Oxygen induced reconstruction of (h11) and (100) faces of copper

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Surface Science 260 (1992) 235-244 North-Holland

Oxygen induced reconstruction

~.:~,,~,~~::,~

p.A

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of ( hll)

and

(

100) faces of copper

M. Sotto Groupe de Physique des Solides, Uniuersite’ de Paris 7, Tour 23, 2 Place Jussieu, 75251 Paris Cedex OS, France

Received 29 April 1991; accepted for publication 23 July 1991

A LEED and AES study on oxygen adsorption on Cu(100) and (hll) faces with 5 2 h I 15 has been performed under various adsorption conditions (220 K I T 5 670 K and 1 X 10-s I P 5 6 X lo-’ Torr of oxygen). The dependence of adsorption temperature on the oxygen surface superstructures is pointed out. At least, three oxygen surface states exist on a Cu(100) face. For low temperature exposures to oxygen, under conditions of slow surface diffusion, on the (100) face, two oxygen surface phases exist: a “four spots” and a c(2 x 2) superstructure, both observed even at saturation coverage; on all the stepped faces, a c(2 X 2) appears and no faceting is observed. For high temperature exposures, on the (100) face, two oxygen superstructures are observed, a “four spots” followed by a (2fi x &JR45 o at higher coverages; on all the stepped faces, surface diffusion is activated and oxygen induced faceting occurs. The appearance of faceting is associated with the onset of the formation of the (26 X &)R45 ’ structure on the (100) face. The oxygen induced faceting and the oxygen surface meshes are reversible with coverages. At saturation coverage, a non-reversible surface transition between the c(2 x 2) and (2fi x aJR45 o superstructures is observed at 420 f 20 K. The importance of impurity traces on the surface meshes is emphasized. Oxygen coverage at saturation is independent of the studied faces and adsorption temperature. Faceting occurs at a critical coverage value, whatever the stepped faces and adsorption temperature are. Models of the oxygen structure on the (h10) stepped faces are discussed.

1. Introduction Oxygen adsorption on Cu(100) surfaces has been extensively studied by a variety of techniques [l-39]. Controversies on many points emerge. However, some of them have been resolved in very recent papers [l-71: (a) it is now admitted that the saturation coverage is l/2 monolayer [1,8] although 3/4 monolayer has been considered [lo-16,19,26,35]; (b) the missing row model of the surface reconstruction of the oxygen covered (100) face where every four [OOl] or [OlO] copper row is missing has been evidenced by STM [5,6], LEED Z(V) [2-41 and X-ray-diffraction studies [7]. Concerning the oxygen superstructures on (100) faces, the debate is not conclusive, although some papers state positively the contrary [l-7,20-22]. Previous LEED studies have identified three superstructures depending on the adsorption conditions: with increased coverages, a “four spots” centred on the (l/2,1/2) positions, a (fi X fi)R45’ (often referred to as 0039-6028/92/$05.00

c(2 X 2)) followed by a (2fi X a)R45 o (two orthogonal domains). This structure sequence has been observed by many authors ([9-19,22-341, and references therein). Recently, several authors [l-7,20,21] claim that only a single oxygen phase exists on the Cu(100) face that is the (26 X a)R45” superstructure. The (l/2,1/2) spots observed on the LEED patterns, would be due to the (2fi x fi)R45 o superstructure, the others quarter-order spots being too weak to be detected and so the c(2 x 2) phase is completely excluded. However, it is worth noting that all their experiments were done at elevated temperature adsorption [20] or at room temperature followed by annealing at relatively high temperature [l] at which a (26 x \/Z)R45 o is formed. With regard to the (110) or (100) low index faces, few works have been undertaken on stepped faces [ll-15,17,19,35-401 although the investigation of oxygen adsorption on these faces may contribute to solve some of the controversies (domain selectivity, reconstruction, stable facets). It

0 1992 - Elsevier Science Publishers B.V. All rights reserved

236

M. Sotto / Oxygen induced reconstruction of Cu(hll) and Cu(100)

is well established now, that under oxygen adsorption, the (410) face only is stable [ll-19,35401. The other stepped faces of copper (hll) [lo] and (h10) [10,14,16,37-401 reconstruct into (410), (100) and (mll) or (n10) facets depending on the primitive orientations of the investigated surfaces

Dll. So, in order to get a better characterization of the thermodynamic range of the different structure stability, we report in this paper, oxygen adsorption studies on several (hll) faces of copper simultaneously with a (100) face under various conditions of exposures (220-670 K, l-7000 L) (1 L = langmuir = 1 X 10e6 Torr . s). This characterization and the influence of impurity traces explain some of the discrepancies between experiments previously reported. Some important data about faceting are reported.

2. Experimental The experimental apparatus consists of a main chamber with facilities for Video-LEED, AES, sample treatments as ion bombardments, annealing and cooling. Two LEED optics have been used, a Riber and an Omicron one. Details of the procedure of sample preparation were described previously for copper stepped faces associated with a (110) face [121. The surface of the sample consists of (100) and stepped faces with variable terrace width. Theses faces are misoriented from the (100) plane along the [Oil] zone axis, e.g., (Ml) faces with 5 2 h 5 15. The unit mesh of the step atoms of (hll) faces with h odd is always a rhomboid mesh and the terrace width corresponds to a half integer number of [Oil] rows. On fig. 1 are represented a (711) (kinkless step edge) and a (410) (kinked step edge) face composed respectively by 3.5 [Oil] and 4 [OOlI atomic rows. The cleaning procedure consists of repeated cycles of Ar bombardment (500 eV, 6 x 10e5 Torr, 10 min) and annealing at about 1000 K. After a large number of cleaning treatments, a sharp LEED pattern is displayed and no residual oxygen, sulphur and carbon could be detected by AES (the sample bulk is sulphur free as no segre-

a

b

Fig. 1. Hard sphere models of (a) a (711) face, (b) a (410) face. Step atom meshes are represented.

gation is noticed after annealing up to 1000 K). The base pressure in the chamber is about 1 x lo-‘” Torr. Gas exposure is carried out by a controllable leak valve. Oxygen is adsorbed in the 1 X lo-‘-6 x lop5 Torr range, at crystal temperatures between 220 + 20 K and 670 f 20 K during 2 min. All oxygen exposures were performed dynamically with oxygen being pumped through a throttled ion pump. Prior each oxygen adsorption, the sample is cleaned by ion bombardments and annealed. During oxygen exposures, activated oxygen formed by the ion-pump and iongauge may influence the uptake rate, although positioned away from the sample (heating is cut off); so, we have operated carefully with the same procedure during each experiment. According to Wiill et al. [6] activated oxygen does not influence the oxygen structures. Auger calibration has been performed by 160(d, p)“O* nuclear microanalysis in our laboratory, leading to an absolute oxygen surface coverage determination [ 13,141.

3. Results 3.1. Stability of the stepped faces The splitting of LEED spots due to periodical step rows and the sequential appearance and disappearance of double and single spots lead to

M. Sotto / Oxygen induced reconstruction of Cu(hll)

0

0 0

0

l 0

0

Fig. 2. Faceting of the (511) face into (311), (410) and (401) facets. LEED pattern (30 eV, incidence angle: 8 “) and its schematic representation (normal incidence). Full circles: (100) spots; small open circles: spots of the (410) and (401) reciprocal step atom lattices; small full circles: (311) spots.

and Cu(100)

237

adsorption at step sites; only one domain of the (2& x fiIR45 o mesh is formed, with the larger side perpendicular to the step edge [ll-15,17, 191. Due to the two domains of the oxygen (2\/2 x fi)R45 ’ superstructure on a (100) face, the probability of forming one orientation or the other during faceting of (hll) faces is identical, so two facets (410) and (401) appear. 3.1.2. Low temperature adsorption For low temperature exposure to oxygen, in the explored range, 220-320 K, no oxygen induced faceting occurs even at saturation coverage. 3.2. AES results

the terrace width and step height of the stepped faces [41]. 3.1.1. High temperature adsorption For high temperature exposures to oxygen (320-670 K), the structural changes induced by oxygen adsorption on the stepped faces associated with the (100) face are dependent on oxygen coverage and initial crystallography of the faces [ll]. All the studied (hll) faces are destabilised by oxygen adsorption and exhibit complex LEED patterns: - As already observed on a (ll,l,l) face [ll], below the saturation coverage, the (hll) faces with h < 7 reconstruct into (4101, (401) and (mll) facets, the last one turning into (100) facets at saturation coverage. - The (711) face has the same behaviour, that is formation of (410), (401) and (mll) facets, but at saturation coverage, (410), (401), (100) and (511) facets are observed. - The (511) face rearranges into (4101, (401) and (311) facets (fig. 2). Since the primitive macroscopical orientation of the stepped faces must be maintained during faceting, facets like (1001, (511) and (311) have to grow up, but the two last faces are not stable and suffer an induced oxygen reconstruction. On the LEED pattern of an oxygen covered (410) surface, the oblique reciprocal lattice of the step atoms appears, revealed by preferential oxygen

Fig. 3 shows the intensity ratio of the oxygen transition at 512 eV to the Cu transition at 61 eV as a function of oxygen exposure for (100) and (711) faces at 470 and 670 K. Curves with similar form have been observed [1,10,11,26]. As expected the adsorption of oxygen on all the stepped faces showed a noticeable increasing rate as compared to the low index plane. For a constant temperature, the larger the step density is, the faster the oxygen uptake is. In other words, for constant adsorption conditions (T and P) the oxygen coverage is higher on the (511) (2.5 [Oil] atomic rows> than on the (15,1,1) (7.5 [Oil] atomic rows) and on the (100) faces. This phenomena is well observed on the LEED patterns: for a constant temperature and with increasing oxygen exposure, the (511) face is the first to reconstruct followed by the (711) face and so on. For lower temperatures, higher oxygen exposure is required to reach the saturation coverage [l]. 3.2.1. Oxygen desorption Heating an oxygen covered sample and measuring by AES the oxygen quantity remaining on the surface, we have observed that oxygen desorption is highly dependent on the coverage: for coverages less than 75% of the saturation value where faceting is already observed, a partial desorption takes place at temperatures lower than the temperature at which the oxygen has been adsorbed; in that case, the LEED pattern corre-

M. Sotto / Oxygen induced reconstruction

238

sponds to the one obtained at the same coverage after an oxygen exposure from a clean surface; if total desorption occurs, the surface is restored to its original unreconstructed state; for coverages near or at the saturation, oxygen desorption occurs at higher temperature, T > 600 K. Such adsorption-desorption cycles and the accompanying structural changes could be repeated indefinitely, indicating that the oxygen induced faceting is completely reversible and only due to the oxygen coverage. 1.2

of Cu(hl1)

and Cu(100)

In other words, the stability of the oxygen structure on (100) and stepped faces is coverage dependent, the more the oxygen coverage is, the more the oxygen structure stability is. Probably oxygen atoms are bonded more strongly to a reconstructed and ordered surface than to a disordered one. 3.2.2. Saturation coverage value We have found that the value of the saturation coverage is independent of the faces and adsorp-

/

I

I I

470

K

a

OXYGEN

DOSIS

[L]

r/

b 12

120

OXYGEN

Fig. 3. Intensity is adsorbed

DOSIS

1200

[L]

ratio of the oxygen transition at 512 eV to the copper transition at 61 eV as a function of the oxygen doses (oxygen in the 1 x lo-‘-6 x lo-’ Torr range, during 2 min). Circle: (100) face; triangle: (711) face. (a) 470 K, (b) 670 K.

M. Sotto f Oxygen induced reconstruction o~~u~h~i~ and Cu(MO)

tion temperature (fig. 3). The ratio of Auger peak intensity, O,,,/Cu,,, equal to (10 of:1) x 10m3(the same value already obtained by Argile and Rhead [IS]) has been calibrated by nuclear microanalysis using the 160(d, p)“O* reaction [13,14]. The ratio of the number of oxygen atoms measured and the atom number of a Cu(lO0) plane corresponds to 0.55 + 0.05 for the (16,1,1) (faceted into (410), (401), (100)) [13], (410) and (100) faces, in agreement with the measurement of Alkemade et al. (0.43 f 0.04) [8]. We have noticed also that the ~$2 X 2) or the “four spots” structures formed at low temperatures evolves into a (2fi x \/2)R45 o structure with annealing, without any oxygen loss. As we have measured the same value of the saturation coverage by AES whatever the faces and whatever the sample temperature, we can conclude as already reported El,83 that the saturation coverage corresponds to half a monolayer. The unit cell of the (2@ x fi)R45” structure contains two oxygen atoms for four copper atoms, moreover on the (100) face and on the (410) face.

3.2.3. Faceting coverage value From all our experimental results, we have noticed that faceting occurs at a critical value of the oxygen coueruge, reached for different exposure conditions depending on the step density and adsorption temperature as mentioned above. This value 8, within the experimental errors is found to be equal to 65 f 5% of the saturation value, that is 0.33 f 0.02 ML. The same value has been obtained for the faceting of the (810) face into (410) and (100) facets below the saturation coverage that is at 0.34 ML, at saturation coverage the (810) face is stable ill]. The (l/2,1/2) reflexes of the (2& x v!?)R45 o structure on the (100) face are observed also at the same value, in complete agreement with measurements of Wuttig et al. [l]. So, oxygen induced faceting of any copper stepped faces seems to occur at a constant value of the coverage and is linked to the appearance of the (2\/2 x J?)R45” structure on the (100) face. Such a measurement has been performed for the first time. It is obvious that oxygen atoms are not distributed uniformly over the surface and

239

(2\ljs x \/Z)R45 o islands may appear, therefore the critical values measured by Wuttig et al. and by us are probably overestimated; due to the electron beam transfer width, very small domains cannot be detected on LEED patterns. 3.3. Oxygen s~rst~ctures The number of oxygen surface phases on (100) and their the~~~arni~ conditions of fo~ation (temperature and pressure) are not clearly defined leading to controversial results. In many papers, two or three oxygen chemisorbed states with increasing coverages have been identified: the “four spots”, the c(2 X 2) followed by the (2fi X 45) superstructures. In some experiments, the “four spots” structure is not observed [10,32]. Recently, the existence of the (y? X a)R45” superstructure has been questioned [l7,20-221. The existence of the ~(2 x 2) phase formed at high temperature exposure is problematic, since the (l/2,1/2) spots are common with the (2fi x &)R45 o structure. 3.3.1. Low t~~rature

a~o~iion:

220 K I T 5

3.50 K

On the (100) face, we have observed a “four spots” or a c(2 x 2) structure for low and high coverages, even for saturator coverage. 2%e formation of either the “four spots” or the c(2 X 2) structure cannot be explained. These structures are stable, the diffraction pattern does not evolve in time at room temperature. In compiete agreement with the work of Spizer and Luth [29], Malcolm et al. [28] and McDonald and Woodruff [23] who have adsorbed oxygen at low temperature in the range 80-300 K, the (2@ X &?)R45 o structure is never observed even for high oxygen coverages. On the stepped faces, our LEED patterns indicate that the primitiue orientation of ail the stepped faces does not change, that is, no faceting occurs. A c(2 X 2) structure is observed on all the stepped faces as already reported by Scheidt et al. [19]. In some experiments, faint streaks in the [OOl] and [NO] directions at very low energy (30 eV) appear with the ~(2 X 2) structure. These

240

M. Sotto / Oxygen induced reconstruction of Cu(hl1) and Cu(lO0)

diffraction features probably can be due to the existence of copper adatoms which begin to rearrange into (410) and (401) facets. On the LEED patterns, an important intensity background is noticed. As already observed 1191, the integral and fractional reflexes of the c(2 X 2) structure on the (100) face are not sharp suggesting a disorder in the oxygen chemisorbed structure. These blurred spots can be explained either by the formation of many domains of c(2 X 2) with antiphase domain boundaries as already suggested [19] or by the small sizes of the domains (size effect). Measuring the AES intensity ratio, we have observed that the oxygen saturation coverage obtained for the c(2 X 2) or the “four spots” structures is the same as that obtained for an adsorption at high temperature, where a (2& x a)R45 o structure is formed. So, annealing such a surface produce two domains of a well ordered (26 x \/Z)R45” structure on the (100) face as already observed [l] and a faceting of all the stepped faces, without any measurable changes in oxygen coverage. A surface transition between the c(2 X 2) formed at low temperature and the (2fi x \/Z)R45” st rut t ure is observed when low barrier surface diffusion occurs. This transition temperature is relatively high, the sample has to be heated up to 420 + 20 K. The formation of the (2fi x &)R45 o structure on the (100) face and the accompanying faceting of the stepped faces are thermally activated. 3.3.2. Influence of the impurity traces We must point out that carbon or any impurities from residual gases in the chamber favour the appearance of the (2fi X fiIR45” superstructure and the reconstruction of the stepped faces. After baking out and after an insufficient number of Ar bombardments, the (2& X a)R45 o superstructure on (100) and faceting of all the stepped faces are observed at low temperature adsorption, even at 220 K. This is noticed systematically after closing the chamber and baking out with the same sample. The c(2 X 2) or the “four spots” superstructure is formed at low temperature exposures after a large number of cycles of argon sputtering, annealing and oxygen expo-

sure. The diffusion of copper atoms seems to be active with the presence of impurity traces as already reported [42] and the (2J?s x &)R45 o structure is formed at the expense of the c(2 X 2) or the “four spots” superstructure. Such an influence only shifts the thermodynamic conditions of superstructure formation and the accompanying faceting to a lower or higher range. The discrepancy between several investigations may be explained by the impurity traces, especially for adsorption at room temperature. 3.3.3. High temperature adsorption: 320 K I T s 670 K

On all the stepped faces, we have observed a faceting above a measured critical value of coverage as mentioned above. On the (100) face, at low coverages a “four spots” superstructure is observed at 0.3 ML, the same value reported by Ascencio et al. [22], followed by a well ordered two orthogonal domains (26 X fi)R45’ structure at higher coverages. The (26 X fi)R45 o patterns could be converted into a “four spots” structure by heating the crystal, indicating the reversibility in the oxygen surface meshes with coverages. Thursgate et al. [32]

noticed the same phenomena but with the (26 X fi)R45 o and c(2 x 2) structures and they observed that subsequent exposure to oxygen retablished the (2fi X &)R45 o structure. The “four spots” structure seems to be a stable thermodynamic oxygen phase on Cu(100) [211 perhaps hardly accessible, due to its small coverage range of formation, especially for high temperature adsorption. For coverage of around 65% of the saturation value, weak spots on the (l/2,1/2) positions are observed and reconstruction of all the stepped faces has already started. If the temperature of such a surface is lowered at around 220 K, streaks along the [OOl] and [OlO] direction with some (l/4,1/4), (3/4,3/4) reflexes are observed indicating the onset of the (26 x I%)R45 o structure. Hence, in this case only, the (l/2,1/2) reflexes are characteristic of the (2fi X &)R45 ’ structure. It is obvious that between the “four spots” structure which corresponds probably to an “unreconstructed” phase and the (2fi X fi)R45 ’

M. Sotto / Oxygeninduced reconstncctionof Cu(hl1) and Cu(100)

structure which corresponds to a high reconstruction state, a disorder phase has to exist as it has been evidenced by EELS [l], STM [5], work function measurements [34] and LEED fine structure [32] but in the later study this disorder state is linked to the c(2 x 2) phase. So, at high temperature, the (2\/2 x &)R45 o structure is connected with a reconstructed surface both on the (100) and stepped faces. At least, two oxygen superstructures exist at high temperature exposures. For adsorption performed at high temperature and oxygen exposure, the state of lowest surface free energy corresponds to the (26 x \/-Z)R45 o structure on a (100) face as already reported and to (4101 facets on the 0~11) stepped faces.

4. Discussions We have shown that the temperature of the surface during oxygen adsorption and surface contaminations have a pronounced effect on the formation of the overlayer surface meshes on Cu(100) face and of the associated faceting of the stepped faces. The surface contamination shifts the formation conditions (T and P) of the different superstructures. Therefore, the controversy found in the literature about the existence of the c(2 x 2) can be explained. At low temperature adsorption, two oxygen surface states exist, a “four spots” and a c(2 x 2) while at high temperature at least two ordered surface states, a “four structures have spots” and a (2& X &)R45” been observed. A disordered state has been evidenced by other measurements [1,5,32,34]. Some authors described the c(2 X 2) structure formed at relatively low temperature as a “metastable” structure [19,32] which reverts to a (2fi x fi)R45 o structure. However, this assumption is not clear since the “four spots” structure formed quasi in the same conditions that the c(2 x 2) at low temperature corresponds to a thermodynamic phase of the oxygen adsorption on Cu(100) face as already reported [22]. At low temperature, the observed c(2 X 2) or “four spots” superstructures seem to be associated with an “unreconstructed” substrate as no faceting is ob-

241

served on the stepped faces, but small relaxations of copper atoms induced by chemisorbed oxygen cannot be excluded in this phase. Concerning the “four spots” structure, using photoelectron diffraction data, Ascencio et al. [22] showed this “four spots” formed at room temperature, before the (2& X fi)R45’ may correspond to disorder (a dynamic or static Debye-Waller effect, essentially equivalent to more than one local site, at least two different local sites). It is worth noting that Wuttig et al. [l] found two adsorption sites for the oxygen surface state before the formation of the (2\/2 X fi)R45 o structure. Lee et al. [9] described the “four spots” superstructure as an oblique mesh (four domains) with unit vectors parallel to [OSi] and [017] directions. Scheidt et al. [19] have reported that the c(2 x 2) formed at room temperature and 300 L is promoted by steps while the “four spots” observed only on the (100) face would be due to molecular oxygen adsorption. In the studies of Malcolm et al. [28] and Spizer and Luth [29], molecular oxygen adsorption have been put into evidence. In our work, and probably in the work of Scheidt et al. [19] where oxygen adsorption has been performed at higher temperature than Malcolm et al. (80 IQ and Spizer et al. (100 K) experiments, molecular 0, adsorption does not occur with the formation of the “four spots” or c(2 x 2) meshes, since the saturation coverage does not depend on the adsorption temperature and no oxygen loss has been measured after annealing. Recently, different models of the (2\/js X \/js)R45” structure have been proposed, especially the missing row models [l-7] where every four 10011or [OlO] copper row is missing. On the (100) face, two (110) microfacets are created if a [OOl] row is removed, that is the configuration of the (410) or (4 x rz, 1, 0) step edges. Only a single adsorption site is populated in the (26 x a)R45 ’ structure [l]. As on a stepped face, the step atoms are preferential sites for adsorption, the oxygen atoms in the missing row model were assumed to occupy the sites adjacent to the missing row. According to the last study of Zeng and Mitchell 131, the oxygen atoms are at a vertical distance of 0.1 A, with 0.30 A lateral relaxations

242

M. Sotto / Oxygen induced reconstruction of Cu(hlI)

for top copper atoms adjacent to the missing row and 0.1 A vertical relaxation in both the first and second copper layers. By RBS and channeling, we have measured the relaxations of an oxygen covered (410) and (100) surfaces [14]. On the (410) face, we found both a vertical and horizontal relaxations of the copper step atoms in the surface plane of +0.20 f 0.03 A. A vertical average relaxation of the first copper plane of +O.ll A on (100) in agreement with the work of Zeng and Mitchell [3] and of +0.075 A on (410) have been measured. A structural relation between the superstructures upon the low index faces and the structure of the stable faces and facets growing during faceting has been put into evidence on several adsorption systems [121. One of the most representative is the Cu(lOO)/O system: the (2fi x fi)R45 o superstructure fits the terrace width of the (410) face. The (410) face is stable whatever the coverage and temperature [ll]. The (810) [ll], (12,1,0) [15] and probably (16,1,0) [13] faces, the terrace width of which are an integer multiple of the (2fi x fi)R45 o unit cell are stable only at saturation coverage. We have no direct evidence for the complete structural arrangement of the oxygen atoms on a (410) face but it may be borne in mind that the model of the (2& X &)R45” structure on Cu(100) must fit the (410) configuration, that is a succession of a (100) facet composed by four [OOl] rows and a (110) microfacet (fig. lb). To accommodate the (2& X a> structure on the (410) face, different models have been proposed: Thompson and Fadley [35] by angle resolved photoelectron diffraction found for low exposures (0.75 L) at room temperature that oxygen lied predominantly on the (410) steps, while at higher exposures 2 40 L both steps and (100) terrace sites are occupied in a modified c(2 X 2) structure, that is two oxygen sites are occupied on the (410) face. Robinson et al. [7] assume a missing row just below the edge, that is the first row in the terrace plane, the oxygen sites being the two (110) sites situated at the same level on the (410) step edge and on the [OOll step edge created by the missing [OOl] row. It is worth noting that the (2& x 6) missing row structure on (100) can be

and Cu(lOO)

fitted by two terraces of the oxygen covered (210) face. However, this face reconstructs into (410) and (530) facets [38,39], the latter being one of the stable facets for the Cu(llO)/O adsorption. What is the status of the missing row model upon (410) state? One answer could be that the step is equivalent to the missing row on (100) and no additional missing row on the terrace is needed, hence, two essentially types of adsorption sites are required. In such a case, the oxygen covered (410) faces could be more stable than (100) faces. It is clear that further investigations are needed to resolve the structural arrangement of the stable (410) stepped face of copper covered by oxygen. We have seen that chemisorption of oxygen atoms can dislodge the substrate atoms from their equilibrium positions and cause the rearrangement of the substrate structure surface if surface diffusion is active. Therefore, the adsorbate-adsorbate interactions are stronger than the adsorbate-substrate interactions. The critical value of coverage for faceting is the same for the straight step edges (hll) faces and for the kink step edge (810) face below the saturation coverage. This critical value can be related to the adsorbate-adsorbate interactions since this value corresponds to a little more than the total filling of the step sites on a (810) face. So, the driving force for faceting seems to be the interaction forces between the oxygen atoms in the (20 X \lZ)R45 o structure. Recently, the presence of 0-Cu-0 chains

which faces chains (PBC)

stabilize the oxygen covered copper surhas been advanced [51; these 0-Cu-0 correspond to the periodic bond chains of Hartman et al. [43,44].

5. Summary We have shown that: (1) For high temperature exposures, at saturation coverage, the (hll) stepped faces with h < 7 reconstruct into (410), (401) and (100) facets; the (711) face reconstructs into (4101, (4011, (100) and (511) and the (511) face into (4101, (401) and (311) facets. (2) High temperature exposures favour the (2fi x \/2)R45” structure on (100) face as al-

M. Sotto / Oxygen induced reconstruction ofCu(hll) and Cu(100)

ready reported and the faceting of all the investigated stepped faces. (3) For low temperature exposures to oxygen and even at saturation coverage, the c(2 X 2) or the “four spots” structures is formed while no reconstruction of the investigated stepped faces occurs. (4) The formation of the (2fi x fi> structure and oxygen induced reconstruction of the stepped faces are thermally activated processes. (5) Faceting is linked to the onset of the (2\/2 x fi)R45 ’ superstructure on the (100) face. (6) At saturation coverage, a surface transition on the (100) face between the c(2 X 2) and (2\/2x \/Z)R45 o structures is observed at 420 f 20 K. (7) Influence of impurity traces on the formation conditions of the diverse oxygen superstructures is pointed out, this effect could be at the origin of the discrepancy found in the literature concerning the existence of the c(2 X 2) structure and the thermodynamic conditions of its formation. (8) Oxygen saturation coverage is independent of the studied faces and the adsorption temperature. (9) Faceting occurs at a constant value of the coverage whatever the copper stepped faces and whatever the adsorption temperature.

References [l] M. Wuttig, R. Franchy and H. Ibach, Surf. Sci. 213 (1989) 103; 224 (1989) L979. [2] H.C. Zeng, R.A. McFarlane and K.A.R. Mitchell, Surf. Sci. 208 (1989) L7. [3] H.C. Zeng and K.A.R. Mitchell, Surf. Sci. Lett. 239 (1990) L578. [4] H.C. Zeng, R.A. McFarlane, R.N. Sodhi and K.A.R. Mitchell, Can. J. Chem. 66 (1988) 38. [5] F. Jensen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. B 42 (1990) 9206. [6] Ch. Wijll, R.J. Wilson, S. Chiang, H.C. Zeng and K.A.R. Mitchell, STM’90 The Fifth Int. Conf. on STM, July 1990, Baltimore. [7j I.K. Robinson, E. Vlieg and S. Ferrer, Phys. Rev. B 42 (1990) 6954. [8] P.F.A. Alkemade, F.H.P.M. Habraken, K. Stap, W.C. Turkenberg and W.F. van der Weg, unpublished results. [9] R.N. Lee and H.E. Farnsworth, Surf. Sci. 3 (1985) 461.

243

DOI F.H.P.M. Habraken, C.M.A.M. Mesters and G.A. Bootsma, Surf. Sci. 97 (1980) 264. t111 J.C. Boulliard, J.L. Domange and M. Sotto, Surf. Sci. 165 (1986) 434. WI J.C. Boulliard and M. Sotto, Surf. Sci. 177 (1986) 139; 182 (1987) 200. [I31 J.C. Boulliard, C. Cohen, J.L. Domange, A. Drigo, A. L’Hoir, J. Moulin and M. Sotto, Phys. Rev. B 30 (1984) 2470. D41 J.C. Boulliard, C. Cohen, A. L’Hoir, J. Moulin, D. Schaums and M. Sotto, Ecoss 9, Luzern, 1987. [I51 J.M. Moison and J.L. Domange, Surf. Sci. 67 (1977) 336. Ml C. Argile and G.E. Rhead, Surf. Sci. 53 (1976) 659. I171 J. Perdereau and G.E. Rhead, Surf. Sci. 24 (1971) 555. I181 C. Argile and G.E. Rhead, J. Phys. C 7 (1974) L 261. [191 A. Scheidt, H. Richter and U. Gerhardt, Surf. Sci. 205 (1988) 38. DOI R. Mayer, Chung-Si Zheng and K.G. Lynn, Phys. Rev. B 31 (1985) 8899. 1211A. Atrei, U. Bardi, G. Rovida and E. Zanazzi, Vacuum 41 (1990) 333. P21 M.C. Asensio, M.J. Ashwin, A.L.D. Kilcoyne, D.P. Woodruff, A.W. Robinson, Th. Lindner, J.S. Somers, D.E. Ricken and A.M. Bradshaw, Surf. Sci. 236 (1990) 1. D31 L. McDonnell and D.P. Woodruff, Surf. Sci. 46 (1974) 505. [241 L. McDonnell, D.P. Woodruff and K.A.R. Mitchell, Surf. Sci. 45 (1974) 1. D51 J.H. Onoferko and D.P. Woodruff, Surf. Sci. 95 (1980) 555. I261 P. Hofman, R. Unwin, W. Wyrobisch and A.M. Bradshaw, Surf. Sci. 72 (1978) 635. P71 S. Kono, SM. Goldberg, N.F.T. Hall and C.S. Fadley, Phys. Rev. 41 (1978) 1831. [281 M.J. Malcolm, J. Braithwaite, R.W. Joyner and M.W. Roberts, Faraday DiscChem. Sot. 60 (1975) 89. [291 A. Spizer and H. Luth, Surf. Sci. 118 (1982) 121. [301 W.S. Yang, F. Jona and P.M. Marcus, Phys. Rev. B 27 (1983) 1394. [311 U. DBbler, K. Baberschke, J. StGhr and D.A. Outka, Phys. Rev. B 32 (1985) 532. [321 S.M. Thurgate and P.J. Jennings, Surf. Sci. 131 (1983) 309. [33] C. Benndorf, B. Egert, B. Keller, H. Seidel and F. Thieme, J. Phys. Chem. Solids 40 (1979) 877. [34] S.P. Holland, B.J. Garrison and N. Winograd, Phys. Rev. L&t. 43 (1979) 220. [35] K.A. Thompson and C.S. Fadley, Surf. Sci. 146 (1984) 261. [36] C.S. McKee, L.V. Renny and M.W. Roberts, Surf. Sci. 75 (1978) 92. [37] A. Oustry, L. Lafourcade and A. Escaut, Surf. Sci. 40 (1973) 545. 1381E. Legrand-Bonnyns and A. Ponslet, Surf. Sci. 53 (1975) 675. [39] L. Trepte, Chr Menzel-Kopp and E. Menzel, Surf. Sci. 8 (1967) 223.

Sotto / Oxygen induced reconsttuction of Cu(hlI) [40] B. Loisel, These, Universitt

Paris 7, 1989. [41] M. Henzler, Surf. Sci. 19 (1970) 159. [42] G.E. Rhead, Int. Mater. Rev. 34 (1989) 261. [43] P. Hartman and W.G. Perdock, Acta Cryst. 8 (1955) 49, 521, 525.

and Cu(lO0)

1441 J.C. Boulliard and M. Sotto, J. Cryst. Growth 110 (1991) 878.