25

CHAPTER - 2

Materials and Experimental Techniques

2.1 Introduction

This chapter provides the complete details about the materials and experimental

techniques used in the present work. The working principle of each experimental

technique is also explained. Detailed descriptions of the following subject matters are

given in this chapter:

Target Materials

Chemical and Electrochemical Synthesis Conducting Polymer/Composites

Pelletron Accelerator

Irradiation (Material Science Beam Line)

Calculation of Ion Range and its Energy Loss using SRIM-2008

X-ray Diffraction

UV-Visible Spectroscopy

Fourier Transforms Infrared (FTIR) Spectroscopy

Scanning Electron Microscopy (SEM)

Raman Spectroscopy

Photoluminescence (PL) Spectroscopy

2.2 Materials

Following target materials have been used in the present work:

2.2.1 Poly Allyl Diglycol Carbonate (PADC)

PADC or CR-39 is made by polymerization of diethyleneglycol bis allylcarbonate

(ADC) in the presence of diisopropyl peroxydicarbonate (IPP) initiator. The presence of

allyl groups allows the polymer to form cross-links resulting into a thermoset resin. It

has a molecular formula (C12H18O7) n, density of 1.129-1.31 g/cm3 and glass transition

temperature of 850C.

The structure PADC is shown in Figure 2.1.

26

Figure 2.1: Structure of PADC

CR-39 is transparent in the visible spectrum and is almost completely opaque in the

ultraviolet range. It has high abrasion resistance. CR-39 is used in Physics as nuclear

track detector in different fields as for example heavy ion collision studies, for the

detection of cosmic ray nuclei, to search for magnetic monopoles. The CR-39 nuclear

track detector can also used for radon and neutron dosimetry.

2.2.2 Polyethylene Terephthalate (PET)

PET is a thermoplastic polymer resin of the polyester family and is used in synthetic

fibers, beverage, food, other liquid containers and thermoforming applications, etc.

Because of its high mechanical strength, PET film is often used in tape applications

such as the carrier for magnetic tape. Depending on its processing and thermal history,

polyethylene terephthalate may exist both as an amorphous (transparent) and as a semi-

crystalline polymer.

It has a molecular formula (C10H8O4) n, density of 1.3-1.4 g/cm3, glass transition

temperature of 700C and melting temperature of 2600C. The structure of PET is shown

in Figure 2.2.

Figure 2.2: Structure of PET

2.2.3 Makrofol-KG

Makrofol-KG, a bisphenol a polycarbonate (PC) is widely used for ion track recording

and to prepare track etched membranes as micro filters. Now polycarbonate particle

27

track etch membranes with pore shape and size, very well controlled within diameters

from 10 to 100nm have been produced. These membranes are used for the

manufacturing of nano tubes and nano wires. The polycarbonates such as Makrofol are

insensitive to charged particles (electron, protons and alpha particles), X-rays and γ-

rays. Thus, they offer a very convenient way of detecting heavy ions in the study of

cosmic rays and nuclear reactions and exploration of super heavy elements which are

most suitable for micro filter technology (Fleisher et al. 1975; Durrani and Bull, 1987).

It has a molecular formula (C16H14O3)n, density of 1.2-1.22 g/cm3, glass transition

temperature of 1500C and melting temperature of 2670C. The structure of polycarbonate

is shown in Figure 2.3.

Figure 2.3: Structure of Polycarbonate

2.2.4 PM-355

PM-355 is a solid state nuclear track detector and has the same chemical composition

that of solid state nuclear tracks detector CR-39 or PADC. Solid state nuclear track

detectors (SSNTDs) have been extensively used for the detection of ions. The PM-355

plastics have high homogeneity, isotropy and high optical transparency.

Nowadays, ion track membranes (ITMs) are also known as nuclear track filters (NTFs)

and have emerged as the main offshoot from SSNTDs. Many applications of SSNTDs

have been developed including biological filters, detection of light ions and dosimetry

for ion track etching, magnetic nano wires as magneto resistive sensors and much more

(Durrani, 1982; Price, 2005; Chakarvarti, 2009).

2.3 Chemical Used

The various chemicals used for undergoing experimental work are presented in the

Table 2.1. Their sources are also mentioned in the Table. All the chemicals used are AR

grade and their purity is ensured more than 99 %.

28

Table 2.1: Chemical Used with their Formula and Origin

Chemical Acronym Source

Aniline Monomer Ani MERCK

Pyrrol Ppy MERCK

Chloroauric Acid HAuCl4 MERCK

Silver Nitrate AgNO3 MERCK

Potassium Chloride KCl MERCK

p-Toluene Sulfonic Acid pTS MERCK

Dodecyl Benzenesulphonic Acid DBS MERCK

Ammonium Persulfate (APS) (NH4)2S2O8 Qualigens, India

Hydrochloric Acid HCl MERCK

Nitric Acid HNO3 MERCK

Sulfuric Acid H2SO4 MERCK

N-Methyl Pyrrolidone (NMP) (CH2) (CH2)2.CO.N.CH3 MERCK

Methanol CH3OH MERCK

2.4 Synthesis of Conducting Polymers/Composites

There are several methods by which conducting polymers/composites can be

synthesized. The most widely accepted methods are chemical oxidative polymerization

and electrochemical. Solid state polymerization, plasma polymerization, precursor

polymer route, template polymerization, etc. are other techniques for the synthesis of

conducting polymers/composites. But, in the present work electrochemical methods for

the synthesis of conducting polymers/composites are used.

2.4.1 Electrochemical Synthesis of Conducting Polymers

Conducting polymers obtained by electrochemical polymerization usually deposits on

the electrode. Electrochemical polymerization of aniline is usually carried out in

strongly acidic aqueous electrolytes through commonly accepted mechanism, which

involves formation of anilinium radical cation by aniline oxidation on the electrode

(Hussain & Kumar, 2003). Mu and Kan, (1995) and Mu et al. (1997) proved that

29

electrochemical polymerization of aniline is auto-catalyzed. The experimental

conditions, such as electrode material, electrolyte composition, dopant anions, pH of the

electrolyte, etc. have strong influence on the nature of the polymerization process

(Camalet et al. 2000; Marjanovic et al. 2006; Cordova et al. 1994; Duic and Mandic,

1992; Giz et al. 2000; Gvozdenovic and Grur, 2009; Inzlet, 2008; Mu and Kan, 1998;

Nunziante and Pistoia, 1989; Okamoto and Kotaka, 1998a, 1998b, 1999; Popovic and

Grur, 2004; Pron et al. 1993; Pron and Rannou, 2010; Wallace et al. 2009). The low pH

is always needed for preparation of the conductive polyaniline in the form of emeraldine

salt. It is clear that at higher pH, the deposited film is consisted of low chain oligomeric

material (Stejskal et al. 2010). The doping anion incorporated into polymer usually

determines the morphology, conductivity, rate of the polyaniline growth during

electrochemical polymerization and has influence on degradation process (Cordova et

al. 1994; Mandic et al. 1997; Pron and Rannou, 2010). Electrochemical synthesis of ppy

was also carried out electrochemically. Following assembly and techniques has been

used to conduct the electrochemical synthesis:

[a] Electrochemical Cell

Electrochemical experiments are normally carried out in a single compartment

electrochemical cell by adopting a standard three electrodes configuration. The cell is

made up of glass with a Teflon lid. The lid was made in such a way that it can

accommodate the working, counter and reference electrodes. The working electrode acts

as a substrate for electro-deposition of polymers. Since, the polymeric films are

deposited by an oxidation process, it is necessary that the electrode should not oxidize

concurrently with the aromatic monomer (Diaz et al. 1982). For this reason only, inert

electrodes like Pt, Au, SnO2, ITO and stainless substrates are used. A counter electrode

which is a metallic foil of Pt, Au and Ni, is used sometimes. A reference electrode like

saturated calomel electrode (SCE), Ag/AgCl electrode etc. can also be used.

The aqueous potassium chloride (KCl) solution in the reference electrode was replaced

very frequently to keep the electrode fresh and to avoid any fluctuation in the potential

values. Before every experiment, the cell was washed with soap powder and then

thoroughly flushed with free flowing water and finally rinsed in double distilled water

and kept in a hot air oven. A general setup for the electrochemical process (ECP) is

shown in Figure 2.4 (Gurunathan et al. 1999).

30

[b] Electrodes

In the present study, ITO coated glass substrate and platinum electrode were used as

working electrodes. A platinum foil of large surface area was used as a counter

electrode. A saturated calomel electrode was used as a reference electrode in the

aqueous medium. The chronopotentiometery was used for the deposition of polyaniline

and polypyrrol films whereas cyclic voltammetery was employed for the deposition of

metal conducting polymer composites.

Figure 2.4: General Setup for Electrochemical Polymerization

[c] Chronopotentiometery

In this experiment, the current flowing through the cell is instantaneously stepped from

zero to some finite value and the potential of the working electrode is monitored as

function of time. This technique comes under the galvanostatic (constant current

31

density) experiment. The chronopotentiometery have been used to monitor the change

of potential during the synthesis of polyaniline.

[d] Cyclic Voltammetery

Cyclic voltammetery widely known as “CV” is a very accepted and most extensively

used electrochemical technique among the potential sweep techniques. The simplest of

potential sweep technique is a linear sweep voltammetery (LSV), which involves

sweeping the electrode potential between the limits E1 and E2 at a known sweep rate ν,

before halting the potential sweep. In case of CV, the waveform applied initially is same

as LSV but on reaching the potential E2, the sweep is reversed usually at the same scan

rate as forward sweep instead of terminating the scan. Typical potential-time profile for

CV is shown in Figure 2.5.

CV related with the scanning of working electrode potential between the potential limit

of V1 and V2 at a known scan rate ν, in both the forward and reverse direction and

measuring the current of the electrochemical cell. The net current of the system involves

the faradic current due to the various electrochemical events take place on the electrode

surface for instance electron transfer redox reaction and adsorption processes as well in

the capacitive current due to the double layer charging at these potential (Bard and

Faulkner, 1980). A plot of recorded current as a function of applied potential is known

as “Cyclic Voltammogram”. It is an electrochemical spectrum representing the

potentials at which a number of processes happen can be achieved rapidly.

Figure 2.5: Potential-time Profiles for Cyclic Voltammetery

32

The conventional experiments of recording voltammograms use a range of sweep rates

vary from mV/s to few hundred V/s and for several values of V1 and V2. Generally,

there will be several peaks over the potential limits and as a function of scan rates. It is

possible to find and conclude, how the processes represented by the peaks are related.

By noting the difference between the first and subsequent cycle’s results into the cyclic

voltammograms and detailed mechanistic information about the electrochemical

reactions can be derived. Normally, the shapes of the cyclic voltammograms depend

upon the type of redox reactions.

2.5 Sources of Irradiation

In order to irradiate the target materials, following sources of irradiation have been used

to conduct the experiments:

2.5.1 The Pelletron Accelerator

A high energy Pelletron Accelerator (Figure 2.6) is running at Inter University

Accelerator Center (IUAC), New Delhi. The same is being used for basic and applied

research in nuclear physics, atomic physics, materials science, biosciences and other

allied fields (Kanjilal et al., 1993; 1999; Blewett, 1991).

A high voltage up to 15 million volts is generated in the center portion of Pelletron

known as terminal. A 15UD Pelletron accelerator is capable of accelerating any ion

from proton to uranium (except inert gases) up to energies of a few hundred MeV

depending upon the nature of the ion. It has been installed by the Electrostatic

International Inc., USA. This is a Tandem Van de Graaff accelerator, in which the

charge carrier belt is replaced by a chain of pellets. The digit 15 stands for 15 MV

terminal voltage and UD stands for Unit Double. It is installed in a vertical

configuration in an insulating steel tank of height 26.57 and width 5.5 m. In order to

attain insulation (to prevent sparking/discharging), the tank is filled with sulfur

hexafluoride (SF6) gas at a pressure of 4.0 Torr. The SNICS (Source of Negative Ion by

Cesium Sputtering) ion source acts as a source of negative ions which are analyzed by

the injector magnets. A high voltage terminal with 1.52 mm diameter and 3.81 mm

length in the middle of the tank can be charged by a high potential varied from 4 to 16

MV using an electrostatic charge transfer device. This terminal is connected to the tank

vertically through ceramic titanium tubes known as the accelerating tubes.

33

Figure 2.6: A Schematic Diagram of 15UD Pelletron Accelerator

A potential gradient is maintained with the help of these tubes. Negative ions from the

ion source are injected towards the terminal and stripped off a few electrons through

stripper foils. The yield is converted into positive ions. These ions are further

accelerated as they proceed towards the bottom of the tank at ground potential. As a

result, the ions from the accelerator gain energy, as given in Equation 2.1.

( 1)E V q MeVπ= + 2.1

Where Vπ is the terminal potential and ‘q’ is the number of positive charges (charge

states) on the ions after stripping. A heavy ion of charge state ‘q’ will attain a final

kinetic energy equal to (q + 1) × 16 MeV. Thus protons accelerated to a full terminal

voltage would have energy of 32 MeV. By using appropriate magnets w.r.t. the charge

34

states and energies, the high energetic ions are analyzed and are bent at 90° to the

vertical position by using analyzer magnet. These redirected ions are directed to the

desired experimental area in the beam hall with the help of multi-port switching magnet.

This switching magnet can redirect the beam to any one of the seven beam lines (NSC

school on accelerator physics, 1989).

2.5.2 Material Science Beam Line

The irradiation process for materials is generally carried out at the material science

beam line of IUAC, New Delhi. The setup of the same is shown in Figure 2.7.

Figure 2.7: An Overview of Material Science Beam Line

This beam line is at 150 angle with respect to the direction of the unswitched direct

beam. The beam line is maintained at ultra-low pressure of the order of 10-9 Torr and

the irradiation is carried out in the high vacuum chamber (HVC). It is fixed in Material

Science beam line of Pelletron. It has an arrangement of temperature control from low

temperature to high temperature, dose control which includes positive bias to the target

for secondary electron suppression (Faraday cup) and proper mechanical support and

alignment. The vacuum in the target chamber is generally maintained below 10−6 Torr.

35

A large number of samples can be mounted on all the four sides of a specially designed

ladder, which is 10cm long copper block of rectangular cross-section. Each sample was

fixed on the ladder with the help of silver paste. Conducting path was provided by using

a line of silver paste from the top surface of the sample to the copper block.

The target ladder is mounted through a Wilson seal from the top flange of the chamber.

This top flange is connected to the chamber through a flexible bellow that can be

expanded up to 11 cm from its minimum position. A stepper motor in conjunction with

a suitable mechanical assembly is used to control the up and down motion of the ladder.

The beam on the ladder can be observed by observing the luminescence of the beam on

the quartz crystal mounted on all sides of the ladder. After the observation of the beam

on the quartz, the sample to be irradiated is brought to the same position as that on the

quartz by moving the ladder in the desirable position. A CCD camera is attached to one

of the ports of the chamber for viewing the sample and the quartz position.

The positions can be monitored using close circuit television (CCTV) in the data

acquisition room. The magnetic scanner that can be swept the beam 25 mm in y-

direction and 10 mm in x-direction to ensure the uniform irradiation of samples. A

cylindrical enclosure of stainless steel surrounds the sample ladder, which is kept at a

negative potential of 120V. This enclosure suppresses the secondary electrons coming

out of the sample during the irradiation. An opening in the suppresser allows the ion

beam to fall on the sample. The total number of particles/charges falling on the sample

can be estimated by a combination of the current integrator and the pulse counter

(Faraday Cup) from which the irradiation fluence can be measured.

2.5.3 Co-60 Gamma Ray Source

The gamma chamber (GC 1200) shown in Figure 2.8 for Co60 gamma rays at IUAC,

New Delhi was used in this work. It consists of radiation source, biological shield for

the source, central drawer including the sample chamber, driving system, control panel

and external cabinet.

The chamber has a compact unit enabling an irradiation volume ~1000 cm3. The central

drawer if required can be raised or lowered by a steel rope passing over a geared motor.

This movement is controlled from the front control panel through an electrical circuit.

The dose rate was 7.32kGy/h at the time of irradiation.

36

Figure 2.8: Gamma Chamber (GC-1200) with a Co-60 Source

2.6 Fluence Calculation

The ion fluence was estimated by time of irradiation and beams current as follows,

Q Dqe AqeIT T T

φ= = = 2.2

AqeTI

φ∴ = 2.3

Where I = ion current (nA)

Q = total charge

D = dose = ion fluence (φ) in ions/cm2 × area (A) of irradiation in cm2

q = charge state

e = electronic charge = 1.6×10-19 C

37

T = time of irradiation

Since, the number of particles per nano ampere of beam current = ( )I pnAqe

( )AT

I pnAφ∴ = 2.4

Using the Equation 2.4, the required time was calculated for all ions fluence. During the

experiment, keeping current of the ion beam constant, the samples were irradiated for

pre-determined time.

2.7 Calculation of Range and Energy Loss

Stopping and Range of Ions in Matter (SRIM) is a group of computer programs, which

calculate the interaction of ions with matter; the core of SRIM is a program Transport of

ions in matter (TRIM) (Ziegler et al. 2008).

The programs were developed by Ziegler and Biersack around 1983 and are being

continuously upgraded. SRIM is based on a Monte Carlo simulation method, namely the

binary collision approximation with a random selection of the impact parameter of the

next colliding ion. As the input parameters, it needs the ion type and energy (in the

range 10 eV - 2 GeV) and the material of one or several target layers. Typical

applications include:

Ion Stopping and Range in Targets: Most aspects of the energy loss of ions in matter

are calculated in SRIM (Ziegler et al. 2008). SRIM includes quick calculations which

produce tables of stopping powers, range and straggling distributions for any ion at any

energy in any elemental target. More elaborate calculations include targets with

complex multi-layer configurations. In the present work, the projected range, nuclear

stopping power and electronic stopping power are calculated using SRIM code by

Ziegler et al. (2008) for all target materials. Details of the samples for heavy ions and

ionizing radiations (gamma rays and neutron radiation) are given in Table 2.2 and 2.3.

Ion Implantation: Ion beams are used to modify samples by injecting atoms to change

the target chemical and electronic properties. The ion beam also causes damage to solid

targets by atom displacement. Most of the kinetic effect is associated with the physics of

this kind of interactions is found in the stopping and range of ions in materials package.

38

Table 2.2: Irradiation Details of the Samples

S.

No. Target

Fluence

(ions/cm2)

Current

(pnA)

Range

(μm)

Se

(eV/A0)

Sn

(eV/A0)

Energy and

Ions

1. Ag-Ppy

1×1011

0.5 287.32 83.05 4.875×

10-3

40 MeV

Li3+ 1×1012

1×1013

2. Au-PANI

1×1011

0.5 59.03 49.57 3.345×

10-2 40 MeV C5+ 1×1012

1×1013

3. PADC

1×1011

0.5 93.08 38.54 0.0218

9 55 MeV C5+ 1×1012

1×1013

4. PET

1×1011

0.5 88.9 40.47 0.0227 55 MeV C5+ 1×1012

1×1013

5. M-KG

1×1011

0.5 137.48 47.46 0.0258 100 MeV

O7+

3×1011

1×1012

3×1012

6. PET

3×1010

0.5 26.96 590.2 0.8373 120 MeV

Ni11+ 3×1011

3×1012

7. M-KG

1×1011

0.5 36.43 502.2 0.5982 150 MeV

Ni11+

3×1011

1×1012

3×1012

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Table 2.3: Irradiation Details of Gamma Rays and Neutron Radiations

S. No. Materials Dose/Fluence Energy and Radiation Agent

1. PM-355

150kGy

1.25 MeV 60Co gamma source

300 kGy

470 kGy

630 kGy

675 kGy

2. Plastic bottle

sheets

150 kGy

300 kGy

470 kGy

675 kGy

3. PANI

15kGy

1.25 MeV 60Co gamma source 21kGy

41.9kGy

4. PADC

2.38×106 n/cm2

4 MeV Am-Be neutron source

3.71×106 n/cm2

1.08×107 n/cm2

1.69×107 n/cm2

5.94×107 n/cm2

Sputtering: The ion beam may knock out target atoms, a process called ion sputtering.

The calculation of sputtering by any ion at any energy level is included in the SRIM

package.

Ion Transmission: Ion beams can be followed through mixed gas/solid target layers as

occurs in ionization chambers or in energy degrader blocks and used to reduce ion beam

energies.

Ion Beam Therapy: Ion beams are widely used in medical therapy, especially in

radiation oncology.

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2.8 Characterization Techniques

2.8.1 X-ray Diffraction (XRD)

The X-ray diffractometer is the most widely used X-ray diffraction technique for

qualitative and quantitative analysis as well as for characterizing a wide range of

materials including polymers, fluids, metals, minerals, plastics, ceramics, solar cells,

semiconductor etc. XRD investigates the crystal structure of an unknown material,

identify the crystalline phase, determine the average spacing between layers or rows of

atoms, determine the orientation of a single crystal or grain, measure the size, shape and

internal stress of small crystalline regions and determine the crystallinity of the thin

films.

In an X-ray diffractometer, X-rays are generated within a sealed tube (anode) consisting

of the metal target (often copper metal) and a tungsten metal filament (cathode). Other

metals such as chromium, iron, nickel, silver and tungsten can also be used as target for

specific purposes. A current (typically 10-15 mA) is applied that heats a filament within

the tube, the higher the current the greater the number of electrons emitted from the

filament. A high potential voltage (typically 15-60 kilovolts), is applied within the tube

so that the generated electrons are accelerated and interact with it to generate X-rays.

The wavelength of these X-rays is the characteristics of that target metal. When

electrons have sufficient energy to dislodge inner shell electrons of the target material,

characteristics X-ray spectra are produced. When the K shell vacancy is filled by an

electron originating from any of the outer shell e.g. L and M, the emitted radiation is

called Kα (8.06keV) and Kβ. (8.93keV) Usually K lines are used in XRD since the

longer wavelength lines are too easily to be absorbed. Kα consists of Kα1 and Kα2 X-

rays. The Kα1 transition will occur almost exactly twice the frequency of Kα2 transition

and the resulting X-rays will have twice the intensity as that of Kα2. Filtering, by foils or

crystal monochrometers is required to produce monochromatic X-rays needed for

diffraction. Kα1 and Kα2 are sufficiently close in wavelength such that a weighted

average of the two can be used.

When a collimated beam of X-rays, with certain wavelength is incident on a crystal, it is

coherently scattered from all atoms and undergoes constructive interference (shown in

Figure 2.9) in certain directions and destructive interference in other directions giving

rise to diffracted beam. It is important to note that only those crystallites or atoms whose

41

reflecting planes are parallel to the specimen surface will contribute to reflected

intensities. The Bragg condition for the angle of the diffraction is thus:

2 sinn dλ θ= 2.5

where n is an integer called the order of diffraction, λ is the wavelength of the x-

radiation, θ is the diffraction angle and d is the interplanar spacing in crystalline

material.

Figure 2.9: Reflection of X-rays from Parallel Planes in a Solid

For homogeneous phase λ is fixed and for a set of lattice planes d is fixed, hence the

extent of diffraction will depend on the glancing angle θ. With the help of Bragg’s

equation it should be possible to determine the spacing d between successive lattice

planes if λ is known and θ is measured.

Determination of Crystallite Size

The main contribution to the XRD peak intensity is due to grain size, lattice vibrations

or strain, instrumental broadening and defect structure. We can estimate the average

crystallite size using Scherre equation (Scherrer, 1918; Patterson, 1939):

.coskL

bλθ

= 2.6

42

Where λ=1.54 nm is the wavelength of the Cu-Kα X-ray radiation used, b is the FWHM

of the diffraction peak and θ is the Bragg angle (in radians), k is the Scherrer constant

(usually taken as unity), L is crystallite size (A0).

Determination of XRD Parameters

The XRD parameters such as interchain separation (R), interplanar distance (d), micro

strain (ε), dislocation density (δ) and distortion parameters (g) can be calculated using

following formulae given by Mallick et al. (2006); Madani, (2011) and Vij et al. (2010).

58 sin

R λθ

= 2.7

2sind λ

θ= 2.8

cos4

b θε = 2.9

12L

δ = and 2.10

tanbgθ

= 2.11

Determination of Percentage Crystallinity

The crystallinity can be calculated by separating intensities due to amorphous and

crystalline phase on diffraction pattern. Percentage of crystallinity (Xc %) is measured

as the ratio of crystalline area to total area (Ramola et al. 2009).

{ } 100%% ×= +AcXc A Aa c

, 2.12

where Ac = area of crystalline phase, Aa = area of amorphous phase and Xc = percentage

of crystallinity.

XRD experiment was performed on thin films by using a Bruker D8 advanced, AXS, X-

ray diffractometer with Cu-Kα radiation in a wide range of Bragg’s angle at

40kV/30mA and having Cu-Kα radiation selected by a graphite monochromator.

43

2.8.2 UV-Visible Spectroscopy

Ultraviolet-visible absorption spectroscopy is the measurement of the attenuation of the

beam of light after it passes through a sample or after reflection from a sample surface.

UV-visible includes transmittance, absorption and reflection measurements in the UV,

visible and near infra red region. The UV-visible spectroscopy is usually applies to

molecules and inorganic ions or complexes in solution. The UV-visible is very useful

for quantitative measurements. The concentration of the sample in a solution can be

determined by measuring the absorbance at some wavelength and applying the Beer-

Lambert Law i.e.0

lNI I e σ−= , where I0 and I are the intensity of the incident light and

transmitted light, respectively; σ is the cross section of light absorption by a single

particle and N is the density of absorbing particles.

The absorption of UV radiation by organic compounds in the visible and ultraviolet

region involves promotion of electrons in σ, π and n-orbitals from the ground state to

higher energy state. These higher energy states are described by molecular orbitals that

are vacant in the ground state and are commonly called anti bonding orbitals. The anti

bonding orbitals associated with σ bond is called the σ* orbital and that associated with

π bond is called the π* orbital.

[a] Principle of UV Visible Spectroscopy

When sample molecules are exposed to UV-Visible light having an energy that matches

a possible electronic transition within the molecule, some of the light energy will be

absorbed as the electron is promoted to a higher energy orbital. An optical spectrometer

records the wavelengths at which absorption occurs, together with the degree of

absorption at each wavelength. Because the absorbance of a sample will be proportional

to the number of absorbing molecules in the spectrometer light beam (e.g. their molar

concentration in the sample tube), it is necessary to correct the absorbance value for this

and other operational factors if the spectra of different compounds are to be compared in

a meaningful way.

The corrected absorption value is called "molar absorptivity" and is particularly useful

when comparing the spectra of different compounds and determining the relative

strength of light absorbing functions. A schematic diagram of the components of a

44

typical spectrometer is shown in Figure 2.10. In the present studies UV Visible

absorption spectra were recorded using a Hitachi U-3300 UV Vis Spectrophotometer.

Figure 2.10 : Schematic of UV-Visible Spectroscopy

[b] Determination of Optical Band Gap Energy

The UV-visible spectral data is used for the determination of the band gap (i.e. the

difference between the conduction band energy and the valence band) in case of various

polymers by using the relation given by Zaki, (2008).

( ) ( - ) /nB h E hgα ν ν ν= 2.13

where hν is the energy of the incident photons, Eg is the value of the optical energy gap

between the valence band and the conduction band and n is the power, which

characterizes the electronic transition, whether it is direct or indirect during the

45

absorption process in the K space. In particular, n is 1/2, 3/2, 2 and 3 for direct allowed,

direct forbidden, indirect allowed and indirect forbidden transitions, respectively.

The factor B depends on the transition probability and can be assumed to be constant

within the optical frequency range. The usual method for the determination of the value

of Eg involves plotting (αhν)1/n against (hν). Indirect transitions in many amorphous

materials is a fit case for n = 2; for a direct transition a reasonable fit with n = l/2 is

achieved.

In this study, satisfactory results were obtained by plotting (αhν)1/2 and (αhν)2 as a

function of the photon energy (hν) respectively, taking into account the linear portion of

the fundamental absorption edge of the UV-visible spectra.

[c] Determination of Urbach Energy

The absorption coefficient α(hv) near the band edge for noncrystalline materials shows

an exponential dependence on the photon energy (hv) which follows the Urbach’s

formula given by Urbach, (1953).

( ) exp( / )0

h Euα ν α ν= 2.14

where α0 is a constant, Eu is an energy that represents the width of the tail of localized

states in the forbidden band gap, ν is the frequency of radiation and h is Planck’s

constant. The origin of Eu is considered as thermal vibrations in the lattice (Urbach,

1953). The values of the Urbach’s energy (Eu) were calculated by taking the reciprocal

of the slopes of the linear portion in the lower photon energy region of the curve.

[d] Determination of Number of Carbon Atoms

Further, the number of carbon atoms per conjugation length N for a linear structure

(Ramola et al., 2008) is given by

2 / gN Eπβ= 2.15

where N is the number of carbon atoms per conjugated length, 2β gives the band

structure energy of a pair of adjacent Π sites. The value of β is taken to be -2.9 eV as it

is associated with π-π* optical transition in the –C=C- structure.

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From the Robertson relation cluster size can be calculated by Nouh et al. (2003) and

then following relation can be used to calculate the number of carbon atoms per cluster

(Gupta et al., 2000):

34.3g

E eVN

= 2.16

2.8.3 Fourier Transforms Infrared Spectroscopy (FTIR)

Infrared spectroscopy is an important technique in organic chemistry. It is an easy way

to identify the presence of certain functional groups in a molecule. Also, one can use

the unique collection of absorption bands to confirm the identity of a pure compound or

to detect the presence of specific impurities. An FTIR spectrometer simultaneously

collects spectral data in a wide spectral range. This confers a significant advantage over

a dispersive spectrometer, which measures intensity over a narrow range of wavelengths

at a time. The term Fourier transform spectroscopy reflects the fact that in all these

techniques, a Fourier transform is required to turn the raw data into the actual spectrum

and in many of the cases in optics involving interferometers is based on the Wiener-

Khinchin theorem. This theorem states that the power spectral density of a wide-sense-

stationary random process is the Fourier transform of the corresponding autocorrelation

function.

In FTIR-spectroscopy, the interference signal of a two-beam interferometer is measured.

The collimated IR beam is partially transmitted to the moving mirror and partially

reflected in the fixed mirror by the beam splitter. These two IR beams are then reflected

back to the beam splitter by the mirrors. The reflected beams interfere constructively or

destructively depending on the wavelength of the light and the optical path difference

between the mirrors. The recombined IR beam passes the sample (or the reference) and

reaches the detector. Helium-Neon laser controls the position and movement of the

movable mirror. The optical schematic of an FTIR Spectrometer is shown in Figure

2.11.

Resolution in an FTIR spectrometer is mainly defined by maximum path difference

between the interferometer arms. It is crucial to maintain the optical alignment of the

interferometer during mirror movement. Hence the efficiency of the device for moving

the mirror (so called scanner) is very important. Spectroscopy is the study of the

interaction of electromagnetic radiation with a chemical substance. When radiation

47

passes through a sample (solid, liquid or gas), certain frequencies of the radiation are

absorbed by the molecules of the substance leading to the molecular vibrations.

Figure 2.11: Optical Schematic of a FTIR Spectrometer

Modern Fourier Transform IR (FTIR) spectrometers are superior to the dispersive IR

spectrometers. An FTIR (Fourier Transform Infrared) is a method of obtaining infrared

spectra by first collecting an interferogram of a sample signal using an interferometer,

then performing a Fourier Transform on the interferogram to obtain the spectrum. An

FTIR Spectrometer is a spectral instrument that collects and digitizes the interferogram,

performs the FT function and displays the spectrum. The main component in the Fourier

Transform Infrared (FTIR) spectrometer is an interferometer. This device splits and

recombines a beam of light such that the recombined beam produces a wavelength-

dependent interference pattern or an interferogram. The Michelson interferometer is

most commonly used. The Michelson interferometer is the heart of all modern FT-IR

48

spectrometers. In the present work, the FTIR measurements on the films were carried

out by using Thermo Nicolet NEXUS 670 FTIR system.

2.8.4 Scanning Electron Microscopy

A scanning electron microscope (SEM) is a type of electron microscope that images a

sample by scanning it with a beam of electrons in a raster scan pattern. The electrons

interact with the atoms that make up the sample producing signals that contain

information about the sample's surface topography, composition and other properties

such as electrical conductivity.

In most applications, data are collected over a selected area of the surface of the sample

and a two-dimensional image is generated that displays spatial variations in these

properties. Areas ranging from approximately 1 cm to 5 microns in width can be imaged

in a scanning mode using conventional SEM techniques (magnification ranging from

20X to approximately 30,000X, spatial resolution of 50 to 100 nm). The SEM is also

capable of performing analyses of selected point locations on the sample; this approach

is especially useful in qualitatively or semi-quantitatively determining chemical

compositions (using EDS), crystalline structure and crystal orientations.

Accelerated electrons in an SEM carry significant amounts of kinetic energy and this

energy is dissipated as a variety of signals produced by electron-sample interactions

when the incident electrons are decelerated in the solid sample. These signals include

secondary electrons (that produce SEM images), backscattered electrons, diffracted

backscattered electrons, photons, visible light and heat. Secondary electrons and

backscattered electrons are commonly used for imaging samples. Secondary electrons

are most valuable for showing morphology and topography on samples and

backscattered electrons are for illustrating contrasts in composition in multiphase

samples.

X-ray generation is produced by inelastic collisions of the incident electrons with

electrons in discrete orbitals (shells) of atoms in the sample. As the excited electrons

return to lower energy states, they yield X-rays that are of a fixed wavelength. Thus,

characteristic X-rays are produced for each element in a mineral that is "excited" by the

electron beam. SEM analysis is considered to be "non-destructive"; i.e. X-rays

generated by electron interactions do not lead to volume loss of the sample, so it is

49

possible to analyze the same materials repeatedly. Schematic diagram of SEM is shown

in Figure 2.12.

Figure 2.12: Schematic of Working Principle of SEM

Because the SEM utilizes vacuum conditions and uses electrons to form an image,

special preparations must be done to the sample. All water must be removed from the

samples because the water would vaporize in the vacuum. All metals are conductive and

require no preparation before being used. All non-metals need to be made conductive by

covering the sample with a thin layer of conductive material. This is done by using a

device called a "sputter coater." In the present studies SEM images were obtained by

using JEOL (JSM-6490 LV) scanning electron microscope.

2.8.5 RAMAN Spectroscopy

Raman spectroscopy is a spectroscopic technique based on inelastic scattering of

monochromatic radiation, usually from a laser source. Inelastic scattering means that the

50

frequency of photons in monochromatic radiation changes upon interaction with a

sample. Photons of the laser light are absorbed by the sample and then re-emitted.

Frequency of the re-emitted photons is shifted up or down in comparison with original

monochromatic frequency, which is called the Raman Effect. This shift provides

information about vibrational, rotational and other low frequency transitions in

molecules. Raman spectroscopy can be used to study solid, liquid and gaseous samples.

Significant scientific and technological interest has focused on polymer over the last

two decades.

The Raman effect is based on molecular deformations in electric field E determined by

molecular polarizability ‘α’. The laser beam can be considered as an oscillating

electromagnetic wave with electrical vector E. Upon interaction with the sample it

induces electric dipole moment P = αE which deforms molecules. Because of periodical

deformation, molecules start vibrating with characteristic frequency νm. Amplitude of

vibration is called a nuclear displacement. In other words, monochromatic laser light

with frequency υ0 excites molecules and transforms them into oscillating dipoles. Such

oscillating dipoles emit light of three different frequencies (Figure 2.13).

Figure 2.13: Energy Level Diagram Showing the States Involved in Raman Signal

51

A Raman system typically consists of four major components:

1. Excitation source (Laser).

2. Sample illumination system and light collection optics.

3. Wavelength selector (Filter or Spectrophotometer).

4. Detector (Photodiode array, CCD or PMT).

A sample is normally illuminated with a laser beam in the ultraviolet (UV), visible (Vis)

or near infrared (NIR) range. Scattered light is collected with a lens and is sent through

interference filter or spectrophotometer to obtain Raman spectrum of a sample.

In Raman spectroscopy, a laser beam is pointed at a sample and shifts in the wave

length of the scattered light are observed. The incident light is scattered by the target

atoms or molecules. In the scattering process, the target system is excited to a 'virtual' or

unstable energy level and it quickly relaxes to either the ground state (Rayleigh

scattering) or an excited vibrational or rotational state (Raman scattering).

The energy difference of the starting and finishing states is manifested as energy shifts

of the scattered light. From the intensities of these shifts information on the structure of

the target material can be deduced. Raman spectroscopy has proven to be a useful tool

in characterizing disorder in materials. In the present studies RAMAN spectra were

measured by Renishaw InVia Raman microscope.

2.8.6 Photoluminescence Spectroscopy

Photoluminescence spectroscopy is a contactless, nondestructive method of probing the

electronic structure of materials. Light is directed onto a sample, where it is absorbed

and imparts excess energy into the material in a process called photo-excitation. This

excess energy can be dissipated by the sample through the emission of light, or

luminescence. In the case of photo-excitation, this luminescence is called

photoluminescence. The intensity and spectral content of this photoluminescence is a

direct measure of various important material properties.

Photo-excitation causes electrons within the material to move into permissible excited

states. When these electrons return to their equilibrium states, the excess energy is

released and may include the emission of light (a radiative process) or may not (a

52

nonradiative process). The energy of the emitted light (photoluminescence) relates to

the difference in energy levels between the two electron states involved in the transition

between the excited state and the equilibrium state. The quantity of the emitted light is

related to the relative contribution of the radiative process.

In the present work, the PL emission spectra were carried out using a Jobin Yvon-Spex

Spectrofluorometer (Fluorolog version-3; Model FL3-11.The 300nm exciting

wavelength of a xenon arc lamp was used.