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Module 5 – NANOTECHNOLOGY
Session 1 : Properties, Approaches, Methods to produce Nanomaterials
Introduction to Nanotechnology
In 1959, Nobel Laureate Richard Feynman presented an idea of building materials atom by
atom. This idea evolved into a new branch of science called Nano-science which deals with
building up complex materials and machines using fine control on the matter at the nano
meter scale. Study of materials and their properties at the length scale of few nano meters is
called nano science.
The techniques involved in the preparation, characterization and use of the properties of nano
materials in different applications are collectively called as nanotechnology.
Nanotechnology is the latest advancement in the world of science. It refers to the engineering
of system functions at nano level length (i.e. at atomic level). It is used to advance the present
systems by implementing the new concepts related to a particular field.
Properties Nanomaterials
Materials when reduced to nano scale, show different properties as compared to the
properties exhibited by them at micro scale. Let us discuss nanomaterials based on optical
properties, Electrical Properties, Magnetic properties, Structural Properties and Mechanical
properties.
1. Optical Properties :
When light is incident on nanomaterial, it can be absorbed or scattered. It depends on the
size of the nanomaterial. If the size of nanomaterial is less than 20 nm, absorption is
significant and if the size greater than 100 nm, scattering is significant. Thus by designing
the nanoparticle of different sizes, optimal amount of absorption or scattering can be
achieved. This may result different colour for the particles of different sizes of
nanoparticles. E.g. opaque substances at the bulk level, become transparent (copper), gold
nano-spheres of 50 nm are green in colour and of 100 nm size appear orange in colour.
2. Electrical Properties
Electrical conductivity of material is altered when it is reduced to nano size. E.g. in
ceramics, the electrical conductivity increases with decrease in nanoparticle size and in
metals, electrical conductivity decreases with decrease in nanoparticle size. It is possible
to invent nano materials having desired conductivity.
3. Magnetic Properties
Nanomaterials are more magnetic than bulk material. Even non-magnetic solids are found
to show magnetic properties when reduced to nano level. Magnetic properties of the
materials can change when reduced to nano level. E.g. Sodium, Potassium which are
paramagnetic at the bulk level become ferromagnetic at the nano level. Iron, Cobalt,
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Nickel which are ferromagnetic at the bulk level become super paramagnetic at the nano
level.
4. Structural Properties
In nanoparticles surface area to volume ratio is very large. Atoms on the surface of a
material are often more reactive than those in the centre, so a larger surface area means
the material is more reactive. Forces of attraction between surfaces can appear to be weak
on a larger scale, but on a nanoscale they are strong. This may lead to different surface
morphology, changes in crystal structure etc.
5. Mechanical Properties
The mechanical properties like hardness, elasticity, adhesion, friction improve as the
material size is decreased to nano scale. Lubrication improves at the nanoscale. Ductility
of nanomaterial may be high at high temperatures.
Thus, at nano scale, optical, thermal, mechanical, electrical, magnetic, dynamic properties of
the materials change. These nano materials, having new properties can be used in variety of
applications in different fields like – food processing, medicine, automobiles, paint
technology, computer technology, robotics, space technology etc. Advances in
nanotechnology have made it possible to build devices and machines like nano-assembler
which assembles the molecules at atomic level very fast.
Surface area to Volume Ratio :
Nanoparticles of a material show different properties compared to larger particles of the same
material. Forces of attraction between surfaces can appear to be weak on a larger scale, but on
a nanoscale they are strong.
One reason for this is the surface area to volume ratio. In nanoparticles this is very large.
Atoms on the surface of a material are often more reactive than those in the centre, so a larger
surface area means the material is more reactive.
Nanoparticles have more surface area to volume ratio than larger particles.
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The diagram shows this idea. The cube on the left has the same volume as the smaller cubes
added together on the right. However, the total surface area is much larger for the smaller
cubes.
NANOTECHNOLOGY APPROACHES
Two major approaches are used to prepare nano materials –
1) Top Down Approach :
It is a physical process. In top down approach, a large scale object is progressively
reduced in dimensions. Nano objects and nano materials are created from larger objects
without affecting its atomic reactions. It consists of ultra fine micro machining of
materials using lithography, epitaxy and etching. This method is time consuming and
relatively costly.
2) Bottom up approach :
This is a chemical process. In bottom up approach, different materials and devices are
constructed from molecular components on their own which do not require any external
agent to assemble them. They chemically assemble themselves by recognising the
molecules of their own type. This approach starts by collection and combination of atoms
and molecules to build complex structures. It consists of chemical synthesis such as soft
chemical methods and self assembly of molecular structures. This approach is relatively
cheaper.
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METHODS TO PRODUCE NANO MATERIALS
The different methods for synthesis of nano materials can be classified into physical,
chemical, biological and hybrid methods.
Physical Methods
Mechanical Method (Ball Milling Method)
Ball Milling
Ball milling is a mechanical process that uses a Top-Down approach. All the structural and
chemical changes are produced by mechanical energy.
The ball milling method consists of balls and a mill chamber. A ball mill contains a stainless
steel container and many small iron, hardened steel, silicon carbide, or tungsten carbide balls
are made to rotate inside a mill (drum).
The powder of a material is taken inside the steel container. This powder will be made into
Nano size using the ball milling technique. A magnet is placed outside the container to
provide the pulling force to the material and this magnetic force increases the milling energy
when milling container or chamber rotates the metal balls. The ball to material mass ratio is
normally maintained at 2 ratio1.
The silicon carbide balls provide very large amount of energy to the material powder and the
powder then get crushed. This process of ball milling is done approximately 100 to 150 hrs to
get uniform fine powder.
Advantages
a) Nanopowders of 2 to 20 nm in size can be produced. The size of nano powder also
depends upon the speed of the rotation of the balls.
b) It is an inexpensive and easy process.
Disadvantages
a) As the process is not so sophisticated, therefore the shape of the nanomaterial is
irregular.
b) There may be contaminants inserted from ball and milling additives.
c) This method produces crystal defects.
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Physical Vapour Deposition Method (Sputtering Method)
Sputtering Method uses a Bottom-up Approach. In this method, the material is evaporated in
a vacuum. Vapour particles directly travel towards the cold target (substrate) where they are
deposited and condense back to a solid state. Different types of evaporators can be used.
a) Resistance evaporators, in which heat is generated to evaporate the material by passing
current through a high resistance coil.
high resistance coil
Substrate
vapour
particles
Vacuum
System
Vapour Deposition
Method (Resistive
evaporator)
Power
Supply
b) Sputtering evaporators, in which atoms are ejected from a solid target due to
bombardment of the target by energetic electrons.
Substrate
vapour
particles
electrons
Magnet
Target
material
electron gun
Vacuum
System
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Vapour Deposition
Method (Sputtering)
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Sol-gel Process
Sol
Dried
Gel
Gel
Sol gel Method
Final
Product
Sifting
Grinding
The sol–gel process is a method for producing solid materials from small molecules. It uses a
Bottom-up approach. Sol-gel is a chemical solution process to make ceramic and glass
materials in the form of film, fibres or powder.
A sol is a colloidal (the dispersed phase in which size of the particles is so small that
gravitational forces do not exist. Only Van der Waals forces and surface charges are present).
It can be a molecular suspension of solid particles of ions in a solvent.
A gel is a semi-rigid mass that forms when the solvent from the sol begins to evaporate and
the particles or the ions left behind begin to join together in a continuous network.
In this chemical method, a "sol" (a colloidal solution) is formed which gradually evolves in
the formation of a gel (containing both a liquid phase and solid phase) whose morphologies
range from discrete particles to continuous polymer networks.
In order to get the gel-like properties, a significant amount of fluid need to be removed
initially. This can be accomplished by sedimentation or centrifugation.
Removal of the remaining liquid (solvent) phase requires a drying process, which is typically
accompanied by a significant amount of shrinkage and densification. Afterwards, a thermal
treatment is often necessary. This may further enhance mechanical properties and structural
stability
The precursor sol can be either deposited on a substrate to form a film, cast into a suitable
container with the desired shape or used to synthesize powders.
Advantages
a) The sol–gel approach is a cheap and low-temperature technique that allows the fine
control of the product’s chemical composition.
b) Even small quantities of dopants can be introduced in the sol and end up uniformly
dispersed in the final product.
c) Rate of reaction can be easily controlled by maintaining temperature.
Chemical Vapour Deposition Method
In a chemical vapour deposition process, the substrate is exposed to one or more volatile
precursors (chemicals) which react and/or decompose on the substrate surface to produce
desired compound. By-products are removed by carrier gas flow through the reaction
chamber. To avoid undesired chemical reactions, the substrate surface temperature,
deposition time, pressure and type of surface is carefully selected.
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Reactive
gas
Pump
Oven
Carrier
gas
Substrate
Evaporator
Chemical Vapour Deposition (CVD)
CVD is used to produce high purity, high performance solid materials. It is widely used to
fabricate semiconductor devices.
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