsorbates (see (c) and (d)) and were shrinking with time because desorption happens at their rims and makes the areas without molecules rough (110); (c) an enlarged image (114 Å × 114 Å, 1.0 V, 0.6 nA) of the ordered adsorbates on facets; (d) a further enlarged image (55 Å × 55 Å, 1.0 V, 0.6 nA) of the ordered adsorbates on facets.

Figure 5. (a) STM image (500 Å × 500 Å, 1.0 V, 0.6 nA) obtained during the desorption process of the Au(110) 1 × 3-glycine surface at room temperature. (b) Large-scale image (800 Å × 800 Å, 1.0 V, 0.6 nA) of the metastable clean Au(110) 1 × 1 surface resulted from the Au(110) 1 × 3-glycine surface after room-temperature desorption. The inset is the schematic side view of the Au(110) 1 × 1 surface.

Au(110) surface are only a few percent higher than that of the 1 × 2 reconstructed surface.24,25,30 This, however, raises an interesting question: why is the barrier of the transition from 1 × 3 to 1 × 1, which, as mentioned above, can happen at room temperature, much smaller than the barrier of the transition from 1 × 3 to 1 × 2? This is seemingly a simple question because one might think that the former case involves only migrations over very short distances, as the upper panel of Figure 6c shows. However, our observation shows that this is not the real mechanism responsible for the transition between 1 × 3 and 1 × 1. C. Fast and Reversible Local Transition between 1 × 3 and 1 × 1. From both parts a and b of Figure 6, one can (30) Garofalo, M.; Tosatti, E.; Ercolessi, F. Surf. Sci. 1987, 188, 321.

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Figure 6. (a) STM image (260 Å × 260 Å, 1.0 V, 0.6 nA) of the metastable clean Au(110) 1 × 1 surface with some 1 × 3-reconstructed rows coexisting. Note the abrupt boundaries between the 1 × 1 and 1 × 3 reconstructed areas. (b) 100 line scans of a same place (with the width being 110 Å and the time for each round-trip scan being 1 s), showing the swift back and forth transition between 1 × 1 and 1 × 3 reconstructions and thus showing clearly that the abrupt boundaries seen in (a) were a result of such transition, instead of real boundaries between 1 × 1 and 1 × 3 reconstructed areas. (c) Schematic side views of Au(110), showing two possible ways for transition from 1 × 3 to 1 × 1 or vice versa. The upper panel shows such a way that only half a monolayer of atoms at the top of the 1 × 3 surface has to diffuse by a short distance to a lower level (as indicated by the arrows) so to form a 1 × 1 surface at a lower level; while in the way shown in the lower panel 1.5 monolayers of atoms have to migrate through long distances, but in this case the surface level is not changed by the transition.

find that the transition between 1 × 3 and 1 × 1 does not result in any, even local, variation of the terrace level, which is expected to happen if the mechanism schematically shown by the upper panel of Figure 6c is responsible for the transition. Consequently, we believe that the transition must involve long-distance migrations of atoms or groups of atoms in and along the 1 × 3 troughs. As the lower panel of Figure 6c shows, to remove or create such a trough and meanwhile not to change the local terrace level, long-distance migration of many atoms in the trough (three in a cross section) must be involved. Moreover, from Figure 6b one can also see that the transition can be not only reversible but also extremely fast: at room temperature it takes less than 1 s for those atoms (at least three) that had a contribution to a line scan to migrate away, or vice versa. Although we do not know the mechanism of the migration, to get an estimation of the diffusion barrier, we assume that the migration process consists of identical single jumps organized in a most efficient way. On the basis of this assumption and according to ED ) kT ln(τυ), the activation barrier ED is estimated to be smaller than 0.73 eV, where τ is the lifetime taken to be 0.33 s, υ is the attempt frequency taken to be 1013/s as usual, and T ) 300 K in the experiment. This activation barrier is indeed small, compared to that of 0.81, 0.89, and 0.91 eV for the single jump,31 long jump,31 and leapfrog32 mechanism for self-diffusion of adatoms or adatom clusters on the Pt(110) 1 × 2 surface, respectively. As for how the barrier for self-diffusion of adatoms in and along a (110) 1 × 3 trough can be lower than that on the (110) 1 × 2 surface, our consideration is the following. First, through many recent theoretical studies about all possible mechanisms of self-diffusion on the (110) 1 × 2 surface of fcc metals,33-35 it has been known that the adatoms can climb up to the (111) facets of the troughs of the “missing-row” reconstructed surface, where they can diffuse fast along the network of metastable minima, and finally are trapped again at the trough bottom and that this mechanism is important for direct mass transport (31) Linderoth, T. R.; Horch, S.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Phys. Rev. Lett. 1997, 78, 4978. (32) Linderoth, T. R.; Horch, S.; Peterson, L.; Helveg, S.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Phys. Rev. Lett. 1999, 82, 1494. (33) Montalenti, F.; Ferrando, R. Phys. Rev. Lett. 1999, 82, 1498; Phys. Rev. B 1998, 58, 3617; 1999, 59, 5881; J. Chem Phys. 2000, 113, 349; Surf. Sci. 1999, 432, 27; 1999, 433-435, 445. Montalenti, F.; Baletto, F.; Ferrando, R. Surf. Sci. 2000, 454, 575. (34) Lorensen, H. T.; Nørskov, J. K.; Jacobsen, K. W. Phys. Rev. B 1999, 60, R5149. (35) Feibelman, P. J. Phys. Rev. B 2000, 61, R2452.

to distant sites. Second, this is supported by, at least in the case of Ir surfaces, the results of a systematic theoretical work36 and experimental studies,37,38 which show that self-diffusion barriers for adatoms or adatom clusters on the (111) surface are much lower than their counterparts on the (110) surface. Third, adatom clusters tend to migrate as a whole rather than via dissociationreassociation processes33-36 and, more importantly, the diffusion barrier of Ir tetramers on the (111) surface has an abrupt drop.36,37 This means that tetramers can move faster than other clusters do in the 1 × 3 troughs and thus must be the majority among all clusters, if not the only one. Finally, the width of the (111) facets on the (110) 1 × 3 surface (see Figure 1c) is just right for the favorable diffusion path for tetramers on the (111) surface (see Figure 18 of ref 36), whereas the (111) facets on the (110) 1 × 2 surface are too narrow for that (see Figure 1c). Knowing these facts, one would not feel surprised to see that transition between the two metastable surfaces Au(110) 1 × 3 and 1 × 1, which involves long distance migration of 1.5 monolayers of atoms, not only is possible but also can be very fast even at room temperature, while, in contrast, the transition from either 1 × 3 or 1 × 1 to the stable 1 × 2 surface, which involves only a halfmonolayer of atoms, can happen only when the temperature is above 170 °C. This, of course, needs to be confirmed by further calculations. Summary In summary, adsorption of glycine on the Au(110) 1 × 2 surface is studied by means of STM and is confirmed to be physisorption, because the adsorbed molecules desorb even at room temperature. However, very similar to those in the chemisorption case, the adsorbed molecules can form a 2D gas phase, chain phase, and solid phase at low, medium, and high coverage, respectively. The difference is that here the binding energies and diffusion barriers are much smaller, and thus in the gas phase the molecules diffuse faster and thereby are invisible by means of STM while in the solid phase the molecules are incommensurate with the substrate. What was absolutely unexpected is that the 2D gas phase can change the clean Au(110) 1 × (36) Chang, C. M.; Wei, C. M.; Chen, S. P. Phys. Rev. B 1996, 54, 17083. (37) Wang, S. C.; Ehrlich, G. Surf. Sci. 1990, 239, 301. (38) Tsong, T. T. In Surface Physics; Li, X., Qin, Z., Shen, D., Wang, D., Eds.; Gordon and Breach: Philadelphia, PA, 1992; p 75.

Adsorption of Glycine on the Au(110) 1 × 2 Surface

2 surface to a well-ordered 1 × 3 reconstructed one and that the surface morphology can be modified dramatically untill becoming completely faceted. As byproducts, the following has been found: (i) Au(110) 1 × 1 is metastable at room temperature whereas 1 × 3 at higher temperatures below 170 °C. (ii) The later consists of narrow and long (111) facets lying in the [11h 0] direction. (iii) The transition between the two metastable structures is reversible and the local transition between the two is very fast, implying that only very small activation barriers are involved. In contrast, the barriers of the transition from the metastable 1 × 3 structure to the stable 1 × 2

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structure are much higher, as evidenced by the required temperature of 170 °C. On the basis of the results of many recent theoretical studies, we suggest that in the fast transition between 1 × 3 and 1 × 1 structures atoms migrate as tetramers on the (111) facets of the 1 × 3 troughs, while this is impossible on the 1 × 2 reconstructed surface because its (111) facets are too narrow for tetramers. Acknowledgment. This work was supported by the National Natural Science Foundation of China. LA011390L