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

On the exploration of cluster, C60 was

discovered

In between bulk and molecular/atomic

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

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

Clusters formation: surface fraction of atoms

surface used to be more reactive and

active for any reactions/catalysis

On Bulk /cm3= 1023 atoms

On the surface of this cube, only 1.5x1016

atoms,

Fraction of atoms on surface is 1 in 1000000

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

This makes a significant change in

the properties

Fraction of atoms F=4/n1/3 , n is the

number of atoms in cluster

F=0.3 if n=1000F=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

Low density (half of bulk)

CBD

What is the CBD?

• Cluster Beam Deposition (CBD)

• Method that deposits nanoparticles as a type of cluster on substrate

• High degree of purity and exact nanoparticle size control

Figure 2. Schematic showing the deposition of patterned films(SEM)

Gas phase Clusters are generated in cluster sourcesIts 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 VaporizationUsing 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.

Control o

n cluste

r growth as d

ensity

reduces d

rastically

in va

cccum

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

1. Particle formation

(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

How can nanoparticles be deposited on a substrate?

The starting material can be vaporized from a hot source into a low density inert gas employing Joule heating, thermal plasma or laser ablation.

: Dimensionless saturation ratio at temperature T

: Molecular volume of the cluster: Boltzmann constant.

: Surface tension

1.Granqvist C G and Buhrman R A (1976)

How can nanoparticles be deposited on a substrate?

It is applied to particle growth in combination with coalescence.

(2)Coagulation (collision):

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.K Wegner and P Piseri (2006)

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)

How can nanoparticles be deposited on a substrate?

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)

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

Surface growth

How can nanoparticles be deposited on a substrate?

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.

Ref. E.Barborini,I.N.Kholmanov(2002)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.

How to get nanoparticles’ formation- (2)Low energy cluster beam deposition

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)

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.

Specific electronic properties

How to get nanoparticles’ formation (method)- (2)Low energy cluster beam deposition

A slight increase of the magnetic anisotropy in the mixed SmCo5cluster films compared to the pure Co-cluster ones.Ref. M. Negrier, J.Tuaillon-Combes (1999)

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