METAL

NANOCLUSTERS

BY:

SUDAMA CHAURASIYA

Ist SEM.

M. TECH (NST)

CONTENTS

INTRODUCTION

NANO CLUSTER SYNTHESIS & MAGIC NUMBERS

THEORETICAL MODELING OF NANAOPARTICLES

STRUCTURES

REATIVITY

MAGNETIC CLUSTERS

BULK TO NANOTRANSITION

CONCLUSION

INTRODUCTION

Nanoclusters are particle that rangein size from a few atoms to severalthousand atoms.

Nanoclusters—near monodispersedparticles that are generally less than10 nm 100 A° . in diameter

Their high fraction of surface atomsgive them properties different frombulk material properties.

Different elements form differentbonds and different nanoclusterstructures.

These bonds and structurescontribute to their uniqueproperties.

Fig. 1. Distinction between molecules, nanoparticles and bulk according to the number of atoms in cluster

NANO CLUSTER SYNTHESIS & MAGIC NUMBERS

Fig 2. Apparatus to make nanoparticles bylaser induced evaporation of atoms fromthe surface of metal

The metal nanoclusters are made usingthe laser vaporization technique.

This technique involves focusing a laserbeam onto a metal sample.

Metal atoms evaporate and are cooledwith a flow of inert gas.

As they cool the atoms combine intonanoclusters of varying sizes.

They are then expanded through anozzle into a vacuum to further coolthem.

Spectrometer gives information aboutthe cluster formed.

SYNTHESIS

MAGIC NUMBERS

ELECTRONIC MAGIC NUMBERIonization potentialIt is the energy that is necessary to remove the outer electron from the atom.

• Maximum ionization potential occurs for the rare gases, because their outer orbital is completely filled.

• Peaks are observed at clusters having two and eight atoms.

• These numbers are referred as electronic magic number.

STRUCTURAL MAGIC NUMBERFor larger clusters the stabilityis determined by structure andmagic number is called asStructural Magic Number.

Fig 3. Plot of ionization potential verses (a)Atomic number (b) Number of atom in cluster

THEORETICAL MODELING OF NANAOPARTICLES

JELLIUM MODEL

It envisions cluster as a largeatom.

Positive nuclear charge ofeach Cluster is assume to beuniformly distributed over asphere the size of the cluster.

Interaction of electron withpositive sphere is described asa spherically symmetricpotential well.

Energy levels can be obtainedby solving Schrodingerequation.

Fig 4. A comparison of energy levels of hydrogen atom and Jellium model of clusters

STRUCTURES

GEOMETRICAL STRUCTURE

Crystal structure of largenanoparticles have same structurewith somewhat different lattice asbulk.

• e.g.80 nm aluminum has FCC unitcell as bulk aluminum have.

Small particles having diameter <5nm may have different structure.

• e.g.Gold nanoparticles of 3-5 nmhave an icosahedral structurerather than the bulk FCC

Fig 5 (a) the unit cell of bulk aluminum (b) three possible structure of Aluminum FCC, HCP, ICOS

Orbital calculation based on thedensity functional method predictthat the Icosahedral form has lowerenergy than other forms.

In late 1970s and early 198s, G.D.Stien determine the structure ofBiN, PbN, and AgN nanoparticles.

Deviation from FCC were observedfor cluster smaller than 8 nm indiameter.

Fig 6. Some calculated structure of small Boron nanoparticles

ELECTRONIC STRUCTURE

Fig.7a. Illustration of how energy levels of metalchanges when no. of atoms of the material is reduced

DENSITY OF STATES: it refers to theno of energy levels in a given intervalof energy.

Moving from bulk to small metalclusters density of states changesdramatically.

The continuous density of statesin band is replaced by a set ofdiscrete energy levels.

Clusters of different sizes will havedifferent electronic structures,and different energy levelseparations.

Fig. 7b. Density functional calculation of excited energy levels of B6, B8, & B12 nanoparticles.

REACTIVITY

The ability of cluster to react with any species should depends on cluster size.

Reactivity with various gases can be studied by the synthesis apparatus by introducing gases such as oxygen into the region of the cluster beam.

Fig. 8b. Mass spectrum of Al nanoparticles before (left) and after (right) exposer to oxygen gas

Fig. 8a. Gas introduction.

Fig. 9 Reaction rate of hydrogen gas with iron nanoparticles versus the particle size.

Fig. shows that the reaction rate ofiron with hydrogen is as a function ofsize of the iron particles.

A group of Oksaka National Researchinstitute in Japan discovered that highcatalytic activity is observed to switchon for Gold nanoparticles smallerthen 3-5 nm, where the structure isicosahedral instead of bulkarrangement.

MAGNETIC CLUSTERS

Magnetic moments arise in atomsfrom the net electron spin zcomponent of the electron angularmomentum.

Hund’s rule states that electrons tendto fill their orbitals in such a way as tomaximize their net spin.

The total magnetic moment of theatom comes from the coupling of theelectronic spin with the z-angularmomentum.

When these atoms combine to formnanoclusters, the atomic magneticmoments can align to form a netmagnetic moment for the cluster.

Fig. 10. Formation of net magnetic moment in a metal nanoclusters.

BULK TO NANOTRANSTION

At what number of atoms does a cluster assume the property of the bulk material ?

In a cluster less than 100 atoms the amount of energy needed to ionize it, or toremove an electron from it is different from the Work function.

Gold nanocluster having 1000 atoms or more is having same melting point as thebulk Gold.

Average separation of Copper atoms in a Copper cluster approaches the value ofthe bulk material when cluster have 100 atoms or more.

• In general it appears that different properties of cluster reaches thecharacteristics value of solids at different cluster sizes.

• The size of the cluster where the transition to bulk behavior occurs appear todepend on the property being measured.

CONCLUSION

The physical, chemical and electronic properties of nanoclustersdepends strongly on the number and kind of atoms that makesthe cluster.

Reactivity, stability and magnetic behavior depends on particlesize.

In some instances entirely new behavior which is not seen in thebulk has been observed in nanoclusters.

Besides providing new research challenges for scientists tounderstand the new behavior, the results have enormouspotential for application, allowing the design of properties bycontrol of particle size

It is clear that nanoscale material can form the basis of new classof automatically engineered materials.