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Boundary structures of the (n×1) added rows of AgO chains on a Ag(110) surface
Volume 192, number 1
CHEMICAL PHYSICS LETTERS
24 April 1992
Boundary structures of the (n x 1) added rows of Ag-0 chains on a Ag( 110) surface Masahiro Tan&hi,
Ken-ichi Tanaka
The Institutefor Solid State Physics, The University of Tokyo, 7-22-l Roppongi, Mnato-ku, Tokyo 106, Japan
Tomihiro Hashizume and Toshio Sakurai Institute for Materials Research, Tohoku University, Sendai, Japan Received 3 December 199 1; in final form 14 February 1992
Linear AgO is grown along the (00 1) direction when a Ag( 110) surface is exposed to O2 at room temperature. After exposure to 760 IIO2 the surface is a mixture of p( 3 x 1)-O and (5 x 1)-O phases; the (5 X 1)domain is composed of an alternate arrangement of phases of (3 x 1) and (2 x 1) periodicity. At the boundary of the p( 3 X 1) and (5 X 1) phases, the two phases can form a smooth junction but the real boundary has a fluctuating structure. When the p( 2 x 1)-O/Ag( I 10) surface was heated to 500 K, the surface was changed to a p(4x 1) structure composed of the four out-of-phase domains. The boundary of the out-of-phase p( 4 x 1) domains takes either nine or ten atomic spacings, and the boundary adopts a fluctuating linear structure. We conclude that energetically degenerate sites inevitably appear when the two phases make contact, and this results in the fluctuation of the domain boundaries.
1. Introduction
Scanning tunneling microscopy (STM), invented by Binnig and Rohrer [ 11, revolutionised our concept of surface phenomena. A large number of STM studies have been performed on semiconductor surfaces. In contrast, only a limited number of investigations have been successful on metal surfaces, such as Au(llO), Au(lOO), Au(ll1) [2-61, Pt(ll0) [7,8], Ni(llO), Ni(lOO) [9-121, and Cu(llO), Cu(lll),Cu(1OO) [ll-22].STMstudiesonmetal surfaces have forced us to change our traditional concept of adsorption. Until now, LEED pattern changes caused by the adsorption of O2 such as p(2xl)/Cu(llO) [14,17,21], ~(3x1) and p(2xl)/Ni(llO) [ll], and p(nXl)/Ag(llO) [ 25 1, have been explained either by a simple model of 0 atom adsorption or by the reconstruction of the substrate surface. However, the real processes taking place on these metal surfaces are entirely different. For example, one-dimensional Cu-0, Ni-0 and Ag0 compounds are produced by the reaction of Cu, Ni and Ag atoms with oxygen on the terrace and form
ordered arrangements of (II X 1) on the ( 110) surfaces. When a Ag( 110) surface is exposed to 02, a series of p( n x 1) LEED patterns, with 2 < n < 7, are observed [ 26,271, but the mechanism is still poorly understood. Silver is especially interesting because of its unique partial oxidation of ethene. It is curious that both the Ag ( 110 ) and Ag ( 111) surface have almost equal activity and selectivity for this reaction and that the Ag( 110) surface, after ethene oxidation, always shows a p(2 x 1) LEED pattern [28,29]. Another distinctive feature is that the sticking probability of 020nAg(110)aswellasonAg(111)issmallerthan 10m3at room temperature and yet silver membranes are used for oxygen purification at high temperature. In order to shed light on these phenomena, we studied the structures of Ag-0 rows on the Ag( 110) surface by STM.
2. Results and discussion A Ag( 110) disk was chemically etched in warm
0009-2614/92/% 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
117
Volume 192, number 1
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(70-80°C) HN03 solution for nearly 1 min, and then it was polished, ultimately with 0.25 urn diamond paste. The sample was sputtered with Ar+ ions (500 V bias potential and 3-4 uA ion current) for 10 min, and then heated in 1 x lo-’ Torr of O2 at 673 K for 10 min. This cleaning process was repeated until the impurities were undetectable by AES, except for a trace amount of oxygen. The sample was then annealed at 750 K in UHV (2x lo-” Tot-r). A Pt tip with ( 111) orientation was used for ob-
taining the STM image of the oxygen-adsorbed Ag( 110) surface. Fig. la shows a STM image of a clean Ag( 110) surface. A large area of the Ag ( 110 ) surface, exposed to O2 for 760 L at room temperature, is shown in fig. lb. It is evident that when the Ag( 110) surface is exposed to O2 at room temperature, a linear AgO compound is grown parallel to the (00 1) direction [ 25 1, which is quite similar to the growth of Cu-0 chains on a Cu( 110) surface [ 14,17,21], although the ordering of Ag-0 chains on Ag( 110) is more
Fig. 1. STM image of clean Ag( 110) surface obtained at 1.2 V of bias voltage. (b) A wide area STM image of Ag( 110) surface exposed to 760 P O2 at room temperature. The upper and lower terraces are shown.
Fig. 2. Expanded STM images. (a) Upper terrace with p( 3 x 1) added rows. (b) Lower terrace with p(3x 1) (i) and (iii), and ( 5 x 1) phases (ii), where the (5 X 1) structure is composed of an alternate arrangement of (2 x 1) and ( 3 x 1) periodicity.
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(phase (i)) and a (5x1) structure (phase (ii)), where the (5 x 1) phase is composed of an alternate arrangement of (2 x 1) and (3 x 1) periodicity. The boundary of phases (i) and (ii) has five atomic spacings and is clearly fluctuating as shown in fig. 3a. The added row model of Ag-0 chains on Ag( 110) surface and its side view are shown in fig. 3b. It is shown that if a Ag-0 chain is grown on site B, phase (i) forms a smooth junction with phase (ii). However, this is not the case and the Ag-0 chain locates on both sites A and B, so that the boundary fluctuates as observed in fig. 3a. It should be pointed out that sites A and B are symmetric with respect to
complex than that of Cu-0 on Cu ( 110 ) . Therefore, it should be emphasized that the dissociation mechanism of O2 on specific sites on the surface, such as assumed in the calculation [ 301, may not be reflected in the actual processes, because the Ag atoms that are diffused on the terrace react with oxygen
[251* The upper and lower terraces in fig. 1b are covered with differently arranged Ag-0 added rows, and each terrace is expanded in figs. 2a and 2b, respectively. It is shown that the p( 3 x 1) structure almost covers the upper terrace. However, the lower terrace is more complex and is composed of the p (3 x 1) structure
(ii)
24 April 1992
ja-5y
(i)
]“55+]
(iii)
(b) Fig. 3. (a) Expanded STM image showing fluctuating boundary. (b) Added row model of p(3X 1) and (5 X 1) phases on Ag( 110). Added Ag-0 row locates on both sites of A and B, where A and B are symmetric with respect to the adjacent Ag-0 rows.
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b)
(4
(i)
I-‘O-4
( ii
)
t4---9-w
( iii 1
Fig. 4. (a) p ( 2 x 1) added Ag-0 rows on Ag ( 110) obtained by exposure to 6000 BO2 at room temperature. (b ) p (4 x 1) surface obtained by heating p( 2 x 1) surface to 500 K. Four out-of-phase p(4x 1) structures, (i), (ii), (iii) and (iv) are divided by fluctuating boundary lines. (c) Model of the four out-of-phase p(4~ 1) domains formed from two p( 2~ 1) phases, whose boundary takes either nine or ten atomic spacings.
the adjacent Ag-0 rows. Therefore, these two sites may be energetically equivalent. If this is correct, we can speculate that the Ag-0 chain on the sites A and/ or B may interact weakly with the Ag-0 rows that form the (n x 1) phase. In other words, the Ag-0 rows on the Ag( 110) surface may have only a weak attractive and/or repulsive force, in contrast to Cu0 chains on Cu( 110) [ 14,17,21] and Ni-0 chain on Ni( 110) [ 111. As a consequence, the boundary of the two phases is not controlled by the ( n x 1) periodicity of the phases but is influenced by the local symmetry of the sites at the boundary. When the p( 2 x 1) surface was heated to 500 K in UHV, the oxygen coverage was halved and the STM 120
image was observed to change from the p (2 x 1) to the p( 4x 1) structure. Taking into account the fact that the carbon impurity on Ag ( 110) undergoes oxidation to CO* at 450 K but that desorption of O2 occurs at about 600 K [ 311, the formation of p(4X 1) might be responsible for the oxidation of the carbon impurity. Fig. 4a shows the p (2 x 1) surface obtained by exposure to 6000 P O2 at room temperature. When this P( 2 x 1) surface was heated (20 K/min) to 500 K in UHV, the p (4 x 1) structure, as shown in fig. 4b, was observed at room temperature. Several fluctuating lines can be recognized in the p (4 x 1) phase in fig. 4b, and it was proved that the p (4 x 1) phases divided by the fluctuating lines are
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mutually out of phase. A ruler superimposed on the image indicates clearly the out-of-phase relation of the two p (4 x 1) phases across the fluctuating line. The expanded STM image of the p (4 X 1) structure shows that the fluctuating line either has nine or ten atomic spacings across the boundary in the ( 110) direction. It should be noted that, by removing every second AgO row, a p (2 x 1) structure will form the two out-of-phase p (4 x 1) structures. In this case, the boundary of the out-of-phase domains has ten atomic spacings, as the model in fig. 4c shows. Taking into account the fact that the two out-of-phase p(2X 1) structures form equally on the Ag( 110) surface, the p( 4x 1) domains originating from the different p (2 x 1) phases will give the boundary with nine atomic spacings, as illustrated in fig. 4c. Therefore, we can conclude that the p (4 x 1) domains having ten atomic boundary spacings originate from the same p (2 x 1) phase but that the p( 4 x 1) domains with nine atomic boundary spacings are produced from the different p (2 x 1) phases. It is shown that symmetric sites necessarily appear in the out-of-phase boundary of nine or ten atomic spacings, as shown in fig. 4c. They are responsible for the fluctuating boundary of the (n x 1) Ag-0 added rows phase because these sites are energetically equivalent. It should be realized that the sticking probability of O2 on Ag ( 110) surface depends on the Ag atoms diffused out from the step edges or defects onto the terrace; therefore it is not surprising that the sticking probability of O2 is as small as 10p3 at room temperature. Acknowledgement
This work was partly supported by the Grant-inAid for Scientific Research on Priority Area (03205026) and a Grant-in-Aid (03640398) of the Ministry of Education, Science and Culture of Japan. References [ 11G. Bin& H. Rohrer, Ch. Gerber and W. Weibel, Phys. Rev. Letters 50 (1983) 120. 121 H.Q. Nsuye,Y. Kuk and P.J. Silverman, J. Phys. (Paris) Colloq. 49 ( 1988) 269. [ 3 I Ch. Wall, S. Chiang, R.J. Wilson and P.H. Lippel, Phys. Rev. B 39 (1989) 7988. [4] M.P. Everson, R.C. Jaklevic and W. Shen, J. Vacuum Sci. Technol. A 8 ( 1990) 3662.
24 April 1992
[ 5 ] J.V. Barth, H. Brune, G. Ertl and R.J. Behm, Phys. Rev. B 42 (1990) 9307. [6] Y. Kuk, M.F. Jarrold, P.J. Silverman, J.E. Bower and W.L. Brown, Phys. Rev. B 39 ( 1989) 11168. [ 71 T. Grit&h, D. Coulman, R.J. Behm and G. Ertl, Appl. Phys. A 49 (1989) 403. [ 81 T. Gritisch, D. Coulman, R.J. Behm and G. Ertl, Phys. Rev. Letters 63 (1989) 386. [ 91 Y. Kuk, P.J. Silverman and T.M. Buck, Materials Research Society Symposium on Initial Stages of Epitaxial Growth (1987) p. 163. [lo] E. Popatzki and R.J. Behm, Surface Sci. 245 ( 1991) 225. [ 111 L. Eierdal, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Proc. STM Conf. ( 199 1) . [ 121 L.P. Nielsen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. B ( 199 I), in press. [ 131 EM. Chua, Y. Kuk and P.J. Silverman, J. Vacuum Sci. Technol. A 8 ( 1990) 305. [ 141 D.J. Coulman, J. Wintterlin, R.J. Behm and G. Ertl, Phys. Rev.Letters64 (1990) 1761. [ 151 F. Jensen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. B 41 (1990) 10233. [ 161 Y. Kuk, F.M. Chua and P.J. Silverman, Phys. Rev. B 41 (1990) 12393. 117 R. Feidenhans’l, F. Grey, M. Nielsen, F. Basenbacher, F. Jensen, E. Laegsgaard, I. Stensgaard and K.W. Jacobsen, Phys. Rev. Letters 65 ( 1990) 2027. [18 D. Coulman, J. Wintterlin, J.V. Barth, G. Ertl and R.J. Behm, Surface Sci. 240 ( 1990) 151. [19 A. Samsaver, E.S. Hirschom, T. Miller, F.M. Leibsle, J.A. Eades and T.C. Chiang, Phys. Rev. Letters 65 ( 1990) 1607. 128 F.M. Chua, Y. Kuk and P.J. Silverman, Phys. Rev. Letters 63 (1989) 386. [ 2 1 ] F. Jensen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. B 42 ( 1990) 9206. [22] C. Wool, R.J. Wilson, S. Chian, H.C. Zeng, K.A.R. Mitchel, Phys. Rev. B 42 ( 1990) 11926. [23 ] R. Schuster, J.V. Barth, G. Ertl and R.J. Behm, Surface Sci. Letters 247 ( 1991) L 229. [ 241 F. Jensen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Surface Sci. Letters 259 ( 1991) L774. [25] K. Tanaka, A. Taniguchi, T. Hashizume, K. Motai, Hua Lu and T. Sakurai, The 5th Mini-Symposium on Surface Science, Sendai ( 199 1) p. 15; T. Hashizume, A. Taniguchi, K. Motai, Hua Lu, K. Tanaka and T. Takurai, Japan. J. Appl. Phys. 30 ( 199 1) L1529. [ 261 H.A. Engelhardt and D. Menzel, Surface Sci. 57 ( 1976) 591. [27] H. Albers, W.J.J. van der Wal, O.L.J. Gijzeman and G.A. Bootsma, Surface Sci. 77 ( 1978 ) 1. [28] C.T. Campbell and M.T. Paffett, Surface Sci. 139 (1984) 396. [29] C.T. Campbell, J. Catal. 94 (1985) 436. [ 301 H. Nakatsuji and H. Nakai, Chem. Phys. Letters 174 ( 1990) 283. [31] C.T. Campbell and M.T. Paffett, Surface Sci. 143 (1984) 517.
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Boundary structures of the (n x 1) added rows of Ag-0 chains on a Ag( 110) surface Masahiro Tan&hi,
Ken-ichi Tanaka
The Institutefor Solid State Physics, The University of Tokyo, 7-22-l Roppongi, Mnato-ku, Tokyo 106, Japan
Tomihiro Hashizume and Toshio Sakurai Institute for Materials Research, Tohoku University, Sendai, Japan Received 3 December 199 1; in final form 14 February 1992
Linear AgO is grown along the (00 1) direction when a Ag( 110) surface is exposed to O2 at room temperature. After exposure to 760 IIO2 the surface is a mixture of p( 3 x 1)-O and (5 x 1)-O phases; the (5 X 1)domain is composed of an alternate arrangement of phases of (3 x 1) and (2 x 1) periodicity. At the boundary of the p( 3 X 1) and (5 X 1) phases, the two phases can form a smooth junction but the real boundary has a fluctuating structure. When the p( 2 x 1)-O/Ag( I 10) surface was heated to 500 K, the surface was changed to a p(4x 1) structure composed of the four out-of-phase domains. The boundary of the out-of-phase p( 4 x 1) domains takes either nine or ten atomic spacings, and the boundary adopts a fluctuating linear structure. We conclude that energetically degenerate sites inevitably appear when the two phases make contact, and this results in the fluctuation of the domain boundaries.
1. Introduction
Scanning tunneling microscopy (STM), invented by Binnig and Rohrer [ 11, revolutionised our concept of surface phenomena. A large number of STM studies have been performed on semiconductor surfaces. In contrast, only a limited number of investigations have been successful on metal surfaces, such as Au(llO), Au(lOO), Au(ll1) [2-61, Pt(ll0) [7,8], Ni(llO), Ni(lOO) [9-121, and Cu(llO), Cu(lll),Cu(1OO) [ll-22].STMstudiesonmetal surfaces have forced us to change our traditional concept of adsorption. Until now, LEED pattern changes caused by the adsorption of O2 such as p(2xl)/Cu(llO) [14,17,21], ~(3x1) and p(2xl)/Ni(llO) [ll], and p(nXl)/Ag(llO) [ 25 1, have been explained either by a simple model of 0 atom adsorption or by the reconstruction of the substrate surface. However, the real processes taking place on these metal surfaces are entirely different. For example, one-dimensional Cu-0, Ni-0 and Ag0 compounds are produced by the reaction of Cu, Ni and Ag atoms with oxygen on the terrace and form
ordered arrangements of (II X 1) on the ( 110) surfaces. When a Ag( 110) surface is exposed to 02, a series of p( n x 1) LEED patterns, with 2 < n < 7, are observed [ 26,271, but the mechanism is still poorly understood. Silver is especially interesting because of its unique partial oxidation of ethene. It is curious that both the Ag ( 110 ) and Ag ( 111) surface have almost equal activity and selectivity for this reaction and that the Ag( 110) surface, after ethene oxidation, always shows a p(2 x 1) LEED pattern [28,29]. Another distinctive feature is that the sticking probability of 020nAg(110)aswellasonAg(111)issmallerthan 10m3at room temperature and yet silver membranes are used for oxygen purification at high temperature. In order to shed light on these phenomena, we studied the structures of Ag-0 rows on the Ag( 110) surface by STM.
2. Results and discussion A Ag( 110) disk was chemically etched in warm
0009-2614/92/% 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
117
Volume 192, number 1
CHEMICAL PHYSICS LETTERS
24 April 1992
(70-80°C) HN03 solution for nearly 1 min, and then it was polished, ultimately with 0.25 urn diamond paste. The sample was sputtered with Ar+ ions (500 V bias potential and 3-4 uA ion current) for 10 min, and then heated in 1 x lo-’ Torr of O2 at 673 K for 10 min. This cleaning process was repeated until the impurities were undetectable by AES, except for a trace amount of oxygen. The sample was then annealed at 750 K in UHV (2x lo-” Tot-r). A Pt tip with ( 111) orientation was used for ob-
taining the STM image of the oxygen-adsorbed Ag( 110) surface. Fig. la shows a STM image of a clean Ag( 110) surface. A large area of the Ag ( 110 ) surface, exposed to O2 for 760 L at room temperature, is shown in fig. lb. It is evident that when the Ag( 110) surface is exposed to O2 at room temperature, a linear AgO compound is grown parallel to the (00 1) direction [ 25 1, which is quite similar to the growth of Cu-0 chains on a Cu( 110) surface [ 14,17,21], although the ordering of Ag-0 chains on Ag( 110) is more
Fig. 1. STM image of clean Ag( 110) surface obtained at 1.2 V of bias voltage. (b) A wide area STM image of Ag( 110) surface exposed to 760 P O2 at room temperature. The upper and lower terraces are shown.
Fig. 2. Expanded STM images. (a) Upper terrace with p( 3 x 1) added rows. (b) Lower terrace with p(3x 1) (i) and (iii), and ( 5 x 1) phases (ii), where the (5 X 1) structure is composed of an alternate arrangement of (2 x 1) and ( 3 x 1) periodicity.
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(phase (i)) and a (5x1) structure (phase (ii)), where the (5 x 1) phase is composed of an alternate arrangement of (2 x 1) and (3 x 1) periodicity. The boundary of phases (i) and (ii) has five atomic spacings and is clearly fluctuating as shown in fig. 3a. The added row model of Ag-0 chains on Ag( 110) surface and its side view are shown in fig. 3b. It is shown that if a Ag-0 chain is grown on site B, phase (i) forms a smooth junction with phase (ii). However, this is not the case and the Ag-0 chain locates on both sites A and B, so that the boundary fluctuates as observed in fig. 3a. It should be pointed out that sites A and B are symmetric with respect to
complex than that of Cu-0 on Cu ( 110 ) . Therefore, it should be emphasized that the dissociation mechanism of O2 on specific sites on the surface, such as assumed in the calculation [ 301, may not be reflected in the actual processes, because the Ag atoms that are diffused on the terrace react with oxygen
[251* The upper and lower terraces in fig. 1b are covered with differently arranged Ag-0 added rows, and each terrace is expanded in figs. 2a and 2b, respectively. It is shown that the p( 3 x 1) structure almost covers the upper terrace. However, the lower terrace is more complex and is composed of the p (3 x 1) structure
(ii)
24 April 1992
ja-5y
(i)
]“55+]
(iii)
(b) Fig. 3. (a) Expanded STM image showing fluctuating boundary. (b) Added row model of p(3X 1) and (5 X 1) phases on Ag( 110). Added Ag-0 row locates on both sites of A and B, where A and B are symmetric with respect to the adjacent Ag-0 rows.
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t.------141A-------_1
-8law
b)
(4
(i)
I-‘O-4
( ii
)
t4---9-w
( iii 1
Fig. 4. (a) p ( 2 x 1) added Ag-0 rows on Ag ( 110) obtained by exposure to 6000 BO2 at room temperature. (b ) p (4 x 1) surface obtained by heating p( 2 x 1) surface to 500 K. Four out-of-phase p(4x 1) structures, (i), (ii), (iii) and (iv) are divided by fluctuating boundary lines. (c) Model of the four out-of-phase p(4~ 1) domains formed from two p( 2~ 1) phases, whose boundary takes either nine or ten atomic spacings.
the adjacent Ag-0 rows. Therefore, these two sites may be energetically equivalent. If this is correct, we can speculate that the Ag-0 chain on the sites A and/ or B may interact weakly with the Ag-0 rows that form the (n x 1) phase. In other words, the Ag-0 rows on the Ag( 110) surface may have only a weak attractive and/or repulsive force, in contrast to Cu0 chains on Cu( 110) [ 14,17,21] and Ni-0 chain on Ni( 110) [ 111. As a consequence, the boundary of the two phases is not controlled by the ( n x 1) periodicity of the phases but is influenced by the local symmetry of the sites at the boundary. When the p( 2 x 1) surface was heated to 500 K in UHV, the oxygen coverage was halved and the STM 120
image was observed to change from the p (2 x 1) to the p( 4x 1) structure. Taking into account the fact that the carbon impurity on Ag ( 110) undergoes oxidation to CO* at 450 K but that desorption of O2 occurs at about 600 K [ 311, the formation of p(4X 1) might be responsible for the oxidation of the carbon impurity. Fig. 4a shows the p (2 x 1) surface obtained by exposure to 6000 P O2 at room temperature. When this P( 2 x 1) surface was heated (20 K/min) to 500 K in UHV, the p (4 x 1) structure, as shown in fig. 4b, was observed at room temperature. Several fluctuating lines can be recognized in the p (4 x 1) phase in fig. 4b, and it was proved that the p (4 x 1) phases divided by the fluctuating lines are
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mutually out of phase. A ruler superimposed on the image indicates clearly the out-of-phase relation of the two p (4 x 1) phases across the fluctuating line. The expanded STM image of the p (4 X 1) structure shows that the fluctuating line either has nine or ten atomic spacings across the boundary in the ( 110) direction. It should be noted that, by removing every second AgO row, a p (2 x 1) structure will form the two out-of-phase p (4 x 1) structures. In this case, the boundary of the out-of-phase domains has ten atomic spacings, as the model in fig. 4c shows. Taking into account the fact that the two out-of-phase p(2X 1) structures form equally on the Ag( 110) surface, the p( 4x 1) domains originating from the different p (2 x 1) phases will give the boundary with nine atomic spacings, as illustrated in fig. 4c. Therefore, we can conclude that the p (4 x 1) domains having ten atomic boundary spacings originate from the same p (2 x 1) phase but that the p( 4 x 1) domains with nine atomic boundary spacings are produced from the different p (2 x 1) phases. It is shown that symmetric sites necessarily appear in the out-of-phase boundary of nine or ten atomic spacings, as shown in fig. 4c. They are responsible for the fluctuating boundary of the (n x 1) Ag-0 added rows phase because these sites are energetically equivalent. It should be realized that the sticking probability of O2 on Ag ( 110) surface depends on the Ag atoms diffused out from the step edges or defects onto the terrace; therefore it is not surprising that the sticking probability of O2 is as small as 10p3 at room temperature. Acknowledgement
This work was partly supported by the Grant-inAid for Scientific Research on Priority Area (03205026) and a Grant-in-Aid (03640398) of the Ministry of Education, Science and Culture of Japan. References [ 11G. Bin& H. Rohrer, Ch. Gerber and W. Weibel, Phys. Rev. Letters 50 (1983) 120. 121 H.Q. Nsuye,Y. Kuk and P.J. Silverman, J. Phys. (Paris) Colloq. 49 ( 1988) 269. [ 3 I Ch. Wall, S. Chiang, R.J. Wilson and P.H. Lippel, Phys. Rev. B 39 (1989) 7988. [4] M.P. Everson, R.C. Jaklevic and W. Shen, J. Vacuum Sci. Technol. A 8 ( 1990) 3662.
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[ 5 ] J.V. Barth, H. Brune, G. Ertl and R.J. Behm, Phys. Rev. B 42 (1990) 9307. [6] Y. Kuk, M.F. Jarrold, P.J. Silverman, J.E. Bower and W.L. Brown, Phys. Rev. B 39 ( 1989) 11168. [ 71 T. Grit&h, D. Coulman, R.J. Behm and G. Ertl, Appl. Phys. A 49 (1989) 403. [ 81 T. Gritisch, D. Coulman, R.J. Behm and G. Ertl, Phys. Rev. Letters 63 (1989) 386. [ 91 Y. Kuk, P.J. Silverman and T.M. Buck, Materials Research Society Symposium on Initial Stages of Epitaxial Growth (1987) p. 163. [lo] E. Popatzki and R.J. Behm, Surface Sci. 245 ( 1991) 225. [ 111 L. Eierdal, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Proc. STM Conf. ( 199 1) . [ 121 L.P. Nielsen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. B ( 199 I), in press. [ 131 EM. Chua, Y. Kuk and P.J. Silverman, J. Vacuum Sci. Technol. A 8 ( 1990) 305. [ 141 D.J. Coulman, J. Wintterlin, R.J. Behm and G. Ertl, Phys. Rev.Letters64 (1990) 1761. [ 151 F. Jensen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. B 41 (1990) 10233. [ 161 Y. Kuk, F.M. Chua and P.J. Silverman, Phys. Rev. B 41 (1990) 12393. 117 R. Feidenhans’l, F. Grey, M. Nielsen, F. Basenbacher, F. Jensen, E. Laegsgaard, I. Stensgaard and K.W. Jacobsen, Phys. Rev. Letters 65 ( 1990) 2027. [18 D. Coulman, J. Wintterlin, J.V. Barth, G. Ertl and R.J. Behm, Surface Sci. 240 ( 1990) 151. [19 A. Samsaver, E.S. Hirschom, T. Miller, F.M. Leibsle, J.A. Eades and T.C. Chiang, Phys. Rev. Letters 65 ( 1990) 1607. 128 F.M. Chua, Y. Kuk and P.J. Silverman, Phys. Rev. Letters 63 (1989) 386. [ 2 1 ] F. Jensen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Phys. Rev. B 42 ( 1990) 9206. [22] C. Wool, R.J. Wilson, S. Chian, H.C. Zeng, K.A.R. Mitchel, Phys. Rev. B 42 ( 1990) 11926. [23 ] R. Schuster, J.V. Barth, G. Ertl and R.J. Behm, Surface Sci. Letters 247 ( 1991) L 229. [ 241 F. Jensen, F. Besenbacher, E. Laegsgaard and I. Stensgaard, Surface Sci. Letters 259 ( 1991) L774. [25] K. Tanaka, A. Taniguchi, T. Hashizume, K. Motai, Hua Lu and T. Sakurai, The 5th Mini-Symposium on Surface Science, Sendai ( 199 1) p. 15; T. Hashizume, A. Taniguchi, K. Motai, Hua Lu, K. Tanaka and T. Takurai, Japan. J. Appl. Phys. 30 ( 199 1) L1529. [ 261 H.A. Engelhardt and D. Menzel, Surface Sci. 57 ( 1976) 591. [27] H. Albers, W.J.J. van der Wal, O.L.J. Gijzeman and G.A. Bootsma, Surface Sci. 77 ( 1978 ) 1. [28] C.T. Campbell and M.T. Paffett, Surface Sci. 139 (1984) 396. [29] C.T. Campbell, J. Catal. 94 (1985) 436. [ 301 H. Nakatsuji and H. Nakai, Chem. Phys. Letters 174 ( 1990) 283. [31] C.T. Campbell and M.T. Paffett, Surface Sci. 143 (1984) 517.
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