Chapter - 2 Basic and Instrumentation Details Of...

Chapter - 2

Basic and Instrumentation Details

Of Experimental Techniques

Chapter – 2 Page | 45

2.1 Introduction

In this chapter, a brief description on surface, structural, optical and thermal

techniques used in the characterization of the synthesized ZnS nanoparticles and

as grown Bi2Se3 single crystals 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. AFM (Atomic Force Microscopy)

8. TGA (Thermogravimetry Analysis)

9. Seebeck ,Hall effect and Resistivity measurement

Energy dispersive analysis of X-ray (EDAX), is used to provide information about

chemical composition of the materials. The structural parameters of ZnS

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 ZnS nanostructures. The optical

properties of the samples were investigated by means of using UV-Vis–NIR

spectroscopy, photoluminescence (PL). TGA measurements are used primarily to

determine the composition of materials and to predict their thermal stability.

Atomic force microscope (AFM) is instrumental in checking the atomic level

arrangement of atoms and molecules of as grown single crystals of Bi2Se3.

Thermoelectric measurement, Hall effect and resistivity measurement are vital

measurements for any thermoelectric material to calculate its figure of merit, The

basic principle working and experimental set up of instruments of above

mentioned techniques are described in detail in this chapter.


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.

Figure 2.1 Experimental setup of EDAX.

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].

2.2.1.2 Experimental Set Up

Figure 2.1 & 2.2 shows an experimental setup and layout diagram of EDAX

attached to an SEM respectively. An EDAX system comprises of 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.

Figure 2.2 Layout diagram of EDAX

Specifications:

Model EDAX : Scanning Electron Microscope XL 30 ESEM with

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


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

X-ray powder diffraction (XRD) is a non-destructive rapid analytical technique

primarily used for phase identification of a crystalline material and can provide

information on unit cell dimensions. It is a common technique for the study of

crystal structures, atomic spacing, crystallite sizes, stress analysis, lattice

parameters and provide quantitative phase analysis. This information is important

for relating the production of a material to its structure and hence its properties. X-

ray diffraction is based on constructive interference of monochromatic X-rays and

a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to


produce monochromatic radiation, collimated to concentrate, and directed toward

the sample. The interaction of the incident rays with the sample produces

constructive interference (and a diffracted ray) when conditions satisfy Bragg's

Law

nλ = 2d sin θ …..(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]. These

diffracted X-rays are then detected, processed and counted. By scanning the

sample through a range of 2θ angles, all possible diffraction directions of the

lattice should be attained due to the random orientation of the powdered material.

Conversion of the diffraction peaks to d-spacing allows identification of the

mineral because each mineral has a set of unique d-spacing. Typically, this is

achieved by comparison of d-spacing with standard reference patterns i.e. JCPDF

files.

Figure 2.3 A schematic of Bragg’s reflection from a crystal


Figure 2.4 and Figure 2.5 shows the experimental set up of X-ray Diffractometer

and Bragg-Brentano geometry respectively, which consist of three basic elements:

X-ray source, the sample under investigation and detector to pick up the diffracted

X-rays.

Figure 2.4 X-ray diffractometer

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

2θ range : 300 to 2100

2θ Measurement range : 00 to 1360

Diffractometer radius : 130 to 230 mm


In XRD, X-rays are generated within a sealed tube that is under vacuum. A

current is applied that heats a filament within the tube, the higher the current the

greater the number of electrons emitted from the filament. A high 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 the signal is then

processed either by a microprocessor or electronically, converting the signal to a

count rate.

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 used to maintain the angle and rotate the sample is termed a

goniometer. The experimental set up is shown in Figure 2.5.

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 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].

2.2.3 Transmission Electron Microscopy (TEM)


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 both physical and biological sciences. TEM finds 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.

The value of the electron microscope lies in its great resolving power.

Transmission Electron Microscopes are capable of resolving objects only 0.2

nanometers apart which is just five times the diameter of a hydrogen atom. The

possibility of achieving high magnifications has made the TEM a valuable tool in

both medical, biological and materials research.

2.2.3.2 Experimental set up

TEM offers two methods of specimen observation as shown in Figure 2.6

1. Image mode

2. Diffraction mode

In image mode, the condenser lens and aperture will control electron beam to hit

the specimen, the transmitted beam will be focused and enlarged by objective and

projector lens and form the image on the screen, with recognizable details related

to the sample microstructure. In diffraction mode, an electron diffraction pattern is

obtained on the fluorescent screen, originating from the sample area illuminated

by the electron beam.

The diffraction pattern is entirely equivalent to an X-ray diffraction pattern. A

single crystal will produce a spot pattern on the screen and polycrystal will

produce a powder or ring pattern. The microstructure, e.g. the grain size, and

lattice defects are studied by use of the image mode, while the crystalline structure

is studied by the diffraction mode.

2.2.3.3 Image Modes of TEM

There are two primary image modes in TEM differ in the manner in which way an

objective aperture is used as a filter in electric optics system are

[1] Bright-field microscopy

[2] Dark-field microscopy


In bright field imaging, the image of a thin sample is formed by the electrons that

pass the film without diffraction, the diffracted electrons being stopped by a

diaphragm. In the corresponding dark field imaging mode, a diffracted beam is

used for imaging. The electron rays corresponding to bright field and dark field

imaging are shown in Figure 2.6.

Figure 2.6 Operations of TEM

The basic components of a TEM system are

1. An electron gun, which produces the electron beam, and the condenser

system, which focuses the beam onto the object.

2. Vacuum system.

3. The image-producing system, consisting of the objective lens, movable

specimen stage, intermediate and projector lenses, which focus the electrons

passing through the specimen to form a real, highly magnified image.


4. The image-recording system, which converts the electron image into some

form perceptible to the human eye. The image-recording system usually

consists of a fluorescent screen for viewing and focusing the image and a

digital camera for permanent records.

Electron beam is produced from an electron gun consisting of tungsten filament or

lanthanum hexaboride (LaB6) as cathode by thermionic or field emission into the

vacuum and are accelerated towards anode by an electric field formed by a

voltage difference of, typically, 200kV.The filament is surrounded by a control

grid, called a Wehnelt cylinder, maintained at a negative potential with a central

aperture arranged on the axis of the column. Condenser lenses are responsible for

primary beam formation to aspot of the order of 1mm on the sample to be

investigated and controlled the intensity and angular aperture of the beam between

the gun and the specimen. The specimen must be extremely thin for the electrons

to pass through it and create an image. In case near atomic resolution is a required

film thickness have to be limited to a few tens of Å.

A vacuum system consisting of turbomolecular or diffusion pump with rotaryvane

as roughing pump are required to increase the mean free path of the electron gas

interaction and to prevent the specimen from any contamination. The

experimental set up of transmission electron microscope is shown in Figure 2.7.

Figure 2. 7 Experimental set up of Transmission Electron Microscope


The specimen grid is carried in a small holder in a movable specimen stage.

Typically, the image producing system consists of three stages of lensing. The

stages are the objective, intermediate and projector lenses. The objective lens is

usually of short focal length focuses the beam down onto the sample and produces

a real intermediate image that is further magnified by the projector lens or lenses.

Projector lenses are used to expand the beam onto the phosphor screen. The lenses

require high degree of stability in power supply for the highest standard of

resolution. The magnification of the TEM is due to the ratio of the distances

between the specimen and the objective lens' image plane.

An analytical TEM is one equipped with detectors that can determine the

elemental composition of the specimen by analysing its X-ray spectrum or the

energy-loss spectrum of the transmitted electrons.

In the most powerful diffraction contrast TEM instruments, crystal structure can

also be investigated by High Resolution Transmission Electron Microscopy

(HRTEM), also known as phase contrast imaging as the images are formed due to

differences in phase of electron waves scattered through a thin specimen. The

ability to determine the positions of atoms within materials has made the HRTEM

an indispensable tool for nanotechnology research and development in many

fields, including heterogeneous catalysis and the development of semiconductor

devices for electronics and photonics [11-15].

2.2.4 Raman Spectroscopy


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. 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.

Figure 2.8 The vibrational level of the material

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.

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.

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 [16-18]. The experimental set up of Raman Spectrometer is

shown in the Figure 2.10.

Figure 2.10 Experimental setup of Raman spectroscopy.


2.2.5 UV-Vis-NIR Spectrophotometer


A spectrophotometer is employed to measure the amount of light that a sample

absorbs. The instrument operates by passing a beam of light through a sample and

measuring wavelength of light reaching a detector. The wavelength gives valuable

information about the chemical structure and the intensity is related to the number

of molecules, means quantity or concentration light is quantized into tiny packets

called photons, the energy of which can be transferred to an electron upon

collision. However, transfer occurs only when the energy level of the photon

equals the energy required for the electron to get promoted onto the next energy

state, for example from the ground state to the first excitation state. This process is

the basis for absorption spectroscopy. Generally, light of a certain wavelength and

energy is illuminated on the sample, which absorbs a certain amount of energy

from the incident light. The energy of the light transmitted from the sample

afterwards is measured using a photo-detector, which registers the absorbance of

the sample. A spectrum is a graphical representation of the amount of light

absorbed or transmitted by matter as a function of wavelength. A UV-Vis

spectrophotometer measures absorbance or transmittance from the UV range to

which the human eye is not sensitive to the visible wavelength range to which the

human eye is sensitive. Bouguer-Beer law as shown in Figure 2.11 is a basic

principle of quantitative analysis, is also called the Lambert-Beer rule. The

following relationship is established when light with intensity Io is directed at a

material and light with intensity I is transmitted.

Figure 2.11 Schematic of Bouguer-Beer law


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 ……..(2.2)

Abs = log(1/T) = log(Io/I) = -kcl ……..(2.3)

Here k is proportionality constant and & l = 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). In

addition, when the optical path is 1 cm and the concentration of the target

component is 1 mol/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 & 2.13 shows the experimental set-up of UV-Vis spectrometer and its

layout diagram respectively .


Figure 2.12 Experimental set up of UV-Vis-NIR spectrophotometer

Figure 2.13 Schematic diagram of UV-VIS-NIR spectrophotometer.

There are two existing light sources within a UV-Vis spectrophotometer – one for

each (UV and visible light) spectrum. The usual light source used to generate

visible light is the tungsten-halogen lamp emitting 200-340nm wavelengths. The

UV source can be either a high-pressure hydrogen lamp or deuterium lamp, the

latter of which is the one found in the lab. When measuring absorbance at the UV

spectrum, the other lamp has to be turned off. The same goes when measuring

visible light absorbance.


This is to prevent interference of unnecessary wavelengths in the incident light on

the sample. 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 400nm,

and the visible portion is from 400 to 800nm. Therefore, this method is excellent

to both determine the concentration and identify 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 [19-23].

In our work Perkin Elmer Lambda 19 UV-Vis-NIR spectrometer is used having

following specifications:

Lamp: Deuterium (UV), Tungsten-Halogen (Vis/NIR)


Detectors: Photomultiplier tube for UV/Vis, Lead-Sulphide cell (PbS) for

NIR

Wavelength range: 185-3200nm for absorbance/transmission and 200-

2500nm for reflectance

Scan Speed: 0.3 to 1200nm/min

Wavelength accuracy: ± 0.15nm for UV/Vis & ± 0.6nm for NIR

Base line flatness: ± 0.001Å, 4nm slit

Ordinate mode: Scan, time drive, wavelength programming, concentration

Photometric accuracy: ± 0.003Å or ± 0.08 %T

2.2.6 Spectrofluorometer – PL (Photoluminescence)


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.


Figure 2.14 shows the experimental set up of FluoroMax - Compact

Spectrofluorometer.

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.

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.


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 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 ultra

trace 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.

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 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.

Figure 2.15 Block diagram of fluorescence spectrometer


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

highsensitivity, 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 50nm 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.

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.

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 Atomic Force Microscopy


Scanning Tunneling Microscope was developed first in 1982 by Binning, Rohrer,

Gerber and Weibel at IBM in Zurich Switzerland and they won Noble prize for

this invention in 1986. AFM (atomic force microscope) is an improvement of

STM which was also developed by Binning and Quade.

Scanning probe microscope (SPM) consists of family of microscopy forms where

a sharp probe is scanned across a surface and some tip sample interactions are

monitored.

SPM consist of many forms like scanning tunneling microscope (STM), Atomic

Force Microscope (AFM), magnetic force microscope (MFM), Electric force

microscope (EFM) etc.

AFM is one type of scanning probe microscope and also known as nanoscope. It

has ability to create three dimensional micrograph of sample surface with

resolution down to the nano-meter and angstrom scale. These are also capable of

measuring or imaging force between tip and sample surface like van der Waal’s

force with resolution in the range of few nano-Newtons.


AFM operates in three modes.

1. Contact mode AFM.

2. Tapping mode AFM.

3. Non contact mode AFM.

AFM consists of microscope force sensor that responds to a force and a detector

that measure the sensor‘s response. In the AFM, sensor is a cantilever with an

effective spring constant k. At the end of the lever, is a sharp tip which is used to

sense the force between the tip and sample. Lever deflects in accordance with the

force acting on the tip. Detector measures the deflection of the lever. In AFM we


use the laser feedback detection system which can then be used to determine the

force on the tip by using Hooke’s law F= k z, where z is cantilever displacement.

If the sample is scanned under the tip, force between tip and sample surface and

image of the sample surface are generated. By scanning the AFM cantilever over a

sample surface and recording the deflection of the cantilever the local height of

the sample is measured, three dimensional topographical maps of the surface are

then constructed by plotting the local sample height verses horizontal probe tip

position. Atomic Force Microscope - Nanoscope is shown in Figure 2.16.

Figure 2.16 Atomic Force Microscope (AFM) – Nanoscope

Contact mode AFM

In the contact region the cantilever is held less than a few angstroms from the

sample surface, the inter-atomic force between the cantilever and the sample is

repulsive. So it is also known as repulsive mode.

AFM tips make soft physical contact with the sample. The tip is attached to the

end of the cantilever with a low spring constant. At the beginning tip and sample

atom are separated by a large distance. As the atoms are gradually brought

together, they first weakly attract each other. This attraction increases until the

atoms are so close together that; their electron clouds begin to repel each other


electro-statically. When the total vander-Waal’s force become positive (repulsion)

the tip and cantilever give deflection. The detection system, detect this deflection.

The cantilever has been set at a set point at a fixed distance from the surface, and

scanning is started. At each (x, y) data point there will be some deflection of the

cantilever because of between tip and sample interaction. This deflection is being

measured by a vertical scanner at each (x, y) data point and the force between tip

and sample is calculated by

F= - k x, where x is the deflection of the lever, k is the spring constant of the lever

which is ranging from 0.01 to 1.0 N/m. The distance by which the scanner moves

vertically at each data point is stored by the computer to the form the topographic

image of the sample surface [25-26].

Tapping mode AFM

Tapping mode AFM operates by scanning a tip attached to the end of oscillating

cantilever across the sample surface. The cantilever is oscillated by a piezo at or

near its resonance frequency with amplitude from 20 nm to 100 nm. The tip

lightly taps on the sample surface during scanning, contacting the surface at the

bottom of its swing. Constant set point amplitude is maintained with the help of

feedback loop maintained by a constant RMS of the oscillation signal acquired by

the split photodiode detector. The vertical position of scanner at each (x, y) data

point in order to maintain a constant set point amplitude is stored by the computer

to form the topographic image of the sample surface. In this mode we can also

form phase and frequency image because of the nature of information available

from phase sensitive detection. This operation can also take place in liquid

environment. In this mode our tip and sample are more safer as compared to the

contact mode, So tapping mode is more useful to generate the image of biological

soft material.

Non contact mode of AFM

In this mode the cantilever is adjusted far away to the sample surface and the set

point height is being set in non-contact region of force distance curve. Means

indirect contact exist between the tip and sample surface. In this mode good

resolution of images is not possible to sub-nanometer level. But in this mode the


sample and tip both are safe. So in this mode the images can be easily formed with

soft samples like biological samples.

Main components of AFM instrument

The various component of AFM are Scanning probe microscope (SPM),

controller, computer, display monitor and control monitor. The main component

of the system is scanning probe microscope (SPM).

The main parts of the SPM are SPM head, scanner, cantilever, tip, control and

feedback system.

SPM head

It contains photodetector system, cantilever and tip. There are many types of

detection systems like tunneling detection system, capacitance detection system,

laser diode photodetection system etc. In AFM laser diode photo detection system

is normally being used. In this a laser light is being used because it is highly

coherent and the spot size is also short. Laser light is focused on the cantilever

supporting the force-sensing tip. And it is reflected back to photodiode. If

cantilever deflect with an angle then laser light reflect with twice the angle. In this

system a four photodiode system is used and is known as quad photodiode.

The four elements of the quad photodiode are combined to provide the different

information depending on the operation mode. At starting the cantilever is fixed at

a constant deflection i.e., set point. And set the laser light with cantilever that after

reflection laser light makes spot at the center of the quad photo diode. Then all

photo diodes collect equal light and give it current that is same. When tip is

deflected the laser light also shows the deflection on the photo diode. These

minute deflections of the lever cause the photo detector to collect more light to

other one. And this current fed to substractor and the output of this substractor is

proportional to the deflection of the lever. This is used to image the force across

the sample and maintained set point.

Cantilever

Cantilever is a beam that is supported at one end and other end is free to move. In

AFM force sensing tip is mounted at the free end of the lever. AFM cantilever is


flexible beam whose geometrical and material property plays an important role in

determining the sensitivity and resolution of AFM.

In contact AFM mode cantilever’s flexibility acts as a nanometeric spring

allowing the tip to measure the surface force. For contact mode AFM imaging, it

is necessary to have a cantilever which is soft enough to be deflected by very

small forces (k is small) and has a high enough resonant frequency to not be

susceptible to vibration instabilities. This is accomplished by making the

cantilever short, to provide a high resonant frequency and thin to provide a small

force constant. The silicon nitride tip and cantilever is being used normally. This

tips exhibit excellent flexibility. For the typical silicon nitride tips, the value of

spring constant k is 0.58, 0.32 0.12 or 0.06 N/m depend upon geometry of

cantilever. Cantilever lengths 100 to 200m. and tip radius of curvature is 20–60

nm.

In tapping mode the cantilever is oscillating up and down at its resonance

frequency while its amplitude are monitored. In the tapping mode we use much

stiffer crystal silicon probe which is oscillated to its resonance frequency. Because

tip describes a high frequency oscillating arc (100 kHz) and larger force constant.

It possesses sufficient energy to break free of surface tension force. This is also

non-conducting material. The probe is considerably stiffer than silicon nitride

making it more brittle and less forgiving .The tip and cantilever are an integrated

assembly of single crystal silicon, produced by etching techniques. For typical

single crystal silicon probes, spring constant k = 20 –100 N/m resonant frequency

200 –400 kHz, tip radius of curvature 5 -10 nm and cantilever length is about

125m.

Scanner

In AFM scanner is used for movement of sample or tip. These scanners are made

from piezo electric material (example ceramic polycrystalline material) which

expands and contracts proportionally to an applied voltage. Piezomaterial expands

or contract depends upon the polarity of the voltage applied. Scanners are of many

sizes and each scanner exhibits its own unique properties. In AFM a scanner tube

is being used, which is constructed by combining independently, operated


piezoelectrodes for x, y and z direction, which can manipulate sample and probes

with extreme precision in three dimensions.

Feedback loop

To produce images the SPM must be capable of controlling the tip sample

interaction with the great precision with the use of feedback loop which

safeguards tip and sample by keeping force between them at a user specified set

point level. Set point refers to how much tip sample force is to be maintained

which means that how much force can be applied on the tip which is just enough

to trace the surface features and not so much that the tip is broken off or sample is

damaged.

In contact AFM mode the deflection of the cantilever, the tip’s height above the

surface can be precisely maintained. To control this we adjust various gains in the

SPM feedback circuit. The main gains are:

Proportional gain

Amount of correction applied in response to the error signal between set point

force and actual force measured by the detection in direct proportion to the error.

Integral gain

Amount of correction applied in response to the average error between set point

force and actual force measured by the detector.

Look ahead gain

Amount of correction applied in response to the error signal between set point

force and actual force measured by the detector based upon recorded information

the adjacent scan line. Computer works as a controller of feed back loop.

2.2.8 Thermo gravimetric Analysis (TGA)


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 2.17.

Figure 2.17 Schematic diagram of Thermogravimetric analyzer


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 2.18.

Figure 2.18 Experimental set-up of Thermogravimetric analysis

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

1000 C. 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].


2.2.9 Seebeck coefficient measurement set up


For Thermoelectric power measurement If a temperature gradient is maintained

between two ends of a conductor than an emf will be developed across it and this

effect is called Seebeck effect or thermoelectric effect. If ΔT is the temperature

gradient across the sample and ΔV is resultant voltage developed because of this

temperature gradient, then thermoelectric power is defined as

S= V/ T ……..(2.4)

Figure 2.19 Schematic diagram of phonon flux in a conductor under

temperature gradient where the energy density of phonon

From the above Figure 2.19 the energy difference between cold and hot end is

given by

ΔU= (U/X) ΔX ……..(2.5)

As the electrons are flowing from hot end to cold end to minimize their energy,

then there will be an electric field generated across the conductor. The electrons

will experience a force by this electric field and force is given by

F= N e E ……..(2.6)

The energy due to this force is

FΔX ……..(2.7)

Considering that the force is acting only on electrons one can get the relation as


Ne E ΔX= X) ΔX ……..(2.8)

or, E = (X)/N e ……..(2.9)

Now S = V/ T …….(2.10)

or, S ΔX

e

S = CV/ Ne …….(2.11)

where CV is the specific heat T).The electronic heat capacity may be written

as CV =T. Hence the thermoelectric power is linear in T. Generally a broad hump

in Thermopower at low temperature is observed. This is called phonon drag effect.

If the phonon-phonon interactions are alone present then the phonons will not

contribute for Thermopower as phonons do not carry any electric charge.

However at low temperatures phonon electron interactions will dominate and a

peak in thermopower temperature curve is observed approximately at about

. The phonon contribution to heat capacity at low temperature may be

approximated to CV = T3 and one finds a T3 variation in S. Schematically one

can show the behavior of S over a temperature range as shown below in

Figure 2.20.

Figure 2.20 Total Thermoelectric power as combination of Selectron and Sphonon

Using the Seebeck effect, thermal energy (heat) can be converted into electric

energy, which is called thermoelectric power generation. When the left side of the

sample is heated, the thermoelectric voltage is induced in proportion to the


temperature difference. If a load is connected to the sample, the electric power is

consumed at the load. Here the thermoelectric material acts as a kind of battery,

where the thermoelectric power corresponds to the electromotive force, and the

resistivity corresponds to the internal resistance. Advantages of thermoelectric

power generation (i) electric power source without maintenance, (ii) energy

recovery from waste heat and (iii) long operating lifetime.


The experimental set up used for determination of thermoelectric power (TEP) is

shown in Figure 2.21. One can measure TEP directly from Thomson coefficient

() but it is not easy to measure , for this reason we highlight the principle being

followed for measurement of TEP. Create a desire temperature gradient (2-3K) in

between the copper block using the heater coil made up of manganin wire, which

is controlled by a temperature controller .Place the sample in between the copper

blocks, The sample can have any size but it should have thickness of about 1-2

mm so that the distance between the copper block and temperature gradient can be

maintained.

Figure 2.21 Block diagram of sample holder of TEP set up.

Measure the temperature gradient with the known calibrated thermocouple which

is made up of Au-Fe (7%) – Chromel thermocouple. To avoid the direct electrical

contact between the thermocouple and Cu block thermocouple must be coated


with an electrically insulating but thermally conductor material. Measure the

voltage developed across the two Cu blocks .For measuring the temperature

gradient the voltage developed across thermocouple has to be measured .By using

a scanner the two voltages are measured sequentially by single voltmeter having

accuracy of nanovolt. Now for achieving low temperatures a CCR (Closed Cycle

Refrigerator) is used.

2.2.10 Hall effect and Resistivity measurements


Consider a metal crystal in the form of a strip having steady current of density jx

flows owing to an electric field (Ex) applied along the x-direction in the strip.

When a uniform static magnetic field (B) directed along the z-direction is

switched over the region of the crystal, a small difference of potential (~V)

develops across the crystal’s faces along the y-direction. This happens because of

the deflection of electrons by the Lorentz force. These electrons create an electric

field along the y-direction in the crystal. As soon as the force owing to this field

on electrons becomes equal to the Lorentz force, the deflection of electrons stops

resulting in the saturation of the field Ey, known as the Hall field. The effect is

known as the Hall effect.


Hall measurements at 300 K (R.T.) are carried out on the newly purchased

sophisticated instrument from Lakeshore Model No. 7504, USA [31] photograph

of the experimental setup is shown in Figure 2.22. As all direct measurements of

the electronic transport properties of a material require adequate electrical contacts

between the sample and the measuring instrument and generally, low resistance

Ohmic contacts are desired. So in the present work annealed indium foil of high

purity provided by the company is used for making the contacts on these samples.

R12, 12 current-voltage measurements

For van der Pauw samples, connection 1 and 3 are opposite to each other (as are

contacts 2 and 4). Measuring R12, 12 sends the excitation current from the switch

card down cable 1 and back on cable 2. The voltage is measured between the same


cables, so the resistance measured includes the cable resistance, contact resistance

at both contacts, and the sample resistance between the two contacts.

The voltage measured across the same contacts used to supply current (e.g. R13, 13

or R24, 24) will tell us the voltage levels at the current source and other instruments

connected to the switch card. Use the voltages measured in this test to keep within

the limits of our measurement configuration.

Figure 2.22 Experimental set-up of Hall effect measurement system

LAKESHORE” model 7504.

Ohmic contacts are required for accurate Hall effect measurements. It is

recommended that we test the current-voltage characteristics between contacts to

verify ohmic behavior before making Hall effect measurements on unknown

sample. With experience, we have found that some samples almost always have

ohmic contacts and do not require contact testing. Measuring both R13, 13 and R24,

24 tests all four contacts on a van der Pauw sample and is the minimum number of

tests required to determine if all four contacts are ohmic. If one or more of the

contacts is non-ohmic, additional test may be required to identify the bad

contact(s). We have tested R12, 12, R23, 23, R34, 34 and R14, 14 to verify ohmic

behavior of all contacts.


Van der Pauw measurements

In the basic van der Pauw contact arrangement, the four contacts made to the

sample are numbered counter clockwise in ascending order when the sample is

viewed from above with the magnetic field perpendicular to the sample and

pointing toward the observer.

Figure 2.23 Geometry of resistivity measurement and Hall coefficient using

van der Pauw method.

The sample interior should contain no contacts or holes. The sample must be

homogeneous and of uniform thickness. Figure 2.23 shows measurement of

resistivity and Hall coefficient using van der Pauw geometry.

Resistivity

Again, let V+ijkl indicate a voltage measured across terminals k and l, with k

positive, while a positive current flows in to terminal i and out of terminal j. In a

similar fashion, let R+ijkl = Vkl/Iij with the voltage measured across terminals k and

l, while a positive current flows into i and out of j. First calculate the two

resistivity:

12,43 12,43 23,14 23,14

12 12 23 23

,,

ln(2)A

A

V V V Vf t m cmm cm

I I I I

…….(2.12)

and

34, 21 34,21 41,23 41,23

34 34 41 41

,,

ln(2)B

B

V V V Vf t m cmm cm

I I I I

…….(2.13)

Geometrical factor fA and fB are functions of resistance ratios QA and QB,

respectively, given by:


12,43 12,43 12,43 12,43 23 23

23,14 23,14 12 12 23,14 23,14

A

R R V V I IQ

R R I I V V

…….(2.14)

and

34, 21 34,21 34,21 34,21 41 41

41,23 41,23 34 34 41,23 41,23

B

R R V V I IQ

R R I I V V

…….(2.15)

If either QA or QB is greater than one, then use the reciprocal instead. The

relationship between f and Q is expressed by the transcendental equation

11 1 ln 2cosh exp

1 ln 2 2

Q f

Q f

.….(2.16)

which can be solved numerically.

The two resistivity QA and QB should agree to within ± 10%. If they do

not, then the sample is too inhomogeneous, or anisotropic, or has some other

problem. If they do agree, the average resistivity is given by

,2

A Bav m cm

..….(2.17)

Hall coefficient

Calculate two values of the Hall coefficient by the following:

31,42 31,42 31,42 31,42 3 1

31 31 31 31

HC

V B V B V B V Bt mR m C

B T I B I B I B I B

31,42 31,42 31,42 31,428 3 1

31 31 31 31

10V B V B V B V Bt cm

cm CB gauss I B I B I B I B

….(2.18)

42,13 42,13 42,13 42,13 3 1

42 42 42 42

HD

V B V B V B V Bt mR m C

B T I B I B I B I B

42,13 42,13 42,13 42,138 3 1

42 42 42 42

10V B V B V B V Bt cm

cm CB gauss I B I B I B I B

.….(2.19)


These two should agree to within ± 10% if they do not, then the sample is too

inhomogeneous, or anisotropic, or has some other problem. If they do, then the

average Hall coefficient can be calculated by

3 1 3 1,2

HC HDHav

R RR m C cm C

…….(2.20)

Hall mobility

The Hall mobility is given by

2 1 1 2 1 1,HavH

av

Rm V s cm V s

….(2.21)

ρav is the magneto resistivity if it was measured, and the zero-field resistivity if it

was not.


2.3 References

[1] J. P. Sibilia, VCH Publishers A Guide to Materials Characterization and

Chemical Analysis, (1998).

[2] D. Brondon and W.D. Kaplan, Microstructural Characterization of

Materials, John- Wiley and Sons (1999).

[3] An introduction of Energy dispersive and wavelength dispersive X-ray

Microanalysis www.microscopy-analysis.com , Microscopy and Analysis

20(4): s-5-s-8 (UK) (2006),

[4] L. F. Vassamillet, J. Appl. Phys., 40 (4),1637,(1969).

[5] C. Suryanarayana, X-Ray Diffraction: A Practical Approach. M. G. Norton

Springer (1998).

[6] B. E. Warren, X-ray Diffraction., Courier Dover Publications (1990)

[7] E. W. Nuffield ,X-ray Diffraction Methods , Published by Wiley (1966)

[8] B. D. Cullity, Elements of X-ray diffraction (2nd edition). 92, 102 Addison-

Wesley (1978).

[9] C. Kittel, Introduction to Solid State Physics , John – Wiley and Sons

(1995) (7th edition).

[10] A. Guinier, X-ray diffraction: in crystals, imperfect crystals and

amorphous bodies.Courier Dover Publications (1994)

[11] L. Ouyang, K.N. Maher, C.L. Yu, J. McCarty, H. Park. Journal of the

American chemical Society, 129, 133, (2007).

[12] Z.Y. Tang, N.A. Kotov. Advanced Materials, 17, 951, (2005).

[13] C. Ma, Z.L. Wang. Advanced Materials, 17, 2635, (2005).

[14] L.L. Zhao, T.Z. Lu, M. Yosef, M. Steinhart, M. Zacharias, U. Gosele, S.

Schlecht. Chem. of Mat., 18, 6094, (2006).

[15] N. Pradhan, H. Xu, X. Peng. Nano Lett., 6, 720, (2006).

[16] I. R. Lewis, H. G. M. Edwards Published by CRC Press Handbook of

Raman Spectroscopy (2001).

[17] E. Smith, G. DentJ. Wiley and Sons Modern Raman Spectroscopy (2005).

[18] J. R. Ferraro, K. Nakamoto, C. W. Brown, Academic Press Introductory

Raman Spectroscopy (2003).


[19] H.H. Willard, L.L. Merrit Jr., J.A. Dean, F.A. Settle Jr. Instrumental

Method of Analysis, Wadsworth Publishing Company, USA (1986).

[20] G.R. Chatwal, S.K. Anand. Instrumental Methods of Chemical Analysis,

Himalaya Publishing House (1979).

[21] H. Gunzzler, A. Williams. Hand Book of Analytical Techniques (Vol.-2),

Wiley VCH, Weinheim (2001).

[22] G.W. Ewing. Analytical Instrumentation Handbook, Marcel Dekker: New

York (1990).

[23] J.A. Dean, The Analytical Chemistry Handbook, McGraw Hill, New York

(1995).

[24] R. A. Meyers (Ed.) John Wiley & Sons Ltd Encyclopedia of Analytical

Chemistry Photoluminescence in Analysis of Surfaces and Interfaces.

[25] G. Binning, C. F. Quate, C. Gerber. Phys. Rev. Lett., 56, 930, (1986).

[26] N. Yao, Z. L. Wang. Handbook of Microscopy for Nanotechnology (1st

Ed.), Springer, (2005).

[27] P. Kent Handbook of Thermal Analysis and Calorimetry: Principles and

Practice (1998).

[28] J. D. Menczel Bruce Prime Thermal Analysis of Polymers, Fundamentals

and Applications (2009).

[29] Wendlandt , John Wiley & Sons, NY Thermal Methods of Analysis

(1974) .

[30] Keattch, C.J. & Dollimore, Heyden & Son Ltd., England An Introduction

to Thermogravimetry,(1975).

[31] User’s Manual-Lakeshore 7500/9500 series Hall System, 1, Lakeshore

Cysotronics, Inc. (1999).

Chapter - 2 Basic and Instrumentation Details Of...

Documents

Transcript of Chapter - 2 Basic and Instrumentation Details Of...