Characterisation of the interaction of glycine with Cu(1 0 0) and Cu(1 1 1)

Surface Science 531 (2003) 304–318 www.elsevier.com/locate/susc

Characterisation of the interaction of glycine with Cu(1 0 0) and Cu(1 1 1) V. Efstathiou a, D.P. Woodruff a

a,b,*

Fritz-Haber-Institut der MPG, Faradayweg 4-6, D14195 Berlin, Germany b Physics Department, University of Warwick, Coventry CV4 7AL, UK Received 7 February 2003; accepted for publication 3 April 2003

Abstract Reflection–absorption infrared spectroscopy (RAIRS) has been used to characterise the interaction of standard and fully deuterated glycine with Cu(1 0 0) and Cu(1 1 1). RAIRS shows clearly that the surface interaction leads to formation of the adsorbed deprotonated glycinate (NH2 CH2 COO–) species, with some evidence for changes in orientation with coverage previously seen on Cu(1 1 0). Qualitative low energy electron diffraction observations were also conducted to characterise the long-range ordering, although effects of electron-beam-induced radiation damage limited the information obtained. Nevertheless, the results do suggest some subtle isotopic-mass-related structural variations. The results are discussed in the context of previously published scanning tunnelling microscopy and photoelectron diffraction measurements. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Infrared absorption spectroscopy; Low energy electron diffraction (LEED); Chemisorption; Copper; Biological molecules – proteins; Low index single crystal surfaces

1. Introduction There has been considerable interest in recent years in the adsorption behaviour of simple amino acids, motivated in part by the growing importance of technologies which are based on the interaction of biological molecules with surfaces. Amino acids are seen as simple models which

*

Corresponding author. Address: Physics Department, University of Warwick, Coventry CV4 7AL, UK. Tel.: +44-2476523378; fax: +44-247-6692016. E-mail address: d.p.woodruff@warwick.ac.uk (D.P. Woodruff).

provide a starting point for research ultimately aimed at understanding much more complex systems, and the simplest amino acid, glycine, NH2 CH2 COOH, has attracted particular attention. In solid glycine the molecule is a zwitterion,  NHþ 3 CH2 COO , but typically when it is adsorbs on even a modestly reactive surface at room temperature, the acid is deprotonated to form adsorbed glycinate, NH2 CH2 COO–. On Cu(1 1 0) there is particularly clear evidence for this reaction from reflection–absorption infrared spectroscopy (RAIRS) [1], which also provided evidence of orientational changes with coverage. In particular, the RAIR spectra were interpreted as indicative of bidentate bonding to the substrate through the two

0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00541-7

V. Efstathiou, D.P. Woodruff / Surface Science 531 (2003) 304–318

equivalent O atoms at low coverage, with the molecular plane Ôstanding upÕ on the surface, a unidentate bonding was inferred at the highest coverage, but modest annealing led to an ordered (3  2) overlayer which appeared to have the C–C molecular backbone parallel to the surface. The orientation in this phase and the implied tridentate substrate bonding through both the carboxylate O atoms and the amino N atom was supported by core level photoabsorption and photoemission together with density functional theory calculations [2,3] while this local bonding geometry, with both the O and the N atoms in local off-atop sites on the Cu(1 1 0) outermost layer Cu atoms was established by scanned-energy mode photoelectron diffraction (PhD) [4–6] (Fig. 1). Scanning tunnelling microscopy (STM) [7] provided some further insight into this long-range ordered glycinate adsorption phase which is generally believed to be stabilised by intermolecular hydrogen bonding. Notice that one interesting feature of glycinate adsorbed in this tridentate lying-down orientation is that it becomes chiral with two inequivalent

Fig. 1. Schematic plan view of the structure of the (3  2)-pg phase of glycinate on Cu(1 1 0). The atomic positions of all the atoms in the molecule (including the H atoms shown as small light circles) are as determined in a theoretical total energy calculation [26], but the positions of the O and N atoms are fully consistent with an experimental PhD structure determination [6].

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mirror-image forms (see Fig. 1); by contrast, glycine is not chiral in the gas phase, and the simplest chiral amino acid in this case is alanine, NH2 CH3 HCCOOH in which one of the H atoms of glycine bonded to the central C atom is replaced by a methyl group. The structure shown in Fig. 1, which contains equal numbers of the molecules of each chirality (i.e. is heterochiral), is generally accepted as correct. However, STM measurements have led to the suggestion [7] that there is also a coexistent homochiral phase, although the proposed associated structure appears to be inconsistent with the PhD measurements [5]. In contrast to the extensive studies of glycine on Cu(1 1 0), there have been fewer studies of the interaction of glycine with Cu(1 0 0) or Cu(1 1 1). An early qualitative low energy electron diffraction (LEED) investigation [8] identified high-coverage long-range ordered adsorption phases on each of these surfaces, with (4  2) and (8  8) periodicities, respectively. More recently, there have been reports of STM studies on the (1 0 0) [9,10] and (1 1 1) surfaces [11], and the results of a PhD investigation of glycinate on Cu(1 0 0) [5,6]. In the case of the STM investigation on Cu(1 1 1) several different surface phases, described as 2-D gas, chain and 2-D solid phases were seen with increasing coverage and appear to be implicitly assumed to be associated with different molecular orientations, the lowest coverage phase being argued to correspond to a standing-up (oxygen bidentate bonded) species which might be expected to be mutually repulsive due to an associated dipole moment, while the 2-D solid phases are thought to involve mainly lying-down species in phases stabilised by hydrogen bonding. Of course, there is no direct evidence for these molecular orientations from STM. In the case of glycinate on Cu(1 0 0), the (4  2) phase is also believed to involve a hydrogen-bonded network of lying-down species, and the PhD results clearly identify nearatop N and O bonding sites; the results favoured tridendate bonding including both O atoms and the N atom (Fig. 2, (4  2)-pg heterochiral), although could not formally exclude bidentate bonding with only one O–Cu bond. The STM results in this case have also provided some evidence for a coexistent c(4  2) homochiral phase

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Fig. 2. Schematic plan view of possible glycinate structures on Cu(1 0 0). The molecular conformations are as found for glycinate on Cu(1 1 0) (Fig. 1). The local O and N sites shown are consistent with PhD experiments [6]. The (4  2)-pg heterochiral structure (top left) is the phase favoured by the PhD and LEED studies, and consistent with STM investigations. The c(4  2) homochiral structure (top right) has been proposed to coexist with the (4  2)-pg phase on the basis of STM measurements. The remaining structures are models which relate to new phases observed in the present work which are discussed in the text.

(Fig. 2), although the evidence for this has been questioned [5].

Notice, incidentally, that the quite different arrangement of the Cu surface atoms on the three

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different low index faces does imply that if the high-coverage 2-D solid phases all involve tridentate glycinate bonding, the actual geometry of these bonding arrangements must differ significantly. On all three surfaces, of course, the Cu–Cu
) but nearest-neighbour spacing is the same (2.55 A on (1 0 0) the Cu atoms form a square mesh of this spacing (Fig. 2), on Cu(1 1 0) the surface comprises rows of Cu atoms at p the same spacings but with an inter-row spacing 2 times larger (Fig. 1), while on Cu(1 1 1) the Cu atoms are significantly more close-packed producing a surface with threefold rotational symmetry (see also Figs. 7 and 8 later in this paper). Indeed, the PhD data indicate that the two O atoms and the N atoms are very close to atop 3 of the 4 Cu atoms which form the unit square on Cu(1 0 0) (Fig. 2, (4  2)-pg heterochiral), whereas on Cu(1 1 0) the O atoms are substantially displaced from atop positions to accommodate the greatly increased inter-row spacing (Fig. 1). On Cu(1 1 1), if a similar tridentate bonding geometry occurs, some intramolecular distortion may be required; alternatively, at least one of the three bonding atoms may have to adopt a more highly coordinated local adsorption site. This issue will be discussed further later in the paper. In this paper we present the results of a new investigation of glycine adsorption on Cu(1 0 0) and Cu(1 1 1) using, particularly, RAIRS to characterise the surface reaction and provide some information on potential changes in molecular orientation with coverage. The results show clearly that the same deprotonation occurs as on Cu(1 1 0), although the evidence for orientation changes is weaker than on that surface. We also present the results of qualitative LEED studies of the longrange ordered phases of glycinate on Cu(1 0 0) and (1 1 1). The well-known problem of electron-beaminduced radiation damage to these surfaces limits the ability of this standard technique to provide a thorough characterisation, but the LEED results do nevertheless provide some useful further information and links to other studies. Curiously, however, we observe some systematic differences between standard glycine and fully deuterated glycine, used primarily to provide additional information on the molecular vibrational modes,

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and the possible significance of these differences is discussed. 2. Experimental details The instrument used in these experiments has been described previously [12]. Briefly, it consists of a commercial FTIR spectrometer (Biorad FTS60A/896), modified for operation under low vacuum (1  103 mbar), interfaced to a UHV chamber equipped with rear-view LEED optics and facilities for argon ion sputtering. Spectra were recorded using a liquid nitrogen-cooled narrow-band MCT detector, and were collected with a spectral resolution of 4 cm1 , each spectrum consisting of 1024 scans using a clean surface background of 2048 scans. The Cu(1 1 1) and Cu(1 0 0) samples were prepared by the usual combination of X-ray Laue alignment, spark machining, surface polishing and in situ argon ion bombardment (500 eV, 15 lA, 15 min) and annealing (750 K) cycles until a well-ordered and clean surface was obtained as judged by the LEED pattern, and by the behaviour in the present FTRAIRS study and in reference data collected for CO adsorption. The samples were mounted (sequentially) on a liquid nitrogen-cooled cold finger and could be resistively heated such that temperatures between 85 and 1000 K could be maintained with high stability. The temperature was measured using a K-type thermocouple spot-welded directly to the top of the crystal and the signal from this thermocouple was fed back into the computercontrolled temperature control unit. The glycine powder was contained in a glass tube which could be heated via a surrounding copper coil and its temperature was measured by a thermocouple attached to a wire mesh within the tube. The doser was held within a small, separately pumped sidearm separated from the upper chamber by a gate valve. The glycine was initially outgassed at 365 K for 48 h. During dosing the glycine was heated to about 385 K and the Cu crystal, at room temperature, was positioned facing the open gate valve to the doser. Typically, it was found that to achieve a saturated overlayer, as judged by the RAIRS, the crystal had to be exposed to the glycine flux for 60 min. The pressure in the preparation chamber

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rose from a base pressure of 1  1010 mbar to around 1  108 mbar during dosing. Experiments were conducted using both standard glycine (99% purity) and fully deuterated glycine (ND2 CD2 COOD) (97% purity), the glycine in the doser being changed by bringing the sample chamber up to atmospheric pressure between the two sets of measurements.

0.0002

3. Results 3.1. RAIRS Figs. 3 and 4 summarise the RAIR spectra from each surface after different exposures and treatments to the two different glycine isotopes, while in Table 1 the frequencies of the main absorption

Cu(100)

1105 1335 1412

963 924 1025

dosed at 300 K 2915

1630 2850

1105 924 970 1030

1415 1335

Glycine 60 mins Heated to 420 K

2915 2850

1630

60 mins

(4x2)-pg 907

1335 965 1017 1102

1412

1630

2850

60 mins

2915

Absorbance

30 mins 10 mins 800

1000

1200

1400

1600

1800 2800

3000

0.0001

3200

3400

D5 -Glycine

930 740 775825 950

1343

1412

1613

2115 2170

60 mins Heated to 420 K

960 1415 743 775 830

1623

1343

2110 2170

1175 743 770820

60 mins 960

1343

1412 1623

1175 730 763 820

955

1340

1412

2115 2163

(2x2) 60 mins

1610

2120 2163

30 mins 10 mins 600

800

1000

1200

1400

1600

1800

2000

2200

2400

-1

Wavenumber (cm ) Fig. 3. RAIR spectra obtained from Cu(1 0 0) following different exposures (and annealing) to glycine, NH2 CH2 COOH and to fully deuterated glycine, ND2 CD2 COOD. LEED observations were not made in all preparations, but in those cases in which an ordered LEED pattern was observed after the RAIRS data collection, the assignment is shown. Note that each spectrum is the result of a different surface preparation.

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Fig. 4. RAIR spectra obtained from Cu(1 1 1) following different exposures (and annealing) to glycine, NH2 CH2 COOH and to fully deuterated glycine, ND2 CD2 COOD. LEED observations were not made in all preparations, but in those cases in which an ordered LEED pattern was observed after the RAIRS data collection, the assignment is shown. Note that each spectrum is the result of a different surface preparation.

lines seen in these spectra are compared with the published results for these species adsorbed on Cu(1 1 0) and the vibrational assignments made in this earlier work [1]. The strong similarity of these spectra, and of the observed frequencies to those seen on Cu(1 1 0), clearly indicates that in all three cases the adsorbed species is glycinate. Notice, in particular, that the spectra are consistent with

deprotonation of the acid, the bands around 1600 and 1400 cm1 being characteristic of the asymmetric and symmetric carboxylate –CO 2 stretches, while there is no evidence of a –COOH carbonyl stretch to be expected around 1720 cm1 . Strictly, this does not exclude the possibility that the acid proton is not lost, but rather is transferred to the amino group to produce –NHþ 3 ; as remarked by

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Table 1 Summary of main absorption band wavenumbers (in cm1 ) seen in this work and in the earlier study of glycine on Cu(1 1 0) [1] Assignment

Cu(1 1 0) low-high-anneal

Cu(1 0 0)

Cu(1 1 1)

(a) Glycine CH2 as str CH2 s str CO 2 as str NH2 sciss CH2 sciss CO 2 s str CH2 wag NH2 w/CN str CC str CH2 r

2906–2910–2920 2860–2860–2858 - - - -–1630–1630 - - - - –1578–1578 - - - - –1441–1441 1417–1422–1414 1332–1320–1323 1088/1024–1105–1102 969–985–985 945–902– - - -

2915 2850 1630 (1580) (1440) 1412 1335 1105/1025 963 924

2915 2850 1643 (1580) (1425) 1410 1333 1110/1033 970 915

(b) Deuterated glycine CD2 as str CD2 s str CO 2 as str CO 2 s str Not assigned ND2 sciss CD2 sciss CN str/CD2 wag ND2 w CC str CD2 r

- - - - –2158–2158 2101–2109–2109 - - - - –1618–1609 1402–1411–1408 - - - - –1350–1350 - - - - – - - - -–1167 - - - - – - - - -–1072 946–968/925–943/922 817–830–827 755–769–760 734– - - - – - - -

2170 2115 1613 1412 1343 (1175)

2115 1635 1412 1335 (1180)

960/930 825 775 740

955/925 830 780 735

Barlow et al. [1], the low symmetry of the molecular species and the mixing of modes at low frequencies makes entirely reliable assignments difficult, so it is difficult to establish from the vibrational spectra alone whether –NH2 or –NHþ 3 is present. However, the fact that tridentate bonding is clearly established on Cu(1 1 0), in particular [4,6], combined with the similarity of the RAIR spectra for the three surfaces, strongly favours an interpretation based on adsorbed glycinate. The low symmetry also formally precludes extracting any orientational information from the RAIR spectra in a rigorous fashion, although some changes in the relative intensities of bands may be indicative of orientation changes as has been suggested by Barlow et al. In particular, they noted that on Cu(1 1 0) the asymmetric COO stretching mode around 1630 cm1 was absent at the lowest coverages, most intense at high coverage, and substantially reduced when the surface was annealed at 420 K with the attendant improvement of the (3  2) ordered LEED pattern. It

was on this basis, primarily, that they argued in favour of a standing-up adsorbate, bonded to the surface through the two O atoms, at low coverage and a lying-down species for the annealed ordered phase, with an intermediate low symmetry orientation. Notice that in the simpler formate (HCOO–) adsorbate on this surface in the symmetric standing-up geometry, the asymmetric COO stretch is symmetry forbidden [13]; the lower symmetry of glycine formally removes this constraint. Qualitatively, this same trend is evident in the spectra from glycine on Cu(1 1 1), the relative intensity of the asymmetric COO stretch (around 1640 cm1 ) to that of the symmetric stretch (1415 cm1 ) growing with increasing coverage and then falling back somewhat on the annealed surface. There is also (much weaker) evidence of the same effect on Cu(1 0 0). Apart from this evidence of orientational changes, for which the systematics of the RAIRS for the three surfaces are similar, but the strength of trends differ, there are a few other points of note. We should perhaps mention that we have

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been unable to make an assignment of the band around 1340 cm1 seen in the RAIRS from the deuterated species. This band has a very similar behaviour on Cu(1 1 0) and was also unassigned in this earlier study [1]. A band at a similar frequency for the hydrogenated glycine is attributed to a CH2 wagging mode, but the equivalent CD2 wag of the deuterated species is expected to occur at much lower frequency (855 cm1 in a copper–glycino complex [14]) and falls in the range of a group of low frequency bands. One further detail of note is the behaviour of the CH2 stretching modes in the hydrogenated and deuterated species. A striking feature of the RAIRS from the deuterated species on Cu(1 1 1) (Fig. 4) is the almost complete absence of the CD2 asymmetric stretching mode expected around 2170 cm1 . This mode is clearly present on Cu(1 0 0), and indeed on Cu(1 1 0). There are, however, some underlying systematic trends of which this effect may be symptomatic. In the hydrogenated species the CH2 asymmetric stretching mode is consistently more intense than the symmetric stretching mode (also on Cu(1 1 0)), whereas this is not true for the CD2 modes; on Cu(1 0 0), the intensities of the asymmetric and symmetric modes are essentially the same, on Cu(1 1 0) the asymmetric mode is weaker, and on Cu(1 1 1) it is almost absent. If the differences in relative amplitudes of the symmetric and asymmetric modes on the different surfaces were consistent for both the hydrogenated and deuterated species, one might suggest that this reflects some difference in the internal conformation of the adsorbed glycinate induced by the different substrate structures. However, it is difficult to see why the two isotomers should behave differently. The general question of isotope effects on the adsorption geometry will be discussed further in the following section but the problem of these vibrational assignments remains unresolved. 3.2. LEED A number of studies of adsorbed amino acids on metal (mainly Cu) surfaces, using LEED (e.g. [8,15]) and STM (e.g. [7,10,11,16]), has shown that a range of long-range ordered overlayer structures may occur, and this is generally supposed to arise

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because of the intermolecular hydrogen bonding which could lead to Ôself-organisedÕ structures even in the absence of a corrugated substrate potential. STM offers the advantage of being able to distinguish locally spatially separated coexistent structural domains, and appears not to produce any significant damage in these molecular layers, but qualitative LEED observations do provide a simple means of obtaining an overview of the longrange ordered phases, and can provide some additional local structural information through symmetry selection rules [17]. The problem of radiation damage does appear to be a rather substantial one for these adsorbed molecules, however, so it proved necessary to take photographs of the LEED patterns quickly, moving the sample laterally in front of the electron beam between observations. The LEED patterns typically faded in a time scale of a few seconds. Especially for the more complex patterns seen here, it was often possible to establish the periodicity only from a succession of fragmentary photographs of the LEED pattern at different incident directions and electron energies (typically 50–100 eV). This problem of radiation damage, combined with the need to keep a fixed sample position during a sequence of RAIRS measurements to ensure a stable background, also made it difficult to cross-correlate the RAIRS and LEED data in a totally reliable fashion. However, as the changes seen in the RAIRS with increasing coverage and annealing were relative modest, this problem did not have any severe consequences. Experiments were conducted using both the hydrogenated and deuterated glycine, and we first consider the results of the experiments using the hydrogenated species. In the case of the Cu(1 0 0) surface, a much earlier LEED study had identified a diffraction pattern which was attributed to a (4  2) real space structure at reasonably high coverage [8]. Subsequent STM studies, however, raised some questions as to whether there exists, either instead or as well, a c(4  2) phase. Furthermore, in the case of a (4  2) structure, it has been suggested that this may be associated with two glycinate species per unit mesh, but with the molecule at the centred position of the (4  2) mesh being a mirror image of those at the corners, producing a heterochiral

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structure (Fig. 2). One important consequence of this structure is that it possesses a glide symmetry plane, which at normal electron incidence will give rise to systematic absences in the LEED pattern. This is illustrated in Fig. 5. Notice that the LEED patterns expected for the (4  2) and c(4  2) structures are quite different; specifically, diffracted beams of the type (12 12) are present in the (4  2) pattern but not in the c(4  2) pattern. Our observations showed clearly (as did the previously published LEED pattern of this phase [8]) that these diffracted beams are present. Fig. 5 also shows the difference to be expected from a simple (4  2) structure and a (4  2)-pg phase having a glide plane; diffracted beams of the type (n/4 0) are missing for odd values of n. Notice, incidentally, that these beams are also absent from the c(4  2) phase, but this pattern contains other systematic absences, such as (12 12) which we have already noted are not absent in the experiment. We should also note that beams of the type (1 12) could also be observed under certain conditions; these beams are not visible in the figure in the original LEED

publication [8] and a recent publication has suggested that their absence may also be characteristic of the LEED pattern of this surface phase [18]. Our LEED observations clearly support the (4  2)-pg interpretation. This result, and its consequences in terms of the clear preference for a structure involving an ordered array of left- and right-handed adsorbed glycinate species, have been discussed in more detail elsewhere [5,6]. A lowerpcoveragepordered structure (suggested to be a Ôð4 5  0:8 5ÞR26:56°)Õ periodicity) reported earlier [8] was not seen in the present work, but no detailed LEED investigation of the low coverage regime was undertaken. On Cu(1 1 1) the previous LEED investigation [8] reported the observation of an (8  8) phase although a more complex pattern, not interpreted, was also seen under conditions of deposition onto a surface at elevated (>375 K) temperature. A very recent STM investigation of glycine on Cu(1 1 1) [11] identified two long-range ordered phases at near-saturation coverage. One of these had an (8  4) periodicity and is consistent with the re-

Fig. 5. Schematic diagram of the real-space unit meshes of some overlayer structures on Cu(1 0 0), associated with possible glycine structures, and their associated LEED patterns. In the LEED patterns the integral-order diffracted beams are shown as larger spots.

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ported (8  8) LEED pattern (Fig. 6); the three rotationally equivalent domains of (8  4) on a cubic (1 1 1) surface give a LEED pattern indistinguishable from that of an (8  8) phase. p In addition the STM study showed a ð2 13  p 2 13ÞR13:9° structure, a phase seen in the LEED investigation of alanine adsorption on this surface [8]. Our own LEED experiments found clear evidence for both the (8  4) phase p (i.e.pan (8  8) LEED pattern) and the ð2 13  2 13ÞR13:9°

313

structure seen in STM. The exact conditions distinguishing these two phases is not completely clear, but the (8  4) phase was generally observed after cleaning the Cu(1 1 1) surface by ion bombardment and annealing followed by deposition of glycine onto p p the sample at room temperature. The ð2 13  2 13ÞR13:9° phase, on the other hand, was observed if the sample was re-cleaned after an earlier glycine adsorption study by heating to 500 K followed by a fresh deposition of glycine; the

Fig. 6. Schematic diagram of the real-space unit meshes of some overlayer structures on Cu(1 1 1), associated with observed glycine structures, and their associated LEED patterns. In the LEED patterns the integral-order diffracted beams are shown as larger spots.

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p (8p 4) phase also transformed to ð2 13  2 13ÞR13:9° on annealing to 330–350 K. In view of the fact that the STM investigation found these two surface phases to co-exist, but with different relative coverages depending on the preparation, it is evident that the two phases must have very similar local coverage and energy. In addition to these LEED investigations of the adsorption of hydrogenated glycine, additional observations were made of the LEED patterns produced following the adsorption of deuterated glycine. Of course, one would not normally expect any isotope effect in the structure of these overlayers, so the primary purpose of these measurements was to try to relate the structural phases to the RAIR spectra. Surprisingly, however, pronounced systematic differences in the structural phases formed were detected. Specifically, on Cu(1 0 0) no clear (4  2)-pg LEED pattern was ever seen for deuterated glycine adsorption, although LEED patterns showing the diffracted beams characteristic of a (2  2) phase were seen. Of course, the diffracted beams of the (2  2) phase are a subset of those of the (4  2)-pg phase, so it is possible that a rather poorly ordered (4  2)-pg phase was formed in which it was never possible to clearly distinguish the additional diffracted beams. p In p addition, however, a quite different ð 5  5ÞR26:5° phase (Fig. 5) was seen, sometimes coexisting with the apparent (2  2) phase and thus making a clear identification p of ap(4  2)-pg phase even more difficult. The ð 5  5ÞR26:5° phase appeared to transform to (2  2) on annealing at 330 K. Deuterated glycine on Cu(1 1 1) also led to distinctly different LEED patterns from those produced by hydrogenated glycine; in this case the only clearly identified ordered phase had a p p ð 21  21ÞR10:9° periodicity (Fig. 6).

4. General discussion and conclusions The strong similarity of our RAIR spectra for both hydrogenated and deuterated glycine on Cu(1 0 0) and Cu(1 1 1) to those obtained previously on Cu(1 1 0) provides clear evidence for the formation of the same glycinate species on all three surfaces, while some changes in the relative in-

tensities of the vibrational bands with coverage, especially of the symmetric and asymmetric C–O stretching modes, provide support for the idea that the molecular orientation changes with coverage, probably consistent with a standing-up species at low coverage and a lying-down species (with tridentate substrate bonding) for the well-ordered high coverage phases. Evidence for this orientation change is particularly clear for (1 1 0) surface originally studied by Barlow et al. and on the (1 1 1) surface, and is least clear on the (1 0 0) surface. The coexistence of different surface phases and the absence of strict symmetry selection rules in the vibrational spectroscopy from this lowsymmetry adsorbate makes it difficult to be more quantitative regarding these effects. While the local geometry of the Cu(1 1 1) surface is quite different from that of Cu(1 0 0) and Cu(1 1 0), and less obviously compatible with tridentate bonding of an essentially undistorted glycinate species, there is no clear evidence in the RAIRS of a marked change in the molecular conformation on Cu(1 1 1). Our qualitative LEED observations on the hydrogenated glycinate adsorbate phases have largely confirmed previous information obtained from earlier LEED or STM measurements. In particular, on the (1 0 0) face our results clearly show that the main ordered phase is a (4  2)-pg structure, with attendant implications for the detailed structural models [5]. On Cu(1 1 1) we have observed not only the (8  8) LEED pattern seen earlier,pbut alsopthe LEED pattern characteristic of the ð2 13  2 13ÞR13:9° phase previously seen only in STM. In addition, however, our LEED observations of the surface phases formed by deuterated glycine on these surfaces reveal a surprising new set of structures. In particular, p we identify two entirely now phases, Cu(1 0 0)ð 5 p p p 5ÞR26:5° and Cu(1 1 1)ð 21  21ÞR10:9°. While the difficulty of obtaining good quality LEED patterns from these beam-sensitive adsorbed molecular layers clearly means that it is possible that not all ordered structural phases were identified in all experiments, our results certainly do indicate some systematic differences in the structural phases identified for the hydrogenated and deuterated forms of glycine. In general one would expect the equilibrium structures to be un-

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influenced by changes in isotopic mass, so it is tempting to consider the possibility that the differences are actually dynamic in origin. Different dynamic behaviour is to be expected from different isotomers; the most obvious example is the vibrational spectra considered here, but one may also see large effects in thermal desorption (e.g. [19]). In addition, however, one might suppose that the two different isotomers could approach their equilibrium structures differently, and that in one or other isotomer one might observe metastable, rather than truly stable phases. To some extent, the fact that there are two different high-coverage ordered phases of (hydrogenated) glycine on Cu(1 1 1), p p (8  4) and ð2 13  2 13ÞR13:9°, which can coexist on the surface and have been seen by both STM and LEED, provides some support for this idea, indicating that the energetic balances are clearly very subtle, and transformations may be kinetically hindered with significantly different kinetics for the two isotomers. Bearing in mind these subtle energy differences, however, it is also possible that there are real differences in the equilibrium structures of the isotomers. For example, Si–H and Si–D equilibrium distances in silane do differ slightly (albeit only about 0.5% [20]) and there is a small difference in C–D and CH bondlengths and in the extent to which these two bonds are polar [21]. Moreover, we might expect these differences between the influence of H and D to be particularly pronounced in hydrogen-bonded systems such as these amino acid layers, and small but significant isotope effects have been seen in the lengths of hydrogen bonds [22]. Indeed, one previous observation of an isotope effect in an adsorption system, that of a water bi-layer on Ru(0 0 0 1), has been reported [23,24]; this is another system in which the ordering is likely to be dominated by hydrogen bonding, and the effect was believed to be a direct consequence of different H and D bond lengths (referred to in these authors as an Ubbelohde effect [25]). We tentatively suggest that there are real differences in the structure due to an isotope effect, either in the surface dynamics influencing our ability to detect different metastable phases in certain situations, or even reflecting real differences in equilibrium phases due to subtle differences in

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the bonding energies, perhaps particularly in the intermolecular hydrogen bonding. Either explanation is surprising, although qualitatively such effects are known to occur. It would be interesting to investigate these deuterated species with STM to gain a clearer understanding of the stability of these phases. Unfortunately, neither the RAIRS nor the LEED results provide sufficient information for clear identification of the structural models associated with the newly identified surface phases, particularly on Cu(1 1 1). On Cu(1 0 0) the RAIRS is certainly consistent with the (4  2)-pg phase involving tridentate bonding of lying-down glycinate species, as favoured by the photoelectron diffraction investigation [6], and the glide symmetry plane favours a model in which equal numbers of left- and right-handed glycinate species form ordered (heterochiral) p p domains [5]. Models for the (2  2) and ( 5  5) phases are more speculative, but one can easily devise models based on the same local geometry as in the (4  2)-pg phase, as seen in Fig. 2. The fact that both structures involve only one molecule per surface unit mesh means that if they are perfectly ordered they must be homochiral. However, as we have already remarked, the diffracted beams characteristic of a (2  2) structure are a subset of those of (4  2), so a poorly ordered (4  2) phase might yield a diffraction pattern which appears to be (2  2). Indeed, an alternative heterochiral (4  2)-pg phase without staggering of the adsorbed glycinate species (centre panel, Fig. 2) has the molecules arranged on a (2  2) mesh, so some disorder of the chirality could certainly lead to a (2  2)-like LEED pattern. In the case of Cu(1 1 1), no comparable quantitative local structural information is available, and the only structural pmodelsp of the relatively complex (8  4) and ð2 13  13ÞR13:9° structural phases are those proposed on the basis of an STM investigation [11]. These are shown in Fig. 7. The local positions and orientations of the adsorbed glycinate are as proposed by the authors of the STM paper, but the internal conformation of the lying-down species are taken from the known geometry on Cu(1 1 0). In view of the total absence of information of the H atoms sites, especially for

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Fig. 7. Schematic plan view of the structural models of the p p (8  4) and ð2 13  2 13ÞR13:9° phases of adsorbed glycinate on Cu(1 1 1) proposed in the STM study [11]. The local positions and orientations are as proposed in the STM study, although the molecular conformations are as found for glycinate on Cu(1 1 0) (Fig. 1). In these models the H atoms have been omitted.

the standing-up glycinate species, these atoms have not been included. These structural models are, of course, largely speculative, but in addition to the observed periodicity, the appearance of the STM images does provide some clue as to the possible molecular arrangements. The models also assume the local coverage is similar to that of the ordered phases on Cu(1 1 0) and Cu(1 0 0). In the case of the (8  4) phase the STM images show broad and essentially featureless circular asperities which approximately define a (4  4) mesh, although a

slight fluctuation in the spacing shows that there are two inequivalent asperities per (8  4) unit mesh. Each such asperity is attributed to a ring of three tridentate glycinate species arranged such as to allow hydrogen bonding. Notice that in order to rationalise the (8  4) periodicity these rings are centred on inequivalent or low symmetry substrate sites. This means that the local adsorption sites of the individual glycinate species differ. Clearly the exact local geometry is not defined by the general model, but the inequivalence is a necessary characteristic. p Thepappearance of the STM images of the ð2 13  2 13ÞR13:9° phase is quite different, although large circular (but more structured) features are again seen. In this case the proposed model involves one ring of three bidentate standing-up glycinate and two rings of three tridentate lying-down glycinate species perpunit mesh. Bearp ing in mind that the ð2 13  2 13ÞR13:9° phase can be formed by annealing the (8  4) phase, the idea that some conversion of tridentate lying-down glycinate to standing-up glycinate occurs in this transition would certainly be opposite to the trend found in other amino acid/Cu surface studies; of course, these structures based on qualitative evaluation of STM images are not in any sense true structure determinations. Notice that one attractive feature of the rings of three molecules on the threefold symmetric surface is that the local adsorption geometry of the three molecules in each ring is equivalent. We should also note that the models of both structures shown in Fig. 7 are homochiral, but in both cases one could easily devise heterochiral alternatives. In particular, both structural models involve two rings of tridentate glycinate species, and these could be of opposite chirality. Indeed, in the case of the (8  4) phase this would overcome the need to propose different local adsorption sites in the two inequivalent rings. Based only on the size of the surface unit mesh obtained from LEED, any discussion p p of the detailed model of the Cu(1 1 1)ð 21  21ÞR10:9°glycinate phase is even more speculative. We may, however, reasonably assume that the surface coverage is similar to that of the other ordered phases; if these ordered phases are stabilised by hydrogen bonding, as is generally believed, there should all have very similar coverages. This does appear to

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be the case. On Cu(1 0 0) the (4  2)-pg and c(4  2) phases have a coverage of 0.25 ML. p This p is also true for a (2  2) phase, while a ð 5  5Þ phase corresponds to a coverage of 0.2 ML. On Cu(1 1 0) the coverage of the (3  2)-pg phase is 0.33 (1 1 0) ML, but taking account of the different atomic densities of the (1 0 0) and (1 1 0) surfaces this corresponds to 0.23 (1 0 0) ML. structures p Thep proposed for the (8  4) and (2 13  2 13) phases on Cu(1 1 1) (Fig. 7) are 0.19 (1 1 1) ML and 0.17 (1 1 1) ML respectively, equivalent in (1 0 0) surface density units to 0.22 (1 0 0) ML and 0.20 (1p0 0) ML. p To achieve a similar coverage in a ð 21  21ÞR10:9° phase on Cu(1 1 1) one requires four molecules per surface unit mesh which gives a coverage of 0.19 (1 1 1) ML or 0.22 (1 0 0) ML. Three or five molecules per surface unit mesh leads to coverages which fall well outside the range of 0.20–0.25 (1 0 0) ML of all the other phases. Fig. 8 shows examples of three differentptypes of p possible structural model for the ð 21 21ÞR10:9° phase. In (a) there is one ring of three bidentate standing-up glycinate species and one isolated tridendate lying-down species per surface unit mesh. In (b) there is one ring of three tridentate lying-down glycinate species and one isolated bidentate standing-up species per surface unit mesh. The relative positions and registries shown are largely arbitrary. In model (c) only tridentate lying-down species are shown and the structure is heterochiral. The model shown has been chosen to allow all the glycinate species to adopt similar local adsorption sites, with the two O atoms in off-atop sites, but the exact relative positions remains arbitrary. The first two models are based on the structural p pelements proposed for the (8  4) and (2 13  2 13) phases, while the third one is chosen to be more like the phases seen on Cu(1 1 0) and (1 0 0). While we can provide very little information on the detailed geometry of these structural phases from our LEED observation, the RAIRS data do provide some additional information. For the (8  4) phase the structural model is based entirely on tridentate lying-down p pspecies, whereas the model for the ð2 13  2 13Þ comprises a 2:1 ratio of lying-down to standing-up molecules, with the latter being supposed to be bidentate, bonding

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Fig. 8. Schematic plan view of three distinct types of structural p p model of the ( 21  21) phases of adsorbed glycinate on Cu(1 1 1), as discussed in the text. The local positions and orientations are as proposed in the STM study, although the molecular conformations are as found for glycinate on Cu(1 1 0) (Fig. 1). In these models the H atoms have been omitted.

through one O atom and the N atom. The fact that our RAIRS data do indicate a more intense

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asymmetric p p C–O stretching vibration for the ð2 13  2 13ÞR13:9° structural phase would actually be qualitatively consistent with this idea. In p p the case of the ( 21  21) deuterated glycinate phase our three models, (a), (b) and (c) have 25%, 75% and 100% tridentate species respectively, the remainder being bidentate. The RAIRS data from deuterated glycinate on the (1 1 1) surface are generally similar to those from the (1 0 0) surface, indicating that the majority of the species must be in the tridendate geometry and excluding the model of panel (a) of Fig. 8. However, the spectrum clearly identified as corresponding to the p p ( 21  21) phase does appear to show an enhanced intensity in the asymmetric C–O stretch, perhaps indicating some occupation of a standingup geometry as seen in panel (b) of Fig. 8. Clearly a proper structure determination of the glycinate phases on Cu(1 1 1), in particular, requires further experiments using truly quantitative structural methods. However, our RAIRS data do provide clear identification of the glycinate species and some indication of its preferred orientation. The surprising influence of isotopic mass on the ordered structural phases seen in LEED certain warrants further investigation by other methods less influenced by the problems of radiation damage.

Acknowledgements DPW acknowledges the support of the Engineering and Physical Sciences Research Council in the form of a Senior Research Fellowship.

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