Modification of Ionic Silver
Clusters Under the Influence of a
High Power CW Laser Beam
H. R. khalesifard
A. Nahal and M. Payami
Institute for Advanced studies in Basic Sciences
Zanjan, IRAN
Contents
• Introduction
• Techniques for studying and producing metallic
clusters
• Ag+-Na+ ion-exchange in glass matrix
• Laser annealing
• Luminescence of laser annealed samples
• Optical absorption spectra of laser annealed
samples
• Experiments results
• Stabilized jellium model and photo-fragmentation.
Introduction
• Composite glasses consisting of small metal
particles  10 nm in diameter embedded in a
dielectric are optically nonlinear materials that
are attractive for optical switching.
• Optical absorption spectroscopy is one of the
most used techniques for the investigation of
colloidal systems formed by such particles.
Techniques to Study the Cluster
Responses
Linear Responses
• Absorption
• Elastic scattering
• Fluorescence
• Luminescence
• Inelastic scattering
(Brillouin, SmekalRaman)
Nonlinear Responses
• Second harmonic
generation (SHG)
• Multiple-wave mixing
• Nonlinear Kerr effect
Destructive methods
• Ionization
• Evaporation of atoms, dimers, …
• Fission of clusters in to fragments
Preparation of Metal Clusters
In beams
On surface
In matrices
gas aggregation
Atom deposition with
subsequent surface
deposition
Co-evaporation of atoms
with matrix material
(matrix isolation0
Field emission
(liquid metal source)
Cluster deposition from
cluster beam
Diffusion in/into matrix
(ion-exchange)
Spraying techniques
nanocrystalline material by
densification
Photoreduction
(photographic material)
Sputtering
Spraying technique into
liquid matrix
Arc discharge
Co-evaporation of clusters
with matrix material
Laser vaporization, ablation
Pressing into porous matrix
Ag+-Na+ Ion-Exchange
Salt Melt
(NO3)-
Glass
Si4+
O2-
Ag+
Na+
•The temperature of the molten salt should be less than the
glass transition temperature.
•The silver ions inside the glass matrix gathers together to
form the ionic silver clusters. The Mie resonances appearing
in the optical absorption spectra of these sample are the
evidence of formation of ionic clusters.
• Usually the ionic clusters con be reduced to the atomic ones
by annealing the ion-exchanged samples in hydrogen
atmosphere in temperatures lower than the ion-exchange
temperature. Surface plasmun resonances at this stage
appear in the optical absorption spectra.
Laser Annealing
•
The high power
Ar+ laser beam
changes the
surface and
index profile of
the sample in the
interaction
region.
The laser beam
induces a
considerable
anisotropy in the
interaction region
A standard set-up for
observing the
luminescence of the
interaction region
The luminescence
of the sample in
the interaction
region can be an
evidence of the
clusters motion in
that region
Absorption spectrum
Annealing in H2 atmosphere
Laser annealing
Exposing the
sample with
laser beam in
two regime]
1. Fast regime: up
t0 67 W/cm2 1n
less than 1 min
2. Slow regime: up
to 67W/cm2 in
1hr
Diffusion limited aggregation
process and the induced anisotropy
A dark background photograph of the interaction area
Experiments Results
• Comparison of absorption spectra after laser annealing and annealing in
hydrogen atmosphere helps us to conclude that the photo-fragmentation of
the ionic clusters by the laser beam can produce neutral clusters which are
smaller in size compared to ionic ones.
• At least two groups of clusters are exist in the interaction region.
1. Nanometer sized clusters which are responsible for the surface plasmun
and Mie resonances in the absorption spectra.
2. Micrometer sized clusters which are positioned in the circumference of the
interaction region and are observable with usual optical microscopes.
• Micrographs and absorption spectra of the interaction region at s and p
polarizations show that the evolution of the atomic clusters is a diffusion
limited aggregation process.
• We hope that to obtain some information about the motion of clusters during
the interaction by observing the luminescence of the interaction region
Stabilized Jellium model and
Photo-Fragmentation
•
If we consider the following possible channels for the photo-fragmentation of
the clusters:
Ag N  Ag N  p  Ag p ;
Ag
2
N
 Ag
2
Np
 Ag p ;
Ag N2  Ag N  p  Ag p ;
•
Evaporation
Evaporation
Fission
We have used the stabilized jellium model and density functional theory to
show that for clusters with N>50, the evaporation processes are dominent.