Disorder and cluster formation during ion irradiation of Au nanoparticles in SiO
2
P. Kluth,
1,
* B. Johannessen,
1
G. J. Foran,
2
D. J. Cookson,
3
S. M. Kluth,
1
and M. C. Ridgway
1
1
Department of Electronic Materials Engineering, Australian National University, Canberra ACT 0200, Australia
2
Australian Nuclear Science and Technology Organization, Menai, Australia
3
Australian Synchrotron Research Program, Building 434, 9700 South Cass Avenue, Argonne, Illinois 60439, USA
Received 23 February 2006; revised manuscript received 3 May 2006; published 11 July 2006
Au nanoparticles NPs have been formed by ion beam synthesis in 600 nm thin SiO
2
. Subsequently the NPs
were irradiated with 2.3 MeV Sn ions at liquid nitrogen temperature. Samples were analyzed using extended
x-ray absorption fine structure EXAFS spectroscopy and small angle x-ray scattering SAXS as a function of
Sn irradiation dose. Transmission electron microscopy shows that the NPs largely retain their spherical shape
upon irradiation. However, we observe a reduction in average NP size and a concomitant significant narrowing
of the size distribution with increasing irradiation dose as consistent with inverse Ostwald ripening. At lower
irradiation doses, significant structural disorder is apparent with an effective bond length expansion as consis-
tent with amorphous material. At higher irradiation doses, EXAFS measurements indicate dissolution of a
significant fraction of Au from the NPs into the SiO
2
matrix as monomers and the formation of small Au
clusters dimers, trimers, etc.. We estimate the volume fraction of such monomers/clusters. Ion irradiation thus
yields disordering then dissolution of Au NPs.
DOI: 10.1103/PhysRevB.74.014202 PACS numbers: 61.46.Df, 61.10.Ht, 61.80.Jh, 61.10.Eq
I. INTRODUCTION
Nanoparticles NPs have received increasing attention
over the last decade due to their interesting properties which
can differ significantly from those of the corresponding bulk
material. Such properties are generally related to the limited
number of atoms within the NPs and the considerable surface
to bulk ratio SBR. This can lead to significant structural
deviations as compared to “infinite” bulk material and a con-
comitant alteration of the NP physical properties.
1
Ion implantation is a very versatile technique and has
been widely used for NP fabrication ion beam synthesis as
well as for structural modification of semiconductor based
materials ion irradiation. Whereas the influence of ion irra-
diation on the structure of bulk materials has been studied
extensively and some understanding of the underlying
mechanisms has been established,
2
only recently has the in-
fluence of ion irradiation on NPs been investigated and re-
vealed a number of interesting phenomena. Simulations
show that ion irradiation can significantly alter the evolution
of the size distribution of NPs embedded in a matrix mate-
rial, i.e., lead to narrowing due to inverse Ostwald ripening,
and some experimental evidence has been reported for irra-
diated Au NPs using transmission electron microscopy
3,4
TEM. At very high energies, where electronic stopping is
dominant, a shape transformation from spherical to elon-
gated rods has been observed for irradiated
5
Co NPs. The
influence of ion irradiation on the structure of metallic NPs,
however, has remained a widely unexplored area.
In this paper, the effect of ion irradiation on size and
structure of Au NPs embedded in SiO
2
is investigated using
a unique combination of extended x-ray absorption fine
structure EXAFS spectroscopy and small angle x-ray scat-
tering SAXS. EXAFS provides a powerful tool for analysis
of particles of nanometer dimensions giving accurate infor-
mation about the average local atomic environment of the
absorbing atom.
6
It is particularly sensitive to interatomic
distances and local disorder and has been successfully uti-
lized to resolve subtle NP structural details.
7
SAXS gives
additional information about the NP size distribution and
thus enables a size-dependent interpretation of the structural
data obtained by EXAFS.
II. EXPERIMENTAL
First, we have grown 600 nm thick SiO
2
on Si 100 sub-
strates using thermal oxidation in O
2
. The amorphous SiO
2
layers were then sequentially implanted with
197
Au ions at
1.4 MeV to an ion dose of 5 10
16
cm
-2
and at 0.6 MeV to
an ion dose of 2.5 10
16
cm
-2
. This yields a peak Au con-
centration of approximately 3 at. %. The implantations were
performed at room temperature. Subsequently samples were
annealed at 1000 ° C for 1 h in a conventional quartz furnace
to promote further precipitation and reduce irradiation-
induced damage of the host material. A first set of samples
series I was annealed in O
2
and a second set of samples
series II was annealed in forming gas 95% N
2
and 5% H
2
.
It has previously been shown that the annealing atmosphere
has a profound influence on the resulting NP size.
8
After
formation the NPs were irradiated with
119
Sn ions at
2.3 MeV to ion doses between 1 10
14
cm
-2
and 1
10
16
cm
-2
. The energy was chosen such that essentially all
Sn ions were stopped within the Si substrate, i.e., Sn-related
impurity effects were negligible. The samples were main-
tained at liquid nitrogen temperature during ion irradiation.
The Au concentration profiles were measured by Rutherford
backscattering spectrometry RBS using 2 MeV
4
He ions
and a scattering angle of 168°. Figure 1 shows RBS measure-
ments of the Au distribution of sample series I prior to and
after irradiation with 1 10
16
cm
-2
Sn ions. It is apparent,
that the Au concentration profile remains essentially un-
changed upon irradiation. Similar results have been observed
for all irradiation doses for both sample series.
PHYSICAL REVIEW B 74, 014202 2006
1098-0121/2006/741/0142029 ©2006 The American Physical Society 014202-1