Download Cluster Beam Evaporation: Bottom Up technique

Cluster Beam Evaporation: Bottom Up technique
what is cluster: Few hundred to few dozens of atoms together(bound
by some forces) are called as cluster( cluster bombing)
Nanoparticles are
nucleated at cluster
stage
Almost everything
forms cluster and the
diversity is vast
In between bulk and
molecular/atomic
On the exploration of
cluster, C60 was
discovered
Properties are
accordingly differ than
bulk and depend on
mole./atomic
constituents
clusters
Can have net
charge
(Ionic cluster)
Or no charge
( neutral cluster)
Aggregate of atoms/mole. Form
cluster are bound by forces, Metallic,
covalent, ionic, hydrogen or Van der
Waals
clusters
Cluster contain well
defined number of
transition metal
atoms
have unique
chemical, electronic
and magnetic
properties
Properties vary dramatically with
number of constituent atoms, type of
element and the charge on the cluster
Clusters formation: surface fraction of
atoms
surface used to be
more reactive and
active for any
reactions/catalysis
On Bulk /cm3= 1023
atoms
Fraction of atoms on
surface is 1 in 1000000
On the surface of this
cube, only 1.5x1016
atoms,
Hence they (fraction of
surface atoms)do not
influence(properties)
much in bulk
Clusters formation: surface fraction of
atoms
On the surface of this
cluster, Fraction of
atoms on surface can
be of the order of 1
Fraction of atoms
F=4/n1/3 , n is the
number of atoms in
cluster
This makes a
significant change in
the properties
F=0.3 if n=1000
F=0.2 if n=10,000
F=0.04 if n=1000,000
What changes with number of constituents
What changes with size of clusters
Cluster assembled
films are formed
by the deposition
of these clusters
on to a solid
substrate
Generally highly
porous
CBD
Low density (half
of bulk)
What is the CBD?
• Cluster Beam Deposition
(CBD)
• Method that deposits
nanoparticles as a type of
cluster on substrate
Figure 2. Schematic showing the
deposition of patterned films(SEM)
• High degree of purity and
exact nanoparticle size
control
Gas phase Clusters are generated in cluster sources
Its possible to produce intense cluster beams of any materials
Cluster include species exist only in gas phase or condensed phase or in both
Laser vaporization
flow condensation
source
Pulsed arc cluster
ion source
Laser ablation
cluster source
Supersonic( free
jet) nozzle source
Knudsen cell
(effusive source)
Ion sputtering
source
Magnetron
sputtering Source
Gas aggregation/
smoke source
Liquid metal ion
source
Vacuum evaporation thin film technique( cluster having high F, are
highly active and will react if not produced in vacuum)
Thin film is deposited at wafer(rotatable to have uniform film) in vacuum ( as clusters are
highly reactive when precursor is evaporated
Laser Vaporization
Using laser energy to heat sample particles
+ highly localized heating(due to small laser spot) and rapid
cooling( with background gases)
Shematic diagram of the Laser reactors
TEM of the iron ultrafine particles
Majima et al. (1994) Jpn. J. Appl. Phys. 33, 4759-4763.
How can nanoparticles be deposited on a
substrate?
Gas phase nanoparticle formation and growth
Formation and growth processes of objects relevant for CBD follow the same
physical and chemical mechanisms as any gas-phase particle synthesis process.
1. Particle formation
2. Particle growth
How can nanoparticles be
deposited on a substrate?
1. Particle formation
The starting material can be vaporized from a hot source into a low density inert
gas employing Joule heating, thermal plasma or laser ablation.
(1)Homogeneous nucleation:
Cooling of the vapor rapidly leads to super-saturation followed by homogeneous
nucleation and the formation of first product clusters.[1] A criterion to determine
the formation path of a cluster is its thermodynamically critical diameter,
the Kelvin diameter d1,C :Qa
: Surface tension
1.Granqvist C G and Buhrman R A (1976)
: Molecular volume of the cluster
: Boltzmann constant.
: Dimensionless saturation ratio at temperature T
How can nanoparticles be
deposited on a substrate?
Critical diameter of cluster << Diameter of a single molecule of the product
species
-A nucleus is stable and there will be no growth or shrinkage by condensation or
evaporation and the particle will form by coagulation
Critical diameter of cluster >> Diameter of a single molecule of the product
species
-A nucleus is unstable and particles are formed by homogeneous nucleation:
balanced condensation and evaporation of molecules to and from clusters of the
product species.
(2)Coagulation (collision): It is applied to particle growth in combination with
coalescence.
2.K Wegner and P Piseri (2006)
How can nanoparticles be
deposited on a substrate?
2. Particle growth
The newly formed particles continue to grow either by surface growth (addition
of atoms or molecules to the particle) or by coagulation (inelastic particle–particle
collisions) which is usually followed by coalescence.
(1).Coagulation (collision) Coagulation describes particle–particle collisions due to
Brownian motion or other mechanisms such as shear or
electrostatic forces. Particle growth by coagulation has to
be distinguished between collisions in the free molecular
regime (particle diameter dp smaller than the mean free path
of the gas λ) and in the continuum regime (dP>>λ).
2.K Wegner and P Piseri (2006)
How can nanoparticles be
deposited on a substrate?
The classical theory for Brownian coagulation of monodisperse spheres in the continuum
regime at temperature T is used to calculate dp [3] (Neglecting the particle morphology and
the spread of the particle size distribution):
: Initial particle diameter
: Initial particle concentration
: Dynamic viscosity of the gas
: Residence time
If the particle diameter is much smaller than the mean free path of the gas, as is usually
the case with clusters, the coagulation theory in the free molecular regime has to be
applied. (Neglecting the spread of the particle distribution and the morphology):
: Total volume of particles per unit volume of
gas
: Density of the particles
3.Friedlander S K (2000)
How can nanoparticles be
deposited on a substrate?
Self-preserving size distributions [3]
:Particles that grow by Brownian coagulation typically reach asymptotic distributions
tSPSD: Time to reach the self-preserving size distribution in the free molecular
regime.[4]
3.Friedlander S K (2000) 4.Wegner K and Walker B(2002)
How can nanoparticles be
deposited on a substrate?
Surface growth
Surface growth consists of a first step of molecule or atom transport to the
surface of an already-formed particle and a second step involving a
chemical reaction or a phase change at the particle surface. Especially
during the first stages of particle formation from supersaturated vapour,
surface growth can be significant as the initially formed clusters act as
condensation seeds for the remaining vapour. By controlling the
supersatura- tion at a low level, particle nucleation can be slowed down
while the rate of surface growth by vapour deposition can be increased
2.K Wegner and P Piseri (2006)
How can nanoparticles be
deposited on a substrate?
The rate of change of the particle diameter dp at temperature T by vapor deposition
in the free molecular regime is given by [3]:
: Volume of the transported molecule in the
particle phase
:Partial pressure of the gas or vapor topical far from
the particle
: Partial pressure at the particle surface obtained
from the equilibrium vapor pressure
: Fuchs–Sutugin factor for bridging the free
molecular with the continuum regime [41]
3.Friedlander S K (2000)
How can nanoparticles be
deposited on a substrate?
In the continuum regime, the rate of change of the particle diameter is [3]:
: Diffusion coefficient of the gas or vapor
: Fuchs–Sutugin factor for bridging the
continuum with the free molecular regime
3.Friedlander S K (2000)
How to get nanoparticles’ formation (method)(1)Supersonic beam cluster
1.Nanostructured TiO2 films were grown by
depositing clusters produced by a pulsed
microplasma cluster source (PMCS), in high
vacuum conditions.
2.Particles with different masses have different
inertia so that a spatial separation takes place and
the particles are deposited in different regions of
the collecting surface, called impactor.
3.Oxidation of titanium clusters constituting the
films takes place immediately after exposition to
the air,due to the high reactivity of titanium and to
the high porosity of the cluster-assembled films.
Fig.2.TEM image showing the nanostructure
of the film:Grains with size below 10 nm are
randomly assembled to constitute a porous
structure such as those typical of the ballistic
aggregation regime.
Ref. E.Barborini,I.N.Kholmanov(2002)
How to get nanoparticles’ formation(2)Low energy cluster beam deposition
Specific electronic properties
The x-ray photoelectron spectroscopy technique
(XPS), associated with the Auger electron
spectroscopy (AES), and the electron energy loss
spectroscopy (EELS) techniques are applied on
samples in ultrahigh vacuum conditions (UHV)
for the investigation of the electronic structure.
1.XPS, Auger, and EELS spectroscopies
confirm that the LECBD technique in
UHV allows us to produce
nanostructured silicon films with a rather
low oxygen contamination. In the
deposited size range under consideration
in our experiments (below 300 atoms) the
general behavior of the film is comparable
to amorphous silicon
2.The silicon nanograins are partially
connected by their dangling bonds
leading to a minimization of their total
numbers. In addition, cluster surface
reconstructions involving the formation of
oddmembered rings is at the origin of
dangling bond minimization.
Ref.P.Melinon P.Keghelian (1997)
How to get nanoparticles’ formation (method)(2)Low energy cluster beam deposition
Magnetic properties
The structural properties and the magnetic behavior of pure SmCo5 cluster
assembled lms prepared by the LECBD technique, and SmCo5 clusters
embedded in a silver matrix
A slight increase of the magnetic anisotropy in the mixed SmCo5
cluster films compared to the pure Co-cluster ones.
Ref. M. Negrier, J.Tuaillon-Combes (1999)