Document 6535987

Chapter III
Experimental Techniques and Sample Preparation
Chapter III
Experimental Techniques and Sample Preparation
3.1 Introduction
ZnO thin films are highly attractive in the development of materials area, due to
their interesting physical properties as high transparency in the visible and nearultraviolet (UV–Vis) spectral regions, as well as their luminescence. The applications of
ZnO have attracted much attention in recent years. Compared to physical coating
processes in a vacuum, the sol-gel method can be used with an enormous range of mostly
commercially available starting materials. This allows multifunctional layers to be
produced. Besides the simple processing of the colloidal solutions (sols), the sol-gel
method is compatible with established coating technologies (dip-coating, spraying,
centrifugation, blade coating, etc.), has low investment costs and the crystalline quality of
the ZnO prepared by the sol–gel process shows hexagonal structure. Notably, the sol–gel
processes with an annealing treatment and doping are intimately affect the crystallization
and physical properties.
3.2 Sol-gel Process
The sol-gel process may be described as: “Formation of an oxide network through
polycondensation reactions of a molecular precursor in a liquid”. A sol is a stable
dispersion of colloidal particles or polymers in a solvent. The particles may be
amorphous or crystalline. An aerosol is particles in a gas phase, while a sol is particles in
a liquid. A sol consists of a liquid with colloidal particles which are not dissolved, but do
not agglomerate or sediment. The method may provide good control over stoichiometry
and reduced sintering temperature. The different sol-gel processing options are given in
Fig 3.1
44
Chapter III
Experimental Techniques and Sample Preparation
Fig 3.1 Sol-Gel Processing options
3.3 Preparation of sol and coating
Fig 3.2 common preparation of sol
In the sol-gel process, the precursors (starting compounds) for preparation of a
colloid consist of a metal or metalloid element surrounded by various ligands [Jeffrey
Brinker, 1990]. For example, the common precursors for zinc oxide include inorganic
45
Chapter III
Experimental Techniques and Sample Preparation
(containing no carbon) salts such as ZnCl2 and organic compounds such as Zinc acetate
(Zn (CH3CO2)2). The later material is the most widely used precursor in zinc oxide (ZnO)
sol gel research.
Metal alkoxides are popular precursors because they react readily with the
solvents (water or alcohol). The reaction is called the hydrolysis, which gives the sol.
This sol is coated through dip or spin coating technique. The spin coater used in the
present work is shown in Fig 3.3
Fig 3.3 Spin coater (HOLMARC) used in this work
The sol prepared was deposited on the cleaned glass substrate using the above
spin coater rotated at the rate of 3000rpm. After each coating the film is heated and the
coating is repeated for few times to get a desired thickness of the film. Finally the film is
annealed at suitable temperature.
46
Chapter III
Experimental Techniques and Sample Preparation
3.4 Characterization Techniques
Thin films are often characterised using a number of different physical
characterisation techniques. The techniques used in this thesis are briefly presented in this
section.
3.4.1 Structure Determination by X-Ray Diffraction (XRD)
X-ray diffraction (XRD) is a versatile, non-destructive technique that reveals
detailed information about the chemical composition and crystallographic structure of
natural and manufactured materials. Atoms of a pure solid are arranged in a regular
periodic pattern called ‘a lattice’. The inter-atomic distance and interaction of atoms in
any crystalline lattice is unique and results in a unique X-ray diffraction (XRD) pattern to
identify its crystal structure. When X-ray radiation with a wavelength, λ, is incident onto
a crystal, a diffraction peak occurs if the Bragg criterion [Bragg W.L., 1913] for
constructive interference is satisfied (the sharpness of this peak is a measure of the degree
of ordering in the crystal)
Nλ = 2d sinθ----------------------------(3.1)
Fig 3.4 View of Bragg´s Law: nλ=2dsinθ
where d is the inter-plane separation of the lattice, n = 0, 1, 2, ... is the interference order
and θ is the angle of incidence as shown in Fig 3.4. In this work, the structure and lattice
parameters of ZnO films were analyzed by a X-ray diffractometer (XRD) with Cu (Kα)
47
Chapter III
Experimental Techniques and Sample Preparation
radiation with  = 1.5405 Å (40 kV, 30 mA). XRD can be used to extract information on
a number of thin film properties, such as crystalline structure, phase composition,
residual stress state, film thickness, grain size, and crystallographic orientation, and is
thus a very powerful technique. The values of lattice constants a and c for various layers
calculated using equation [Yasemin Caglar et al., 2009]
2
1 4  h2  hk  k 2  l
 
  2            (3.2)
d2 3 
a2
 c
where ‘d’ is the interplanar spacing and h, k, and l are the Miller indices. The preferential
growth orientation was determined using a texture coefficient TC(hkl). This factor is
calculated using the following relation [Caglar et al., 2006]:
TChkl 
I ( hkl ) / I 0( hkl )
N 1  n  I ( hkl ) / I 0( hkl ) 
         (3.3)
where I(hkl) is the measured relative intensity of a plane (hkl),Io(hkl) is the standard
intensity of the plane (hkl) taken from the JCPDS data, N is the reflection number and n
is the number of diffraction peaks. A sample with randomly oriented crystallite presents
TC(hkl) = 1, while the larger this value, the larger abundance of crystallites oriented at the
(h k l) direction. From the XRD pattern, the average crystallite size could be calculated
using the Debye Scherrer’s formula [Cullity, B.D., 1978]
D
0.9
               (3.4)
 cos 
where D is the grain size, λ is the wavelength of the x-ray radiation used, β is the full
width at half maximum (FWHM) of the diffraction peak and θ is the Bragg diffraction
angle of the XRD peak. The relative percentage error for the observed and JCPDS
standard d –value for all the films is calculated using the formula [Shinde et al., 2008],
48
Chapter III
Experimental Techniques and Sample Preparation
Relative percentage error, d % 
ZH  Z
Z
100            (3.5)
where ZH is the observed d-value and Z is the standard d-value from JCPDS data file.
3.4.2 Surface Morphology by Field Emission Scanning Electron Microscopy
(FESEM)
Figure 3.5 Schematic diagram of a FESEM.
Electron microscopes use a beam of highly energetic electrons to probe objects on
a very fine scale. In standard electron microscopes, electrons are mostly generated by
“heating” a tungsten filament (electron gun). They are also produced by a crystal of
LaB6. The use of LaB6 results in a higher electron density in the beam and a better
resolution than that with the conventional device. In a field emission (FE) electron
microscope (Fig 3.5), on the other hand, no heating but a so-called "cold" source is
employed. Field emission is the emission of electrons from the surface of a conductor
caused by a strong electric field. An extremely thin and sharp tungsten needle (tip
diameter 10–100 nm) works as a cathode. The FE source reasonably combines with
scanning electron microscopes (SEMs) whose development has been supported by
advances in secondary electron detector technology. The acceleration voltage between
cathode and anode is commonly in the order of magnitude of 0.5 to 30 kV, and the
49
Chapter III
Experimental Techniques and Sample Preparation
apparatus requires an extreme vacuum (~10–6 Pa) in the column of the microscope.
Because the electron beam produced by the FE source is about 1000 times smaller than
that in a standard microscope with a thermal electron gun, the image quality will be
markedly improved; for example, resolution is on the order of ~2 nm at 1 keV and ~1 nm
at 15 keV. Therefore, the FE scanning electron microscope (FE-SEM) is a very useful
tool for highresolution surface imaging in the fields of nanomaterials science.
3.4.3 Chemical Composition by EDAX and XPS
EDAX
Energy dispersive X-ray spectroscopy (EDS, EDX or EDXRF) is an analytical
technique used for the elemental analysis or chemical characterization of a sample. It is
one of the variants of XRF. As a type of spectroscopy, it relies on the investigation of a
sample through interactions between electromagnetic radiation and matter, analyzing xrays 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 each other. To stimulate the
emission of characteristic X-rays from a specimen, a high energy beam of charged
particles such as electrons or protons (see PIXE), 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, higherenergy 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 (Fig 3.6). 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 are 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.
The excess energy of the electron that migrates to an inner shell to fill the newly-created
hole can do more than emit an X-ray. Often, instead of X-ray emission, the excess energy
50
Chapter III
Experimental Techniques and Sample Preparation
is transferred to a third electron from a further outer shell, prompting its ejection. This
ejected species is called an Auger electron, and the method for its analysis is known as
Auger Electron Spectroscopy (AES).
Fig 3.6 Elements in an EDX spectrum are identified based on the energy content of the
X-rays emitted by their electrons as these electrons transfer from a higher-energy
shell to a lower-energy one.
XPS
Information on the quantity and kinetic energy of ejected electrons is used to
determine the binding energy of these now-liberated electrons, which is element-specific
and allows chemical characterization of a sample. X-ray photoelectron spectroscopy
(XPS) is a surface sensitive technique, which provides information both about chemical
composition and chemical bonding. The phenomenon is based on the photoelectric effect
outlined by Einstein in 1905 where the concept of the photon was used to describe the
ejection of electrons from a surface when photons impinge upon it. For XPS, Al Kα
(1486.6eV) or Mg Kα (1253.6eV) are often the photon energies of choice. Other X-ray
lines can also be chosen such as Ti Kα (2040eV). The XPS technique is highly surface
specific due to the short range of the photoelectrons that are excited from the solid. The
energy of the photoelectrons leaving the sample are determined and this gives a spectrum
with a series of photoelectron peaks. The binding energy of the peaks are characteristic of
each element. The peak areas can be used (with appropriate sensitivity factors) to
determine the composition of the materials surface. The shape of each peak and the
51
Chapter III
Experimental Techniques and Sample Preparation
binding energy can be slightly altered by the chemical state of the emitting atom. Hence
XPS can provide chemical bonding information as well. XPS is not sensitive to hydrogen
or helium, but can detect all other elements. XPS must be carried out in ultra high
vaccum (UHV).
Fig 3.7 Diagram of the Side View of XPS System.
The analysis of composition has been widely applied in the thin film research fields. One
can obtain the composition of thin films from XPS spectra. The information of all
elements in thin film can be gained from the survey scan spectrum of XPS. The detailed
information of each element in the thin film can be obtained from the narrow scan
spectrum of XPS.
3.4.4 Transmission Electron Microscope
TEMs 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. A "light
source" at the top of the microscope emits the electrons that travel through vacuum in the
column of the microscope. Instead of glass lenses focusing the light in the light
microscope, the TEM uses electromagnetic lenses to focus the electrons into a very thin
beam. The electron beam then travels through the specimen you want to study.
Depending on the density of the material present, some of the electrons are scattered and
disappear from the beam. At the bottom of the microscope the unscattered electrons hit a
fluorescent screen, which gives rise to a "shadow image" of the specimen with its
different parts displayed in varied darkness according to their density. The image can be
52
Chapter III
Experimental Techniques and Sample Preparation
studied directly by the operator or photographed with a camera. The possibility for high
magnifications has made the TEM a valuable tool in both medical, biological and
materials research.
Fig 3.8 A schematic diagram of the transmission electron microscope
3.4.5 UV-Vis Absorption Spectrophotometer
The purpose of this instrument is to determine the amount of light of a specific
wavelength absorbed by an analyte in a sample. The samples may be gases or liquids or
solid materials, an analyte dissolved in a solvent.
The
double
beam
spectrometer
consists of multifrequency source, grating, beam splitter, sample and reference holder
then optical detector.
53
Chapter III
Experimental Techniques and Sample Preparation
Fig 3.9 Double beam spectrophotometer.
The optical transmittance of all samples was measured by this spectrometer in the
range of wavelength 300–1000 nm. Transmittance of the films are connected with the
absorption coefficient (α) by the relation [Manifacier et al., 1976]
1
t
1
T
  (ln )          (3.6)
or the absorption coefficient (α) could be evaluated by the relation [Suwanboon et al.,
2008)

A
           (3.7)
d s'
where A is the measured absorbance and d s' is the thickness of sample in UV–Vis cell.
The bandgap of semiconductor materials will increase with the decrease in particle size(S
C SINGH et al., 2010), which leads to the shift of the absorption edge towards high
energy side, which is the so called quantum size effect. Band gap values are obtained by
extrapolating the linear portion in the plots of (αhν)2 versus (hν) to cut the x-axis, i.e., at
(αhν)2 =0, which is obtained by the Tauc’s relationship [Tauc, 1970]. The optical
bandgap, Eg, is determined from the absorbance spectra, where a steep increase in the
absorption is observed due to the band–band transition using the general relation
 h  A(h  Eg )n      (3.8)
54
Chapter III
Experimental Techniques and Sample Preparation
where A is a constant related to the effective masses of charge carriers associated with
valence and conduction bands, Eg the bandgap energy, hν the photon energy, and n = 2
or 1/2, depending on whether the transition is indirect or direct, respectively.
 h  A(h  Eg )2      (3.9)
 h  A(h  Eg )1/2      (3.10)
3.4.6 Photoluminescence
Photoluminescence spectroscopy (PL) is a powerful optical method used for
characterizing materials. It can be used to find impurities and defects in semiconductors,
and to determine semiconductor band gaps. A material absorbs light, creating an electron
hole pair; an electron from the valence band jumps to the conduction band leaving a hole.
The photon emitted upon recombination corresponds to the energy difference between
the valence and conduction bands, and is hence lower in energy than the excitation
photon.
Fig 3.10 Photoluminescence Spectrophotometer.
55
Chapter III
Experimental Techniques and Sample Preparation
Luminescence is light that accompanies the transition from an electronically
excited atom or molecule to a lower energy state. The forms of luminescence are
distinguished by the method used to produce the electronically excited species. When
produced by absorption of incident radiation, the light emission is known as
photoluminescence. Photoluminescence that is short-lived (10−8 s or less between
excitation and emission) is known as fluorescence. Photoluminescence that is longerlived (from 10−6 s all the way up to seconds) is known as phosphorescence. The reason
for the difference in lifetime is that fluorescence involves an allowed, high-probability
transition while phosphorescence involves a forbidden, low-probability transition.
Photoluminescence excitation spectra are determined by measuring emission intensity at
a fixed wavelength while varying the wavelength of the incident light used to produce the
electronically excited species responsible for emission. The excitation spectrum is a
measure of the efficiency of electronic excitation as a function of excitation wavelength.
Photoluminescence emission spectra are determined by exciting at a fixed wavelength
and varying the wavelength at which emission is observed. Between excitation and
emission, electronically excited molecules normally lose some of their energy because of
relaxation processes. As a consequence, the emission spectrum is at longer wavelengths,
that is, at lower energy, than the excitation spectrum.
3.4.7 Vibrating Sample Magnetometer (VSM)
A vibrating sample magnetometer or VSM is a scientific instrument that measures
magnetic properties of material. If a sample of any material is placed in a uniform
magnetic field, created between the poles of a electromagnet, a dipole moment will be
induced. If the sample vibrates with sinusoidal motion a sinusoidal electrical signal can
be induced in suitable placed pick-up coils. The signal has the same frequency of
vibration and its amplitude will be proportional to the magnetic moment, amplitude, and
relative position with respect to the pick-up coils system.
56
Chapter III
Experimental Techniques and Sample Preparation
Some of the most common measurements done are: hysteresis loops,
susceptibility or saturation magnetization as a function of temperature (thermo magnetic
analysis), magnetization curves as a function as a function of angle (anisotropy), and
magnetization as a function of time.
Fig 3.11 Vibrating Sample Magnetometer (Lakeshore VSM 7410)
All the above instrumentations are used in the present work to study the structural,
optical, and magnetic properties of the pure and doped zinc oxide films.
57
Chapter III
Experimental Techniques and Sample Preparation
References
Bragg W.L., 1913, The Diffraction of Short Electromagnetic Waves by a Crystal, Proc.
Cambridge Phil. Soc., 17, 43-57.
Caglar.M, Y. Caglar, S. Ilican, Journal of Optoelectronics and Advanced Materials,8 (4)
(2006), 1410 – 1413.
Cullity, B.D., “Elements of x-ray diffraction”, 2nd edn.. Addison-Wesley, Reading,
MA.1978.
Jeffrey Brinker.C, George W. Scherer, “The Physics and Chemistry of Sol-Gel
processing”, 1990.
Manifacier.J.C, J Gasiot and J P Fillard, J. Phys. E: Sci. Instrum. 9, 1002 (1976).
Shinde.S.S, P S Shinde, C H Bhosale and K Y Rajpure, J. Phys. D: Appl. Phys., 41
(2008), P. 105.
Singh.S.C, R.K.Swarnkar and R. Gopal, Bull. Mater. Sci., 33 (1) (2010), 21–26.
Suwanboon.S, P. Amornpitoksuk, A. Haidoux, J.C. Tedenac , Journal of Alloys and
Compounds, 462 (2008), 335–339.
Tauc.J, The optical properties of solids,(North-Holland, Amsterdam, 1970.
Yasemin Caglar, Seval Aksoy, Saliha Ilican, Mujdat Caglar, Superlattices and
Microstructures, 46 (2009), 469-475.
58