Structural evolution of W nano clusters with increasing cluster size

Article abstract of DOI:10.1063/1.480063

We have recorded.the x-ray diffraction (XRD) patterns of nanometer-size W metal clusters prepared at different average cluster sizes. Nanometer-size W metal clusters were produced through a collision induced clustering mechanism of W metal atoms generated by decomposing W(CO)₆ vapors. The XRD patterns clearly showed that structure changed from amorphous→face-centered-cubic (fcc)→body-centered-cubic (bcc) with increasing average cluster size. This implies that W metal clusters do not simply approach the bulk bcc structure but pass through an intermediate fcc structure before they reach the bulk structure, as predicted by Tomanek, Mukherjee, and Bennemann [Phys. Rev. B 28, 665 (1983)].

Full text of DOI:10.1063/1.480063

Structural evolution of W nano clusters with increasing cluster size
S. J. Oh, S. H. Huh, H. K. Kim, J. W. Park, and G. H. Lee
Citation: The Journal of Chemical Physics 111, 7402 (1999); doi: 10.1063/1.480063
View online: http://dx.doi.org/10.1063/1.480063
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 16
22 OCTOBER 1999
Structural evolution of W nano clusters with increasing cluster size
S. J. Oh, S. H. Huh, H. K. Kim, J. W. Park, and G. H. Leea)
Department of Chemistry, College of Natural Sciences, Kyungpook National University,
Taegu 702-701, South Korea
͑Received 8 June 1999; accepted 3 August 1999͒
We have recorded the x-ray diffraction ͑XRD͒ patterns of nanometer-size W metal clusters prepared
at different average cluster sizes. Nanometer-size W metal clusters were produced through a
collision induced clustering mechanism of W metal atoms generated by decomposing W͑CO͒
6
vapors. The XRD patterns clearly showed that structure changed from amorphous→face-
centered-cubic ͑fcc͒→body-centered-cubic ͑bcc͒ with increasing average cluster size. This implies
that W metal clusters do not simply approach the bulk bcc structure but pass through an intermediate
´
fcc structure before they reach the bulk structure, as predicted by Tomanek, Mukherjee, and
Bennemann ͓Phys. Rev. B 28, 665 ͑1983͔͒. © 1999 American Institute of Physics.
͓S0021-9606͑99͒02540-4͔
The cluster is an ideal model to study the evolution of a
variety of properties of clusters towards the corresponding
bulk properties with increasing cluster size. In this paper, we,
for the first time, directly observed structural evolution from
amorphous ͑perhaps, molecular͒ to bulk structures in
nanometer-size W metal clusters with increasing cluster size.
Structural evolution of metal clusters with cluster size of
nϽ10 has been well studied by various theoretical
calculations1 relying on spectroscopic results.2 For metal
clusters of nϽ10, structure changes dramatically with cluster
size. It is expected that the cluster will show a crystal struc-
ture at a certain cluster size and eventually possess a bulk
structure as cluster size approaches the bulk limit. A question
arises regarding whether or not the cluster will monotoni-
cally approach the bulk structure as cluster size increases.
carbonyl vapor (40–150 Torr͒/SF6 ͑50 Torr͒/Ar ͑200 Torr͒
was used. An appropriate amount of solid sample of W͑CO͒
6
which corresponds to a certain vapor pressure at room tem-
perature was weighed and then loaded into the chamber. The
chamber was heated up to 150–200 °C to vaporize the solid
sample. Metal clusters were produced through reaction
which began by photosensitized SF6 molecules which ab-
sorbed 10.6 ␮m photons from the CO2 laser and transferred
vibrational energies to metal carbonyl vapors through
collisions.5 Vibrationally excited metal carbonyl vapors be-
came thermally excited after further collisions with other
molecules and then finally decomposed into neutral metal
atoms and COs. As presented in Fig. 1 ͑b͒ ͑thermal decom-
position method͒, the reaction chamber is identical to that
used for the multiphoton decomposition method except for
the top flange, which is equipped with two filament
feedthroughs instead of a KBr window. The filament con-
nected to the top flange electrical feedthroughs is a resistive
heater and was used to decompose metal carbonyl vapors. A
1 mm thick nichrome ͑Ni/Cr/Fe alloy͒ wire with a total re-
sistance of 2.3 ohms was used in this experiment. Typical
filament voltage was 20–25 VAC, at which only pure W
metal clusters were produced. Metal carbides were, however,
also produced when a high filament voltage ͑40 VAC͒ was
used.6 In both methods, black and nanometer-size metal clus-
ters were produced as final products through collisions be-
tween decomposed neutral metal atoms. Cluster size could
be changed by varying production conditions, which were
metal carbonyl vapor and Ar pressures in the multiphoton
decomposition method and metal carbonyl vapor pressure
and filament voltage in the thermal decomposition method.
Cluster size produced by the thermal decomposition method
was generally larger than that produced by the multiphoton
decomposition method, which we think is probably because
metal density produced by the thermal decomposition
method is higher than that by the multiphoton decomposition
method. Purity of W metal clusters was more than 95% in
both methods. However, impurity particles ͑i.e., metal car-
bides͒ other than W metal clusters were also produced at a
´
Tomanek et al. had theoretically predicted that metal clusters
with the bulk body-centered-cubic ͑bcc͒ structure would
have an intermediate face-centered-cubic ͑fcc͒ structure be-
fore they reach the bulk structure in order to decrease a large
surface free energy when cluster size is small.3 Here, we
experimentally addressed this issue by investigating the
structural evolution of nanometer-size W metal clusters with
increasing cluster size.
W metal clusters were produced using both the multi-
photon decomposition method4 and thermal decomposition
method of W͑CO͒ vapors. As presented in Fig. 1 ͑a͒ ͑mul-
6
tiphoton decomposition method͒, the experimental setup
consists of the stainless-steel chamber and the CO2 laser.
The chamber is a pipe type with a dimension of length ͑6 in.͒
ϫinner diameter ͑1 in.͒. The KBr window is epoxy-glued to
the center of the 3 in. conflat flange to pass the laser beam.
The chamber is evacuated by a mechanical pump down to
10Ϫ3 Torr. Even at this vacuum level, impurities such as
metal oxides were minimal. The sealed CO2 laser ͑10.6 ␮m͒
was operated with a repetition rate of 1 Hz, a pulse energy of
1 mJ, and a pulse duration of 1 ms. A gas mixture of metal
a͒
Author to whom correspondence should be addressed. Electronic mail:
ghlee@bh.kyungpook.ac.kr
0021-9606/99/111(16)/7402/3/$15.00
7402
© 1999 American Institute of Physics
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J. Chem. Phys., Vol. 111, No. 16, 22 October 1999
Structural evolution of W nano clusters
7403
FIG. 1. ͑a͒ A schematic experimental setup of multiphoton decomposition method. ͑b͒ A schematic experimental setup of thermal decomposition method.
high filament voltage in thermal decomposition method ͓see
Fig. 2 ͑VI͔͒.
Figures 2 ͑I͒–͑VI͒ represent the x-ray diffraction ͑XRD͒
patterns of W nano clusters produced at different conditions.
Metal clusters in ͑I͒–͑IV͒ were produced by the multiphoton
decomposition method, whereas those in ͑V͒ and ͑VI͒ were
produced by the thermal decomposition method. Clusters in
͑I͒ mostly consist of amorphous W clusters. Clusters in ͑II͒–
͑IV͒ consist of both amorphous and fcc W clusters. Clusters
in ͑V͒ consist of only fcc W clusters. Clusters in ͑VI͒ consist
of fcc and bcc W clusters with additional hexagonal ␣-W2C
clusters. Cluster size distribution of each structure in each
XRD pattern ͓͑I͒–͑V͔͒ is expected to be nearly homogeneous
because clusters were produced under supersaturation condi-
tion of W metal atoms in both production methods.7 We also
noticed that cluster size of a given structure did not much
depend on source condition but on production method, as
can be noticed from the full width at half maximum
͑FWHM͒ of fcc structure as discussed below ͑also see Table
I͒.
The full width at half maximum ͑FWHM͒ of the peak is
related to cluster size through Scherrer’s formula because of
somewhat homogeneity in cluster size distribution for each
structure in each XRD pattern.8 A wider FWHM indicates a
smaller cluster size. Peak positions ͑2␪) and the FWHMs of
the peaks were determined by fitting the peaks to Lorentzian
function. In ͑II͒ and ͑IV͒, the 2␪ and the FWHM of the
amorphous peak were fixed to the values in ͑I͒ because their
values are nearly constant. As provided in Table I, FWHM
͓amorphous, ͑I͔͒ ϾFWHM ͓fcc, ͑II͔͒уFWHM ͓fcc,
͑III͔͒уFWHM ͓fcc, ͑IV͔͒ϾFWHM ͓fcc, ͑V͔͒уFWHM ͓fcc,
͑VI͔͒ϾFWHM ͓bcc, ͑VI͔͒. As presented in Fig. 2, the amor-
phous peak intensity decreases from ͑I͒ to ͑IV͒, whereas the
fcc peak intensity increases and finally, only an fcc peak and
both fcc and bcc peaks appear in V and VI, respectively.
These results clearly show that structure changes from
amorphous→fcc→bcc with increasing cluster size. Thus, W
metal clusters do not simply approach the bulk bcc structure
but pass through an intermediate fcc structure before they
reach the bulk structure.
FIG. 2. The XRD patterns of W metal clusters. The assignments are the
Miller indices ͑hkl͒. The W metal cluster size generally increases in the
order of ͑I͒→͑II͒→͑III͒→͑IV͒→͑V͒→͑VI͒ in the XRD patterns. Metal clus-
ters in the ͑I͒–͑IV͒ XRD patterns were produced at W͑CO͒ vapor pressure
6
We estimated the average cluster diameter (d) of each
structure in each XRD pattern using Scherrer’s formula and
finally obtained the average cluster size (n) using a relation-
ship nХcluster volume/atomic volumeϭ(d/2ro)3 f in which f
is the packing fraction ͑0.74 and 0.68 for fcc and bcc struc-
tures, respectively͒ and ro is the atomic radius ͑ϭ 0.148 nm
of 44, 66, 99, and 142 Torr, respectively, and at laser condition of 1 Hz
repetition rate, 1 mJ pulse energy, and 1 ms pulse duration using multipho-
ton decomposition method. Metal clusters in the ͑V͒ and ͑VI͒ XRD patterns
were produced at W͑CO͒ vapor pressure of 200 Torr and filament tempera-
6
ture of ϳ400 and ϳ800 °C, respectively, ͑see Ref. 6͒ using thermal decom-
*
position method. In the ͑VI͒ XRD pattern, indicates the ͑101͒ plane of
hexagonal ␣-W2C particles.
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7404
J. Chem. Phys., Vol. 111, No. 16, 22 October 1999
Oh et al.
TABLE I. Peak position ͑2␪͒ in deg, FWHM in deg, average cluster diameter (d) in nm, and average cluster
size (n).
XRD pattern
2␪ ͑hkl͒
FWHM
d a
na
͑I͒
͑II͒
39.38Ϯ0.10 ͑amorphous͒
37.67Ϯ0.10 ͑111͒
37.60Ϯ0.07 ͑111͒
37.35Ϯ0.03 ͑111͒
37.53Ϯ0.02 ͑111͒
37.21Ϯ0.03 ͑111͒
40.30Ϯ0.05 ͑110͒
5.33Ϯ0.86
1.52Ϯ0.07
1.44Ϯ0.04
1.42Ϯ0.01
0.76Ϯ0.01
0.67Ϯ0.03
0.36Ϯ0.05
0.91Ϯ0.17
3.16Ϯ0.16
3.34Ϯ0.09
3.38Ϯ0.02
6.32Ϯ0.10
7.16Ϯ0.32
13.46Ϯ2.69
30Ϯ17
900Ϯ140
1060Ϯ90
͑III͒
͑IV͒
͑V͒
͑VI͒
͑VI͒
1100Ϯ20
7200Ϯ400
10 470Ϯ1440
63 940Ϯ38 850
aError in d and n indicates size distribution estimated from error in FWHM.
1
ˇ ´ ´
V. Bonacic-Koutecky, J. Pittner, C. Fuchs, P. Fantucci, M. F. Guest, J.
taken from Mo atomic radius because Mo and W have a
similar atomic radius4͒. Values of 2␪, FWHM, d, and n,
respectively, are provided in Table I.
´
Koutecky, J. Chem. Phys. 104, 1427 ͑1996͒; C. Gatti, S. Polezzo, and P.
Fantucci, Chem. Phys. Lett. 175, 645 ͑1990͒; W. Ekardt, Z. Penzar, and
M. Sunjic, Phys. Rev. B 33, 3702 ͑1986͒; C. Yannouleas, E. Vigezzi, and
R. A. Broglia, ibid. 47, 9849 ͑1993͒; R. Kishi, S. Iwata, A. Nakajima, and
K. Kaya, J. Chem. Phys. 107, 3056 ͑1997͒; D. M. Lindsay, Y. Wang, and
T. F. George, ibid. 86, 3500 ͑1987͒.
´
In comparison to the theory by Tomanek et al. who pre-
dicted a critical cluster size, i.e., 5660 at which cluster struc-
ture changes from fcc to bulk bcc structures,3 the structural
transition from fcc to bcc is qualitatively consistent with our
observation. However, the theoretical critical cluster size is
somewhat smaller than our measured value, i.e., 10 470,
which is the lower bound of critical cluster size because a
small bcc peak begins to appear as presented in ͑VI͒ XRD
pattern.
Regarding the observed amorphous and fcc structures,
we speculate about them in the following way. The amor-
phous pattern is resultant from small clusters which possess a
half-filled second shell structure ͑with nϽ55, which corre-
sponds to the fully filled second shell cluster size͒. That is,
there exist many isomers in the half-filled second shell struc-
ture, which thus eventually can yield an amorphous XRD
pattern. Remember that the amorphous W metal clusters had
ϳ30 atoms in one cluster as provided in Table I and as
discussed in the previous work.4 Regarding the fcc structure,
the fcc structure is more compact than the bcc structure be-
cause the packing fraction of fcc structure ͑i.e., 0.74͒ is
larger than that of bcc structure ͑i.e., 0.68͒. As a result, the
fcc structure has less surface area than the bcc structure; the
ratio of surface area of fcc structure to that of bcc structure is
roughly (0.68/0.74)2/3ϭ0.945.9 Thus, the observed fcc struc-
ture is likely a consequence for clusters to minimize their
surface free energy ͑or maximum cohesion energy͒ by hav-
ing a more compact structure.
2 K. Selby, V. Kresin, J. Masui, M. Vollmer, W. deHeer, A. Scheidemann,
and W. D. Knight, Phys. Rev. B 43, 4565 ͑1991͒; C. Wang, S. Pollack, T.
Dahlseid, G. M. Koretsky, and M. M. Kappes, J. Chem. Phys. 96, 7931
¨
͑1992͒; M. Broyer, J. Chevaleyre, Ph. Dugourd, J. P. Wolf, and L. Woste,
Phys. Rev. A 42, 6954 ͑1990͒; W. Harbich, S. Fedrigo, J. Buttet, and D.
M. Lindsay, Z. Phys. D 19, 157 ͑1991͒; R. Kishi, H. Kawamata, Y.
Negishi, S. Iwata, A. Nakajima, and K. Kaya, J. Chem. Phys. 107, 10029
͑1997͒; D. G. Leopold, J. Ho, and W. C. Lineberger, ibid. 86, 1715
͑1987͒; R. L. Whetten, M. R. Zakin, D. M. Cox, D. J. Trevor, and A.
Kaldor, ibid. 85, 1697 ͑1986͒; L.-S. Zheng, C. M. Karner, P. J. Brucat, S.
H. Yang, C. L. Pettiette, M. J. Craycraft, and R. E. Smalley, ibid. 85, 1681
͑1986͒; M. Iseda, T. Nishio, S. Y. Han, H. Yoshida, A. Terasaki, and T.
Kondow, ibid. 106, 2182 ͑1997͒; K. M. McHugh, J. G. Eaton, G. H. Lee,
H. W. Sarkas, L. H. Kidder, J. T. Snodgrass, and K. H. Bowen, ibid. 91,
6536 ͑1989͒.
3
´
D. Tomanek, S. Mukherjee, and K. H. Bennemann, Phys. Rev. B 28, 665
͑1983͒.
4 G. H. Lee, S. H. Huh, and H. I. Jung, J. Mol. Struct. 440, 141 ͑1998͒.
5 T. Majima, Y. Matsumoto, and M. Takami, J. Photochem. Photobiol., A
71, 213 ͑1993͒; T. Majima, T. Ishii, Y. Matsumoto, and M. Takami, J.
Am. Chem. Soc. 111, 2417 ͑1989͒.
6 Technical data obtained from Pelican Wire Company, Inc., 6266 Taylor
Road, Naples, FL 34109-1896. The filament used in this experiment ͑1
mm thick, 2.3 ohms, and nichrome wire ͑Ni/Cr/Fe alloy͒͒ provides
ϳ400 °C and ϳ800 °C filament temperature at 20 VAC and 40 VAC
filament voltages, respectively, according to the technical data.
7 Here, cluster size means ‘‘average’’ cluster size because metal clusters
produced in this experiment have a size distribution around the average
cluster size.
8 B. D. Cullity, Elements of X-ray Diffraction ͑Addison-Wesley, Reading,
MA, 1978͒, p. 102. The Scherrer’s formula is DhklϭK␭/ ␦ cos ␪ in
͓
͔
which Dhkl is the cluster diameter (d), K is a constant ͑ϭ0.9͒, ␭ is the
x-ray wavelength ͑ϭ0.154 25 nm͒, and ␦ is the FWHM. Dhkl was cali-
brated against the Fe͑110͒ peak (dϭ6 nm measured from TEM micro-
graph, 2␪ϭ44.8°, and FWHMϭ0.82°͒. That is, DhklϭDhkl͑Fe͒␦͑Fe͒ cos ␪
͑Fe͒/͓␦ cos ␪͔.
G.H.L. thanks Professor Kit Bowen at Johns Hopkins
University and Professor Y. H. Lee at Cheonpook National
University for an insightful discussion in interpreting the
data. He acknowledges the partial supports by the ‘98
Kyungpook National University Research Fund, and thanks
KBSI for allowing us to use the XRD apparatus at a mem-
bership rate.
9 For a spherical cluster, nϭϳ(R/ro)3 f ͑See Ref. 4͒. For the same n in both
fcc and bcc clusters, (Rfcc /ro)3ϫ0.74ϭ(Rbcc /ro)3ϫ0.68. Thus,
(Rfcc /Rbcc)3ϭ0.68/0.74. Finally, the ratio of surface area of fcc structure
to that of bcc structure is (Rfcc /Rbcc)2ϭ0.945.
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