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Faceting of copper (10,10) and (510) surfaces under lead adsorption: A LEED-AES study
338
Surface
Science 177 (1986) 338-352 North-Holland, Amsterdam
FACEtiNG OF COPPER (1410) AND (510) SURFACES ADSORPTION: A LEED-AES STUDY M.-G.
BARTHES-LABROUSSE
UNDER LEAD
*
Groupe de Ph.vsrque des Solides de I’ENS, UnrversrtP de Paris VII, Tour 23, 2 Place Jussieu, F-75005 Paris. Frunce Received
11 March
1986; accepted
for publication
28 July 1986
Low-energy electron diffractron and Auger electron spectroscopy have been used to examine the deposition of (sub)monolayer quantities of lead onto (10.10) and (510) copper surfaces. Faceting into (100) and more complex facets is observed at ambient temperature. For the (10.10) crystal, (210), (710) and (510) facet orientations are successively identified with increasing lead coverage. For the (510) crystal, decomposition into (210) and (100) facets is followed by restoration of the initial macroscopic (510) orientation. These results are discussed in terms of the correlation between the terrace width of the observed facets and the adsorbate superstructure on the corresponding low-index plane. It is shown that the orientatrons stabilised at monolayer coverage cannot be entirely explained by such a correlation but the whole succession of adsorption induced facets must be taken into account.
1. Introduction
Since 1968 adsorption on stepped surfaces has been the subject of a number of studies [l]. Both structural and kinetic information were obtained using routine surface-science techniques. In particular LEED has proved to be a simple and useful tool for investigations of the ordering of the adsorbate layer and the topographical stability of the substrate. Although only a few systematic investigations have been undertaken, the existence of a correlation between the stability of stepped surfaces under adsorption and the ratio of the crystalline parameter of the adsorbate unit mesh to the width of the substrate terrace has been suggested [2-51. The aim of this paper is to obtain complementary information on such a correlation. We report here the results of a LEED-AES study of the deposition of lead on the copper (10,lO) and (510) surfaces. The system lead-on-copper has been chosen because several successive structures can be observed during the formation of the lead monolayer - thus permitting a study of the influence * Permanent address: Laboratoire de Physico-Chimre des Surfaces, Pierre et Marie Curie, F-75231 Paris Cedex 05, France
0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
UA 425, ENSCP-11,
rue
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of the variation of the adlayer unit mesh on the stability of a particular substrate orientation - and because there are available sets of data obtained for different substrate crystallographic orientations [3,6-121. In particular, Moison and Domange [3] have studied all copper vicinal surfaces 4O off the (100) plane. They have shown that two-dimensional faceting occurred under adsorption of a monolayer of lead and the (15,lO) copper face was stabilised. It was noted that three-unit-mesh-wide bands of the lead monolayer structure could fit exactly with the geometry of the terraces of a (15,lO) copper crystal. By analogy (10,lO) and (510) copper surfaces can be expected to be stabilised by respectively two-unit-mesh-wide and one unit-mesh-wide bands of the lead monolayer structure. Because the (10,lO) and (510) surfaces also consist of { lOO} terraces and fully kinked monoatomic (10) steps, the interaction of lead atoms with a step should be similar to the (15,lO) case. But some variations can be introduced by differences in step density and in terrace widths. Moreover, the primitive mesh is oblique in the case of the (10,lO) surface while it is rectangular for the (510) and (15,lO) orientations.
2. Experimental The experiments were performed in a conventional Riber ultrahigh vacuum chamber (base pressure in the lo-* Pa range) in which the 120° retarding grids of the LEED system were used to obtain the Auger spectra (primary excitation, 2.5 keV at glancing incidence; grid modulation, 4.5 V peak-to-peak). In the work described here two copper specimens were employed, each of them containing a portion of (100) face together with the (10.10) or (510) face. These specimens were prepared in the usual way by spark cutting after orientation by means of Laue X-ray diffraction patterns. The specimens were first mechanically and then electrochemically polished immediately before being mounted on the manipulator. A chromel-alumel thermocouple was inserted into a hole in the side of the crystals. The specimen surfaces were cleaned by alternate cycles of argon ion bombardment and heating (350 eV, 3 PA cm-*, 10 min and 550°C, 20 min). Carbon could not be completely eliminated. According to a nuclear microanalysis calibration [13], the residual contamination was estimated to be less than 4% of a copper (100) plane. High purity lead, heated in an alumina crucible maintained at a stabilised temperature, was used as the source of the lead vapour. In a typical experimental run the leaned substrate was exposed at ambient temperature to a constant flux of lead vapour that could be interrupted by a rotating shutter. After each exposure the LEED pattern was observed and/or the differentiated Auger spectrum was recorded. Taking peak-to-peak heights for the substrate and the adsorbate, Auger signal versus deposition time (AS-t) plots were obtained.
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3. Results and discussion
3. I. Clean surfaces The LEED patterns from the clean (10,lO) and (510) faces exhibit well-defined and sharp doublets characteristic of stepped surfaces [.14,15]. Within the experimental error, the mean terrace widths (as deduced from the intra-doublet separations) are, respectively, 10 and 5 [OOl] rows, which are in good agreement with a sjmple terrace-ledge-kink (TLK) model. As theoretically expected the step edge orientation - deduced from the direction of the spot splitting - was (10) for the (510) face. A small misorientation of about 5” was found for the (10,lO) crystal for which the steps were orientated in the (12.1) direction. We think this misorientation was introduced by the polishing process, which had to be repeated several times. 3.2. Adsorption
of lead.
Figs. 1 and 2 show the AS-t plots obtained for deposition of lead onto the (10,lO) and (510) substrates. As recently discussed [16] the two plots presented (obtained for different vapour fluxes) are typical of the Stranski-Krastanov growth mode. For the two faces a sharp knee (q) ascribed to the completion of the dense lead monolayer (19= 1) is followed for both the adsorbate and substrate signals by a plateau due to crystallite formation. An early break at (p) is also observed in the lead signal before the monolayer break and can be associated with the appearance of the first two-dimensional ordered structure. As discussed by Sepulveda and Rhead [S], the appearance of such a break in the lead signal alone might be interpreted in terms of a change in the effective Auger sensitivity, i.e. a difference in contribution to the Auger emission between an isolated adsorbed atom and an atom inserted in a dense layer. The difference in shape observed in figs. 1 and 2 between the AS-t plots is due to the copper signal which has lower values for the (510) face. In fact, this is an artefact of the experimental procedure. The copper signal appears on the low-energy part of the spectrum (63 eV), where the background has a high slope, strongly dependent of the secondary emission. Because of contamination of the screen, which occurred between the experiments carried out on the (10,lO) and (510) crystals, the background of that part of the spectrum was strongly modified, thus leading to a systematic error in the copper peak-to-peak measurements on the (510) specimen. When a comparison is made with the data available for adsorbed lead overlayers on copper (100) and (410) substrates [17] it is found that both the attenuation of the substrate signal and the plateau value of the adsorbate signal obtained for the (10,lO) substrate are consistent with the published data. For the low index surface we observed the same sequence of LEED
M.-G. Barth&Lubrouwe
1
I21
3 LEED
Fig. 1. AS-t
141
6
patterns Pb/Cu
2
341
/ Faceting of Cu(lO,lO) and Cu(510)
4
(10,&O)
6
8
t,min
I
plots for the deposition of lead onto Cu(10.10)
patterns that has been reported in earlier papers (see ref. [12]). The stepped surfaces were topographically unstable under lead adsorption and different LEED patterns appeared, at lead coverages which are indicated in figs. 1 and 2. During the first stages of deposition the first significant change observed for both the (10,lO) and (510) copper faces is the replacement of the sharp doublets in the LEED patterns (structures 1 in figs. 1 and 2) by streaks in the same direction (structure 2). This indicates changes in the terrace widths. Step migration might produce a distribution of terrace widths with the same initial step orientation. Further adsorption of lead results in decomposition into facets of two alternating types: (100) planes and complex planes. Although this decomposition occurred instantaneously even at room temperature, the resulting patterns were very streaky with high background intensity. No significant improvement in the quality of the patterns was observed when the crystal was left at room temperature overnight. However, flash heating at
342
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Barth&L&m.sse
112131 LEED
4
/ Focetmg
1
of Cu(lOJ0)
and Cu(5IO)
5
patterns Pb/Cu
(5,1,0)
. 5
10
15
t,min
Fig. 2. AS-t plots for the deposition of lead onto Cu(510). - 280°C gives rise to well contrasted patterns with sharp spots. The annealing temperature was chosen to avoid lead evaporation which starts very slowly at about 300°C [9]. No systematic investigation of the kinetics of this faceting was undertaken and, in particular, annealing lower lead coverages was not tried when no evidence of a change was observed in the LEED pattern obtained at room temperature. It must be noted that the appearance of a new LEED pattern is not easy to detect accurately because streaky patterns can be ascribed either to a disordered layer or to the coexistence of two structures over a range of coverage. Moreover, small fractions of a structure (or facet) may be overlooked if too small to be seen by LEED (coherence length for the primary beam is 500 to 1000 A). For the low-index facets the same adsorbate meshes - (2& x 2fi)R45” and c(Sfi X fi)R45O - were observed as for deposition of lead onto the macroscopic Cu(100) plane but, as reported earlier [3], the presence of steps
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leads to the preferential growth of one of the two possible domain structures which are equivalent on the (100) plane. For the (10,lO) substrate, three distinct domains - labelled 3, 4 and 5 in fig. 1 - were successively observed and the corresponding LEED patterns are shown in fig. 3. At low lead coverages (0.52 < 8 < 0.84) decomposition into (210) and (100) facets occurred together with the appearance of the (2fi X 2fi)R45O structure on the (100) facets (figs. 3a and 3b). At intermediate coverages (0.84 < 0 < 0.94) evidence of the (210) beams has vanished and bright spots appear corresponding to (710) facets (figs. 3d and 3e). Fading of the (2fi x 2fi)R45O structure is simultaneously observed on the (100) facets. The pattern of fig. 3f is obtained just before the monolayer break in the AS-f plots and can be ascribed to two sets of spots corresponding to (510) and (100) facets with a c(5& x fi)R45O adsorbate structure (fig. 3g). Although the presence of faceting in our study was readily detected by the crossing of LEED beams from two different facets as the electron wavelength was varied, the orientation of the facets was difficult to determine. Usual methods based on movement of spots with changing electron wavelength [14,18,19] were difficult to use because of the large number of spots. Alternative methods like RHEED or crystal rotation [4] were not available. Identification of complex facet planes is therefore mainly based on unit cell geometries with the electron beam incidence normal to the (100) facets. In the case of the (210) facets, orientated 26O34’ off the (100) plane, distortion of the reciprocal lattice due to non-normal incidence with the Ewald sphere has to be taken into account, as shown in fig. 3c. For the (510) face a slightly different succession of patterns was observed after the streaky pattern. For 0.25 < 19< 0.61 faceting into (210) and (100) orientation occurred (domains 3 and 4 in fig. 2) but the (2fi X 2fi)R45O structure appeared on the (100) facets only for 0 > 0.38 (domain 4). The existence of the (100) facets without any adsorbate coincidence mesh in the coverage region 0.25 < 8 < 0.38 is not surprising since the (2& x 2@)R45O structure is only visible for B > 0.36 on a copper (100) macroscopic crystal [12]. For 8 > 0.61 the (510) face was restored and the pattern shown in fig. 4 was observed and persisted over the later stages of deposition into the plateau region of the AS-r plots. This pattern can be ascribed to a structure with a periodicity equal to the kink spacing along the steps and extending across two (510) terraces - i.e. a p(1 X 2) structure. It must be noted that faint spots due to the c(5fi X fi)R45” structure on (100) facets could still be observed, thus indicating that restoration of the initial (510) orientation was not completely achieved. Because the destruction of facets involves step mobility, it must be strongly affected by the presence of adsorbates which can alter surface diffusion [20]. Complete restoration of the initial (510) face may require heating the specimen to a higher temperature than used in this study. But such an increase in temperature can lead to lead desorption [9].
Fig. 3. LEED patterns (and schematic representations) observed at various stages of adsorption of lead on “initialiy clean” Cu(10.10): (t f spots characterising the complex facet: (0) spots characterising the substrate on the (100) facets; (*) extra spots on the (100) facets. (a), (b) Domain 3 in fig. 1, 127 eV. (c) Ewald construction showing the distortion induced by the non-normal incidence of the electron beam on the (210) facet (corresponding to the row indicated by an arrow in (b). (d), (e) Domain 4 m fig. 1,134 eV. (0. (g) Domain 5 m fig. 1,129 eV.
M. 4.
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Fig. 4. LEED pattern (and schematic representation) observed during adsorption of lead on Cu(510); domain 5 in fig. 2, 127 eV: (+) spots characterising the complex facet; (0) spots characterising
the substrate on the (100) facets; (-) extra spots on the (100) facets.
Unique interpretations of the LEED patterns cannot be made without an accurate determination of the content of the unit mesh. However, the appearance (or persistence) of the c(5fi x fi)R45” LEED pattern on the (100) facets at completion of the monolayer, together with comparison with Auger data available for lead overlayers on Cu(100) macroscopic substrates [17], seem to indicate that the monolayer densities on the (510) and faceted (10,lO) crystals are similar to those on the low-index plane. For the (100) facets the observed LEED patterns can be interpreted by the same models as proposed for the macroscopic (100) copper crystal (see fig. 3 in ref. [12]). In these models the lead atoms are adsorbed in four-fold coordination copper sites and the whole succession of structures is derived from a (\/z X @)R45O structure. Because all the complex facets observed in this study are oriented in the (001) zone it is reasonable to assume that the lead atoms will also adsorb in hollow sites of the (100) terraces including kink sites. The ratio between the diameters of the lead atoms to the copper atoms being 1.37, it might appear unrealistic to assume that lead atoms are adsorbed in four-fold symmetry sites. However, it has been shown [7,11] that either rumpling or small shifts (0.3 A) off the hollow sites of the lead atoms could release the stress in the overlayer with respect to the symmetry. For the sake of clarity, only the centers of the lead atoms (in hollow sites) are schematically represented by small black dots in figs. 5 and 6. For the (210) facet, observed for adsorption of lead on both clean (10,lO) and (510) surfaces, the interstep distance is only two (10) copper rows, so that single rows of lead can be adsorbed in kink sites as shown in fig. 5a. On figs. 5b and 5c are shown the structural models proposed to interpret the p(1 X 1) coincidence meshes observed at saturation on the (710) and (510) facets for the “initially clean” (10,lO) surface. It is worth pointing out that
346
M. 4.
Barth&Lahrousse
/ Facetmg of Cu(lO.10) and Cu(510)
b
C Fig. 5. Proposed models for the structures of lead formed on: (a) Cu(210) facets, (b) Cu(710) facets, (c) Cu(510) facets, (0) center of the adatoms.
M.-G. Barth&Labrouwe
/ Faceting of Cu(lO,lO) and Cu(510)
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b Fig. 6. Proposedmodels for the structuresof lead on Cu(510) at differentcoverages: (a) B = 0.66, (b) 8 = 1 (0 = fraction of a completemonolayer).(0) Center of the adatoms.
these structures
are modifications
of the (fi
X fi)R45“
arrangement
and that
the model proposed in fig. 5c has the same density as the monolayer arrangement proposed on the macroscopic (100) crystal [12].For the (510) crystal the p(1 x 2) patterns observed towards the plateaux in the AS-t plots can be interpreted as shown in fig. 6b. Again the density of this model is identical to
the density of the lead monolayer on a (100) crystal. Because the p(1 X 2) pattern was also observed at a relatively low coverage (6 = 0.61) it is reasonable to assume - as for the (100) surface - that another structure with the same mesh is formed at this low coverage by adsorption of a strip of the (fi X fi)R45O arrangement on each terrace (fig. 6a). Again such an arrangement has the same density as the first c(5& x &)R45” structure observed for the (100) crystal at the corresponding value of B = 0.6.
4. Further discussion The stability of clean crystal surfaces is closely related to the orientation dependence of the surface free energy y. For copper, McLean has shown that
348
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/ Faceting
of Cu(lO, IO) und Cu(5IO)
y varies over the stereographic triangle by only a few percent [21] so that most orientations of clean surfaces are expected to be stable. Therefore, the stability of clean copper stepped surfaces observed in many LEED studies, including this work, is not surprising. Adsorption can cause drastic changes in equilibrium shape and faceting has been observed in many LEED experiments of adsorption on metal surfaces, even though it is not clear whether the phenomenon is due to a surface energy effect or to selective evaporation which may prevail under vacuum conditions. As shown by Gjostein [22] faceting may result from preferential adsorption either in edge positions or on terraces. In the first case the angular dependence of the surface free energy will be modified, while in the second case the y-plot will become more sharply cusped due to the lowering of the surface free energy of the terraces. Preferential adsorption of lead at step sites has been observed during the very first stage of deposition onto copper (hll) [8] and (410) [lo] surfaces. In the latter case, evidence of this preferential adsorption was given by the observation of a sharp p(1 x 1) LEED pattern - i.e. with the same mesh as the primitive mesh of the (410) surface - correlated with the appearance in the AS-t plots of a premonolayer break which was ascribed to a change in the sticking probability after saturation of the steps. None of these features was observed in our study. Moreover, if faceting was to result from preferential adsorption on kinks it should occur for a lower coverage on the (10,lO) than on the (510) face because the steps, having a lower density on (lOJO), would become saturated earlier on this surface. This is in contradiction with our observations. An alternative explanation could be that faceting occurs before saturation of the steps. However, the value of 6 = 0.30 corresponding to the first appearance of streaks on the (10,lO) face is too high to be ascribed to adsorption on kink sites alone. We can thus conclude that adsorption occurs first either preferentially on terrace sites or randomly on terrace and kink sites. It seems difficult to assess unambiguously if the initial faceting observed in this study is induced by adsorption on terraces or on kinks. In both cases the (510) face is expected to facet for lower lead coverages because of the differences either in terrace width or in step density between the two orientations. However, stabilisation of (210) facets with the adsorbate structure proposed in fig. 5a is easier to understand if we imagine a mechanism where a lead atom removes a copper atom from the ledge and diffuses with it across terraces and over ledges as a Pb-Cu complex. It must be also noted that carbon contamination can play a major role in step diffusion. Assuming that all the carbon atoms are adsorbed on steps, a residual contamination of 4% of a copper (100) plane corresponds to saturation of about l/3 of the kink sites of the (10,lO) face. Any attempt to predict the orientation of the facets appearing during adsorption is rather speculative when the adsorbate is vapour deposited because complete equilibrium is generally not properly achieved. Faceting can thus be related either to lowering of y or to stabilisation of terraces having a
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/ Faceting of Cu(IO,IO) and Cu(510)
349
specific size in relation with the mesh of the adsorbate structure. As pointed out in the previous section, the distributions of atoms in the unit cells proposed for the complex facets are closely related to the (6 X fi)R45”-(100) structure. Indeed all the arrangements in figs. 5 and 6 are combinations of (10) oriented lead chains forming (@ X fi)R45” microdomains and antiphase chains. For the (710) facet the structure proposed in fig. 5b is obtained by adsorption on each terrace of three lead chains forming (a X fi)R45” microdomains and an antiphase chain on top of steps. Further adsorption can be interpreted by an increase of the density of the antiphase chains and the following structure is obtained by introducing an antiphase chain every two lead chains (fig. 5~). Such an arrangement fits exactly into terraces five (10) copper rows wide and hence it is expected to stabilise (510) facets, as experimentally observed. It must be noted that this structure is deduced from the c(5fi x &)R45” structure proposed for a lead monolayer adsorbed on Cu(100) by a small translation of lead atoms along the (10) axis every two terraces to accommodate the steps. At first sight there seems to be no obvious reason why the particular (710) orientation should be favoured. However, it must be pointed out that the primitive mesh of the (710) surface is the smallest rectangular mesh in the (001) zone which can contain a cell of the (2& X 2fi)R45O lead structure observed on the (100) facets before and during the existence of the (710) facets (see ref. [12], fig. 3). The complete filling of the vacant sites of the (2\/2 x 2fi)R45°-(100) structure and the introduction of antiphase chains lead to the model presented in fig. 5b. Similarly, for the adsorption of lead onto the (510) face, the models presented in figs. 6a and 6b are combinations of lead chains. For this face stabilisation of (710) facets - PO8 off the (100) plane - was not observed. In fact the existence of these facets would imply the creation of more complex facets to keep the initial surface orientation - ll”19’off the (100) plane - and such a process may be not energetically favourable. Instead we observed the restoration of the initial (510) orientation which is stabilised by adsorption of a strip of the (fi X fi)R45”-(100) structure on each terrace (fig. 6a). The dense model of fig. 6b is then obtained by introducing antiphase chains on top and bottom of steps every two steps. Although the substrate orientation is identical in figs. 5c and 6b - i.e. (510) plane - the proposed structural arrangements are different. In fact the monolayer arrangements are deduced from the preceding structures. For the macroscopic (510) face it would be easy to imagine a p(1 X 1) lead structure with the same density as in fig. 6a by saturating steps with adjacent rows of lead. However, it is worth noticing that the arrangement proposed in fig. 6a has the same unit mesh as the first c(5fi X fi)R45O lead overlayer observed on the (100) face and can thus be more stable.
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M.-G. Barth&-Lnbrowse
/ Fcrcetrng of Cu(lOJ0)
und Cu(510)
In this study we have shown that copper (10,lO) and (510) surfaces were faceting under adsorption of (sub)monolayer quantities of lead. These results are in contradiction with the stability of copper (410) and (1510) planes during the whole range of a lead monolayer deposition as respectively observed by Argile and Rhead [lo] and Moison and Domange [3]. Preferential stability of given orientations has been ascribed to a strong correlation between the terrace widths of the stable faces and the adsorbate superstructure of the corresponding low-index plane. Originally proposed by Perdereau and Rhead [2], this explanation was used by Moison and Domange to explain the stability of the (15,lO) copper face by the adsorption of three-unit-mesh-wide bands of the monolayer lead structure onto the (100) terraces of the vicinal surface. In fact similar arguments should lead to stabilisation of (n X 5,lO) surfaces (n integer) by n-unit-mesh-wide bands of the monolayer structure. Our observations for (510) and (10,lO) specimens as well as the proposed monolayer arrangements suggest that the monolayer unit mesh dimension may not be the main factor in favouring a particular orientation. In fact the geometry of all the successive adlayer structures and the orientation of the different facets appearing at submonolayer coverages must also be taken into account. Indeed the stabilisation of the (510) orientation by adsorption of a monolayer of lead on the “initially clean” (10,lO) surface can be understood by the intermediate appearance of the (710) facet. From a simple ball model it can be seen that the transition from a (710) to a (510) face requires only the transfer of two (10) rows whereas the restoration of the (10,lO) face would involve three atomic rows. If the diffusion process is the driving mechanism as already suggested [20] the favouring of the (510) orientation can then be easily understood. On the other hand, stabilisation of the “initially clean” (510) surface by a structure which extends across two terraces can be related to the geometry of the first c(5& x 6) structure proposed for adsorption of lead on Cu(100). This structure has a rectangular mesh with the long side just equal to 10 atomic copper rows and can thus fit exactly into two terraces of the (510) surface while there is no possible coincidence with the oblique primitive mesh of the (10,lO) surface. In view of these considerations, the stability of the (15,lO) surface under lead adsorption as observed by Moison and Domange [3] is difficult to explain. However it must be noted that only the monolayer coverage was extensively studied by these authors and experiments were performed on a conical sample. Two-dimensional faceting of all the orientations 4” off (100) into (15,lO) facets was ascribed to a zipper-like mechanism. The defects generated in the steps by such a mechanism could block further diffusion and stab&e this particular orientation. Moreover, because the density of steps is very low for this orientation protective overlayer structures can be formed before the adsorbate can attack step edges. For the (410) face the monolayer p(1 x 2) structure has been interpreted by
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/ Faceting of Cu(I 410) and Cu(5IO)
351
Argile and Rhead [lo] in terms of dense atomic rows parallel to the step edges. It must be noticed that the terraces of the (410) face, consisting of only four copper (10) rows, are not wide enough to accommodate any of the adsorbate superstructure observed on the low-index copper plane. The structure of the adlayer is hence strongly modified by the presence of steps.
5. Conclusion In this study three-dimensional faceting of (510) and (10,lO) copper crystals has been observed during adsorption of (sub)monolayer quantities of lead. For the (10,lO) surface decomposition into (100) and more complex facets was obtained and (210), (710) and (510) orientations successively appeared as the lead coverage increased. In the case of the (510) surface, (210) facets were followed by restoration of the initial macroscopic (510) orientation. It has been shown that, although a strong correlation does generally exist between the terrace width of the observed facets and the adsorbate superstructure of the corresponding low-index plane, the appearance of intermediate facets at low lead coverages can modify the structure of the adsorbate and influence the orientation of the higher coverage facets. Thus the whole succession of adsorption induced facets must be taken into account to explain the favouring of a given orientation at saturation.
Acknowledgements This work was performed during a year on leave at the Groupe de Physique des Solides de 1’Ecole Normale Superieure (Universite Paris VII). It is a pleasure to thank M. Sotto and J.-C Boulliard for constant and stimulating discussions on fascinating facetings. The author is grateful to Mme J. Xavier at the Pcole Nationale Superieure de Paris for her skillful preparation of the copper crystals.
References [l] See, for example, G.E. Rhead, Surface Sci. 68 (1977) 20; H. Wagner, in: Solid State Physics, Springer Tracts in Modem Physics, Vol. 85, Ed. G. Hijhler (Springer, Berlin, 1979) p. 151; and references therein. [2] J. Perdereau and G.E. Rhead, Surface Sci. 24 (1971) 555. [3] J.M. Moison and J.L. Domange, Surface Sci. 97 (1980) 1. [4] R.E. Kirby, C.S. McKee and L.V. Renny, Surface Sci. 97 (1980) 457. [5] J.C. Boulliard and M. Sotto, Surface Sci. 152/153 (1985) 392.
352 [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] (221
M. 4.
Barrhb-L&muse
/ Fucermg of Cu(lO.10) und Cu(510)
J. Henrion and G.E. Rhead, Surface Sci. 29 (1972) 20. J.P. Biberian and M. Huber, Surface Sci. 55 (1976) 259. A. Sepulveda and G.E. Rhead, Surface Sci. 66 (1977) 436. M.G. Barth& and G.E. Rhead, Surface Sci. 80 (1979) 421. C. Argile and G.E. Rhead, Thin Solid Films 87 (1982) 265. W. Hoesler and W. Moritz, Surface Sci. 117 (1982) 196. C. Argile, M.G. Barth&+Labrousse and G.E. Rhead, Surface Sci. 138 (1984) 181. J.C. Boulliard, C. Cohen, J.L. Domange, A.V Drigo, A. L’Hoir, J. Moulin and M Sotto, Phys. Rev. B30 (1984) 2470. G.E. Rhead and J. Perdereau, m: Colloq. Intern. sur la Structure et les Proprietts des Surfaces des Solides (CNRS, Paris, 1969). M. Henzler, Surface Sci. 19 (1970) 159. G.E. Rhead, M.G. Barth& and C. Argile, Thin Solid Films 82 (1981) 201. G.E. Rhead, C. Argile and M.G. Barth& Surface Interface Anal. 3 (1981) 165. J.C. Tracy and J.M. Blakely, Surface Sci. 13 (1969) 313. C.W. Tucker, J. Appl. Phys. 38 (1967) 1988. G.E. Rhead, Surface Sci. 47 (1975) 207. M. McLean, Acta Met. 19 (1971) 387. N.A. Gjostein, Acta Met. 11 (1963) 969.
Surface
Science 177 (1986) 338-352 North-Holland, Amsterdam
FACEtiNG OF COPPER (1410) AND (510) SURFACES ADSORPTION: A LEED-AES STUDY M.-G.
BARTHES-LABROUSSE
UNDER LEAD
*
Groupe de Ph.vsrque des Solides de I’ENS, UnrversrtP de Paris VII, Tour 23, 2 Place Jussieu, F-75005 Paris. Frunce Received
11 March
1986; accepted
for publication
28 July 1986
Low-energy electron diffractron and Auger electron spectroscopy have been used to examine the deposition of (sub)monolayer quantities of lead onto (10.10) and (510) copper surfaces. Faceting into (100) and more complex facets is observed at ambient temperature. For the (10.10) crystal, (210), (710) and (510) facet orientations are successively identified with increasing lead coverage. For the (510) crystal, decomposition into (210) and (100) facets is followed by restoration of the initial macroscopic (510) orientation. These results are discussed in terms of the correlation between the terrace width of the observed facets and the adsorbate superstructure on the corresponding low-index plane. It is shown that the orientatrons stabilised at monolayer coverage cannot be entirely explained by such a correlation but the whole succession of adsorption induced facets must be taken into account.
1. Introduction
Since 1968 adsorption on stepped surfaces has been the subject of a number of studies [l]. Both structural and kinetic information were obtained using routine surface-science techniques. In particular LEED has proved to be a simple and useful tool for investigations of the ordering of the adsorbate layer and the topographical stability of the substrate. Although only a few systematic investigations have been undertaken, the existence of a correlation between the stability of stepped surfaces under adsorption and the ratio of the crystalline parameter of the adsorbate unit mesh to the width of the substrate terrace has been suggested [2-51. The aim of this paper is to obtain complementary information on such a correlation. We report here the results of a LEED-AES study of the deposition of lead on the copper (10,lO) and (510) surfaces. The system lead-on-copper has been chosen because several successive structures can be observed during the formation of the lead monolayer - thus permitting a study of the influence * Permanent address: Laboratoire de Physico-Chimre des Surfaces, Pierre et Marie Curie, F-75231 Paris Cedex 05, France
0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
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of the variation of the adlayer unit mesh on the stability of a particular substrate orientation - and because there are available sets of data obtained for different substrate crystallographic orientations [3,6-121. In particular, Moison and Domange [3] have studied all copper vicinal surfaces 4O off the (100) plane. They have shown that two-dimensional faceting occurred under adsorption of a monolayer of lead and the (15,lO) copper face was stabilised. It was noted that three-unit-mesh-wide bands of the lead monolayer structure could fit exactly with the geometry of the terraces of a (15,lO) copper crystal. By analogy (10,lO) and (510) copper surfaces can be expected to be stabilised by respectively two-unit-mesh-wide and one unit-mesh-wide bands of the lead monolayer structure. Because the (10,lO) and (510) surfaces also consist of { lOO} terraces and fully kinked monoatomic (10) steps, the interaction of lead atoms with a step should be similar to the (15,lO) case. But some variations can be introduced by differences in step density and in terrace widths. Moreover, the primitive mesh is oblique in the case of the (10,lO) surface while it is rectangular for the (510) and (15,lO) orientations.
2. Experimental The experiments were performed in a conventional Riber ultrahigh vacuum chamber (base pressure in the lo-* Pa range) in which the 120° retarding grids of the LEED system were used to obtain the Auger spectra (primary excitation, 2.5 keV at glancing incidence; grid modulation, 4.5 V peak-to-peak). In the work described here two copper specimens were employed, each of them containing a portion of (100) face together with the (10.10) or (510) face. These specimens were prepared in the usual way by spark cutting after orientation by means of Laue X-ray diffraction patterns. The specimens were first mechanically and then electrochemically polished immediately before being mounted on the manipulator. A chromel-alumel thermocouple was inserted into a hole in the side of the crystals. The specimen surfaces were cleaned by alternate cycles of argon ion bombardment and heating (350 eV, 3 PA cm-*, 10 min and 550°C, 20 min). Carbon could not be completely eliminated. According to a nuclear microanalysis calibration [13], the residual contamination was estimated to be less than 4% of a copper (100) plane. High purity lead, heated in an alumina crucible maintained at a stabilised temperature, was used as the source of the lead vapour. In a typical experimental run the leaned substrate was exposed at ambient temperature to a constant flux of lead vapour that could be interrupted by a rotating shutter. After each exposure the LEED pattern was observed and/or the differentiated Auger spectrum was recorded. Taking peak-to-peak heights for the substrate and the adsorbate, Auger signal versus deposition time (AS-t) plots were obtained.
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3. Results and discussion
3. I. Clean surfaces The LEED patterns from the clean (10,lO) and (510) faces exhibit well-defined and sharp doublets characteristic of stepped surfaces [.14,15]. Within the experimental error, the mean terrace widths (as deduced from the intra-doublet separations) are, respectively, 10 and 5 [OOl] rows, which are in good agreement with a sjmple terrace-ledge-kink (TLK) model. As theoretically expected the step edge orientation - deduced from the direction of the spot splitting - was (10) for the (510) face. A small misorientation of about 5” was found for the (10,lO) crystal for which the steps were orientated in the (12.1) direction. We think this misorientation was introduced by the polishing process, which had to be repeated several times. 3.2. Adsorption
of lead.
Figs. 1 and 2 show the AS-t plots obtained for deposition of lead onto the (10,lO) and (510) substrates. As recently discussed [16] the two plots presented (obtained for different vapour fluxes) are typical of the Stranski-Krastanov growth mode. For the two faces a sharp knee (q) ascribed to the completion of the dense lead monolayer (19= 1) is followed for both the adsorbate and substrate signals by a plateau due to crystallite formation. An early break at (p) is also observed in the lead signal before the monolayer break and can be associated with the appearance of the first two-dimensional ordered structure. As discussed by Sepulveda and Rhead [S], the appearance of such a break in the lead signal alone might be interpreted in terms of a change in the effective Auger sensitivity, i.e. a difference in contribution to the Auger emission between an isolated adsorbed atom and an atom inserted in a dense layer. The difference in shape observed in figs. 1 and 2 between the AS-t plots is due to the copper signal which has lower values for the (510) face. In fact, this is an artefact of the experimental procedure. The copper signal appears on the low-energy part of the spectrum (63 eV), where the background has a high slope, strongly dependent of the secondary emission. Because of contamination of the screen, which occurred between the experiments carried out on the (10,lO) and (510) crystals, the background of that part of the spectrum was strongly modified, thus leading to a systematic error in the copper peak-to-peak measurements on the (510) specimen. When a comparison is made with the data available for adsorbed lead overlayers on copper (100) and (410) substrates [17] it is found that both the attenuation of the substrate signal and the plateau value of the adsorbate signal obtained for the (10,lO) substrate are consistent with the published data. For the low index surface we observed the same sequence of LEED
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(10,&O)
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patterns that has been reported in earlier papers (see ref. [12]). The stepped surfaces were topographically unstable under lead adsorption and different LEED patterns appeared, at lead coverages which are indicated in figs. 1 and 2. During the first stages of deposition the first significant change observed for both the (10,lO) and (510) copper faces is the replacement of the sharp doublets in the LEED patterns (structures 1 in figs. 1 and 2) by streaks in the same direction (structure 2). This indicates changes in the terrace widths. Step migration might produce a distribution of terrace widths with the same initial step orientation. Further adsorption of lead results in decomposition into facets of two alternating types: (100) planes and complex planes. Although this decomposition occurred instantaneously even at room temperature, the resulting patterns were very streaky with high background intensity. No significant improvement in the quality of the patterns was observed when the crystal was left at room temperature overnight. However, flash heating at
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Fig. 2. AS-t plots for the deposition of lead onto Cu(510). - 280°C gives rise to well contrasted patterns with sharp spots. The annealing temperature was chosen to avoid lead evaporation which starts very slowly at about 300°C [9]. No systematic investigation of the kinetics of this faceting was undertaken and, in particular, annealing lower lead coverages was not tried when no evidence of a change was observed in the LEED pattern obtained at room temperature. It must be noted that the appearance of a new LEED pattern is not easy to detect accurately because streaky patterns can be ascribed either to a disordered layer or to the coexistence of two structures over a range of coverage. Moreover, small fractions of a structure (or facet) may be overlooked if too small to be seen by LEED (coherence length for the primary beam is 500 to 1000 A). For the low-index facets the same adsorbate meshes - (2& x 2fi)R45” and c(Sfi X fi)R45O - were observed as for deposition of lead onto the macroscopic Cu(100) plane but, as reported earlier [3], the presence of steps
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leads to the preferential growth of one of the two possible domain structures which are equivalent on the (100) plane. For the (10,lO) substrate, three distinct domains - labelled 3, 4 and 5 in fig. 1 - were successively observed and the corresponding LEED patterns are shown in fig. 3. At low lead coverages (0.52 < 8 < 0.84) decomposition into (210) and (100) facets occurred together with the appearance of the (2fi X 2fi)R45O structure on the (100) facets (figs. 3a and 3b). At intermediate coverages (0.84 < 0 < 0.94) evidence of the (210) beams has vanished and bright spots appear corresponding to (710) facets (figs. 3d and 3e). Fading of the (2fi x 2fi)R45O structure is simultaneously observed on the (100) facets. The pattern of fig. 3f is obtained just before the monolayer break in the AS-f plots and can be ascribed to two sets of spots corresponding to (510) and (100) facets with a c(5& x fi)R45O adsorbate structure (fig. 3g). Although the presence of faceting in our study was readily detected by the crossing of LEED beams from two different facets as the electron wavelength was varied, the orientation of the facets was difficult to determine. Usual methods based on movement of spots with changing electron wavelength [14,18,19] were difficult to use because of the large number of spots. Alternative methods like RHEED or crystal rotation [4] were not available. Identification of complex facet planes is therefore mainly based on unit cell geometries with the electron beam incidence normal to the (100) facets. In the case of the (210) facets, orientated 26O34’ off the (100) plane, distortion of the reciprocal lattice due to non-normal incidence with the Ewald sphere has to be taken into account, as shown in fig. 3c. For the (510) face a slightly different succession of patterns was observed after the streaky pattern. For 0.25 < 19< 0.61 faceting into (210) and (100) orientation occurred (domains 3 and 4 in fig. 2) but the (2fi X 2fi)R45O structure appeared on the (100) facets only for 0 > 0.38 (domain 4). The existence of the (100) facets without any adsorbate coincidence mesh in the coverage region 0.25 < 8 < 0.38 is not surprising since the (2& x 2@)R45O structure is only visible for B > 0.36 on a copper (100) macroscopic crystal [12]. For 8 > 0.61 the (510) face was restored and the pattern shown in fig. 4 was observed and persisted over the later stages of deposition into the plateau region of the AS-r plots. This pattern can be ascribed to a structure with a periodicity equal to the kink spacing along the steps and extending across two (510) terraces - i.e. a p(1 X 2) structure. It must be noted that faint spots due to the c(5fi X fi)R45” structure on (100) facets could still be observed, thus indicating that restoration of the initial (510) orientation was not completely achieved. Because the destruction of facets involves step mobility, it must be strongly affected by the presence of adsorbates which can alter surface diffusion [20]. Complete restoration of the initial (510) face may require heating the specimen to a higher temperature than used in this study. But such an increase in temperature can lead to lead desorption [9].
Fig. 3. LEED patterns (and schematic representations) observed at various stages of adsorption of lead on “initialiy clean” Cu(10.10): (t f spots characterising the complex facet: (0) spots characterising the substrate on the (100) facets; (*) extra spots on the (100) facets. (a), (b) Domain 3 in fig. 1, 127 eV. (c) Ewald construction showing the distortion induced by the non-normal incidence of the electron beam on the (210) facet (corresponding to the row indicated by an arrow in (b). (d), (e) Domain 4 m fig. 1,134 eV. (0. (g) Domain 5 m fig. 1,129 eV.
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Fig. 4. LEED pattern (and schematic representation) observed during adsorption of lead on Cu(510); domain 5 in fig. 2, 127 eV: (+) spots characterising the complex facet; (0) spots characterising
the substrate on the (100) facets; (-) extra spots on the (100) facets.
Unique interpretations of the LEED patterns cannot be made without an accurate determination of the content of the unit mesh. However, the appearance (or persistence) of the c(5fi x fi)R45” LEED pattern on the (100) facets at completion of the monolayer, together with comparison with Auger data available for lead overlayers on Cu(100) macroscopic substrates [17], seem to indicate that the monolayer densities on the (510) and faceted (10,lO) crystals are similar to those on the low-index plane. For the (100) facets the observed LEED patterns can be interpreted by the same models as proposed for the macroscopic (100) copper crystal (see fig. 3 in ref. [12]). In these models the lead atoms are adsorbed in four-fold coordination copper sites and the whole succession of structures is derived from a (\/z X @)R45O structure. Because all the complex facets observed in this study are oriented in the (001) zone it is reasonable to assume that the lead atoms will also adsorb in hollow sites of the (100) terraces including kink sites. The ratio between the diameters of the lead atoms to the copper atoms being 1.37, it might appear unrealistic to assume that lead atoms are adsorbed in four-fold symmetry sites. However, it has been shown [7,11] that either rumpling or small shifts (0.3 A) off the hollow sites of the lead atoms could release the stress in the overlayer with respect to the symmetry. For the sake of clarity, only the centers of the lead atoms (in hollow sites) are schematically represented by small black dots in figs. 5 and 6. For the (210) facet, observed for adsorption of lead on both clean (10,lO) and (510) surfaces, the interstep distance is only two (10) copper rows, so that single rows of lead can be adsorbed in kink sites as shown in fig. 5a. On figs. 5b and 5c are shown the structural models proposed to interpret the p(1 X 1) coincidence meshes observed at saturation on the (710) and (510) facets for the “initially clean” (10,lO) surface. It is worth pointing out that
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C Fig. 5. Proposed models for the structures of lead formed on: (a) Cu(210) facets, (b) Cu(710) facets, (c) Cu(510) facets, (0) center of the adatoms.
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b Fig. 6. Proposedmodels for the structuresof lead on Cu(510) at differentcoverages: (a) B = 0.66, (b) 8 = 1 (0 = fraction of a completemonolayer).(0) Center of the adatoms.
these structures
are modifications
of the (fi
X fi)R45“
arrangement
and that
the model proposed in fig. 5c has the same density as the monolayer arrangement proposed on the macroscopic (100) crystal [12].For the (510) crystal the p(1 x 2) patterns observed towards the plateaux in the AS-t plots can be interpreted as shown in fig. 6b. Again the density of this model is identical to
the density of the lead monolayer on a (100) crystal. Because the p(1 X 2) pattern was also observed at a relatively low coverage (6 = 0.61) it is reasonable to assume - as for the (100) surface - that another structure with the same mesh is formed at this low coverage by adsorption of a strip of the (fi X fi)R45O arrangement on each terrace (fig. 6a). Again such an arrangement has the same density as the first c(5& x &)R45” structure observed for the (100) crystal at the corresponding value of B = 0.6.
4. Further discussion The stability of clean crystal surfaces is closely related to the orientation dependence of the surface free energy y. For copper, McLean has shown that
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y varies over the stereographic triangle by only a few percent [21] so that most orientations of clean surfaces are expected to be stable. Therefore, the stability of clean copper stepped surfaces observed in many LEED studies, including this work, is not surprising. Adsorption can cause drastic changes in equilibrium shape and faceting has been observed in many LEED experiments of adsorption on metal surfaces, even though it is not clear whether the phenomenon is due to a surface energy effect or to selective evaporation which may prevail under vacuum conditions. As shown by Gjostein [22] faceting may result from preferential adsorption either in edge positions or on terraces. In the first case the angular dependence of the surface free energy will be modified, while in the second case the y-plot will become more sharply cusped due to the lowering of the surface free energy of the terraces. Preferential adsorption of lead at step sites has been observed during the very first stage of deposition onto copper (hll) [8] and (410) [lo] surfaces. In the latter case, evidence of this preferential adsorption was given by the observation of a sharp p(1 x 1) LEED pattern - i.e. with the same mesh as the primitive mesh of the (410) surface - correlated with the appearance in the AS-t plots of a premonolayer break which was ascribed to a change in the sticking probability after saturation of the steps. None of these features was observed in our study. Moreover, if faceting was to result from preferential adsorption on kinks it should occur for a lower coverage on the (10,lO) than on the (510) face because the steps, having a lower density on (lOJO), would become saturated earlier on this surface. This is in contradiction with our observations. An alternative explanation could be that faceting occurs before saturation of the steps. However, the value of 6 = 0.30 corresponding to the first appearance of streaks on the (10,lO) face is too high to be ascribed to adsorption on kink sites alone. We can thus conclude that adsorption occurs first either preferentially on terrace sites or randomly on terrace and kink sites. It seems difficult to assess unambiguously if the initial faceting observed in this study is induced by adsorption on terraces or on kinks. In both cases the (510) face is expected to facet for lower lead coverages because of the differences either in terrace width or in step density between the two orientations. However, stabilisation of (210) facets with the adsorbate structure proposed in fig. 5a is easier to understand if we imagine a mechanism where a lead atom removes a copper atom from the ledge and diffuses with it across terraces and over ledges as a Pb-Cu complex. It must be also noted that carbon contamination can play a major role in step diffusion. Assuming that all the carbon atoms are adsorbed on steps, a residual contamination of 4% of a copper (100) plane corresponds to saturation of about l/3 of the kink sites of the (10,lO) face. Any attempt to predict the orientation of the facets appearing during adsorption is rather speculative when the adsorbate is vapour deposited because complete equilibrium is generally not properly achieved. Faceting can thus be related either to lowering of y or to stabilisation of terraces having a
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specific size in relation with the mesh of the adsorbate structure. As pointed out in the previous section, the distributions of atoms in the unit cells proposed for the complex facets are closely related to the (6 X fi)R45”-(100) structure. Indeed all the arrangements in figs. 5 and 6 are combinations of (10) oriented lead chains forming (@ X fi)R45” microdomains and antiphase chains. For the (710) facet the structure proposed in fig. 5b is obtained by adsorption on each terrace of three lead chains forming (a X fi)R45” microdomains and an antiphase chain on top of steps. Further adsorption can be interpreted by an increase of the density of the antiphase chains and the following structure is obtained by introducing an antiphase chain every two lead chains (fig. 5~). Such an arrangement fits exactly into terraces five (10) copper rows wide and hence it is expected to stabilise (510) facets, as experimentally observed. It must be noted that this structure is deduced from the c(5fi x &)R45” structure proposed for a lead monolayer adsorbed on Cu(100) by a small translation of lead atoms along the (10) axis every two terraces to accommodate the steps. At first sight there seems to be no obvious reason why the particular (710) orientation should be favoured. However, it must be pointed out that the primitive mesh of the (710) surface is the smallest rectangular mesh in the (001) zone which can contain a cell of the (2& X 2fi)R45O lead structure observed on the (100) facets before and during the existence of the (710) facets (see ref. [12], fig. 3). The complete filling of the vacant sites of the (2\/2 x 2fi)R45°-(100) structure and the introduction of antiphase chains lead to the model presented in fig. 5b. Similarly, for the adsorption of lead onto the (510) face, the models presented in figs. 6a and 6b are combinations of lead chains. For this face stabilisation of (710) facets - PO8 off the (100) plane - was not observed. In fact the existence of these facets would imply the creation of more complex facets to keep the initial surface orientation - ll”19’off the (100) plane - and such a process may be not energetically favourable. Instead we observed the restoration of the initial (510) orientation which is stabilised by adsorption of a strip of the (fi X fi)R45”-(100) structure on each terrace (fig. 6a). The dense model of fig. 6b is then obtained by introducing antiphase chains on top and bottom of steps every two steps. Although the substrate orientation is identical in figs. 5c and 6b - i.e. (510) plane - the proposed structural arrangements are different. In fact the monolayer arrangements are deduced from the preceding structures. For the macroscopic (510) face it would be easy to imagine a p(1 X 1) lead structure with the same density as in fig. 6a by saturating steps with adjacent rows of lead. However, it is worth noticing that the arrangement proposed in fig. 6a has the same unit mesh as the first c(5fi X fi)R45O lead overlayer observed on the (100) face and can thus be more stable.
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In this study we have shown that copper (10,lO) and (510) surfaces were faceting under adsorption of (sub)monolayer quantities of lead. These results are in contradiction with the stability of copper (410) and (1510) planes during the whole range of a lead monolayer deposition as respectively observed by Argile and Rhead [lo] and Moison and Domange [3]. Preferential stability of given orientations has been ascribed to a strong correlation between the terrace widths of the stable faces and the adsorbate superstructure of the corresponding low-index plane. Originally proposed by Perdereau and Rhead [2], this explanation was used by Moison and Domange to explain the stability of the (15,lO) copper face by the adsorption of three-unit-mesh-wide bands of the monolayer lead structure onto the (100) terraces of the vicinal surface. In fact similar arguments should lead to stabilisation of (n X 5,lO) surfaces (n integer) by n-unit-mesh-wide bands of the monolayer structure. Our observations for (510) and (10,lO) specimens as well as the proposed monolayer arrangements suggest that the monolayer unit mesh dimension may not be the main factor in favouring a particular orientation. In fact the geometry of all the successive adlayer structures and the orientation of the different facets appearing at submonolayer coverages must also be taken into account. Indeed the stabilisation of the (510) orientation by adsorption of a monolayer of lead on the “initially clean” (10,lO) surface can be understood by the intermediate appearance of the (710) facet. From a simple ball model it can be seen that the transition from a (710) to a (510) face requires only the transfer of two (10) rows whereas the restoration of the (10,lO) face would involve three atomic rows. If the diffusion process is the driving mechanism as already suggested [20] the favouring of the (510) orientation can then be easily understood. On the other hand, stabilisation of the “initially clean” (510) surface by a structure which extends across two terraces can be related to the geometry of the first c(5& x 6) structure proposed for adsorption of lead on Cu(100). This structure has a rectangular mesh with the long side just equal to 10 atomic copper rows and can thus fit exactly into two terraces of the (510) surface while there is no possible coincidence with the oblique primitive mesh of the (10,lO) surface. In view of these considerations, the stability of the (15,lO) surface under lead adsorption as observed by Moison and Domange [3] is difficult to explain. However it must be noted that only the monolayer coverage was extensively studied by these authors and experiments were performed on a conical sample. Two-dimensional faceting of all the orientations 4” off (100) into (15,lO) facets was ascribed to a zipper-like mechanism. The defects generated in the steps by such a mechanism could block further diffusion and stab&e this particular orientation. Moreover, because the density of steps is very low for this orientation protective overlayer structures can be formed before the adsorbate can attack step edges. For the (410) face the monolayer p(1 x 2) structure has been interpreted by
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Argile and Rhead [lo] in terms of dense atomic rows parallel to the step edges. It must be noticed that the terraces of the (410) face, consisting of only four copper (10) rows, are not wide enough to accommodate any of the adsorbate superstructure observed on the low-index copper plane. The structure of the adlayer is hence strongly modified by the presence of steps.
5. Conclusion In this study three-dimensional faceting of (510) and (10,lO) copper crystals has been observed during adsorption of (sub)monolayer quantities of lead. For the (10,lO) surface decomposition into (100) and more complex facets was obtained and (210), (710) and (510) orientations successively appeared as the lead coverage increased. In the case of the (510) surface, (210) facets were followed by restoration of the initial macroscopic (510) orientation. It has been shown that, although a strong correlation does generally exist between the terrace width of the observed facets and the adsorbate superstructure of the corresponding low-index plane, the appearance of intermediate facets at low lead coverages can modify the structure of the adsorbate and influence the orientation of the higher coverage facets. Thus the whole succession of adsorption induced facets must be taken into account to explain the favouring of a given orientation at saturation.
Acknowledgements This work was performed during a year on leave at the Groupe de Physique des Solides de 1’Ecole Normale Superieure (Universite Paris VII). It is a pleasure to thank M. Sotto and J.-C Boulliard for constant and stimulating discussions on fascinating facetings. The author is grateful to Mme J. Xavier at the Pcole Nationale Superieure de Paris for her skillful preparation of the copper crystals.
References [l] See, for example, G.E. Rhead, Surface Sci. 68 (1977) 20; H. Wagner, in: Solid State Physics, Springer Tracts in Modem Physics, Vol. 85, Ed. G. Hijhler (Springer, Berlin, 1979) p. 151; and references therein. [2] J. Perdereau and G.E. Rhead, Surface Sci. 24 (1971) 555. [3] J.M. Moison and J.L. Domange, Surface Sci. 97 (1980) 1. [4] R.E. Kirby, C.S. McKee and L.V. Renny, Surface Sci. 97 (1980) 457. [5] J.C. Boulliard and M. Sotto, Surface Sci. 152/153 (1985) 392.
352 [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] (221
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J. Henrion and G.E. Rhead, Surface Sci. 29 (1972) 20. J.P. Biberian and M. Huber, Surface Sci. 55 (1976) 259. A. Sepulveda and G.E. Rhead, Surface Sci. 66 (1977) 436. M.G. Barth& and G.E. Rhead, Surface Sci. 80 (1979) 421. C. Argile and G.E. Rhead, Thin Solid Films 87 (1982) 265. W. Hoesler and W. Moritz, Surface Sci. 117 (1982) 196. C. Argile, M.G. Barth&+Labrousse and G.E. Rhead, Surface Sci. 138 (1984) 181. J.C. Boulliard, C. Cohen, J.L. Domange, A.V Drigo, A. L’Hoir, J. Moulin and M Sotto, Phys. Rev. B30 (1984) 2470. G.E. Rhead and J. Perdereau, m: Colloq. Intern. sur la Structure et les Proprietts des Surfaces des Solides (CNRS, Paris, 1969). M. Henzler, Surface Sci. 19 (1970) 159. G.E. Rhead, M.G. Barth& and C. Argile, Thin Solid Films 82 (1981) 201. G.E. Rhead, C. Argile and M.G. Barth& Surface Interface Anal. 3 (1981) 165. J.C. Tracy and J.M. Blakely, Surface Sci. 13 (1969) 313. C.W. Tucker, J. Appl. Phys. 38 (1967) 1988. G.E. Rhead, Surface Sci. 47 (1975) 207. M. McLean, Acta Met. 19 (1971) 387. N.A. Gjostein, Acta Met. 11 (1963) 969.