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Monodispersed Cr cluster formation by
plasma-gas-condensation
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S. Yamamuro, Kenji Sumiyama, K. Suzuki
JOURNAL OF APPLIED PHYSICS
85
1
483-489
1999-01-01
http://id.nii.ac.jp/1476/00004609/
doi: 10.1063/1.369476(http://dx.doi.org/10.1063/1.369476)
Copyright (1999) American Institute of Physics. This article may be downloaded for personal use only.
Any other use requires prior permission of the author and the American Institute of Physics.The
following article appeared in Journal of Applied Physics, 85(1), pp.483- 489 ; 1999and may be found at
http://link.aip.org/link/?jap/85/483
JOURNAL OF APPLIED PHYSICS
VOLUME 85, NUMBER 1
1 JANUARY 1999
Monodispersed Cr cluster formation by plasma-gas-condensation
S. Yamamuro,a) K. Sumiyama, and K. Suzuki
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
~Received 7 July 1998; accepted for publication 28 September 1998!
Nanometer-sized Cr clusters in the size range of 7.6–13 nm have been produced by a
plasma-gas-condensation-type cluster deposition apparatus, which combines a grow-discharge
sputtering technique with an inert gas condensation technique. We have studied influences of the Ar
gas pressure, P Ar , and the Ar gas flow rate, V Ar , on the size distribution of Cr clusters by
transmission electron microscopy. Monodispersed Cr clusters are formed at both low P Ar and low
V Ar . At low P Ar , Cr clusters nucleate and grow only in the liquid-nitrogen-cooled growth region,
and the deposition rate is rather low. At high P Ar , on the other hand, a large amount of Cr clusters
are formed even near the sputtering source, and the nucleation and growth occur over a wide region
between the sputtering source and the growth region. Under this condition, the deposition rate is
relatively high. Consequently, the formation mechanism of the monodispersed clusters is similar to
that of monodispersed colloidal particles: The nucleation and growth processes are definitely
separated and the coagulation of growing particles is prohibited. In the present experiments, these
conditions are effectively attained by using a carrier gas flow and liquid-nitrogen-cooling of the
cluster growth region. © 1999 American Institute of Physics. @S0021-8979~99!03701-9#
condensation-type ~PGC! cluster deposition apparatus7,8
which is similar to the one originally developed by Haberland et al.9 The PGC method combines a grow-discharge
sputtering technique with an inert gas condensation technique: Metal vapor is generated by sputtering at about 150
Pa pressure and condenses into clusters in a carrier gas flow.
In contrast to conventional thermal evaporation sources, this
method is advantageous to generating clusters consisting of
refractive or low-vapor-pressure metals because any kind of
metals can be vaporized by sputtering. In the present report,
we demonstrate monodispersed Cr cluster formation using
the PGC method, and elucidate its process by studying the
influences of process parameters ~the Ar gas pressure and the
Ar gas flow rate! on the formed clusters with transmission
electron microscopy ~TEM!.
I. INTRODUCTION
Nanostructured materials are topical in both fundamental
and applied sciences, because their nanometer-scale compositional and structural heterogeneities give rise to several
unique properties in contrast to homogeneous bulk materials.
Granular solids1 and nanocrystalline materials,2 for example,
exhibit fascinating magnetic, transport, and mechanical properties. These materials have been usually produced by precipitation from supersaturated solid solution or crystallization of amorphous alloys via annealing, or by consolidation
of fine particles produced by an inert gas condensation. In
these methods, however, it is difficult to control the nanostructural parameters precisely: For instance, the particle size
and its dispersion cannot be changed independently in the
granular solids.
Assembling of free nanometer-sized clusters as building
blocks is a good alternative process for constructing the
nanostructured functional materials.3 For this purpose, it is
crucial to produce monodispersed clusters. There have been
several reports on the formation of monodispersed metal,
polymer, and composite particles from submicro to micrometer in size by using a colloidal method,4,5 where surfactants are used for suppressing coagulation and coalescence growth. Controlling the concentration of dissolved
spices and the temperature during chemical reaction is also
effective to produce the monodispersed particles. In the conventional gas condensation method, however, there have
been only a few reports on the monodispersed cluster
formation,6 because the particle formation process is rather
complex.
Recently, we have constructed an intensive and sizecontrollable cluster deposition system, i.e., a plasma-gas-
II. EXPERIMENT
Figure 1 shows a schematic drawing of the PGC apparatus. It mainly consists of three components: a sputtering
chamber, a growth region, and a deposition chamber. The
growth region was cooled with liquid nitrogen. Comparing
with a previous apparatus,7 we put an additional differentialpumping chamber prior to the deposition chamber to keep
the deposition chamber in a better vacuum condition. A
facing-target-type direct current dc sputtering source was
used for vaporizing Cr targets, whose sizes are 70 mm in
diameter. The distance between the two targets is 100 mm.
The sputtering source, which is a combination of a hollowcathode discharge mode and a magnetron mode, was operated at high Ar gas pressure, P Ar , of 70–360 Pa. The magnetic field of about 20 mT was applied to the space between
the two targets to get a high ionization rate of Ar gas and a
high sputtering rate. A large amount of Ar gas ~the Ar gas
flow rate, V Ar51.731026 – 2.031025 m3/s was injected
a!
CREST Fellow; electronic mail: [email protected]
0021-8979/99/85(1)/483/7/$15.00
483
© 1999 American Institute of Physics
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484
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Yamamuro, Sumiyama, and Suzuki
FIG. 1. Schematic drawing of PGC-type cluster deposition apparatus. TMP,
MBP, and CMP represent turbomolecular pump, mechanical booster pump,
and compound molecular pump, respectively.
continuously into the sputtering chamber from a gas inlet,
and effectively evacuated by a mechanical booster pump
~MBP! through a small nozzle whose typical diameter was
3.5 mm. Clusters were formed in a carrier gas flow, and
ejected through the small nozzle and two skimmers by differential pumping, and then deposited onto a substrate fixed
on a sample holder in the deposition chamber (;1
31022 Pa). The substrate temperature was at about 300 K
during the deposition. If the nozzle diameter of the growth
region is fixed, P Ar depends on V Ar . If the nozzle diameter is
varied adequately, on the other hand, we can control P Ar and
V Ar independently. The input power for sputtering was 300
W; the typical current and voltage values were 1 A and 300
V, depending strongly on P Ar .
The microgrid, which consists of carbon-coated colodion
film supported by a Cu grid, was used as a substrate for TEM
observation. The Cr clusters were usually deposited in the
deposition chamber with the weight thickness of 2 nm,
which was measured by a quartz oscillator-type thickness
monitor. To clarify the cluster formation region, we also deposited for 5 min at three positions: the sputtering chamber,
and the entrance and the exit of the growth region. We observed cluster images using a 200 kV Hitachi HF-2000
TEM, and stored them as digital data with a slow scan
charge coupled device camera installed in TEM. Using an
image-analysis software ~Image-Pro PLUS: Media Cybernetics!, we measured the cluster size distributions from these
digitized images in the area of 3503350 nm2.
III. RESULTS
In the inert gas condensation under the carrier gas flow,
it has been pointed out that the cluster size strongly depends
on V Ar ~or P Ar!.10 Thus, we first examined the influence of
V Ar on the cluster size distribution. Figures 2 and 3 show the
bright-field TEM images and the size distributions of Cr
clusters produced at several V Ar values of 3.331026 – 1.67
31025 m3/s. In these experiments, P Ar also increases from
120 to 360 Pa with increasing V Ar from 3.331026 to 1.67
31025 m3/s. For the low V Ar of 3.331026 – 6.731026
FIG. 2. Bright-field TEM images of Cr clusters produced at several Ar gas
flow rates, V Ar : ~a! 3.331026 m3/s, ~b! 6.731026 m3/s, ~c! 1.0
31025 m3/s, ~d! 1.3331025 m3/s, and ~e! 1.6731025 m3/s. The Ar gas
pressure, P Ar , was also varied with V Ar .
m3/s, uniform-sized Cr clusters were obtained. However, the
size distribution suddenly becomes broad at V Ar51.0
31025 m3/s and broader with further increasing V Ar . At
V Ar>1.031025 m3/s, in particular, there are bimodal peaks
locating at about 5 and 20 nm ~or more!. These size distributions indicate that the clusters corresponding to each peaks
are formed through different processes, because the cluster
size distribution is crucially affected by its formation process. The mean cluster diameter and the standard deviation
estimated from Figs. 3~a! to 3~e! and the other two samples
are shown in Fig. 4~a!. With increasing V Ar , the mean cluster diameter initially increases from 7.6 nm at V Ar53.3
31026 m3/s, and shows a maximum value of 13 nm at
around V Ar51.031025 m3/s. The standard deviation shows
a constant value of about 0.8 nm for V Ar53.331026 – 6.7
31026 m3/s. This value is less than 10% of the mean cluster
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Yamamuro, Sumiyama, and Suzuki
485
FIG. 4. ~a! Mean cluster diameter and standard deviation estimated from
Figs. 3~a! to 3~e! and other two samples, and ~b! typical deposition rate, as
a function of the Ar gas flow rate, V Ar .
31025 m3/s with varying P Ar from 120 to 280 Pa. The
monodispersed Cr clusters were prepared at both P Ar5120
and 170 Pa, whereas the size distribution becomes markedly
broad at P Ar5280 Pa. The mean cluster diameter and the
standard deviation evaluated from Figs. 6~a!–6~c! are shown
in Fig. 7~a!. With increasing P Ar from 120 to 280 Pa, the
FIG. 3. Size distributions of Cr clusters estimated from Figs. 2~a! to 2~e!.
diameter, showing a good monodispersivity. At V Ar>1.0
31025 m3/s, however, the standard deviation rapidly increases with V Ar . Similar V Ar dependence of the mean cluster diameter and the standard deviation has been obtained in
the previous experiments performed with a diode dc sputtering source.7 Therefore, these features are characteristic for
the PGC apparatus. Figure 4~b! shows a typical deposition
rate as a function of V Ar . The deposition rate shows a maximum value at around V Ar51.331025 m3/s. At V Ar,3.3
31026 m3/s, the deposition rate is almost negligible, indicating that clusters are hardly formed under such low V Ar
conditions.
It has not been clear which process parameter affects the
size distribution in the above mentioned results, because P Ar
and V Ar varied simultaneously. Thus, we controlled them
independently by varying the nozzle diameter of the growth
region, and examined their effects on the cluster size distribution. Figures 5 and 6 show the TEM images and the size
distributions of Cr clusters deposited at constant V Ar51.0
FIG. 5. Bright-field TEM images of Cr clusters produced by varying the Ar
gas pressure, P Ar : ~a! 120 Pa, ~b! 170 Pa, and ~c! 280 Pa. The Ar gas flow
rate, V Ar , was constant at 1.031025 m3/s.
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486
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Yamamuro, Sumiyama, and Suzuki
FIG. 6. Size distributions of Cr clusters estimated from Figs. 5~a! to 5~c!.
mean cluster diameter increases from 8.6 to 13.2 nm and the
standard deviation increases from 0.7 to 5.8 nm, indicating
that small and monodispersed clusters can be formed at low
P Ar . Figure 7~b! shows a typical deposition rate as a function of P Ar , indicating that the deposition rate increases
monotonically with P Ar . Thus, it can be said that the cluster
FIG. 8. Bright-field TEM images of Cr clusters produced by varying the Ar
gas flow rate, V Ar : ~a! 5.031026 m3/s, ~b! 1.031025 m3/s, ~c! 1.42
31025 m3/s, and ~d! 2.031025 m3/s. The Ar gas pressure, P Ar , was constant at 170 Pa.
FIG. 7. ~a! Mean cluster diameter and standard deviation estimated from
Figs. 6~a! to 6~c!, and ~b! typical deposition rate, as a function of the Ar gas
pressure, P Ar , at constant Ar gas flow rate, V Ar , of 1.031025 m3/s.
formation is promoted at higher P Ar as reported in the conventional gas evaporation method.11,12
Figures 8 and 9 show the TEM images and the size
distributions of Cr clusters deposited at constant P Ar
5170 Pa with varying V Ar from 5.031026 to 2.0
31025 m3/s. The monodispersed Cr clusters were formed at
low V Ar (<1.031025 m3/s), whereas the cluster size distribution becomes broad at high V Ar (>1.4231025 m3/s). The
mean cluster diameter and the standard deviation estimated
from Figs. 9~a! to 9~d! are shown in Fig. 10~a!. As V Ar increases from 5.031026 to 2.031025 m3/s, the mean cluster
diameter does not vary much at around 10 nm, while the
standard deviation increases monotonically from 0.54 to 3.5
nm. As shown in Fig. 10~b!, the deposition rate reveals no
simple V Ar dependence, however, it obviously decreases at
low V Ar .
For understanding the cluster formation process, it is
necessary to clarify where the clusters nucleate and grow.
Thus, we deposited the growing clusters onto the TEM microgids in the sputtering chamber and the growth region, and
then observed them by TEM. Figure 11 shows the TEM
images of the samples deposited at different positions and
V Ar values. In these experiments, P Ar also increases from 70
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Yamamuro, Sumiyama, and Suzuki
487
FIG. 11. Bright-field TEM images of Cr clusters deposited at different Ar
gas flow rates, V Ar , of 1.731026 – 1.031025 m3/s and different positions.
The samples were deposited near the sputtering target, at the entrance to the
growth region, and the exit from the growth region. The Ar gas pressure,
P Ar , was also varied with V Ar .
FIG. 9. Size distributions of Cr clusters estimated from Figs. 8~a! to 8~d!.
to 280 Pa with increasing V Ar from 1.731026 to 1.0
31025 m3/s. At V Ar51.731026 m3/s, the deposit forms island morphology both near the sputtering target and at the
entrance to the growth region, while there is almost no deposit at the exit from the growth region. Under this condition, no clusters are observed at all the deposited positions.
At V Ar55.031026 m3/s, there exist only the island morphology without any clusters at the positions of both the
sputtering target and the entrance to the growth region. However, small clusters are observed at the exit from the growth
region. These results indicate that the clusters nucleate and
grow only in the liquid-nitrogen-cooled growth region. At
V Ar51.031025 m3/s, large particles ~some of them exceed
100 nm in diameter! are formed even near the sputtering
target and at the entrance to the growth region. However,
such large particles suddenly disappear at the exit from the
growth region, probably because they drop out from the carrier gas stream owing to their high gravity. By contrast, a
large number of small clusters are observed there. This indicates that the nucleation and growth occur in a wide region at
high V Ar .
IV. DISCUSSION
FIG. 10. ~a! Mean cluster diameter and standard deviation estimated from
Figs. 9~a! to 9~d!, and ~b! typical deposition rate, as a function of the Ar gas
flow rate, V Ar , at constant Ar gas pressure, P Ar , of 170 Pa.
In the colloidal method, a particle formation process is
generally explained by a LaMer’s figure,13 which provides a
concentration evolution of dissolved species as a function of
the reaction time. There are incubation, nucleation, and
growth stages in the particle formation process. In the incubation stage, the concentration of the dissolved species increases with the reaction time by adding the solute or changing the temperature. Since the concentration is lower than a
critical supersaturation, the nucleation can be negligible in
this stage. On reaching the critical supersaturation concentration, the clusters start to nucleate ~the nucleation stage!.
Since the formed nuclei grow by absorbing the dissolved
atoms or molecules, the concentration of the dissolved species decreases rapidly, suppressing the nucleation probabil-
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488
J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
ity. After the concentration of the dissolved species becomes
lower than the critical supersaturation concentration, the
nucleation process almost stops and only the stable nuclei
grow by diffusion of the dissolved species to the nuclei and
coagulation of the growing particles ~the growth stage!.
In order to produce the monodispersed colloidal particles, the following two prerequisites must be satisfied.4
First, the nucleation and growth stages must be separated
distinctly. Since the nuclei grow stocastically also in the
nucleation stage, the long nucleation stage leads to the broad
size distribution. Thus, for the monodispersed particle formation, the nucleation must be finished in the early stage of the
particle formation process, and then only the initially formed
nuclei must be grown without further nucleation. Second, the
coagulation of the growing particles must be prohibited because it significantly broadens the particle size distribution.
It is important to control the nucleation and growth processes precisely for the monodispersed cluster formation in
the gas condensation method as well as in the colloidal
method. However, It is difficult to control them in the conventional gas evaporation method, because the particles
nucleate and grow rapidly in a narrow region just above an
evaporation source.11,12 When we use the carrier gas flow,
we can extend the nucleation and growth regions.14 Indeed,
the nucleation does not occur near the sputtering source at
low P Ar in the present experiments as shown in Fig. 11.
Hence, we can control the nucleation and growth processes
independently in the present PGC apparatus.
As shown in Fig. 11, it is revealed that the nucleation
and growth are restricted only in the liquid-nitrogen-cooled
growth region under the monodispersed cluster formation
condition ~V Ar55.031026 m3/s, i.e., P Ar5170 Pa! and it
extends over a wide region under the broad-sized cluster formation condition ~V Ar51.031025 m3/s, i.e., P Ar5280 Pa!.
These results can be understood in terms of the first prerequisite mentioned above. At low P Ar , the sputtered metal atoms do not aggregate quickly into clusters, because of low
probability of three-body collisions among metal and inert
gas atoms. Thus, we can control the nucleation process effectively by liquid nitrogen cooling of the growth region, by
which the supersaturation ratio of the sputtered metal vapor
increases rapidly. This promotes the nucleation process and
rapidly reduces the number density of sputtered metal atoms
by absorption into the nuclei. Therefore, the nucleation process is terminated instantaneously, and the nucleation and the
growth stages are clearly separated from each other. Hence,
this leads to the monodispersed cluster formation. We have
confirmed that the liquid nitrogen cooling of the growth region reduces a width of the size distribution.15 At high P Ar ,
on the other hand, clusters nucleate and grow even near the
sputtering source, where the formed clusters grow by both
atom absorption and coagulation because the number densities of the sputtered metal atoms and the nuclei are significantly large near the target. Thus, it is difficult to control the
nucleation process precisely. Moreover, the cluster formation
process extends over the wide region from the sputtering
source to the growth region. Hence, the nucleation and
growth stages do not separate clearly, and the size distribution becomes broad.
Yamamuro, Sumiyama, and Suzuki
The second prerequisite mentioned above requires the
low number density of nuclei ~i.e., clusters! in the carrier gas
flow. As shown in Figs. 4~b!, 7~b!, and 10~b!, the deposition
rates are low when we obtained the monodispersed clusters
at low P Ar . The number of clusters deposited at the exit
from the growth region at low P Ar ~170 Pa! is also smaller
than that at high P Ar ~280 Pa! as shown in Fig. 11. These
results indicate that the number density of nuclei is low under the monodispersed cluster formation condition. In fact,
the number density of the sputtered metal atoms in the
present experiments is roughly estimated to be about
1020 atoms/m3 with referring the sputtering rate at P Ar
5133 Pa. 16 Since this value is 102 – 103 times smaller than
the number density of Ar gas atoms, the number density of
nuclei is also low. Therefore, the collisions among clusters or
nuclei are hard to occur, preventing the coagulation of the
clusters. At high P Ar , on the other hand, the deposition rate
is higher than that at low P Ar . Figure 11 also shows that a
large number of clusters are deposited at the exit from the
growth region at high P Ar ~280 Pa!. These results indicate
that the number density of nuclei is high. Therefore, the collisions among clusters or nuclei will be easy to occur, promoting the coagulation. Consequently, we conclude that the
formation process of the monodispersed clusters in the gas
phase is similar to that of the colloidal particle formation.
The carrier gas flow rate also influences the dispersion of
the cluster size distribution as shown in Fig. 10~a!: The size
distribution becomes broad with increasing V Ar . This result
will be related to the state of the carrier gas stream. With
increasing V Ar , the number density of Ar gas atoms is constant at the same P Ar , but the thermalization and cluster
formation processes are disturbed in the faster carrier gas
stream because these processes are stochastic. Therefore, the
size distribution becomes broad, though the mean cluster size
is not sensitive to V Ar . For a detailed understanding, it is
necessary to perform a computer simulation of the gas
stream including metal clusters formed in the sputtering
chamber and the growth region. Moreover, the low temperature process of the sputtering technique will be also favorable to produce monodispersed clusters, because it suppresses the coalescence growth during the coagulation.
V. CONCLUSION
We have deposited nanometer-sized Cr clusters on TEM
microgrids by the PGC method and observed them by TEM.
The monodispersed Cr clusters were obtained in the size
range of 7.6–13 nm by varying the Ar gas pressure, P Ar , and
the Ar gas flow rate, V Ar . In order to clarify the cluster
formation process, we studied the effects of these process
parameters on the cluster size distributions. The size distribution becomes narrower with decreasing both P Ar and V Ar :
The monodispersed clusters can be formed at low P Ar and
low V Ar . At low P Ar , the nucleation and growth occur only
in the liquid-nitrogen-cooled growth region, and the deposition rate is rather low. At high P Ar , on the other hand, a
number of large clusters are formed even near the sputtering
source, and the nucleation and growth region extend over a
wide region from the sputtering source to the growth region.
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J. Appl. Phys., Vol. 85, No. 1, 1 January 1999
Under the latter condition, the deposition rate is relatively
high. These results demonstrate that the monodispersed cluster formation process in the PGC method is analogous to the
monodispersed colloidal particle formation: The nucleation
and growth processes are separated definitely and the coagulation of growing clusters is prohibited. In the present experiments, these conditions are achieved effectively by using the
carrier gas flow and liquid nitrogen cooling of the cluster
growth process.
ACKNOWLEDGMENT
The authors thank Dr. M. Sakurai and Dr. T. J. Konno
for useful comments. This work was supported by Core Research for Evolutional Science and Technology ~CREST! of
Japan Science and Technology Corporation ~JST!. They are
also indebted to Laboratory for Developmental Research of
Advanced Materials in our Institute ~IMR! for its support.
1
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See, for example, Y. Yoshizawa, S. Oguma, and K. Yamauchi, J. Appl.
Yamamuro, Sumiyama, and Suzuki
489
Phys. 64, 6044 ~1988!; H. Gleiter, Prog. Mater. Sci. 33, 223 ~1989!; R. W.
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3
P. Melinon et al., Int. J. Mod. Phys. B 9, 339 ~1995!.
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T. Sugimoto, Adv. Colloid Interface Sci. 28, 65 ~1987!.
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See, for example, E. Matijevic, Annu. Rev. Mater. Sci. 15, 483 ~1985!;
Fine Particles Science and Technology: From Micro to Nanoparticles,
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M. Oda and N. Saegusa, Jpn. J. Appl. Phys., Part 2 24, L702 ~1985!.
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S. Yamamuro, M. Sakurai, K. Sumiyama, and K. Suzuki, AIP Conf. Proc.
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S. Yamamuro, K. Sumiyama, and K. Suzuki ~unpublished!.
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