2. CONTENTS
INTRODUCTION
NANO CLUSTER SYNTHESIS & MAGIC NUMBERS
THEORETICAL MODELING OF NANAOPARTICLES
STRUCTURES
REATIVITY
MAGNETIC CLUSTERS
BULK TO NANOTRANSITION
CONCLUSION
3. INTRODUCTION
Nanoclusters are particle that range
in size from a few atoms to several
thousand atoms.
Nanoclusters—near monodispersed
particles that are generally less than
10 nm 100 A° . in diameter
Their high fraction of surface atoms
give them properties different from
bulk material properties.
Different elements form different
bonds and different nanocluster
structures.
These bonds and structures
contribute to their unique
properties.
Fig. 1. Distinction between molecules,
nanoparticles and bulk according to the
number of atoms in cluster
4. NANO CLUSTER SYNTHESIS & MAGIC NUMBERS
Fig 2. Apparatus to make nanoparticles by
laser induced evaporation of atoms from
the surface of metal
The metal nanoclusters are made using
the laser vaporization technique.
This technique involves focusing a laser
beam onto a metal sample.
Metal atoms evaporate and are cooled
with a flow of inert gas.
As they cool the atoms combine into
nanoclusters of varying sizes.
They are then expanded through a
nozzle into a vacuum to further cool
them.
Spectrometer gives information about
the cluster formed.
SYNTHESIS
5. MAGIC NUMBERS
ELECTRONIC MAGIC NUMBER
Ionization potential
It 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 NUMBER
For larger clusters the stability
is determined by structure and
magic number is called as
Structural Magic Number.
Fig 3. Plot of ionization potential verses (a)Atomic
number (b) Number of atom in cluster
6. THEORETICAL MODELING OF NANAOPARTICLES
JELLIUM MODEL
It envisions cluster as a large
atom.
Positive nuclear charge of
each Cluster is assume to be
uniformly distributed over a
sphere the size of the cluster.
Interaction of electron with
positive sphere is described as
a spherically symmetric
potential well.
Energy levels can be obtained
by solving Schrodinger
equation.
Fig 4. A comparison of energy levels of
hydrogen atom and Jellium model of
clusters
7. STRUCTURES
GEOMETRICAL STRUCTURE
Crystal structure of large
nanoparticles have same structure
with somewhat different lattice as
bulk.
• e.g.
80 nm aluminum has FCC unit
cell as bulk aluminum have.
Small particles having diameter <5
nm may have different structure.
• e.g.
Gold nanoparticles of 3-5 nm
have an icosahedral structure
rather than the bulk FCC
Fig 5 (a) the unit cell of bulk aluminum (b) three
possible structure of Aluminum FCC, HCP, ICOS
8. Orbital calculation based on the
density functional method predict
that the Icosahedral form has lower
energy than other forms.
In late 1970s and early 198s, G.D.
Stien determine the structure of
BiN, PbN, and AgN nanoparticles.
Deviation from FCC were observed
for cluster smaller than 8 nm in
diameter.
Fig 6. Some calculated structure
of small Boron nanoparticles
9. ELECTRONIC STRUCTURE
Fig.7a. Illustration of how energy levels of metal
changes when no. of atoms of the material is reduced
DENSITY OF STATES: it refers to the
no of energy levels in a given interval
of energy.
Moving from bulk to small metal
clusters density of states changes
dramatically.
The continuous density of states
in band is replaced by a set of
discrete energy levels.
Clusters of different sizes will have
different electronic structures,
and different energy level
separations.
Fig. 7b. Density functional calculation of excited
energy levels of B6, B8, & B12 nanoparticles.
10. 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.
11. Fig. 9 Reaction rate of hydrogen gas with iron
nanoparticles versus the particle size.
Fig. shows that the reaction rate of
iron with hydrogen is as a function of
size of the iron particles.
A group of Oksaka National Research
institute in Japan discovered that high
catalytic activity is observed to switch
on for Gold nanoparticles smaller
then 3-5 nm, where the structure is
icosahedral instead of bulk
arrangement.
12. MAGNETIC CLUSTERS
Magnetic moments arise in atoms
from the net electron spin z
component of the electron angular
momentum.
Hund’s rule states that electrons tend
to fill their orbitals in such a way as to
maximize their net spin.
The total magnetic moment of the
atom comes from the coupling of the
electronic spin with the z-angular
momentum.
When these atoms combine to form
nanoclusters, the atomic magnetic
moments can align to form a net
magnetic moment for the cluster.
Fig. 10. Formation of net magnetic moment
in a metal nanoclusters.
13. 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 to
remove an electron from it is different from the Work function.
Gold nanocluster having 1000 atoms or more is having same melting point as the
bulk Gold.
Average separation of Copper atoms in a Copper cluster approaches the value of
the bulk material when cluster have 100 atoms or more.
• In general it appears that different properties of cluster reaches the
characteristics value of solids at different cluster sizes.
• The size of the cluster where the transition to bulk behavior occurs appear to
depend on the property being measured.
14. CONCLUSION
The physical, chemical and electronic properties of nanoclusters
depends strongly on the number and kind of atoms that makes
the cluster.
Reactivity, stability and magnetic behavior depends on particle
size.
In some instances entirely new behavior which is not seen in the
bulk has been observed in nanoclusters.
Besides providing new research challenges for scientists to
understand the new behavior, the results have enormous
potential for application, allowing the design of properties by
control of particle size
It is clear that nanoscale material can form the basis of new class
of automatically engineered materials.
