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STUDIES ON THE GROWTH AND CHARACTERIZATION
OF SOME OPTICAL CRYSTALS
Thesis of the research work submitted to
Bharathidasan University, Thiruchirappalli in partial fulfillment
of the requirements for the award of the degree of
DOCTOR OF PHILOSOPHY
IN
PHYSICS
Submitted by
P.PARAMASIVAM
Under the Supervision of
Dr. C. RAMACHANDRA RAJA, Ph.D., Associate Professor in Physics
POSTGRADUATE & RESEARCH DEPARTMENT OF PHYSICS
GOVERNMENT ARTS COLLEGE (AUTONOMOUS)
KUMBAKONAM - 612001
TAMIL NADU, INDIA
FEBRUARY 2012
Dr. C. Ramachandra Raja, Ph.D.,
Associate Professor in Physics,
Department of Physics,
Government Arts College (Autonomous), Phone : +91 4364221751
Kumbakonam 612001, CelL: +91 9976696277
Tamil Nadu, India. Email: crraja_phy@yahoo.com
CERTIFICATE
This is to certify that the thesis entitled STUDIES ON THE GROWTH
AND CHARACTERIZATION OF SOME OPTICAL CRYSTALS submitted by
Mr. P.PARAMASIVAM is a bonafide record of the research work done by him
during the period of study from 2004 to 2011 under my supervision in the Department
of Physics, Government Arts College (Autonomous), Kumbakonam and that the thesis
has not previously formed the basis for the award of any Degree, Diploma,
Associateship, Fellowship or any other similar title. This thesis represents an
independent work on the part of candidate.
Kumbakonam C. Ramachandra Raja
(Research Supervisor)
Mr. P.Paramasivam,
Research Scholar (Part - Time),
Department of Physics,
Government Arts College (Autonomous),
Kumbakonam 612001,
Tamil Nadu, India.
DECLARATION
I hereby declare that the work presented in this thesis entitled STUDIES
ON THE GROWTH AND CHARACTERISATION OF SOME OPTICAL
CRYSTALS has been originally carried out by me under the guidance and
supervision of Dr.C.Ramachandra Raja, Associate Professor, Department of
Physics, Government Arts College (Autonomous), Kumbakonam. This work has not
been submitted either in whole or in part for any other Degree or Diploma at any
Universities or Research Institutes.
Kumbakonam P.Paramasivam
ACKNOWLEDGEMENT
The author deeply expresses his wholehearted gratitude to his respectful guide
and supervisor Dr. C. RAMACHANDRA RAJA, Associate Professor, Department
of Physics, Government Arts College (Autonomous), Kumbakonam, India for his
effective guidance, continuous encouragement, and who has had a profound influence
to complete the research and thesis work. This thesis shall always bear testimony to
my respect and gratitude towards my mentor.
The author expresses his sincere gratitude to Dr.J.Govindhadas, Principal,
Government Arts College, Kumbakonam, India and extends his profound thanks to
Dr.K.C.Srinivasan, Head of the Department of Physics, Government Arts College,
Kumbakonam, India for providing this opportunity.
The author is deeply thankful to Dr.R.Jayavel, Director, Department of Nano
Technology, Anna University, Chennai, India, Dr.R.Mohan Kumar, Professor,
Presidency College, Chennai, India, Dr.N.Vijayan, Scientist, NPL, New Delhi, India,
Dr.V.Manivannan, Addl. Director (CRD), PRIST University, Thanjavur,
Prof.R.S.Sundararajan, Department of Physics, Govt. Arts College, Kumbakonam,
India, Mr.B.Vijayabhaskaran, Assistant Professor of Physics, Anjalai Ammal-
Mahalingam Engineering College, Kovilvenni, Tiruvarur, India and Dr.A.Antony
Joseph, Assistant Professor of Physics, Annai Enggineering college, Kumbakonam,
India for their valuable suggestions, fruitful discussions and immense help at various
phases of the research.
iv
The author records his immense gratitude to Dr.P.K.Das, Professor, IPC, IISc,
Bangalore, India for providing the opportunity to do the NLO study. The author also
gratefully acknowledges the valuable help extended by the authorities of SAIF, IIT,
Chennai, India, ICP, CECRI, Karaikudi, India and ACIC, St. Josephs College,
Tiruchirappalli, India to carryout the desired studies.
The author expresses his deep sense of gratitude to Dr.M.Arivazhagan,
Assistant Professor of Physics, A.A. Government Arts College, Musiri, India,
Dr.I.Vethapothakar, Assistant Professor of Physics, Anna University of Technology,
Thiruchirappalli, Mr.M.Chitravel, Assistant Professor of Chemistry, T.R.P. Engg.
College, Thiruchirappalli, India, Ms.D.Trixy Nimmy Priscilla, Lecturer, Department
of Physics, Anjalai Ammal-Mahalingam Engineering College, Kovilvenni, Tiruvarur,
India for their consistent support throughout this work.
The author is also thankful to Prof. M.Arulanandasamy Department of
English, Anjalai Ammal-Mahalingam Engineering College, Kovilvenni, Tiruvarur and
Dr.P.Arangamsamy, Head of the Department of English, periyar Maniammai
University, Thanjavur for their careful revision and proof- reading of the text at every
stage of its preparation.
The author extends his sincere thanks to all the teaching and non teaching staff
members of the Department of Physics, Government Arts College, Kumbakonam for
their continued support.
v
Lastly, and most importantly, the author wants to thank his parents, wife and
sons without whom this work could not have been accomplished successfully. Their
support, even at the cost of their personal comfort and needs, is worth the whole
world. The author also thanks his entire extended family and friends for having
provided a loving environment for him during the whole course of his research work.
The Author
vi
TABLE OF CONTENTS
Chapter No Title Page No
Preface xii
List of Publications xvi
Conferences / Seminars xvii
List of Tables xviii
List of Figures xix
List of Symbols xxi
List of Abbreviations xxii
1 INTRODUCTION TO CRYSTAL GROWTH AN OVERVIEW
1.1 INTRODUCTION 1
1.2. NUCLEATION 3
1.2.1. Kinds of Nucleation 4
1.2.2. Classical Theory of Nucleation 5
1.2.3. Kinetic Theory of Nucleation 5
1.3. STABILITY OF NUCLEUS 6
1.4. ENERGY FORMATION OF SPHERICAL NUCLEUS
7
1.5. SUPERSATURATION AND ITS EXPRESSION 10
1.6. CLASSIFICATION OF CRYSTAL GROWTH 11
1.6.1. Growth from Melt 12
1.6.2. Growth from Vapour 15
1.6.3. Growth from Solution 17
1.7. GEL GROWTH 18
vii
Chapter No Title Page No
1.8. HYDROTHERMAL GROWTH 19
1.9. FLUX GROWTH 19
1.10. LOW TEMPERATURE SOLUTION GROWTH 20
1.10.1. Slow Cooling Method 21
1.10.2. Temperature Gradient Method 22
1.10.3. Slow Evaporation Method 22
1.11. CRITERIA FOR OPTIMIZING SOLUTION GROWTH
23
1.11.1. Material Purification 23
1.11.2. Solvent Selection 24
1.11.3. Solubility 24
1.11.4. Solution Preparation and Crystal Growth 25
1.11.5. Crystal Habit 25
1.12 ADVANTAGES OF LOW TEMPERATURE SOLUTION GROWTH TECHNIQUE
26
2 AN OVERVIEW OF OPTICAL MATERIALS
2.1. INTRODUCTION 28
2.2 IMPORTANCE OF CRYSTALS AS OPTICAL MATERIALS
30
2.3 NONLINEAR OPTICAL MATERIALS 31
2.4. THEORETICAL EXPLANATION OF NONLINEAR OPTICS
33
2.5. VARIOUS TYPES OF NLO EFFECTS 36
2.5.1. Second Harmonic Generation 37
2.5.2. Sum Frequency Generation 38
2.5.3. Difference Frequency Generation 39
viii
Chapter No Title Page No
2.5.4. Optical Parametric Generation 39
2.5.5. Linear Electro Optic Effect 40
2.5.6. Optical Rectification 40
2.6. NONLINEAR OPTICAL MATERIALS 40
2.7. DEVELOPMENT OF NLO MATERIALS 41
2.7.1. Organic Crystals 42
2.7.2. Semi-Organic Crystals 44
2.7.3. Inorganic Crystals 45
2.8. SQUARIC ACID, L-PROLINE, GLYCINE AND THIOCYANATE BASED OPTICAL CRYSTALS
46
2.9. SCOPE OF THE RESEARCH WORK 50
3 CHARACTERIZATION TECHNIQUES
3.1. INTRODUCTION 52
3.2. SINGLE CRYSTAL XRD STUDIES 53
3.2.1. Principle of X-ray diffraction 53
3.2.2. Sample Selection and Preparation 55
3.2.3. Sample Mounting 55
3.2.4. Sample Centering 55
3.3 POWDER X-RAY DIFFRACTION STUDIES 57
3.3.1. X-ray Powder Diffractometer 58
3.4. FT-IR SPECTRAL ANALYSIS 60
3.4.1. Preparation of Liquid Sample 64
3.4.2. Preparation of Solid Sample 64
3.5 NUCLEAR MAGNETIC RESONANCE ANALYSIS 65
ix
Chapter No Title Page No
3.5.1 Introduction 65
3.5.2 NMR Spectroscopy - Principle 66
3.5.3 Nuclear spins 66
3.5.4 NMR Spectrometer - Construction 68
3.5.5 NMR Spectrometer - Working 69
3.5.6 Applications of NMR Spectroscopy 69
3.6. UV-Vis-NIR SPECTROSCOPY 70
3.7. THERMAL STUDIES 72
3.7.1. Differential Thermal Analysis 74
3.7.2. Thermogravimetry Analysis 75
3.8. KURTZ POWDER METHOD 77
3.8.1. Introduction 77
3.8.2. Experimental Procedure 77
4 SYNTHESIS, GROWTH AND CHARACTERIZATION OF A NEW NONLINEAR OPTICAL MATERIAL: 4-PHENYLPYRIDINIUM HYDROGEN SQUARATE (4PHS)
4.1. INTRODUCTION 80
4.2. EXPERIMENTAL PROCEDURE 81
4.3. CHARACTERIZATION STUDIES
4.3.1. Single Crystal X-RD Analysis 82
4.3.2. FT-IR Spectral Analysis 83
4.3.3. Nuclear magnetic resonance 86
4.3.4 Optical transmission spectrum analysis 88
4.3.5. Second Harmonic Generation Analysis 89
x
Chapter No Title Page No
4.3.6. Thermal Analysis 90
4.4. CONCLUSION 91
5 GROWTH AND CHARACTERIZATION OF A NEW NONLINEAR OPTICAL CRYSTAL: GHS
5.1. INTRODUCTION 93
5.2. EXPERIMENTAL PROCEDURE 94
5.3. CHARACTERIZATION STUDIES 96
5.3.1. Single Crystal X-RD Analysis 97
5.3.2. Powder XRD Analysis 97
5.3.3. FT-IR Spectral Analysis 100
5.3.4. Optical Transmission Spectrum Analysis 101
5.3.5. Nuclear magnetic resonance 102
5.3.6. Second Harmonic Generation Analysis 105
5.3.7 Thermal Analysis 106
5.4. CONCLUSION 107
6 CRYSTALLIZATION AND CHARACTERIZATION OF A NEW NONLINEAR OPTICAL CRYSTAL: LPS
6.1. INTRODUCTION 109
6.2. EXPERIMENTAL PROCEDURE 110
6.3. CHARACTERIZATION STUDIES 111
6.3.1. Single Crystal X-RD Analysis 112
6.3.2. FT-IR Spectral Analysis 112
6.3.3. Optical Transmission Spectrum Analysis 114
6.3.4. Second Harmonic Generation Analysis 115
6.3.5. Thermal Analysis 116
6.4. CONCLUSION 117
xi
Chapter No Title Page No
7 GROWTH AND CHARACTERIZATION OF CADMIUM MANGANESE THIOCYANATE (CMTC) CRYSTAL
7.1. INTRODUCTION 119
7.2. EXPERIMENTAL PROCEDURE 120
7.3. CHARACTERIZATION 121
7.3.1. Single Crystal X-RD Analysis 122
7.3.2. FT-IR Spectral Analysis 122
7.3.3. Optical Transmission Spectrum Analysis 124
7.3.4. Thermal Analysis 125
7.4. CONCLUSION 126
8 GROWTH AND CHARACTERIZATION OF ZINC MANGANESE THIOCYANATE (ZMTC) CRYSTAL
8.1. INTRODUCTION 127
8.2. EXPERIMENTAL PROCEDURE 128
8.3. CHARACTERIZATION STUDIES
8.3.1. Single Crystal X-RD Analysis 130
8.3.2. FT-IR Spectral Analysis 130
8.3.3. Optical Transmission Spectrum Analysis 132
8.3.4. Thermal Analysis 133
8.4. CONCLUSION 134
9 SUMMARY AND SUGGESTIONS FOR FUTURE WORK
9.1. SUMMARY 135
9.2. SUGGESTIONS FOR FUTURE WORK 137
REFERENCES 139
ANNEXURE
xii
PREFACE
During the last decades the growth of single crystals has assumed enormous
importance for both academic research and technology. Atomic arrays that are
periodic in three dimensions with repeated distances are called single crystals. It is
clearly more difficult to prepare single crystals than poly-crystalline material and extra
effort is justified because of the outstanding advantages of single crystals. Nonlinear
optical materials are gaining attention due to their enormous applications in
telecommunication activities such as optical computing, laser remote control, optical
modulators, data processing, color display and medical diagnostic. Both organic
materials and inorganic materials were used for research work. Second harmonic
generation is a nonlinear optical process in which photons interacting with a nonlinear
material are effectively combined to form new photons with twice the energy and
therefore, twice the frequency and half the wavelength of the initial photons. In the
present research work, the optical property arises due to donor and acceptor groups at
the opposite ends of the molecule which produces dipolar structure. It has been long
recognized that the electronic structure and the strength of donor and acceptor groups
are responsible for achieving optical properties.
The thesis comprises of nine chapters. The first chapter is an over view of
crystal growth and nonlinear optical phenomenon. An overview of optical materials is
discussed in the second chapter. The third chapter describing the different
characterization techniques involved in this thesis.
xiii
The fourth chapter deals with growth and characterization of
4-phenylpyridinium hydrogen squarate (4PHS) crystal. Single crystals of 4PHS have
been successfully synthesized by slow evaporation solution growth method. The
measurements from the single crystal XRD indicates that the crystal belongs to
monoclinic crystal system and its unit cell parameters have been determined. The
vibrational frequencies have been reported using FTIR technique. The presence of
carbon and protons has been confirmed from the 13C and 1H NMR analyses. It is
found that the crystal is transparent in the range of wavelength 240-2000 nm. The UV
transparency cut-off wavelength of 4PHS crystal occurs at 240 nm. The relative SHG
efficiency has been determined by Kurtz powder technique and found to be five times
greater than that of KDP. The presence of SHG exhibits the NLO property of the
grown crystal. The sharp endothermic peak at around 2600C is assigned as the melting
point of 4PHS crystal.
The fifth chapter presents the growth and characterization of glycinium
hydrogen squarate (GHS) crystal. Single crystals of glycinium hydrogen squarate
were grown by adopting the slow evaporation solution growth method using de-
ionized water as solvent at room temperature. From the single crystal XRD and
Powder XRD measurements, it is observed that the crystal belongs to monoclinic
system. The functional groups were confirmed by FTIR technique. The material has
extended its transmission greater than 90% for light with incident wavelengths from
390-1100 nm. The UV cut-off wavelength of GHS crystal occurs at 342 nm. The
chemical structure has been confirmed from 1H and 13C-NMR analysis. The relative
efficiency of SHG has been determined from Kurtz powder technique and found to be
xiv
17% of that of KDP. From the DTA/TGA curve, it is observed that the material is
stable upto 1500C, which denotes the melting point of the substance.
The sixth chapter describes the growth and characterization of L-Proline
succinate (LPS) crystal. A new non-linear optical crystal with an interesting
hydrogen bonding network that holds together the L-Proline and succinic acid
molecules was synthesized. The grown crystals were characterized by different
instrumental techniques. The dimension of the grown crystal is 8x5x7mm3. The
single crystal XRD studies proved that the grown LPS crystals belong to monoclinic
system. The presence of the functional groups of the grown crystal was confirmed by
FTIR analysis. From the UV-Vis-NIR spectrum, it is seen that the transmission is
greater than 90% for light with incident wavelengths from 204-1100 nm. The UV
transparency cut-off wavelength of LPS crystal occurs at 204 nm. The SHG study
shows that its relative efficiency which was determined by Kurtz powder technique is
found to be 23% of that of KDP crystal. The DTA and TGA studies reveal that the
crystal is thermally stable upto 1600C.
The seventh chapter deals with the cadmium manganese thiocyanate (CMTC)
crystal. A new optical crystal CMTC has been successfully synthesized and grown by
slow evaporation solution growth method at room temperature. The dimension of the
grown crystal is 30x20x30 mm3. From the XRD measurements, it has been proved
that the crystal is of tetragonal crystallographic system. The presence of functional
groups was confirmed by the FTIR techniques. The optical behaviour has been studied
using UV-Vis-NIR analysis and found that the crystal transparency is in the range
xv
from 380 to 1170nm, which highlights its prospects of application in opto-electronic
devices. The UV cut-off wavelength of the grown crystal is 380nm. The thermal
behaviour of CMTC crystal was studied by TGA/DTA analysis which confirms the
melting point of the crystal at 4300C
The eighth chapter deals about the zinc manganese thiocyanate (ZMTC)
crystal. Single crystals of ZMTC have been conveniently grown by slow evaporation
at room temperature. The measurements from the single crystal XRD indicates that
the crystal belongs to tetragonal crystal system and its unit cell parameters have been
determined. It is seen that the crystallographic data agree well in comparison with the
results of X-ray powder diffraction pattern. The absorption bands assigned to the
particular vibrations have been predicted by FTIR technique. From the recorded UV-
Vis-NIR spectrum it is observed that the crystal is transparent in the wavelength range
380-1193 nm and the UV transparency cut-off wavelength is found to occur at 380
nm. The exceptional thermal stability of ZMTC crystal is much higher than the
inorganic molecular crystals which were determined by TGA-DTA investigations.
The crystal is thermally stable upto 8060C.
The chapter nine describes the summary of the present investigation and
suggestions for the future work.
xvi
LIST OF PUBLICATIONS
1. Synthesis, growth and characterization of a new nonlinear optical material:
4-Phenylpyridinium hydrogen squarate (4PHS),
C. Ramachandra Raja, P. Paramasivam and N. Vijayan,
Spectrochimica Acta A, 69 (2008) 1146 1149.
2. Synthesis, growth and characterization of cadmium manganese thiocyanate
(CMTC) crystal
P. Paramasivam and C. Ramachandra Raja
Spectrochimica Acta A, 79 (2011) 1109 1111
3. Synthesis, growth and characterization of zinc manganese thiocyanate crystal
P. Paramasivam, M. Arivazhagan and C. Ramachandra Raja.
Indian Journal of Pure & Applied Physics, Vol.49 (June 2011) 394 397.
4. Crystallization and characterization of a new nonlinear optical crystal:
L-Proline succinate (LPS)
P.Paramasivam and C. Ramachandra Raja.
Journal of Crystallization Process and Technology, 2(2012) 21-25
Paper under review
1. Synthesis, growth and characterization of a new nonlinear optical crystal:
Glycinium hydrogen squarate (GHS)
P.Paramasivam and C. Ramachandra Raja.
Spectrochimica Acta A
xvii
CONFERENCES / SEMINARS
1. Symposium on nonlinear optical crystals and modelling in crystal growth,
February 28-March1, 2005, Department of Physics, Anna University, Chennai.
2. Growth and Characterization of a new nonlinear optical material:
4-Phenylpyridinium hydrogen squarate (4PHS)
C. Ramachandra Raja, P.Paramasivam and N. Vijayan
18th AGM, Materials Research Society of India, Theme Symposium on
Materials for Energy Generation, Conservation and Storage, February 12-14,
2007. National Physics Laboratory, New Delhi, India.
3. Synthesis, Growth and Characterization of a new nonlinear optical crystal:
Glycinium hydrogen squarate (GHS) crystal.
C. Ramachandra Raja and P.Paramasivam
International Conference on Advances in Engineering and Technology 2011,
May 27th & 28th 2011, E.G.S. Pillay Engineering College, Nagapattinam.
xviii
LIST OF TABLES
Table No Title Page No
2.1 Optical effects of nonlinear optical materials 35
4.1 Assignments of FT-IR bands observed for 4PHS crystal 86
5.1 Comparative statement of glycine, squaric acid and GHS 97
5.2 Cell parameters of GHS crystal 98
5.3 Powder XRD data of GHS crystal 99
5.4 FT-IR spectral assignments of GHS crystal 100
5.5 Chemical shift assignments of proton of GHS crystal 103
5.6 Chemical shift assignments of carbon of GHS crystal 104
6.1 FT-IR spectral assignments of LPS crystal 114
7.1 FT-IR spectral assignments of CMTC crystal 124
8.1 FT-IR spectral assignments of ZMTC crystal 131
9.1 Comparative statement of the grown crystals 136
xix
LIST OF FIGURES
Figure No Title Page No
1.1 Free energy diagram 08
2.1 Schematic diagram of SHG 37
2.2 Schematic diagram of sum frequency generation 38
2.3 Schematic diagram of difference frequency generation 39
2.4 Schematic diagram of optical parametric generator 40
3.1 Experimental setup for single crystal X- ray diffractometer 56
3.2 Schematic diagram of Guinier geometry 59
3.3 Schematic diagram of FT-IR spectrometer 63
3.4 Nuclear spin 67
3.5 Schematic diagram of NMR spectrometer 68
3.6 Schematic diagram of TGA equipment 76
3.7 Experimental setup for SHG efficiency measurement 78
4.1 Photograph of 4PHS single crystal 82
4.2 FT-IR spectrum of 4PHS crystal 85
4.3 Indication of NMR spectra analysis of 4PHS crystal 88
4.4 UV-Vis-NIR spectrum of 4PHS crystal 89
4.5 TGA / DTA curve of 4PHS crystal 91
5.1 Photograph of GHS single crystal 96
5.2 Powder X-Ray Diffraction of GHS crystal 99
xx
Figure No
Title
Page No
5.3 FT-IR Spectrum of GHS crystal 101
5.4 UV-Vis-NIR spectrum of GHS crystal 102
5.5 1H NMR spectrum of GHS crystal 104
5.5 13C NMR spectrum of GHS crystal 105
5.6 TGA/DTA curve of GHS crystal 107
6.1 Photograph of LPS single crystal 111
6.2 FT-IR spectrum of LPS crystal 113
6.3 UV-Vis-NIR spectrum of LPS crystal 115
6.4 TGA / DTA curve of LPS crystal 117
7.1 Photograph of CMTC crystal 121
7.2 FT-IR spectrum of CMTC crystal 123
7.3 UV-Vis-NIR spectrum of CMTC crystal 125
7.4 TGA / DTA curve of CMTC crystal 126
8.1 Photograph of ZMTC single crystal 129
8.2 FT-IR spectrum of ZMTC crystal 131
8.3 Optical transmission spectrum analysis 132
8.4 TGA / DTA curve of ZMTC crystal 134
xxi
LIST OF SYMBOLS
Symbols Descriptions
Angstrom Unit
G Gibbs free energy change
GV Volume excess Free energy
GS Surface excess Free energy
surface energy change per unit area
E Electric field vector
P Polarization
Linear susceptibility
2 , 3 Non linear susceptibilities
0 Permittivity of free space
OC Degree Celsius
m micrometer
nm nanometer
Frequency of incident radiation
a, b and c Cell parameters
, and Interfacial angles
cm-1 per centimeter
ns nanosecond
mJ/pulse milli Joule per pulse
MHz Mega hertz
Wavelength
xxii
LIST OF ABBREVIATIONS
Abbreviations Descriptions
NLO Nonlinear Optics
SHG Second Harmonic Generation
4PHS 4- Phenylpyridinium Hydrogen Squarate
GHS Glycinium Hydrogen Squarate
LPS L- Proline Succinate
CMTC Cadmium Manganese Thiocyanate
ZMTC Zinc Manganese Thiocyanate
AR Analytical Reagent
XRD X-ray Diffraction
UV-Vis-NIR Ultra Violet- Visible- Near Infra Red
FT-IR Fourier Transform Infrared
TGA Thermo Gravimetric Analysis
DTA Differential Thermal Analysis
KDP Potassium dihydrogen orthophosphate
Nd:YAG Neodymium: Yttrium Aluminium Garnet
CHAPTER 1
INTRODUCTION TO CRYSTAL GROWTH - AN OVERVIEW
1.1. INTRODUCTION
A short history of observations on the shapes of snow crystals in ancient
China was summarized by Kepler in 1611. During 16th 19th centuries, quartz
to sapphire crystals was used as gems and precious stones. The largest event
that showed the importance of the crystals was the invention of transistor. In
the 20th century, contributions of crystal growth in the fabrication of the
electronic and optical devices have thrown more light on the importance of
crystals. Crystal growth is an interdisciplinary subject covering physics,
chemistry, material science, electrical engineering, mineralogy, metallurgy etc.
Nowadays, crystals are produced artificially to satisfy the needs of jewelers,
science and technology.
In the past few decades, there has been a growing interest in crystal
growth process, particularly in view of the increasing demand for materials for
technological applications [1-3]. New materials are the life blood of solid state
research and device technology. New materials are not usually discovered by
device engineers or solid state theorists; they are mostly grown by crystal
growers.
An ideal crystal is one, in which the surroundings of any atom would be
exactly the same as the surroundings of every similar atom. Real crystals are
2
finite and contain defects. However, single crystals are solids in the most
uniform condition that can be attained and this is the basis for most of the uses
of these crystals. The uniformity of single crystals can allow the transmission
without the scattering of electromagnetic waves. The strong influence of single
crystals in the present day technology led to the recent development and
advancement in the fields of semiconductors, solid state lasers, ultrasonic
amplifiers, infrared detectors, transducers, nonlinear optic, piezoelectric,
photosensitive materials, thin films and computer industries.
All these developments could be achieved due to the availability of
single crystals like silicon, germanium, gallium arsenide and also with the
invention of nonlinear optical properties in some inorganic, semi-organic and
organic crystals. The desired physical phenomena for the fabrication of
devices are exhibited only by certain single crystals. Hence in order to achieve
high performance, good quality single crystals are needed.
Therefore, researchers worldwide have always been in the search of
new materials through their single crystal growth. The methods of growing
crystals are very wide and mainly dictated by the characteristics of the material
and its size [4-5]. In this chapter, the fundamentals of the various methods to
grow quality single crystals and, in particular, the solution growth method is
discussed.
3
1.2. NUCLEATION
Nucleation is an important event in crystal growth. A comprehensive
study on the growth of crystals should start from an understanding of
nucleation process [6]. Nucleation is the physical reaction which occurs when
components in a solution start to precipitate out forming nuclei which attracts
more precipitate. In a supersaturated or super-cooled system when a few atoms
or molecules join together, a change in energy takes place in the process of
formation of the cluster. The cluster of such atoms or molecules is termed
embryo. An embryo may grow or disintegrate and disappear completely. If
the embryo grows to a particular size, critical size known as critical-nucleus,
then greater is the possibility for the nucleus to grow into a crystal. There are
four stages involved in the formation of stable nucleus:
(a) The first stage is the development of supersaturation:
Supersaturation may be attained due to a chemical reaction, changes
in temperature, pressure or any other physical or chemical condition.
(b) The second stage is the generation of embryo:
The formation of embryo may be either homogeneous (the atoms or
molecules build themselves in the interior of the parent system) or
heterogeneous (the molecules build up on dust particles or on the
surface of the container or any other imperfections).
(c) The third stage is the growth of the embryo from the unstable critical
state to stable state.
4
(d) The fourth stage is the relaxation process, where, the texture of the
new born nucleus changes.
1.2.1. Kinds of Nucleation
Nucleation is broadly classified into two types. These two types are
primary and secondary nucleation. The former occurs either spontaneously or
induced artificially.
The primary nucleation is further divided into homogeneous and
heterogeneous nucleation. The spontaneous formation of crystalline nuclei
within the interior of parent phase is called homogeneous nucleation. The
formation of nuclei in the bulk of supersaturated system is a comparatively rare
occurrence; it gives the basic principles for understanding the numerous
processes in science and technology as well as in nature where phase
transitions are involved. On the other hand, if the nuclei form heterogeneously
around ions, impurity molecules or on dust particles or on the surface of the
container or at structural singularities such as dislocation or imperfection, it is
called heterogeneous nucleation.
If the nuclei are generated in the vicinity of crystals present in
supersaturated system, then this phenomenon is often referred to as secondary
nucleation [7]. Nucleation can often be induced by external influence like
agitation, mechanical shock, friction, spark, extreme pressure, electric and
magnetic fields, UV - rays, X - rays, gamma rays and so on.
5
1.2.2. Classical Theory of Nucleation
The formation of the crystal nuclei is a difficult and complex process,
because the constituent atoms or molecules in the system have to be oriented
into a fixed lattice. In practice, a number of atoms or molecules may come
together to form an ordinary cluster of molecules known as embryo. The
energetic considerations show that this embryo is likely to re-dissolve unless it
reaches a certain critical size. If it does not dissolve it means that the assembly
is stable under the prevailing conditions.
1.2.3 Kinetic Theory of Nucleation
The main aim of the nucleation theory is to calculate the rate of
nucleation. Rate of nucleation is nothing but the number of critical nuclei
formed per unit time per unit volume. In kinetic theory, nucleation is treated as
the chain reaction of monomolecular addition to the cluster and ultimately
reaching macroscopic dimensions.
Two monomers collide with one another to form a dimer. A monomer
joins with a dimer to form a trimer. This reaction builds a cluster having
i-molecules known as i-mer. As the time increases, the size distribution in the
embryos changes and larger ones increases in size. As the size attains a critical
size Aj*, further growth into macroscopic size is guaranteed, and there is also a
possibility for the reverse reaction i.e., the decay of a cluster into monomers.
6
The reaction is represented as follows:
A1 + A1 A2
A2 + A1 A3
Ai-1 + A1 Ai
Ai + A1 Ai+1
Aj-1 + A1 Aj*
1.3. STABILITY OF NUCLEUS
The total free energy of a crystal in equilibrium with its surrounding at
constant temperature and pressure would be a minimum for a given volume [7].
Since the volume free energy per unit volume is a constant, then
ai i = minimum 1.1
where ai - area of ith face and
i - surface energy per unit area
7
1.4. ENERGY FORMATION OF SPHERICAL NUCLEUS
Energy is quite essential for the creation of a new phase. When a droplet
nucleus forms due to supersaturation of vapour, certain quantity of energy is
spent in the creation of a new phase. The free energy change associated with
the formation of a nucleus can be written as
G = GS + GV 1.2
G can be represented as a combination of surface excess free energy
(GS) and volume excess free energy (GV). For the spherical nucleus,
G = 4 r2 + 4/3 r3Gv 1.3
Where Gv is the free energy change per unit volume which is a
negative quantity and is the surface energy change per unit area. The
quantities G, GS and GV are represented in Fig. 1.1.
The surface excess free energy increases with r2 and the volume
excess free energy GV decreases with r3. So, the net free energy change
increases with the increase in size, attains the maximum and then decreases for
further increase in the size of nucleus.
The size corresponding to the maximum free energy change is called
critical nucleus. The radius of the critical nucleus is obtained by setting the
condition,
8
Fig. 1.1.
Free energy diagram
Surface Term GS
GV Volume Term
G*
G
r* Radius
9
i.e. 0=dr
Gd
when r = r*(radius of critical nucleus)
r* = -2GV
1.4
The free energy change associated with the formation of critical nucleus
can be estimated by substituting equation 1.4 in equation 1.3.
G* = 163 / 3 Gv2 1.5
In terms of r* the above equation can be written as,
G* = 4/3
G* = 1/3 S. 1.6
Where, S is the surface area of the critical nucleus. The crucial
parameter between a growing crystal and the surrounding mother liquid is the
interfacial tension (). Interfacial tension is a measurement of the excess energy
present at an interface arising from the imbalance of forces between molecules
at an interface (gas/liquid, liquid/liquid, gas/solid, and liquid/solid). This
complex parameter can be determined by conducting the nucleation
experiments. The significant nucleation parameters have been estimated for
10
KTP and LAP crystals, which are grown from high temperature and low
temperature solutions respectively [8-9].
Though the present phase is at constant temperature and pressure, there
will be variation in the energies of the molecules. The molecules having higher
energies temporarily favour the formation of the nucleus. The rate of nucleation
can be given by Arrhenius reaction [10] which is a velocity equation since the
nucleation process is basically a thermally activated process. The nucleation
rate J is given by
=
KT*G AexpJ 1.7
where, A- pre-exponential constant
K- Boltzmann constant
T- absolute temperature
1.5. SUPERSATURATION AND ITS EXPRESSION
The concentration of the solution is more than the equilibrium
concentration is called super saturation. In supersaturation the solution has
exceeded its solubility limit. In order to grow crystals, the solution must be
supersaturated; the concentration of the solution is more than the equilibrium
concentration. Usually the concentration is defined as the mass of the solute
11
dissolved in one litre of the solvent. Supersaturation is the driving force, which
controls the rate of crystal growth.
The driving force *CCC =
where C is the concentration of the dissolved substance
C* is the solubility limit
The supersaturation ratio (S) is defined as the ratio between the
concentration of the dissolved substance and the solubility limit.
*CCS =
1.6. CLASSIFICATION OF CRYSTAL GROWTH
Crystal growth is a controlled phase transformation, either from solid or
liquid or gaseous phase to solid phase. The choice of a particular method for
growing a desired single crystal critically depends on the physical and chemical
properties of the substances. The consistency in the characteristics of devices
fabricated from the crystals depends mainly on the homogeneity and defect
present in the crystals. Thus, the process of producing single crystals, from
homogeneous media with directional properties, attracts more attention and
gains more importance than any other process. The method of crystal growth
may range from a small inexpensive technique to a complex sophisticated
technique. The basic methods of growing single crystals are:
12
(a) growth from melt
(b) growth from vapour
(c) growth from solution
The basic methods to grow single crystals have been discussed in detail by
several authors [2, 11-12]. In the solid growth of crystals, the important factor
is conversion of a polycrystalline piece of a material into a single crystal by
causing the grain boundaries to sweep through and pushed out of the crystal
[13]. The basic methods of growing single crystals are explained below.
1.6.1. Growth from Melt
A gas is cooled until it becomes a liquid, which is then cooled further
until it becomes a solid. Polycrystalline solids are typically produced by this
method unless special techniques are employed. In any case, the temperature
must be controlled carefully. Knowledge of how crystals grow from the melt
and the effects of the various factors which may influence crystal growth is a
potentially important tool in interpreting textural and chemical features and
crystallization histories of igneous rocks.
The first detailed study of crystal-growth phenomena was explained by
Tamman (1899), who measured the rate of crystal growth from a melt. He
found that the rate is zero at the liquid state, increases to a maximum, and then
decreases with decrease in temperature. Later in the year 1931, Volmer and
Marder developed a simple theory to explain this relationship. Depending on
13
the thermal characteristics, the following techniques are employed for the
crystal growth:
(a) Czochralski technique
(b) Bridgman technique
(c) Kyropoulos technique
(d) Zone melting technique
(e) Verneuil technique
Large crystals can be grown rapidly from the liquid elements using a
popular method invented in 1918 by the Polish scientist Jan Czochralski [14].
One attaches a seed crystal to the bottom of a vertical arm such that the seed is
barely in contact with the material at the surface of the melt.
The arm is raised slowly, and a crystal grows underneath at the interface
between the crystal and the melt. Usually the crystal is rotated slowly, so that
inhomogeneities in the liquid are not replicated in the crystal. Large diameter
crystals of silicon are grown in this way for use as computer chips. Based on
measurements of the weight of the crystal during the pulling process, computer
controlled apparatus can vary the pulling rate to produce any desired diameter.
Crystal pulling is the least expensive way to grow large amounts of pure
crystal. Synthetic sapphire crystals can be pulled from molten alumina. Special
care is required to grow binary and other multi-component crystals; the
temperature must be precisely controlled because such crystals may be grown
14
only at a single, extremely high temperature. The melt has a tendency to be
inhomogeneous, since the two liquids may try to separate by gravity.
The Bridgman method [15-16] is also widely used for growing large
single crystals. The molten material is put into a crucible, often of silica, which
has a cylindrical shape with a conical lower end. Heaters maintain the molten
state. As the crucible is slowly lowered into a cooler region, a crystal starts
growing in the conical tip. The crucible is lowered at a rate that matches the
growth of the crystal, so that the temperature at the interface between crystal
and melt is always same. The rate of moving the crucible depends on the
temperature and the material. Then, the entire molten material in the crucible
grows into a single large crystal. One disadvantage of this method is that,
impurities are pushed out of the crystal during growth. A layer of impurities
grows at the interface between melt and solid as this surface moves up the melt,
and the impurities become concentrated in the higher part of the crystal.
In Kyropoulos technique, the crystal is grown in a large diameter. As in
the Czochralski method, here also the seed is brought in contact with the melt
and is not raised much during the growth, i.e. part of the seed is allowed to melt
and a short narrow neck is grown. After this, the vertical motion of the seed is
stopped and growth proceeds. The major use of this method is the growth of
alkali halides to make optical components.
15
Zone refining was developed by William Gardner Pfann [17] in Bell
Labs as a method to prepare high purity materials for manufacturing transistors.
In the zone melting technique, the feed material is formed into a mass by heat
and pressure then the seed is attached to one end. A small molten zone is
maintained by surface tension between the seed and the feed. The zone is
slowly moved towards the feed. Single crystal is obtained over the seed. This
method is applied to materials having large surface tension. The main reasons
for the impact of zone refining process to modern electronic industry are the
simplicity of the process, the capability to produce a variety of organic and
inorganic materials of extreme high purity, and to produce dislocation free
crystal with a low defect density.
In the Verneuil technique, a fine dry powder of size 1-20 microns of the
material to be grown is shaken through the wire mesh and allowed to fall
through the oxy-hydrogen flame. The powder melts and a film of liquid is
formed on the top of the seed crystal. This freezes progressively as the seed
crystal is slowly lowered. The art of the method is to balance the rate of charge
feed and the rate of lowering of the seed to maintain a constant growth rate and
diameter. This technique is widely used for the growth of synthetic gems.
1.6.2. Growth from Vapour
Crystals can be grown from vapour when the molecules of the gas attach
themselves to a surface and move into the crystal arrangement. Several
16
important conditions must be met for this to occur. At constant temperature and
equilibrium conditions, the average number of molecules in the gas and solid
states is constant; molecules leave the gas and attach to the surface at the same
rate that they leave the surface to become gas molecules [18].
For crystals to grow, the gas solid chemical system must be in a non-
equilibrium state such that there are too many gaseous molecules for the
conditions of pressure and temperature. This state is called supersaturation.
Molecules are more prone to leave the gas than to rejoin it, so they get
deposited on the surface of the container. Supersaturation can be induced by
maintaining the crystal at a lower temperature than the gas. A critical stage in
the growth of a crystal is seeding, in which a small piece of crystal of proper
structure and orientation, called a seed, is introduced into the container. The
gas molecules find the seed a more favourable surface than the walls and
preferentially deposit there. Once the molecule is on the surface of the seed, it
wanders around this surface to find the preferred site for attachment. Growth
proceeds as one molecule at a time and one layer at a time. The process is slow;
it takes days to grow a small crystal. The advantage of vapour growth is that
very pure crystals can be grown by this method, while the disadvantage is that
it is slow.
Most clouds in the atmosphere are ice crystals that form by vapour
growth from water molecules. In the laboratory, vapour growth is usually
accomplished by sending a supersaturated gas over a seed crystal. Quite often a
17
chemical reaction at the surface is needed to deposit the atoms. Crystals of
silicon can be grown by moving chlorosilane (SiCl4) and hydrogen (H2) over a
seed crystal of silicon. Hydrogen acts as the buffer gas by controlling the
temperature and rate of flow. The molecules dissociate on the surface in a
chemical reaction that forms hydrogen chloride (HCl) molecules. Hydrogen
chloride molecules leave the surface, while silicon atoms remain to grow into a
crystal. Binary crystals such as gallium arsenide (GaAs) are grown by a similar
method.
1.6.3. Growth from Solution
The essential technique which produces large single crystals suitable for
lot of applications at minimum cost is vital for research and commercial
purpose. The selection of growth method is also important because it suggests
the possible impurity and other defect concentrations to improve the physical
and chemical properties of the material.
The crystal growth from solution falls into:
(a) gel growth
(b) flux growth
(c) hydrothermal growth
(d) low temperature solution growth
18
Growth of crystals from solution is an important process that can be
used in laboratory, industry, research and development. In order to grow good
quality single crystals by solution growth, the material should have high
solubility and variation in solubility with temperature. The viscosity of the
solvent solute system should be low [19]. The materials used for the growth of
crystal must not be a flammable one. Another aspect to consider is that the
container and stirrer should be non-reactive with material.
Among the various methods, growth from solution at low temperature
occupies a prominent place owing to its versatility, simplicity and used to
produce technically important crystals. Growth from solution at low
temperature occurs close to equilibrium conditions and hence good quality bulk
single crystals of utmost perfection can be grown easily.
1.7. GEL GROWTH
It is an alternative technique to solution growth with controlled diffusion
and the growth process is free from convection. Gel is a two component system
of a semisolid rich in liquid and inert in nature. The material, which
decomposes before melting, can be grown in this medium by counter diffusing
two suitable reactants. Crystals with dimensions of several millimeters can be
grown in a period of 3 to 4 weeks. The crystals grown by this technique have
high degree of perfection and fewer defects since the growth takes place at
room temperature.
19
1.8. HYDROTHERMAL GROWTH
Hydrothermal implies conditions of high pressure as well as high
temperature. Substances like calcite, quartz is considered to be insoluble in
water but at high temperature and pressure, these substances are soluble. This
method of crystal growth at high temperature and pressure is known as
hydrothermal method. Temperatures are typically in the range of 400C to
600C and the pressure involved is high.
Growth is usually carried out in steel autoclaves with gold or silver
linings. Depending on the pressure the autoclaves are grouped into low,
medium and high-pressure autoclaves. The concentration gradient required to
produce growth is provided by a temperature difference between the nutrient
and growth areas. The requirement of high pressure presents practical
difficulties and there are only a few crystals of good quality and large
dimensions are grown by this technique. Quartz is the outstanding example of
industrial hydrothermal crystallization. One serious disadvantage of this
technique is the frequent incorporation of OH-
ions into the crystal, which
makes them unsuitable for many applications.
1.9. FLUX GROWTH
In this method of crystal growth, the components of the desired
substance are dissolved in a solvent (flux). The method is particularly suitable
for crystals needing to be free from thermal strain and it takes place in a
20
crucible made of non reactive metals. Crucibles are normally sealed in
evacuated quartz ampoules or reactions take place in controlled atmosphere
furnaces. A saturated solution is prepared by keeping the constituents of the
desired crystal and the flux at a temperature slightly above the saturation
temperature long enough to form a complete solution. Then the crucible is
cooled in order to cause the desired crystal to precipitate. Nucleation happens
in the cooler part of the crucible. A disadvantage is that most flux method
syntheses produce relatively small crystals.
1.10. LOW TEMPERATURE SOLUTION GROWTH
In the present investigation, the low temperature solution growth
technique is employed and the fundamentals of the same are given below:
Solubility and supersaturation are the two important parameters for the
solution growth process. Solubility is defined as the maximum amount of
substance dissolved in a particular solvent at a given temperature. Before
starting the solution growth process, the solubility of the solute must be
determined by dissolving the solute in the solvent at a constant temperature
with continuous stirring. Solubility of the substance increases with increase in
temperature for most of the materials. Either by cooling or evaporating the
solvent, the solution attains its supersaturation. The solution is said to be in
supersaturated state, if the concentration of the solution is greater than the
equilibrium concentration.
21
When the starting materials are unstable at high temperatures, low
temperature solution growth is the most widely used method for the growth of
crystals [20]. The supersaturation is achieved either by temperature lowering
or by solvent evaporation. This method is widely used to grow bulk crystals,
from materials, which have high solubility and have variation in solubility with
temperature [21-22].
Growth of crystals from solution at room temperature has many
advantages over other growth methods. But the rate of crystallization is slow in
this method. Since growth is carried out at room temperature, the structural
imperfections in the grown crystals are relatively low [23]. The ambient
temperature of growth, the pH of the solution and the presence of deliberately
added impurities are the essential additional parameters that determine the rate
of growth and morphology of the crystal. Low temperature solution growth
(LTSG) can be subdivided into the following categories:
(a) slow cooling method
(b) slow evaporation method
(c) temperature gradient method
1.10.1. Slow Cooling Method
Slow cooling method is one in which the solution is allowed to cool to a
lower temperature in order to achieve supersaturated solution and the
temperature of the solution is reduced in small steps. By doing so, the solution
22
which is just saturated at the initial temperature will become a supersaturated
solution. Once supersaturation is achieved, growth of single crystal is possible.
The main disadvantage of slow cooling method is the need to use a range of
temperature. The temperature at which such crystallization can begin is usually
within the range 45-75C and the lower limit of cooling is the room
temperature. Wide range of temperature may not be desirable because the
properties of the grown crystal may vary with temperature. Even though this
method has technical difficulty of requiring a programmable temperature
control, it is widely used with great success. The crystals produced by this
method are small and possess unpredictable shape.
1.10.2. Temperature Gradient Method
This method involves the transport of the materials from hot region to a
cooler region, where the solution is supersaturated and the crystal grows. The
advantages of this method are that the crystal is grown at fixed temperature,
this method is insensitive to changes in temperature (provided both the source
and the growing crystal undergo the same change) and the cost of the basic
materials are low. On the other hand, small changes in temperature difference
between the source and the crystal zones have a large effect on the growth rate.
1.10.3. Slow Evaporation Method
In this process the temperature of the solution is not changed, but the
solution is allowed to evaporate slowly. When the solvent begins to evaporate,
23
the concentration of solute is increased and, therefore, supersaturation is
achieved. The advantage of using this method is that the crystals grow at a
fixed temperature. This method can effectively be used for materials having
very low temperature coefficient of solubility. But inadequacies of the
temperature control system still have a major effect on the growth rate. In
order to control the temperature of the system, constant temperature bath can
be used. In spite of some of the disadvantages, this method is simple and
convenient to grow bulk single crystals.
1.11. CRITERIA FOR OPTIMIZING SOLUTION GROWTH
The growth of good quality single crystals requires optimized
conditions; this may be achieved with the help of the following criteria:
(a) material purification
(b) solvent selection
(c) solubility
(d) solution preparation
(e) crystal habit
1.11.1. Material Purification
Availability of the material with highest purity is an essential
requirement for success in crystal growth. The impurity included into crystal
lattice may lead to the formation of flaws and defects. Some times, impurities
24
may slowdown the crystallization process. To harvest good quality crystals,
material purification is a must. A careful repetitive use of standard purification
methods of re-crystallization followed by filtration of the solution would
increase the level of purity.
1.11.2. Solvent Selection
Solution is a homogeneous mixture of a solute in a solvent. Solute is the
component present in a smaller quantity. For a given solute, there may be
different solvents. Apart from high purity starting materials, solution growth
requires a good solvent. The solvent must be chosen taking into account the
following factors:
high solubility for the given solute
low viscosity
low volatility
low corrosion
low cost
high purity
1.11.3. Solubility
Solubility is an important parameter which dictates the growth
procedure. If the solubility is too high, it is difficult to grow bulk crystals and
too low solubility restricts the size and growth of bulk crystals. Hence
25
solubility of the solute in the chosen solvent must be determined before starting
the growth process [24].
1.11.4. Solution Preparation and Crystal Growth
After selecting the desirable solvent with high purity solute to be
crystallized, the next important part is preparation of the saturated solution. To
prepare a saturated solution, it is necessary to have an accurate solubility-
temperature data of the material. The saturated solution at a given temperature
is placed in the constant temperature bath. Wattman filter papers are used for
solution filtration. The filtered solution is transferred to crystal growth vessel
and the vessel is sealed by polythene paper in which 1520 holes are made for
slow evaporation. Then the crystallization is allowed to take place by slow
evaporation at room temperature or at a higher temperature in a constant
temperature bath. As a result of slow evaporation of solvent, the excess of
solute which has got deposited in the crystal growth vessel results in the
formation of crystals.
1.11.5. Crystal Habit
The growth of a crystal at approximately equivalent rates along all the
directions is a prerequisite for its accurate characterization. This will result in a
large bulk crystal. Such large crystals should also be devoid of dislocation and
other defects. These imperfections become isolated into defective regions
surrounded by large volumes of high perfection. In the crystals the
26
imperfections grow as needles or plates, the growth dislocations propagate
along the principle growth directions and the crystals remain imperfect [20].
Change of habit in such crystals which naturally grow as needles or plates can
be achieved by any one of the following ways:
changing the temperature of the growth
changing the pH of the solution
adding a habit modifying agent
changing the solvent
Achievement of the above parameters is of great industrial importance,
where such morphological changes are induced during crystallization to yield
crystals with better perfection and packing characteristics.
1.12. ADVANTAGES OF LOW TEMPERATURE SOLUTION
GROWTH TECHNIQUE
Low temperature solution growth is utilized for crystal growth due to its
simplicity and versatility. Following are the important advantages of using low
temperature solution growth technique:
(a) simple growth apparatus
(b) growth of strain and dislocation free crystals
(c) permits the growth of prismatic crystals by varying the growth
conditions
27
(d) this is the only method which can be used for substances that undergo
decomposition before melting
Following are the disadvantages of this technique:
(a) the growth substance should not react with solvent
(b) this method is applicable for substances fairly soluble in a solvent
(c) inclusions of solvent may present in the grown crystal
(d) growth rate of this method is low
28
CHAPTER 2
AN OVERVIEW OF OPTICAL MATERIALS
2.1. INTRODUCTION
An optical material is one which is transparent to light or to infrared,
ultraviolet, or X-ray radiation, such as glass, certain single crystals,
polycrystalline materials, and plastics. All substances used in the construction
of devices or instruments whose function is to alter or control electromagnetic
radiation in the ultraviolet, visible, or infrared spectral regions. Optical
materials are fabricated into optical elements such as lenses, mirrors, windows,
prisms, polarizers, detectors, and modulators. These materials serve to refract,
reflect, transmit, disperse, polarize, detect, and transform light. The term
light refers here not only to visible light but also to radiation in the adjoining
ultraviolet and infrared spectral regions. At the microscopic level, atoms and
their electronic configurations in the material interact with the electromagnetic
radiation (photons) to determine the material's macroscopic optical properties
such as transmission and refraction. These optical properties are functions of
the wavelength of the incident light, the temperature of the material, the applied
pressure on the material, and in certain instances the external electric and
magnetic fields applied to the material.
The ability to focus the optical field to deeply sub-wavelength
dimensions opens the door to an entirely new class of photonic devices. If one
29
could combine the imaging powers of X-ray wavelengths with the economy
and maturity of visible light sources, one could greatly broaden the practical
engineering toolbox. Imagine focusing visible photons to spatial dimensions
less than ten nanometers. By doing so, electron beam microscopy is
immediately displaced by optical microscopy, replacing expensive electron
beam sources with inexpensive visible lasers. Beyond simple economics,
though, this achievement would extend the range of nanometer scale
microscopy to living biological samples and highly insulating surfaces.
There is a wide range of substances that are useful as optical materials.
Most optical elements are fabricated from glass, crystalline materials,
polymers, or plastic materials. In the choice of a material, the most important
properties are often the degree of transparency and the refractive index, along
with each property's spectral dependency. The uniformity of the material,
temperature limits, hygroscopicity, chemical resistivity, and availability of
suitable coatings should also be considered. Fused silica, which transmits to
about 180 nm, is well suited for the lithography in the ultraviolet region.
However, the crystalline material calcium fluoride, which transmits into the
ultraviolet region to about 140 nm, outperforms any glass in printing
microchips using fluorine excimer lasers. Deep-ultraviolet applications of
fused-silica glasses include high-energy lasers, spacecraft windows, blanks for
large astronomical mirrors, optical imaging, and cancer detection using
ultraviolet-laser-induced autofluorescence.
30
The need for an inexpensive, unbreakable lens that could be easily mass-
produced precipitated the introduction of plastic optics in the mid-1930s.
Although the variety of plastics suitable for precision optics is limited
compared to glass or crystalline materials, plastics are often preferred when
difficult or unusual shapes, lightweight elements, or economical mass-
production techniques are required. The softness, inhomogeneity, and
susceptibility to abrasion intrinsic to plastics often restrict their application.
Haze (which is the light scattering due to microscopic defects) and
birefringence (resulting from stresses) are inherent to plastics. Plastics also
exhibit large variations in the refractive index with changes in temperature.
Shrinkage resulting during the processing must be considered.
2.2 IMPORTANCE OF CRYSTALS AS OPTICAL MATERIALS
Although most of the early improvements in optical devices were due to
advancements in the production of glasses, the crystalline state has taken on
increasing importance. Synthetic crystal-growing techniques have made
available single crystals such as lithium fluoride (of special value in the
ultraviolet region, since it transmits at wavelengths down to about 120 nm),
calcium fluoride, and potassium bromide (useful as a prism at wavelengths up
to about 25 m in the infrared). Many alkali-halide crystals are important
because they transmit into the far-infrared. Single crystals are indispensable for
transforming, amplifying, and modulating light. Birefringent crystals serve as
31
retarders, or wave plates, which are used to convert the polarization state of the
light. In many cases, it is desirable that the crystals not only be birefringent, but
also behave nonlinearly when exposed to very large fields such as those
generated by intense laser beams. A few examples of such nonlinear crystals
are ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate
(KDP), beta barium borate (BBO), lithium borate (LBO), and potassium titanyl
phosphate (KTP).
2.3 NONLINEAR OPTICAL MATERIALS
Optics is the study of interaction of electromagnetic radiation and
matter. Nonlinear optics is the study of the phenomena that occurs as a
consequence of the modification of optical properties of a material system by
the presence of light [25-26]. Nonlinear optics (NLO) has been an active field
of research since the late 1960s with the advent of lasers followed by the
demonstration of harmonic generation in quartz [27]. Nonlinear optics extends
the usefulness of lasers by increasing the number of wavelengths available.
Nonlinear optical material is the medium on which a laser beam interacts. After
the invention of laser, frequency conversion by nonlinear optical materials has
become an important and widely used technique.
Nonlinear optics is the study of the interaction of intense
electromagnetic field with materials to produce modified fields that are
32
different from the input field in phase and frequency. Nonlinear optics is
completely a new effect in which the light of one wavelength is transformed to
the light of another wavelength.
In a linear material, electrons are bound inside a potential well, which
acts like a spring, holding the electrons to lattice point in the crystal. If an
external force pulls an electron away from its equilibrium position the spring
pulls it back with a force proportional to the displacement. The springs
restoring force increases linearly with the electron displacement from its
equilibrium position. In an ordinary optical material, the electrons oscillate
about their equilibrium position at the same frequency of the electric field (E).
Hence, these electrons in the crystal generate light at the frequency of the
original light wave.
In the nonlinear material, if the light passing through the material is
intense enough, its electric field can pull the electrons so far that they reach the
end of their springs. The restoring force is no longer proportional to the
displacement and then it becomes nonlinear. The electrons are jerked back
rather than pulled back and they oscillate at frequencies other than the driving
frequency of the light wave. So, the electrons radiate at the new frequencies,
generating the new wavelength of light [28].
Nonlinear optics is now established as an alternative field to electronics
for the future photonic technologies. The fast-growing development in optical
33
fiber communication systems has stimulated the search for new, highly
nonlinear materials capable of fast and efficient processing of optical signals.
In recent years, many significant achievements have been realized in this field
because of the development of new nonlinear optical organic, semi-organic and
inorganic materials. Among the nonlinear crystals studied so-far, only a few
crystals satisfy the major requirements. For the development of new
technologies, the emergence of new nonlinear materials with superior quality is
needed.
2.4. THEORETICAL EXPLANATION OF NONLINEAR OPTICS
When a beam of electromagnetic radiation propagates through a solid,
the nuclei and associated electrons of the atoms create electric dipoles. The
electromagnetic radiation interacts with these dipoles causing them to oscillate,
which by the classical laws of electromagnetism, results in the dipoles
themselves acting as sources of electromagnetic radiation. If the amplitude of
vibration is small, the dipoles emit radiation of the same frequency as the
incident radiation.
As the intensity of the incident radiation increases, the relationship
between irradiance and amplitude of vibration becomes nonlinear resulting in
the generation of higher harmonics in the frequency of radiation emitted by the
oscillating dipoles. Thus, frequency doubling or second harmonic generation
34
(SHG) and, indeed, higher order frequency effects occur as the incident
intensity is increased.
In a nonlinear medium, the induced polarization is a nonlinear function
of the applied electric field. A medium exhibiting SHG is composed of
molecules with asymmetric charge distributions arranged in the medium in
such a way that a polar orientation is maintained throughout the crystal.
At very low fields, the induced polarization is directly proportional to
the electric field [3].
P = 0 . E 2.1
Where is the linear susceptibility of the material, E is the electric
field vector, 0 is the permittivity of free space.At high fields, polarization
becomes independent of the electric field and the susceptibility becomes field
dependent. Therefore, this nonlinear response is expressed by writing the
induced polarization as a power series in the fields.
P = 0 1 E + 2 E2 + 3 E3 + . 2.2
35
Table 2.1
Optical effects of linear and nonlinear optical materials
Order Susceptibility Optical Effects Applications
Linear effect
1 1 Refraction
Absorption
Transmission
Optical fibers
Colour Filter
Photolithography
Nonlinear effect
2 2 SHG
(=2) Frequency doubling
Frequency mixing
(1 2 =3) Optical parametric oscillations
Pockels effect
( + 0 =) Electro optical modulators
3 3 Four wave mixing Raman coherent spectroscopy
Phase gratings Real time holography
Kerr effect Ultra high speed optical gates
Optical amplitude Amplifiers, Choppers etc.
36
Where 2 , 3 .. are the nonlinear susceptibility of the medium. 1 is the
term responsible for materials linear optical properties like, refractive index,
dispersion, birefringence and absorption. 2 is the quadratic term which
describes second harmonic generation in non-centrosymmetric materials. 3 is
the cubic term responsible for third harmonic generation, stimulated Raman
scattering, phase conjugation and optical instability. Hence the induced
polarization is capable of multiplying the fundamental frequency to second,
third and even higher harmonics. The co-efficient of 1, 2, and 3 produces
certain optical effects, which are listed in Table 2.1.
2.5. VARIOUS TYPES OF NLO EFFECTS
Some nonlinear optical processes are familiar to physicists, chemists and
other scientists because they are in common use in the laboratories. Second
harmonic generation is a nonlinear optical process that results in the conversion
of an input optical wave into an output wave of twice as that of the input
frequency. The process occurs within a nonlinear medium, usually a crystal
(KDP-Potassium Dihydrogen Phosphate, KTP-Potassium Titanyl Phosphate,
etc.). Such frequency doubling processes are commonly used to produce green
light (532nm) using, a Nd:YAG (Neodymium:Yttrium Aluminum Garnet) laser
operating at 1064 nm [29]. Some of the NLO processes are given below:
(a) second harmonic generation
(b) sum frequency generation
37
(c) difference frequency generation
(d) optical parametric generation
(e) linear electro optic effect or Pockels effect
(f) optical rectification
2.5.1 Second Harmonic Generation (SHG)
The process of transformation of light with frequency into light with
double frequency 2 and half the wavelength (Fig. 2.1) is referred to second
harmonic generation. The process is spontaneous and involves three photon
transitions. Second harmonic generation has been of practical interest ever
since after it was demonstrated because of its efficient conversion from
fundamental to second harmonic frequencies. This can be achieved by the
available powerful sources of coherent radiation at higher to unattainable
wavelengths [30].
Fig. 2.1
Schematic diagram of SHG
The most extensively studied conversion process of all has been the
doubling of the 1.064 m line obtained from the neodymium ion in various
NLO crystal
2
38
hosts. In particular, the doubling of the continuous wave Nd:YAG laser source
has recently been the subject of intensive study, because the laser light itself is
efficient and powerful so that the green light obtained by doubling is well
placed spectrally for detection by photomultipliers.
2.5.2. Sum Frequency Generation
It is a nonlinear optical process. Crystal materials with inversion
symmetry can exhibit nonlinearity. In such NLO materials the sum frequency
generation can occur. Fig. 2.2 illustrates the sum frequency generation.
1 + 2 = 3 2.3
When two electromagnetic waves with the frequency 1 and 2 interact
in a NLO medium, a nonlinear polarizability can be induced. The NLO
material generates an optical wave of frequency 3 which is equal to the sum of
the two input wave frequency 1 and 2. The energy of output wave is
represented in the equation 2.3.
Fig. 2.2
Schematic diagram of sum frequency generation
NLO crystal 1
2 3 = 1 +2
39
2.5.3. Difference Frequency Generation
The process of difference-frequency generation is described by the
following equation 2.4.
1 - 2 = 3 2.4
Fig. 2.3
Schematic diagram of difference frequency generation
Fig. 2.3 illustrates the difference frequency generation. Here the
frequency of the generated wave is the difference of those of the input
frequencies.
2.5.4. Optical Parametric Generation
Optical parametric generation (Fig. 2.4) is an inverse process of sum
frequency generation and described by the following equation 2.5. It splits one
high-frequency photon (pumping wavelength p) into two low-frequency
photons (signal wavelength s and idler wavelength i)
s + i = p 2.5
NLO crystal 1
2 3 = 1 - 2
40
Fig. 2.4
Schematic diagram of optical parametric generation
2.5.5. Linear Electro Optic Effect
The Pockels effect is a linear change in the refractive index of a
medium in the presence of an external electric field. Here a dc field is applied
to a medium through which an optical wave propagates. The change in the
polarization due to the presence of these two interacting field components
effectively alters the refractive index of the medium.
2.5.6. Optical Rectification
The optical rectification is defined as the ability to induce a dc voltage
between the electrodes placed on the surface of the crystal when an intense
laser beam is directed into the crystal.
2.6. NONLINEAR OPTICAL MATERIALS
The search for new and efficient materials has been very active since
second harmonic generation (SHG) was first observed in single crystal quartz
[27]. The discovery of inorganic photorefractive crystals such as potassium
NLO crystal
p s
i
41
niobate (KNbO3), potassium dihydrogen phosphate (KH2PO4), barium titanate
(BaTiO3), lithium niobate (LiNbO3) and their optimization during the last thirty
five years have led to numerous demonstration of variety of optical
applications.
At the end of 1968, Kurtz and Perry SHG method was introduced and a
powdered sample is irradiated with a laser beam and scattered light is collected
and analyzed for its SHG efficiency. So, the stage was set for a rapid
introduction of new materials, both inorganic and organic [31]. For the optical
applications, a non linear material should have the following requirements [3]:
(a) wide optical transparency range
(b) ease of fabrication and high nonlinearity
(c) high laser damage threshold
(d) ability to process into crystals and thin films
(e) good environmental stability
(f) fast optical response time
(g) high mechanical and thermal stability
2.7. DEVELOPMENT OF NLO MATERIALS
In recent years, the extensive investigations carried out on NLO
materials have been very much helpful to identify different types of NLO
crystals. New techniques applied to the fabrication of ultra glass that enabled
the fabrication of fibers with ultra-low loss, provided the main stimulus to
42
optical fiber communication. The recent emergence of Erbium doped glasses
and the fabrication of fiber amplifiers, another major milestone in this area,
enabled 50 gigabits per second transmission rates. Such high amplification
rates can not be achieved with standard electronic amplifiers. The high speed,
high degree of parallelism of optics will lead gradually to optoelectronic
systems where an increasing number of functions will be implemented
optically. In that respect, materials with a nonlinear optical (NLO) response are
expected to play a major role in enabling optoelectronics and photonic
technologies.
The nonlinear optical materials are broadly classified into:
(a) organic crystals
(b) semi-organic crystals
(c) inorganic crystals
2.7.1. Organic Crystals
The search for new NLO materials over the past two decades has
concentrated primarily on organic compounds because of their high
nonlinearity. Nonlinear organic crystals have proven to be interesting
candidates for a number of applications like, second harmonic generation,
frequency mixing, electro-optic modulation, optical parametric oscillation etc.
The superiority of organic NLO materials results from their versatility and the
possibility of tailoring them for a particular end use [3]. The NLO properties of
43
large organic molecules and polymers have been the subject of extensive
theoretical and experimental investigations during the past two decades and
have been investigated widely due to their high nonlinear optical properties,
rapid response in electro optic effect and large second or third order
polarizability.
Rosker and Tank [32] have reported that urea has been used in an optical
parametric oscillator to generate tunable radiation throughout the visible
region. Intrinsic absorption and phase matching considerations make urea
unsuitable for wavelengths greater than 1000nm. The efforts made to resolve
the problems associated with urea have not been successful. The newly grown
binary urea and m.nitrobenzoic acid (UNBA) crystal amplifier [33] is thermally
and mechanically harder than the crystal of the parent components.
Manivannan and Dhanuskodi [34] have grown a new organic crystal
3-[(1E)-N-ethylethanimidoyl]-4-hydroxy-6-methyl-2H-pyran-2-one and found
that its SHG efficiency is close to urea. Haja Hameed et al [35] have obtained
trans-4-(dimethylamino)-N-methyl-4-stilbazolium tosylate (DAST) crystals
and the crystal surfaces were analyzed with the help of optical and scanning
electron microscope.
Modified hippuric acid (HA) single crystals have been grown from
aqueous solution of acetone by doping with NaCl and KCl, with the vision to
improve the physicochemical properties of the sample [36]. A new nonlinear
optical organic single crystal 4-Phenylpyridinium hydrogen squarate (4PHS)
44
has been grown by Ramachandra Raja et al [37] and showed that the SHG
efficiency of the grown crystal is five times greater than that of KDP crystal.
L-alanine nitrate (LAAN) [38] an organic nonlinear optical material was grown
by slow evaporation method at room temperature from aqueous solution. The
transmission spectrum reveals that the crystal has a low UV cut-off wavelength
and has a good transmittance in the entire visible region.
2.7.2. Semi-Organic Crystals
The widest search for new compounds and crystals led to the
development of many amino acids based semi-organic single crystals. In
comparison with inorganic crystals, semi-organic crystals are less hygroscopic
and can be easily grown as single crystals. L-arginine phosphate monohydrate
(LAP) is one of the potential nonlinear optical crystals among the amino acid
based semi-organic materials. Monaco et al [39] synthesized LAP and its
chemical analogs are the strongly basic amino acid and various other acids. All
the compounds in this class contain an optically active carbon atom, and
therefore all of them form acentric crystals. All the crystals are optically biaxial
and several among them give second harmonic signals greater than quartz.
Different organic and inorganic acids were introduced into L-alanine
and L-hystidine and many new nonlinear optical materials were reported with a
better NLO efficiency compared to inorganic KDP crystals. LAP crystals are
usually grown from aqueous solution by temperature lowering technique. LAP
45
crystals possess high nonlinearity, wide transmission range (220-1950nm), high
conversion efficiency (38.9%) and high laser damage threshold. Metal-organic
crystals form a new class of materials under semi-organics. Compared to
organic molecules, metal complexes offer a larger variety of structures, the
possibility of high environmental stability, and a diversity of electronic
properties by virtue of the coordinated metal center.
2.7.3. Inorganic Crystals
Inorganic materials are mostly ionic bonded and have high melting point
and high degree of chemical inertness. Investigations on nonlinear optical
phenomena in single crystals were initially focused on purely inorganic
materials such as quartz, lithium niobate (LiNbO3), potassium niobate
(KNbO3), potassium titanyl phosphate (KTiPO4), lithium iodate (LiIO3),
borates and semiconductor crystals.
Various borate crystals including -BaB2O4 (BBO), LiB3O5 (LBO),
have been reported as promising NLO crystals. The family of the various
borate crystals plays a very important role in the field of nonlinear optics [40].
Ravi et al [41] have reported the optimized growth condition of tetragonal
phase deuterated potassium dihydrogen phaspate (DKDP) with higher
deuterium concentration for growing large size crystals. D.Xue et al [42] have
studied the second order nonlinear optical properties of doped lithium
niobate(LN) crystals. It was observed that the second order NLO response of
46
doped LN crystals decreases with increased doping concentration in the crystal.
Successful growth of a new NLO crystal Ca5(BO3)3 with UV cut-off 190nm
was presented by Guojun Chen et al [43]. Zhoubin Lin et al [44] have found
that the SHG efficiency of YCa9 (VO4)7 single crystal is 4.7 times as large as
that of KDP crystal. The structure and NLO efficiency of the non-
centrosymmetric borate chloride Ba2TB4O9Cl (T=Al, Ga) crystals have been
explained by Jacques Barbier [45].During the last few years, various borate
crystals like GdCa4O(BO3)3, YCa4O(BO3)3 and LaCa4O(BO3)3 have been
reported as promising NLO crystals.
2.8 SQUARIC ACID, L-PROLINE AND THIOCYANATE BASED
OPTICAL CRYSTALS
Squaric acid (3, 4 dihydroxy-3-cyclobutene-1-2 dione) was first
prepared by S. Cohen et al [46] has been the subject of great attention. It is a
chemically stable highly acidic colourless crystalline substance, which melts at
about 566 K with decomposition. The squaric acid (C4H2O4) at room
temperature consists of ordered layer of C4O4 groups [47-49]. Each C4O4 group
is linked by four O-H-O bonds to neighbouring molecules within the same
layer, thus forming a pseudo-two dimensional structure. The layers are held
together by Vander Waals forces. To our knowledge, most of the experimental
and theoretical investigations of squaric acid have been concentrated on
structural analysis. The structure of the following compounds
4-phenylpridinium betaine of squaric acid, (8-hydroxyquinolinium) squarate,
47
4-phenylpridinium hydrogen squarate and 4-Dimethylaminopyridinium-1-
squarate belongs to monoclinic crystal system [50-53]. Our present work
focuses mainly on the growth and characterization of squaric acid and amino
acid based single crystals.
In squaric acid, the motion of the protons between the two equilibrium
sites in O-H-O creates an anion so that it acts as hydrogen donor which was
detected by 17O proton magnetic dipolar coupling measurement [54]. Squaric
acid when mixed with proton acceptor groups like NO2, NO, CN results in the
formation of dipole. This dipole is responsible for NLO activity of the
compound which is observed in non-centro symmetric crystals [55].
Vibrational spectral analysis of the nonlinear optical material, L-
prolinium tartrate (LPT) was carried out using NIR-FT-Raman and FT-IR
spectroscopy by Padmaja et al (2006). Also the single crystals of LPT were
grown by Martin Britto
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