PHYSICAL REVIEW B

VOLUME 60, NUMBER 24

15 DECEMBER 1999-II

Adsorption and growth of Xe adlayers on the Cu„111… surface
Ji-Yong Park, S.-J. Kahng, U. D. Ham, and Y. Kuk*
Department of Physics and Center for Science in Nanometer Scale, Seoul National University, Seoul 151-742, Korea

K. Miyake and K. Hata
Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,
Tsukuba 305-8573, Japan

H. Shigekawa
Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba, Tsukuba 305-8573,
Japan
and Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo,
Tokyo 113-8656, Japan
共Received 24 May 1999兲
The adsorption and growth of Xe layers on the Cu共111兲 surface were studied with a low-temperaturescanning tunneling microscope. Initially, Xe atoms preferentially adsorb at the step, revealing two different
wetting behaviors at the upper and the lower step edges at the coverage of ⬍0.1 monolayer. Three-dimensionalisland growth is followed on the terrace at the coverage of ⬎0.2 monolayer when grown at ⬍20 K. The island
growth is attributed to inhomogeneous nucleation and lower diffusivity of Xe on the Xe monolayer than on the
Cu共111兲 surface. The diffusion barriers, the two-dimensional barrier on a terrace and the one-dimensional
barrier along a step, and the step-down barrier determine the growth morphology of Xe layers as the substratetemperature was raised. 关S0163-1829共99兲00248-9兴

I. INTRODUCTION

Rare-gas layers on various surfaces have been regarded as
two-dimensional 共2D兲 model systems due to their closedshell electronic structure and weak interaction with substrates and among themselves. Because of their simple electronic structures, the adsorbed structures and theirtransformations with coverage and temperature have beensuccessfully described only with the consideration of simple
mutual interaction such as van der Waals interaction. In
many experiments, graphite has often been chosen as a substrate due to its weak interaction with rare-gas atoms. Rather
complex adsorbed structures were observed when rare gases
adsorbed on metal surfaces such as Pt, Ag, Au, Cu, Pd, andRu.1 2D phase transitions such as the commensurateincommensurate transition 共C-I transition兲, the roughening
transition, and the wetting transition are just a few examples
of topics which have been studied on various substrates.1
The wetting behavior has been one of the most studied subjects, as it can be explained as a growth kinetics.2 When anadsorbate wets the surface and grows in a layer-by-layermode, the growth behavior is called ‘‘complete wetting’’ 关orFrank–van der Merwe 共FM兲 growth兴. When an adsorbate
grows in a layer-by-layer for initial several layers before
three-dimensional 共3D兲 islands are formed on top of them, it
is called ‘‘incomplete or partial wetting’’ 关or StranskiKrastanov 共SK兲 growth兴. When 3D islands are formed on topof the substrate, it is called ‘‘nonwetting’’ 关or Volmer-Weber
共VM兲 growth兴. It was shown that the competition between
the adatom-substrate interaction and the adatom-adatom interaction determines the growth behavior of rare-gas atoms.3
Although many experimental tools have been used to ex0163-1829/99/60共24兲/16934共7兲/$15.00

PRB 60plore these systems and succeeded in understanding thephysics of the rare-gas adsorption, the real-space informationis needed for better understanding. Eigler et al.4 first showedisolated Xe atoms adsorbed on Ni共110兲 and explained themechanism of how Xe atoms can be imaged with scanningtunneling microscopy 共STM兲 without the apparent electronic
states near the Fermi level. The following studies of rare-gas
adsorptions have mainly dealt with the subject of atomic
manipulation by the STM tip.5 The adsorption and growth of
Xe atoms on Pt共111兲 were studied by two groups, showing
heterogeneous nucleation at defects, followed by layer-bylayer growth.6,7
In this paper, the adsorption and growth mode of Xe atoms were studied as functions of coverage and temperature.
Xe atoms initially nucleate at the lower step edges and form
one-dimensional 共1D兲 chains, and then wet both the lower
and upper step edges. With increasing coverage, the Xe layer
grows into 3D island structure at the temperature range of
10–20 K.
II. EXPERIMENTThe experiment was performed with a low-temperature
STM with a base pressure of ⬍1⫻10⫺10 torr. The whole
STM unit can be cooled down to 6 K. A Cu共111兲 singlecrystal sample was cleaned by repeated cycles of Ar⫹ ion
sputtering and annealing up to 900 K. High-purity Xe gas
was dynamically dosed through a stainless-steel tube with adiameter of 161 in., which is opened at ⬃1 cm away from thecold sample surface by a precision leak valve. This dynamic
supply ensured high partial pressure of Xe only around thesample surface, but not in the whole chamber. Although theabsolute Xe exposure could not be determined with othersurface-science tools, the coverage could be determined with
16 934©1999 The American Physical Society

PRB 60ADSORPTION AND GROWTH OF Xe ADLAYERS ON THE . . .

16 935the dose rate of Xe, the arrival rate multiplied by the exposure time, and STM images. The sample can be annealed bycontrolling the liquid-helium supply or using a heater attached on the back of the sample. All STM images in thispaper were obtained after dosing Xe gas to the Cu surface at
10–20 K.
III. RESULTS AND DISCUSSIONS
A. Growth at low coverage „<0.3 ML…
1. One-dimensional growth at lower step edge

Two different adsorption behaviors were observed with
STM when Xe atoms of ⬍0.1 monolayer 共ML兲 were introduced to the metal surface. On a close-packed surface suchas Pt共111兲, they first decorated step edges even at 4 K.6 On
an open surface such as Ni共110兲, isolated Xe atoms nucleateon the terrace at the low temperature.4 Figure 1 shows theinitial adsorption behavior of Xe atoms on the Cu共111兲 surface at 10 K. At this low coverage, most of the Xe atoms
appear along the step edges, except the ones nucleated at
isolated defects or impurities on the terrace. As previously
observed in the case of Xe on Pt共111兲,6 Xe atoms seem tohave large enough transient mobility on the Cu共111兲 to reachthe step edges. However, unlike on the Pt共111兲 surface,7 thelower step edges are the first adsorption sites. Figures 1共a兲and 1共b兲 show the first adsorption at the step edges at acoverage of Ⰶ0.1 ML. In these images, the center of theadsorbed Xe atoms is located at 1.5⫾0.3 Å away from the
Cu lower step edges. Even with a simple atomic superposition calculation,8 it is easy to conclude that the Xe atoms arelocated at the lower step edges. There are several missing Xe
atoms in the nearly 1D Xe chains. The height of the missing
parts is the same as the lower terrace, which is more evidence of the adsorption at the lower step edges. As marked
by arrows in Fig. 1共b兲, some Xe atoms started to occupy theupper step edges with increasing coverage. Figure 1共c兲 is aperspective view of Fig. 1共b兲. The arrows in Fig. 1共c兲 indicate the same Xe atoms that are indicated by arrows in Fig.
1共b兲. From these images, it can be concluded that most of the
Xe atoms adsorb at the lower step edges initially. By further
increasing the coverage, the growth of the Xe 1D chain iscompleted at the lower step edges, but an additional chain is
not grown until the first atomic row of Xe is completed. The
growth mode of Xe atoms at the lower step edges of Cu共111兲
can be typified as a ‘‘row-by-row’’ growth. A similar growthmode was observed up to two rows at the step edges on
Pt共997兲 with low-energy helium scattering.9
2. Wetting of surface steps

As shown in the preceding section, Xe atoms start to occupy the upper step edges after the growth of one Xe atomic
row is almost completed. With increasing coverage 共⬎0.08
ML兲, the upper step edges are wet as shown in Fig. 2. Thesechanges of the adsorption sites with coverage can be explained with the change of potential well at the lower andupper step edges for Xe atoms. At a very low coverage of
Xe, they only decorate the lower step edge, meaning thepotential well at the upper step edge is shallower than thethermal energy and the step-edge barrier 关Ehrlich-Schwoebel
共ES兲 barrier10兴. Arriving Xe atoms have enough energy to

FIG. 1. STM image of the low-coverage adsorption stage for Xeon the Cu共111兲 surface. Xe was dosed and imaged at 10 K. 共a兲 All
Xe atoms adsorb at the lower step edge (200⫻200 Å 2 , V s⫽0.4 V, I t ⫽0.2 nA). 共b兲 Almost all Xe atoms appear along Cu
steps. There are some Xe atoms adsorbed at the upper step edges asmarked by arrows 共300⫻300 Å2, V s ⫽0.4 V, I t ⫽0.2 nA兲. 共c兲 Perspective view of 共b兲. Arrows indicate the same Xe atoms as in 共b兲which adsorb at upper step edges. Z scale is exaggerated for easiercomparison.

16 936

JI-YONG PARK et al.

PRB 60

FIG. 2. Images showing wetting of Cu steps by Xe atoms atcoverages of 共a兲 0.08 ML, 共b兲 0.2
ML, 共c兲 0.3 ML, and 共d兲 0.35
ML (800⫻800 Å 2 , V s ⫽0.4 V,
I t ⫽0.2 nA, and 10 K兲. Note thatthe morphology at the lower stepedges is in sharp contrast with theupper ones. The morphology at
the upper step edges changes withincreasing coverage, while it remains the same at the lower stepedge.

overcome the ES barrier and are trapped at the potential wellat the lower step edges. Once they wet the lower step edges
with one atomic row, arriving Xe atoms start to occupy upper step edges and form multiple atomic lines as shown in
Figs. 1共b兲 and 2. In the presence of adsorbed Xe atoms at thelower step edges, the potential well at the upper step edge ismodified and can accommodate Xe atoms. That may be explained with a modification of the electron density at the stepedges with the adsorption of Xe atoms at the lower stepedge. Further confirmation may be needed with theoretical
calculation.11
In Fig. 2, original Cu steps are easily discernible withdifferent growth morphology of Xe atoms between at lower
step edges and upper step edges. These growth morphologies
can be explained with the 1D wetting behavior with a step
acting as a 1D substrate. As can be seen in Fig. 2共a兲, Xe
atoms fully wet the lower step edges to form a thin stripe
phase, while Xe atoms ‘‘nonwet’’ the upper step edges toform clusters dispersed along the steps at the low coverage.
From Fig. 2, we can see that the wetting characteristics at theupper and lower step edges are quite different from that of
the Xe/Pt共111兲 system, where Xe atoms first adsorb at theupper step edges as a 1D chain, then at the lower step edges.7
As shown in Fig. 2, the width of the Xe stripe phase at thelower Cu step edges ranges 10–20 Å 共3–6 monorows兲 andremains almost the same even at the higher coverage of 0.1–
0.4 ML. Unlike at the lower step edge, Xe atoms show mutually attractive interaction at the upper step edges, resulting

in the cluster formation. By further increasing the coverage,
Xe layers can form a stripe phase even at the upper stepedges similar to the ones at the lower step edges, but muchthicker with nonwetting behavior 关see Figs. 2共b兲–2共d兲兴. This
striking difference between wetting behaviors of Xe between
the upper and lower step edges is in sharp contrast to theXe/Pt共111兲 case. In one theoretical consideration, Bertel12
suggested that the Xe atoms at the upper step edge arestrongly repulsive in the presence of the Shockley surface
state since attractive interaction among Xe atoms through thesurface state is absent. As demonstrated in a previous
work,13 a dipole-dipole repulsion created by Xe adsorption iseffective in preventing rare-gas atoms from clustering on
metal surfaces. Since both Pt共111兲 and Cu共111兲 have a surface state with a nearly s electron nature 关m * ⫽1.3m e ,¯ )⫽400 meV for Pt共111兲,14 m * ⫽0.46m e , E B (⌫¯)
E B (⌫
15⫽390 meV for Cu共111兲, respectively, m * is the effective
mass in the free-electron-like dispersion curve, m e the elec¯ ) the surface state energy at the ¯⌫ point
tron mass, and E B (⌫of the surface Brillouin zone兴, observed contrast between
two systems seems unusual. At the same time, the lower stepedge is usually the preferred adsorption site due to its highcoordination number. In view of the similar surface-state
properties of these two systems, we speculate that the dipoledipole interaction around step edges may vary in these twosystems.16 The coverage-independent width of the thin stripe
phase at the lower step edges and different wetting behaviors

PRB 60ADSORPTION AND GROWTH OF Xe ADLAYERS ON THE . . .

16 937

universal curve since Xe atoms have large diffusivity and are
weakly bound to the substrate. In this type of system, theisland density of size s, N s , is scaled as

N s⫽FIG. 3. Normalized density of Xe islands as a function of theirrelative sizes for three different coverages.

are believed to be the result of the balance between dipoledipole repulsion and attraction due to its high coordination
environment.
B. Growth at higher coverage
1. Nucleation on the terrace

As the coverage exceeds 0.2 ML, Xe islands begin to
nucleate on the terraces. Growth behavior can be scaled to a

冉 冊

s
⍜
,
2g
s
s
具 典
具 典

共1兲

where ⌰ denotes coverage, 具s典 the average island size, and g
the universal scaling function, respectively.17,18 The scaling
function is plotted at three different coverages in Fig. 3.
These three curves, however, do not exactly follow a universal curve as shown in Fig. 3. This slight deviation cannot be
explained by strong interactions with the substrate or intermixing between Xe and the substrate. This deviation ismainly due to the fact that Xe atoms nucleate on the Cu
terrace only around defects: inhomogeneous nucleation. If
there exist defects with large concentration 共larger than thedensity of islands that would otherwise nucleate homogeneously兲, N s is solely determined by the concentration of
defects, therefore the deviation from a universal scaling is
expected.17 It was also reported in the Xe/Pt共111兲 system that
Xe islands nucleate around defects or impurities on theterrace.4,7 It can be concluded that the inhomogeneous nucleation accounts for the clustering of Xe islands on the terrace.

FIG. 4. STM images of morphological changes of Xe as thetemperature rises from 10 to 21 K
at the same place. For details, seetext. 共a兲 10 K, 共b兲 15 K, 共c兲 17 K,
zoomed image of the region enclosed by a square in 共b兲, and共d兲 21 K 关800⫻800 Å 2 except
200⫻200 Å 2
for
共c兲,
Vs
⫽0.4 V, I t ⫽0.2 nA兴.

JI-YONG PARK et al.

16 938

PRB 602. Temperature-dependent evolution of morphology

Neither noticeable island coarsening nor cluster diffusion
was observed in the time scale of several hours at 10 K. We
followed the morphological changes at the same location as
raising the temperature of the substrate. Comparing the images taken at the same place at the different temperatures
below 15 K as in Figs. 4共a兲 and 4共b兲, only very smallchanges due to the diffusion of Xe atoms along the edges of
the islands were observed. But as the temperature was raised⬎15 K, but ⬍20 K, the shape of the Xe islands began to takeon a hexagonal shape as shown in Fig. 4共c兲. Figure 4共c兲 is anenlarged image of the lower left region of Fig. 4共b兲 at 17 K.
The edges of the Xe islands became sharp at this temperatureand the islands began to show hexagonal shapes. As Cu共111兲
and Xe共111兲 surfaces have threefold symmetries, the angle
between two adjacent 兵211其 planes should be 120° at equilibrium below the roughening transition temperature. In
equilibrium, it may show an equilateral hexagon, but manyof these are pinned by defects on the Cu substrate, showing aquasiequilibrium, threefold symmetry. Neither noticeablemass transport such as island coarsening nor cluster diffusion
was observed at this temperature, suggesting that this equilibrium shape is the result of the diffusion of Xe atoms along
the island perimeters. At a temperature of ⬎20 K as in Fig.
4共d兲, the sharp edges of the hexagonal islands are smoothed,suggesting the roughening transition. All Xe clusters originally adsorbed at the upper step edge diffuse 共or move overthe ES barrier兲 to the lower step edges and island coarsening
was also observed. The images at higher temperature are notshown here, but at ⬎25 K, Xe atoms began to diffuse fromXe terrace islands to the lower step edges. From these observations, we are able to estimate the order of various barrierheights that Xe atoms experience on the Cu共111兲 surface.
From the temperature dependence, it can be suggested thatthe barrier for evaporation (⌬E ev) from the island is higher
than the step-edge barrier 共ES barrier, ⌬E ES) and the edge
diffusion barrier (⌬E ED): ⌬E ev⬎⌬E ES⬎⌬E ED . This order of barrier height is in agreement with the recent result ofa calculation for homogeneous diffusion of Al on the
Al共111兲 surface.19
3. Multilayer growth

By further increasing Xe coverage, 3D island growth wasobserved at a temperature range of 10–20 K. The secondand third-layer Xe islands are shown in Figs. 5共a兲 and 5共b兲.
The STM tip often drags Xe atoms at the third layer, resulting in frequent tip changes or degradation of the STM images. As Xe atoms physisorb on the surface, the atoms canbe easily pulled or pushed depending on the polarity of the
STM tip. It has been reported that the STM tip could be used
to manipulate weakly bound Xe atoms on metal surfaces.5 In
order to make sure that our scanning process did not disturb
the morphology, we checked the images by scanning withvarious bias voltages and tunneling currents at the sameplace. The tunneling conditions without any changes on themorphology before and after the scan were chosen.
From observation of this multilayer growth, Xe atoms
seem to ‘‘nonwet’’ the Cu共111兲 surface at 10 K. As stated in
the Introduction, it has been shown that the growth behaviorof rare gas on a substrate can be scaled according to theinteraction strength.2 The growth behavior can be explained

FIG. 5. STM images of 3D island growth. 共a兲 Second-layer Xeislands (800⫻800 Å 2 , V s ⫽0.4 V, I t ⫽0.2 nA, and 10 K兲. 共b兲
Third-layer
Xeislands(2000⫻2000 Å 2 , V s ⫽0.4 V, I t⫽0.2 nA, and 16 K兲.

by the ratio u/h 共where u is the adatom-substrate interaction
with which isosteric heat of adsorption is taken and h is theadatom-adatom interaction with which 0 K cohesive energyof the bulk phase is taken兲.2 Complete wetting 共FM growth兲takes place only when the ratio u/h is around 1. The incomplete wetting 共SK growth兲 occurs when the ratio is less thanor more than 1. Nonwetting 共VM growth兲 is observed when
the ratio is much less than 1.2 Although there are some exceptions to this theory,20 many known systems follow thistrend. In the case of Xe on the Cu共111兲 surface, layer-bylayer growth of the Xe multilayer on the Cu共111兲 surface
was observed at ⬎20 K 共Ref. 21兲 关15 K, in the case ofpolycrystalline Cu 共Ref. 22兲兴. Despite the nonwetting of Xeon the Cu共111兲 surface at 10 K as observed here, our results
agree well with the earlier results of complete wetting behavior at higher temperature with an enhanced interlayer masstransport at ⬎20 K as shown in Fig. 4共d兲. It can be suggested
that there should be a phase transition from nonwetting toincomplete or complete wetting at a temperature range of
15–25 K, based on our observation.
4. Difference in diffusivity

As described in Sec. III B 1, the first-layer Xe islands
nucleate around the defects on the Cu surface. These defects

PRB 60ADSORPTION AND GROWTH OF Xe ADLAYERS ON THE . . .

16 939

FIG. 6. 共a兲 One-monolayer-high Xe patches cover the half of the surface. The image was taken after Xe was first dosed at 10 K, annealed
to ⬎30 K, and quenched to 10 K again. A denotes the Xe monolayer patch and B denotes the clean Cu surface, for example. 共b兲 Xe of⬃0.05 ML was additionally dosed on the surface similar to 共a兲 at 10 K (800⫻800 Å 2 , V s ⫽0.4 V, I t ⫽0.2 nA). 共c兲 Prepared in thesame way as 共b兲, but about 0.7 ML on the larger terrace (1600⫻1600 Å 2 , V s ⫽0.4 V, I t ⫽0.2 nA, and 14 K兲. 共d兲 Island density
distribution as a function of island size for both the ‘‘Xe on Xe’’ and ‘‘Xe on Cu’’ cases. Solid lines are Gaussian fits for each datum.

can accommodate potential wells for Xe atoms to nucleate
not only on the Cu surface but also on the Xe layer. However, this inhomogeneous nucleation mechanism alone cannot explain the observed different shapes of multilayer islands. As seen in Fig. 5, the shapes of second-layer Xeislands are quite different from those of the first-layer Xeislands; the first-layer islands are those of nearly equilibrium,
but the second-layer islands are those of diffusion-limited
aggregation. Usually the layer-by-layer growth mode is determined by the competition between in-plane diffusivity andthe ES barrier.23 In the present observation, the second-layer
islands nucleate at the center of the first layer and grow toirregularly shaped islands. This can be explained by the factthat diffusivity is the determining factor in 3D island growth.
The large difference in diffusivity between ‘‘Xe on Cu’’ and‘‘Xe on Xe’’ was observed at a temperature range of 10–20
K. To elucidate this effect further, about 0.5 ML of Xe wasdosed at 10 K, resulting in 3D island growth, and the sample
was annealed to ⬎30 K. After it was cooled down to 10 K,
additional Xe was introduced on the surface again to grow
Xe islands on the bare Cu substrate and the large Xe firstlayer at the same time. Figure 6共a兲 shows a typical image justafter cooling. As expected, about half of the surface is cov-

ered with a Xe monolayer while the other half exposes thebare Cu surface. White spots are additional Xe clusters,
strongly pinned at defects during the annealing procedure.
As an additional ⬃0.05 ML of Xe gas was introduced to thesurface at 10 K, the image became quite different from theone just after the cooling. Because this imaged region has ahigher step density than other regions imaged previously,
only a few Xe islands are visible here. The region around theboundary between bare Cu terraces and Xe terraces shows
substantial differences. The Xe atoms spread along theboundary on the Cu terrace to form a stripe phase. However,isolated Xe cluster structures grow from the lower stepedges, indicating reduced diffusivity along the lower edgesof the Xe layer. When we have large terraces, we can compare the difference of diffusivity by measuring island density
as in the case of Fig. 6共c兲. The sizes of the Xe islands on the
Xe terraces are much smaller and the densities are higher
than those on the Cu terraces, meaning lower diffusivity of
Xe atoms on top of Xe terraces than on the Cu substrate.
According to a model which relates the density and size ofthe islands to diffusivity,24 the island density N is proportional to D ⫺1/3, where D is the diffusion coefficient. From
the observed island distribution on each layer as in Fig. 6共d兲,JI-YONG PARK et al.

16 940we were able to estimate that D Xe⫺Cu, the diffusion coefficient of ‘‘Xe on Cu,’’ is about 350 times larger than D Xe⫺Xe,the diffusion coefficient of ‘‘Xe on Xe.’’ This large difference in diffusivity prevents Xe atoms that adsorb on top of
the existing first-layer Xe islands from diffusing to the edge
and stepping down to the Cu terrace, resulting in the observed 3D island growth. As the binding energy of Xe on
first-layer Xe is smaller than on the Cu substrate, this small
diffusivity is unusual. The observed diffusivity difference
may be explained with two possible mechanisms. First, thebinding energy among Xe atoms is much lower on the Xemonolayer than on the Cu substrate, explaining the small Xe
island size on top of the first layer. Second, the difference
may be explained with transient mobility. As we showedearlier in this paper about the low-coverage growth, Xe atoms show high transient mobility on Cu共111兲 at around 10
K. We think that the low diffusivity on the Xe monolayer
can result from the fact that the transient mobility of the Xeatoms is more easily dissipated on the Xe layer than on theCu surface.
IV. CONCLUSION

We have presented growth modes and mechanisms of Xe
atoms adsorbed on the Cu共111兲 surface with low-temperature
STM. We have identified the first adsorption site as the lower*Author to whom correspondence should be addressed. Electronic
address: ykuk@phya.snu.ac.kr
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PRB 60step edge at low temperature, in contrast to the Pt共111兲 surface. It was shown that the upper step edge could be an
attractive site for Xe atoms, possibly due to the reduced
dipole-dipole repulsion resulting from the redistribution ofelectrons. We found that the density of Xe islands does notfollow the scaling behavior due to the inhomogeneous nucleation. The relative order of barrier heights was determined
共the barrier for evaporation from island edges ⬎ES barrier⬎edge diffusion barrier兲 from the temperature-dependentmorphology. We found that Xe islands on top of the Cu
surface at around 17 K reveal quasi-two-dimensional thermal
equilibrium shape. For high coverage growth, 3D island
growth was found for Xe atoms on top of the Cu surface
around 10 K. This 3D island growth is thought to occur due
to much reduced diffusivity for the ‘‘Xe on Xe’’ compared tothe ‘‘Xe on Cu’’ case at low temperature and inhomogeneous nucleation.

ACKNOWLEDGMENTS

This work was partially supported by Korean Ministry of
Science and Technology through the Creative Research Initiative Program. Support from a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sport andCulture of Japan is also acknowledged.

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