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NANO TECHNOLOGY

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Origin of Nanotechnology, Nano Scale, Surface to Volume Ratio,Quantum Confinement, Bottom-up Fabrication: Sol gel, Precipitation,

Combustion Methods;Top-down Fabrication: Chemical Vapour Deposition, Physical Vapour Deposition, Pulsed Laser Vapour Deposition Methods, Characterization(XRD&TEM) and Applications.

Nano science can be defined as the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at a larger scale.

Nano is one billionth part of a meter.A material is called nano material if the

size of the material is 100nm. We may wonder why 100nm but not 1000nm or 10nm?

This is because many physical and chemical properties of a material starts changing significantly when the size of grains is below 100nm.

INTRODUCTION:

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Nano materials have enhanced surface area to volume ratio

BASIC PRINCIPLES OF NANO MATERIALS

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1. INCREASE IN SURFACE AREA TO VOLUME RATIO:

Ex: For a cube

Due to increase in surface area, more number of atoms will appear at the surface compared to those inside.

This makes Nano materials more chemically reactive.

Ex: Size of 10nm has 20% of its atoms on its surface. & Size at 3nm has 50% of its atoms on its surface.

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2. QUANTUM CONFINEMENT

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Quantum dots can absorb higher frequency light and reemit a lower frequency light characteristic to their size

When material size is reduced to Nano scale energy levels of electrons changes to intermediate of both discrete energy levels and energy bands.

Quantum confinement refers to keeping of electrons trapped in a very small area.

This effects the Optical, Electrical, Magnetic & Mechanical properties of Nano materials.

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BOHR’S EXCITON Very small semiconductor particles with a size comparable to the Bohr radius of the excitons.

(An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Columbic force. i.e., separation of electron and hole).

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•One dimension•confinement•QUANTUM WELLBohr Exction

•Two dimension•confinement•QUANTUM WIREBohr Exction

•Three dimension•Confinement•QUANTUM DOT

Bohr Exction

PROPERTIES OF NANO TECHNOLOGY

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The change in inter atomic spacing and large surface to volume ratio have a combined effect on material properties . Variation in the surface, free energy changes the chemical potential .

As the result the melting point shows variation for a change in particle size for gold particles.

PHYSICAL PROPERTIES

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When a particle size is reduced from the bulk, the electronic band in the metals becomes narrow, leading to the transformation of the delocalized electronic states into more localized molecular bonds. As a result, the ionization potential increases.

CHEMICAL PROPERTIES:

MECHANICAL PROPERTES :At Low temperature:We know that green refinement leads to an improvement in

the properties of the metals and alloys. For ex, a reduction in grain size lowers the transition

temperature in steel from ductile to brittle .

At High temperature:In case of nanophase materials, the occurrence of super

plastic temperature is decreased due to the decrease in grain size , which results in an increase in the strain rate .

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ELECTRICAL PROPERTIES The electrical conductivity and energy bandwidth

changes when some materials are converted into Nano materials.

Bulk silicon is an insulator which becomes a conductor in Nano phase.

If semiconductor particles are made small enough , quantum effect come into play which limits the energies at which, electrons and holes can exists in the particles .

As energy is related to wavelength (or colour) this mean that the optical properties of the particle can be tuned depending on the size.

Thus , the particle can be made to emit or absorb specific wave lengths (colors) of light by controlling their size.

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OPTICAL PROPERTIES:

MAGNETIC PROPERTIES:At nano scale , the magnetic material has a

single magnetic domain .As the result , there is no hysteresis loss.

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Top –down approach :It implicates the breaking down of the bulk material into nano sized structures or particles. “Top down‟ is realized by breaking, cutting or etching techniques, which is achieved by bulk or film machining, surface machining and mold machining occupying lithography. Bulk machining makes use of photolithography, which applies the etching process while mold machining uses soft lithography

Top –down approach involves:1. High energy milling2. Chemical mechanical milling3. Vapour phase condensation4. Electro-explosion5. Laser ablation6. Sputtering

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Bottom- up approachIt is about the build up of a material from the bottom, atom-by-atom, molecule-by molecule, or cluster-by-cluster. The bottom up approach of nano materials synthesis first forms the nano structured building blocks (nano particles) and then collects these into the final material, The buildings blocks may be manipulated through controlled chemical reactions to self assemble and make nanostructures such as nano tubes and quantum dots

Bottom–up approach involves:1. Solution combustion method2. Sol –gel method3. Micro emulsion4. Reverse micelle synthesis5. Chemical precipitation synthesis

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SOL GEL PROCESSTypical precursors are metal alkoxide and metal chlorides, which undergo hydrolysis and polycondensation reactions to form either a network "elastic solid" or a colloidal suspension (or dispersion)

MOR + H2O MOH + ROH (hydrolysis)Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution.

MOH+ROM M-O-M+ROH (condensation)Thus, the sol evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks.

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Nanoparticles can be generated bottom-up in liquids by means of precipitation. In this a supersaturated mixture starts forming solid particles out of the highly concentrated material that will grow and finally precipitate.

‘Banert’ used an optimized sono-chemical reactor right picture, feed 1: iron solution, feed 2: precipitation agent to produce the magnetite nano particles "by co-precipitation of an aqueous solution of FeCl3.6H2O & FeSO4.6H2O with a molar ratio of Fe3+/Fe2+ = 2:1. Iron solution is precipitated with concentrated ammonium hydroxide and sodium hydroxide respectively. In order to avoid any pH gradient,

PRECIPITATION OF NANO PARTICLES

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Physical vapour deposition (PVD) methodIn this method depositing of thin films by condensation of a

vaporized form of a material onto various surfaces under high vacuum conditions.

The coating method involves purely physical processes rather than chemical reactions.

Various methods of PVD are

• Evaporative Deposition.• Electron Beam Physical Vapour

Deposition.• Sputter Deposition.• Cathodic Arc Deposition. • Pulsed Laser deposition.

Thus in all these methods we find different techniques used to convert the material into vapour phase for further deposition on the substrate.

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Physical vapour deposition (PVD) method

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Chemical Vapour Condensation (CVC)• The metal-organic precursor is

introduced into the hot zone of the reactor employing flow controller.

• The precursor is vaporized by resistive eating method.

• An inert gases are used as carrier gas in this process.

• The hot atoms which undergoes collision with the atoms in the cold gas and hence loses its energy.

• Thus the colloidal atoms undergoes condensation into small clusters through a homogeneous nucleation.

• The condensed clusters are allowed to pass through cold finger. Then nano particles are collected using scrapper.

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Pulsed Laser vapour Deposition method (PLD)

PLD was the first technique used to successfully deposit a superconducting YBa2Cu3O7-x thin film. Since that time, many materials that are normally difficult to deposit by other methods, especially multi-element oxides, have been successfully deposited by PLD.PLD is a photon interaction to create an ejected plume of material from any target.

The vapor (plume) is collected on a substrate placed a short distance from the target. Though the actual physical processes of material removal are quite complex, one can consider the ejection of material to occur due to rapid explosion of the target surface due to superheating.

In thermal evaporation, which produces a vapor composition dependent on the vapor pressures of elements in the target material, the laser-induced expulsion produces a plume of material with stoichiometry similar to the target. It is generally easier to obtain the desired film stoichiometry for multi-element materials using PLD than with other deposition technologies.

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CHARECTERISATION OF NANOPARTICLES XRD Analysis

XRD study indicates the formation of silver (Ag) Nano particles

Experimental diffraction angle [2θ in degrees]

Standard diffraction angle [2θ in degrees]

45 44.3

From this study, considering the peak at 45 degrees, average particle size has been estimated by using Debye-Scherrer formula

Where• 'λ' is wave length of X-Ray (0.1541 nm), • 'W' is FWHM (Full Width at Half Minimum) • ‘θ' is the diffraction angle and • 'D' is particle diameter (size).

Diffraction angle

[degree]

FWHM [radians]

d spacing [nm]

Diffraction plane Size [nm]

45 0.011 0.1788 200 14

The average particle size is calculated to be around 14 nm. Thus using XRD pattern we are able to calculate the particle size.

To see the particle size, we have to go for TEM.

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Transmission electron microscopy(TEM)

TEM gives the following useful informationMorphologyThe size, shape and arrangement of the particles which make up the specimen as well as their relationship to each other on the scale of atomic diameters. Crystallographic InformationThe arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects in areas a few nanometers in diameter Compositional Information (if so equipped) The elements and compounds the sample is composed of and their relative ratios, in areas a few nanometers in diameter

A TEM image of the prepared silver nano particles is shown in the fig.. The Ag nano particles are spherical in shape with a smooth surface morphology. The diameter of the nano particles is found to be approximately 16 nm. TEM image also shows that the produced nano particles are more or less uniform in size and shape.

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• In the transmission electron microscope (TEM), electrons are transmitted through a thinly sliced specimen and form an image on a fluorescent screen or photographic plate. 

• Those areas of the sample that are more dense will transmit fewer electrons (ie will scatter more electrons) and will therefore appear darker in the image. 

• TEM’s can magnify up to one million times and are used extensively in Biology and Medicine to study the structure of viruses and the cells of animals and plants.  This diagram shows the basic structure of a TEM.

BASIC STRUCTURE OF A TEM

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CARBON NANOTUBES (CNTs)The carbon nano tubes are man made nano materials

which cannot be found in nature, and having many interesting physical and chemical properties with great potential device applications.

Carbon nano tubes are fabricated by rolling up graphite sheet as a cylinder like a straw.

Different ways of rolling graphite sheets

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It is an allotrope of carbon, whose structure is one-atom-thick planarsheets of sp2-bonded carbon atoms that are densely packed in a honey comb crystal lattice.

The C-C bond length in graphene is about 0.142 nanometers.

Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes and fullerenes 31

GRAPHENE

BUCKMINSTER FULLERENES:• It is a spherical fullerene molecule with the formula C60

made of 20 hexagonal and 12 pentagons with a carbon atom at the vertices of each polygon.

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APPLICATIONS OF NANO MATERIALS

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CARBON NANO TUBES:These are extremely useful in the preparation of artificial

muscles. Used as connecters in computer industries .These electrical current made using CNT in field effect

transistor.CNTs have been used as a storage device in battery , fuel

cells, etc.,

AS A SUNSCREEN:Nano sized titanium dioxide and zinc oxide are used in

sunscreens to absorb and reflect UV-rays .

IN TREATMENT OF CANCER:Nano sized particles Fe2O3 is injected into the cancer tumor in

the time varying field.The temperature of nano sized Fe2O3 will rises to 45 oC, which

will kill the cancer cells, leaving normal cell intact.

IN DRUG DELIVERY:Bio chips can be used for control release of drug at the site in

any part of the body and it can control the delivery and the dosage of drug when ever it is needed.

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In medicine