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Physica B 353 (2004) 92–97
www.elsevier.com/locate/physb
Metal alloy and monoelemental nanoclusters in silica
formed by sequential ion implantation and annealing in
selected atmosphere
F. Rena,b, C.Z. Jianga,b,, H.B. Chenc, Y. Shia, C. liua, J.B. Wanga,b
a
Department of Physics, Wuhan University, Wuhan 430072, China
Center for Electron Microscopy, Wuhan University, Wuhan 430072, China
c
State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences,
Beijing 100083, China
b
Received 25 August 2004; received in revised form 3 September 2004; accepted 5 September 2004
Abstract
The preparation of metal alloy and monoelemental nanoclusters in silica by Ag, Cu ion sequential implantation and
annealing in selected oxidizing or reducing atmosphere is studied. The formation of metastable Ag–Cu alloy is verified
in the as-implanted samples by optical absorption spectra, selected area electron diffraction and energy dispersive
spectrometer spectrum. The alloy is discomposed at elevated annealing temperature in both oxidizing and reducing
atmospheres. The different effects of annealing behaviors on the Ag–Cu alloy nanoclusters are investigated.
r 2004 Elsevier B.V. All rights reserved.
PACS: 61.46.+W; 61.72.Ww; 78.67.Hc; 61.16.C; 81.05.Bx
Keywords: Ion implantation; Nanocluster composites; Alloy; Annealing
1. Introduction
The metal nanocluster composite is a promising
candidate of optical switch, which is a key device
in all-optical communication, because of the large
Corresponding author. Department of Physics, Wuhan
University, Wuhan 430072, China. Tel.: +86 27 68752567;
fax: +86 27 68752569.
E-mail address: [email protected] (C.Z. Jiang).
third-order nonlinear susceptibility and picosecond nonlinear response times. As ion implantation is a useful technique to obtain nanocluster
composite materials [1], noble metals implanted in
sequence may form multicomponent metal nanoclusters in silica glass. The composition of the
metal nanoparticles and then both the linear and
nonlinear optical prosperities of the composites
can be controlled by the implantation [2,3]. The
metal vapor vacuum arc (MEVVA) ion source,
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2004.09.005
ARTICLE IN PRESS
F. Ren et al. / Physica B 353 (2004) 92–97
which provides large current and broad beams of
metal ions, is suitable for high dose ion implantation.
In this paper, Ag–Cu metastable alloy nanoclusters are prepared by sequential ion implantation
with a MEVVA source. The effects of annealing
on the metal nanocluster composite glass at
different temperatures in oxidizing or reducing
atmosphere are studied. The remarkable difference
between the annealing behaviors in different
atmospheres indicates that the electronic structure
and the optical characteristics of the nanoclusters
can be tailored by implantation and annealing.
93
velengths from 900 to 200 nm. The absorption
spectra for all samples were measured with an
unimplanted sample in the reference beam.
3. Results and discussion
3.1. XPS spectra
Fig. 1 shows the XPS Ag3d and Cu2p spectra of
the as-implanted sample. XPS results show that
Ag3d5=2 binding energy is 368.2 eV, which is
attributed to the metal state of Ag in the
composite. The Cu2p3=2 spectrum can be deconvoluted into two spectra with peaks at 932.7 and
2. Experiment
Ag and Cu ions were sequentially implanted
into silica glass at room temperature with a
MEVVA implanter. The ion flux densities of Ag
and Cu were both about 1 mA/cm2 and the doses
of Ag and Cu were both 5 1016 ions/cm2. The
extracted voltages of Ag and Cu were 43 and
30 kV, respectively. Then the projected ranges for
Ag and Cu in silica were similar and the ion ratio
of Ag/Cu was 1. The valence states of Ag and Cu
in the composite were investigated by X-ray
photoelectron spectroscopy (XPS), which were
measured in a Kratos XSAM800 spectrometer
operated in fixed retarding ratio or fixed analyzer
transmission mode using Mg Ka1,2 (1253.6 eV)
excitation. Transmission electron microscopy
(TEM) observations were finished with a JEOL
JEM 2010 (HT) and a JEOL JEM 2010FEF
(UHR) equipped with an EDAX energy dispersive
X-ray spectrometer (EDS). Both the microscopes
were operated at 200 kV. Selected area electron
diffraction (SAED), bright field (BF) and dark
field (DF) imaging techniques were used to
determine the crystal structure, size distribution,
and shape of nanoclusters. The implanted samples
were then heated to different temperatures from
300 to 800 1C, with a rate of 1 h per step at an
interval of 100 1C in either oxidizing (air) or
reducing (30% H2+70% Ar gas mixture, gas
pressure 20 Pa) atmosphere. Optical absorption
spectra were recorded at room temperature using a
UV–VIS dual-beam spectrophotometer with wa-
Fig. 1. Ag3d (a) and Cu2p (b) XPS spectra of the Ag/Cu
implanted sample.
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F. Ren et al. / Physica B 353 (2004) 92–97
933.7 eV, corresponding to Cu–Cu and Cu–O
bonds, respectively. So Cu carries both the metal
and the +2 oxidation states in the as-implanted
sample.
During the process of implantation, the implanted ions and the host atoms compete for the
oxygen in the SiO2 matrix. The free energy for the
formation of SiO2 is lower than that for the
formation of the silver oxides. However, the free
energy of formation of CuO is also low, so Ag is
expected to be preserved in metallic form, while a
part of Cu is in oxidation form.
3.2. Formation of Ag–Cu alloy nanoclusters
Fig. 2(a) is the TEM image of implanted sample,
which indicates that spherical particles have been
formed. SAED for the as-implanted samples is
shown in Fig. 2(b). The SAED pattern shows rings
that are typical of crystalline clusters with mutual
random orientation. The SAED pattern can be
indexed according to a single face-centered-cubic
(FCC) phase with a lattice constant of
0.39670.002 nm. This value is not consistent with
the experimental values of the pure bulk phases of
either Ag (aAg ¼ 0:4079 nm) or Cu (aCu ¼
Fig. 3. Energy dispersive X-ray spectra for a large nanocluster.
0:3608 nm), which indicates that an intermetallic
Ag–Cu alloy may have been formed. The composition of larger clusters is further examined by
EDS. The spectrum shown in Fig. 3 indicates that
there are both Ag and Cu elements in the same
nanoclusters, which provides another evidence for
the formation of Ag–Cu alloy nanoclusters [4,5].
A possible mechanism of the formation of alloy
is related to the enhanced diffusion of Cu in small
Ag clusters, just like adding Cu to Ag with the heat
generated by the implantation, where high local
temperatures is achieved [2,6,7]. Cu ion implantation will create a lot of displacement cascades in
the formed Ag nanoclusters. Molecular dynamic
simulation on the displacement cascade has
suggested that during the thermal spike of the
cascade formation, the temperature in the cascade
core can be extremely high and atoms inside the
displacement cascade may achieve a ‘liquid-like
state’ [8]. The Ag–Cu metastable alloy may be
formed, as the cooling rate of collision cascade is
sufficiently high (1014 K/s) [9,10]. There is no
ordered phase for Ag–Cu alloy, so these nanoclusters are indeed Ag–Cu solid solution.
3.3. Influence of annealing in oxidizing atmosphere
on nanoclusters
Fig. 2. (a) TEM and (b) SAED image for the Ag/Cu
sequentially implanted sample.
The linear absorption for noninteracing spherical colloids with diameters less than l/20, is
described by Mie scattering theory in the electric
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F. Ren et al. / Physica B 353 (2004) 92–97
95
dipole approximation [11] and is given by
a¼
18ppn3
2
;
l
ð1 þ 2n2 Þ2 þ 22
(1)
where a is the absorption coefficient, ðlÞ ¼ 1 þ
i2 is the dielectric constant of the metal, p is the
volume fraction of the metal particles and n is the
index of refraction of the dielectric host. The
absorption is expected to exhibit a peak at the
surface plasmon resonance (SPR) frequency for
which the condition 1 þ 2n2 ¼ 0 is met. The SPR
frequency depends explicitly on metal cluster
constitution, size, volume faction, shape, and
dielectric function of matrix. So changing the
composition of composite or the size of clusters
will change the SPR spectra.
Fig. 4(a) shows the optical absorption spectra of
the samples before and after annealing in air from
300 to 800 1C. For the optical absorption spectrum
of the as-implanted sample, the position of SPR
peak is at 442 nm, which lies between that of pure
Ag (410 nm) and Cu (560 nm) nanoclusters. Therefore, it indicates that intermetallic Ag–Cu alloy
nanoclusters have been formed instead of two
separated Ag and Cu nanoclusters, which on the
contrary would give rise to double-peaked spectroscopy [6].
When the as-implanted sample was annealed in
air, the SPR peaks shift rapidly toward shorter
wavelength and approaches the SPR peak position
of pure Ag nanoclusters with the increase of
annealing temperature to 400 1C. This means that
alloy nanoclusters have been discomposed and Ag
monoelemental clusters are formed, whereas Cu
migrates toward the surface of the sample, where it
is oxidized. Four mechanisms may be responsible
for this alloy dissociation and the formation of
larger Ag clusters. Firstly, the Ag–Cu metastable
alloy is not stable and easy to decompose at
elevated temperature [12]; secondly, the solubility
of Cu in Ag is low, so Cu will escape from the alloy
clusters after annealing; thirdly, oxygen permeates
in the sample and reacts strongly with Cu,
enhancing the Cu diffusion out of the alloy clusters
and migration toward the sample surface;
fourthly, the migration of oxygen in-depth also
increases Ag mobility [13].
Fig. 4. Optical absorption spectra of the Ag/Cu sequentially
implanted sample annealing in (a) oxidizing or (b) reducing
atmosphere.
The sharpening of SPR peaks shows that the
sizes of nanoclusters became larger with the
increase of annealing temperature. But the intensity of the SPR peaks weakens at higher annealing
temperature, which may be due to the break of the
already presented large Ag clusters and the
reduction of Ag cluster number because Ag
diffuses atomistically into the substrate or move
toward the surface, where it evaporates. This will
also lead to the decrease of Ag radius at
temperature higher than 600 1C.
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F. Ren et al. / Physica B 353 (2004) 92–97
3.4. Influence of annealing in reducing atmosphere
on nanoclusters
For the samples annealed in reducing atmosphere, the SPR peaks also shift toward that of the
pure Ag clusters. However, a shoulder peak
appears at 567 nm at 500 1C, approaching that of
pure Cu clusters. The two absorb bands become
sharper and more intense in the subsequent
annealing. Therefore, Ag–Cu alloy is also decomposed and Ag clusters are formed with temperature lower than 500 1C. With the further increase
of annealing temperature, Cu ions in oxidation
state are reduced by H2 and aggregate to form Cu
nanoclusters together with the existed Cu atoms in
metal state, which brings about the SPR peak of
Cu. This was confirmed by the SAED patterns of
the sample annealed in reducing atmosphere. Fig.
5 is the TEM, BF and SAED images of the sample
directly annealing in reducing atmosphere at
800 1C for 1 h. The SAED pattern displays two
sets of diffraction rings characteristic of FCC Ag
and Cu.
4. Conclusion
Metastable Ag–Cu alloy and monoelemental
nanoclusters have been formed by Ag and Cu
sequential ion implantation in SiO2 matrix and
annealing. In the case of annealing in air, Ag–Cu
alloy is discomposed; Cu migrates to the surface of
the sample and oxidizes. Ag nanoclusters are
formed due to weak oxygen–silver interaction.
For the sample annealed in reducing atmosphere,
the Ag–Cu alloy discomposes into Ag and Cu
nanoclusters.
Acknowledgments
The authors would like to thank Professor J.N.
Gui for useful discussions. This work was partially
supported by the National Natural Science Foundation of China (No. 10005005, 10205010,
10375044).
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Fig. 5. (a) TEM and (b) SAED image for the sample annealed
in reducing atmosphere at 800 1C.
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