Jpn. J. Appl. Phys. Vol. 40 (2001) pp. 4399–4402
Part 1, No. 6B, June 2001c2001
The Japan Society of Applied Physics

Adsorption and Wetting Structures of Kr on Pt(111) at 8 K and 45 K
Studied by Scanning Tunneling Microscopy
Yan Jun L I1 , Koji M IYAKE1, ∗ , Osamu TAKEUCHI1 , Don Norimi F UTABA1 , Masuaki M ATSUMOTO2 ,Tatsuo O KANO2 and Hidemi S HIGEKAWA1, †1 Institute of Applied Physics, CREST, Japan Science and Technology Corporation (JST), University of Tsukuba, Tsukuba 305-8573, Japan
2 Institute of Industrial Science, University of Tokyo, Minato-ku, Tokyo 106-8558, Japan(Received March 8, 2001; accepted for publication March 27, 2001)

Adsorption and wetting structures of Kr on Pt(111) were studied using scanning tunneling microscopy. The characteristics
were different from those of Xe/Pt(111) and Xe/Cu(111). At 8 K, Kr atoms adsorbed on terraces and preferential adsorption
to step edges was not observed. Kr atoms were imaged with 0.12 nm height and 1.0 nm full width at half maximum. With theadditional exposure, adsorption at step edges occurred, with a slight preference for the upper step edges. However, no evidence
for the characteristic properties, such as repulsive interaction between adsorbates or modification of the electronic structures atstep edges, was observed. With the rise in temperature to 45 K, Kr atoms on the terraces increased in size with hexagonal shape,
and wetting of the step edges began. In the wetting process, the lower step edge was more preferential, and the wetting region
on both step edges grew with a stripe shape; the edge of the wetting region was not irregular but straight and parallel to thesteps. In addition, the height of the Kr images decreased and showed two different values, namely, Kr stripes grown from thelower step edges were observed to have 0.10 nm height, 0.02 nm higher than those with a height of 0.08 nm grown on terraces
or at upper step edges. The obtained results indicate the existence of two different structures, which may be attributed to thetwo structures previously observed by the macroscopic analyses.
KEYWORDS: STM, Kr, Pt(111), adsorption, terrace, step edge, wetting

1. Introduction
Since rare gas atoms have a simple closed shell electronic structure, they have been widely used as ideal twodimensional model systems to clarfy the wetting process ofsolid surfaces.1–12) Structural aspects have been basically investigated using reciprocal space methods in the past: lowenergy electron diffraction,1) spin-polarized low-energy electron diffraction,2) He diffraction,3) elastic and inelastic Hescattering and so on.4) However, in the wetting process, microscopic structures such as steps or defects have been considered from the beginning to play important roles. Recent scanning tunneling microscopy (STM) studies on an atomic scale
revealed the existence of interesting characteristics during theinitial adsorption and growth processes of the rare gases on
metal surfaces.
For example, Xe atoms on Pt(111) and Cu(111) surfaces show completely different behaviors at step edges.5–12)
Namely, Xe atoms on the Pt(111) surface are preferentiallyadsorbed on the upper step edges which results in the formation of one-dimentional (1D) chains of Xe atoms. These
chains lead to a local repulsive interaction with subsequent
Xe adatoms, and Xe atoms do not serve as nuclei for theformation of two-dimensional (2D) islands at the upper stepedges. With increasing coverage, Xe islands grow from thelower step edges to the lower terraces. On the other hand, Xe
atoms on the Cu(111) surface are preferentially adsorbed at
the lower step edges and form 1D chains. Along the chains,the interaction between Xe atoms is repulsive. Once Xe
atoms occupy the lower step edges, they modify the EhrlichSchwoebel barrier, resulting in wetting at the upper step edges∗ Present address: Tribology group, Institute of Mechanical Systems Engi-neering, National Institute of Advanced Industrial Science and Technology(AIST), 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan.
† Corresponding author, E-mail address: hidemi@ims.tsukuba.ac.jp
http://dora.ims.tsukuba.ac.jp

with attractive interaction.
Therefore, despite the stable electronic structure of the raregas atoms, their initial adsorption characteristics on metals arecomplex, and their mechanism has not been clarified completely. In order to further investigate the characteristics ofthe rare gases on metals, we investigated the adsorption andwetting processes of Kr on the Pt(111) surface on an atomic
scale using STM, and the results were compared with thoseobtained for the cases of Xe/Pt(111) and Xe/Cu(111).
2. Experiments
A Pt single crystal was prepared by repeated argon ion
sputtering (770 K) and annealing (∼ 1400 K) cycles. Carbon
contamination on the surface was removed by annealing at
1100 K in an O2 environment (∼ 1 × 10−5 Pa). The base pressure was 7 × 10−9 Pa. An electrochemically etched tungsten
tip was used. Krypton dosing was performed using an electronically controlled pulse valve. In order to eliminate the influence of the environmental effect, the STM tip was extracted
during the Kr dose. Therefore, observation of the same place
was difficult, but reproducibility was confirmed by repeating
the experiment. According to the results of the diffraction
measurement, Kr atoms are mobile at 25 K on the Pt(111)
surface and form islands,6) and annealing up to 50 K improves
the order slightly. Therefore, in this study, Kr atoms were adsorbed at 8 K first, and change due to annealing at 45 K wasobserved. The sample temperature was monitored using anAu+0.07%Fe-Cr thermocouple located near the sample.
3. Results and Discussion
Figure 1 shows a series of STM images with different Krcoverages at 8 K. As evident in Fig. 1(a), Kr atoms were adsorbed on the Pt(111) terraces, and they were imaged with
0.12 nm height and 1.0 nm full width at half maximum (Fig.

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Y. J. L I et al.

Fig. 1. (a), (c), (d) Series of STM images (29 nm × 29 nm) taken at 8 K with different Kr coverage (sample bias: Vs = 0.5 V, tunnelingcurrent: It = 0.5 nA). (b) Cross section along A–B in (a).

1(b)). The observed width is large, but each protrusion wasresolved even when clusters were formed. In addition, some
STM images of the protrusions changed, as indicated by anarrow in Fig. 2(a), during the scan, which can be explained asin the schematic shown in Fig. 2(b); a single Kr atom moved
during the scan under the influence of the STM tip. Therefore, we attributed the protrusions in Fig. 1(a) to the single Kratoms. No preferential adsorption at step edges was observed
contrary to the cases of Xe/Pt(111) and Xe/Cu(111).
With additional exposure, adsorption at step edges appeared, with a slight preference for the upper step edges asindicated by arrows in Figs. 1(c) and 1(d). Therefore, the upper step edge does not act as a repulsive barrier against Kratoms. In addition, as shown in Figs. 1(c) and 1(d), successive adsorption on the upper step edges near the Kr atoms
was observed, which indicates that the Kr atoms at the upper
step edges do not have repulsive interaction with the Kr atoms
which follow. On the basis of the analyses of the diffraction
method, annealing at ∼ 25 K is considered necessary to form
well-ordered Kr structures on Pt(111). The observed size ofeach Kr island is small, as shown in Fig. 1, indicating the short
diffusion length of Kr on the Pt(111) surface at this temperature.
Then, the sample temperature was raised to 45 K. Figure 3
shows a typical STM image taken at 45 K for Kr/Pt(111).
There exist larger islands of hexagonal shape on the terraces.
In addition, as shown in Fig. 3, the step edges are wetted by
Kr atoms. In particular, the lower step edge side is completely
wetted and the shape of the wetting region is not irregular but

striped; the edge of the wetting region is straight and parallelto the step. A remarkable feature is that the observed heightof the Kr images decreased and showed two values, namely,
Kr stripes grown from lower step edges were observed to have
0.10 nm height, 0.02 nm higher than those of height 0.08 nm
grown on terraces or upper step edges (Fig. 3(b)).
In order to examine the details more closely, the sample
was further annealed, and a typical STM image is shown in
Fig. 4. Here, the surface is more deeply wetted, and thestraight shape of the wetting layer edge is more clearly evident. The wetting regions grown at the upper and lower stepedges, are indicated by arrows with the characters U and L,respectively. The shape is extremely straight and the width isgreater on the lower step edge side. In addition, there still exist hexagonal islands on the terraces, and in some areas, theybridge the wetting regions at the upper and lower step edges
on the same terraces. However, as shown in the cross section(Fig. 4(b)), the height difference remains between the Kr islands on the terraces and the Kr stripes grown from the lower
step edges, and a domain boundary exists between them. On
the basis of the macroscopic analyses, it is thought that the existence of the step has influence on the growth mode of Kr on
the Pt(111) surface, and that this causes the discrepancies between theory and experiments.6) The observed difference may
be attributed to the difference between the two structures, but
further experiments with higher resolution are necessary toclarify this point.
Next, the observed wetting process of the Pt(111) surface by Kr atoms is compared with the characteristics of theJpn. J. Appl. Phys. Vol. 40 (2001) Pt. 1, No. 6B

Y. J. L I et al.

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(a)
Anm

B

nm

20

1.0
0.5

1020

Height (nm)

(b)

1000A

0.30
0.25
0.20
0.15
0.10
0.05

0.08 nm

B0.10 nm

02468Distance (nm)1012Fig. 3. (a) STM image of Kr/Pt(111) surface taken after annealing at 45 K.
(Vs = 0.5 V, It = 0.5 nA, 29 nm × 29 nm). (b) Cross section along A–B in(a).

(a)

Xe/Pt(111) and Xe/Cu(111) cases. In both cases of the initialadsorption of Xe/Pt(111) and Xe/Cu(111) at ∼ 10 K, the first
adsorption of Xe atoms at the upper or lower step edge modifies the electronic structures of the steps, and the Xe atoms
which follow start to wet the other side of the step edge. From
the observation at 8 K, it is evident that Kr atoms prefer to adsorb at the upper step edges rather than at the lower step edge,
similar to the case of Xe/Pt(111). However, contrary to thecase of Xe/Pt(111), Kr atoms on the upper step edges do not
induce repulsive interaction with the Kr atoms which follow,
and do not atract Kr atoms to the lower step edges. Considering these facts, Kr atoms appear to modify the electronic
structure of the step edges less than Xe. This difference may
be related to the fact that Xe atoms have a stronger dipolemoment.
Even at 45 K, the obtained results for Kr/Pt(111) were quite
different from those for Xe/Pt(111) and Xe/Cu(111). With annealing at ∼ 40 K, in the case of Xe/Pt(111), Xe islands growlarger on the terraces, but the wetting of the step edges occurs
only at the lower step edge. The monoatomic chain on the upper step edge remains repulsive even at temperature as highas 40 K. On the other hand, in the case of Xe/Cu(111), theupper step edges are wetted from the beginning at 10 K, andLULBULAUL(b)1.3Height (nm)Fig. 2. (a) STM image of Kr/Pt(111) at 6 K (Vs = 0.5 V, It = 0.5 nA,14.5 nm × 14.5 nm). (b) Schematic model for the STM image of the protrusion indicated by an arrow in (a).

A1.2
1.10.10 nm

0.08 nm1.00.9B

0.8
0510Distance (nm)15Fig. 4. (a) STM image of Kr/Pt(111) after further annealing of the sample
shown in Fig. 3 at 45 K. (Vs = 0.5 V, It = 0.5 nA, 45 nm×30 nm). Arrows
with the characters U and L indicate the wetting layer at the upper andlower step edges. (b) Cross section along A–B in (a).

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Jpn. J. Appl. Phys. Vol. 40 (2001) Pt. 1, No. 6B

the lower step edges become wet as the temperature is raised.
Since Kr islands grow both on the upper and lower step edges,
the observed wetting process at 45 K appears to be more similar to the case of Xe/Cu(111) than that of Xe/Pt(111), although it differs on several details. Namely, the wetting region on both step edges, particularly on the lower step edge
side, grew with a stripe shape, contrary to the irregular structure observed for the cases of Xe/Pt(111) and Xe/Cu(111).
In particular, in the case of Xe/Pt(111), the observed structure of the wetting region is very irregular and is considered
to have a curly shape.9–12) The result suggests that the interaction between Kr atoms is weaker than in the case of Xe atoms;
Kr atoms are adsorbed at the high coordination sites, whichmakes the edges of the islands straight. This is in good agreement with the observed result at 8 K; Kr atoms do not form
islands as described above.
As has been discussed, the observed wetting process ofKr/Pt(111) showed a different characteristic properties fromthose of Xe/Pt(111) and XeCu(111). Since interactions between rare gases on metal surfaces are considered to be related to the interactions between the adsorbates and the substrates,13) a comparative theoretical calculation for the caseof Kr, including the electronic structures of the substrate, isstrongly needed to understand the wetting process of the raregases on metals comprehensively.
4. Conclusions
Adsorption and wetting structures of Kr on Pt(111) werestudied using STM for the first time. At 8 K, Kr atoms adsorbed on terraces and preferential adsorption to step edges
was not observed. With additional exposure, adsorption atstep edges occurred, with a slight preference for the upper
step edges. However, no evidence for the characteristic properties, such as repulsive interaction between adsorbates ormodification of the electronic structures of the step edges wasobserved. With the increase in temperature up to 45 K, KrY. J. L I et al.

atoms on terraces increased in size with hexagonal shape, andwetting of the step edges began. During the wetting process,the lower step edge was more preferential, and the wetting
region on both step edges grew with a stripe shape; the edgeof the wetting region was not irregular but straight and parallel to the steps. In addition, the height of the Kr images
decreased and showed two types, namely, Kr stripes grownfrom the lower step edges were observed to have 0.10 nm
height, 0.02 nm higher than those of 0.08 nm height grownon terraces or upper step edges. The obtained results indicate the existence of two different structures, which may be
attributed to the two structures previously obtained by macroscopic analyses.
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.

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