Chapter - I

1

1.1 Introduction

1.1.1 General Approach of Cadmium Chalcogenide thin films

Cadmium chalcogenides (CdS, CdSe and CdTe) belongs to II-VI

compound semiconductor material, which has been popular in the field of solar

cells, optoelectronic devices, solar selective coating, etc. Particularly cadmium

chalcogenides have received considerable attention during years 2000, because of

their proven and potential applications in a variety of semiconducting devices.

Cadmium chalcogenides (CdS, CdSe and CdTe) in the forms of single crystals,

sintered pellets and polycrystalline materials have been employed in

photoelectrochemical (PEC) cells. The stable PEC cells are obtained with S2-

/S22-

redox couple [1-3]. Resonable efficiencies (~8-9%) have been obtained with

polycrystalline films by many workers using polysulphide electrolyte[4-6].

Photoetching removes some of the recombination centres resulting in an increase

in efficiency [7,8]. Lokhande reported that the efficiency and stability of PEC

cells are strongly dependent on the preparation condition of the electrodes,

electrolytes and on experimental conditions during test [9].

Solar cells from cadmium chalcogenide single crystals are very expensive;

therefore the use of polycrystalline metal chalcogenide thin films is a desirable

alternative for cost reduction [10].

1.1.2 Cadmium Sulfide

Cadmium sulfide (CdS) is an hexagonal, yellowish crystal with specific

gravity of 4.7. with molecular weight 144.46 gmol-1

and density 4.82 g cm-3

. The

reported Melting point is 17500C and Boiling point is 980

0 C. Synthetic cadmium

pigments based on cadmium sulfide are valued for their good thermal stability in

many polymers. Cadmium sulfide is a direct bandgap semiconductor with a

bandgap of 2.42 eV.

Cadmium sulfide (CdS) film is a n-type material which has shown the

potential to be used as a window material in the photovoltaic solar cells,

electrochromic devices and display screens [11].

Chapter - I

2

Polycrystalline thin films of CdS have received considerable attention

during years 1998 because of their proven and potential applications in a variety

of semiconducting devices such as solar cells, transistors, light activated valves,

etc. [12]. The conversion efficiencies of the photoelectrochemical (PEC) cells with

configuration CdS /NaOH-Na2S-S/C have been reported[13,14]. The efficiency of

these cells were found to improve with an increase in electrical conductivity of the

CdS films[15-17].

1.1.3 Cadmium Selenide

Cadmium Selenide (CdSe) is appearance in Greenish-brown or dark red

solid powder with molecular weight 191.37 g/mol and density 5.816 g/cm3. It’s

Melting point is 1268 0 C with band gap 1.74 eV and refractive index 2.5.

Cadmium Selenide (CdSe ) is the member of the family of group II and VI

compounds and it is one of the best photoconducting materials, It is widely used in

solar cells as well as opto-electric and photoconductive devices. As well as CdSe

are considered to be very important materials for its potential applications in photo

electrochemical (PEC) solar cells, thin film transistor and gamma ray detectors[

18-21]. Various nanodevices like logic circuits, nanosensors, and

nanothermometers have been assembled using nanoscale materials[22,23].

Certain nanocrystallites shows size dependant structural, morphological,

optical and electrical properties, which make intrinsic candidates for different

applications, such as light emitting diodes, solar cells, non-linear optical and

luminescent devices[24]. Developments of such materials, whose structural,

morphological and optoelectronic properties can be tuned, are useful in many

applications[25-31].

1.1.4 Cadmium Telluride

Cadmium Telluride (CdTe) is an hexagonal, grayish black colored crystal

with specific gravity of 4.7 with molecular weight 240.01 gmol-1

and density 5.85

g.cm-3

. It’s Melting point is 1092 0C and Boiling point is 1130

0 C. Cadmium

Telluride is a direct band gap semiconductor with a band gap of 1.45 eV.

Chapter - I

3

Cadmium telluride, CdTe is one of the corresponds to sunlight spectrum

II-VI group compound used as absorber for solar cells. CdTe has a direct transition

type band structure, so the absorption coefficient is larger for light with

wavelength below the absorption edge. Therefore, CdTe is an important candidate

material for the fabrication of high efficiency solar cells.

Cadmium Telluride (CdTe ) is recognized as a highly versatile binary

compound semiconductor (32), from which one can expect a high solar energy

conversion efficiency. This compound is the only binary II and VI material which

can be in both n-and p- conductivity type (33). Its applications range from solar

cells to gamma-ray and infrared detectors(34).

1.2 Introduction to Thin Films

Any solid or liquid system which possesses at most two dimensional order

or periodicity and whose third dimension is negligibly small , below 1µm is called

as thin films. Thin films are thin material layers ranging from fractions of a

nanometer to few micrometers in thickness. In years 1998, thin film science has

grown world-wide into a major research area. Currently, this development goes

increases with the explosion of scientific and technological breakthroughs in

microelectronics, optics and nanotechnology [35]. A second major field comprises

process technologies for films with thicknesses ranging from one to several

microns. These fims are essential for a multitude of production areas, such as

thermal barrier coatings and wear protections, enhancing service life of tools and

to protect materials against thermal and atmospheric influences [36,37]. Presently,

rapidly changing needs for thin film materials and devices are creating new

opportunities for the development of new processes, materials and technologies.

Therefore, basic research activities will be necessary in the future, to increase

knowledge, understanding, and to develop predictive capabilities for relating

fundamental physical and chemical properties to the microstructure and

performance of thin films in various applications.

Chapter - I

4

1.3 Deposition techniques

Thin-film technology is simultaneously one of the oldest arts and one of the

newest sciences. Involvement with thin films dates to the metal ages of antiquity.

There exists a huge variety of thin film deposition processes and

technologies which originate from purely physical or purely chemical

processes[38]. The broad classification of thin film deposition techniques is

outlined in Chart 1.2. The more important thin film processes are based on liquid

phase chemical techniques, gas phase chemical processes, glow discharge

processes and evaporation methods [39]. The common factor in thin film

deposition is that they are atomistic in nature i.e. films are grown atom-by-atom.

Physical methods are expensive but result in the formation of very pure and well-

defined films. Most of the chemical methods are cost-effective, but their full

potential for obtaining device quality films has not been fully explored in many

cases. The choice of the particular method depends on several factors like material

to be deposited, nature of substrate, film thickness requirement, structure of the

film and application of the film. Amongst chemical deposition techniques, the

electrodeposition technique is the most popular today to deposit conducting and

semiconducting thin films because it is easy, attractive and less expensive. The

classification of thin film deposition techniques is shown in following chart1.1

Chapter - I

5

Chart 1.1 Classification of thin film deposition techniques

1.4 History of electroplating

Electroplating starts with a frog and ends with a chip in 1800 Allesandro

volta disproves Galvani’s theory by inventing the frogless electric pile. Then in

1805 Brugnatelli plates Gold onto Silver using Volta’s Electric Pile. Brugnatelli used

his colleague Alessandro Volta's invention of five years earlier, the voltaic pile, to

facilitate the first electrodeposition. Unfortunately, Brugnatelli's inventions were

repressed by the French Academy of Sciences and did not become used in general

industry for the following thirty years[40].

By 1839, scientists in Great Britain and Russia had independently

devised metal deposition processes similar to Brugnateli's for the copper

electroplating of printing press plates. Soon after, John Wright of Birmingham,

England discovered that potassium cyanide was a suitable electrolyte for gold and

silver electroplating. Wright's associates, George Richards Elkington and Henry

Chapter - I

6

Elkington were awarded the first patents for electroplating in 1840. These two

then founded the electroplating industry in England from where it spread around

the world.

In 1837 Boris Semionovich Yakobi invents electrotyping and in 1840 George

and Henry Elkington awarded first patents for industrial silver-plating. In the recorded

literature, Bunsen and Grove obtained metal films in 1852 by means of chemical

reaction and by glow-discharge sputtering, respectively. Faraday obtained metal

films in 1857 by the thermal evaporation on explosion of a current carrying metal

wire. In 1876 The Norddeutsche Affinerie was starting the first modern

electroplating plant in Hamburg[41-43] .

After several years experience in Thin Head manufacture, IBM announces

they have a copper metallization process for microprocessors. IBM makes a big

announcement and the age of copper damascene begins. Interestingly, Motorola

(who independently developed a copper damascene process) announces copper

manufacturing just days before IBM. However, they choose to make a quiet

announcement and so it goes unnoticed.

1.5 Fundamental concepts of Electrodeposition technique

Historically, the discovery of electroplating can be traced back to Michael

Farad and his famous laws of electrolysis.

Electrolysis was first studied quantitatively by Michael Faraday, who established

as a result of his investigations, the following laws of electrolysis, known as

Faraday’s laws [44].

First law : The total amount of chemical changes produced by a current is

proportional to the charge passing through the electrolyte..

If W is the amount of substance liberated or deposited on the electrode in

grams, and Q is the quantity of electricity passed through electrolyte in coulombs,

then

W α Q (1.1)

Chapter - I

7

If current strength I in ampere is passed for t seconds, then the quantity of

electricity

Q = I t (1.2)

W α It (1.3)

Or

W = Z I t (1.4)

Here Z is the proportionality constant, known as electrochemical equivalent.

Second law : The masses of the different substances liberated in the electrolysis

are proportional to their chemical equivalent weights..

An important implication of Faraday’s second law is that the ratio of the

mass of the electrodeposits to its gram-equivalent weight is a constant equal to 1

faraday or 96,500 coulombs or 26.8 ampere-hours.

1.5.1 Basic components in Electrodeposition technique

The schematic experimental setup explaining the electrodeposition is

shown in Fig. 1.2. The typical electrodeposition set up consists of the following

components.

1. Electrolyte - A chemical compound (salt, acid, or base) that dissociates into

electrically charged ions when dissolved in a solvent. The resulting electrolyte (or

electrolytic) solution is an ionic conductor of electricity. Very often, the so formed

solution itself is simply called an "electrolyte". Also, molten salts and molten salt

solutions are often called "electrolyte"

2. Electrode - The two electronically conducting parts of an electrochemical cell

are called anode and cathode act as electrodes. An applied electric field across

these provides the main ‘driving force’ for the ions. The positive and negative ions

deposit at the cathode and anode, respectively. Cathodic deposition is more

popular in electroplating cause of most metal ions are positive ions and anodic

deposition has been found to give poor stoichiometry and adhesion.

3. Electrical source (Power supply) - A source of electrical power supply is a

device that supplies direct current at constant voltage, which leads to potentiostatic

Chapter - I

8

deposition and direct current at constant current, which leads to galvanostatic

deposition.

1.5.2 Advantages of electrodeposition technique

1. It is possible to grow uniform films over large area, as well as irregularly

shaped surfaces.

2. Compositionally modulated structures of non-equilibrium alloys can be

electroplated.

3. The deposition rate is higher than other physical and chemical methods..

4. It is especially attractive in terms of cost, high throughput and scalability.

5. It is an isothermal process in which, the thickness and morphology of the

films can be easily controlled by electrochemical parameters such as

electrode potential and current.

Above advantages, electrodeposition has interesting feature that, direct

cathodic electrodeposition from aqueous and non aqueous baths is possible and

can be employed as one of the steps in the preparation of semiconductors or

oxides.

1.5.3 Limitation of electrodeposition technique.

1. It is not possible to grow the films other than metallic or conducting.

2. It requires the substrates to be conductive deposition on non-conducting

substrates such as glass, quartz, ceramics etc. is not possible.

3. Thickness is limited.

1.6 Experimental setup

Fig.1.2 shows the experimental setup of a simple electrodeposition

technique. Electrodeposition should be defined as the process in which the

deposition takes place in the form of thin layer on a substrate. The bath is specially

designed chemical solution that contains the desired metal dissolved in a form of

submicroscopic positively charged metallic particles.

Chapter - I

9

The object that is to be plated is submerged into the electrolyte

(electroplating bath). Placed usually at the center of the bath, which acts as a

negatively charged cathode.

Fig.1.2 Experimental setup of a simple electrodeposition technique

The positively charged anode completes the electric circuit; those may be at

opposite edges of the plating tank, thus causing film deposit on both sides of the

cathode. A power source in the form of a battery is providing the necessary

current. This type of circuit arrangement directs electrons (negative charge

carriers) into a path from the power supply to the cathode (the object to be plated).

Now, in the bath the electric current is carried largely by the positively charged

ions from the anode towards the negatively charged cathode. This movement

makes the metal ions in the bath to migrate towards extra electrons that are located

at or near the cathode's surface outer layer. Finally, by way of electrolysis the

metal ions are removed from the solution and are deposited on the surface of the

object as a thin layer [45].

1.7 Characterization of Thin Films

Scientific disciplines are identified and differentiated by the equipments

and measurement techniques they employ. The same is true to thin-film science

and technology. For the first half of this century the interest in thin films centered

Chapter - I

10

on optical applications. At first, single films on thick substrates were involved.

However, with the explosive growth of thin-film utilization in microelectronics,

there was an important need to understand the intrinsic nature of films in more

complex materials environments. Increasingly, the benefits of multilayer film

structures have been realized in an assortment of high- technology applications. It

is necessity that, drove the creativity and inventiveness that culminated in the

development of an impressive array of commercial analytical instruments. These

are now ubiquitous in the thin-film, coating, and broader scientific communities.

A partial list of the modern techniques employed in the characterization of thin-

film materials. Among their characteristics are the unprecedented structural

resolution and chemical analysis capabilities over both small lateral and depth

dimensions. Some techniques only sense and provide information on the first few

atom layers of the surface. Others probe more deeply but in most cases depths of a

micron or less are analyzed. Virtually all of these techniques require a higher

ultrahigh-vacuum ambient. Some are nondestructive while others are not. All of

them utilize incident electron, or ion, or photon beams. These interact with the

surface and excite it in such a way that some combination of secondary beams of

electrons, ions, or photons are emitted, carrying off valuable structural and

chemical information in the process. A rich collection of acronyms a savory

alphabet soup has emerged to differentiate the various techniques[46].

1. Size :- This varies from a portable desktop interferometer to the 50-foot long

accelerator and beam line of a Rutherford backscattering (RBS) facility.

2. Cost:- There is a wide variation in cost from the modest levels for test

instruments required to measure electrical resistivity of films, to the near million-

dollar price tag of an Auger spectrometer.

3. Operating environment:- This varies from the ambient in the measurement of

film thickness to the 10~10

torr vacuum required for the measurement of film-

surface composition.

Chapter - I

11

4. Sophistication:- At one extreme is the manual Scotch-tape film peel-test to

evaluate adhesion;at the other is an assortment of electron microscopes and

surface analytical equipment where operation, data gathering, display, and analysis

are computer controlled.

What is remarkable is that films can be characterized structurally, chemically, and

with respect to various properties with almost the same ease and precision that we

associate with bulk measurements. This despite the fact that there are many orders

of magnitude fewer atoms available in films.

This chapter will only address, with roughly equal coverage, the

experimental techniques and applications associated with determination of:

i. Film thickness

ii. Film and surface morphology and structure

iii. Film and surface composition

These represent the common core of information required of all films and

multilayer coatings irrespective of ultimate application. Within each of these three

categories only the most important techniques will be discussed. Beyond these

characteristics there are a host of individual properties (e.g., hardness, adhesion,

stress, electrical conductivity) which are specific to the particular application.

1.7. 1 X- Ray Diffractograms

X-Ray diffraction is a very powerful and suitable technique for

characterizing the microstructure of the thin film. The basic principles of X- ray

diffraction are explained by Buerger , Culity , and Warren [47,48].

Diffraction effects are observed when electromagnetic radiation impinges

on periodic structures with geometrical variations on the length scale of the

wavelength of the radiation. The interatomic distances in crystals and molecules

amount to 0.15–0.4 nm which correspond in the electromagnetic spectrum with

the wavelength of X-rays having photon energies between 3 and 8 keV.

Accordingly, phenomena like constructive and destructive interference should

become observable when crystalline and molecular structures are exposed to X-

Chapter - I

12

rays. In the following sections, firstly, the geometrical constraints that have to be

obeyed for X-ray interference to be observed are introduced. Secondly, the results

are exemplified by introducing the θ/2θ scan, which is a major X-ray scattering

technique in thin-film analysis. Thirdly, the θ/2θ diffraction pattern is used to

outline the factors that determine the intensity of X-ray ref lections. We will

thereby rely on numerous analogies to classical optics and frequently use will be

made of the fact that the scattering of radiation has to proceed coherently, i.e. the

phase information has to be sustained for an interference to be observed.

1.7. 2 The Basic Phenomenon

Before the geometrical constraints for X-ray interference are derived the

interactions between X-rays and matter have to be considered. There are three

different types of interaction in the relevant energy range. In the first, electrons

may be liberated from their bound atomic states in the process of photoionization.

Since energy and momentum are transferred from the incoming radiation to the

excited electron, photoionization falls into the group of inelastic scattering

processes. In addition, there exists a second kind of inelastic scattering that the

incoming X-ray beams may undergo, which is termed Compton scattering. Also in

this process energy is transferred to an electron, which proceeds, however, without

releasing the electron from the atom. Finally, X-rays may be scattered elastically

by electrons, which is named Thomson scattering. In this latter process the

electron oscillates like a Hertz dipole at the frequency of the incoming beam and

becomes a source of dipole radiation. The wavelength λ of X-rays is conserved for

Thomson scattering in contrast to the two inelastic scattering processes mentioned

above. It is the Thomson component in the scattering of X-rays that is made use of

in structural investigations by X-ray diffraction.

X- ray diffraction is well known technique for the structure characterization

of the material, structure identification, determination of lattice parameter and

grain size is based on the interpretation of X-ray diffraction pattern. This

technique is based on the monochromatic radiation. The phenomenon of X- ray

Chapter - I

13

diffraction can be considered as reflection of X- rays from the crystallographic

plane of the material and governed by the Bragg’s law,

λθ nd =sin2 (1.5)

Where, d - lattice spacing

λ - wavelength of used X- ray

n - order of diffraction

θ - diffraction angle

The ‘d’ values can be calculated using above relation for known λ, θ and n. The

obtained XRD data is compared with Joint Committee Powder Diffraction

Standard (JCPDS) to identify the unknown material. The Crystallite size of the

deposited material is estimated from the Full Width at Half Maximum (FWHM) of

the most intense diffraction line by Scherer’s formula [49],

θβ

λ

cos

⋅=

kD (1.6)

Where, D - Crystallite size

λ - wavelength of used x- ray

β - FWHM of the peak and

θ - Bragg’s angle.

k -constant taken to be 0.94,

The value of k varies from 0.89 to 1.39 but for most cases it is closer to 1[50].

1.7.3 Optical absorption

The absorption spectroscopy is involved in the operation of UV- VIS-

NIR spectrophotometer. Absorption spectroscopy based on the principle that

amount of absorption that occurs, is dependent on number of molecules present in

the absorbing material. Therefore, the intensity of the radiation leaving the

substance may be used as indicator of concentration of material.

The UV/visible electromagnetic radiation causes electronic transitions

within a molecule, promoting bonding and non-bonding electrons to higher, less

stable antibonding orbital. The molecule then loses this excess energy by rotation

Chapter - I

14

and vibrational relaxation. In principle the technique is similar to IR-absorbance

i.e. when a sample of an unknown compound is exposed to light, certain functional

groups within the molecule absorb light of different wavelengths in the UV or

visible or NIR region. UV-VIS-NIR spectroscopy is used for qualitative and

quantitative analysis of materials[51,52].

1.7.4 Surface wettability

When clean glass plate is dipped in water, water molecules sticks on glass

surface i.e. it becomes wet. Wetting refers to the study of how a liquid deposited

on a solid or a liquid spreads out. Understanding of wetting enables us to explain

why water spreads readily on a clean glass but not on a plastic sheet. ‘Controlling’

it, means being able to modify a suitable surface to turn a non-wettable solid into

one that is wettable or vice-versa [53].

Fig. 1.3 The angle formed by a liquid at the three phase boundary

Contact angle θ is a quantative measure of the wetting of a solid by a liquid. It is

defined geometrically as the angle formed by a liquid at the three phase boundary

where a liquid, gas and solid intersect as shown in Fig. 1.3 above:

1. Hydrophobic Surfaces:

“water-fearing surface” water tries to minimize contact with surface.

i. Surfaces with a contact angle θc > 90°

ii. Water beads-up on the surface

1. Hydrophilic Surfaces:

“water-loving surface” water tries to maximize contact with surface.

Chapter - I

15

i. Surfaces with a contact angle θc < 90°

ii. Water spreads out on surface

If a drop of liquid is placed on a horizontal solid surface in equilibrium with

vapour phase, then the drop spreads on the solid surface till the three interfacial

forces balance each other shows three interfaces-solid-liquid, liquid-vapour, solid-

vapour interface. Contact angle (θ) is the angle made by the tangent to the liquid-

vapour interface drawn at the contact line makes an angle with the solid surface,

which is the characteristic of the three-phase system (solid-liquid-vapour). The

contact angle therefore is a thermodynamic property. This is governed by Young's

equation [54].

( )θγγγ coslvslsv

+= (1.7)

Where,

γγγγsv, γγγγsl , γγγγlv are solid-vapour, solid-liquid and liquid-vapour interfacial

energies, respectively and θ is the contact angle.

3. Using travelling microscope

The travelling microscope was used to measure the base contact length (b) and

the height of the water drop (h) and θ was calculated using the following formula,

θ = 2 tan-1

(2h/b) (1.8)

4. Using contact angle meter

In this method, the contact angle θ was directly measured using a contact

angle meter (rame-hart instrument.). A water drop is kept on the hydrophobic

aerogel surface and the image of the drop is projected on the screen of the monitor

with CCD camera. This gives directly the value of θ after the proper adjustment of

the tangent to the water drop at the point of contact with the solid surface. Good

agreement, in contact angle (θ) values, has been observed by both the methods.

1.7.5 Surface morphology

SEM stands for scanning electron microscope. The SEM is a microscope

that uses electrons instead of light to form an image. Since their development in

Chapter - I

16

the early 1950's, scanning electron microscopes have developed new areas of

study in the medical and physical science communities. The SEM has allowed

researchers to examine a much bigger variety of specimens. The scanning electron

microscope has many advantages over traditional microscopes. The SEM has a

large depth of field, which allows more of a specimen to be in focus at one time.

The SEM also has much higher resolution, so closely spaced specimens can be

magnified at much higher levels. Because the SEM uses electromagnets rather

than lenses, the researcher has much more control in the degree of magnification.

All of these advantages, as well as the actual strikingly clear images, make the

scanning electron microscope one of the most useful instruments in research today

[55].

The SEM is an instrument that produces a largely magnified image by

using electrons instead of light to form an image. A beam of electrons is produced

at the top of the microscope by an electron gun.

1.7.6 Fourier Transform Raman Spectroscope

The use of the FT spectrometer has had a considerable impact on infrared

spectroscopy in recent years. This is due to the three principal advantages of FT

spectrometry: high throughput, wavelength accuracy, and multiplexing, that is, the

simultaneous detection of all wavelengths. The first FT-Raman experiment was

performed by Chandry in 1964 but went largely unnoticed until Hirschfeld and

Chase and Murphy and co-workers reintroduced FT-Raman spectroscopy in 1986.

Several manufacturers of FT-IR instrumentation have recently adapted their

spectrometers to perform FT-Raman measurements. Near-IR excited FT- Raman

spectroscopy was developed only recently (1986). Within a short time, however, it

has become a very useful technique and is well suited to studies of research and

industry samples. The reason why FT Raman is so successful compared to

conventional Raman spectroscopy (visible laser excitation) is that:

(i) for most samples, spectra are free of fluorescence, so that it’s applicable to

many samples that could not be examined by conventional Raman spectroscopy,

Chapter - I

17

(ii) spectra can be acquired rapidly,

(iii) spectral subtraction is accurate. In the near-ill excited FT Raman, sample

fluorescence is suppressed (or eliminated) due to sample excitation at 1064 nm

where most materials do not absorb. Spectra are obtained rapidly due to the well

known signal-to-noise advantages associated with FT instruments[56-58].

1.7.7 FT-IR Spectroscopy

Studies of the spontaneous orientation of dipole moment in semiconductors

are carried out with a non destructive tool of analysis by infrared spectroscopy

which gives information on atomic arrangement and inter atomic forces in the

crystal lattice itself. It is possible to investigate how the infrared vibrational

frequencies and thus the inter-atomic forces are affected by the onset of the

semiconductor states. If the two energy levels E1 and E2 are placed in an

electromagnetic field and the difference in the energy between the two states is

equal to a constant 'h' multiplied by the frequency of the incident radiation ν, a

transfer of energy between the molecules can occur, giving therefore

νh E =∆ (1.9)

Where, the symbols have their usual meanings. When the ∆E is positive the

molecule absorbs energy; when ∆E is negative, radiation is emitted during the

energy transfer and emission spectra are obtained. When the energies are such that

the equation (1.9) is satisfied, a spectrum unique to the molecule under

investigation is obtained[59].

Chapter - I

18

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