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CHAPTER 2
Basic Principles of Experimental
Techniques
Used for Characterizing ZnO
Nanostructures
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2.1. Introduction
In this chapter, a brief description of surface, structural, optical and thermal
techniques used in the characterization of the synthesized ZnO nanostructures are
given.
The following techniques were used for characterization purpose.
1. EDAX (Energy Dispersive Analysis of X-rays)
2. XRD (X-ray powder diffraction)
3. TEM (Transmission Electron Microscopy)
4. Raman Spectroscopy
5. UV-Vis-NIR Spectroscopy
6. Spectrofluorometer- PL (Photoluminescence)
7. FTIR (Fourier Transform Infrared Spectroscopy)
8. XPS (X- ray Photoelectron Spectroscopy)
9. TGA (Thermogravimetry Analysis)
Energy dispersive analysis of X-ray (EDAX), it can provide information about
chemical composition of the materials. The structural parameters of ZnO
nanostructures are determined by XRD, selected area electron diffraction patterns
(SAED) and Raman spectroscopy. XRD and TEM were used to identify the
crystalline phases and crystal sizes of the ZnO nanostructures. The optical properties
of the samples were investigated by means of using UV-Vis–NIR spectroscopy,
photoluminescence (PL) and FTIR. To analyze the chemical states of the constituent
elements, XPS measurements were performed. TGA measurements are used primarily
to determine the composition of materials and to predict their thermal stability at
temperatures. The basic principle, working and experimental set up of instruments of
above mentioned techniques are described below:
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2.2 Characterization Methods
2.2.1 Energy Dispersive Analysis of X-rays (EDAX)
2.2.1.1 Basic principle
Energy dispersive analysis of X-ray (EDAX) is an analytical technique used
for the elemental analysis or chemical characterization of a sample. It is one of the
variants of X-ray fluorescence spectroscopy which relies on the investigation of a
sample through interactions between electromagnetic radiation and matter, analyzing
X-rays emitted by the matter in response to being hit with charged particles. Its
characterization capabilities are due in large part to the fundamental principle that
each element has a unique atomic structure allowing X-rays that are characteristic of
an element's atomic structure to be identified uniquely from one another. To stimulate
the emission of characteristic X-rays from a specimen, a high energy beam of charged
particles such as electrons 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 [1-2]. The experimental set up for EDAX is shown in figure 2.1.
2.2.1.2 Experimental Set Up
Figure 2.2 shows a layout diagram EDAX attached to an SEM. An EDAX
system comprises three basic components that must be designed to work together to
achieve optimum results: the X-ray detector or spectrometer, the pulse processor, and
the analyzer.
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Figure 2.1 Experimental set up of Energy Dispersive analysis of X-rays
Specifications:
Model : Scanning Electron Microscope XL 30 ESEM with EDAX
Resolution : With LaB6 filament 2 nm at 30 kV, With W filament 3.5 nm at
30 kV
Accelerating Voltage : 0.2 to 30 kV
Magnification : up to 2,50,000 X
Figure 2.2 Layout diagram of EDAX
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The Energy-Dispersive Spectrometer
The ED spectrometer converts the energy of each individual X-ray into a
voltage signal of proportional size. This is achieved through a three stage process.
Firstly the X-ray is converted into a charge by the ionization of atoms in a
semiconductor crystal. Secondly this charge is converted into the voltage signal by the
FET preamplifier. Finally the voltage signal is input into the pulse processor for
measurement. The output from the preamplifier is a voltage „ramp‟ where each X-ray
appears as a voltage step on the ramp. EDAX detectors are designed to convert the
X-ray energy into the voltage signal as accurately as possible. At the same time
electronic noise must be minimized to allow detection of the lowest X-ray energies.
The Role of the Pulse Processor
The charge liberated by an individual X-ray photon appears at the output of
the preamplifier as a voltage step on a linearly increasing voltage ramp. The
fundamental job of the pulse processor is to accurately measure the energy of the
incoming X-ray, and give it a digital number that is used to add a count to the
corresponding channel in the computer. It must also optimize the removal of noise
present on the original X-ray signal. It needs to recognize quickly and accurately a
wide range of energies of X-ray events from 110 eV up to 80 keV. It also needs to
differentiate between events arriving in the detector very close together in time;
otherwise the combination produces the spectrum artifact called pulse pile-up [3].
2.2.2 X-ray Diffractometer (XRD)
2.2.2.1 Basic Principle
Radiations striking a material may be scattered or absorbed. X-rays, high
energy electrons and neutrons are used to extract structural information of the crystals
lattice. Incident radiations of sufficiently smaller wavelength interact elastically with
the regular arrays of atoms in a crystal lattice to yield a diffraction pattern. Both
diffraction angles and the intensities in various diffracted beams are a sensitive
function of the crystalline structure. The diffracted angles depends on the Bravais
points lattice and the unit cell dimensions, while the diffracted intensities depends on
the atomic numbers of constituent atoms and their geometrical relationship with
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respect to the lattice points. The condition for a crystalline material to yield a discrete
diffraction pattern is that the wavelength of incident radiation should be comparable
to, or less than the determining the angular distribution of the peak intensities in the
diffraction pattern from a regular crystal lattice is the Bragg‟s equation:
n λ = 2 d sinθ ------- (2.1)
where „n‟ is an integer referring to the order of reflection, „λ‟ is wavelength of
the radiation, „d‟ is the spacing between the crystal lattice planes responsible for a
particular diffracted beam, and „θ‟ is the angle that incident beam makes with lattice
planes. The path difference between the incident beam and the beams reflected from
two consecutive crystal planes is shown in figure 2.3. An X-ray diffractometer
comprise of a source of X-rays, the X-ray generator, a diffractometer assembly, and
X-ray data collection and analysis system. The diffractometer assembly controls the
alignment of the beam, as well as the position and orientation of the specimen and the
X-ray detector [4-7].
Figure 2.3 A schematic of Bragg’s reflection from a crystal
2.2.2.2 Experimental Set Up
Figure 2.4 shows the experimental set up of X-ray Diffractometer, which
consist of three basic elements: X-ray source, the sample under investigation and
detector to pick up the diffracted X-rays. In XRD, X-rays are generated within a
sealed tube that is under vacuum. A current is applied that heats a filament within the
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tube, the higher the current the greater the number of electrons emitted from the
filament. A high voltage, typically 15-60 kilovolts, is applied within the tube. This
high voltage accelerates the electrons, which then hit a target, commonly made of
copper. When these electrons hit the target, X-rays are produced. The wavelength of
these X-rays is characteristic of that target. These X-rays are collimated and directed
onto the sample, which has been ground to a fine powder. As the sample and detector
are rotated, the intensity of the reflected X-rays is recorded. When the geometry of the
incident X- rays impinging the sample satisfies the Bragg Equation, constructive
interference occurs and a peak in intensity occurs. A detector detects the X-ray signal;
the signal is then processed either by a microprocessor or electronically, converting
the signal to a count rate.
Figure 2.4 Experimental set up of X-ray Diffractometer
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Specifications:
Model : XRD Diffractometer (powder) Philips Xpert MPD
Source : Cu target X-Ray tube
Operating power of the tube : 2 kW
Detector : Xe-filled Count rate or Proportional detector
Software : JCPDS database for powder diffractometry
Operation Modes : Vertical & Horizontal
Accuracy : ± 0.0025
2q range : 300 to 2100
2q Measurement range : 00 to 1360
Diffractometer radius : 130 to 230 mm
Figure 2.5 Schematic representation of sample mounted on a goniometer stage,
which can be rotated about one or more axes, and a detector which
travels along the focusing circle in the Bragg-Brentano geometry
The geometry of an X-ray diffractometer is such that the sample rotates in the
path of the collimated X-ray beam at an angle θ while the X-ray detector is mounted
on an arm to collect the diffracted X-rays and rotates at an angle of 2θ. The instrument
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used to maintain the angle and rotate the sample is termed a goniometer. The
experimental set up is shown in figure 2.4. The intensity of diffracted X-rays is
continuously recorded as the sample and detector rotate through their respective
angles. A peak in intensity occurs when the lattice planes with d-spacing are
appropriate to diffract X-rays at that value of θ. Although each peak consists of two
separate reflections (Kα1 and Kα2), at small values of 2θ the peak locations overlap.
Greater separation occurs at higher values of θ. These combined peaks are treated as
one. The 2λ position of the diffraction peak is typically measured as the center of the
peak at 80% peak height [8-10]. Bragg-Brentano geometry is shown in figure 2.5.
2.2.3 Transmission Electron Microscopy (TEM)
2.2.3.1 Basic Principle
Transmission electron microscopy (TEM) is a microscopy technique
whereby a beam of electrons is transmitted through an ultra thin specimen, interacting
with the specimen as it passes through. An image is formed from the interaction of the
electrons transmitted through the specimen; the image is magnified and focused onto
an imaging device, such as a fluorescent screen, on a layer of photographic film, or to
be detected by a sensor such as a CCD camera.
The transmission electron microscope (TEM) operates on the same basic
principles as the light microscope but uses electrons instead of light. What you can
see with a light microscope is limited by the wavelength of light. TEM use electrons
as "light source" and their much lower wavelength make it possible to get a resolution
a thousand times better than with a light microscope. The possibility for high
magnifications has made the TEM a valuable tool in both medical, biological and
materials research.
TEMs are 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 tens of thousands times smaller than the smallest resolvable object in a light
microscope. TEM forms a major analysis method in a range of scientific fields, in
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both physical and biological sciences. TEMs find application in cancer research,
virology, materials science as well as pollution and semiconductor research.
At smaller magnifications TEM image contrast is due to absorption of
electrons in the material, due to the thickness and composition of the material. At
higher magnifications complex wave interactions modulate the intensity of the image,
requiring expert analysis of observed images. Alternate modes of use allow for the
TEM to observe modulations in chemical identity, crystal orientation, electronic
structure and sample induced electron phase shift as well as the regular absorption
based imaging.
2.2.3.2 Experimental Set Up
From the top down, 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 electron
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. The layout diagram of basic TEM is shown in
figure 2.6. 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 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.
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Figure 2.6 Layout of optical components in a basic TEM
TEM the ability to change magnification simply by modifying the amount of
current that flows through the coil; quadrupole or hexapole lenses. The quadrupole
lens is an arrangement of electromagnetic coils at the vertices of the square, enabling
the generation of a lensing magnetic fields, the hexapole configuration simply
enhances the lens symmetry by using six, rather than four coils. Typically a TEM
consists of three stages of lensing. The stages are the condenser lenses, the objective
lenses, and the projector lenses. The condenser lenses are responsible for primary
beam formation, whilst the objective lenses focus the beam down onto the sample
itself. The projector lenses are used to expand the beam onto the phosphor screen or
other imaging device, such as film. The magnification of the TEM is due to the ratio
of the distances between the specimen and the objective lens' image plane. Figure 2.7
shows experimental set up for TEM. Imaging systems in a TEM consist of a phosphor
screen, which may be made of fine (10-100 μm) particulate zinc sulphide, for direct
observation by the operator. Optionally, an image recording system such as film based
or doped YAG (Yttrium aluminium garnet) screen coupled CCDs [11-17].
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Figure 2.7 Experimental set up of Transmission Electron Microscope
Specifications:
Model : TEM with CCD camera Philips, Tecnai 20
Electron Source : W emitter and LaB6
Accelerating Voltage : 200 kV
Objective lens : S- TWIN
Point Resolution : 0.27 nm or better
Line Resolution : 2.0 nm or better
Magnification : 25X to 750000X or higher
2.2.4 Raman Spectroscopy
2.2.4.1 Basic Principle
Raman spectroscopy is a useful technique for the identification of a wide
range of substances – solids, liquids and gases. It is a straightforward, non-destructive
technique requiring no sample preparation. Raman spectroscopy involves illuminating
a sample with monochromatic light and using a spectrometer to examine light
scattered by the sample.
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At the molecular level photons can interact with matter by absorption or
scattering processes. Scattering may occur either elastically or inelastically. The
elastic process is termed Rayleigh scattering, whilst the inelastic process is termed
Raman scattering. The electric field component of the scattering photon perturbs the
electron cloud of the molecule and may be regarded as exciting the system to a
„virtual‟ state. Raman scattering occurs when the system exchanges energy with the
photon and the system subsequently decays to vibrational energy levels above or
below that of the initial state. The frequency shift corresponding to the energy
difference between the incident and scattered photon is termed the Raman shift.
Depending on whether the system has lost or gained vibrational energy, the Raman
shift occurs either as an up- or down- shift of the scattered photon frequency relative
to that of the incident photon. The down-shifted and up-shifted components are called
respectively the Stokes and anti-Stokes lines. Stokes radiation occurs at lower energy
(longer wavelength) than the Rayleigh radiation, and anti-stokes radiation has greater
energy. Figure 2.8 shows the energy of the vibrational level of the sample material. A
plot of detected number of photons versus Raman shift from the incident laser energy
gives a Raman spectrum. Different materials have different vibrational modes, and
therefore characteristic Raman spectra. This makes Raman spectroscopy a useful
technique for material identification.
A molecular polarizability change or amount of deformation of the electron
cloud, with respect to the vibrational coordinate is required for the molecule to exhibit
the Raman effect. The amount of the polarizability change will determine the
intensity, whereas the Raman shift is equal to the vibrational level that is involved.
Homonuclear diatomic molecules such as H2, N2, O2, etc which do not show infrared
spectra since they do not possess a permanent dipole moment do show Raman spectra
since their vibration is accompanied by a change in polarizability of the molecule.
Thus, Raman spectroscopy permits us to examine the vibrational spectra of
compounds that do not lend themselves to IR absorption spectroscopy.
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Figure 2.8 The vibrational level of the material
2.2.4.2 Experimental Set-Up
The Raman measurements are made using the instrumentation shown in
Figure 2.9. The output light from a laser is focused on the sample cell. The scattered
light is collected at right angles to the excitation laser beam and focused onto the
polychromator where it is dispersed and detected by a charge coupled device camera.
In recent years Raman spectroscopy has become even more accurate and
easier due to advancements in optics, laser and computer technology. Charge Coupled
Device (CCD) detectors have enormously helped the use of Raman spectroscopy by
allowing scientist to take data quicker and with more precision that they were able to
with the older photomultiplier tubes. The CCD has an array of detectors that can look
at a range of wavelengths at one time greatly reducing the collection time. In older
spectrometers with photomultiplier tubes the grating of the spectrometer would
physically move in small increments over a period of time to take a scan of the
spectrum which is a very time consuming process.
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Figure 2.9 Schematic diagram of Raman spectrometer
Raman spectroscopy can be used on liquids, solids and gases making it very
versatile for studying various materials. Because of the distinct spectra that certain
classes of materials give off, due to their structural arrangement, Raman spectroscopy
can be used to determine the composition of unknown substances. This also makes
Raman spectroscopy ideal for qualitative analysis of materials. In Raman
spectroscopy no probe physically touches the material the laser light is the only thing
to disturb the sample, this means that the material is not disturbed by the probe
physically touching it and in some cases is the only way to accurately study a
material.
Surface Enhanced Raman Spectroscopy (SERS) and Resonance Raman Effect
(RRE) are different types of Raman spectroscopy. The goal of these two processes is
to enhance the weak signal of the Raman spectra. Micro Raman spectroscopy (MRS)
is another type of Raman spectroscopy and this process reduces the spot size of the
light source on the sample, which is helpful if a small area of the sample is to be
observed. It is also used to reduce damage or heating of the sample by the laser light
[18-20]. The experimental set up of Raman Spectrometer is shown in the figure 2.10
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Figure 2.10 Experimental set up of Raman Spectrometer
2.2.5 UV-Vis-NIR Spectrophotometer
2.2.5.1 Basic Principle
When a beam of radiation strikes any object it can be absorbed, transmitted,
scattered, reflected or it can excite fluorescence. With scattering it can be considered
that the radiation is first absorbed then almost instantaneously completely re-emitted
uniformly in all directions, but otherwise unchanged. With fluorescence a photon is
first absorbed and excites the molecule to a higher energy state, but the molecule then
drops back to an intermediate energy level by re-emitting a photon. Since some of the
energy of the incident photon is retained in the molecule (or is lost by a non-radiative
process such as collision with another molecule) the emitted photon has less energy
and hence a longer wavelength than the absorbed photon. Like scatter, fluorescent
radiation is also emitted uniformly in all directions. The processes concerned in the
absorption spectroscopy are absorption and transmission. The condition under which
sample is examined are absorption are chosen to keep reflection, scatter and
fluoresces to a minimum. An optical spectrometer records the wavelength at which
absorption occurs together with degree of absorption at each wavelength. Ultraviolet
and visible spectroscopy is used for quantitative analysis of the sample. The
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absorption of radiation in a sample follows the Beer-Lambert law which states that the
concentration of a substance in a sample (thin film/ solution) is directly proportional
to the absorbance “A”.
Absorbance A = constant × concentration × cell length -------- (2.2)
The law is only true for monochromatic light that is light of a single
wavelength or narrow band of wavelengths, and provided that the physical or
chemical state of the substance does not change with concentration.
Figure 2.11 Shows Bouguer-Beer Rule
The following relationship is established when light with intensity „Io‟ is
directed at a material and light with intensity „I‟ is transmitted. In this instance the
value I/Io is called transmittance (T) and the value I/Io*100 is called transmission rate
(T %). The value log (1/T) = log (Io/I) is called absorbance (Abs).
T = I/Io = 10 – kcl -------- (2.3)
Abs = log (1/T) = log (Io/I) = – kcl -------- (2.4)
here „k‟ is proportionality constant
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„L‟ is length of light path through the cuvette in cm
As can be seen from the above formulas, transmittance is not proportional to
sample concentration. However, absorbance is proportional to sample concentration
(Beer's law) along with optical path (Bouguer's law). Figure 2.11 shows simple
pictorial representation of Bouguer-Beer rule. In addition, when the optical path is
1cm and the concentration of the target component is 1mol/l, the proportionality
constant is called the molar absorption coefficient and expressed using the symbol ε.
The molar absorption coefficient is a characteristic value of a material under certain,
specific conditions. Finally, stray light, generated light, scattered light, and reflected
light must not be present in order for the Bouguer-Beer rule to apply.
2.2.5.2 Experimental Set Up
Visible and UV-Visible spectrophotometers consist of a number of
fundamental components
1. Light Sources (UV and visible)
2. Wavelength selector (monochromator)
3. Sample containers
4. Detector
5. Signal processor and readout
Figure 2.12 and figure 2.13 shows the experimental set up of UV-Vis
Spectrometer and its layout diagram respectively.
Figure 2.12 UV-Vis-NIR Spectrophotometer
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Specifications:
Model : UV-Vis-NIR Spectrometer Perkin Elmer Lambda 19
Lamp : Deuterium (UV), Tungsten-Halogen (VIS/NIR)
Detectors : Photomultiplier tube for UV/Vis, Lead-Sulphide cell (PbS)
for NIR
Wavelength Range : 185-3200 nm for Absorbance
Scan Speed : 0.3 to 1200 nm/min
Wavelength Accuracy : ± 0.15 nm for UV/VIS & ± 0.6 nm for NIR
Base line flatness : ± 0.001 Ǻ, 4 nm slit
Photometric Accuracy : ± 0.003 Ǻ
Figure 2.13 Optical diagram of spectrophotometer
Besides the sun, the most conveniently available source of visible radiation
with which we are familiar is the tungsten lamp. If the current in the circuit supplying
such a lamp is gradually increased from zero, the lamp filament at first can be felt to
be emitting warmth, then glows dull red and the gradually brightens until it is emitting
an intense white light and a considerable amount of heat. For the UV region the most
common source is the deuterium lamp and a UV-Visible spectrometer will usually
have both lamp types to cover the entire wavelength range.
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In complex molecules the energy levels are more closely spaced and photons
of near ultraviolet and visible light can affect the transition. These substances,
therefore, will absorb light in some areas of the near ultraviolet and visible regions.
The vibrational energy states of the various parts of a molecule are much closer
together than the electronic energy levels and thus photons of lower energy
(longer wavelength) are sufficient to bring about vibrational changes. Light
absorption due to only to vibrational changes occurs in the infrared region. The
rotational energy states of molecules are so closely spaced that light in the far infrared
and microwave regions of the electromagnetic spectrum has enough energy to cause
these small changes.
Following the light source is a monochromator, the purpose of which is to
filter light and select a specific wavelength by using either a prism or a diffraction
grating. Each monochromatic (single wavelength) beam in turn is split into two equal
intensity beams by a half-mirrored device. One beam, the sample beam, passes
through a small transparent container (cuvette) containing a solution of the compound
being studied in a transparent solvent. The other beam, the reference, passes through
an identical cuvette containing only the solvent. The containers for the sample and
reference solution must be transparent to the radiation which will pass through them.
Quartz or fused silica cuvettes are required for spectroscopy in the UV-Vis region.
The light sensitive detector follows the sample chamber and measures the intensity of
light transmitted from the cuvettes and passes the information to a meter that records
and displays the value to the operator on an LCD screen. The intensities of these light
beams are then measured by electronic detectors and compared. Today two kinds of
detectors are of use in UV/Vis spectrophotometry the phototube and the
photomultiplier tube. The phototube or photocell functions by generating an electric
current and the photomultiplier tube, which is more sensitive, relies on Planck‟s
photoelectric effect. The intensity of the reference beam, which should have suffered
little or no light absorption, is defined as I0. The intensity of the sample beam is
defined as I. Over a short period of time, the Spectrometer automatically scans all the
component wavelengths in the manner described. The ultraviolet (UV) region scanned
is normally from 200 to 400 nm, and the visible portion is from 400 to 800 nm.
Therefore, this method is excellent to both determine the concentration and identify
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the molecular structure or the structural changes. Spectrophotometers are also useful
to study the changes in the vibration and conformation energy levels after and before
an interaction with a substrate, or another molecule [21-23].
2.2.6 Spectrofluorometer – PL (Photoluminescence)
2.2.6.1 Basic Principle
When light of sufficient energy is incident on a material, photons are
absorbed and electronic excitations are created. Eventually, these excitations relax and
the electrons return to the ground state. If radiative relaxation occurs, the emitted light
is called photoluminescence (PL). This light can be collected and analyzed to yield a
wealth of information about the photo excited material. The PL spectrum provides the
transition energies, which can be used to determine electronic energy levels. The PL
intensity gives a measure of the relative rates of radiative and non radiative
recombination. Variation of the PL intensity with external parameters like
temperature and applied voltage can be used to characterize further the underlying
electronic states and bands.
2.2.6.2 Experimental Set Up
Figure 2.14 shows the experimental set up of spectrofluorometer, which
having the following apparatus.
The Source
Starting with a Xenon source that supplies prime UV performance, we mount
the bulb vertically, since horizontal mounting leads to sagging of the arc that increases
instability and decreases the useful life.
The Xenon source is focused onto the entrance slit of the excitation
monochromator with an elliptical mirror. Besides ensuring efficient collection, the
reflective surface keeps all wavelengths focused on the slit, unlike lenses with
chromatic aberrations that make them totally efficient only at one wavelength.
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The Slits
The slits themselves are bilaterally, continuously adjustable from the computer
in units of band pass or millimeters. This preserves maximum resolution and instant
reproducibility.
Figure 2.14 Experimental set up of FluoroMax - Compact
Spectrofluorometer
The Excitation Monochromator
The excitation monochromator is an aspheric design which ensures that the
image of the light diffracted by the grating fits through the slit. The gratings
themselves are plane, blazed gratings that avoid the two major disadvantages of the
more common concave holographic gratings: poor polarization performance and
inadequate imaging during scans that throws away light. The unique wavelength drive
scans the grating at speeds as high as 80nm/s. The grating grooves are blazed to
provide maximum light in the UV and visible region.
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The Reference Detector
Before the excitation light reaches the sample, a photodiode reference detector
monitors the intensity as a function of time and wavelength. The photodiode detector
has a wider wavelength response than the older, traditional rhodamine-B quantum
counter, and requires no maintenance.
The Sample Chamber
A spacious sample chamber is provided to allow the use of a long list of
accessories for special samples, and encourages the user to utilize a variety of sample
schemes.
The Emission Monochromator
All the outstanding features of the excitation monochromator are also
incorporated into the emission monochromator. Gratings are blazed to provide
maximum throughput in the visible region.
The Emission Detector
Emission detector electronics employ photon-counting for the ultimate in low
light level detection. Photon-counting concentrates on signals that originate from
fluorescence emission, ignoring smaller signals originating in the detector tube
(PMT). The more common method of analog detection (used by lower performance
fluorometers) simply adds noise and signal together, masking weak emissions. The
emission detector housing also contains an integral high-voltage supply which is
factory set to provide the signal-to-noise ratio.
Computer Control
The entire control of the FluoroMax-4 originates in your PC with our
revolutionary new Fluor Essenc software and is transmitted through a serial link. On
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start-up, the system automatically calibrates and presents itself for new experiments or
stored routines instantly called from memory.
Figure 2.15 shows the block diagram of fluorescence spectrometer.
Fluorescence spectrometers use laser sources, which contains wavelength selectors,
sample illumination, detectors and corrected spectra.
Wavelength Selectors
Portable, inexpensive fluorescence spectrometers use filters as wavelength
selectors. Such instruments (filter fluorometers) are used when it is sufficient to
measure fluorescence intensity at a single excitation and emission wavelength.
Moreover filters can transmit a very large number of photons from source to sample
and from sample to detector. Thus, filter instruments may be used in ultratrace
analysis, wherein it is crucial to maximize the fluorescence signal that impinges on
the detector, at the cost of decreased selectivity. Most fluorometers in laboratory
environments use grating monochromators as excitation and emission wavelength
selectors. Usually, only moderate spectral resolution (1 to 2 nm) is needed.
Figure 2.15 Block diagram of fluorescence spectrometer
Sample Illumination
The most common arrangement is the right-angle geometry in figure 2.15,
wherein fluorescence is viewed at a 90° angle relative to the direction of the exciting
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light beam. This geometry is suitable for weakly absorbing solution samples. For
solutions that absorb strongly at the excitation wavelength, and for solids (or samples
adsorbed on solid surfaces, such as thin-layer chromatography plates), a front surface
geometry often is preferable; here, fluorescence is viewed from the face of the sample
on which the exciting radiation impinges. For solution samples, rectangular 1-cm
glass or fused silica cuvettes with four optical windows are usually used.
Detectors
The fluorescence signal for an analyze present at low concentration is small;
thus, a key requirement for a detector is its ability to detect weak optical signals.
A photomultiplier tube (PMT) is used as the detector in most fluorescence
spectrometers. PMTs used in fluorometry are chosen for low noise and high
sensitivity, and are sometimes operated at sub ambient temperatures to improve their
signal-to-noise ratios. The main shortcoming of a PMT is that it is a single-channel
detector. To obtain a spectrum, one must mechanically scan the appropriate
monochromator across the wavelength range encompassed by the spectrum, which
may be 50 nm or more. Thus, it is difficult to obtain spectra of transient species or
analysts that remain in the observation region for a short time (such as elements from
chromatographic columns). It has long been recognized that a multichannel
instrument using an array of detectors would be preferable for such applications
because the entire spectrum could be viewed at once. UV/Vis absorption
spectrometers with array detectors are commercially available and widely used. Until
recently, no electronic array detector has been competitive with a PMT in the
detection of weak optical signals. That situation is changing as new classes of
electronic array detectors are developed and improved. At present, the most promising
electronic array detector for fluorometry is the charge-coupled device (CCD).
Fluorescence instruments using CCDs or other high-performance array detectors are
not numerous, but will become more common in the future.
79
Corrected Spectra
Most fluorometers are single-beam instruments. Excitation and fluorescence
spectra obtained using such an instrument are distorted, due to variation of source
power or detector sensitivity with wavelength. Spectra of the same sample obtained
using two different fluorometers may therefore be quite dissimilar; even changing the
source or detector in a fluorometer may alter the apparent fluorescence or excitation
spectrum of a compound. It is possible instrumentally to eliminate these artifacts, and
several manufacturers offer instruments that can generate corrected spectra. Because
most published fluorescence spectra are uncorrected, they cannot readily be
reproduced by other investigators. Hence, there are few extensive and broadly useful
data bases of fluorescence spectra. That a fluorescence spectrometer is a single-beam
instrument also means that fluctuations in the power output of the excitation source
produce noise. This problem may be solved by splitting off a portion of the source
output and viewing it with a second detector, and electronically rationing the observed
fluorescence signal to the output of the detector that is used to monitor the source
power. High-performance commercial fluorometers have this capability.
Photoluminescence Uses:
Band Gap Determination
The most common radiative transition in semiconductors is between states in the
conduction and valence bands, with the energy difference being known as the
band gap. Band gap determination is particularly useful when working with new
compound semiconductors.
Impurity Levels and Defect Detection
Radiative transitions in semiconductors also involve localized defect levels. The
photoluminescence energy associated with these levels can be used to identify
specific defects, and the amount of photoluminescence can be used to determine
their concentration.
80
Recombination Mechanisms
The return to equilibrium, also known as "recombination," can involve both
radiative and non radiative processes. The amount of photoluminescence and its
dependence on the level of photo-excitation and temperature are directly related
to the dominant recombination process. Analysis of photoluminescence helps to
understand the underlying physics of the recombination mechanism.
Material Quality
In general, non-radiative processes are associated with localized defect levels,
whose presence is detrimental to material quality and subsequent device
performance. Thus, material quality can be measured by quantifying the amount
of radiative recombination [24].
2.2.7 Fourier Transform Infrared Spectroscopy (FTIR)
2.2.7.1 Basic Principle
FTIR stands for Fourier Transform Infrared, the preferred method of infrared
spectroscopy. It is an easy way to identify the presence of certain functional groups in
a molecule. Analysis by infrared spectroscopy is based on the fact that molecules have
specific frequencies of internal vibrations. These frequencies occur in the infrared
region of the electromagnetic spectrum: ~ 4000 cm-1
to ~ 200 cm-1
.
When a sample is placed in a beam of infrared radiation, the sample will
absorb radiation at frequencies corresponding to molecular vibrational frequencies,
but will transmit all other frequencies. The frequencies of radiation absorbed are
measured by an infrared spectrometer, and the resulting plot of absorbed energy vs.
frequency is called infrared spectrum of the material [1].
Identification of a substance is possible because different materials have
different vibrations and yield different infrared spectra. Furthermore, from the
frequencies of the absorptions it is possible to determine whether various chemical
81
groups are present or absent in a chemical structure. In addition to the characteristic
nature of the absorptions, the magnitude of the absorption due to a given species is
related to the concentration of that species.
Fourier Transform Infrared (FTIR) spectrometry was developed in order to
overcome the limitations encountered with dispersive instruments. The main difficulty
was the slow scanning process. A method for measuring all of the infrared frequencies
simultaneously, rather than individually, was needed. A solution was developed
which employed a very simple optical device called an interferometer. The
interferometer produces a unique type of signal which has all of the infrared
frequencies “encoded” into it. The signal can be measured very quickly, usually on
the order of one second or so. Thus, the time element per sample is reduced to a
matter of a few seconds rather than several minutes. Most interferometers employ a
beam-splitter which takes the incoming infrared beam and divides it into two optical
beams. One beam reflects off of a flat mirror which is fixed in place. The other beam
reflects off of a flat mirror which is on a mechanism which allows this mirror to move
a very short distance (typically a few millimeters) away from the beam-splitter. The
two beams reflect off of their respective mirrors and are recombined when they meet
back at the beam-splitter. Because the path that one beam travels is a fixed length and
the other is constantly changing as its mirror moves, the signal which exits the
interferometer is the result of these two beams “interfering” with each other. The
resulting signal is called an interferogram which has the unique property that every
data point (a function of the moving mirror position) which makes up the signal has
information about every infrared frequency which comes from the source. This means
that as the interferogram is measured; all frequencies are being measured
simultaneously. Thus, the use of the interferometer results in extremely fast
measurements. Because the analyst requires a frequency spectrum (a plot of the
intensity at each individual frequency) in order to make identification, the measured
interferogram signal cannot be interpreted directly. A means of “decoding” the
individual frequencies is required. This can be accomplished via a well-known
mathematical technique called the Fourier transformation. This transformation is
performed by the computer which then presents the user with the desired spectral
information for analysis.
82
It can identify unknown materials
It can determine the quality or consistency of a sample
It can determine the amount of components in a mixture
2.2.7.2 Experimental Set Up
The Sample Analysis Process
The normal instrumental process is as follows:
1. The Source: Infrared energy is emitted from a glowing black-body source.
This beam passes through an aperture which controls the amount of energy
presented to the sample (and, ultimately, to the detector).
2. The Interferometer: The beam enters the interferometer where the “spectral
encoding” takes place. The resulting interferogram signal then exits the
interferometer.
3. The Sample: The beam enters the sample compartment where it is transmitted
through or reflected off of the surface of the sample, depending on the type of
analysis being accomplished. This is where specific frequencies of energy,
which are uniquely characteristic of the sample, are absorbed.
4. The Detector: The beam finally passes to the detector for final measurement.
The detectors used are specially designed to measure the special interferogram
signal.
5. The Computer: The measured signal is digitized and sent to the computer
where the Fourier transformation takes place. The final infrared spectrum is
then presented to the user for interpretation and any further manipulation.
83
Figure 2.16 Block diagram of FTIR spectrometer
Figure 2.16 shows the block diagram of FTIR spectrometer. Because there
needs to be a relative scale for the absorption intensity, a background spectrum must
also be measured. This is normally a measurement with no sample in the beam. This
can be compared to the measurement with the sample in the beam to determine the
“percent transmittance”. This technique results in a spectrum which has all of the
instrumental characteristics removed. Thus, all spectral features which are present are
strictly due to the sample. A single background measurement can be used for many
sample measurements because this spectrum is characteristic of the instrument itself
[25]. Figure 2.17 shows the experimental set up of FTIR spectrometer.
84
Figure 2.17 Experimental set up of FTIR spectrometer
Specifications:
Model : Perkin Elmer Spectrum GX
Sample : Solid, Liquid or Gas
Operating Mode : NIR and MIR
Scan Range : 15600 to 30 cm-1
Optical system : Source NIR: 15,200 – 1,200 cm-1
Beam splitter KBr : 7,800 - 370 cm-1
Detector MIRTGS : 10,000 - 220 cm-1
Optimum Range : 7,800 - 1,200 cm-1
OPD Velocity : 0.20 cm/s
Inferferogram Direction : Bi-Direction
Scan Time : 20 scan/second
Resolution : 0.15 cm-1
Single Beam/Ratio : Single
Detector : MIRTGS
85
2.2.8 X- ray Photoelectron Spectroscopy (XPS)
2.2.8.1 Basic Principle
XPS (X-ray Photoelectron Spectroscopy) or ESCA (Electron Spectroscopy
for Chemical Analysis) is based on the principle that X-rays hitting atoms generate
photoelectrons. It is a typical example of a surface-sensitive technique. Only electrons
that are generated in the top few atomic layers are detected. In this way quantitative
information can be obtained about the elemental composition of the surface of all
kinds of solid material (insulators, conductors, polymers). An important strength of
XPS is that it provides both elemental and chemical information.
Bombarding a sample in vacuum with X-rays gives rise to the emission of electrons.
If monochromatic X-rays are used with a photon energy hν, the kinetic energy of the
emitted electrons Ke is given by:
Ke = hν – Be – φ ------- (2.5)
where „Be‟ is the binding energy of the atomic orbital from which the electron
originates and „φ‟ is the work function. The work function is the minimum amount of
energy an individual electron needs to escape from the surface. Each element
produces a unique set of electrons with specific energies. By measuring the number of
these electrons as a function of kinetic (or binding) energy, an XPS spectrum is
obtained. All elements can be detected, except H and He. Binding energies of
photoelectrons depend on the chemical environment of the atoms [26]. Accurate
measurement of the exact peak position of the elements present gives information on
the chemical state of these elements. Figure 2.18 shows the photoemission process.
86
Figure 2.18 Photoemission process
2.2.8.2 Experimental Set Up
Vacuum System
The UHV system of the experiment is based on a turbo molecular pump with
an upstream rotary vane pump. The turbo molecular pump works very efficiently for
heavy gases. Light gases, mainly H2, will be pumped slowly because of high particle
speeds. After some time the turbo pump reaches a final pressure of about 10-9
mbar.
As a residual gas in the recipient remains mainly water, that sticks on the steel walls
due to its dipole structure. Therefore, the chamber is baked out under UHV conditions
for some hours up to days. After cooling a final pressure down to 10-11
mbar can be
reached.
X-ray source
The X-ray source consists of a cathode (filament), which emits thermal
electrons through heating (usual emission current of about 30 mA), and an anode to
which the electrons are accelerated by applying a high voltage of typically 9 kV to
12 kV. The construction of the X-ray source allows to choose between Mg and Al as
the material of the anode, each with a characteristic emission spectrum. This spectrum
includes Bremsstrahlung and characteristic radiation. The short-wave radiation of the
bremsstrahlung will be largely absorbed by an approximately 1 micron thick Al
87
window. Figure 2.19 shows the schematic design of a hemispherical analyzer with
channeltron.
Electron energy analyzer
The analyzer measures the kinetic energy of the photoelectrons by a
hemispherical capacitor. It consists of an electrical lens system and two metallic
hemispheres, (figure 2.19). One crucial parameter of such a setup is the pass energy
Epas. It has influence on the resolution and the background noise. The highest
contribution to the noise in the spectrum stems from electrons excited by secondary
processes which have low kinetic energy. To reduce the background we select out the
electrons with low kinetic energy by applying a pass voltage. This can be realized in
many ways for example: slowing down the electrons before they enter the electrical
lens. Such that only electrons with Ekin > Epas can pass. In our setup the selection is
realized through a voltage difference (K+ – K
–) between the hemispherical plates
(figure 2.19). In this experiment a channeltron is used as a detector consisting of a
glass tube which is covered with a conducting material as high electrical resistance.
This works on the principle of a secondary electron multiplier. The multiplication
level is typically 106
to 108 with the corresponding electronics the detector amplifies
pulses and recorded them.
Figure 2.19 Schematic design of a hemispherical analyzer with channeltron
88
2.2.9 Thermo gravimetric Analysis (TGA)
2.2.9.1 Basic Principle
Thermogravimetric analysis is used to determine changes in sample weight
which may result from chemical or physical transformation, as a function of
temperature or time. Isothermal TG measure weight change as a function of time at a
constant temperature. As materials are heated, they can lose weight from a simple
process such as drying, or from chemical reactions that liberate gases. Some materials
can gain weight by reacting with the atmosphere in the testing environment. Since
weight loss and gain are disruptive processes to the sample material, knowledge of the
magnitude and temperature range of those reactions are necessary in order to design
adequate thermal ramps and holds during those critical reaction periods. Such analysis
relies on a high degree of precision in three measurements: weight, temperature, and
temperature change. As many weight loss curves look similar, the weight loss curve
may require transformation before results may be interpreted. A derivative weight loss
curve can be used to tell the point at which weight loss is most apparent. A schematic
diagram of TGA shown in figure 2.20.
Figure 2.20 Schematic diagram of Thermogravimetric analyzer
89
2.2.9.2 Experimental Set Up
The apparatus used for obtaining TG curves is referred to as a thermo
balance. It consists of a continuously recording balance, furnace, temperature,
programmer and a recorder. The high-precision balance with a pan is loaded with the
sample. The sample is placed in a small electrically heated oven with a thermocouple
to accurately measure the temperature. Temperature can vary from 25°C to 900°C
isothermally. The maximum temperature is 1000°C. Sample weight can range from
1 mg to 150 mg. Sample weights of more than 25 mg are preferred, but excellent
results are sometimes obtainable on 1 mg of material. The atmosphere may be purged
with an inert gas to prevent oxidation or other undesired reactions. A computer is used
to control the instrument. The experimental arrangement is shown in figure 2.21.
Figure 2.21 Experimental set-up of Thermogravimetric analysis
90
Analysis is carried out by raising the temperature gradually and plotting weight
against temperature. After the data is obtained, curve smoothing and other operations
may be done such as to find the exact points of inflection. The sensitivity of this
equipment is 0.0001 mg with temperature range from ambient to 10000C. TGA curves
can provide us to determine:
Temperature and weight change of decomposition reactions which often allows
quantitative composition analysis. May be used to determine water content.
Allows analysis of reactions with air, oxygen, or other reactive gases
Can determine the purity of a mineral, inorganic compound, or organic material
[27-30].
91
2.3 References
[1] J. P. Sibilia
VCH Publishers (1998)
A Guide to Materials Characterization and Chemical Analysis.
[2] D. Brondon and W.D. Kaplan
John- Wiley and Sons (1999)
Microstructural Characterization of Materials.
[3] Microscopy and Analysis 20(4): s-5-s-8 (UK) (2006)
An introduction of Energy dispersive and wavelength dispersive X-ray
Microanalysis
www.microscopy-analysis.com
[4] L. F. Vassamillet
J. Appl. Phys. 40 (4) (1969) 1637.
[5] C. Suryanarayana, M. G. Norton
Springer (1998)
X-Ray Diffraction: A Practical Approach.
[6] B. E. Warren
Courier Dover Publications (1990)
X-ray Diffraction.
[7] E. W. Nuffield
Published by Wiley (1966)
X-ray Diffraction Methods.
[8] B. D. Cullity
Addison-Wesley (1978) 102
Elements of X-ray diffraction (2nd
edition).
92
[9] C. Kittel
John – Wiley and Sons (1995)
Introduction to Solid State Physics (7th
edition).
[10] A. Guinier
Courier Dover Publications (1994)
X-ray diffraction: in crystals, imperfect crystals and amorphous bodies.
[11] B. Fultz, J. M. Howe (2007)
Transmission Electron Microscopy and Diffractometry of Materials (3rd
edition).
[12] D. B. Williams, C. B. Carter
Springer (1996)
Transmission electron microscopy.
[13] P. J. Goodhew, F. J. Humphreys, R. Beanland
Taylor & Francis (1975)
Electron microscopy and analysis.
[14] M. D. Graef
Cambridge University Press
Introduction to Conventional Transmission Electron Microscopy.
[15] X. F. Zhang, Z. Zhang
Springer (2001)
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[16] R. D. Heidenreich
Interscience Publishers (1964)
Fundamentals of Transmission Electron Microscopy.
93
[17] D. K. Racker
Thomas (1983)
Transmission Electron Microscopy: Methods of Application.
[18] I. R. Lewis, H. G. M. Edwards
Published by CRC Press (2001)
Handbook of Raman Spectroscopy.
[19] E. Smith, G. Dent
J. Wiley and Sons (2005)
Modern Raman Spectroscopy.
[20] J. R. Ferraro, K. Nakamoto, C. W. Brown,
Academic Press (2003)
Introductory Raman Spectroscopy.
[21] Skoog, et al.
6th
edition Thomson Brooks/Cole (2007) 169-173
Principles of Instrumental Analysis.
[22] "Beer's Law Alisdair Boraston". Retrieved (2009)
http://www.brewingtechniques.com/brewingtechniques/beerslaw/boraston.html.
[23] Thermo Spectronic “Basic UV-Vis Theory, Concepts and Applications”
http://www.plant.uoguelph.ca/research/homepages/raizada/Equipment/Raizada
eb Equipment/PDFs/5B.UV VIS theory ThermoSpectric.pdf.
[24] R. A. Meyers (Ed.)
John Wiley & Sons Ltd Encyclopedia of Analytical Chemistry
Photoluminescence in Analysis of Surfaces and Interfaces.
[25] Introduction to Fourier Transform Infrared Spectrometry
www.thermonicolet.com.
94
[26] “X-Ray Photoelectron Spectroscopy (XPS/ESCA)”
http://www.research.philips.com/technologies/matanalysis/downloads/3-
xpstn.pdf.
[27] P. Kent (1998)
Handbook of Thermal Analysis and Calorimetry: Principles and Practice.
[28] J. D. Menczel
Bruce Prime (2009)
Thermal Analysis of Polymers, Fundamentals and Applications.
[29] Wendlandt
John Wiley & Sons, NY (1974)
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[30] Keattch, C.J. & Dollimore
Heyden & Son Ltd., England (1975)
An Introduction to Thermogravimetry.
