CHAPTER I - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/100897/6/06_chapter1… · 2. High thermal conductivity of substrates minimizes thermal gradient between components,

CHAPTER I

INTRODUCTION AND REVIEW OF LITERATURE

Chapter-I

1

1.1 Introduction to thin film technology

Technological progress of modern society depends on the material science and

engineering community's ability to conceive the novel materials with extraordinary

combination of physical and mechanical properties. Modern technology requires thin films

for different applications. Thin film technology is a relatively young and ever growing field

in the physical and chemical sciences which are confluence of materials science, surface

science, applied physics and chemistry. Thin film is a layer of solid, liquid or a gaseous

phase. Thin films can also be defined as any solid or liquid with one of its dimension very

much less than that of the other two

Thin = less than about one micron.

Film = layer of material on a substrate. (If no substrate, it is a "foil")

The invention of thin films improved the science and technology with their extensive

applications in various fields1. Thin film layers can be achieved through the deposition of one

or more thin layers of material on to a substrate (usually glass). This is most often done using

physical vapor deposition process, such as evaporation or sputter deposition or a chemical

process such as chemical vapor deposition2.

A microscopically thin layer of material that is deposited onto a metal, ceramic,

semiconductor or plastic base can be conductive or dielectric and are used in various

applications. Thin films of photovoltaic material using silicon, cadmium telluride and other

elements are used to make solar panels and solar roof shingles. It needs substrate, it cannot

stand by alone, its characteristics depend on the surface condition of the substrate, it shows

enough thin to change properties. Thin film features mostly come from its thickness. It is

formed through atom/molecular state and/or formed after decomposing the materials into

atomic/molecular scale by physical and/or chemical measures3.

An ideal film can mathematically be defined as a homogenous solid material

contained between two parallel planes and extended infinitely in two directions (x, y) but

restricted along the third direction (z), which is perpendicular to the x-y plane. The dimension

along z-direction is known as the film thickness (d or t). Its magnitude may vary from a limit

d→0 to any arbitrary value say to 10 μm or more but always remaining much less than those

along the other two directions i.e., x and y. Thickness of thin films is usually discussed in

terms of Angstrom (Å) units and is of the same order of magnitude as the dimension of a

single atom. The thickness ranging up to 100 Å are called ultra-thin films, ranging between

100 and 10000 Å are thin films and above 10000 Å are thick films. They have large surface

Introduction and Review of Literature

2

to volume ratio and so the surface plays an important role in determining the film properties4.

Thin film materials are the key elements of continued technological advances made in the

fields of optoelectronic, photonic and magnetic devices. The processing of materials into thin

films allows easy integration into various types of devices. The properties of material

significantly differ when it is converted into thin films. Most of the functional materials are

rather applied in thin film form due to their specific electrical, magnetic, optical properties or

wear resistance5. Thin film technologies make use of the fact that the properties can

particularly be controlled by the thickness parameter. Thin films, both crystalline and

amorphous, have immense importance in the age of high technology. Few of them are:

microelectronic devices, magnetic thin films in recording devices, magnetic sensors, gas

sensors, photoconductors, IR detectors, interference filters, solar cells, polarisers, temperature

controller in satellites, super conducting films, anticorrosive and decorative coatings6-12

.

The phenomenal rise in thin film researches is due to their extensive applications in

the diverse fields of electronics, optics, space science, aircrafts, defense and other industries.

These investigations have led a numerous inventions in the form of active devices and

passive components, piezo-electric devices, micro-miniaturization of power supply,

rectification and amplification, sensor elements, storage of solar energy and its conversion to

other forms, magnetic memories, superconducting films, interference filters, reflection and

antireflection coatings and many others. The present developmental trend is towards newer

types of devices, monolithic and hybrid circuits, field effect transistors (FET), metal oxide

semiconductor transistors (MOST), sensors for different applications, switching devices,

cryogenic applications and high density memory systems for computers etc13-17

.

Ceramic thin films are also in wide use. The relatively high hardness and inertness of

ceramic materials make this type of thin coatings of interest for protection of substrate

materials against corrosion, oxidation and wear. In particular, the use of such coatings on

cutting tools may extend the life of these items by several orders of magnitude. The

engineering of thin films is complicated by the fact that their physics is in some cases not

well understood. In particular, the problem of rewetting may be hard to solve, as there is

ongoing debate and research into some processes by which this may occur. So a thin film is

defined as a low dimensional material created by condensing, one by one,

atomic/molecular/ionic species of matter18

.

Chapter-I

3

It may be possible to induce electrical conduction in an insulator by introducing donor

impurity levels close to the conduction band or acceptor impurity levels close to the valence

band. If the magnitude of such doping is close to the extent of creating a high carrier density

comparable to that present in a metal, then the insulator is changed into an electrical

conductor. When this heavy doping is done in an insulator whose band gap is greater than

3.0 eV, the material transmits most part of the visible radiation with energy between 1.66 and

3.21 eV and hence is called as “Transparent Conductors”19

.

1.2 Special features of thin films

Some special properties of thin films are different from bulk materials. This will

change the electrical, magnetic, optical, thermal, and mechanical properties. Typical steps in

making thin films are emission of particles from source, transport of particles to substrate,

condensation of particles on substrate.

High Stability

Uniformity between neighboring elements

Temperature stability, long term stability, high

precision

High frequency

Minimum and uniform floating capacitance

Low noise, wide frequency characteristics

Thin and small, stable termination

Environmental

Longer term stability, temperature stability

Tough terminal strength

High stable protection thin film

Miniature size

Micro processing, precise processing

New functions by multi-layer

Low profile thin film multi-layer

Cost effectiveness Total cost saving by energy saving, high precision,

high stability

1.3 Advantages and limitations of thin films

The advantages of thin film technology are

higher Performance

greater Flexibility

outstanding Reliability and

cost-effectiveness

Performance

1. Thick film technology has the advantage of reduced parasitic capacitive

coupling between components and minimized lead resistance and inductance.

This allows going for improved high-speed and high frequency performance.

Introduction and Review of Literature

4

2. High thermal conductivity of substrates minimizes thermal gradient between

components, which leads to improved stability of circuits at high temperature.

3. Resistors having similar Temperature coefficient of resistance (TCR) can be

made using pastes with same resistivity.

4. Resistor power ratings are high (40 to 50 W/in2).

5. Resistors can be precisely trimmed to achieve accurate values making it possible

to obtain close resistor tolerances and ratio matching. Functional trimming of

circuits is also possible.

6. Thin film transistors (TFT) provide high dielectric isolation useful in high

frequency, high voltage and radiation ambient applications.

Flexibility

1. Existing designs from bread-boards or printed circuit boards (PCB) can be

converted on direct one-one basis with minimum design changes.

2. Fast turnaround times are possible and design changes can be achieved with

minimum time and effort.

3. Wide selections of active and passive components with closely controlled

parameters, in packaged and unpackaged forms are possible.

4. Custom built circuits for specific needs and variety of patterns can be achieved

with no variation in the manufacturing process.

Reliability

1. Increased reliability is due to the reduction of physical interconnections.

2. Since solder connections are replaced by chemically bonded material interfaces,

the susceptibility to wiring errors, shock, vibration and acceleration damages are

reduced.

3. Close bounding between resistive elements and hot spots in resistors.

Cost effectiveness

1. Initial investments in equipment, personal training and development costs are

very low.

2. Thick film process is suited for mass production. Prototype and evaluation

modules can be assembled at minimal costs.

3. Circuit changes can be easily accomplished with reduced final product assembly

time.

Limitations

1. In most of the practical circuits, the value of resistances is limited in the range

10 Ω to 10 MΩ.

Chapter-I

5

2. Through trimming any tolerance can be obtained. The practical limit is 0.25%

using air abrasive trimming.

3. High voltage and high power circuits cannot be realised occupy space.

4. Even though there is size reduction, it is not comparable to that of monolithic

process. Particularly this technology is component limited; too many

components cannot be accommodated on a single substrate.

5. Mounting capacitance more than 22 μF is space consuming and hence is not

economical. Transformers, large inductors and large capacitors are to be kept

out of the circuit20

.

1.4 Rare earth materials

For decades, a broad family of functional materials has been extensively investigated

for their physico-chemical properties and useful applications by solid-state chemists. Metal

oxides including the transition metals and rare earths are able to form a large diversity of

oxide compounds, giving the inspiration for designing new materials21

. The crystal structures

ranging from simple rock salt to complex oxide are often built by the metal-oxygen bonds

varying nearly ionic to covalent or metallic. The oxide materials exhibit fascinating electronic

and magnetic properties associated with the changes in electronic structure and bonding22

.

Rare earth (RE) elements are the 15 elements of the Periodic Table (La, Ce, Pr, Nd, Pm, Sm,

Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) with atomic numbers from 57 through 71. Among

them, Pm is radioactive and does not occur naturally, but might be prepared synthetically. In

the outer electronic configuration of the RE element row, the 6s2 shell is always occupied, the

5d1 configuration appears in La, Ce, Gd and Lu and finally the 4f shell is progressively filled

as the atomic number increases. The degree of filling of the 4f shell is therefore the

distinctive characteristics of the RE elements. In particular, the half-filled (Gd, with 7

electrons in the 4f shell) and the totally filled (Lu, with 14 electrons in the 4f shell)

configurations seem particularly stable as shown in Fig. 1.1. In the solid state, all 15 RE

elements have the oxidation state +3, but some are stable also in the oxidation state +4

(Ce, Pr, and Tb), and others in the oxidation state +2 (Sm, Eu, Tm, and Yb). The

classification of di-, tri-, and tetravalent RE elements and the RE-O bond lengths are

summarized in Table 1.1. It is noteworthy that the +4 oxidation state appears in elements that

follow one with a stable configuration, whereas the +2 oxidation states appears in elements

that proceed one with a stable configuration. These observations suggest that there might be

“periodic” properties in the RE oxides23

.

Introduction and Review of Literature

6

Fig. 1.1 Energy level diagram of cerium oxide

Table 1.1 Bond length and Ionic radius of various rare earth oxide materials * trivalent monoxides,

+ intermediate valent oxides

Rare earth

materials REO Å RE2O3 Å REO2 Å

Ionic Radius (3+)

nm

La 0.257* 0.246 - 0.117

Ce 0.254* 0.244 0.234 0.115

Pr 0.252* 0.238 0.233 0.113

Nd 0.250* 0.238 - 0.112

Pm - 0.236 - -

Sm 0.249+ 0.235 - 0.110

Eu 0.257 0.233 - 0.109

Gd - 0.232 - 0.108

Tb - 0.230 0.226 0.106

Dy - 0.229 - 0.105

Ho - 0.228 - 0.104

Er - 0.227 - 0.103

Tm - 0.225 - 0.102

Yb 0.244 0.224 - 0.101

Lu - 0.223 - 0.100

Chapter-I

7

1.5 Brief history of Cerium Oxide

Cerium is the most abundant element in lanthanides or RE elements in the Periodic

Table in which the inner 4f electron shell is being filled. Cerium was named for the asteroid

Ceres, which was discovered in 1801. The element was discovered two years later in 1803 by

Klaproth, Berzelius and Hisinger. In 1875 Hillebrand and Norton prepared the metal. Cerium

is the most abundant so-called rare-earth metal. It is found in number of minerals including

allanite (also known as orthite), monazite, bastnasite, certie and samarskite. Monazite and

bastnasite are presently the two important sources of cerium. Large deposits of monazite

(found on the beaches of Travancore, India and in river sands in Brazil), allanite (in the

western United States), and bastnasite (in Southern California) will supply cerium, thorium,

and the other rare-earth metals for many years to come. Metallic cerium is prepared by

metallothermic reduction techniques, such as reducing cerous fluoride with calcium, or using

electrolysis of molten cerous chloride or others processes. The metallothermic technique

produces high-purity cerium. Cerium is especially interesting because of its variable

electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or

valence electrons and only small amount of energy is required to change the relative

occupancy of these electronic levels. Cerium is an iron-gray lustrous metal. It is malleable

and oxidizes very readily at room temperature, especially in moist air. Except for europium,

cerium is the most reactive of the rare-earth metals. It decomposes slowly in cold water and

rapidly in hot water. Alkali solutions and dilute and concentrated acids attack the metal

rapidly. The pure metal is likely to ignite if scratched with a knife. Ceric slats are orange red

or yellowish; cerous salts are usually white. It is also finding use as an important catalyst in

petroleum refining and in metallurgical and nuclear applications.

1.6 Cerium oxide: Structure, Properties and Applications

Cerium (atomic number 58, atomic weight 140.12) exist in two stable valence states,

Ce4+

(ceric) and Ce3+

(cerous) and this property triggers several technological uses24

. The

most stable oxide of cerium is cerium dioxide, CeO2, also called ceria or ceric oxide. Cerium

(IV) Oxide has a long history in materials science. Cerium oxide is an insulating high

dielectric material (k~26) that is transparent over the visible and infrared. Bulk CeO2 has a

cubic fluorite with an experimentally determined lattice constant 0.5411 nm at 300 K. In the

fluorite structure each unit cell contains 4 cerium atoms and 8 oxygen atoms as represented in

Fig. 1.2. A notable feature of this crystal structure is that both interstitial and cation

substitutional sites have eight fold cubic coordination by oxygen only differing in their radii.

Introduction and Review of Literature

8

The unique redox properties of cerium oxide have led to many different applications

including use in the anodes of solid oxide fuel cells catalysis and oxygen sensors. The

electronic band structure in cerium oxide is derived primarily from the full oxygen 2p states

that form the valence band and from the empty cerium 5d states that form the conduction

band. The Ce 4f band lies between these two bands. In defect free material, the Ce 4f band is

completely empty because all Ce are in the 4+ state. The approximate energy gap band

diagram of CeO2-γ shows the shift of the Ce 4f (Fig. 1.1) state due to defects or core

excitation. In oxygen deficient cerium oxide the empty Ce 4f band becomes partially filled25

.

Ceria has the fluorite (CaF2) structure with space group Fm3m having 8-coordinate

cations and f-coordinate anions. In other words, each cerium cation is coordinated to eight

equivalent nearest oxygen anions at the corner of a cube, each anion being tetrahedrally

coordinated by four cations (Fig. 1.2). Extending this structure by drawing cubes of oxygen

ions at each corner reveals the eight-fold cubic coordination of each cerium, which alternately

occupies the centre of the cube. Ceria has only one crystallographic form for a wide range of

temperature. It has a strong tendency to remain in the fluorite-structured lattice even after

losing considerable amount of oxygen, thus stabilizing a structure with an elevated number of

oxygen vacancies26

.

Fig. 1.2 Cubic fluorite structure of cerium oxide

Powdered ceria is slightly hygroscopic and will absorb a small amount of carbon

dioxide from the atmosphere. Cerium also forms cerium (III) oxide but CeO2 is the most

stable phase at room temperature and under atmospheric conditions. Oxygen atoms in CeO2

units are very mobile and easily leave the ceria lattice, giving rise to a large variety of

non-stoichiometric oxides with two limiting cases being CeO2 and Ce2O327

.

Chapter-I

9

1.6.1 Physical properties

Stoichiometric CeO2 is pale yellow due to Ce(IV)-O charge transfer. However, since

CeO2 can be reduced in a reducing environment and Ce2O3

can be partially converted to

CeO2. It is usual that cerium oxide is an oxygen-deficient and non-stoichiometric oxide

(CeO2-x with 0<x<=0.5). The non-stoichiometric cerium oxides appear with darker colors.

The basic physical properties for CeO2 are listed in Table 1.2

28.

Table 1.2 Physicochemical properties of pure stoichiometric CeO2

Property Value

Color Yellow-White

Molecular formula CeO2

Molar mass 172.115 g/mole

Solubility in water insoluble

Crystal structure cubic (fluorite)

Density 7.215 g/cm3

Melting point 2400°C

Boiling point 3500°C

Formation Heat -246 kcal/mol

Specific Heat Cal. 460 J/Kg/K

Conductivity 1.2-2 X 10-8

S/cm

Thermal Conductivity Cal. 12 W/m/K

Refractive Index Cal. 2.1 Visible

Cal. 2.2 Infrared

Absorption Edge ~ 420 nm

Relative Dielectric Constant 11

Bandgap ~ 2.95 eV (UV)

5.5 eV (electronic cal.)

Introduction and Review of Literature

10

1.6.2 Optical properties

Electronic structure calculations of CeO2 result in a band gap of 5.5 eV.

A 1 eV 4f

band that is around 3 eV above the valence band separates the conduction band (Ce 5d) and

valence band. No observation of absorption in visible or IR region is existent for pure

stoichemetric CeO2. The optical properties of cerium oxide are usually investigated with

spectral absorption and reflectance in blue/violet region. Absorptions are observed at photon

energies of 3eV, 90~190 eV. A 3 eV sharp absorption is attributed to transition between 2p

oxygen valence band and cerium 4f band that lies above the Fermi level of the material29

.

1.6.3 Electrical Properties

It was recognized a long time ago that cerium oxide is an electrical conductor.

Estel30

reviewed the electrical properties of ceria material and also discussed the electrical

conductivity and diffusivity properties of ceria-based solid electrolytes from both theoretical

and experimental views.

Pure CeO2-x is a mixed conductor made up of almost same partial

conductivities from oxygen ions, electron and hole conductivities. Hole conductivity of CeO2

is negligible in the region of oxygen pressure of 10-26

~1 atm. Therefore, electrical

conductivity usually refers to the summation of electronic conductivity (σe) and ionic

conductivity (σi).

Pure cerium oxide is basically an electronic conductor with n-type

semiconductor characteristics and this property occurs through the transport of polarons that

are oxide vacancies. The percentage of ionic conductivity increases significantly with the

oxides of two or three-valent metal doping, since doping introduces more oxide ion

vacancies. Doped ceria acts as an ionic electrolyte in which the conductivity is derived

mainly from the transport of oxide ions to the vacant sites. Combining ionic and electronic

conductivities, it is inferred that electrical conductivity increases with decrease of

temperature. At higher concentrations of Ce3+

, electronic conductivity contributes more than

at lower concentration of Ce3+

. It is recognized that by controlling temperature, oxygen

activity, chemical structure and composition, it is possible to vary the composition of the

different conductivities31

.

1.6.4 Applications

Based on the above properties, the fabricated CeO2 thin films can be effectively

employed to various technological applications discussed below:

i. Corrosion production

Generally, surface modification has been recognized as a very important approach to

enhance the corrosion resistance of many materials. Surface modification only covers or

Chapter-I

11

alters the surface layers of the materials but does not interfere with the physical, chemical and

mechanical properties of bulk materials underneath. It is found that the cerium species can be

applied to protect zinc,

aluminum and aluminum containing alloys,

stainless steel,

magnesium

containing alloys,

tin-containing alloys and even SiC/Al metal-matrix composites from

corrosion. It can be used to reduce the rate of general corrosion, pitting, crevice corrosion and

stress corrosion32,33

. Cerium species have been used in two ways for metal corrosion

protection. The first one is low concentrations of cerium ions to be added into an aqueous

solution in which the metal is immersed as an electrode.

Another one is that the cerium

species will be incorporated directly into passive oxide film on metal surfaces or

electroplated on the surface of the metal to provide a protective coating.

The immersion

approach has been applied to aluminum and stainless steel.

The corrosion inhibition is explained as follows: pH at cathodic site increases so that

the cerium ions in solution can form cerium oxide or hydroxide on the surface. The layer

blocks cathodic reduction of oxygen.

Cathodic inhibition can also be observed by the

observation of negative shifts of the corrosion potential in cerium-containing solution.

Cerium species also can be an anodic inhibitor to prevent metals from corrosion. Cerium

combined with yttrium is also used on the corrosive wear of stainless steel, aluminum alloy as

well and laser surface cladding of ceria is used for nickel-based alloys34,35

.

ii. Photocatalyst

Cerium dioxide to a certain degree is a semiconductor that can adsorb light between

UV and visible region. The cations of cerium are well-known as electron-acceptors due to the

various valence states of cerium. Bamwenda et al.,36

investigated the possibility of cerium

oxide as a photocatalyst to convert water to oxygen.

The results indicated that cerium dioxide

demonstrated photostability and activity as a photocatalyst and further exploration of oxygen

production in this photocatalysized reaction strongly depends on the concentration of cerium

dioxide. It has been established that cerium dioxide could be a very promising material for

photocatalysis.

iii. Polishing agent

It was used as a main component for polishing powders as early as 1940s. A detailed

process of producing polishing powder based on cerium dioxide along with the development

and history of cerium dioxide based polishing powder are presented in the report of

Kosynkin’s37

. Before 1992, mainly two types of cerium dioxide based polishing materials

(cerite and photopol) were used in industries. These two polishing powders had as high as

Introduction and Review of Literature

12

90% of cerium dioxide in their contents. Relatively new types of polishing powder were

released in 1997 with improved properties38

.

iv. Optical and Electrochromic applications

Cerium oxide is a relatively very efficient UV radiation absorber and it can be used as

a protection film for some UV sensitive materials. The most impressive application of CeO2

film in this area is the utilization of cerium oxide as the counter electrode in electrochromic

devices39

.

Electrochromic devices are usually composed of conducting lithium as electrolytes

and amorphous tungsten oxide (WO3) as the electrochromic layer.

CeO2 meets the

requirement for the counter electrode because of its high optical transparency property in the

visible region combined with excellent ability of charge delivery and storage in both reduced

and oxidized state. As the insertion site size in CeO2 (1.02 Å) is much larger than the radius

of lithium (0.6 Å), the structure limits the diffusion of lithium which affects the cell

performance. The cation of smaller size replacing the cerium ion in the cubic structure is

favorable for the insertion/release of lithium from the counter electrode. Titanium40

,

zirconium41

,

tin42

,

tungsten43

and vanadium44

have been doped with cerium oxide for this

purpose. It shows that the counter electrode made by mixed oxide films of cerium and

titanium or tin have shown good transmission in both reduced and oxidized states and better

performance in electrochemical properties. Further, ZrO2-CeO2 shows good promise in this

area due to large charge capacity and good optical property of V2O5, CeO2-V2O5.

v. Electronic devices

Cerium dioxide is an attractive buffer layer material for silicon technology because it

exhibits high insulating resistance and great chemical stability even at high temperatures. It is

demonstrated that the epitaxial grown CeO2 can be used in silicon-on-insulator (SOI) and the

silicon-based devices of superconduction, colossal-magnetoresistance and ferroeletrics13

.

The

silicon-on-insulator (SOI) structure is composed of a thin layer of low-defect epitaxial silicon

and an insulating layer. This structure has potential application in the high-speed devices and

Very Ultra-Large-Scale Integrated (VULSI) circuits14

.

Choosing an insulator layer material is

a cumbersome process. Two types of problems exist in the previously explored material: the

mismatch of lattice between silicon and the insulator (zirconia, spinel, sapphire, etc.) and the

silicon-insulator (CaF2) interface reaction induced contamination to the silicon. Cerium oxide

has the fcc symmetrical structure with lattice parameter 0.5411 nm, which matches well with

the lattice of silicon (0.5431 nm) so that it is possible to deposit epitaxial CeO2 thin layers on

silicon surface without twinning and any other imperfection between silicon and the

Chapter-I

13

insulating layers. Simultaneously, cerium oxide is inert to silicon so contamination in silicon

is also not a problem for the SOI structure. Techniques that have been applied to the

deposition of this structure include electron beam assisted evaporation45

,

ion beam

deposition14

,

laser ablation46

, spray pyrolysis47

and atomic layer deposition48

.

More work has

been focused on the characterization of the grown films and the Si/CeO2 interface to control

and improve the quality of the SOI structure14,29,46,47

.

Oxidation of silicon at the interface of

Si/CeO2 is the biggest problem during the growth of CeO2

on silicon and production of

electronic device.

Cerium oxide has also been fabricated as the buffer layer for the superconductor-

related structures. Conventionally, the epitaxial deposition of the high quality and high-

temperature superconducting YbaCu3O7-δ (YBCO)

thin films are obtained at the substrates of

YSZ, SrTiO3, LaAlO3, MgO49,50

. The new development of CeO2 in the superconductor

industry is the inclusion of CeO2-SnO2 combination in YBCO to improve the critical current

density of the superconductor. The epitaxial deposited film is a promising material for the

capacitor structure due to the high dielectric constant of cerium oxide (26)

51.

vi. pH sensors and Gas sensors

Oxygen vacancy is a well-understood property for nonstoichiometric cerium oxide.

Oxygen vacancy diffusion coefficient of cerium oxide is reported as 10-5

cm2/s at 970

oC

52.

In

addition, cerium oxide has high chemical stability at high temperature. Therefore, cerium

oxide or ceria based films have been used to act as oxygen gas sensors at high temperature.

The application examples include sputtered CeO2 film and mist pyrolysized CeO2

powder

based thick films53

.

CeO2 based sensors can be applied to detect other reductive gases because

the oxygen vacancies can react with CO, NO and acetone54

.

To evaluate the sensitivity of

ceria-based gas sensors, Stefanik studied the relationship between the electrical conductivity

of the Pr-Ce-O system with oxygen partial pressure and temperature55

.

Outline of the work

The work on “Spectroscopic investigations on the Structural, Optical and Electrical

Properties of Pure and doped Cerium Oxide Thin Films and the Feasibility Study of Diode and Solar

Cell Fabrication” has been presented in nine chapters followed with a summary and

conclusion. Attempts have also been made to fabricate and characterize p-n junction

photodiodes and hetero-junction solar cells as a part of device development. The entire work

is depicted in the following flow chart.

Introduction and Review of Literature

14

Fig. 1.3 Flow chart of the work

Start

Chapter-I: Introduction and Review of Literature

Chapter-II: Deposition Methods and Characterization

Techniques

Chapter-III: Optimization and characterization of pure

cerium oxide thin films

Chapter-IV: Influence of Al Concentration on the properties of

cerium oxide thin films

Chapter-V: Impact of In doping on the properties of cerium oxide

thin films

Chapter-VI: Preparation and characterization of CeO2:Co

thin films

Chapter-VII: Consequence of Mn concentration on the properties

of cerium oxide thin films

Chapter-VIII: Incorporation of Y doping on the properties of

cerium oxide thin films

Chapter-IX: Fabrication of Diodes and Solar cells

Summary and Conclusion

Chapter-I

15

1.7 Review on cerium oxide thin films

Thin films are extremely useful in both present and future work in various

technological aspects. Most of the techniques are frequently used to produce thin film

coatings including spray pyrolysis, vacuum evaporation, chemical vapour deposition, pulsed

lased deposition and sputtering. Of late increasing interest towards the spray and sol-gel

preparation of semiconducting thin films has been observed. As the preparation of cerium

oxide thin films and their change in properties due to different doping elements is the area of

research, a systematic review of the same is presented here.

1.7.1 Atomic Layer Deposition (ALD)

E. Gourba et al., (2003)56

investigated the preparation and characterization of cerium

gadolinium oxide (CGO) thin films by atomic layer deposition and the feasibility study of

fuel cells. SEM micrographs of CGO films show the presence of columnar structure with low

surface roughness of the films. XRD pattern show the increase of lattice parameters in

pristine CeO2 compared to CGO due to the increase of Gd/Ce ratio. The calculated growth

rate is 11 times lower than that of sputtered films. Films deposited on lanthanum strontium

magnesium (LSM) substrates show the well covered films over the substrate surface.

Impedance measurements revealed that the conductivity of ALD grown CGO layer 30 times

lower than that of sputtered films. This difference may attributed to the apparent difference

on the microstructure.

P.J. King et al., (2011)57

have reported the effect of deposition temperature on the properties

of cerium oxide thin films grown on n-Si (1 0 0) substrates by atomic layer deposition. The

resulting films exhibit permittivities in the range 25-42 at 1 MHz with a strong dependency

on the deposition temperature. The microstructural origin of this behaviour has also been

investigated. The as-deposited films are found to be crystalline and they exhibited the cubic

fluorite structure for the deposition temperature 150°C to 350°C. Variations in the crystallite

sizes are governed by the deposition temperature which is estimated through Debye-Scherrer

analysis of the XRD patterns. The changing crystallite size correlates with changes in the

triply-degenerate F2g first-order Raman line half-width at 465 cm-1

. It is concluded that the

frequency dependency of the film dielectric properties is strongly influenced by the crystallite

size which in turn is governed by the growth temperature.

Chun Zhao et al., (2013)58

investigated the grain size dependence of dielectric relaxation in

cerium oxide thin films. XRD and Raman spectra show an increase in grain size for

increasing growth temperatures. From the capacitance-voltage (C-V) measurement of the

Introduction and Review of Literature

16

samples, strong frequency dispersion is observed. In order to further investigate the dielectric

relaxation, the normalized dielectric constant is utilized for the CeO2 samples of different

grain sizes. The CeO2 samples have better dielectric relaxation behaviour after annealing

since the annealed samples have a larger grain size. The mechanism of grain size effects is

attributed to the alignment enhancement of the polar nanodomains.

1.7.2 Chemical Vapour Deposition (CVD)

D. Barreca et al., (2003)59

elaborated the interrelations between the nanostructure and

optical properties of cerium oxide thin films deposited by plasma-enhanced chemical vapour

deposition (PE-CVD). The layers are synthesized in Ar and Ar-O2 plasma on Si (1 0 0)

substrates at temperatures lower than 300oC. Particular attention is devoted to the influence of

synthesis conditions and sample properties, taking into account the effects of surface

roughness and SiO2 interface layer on Si. In particular, the fundamental transition energies

evidenced a blue shift as a function of nanocrystal size. The absorption onset depend on

oxygen defects (i.e., Ce(III) content) and could be related to the PE-CVD deposition

conditions, especially to O2 presence/absence in the plasma. As a consequence, a suitable

choice of processing conditions might enable the control of nanocrystalline CeO2 optical

response.

Roberta G. Toro et al., (2004)60

focussed on the correlation between microstructures and

optical properties of epitaxial CeO2 thin films by metal organic CVD (MOCVD) method.

XRD pattern show the FWHM value observed in TiO2 (0 0 1) and is greater than that of CeO2

films on Al2O3 due to misalignment between two substrates. AFM images showed that the

samples are well shaped grains with 100-130 nm abrupt edges in single-columnar structure.

The cross-sectional SEM micrographs show the thickness range from 800-1500 nm. Optical

spectra of CeO2 films exhibit that the value of high-frequency dielectric constant is an

important factor for the evaluation of film quality and porosity. The band gap energy is found

to be increased to 3.000.05 and 3.050.04 eV for Al2O3 and TiO2 substrates respectively.

Yinzhu Jiang et al., (2009)61

have reported the effect of film thickness on the structural and

optical properties of nanocrystalline CeO2 films. The prepared films are characterized by

structural, morphological and optical properties. XRD pattern showed that the CeO2 films

have cubic fluorite structure compared with JCPDS data 81-0792. The peak (2 0 0)

orientation is significantly enhanced and grain size also increases from 6.2 to 32.3 nm when

film thickness increases from 72 to 334 nm due to nucleation and subsequent growth in a

random mode. Raman spectra showed that the films exhibit a triply degenerate Raman active

region at 460 cm-1

red shift with respect to bulk material. AFM images revealed that the films

Chapter-I

17

are highly coherent well aligned sub-micrometer sized taper-like structure due to the

agglomeration of tiny particles with an average size of ~50nm. UV-Vis spectra show the

films are largely transparent in the visible region and the transmittance decreases with

thickness increases.

J.R. Vargas-Garcia et al., (2010)62

studied the highly (1 0 0) oriented CeO2 films grown on

amorphous substrates by laser chemical vapour deposition technique. XRD patterns

confirmed that the CeO2 films exhibit a high (1 0 0) preferred orientation at 2θ=33.13o, when

film thickness increases and the FWHM decreased due to the alignment of the columnar

structure normal to the substrate. When laser power is increased upto 150 W the intensity of

orientation (1 0 0) increases, laser power reaches 200 W the same decreases. SEM and TEM

images showed that the CeO2 films exhibits both pyramidal and flat top ending columns

depend on the laser power. Pl=100 W, the columnar grains become larger, more dense

aligned in two perpendicular orientation, Pl=150 W, pyramidal ending columns with four

layered triangular facets.

A.M. Torres-Huerta et al., (2010)63

reported the structural and optical properties of

ZnO:CeO2-x thin films by atmospheric pressure-MOCVD technique. The correlation among

crystallinity, surface morphology and optical properties of the as-prepared films are analysed

by XRD, SEM, AFM, TEM and UV-Vis spectroscopy. The synthesized films showed

different crystallographic orientations depending on the ZnO and CeO2 lattice mismatch,

cerium content and growth rate. Both pure and ZnO:CeO2-x films obtained have hexagonal

structure and highly preferred orientation with the c-axis perpendicular to both substrates

under the optimal deposition conditions. The microstructure is modified from dense, short

round columns to round structures with cavities and the typical ZnO morphology by

controlling the cerium doping the film and substrate nature. High optical transmittance

(>87%) is observed in the pure ZnO films.

O. Pena Roudriguez et al., (2013)64

investigated the optical properties of ceria-zirconia

epitaxial films grown on YSZ substrates by chemical solution deposition method. The optical

properties as well as the film homogeneity of the films have been studied as a function of the

cerium concentration. XRD and AFM analysis have demonstrated that smooth, high density

epitaxial films grown for all the series. A Tauc-Lorentz (TL) model describes efficiently the

optical properties over the entire concentration range. The fitting revealed that the optical

properties of the solid solutions vary smoothly with composition. Likewise, the parameters of

the TL model exhibited a linear dependency on the cerium concentration. The reported results

Introduction and Review of Literature

18

constitute a valuable database, at least as a starting point, for the analysis of CexZr1-xO2 thin

films of unknown thicknesses for which the optical constants are unknown.

1.7.3 Electron Beam Evaporation

Deposition and characterization of single layer CeO2 and SiO2 thin films are reported by

K. Narasinha Rao et al., (2003)65

. The films are polycrystalline with a strong (1 1 1)

reflection at ambient to 300oC. However, the grain size is decreased at 400

oC. The refractive

indices and UV, Vis and IR transmittance of the films have been measured by varying

substrate temperature in the range ambient to 400oC. The refractive index (n) of CeO2 films

(at 550 nm) increased from 2.00 to 2.41 as the substrate temperature is increased from

ambient to 400oC. The extinction coefficient (k) of these films is negligibly small even at

elevated substrate temperatures. The refractive index of SiO2 films is increased marginally

from 1.46 to 1.48. The density of CeO2 and SiO2 films has also been estimated with substrate

temperature.

S. Debnath et al., (2007)66

discussed the optical characteristics of CeO2 thin films by e-beam

evaporation technique. Optical spectra of CeO2 thin films show high transmittance probably

due to the existence of an interfacial layer with low refractive index between CeO2 and glass.

Absorbance spectra showed that the films have high value in UV region and decreases

sharply with increasing wavelength becomes almost constant in the visible region. The

absorption and extinction coefficient are found to be decreased with increased film thickness.

The refractive index is calculated as 0.820.02 at λ=600 nm which is slightly less than

reported value; this low refractive index may be attributed to the singular ear structure, which

could grow from first nucleation layer. XRD pattern showed that the crystalline size of

(1 1 1), (2 0 0) and (2 2 0) planes as 20.66, 34.99 and 14.90 nm, respectively.

Preparation and photoelectron characteristics of Sn doped ceria thin films are reported by

V. Matolin et al., (2008)67

. RPES show the Ce 4d-4f photo absorption region at hυ=122.0

and 124.5 eV. The resonant energy states are observed at 122.0 eV due to Ce3+

and 124.5 eV

for Ce4+

. XPS spectra of Sn doped films show the presence of two peaks observed at 484.6

and 486.5 eV. The first one is associated with metallic Tin and other one associated with

Sn-Ce.

Yinghui Zhou et al., (2010)68

have reported the effect of nanostructures and redox properties

of Pt nanoparticles on CeO2 (1 1 1) thin films. The CeOx films are prepared at three oxidation

states viz., CeO2, CeO1.88 and CeO1.77. XPS spectra revealed that the ceria exhibits two

reduced states contains 76 and 53% of Ce4+

ions and are assigned to CeO1.88 and CeO1.77.

Chapter-I

19

STM images of CeOx (1 1 1) films consist of atomically flat terraces and the measured

distance between two adjacent Ce atoms is 0.3 nm. The surface roughness of flat terraces

increases upto 0.07 nm for CeO1.77 from 0.03 and 0.05 nm for CeO2 and CeO1.88 respectively.

Incorporation of Pt particles on CeO2 grows faster in size while Pt particle density

significantly increases on CeO1.88. The power density decreases upto 3.1x1012

due to

aggregation of Pt.

Jason A. Farmer et al., (2010)69

elaborated the Ag adsorption characteristics of reduced

CeO2 (1 1 1) thin films. CeO2-x (1 1 1) films are deposited on Pt (1 1 1) anodes with different

thickness as 1, 2, 3 and 4 nm. LEED pattern shows only 1.4x1.4 spots for continuous CeO2-x

(1 1 1) films. XPS spectra were used to measure the reduced and oxidized states of ceria.

STM images showed rough island formation and their eventual growth together to cover

whole surface by Pt (1 1 1). The growth of Ag on CeO2-x (1 1 1) is characterized by LEED,

AES and ISS. When increasing Ag coverage, the normalized Ag AES intensity approaches

unity slowly and O AES signal approaches zero more slowly than Ag AES.

F. Dvorak et al., (2011)70

studied the correlation between ion surface reduction and surface

morphology of CeO2 thin films on Cu (1 1 1) substrates. STM images showed that the CeO2

thin films prepared at low temperature (250oC), exhibit the interfacial layer islands have open

dendrite-like shapes and nucleate with higher density than the interfacial layer islands at

450oC. The surface of interfacial layer of CeO2 (1 1 1) and Cu (1 1 1) observed a honey-comb

surface corrosion with a periodicity about 60 Å with lattice mismatch of 5.41 Å and 3.41 Å

respectively. Ceria exhibits two oxidized and reduced states like Ce3+

and Ce4+

. The degree of

reduction increases with decrease of Ts as 0.03, 0.05 and 0.08 on the layer at 450, 250 and

150oC, respectively.

F. Pagliuca et al., (2013)71

elaborated the interfacial interaction between cerium oxide and

silicon surfaces by e-beam evaporation technique. All the silicate phases are characterized by

the presence of cerium in its 3+ valence state, while Si presents different valence states

ranging up to 4+. After thermal treatments at 1040 K in O2 the silicate phase evolves its

average stoichiometry within the XPS probing depth and increases in overall thickness. The

presence of a thermal oxide on the silicon surface partially limits the reaction. The silicate

phase formed after the thermal treatment is very stable towards the different oxidizing and

thermal treatments performed. A cerium oxide film with CeO2 stoichiometry can grow on the

silicate phase, but the presence of the interfacial silicates hinders the long range epitaxial

growth of the oxide.

Introduction and Review of Literature

20

1.7.4 Electrodeposition (ELD)

L. Yang et al., (2011)72

elaborated the electrodeposition of cerium oxide films and

composites. This method has been developed for the deposition of poly(2-vinylpyridine)

(PVP) films. The thickness of the films is varied in the range 0.1-3 μm. The deposition

mechanism is based on the pH increase in the cathodic reactions, electrophoresis of the

protonated PVP macromolecules and neutralization of their charge and film formation at the

cathode surface. The deposition yield is studied by the quartz crystal microbalance method.

Two electrochemical strategies are developed for the fabrication of composite PVP-CeO2

films, which are based on the electrodeposition of PVP and ELD or EPD of CeO2. The

composite films are studied by electron microscopy, XRD, thermogravimetric and differential

thermal analysis (TG-DTA).

H. Hasannejad et al., (2011)73

investigated the electrochemical impedance mechanism of

Ni-Cerium oxide thin films deposited by electrodeposition technique. The novelty of this

method lies in the fact that the metal and the oxide are both deposited simultaneously on the

substrate, directly from the plating bath containing Ni and Ce ions with no oxide powder

addition. Electrochemical impedance spectroscopy (EIS), SEM, XRD and EDS are used to

study the mechanisms of Ni-CeO2 nanocomposite coating deposition. The results indicated

that the morphology of Ni-cerium oxide coatings varied based on the Ni:Ce ion ratio. When

this ratio exceeds 100, sporadic distribution of cerium oxide in the Ni matrix occurred. When

the ratio is less than 100, it is found that Ni species are dispersed in a continuous film of

cerium oxide.

Deposition and characterization of samaria doped cerium oxide thin films are investigated by

V. Lair et al., (2011)74

. Structural and morphological properties of electrodeposited films

have been studied by XRD, SEM and Raman spectroscopy. Special attention has been

focused on the Raman spectroscopy study to emphasize the effect of heat treatment and

samarium doping. Despite cracks, single SDC phase is obtained in a cubic symmetry. The

evolution of the crystal structure is highlighted and the change in peak position and intensity

is discussed in the light of Sm and Ce content.

Synthesis and characterization of porous electrodeposited cerium oxide thin films are

reported by D. Chu et al., (2012)75

. The size and shape of the pores are determined by the

morphology of ZnO templates. The as-prepared CeO2 films are crystalline and both Ce 3d

and Ce 4d states are confirmed by XPS analysis. In addition, highly sensitive

photoluminescence performances of CeO2 films are obtained, indicating they are an attractive

material for future applications.

Chapter-I

21

Geping He et al., (2013)76

studied the fabrication of AAO/CeO2 membrane system by

electrodeposition technique. The larger the intersection angles of the cathode, the more the

charge storage of the fabricated system. A lower potential scan rate is beneficial to the

electrochemical charge storage of the system. The hydrophobicity of the fabricated

nanosystem is considerably improved with the cathode intersection angle increasing during

the subsequent anodic oxidation. The electrochemical charge storage density of the system is

likewise increased. Lower potential scan rate is favourable to the electrochemical charge

storage of the nanosystem. The cathode intersection angle has an essential effect on either

electrochemical charge storage or hydrophobicity of the nanosystem. With the cathode

intersection angle increasing, the hydrophobicity of the AAO/CeO2 system is greatly

improved.

1.7.5 Electrostatic Spray Assisted Vapour Deposition (ESAVD)

Ming Wei et al., (2005)77

elaborated the deposition of CeO2 thin films over Si (1 0 0) and

glass substrates by ESAVD method. XRD pattern showed that the prepared films have no

preferred orientation. The average crystalline size depends on the both deposition temperature

and mole concentration. AFM images showed that the dense microstructure with some over-

grow grains, due to coalescence of spray droplets while approaching the stationary substrate.

In Si (1 0 0) substrates, results showed that the films were crystallised in the cubic structure

with a preferred orientation along (1 0 0) direction, better alignment is achieved with high

electric field. XRD and SEM results showed that the electric field plays an important role to

form a uniform, fine structured and highly oriented CeO2 films on Si substrate. The silicon

substrates may be chosen as the best substrate due to better crystallinity, uniform and fine-

grain structure compared to glass substrates, so it can be used as a buffer layer on high

temperature superconducting oxides.

1.7.6 Ion Beam Deposition

C. Chunlin et al., (2003)78

correlated the emission properties of CeO2 thin films prepared by

ion beam epitaxial deposition. A nonstoichiometric phase of CeO2 is inferred and some

oxygen ion is sputtered during deposition and resulted in the formation of less-oxygen phase.

The fluorite crystal structure of the material like CeO2 could remain intact over a relatively

wide range of nonstoichiometric compositions through the inclusion of oxygen vacancies. It

can be inferred that the origin of CeO2 PL is due to the transition by Ce 4f-O 2p and defects

level→O 2p. These defects levels are located in the range of 1 eV around Ce 4f band.

Introduction and Review of Literature

22

S.M.A. Durrani et al., (2008)79

studied the carbon monoxide gas-sensing properties of

electron-beam deposited cerium oxide thin films. XPS survey scan clearly showed five peaks

at binding energies of 882.2, 888.09, 897.72, 900.5 and 907.5 eV. The first three peaks

correspond to Ce 3d5/2 and other peaks correspond to Ce 3d3/2. The lower energy component

with a binding energy of 528.8 eV corresponds to Ce-O bond. AFM images evident that the

CeO2 thin film exhibits large nicely separated conical columnar microstructure with

measured roughness Ra=2.38 nm. Gas sensing properties depend on porosity of the films.

Optical, XPS and AFM results revealed that the prepared films are highly porous. It is

evident that the pure CeO2 films are highly sensitive to CO gas. Gas sensing response mainly

depends on bias voltage, gas concentration and operating temperature.

Deposition and microstructural characterization of gadolinium doped ceria (GDC) electrolyte

thin films by e-beam technique is reported by G. Laukaitis et al., (2008)80

. XRD patterns of

GDC powders indicate sharp (1 1 1) and minor (2 0 0), (2 2 0) and (3 1 1) orientations with

cubic fluorite structure. The growth rate of the film is increased by increasing gun power and

also affects the crystalline size. The crystalline size of GDC films is also increased by

increasing Ts from 100-500oC as 12-56 nm, respectively. SEM images showed that the GDC

films have a large number of interfaces, grain boundaries, nanoporosity and randomly

oriented grains. It shows that the porous substrates consist of columns with feathery structure

at lower Ts 300oC, at higher Ts 500

oC having 10% porosity and more dense films are

observed. This morphology changes may be due to kinetic energy of atoms, speed of

coalescence of islands, surface potential and interaction between particles and substrate

surface.

Catalina Mansilla et al., (2009)81

elaborated the influence of experimental conditions on the

characteristics of CeO2 thin films prepared by ion beam deposition. XRD pattern showed that

the CeO2 films have cubic fluorite structure compared with JCPDS data (34-0394). There is

no significant changes in atmosphere (O2, Ar), at 500oC and the preferred orientation is

observed at (2 2 0) reflections. In argon atmosphere, the (2 2 0) reflection starts to disappear,

in oxygen atmosphere, the (1 1 1) reflection starts to disappear. SEM micrographs showed

that the distribution of grains are more homogenous for both cases, cross sectional images of

the samples confirmed that piling slim triangular shaped crystallites with random orientation.

The effect of bombardment is to reduce the grain size and increase the film density, due to an

increase of the nucleation process. Optical analysis confirmed that the effect of ion

bombardment and substrate temperature increases the refractive index of the films.

Chapter-I

23

G. Laukaitis et al., (2009)82

elaborated the affecting parameters on the substrate surface of

CeO2 films deposited by e-beam technique. XRD pattern of SDC films exhibits sharp (1 1 1)

and minor (2 2 0) (2 0 0) and (3 1 1) reflections. The crystalline size depends on gun power

and substrates. Crystalline size of SDC films is varied from 5-9 nm and 7-18 nm for optic

quartz and Alloy 600 substrates respectively. The crystalline size of SDC films is increased

by increasing gun power from 0.12-0.8 kW. SEM images showed that the films exhibit

columnar structure with all columnar grains oriented in same direction. The bias voltage

affects the density of films. The porosity decreased upto 12% without applying bias voltage.

Photoemission and catalytic properties of ion beam deposited Rh/CeO2 system is studied by

V. Nehasil et al., (2010)83

. The surface reduction of cerium oxide before Rh deposition

seems to be crucial for the reactivity rate. Without this reduction, the production of CO2 is

negligible. The shape of the Ce 3d peak is observed by XPS. Its development as a function of

the reaction conditions is used to explain the reaction mechanism, especially the role of

oxygen released from cerium oxide during CeO2→Ce2O3 transformation.

A. Kumar et al., (2012)84

studied the effect of radiation-induced reduction of high quality

ceria thin films deposited by ion beam deposition. The surface chemistry of the irradiated

ceria thin films is characterized by in situ X-ray photoelectron spectroscopy (XPS). Upon

irradiation, the concentration of Ce3+

increased by 13% and 19% in single and polycrystalline

ceria, respectively. Molecular dynamics simulation of thermal spikes and displacement

cascade damage provide details of radiation induced defects at the end of the range of the

ions that can contribute to the observed reduction. The simulation study suggests that thermal

spikes arising from electronic stopping due to 2 MeV He+ is not responsible for the observed

reduction.

1.7.7 Physical Vapour Deposition (PVD)

I. Porqueras et al., (2003)85

investigated the structural, compositional and electrochromic

properties of CeO2 thin films by e-beam PVD technique. XRD pattern indicated that the films

are polycrystalline nature with preferred orientation along (1 0 0) direction. The crystalline

size increases with the increase of ion bombardment and oxygen flow and strongly controlled

by ion bombardment. TEM images showed that the columnar structure in (1 0 0) direction.

The shape of the main column and 45o

oriented branches remain of a kind of wheat ear.

Selected area electron diffraction (SAED) pattern presented circular sectors instead of the

complete ring characteristics of disoriented crystals and the presence of nanometric order.

This interfacial layer may be attributed to the singular ear structure. CV curve states that the

Introduction and Review of Literature

24

charge density becomes stable at 400-500 cycles. Pure CeO2 film shows poor charge storage

capacity.

G. Laukaitis et al., (2008)86

investigated the effect of substrates in microstructure of GDC

electrolyte thin films by EB-PVD technique. XRD pattern of the films exhibits cubic

structure with sharp (1 1 1) and minor (2 0 0), (2 2 0), (3 1 1) and (2 2 2) orientations

respectively. The crystalline size decreases linearly from 20.9 to 9.8 nm with the increase of

e-beam gun power from 0.66 to 1.05 kW. The crystalline size increases from 22 to 33 nm by

decreasing bias voltage from 0 to -100 V. SEM micrographs showed that the density

increases upto 25% compared to the density of GDC thin films formed without bias voltage.

Ion bombardment can disturb the columnar growth by enhance of atom mobility and leads to

more dense film formation.

M. Hartmanova et al., (2009)87

correlated the effect of deposition temperature and

composition of Sm doped cerium oxide thin films prepared by physical vapour deposition

technique. The X-ray photoelectron spectroscopy of the surface and electron probe

microanalysis of the bulk of the film revealed the dominant occurrence of Ce4+

oxidation

state, suggesting the presence of CeO2 phase confirmed by X-ray diffraction. The Ce3+

oxidation states corresponding to Ce2O3 phase are in minority. The X-ray diffraction and

scanning electron microscopy showed the polycrystalline columnar structure and rooftop

morphology of the surface. Effects of the preparation conditions (temperature, composition,

IBAD) on the lattice parameter, grain size, perfection of the columnar growth and its impact

on the surface morphology are analysed and discussed.

J. Huang et al., (2013)88

elaborated the microstructure and thermal properties of cerium

oxide thin films deposited by physical vapour deposition. The grain orientation, morphology,

hardness and thermal cycling oxidation behaviour of CeO2 coatings are systematically

studied. The deposition power density has remarkable influence on the preferred crystal

orientation and morphology of the coatings. The heating-cooling test cycles from 1000°C to

room temperature indicate that the CeO2 coatings with the columnar structure show excellent

thermal shock resistance. The hardness of the CeO2 coating varies with thermal cycling. The

thickness of thermal grown oxide (TGO) layer containing dominant Ce2O3 is basically linear

to the thermal cycles. The hardness of the coatings increases initially and decreases finally

during thermal cycles. The EB-PVD derived CeO2 coating is promising in protection of metal

components undergoing thermal cycling.

Chapter-I

25

1.7.8 Pulsed Laser Deposition (PLD)

Jennifer L.M. Rupp et al., (2006)89

reported the grain growth and microstrain evaluation in

nanocrystalline ceramics of doped and undoped ceria films. Thermogravimetric and

differential scanning calorimetric (TG-DSC) measurements showed a large endothermic peak

at 180oC due to removal of water and a broad exothermic peak between 400-1000

oC due to

carbon combustion. XRD pattern showed that the CGO films exhibit cubic fluorite structure

with preferred orientation along (2 0 0) reflections. Ts below 500oC, shows amorphous

nature, above 500oC, show well-crystallized domains. At higher temperature, the (2 0 0) peak

dominates with respect to (1 1 1) reflection. SEM micrographs of the prepared films showed

that the grains have globular shape and have not developed flat habitus planes on the surfaces

at higher temperature. The grain growth occurs mainly in the 5-10 hours of dwell for an

average grain size of below 140 nm with dwell temperature below 1100oC. The grain growth

of CGO films demonstrates the self-limited grain growth at low annealing temperature

independent on preparation method, which means independent of residual carbon impurities.

S. Selladurai et al., (2006)90

investigated the microstructural variations of GDC thin films by

pulsed laser deposition. XRD pattern showed that the films are single phase with no other

impurities. The lattice constant is calculated as 0.542 nm with various laser energies. The

grain size also varies from 12 to 17 nm due to the variation of laser energy 100-600 mJ/pulse.

The intensity ratio increases slowly and reaches maximum at 400 mJ/pulse and decreases

thereafter. TEM images showed that the films deposited in the range 100-200 mJ/pulse show

a fine distribution of nanocrystalline grains. The dark field images are observed in centred

objective aperture in the laser energy 200-400 mJ/pulse. The size of nanocrystalline films

does not increase significantly with increasing laser energy. Incorporation of laser energy

controls the grain growth defects and microstructure.

Ulrich P. Muecke et al., (2008)91

fabricated the NiO/CGO thin film anodes by spray and

pulsed laser deposition method and studied its electrochemical behaviour. Impedance spectra

showed that the activation energies achieved with different electrodes (Ni/CGO anode on

CGO electrode and Ni/CGO anode on YSZ electrode) due to the conduction of electrodes.

SEM micrographs showed that the films are adhered well to the electrolyte surface with

ceramic grains of 16 and 53 nm annealed to 800oC. The polarization resistance decreases

with decreasing grain size. The electrochemical performance of the PLD electrode is slightly

better than that of a sprayed electrode annealed at the same temperature 1000oC. The

activation energy of PLD electrode is 1.46 eV and for spray 1.45 eV.

Introduction and Review of Literature

26

G. Balakrishnan et al., (2013)92

have reported the influence of deposition parameters on the

epitaxial growth of CeO2 thin films by pulsed laser deposition. The films are characterized by

X-ray diffraction and atomic force microscopy to study the influence of substrate

temperature, laser fluency and repetition rate on epitaxy, growth mode and surface

morphology. XRD studies revealed the epitaxial nature of CeO2 (2 0 0) films on yttria

stabilized zirconia (1 0 0) substrate (CeO2 (2 0 0)/YSZ (1 0 0)) deposited in the temperature

range 673-973 K. The films prepared at low energy densities (1-3 J/cm2) and low repetition

rates (1-25 Hz) also indicated the epitaxial nature, whereas the films prepared at high energy

density (≥ 4 J/cm2) and repetition rate (30 Hz) indicated deviation from epitaxy.

G. Balakrishnan et al., (2014)93

investigated the XRD, Raman and PL studies of cerium

oxide thin films grown on Si (1 0 0) and glass substrates by pulsed laser deposition. XRD

analysis revealed the polycrystalline and cubic structure of the CeO2 films with preferred

orientation (2 0 0) as the increase of oxygen partial pressure. The Raman studies indicated the

formation of Ce-O with the systematic variation of peak intensity and FWHM with oxygen

partial pressures. High resolution transmission electron microscopy (HRTEM) investigation

confirmed the polycrystalline and cubic nature of the CeO2 films with (2 0 0) preferred

orientation. AFM studies showed the increased roughness (RMS) value of the films from 0.8

to 4.6 nm with increasing oxygen partial pressure from 2x10-5

to 3x10-1

mbar. The

photoluminescence investigation indicated the bandgap values in the range 3.05-3.10 eV with

increasing oxygen partial pressures. The UV-Visible spectroscopy analysis demonstrated the

reduction of refractive index from 2.41 to 1.72 with increasing oxygen partial pressures.

1.7.9 Screen Printing Technique

Y.J. Leng et al., (2004)94

studied the performance of SOFC with Ni-GDC as anode and

LSCF-GDC as cathode by screen printing technique. SEM images indicate the existence of

mostly closed packing of pores. XRD pattern exhibits well crystallized cubic fluorite

structure with sharp lines of peaks. The open circuit voltage (OCV) obtained at 500 and

600oC as 0.901 and 0.863 V, respectively are compared to theoretical OCV 1.155 and

1.138 V. The difference may be due to the ionic conductivity of doped ceria electrolyte. The

apparent activation energies of electrode polarization resistance and ohmic resistance are 115

and 48 kJ/m respectively, at low temperature where the effect of electrode resistance

predominate the total cell impedance.

Chapter-I

27

1.7.10 Spray Pyrolysis

K. Konstantinov et al., (2000)95

investigated the subsequent thermal treatment on the

morphology of Ceria thin films by spray. XRD pattern confirmed that the films are cubic

fluorite structure compared with JCPDS data (34-0394) and the lattice constant is calculated

as a=0.5419 nm. SEM Photographs showed that the ceria films are polycrystalline nature.

The film represents a fine-grain matrix and rather uniform morphology up to 400oC, beyond

400oC some of numerous cracks appeared in films. According to this concept, the number

and size of the crystal generated from the evaporation of solution droplets depends on the

degree of super saturation with rising temperature. The rate of droplet and degree of super

saturation increase lead to the formation of large number of nanometer sized crystallites.

Among these prepared films (300-500oC) the best film is formed at 400

oC annealed at 500

oC

due to the formation of uniform fine-grain microstructure.

B. Elidrissi et al., (2000)38

reported the structural and optical properties of CeO2 thin films

by spray pyrolysis. The influence of mole concentration and substrate temperature on the

structural, optical and elemental properties of the films has been investigated. XRD pattern

showed that the prepared films are mainly amorphous upto 350oC. When the temperature is

increased above 350oC, crystallinity of the film also increased due to the increase of (2 0 0)

peak intensity. The well crystalline film is obtained at 500oC. As the mole concentration

increases, the crystallinity also increases due to film thickness. Films deposited at high

concentration (0.1 M) show poor crystallinity with some cracks due to internal stress applied

to films. Similar effects are also observed in CeO2 using cerium nitrate. SEM micrographs

showed that the films prepared from chloride solution have large grain size and high density

of porosity. On the other hand the nitrate solution leads to small grain size and no pores. XPS

spectra showed that the films have (Ce:O-64.29:35.21) slight deficiency in oxygen. Films

show high optical transmittance (80%) can be employed as humidity sensor and counter

electrode in smart window devices.

I. Taniguchi et al., (2003)96

elaborated the effect of preparation conditions on the

morphology of CGO and lanthanum strontium gallium magnesium (LSGM) thin films by

electrostatic spray deposition (ESD) technique. SEM images showed that when Ts increases,

the droplets landing on the substrates may be semidry and slightly spread on it and deep

cracks disappear. The size of the spray droplets increases with the flow rate of precursor

solution. Formation of cracks may due to the thermal stress during drying process. The

deposition time increase shows porous super facial part and a relative dense part close to the

substrate. SEM images of LSGM films show highly porous and 3D interconnected structure

Introduction and Review of Literature

28

with a narrow pore size. XRD pattern showed that the films prepared at 573 K exhibit

amorphous nature; the crystal structure transforms into desired perovskite structure when

annealed upto 1173 K.

J.L.M. Rupp et al., (2006)97

studied the relation between microstructures and electrical

conductivity of nanocrystalline ceria-based thin films by air blast spray. SEM micrographs of

CeO2 and CGO films exhibit dense and crack-free microstructure with no abnormal grain

growth. The grain growth of CeO2 films proceeds faster than grain growth of CGO films. The

addition of Gd host lattice becomes asymmetric and acts as drag force on grain growth. Spray

deposition of CeO2 films exhibits increase of average grain size from 37 to 162 nm

corresponding decrease of total conductivity from 0.13-0.026 S/m at 700oC. All CGO films

show higher activation energy than sintered films and decrease of total conductivity with

decreasing grain size.

N.L. Petrova et al., (2006)98

reported the influence of starting materials on spray deposition

of CeO2 thin films. The CeO2 thin films are deposited on fused-silica and ß-quartz substrates

with different temperature (300-500oC) and annealed at 600

oC and 800

oC for 1h, show

relatively poor crystalline peaks at higher temperature due to lower thickness of the films.

The CeO2 films prepared with nitrate salts and citric acid add little more stability than

chloride salt compared with JCPDS data 43-1002. SEM images showed that the films are

uniform with square or rectangular shape of crystallites. Films produced from Ce (N) cit are

more dense and uniform than Ce (Cl) tar.

Jennifer L.M. Rupp et al., (2007)99

investigated the chemical homogeneity of Gadolinia

doped ceria SOFC electrolytes thin films by spray pyrolysis. DSC curves revealed that the

3 wt% of mass loss observed at 180oC is due to desorption of water and broad exothermic

peak observed between 400-1000oC is due to the formation of crystallization from the

amorphous phase. XPS curves revealed that the increasing sputtering time, the Ce 3d valance

band structure of ceria changes from substiochiometric Ce4+

/Ce3+

which is probably oxidized

to mostly reduced ceria after sputtering. The presence of Ce2O3 or Gd2O3 doping in the CeO2

host lattice results in the formation of point defects with oxygen vacancies and electrons as

charge carriers. It exhibits small polaron hopping conduction mechanism.

J. De Souza et al., (2007)100

elaborated the preparation and characterization of cerium oxide

thin films deposited by spray pyrolysis technique. The prepared films are analysed by

structural, morphological, optical and electrical properties. SEM images reveal the presence

of cracks in the films that depend on substrate temperature and deposition time. Films

deposited at temperatures between 400 and 500oC with deposition time upto 10 minutes are

Chapter-I

29

crack free and also present high optical transmittance upto 90% in the visible range and close

to infrared. XRD shows that all films are polycrystalline and the growth preferential direction

is altered from (1 1 1) to (2 0 0) with the increase of the deposition temperature. The

activation energy of the electrical conduction process is 0.67 ± 0.03 eV.

Ulrich P. Muecke et al., (2008)101

studied the microstructure and electrical conductivity of

NiO/CGO electrolyte thin films by air blast spray pyrolysis. The average grain size of

NiO/CGO grains mainly defends on the annealing temperature and is found to be 5, 16, 53

and 260 nm for temperatures 600, 800, 1000 and 1200oC, respectively. The percolation limit

of the metallic conductivity in the thin film cermets is found to be dependent not only on the

film composition but also the grain size. The stability of CGO network can be enhanced by

annealing at higher temperatures. A stable CGO network and a stable nickel network are

formed when the annealing temperature and grain size are further increased.

I. Taniguchi et al., (2008)102

elaborated the surface morphology control of SDC and

NiO-SDC thin films grown on SDC substrates by ESD technique. The as-deposited thin films

are amorphous at the deposition temperature (350oC). Subsequently, the thermal treatments

are done for ranging from 700 to 900oC in air. As the result, the crystal structure transformed

into the desired cubic fluorite one after the sample is annealed over 700oC in the as-deposited

SDC thin films and 900oC in the as-deposited NiO-SDC thin films, respectively. To confirm

the composition of the as-sintered SDC thin films, ICP-OES analysis is studied for the films

annealed at 900oC in air for 2 hours. The observed chemical composition is found to be close

to that of the precursor solution within experimental errors.

Changsheng Ding et al., (2009)103

have reported the preparation of dense ceria electrolytes

thin films by spray coating method. Sedimentation test exhibits the well dispersed and stable

suspension with 2% dispersant. FESEM images exhibit the particles well dispersion in the

suspension. The agglomeration of particles becomes larger with increasing dispersion

constant. Dispersion could be improved with the addition of Triton X-100. FESEM images of

sintered films have uniform thickness and well bonded to the substrate without any interfacial

flaws between the coating layer and substrate.

A.K. Bhosale et al., (2009)104

reported the incorporation of WO3 on the structural,

morphological, optical and electrochemical properties of CeO2 thin films by spray. TGA

showed that the formation of CeO2 phase starts at 300oC and completes at 400

oC hence,

deposition temperature range between 300-450oC. XRD pattern showed that the obtained

films are cubic fluorite structure compared with JCPDS data 81-0792. All the samples are

polycrystalline with prominent peaks along (1 1 1) reflections. UV-Vis Spectra recorded that

Introduction and Review of Literature

30

the films have high optical transmittance (80%) in the visible region and band gap varies

from 3.01-3.55 eV for direct and indirect transitions. CeO2 film prepared at 400oC showed

negligible optical modulation with WO3 in electrochromical device having enhanced

efficiency 47-53 cm2/C. This indicates that the film can be used as optically passive counter

electrode in electrochromical device.

J.L.M Rupp et al., (2009)105

elaborated the impact of crystallization and growth kinetics of

amorphous ceria thin films deposited by spray pyrolysis technique. Thin films are produced

with only one kind of cation to avoid the influence of dopants and unpredictable influence of

too many organics on crystallization and grain growth via spray pyrolysis. Just above the

processing temperature the as-prepared metal oxide transforms from amorphous into a

crystalline phase. The driving force for grain growth is the reduction of free volume and

crystallization enthalpy during an isothermal hold for these biphasic amorphous-crystalline

metal oxides. Once the crystallization enthalpy equals zero, stable microstructures are

established and grain growth ceases. Self-limited grain growth kinetics prevails for the

biphasic ceramics upto 70-80% crystallized material. Increasing the temperature above the

crystallization temperature leads to fully crystalline microstructures.

Ulrich P. Muecke et al., (2009)106

fabricated the NiO-CGO thin films by air blast spray and

study their formation parameters. The maximum deposition temperature is not only a

function of the boiling point of the solvent but also strongly depends on the substrate

material. The maximum deposition temperature is found for sapphire at 430 10oC, for

silicon at 390 10oC, for foturan >470

oC for YSZ and CGO>500

oC. Light microscope

revealed the presence of small splats in the films with diameters around 0.5-2 μm, 5-10 μm,

>10μm for different working distance 20, 30 and 39 cm respectively. The Leiden frost point

(LFP) does not only depend on the solvent boiling point and salt concentration but also the

type of salt used.

A.K. Bhosale et al., (2010)107

improved the optical modulation and coloration efficiency of

electro chromic properties of CeO2-ZrO2 passive counter electrodes. XRD patterns confirmed

that the obtain films having cubic fluorite structure compared with JCPDS data 81-0792. Four

distinct peaks are observed, besides prominent reflection (1 1 1). There is no evidence of

zirconia content in the CeO2-ZrO2 two space composite films. The crystalline size of the films

increases in range from 9-17 nm with increases of ZrO2 content. SEM micrographs showed

that the pristine CeO2 films consist of golf-ball bulky porous structure of 5-12μm, the density

of gulf-ball like structure and porosity decrease when Zr content increases. The golf-ball like

structure disappeared slowly for high % of Zr content. CV confirmed that the better

Chapter-I

31

ion-storage capacity and electrochemical stability are observed in 5% of Zr content. The

optical spectra recorded that the films have high optical transmittance in the visible region.

The addition of Zr content in ceria affects the refractive index, band gap and surface

roughness of the films.

B.B. Patil et al., (2011)108

investigated the structural morphological and electrical properties

of spray deposited nanocrystalline CeO2 thin films. DTA curves showed the presence of

flatted exothermic peak at 279oC due to removal of nitrate from the precursor solution, TGA

revealed that the weight loss of 2.8% upto 350oC, beyond this not much loss is observed.

XRD pattern of CeO2 thin films exhibits prominent orientation in (1 1 1) reflection compared

to minor (2 2 0), (2 0 0) and (3 1 1) peaks. SEM images showed that the CeO2 thin films

exhibit smooth surface with some cracks due to shrinkage of film upon annealing. AFM

images showed that the films are made up of small granules having nearly spherical shape

with the surface roughness 1.6 nm. DC and impedance measurements showed that the

resistivity calculated is in the order of 107 Ω/cm and Ea 0.78 to 0.79 for annealed at 300 and

400oC respectively. The ionic conductivity may be significantly enhanced in nanocrystalline

to the microcrystalline films.

1.7.11 Sol-Gel Spin Coating

A. Pawlicka et al., (2000)109

studied the electrochromic characteristics of CeO2- ZrO2 mixed

oxide thin films by Sol-Gel Dip coating technique. XRD pattern showed that the films have

amorphous structure upto 100oC and when temperature increases to 235

oC, the crystalline

peaks (1 1 1) (2 0 0) and (2 2 0) appear. The absence of ZrO2 peaks suggest that the

crystalline peaks of CeO2 dispersed in amorphous phase of ZrO2. TG/DTA curves showed

that the large endothermic observed at 90-135oC is due to cerium salt decomposition and a

sharp exothermic peak observed 235-450oC due to final decomposition of cerium salt. CV

curves states that the cathodic wave maximum at -1.1 V corresponds to the Li+ insertion and

the anodic wave due to the Li+ extraction at -0.1 V. The charge density increased from

3.3 to 4.0 mC/cm2 for first 400 cycles and then stabilizes upto 500 cycles.

I. Kosacki et al., (2001)110

investigated the dynamical properties of nanocrystalline CeO2

thin films using Raman spectroscopy. Raman spectra revealed that the Raman active mode in

this material corresponds to 466 cm-1

which may be attributed to a symmetrical stretching

mode of CeO2 vibrational unit. The influence of microstructure of CeO2 is observed by the

line broadening and increases in its asymmetry due to reduction of phonon life-time in the

nanocrystalline regime. The correlation length depends on grain size; it is possible that the

change of correlation length attributed to oxygen vacancies.

Introduction and Review of Literature

32

Fabrication of ceria thin films prepared with sol-gel method was elaborated by Nilgun Ozer

et al., (2001)39

. SEM images showed that the surface is smooth and composed of very fine

grains with 200 nm thick. XRD pattern showed that the films are annealed at 300oC having

no peaks indicate amorphous nature, annealed above 400oC exhibit fcc cerianite structure

with (1 1 1) (2 0 0) (2 2 0) and (3 1 1) reflections. UV spectra revealed that the transmittance

of amorphous is higher than crystalline film, the calculated optical parameters like refractive

index 1.78±0.02 at λ=650 nm and extinction coefficient 0.008±0.003 are in good agreement

with reported values. The band gap energy calculated is 3.03-3.07 eV which is smaller than

sputtered films (3.15-3.25 eV). CV curves state that the ratio of inserted/extracted charges

improve from 0.91-0.96 for 1000 cycles.

Toshio Suzuki et al., (2002)111

reported the defect and conductivity in nanocrystalline ceria

thin films grown on sapphire substrates by polymeric precursor spin coating technique. The

electrical conductivity is studied as a function of temperature and oxygen activity and

correlated with the grain size. For nanocrystalline Gd-doped CeO2 thin films, the ionic

conductivity increased with decreasing activation energy as the grain size decreased. A

conductivity model is developed to analyse Poz behaviour of the electrical conductivity.

Using the conductivity model, the hopping energy of electron conduction and the enthalpy of

oxygen vacancy formation are determined for different microstructures.

I. Kosacki et al., (2002)112

correlated the relation between the disorder and microstructure of

nanocrystalline ceria and zirconia thin films by polymeric precursor spin coating technique.

The Raman spectra are described using the spatial correlation model, from which the

correlation length determined. This parameter is related to the average distance between

defects in the oxygen sub-lattice. It has been found that the Raman spectra are influenced by

the defects attributed to grain-size-controlled non-stoichiometryor acceptor dopants. A direct

comparison between the defects related to the spatial correlated effects and the concentration

of oxygen vacancies in CeO2 and YSZ specimens has been achieved. Therefore, Raman

spectroscopy can be an effective method to determine the concentration of oxygen vacancies

in oxides with the fluorite structure.

T. Suzuki et al., (2002)113

have studied the influence of the microstructure on the material

nonstiochiometry of undoped and doped ceria thin films. SEM images showed that the films

are very uniform texture annealed at 600oC, but annealed at 400

oC, show uneven grain

distribution with neck growth. Influence of thickness changes the grain size from 10-16 nm.

It indicates that the stable microstructure can be obtained by polymeric precursor spin coating

technique. XRD pattern confirmed that the presence of (1 1 1) (2 0 0) orientations in both

Chapter-I

33

undoped and doped ceria films. The grain size strongly depends on the annealing temperature

and gadolinium. Incorporation of Gd strongly reduces the grain size from 250 to 50 nm. The

measurement of electrical conductivity as a function of oxygen partial pressure and

temperature provide the information about the type of conductivity and thermodynamic

parameters of the defects. The activation energy calculated is 1.3 0.05 eV.

Piotr Jasinski et al., (2003)114

studied the sensing behaviour of nanocrystalline undoped

ceria thin films by polymeric spin coating technique. The films are characterized by XRD,

SEM, AFM and impedance analyser. Results confirmed that the relationship (POz)-1/4

corresponds to extrinsic charge compensation. In other words the concentration of O2

vacancies is related to the acceptor impurity concentration and the relationship (POz)-1/6

corresponds to intrinsic charge compensation. This difference may be due to different levels

of purity of ceria and different preparation technology. Inter digital electrodes allow a

decrease of resistance in the temperature range 700-750oC which can be used for the

operation of sensor. The poisoning effect changes the film structure in electrode-electrolyte

interface.

Marcos A.C. Berton et al., (2003)115

have reported the optical and electrochemical

characterization of a new counter electrode layer of CeO2-SiO2 prepared by sol-gel process.

CV curves state that the addition of Si increases the charge density for one layer film. A well-

defined reduced and oxidized peak at -0.5 V and +1.0 V are observed at 10th

500th

1000 cycles

and charge densities ratios are closed 1.0 upto 1000 cycles. CeO2 exhibits poor

electrochemical behaviour compared to CeO2-SiO2 films. The difference in the energy could

be seen as a difference in the depth of the trapping levels for the Li+ ions within the Ce/Si

oxide films. XRD indicates that the Si doped CeO2 shows less crystallinity than CeO2 film

due to a spynoidal transformation in small size Si to Ce atom.

Cesar O. Avellaneda et al., (2004)116

determined the chemical diffusion coefficient of Li

intercalation in CeO2-TiO2 thin films by Sol-Gel process. Lithium insertion is performed by

GITT method. It shows the concentration of all species is uniform and increase with applied

potential. At 0.63 V, only a small amount of Li inserted as X=0.03 into the electrode and at

0.86 V, higher amount of Li inserted as X=0.15. The chemical diffusion coefficient either

increases or decreases with increasing content of species and also depends on the nature of

long range interaction between species. D increases with increasing Li concentrations as

1.66x10-13

cm2/s at X=0.03 and 2.14x10

-11 cm

2/s at X=0.15.

Ajeet Kaushik et al., (2009)117

investigated the sensing behaviour and co immobilization of

CH-nanoCeO2 biocomposite films. XRD pattern confirmed that the presence of cubic fluorite

Introduction and Review of Literature

34

structure with an average size 10 nm. The incorporation of CeO2 in CH matrix changes the

regular planar morphology to the globular nanoporous morphology. IR spectra revealed that

the band at 465 cm-1

shifts to 505 cm-1

due to electrostatic interaction between OH and nano

CeO2. The presence of nano CeO2 in CH matrix improves the electronic and ionic transport

due to uniformly distributed nano CeO2 in CH matrix. The sensing behaviour of nano CeO2

improves with the addition of CH and immunoelectrodes.

Fabrication and characterization of mesoporous rare earth oxide binaries are reported by

Jessie Hierso et al., (2009)118

. The films are characterized by AFM, TEM, FTIR, XRD, XPS

and Impedance spectra. Gd-CeO2 films exhibits a decrease in refractive index between

20-200oC while the thickness almost constant, then sharp increase and decrease takes place at

200-300oC due to the decomposition of ps-b-PEO block copolymer. SEM images showed

that the films are porous structure which is perpendicular to the film layer forming 3D-porous

network. XPS spectra confirmed the presence of Ce(IV) oxide in Ce 3d peak with a small

amount of Ce(III) oxide. Impedance analyser exhibits two frequency states as incomplete

semicircle at high frequency and well defined semicircle at low frequency. Gd-CeO2

3D-nanoarchitexture exhibits low grain boundary blocking effect is attributed to high quality

of grain boundary region.

Torsten Brezesinski et al., (2010)119

studied the mechanical, electrochemical behaviour of

polymer-templated mesoporous ceria thin films. The homogeneity of CeO2 thin films are

characterized by TEM, FESEM and HRTEM. It shows cubic architecture with 14 mm

diameter pores, AFM images show the RMS roughness of 2 nm, HRTEM establishes 15 nm

thick pore walls which are highly crystalline with randomly oriented cerianite grains. SXAS

and WXAS spectra show the crystallization begins at 300oC, leading to further small

crystallites with an average size 2 nm. The peak intensity increases slowly and FWHM

decreases with higher annealing temperature suggest gradual nanocrystal growth in the pore

walls. The area under CV curves represents the total amount of stored charges which arises

from both faradaic and non-faradaic process. This indicates that the charge storage is strongly

dependent on films and substrate morphology. Electronic conductivity increases from

electron hopping processes and the films reveal significant amounts of increased capacitance

with decreased voltage. The calculated storage capacitance observed is 61 and 41 mC/m2 for

templated and non-templated films respectively.

C.S. Naveen et al., (2013)120

investigated the antireflective properties of nanostructured

CeO2-SiO2 composite thin films prepared by sol-gel method. Structural studies confirmed the

presence of CeO2 with (1 1 1) and (2 0 0) face centered fluorite orientations. The pure CeO2

Chapter-I

35

thin film exhibits maximum transmittance 85% whereas CeO2-SiO2 composite (2:1) film

exhibits excellent antireflective properties with reflectance of 1.08%. Pure CeO2 shows

superior antireflective properties (0% reflectance) on silicon substrate. The thickness and

refractive index of the films are calculated through transmission and reflection measurements.

The refractive index 2.05 is observed for CeO2 films deposited on glass. These CeO2 thin

films find their applications in the fabrication of efficient photovoltaic and optoelectronic

devices.

1.7.12 Sputtering

D.G. Lim et al., (2004)121

have studied the improvement of dielectric and interface properties

of CeO2 buffer layer by using metal seed layer and N2 plasma treatment. The dielectric

constant decreases from 13.7-4.5, when Ts increases from 200-400oC. AES observations

indicate that the interface transition regions are divided into O-rich and Ce-rich regions. XRD

pattern exhibits the presence of major peak (1 1 1) and weak peaks (3 1 1) and (2 0 0). The

growth temperature increases the texture coefficient of (1 1 1) peak, which is probably due to

the increase of diffusion velocity of atoms on the substrate stabilizes on (1 1 1) planes. The

surface roughness got reduced when Ts is increased due to the relaxation of built-in-stresses.

The lowest surface roughness is achieved at 400oC. Incorporation of Ce seed layer increases

the dielectric constant and crystallinity due to its improved chemical stability and interface

property. CV states that the higher capacitance is achieved after N2 plasma treatment.

G. Linker et al., (2005)122

investigated the growth stages of ultra-thin CeO2 films on r-plane

sapphire substrates by sputtering technique. XRD pattern showed that the epitaxial CeO2

films having three-well developed Laue oscillations observed in addition to (1 1 1) and

(1 0 0) main peaks. The -scan of mosaic distribution consists of two contributions as very

sharp high peak on the top of a shallow broad back grown and a small sharp observed left to

the peak. The lattice constant is calculated using XRD with partially and fully covered

epitaxial CeO2 films as 0.5411 and 0.5620 nm, respectively.

C. Brahim et al., (2006)123

have studied the influence of YSZ layer on electrical properties

of GDC thin films by sputtering deposition. XRD pattern indicates the existence of cubic

phase in both YSZ and GDC layers. SEM images show that the GDC layer seems more

compact with finer microstructure than YSZ layer. Impedance measurements show the

pseudo conductivity variations clearly and reveal the important contribution of YSZ layer.

Incorporation of YSZ layer decreases the conductivity of GDC layer.

Introduction and Review of Literature

36

H.Tollefsen et al., (2008)124

investigated the f-d hybridization and mixed vacancy in Ce thin

films on Ru (0 0 0 1) substrates. XPS spectra revealed that the absence of 4f0 peak near a

binding energy of 915 eV indicates the presence of trivalent Ce2O3 on the substrate. This

intensity increases with increasing oxygen exposure. At 681 eV, the XPS spectrum shows a

mixture of CeO2 and Ce2O3. At low oxygen exposure 4f2 is absent. This feature shows a

shake-down satellite in the presence of 3d core hole. The core level shift strongly depends on

hybridization between f and d states. Oxidation of Ru (0 0 0 1) exhibits the presence of both

trivalent Ce2O3 and tetravalent CeO2.

Yiguang Wang et al., (2009)125

investigated the synthesis and characterization of high

quality multilayer Gd-doped ceria and zirconia thin films by sputtering technique. The

Gd-doped ceria and zirconia films exhibit single phase with preferred orientation (1 1 1)

parallel to the α-Al2O3 substrate as per the XRD pattern. AFM images showed that the

surface roughness depends on Ts and number of layers, when Ts increases; surface roughness

of the films decreases and at higher temperature smoothest surface has been observed. No

significant difference between binding energies associated with Ce 3d7/2 and Ce 3d5/2 has

been confirmed from XPS spectra. Also observed the absence of Ce3+

state in binding energy

indicates that the films are fully oxidized into Ce4+

, Zr films clearly indicates the presence of

Zr4+

oxidation states. DC conductivity measurements show higher oxygen ionic conductivity

at lower temperature.

M. Baron et al., (2009)126

studied the interaction of gold with cerium oxide. STM images

showed that the films having flat terraces with various size. As prepared films have more

terraces and annealed films fewer terraces due to the oxidation of films. STM also suggests

that gold particles nucleates exclusively on the ceria, Au species are interacting with ceria

rather forming Au. XPS revealed that the highest binding energy peak is observed at 917 eV

due to the existence of Ce 3+

species. IRAS spectra revealed the presence of two absorption

bands at 2177 and 2156 cm-1

for CO stretching vibration. The interaction of Au with CeOx

involves strong charge redistribution in the ceria nanoparticles.

N. Tusd et al., (2010)127

investigated the photoemission properties of tin doped cerium oxide

thin films prepared by rf magnetron sputtering technique. The tin addition in the CeO2 thin

film causes growth of Ce3+

rich films whilst pure cerium oxide sputtering results in

stoichiometric CeO2 layers. It is shown that the quantity of Ce3+

depends on the tin

concentration in the film. An extra Ce 3d component in the tin loaded samples has been

reported. The formation of partially reduced ceria films by rf sputtering is of great importance

for the development of ceria-based catalysts with higher oxygen storage capacity. Magnetron

Chapter-I

37

sputtering is an interesting method for the preparation of ceria and mixed oxide layers for

both real and model catalysts since their characterisation revealed similar properties as in the

case of model systems prepared in situ. It is shown that the mixed oxide film properties are

stable towards air exposure, which allows studying them by various methods accessible only

by ex-situ.

Fabrication of MFIS using three different high K-insulating materials HfO2, Y2O3 and CeO2

are reported by Woo-Sic Kim et al., (2011)128

. XRD pattern of Y2O3 films shows only (1 1 1)

peaks, in CeO2 it shows strong (1 1 1) and weak (2 0 0), (2 2 0) peaks and in HfO2 shows

polycrystalline nature of growth orientation. C-V curves revealed that the permittivities of

Y2O3, HfO2 and CeO2 films got reduced by 13.5, 32.4 and 31.5% and the same may be

attributed to the interfacial reaction between insulating layers and Si substrates. The highest

breakdown voltage observed is 10 mV/cm for HfO2-MFIS. The HfO2-MFIS structure has the

best retention property which may be due to the smallest initial decrement and shortest

saturation time. This property might depend on leakage current behaviour.

1.7.13 Ultrasonic Spray Pyrolysis

Shengyne Wang et al., (2000)47

deposited cerium oxide thin films over silicon substrates by

ultrasonic spray. The films exhibit cerianite structure with prominent peak along (1 1 1)

reflection. The crystalline size also increases with decrease of tp/ti in the range 4.2-20 nm.

SEM images confirmed the uniform and smooth morphology. The films deposited in the

5-15s intervals are much smoother and uniform than that of 30-10s pulses at 5s intervals.

AFM images show the effect of short pulse time with relative long dip interval time to

provide enough time for crystallization of smooth and high density films. The results show

that low ratios of tp/ti are very much needed to obtain rich crystalline nanosized thin films.

Besides, the 2D and 3D growth of the films can be controlled by different ratios of tp/ti.

M.F. Garcia-Sanchez et al., (2010)129

elaborated the synthesis and characterization of

nanostructured cerium oxide thin films grown on Si substrates by ultrasonic spray pyrolysis.

The morphology, structure, optical index and electrical properties are studied by XRD, SEM,

AFM, ellipsometry and impedance spectroscopy techniques. The use of additives is very

important to obtain crack-free films. The substrate temperature and flow rate are optimized

for obtaining smooth, dense and homogeneous nanocrystalline films with grain size as small

as 10 nm. The influence of thermal annealing on the structural properties of films is also

studied. The low activation energy calculated for total conductivity (0.133 eV) is attributed to

the nanometric size of the grains.

Introduction and Review of Literature

38

1.8 Review on Al doped cerium oxide thin films

B. Zhu et al., (2001)130

fabricated the transparent two-phase composite thin films with high

conductivity by sol-gel technique. Film conductivity has been determined by impedance

analysis and electrochemical property is investigated by cyclic voltammetry. The

conductivity of the composite films is in the range 10-2

-0.2 S/cm for the temperature range

500-700oC. Binary films containing 30% alumina and 50% silica with thickness 35 and

30 nm are found to have highest amount of charge exchange for 1.0 Li/Ce and 0.7 Li/Ce,

compounds respectively. After 380 cycles the charge capacity of CeO2/Al2O3 reached the

same value that of the beginning. The films show a high transmittance for visible light.

Unusual properties for these composite thin films are due to the special thin film

nanostructure along with two-phase regions and interfaces. The ionic transport and

insertion/extraction mechanisms in these new composite film materials are discussed based

on a proposed physical model.

Preparation and characterization of Ce-Ti mixed oxide thin films deposited by DC magnetron

sputtering are studied by J. Purans et al., (2001)131

. Spectra results confirmed the presence

of Ce3+

, Ce4+

and Ti in the mixed oxide films which shows the crystalline behaviour of Ti and

amorphous behaviour of Ce-Ti. The 4f1 5d and 4f

0 5d peaks shift their position to 1.5 eV for

Ti and 2 eV for CeO2. EXAFS spectra show the presence of Ce4+

in 0.6 Ce/Ti films and Ce3+

in 0.1 Ce/Ti films respectively.

P. Muhamed Ashraf et al., (2007)132

have analysed the cerium oxide based metal matrix

composites of aluminium in marine environment. SEM images show the existence of

homogeneous CeO2- reinforced aluminium which acts as cathodic zone to suppress corrosion

of the mass. CV study states that a gradual increase in the stability of open circuit potential

(OCP) could be achieved with increased amounts of CeO2 reinforcement. The presence of

CeO2 in aluminium not only improves the electrochemical characteristics of the reinforced

aluminium, but also provides a stable protective oxide film to the surface.

T. Dhannia et al., (2009)133

studied the effect of aluminium doping on the structural and

optical properties of cerium oxide nanocrystals. The prepared samples are characterized by

XRD, TEM and DRS. The particle size of Al doped CeO2 samples are found to decrease with

Al concentration and increases from 6 to 20 nm with annealing temperature upto 900oC.

XRD and TEM studies confirm the cubic-phase Ce1-xAlxO2-γ nanocrystalline particle

formation. Lattice parameter is found to be increased with doping concentration of

aluminium and decreased with annealing temperature. Due to band tailing effect both direct

Chapter-I

39

and indirect bandgap energies are found to decrease with aluminium doping concentration

and decrease with increase in particle size due to size effect.

Madhana Sunder et al., (2009)134

have investigated the effect of substrate surface geometric

transformation in the growth of (0 0 1) CeO2 thin films prepared by rf sputtering. X-ray

reflectivity measurements showed that the ceria films having mixed (0 0 1) (1 1 1)

orientations exhibit the broadest (0 0 2) rocking curve with FWHM of 1.07Å. The annealed

films exhibit lower rocking curve of FWHM 1.580 and 1.46 Å at 830 and 1430oC

respectively. The as-prepared films exhibit well-defined atomic terraces with RMS surface

roughness of 1.9 nm and the annealed films weakly defined narrow terraces with RMS

roughness of 0.15 nm. The annealing temperature promotes the nucleation and growth of

single-crystal atomic (0 0 1) CeO2 epitaxially instead of multi domain (1 1 1) oriented CeO2.

W. Pinc et al., (2009)135

deliberated the addition of gelatine to the deposition of cerium based

conversion coatings and study their morphology and electrical properties. EIS and salt spray

observed that the films have better corrosion resistance 800 ppm compared to other samples.

SEM micrographs showed that the larger degree of cracking and spalling observed in the

lower gelatine concentration due to the film thickness. OCP measurements showed that the

gelatine concentration (50 to 100 ppm) increases up to 100ppm where the open circuit

potential reaches a (-0.42 V) nearly constant value. XRD analysis confirmed that the films are

having broad peak at 20-30 2θ due to the presence of hydrated Ce(IV) oxide. No peaks are

observed for CePO4.H2O up to 400 ppm, after 400 ppm there occurs some relatively sharp

peaks about CePO4 .H2O.

C. Baristiran Kaynak et al., (2012)136

focussed the effect of annealing on the properties of

Al doped cerium oxide thin films for MIM applications. The detailed physical

characterization on CexAlyOz/TiN stack upon annealing at different temperatures (600°C and

850°C) through different deposition methods AVD and PVD for possible Metal-Insulator-

Metal applications. XRD results exhibit that the dielectric and TiN(AVD) are amorphous,

while TiN (PVD) is crystalline in the as deposited stacks. In CexAlyOz/TiN (PVD) stack, the

dielectric remains in its amorphous state until 850°C. However, TiO2 crystallization is formed

at 600°C in CexAlyOz/TiN (PVD). The non-stoichiometric character due to the existence of

Ce4+

become more pronounced on the dielectric deposited on TiN (AVD) with annealing

temperature from 600 to 850°C.

Ivalina Avramova et al., (2013)137

investigated the dynamic X-ray photoelectron

spectroscopy of CeOx/Al2O3 films grown on stainless steel substrates by sputtering technique.

The resulting binding energy differences are derived from the frequency dependence of the

Introduction and Review of Literature

40

corresponding Al 2p, Ce 3d and O 1s peaks. At low ceria loadings the main constituent on the

surface is CeAlO3 phase, while for high ceria loading the film is constructed from CeO2 and

CeAlO3 phases spread over the Al2O3. Accordingly, it is observed that the ceria loading

determines the conductivities of the investigated thin oxide films. The main constituent on the

surface is CeAlO3 phase for low ceria loading sample, while for high ceria loading the film is

constructed from CeO2 and CeAlO3 phases spread over the Al2O3. It has been confirmed that

the film conductivity could be ruled by ceria loading. The conductivity increases with

increasing amount of cerium deposited. It is found out that the frequency response depends of

host oxide matrix.

L. Qu et al., (2014)138

reported the support modification for improving the performance of

MnOx-CeOy/γ-Al2O3 in selective catalytic reduction of NO and NH3. Characterization for the

samples involved N2adsorption-desorption, XRD, XPS, H2 temperature programmed

reduction and FT-IR. In the selective catalytic reduction (SCR) tests, MnOx-CeOy/γ-Al2O3-

ZrO2 (MCAZ) showed outstanding NO removal efficiency and could abate the deactivation

brought by SO2 and H2O. Moreover, the fluctuation of gas hourly space velocity (GHSV) has

a bit of influence on the activity at middle temperature. The characterization results exhibited

that MnOx-CeOy/γ-Al2O3-ZrO2 owned bigger specific surface area and appropriate pore

diameter, highly dispersed amorphous Mn2O3 as well as rational ratio of Ce4+

/Ce3+

. The

promotion of the activity was partially due to the stronger oxidation ability at low

temperature per the H2-temperature programmed reduction (H2-TPR) results presented.

1.9 Review on Co doped cerium oxide thin films

T. Yoshino et al., (2003)139

analysed the characterization of nano-structured Ce-Co mixed

oxide thin films prepared by electrodeposition technique. Films are deposited in equimolar

mixture show highest transmittance and little spectral change during charging and

discharging processes in 0.05 M NaOH solution. CV showed that the Ce-Co composite film

co-deposited at 30 jC yielded larger charge capacity in the potential range from -900 to

600 mV vs. Ag in 0.05 M NaOH solution. TEM observations and EDS measurements of the

Ce–Co composite films revealed that they consist of CeO2 nano-crystals in amorphous host

structures. It is concluded that these Ce–Co composite films (Ce/Co=1.0) are most suitable

for transparent counter electrodes in electrochromic (EC) devices.

T.S. Zhang et al., (2006)140

studied the effect of incorporation of Fe2O3 on the densification,

microstructure and electrical properties of Ce0.9Gd0.1O2-δ ceramics. TG-DTA curves showed

that the peak at 120oC due to loss of chemically absorbed water which decomposed into

oxides at 400oC. XRD pattern indicates that the increase in lattice constant with sintering

Chapter-I

41

temperature below 1600oC is due to dissolution of Gd2O3 in in CeO2. The maximum lattice

constant obtained at 1600oC confirms the dissolution completion of Gd2O3 in CeO2 in the

mixture. TEM images showed that the powder has nearly spherical shape with narrow

particle size distribution. Addition of Fe2O3 strongly affects the densification, microstructure

and grain growth of mixed powders, without Fe shows densification upto 78% and addition

of Fe improves the densification upto 87%. Addition of Fe improves the GB conductivity

without changing the Gi conductivity.

A. Samson Nesaraj et al., (2007)141

studied the physico-chemical properties of LaCoO3

based cathode and GDC/SDC electrolyte materials by glycine-nitrate combustion method.

XRD pattern showed that the LSC cathode powder has single-phase perovskite

(rhombohedral-hexagonal) structure, whereas the GDC/SDC has cubic fluorite structure

compared with JCPDS data 34-0394. Conductivity measurements state that the LSC

specimen is metallic at all temperature due to the conductivity decrease when temperature

increases. These results showed that LSC exhibits high electrical conductivity (3038 S/cm)

and is suitable for making cathode in SOFC. Chemical compatibility of LSC+GDC and

LSC+SDC composites is found to be good. SEM images showed the well-defined boundary

between the two phases (LSC/GDC), the LSC grain size having 50 μm and GDC grain size

25 μm.

S.Y. Qiang et al., (2007)142

deliberated the ferromagnetic properties of Co doped cerium

oxide thin films grown on Si (1 0 0) substrates by pulsed laser deposition method. XRD

reveals that CCO films grown on Si have (1 1 1) preferential orientation, while the film on

the glass is polycrystalline with nanocrystals. As confirmed by VSM, XPS shows that the Co

exists in high spin state displaces the Ce atom which contributes to the room temperature

ferromagnetism. Films grown on Si and glass substrates are different state of ferromagnetism,

which is believed to be induced by different film microstructures. Based on these results, the

possible ferromagnetism of their insulating film has been discussed. The successful

fabrication of CCO films having room temperature ferromagnetism on Si substrates is of

great importance in both technological and theoretical aspects.

Fabrication of cerium oxide based anode materials for SOFC is reported by L. Nie et al.,

(2010)143

. Examination of the microstructures reveals that small SDC particles are formed on

the surface of LSCF grains with a relatively narrow size distribution. Impedance analysis

indicates that the SDC infiltration has dramatically reduced the polarization of LSCF cathode,

reaching interfacial resistances of 0.074 and 0.44Ω cm2

at 750 and 650oC, respectively. The

activation energies of the SDC infiltrated LSCF cathodes are in the range 1.42-1.55 eV,

Introduction and Review of Literature

42

which is slightly lower than those for a blank LSCF cathode. The SDC infiltrated LSCF

cathodes have also shown improved stability under typical SOFC operating conditions,

suggesting that SDC infiltration improves not only power output but also performance

stability and operational life.

M.G. Chourashiya et al., (2010)144

fabricated Ni-GDC based electrolytes for IT-SOFC by

spray pyrolysis. XRD pattern confirmed the presence of gadolinium in the composite films in

NiO ceramic substrates. The lattice constant is calculated as 5.420, 4.169 and 3.540 Å for

GDC, NiO and Ni phases respectively. The average grain size and density of Ni-GDC is

comparatively lower than GDC on NiO/GDC ceramic substrates which may be attributed to

the phase reduction of NiO to Ni. SEM images indicate that the interface of films and

substrate appears physically separate with discontinuous grains in the films; such interface

leads to interfacial resistance in the total system. Impedance spectra revealed that the GDC

films exhibit three semicircles classified as slow, intermediate and fast electrical processes.

The capacitance value is observed in the order 10-8

and 10-11

F/cm2 for intermediate and fast

processes respectively. The conductivity of GDC films on ceramic substrates have shown

decreased value than NiO-GDC ceramic substrate.

M.G. Chourashiya et al., (2010)145

investigated the microstructure and electrical properties

of GDC thin films by spray method. XRD patterns of GDC films confirmed the presence of

Gd in the composite films. The average grain size and density of Ni-GDC is comparatively

lower than GDC on NiO/GDC ceramic substrates. The increased porosity may be attributed

to the raising temperature of NiO-GDC in reduced environment which eliminates oxygen

from NiO phase by forming H2O. Impedance spectra revealed that the GDC films exhibit

three semicircles classified as slow, intermediate and fast electrical processes. The

capacitance value is observed in the order 10-8

and 10-11

F/cm2 for intermediate and fast

processes respectively. The conductivity of GDC films on ceramic substrates showed

decreased value than NiO-GDC ceramic substrates.

T. Ivas et al., (2012)146

calculated the thermodynamic properties of CeO2-CoO

multicomponent system. The nano-phase diagram for 5 nm CoO particles show significant

lowering of the eutectic from 1644 to 1327°C. With this approach, the lowest melting

temperature for a 1 nm thick particle neck at the eutectic composition to TE=1180°C has

been derived. This „„size depending eutectic‟‟ is close to the previously reported unusual low

sintering temperature of CoO doped ceria with a shrinkage rate maximum at Ts ∼1023°C for

sintering 47 nm sized CeO2 nanoparticles with CoO (1 at.%).

Chapter-I

43

1.10 Review on In doped cerium oxide thin films

Enhanced mesostructural Order and optical and electrochemical property modification by the

addition of cerium (III) to mesoporous titania thin films have been reported by Karen L.

Frindell et al., (2004)147

. The photocurrent response of the mesoporous thin films dropped

dramatically as the amount of added cerium was increased. The loss in photocurrent can be

explained by the fact that the cerium-titanium oxide films have smaller anatase

nanocrystallites and also that cerium ions trap photogenerated electrons. Electrochemical

characterization revealed several effects to increase the content of cerium in the

mesostructure and presented significant differences between mesoporous and bulk (sol-gel or

sputtered) cerium doped titania materials. The large number of interfacial surface states is the

greatest structural difference between the mesoscopically ordered thin films and the denser

sol-gel films. The ordered porous networks, high surface area and surface accessible porosity

associated with this type of mesoporous thin film suggest great potential for their use in

device applications such as solar cells, electrochromic devices and batteries.

Ruigang Wang et al., (2006)148

investigated the low-temperature redox properties and

nanoscale heterogeneity of CeZrO. XRD pattern showed that the peaks are slightly shifted to

higher angles with increasing calcinated temperature. TEM images confirmed that the

particles are well crystallized with irregular surfaces. TGA curves revealed that the mass loss

may be due to the loss of oxygen associated with the reduction of Ce4+

to Ce3+

oxidation

states. High resolution images showed that the domains of crystallographically distinct

second phases are coherently embedded in the fluorite type matrix. Reduction properties

suggest that the formation of phases with Zr and Ce is due to improved uniformity within

individual grains which lead to great stability and lower redox activity.

A. K. Bhosale et al., (2010)149

studied the structural, morphological, optical and

electrochromic properties of silica mixed ceria thin films by spray pyrolysis. XRD pattern

showed that the CeO2-SiO2 thin films are having cubic fluorite structure with prominent

reflections (1 1 1) compared with JCPDS data 81-0792. The decrease in crystallinity and

intensity of reflection are observed with the increase of Si content. SEM images showed that

the larger portion of the CeO2 film is crack free, when Si content increases numerous cracks

and trenches are observed with increased porosity compared with other samples. The

electrical resistivity of the films varies from 1.05x1010

Ω/cm for CeO2 and 1.13x1010

Ω/cm for

CeO2-SiO2 thin films. CV confirmed the cathodic/anodic behaviour of CeO2-SiO2thin films.

The anodic wave different for different samples, indicates that similar intercalation process,

Introduction and Review of Literature

44

dissimilar deintercalation process and maximum reversibility for SiO2 films which exhibit

better ion storage capacity (ISC) than other samples.

1.11 Review on Mn doped cerium oxide thin films

Investigation of factors affecting the film formation and film quality of MnO2 and CeO2 by a

chemical bath method are reported by H. Unuma et al., (2003)150

.The optimum bath

temperature is found to be 333 K for MnO2 at 323 K. The deposition rate is very slow at

343 K and the surface become rough to predominant homogeneous precipitation. SEM

images showed that the tightly packed fusiform particles of MnO2 have the pH variation from

5.0-2.0 for MnO2 phase and 7.5-7.0 for Mn3O4 and Mn2O3 phase. XRD pattern of CeO2 films

formed at 323 K exhibit polycrystalline nature with very poor adherent to the substrate.

D. Pavlopoulos et al., (2008)151

prepared buffer layers using CeO2 and MgO thin films by

spray pyrolysis technique. XRD pattern of CeO2 films exhibit amorphous nature at 200oC,

but when Ts increases to 250oC, starts to show some of the weak peaks. At Ts 350

oC (1 1 1)

plane becomes most intense than other peaks and when the temperature is in between 450 and

600oC, the (2 0 0) peak becomes very strong. SEM images showed three different

morphologies; at low Ts (250-350oC), extremely rough films are observed with spaghetti-like

morphology, at medium Ts (350-550oC), smoothest films are formed and at higher Ts

(600oC), the film surface is quite rough with varying crystallite size 1 μm to 3 μm. XRD and

SEM image of MgO films indicates that a dense and homogeneous layer with thickness

ranges from 1650-1750 nm and film thickness got reduced to 650-750 nm due to the

reduction of deposition time.

F.E. Ghodsi et al., (2008)152

have investigated the optical, electrochromic and structural

properties of sol-gel derived Ce/Ti/Zr mixed oxide thin films by Dip coating method. AFM

images showed the formation of smoothest film having a mole ratio of 40/10 Ce/Ti/Zr in

mixed oxides. XRD pattern observed that the mixed oxide thin films are mainly amorphous

and no corresponding peak of any crystalline phase CeO2, ZrO2 and TiO2. UV-ViS spectra

revealed that the highest (45/5) and lowest (20/30) value of refractive index of the films is in

the visible region. It means that the refractive index of the film increases with Zr content in

the composition. CV analyse the ion storage capacity of Ce/Ti/Zr mixed oxide and the same

increases with increasing ratio of Zr in composition except 40/10 mole ratio.

Investigation on XPS spectra of cerium based mixed oxide thin films are discussed by

H. Yang et al., (2010)153

. XPS results indicate that Ti exists mainly in the form of Ti4+

. Ce4+

and Ce3+

coexist and Si exists in the form of Si4+

on the surface of the CST films. AFM

Chapter-I

45

analysis reveals that no spherical particles are observed for the CST film due to its smaller

particles, and it has lower surface roughness compared with pure TiO2 film. The UV-Vis

spectra demonstrate that absorption edges for the CST film with Ce/Ti=5% exhibits obvious

red shift compared with pure TiO2 film. Especially for Si/Ti=10% samples, band gap energy

reduces to a minimum value 2.87 eV. Moreover, CST films have much smaller water contact

angle less than 10o, while the pure TiO2 film shows a water contact angle 60

o. The Si/Ti=20%

samples reach super hydrophilicity with water contact angle of 31o with UV irradiation for

30 minutes.

I. Kozjek skofic et al., (2011)154

have reported the structural and electrochemical

investigation of Ce-Cu mixed oxide thin films. Fourier transform of the EXAFS confirmed

that only Ce4+

and Cu2+

species are present in both CeO2 and CuO samples and there is no

evidence of Ce3+

and Cu1+

peaks. Peaks in FT at a distance beyond 3 Å in Ce and 2 Å in Cu

are much lower compared to oxides. Extended X-ray absorption fine structure (EXAFS)

spectra showed that the low signals of Ce-Ce neighbours in the films relative to the bulk

suggested that CuO2 phase has crystallites about the average size of nanocrystals. Optical

spectra show that both the films are transparent in the visible region.

1.12 Review on Y doped cerium oxide thin films

B. Elidrissi et al., (2001)155

have investigated the structural, optical and electrochemical

properties of Ce1-xZrxO2 thin films by spray pyrolysis technique. XRD pattern showed that the

films having cubic fluorite structure with low (1 1 1) diffraction when Ce content is

maximum (x≠1) and the cubic fluorite structure changed to monoclinic structure at high Zr

content. It is well known that ZrO2 have three possible structures; monoclinic, tetragonal and

cubic. Raman spectra revealed the presence of three important bands located at 465, 556 and

1100 cm-1

. The bands at 556 and 1100 cm-1

are attributed to Si-O stretching and the low

energy band located at 465 cm-1

is due to cubic phase of CeO2 fluorite structure. UV-Vis

spectra showed that the films have high optical transmittance (80%) in visible region, when

Zr content increases absorption edges shift to lower wavelength.

C. Tian et al., (2002)156

elaborated the electrical properties of Y doped cerium oxide thin

films grown on sapphire and amorphous silica substrates by pulsed laser ablation technique.

The ionic conductivities of the films are found to be dominated by grain boundaries of high

conductivity as compared with that of the bulk ceramic of the same dopant concentration

sintered at 1500°C. The film grain-boundary conductivities are investigated with regard to

grain size, grain-boundary impurity segregation, space charge at grain boundaries and

grain-boundary microstructures.

Introduction and Review of Literature

46

D.C. Sayle et al., (2002)157

investigated the oxygen storage properties of YSZ supported

cerium oxide nanoparticles. Much effort has been focused on increasing the oxygen storage

capacity (OSC) of ceria, and one avenue of exploration is the ability to fabricate CeO2-based

catalysts, which can expose reactive surfaces. This is achieved by supporting the CeO2 thin

film on an yttrium-stabilized zirconia substrate using a simulated amorphization and

recrystallization strategy. In particular, the methodology generates models which reveal the

atomistic structures present on the surface of the reactive faces and provide details of the

grain boundary structures, defects (vacancies, substitutionals, and clustering) and epitaxial

relationships. Such models are much important to understand the active sites at the surface of

a catalytic material.

K. Sato et al., (2004)158

have investigated the microstructure and mechanical properties of

doped ceria ceramics. XRD pattern of cubic ceria shows slight shift in the lower angles. The

grain shape of all samples are either regular pentagon or hexagonal. Pure Sm2O3 shows

inhomogeneous structure. The grain size decreases at low level dopant and the same increases

at high level dopant (20-30 mol%). The colour of samples also changes from brown to yellow

and then white with the increase of dopant concentration which depends on valance state of

CeO2. Young‟s modulus of pure CeO2 is of low value compared with the doped one. Fracture

toughness and transgranular fracture ratio increase with dopant contents.

D. Perednis et al., (2004)159

discussed the electrochemical performance and affecting

parameters of Solid Oxide Fuel Cells prepared by spray pyrolysis. SEM images of single

layer YSZ electrolyte showed that the film is uniform, continuous and well adherence to the

anode and cathode. YSZ layer deposited by air-blast atomized process showed well

adherence to anode and delamination from smooth electrolyte due to bad adhesion to cathode.

The performance of the cells strongly depends on deposition temperature. The OCV is found

to be decreased from 880 to 810 mV with the Ts increase from 317 to 335oC. The OCV

achieved is 880 mV for ESD and 970 mV for air-blast atomizer. The highest OCV achieved

for multilayer electrolyte (CYO/YSZ/CYO) is 0.85 V at 700oC. The use of multilayer

electrolyte reduces the operating temperature upto 70oC.

B.B. Patil et al., (2007)160

studied the subsequent thermal treatment of samarium doped ceria

(SDC) thin films prepared by spray pyrolysis technique. The prepared films are characterized

by TG-DTA, XRD, SEM-EDAX and conductivity studies. DTA curve showed the presence

of broad exothermic peak at 279oC. XRD pattern showed that the obtained films are

polycrystalline in nature. Among these (300-450oC) films, the best one is formed at 400

oC

which has uniform, smooth and adhesive properties. The crystalline size of the deposited film

Chapter-I

47

is found to be increased with increase of annealing temperature. SEM images showed the

formation of uniform and dense films with numerous cracks due to the shrinkage and

compaction of the films. Annealed above 550oC, the substrate starts melting. Electrical

studies confirmed the semiconducting nature as the resistivity of all films decrease with

increasing temperature.

F.E. Ghodsi et al., (2008)161

studied the effect of heat treatment on electrochromic and

optical properties of Ce-Ti-Zr mixed oxide thin films by sol-gel Dip coating technique. XRD

pattern of heat treated films at 100 and 300oC show amorphous structure and do not having

any peaks of Ce, Ti, Zr. At 500oC, the prepared films show amorphous structure with low

intensity of (1 1 1) orientation. AFM images of Ce-Ti-Zr mixed oxide films show crack free

and homogeneity morphology with roughness range from 8.5-3.6 nm. If heat treatment is

below 300oC, the amount of organic compound is reduced. This effect converts the

electrostatic attraction between OH dipoles into repulsion. CV curves states that the films

exhibit anodic charge density of 2.72, 4.86 and 7.19 mC/cm2 when heat treated at 100, 300

and 500oC respectively. Films heat treated at 500

oC show better reversibility and well defined

reduction and oxidation peaks.

M.G. Chourashiya et al., (2008)162

fabricated CGO (Gd0.1Ce0.9O1.95) thin films by spray

pyrolysis method. XRD pattern revealed that even the deposition temperature reaches 300oC,

no material peaks appeared in the pattern. Elemental analysis confirmed that the atomic ratio

of Ce:Gd is 0.889:0.110 in 0.04 M for the deposited films and the same is 0.883:0.116 for

0.05 M which is in good agreement with precursor solution 0.9:0.1. AFM images showed that

the CGO films prepared from 0.04 M precursor solution are rougher than that of 0.05 M

precursor solution due to their large grain size and decreased number of nucleation centre and

cluster. Electrical measurements revealed that the conductivity of films at 623 K is 0.5 S/cm

and activation energy calculated is 0.697 eV.

C.Y. Chen et al., (2009)163

elaborated the effect of precursor characteristics on the surface

morphology of ceria and zirconia mixed oxides. ZHA exhibits four weight loss mechanisms

in the temperature range 50-150, 150-300, 300-400 and 400-600oC respectively and the

weight losses during the temperature ranges 50-150 and 400-600oC are attributed to water

evaporation and conversion into ZrO2 respectively as per the TGA analysis. XRD pattern

showed that the dominant YSZ phase produced tetragonal structure with partial monoclinic

structure. SEM images showed that the YSZ particles are spherical, agglomerated and

responsible for smooth surfaces; CeO2 particles have bowl like structure in uneven surfaces.

Introduction and Review of Literature

48

The difference in particle morphology at low concentration can be attributed to drastic

difference in precursor solubility.

Peter Blennow et al., (2009)164

focussed on synthesis and characterization of CGO powders

with high surface area by spray drying. TGA showed two major weight losses in the curve.

The first one started from 80-110oC due to the evaporation of residual water from powder.

The second one is 230-450oC due to the decomposition of nitrates. XRD pattern showed that

when the calcination temperature increases the peaks become slightly narrower and the

crystalline size varies from 9.8 to 13.3 nm for the calcination temperature 300 to 600oC. SEM

micrographs showed that the agglomeration is believed to be formed out of clustering of

small particles as observed from XRD and TGA.

Ulrich P. Muecke et al., (2009)165

have investigated the spatial distribution of the substrate

surface of NiO-CGO thin films formed by spray pyrolysis method. Light Microscope

revealed that the NiO films having the ring and disc type regular shape for acetate salt and

irregular shape for nitrate and perchlorate salts. The morphology is changed from ring type to

irregular (bubbled) shape with lowered salt saturation and the same from bubbled to ring type

shape with increasing amount of low boiling point solvent. NiO-CGO thin films are prepared

to demonstrate the feasibility of developing dense and crack-free two phase composite

anodes.

Chang Sheng Ding et al., (2010)166

have reported the electrochemical performance of anode

supported SOFC with GDC electrolyte thin films by spray drying process. SEM images

showed that the GDC electrolyte thin film is uniform and well adhered to the porous anode

substrate. The estimated porosity of GDC films and Ni-GDC substrate is about 2.5 and 26.5%

respectively. I-V and I-P characteristics showed that the cell voltage decreases linearly with

current density increase, which indicates the good electrode performance. The OCV depends

on the density of electrolyte and operating temperature.

Nikolaos I. Karageorgakis et al. (2011)167

investigated the flame spray deposition of CGO

thin films and study their deposition mechanism and characteristics. The cross-sectional view

of the CGO thin films prepared with three different solvents showed dense, crack-free

microstructure and the smoothness varies significantly with solvent. Nitrates dissolved in di

methyl formamide (DMF) at 200oC are chosen as the smoothest film. It is also important to

identify the size of the droplet that is responsible for the thin film formation, in order to

control and optimize the deposition technique and layer properties. XRD patterns of as

deposited and annealed (600-1200oC) CGO thin films showed that as deposited films exhibit

biphasic amorphous nature which is indicated by broad reflection at 28o and when the

Chapter-I

49

annealing temperature increases from 600-1200oC, the intensity of the peaks (1 1 1) and

(2 0 0) increases sharply.

Chin-Yi Chen et al., (2012)168

fabricated NiO-YDC anode material for solid oxide fuel cells

by spray pyrolysis. The grain size of as-pyrolyzed YDC particles decreases with NiO

addition; however, the grain size of sintered YDC composite increases by a small addition of

NiO. The AC impedance data revealed that the precipitated NiO may reduce the activation

energy of the YDC electrolyte and increases the conductivity of the YDC composite. The

addition of NiO intended to make the surfaces of the spray pyrolyzed particles significantly

uneven resulting in poor densification of the sintered YDC ceramics.

Based on the above reviews, the work on cerium oxide thin film preparation with

suitable dopants and devices development viz., photodiode and hetero-junction solar cell has

been conceived, realized and reported with required optimization and characterization.

Scope and objective of the work

The main aim of the present work is to prepare good quality semiconducting

CeO2:(Al, In, Co, Mn, Y) thin films with the following objectives:

To prepare good quality semiconducting pure and doped CeO2 thin films.

To carry out characterization for the deposited thin films.

To investigate the effect of various dopants like Al, Co, In, Mn and Y on the

structural, optical, morphological and electrical properties of the nanocrystalline

CeO2 thin films.

To appraise the rectifying behaviour of pure and doped CeO2 thin films.

To identify the most suitable dopant and its composition to go for device

fabrication.

To assess the employability of CeO2 thin films in diode and solar cell fabrication.

Introduction and Review of Literature

50

References

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2. J. George, “Preparation of Thin Films”, Marcel Dekker, Inc., New York (1992).

3. Moreno, Ivan, Optical Letters, 30 (2005) 914.

4. D.S. Sze, Physics of Semiconductor Devices, wiley, New York (1981).

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