1

CHAPTER 1

INTRODUCTION TO CRYSTAL GROWTHMETHODS

AND CHARACTERIZATION

1.1 INTRODUCTION

Single crystal growth has prominent role to play in the present era

of rapid technical and scientific advancement, where the application of

crystals has unbounded limits. A single crystal is a periodic array of atoms

arranged in a three dimensional structure with equally repeated distance in a

given direction (Dryburgh 1986). Single crystals are the fundamental building

blocks for modern technology. The properties of the material can be

extensively studied when the material is prepared in single crystal form. The

uniformity of the single crystals allows transmission of electromagnetic

waves without scattering.

Crystal growth is an important field of materials science, which

involves controlled phase transformation. Significant advancement in crystal

growth technology has allowed the development of many excellent crystals to

meet the ever growing applications in lasers, optical communication and data

storage technology. Hence, growth of single crystals has become inevitable

for further research and technology.

The search of new materials is primarily focused on increasing the

nonlinearity. With progress in crystal growth technology, materials having

attractive nonlinear optical properties are being discovered at a rapid pace

2

(Baumert et al 1987; Chemla and Zyss 1987; Gunter et al 1987). This has

enabled the commercial development of single crystals with promising

nonlinear optical properties. Large size crystals are essential for device

fabrication (Brice 1986; Laudise 1970) and efforts are taken to grow large

crystals in short durations by fast growth techniques. The present day demand

is for large and high quality NLO, ferro-electric, piezo-electric single crystals

with minimum defects.

The important goal of crystal growth is the improvement of

microscopic and macroscopic homogeneity, which is a necessity for any

application. The production of most single crystals is a difficult process

requiring significant technical skills in the synthesis of materials, growth,

processing and characterization (Byrappa and Ohachi 2003). It acts as a link

between science and technology for the practical device applications of single

crystals as can be seen from achievements in the modern microelectronics

industry. Organic nonlinear optical crystals have proven to be interesting

candidates for a number of applications: second harmonic generation,

frequency mixing, electro-optic modulation, optical parametric oscillation,

optical bistability etc., (Badan et al 1983). Due to the technological

importance of these nonlinear optical crystals, the need for high quality

organic crystals has grown dramatically in the last decade (Zyss et al 1985).

Crystal growth involves phase transformation to solid phase from

supersaturated mother phase. Diffusion of growth unit occurs at the growth

site and they are orderly arranged with time in the lattice. It is essential to

have a better understanding of the nucleation which occurs during the initial

stages of crystal growth. Nucleation may occur either spontaneously due to

the conditions prevailing in the parent phase or it may be induced artificially.

In case of organic crystals, delocalized -electrons can easily move between

electron donor and electron acceptor groups on opposite sides of the

3

molecule, inducing a molecular charge transfer (Gallagher et al 2003,

Bosshard et al 1993). Growth from aqueous solution provides information on

the fundamental process applicable to both low and high temperature solution

methods (Bordui 1987). Since an understanding of the various crystal growth

methods is very much essential for the growth of nonlinear optical single

crystal, the materials of choice for this investigation, the author discusses in

the following sections the fundamentals of the various methods of growing

single crystals.

1.2 METHODS OF CRYSTAL GROWTH

Crystal growth process and size of the grown crystal differ widely

and are determined by the characteristics of the material (Buckley 1951;

Mullin 1976). The principal methods of crystal growth can be classified as

follows

1. Growth from melt

2. Growth from vapour

3. Growth from solution

There are number of growth methods in each category. A brief

outline on various important techniques of crystal growth has been presented

below.

1.3 GROWTH FROMMELT

Melt growth is the process of crystallization by fusion and

resolidification of the pure material. In this technique, apart from possible

contamination from crucible material and surrounding atmosphere, no

impurities are introduced in the growth is normally much higher than the

4

other methods. In principle, all materials can be grown into a single crystal

from the melt, provided they melt congruently, they do not decompose before

melting and they do not undergo a phase transition between the melting point

and room temperature. The melt growth can be classified as follows:

i. Bridgman – Stockbarger technique

ii. Czochralski technique

iii. Kyropoulos technique

iv. Zone melting technique and

v. Verneuil technique

The important feature of Bridgman technique is the steady motion

of a freezing solid – liquid interface along an ingot which is mounted either

vertically or horizontally. The material is melted in a vertical cylindrical

container. The container is lowered slowly from the hot zone of the furnace

into the cold zone (Bridgman 1924). Crystallization begins at the tip of the

container by forming a nucleus and crystal to grow from the nucleus. One of

the constraints of this technique is the choice of the crucible. The crucible

should not contaminate the melt. The crystal should not adhere to the crucible

as this also can introduce excessive strains during cooling. This technique

cannot be used for materials which decompose before melting. This technique

is best suitable for low melting point materials. Germanium, Gallium arsenide

and such other materials expand on solidification and hence this method is not

useful to grow such crystals.

In Czochralski method, the material is taken in a crucible and is

kept in a furnace. By controlling the furnace temperature, the material is

melted (Zulehner 1983). A seed crystal is lowered to touch the molten charge

which has been maintained at its melting point. When the temperature of the

5

seed is maintained very low compared to the temperature of the melt, by

suitable water cooling arrangement, the molten charge in contact with the

seed will solidify on the seed. Then the seed is pulled with simultaneous

rotation of the seed rod and the crucible in order to grow perfect single

crystals. Liquid Encapsulated Czochralski abbreviated as LEC technique

makes it possible to grow single crystals of materials which consist of

components that produce high vapour pressure at the melting point. This

refined method of Czochralski technique is widely adopted to grow the

III – V compound semiconductors.

In Kyropoulos technique, the crystal is grown in a larger diameter.

From the larger diameter crystal, we can make windows, prisms, lenses and

other optical components. As in the Czochralski method, here also the seed is

brought into 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 by

decreasing the power into the melt. The major use of this method is for alkali

halides to make optical components.

In the Zone melting technique, a liquid zone is created by melting a

small amount of material in a relatively large or long solid charge or ingot. It

is then made to traverse through a part or the whole of the charge. It is a more

advantageous method than the other methods due to the removal or addition

of impurities from or to the crystal as the crystal is growing. In this method,

the rate of zone movement depends on the orientation of the two solids

binding the liquid zone as well as the thickness and temperature of the zone

(Keller and Muhlbauer 1981).

In the Verneuil technique, fine dry powder of the material to be

grown is showered through a wire mesh and allowed to fall through the

6

oxy – hydrogen flame. The powder melts and a film of liquid is formed on the

top of the seed crystal, maintained on a pedestal at the bottom of the flame

(Hopper et al 1980). This freezes progressively as the seed crystal is slowly

lowered. The art of the method is to balance the rate of lowering of the seed to

maintain a constant growth rate and diameter. By this method, ruby crystals

are grown for use in jewelled bearing and lasers. This technique is widely

used for the growth of synthetic gems.

1.4 GROWTH FROM VAPOUR

The growth of single crystal materials from the vapour phase is

probably the most versatile of all crystal growth processes, although the large

number of variables involved tends to make it a relatively difficult process to

control. In addition, the lower density of molecules in the crystal growth

environment leads to lower growth rate than achieved from stoichiometric

melts. The driving force for the great evolution of the vapour growth methods

in the past years has been unquestionably the demand of physics and

electronics for single crystals of all possible materials (Santhanaraghavan and

Ramasamy 2002).

The fundamental aspects of vapour phase crystal growth might be

divided into four major areas. They are as follows,

i. Thermodynamics, which controls the driving force for the

chemical reaction occurring at the vapour-solid interface.

ii. Mass transport by which the reactants reach the growing

surface and the products are removed.

7

iii. Surface kinetics, including adsorption of reactants onto the

surface, surface diffusion, step generation, surface chemical

reactions between adsorbed reactants at steps and desorption

of products.

iv. Structural aspects of growth, especially epitaxial growth such

as defect generation and surface morphology of homoepitaxial

growth and heteroepitaxial layers.

Precisely, crystal of a substance can be grown from its vapour

phase by sublimation at atmospheric pressure. By this method small crystals

of high purity can be obtained. There are two methods by which crystals can

be grown from vapour phase. They are

a) Physical Vapour Transport (PVT) technique

b) Chemical Vapour Transport (CVT) technique

1.4.1 Physical Vapour Transport Technique

Crystals of substances having high vapour pressure at temperature

below their melting point are grown by this method. The substance is

enclosed in a capsule and is placed in the furnace. The substance evaporates

and diffuses from the hot end to the colder growth end. It then deposits in the

form of single crystals.

1.4.2 Chemical Vapour Transport Technique

In this method the material to be grown is made to react with a

transporting agent and is transported from the source region to the growth

8

region by means of the transporting agent. The chemical reaction involved

can be given as

A(s) + B(g) AB(g) (1.1)

When the solid or liquid substance reacts with a gaseous

transporting agent at temperature T1 to form vapour phase reaction product

(AB), which in turn undergoes a reverse reaction at low temperature T2

resulting in the reformation of A in the crystalline form and the release of B.

The reversible reaction is an important condition for crystallization to take

place. Substances like SbSi, BiBr etc., can be grown using this method.

1.5 GROWTH FROM SOLUTION

This method is widely used to grow the crystals which have high

solubility and have variation in solubility with temperature (Chernov 1984).

The vapour pressure at the growth temperature should be small. The viscosity

of the solvent – solute system 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 inflammable and less

toxic. Easy separation of grown crystal by chemical or physical means and

low melting point are citied as additional criteria for flux growth or high

temperature solution growth.

There are two methods in solution growth depending upon the

solvents and the solubility of the solute. They are classified into

(i) High temperature solution growth

(ii) Low temperature solution growth

9

1.5.1 High Temperature Solution Growth

The high temperature solution growth can be further sub-divided

into

(a) Flux method

(b) Hydrothermal growth

1.5.1.1 Flux method

In this method, a solid (molten solid / flux) is used as the solvent

instead of liquid and the growth process takes place well below the melting

temperature (Hubner 1969). Molten salt is 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 do have higher concentration of impurities

than these grown from melt. Since the growing crystal is not exposed to steep

temperature gradient, strain free crystals can be obtained. The flux growth is

preferably used for the following reasons:

(i) The material melts incongruently

(ii) The melting point of the material is too high and

(iii) The material is non-stoichiometric at its melting point due to a

high vapour pressure of one or more constituents.

A number of metals, metal oxides and other compounds, practically

insoluble in water up to its boiling point, show an appreciable solubility when

the temperature and pressure is increased well above 373 K and 1 atmosphere

pressure respectively.

10

1.5.1.2 Hydrothermal growth

The hydrothermal technique is a very important technique for its

technological efficiency in developing purer, bigger and dislocation free

single crystals. Although large number of materials can be grown by this

technique, it is primarily used for the growth of high quality quartz crystals.

A number of metals, metal oxides and other compounds practically insoluble

in water up to its boiling point, show an appreciable solubility when the

temperature and pressure increase well above 100ºC and 1 atmosphere

respectively. Growth is usually carried out in steel autoclaves with gold or

silver linings. The liquids from which the process starts are usually alkaline

aqueous solutions. Pressure is typically in the range of hundreds or thousands

of atmosphere. The requirement of high pressure presents practical difficulties

and there are only few crystals of good quality and large size grown by this

technique (Ballaman and Laudise 1963). Materials like quartz, calcite,

alumina and antimony sulphite can be grown by this method.

1.5.2 Low Temperature Solution Growth

Among the various methods of growing single crystals, solution

growth at low temperature occupies a prominent place owing to its versatility

and simplicity. Hence the crystals grown by this technique are free from

thermal strains and have well defined facets. Growth of crystals from solution

at room temperature has many advantages over the melt growth though the

rate of crystallization is very slow. Since growth is carried out at room

temperature, the concentration of structural imperfection in solution grown

crystals is relatively low (Brice 1972).

11

Low temperature solution growth can be further classified into the

following methods.

(a) Slow evaporation method

(b) Slow cooling method

(c) Temperature gradient method and

(d) Gel method

1.5.2.1 Slow evaporation method

This method is also called solvent evaporation method.

The temperature is fixed constant and provision is made for evaporation of

solvent. With non-toxic solvents like water, it is permissible to allow

evaporation into the atmosphere. Typical growth conditions involve

stabilization to about ± 0.01ºC. The evaporation techniques of crystal growth

have the advantage that the crystals grow at a fixed temperature. In this

method the solution loses particles, which are weekly bound to other

components, and therefore the volume of the solution decreases. In almost all

the cases, the vapour pressure of the solvent above the solution is higher than

the vapour pressure of the solute and therefore the solvent evaporates more

rapidly and the solution becomes supersaturated (Petrov 1969). This method

can effectively be used for the materials having moderate solubility

coefficient. Many organic and inorganic crystals were grown by slow

evaporation technique.

1.5.2.2 Slow cooling method

This method is suitable to grow bulk single crystals in short

duration. In this technique, supersaturation is achieved by changing

temperature usually throughout the period of crystal growth. The

12

crystallization process is carried out in such a way that the point on the

temperature dependence on the concentration moves into the metastable

region along the saturation curve in the direction of lower stability. The main

disadvantage is the need to use a range of temperature. The possible range of

temperature is usually small so that much of the solute remains in the solution

at the end of the run. To compensate these effects, large volumes of solution

are required. Even though the method has technical difficulty of requiring a

programmable temperature control, it is widely used with great success

(Brice 1973).

1.5.2.3 Temperature gradient method

This method involves the transport of the 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. On the other hand, changes in

the small temperature difference between the source and the crystal zones

have a large effect on the growth rate. The main advantages of this method

are:

(i) Crystal grows at fixed temperature

(ii) Insensitive to changes in temperature provided both the source

and growing crystal undergo the same change and

(iii) Economy of solvent and solute

1.5.2.4 Gel growth method

It is an alternative technique to solution growth with controlled

diffusion and the growth process is free from convection. When the growth of

single crystals by conventional technique has a problem such as

decomposition before melting or non-availability of suitable flux, gel growth

13

serves as an excellent alternative method. Gel is a two component system of a

semi solid 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 of several millimetre dimensions can be grown in a few

days by using a simple test tube apparatus. The crystals grown by this

technique have high degree of perfection and fewer defects since the growth

takes place at room temperature.

1.6 UNIDIRECTIONAL GROWTH METHOD FROM

SOLUTION

To grow high quality crystal with a reasonable yield, the

investigation of a possible single crystal growth technique is needed.

Nonlinear optical interactions in bulk single crystals usually require

phase-matching orientations in acentric crystals to maximize the frequency

conversion efficiency. To attain high wavelength conversion efficiency,

nonlinear optical crystals have to be long enough in the phase-matched

direction. Therefore, method of growing nonlinear optical crystals with a

large size in a phase-matched direction is strongly required. Further, growth

of organic or inorganic crystal with specific orientation has tremendous value

in terms of its significance towards device application. From this point of

view, a novel Sankaranarayanan-Ramasamy crystal growth method has more

advantages when compared to conventional solution growth techniques

(Sankaranarayanan and Ramasamy 2005) and this can be employed to grow

unidirectional crystal from solution.

1.6.1 Sankaranarayanan-Ramasamy (SR) Method

It is one of the methods to grow the crystals from solution.

Unidirectional crystals are very important for the preparation of functional

14

crystals. For example, as the conversion efficiency of second harmonic

generation is always highest along the phase-match direction for nonlinear

optical crystals, the unidirectional crystal growth method is most suitable for

the crystal growth along that direction. In addition, the unidirectional solution

crystallization usually occurs at around room temperature; much lower

thermal stress is expected in these crystals over those grown at high

temperatures. This is particularly helpful for growth of mixed crystals because

thermal stresses can cause these crystals to crack easily. Crystals with all

facets and different morphology are grown by conventional solution growth

technique but from application point of view, orientation controlled good

quality, large size SHG crystals are needed. In all the methods of growth by

solution, planar habit faces contain separate regions common to each facet

having their own sharply defined growth direction known as growth sectors.

The boundaries between these growth sectors are more strained than the

extended growth sectors due to mismatch of lattices on either side of the

boundary as a result of preferential incorporation of impurities into the lateral

section (Gallagher et al 2003).

A unidirectional crystal growth method was reported by

Sankaranarayanan-Ramasamy (Sanakaranarayanan and Ramasamy 2005).

The main concept of the method is gravity driven concentration gradient. The

solutions at the bottom of the ampoule have more concentration compared to

top solutions. The concentration gradient is directly proportional to time.

In Sankaranarayanan-Ramasamy (SR) method, a glass ampoule

was made up of an ordinary hollow borosilicate-glass with a tapered

V-shaped or flat bottom portion to mount the seed crystal and a U-shaped top

portion to fill a good amount of saturated solution to grow a good size crystal.

The middle portion was cylindrical in shape with lesser diameter than that of

the U-shaped portion, wherein one can get a cylindrical shaped crystal.

15

1.6.2 Salient Features of Sankaranarayanan-Ramasamy (SR) Method

The salient features of SR method are listed below:

(i) Single crystal with desired orientation is possible at room

temperature.

(ii) It is easy to adjust the growth rate as per our need.

(iii) Scaling up is relatively very simple.

(iv) With a thin plate as seed, growth of large size crystal is

possible.

(v) Microbial growth has been causing serious concern in

solution growth experiments largely due to aging of the

solution. However, as fresh solution can be constantly fed as

crystal growth proceeds the problem associated with

microbial growth can be avoided.

(vi) Simple experimental set up offers the feeding of the growth

solution at a definite interval which depends on the growth

rate of the crystal, thereby minimizing the exposure of the

growth solution to the environment.

(vii) The achievement of solute-crystal conversion efficiency of

100% reduces the preparation and maintenance of growth

solution to a large extent because in conventional solution

growth method, to grow such a large size crystal, a large

quantity of solution in a large container is normally used and

only a small fraction of the solute is converted into bulk

single crystal. But, in the present method, the size of the

growth ampoule is the size of the crystal.

16

(viii) The results obtained from the characterization techniques

such as XRD, phase-matching study and laser damage

threshold measurement demonstrate the suitability of this

method to obtain nonlinear elements right during crystal

growth thus decreasing material consumption when making

products for nonlinear optical applications.

(ix) In the case of thread hanging technique, inclusion appears

and the quality of the crystal is poor if a suspension thread is

used. This situation is avoided in this method.

(x) Usually in solution growth it is difficult to control the shape

and in this method by changing the ampoule shape it is

possible to change the shape of the crystal.

(xi) The crystal quality is always higher compared to the

conventional method grown crystals.

1.6.3 Experimental setup of Sankaranarayanan-Ramasamy (SR)

Method

The main concept of the method is gravity driven concentration

gradient. The solution at the bottom of the ampoule have more concentration

compared to top solutions. In SR method a glass ampoule was made up of an

ordinary hollow borosilicate-glass with a tapered V-shaped or flat bottom

portion to mount the seed crystal and a U-shaped top portion to fill a good

amount of saturated solution to grow a good size crystal. The middle portion

was cylindrical in shape with lesser diameter than that of the U-shaped

portion, wherein one can get a cylindrical shaped crystal. The schematic

diagram of experimental setup of SR method is shown in the Figure 1.1.

17

It consists of temperature controllers, ammeters, transformers, ring

heaters at top and bottom portions, sensors, glass ampoule and water bath.

Ring heater was directly connected to the temperature controller to maintain

the heater voltage and it provides the necessary temperature for solvent

evaporation and for growing crystals. The growth ampoule was placed inside

the water bath for avoiding temperature fluctuations in the growth portion.

Growth condition of this method depends on the temperatures of the heating

coils. Mercury thermometers show the temperatures near the heating coils.

The entire experimental setup is porously sealed and placed in a dust free

zone.

1.Thermometers 2. Heating coils 3. Top portion 4. Water

5. Bottom portion 6. Saturated solution 7. Bath 8. Seed

Figure 1.1 Schematic diagram of SR experimental setup

18

In SR method the following main points have to be considered i.e.

concentration of the solution, size of the ampoule, selection of seed, seed

orientation and mounting, temperature at top and bottom portion, evaporation

rate and growth rate. According to the solubility data, saturated solution was

prepared and transferred to crystallizer for collecting the seed crystal by slow

evaporation solution growth technique (SEST). A suitable seed crystal having

a reasonable size was selected for SR method of crystal growth with specific

orientation. The saturated solution fed into the SR glass ampoule. In the

freshly prepared solution, the solute concentration was deliberately kept

slightly undersaturated in order to avoid any possible physical instability at

the growth interface. For controlled evaporation, the top portion was closed

with some opening at the middle using thick plastic cover. Due to the

transparent nature of the solution and the experimental setup, real-time

close-up observation revealed the solid-liquid interface which was found to be

flat. In contrast to the SEST method, in the SR method the crystal was

restricted to grow with a specific direction inside a growth ampoule. The

entire arrangement was placed in the water bath to reduce the temperature

fluctuation.

1.7 GROWTH TECHNIQUES FOR THE PRESENT

INVESTIGATION

In the present investigation two different types of low temperature

solution growth techniques were used. They are low temperature conventional

slow evaporation solution method and unidirectional growth by

Sankaranarayanan-Ramasamy (SR) method. The SR method is superior to the

other low temperature solution growth techniques, due to its simple

experimental setup, less cost, flexibility to optimise for improving the

perfection of the crystals and ease in growing bulk size unidirectional crystals.

19

1.7.1 Solution and Solubility

The important process in the growth of crystals from solution is the

preparation of the solution. It is essential to prepare the solution, from pure

solute and solvent. In recent years, an appreciable attention is given to grow

crystals from solutions with faster growth rate. A solution is a homogeneous

mixture of a solute in a solvent. Solute is the component, which is present in a

smaller quantity and that one which gets dissolved in the solution

(Santhanaraghavan and Ramasamy 2000).

Solubility of the material in a solvent decides the amount of the

material, which is available for the growth and hence defines the total size

limit. Solubility gradient is another important parameter, which dictates the

growth procedure. Neither, a flat nor steep solubility curve will enable the

growth of bulk crystals from solution, while the level of supersaturation could

not be varied by reducing the temperature in the former. Even a small

fluctuation in the temperature will affect the supersaturation to grow the good

quality bulk crystals in both cases. If the solubility gradient is very small,

small evaporation of the solvent is the best option for crystal growth in order

to maintain a constant supersaturation in the solution.

Supersaturation is an important parameter for the solution growth

process. The crystal grows by the accession of the solute in the solution where

the degree of supersaturation is maintained. For the purification of the solute,

one of the simple processes is the repeated fractional recrystallization of the

solute using pure solvent. Any standard procedure such as distillation and

deionization can be adopted to purify the solvent. Recently, it has been proved

that the biological contamination such as bacteria also affects the growth.

Hence the solvent must be made free from all impurities.

20

1.7.2 Expression for Supersaturation

The supersaturation of the system may be expressed in number of

ways. The basic units of concentration as well as temperature must be

specified. The concentration driving force ( C), the supersaturation ratio (S)

and relative supersaturation ( ) are related to each other as follows:

The concentration driving force C = C – C* (1.2)

where C is the actual concentration of the solution and C* is the equilibrium

concentration at a given temperature. Then,

Supersaturation ratio S = C/ C* (1.3)

Relative supersaturation = (C - C*)/ C* (1.4)

= S-1 (1.5)

If the concentration of the solution can be measured at a given

temperature and the corresponding equilibrium saturation concentration is

known, then it is easy to calculate the supersaturation.

1.7.3 Nucleation

Nucleation is an important phenomenon in crystal growth. The

formation of crystals in liquid solutions begins with nucleation. Nucleation

may occur spontaneously or it may be induced artificially. These two cases

are called homogeneous and heterogeneous nucleation, respectively. Both the

nucleation come under primary nucleation and occurs in systems that do not

contain crystalline matter.

21

When nucleation is generated in the vicinity of crystals present in

the supersaturated system, it is referred to as secondary nucleation. The

attainment of the supersaturated state is an essential criterion for any

crystallization process. In general crystallization process takes place by three

steps:

1. Achievement of supersaturation or supercooling

2. Formation of crystal nuclei of microscopic size

3. Successive growth of crystal to yield distinct faces

All the above steps may occur simultaneously at different regions

of a crystallization unit. However an ideal crystallization process consists of a

strictly controlled stepwise program. Meirs and Issac (1987) conducted a

detailed investigation on the relationship between supersturation and

spontaneous crystallization, the results of which are shown in Figure 1.2.

It shows three zones, which are termed as region I, II, III respectively.

Region I corresponds to the undersaturated zone, where the

crystallization is not possible. The region between solubility curve and

supersolubility curve is termed as metastable zone (region II). Seeded growth

can be initiated from this region. The unstable or labile zone where the

spontaneous nucleation is probable which is termed as region III.

The formation of nucleation is insufficient to initiate

crystallization. The formation of embryos or nuclei with number of minute

solid particles present in the solution often termed as centres of

crystallization, is a prerequisite for the growth of crystals. The formation of

stable nuclei occurs only by the addition of a number of molecules (A1) until a

critical cluster is formed. In general

An-1 + A1 An (critical) (1.6)

22

Figure 1.2 Meirs solubility Curve

Any further addition to the critical nucleus results in nucleation

followed by growth. Once these nuclei grow beyond a certain size, they

become stable under the average conditions of supersaturation of the solution.

1.7.4 Solvent Selection

In solution growth, it is very important to choose the correct solvent

to grow the crystals. Solvents may be classified as being polar or non-polar.

The former description is given to liquids which have high dielectric constant

such as water, acids, alcohols and the latter refer to liquids of low dielectric

constant like aromatic hydrocarbons. A non-polar solute (anthracene) is

usually more stable than in a polar solvent (water). However, close chemical

similarity between solute and solvent should be avoided, because their mutual

solubility will in all probability be high and crystallization may prove difficult

MetastableLabile

BB’ Solubility Curve

AB”C” Evaporation and Cooling CC’ Super Solubility Curve

23

or uneconomical (Mullin 1961). The solvent must be chosen taking into

account the following factors:

(i) Good solubility for the given solute

(ii) Good temperature coefficient of solute solubility

(iii) Non toxicity

(iv) Non corrosiveness

(v) Non volatility

(vi) Non flammability

(vii) Maximum stability

(viii) Less viscosity

(ix) Small vapour pressure at the growth temperature

(x) Cost advantage

Almost 90% of the crystals produced from low temperature

solutions are grown by using water as a solvent. Probably no other solvent is

as generally useful for growing crystal as is water. Because of its higher

boiling point than most of the organic solvents commonly used for growth, it

provides a reasonably wide range for the selection of growth temperature.

Moreover, it is chemically inert to a variety of glasses, plastics and metals

used in crystal growth equipment (Buckley 1951, Santhanaraghavan and

Ramasamy 2000).

1.7.5 Materials Purification

High purity of material is an essential prerequisite for crystal

growth. Therefore the first step in crystal growth is the purification of

material in appropriate solvents. Impurities as low as possible at the scale of

10-100 ppm are required. Purification needs repetition of the crystallization

process in an appropriate solvent. For the purification of the solute, one of the

24

simple processes is the repeated fractional recrystallization of the solute using

pure solvent. Any standard procedure such as distillation and deionization can

be adopted to purify the solvent. Recently, it has been proved that the

biological contamination such as bacteria also affects the growth. Hence the

solvent must be made free from all impurities. Although the chromatographic

techniques like high performance liquid chromatography or gas

chromatography can be used for purification, they yield very small quantity or

purified product per cycle. The process like recrystallization followed by

filtration of the solution would increase the level of purity.

1.7.6 Preparation of the Solution

The important process in the growth of crystals from solution is the

preparation of the solution. It is essential to prepare the solution, from pure

solute and solvent. For solution preparation it is essential to have the

solubility data of the material at different temperatures. Sintered glass filters

of different pore size are used for solution filtration. The clear solution,

saturated at the desired temperature is taken in a growth vessel.

1.7.7 Harvesting of Grown Crystals

The extraction of a crystal from its mother liquor requires some

care because any damage may destroy completely the scientific value of the

crystal or even fracture it altogether. If a crystal is extracted from a solution

kept close to room temperature, it can be simply dried by means of high

quality Whatman filter paper (grade 1). Filter paper must be used to rub the

surface. The majority of crystals prepared from low temperature solutions are

easily scratched. The surface of a carelessy treated crystal may acquire many

defects.

25

The quality of the harvesting crystal depends on

i. Purity of the starting material

ii. Quality of the seed crystal

iii. Cooling rate employed

iv. Efficiency of agitation

1.8 CHARACTERIZATION TECHNIQUES

The role of characterization of the crystal gains significance in the

context that the results of such studies will give a feedback about the quality

of the grown crystals so that the growth parameters will be standardized. For

proper understanding of the grown crystals and to use them for technological

applications, they have to be characterized accurately. The four basic

parameters that control the properties of the single crystals are composition,

purity, crystallographic structure and crystal defects (Laudise et al 1975).

Characterization of a crystal essentially consists of its chemical

composition, structure, defects and the study of its electrical, mechanical,

thermal and optical properties.

In the present investigation the grown crystals were subjected to

crystallographic analysis like X-ray diffraction (XRD) to determine the

crystal system and its lattice parameters. High Resolution Scanning Electron

Microscope (HR-SEM) and the Energy Dispersive X-ray Analysis (EDX)

shows the nature of growth and the presence of constituent elements,

respectively. The functional groups and vibrational frequencies were

identified using Fourier transform infrared (FT-IR) and Fourier transform

Raman (FT-Raman) spectral analyses. The UV-Visible absorbance spectra

were recorded to find out the cut-off wavelength and suitability of the crystal

26

for NLO application. Photoluminescence studies were carried out to analyse

the structural perfection of the grown crystals. Thermal characteristics were

analyzed using the Thermogravimetric (TG), Differential Thermal (DT) and

Differential Scanning Calorimetry (DSC) studies. Microhardness test was

performed to evaluate the mechanical behaviour. Consequently it is the

purpose of this section to briefly outline some of the basic as well as applied

characterization methods.

1.8.1 X-Ray Diffraction

X-ray diffraction (XRD) is versatile, non-destructive technique that

reveals detailed information about the chemical composition and

crystallographic structure of manufactured materials. A crystal lattice is a

regular three dimensional distribution of atoms in space. These are arranged

so that they form a series of parallel planes separated from one another by a

distance d, which varies according to the nature of the material. For any

crystal, planes exist in a number of different orientations, each with its own

specific d-spacing.

1.8.1.1 Single Crystal X-ray Diffraction

X-ray diffraction studies for the grown crystals were carried out

using a Bruker AXS Kappa APEX II single crystal CCD diffractometer

(Figure 1.3) equipped with graphite monochromated MoK ( = 0.7107 Å)

radiation. The goniometer equipped with the diffractometer is four circle

goniometer with , , and 2 axes by which the crystal is rotated. The

crystals of 0.30 × 0.25 × 0.25 mm3 were cut by sharp blade and mounted on

a glass fibre using cyanoacrylate. The unit cell parameters were determined

by collecting the diffracted intensities from 36 frames measured in three

different crystallographic zones.

27

Figure 1.3 Single Crystal X-ray diffractometer

1.8.1.2 Powder X-ray Diffraction

Powder X-ray diffraction is a non-destructive technique widely

applied for the characterization of crystalline materials. The method has been

used for phase identification, quantitative analysis, extraction of three

dimensional micro structural properties, determination of structure

imperfections, crystal structures of a crystalline material and can provide

information on unit cell dimensions. The XRD works on the principle of

Bragg’s law. According to this law, when a beam of monochromatic X-rays

falls on a crystal each atom becomes a source of scattering radiations.

28

In a crystal there are certain planes which are particularly rich in atoms. The

combined scattering of X-rays from these planes can be considered as a

reflection from these planes. At certain glancing angles, the reflections from

these set of parallel planes are in phase with each other, and hence they

reinforce each other to produce maximum intensity. For other angles, the

reflections from different planes are out of phase and hence they reinforce to

produce either zero intensity or feeble intensity.

In the present work, powder X-ray diffraction analysis was carried

out using BRUKER D2 PHASER spectrometer with CuK ( = 1.5418Å)

radiation is shown in the Figure 1.4. In the powder X-ray diffraction method

the sample is grounded to a fine powder which contains thousands of grains

with random orientations. By scanning through 10° to 80° at a rate of 1°/min,

we can find where the diffraction has occurred and each of them will be

associated with different atomic spacing.

To detect the diffracted X-rays from the sample an electronic

detector is placed on the other side of the sample and it is allowed to rotate it

from 10° to 80°. It keeps track of the angle and sends the information to a

computer. The graph is plotted between X-ray intensity and angle 2 values.

Angle 2 for each diffraction peak can be converted to ‘d’ spacing using

Bragg’s law, which is used to find out the crystal structure.

29

Figure 1.4 Photograph of Powder X-ray diffraction instrument

1.8.2 Elemental Analysis

1.8.2.1 CHN analysis

The elemental analysis of a compound is particularly useful in

determining the empirical formula of the compound. The empirical formula is

the formula for a compound that contains the smallest set integer ratio for the

elements in the compound that gives the correct elemental composition by

mass. The most common form of elemental analysis, CHN analysis, is

accomplished by combustion analysis. In this technique, a sample is burned

with excess of oxygen and various traps collect the combustion products-

carbon dioxide, water and nitric oxide. The weights of these combustion

products can be used to calculate the composition of the unknown sample.

30

The analysis of results is performed by determining the ratio of

elements from within the sample, and working out a chemical formula that

fits with those results. This process is useful as it helps to determine if the

sample examined has the desired composition and confirms the purity of a

compound. The accepted deviation of elemental analysis from the calculated

is 0.4 %.

Figure 1.5 Photograph of CHNS analyzer

In our study, chemical composition of the grown crystals

determined by Carbon, Hydrogen and Nitrogen (CHN) analysis using

PERKIN ELMER 2400 Series CHNS analyser (Figure 1.5) is compared with

the theoretical values of carbon, hydrogen and nitrogen present in the crystals.

31

1.8.2.2 Energy Dispersive X-ray (EDX) analysis

Energy dispersive spectroscopy is a chemical microanalysis

technique performed in combination with Quanta 200 FEG scanning electron

microscope (Figure 1.6). The interaction of an electron beam with a specimen

in the scanning electron microscope produces many signals, including X-rays.

Some of the X-rays produced in this manner have wavelength and energies

that are characteristics of the element in that specimen.

It is a technique used for identifying the elemental composition of

the specimen. Elemental evaluation by the analytical microscopy may be

accomplished by analysing the emitted radiation and showing that these

specific wavelengths are present. The analysis is carried out in an X-ray

spectrometer by two methods, (i) wavelength-dispersive analysis and

(ii) energy-dispersive spectroscopy. In the present investigation the chemical

analysis has been carried out by Energy Dispersive spectroscopy. During

Energy Dispersive X-ray (EDX) analysis, the specimen is bombarded with an

electron beam inside the scanning electron microscope. The bombarding

electrons collide with the specimen atom’s own electrons, knocking some of

them off in the process. The EDX spectrum is a plot of intensity of X-rays

with energy of the emitted X-rays. An EDX spectrum normally displays peaks

corresponding to the energy levels for which the most X-rays have been

received. Each of these peaks is unique to an atom, and therefore corresponds

to a single element. The higher the intensity of peak in a spectrum, the more

concentrated is the element in the specimen.

32

Figure 1.6 Photograph of HR-SEM with EDX instrument

1.8.3 Vibrational Analysis

Vibrational spectroscopy is an extremely useful tool in the

elucidations of molecular structure. The spectral bands can be assigned to

different vibrational modes of the molecule. The various functional groups

present in the molecule can be assigned by a comparison of the spectra with

characteristic functional group frequencies. As the positions of the bands are

directly related to the strength of the chemical bond, a large number of

investigations including intermolecular interactions, phase transitions and

chemical kinetics can be carried out using this branch of spectroscopy.

1.8.3.1 Fourier Transform Infrared (FT-IR) spectroscopic analysis

Infrared spectroscopy involves study of the interaction of

electromagnetic radiation with matter. Due to this interaction, electromagnetic

33

radiation characteristic of the interacting system may be absorbed or emitted.

The experimental data consist of the nature (frequency of wavelength) and the

amount (intensity) of the characteristic radiation absorbed or emitted. These

data are correlated with the molecular and electronic structure of the

substance with intra- and inter molecular interactions. It is one of the most

widely used tools for the detection of functional groups in pure compounds

and mixtures, and for the comparison of compounds.

There are two types of infrared spectrometers characterized by the

manner in which infrared frequencies are handled. In the first type called the

dispersive type, the infrared light is dispersed into individual frequencies

using a grating monochromator whereas in the second called the Fourier

transform infrared where, the infrared frequencies interact to produce an

interference pattern and this pattern is then analysed mathematically using

Fourier Transform to determine the individual frequencies and their

intensities (Willard et al 1986, Silverstein et al 1998).

When a sample is placed in the path of the beam, it absorbs the

characteristic frequencies so that their intensities are reduced in the

interferometer and the ensuing Fourier transform is the infrared absorption

spectrum of the sample. The scan time for the moving mirror dictates the

speed with which the infrared spectrum can be recorded. Digitalization of the

data and calculation of the Fourier Transform take few seconds more, but the

information which constitutes the spectrum, can be acquired in exceedingly

short time even in a few milliseconds.

34

Figure 1.7 FT-IR spectrophotometer

The FT-IR spectra of the sample are recorded in the wavelength

region of 4000 - 400 cm-1. The photograph of PERKIN-ELMER FT-IR

spectrometer is shown in Figure 1.7. This spectrometer consists of an

interferometer between source (Globar) and sample compartment. The sample

compartment is located in-between the interferometer and the detector. The

KBr pellet method is used for recording spectrum in the present work. The

background is first recorded with KBr pellets. The sample is then pelletized

with KBr and the signals are recorded.

1.8.3.2 Fourier Transform Raman (FT-Raman) spectroscopic analysis

Raman spectroscopy concerns with the scattering of radiation by

the sample, rather than an absorption process. It is based on the principle of

inelastic scattering of photons by molecules. For a transition to be Raman

active there must be a change in polarizability of the molecule. Raman

spectroscopy is true for heteronuclear diatomic molecules. Diatomic

35

molecules do not possess a permanent electric dipole, and so they are

undetectable. The energy of the exciting radiation will determine as to which

type of transition occurs. The energy of the rotational transitions are lower

than the vibrational transitions (Clark 2007).

1.8.4 Optical Studies

1.8.4.1 UV-visible transmittance studies

UV-Visible spectroscopy is the measurement of the attenuation of

the light beam after it passes through a sample or after reflection from a

sample surface. The optical transmission spectra are recorded for polished

plates of grown crystals using VARIAN CARY 5E UV-Vis-NIR

Spectrophotometer (Figure 1.8). The spectrophotometer consists of a light

source, monochromator which resolves radiation into its component

wavelengths, sample compartment and detector which measures the amount

of light transmitted by the sample. Two light sources are used to cover entire

range of spectrum from 200-2500 nm. A narrow bandwidth was selected with

the help of band pass filters. Then the radiation was passed through the cells

of the given solution and solvent alternatively and respectively. This process

is double beam process and it eliminates the fluctuations of intensity of

radiation scattering effect and solvent effect. The two rays were combined and

differences in intensities of those two radiations are measured electrically.

The movement of the monochromators and the recorder were synchronized

and adjusted so that the record shows the intensity of appropriate radiation. In

the transmission study, the intensity was recorded in terms of percentage of

transmittance (%) along Y-axis and the wavelength (nm) along X-axis.

The UV-Visible spectrum gives information about the useful range of

wavelength in which the NLO crystals can be operational.

36

Figure 1.8 UV-Visible spectrophotometer

1.8.4.2 Photoluminescence (PL) studies

Photoluminescence (PL) is a commonly used characterization tool

for material analysis. In Photoluminescence, absorption of light results in a

transition from the ground state to an excited state of an atom or molecule;

then the system undergoes a non radiative internal relaxation and the excited

electron moves to a more stable excited level; after a characteristic lifetime in

the excited state, the electronic system will return to the ground state. The

energy is released in the form of light, and this emitted light is detected as

photoluminescence.

37

Figure 1.9 Photograph of Spectrofluorometer

The photoluminescence measurement was made using a Jobin

Yvon Spex spectrofluorometer (Figure 1.9). The sample compartment module

equipped with a Xenon lamp operates at 450 Watts. This high-pressure Xenon

lamp is typically used in the instruments because it provides a continuous

output from 200 nm to 700 nm. From the Photoluminescence spectrum, the

spectral dependence of its intensity can provide information about the

properties of the material; the time dependence of the emission can provide

information about energy level coupling and lifetimes (Ricbard Brundle et al

1992). The presence of defects in the materials can also be estimated by the

PL emission involving defect level transitions, so the quality of materials can

be determined.

1.8.5 Hardness Measurements

Hardness is an important solid state property, which determines the

mechanical strength of the materials and the resistance to local deformation.

38

In the present study, hardness of the grown crystals is measured using

Vicker’s microhardness tester. To understand the mechanical behaviour of

crystalline materials and identify the mode of deformation micro indentation

has been made on a smooth surface of the selected specimen with the Vickers

diamond pyramidal indenter. The indentation is performed for various loads

at a constant indentation time of 5s. Vickers microhardness number (Hv) was

determined using the formula is given by

2v 2

1.8544PH kg / mm

d= (1.7)

where P is the test load in gram, d is the mean diagonal length of indentation

in m. Generally the hardness of the material varies with the applied test load.

Its value depends on the elastic and plastic behaviour of the solid and the test

conditions. It also facilitates to study the behaviour of dislocation when the

crystal is subjected to a stress. It is a technique of subjecting a crystal to

relatively high pressures within a localized area.

1.8.6 Etching Studies

The study of the identification, origin and characteristic of

crystalline defects such as grain boundaries, slip planes, dislocation and

plastic flow relies heavily on etching phenomena (Sangwal 1987). Etching is

the selective dissolution of the crystal, a reverse phenomenon of growth.

When a crystal phase is exposed to a solvent, dissolution begins by the

nucleation of unit pits of one molecular depth which then grows in the size by

the retreating steps across the crystal surface.

Etching is however a surface technique and care must be exercised

in the interpretation of etching studies, since densities and properties in the

39

surface and bulk regions of crystals frequently show marked difference. For

any defect etchant, it is essential to assess the extent to which a

correspondence exists between etch features and dislocations. In general, it

should be noted that not all etch pits are necessarily formed at emergent

dislocations and that not all dislocations give rise to etch features.

An etchant is a solvent of the testing sample. But all the solvents

are not the best etchant for the same sample. An etchant should satisfy the

following requirement: continued etching of a surface should, in general

result in no net change in the number of pits other than where pits are

associated with, for example, dislocation loops. In such cases, the pits should

appear or disappear in pairs. The occurrence of etch pits is an illustration of

the enhanced chemical reactivity of dislocation, while in this case the

different etch pit shapes on the same crystal face demonstrate the dependence

of this behaviour upon dislocation character. This technique requires only

very basic equipment and yields much valuable information. The three

important parameters for performing chemical etching of a material are;

etchant, temperature of etching and time of etching.

1.8.7 Thermal Analysis

The Thermogravimetric (TG), Differential thermal (DT) and

Differential scanning calorimetry (DSC) are used for thermal analysis in the

present investigation. The Perkin Elmer thermal analyser was used to study

the thermal behaviour of the grown samples between 35°C to 800°C in

nitrogen atmosphere at a heating rate of 10°C/min. TG records the change in

weight of the sample as a function of time or temperature. DT measures the

difference in the temperature of the sample and an inert reference material as

a function of temperature, thus detects the changes in the heat content of the

sample. The differential thermal curve would be parallel till the sample

40

undergoes physical or chemical change of state. However as soon as the

sample has reached the temperature, a change of state occurs which shows a

differential signal. Both the shape and size of the peak furnish the nature of

the sample. In DSC, the cell is designed so as to enable one to measure the

enthalpy changes quantitatively as a function of time or temperature. The

thermal analyser with the nitrogen atmosphere is used to get information

about the melting point, dehydration process and decomposition temperature

of the sample. Differential scanning calorimetry (DSC) is the most often used

thermal analysis method. In DSC, a sample and a reference are placed in

holders in the instrument. The instrument measures the difference in the heat

flow between the sample and the reference. The events which do not involve

weight changes like polymorphic transformations, para-ferroelectric phase

transitions can be detected by differential scanning calorimetry (DSC).

1.9 GENERAL DESCRIPTION OF NONLINEAR OPTICAL

PHENOMENA

Nonlinear optics is the study of phenomena that occurs as a

consequence of the modification of the optical properties of a material system

by the presence of light (Prasad and Williams 1991). The laser light is

sufficiently intense to modify the optical properties of a material system. The

beginning of the field of nonlinear optics is often taken to be discovery of

second harmonic generation by Franken et al (1961), after the demonstration

of the first working laser by Maiman (1960). The existence of harmonic light

waves at the boundary of a nonlinear dielectric medium was predicted

(Bloembergen and Pershan 1962) by discovery of other nonlinear effects

followed the discovery of second harmonic generation in quartz

(Bloembergen 1965). The field of nonlinear optics and photonics are rapidly

emerging as the technology for the twenty first century.

41

Photonics is the technology in which a photon instead of an

electron is used to acquire, store, process and transmit information. A

photonic circuit is equivalent to electronic circuit, in which photons are

conducted through channels. Light can be switched from one channel to

another at certain junction points. For optical switching at these junctions, one

needs to use a material that allows the manipulation of an electric field or

laser pulse. The materials which manipulate the light at these junction points

are known as NLO materials and these are gaining importance in technologies

such as optical communication, optical computing and dynamic image

processing (Boyd 1992). Photonics has many distinct merits over electronics.

The most important advantage is the gain in speed due the fact that a photon

travels much faster than electron. Other advantages are that there is no

electrical and magnetic interference, thereby the photonic circuits are fully

compatible with the existing fibre optics networks.

Before the advent of the lasers, optics assumed that optical

parameters of the medium are independent of the intensity of the light

propagating in these media. The reason is that, the electric field strength

generated by the non-laser light sources is of the order of 103 V/cm, is very

much smaller than the interatomic field strength that is 107 to 1010 V/cm of

the media, which is unable to affect the atomic fields of the medium and

thereby the optical properties of the medium. Lasers have drastically changed

the situation as they generate electric field strength varying from

105 to 1010 V/cm, which is close to that of the atomic electric fields of the

medium and thereby affect the optical properties of the medium and thus

generate new electromagnetic fields altered in phase, frequency and

amplitude. This is the domain of NLO. Each NLO processes may consist of

two parts. The intense light first induces a nonlinear response in a medium.

Then the medium in reaction modifies the optical fields in a nonlinear way.

42

The process is governed by Maxwell’s equations (Shen 1984, Butcher and

Cotter 1990).

A dielectric medium, when placed in an electric field is polarized.

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 field is to induce a polarization (Dmitriev et al 1991).

A light wave consists of electric and magnetic fields which vary sinusoidal to

optical frequencies. The motion of the charged particles in a dielectric

medium in response to an electric field is therefore oscillatory and form

oscillatory dipoles. Each constituent molecule acts as a dipole. At very low

fields, the induced polarization is directly proportional to the electric field.

P(t) = 0 E(t) (1.8)

where is the linear susceptibility of the material, E is the electric field

vector, 0 is the permittivity in free space.

At high fields polarization becomes independent of the field and the

susceptibility becomes field dependent. Therefore, the nonlinear response is

expressed by writing the induced polarization P(t) as a power series in the field

strength E(t) as,

P(t) = 0(1) E(1)(t)+ 0

(2)E(2)(t) + 0(3) E(3)(t) + … (1.9)

where the (2) and (3) coefficients represent the second and third order

nonlinear optical susceptibilities of the medium, respectively. To treat the

vector nature of the fields, in such a case (1) becomes a second rank tensor,(2) becomes a third rank tensor and so on.

43

(1) is the linear term responsible for the refractive index,

dispersion, birefringence and absorption. (2) is the quadratic term which

describes second harmonic generation, optical mixing and optical parametric

oscillation. (3) is the cubic term which is responsible for observing

phenomena such as stimulated Raman scattering, third harmonic generation,

phase conjugation and optical bistability.

In the case of centrosymmetric crystal (2) = 0, the material can

exhibit harmonic generation of third and fifth order. Two stage processes

occur during harmonic generation. A polarization wave at second harmonic

2 1 is produced in the first stage. The phase velocity and wavelength in the

medium are determined by n1, the index of refraction. In the second stage,

transfer of energy from the polarization wave to electromagnetic wave occurs

at frequency 2 2. The index of refraction n2 defines the phase velocity and

wavelength for the doubled frequency.

For efficient energy transfer, the two waves, should remain in

phase, i.e., n1 = n2. Due to normal dispersion occurring in the materials in the

optical region, the radiation will generally lag behind the polarization wave.

The phase mismatch between the polarization and electromagnetic

wave is given by

1 2

4k (n n )

π

λ

� �∆ = − !

" #(1.10)

For improving the efficiency of the doubled frequency, the crystal

has to be phase matched (Giordmaine 1962). The dispersion in the materials

can be offset by using the natural birefringence. There exist two indices of

refraction for a given direction of propagation, corresponding to the two

allowed orthogonally polarised modes. By an appropriate choice of

polarization and direction of propagation, it is often possible to obtain phase

44

matching or index matching, where k = 0. To realize the nonlinear effect a

suitable medium is required. To be a potential candidate for practical

applications, the materials need to exhibit large second order optical

nonlinearities.

1.9.1 Nonlinear Optical Crystals

Nonlinear optical (NLO) materials play a major role in nonlinear

optics and in particular they have a great impact on information technology

and industrial application. The understanding of the nonlinear polarization

mechanisms and their relation to the structural characteristics of the materials

has been considerably improved. The aim is to develop materials presenting

large nonlinearities and satisfying, at the same time, all the technological

requirements for applications such as wide transparency range, fast response

and high damage threshold.

Novel materials having attractive properties are being discovered at

a rapid pace, which advances in crystal growth technology making possible

the commercial development of promising materials such as Urea, KDP,

ADP, Lithium niobate, Potassium niobate, KTP (Wang et al 2009), YAB

(Leonyuk et al 2005) and -BBO (Sabharwal and Sangeeta 1998). Several

thousand nonlinear crystals and their closely isomorphs like Li6CuB4O10,

BiCu5B4O14 etc., have been developed (Pan et al 2006, Pan et al 2008).

1.9.2 General Requirements of NLO Crystals

An ideal nonlinear optical material should possess the following

characteristics:

45

1. High nonlinear coefficient

2. Wide transparency region

3. High laser damage threshold

4. Wide phase matching angle

5. High mechanical strength and thermal stability

6. Fast optical response time

7. Nontoxicity and good environmental stability

8. Non-hygroscopic nature

9. Architectural flexibility for molecular design and morphology

10. Ease of device fabrication

1.9.3 Phase Matching

The intense development of research on the mechanism of

generation of optical harmonic in crystal and media in which such generation

is effectively realizable, has indicated the importance of phase relation

between the fundamental and generated harmonic, as they propagate in crystal

having optical dispersion (Bloembergen 1963, Franken and Ward 1963). It

was observed that the efficiency of the generation of harmonics depends not

only on the intensity of the exciting radiations, but also on its direction of

propagation in crystals.

Phase matching means that the wave generating the polarization

and the generated waves (the three interacting waves) are in phase over the

interaction region, so the microscopic contributions of the generated

polarization of each individual dipole in the crystal can interfere

constructively, adding up to a macroscopic field. Only after this constructive

interference the nonlinear effect can be observed. To achieve phase-matching,

the phase velocity of the generated waves (while travelling through the

crystal) should equal the phase velocity of the pump wave in a parametric

46

process. This can be achieved in birefringent crystals, waves with different

wavelength can travel at the same speed (so in phase) when their polarization

directions are along different crystal axes. To fulfil phase matching, the

generated waves and the applied wave must have different polarization to

control their propagation velocities.

Certain asymmetric crystals are birefringent (doubly refracting)

because through them, light can travel at two different velocities, described as

ordinary (o) and extra ordinary (e). These velocities actually vary with

propagation direction and polarization as well as with wavelength. In certain

direction the ordinary fundamental light travels at exactly the same velocity as

extra ordinary harmonic light. When this happens, the SHG is greatly

enhanced and the system is said to be phase matched.

1.9.4 Types of NLO Materials

The nonlinear optical (NLO) crystals which provide the active

medium for nonlinear processes can be divided into three broad categories.

(i) Organic crystals

(ii) Inorganic crystals

(iii) Semiorganic crystals

1.9.4.1 Organic crystals

For the past two decades, extensive investigations have been

carried out on organic NLO materials for their nonlinear susceptibilities

compared with the inorganic materials (Chemla and Zyss 1987). Large

molecular first order susceptibilities ‘ ’ are associated with structures that

have a large difference between the ground and excited state dipole moments

47

and low energy charge transformation (Nalwa et al 1998). The preparation of

organic crystals is less expensive, nonlinear susceptibilities are high and

bifringence is sufficient for use in frequency converters. In general, organic

molecules exhibit very special properties originating from the versatility of

the carbon atoms, which is capable of engaging in various types of covalent

bonds and in a wide range of combinations. The molecules in pure organic

crystals are often bounded by weak van der Waals force of hydrogen bonds

which results in a high degree of delocalization of charges. The prototype

organic NLO materials contain one or more delocalized bonds, typically a

ring structure like benzene. Recent growing efforts in molecular engineering

suggest that organic NLO materials possess comparably better NLO

properties than inorganic, hence their tremendous practical potentials have

been anticipated.

1.9.4.2 Inorganic crystals

Investigations of nonlinear optical effects were initially focused on

pure inorganic systems and the first solid to demonstrate the second order

NLO properties was a quartz crystal. Many efficient inorganic materials such

as potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate

(ADP), -barium borate (BBO), potassium niobate (KNbO3), lithium niobate

(LiNbO3), lithium iodate (LiIO3), potassium titanyl phosphate (KTP) and

lithium triborate (LBO) were developed in the past decades for NLO

applications (Dmitriev et al 1999, Yatriv 1988, Prasad and Williams 1991,

Sasaki et al 2000, Nikogosyan 2005). Inorganic NLO materials are mostly

ionic bonded and have high melting point and high degree of chemical

inertness. Indeed most of the commercial frequency doublers use these

inorganic materials. Among the inorganic materials, KDP is one of the

popular nonlinear optical materials used for frequency doubling phenomena.

48

1.9.4.3 Semiorganic crystals

A combination of organic and inorganic materials provides a

potentially useful approach to more efficient and stable NLO crystals. Metal

complexes satisfy very different demands of second order NLO materials

such as switchable, tunable and multi dimensional properties depending on

the suitable interplay of structure property relationships. It offers a wide range

of metals with different oxidation states and ligands, which can give rise to

tunable electronic properties. Semiorganic NLO crystals are expected to

possess the advantages of both organic and inorganic materials. There are

three different types of semiorganic crystals

(i) Organic-Inorganic salts such as L-arginine Phosphate

monohydrate (LAP), which was explored in the early 1980s

(Xu et al 1983).

(ii) Metal-Organic coordination complexes which were proposed

in 1985.

(iii) Organometallic compounds

Thus the organic-inorganic hybrid complexes present a new

promising type of materials for various applications. The benefits are due to

the materials such as wide range of electronic characteristics, mechanical

hardness and thermal stability and on the other hand structural variety, large

polarizability and easy processing of organics. The refractive indices of the

crystal could also be tuned due to exchange ability of metal and halogen

species within anions.

49

1.9.5 Objectives of the Thesis

The present thesis focuses the growth of complex organic

molecular crystals for nonlinear optical applications and a comparative study

on growth and characterization of Methyl 2-amino-5-bromobenzoate single

crystals by Conventional and Unidirectional methods.

The first chapter presents an outline to various crystal growth

methods and characterization and a brief description of nonlinear optics.

The second chapter deals with the growth of L-Serine methyl ester

hydrochloride single crystals by conventional slow evaporation solution

growth technique using methanol as solvent. The solubility was estimated in

different temperatures and was purified by repeated recrystallization

processes. The grown single crystals were subjected to various

characterization studies such as Fourier Transform Infrared (FT-IR), Optical,

Thermal (TG/DTA) and Second Harmonic Generation (SHG). The refractive

indices nx, ny and nz along the principal directions of the title crystal were

measured using Brewster’s angle method. The suitability of material for

Second Harmonic Generation (SHG) was confirmed by Kurtz and Perry

powder technique. The linear increase of SHG with increase of particle size

confirms the phase matching behaviour of the crystal.

The third chapter presents the growth and characterization of

nonlinear optical single crystals of L-Tyrosine methyl ester hydrochloride.

The cell parameters were determined using single crystal X-ray diffraction

studies. The presence of constituent elements in the material was confirmed

by the occurrence of their respective peaks in the EDX spectrum. A single

sharp high intensity peak was observed in the Photoluminescence spectrum

50

shows the crystalline perfection of the title compound. The thermal,

mechanical and second harmonic generation studies were carried out.

The fourth chapter discusses the synthesis and growth of a

nonlinear optical single crystal of D-Phenylglycinium bromide by

conventional solution method at 35°C using water as solvent. The crystal

structure was solved by direct method and refined by full matrix least squares

with SHELXL 97 program. The title crystal belongs to orthorhombic crystal

system with space group P212121. The lattice parameters are a = 5.5240(5) Å,

b = 7.4735(5) Å and c = 23.1229(18) Å. The photoluminescence study asserts

the suitability of the compound for optical applications. The thermal

behaviour of the crystal was studied by TG-DTA response curves. The

Vicker’s microhardness test clearly revealed that the grown crystal obeys the

reverse indentation size effect. The emission of green radiation of wavelength

532 nm from the crystalline powder was confirmed using Kurtz and Perry

powder technique.

The fifth chapter presents the bulk single crystal growth of

Ethyltriphenyl phosphonium bromide dihydrate by the slow evaporation

solution growth technique at room temperature using mixed solvent of double

distilled water and ethanol. It belongs to orthorhombic crystal system with

space group P212121. The optical behaviour was studied using

UV-Visible spectral analysis and the optical band gap is found to be 3.13 eV.

The suitability of the grown crystal in the blue emission region was confirmed

by Photoluminescence studies. The thermal, mechanical and SHG behaviour

were tested.

The sixth chapter deals with the comparative study on growth and

characterization of Methyl 2-amino-5-bromobenzoate by conventional

solution growth and Unidirectional Sankaranarayanan-Ramasamy (SR)

51

methods using methanol as solvent. The grown crystal was confirmed by

single crystal X-ray diffraction studies. The FT-IR and FT-Raman spectral

analyses confirm the functional groups present in the crystal. The refractive

indices (nx, ny and nz) of the title crystal were found using Brewster’s angle

method. Photoconductivity study confirms the negative photoconductivity

nature of the crystal. SHG studies were carried out with different particle

sizes of the powdered material and the phase matching behaviour was

confirmed.

Bulk size cylindrical shaped transparent <001> oriented single

crystal of Methyl 2-amino-5-bromobenzoate of size 60 mm length and 15 mm

diameter was successfully grown by Unidirectional growth method of

Sankaranarayanan-Ramasamy (SR). Identical samples prepared with similar

orientation using conventional and SR methods were subjected for various

characterization studies such as UV-Visible transmittance analysis,

Photoluminescence, Microhardness and Etching studies. The results of SR

method grown crystal and conventional grown crystal were compared. It was

found that the quality of crystal grown by SR method is better than

conventional method grown crystal.

The seventh chapter presents the summary of the work carried out

and suggestions for future work.