OPTICAL PROPERTIES OF DYE SENSITIZED ZINC OXIDE THIN FILM
DEPOSITED BY SOL-GEL METHOD
FARHATUL MU’MINAH BINTI ASAD
A thesis submitted in partial fulfillment of the
requirements for the award of the degree of
Master of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
JULY 2010
OPTICAL PROPERTIES OF DYE SENSITIZED ZINC OXIDE THIN FILM
DEPOSITED BY SOL-GEL METHOD
FARHATUL MU’MINAH BINTI ASAD
UNIVERSITI TEKNOLOGI MALAYSIA
iii
With lots of love and gratitude, I dedicate this to my beloved parent and siblings and
my closest friend.
Each and every one of you always makes me feel special and thank you for
believing me.
iv
ACKNOWLEDGEMENT
While preparing for this thesis, I was in contact with many people including
lecturers, researchers, and practitioners. I would like to express my deepest gratitude
to my supervisor, Professor Dr. Madzlan Aziz for his guidance, critics, and
motivation. Without your continued support and interest, this thesis would not have
been the same as presented here.
I’m also indebted to Universiti Teknologi Malaysia UTM for funding my
research study. Librarian of UTM and the staff in UTM also deserve special thank
for their assistance. I also would like to thank the staff of Universiti Tun Hussein
Onn for allow me to use their equipment while doing my research. That is really a
big help to finish my thesis.
Last but not least, I would like to thank my family member and friends who
has provided assistance at various occasion. Thank you for all people who has
involve while finishing my thesis. Thank you so much.
v
ABSTRACT
Metal oxide semiconductor has been widely studied due to it varied
properties and application. Among metal oxide, ZnO semiconductor is of great
interest due to wide band gap (3.2-3.4eV) with hexagonal wurtzite structure. This
study involves the comparison of optical properties of ZnO thin films when doped
with
erbium
and
terbium.
The
optical
properties
were
studied
using
Photoluminescence and Direct Reflector ultraviolet (DR-UV). FESEM image shows
that the irregular shape and there is variation in the surface morphology of undoped
and doped ZnO thin film. The grain size increased with addition of erbium and
terbium. The transmission spectra for all samples show high transmission with more
than 90%. However the band gap increase for doped ZnO thin film sensitized with
eosin B. The band gap value also decreases with the addition of erbium and terbium
for sensitized eosin Y. The XRD patterns of these samples are in close agreement
with the JCPDS standard (No. 36-1451). It was observed that all deposited films
were polycrystalline with hexagonal wurtzite structure with diffraction peaks
oriented along the (100),(002),(101),(102),(110) and (103) planes. The preferred
orientation for ZnO thin film sensitized with eosin Y and B is (101) plane. The
discrepancy may be caused by different preparation condition. The PL emission in
the UV bands was observed, peaking at the range of 365-387nm, centered at 380 at
near band edge emission, due to Er3+ transition from 4F7/2→4I15/2 and Tb3+ transition
from 5D4→7F1.
vi
ABSTRAK
Semikonduktor logam oksida telah dipelajari dengan meluas kerana
kepelbagaian sifat dan aplikasinya. Antara logam oksida, semikonduktor zink
oksida dipelajari kerana luang jalur yang luas (3.2-3.4eV) dengan struktur
heksagonal wutzit. Kajian ini melibatkan perbandingan sifat optikal zink oksida
saput tipis apabila didop dengan erbium dan terbium. Sifat optikal ini dikaji
menggunakan Photoluminescence dan Direct Reflector ultraviolet (DR-UV). Imej
FESEM menunjukkan bentuk tak tersusun dan terdapat variasi pada sifat permukaan
zink oksida dan dop-zink oksida saput tipis. Saiz partikel meningkat dengan
pertambahan erbium dan terbium. Transmisi spektra bagi semua sample
menunjukkan transmisi tinggi lebih 90%. Walaubagaimanapun, luang jalur
meningkat untuk dop zink oksida saput tipis yang dipekakan dengan eosin B.
Namun begitu luang jalur zink oksida berkurang dengan pertambahan erbium dan
terbium yang dipekakan dengan eosin Y.Bentuk XRD untuk semua sampel ini
mengikuti taraf JCPDS 36-1451. Ia menunjukkan semua saput tipis adalah
polikristal dengan struktur heksagonal wutzit degan puncak tersusun mengikut
tapak (100), (002), (101), (102), (110), dan (103). Orientasi terpilih intuk zink
oksida saput tipis yang dipekakan dengan eosin Y dan B adalah tapak (101).
Perbezaan orientasi terpilih mungkin disebabkan perbezaan penyediaan bahan.
Pancaran PL pada jalur UV diperhatikan dan puncaknya wujud dalam lingkungan
365-387nm,berpusat pada 380nm dekat dengan pancaran jalur disebabkan transisi
Er3+ dari 4F7/2→4I15/2 dan transisi Tb3+ dari 5D4→7F1.
vii
TABLE OF CONTENTS
CHAPTER
1
2
3
TITLE
PAGE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF FIGURES
x
LIST OF SYMBOLS / TERMS
xii
LIST OF APPENDICES
xiii
INTRODUCTION
1
1.1
Research Background
1
1.2
Background of Problem
5
1.3
Statement of Problem
6
1.4
Research Objectives
7
1.5
Scope of Study
8
LITERATURE REVIEW
9
2.0
Introduction
9
2.1
Feature of Zinc Oxide thin film
9
2.2
Effect of Doping
10
2.3
Effect of Annealing Temperature
12
2.4
Effect of Sensitizer
14
EXPERIMENTALS
16
viii
3.1
4
Introduction
15
3.2 Chemicals
17
3.3
17
Apparatus
3.4 Preparation of Zinc sol
17
3.5
Preparation of doped zinc oxide thin films
18
3.6
Dye sensitized zinc oxide thin films
18
3.7
Characterization
18
3.7.1
FESEM
19
3.7.2
XRD
19
3.7.3
DR-UV
20
3.7.4
Photoluminescence
20
RESULT AND DISCUSSION
21
4.1
21
Analysis by FESEM with microscope of
Zeiss SupraTM 35VP.
4.1.1 Zinc oxide thin films sensitized
22
by 0.010M eosin Y
4.1.2
Zinc oxide thin films sensitized
24
by 0.010M eosin B
4.2
Analysis by DR-UV Spectrophotometer
26
4.2.1 Zinc oxide thin films sensitized
26
by 0.010M eosin Y
4.2.2
Zinc oxide thin films sensitized
29
by 0.010M eosin B
4.3 Analysis by X-Ray Diffractor (XRD)
32
Bruker DS Advance
4.3.1 Zinc oxide thin films sensitized
33
by 0.010M eosin Y
4.3.2 Zinc oxide thin films sensitized
36
by 0.010M eosin B
4.4 Analysis by Photoluminescence of Perkin
39
Elmer Luminescence Spectrometer LS50B
4.4.1 Zinc oxide thin films sensitized by
0.010M eosin Y
39
ix
4.4.2 Zinc oxide thin films sensitized by
43
0.010M eosin B
5
CONCLUSIONS AND SUGGESTIONS
47
5.1
Conclusions
47
5.2
Suggestions for Further Work
48
REFFERENCES
49
APPENDICES A-C
52
x
LIST OF FIGURES
FIGURE NO
TITLE
PAGE
1.1
Eosin Y
4
1.2
Eosin B
4
3.1
Schematic representation of experimental procedure
16
4.1
Undoped zinc oxide thin films sensitized by eosin Y
22
4.2
Er-doped zinc oxide thin film sensitized by eosin Y
22
4.3
Tb-doped zinc oxide thin film sensitized by eosin Y
23
4.4
Undoped zinc oxide thin film sensitized by eosin B
24
4.5
Er-doped zinc oxide thin film sensitized by eosin B
24
4.6
Tb-doped zinc oxide thin film sensitized by eosin B
25
4.7
Optical transmittance spectra of undoped zinc oxide
26
thin film sensitized by eosin Y annealed at 600oC.
4.8
Optical transmittance spectra of Er-doped zinc oxide
27
thin film sensitized by eosin Y annealed at 600oC.
4.9
Optical transmittance spectra of Tb-doped zinc oxide
28
o
thin film sensitized by eosin Y annealed at 600 C.
4.10
Optical transmittance spectra of undoped zinc oxide
29
thin films sensitized by eosin B annealed at 600oC.
4.11
Optical transmittance spectra of Er-doped zinc oxide
30
thin films sensitized by eosin B annealed at 600oC
4.12
Optical transmittance spectra of Tb-doped zinc oxide
31
o
thin films sensitized by eosin B annealed at 600 C
4.13
XRD pattern of undoped ZnO films sensitized by
33
eosin Y annealed at 600oC for 1h.
4.14
XRD pattern of Er-doped ZnO films annealed sensitized
by eosin Y at 600oC for 1h.
34
xi
4.15
XRD pattern of Tb-doped ZnO films sensitized
35
by eosin Y annealed at 600oC for 1h.
4.16
XRD pattern of undoped ZnO films sensitized by eosin B
36
annealed at 600oC for 1h
4.17
XRD pattern of Er-doped ZnO films sensitized by eosin
37
o
B annealed at 600 C for 1h
4.18
XRD pattern of Tb-doped ZnO films sensitized by
38
eosin B annealed at 600oC for 1h
4.19
PL spectra of 600oC annealed undoped ZnO thin films
40
sensitized by eosin Y; with excitation 230nm
4.20
PL spectra of 600oC annealed Er-doped ZnO thin films
41
sensitized by eosin Y; with excitation 230nm
4.21
PL spectra of 600oC annealed Tb-doped ZnO thin films
42
sensitized by eosin Y; with excitation 230nm
4.22
PL spectra of 600oC annealed undoped ZnO thin films
43
sensitized by eosin B; with excitation 230nm
4.23
PL spectra of 600oC annealed Er-doped ZnO thin films
44
sensitized by eosin B; with excitation 230nm
4.24
PL spectra of 600oC annealed Tb-doped ZnO thin films
sensitized by eosin B; with excitation 230nm
45
xii
LIST OF SYMBOLS
DRUV
-
Direct reflector-ultraviolet
Er
-
Erbium
FESEM
-
Field emission scanning electron microscope
Nm
-
Nanometer
Tb
-
Terbium
XRD
-
X-ray diffraction
xiii
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
Lanthanide transition energy
52
B
Calculation of chemical used in experiment
53
C
Calculation of band gap using UV data
55
CHAPTER 1
INTRODUCTION
1.1
Research Background
Zinc comes from d-block element from periodic table. Within this group, in
each case the d orbital are filled. Group IIB elements decrease in electropositive
tendencies with increasing atomic number, although the change is not marked
between zinc and cadmium. These two metals oxidize only superficially in air at
room temperature, and are used as plating metals. They will however, oxidize
readily in dilute temperatures. Zinc normally dissolves readily in dilute acids but
pure samples are much more resistant to attack than commercial metal. Zinc and
cadmium form many compounds involving direct links to carbon, as well as
nitrogen, sulphur, etc., but zinc appears to form more stable links with oxygen than
cadmium (Cooper, 1968). ZnO belongs to a group of hexagonal wurtzite, 6 -mm
symmetry, wide band gap (Eg =3.35 eV, 300K). ZnO has been used in a wide range
of device applications due to its semiconducting electrical, optical and piezoelectric
properties (Wen and Jing, (2007).
Metal Oxide semiconductor films have been widely studied and have
received considerable attention in recent years due to their optical and electrical
properties. Some of them are good candidates for transparent conductive oxide
films. Among them ZnO is one of the metal oxide semiconductors suitable for use
in optoelectric devices. It is an alternative material to tin oxide and indium tin oxide,
which have been most used to date (Jin Hong and Byong Ok, (2003). Zinc oxide is
an important oxide semiconductor for toxic and combustible gas sensing
2
applications. ZnO gas sensor elements have been fabricated in various forms, such
as, single crystals, wintered pellets, thick films, thin films and heterojunctions
(Mitra et al., 1998). Among the metal oxides, SnO2 semiconductor gas sensor is of
great interest from a scientific and technological point of view. In fact,
commercially available gas sensors are mainly based on SnO2 in the form of thick
films, pellets or thin films (Rella et al., 1997).
The development solid-state chemical sensors, and more specifically gas
sensors, is currently being carried out intensively since environmental pollution and
security in the work and domestic ambient atmospheres represent acute problems
with a high social impact (Rella et al.,1997). Materials that change their properties
depending on the ambient gas can be utilized as gas sensing materials. Usually
changes in electrical conductance in response to environmental gases are monitored.
Many metal oxides are suitable for detecting combustible, reducing, or oxidizing
gases. However, the most commonly used gas sensing materials are ZnO and SnO2
(Matthias and Ulrike, 2005).
Undoped and doped zinc oxide thin films have a wide range of applications
as an important semiconductor material such as solar cells, electrical, piezoelectric
or luminescent devices and also as gas sensors and chemical sensors (Musat et al.,
2008). It is well known that chemical doping greatly influences the electronic and
optical properties of ZnO. Al, In and Ga-doped ZnO thin films are one of the
promising transparent conductive oxides for advanced applications such as displays
and electro chromic devices (Ka Eun et al., 2009).
Terbium was discovered by C.G. Mosander in 1843 in Stockholm, Sweden.
Terbium with the symbol Tb and atomic number 65 is a silver-grey metal,
malleable, ductile, and soft enough to be cut with a knife. Terbium can be recovered
from the mineral monazite by ion exchange and solvent extraction, and from
euxenite, a complex oxide containing 1% or more of terbium. It is usually produced
commercially by reducing the anhydrous fluoride or chloride with calcium metal.
Terbium is used to dope calcium fluoride, calcium tungstate and strontium
3
molybdate, all used in solid-state devices. Terbium salts are used in laser devices,
but otherwise this element is not widely used.
Erbium is a chemical element in the lanthanide series with the symbol Er
and atomic number 68. Natural erbium is always found in chemical combination
with other elements on earth. Erbium’s principle uses involves its pink-colored ions,
which have optical fluorescent properties particularly useful in certain laser
application. Erbium is a trivalent element, pure erbium metal is malleable (easily
shaped), soft yet stable in air and does not oxidize quickly as some other rare-earth
metals. The principle commercial sources of erbium are from the minerals xenotime
and euxenite same as other lanthanides. Erbium compounds are of low to moderate
toxicity. Erbium-doped optical silica glass fibers are the active element in erbiumdoped fiber amplifiers (EDFAs), which are widely used in optimal communication.
Dye-sensitized solar cell (DSSC) is a relatively new class of low-cost solar
cell, which belongs to the group of thin film solar cells. It’s based on a
semiconductor formed between a photo-sensitized anode and an electrolyte, a
photoelectrochemical system. A dye can generally be described as a colored
substance that has an affinity to the substrate to which it is being applied. The dye is
generally applied in an aqueous solution, and may require mordant to improve the
fastness of the dye on the fiber.
Eosin is a fluorescent red dye resulting from the action of bromine on
fluorescein. There are actually two much related compounds commonly referred to
as eosin. Most often used is eosin Y. It has a very slightly yellowish cast. The other
eosin compound is eosin B; it has a very faint bluish cast. The two dyes are
interchangeable where Eosin Y is a tetrabromo derivate of fluorescein and eosin B
is a dibromo dinitro derivate of fluorescein.
4
Figure 1.1: Eosin Y
Figure 1.2: Eosin B
The properties of films are strongly dependent upon the method of
preparation. As a result, a wide range of different deposition techniques have been
investigated. These include chemical vapor deposition, spray pyrolysis, electron
beam evaporation, and sputtering. The sol gel technique offers many advantages
over other method, such as low temperature processing, precise control of the
doping level, and simplicity not requiring expensive deposition facilities. The solgel method is a “wet chemistry” approach to make thin films. A sol is a colloidal
mixture of solid particles dissolved in a solvent. After a stable sol is prepared with
the desired composition, this precursor solution can be easily deposited onto surface
and spin-coated to make a thin film. The film is then heated to drive the formation
of a gel. Subsequent high temperature annealing removes organic residues and
makes solid thin film denser (Ken, 2004). The films obtained by the sol gel
technique have a porous structure consisting of ultra-fine particles, thereby offering
a large specific surface area. Such a structure is desirable for gas sensor application
(Racheva and Critchlow, 1997).
5
1.2
Background of Problem
Metal Oxide Semiconductor have been widely studied the properties and
application. Zinc oxide thin film is extensively studied because of its potential
application in various fields such as gas sensor, solar cells, photo-detector, light
emitting diodes and laser system. Due to its non-toxicity, abundance and chemical
stability even under a reducing environment, zinc oxide thin film could surpass the
performance of indium oxide and tin oxide in some applications. Zinc oxide thin
film with band gap value of 3.42eV at 300K, and as result of oxygen efficiency
and/or zinc interstitials shows intrinsic n-type conductivity.
Magnetron sputtering is generally accepted as an optimum method for
preparing zinc oxide based film. However, sputtering requires a complex and
expensive vacuum technique and has its distinct challenges when it comes to large
area coating and film deposition on substrates featuring a complex geometry. An
alternative deposition technique that can easily overcome these issues is sol-gel
method. The sol-gel method has distinct potential advantages over other technique
due to its lower crystallization temperature, ability to tune microstructure via sol-gel
chemistry, conformal deposition ability, compositional control and large surface
area coating capability (Shane et al., 2007).
The numerous factors that affect the properties of sol gel based ZnO films
fall into three main categories. They are: (1) the chemistry of the sol gel method, (2)
the interaction between the film and the substrate during the coating process, and (3)
post heat treatment. All these factors have a distinct effect on the films
microstructure. This especially holds true in the case of sol-gel based multi-layer
coatings which are utilized to overcome cracking issues in thick layers. The main
parameters of interest have been heating temperature and heating time.
The used of anhydrous zinc acetate, which avoids the introduction of large
amounts of water to the sol-gel, enabling control over reactions taking place within
the sol-gel method (Shane et al., 2007).One of the most promising materials for
room temperature
ferromagnetism
is
transition
metal-doped
zinc
oxide.
6
Ferromagnetism operates via mechanism by which itinerant carriers are spin
polarized and mediate ferromagnetic ordering between the widely spaced dopant
ions. Nowadays, Mn-doped ZnO has been extensively studied for it expected
ferromagnetism and magneto-optical application at room temperature. But Mn
doping severely suppresses the luminescence of Zn1-xMnxO even at a very low
doping level (Srinivasan and Kumar, 2008).
Photoactive materials in dye-sensitized solar cells are typically prepared by
stepwise processing start with the formation of porous films of inorganic
semiconductors by colloid coating and heat treatment (around 450oC) followed by
dye adsorption by dipping the film in the dye solution.
1.3
Statement of Problem
The used of anhydrous zinc acetate, which avoids the introduction of large
amounts of water to the sol-gel, enabling control over reactions taking place within
the sol-gel method. Magnetron sputtering is generally accepted as an optimum
method for preparing ZnO based film. Sputtering requires a complex and expensive
vacuum technique and has its distinct challenges when it comes to large area
coating and film deposition on substrates featuring a complex geometry. The sol-gel
method is preferred due to its lower crystallization temperature, ability to tune
microstructure via sol-gel chemistry, conformal deposition ability, compositional
control and large surface area coating capability. Even though Mn-doped has been
extensively studied before it severely suppresses the luminescence of Zn1-xMnxO at
very low doping. The quick decrement of isolated Mn2+ with increasing x is
considered to be responsible for the severe suppression of the blue and ultraviolet
luminescence. The green emission is also quenched, which is generally attributed to
the passivating action of Mn on surface defects (Srinivasan and Kumar, 2008).
The two main drawbacks of the colloidal process is an active control of the
crystallographic structure and pore sizes of the inorganic phase is difficult which
7
resulting in electron traps and resistance to the electron percolation. Besides that
heat treatment limit the substrate choice to heat-resistant materials, thus excluding
temperature-sensitive materials such as polymers. Sol gel technique has it own
problem concerning annealing temperature because too high temperature may lead
to overgrown dopant precipitates, while annealing at too low temperature would
lead to in sufficient pyrolysis, crystallization thus poor film properties. The problem
is rather serious for TiO2 composite films because a relatively high temperature is
required to remove the organic byproduct and to crystallize the oxide. In this
experiment this problem is not serious since the main solvent is volatile and ZnO
can easily crystallize out at relatively low temperature (Xiang Hua et al., 2003).
1.4
Research Objectives
Based on the research background and problem statement mentioned above,
the objective of this study is to investigate the influence of addition of dopants in
zinc oxide thin films as well as their optical properties when sensitized with Eosin Y
and Eosin B.
1.5
Scope of Study
In order to achieve the above objective, this study starts with the preparation
of zinc sol precursor. Zinc sol precursor was prepared by sol gel method. Zinc sol
was prepared by using zinc acetate dehydrate as starting material and dissolved in
absolute ethanol and diethanolamine (DEA) at room temperature. The solution was
stirred vigorously and yields a clear and homogenous solution. The solution was
prepared in three conditions as undoped zinc oxide thin films, erbium doped zinc
oxide thin films and terbium doped zinc oxide thin films. Each solution was
prepared in two sample as each the sensitized with eosin Y and eosin
B.respectively. Characterization of structure, morphology and optical properties was
8
measured by X-Ray diffraction (XRD), field-emission scanning electron
microscope (FESEM), photoluminescence (PL), and direct reflector ultraviolet
(DRUV). Comparison of characterization will be made between zinc oxide thin
films, erbium doped zinc oxide thin film and terbium doped zinc oxide thin films
with different dye.
CHAPTER 2
LITERATURE REVIEW
2.0
Introduction
The modern industrial process is based on the capacity of measuring and
controlling physical and chemical parameters, as temperature pressure, and
chemical composition (Hong Xia et al., 2005). A wide variety of electronic and
chemical properties of metal oxides makes them exciting materials for basic
research and technological applications alike. Oxides span a wide range of electrical
properties from wide band –gape insulators to metallic and superconducting.
Another field in which oxides play a dominant role is in solid state gas sensors
(Ohyama et al., 1996).
2.1
Feature of Zinc Oxide thin films
Among them ZnO is one of the metal oxides semiconductors suitable for use
in optoelectric devices. Zinc Oxides is extensively studied because of its potential
applications in various fields such as gas sensor, solar cells, photo-detector, light
emitting diodes (LEDs) and laser systems, etc. The properties of ZnO thin films are
much influenced by not only the growth methods but also the heat treatment
parameters, especially the thermal annealing (Musat et al., 2008). ZnO is usually an
n-type compound semiconductor with hexagonal wurtzite structure and a wide band
gap (3.2–3.4 eV) at room temperature (Bhattacharyya et al., 2007)
10
Pure ZnO exceeds the field effect mobility of hydrogenated amorphous
silicon (a-Si:H) in its carrier mobility, which serves as the active channel layer in
typical TFT arrays. Furthermore, polycrystalline ZnO films can be prepared in a
normal atmosphere and possess low photosensitivity. Therefore, ZnO may replace
a-Si:H as an active layer (Chien Yie et al., 2008). Due to large amount of oxygen
(O) vacancy the pure ZnO films have a lower stability in corrosive and humid
ambient. The properties of such zinc oxide films are often altered by adsorption of
O2 and the water. Thermal instability is another factor that limit the application of
pure ZnO as a transparent conductive film The properties of ZnO films are
controlled by appropriate doping either by cationic (A, In) or anionic (F)
substitution and post-growth annealing to overcome this disadvantages (Lupan et
al., 2009).
2.2
Effect of Doping
The conductivity of ZnO can be control by it stoichiometry and doping
(Ohyama et al., 1996). Many studies about effect of doping on the properties of
undoped and doped ZnO films have been widely reported. Undoped ZnO thin films
are not stable especially at high temperatures so by doping the zinc oxide can
overcome the problem. Doping leads to an increase in the conductivity of the ZnO
thin films. The ZnO doping is achieved by replacing Zn2+ atoms with atoms of
elements of higher valence such as indium, aluminium and gallium
and the
efficiency of the dopant element depends on its electronegativity and difference
between its ionic radius and the ionic radius of zinc (Nunes et al., 2002). One of
them is effect of fluorine which is group III elements in ZnO. It is a good candidate
to be used as a high quality transparent electrode in the solar cell application.
Fluorine- Zinc oxide thin films have a good crystallinity with a proper hexagonal
wurtzite phase formation but its crytallinity was deteriorated with the increasing of
fluorine content. Its preferential growth strongly affected by the variation in the
fluorine content (Saliha et al., 2008).
11
Another transition metal doped Zinc oxide that has been studied is Mndoped ZnO. But the addition of Mn doping severely suppresses the luminescence of
Zn1-xMnxO at very low doping level. Mn- doped ZnO film shows the formation of
many long nanoside rods all over the surface. The optical transmission spectra
found to be maximum for undoped ZnO and decreased within Mn-doped ZnO film.
The decrease in optical transmission is associated with the loss of light due to the
oxygen vacancies and scattering at grain boundaries. The increase in band gap from
bulk ZnO with dopant concentration (x) can be attributed to the sp-d spin exchange
interaction between the band electrons and localized spin of the transition metal
ions. The substitution of Mn2+ into the lattice caused the decrease in the lattice
parameter and ZnO crystalline size (Srinivasan and Kumar, 2008).
The effect of concentration of Mn doping on the properties of ZnO thin
films has also been reported. From the report it shows that Mn impurities do not
change the wurtzite structure of ZnO. From the XRD pattern it shows that (002)
peak intensity of Mn-doped ZnO is higher than that of the undoped film. However,
more doping concentration deteriorated the crystallinity of films which may due to
the formation of stresses by the difference in ion size between zinc and manganese,
and the segregation dopants in grain boundaries for high doping concentration. The
electrical properties also highly affected by doping concentration. It is clear that the
resistivity of ZnO films rapidly increase with the concentration of Mn doping (Wen
and Jing, 2007).
Doped zinc oxide films have high conductivities and result in producing
interesting structural, optical and electrical properties (Yung et al., 2009). Much
effort have been made to modify and tailor the visible emissions of ZnO by
introducing impurities including Al, Mg, Li,V, In, and trioctylamine /
trioctylphosphine oxide. Ohashi et al. prepared Li and Al codoped ZnO (LAZO)
powders by mixing ZnO powder with aqueous solution of LiCl and Al(NO)3 , then
drying the mixture and annealing it at 900 °C in oxygen. They observed yellowishwhite luminescence from the LAZO powders. The incorporation of Al into ZnO led
to a slight degeneration in crystallinity due to trivalent cation doping. The
concentration of the zinc interstitials is reduced for charge compensation, resulting
12
in suppressed ZnO grain growth and degenerated crystallinity. Intensity and full
width at half-maximum (FWHM) of diffraction peaks were not sensitive to the Lidoping, indicating that the film crystallinity was not degenerated with increasing Li
concentration.
The UV and green emissions from the pure ZnO thin films are attributed to
free exciton emission and oxygen vacancy, respectively. Doping ZnO with Al leads
to green emission, while Li-doping causes yellow emission. The green and yellow
emissions of the LAZO films are attributed to donor–acceptor-pair (DAP)
transitions involving a Zn vacancy and Li as the acceptor state, respectively, and the
donor responsible for both emissions to oxygen vacancies. Al-doping causes
neutralization of the deep donor levels. An improvement in film crystallinity at
higher annealing temperatures causes enhanced emissions. Moreover, the yellowishwhite emission is enhanced with increasing Li-doping concentration at a fixed
annealing temperature because increased Li-doping leads to the formation of more
oxygen vacancies thus enhancing the DPA transitions (Hongen et al., 2010).
.
2.3
Effect of Annealing Temperature
One of the other sensitive factors that affect the crystal quality, oxygen
defect and local structure of semiconductor oxides during sample preparation
process is annealing temperature. Moreover, there is report about the annealing
effects for Er-doped ZnO films and bulk. When ZnO:Er annealed at different
temperature they exhibit only at peak (002) which imply a polycrystalline
hexagonal wurtzite crystal structure with preferred c-axis orientation. Intensity of
(002) peak when annealed lower than 800oC and higher annealing temperature leads
to better crystallinity and the films that annealed at 800oC shows the highest degree
of preferential orientation. The improvement of crystallinity is very slight when
annealing temperature is higher than 800oC but the preferential orientation becomes
weak. Besides that, the grain size became enlarges as the annealing temperature
13
increases. At very high temperature, the grains grow and aggregate quickly
(Fanyong et al., 2008).
There is the transparent conductive oxide (TCO) film that widely used
which is indium tin oxide because of their good electrical and optical properties.
However because of the problem of high cost, low stability in hydrogen plasma and
toxicity appear make Al-doped ZnO as another alternative to replace indium tin
oxide because of it competitive properties and low fabrication cost. The films were
annealed at different temperature. The resistivity of film decreases as the drying
temperature increases. The crystalline orientation has a trivial effect on the electrical
property of the Al-doped ZnO films at lower drying temperatures. Intensity of the
peak (002) increases with the increasing of annealing temperature which indicates
that the crystallinity was better at high annealing temperature. But intensity peak of
the (002) decrease as the cooling rate increase which shows that crystallinity of Aldoped ZnO deteriorates as cooling rate increase. This showed the influence of
cooling rates on the contribution of the oxygen vacancies in the films. The oxygen
vacancies may form when the oxygen atoms run out from the films at high
temperature. The crystallite size has better crystallinity when annealed at higher
temperature (Mei Zhen et al., 2009).
For undoped ZnO thin films, annealing temperature still play important role
to determine the optical constant. Before, Senadium et al. investigate the annealing
effect on the properties of ZnO films prepared by pulsed filtered cathodic vacuum
arc deposition. From there they got that the refractive index of ZnO films increases
with increasing annealing temperature from 200 to 600oC. Only few reported the
effect of annealing temperature above 600oC on optical constant of ZnO film. There
is a report of prepared film was annealed at different temperatures from 600 to
950oC in argon ambient. It showed that the optical transmittance decreases in the
visible region, while it increases in the UV region with the increasing annealing
temperature. Aly et al. found that the thermal annealing in air could improve the
optical transmittance of ZnO films attributing to the oxygen reaction with ZnO.
When annealing temperature reach 750oC, the stoichiometry of the thin film
improved, but after annealing temperature exceeds 750oC, thermal induced defects
14
increase dramatically thus generates low quality of ZnO film. The optical constant
of ZnO film decrease in the ultraviolet region while increase in visible region (Xue
et al., 2008).
2.4
Effect of Sensitizer
By introducing dyes with photoactive layer, the light absorption in the active
layer can be increased (Suresh et al., 2008). The dye is promoted into an excited
state upon light absorption from where an ultrafast electron injection process takes
places into the conduction band of the semiconductor. The present of the redox
couple in the electrolyte and suitable energy band positioning ensure the
regeneration of the dye as well as diffusion of the charge carriers (Frederic et al.,
2009). Alzarine dyes possess high electron affinity and high absorption coefficient.
There were a report that these dyes were used as electron acceptors in conjugated
polymer/ dye composite photovoltaic devices and dye-sensitized solar cells (Suresh
et al., 2008).
Recently, a one-step low temperature cathodic electro deposition of dyemodified zinc oxide films process has been proposed by combine ease of
preparation and high dye loading due to high porosity of the film. There are a few
successful adsorbed dyes on zinc oxide thin films by such method including
riboflavin
5
phosphate,
tetrabromophenol
blue,
tetrasulfonated
metallophthalocyanines, N3, and D149 or eosin-Y. Eosin Y has been recently used
in development trend due to the potential lower cost and easier recycling of metal
free dye-based cells. Characterized eosin Y/ZnO gives high electron efficiency,
better electron transport properties in the electrodeposited films that their
counterparts and automatic formation of the desired porous structure of the film by
hindering of the crystal growth of ZnO. It definitely gives advantage from
experimental point of view because it does not involve high temperature, high
mechanical stress, or aggressive chemicals therefore it is environmental friendly and
compatible with conductive plastic film substrates (Frederic et al., 2009).
CHAPTER 3
EXPERIMENTALS
3.1
Introduction
In this chapter, the experimental work on preparation of zinc oxide thin
films is described in detail. It consists of several stages as shown in figure 3.1. The
method used in preparation of zinc oxide thin films is by sol gel method. The initial
stage involved preparation of precursor zinc sol, followed by addition of dopant,
sensitized by eosin Y and B and finally characterization of the thin films.
16
Preparation of
Precursor
Zinc acetate dehydrate
Absolute ethanol
Diethanolamine
Doped Zinc oxide thin films
Erbium
Terbium
Dye
Characterization
XRD
FESEM
PL
Figure 3.1: Schematic representation of experimental procedure.
3.2
Chemicals
DRUV
17
The chemicals used to produce ZnO thin films were zinc acetate dehydrate
(G.E), absolute ethanol (C2H5OH, 99.7% v/v min, Hayman Limited), polyethelene
glycol 300 (Fluka Biochemica), diethanolamine (C4H11NO2), terbium (III) nitrate
pentahydrate (Tb (NO3)3.5H2O, 99.9% purity, Sigma-Aldrich Chemie), erbium (III)
nitrate pentahydrate (Er (N03)3. 5H20, 99.9% purity, Sigma-Aldrich Chemie), eosin
Y (C20H6Br4Na2O5) and eosin B (C20H6Br2N2Na2O9).
3.3
Apparatus
The apparatus that has been used in this experiment are crucible, dropper,
measuring cylinder, electronic balance, filter tunnel, spatula, plastic wash bottle,
beaker, electronic bath, petri dish, spin coater, desiccator, hotplates, filter paper,
forceps, gloves, containers, pyrex glasses substrate..
3.4
Preparation of Zinc Sol
Sol gel method was used to prepare zinc oxide thin films. Zinc acetate
dehydrate (Zn (CH3COO)2. 2H2O), absolute ethanol and diethanolamine
(HN(CH2CH2OH)2) were used as starting material, solvent and stabilizer
respectively. 0.5M methanolic solution was prepared by dissolving 0.05 mol of zinc
acetate dehydrates in 0.1L of absolute ethanol. After vigorously stirred for 30
minute DEA was added drop by drop under stirring. The molar ratio of DEA to zinc
acetate was maintained at unity. The solution became homogeneous and clear after
stirred about 20 minute.
3.5
Preparation of doped zinc oxide thin films
Erbium 5 % (Er (NO3)3.5H2O) was added in the zinc sol. Then the sol was
stirred vigorously for 30 minute. Polyethylene Glycol (PEG) was added in the
18
solution and stirred about 30 minute. After the sol was prepared, it was filtered
through a syringe filters and dropped onto pyrex glass substrate which were rotated
at 3000 rpm for 30 second. After being deposited by spin coating, the films were
dried at 100oC for 30 minute to evaporate the solvent and remove organic residuals.
The procedure was repeated for 6 times. After that the thin films were annealed at
600oC. The same procedures were used to prepare terbium doped zinc oxide thin
films.
3.6
Dye sensitized zinc oxide thin films
Two types of dyes were used which is Eosin Y and Eosin B. 0.010M.
EosinY was prepared by dissolving 0.69186 g of Eosin Y in 25ml of distilled water
and 25 ml of absolute ethanol. The solution was stirred vigorously. After that eosin
Y were rinsed on the zinc oxide thin films. The films were leave for 1 hour at room
temperature. Then dye- sensitized zinc oxide thin films were dried at 100oC for 1
hour. Same procedure was used for 0.010M of eosin B.
3.7
Characterization
Characterization of zinc oxide thin films and doped zinc oxide thin films and
doped zinc oxide thin films when sensitized with eosin Y and eosin B was undergo
to determine the morphology, structure and optical properties of this thin films. The
instrument used in this experiment is field-emission scanning electron microscope
(FESEM) with microscope of Zeiss SupraTM 35VP, X- Ray Diffractor (XRD)
Bruker D8 Advance, DR-UV Spectrophotometer and Photoluminescence of Perkin
Elmer Luminescence Spectrometer LS50B.
19
3.7.1
Field-emission scanning electron microscope (FESEM) with microscope
of Zeiss SupraTM 35VP
A field-emission cathode in the electron gun of a scanning electron
microscope provides narrower probing beams at low as well as high electron
energy, resulting in both improved spatial resolution and minimized sample
charging and damage. The application of FESEM includes semiconductor device
cross section analyses for gate widths, gate oxides, film thicknesses, and
construction details. FESEM produces clearer, less electrostatically distorted images
with spatial resolution down to 1 1/2 nm and it is 3 to 6 times better than
conventional scanning electron microscope (SEM).
3.7.2
X- Ray Diffractor (XRD) Bruker D8 Advance
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. When a monochromatic X-ray beam with
wavelength lambda is projected onto a crystalline material at an angle theta,
diffraction occurs only when the distance traveled by the rays reflected from
successive planes differs by a complete number n of wavelengths. Based on the
principle of X-ray diffraction, a wealth of structural, physical and chemical
information about the material investigated can be obtained. A host of application
techniques for various material classes is available, each revealing its own specific
details of the sample studied.
3.7.3
DR-UV Spectrophotometer
It measures the intensity of light passing through a sample (I), and compares
it to the intensity of light before it passes through the sample (Io). The ratio I / Io are
called the transmittance, and are usually expressed as a percentage (%T). The basic
20
parts of a spectrophotometer are a light source, a holder for the sample, a diffraction
gratings or monochromator to separate the different wavelengths of light, and a
detector. The radiation source is often a tungsten filament (300-2500 nm), a
deuterium arc lamp, which is continuous over the ultraviolet region (190-400 nm)
— or more recently, light emitting diodes (LED) and Xenon Arc Lamps for the
visible wavelengths. The detector is typically a photodiod or CCD. Photodiodes are
used with monochromators, which filter the light so that only light of a single
wavelength reaches the detector. Diffraction gratings are used with CCDs, which
collects light of different wavelengths on different pixels.
3.7.4
Photoluminescence of Perkin Elmer Luminescence Spectrometer
LS50B.
Photoluminescence is an important technique for measuring the purity and
crystalline quality of semiconductors. Photoluminescence (abbreviated as PL) is a
process in which a substance absorbs photons (electromagnetic radiation) and then
re-radiates photons. Quantum mechanically, this can be described as an excitation to
a higher energy state and then a return to a lower energy state accompanied by the
emission of a photon. This is one of many forms of luminescence (light emission)
and is distinguished by photo excitation (excitation by photons).
CHAPTER 4
RESULT AND DISCUSSION
Zinc oxide thin films have gone through characterization by field-emission
scanning electron microscope (FESEM) with microscope of Zeiss SupraTM 35VP,
X- Ray Diffractor (XRD) Bruker D8 Advance, DR-UV Spectrophotometer and
Photoluminescence of Perkin Elmer Luminescence Spectrometer LS50B to
determine the morphology, structure and optical properties of undoped ZnO thin
film, doped ZnO thin film and the properties if the films when sensitized with dye.
4.1
Analysis by field-emission scanning electron microscope (FESEM) with
microscope of Zeiss SupraTM 35VP
All the samples that have been prepared will undergo characterization by
FESEM to determine the morphology and the grain size of each sample.
22
4.1.1
Zinc Oxide thin films sensitized by 0.010M Eosin Y
Figure 4.1: Undoped zinc oxide thin films sensitized by eosin Y; grain size=
42.80nm.
Figure 4.2: Er-doped zinc oxide thin films sensitized by eosin Y; grain
size=61.80nm
23
Figure 4.3: Tb-doped zinc oxide thin films sensitized by eosin Y; grain size=
69.77nm.
From the figures above it shows that the grain size for undoped zinc oxide,
erbium doped zinc oxide and terbium doped zinc oxide thin films are 42.80nm,
61.8nm and 69.77nm respectively. The grain size of doped zinc oxide thin films is
bigger that undoped zinc oxide thin films. All the films were annealed at 600oC
respectively. FESEM image show irregular shape for undoped zinc oxide film and
became more agglomerate for doped zinc oxide thin films.
24
4.1.2
Zinc Oxide thin films sensitized by 0.010M Eosin B
Figure 4.4: Undoped zinc oxide thin film sensitized by eosin B; grain size=84.06nm
Figure 4.5: Er-doped zinc oxide thin films sensitized by eosin B; grain
size=116.45nm.
25
Figure 4.6: Tb-doped zinc oxide thin films sensitized by eosin B; grain
size=131.83nm.
From the figures above, it shows that the grain size for undoped zinc oxide,
erbium doped zinc oxide and terbium doped zinc oxide thin films are 84.6nm,
116.45nm and 131.83nm respectively. The grain size of doped zinc oxide thin films
is bigger that undoped zinc oxide thin films. All the films were annealed at 600oC
respectively.
From the range of the grain size between zinc oxide thin films sensitized
with eosin Y and eosin B it show that the grain size for zinc oxide thin films
sensitized with eosin B is much bigger than zinc oxide thin films sensitized with
eosin Y.A granular and porous structure could be seen.
26
4.2
Analysis by DR-UV Spectrophotometer
All samples have been characterized by DR-UV spectrophotometer to
determine the optical properties as well as their conductivity. All samples have been
prepared by sol-gel method.
4.2.1
Zinc oxide thin films sensitized by 0.010M Eosin Y
200.0
180
160
140
379.93,114.20
120
%T
100
80
60
40
20
0.0
340.0
350
360
370
380
390
400
410
420
430
440
450.0
NM
Figure 4.7: Optical transmittance spectra of undoped zinc oxide sensitized by eosin
Y annealed at 600oC
The above figure show the optical transmittance spectra of undoped zinc
oxide thin film annealed at 600oC and measured in the range 340-450nm. The film
exhibit a transmittance 114.2% in the visible light region with sharp adsorption edge
around 379.93nm. The value of the band gap is 3.266eV.
27
150.0
140
130
120
110
381.51,94.385
100
90
80
%T 70
60
50
40
30
20
10
0.0
340.0
350
360
370
380
390
400
410
420
430
440
450.0
NM
Figure 4.8: Optical transmittance spectra of Er-doped zinc oxide sensitized by eosin
Y annealed at 600oC
The above figure show the optical transmittance spectra of erbium doped
zinc oxide thin film annealed at 600oC and measured in the range 340-450nm. The
film exhibit a transmittance 94.385% in the visible light range with a sharp
absorption edge around 381.51nm. The value of the bandgap is 3.252eV.
28
200.0
180
380.56,152.14
160
140
120
%T
100
80
60
40
20
0.0
340.0
350
360
370
380
390
400
410
420
430
440
NM
Figure 4.9: Optical transmittance spectra of Tb-doped zinc oxide sensitized by
eosin Y annealed at 600oC
The above figure show the optical transmittance spectra of terbium doped
zinc oxide thin film annealed at 600oC were measured in the range 340-450nm. The
film exhibit a transmittance 152.14nm in the visible light region with a sharp
adsorption edge around 380.56nm. The value of the bandgap is 3.260eV.
Optical transmittance spectra of undoped zinc oxide, Er-doped ZnO and Tbdoped ZnO thin films sensitized by eosin Y in the wavelength range (340-450nm)
are compared. The transmission spectra for all zinc oxide thin films which are
sensitized with 0.010M eosin Y was higher than 90%. The transmission spectra for
Tb-doped ZnO thin films are higher than Er-doped ZnO thin films and undoped
ZnO thin film. Surprisingly, undoped zinc oxide thin film has higher transmittance
spectra than Er-doped ZnO thin films.
There is a different in the bandgap for undoped ZnO, erbium-doped ZnO
and terbium-doped ZnO thin films. It shows that the bandgap of undoped ZnO thin
film is higher than erbium-doped ZnO and terbium-doped ZnO. Even though
450.0
29
terbium-doped ZnO thin film shows highest transmittance among them, the value of
bandgap is still lower than undoped zinc oxide thin film. It also shows that the
addition of dopant in the zinc oxide thin film caused the bandgap to decrease.
Oxygen vacancies and scattering at grain boundaries cause the decreased in optical
transmittance which associated with the loss of light (Srinivasan and Kumar, 2008).
4.2.2
Zinc oxide thin films sensitized by 0.010M Eosin B
200.0
180
160
382.30,143.01
140
120
%T
100
80
60
40
20
0.0
340.0
350
360
370
380
390
400
410
420
430
440
450.0
NM
Figure 4.10: Optical transmittance spectra of undoped zinc oxide thin films
sensitized by eosin B annealed at 600oC.
Figure 4.10 show the optical transmittance spectra of undoped zinc oxide
thin film annealed at 600oC were measured in the range 340-450nm. The film
exhibit a transmittance 143.01% in the visible light-light region with a sharp
absorption edge around 382.30nm. The value of the bandgap is 3.245eV.
30
200.0
180
380.03,151.69
160
140
120
%T
100
80
60
40
20
0.0
340.0
350
360
370
380
390
400
410
420
NM
Figure 4.11: Optical transmittance spectra of Er-doped zinc oxide thin films
sensitized by eosin B annealed at 600oC
The above figure show the optical transmittance spectra of erbium doped
zinc oxide thin film annealed at 600oC were measured in the range 340-430nm. The
film exhibit a transmittance 151.69% in the visible-light region with a sharp
absorption edge around 380.03nm. The value of the bandgap is 3.265eV.
430.0
31
200.0
180
160
381.66,136.05
140
120
%T
100
80
60
40
20
0.0
340.0
350
360
370
380
390
400
410
420
430
440
450.0
NM
Figure 4.12: Optical transmittance spectra of Tb-doped zinc oxide thin films
sensitized by eosin B annealed at 600oC
Figure 4.12 show the optical transmittance spectra of terbium doped zinc
oxide thin film annealed at 600oC measured in the range 340-450nm. The film
exhibit a transmittance 136.05% in the visible light region with a sharp absorption
edge around 381.66nm. The value of the bandgap is 3.251eV. Optical transmittance
spectra of undoped zinc oxide, Er-doped ZnO and Tb-doped ZnO thin films
sensitized by eosin B in the wavelength range (340-450nm) are compared. It shows
that the transmission spectra of erbium-doped zinc oxide thin film are higher than
undoped zinc oxide and terbium doped zinc oxide thin film. It also consistent with
the bandgap value where erbium doped zinc oxide thin film has higher value which
is 3.265eV compare to undoped zinc oxide and terbium doped zinc oxide thin film
sensitized by eosin B.
The transmission spectra for all zinc oxide thin films which are sensitized
with 0.010M eosin B was higher than 95%. Obviously all zinc oxide thin films that
sensitized with Eosin B show high transmission spectra which are more than 100%
transmittance spectra. Even though the transmission spectra for zinc oxide thin film
sensitized with eosin B is more than 100%, it shows that undoped zinc oxide thin
32
film has higher transmission spectra then Tb-doped ZnO which is 143.01% and
136.05%.
The figures of undoped ZnO, erbium-doped ZnO and terbium-doped ZnO
thin films sensitized with 0.010M eosin B also show that the bandgap increase with
the addition of erbium and terbium. However, the bandgap for undoped ZnO,
erbium-doped ZnO and terbium-doped ZnO thin films sensitized with 0.010M eosin
B show a decrease with the addition of erbium and terbium. When the annealing
temperature is changed there is no obvious change of optical transmittance. But, the
optical transmittance decreases slightly with increasing cooling rate. These may
related to the increase of optical scattering resulted from the deteriorated
crystallinity when the ZnO films are cooled with a rapid cooling rate (Mei Zhen et
al., 2009). Optical transmittance decreased in the visible region, while it increases in
the UV region with increasing annealing temperature. Aly et al. report that thermal
annealing in the air could improve the optical transmittance of ZnO films,
attributing to the oxygen reaction with ZnO. It also suggests that annealing ambient
has a great influence on the optical transmittance of ZnO film (Xue et al., 2008).
4.3
Analysis by X- Ray Diffractor (XRD) Bruker D8 Advance
All the samples have to undergo characterization by XRD to determine the
chemical composition and crystallographic structure of natural and manufactured
materials.
33
4.3.1
Zinc oxide thin films sensitized by 0.010M Eosin Y
ZnOY
60
(100)
(101)
(002)
50
(102)
L in (C o u n ts )
40
(110)
(103)
30
20
10
0
25
30
40
50
60
2-Theta - Scale
ZnOY - File: ZnOY.1.raw - Type: 2Th alone - Start: 25.000 ° - End: 70.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 7 s - 2-Theta: 25.000 ° - Theta: 2.000 ° - Chi: 0.00 ° - P
Operations: Import
00-036-1451 (*) - Zincite, syn - ZnO - Y: 83.36 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 3.24982 - b 3.24982 - c 5.20661 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63mc (186) - 2 - 4
Figure 4.13: XRD pattern of undoped ZnO films sensitized by eosin Y annealed at
600oC for 1h.
34
(101)
ZnOErY
(002)
50
(100)
40
(102)
Lin (Counts)
(110)
(103)
30
20
10
0
25
30
40
50
60
2-Theta - Scale
ZnOErY - File: ZnOErY.raw - Type: 2Th alone - Start: 25.000 ° - End: 70.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 11 s - 2-Theta: 25.000 ° - Theta: 2.000 ° - Chi: 0.00 ° - Phi: 0.00 ° Operations: Import
00-036-1451 (*) - Zincite, syn - ZnO - Y: 133.39 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 3.24982 - b 3.24982 - c 5.20661 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63mc (186) - 2 - 47.6216 - I/Ic
Figure 4.14: XRD pattern of Er-doped ZnO films annealed sensitized by eosin Y at
600oC for 1h.
70
35
ZnOTbY
(101)
50
(002)
Lin (C ounts)
40
(100)
(102)
30
(110)
(103)
20
10
0
25
30
40
50
60
2-Theta - Scale
ZnOTbY - File: ZnOTbY.raw - Type: 2Th alone - Start: 25.000 ° - End: 70.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 7 s - 2-Theta: 25.000 ° - Theta: 2.000 ° - Chi: 0.00 ° - Phi: 0.00 ° Operations: Import
00-036-1451 (*) - Zincite, syn - ZnO - Y: 115.09 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 3.24982 - b 3.24982 - c 5.20661 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63mc (186) - 2 - 47.6216 - I/Ic
Figure 4.15: XRD pattern of Tb-doped ZnO films sensitized by eosin Y annealed at
600oC for 1h.
70
36
4.3.2
Zinc oxide thin films sensitized by 0.010M Eosin B
(002)
(100)
ZnOB
(101)
50
40
Lin (C ounts)
(102)
(110)
(103)
30
20
10
0
25
30
40
50
60
2-Theta - Scale
ZnOB - File: ZnOB.raw - Type: 2Th alone - Start: 25.000 ° - End: 70.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 7 s - 2-Theta: 25.000 ° - Theta: 2.000 ° - Chi: 0.00 ° - Phi: 0.00 ° - X: 0.
Operations: Import
00-036-1451 (*) - Zincite, syn - ZnO - Y: 123.69 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 3.24982 - b 3.24982 - c 5.20661 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63mc (186) - 2 - 47.6216 - I/Ic
Figure 4.16: XRD pattern of undoped ZnO films sensitized by eosin B annealed at
600oC for 1h
70
37
ZnOErB
(101)
(002)
50
(100)
Lin (C ounts)
40
(102)
(110)
30
(103)
20
10
0
25
30
40
50
60
2-Theta - Scale
ZnOErB - File: ZnOErB.raw - Type: 2Th alone - Start: 25.000 ° - End: 70.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 8 s - 2-Theta: 25.000 ° - Theta: 2.000 ° - Chi: 0.00 ° - Phi: 0.00 ° Operations: Import
00-036-1451 (*) - Zincite, syn - ZnO - Y: 101.57 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 3.24982 - b 3.24982 - c 5.20661 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63mc (186) - 2 - 47.6216 - I/Ic
Figure 4.17: XRD pattern of Er-doped ZnO films sensitized by eosin B annealed at
600oC for 1h
70
38
ZnOTbB
(002)
(100)
50
(101)
40
(102)
(103)
Lin (C ou n ts)
(110)
30
20
10
0
25
30
40
50
60
2-Theta - Scale
ZnOTbB - File: ZnOTbB.raw - Type: 2Th alone - Start: 25.000 ° - End: 70.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 6 s - 2-Theta: 25.000 ° - Theta: 2.000 ° - Chi: 0.00 ° - Phi: 0.00 ° Operations: Import
00-036-1451 (*) - Zincite, syn - ZnO - Y: 139.66 % - d x by: 1. - WL: 1.5406 - Hexagonal - a 3.24982 - b 3.24982 - c 5.20661 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P63mc (186) - 2 - 47.6216 - I/Ic
Figure 4.18: XRD pattern of Tb-doped ZnO films sensitized by eosin B annealed at
600oC for 1h
All the above figures show x-ray diffraction patterns of zinc oxide thin films
deposited on pyrex glass by spin coating at 3000rpm for 30s and annealed at 600oC.
Undoped zinc oxide and doped zinc oxide then sensitized with eosin Y and eosin B
respectively. It is observed that all the deposited films were polycrystalline with a
hexagonal wurtzite structure (JCPDS 36-1451) with the c-oriented preferentially
normal to the substrate surface.
The 600oC annealed films, undoped and doped ZnO shows six relatively
higher diffraction peaks oriented along the (100),(002),(101),(102),(110) and (103)
planes. This indicates a random orientation of crystallites in ZnO film. The
70
39
preferred orientation for ZnO thin film sensitized with eosin Y and B is (101) plane.
The discrepancy may be caused by different preparation condition (Xue et al.,
2008). All peaks match the hexagonal ZnO structure with lattice constant a=3.250Å
and c=5.206 Å. Some group obtained ZnO thin films with strongly preferred (002)
orientation by sol gel method.
It is observed that for sol-gel zinc concentration between 0.3 and 0.7M with
a degree of c-axis orientation of 0.96 obtained from the 0.3M sol-gel, decreasing to
a degree of c-axis orientation of 0.5 for the 0.7M sol-gel, the samples were
preferentially oriented in the c-axis or (002) plane. Preferentially oriented in the
(101) plane resulted by an increased of zinc concentration to 1.3M (Shane et
al.,2007).
4.4
Analysis by Photoluminescence of Perkin Elmer Luminescence
Spectrometer LS50B.
All samples have to undergo characterization by Photoluminescence to
measuring the purity and crystalline quality of semiconductors
40
4.4.1
Zinc oxide thin films sensitized by 0.010M Eosin Y
PL intensity (abs unit)
800.0
750
700
455.87nm
650
600
550
500
450
400
350
300
250
481.01nm
365.5nm
200
150
683.29nm
100
50
0.0
200.0
250
300
350
400
450
500
nm
550
600
650
700
750
Figure 4.19: PL spectra of 600oC annealed undoped ZnO thin films sensitized by
eosin Y; with excitation 230nm
Figure 4.19 show the PL spectra of undoped ZnO thin film annealed at
600oC in the wavelength range 200-800nm. We can find 3 luminescence centers.
One is UV emission of undoped ZnO thin film at 365.5nm corresponding to the
near band edge emission. Another luminescence center is the blue emission at
455.87nm. The third luminescence center is the red emission at 682.780nm but the
intensity is not high.
800.5
41
PL intensity (abs unit)
250.0
240
220
200
180
160
450.25
140
380.73nm
120
100
417.51nm
429.48
80
60
40
480.41nm
20
0.0
300.0 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490 500.0
nm
Figure 4.20: PL spectra of 600oC annealed Er-doped ZnO thin films sensitized by
eosin Y; with excitation 230nm
Figure 4.20 show the PL spectra of erbium doped ZnO thin film annealed at
600oC with the wavelength range 300-500nm. We can find 2 luminescence centers.
One is occur at 380.73nm corresponding to the near band edge emission and another
luminescence center is the blue emission and show high intensity at 450.25nm.
There is also emission at 480.41nm but the intensity is not high.
42
100.0
PL intensity (abs unit)
95
90
85
80
75
70
65
60
455.38nm
55
50
45
40
35
30
25
387.48nm
20
15
360.06nm
481.50nm
682.15nm
10
5
0.0
300.0 320
340
360
380
400
420
440
460
480
500
nm
520
540
560
580
600
620
640
660
680 700.0
Figure 4.21: PL spectra of 600oC annealed Tb-doped ZnO thin films sensitized by
eosin Y; with excitation 230nm
Figure 4.21 show the PL spectra of terbium doped ZnO thin film annealed at
600oC in the wavelength range 300-700nm. We can find three luminescence
centers. The first one is the UV emission of terbium doped ZnO thin films at
387.48nm and correspond to the near band edge emission. The second one is the
blue emission at 455.38nm. The third emission happen at green emission 481.51nm
but the intensity is not high. There is also red emission peak at 682.15nm but the
intensity is not high. From all figure for ZnO thin film sensitized with eosin Y it
shows that undoped ZnO thin film show highest intensity at UV emission
correspond to the near band edge emission. The addition of erbium and terbium also
show a decrease in the intensity of photoluminescence.
43
4.4.2
Zinc oxide thin films sensitized by 0.010M Eosin B
PL intensity (abs unit)
850.0
456.14nm
800
750
700
650
600
550
500
450
400
350
300
250
200
378.29nm
150
683.35nm
100
480.63nm
50
0.0
250.0
300
350
400
450
500
550
600
650
700.0
nm
Figure 4.22: PL spectra of 600oC annealed undoped ZnO thin films sensitized by
eosin B; with excitation 230nm
Figure 4.22 show the PL spectra of undoped ZnO thin film annealed at
600oC in the wavelength range 250-700nm. There are three luminescence centers
that obviously occur. The first on is the UV emission of undoped ZnO thin films at
378.29nm correspond to the near band edge emission. The other one is the blue
emission at 456.14nm and the intensity was really high. There is a slight red peak
emission at 683.35nm but the intensity was not high.
44
PL intensity (abs unit)
100.0
95
90
85
80
75
70
65
60
55
384.87nm
50
456.69nm
45
40
35
441.02nm
30
25
20
15
481.17nm
10
5
0.0
300.0 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490 500.0
nm
o
Figure 4.23: PL spectra of 600 C annealed Er-doped ZnO thin films sensitized by
eosin B; with excitation 230nm
Figure 4.23 shows the PL spectra of erbium-doped ZnO thin film annealed
at 600oC in the wavelength range 300-500nm. From the above figure it shows two
centers of luminescence. The first one is the UV emission at 384.87nm correspond
to the near band edge emission. There are two peaks that occur in the blue emission
but peak at 456.69nm shows higher intensity than the other one at 441.02nm. The
intensity from these two luminescence center is almost same even though peak of
blue emission just slightly higher than the peak at near band edge emission.
45
PL intensity (abs unit)
550.0
500
456.69nm
450
400
350
300
250
200
150
100
383.13nm
50
683.46nm
0.0
300.0 320
340
360
380
400
420
440
460
480
500 520
nm
540
560
580
600
620
640
660
680 700.0
Figure 4.24: PL spectra of 600oC annealed Tb-doped ZnO thin films sensitized by
eosin B; with excitation 230nm
Figure 4.24 shows the PL spectra for terbium doped ZnO thin films
annealed at 600oC in the wavelength range 300-700nm. There are 3 luminescence
centers observed. One is the UV emission at 383.13nm correspond to the near band
edge emission. The second one occur blue emission at 456.69nm and the intensity
was really high. The third emission occur red peak emission at 683.46nm but the
intensity is not high. It shows that undoped ZnO thin film has highest intensity
among ZnO thin film sensitized with 0.010M eosin B.
It is observed that the increased of annealed temperature improved the
optical properties of ZnO films and the ZnO films annealed at 600oC reveals the
strongest UV emission intensity. In this work, we used the optimum temperature of
46
600oC (Hu et al., 2008). The PL emission in the UV bands was observed, peaking at
the range of 365-387nm and is consistent with the result reported in the previous
literatures that ZnO material display a strong UV band assigned to the near-bandedge emission around 380nm, due to Er3+ transition from 4F7/2→4I15/2 and Tb3+
transition from 5D4→7F1. Two emission peaks at room temperature were noted. The
emission peak at 450-460nm is attributed to the transition of electron from defect
level of Zn interstitial atoms to top level of valence band. But the research of Wang
et al. indicates that oxygen vacancy is responsible for the blue emission and can
produce two defect donor levels (Wei et al., 2007). The blue emission peaks shows
a decrease in intensity when erbium and terbium was added in the ZnO films.
The growth temperature and availability of oxygen during sample
preparation greatly affect the green and/or orange visible luminescence of pure zinc
zinc oxide. The recombination of electrons with holes trapped in singly ionized
oxygen vacancies and is commonly seen in ZnO materials synthesized under
oxygen-deficient conditions caused the green emission. Also can be seen in ZnO
grown electrochemically, hydrothermally, pulsed laser deposition and spray
pyrolysis is orange PL (Shane et al., 2007).
CHAPTER 5
CONCLUSIONS AND SUGGESTIONS
5.1
Conclusions
Metal oxide semiconductor has been widely studied due to its varied
properties and application. Among metal oxides, zinc oxide semiconductor is of a
great interest from scientific and technological point of view due to its wide band
gap (3.2-3.4eV) with hexagonal wurtzite structure. This study involves the
comparison of optical properties of zinc oxide thin film when it doped with erbium
and terbium then dye sensitized with eosin Y and B respectively. The preparation of
Zn sol was deposited by sol gel method due to its low temperature processing,
control of doping level and not requires expensive deposition facilities.
X-ray diffraction (XRD), Field-Emission Scanning Electron Microscopy
(FESEM), Photoluminescence (PL), and Direct reflector ultraviolet (DRUV) were
used to characterize the structure, morphology and optical properties of zinc oxide
thin film. When undoped ZnO, Er-doped ZnO and Tb-doped ZnO were
characterized by FESEM it shows that the irregular shape of the particle and there is
variation in the surface morphology of undoped and doped ZnO thin film. The grain
size of doped ZnO is bigger than undoped ZnO thin film when sensitized with eosin
Y and B. However the grain size of ZnO thin film sensitized by eosin B is larger
than eosin Y.
48
The transmission spectra for undoped ZnO, Er-doped ZnO and Tb-doped
ZnO thin film sensitized by eosin Y and B were higher than 90%. However, the
bandgap of undoped ZnO thin film sensitized by eosin Y is higher than Er-doped
ZnO and Tb-doped ZnO. It also shows that the addition of erbium and terbium in
the zinc oxide thin film caused the bandgap to decrease. The decrease in bandgap
shows that the increase in the conductivity. Oxygen vacancies and scattering at
grain boundaries cause the decreased in optical transmittance which associated with
the loss of light. The XRD pattern of all sample follow the JCPDS 36-1451
standard. It was observed that all sample annealed at 600oC were polycrystalline
with hexagonal wurtzite structure oriented along the (100), (002), (101), (102),
(110), and (103) planes. The preferred orientation for all samples is (101) plane.
Some group obtained ZnO thin film with strongly preferred (002) orientation by sol
gel method. This discrepancy may be caused by different preparation condition.
The PL spectra was observed and peeking at the range 365-387nm and
consistent with the result reported in the previous literatures that ZnO material
display a strong near band-edge emission around 380nm due to due to Er3+
transition from 4F7/2→4I15/2 and Tb3+ transition from 5D4→7F1. The PL intensity
decrease with the addition and terbium due to the oxygen vacancy. Sensitization
with eosin Y and B does not give much different in term of the optical properties of
all samples. It’s caused by the bromo and nitro in the eosin Y and B does not give
much effect to the luminescence properties.
5.2
Suggestions for Further Work
It is suggested that future study in this area can be focus on other
semiconductor such as titanium oxide or change the dye used for sensitization such
as Ruthenium Bipyridyl. More parameters should be used to determine the
efficiency for its optical properties such as annealing temperature and the sol
concentration.
49
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53
APPENDIX B
Calculation of chemicals used in the experiment
1. Zinc acetate dehydrate (CH3COO)2. 2H2O)
0.05 mol zinc acetate dehydrate × 219.50 g/mol = 10.975 g
2. Erbium (Er (N03)3. 5H20)
100 % = 443.35g/mol
99 % = 442.91g/mol
Mass of Er = 0.0005 mol × 442.91 g/mol
= 0. 221455g × 5 %
= 1.11g
3. Terbium (Tb (NO3)3.5H2O)
100 % = 435.02 g/mol
99 % = 434.58 g/mol
Mass of Tb = 0.005 mol × 434.58 g/mol
= 0.21729 g × 5 %
= 1.10 g
4. Eosin Y (E-514)
0.010M = mol/ 0.1L
= 0.001 mol
0.001mol = mass/ 691.86 g/mol
mass
= 0.69186 g
54
5. Eosin B
0.010M = mol/ 0.1L
= 0.001 mol
0.001mol = mass/ 624.09 g/mol
mass
= 0.62409 g
APPENDIX A
Energy Level of lanthanide
52
55
APPENDIX C
Calculation of band gap using UV data
For example: Wavelength of PL emission ZnO = 379.93nm
Planck’s equation,
∆E = hν = hc/ λ
Where
H = Planck’s constant (6.62607 × 10-34 Js)
C = Speed of light in vacuum (3.0 × 108 m/s)
λ = Wavelength (nm)
ν = frequency
E is energy (J)
∆E = (6.62607 × 10-34 Js) (3.0 × 108 m/s)
379.93 × 10-9m
1J → 6.2415 × 1018 eV (electron volt)
5.23207 × 10-19→ 3.266 eV
~ Thus 379.93nm = 3.266 eV