NANO PHYSICSUnit-6Sub: Engineering Physics

By: Pankaj Sorathiya

Introduction:Nano Science: It is the study of atoms, molecules and objects whose size is of the nanometer scale.

Nano Technology: It is the technique of design, production of devices and systems by controlling the shape and size at the nanometer scale.

Nanoparticle: Particle with size in the range of 1-100nm is called nano –particle.

Nano Materials are the materials containing nano crystals i.e their grain size is in the 100nm range. The nano materials may be metals, alloys, ceramics.. These materials can be used in many electronic devices, textiles, tyres, paint, in medical fields etc..

Classification of Nano StructuresAccording to their dimensions they are classified in following:

1. Zero- dimensional (0D) nanostructures: In this, all of the three dimensions are in the nano metric range.Ex. Nano particles or well separated nano powders.

2. One- dimensional (1D) nanostructure: In this, two dimensions are in the nano metric range and third dimension remains large. These structures have shape like rods.Ex. Nano tubes, Nano rods etc.

3. Two- dimensional (2D) nanostructure : In this, only one dimension is in the nano metric range while other two dimensions remain large.These display plane like structures.Ex. Nano thin films, Nano coating, Nano layers etc..

Three- dimensional (3D) nanostructure : In this, all three dimensions are outside the nano metric size

range. It may consist of group of nano wires, nano tubes, or different

distributionOf nano particles.

Based on their size they have applications in the different areas of science and technology.

Quantum Size effect:When the particle are in the nano scale range of about 1-100 nm, The materials properties like melting point, electrical conductivity,Chemical reactivity, magnetic permeability changes significantly from those at Larger scales. This is known as quantum size effect.

For example, bulk copper like in form of wires , bending occurs Because of the movement of atoms at about 50 nm scale. But thisBehavior is not seen in nano copper particles as they are smaller Than 50 nm and behaves as a hard materials.

i.e. Nano copper don’t exhibit the ductility property as that of bulkCopper.

Basically there are two types of size effect:(i) Increase in the surface area to volume ratio. These effect can

make nano material chemically reactive and also affect the melting point.

(ii) Quantum effect. It shows discontinuous behavior due tocompletion of shells with delocalized electrons.It may affect the optical, electrical and magnetic behavior of The material.

Surface area to volume RatioIt gives the surface area per unit volume of an object. It is the great way to measure the efficiency of the nano material

Let us assume that the nano particles are spherical in shape. Let us considerthat radius of the atom be r.

So, surface area of the spherical atom = 4r²The volume of the spherical atom = 4/3 r³

Therefore, SA:V = 4r² = 3/r 4/3 r³

The above value shows that this ratio increases with decrease in radius.

Electron confinementElectron or quantum confinement describes how theElectronic and optical properties change when the size of Material is at nano scale.

When material size is reduced the energy spectrum becomes Discreet rather than a continuous spectrum as in the bulk.This situation of discreet energy level is known as quantum confinement.Thus in nano material, the electrons are confined in space rather Than free to move in case of bulk materials.

Nano scale quantum confinement can be 0D, 1D or 2D.

NanotechnologyProgram Focuses on -> design, fabricating and controlling

materials, components and machinery with dimensions on the nanoscale (0.1 – 100 nm)

Specialization in: -> Nanophysics and Nanomaterial

-> Nanobiotechnology

http://nano.aau.dk

SYNTHESISNano materials are basically carbon structure

i.e. fullerenes, carbon nanotubes and other nano particles .

Using a variety of synthesis method ,it is possible to produce nano structured material in the form of thin film ,coating ,powder and as a bulk material.

The methods used for the synthesis of nano particles can be broadly classified in to physical, chemical, biological, self assembly and hybrid method

CLASSIFICATION OF SYTHESIS METHODS It is classified in to two process(1) top down process (2)

bottom up process Top down process: it involves , the breaking down of a

large piece of a bluk material to generate required smaller nano structure through etching or milling from the bulk material . The schematic representation of the process

The bottom up process : it involves the process of building up of the atom or molecular constituent into a large nano structured material . The bottom approach is the powerful approach of creating identical structures with atomic precision

Techniques used in synthesis of nanomaterial Top to down process (1)milling(2)lithographics(3)maching

Bottom –up process (1) Plasma assited deposition process(2)Vapour deposition (3)Liquide phase process (4)Molecular beam epitaxy(5) Laser synthesis

Plasma –arcing Plasma is a hot ionized gas containing positive ions and

electrons.Thermal plasma can also deliver the energy necessary to

cause evaporation of small micrometer size particles . The main types of thermal plasma torches used to produced nano particles are dc plasma, discharge and radio frequency (RF) induction plasma .

Description and working the arc plasma set up consists of a pair electrodes with a gap

of approximately 1 mm.The anode electrode itself acts as a source (target) of

material and other electrode acts as a substrate .The electrodes should kept at high vacuum or ultra high

vacuum enclosure filled with an inert gas (He or Ar) at 100-500 torr .

When a high current approximately 50to100 is passed from a low voltage power supply , a high discharge is generated between the two electrodes

The temp is high approximately 3500 degree C reaches as arc discharge takes place .Thus when an arc is set up, anode material evaporates .These positive ions are attracted towards the other electrode and gets deposited over it to form tin film of nano particles .The anode material evaporation continues as long as discharge maintained.The adjustment of the electrode gap without breaking the vacuum becomes essential as one of the electrode burns and gap increases .This method mostly found to be suitable for fullerenes or carbon nano tubes deposition

Vapor Phase Deposition Method (VPD) To fabricate thin films, multilayer's , nano tubes , nano filaments

and nano size d particles of different materials (organic or in organic)

There are two types VPD (1) Physical vapor deposition<PVD>(2) Chemical vapor deposition <CVD>

PVD process involves the direct deposition of gaseous over the substrate surface but CVD involves either chemical reaction or thermal decomposition of a gas phase at elevated temp and depositing on the substrate CVD process is more complex than PVD .

CHEMICAL VAPOUR DEPOSITION(CVD)PRINCIPLE : THE FLOW OF A GAS WITH DIFFUSE

REACTANTS OVER A HOT SUBSTRATEDescription and working: A typical thermal CVD

comprises of a gas supplying system , deposition chamber and exhaust system . The entire system called a reactor . There are two types of thermal CVD reactors which are used to perform the process.

1. Horizontal reactor 2. Vertical reactor The substrate surface is raised to a very high temp with the

help of resistance heaters . The resistance heaters either surround the chamber or lie directly under the susceptor that hold the substrates.

In the thermal CVD process the chemical reaction is activated by a high temp above 900 degree C . In this technique the material to be deposited is heated to form a gas phase and is deposited on to a heated substrate.

While the gas flow over the hot solid substrate , the heat energy activates the chemical reaction at appropriate site , nucleate and grow to form the desired material film . The byproduct created on the substrate are then properly vented with the help of an outlet

The gas that carries the reactants is called the carrier gas

1) In thermal CVD : reaction is activated by high temp 900 degree C.

2) In plasma CVD : reaction is activated by a plasma at a temp between 300 to 700 degree C .

3) In LASER : the pyrolysis occurs by a LASER beam .

4) In photo LASER CVD : by uv radiation

MERITS Simple in design so we can

use it in mass production . Used for defect free nano

particle

Sol Gel Method In this technique , nitrates or carbides are taken pre

cursors which are dissolved in deionized water . The solution is kept at a suitable temp and some amount of gelling agents are added to it. Thus viscosity , temp and PH of solution is controlled in this technique. The nano materials in the form of thin film coating are made by this technique.

For thin film coating substrates like copper , nickel or glass are taken and dipped in the solution before gel formation . Finally annealing has to be done to get thin film.

The sol gel synthesis is most widely used due to following reason.

1. In this technique; materials both ceramic and metals can be produced at ultra low temp.

2. Any type of material can be synthesized in enlarge quantity very cheaply.

3. Extremely homogeneous alloys can be produced.

4. High purity in synthesized material can be obtained.

5. In this, all the properties can be controlled.

Fabrication of Carbon Nano Tubes

CNT’s are fabricated primarily using(1) electric arc (2) chemical Vapor deposition and (3) pulsed laser deposition technique.

1) Electric Arc Method: It is evaporation method gives

best quality of nano tubes.This is also known as Plasma arc method.

2) Chemical Vapour Deposition Method:

The chemical vapor deposition method involves decomposing aHydrocarbon gas such as methane at 1100 °C. For large scale production of carbon fibers and filaments.

Description & Working:

Inside the furnace, a substrate prepared with a layer of metalCatalyst such as Fe, Co or Ni is placed.

There is provision in the furnace to allow two types of gases toInitiate the growth of CNT’s. One type of gas such as ammonia, Nitrogen, hydrogen etc.. And other a carbon containing such as Methane, ethane, ethylene etc.

The hydrogen gas such as methane is passed in furnace. Furnace is heated approximately 750°C to 1100 °C.At this high temperature the gas decomposes producing carbonCarbon atoms which get deposited over the substrate formingnano tubes. This method of production allows continues fabrication of nanoTubes with open ends which doesn’t occur in other methods.Merits: 1) Both SWCNT and MWCNT are possible to obtain by

This method. 2) High purity CNT’s can be produced due to the non- formation of nano particles or amorphous carbon.

Pulsed Laser Deposition:

Water cooled Cu collector

Here Laser source is incident on the target, it evaporates carbon from the target graphite.The Argon gas sweeps the evaporate carbon atom from high temperature zone to the low temperature zone. Where they condensed in to nano tubes.

The initial laser vaporization pulsed laser is followed by second pulse to vaporize the target more uniformly. The second laser pulse break up the larger particles ablated by first one and feeds them into the growing nano tube structure.

Merits: In this method, CNT ropes of 10-20 mm diameter and length of 100 mm can be produced.

The tube diameter can be controlled by the reaction temperature.

More than 85% of graphite is converted in to CNT’s

Properties of Carbon Nano Tubes:

1. The carbon nano tubes have extremely low resistance.2. The conductivity of nano tube is a function of its diameter.3. CNT’s are metallic or semiconducting depending on diameter4. Metallic nano tubes can carry current which is more than 100

times larger than metal copper.5. CNT’s have high thermal conductivity which increase with

decrease in diameter.6. CNT’s melting temperature is three times higher than melting

point of copper.7. CNT’s have ability to withstand extreme strain.8. Nano tubes are highly resistant to chemical attack.

Applications of Carbon Nano Tubes:

1. CNT’s are used in constructing nano scale electronic devices.2. They are used in battery electrodes, electrical devices.3. Nano tube tips can be used as nano probes.4. Nano tubes serves as catalyst for some chemical reaction.5. Semiconducting CNT’s are used in switching circuits.6. They are used as a sensitive detector of various gases.7. CNT’s are used as interconnects in chip due to their extremely

low resistivity.