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CHAPTER 1

INTRODUCTION TO CRYSTAL GROWTH, BRIDGMAN

TECHNIQUE AND THE CHARACTERIZATION

TECHNIQUES

1.1 INTRODUCTION

Crystals are the unacknowledged pillars of modern technology. The

modern technological development depends greatly on the availability of

suitable single crystals, whether it is for lasers, semiconductors, magnetic

devices, optical devices, superconductors, telecommunication, etc. The

growth of single crystals has been developed over the years to meet the needs

of basic research and applications. Crystal growth is an art of science and the

subject of the growth of crystals is an interdisciplinary one which contributes

many professional fields; solid state physicists, mineralogists,

crystallographers, physical chemists, mathematicians, chemical engineers,

metallurgists etc. Though it has been studied extensively more than 100 years,

crystal growth still plays an important role in both theoretical and

experimental research fields, as well as in applications. The crystalline state is

the regular periodic arrangement of the constituent molecules, atoms or ions

in three dimensions. The crystals are anisotropic in nature. The mechanical,

electrical, magnetic and optical properties can vary according to the direction

in which they were measured (Mullin 1993). Crystals are used for devices

because the charge carriers are electrons and holes which can move freely.

Modern devices are much smaller and are made with higher yields. The

improved yields come partly from device fabrication techniques and partly

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from improved materials. Although single crystals are the basic materials

supporting the present advanced technologies and such research activities

play an important role in manufacturing high-quality single crystals. Many

researchers are involved in such research activities.

The relevant scientific research in the field of crystal growth covers

a wide spectrum of work on nucleation, growth rates, translation rates,

segregation, growth interfaces, composition morphology, stability and

crystalline defects. The crystal growth and characterization helps particularly

for some of the challenges and opportunities for fundamental research,

development and technological applications. With the discovery of the

transistor in 1948 and the emergence of a revolutionary in solid state

electronic technology which depends on single crystal technology, for the

basic and applied studies on crystal growth become a significant part of

contemporary materials science. The advanced technology is very much

dependent upon materials or single crystals such as semiconductors,

polarizers, radiation detectors, transducers, ferrites, ultrasonic amplifiers,

magnetic garnets, solid state lasers, scintillator, nonlinear optic (NLO),

dielectric, piezoelectric, acousto-optic, photosensitive materials and

crystalline films for microelectronics and computer industries.

Nowadays, crystalline organic compounds represent a new class for

technological applications (Figi et al 2008). Organic materials have been of

particular interest because the NLO responses in this broad class of materials

are microscopic in origin, offering an opportunity to use theoretical modeling

coupled with synthetic flexibility to design and produce novel materials with

low cost (Chemla et al 1987, Ledoux et al 1987 and Prasad et al 1991). The

relevance of organic materials in this context is because the delocalized

electronic structure of –conjugated organic compound offers a number of

tantalizing opportunities in applications as NLO materials. For future

applications in electronics and optoelectronics the researchers have prompted

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to look for newer promising materials. The search for new materials with high

optical nonlinearities is an important area due to their practical applications.

With the rapid advancement of the microelectronic and the optoelectronic

industry in the country, the demand for crystals has increased dramatically

during the past two decades. The requirement for better and well

characterized single crystals has been the driving force behind the extensive

research and development in crystal growth. For the commercial use and for

the technological applications large size good quality transparent single

crystals are needed. To meet with the increased need of crystals in various

fields, scientists have started growing crystals for different techniques.

1.2 TYPES OF CRYSTAL GROWTH

The method of growing crystals varies widely; it is mainly dictated

by the characteristics of the material and its size. Crystal growth techniques

are generally classified in to three categories; they are growth from solution,

growth from vapor and growth from melt. Each growth techniques has

numerous variations, all materials cannot be grown by all the above three

methods.

1.3 GROWTH FROM SOLUTION

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

solubility and have variation in solubility with temperature. The mechanism

of crystallization from solutions governed, in addition to other factors, by

the interaction of the ions or molecules of the solute and the solvent which is

based on the solubility of substance on the thermodynamical parameters of

the process; temperature, pressure and solvent concentration (Chernov 1984).

The techniques such as slow cooling and slow evaporation are mainly used in

solution growth technique.

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1.3.1 Slow Cooling Technique

This is one of the best techniques to grow bulk single crystals in the

solution growth. In this technique, supersaturation is produced by a change in

temperature usually throughout the crystallizer. The crystallization process

involves the temperature and the concentration of the solution which moves in

to the metastable region along with the saturation curve in the section of

lower solubility. Since the volume of the crystallizer is finite and the amount

of substance placed in it is limited, the supersaturation requires systematic

cooling. It is achieved by using a temperature controlled water bath to control

the temperature with 0.01oC. The volume of the crystallizer is selected

based on the desired size of the crystals and the temperature dependence of

the solubility of the substance. The main disadvantage of slow cooling

technique is only the particular range of temperature is used. The possible

range of temperature is usually small and much of the solute remains in the

solution at the end of the growth period.

1.3.2 Slow Evaporation Technique

In this technique, an excess of a given solute is established by

utilizing the difference between rates of evaporation of the solvent and the

solute. A solution of the compound in a suitable solvent is prepared. The

water is mainly used as a natural solvent. If the compound is not dissolved in

water then organic solvents such as acetone, ethanol, methanol etc are used.

Unlike the cooling method, in which the total mass of the system remains

constant, the solvent evaporation technique, the solution loses particles, which

are weakly bound to other components and therefore the volume of the

solution decreases. In almost all cases, the vapor pressure of the solvent above

the solution is higher than the vapor pressure of the solute and therefore the

solvent evaporates more rapidly and the solution becomes supersaturated

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(Petrov 1969). Usually, it is sufficient to allow the vapor formed above the

solution to escape freely into the atmosphere. This is the oldest technique of

crystal growth and technically, it is very simple.

1.4 GROWTH FROM VAPOR

The growth of single crystal materials from the vapor 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 a lower growth rate than achieved from stoichiometric

melts. Crystal growth from a vapor by direct condensation, without the

intervention of the liquid phase, can be used to produce small strain free

single crystals of substances that sublime readily. Large single crystals cannot

be grown by this method. Sublimation in sealed tubes is also used for the

preparation of single crystals of metals, such as Zinc and cadmium, and

non-metallic sulphides. A quantity of the material is placed at one end of the

tube, along which a temperature gradient is maintained, so that sublimation

occurs at the hot end and crystallization at the other. An electric furnace with

a number of independently controlled windings is used to maintain the

temperature gradient to give a rate of sublimation sufficiently slow for the

grown crystals to be single and not polycrystalline. The growth of crystal

from vapor phase can be divided into two categories; they are physical vapor

deposition (PVD) and chemical vapor deposition (CVD).

1.4.1 Physical Vapor Deposition

In PVD, there is a transfer of subliming molecules/atoms by

(saturated) carrier gas to a colder growth zone. It is a process of growth

species from a source or target and deposits them on a substrate to form a

film. The process proceeds atomistically and mostly involves no chemical

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reactions. Various methods have been developed for the removal of growth

species from the source or target. The thickness of the deposits can vary from

angstroms to millimeters. In general, those methods can be divided into two

groups: evaporation and sputtering. In evaporation, the growth species are

removed from the source by thermal means. In sputtering, atoms or molecules

are dislodged from solid target through impact of gaseous ions. Each group

can be further divided into a number of methods, depending on the specific

techniques applied to activate the source or target atoms or molecules and the

deposition conditions applied.

1.4.2 Chemical Vapor Deposition

In chemical vapor deposition gaseous compounds of the materials

to be deposited are transported to a substrate surface, where a thermal

reaction/deposition occurs. Reaction byproducts are then exhausted out of the

system. CVD is the process of chemically reacting a volatile compound of a

material to be deposited, with other gases, to produce a nonvolatile solid that

deposits atomisitically on a suitable placed substrate. Basically, chemical

vapor deposition involves exposure of the substrate to be coated to one or

several vaporized compounds or reagent gases; some or all of which contain

constituents of the material to be deposited. A chemical reaction is then

initiated, preferably near or on the substrate surface producing the desired

material as a solid phase reaction product, which condenses on the substrate.

1.5 GROWTH FROM MELT

The growth processes of crystals from the melt have gained much

interest, which has resulted in considerable success in the solution of some

problems. The melt growth techniques such as Czochralski technique and

Bridgman techniques are used to grow technological important single crystals.

The rate of crystallization from a melt depends on the rate of heat transfer

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from the crystal face to the bulk of the liquid. The process is generally

accompanied by the liberation of heat of crystallization; the surface of the

crystal will have a slightly higher temperature than the supercooled melt

(Mullin 1993). The driving force for the great evolution of melt growth

techniques in the past years has been unquestionably, the demand of physics

and electronics for single crystals of all possible materials.

1.5.1 Czochralski Technique

In melt growth, the Czochralski technique is used to grow single

crystals of silicon, semiconductors, metals, oxides and halides. The charge

material is contained in a crucible which is heated to above the melting point

of the charge. A pull rod with a chuck containing a seed crystal at its lower

end is positioned above the crucible. The seed crystal is dipped into the melt

and the melt temperature is adjusted until a meniscus can be supported by the

seed crystal. The pull rod is then slowly rotated and lifted and by carefully

adjusting the power supplied to the melt a crystal of the desired diameter can

be grown. The whole assembly is maintained in an envelope which permits

control of the ambient gas and enables the crystal to be observed visually.

This technique has been applied to an extremely wide range of materials. The

Czochralski process lends itself to produce crystals with length to diameter

ratios which greatly exceed unity and the diameter of the crystals rarely

exceed half the crucible diameter. Attempts to grow crystals with larger

diameter almost fail; it is extremely difficult to hold constant diameter

crystals (Brice 1986).

1.5.2 Bridgman Technique

This growth technique was developed by Bridgman in 1925.

Bridgman technique is the simplest technique for growth of crystal from

melts. The range of materials grown by this technique is very large. The

reasons for this popularity are that the technique produce crystals with good

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dimensional tolerance quickly, fast growth and employ relatively simple

technology. The materials which melt congruently do not decompose before

melting and those do not undergo phase transformation between the melting

point and room temperature can be grown as single crystal by Bridgman

technique. The material to be grown is encapsulated in glass or quartz tube

and suspended in the furnace having suitable gradient for growth. The tip of

the ampoule is mainly conical shaped to enhance nucleation of a single

crystal. After melting of the substance, growth ampoule is moved from hot

zone to cold zone gradually. The lower part of the ampoule has capillary tip in

which the melt is filled and during lowering in the gradient, seed initiates and

gradually grows up the entire melt region of the ampoule. The rate of

lowering of the ampoule, required to produce a crystal of reasonable quality

varies from material to material. It depends largely on materials molecular

and crystalline complexity.

The organic materials which have spherical molecules in cubic

lattice can be quite rapidly grown. The average growth rate to start with for

most materials is 1 mm/h. The organic crystals have grown with low

translation rate because of its low thermal conductivity and the slow growth

rate. Low temperature gradients are used to grow the organic single crystals;

the temperature gradients can be produced by providing graded winding of

the furnace. The temperature gradient of solid-liquid interface in the

Bridgman furnace is usually lower than that in Czochralski growth; therefore

the cracking can be easily avoided during Bridgman growth. Bridgman

technique has been found to be an attractive technique for the growth of large

size good quality transparent single crystals in organic and inorganic

materials. In the present study, organic single crystals are successfully grown

by using the single zone transparent modified vertical Bridgman technique.

The single zone transparent modified vertical Bridgman setup is designed and

the temperature profile is optimized. The construction is briefly discussed in

the chapter 2.

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1.5.2.1 Ampoule Shape

The vertical Bridgman technique ampoule design is a one of the

important parameter to grow the good quality organic single crystals. The

growth of organic crystals is difficult, compared to the growth of inorganic

crystals, mainly, due to their low thermal conductivity and great supercooling

tendencies (Reynolds et al 1963). Tamman used ampoule with long capillary

and experiments were conducted with the growth ampoule shown in

Figure 1.1(a). But it was not successful in giving the single crystal

(Sherwood et al 1960). The modified design (Figure 1.1(b)) had a bulb on the

capillary and produced about six single crystals in the ampoule. While

investigating the growth of stilbene crystals, Scott Hutchinson and Lappage

used bent capillary as shown in Figure 1.1(c) and single crystals were

produced. With the same type of growth ampoule, anthracene single crystals

grown in the upper part of the ampoule formed inverted wedges and the

wedge was surrounded by polycrystals. These might be due to seeding in the

lower corners of the ampoules either because of temperature fluctuations or

because of outward growth of the single crystals being slow. Design shown in

Figure 1.1(d) was introduced to remove the shoulders of the growth ampoule

but once again polycrystals were formed. To reduce their interference with the

single crystal, a baffle was used in the design shown in Figure 1.1(e). It

resulted in larger single crystals in the base region with polycrystals initiation

at the top of the baffle, perhaps because of local cooling.

In the final successful design (Figure 1.1(f)), ampoule was

constructed with an inner growth tube. The anthracene in the annular space

between the inner and outer ampoules acted as a thermal insulator. The bent

capillary aided single crystal initiation and the conical shape obviated

“corner” effects as the single crystal grew into the wider tube. The orientation

of crystal can be changed by varying the angle of capillary with the vertical

axis (Sherwood et al 1960).

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Figure 1.1 Ampoule designs used by different authors (Sherwood 1960)

Brice has given some models of ampoules used for the growth of

crystal (Figure 1.2). Initially the researchers using the vertical Bridgman

technique relied on random nucleation to produce a single crystal, which then

propagated to form just one crystal including whole frozen charge. The

formation of one nucleus is more probable if the supercooled volume is small.

Thus traditionally crucibles have tapered tips. To ensure that only one crystal

propagates, various shapes are used to grow crystals.

Figure 1.2 Ampoule design for single crystal growth (Brice 1986)

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The growth of large size organic crystal is still a challenge and the

quality is not uniform throughout the grown crystal. The non-uniformity in

the quality of the grown crystal highly depends on the method and material

used and this poses severe problems in the crystal growth. The double walled

ampoule growth was tried in many reports earlier; the effect of the length of

capillary used for seeding has not been studied much. The long capillary

ampoule is accompanied by thermal insulation and with necking seems to

enhance the quality of crystals grown and gives maximum possibility of

obtaining the single crystal in a given run (Arulchakkaravarthi et al 2002).

The vacuum in the inner annular space and the outer tube gives the thermal

insulation. This vacuum helps in avoiding any thermal shocks and diffuses

any thermal spikes before it reaches the inner tube. Figure 1.3 shows the

double wall ampoule designed by Arulchakkaravarthi (2002).

Figure 1.3 Double wall ampoule designed for single crystal growth

(Arulchakkaravarthi 2002)

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In the present study, the organic single crystals are grown by using

single and double wall ampoules. The various types of single and double wall

ampoules are designed with different cone length and optimized the cone

angle to grow the crystals. The ampoule designs are discussed in the chapter 2

to chapter 6.

1.6 REVIEW OF MELT GROWN ORGANIC CRYSTALS

The organic structured molecular crystals can be grown as single

crystals more easily by melt compared to other techniques. The low solubility

with probability of solvent inclusion and slow growth rate rejects use of

solution growth techniques. Vapor growth technique yields crystals of small

size with large time consumption. In the melt technique such as Bridgman

technique and Czochralski technique are suitable to grow large size crystals.

The Czochralski technique is mainly used to grow inorganic crystals. The

organic crystal growth vertical Bridgman technique is mainly used because

the temperature gradient of solid-liquid interface is usually lower than that in

Czochralski growth; therefore the cracking is easier to be avoided during

Bridgman growth. Generally the organic materials have low melting point and

the glass furnace can be used to grow the crystals. The advantage of using this

furnace is that the solidification can be directly observed. If multinucleation

occur means solidification may be restarted. For low thermal conductivity,

soft nature and slow growth rate of organic materials the low temperature

gradient furnace can be used. Many applications like scintillator,

piezoelectric, semiconductor, dielectric, NLO single crystals can be

successfully grown by vertical Bridgman technique. For growing good quality

crystal every growth technique have some important parameter to be change.

In the Bridgman technique, important parameter to be change is the

temperature gradient of the furnace, the ampoule size and shape and the

translation rate.

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The birth of single nuclei for subsequent growth of single crystal

(seed formation) is very difficult in the case of organic molecular single

crystals. Many authors stress that the growth of single crystal by vertical

Bridgman technique depends on the crystal growth ampoule design

(Lipsett 1957, Sherwood et al 1960, Marciniak et al 1981). The organic

material anthracene was grown by (Sherwood et al 1960, Marciniak et al 1981

and Arulchakkaravarthi et al 2002 a) using various types of ampoules and

optimized the ampoule for the growth of anthracene single crystals. The

organic material naphthalene was grown by vertical Bridgman technique

(Selvakumar et al 2005 and Balamurugan et al 2006). The organic materials

good crystal can be grown by using the temperature gradient of the furnace to

change gradually between the upper and lower ends. In the organic crystals

if , the angle of inclination of the capillary to the vertical, was less than

45o, then the crystals grown in the ampoule had the 001 cleavage plane

vertical. If is greater than or equal to 45o, re-orientation occurred and

the cleavage planes of the crystal were horizontal (Sherwood et al (1960).

Due to the low thermal conductivity of organic substances, good

single crystals can only be obtained when growth rates are relatively low.

Growth rates of such substances do not usually exceed 1 mm/h, while growth

rates for metals and inorganic substances may reach 20 mm/h (Jones 1974).

Many organic crystals are grown by slow growth rates such as the growth rate

for anthracene single crystals doped with 7,7’,8,8’-tetracyano-p-

quinodimethane is 0.2 mm/h (Marciniak et al 1981), meta-nitroaniline is 0.4

mm/h (Singh et al 1989), 3-methoxy-4-methoxy-4'-nitrostilbene is 0.5 mm/h

(Pan et al 1995), 2-dicyanovinylanisole is 0.1-0.3 mm/h (Aggarwal et al

1996), 2, 3-dimethylnaphthalene is 0.4 mm/h (Tachibana et al 1999),

benzimidazole is 1 mm/h (Vijayan et al 2005), benzophenone is 1 mm/h

(Arivanandhan et al 2005) etc.

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The double zone furnace with each zone have 30 cm long was used

to grow the organic material 2,5-diphenyloxazole-doped naphthalene crystal

by Bridgman technique. In this study small size crystals are obtained

(Balamurgan et al 2008). The melt growth of a bulk single crystal is carried

out under such thermal conditions; large thermal stress is induced in a crystal

during the growth process. Such thermal stress causes the generation and

multiplication of dislocations that affect the performance of electronic or

optical devices (Miyazaki 2007). In ampoule, as the cone length increases the

crystal quality increases and many researchers suggests the ampoule of the

prescribed cone angle was found to be the most optimum to obtain inclusion-

free good quality transparent crystals (Mohan et al 2000 and Balamurugan et

al 2008). The single and double wall ampoules with cone angle were used to

grow good quality transparent organic crystals. Multiple twinning,

multinucleation, impurity effects on nucleation were all sorted out by the

above-proposed ampoules.

The development of organic materials for the use in nonlinear

optical (NLO) devices is of interest because their optical nonlinearities

are in orders of magnitude larger than those of conventional inorganic

materials such as lithium niobate (LiNbO3) and potassium dihydrogen

phosphate (KDP). Moreover, organic materials offer flexibility of

molecular design, virtually an unlimited number of crystalline structures,

purification by conventional methods, and a high damage resistance to

optical radiation. Consequently, these materials might make it desirable to

replace electronic switching circuits in computing and telecommunication

systems with purely optical devices. In optical information processing, this

offers the potential for extremely high throughput, compact information

processing systems. Moreover, since optical beams are less affected by

electromagnetic interference than their electric signal counterpart, optical

technology can be used advantageously in those circumstances where a

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high degree of connectivity or interconnectivity is required. An example

of such application is the interconnection of different parts of computing

systems.

In recent years many organic NLO crystals are grown by vertical

Bridgman technique. They are Methyl 2-(2,4-dinitrophenyl)aminopropanoate

(Perigaud et al 1986), meta-nitroaniline and para-nitroaniline (Henningsen

et al 1994), 8-( 4’-acetylphenyl) -1 ,4-dioxa-8-azaspiro[4.5]decane (Kagawa

et al 1994), 3-methoxy-4-methoxy-4'-nitrostilbene (Pan et al 1995),

2-dicyanovinylanisole (Aggarwal et al 1996), meta-Dinitrobenzene

(Stanculescu et al 1999), benzil (Rai et al 2001), acenaphthene (Ramesh babu

et al 2005), benzimidazole (Vijayan et al 2006), pentachloropyridine (Rai et

al 2005), 4-hydroxy-3-methoxybenzaldehyde (Kannan et al 2006),

4-aminobenzophenone (Bhowmik et al 2007) etc. It is well established that

high quality crystals can also be grown from Bridgman technique as

Bridgman grown crystals do not exhibit facets and impurity striations and

possess better quality (Zhou et al 1993 and Dutta et al 1997). For

technological applications need large size organic crystals, in the present

study organic single crystals grown by modified vertical Bridgman technique

using single and double wall ampoules with nano translation.

1.7 CHARACTERIZATION TECHNIQUES

1.7.1 Single Crystal X-ray Diffraction

Single crystal X-ray diffraction analysis is a powerful technique for

the determination of crystal structures. It is considered today as a routine

technique for the structural characterization in organic, inorganic and

organometallic research. The molecular structure, atomic coordinates, bond

lengths, bond angles, molecular orientation and packing of molecules in single

crystals can be determined by X-ray crystallography. Single crystal

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X-ray diffractometer collects intensity data required for structure determination.

The monochromatic X-rays incident on a plane of single crystal at an angle

theta are diffracted according to Bragg's relation 2d sin( ) = n where d is the

interplanar spacing of the incident plane, is the wavelength of X-rays and n

is a positive integer. The intensity of the diffracted rays depends on the

arrangement and nature of atoms in the crystal. Collection of intensities of a

full set of planes in the crystal contains the complete structural information

about the molecule. Fourier transformation techniques are used to determine

the exact coordinates of atoms in the unit cell from this data.

Enraf Nonius CAD4-MV31 single crystal X-ray diffractometer is a

fully automated four circle instrument controlled by a computer. It consists of

an FR 590 generator, a goniometer, CAD4F interface and a microVAX3100

equipped with a printer and plotter. The detector is a scintillation counter. A

single crystal is mounted on a thin glass fiber fixed on the goniometer head.

The unit cell dimensions and orientation matrix are determined using 25

reflections and then the intensity data of a given set of reflections are

collected automatically by the computer. An IBM compatible PC/AT 486 is

attached to microVAX facilitating the data transfer on to a DOS floppy of

5.25" or 3.5". Mo and Cu targets are available. Maximum X-ray power is 40

mA x 50 KV. Polaroid camera is available. UPS backed power supply takes

care of the instrument during power failure for short periods. The ideal

dimensions of the single crystal required are approximately 0.3 x 0.3 x 0.3 mm3.

However, these dimensions may vary based on the habit of crystal growth and

the diffracted beam intensity. Single crystal XRD is used to identify the lattice

parameters, crystal structure and the space groups. The single crystal XRD

analysis was carried out at using Enraf Nonius CAD4 single crystal X-ray

diffractometer.

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1.7.2 Powder X-ray Diffraction

Max von Laue, in 1912, discovered that crystalline substances act

as three-dimensional diffraction gratings for X-ray wavelengths similar to the

spacing of planes in a crystal lattice. X-ray diffraction is now a common

technique for the study of crystal structures and atomic spacing. X-ray

diffraction is based on constructive interference of monochromatic X-rays and

a crystalline sample. These X-rays are generated by a cathode ray tube,

filtered to produce monochromatic radiation, collimated to concentrate, and

directed toward the sample. The interaction of the incident rays with the

sample produces constructive interference (and a diffracted ray) when

conditions satisfy Bragg's Law (n =2d sin ). This law relates the wavelength

of electromagnetic radiation to the diffraction angle and the lattice spacing in

a crystalline sample. These diffracted X-rays are then detected, processed and

counted. By scanning the sample through a range of 2 angles, all possible

diffraction directions of the lattice should be attained due to the random

orientation of the powdered material. Conversion of the diffraction peaks to d-

spacings allows identification of the mineral because each mineral has a set of

unique d-spacings. Typically, this is achieved by comparison of d-spacings

with standard reference patterns.

All diffraction methods are based on generation of X-rays in an

X-ray tube. These X-rays are directed at the sample, and the diffracted rays

are collected. A key component of all diffraction is the angle between the

incident and diffracted rays. Powder and single crystal diffraction vary in

instrumentation beyond this. X-rays are generated in a cathode ray tube by

heating a filament to produce electrons, accelerating the electrons toward a

target by applying a voltage, and bombarding the target material with

electrons. When electrons have sufficient energy to dislodge inner shell

electrons of the target material, characteristic X-ray spectra are produced.

These spectra consist of several components, the most common being K and

K . K consists, in part, of K 1 and K 2. K 1 has a slightly shorter wavelength

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and twice the intensity as K 2. The specific wavelengths are characteristic of

the target material (Cu, Fe, Mo, Cr and Co). Filtering, by foils or crystal

monochrometers, is required to produce monochromatic X-rays needed for

diffraction. K 1 and K 2 are sufficiently close in wavelength such that a

weighted average of the two is used. Copper is the most common target

material for single-crystal diffraction, with CuK radiation = 1.54056 Å.

These X-rays are collimated and directed onto the sample. As the sample and

detector are rotated, the intensity of the reflected X-rays is recorded. When

the geometry of the incident X-rays impinging the sample satisfies the Bragg

Equation, constructive interference occurs and a peak in intensity occurs. A

detector records and processes this X-ray signal and converts the signal to a

count rate which is then output to a device such as computer monitor. For

typical powder patterns, data is collected at 2 from 10° to 70°, angles that are

preset in the X-ray scan. The powder XRD analysis was carried out at using

XPERT-PRO analytical X-ray powder diffractometer.

1.7.3 Fourier Transform Infrared Spectroscopy

Fourier Transform-Infrared (FTIR) Spectroscopy is an analytical

technique used to identify organic and inorganic materials. This technique

measures the absorption of infrared radiation by the sample material versus

wavelength. The infrared absorption bands identify molecular components

and structures. When a material is irradiated with infrared radiation, absorbed

IR radiation usually excites molecules into a higher vibrational state. The

wavelength of light absorbed by a particular molecule is a function of the

energy difference between the at-rest and excited vibrational states. The

wavelengths that are absorbed by the sample are characteristic of its

molecular structure. The FTIR spectrometer uses an interferometer to

modulate the wavelength from a broadband infrared source. A detector

measures the intensity of transmitted or reflected light as a function of its

wavelength. The signal obtained from the detector is an interferogram, which

must be analyzed with a computer using Fourier transforms to obtain a single-

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beam infrared spectrum. The FTIR spectra are usually presented as plots of

intensity versus wavenumber (in cm-1). Wavenumber is the reciprocal of the

wavelength. The intensity can be plotted as the percentage of light

transmittance or absorbance at each wavenumber. To identify the material

being analyzed, the unknown IR absorption spectrum is compared with

standard spectra in computer databases or with a spectrum obtained from a

known material. Spectrum matches identify the material or other constituent(s)

in the sample. Absorption bands in the range of 4000 - 400 cm-1 are used to

identify the functional groups present in crystal. The FTIR analysis was carried

out at using Perkin-Elmer FTIR Spectrum RXI Spectrometer.

1.7.4 High Resolution X-ray Diffraction

Description of the Multicrystal X-ray Diffractometer designed,

developed and fabricated at National Physical Laboratory (NPL), New Delhi.

Figure 1.4 Schematic line diagram of Multi crystal X-ray

diffractometer designed, developed and fabricated at NPL

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A multicrystal crystal X-ray diffractometer designed and developed

at National Physical Laboratory (Lal and Bhagavannarayana 1989) has been

used to study the crystalline perfection of the single crystal(s). Figure 1.4

shows the schematic diagram of the multicrystal X-ray diffractometer. The

divergence of the X-ray beam emerging from a fine focus X-ray tube (Philips

X-ray Generator; 0.4 mm x 8 mm; 2kWMo) is first reduced by a long

collimator fitted with a pair of fine slit assemblies. This collimated beam is

diffracted twice by two Bonse-Hart (Bonse et al 1965) type of

monochromator crystals and the thus diffracted beam contains well resolved

MoK 1 and MoK 2 components. The MoK 1 beam is isolated with the help

of fine slit arrangement and allowed to further diffract from a third (111) Si

monochromator crystal set in dispersive geometry (+, -, -). Due to dispersive

configuration, though the lattice constant of the monochromator crystal and

the specimen are different, the dispersion broadening in the diffraction curve

of the specimen does not arise. Such an arrangement disperses the divergent

part of the MoK 1 beam away from the Bragg diffraction peak and there by

gives a good collimated and monochromatic MoK 1 beam at the Bragg

diffraction angle, which is used as incident or exploring beam for the

specimen crystal. The dispersion phenomenon is well described by comparing

the diffraction curves recorded in dispersive (+,-,-) and non-dispersive (+,-,+)

configurations. This arrangement improves the spectral purity ( / 10-5)

of the MoK 1 beam. The divergence of the exploring beam in the horizontal

plane (plane of diffraction) was estimated to be 3 arc sec. The specimen

occupies the fourth crystal stage in symmetrical Bragg geometry for

diffraction in (+, -, -, +) configuration. The specimen can be rotated about a

vertical axis, which is perpendicular to the plane of diffraction, with minimum

angular interval of 0.4 arc sec. The diffracted intensity is measured by

scintillation counter. To provide two-theta (2 B) angular rotation to the

detector (scintillation counter) corresponding to the Bragg diffraction angle

21

( B), it is coupled to the radial arm of the goniometer of the specimen stage.

The rocking or diffraction curves were recorded by changing the glancing

angle (angle between the incident X-ray beam and the surface of the

specimen) around the Bragg diffraction peak position B (taken as zero for the

sake of convenience) starting from a suitable arbitrary glancing angle. The

detector was kept at the same angular position 2 B with wide opening for its

slit, the so-called scan. The High resolution X-ray diffraction (HRXRD)

analysis was carried out using Multicrystal X-ray diffractometer.

1.7.5 UV-vis-NIR Studies

The UV-visible-NIR spectrophotometer is used to determine the

absorption or transmission of light by a sample. A double beam UV-visible-

NIR spectrophotometer consists of deuterium lamp which gives continuous

radiation and a tungsten halogen lamp produces a continuous spectrum of

light, from near ultraviolet to deep into the infrared. The instrument has an

arrangement for switching on either radiation source as required. A narrow

bandwidth was selected with the help of band pass filters. The two rays were

combined and differences in intensities of those two radiations are measured

electrically. In the transmission study the intensity was recorded in terms of

percentage of transmittance (%) along Y-axis and the wavelength (nm) along

X-axis. In this investigation, Perkin-Elmer Lambda 35 UV-vis-NIR

spectrophotometer was used to study the optical property of the grown

crystals in the wavelength range of 190 to 1100 nm at room temperature. The

UV-vis-NIR analysis was carried out at using Perkin-Elmer Lambda 35

UV-vis-NIR spectrophotometer.

22

1.7.6 Photoluminescence

Photoluminescence (PL) analysis is widely used in science and

industry to evaluate the properties of a material. This technique light is

directed onto a sample, where it is absorbed and where a process called photo-

excitation can occur. The photo-excitation causes the material to jump to a

higher electronic state, and will then release energy (photons) as it relaxes and

returns to back to a lower energy level. The emission of light or luminescence

through this process is photoluminescence. The excitation energy and

intensity are chosen to probe different regions and excitation concentrations in

the sample. The period between absorption and emission is typically

extremely short, in the order of 10 nanoseconds. Under special circumstances,

however, this period can be extended into minutes or hours. The emission

spectrum shows the luminescence property of the material. The intensity of

the photoluminescence signal provides important information on the purity

and crystalline quality of the materials. The photoluminescence studies were

carried out at using Spectro urometer with 450W high pressure Xenon lamp

as excitation source.

1.7.7 Thermogravimetric Analysis

Thermogravimetric analysis (TGA) provides testing for a wide

range of organic and inorganic materials. The TGA uses heat to force

reactions and physical changes in materials. TGA provides quantitative

measurement of mass change in materials associated with transition and

thermal degradation. Place the test material in the specimen holder and raise

the furnace. Set the initial weight reading to 100%, and then initiate the

heating program. TGA records change in mass from dehydration,

decomposition, and oxidation of a sample with time and temperature in the

inert nitrogen atmosphere at a uniform heating rate of 10oC min-1.

Characteristic thermogravimetric curves are given for specific materials and

23

chemical compounds due to unique sequence from physicochemical reactions

occurring over specific temperature ranges and heating rates. These unique

characteristics are related to the molecular structure of the sample. The

organic materials have mainly low melting point and decomposition points so

used the temperature range from 30°C to 600°C. Weight change occur

sensitivity of 0.01 mg. The obtained results plot with percent weight loss

versus temperature. The thermogravimetric analysis was carried out at using

SDT Q600 simultaneous thermal analyzer.

1.7.8 Differential Thermal Analysis

Differential Thermal Analysis (DTA) is a very popular thermal

analysis technique it measures endothermic and exothermic transitions as a

function of temperature. The DTA is used to characterize organic and

inorganic materials. And this technique, recording the temperature and heat

flow associated with thermal transitions in a material such as melting,

crystallization, sublimation and the decomposition. DTA detects the release or

absorption of heat, which is associated with chemical and physical changes in

materials as they are heated or cooled. Such information is essential for

understanding thermal properties of materials. In DTA the observed

endothermic or exothermic peak depends on the sample weight and the

heating rate. The sharpness of the peaks indicates improved resolution.

Lowering the heating rate is roughly equivalent to reducing the sample

weight. The organic materials have mainly low melting and decomposition

points so used the temperature range from 30°C to 600°C in inert nitrogen

atmosphere at a uniform heating rate of 10oC min-1. The obtained results plot

with percent heat flow versus temperature. The differential thermal analysis

was carried out at using SDT Q600 simultaneous thermal analyzer.

24

1.7.9 Dielectric Studies

The term “dielectric” was coined by William Whewell (from “dia-

electric”) in response to a request from Michael Faraday (Daintith 1994). The

dielectric study was carried out using the parallel plate capacitor method. The

study of dielectric properties is concerned with the storage and dissipation of

electric and magnetic energy in materials. It is important to explain various

phenomena in electronics, optics, and solid-state physics. A dielectric is an

electrical insulator that may be polarized by an applied electric field. When a

dielectric is placed in an electric field, electric charges do not flow through

the material, as in a conductor, but only slightly shift from their average

equilibrium positions causing dielectric polarization. Because of dielectric

polarization, positive charges are displaced toward the field and negative

charges shift in the opposite direction. This creates an internal electric field

that partly compensates the external field inside the dielectric. If a dielectric

is composed of weakly bonded molecules, those molecules not only become

polarized, but also reorient so that their symmetry axis aligns to the field.

The term “insulator” refers to a low degree of electrical conduction,

the term “dielectric” is typically used to describe materials with a high

polarizability. The latter is expressed by a number called the dielectric

constant. A common, yet notable example of a dielectric is the electrically

insulating material between the metallic plates of a capacitor. The polarization

of the dielectric by the applied electric field increases the capacitor's surface

charge. Dielectric material is popularly used in capacitors to reduce the size of

the capacitor. The dielectric materials are also used in various other electrical

and electronic components. In both the vertical and horizontal dimensions the

reduction in spacing of metal interconnects has created the need for low

dielectric constant materials that serve as interlevel dielectrics to offset the

increase in signal propagation time between transistors, known as RC delay

25

(R is metal wire resistance and C is interlevel dielectric capacitance).

To fulfill these requirements at 32 nm and lesser IC fabrication nodes,

innovation in dielectric materials is must if the device density of ICs has to

continue at Moore's Law rate. Low dielectric constant materials are used in

multi level interconnects, interlayer dielectrics, and for microelectronic

industry. The dielectric studies were carried out at using Agilent 4284A LCR

meter.

1.7.10 Microhardness Studies

The Vickers hardness test method consists of indenting the test

material with a diamond indenter, in the form of a right pyramid with a square

base and an angle of 136o between opposite faces subjected to a load of 1 to

100 kilograms-force (kgf). The full load is normally applied for 10 to 15

seconds. The two diagonals of the indentation left in the surface of the

material after removal of the load are measured using a microscope and their

average calculated. The area of the sloping surface of the indentation is

calculated. The Vickers hardness is the quotient obtained by dividing the kgf

load by the square mm area of indentation. The advantages of the Vickers

hardness test are that extremely accurate readings can be taken, and just one

type of indenter is used for all types of materials and surface treatments. The

Vickers hardness number (Hv) is calculated using the relation

Hv = 1.8544 × (p/d2)

Where p is the applied load and d is the average diagonal length of

the indentation. The microhardness study is used to find the nature of the

material either soft or hard. The microhardness measurements were carried

out at using Leitz Wetzlar hardness tester.

26

1.7.11 Second Harmonic Generation Studies

Second harmonic generation (SHG) also called frequency doubling

is a nonlinear optical process, in which photons interacting with a nonlinear

material are effectively “combined” to form new photons with twice the

energy, and therefore twice the frequency and half the wavelength of the initial

photons. It is a special case of sum frequency generation. Second harmonic

generation was first demonstrated by P. A. Franken, A. E. Hill, C. W. Peters, and

G. Weinreich at the University of Michigan, Ann Arbor, in 1961. The

demonstration was made possible by the invention of the laser, which created the

required high intensity monochromatic light. They focused a ruby laser with a

wavelength of 694 nm into a quartz sample. They sent the output light through a

spectrometer, recording the spectrum on photographic paper, which indicated the

production of light at 347 nm (Franken et al 1961). The ability of non-

centrosymmetric crystals to produce SHG has led to their implementation as

frequency doublers in a wide variety of laser systems.

Nowadays the active tool of measuring the nonlinear optical property

of the crystal is Kurtz and Perry powder SHG test. The powder SHG

measurement was done using Nd:YAG laser fundamental ( =1064 nm)

radiation. To measure the SHG ef ciency of the grown crystal, the powder

sample was packed in a triangular cell and was kept in a cell holder, 1064 nm

laser from Nd:YAG irradiates the sample. The monochromator was set at

532 nm. NLO signal was captured by the oscilloscope through the

photomultiplier tube. The Nd:YAG laser source produces nanosecond pulses (8 ns)

of 1064 nm light and the energy of the laser pulse was around 30 mJ. The beam

emerging through the sample was focused on to a Czerny-Turner monochromator

using a pair of lenses. The detection was carried out using a Hamamatsu R-928

photomultiplier tube. The signals were captured with an Agilent in nium digital

storage oscilloscope interfaced to a computer. The emission of green light from

the sample confirms the second harmonic generation in the crystal. The

potassium dihydrogen phosphate (KDP) sample was used as the reference

material in the SHG measurements.

27

1.8 SCOPE OF THE THESIS

The present thesis is aimed at the growth and characterization of

organic single crystals by the modified vertical Bridgman technique using the

single and double wall ampoules with nano translation. The present study

designed the single zone transparent modified vertical Bridgman setup and the

temperature profile is optimized. The nano stepper motor drive is used for the

translation purpose. For the potential scientific interest on the organic single

crystals and the increasing demand for applications large size transparent

good quality crystals are needed. The large size transparent organic crystals

are grown by different types of single and double wall ampoules with the

different cone length are designed and the suitable cone angle is optimized to

grow the crystals. The present study, different translation rates are used to

grow the crystals and optimized the translation rate for better crystal growth.

As the translation rate decreases the crystal quality increases.

The organic materials naphthalene, benzil, 4-nitrobenzalddehyde, 3-

hydroxybenzaldehyde (3HBA), 2-hydroxypyridine, 2-methylamino-5-

chlorobenzophenone (MACB) and 2-hydroxy-4-methoxybenzophenone single

crystals are successfully grown by modified vertical Bridgman technique. The

grown crystals were confirmed by single crystal X-ray diffraction (XRD) and

powder XRD. Fourier transform infrared (FTIR) analysis was used to identify

the functional groups present in the grown crystals. High resolution X-ray

diffraction (HRXRD) has been performed to analyze the crystalline perfection

of the grown single crystals. The optical property of the grown crystals was

analyzed by UV-vis-NIR and photoluminescence (PL) spectral measurements.

The thermal characteristics of the grown crystals were analyzed by

thermogravimetric (TG) and differential thermal analyses (DTA). The

dielectric measurements were carried out using the conventional parallel plate

capacitor method. The mechanical and nonlinear optical (NLO) properties of

the grown crystals were analyzed by microhardness and second harmonic

generation (SHG) studies. The results are discussed detail in the thesis.