Available online at www.sciencedirect.com
Surface Science 601 (2007) 5162–5169
www.elsevier.com/locate/susc
Asymmetric adsorption-site of potassium atoms in the
(3 · 2)-p2mg structure formed on Cu(0 0 1)
Ming-Shu Chen 1, Seigi Mizuno, Hiroshi Tochihara
*
Department of Molecular and Material Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
Available online 4 May 2007
Abstract
The adsorption of K atoms on Cu(0 0 1) has been studied by low-energy electron diffraction (LEED) at room temperature (RT) and
130 K. At RT, a (3 · 2)-p2mg LEED pattern with single-domain was observed at coverage of 0.33, whereas the orthogonal two-domain
was found at 130 K. At 130 K, a c(4 · 2) pattern with orthogonal two-domain was observed at coverage 0.25. Both the (3 · 2)-p2mg and
c(4 · 2) structures have been determined by a tensor LEED analysis. It is demonstrated that K atoms are adsorbed on surface fourfold
hollow sites in the c(4 · 2), while in the (3 · 2) structure two K atoms in the unit cell are located at an asymmetric site with a glide-reflection-symmetry. The asymmetric site is at near the midpoint between the exact hollow site and bridge-site but slightly close to the hollow
site. A rumpling of 0.07 Å in the first Cu layer was confirmed, which might stabilize K atoms at the asymmetric site. Surface structures
appearing in a coverage range 0.25–0.33 are discussed in terms of the occupation of the asymmetric site with increase of coverage.
2007 Published by Elsevier B.V.
Keywords: Surface structure, morphology, roughness, and topology; Potassium; Cu(0 0 1)
1. Introduction
The adsorption of alkali-metal atoms on metal surfaces
has been extensively studied as a prototype of chemisorption [1–4]. Especially, the adsorption system of K atoms
on fcc(0 0 1) surfaces has been typical of the individual
adsorption of alkali metals [2,3] and studied intensively
by theory [5,6] and experiment [7–13], due to its technological application to catalysis and scientific interest. One of
interesting points of this adsorption system is that the formation of so-called higher-order commensurate (HOC)
structures [2,9,12] formed at coverages larger than 0.25.
(Hereafter, the coverage is defined as the ratio of density
of adsorbate atoms with respect to that of substrate atoms
in the ideal top layer.) Since the size of K atoms is larger
than Cu or Ni atoms (the atomic size ratios of K with respect to Cu and Ni are 1.8 and 1.9, respectively), the second
nearest-neighbor hollow sites are inhibited for K atoms to
occupy. Therefore, a c(2 · 2) overlayer structure is not
formed on these surfaces. On Ni(0 0 1), the c(4 · 2) structure was determined by low-energy electron diffraction
(LEED) analysis [11]: K atoms are located at fourfold hollow sites at coverage 0.25, where the third and the fourth
nearest-neighbor sites are occupied. On Cu(0 0 1), it was
confirmed that K atoms occupy hollow sites randomly at
coverages lower than 0.25 at 330 K from a study using surface X-ray diffraction (SXRD) [8]. Thus, it has been considered that the fourfold hollow site is the preferable site for K
atoms on Cu(0 0 1) and Ni(0 0 1) at coverages 0.25 and
below.
As the HOC structures in the system of K on Cu(0 0 1),
so far, c(10 · 2)2 and (3 · 2)3 structures were demonstrated
at coverages 0.30 and 0.33, respectively, by LEED observation. The c(10 · 2) pattern was observed also in the system
of K on Ni(0 0 1), but the (3 · 2) was not found [12]. The
*
Corresponding author.
E-mail address: [email protected] (H. Tochihara).
1
Present address: Department of Chemistry, Texas A & M University
College Station, Texas 77843, US.
0039-6028/$ - see front matter 2007 Published by Elsevier B.V.
doi:10.1016/j.susc.2007.04.167
2
3
In Refs. [2,9,12], the c(10 · 2) was represented by a matrix.
In Ref. [9], the (3 · 2) was shown by a matrix.
M.-S. Chen et al. / Surface Science 601 (2007) 5162–5169
(3 · 2) pattern was observed also in the adsorption of K on
Ir(0 0 1) at 100 K [14], and on Ag(0 0 1) at 90 K [15]. And a
structural model for the (3 · 2) was proposed on Cu(0 0 1)
and Ir(0 0 1) [9,14]: one K atom at the hollow site and another at the bridge-site in the unit cell. The bridge-site
seems to be unlikely but there has been no report on the
determination of these structures.
In the present study, we focus on the (3 · 2) structure
formed on Cu(0 0 1) by K adsorption. We observed singledomain (3 · 2) LEED patterns at room temperature (RT),
while orthogonal two-domain (3 · 2) patterns were found
at 130 K. It is found for the first time that there exist missing
spots in the (3 · 2) patterns due to a glide-reflection-symmetry, whose structure is correctly represented by the (3 · 2)p2mg. We have determined the (3 · 2)-p2mg structure by a
tensor LEED analysis. It is demonstrated that K atoms
are located at an asymmetric site in the (3 · 2) unit cell:
one is at near the midpoint between the hollow- and
bridge-site and that another occupies an equivalent asymmetric site but at the position with a glide-reflection symmetry. In addition, it is found that a rumpling in the top Cu
layer takes place. The previously proposed structure is
incorrect, and it is strongly demonstrated that reliable structural analysis is essential to obtain a complete picture of the
adsorption of alkali metal atoms on metal surfaces.
2. Experiment and calculation
The experiments were carried out in a UHV chamber
(base pressure of 1 · 1010 Torr) equipped with a commercial LEED system. The cleanliness of the Cu(0 0 1) surface
was achieved by repeated cycles of Ar+ bombardment
(0.5 keV, 1.5 lA) and annealing to 910 K. K atoms were
evaporated from a commercial source (SAES Getters
Inc.). The K coverage was calibrated by the presence of
the clearest c(4 · 2) LEED pattern at 130 K as a coverage
of 0.25, whose structure has been determined in the present
study below. LEED spot intensities were recorded by our
computer controlled video LEED system at a sample temperature of 130 K. Details of the experiment were similar to
the previous study [16].
A Barbieri/Van Hove symmetrized automated tensor
LEED package was used to calculate I–V curves for structure models [17]. Thirteen phase shifts were used to calculate atomic scattering. The programs searched for
agreement with experiment by minimizing the Pendry Rfactor, RP [18]. The real part of the inner potential was
determined during the course of the theory-experiment
fit. The damping was represented by an imaginary part of
the potential, Voi, of 5.0 eV.
*: splitting pattern,
130 K
disorder
disorder
RT
0
0.1
5163
: single domain (3x2), #: two-domain (3x2)
c(4x2)
*
halo
*
0.2
K coverage
0.3
#
hex.
hex.
0.4
Fig. 1. LEED pattern changes as a function of nominal K coverage on
Cu(0 0 1) at 130 K and RT.
the deposition was not continuous, nominal coverages are
determined with an accuracy of 0.02 as shown by tilted
bars separating different LEED patterns in Fig. 1. At
130 K, an orthogonal two-domain pattern of the c(4 · 2)
was observed at coverages 0.20–0.25, and intensities of
equivalent spots from the two domains were similar. With
increasing K coverage, splitting patterns appeared. That is,
half-order spots split into two. For example, the (1/2, 0)
spot in the c(4 · 2) pattern split in two spots at ((1/
2 ± x), 0) and their distance (denoted splitting distance
s = 2x) increased with increasing coverage. Hereafter, these
patterns are denoted as the splitting patterns. Actually, the
splitting patterns can be explained by double diffraction.
The distance s continuously increased to 1/3 of an integer
spot distance ((0, 0) to (1, 0)) at coverage of 0.33, where a
(3 · 2) pattern with orthogonal two-domain was observed
as shown in Fig. 2a. There are missing spots such as (1/
2, 0) in the (3 · 2) patterns. Eventually, hexagonal LEED
patters were observed, which were reported as the rotational epitaxy [10]. On the other hand, at RT, halo patterns
3. Results
3.1. LEED observations
Changes of LEED patterns in the adsorption of K on
Cu(0 0 1) at 130 K and RT are summarized in Fig. 1. Since
Fig. 2. Normal incident LEED patterns of the (3 · 2) formed on Cu(0 0 1)
by K deposition (a) at 130 K, (b) and (c) at RT. (d) A schematic
illustration of (c).
5164
M.-S. Chen et al. / Surface Science 601 (2007) 5162–5169
appeared at coverages 0.18–0.26. The c(4 · 2) pattern was
not observed probably due to thermal fluctuation. With
further deposition, two-domain splitting patterns appeared
with highly unequal spot-intensity. The splitting distance
increased with increasing coverage as observed at 130 K.
Then, the single-domain (3 · 2) pattern was observed with
missing spots in Fig. 2b and c. The I–V curves from this
single-domain were identical with those from the two-domain formed at 130 K. The formation of such single-domain is an unusual phenomenon on the fcc(0 0 1) surface.
Therefore, we repeated the sample cleaning several times,
but always the single-domain was observed in the same
direction, i.e., always (3 · 2) but no (2 · 3). The changes
of the LEED patterns at RT are basically similar to those
previously observed [9], but we have found very important
different features in the (3 · 2) LEED patterns: the missing
spots and the single-domain formation.
placement in the top layer Cu atoms from the bulk
position. The obtained parameters give a K–Cu bond distance of 3.25 ± 0.02Å, resulting in an effective K radius
of 1.97 Å. This value is similar to that in the Ni(0 0 1)c(4 · 2)-K structure determined by a dynamical LEED
analysis [10], in which an effective K radius of 1.96Å was
obtained. The interlayer distance 2.70 ± 0.02Å is obviously
larger than 2.25 ± 0.15Å obtained by SXRD [8] at lower
coverages on Cu(0 0 1) at RT, where K atoms are randomly
located at hollow sites. Different coverages might cause
such the significant difference, since it was reported that
the interlayer distance increases with increasing coverages
of alkali metals on metal surfaces [13].
3.3. Determination of the (3 · 2)-p2mg structure
As mentioned above, single-domain (3 · 2) LEED patterns with missing spots are shown in Fig. 2b and c.
Fig.
2d isa schematic of (c). The missing spots are located
at 0; 2n1
, where n is integer, along the short-lattice direc2
tion of the (3 · 2) periodicity (denoted the [1 1 0] direction
hereafter) as shown in Fig. 2d. Such the regular absence
of spots leads to the conclusion that there exists one glide
plane along the [1 1 0] direction. The comparison of I–V
curves of corresponding spots shows that there is a mirror
plane along the [1 1 0] direction. Thus, the observed LEED
pattern can be assigned to the single-domain of the (3 · 2)p2mg structure. The (3 · 2) pattern observed at 130 K is assigned to the double-domain (3 · 2)-p2mg structure. We
used I–V curves obtained from the single-domain for the
determination of the (3 · 2)-p2mg structure.
Fig. 4b shows the previously proposed (3 · 2) structure
[9]: in the unit cell, one K atom is located at the fourfold
3.2. Determination of the c(4 · 2) structure
First, we have determined the c(4 · 2) structure formed
at 130 K. RP = 0.172 is achieved for a structure shown in
Fig. 3a by allowing the displacement of the second layer
Cu atoms and optimizing the Debye temperatures for K,
Cu in the first layer, second layer and bulk to 180, 240,
280 and 335 K, respectively. Eleven inequivalent beams
with total energy range of 1850 eV were used in the tensor
LEED calculation. The comparison of the experimental I–
V curves with theoretical ones is shown in Fig. 3b. The
good agreement of I–V curves and a low RP value indicate
the structure in Fig. 3a is correct. K atoms were located at
surface fourfold hollow sites at a height above the top layer
Cu atoms, dh = 2.70 ± 0.02 Å. There is no obvious dis-
Exp.
Theo.
Cu
Exp.
Theo.
Cu
(1/2,1)
K
(1/4,1/2)
(5/4,1/2)
(1,1)
(3/2,0)
top-view
(2,1)
(2,0)
dh
(1/2,0)
(1/4,3/2)
(3/4,1/2)
(1,0)
A'
A
side-view
50
100
150
200
250
Energy (eV)
300 50
100
150
200
250
Energy (eV)
300
Fig. 3. (a) Top- and side-views of the best-fit Cu(0 0 1)-c(4 · 2)-K structure. (b) Comparison between experimental (solid) and theoretical (dashed) I–V
curves for the best fit Cu(0 0 1)-c(4 · 2)-K structure.
M.-S. Chen et al. / Surface Science 601 (2007) 5162–5169
m
5165
[110]
m1
g
[110]
m2
Fig. 4. (a) Top- and (c) side-views of the (3 · 2)-p2mg structure (GM model), and the arrows therein show the possible displacement-directions. (b) Topview of the (3 · 2) structure with one K at bridge-site and another one at hollow site (MM model). Smaller blank and gray spheres are Cu atoms in the first
and second layers, respectively. Larger blank spheres are K atoms with smaller circles indicating the atomic centers. See text in detail.
hollow site and another one at the twofold bridge-site. Two
K atoms should be in the (3 · 2) unit cell, because the K
coverage is about 0.33 as shown in Fig. 1. This model
has two mirror planes, m and m 0 in Fig. 4b, but no glide
plane. Therefore, this model does not satisfy the p2mg symmetry, and is denoted MM model. Here, we propose a new
model in Fig. 4a, where two K atoms in the unit cell are located at an asymmetric site between the hollow- and
bridge-site. In this structure, there are one mirror plane
m along the [1 1 0] direction and one glide plane g along
the [1 1 0] direction, which satisfy the p2mg symmetry. This
model is denoted MG model.
We have performed the tensor LEED calculation under
the p2m symmetry for comparison of MM and GM model,
because the present tensor LEED calculation program [19]
still does not fully implement the determination of structures including a glide plane. Displacements of the second
layer Cu atoms were allowed, and Debye temperatures were
optimized to 180, 240, 250, 290 and 335 K for K, Cu1, Cu2
(see Fig. 4c), Cu atoms in the second layer and Cu atoms in
the bulk, respectively. A total energy range of 3800 eV over
twenty-four inequivalent spots was used in the theoretical
comparison. Thus obtained RP is 0.16 for GM model, while
0.19 for MM model. GM model shows better agreement
than MM model. It is noted again that MM model does
not satisfy the p2mg symmetry. The calculated I–V curves
of GM model are compared with experimental ones in
Fig. 5. The agreement between theory and experiment is
good. Thus, it is concluded that GM model is correct.
However, the above calculation for GM model was done
under the p2m symmetry, in which the number of structural
parameter is 26, while it should actually be only 12 under
the p2mg symmetry. Possible displacements under the
p2mg symmetry for the twelve structural parameters are
shown by arrows in Fig. 4a and c–e. For getting more reliable structural parameters, we have performed a fixedparameter test reflecting the p2mg symmetry as follows:
one structural parameter among the twelve in the p2mg
symmetry was fixed, while other eleven parameters were allowed to displace to optimize under the p2m symmetry.
Then, we slightly changed the initial fixed value, and similarly an optimized RP value was obtained. Finally, a plot of
RP with respect to that parameter was acquired, in which
the smallest RP corresponds to the most suitable value
for that parameter. The error
range was obtained from
qffiffiffiffiffiffiffiffi
the variance of RP, DR ¼ Rm 8jVDEoi j, where Rm is the minimum RP achieved in each structural parameter test [17]
and DE is the total energy range.
First, we determined the height and the lateral position
of K atoms following the procedure mentioned above. In
the (3 · 2) unit cell, two K atoms exist as shown in
Fig. 4a. Their heights above the top layer Cu atoms should
be the same and their lateral positions should shift by the
same distance but towards opposite directions, i.e., [1 1 0]
and [1 10], due to the requirement of the p2mg symmetry.
Thus obtained RP values are plotted against fixed heights
in Fig. 6a as well as against fixed lateral displacements from
the exact hollow site. The height of K atoms above the
ideal position of the top layer Cu atoms was thus determined to be 2.78 ± 0.05 Å. And K atoms are located at
1.89 ± 0.07 Å from the glide plane g, corresponding to
the distance of 0.61 Å from the exact hollow site. Therefore, K atoms are located at near the midpoint (0.638 Å)
between the exact hollow site and bridge-site. In detail, K
atom is slightly closer to the hollow site than the midpoint
by 0.028 Å.
Second, the first layer Cu atoms are classified into two
groups according to the p2mg symmetry: two Cu1 atoms
and four Cu2 atoms (see Fig. 7). The RP values are displayed in Fig. 6b as a function of height of Cu1 or Cu2
atoms. Cu1 atoms are located outward by 0.06 ± 0.03Å
from the ideal bulk position, and Cu2 slightly inward by
0.01 ± 0.02 Å, resulting in a net rumpling of 0.07 Å for
the first Cu layer. The lateral displacement of two Cu1
atoms is symmetrically inhibited. But four Cu2 atoms are
allowed to displace along both the [1 1 0] and [1 1 0] directions, and obtained RP values are shown in Fig. 6c. Cu2
atoms are slightly displaced away from K atoms along both
the [1 1 0] and [1 1 0] directions by (0.02 ± 0.05) Å from the
ideal position.
5166
M.-S. Chen et al. / Surface Science 601 (2007) 5162–5169
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
(1,1/2)
(1,1)
(4/3,0)
(1/3,3/2)
(1,3/2)
Intensity (arb. units)
(4/3,1/2)
(2/3,1/2)
(1/3,0)
(2/3,3/2)
(0,2)
(4/3,3/2)
(1,2)
(2,0)
(0,1)
(1/3,1)
(2,1)
(5/3,1/2)
(5/3,0)
(1/3,1/2)
(4/3,1)
(2/3,0)
(1,0)
(2/3,1)
50
100 150 200 250 300 50
Energy (eV)
100 150 200 250 300
Energy (eV)
50
100 150 200 250 300 50
Energy (eV)
(5/3,1)
100 150 200 250 300
Energy (eV)
Fig. 5. Comparison between experimental (solid) and the best fit theoretical (dashed) I–V curves for the Cu(0 0 1)-(3 · 2)-p2mg structure.
Third, the second layer Cu atoms (Cu3, Cu4, Cu5 in
Fig. 7) can be displaced, and RP values are plotted against
heights and lateral displacements in Fig. 6d and e, respectively. Slight increase of the heights from the ideal position
is discernible: 0.04 ± 0.03, 0.02 ± 0.03 and 0.01 ± 0.03 Å
for Cu3, Cu4 and Cu5, respectively. The lateral displacements are within errors (Fig. 6e).
Thus, the twelve structural parameters of the (3 · 2)p2mg structure were obtained, and are summarized in
Table 1 and Fig. 7.
4. Discussion
The present LEED analysis of the (3 · 2)-p2mg structure
demonstrates that K atoms are adsorbed at the asymmetric
site on Cu(0 0 1) at coverage 0.33, indicating that GM model is more reasonable than MM model. Here, we briefly
consider the reason why the asymmetric site is favored.
Here, we assume that the lateral variation in the adsorbate–substrate interaction energy along the [1 1 0] direction
through hollow sites is represented by a sinusoidal potential having the symmetry of the Cu(0 0 1) surface as shown
in Fig. 8. In MM model (cf. Fig. 8b), the adsorption energy
of K atoms on the Cu(0 0 1) surface is an average of those
at the hollow site and the bridge-site, corresponding to the
energy at the midpoint between the hollow- and bridge-site
(cf. broken line in Fig. 8b). On the other hand, in GM mod-
el (cf. Fig. 8a), the potential energy at the asymmetric site
in the present study is less than that at midpoint as shown
by a solid line in Fig. 8a, because K atoms are located between the midpoint and the exact hollow-site (denoted by
small circles). Thus, the asymmetric site provides a larger
adsorption energy, and MM model is not favored.
Here, we note structural features of the atomic geometry
of K adatoms at the asymmetric site. Two K atoms in the
unit cell are located at positions where they satisfy the
glide-reflection-symmetry as shown in Fig. 7a. Their local
structures are equivalent. Therefore, each K atom bonds
with four surface Cu atoms, resulting in the two pairs of
K–Cu bond distances, l1 and l2, shown in the side view of
Fig. 7a. Values of l1 and l2 are obtained to be 3.55 ± 0.03
and l2 = 3.15 ± 0.03 Å, respectively. It should be noted
that the rumpling 0.07 Å in the first layer Cu atoms mentioned above is taken into account for obtaining l1 and
l2. Suppose there is no rumpling, then l1 = 3.57 Å and
l2 = 3.11 Å. Thus, the rumpling slightly contributes to stabilize K atoms at the asymmetric site, because l2 = 3.11 Å
is too short as the bond distance between Cu and K atoms
(note that it is 3.25 Å in the c(4 · 2) structure mentioned
below). Small displacements of surface Cu atoms shown
by arrows in Fig. 7c are also helpful for occupying the
asymmetric sites: Surface Cu1 atoms change their positions
slightly to accommodate K atoms more effectively in nearly
hollow sites. Similar rumpling of substrate surface atoms
M.-S. Chen et al. / Surface Science 601 (2007) 5162–5169
0.32
Lateral distance ( A )
1.72 1.77 1.82
1.87
1.92 1.97
5167
0.26
2.02
Cu2
0.33
0.28
0.24
Cu1
Pendry R-factor
0.30
[110]
0.27
0.24
0.22
0.20
0.20
[110]
0.24
0.21
0.18
2.70 2.73
2.76 2.79
2.82
2.85 2.88
0.16
-0.07 -0.03
0.01
0.05 0.09
Height ( Å )
Height ( Å )
d
Pendry R-factor
0
0.08
Lateral displacement ( Å )
0.21
Cu4
Cu4
Cu5
0.20
Cu3
0.19
0.20
Cu3
0.18
Cu5
0.18
0.16
-0.08
e
0.24
0.22
0.18
-0.16
0.13
0.17
-0.04
0.04
0
Height ( Å)
0.08
0.16
-0.12 -0.08 -0.04 0 0.04 0.08 0.12
Lateral displacement ( Å)
2
2 1
1
2
2
1
4
3
4
3
5
4
3
5
top-view
g
4
3
5
4
3
5
4
3
3
4
5
3
4
5
3
4
m
4
3
5
4
3
5
4
3
[110]
Fig. 6. Plots of Pendry R-factors vs. given values in the fixed-test for twelve structural parameters in the Cu(0 0 1)-(3 · 2)-p2mg structure. (a) Height and
lateral displacement of K atoms; (b) vertical displacements of Cu1 and Cu2 atoms in the first Cu layer; (c) lateral displacements of Cu2 atoms; (d) vertical
displacements of Cu3, Cu4 and Cu5 atoms in the second Cu layer; and (e) lateral displacements of Cu3, Cu4 and Cu5 atoms.
3
4
5
3
4
5
3
4
4
3
5
4
3
5
4
3
[110]
c
dh2
side-view
l2 l1
dh1
1
2
2 1
2
2 1
1
2
2 1
2
2 1
1
2
2 1
2
2 1
1
2
2 1
2
2 1
Fig. 7. (a) Top- and side-views of the best fit Cu(0 0 1)-(3 · 2)-p2mg-2K structure. Top-views of (b) the second and (c) the first layer Cu atoms. Small blank
and gray spheres represent Cu atoms in the first and second Cu layers, large blank circles are K atoms. In-equivalent Cu atoms are numbered. Arrows
indicate determined displacement-directions. See text in detail.
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M.-S. Chen et al. / Surface Science 601 (2007) 5162–5169
Table 1
Optimum parameters of the Cu(0 0 1)-(3 · 2)-p2mg structure illustrated in
Fig. 7a
Atom
Lateral displacement (Å)
Height (Å)
Interlayer distance (Å)
K
Cu1
Cu2
Cu3
Cu4
Cu5
0.61 ± 0.07
6.39 ± 0.03
3.67 ± 0.03
3.60 ± 0.02
1.85 ± 0.03
1.83 ± 0.04
1.82 ± 0.03
dh = 2.75
0.03 ± 0.05
0.00 ± 0.09
0.02 ± 0.07
0.02 ± 0.05
‘‘Lateral displacement’’ refers to displacements from the center of the
hollow site (the directions are shown in Fig. 7). ‘‘Height’’ is measured from
the plane A–A 0 in Fig. 7a. ‘‘dh’’ is the averaged spacing between K and the
top layer Cu atoms.
a
creased with increasing coverage as in the literature [9,12].
Previously, it had been suggested that such splitting were
due to the continuous, uniaxial compression of the HOC
structure along the [1 1 0] direction, forming an equal K–
K distances along that direction [9,12]. Such suggestion
inevitably results in the occupation of various sites for K
atoms, and we denote such occupation of various sites as
various sites (VS) model. For example, in the (7 · 2) VS
model (see Fig. 9c) at a coverage of 2/7 = 0.286, there are
four K atoms in the unit cell, in which one K atom at hollow site, one at bridge-site, and two at the midpoint between bridge- and hollow-sites. In the c(10 · 2) VS model
with a coverage of 0.30 [9,12] shown in Fig. 9d, two K
atoms are at hollow sites, but another four atoms at positions near the bridge-sites. Thus, MM model of the (3 · 2)
structure in Fig. 4b is classified into VS model. As men-
midpoint
asymmetry
site
a
b
bridge-site
hollow-site
b
Fig. 8. Variation of adsorption energy between K and surface Cu atoms
along [1 1 0] through hollow sites for (a) GM model, (b) MM model.
has been reported in the system of K on Al(1 1 1) at 90 K,
where K atoms are at the on-top sites [21].
Next, we note K–K bond distances in the (3 · 2)-p2mg
structure. The first and second nearest-neighbor K–K distances are 4.56 ± 0.07 and 4.64 ± 0.07 Å, respectively.
The distance 4.56 Å may be the shortest attainable in K
monolayers on Cu(0 0 1), because further deposition of K
on the (3 · 2)-p2mg structure led to the formation of hexagonal structures (cf. Fig. 1). That is, the K overlayer begins
to rotate with respect to the Cu(0 0 1) surface (known as
the rotational epitaxy) obeying the theory of NovacoMcTague [20]. In the densest rotated K monolayer, the
shortest K–K distance was reported to be 4.57 Å [10],
which is in good agreement with the shortest K–K distance
in the (3 · 2)-p2mg structure. Therefore, K atoms at the
asymmetric site keep this position balancing two opposite
interactions: repulsive K–K interaction and attractive
interaction of K with the hollow site of the substrate surface. On Ni(0 0 1), the (3 · 2) structure is not formed [12].
This can be explained as follows: the shortest K–K distance
becomes less than 4.56 Å, suppose that K atoms occupy the
equivalent asymmetric site on Ni(0 0 1) whose lattice constant is smaller than that of Cu(0 0 1).
Having determined the c(4 · 2) and (3 · 2)-p2mg structures, we discuss K monolayer structures formed at coverages 0.25–0.33. As described above, the splitting LEED
patterns appeared there and the splitting s continuously in-
c
d
e
f
[110]
Fig. 9. Schematic illustration of K atoms (small open circles) on grids
representing the Cu(0 0 1) surface. Surface Cu atoms are located at cross
points of the grids. (a) c(4 · 2), (b) (3 · 2)-p2mg, (c) (7 · 2) VS model, (d)
c(10 · 2) VS model, (e) (7 · 2) AS model, and (f) c(10 · 2) AS model.
M.-S. Chen et al. / Surface Science 601 (2007) 5162–5169
tioned above, the (3 · 2) VS model was rejected, so it seems
that other VS models are also inappropriate.
We propose asymmetric site (AS) model based on the
structural feature of the (3 · 2)-p2mg structure obtained
in this study: the asymmetric site is occupied by K atoms
as well as the hollow site. We assume that the asymmetric
site in AS model is the same position as that determined in
the (3 · 2)-p2mg structure. The (7 · 2) and c(10 · 2) AS
models are shown in Fig. 9e and f, respectively. In the
(7 · 2) AS model, two K atoms at the hollow site and other
two at the asymmetric site; In the c(10 · 2) AS model, one
at the hollow site and other four at the asymmetric site. It is
noted that identical sites are occupied by K atoms along
the [1 1 0] direction as shown in Fig. 9. Therefore, structural
changes of the K overlayer in the coverage range 0.25–0.33
can be explained by introduction of the asymmetric-site
column into the c(4 · 2) structure in Fig. 9a where only
the hollow-site columns exist. That is, with the increase
of coverage, the number of K atoms at the asymmetrical
site increases at cost of the hollow site (see Fig. 9e and f).
Eventually, no hollow site is taken at coverage 0.33, where
the (3 · 2)-p2mg structure in Fig. 9b is completed. It should
be noted that the asymmetric site column always makes a
pair, in which K atoms are located at positions of the
glide-reflection-symmetry.
Finally, we discuss the reason that the single-domain
(3 · 2)-p2mg structure is formed at RT. As described
above, the adsorption of K at the asymmetric site induces
the rumpling in the first layer Cu atoms, resulting in the
formation of ridges of Cu1 in Fig. 7a along the [1 1 0] direction on the Cu(0 0 1) surface. At the domain boundaries the
ridges are not always coincide with those in other domains,
and they become energetically unstable there. Therefore,
such boundaries are canceled and the single-domain
(3 · 2)-p2mg structure prevails the surface. At 130 K, the
two-domain (3 · 2)-p2mg structure was formed, implying
that there is an activation barrier to rearrange the rumpling
to single-domains.
5. Conclusion
A single-domain (3 · 2)-p2mg structure formed by the
adsorption of K on Cu(0 0 1) at RT has been found and
determined by LEED. K atoms are located at a position
near to the exact hollow sites but displaced by 0.61 Å along
5169
[1 1 0]. Two K atoms in the unit cell are located at positions
satisfying a glide-reflection-symmetry. A rumpling of
0.07 ± 0.02 Å is found in the first layer Cu atoms, which
helps K atoms to be stabilized at the asymmetric site. At
130 K, the c(4 · 2) and (3 · 2)-p2mg structures with equivalent orthogonal two-domains were formed at coverages of
0.25 and 0.33, respectively. In the coverage range 0.25–
0.33, we suggest that K atoms occupy the asymmetrical site
at the cost of the hollow site with increasing coverage.
Acknowledgement
One of the authors (H.T.) appreciates partial financial
support from Iketani Science and Technology Foundation.
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