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Bismuth on copper (110): analysis of the c(2 × 2) and p(4 × 1) structures by surface X-ray diffraction
i
surface s c i e n c e ELSEVIER
Surface Science 373 (1997) 11-20
Bismuth on copper (110): analysis of the c(2 × 2) and p(4 × 1) structures by surface X-ray diffraction L. L o t t e r m o s e r a,,, T. B u s l a p s a, R i . J o h n s o n a, R. F e i d e n h a n s ' l b M. N i e l s e n b D. Smilgies b, E. L a n d e m a r k b, H . L . M e y e r h e i m c a H. Institutfar Experimentalphysik, Universitat Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany b Riso National Laboratory, DK-4000 Roskilde, Denmark c Institutfar Kristallographie und Mineralogie, Universitgit Mfinchen, Theresienstrasse 41, D-80333 Manchen, Germany Received 9 May 1996; accepted for publication 9 September 1996
Abstract
Surface X-ray diffraction has been used to analyze the atomic structures of the Cu(ll0)-c(2x2)-Bi and Cu(ll0)-p(4x 1)-Bi reconstructions with submonolayer coverages. A quasi-hexagonal c(2 x 2) adlayer structure is formed when half a monolayer of bismuth is deposited; the coverage corresponds to 1.08 x 10 -15 atoms cm -2. There is one Bi atom per c(2 x 2) surface unit cell, and the nearest-neighbor distance on the planar overlayer was found to be 4.43 .~. In the case of the p(4 x 1) reconstruction formed at a coverage of 0.75 monolayers, both the in-plane and out-of-plane data are in excellent agreement with a model in which every fourth Cu row in the [001] direction of the topmost layer is replaced by Bi atoms to form a substitutional surface alloy. © 1997 Elsevier Science B.V. All rights reserved.
Keywords: Bismuth; Copper; Low index single crystal surfaces; Metallic surfaces; Surface relaxation and reconstruction; Surface structure, morphology, roughness, and topography; X-ray scattering, diffraction, and reflection
1. Introduction
Copper and bismuth are usually regarded as immiscible metals, and hence one would expect that adsorbate layers of Bi on copper should form simple overlayer structures. However, it has been found recently that even immiscible metals can form surface alloys [1-5]. In this paper we will demonstrate that surface alloying also occurs in the case of Bi on Cu(ll0), which indicates that surface alloying is probably a very widespread phenomenon. Previously, the interaction of vapour-deposited Bi with Cu(110) has been studied * Corresponding author. Fax: +49 40 89982787; e-mail: [email protected]
by Clendening and Campell [6] with X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), and by low-energy electron diffraction (LEED). They found that adlayers of Bi on Cu(ll0) surfaces formed a series of superstructures at submonolayer coverages: at a coverage of 0.5 ML a c(2 x 2) LEED pattern, for Bi coverages in the range between 0.5 and 0.73 ML streaked patterns were observed, and at a coverage of 0.75 ML a p(4 x 1) LEED pattern was observed. The c(2x2) superstructure phase has been described in Ref. [6] as a quasi-hexagonal arrangement in which the Bi atoms were assumed to be placed in the four-fold hollow sites. The streaked LEED patterns were interpreted by Clendening and Campbell [6] as being due to the coexistence
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved 1)11 S0039-6028 ( 9 6 ) 0 1 1 5 1 - X
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L . Lottermoser et al. / Surface Science 373 (1997) 11-20
of p ( n x l ) - B i domains, where n=5-7. Each domain consisted of simple rows of Bi atoms located in adjacent Cu(ll0) troughs, and the Bi-Bi distance within the troughs decreased with increasing Bi coverage. The p ( 4 x l ) structure was explained in the same manner. Using a hard-sphere model, the Bi-Bi distance within the adjacent Cu troughs at the saturation coverage of 0.75 ML was calculated to be 3.41 .~. However, no previous crystallographic studies of the atomic geometry have been published. In this paper, we derive new structural models for the Cu(110)-c(2 x 2)-Bi and Cu( 110)-p(4 x 1)-Bi reconstructions, based on surface X-ray diffraction data. 2. Experimental
The experiments were performed at the Hamburg Synchrotron Radiation Laboratory (HASYLAB/DESY), and the samples were prepared at the Flipper II photoemission beamline. The Cu(110) crystal was cleaned by repeated cycles of Ar ÷ ion sputtering (500 eV) and annealing to 700°C until the characteristic (1 x 1) LEED pattern of the clean surface with sharp spots was observed and the photoemission spectra showed no traces of impurities. A Knudsen effusion cell with a boron nitride crucible was used to deposit Bi, and the deposition rate, calibrated with a quartz crystal monitor, was ~ 1.5 x 10 -3 A min -1. The pressure in the chamber during deposition was <2 × 10 -1° mbar. In the first experiment we prepared the c(2 x 2) structure at a Bi coverage of 0 = 0.5 ML, and in the second experiment the p(4 x 1) superstructure at a Bi coverage of 0=0.75 ML. Both samples were post-annealed at ~600 K, which improved the LEED pattern. The coverages quoted here were determined from the structural analysis. After preparation the sample was transferred to a portable ultra-high vacuum (UHV) chamber with a hemispherical beryllium window, which was mounted on the surface X-ray diffractometer at the wiggler beamline BW2.
fixed angle of incidence. To avoid fluorescence from the Cu crystal, the wavelengths of the X-ray beam were chosen to be 1.61 and 1.42 A, and the angle of grazing incidence was 0.43 ° for the c(2 x 2) reconstruction and 0.21° for the p(4x 1) superstructure, respectively. Integrated intensities were measured by rocking scans around the axis normal to the surface. In total, for the c(2 x 2) structure 25 fractional-order in-plane intensities (11 nonequivalent) and three fractional-order rods were measured. For the p(4 x 1) structure 126 fractionalorder and nine integer-order in-plane peaks were measured, out of which 42 and six, respectively, were non-equivalent. Subsequently, five fractionalorder and four crystal truncation rods were collected. The rod scans were measured by keeping the angle of incidence to the surface fixed and varying the exit angle with respect to the surface
[7]. To reduce systematic errors, the intensities of the symmetry-equivalent reflections were averaged, and the uncertainties were calculated from the counting statistics and the reproducibility of the symmetry-related reflections. The reproducibility between equivalent fractional-order in-plane intensities was found to be about 10%, probably due to sampling over different parts of the surface and small changes in the angle of incidence. The structure-factor intensities were obtained by correcting the measured integrated intensity with a Lorentz factor, for the variation in active sample area, and with polarization factors for the out-of-plane data [7]. The reflections are indexed with respect to a surface unit cell given by bl=[ll0]btak, b2= [001]bulk in the surface plane, and b3=[ll0]b~k normal to the surface plane. Hence, the real-space unit cell is given by al =l[-1T0]b~, a2= [001]b~, and a3 = ½[, 110]b~ak.
4. C u ( l l 0 ) - c ( 2 x 2)-Bi structure
A contour map of the Patterson function
P(x,Y)=Y~h.kIF[2 cos(2n(hx+ky)) provides a map 3. Data collection
The optical surfaces of the samples were aligned by total external reflection in order to ensure a
of the interatomic vectors within the unit cell [-7]. The contour map P(x,y), based on the fractionalorder structure reflections of the c(2 x 2) structure only, has a peak at the origin which suggests a
L. Lottermoser et aL / Surface Science 373 (1997) 11-20
simple quasi-hexagonal overlayer model with only one Bi atom in the surface unit cell. In the data analysis the goodness of fit between the model structure-factor intensities ]F~hk°d°X12 and the experimental structure-factor intensities tPhhk°del[2 and the experimental structure-factor intensities IF~Pl 2 was evaluated using either an R factor or g 2 given by (1)
Z2=
(2)
1
(Thk
/~2/.~2. 2 --87~ 2 " t'lUll
+k
2 2 2
2 2
/
where ahk are the uncertainties in the structurefactor intensities IFT,~Pl2, g is the number of data points and p the number of free parameters. The least-squares analysis of the in-plane fractional-order structure-factor intensities with a model including only the Bi overlayer (see Fig. 1) gave Z2 and R of 3.3 and 0.20, respectively, refining only an overall Debye-Waller factor for the Bi atom and a scale factor. By including an anisotropic Debye-Waller factor for the Bi atom, the fit could be significantly improved to Z2= 1.9 and R = 0.10. The agreement between the measured and calculated fractional-order structure-factor inten-
2 x
b2u22+/b3u33
4
(]f~'~Pl2--2--F~hk°dell2~2,Y,
N- p ~ \
sities are shown in Fig. 2. The calculated values were obtained using (u2t)=0.037+0.003.~2 and (U2z) = 0.023 _ 0.003 ~2 for the mean square vibration amplitudes of the Bi atom along the [110] and [001] directions, respectively. In the leastsquares analysis the mean square amplitudes were refined using the expression exp
R = ~h,k IIF~,~Pl2 - IPhhk°dell2I ~h.k IF~,~Pl2 '
13
(3)
)
for the anisotropic Debye-Waller factor, where a is the lattice constant of copper and hkl (l--0) are the Miller indices. From in-plane measurements alone it is not possible to obtain information about atomic displacements in the direction normal to the surface; out-of plane measurement are necessary. For the c(2 x 2) superstructure three fractional-order rodscans were measured, namely at (3/2,1/2,1), (3/2,3/2,1 ) and (1/2,5/2,1 ). The rod scans are shown in Fig. 3. The perpendicular momentum transfer l is in units of the reciprocal lattice vector ba. The monotonic variation of the measured rod-scans with momentum transfer perpendicular to the surface is clear evidence of a planar Bi overlayer. Using the structural parameters determined from
~k
3
.
.
.
.
2 ...............................
1 ...............
.,,-'-
.
.
.
'. . . . . . . . . . . . . . . . . . .
:
.......................
1
2
al
=
1[li0]
a 2 = [0011
Fig. 1. Top view of the Cu(ll0)-c(2 x 2)-Bi structural model. The dark circles are Bi atoms and lighter circles are Cu atoms. The dashed lines indicate a c(2 x 2) surface unit cell.
3
~1
Fig. 2. Comparison between the measured (filled semicircles) and calculated (open semicircles) in-plane structure-factor intensities for the c(2 x 2) overlayer model. The areas of the semicircles are proportional to the intensities.
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
14
40
+
r r
20 311) (~ 0 ,..i
--
J
I
I
I
I
I
J
I
,
I O.O
40
20
0
t
I
i
,
I 0.2
i
40
20 0
r 0.4 Qt[ree. lattice units]
Fig. 3. Structure-factor intensities of three fractional-order superlattice rods of the c(2 x 2)-Bi reconstructionas a function of momentum transfer Q, in the direction normal to the surface in reciprocallattice units of b 3. The best fit is indicated by the solid line. the least-squares analysis of the in-plane data, together with the planar Bi overlayer, yielded X2= 0.6 and R = 0.06 for the three fractional-order rods. The fit is indicated by the solid line in Fig. 3, and the mean square vibrational amplitudes for the Cu(110)-c(2 x 2)-Bi model are listed in Table 1.
5. Cu(ll0)-p(4 × 1)-Bi structure
5.1. In-plane data Fig. 4 shows a contour map of the Patterson function for the p(4 x 1) structure using the meaTable 1 Mean-square vibrational amplitudes h e x a g o n a l C u ( 110)-c(2 x 2 ) - B i m o d e l
c(2 x 2) Atom (u21)(A,z) structure Bi
for t h e p l a n a r
(u22)(A,2)
quasi-
(ua23)(A,z)
0.037±0.003 0.023+0.003 0.03+0.01
sured intensities of the in-plane fractional-order reflections. Two interatomic vectors, each represented by a peak of comparable amplitude, are seen in the Patterson map. The interatomic vectors of the Patterson map suggest an in-plane arrangement of the Bi atoms as shown in Fig. 4. In the first stage of the structural refinement, three Bi atoms with a common isotropic Debye-WaUer factor, an overall scale-factor and one positional coordinate were adjusted. The bismuth atoms on the mirror line in the al direction are only allowed to move along this line due to the symmetry (p2mm). The refinement yielded Z2=8.1 and R = 0.25. The high values of R and Xz indicate that something is missing in the trial model. The discrepancy between the trial model and the real structure was found using electron-density difference Fourier-synthesis, which is based on the assumption that the phases of the experimental structure factors can be approximated by the model phases, ~od~ [7,8]. This is normally valid provided the essential features of the structure are correct. The map of the electron-density difference can be calculated using
Ap(x,y) = p~Xp(x,y)- pm°a~t(x,y) 1 a h,k X COS [2~(hx + k y ) - ~od~l],
(4)
where a is the area of the unit cell. In Fig. 5 the electron-density difference map Ap is shown using IF~Pl and the calculated phases derived from the starting structural model. The positive peaks in the electron-density difference plot indicate electron density which is missing in the structural model, whereas the negative peaks indicate excess electron density in the trial model. The electron-density difference map shows a reduction in the electron density at the origin relative to a full occupation of the adatom sites by bismuth. The difference plot can be explained by two substantially different p(4 x 1) structural models. Starting from the simple overlayer model, one might imagine that the charge reduction is caused by partial occupation of the adatom sites at the origin by Bi atoms. By reducing the occupancy of the Bi atom at the origin to 0.68 we found that X2 improved to 4.9.
L. Lottermoser et aLI Surface Science 373 (1997) 11-20
I
....
:-
"-:
15
.-.!!!!!-77"..... ..
Iv
o,=½[liol
ID
I
O ..............................O Fig. 4. Contour map of the Patterson function of the p(4 x 1)-Bi reconstruction calculated from the in-plane fractional-order data. Positive contours are indicated by solid lines, and negative contours are dashed. The positive maxima suggest as a first approximation an atomic arrangement with three bismuth atoms per p(4 x 1) unit cell, located at (0,0), (1.4,0.5) and (2.6,0.5).
*,=½DTol
i
Fig. 5. Electron-density difference plot between the initial model and the experimental data. Positive contours are marked with solid lines, and negative contours are dashed.
T h e c o n t o u r m a p is also consistent with a structural m o d e l in which every f o u r t h C u r o o l ] r o w in the t o p m o s t l a y e r is r e p l a c e d b y Bi a t o m s , as s h o w n in Fig. 6. T h e positive p e a k s in the electrondifference p l o t are then due to the three s u b s t r a t e C u a t o m s which are missing in the model.
A s s u m i n g this substitution, b y s y m m e t r y the m o d e l has two free d i s p l a c e m e n t p a r a m e t e r s ; one is the i n t e r a t o m i c distance b e t w e e n the Bi a t o m s at p o s i t i o n s (1.39,0.5) a n d (2.61,0.5) a n d the second is the i n t e r a t o m i c distance between the C u a t o m s at p o s i t i o n s (1,0) a n d (3,0) within the surface unit
16
L. Lottermoser et al. /Surface Science373 (1997) 11-20
p(4xl)
f
0B~=0.75
it I .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
e
~
1.2 (R=0.12), respectively. In all the refinements the Debye-Waller factor of the Cu atoms was fixed at its bulk value (Be, = 0.55 A 2) [ 9 ] . The in-plane displacements of 0.06 A for the C u atoms, indicated by the short arrows in Fig. 6, are small but necessary in order to obtain g o o d fits with both models. Excluding these in-plane relaxations causes X2 to increase significantly to 3.3 and 3.02 for the substitutional and the overlayer model, respectively. To distinguish between the two models, the six in-plane integer-order reflections were included in the in-plane data analysis. These reflections contain contributions from b o t h the p(4 x 1) reconstructed surface layers and the bulk. Inclusion of the integerorder peaks yielded a X2 of 1.2 ( R = 0 . 1 2 ) for the substitutional model. The resulting agreement between the final substitutional model IF~hk°dell2and experimental structure-factor intensities IF~Pl 2 are shown in Fig. 7. The final overlayer model also agrees with the in-plane structure factor intensities to the same accuracy (Z 2 = 1.3 and R =0.11). Hence, the analysis of in-plane data alone is insufficient to distinguish between the two models. An unambiguous description of the structure can only be extracted by including the analysis of the crystal truncation rods in the refinement, as discussed in Section 5.2.
a1
Fig. 6. Top and side view of the final substitutional model of the Cu(ll0)-p(4 x 1)-Bi surface structure. The dark circles are Bi atoms and lighter circles are Cu atoms. The directions of the sub-surface displacements of the Cu atoms are indicated by arrows. The dashed lines indicate a (4 x 1) unit cell. cell. Including independent isotropic Debye-Waller factors for each of the two inequivalent Bi atoms, a c o m m o n scale factor and the two displacement parameters in the refinement improves g 2 to 3.62 and R to 0.15. In order to further improve the fit, anisotropic Debye-Waller factors for the Bi atoms must be taken into account in the structural analysis. This significantly improved the agreement between the measured and calculated in-plane intensities. We obtain for the simple overlayer with the partially filled adsorption sites at the origin and for the substitutional model z 2 = l . 1 ( R = 0 . 1 2 ) and Z2=
4 k,
-~D
t)--t)---o
;
- 43..........
: ~
® l ) - e ........ ~ ......... i
...... e
2
. . . . . . .
3
Fig. 7. Comparison between the measured (filled semicircles) and calculated (open semicircles) in-plane structure-factor intensities of Cu(ll0)-p(4× 1)-Bi for the final substitutional model. The areas of the semicircles are proportional to the intensities.
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
17
with corresponding values for ~od 2 = 1.13 and Rro d = 0.09. The directions of the atomic displacements of the Cu atoms, which were used in the least-squares analysis, are shown schematically in Fig. 6. The best fit for the overlayer model is indicated by the dashed curves in Fig. 8. The overlayer model has Z2= 3.7 with the same free parameters, which is significantly worse than for the substitutional model. The crystal truncation rods are very sensitive to the change in electron density in the near-surface region, and hence are expected to be significantly different for the two different models [7,10]. The overall scale factors of the two models were fixed at the values found in the fractional out-of plane refinements and the structural parameters were
5.2. Rod-scan analysis Five fractional-order rod-scans, as shown in Fig. 8, were used for the out-of-plane analysis of the p(4x 1) Bi structure. The starting structural models were those derived from the in-plane analysis and the Debye-Waller factors, and in-plane relaxations of the Cu atoms were fixed at the values determined by the in-plane refinement. In the least-squares analysis only the distances normal to the surface were allowed to vary. However, to obtain satisfactory agreement with the observed intensities, sub-surface relaxations and two different mean-square vibration amplitudes (u~3) of the Bi atoms along the surface normal had to be taken into account. Including these parameters yields the solid curve in Fig. 8,
Rod (-3/4,2,1)
Rod (3/4,0,1)
9 6 3 0
I
I
g
I
I
Rod (112,1,1)
Rod (7/4,1,1)
3 ~
2
0
I
I
I
I
Rod (514,0,1)
1,5
0,1
0,3
0,5
0,7
0,9
1,1
Qz [r. I. u.]
1,0 0,5
0,1
0,3
0,5
0,7
Qz [r. I. u.] Fig. 8. Structure-factor intensities of five fractional-order rods of the p(4 × 1)-Bi reconstruction as a function of Q, in reciprocal lattice units of b 3. The solid and dashed lines are the calculated intensities for the substitutional and the overlayer model, respectively, by including two different mean-square vibration amplitudes (u33)2 of the Bi atoms and sub-surface relaxations in the structural analysis.
18
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
fixed at the values listed in Table 2; hence there are no adjustable parameters. Fig. 9 shows the structure-factor intensities measured along four crystal truncation rods. The dashed curves correspond to the best overlayer model (Z2= 33). The solid curves were obtained with the substitutional model, and only this model gives a satisfactory fit to all of the measured integer-order rods (Z2= 3.9). One aspect which has not been incorporated in our structural model but is always present on metal surfaces is surface roughness. We have included an exponential distribution of heights I-8] in the final refinement to model the effect of statistical surface roughness on the intensities along the rods. The additional fit parameter fl corresponds to a rough surface in which layer 0 is assumed to be fully occupied, layer 1 above it has a fraction of/3 sites filled, layer 2 has fraction/32, and so on. For the ultimate refinement, the substitutional model including surface roughness was fitted to the merged dataset which included all the in-plane reflections, the crystal-truncation rods and the fractional-rods. With the free geometrical parameters and with the anisotropic Debye-Waller factors for the Bi atoms as listed in Table 2, a X2 of 1.6 (R = 0.08) was obtained. Replacing the anisotropic Debye-Waller factors with isotropic factors cause Z2 to increase significantly to 2.8. The details of the final substitutional model are listed in Table 2.
6. Discussion and conclusion On the basis of surface X-ray diffraction data we have established two new structural models
which describe the C u ( l l 0 ) - c ( 2 x 2) structure at 0 . 5 M L Bi coverage and the C u ( l l 0 ) - p ( 4 x l ) reconstruction at 0.75 M L Bi coverage. The results of the analysis of the C u ( l l 0 ) p(4 x 1)-Bi data indicates a massive rearrangement of the surface atoms similar to the alkali-metal induced reconstructions of fcc (110) metal substrates [11,12] and the structures of Pb on lowindex copper surfaces I-1,3-5]. For steric reasons it is not possible for all of the Bi atoms to be located in a highly coordinated adsorption site on the unreconstructed Cu(110) surface at a coverage of 0.75 M L - the Bi atoms are simply too large. The removal of every fourth [001] row of Cu atoms in the outermost substrate layer transforms the Bi position at the origin into a energetically preferred highly coordinated adsorption site. The two equivalent Bi atoms at the positions (1.39,0.5) and (2.61,0.5) within the surface unit cell are displaced laterally by 0.28 ,~ from an ideal four-fold hollow site of the unreconstructed Cu lattice. The energy cost for such displacements must be compensated by the interaction between these Bi atoms. The interatomic distance of the two Bi atoms of 3.12 ,~, obtained by our structural analysis, is identical to the bulk value 1-13]. The energy gained by forming a missing-row structure with all three Bi atoms in highly coordinated adsorption sites seems to outweigh the effect of displacing the Cu atoms to form this reconstruction. The corrugation of the missing-row layer induced by Bi adsorption was found to be 0.20 ,~, as shown in Table 2, and is probably associated with charge redistribution in the surface due to the Bi atoms at the (1.39,0.5) and (2.61,0.5) positions.
Table 2 Atomic coordinates of the best Cu( 110)-p(4 x 1)-Bi substitutional model, including statistical surface roughness (fl= 0.06); an asterisk indicates that the parameter was fixed by symmetry;two asterisks indicate parameters which were fixed in the least-squares structural refinement Atom
x
y*
z
Bi1 Bi2 Bi3 Cul Cu2 Cu3
0* 1.389___0.001 2.611 ___0.001 1.024_+0.002 2* 2.976 + 0.002
0 0.5 0.5 0 0 0
0.28 _ 0.02 0.72 ___0.02 0.72 + 0.02 0.00 0.08 + 0.01 0.00
B (A2)
0.55** 0.55** 0.55**
(u~l) (A2)
(u~2) (A2)
(u]3) (A2)
0.014 ___0.002 0.033 _ 0.002 0.033 __+0.002
0.022__+0.001 0.020 ___0.002 0.020 + 0.002
0.04 ___0.02 0.09 +__0.02 0.09 _ 0.02
19
L. Lottermoser et al. / Surface Science 373 (1997) 11-20 i
lO3 !
!
|
|
•
I
!
Rod (0,-2,1)
Rod (1,0,1) i!
I [
ll!
10 2
IE
.m
101 L
¢~_
10 0 %
10 -1
I
I
I
I
I
I
I
I
Rod (0,-1,1)
Rod (1,1,1) i0 z
•
101
10 °
10 -1
I
0,0
i
|
I
I
I
I
I
0,3
0,6
0,9
0,0
0,2
0,4
0,6
Qz [r. I . u . ]
Qz [r. !. u.]
Fig. 9. Structure-factor intensities of four crystal truncation rods of the p(4 × 1)-Bi reconstruction as a function of Qz in reciprocal lattice units of b3. The dashed curves were obtained with the overlayer model and the solid lines correspond to the final substitutional model.
20
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
This would produce a reduction in the force constant between the Cu atoms localized close to these Bi positions and the Cu atoms localized close to the Bi atom at the origin. The weakening of the C u - C u bonds corresponds to a lowering of the energy threshold for a displacement of the copper row underneath the Bi atom at the origin during the initial formation of the p(4 × 1) reconstruction. Similar corrugation enhancement and redistribution of the surface charge density induced by lowcoverage Pb and Bi adsorption on Cu(001) have recently been reported [ 14], but without any substrate reconstruction. The relatively large vibrations amplitudes of the bismuth atoms of 0.04 and 0.09 ~2 along the surface normal are also consistent with the corrugation enhancement. The lateral compression of 0.06 A of the missingrow layer with respect to the bulk lattice parameter is probably caused by a complex interaction due to chemisorption of a bismuth atom in the highly coordinated adsorption site at the origin and the electrostatic repulsion between this bismuth atom and the Cu atom at (1.024,0). The Cu-Bi bond lengths within the p(o4 x 1) superstructure are all between 2.7 and 2.9 A, which corresponds closely to the simple sum of the metallic radii of Cu and Bi, and also indicates that these atoms are close to being fully coordinated due to the formation of a Bi/Cu surface alloy. In the c(2 × 2) structural model, the bismuth atoms form a planar quasi-hexagonal overlayer with only one Bi atom per surface unit cell and a nearest Bi-Bi distance of z~.43 A. By comparison with the p(4 x 1) model, we infer that the Bi atoms are adsorbed in every second of the four-fold hollow sites of the substrate, as shown in Fig. 1. In conclusion, by performing extensive surface X-ray diffraction measurements we have been able to establish the detailed atomic coordinates of two
new structural models which describe the c(2 x 2) and p(4 × 1) reconstructions of Bi on Cu(110).
Acknowledgements We wish to thank the staff of HASYLAB for their technical assistance. Financial support from the Danish Natural Science Research Council and from the Bundesminister ftir Forschung und Technologic (BMBF) under project No. 05622GUAB1 is gratefully acknowledged. One of us ( E i . ) acknowledges the Swedish Natural Science Research Council (NFR) for financial support.
References [1] C. Nagl, E. Platzgummer, O. Hailer, M. Schmid and P. Varga, Surf. Sci. 331-333 (1995) 831. I-2] L. Pleth Nielsen, F. Besenbacher, I. Stensgaard, E. L~egsgaard, C. Engdahl, P. Stoltze, K.W. Jacobsen and J.K. Norskov, Phys. Rev. Lett. 71 (1993) 754. I-3] C. de Beauvais, D. Rouxel, M. Michailov and B. Mutaftschiev, Surf. Sci. 324 (1995) 1. [4] C. Nagl, E. Platzgummer, O. Hailer, M. Schmid and P. Varga, Surf. Sci. 321 (1994) 237. [5] Y. Gauthier, W. Moritz and W. H6sler, Surf. Sci. 345 (1996) 53. [6] W.D. Clendening and C.T. Campbell, J. Phys. Chem. 90 (1989) 6656. 1-7] R. Feidenhans'l, Surf. Sci. Rep. 10 (1989) 105. ['81 I.K. Robinson and D.J. Tweet, Rep. Prog. Phys. 55 (1992) 599. 1,9] P.A. Flynn, G.M. McManus and H.A. Rayne, Phys. Rev. 123 (1961) 809. 1-10] I.K. Robinson, Phys. Rev. B 33 (1986) 3830. [11] W.C. Fan and A. Ignatiev, Phys. Rev. B 38 (1988) 366. 1-12] C.J. Barnes, M.Q. Ding, M. Lindroos, R.D. Diehl and D.A. King, Surf. Sci. 162 (1985) 59. [ 13] R.W.G.Wyckoff,Crystal Structures, 2nd ed. (Wiley, New York, 1967). [14] W. Li and G. Vidali, Phys. Rev B 47 (1992) 4356.
surface s c i e n c e ELSEVIER
Surface Science 373 (1997) 11-20
Bismuth on copper (110): analysis of the c(2 × 2) and p(4 × 1) structures by surface X-ray diffraction L. L o t t e r m o s e r a,,, T. B u s l a p s a, R i . J o h n s o n a, R. F e i d e n h a n s ' l b M. N i e l s e n b D. Smilgies b, E. L a n d e m a r k b, H . L . M e y e r h e i m c a H. Institutfar Experimentalphysik, Universitat Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany b Riso National Laboratory, DK-4000 Roskilde, Denmark c Institutfar Kristallographie und Mineralogie, Universitgit Mfinchen, Theresienstrasse 41, D-80333 Manchen, Germany Received 9 May 1996; accepted for publication 9 September 1996
Abstract
Surface X-ray diffraction has been used to analyze the atomic structures of the Cu(ll0)-c(2x2)-Bi and Cu(ll0)-p(4x 1)-Bi reconstructions with submonolayer coverages. A quasi-hexagonal c(2 x 2) adlayer structure is formed when half a monolayer of bismuth is deposited; the coverage corresponds to 1.08 x 10 -15 atoms cm -2. There is one Bi atom per c(2 x 2) surface unit cell, and the nearest-neighbor distance on the planar overlayer was found to be 4.43 .~. In the case of the p(4 x 1) reconstruction formed at a coverage of 0.75 monolayers, both the in-plane and out-of-plane data are in excellent agreement with a model in which every fourth Cu row in the [001] direction of the topmost layer is replaced by Bi atoms to form a substitutional surface alloy. © 1997 Elsevier Science B.V. All rights reserved.
Keywords: Bismuth; Copper; Low index single crystal surfaces; Metallic surfaces; Surface relaxation and reconstruction; Surface structure, morphology, roughness, and topography; X-ray scattering, diffraction, and reflection
1. Introduction
Copper and bismuth are usually regarded as immiscible metals, and hence one would expect that adsorbate layers of Bi on copper should form simple overlayer structures. However, it has been found recently that even immiscible metals can form surface alloys [1-5]. In this paper we will demonstrate that surface alloying also occurs in the case of Bi on Cu(ll0), which indicates that surface alloying is probably a very widespread phenomenon. Previously, the interaction of vapour-deposited Bi with Cu(110) has been studied * Corresponding author. Fax: +49 40 89982787; e-mail: [email protected]
by Clendening and Campell [6] with X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), and by low-energy electron diffraction (LEED). They found that adlayers of Bi on Cu(ll0) surfaces formed a series of superstructures at submonolayer coverages: at a coverage of 0.5 ML a c(2 x 2) LEED pattern, for Bi coverages in the range between 0.5 and 0.73 ML streaked patterns were observed, and at a coverage of 0.75 ML a p(4 x 1) LEED pattern was observed. The c(2x2) superstructure phase has been described in Ref. [6] as a quasi-hexagonal arrangement in which the Bi atoms were assumed to be placed in the four-fold hollow sites. The streaked LEED patterns were interpreted by Clendening and Campbell [6] as being due to the coexistence
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved 1)11 S0039-6028 ( 9 6 ) 0 1 1 5 1 - X
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L . Lottermoser et al. / Surface Science 373 (1997) 11-20
of p ( n x l ) - B i domains, where n=5-7. Each domain consisted of simple rows of Bi atoms located in adjacent Cu(ll0) troughs, and the Bi-Bi distance within the troughs decreased with increasing Bi coverage. The p ( 4 x l ) structure was explained in the same manner. Using a hard-sphere model, the Bi-Bi distance within the adjacent Cu troughs at the saturation coverage of 0.75 ML was calculated to be 3.41 .~. However, no previous crystallographic studies of the atomic geometry have been published. In this paper, we derive new structural models for the Cu(110)-c(2 x 2)-Bi and Cu( 110)-p(4 x 1)-Bi reconstructions, based on surface X-ray diffraction data. 2. Experimental
The experiments were performed at the Hamburg Synchrotron Radiation Laboratory (HASYLAB/DESY), and the samples were prepared at the Flipper II photoemission beamline. The Cu(110) crystal was cleaned by repeated cycles of Ar ÷ ion sputtering (500 eV) and annealing to 700°C until the characteristic (1 x 1) LEED pattern of the clean surface with sharp spots was observed and the photoemission spectra showed no traces of impurities. A Knudsen effusion cell with a boron nitride crucible was used to deposit Bi, and the deposition rate, calibrated with a quartz crystal monitor, was ~ 1.5 x 10 -3 A min -1. The pressure in the chamber during deposition was <2 × 10 -1° mbar. In the first experiment we prepared the c(2 x 2) structure at a Bi coverage of 0 = 0.5 ML, and in the second experiment the p(4 x 1) superstructure at a Bi coverage of 0=0.75 ML. Both samples were post-annealed at ~600 K, which improved the LEED pattern. The coverages quoted here were determined from the structural analysis. After preparation the sample was transferred to a portable ultra-high vacuum (UHV) chamber with a hemispherical beryllium window, which was mounted on the surface X-ray diffractometer at the wiggler beamline BW2.
fixed angle of incidence. To avoid fluorescence from the Cu crystal, the wavelengths of the X-ray beam were chosen to be 1.61 and 1.42 A, and the angle of grazing incidence was 0.43 ° for the c(2 x 2) reconstruction and 0.21° for the p(4x 1) superstructure, respectively. Integrated intensities were measured by rocking scans around the axis normal to the surface. In total, for the c(2 x 2) structure 25 fractional-order in-plane intensities (11 nonequivalent) and three fractional-order rods were measured. For the p(4 x 1) structure 126 fractionalorder and nine integer-order in-plane peaks were measured, out of which 42 and six, respectively, were non-equivalent. Subsequently, five fractionalorder and four crystal truncation rods were collected. The rod scans were measured by keeping the angle of incidence to the surface fixed and varying the exit angle with respect to the surface
[7]. To reduce systematic errors, the intensities of the symmetry-equivalent reflections were averaged, and the uncertainties were calculated from the counting statistics and the reproducibility of the symmetry-related reflections. The reproducibility between equivalent fractional-order in-plane intensities was found to be about 10%, probably due to sampling over different parts of the surface and small changes in the angle of incidence. The structure-factor intensities were obtained by correcting the measured integrated intensity with a Lorentz factor, for the variation in active sample area, and with polarization factors for the out-of-plane data [7]. The reflections are indexed with respect to a surface unit cell given by bl=[ll0]btak, b2= [001]bulk in the surface plane, and b3=[ll0]b~k normal to the surface plane. Hence, the real-space unit cell is given by al =l[-1T0]b~, a2= [001]b~, and a3 = ½[, 110]b~ak.
4. C u ( l l 0 ) - c ( 2 x 2)-Bi structure
A contour map of the Patterson function
P(x,Y)=Y~h.kIF[2 cos(2n(hx+ky)) provides a map 3. Data collection
The optical surfaces of the samples were aligned by total external reflection in order to ensure a
of the interatomic vectors within the unit cell [-7]. The contour map P(x,y), based on the fractionalorder structure reflections of the c(2 x 2) structure only, has a peak at the origin which suggests a
L. Lottermoser et aL / Surface Science 373 (1997) 11-20
simple quasi-hexagonal overlayer model with only one Bi atom in the surface unit cell. In the data analysis the goodness of fit between the model structure-factor intensities ]F~hk°d°X12 and the experimental structure-factor intensities tPhhk°del[2 and the experimental structure-factor intensities IF~Pl 2 was evaluated using either an R factor or g 2 given by (1)
Z2=
(2)
1
(Thk
/~2/.~2. 2 --87~ 2 " t'lUll
+k
2 2 2
2 2
/
where ahk are the uncertainties in the structurefactor intensities IFT,~Pl2, g is the number of data points and p the number of free parameters. The least-squares analysis of the in-plane fractional-order structure-factor intensities with a model including only the Bi overlayer (see Fig. 1) gave Z2 and R of 3.3 and 0.20, respectively, refining only an overall Debye-Waller factor for the Bi atom and a scale factor. By including an anisotropic Debye-Waller factor for the Bi atom, the fit could be significantly improved to Z2= 1.9 and R = 0.10. The agreement between the measured and calculated fractional-order structure-factor inten-
2 x
b2u22+/b3u33
4
(]f~'~Pl2--2--F~hk°dell2~2,Y,
N- p ~ \
sities are shown in Fig. 2. The calculated values were obtained using (u2t)=0.037+0.003.~2 and (U2z) = 0.023 _ 0.003 ~2 for the mean square vibration amplitudes of the Bi atom along the [110] and [001] directions, respectively. In the leastsquares analysis the mean square amplitudes were refined using the expression exp
R = ~h,k IIF~,~Pl2 - IPhhk°dell2I ~h.k IF~,~Pl2 '
13
(3)
)
for the anisotropic Debye-Waller factor, where a is the lattice constant of copper and hkl (l--0) are the Miller indices. From in-plane measurements alone it is not possible to obtain information about atomic displacements in the direction normal to the surface; out-of plane measurement are necessary. For the c(2 x 2) superstructure three fractional-order rodscans were measured, namely at (3/2,1/2,1), (3/2,3/2,1 ) and (1/2,5/2,1 ). The rod scans are shown in Fig. 3. The perpendicular momentum transfer l is in units of the reciprocal lattice vector ba. The monotonic variation of the measured rod-scans with momentum transfer perpendicular to the surface is clear evidence of a planar Bi overlayer. Using the structural parameters determined from
~k
3
.
.
.
.
2 ...............................
1 ...............
.,,-'-
.
.
.
'. . . . . . . . . . . . . . . . . . .
:
.......................
1
2
al
=
1[li0]
a 2 = [0011
Fig. 1. Top view of the Cu(ll0)-c(2 x 2)-Bi structural model. The dark circles are Bi atoms and lighter circles are Cu atoms. The dashed lines indicate a c(2 x 2) surface unit cell.
3
~1
Fig. 2. Comparison between the measured (filled semicircles) and calculated (open semicircles) in-plane structure-factor intensities for the c(2 x 2) overlayer model. The areas of the semicircles are proportional to the intensities.
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
14
40
+
r r
20 311) (~ 0 ,..i
--
J
I
I
I
I
I
J
I
,
I O.O
40
20
0
t
I
i
,
I 0.2
i
40
20 0
r 0.4 Qt[ree. lattice units]
Fig. 3. Structure-factor intensities of three fractional-order superlattice rods of the c(2 x 2)-Bi reconstructionas a function of momentum transfer Q, in the direction normal to the surface in reciprocallattice units of b 3. The best fit is indicated by the solid line. the least-squares analysis of the in-plane data, together with the planar Bi overlayer, yielded X2= 0.6 and R = 0.06 for the three fractional-order rods. The fit is indicated by the solid line in Fig. 3, and the mean square vibrational amplitudes for the Cu(110)-c(2 x 2)-Bi model are listed in Table 1.
5. Cu(ll0)-p(4 × 1)-Bi structure
5.1. In-plane data Fig. 4 shows a contour map of the Patterson function for the p(4 x 1) structure using the meaTable 1 Mean-square vibrational amplitudes h e x a g o n a l C u ( 110)-c(2 x 2 ) - B i m o d e l
c(2 x 2) Atom (u21)(A,z) structure Bi
for t h e p l a n a r
(u22)(A,2)
quasi-
(ua23)(A,z)
0.037±0.003 0.023+0.003 0.03+0.01
sured intensities of the in-plane fractional-order reflections. Two interatomic vectors, each represented by a peak of comparable amplitude, are seen in the Patterson map. The interatomic vectors of the Patterson map suggest an in-plane arrangement of the Bi atoms as shown in Fig. 4. In the first stage of the structural refinement, three Bi atoms with a common isotropic Debye-WaUer factor, an overall scale-factor and one positional coordinate were adjusted. The bismuth atoms on the mirror line in the al direction are only allowed to move along this line due to the symmetry (p2mm). The refinement yielded Z2=8.1 and R = 0.25. The high values of R and Xz indicate that something is missing in the trial model. The discrepancy between the trial model and the real structure was found using electron-density difference Fourier-synthesis, which is based on the assumption that the phases of the experimental structure factors can be approximated by the model phases, ~od~ [7,8]. This is normally valid provided the essential features of the structure are correct. The map of the electron-density difference can be calculated using
Ap(x,y) = p~Xp(x,y)- pm°a~t(x,y) 1 a h,k X COS [2~(hx + k y ) - ~od~l],
(4)
where a is the area of the unit cell. In Fig. 5 the electron-density difference map Ap is shown using IF~Pl and the calculated phases derived from the starting structural model. The positive peaks in the electron-density difference plot indicate electron density which is missing in the structural model, whereas the negative peaks indicate excess electron density in the trial model. The electron-density difference map shows a reduction in the electron density at the origin relative to a full occupation of the adatom sites by bismuth. The difference plot can be explained by two substantially different p(4 x 1) structural models. Starting from the simple overlayer model, one might imagine that the charge reduction is caused by partial occupation of the adatom sites at the origin by Bi atoms. By reducing the occupancy of the Bi atom at the origin to 0.68 we found that X2 improved to 4.9.
L. Lottermoser et aLI Surface Science 373 (1997) 11-20
I
....
:-
"-:
15
.-.!!!!!-77"..... ..
Iv
o,=½[liol
ID
I
O ..............................O Fig. 4. Contour map of the Patterson function of the p(4 x 1)-Bi reconstruction calculated from the in-plane fractional-order data. Positive contours are indicated by solid lines, and negative contours are dashed. The positive maxima suggest as a first approximation an atomic arrangement with three bismuth atoms per p(4 x 1) unit cell, located at (0,0), (1.4,0.5) and (2.6,0.5).
*,=½DTol
i
Fig. 5. Electron-density difference plot between the initial model and the experimental data. Positive contours are marked with solid lines, and negative contours are dashed.
T h e c o n t o u r m a p is also consistent with a structural m o d e l in which every f o u r t h C u r o o l ] r o w in the t o p m o s t l a y e r is r e p l a c e d b y Bi a t o m s , as s h o w n in Fig. 6. T h e positive p e a k s in the electrondifference p l o t are then due to the three s u b s t r a t e C u a t o m s which are missing in the model.
A s s u m i n g this substitution, b y s y m m e t r y the m o d e l has two free d i s p l a c e m e n t p a r a m e t e r s ; one is the i n t e r a t o m i c distance b e t w e e n the Bi a t o m s at p o s i t i o n s (1.39,0.5) a n d (2.61,0.5) a n d the second is the i n t e r a t o m i c distance between the C u a t o m s at p o s i t i o n s (1,0) a n d (3,0) within the surface unit
16
L. Lottermoser et al. /Surface Science373 (1997) 11-20
p(4xl)
f
0B~=0.75
it I .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
e
~
1.2 (R=0.12), respectively. In all the refinements the Debye-Waller factor of the Cu atoms was fixed at its bulk value (Be, = 0.55 A 2) [ 9 ] . The in-plane displacements of 0.06 A for the C u atoms, indicated by the short arrows in Fig. 6, are small but necessary in order to obtain g o o d fits with both models. Excluding these in-plane relaxations causes X2 to increase significantly to 3.3 and 3.02 for the substitutional and the overlayer model, respectively. To distinguish between the two models, the six in-plane integer-order reflections were included in the in-plane data analysis. These reflections contain contributions from b o t h the p(4 x 1) reconstructed surface layers and the bulk. Inclusion of the integerorder peaks yielded a X2 of 1.2 ( R = 0 . 1 2 ) for the substitutional model. The resulting agreement between the final substitutional model IF~hk°dell2and experimental structure-factor intensities IF~Pl 2 are shown in Fig. 7. The final overlayer model also agrees with the in-plane structure factor intensities to the same accuracy (Z 2 = 1.3 and R =0.11). Hence, the analysis of in-plane data alone is insufficient to distinguish between the two models. An unambiguous description of the structure can only be extracted by including the analysis of the crystal truncation rods in the refinement, as discussed in Section 5.2.
a1
Fig. 6. Top and side view of the final substitutional model of the Cu(ll0)-p(4 x 1)-Bi surface structure. The dark circles are Bi atoms and lighter circles are Cu atoms. The directions of the sub-surface displacements of the Cu atoms are indicated by arrows. The dashed lines indicate a (4 x 1) unit cell. cell. Including independent isotropic Debye-Waller factors for each of the two inequivalent Bi atoms, a c o m m o n scale factor and the two displacement parameters in the refinement improves g 2 to 3.62 and R to 0.15. In order to further improve the fit, anisotropic Debye-Waller factors for the Bi atoms must be taken into account in the structural analysis. This significantly improved the agreement between the measured and calculated in-plane intensities. We obtain for the simple overlayer with the partially filled adsorption sites at the origin and for the substitutional model z 2 = l . 1 ( R = 0 . 1 2 ) and Z2=
4 k,
-~D
t)--t)---o
;
- 43..........
: ~
® l ) - e ........ ~ ......... i
...... e
2
. . . . . . .
3
Fig. 7. Comparison between the measured (filled semicircles) and calculated (open semicircles) in-plane structure-factor intensities of Cu(ll0)-p(4× 1)-Bi for the final substitutional model. The areas of the semicircles are proportional to the intensities.
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
17
with corresponding values for ~od 2 = 1.13 and Rro d = 0.09. The directions of the atomic displacements of the Cu atoms, which were used in the least-squares analysis, are shown schematically in Fig. 6. The best fit for the overlayer model is indicated by the dashed curves in Fig. 8. The overlayer model has Z2= 3.7 with the same free parameters, which is significantly worse than for the substitutional model. The crystal truncation rods are very sensitive to the change in electron density in the near-surface region, and hence are expected to be significantly different for the two different models [7,10]. The overall scale factors of the two models were fixed at the values found in the fractional out-of plane refinements and the structural parameters were
5.2. Rod-scan analysis Five fractional-order rod-scans, as shown in Fig. 8, were used for the out-of-plane analysis of the p(4x 1) Bi structure. The starting structural models were those derived from the in-plane analysis and the Debye-Waller factors, and in-plane relaxations of the Cu atoms were fixed at the values determined by the in-plane refinement. In the least-squares analysis only the distances normal to the surface were allowed to vary. However, to obtain satisfactory agreement with the observed intensities, sub-surface relaxations and two different mean-square vibration amplitudes (u~3) of the Bi atoms along the surface normal had to be taken into account. Including these parameters yields the solid curve in Fig. 8,
Rod (-3/4,2,1)
Rod (3/4,0,1)
9 6 3 0
I
I
g
I
I
Rod (112,1,1)
Rod (7/4,1,1)
3 ~
2
0
I
I
I
I
Rod (514,0,1)
1,5
0,1
0,3
0,5
0,7
0,9
1,1
Qz [r. I. u.]
1,0 0,5
0,1
0,3
0,5
0,7
Qz [r. I. u.] Fig. 8. Structure-factor intensities of five fractional-order rods of the p(4 × 1)-Bi reconstruction as a function of Q, in reciprocal lattice units of b 3. The solid and dashed lines are the calculated intensities for the substitutional and the overlayer model, respectively, by including two different mean-square vibration amplitudes (u33)2 of the Bi atoms and sub-surface relaxations in the structural analysis.
18
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
fixed at the values listed in Table 2; hence there are no adjustable parameters. Fig. 9 shows the structure-factor intensities measured along four crystal truncation rods. The dashed curves correspond to the best overlayer model (Z2= 33). The solid curves were obtained with the substitutional model, and only this model gives a satisfactory fit to all of the measured integer-order rods (Z2= 3.9). One aspect which has not been incorporated in our structural model but is always present on metal surfaces is surface roughness. We have included an exponential distribution of heights I-8] in the final refinement to model the effect of statistical surface roughness on the intensities along the rods. The additional fit parameter fl corresponds to a rough surface in which layer 0 is assumed to be fully occupied, layer 1 above it has a fraction of/3 sites filled, layer 2 has fraction/32, and so on. For the ultimate refinement, the substitutional model including surface roughness was fitted to the merged dataset which included all the in-plane reflections, the crystal-truncation rods and the fractional-rods. With the free geometrical parameters and with the anisotropic Debye-Waller factors for the Bi atoms as listed in Table 2, a X2 of 1.6 (R = 0.08) was obtained. Replacing the anisotropic Debye-Waller factors with isotropic factors cause Z2 to increase significantly to 2.8. The details of the final substitutional model are listed in Table 2.
6. Discussion and conclusion On the basis of surface X-ray diffraction data we have established two new structural models
which describe the C u ( l l 0 ) - c ( 2 x 2) structure at 0 . 5 M L Bi coverage and the C u ( l l 0 ) - p ( 4 x l ) reconstruction at 0.75 M L Bi coverage. The results of the analysis of the C u ( l l 0 ) p(4 x 1)-Bi data indicates a massive rearrangement of the surface atoms similar to the alkali-metal induced reconstructions of fcc (110) metal substrates [11,12] and the structures of Pb on lowindex copper surfaces I-1,3-5]. For steric reasons it is not possible for all of the Bi atoms to be located in a highly coordinated adsorption site on the unreconstructed Cu(110) surface at a coverage of 0.75 M L - the Bi atoms are simply too large. The removal of every fourth [001] row of Cu atoms in the outermost substrate layer transforms the Bi position at the origin into a energetically preferred highly coordinated adsorption site. The two equivalent Bi atoms at the positions (1.39,0.5) and (2.61,0.5) within the surface unit cell are displaced laterally by 0.28 ,~ from an ideal four-fold hollow site of the unreconstructed Cu lattice. The energy cost for such displacements must be compensated by the interaction between these Bi atoms. The interatomic distance of the two Bi atoms of 3.12 ,~, obtained by our structural analysis, is identical to the bulk value 1-13]. The energy gained by forming a missing-row structure with all three Bi atoms in highly coordinated adsorption sites seems to outweigh the effect of displacing the Cu atoms to form this reconstruction. The corrugation of the missing-row layer induced by Bi adsorption was found to be 0.20 ,~, as shown in Table 2, and is probably associated with charge redistribution in the surface due to the Bi atoms at the (1.39,0.5) and (2.61,0.5) positions.
Table 2 Atomic coordinates of the best Cu( 110)-p(4 x 1)-Bi substitutional model, including statistical surface roughness (fl= 0.06); an asterisk indicates that the parameter was fixed by symmetry;two asterisks indicate parameters which were fixed in the least-squares structural refinement Atom
x
y*
z
Bi1 Bi2 Bi3 Cul Cu2 Cu3
0* 1.389___0.001 2.611 ___0.001 1.024_+0.002 2* 2.976 + 0.002
0 0.5 0.5 0 0 0
0.28 _ 0.02 0.72 ___0.02 0.72 + 0.02 0.00 0.08 + 0.01 0.00
B (A2)
0.55** 0.55** 0.55**
(u~l) (A2)
(u~2) (A2)
(u]3) (A2)
0.014 ___0.002 0.033 _ 0.002 0.033 __+0.002
0.022__+0.001 0.020 ___0.002 0.020 + 0.002
0.04 ___0.02 0.09 +__0.02 0.09 _ 0.02
19
L. Lottermoser et al. / Surface Science 373 (1997) 11-20 i
lO3 !
!
|
|
•
I
!
Rod (0,-2,1)
Rod (1,0,1) i!
I [
ll!
10 2
IE
.m
101 L
¢~_
10 0 %
10 -1
I
I
I
I
I
I
I
I
Rod (0,-1,1)
Rod (1,1,1) i0 z
•
101
10 °
10 -1
I
0,0
i
|
I
I
I
I
I
0,3
0,6
0,9
0,0
0,2
0,4
0,6
Qz [r. I . u . ]
Qz [r. !. u.]
Fig. 9. Structure-factor intensities of four crystal truncation rods of the p(4 × 1)-Bi reconstruction as a function of Qz in reciprocal lattice units of b3. The dashed curves were obtained with the overlayer model and the solid lines correspond to the final substitutional model.
20
L . Lottermoser et al. / Surface Science 373 (1997) 11-20
This would produce a reduction in the force constant between the Cu atoms localized close to these Bi positions and the Cu atoms localized close to the Bi atom at the origin. The weakening of the C u - C u bonds corresponds to a lowering of the energy threshold for a displacement of the copper row underneath the Bi atom at the origin during the initial formation of the p(4 × 1) reconstruction. Similar corrugation enhancement and redistribution of the surface charge density induced by lowcoverage Pb and Bi adsorption on Cu(001) have recently been reported [ 14], but without any substrate reconstruction. The relatively large vibrations amplitudes
new structural models which describe the c(2 x 2) and p(4 × 1) reconstructions of Bi on Cu(110).
Acknowledgements We wish to thank the staff of HASYLAB for their technical assistance. Financial support from the Danish Natural Science Research Council and from the Bundesminister ftir Forschung und Technologic (BMBF) under project No. 05622GUAB1 is gratefully acknowledged. One of us ( E i . ) acknowledges the Swedish Natural Science Research Council (NFR) for financial support.
References [1] C. Nagl, E. Platzgummer, O. Hailer, M. Schmid and P. Varga, Surf. Sci. 331-333 (1995) 831. I-2] L. Pleth Nielsen, F. Besenbacher, I. Stensgaard, E. L~egsgaard, C. Engdahl, P. Stoltze, K.W. Jacobsen and J.K. Norskov, Phys. Rev. Lett. 71 (1993) 754. I-3] C. de Beauvais, D. Rouxel, M. Michailov and B. Mutaftschiev, Surf. Sci. 324 (1995) 1. [4] C. Nagl, E. Platzgummer, O. Hailer, M. Schmid and P. Varga, Surf. Sci. 321 (1994) 237. [5] Y. Gauthier, W. Moritz and W. H6sler, Surf. Sci. 345 (1996) 53. [6] W.D. Clendening and C.T. Campbell, J. Phys. Chem. 90 (1989) 6656. 1-7] R. Feidenhans'l, Surf. Sci. Rep. 10 (1989) 105. ['81 I.K. Robinson and D.J. Tweet, Rep. Prog. Phys. 55 (1992) 599. 1,9] P.A. Flynn, G.M. McManus and H.A. Rayne, Phys. Rev. 123 (1961) 809. 1-10] I.K. Robinson, Phys. Rev. B 33 (1986) 3830. [11] W.C. Fan and A. Ignatiev, Phys. Rev. B 38 (1988) 366. 1-12] C.J. Barnes, M.Q. Ding, M. Lindroos, R.D. Diehl and D.A. King, Surf. Sci. 162 (1985) 59. [ 13] R.W.G.Wyckoff,Crystal Structures, 2nd ed. (Wiley, New York, 1967). [14] W. Li and G. Vidali, Phys. Rev B 47 (1992) 4356.