Properties of Multifunctional Oxide Thin Films Deposited by Ink-jet Printing

Properties of Multifunctional Oxide Thin Films Deposited by Ink-jet Printing
Properties of Multifunctional
Oxide Thin Films Deposited
by Ink-jet Printing
MEI FANG
Doctoral Thesis in
Engineering Materials Physics
Stockholm, Sweden 2012
Properties of Multifunctional Oxide Thin Films
Deposited by Ink-jet Printing
Mei Fang
Doctoral Thesis, 2012
KTH-Royal Institute of Technology
School of Industrial Engineering and Management
Department of Materials Science and Engineering
Division of Engineering Materials Physics
SE-10044 Stockholm, Sweden
Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm, framlägges
för offentlig granskning för avläggande av Teknologie doktorsexamen, tisdag den
25 sept. 2012, kl. 10:00 i Sal F3, Lindstedtsvägen 26, Kungliga Tekniska Högskolan, Stockholm
Mei Fang Properties of Multifunctional Oxide Thin Films
Deposited by Ink-jet Printing
KTH-Royal Institute of Technology
School of Industrial Engineering and Management
Department of Materials Science and Engineering
Division of Engineering Materials Physics
SE-10044 Stockholm, Sweden
ISRN KTH/MSE--12/23--SE+TMFY/AVH
ISBN 978-91-7501-477-7
© Mei Fang(方梅), September 2012
Thesis Abstract
Ink-jet printing offers an ideal answer to the emerging trends and demands of depositing at
ambient temperatures picoliter droplets of oxide solutions into functional thin films and
device components with a high degree of pixel precision. It is a direct single-step mask-free
patterning technique that enables multi-layer and 3D patterning. This method is fast, simple,
easily scalable, precise, inexpensive and cost effective compared to any of other methods
available for the realization of the promise of flexible, and/or stretchable electronics of the
future on virtually any type of substrate. Because low temperatures are used and no aggressive
chemicals are required for ink preparation, ink-jet technique is compatible with a very broad
range of functional materials like polymers, proteins and even live cells, which can be used to
fabricate inorganic/organic/bio hybrids, bio-sensors and lab-on-chip architectures. After a
discussion of the essentials of ink-jet technology, this thesis focuses particularly on the art of
designing long term stable inks for fabricating thin films and devices especially oxide
functional components for electronics, solar energy conversion, opto-electronics and
spintronics. We have investigated three classes of inks: nanoparticle suspension based, surface
modified nanoparticles based, and direct precursor solution based. Examples of the films
produced using these inks and their functional properties are:
1) In order to obtain magnetite nanoparticles with high magnetic moment and narrow size
distribution in suspensions for medical diagnostics, we have developed a rapid mixing
technique and produced nanoparticles with moments close to theoretical values (APL 2011
and Nanotechnology 2012). The suspensions produced have been tailored to be stable over
a long period of time.
2) In order to design photonic band gaps, suspensions of spherical SiO2 particles were
produced by chemical hydrolysis (JAP 2010 and JNP 2011 - not discussed in the thesis).
3) Using suspension inks, (ZnO)1-x(TiO2)x composite films have been printed and used to
fabricate dye sensitized solar cells (JMR 2012). The thickness and the composition of the
films can be easily tailored in the inkjet printing process. Consequently, the solar cell
performance is optimized. We find that adding Ag nanoparticles improves the ‘metalbridge’ between the TiO2 grains while maintaining the desired porous structure in the
films. The photoluminescence spectra show that adding Ag reduces the emission intensity
by a factor of two. This indicates that Ag atoms act as traps to capture electrons and inhibit
recombination of electron-hole pairs, which is desirable for photo-voltaic applications.
4) To obtain and study room temperature contamination free ferromagnetic spintronic
materials, defect induced and Fe doped MgO and ZnO were synthesized ‘in-situ’ by
precursor solution technique (preprints). It is found that the origin of magnetism in these
materials (APL 2012 and MRS 2012) is intrinsic and probably due to charge transfer hole
doping.
Abstract Contd. ...
i
5) ITO thin films were fabricated via inkjet printing directly from liquid precursors. The
films are highly transparent (transparency >90% both in the visible and IR range, which is
rather unique as compared to any other film growth technique) and conductive (resistivity
can be ~0.03 Ω•cm). The films have nano-porous structure, which is an added bonus from
ink jetting that makes such films applicable for a broad range of applications. One example
is in implantable biomedical components and lab-on-chip architectures where high
transparency of the well conductive ITO electrodes makes them easily compatible with the
use of quantum dots and fluorescent dyes.
In summary, the inkjet patterning technique is incredibly versatile and applicable for a
multitude of metal and oxide deposition and patterning. Especially in the case of using acetate
solutions as inks (a method demonstrated for the first time by our group), the oxide films can
be prepared ‘in-situ’ by direct patterning on the substrate without any prior synthesis stages,
and the fabricated films are stoichiometric, uniform and smooth. This technique will most
certainly continue to be a versatile tool in industrial manufacturing processes for material
deposition in the future, as well as a unique fabrication tool for tailorable functional
components and devices.
Keywords
inkjet printing, oxides, thin film, ink, suspension, dye sensitized solar cell, Ag/TiO2, Fe3O4,
magnetism, acetates, morphology, Fe-doped ZnO, Fe-doped MgO, RT-ferromagnetism, ITO,
resistivity.
ii
Contents
Thesis Abstract ......................................................................................................................................... i
List of papers/manuscripts in the thesis....................................................................................................v
Publications not included in the thesis .................................................................................................... vi
Acknowledgements ................................................................................................................................ vii
Nomenclature, Abbreviations and denotations........................................................................................ ix
Part I Thesis
Chapter 1: Introduction .................................................................................................................... 1
1.1 Motivation ................................................................................................................................. 1
1.2 Framework of the thesis............................................................................................................. 2
1.3 Characterization techniques ....................................................................................................... 3
Chapter 2: Magnetic properties ...................................................................................................... 4
2.1 Brief history ............................................................................................................................... 4
2.2 Magnetism of nanoparticles ....................................................................................................... 6
2.2.1 Blocking temperature ...................................................................................................... 6
2.2.2 Susceptibility ................................................................................................................... 7
2.3 Magnetism in diluted magnetic semiconductors ....................................................................... 8
2.4 d0 ferromagnetism...................................................................................................................... 9
2.5 Prospects .................................................................................................................................. 10
Chapter 3: Inkjet printing technology ......................................................................................... 11
3.1 Background .............................................................................................................................. 11
3.2 Essentials of inkjet technology ................................................................................................ 14
3.3 EPU inkjet system ................................................................................................................... 16
3.3.1 The set-up of EPU system ............................................................................................. 16
3.3.2 Operation manual ......................................................................................................... 16
3.3.3 The principles of EPU system ....................................................................................... 17
3.4 Ink preparations of oxide materials ......................................................................................... 21
3.4.1 Properties of the ink ...................................................................................................... 21
3.4.2 Preparations of different inks ....................................................................................... 22
3.4.3 Stability of different inks ............................................................................................... 23
3.5 Processing conditions of inkjet printing .................................................................................. 26
3.5.1 Temperature compensation ........................................................................................... 26
3.5.2 Substrate pretreatments ................................................................................................ 27
3.5.3 Post-treatments ............................................................................................................. 28
3.6 Applications of inkjet technology ............................................................................................ 29
Chapter 4: Oixide films printed from suspensions ................................................................... 30
4.1 Printing of spherical silica ....................................................................................................... 30
4.1.1 Preparation of spherical silica ..................................................................................... 30
4.1.2 Patterns from inkjet printing ......................................................................................... 32
4.1.3 Coffee ring effect ........................................................................................................... 34
4.2 (ZnO)1-x(TiO2)x composite films for DSSC applications ......................................................... 36
4.2.1 Dye-sensitized solar cell ............................................................................................... 37
4.2.2 (ZnO)1-x(TiO2)x composite films .................................................................................... 39
4.2.3 DSSC performance........................................................................................................ 41
iii
4.3 Ag/TiO2 composite films ......................................................................................................... 44
4.3.1 Ag/TiO2 film preparations............................................................................................. 45
4.3.2 Effect of silver in TiO2 films .......................................................................................... 45
4.4 Summary .................................................................................................................................. 47
Chapter 5: Ferrofluid ...................................................................................................................... 48
5.1 Introduction of ferrofluid ......................................................................................................... 48
5.1.1 What is a ferrofluid? ..................................................................................................... 49
5.1.2 The properties of ferrofluids ......................................................................................... 50
5.1.3 Some applications of ferrofluids ................................................................................... 51
5.2 Magnetite nanoparticles ........................................................................................................... 52
5.2.1 Structure of magnetite ................................................................................................... 52
5.2.2 Properties of magnetite ................................................................................................. 53
5.2.3 Preparations of magnetite nanoparticles ...................................................................... 54
5.3 How to prepare ferrofluids? ..................................................................................................... 56
5.4 Summary .................................................................................................................................. 58
Chapter 6: Oxides films printed from acetate solutions .......................................................... 59
6.1 Why are acetates solutions? ..................................................................................................... 59
6.2 Inkjet printing of DMO............................................................................................................ 61
6.2.1 DMS: Fe-doped ZnO thin films..................................................................................... 61
6.2.2 DMI: Fe-doped MgO thin films .................................................................................... 68
6.3 Phase separation ...................................................................................................................... 72
6.4 Summary .................................................................................................................................. 74
Chapter 7: Inkjet printing ITO films ........................................................................................... 75
7.1 Introduction ............................................................................................................................. 75
7.2 Inks for printing ITO films ...................................................................................................... 76
7.3 ITO films from inkjet printing ................................................................................................. 77
7.3.1 Comparison of films printed from ink A and ink B ....................................................... 77
7.3.2 Lattice structure of ITO films ........................................................................................ 78
7.3.3 Electrical properties of ITO films ................................................................................. 79
7.4 Summary .................................................................................................................................. 81
Chapter 8: Conclusions &Future scope ...................................................................................... 82
Bibliography .......................................................................................................................................... 84
Appendix I: Units: magnetic properties ................................................................................................ 94
Appendix II: The printing rate determination of the EPU printer ......................................................... 95
Appendix III: Physical and chemical properties of solvents ...................................................96
Appendix IV: Physical constants.....................................................................................97
Appendix V: Recipe for preparaing kerosen based ferrofluid ............................................................... 98
Part II Attached papers/manuscripts
iv
List of papers/manuscripts in the Thesis
I.
II.
The art of tailoring inks for inkjet printing metal oxides *
Mei Fang, Lyubov Belova, K. V. Rao. Manuscript (2012)
Inkjet-printed (ZnO)1-x(TiO2)x composite films for solar cell applications **
Emad Girgis, Mei Fang, E. Hassan, N. Kathab, K. V. Rao.
Journal of Materials Research. Accepted (2012)
III.
Thermal annealing effects on Ag/TiO2 thin films prepared by ink-jet printing *
Mei Fang, Lyubov Belova, K. V. Rao. Manuscript (2012)
IV.
Rapid mixing: A route to synthesize magnetite nanoparticles with high moment *
Mei Fang, Valter Stöm, Richard T. Olsson, Lyubov Belova, K. V. Rao.
Appl. Phys. Lett. 99, 222501 (2011)
V.
Particle size and magnetic properties dependence on growth temperature for rapid
mixed co-precipitated magnetite nanopartices *
Mei Fang, Valter Stöm, Richard T. Olsson, Lyubov Belova, K. V. Rao.
Nanotechnology 23, 145601 (2012)
VI.
Room temperature ferromagnetism of Fe-doped ZnO and MgO thin films prepared
by ink-jet printing *
Mei Fang, Wolfgang Voit, Adrica Kyndiah, Yan Wu, Lyubov Belova, K. V. Rao
Mater. Res. Soc. Symp. Proc. 1394, (2012)
VII.
Magnetic properties of inkjet printed Fe-doped ZnO thin films *
Mei Fang, Anastasia V. Riazanova, Lyubov Belova, K. V. Rao. Manuscript (2012)
VIII.
Magnetism of Fe-doped MgO thin films prepared by inkjet printing *
Mei Fang, Anastasia V. Riazanova, Lyubov Belova, K. V. Rao. Manuscript (2012)
IX.
Electronic structure of room-temperature ferromagnetic Mg1-xFexOy thin films ***
Mukes Kapilashrami, Hui Zhang, Mei Fang, Xin Li, Xuhui Sun, K. V. Rao,
Lyubov Belova, Yi Luo, and Jinghua Guo.
Appl. Phys. Lett. 101, 082411 (2012)
Contribution Statement:
*
Mei Fang performed the literature survey, experiments, data analysis, and wrote the draft
of the manuscripts.
** Mei Fang preformed the films preparations, data analysis and wrote the draft of the
manuscripts jointly.
*** Mei Fang prepared the samples and performed XRD, SEM and SQUID measurements.
v
Publications not included in the Thesis
I.
II.
Effect of embedding Fe3O4 nanoparticles in silica spheres on the optical
transmission properties of three-dimensional magnetic photonic crystals.
Mei Fang, Tarja T. Volotinen, S. K. Kulkarni, Lyubov Belova, K. V. Rao.
J. Appl. Phys. 108, 103501 (2010)
Designing photonic band gaps in SiO2-based face-centered-cubic-structured crystals
Mei Fang, Tarja T. Volotinen, S. K. Kulkarni, Lyubov Belova, K. V. Rao.
Journal of Nanophotonics 5, 053514 (2011)
III.
Rapid and direct magnetization of goethite ore roasted by biomass fuel
Yan Wu, Mei Fang, Lvdeng Lan, Ping Zhang, K. V. Rao, Zhengyu Bao.
Separation and Purification Technology 94, 34-38 (2012)
IV.
‘In-situ’ solution processed room temperature ferromagnetic MgO thin films
printed by inkjet technique.
Yan Wu, Yiqiang Zhan, Mats Fahlman, Mei Fang, K. V. Rao, Lyubov Belova.
Mater. Res. Soc. Symp. Proc. 1292, 105-109 (2011)
vi
Acknowledgements
During the last four years, I have worked on exploiting ink-jet technology to fabricate and
study the physical properties of various technologically important different materials from
solar cells to dilute magnetic semiconductors. It has been a good opportunity for me to train
myself to gain experience on the role of a deposition technique and its competitiveness in
producing novel materials. I would like to thank everyone of my departmental colleagues for
their support in this effort.
First and foremost, I wish to express my deepest gratitude to my supervisor Prof. K.V. Rao
for his professional tutoring, continuous support and fruitful discussions. With his immense
experience, he has been a positive critic suggesting ways to solve problems that constantly
arise. With his strict attitude and bright ideas, my work has improved a lot. I would also like
to thank my supervisor Assoc. Prof. Lyubov Belova especially for her training in using
sophisticated microscopies, tutoring and supporting the goal of the project. I appreciate her
trust and encouragement from which I benefited a lot especially in developing a healthy
meticulous attitude toward science and research.
I am grateful to: Dr. Tarja T. Volotinen, the first people I worked with at KTH, for guiding
me into the fields of chemical synthesis; Dr. Valter Ström who has given me many
suggestions and helping me with all the magnetism related experiments; Mr. Wolfgang Voit
who taught me the inkjet technology and gave me frequent advice and technical support; and
Ms. Anastasia Riazanova for helping me a lot in using ion beam techniques to characterize the
ink-jet printed thin films. Heartfelt gratitude to Dr. Sandeep Nagar, Mr. Ansar Masood, Mr.
Kaduvallilsreekanth Sharma, Dr. Anis Biswas, Dr. Jingcheng Fan, Dr. Zhiyong Quan, Ms.
Adrica Kyndiah, Ms. Deepika Mutukuri, Ms. Maryam Salehi, Mr. Shirong Wang, Ms.
Maryam Beyhaghi, and many other colleagues in our group, Tmfy-MSE-KTH, for helping me
with many practical issues and providing me such a nice research company.
Special thanks to Prof. Takahiko Tamaki, Prof. Jun Xu, Prof. G. Gehring, Prof. Dan Dahlberg,
Prof. S. K. Kulkarni, Asst. Prof. R. T. Olsson, Prof. Emad Girgis, Prof. J.H. Guo, Dr. Yan Wu,
Dr. Mukes Kapilashrami, Ms Wenli Long, Assoc. Prof. Weihong Yang and Prof. Zhijian Shen,
for their helpful suggestions in my work and fruitful discussions in our collaborations.
vii
I would like to thank Hualei, Haiyan, Yanyan, Xiaolei, Shidan, Jun Li and Jianhui Liu with all
my heart, as they helped me a lot in my life abroad. Thanks also to my friends at KTH, for
giving me great happiness during my study in Sweden. I enjoy the friendship with them so
much!
I am so lucky and happy to have Xiao Zhu who always keeps company with me. His love,
encouragements and suggestions always have been a great help for me to find the way out of
life’s mess on occasions.
I would also like to express my heartfelt gratitude to my family. Over all these years, my
parents tried their best to support me. Their love is the most valuable treasure in all my life.
I’d also like to thank my sisters and brothers, and all of my relatives, for their love, support
and encouragement!
Thanks to China Scholarship Council for the support of my Ph.D study in Sweden. Kunt and
Alice Wallenbergs fund at KTH is acknowledged for the financial support which allowed me
to attend some conferences.
I would like to dedicate this thesis to all these people who ever gave me their support.
Thank you for being there, and thank you for your kindness!
I will keep moving forward and remember your care!
Mei Fang
viii
Nomenclature, Abbreviations and denotations
AFM
atomic force microscope
ATA
acetylacetone
BMP
bound magnetic polaron
c
velocity of light in vacuum
CIJ
continuous inkjet
CM
classic mixing
CVD
chemical vapor deposition
CW
channel wall
DMO
diluted magnetic oxide
DMS/DMI
diluted magnetic semiconductor/insulator
DOD
drop on demand
DSSC
dye sensitized solar cell
EB
energy barrier
EDXS
energy-dispersive X-ray spectroscopy
Eg
band gap energy
EPU
experimental printing unit
EVA
evaluation
FAM
formamide
FC
field cooling
FF
fill factor
FIB
focused ion beam
h
Planck’s constant
HC
coercivty
IPE
2-isopropoxyethanol
ISC
short circuit current
ITO
indium tin oxide
j
total angular momentum
JCPDS
Joint Committee on Powder Diffraction Standards
JSC
short circuit current density
K
anisotropy constant
kB
Boltzmann constant
LCP
left circularly polarized
MRI
magnetic resonance image
MS
saturation magnetization
ix
N
number of print passes
Oh
octahedral
PCA
photocatalytic activity
PL
photoluminescence
PVD
physical vapor deposition
PZT
Pb(Zr0.53Ti0.47)O3
RCP
right circularly polarized
RM
rapid mixing
SEM
scanning electron microscope
SQUID
superconducting quantum interference device
Rs
series resistance
Rsh
shunt resistance
RTFM
room temperature ferromagnetism
TB
blocking temperature
TC
temperature compensation
TC
Curie temperature
TCO
transparent conducting oxide
Td
tetrahedral
TEM
transmission electron microscope
TEOS
tetraethylorthosilicate
TGA
thermogravimetric analysis
TM
transition metal
TMAH
tetramethylammonium hydroxide
TN
Néel temperature
VOC
open circuit voltage
VSM
vibrating sample magnetometer
XAS
X-ray absorption spectroscopy
XES
X-ray emission spectroscopy
XMCD
X-ray magnetic circular differences
XPS
X-ray photoelectron spectroscopy
XRD
X-ray diffraction
ZFC
zero field cooling
λ
wavelength
η
power conversion efficiency
ρ
resistivity
χ
susceptibility
x
Part I: Thesis
Chapter 1: Introduction

Inkjet technique is a promising, efficient, inexpensive and scalable technique for
material deposition and mask-less patterning in many device applications. The thesis
focuses on inkjet printing of different functional metal-oxides thin films. The
preparation methods of inks for different oxides are presented, and their physical
properties are discussed.
1.1 Motivation
Metal-oxide thin films have wide applications in emerging electronics and renewable energy
technologies, such as thin film transistors, spin-based devices and solar cells, because of their
fundamental structure-properties.[1] Many techniques like evaporation, sputtering, pulsed
laser deposition, electron beam deposition, chemical vapor deposition, spin coating, spraying,
dip coating and inkjet printing, etc. have been used to prepare different metal-oxide films.
Among these, inkjet printing can be considered to be a ‘smart’ materials deposition technique:
the printing process and the deposition pattern can be controlled by a digital computer. Thus
any pattern can be printed directly without mask and the pattern is repeatable. The directpatterning ability reduces the waste of chemicals during fabrication. The process is costeffective with efficient usage of precursor materials and certainly environmentally friendly. In
addition, the printer generates tiny volume (in picoliter range) of the ink droplet for deposition,
enabling accurate deposition of minute quantities of materials with a high resolution in the
micrometer to submicrometer range for patterning. Layer by layer deposition offers the inkjet
technique the ability for 3-dimensional prototyping. Furthermore, inkjet printing is a liquid
deposition technique which doesn’t require vacuum system and gas system comparing to
vapor phase deposition techniques. It is a simple equipment, and the printing process is
inexpensive and easy to operate, enabling the possibility of materials deposition at home and
office. These advantages make inkjet technique attractive for patterning and thin film
deposition.
For inkjet deposition of metal-oxides, preparation of inks suitable for printing is long standing
challenge especially because every material we need to deposit has its own requirements and
the methods used are not easily transferable. There are three types of inks in common use:
1
dispersion of suspensions in a solution with limited stability, surface modified particles in
solution to suppress aggregations and sedimentations, and direct precursors solution based
inks. Commonly, the ink is prepared by dispersing the oxide particles in a liquid to form a
suspension. The particle-laden ink is transported to the substrate by printing, which is an ‘exsitu’ process. The films printed from the suspension-based ink are typically loose with weak
links among the particles, and can be porous. The morphology of the films depends on the
size and shape of the oxides source. The aggregates and sediments of particles in suspensions
affect the repeatability of the printing process, and may even cause nozzle clogging.
Suspension technique thus has limitations that need constant attention.
To implement inkjet technique in metal oxide deposition, we have developed metal-acetates
solutions approach to produce inks for printing oxide films. Unlike suspension-based inks,
solution-based inks are homogenous and stable over long time scale. The as-deposited films
are acetate precursors, which can decompose into metal-oxides during calcination. Thus the
oxide films are prepared as required on the substrate itself, i.e. ‘in-situ’. In this way, the
nucleation and the growth of oxides can be controlled during the calcination process and thus
the morphology and the properties of the films can be tuned efficiently.
The thesis discusses different examples of oxide films printed by using a custom made dropon-demand (DOD) inkjet printer for specific applications. For instance, fabrication of (ZnO)1x(TiO2)x
composite films for dye sensitized solar cell fabrications, Ag/TiO2 composite films
for photocatalysis, Fe-doped ZnO and Fe-doped MgO films for spintronic applications and
ITO for transparent conductive coatings, etc. Details of the ink preparations and the
characterizations of the obtained films are presented. The work addresses the merit and
inadequacies of different types of inks and the quality of the metal-oxides films obtained. The
use of acetate solutions as inks developed in this thesis could pave the way to extend the
inkjet printer into a versatile tool in industrial manufacturing processes.
1.2 Framework of the thesis
There are eight chapters in the thesis. After a brief overview of the content of the thesis in
Chapter 1, in Chapter 2 some basic essential concepts in magnetism will be presented. Then
the essentials of inkjet printing technique will be introduced in Chapter 3. Based on these,
Chapter 4 will discuss the oxide films printed from suspension inks, including SiO2, (ZnO)12
x(TiO2)x,
Ag/TiO2. In Chapter 5, Ferrofluid, an example for producing special suspensions
without sediments and aggregates achieved by surface modification, is presented. In Chapter
6, the ‘in-situ’ preparations of oxides films from acetate solution inks will present MgO, ZnO,
Fe-doped MgO and Fe-doped ZnO thin films. In Chapter 7, inkjet printing ITO thin films will
be introduced, which are printed from a chelated acetate solution ink. Chapter 8 will
summarize the work in the thesis, and give some suggestions for the future work.
1.3 Characterization techniques
Our inkjet printer used in the thesis is a custom-made piezoelectric shear mode drop on
demand printer, using the printhead from Xaar (XJ126/50). The visual photograph of the inks
and the setups were taken by a digital camera. The decomposition temperatures of the
precursors in the inks were measured in a Perkin-Elmer TGS-2 thermogravimetric analysis
(TGA) facility. X-ray diffraction intensity patterns were collected by SIEMENS D5000 XRay diffractometer (XRD). ‘Celref3’ was used to refine the lattice parameters of the films.
The morphology of the samples was characterized by optical microscope, Hitachi 3000N
scanning electron microscope (SEM) and JSPM-4000 atomic force microscope (AFM). High
resolution SEM images and film cross-section images were taken in a Nova600-Nanolab
SEM/FIB system. The morphology of nanoparticles was investigated in a Philips, Tecnai 10
transmission electron microscope (TEM), and the size and size distribution of the particles
was detected by ‘Image J’. The magnetic hysteresis loops were determined by Model 155
EG&G Princeton Applied Research Vibrating Sample Magnetometer (VSM) and Quantum
Design MPMS2 superconducting quantum interference device (SQUID). The temperature
dependencies of the magnetization were measured using the SQUID. The ac- susceptibilities
were obtained with a custom-built high sensitivity susceptometer. The photoluminescence (PL)
emission spectra were detected by means of Perkin Elmer LS55 luminescence spectrometry.
The electronic structures of the films were determined at Lawrence Berkerley National
Laboratory: the X-ray absorption (XAS) and emission spectroscopy (XES) were performed on
BL7; the X-ray photoelectron spectroscopy (XPS) was determined by a PHI 5400 ESCA; and
the X-ray magnetic circular differences (XMCD) were measured from the difference of XAS
between the right circularly polarized light and the left circularly polarized light.
3
Chapter 2: Magnetic properties

This chapter introduces some relevant basics of magnetism and some new challenges in
magnetism and magnetic materials like dilute magnetic semiconductors, and the so
called “d0 magnetism”.
2.1 Brief history
The story of magnetic properties began as early as 7th century BC in ancient China, when
Guan Zhong (管仲) first described the lodestone as “慈石”, followed by the discovery of
attraction between the lodestone and the iron at 4th century BC in “Book of the Devil Valley
Master” (《鬼谷子》). In 11th century, Sheng Kuo (沈括) first recorded the usage of
magnetic materials as compass. He used the compass and employed the astronomical concept
of true north of the earth in “Dream Pool Essays” (《梦溪笔谈》). Since then, the magnetic
compass was developed and used in navigation all over the world. In 16th century, William
Gilbert studied the terrestrial magnetism and concluded the earth itself was magnetic, which is
the reason that the compass pointed north. [2]
With the discovery of the electricity, the relation between magnetic field and electric field has
been widely investigated since 19th century: Hans Christian Oersted found that the electric
current could influence the compass (1819); André-Marie Ampère, Carl Friedrich Gauss,
Jean-Baptiste Biot, Félix Savart developed the relation of induced magnetic field from electric
currents (1820); Michael Faraday found the electromagnetic induction (Faraday’s Law, 1831),
etc. The effect of magnetic field on optical properties was first observed by Michael Faraday
(Faraday rotation, 1845). With the experimental observations, the theory of magnetism was
developed: James Clerk Maxwell (1860s) gave the mathematic forms for Faraday’s
discoveries. He unified electricity, magnetism and optics into the field of electromagnetism,
which was formulated as “Maxwell’s equations” and formed the foundation of classical
electrodynamics. With the development of quantum mechanics, quantum electrodynamics
(QED) attained its present form in 1975 by H. David Politzer, Sidney Coleman, David Gross
and Frank Wilczek. Erwin Schrödinger, Paul Dirac, Wolfgang Pauli, Gerald Guralnik, Dick
Hagen, Steven Weinberg and others have contributed pioneering works of QED. Modern
4
history of magnetism is listed in Fig. 2-1, for both the theoretical and the experimental
breakthroughs. [3]
Fig. 2-1 Modern history of magnetism.
Of late with the attention drawn towards nanoscience and magnetic nanoparticles, some
special features relevant to understand them have become important. Besides, some features
particularly relevant to characterize the magnetism of oxide materials will be considered.
5
2.2 Magnetism of nanoparticles
Magnetism of nanoparticles is dominated by size effect when the particles are small enough to
contain only single domain. In a single domain, thermal energy can easily randomize the spins
at zero fields, and any external magnetic field will easily align the domains along its direction.
The competition between the thermal and intrinsic energy of ferromagnetic nanoparticles
which behave like ‘superparamagnets’ below a particular characteristic temperature results in
a rather informative parameter called blocking temperature TB.
2.2.1 Blocking temperature
The temperature dependent magnetization of superparamagnetic materials is usually
determined under a weak external magnetic field during the warming run after zero-fieldcooling (ZFC) from room temperature, followed by a measurement after cooling in the same
external field ( the so called field-cooling (FC) curve). During the warming run above a
critical temperature, spins in the single domain superparamagnetic materials can fluctuate
randomly due to thermal vibration. This means the spin fluctuation can remove the remanence
left in the material after magnetization. Below this critical temperature, however, the spin
remains frozen in a particular direction since the thermal energy is small and the magnetic
moments are blocked. This critical temperature is defined as blocking temperature (
ZFC curve,
). In
is determined as the temperature for the peak point of ZFC curve. It is the
critical temperature at which the thermal fluctuation and the interaction of the atomic spins
with the local field hold the balance: when
, superparamagnetic material loses its
preferred direction of magnetization in zero magnetic field (zero remanence and thus zero
coercivity) because of the thermal fluctuation; when
, the thermal energy is too small
to randomly align the spins and the magnetic moments are blocked.
The blocking temperature shows particle size dependence, described as:[4]
EB  K  V  (1  H / H K )
(2-1)
Where EB is the energy barrier, K is the anisotropy constant, V is the volume of the particles,
H is the applied magnetic field, and H K  2K / M S is the anisotropy field. For small particles,
they have small volume and thus low energy barrier and low blocking temperature. The size
distribution influence the shape of the M(T) curve. Narrow peaks in ZFC curve can be
6
obtained for particles with narrow size distribution, while wide peaks indicate large size
distribution of the particles.
2.2.2 Susceptibility
Since the induced moment in a sample is time-dependent, the AC magnetic measurements can
provide more dynamic information than DC measurements.[5] The induced AC moment
( mAC ) can be written as:
mAC    H AC sin(t )
(2-2)
where  is the susceptibility, H AC is the amplitude and  is the driving frequency of the AC
field. The slope of m(H ) curve can be expressed as:
  dm dH .
(2-3)
At a higher frequency, this magnetic response may lag behind the drive AC field and yields
phase shift (φ) in susceptibility. In this case, in-phase (   ) and out-of phase (   )
susceptibilities are used:
    cos  ,
    sin 
(2-4)
   2   2 ,
  arctan(    ) .
(2-5)
and
The blocking temperature (TB) determined at the peak of   and show frequency dependence,
which can be explained by Néel-Arrhenius law:[6]
TB 
EB
(2-6)

k B ln( )
0
Since the energy barrier ( EB ), attempt time ( ) and the Boltzmann constant ( k B ) can be the
same in measurements, while the measurement time  is the reciprocal of the frequency. The
Néel-Arrhenius law can be written as:[6]
ln( ) 
EB 1
  ln( 0 ) ,
k B TB
(2-7)
7
indicating the linear relationship of
and
. The relation is modified and expressed as
Vogel-Fucher law:[7, 8]
ln( ) 
where
EB
1

 ln( 0 )
k B TB  T0
(2-8)
is an effective temperature which accounts for the particle interactions in the system.
The slope can be used to determine the energy barrier ( EB ) and the anisotropy energy density
(K, Eq. 2-1), which indicates the magnetic properties of nanoparticles.
2.3 Magnetism in diluted magnetic semiconductors
Semiconductor devices use the charge of electrons, whereas magnetic materials are based on
the functional performance of electron spins. To combine both the charge and the spin of
electrons in semiconductors, diluted magnetic semiconductors (DMS) were developed by
introducing transition elements (Fe, Co, Ni, Mn, V, et al., with partially filled 3d orbitals) into
semiconductors to further enhance the performance of devices. By introducing +2 valence
magnetic ions into III-V semiconductor lattice or +3 valence magnetic cations into II-VI
semiconductors, DMS change their properties to create p- or n- type for attractive electronic
device applications.
The origin of ferromagnetism in DMS materials is still a topic of much discussion. The basic
magnetic interactions considered in understanding a DMS can be classified into:[9]
 Direct exchange and super-exchange interactions among the transition metal (TM)
atoms. For electrons in a free atom, the coupling interaction aligns the spins in parallel
according to Hund’s rules. Due to this interaction occurs between the electrons localized
in different neighboring atoms, the spins of the electrons would align antiparallel to form
covalent bonds. This is direct exchange which requires a close distance of the two
neighboring atoms. The exchange interaction can also be mediated by anions (e.g.,
oxygen) via metal-anion-metal bonds. This anion mediated magnetic coupling is known
as super-exchange.
 Carrier-mediated exchange: the localized magnetic moments interact with each other
through free carriers. Carrier-mediated exchange can be explained via (i) RKKY
interaction (Ruderman-Kittel-Kasuya-Yosida interaction) which describes the coupling
8
mechanism of a single localized magnetic moment (e.g., nuclear, or d or f shell electron
spins) and a free electron gas;[10] (ii) Zener carrier-mediated interaction. In systems
with both local magnetic moments and itinerant carriers, the interaction between a local
moment and a carrier is antiferromagnetic.[11] When the itinerant carrier encounters with
another local moment, the interaction will be again antiferromagnetic. These result in an
indirect ferromagnetic coupling between two local moments through the itinerant carriers.
 Bound magnetic polarons (BMP) exchange: oxygen vacancies in a system can act as
electron donors as well as electron traps. Because of the Coulomb attraction within a
Bohr orbit, a magnetic cloud forms surrounding a carrier (magnetic polaron). The
formation of magnetic polaron is easier in a system with donors/acceptors than that with
free carriers.[12] When the electron is trapped, its magnetic polaron couples the spins in
orbit of host material ferromagnetically. It forms a bound polaron, leading to a large net
magnetic moment. This exchange interaction depends on the distance of two BMP,
typically in the order of Bohr radii.
In diluted magnetic materials, the exchange interaction is weaker than that in magnetic
materials. It is because the larger distance between the spins of the dopants in DMS. Less
thermal energy is needed to randomly align the magnetic spins of the doped atoms. Many
DMSs were observed with Curie temperature less than 200 K.[10, 13] For most of the
applications, however, the robust room temperature ferromagnetism (ferromagnetism at and
well above room temperature, RTFM), is required for room temperature devices.[14, 15]
2.4 d0 ferromagnetism
In classical magnetism theory, the origin of ferromagnetism is the un-paired electrons in d or f
shells. Electrons in these orbitals are localized in narrow energy bands. Under an applied
magnetic field, the spins of unpaired electrons align in the same direction with the field and
show a net magnetic moment. However, recent studies show ferromagnetism from materials
without unpaired electrons in d or f orbitals. This phenomenon is called “d0 ferromagnetism”.[16] This type of magnetism was observed in carbon (e.g., C60 and graphite),
hexaborides (e.g., CaB6, BaB6 and SrB6), hafnium dioxide (HfO2) and oxides materials (e.g.,
MgO and ZnO), etc. One common factor in all these materials is the presence of defects, like
atomic vacancies or interstitials, surfaces or grain boundaries and dislocations or bond defects.
Most of the studies introduced dopant (e.g., C, N) into the lattice to investigate the
9
mechanisms of d0 magnetism. The origin of this type ferromagnetism is still not well
understood. Some interesting relevant concepts to understand magnetism in oxides are:
 p-type ferromagnetism. Typically electrons in p orbitals are itinerant with wide bands.
Even with partially filled p orbital, the exchange interaction between neighbor atoms
couples electron spins into pairs and cancels the magnetic moment (paramagnetism).
However, holes (defects) in 2p orbitals can act as electron traps and thus localize
electrons in some degree which contributes to the ferromagnetic ordering.[17, 18]
 Distortion of magnetic polaron of a free carrier. The defects (e.g., vacancies, substitution
defects) alter the symmetry of a charge with a local distortion which introduces potentials
in the atomic lattices. It represents a total spin around the defects, combining with
Coulomb correlations to create an extended magnetic moment on neighbor atoms.
 Resonance of the spin-polarization. With the introduction of open-shell impurities (the
valence shell has free-electrons, e.g., C, N) the molecular magnetism resonates with the
electrons in the host conduction band to form d0 ferromagnetism.[19] Hence the
presented magnetism is related to the band structure of host materials.
 Long range correlations. The defects induce local magnetic moments, while couplings
between two defects within a super-cell, or correlations between two impurities suggest
long-range ferromagnetic order. [20, 21]
 Spin-split of impurity bands. The defects in a semiconductor create states in the gap and
when the density of the states is sufficiently great, they form impurity band and
spontaneous spin splitting may occur. It can propagate the exchange interactions by
providing the localization length greater than the spacing between magnetic centers,
which results in ferromagnetism.[16]
2.5 Prospects
This thesis presents the studies of magnetism for (i) magnetite nanoparticles which have high
magnetization and almost zero coercivity prepared from co-precipitation by rapid mixing the
reactants; (ii) semiconductor (ZnO) and insulator (MgO) thin films which shows intrinsic
room temperature ferromagnetism and (iii) diluted magnetic oxides (Fe-doped ZnO and MgO)
thin films with enhanced RTFM by Fe-doping.
10
Chapter 3: Inkjet printing technology

Thin films can be fabricated by many techniques, such as physical vapor depositions
(PVD, including evaporation, sputtering, pulsed laser deposition and electron beam
deposition, etc.), chemical vapor depositions (CVD) and wet-chemical depositions (e.g.,
dip coating, spin coating, spraying and inkjet printing, etc.). With the developments of
electronics and optics, films with different patterns are required for the fabrications of
devices, like solar cells, thin film transistors, sensors, electrodes, electric circuits and so
on. Thus, film patterning techniques have been developed. The typical process is
photolithography in electronic applications, where the patterns are defined by the resist
masks. Imaging the office printing of the graphic-arts, can we use the direct digital
printing for patterning of functional materials? This chapter gives the answer: Yes, we
can!
3.1 Background
From the well-known office-printer, the advantages of inkjet printing functional materials can
be concluded but not limited to:

Simple equipment: no cumbersome vacuum and gas systems are needed. Ability to
directly pattern without the need for resist mask is an added feature.

Low cost, widely available and easy operation, enabling the home and office desktop
printing of functional thin films is especially attractive.

Computer controlled digital printing endows the possibility to print any type of patterning.

High productivity and repeatability promise their applications in industry.

High efficient usage of precursor materials and minimum waste during the printing
process make it environmentally friendly.

The film thickness can be controlled by choosing the concentration of droplets from the
ink and the number of print passes.
Because of these merits, inkjet printing technique has attracted much interest for fabrications
of different materials, including polymers, proteins, metals, ceramics and nanomaterials, etc.,
for developing new functional applications. Figure 3-1 shows the history of the inkjet printer.
11
Printers based on different principles have developed since 1948 and they are now widely
available in our daily work, ranging from desktop printers in office to professional machines
in industry. Recent developments in inkjet technology promise the possibility of printing
functional materials, not just printing graphic-arts.
Fig. 3-1 The phylogeny of inkjet technology.
The thesis focuses on inkjet printing of non-graphic-art: the deposition of functional oxide
materials. With the impressive tempo in the field of graphic-art, the potential of inkjet
material deposition can be imaged. The remaining challenges are:
12
 Ink preparation. There exists no general formula of inks for different materials.
Commercial inks are available only for a few materials.
 The precise patterning on defined locations depends on the geometry accuracy of the
printhead, especially on requirements in micron level.
 Either the spreading of the ink or the forming of the ink beads would change the
geometry of the film on the substrate from the digital defined pattern. Suitable
pretreatments for the substrates are necessary to avoid the distortion of the patterns, like
adjusting the wettability of the liquid-solid interface.
 Post-treatment is usually needed to obtain the solid films from the precursor liquid phase
materials, e.g., evaporation of the solvents and chemical decomposition, etc.
 The coffee-ring strain during the drying process might affect the uniformity of the film
and the final geometry of the printed patterns.
An important merit of inkjet material deposition is patterning. Traditional techniques of
material patterning use resist mask, either by subtractive process (e.g., etching) or additive
process (e.g. sputtering). The direct patterning can be achieved in focused ion beam (FIB)
system, either by milling extra materials or by depositing films with desired geometry.
Compared to these techniques, inkjet printing is a liquid deposition technique which can
achieve directly patterning with simple procedures, no-mask required, fast process and
computer controlled digital patterns. The typical patterning techniques for comparison are:
Photolithography: It is a high pattern definition micro-fabrication technique. The geometry of
the photo-mask is transferred to a photoresist layer which is light-sensitive. It can either
engrave the exposure pattern on the substrate by etching, or enable the deposition of a
new material in the desired pattern. The technique requires multi-steps which are not
efficient for small series production.
e-beam lithography: To avoid the diffraction limit of light, electron beam is used in e-beam
lithography. It can create architectures in nanometer range. The electron beam is emitted
in a patterned fashion across a resist film. The exposed part of which is removed and the
non-exposed regions are developed. Followed by either etching or depositing, the
pattern can be transferred to the substrate material. The disadvantages are time
consuming and the drift of the electron beam during the exposure, both of which would
affect the accuracy of the final geometry.
13
Focused ion beam technique: Comparing to electrons, ions are heavier and less scattering
with straight paths during the propagation. These endow the potential of a higher
resolution. Because of the high momentum of ion beam, atoms can be sputtered
physically from the surface. By scanning the beam over the specimen surface, an
arbitrary shape can be etched. Because of the high energy of ion beams, FIB can also
deposit materials via chemical vapor deposition process: the high energy of the beam
deposit atoms of precursor gases on the substrate, with high resolution and accurate
position. The feature of the pattern can be ~100 nm with thickness down to ~10 nm.
Inkjet printing: This is a liquid deposition technique, where the pattern can be designed on
demand by a computer. It is a direct patterning technique, with the process similar with
the office printer used to print graphic-arts. The resolution is typically at micro-level,
depending on the accuracy of the printhead. The basic principles of inkjet printing will
be presented in the following section.
3.2 Essentials of inkjet technology
As introduced in the phylogeny, different principles of inkjet technology have been developed
independently since the middle of 20th century. Figure 3-2 shows the inkjet family, with brief
annotations. According to the generation of droplets, they can be classified into two types:
continuous inkjet (CIJ) and drop on demand (DOD) inkjet. Figure 3-3 shows the schematic
diagrams of ink droplets generation and deposition of the CIJ and DOD printers.
In CIJ technique, the droplets are generated continuously and charged selectively. When they
pass through a high voltage deflection plate, the charged droplets are deflected and the
uncharged droplets are not affected to achieve either the pattern deposition on a substrate or
the collection of the ink for recirculation. The primary advantage of CIJ is the large number of
drops per unit time available per element. However, only a small fraction of the drops are
used for printing and the majority are directed into a catcher or gutter and re-circulated.
Besides, it is difficult to control the alignment and the precise position of the drops generated
from CIJ printers.
For DOD inkjet, the ink is jetted out only when it required. The transducer driver of the
printer is controlled by the data from the pattern for printing. Only when a pixel exists in the
pattern, can a drop be generated and deposited on the substrate. To achieve the drop on
14
demand, the transducer can be made from: (i) a resistor. By applying an electric current, the
resistor can be heated up and create bubbles in the ink to jet out a droplet (thermal inkjet). (ii)
a piezoelectric material. By applying a voltage/current, the shape of the piezoelectric material
can be changed, leading to the geometry changes of the ink chamber to jet out an ink droplet
(piezoelectric inkjet).
Fig. 3-2 Typical technologies of inkjet printing.
Fig. 3-3 Schematic diagrams of ink generation and deposition of CIJ and DOD printers.
15
3.3 EPU inkjet system
3.3.1 The set-up of EPU system
In our group, we constructed a desktop “Experimental Printing Unit” (EPU) for inkjet
deposition of functional thin film materials. Figure 3-4 shows the configuration of the EPU
inkjet station: both of the object picture (a) and the schematic diagram (b) of the system are
shown. This is a DOD inkjet printer. The printing patterns can be digitally designed by a
computer. The data of the pattern is processed by the core unit in the system: evaluation
system (EVA). It was designed from XaarJet for their printheads to achieve the printing of
different patterns by a computer-controlled user interface. The printhead in EPU is fixed
while the substrate is moving during printing. The substrate stage can be moved on x- and ydirections by the table controller. The ink droplets are jetted out according to the input signal
of the pattern, while the stage is moving the substrate to the printhead for deposition.
(b)
(a)
Printhead
Z-controller
EVA
Ink
Substrate table
Fig. 3-4 EPU inkjet station: (a) a real photo of the set-up and (b) the schematic diagram of the
system.
3.3.2 Operation manual
The operation manual for the inkjet station can be described as:
 Preparation: The substrate should be well cleaned and dried before being mounted on the
stage. The printhead ready for printing should be fully filled with ink.
 Printing: Move the substrate to the printhead by the table controller. Input the printing
pattern and start the printing. At this moment, the substrate is moving with the table stage
16
while the printhead is generating droplets according to the input signal, to achieve the
deposition of the patterns on the substrate.
 Post-treatments: The solvents in the ink should be evaporated to achieve solid films. For
multi-pass printed films, a new layer of the pattern is deposited on top of the dried layer.
3.3.3 The principles of EPU system
3.3.3.1 Printhead
The printhead used in this work is Xaar XJ126/50, representing 126 channels and droplet
volume of 50 pl (picoliter, 1 pl = 10-12 l =10-15 m3). Figure 3-5 shows the abridged view and
the sectional views of the printhead. There are 126 channels inside the printhead distributed
side by side in the 17.14 mm length. The channel walls (CWs) are made from piezoelectric
materials: Pb(Zr0.53Ti0.47)O3 (PZT), and plated on the upper half of both sides with metal
electrodes.[22] With the PZT cover plate, the ink channels are formed for ink. At the front
surface, a nozzle plate is assembled on the actuator for shaping the droplets. It is a shear
mode actuator, in which the driving voltage signals applied on the electrodes produce shear
deformation of the upper halves of the CWs. Latter, the lower halves follow the motion of the
upper halves and the ink channels are deformed into shapes (shown as the dash lines in Fig. 35c), and the droplet is jetted out from the b channel.
(b)
(a)
(c)
Fig. 3-5 (a) the abridged view and (b, c) the sectional views [22] of the printhead.
3.3.3.2 Piezoelectric effect
In EPU system, the printhead uses the piezoelectric actuator to generate ink droplets.
Piezoelectric effect describes the reversible linear electro-mechanical relationship of
crystalline materials. By applying a voltage on the piezoelectric channel walls, the shape of
the channel can be changed. The nature is the formation of electric dipole moment in the solid
which creates strains on the lattice to deform the shape of the substance. Figure 3-6 shows
17
examples of the changes of the positive/negative charge centers caused by the applied electric
field. The substance contracts, expands and shears into different geometry or shapes.
V
Charge
centers
V
V
V
(a)
(b)
(d)
(c)
Fig. 3-6 The schematic diagrams of shapes changed by an applied electric field: (a) the
distribution of positive/negative charge centers of a piezoelectric material; (b) contract, (c)
expansion and (d) shears of the piezoelectric materials under the applied voltages (V).
The PZT ceramics are used to fabricate channel walls of the printhead because they have:
(i) high curie temperature (TC, ~520 K).[22] For piezoelectric materials, TC is the temperature
above which spontaneous polarization lost. Figure 3-7 shows the two lattice structures of
PZT below and above TC.[23] At low temperature, the spontaneous polarization caused by
the deformation of the symmetric cubic unit cells endows the PZT crystallites piezoelectric
properties (the separation of the positive and negative charge centers, see Fig. 3-6a). At
temperature above TC, the charge centers overlap with each other leading to a net zero
spontaneous polarization, and thus PZT loses the piezoelectric effect.
(b)
(a)
Fig. 3-7 The lattice structures of PZT ceramic: (a) the distorted and asymmetric cubic
structure below TC and (b) the perovskite symmetric cubic structure above TC.[23]
18
(ii) high coupling coefficient. It is defined as the root of energy conversion ratio:
√
(3-1)
For the energy conversion in a thickness shear vibration in the printhead,
is the
coupling coefficient according to the direction definitions for piezoceramics (the subscript
‘15’ represents the shear stress and the perpendicular electric field to the polarization
axis).[24] The high
endows a good performance of inkjet actuators.
3.3.3.3 Generation of ink droplets
Figure 3-8 shows the waveform of voltage applied on CWs and the respective geometry of the
channel to generate an ink droplet.[25, 26] The voltage is applied on the metal electrodes
coated on the PZT channel walls (Fig. 3-8a). Because of the high coupling factor of PZT
ceramics, the perpendicular electric field causes the shear motion of the CWs and changes the
geometry of the channel. For example, the CWs with an applied positive voltage shear and
bend-out with extra ink filled inside the channel (Fig. 3-8b). Then change the direction of the
voltages to shear the CWs in the opposite direction (Fig. 3-8c). The ink is pushed out from the
nozzle due to the changes of the geometry of the chamber. A reset pulse is then applied on the
CWs to reset the shape of the channel wall. The whole process takes ~50 μs.
Fig. 3-8 The waveform of the applied voltage on channel walls for ink droplet generation: (a)
the structure diagram of applying voltage on CWs; (b) the bulge-out of the CWs; (c) the bendin of the CWs; (d) reset of the channel to the standard state.[25, 26]
19
To jet out an ink droplet, three neighbor channels are required. When a channel is ejecting a
droplet, its neighbored channels are at the stage with extra ink and ready for the next firing
process. The neighbored channels cannot generate droplets at the same time. Considering the
time requirement for the ink jetting out from the neighbored channels, the printhead is
assembled with an angle of 30.96° in our system (see the configuration of the EPU system
shown in Fig. 3-4a).
Figure 3-9 shows a sequence of snapshots of an ink drop at different times after the ejection
from a nozzle orifice.[22] The velocity of the drop is ~7 m/s. The break-off of the ink from
the nozzle plate depends on the firing frequency of the CWs, the driving voltage applied on
the printhead, and the physical-chemical properties of the ink. The images show that the inkjet
droplet consists of a “lead drop” and an elongated tail. To decrease the surface energy, the tail
contracts and breaks into small satellites. These satellites can deposit on substrate, deviate
from desired position during the movement of the substrate, or splash the deposited main drop
and change its geometry. In some cases but not all, the satellites can merge into the lead drop
to form a single drop for deposition. It is important to control the distance between the
printhead and substrate, the applied driving voltage and the properties of the ink to avoid
satellites deposition.
Ink droplet
Nozzle plate
Fig. 3-9 Sequence of snapshots at different instants after the ejection of an ink droplet from a
nozzle.[22]
3.3.3.4 Technique information
Table 3-1 lists the technique information for the moveable x, y-table and the Xaar XJ126/50
printhead used in the EPU system.[27] The pixel of the printing pattern is 126 in width
20
according to the number of the ink channels/nozzles. Narrow lines can be achieved, typically
around 50 μm and possible down to 5~10 μm in width. The resolution of printing depends on
the volume of the ejected droplets and the wetting properties of the ink on the substrate. The
printing rate can be calculated, 1.6~2.8 cm2/s with the table velocity of 2 cm/s and the
geometry of Xaar 126 printhead (see Appendix II).
Table 3-1 Technique information of the printhead and the x, y-table used in the EPU system.
Xaar 126/50 a
a
b
x, y-table
Active nozzles
126
Velocity (automatic)
0~3.6 cm/s
Print width
17.2 mm
Velocity (joystick)
0~2.2 cm/s
Nozzle pitch
137.3 μm
encoder resolution (x-)
1 μm
Nozzle density
185 nozzles/inch
encoder resolution (y-)
0.5 μm
Drop velocity
~6 m/s
z- height accuracy
10 μm
Temperature
RT~240 °C
b
Drop volume
50 pl
Firing frequency
7.5 kHz
Weight
22 g
Dimension
45×13×43
The technical information for the printhead referred to Xaar 126 printhead datasheet.
Image the droplet is spheres, the diameter is ~22.9 μm for 50 pl.
3.4 Ink preparations of oxide materials
Ink preparation is still one of the major challenges of inkjet deposition of different materials,
especially for oxide materials. There are some specified physicochemical properties of the
inks for inkjet printing:[28]

Stability.

Specified viscosity range (typically 1~25 mPa·s for DOD printer).

Specified surface tension range (typically 20~50 mN·m-1 for DOD printer).

Chemical compatible with the printing system (e.g., pH value).
3.4.1 Properties of the ink
The stability requires that all physical and chemical properties of the ink remain constant over
time. Instability of the ink is usually caused by the interactions between the ink components,
the precipitation and the phase separation due to the solubility, the chemical reactions
21
between the ink and the printer system, and so on. Typical examples are the aggregates of
particles in colloidal systems and the particle precipitation in suspension inks.
Viscosity describes how easy the ink can be deformed. In piezoelectric inkjet system, the ink
is jetted out because of the geometry changes of the ink chamber. Low viscosity of the ink is
required to achieve the generation of the droplets by the deformation of the channel walls.
Besides, the spreading of the ink on the substrate also depends on the viscosity of the ink. In
the EPU system, the suggested value range is ~8-15 mPa·s (the room temperature viscosity is
0.9 mPa·s for water and 81 mPa·s for olive oil). The viscosity of the ink depends on: (i) the
solvents used in the ink; (ii) the solute concentration in the ink; and (iii) the temperature.
Surface tension describes how large an external force can be applied on a liquid surface.
Taken the floating objects on water as examples, the surface tension of water is large enough
to resist the force from the objects. The surface tension of the ink affects the formation of the
droplets and the spreading of the ink on substrate. The ideal value of surface tension of the ink
is in range of 25~35 mN·m-1 (the RT surface tension is ~72 mN·m-1 for water and ~32 mN·m-1
for olive oil) for the EPU system. By adjusting the solvents, the surfactants and the
composition of the liquid, the surface tension of the ink can be tuned to the target value.
pH value of the ink is another important factor for inkjet printing. Firstly, it affects the
solubility of the solutes. For instance, the ink which contains a polymeric binder is insoluble
at low pH.[28] Secondly, the stability can be affected by the pH value, especially for particles
colloids. In many cases the stabilization is caused by the charge which is dependent on the pH
value. A well-known example is the aqueous ferrofluid prepared from tetramethylammonium
hydroxide (TMAH).[29] Thirdly, the pH value of the ink affects the longevity of the printhead.
The corrosion of the channel walls by the ink influences the function and the accuracy of the
printing process. The suggested pH value of the ink used for Xaar 126/50 printhead is 6.5~7.5.
3.4.2 Preparations of different inks
The inkjet inks used for printing oxide materials can be prepared in forms of colloids,
suspensions and solutions. Both colloids and suspensions consist of an internal phase and a
dispersion medium. For colloids, the internal phase is microscopically dispersed throughout
the continuous medium phase uniformly. The dispersion is strongly affected by the surface
chemistry. For suspensions, the internal phase is typically solid particles. These particles
disperse in the fluid through mechanical agitation, but they will settle sooner or later.
22
Different from the colloids and suspensions, solutions are homogenous without a second
phase. They are prepared by dissolving solutes in solvents. Different types of the inks
discussed in the thesis are listed as follows:
 Oxide particles suspensions (Chapter 4). The suspension is prepared by dispersing oxide
particles in a liquid. To avoid the blocking of the nozzles, the oxide particles should be
with size as small as possible (smaller than a few μm, because the typical channel width
is ~75 μm and the nozzle diameter is ~50 μm). The major problem of suspension ink is
the instability. The oxide particles will eventually settle, either caused by the aggregation
or the precipitation of the particles. The sediments of oxide particles can change the ink
concentration, viscosity, surface tension, and may even give rise to nozzle clogging.
 Colloids (Chapter 5). Surfactants coated particles are usually used to prepare colloids.
With the surfactant, the particles-solvent interaction or the inter-particle interactions
contribute to the dispersion of the internal phase in the medium without sediments over
long time spans.
 Metal salts solutions (Chapters 6&7). The inks are prepared by dissolving the metal salts
into a solvent. After the evaporation of the solvents, the as-printed films are metal salts
and post-treatments are required to generate oxide materials. In the thesis, metallic
acetates are used as the precursors of different oxide materials, since (i) they are widely
available for many metals; (ii) they are soluble in water and many organic solvents; (iii)
the introduced impurity elements (carbon and hydrogen) can be easily burned off to
produce high purity oxides.
3.4.3 Stability of different inks
Comparing with the above inks, it can be seen that suspension-based inks are less stable than
others. Figure 3-10 shows a comparison of the stability of a solution-based ink and a
suspension-based ink. The suspension-based TiO2 ink was prepared by dispersing TiO2
particles in 2-isopropoxyethanol (IPE) to form the concentration of 0.25 M [TiO2], and the
solution-based ink for ZnO was prepared by dissolving zinc acetates in IPE solvent with the
[Zn] concentration of 0.25 M. The color of the as-prepared suspensions depends on the oxide
particles, e.g. milky for TiO2, ZnO, SiO2 etc. and black for Fe3O4, while the solutions are
typically transparent, e.g. yellowish for zinc acetates and reddish for iron acetates.[30] After 1
day aging, precipitations can be seen from the suspension based ink (TiO2 ink) while the
solution based ink (ink for printing ZnO) can be stable for years.
23
(a)
(b)
Fig. 3-10 (a) The as-prepared and (b) the 1 day aged suspension-based TiO2 inks and the
metallic acetates solution-based ink for printing ZnO.
The stability of the suspension-based ink can be improved by reducing the particle size.
Figure 3-11 shows the as-prepared and the 1 week aged inks prepared from SiO2 particles
with average diameter of ~520 nm and ~260 nm (±10% size distribution), respectively. The
solvent is formamide (FAM). The as-prepared inks are microscopically uniform, while the 1week-aged inks show sediments: the increased transparency at the top and the reduced
transparency at the bottom of the ink. For the larger size SiO2 in suspension, the particles are
almost precipitated at the bottom and the solution at the top is almost transparent. For smaller
size SiO2 in suspension, however, the transparency changed slightly, indicating the slow
sedimentation rate.
~260 nm ~520 nm
~260 nm
(a)
~520 nm
(b)
Fig. 3-11 (a) The as-prepared and (b) the 1 week aged silica suspensions prepared from
spheres with average size of ~260 nm and ~520 nm (±10% size distribution).
24
In a suspension, the stability is related to Brownian motion of the particles: the random
drifting of particles in the fluid. The amplitude of Brownian motion depends on the particles
size: smaller particles have grater amplitude. Another important factor is the difference in
density of the dispersed phase and the dispersion medium: if the density of the particles is
very close to that of the solvent, the suspension is stable. It can be explained by the force
analysis of the buoyancy and the gravity of particles in a fluid.
Considering the inter-particles interaction, the stability of suspensions can be even worse.
Figure 3-12 shows the as-prepared and the 1 day aged aqueous inks prepared from coprecipitated magnetite nanoparticles.[29, 31] Because of the magnetic interactions among the
Fe3O4 particles, the particles aggregate and settle in one day even though they were prepared
with several nanometers in size. Instead of direct dispersing Fe3O4 particles in water, drops of
tetramethylammonium hydroxide (TMAH) were added into the particles and then mixed
together before dispersing in water. The hydroxide anions were coated on Fe3O4 particles and
the tetramethylammounium cations formed a shell for each particle, resulting in electrostatic
repulsion among particles.[32] This is aqueous ferrofluid. Because of the repulsions among
the particles, the ferrofluid can be stable without any sediment for months or even longer.
(a)
(b)
Fig. 3-12 (a) The as-prepared and (b) the 1 day aged inks prepared from magnetite
nanoparticles without (Fe3O4 suspension) and with surfactant (ferrofluid).
More inks for printing oxide materials are introduced in the attached paper (Paper I: The art of
tailoring inks for inkjet printing metal oxides). The physical and chemical properties of the
solvents used in ink preparation are listed in Appendix III.
25
3.5 Processing conditions of inkjet printing
When using inkjet printer, we intent to print the designed pattern on precise position of the
substrate. The final geometry of the printed patterns depends on the shape of the droplet and
the dispersing of the ink on substrate. Both the properties of the inks and the processing
conditions are critical to the final geometry of the printed patterns. In this section, the
processing conditions of inkjet printing films will be discussed.
3.5.1 Temperature compensation
In the printing processes, the applied electric energy is transferred into the mechanical energy
of the piezoelectric materials. This is just the driving force for ink droplets to jet out. Note the
possible transfer of electric energy into thermal energy. It would introduce the increasing
ambient temperature of the printhead. The consequent temperature effects include:
 the changes of physicochemical properties of the ink;
 the thermal expansion of channel walls introduces unexpected shape changes of the
channels, resulting in the variation of the volume of the generated droplets;
 the behaviors of piezoelectric effects which show temperature dependency. The
temperature dependent polarization of piezoelectric materials could be related to the
variations of permittivity, channel wall capacitance, strain/stress in the lattice and
impedance magnitude, etc.[22, 33]
In EPU system, the temperature compensation (TC) can be adjusted accordingly by a factor
ranging from 0.5 to 1.0 in the evaluation system (see Fig. 3-4). Figure 3-13 shows the SEM
images of Fe-doped ZnO thin films, which reveal the TC effect on film thickness. The films
were printed from 0.25 M cation concentration ink with [Fe]:[Zn]=1:9. The film printed at TC
= 0.5 is ~27 nm thick, and the one printed at TC = 0.6 is ~45 nm thick, as shown in the FIB
cross-section SEM images. By adjusting TC, the applied voltages on CWs are changed, which
results in the changes in the volume of the ejected droplets and the thickness of the films.
The thickness of the printed films can be estimated from the technical information of the
printhead and the settings of the x, y-table (see Table 3-1). Figure 3-14 shows the process step
by step under the conditions of the printing resolution of XaarJet 126/50 printhead (resolution
of 360 drop per inch, 360 dpi).[26] The thickness is calculated to be ~46 nm using bulk ZnO
density, which is very close to the determined thickness of the printed films. From the
26
topography of the films (Fig. 3-13 c), nano-porous structure is observed in the printed films,
indicating the density of the films should be smaller than that of bulk materials. Consequently,
the thicker films should be obtained. This is complementary with the TC effects: the
suggested TC value for the printhead is typically ~0.8. The low TC factor is used in the thesis
to arrest the final geometry of the film in a short time.
(a)
(b)
(c)
Fig. 3-13 SEM images of Fe-doped ZnO thin films prepared from 0.25 M acetates solution
ink: (a) and (b) cross-section SEM images of films printed at TC = 0.6 and 0.5, respectively;
(c) the topography of the film printed at TC = 0.5.
360 dpi, for each droplet deposit on substrate:
Diameter =70.56 μm, Area =3910 μm2, Volume = 50 pl
Average thickness of liquid = 12.8 μm
For 0.25 M, 50 pl ink, the mass of Fe-doped ZnO = 1.0125×10-9 g
Density of solid ZnO: 5.6 g/ml (bulk) →volume = 180.8 μm3
Thickness of solid Fe-doped ZnO = 46 nm
Fig. 3-14 Flow chart of thickness estimation of films printed by inkjet technology.
3.5.2 Substrate pretreatments
The ejected ink droplet would move to and deposit on a substrate. Besides the ink droplet, the
surface property of the substrate is another important factor to achieve uniform films. The
substrates used in the thesis are glass (microscope slides, Knittel Glaser, 76×26×1 mm) and
silicon. The pretreatments for the substrates are listed as following:
Cutting: the substrates were cut into certain size by a diamond knife.
27
Cleaning: the substrates were firstly ultrasonically cleaned in acetone for 10 minutes. Then
rinsed by 2-propanol and blow-dried by nitrogen gun. The cleaned substrates were
immersed in 2-propanol to avoid any dust.
Etching: for Si substrates, an atomic layer of SiO2 exists because of the dangling Si-bond on
the surface. 10% diluted hydrofluoric (HF) acid is used to remove the oxide layer by
etching and to produce H-termination of Si surface.[34] Beside, HF etching can create
microstructures at the surface. The adhesion of film-substrate can be improved via
molecule bonding and mechanical occlusion.[35] The time for etching is ~5 minutes.
Longer time etching may increase the roughness of the substrate surface and affect the
uniformity of the inkjet printed films. After etching, the substrates were rinsed in deionized water, followed by rinsed by 2-propanol to remove HF acid.
Drying: the substrates were blow-dried by N2 gun just before printing to remove the liquid on
the surface.
Pre-heating: the substrates were typically pre-heated to certain temperature, e.g., 60 °C for
inkjet printing ZnO thin films, to adjust the evaporation rate of the printed ink. The
drying rate of the deposited ink is an important factor for the uniformity of the films and
the accuracy of the geometry.
3.5.3 Post-treatments
As a liquid deposition technique, post-treatments are required to transfer the liquid into target
solid films. Basically, the solvents should be evaporated. Because of the pre-heating of the
substrate, the drying time of the printed ink is 1~100 seconds, depending on substrate
temperature and the evaporation rate of the solvent. The dried films are then condensed on a
hot-plate setting at high temperature for each print pass to produce dense films.
 For oxide-suspension inks, the oxide films are obtained after drying. The films are loose
powders of oxides in this case. To condense the film, two methods are introduced in the
thesis: (i) sintering at a high temperature and (ii) adding another material as adhesive (e.g.,
Ag in TiO2).
 For acetates-solution inks, the as-dried solid films are acetates rather than oxides.
Therefore, calcination is required to achieve oxide films. The decomposition temperature
of the acetates is typically in range of 200~500 °C. Since the oxides are formed during
the calcination, the structure, morphology and properties of the oxide films can be tuned
by adjusting parameters in the calcination process.
28
3.6 Applications of inkjet technology
With the development of the inkjet technology in the mid-20th century (Fig. 3-1), a wide
range of applications have been developed: affordable color inkjet printer in 1995; office and
personal applications with computer control printing before 2000; industrial coating
productions and commercial printings including display panels, filters, large format, textile
and security printing, etc. before 2005; market growth of industrial applications and genetic
engineering developments in recent years. Figure 3-15 shows an overview of various
applications of inkjet printing in respect to the development of inkjet technology and the
market demands.[25] The thesis focuses on developing techniques for inkjet printing of metal
oxides (ceramics).
Fig. 3-15 Current and evolving applications of inkjet printing technology.
29
Chapter 4: Oxides films printed from suspensions

Suspensions can be used as inkjet inks for materials deposition when (i) they are stable
enough (the particles are well dispersed without aggregate or sediment); (ii) the size of
the particles is small enough to avoid nozzle clogging during printing; (iii) the viscosity
and surface tension of the liquid are suitable for inkjet printing; and (iv) they are
compatible with the printer system. In this chapter, particle suspensions are introduced
as inkjet inks to print different oxide films.  Different patterns printed from SiO2
suspensions are studied to reveal the features of the films prepared from inkjet printing;
 (ZnO)1-x(TiO2)x composite films are printed and used for dye sensitized solar cell
(DSSC) fabrications;  Ag/TiO2 composite films are prepared for photovoltaic
applications. Possible ways to modify the microstructure and improve the properties are
also discussed for inkjet deposition of oxide thin films from suspension inks.
4.1 Printing of spherical silica
Monodisperse silica spheres have attracted increasing attentions because of their widespread
applications in optical field (e.g. self-assembly photonic crystals), industrial products (e.g.
colloidal particles used in paints, inks and cosmetics), materials science (e.g. aerosols and
slurry), chemistry, and biology (e.g. catalysis), etc.[36, 37] They can be prepared in a narrow
size distribution with average size in range of 50 nm to 2 μm. The scale of visible light
wavelength is inclusive in this range, which makes silica be attractive for optical devices.[38]
Spherical silica is typical slurry materials for chemical mechanical polishing. The spherical
shape of silica can decrease defects and scratches on the polished surface.[39] In the field of
inkjet printing, Perelaer et al. used microspheres silica to study the ‘coffee ring’ effect.[40] In
this section, spherical silica prepared from hydrolysis of tetraethylorthosilicate (TEOS) are
introduced and used as the internal phase in preparing suspension ink. The features of the
printed patterns are characterized.
4.1.1 Preparation of spherical silica
Spherical silica were prepared by a sol-gel method discovered by Werner Stöber in 1968.[41]
Stöber process can generate monodisperse silica particles by the hydrolysis of silyl ether to a
30
silanol followed by condensation reactions. The reactions of using TEOS as precursors of
SiO2 can be written as:
→
(4-1)
→
(4-2)
In the Stöber process, the concentrations of TEOS, water and ammonia, and the reaction
temperature are critical factors to control the final size and size distribution of silica.
According to Bogush and his colleagues’ study,[42] silica spheres with different sizes are
prepared in the thesis. The synthesis conditions are listed in Table 4-1.
Table 4-1 Synthesis conditions and the size of spherical SiO2 prepared from Stöber process.
Water a Ammonium b TEOS Volume c Predicted
(M)
(M)
(M)
(ml)
size d (nm)
SEM size
(nm)
Stirring
time (hrs)
ID e
2.5
0.5
0.3
100
~100
120± 30
2
P1
5
0.5
0.3
100
~280
260 ± 30
2
P2
10
2.0
0.17
100
~600
460 ± 50
2
P3
7.5
1.0
0.3
80
~500
495 ± 50
2
P41
520 ± 50
4
P42
2.5
0.5
0.3
100
~100
90 ± 15
2
M11
100 ± 15
4
M12
5
0.5
0.17
100
~200
250 ± 30
2
M21
280 ± 30
4
M22
7.5
0.5
0.17
100
~300
490 ± 30
2
M31
510 ± 30
4
M32
5
1.0
0.17
100
~450
630 ± 40
2
M41
640 ± 40
4
M42
10
2.0
0.17
100
~600
840 ± 50
2
M51
830 ± 50
4
M52
10
2.0
0.3
100
~700
830 ± 50
2
M61
850 ± 50
4
M62
a
The water contained in other reactants, like ammonia and the base solvent, is included.
The concentration of NH3•H2O in 25~30 wt.% ammonia solution. c The total volume,
achieved by adding 99.5% ethanol. d The predicted size of the spheres is referred to
Bogush et al.’s study. e Sample P1~P42 are pure silica; magnetite nano-particles are
added in the base ethanol to prepare samples M11~M62.
b
31
The steps of preparing spherical silica are listed as:
 High purity ethanol (99.5%), ammonia solution (25~35 wt.%) and water (plus other
additives like magnetite nanoparticles) were mixed in a flask by mechanical stirring.
 TEOS were then added drop by drop into the mixture and keep stirring for hours to
achieve spherical silica. For pure silica samples, the color of the liquid changed from
transparent to milky in the first few minutes. Ammonia solution was added drop by drop
to compensate the evaporation during the synthesis process.
 The resultants were collected by centrifuging at 5300 rpm for 4 minutes, and washed by
ethanol for three times to remove the reactants left over.
Some tips for the synthesis are suggested: (i) the compensation of ammonia evaporation
during mixing. A closed reaction system is suggested to lower down the evaporation rate of
ammonia. (ii) The pH value of the system (before adding TEOS) can be used as a reference
for the compensation of ammonia evaporation in the subsequent steps. (iii) Long time stirring
can achieve spheres with narrow size distribution because of Oswald ripening. The adding of
ammonia (dropwise) in this stage is useful to narrow the size distribution, especially for the
reactions with low ammonia concentration (i.e., 0.5 M and 1.0 M). (iv) The stirring rate. In
principle, high stirring rate is beneficial to prepare monodisperse spheres. Considering the
splash of the liquid with silica, the rate is suggested to be 500 rpm.
4.1.2 Patterns from inkjet printing
The printing patterns have been designed using a digital computer. Figure 4-1 shows the
patterns of dots and lines printed using SiO2 suspension inks. The suspension was prepared by
dispersing 2 mmol SiO2 spheres (P42 in Table 4-1) in formamide (FAM) via 10 minutes
ultrasonic agitation to form 20 ml 0.1 M ink. The ink was then printed on glass substrate with
designed dots and lines patterns. The liquid was slowly evaporated after printing, leaving
solid SiO2 spheres in the printed regions. Each dot in diameter of 90 ± 10 µm was printed
from a single droplet which was ejected from an individual nozzle (Fig. 4-1a), and the lines
were formed by many droplets aligned side by side. According to the input pattern signal, the
lines printed from 1, 2, 3 neighbored channels on the lateral direction are the narrow (85 ± 10
µm), medium (130 ± 10 µm) and wide (260 ± 10 µm) lines, respectively (Fig. 4-1b). Because
of the spreading of the inks on the substrate, the width of the line shows the non-linear
relationship with the number of the droplets. More complex patterns can be printed, as
examples shown in Fig. 4-2. They are UV-curable polymers printed on Si wafer.
32
(a)
(b)
Fig. 4-1 Optical microscopy images of patterns: (a) dots and (b) lines, with the enlarged views
shown in the right.
Fig. 4-2 Some examples of complex patterns printed by Xaar inkjet printhead.
33
4.1.3 Coffee ring effect
As one may notice in Fig. 4-1, the SiO2 spheres are accentuated at the contact edge of the
droplet for both circles and lines, which is known as “coffee-ring”. In physics, a coffee-ring is
the trace of liquid with solid particles after drying. It is called coffee ring because it was first
observed from coffee with 1 wt % solids in 1997 by Deegan et al. [43]. They found that the
solid particles in the coffee were accentuated in regions of high curvature (shown in Fig. 4-3a).
1 wt % spherical silica in water (molar concentration is ~0.188 M) was prepared and a single
droplet from inkjet printhead was printed on glass substrate. After drying, the silica spheres
were accentuated in the edges of the circle with high curvature. The optical microscopy image
is shown in Fig. 4-3(b). This is the coffee ring observed in inkjet printing. It is very typical for
liquid deposition techniques. The formation of coffee ring alters the distribution of the solid
from uniform depositions, which is undesirable in inkjet printing for device applications.
(a)
(b)
Fig. 4-3 (a) The pattern left by a 2 cm diameter drop of coffee containing 1 wt % solid [43]
and (b) coffee-ring observed from an inkjet droplet of ~1 wt % SiO2 suspension ink.
According to Deegan et al., the formation of coffee ring is attributed to the pinned contact line
with a fixed edge of the droplet.[43, 44] Figure 4-4 shows the schematic diagram of the
formation of a coffee ring. When there is a particle-laden liquid on a surface of solid with
pinned edges (Fig. 4-4a), the contact area of liquid-solid will be maintained during the whole
process of drying. The loss of the liquid during evaporation leads to an outward flow (a kind
of capillary flow) inside the droplet to maintain the liquid-solid contact edges (Fig. 4-4b). The
outward flow takes the particles to the contact edges where the particles deposited. After the
liquid is dried, the ‘coffee-ring’ is formed on the substrate (Fig. 4-4d). Hu et al. [45] claimed
that the formation of coffee-ring requires:  a pinned contact line;  adhesive particles to the
34
substrate;  high evaporation rate at the edge of the liquid pattern; and  the suppressed
Marangoni effect. The Marangoni effect is a phenomenon that the liquid flow away from
regions of low surface tension. It is caused by the surface tension gradient. From flow field
theory, Marangoni flow is observed along the surface of the droplet, which takes the particles
to the center of the droplet rather than to the edge.
Liquid droplet
Liquid flow outward
Liquid evaporation
Coffee ring: the particles
accentuated at the edges.
Solid surface
(a)
(b)
(c)
(d)
Fig. 4-4 The schematic diagrams of the formation of a coffee ring: (a) a particle-laden liquid
droplet on a solid surface; (b) inside the droplet, the liquid flow outward during evaporation;
(c) the loss of the liquid but the liquid-solid contact line maintains; and (d) coffee ring left on
the solid surface.
Coffee ring is unwanted in inkjet printing films because of the uneven materials deposition. In
inkjet printing, especially for patterning, the boundary of the printed droplet needs to be fixed
to obtain the desired patterns. Based on this, the printing process can be controlled to avoid
the coffee ring by adjusting:
 The contact angle. The contact angle is defined as the angle between the liquid–vapor
interface and solid–liquid interface. It affects the geometry of the printed liquid on
substrate. With a lower contact angle, the difference of evaporation rate is smaller and
less outward flow is needed to maintain the geometry during drying process. To decrease
the contact angle, the surface tension of the ink should be low (by selecting solvents or by
adding surfactant agents) and the wettability of liquid–solid interface should be high. The
wettability is the ability of a liquid to maintain the contact with a solid surface, which
depends on properties of both the solid and the liquid.
 The size and the shape of the particles. Large particles deposited on the substrate cannot
be moved with the outward flow of the liquid because of the gravity. It is reported that
spherical particles are more easily to accentuate at the edges and form coffee ring. The
reason is that they can detach from the interface and move past one another more easily
than anisotropic shaped particles, e.g. ellipsoid particles.[46] Thus to avoid the coffee
ring, anisotropic shape particle is suggested for materials deposition.
35
 Evaporation rate of the liquid during the drying process. It is reported that coffee ring
cannot be observed with a very short evaporation time. The liquid droplet is dried before
the diffusion of the particles.[47] Thence substrate temperature can be adjusted to control
the coffee ring effect.
4.2 (ZnO)1-x(TiO2)x composite films for DSSC applications
With the development of human civilization, the energy consumption increases year by year
for both industry and daily life. Figure 4-5 shows global energy consumption in recent years
and the regional consumption in 2011.[48] The statistics figure shows:
 The large and increasing requirement of energy (e.g. 12274.6 million tones oil equivalent
energy were consumed in 2011).
 The majority energy consumption is from fossil fuels, including coal, oil and nature gas.
The formation of fossil fuels takes millions of years, and the combustion produces carbon
dioxide which contributes to the ‘global warming’. They are non-renewable energy resources.
According to current proved reserves and flows, the fossil fuels will be used up in 148 years
for coal, 43 years for oil and 61 years for natural gas. Thus it is extremely important and
emergent to develop renewable energy resources. Renewable energy comes from natural
resources such as sunlight, wind, rain, tide and geothermal heat, etc. Among them, solar
energy is affordable, inexhaustible and clean. The total solar energy absorbed by earth’s
atmosphere, ocean and land in one hour is estimated to be enough for the energy consumption
all over the world for one year! However, the solar technology is currently very expensive,
which limits its applications.
(a)
(b)
Fig. 4-5 World energy consumption: (a) energy consumption from 1986 to 2011, and (b)
regional energy consumption in 2011.[48]
36
An important way of using solar energy is to convert the solar energy into electric energy
using semiconductor devices, i.e. solar cells. Under sunlight exposure, the electrons in the
valence band of semiconductor can be excited into conduction band to create a voltage or an
electric current in the semiconductor materials, known as photovoltaic effect. The sun emits
electromagnetic radiation across most of the electromagnetic spectrum while only waves with
wavelength ranging from 100 nm to 1 mm strike the earth’s atmosphere. The photon energy
(E, in unit of eV) of sunlight corresponding to the wavelength (λ, in unit of μm) can be
calculated from:
(4-3)
is Planck’s constant (
) and is the speed of
light in vacuum (
less than the energy band gap (
, see Appendix IV). Sunlight with photon energy
) of the semiconductor cannot contribute to the cell output.
Waves have energy greater than
energy (
contribute energy of
to the cell output. The extra
) is wasted as heat.
For the wide applications, the solar cells should be prepared with high energy conversion
efficiency and low cost to allow it to compete with fossil fuel electric generation. Multi types
of solar cells have been developed in order to improve the power conversion efficiency and to
decrease the price. Currently, the best research efficiency is 43.5% for multijunction cells,
28.8% for single junction GaAs thin film crystal cells, 27.6% for single crystal Si cells, 20.3%
for Cu(In,Ga)Se2 thin film cells, 11.8% for dye-sensitized solar cells, etc.[49] In Germany, 18
TWh electricity was generated in 2011. It contributed to 3.2% of total energy consumption,
with the charge of 3.6 euro cents per kWh which is approximately 10% of the total domestic
price of electricity.[50] The success example in Germany confirms the dominate role of solar
energy for the world energy consumption in the near future.
4.2.1 Dye-sensitized solar cell
Dye-sensitized solar cell (DSSC) is a low cost solar cell because of its simple manufacturing
process. In 1968 Gerischer and his colleagues presented the experimental data and theoretical
model of dye sensitized photocurrent, which was generated by organic dyes at oxide electrode
with large band gap (ZnO,
).[51] The modern DSSC was invented by O′Regan
and Grätzel et al. in 1991, known as Grätzel cell.[52] It is constructed by a transparent
conductive anode (e.g. tin doped indium oxide (In2O3:Sn) or SnO2:F), a layer of porous
37
semiconductor materials (such as ZnO, TiO2) covered with photosensitive dye molecules,
electrolyte and a platinum cathode. Figure 4-6 shows the schematic diagram of the DSSC.
The principle of electricity generation in a DSSC can be described as:

The sunlight pass through the transparent conductive anode to the semiconductor layer
covered with photosensitive dye.

Photons with enough energy are absorbed and excite an electron from the ground state of
the dye. The excited electron is injected directly into the conduction band of the
semiconductor and moves toward to the anode because of the electron concentration
gradient. Photocurrent inside the cell is formed. The dye molecule is oxidized.

The oxidized dye strips another electron from iodide in electrolyte immediately after the
loss of the electron. It is reverted into dye molecule which is ready for the generation of
next photoelectron. The iodide is oxidized into triiodide. This process is very fast (in the
time scale of 10-6 s) and can prevent the recombination (in the time scale of 10-4 s) of the
injected electron in the conduction band with the oxidized dye.[53, 54]

The triiodide is then reverted into iodide, where platinum electrode can act as catalysis
for the reaction. The whole process accentuates photoelectrons to the top anode and form
voltage/current with an external circuit.
Sunlight
TiO2 porous film
covered with dye
molecules
Glass substrate
Transparent conductive film
Dye
TiO2
Electrolyte
Platinum cathode
Fig. 4-6 The schematic diagram of a dye-sensitized solar cell.
The semiconductor layer plays important roles in (i) build-up dye molecules and (ii) transport
photoelectrons to the carrier collection anode. To increase the photo-induced electrons in a
given area, the number of dye molecules loaded on the semiconductor layer could be
increased by using porous nanostructured films which have large specific surface for dye
38
absorption. Simultaneously, the electron transport in the semiconductor layer should be
considered. The porous film should be with enough electronic conductivity. Generally
speaking, the photon-generated electrons diffuse to the anode along the gradient of electron
concentration. The drift of the electrons is prevented by coupling the moving electrons
strongly with the ions in the electrolyte (neutral carriers).[55]
The processes of DSSC fabrication are listed step by step as following:
 Print films (20~50 μm) of semiconductor ((ZnO)1-x(TiO2)x) layer by layer.
 Sinter the printed films at 600 °C for 1 hour to improve the link among the particles and
the electrical conductivity.
 Immerse the films in N3 dye solution (3×10-4 M in isopropanol) over night to form a thin
layer of the dye covalently bonded to the surface of the particles. The N3 dye molecules
are adsorbed on TiO2 surface.[56, 57]
 Wash the semiconductor layer covered with dye molecules by de-ionized water and
ethanol sequentially and left to dry.
 Sandwich liquid electrolyte (0.5 M KI and 0.05 M I2 in ethylene glycol) between the
photoelectrode and the Pt counter electrode prepared by magnetron sputtering.
4.2.2 (ZnO)1-x(TiO2)x composite films
TiO2 is a widely available, low cost, non-toxic and biocompatible material with chemical and
electrochemical properties sensitive to photons. It becomes one of the best choice of
semiconductor layer in DSSC.[57] The wide band gap and high conduction band edge energy
of TiO2 promise the ability of charge separation, which leads to a high open circuit potential
(VOC) in a DSSC. The chemisorption of dye molecules on the surface of TiO2 is also an
advantage for its applications in DSSC device. Further improvement in power conversation
efficiency higher than ~11% is a challenge. The balance between electron transport and
recombination is delicate in DSSCs. The lack of a depletion layer on the surface of TiO 2 leads
to the recombination, especially when the TiO2 film is thick.
Wide band gap ZnO has been explored as an alternative semiconductor in DSSC. It has
similar energy band gap with TiO2 but much higher electronic mobility (hundred times
higher), which can improve the electron transport and reduce the recombination loss.[54, 58]
Additionally, the easy crystallization and anisotropic growth of ZnO endows the possibility of
producing various nanostructures such as nanotubes, nanotips and nanowires, etc. However,
the overall conversion efficiency of ZnO based DSSC is lower than that of TiO2 (the highest
39
conversion efficiency of ZnO solar cell is 5.4%). The reasons could be the instability of ZnO
in dye solution, and the slow electron injection from dye molecules to the conduction band of
ZnO.[58, 59]
To combine the advantages of TiO2 and ZnO as the photoelectrode, ZnO-TiO2 core-shell
nanowire was developed for DSSC applications. The overall cell efficiency of the core-shell
cell is higher than that of ZnO nanowires without TiO2 shell cell. It is related to the
suppressed interfacial charge recombination which improves the open circuit voltage and fill
factor.[60, 61] It is valuable to get the performance of solar cells prepared from (ZnO)1x(TiO2)x
composite films.
In the thesis, (ZnO)1-x(TiO2)x composite films were prepared by inkjet printing the oxide
particles suspensions with x = 0, 0.2, 0.5, 0.8 and 1. The composite films were printed with 20
and 30 passes to make the thickness different. The average thickness of each printed pass is
~1 μm, observed from focused ion beam (FIB) cross-section scanning electron microscope
(SEM) images. Figure 4-7 shows the X-ray diffraction (XRD) intensity patterns of the printed
composite films with different molar concentration of TiO2. At x = 1, the pure TiO2 film
shows diffraction peaks matched well with anatase TiO2 (Joint Committee on Powder
Diffraction Standards, JCPDS no. 00-002-0387). It also shows diffraction peaks of other
titanium oxide phase (TinO2n-1. The marked peak is from Ti6O11, JCPDS no. 01-071-0628)
which comes from the chemical source of oxide particles. With the increase of molar
concentration of ZnO particles in the films, the diffraction peaks of ZnO (JCPDS no. 01-0792205) become dominated.
:ZnO
:from ITO glass
Intensity (a.u.)
:TiO2
:TinO2n-1
x=0
x=0.2
x=0.5
x=0.8
x=1
20
30 40 50 60
2 Theta (degree)
70
Fig. 4-7 XRD intensity patterns of inkjet printed (ZnO)1-x(TiO2)x composite films. Each
spectrum is shifted along y-axis with respect to each other for clarity.
40
4.2.3 DSSC performance
The (ZnO)1-x(TiO2)x composite films were used to fabricate dye-sensitized solar cells, and the
performance of these DSSCs were characterized. The performance of a DSSC is affected by
many factors such as the transmittance and the conductivity of the anode, the structure and
morphology of the semiconductor layer, the types of dye sensitizer, the electrolyte
composition, and the efficiency of the Pt catalyst, etc. To improve the power conversion
efficiency of DSSC, three potential ways are under investigations: [61, 62]
 New photon sensitizer with higher molar extinction coefficient and broader spectral
response than the existing dyes. This will fundamentally increase the number of
generated photoelectrons in a solar cell.
 Better electrolyte with redox couple closely matches to the energy of the oxidized dye to
increase the open circuit voltage (VOC). The VOC is the energy difference between the
Quasi-Fermi level of the electrons in the semiconductor and the potential of redox
reaction in electrolyte. This is a less effective way to improve the conversion efficiency.
 Structure tuning of the semiconductor layer to reduce the energy loss in a solar cell. The
energy loss can be caused by charge recombination, electron trapping, and optical
reflection, etc. For example, nanocrystalline materials can be used to increase the
diffusion length of electrons to maximize red light conversion in a DSSC.
The power conversion efficiency (η) of solar cells prepared from (ZnO)1-x(TiO2)x composite
films was determined by current-voltage (I-V) characteristics.[63] Figure 4-8 shows the
schematic diagram of I-V measurements used in the thesis.
(b)
(a)
(c)
Fig. 4-8 (a) schematic symbol of a solar cell; (b) the equivalent circuit model for a real solar
cell; (c) the I-V curves measured for the solar cell.
DSSCs convert solar energy into electric energy. They create a voltage or an electric current
(as the symbol shown in Fig. 4-8a) for external circuits. The process can be modeled as a
41
current source in parallel with a diode. Considering the resistance in a real solar cell, a series
resistance (Rs) and a shunt resistance (Rsh) are used in its equivalent circuit (Fig. 4-8b). For a
high efficiency solar cell, the Rs is small and Rsh is large. By measuring the output current (I)
and voltage (V), the I-V curve of the solar cell can be determined (shown in Fig.4-8c).
When the cell is operated at open circuit (i.e., the Rx is infinite and I = 0), the detected voltage
is the open circuit voltage (VOC); when the cell is operated at short circuit (i.e., the Rx = 0), the
short circuit current (ISC, or the short circuit current density JSC by average the current with
the area of the solar cell) is the maximum current value, i.e., the photon current. The
theoretical power of the solar cell is
. However, the maximum power (
) can be achieved by a solar cell is lower than the theoretical value. The ratio of
maximum power to theoretical power of the solar cell is fill factor (FF):
(4-4)
The power conversion efficiency of the solar cell is the ratio of the electrical power output
(
) and the solar power input (
):
.
(4-5)
Considering the maximum output power and Eq. (4-4), the maximum efficiency of a solar cell
can be presented as:
.
(4-6)
Table 4-2 shows the photovoltaic properties of solar cells fabricated from the inkjet printed
(ZnO)1-x(TiO2)x composite films. The results indicate the effects of component and the
thickness of the semiconductor layer on the performance of solar cells.
 DSSC prepared from thinner (ZnO)1-x(TiO2)x films have higher short circuit current
density (JSC) but lower open current voltage (VOC).
Wang et.al [64] found the JSC initially increased with the film thickness (11~18 μm) of the
semiconductor layer but decreased with further increasing in their DSSCs. The phenomena
were attributed to: (i) the build-up of dye molecules in the film for photoelectron generation
(the initial increase of JSC), and (ii) the charge recombination between the ejected electrons
and the
ions in the electrolyte, especially for thick films (the decrease of JSC). The value of
VOC is determined by the difference between Fermi level for electrons in the semiconductor
42
layer and the redox potential of
in the electrolyte.[62, 65] High electron density in
the conduction band of (ZnO)1-x(TiO2)x can shift the Fermi level and obtain a high VOC value,
while the charge recombination and the restricted mass transport can reduce the VOC value.
We observed increasing values of VOC with the thickness in all composite cells, which implies
the increased charge generation rate in the DSSC with thicker (ZnO)1-x(TiO2)x film.
Table 4-2 Photovoltaic properties of DSSCs based on inkjet printed (ZnO)1-x(TiO2)x films.
JSC (Am-2)
VOC (mV)
Fill Factor
Efficiency (%)
20
1.60
36.7
0.516
0.005
30
1.36
202.0
0.780
0.034
20
0.42
15.5
0.902
0.001
30
0.19
34.0
0.721
0.001
20
1.14
32.2
0.974
0.005
30
0.65
399.8
0.858
0.033
20
10.14
379.0
0.823
0.460
30
9.37
382.7
0.931
0.508
20
11.43
353.4
0.848
0.521
30
11.71
399.6
0.602
0.375
(ZnO)1-x(TiO2)x Printed layers
x=0
x=0.2
x=0.5
x=0.8
x=1
 The component of the semiconductor layer plays a dramatic role on the power conversion
efficiency (η) of the DSSCs. The ZnO dominated cells (
efficiency than TiO2 dominated cells (
) have much lower
).
The dramatic influences of the component of the semiconductor layer on the performance of
DSSCs could be attributed to: (i) the instability of ZnO in dye solution. It is reported that the
acidic dye could dissolve Zn atoms and form Zn2+/dye complex. This results in insufficient
dye adsorption and thence photoelectron generation, which leads to the poor performance of
DSSCs.[58] (ii) Even though the TiO2 and ZnO have similar band gap energy, their
conduction band structure are different: Ti4+ has empty 3d orbitals (1s2 2s22p6 3s23p6, loss 4
electrons from 1s2 2s22p6 3s23p6 4s2 3d2) while Zn2+ has empty 4s orbitals (1s2 2s22p6 3s23p6
3d10, loss 2 electron from 1s2 2s22p6 3s23p6 4s2 3d10). Therefore, the density states and the
electronic coupling strength with dye adsorbate are different between TiO2 and ZnO. This
results in the differences in electron injection rate from dye to their conduction band. For
instance, the ejection time scale of photoelectron into ZnO conduction band is 1.5~150 ps
(10-12 second), while it takes <100 fs (10-15 second) for TiO2.[66] Faster electrons injection
into the conduction band of TiO2 can reduce the recombination of photoelectrons with
43
ions
in the electrolyte and thus the electron injection efficiency is higher and the cell performance
is better. (iii) The advantage of using ZnO in DSSC is the higher electron mobility than TiO 2
(2 orders of magnitude higher), which may improve the performance of the cell by
minimizing the charge recombination during the transport process of the photoelectrons.[67]
The study employed inkjet printing technique to prepare (ZnO)1-x(TiO2)x films used for solar
cell fabrications. It is low cost and easy for mass production, which can be applied in large
scale industrial DSSC productions.
4.3 Ag/TiO2 composite films
Beside the applications in photovoltaic devices, TiO2 has also been widely used in
environment purification and water splitting because of its photocatalytic activity (PCA).[68,
69] As a photocatalysis, TiO2 can accelerate photoreactions in these applications, either by
catalyzing photolysis on a light adsorbed substance (e.g., DSSCs) or by creating photoinduced electron-hole pairs for secondary reactions (e.g., water splitting). A high photocatalytic efficiency of TiO2 is important for a good device performance. The PCA of TiO2 can
be improved by means of: [70]
 Shift the absorption wavelength range of sunlight to the red by engineering the band gap
edge of TiO2. This could enhance the activities of photons in visible portion.
 Increase the efficiency of electron-hole separation.
 Increase the interfacial electron transfer rate.
The band gap of TiO2 is ~3.0 eV for rutile and ~3.3 eV for anatase. It implies that only
ultraviolet (UV) light with energy higher than the band gap energy can be absorbed to initiate
the photocatalytic mechanism. The energy fraction of UV light in solar radiation is only 5%,
illustrating the development space to improve PCA by extending the photoresponse to visible
light region (~50% energy of solar spectrum). It can be achieved by:
 Surface structure modification;
 Band gap narrowing via element doping, either by hybrids of sp states, or by introducing
impurity energy level or oxygen vacancies.[71, 72]
Noble metal elements can trap electrons, separate electrons and holes to avoid recombination,
and thus allow an efficient interfacial electron transfer. [73, 74] In this section, Ag/ TiO2 films
44
prepared by inkjet printing are introduced. The effects of Ag on morphology of the films are
presented, and the photoluminescence performances of the Ag/ TiO2 films are discussed.
4.3.1 Ag/TiO2 film preparations
Ag/TiO2 thin films are prepared by inkjet printing. The inks for printing are:

Ink A: 20 wt % silver ink (SunJET). Commercially purchased.

Ink B: TiO2 suspension ink. The ink was prepared by dispersing 0.01 molar (10 mmol)
TiO2 particles in 2-isopropoxyethanol (IPE, SIGMA-ALDRICH) to form 20 ml
suspension via ultrasonic agitation.

Ink C: 2 at.% Ag/TiO2 ink. The ink was prepared by adding 0.2 mmol silver from ink A
and 9.8 mmol TiO2 particles into IPE to form 20 ml suspension via ultrasonic agitation.

Ink D: 10 at.% Ag/TiO2 ink. The ink was prepared by adding 1 mmol silver from ink A
and 9 mmol TiO2 particles into IPE to form 20 ml suspension via ultrasonic agitation.
The films studied in the thesis are printed from:
 TiO2 films: directly printed from ink B.
 2 at.% Ag/TiO2 thin films: directly printed from ink C.
 10 at.% Ag/TiO2 thin films: directly printed from ink D.
 Ag coated TiO2 thin films: firstly printed TiO2 layer from ink B; dried the TiO2 layer, and
then printed a layer of silver ink (ink A) on top of the TiO2 layer. The liquid silver ink
infiltrated into the porous of TiO2 films and formed a composite film.
To improve the link among TiO2 particles, the printed films were thermally annealed at
600 °C for 1 hour. It is important to notice that the annealing conditions affect the distribution
and the concentration of Ag in the films.[74]
4.3.2 Effect of silver in TiO2 films
Among noble elements, Ag is cost-effective. Ag nanoparticles can absorb visible light due to
the localized surface plasmon resonance[75] and can be active catalysts in various reaction
system.[76] The surface morphology of the Ag/TiO2 composite films compared with pure
TiO2 thin films is shown in Fig. 4-9. The atomic Ag concentration was determined by energydispersive X-ray spectrum (EDXS). The detected Ag concentration in the films is lower than
the designed concentration in the ink. It could be attributed to the evaporation of metallic Ag
during the annealing process since the Ag particles are nanometer in size. The high surface
energy of nanoparticles in the system contributes to the observed loss of Ag element in the
45
films. As we can see from the SEM images, the inkjet printed films are porous. Silver
nanoparticles and TiO2 particles in the film can be sintered together during the annealing
process, and the porous structure remains. With the increase of Ag concentration, the
correlations between TiO2 particles are improved (Fig. 4-9 b&c). By depositing a layer of Ag
ink on top of TiO2 film, the pores among TiO2 particles may be filled by Ag nanoparticles
(Fig. 4-9 d). For many applications of TiO2, the porous structure with large specific surface
area is desired because it can increase the efficiency of devices. Thus it is important to control
the concentration and the distribution of Ag for a better device performance.
(b) 1.70 at.% Ag
(c) 5.98 at.% Ag
(d) 25.78 at.% Ag
Fig. 4-9 High resolution SEM images of (a) TiO2 film, (b) 2 at.% Ag/TiO2 film, (c) 10 at.%
Ag/TiO2 film, and (d) a layer of Ag coated on top of TiO2 film. The EDX spectra of
the films (b), (c) and (d) are shown in the right.
To inspect the photoresponse of the inkjet printed films, photoluminescence (PL) of the
Ag/TiO2 composite films were investigated. Photoluminescence describes the ability of a
substance to absorb the incident photons and then radiate photons. In quantum mechanical, it
is because some electrons are excited into higher energy state by the incident photons,
followed by returning to a lower energy state accompanied with photon emission caused by
the energy changes in the system. PL spectrum is closely related to the micro-structure,
electronic state, defect state and energy level structure of the materials.
46
Fig. 4-10 PL emission spectra of (a) TiO2 film excited by different wavelengths; (b) Ag/TiO2
composite films compared to pure TiO2 film excited by 330 nm wavelength light.
For Ag/TiO2 nanostructured films, the PL spectrum is related to the transfer behavior of the
photoinduced electrons. Figure 4-10 shows the PL emission spectra of the printed TiO2 and
Ag/TiO2 films. There are three PL peaks (p1~p3) which show a linear red-shift with the
increasing excitation wavelength (Fig. 4-10a). They are suggested to be related to the particle
size and size distribution.[77, 78] There are five identifiable peaks (λ1~λ5) at ~378 nm
(energy ε1=3.28 eV), ~420 nm (ε2=2.95 eV), ~458 nm (ε3=2.71 eV), ~483 nm (ε4=2.57 eV)
and ~527 nm (ε5=2.35 eV), respectively, which are independent on the excitation wavelength.
The first two are corresponding to the direct transition of electrons from the conduction band
to the valence band of anatase (ε1) and rutile (ε2) structured TiO2, which is related to their
band gap energy. The other three peaks are attributed to the defects based emissions and
band-band electron transition. The PL intensity of the emission spectra shows decreases with
Ag doping (Fig. 4-10b), which indicates that Ag can trap photo-induced electrons in TiO2
lattice.[79] The PL results promise a good photoresponse of Ag/TiO2 films, which could
enhance the photocatalysis activities of TiO2 for many device applications.
4.4 Summary
Different oxides films have been prepared from inkjet printing suspension inks, such as SiO 2,
(ZnO)1-x(TiO2)x composite films for DSSC applications, and Ag/TiO2 films for photoresponse
investigations. The oxide films prepared by this method have micro-porous structure with
large specific surface area, which is attractive for many applications. The major problem is
the poor correlation among oxide particles in the films, which affects the transport properties
of the films. By adding Ag nanoparticles, the links among particles can be improved via the
metal-bridges inside the films.
47
Chapter 5: Ferrofluid

A ferrofluid (from Latin ferrum, means iron, and fluid) is a colloidal liquid made from
nanoscale magnetic particles (e.g., magnetite, maghemite, or some other magnetic
compound containing iron) suspended in a carrier fluid. They can be strongly
magnetized in the presence of a magnetic field and demagnetized to zero in the absence
of the external magnetic field, because of the ‘superparamagnetism’ caused by the
nanosize effect. As liquids with superparamagnetic properties, ferrofluids are attractive
for many applications, such as damping undesired vibrations and cooling in
loudspeakers, identification purposes, and drug targeting in medicine. They can be
controlled by an external magnetic field.[32] In this chapter, the background, basic
physics, properties and applications of ferrofluids are introduced. Magnetite
nanoparticles have been synthesized from co-precipitation method utilizing “rapidmixing” of reactants. Both water-based and kerosene-based ferrofluids are presented.
5.1 Introduction of ferrofluid
Ferrofluid was first invented by Stephen Pappell at National Aeronautics and Space
Administrator (NASA) in 1965. It was used for a rocket propellant fluid in space under zero
gravity conditions.[80] In the patent, small magnetite particles (<0.25 μm) prepared by long
time grinding (i.e., 15~19 days of milling) in oleic acid agent were dispersed in organic
particle carrier with 0.1~10 wt % iron oxide particles. Its magnetic properties made it possible
to control the performance of the liquid even under zero gravity by manipulating the magnetic
force under an external magnetic field. Later, it was used by NASA as rotating shaft seals in
satellites.[81] Nowadays, ferrofluids are utilized in many machines, such as in centrifuges and
computer hard devices as exclusion seals.[82, 83] Their applicability attracts scientists in
different fields: chemists want to develop the methods to produce them; physicists try to
explain the theory behind them; engineers want to apply them in technological productions;
and biologists try to use them in biological area……The properties of ferrofluids should be
inspected corresponding to their different applications: Are they stable over time? Is their
magnetism strong enough for localization? Are they biocompatible? How much energy can
they absorb to cure local cancer cells? Let’s start from the basic: what is a ferrofluid?
48
5.1.1 What is a ferrofluid?
A ferrofluid is a colloidal liquid with magnetic particles suspended in a carrier fluid. Because
of the magnetic particles, it can be magnetized by an external magnetic field, which endows
the ferrofluid with magnetic field functionalization. And since it is a liquid, the low viscosity
makes it possible to be continuously deformed (flow), while the free surface makes it possible
to be in any geometry. It combines the magnetic properties with the liquid performance and
thus is attractive for many applications. Different from melting the solid magnetic materials
into liquid, which requires a high temperature for melting and a degenerated magnetism due
to the thermal effect, ferrofluid is a magnetic-particle-laden liquid. It is available at a low
temperature such as room temperature. Therefore, it maintains the strong magnetism.
A ferrofluid contains three components: the magnetic particles, surfactant molecules coated
on the surface of the magnetic particles, and the carrier liquid. In principle, the magnetic
particles can be various magnetic materials, such as nickel, cobalt, iron and iron oxides,
whose liquids are called “magnetic fluids”. Magnetite (Fe3O4) was found to be the most
satisfactory [80] and widely studied, and subsequently came the name “ferrofluid”. The
structure and the properties of Fe3O4 will be introduced briefly in the coming section. The
particles should be in small size, so that the Bownian motion (with large motion amplitude for
small particles) could overcome their gravimetric force (driving force for sediment) to achieve
the suspension of the magnetic particles in the liquid.
To lower down the large surface energy in the system introduced by the small particle size,
the particles agglomerate. The agglomeration becomes severe when the particles are magnetic
because of the magnetic interaction among the particles. To avoid particle agglomeration in
ferrofluid, surfactant coated on the surface of the magnetic particles is used. The surfactant
molecules create Van der Waals force or electrostatic force to repulse the particles from each
other and prevent clumping. Take the oleic acid (CH3(CH2)7CH=CH(CH2)7COOH) and
tetramethylammonium hydroxide ((CH3)4NOH, TMAH) used as surfactants for magnetite
nanoparticles as examples, the surface modification mechanisms in ferrofluid fabrications are
described. Their schematic diagrams are shown in Fig. 5-1.
 Oleic acid: magnetite nanoparticles have hydrophilic surface, where the polar head of
oleic acid molecules (-OH radical) will attach. The non-polar tails (the carbonic chain) of
the surfactant form a shell for the particles and prevent the particles from clumping.
When the fluid carrier is non-polar solvent, like kerosene, the highly soluble non-polar
49
shell helps to disperse the particles in the liquid; while when the fluid carrier is polar
solvent, like water, the non-polar tails repel the particles from the solvent, which leads to
the phase separation in the liquid. By coating another layer of surfactant to form polar
head shell, it is possible to disperse the particles in polar solvent.
 TMAH: the hydroxide anions are attached to the hydrophilic Fe3O4 surface. With the
electrostatic attraction from anions, the tetramethylammonium cations form a diffuse
shell on each Fe3O4 particles. The steric electrostatic repulsion of cations among particles
prevents the particles from agglomeration and stabilizes the aqueous colloid.
(a)
(b)
Fig. 5-1 Surface modification of magnetite nanoparticles using (a) oleic acid and (b) TMAH
as surfactants for ferrofluids fabrications. Note in fact the coating is steric.
The third component in a ferrofluid is the fluid carrier, such as the kerosene for oleic acid
coated particles and the water for TMAH modified particles. The fluid carrier should match
with the surfactants used for coating: for a non-polar solvent, non-polar surface modification
should be used for fabrications of ferrofluids, vice versa.
5.1.2 The properties of ferrofluids
Properties of ferrofluids such as stability (or operating life), thermodynamic properties,
transport properties, magnetic properties, mechanical properties and biocompatibility, are
important factors to be inspected for specific performance in device applications. The
selection of ferrofluid in device applications should also consider the environment factors:
How do the ferrofluids perform in the working conditions of the device?
50
The stability of ferrofluid means how long it can maintain its properties for a stable
performance in a device. Stable ferrofluids require that:  the magnetic particles should be
prevented from clumping and settling over time;  the evaporation rate of the liquid should
be very low, and  the magnetic properties of the fluid should not degenerate with time. By
coating surfactants over particle surface, the magnetic particles are protected from
agglomeration. It provides a steric repulsion force among particles to avoid sediments. By
selecting the carrier liquid, the evaporation rate can be tuned. The carrier liquid dominates the
thermodynamic properties, such as density, heat capacity, thermal expansion, and the
transport properties like rheological performance (kinematic viscosity and vortex viscosity)
and thermal conductivity. Of course they are affected by the concentration of the magnetic
nanoparticle in ferrofluids. For instance, high particle-laden fluid has higher viscosity than
low particle-laden fluid, and thermal conductivity shows linearly dependence on the
concentration of solid loading. Magnetic properties like magnetic susceptibility, saturation
magnetization, magneto-optical properties, and relaxation time constant of magnetic moment,
are dominated by magnetic particles. The material, the shape of the particles, the particle size
and size distribution, contribute to the final magnetic performance of a ferrofluid. Mechanical
properties such as radial stiffness in ferrofluid seals used as radial bearings,[84] are also
important. Besides, for many applications the environments effects on the performance of
ferrofluids should be considered, such as temperature (e.g. in winter the outdoor temperature
can be -20 °C in Sweden and at space it can be -55°C; for some applications, ferrofluids are
expected to work at 150~200 °C), chemical compatibility with the contact materials in the
system, and biocompatibility for biomedical applications, etc.
5.1.3 Some applications of ferrofluids
Because fluid performance can be controlled by an external magnetic field, ferrofluids are
attractive for many applications ranging from space applications to biomedicine fields. In
space, ferrofluid was developed as rocket engine propellant because its performance can be
controlled using magnetic field even at zero gravity. [80] It can be vented from the tank
effectively. The orientation of propellant during pump inlet can be well controlled to
eliminate cavitation and surging. For technological applications, ferrofluids are applied for
dynamic sealing, heat dissipation, damping and micro-pumping.[82, 83, 85] For instance,
many types of equipment have a shaft to carry energy (rotation) from the ambient air to an
absolutely clean space, where seal hermetically the hole through which the axle passes is
needed. Ferrofluids can seal and would not move with the shaft but leave the axle free to
51
rotate. For biomedical applications, ferrofluid has been developed for:  magnetic drug
targeting by which the drug can be carried and released to the target tissue under the control
of an external magnetic field;[86-88]  hyperthermia, for instance to kill cancer tumors by
absorbing electromagnetic energy and heating the cancer cells;[89-91] and  contrast
enhanced agent of magnetic resonance imaging (MRI) for distinguishable images among
tissues, etc.[92-94]
5.2 Magnetite nanoparticles
Magnetite is a well-known naturally occurring mineral, the first magnetic material people
known. At 4th century BC, Chinese have already noticed that this mineral can attract iron.
They developed magnetic needle compass at the beginning of 11th century. At 12th century,
European used the compass in navigation, and discovered the new continent. Nowadays, more
and more applications of magnetite in different fields have been developed, like ferrofluids
presented in the thesis. In this section, structure, properties and preparation methods of
magnetite nanoparticles will be presented.
5.2.1 Structure of magnetite
With the development of science, the structure of the magnetite becomes clear today,
thousands years after the discovery of the magnetic properties. Magnetite, one oxide of iron,
can be written as Fe3O4 in chemical formula. It belongs to a family of magnetic ceramic called
ferrites, which can be written as MO•Fe2O3 (M: metal, such as Ni, Co, Cu, Mn, etc.). Ferrites
have either the normal spinel structure or inverse spinel structure, in which oxygen atoms
construct a face centered cubic (fcc) lattice and form tetrahedral interstices and octahedral
interstices for iron ions. Figure 5-2 shows the examples of octahedral site and tetrahedral site
in cubic close packed lattices. The volume of octahedral site is larger than tetrahedral site. In
each ferrites crystal lattice, there are 4 oxygen atoms, 1 M2+, 2 Fe3+, 4 octahedral sites and 8
tetrahedral sites.
 For normal spinel structure, the two Fe3+ ions are at two of the four octahedral sites
because of charge factors, and the M2+ ion is at one of the eight tetrahedral sites. The
ferrites can be written as M2+(tetra.)Fe3+2(octa.)O4.
 For inverse spinel structure, like magnetite, the Fe2+ ion has high spin state and has a
larger ionic radius. It substitutes one of Fe3+ ion in octahedral sites and forms the inverse
52
spinel structure: Fe2+ and half of Fe3+ ions are at octahedral sites and the other half of
Fe3+ ions are at tetrahedral sites, written as Fe3+(tetra.)Fe2+Fe3+(octa.)O4.[95-97]
Fig. 5-2 Example of the octahedral interstice (the left) and the tetrahedral interstice (the right)
in a cubic close packed crystal lattice.
5.2.2 Properties of magnetite
The electron configuration in iron atom and ions can be written as:
Fe atom:
1s22s22p63s23p64s23d6,
Fe3+ ion:
1s22s22p63s23p63d5,
Fe2+ ion:
1s22s22p63s23p63d6.
In Fe3+ ion, there are 5 unpaired electrons in 3d orbital. In Fe2+ ion, there is an extra electron
in 3d orbital which can be coupled with any of the other 5 unpaired electrons and form a pair.
The itinerant electron in Fe2+ ions can hop among Fe2+ and Fe3+ ions in the octahedral site,
which makes magnetite conductive for electrons.
The magnetic and electric properties of Fe3O4 show dependence on temperature: at very low
temperature, the magnetite has a special transition, Verwey transition, which is an abrupt
change of crystal structure at a critical temperature (~120K). It is accompanied by changes of
variety of related parameters controlling the magnetic, electric, thermodynamic and
mechanical properties.[95, 98, 99] At a temperature higher than Curie temperature (~840K),
magnetite loss the magnetic properties and become paramagnetic, while the electric properties
become metal-conductive.[100] Fe2+ is metastable since the itinerant electron can be easily
lost and form stable Fe3+. Thus, magnetite can be easily oxidized into γ-Fe2O3 which has
lower magnetization. The degeneration of magnetic properties is explained as the introduction
of vacancies in the inverse spinel structure.[101]
53
5.2.3 Preparations of magnetite nanoparticles
The properties of magnetite nanoparticles depend on the preparation methods. Many methods
like chemical reaction (co-precipitation), hydrothermal reaction, thermal decomposition and
micro-emulsion, etc. have been developed to get nano-sized magnetite.[102, 103] However, it
is still a challenge to prepare pure magnetite with controllable particle size and narrow size
distribution for a good magnetic performance. In theory, the saturation magnetization (M S) of
Fe3O4 can be 96 Am2•kg-1 at 0 K and 92 Am2•kg-1 at room temperature (273 K),[101] while
the experimental MS values of Fe3O4 nanoparticles are typically 30~70 Am2•kg-1 from coprecipitation,[104-108] 60~90 Am2•kg-1 from thermal decomposition,[109-111] and 50~80
Am2•kg-1 from hydrothermal synthesis, respectively.[112-114] In the thesis, magnetite
nanoparticles with controllable particle size and narrow size distribution are prepared from
co-precipitation via rapid mixing of reactants. The particles show high MS value (~87
Am2•kg-1) and extremely low coercivity (HC, ~12 Am-1), which are desirable for many
applications.
Co-precipitation is probably the simplest, most common and efficient chemical method to
produce nanosized magnetite using iron salts as precursors. The chemical reaction formula
can be written as:
Fe2+ + 2 Fe3+ + 8 OH- Fe3O4 + 4H2O
(5-1)
From the thermodynamics point of view, the reaction requires an alkaline environment.
Commonly, ammonia or sodium hydroxide is used. The characteristics of the co-precipitated
nanoparticles, including the shape, the composition, the size and size distribution, and
consequently properties, depend a lot on the precursor salts (chlorides, sulfates or nitrates),
the ion ratio of Fe2+ and Fe3+, the pH value, the temperature, the mixing rate, and so on. The
saturation magnetization of Fe3O4 nanoparticles obtained from this method is typically in
range of 30-70 Am2•kg-1, which is lower than the theoretical value. It is probably related to
the impurities, such as maghemite (γ-Fe2O3) and Fe(OH)x, formed during the co-precipitation
process. During mixing, the pH value of iron ion source increased from ~2 to ~11. In this
process, hydrolysis of Fe3+ requires a low pH value (pH of 2)[115] to convert into Fe(OH)3,
while Fe2+ requires a high pH value (>7) [116] for Fe(OH)2 precipitation. Consequently, a
slow mixing of reactants creates more Fe(OH)3 than a rapid mixing, and precipitates γ-Fe2O3
instead of pure Fe3O4. Rapid mixing increases the pH value of iron source rapidly, which
limits the priority precipitation of Fe3+ and co-precipitate Fe2+ and Fe3+ ions to produce high
purity Fe3O4 nanoparticles.
54
In our group, we developed a high speed method to mix the reactants in co-precipitation.
Figure 5-3 shows the schematic diagram of the rapid mixed (RM) co-precipitation set-up and
the classic mixed (CM) co-precipitation.[31] In RM co-precipitation, the reactants are mixed
in the time scale of milliseconds,[106] by letting equal volume flows of iron ion source and
base solution from two jets (diameter of 0.19 mm) impinge to each other at 45 ° in a laminar
flow mode. The reactants in the jets are discharged by means of a high pressure pneumatic
cylinder to achieve a flow velocity of ca. 8 m/s for impinging. The mixture (resultants) is then
directed into a tube to collect nanoparticles. For CM co-precipitation, the iron ion source is
added into the base solution under stirring, which takes a few seconds for mixing. By refining
the lattice parameter according to the positions of X-ray diffraction peaks, the lattice
parameter of the RM particles is 0.8393 nm which is very close to the standard value of
magnetite (0.8396 nm). For CM particles, however, it is 0.8374 nm which is somewhere
between the values of standard magnetite and maghemite (0.8352 nm), indicating the
existence of γ-Fe2O3 in the products.[31]
Fig. 5-3 Set-ups of (a) RM and (b) CM co-precipitations. (c) shows the XRD intensity
patterns of the particles obtained from RM and CM co-precipitation.[31]
Since the nucleation of Fe3O4 in co-precipitation process can be well controlled by the rapid
mixing of reactants, the size of the particles can be tuned by controlling the growth of Fe3O4
after mixing, for instance, by controlling the growth temperature. Table 5-1 shows the size
and magnetic properties of magnetite nanoparticles that grew for 2 hours in ice water bath (1
± 1 °C), RT (22 ± 1 °C), warm water (53 ± 1 °C), hot water (74 ± 1 °C) and boiling water (95
± 1 °C) bath, respectively.[29] With the increase of the temperature, the size of the particles
increases and consequently coercivity of the particles increases. The temperature dependent
size distribution follows the same trend, which can be explained by Ostwald ripening. The
lower magnetization determined for larger particles is attributed to the oxidation of Fe3O4
under the high growth temperature.
55
Table 5-1. Size and magnetic properties of nanoparticles grown at different temperatures.
Temperature (°C)
Size a (nm)
Magnetization b (Am2•kg-1)
Coercivity c (Am-1)
1±1
3.82 ± 0.97
87.85
12
22 ± 1
4.46 ± 1.04
86.81
22
53 ± 1
5.55 ± 1.28
87.60
71
74 ± 1
7.81 ± 1.65
79.42
348
95 ± 1
10.87 ± 2.22
74.37
832
Notice: a The size and size distribution were determined from manual measurements of
over 1000 particles from TEM images; b the magnetizations were determined at a magnetic
field of 500 kAm-1; c the coercivity were determined from the average value of the fields at
zero magnetization.
5.3 How to prepare ferrofluids?
In this section, the preparations of ferrofluids will be introduced in details. The aqueous
ferrofluids from RM particles and kerosene based ferrofluids prepared from CM particles will
be presented as examples.
The preparation of the reactants:
 50 ml 0.2 molar/liter (M) iron ion source with [Fe2+]:[Fe3+] = 1:2
Dissolve 0.6627 g iron (II) chloride tetrahydrate (≥98%, Fluka) and 1.8020 g iron (III)
chloride hexahydrate (≥98%, Fluka) in de-ionized water to form 50 ml transparent
solution.
 50 ml ~4 M NH3•H2O base solution
Dilute ~31 ml ammonia hydroxide (25%~35%, Fluka) in de-ionized water to form 50 ml
solution. (Using the concentration of 25% and density of 0.91 g•cm-3 for calculation.)
Aqueous ferrofluids from RM particles:
 Prepare 3.895×10-3 g (3.895 mg) RM magnetite nanoparticles
The particles are prepared from rapid mixing 0.25 ml 0.2 M iron ion source with 0.25 ml
4 M ammonia solution. The mixture is kept in room temperature for 2 hours for the
growth of the nanoparticles. Then the resultants are washed by de-ionized water for three
times to remove the residues of reactants, using a magnet to separate the particles from
the liquid as much as possible.
56
 TMAH surface modification
Add a drop of tetramethylammonium hydroxide (TMAH, 25 wt % aqueous solution) into
the slurry of RM particles. Use a magnet to move the particles in TMAH liquid to mix
them fully. The obtained suspension is ferrofluid which is stable for months. The Fe3O4
concentration of the ferrofluid is ~0.42 M (calculated from 0.1 ml ferrofluid), or ~7.2 wt %
(calculated from adding ~0.05 ml TMAH).
 Concentration adjusting of aqueous ferrofluid
The obtained ferrofluid is aqueous based and can be diluted by adding water. When
adding non-polar solvents, phase separation of the above ferrofluid with the non-polar
solvents will be observed.
Kerosene based ferrofluids from CM particles: (see the origin recipe: Appendix V)
 Prepare 0.15436 g CM magnetite nanoparticles
The CM particles are prepared by adding 10 ml 0.2 M iron ion source into 10 ml 4 M
ammonia solution. The reactants are mixed by magnetic stirring at RT (Fig. 5-3 b). After
2 hours stirring, the mixture is washed thrice by de-ionized water.
 Oleic acid surface modification
Disperse the CM particles into 20 ml de-ionized water under stirring (~10 min) at RT in a
100 ml beaker. Add ammonia solution to adjust the pH value to 11 (operated in fume
chamber). Add 1 ml oleic acid and continuous stirring for 1 hour. Then put the beaker
into boiling water (95~97 °C) bath for 3 minutes followed by cold water bath to cool it
down to RT, keeping stirring. Add diluted HCl (0.01 M) dropwise to adjust the pH value
to 5. At this moment, the coated particles are separated from other impurities and
coagulate into clusters. Wash the clusters thrice by warm water (70~80 °C, 10 ml each) to
remove impurities, and then four times by acetone (10 ml each) to remove water and
separate individual particles from clusters under vigorous stirring. Remove the acetone
from the particles as much as possible by magnetic sedimentation.
 Preparation of kerosene based ferrofluid
Add 6 ml kerosene into the wet slurry and disperse the coated particles by hand stirring.
Heat the dispersion at 60~65 °C to evaporate acetone. Kerosene based ferrofluid is
prepared.
57
5.4 Summary
This chapter introduces the basics of ferrofluids, with emphasis on the magnetite and the
preparation of ferrofluids. Note the difference between RM and CM co-precipitation is the
mixing rate of reactants, which controls the pH value evolution and consequently leads to
different component, structure, morphology and properties of the precipitated magnetic
particles. There is no difference on the process of surface modification: RM particles can be
coated with oleic acid for kerosene based ferrofluids and CM particles can be coated with
TMAH for aqueous ferrofluids. The recipe is an example for ferrofluid preparation.
58
Chapter 6: Oxide films printed from acetate solutions

…… A method for the ‘in-situ’ preparations of functional oxide films and devices
The oxide particles in suspensions and colloids are used in ‘ex situ’ preparations and are
not directly synthesized on the substrates. Thus the morphology and properties of the
printed oxide films depend on the source of the oxide particles. In this chapter, ‘in situ’
preparations of oxide films will be introduced, from which the oxides are formed
directly on the substrate after printing. In this approach instead of using oxide particle
suspensions, metal-acetates (MeOAc) solutions are used as inks for printing. The asprinted films are acetates precursors which decompose into oxides during calcination
process. Therefore, the oxide films are prepared in situ at the desired spot on the
substrate. This ‘in-situ’ method makes it possible to control the morphology and the
structure of oxide films in inkjet oxide deposition process. Inkjet printing of doped and
undoped ZnO, MgO films are presented as examples.
6.1 Why are acetates solutions?
Metal-acetates are used as precursors in preparing solution inks for DOD inkjet deposition of
oxides films. The reasons of using acetates are listed as following:

Acetates are available for many metals. They can be prepared from the reaction of metal
hydroxides with acetic acid. Examples like Mg(OAc)2, Fe(OAc)2, Zn(OAc)2, In2(OAc)3
and Sn(OAc)4 are used in the thesis for preparing MgO, Fe-doped MgO, ZnO, Fe-doped
ZnO and Sn-doped In2O3 films, respectively.

The metal acetate salts are ionic. They can be dissolved in water and many organic
solvents to prepare stable single phase inks for inkjet printing.

Acetates have a low decomposition temperature, typically lower than 500 °C.[117]
Oxides can be obtained in oxidation atmospheres during calcination.

The introduced impurity elements like carbon and hydrogen in acetates can be easily
burned off. Therefore, oxides with high purity can be prepared.
The inks prepared from metal-acetates are single phase solutions. Unlike oxide particle
suspensions, there is no solid internal phase in acetate solutions. Therefore, they can be stable
59
for years without sediments. For normal aqueous ionic solution, for instance dissolving NaCl
in water, the solutes nucleate and precipitate during the evaporation of water because of the
solubility. In inkjet inks, organic solvents are used not only for suitable physicochemical
properties (such as viscosity and surface tension) for DOD inkjet printing, but also for
forming an intermediate gel phase to achieve uniform deposition of the precursors. During
the evaporation of the liquid, the acetate solutes become saturated; instead of precipitating
acetate crystals, the solution forms a gel phase due to the hydrolysis and polycondensation
reactions between the ionic metal acetates and the organic solvents.[118] This process is well
known as sol-gel. Generally, alkoxides (ROH, R stands for alkyl group) are used as solvents
in the polymer reaction process to evolve the solution towards the gel.[119]
When using oxide particles suspensions as inks, it is difficult to dope secondary metal element
into the matrix metal oxide lattice, because the diffusion of atoms requires high energy and
the diffusion of atoms among oxide particles is limited in solid state. Therefore, the obtained
films from oxides suspension are typical the mixture of the oxides.
When using acetates solutions as inks, it is possible to dope secondary element into the matrix
oxide lattice. The obtained films can be dense and uniform. During the ink preparation
process, the secondary acetates can be added into the matrix metal-acetates and dissolved into
the solvent to form a signal phase solution. The as-printed films are acetate precursors
containing two or even more metal elements. During calcination, the precursors decompose
into oxides, in which the secondary element atoms can be incorporated into the matrix oxide
lattice without forming impurity oxide phases. For instance, the ink prepared from iron
acetates and zinc acetates with cation concentration of iron less than 10 at.% (i.e. [Fe2+]:[Fe2+
+ Zn2+]≤0.1) can be used to print Fe-doped ZnO films. In the final oxide products, there is no
iron oxide phases, indicating that the iron atoms are incorporated into the matrix ZnO
lattice.[120] Besides, because of the special ‘in-situ’ process, the doped element concentration
in the prepared oxide films is the same with the value in the ink. It is different from using
vapor deposition techniques, that the concentration of the doped elements in the final products
depends not only on the composition of the target, but also the deposition conditions
including vacuum, power and deposition rate, etc. These advantages make inkjet printing
acetate solutions applicable for many oxide materials, like diluted magnetic oxides (DMO)
discussed in the thesis.
60
6.2 Inkjet printing of DMO
The diluted magnetic oxides (DMOs) discussed here are Fe-doped ZnO and Fe-doped MgO
thin films. Since the semi-conductivity of ZnO (with a direct band gap energy (Eg) of ~3.3 eV)
and insulation of MgO (Eg of ~7.8 eV), the transition metal (TM, such as Mn, Fe, Co and Ni)
doped ZnO and MgO is a type of diluted magnetic semiconductor (DMS) and diluted
magnetic insulator (DMI), respectively. These compounds have combined properties of
semiconductor/insulator with ferromagnet. It is possible to fabricate quantum structures of
semiconductor/insulator with confined electrons or photons to achieve remarkable properties
and functionalities via magnetic interactions.[121] Figure 6-1 shows the flow chart of DOD
inkjet deposition of oxide thin films, where acetate solutions are used as inks (see Chapter 3).
Fig. 6-1 Flow chart of inkjet printing acetates solution inks to prepare oxide thin films.
6.2.1 DMS: Fe-doped ZnO thin films
6.2.1.1 What is DMS?
A magnetic semiconductor contains a periodic array of magnetic element, like magnetite (a
semiconductor, with band gap energy of 0.14 eV). Most of semiconductors contain no
magnetic ions (such as GaAs, ZnO and PbTe, etc.) and are non-magnetic. To make
nonmagnetic semiconductors ferromagnetic, magnetic elements (e.g., TM elements) can be
introduced into the nonmagnetic semiconductors. This is a diluted magnetic semiconductor,
an alloy of the nonmagnetic semiconductor and the magnetic element. Figure 6-2 shows
schematic pictures of the above three types of semiconductors.[9, 10]
61
(b)
(a)
(c)
Fig. 6-2 Schematic structures of three types of semiconductors: (a) magnetic semiconductors
with magnetic elements (atoms with spins) periodically arrayed; (b) diluted magnetic
semiconductors with small fraction of magnetic elements in the lattice; (c) nonmagnetic
semiconductors which contain no magnetic ions.
When using DMS in a device, it is important to inspect both the electric and the magnetic
properties. Typically, carrier density, electron mobility, band gap energy, Curie temperature
(TC) and magnetism should be considered.
6.2.1.2 Semiconductors for DMS
Since the discovery of ferromagnetism in Mn-doped IV-VI,[122] III-V,[123] and II-VI [124]
compounds, many semiconductors have been developed for fabrications of DMS. Their merits
and demerits are listed as following:[10]
 II-VI-based DMS can be easily prepared, where the valance of cations matches with that
of the common magnetic ions, such as Co2+ and Mn2+ doped in ZnO, CdTe and ZnSe
lattice. Their disadvantage is difficult to form p- or n- types, which limits carrier density
in DMS and is therefore less attractive for applications.
 III-V-based DMS: III-V semiconductors such as GaAs have already been widely used in
a lot of optical and electronic applications; by introducing magnetic properties, it is
possible to functionalize a variety of optical and electrical responses in these devices via
an external magnetic field. The major obstacle in using III-V semiconductor in DMS
fabrication is the low solubility of magnetic elements, which limits the effect of TMdoping on magnetism.
 The band gap energy: for most of the applications of spintronics, the DMS should be
functional at ambient temperatures. This requires DMS materials with high intrinsic
Curie temperature. The theoretical TC of semiconductors were predicted,[125] which
shows dependence on band gap of materials. Figure 6-3 shows both the theoretical and
experimental TC values of DMS fabricated from different semiconductors. [126]
62
(a)
(b)
Fig. 6-3 The effect of band gap on Curie temperature: (a) theoretical prediction and (b)
experiment results of different materials.[126]
ZnO itself is attractive as a low-cost and wide, direct band gap (~3.3 eV) semiconductor. It
has already been developed for many applications, such as transparent conductive thin films,
UV sensors, piezoelectric transducers and solar cells, etc.[127] According to the prediction
and the experiment results of Curie temperatures (Fig. 6-3), ZnO is a candidate semiconductor
for fabricating room temperature DMS. In recent years, robust room temperature
ferromagnetism (RTFM) of TM-doped ZnO has been extensively studied for both
fundamental science and industry applications.[14, 128-131] Among these studies, Fe-doped
ZnO was reported to have a mixed valence state (Fe2+ and Fe3+) in ZnO lattice. The trivalent
iron in Zn site can capture an electron and become divalent, creating a hole-doping (p-type)
DMS.[132] Besides, the different oxidation states of iron could give rise to double exchange
magnetic interaction which is suggested to be a stabilizer of the ferromagnetic state.[133, 134]
6.2.1.3 Fe-doped ZnO thin films from inkjet printing
Fe-doped ZnO thin films with different iron concentration were prepared from solutions of
acetates by DOD inkjet printing.[120] Figure 6-4 shows the typical morphology of ZnO and
10 at.% Fe-doped ZnO thin films. The films are uniform with smooth surface: the root mean
square surface roughness (Rq) determined from atomic force microscope (AFM) is 3.49 nm
and 1.80 nm for ZnO and 10 at. % Fe-doped ZnO thin films, respectively (Figs. 6-4 b&d).
Fe-doping decreases the grain size of the film: From the morphology of ZnO and Fe-doped
ZnO films, the grain size of the Fe-doped ZnO film is found to be smaller than that of the pure
ZnO film. This occurrence is attributed to the enhanced nucleation of ZnO on Fe-doping: the
doped Fe atoms act as seed crystals for the nucleation of ZnO. This heterogeneous nucleation
63
has larger nucleation rate than the homogeneous nucleation in ZnO without Fe seed crystals.
The competitive growth mechanism of the large number nuclei results in the small grain size
observed in the Fe-doped ZnO films.
(a)
(c)
(b)
(d)
Fig. 6-4 Morphology of inkjet printed ZnO and Fe-doped ZnO films: (a) SEM and (b) 3-D
AFM images of ZnO film; (c) SEM and (d) 3-D AFM images of 10 at.% Fe-doped ZnO film.
The printing process is found to be reliable and repeatable. Figure 6-5 shows the focuses ion
beam (FIB) cross-section SEM images of ZnO (a~c) and 10 at.% Fe-doped ZnO (d~f) thin
films prepared from different passes printing: (a, d) single pass (N =1); (b, e) 3-passes (N =3)
and (c, f) 5-passes (N =5). The top morphology of the films is insensitive with the thickness of
the films: ZnO films have similar morphology with each other, as the example shown in Figs.
6-4 a&b, and 10 at.% Fe-doped ZnO films have similar morphology with each other, as the
example shown in Figs. 6-4 c&d. Both of the two systems show that the thickness of the films
increases linearly with the number of the print-passes. These results indicate that the printing
process is reliable and repeatable.
64
 Thickness of ZnO thin films: ~30 nm for the single pass printed film, ~90 nm for the 3
passes printed film, and ~150 nm for the 5 passes printed films, respectively. The
thickness of the ZnO films is ~30×N nm.
 Thickness of 10 at.% Fe-doped ZnO thin films: ~45 nm for the single pass printed film,
~125 nm for the 3 passes printed film, and ~225 nm for the 5 passes printed films,
respectively. The thickness of the films is ~45×N nm.
(a)
(b)
(c)
(e)
(d)
N =5
N =3
N =1
(f)
N =1
N =3
N =5
Fig. 6-5 FIB cross-section SEM images of: (a~c) ZnO and (d~f) 10 at.% Fe-doped ZnO thin
films, printed with different number of passes (N).
The thickness of each single pass printing (e.g., for the single pass printed films) depends on:
 the solute concentration in the ink (0.25 M for both);  the temperature compensation
factor of the DOD printhead which affects the volume of each ejected drops (0.5 for printing
ZnO and 0.6 for printing Fe-doped ZnO); and  the density of the solid films. The thickness
difference between single pass printed ZnO and 10 at.% Fe-doped ZnO is majorly caused by
the different temperature compensation factors used during the printing process.
6.2.1.4 RTFM of Fe-doped ZnO thin films
The prepared films show ferromagnetism at room temperature. The RTFM can be enhanced
by Fe-doping, as seen for Fe-doped vs. pure ZnO thin films. Figure 6-6 shows the magnetic
loops of ZnO and 10 at.% Fe-doped ZnO thin films with comparable thickness (~30 nm) to
address the effect of Fe-doping on RTFM. The data are collected using a superconducting
quantum interference device (SQUID, Quantum Design MPMS2) at room temperature.
65
 A small value of coercivity (~75 Oe) is obtained for both ZnO and 10 at.% Fe-doped ZnO
thin films, indicating that it exhibits robust RT ferromagnetism with finite HC.
 The saturation magnetization (MS) values are found to be 0.9 emu/g for the pure ZnO thin
film and 3.8 emu/g for the 10 at.% Fe-doped ZnO thin film. By Fe-doping, the Ms value
has increased by ~4 times.
The enhancement of magnetization is attributed to (i) the changes of structure with a high
defect concentration. For the pure ZnO film, the magnetism is attributed to “d0 magnetism”
which originates from the defects in the films. The concentration of these defects can be
increased by Fe-doping. For instance, the grain size of the Fe-doped film is smaller (see Fig.
6-4), which results in more defects at grain boundaries in the films. (ii) The doped iron atoms
introduce unpaired electrons in localized 3d orbitals that interact magnetically with each
(a)
5.0
ZnO
FeZnO
2.5
0.0
-2.5
-5.0
-10
-5
0
5
10
Magnetic field (kOe)
Magnetization (emu/g)
Magnetization (emu/g)
other, and contribute to the magnetic moment in the Fe-doped ZnO films (see Chapter 2).
(b)
2
1
ZnO
FeZnO
0
-1
-2
-500
HC: ~75 Oe
-250
0
250
Magnetic field (Oe)
500
Fig. 6-6 (a) Room temperature magnetic loops of ZnO and 10 at.% Fe-doped ZnO thin films;
(b) the magnified loops around origin. The thicknesses of the two films are ~30 nm. The
density used in calculation of mass magnetization of the films is 5.6 g•cm-1.
The effects of calcination temperature and the film thickness on magnetic properties of Fedoped ZnO thin films have also been studied. Figure 6-7 shows the dependence of saturation
magnetization on (a) calcination temperature (the thickness of the films is ~225 nm) and (b)
film thickness (the calcination temperature is 450 °C for all the films), detected from 10 at.%
Fe-doped ZnO thin films:
 For ~225 nm thick films: with the increase of the calcination temperature, the MS value
initially increases and then decreases. The peak value is obtained from the film calcined
at 500 °C. The observed trend is explained to arise from the effect of calcination
66
temperature on the crystal structure of the films: (i) higher calcination temperature helps
the diffusion of Fe-atoms to form direct magnetic exchange; (ii) desorption of oxygen
atoms at high temperature creates oxygen vacancies which can trap and donate electrons.
They can mediate the magnetic interaction among localized magnetic moments, known as
carrier-mediated exchange; (iii) The diffusion of atoms can also relax the strain in the
lattice and eliminate the defects inside the structure, which results in a decrease in
magnetization (defect induced magnetism). The overall effect of the three factors leads to
the final dependence of MS on calcination temperature, as shown in Fig. 6-7(a).
 For films calcined at 450 °C: with the increase of film thickness, the MS value increases
from 2.35 emu/g to 4.44 emu/g and then decreases. The peak MS value (4.44 emu/g) is
obtained for the ~45 nm thick film. The film thickness dependence of MS is attributed to:
(i) the defect concentration in the films.[135, 136] With the increase thickness of the
films, the initial increase of MS value (thickness from 18 to 45 nm) is attributed to the
increasing density of the defects, while the following decrease of MS value
(thickness >45 nm) is caused by bulk effect: the defects might be annealed out inside the
films through lattice relaxation during calcination. (ii) Strain in the films. In thicker films,
less lattice distortion was observed, which could be the reason for the weaker
magnetization.[137] (iii) It is reported that the areal magnetization of the films was
insensitive to the thickness, while the volume magnetization decreased with the
increasing thickness. [138] This observation suggests that the magnetization may reside
at the surface. The surface induced magnetism is expected to originate from the unpaired
electron spin in the O 2p orbitals at the surface, which are different from the ones with all
O orbital fully filled in bulk.
5
(a)
MS (emu/g)
MS (emu/g)
0.8
0.6
0.4
0.2
0.0
(b)
4
3
2
1
0
400 450 500 550 600
Annealing temperature (°C)
0
50
100 150 200
Thickness (nm)
250
Fig. 6-7 Dependences of saturation magnetization on (a) calcination temperature (film
thickness of ~225 nm) and (b) film thickness (films calcined at 450 °C for 1 hour).
67
6.2.2 DMI: Fe-doped MgO thin films
Since Dietl et al.[125] predicted that room temperature ferromagnetism may exist in wideband-gap semiconductor, semiconductors such as ZnO (band gap energy of ~3.3 eV) and
TiO2 (band gap energy of ~3.0 eV for rutile and ~3.3 for anatase), etc. have been extensively
studied for spintronics applications. Compared to ZnO or TiO2, MgO is a rocksalt insulator
with wider energy band gap (~7.8 eV). It shows the possibility of ferromagnetism at or above
room temperature when doped with TM atoms. Accordingly, the Fe-doped MgO is called
diluted magnetic insulator (DMI).
The electron configuration of magnesium is different from that of zinc: there is no electron in
3d orbital of Mg while the 3d orbital of Zn is fully occupied. The electron configurations of
atomic and ionic magnesium and zinc are listed as:
Mg atom: 1s2 2s22p6 3s2
Mg2+ ion:
1s2 2s22p6
Zn atom: 1s2 2s22p6 3s23p6 4s2 3d10
Zn2+ ion:
1s2 2s22p6 3s23p63d10
It is important to study the RTFM in MgO to reveal the origin of magnetism in diluted
magnetic oxides. Besides, MgO is of huge interest as a tunnel barrier in magnetic tunnel
junctions, which shows giant room temperature tunneling magneto-resistance.[139, 140]
6.2.2.1 Thin films prepared by inkjet printing
Fe-doped MgO thin films were prepared by inkjet printing. The ink was prepared by
dissolving acetates in 2-methoxyethanol.[141] Figure 6-8 shows the characterizations of the
10 at.% Fe-doped MgO films and the pure MgO films. The films were printed from 0.25 M
([Fe] + [Mg]) inks for 3 passes, and calcined at 450 °C for 2 h followed by 600°C for 2 h in
air.
(a)
(b)
(c)
Fig. 6-8 (a) XRD intensity patterns of MgO and 10 at.% Fe-doped MgO thin films; the SEM
morphology of (b) MgO and (c) 10 at.% Fe-doped MgO thin films. The insert figures in
(b)&(c) show the FIB cross-section SEM images of the two films.
68
Both the XRD intensity pattern and the SEM morphology show the amorphous structure of
the pure MgO film but crystal features of the Fe-doped MgO film. As we can see from the
cross-section images, the average thickness is ~270 nm for the MgO film and ~90 nm for the
10 at.% Fe-doped MgO film, respectively. This may be related to the carbides left in the films.
Similar to the effect in ZnO, Fe-doping can enhance the nucleation of MgO during
calcination: the iron atoms can act as seed crystals for the nucleation of MgO, leading to the
crystal structure of Fe-doped MgO thin films rather than the amorphous structure in pure
MgO films.
6.2.2.2 Room temperature ferromagnetism
The thickness of the films was determined from FIB cross-section SEM images, as examples
of 3-passes printed films shown in Fig. 6-8. For each pass printing, the thickness is ~90 nm
for pure MgO films (because of the existence of carbides) and ~30 nm for the 10 at.% Fedoped MgO films. To study the effect of Fe-doping on magnetism, the films prepared from
single pass and the films with comparable thickness (~90 nm) were characterized using
SQUID. Figure 6-9 shows the room temperature magnetic hysteresis loops of the 10 at.% Fedoped MgO thin films and pure MgO films: (a) films are single pass printed and (b) films
have comparable thickness (~90 nm, single pass for MgO and 3-passes for Fe-doped MgO).
 Fe-doped MgO films have higher magnetization than pure MgO films. For single pass
printed films (Fig. 6-9a), the saturation magnetization obtained from the loops is 0.8
emu·cm-3 for the MgO (thickness of ~90 nm) and 6.3 emu·cm-3 for the 10 at.% Fe-doped
MgO (thickness of ~30) films, respectively. Considering the thickness effect, the M S is
4.2 emu·cm-3 for the ~90 nm thick 10 at.% Fe-doped MgO film (Fig. 6-9b). It is about ~5
times as much as the MS value of pure MgO film with the same thickness.
 From the enlarged view around origin (the insert curves shown in Fig. 6-9a), small
coercivity can be observed, which indicates robust room temperature ferromagnetism.
Because of the different electron configurations and the band gap energy of ZnO and MgO,
the magnetic exchange interactions in ZnO and MgO and in their Fe-doped films are different:
 In ZnO the oxygen vacancies contribute to the defect induced magnetism. The
experimental evidence is low magnetization obtained from the films annealed in
oxygen.[142] By doping with iron, the oxygen vacancies can donate and trap electrons
and contribute to the carrier-mediated magnetic exchange among the localized magnetic
moments.
69
 In MgO, oxygen vacancies don’t contribute to magnetism while magnesium vacancies
generate magnetism.[143, 144] The observed experiment results show higher
magnetization for MgO films annealed in oxygen.[145] By doping with iron, the oxygen
anions in the films mediate the superexchange interactions between two neighboring iron
atoms (see Chapter 2 for the mechanisms of magnetism in DMS materials).
(a)
Single-pass
-3
4
0
4
MgO
-4
0
-4
MgO
Fe-doped MgO
(b)
8
M (emu*cm )
-3
M (emu*cm )
8
0.5
Fe-doped MgO -0.5 0.0
-8
-10
-5
0
5
10
Magnetic field (kOe)
4
~90 nm thick
0
MgO
-4
-8
Fe-doped MgO
-10
-5
0
5
Magnetic field (kOe)
10
Fig. 6-9 Magnetic hysteresis loops measured at room temperature for (a) single pass printed
MgO and Fe-doped MgO thin films and (b) ~90 nm MgO (single pass) and Fe-doped MgO
(three passes) thin films. The insert figure in (a) is the magnified loops around the origin of
the single pass printed films, which show small coercively (~50 Oe). The data were corrected
from the diamagnetic signal of the Si substrates.
6.2.2.3 X-ray magnetic circular differences (XMCD)
The energy of X-ray is in the same range with the energy levels of electrons in atoms.
Therefore, the incident X-ray can be absorbed and interact with a sample to excite inner shell
electrons in atoms. The absorption of X-ray as a function of the wavelength is determined as
X-ray absorption spectra (XAS), which provides electronic structure information of matters.
Because of the asymmetry of molecules, the absorption of left circularly polarized (LCP) and
right circularly polarized (RCP) light would be different from each other (circular differences,
CD). This is nature optical activity, but rare because the differences are tiny. Under a static,
uniform magnetic field, the magnetic optical activity makes the difference in absorption
between LCP and RCP light. X-ray magnetic circular difference (XMCD) is the difference
between LCP and RCP XAS measured in a magnetic field, from which the magnetic
properties of atom, such as spins and orbital magnetic moment, can be deduced.
70
For transition metals, such as Mn, Fe, Co, Ni, the magnetic properties are attributed to the
unpaired electrons in localized 3d orbitals. The special advantages of using 2p absorptions for
TM ions are:[146]
 Sharp multiplet (the number of the possible values of spin component) structure can be
observed because the core-hole lifetime broadening is only 100~300 meV.
 The spin-orbital couplings with opposite spin polarizations (L2,3-edges) are clearly
separated in 2p orbital.
 The excitation of 2p electrons into localized 3d orbital contributes to magnetic moment.
Because of the spin-orbital coupling of 2p orbitals, the excitation of 2p electrons to unfilled
3d orbital creates a core hole in 2p orbital (l=1) and two opposite spin angular momentums
⁄ and
(spin up and spin down, s = ±½). They produce total angular momentum (j) of
⁄ . These states are regarded as the L3-edge (
⁄
) and L2-edge (
⁄
) in the XAS
spectra. Besides, the spectra are related to the charge transfer (carrier hopping), the crystalfield splitting and the intra-cationic electronic repulsion (electron interactions and electroncore hole interactions).
Figure 6-10 shows the XAS and the XMCD of 10 at.% Fe-doped MgO thin films with
different thickness. L3-edge (at lower energy, ~707 eV) and L2-edge (at higher energy, ~720
eV) are clearly observed from XAS. They split into two peaks because of the interplay of
crystal-field and electronic interactions.[147] L3-edge shows tails, indicating the charge
transfer through hopping, which can be the transition of Fe2+ ↔ Fe3+. The quite symmetric L2edge is related to the iron ions at symmetrical Oh sites.
In Oh symmetry, d orbitals split into two sets: higher energy states (eg:
energy states (t2g:
,
,
,
) and lower
). They can form low spin state (spins paired and occupied at
t2g states) or high spin state (un-paired spins occupied both eg and t2g states). Three identified
peaks on L3-edge were observed in XMCD. They are corresponding to Fe2+ at Oh sites (3d
6
Oh ), Fe3+ at Td sites (3d 5 Td ) and Fe3+ at Oh sites (3d 5Oh), starting from the lower energy.
 The peaks shift of ~2 eV from 3d 6Oh to 3d 5Oh. It is due to the change of cation valence
(lower energy peak for Fe2+ and higher energy peak for Fe3+) in the Oh sites.
 The shift of ~1 eV from 3d 5 Td to 3d 5Oh is caused by the change in sites.[148]
71
The ferromagnetic alignment of the spins within Oh lattice results in the same polarization
direction of 3d 6Oh and 3d 5Oh edges, while the antiferromagnetically aligned spins between
the two lattices (Oh lattice and Td lattice) lead to the opposite polarization of the two sites.[149,
150] A small peak of dichroic signal (at lower energy than the 3d 6Oh peak) can be observed
in thick films, which might be originated from Fe2+ at Td sites (3d 6 Td).
Fig. 6-10 XAS and the XMCD of Fe-doped MgO thin films with different thickness.
6.3 Phase separation
Uniform and smooth films were obtained without impurity phases for ZnO, MgO and their
Fe-doped thin films.[120, 141, 151] However, the simple mixing of acetates in solutions may
not be suitable for printing all oxide materials. For some of the acetates solution inks, such as
the solution of zinc acetates with small fraction of magnesium acetates (5 at.% [Mg] in the
total of [Mg]+[Zn]), phase separation can be observed in the prepared films. They formed
during the calcination process (separation of MgO and ZnO phases), or even during the liquid
evaporation process (separation of the acetates). Figure 6-11 shows an example of phase
separation observed in 5 at.% Mg-doped ZnO thin films. The films were calcined at 450 °C
for 1 hour after printing. Different from the uniform and smooth ZnO or Fe-doped ZnO thin
films, the Mg-doped ZnO thin films show the reticular micro-structures. These reticular
structures are determined to be ZnO by X-ray element mapping. The concentration of Mg is
too low to get enough information for mapping its distribution. Figure 6-12 shows different
stages of the these reticular structure duing formation.
72
(a)
(b)
(c)
Si
O
Zn
Mg
Fig. 6-11 Phase separation observed from 5 at.% Mg-doped ZnO thin films: (a, b) SEM
images (c) X-ray element maps of Si, O, Zn and Mg, for the same area shown in (b).
(a)
(b)
(c)
(d)
Fig. 6-12 The reticular structure of Mg-doped ZnO thin films at different stages: (a) ~ (d) the
growth of the reticular structures.
73
The ZnO crystals nucleate here and there (Fig. 6-12a), followed by the Oswald coarsening.
Because magnesium acetates have higher decomposition temperature than the zinc
acetates,[151, 152] they will remain while ZnO starts to nucleate and grow. Besides, the
energy barrier of MgO nucleation is higher than that of ZnO. It can be seen from Figs. 6-4&8:
ZnO shows good crystallinity after calcination at 450 °C for 1 hour, while MgO is amorphous
even after calcination at 450 °C for 2 hours followed by 600 °C for 2 hours. Unlike Fe-doping,
the doped magnesium acetates act as obstacles for the growth of ZnO phase along the
substrate surface. Thus ZnO would grow in the direction perpendicular to the film surface
which has less obstacle, to form the reticular structure.
6.4 Summary
From the features and the properties of the prepared oxide films, the merit and demerit of
using acetate solution as inks for DOD inkjet printing can be concluded as:
 Dense, uniform and smooth metal oxide films can be prepared from the ‘in-situ’
decomposition of the metal-acetates.
 Doping of secondary elements into the metal oxide lattice is possible during the ‘in-situ’
preparation process of metal oxides. The microstructure and morphology of the films can
be tuned.
 The printed DMS and DMI films show room temperature ferromagnetism, indicating that
it is possible to apply inkjet printing acetate solution inks in spintronic devices.
 In some cases, phase separation may be observed in the inkjet printed acetate precursors
or the final oxide films. Further study on suppressing the phase separation during the
printing process and the calcination is needed.
74
Chapter 7: Inkjet printing ITO films

Transparent conducting oxides (TCO) are widely used as electrodes on light emitting
diodes and photovoltaic devices. They are doped metal oxides which are optically
transparent and electrically conductive. To date, tin doped indium oxide (ITO) is one of
the most common and standard TCO in practical applications. It has low resistivity
(~10-4 Ω•cm) and high transmittance (>80%) in films. The coming problem is the
exhaust of indium sources in the earth and consequently the increasing prices of indium.
It is emergent to reduce the chemical waste during the preparation of ITO. In this
chapter, preparations of the inks and characterizations of structure and property of inkjet
printed ITO films are introduced.
7.1 Introduction
ITO films can be prepared by physical vapor depositions (PVD, including pulsed laser
deposition,[153, 154] sputtering[155, 156] and evaporation[157-159]) and chemical routes
(e.g. sol-gel process[160] and dip-coating,[161] etc.). PVD techniques can produce high
quality ITO films, but the limited usage of the target materials causes the chemical wastes.
Wet chemical routes have a problem in controlling the uniformity and thickness of the films.
As an alternative technique, DOD inkjet printing is promising in fabrication of ITO films:
 Minimum waste of chemicals. This is especially great for an efficient usage of indium:
both in saving indium source and in protecting environment since indium is toxic.
 Patterning. For some devices, the electrode is required in certain geometry. By using
inkjet printing, any pattern can be digitally designed and directly printed.
The problem is how to prepare suitable inks for printing. Recently, there are some studies on
printing ITO films.[162-165] The inks are prepared from dispersing ITO nanoparticles in
solvent with suitable surfactant. To obtain good conductivity, it is necessary to anneal the
printed films at high temperature to improve the links among the ITO particles. The high
temperature annealing process limits the substrates of the films, and cannot be used in some
applications. Besides, even though under high temperature annealing, the resistivity of the
printed films is higher than the reported values of PVD films. Loose and porous structure of
the printed films leads to the high resistivity.
75
To improve the quality of the printed ITO films, acetate solution inks are used in the study.
Comparing the two types of inks in printing ITO films,

ITO oxides suspension: ITO particles should be synthesized for the preparation of the ink.
The printing process transports the ITO particles from the suspension ink on the substrate
and form films. It is an “ex-situ” process. The size of the particles and the structure of the
films depend on the ITO source rather than the printing process.

Acetate solutions: The as-printed films are acetates precursors. To obtain target ITO
films, it is necessary to decompose the acetate into oxides. Thus, the oxides could be
formed “in-situ” on the substrate after printing. This makes it possible to tune the
structure and morphology of the ITO films during calcination.
7.2 Inks for printing ITO films
In the thesis, two types of inks for printing ITO films are introduced. The two inks were
prepared with 0.1 M cations ([Sn]+[In]) concentration with molar ratio of [Sn]:[In]=1:9. The
preparations of the two inks are listed as follows:
A: The ink is prepared by dissolving indium acetate and tin acetate in acetylacetone (ATA).
B: The ink is prepared by first dissolving In- and Sn- acetates in ATA, followed by heating
the solution at ~120 °C in the presence of tetramethylammonium hydroxide (TMAH, 25
wt % aqueous solution) and hydrogen peroxide (~30 wt % in water).
The stability of the two inks is demonstrated by visually monitoring the ink over time with a
digital camera. Figure 7-1 shows the visual images of the fresh and the aged inks.
A
B
(a)
A
A
B
(b)
B
(c)
B
(d)
Fig. 7-1 The stability of inks A and B: (a) the fresh inks; (b) the 1-day aged inks; (c) the 1month aged inks, and (d) the re-treated ink B.
76
 The fresh inks are transparent (Fig.7-1a). The color of the ink is yellowish for ink A and
red-brown for ink B, respectively.
 Ink A is stable over long time scale without any sediment (>6 months). Ink B shows
sediments during aging: 1-day after preparation, the transparency of the ink at the bottom
has decreased (Fig. 7-1b); 1-month after preparation, the flocculent sediments can be seen
in the bottom of the ink (Fig. 7-1c).
 The aged ink B can be retreated by hot-water (70~90 °C) bath for a fresh ink, and the retreated ink is shown in Fig. 7-1(d).
7.3 ITO films from inkjet printing
Both ink A and ink B were used to prepare ITO films by inkjet printing. By comparing the
morphology of the films prepared from the two inks, ink B was selected for further studies. In
this section, the ITO films are characterized to qualify the inks.
7.3.1 Comparison of films printed from ink A and ink B
Figure 7-2 shows the SEM morphology of the films printed from ink A and ink B. The film
printed from ink A shows micro-dendritic structure (Fig. 7-2a), leading to rough surface. The
deposited film is uneven along the surface of the substrate. From the X-ray mapping of Si, In
and O, the distribution of the ITO films can be seen (Fig. 7-2b). The Si is from glass substrate,
In is from the printed film, and O can be from the both. The formation of this micro-dendritic
structure of ITO films could be caused by the nucleation and growth of acetates during the
drying process. The acetates were dissolved in acetylacetone (ATA). Unlike alkoxides (ROH,
such as 2-isopropoxyethanol and 2-methoxyethanol) used for printing ZnO and MgO in
chapter 6, the ATA cannot bond with metal ions to form cross-links in the solution. Thus
during the evaporation of the liquid, the solution becomes saturated, and the acetates
precipitate instead of forming gel for uniform depositions. The acetate crystals grow into the
dendritic structure, resulting in the observed micro-structure of ITO films.
To suppress the nucleation and the growth of acetates in a saturated solution, tetramethylammonium hydroxide (TMAH, 25 wt.% aqueous solution) and hydrogen peroxide (~30 wt.%
in water) were added into ink B. At ~120 °C, the tin acetates and indium acetates were
chelated with acetylacetone and form complexes. The acetylacetone complexes form crosslinks through the solution to achieve the “sol-gel” process. As the morphology shown in Fig.
77
7-2(c), the printed films are uniform with smooth surface. The ITO films show nano-feathery
structure with nanoporous (Fig. 7-2d). The nanoporous could be formed during the calcination
of the precursors: the carbon burns and produces CO2/CO gas during the whole process of
nucleation and growth of ITO films.
SE
Si
In
(a)
O
(b)
(c)
(d)
Fig. 7-2 (a) SEM morphology and (b) X-ray element maps of Si, In and O for the film printed
from ink A; (c) and (d) the surface SEM morphology of the ITO films printed from ink B.
7.3.2 Lattice structure of ITO films
To obtain enough signals, the ITO films were printed with 4 passes for XRD analysis. Figure
7-3 shows the XRD intensity patterns of ITO films which were annealed in oxygen gas at
300°C, 350°C, 400°C, 450°C and 500°C for 2 hours. The diffraction peaks are matched with
the standard peaks of In2O3 (JCPDS No. 00-006-0416, body-centered cubic with a = 10.118
Å). There is an extra peak at 2θ around 27.5°, which is matched with (111) diffraction peak of
calcium (JCPDS No. 01-089-3683). It probably comes from substrate since the films are
78
rather thin and the X-ray can penetrate through the film. The data have been shifted along Yscale for clarity.
 ITO thin films can be obtained at calcination temperature as low as 300 °C.
 The diffraction peaks shift with the calcination temperature, like (222) and (440)
diffraction peaks shown in Fig. 7-3. This indicates that the lattice structure of the films
depends on the calcination temperature. The lattice parameter of the films can be refined
by Ceref3.
 For films annealed at temperature lower than 450 °C, a = 10.111 ± 0.005 Å, which is
smaller than that of cubic indium oxide (a = 10.118 Å). Since the Sn4+ has smaller
Pawling radii (71 pm) than In3+ ion (81 pm), the substitution of Sn ions for In in cubic
In2O3 structure would contract the lattice.[166]
 For the film annealed at 500 °C, a = 10.117 ± 0.005 Å, which is very close to the value
for bulk In2O3 lattice. The reason could be the expansion of the ITO lattice at high
temperature. The expanded lattice can be maintained after cooling because of the
(622)
(440)
Intensity (a.u)
500 °C
450 °C
400 °C
350 °C
20
29
30
31
32
(440)
300 °C
Standard In2O3
10
(222)
Peak from substrate
(400)
(411)
(332)
(431)
(211)
(222)
expansion coefficient of the glass substrate.[153]
30
40
50
2 Theta (degree)
60
70
49 50 51 52 53
Fig. 7-3 XRD intensity patterns of ITO thin films annealed at different temperatures.
(Standard peaks of In2O3 refer to JCPDS No. 00-006-0416). The insert patterns are the
magnified diffraction peaks from (222) and (440) crystal planes of ITO films.
7.3.3 Electrical properties of ITO films
The resistances (R) of the prepared ITO films were determined by Four Probe Method. The
Current-Voltage (I-V) curves were measured. Since the film thickness (h, from FIB cross79
section SEM images, the thickness of each printing pass is ~20 nm) is smaller than the space
between probes (~2.5 mm), the resistivity (ρ) of the films can be calculated by:[167]


ln( 2)
hR
(7-1)
Figure 7-4 shows the resistance and the resistivity of the ITO films printed for different passes
(N = 1, 2, 4) and calcined at different temperature (T =300, 350, 400, 450 and 500 °C). The
values of resistance are determined by the I-V curves and the values of resistivity of the films
are calculated from Eq. (7-1) using the thickness of ~20 nm for each printing pass.
 The thicker films have lower resistance. The increase of the thickness can condense the
films and improve the continuity, which can improve the electron conductivity of the
films. However, the resistivity of thick films is larger than that of thin films. The reason
could be the remained carbides in thick films. Besides, because of the densification of the
films, the thickness of thicker films may be less than 20×N nm.
 The films calcined at higher temperature show lower resistance and lower resistivity. The
explanation is that the decomposition of the precursors requires thermal energy to form
pure phase ITO thin film (without carbides). The improved crystal structure at higher
annealing temperature could also improve the electron conductivity.
 Annealing in N2 can reduce the resistance and resistivity of the films to half. It is
attributed to the increase oxygen vacancies in the films (Fig. 7-4c).
 Comparing to the reported resistivity of ITO films prepared from PLD (~10 -4 Ω•cm), the
resistivity of the inkjet printed ITO films is higher. It could be related to the nanoporous
structure (see Fig. 7-2d) and the carbides impurities inside the films.
300
350 400 450 500
Temperature (°C)
0.1
300
350 400 450 500
Temperature (°C)
Before N2 annealing
0.09
0.06
After N2 annealing
0.03
1k
20
40
60
Thickness (nm)
80
Fig. 7-4 Electric transport properties of the printed ITO films: (a) the resistance and (b) the
resistivity of the films calcined at different temperatures, (c) comparison of the resistance and
resistivity before and after annealing in N2.
80
Resistivity (Ohm*cm)
10k
N =1
N =2
N =4
1
(c)
10k
Resistance (Ohm)
Resistance (Ohm)
N =1
N =2
N =4
(b)
Resistivity (Ohm*cm)
(a)
100k
7.4 Summary
As an alternative fabrication technique with high usage efficiency of chemicals, inkjet
printing has been developed for printing ITO films in recent years. In this work, instead of
using oxide particle suspensions as the inkjet inks, acetate solution is introduced as the ink to
“in-situ” printing ITO films. However, the simple mixing of acetates in solvents does not
work in this case: the acetates precipitate during the drying process. This leads to the observed
micro-dendritic structure of the films with rough surface. A chelating treatment is developed
to bond the metal ions with the solvent molecules in the ink. Instead of acetates precipitation,
the saturated ink becomes a gel to achieve uniform deposition. The obtained ITO films are
uniform and the surface is smooth. From the high resolution SEM images, the films show
nano-feathery structure with nano-pores. The electron conductivity of the films shows
dependence on the film thickness, the calcination and the annealing conditions. The resistivity
of ~40 nm ITO film is obtained to be 0.029 Ω•cm. The results could be useful for preparing
ITO films at low temperature especially on some special substrate like plastics, or producing
ITO films with large area which is limited in PVD systems.
81
Chapter 8: Conclusions & Future scope

Different oxides materials have been deposited by inkjet printing technique. Three types of
inks: particle suspension, colloid and acetate solution were introduced for printing functional
oxides films. From the observed results, the conclusions can be drawn as:
 Inkjet printing of (ZnO)1-x(TiO2)x composite films can be used in fabrications of DSSCs.
It is a very low cost technique and easy for mass production, which is expected to be
applied in large scale industrial productions.
 The TiO2 films printed from suspension-based ink are porous and loose with weak link
among particles. By doping Ag nanoparticles, the correlations among TiO2 particles can
be improved by the metal-bridge. The reduced intensity of PL in emission spectra
indicates that Ag atoms can act as traps to capture electrons and inhibit the recombination
of electron-hole pairs, which is desirable in many photoresponse applications.
 For the suspension-based inks, aggregates and sediments of the particles affect the
repeatability of the printing process. Because of Brownian motion, smaller particles have
a lower sediment rate and thus the suspension is more stable.
 For magnetic particles, even in nanoscale, the magnetic interaction among them
accelerates the formation of aggregates and sediments. We fabricated magnetite (Fe3O4)
nanoparticles from co-precipitation. With rapid mixing the reactants, magnetite
nanoparticles with high saturation magnetization (~88 Am2•kg-1) and low coercivity (~12
A•m-1) can be prepared. Because of the strong magnetic interaction, the aqueous
suspension settled in a short time. By surface modifying the nanoparticles, a charged
‘shell’ can form for each particle. The electrostatic repulsion among the charged ‘shell’
prevents the particles from agglomeration and stabilizes the suspension. It can be stable
for months without any sediment.
 By using acetate solutions as inks, the oxide films can be prepared ‘in-situ’ on substrate.
The structure and morphology of the films can be tuned. For instance, the size of crystal
grains decreases by doping Fe in ZnO system; under the same preparation conditions,
pure MgO is amorphous while Fe-doped MgO is crystal. It is related to the nucleation
and growth of crystals in the calcination process. These semiconductor/insulator films
show robust RT ferromagnetism, implying that the inkjet printing technique is a reliable
method to produce magnetic films applicable in the field of spintronics.
82
 One problem in printing acetates solution-based ink is phase separation of oxides. Phase
separation can be caused by (i) the separation of the acetates in the ink because of the
solubility; (ii) the difference of energy barrier for oxides nucleation and growth during
calcination process. By using alkoxide (ROH) as solvent, the alkoxy group (-OR) can
bond with metal ions in acetates and form cross links through the solvents to achieve the
sol-gel process during drying. The sol-gel process deposits the acetate precursors
uniformly on the substrate. Or by chelating the acetates with the solvents, the acetates
phase separation can be avoided (e.g., the deposition of ITO films). For the second issue,
new metal complexes may be needed to avoid the phase separation.
Based on this study, in the future the following suggestions could be further carried out:
 Test and print ferrofluid to prepare Fe3O4 thin films. It will be interesting to compare the
Fe3O4 films prepared from ferrofluid with the films prepared from iron acetates. It would
explore inkjet printing technique in fabrications of magnetic oxide thin films.
 Try other organic solvents to form Mg- and Zn- complexes which can decompose and
form oxides simultaneously. Prepare different concentration of Mg doped in ZnO thin
films for optoelectronic applications.
 Further work in magnetism of Fe-doped ZnO/MgO films: print films with comparable
thickness and anneal them in different environment: reducing gas and oxidizing gas, and
study the RTFM to reveal the effects of oxygen vacancies/anions in the two materials
with different energy band gaps.
 Annealing the films in reducing gas may improve the electric conductivity of ITO films.
 The films prepared from acetate solution-based inks show nanoporous structure. It will be
interesting to known its special effects on magnetic, optical, or electrical properties of
materials.
 Develop more acetates solutions as inks for inkjet printing other metal oxide films for
industrial applications or special demands.
83
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93
Appendix I:
Units: magnetic properties
Quantity
Symbol
Gaussian &
cgs emu a
SI & rationalized
mks b
Conversion
Magnetic flux
Φ
gauss (G)
tesla (T), Wb·m-2
1 G =10-4 T = 10-4 Wb·m-2
Magnetic flux density,
magnetic induction
B
maxwell (Mx),
G·cm2
weber (Wb),
volt second (V·s)
1 Mx = 10-8 Wb
= 10-8 V·s
Magnetic field strength,
magnetizing force
H
oersted (Oe),
Gb·cm-1
ampere-turn per
meter (A·m-1)
1 Oe = 103/(4π) A·m-1
Volume magnetization
M
emu·cm-3 c
A·m-1
1 emu·cm-3= 103 A·m-1
Mass magnetization
σ, M
emu·g-1
A·m2·kg-1, Wb·m·kg-1
1 emu·g-1 = 1 A·m2·kg-1
= 4π × 10-7 Wb·m·kg-1
Magnetic moment
m
emu, erg·G-1
A·m2,
joule per tesla (J·T-1)
1 emu = 1 erg·G-1
= 10-3 A·m2 = 10-3 J·T-1
Magnetic polarization,
intensity of magnetization
J, I
emu·cm-3
T, Wb·m-2 d
1 emu·cm-3 = 4π × 10-4 T
= 4π × 10-4 Wb·m-2
Magnetic dipole moment
j
emu, erg·G-1
Wb·m
1 emu = 4π × 10-10 Wb·m
dimensionless,
1→4π,
emu·cm-3
henry per meter
(H·m-1), Wb·(A·m)-1
1 emu·cm-3 = (4π)2 × 10-7 H·m-1
= (4π)2 × 10-7 Wb·(A·m)-1
dimensionless,
Volume susceptibility
χ, κ
Mass susceptibility
χρ, κρ
cm3·g-1,
emu·g-1
m3·kg-1, H·m2·kg-1
1 cm3·g-1 = 4π × 10-3 m3·kg-1
1 emu·g-1 = (4π)2 × 10-10 H·m2·kg-1
Permeability
μ
dimensionless
(H·m-1), Wb·(A·m)-1
1→4π × 10-7 H·m-1 = 4π × 10-7
Wb·(A·m)-1
Volume energy density,
energy product
W
erg·cm-3
J·m-3
1 erg·cm-3 = 10-1 J·m-3
a
Gaussian units and cgs emu are the same for magnetic properties.
b
SI (Système International d’ Unités) have been adoped by the National Bureau of Standards.
c
The designation ‘emu’ is not a unit.
d
Recognized under SI.
R. B. Goldfarb and F.R. Fickett, U.S. Department of Commerce, National Bureau of Standards, Boulder, Colorado 80303, March 1985
NBS Special Publication 696 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
94
Appendix II:
The printing rate determination of the EPU printer
1. The printing process of the EPU system
Printhead, start stage
L = v·t
Printed film
α =30.96 °
Printhead, after printing
Fig.1 The schematic daigram of EPU printing process.
2. The calculation of the printing rate
400
2
Printing rate (mm /s)
3. The printing rate vs time
300
v = 20 mm/s
w = 17.2 mm
°
200
100
0
0
20
40 60
Time (s)
80
100
Fig.2 The printing rate of EPU system.
95
96
97
Appendix V:
Recipe for preparing kerosen based ferrofluid
1. Chemicals
FeCl3•6H2O(molar weight: 270.3 g/mol)
FeCl2•4H2O(molar weight: 198.8 g/mol)
Ammonia soluiton (25~35%)
Oleic acid
1 ml
De-ionized water
250 ml
2.703 g
0.9938 g
~20 ml
Acetone
Kerosene
40 ml
6 ml
2. Co-precipitation of magnetite particles
 Weigh 0.9938 g FeCl2•4H2O and 2.703 g FeCl3•6H2O and dissolve them in 40 ml de-ionzed





water. (iron ion source, pH value of 2~3)
Dissolve 10 ml of ammonia in 30 ml de-ionized water. (base solution, pH value of ~10.5)
Stir the basic solution in 250 ml beaker in fume champer using a magnetic bar and raise the
temperature up to 40 °C and maintain this temperature.
Add the salt solution into the basic solution at a constant rate at 40 °C while stirring. Black
precipitate formed. Adjust the pH value of the mixture to be ~10.5 by adding ammonia dropwise.
Allow stirring for 30 minutes at 40 °C.
Remove the impurites by hot-water (~70 °C) wash (3 times, 30 ml each) and decant the supernant
by mangetic sedimentation.
3. Surface coating of the particles
 Add 40 ml water to the as-prepared particles and stir it in 250 ml beaker. Ajust the pH value to




~10.5 by adding ammonia dropwise.
Add 1 ml oleic acid and stirring for 1 hour at ambient temperature.
Heat the temperature of the mixture to 92~95 °C maintain this temperature for 3 minutes and then
remove the mixture from hot source and cool down to room temperature while continuous stirring.
Add diluted HCl (0.01 M conc) dropwise to adjust the pH value to 5 while keeping stirring. At
this pH, the coated particles become separated from other impurites and coagulate to make
clusters.
Wash the clusters with warm water (3 times, 20 ml each) to remove excess surfactant and
impurites, followed by wash by acetone (4 times, 10 ml each) to remove water as well as extra
surfactant. During acetone wash, try to separate each individual particles from cluster by
virgorous stirring.
4. Disperse coated particles into non-magnetic matrix
 Add 6 ml kerosene into acetone wet slurry in 50 ml beaker and disperse the particles by hand
stirring.
 Heat the dispersion at 60~65 °C to evaporate acetone.
 Confirmation test: (i) with acetone, the surface of beaker doesn’t heat up. It will be heated only
after the acetone evaporated; (ii) add a few drops of kerosene to the surface of the beak: if drops
slide to the bottom with clean surface, the acetone has evaporated completely; (iii) The bottom of
the beaker looks clear when it is slanted; (iv) smell it: kerosene smells different between with and
without acetone.
Now kerosene-based ferrofluid is ready for use!
98
Part II: Attached Papers
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