when light enters one end of the fibre it undergoes ... · reduces the cone of acceptance and...

Department Of Applied Physics, ACET. BE-Second Semester – Advanced Physics

UNIT-IV

OPTICAL FIBER AND

NANOSCIENCE


OPTICAL FIBER

Definition: Optical fibres are made up of glass or plastic conduits as thin as a human hair, designed to

guide light waves along their length based on the principle of total internal reflection.

An optical fibre works on the principle of total

internal reflection (TIR). When light enters at one end of the

fibre it undergoes successive total internal reflections and

travels down the length of the fibre along a zigzag path.

(fig.(1)). A small fraction of light may escape through

sidewalls but a major fraction emerges out from the other end

of the fibre.

Principle : Total internal reflection:

The phenomenon in which light is totally reflected from a denser-to-rarer medium boundary, at

an angle greater than critical angle, is known as total internal reflection.

A medium having lower refractive index is said to be optically rarer medium and a medium having

higher refractive index is called an optically denser medium. When a light ray passes from rarer medium to

denser medium, it is bent toward the normal in the denser medium. Incident ray & refracted ray are

reversible. Hence when a ray passes from a denser medium to a rarer medium, it is bent away from the

normal.

Let θi is angle of incidence θr is the angle of refraction, then by Snell’s law,

1

2

sin

sin

n

n

r

i

-----(1)

ri nn sinsin 21 Transmission or refraction angle θr increases faster than the angle of incidence θi and refracted rays

bent more and more away from the normal. At some particular angle of incidence θi=φc the refracted ray


glides along boundary surface so that θr =90º. At angles greater than the (θi> φc ,) there is no refracted rays

i.e rays are reflected back into the denser medium .

Rays experiencing total internal reflection obey the law of reflection.(i.e. angle of incidence = angle

of reflection)

Critical angle φc can be found from Snell’s law, From (1)

(When θi=φc and θr=90º)

90sin)(sin

90sin

sin

1

2

1

2

n

nc

n

nc

φc=sin-1

( )1

2

n

n

Total internal reflection does not takes place when light propagates from a rarer medium to denser

medium.

Definition of critical angle ( c ): It is that angle of incidence in denser medium for which angle of

refraction is 900.

The ray whose angle of incidence is above critical angle Фc undergoes total internal reflection.

Structure of Optical fibre: A practical optical fibre has in general three co-axial regions.

Fig.2: Side view and cross sectional view of a typical optical fibre.

1. Core: The innermost cylindrical region is the light guiding region known as the core. In general, the

diameter of the core is of the order of 8.5 µm to 62.5 µm.

2. Cladding : It is (core) surrounded by a Co-axial middle region called as the Cladding. The diameter

of the cladding is of the order of 125 µm.

The refractive index of cladding n2 is lower than that of the core n1(μclad < μcore or n1>n2) The purpose

of cladding is to make the light to be confined to the core.

Light launched into the core and striking the core to cladding interface at angle > critical angle, will be

reflected back into the core. Since the angles of incidence and reflection are equal, the light will continue to

rebound and propagate through the fibre

3. Sheath/Buffer : The outermost region is called the Sheath.

Sheath protects the cladding and the core from abrasions, contamination and the harmful influence

of moisture. It also increases the mechanical strength of the fibre.

Necessity of cladding:


The cladding maintains uniform size of the fibre, protects the walls of fibre from chipping, and reduces the

size of the cone of light that will be trapped in the fibre.

Thus, the cladding performs the following important functions:

Keeps the size of the fibre constant and reduces loss of light from the core into the

surrounding air

Protects the fiber from physical damage and absorbing surface contaminants

Prevents leakage of light energy from the core through frustrated total internal

reflection.

Reduces the cone of acceptance and increases the rate of transmission of data.

Acceptance angle :

Def : Acceptance angle is defined as the maximum angle that a light ray can have relative to the axis of

the fibre and propagate down the fibre.

Fig. Illustration of the path of a light ray incident on the end of an optical fibre at angle θi to the fibre axis.

Consider a step-index optical fibre into which light is launched at one end as in fig. The end at which light

enters the fibre is called Launching end.

Let Refractive index of core = n1 or μ1,

Refractive index of cladding = n 2 or μ2

Refractive index of the medium from which light is launched into the fibre= n 0

Where n2 < n1( or μ2< μ1 ),

Let a light ray enters the fiber at an angle θi to the axis of the fibre. The ray refracts at an angle θr and strikes

the core – cladding interface at angle of Φ. If Φ > Φc the ray undergoes total internal reflection at the

interface because n 1 > n2 .

As long as Φ > Φc , the light will stay within the fibre.

By Snell’s law, apply it to the launching zone of the fiber

)1....(..........sin

sin

0

1

n

n

r

i

If i increases , r will also increases hence Φ decreases ,when Φ will drop below the critical angle Φc

then the ray will escape from the side wall of the fibre.


The largest value of θi occurs when Φ = Φc

from ΔABC,

rsin =sin (90- Φ)

rsin = cos Φ ---------(2)

From eq 1 & eq2,

1

0

1

0

1

0

max

1max

0

2

1

2 221 22 2

2

1 1

2 2

1 2

1

2 2

1 21max

0 1

2 2

1 2

(max)

0

sin(1)

sin

sin

cos

sin cos

,

sin( ) cos

sin

cos (1 sin ) 1

cos

sin ( )

sin( )

c i

i c

c

c

c

i

i

ni

r n

ni

n

ni

n

when i

n

n

nBut

n

n nnc

n n

n n

n

n nn

n n

n n

n

(n0=1 since air medium.)

Quite often the incident ray is launched from air medium, for which n0=1= μair

Let

)(sin

sin

2

2

2

1

1

0

2

2

2

10

0max

nn

nn

i

θ0 is called the acceptance angle of the fibre.

Acceptance Cone :

Def : The light rays contained within the cone having full angle 2 θ0 are accepted and transmitted along the

fibre. Therefore, the cone is called the Acceptance cone.


Acceptance cone = 2θ0= 2 )(sin 2

2

2

1

1 nn

Light incident at an angle beyond θ0 i.e. i > θ0, refract through the cladding c as r increases

decreases since i increases and the corresponding optical energy is lost. Larger the diameter of the core

larger the acceptance angle.

An optical fibre accept only those rays which are incident with in a cone having a semi-angle θ0.

Fractional Refractive index change:- The fractional difference Δ between the refractive indices of the core and the cladding is known as

fractional refractive index change. It is expressed as

1

21

n

nn

Δ is always positive because n1 must be larger than n2 for the total internal reflection condition. To guide the

light rays effectively through a fibre Δ<<1, i.e. of the order of 0.01.

Q: Derive an expression for numerical aperture of a step index fibre in terms of Δ.

Numerical Aperture: - (NA) :-

The numerical aperture (NA) is defined as the sine of the acceptance angle.

The main function of an optical fiber is to accept & transmit as much light from the source as possible. The

light gathering ability of a fiber depends on two factors, namely core size & the numerical aperture. The

acceptance angle & fractional R.I. change determine the numerical aperture of fiber.

)2....(....................2..

2

)2(

)2()(

)(2

2))(2

(

))((

)1(.

sin.

1

1

2

2

2

1

2

1

11

2

2

2

1

21

1

211

21

1

1

2121

2121

2

2

2

1

2

2

2

1

0

nAN

nnn

n

nnnn

nnn

nnandn

nnelyApproximat

nn

nnnn

nnnnnn

nnAN

AN


Numerical aperture determines the light gathering ability of the fibre. It is a measure of the amount of light

that can be accepted by a fibre. From equation (1)it is seen that N.A.=2

2

2

1 nn N.A. is dependent only on

refractive indices of the core and cladding materials. Its value ranges from 0.13 to 0.5. Larger N.A. means a

fibre will accept large amount of light from the source

Modes of propagation:

Modes are the possible number of allowed paths of light in an optical fibre.

The light ray paths along which the waves are in phase inside the fibre are known as modes.

Modes are designated by an order number ‘m’. In a fibre of fixed thickness higher order modes

propagate at angles close to the critical angle Φc and lower order modes propagate with angle much

higher than the critical angle Φc.

Fig : Low and high order ray path in multimode fibre Fig :Zero order ray path (Axial ray) in single mode fibre

Zero order ( mode) ray travel along the axis and is known as axial ray.

Light propagates as an electro-magnetic wave through an optical fiber, thus the waves travel in

number of directions and not all waves are trapped within the optical fibre.

The allowed directions corresponds to the modes in optical fibre .

The light rays traveling through a fibre are classified as

1.Axial rays

2.Zigzag rays.

Classification of optical fibre:-

Optical fibres are classified as follows.

1. Classification On the basis of refractive index profiles, they are classified as

(a) Step index (SI)and (b) GRaded INdex (GRIN)

Refractive Index profile is a plot of R.I. drawn on horizontal axis verses the distance from core

axis drawn on the vertical axis.

2. Classification On the basis of modes of light propagation as

(a) A Single Mode Fibre: A single mode has a smaller fiber (SMF) core diameter & can support only

one mode of propagation.


(b) Multimode fibres: A multimode fiber(MMF) has a larger core diameter & supports a number of

modes.

3. Classification On the basis of material used for core and cladding

(a) Glass/ Glass (All glass fibres) (b) Glass/ Plastic(PCS) (c) Plastic/ Plastic fibres (All Plastic

Fibres)

1. Classification Of Optical Fibres:

Que. Explain what is step index single mode , Step index multimode and Graded index multimode

fibre. Draw relevant sketches.

1) Single Mode Step Index Fibre:

Fig.1 : Single mode step index fibre (a) RI. profile (b) ray paths (c) typical dimensions.

Structure

A single mode step index fibre has a very fine thin core of diameter of 8 µm to 12 µm (see Fig.8c).

The core is surrounded by a thick cladding of lower refractive index. The cladding is composed of

silica lightly doped with phosphorous oxide. The external diameter of the cladding is of the order of

125 µm.

The refractive index of the core is maximum and constant throughout the core. The refractive

index of the cladding is less than that of the core and changes abruptly at the core-cladding


boundary, in a stepwise manner as shown in Fig 1(a). Since the refractive index profile is step-

type, the fibre is called a step index fibre.

Propagation of light in SMF:

Only one mode can propagate through a single mode step index fibre. It means that light travels in

SMF along a single path that is along the axis (Fig.8.llb). This mode is known as the zero order

mode.

Both ∆ and NA are very small for single mode fibres. ∆ is of the order of 0.002.

Costly laser diodes are needed to launch light into the SMF.

2) Multimode Step Index Fibre:

Fig. 2: Multimode step index fibre (a) RI. Profile (b) Ray paths (c) Typical dimensions.

Structure:

A multimode step index fibre is very much similar to the single mode step index fibre except that its

core is of larger diameter. The core diameter is of the order of 50 to 100 µm, which is very large

compared to the wavelength of light. The external diameter of cladding is about 150 to 250 µm

(Fig.9c).

The refractive index of the core is maximum and constant throughout the core. The refractive

index of the cladding is less than that of the core and changes abruptly at the core-cladding

boundary, in a stepwise manner as shown in Fig. 2(a). Since the refractive index profile is step-

type, the fibre is called a step index fibre.

Propagation of light in MMF:

Multimode step index fibres allow finite number of guided modes.

The direction of polarization, alignment of electric and magnetic fields will be different in rays of

different modes.

In other words, many zigzag paths of propagation are permitted in a MMF.

The path length along the axis of the fibre is shorter while the other zigzag paths are

longer.

2) Graded Index (GRIN) Fibre:

3)


Fig. 3: GRIN fibre (a) R.I. Profile (b) Ray paths (c) Typical dimensions.

Structure:

The size of the graded index fibre is about the same as the step index fibre (Fig.3 c).

A graded index fibre is a multimode fibre with a core consisting of concentric layers of

different refractive indices.

The refractive index of the core has a high value at the centre and falls of with increasing

radial distance from the axis. Atypical structure and its index profile are shown in Fig.10 (a).

The index profile is parabolic and is preferred for different applications.

Propagation of light:

A GRIN fibre supports a finite number of guided modes.

As a light ray goes from a region of higher refractive index to a region of lower refractive index, it is

bent away from the normal. In the graded index fibre, rays making larger angles with the axis

traverse longer path but they travel in a region of lower refractive index and hence at a higher speed

of propagation. Thus, there occurs a self focusing effect.

2. Classification On the basis of materials used for core and cladding

Optical fibres are fabricated from glass or plastic which are transparent to optical frequencies. Step

Index Fibres are produced in three common forms.

(a) Glass/ Glass (All glass fibres)

The basic material for fabrication of optical fibres is silica (SiO2). It has a refractive index of 1.458 at

λ=850 nm. Materials having slightly different refractive index are obtained by doping the basic silica

materials with small quantities of various oxides. If the basic silica material is doped with Germania

GeO2 or Phosphorous pentoxide (P2O5), the refractive index of the material increases.Such materials are

used as core materials and pure silica is used as cladding material in these cases.When pure silica is

doped with Boria (B2O3) or Fluorine its refractive index decreases. These materials are used for cladding

when pure silica is used as the core materials.Examples of fibre compositions are;

SiO2 Core - B2O3 Cladding

GeO2. SiO2 Core -SiO2 Cladding

The glass optical fibres exhibit very low losses and are used in long distance communications.

(b) Plastic/ Plastic fibres (All Plastic Fibres)

In these fibres perspex (PMMA)and polysterene are used for core. Their refractive indices are 1.49 and

1.59 respectively. A Flurocarbon Polymer or a silicone resin is used as a cladding material . A high

refractive index differenceis achieved between the core and the cladding materials. Therefore plastic

fibres have large NA of the order of 0.6 and large acceptance angles upto 700 .The main advantages of


the plastic fibres are low cost and higher mechanical flexibility. However they are temperature sensitive

and exhibit very high loss. Therefore they are used in low cost applications and at ordinary temperature

(below 800) . Examples of plastic fibre composition are :

Polysterene core n1=1.60 NA=0.60

Methyl methacrylate cladding n2=1.49

Polymethyl methacrylate core n1=1.49 NA=0.50

Copolymer cladding n2=1.40

(c) Glass/ Plastic(PCS Fibres)

The Plastic Clad silica (PCS) fibres are composed of silica cores surrounded by a low refractive index

transparent polymer as a cladding . The core is made from high purity quartz. The cladding is made up of

silicone resin having a refractive index of 1.405 or of perfluoronated ethylene propylene(Teflon) having a

refractive index of 1.338. Plastic claddings are used for step Index fibres only. The PCS fibres are less

expensive but have high losses. Therefore they are mainly used for short distance communications.

V-number/Normalized frequency:- An optical fibre is characterized by one more important parameter called as V-number which is called

as Normalized frequency of the fibre. It is given by ,

aV

2 )(

2

212 nn --------(1)

Normalized frequency (V) is a relation among the fibre size, the refractive indices (n1,n2) & the

wavelength(λ).

In equation (1), ‘a’ is the radius of the core, λ is the free space wavelength, n1 & n2 is R.I. of the core &

cladding.

aV

2 (NA) ---------------(2) ( ..AN 2

21

2 nn )

a2 (n1 2 )

d (n1 2 )

V- number determines the number of modes Nm that can propagate through a fiber.

According to equation (2), the number of modes that propagate through a fibre increases with increase in

numerical aperture.

The maximum number of modes Nm supported by SI fibre (Step Index) is determined by

Nm = 2

2

1V

While the number of modes in the GRIN fibre is about half than that of step-index fibre.

Nm = 2

4

1V

For V<2.405 , the fibre can support only one mode and is classified as a Single mode fibre (SMF)

For V>2.405 , the fibre can support many modes and is classified as a Multi mode fibre (MMF).

The wave length corresponding to the value of V=2.405 is known as Cutoff wavelength λc of the fibre.

FIBRE LOSSES:

ATTENUATION:

As a light signal propagates through a fibre, it suffers loss of amplitude and change in shape. The loss of

amplitude is referred to as attenuation and the change in shape as distortion.


The attenuation of optical signal is defined as the ratio of the optical output power from a fibre of length

L to the input optical power. It is expressed in decibel per kilometer (dB/Km).

……………… (1)

Where Pi is the power of optical signal launched at one end of the fibre and P0 is the power of the optical

signal emerging from the other end of the fibre.

In case of an ideal fibre, P0 = Pi, and the attenuation would be zero.

Different Mechanisms of Attenuation:

There are several loss mechanisms responsible for the signal attenuation in optical fibers. They are

broadly divided into two categories.

1. Intrinsic losses

2. Extrinsic losses

1. Intrinsic losses:-Intrinsic losses are influenced by the material composition and purification

level. Impurities and inhomogeneities in material cause signal absorption and scattering.

a) Absorption by material:- Absorption accounts for 3-5% of fiber attenuation. This

phenomenon causes a light signal to be absorbed by natural impurities in the glass, and

converted to vibrational energy or some other form of energy.

b) Rayleigh scattering:- Rayleigh scattering accounts for 96% of fiber attenuation. The

local microscopic density variations in glass cause local variations in refractive index.

These variations which are inherent in manufacturing process and cannot be eliminated

and act as obstructions and scatter light in all directions. This is known as Rayleigh

scattering

2. Extrinsic losses:- They are cause by geometric effects. Irregularities of geometric nature

cause light energy losses. Any bends in the optical fiber produce radiative losses.

a) Microbend losses:- Microbend is a small-scale distortion. It is generally form due to

pressure on the fiber. The bend may not be clearly visible on inspection. Microbends

may be introduced during manufacturing or installation processes. Microbending may

occur, for example, due to winding of optical fiber cable over spools. Light rays get

scattered at the small bends and escape into the cladding. Such losses are known

as microbend losses.

b) Macrobend losses:- A macrobend is a large-scale bend that is visible. When a fibre is bent

through a large angle, strain is placed on the fiber along the region that is bent. The bending

strain will affect the refractive index and the critical angle of the light ray in that specific area. As

a result, light traveling in the core can refract out, and loss occurs.

c) Waveguide losses:- Due to irregularities in the optical fiber geometry, the incident angle

becomes less than the critical angle for higher order modes. As a result, part of the light ray will

be refracted into the cladding. They are known as waveguide losses.

d) Mode coupling losses:- The power launched into a propagating mode may get coupled into a

leaky mode at some points of the fiber. The coupling occurs due to the small imperfections

present in the core and imperfectly aligned connectors.


DISTORTION/ DISPERSION:

A light pulse launched into a fibre decreases in amplitude, as it travels along the fibre, due to losses in the

fibre. It also spreads during its travel. The pulse received at the output is wider than input pulse, as shown in

Fig.1. It means that the pulse becomes distorted as it propagates through the fibre. Such a distortion arises

due to dispersion effects. Dispersion is typically measured in nanoseconds per kilometer (ns/km.)

Fig. 1 Broadending of the signal due to dispersion.

There are three mechanisms in the distortion of the light pulse in a fibre. They are known as (i)

material dispersion, (ii) waveguide dispersion and (iii) intermodal dispersion.

Applications of the Optical Fibre:

Optical fibers have different applications as follows

1) They are used for illumination and short distance transmission of images.

2) They are used as fiber optics sensors.

3) They are used as waveguides in telecommunications.

4) They are used in medical diagnostics and military applications

FIBRE OPTIC SENSORS:

a) Temperature sensors

Principle:- Temperature sensor is based on the 1 ¼ m (1 μm) wavelength light-absorption characteristics of

silicon as a function of temperature.

Temperature Sensor


Construction: Figure shows a temperature sensor with a multimode fiber. The fiber is coated at one end

with a thin silicon layer. The silicon layer is in turn coated with a reflective coating at the back. The silicon

layer acts as the sensing element.

Working: The light from a light source is launched into the fibred from one of the ends of one of its

branches. It passes first through the fiber and then through the silicon layer. The mirror coating at the other

end of the silicon layer reflects the light back which again travels through the silicon layer. The reflected

light emerges out through another branch of multimode fiber and is collected by a photo-detector. The

amount of the reflected light is converted into voltage by the photo-detector. The absorption of light by the

silicon layer varies with temperature and the variation modulates the intensity of the light received at the

detector. Temperature measurements can be made with a sensitivity of 0.001°C.

b) Liquid Level Detector:-

Principle:- The liquid level detector is based on the principle of total internal reflection.

Construction:- A simple liquid level detector is shown in figure. A notch is made at one end of a multimode

optical fiber and its other end is chamfered. A light source sends light on to the fiber and a photodetector on

the other side notes light emerging out from the fiber.

Working:- The optical fiber is arranged at the desired height in a vessel. The refractive index of the fiber is

chosen to be less than that of the liquid whose level is to be detected. Light from the light source is made to

be incident on one of the inclined faces of the notch. The light turns through 900 and travel through the fiber.

On reaching the chamfered end of the fiber, it gets internally reflected at the fiber-air boundary, if the liquid

is below the desired level. Then it is again turned through 900 at the opposite face, travels back through the

fiber to be turned once again through 900 and is detected at the detector as shown in fig. (a)

When the liquid rises and touches the fiber end, total internal reflection stops and the light is

transmitted into liquid. Hence, the photodetector does not receive any light as shown in fig. (b). Thus, an

indication of the liquid level is obtained at the detector.


ADVANTAGES / MERITS OF OPTICAL FIBRES OVER CONVENTIONAL

CABLE :-

Optical fibres have many advantages over the conducting wires.

(1) Cheaper: Optical fibres are made from silica(SiO2) which is one of the most abundant material on

the earth. The overall cost of a fibre optic communication is lower than that of an equivalent cable

communication system.

(2) Smaller in size, lighter in weight, flexible yet strong: The cross section of an optical fibre is about

a few microns. Hence, the fibres are less bulky. Optical fibres quite flexible & strong.

(3) Not Hazardous : A wire communication link could accidentally short circuit high voltage lines and

the sparking occurring thereby could ignite combustible gases in the area leading to a great damage .

Such accidents cannot occur with fibre links sinces fibres are made of insulating material.

(4) Immune to electromagnetic interference(EMI)& radiofrequency interference ( RFI).In optical

fibres information is carried by photons. Photons are electrically neutral and can not be disturbed by

high voltage fields , lightening, etc.Therefore , fibres are immune to externally caused background

noise generated through EMI & RFI.

(5) No cross talk : The light waves propagating along the optical fibre are completely trapped within

the fibre and can not leak out further , light can not couple into the fibre from sides . Therefore the

possibility of cross talk is minimized when optical fibre is used. Thus transmission is more secure

and private.

(6) Wider band-width: Optical fibres have ability to carry large amounts of information . A 1 mm

optical fibre can transmit 50,000 calls .

(7) Low loss per unit length: The transmission loss per unit length of an optical fibre is about 4 dB/km.

Therefore, longer cable-runs between repeaters and feasible.

DISADVANTAGES OF OPTICAL FIBRE:

Installation and maintenance of optical fibres require a new set of skills. They required specialized and

costly equipment like optical time domain reflectometers etc.


NANOSCIENCE

NANOSCIENCE AND NANOTECHNOLOGY:

The word "nano" is derived from a Greek word meaning dwarf or extremely small and means a

billionth (10-9

) part of a unit. A nanometer or nm is one thousand millionth of a meter, i.e., 1 nm =10-

9 m = 10

-3 m = 10 Å.

One nanometer spans 3 to 5 atoms lined up in a row. For comparison, a single human hair is about

80,000 nm wide and a red blood cell is approximately 7,000 nm wide. Scientists and engineers are

nowadays interested in the nanoscale, which may be taken as 100 nm to 0.2 nm approximately. At

the nanoscale the properties of materials can be very different from those at a larger scale. Therefore,

the nano-world can be considered as a borderland between the quantum world and the macro world.

Nanoscience is the study of the fundamental principles of molecules and structures with at least one

dimension roughly between 1 and 100 nanometers. Nanotechnology is the design, characterization,

production and application of structures, devices and system by controlling shape and size at the

nanometer scale.

NANOMATERIALS:

Nanomaterials are those which have structured components with at least one dimension less than l00

nm.

Materials that have one dimension in the nanoscale (and are extended in the other two dimensions)

are layers, such as a thin films or surface coatings.


Materials that are nanoscale in two dimensions (and extended in one dimension) include nanowires

and nanotubes.

Material that are nanoscale in three dimensions are particles, for example precipitates, and colloids.

Nanocrystalline materials, made up of nanometer-sized grains, also fall into this category.

Q. 1. Explain any one method of preparation of nanomateials

Ans.:- There are a wide variety of methods of preparation nanomaterials

1) Chemical Vapor Deposition (CVD) method:

CVD is a well known process in which a solid is deposited on a heated surface via a chemical

reaction from the vapour or gas phase.

CVC reaction requires activation energy to proceed, in thermal CVD the reaction is activated by a

high temperature. A typical apparatus comprises of gas supply system, deposition chamber and an

exhaust system.

The deposition chamber is an evacuated chamber. A wafer is kept on a carrier and heated to a

temperature between 350 °C and 800°C.

One or several species of gases are admitted into the chamber through an inlet till a medium gas

pressure is built up in the chamber. Now a dissociation or reaction between two species takes place.

In both cases, a newly formed molecule adheres to the wafer surface and participates in the

formation of a nanolayer. Example. let us take silane SiH4. The gas dissociates into elementary

silicon, which partly adheres to the wafer surface and partly to hydrogen which is removed by the

exhaust pump.

2) Sol-gel method:- Sol-gel method of synthesizing nanomaterials is very popular amongst chemists

and is widely employed to prepare oxide materials. A sol is a solution with particles suspended in it. When

the particles in the sol form long polymers (chains) that span the entire sol, a gel is formed. The sol-gel

process is a bottom-up approach technique. In this process, the starting material is processed to form a

dispersible oxide and a colloidal suspension (sol) of the particles of the metal compound is prepared first and

then converted into a gel. The gel so formed is a network in a continuous liquid phase. Removal of the liquid

from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Calcination of the

gel produces the oxide.

The sol-gel formation occurs in four stages.

Hydrolysis

Condensation

Growth of particles

Agglomeration of particles

Production of SiO2 is an example of this process. The sol-gel process may be summarized in

Fig.


Fig. Schematic representation of sol-gel process of synthesis of nanomaterials

Step 1: A stable solution of the alkoxide or solvated metal precursor (the sol) is

formed.

Step 2: An oxide- or alcohol- bridged network (the gel) forms by a polycondensation

or polyesterification reaction.

Step 3: The polycondensation reactions continue until the gel transforms into a solid

mass, accompanied by contraction of the gel network and expulsion of

solvent from gel pores.

Step 4: Drying of the gel, when water and other volatile liquids are removed from the

gel network. If the solvent (such as water) is extracted under supercritical or

near super critical conditions, the product is an aerogel. If isolated by thermal

evaporation, the resulting product is termed a xerogel.

Step 5: Calcining the xerogel at temperatures up to 8000C stabilizes the gel against

rehydration.

Aerogels are porous and extremely light, but they can withstand 100 times their weight. If the

gelled spheres are calcined, one obtains powder. If the gel is collected on a surface, a thin film is obtained.

The interest in the sol-gel synthesis method arises due to the possibility of synthesizing nonmetallic

inorganic materials like glasses, glass ceramics or ceramic materials at very low temperatures compared to

the high temperature process required by melting glass or firing ceramics.

The major technical difficulties to overcome in developing a successful bottom-up approach is

controlling the growth of the particles and then stopping the newly formed particles from

agglomerating.

Sol-gel synthesis is superior of all the available processes because it can produce large quantities of

nanomaterials at relatively low cost. It synthesizes almost any material, co-synthesize two or more materials

simultaneously, coat one or more materials onto other materials (metal or ceramic particulates, and three-

dimensional objects), produce extremely homogeneous alloys and composites, synthesize ultra-high purity

(99.9999%) materials, tailor the composition very accurately even in the early stages of the process,

precisely control the microstructure of the final products, and precisely control the physical, mechanical, and

chemical properties of the final products.


Q. 2. Write down the important applications of Nanomaterial.

Ans.:- Nanomaterials have wide range of applications in the field of electronics, fuel cells,

batteries, construction, textile, agriculture, food industry and medicine, etc.

1) Fuel Cell:- A fuel cell is an electrochemical energy conversion device that converts the chemical

energy from fuel (on the anode side) and oxidant (on the cathode side) directly into electricity. The

performance of a fuel cell electrode can be optimized in two ways: by improving the physical

structure and by using more active electro catalyst. A good structure of electrode must provide ample

surface area, provide maximum contact of catalyst, reactant gas and electrolyte, facilitate gas

transport and provide good electronic conductance, which can be achieved by nano materials. Such

structure can minimize losses.

2) Construction:- nanotechnology has potential to make construction of buildings faster, cheaper, safer

and more varied. Light weight and stronger nanomaterials can be used to build skyscrapers much

more quickly and at a much lower cost. In near future, nanotechnology can be used to detect cracks

in the foundation of architecture of a building and it can also used to repair the cracks by sending

nonorobots.

3) Textile:- The fabric made up of nanofibres is water and stain repellent. It is also wrinkle free. Such

fabric need not be washed frequently.

4) Agriculture and food industry: - Nanoscience concepts and nanotechnology applications can

revolutionize these fields at production, conservation, processing, packaging, transportation and

waste management levels.

Q. 3. Write short note on Graphene.

Ans.:-

Graphene is a 2-dimensional, crystaline allotrope of carbon.

In graphene, carbon atoms are densely packed in a regular sp2-bonded atomic-scale chicken

wire (hexagonal) pattern.

Graphene can be described as a one-atom thick layer of graphite.

It is the basic structural element of other allotropes, including graphite, charcoal, carbon

nanotubes and fullerenes.

It can also be considered as an indefinitely large aromatic molecule, the limiting case of the family of

flat polycyclic aromatic hydrocarbons.

High-quality graphene is strong, light, nearly transparent and an excellent conductor of heat and

electricity.

http://en.wikipedia.org/wiki/Crystal

http://en.wikipedia.org/wiki/Allotrope

http://en.wikipedia.org/wiki/Carbon

http://en.wikipedia.org/wiki/Sp2_bond

http://en.wikipedia.org/wiki/Chicken_wire_(chemistry)

http://en.wikipedia.org/wiki/Chicken_wire_(chemistry)

http://en.wikipedia.org/wiki/Hexagonal_tiling

http://en.wikipedia.org/wiki/Graphite

http://en.wikipedia.org/wiki/Charcoal

http://en.wikipedia.org/wiki/Carbon_nanotube



http://en.wikipedia.org/wiki/Fullerene

http://en.wikipedia.org/wiki/Aromaticity

http://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon


Its interactions with other materials and with light and its inherently two-dimensional nature produce

unique properties, such as the bipolar transistor effect, ballistic transport of charges and large

quantum oscillations.

Application of Graphene:

Super-dense data storage

Energy storage

Optical devices: solar cells and flexible touch screens.

High –energy particle physics.

Q. 4. How does the properties of nanomaterials differ from bulk materials ?

Ans.:- The properties of nanomaterials are significantly different from the properties of

bulk materials. The difference is due to large fraction of surface atom, large surface energy, spatial

confinement and the reduced imperfection.

i) Physical properties of nanomaterials:- Nanomaterials may have a significantly lower melting point

or phase transition temperature and appreciably reduced lattice constant due to the huge fraction

of surface atom in the total amount of atom.

ii) Mechanical properties:- The large amount of grain boundaries the bulk materials made of nano-

particles allows extended grain boundary sliding leading to high plasticity

iii) Magnetic properties:- In magnetic nano-particles the energy of magnetic anisotropy may be that

small that the vector of magnetization fluctuates thermally; this is called super magnetism. Such

a material is a free of permanence and coercitivity. Touching super paramagnetic particles are

losing their special property by interaction, except the particles are kept at distance. The

combining particle with high energy of the anisotropy with super paramagnetic once lead to new

class of permanent paramagnetic material.

iv) Catalytic properties:- Due to their large surface, nano-particles made of transition element oxides

exhibit interesting catalyst properties. In a special case, the catalysis may be enhanced and more

specific by decorating these particles with gold or platinum clusters.

Q. 5. What are Zeolites ? Give their applications.

Ans.:- Zeolites are the microporous crystalline solid with well defined structures. Generally they

contain silicon, aluminium and oxygen in their framework and cations, water and other

molecules within pores.

Many occur naturally as minerals, and are extensively mined in many parts of the world.

Others are synthetic, and are made commercially for a specific use, or produced by research scientist

trying to understand more about their chemistry.

http://en.wikipedia.org/wiki/Bipolar_junction_transistor

http://en.wikipedia.org/wiki/Ballistic_transport


Applications of Zeolite: - Because of their unique porous properties, zeolites are used in a variety of

applications with the global market of several million tonnes per annum. In the western world major

uses are in petrochemical in cracking, ion exchange (water softening and purification) and in the

separation of removal of gases and solvent.

Other applications are in agriculture, animals and husbandry and construction. They are

obtained also referred to as molecular sieves.

QUESTIONS

1. What are nanomaterials? Why do they exhibit different properties?

2. Why do properties of materials change at nanoscale?

3. How are nanomaterials synthesized? Describe any two methods.

4. Explain how some of the physical properties change at nanoscale in case of nanoclusters.

5. How are optical, physical and chemical properties of nanoparticles vary with their size?

6. How are electrical, magnetic and mechanical properties of nanoparticles vary with their size?

7. Discuss any four applications of nanomaterials.

8. What is a carbon nanotube? Explain the different types of carbon nanotubes.

9. Discuss the characteristics and properties of carbon nanotubes.

10. How is carbon nanotubes synthesized? Describe any two methods.

11. What are the different types of carbon nanotubes? What are their properties?

12. Explain the physical properties of nanotubes.

13. Write a note on carbon nanotubes.

14. What are the important applications of nanotubes.

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