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* Corresponding author: Suresh Sagadevan Department of Physics, Sree Sastha Institute of Engineering and Technology, Chembarambakkam, Chennai-600 123, India

ISSN: 0976-3031 REVIEW ARTICLE

CRYSTAL GROWTH TECHNIQUES AND NONLINEAR OPTICAL APPLICATIONS: A REVIEW Suresh Sagadevan*

* Department of Physics, Sree Sastha Institute of Engineering and Technology, Chembarambakkam, Chennai-600 123, India

ARTICLE INFO ABSTRACT The importance of crystal growth in material science and the various techniques of crystal growth are discussed. Various types of defects and imperfections that get incorporated into the crystal lattice during growth are considered. The various methods of crystal growth, in particular, the growth from solution and an up-to date level of achievements in the development of NLO materials are also discussed. This paper deals with the fundamental theories of crystal growth methods and nonlinear optics applications.

INTRODUCTION

The regular surface geometry and the shiny and often spectacular appearance have made crystals from the mineral kingdom fascinating objects for everybody. Natural crystals have often been formed at relatively low temperatures by crystallization from solutions, sometimes in the course of hundreds and thousands of years. Nowadays, crystals are produced artificially to satisfy the needs of science, technology and jewellers. The ability to grow high quality crystals has become an essential criterion for the competitiveness of nations. Crystal growth specialists have been moved from the periphery to the center of the materials-based technology. An interdisciplinary crystal growth science has developed with scientific journals, conventions and societies. International networks of crystal growth laboratories and materials science centers have been formed. Crystal laboratories operate in large number to satisfy the needs of research and technology for high-quality, tailor – made crystals of all kinds. New materials are the life blood of solid – state research and device technology. Contrary to what many believe, new materials are not usually discovered by device engineers, solid state theorists, or research managers; they are mostly discovered by crystal chemists who are crystal growers. Some physical phenomena are only exhibited in single crystals and can only be studied and understood in single crystals. Thus, the crystal grower especially if he develops a proficiency in relating structure, bonding and other chemo physical considerations to properties of interest is in a key position in determining the direction and success of solid state research and ultimately technology.

Classifications of Crystals

The particles in a crystal occupy positions with definite geometrical relationships to each other. The atomic occupancies of lattice positions are determined by the chemical composition of the substance. A crystalline substance is uniquely defined by the combination of its chemistry and the

structural arrangement of its atoms. In all crystals of any specific substance the angles between corresponding faces are constant. Crystalline substances are grouped, according to the type of symmetry they display, into 32 classes. These in turn are grouped into seven systems on the basis of the relationships of their axes, i.e., imaginary straight lines passing through the ideal centers of the crystals. Crystals may be symmetrical with relation to planes, axes, and centers of symmetry. Planes of symmetry divide crystals into equal parts (mirror images) that corresponding point for point, angle for angle, and face for face. Axes of symmetry are imaginary lines about which the crystal may be considered to rotate, assuming, after passing through a rotation of 60 o, 90 o, 120 o or 180 o the identical position in space that it originally possessed. Centers of symmetry are points from which imaginary straight lines may be drawn to intersect identical points equidistant from the center on opposite sides. The crystalline systems are cubic or isometric (three equal axes, intersecting at right angles); hexagonal (three equal axes, intersecting at 60 o angles in a horizontal plane, and a fourth, longer or shorter axis, perpendicular to the plane of the other three); tetragonal (two equal, horizontal axes at right angles and one axis longer or shorter than the other two and perpendicular to their plane); orthorhombic (three unequal axes intersecting at right angles); monoclinic (three unequal axes, two intersecting at right angles and the third at an oblique angle to the plane of the other two); trigonal or rhombohedral (three equal axes intersecting at oblique angles); and triclinic (three unequal axes intersecting at oblique angles).

Physical Properties of Crystals

Crystals differ in physical properties, i.e., in hardness, cleavage, optical properties, heat conductivity, and electrical conductivity. These properties are important for crystals which are to be used in the industry. For example, crystalline substances that have special electrical properties are much used in communication equipments. These include quartz and Rochelle salt, which supply voltage on the application of

Available Online at http://www.recentscientific.com International Journal

of Recent Scientific Research

International Journal of Recent Scientific Research

Vol. 4, Issue, 7, pp.1060– 1071, July, 2013

Article History:

Received 11th, June, 2013 Received in revised form 24th, June, 2013 Accepted 18th, July, 2013 Published online 30th July, 2013

© Copy Right, IJRSR, 2013, Academic Journals. All rights reserved.

Key words: Crystal growth, NLO materials, Crystal defects, Solution growth method

International Journal of Recent Scientific Research, Vol. 4, Issue, 7, pp. 1060 - 1071, July, 2013

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mechanical force and germanium, silicon, galena, and silicon carbide, which carry current unequally in different crystallographic directions, as semiconductor rectifiers.

Defects in Crystals

A perfect crystal with regular arrangement of atoms cannot exist. There are always defects, and the most common defects are characterized as below. It is showed that the crystals are imperfect, strained and inhomogeneous. In order to minimize the amount of imperfection, we need to know what defects can be present. Essentially there are three classes of defects.

Point defects

Atoms missing or in irregular places in the lattice (lattice vacancies, substitution and interstitial impurities, self-interstitials) are called point defects. They are referred to as zero dimensional defects

Linear defects (Dislocation)

Groups of atoms in irregular positions (e.g. screw and edge dislocations) are known as linear defects. They are referred to as one dimensional defects. Dislocations are line-defects the inter atomic bonds are significantly distorted only in the immediate vicinity of the dislocation line. This area is called the dislocation core. Dislocations also create small elastic deformations of the lattice at large distances. Dislocations that have been considered until now have Burgers vector directed perpendicular to the dislocation line. These dislocations are called edge dislocations. There is a second basic type of dislocation, called screw dislocation, where the atomic planes forms a spiral ramp winding around the line of dislocation.

Planar defects

The interfaces between homogeneous regions of the material (grain boundaries, stacking faults, external surfaces). The defects have an extension area and are confined to a small region. Some of the planar defects are, Line boundary Plane boundary Stacking fault

Crystal Growths

Technological advances have been accompanied by rapid studies in crystal growth. The beauty, symmetry, structure, crystallinity and rarity of crystals have kindled man and the large-scale uses of crystals are brought out mainly by the demands of solid – state physicist for material research and devices. Crystals are nothing but solids in the most ordered form. Many types of crystals find application in lasers, optical components for communication, light emitting diodes, thermal imaging, piezoelectric detectors etc. Semiconductor laser, piezoelectric, ferroelectric and IR sensitive crystals are part of several solid state devices in use today. The fast development of solid-state research requires single crystals of near perfection. With the absence of crystals there would be no electronic industry and fiber optic communication. New materials are always investigated and the list of applications for crystals is on the rise. Hence growth of crystals has become inevitable for any further development in material science research. Crystals are grown commercially by various growth methods, which include melt growth, solution growth, gel growth and other related growths. The solution growth

technique is discussed in detail in this chapter. In recent years, crystals such as quartz, calcite, ammonium dihydrogen phosphate and halide crystals find applications in frequency controlled oscillators, polarizers, transducers and radiation detectors. Crystals which lack center of symmetry can be used in piezoelectric devices, high capacity condensers and electronic components. Bulk single crystals play a major role in most of these applications. The growth of single crystals with their characterization towards device fabrication has assumed great impetus due to their applications for both academic research and applied research. Although there is a strive to understand the properties of the materials that are grown, all properties cannot easily be predicted while some could be predicted. Hence a thorough understanding of the basic properties enables us to fabricate devices with high product efficiency. The solid-state materials are classified as single crystals, polycrystals and amorphous depending upon the arrangement of constituent molecules, atoms or ions. An ideal crystal may be defined as the solid in which the atoms or molecular are arranged in the most ordered form. In the ordered form infinite lattice of atoms or molecules are arranged in a pattern, which repeats itself in all three directions. However, real crystals are with finite repetitions and do contain defects.

Nucleation

Nucleation is an important phenomenon in crystal growth and is the precursor of the overall crystallization process. Nucleation is the process of generating within a metastable mother phase, the initial fragments of a new and more stable phase capable of developing spontaneously into gross fragments of the stable phase. Nucleation is consequently a study of the initial stages of the kinetics of such transformations. Nucleation may occur spontaneously or it may be induced artificially. These two cases are referred to as homogeneous and heterogeneous nucleation respectively. Both these nucleations are called primary nucleation and occur in systems that do not contain crystalline matter. On the other hand, nuclei are often generated in the vicinity of crystals present in the supersaturated system. This phenomenon is referred to as secondary nucleation [1]. The growth of crystals from solutions can occur if some degree of supersaturation or supercooling has first been achieved in the system. There are three steps involved in the crystallization process:

1. Achievement of supersaturation or supercooling 2. Formation of crystal nuclei 3. Successive growth of crystals to get distinct faces

Expression for Supersaturation

The supersaturation of a system can be expressed in a number of ways. A basic unit of concentration as well as temperature must be specified. The concentration driving force (ΔC), the supersaturation ratio (S) and relative supersaturation (σ) are related to each other as follows: The concentration driving force ΔC = C-C* where, C is the actual concentration of the solution and C* is the equilibrium concentration at a given temperature.

Supersaturation ratio *C

CS

Relative supersaturation, *

*)(C

CC

or 1 S


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If the concentration of a solution can be measured at a given temperature and the corresponding equilibrium saturation concentration is known, then the supersaturation can be estimated. The required supersaturation can be achieved either by cooling/evaporation or addition of a precipitant. Meirs and Issac reported a detailed investigation on the relationship between supersaturation and spontaneous crystallization. The results of their analyses are shown in Fig.1. It shows three zones, which are termed as region I, II and III. The lower continuous line is the normal solubility curve for the salt concerned. Temperature and concentration, at which spontaneous crystallization occurs, are represented by the upper broken curve, generally referred as the super-solubility curve. The three zones are defined as follows:

1. The stable (undersaturated) zone, where crystallization is not possible

2. The metastable zone, where spontaneous crystallization is improbable. However, if a seed crystal is placed in such a metastable solution, growth will occur.

3. The unstable or labile (supersaturation) zone, where spontaneous crystallization is more probable. The achievement of supersaturation is not sufficient to initiate the crystallization. The formation of embryos or nuclei with number of minute solid particles present in the solution, often termed as centers of crystallization is a prerequisite. The change in Gibbs free energy (ΔG) between the crystalline phase and the surrounding mother liquor results in a driving force, which stimulates crystallization. This ΔG is the sum of surface free energy and volume free energy.

Vs GGG (1) where ΔGs is the surface free energy change and ΔGV is the volume free energy change. For a spherical nucleus

vGrrG 32

344 (2)

where r is the radius of nuclei, γ is the interfacial tension and

vG is the free energy per unit volume which is a negative quantity. For rapid crystallization, ΔGv<0; the first term express the formation of new surface and second term express the difference in chemical potential between the crystalline phase (µ) and the surrounding mother liquor (µ

). As the size

(r) of the nucleus grows, the free energy change (ΔG) increases, attains maximum and then starts decreasing. The size corresponding to the maximum free energy change is called critical radius. At the critical condition, the free energy formation obeys the condition 0/ drGd . Hence the radius of the critical nucleus is expressed as,

vGr

2* (3)

The critical free energy barrier,

2

3

316*

vGG

(4)

The number of critical nuclei per unit volume per unit time is called nucleation rate J and is written as,

)T

*exp(k

GAJ (5)

where A is the pre-exponential factor. The critical supersaturation Sc corresponding to J=1 can be fixed to optimize the growth conditions to obtain good quality crystals.

Importance of the Crystal Growth

Crystals have been admired by man from ancient times because of fantastic arrangements of atoms or molecules or ions in the crystals. Naturally occurring stone crystals were priced along with gold in antiquity. The scientific approach in crystal growth was born during early 17th century when Kepler started accounting for the morphology and structure of the crystal, followed by Nicolous Steno who explained variety of external forms exhibited by natural quartz crystals in terms of different growth rates in different crystallographic directions. The work carried out during 19th century laid a firm foundation for the modern scientific and technology developments in crystal growth. Crystals are the pillars of modern technology. Crystals play a vital role in electronic industry, photovoltaic solar cells, fibre optic communications, detecting instrument scintillators and in space technology. Integrated micro-electronics and opto electronics necessitate improved crystal growth technology for obtaining large diameter silicon and GaAs crystals with optimized defect and property control on submicron scale. Laser for technology depends on high power laser crystals and oxide crystals. Crystal growth is a vital and fundamental part of materials science and engineering. Since crystals of suitable size and perfection are required for practical devices such as detectors, integrated circuits and for other millions of applications. The ever increasing application of semiconductors in electronics creates an enormous demand for high quality semiconducting, ferroelectric, piezoelectric, oxide single crystals. Since there is a vast market for solid state devices, efforts have been made in recent years on producing large size, good quality single crystals. Nowadays the growth of single crystals has assumed great impetus due to their importance in the academic research and technology.

Crystal growth is an interdisciplinary subject covering physics, chemistry, electrical engineering, metallurgy, materials science, crystallography, mineralogy, etc. The applications of the crystals are possible only by growing single crystals. Crystallography is concerned with the nature of the regular atomic arrangements within the crystal. Crystallographers had made remarkable studies about the crystal before the discovery of X-rays. However, only after that, it became possible to know about internal arrangement of atoms in the crystals, in a more developed way. As there was a remarkable achievement in the study of internal atomic arrangements, it leads to the study of more physical properties. This interest shifted from the study of natural crystals to the laboratory grown crystals. The modern technology is very much dependent upon materials/crystals such as semiconductors, polarizers, transducers, radiation detectors, ultrasonic amplifiers, ferrites, magnetic garnets, solid state lasers and nonlinear optic, piezoelectric, acousto-optic and photo sensitive materials and crystalline films for microelectronics and computer industries. All these involve research in crystal growth and characterization studies of new materials.


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Growth Methods

Growth of crystals may be considered as the formation of bonds between growth units (atoms, ions and molecules). The growth methods for obtaining single crystals may be classified according to their phase transition as mentioned in Table 1. The mode of selection of a particular method of crystal growth depends on the characteristic properties of the materials such as melting point, vapour pressure, decomposition and solubility in solvents etc. The growth methods are also heavily dependent on growth kinetics involved, crystal size, and requirement, shape of the crystal, purity and nature of application of the crystal. The conversion of a polycrystalline piece of material into single crystal by causing the grain boundaries to be swept through and pushed out of the crystal takes place in the solid growth of crystals [1]. The above methods have been discussed in detail by several authors [2-4]. The different techniques of each category are found in reviews and books on vapour growth [5], on melt growth [6], gel growth [7], solution growth [8] and high temperature solution growth [9].

Melt Growth

If the material to be grown is melted and allowed to cool carefully, then solidification occurs in the direction of cooling or in the direction of relatively low temperature. This principle is adopted in melt growth. Melt growth is the best method for growing large single crystals of high perfection. It is suitable for materials, which melts congruently relatively at low vapour pressure and does not undergo phase transition from its melting point to room temperature. This method is hence not recommended for materials, which undergo polymorphic phase changes, development of high vapour pressure at the melting point and for materials in which thermal strains are produced. Again if the melting point is very high it makes growth impractical. The growth of single crystals from melt may further be subdivided into,

Czochralski technique Bridgmann – Strockbarger technique Verneuil technique Zone melting technique

Czochralski technique

This technique involves the relative motion of a seed and melt. This method is mainly recommended for semiconductors, inorganic and organic materials. This method has many advantages for the growth of single crystals due to the following reasons,

The grown crystal is free from container contact. The orientation of the growing crystal can be preselected by using suitably oriented seed crystal. Growth time is much less, when compared to other crystal growth techniques. High quality crystals can be produced by this method.

In this method a seed crystal of desired orientation is wired to the bottom of the seed holder. The seed holder is either water-cooled or air-cooled. The seed crystal is slowly lowered into the crucible containing the melt and allowed to touch the melt. Since the seed holder is cooled, the temperature of the seed is very much lower than the melt. Hence the melt solidifies on the tip of the seed and thus the crystal grows. The advantage of

this method is that though the crucible is used, the grown crystal is not in contact with the crucible. This important feature is that it helps in getting dislocation free crystals. The experimental setup of Czochralski technique is shown in Fig. 2.

Bridgmann – Stockbarger technique

In this method, the melt along with the crucible is slowly cooled to yield a single crystal. This method is used to grow crystals, which will withstand considerable thermal stress. An important criterion is that neither the melt nor the vapour should attack the crucible. The thermal expansion coefficient of both the material and the ampoule should be same so that thermal stresses in the crystal will be avoided. The ampoule should have a tapered end at the bottom. The important feature of this technique is the steady motion of the solid – liquid interface. The motion of the interface is achieved in many ways. The routine and advantageous method is to lower the molten materials vertically through the furnace. The material is taken in a cylinder tube and lowered into a double walled electrically heated furnace. The furnace is divided into two zones namely upper and lower zones. The temperatures of these zones are independently controlled. The temperature of the top zone of the furnace is maintained at a temperature higher than the melting point of the material and lower zone is slightly lower than the melting point. As the substance passes through the hotter region, it melts. When lowered gradually, into the cooler region, nucleation occurs at the bottom of the tip of the ampoule and solidifies. This acts as the seed for further growth. The seed grows when the ampoule is lowered. In this method, the solid liquid interface is completely enclosed within the crucible so that the latent heat is removed only by thermal conduction. This technique is best suited for materials of low melting point. The Bridgmann experimental setup is shown in Fig. 3. This process has been used for a large number of materials with high melting points. In this technique there is no problem of selecting suitable crucible material because it does not require a crucible. The material experiences a steep temperature gradient. This is a serious problem because this produces strains in the crystals. An oxy-hydrogen flame is used to heat the seed crystal. Powder from the hopper is shaken through the sieve-using vibrator at low amplitude. The powder melts during its transit through the flame. Though the volume of the growth rate depends on the powder feed, the

Fig. 1 Meirs and Issac solubility curve


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linear growth rate is approximately equal to rate of lowering of seed. The schematic diagram of verneuil technique is shown in Fig.4.

Verneuil Technique

Zone Melting

Originally this technique is used only to purify the materials and this can also be used to grow single crystals. In this method, a small liquid zone is created by melting small amount of materials in a long solid ingot and moved up or down. For doping an impurity the ingot is repeatedly moved up and down. Germanium and silicon are few examples of the crystals grown by this technique. Advantages of zone melting are due to not using crucible and simultaneous purification of the material during the growth process.

Growths From Solution

Of all methods of crystal growth, low temperature solution growth is practiced next to melt growth. The main advantage of the solution growth is that the crystals are grown at a temperature well below the melting point. Hence, the detailed knowledge about certain parameters of the material like melting point, melting behavior, stability in the reduced

pressure and atmosphere are not necessary. Though the problems usually present in other methods like presence of substitutional or interstitial incorporation of ions and inclusions are not overcome, the solution growth can be considered as a superior method because good optical transparency of crystal and uniform mixing of dopant in the lattice is achievable easily.

However, the disadvantages are due to the slow growth rate and container problem. For growing good quality large single crystals by solution growth method, the materials should have high solubility and variation in solubility with temperature. The vapour pressure at the growth temperature should be small. The viscosity of the solvent – solute systems should be low. Another aspect to be seen while employing solution growth method is that the container and stirrer should be non-reactive with the material. The materials must be less toxic. Solution growth techniques can be broadly subdivided as:

Low temperature solution growth. High temperature solution growth. Hydrothermal growth. Gel growth.

Since in the present investigation low temperature solution growth technique is employed, the fundamentals of the above technique are discussed below.

Low temperature solution growth

The method of crystal growth from low-temperature aqueous solution is extremely popular in the production of many technologically important crystals. The growth of crystals by low temperature solution growth involves weeks, months and sometimes years. This method is well suited to those materials, which suffer from decomposition in the melt, or the solid at high temperatures and which undergo structural deformations while numerous organic and inorganic materials, which fall in this category, can be crystallized using this technique. This technique also allows variety of different morphologies and polymorphic forms of the same substance which can be grown by varying the growth conditions of the solvent. Low temperature solution growth can be subdivided into the following methods:

i. Slow cooling method ii. Slow evaporation method

iii. Temperature gradient method

Fig.2 The experimental setup of Czochralski technique

Fig.3. A Typical Bridgmann System

Fig.4 Schematic Diagram of Verneuil technique


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Crystallization by slow cooling of solution

This is the most suitable method among the various methods of solution growth process. However, the main difficulty of slow cooling method is the need of various temperature ranges. The possible range of temperature is usually narrow and hence much of the solute remains in the solution at the end of the growth run. To compensate this effect, large volume of solution is required .The use of wide range of temperature may not be desirable because the properties of the grown crystals may vary with temperature. Achieving the desired rate of cooling is a major technological difficulty in this technique after fixing the temperature. Even though this method has technical difficulties, using a programmable temperature controller, it is widely used with great success. Ferro electric Bi2WO6 (BWO) single crystals have been grown by a flux method below the phase transition. Sodium chloride, LiBO2, Li2B4O7 (LBO), Na2B4O7 (NBO) and Na2BO7.10H2O were selected as the flux of which LBO and NBO are suitable material for crystal growth [10]. KDP crystals were grown from aqueous solution added with organic additives by the slow cooling method for obtaining better non-linear properties. The influences of the organic additives on the optical and mechanical properties have been studied and the study concludes that the addition of organic additives improves the mechanical stability and optical transmission property of the crystal [11]. L-pyroglutamic acid showed different morphological characteristics and growth rate in different solvents with different crystallographic orientations [12]. The large grown protein crystals by slow cooling method have made it possible to remove them without any mechanical damage [13]. The effect of deuterium on morphology and crystal properties of deuterated potassium acid phthalate (DKAP) grown from aqueous solution by slow cooling method has beens tudied[14]. DAST [4-dimethylamino-4-methyl-4-stilbazolium tosylate] has been successfully grown using saturated methanol solution by a slow cooling method in a Teflon vessel [15]. L-threonine and dl-threonine grown by slow cooling solution growth technique are exhibiting prominent changes in the physical and optical properties [16]. It is observed that the crystal grown by cooling method reduces the number of secondary nucleation in the solution medium and changes the growth rate of the crystal [17]. The growth kinetics of different faces of ammonium oxalate monohydrate single crystals doped by Co and Ni ions using slow cooling method has been reported [18]. A simple method to correlate dislocation observed in X-ray topograph with etch pits is observed and has been applied to the case of cyclotrimethylene trinitramine crystals grown from solution by slow cooling [19]. Growth of 4-nitro-4-methoxy benzylidene aniline (NMOBA) by employing restricted evaporation and slow cooling method has been reported [20]. Slow cooling of a saturated solution enables precise control of supersaturation in the solution and also allows reuse of the solution because it maintains its composition [21].

Crystallization by solvent evaporation

In this method, an excess of a given solute is established by utilizing the difference between the rates of evaporation of the

solvent and the solute, in contrast to the cooling method, in which the total mass of the system remains constant. In the solvent evaporation method, the solution loses particles, which are weakly bound to their components and therefore, the volume of the solution decreases. This is oldest method of crystal growth and technically it is very simple. Typical growth conditions involve temperature stabilization to about ± 0.005oC and rates of evaporation of a few mm/hr. The basic apparatus (Manson Jar Crystallizer) used for the solution growth technique is shown in Fig.5. The evaporation technique has an advantage viz. the crystals grow at a fixed temperature. But, inadequacies of the temperature control system still have a major effect on the growth rate. This method can effectively be used for materials having very low temperature coefficient of solubility. But the crystals tend to be less pure than the crystals produced by slow cooling technique because, as the size of the crystal increases, more impurities find place in the crystal faces. Evaporation of solvent from the surface of the solution produces high local supersaturation and formation of unwanted nuclei.

Small crystals are also formed on the walls of the vessel near the surface of the liquid from the material left after evaporation. These tiny crystals fall into the solution and hinder the growth of the crystal. Another disadvantage lies in controlling the rate of evaporation. A variable rate of evaporation may affect the quality of the crystal. In spite of all

Table 1 Various Phase Transitions Melt growth Liquid to solid phase transition Solid growth Solid to solid phase transition Solution growth Liquid to solid phase transition Vapour growth Vapour to solid phase transition

Fig.6 A constant temperature bath used for bulk growth of crystal from aqueous solution

Fig.5 Manson Jar Crystallizer


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these disadvantages, this is a simple and convenient method for growing single crystals of large size.

The growth of SeS and SeSnS2, microcrystals by slow evaporation technique was established and rod like SeS and platy SeSnS2 were identified by optical microscopy [22]. Crystals of HgCl2.2KCl:H2O [23] and ammonium pentaborate [24] have been grown from aqueous solution by slow evaporation technique. The grown crystals of Na3BaCl5.2H2O by slow evaporation method were hydrated and were confirmed from the appearance of bands due to stretching and bending modes of water molecules in the IR spectra [25]. Single crystals of acetoacetanilide can be easily grown by slow evaporation method [26].Single crystals of poly (-propiolactone) grown by slow evaporations, under four different solvents have been subjected to morphological studies [27].Crystal perfection of 4-ethoxy benzaldehyde-n-methyl-4-stilbazolium tosylate [EBST] was studied and the results reveal that the quality of the grown crystal by slow evaporation method was quite good without having any internal structural grain boundary [28]. Mixed crystals of two nonlinear optical materials, L-arginine hydrochloride monohydrate (LAHCl) and L-arginine hydro bromide monohydrate (LAHBr) have been grown from aqueous solution by using slow solvent evaporation method [29].

The quality and size of L-arginine maleate dihydrate are found to be dependent on pH of the solution and best crystals were obtained at pH = 4 [30]. Single crystals of L-histidinium bromide (L-HBr) have been grown by temperature lowering and slow evaporation techniques [31]. The [010] direction of TGS is very important and it is used as infrared detectors [32]. The metastable zone width and nucleation

parameters of L-tartaric acid-nicotinamide, an organic NLO crystal under slow evaporation method of solvent have been reported [33]. L-pyrrolidone-2-carboxylic acid (L-PCA) single crystal is grown by slow evaporation of the solvent at ambient temperature [34]. L-PCA has been found to possess laser damage threshold of 17 GW/cm2. Single crystals of 2-amino-5-chlorobenzophenone (2A-5CB) were grown by employing slow evaporation technique using acetone as solvent [35]. The UV-Vis spectrum of (2A-5CB) shows a cut-off wavelength of 440 nm.

Temperature Gradient Method

This method involves the transport of materials from a hot region containing the source material to be grown to a cooler region where the solution is supersaturated and the crystal grows. The main advantages of this method are that,

a. Crystal grows at fixed temperature. b. This method is insensitive to change in temperature

provided both the source and the growing crystal undergo the same change.

c. Economy of solvent and solute On the other hand, a small change in the temperature between

the source and the crystal zone has a large effect on the growth rate.

High Temperature Solution Growth

This technique is also known as flux method. Molten salt called flux is used as a solvent and the growth process takes place well below the melting point of the solute. The crystals grown by this technique have higher concentration of impurities than those grown from melt. Further this technique can be used for the crystallization of oxide compounds, which generally have high melting points as well as for materials, which have phase transitions below the melting points [36]. Since the growing crystal is not exposed to steep temperature gradient, strain free crystals can be obtained. For growing crystals at high temperatures this method is preferred next only to melt growth.

1. Perforated closed lid 2.Crystallization vessel 3. Supersaturated solution 4. Seed Crystal

Fig.7. Apparatus for the preparation of seed crystals

Fig. 8 Electrons in a nonlinear crystal are bound in a potential

well, holding the electrons to lattice points in a crystal

Table 2 Parameters for selecting a NLO crystal

Laser parameters Crystal parameters

NLO process Types of phase matching

Power, Repetition rate Damage threshold

Divergence Acceptance angle

Band width Spectral acceptance

Beam size Crystal size, Walk-Off angle

Pulse width Group velocity mismatching Environment Moisture, temperature

Fig. 9 Two photons are welded together to produce a single photon with

the energy of both original photons


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Hydrothermal Growth

Although large number of materials can be grown by this technique, it is primarily used for the growth of high quality quartz (SiO2) crystals. Though this method is classified under high temperature crystallization, in reality the temperature is low compared to the melting point of the material. It is an excellent method for the growth of refractory materials and nanocrystals. The requirements of high pressure pose practical difficulties and there are only a few crystals of good quality which can be grown by this technique [37]. Quartz is the first crystal grown, industrially by this technique. Rapid rate of growth is also considered as another advantage. For example Quartz crystals grow as fast as 3 cm/day on the basal plane.

Gel Growth

This is a convenient laboratory process. Only small crystals can be grown through this technique. In this method, solutions of suitable compounds say AX and BY are allowed to diffuse through a gel medium to give rise to insoluble substance AB and the waste XY as follows:

AX + BY → AB + XY (6)

Lead (II) chloride, lead (II) bromide and lead chlorobromide crystals are grown recently by gel method [38].

Thermodynamic considerations reveal that the grown crystals would contain less concentration of equilibrium defects since growth is carried out at near ambient temperature.

The Crystals can be practically observed in the transparent medium in low temperature solution growth and gel growth.

The grown crystals can be easily harvested without much damage to the crystal surface.

Growth procedure is simple, economical and yields good quality crystals.

Criteria For Optimizing Solution Growth Parameters

The growth of good quality single crystals by slow evaporation and slow cooling techniques requires the optimized conditions and the same may be achieved with the help of the following criteria: (i) material purification, (ii) solvent selection, (iii) solubility, (iv) solution preparation, (v) seed preparation, (vi) agitation, (vii) crystal habit and (viii) cooling rate.

Material purification

An essential prerequisite for success in crystal growth is the availability of material of the highest purity attainable. Solute and solvents of high purity are required, since impurity may be incorporated into the crystal lattice resulting in the formation of flaws and defects. Sometimes impurities may slow down the crystallization process by being adsorbed on the growing face of the crystal, which changes the crystal habit. A careful repetitive use of standard purification methods of recrystallization followed by filtration of the solution would increase the level of purity.

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.

(i) High solubility for the given solute (ii) Good solubility gradient (iii) Low viscosity (iv) Low volatility and (v) Low corrosion

Solubility

Solubility is an important parameter, which dictates the growth procedure. If the solubility is too high, it is difficult to grow bulk single crystals and too low solubility restricts the size and growth rate of the crystals. Neither a flat nor a steep solubility curve will enable the growth of bulk crystals from solution. If the solubility gradient is very small, slow evaporation of the solvent is the other option for crystal growth to maintain the supersaturation in the solution. Low temperature solution growth is mainly a diffusion-controlled process; the medium must be less viscous to enable faster transfer of the growth units from the bulk solution by diffusion. Hence a solvent with less viscosity is preferable. Supersaturation is an important parameter for the solution growth process. The solubility data at various temperatures is essential to determine the level of supersaturation. Hence, the solubility of the solute in the chosen solvent must be determined before starting the growth process [39].The solubility of the solute can be determined by dissolving the solute in the solvent maintained at a constant temperature with continuous stirring. On reaching saturation, equilibrium concentration of the solute can be determined gravimetrically. A known quantity of the clear sample is analyzed. The solubility curve can then be plotted from the amount of solute dissolved and temperature by repeating the measurements for different temperatures. The solubility of most substances increases with temperature. Crystals can be grown only from supersaturated solutions, which contain an excess of the solute above the equilibrium value. The region of supersaturated solutions can be divided into two sub regions, metastable (stable) and labile (unstable) zones. Nucleation will occur spontaneously in the labile zone. Metastable zone refers to the level of super saturation where spontaneous nucleation cannot occur and a seed crystal is essential to facilitate growth.

Solution preparation and crystal growth

For solution preparation, it is essential to have the solubility data of the material at different temperatures. Filter papers of different pore size are used for solution filtration. The clear solution, saturated at the desired temperature is taken in a growth vessel. For growth by slow cooling, the vessel is sealed to prevent the solvent evaporation. Solvent evaporation at constant temperature can be achieved by providing a controlled vapour leak. A small crystal suspended in the solution is used to test the saturation. By varying the temperature, a situation where neither the occurrence of growth nor dissolution is established. The test seed is replaced with a good quality seed. All unwanted nuclei and the surface damage on the seed are removed by dissolving at a temperature above the saturation point. Growth is initiated after saturation. Solvent evaporation can also be helpful in initiating the growth. The quality of the grown crystal depends on the nature of seed, cooling rate employed and agitation of the solution.


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Crystal Growth Apparatus

The constant temperature bath used in the present study is shown in Fig.6. It consists of a large tank (constant temperature bath) heated at the base using an infrared lamp. The IR lamps are energized through a relay switch. The control is affected by a jumo contact thermometer coupled to an on-off controller, which has a controlling accuracy of 0.01°C. The temperature of the constant temperature bath is converted into a signal by a suitable sensor. The controller is contacted with an on-off switch. It gets activated when the process variable (bath temperature) crosses the set point. There are only two stable states in an on-off controller. On state is enabled when the temperature is below the set point. As the desired set point is arrived, the controller goes to the off state. To get change in the state, the temperature must cross the set point. Set point variations, which occur due to electrical noise interference and process disturbances, seriously affect the practical applications of the controller. Contrary to this, a proportional controller continuously manipulates the process variable so that the heat input is in balance with the heat demand. The controller consists of a power supply, processor, booster and proportional controllers. In the present investigation, the growth instrument was modified by replacing the infrared lamp using a programmable controller heater.

Seed preparation

Seed crystals are prepared by self-nucleation under slow evaporation from a saturated solution (Fig.7). Seeds of good visual quality, free from any inclusion and imperfections are chosen for growth. Since strain free refaceting of the seed crystal results in low dislocation content, a few layers of the seed crystal are dissolved before initiating the growth.

Agitation

To have a regular and even growth, the level of supersaturation has to be maintained equally around the surface of the growing crystal. An uneven growth leads to localized stresses at the surface generating imperfection in the bulk crystals. Moreover, the concentration gradients that exist in the growth vessels at different faces of the crystal cause fluctuations in supersaturation, seriously affecting the growth rate of individual faces. The gradient at the bottom of the growth vessel exceeds the metastable zone width, resulting in spurious nucleation. The degree of formation of concentration gradients around the crystal depends on the efficiency of agitation of the solution. This is achieved by agitating the saturated solution in either direction at an optimized speed using a stirrer motor.

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 from which samples of any desired orientation can be cut. Further, such large crystals should also be devoid of dislocations and other defects. These imperfections become isolated into defective regions surrounded by large volumes of high perfection, when the crystal grows with a bulk habit. In the crystals, which grow as needles or plates, the growth dislocations propagate along the principal growth directions and the crystals remain imperfect [40]. Needle like crystals have very limited applications and plate-like crystals need to be favorably oriented.

Changes of habit in such crystals, which naturally grow as needles or plates, can be achieved by any one of the following ways

(i) Changing the temperature of growth (ii) Changing the pH of the solution (iii) Adding a habit modifying agent and (iv) Changing the solvent

Achievement in the above parameters is of great industrial importance where such morphological changes are induced during crystallization to yield crystal with better perfection and packing characteristics.

Cooling rate

Supersaturation is the driving force which governs the growth of a crystal. This can be achieved by lowering the temperature of a solution. Temperature and supersaturation have to be precisely controlled for desirable results. The growth rate is maintained linear in order to grow large crystals. This requires an increase in the supersaturation level and linear cooling will not provide this. Hence, after the initial growth, the rate of temperature lowering is increased. Operation within the metastable limit occurs without any spurious nucleation in the solution. A large cooling rate changes the solubility beyond the metastable limit. Further, fluctuations in supersaturation may encourage solution inclusion flaws in growing crystals [41]. Hence, a balance between the temperature lowering rate and the growth rate has to be maintained.

Crystal perfection

The perfection of the final crystal is based on (i) The purity of the starting materials (ii) The quality of the seed crystal (iii) Cooling rate employed and (iv) The efficiency of agitation

Hence, high quality single crystals can be grown from quality seeds in an efficiently stirred solution.

Concept of Nonlinear Optics

As this thesis contains growth and characterization of some nonlinear optical materials, knowledge of nonlinear optics is very essential to discuss the NLO activity of the materials. Nonlinear optics is a topic of much current interest that exhibits a great diversity. This is due to the technological potentials of certain nonlinear optical effects for photonic based technologies. Applications of nonlinear optics include the frequency doubling of semiconductor lasers, the generation of ultra short laser pulses, optical information processing, telecommunications and integrated optics. Nonlinear optics is completely, a new effect in which light of one wavelength is transformed to light of another wavelength. The creation of light of new wavelength can be best understood, as we think about the electrons in nonlinear crystal are bound in potential well, which acts like a spring, holding the electrons to lattice points in the crystal (Fig.8). 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. The electric field in a light wave passing through the crystal exerts a force on the electrons and pulls them away from their equilibrium position. In an ordinary optical material, the electrons oscillate about their equilibrium position at the frequency of this electronic


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field. According to the fundamental law of physics an oscillation change will radiate at its frequency of oscillation, hence these electrons in the crystal “generate” light at the frequency of the original light wave.

The nonlinear material is different from the linear material in several aspects. We can think of a nonlinear material as the one whose electrons are bound by very short springs. 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 non-linear. The electrons are jerked back roughly rather than pulled back smoothly and they oscillate at frequencies other than the driving frequency of the light wave. These electrons radiate at the new frequencies, generating the new wavelength of light. The exact values of the new wavelengths are determined by conservation of energy. The energy of the new photons generated by the nonlinear interaction must be equal to the energy of the photon used. Fig.9 shows the photons involved in the second harmonic generation process.

Origin of Optical Nonlinearity

The origin of nonlinear optical effects lies in the nonlinear response of a material to an electric field. In a dielectric medium, the charged particles are bound together. When an electric field is applied, they are slightly displaced from their usual positions. This small movement of positive charges in one direction and negative charges in the other, results in a collection of induced electric dipole moments. The effect of the field is to induce a polarization. The polarization of the molecule induced by an external field can be described by

....., lkjijklkjijkjiji EEEEEEP (7) where Pi is the component of the induced dipole moment in the coordinate direction I, αij is the second-order polarizability or first-order hyperpolarizability tensor, E is the electric field strength at the locations of the molecule. The expression parallels that for the macroscopic polarization of a material,

.....][ )3()2()1(0 lkjkjji EEEEEEP

(8) In both equations the tensors are frequency dependent and should be written as functions of the frequency of the fields and the polarization produced. The linear term involving χ(1) gives to the index of refraction, absorption, dispersion and birefringence of a medium. Most of the interesting nonlinear optical effects however arise from the terms of electric polarization which are quadratic or cubic in the electric field. The coefficients of the nonlinear terms are extremely small, but under proper circumstances they lead to striking effects. The quadratic polarization χ(2) gives rise to the phenomena of second harmonic generation, sum and difference frequency mixing, linear electro-optic modulation and parametric generation, etc., while the cubic term χ(3) is responsible for third harmonic generation, stimulated Raman scattering, optical bistability and phase conjugation. Even order terms such as χ(2) are nonzero only in non centrosymmetric media, whereas the χ(1) and χ(3) terms are nonzero in all media. Therfore centrosymmetric media do not exhibit second-order effects, although third-order processes are possible in such media. The high intensity available in the laser beam has made possible the observation of nonlinear effects at optical frequencies. Franken and co-workers generally identified the

birth of nonlinear optics with the experiment on second harmonic generation (SHG) of light by a ruby laser pulse in a quartz crystal. This discovery propelled the field of modern nonlinear optics and initiated intensive research in material science and crystal technology. In a short period following this discovery, several other nonlinear optical phenomena including parametric amplification and frequency mixing were identified and many important concepts such as phase matching were quickly developed.

Nonlinear optical materials

Materials in crystalline form have special optical and electrical properties, in many cases improved properties over randomly arranged materials. Many organic and inorganic materials are highly polarizable and are good candidates for NLO study. However, the net polarization of a material depends on its symmetry properties, with respect to the orientation of the impinging fields. It can be shown that the odd order terms in equation (8) are orientationally independent, but the even terms vanish in a centrosymmetric environment. Thus materials for second order NLO application must be orientationally non-centrosymmetric to be functional. No such restriction applies to third order materials. Advances in the development of NLO materials can be divided into three different areas.

1. Discovery of new NLO materials 2. Growth of promising NLO crystals 3. Improving the characteristics of NLO crystals

Nonlinear optical materials will be the key elements for future photonic technologies based on the fact that photons are capable of processing information with speed of light. The search for new and efficient materials has been very active since SHG was first observed in single crystal quartz by (Franken and co-workers in 1961) [42]. In beginning, studies were concentrated on inorganic materials such as quartz, potassium dihrogen phosphate (KDP), lithium niobate (LiNbO3), and semiconductors such as cadmium sulfide, selenium and tellurium. Table 2 lists the laser and crystal parameters for selecting a NLO crystal for a particular application. At the end of 1968, the Kurtz and Perry powder SHG method was introduced. In this method, a powdered sample is irradiated with a laser beam and scattered light is collected and analyzed for its harmonic content with the use of suitable filters. For the first time, rapid, qualitative screening for second order NLO effect was possible. The stage was set for a rapid introduction of new materials, both inorganic and organic. Second order NLO materials are used in optical switching (modulation), frequency conversion (SHG, wave mixing), and electro-optic applications, especially in EO modulators. All of these applications rely on the manifestation of the molecular hyperpolarizability of the materials. For optical applications, a nonlinear material should have the following characteristics [43]

1. A wide optical transparency domain 2. Large nonlinear figure of merit for frequency

conversion 3. High laser damage threshold 4. Be readily available in large single crystals 5. Wide phase matchable angle 6. Ability to process into crystals, thin films, etc. 7. Ease of fabrication 8. Non toxicity and good environmental stability


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9. High mechanical strength and thermal stability 10. Fast optical response time

For exhibiting SHG, in materials, there are two factors, which determine the existence or absence of efficient SHG. Firstly, and fundamentally, the material should crystallize with a non-centrosymmetric crystal structure. Secondly, for maximum SHG efficiency, crystals should possess phase matching properties.

CONCLUSION

Growth of crystals from aqueous solution, one of the methods of crystal growth, which is extremely popular in the production of many technologically important crystals, was adopted to grow the NLO crystals. Various physical properties of the crystals are studied from application point of view. This review summarizes the main results obtained in this work along with further experiments that can be carried out on these crystals. 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 NLO single crystals.

Acknowledgement

The author thanks the Management and Principal of Sree Sastha Institute of Engineering and Technology, Chembarambakkam, Chennai-600123 for their encouragements throughout this work.

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