Modification of Ionic Silver Clusters Under the Influence of a High Power CW Laser Beam

H. R. khalesifardA. 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 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.

10

Techniques to Study the Cluster Responses

Linear Responses• Absorption• Elastic scattering• Fluorescence• Luminescence• Inelastic scattering

(Brillouin, Smekal-Raman)

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

GlassSalt Melt(NO3)-

Ag+

Na+

O2-

Si4+

•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

Laser annealingAnnealing in H2 atmosphere

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:

• We have used the stabilized jellium model and density functional theory to show that for clusters with N>50, the evaporation processes are dominent.

;

;AgAgAg

;AgAgAg

2

p2pN

2N

ppNN

ppNN AgAgAg

Evaporation

Evaporation

Fission