Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100)
The structure and growth of two-dimensional crystals of Kr on Ag(111) and Ag(100) have been investigated by means of ellipsometry and eXtremely-low-current Low Energy Electron Diffraction (XLEED) under the quasi-equilibrium condition. The layering growth of a Kr film was observed up to the third lay...
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Aki Tosaka Taro Mitake Takashi Miura Ichiro Arakawa 2018-01-14T12:46:43Z 2018-01-14T12:46:43Z 2003 Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) / Aki Tosaka, Taro Mitake, Takashi Miura, Ichiro Arakawa // Физика низких температур. — 2003. — Т. 29, № 9-10. — С. 1027-1031. — Бібліогр.: 8. назв. — англ. 0132-6414 PACS: 68.10.Cr, 68.10.Gw https://nasplib.isofts.kiev.ua/handle/123456789/128910 The structure and growth of two-dimensional crystals of Kr on Ag(111) and Ag(100) have been investigated by means of ellipsometry and eXtremely-low-current Low Energy Electron Diffraction (XLEED) under the quasi-equilibrium condition. The layering growth of a Kr film was observed up to the third layer by ellipsometry while the crystal geometry by XLEED. Kr overlayer on Ag(100) has two types of alignment. In the predominant alignment, one of the unit vectors aligns with <001> of Ag substrate, while in the other alignment with <011>. Kr–Kr spacing in monolayer on Ag is 10% larger than that of bulk. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Low-Temperature Thermodynamics and Structure Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) Article published earlier |
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Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) |
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Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) Aki Tosaka Taro Mitake Takashi Miura Ichiro Arakawa Low-Temperature Thermodynamics and Structure |
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Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) |
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Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) |
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Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) |
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Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) |
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observation of growth and structure of kr films physisorbed on ag(111) and ag(100) |
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Aki Tosaka Taro Mitake Takashi Miura Ichiro Arakawa |
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Aki Tosaka Taro Mitake Takashi Miura Ichiro Arakawa |
| topic |
Low-Temperature Thermodynamics and Structure |
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Low-Temperature Thermodynamics and Structure |
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2003 |
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English |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Article |
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The structure and growth of two-dimensional crystals of Kr on Ag(111) and Ag(100) have been investigated by means of ellipsometry and eXtremely-low-current Low Energy Electron Diffraction (XLEED) under the quasi-equilibrium condition. The layering growth of a Kr film was observed up to the third layer by ellipsometry while the crystal geometry by XLEED. Kr overlayer on Ag(100) has two types of alignment. In the predominant alignment, one of the unit vectors aligns with <001> of Ag substrate, while in the other alignment with <011>. Kr–Kr spacing in monolayer on Ag is 10% larger than that of bulk.
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0132-6414 |
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https://nasplib.isofts.kiev.ua/handle/123456789/128910 |
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Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100) / Aki Tosaka, Taro Mitake, Takashi Miura, Ichiro Arakawa // Физика низких температур. — 2003. — Т. 29, № 9-10. — С. 1027-1031. — Бібліогр.: 8. назв. — англ. |
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2025-11-26T01:42:50Z |
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Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10, p. 1027–1031
Observation of growth and structure of Kr films
physisorbed on Ag(111) and Ag(100)
Aki Tosaka, Taro Mitake, Takashi Miura, and Ichiro Arakawa
Department of Physics, Gakushuin University
151-8588, 1-5-1 Mejiro Toshima-ku, Tokyo, Japan,
E-mail: ich.arakawa@gakushuin.ac.jp
The structure and growth of two-dimensional crystals of Kr on Ag(111) and Ag(100) have
been investigated by means of ellipsometry and eXtremely-low-current Low Energy Electron Dif-
fraction (XLEED) under the quasi-equilibrium condition. The layering growth of a Kr film was
observed up to the third layer by ellipsometry while the crystal geometry by XLEED. Kr overlayer
on Ag(100) has two types of alignment. In the predominant alignment, one of the unit vectors
aligns with <001> of Ag substrate, while in the other alignment with <011>. Kr–Kr spacing in
monolayer on Ag is 10% larger than that of bulk.
PACS: 68.10.Cr, 68.10.Gw
1. Introduction
The system of rare gas physisorbed on a metal sur-
face has been studied for its interesting structure and
phase transition in two-dimension since 1970's.
Xe/Ag(111) has been intensively investigated to re-
veal the structure, the isosteric heat of adsorption and
the two-dimensional phase diagram, whereas a few re-
sult of a Kr films on a metal surface has been reported.
Roberts and Pritchard observed a Kr monolayer
film adsorbed on Ag(111) using low energy electron
diffraction (LEED) and revealed that the Kr
overlayer has incommensurate hexagonal structure
with the substrate [1]. Ungris et al. found the struc-
ture and the thermodynamic property of Kr/Ag(111):
the phase diagram, the monolayer lattice constant, la-
tent heats of adsorption and isosteric heats [2].
In the case of a Kr film on a metal surface,
desorption of Kr atoms by an electron beam is a serious
problem in a conventional LEED system whose inci-
dent electron current is typically 1 �A. Roberts and
Pritchard reported that the diffraction spots disap-
peared within 10–15 min [1]. Ungris et al. also re-
ported that a Kr monolayer film at 40 K would be com-
pletely desorbed by an electron beam in several minutes
[2]. Therefore, they either had to make experiments un-
der the constant beam flux of Kr atoms which was suf-
ficient to maintain an adsorbed layer or to measure the
diffraction patterns before the coverage decreased due
to electronic stimulated desorption (ESD).
The structure and layering growth of rare gas films
on metal surfaces are determined by the balance of the
adatom-adatom and adatom-surface interaction. The
comparison between the same adsorbate on the differ-
ent substrates and between the different adsorbates on
the same substrate is a promising way to reveal how
the subtle balance between the lateral and vertical in-
teractions affects the growth and the structure of the
overlayer. We have observed the system of Kr and Xe
on Ag(111), Ag(110) and Ag(100). In our study, the
thickness of the overlayer was observed by
ellipsometry and the surface geometry by LEED.
Ellipsometry has no restriction on surrounding pres-
sure and does not destroy a physisorbed layer. Our
LEED system was prepared to be free from ESD prob-
lem as described in Sec. 2.
We present here the results on the growth and the
structure of Kr films on Ag(111) and Ag(100) obtained
using these two techniques and discuss how the sub-
strate geometry affects the structure of the overlayer.
Comparison between the results on Xe/Ag(111),
Xe/Ag(100) and Kr/Ag(111), Kr/Ag(100) systems
is also made.
2. Experimental
Our experimental system makes it possible to ob-
serve the surface geometry by XLEED and the film
growth by ellipsometry simultaneously [3]. The sche-
matic diagram is shown in Fig. 1. The observation of
© Aki Tosaka, Taro Mitake, Takashi Miura, and Ichiro Arakawa, 2003
the layer growth can be made under the quasi-equilib-
rium condition because ellipsometry has no restriction
on surrounding pressure.
Our XLEED system utilizes the position sensitive
detector using a micro channel-plate (MCP) electron
multiplier in a pulse counting mode [3]. The detection
area is 75 mm in diameter. The incident electron beam
current was about 1 pA which was adjusted so that the
total electron counts became 105–106 over the whole
detector area for one minute data accumulation which
yielded a sharp diffraction pattern with enough statis-
tics for analysis.
The thickness of an adsorbed film was determined
by the ellipsometric parameters; «relative phase shift»
� and «amplitude reflectance ratio» �. The relative
change in �, �� � �� � 0 , where �
�
is the shift for the
bare substrate, is proportional to the overlayer cover-
age and has been used as a monitor of the amount of ad-
sorption in the ellipsometric studies of physisorbed
films on metal surfaces [4]. The ellipsometer is a polar-
izer-compensator-sample-analyzer (PCSA) automatic
null system [3].
Silver substrates were prepared by repeated cycles
of sputtering by a Kr ion (1 keV, 5–8 �A, 2 min) and
annealing (700 K, 3 hours). The base pressure of the
main chamber was 10–8 Pa or lower. The substrates of
Ag(111) and Ag(100) are mounted side by side on the
sample holder which was cooled by cold He gas.
3. Results
3.1. The growth and the structure of Kr/Ag(111)
The diffraction patterns of Kr/Ag(111) are shown
in Fig. 2,a, which demonstrates a hexagonal structure
of the Kr monolayer film whose unit vectors align
with those of the substrate. The ellipsometric isotherm
of Kr/Ag(111) at 50.5 K is shown in Fig. 2,b, where
the isotherm is represented by the change in relative
phase shift �� due to adsorption as a function of Kr
pressure. The isotherm for Kr on Ag(111) has several
steps. Each step is due to a first-order phase condensa-
tion where 2D gas and solid coexist and corresponds to
the formation of one atomic layer. This isotherm for
Kr—Ag(111) at 50.5 K plotted in Fig. 2,b shows layer
by layer growth up to three atomic layers at least. A
Kr–Kr spacing in the monolayer, measured by
XLEED simultaneously, is also shown in Fig. 2,b. The
interatomic distance was calculated using the spot dis-
tances in the diffraction pattern, the incident electron
energy, the distance between the sample and the
MCP, which was determined by the Ag spot distance
in the diffraction pattern of bare Ag surfaces. The
Kr–Kr spacing is 0.440 nm just after the first layer
condensation and 0.436 nm before the second layer
condensation; the monolayer compression ratio was
less than 1%. The XLEED pattern of the monolayer
Kr which has both Ag and Kr spots, as shown in
1028 Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10
Aki Tosaka, Taro Mitake, Takashi Miura, and Ichiro Arakawa
FC1 FC2
Laser
lens
Polarizer
Compensator
Vacuum
gauge
XLEED
PMT
AnalyzerIon gun
Gas inlet
system
Diaphragm
pumpTMP
TMP
MV
0 10 20 [cm]
Fig. 1. Schematic diagram of our experimental system.
à
3.0
2.5
2.0
1.5
1.0
0.5
0
0.46
0.44
0.42
0.40
K
r—
K
r
sp
a
ci
n
g
, n
m
�
�
, d
e
g
10
–7
10
–5
10
–3
10
–1
p, Pa
bulk Kr
b
Fig. 2. (a) XLEED pattern of Kr monolayer on Ag(111).
Ei = 104 eV, P = 3·10–5 Pa, T = 51.3 K. The incident
electron energy was adjusted to the spots both Ag and Kr
appeared: outside six-fold spots shows Ag and inside spots
Kr. (b) Adsorption isotherm of Kr/Ag(111) (stepwise
line) and Kr–Kr spacing (solid circle) at 50.5 K.
Fig. 2,a, was examined to confirm the absolute value
of Kr–Kr spacing by direct comparison with Ag inter-
atomic distance of 0.288 nm. It was confirmed that
Kr–Kr spacing is about 8–9% larger than Kr bulk
value of 0.403 nm [5]. The experimental errors were
respectively about 1% for the Ag interatomic distance
and 3% for Kr–Kr spacing because the spots of Kr
were broader than the spots of the bare Ag.
3.2. The structure of Kr/Ag(100)
The structure of Kr films on Ag(100) was observed
by XLEED. It was found that the Kr film on Ag(100)
has two types of alignment; the corresponding dif-
fraction patterns are shown in Fig. 3,a and Fig. 4,a.
The scheme of the diffraction patterns and the ar-
rangement of the atoms in the real space are shown in
Figs. 3,b,c and Figs. 4,b,c, respectively. As shown in
Fig. 3 one of the unit vectors of the Kr film aligns
with <001> direction of Ag. We call this type of
alignment «along Ag <001> type». On the other
hand, in Fig. 4 one of the unit vectors of the Kr film
aligns with <011> of Ag: «along Ag <011> type».
The systematic observation by scanning the sample
surface with an incident electron beam revealed that
the Kr film has both types of alignment but «along Ag
<001> type» is predominant.
We observed that Kr–Kr spacing in the «along Ag
<001> type» monolayer was 0.450 nm, which is 11%
larger than that of bulk, as it was also observed for
Kr/Ag(111) case.
4. Discussion
4.1. Kr–Kr spacing of Kr/Ag(111) and
Kr/Ag(100)
The fact that Kr–Kr spacing is about 10% larger
than that of bulk must be a matter of discussion. We
obtained the similar results on both Ag(111) and
Ag(100). Roberts and Pritchard studied the Kr–Kr
Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100)
Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10 1029
a b
[001][010]
c
Fig. 3. (a) XLEED pattern of Kr/Ag(100). Ei = 66.2 eV, P = 3.0�10–5 Pa, T = 53.6 K. (b) Schematic of diffraction
pattern shown in (a). Open circles show Kr spots and crosses show the substrate spots. (c) The illustration in the real
space.
a b
[011]
[011]
c
Fig. 4. (a) XLEED pattern of Kr/Ag(100). Ei = 66.2 eV, P = 3.0·10–5 Pa, T = 52.3 K. (b) Schematic of diffraction pat-
tern shown in (a). Open circles show Kr spots and crosses show the substrate spots. (c) The illustration in the real
space.
spacing on Ag(111) at 55 K and found that it was
0.419 nm [1]. Ungris et al. investigated Kr–Kr spac-
ing by LEED in the temperature range between 10 and
60 K and obtained value 0.401–0.407 nm [2]. There is
a considerable discrepancy in the absolute value of
Kr–Kr spacing between our results and those reported
by Roberts and Pritchard and by Ungris et al. This
discrepancy cannot be attributed only to our experi-
mental error of about 3%.
In the case of Xe/Ag(111), it was observed that
Xe–Xe spacing just after the first layer condensation
is about 3% larger than that of bulk. Xe–Xe spacing in
the monolayer decreases gradually with increasing the
pressure or lowering the temperature and reaches the
bulk value before the second layer condensation [3].
We observed that in the case of Kr monolayer on Ag
the monolayer compression ratio was no more than
1%. This result implies that the Kr film has a structure
commensurate with the substrate.
It is known that rare gas films on Ag have struc-
tures incommensurate with the substrate. However, if
a Kr film on Ag(111) may have Ag(111) 1.5 1.5 com-
mensurate structure, Kr–Kr spacing will be 0.432 nm
which is 7% larger than that of bulk Kr. This value is
between our result and that of Roberts and Pritchard
[1]. However, there is no adequate commensurate
structure of Kr/Ag(100).
4.2. The structure of Kr/Ag(100)
It is well known that the alignment of a rare gas
film on a metal surface is affected by the substrate
steps. A Xe film on Ag(111) has incommensurate
structure but the unit vectors of the overlayer align
with the substrate ones. The reason for this alignment
is explained by the inference that the overlayer is
pinned by substrate steps and its alignment is deter-
mined by the step direction. Leatherman et al. has
shown experimentally the effect of the surface steps
on the alignment; when a small amount of CO or K is
put on the surface to block the steps for rare gas
adsorption, the nucleation occurs on a terrace and the
overlayer grows in orientation rotated at a non-
symmetric angle [6].
In the present study, we found that a Kr film on
Ag(100) has two types of alignment and that «along Ag
<001> type» is predominant. There are some possible
reasons why «along Ag <001> type» is predominant.
The first inference is that most surface steps on
Ag(100) are directed along <001> and Kr atoms nucle-
ate at the step edge of the substrate. The second reason
is that the steps of Ag(100) are directed along both
<011> and <001>, and the step-pinning energy for the
Kr film on <001> Ag step edge is larger than that on
<011> step. The third circumstance is that Kr atoms
nucleate on the terrace of Ag(100) and its alignment is
determined by Novaco and McTague effect [7].
It was also observed that a Xe film on Ag(100) has
two types of alignment: «along Ag <001> type» and
«along Ag <011> type», and there are, however, no
preference between them.
The result that «along Ag <011> type» was ob-
served in the case of Xe/Ag(100) contradicts the sup-
position that surface steps of Ag(100) are oriented only
along <001> direction. The coexistence of two types of
alignment can be interpreted by two possible reasons.
The first reason is the combination of the step effect
and the Novaco and McTague effect [7]: one type of
alignment is determined by the step direction and the
other by the Novaco and McTague effect. The second
one is that the surface steps are along both <001> and
<011> direction on Ag(100) and its alignment within
the overlayer is determined by the step-pinning energy
of overlayer atoms at the step edges.
The one-dimensional structure period of Ag(100)
along <001> direction is 0.407 nm which is close to
the Kr bulk value. This agreement should have a close
relation to the preference of «along Ag <001> type»
in the case of Kr/Ag(100). However, the value of
0.407 nm does not accord with our result described in
Sec. 3.2. It is considered that, except for the agree-
ment with one-dimensional periodicity, there are more
complicated origins of Kr alignment.
Similar bi-structure system was found by Kiguchi
et al.: an alkali halide monolayer film on a fcc metal
surface. They reported that overlayer on the fcc(100)
surface has two types of alignment: one of them is the
expected growth and the other is not [8]. They con-
cluded that the alignment is determined by the degree
of surface diffusion. When the surface diffusion con-
stant is large, alkali halide atoms diffuse on metal sur-
face and nucleate at the step edges of the substrate,
which cause the unexpected growth. They anticipated
that the most surface steps of a fcc(100) surface would
be along <011> direction of the substrate. However,
the direct observation of the surface steps on fcc(100)
has not been performed.
5. Summary
The structure and layer by layer growth up to the
third layer of Kr films were investigated simulta-
neously by extremely-low-current low energy electron
diffraction and ellipsometry. The XLEED pattern
showed that a Kr monolayer film on Ag(100) has two
types of alignment: «along Ag <001> type» and
«along Ag <011> type». In the case of Kr/Ag(100),
«along Ag <001> type» is predominant. The existence
of two types of alignment was explained by two possi-
ble reasons: one is the combination of the step edge ef-
1030 Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10
Aki Tosaka, Taro Mitake, Takashi Miura, and Ichiro Arakawa
fect and the Novaco and McTague effect and another
is the existence of surface steps along both <001> and
<011> directions.
1. R.H. Roberts and J. Pritchard, Surf. Sci. 54, 687
(1976).
2. J. Ungris, L.W. Bruch, E.G. Moog, and M.B. Webb,
Surf. Sci. 109, 522 (1981).
3. S. Igarashi, Y. Abe, Y. Irie, T. Hirayama, and
I. Arakawa, J. Vac. Sci. Technol. A16, 974 (1998).
4. A. Itakura and I. Arakawa, J. Vac. Sci. Technol. A9,
1779 (1991).
5. M.L. Klein and J.A. Venables, in: Rare Gas Solid,
vol. II, Academic Press (1977), p. 783.
6. G.S. Leatherman, R.D. Diehl, M. Karimi, and
G. Vidali, Phys. Rev. B56, 6970 (1977).
7. A.D. Novaco and J.P. McTague, Phys. Rev. Lett. 38,
1286 (1977).
8. M. Kiguchi, S. Entani, K. Saiki, H. Inoue, and
A. Koma, Phys. Rev. B, to be published.
Observation of growth and structure of Kr films physisorbed on Ag(111) and Ag(100)
Fizika Nizkikh Temperatur, 2003, v. 29, Nos. 9/10 1031
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