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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.
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