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CHAPTER 2

INSTRUMENTATION AND CHARACTERIZATION TECHNIQUES

2.1 INTRODUCTION

Over the past several years, a number of techniques have been

developed for the production of ceramic nanoparticles and they include: laser

ablation, microwave plasma synthesis, spray pyrolysis, plasma arc synthesis,

hydrodynamic cavitation and wire explosion techniques, the polymerizable

complex method, flame synthesis of nanoparticles, microemulsion techniques,

hydrothermal treatments, the sonochemical method, combustion synthesis,

solid state reaction, precipitation and co-precipitation from a solution and sol–

gel processing. In the present study, sonochemical synthesis (ultrasound

assisted simple precipitation method) was employed to prepare the

nanoparticles and nanocomposites.

Characterization of nanomaterials is performed at different levels.

Some characterization methods are used to study the sizes, shapes, and

morphology of nanostructures, whereas others are used to obtain detailed

structural information. The structures of materials can be studied at various

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levels of sophistication, including crystal structure, microstructure, atom-level

structure, and electronic structure. This chapter discusses in brief the various

characterization techniques and analysis carried out to probe into the internal

structure, surface morphology and properties of the synthesized material.

2.2 SONOCHEMICAL SYNTHESIS TECHNIQUE

Figure 2.1 Frequency range of ultrasound

One of the most widely used solution techniques for synthesis of

nanostructured materials is the co-precipitation method. The major negative

aspect of the process is the inability to control the size of the precipitating

particles and their subsequent aggregation. To overcome this problem, a

secondary aid such as surfactant assisting is required during synthesis process.

However, removal of surfactant from the material is difficult. Alternatively, the

better choice to control the particle growth during precipitation is physical

agitation through ultrasonication process. It is believed that precipitation under

ultrasonication process can make the changes in the particle size, crystallinity

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and morphology. Sonochemical synthesis under the irradiation of ultrasound in

solution is thus an effective method that can be used to prepare various

dimensional nanostructures.

If the precipitation technique could be tailored with ultrasonication

such that the de-agglomeration of the synthesized nanomaterial could be

accomplished then that hybrid method would be able to produce fine grained

crystalline nanomaterials. The frequency range of ultrasound is given in Figure

2.1. During the sonochemical precipitation process, ultrasonic waves

consisting of compression and rarefaction cycles during their propagation

through the media produce cavitation bubbles in the media. After several

acoustic cycles, the cavitation bubbles collapse violently and adiabatically

generating extremely high temperatures and pressures. (Patil and Pandit 2007)

Thus, such extreme temperatures and pressures within a small reactor can

induce many changes in the morphology of the nanoparticles during its

precipitative formation. The effect of ultrasonic irradiation on chemical

reactions is to accelerate them and to initiate new reactions that are difficult to

carry about under normal conditions. The schematic diagram of sonochemical

experimental set-up and the sonochemical experimental apparatus are shown in

Figure 2.2 and 2.3 respectively.

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Figure 2.2 Schematic diagram of sonochemical experimental set-up

Figure 2.3 Sonochemical experimental apparatus

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2.3 CHARACTERIZATION TECHNIQUES AND ANALYSIS

The prepared samples in the present research work are

characterized by X-ray powder Diffraction (XRD), Fourier Transform Infrared

Spectroscopy (FTIR), Ultraviolet Diffuse Reflectance Spectroscopy (UV-

DRS), Photoluminescence Spectroscopy (PL), Scanning Electron Microscopy

(SEM)/ Field Emission Scanning Electron Microscopy (FESEM) coupled with

Energy Dispersive X-ray Spectroscopy (EDAX) and Transmission Electron

Microscopy (TEM) with Selective Area Electron Diffraction (SAED) inorder

to analyze their structural, optical, electronic and morphological

characteristics. From the XRD data, the lattice parameters namely mean

crystallite size, strain, dislocation density, lattice constant and unit cell volume

was calculated. The phase identification was done by analyzing the XRD data

by comparing the interplanar distances and intensity values with the standard

peaks using JCPDS files and peaks are indexed to the corresponding hkl

planes. The peaks in the FTIR spectra were analyzed and designated to the

corresponding characteristic vibrational modes of the materials. The Kubelka

Munk plot was plotted using the reflectance data to determine the energy band

gap. The optical properties and electronic properties were analyzed from the

UV DRS and photoluminescence spectroscopy. The morphological

characterization was done by SEM/FESEM and TEM by analyzing the images

in detail. EDAX analysis was done along with SEM to confirm the

composition of the nanomaterials.

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2.3.1 X-Ray Powder Diffraction

X-rays are electromagnetic radiation of exactly the same nature as

light but of very much shorter wavelength about 1 Å. Max Von Laue in 1912,

discovered that the crystalline substances act as three-dimensional diffraction

gratings for X-ray wavelengths similar to the spacing of planes in a crystal

lattice. X-ray powder diffraction (XRD) is a powerful technique used to

uniquely identify the crystalline phases present in materials and to measure the

structural properties of these phases. XRD is also used to determine the

thickness of thin films and multilayer and atomic arrangements in amorphous

materials (including polymers) and at interfaces. The intensities measured with

XRD can provide quantitative, accurate information on the atomic

arrangements at interfaces (e.g., in multilayers). It may be used to determine

its structure, average particle size, unit cell dimensions and sample purity.

X-ray powder diffraction is based on the constructive interference

of monochromatic X-rays and a crystalline material. These X-rays are

generated when electrons moving at high speed are directed to a metal target;

a small percentage of their kinetic energy is converted into X-rays. The X-rays

emitted by the target consist of continuous range of wavelength and is called

white radiation. The minimum wavelength in continuous spectrum is inversely

proportional to the applied voltage, which accelerates the electron towards the

target. If the applied voltage is sufficiently high in addition to the white

radiation, the target also emits a characteristic radiation of specific wavelength

and high intensity. The radiation emitted by a target includes both types of

radiation. In spectroscopic notation, the characteristics radiations are named as

KαKβKγ etc. Kα radiation has high intensity and is commonly used for

diffraction studies. The wavelength of this radiation for a typical copper metal

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target is 1.54056 Å. A beam of X-rays directed at a crystal interacts with the

electron of the atom that constitutes the crystal.

The diffraction effect produced by the three dimensional grating

provided by the crystal obeys Bragg’s law. X-rays penetrate into the solid non-

destructively and provides the information about the internal structure of

solids. Crystal acts as a natural diffraction grating for the diffraction of X-ray

beam incident upon it in all directions. The X-rays are diffracted in accordance

with the Bragg’s law given by n� = 2d sin�, where ‘n’ is an integer referring to

the order of reflection, ‘�’ is the wavelength, ‘d’ is the spacing between the

crystal lattice planes responsible for particular diffracted beam and ‘�’ is the

angle that incident beam makes with lattice planes.

Figure 2.4 Schematic of Bragg’s reflection from a crystal

The path difference (2x) between the incident beam and the

reflected beam in the consecutive lattice planes is shown in Figure 2.4. The

width of the Bragg’s reflection in a standard X-ray powder diffraction pattern

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can provide the information of the average grain size. The peak breadth

increases as the grain size decreases, because of the reduction in the

coherently diffracting domain size, which can be assumed to be equal to the

average crystallite size. The average crystallite size can be estimated by using

Scherrer’s relation (2.1) (Cullity 1956).

D = k � / (� cos �) (2.1)

where D is the crystallite size; k = 0.9, a correction factor to account for

particle shape; � is the full width at half maximum (FWHM) of the most

intense diffraction plane; � is the wavelength of Cu target = 1.54 Å; and � is

the Bragg angle.

The microstrain and dislocation density can be calculated using the

equation (2.2) and (2.3) respectively

� = [(�/D cos �) - �] 1/tan � (2.2)

� = 1/D2 (lines/m2) (2.3)

where � is the microstrain, � is the dislocation density; � is the

wavelength of Cu target = 1.54 Å; D is the crystallite size; � is the Bragg

angle; and � is the full width at half maximum (FWHM) of the most intense

diffraction plane.

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In this dissertation work, the structure and the average particle size

of all samples were identified by using X-ray powder diffraction (XRD) at

room temperature on a PANalytical X’pert PRO X-ray diffractometer using

CuK�1 radiation (�= 1.54056 Å) as the X-ray source. X-ray powder diffraction

(XRD) is a rapid analytical technique primarily employed for phase

identification of a crystalline material, quantify the crystalline nature of the

material and can provide information on unit cell dimensions and spacing

between lattice planes. The phase identification and peak indexation

corresponding to the (hkl) planes can be done by analyzing the XRD data with

the standard database (JCPDS). The lattice parameters namely lattice constant,

volume of unit cell, crystallite size, strain and dislocation density can be

calculated from the XRD data.

2.3.2 Fourier Transform Infrared Spectroscopy

Infrared (IR) refers broadly to that part of the electromagnetic

spectrum between the visible and microwave regions. Of greatest practical use

to the organic chemist is the limited portion between 4000 cm-1 and 400 cm-1.

There has been some interest in the near-IR (14,290-4000 cm-1) and the far-IR

regions, 700-200 cm-1. FTIR is conceivably the most powerful tool for

identifying the functional groups or the types of chemical bonds.

FTIR Spectrum is often called as the finger print of the sample and

is the characteristic of each material. The spectrum represents the molecular

absorption and transmissions, creating a molecular finger print of the material.

Like a finger print no two unique molecular structures produce the same

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infrared spectrum. An infrared spectrum represents a finger print of a material

with absorption peaks which correspond to the frequencies of vibrations

between the bonds of the atoms making up the material. Because each

different material is a unique combination of atoms, no two compounds

produce the exact same infrared spectrum. Therefore, infrared spectroscopy

can result in a positive identification (qualitative analysis) of every different

kind of material. In addition, the size of the peaks in the spectrum is a direct

indication of the amount of material present. Infrared spectroscopy gives

information on the vibrational and rotational modes of motion of a molecule

and hence an important technique for identification and characterization of a

substance. The peaks exhibited in the FTIR spectrum are analyzed and

correlated to their respective rotational and vibrational modes of molecules.By

interpreting the infrared absorption spectrum, the chemical bonds in a

molecule can be determined. Organic compounds have very rich, detailed

spectra but inorganic compounds are usually much simpler. For most common

materials, the spectrum of an unknown material can be identified by

comparison to a library of known compounds (Nakamoto 1986, Richard

Brundle et al. 1992).

A beam of infrared light (wavelength ~ 0.7-500 �m) is focused on

the samples using all-reflective optics. Depending on the sample composition,

differing amounts of light are absorbed at different wavelengths. This pattern

of light absorption is unique for almost every organic compound (except

optical isomers) and many inorganics. From the pattern of light absorbed,

identification of the composition (qualitative analysis) can be made. With

additional control over the sample thickness or sampling depth, the intensity of

the individual absorbing components can be used to perform quantitative

analysis (amount of each compound present). User-provided reference

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samples aid in positive substance identification and compositional verification.

Figure 2.5 shows the schematic diagram of the arrangement of FTIR.

Figure 2.5 Schematic representation of FTIR spectrometer

The energy corresponding to the transitions between molecular

vibrational states is generally 1-10 kilocalories/mole which corresponds to the

infrared of the electromagnetic spectrum.

Difference in Energy States = Energy of Light Absorbed

E1 - E0 = hc/� (2.4)

where h is Planck’s constant, c is the speed of light and � is the wavelength of

light.

FTIR analysis was done in all the prepared samples with the

instrument of FTIR spectrometer (Thermo Scientific Nicolet IS-10).

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2.3.3 UV- Visible Diffuse Reflectance Spectroscopy

In this research work, the optical reflectance spectra of the prepared

samples were recorded using UV-Visible diffuse reflectance

spectrophotometer (UV-2102 PCS spectrophotometer). Since light cannot

penetrate through opaque (solid) samples, it is reflected on the surface of the

samples. As shown in Figure 2.6, the incident light reflected symmetrically

with respect to the normal line is called specular reflection, while incident

light scattered in different directions is called diffuse reflection (Wendlandt

and Hecht 1966, Kortüm 1969). The light is diffusely reflected from the

randomly oriented crystals in the nanopowder.

Figure 2.6 Schematic representation of reflection mechanisms

With integrating spheres, measurement is performed by placing the

sample in front of the incident light window and concentrating the light

reflected from the sample on the detector using a sphere with barium sulfate-

coated inside. The obtained value becomes the reflectance (relative

reflectance) with respect to the reflectance of the reference standard.

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The Diffuse Reflectance of the samples is recorded in the

wavelength range of 200 – 800 nm with barium sulphate as the reference

material. The electronic band gap is usually determined by using Tauc

equation from the UV-Vis spectroscopic data, but in the case of Diffuse

Reflectance where the reflectance of solid sample is recorded some

modification has to be done in Tauc equation. This modified Tauc equation

called as Kubelka-Munk equation is used to determine the electronic band gap

of solid samples characterized by diffuse reflectance technique.The relational

expression proposed by Tauc, Davis, and Mott is as follows:

(�h)n = A(h-Eg) (2.5)

where h is the Planck's constant; is the frequency of vibration; � is

the absorption coefficient; Eg is the energy band gap; A is a proportional

constant. The value of the exponent n denotes the nature of the sample

transition. The absorption co-efficient (�) in Tauc equation is replaced by the

function of reflectance F(R) in Kubelka-Munk equation. The Kubelka-Munk

equation is expressed as follows:

[F(R)h]n = A(h-Eg) (2.6)

The Kubelka-Munk function is expressed as F(R) = (1-R)2/2R

where R is the reflectance.The energy band gap is obtained by extrapolating

the linear portion of the curve to the X-axis of the Kubelka-Munk plot which

is plotted with the nth power of the product of the function of reflectance and

photonic energy in the Y-axis against the photonic energy in the X-axis. The

power factor n depends upon the nature of band gap structure and transition.

The value of n = 2 for allowed transitions and n = 2/3 for forbidden transitions

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in direct band gap semiconductor; n = ½ for allowed transitions and n = 1/3

for forbidden transitions in indirect band gap semiconductors (Morales et al.

2007). The tangent line is drawn tangent to the point of inflection on the curve.

The point of inflection is found by taking the first derivative of the curve. The

point at which the value of the first derivative coefficient begins to decrease

after increasing is the point of inflection. The Eg values can be easily obtained

using UVProbe software. This method thus enables to determine the band gap

of powder samples without the influence of any solvent. There are chances of

the nanopowder to interact with the solvent in UV-Visible Spectroscopy where

the nanopowders are dispersed in a solvent medium. The determination of

energy band gap of nanoceramic material helps to find the excitation

wavelength of photoluminescence as given by Plank’s relation Eg = h.

2.3.4 Photoluminescence Spectroscopy

Photoluminescence (abbreviated as PL) is a process in which a

substance absorbs photons (electromagnetic radiation) and then re-radiates

photons. Quantum mechanically, this can be described as an excitation to a

higher energy state and then a return to a lower energy state accompanied by

the emission of a photon. This is one of many forms of luminescence (light

emission) and is distinguished by photo excitation (excitation by photons),

hence the prefix photo. The period between absorption and emission is

typically extremely short, in the order of 10 nanoseconds. Under special

circumstances, however, this period can be extended into minutes or hours.

The simplest photoluminescence processes are resonant radiations, in which a

photon of a particular wavelength is absorbed and an equivalent photon is

immediately emitted. This process involves no significant internal energy

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transitions of the chemical substrate between absorption and emission and is

extremely fast, of the order of 10 nanoseconds. More interesting processes

occur when the chemical substrate undergoes internal energy transitions

before re-emitting the energy from the absorption event. The most familiar of

such effect is fluorescence, which is also typically a fast process, but in which

some of the original energy is dissipated so that the emitted light photons are

of lower energy than those absorbed. The generated photon in this case is said

to be red shifted, referring to the loss of energy.

Photoluminescence (PL) spectroscopy is a contact-less,

nondestructive method to probe the electronic structure of materials. The

spectral distribution of PL from a semiconductor can be analyzed to

nondestructively determine the electronic band gap. This provides a means to

quantify the elemental composition of compound semiconductor and is vitally

important material parameter influencing solar cell device efficiency. The PL

spectrum at low sample temperatures often reveals spectral peaks associated

with impurities contained within the host material. The high sensitivity of this

technique provides the potential to identify extremely low concentrations of

intentional and unintentional impurities that can strongly affect material

quality and device performance. The quantity of PL emitted from a material is

directly related to the relative amount of radiative and non-radiative

recombination rates. Non-radiative rates are typically associated with

impurities and thus, this technique can qualitatively monitor changes in

material quality as a function of growth and processing conditions.

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2.3.5 Scanning Electron Microscope (SEM)

Scanning electron microscope (SEM) is one of the most widely

used techniques in characterization of nanomaterials and nanostructures. The

signals that derive from electron-sample interactions reveal information about

the sample including surface morphology (texture) and chemical composition

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

the sample surface and a two dimensional image is generated that displays

spatial variations in these properties. The resolution of the SEM approaches a

few nanometers, and the instruments can operate at magnifications that are

easily adjusted from - 10 to over 3,00,000. Not only does the SEM produce

topographical information as optical microscopes do, it also provides the

chemical composition information near the surface. As well as, it is capable of

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

especially useful in qualitatively or semi-quantitatively determining chemical

compositions.

Figure 2.7 illustrates the typical SEM instrumentation. Electrons

are generated in the electron gun. The tungsten-hairpin gun is commonly used,

in which a tungsten filament serves as the source of electrons. By applying a

current through the filament the tungsten wire will heat up and emission of

electrons can be achieved. Generated electrons will be focused in front of an

anode. To move the electrons down the column, a voltage difference between

the tungsten filament and the anode is applied. This voltage differences is

called the accelerating voltage and can be varied between 0.2 and 40 keV

determining the energy and wavelength of the electrons within the beam. The

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beam of electrons to be condensed and focused as a fine spot on the specimen

by 2 to 3 electromagnetic lenses located in the microscope column.

The main functions of first two lenses namely condenser lens 1

(C1) and condenser lens 2 (C2) are to control the beam current (number of

electrons striking the specimen) and the final size of the area illuminated on

the specimen (spot size). The third condenser lens (C3) also called the final

lens, is used primarily to focus the beam of electrons on the surface of the

specimen. The final lens usually contains deflecting coils and stigmator coils

(Richard Brundle et al 1992).

Accelerated electrons in a SEM carry significant amount 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 (EBSD that are used to determine crystal

structures and orientations of minerals). 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 most valuable for illustrating contrasts

in composition in multiphase samples

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Figure 2.7 Schematic diagram of scanning electron microscope

2.3.6 Field Emission Scanning Electron Microscope (FESEM)

FESEM produces clearer, less electrostatically distorted images

with spatial resolution down to 1.5 nm which is 3 to 6 times better than

conventional SEM. Reduced penetration of low kinetic energy electrons

probes closer to the immediate material surface. High quality, low voltage

images can be obtained with negligible electrical charging of samples

(Accelerating voltages range from 0.5 to 30 kV). The need for placing

conducting coatings on insulating materials is virtually eliminated. A field-

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emission cathode in the electron gun of a scanning electron microscope

provides narrower probing beams at low as well as high electron energy,

resulting in both improved spatial resolution and minimized sample charging

and damage. The function of the electron gun is to provide a large and stable

current in a small beam. There are two classes of emission source: thermionic

emitter and field emitter. Emitter type is the main difference between the

Scanning Electron Microscope (SEM) and the Field Emission Scanning

Electron Microscope (FESEM). Thermionic Emitters use electrical current to

heat up a filament; the two most common materials used for filaments are

Tungsten (W) and Lanthanun Hexaboride (LaB6). When the heat is enough to

overcome the work function of the filament material, the electrons can escape

from the material itself. Thermionic sources have relative low brightness,

evaporation of cathode material and thermal drift during operation. Field

Emission is one way of generating electrons that avoids these problems. A

Field Emission Gun (FEG); also called a cold cathode field emitter, does not

heat the filament. The emission is reached by placing the filament in a huge

electrical potential gradient. The FEG is usually a wire of Tungsten (W)

fashioned into a sharp point. The significance of the small tip radius (~ 100

nm) is that an electric field can be concentrated to an extreme level, becoming

so big that the work function of the material is lowered and electrons can leave

the cathode. FESEM uses Field Emission Gun producing a cleaner image, less

electrostatic distortions and spatial resolution.

2.3.7 Energy-Dispersive X-Ray Spectroscopy (EDAX)

Energy-dispersive X-ray spectroscopy (EDS, EDX, or XEDS) is an

analytical technique used for the elemental analysis or chemical

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characterization of a sample. It relies on the investigation of the interaction of

some source of X-ray excitation and a sample. Its characterization capabilities

are due in large part to the fundamental principle that each element has a

unique atomic structure allowing unique set of peaks on its X-ray

spectrum. To stimulate the emission of characteristic X-rays from a specimen,

a high energy beam of charged particles such as electrons or protons or a beam

of X-rays, is focused into the sample being studied. At rest, an atom within the

sample contains ground state (or unexcited) electrons in discrete energy levels

or electron shells bound to the nucleus. The incident beam may excite an

electron in an inner shell, ejecting it from the shell while creating an electron

hole where the electron was. An electron from an outer, higher-energy shell

then fills the hole, and the difference in energy between the higher-energy

shell and the lower energy shell may be released in the form of an X-ray. The

number and energy of the X-rays emitted from a specimen can be measured by

an energy-dispersive spectrometer. As the energy of the X-rays is

characteristic of the difference in energy between the two shells, and of the

atomic structure of the element from which they were emitted, this allows the

elemental composition of the specimen to be measured (Rao and Biswas

2009).

The limitations are summarized as follows. Samples must be solid

and they must fit into the microscope chamber. Maximum size in horizontal

dimensions is usually on the order of 10 cm; vertical dimensions are generally

much more limited and rarely exceed 40 mm. For most instruments samples

must be stable in vacuum on the order of 10-5 - 10-6 torr. EDS detectors on

SEM's cannot detect very light elements (H, He, and Li), and many

instruments cannot detect elements with atomic numbers less than 11 (Na).

Most SEMs use a solid state x-ray detector (EDS), and while these detectors

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are very fast and easy to utilize, they have relatively poor energy resolution

and sensitivity to elements present in low abundances when compared to

wavelength dispersive x-ray detectors (WDS) on most electron probe

microanalyzers (EPMA). An electrically conductive coating must be applied

to electrically insulating samples for study in conventional SEM's, unless the

instrument is capable of operation in a low vacuum mode. However, gold can

serve as a conducting layer on the sample and coated on top of the sample.

2.3.8 Transmission Electron Microscopy (TEM)

TEM is a vital characterization tool for direct imaging of

nanomaterials to obtain quantitative measures of particle and/or grain size,

size distribution and morphology. TEM is capable of imaging at a significantly

higher resolution than light microscopes, owing to the small de Broglie

wavelength of electrons. This enables the instrument's user to examine fine

detail even as small as a single column of atoms, which is thousands of times

smaller than the smallest resolvable object in a light microscope. TEM images

are formed using transmitted electrons (instead of the visible light) which can

produce magnification details up to 1,000,000x with resolution better than

10Å. TEM forms a major analysis method in a range of scientific fields, in

both physical and biological sciences and especially material science. Selected

Area Electron Diffraction (SAED) is a crystallographic experimental

technique that can be performed inside a transmission electron

microscope (TEM). As a diffraction technique, SAED can be used to identify

crystal structures and examine crystal defects. It is similar to X-ray powder

diffraction, but unique in that area as small as several hundred nanometers in

size can be examined, whereas in X-ray diffraction typically samples areas

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extend several centimeters in size. SAED pattern of nanoparticles or

nanocrystals gives ring patterns analogous to those from X-ray powder

diffraction, and can be used to identify texture and discriminate

nanocrystalline from amorphous phases. The electron diffraction pattern yield

information about the orientation, atomic arrangement and structure of narrow

regions of interest in the nanomaterial (Rao and Biswas 2009).

Figure 2.8 shows the schematic layout of optical components in a

basic TEM. The TEM consists of an emission source, which may be

a tungsten filament, or a lanthanum hexaboride (LaB6) source. For tungsten,

this will be of the form of either a hairpin-style filament, or a small spike-

shaped filament. LaB6 sources utilize small single crystals. By connecting this

gun to a high voltage source (typically ~100–300 kV) the gun will, given

sufficient current, begin to emit electrons either by thermionic or field electron

emission into the vacuum. This extraction is usually aided by the use of

a Wehnelt cylinder. Once extracted, the upper lenses of the TEM allow for the

formation of the electron probe to the desired size and location for later

interaction with the sample.

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Figure 2.8 Schematic layout of optical components in a basic TEM

Manipulation of the electron beam is performed using two physical

effects. The interaction of electrons with a magnetic field will cause electrons

to move according to the right hand rule, thus allowing for electromagnets to

manipulate the electron beam. The use of magnetic fields allows for the

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formation of a magnetic lens of variable focusing power, the lens shape

originating due to the distribution of magnetic flux. Additionally, electrostatic

fields can cause the electrons to be deflected through a constant angle.

Coupling of two deflections in opposing directions with a small intermediate

gap allows for the formation of a shift in the beam path. The optical

configuration of a TEM can be rapidly changed, unlike that for an optical

microscope, as lenses in the beam path can be enabled, have their strength

changed, or be disabled entirely simply via rapid electrical switching, the

speed of which is limited by effects such as the magnetic hysteresis of the

lenses.