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Oxygen-induced step-edge faceting; a precursor to (410) planar faceting of Cu(100) vicinal surfaces
"
ELSEVIER
13 September 1996
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 259 (1996) 503-507
Oxygen-induced step-edge faceting; a precursor to (410) planar faceting of Cu(100) vicinal surfaces P.J. Knight, S.M. Driver, D.P. Woodruff Physics Department, University of Warwick, Coventry CV4 7AL, UK Received 20 May 1996; in final form 27 June 1996
Abstract
Scanning tunneling microscopy has been applied to an atomistic scale investigation of the oxygen-induced restructuring of a Cu(100) vicinal surface, close to (610) but with average step orientations l0 ° away from [001]. The initial changes upon oxygen adsorption are found to be step-edge faceting to [001] and [010], and [001] step coalescence to form small (410) facets. Annealing enlarges these (410) facets while the much shorter [010] steps coalesce to form (401) facets. The implied strong energetic favouring of (100) step-edges in the presence of oxygen is attributed to the formation of Cu-O-Cu-O linear chains, known to be the stable element of oxygen-induced reconstruction of the (110) and (100) copper surfaces.
Although the basic phenomenon of faceting of planar surfaces into 'hill-and-valley' structures is well-known and understood to be a consequence of the anisotropy of the surface free energy, most studies until recently have involved either optical microscopy or diffraction methods [1]. It is only with the advances of atomic-scale scanning probe microscopies that such phenomena can be investigated at an atomistic scale. Here we present the results of a scanning tunneling microscopy (STM) investigation of the early stages of oxygen-induced faceting of a Cu(100) vicinal surface. Our results provide a clear linkage between the known microscopic tendency of such surfaces to facet to {410} in the presence of adsorbed oxygen [2-6], and the known stability at an atomic scale of Cu-O-Cu-O chains with a periodicity equal to that in the (100) directions of metallic
copper which occur in several oxygen chemisorption structures, e.g., [7-19], and indicate that the formation of such preferred structures at the step edges is a precursor to the formation of the three-dimensional facet planes. Our experiments were conducted on a sample with a surface orientation some 9.5 ° from (100), nominally in the [001] zone to produce a (610) surface, although in fact the cut was some 10° out of this azimuthal direction, leading to a surface on which the average step direction between the (100) terraces which comprise the surface is approximately 10° away from [001] towards [011]. This slight twist from the nominal (610) face proved of especial value in understanding the general restructuring phenomena induced by oxygen adsorption. Note that an ideal Cu(610) surface is composed of 10.8 ,~ wide (100)
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 0 7 5 3 - 1
504
P,I. Knight et a L / Chemical Physics Letters 259 (1996) 503-507
terraces separated by mono-atomic steps along [001] whereas a (410) surface has a similar structure with 7.2 ,~ wide terraces. The sample was cleaned in situ by the usual combination of argon ion bombardment and annealing cycles to produce a surface with a LEED pattern corresponding to that expected from this clean surface, and which Auger electron spectroscopy indicated to be clean. STM measurements were conducted with the sample nominally at room temperature in constant current mode using an Omicron Micro-STM mounted on a magnetically damped spring suspension system. In order to monitor the earliest stages of any structural rearrangement induced by oxygen chemisorption, dynamic imaging studies were conducted while exposing the surface to pressures in the range 10 -9 to 10 -8 mbar of oxygen. Fig. 1 shows results from such a study comparing images before and after approximately 2 L exposure. A major limitation with this experiment was our inability to obtain clear atomic resolution images from the clean surface; we believe the reason for this to be largely fundamental rather than instrumental. Previous studies of C u ( n l l ) surfaces ( n = 7 to 19) by STM, which are also vicinal to Cu(100), show step edges which are characteristically 'frizzy', over distances of several atomic spacings, due to the rapid migra-
Fig. 1. STM images from areas of approximately 45 ,~ × 70/1, of the nominal Cu(610) surface before (a) and after (b) 2 L oxygen exposure. Note the anticlockwise rotation of the average step edges by ~ 10° to [001] and the resolution of some atomic detail along the edges following the oxygen exposure. These images were recorded with - 4 6 0 mV sample bias and 1.1 nA tunnel current.
tion of step-edge kinks [20]. Notice, however, that while ( n l l ) surfaces also have (100) terraces, the step edges of these orientations correspond to the close-packed [01 l] direction which would normally be regarded as 'kinkless' at low temperature. For these surfaces there is evidence that, at least for smaller values of n (for which the order should be most stable), the roughening temperature [21,22] at which kinks are formed spontaneously in large numbers probably lies somewhat above room temperature. (nl0) surfaces, on the other hand, have nominal [001] step edges which are maximally kinked, so the roughening transition is expected at much lower temperatures; even for Cu(310) there is evidence that the roughening temperature lies below room temperature [23]. We therefore believe that Fig. l(a), which shows an STM image of the clean surface of our sample, lacks any clearly resolved features for this reason. What is visible, however, is a general streaking of light and dark regions aligned along the average step direction of the sample. Notice, incidentally, that the STM grey-scale is levelled to the average face, so each (100) terrace should appear dark at its down-step end and bright at its up-step end. Oxygen adsorption (e.g. Fig. l(b)) leads to the emergence of more clearly defined step edges which show some atomic resolution along the step-edges, although much of the image still appears very noisy. A key observation at this stage, however, is a change in the alignment of the step edges to be [001]. Of course, the average step direction must remain constant, and this is actually achieved by the formation of sections of step edges at right angles to the dominant [001] step edges in the [010] direction. This effect can only be faintly discerned from Fig. l(b). Comparison of this figure with the image of Fig. 2, recorded in a separate experiment after saturating the surface at room temperature with 1000 L oxygen prior to imaging, shows this much more clearly. The image shows a surface comprising 'laths' or rectangular plates inclined at a slight angle to the surface whose long sides are terminated in [001] steps and bright ends are terminated in [010] steps. Within the laths one can see additional [001] step-edges whose separation is predominantly that of the same step edges within the stable (410) facets known to form on this surface when well-annealed following oxygen exposure. On subsequent annealing of such a
P.I. Knight et al./ Chemical Physics Letters 259 (1996) 503-507
lOlOl i
....
10011
Fig. 2. STM image of an area of the nominal Cu(610) surface of approximately 160 /~×230 /~ following exposure to 1000 L oxygen at room temperature. Image recorded at - 670 mV sample bias and 1.5 nA tunnel current.
surface to temperatures of 250-280°C, the (410) facets grow considerably (by a factor of 5 0 - 1 0 0 ) in size producing regular arrays of [001] step edges at the appropriate spacing. In addition, however, the [010] step edges also coalesce to produce smaller (401) facets. Fig. 3 shows a high magnification image of a region of intersection of these two {410} facet planes and a schematic model of such a comer showing the [001]/[010] step-edge comer. The general observation of step-edge faceting on chemisorption is well-known. The 'frizzy' step edges seen on many metal surfaces at room temperature are characteristic of the clean surface, and in the particular case of the C u ( l l l l) surface, for example, chemisorbed sulphur has been shown to cause stepedge faceting [24]. The (2 x 2)-S phase seen on this surface is found to stabilise the 'natural' [011] step edge direction, but a c(4 X 2)-S phase has also been observed which appears to stabilise (310) step-edges; these latter orientations (at 26 ° to [011]) correspond to one edge of the primitive unit mesh of the c(4 x 2) phase and can be rationalised as involving one sulphur atom at each kink on the step edge. In the
505
present case, however, we note that the clean surface (100) steps are maximally kinked (and thus potentially least stable), and that while step-edge faceting of our surface to [001] involves a step rotation of only 10° relative to the average value for the surface, the [010] step-edges are 80 ° away, so the increase in step-length incurred by these step-edge facets is considerable. Clearly the (100) step edges must be strongly favoured energetically in the presence of chemisorbed oxygen. Our STM data, of course, do not permit us to determine the atomic geometry of the step edges. We cannot tell, for example, whether the atomic features we see are due to Cu atoms, O atoms, or indeed some other intermediate location; previous STM studies of oxygen adsorption on copper surfaces have generally been interpreted in terms of imaging of Cu atoms alone (although this need not always be the case [25]). On the other hand, we do know from a~
Fig. 3. Schematic atomic model (a) and STM image (45 ~, × 70 A) recorded at a sample bias of - 40 mV and a tunnel current of 6.5 nA of the junction of a (410) and a (401) facet from the nominal Cu(610) surface after exposure to 1000 L oxygen and annealing to approximately 265°C. The schematic model shows ideal unreconstructed Cu{410}surfaces without any oxygen atoms, but with the step edge atoms darkened for clarity.
506
P.I. Knight et al./ Chemical Physics Letters 259 (1996) 503-507
quantitative structural studies of oxygen adsorption on Cu(ll0) and (100) surfaces that the preferred oxygen adsorption sites are bridging Cu atoms along (100) directions to produce Cu-O-Cu-O linear chains. In the case of the Cu(110) surface, the (2 × 1)-O structure is formed by the production of such surface chains separated from one another by a distance equal to twice that of the (110) Cu-Cu nearest neighbour direction in the underlying substrate. In the case of the Cu(100) surface, the most stable oxygen-induced structure is the (2V~× V~-)R45°-O phase which involves every fourth (100) Cu atom row being expelled from the surface, with the adsorbed oxygen atoms occupying sites along the edges of these missing rows approximately co-linear with the edge (100) Cu atom chains. These missing row edges are, of course, essentially identical to the step edges of the (nl0) surfaces, so one can surmise that these step edge kink sites are the oxygen adsorption sites on our surface. There is one specific experimental study using X-ray photoelectron diffraction [26] which concludes that these are, indeed, the occupied sites; this early study predates much of the more detailed understanding of copper-oxygen chemisorption structures, however, and should probably be regarded as indicative rather than definitive. Nevertheless, it is clear that there is sufficient evidence from these other surfaces to suggest that the oxygen-covered (100) step edges of our surface do indeed have this approximately co-linear Cu-O-Cu-O geometry. It is also clear that this structural interpretation provides a qualitative understanding of the step-edge faceting seen in our studies; despite the increase in step-edge length incurred by the faceting, it is less drastic a reconstruction than the missing or added row reconstructions of the Cu(100) and (110) surfaces involving the same structural element. Of course, this understanding of the early stages of surface modification leading to oxygen-induced (410) faceting does not account for the special stability of the (410) face relative to other (nl0) surfaces. As yet, however, the detailed structure of the (100) terraces on the oxygen-covered (410) surface is still in doubt [27], and it is clear that the special energetic stability of this particular face must relate to the step periodicity and thus may well be related to the terrace structure.
The authors are pleased to acknowledge the SERC, and latterly the EPSRC (Engineering and Physical Science Research Council) for a research grant for instrumentation and the support of PJK as a research assistant, and for a research studentship for SMD. They also acknowledge the valuable assistance of Peter Varga, Michael Schmidt and their colleagues at the TU Wien for their valuable contribution to the effective commissioning of our STM instrument.
References [1] J.M. Blakely, in: Introduction to the properties of crystal surfaces (Pergamon, Oxford, 1973); H. Mykura, in: Solid surfaces and interfaces (Routledge and Kegan Paul, London, 1966) p. 25. [2] W. Bemdt, Z. Naturforsch. 22a (1967) 1655. [3] L. Trepte, C. Menzel-Kopp and E. Menzel, Surf. Sci. 8 (1967) 223. [4] J. Perdereau and G.E. Rhead, Surf. Sci. 24 (1971) 555. [5] J.C. Boulliard, J.L. Domange and M. Sotto, Surf. Sci. 165 (1986) 434. [6] M. Sotto, Surf. Sci. 260 (1992) 235. [7] D.P. Woodruff, J. Phys.: Condens. Matter 6 (1994) 6067 and references therein. [8] M. Bader, A. Puschmann, C. Ocal and J. Haase, Phys. Rev. Leu. 57 (1986) 3273. [9] R. Feidenhans'l, F. Grey, R.L. Johnson, S.G.J. Mochrie, J. Bohr and M. Nielsen, Phys. Rev. B 41 (1990) 5420. [10] S.R. Parkin, H.C. Zeng, M.Y. Zhou and K.A.R. Mitchell, Phys. Rev. B 41 (1990) 5432. [11] DJ. Coulman, J. Wintterlin, R.J. Behm and G. Ertl, Phys. Rev. Lett. 64 (1990) 1761. [12] F. Jensen, F. Besenbacher, E. l.~gsgaard and I. Stensgaard, Phys. Rev. B 41 (1990) 10233. [13] H.C. Zeng, R.A. McFarlane and K.A.R. Mitchell, Surf. Sci. 208 (1989) L7. [14] Ch. W~il, R.J. Wilson, S. Chiang, H.C. Zeng and K.A.R. Mitchell, Phys. Rev. B 42 (1990) 11926. [15] I.K. Robinson, E. Vlieg and S. FetTer, Phys. Rev. B 42 (1990) 6954. [16] F. Jensen, F. Besenbacher, E. L~egsgaard and I. Stensgaard, Phys. Rev. B 42 (1990) 9206. [17] H.C. Zeng and K.A.R. Mitchell, Surf. Sci. 239 (1990) L571. [18] A. Atrei, U. Bardi, G. Rovida, E. Zanazzi and G. Casalone, Vacuum 41 (1990) 333. [19] M.C. Asensio, M.J. Ashwin, A.L.D. Kilcoyne, D.P. Woodruff, A.W. Robinson, Th. Lindner, J.S. Somers, D.E. Ricken and A.M. Bradshaw, Surf. Sci. 236 (1991) 1. [20] J. Frohn, M. Giesen, M. Poensgen, J.F. Wolf and H. Ibach, Phys. Rev. Lett. 67 (1991) 3543. [21] J. Lapujoulade, Surf. Sci. Rep. 20 (1994) 191. [22] K. Kern, in: The chemical physics of solid surfaces, eds.
P.I. Knight et a l . / Chemical Physics Letters 259 (1996) 503-507
D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1994) pp. 7, 328. [23] cited in ref. [22] as B. Loisel, Thesis, Paris (1989). [24] S. Rousset, S. Ganthier, O. Siboulet, W. Sacks, M. Belin and J. Klein, Phys. Rev. Lett. 63 (1989) 1265.
507
[25] L. Ruan, F. Besenbacher, I. Stensgaard and E. Leegsgaard, Phys. Rev. Lett. 70 (1993) 40/9. [26] K.A. Thompson and C.S. Fadley, Surf. Sci. 146 (1984) 281. [27] G.W. Lloyd and D.P. Woodruff, Surf. Sci. 285 (1993) L503.
ELSEVIER
13 September 1996
CHEMICAL PHYSICS LETTERS Chemical Physics Letters 259 (1996) 503-507
Oxygen-induced step-edge faceting; a precursor to (410) planar faceting of Cu(100) vicinal surfaces P.J. Knight, S.M. Driver, D.P. Woodruff Physics Department, University of Warwick, Coventry CV4 7AL, UK Received 20 May 1996; in final form 27 June 1996
Abstract
Scanning tunneling microscopy has been applied to an atomistic scale investigation of the oxygen-induced restructuring of a Cu(100) vicinal surface, close to (610) but with average step orientations l0 ° away from [001]. The initial changes upon oxygen adsorption are found to be step-edge faceting to [001] and [010], and [001] step coalescence to form small (410) facets. Annealing enlarges these (410) facets while the much shorter [010] steps coalesce to form (401) facets. The implied strong energetic favouring of (100) step-edges in the presence of oxygen is attributed to the formation of Cu-O-Cu-O linear chains, known to be the stable element of oxygen-induced reconstruction of the (110) and (100) copper surfaces.
Although the basic phenomenon of faceting of planar surfaces into 'hill-and-valley' structures is well-known and understood to be a consequence of the anisotropy of the surface free energy, most studies until recently have involved either optical microscopy or diffraction methods [1]. It is only with the advances of atomic-scale scanning probe microscopies that such phenomena can be investigated at an atomistic scale. Here we present the results of a scanning tunneling microscopy (STM) investigation of the early stages of oxygen-induced faceting of a Cu(100) vicinal surface. Our results provide a clear linkage between the known microscopic tendency of such surfaces to facet to {410} in the presence of adsorbed oxygen [2-6], and the known stability at an atomic scale of Cu-O-Cu-O chains with a periodicity equal to that in the (100) directions of metallic
copper which occur in several oxygen chemisorption structures, e.g., [7-19], and indicate that the formation of such preferred structures at the step edges is a precursor to the formation of the three-dimensional facet planes. Our experiments were conducted on a sample with a surface orientation some 9.5 ° from (100), nominally in the [001] zone to produce a (610) surface, although in fact the cut was some 10° out of this azimuthal direction, leading to a surface on which the average step direction between the (100) terraces which comprise the surface is approximately 10° away from [001] towards [011]. This slight twist from the nominal (610) face proved of especial value in understanding the general restructuring phenomena induced by oxygen adsorption. Note that an ideal Cu(610) surface is composed of 10.8 ,~ wide (100)
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 0 7 5 3 - 1
504
P,I. Knight et a L / Chemical Physics Letters 259 (1996) 503-507
terraces separated by mono-atomic steps along [001] whereas a (410) surface has a similar structure with 7.2 ,~ wide terraces. The sample was cleaned in situ by the usual combination of argon ion bombardment and annealing cycles to produce a surface with a LEED pattern corresponding to that expected from this clean surface, and which Auger electron spectroscopy indicated to be clean. STM measurements were conducted with the sample nominally at room temperature in constant current mode using an Omicron Micro-STM mounted on a magnetically damped spring suspension system. In order to monitor the earliest stages of any structural rearrangement induced by oxygen chemisorption, dynamic imaging studies were conducted while exposing the surface to pressures in the range 10 -9 to 10 -8 mbar of oxygen. Fig. 1 shows results from such a study comparing images before and after approximately 2 L exposure. A major limitation with this experiment was our inability to obtain clear atomic resolution images from the clean surface; we believe the reason for this to be largely fundamental rather than instrumental. Previous studies of C u ( n l l ) surfaces ( n = 7 to 19) by STM, which are also vicinal to Cu(100), show step edges which are characteristically 'frizzy', over distances of several atomic spacings, due to the rapid migra-
Fig. 1. STM images from areas of approximately 45 ,~ × 70/1, of the nominal Cu(610) surface before (a) and after (b) 2 L oxygen exposure. Note the anticlockwise rotation of the average step edges by ~ 10° to [001] and the resolution of some atomic detail along the edges following the oxygen exposure. These images were recorded with - 4 6 0 mV sample bias and 1.1 nA tunnel current.
tion of step-edge kinks [20]. Notice, however, that while ( n l l ) surfaces also have (100) terraces, the step edges of these orientations correspond to the close-packed [01 l] direction which would normally be regarded as 'kinkless' at low temperature. For these surfaces there is evidence that, at least for smaller values of n (for which the order should be most stable), the roughening temperature [21,22] at which kinks are formed spontaneously in large numbers probably lies somewhat above room temperature. (nl0) surfaces, on the other hand, have nominal [001] step edges which are maximally kinked, so the roughening transition is expected at much lower temperatures; even for Cu(310) there is evidence that the roughening temperature lies below room temperature [23]. We therefore believe that Fig. l(a), which shows an STM image of the clean surface of our sample, lacks any clearly resolved features for this reason. What is visible, however, is a general streaking of light and dark regions aligned along the average step direction of the sample. Notice, incidentally, that the STM grey-scale is levelled to the average face, so each (100) terrace should appear dark at its down-step end and bright at its up-step end. Oxygen adsorption (e.g. Fig. l(b)) leads to the emergence of more clearly defined step edges which show some atomic resolution along the step-edges, although much of the image still appears very noisy. A key observation at this stage, however, is a change in the alignment of the step edges to be [001]. Of course, the average step direction must remain constant, and this is actually achieved by the formation of sections of step edges at right angles to the dominant [001] step edges in the [010] direction. This effect can only be faintly discerned from Fig. l(b). Comparison of this figure with the image of Fig. 2, recorded in a separate experiment after saturating the surface at room temperature with 1000 L oxygen prior to imaging, shows this much more clearly. The image shows a surface comprising 'laths' or rectangular plates inclined at a slight angle to the surface whose long sides are terminated in [001] steps and bright ends are terminated in [010] steps. Within the laths one can see additional [001] step-edges whose separation is predominantly that of the same step edges within the stable (410) facets known to form on this surface when well-annealed following oxygen exposure. On subsequent annealing of such a
P.I. Knight et al./ Chemical Physics Letters 259 (1996) 503-507
lOlOl i
....
10011
Fig. 2. STM image of an area of the nominal Cu(610) surface of approximately 160 /~×230 /~ following exposure to 1000 L oxygen at room temperature. Image recorded at - 670 mV sample bias and 1.5 nA tunnel current.
surface to temperatures of 250-280°C, the (410) facets grow considerably (by a factor of 5 0 - 1 0 0 ) in size producing regular arrays of [001] step edges at the appropriate spacing. In addition, however, the [010] step edges also coalesce to produce smaller (401) facets. Fig. 3 shows a high magnification image of a region of intersection of these two {410} facet planes and a schematic model of such a comer showing the [001]/[010] step-edge comer. The general observation of step-edge faceting on chemisorption is well-known. The 'frizzy' step edges seen on many metal surfaces at room temperature are characteristic of the clean surface, and in the particular case of the C u ( l l l l) surface, for example, chemisorbed sulphur has been shown to cause stepedge faceting [24]. The (2 x 2)-S phase seen on this surface is found to stabilise the 'natural' [011] step edge direction, but a c(4 X 2)-S phase has also been observed which appears to stabilise (310) step-edges; these latter orientations (at 26 ° to [011]) correspond to one edge of the primitive unit mesh of the c(4 x 2) phase and can be rationalised as involving one sulphur atom at each kink on the step edge. In the
505
present case, however, we note that the clean surface (100) steps are maximally kinked (and thus potentially least stable), and that while step-edge faceting of our surface to [001] involves a step rotation of only 10° relative to the average value for the surface, the [010] step-edges are 80 ° away, so the increase in step-length incurred by these step-edge facets is considerable. Clearly the (100) step edges must be strongly favoured energetically in the presence of chemisorbed oxygen. Our STM data, of course, do not permit us to determine the atomic geometry of the step edges. We cannot tell, for example, whether the atomic features we see are due to Cu atoms, O atoms, or indeed some other intermediate location; previous STM studies of oxygen adsorption on copper surfaces have generally been interpreted in terms of imaging of Cu atoms alone (although this need not always be the case [25]). On the other hand, we do know from a~
Fig. 3. Schematic atomic model (a) and STM image (45 ~, × 70 A) recorded at a sample bias of - 40 mV and a tunnel current of 6.5 nA of the junction of a (410) and a (401) facet from the nominal Cu(610) surface after exposure to 1000 L oxygen and annealing to approximately 265°C. The schematic model shows ideal unreconstructed Cu{410}surfaces without any oxygen atoms, but with the step edge atoms darkened for clarity.
506
P.I. Knight et al./ Chemical Physics Letters 259 (1996) 503-507
quantitative structural studies of oxygen adsorption on Cu(ll0) and (100) surfaces that the preferred oxygen adsorption sites are bridging Cu atoms along (100) directions to produce Cu-O-Cu-O linear chains. In the case of the Cu(110) surface, the (2 × 1)-O structure is formed by the production of such surface chains separated from one another by a distance equal to twice that of the (110) Cu-Cu nearest neighbour direction in the underlying substrate. In the case of the Cu(100) surface, the most stable oxygen-induced structure is the (2V~× V~-)R45°-O phase which involves every fourth (100) Cu atom row being expelled from the surface, with the adsorbed oxygen atoms occupying sites along the edges of these missing rows approximately co-linear with the edge (100) Cu atom chains. These missing row edges are, of course, essentially identical to the step edges of the (nl0) surfaces, so one can surmise that these step edge kink sites are the oxygen adsorption sites on our surface. There is one specific experimental study using X-ray photoelectron diffraction [26] which concludes that these are, indeed, the occupied sites; this early study predates much of the more detailed understanding of copper-oxygen chemisorption structures, however, and should probably be regarded as indicative rather than definitive. Nevertheless, it is clear that there is sufficient evidence from these other surfaces to suggest that the oxygen-covered (100) step edges of our surface do indeed have this approximately co-linear Cu-O-Cu-O geometry. It is also clear that this structural interpretation provides a qualitative understanding of the step-edge faceting seen in our studies; despite the increase in step-edge length incurred by the faceting, it is less drastic a reconstruction than the missing or added row reconstructions of the Cu(100) and (110) surfaces involving the same structural element. Of course, this understanding of the early stages of surface modification leading to oxygen-induced (410) faceting does not account for the special stability of the (410) face relative to other (nl0) surfaces. As yet, however, the detailed structure of the (100) terraces on the oxygen-covered (410) surface is still in doubt [27], and it is clear that the special energetic stability of this particular face must relate to the step periodicity and thus may well be related to the terrace structure.
The authors are pleased to acknowledge the SERC, and latterly the EPSRC (Engineering and Physical Science Research Council) for a research grant for instrumentation and the support of PJK as a research assistant, and for a research studentship for SMD. They also acknowledge the valuable assistance of Peter Varga, Michael Schmidt and their colleagues at the TU Wien for their valuable contribution to the effective commissioning of our STM instrument.
References [1] J.M. Blakely, in: Introduction to the properties of crystal surfaces (Pergamon, Oxford, 1973); H. Mykura, in: Solid surfaces and interfaces (Routledge and Kegan Paul, London, 1966) p. 25. [2] W. Bemdt, Z. Naturforsch. 22a (1967) 1655. [3] L. Trepte, C. Menzel-Kopp and E. Menzel, Surf. Sci. 8 (1967) 223. [4] J. Perdereau and G.E. Rhead, Surf. Sci. 24 (1971) 555. [5] J.C. Boulliard, J.L. Domange and M. Sotto, Surf. Sci. 165 (1986) 434. [6] M. Sotto, Surf. Sci. 260 (1992) 235. [7] D.P. Woodruff, J. Phys.: Condens. Matter 6 (1994) 6067 and references therein. [8] M. Bader, A. Puschmann, C. Ocal and J. Haase, Phys. Rev. Leu. 57 (1986) 3273. [9] R. Feidenhans'l, F. Grey, R.L. Johnson, S.G.J. Mochrie, J. Bohr and M. Nielsen, Phys. Rev. B 41 (1990) 5420. [10] S.R. Parkin, H.C. Zeng, M.Y. Zhou and K.A.R. Mitchell, Phys. Rev. B 41 (1990) 5432. [11] DJ. Coulman, J. Wintterlin, R.J. Behm and G. Ertl, Phys. Rev. Lett. 64 (1990) 1761. [12] F. Jensen, F. Besenbacher, E. l.~gsgaard and I. Stensgaard, Phys. Rev. B 41 (1990) 10233. [13] H.C. Zeng, R.A. McFarlane and K.A.R. Mitchell, Surf. Sci. 208 (1989) L7. [14] Ch. W~il, R.J. Wilson, S. Chiang, H.C. Zeng and K.A.R. Mitchell, Phys. Rev. B 42 (1990) 11926. [15] I.K. Robinson, E. Vlieg and S. FetTer, Phys. Rev. B 42 (1990) 6954. [16] F. Jensen, F. Besenbacher, E. L~egsgaard and I. Stensgaard, Phys. Rev. B 42 (1990) 9206. [17] H.C. Zeng and K.A.R. Mitchell, Surf. Sci. 239 (1990) L571. [18] A. Atrei, U. Bardi, G. Rovida, E. Zanazzi and G. Casalone, Vacuum 41 (1990) 333. [19] M.C. Asensio, M.J. Ashwin, A.L.D. Kilcoyne, D.P. Woodruff, A.W. Robinson, Th. Lindner, J.S. Somers, D.E. Ricken and A.M. Bradshaw, Surf. Sci. 236 (1991) 1. [20] J. Frohn, M. Giesen, M. Poensgen, J.F. Wolf and H. Ibach, Phys. Rev. Lett. 67 (1991) 3543. [21] J. Lapujoulade, Surf. Sci. Rep. 20 (1994) 191. [22] K. Kern, in: The chemical physics of solid surfaces, eds.
P.I. Knight et a l . / Chemical Physics Letters 259 (1996) 503-507
D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1994) pp. 7, 328. [23] cited in ref. [22] as B. Loisel, Thesis, Paris (1989). [24] S. Rousset, S. Ganthier, O. Siboulet, W. Sacks, M. Belin and J. Klein, Phys. Rev. Lett. 63 (1989) 1265.
507
[25] L. Ruan, F. Besenbacher, I. Stensgaard and E. Leegsgaard, Phys. Rev. Lett. 70 (1993) 40/9. [26] K.A. Thompson and C.S. Fadley, Surf. Sci. 146 (1984) 281. [27] G.W. Lloyd and D.P. Woodruff, Surf. Sci. 285 (1993) L503.