- Email: [email protected]
The structural relations between the stable stepped copper faces upon oxygen adsorption and the oxygen superstructures on Cu(100) and Cu(110)
200
Surface
Science 182 (1987) 200-212 North-Holland. Amsterdam
THE STRUCTURAL RELATIONS BETWEEN THE STABLE STEPPED COPPER FACES UPON OXYGEN ADSORPTION AND THE OXYGEN SUPERSTRUCTURES ON Cu(100) AND Cu(ll0) J.C. BOULLIARD
and M.P. SOTTO
Group de Ph_vsiquedes Solides de I’Ecole Normale SqGrieure, UniuersztBParis VII, Tour 33. 2, Pluce Juuieu, 75251 Paris Cede.%05, France Received
17 July 1986: accepted
for publication
21 October
1986
This paper is concerned with the general structural relations we have deduced after LEED studies of oxygen adsorption on the (hk0) and (hkk) stepped faces associated with Cu(100) and the (hk0) faces associated with Cu(ll0). Strong structural relations between the oxygen superstructures on the low index faces and the structures of the stable complex Cu faces are evidenced. Such relations imply some remarks about the model of the oxygen settlement on the low index and stable stepped faces.
1. Introduction As early as in the first studies on the stepped faces of copper [l-8], it has been shown that three-dimensional faceting upon oxygen adsorption occurs. Although the experimental conditions of exposure vary greatly from one author to another, there is some kind of general agreement on the stable stepped faces or facets. These results are summarized as follows: ~ all the high index faces associated with (100) facet into (410) and (100) [l-41; - all the high index faces deduced from (110) facet into (530) and (110) [2,4,8]; _ only a few stepped faces deduced from (111) have been previously studied [2,6,8]. These works have concluded with the stability of (111) (311) and (322). Nevertheless there are some points of disagreement about this simple schema: the first one concerns the (210) face which is, according to Trepte [2] and Legrand-Bonnyns and Ponslet 141, decomposed into wide (410) and (530) facets and stable with the setting up of the p(2 x 1) and p(3 x 1) superstructures according to McKee et al. [5]. Another point is the stability, at saturation coverage. of the (12,1,0) face, evidenced by Moison and Domange [7], on a conical sample presenting all the vicinal faces inclined at 4” from the Cu(100) pole. These authors concluded faceting of all the faces into {12,1,0} facets at saturation coverage occurs. Lastly our works on the vicinal and complex faces 0039-6028/87/$03.50 ‘0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J.C. Boulliurd, M.P. Sotto / Stable stepped Cu facesand 0 superstructures on CM
201
associated with Cu(100) and Cu(ll0) [9,10] show that the structural changes induced by oxygen are more complex with increasing oxygen coverage. In this paper we intend to show that structural relations link the structures of the stable stepped faces to the oxygen superstructures settled on the low index faces. Moreover, the structure of the complex face may present some information about the models of the superstructures on the low index face.
2. Crystallography Cu(ll0)
of the studied stepped faces associated
with Cu(100) and
Concerning the faces associated with Cu(100) we have studied two extreme kinds of faces characterized by the step structure. _ The faces inclined along the [OOl] zone axis (indexed (hk0)) (fig. 1). The adjacent step atoms lie on a kinked line. Each terrace width is equal to an integer number of [OOl] inter-row distances. The superlattice describing the positions of the step atoms, has a rhombic primitive mesh for the faces with h = 2n (n an integer number) and k = 1 and a rectangular primitive for the faces with h = 2n + 1 and k = 1. The stacking fault introduced by a step in a pure two-dimensional (100) lattice, is characterized by the vector joining a step atom with its nearest neighbour on the terrace below. Neglecting any relaxation, the value of this vector is equal tog, = $a,( $-aLWil - a,roo,), (aIhk,] being the unit vector in the [hkl] direction and a, the Cu fee mesh parameter equal to 3.61 A at room temperature). _ The faces inclined along the [Oil] zone (indexed (hkk)) (fig. 2). Their steps are straight. Each terrace width is equal to a half-integer number of [Oil] inter-row distances. The superlattice of the step atoms has always a rhombic
Fig. 1. Top view of the hard sphere model of a (hk0) face associated with a kc (100) face (here (810)). (A) Centred and symmetrical unit meshes of the (100) face. (B) Centred and symmetrical unit meshes of the step atom superlattice. The g1 vector is the stacking fault vector characteristic of the steps. (C) Step site (here the (110) face unit mesh).
202
.I. C. Boullmd,
M. P. Sotto / Stable stepped Cu faces and 0 supemtructures
0
[loo]
jbiil “\colol
01, Cu
J[oo11 +bWJ
Fig. 2. Top view of the hard sphere model of a (hkk) face associated with a fee (100) face (here (ll,l,l)). (A) and(B): as fig. 1. (C) Step site (here the (111) face unit mesh).
primitive mesh. The stacking fault vector of the step is identical to the (MO) one. Concerning the vicinal and complex faces associated with Cu(ll0) we have only studied the (MO) faces between (110) and (530) (i.e. faces inclined along the [OOl] zone axis) (fig. 3). Each terrace width is equal to a half integer number of [OOl] inter-row distances. The superlattice of the step atoms has always a rhombic primitive mesh. The fault vector of the step is equal to g,
=
f%l
( *a[,,]
+
mqlio]
-
Ma,,,,,).
3. Survey of the vicinal and complex copper faces associated (lOtI), stable upon oxygen adsorption (fig. 4)
with (110) and
3.1. Vicinal and complex faces associated with Cu(lO0) It is well known that the (410) face is stable at all oxygen coverages [l-4]. In addition to this face, the stability, at saturation, of the (12,1,0) [7] and the
0 [Ilo]
1[oofl
Fig. 3. Top view of the hard sphere model of a (hk0) face associated with a fee (110) face (here (320)). (A) Symmetrical unit mesh of the (110) face. (B) Symmetrical unit mesh of the step atom superlattice. (C) Step site (here the (100) face unit mesh).
J.C. Boulliard, M.P. Sotto / Stable stepped CUfacesand 0 superstructureson CU
203
1320) 1lOOl
Fig. 4. Stereographic
03101
triangle
(410)
(210)
(530)
(750)
with the (MO) and (hkk) facets (refs. [l-lo]).
detected
(1lOJ
upon oxygen
adsorption
(810) face [9] has been evidenced. Moreover, in a peculiar case (i.e. faceting of (711) into (410), (401), (100) and (511) facets), the (511) facet occurs, it is worth noting (see next section) that the faceting phenomena occur for exposures implying the setting up of the (2fi X &)R45” oxygen superstructure on the Cu(100) face (i.e. a relative coverage greater than - 0.65). 3.2. Vicinal and complex faces deduced from Cu(lI0) The only one face previously known to be stable was (530) [2,4,8]. Our previous work [lo] has led up to more complex results, for exposures at 300°C. Neglecting intermediate and complex structures, the results are summarized as follows: - Stability of the (320) face and faceting of all the other faces (up to (530)) into (320) facets. The required exposures correspond to the first half of the stability range (in oxygen coverage) of the p(2 X 1) superstructure on Cu(llO), i.e. a relative coverage range equal to 0.2 ,< 0 ,< 0.45. - Stability of the (750) face for exposures corresponding to the third last part of the stability of p(2 X 1) onto Cu(ll0) and first stages of the c(6 x 2) settlement (0.6 ,( B < 0.75). - Stability of the (530) face and faceting of all the other faces into (530) and (110) facets, connected with the appearance of the c(6 X 2) superstructure on Cu(ll0) (0.75 2 e < 1). In addition to these results the coalescence of the steps into double height steps has been established as early as exposures corresponding to the last third
204
J.C. Roullrurd, M.P. Sotto / Stuhle stepped Cu
part of the stability I9 ,, 0.57).
domain
of the p(2
x 1)
facesand
0 superstructures
superstructure
on Cu
on Cu(ll0)
(from
4. Discussion 4.1. Relation between the structure of the vicinal and complex stable face and the oxygen superstructure on the low index faces An accurate study of the structural characteristics of the vicinal and complex faces stable upon oxygen adsorption gives evidence of some structural relations with the superstructures established on the low index faces. These relations were already suspected by Perdereau and Rhead [3] and Milne [8] concerning the (410) and (530) faces but no generalization could be made on the basis of those few experimental results. The later results [7,9,10] give rise to the following structural relation:
Fig. 5. Relation between the (2v’? X fi)R45” structure and the structure of the (410), (810) and (12.1,O) faces. (a) Vectors of the step atom symmetrical unit mesh. (b) ProJection of. respectively. one, two and three times the (26 X fi)R45” mesh. (c) Asymmetrical step atom unit mesh showing the relation with the (26 x fi)R45O structure.
J.C. Bouliiard, M.P. Sotto / Siahle stepped Cu faces and 0 superstructures
on Cu
205
13201
Fig. 6. Relation between the p(2 x 1) structure and the structure of the (320) face. (a) Vectors of the step atom symmetrical unit mesh. (b) Projection of the p(2 >: 1) mesh. (c) Asymmetrical step atom unit mesh showing the relation with the p(2 x 1) structure.
- The vectors of the unit mesh of the superlattice describing the step atom positions are equal to one or several times a vector of the mesh describing the superstructure on the low index face, added to the stacking fault vector of the steps. This relation is particularly well represented by the (12,1,0), (810) and (410) faces. In these cases the vectors of the symmetrical unit mesh of the superlattice of the step atoms are, respectively, equal to three, two and one time the bigger vector of the rectangular (26 X fi)R45’ mesh added to the step stacking fault vector g, = +a,( +a[,,] - apool) (fig. 5). - Similarly, the vectors of the symmetrical unit mesh of the step atoms superlattice of the (320) face are equal to the greater vector of the rectangular p(2 X: 1) mesh added to the fault vector g, = +~,(+a[,,, + tfiaL,iol - ~J~Lz[,,,~) (fig. 6). With regard to the (511) face, the step of which is not parallel to a direction of the (2fi x fi)R45’ mesh, we can consider an asymmetrical unit mesh of the step atom superlattice (fig. 7). The bigger vector of this mesh is equal to the bigger vector of the (2fi X fi)R45” mesh added tog,.
Fig. 7. Relation between the (26 x fi)R45O structure and the structure of the (511) face. (a) Vectors of the step atom symmetrical unit mesh. (b) Projection of the (26 x &)R45O mesh. (c) Asymmetrical step atom unit mesh showing the relation with the (2& X fi)R45’ structure.
206
J.C. Boullturd, M.P. Sotto / Stcrhlestepped Cu Jutes md 0 super.wuctures on Cu
In the case of the c(6 X 2) oxygen superstructure on Cu(ll0) we remark that the presence of steps breaks the c(6 X 2) symmetry, i.e. the centre disappears. In this case, it is better considering the p(3 X 2) or the p(6 x 2) superstructure rather than the c(6 x 2) one. In fact the experimental LEED patterns show that the two-fold periodicity along the [OOl] axis disappears, so we will consider the p(3 X 1) and the p(6 X 1) superstructures. Moreover taking into account the double step height and the correlated double terrace width, there are some uncertainties as to the exact form of the reconstructed (530) and (750) faces. These uncertainties are due to the possible configurations of the step, which could be described either as a (100) or a (1x0) microfacet. These are evidenced by the notation of Van Hove et al. [ll]: (530),,,,,,tructed = 4(110) + l(110) or 3(110) + 2(100) and (750),,,,,,tm,_ted = 6(110) + l(li0) or S(110) + 2(100). The LEED patterns do not permit discrimination of these two step structures. Nevertheless, supposing the validity of the structural relation, a satisfactory explanation can be found: the rectangular mesh of the reconstructed (530) face is equal to the rectangular p(3 X 1) mesh added to the stacking fault vector g, = 2g, = uOi fi( a tiiol - a Iin,l) so that (530) reconstructed= 3(110) + 2(100) (fig. 8) and the rectangular mesh of the reconstructed (750) face is equal to the rectangular p(6 x 1) mesh (or three times the p(2 x 1) one) added to the stacking fault vector g, = - a,%fia(,,,) so that (750),,,,,,t,,,cted = with c(6 X 2) (rather 6010) + I(li0) (fig. 9). If (750)reconstmcted is connected than 3p(2 X l)), such configurations would explain why a face, with a terrace width twice greater than the (530)reconstructed one and a step vector equal to g, (i.e. 6(110) + 2(100)), is not detected, contrary to the (810)-(410) case. The step differences could be related to an oxygen dependent reconstruction. In summary, the structural relation is well established for most of stable stepped faces and its extension to the reconstructed (750) and (530) faces gives rise to the step structure. Moreover, this relation is also verified in our previous works, e.g. the (hk0) vicinal and complex faces associated with Ni(lOO). In this case the oxygen superstructure on Ni(lOO) is c(2 X 2) and the stable facets detected are (210) and (410) [12]. Another case is given by the
I5301
Fig. 8. Relation between the p(3 x 1) structure and the structure of the “(530)” face (with a (100) type double step height). (a) Step atom symmetric unit mesh. (b) Projection of the p(3 x 1) structure. (c) Relation between the step atom mesh and the p(3 x 1) structure.
J.C. Boulliard, M.P. Sotto / Stahle stepped Cu fares and 0 superstructures on CM
207
17501
Fig. 9. Relation between the p(6 X 1) structure and the structure of the “(750)” face (with a (110) type double step height). (a) Projection of three p(2 X 1) mesh (similar to the p(6 X 1) mesh). (b) Step atom symmetric unit mesh showing the relation with the p(6 x 1) superstructure.
faceting into (510) and (100) of the vicinal faces associated with Ag(lOO) under sulphur adsorption. The appearance of the (510) face is linked up to the setting up of the sulphur (T i) superstructure on (100) [13]. Concerning the exposure conditions required for the setting up of the stable face or facets, it appears that some of them are easy to obtain while others require special conditions (i.e. low coverage range, special initial face or high temperature during exposure). These observations may be related to the surface energy of the stable faces and permit one to establish the degree of stability of the different structural configurations. So, considering (12,1,0), (810) and (410) it appears that the stepped face, corresponding to one mesh of the superstructure of low index face is more stable than the faces linked up to several meshes. Another even more important remark concerns the structure of the step; the most stable steps are parallel to the most dense direction of the mesh of the superstructure onto the low index face. This fact is well illustrated by the fact that the (511) facet appears on a peculiar initial face [9] and that the studied (hkk) faces mostly facet into (410) and (100) facets. 4.2. Models of the oxygen arrangement
on the stable complex face
In the preceding section we have seen that the structures of the most stable complex faces (i.e. (410), (320) and (530)) are strictly similar to the oxygen superstructures established on the low index faces (i.e. (100) and (110)). The only crystallographic modifications are exclusively due to the addition (to the low index face supermesh) of a vector characterizing the stacking fault induced by a step. Such structural similarities must implicate that there are (at least in first order) similarities between the atom localization on the low index faces and the terraces of the stable stepped faces. 4.2.1. (410) face The oxygen superstructure. [14-171 consists
and (2fi x fi)R45O superstructure on Cu(lO0) covered (410) face is related to the (2fi x fi)R45”-(100) The model of which based on the hypothesis of a four-fold site in alternating three [OlO] or [OOl] rows of occupied four-fold
208
J.C. Bouhrd,
M.P. Sotto / Stuble stepped CMfucesand0 superstructures
Fig. 10. Model of the (2fi (2fi
OH Cu
X &)R45O structure. (a) Symmetric unit mesh of the (100) plane. X V!?)R45’= mesh. The oxygen atoms are dotted.
(b)
sites and one row of empty sites (fig. IO). Another model proposed by Onuferko and Woodruff [18] and Godfrey and Woodruff [19], is based on the existence of one-third of two-fold sites (in addition to four-fold sites). But we will prefer the first model because the two-fold site symmetry may induce phase-antiphase boundaries which were not detected during our work. The saturation coverage on (410) was measured by nuclear microanalysis in our laboratory [20]. It is equal to - 0.75 (i.e. 3 oxygen atoms for 4 copper atoms); this value is the same as the one measured on Cu(100). With the hypothesis of four-fold sites (and neglecting the relaxations) the model of the oxygen covered Cu(410) face is rather obvious and consists in the occupation of all the four-fold sites of the terraces (fig. 11). 4.2.2. p(2 X I) superstructure on Cu(ll0) and Cu(320) faces Concerning the adsorption site of the p(2 x 1) superstructure on Cu(llO), most of the previous works [21-231 have concluded with a two-fold site, i.e. a long bridge site between two successive (110) rows. Feidenhans’l and Steengard [24] measured by RBS the oxygen coverage, and found out that it is equal to 0.5. If we consider that the (320) face appears during the first stages of the establishment of p(2 x 1) on (110) we may conclude that the coverage on (320) is lower than 0.5. Following this hypothesis the model of the oxygen arrangement is rather simple and consists in one [OOl] row of occupied
Fig. 11. Model of the oxygen
structure on the Cu(410) face (the relaxations oxygen atoms are dotted.
are neglected).
The
J. C. Boulliard, M. P. Soito / Stable stepped Cu faces and 0 superstructures on Cu
Fig. 12. Model of the oxygen structure
two-fold coverage
sites on each terrace is equal to 0.4.
on the Cu(320)
(fig. 12). In such a model
209
face
the maximum
of the
4.2.3. p(2 x I) superstructure and change of the step structure The reconstruction of the Cu(ll0) surface by the oxygen p(2 X 1) structure is currently subject to controversy and two models have been proposed to explain the resulting surface geometry. Some studies [22,25,26] argue that the reconstruction results of alternating missing rows of Cu atoms in the [OOl] directions (the so called “missing row” model), while other experiments [23,27,28] favor a slight outward displacement of alternating [OOl] rows (the “buckled row” model). We can note that these two models have not the same consequences for faces with steps. The “missing row” model is based on the presence of steps which permit the expanding of the initial terraces in order to give rise to the lower coverage of Cu atoms on the surface. With faces initially presenting a high rate of periodic steps (i.e. vicinal or complex faces) such a behaviour leads to a doubling of the terrace width and correlatively to a double step height (fig. 13). Such a doubling is well established, in our experiments, on all stepped faces, especially at exposures corresponding to high coverages of p(2 X 1) on Cu(ll0) [lo]. On the contrary, the “buckled-row”
Fig. 13. (A) Clean (530) face. (B) The p(2 X 1) missing row model (on the (530) face) showing the apparent doubling of the step height. The dotted circles represent the supposed copper atoms displaced from the step sites (here, the copper atoms only are drawn).
210
J.C. Boulliurd,
M.P.
Sotto / Stuhle stepped CM facesand 0 superstructures
on Cu
Fig. 14. The p(2 x 1) buckled-row model on a face with constant terrace width (here (320)). The supposed highly relaxed copper row is represented by the dotted circles. Since the superlattice is the same as on the clean face, there is no doubling effect of the step height (contrary to the “ missing row” model).
model singularizes one of two consecutive steps, only on particular faces, i.e., faces composed of a SO%-50% mixing of different terraces as (530), for example. For faces with constant terrace width the doubling effect is not, a priori, expected (fig. 14). So, the experimental detection of the step height doubling argues in favour of the “missing row” model. 4.2.4. (210) Face As mentioned above, the behaviour of this face under oxygen adsorption is currently not well established. Trepte [2], using drastic exposure conditions has detected its decomposition into wide (410) and (530) facets. McKee et al. [5], using slight exposure conditions, concluded its stability with the setting up of the p(2 X 1) and further the p(3 X 1) superstructures (i.e. superstructures respectively established on two and three successive terraces). From our arguments described above we can suggest an explanation conciliating these two results. If we consider that the superstructures are due to step coalescence (this explanation being compatible with the LEED patterns), the p(2 x 1) superstructure is attributed to a doubling of the step height (effectively detected in our studied faces) and the p(3 X 1) superstructure is attributed to a tripling of the step height. In this last case, we remark that the (210) face is altered by a periodical microfacetting described by the alternation of (410) and (530) terraces (fig. 15). In this hypothesis the superstructures detected on
Fig. 15. (a) p(3 X 1) asymmetrical unit mesh on (210) or step atom asymmetrical unit mesh of a (210) face after tripling of the step height. (b) (410) terrace. (c) “(530)” terrace (with a (100) type double step height).
J.C. Boulliard, M.P. Sotto / Stable stepped Cu faces and 0 superstructures
Cu(210) are interpreted explanation is consistent al. [5].
on Cu
211
as being the first stages of faceting of this face. This with the lower exposure conditions used by McKee et
5. Conclusions ‘The oxygen adsorption on vicinal and complex copper faces leads to three-dimensional faceting into well-defined facets. We show that the structures of the facets are, undoubtedly, strongly correlated with the oxygen superstructures established on the low index faces from which the stepped faces are deduced. These correlations may permit one on the one hand to foresee which facets might appear, and on the other hand, to get some information about the models of the superstructures on the low index face. These correlations seem to be more general, as suggested by our other works on nickel and silver [12,13] and might be of some help in other experimental fields like crystals or thin film growth processes in low contaminated atmosphere.
References [l] [2] (31 [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
W. Bemdt, Z. Naturforsch. 22a (1967) 1655. L. Trepte, Z. Naturforsch. 23a (1968) 1241. J. Perdereau and G.E. Rhead, Surface Sci. 24 (1971) 555. E. Legrand-Bonnyns and A. Ponslet, Surface Sci. 53 (1975) 675. C.S. McKee, L.V. Renny and M.W. Roberts, Surface Sci. 75 (1978) 92. R.C. Baetzold, Surface Sci. 95 (1980) 286. J.M. Moison and J.L. Domange, Surface Sci. 97 (1980) 1. R.H. Milne, Surface Sci. 121 (1982) 347. J.C. Boulliard, J.L. Domange and M.P. Sotto, Surface Sci. 165 (1986) 434. J.C. Boulliard and M.P. Sotto, Surface Sci. 177 (1986) 139. M.A. Van Hove and G.A. Somojai, Surface Sci. 92 (1980) 489. J.C. Boulliard and M.P. Sotto, Surface Sci. 152/153 (1985) 392. M.P. Sotto and J.C. Boulliard, Surface Sci. 162 (1985) 285. J.E. Demuth, D.W. Jepsen and P.M. Marcus, Phys. Rev. Letters 31 (1973) 540. J.R. Noonan, D.M. Zehner and L.J. Jenkins, Surface Sci. 69 (1977) 731. S. Kono, SM. Goldberg, N.F.T. Hall and C.S. Fadley, Phys. Rev. Letters 41 (1978) 1831 S.P. Holland, B.J. Garrison and M. Winograd, Phys. Rev. Letters 43 (1979) 220. J.P. Onuferko and D.P. Woodruff, Surface Sci. 95 (1980) 555. D.J. Godfrey and D.P. Woodruff, Surface Sci. 105 (1981) 459. C. Cohen, M.P. Sotto, J.C. Boulliard, D. Schmaus and A. L.‘Hoir, to be published. J.F. Wendelken, Surface Sci. 108 (1981) 605. R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 112 (1981) 133. SM. Thurgate and P.J. Jennings, Surface Sci. 131 (1983) 309. R. Feidenhans’l and I. Stengaard, Surface Sci. 133 (1983) 453.
212 [25] [26] [27] [28]
J.C. Boulliard. M. P. Sotto / Stable stepped Cu faces und 0 superstructures
on Cu
.T. Lapujoulade, Y. Le Cru$, M. Lefort, Y. Lejay and E. Maurel, Surface Sci. 118 (1982) 103. H. Niehus and G. Comsa, Surface Sci. 140 (1984) 18. R.A. Didio, D.M. Zehner and E.W. Plummer, J. Vacuum Sci. Technol. A2 (1984) 852. H.L. Davis and J.R. Noonan, 41st Physical Electronics Conference. Bozeman, Montana, 1981.
Surface
Science 182 (1987) 200-212 North-Holland. Amsterdam
THE STRUCTURAL RELATIONS BETWEEN THE STABLE STEPPED COPPER FACES UPON OXYGEN ADSORPTION AND THE OXYGEN SUPERSTRUCTURES ON Cu(100) AND Cu(ll0) J.C. BOULLIARD
and M.P. SOTTO
Group de Ph_vsiquedes Solides de I’Ecole Normale SqGrieure, UniuersztBParis VII, Tour 33. 2, Pluce Juuieu, 75251 Paris Cede.%05, France Received
17 July 1986: accepted
for publication
21 October
1986
This paper is concerned with the general structural relations we have deduced after LEED studies of oxygen adsorption on the (hk0) and (hkk) stepped faces associated with Cu(100) and the (hk0) faces associated with Cu(ll0). Strong structural relations between the oxygen superstructures on the low index faces and the structures of the stable complex Cu faces are evidenced. Such relations imply some remarks about the model of the oxygen settlement on the low index and stable stepped faces.
1. Introduction As early as in the first studies on the stepped faces of copper [l-8], it has been shown that three-dimensional faceting upon oxygen adsorption occurs. Although the experimental conditions of exposure vary greatly from one author to another, there is some kind of general agreement on the stable stepped faces or facets. These results are summarized as follows: ~ all the high index faces associated with (100) facet into (410) and (100) [l-41; - all the high index faces deduced from (110) facet into (530) and (110) [2,4,8]; _ only a few stepped faces deduced from (111) have been previously studied [2,6,8]. These works have concluded with the stability of (111) (311) and (322). Nevertheless there are some points of disagreement about this simple schema: the first one concerns the (210) face which is, according to Trepte [2] and Legrand-Bonnyns and Ponslet 141, decomposed into wide (410) and (530) facets and stable with the setting up of the p(2 x 1) and p(3 x 1) superstructures according to McKee et al. [5]. Another point is the stability, at saturation coverage. of the (12,1,0) face, evidenced by Moison and Domange [7], on a conical sample presenting all the vicinal faces inclined at 4” from the Cu(100) pole. These authors concluded faceting of all the faces into {12,1,0} facets at saturation coverage occurs. Lastly our works on the vicinal and complex faces 0039-6028/87/$03.50 ‘0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J.C. Boulliurd, M.P. Sotto / Stable stepped Cu facesand 0 superstructures on CM
201
associated with Cu(100) and Cu(ll0) [9,10] show that the structural changes induced by oxygen are more complex with increasing oxygen coverage. In this paper we intend to show that structural relations link the structures of the stable stepped faces to the oxygen superstructures settled on the low index faces. Moreover, the structure of the complex face may present some information about the models of the superstructures on the low index face.
2. Crystallography Cu(ll0)
of the studied stepped faces associated
with Cu(100) and
Concerning the faces associated with Cu(100) we have studied two extreme kinds of faces characterized by the step structure. _ The faces inclined along the [OOl] zone axis (indexed (hk0)) (fig. 1). The adjacent step atoms lie on a kinked line. Each terrace width is equal to an integer number of [OOl] inter-row distances. The superlattice describing the positions of the step atoms, has a rhombic primitive mesh for the faces with h = 2n (n an integer number) and k = 1 and a rectangular primitive for the faces with h = 2n + 1 and k = 1. The stacking fault introduced by a step in a pure two-dimensional (100) lattice, is characterized by the vector joining a step atom with its nearest neighbour on the terrace below. Neglecting any relaxation, the value of this vector is equal tog, = $a,( $-aLWil - a,roo,), (aIhk,] being the unit vector in the [hkl] direction and a, the Cu fee mesh parameter equal to 3.61 A at room temperature). _ The faces inclined along the [Oil] zone (indexed (hkk)) (fig. 2). Their steps are straight. Each terrace width is equal to a half-integer number of [Oil] inter-row distances. The superlattice of the step atoms has always a rhombic
Fig. 1. Top view of the hard sphere model of a (hk0) face associated with a kc (100) face (here (810)). (A) Centred and symmetrical unit meshes of the (100) face. (B) Centred and symmetrical unit meshes of the step atom superlattice. The g1 vector is the stacking fault vector characteristic of the steps. (C) Step site (here the (110) face unit mesh).
202
.I. C. Boullmd,
M. P. Sotto / Stable stepped Cu faces and 0 supemtructures
0
[loo]
jbiil “\colol
01, Cu
J[oo11 +bWJ
Fig. 2. Top view of the hard sphere model of a (hkk) face associated with a fee (100) face (here (ll,l,l)). (A) and(B): as fig. 1. (C) Step site (here the (111) face unit mesh).
primitive mesh. The stacking fault vector of the step is identical to the (MO) one. Concerning the vicinal and complex faces associated with Cu(ll0) we have only studied the (MO) faces between (110) and (530) (i.e. faces inclined along the [OOl] zone axis) (fig. 3). Each terrace width is equal to a half integer number of [OOl] inter-row distances. The superlattice of the step atoms has always a rhombic primitive mesh. The fault vector of the step is equal to g,
=
f%l
( *a[,,]
+
mqlio]
-
Ma,,,,,).
3. Survey of the vicinal and complex copper faces associated (lOtI), stable upon oxygen adsorption (fig. 4)
with (110) and
3.1. Vicinal and complex faces associated with Cu(lO0) It is well known that the (410) face is stable at all oxygen coverages [l-4]. In addition to this face, the stability, at saturation, of the (12,1,0) [7] and the
0 [Ilo]
1[oofl
Fig. 3. Top view of the hard sphere model of a (hk0) face associated with a fee (110) face (here (320)). (A) Symmetrical unit mesh of the (110) face. (B) Symmetrical unit mesh of the step atom superlattice. (C) Step site (here the (100) face unit mesh).
J.C. Boulliard, M.P. Sotto / Stable stepped CUfacesand 0 superstructureson CU
203
1320) 1lOOl
Fig. 4. Stereographic
03101
triangle
(410)
(210)
(530)
(750)
with the (MO) and (hkk) facets (refs. [l-lo]).
detected
(1lOJ
upon oxygen
adsorption
(810) face [9] has been evidenced. Moreover, in a peculiar case (i.e. faceting of (711) into (410), (401), (100) and (511) facets), the (511) facet occurs, it is worth noting (see next section) that the faceting phenomena occur for exposures implying the setting up of the (2fi X &)R45” oxygen superstructure on the Cu(100) face (i.e. a relative coverage greater than - 0.65). 3.2. Vicinal and complex faces deduced from Cu(lI0) The only one face previously known to be stable was (530) [2,4,8]. Our previous work [lo] has led up to more complex results, for exposures at 300°C. Neglecting intermediate and complex structures, the results are summarized as follows: - Stability of the (320) face and faceting of all the other faces (up to (530)) into (320) facets. The required exposures correspond to the first half of the stability range (in oxygen coverage) of the p(2 X 1) superstructure on Cu(llO), i.e. a relative coverage range equal to 0.2 ,< 0 ,< 0.45. - Stability of the (750) face for exposures corresponding to the third last part of the stability of p(2 X 1) onto Cu(ll0) and first stages of the c(6 x 2) settlement (0.6 ,( B < 0.75). - Stability of the (530) face and faceting of all the other faces into (530) and (110) facets, connected with the appearance of the c(6 X 2) superstructure on Cu(ll0) (0.75 2 e < 1). In addition to these results the coalescence of the steps into double height steps has been established as early as exposures corresponding to the last third
204
J.C. Roullrurd, M.P. Sotto / Stuhle stepped Cu
part of the stability I9 ,, 0.57).
domain
of the p(2
x 1)
facesand
0 superstructures
superstructure
on Cu
on Cu(ll0)
(from
4. Discussion 4.1. Relation between the structure of the vicinal and complex stable face and the oxygen superstructure on the low index faces An accurate study of the structural characteristics of the vicinal and complex faces stable upon oxygen adsorption gives evidence of some structural relations with the superstructures established on the low index faces. These relations were already suspected by Perdereau and Rhead [3] and Milne [8] concerning the (410) and (530) faces but no generalization could be made on the basis of those few experimental results. The later results [7,9,10] give rise to the following structural relation:
Fig. 5. Relation between the (2v’? X fi)R45” structure and the structure of the (410), (810) and (12.1,O) faces. (a) Vectors of the step atom symmetrical unit mesh. (b) ProJection of. respectively. one, two and three times the (26 X fi)R45” mesh. (c) Asymmetrical step atom unit mesh showing the relation with the (26 x fi)R45O structure.
J.C. Bouliiard, M.P. Sotto / Siahle stepped Cu faces and 0 superstructures
on Cu
205
13201
Fig. 6. Relation between the p(2 x 1) structure and the structure of the (320) face. (a) Vectors of the step atom symmetrical unit mesh. (b) Projection of the p(2 >: 1) mesh. (c) Asymmetrical step atom unit mesh showing the relation with the p(2 x 1) structure.
- The vectors of the unit mesh of the superlattice describing the step atom positions are equal to one or several times a vector of the mesh describing the superstructure on the low index face, added to the stacking fault vector of the steps. This relation is particularly well represented by the (12,1,0), (810) and (410) faces. In these cases the vectors of the symmetrical unit mesh of the superlattice of the step atoms are, respectively, equal to three, two and one time the bigger vector of the rectangular (26 X fi)R45’ mesh added to the step stacking fault vector g, = +a,( +a[,,] - apool) (fig. 5). - Similarly, the vectors of the symmetrical unit mesh of the step atoms superlattice of the (320) face are equal to the greater vector of the rectangular p(2 X: 1) mesh added to the fault vector g, = +~,(+a[,,, + tfiaL,iol - ~J~Lz[,,,~) (fig. 6). With regard to the (511) face, the step of which is not parallel to a direction of the (2fi x fi)R45’ mesh, we can consider an asymmetrical unit mesh of the step atom superlattice (fig. 7). The bigger vector of this mesh is equal to the bigger vector of the (2fi X fi)R45” mesh added tog,.
Fig. 7. Relation between the (26 x fi)R45O structure and the structure of the (511) face. (a) Vectors of the step atom symmetrical unit mesh. (b) Projection of the (26 x &)R45O mesh. (c) Asymmetrical step atom unit mesh showing the relation with the (2& X fi)R45’ structure.
206
J.C. Boullturd, M.P. Sotto / Stcrhlestepped Cu Jutes md 0 super.wuctures on Cu
In the case of the c(6 X 2) oxygen superstructure on Cu(ll0) we remark that the presence of steps breaks the c(6 X 2) symmetry, i.e. the centre disappears. In this case, it is better considering the p(3 X 2) or the p(6 x 2) superstructure rather than the c(6 x 2) one. In fact the experimental LEED patterns show that the two-fold periodicity along the [OOl] axis disappears, so we will consider the p(3 X 1) and the p(6 X 1) superstructures. Moreover taking into account the double step height and the correlated double terrace width, there are some uncertainties as to the exact form of the reconstructed (530) and (750) faces. These uncertainties are due to the possible configurations of the step, which could be described either as a (100) or a (1x0) microfacet. These are evidenced by the notation of Van Hove et al. [ll]: (530),,,,,,tructed = 4(110) + l(110) or 3(110) + 2(100) and (750),,,,,,tm,_ted = 6(110) + l(li0) or S(110) + 2(100). The LEED patterns do not permit discrimination of these two step structures. Nevertheless, supposing the validity of the structural relation, a satisfactory explanation can be found: the rectangular mesh of the reconstructed (530) face is equal to the rectangular p(3 X 1) mesh added to the stacking fault vector g, = 2g, = uOi fi( a tiiol - a Iin,l) so that (530) reconstructed= 3(110) + 2(100) (fig. 8) and the rectangular mesh of the reconstructed (750) face is equal to the rectangular p(6 x 1) mesh (or three times the p(2 x 1) one) added to the stacking fault vector g, = - a,%fia(,,,) so that (750),,,,,,t,,,cted = with c(6 X 2) (rather 6010) + I(li0) (fig. 9). If (750)reconstmcted is connected than 3p(2 X l)), such configurations would explain why a face, with a terrace width twice greater than the (530)reconstructed one and a step vector equal to g, (i.e. 6(110) + 2(100)), is not detected, contrary to the (810)-(410) case. The step differences could be related to an oxygen dependent reconstruction. In summary, the structural relation is well established for most of stable stepped faces and its extension to the reconstructed (750) and (530) faces gives rise to the step structure. Moreover, this relation is also verified in our previous works, e.g. the (hk0) vicinal and complex faces associated with Ni(lOO). In this case the oxygen superstructure on Ni(lOO) is c(2 X 2) and the stable facets detected are (210) and (410) [12]. Another case is given by the
I5301
Fig. 8. Relation between the p(3 x 1) structure and the structure of the “(530)” face (with a (100) type double step height). (a) Step atom symmetric unit mesh. (b) Projection of the p(3 x 1) structure. (c) Relation between the step atom mesh and the p(3 x 1) structure.
J.C. Boulliard, M.P. Sotto / Stahle stepped Cu fares and 0 superstructures on CM
207
17501
Fig. 9. Relation between the p(6 X 1) structure and the structure of the “(750)” face (with a (110) type double step height). (a) Projection of three p(2 X 1) mesh (similar to the p(6 X 1) mesh). (b) Step atom symmetric unit mesh showing the relation with the p(6 x 1) superstructure.
faceting into (510) and (100) of the vicinal faces associated with Ag(lOO) under sulphur adsorption. The appearance of the (510) face is linked up to the setting up of the sulphur (T i) superstructure on (100) [13]. Concerning the exposure conditions required for the setting up of the stable face or facets, it appears that some of them are easy to obtain while others require special conditions (i.e. low coverage range, special initial face or high temperature during exposure). These observations may be related to the surface energy of the stable faces and permit one to establish the degree of stability of the different structural configurations. So, considering (12,1,0), (810) and (410) it appears that the stepped face, corresponding to one mesh of the superstructure of low index face is more stable than the faces linked up to several meshes. Another even more important remark concerns the structure of the step; the most stable steps are parallel to the most dense direction of the mesh of the superstructure onto the low index face. This fact is well illustrated by the fact that the (511) facet appears on a peculiar initial face [9] and that the studied (hkk) faces mostly facet into (410) and (100) facets. 4.2. Models of the oxygen arrangement
on the stable complex face
In the preceding section we have seen that the structures of the most stable complex faces (i.e. (410), (320) and (530)) are strictly similar to the oxygen superstructures established on the low index faces (i.e. (100) and (110)). The only crystallographic modifications are exclusively due to the addition (to the low index face supermesh) of a vector characterizing the stacking fault induced by a step. Such structural similarities must implicate that there are (at least in first order) similarities between the atom localization on the low index faces and the terraces of the stable stepped faces. 4.2.1. (410) face The oxygen superstructure. [14-171 consists
and (2fi x fi)R45O superstructure on Cu(lO0) covered (410) face is related to the (2fi x fi)R45”-(100) The model of which based on the hypothesis of a four-fold site in alternating three [OlO] or [OOl] rows of occupied four-fold
208
J.C. Bouhrd,
M.P. Sotto / Stuble stepped CMfucesand0 superstructures
Fig. 10. Model of the (2fi (2fi
OH Cu
X &)R45O structure. (a) Symmetric unit mesh of the (100) plane. X V!?)R45’= mesh. The oxygen atoms are dotted.
(b)
sites and one row of empty sites (fig. IO). Another model proposed by Onuferko and Woodruff [18] and Godfrey and Woodruff [19], is based on the existence of one-third of two-fold sites (in addition to four-fold sites). But we will prefer the first model because the two-fold site symmetry may induce phase-antiphase boundaries which were not detected during our work. The saturation coverage on (410) was measured by nuclear microanalysis in our laboratory [20]. It is equal to - 0.75 (i.e. 3 oxygen atoms for 4 copper atoms); this value is the same as the one measured on Cu(100). With the hypothesis of four-fold sites (and neglecting the relaxations) the model of the oxygen covered Cu(410) face is rather obvious and consists in the occupation of all the four-fold sites of the terraces (fig. 11). 4.2.2. p(2 X I) superstructure on Cu(ll0) and Cu(320) faces Concerning the adsorption site of the p(2 x 1) superstructure on Cu(llO), most of the previous works [21-231 have concluded with a two-fold site, i.e. a long bridge site between two successive (110) rows. Feidenhans’l and Steengard [24] measured by RBS the oxygen coverage, and found out that it is equal to 0.5. If we consider that the (320) face appears during the first stages of the establishment of p(2 x 1) on (110) we may conclude that the coverage on (320) is lower than 0.5. Following this hypothesis the model of the oxygen arrangement is rather simple and consists in one [OOl] row of occupied
Fig. 11. Model of the oxygen
structure on the Cu(410) face (the relaxations oxygen atoms are dotted.
are neglected).
The
J. C. Boulliard, M. P. Soito / Stable stepped Cu faces and 0 superstructures on Cu
Fig. 12. Model of the oxygen structure
two-fold coverage
sites on each terrace is equal to 0.4.
on the Cu(320)
(fig. 12). In such a model
209
face
the maximum
of the
4.2.3. p(2 x I) superstructure and change of the step structure The reconstruction of the Cu(ll0) surface by the oxygen p(2 X 1) structure is currently subject to controversy and two models have been proposed to explain the resulting surface geometry. Some studies [22,25,26] argue that the reconstruction results of alternating missing rows of Cu atoms in the [OOl] directions (the so called “missing row” model), while other experiments [23,27,28] favor a slight outward displacement of alternating [OOl] rows (the “buckled row” model). We can note that these two models have not the same consequences for faces with steps. The “missing row” model is based on the presence of steps which permit the expanding of the initial terraces in order to give rise to the lower coverage of Cu atoms on the surface. With faces initially presenting a high rate of periodic steps (i.e. vicinal or complex faces) such a behaviour leads to a doubling of the terrace width and correlatively to a double step height (fig. 13). Such a doubling is well established, in our experiments, on all stepped faces, especially at exposures corresponding to high coverages of p(2 X 1) on Cu(ll0) [lo]. On the contrary, the “buckled-row”
Fig. 13. (A) Clean (530) face. (B) The p(2 X 1) missing row model (on the (530) face) showing the apparent doubling of the step height. The dotted circles represent the supposed copper atoms displaced from the step sites (here, the copper atoms only are drawn).
210
J.C. Boulliurd,
M.P.
Sotto / Stuhle stepped CM facesand 0 superstructures
on Cu
Fig. 14. The p(2 x 1) buckled-row model on a face with constant terrace width (here (320)). The supposed highly relaxed copper row is represented by the dotted circles. Since the superlattice is the same as on the clean face, there is no doubling effect of the step height (contrary to the “ missing row” model).
model singularizes one of two consecutive steps, only on particular faces, i.e., faces composed of a SO%-50% mixing of different terraces as (530), for example. For faces with constant terrace width the doubling effect is not, a priori, expected (fig. 14). So, the experimental detection of the step height doubling argues in favour of the “missing row” model. 4.2.4. (210) Face As mentioned above, the behaviour of this face under oxygen adsorption is currently not well established. Trepte [2], using drastic exposure conditions has detected its decomposition into wide (410) and (530) facets. McKee et al. [5], using slight exposure conditions, concluded its stability with the setting up of the p(2 X 1) and further the p(3 X 1) superstructures (i.e. superstructures respectively established on two and three successive terraces). From our arguments described above we can suggest an explanation conciliating these two results. If we consider that the superstructures are due to step coalescence (this explanation being compatible with the LEED patterns), the p(2 x 1) superstructure is attributed to a doubling of the step height (effectively detected in our studied faces) and the p(3 X 1) superstructure is attributed to a tripling of the step height. In this last case, we remark that the (210) face is altered by a periodical microfacetting described by the alternation of (410) and (530) terraces (fig. 15). In this hypothesis the superstructures detected on
Fig. 15. (a) p(3 X 1) asymmetrical unit mesh on (210) or step atom asymmetrical unit mesh of a (210) face after tripling of the step height. (b) (410) terrace. (c) “(530)” terrace (with a (100) type double step height).
J.C. Boulliard, M.P. Sotto / Stable stepped Cu faces and 0 superstructures
Cu(210) are interpreted explanation is consistent al. [5].
on Cu
211
as being the first stages of faceting of this face. This with the lower exposure conditions used by McKee et
5. Conclusions ‘The oxygen adsorption on vicinal and complex copper faces leads to three-dimensional faceting into well-defined facets. We show that the structures of the facets are, undoubtedly, strongly correlated with the oxygen superstructures established on the low index faces from which the stepped faces are deduced. These correlations may permit one on the one hand to foresee which facets might appear, and on the other hand, to get some information about the models of the superstructures on the low index face. These correlations seem to be more general, as suggested by our other works on nickel and silver [12,13] and might be of some help in other experimental fields like crystals or thin film growth processes in low contaminated atmosphere.
References [l] [2] (31 [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
W. Bemdt, Z. Naturforsch. 22a (1967) 1655. L. Trepte, Z. Naturforsch. 23a (1968) 1241. J. Perdereau and G.E. Rhead, Surface Sci. 24 (1971) 555. E. Legrand-Bonnyns and A. Ponslet, Surface Sci. 53 (1975) 675. C.S. McKee, L.V. Renny and M.W. Roberts, Surface Sci. 75 (1978) 92. R.C. Baetzold, Surface Sci. 95 (1980) 286. J.M. Moison and J.L. Domange, Surface Sci. 97 (1980) 1. R.H. Milne, Surface Sci. 121 (1982) 347. J.C. Boulliard, J.L. Domange and M.P. Sotto, Surface Sci. 165 (1986) 434. J.C. Boulliard and M.P. Sotto, Surface Sci. 177 (1986) 139. M.A. Van Hove and G.A. Somojai, Surface Sci. 92 (1980) 489. J.C. Boulliard and M.P. Sotto, Surface Sci. 152/153 (1985) 392. M.P. Sotto and J.C. Boulliard, Surface Sci. 162 (1985) 285. J.E. Demuth, D.W. Jepsen and P.M. Marcus, Phys. Rev. Letters 31 (1973) 540. J.R. Noonan, D.M. Zehner and L.J. Jenkins, Surface Sci. 69 (1977) 731. S. Kono, SM. Goldberg, N.F.T. Hall and C.S. Fadley, Phys. Rev. Letters 41 (1978) 1831 S.P. Holland, B.J. Garrison and M. Winograd, Phys. Rev. Letters 43 (1979) 220. J.P. Onuferko and D.P. Woodruff, Surface Sci. 95 (1980) 555. D.J. Godfrey and D.P. Woodruff, Surface Sci. 105 (1981) 459. C. Cohen, M.P. Sotto, J.C. Boulliard, D. Schmaus and A. L.‘Hoir, to be published. J.F. Wendelken, Surface Sci. 108 (1981) 605. R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 112 (1981) 133. SM. Thurgate and P.J. Jennings, Surface Sci. 131 (1983) 309. R. Feidenhans’l and I. Stengaard, Surface Sci. 133 (1983) 453.
212 [25] [26] [27] [28]
J.C. Boulliard. M. P. Sotto / Stable stepped Cu faces und 0 superstructures
on Cu
.T. Lapujoulade, Y. Le Cru$, M. Lefort, Y. Lejay and E. Maurel, Surface Sci. 118 (1982) 103. H. Niehus and G. Comsa, Surface Sci. 140 (1984) 18. R.A. Didio, D.M. Zehner and E.W. Plummer, J. Vacuum Sci. Technol. A2 (1984) 852. H.L. Davis and J.R. Noonan, 41st Physical Electronics Conference. Bozeman, Montana, 1981.