Studies on the Growth and Characterization of Some Optical Crystals

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: [email protected]

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. Joseph’s 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.

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

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 7

NUCLEUS 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

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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 23

GROWTH 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 26

SOLUTION GROWTH TECHNIQUE 2 AN OVERVIEW OF OPTICAL MATERIALS 2.1. INTRODUCTION 28 2.2 IMPORTANCE OF CRYSTALS AS OPTICAL 30

MATERIALS 2.3 NONLINEAR OPTICAL MATERIALS 31 2.4. THEORETICAL EXPLANATION OF 33

NONLINEAR OPTICS 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

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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 46

THIOCYANATE BASED OPTICAL CRYSTALS 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

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

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

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

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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.

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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 deionized 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

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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

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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

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

Å Angstrom Unit

ΔG Gibbs free energy change

ΔGV Volume excess Free energy

ΔGS Surface excess Free energy

σ surface energy change per unit area

E Electric field vector

P Polarization

χ Linear susceptibility

2 3 χ , χ Non linear susceptibilities

ε0 Permittivity of free space

OC Degree Celsius

µm micrometer nm nanometer

ω Frequency of incident radiation a, b and c Cell parameters

α , β and γ Interfacial angles cm-1 per centimeter ns nanosecond mJ/pulse milli Joule per pulse

MHz Mega hertz

λ Wavelength

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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.

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).

state to stable state.

(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.

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.

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 σi = minimum 1.1

th where ai - area of i face and

σi - surface energy per unit area

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

ΔG = ΔGS + ΔGV 1.2

ΔG can be represented as a combination of surface excess free energy

(ΔGS) and volume excess free energy (ΔGV). For the spherical nucleus,

2 3 ΔG = 4 πr σ + 4/3 π r ΔGv 1.3

Where ΔGv is the free energy change per unit volume which is a negative quantity and ‘σ’ is the surface energy change per unit area. The quantities ΔG, ΔGS and ΔGV are represented in Fig. 1.1.

The surface excess free energy σ increases with r2 and the volume

3 excess free energy ΔGV decreases with r . 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,

Surface Term

ΔGS

ΔG*

ΔG

r* Radius

Volume Term

ΔGV

Fig. 1.1.

Free energy diagram

dΔG i.e. =0 dr when r = r*(radius of critical nucleus)

-2σ r* = 1.4 ΔGV

The free energy change associated with the formation of critical nucleus can be estimated by substituting equation 1.4 in equation 1.3.

* 3 2 ΔG = 16πσ / 3 ΔGv 1.5

In terms of r* the above equation can be written as,

ΔG* = 4/3π σ

ΔG* = 1/3 S.σ 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 (γ). 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

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

− ΔG * J = Aexp   1.7  KT  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 ΔC= C − C* 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.

C S = C *

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:

(a) growth from melt

(b) growth from vapour

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

(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.

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

(d) low temperature solution growth

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.

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 400°C to

600°C 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.

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

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 45-75°C 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

(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 15–20 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

conditions

(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

(d) growth rate of this method is low

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.

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 μm 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 1960’s 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 spring’s 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

(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 = ε0 χ. E 2.1

Where ‘χ’ is the linear susceptibility of the material, ‘E’ is the electric field vector, ‘ε0’ 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.

1 2 2 3 3 P = ε0 χ E + χ E + χ E + …. 2.2

Table 2.1

Optical effects of linear and nonlinear optical materials

Order Susceptibility Optical Effects Applications

Linear effect

1 χ1 Refraction Optical fibers

Absorption Colour Filter

Transmission Photolithography

Nonlinear effect

2 χ2 SHG Frequency doubling (ω=2ω)

Frequency mixing Optical parametric oscillations (ω1 ± ω2 =ω3)

Pockel’s effect Electro optical modulators (ω + 0 =ω)

3 χ3 Four wave mixing Raman coherent spectroscopy

Phase gratings Real time holography

Kerr effect Ultra high speed optical gates

Optical amplitude Amplifiers, Choppers etc.

Where χ2 , χ3 ….. are the nonlinear susceptibility of the medium. χ1 is the term responsible for material’s linear optical properties like, refractive index, dispersion, birefringence and absorption. χ2 is the quadratic term which describes second harmonic generation in non-centrosymmetric materials. χ3 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 χ1, χ2, and χ3 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

(d) optical parametric generation

(e) linear electro optic effect or Pockel’s effect

(f) optical rectification

2.5.1 Second Harmonic Generation (SHG)

The process of transformation of light with frequency ‘ω’ into light with double frequency 2ω 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].

NLO crystal

ω 2ω

Fig. 2.1

Schematic diagram of SHG

The most extensively studied conversion process of all has been the doubling of the 1.064 µm line obtained from the neodymium ion in various

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.

ω1 + ω2 = ω3 2.3

When two electromagnetic waves with the frequency ω1 and ω2 interact in a NLO medium, a nonlinear polarizability can be induced. The NLO

material generates an optical wave of frequency ω3 which is equal to the sum of the two input wave frequency ω1 and ω2. The energy of output wave is represented in the equation 2.3.

NLO crystal ω 1

ω ω ω ω 3 = 1 + 2 2

Fig. 2.2

Schematic diagram of sum frequency generation

2.5.3. Difference Frequency Generation

The process of difference-frequency generation is described by the following equation 2.4.

ω1 - ω2 = ω3 2.4

NLO crystal ω 1

ω3 = ω1 - ω2 ω2

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 λp) into two low-frequency photons (signal wavelength λs and idler wavelength λi)

ωs + ωi = ωp 2.5

NLO crystal ω s ωp

ω i

Fig. 2.4

Schematic diagram of optical parametric generation

2.5.5. Linear Electro Optic Effect

The Pockel’s 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

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

(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

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 β-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,

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 Dhas and Natarajan (2007a) using submerged seed solution growth method. The SHG conversion efficiency is about 90% of that of the standard KDP crystal. Laser damage threshold study was also carried out and found to be in a higher range. Growth and characterization of L-prolinium picrate single crystals (LPP) were grown by T. Uma devi et al. (2008) using temperature reduction method. The UV cut off wavelength was found to be around 480 nm. The transmittance between 500 and 1000 nm is approximately

75%. The SHG efficiency is about 52 times greater than that of KDP. The mechanical strength was tested by Vicker’s microhardness test. The same LPP

48 crystal was also grown at room temperature using slow evaporation technique by Martin Britto Dhas et al. (2008). L-Proline lithium chloride monohydrate single crystal and grown and characterized by T. Uma devi et al. (2009). Its

SHG efficiency is about 0.2 times that of KDP. The UV cut off wavelength occurs at 350 nm.

In the thiocyanate complex crystals, as a ligand, the ambidentate SCN ion, which is usually S-bonded to a soft and N-bonded to a hard metal centre, can also act as a bridging bidentate ligand to satisfy the coordination number of the metal. In the AB (SCN)4 molecules , metal ions can bind to SCN ion through either S or N. i.e. B ion binds with SCN through the S atom and A ion through N atom. The most striking feature is the –S=C=N– bridges between the

A and B atoms that leads to the formation of an infinite three dimensional –B–

S=C=N–A– network. The three dimensional network provides the crystal with larger domain for polarization, which in turn induces a larger macroscopic nonlinearity, high physicochemical stability [56] and large variety of structures with high environmental stability. AB (SCN)4 series of thiocyanate crystalline complex crystals have been known for over a century in analytical chemistry for their characteristic shapes and colours [Rosenheim et al 1901], but their crystal structures were reported only after another 46 years later. [Jeffery 1947,

Rose 1968, Porai-Kosic 1963, Izuka, Sudo 1968, Yuan et al 1997, Yan et al

1999].

Guanghui Zhang et al [57] presented the refractive index and second harmonic generation phase matching angles of zinc cadmium thiocyanate

(ZCTC) crystal and also they showed ZCTC is a promising nonlinear optical material for frequency doubling.

D.R.Yuan et al [58] explained the analysis of a regular crystalline morphology, homogeneous growth rate at different direction and the effect of impurities on the quality of the cadmium mercury thiocyanate crystal. With aging and elevated temperature the solution becomes unstable. The hydrolysis, decomposition reactions and the reaction rate are influenced by various factors, such as pH value, temperature and time. D.R.Yuan et al [59] studied the stability of CdHg(SCN)4 crystal and analysed the various effects of temperature, pH value, time and KCl concentration in the crystal. The pH value was selected between 2.5 – 3.8. When KCl concentration was increased greater than 27%, the crystal was not generated in the solution. When the pH value was maintained, other than 3 – 4 the crystal growth was not good. For better growth of single crystals the KCl concentration was maintained greater than 27% and pH value of the saturated solution was maintained at 3.8. Growth of cadmium mercury thiocyanate single crystals using acetone-water mixed solvent was studied by R.Jayavel et al [60] and the study showed that the laser damage threshold value of cadmium mercury thiocyanate crystal was found to be

4.4GW/cm2, which is moderately good.

Ravi et al [62] have reported the optimized growth condition of tetragonal phase DKDP with higher deuterium concentration for growing large size crystals. Xue et al [63] have studied the second order nonlinear optical properties of doped lithium niobate(LN) crystals. It was observed that the second order NLO response of 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 190 nm was presented by Guojun Chen et al [64].

Zhoubin Lin et al [65] have found that the SHG efficiency of Yca9 (VO4)7 single crystal is 4.7 times as that of KDP crystal.

In addition, during the last few years, newly developed nonlinear optical

(NLO) crystals GdCa4O (BO3)3 , YCa4O(BO3)3 and LaCa4O(BO3)3 have been studied as promising NLO crystals.

2.9. SCOPE OF THE RESEARCH WORK

Crystals are the unacknowledged pillars of modern technology. Without crystals, there would be no electronic industry, no photonic industry, no fiber- optic communications. The reason for growing single crystals is that many physical properties of solids are obscured or complicated by the effect of grain boundaries. The chief advantages are the anisotropy, uniformity of composition

51 and the absence of boundaries between individual grains, which are inevitably present in polycrystalline materials. In order to achieve high performance from the device, good quality single crystals are needed. Growth of single crystals and their characterization towards device fabrication have assumed great impetus due to their importance for both academic and applied research.

The present investigation is aimed at the growth and characterization of some optical crystals, due to their potential applications in opto-electronic devices. In the present work, single crystals of 4-phenylpyridinium hydrogen squarate(4PHS), glycinium hydrogen squarate(GHS), L-proline succinate(LPS), cadmium manganese thiocyanate(CMTC), zinc manganese thiocyanate(ZMTC) have been grown from low temperature solution growth.

The present investigation is aimed at

(i) synthesizing the chosen material for the growth of single crystals

(ii) growing bulk size single crystals by purification and re-

crystallisation

(iii) estimating the lattice parameters of the grown crystals using

powder and single crystal X-ray diffraction method

(iv) characterizing the grown crystals by FT-IR, NMR and optical

transmission studies

(v) studying the thermal stability of the grown crystals

(vi) determining the SHG efficiency of the crystals by Kurtz-Perry

powder technique

CHAPTER 3

CHARACTERIZATION TECHNIQUES

3.1 INTRODUCTION

Characterization is a tool for the measurement of physical and chemical properties of materials. Characterization provides a basis for understanding and improving the characteristics of material for specific applications.

Characterization of a material essentially depends on the characterization and experimental techniques involved with tools of sophisticated technology.

Today scientists and researchers have powerful and elegant tools for obtaining qualitative and quantitative information about the composition and structure of matter. The development of these tools began over two centuries ago and the search still continues. The use of instrumentation is an exciting and fascinating part of any analysis that interacts with all the areas of chemistry and with many other fields of pure and applied science.

Characterization of a crystal essentially consists of determination of chemical composition, structure, defects and study of their optical properties [61]. Crystal studies such as structural analysis, investigation of growth defects, and measurement of linear and nonlinear optical properties are essential in understanding the nature and properties of the grown crystals. In order to obtain good quality single crystals pertaining to specific applications enhancement of the desired properties must be done.

Hence, in this chapter, the various instruments with different techniques involved in the characterization of the grown crystals with their working principles are discussed.

3.2. SINGLE CRYSTAL X-RAY DIFFRACTION (XRD) STUDIES

Single crystal X-ray Diffraction is a non-destructive analytical technique which provides detailed information about the internal lattice of crystalline substances, including unit cell dimensions, bond lengths, bond angles, and details of site ordering. Directly related is single crystal refinement, where the data generated from the X-ray analysis is interpreted and refined to obtain the crystal structure.

3.2.1. Principles of X-ray Diffraction

Max von Laue, in 1912, discovered that crystalline substances act as three dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-ray diffraction is now a common technique for the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference and a diffracted ray,

54 when conditions satisfy Bragg’s Law. This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. The Bragg’s condition for reflection can therefore be written as

2d sinθ = nλ where,

n - order of diffraction

λ - wavelength of the X-ray used

θ - Bragg’s angle

d - inter planar spacing

These diffracted X-rays are then detected, processed and counted. By changing the geometry of the incident rays, the orientation of the centered crystal and the detector, all possible diffraction directions of the lattice should be attained. In the study of crystals, the single crystal X-ray diffraction is used for four purposes:

 to prove the crystallinity

 to determine the structure

 to determine the perfection

 to determine the lattice parameters

3.2.2. Sample Selection and Preparation

Samples for single-crystal diffraction should be selected from unfractured, optically clear crystals. This can be determined by viewing the samples under crossed polars on a petrography microscope. Crystals can be broken off a larger sample and the best fragment selected. Samples should be between 30 and 300 microns, with ideal crystals averaging 150-250 microns in size.

3.2.3. Sample Mounting

Samples are mounted on the tip of a thin glass fibre using an epoxy or cement. Care should be taken to use just enough epoxy to secure the sample without embedding it in the mounting compound. Then the fibre is attached to a brass mounting pin, usually by the use of modeling clay, and the pin is then inserted into the goniometer head.

3.2.4. Sample Centering

The goniometer head and sample are then affixed to the diffractometer.

Samples can be centered by viewing the sample under an attached microscope or video camera and adjusting the x, y and z directions until the sample is centered under the cross-wire for all crystal orientations.

All diffraction methods are based on generation of X-rays in an X-ray tube. These X-rays are directed at the sample, and the diffracted rays are

56 collected. A key component of all diffraction is the angle between the incident and diffracted rays.

φ Rotation

Beam trap χ Rotation

Incident X-ray 2θ

beam Diffracted beam Counter

Ω Rotation

2θ Rotation

Fig. 3.1.

Experimental setup of single crystal X-ray diffractometer

Single crystal diffractometers use either 3 or 4 circle goniometers

(Fig.3.1). These circles refer to the four angles (2θ, χ, φ, and Ω) that define the relationship between the crystal lattice, the incident ray and detector. Samples are mounted on thin glass fibers which are attached to brass pins and mounted onto goniometer heads. Adjustment of the x, y and z directions allow centering of the crystal within the X-ray beam.

X-rays leave the collimator and are directed at the crystal. Rays are either transmitted through the crystal, reflected off the surface or diffracted by

57 the crystal lattice. A beam stop is located directly opposite the collimator to block transmitted rays and prevent burn-out of the detector. Reflected rays are not picked up by the detector due to the angles involved. Diffracted rays at the correct orientation for the configuration are then collected by the detector.

Modern single crystal diffractometers use CCD (charge-coupled device) technology to transform the X-ray photons into an electrical signal which are then sent to a computer for processing. In the present study, single crystal

X ray diffraction analyses were carried out using ENRAF NONIUS CAD 4 /

Brucker Kappa APEX II CCD diffractometer.

3.3 POWDER X-RAY DIFFRACTION (XRD) STUDIES

The discovery of diffraction of X-rays by crystals led to the development of a powerful and precise method for the exploration of the internal arrangement of atoms in a crystal. X-rays are still the principle source of new information about the crystallography of solids and are supplemented by electron and neutron diffraction. The diffraction of X-rays by the atoms in a solid is a completely analogous phenomenon, the wavelength of electromagnetic radiation in the case being of the order of inter atomic distance in solids is 1Å. X-rays are used as a tool for investigating the crystal structure was first suggested by Von Laue in 1912 and further developed by Bragg and

W.L.Bragg. X-ray diffraction gives the appropriate internal structure of the

58 crystal. There are three main X-ray diffraction methods by which the crystal structure can be analyzed.

1. The Laue method which is applicable to single crystals.

2. The rotating crystal method which is also applicable to single

crystals.

3. The powder method applicable to finely divided crystalline or

polycrystalline powdered specimen

The principle of powder X-ray diffraction is a tool for accurate determination of lattice parameters in crystals of known structure and for the identification of elements and compounds. If one uses X-rays having strong monochromatic components and a sample consisting of many tiny, randomly oriented crystals, the equation,

n λ = 2d sinθ will be fulfilled because some of the crystals will be oriented to give reflections with out need of rotation. The most common powder technique is the Debye seherrer-Hull method.

3.3.1 X-ray Powder Diffractometer

The powder method of diffraction was devised independently by Debye and Scherrer. It is the most useful of all diffraction methods and when properly

59 employed, can yield a great deal of structural information about the material under investigation. Powder diffraction method involves the diffraction of monochromatic X-rays by a powder specimen. Monochromatic usually means a strong Kα characteristic component of the filtered radiation from an X- ray tube operated above the Kα excitation potential of the target material.

Fig. 3.2

Schematic of Guinier geometry

Selection of Kα renders the incident beam to be a highly monochromatised one. The focusing monochromatic geometry results in narrower diffracted peaks and low background at low angles. The sample is mounted vertically to the Seemann-Bohlin focusing circle with the scintillation counter tube moving along the circumference of it. It is possible to record the

60 diffracted beam from 2 to 160 degrees. The diffractometer is connected to a computer for data collection and analysis. The scintillation counter tube can be moved in step of 0.01 degree by means of a stepper motor and any diffracted beam can be closely scanned to study the peak profile. A high resolution powder diffractometer - RICH SIEFERT & CO with Guinier geometry was employed for characterization and grain size determination. Schematic of

Guinier geometry is shown in Fig. 3.2.

3.4. FOURIER TRANSFORM INFRARED (FT-IR) SPECTRAL

ANALYSIS

Fourier transform spectroscopy is a simple mathematical technique to resolve a complex wave into its frequency components. Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a sample positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Different functional groups absorb characteristics frequencies of IR radiation. Using various sampling accessories,

IR spectrometers can be used for a wide range of sample types such as gases, liquids and solids. Thus, IR spectroscopy is an important and popular tool for compound identification.

The modern era of spectroscopy began with the observation of the spectrum of the Sun by Sir Isaac Newton in 1672. Fourier transform spectroscopy was first developed by astronomers in the early 1950s in order to study the infrared spectra of distant stars. The first chemical applications of

Fourier transform spectroscopy, which was reported approximately a decade later. Commercial instruments for chemical studies in both the far-infrared and the ordinary infrared regions were introduced in the late 1960s.

Conventional spectroscopy or frequency domain spectroscopy records the radiant power as a function of frequency. In time domain spectroscopy, the change in radiant power is recorded as a function of time. In Fourier transform spectrometer, a time domain plot is converted into a frequency domain spectrum. The actual calculation of the Fourier transform of such systems is performed by high speed computers.

The IR absorption information is generally presented in the form of a spectrum with wavelength or wavenumber as the x-axis and absorption intensity or transmittance percentage as the y-axis. The transmittance spectra provide better contrast between intensities of strong and weak bands because transmittance ranges from 0 to 100%T, whereas absorbance ranges from infinity to zero.

The IR region is commonly divided into three smaller areas. They are near IR, mid IR and far IR. The most frequently used mid IR region ranges

62 between 4000 and 450cm-1. The far IR requires the use of specialized optical materials and sources. It is used for analysis of organic, inorganic and organometallic compounds involving heavy atoms (mass number over 19). It provides useful information to structural studies. Near IR spectroscopy needs minimal or no sample preparation. It offers high-speed quantitative analysis without consumption or destruction of the sample. Near IR spectroscopy has gained increased interest, especially in process control applications.

The FT-IR spectrometer consists of an infrared source, a sample chamber with a provision for holding solids, liquids and gases, monochromator, a detector and a recorder, which are integrated with a computer. At present, all commercially available infrared spectrophotometers employ reflection gratings rather than prisms as dispersing elements. The schematic diagram of a FT-IR spectrometer is shown in Fig. 3.3.

FT-IR spectroscopy finds widespread application in qualitative and also quantitative analyses [62-64]. Its important use has been for the identification of organic compounds whose mid-infrared spectra are generally complex and provide numerous maxima and minima that are useful for the comparison purpose.

Collimated lamp for visual alignment

IR source

Oscillating mirror ‘A’

Oscillating mirror ‘B’ Interferometer

Reference holder Sample Sample component holder

Oscillating mirror ‘C’

TGS Detector

Fig. 3.3.

Schematic experiment setup for FT-IR spectrometer

The advantage of an FT-IR instrument is that it acquires the interferogram in less than a second. Thus, it is possible to collect dozens of interferograms of the same sample and accumulate them in the memory of a computer. In the present work, FT-IR studies were carried out using Perkin

Elmer 783 / Perkin Elmer IF 66 spectrophotometer.

3.4.1. Preparation of Liquid Samples for FT-IR Spectroscopy

A drop of a liquid compound is placed between a pair of polished sodium chloride or potassium bromide plates, referred to as salt plates. When the plates are squeezed gently, a thin liquid film forms between them. A spectrum determined by this method is referred to as a neat spectrum since no solvent is used. The disadvantage of this method is that the salt plates break easily and are water soluble. So, the compounds analyzed by this technique must be free from water. The pair of plates is inserted into a holder which fits into the spectrometer.

3.4.2. Preparation of Solid Samples for FT-IR Spectroscopy

There are at least three common methods for preparing a solid sample for spectroscopy. The first method involves mixing the finely ground solid sample with powdered potassium bromide and pressing the mixture under high pressure. Under pressure, the potassium bromide melts and seals, the result is a

KBr pellet. Then the KBr pellet is inserted into a holder in the spectrometer.

The main disadvantage of this method is that potassium bromide absorbs water, which may interfere with the spectrum that is obtained. If a good pellet is prepared, the spectrum obtained will have no interfering bands since potassium bromide is transparent down to 400cm-1.

The second method, a Nujol mull, involves grinding the compound with mineral oil to create a suspension of the finely ground sample dispersed

65 in the mineral oil. The thick suspension is placed between salt plates. The main disadvantage of this method is obscure bands may be present in the analyzed compound.

The third common method used with solids is to dissolve the sample in a solvent, most commonly carbon tetrachloride (CCl4). Again, as was the case with mineral oil, bands in the solvent obscure some regions of the spectrum.

3.5 NUCLEAR MAGNETIC RESONANCE (NMR) ANALYSIS

Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum mechanical magnetic properties of an atom's nucleus.

3.5.1. Introduction

Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as NMR, has become the predominent technique for determining the structure of organic compounds. Of all the spectroscopic methods, it is the only one for which a complete analysis and interpretation of the entire spectrum is normally expected. Although larger amounts of sample are needed than for mass spectroscopy, NMR is non-destructive, and with modern instruments good data may be obtained from samples weighing less than a milligram.

If a sample is placed in a magnetic field and is subjected to radiofrequency (RF) radiation (energy) at the appropriate frequency, nuclei in

66 the sample can absorb the energy. The frequency of the radiation necessary for absorption of energy depends on three things. First, it is characteristic of the type of nucleus (e.g., 1H or 13C). Second, the frequency depends on chemical environment of the nucleus. For example, the methyl and hydroxyl protons of methanol absorb at different frequencies, and amide protons of two different tryptophan residues in a native protein absorb at different frequencies since they are in different chemical environments. Thirdly, the NMR frequency also depends on spatial location in the magnetic field, if that field is not everywhere uniform.

3.5.2 NMR Spectroscopy – Principle

Principle of NMR is based upon the spin of nuclei in an external magnetic field. In absence of magnetic field, the nuclear spins are oriented randomly. Once a strong magnetic field is applied they reorient their spins i.e. aligned with the field or against the field. Orientation parallel to alignment of applied force is lower in energy. When nuclei are irradiated with RF radiation the lower energy nuclei flip to high state and nuclei said to be in resonance, hence the term nuclear magnetic resonance.

3.5.3 Nuclear spins

Nuclei have positive charges. Many nuclei behave as though they were spinning. Anything that is charged and moves has a magnetic moment and produces a magnetic field. Therefore, a spinning nucleus acts as a tiny bar

67 magnet oriented along the spin rotation axis (Fig.3.4). This tiny magnet is often called a nuclear spin. If we put this small magnet in the field of a much larger magnet, its orientation will no longer be random. There will be one most probable orientation. However, if the tiny magnet is oriented precisely 180° in the opposite direction, that position could also be maintained. The most favorable orientation would be the low-energy state and the less favorable orientation the high-energy state.

Fig.3.4

Nuclear spin

The charged nucleus (e.g., 1H) rotating with angular frequency (ω=2πγ) creates a magnetic field B and is equivalent to a small bar magnet whose axis is coincident with the spin rotation axis.

3.5.4 NMR Spectrometer - Construction

To get the nuclei in a molecule to all align in the same direction, a very strong magnetic field is generated using a superconducting electromagnet, which requires very low temperatures to function.

Fig. 3.5.

Schematic diagram of NMR spectrometer

The schematic diagram of NMR spectrometer shown in fig.3.5. The coils of the magnet are surrounded by liquid helium (4K, or -269°C), which is prevented from boiling off too quickly by a surrounding layer of liquid nitrogen

(-77°C). These coolants are all contained in double-layer steel with a vacuum between the layers, to provide insulation just like a thermos. There is a narrow hole through the middle of the magnet, and the sample tube and radio frequency coils ("probe”) are located there.

3.5.5 NMR Spectrometer - Working

All nuclear magnetic resonance technology, including NMR spectroscopy, is based on two physical phenomena:

Chemical Shift: All nuclei containing an odd number of protons and neutrons

have an intrinsic type of movement (also called resonance). This movement

is slightly different for different protons in a given molecule, and can change

depending on the chemical environment.

The Zeeman Effect: This is what happens when a spectral line (an emission

or absorption point in an otherwise uniform spectrum) is split into

components while in the presence of a magnetic field.

In NMR spectroscopy, magnetic nuclei are aligned with a constant magnetic field. This alignment is then disturbed by applying an alternating magnetic field to the nuclei. Both NMR spectroscopy and NMR imaging (often used in medical diagnosis) analyze the response of the nuclei to the alternating magnetic field to gather information about the composition of the sample under analysis.

3.5.6 Applications of NMR Spectroscopy

Today, NMR has become a sophisticated and powerful analytical technology that has found a variety of applications in many disciplines of scientific research, medicine, and various industries. Modern NMR

70 spectroscopy has been emphasizing the application in biomolecular systems and plays an important role in structural biology. Some of the applications of

NMR spectroscopy are listed below:

Structural Biology: Proteins, DNA/RNA, Protein-DNA/RNA complexes,

Protein-lipid complexes, and Polysaccharides

Medicine: Magnetic resonance imaging (MRI) helps doctors to make their diagnosis.

Chemistry: synthesis, pharmaceutical, quality control, structure conformation, dynamics, kinetics, chemical exchange, equilibrium, molecular tumbling, etc…

Drug design: Structure Activity Relationships (SAR)

Petroleum industry: test water, oil, organic, inorganic composition.

Food industry: drinks, solid foods, quality, contents, contamination.

Natural product analysis: extracts, potential drug leads

3.6. UV-Vis-NIR SPECTROSCOPY

UV-Vis-NIR spectroscopy is helpful for the measurement of absorption or emission of radiation associated with changes in the spatial distribution of electrons in atoms and molecules. In practice, the electrons involved are usually the valence electrons, which can be excited by absorption of UV or visible or near IR radiation. Excitation of a bound electron from the highest occupied molecular orbit increases the spatial extent of the electron

71 distribution, making the total electron density larger and more diffuse, and often more polarizable.

Ultraviolet spectrum of a molecule results from transitions between electronic energy levels accompanied by changes both in vibrational and rotational states. The probability for electronic transitions determines the intensity of spectral lines. There must be large overlap between the vibrational states in the initial and the final electronic states to have a large absorption cross-section, or high probability that the molecule will absorb/emit radiation.

Electronic transitions are possible for a wide range of vibrational levels within the initial and final electronic states. Saturated hydrocarbons and compounds containing only alkyl groups, alcohol groups and ether groups are transparent

(i.e. they show no absorption) in the region 200-1000nm. Such compounds are useful as solvents for spectral determination, using solutions of the specimen in this region.

An isolated functional group not in conjugation with any other group is said to be a chromophore, if it exhibits absorption of a characteristic nature in the ultraviolet or visible region. If a series of compounds have the same functional group and no complicating factors are present, all of them will generally absorb at very nearly the same wavelength. Thus, it is readily seen that the spectrum of a compound, when correlated with data from the literature for known compounds, can be a very valuable aid in determining the functional

72 groups present in the molecule. The absorption (A) of a solution at a particular wavelength is given by Beer-Lambert’s law.

A = e c t

Where t is the thickness of the cell, c is the concentration of the compound and e is the molar extinction coefficient characteristic of the compound at a given wavelength. This principle is used for quantitative measurements.

The UV spectrum consists of far or vacuum ultraviolet region, near or quartz ultraviolet region and visible region. The region between 10-200nm is termed as vacuum ultraviolet region, which can be studied in evacuated systems. The region between 200 - 380nm is called quartz ultraviolet region, normally termed as ultraviolet region. The region or spectral range most accessible for the instruments, is from 200 to 800nm includes the visible region lying between 380 - 780nm. In the present work, UV-Vis-NIR studies were carried out using a Varian Cary 500 / Lambda 35 spectrophotometer.

These spectrophotometers are controlled by a microprocessor, which is used to study the electronic spectrum of solution, single crystals and powder samples.

3.7. THERMAL STUDIES

Recent trends indicate that thermal analysis has become an established method in the study of the thermal behavior of materials and finds widespread

73 application in diverse industrial and research fields. Thermal analysis is general term, which covers a group of related techniques in which the temperature dependence of the parameters of any physical property of a substance is measured. In addition to providing valuable information on the thermal stability of the compounds and the decompositions products, these studies often provide an insight into their mode of decomposition. The importance of thermal analysis in quality control, failure analysis and material research and development is well established.

Thermoanalytical methods involve the measurement of various properties of materials subjected to dynamically changing environments under predetermined conditions of heating rate, temperature range and gaseous atmosphere or vacuum. In many cases, the use of a single thermoanalytical technique may not provide sufficient information to solve the problem on hand and hence the use of other thermal techniques, either independently or simultaneously for complementary information becomes necessary. Both differential thermal (DTA) and thermogravimetry analysis (TGA) are widely used in studies involving physicochemical changes accompanied by variation in the heat content and the weight of the material.

Among the thermal methods, the most widely used techniques are TGA,

DTA and differential scanning calorimetry (DSC) which find extensive use in all fields of inorganic and organic chemistry, metallurgy, mineralogy and many other areas.

3.7.1. Differential Thermal Analysis (DTA)

Differential thermal analysis (DTA), often considered an adjunct to

TGA is, in fact, far more versatile and yields data of a considerably more fundamental nature. The technique is simple as it involves the measurement of the temperature difference between the sample and an inert reference materials, as both are subjected to identical thermal regimes, in an environment heated or cooled at a constant rate. The origin of the temperature difference in the sample lies in the energy difference between the products and the reactants or between the two phases of a substance. This energy difference is manifested as enthalpy changes either exothermic or endothermic.

The differential thermal curve would be parallel to the temperature axis till the sample undergoes any physical or chemical change of state. However, as soon as the sample has reached the temperature of this change of state, the additional heat flux reaching the sample will not increase the sample temperature at the same rate as that of the reference and the differential signal appears as a peak. The differential signal would return to the base line only after the change of state of the sample is completed and the temperature becomes equal to that of the reference material. The thermal effects are observed as peaks whose sequence, sign, magnitude and shape reflect the physical or chemical changes taking place.

Since any change in the chemical or physical state of a substance is accompanied by changes in energy which are manifested as heat changes, the

DTA method is applicable to all the studies listed for TGA and also to phase transformations including polymerization, phase equilibrium and chemical reactions. The DTA apparatus consists of a furnace for heating the sample and reference in an identical environment, linear temperature programmer controller, sample holder, differential temperature detector with preamplifier and a recorder.

3.7.2. Themogravimetry Analysis (TGA)

Thermogravimetry is a technique in which the mass of a substance is measured as function of temperature or time while the substance is subjected to a controlled temperature program. The curve obtained in a thermogravimetric analysis is called a thermogram(TG). It is important to note that the term thermogravimetric analysis and the abbreviation TGA are in common use.

Modern commercial TGA instrument (Fig. 3.6) consists of:

 a sensitive analytical balance

 a temperature programmable furnace

 a gas system for providing suitable gas atmosphere

 a microprocessor for instrument control, data acquisition and display

Light beam Light source Shutter

Taut band Photomultiplier tubes

Weight Beam

Magnet

Suspension wire Feedback coil

Sample Temperature Amplifier Adjustor for weight Furnace measurement

Recorder Thermocouple

Fig. 3.6.

Schematic experiment setup of TGA equipment

Even though different types of balance mechanism are available today, those employing null point weighing mechanism is favoured as the sample remains in the same zone of furnace irrespective of changes in mass. The furnace is normally an electrical resistive heater and the temperature range for most of the furnace is from room temperature to 2000°C. Thermogravimetry is

77 widely used to determine the thermal stability, decomposition temperature, temperature of desorption and drying, oxidative stability etc.

Heating rate affect the results of thermal analysis, so, heating rate is very important. The rate of heat exchange between the furnace and the sample depends upon the heating rate. A slower heating rate gives a better resolution of the closely lying steps, while the faster heating rate merges such steps. In the present work, the thermal studies were carried out using NETZSCH STA

409C-CD / SDT Q 600 V8.3 Build 101 instrument.

3.8. KURTZ POWDER METHOD – SHG EFFICIENCY TEST

3.8.1. Introduction

Growth of large single crystal is a slow and difficult process. Hence, it is highly desirable to have some technique of screening crystal structures to determine whether they are non-centrosymmetric and it is also equally important to know whether they are better than those currently known. Kurtz and Perry proposed a powder SHG method for comprehensive analysis of the second order nonlinearity. Employing this technique, Kurtz surveyed a very large number of compounds.

3.8.2. Experimental Procedure

The nonlinear optical property of the grown single crystal is tested by passing the output of Nd:YAG laser light through the crystalline powder

sample. The schematic diagram of the experimental setup used for SHG study

is shown in Fig. 3.78. A Q-switched, Nd:YAG laser light was used to generate

about 6.5mJ/pulse at a wavelength of 1064nm.

IR reflector Nd-YAG Laser

Glass Plates

Photodiode

n CuSo4 Sol Oscilloscope Beam splitter 50/50 Sample holder Interference Filter

Fig. 3.7.

Schematic experiment setup for SHG efficiency measurement

This laser can be operated in two modes. In the single shot mode the

laser emits a single 8ns pulse. In the multi shot mode the laser produces a

continuous pulse of 8ns at a repetition rate of 10Hz. In the present study, a

single shot mode of 8ns laser pulse with a spot radius of 1mm was used. This

experimental setup used a mirror and a 50/50 beam splitter (BS) to generate a

beam with pulse energies about 6.5mJ. The input laser beam was passed

79 through an IR reflector and then directed on the microcrystalline powdered sample packed in a capillary tube of diameter 0.154 mm.

The photodiode detector and oscilloscope assembly measure the light emitted by the sample. Microcrystalline powder of urea or KDP is taken in a similar capillary tube sealed at one end for comparison. The intensity of the second harmonic output from the sample is compared with that of either urea or

KDP. In the present study the grown crystals are compared with KDP crystals and the figure of merit of SHG of the crystals are estimated.

CHAPTER – 4

SYNTHESIS, GROWTH AND CHARACTERIZATION OF A NEW NONLINEAR OPTICAL MATERIAL: 4-PHENYLPYRIDINIUM HYDROGEN SQUARATE (4PHS)

4.1 INTRODUCTION

In the recent past non-linear optical materials gaining attention due to their enormous applications in telecommunication activities such as optical computing, optical information processing, optical disk data storage, laser remote sensing, laser driven fusion, color display and medical diagnostic, etc.

[31,71]. The above said facts motivated the materials scientists to search the novel materials for nonlinear optical applications. The large second order optical nonlinearities, short transparency cut-off wavelength and stable physiochemical performance are needed in the realization of most of the recent electronic applications. Since there is a large demand of crystals in the revolution of electronic industries, it is required to be improved in both technical as well as economic aspects. By keeping the above facts in mind, we have synthesized a new nonlinear optical material of 4-phenylpyridinium hydrogen squarate (4PHS).

This analysis focuses on pure organic materials and in the emerging field of opto-electronic materials. The synthesized 4PHS has been successfully grown by slow evaporation solution growth technique and the grown single crystals have been analyzed by different instrumentation methods. The title compound has high nonlinearity, wide transparency range and high physicochemical

81 stability.

4.2. EXPERIMENTAL PROCEDURE

The commercially available 4-phenylpyridine and squaric acid (AR grade) have been used to synthesize the 4PHS. A solution of 4-phenylpyridine has been dissolved in water ethanol (1:1) mixture and the prepared solution was poured into the dissolved squaric acid mixture. Immediately the reaction was taking place between 4-phenylpyridine (pyridine-based) and squaric acid (acid- based) through hydrogen transfer. Crystal growth process was carried out in an aqueous medium. The formed ionic compound of 4-phenylpyridinium hydrogen squarate is represented in the following equation:

+ C11H9N + C4H2O4 → C11H10N C4HO4−

– O O O O

N N+ –H

O O O O

By adopting the solution growth method, single crystal of

4-phenylpyridium hydrogen squarate was grown from supersaturated solution.

The aqueous solution was maintained in the undisturbed condition over the required duration. The solution was periodically inspected and at the end of the twentieth day, the crystal was started growing. Further the crystal was permitted to grow for another 20 days in order to obtain a nominal size suitable for characterization. The 4-phenylpyridine interacts with squaric acid through a

82 single N-H-O hydrogen bond [72]. The crystal thus obtained appears pale yellow in color. The photograph of the grown crystals is shown in Fig. 4.1.

Fig. 4.1.

Photograph of 4PHS single Crystals

4.3. CHARACTERISATION STUDIES

4.3.1. Single crystal XRD analysis

The lattice dimensions and the crystal system have been determined from the single crystal X-ray diffraction analysis (Model: ENRAF NONIUS CAD 4).

The observed results indicates that the crystal belong to monoclinic crystal system, and the determined unit cell parameters are a = 11.781 Å, b = 8.994 Å and Å c=12.988, α = 89.72º, β = 113.91º and γ

= 89.99º, Volume = 1258 Å3 and Z = 4.

The observed results are good in agreement with the reported literature [72].

4.3.2. Fourier transform infrared (FT-IR) analysis

The functional groups presented in the title compound have been identified from the FTIR analysis using KBr pellet technique in the region

450-4000cm−1 (Bruker IFS 66V model FTIR spectrometer). The recorded

FTIR spectrum is shown in Fig. 4.2. The detailed analyses were given in the following sections. The observed bands along with their vibrational assignments are summarized in Table 4.1.

O-H vibrations

Bands in the region 3500-3600 cm−1 are usually due to various O-H stretching vibrations. The bonded O-H group generally gives rise to a broader band than the N-H group [73]. Hence, in this study, the band observed in the region 3534 cm−1 in 4-phenylpyridinium hydrogen squarate compound is assigned to O-H stretching vibration.

N-H vibrations

According to Bellamy [74], the characteristic region for this group of vibration is around 3000-3500 cm−1. Hence in this study, the FTIR band appeared at 3155 cm−1 is assigned to N-H stretching vibrations. The in plane and out of plane bending vibrations of N-H group have also been

84 identified for the title compound and they are given in Table 1.

C-H vibrations

The hetero aromatic structure shows the presence of C-H stretching vibrations in the region 3000-3100 cm−1 which is the characteristic region for the ready identification of C-H stretching vibrations [75]. Hence in the present investigations, the peaks observed at 3105 cm−1, 3058 cm−1 and 3004 cm−1 for 4-phenylpyridinium hydrogen squarate compound are assigned to

C-H stretching vibrations. The in plane and out-plane bending vibrations of C-H group have also been identified and they are given in

Table 4.1.

C-N vibrations

The assignments of the bands in the region 1200-1400 cm−1 are not easy, since the group frequency approximation does not work in this region. Moreover, a large number of coupling interaction between intra and extra ring of C-N stretching vibration takes place in this region. From the figure, the peaks appeared at 1292 cm−1 and 1207 cm−1 may be attributed to C-N stretching vibrations

[76,77].

Fig. 4.2.

FT-IR Spectrum of 4PHS Crystal

Table 4.1

Assignments of FT-IR bands observed for 4-PHS crystal

Wave number (cm-1) Assignments 3534 O-HStretching 3155 N-HStretching 3105 C-HStretching 3058 C-HStretching 3004 C-HStretching 1803 C-NStretching 1668 C-CStretching 1634 C-CStretching 1292 C-NStretching 1207 C-NStretching 1145 N-H in-plane bending 1016 C-H in-plane bending 1003 C-H in-plane bending 990 C-H in-plane bending 880 C-H out-of-plane bending 851 N-H out-of-plane bending 832 C-N-out-of-plane bending 769 C-O-in-plane bending 744 C-H-out-of-plane bending 726 C-H-out-of-plane bending 692 C-O-in-plane bending 642 C-C-C-in-plane bending 606 C-C-C-out-of-plane bending 552 C-N-out-of-plane bending 477 C-O-out-of-plane bending

4.3.3. Nuclear magnetic resonance

1H and 13C NMR analyses

The 300 MHz proton NMR spectrum of 4-phenylpyridinium hydrogen squarate measured in DMSO using Bruker instrument is integrated for a total of 10 protons [78]. A broad diffused signal centered at δ 7.59 ppm integrated for four protons was simplified into three proton multiplet signal suggested the

87 presence of one exchangeable proton in the compound. Even though the compound contains two exchangeable protons viz. N-H and O-H, N-H protons are not much observed in proton NMR spectra. Hence, the 1H NMR for the compound in DMSO is totally integrated for only 10 protons instead of 11 protons. Thus the exchangeable proton signal is attributed to O-H group of squaric acid. The compound contains four sets of protons viz. H2 and H6, H3 and H5, H8 and H12 and H9, H10 and H11. The multiplet signal at δ 7.50 ppm is attributed to H9, H10 and H11 protons; a multiplet at δ 7.80 ppm integrated for two protons can be assigned to H8 and H12 protons of benzene ring. Two doublets at δ 8.17 and δ 8.62 ppm were attributed to H2 and H6 and H3 and H5 protons, respectively.

Appearance of two doublets for the pyridine protons confirms the para substitution. Structure of the compound is further confirmed from its 13C NMR spectrum. There are eight sets of carbon atoms present in the compound. Two equivalent carbonyl carbons C-3′ and C-4′ resonate at δ 194 ppm, equivalent

C-1′ and C-2′ were resonate at δ 123 ppm. Other carbons resonance signals can be assigned as C2 and C6 at δ 135 ppm, C3 and C5 at δ 144 ppm, C4 at δ 153 ppm, C7 at δ 131 ppm, C8 and C12 at δ 127 ppm and more intense resonance signal at δ 129 ppm is attributed to C9, C10 and C11 carbons (Fig. 4.3).

Fig 4.3

Indication of NMR spectra analysis of 4PHS crystal

4.3.4. Optical transmission spectrum analysis

Since single crystal is mainly used in optical applications, the optical transmission range and the transparency cut-off wavelength are essential.

Optical behaviour of 4-phenylpyridinium hydrogen squarate was measured by Varian Cary 500 Spectrophotometer in the wavelength range of

200-2000nm. The recorded spectrum is shown in Fig. 4.4. From the measurement we observed that the crystal is transparent in the range of wavelength 240-2000nm. The UV transparency cut-off wavelength of

4-phenylpyridinium hydrogen squarate crystal occurs at 240 nm. There is infrared absorption peaks at 1710nm which is an overtone of the CH stretching vibrations [79].

Fig. 4.4.

UV-Vis-NIR Spectrum of 4PHS Crystal

4.3.5. Second harmonic generation

The SHG of the crystal was checked using the powder SHG technique developed by Kurtz and Perry [80]. The crystal was ground into powder and densely packed in a capillary tube. An Nd:YAG [DCR 11] laser beam of wavelength 1064 nm was made to fall normally on the sample tube. The ini cident beam focused on the capillary tube by 10 cm lens. The scattered light collected at

90◦ to the incident beam. The emission of green light confirms the second harmonic generation on 4-phenylpyridinium hydrogen squarate. Its conversion efficiency was found to be five times higher than that of standard potassium dihydrogen phosphate (KDP).

4.3.6. Thermal analysis

The TG/DTA analysis of 4PHS was carried out using NETZSCHSTA

409C between 30◦C and 450◦C at a heating rate of 25 ◦C/min. The recorded

TGA trace and its differential thermogravimetric trace are shown in Fig. 4.5.

There is an intensive weight loss between 250◦C and 300◦C. The study therefore reveals that the crystal is devoid of any lattice water and it is stable upto 250◦C. The DTA analysis was carried out between 30◦C and 450◦C in the nitrogen atmosphere. There is an endothermic transition between 250◦C and

300◦C, which is good in agreement with the TGA trace. However, the sharp endothermic peak at around 260◦C is assigned to melting point of the

4-phenylpyridinium hydrogen squarate compound. It is followed by decomposition and volatilization of the compound above 310◦C. TG/DTA analysis done by Srinivasan [81] proves that the developed (4-nitro-4′- methyl benzylidene aniline) crystal is stable up to 224.02◦C, where as the other researchers like Tapati Mallik [82] developed the l-arginine maleate dehydrate crystals with a maximum temperature stability of considerably lower than that of 224.02◦C. It may be useful for making the NLO devices below

250◦C.

Fig.4.5.

TGA/DTA curve of NMTC crystal

4.4. CONCLUSION

The title compound of 4PHS has been successfully synthesized by chemical reaction and the single crystals have been grown by slow evaporation solution growth method. From the single crystal XRD measurements the crystal belongs to monoclinic crystal system and its unit cell parameters have been determined. The functional groups have been identified from FTIR analysis. The presence of carbon and protons has been confirmed from the 13C and 1H NMR analyses. The sharp endothermic peak at around 260◦C is assigned to melting point of the

4-phenylpyridinium hydrogen squarate compound. It is followed by

92 decomposition and volatilization of the compound above 310◦C. It may be useful for making the NLO devices below 250◦C. Its relative SHG efficiency has been determined from the Kurtz powder technique and found that five times greater than that of KDP.

CHAPTER – 5

SYNTHESIS, GROWTH AND CHARACTERIZATION OF A NEW

NONLINEAR OPTICAL CRYSTAL: GLYCINIUM HYDROGEN

SQUARATE (GHS)

5.1. INTRODUCTION

Nonlinear optical (NLO) materials are used in electro-optic switching elements for telecommunication, optical information processing, optical parametric oscillator, degenerate four wave mixing, optical disk data storage, laser remote sensing, laser driven fusion, colour display and medical diagnostic, etc. [31]. The NLO process requires materials that manipulate the amplitude, phase, polarization and frequency of optical beams. At present, the aim is to develop materials satisfying all the technological requirements such as wide transparency range, fast response, and high damage threshold [83]. It is well known that certain classes of organic compounds show very high NLO and electro-optical effects. The linearity is based on molecular units, containing donor and acceptor groups at the opposite ends of the molecule which produces dipolar structure [84]. It has been long recognized that the electronic structure and the strength of donor and acceptor groups are responsible for achieving large molecular hyperpolarizabilities. Optimization of polarizability on molecular level is directed towards the synthesis of chromophores exhibiting intense low-lying absorption maxima with high dipoles of the charge transfer transition and large difference between ground and excited state dipole

94 moments. These molecular parameters could be used as characteristics for their

NLO potential [85].

The interest towards squaric acid derivatives is dictated both by the numerous potential fields of applications and by the simplicity of their pattern for theoretical investigations. Squaric acid (3, 4 dihydroxy-3-cyclobutene 1, 2- dione) is a typical two dimensional hydrogen bonded system with large isotope effect [86]. The transfer of hydrogen atom from its equilibrium position is responsible for second harmonic generation (SHG) in squaric acid. The SHG intensity is relatively strong and seems to increase linearly at temperature below about 340K [87]. A new nonlinear optical material known as glycinium hydrogen squarate (GHS) has been synthesized and the characteristics of the grown crystals have been studied by different instrumentation methods. The results are reported for the first time.

5.2. EXPERIMENTAL PROCEDURE

Glycine and squaric acid (AR grade) were used as starting materials for growing GHS single crystals. Glycine and squaric acid were dissolved separately in deionized water and ethanol in the ratio 1:1 and were mixed together to prepare a supersaturated solution. This solution is filtered twice to ascertain the growth of pure crystals. Crystal growth process was carried out in an aqueous medium.

The formed ionic compound of glycine hydrogen squarate is represented by the following equation.

+ - NH2CH2COOH + C4H2O4 NH3CH2COOH C4HO4

+ ---- - CH2 – NH2 OH O CH2 – NH3 O O

COOH + COOH OH O OH O

The saturated solution was maintained in an undisturbed condition and the beaker was covered with a polythene paper. A few holes were made on the polythene cover to facilitate slow evaporation. By adopting the solution growth method, single crystals of glycinium hydrogen squarate (GHS) were grown from the supersaturated solution at room temperature. Glycine interacts with squaric acid through a single N-H-O hydrogen bond. The solution was periodically inspected and on the 25th day, the crystal started growing. Further, the crystal was permitted to grow for another 20 days in order to obtain a nominal size suitable for characterization. The single crystals of dimension upto 10 X 3 X 2 mm3 were harvested and the photograph of GHS crystal is shown in Fig 5.1.

Fig. 5.1.

Photograph of GHS single Crystal

5.3. CHARACTERIZATION

The lattice parameters and the crystal systems have been determined

using single crystal X-ray diffraction analysis (Model: Bruker AXS Kappa

APEX II single crystal CCD diffractometer) and the result is compared with

powder X-ray diffraction analysis (Rich Seifert diffractometer). The functional

groups presented in the title compound have been identified from the FTIR

analysis using KBr pellet technique in the region from 400cm-1 to 4000cm-1

(Bruker IFS 66V model FTIR spectrometer). Optical behaviour of GHS was

measured by Perkin Elmer Lambda 35 UV-VIS-NIR Spectrophotometer in the

wavelength range of 190-1100nm. The 1H-NMR and 13C-NMR of the grown

sample have been taken using Bruker 300MHz ultrashield NMR spectrometer.

The SHG test was carried out using Kurtz Perry technique. The thermal

stability of GHS was studied by thermo gravimetric analysis (TGA) and

differential thermal analysis (DTA) by using SDT Q600 V8.3 Build 101

thermal analyzer instrument ranging from room temperature to 11000C at a

heating rate of 200C per minute under nitrogen atmosphere.

5.3.1 Single crystal XRD

Precise single crystal X-ray diffraction data for the GHS crystal was

collected using X-ray diffractometer equipped with graphite monochromated

Mo(Kα) (λ= 0.7107Å) radiation. The observed results indicate that the crystal

belongs to monoclinic crystal system and the determined unit cell parameters

are given below in a comparative statement:

Table. 5.1

Comparative statement for lattice parameters of glycine, squaric acid and

GHS crystals

a b c Crystal Crystal α β γ (Å) (Å) (Å) System

Glycine 6.996 6.996 5.471 900 900 1200 Hexagonal

Squaric acid 6.13 5.27 6.14 900 900 900 Tetragonal

GHS 16.78 8.27 15.73 900 1000 900 Monoclinic

5.3.2 Powder XRD analysis of GHS

Powder X- ray diffraction analysis of grown GHS crystals have been

carried out using Rich Seifert diffractometer with CuKα (λ = 1.54060 Ǻ)

radiation on crushed powder of GHS crystals. The recorded powder X-ray

pattern is shown in Fig. 5.2. The differences in amplitude of the peak can be

attributed to the difference in grain size and orientation of the powdered grains

of GHS crystal. The observed diffraction pattern is indexed by Reitveld index

software package. These lattice parameters calculated by Reitveld unitcell

software package are compared with cell parameters observed from single

crystal x-ray diffraction method and tabulated in Table 5.1. The data obtained

by powder XRD analysis are in accordance with the single crystal XRD data.

The values of h, k, l and 2 theta values are stacked in Table 5.2.

Table 5.2 Cell parameters of GHS crystal a b c α β γ Volume XRD Å Å Å deg deg deg Å3

Single crystal 16.78 8.27 15.73 90 100.39 90 2147

Powder 16.81 8.27 15.75 90 100.22 90 2155

Fig.5.2

Power X-Ray Diffraction of GHS crystal

Table 5.3 Powder XRD data of GHS crystal Position d- spacing ( h k l ) °2θ Å 21.3592 4.1601 ( 3 0 2 ) 23.3386 3.8115 ( 4 0 1 ) 25.6290 3.4759 ( 4 1 1 ) 27.2138 3.2769 ( -4 1 3 ) 27.8582 3.1999 ( 3 1 3 ) 32.4115 2.7626 ( -3 1 5 ) 35.2786 2.5441 ( 1 3 2 ) 38.1194 2.3608 ( 5 2 2 ) 42.3941 2.1321 ( 1 2 6 ) 43.5519 2.0781 ( 1 1 7 ) 55.7455 1.6490 ( 4 0 8 ) 57.6359 1.5980 ( 3 4 5 )

100

5.3.3. FTIR spectral analysis

FTIR (Fourier transform Infrared) Spectroscopy is one of the most reliable methods for identification and characterization of organic and inorganic materials. The FTIR absorption spectrum of the title compound is shown in Fig. 5.3. In the present study, a strong and broad FTIR band observed at 3403 cm-1 is assigned to OH stretching vibration. The C=O stretching vibrational mode at 1609 show excellent agreement with the literature value

-1 [88]. The peak at 1502 cm is due to C-O stretching vibration. The CH2 wagging vibration at 920 cm-1 is excellent with expected characteristic value

-1 [89-90]. The mode that appears in the region 3110 cm is assigned to NH3 stretching mode of vibrations [91]. The observed bands along with their vibrational assignments are presented in Table 5.3.

Table 5.4

The observed frequencies and corresponding assignments

Observed FTIR frequencies Assignments 3403 OH stretching

3110 NH3 stretching 1609 C = O stretching 1502 C – O stretching

920 CH2 wagging

101

Fig.5.3

FTIR Spectrum of GHS

5.3.4. Optical transmission spectrum

The UV-Visible NIR spectrum for grown GHS crystal was recorded in the wavelength range of 190 to 1100 nm. The percentage of transmittance vs. wavelength is shown in Fig. 5.4. Since the single crystal is mainly used in opto- electronic applications, the optical transmission range and the transparency cut- off wavelength are essential. From this measurement, it is observed that the crystal is transparent more than 90% in the wavelength range of 390-1100 nm.

The UV transparency cut-off wavelength of GHS crystal occurs at 342 nm.

Due to high transmittance in the visible and infra red region, the GHS crystal could well be used as a potential material for NLO applications.

102

Fig.5.4

UV-Vis-NIR spectrum of GHS crystal

5.3.5 Nuclear magnetic resonance

1H NMR and 13C NMR analysis

The commonly used NMR spectroscopy techniques are notably 1H-

NMR and 13C- NMR. Hydrogen and carbon are the core elements in organic chemistry and hence 1H NMR and 13C-NMR play an important role in determining the structure of unknown organic molecules. The 1H NMR spectrum of GHS is shown in Fig. 5.5. In 1H-NMRspectrum of GHS, the peak observed at δ 7.850 ppm is assigned to the OH proton of squaric acid in the title compound. The sharp and intense peak at δ 4.712 ppm was due to the presence

103 of undeuteriated D2O as solvent. The singlet at δ 4.177 ppm was assigned to methylene proton (CH2) of glycine which is tabulated in Table 5.4.

The structure of the compound is further confirmed from its 13C-NMR spectrum. The presence of multiple carbonyl carbons in the compound was revealed by the number of signals that arrive in the spectrum. A peak at δ

197.18 ppm is due to carbonyl carbon of the carboxylic group present in the glycine moiety [92]. A signal at δ 170.88 ppm with large intensity can be assigned to two carbonyl carbons of the “squaric” acid moiety. Two more signals that arrived at δ 174.07 ppm and δ 181.15 ppm support the existence of squarate anion in the compound. The reason for obtaining two signals can be explained with the possible proton exchange from neighboring OH group to the anionic oxygen. The methylene proton signal was obtained at 40 ppm. The above said evidences from proton and 13C–NMR spectra support the formation of an ionic compound between glycine and squaric acid. The 13C-NMR spectrum of GHS is shown in Fig. 5.6. The chemical shift assignments of carbon of GHS are given in Table 5.5.

Table 5.5

Chemical Shift assignments of proton of Glycinium hydrogen squarate

Protons δ, ppm

-CH2- (glycine) 4.177

-OH- (squaric acid) 7.850

D2O (solvent) 4.712

104

Table 5.6

Chemical Shift assignments of carbon of Glycinium hydrogen squarate

Carbon δ, ppm

C = O (glycine) 197.18

C = O (squaric acid) 170.88

C O- (squaric acid) 174, 181

CH2 (glycine) 40

Fig.5.5

1H-NMR spectrum of GHS crystal

105

Fig.5.6

13C NMR spectrum of GHS

5.3.6. Second harmonic generation

The second harmonic generation (SHG) efficiency has been carried out using the powder technique of Kurtz and Perry [80]. A Q-switched Nd:YAG laser beam of wavelength 1064 nm, with a beam energy of 4.5 mJ/pulse, and a pulse width of 8 ns with a repetition rate of 10Hz was used. The grown single crystal was crushed to fine powder of about 125 μm to 150 μm and then packed in a micro capillary of uniform bore and exposed to laser radiations. The 532 nm radiation was collected by a monochromater after separating the 1064 nm pump beam with an infra-red blocking filter. The second harmonic radiation generated by the randomly oriented micro crystals was focused by a lens and detected by a photo multiplier tube (Hamamatsu

R2059). The second harmonic generation was confirmed by the emission of green

106 light. The measured SHG value of the grown crystal is 3.4 mV. For the same input, the SHG value of KDP is 20 mV. The SHG efficiency of the grown crystal was found to be 17% as that of KDP.

5.3.7. Thermal analysis

The thermal behaviour of GHS crystal has been studied from room temperature to 11000C at a heating rate of 200C per minute under nitrogen atmosphere. The TG-DTA curve of glycinium hydrogen squarate is shown in

Fig. 5.7. From the TGA curve, it is observed that the title compound is thermally stable upto 1500C, the same is confirmed by the peak observed at

1500C from the DTA curve. The thermal stability of GHS is low compared to glycine (2330C) and squaric acid (2930C). There are many stages of weight loss in TGA curve, during the first stage 11.28% of weight loss is observed. This weight loss may be due to the loss of OH molecule from the title compound which was compared with the theoretical value (9%). Corresponding peaks which were observed upto 295.760C in DTA curve reveals the loss of OH and

COOH molecules. In the next stage an amount of 23.03% of weight loss occurred due to the loss of COOH. Theoretically it was found to be 23.8%.

Further, 9.98%, 8.62% and 10.29% of weight losses were observed due to the loss of CH4, NH3 and OH respectively. An exothermic peak observed between

295.760C and 730.680C confirms the loss of the above mentioned compounds.

In the last stage of decomposition, 10.68% and 21.18% of weight losses are observed, which may be due to the release of carbon and CO2. Finally, 0.138% of residue is observed which may be a fraction amount of hydrogen and carbon.

107

The thermal stability of the title compound is more than glycine nitrate [93] and is lower than 4-phenyl pyridinium hydrogen squarate, gamma glycine, glycine acetamide [37,94-96].

Fig.5.7.

TGA/DTA curves of GHS crystal

5.4. CONCLUSION

The compound of GHS has been conveniently synthesized by chemical reaction and grown by slow evaporation solution growth method at room temperature. From the single crystal XRD and Powder XRD measurements, it is known that the crystal belongs to monoclinic crystal system. The functional groups were confirmed by FTIR technique. The transparency range of GHS was observed between 390-1100nm. The structure of the grown crystal was established from the 1H and 13C NMR analyses. The second harmonic generation was checked by Kurtz and Perry technique which showed positive

108 results. From the DTA curve, it is observed that the material is stable upto

1500C which is the melting point of the substance. It may be useful for making the NLO devices below 1500C.

109

CHAPTER – 6

CRYSTALLIZATION AND CHARACTERIZATION OF A NEW NON

LINEAR OPTICAL CRYSTAL: L PROLINE SUCCINATE (LPS)

6.1. INTRODUCTION

One of the most important applications of NLO materials is their use for fast data transfer, combined with a very high Signal-to-Noise ratio, even over long distances. In recent years, different applications of NLO and photorefractive materials have been developed, for example, optical frequency conversion, electro-optical modulation, dynamic holography, optical writing and optical guiding of laser beams [97]. It is seen that L-proline and (4R)- hydroxy-L-proline derivatives, containing donor groups are chiral carriers [98].

The introduction of chirality by means of an asymmetrically substituted carbon should in addition respect the molecular features leading to a high nonlinear behaviour [99]. Proline and its derivatives are often used as asymmetric catalysts in organic reactions. Only noncentrosymmetric alignment of the chromophores in the crystal lattice leads to an observable bulk second-order

NLO response [100]. In order to obtain the adjustment of the nonlinear efficiency/transparency, based on the molecular engineering and crystal engineering approach, it is tried to develop a new method to design organic non-linear optical second-harmonic generation materials such as organic inclusion complex [101]. In this study, the crystal growth of a new NLO crystal

110 of L-Proline succinate by slow evaporation technique and its characterization along with its optical properties is reported.

6.2. EXPERIMENTAL DETAILS

Equal proportions of L-Proline and succinic acid were taken and were dissolved separately in deioinized water. Then the solution of L-Proline was poured into the dissolved succinic acid mixture. The solution thus arrived was filtered twice to remove dust particles and undissolved materials. The reaction takes place between L-Proline and succinic acid (acid-based) through hydrogen transfer. Thus formed ionic compound of L-Proline succinate is represented in the following equation:

+ - C5H9NO2 + C4H6O4 → C5H10NO2 C4H5O4

The saturated solution was maintained in the undisturbed condition and the beaker was covered by polythene paper. Few holes were made on the polythene cover for slow evaporation. By adopting the solution growth method, single crystal of L-Proline succinate (LPS) was grown from supersaturated solution at room temperature. Then this solution was periodically inspected and

111 from the 20th day onwards the crystal started growing and it was permitted to grow for another 10 days in order to get a nominal size suitable for characterization. The single crystal of LPS with dimensions of 8mm x 5mm x

10mm was thus obtained. The L-Proline interacts with succinic acid through a single N-H-O hydrogen bond. A single crystal of LPS which has been grown by this process is shown in Fig 6.1.

Fig. 6.1.

Photograph of LPS crystal

6.3. CHARACTERIZATION

The lattice parameters and the crystal systems have been determined using single crystal X-ray diffraction analysis (Model: Bruker AXS Kappa

112

APEX II single crystal CCD diffractometer). The functional groups presented in the LPS compound have been identified by Bruker IFS 66V model FTIR

Spectrometer using KBr pellet technique in the region 400 – 4000 cm-1. Optical behaviour of LPS was measured by Perkin Elmer Lambda 35 UV-VIS-NIR

Spectrophotometer in the wavelength range of 190-1100nm. The thermal stability of LPS was studied by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) by using SDT Q600 V8.3 Build 101 thermal analyzer instrument ranging from room temperature to 1100oC at a heating rate of 20oC per minute under nitrogen atmosphere.

6.3.1. Single crystal X-Ray Diffraction Analysis

Single crystal X-ray diffraction studies were carried out on the grown crystals. The X-ray data were collected using X-ray diffractometer (Model:

Bruker AXS Kappa APEX II single crystal CCD). The observed results indicate that the crystal belongs to monoclinic crystal system and the determined unit cell parameters are:

a = 5.07 Å, b = 8.84 Å, c = 5.48 Å, α = 90o, β = 91.60o, γ = 90o and V = 246Å3.

6.3.2 FTIR Spectroscopy

The functinal groups presented in the LPS compound have been identified by Bruker IFS 66V model FTIR Spectrometer using KBr pellet technique in the region 400 – 4000 cm-1. The FTIR spectrum of title compound

113 is shown in Fig. 6.2. The peaks obtained are 3419 cm-1 , due to stretching vibration of CH and the peak at 1600 cm-1 is due to the stretching vibration of

C=O. The bands appeared at 793 cm-1 is assigned unambiguously to the wagging of NH2 modes. The OH stretching vibrations is assigned in the range

-1 -1 - of 2565 cm . The peak at 1398 cm is due to the symmetric stretching of COO

These assignments are also supported in the literature [102-106]. The observed bands along with their vibrational assignments are given in Table 6.1.

Fig. 6.2.

FT-IR Spectral Analysis of LPS Crystal

114

Table 6.1

FT-IR spectral assignments of LPS crystal

Observed FTIR frequencies Assignments

3419 CH stretching

1600 C=O stretching

1398 Symmetric stretching of COO-

793 NH2 wagging

2565 O–H stretching

6.3.3. Optical Transmission Spectrum Analysis

Good optical transmittance and lower cut-off wavelengths are very important properties for NLO crystals. Optical behaviour of LPS was measured by Perkin Elmer Lambda 35 UV-VIS-NIR spectrophotometer in the wavelength range of 190-1100 nm.The recorded spectrum is shown in Fig. 6.3.

The crystals are broadly transparent possessing a transmission of greater than

90% for light with incident wavelengths from 236-1100 nm. The UV transparency cut-off wavelength of LPS crystal occurs at 204 nm which is better than L-Prolinium tartrate and 4-phenylpridinium hydrogen squarate

[107,37]. It is observed that in the LPS crystal, there is high transmittance in the far ultraviolet, visible and infra red region. Hence, the title compound may

115 be used for the nonlinear optical applications in the above mentioned wavelength range.

Fig. 6.3.

UV-Vis-NIR Spectrum of LPS Crystal

6.3.4 Second harmonic generation

The SHG of the crystal was checked using the powder SHG technique developed by Kurtz and Perry15. A Q-switched Nd:YAG laser beam of wavelength 1064 nm, with beam energy of 4.5 mJ/pulse, and pulse width of 8 ns with a repetition rate of 10Hz were used. The grown single crystal was

116 crushed to fine powder and then packed in a micro capillary of uniform bore and exposed to laser radiations. The 532 nm radiation was collected by a monochromator after separating the 1064 nm pump beam with an infra-red blocking filter. The second harmonic radiation generated by the randomly oriented micro crystals was focused by a lens and detected by a photo multiplier tube (Hamamatsu R2059). The second harmonic generation is confirmed by the emission of green light.

6.3.5 Thermal analysis

The thermal behaviour of LPS had been studied by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) using SDT Q600 V8.3

Build 101 thermal analyzer instrument, ranging from room temperature to

1100oC at a heating rate of 20oC per minute under nitrogen atmosphere. TGA-

DTA curve of L-Proline succinate is shown in Fig. 6.4. From the TGA curve, the material starts to decompose around 160oC, which is also confirmed by the peak which appears at 154oC in the DTA curve. The thermal stability of LPS is low compared to L-Proline (205oC) and succinic acid (240oC). The TGA curve shows a major loss of weight and two losses of smaller weight. The major weight loss of 86.58% is observed between 169.91oC and 243.16oC. The decomposition of L-Proline succinate (87.53%) leads to the major loss of weight in the above mentioned temperature region. Further, a small loss of weight of 10.24% is observed, which may be due to the decomposition of C2H2

(11.16%). During the next stage of decomposition, fraction amount of

117 hydrogen may be decomposed. A residue of 0.9018% which may be due to presence of some fraction of carbon is observed. The thermal stability of the

LPS single crystal is more than glycine nitrate [108] and is lower than gamma glycine and glycine acetamide [95, 96].

Fig. 6.4.

TGA/DTA curve of LPS crystal

6.4. CONCLUSION

We have synthesized a new non-linear optical crystal with an interesting hydrogen bonding network that holds together the L-Proline and succinic acid molecules. The grown crystals are characterized by different instrumental techniques. The single crystal XRD studies prove that the grown LPS crystals belong to monoclinic crystal system. The presence of the functional groups of

118 the grown crystal has been confirmed by FTIR analysis. From the optical transmittance spectrum, it is observed that there is high transmittance in the far ultraviolet, visible and near infra red regions. The UV transparency cut-off wavelength of LPS crystal occurs at 204 nm. The Kurtz Perry technique for second harmonic generation has showed positive results. It is well known that the DTA and TGA studies reveal that the crystal is thermally stable upto 160oC

119

CHAPTER – 7

GROWTH AND CHARACTERIZATION OF CADMIUM

MANGANESE THIOCYANATE (CMTC) CRYSTAL

7.1. INTRODUCTION

In the emerging field of communication, opto-electronic

devices play a very important role. Materials with high transmission

range in optical fiber communication are coming up fastly. Initially pure

organic materials were used, because of low cost, fast transmission and

non linear response over a broad bandwidth of frequencies with high

optical damage threshold but in these materials, there are problems

due to volatility, low thermal stability, mechanical weakness etc. Hence

attention has been turned towards inorganic compounds and coordination

complexes [109]. Metal complexes offer a large variety of environmental

stability and tunable electronic properties.

In recent years, thiocyanate compounds have attracted the attention of material scientists, due to the NLO properties exhibited by them, when they are forming complexes with metals of the form AB(SCN)4.

Several studies with different AB compounds in different proportions and also with different dopant percentages have exhibited remarkable optical properties. The materials are relevantly chosen that CN and CS stretching

120 vibrations are enhanced and SCN bending is reduced for obtaining better optical properties. The cadmium mercury thiocyanate crystal consists of two slightly flattened tetrahedra CdN4 and HgS4. The noteworthy factors in this compound namely –N=C=S– bridge which connects the central atom of the flattened tetrahedra –Cd–N=C=S–Hg– networks. These structural features provide the good optical activities [110-111]. In the present work, an attempt has been made to replace Hg by Mn. The characterizations of grown crystals have also been studied here.

7.2. EXPERIMENTAL PROCEDURE

The commercially available AR grade potassium thiocyanate,

cadmium chloride and manganese chloride have been used to synthesize

the CMTC crystal. All the preparation and growth process were carried

out in aqueous solution. The CMTC crystals were thus prepared by taking

the raw materials in molar ratio 1:1:1 and then dissolved in the de- ionised

water.

4K (SCN) + CdCl2 + MnCl2 Cd Mn (SCN)4 + 4KCl

The solution was filtered to remove dust particles and undissolved

materials. The saturated solution was maintained in the undisturbed condition

and the beaker was covered by polythene paper. Few holes were made on the

polythene cover for slow evaporation. By adopting the solution growth

method, single crystal of CMTC was grown from supersaturated solution at

121

room temperature. The aqueous solution was maintained in the undisturbed

condition. The solution was periodically inspected and at the 30th day the

crystal started growing. Further the crystal was permitted to grow for another

20 days in order to obtain a nominal size suitable for characterization. The

large single crystal of CMTC was grown out with dimensions of

30 x 20 x 30 mm3, which is shown in Fig.7.1.

Fig.7.1.

Photograph of CMTC crystal

7.3. CHARACTERIZATION

The lattice parameters and the crystal systems have been determined by using single crystal X-ray diffraction analysis (Model: ENRAF NONIUS

122

CAD 4). The functional groups presented in the title compound have been identified from the FTIR analysis using KBr pellet technique in the region from 450cm-1 to 4000cm-1. In order to come to this decision, Bruker IFS 66V model FTIR spectrometer was used. Optical behaviour of CMTC was measured by Varian Cary 500 Spectrophotometer in the wavelength range of 200-2000nm. The thermal stability of CMTC was studied by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) by using STA 409 C instrument in the temperature range upto 1200ºC at a heating rate of 10ºC per minute under nitrogen atmosphere.

7.3.1. Single crystal XRD

The observed results indicate that the crystal belongs to tetragonal crystal system and the determined unit cell parameters are a = 12.0835 Ǻ, b = 12.1349 Ǻ , c = 8.5574 Ǻ and α = 89.7128, β = 90.053, γ = 90.0023 and

Volume = 1254 Ǻ3. The family of crystals such as cadmium mercury thiocyanate (CMTC), ferrous mercury thiocyanate (FMTC), zinc mercury thiocyanate (ZMTC) and zinc cadmium thiocyanate (ZCTC) also belong to tetragonal crystallographic system [112].

7.3.2. FT-IR Spectral Analysis

The recorded FT-IR spectrum is shown in Fig. 7.2. The frequencies

were observed and the corresponding assignments are given in

123

Table 7.1. The absorption peak 783 cm-1 refers to the CS stretching vibration. The peaks at 2077 cm-1, 1397 cm-1 and 586cm-1 correspond to the CN stretching, CN asymmetric stretching and SCN symmetric stretching vibration [113-116] respectively.

Fig. 7.2.

FT-IR Spectrum of CMTC Crystal

124

Table 7.1

FT-IR spectral assignments of CMTC crystal

Wave number (cm-1) Tentative assignments

586 (w) γ ss SCN

783 (vw) γ CS

1397 (s) γ as CN

2077 (vs) γ CN w – weak ; vw – very weak ; s – strong ; vs – very strong ; γ – stretching ; γ ss - symmetric stretching ; γ as – asymmetric stretching .

7.3.3. Optical transmission spectrum Analysis

When single crystal is mainly used in opto-electronic applications,

the optical transmission range and the transparency cutoff wavelength are

essential. The spectrum is shown in Fig. 7.3. It is observed that the

crystal is transparent in the wavelength range of 390 - 1170 nm. The UV

transparency cutoff wavelength of CMTC crystal occurs at 380 nm which

is very near to that of MMTD crystal (375nm) [117].

125

Fig. 7.3.

UV-VIS-NIR Spectrum of CMTC Crystal

7.3.4. Thermal Analysis

The TGA/DTA thus observed is shown in Fig. 7.4. There is a first weight loss starting at about 230ºC. It is due to the loss of water which was embedded within the crystal. The second weight loss is observed between 800 and 1000° C. It is due to a decomposition of CMTC crystals. From the DTA curve, it is observed that the material is stable upto 430°C, which is the melting point of the substance. The thermal stability of CMTC crystal is greater than 4PHS, DSTC, CMTC, NMTC [94, 118-120].

126

Fig. 7.4.

TGA/DTA curves of CMTC Crystal

7.4. CONCLUSION

The single crystals of CMTC have been successfully synthesized by chemical reaction and grown by the technique of slow evaporation. The XRD measurements have proved that the crystal is of tetragonal system. Its unit cell parameters were also determined. The presence of functional groups was conformed by the FTIR technique. The optical studies show that the crystal transparency is in the range of 390-1170nm, which highlights its prospects of application in opto-electronic devices. The thermal behavior of grown crystal was studied by TGA/DTA analysis and the melting point is thus confirmed as 430ºC.

127

CHAPTER – 8

GROWTH AND CHARACTERIZATION OF ZINC MANGANESE

THIOCYANATE (ZMTC) CRYSTAL

8.1. INTRODUCTION

In opto-electronic technologies and communication, fibre optics play a vital role. In optic fiber communication for proper transmission and reception of laser light communication link is very essential. Researchers are trying to fabricate materials with high transmitting range. Initially investigations were focused on purely inorganic materials; but later on attention has been drawn towards organic materials due to their low cost, fast and non-linearity response over a wide range of frequencies and high optical damage threshold. However, organic materials may suffer from problems such as volatility, low thermal stability, mechanical weakness etc [109]. The main advantage of inorganic material is that it has short cut off wavelength, non-linearity and good physico chemical stability. Recently it is found that transition metal, inorganic compounds and coordinated complex have emerged as extremely promising building block for opto-electronic materials [111]. Compared to organic molecules, metal complexes offer a larger variety of structures of similar environmental stability or in some cases higher stability and a much greater diversity of tunable electronic properties.

128

In recent days, cyanide group of materials are gaining attention due to their good optical properties. As per the general formula of the crystal growth

AB (SCN)4 , the various A group and B groups of materials are suitably selected by considering different materials percentage of constituent materials and different percentages of different dopants which were not tried. The parent materials are selected to increase the electron transformation which increases the CN and CS stretching vibrations and reducing the SCN bending vibration for obtaining better optical properties [121]. The ZMTC crystal consists of two kinds of slightly flattened tetrahedra CdN4 and HgS4. The most striking feature is that – N = C = S – bridge which connect the central atom of the flattened tetrahedra – Cd – N = C = S – Hg – networks. These structural features provide the group optical activities [110]. In the present work, an attempt has been made to replace Hg by Mn, Cd by Zn and the characteristics of the grown crystals were studied and has been reported for the first time.

8.2. EXPERIMENTAL PROCEDURE

The commercially available Potassium thiocyanate, Zinc Chloride and

Manganese Chloride (AR grade) have been used to synthesize the ZMTC crystal. All the preparation and growth process are carried out in aqueous solution. ZMTC are prepared by taking the raw material in 1:1:1 molar ratio and then dissolved in the de-ionised water.

4KSCN + ZnCl2 + MnCl2 ZnMn(SCN)4 + 4KCl

129

The solution was filtered to remove dust particles and undissolved materials. The saturated solution was maintained in the undisturbed condition and the beaker was covered by polythene paper. Few holes were made on the polythene cover for slow evaporation. By adopting the solution growth method, single crystal of ZMTC was grown from supersaturated solution at room temperature. The aqueous solution was maintained in the undisturbed condition. The solution was periodically inspected and at the 30th day the crystal started growing. Further the crystal was permitted to grow for another

20 days in order to obtain a nominal size suitable for characterization. The large single crystal of ZMTC was grown out. Fig. 8.1. exhibits a transparent and colourless single crystal of ZMTC which has been grown by this process.

Fig. 8.1.

Photograph of ZMTC single Crystal

130

8.3. CHARECTERIZATION STUDIES

8.3.1 Single crystal XRD

The lattice parameters and the crystal system have been determined using the single crystal X-ray diffraction analysis by using X-ray diffractometer

(Model: ENRAF NONIUS CAD 4). The observed results indicate that the crystal belongs to tetragonal crystal system and the determined unit cell parameters are a= 12.0835Ao, b = 12.131 ÅÅ c = 8.569 Å α = 89.715o,

β = 90.018o and γ = 89.99o, V = 1256 (Å)3. The crystal such as CMTC, FMTC,

ZMTC and ZCTC along with the reported crystal ZnMn(SCN)4 belongs to tetragonal crystallographic system [112].

8.3.2 FT-IR Spectral Analysis

The functional groups presented in the title compound have been identified by Bruker IFS 66V model FTIR spectrometer using KBr pellet technique in the region from 450cm-1 to 4000cm-1. The detailed analyses were given below. The infrared absorption spectrum of ZMTC crystal is shown in

Fig. 8.2. ZMTC exhibits characteristic peaks at 673cm-1 and at 2347cm-1. The and at 673cm-1 is due to stretching vibration of SCN and the peak at 2347cm-1 is due to the stretching vibration of CN. The bands at 1417cm-1 and 1625cm-1 are attributed to asymmetric stretching of CN and symmetric stretching of CS respectively [113-114,122]. The assignment of IR absorption of ZMTC is given in the table 8.1.

131

Fig. 8.2.

FT-IR Spectrum of ZMTC Crystal

______

Table 8.1 –Assignment of IR absorption of ZMTC

Wave number (cm-1) Tentative assignments

673 (vw) γ SCN

γ 1417 (s) as C-N

γ 1625 (s) ss C-S

2347 (vw) γ C-N

γ – stretching γss – symmetric stretching γas – asymmetric stretching

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8.3.3 Optical Transmission Spectrum Analysis

Since single crystal is mainly used in opto-electronic applications, the optical transmission range and the transparency cutoff wavelength are essential.

Optical behaviour of ZMTC was measured by Varian Cary 500

Spectrophotometer in the wavelength range of 200 to 2000nm. The recorded spectrum is shown in Fig. 8.3. From this measurement, we observe that the crystal is transparent in the range of wavelength 390nm to 1193nm. The UV transparency cutoff wavelength of ZMTC crystal occurs at 380 nm which is nearly equal to MMTD crystal [117].

Fig. 8.3.

UV-Vis-NIR Spectrum of ZMTC Crystal

133

7.3.4 Thermal Analysis

The thermal stability of ZMTC was studied by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) using STA 409C instrument in the temperature range upto 12000 C at a heating rate of 100C per minute under nitrogen atmosphere. The TGA and DTA traces of ZMTC are shown in Fig. 8.4. There is a major weight loss between 800oC and 1168oC which occurs in a single stage. It is assigned to the decomposition of ZMTC crystals. There is a sharp endothermic peak at 806o C. It coincides with the weight loss in the TGA trace. Based on the results of TGA and DTA, the maximum temperature for opto electronic application of ZMTC crystal is limited to 806o C. It is found that melting point of the grown ZMTC crystal is

806oC. The thermal stability is more than DSTC, CMTC, NMTC

[123,116,120].

134

Fig. 8.4.

TGA/DTA curve of ZMTC crystal

8.4. CONCLUSION

The optically transparent single crystals of ZMTC were conveniently grown by slow evaporation technique at room temperature. The single crystal

X-ray diffraction studies confirm the system as tetragonal. The presence of the fundamental functional groups of the grown sample was confirmed by FTIR analysis. The transparency range of ZMTC was observed in between 390nm and 1193nm, which highlights its prospects of application in opto electronic applications. TGA/DTA studies reveal the single stage of weight loss occurring to the sample and it proves that ZMTC is thermally stable up to 806oC.

135

CHAPTER – 9

SUMMARY AND SUGGESTIONS FOR FUTURE WORK

9.1. SUMMARY

Organic and inorganic optical materials have been under extensive investigation due to their promising applications in opto-electronic devices, telecommunication, optical computing, optical data storage and optical information processing, The growth of organic and inorganic single crystals plays a vital role in linear and nonlinear optical (NLO) applications. Growth of high quality crystals with fewer defects is important for the industries involved in the production of bulk crystals.

The thesis mainly deals with the synthesis, growth and characterization of some squaric, amino acid and thiocyanate based optical single crystals. The materials having moderate to high solubility can be grown by low temperature solution growth method. The low temperature solution growth technique is well suited for growing organic and inorganic materials.

In the present investigation single crystals of 4-phenylpyridinium hydrogen squarate(4PHS), glycinium hydrogen squarate(GHS), L-proline succinate(LPS), cadmium manganese thiocyanate(CMTC), zinc manganese thiocyanate(ZMTC) have been grown from slow evaporation technique.

Various physical properties of the grown crystals are studied for the application point of view.

136

Table 9.1.

Comparative statement of the grown crystals

UV cut-off Thermal Grown Crystal Wavelength Stability SHG efficiency Crystals System (nm) (°C)

5 times greater than 4PHS Monoclinic 240 260 that of KDP

GHS Monoclinic 342 150 17% of that of KDP

LPS Monoclinic 204 160 23% of that of KDP

CMTC Tetragonal 380 430 0

ZMTC Tetragonal 380 806 0

The comparative statement of the grown crystals is shown in the

Table 9.1. Among the five materials reported in the present investigation, three crystals belong to monoclinic system and the remaining two belong to tetragonal crystal system.

137

Since the single crystal is mainly used in opto-electronic applications, the optical transmission range and the transparency cut-off wavelength are essential. The UV cut-off wavelengths of the grown crystals are also given in the Table 9.1. The optical spectrum reveals that the grown crystals have good optical transparency in the entire visible and IR region. From the table it is observed that the UV transparency cut-off wavelength is low for organic crystals compared to the inorganic crystals.

Considering the thermal stability of the grown crystals, compared with the organic crystals the thermal stability of the inorganic crystal (CMTC and

ZMTC) is high.

From the Table 9.1, it is clear that 4PHS have good SHG conversion efficiency than that of the remaining organic grown crystals. All the grown crystals can be considered for opto-electronic device fabrication.

9.2. SUGGESTIONS FOR FUTURE WORK

The grown organic and inorganic optical crystals can be used to fabricate opto-electronic and NLO applications. A detailed study on the determination of crystal structure can be made for the crystals reported in this investigation. It will explain the cause for variation of thermal stability, UV cut-off wavelength and SHG efficiency of the doped crystals. The effect of impurities with different percentage on the growth of these crystals can be carried out to study the change in their properties. Further possible derivatives

138 of the crystals may be synthesized and investigated for the improvement on their SHG efficiencies.

The particle size may be one of the factors which influence the SHG efficiency. The nano-particle size of the grown crystal may change the NLO efficiency of the crystal. Hence, attempts could be made in future to synthesis the crystals in nano-crystalline form. The physicochemical properties of the nano-crystalline materials can be investigated and the influence of their size, in particular, can be studied for NLO efficiency.

139

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Annexure Journal of Crystallization Process and Technology, 2012, 1, 21-24 21 doi:10.4236/jcpt.2012.21004 Published Online January 2012 (http://www.SciRP.org/journal/jcpt)

Crystallization and Characterization of a New NonLinear Optical Crystal: L Proline Succinate (LPS)

P. Paramasivam1, C. Ramachandra Raja2* 1Anjalai Ammal Mahalingam Engineering College, Kovilvenni, India; 2Department of Physics, Govt Arts College (Autonomous), Kumbakonam, India. Email: *[email protected]

Received November 7th, 2011; revised December 9th, 2011; accepted December 18th, 2011

ABSTRACT In this analysis, the single crystal of L-Proline Succinate (LPS) has been successfully synthesized and the purity of material has been increased by repeated recrystallization process. Single crystal was grown by adopting the method of growing in a slow evaporation solution using water as solvent at room temperature. The LPS single crystal has been synthesized by taking equimolar quantity of L-Proline and succinic acid, by mixing them thoroughly using deionized water. The prepared concentrated solution was placed in an undisturbed condition, and the solution was inspected regu- larly. The single crystal has been harvested over a period of 1 month. The same crystal was characterized by different techniques for finding its suitability for device fabrications. The grown crystal was characterized by Single crystal XRD, Powder XRD, FTIR, UV-vis-NIR, DTA/TGA and SHG analyses, respectively. The observed results from various characterization show the suitability for NLO application. The second harmonic generation of this grown crystal was checked using Kurtz Perry technique which showed positive results. The UV cut-off wavelength and the decomposition temperature of this grown crystal were found to be good when compared with the existing organic crystals.

Keywords: Slow Evaporation; Crystal Growth; X-Ray Diffraction; Fourier Transform Infrared Spectroscopy; Second Harmonic Generation; LPS Crystal

1. Introduction as organic inclusion complex [5]. In this study, the crystal growth of a new NLO crystal of L-Proline succinate by One of the most important applications of NLO materials slow evaporation technique and its characterization along is their use for fast data transfer, combined with a very high with its optical properties is reported. Signal-to-Noise ratio, even over long distances. In recent years, different applications of NLO and photorefractive materials have been developed, for example, optical fre- 2. Experimental Details quency conversion, electro-optical modulation, dynamic Equal proportions of L-Proline and succinic acid were ta- holography, optical writing and optical guiding of laser ken and were dissolved separately in deioinized water. Then beams [1]. It is seen that L-proline and (4R)-hydroxy-L- the solution of L-Proline was poured into the dissolved suc- proline derivatives, containing donor groups are chiral car- cinic acid mixture. The solution thus arrived was filtered riers [2]. The introduction of chirality by means of an asym- twice to remove dust particles and undissolved materials. metrically substituted carbon should in addition respect The reaction takes place between L-Proline and succinic the molecular features leading to a high nonlinear beha- acid (acid-based) through hydrogen transfer. Thus formed viour [3]. Proline and its derivatives are often used as asym- ionic compound of L-Proline succinate is represented in metric catalysts in organic reactions. the following equation: Only noncentrosymmetric alignment of the chromophores in the crystal lattice leads to an observable bulk second- order NLO response [4]. In order to obtain the adjustment of the nonlinear efficiency/transparency, based on the molecular engineering and crystal engineering approach, it is tried to develop a new method to design organic non- The saturated solution was maintained in the undistur- linear optical second-harmonic generation materials such bed condition and the beaker was covered by polythene paper. Few holes were made on the polythene cover for *Corresponding author. slow evaporation. By adopting the solution growth method,

Copyright © 2012 SciRes. JCPT 22 Crystallization and Characterization of a New Non Linear Optical Crystal: L Proline Succinate (LPS) single crystal of L-Proline succinate (LPS) was grown from supersaturated solution at room temperature. Then this solution was periodically inspected and from the 20th day onwards the crystal started growing and it was permitted to grow for another 10 days in order to get a nominal size suitable for characterization. The single crystal of LPS with dimensions of 8 mm × 5 mm × 10 mm was thus obtained. The L-Proline interacts with succinic acid through a single N-H-O hydrogen bond. A single crystal of LPS which has been grown by this process is shown in Figure 1.

2.1. Characterization Figure 1. Photograph of LPS crystal. The lattice parameters and the crystal systems have been determined using single crystal X-ray diffraction analysis (Model: Bruker AXS Kappa APEX II single crystal CCD diffractometer). The functional groups presented in the LPS compound have been identified by Bruker IFS 66V model FTIR Spectrometer using KBr pellet technique in the region 400 - 4000 cm–1. Optical behaviour of LPS was measured by Perkin Elmer Lambda 35 UV-VIS-NIR Spectro- photometer in the wavelength range of 190 - 1100 nm. The thermal stability of LPS was studied by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) by using SDT Q600 V8.3 Build 101 thermal analyzer instrument ranging from room temperature to 1100˚C at a heating rate of 20˚C per minute under nitrogen atmosphere.

3. Results and Discussion Figure 2. FTIR spectrum of LPS crystal.

3.1. Single Crystal X-Ray Diffraction Analysis Table 1. The observed frequencies and corresponding as- Single crystal X-ray diffraction studies were carried out signments. on the grown crystals. The X-ray data were collected using Observed FTIR frequencies Assignments X-ray diffractometer (Model: Bruker AXS Kappa APEX II single crystal CCD). The observed results indicate that 3419 CH stretching the crystal belongs to monoclinic crystal system and the 1600 C=O stretching determined unit cell parameters are a = 5.07 Å, b = 8.84 1398 Symmetric stretching of COO– Å, c = 5.48 Å, = 90˚, = 91.60˚, = 90˚ and V = 246 Å3. 793 NH2 wagging 3.2. FTIR Spectroscopy 2565 O-H stretching The functional groups presented in the LPS compound have been identified by Bruker IFS 66V model FTIR Spec- 3.3. UV-Visible Spectroscopy trometer using KBr pellet technique in the region 400 - Good optical transmittance and lower cut-off wavelengths –1 4000 cm . The FTIR spectrum of title compound is shown are very important properties for NLO crystals. Optical be- in Figure 2. The peaks obtained are 3419 cm–1 , due to stret- haviour of LPS was measured by Perkin Elmer Lambda ching vibration of CH and the peak at 1600 cm–1 is due to 35 UV-VIS-NIR spectrophotometer in the wavelength the stretching vibration of C=O. The bands appeared at range of 190 - 1100 nm. The recorded spectrum is shown –1 793 cm is assigned unambiguously to the wagging of NH2 in Figure 3. The crystals are broadly transparent possess- modes. The OH stretching vibrations is assigned in the ing a transmission of greater than 90% for light with in- range of 2565 cm–1. The peak at 1398 cm–1 is due to the cident wavelengths from 236 - 1100 nm. The UV trans- - symmetric stretching of COO . These assignments are also parency cut-off wavelength of LPS crystal occurs at 204 supported in the literature [6-10]. The observed bands along nm which is better than L-Prolinium tartrate and 4- with their vibrational assignments are given in Table 1. phenylpridinium hydrogen squarate [11,12]. It is observed

169.91˚C and 243.16˚C. The decomposition of L-Proline

Figure 3.Transmission spectrum of LPS crystal. Figure 4. TGA/DTA curve of LPS crystal. that in the LPS crystal, there is high transmittance in the far ultraviolet, visible and infra red region. Hence, the title succinate (87.53%) leads to the major loss of weight in the compound may be used for the nonlinear optical applica- above mentioned temperature region. Further, a small loss tions in the above mentioned wavelength range. of weight of 10.24% is observed, which may be due to the decomposition of C2H2 (11.16%). During the next stage 3.4. Second Harmonic Generation of decomposition, fraction amount of hydrogen may be decomposed. A residue of 0.9018% which may be due to The SHG of the crystal was checked using the powder presence of some fraction of carbon molecule is observed. SHG technique developed by Kurtz and Perry [13]. A Q- The thermal stability of the LPS single crystal is more than switched Nd:YAG laser beam of wavelength 1064 nm, glycine nitrate [14] and is lower than gamma glycine and with beam energy of 4.5 mJ/pulse, and pulse width of 8 ns glycine acetamide [13,15]. with a repetition rate of 10Hz were used. The grown single crystal was crushed to fine powder and then packed in a 4. Conclusion micro capillary of uniform bore and exposed to laser radiations. The 532 nm radiation was collected by a mono- We have synthesized a new non-linear optical crystal with chromater after separating the 1064 nm pump beam with an interesting hydrogen bonding network that holds to- an infra-red blocking filter. The second harmonic radia- gether the L-Proline and succinic acid molecules. The tion generated by the randomly oriented micro crystals grown crystals are characterized by different instrumen- was focused by a lens and detected by a photo multiplier tal techniques. The single crystal XRD studies prove that tube (Hamamatsu R2059). The second harmonic genera- the grown LPS crystals belong to monoclinic crystal sys- tion is confirmed by the emission of green light and its tem. The particle size of the grown crystal is character- efficiency is found to be 23% of that of KDP crystal. ized by Powder XRD analysis. The presence of the functional groups of the grown crystal has been confirmed by 3.5. Thermal Analysis FTIR analysis. From the optical transmittance spectrum, it is observed that there is high transmittance in the far The thermal behaviour of LPS had been studied by ther- ultraviolet, visible and near infra red regions. The UV trans- mo gravimetric analysis (TGA) and differential thermal parency cut-off wavelength of LPS crystal occurs at 204 analysis (DTA) using SDT Q600 V8.3 Build 101 thermal nm. The Kurtz Perry technique for second harmonic ge- analyzer instrument, ranging from room temperature to neration has showed positive results. It is well known that 1100˚C at a heating rate of 20˚C per minute under nitro- the DTA and TGA studies reveal that the crystal is ther- gen atmosphere. TGA-DTA curve of L-Proline succinate mally stable up to 160˚C. is shown in Figure 4. From the TGA curve, the material starts to decompose around 160˚C, which is also confir- 5. Acknowledgements med by the peak which appears at 154˚C in the DTA curve. The thermal stability of LPS is low compared to L-Proline The authors are thankful to Prof. P.K. Das, IISc, Banga- (205˚C) and succinic acid (240˚C). The TGA curve shows lore, India for the SHG test. They also express their gra- a major loss of weight and two losses of smaller weight. titude to the authorities of SAIF, IIT, Chennai, India and The major weight loss of 86.58% is observed between CECRI, Karaikudi, India for providing spectral facilities,

Copyright © 2012 SciRes. JCPT 24 Crystallization and Characterization of a New Non Linear Optical Crystal: L Proline Succinate (LPS) to undertake this study. [8] G. Varaasyi, “Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives,” Wiley, New York, 1974. REFERENCES [9] S. Gunasekaran and D. Uthra, “Fourier Transform Infra- [1] T. M. Kolev, D. Y. Yancheva and S. I. Stoyanov, “Syn- red and Fourier Transform Raman Spectra and Normal thesis and Spectral Elucidation of Some Pyridinium Be- Coordinate Analysis of Ethyleneimine,” Indian Journal of taines of Squaric Acid: Potential Materials for Nonlinear Pure & Applied Physics, Vol. 46, No. 2, 2008, pp. 100- Optical Applications,” Advanced Functional Materials, 105. Vol. 14, No. 8, 2004, pp. 799-805. [10] S. Krishnan, C. J. Raj, R. Robert, A. Ramanand and S. J. [2] B. Gutierrez, N. Rubio and C. Minguillon, “Evaluation of Das, “Growth and Characterization of Succinic Acid Sin- L-Proline Derivatives as Chiral Carriers in the Separation gle Crystals,” Crystal Research Technology, Vol. 42, No. of Enantiomers by Membrane Techniques,” Journal of 11, 2007, pp. 1087-1090. doi:10.1002/crat.200710981 desalination, Vol. 200, No. 1-3, 2006, pp. 117-119. [11] S. A. M. Britto Dhas and S. Natarajan, “Growth and Cha- [3] R. Hierle, J. Badan and J. Zyss, “Growth and Characteri- racterization of L-Prolinium Tartrate—A New Organic zation of a New Material for Nonlinear Optics: Methy NLO Material,” Crystal Research Technology, Vol. 42, l-3-Nitro-4-Pyridine-1-Oxide (POM),” Journal of Crystal No. 5, 2007, pp. 471-476. doi:10.1002/crat.200610850 growth, Vol. 69, No. 2-3, 1984, pp. 545-554. [12] C. R. Raja, P. Paramasivam and N. Vijayan, “Synthesis, doi:10.1016/0022-0248(84)90366-X Growth and Characterization of a New Nonlinear Optical [4] T. kolev, B. Stamboliyska and D. Yancheva, “Spectral Material: 4-Phenylpyridinium Hydrogen Squarate (4PHS),” and Structural Study of Two Acceptor-Substituted Pyri- Spectrochimica Acta Part A, Vol. 69, No. 4, 2008, pp. dinium-Betaines of Squaric Acid: Promising Chromo- 1146-1149. doi:10.1016/j.saa.2007.06.014 phores for Nonlinear Optical Applications,” Chemical Phy- [13] V. Krishnakumar, L. G. Prasad, R. Nagalakshmi, P. Mut- sics, Vol. 324, No. 2-3, 2006, pp. 489-496. husamy, “Physicochemical Properties of Organic Non- doi:10.1016/j.chemphys.2005.11.014 linear Optical Crystal for Frequency Doubling: Glycine [5] F. Q. Meng, M. K. Lu, J. Chen, S. J. Zhang and H. Zeng, Acetamide,” Materials Letters, Vol. 63, No. 15, 2009, pp. “Characterization of Linear and Nonlinear Optical Prop- 1255-1257. doi:10.1016/j.matlet.2009.02.052 erties of a New Single Crystal,” Solid State Communica- [14] S. A. M. Britto Dhas and S. Natarajan, “Growth and tions, Vol. 101, No. 12, 1997, pp. 925-928. Characterization of a New Organic NLO Material: Gly- doi:10.1016/S0038-1098(96)00702-8 cine Nitrate,” Optics Communications, Vol. 278, No. 2, [6] K. V. Krishna and M. Arivazhagan, “Vibrational and Nor- 2007, pp. 434-438. doi:10.1016/j.optcom.2007.06.052 mal Coordinate Analysis of Xanthine and Hypoxanthine,” [15] T. Balakrishnan, R. R. Babu and K. Ramamurthi, “Grow- Indian Journal of Pure & Applied Physics, Vol. 42, No. th, Structural, Optical and Thermal Properties of -Gly- 12, 2004, pp. 411-418. cine Crystal,” Spectrochimica Acta Part A, Vol. 69, No. 4, [7] G. Socrates, “Infrared and Raman Characteristic Group 2008, pp. 1114-1118. doi:10.1016/j.saa.2007.06.025 Frequencies,” 3rd Edition, Wiley, Chichester, 2001.

 . . . .. ..

..... .... ..               .  ....  .         .          .           .          . ....

 .                  .         .  .          .         .            .           .                       . .  .                           .       .     .                         .   .   . .. ..    .         ..      .            .       .   .     .      .            . .     .                                   ..      .    . .                  .       .  .    . .. .      .              .      .  

.  ..      . .  .  .             .     ..            .           .  .             .      ..         .  .         .             . .                          .                  .                .    .                 .      .    . .                   . .        .          

  .                  .  . . .                         . ..               .  .      .  . .  .                            .            .       .    . ..           .          .. .   .      .       .     .                ..  .   .  .               .   .  .     .              .            .   . ..    .                            .                                   .                    .       .              .  ..       .  .   .   .

    

            .

  

.       .  .

      

               .          

  

 .  .    .     . .        .                     .   .      .           .   . .            .            . .                 .             .   .        .         .     . 

                    .              .      .  .               . .                        .     .                    .     .   .     .                        .            .             .   .         

  . .                        .   .                                                     .  .            .   .      .  .           .                              . . .    .       .       .                        . .     .   . .  .                          .      ..             

       .

          

      .

    

      

      

     .       .     .   .

     .

                   .  .            .     .    .     .  .                     

   

  . 

      .    . .                              .  .    . .   .       . .   . .      .      . .        .    .    .    .

                                  .  .                      .               .            .     .       .      .   

      .       . .

          

  . 

           .                                    .                             .                           .   .                   .                 .                .              .        .     . .            .     .               .   .  .            . .                  .    .         . .          .                            .                        .     .   .  .           .  .           



    

.          .. .           .                 .       . .       .            

        .  .. .     .  .     .               . .   .  ..      .                       .    .  .              .     .    .  .      . .       .   .            .    

    .   .       .         

.           

      .       .. .    .                     .             .  .                   .   . .     .                        .  .    .          .      .     .                        .      .  ..                     .           .       ..                      .             .. .     .    .  .   ..  .           

                 

  .. .                   .                 

.       .  .           .. .         .                   .. .                   .         .  .      .         .                ..  .                          .     .     .   .                               .        .             . .         . .  .                       .    . .                                             .. .                                    .          

                                                .                                                                                                                            

      

               .    .                         

     .        

          .

   .                    ..        .                                       .                   .   .   .              .    .                                      .   .           .           .  .                                     .. .                           .                                  .       .        .                                                 .                           .     .    .                      .                                                          

  

  .  .           .         .. .    .  .                          .            .     .. .  .                     

 .    .     . .       

 .           .                .. .      .           .         

          

                          .       .  .        .       . .  .         .  .                          .         . .            .     .  .                 .                  ..    .      .      .       .             .      .     .                       .  

      .            .         .   .           . .         .  .  .      .        .            .     