- Email: [email protected]
LEED study of Na on W(112)
SURFACE SCIENCE 21 (1970) 377-389 © North-Holland Publishing Co.
L E E D S T U D Y O F Na O N W ( l 1 2 ) J. M. CHEN and C. A. PAPAGEORGOPOULOS Bartol Research Foundation of The Franklin Institute, Swarthmore, Pennsylvania 19081, U.S.A.
Received 12 February 1970 The structures of Na ovedayer on W(112) surfaces have been studied with LEED and work function measurements. It was found that in the 0 to 0.5 monolayer range the Na atoms formed a p(2 × 1) structure. Addition of Na beyond 0.5 monolayer forced the 2 x 1 array out of registry with the W substrate and the columns of Na on neighboring troughs of W(112) became randomly shifted with respect to each other. This gives the effect of one-dimensional diffraction with the positions of the diffraction lines changing continuously with changing Na coverage, similar to that observed by Gerlach and Rhodin~). Above 0.8 monolayer of Na coverage a second layer formed. The second layer atoms are believed to reside along troughs formed by the first layer with the Na-Na spacing along the trough equal to the atomic diameter of Na. The second layer is diperiodic but not in registry with the substrate. Heating ofa Na-covered W(112) surface to 500:K removed the second layer and caused the columns of the first layer Na to shift relative to each other to form a diperiodic array. Continued heating then gradually removed some Na and changed the surface back to a well ordered p(2 × 1). Further heating converted the surface to clean W(112).
1. Introduction Recent L E E D results 1-5) from alkali-covered metal surfaces have given a wealth of interesting i n f o r m a t i o n . Several alkali-on-metal systemst, 4) appear to be most promising in providing intimate correlation between structural and electronic properties of surfaces. Gerlach a n d R h o d i n 1) reported results of o n e - d i m e n s i o n a l diffraction for Na on Ni (110) which appear to be u n a m b i g u o u s in interpretation. The o n e - d i m e n s i o n a l behavior of N a was due to the trough nature of Ni(110). Since the (112) surface of W also has this trough nature, a n d it has been shown from gas a d s o r p t i o n work on W(112) 6-11) that the troughs indeed play essential roles in determ i n i n g the structure of overlayers, it was decided to study the W ( 1 1 2 ) - N a system for c o m p a r i s o n . O u r observations are similar to those of Gerlach a n d R h o d i n 1). 2. Experimental The experiment was performed in o u r U H V L E E D system as shown in fig. 1. The L E E D optics is of the full-hemisphere post-acceleration type. N a 377
378
J.M. CHEN AND C. A. PAPAGEOR(;OPOULOS
was deposited from a Na-aluminosilicate (zeolite) ion source, these ionic Na sources have been shown I'-') to emit pure Na at 1300~K. Emission of neutral Na atoms from the ion gun was found negligible. The rate o f Na deposition was determined by integrating the ion current to the target. A monolayer on W(112) was deposited in ten minutes. The kinetic energy o f ions was kept below 4 eV. The change in work function o f the surface was LEED
..... l~/nl ..... III , .... i
J / ~
< -'-
, FUNCTION illG°N
VIEWPORT
Fig. I. A schematic drawing of the experimental system. measured by a Farnsworth type electron gunt:~). The background pressure during Na deposition and other measurements was below 5 × 10-'1 torr as measured by a nude ionization gauge. Na deposition and all other measurements were made at r o o m temperature. Temperature of the crystal was mcasured by a W - R h thermocouple. Initial heating o f the W(112) crystal produced two carbon structures rl) as evidenced by a large carbon peak in the Auger spectrum. Repeated oxidation at I I00~K in 10 - 7 torr o f oxygen followcd by flashing to 2500°K gradually removed the carbon as indicated by a W(112) 1 × 1 pattern and a clean W Auger spectrum")). A model of this surface and a clean W(112) 1 × 1 pattern are shown in fig. 2. The W atoms on (112) lie in close-packed columns with troughs in between. Notc that this surface has mirror symmetry about the [/]-1] axis but is asymmetrical about the [/10] axis. 3. Results
3.1. L E E D PATTERN OBSERVATION 3. I. 1. 0 to 0.5 monola),er o/" Na coverage
Initial deposition o f Na produced a p(2 x 1) pattern as shown in fig. 3. Thc ~, ordcrs were at first weak and diffuse but gradually coalesced into clear
LEEDSTUDYOF Na ON W (112)
(a)
379
(b)
Fig. 2. (a) Model of a W(112) surface. (b) LEED pattern of W(112) at 130 eV.
(a)
(b)
Fig. 3. W(l12) p(2× l)-Napattern, 130eV.(a) 0 - - 0 . 2 5 ; ( b ) 0 : 0 . 5 .
spots. At m a x i m u m p(2 x I) p a t t e r n intensity the N a coverage as m e a s u r e d by the integrated ion c u r r e n t was between 0.4 to 0.5 m o n o l a y e r . N o t e that the p(2 × 1) structure forms w i t h o u t heating the crystal, while in the case o f N a on Ni(110) 1) it was necessary to heat the N a - c o v e r e d surface to 100°C to p r o d u c e o r d e r e d structures. A m o d e l o f the W(112) surface at 0.5 m o n o l a y e r N a coverage is shown in fig. 4. Because o f the trough nature o f the W(112) surface, the N a a t o m s
380
3. M. C H E N
AND
C . A. P A P A G E O R G O P O U L O S
most likely reside along the troughs. To explain the observed 2 x 1 periodicity, the N a - N a spacing along the trough is assumed to be twice that of W atoms in the [111] direction. In fig. 4 the Na atoms are assumed to be bonded to four W atoms.
[Tr,]
f.--'.
- .
-
-
.
.
_
.
/" ,i i
[ t
.
'
/,
•
4---4.471---Id
Fig. 4.
,,[T 10]
,,
I
Surface model for the W(112) p(2 × l)-Na pattern.
3.1.2. 0.5 to 0.8 monolayer of Na coverage Deposition of Na above 0.5 monolayer caused the background to increase, the ½ order beams to become weak and diffuse again, and the formation of a set of lines parallel to the [TI0] direction. This is shown in fig. 5. At low coverage the lines are very weak and diffuse. They lie just outside the h = +_~, positions. As the Na coverage is increased, these lines become sharper and more intense, and move continuously away from the 00 beam. The motion stopped near 0.8 monolayer of Na as determined by the integrated ion current, with the tinal positions of the lines at h = _+0.78. As shown in fig. 5d, the lines are quite sharp and intense at 0.8 monolayer coverage. The second
LEEDSTUDYOF Na ON W (112)
381
lid
(i
•
•
•
o
g
•
e
(a)
(b)
O ~r 6
Ib
(c) Fig. 5.
(d)
LEED pattern of Na-covered W(112) at 130 eV, 0.5 < 8 < 1. (a) 0 - 0 . 5 5 , note the diffuse ½ order spots; (b) 0 ~ 0.6; (c) O :- 0.7; (d) 8 0.8.
o r d e r lines, as well as lines due to multiple diffraction, can also be observed. All these changes did not require any heating o f the crystal. A model o f the structure is shown in fig. 6a. All N a a t o m s are assumed to reside a l o n g the troughs, the s e p a r a t i o n o f N a a t o m s within a trough is d e t e r m i n e d by the N a coverage. The c o l u m n s o f N a in neighboring troughs are assumed to be r a n d o m in phase. Thus we can c o n s i d e r the observed lines as due to the diffraction o f a set o f one d i m e n s i o n a l crystals with individual c o l u m n s o f Na a t o m s scattering incoherently. Since the c o l u m n s o f N a have c o n s t a n t spacing in the [i10] direction, the diffraction p a t t e r n
382
J . M . C H E N A N D C. A. P A P A G E O R G O P O U L O S
[Tn] ,f
....
f.
",.
--~
....
\
'\._J (o)
~ +
+
+oo
+
Multiple / Dif fraction Line
+o2 #~
fst Order Diffraction Line
~J J
2nd Order Diffroclion Line
,'o
(b) Fig. 6. (a) Surface model for the pattern observed in fig. 5d. (b) Sketch of the expected pattern from the model o f (a), (-- - - , t ) diffraction from Na:, ( Q ) diffraction from W ; (. . . . ) double diffraction.
LEED STUDY OF N a ON W (112)
383
has a set of (0, k) spots rather than a h = 0 diffraction line through the 00 beam. A sketch of the diffraction pattern is shown in fig. 6b. Similar diffraction lines and interpretation have been given by Gerlach and Rhodinl). We believe that in the range 0.5 to 0.8 monolayer the Na atoms are added to the first layer ~ather than that they form a second layer on top of the 2 x 1 array. This is because: (a) The Na coverage, calculated from the initial and final positions of the diffraction lines, is consistent with our coverage measurement if all Na atoms are assumed to form a single layer. (b) The ½ order spots which are sharp at 0.5 monolayer, become diffuse at higher coverage. This means that the 2 × I structure is destroyed by the additional Na. It is not simply covered by another layer. (c) Some of the ½ order beams at 0.5 monola2~er (fig. 3b) are nearly as intense as the W spots, l f a second layer of Na were formed then the ½ order beams would be weaker but should still be observable because the W spots at 0.8 monolayer of Na are still very clear. The line pattern of fig. 5 and that of Ni(110)-Na system has one basic difference. In the Ni case, besides the lines, there were also intense -1, order spots in the direction perpendicular to the troughs. This led to the proposed model that Na resided in every other trough of N i ( l l 0 ) . In the present case, there are no extra diffraction spots so we assume that there are Na atoms in all troughs of W(112). Also the diffraction lines start to form on N i ( l l 0 ) at 0.25 monolayer whereas they start to form on W ( I I 2 ) at 0.5 monolayer. The spacing between tloughs on W(112) (4.47 A) is 0.95 A more than on Ni(110) (3.52 A). This larger spacing for W(112) is the probable explanation for the allowed occupation of adjacent troughs beginning at low coverages, since the larger spacing leads to a smaller dipole repulsion between atoms in adjacent troughs than for Na on N i ( l l 0 ) at the same coverage. 3.1.3. Higher than 0.8 monolayer of Na coverage Deposition of Na beyond 0.8 monolayer apparently caused the formation of a second layer. The LEED pattern showed a high background and some small weak spots as shown in fig. 7a. A sketch of the pattern is shown in fig. 7b. The sticking probability of Na above one monolaye~ appears to be very low; the LEED pattern was the same at a deposition equivalent to 1.5 monolayers as at an equivalent of 10 monolayers. Therefore it is not known how much of the surface was covered by a second layer of Na. The high background appears to have masked the underlying line pattern, when the second layer was rcmoved by gentle heating to 320°K, the line pattern reappeared.
384
J. M , C H t { N A N D
C. A. PAPAGEORGOPOUI.OS
0
0
0
0
0
X
X
X
X
×
•
•
• oo
•
0o2
X
X
X
X
X
0
0
0
0
0
Q
•
•
•
•
o
o
X
X
o X
x
X
2O (a)
(b)
Fig. 7. (a) LEED pattern due to a second layer of Na at 130 eV. (b) A sketch of the expected pattern from the model of fig. 8, ( 0 ) diffraction from W; (~,) diffraction from second layer Na; ( × ) double diffraction.
[Tr ] Ii
.' ( .~~
T
.
.I "x.
~
-i-
.2 i....,
-
"
i; i,._
o- z
I st foyer No
Fig. 8.
Model for the second layer Na.
LEEDSTUDYOF Na ON W (112)
385
The pattern of fig. 7a is approximately p(3 x 1) except that some ~- order spots are replaced by doublets. A model of the surface structure is shown in fig. 8. The Na atoms are assumed to reside in the troughs formed by the first layer of Na, so they are directly above the W surface atoms. The second layer Na atoms are assumed to form a two-dimensional periodic array, but not in registry with the substrate. A diffraction pattern from such a surface, including single and double scattering, would have all the observed spots. The N a - N a spacing in the [111] direction, as estimated from fig. 7a, is 3.91 A, close to twice the atomic radius of Na (3.8 A). 3.1.4. Heat treatment
The results of heating of a W(112) surface covered with 0.8 monolayer of Na are shown in fig. 9. The crystal was maintained at 500°K for a few minutes and then cooled to room temperature for LEED pattern observation. The results were the same for any coverage above 0.8 monolayer. As shown in fig. 9a, brief heating causes the diffraction lines of fig. 5d to coalesce into diffuse spots. This means that the originally randomly shifted Na columns now shift with respect to each other to form a diperiodic array. All Na columns are now in phase. Further heating apparently removes some Na atoms, so the N a - N a spacing along the trough increases continuously, which causes a continuous motion of the diffuse spots toward the ½ order positions. After 15 min of heating at 500°K, a clear p(2 x 1) pattern was obtained. Further heating on the p(2 x I) structure to higher temperature gradually removed all adsorbed Na atoms. No new structure was observed in the conversion from p(2 x I) to clean. 3.2. INTENSITYANALYSIS The intensity of the diffraction spots in the 2 x 1 pattern and the intensity of the one-dimensional diffraction lines appear to provide some quantitative information of the processes occurring at the surface. A plot of the intensity of one of the ½ order beams, the (½, - 3 ) spot, in the p(2 x l) pattern as a function of Na coverage is shown in fig. 10. As seen, the 2 x I pattern is built up to its maximum intensity in 0.5 monolayer of Na, but is destroyed by 0.25 monolayer of Na. This shows the interesting phenomenon that beyond 0.5 monolayer the addition of Na atoms to a trough pushes a row of several Na atoms out of 2 x l registry. The intensity of the diffraction lines of fig. 5 was also found to vary with primary beam energy. A plot of the intensity curve of the first order lines and that of the 00 beam is shown in fig. I I. Although the W(112) substrate does not have reflection symmetry about a Ill0] axis, the intensity curves of the two first order diffraction lines appears to be identical. Also, no ap-
386
J . M . C H I : N A N D (?. A. P A I ~ A G E O R ( ; O P O U L O S
(a)
(c)
(b)
(d)
Fig. 9. LEED pattern of the Na-covered W(112) surface at 130 eV, the W(112) surface was first covered with 0.8 monolayer of Na then heated to 500~K. Pattern observed with the crystal near room temperature. (a) Heating time T - 2 rain: (b) T - 4 rain; (c) T l0 rain; (c) T - 15 rain. preciable shift in energy o f the main peaks in the intensity curve was observed when the Na coverage, or the N a - N a spacing in the troughs, was varied. As shown in fig. I I , t h e 9 0 e V and 190 eV peaks o f the diffraction line can be c o r r e l a t e d to peaks in the 00 beam intensity curve. Therefore, it is possible that at these two energies the intensity o f the diffraction lines does not c o m e from back scattering o f the incident beam but rather from f o r w a r d scattering o f the 00 beam. Such strong f o r w a r d scattering o f partially ionized low Z a t o m s has been suggested by Bauer ~9) and observed by Estrup and A n d e r -
LEED STUDY OF N a ON W
(112)
387
line p ~ e r n
"E J
~
Bockground of cleon Ixl pottern
'
012
~/-f
'
0.4
0.6
0.8
Coveroge (Monoloyer) Fig. 10.
Intensity of the (J, --3) beam of the W(112) p(2 x I)-Na pattern versus Na coverage. Primary beam energy 130eV.
intensity of diffraction lines
I-D >rr nl--
rr
<~
>I-Z ILl I-Z
60
I00
150
ENERGY Fig. 11.
ZOO
(eV)
Intensity versus energy curves for the O0 beams and diffraction lines.
388
J.M. CHEN AND C. A. PAPAC;EORGOPOULOS
son 2°) for H2 on W. There is one (132 eV) peak in the intensity curve of the diffraction lines which approximately coincides with a minimum of the 00 beam intensity. Since this peak did not move when the N a - N a spacing was changed, it cannot come from multiple scattering among Na atoms in the same trough. It might be that the atomic scattering factor of Na happens to have a maximum in the backward direction at this energy. 4. Discussion
All Na overlayer structures observed in the present work can be considered as a result of competition between the N a - N a binding energy with the N a - W binding energy. The formation of the observed p(2 × 1) structure means that the partially ionized Na atoms are bonded quite strongly to the W atoms so that the overlayer is in registry with the substrate; it also means that the long range Coulomb interaction of Na atoms in neighboring troughs is also strong enough to produce the two-dimensional periodicity. The experimental results suggest that deposition of Na above 0.5 monolayer causes the surface Na atoms to be forced out of 2 x 1 registry with a compression of the overlayer along the [1 I I] direction. The N a - N a spacing within a trough decreases with increasing Na coverage until near 0.8 monolayer the one-dimensional crystals reach their limit of compression. The loss of registry of the Na overlayer with substrate indicates that the N a - W interaction is now weaker than the N a - N a interaction. Tracy and Palmberg 16) found a drastic decrease in C O - P d binding energy when the CO layer was forced out of registry. Similar effect was also observed by Gerlach for alkali on Ni17). The minimum N a - N a spacing, as determined by the tinal positions of the diffraction lines, was 3.54_+0.05 A which is smaller than the nearest neighbor distance of metallic Na (3.71 A). So the one-dimensional Na crystals on W ( 1 1 2 ) a r e more compressed than metallic Na. Gerlach and Rhodin '~') found that in non-registry cases, the minimum N a - N a spacing on Ni(110) and N i ( l l l ) were 3.50 A and 3.54 A respectively. So the minimum N a - N a spacing appears to be independent of substrate. Furthermore, the closepacking is one-dimensional for N a - W ( I I 2 ) and N a - N i ( l l 0 ) but twodimensional for N a N i ( l l l ) . Compressed overlayers in registry with the substrate have been reportedlS). Presumably this is made possible by the strong binding force of the substrate. But for the non-registry cases the contraction of the Na arrays is not due to strong Na-substrate binding but rather due to N a - N a interaction. The similar behavior of Na on Ni(110), Ni(11 I) and W(112) suggests that the N a - N a spacing is mainly determined by a short range repulsive force. Presumably, the contraction of Na array
LEED STUDY OF N a ON W (112)
389
is m a d e p o s s i b l e by r e d u c e d e l e c t r o n i c c h a r g e a r o u n d e a c h N a ion w h i c h r e d u c e d the e l e c t r o n - e l e c t r o n r e p u l s i o n f o r c e p r o d u c e d by the Pauli principle.
Acknowledgements
W e are g r a t e f u l to D r s . J. W. M a y a n d C. E. C a r r o l l for helpful discussions.
References
I) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20)
R. L. Gerlach and T. N. Rhodin, Surface Sci. 10 (1968) 446. J. J. Lander and J. Morrison, Surface Sci. 14 (1969) 465. A. U. MacRae, K. Muller, J. ,1. Lander and J. Morrison, Surface Sci. 15 (1969) 483. A. U. MacRae, K. Muller, J. J. Lander, J. Morrison and J. C. Phillips, Phys. Rev. Letters 22 (1969) 1048. R. L. Gerlach and T. N. Rhodin, Surface Sci. 17 (1969) 32. L. H. Germer and C. C. Chang, Surface Sci. 4 (1966) 498. C. C. Chang and L. H. Germer, Surface Sci. 8 (1967) 115. C. C. Chang, J. Electrochem. Soc. 115 (1968) 354. J. W. May, R. J. Szostak and L. H. Germer, Surface Sci. 15 (1969) 37. J. C. Tracy and J. M. Blakely, Surface Sci. 15 (1969) 257. J. C. Tracy and J. M. Blakely, in : Structure and Chemistry of Solid Surfaces, Ed. G. A. Somorjai (Wiley, New York, 1969) p. 65-I. R. E. Weber and L. F. Cordes, Rev. Sci. Instr. 37 (1966) 112. H. E. Farnsworth, J. Opt. Soc. Am. 15 (1927) 290. J. M. Chen and C. A. Papageorgopoulos, Surface Sci. 20 (1970) 195. L. Pauling, The Nature of the Chemical Bond (Cornell University Press, 1960) p. 93. J. C. Tracy and P. W. Palmberg, J. Chem. Phys. 51 (1969) 4852. R. L. Gerlach, Ph.D. Thesis, Cornell University (1969) fig. 37. See for example, J. H. Pollard and W. E. Danforth, J. Appl. Phys. 39 (1968) 4019. E. Bauer, Phys. Rev. 123 (1961) 1206. P.J. Estrup and J. Anderson, J. Chem. Phys. 45 (1966) 2254.
L E E D S T U D Y O F Na O N W ( l 1 2 ) J. M. CHEN and C. A. PAPAGEORGOPOULOS Bartol Research Foundation of The Franklin Institute, Swarthmore, Pennsylvania 19081, U.S.A.
Received 12 February 1970 The structures of Na ovedayer on W(112) surfaces have been studied with LEED and work function measurements. It was found that in the 0 to 0.5 monolayer range the Na atoms formed a p(2 × 1) structure. Addition of Na beyond 0.5 monolayer forced the 2 x 1 array out of registry with the W substrate and the columns of Na on neighboring troughs of W(112) became randomly shifted with respect to each other. This gives the effect of one-dimensional diffraction with the positions of the diffraction lines changing continuously with changing Na coverage, similar to that observed by Gerlach and Rhodin~). Above 0.8 monolayer of Na coverage a second layer formed. The second layer atoms are believed to reside along troughs formed by the first layer with the Na-Na spacing along the trough equal to the atomic diameter of Na. The second layer is diperiodic but not in registry with the substrate. Heating ofa Na-covered W(112) surface to 500:K removed the second layer and caused the columns of the first layer Na to shift relative to each other to form a diperiodic array. Continued heating then gradually removed some Na and changed the surface back to a well ordered p(2 × 1). Further heating converted the surface to clean W(112).
1. Introduction Recent L E E D results 1-5) from alkali-covered metal surfaces have given a wealth of interesting i n f o r m a t i o n . Several alkali-on-metal systemst, 4) appear to be most promising in providing intimate correlation between structural and electronic properties of surfaces. Gerlach a n d R h o d i n 1) reported results of o n e - d i m e n s i o n a l diffraction for Na on Ni (110) which appear to be u n a m b i g u o u s in interpretation. The o n e - d i m e n s i o n a l behavior of N a was due to the trough nature of Ni(110). Since the (112) surface of W also has this trough nature, a n d it has been shown from gas a d s o r p t i o n work on W(112) 6-11) that the troughs indeed play essential roles in determ i n i n g the structure of overlayers, it was decided to study the W ( 1 1 2 ) - N a system for c o m p a r i s o n . O u r observations are similar to those of Gerlach a n d R h o d i n 1). 2. Experimental The experiment was performed in o u r U H V L E E D system as shown in fig. 1. The L E E D optics is of the full-hemisphere post-acceleration type. N a 377
378
J.M. CHEN AND C. A. PAPAGEOR(;OPOULOS
was deposited from a Na-aluminosilicate (zeolite) ion source, these ionic Na sources have been shown I'-') to emit pure Na at 1300~K. Emission of neutral Na atoms from the ion gun was found negligible. The rate o f Na deposition was determined by integrating the ion current to the target. A monolayer on W(112) was deposited in ten minutes. The kinetic energy o f ions was kept below 4 eV. The change in work function o f the surface was LEED
..... l~/nl ..... III , .... i
J / ~
< -'-
, FUNCTION illG°N
VIEWPORT
Fig. I. A schematic drawing of the experimental system. measured by a Farnsworth type electron gunt:~). The background pressure during Na deposition and other measurements was below 5 × 10-'1 torr as measured by a nude ionization gauge. Na deposition and all other measurements were made at r o o m temperature. Temperature of the crystal was mcasured by a W - R h thermocouple. Initial heating o f the W(112) crystal produced two carbon structures rl) as evidenced by a large carbon peak in the Auger spectrum. Repeated oxidation at I I00~K in 10 - 7 torr o f oxygen followcd by flashing to 2500°K gradually removed the carbon as indicated by a W(112) 1 × 1 pattern and a clean W Auger spectrum")). A model of this surface and a clean W(112) 1 × 1 pattern are shown in fig. 2. The W atoms on (112) lie in close-packed columns with troughs in between. Notc that this surface has mirror symmetry about the [/]-1] axis but is asymmetrical about the [/10] axis. 3. Results
3.1. L E E D PATTERN OBSERVATION 3. I. 1. 0 to 0.5 monola),er o/" Na coverage
Initial deposition o f Na produced a p(2 x 1) pattern as shown in fig. 3. Thc ~, ordcrs were at first weak and diffuse but gradually coalesced into clear
LEEDSTUDYOF Na ON W (112)
(a)
379
(b)
Fig. 2. (a) Model of a W(112) surface. (b) LEED pattern of W(112) at 130 eV.
(a)
(b)
Fig. 3. W(l12) p(2× l)-Napattern, 130eV.(a) 0 - - 0 . 2 5 ; ( b ) 0 : 0 . 5 .
spots. At m a x i m u m p(2 x I) p a t t e r n intensity the N a coverage as m e a s u r e d by the integrated ion c u r r e n t was between 0.4 to 0.5 m o n o l a y e r . N o t e that the p(2 × 1) structure forms w i t h o u t heating the crystal, while in the case o f N a on Ni(110) 1) it was necessary to heat the N a - c o v e r e d surface to 100°C to p r o d u c e o r d e r e d structures. A m o d e l o f the W(112) surface at 0.5 m o n o l a y e r N a coverage is shown in fig. 4. Because o f the trough nature o f the W(112) surface, the N a a t o m s
380
3. M. C H E N
AND
C . A. P A P A G E O R G O P O U L O S
most likely reside along the troughs. To explain the observed 2 x 1 periodicity, the N a - N a spacing along the trough is assumed to be twice that of W atoms in the [111] direction. In fig. 4 the Na atoms are assumed to be bonded to four W atoms.
[Tr,]
f.--'.
- .
-
-
.
.
_
.
/" ,i i
[ t
.
'
/,
•
4---4.471---Id
Fig. 4.
,,[T 10]
,,
I
Surface model for the W(112) p(2 × l)-Na pattern.
3.1.2. 0.5 to 0.8 monolayer of Na coverage Deposition of Na above 0.5 monolayer caused the background to increase, the ½ order beams to become weak and diffuse again, and the formation of a set of lines parallel to the [TI0] direction. This is shown in fig. 5. At low coverage the lines are very weak and diffuse. They lie just outside the h = +_~, positions. As the Na coverage is increased, these lines become sharper and more intense, and move continuously away from the 00 beam. The motion stopped near 0.8 monolayer of Na as determined by the integrated ion current, with the tinal positions of the lines at h = _+0.78. As shown in fig. 5d, the lines are quite sharp and intense at 0.8 monolayer coverage. The second
LEEDSTUDYOF Na ON W (112)
381
lid
(i
•
•
•
o
g
•
e
(a)
(b)
O ~r 6
Ib
(c) Fig. 5.
(d)
LEED pattern of Na-covered W(112) at 130 eV, 0.5 < 8 < 1. (a) 0 - 0 . 5 5 , note the diffuse ½ order spots; (b) 0 ~ 0.6; (c) O :- 0.7; (d) 8 0.8.
o r d e r lines, as well as lines due to multiple diffraction, can also be observed. All these changes did not require any heating o f the crystal. A model o f the structure is shown in fig. 6a. All N a a t o m s are assumed to reside a l o n g the troughs, the s e p a r a t i o n o f N a a t o m s within a trough is d e t e r m i n e d by the N a coverage. The c o l u m n s o f N a in neighboring troughs are assumed to be r a n d o m in phase. Thus we can c o n s i d e r the observed lines as due to the diffraction o f a set o f one d i m e n s i o n a l crystals with individual c o l u m n s o f Na a t o m s scattering incoherently. Since the c o l u m n s o f N a have c o n s t a n t spacing in the [i10] direction, the diffraction p a t t e r n
382
J . M . C H E N A N D C. A. P A P A G E O R G O P O U L O S
[Tn] ,f
....
f.
",.
--~
....
\
'\._J (o)
~ +
+
+oo
+
Multiple / Dif fraction Line
+o2 #~
fst Order Diffraction Line
~J J
2nd Order Diffroclion Line
,'o
(b) Fig. 6. (a) Surface model for the pattern observed in fig. 5d. (b) Sketch of the expected pattern from the model o f (a), (-- - - , t ) diffraction from Na:, ( Q ) diffraction from W ; (. . . . ) double diffraction.
LEED STUDY OF N a ON W (112)
383
has a set of (0, k) spots rather than a h = 0 diffraction line through the 00 beam. A sketch of the diffraction pattern is shown in fig. 6b. Similar diffraction lines and interpretation have been given by Gerlach and Rhodinl). We believe that in the range 0.5 to 0.8 monolayer the Na atoms are added to the first layer ~ather than that they form a second layer on top of the 2 x 1 array. This is because: (a) The Na coverage, calculated from the initial and final positions of the diffraction lines, is consistent with our coverage measurement if all Na atoms are assumed to form a single layer. (b) The ½ order spots which are sharp at 0.5 monolayer, become diffuse at higher coverage. This means that the 2 × I structure is destroyed by the additional Na. It is not simply covered by another layer. (c) Some of the ½ order beams at 0.5 monola2~er (fig. 3b) are nearly as intense as the W spots, l f a second layer of Na were formed then the ½ order beams would be weaker but should still be observable because the W spots at 0.8 monolayer of Na are still very clear. The line pattern of fig. 5 and that of Ni(110)-Na system has one basic difference. In the Ni case, besides the lines, there were also intense -1, order spots in the direction perpendicular to the troughs. This led to the proposed model that Na resided in every other trough of N i ( l l 0 ) . In the present case, there are no extra diffraction spots so we assume that there are Na atoms in all troughs of W(112). Also the diffraction lines start to form on N i ( l l 0 ) at 0.25 monolayer whereas they start to form on W ( I I 2 ) at 0.5 monolayer. The spacing between tloughs on W(112) (4.47 A) is 0.95 A more than on Ni(110) (3.52 A). This larger spacing for W(112) is the probable explanation for the allowed occupation of adjacent troughs beginning at low coverages, since the larger spacing leads to a smaller dipole repulsion between atoms in adjacent troughs than for Na on N i ( l l 0 ) at the same coverage. 3.1.3. Higher than 0.8 monolayer of Na coverage Deposition of Na beyond 0.8 monolayer apparently caused the formation of a second layer. The LEED pattern showed a high background and some small weak spots as shown in fig. 7a. A sketch of the pattern is shown in fig. 7b. The sticking probability of Na above one monolaye~ appears to be very low; the LEED pattern was the same at a deposition equivalent to 1.5 monolayers as at an equivalent of 10 monolayers. Therefore it is not known how much of the surface was covered by a second layer of Na. The high background appears to have masked the underlying line pattern, when the second layer was rcmoved by gentle heating to 320°K, the line pattern reappeared.
384
J. M , C H t { N A N D
C. A. PAPAGEORGOPOUI.OS
0
0
0
0
0
X
X
X
X
×
•
•
• oo
•
0o2
X
X
X
X
X
0
0
0
0
0
Q
•
•
•
•
o
o
X
X
o X
x
X
2O (a)
(b)
Fig. 7. (a) LEED pattern due to a second layer of Na at 130 eV. (b) A sketch of the expected pattern from the model of fig. 8, ( 0 ) diffraction from W; (~,) diffraction from second layer Na; ( × ) double diffraction.
[Tr ] Ii
.' ( .~~
T
.
.I "x.
~
-i-
.2 i....,
-
"
i; i,._
o- z
I st foyer No
Fig. 8.
Model for the second layer Na.
LEEDSTUDYOF Na ON W (112)
385
The pattern of fig. 7a is approximately p(3 x 1) except that some ~- order spots are replaced by doublets. A model of the surface structure is shown in fig. 8. The Na atoms are assumed to reside in the troughs formed by the first layer of Na, so they are directly above the W surface atoms. The second layer Na atoms are assumed to form a two-dimensional periodic array, but not in registry with the substrate. A diffraction pattern from such a surface, including single and double scattering, would have all the observed spots. The N a - N a spacing in the [111] direction, as estimated from fig. 7a, is 3.91 A, close to twice the atomic radius of Na (3.8 A). 3.1.4. Heat treatment
The results of heating of a W(112) surface covered with 0.8 monolayer of Na are shown in fig. 9. The crystal was maintained at 500°K for a few minutes and then cooled to room temperature for LEED pattern observation. The results were the same for any coverage above 0.8 monolayer. As shown in fig. 9a, brief heating causes the diffraction lines of fig. 5d to coalesce into diffuse spots. This means that the originally randomly shifted Na columns now shift with respect to each other to form a diperiodic array. All Na columns are now in phase. Further heating apparently removes some Na atoms, so the N a - N a spacing along the trough increases continuously, which causes a continuous motion of the diffuse spots toward the ½ order positions. After 15 min of heating at 500°K, a clear p(2 x 1) pattern was obtained. Further heating on the p(2 x I) structure to higher temperature gradually removed all adsorbed Na atoms. No new structure was observed in the conversion from p(2 x I) to clean. 3.2. INTENSITYANALYSIS The intensity of the diffraction spots in the 2 x 1 pattern and the intensity of the one-dimensional diffraction lines appear to provide some quantitative information of the processes occurring at the surface. A plot of the intensity of one of the ½ order beams, the (½, - 3 ) spot, in the p(2 x l) pattern as a function of Na coverage is shown in fig. 10. As seen, the 2 x I pattern is built up to its maximum intensity in 0.5 monolayer of Na, but is destroyed by 0.25 monolayer of Na. This shows the interesting phenomenon that beyond 0.5 monolayer the addition of Na atoms to a trough pushes a row of several Na atoms out of 2 x l registry. The intensity of the diffraction lines of fig. 5 was also found to vary with primary beam energy. A plot of the intensity curve of the first order lines and that of the 00 beam is shown in fig. I I. Although the W(112) substrate does not have reflection symmetry about a Ill0] axis, the intensity curves of the two first order diffraction lines appears to be identical. Also, no ap-
386
J . M . C H I : N A N D (?. A. P A I ~ A G E O R ( ; O P O U L O S
(a)
(c)
(b)
(d)
Fig. 9. LEED pattern of the Na-covered W(112) surface at 130 eV, the W(112) surface was first covered with 0.8 monolayer of Na then heated to 500~K. Pattern observed with the crystal near room temperature. (a) Heating time T - 2 rain: (b) T - 4 rain; (c) T l0 rain; (c) T - 15 rain. preciable shift in energy o f the main peaks in the intensity curve was observed when the Na coverage, or the N a - N a spacing in the troughs, was varied. As shown in fig. I I , t h e 9 0 e V and 190 eV peaks o f the diffraction line can be c o r r e l a t e d to peaks in the 00 beam intensity curve. Therefore, it is possible that at these two energies the intensity o f the diffraction lines does not c o m e from back scattering o f the incident beam but rather from f o r w a r d scattering o f the 00 beam. Such strong f o r w a r d scattering o f partially ionized low Z a t o m s has been suggested by Bauer ~9) and observed by Estrup and A n d e r -
LEED STUDY OF N a ON W
(112)
387
line p ~ e r n
"E J
~
Bockground of cleon Ixl pottern
'
012
~/-f
'
0.4
0.6
0.8
Coveroge (Monoloyer) Fig. 10.
Intensity of the (J, --3) beam of the W(112) p(2 x I)-Na pattern versus Na coverage. Primary beam energy 130eV.
intensity of diffraction lines
I-D >rr nl--
rr
<~
>I-Z ILl I-Z
60
I00
150
ENERGY Fig. 11.
ZOO
(eV)
Intensity versus energy curves for the O0 beams and diffraction lines.
388
J.M. CHEN AND C. A. PAPAC;EORGOPOULOS
son 2°) for H2 on W. There is one (132 eV) peak in the intensity curve of the diffraction lines which approximately coincides with a minimum of the 00 beam intensity. Since this peak did not move when the N a - N a spacing was changed, it cannot come from multiple scattering among Na atoms in the same trough. It might be that the atomic scattering factor of Na happens to have a maximum in the backward direction at this energy. 4. Discussion
All Na overlayer structures observed in the present work can be considered as a result of competition between the N a - N a binding energy with the N a - W binding energy. The formation of the observed p(2 × 1) structure means that the partially ionized Na atoms are bonded quite strongly to the W atoms so that the overlayer is in registry with the substrate; it also means that the long range Coulomb interaction of Na atoms in neighboring troughs is also strong enough to produce the two-dimensional periodicity. The experimental results suggest that deposition of Na above 0.5 monolayer causes the surface Na atoms to be forced out of 2 x 1 registry with a compression of the overlayer along the [1 I I] direction. The N a - N a spacing within a trough decreases with increasing Na coverage until near 0.8 monolayer the one-dimensional crystals reach their limit of compression. The loss of registry of the Na overlayer with substrate indicates that the N a - W interaction is now weaker than the N a - N a interaction. Tracy and Palmberg 16) found a drastic decrease in C O - P d binding energy when the CO layer was forced out of registry. Similar effect was also observed by Gerlach for alkali on Ni17). The minimum N a - N a spacing, as determined by the tinal positions of the diffraction lines, was 3.54_+0.05 A which is smaller than the nearest neighbor distance of metallic Na (3.71 A). So the one-dimensional Na crystals on W ( 1 1 2 ) a r e more compressed than metallic Na. Gerlach and Rhodin '~') found that in non-registry cases, the minimum N a - N a spacing on Ni(110) and N i ( l l l ) were 3.50 A and 3.54 A respectively. So the minimum N a - N a spacing appears to be independent of substrate. Furthermore, the closepacking is one-dimensional for N a - W ( I I 2 ) and N a - N i ( l l 0 ) but twodimensional for N a N i ( l l l ) . Compressed overlayers in registry with the substrate have been reportedlS). Presumably this is made possible by the strong binding force of the substrate. But for the non-registry cases the contraction of the Na arrays is not due to strong Na-substrate binding but rather due to N a - N a interaction. The similar behavior of Na on Ni(110), Ni(11 I) and W(112) suggests that the N a - N a spacing is mainly determined by a short range repulsive force. Presumably, the contraction of Na array
LEED STUDY OF N a ON W (112)
389
is m a d e p o s s i b l e by r e d u c e d e l e c t r o n i c c h a r g e a r o u n d e a c h N a ion w h i c h r e d u c e d the e l e c t r o n - e l e c t r o n r e p u l s i o n f o r c e p r o d u c e d by the Pauli principle.
Acknowledgements
W e are g r a t e f u l to D r s . J. W. M a y a n d C. E. C a r r o l l for helpful discussions.
References
I) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20)
R. L. Gerlach and T. N. Rhodin, Surface Sci. 10 (1968) 446. J. J. Lander and J. Morrison, Surface Sci. 14 (1969) 465. A. U. MacRae, K. Muller, J. ,1. Lander and J. Morrison, Surface Sci. 15 (1969) 483. A. U. MacRae, K. Muller, J. J. Lander, J. Morrison and J. C. Phillips, Phys. Rev. Letters 22 (1969) 1048. R. L. Gerlach and T. N. Rhodin, Surface Sci. 17 (1969) 32. L. H. Germer and C. C. Chang, Surface Sci. 4 (1966) 498. C. C. Chang and L. H. Germer, Surface Sci. 8 (1967) 115. C. C. Chang, J. Electrochem. Soc. 115 (1968) 354. J. W. May, R. J. Szostak and L. H. Germer, Surface Sci. 15 (1969) 37. J. C. Tracy and J. M. Blakely, Surface Sci. 15 (1969) 257. J. C. Tracy and J. M. Blakely, in : Structure and Chemistry of Solid Surfaces, Ed. G. A. Somorjai (Wiley, New York, 1969) p. 65-I. R. E. Weber and L. F. Cordes, Rev. Sci. Instr. 37 (1966) 112. H. E. Farnsworth, J. Opt. Soc. Am. 15 (1927) 290. J. M. Chen and C. A. Papageorgopoulos, Surface Sci. 20 (1970) 195. L. Pauling, The Nature of the Chemical Bond (Cornell University Press, 1960) p. 93. J. C. Tracy and P. W. Palmberg, J. Chem. Phys. 51 (1969) 4852. R. L. Gerlach, Ph.D. Thesis, Cornell University (1969) fig. 37. See for example, J. H. Pollard and W. E. Danforth, J. Appl. Phys. 39 (1968) 4019. E. Bauer, Phys. Rev. 123 (1961) 1206. P.J. Estrup and J. Anderson, J. Chem. Phys. 45 (1966) 2254.