1

CHAPTER 1

INTRODUCTION

1.1 PREAMBLE

In recent years nanotechnology has become one of the most important and exciting

forefront field in Physics, Chemistry, Engineering and Biology. Nanotechnology is

based on the fact that a particle less than the size of 100 nanometers impart novel

properties and behavior to the nanostructures. The dependence of the behavior on the

particle sizes can allow one to engineer their properties [Feynman, 1992]. Quantum dots

(QDs) are a special class of nanomaterials, composed of periodic groups of II-VI, III-V,

or IV-VI materials, ranging from 2-10 nanometers (10-50 atoms) in diameter. The

optical properties of quantum dots can be easily tuned with the alteration of particle

size. The band gap can be controlled with the change in size of the nanomaterials, thus

different colored emissions may be observed from the same material. Core-shell

quantum dots are highly functional materials with tailored properties that are quite

different than either of the core or of the shell material. In fact they exhibit modified and

improved properties than their single component counterparts or quantum dots of the

same size. ZnS has attracted a lot of attentions from research community due to its

versatile luminescent properties. By controlling the size of ZnS in nanometer range

through capping and/or doping the luminescent efficiency can be improved in

comparison to bulk ZnS. Doped core-shell ZnS quantum dots are extremely promising

candidates as luminescent probes and labels in biological applications as they are

nontoxic and also shows stable luminescence.

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1.2 NANOSTRUCTURED MATERIALS

The word structure in context of nanostructures refers to the chemical composition,

arrangement of the atoms and the size of a solid in one, two or three dimensions. The

various effects controlling the properties of nanostructured materials include size effects

(where critical length scales of physical phenomenon become comparable with the

characteristic size of the building blocks of the micro structure), changes of the

dimensionality of the structure, changes of the atomic structure and alloying of

components. The synthesis, processing and characterization of nanostructured materials

are part of an emerging and rapidly growing field. Nanostructured materials may be

categorized as nanoparticles, nanointermediates, and nanocomposites. They can also be

classified as quantum dots, quantum wells and quantum wires depending upon the

confinement direction.

The nanomaterials can be synthesized through top down approach and bottom-up

approach. Top down approach generally used the traditional workshop or micro

fabrication methods where externally-controlled tools are used to cut, mill and shape

materials into desired shape and order e.g. lithography and ball milling. These

techniques are costly, complex and have limitations to the nanoscale that can be

achieved. However bottom-up approach is to collect, consolidate and fashion individual

atoms and molecules into the structure. Bottom-up methods are able to produce devices

in parallel and much cheaper than top-down methods, with low cost, simple apparatus,

efficient reproducibility and high yield. We can synthesize nanoparticles of size ≈ 2 nm

by bottom-up approach. These techniques include: chemical precipitation, sol-gel,

solvo/hydrothermal, chemical vapor deposition, template synthesis etc.

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Now a day, the research in the field of nanomaterials is largely based on the

investigating fundamental properties of matter in nanoscale regime, however, the timely

attention and efforts are essential for the transformation of these new findings into

technological products so that the well assumed technology revolution will become the

day light reality. So far, a variety of chemical, biological as well as physical processes

are established for the preparation of different kinds of nanoparticles systems and the

ground breaking inventions such as scanning tunneling microscopy (STM), atomic force

microscopy (AFM), transmission electron microscopy (TEM) etc. made the

characterization and atomic scale manipulation of nanostructures materials a practical

reality. Various types of nanostructures include quantum dots, quantum wells, quantum

wires, nanoparticles, nanowires, nanofilms etc. depending upon shape and size. Further

they can be semiconductors, metals or insulators.

1.3 SEMICONDUCTOR QUANTUM DOTS

Quantum dots(QDs) are a special class of nanomaterials, because they are so small,

ranging from 2-10 nanometers (10-50 atoms) in diameter which is a crystal, composed

of periodic groups of II-VI, III-V, or IV-VI materials. Semiconductors derive their great

significance from the fact that their electrical conductivity can be greatly altered via an

external stimulus (voltage, photon flux, etc.), which makes semiconductors suitable for

the critical parts of different kinds of electrical circuits and optical applications. Their

small size results in novel electronic and optical properties attributed to quantum

confinement. The material in such a small size range behaves differently, giving

quantum dots unprecedented tunability and enabling never before seen applications to

science and technology [Sambasivam et al., 2008; Unni et al., 2009]. The optical

properties of quantum dots can be easily tuned with the alteration of particle size. The

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band gap can be controlled with the change in size of the nanomaterials, thus different

colored emission may be observed from the same material. Figure 1.1 shows the size

dependent properties of the CdSe quantum dots with the particle size getting smaller

from left to right.

Figure 1.1: Size dependence of CdSe quantum dots

The quantum dots of same material can be used for fabrication of LEDs having

emission over the whole visible spectrum. Semiconductors with widely tunable energy

band gap are considered to be the materials for next generation, for the production of

flat panel displays, photovoltaic, optoelectronic devices, laser, sensors, photonic band

gap devices etc. Quantum dots have some other promising potential applications also.

The more prolific research involves using quantum dots for biological imaging [Bruchez

et al., 1998; Chan and Nie, 1998; Luan et al., 2012; Sotelo-Gonzalez et al., 2012] and

computing applications [Li et al., 2003]. Biological labeling exploits the luminescent

properties of the quantum dots and requires attaching a functional group to the surface

of the QD. The functional group preferentially binds itself to a specific organism, cell or

protein, once the system is injected into a biological system. The QDs are then caused to

emit light through luminescence or fluorescence allowing the detection and tracing of

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the biological targets inside the body [Chan and Nie, 1998; Tran et al., 2002; Gao et al.,

2004]. The advantage of using quantum dots for this application is that QDs are brighter

and more resistant to photo bleaching as opposed to organic dyes, which is currently

used [Smith et al., 2004].

1.3.1 Quantum Confinement in Semiconductors

The electrons in the quantum dots have a range of energies. The excitons have an

average physical separation between the electron and hole known as the Exciton Bohr

radius, which is different for each material. The dimensions of the bulk semiconductor

crystal are much larger than the Exciton Bohr radius, allowing the exciton to extend to

its natural limit. However, when the size of a semiconductor crystal becomes small

enough that it approaches the size of the material's Exciton Bohr radius, the electron

energy levels can no longer be treated as continuous. They must be treated as discrete

i.e. there is a small and finite separation between energy levels. This property of discrete

energy levels is called quantum confinement, and under these conditions, the

semiconductor material ceases to resemble bulk and termed as quantum dot. This has a

major influence on the absorptive and emissive behavior of the semiconductor materials

[Bube, 1950; Poelman et al., 1997; Gilbert et al., 2006; Saravanan et al., 2012]. Figure

1.2 shows a schematic of the electron energy levels of a typical bulk semiconductor and

of a nanocrystal QDs in which Eg represents the energy bandgap, CB is the conduction

band and VB is the valence band. For example, in II-VI zinc blende crystals such as

ZnS, each Zn atom is surrounded tetrahedrally by four S atoms and vice versa. With

three p and one s atomic orbital on each cation and anion, we will obtain four sp3

hybridized atomic orbitals. When these atoms assemble in a cluster these orbitals should

be treated as a set of band, rather than atomic orbitals between nearest neighbour atoms.

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They so form a set of bonding orbitals σ and a set of antibonding orbitals σ*[Bawendi et

al., 1990].

Figure 1.2: Schematic representation of a typical bulk semiconductor and a

nanocrystal QD and their associated electron energy levels

As the number of atoms in the crystallite grows, each of the localized bond orbital sets

form molecular orbitals extended over the crystallite that finally develop into conduction

and valence bands. The highest occupied molecular orbital (HOMO) becomes the top of

the valence band and the lowest unoccupied molecular orbital (LUMO) becomes the

bottom of the conduction band. The HOMO-LUMO spacing tends to the band gap

energy of the bulk nanocrystals. In Figure 1.3, it can be seen that with decreasing

number of atoms in the particle the band gap increases and we obtain discrete energy

levels [Wang and Herron, 1987].

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Figure 1.3: Evolution from molecular orbitals into bands [Bawendi et al., 1990]

As quantum dots can be very useful for different applications because of quantum

confinement effects but there are certain problems related with the use of nascent

quantum dots. Most of the physical or chemical properties exhibited by these

nanoparticles are due to their crystallites. Further growth in their size is due to

agglomeration of these crystallites to form primary particles. If this growth of particles

is not controlled, they may agglomerate and settle down due to Ostwald ripening and

Vander-Waals interactions between particles [Yao et al., 1993; Dutta et al., 2004].

Secondly Due to the high surface-to-volume ratio of nanocrystals‚ the number of defects

on the surface increases. In order to control the growth and to passivate their surface

capping is needed. Also the structural‚ optical‚ and electronic features of semiconductor

quantum dots are heavily influenced by their surface properties so capping is required to

enhance their stability‚ processability‚ and optical properties. We can use different

organic and inorganic capping agents to passivate the free QDs.

For optoelectronic applications, band gap tuning is very important but for biomedical

applications, dispersion and surface passivation is very important to reduce

luminescence quenching. As in the present work, more inclination is towards

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biomedical applications, so the dispersibility and surface passivation was kept in mind

during the whole work. Recently to achieve fine dispersion and surface passivation, the

research on core-shell quantum dots comes in to the picture [Koneswaran and

Narayanaswamy, 2009]. However, very less work is going on the synthesis of core-shell

quantum dots especially for biomedical applications. As capping can be done by using a

polymer, organic material or inorganic material, still lot of scope is there in this research

area, which mostly deals with core-shell nanostructures.

1.4 CORE-SHELL QUANTUM DOTS

Core-shell quantum dots constitute a special class of nanocomposite materials, in which

a thin layer of one material is coated on to the particles of the other material using the

emerging technologies [Kalele et al., 2006]. Core-shell quantum dots are highly

functional materials with tailored properties that are quite different than either of the

core or of the shell material. In fact they exhibit modified and improved properties than

their single component counterparts or quantum dots of the same size. The core-shell

quantum dots are generally preferred over quantum dots as their properties can be

modified by changing either the constituting materials or core-to-shell ratio [Bae and

Mehra, 1998; Kalele et al., 2006]. The term nanoshell is used particularly because

thickness of the shell is generally of the order 1–20 nm. Thickness of shell materials in

nanometers becomes important when they are coated on the core material to achieve

higher surface area. Thicker shells can also be synthesized but their preparation is

restricted primarily to achieve some specific goal, such as providing thermal stability to

core particles. Synthesis of nanoshells may be useful for creating novel materials with

various morphologies, as it is not possible to synthesize all the materials in the desired

morphologies. These materials are also of economic interest, as inexpensive cores can

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be coated with precious materials [Chan and Liu, 1999]. Thereby expensive material is

required in lesser amount than usual. The core-shell nanoparticles are synthesized for a

variety of purposes like enhancing luminescence properties, providing chemical stability

to colloids, biosensors drug delivery, engineering band structures etc. [Matijevic, 1986;

Everett and Everett, 1988; Kalele et al., 2006]. Nanoshell particles may be synthesized

in various combinations such as (core-shell) dielectric–metal, dielectric–semiconductor,

dielectric-dielectric, semiconductor–metal, metal–metal, semiconductor–semiconductor,

semiconductor–dielectric, metal– dielectric, dye–dielectric, etc. [Kalele et al., 2006].

Core shell quantum dots can be further utilized for creation of another class of novel

materials like quantum bubbles or colloidal crystals. It is indeed possible to synthesize

unique core shell structures having multiple shells. In the multishells firstly the core

particles are coated with a shell to obtain a single nanoshell, there after these

combinations of core and shell are repeated again to get multishells. By choosing

different combinations of core and shell materials these structures show tunable optical

properties from the visible to infrared region of the electromagnetic spectrum. A

schematic diagram showing a variety of core shell quantum dots is depicted in Figure

1.4. Surface of the core particle can be modified using bi-functional molecules and then

small particles can be anchored on it (Figure 1.4a). Nanoparticles grow around the core

particle and form a complete shell (Figure 1.4b). In some cases a smooth layer of shell

material can be deposited directly on the core by co-precipitation method (Figure 1.4c).

Small core particles such as gold or silver (10–50 nm) can be uniformly encapsulated

with silica (Figure 1.4d). Also a number of colloidal particles can be encapsulated inside

a single particle (Figure 1.4e). Core particles can be removed either by calcinations or

by dissolving them in a proper solvent. This gives rise to hollow particles also known as

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quantum bubbles (Figure 1.4f). Concentric shells (Figure 1.4 g) also can be grown on

core particles to form a novel structure known as multishell [Kalele et al., 2006].

Figure 1.4: Variety of core shell particles (a) Surface-modified core particles

anchored with shell particles (b) More shell particles reduced onto core to form a

complete shell (c) Smooth coating of dielectric core with shell (d) Encapsulation

of very small particles with dielectric material (e) Embedding number of small

particles inside a single dielectric particle (f) Quantum bubble (g) Multishell

particle [Kalele et al., 2006]

Semiconductor nanoparticles are well established fluorescent materials, which are

generally coated with silica to reduce photo bleaching [Kalele et al., 2006]. The

semiconductor nanoparticles coated with another layer of semiconductor have also

proved to be of great significance to enhance the luminescence [Matijevic, 1986; Everett

and Everett, 1988]. The selection of shell material is important for localization of the

electron–hole pair. There are type-I nanostructures such as CdSe@CdS or CdSe@ZnS

in which the conduction band of the shell material (which is a higher band-gap material)

is at higher energy than the core and valence band of the shell is at lower energy than

that of the core (see Figure 1.5). In these materials, electrons and holes are confined in

the core. In type-II nanostructures such as (CdSe@ZnTe or CdTe@CdSe), both valence

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and conduction bands of the core material are at higher (or lower) energy than in the

shell (Figure 1.5). In this case one carrier is confined in the core and the other in the

shell. Type-I and type-II nanostructures have properties different from each other

because of the spatial separation of carriers.

Figure 1.5: Type-I and type-II semiconductor core shell structures

The most commonly synthesized semiconductor core-shell nanocrystals are composed

of atoms from groups II-VI (e.g. CdSe‚ CdTe‚ CdS‚ ZnSe and ZnS) and groups III-V

(e.g. InP and InAs) of the periodic table [Alivisatos, 1996; Nirmal and Brus, 1999;

Manna et al., 2002; Wuister et al., 2003]. In particular‚ rapid advancements in synthesis

and characterization techniques have led to the development of highly luminescent and

monodisperse core-shell CdSe quantum dots [Murray et al., 1993; Hines and Guyot-

Sionnest, 1996; Dabbousi et al., 1997; Peng et al., 1997].

1.5 CORE-SHELL QUANTUM DOTS IN TERMS OF CAPPING

Core shell quantum dots can be capped either with the organic capping agents or the

inorganic capping agents. The following section describes the brief introduction along

with the literature insight about both types of capping agents.

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1.5.1 Organic Capping

Coating of quantum dots surfaces with organic ligands typically facilitates the

production of stable‚ high-quality quantum dots in organic media. A classic example of

this effect is observed in the synthesis of CdSe quantum dots [Talapin et al., 2001] in

hot solutions of amphiphilic tri-n-octylphosphine oxide (TOPO). In these solutions‚ the

hydrophilic phosphine oxide groups coordinate to Cd sites on the quantum dot surface

while the hydrophobic alkyl chains form a densely packed surface coating that stabilize

nanocrystals against aggregation. The non-polar alkyl chains at the solid-liquid interface

between the nanocrystals and solvent cause the resulting quantum dots to be soluble in

organic solvents‚ such as chloroform and hexane. The capping of organic ligands on the

quantum dots surfaces tends to improve their overall quantum yields. The fluorescence

quality of quantum dots is often quantified in terms of a fluorescence quantum yield -

the ratio between the number of fluorescence photons emitted when mobile electrons

recombine with holes to the number of photons absorbed upon excitation. Defect

structures, both in the internal structure and on the surface of quantum dots can produce

competing energy states that trap the excited electrons and holes‚ resulting in lower

quantum yields [Bowen Katari et al., 1994]. Internal defect structures can be removed

by altering the solvent reaction conditions. Apart from internal defect structures‚ a

major challenge to improving the quantum yield of quantum dots is to remove surface

defect sites. Quantum dots have a large surface area-to-volume ratio and therefore‚ a

large population of atoms on their surface. Removal of dangling bonds with organic

stabilizing molecules had dramatically improved the overall quantum yield of the

quantum dots [Dabbousi et al., 1997].

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1.5.2 Inorganic Capping

The fluorescence efficiency of quantum dots can be greatly enhanced by the capping of

the nanocrystals inside an inorganic shell. In order for the capping layer, to produce

increased quantum yields of quantum dots‚ the inorganic layer must be composed of a

semiconductor with larger band gap energy than the core quantum dot. In addition‚ for

efficient capping‚ the bond length of the semiconductor comprising the capping layer

must be similar to that of the core. The observed increase in quantum yield upon growth

of an inorganic shell can be attributed to the removal of surface defects (trap states) on

the nanocrystals‚ a process referred to as electronic passivation. For CdSe nanocrystals

these surface defects typically result in broad emission at 700-800 nm‚ which are

removed with the growth of an inorganic shell with a wider band gap than the CdSe

core [Hao et al., 1999]. The removal of these trap states increases the number of

electrons that undergo radiative recombination and exhibit an increase in fluorescence

efficiency. Furthermore‚ capping with a semiconductor layer also prevents the photo-

oxidation of the core quantum dot while uncapped quantum dots may degrade within a

month or two when stored in air at room temperature. Long-term storage of capped

quantum dots under similar conditions can often be achieved with minimal effect on

their optical properties.

1.6 CLASSIFICATION OF SURFACTANTS (CAPPING AGENTS)

The organic and inorganic surfactants are classified by three different ways according to

their chemical structure (head and tail) and their counter ion.

According to the composition of their tail

According to the composition of their head

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According to the composition of their counter-ion

1.6.1 Classification According to Composition of the Tail

The tail of surfactants can be: A hydrocarbon chain [aromatic hydrocarbons (arenes),

alkanes (alkyl), alkenes, cycloalkanes, alkyne-based], an alkyl ether chain [Ethoxylated

surfactants: polyethylene oxides are inserted to increase the hydrophilic character of a

surfactant, Propoxylated surfactants ( polypropylene oxides are inserted to increase the

lipophilic character of a surfactant)], A fluorocarbon chain[fluorosurfactants], a siloxane

chain [siloxane surfactants]. A surfactant can have one or two tails; these are called

double-chained.

1.6.2 Classification According to Composition of the Head

A surfactant can be classified by the presence of formally charged groups in its head.

They are classified as nonionic, and ionic surfactant (see Figure 1.6).

Figure 1.6: Surfactant classification according to the composition of their head:

nonionic, anionic, cationic, zwitterionic (amphoteric)

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1.6.2.1 Nonionic surfactants

A non-ionic surfactant has no charge groups in its head. They do not ionize in aqueous

solution because their hydrophilic group is of a non dissociable type such as alcohol,

phenol, ether, ester, or amide. A large proportion of these nonionic surfactants are made

hydrophilic by the presence of a polyethylene glycol chain, obtained by the

polycondensation of ethylene oxide. They are called polyethoxylated nonionics. As far

as the lipophilic group is concerned, it is often of the alkyl or alkylbenzene type, the

former coming from fatty acids of natural origin. The polycondensation of propylene

oxide produce a polyether which (in oposition to polyethylene oxide) is slightly

hydrophobic. This polyether chain is used as the lipophilic group in the so-called

polyEO-polyPO block copolymers, which are most often included in a different class,

e.g. polymeric Surfactants.

1.6.2.2 Ionic surfactants

The head of an ionic surfactant carries a net charge. Depending upon the type of charge

they are classified as anionic and cationic surfactant.

(i) Anionic surfactant: If the charge is negative the surfactant is more specifically called

anionic surfactant. Anionic Surfactants are dissociated in water in an amphiphilic anion

and a cation which is in general an alkaline metal (Na+, K

+) or a quaternary ammonium.

They are the most commonly used surfactants. They include alkylbenzene sulfonates

(detergents, fatty acid), soaps, lauryl sulfate (foaming agent), di-alkyl sulfosuccinate

(wetting agent), lignosulfonates, phosphates (Alkyl aryl ether phosphate, Alkyl ether

phosphate) etc.

(ii) Cationic Surfactant: If the charge is positive, it is called cationic surfactant.

They are dissociated in water into an amphiphilic cation and an anion, most often of the

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halogen type. A very large proportion of this class corresponds to nitrogen compounds

such as fatty amine salts and quaternary ammoniums with one or several long chain of

the alkyl type often coming from natural fatty acids. They include pH-dependent

primary, secondary or tertiary amines (Primary amines become positively charged at pH

< 10, secondary amines become charged at pH < 4), Octenidine dihydrochloride,

Cetylpyridinium chloride (CPC), Benzethonium chloride (BZT) etc.

(iii) Zwitterionic (amphoteric): If a surfactant contains a head with two oppositely

charged groups, it is termed zwitterionic or when a single surfactant molecule exhibit

both anionic and cationic dissociations it is called amphoteric or zwitterionic. This is the

case of synthetic products like betaines (cocamidopropyl betaine) or sulfobetaines and

natural substances such as aminoacids and phospholipids. Some amphoteric surfactants

are insensitive to pH, whereas others are cationic at low pH and anionic at high pH with

an amphoteric behavior at intermediate pH. Amphoteric surfactants are generally quite

expensive and consequently their use is limited to very special applications such as

cosmetics where their high biological compatibility and low toxicity is of primary

importance.

1.6.3 Classification According to Composition of Counter-ion

In the case of ionic surfactants, the counter-ion can be:

1.6.3.1 Monoatomic / Inorganic

Cations: metals : alkali metal, alkaline earth metal, transition metal

Anions: halides: chloride (Cl−), bromide (Br

−), iodide (I

−)

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1.6.3.2 Polyatomic / Organic

Cations: ammonium, pyridinium, triethanolamine (TEA)

Anions: tosyls, trifluoromethanesulfonates, methylsulfate

The important concepts that are involved in the stabilization of the surface of quantum

dots are as follows.

1.7 STABILIZATION OF COLLOIDAL SYSTEMS

It is commonly accepted that the stability of colloidal systems is, in most cases the result

of an extremely slow aggregation process. The main reason for such a slow aggregation

is a high electrostatic energy barrier and in some cases a protective layer of a surfactant

[Kallay and Zalac, 2002].

1.7.1 Forces Between Atoms and Molecules

Since the work of Vander Waals in 1873, it is known that there exist attractive forces

due to fluctuating dipoles resulting from the movement of the electrons within the

electron cloud of the atoms. He calculated the free attractive energy ∆Gatt

between two

atoms or molecules as:

6(1.1)att

aa

AG

d

where A is a molecule or atom specific constant and daa is the distance between two

atoms or molecules. When the atoms get below a certain distance their electron clouds

begin to interact, which creates repulsive forces and lead to an increase of ∆Gatt

. This

free repulsive energy ∆Grep

can be expressed with the following equation:

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12

(1.2)rep

aa

BG

d

where ‘B’ is a molecule or atom specific constant. The sum of these interactions is the

so called Lennard-Jones-Potential ∆GLJ [Axmann, 2004].

12 6

(1.3)rep att

LJ

aa aa

B AG G G

d d

1.7.2 Forces between Particles

To calculate the attractive forces between two particles one assumes that every atom of

one particle interacts with every atom of the other particle following the Lennard-Jones-

Potential and the free energy of these interactions is the sum over the contribution of all

possible atom pairs. Since the repulsive forces are of short range, only the surface atoms

are taken into account. The easiest way to solve this problem is to calculate it for two

infinite surfaces separated by a distance H, which was done by Hamaker and leads to the

free energy between the surfaces, with AH being the so called Hamaker constant.

2

(1.4)12

att HAG

H

A similar approach for two spheres of the same radius r leads to

2

31 .... (1.5)

12 4

att HA r HG

H r

where, ‘H’ represents the distance between the centers of the two spheres.

The higher terms can be neglected for most cases. Equations 1.4 and 1.5 show clearly

that for bigger particles the attractive forces decrease considerably slower than for

single molecules and atoms which has a significant influence on the stability of particle

dispersions.

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1.7.3 Electrostatic Stabilization

Particles in aqueous dispersion normally show a surface charge that can be due to the

ionization of surface groups or due to the adsorption of charged molecules or simply

ions. Following Coulomb’s law, every charge creates an electric potential that will be

compensated by charges of opposite sign in its proximity which means that around

every negatively charged ion an excess of positive charges will be present and vice

versa. This cloud of counter ions is called electric double layer.

The concept of double layer was first introduced by Helmholtz. He imagined it as a

molecular condensator in which the potential decreases linearly. But thermal movement

leads to a distribution of the counter ions, so that a diffuse double layer is formed. This

double layer results in a Coulombic repulsion ( ) between two particles which decays

approximately exponentially with the interparticle distance z as given below:

(1.6)k

o ze

The net result is shown schematically in Figure 1.7. The weak minimum in the potential

energy at moderate inter particle distances defines a stable arrangement for the colloidal

particles which is easily disrupted by medium effects and by the thermal motion of the

particles at room temperature. Thus if the electric potential associated with the double

layer is sufficiently high then electrostatic repulsion will prevent particle agglomeration.

Waals forces are outweighed by repulsive electrostatic forces between adsorbed ions

and associated counter ions at moderate interparticle separation [Schmid, 2008].

According to the Debye-Huckel-theory for electrolytes the thickness of the double layer

is defined by:

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0

2

1(1.7)

2

r B

A

k T

k N e I

with εr and ε0 being the electric permittivity of the medium and the vacuum, ‘kB’

Boltzmann’s constants, ‘NA’ Avogadro’s constant, ‘T’ temperature, ‘e’ the elementary

charge and ‘I’ the ionic strength. In Equation 1.7 one can see that the thickness of the

double layer depends strongly on the ionic strength in the solution. This shows that an

electrostatically stabilized solution can be coagulated if the ionic strength of the

dispersing medium is increased sufficiently, since this compresses the double layer and

shortens the range of the repulsion [Schmid, 2008].

Figure 1.7: Electrostatic stabilization of colloidal particles.

Two particles will not “see” one another at long distances because of their neutralizing

double layers. When they get closer these double layers act like the electron clouds

around an atom. When the two double layers are charged in the same way they will

repel one another [Wedler and Freund, 1985]. This is the basis of the DVLO theory. The

DVLO theory takes only two interactive energies into account, the Vander-Waal

attraction (see Equation 1.4) and the repulsive potential due to the overlapping of the

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double layers. When the double layers of two charged surfaces overlap their electric

potentials add up. This results in an increase of the electric Gibbs energy.

064(1.8)kHB

elst

c k TG e

k

with ‘c0’ being the ion concentration without potential. This concentration dependence

mainly comes from the dependence of the double layer thickness 1/k on the ionic

strength. Added with the term for the Vander-Waals attraction we get an expression for

the potential interactive energy ∆Gpot per surface unit between two flat surfaces:

0

2

64. (1.9)

12

kHB Hpot

c k T AG e

k H

The stabilizing effect of surface ions is not only dependent on the concentration of the

ions in solution but also on the concentration of ions adsorbed at the particle surface. If

the surface charge is reduced by the displacement of adsorbed ions by a more strongly

binding neutral adsorbate, the colloidal particles can collide and agglomerate under the

influence of the Vander Waals attractive forces.

1.7.4 Steric Stabilization

Colloidal dispersions can also be stabilized by the addition of a surfactant that is in most

cases a polymer or an organic material. The surfactant forms a protecting layer around

the particle and so prevents flocculation. The adsorbed layers can influence the Vander-

Waal’s attractions or induce repulsion of the particles. This is called steric stabilization.

We have to distinguish between three cases:

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1.7.4.1 Anchored polymers/organic materials

In the case of a compact adsorbed layer the particles can be regarded as a hard sphere

with the diameter 2(r + δ), where r is the particle radius and δ the thickness of the

adsorbed layer. So this adsorbed layer prevents that the centers of the particles can get

closer than 2(r + δ).

When the protective layer resembles the dispersing medium, the attraction between the

particles for a distance H without contact will not be influenced by the adsorbed layer.

But since the particles cannot get closer than 2(r + δ) on contact, the attractive potential

on contact will be smaller than for particles without protective layers. On the other

hand, when the layer resembles the particle material, it will act like a bigger particle and

so result in a higher attractive potential than a non coated particle (see Equation 1.5).

But normally the adsorbed polymer/organic material does not act like a hard surface.

The polymer chains will extend into the dispersing medium in a degree that depends on

the interaction between polymer and dispersing medium. More the surfactant resembles

the dispersing medium, more the polymer chains will be directed outward into the

solvent. When two particles collide, the surfactant or polymer chains will penetrate one

another. This has two consequences. First, the local density of the polymer will be

increased so that the osmotic effect will cause a diffusion of solvent molecules between

the two surfaces to diminish the surfactant concentration. This will push apart the

particles. Second, the higher surfactant concentration between the particles will lead to a

reduced number of possible arrangements of the chains. So the entropy decreases by a

value ∆S and so the free repulsive energy increases by |T∆S|. This is called entropic

repulsion. In this case the polymer chains of one layer penetrate in the layer of another

particle, so the center-center distance of the particles can be smaller than 2(r + δ) but

the repulsive potential increases strongly (Figure 1.8). So to achieve a good steric

INTRODUCTION

23

stabilization the surfactant should be different from the nature of the particle but similar

to the one of the dispersing agent. The longer the chain length, the better will be

stabilization because the repulsive strength depends strongly on the thickness of the

surfactant layer around the particle. The surfactant can be attached to the surface by

chemical bonding or by a strong and specific binding [Russel et al., 1992].

Figure 1.8: Steric stabilization of colloidal particles with anchored polymers a) in

the interparticle space the configurational freedom of the polymer chains of two

approaching particles is restricted causing a lowering of entropy b) the local

concentration of polymer chains between the approaching particles is raised and

the resulting higher local activity is osmotically counteracted by solvation

[Schmid, 2008]

INTRODUCTION

24

1.7.4.2 Non adsorbing polymers/organic material

Dissolved, non-adsorbing polymer/organic material molecules must alter their

configuration if their centre of mass is to approach a particle surface. This leads to the

formation of a so called depletion layer around each particle. That means a layer from

which the polymer is excluded.

A particle immersed in a polymer solution experiences an osmotic pressure acting

normal to its surface. For an isolated particle, the integral of the pressure over the entire

surface nets zero force. But when the depletion layers of two particles overlap, polymer

will be excluded from a portion of the gap between the particles. Consequently, the

pressure due to the polymer becomes unbalanced. The polymer concentration between

the particles is lower than in the volume, so the dispersing agent diffuses in the volume

leading to an attractive force and particle flocculation. This is called depletion

flocculation [Russel et al., 1992].

1.7.4.3 Adsorbing polymers/organic material

Thus far, we have considered only the extremes of irreversibly anchored and non

adsorbing polymers. More commonly through polymers/organic materials adsorb at

random points along their backbone to a degree that depends on the nature of the

polymer, solvent and the surface. The interaction between surfaces is more complicated

with adsorbed polymer layers than with terminally anchored chains for two reasons:

1. Chains originally attached to one surface can adsorb at the same time to the

surface of a second particle to create a bridging between two particles.

2. Given enough time, chains can desorbs from the surface and migrate out of

the gap between two particles.

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25

With strong adsorption at full coverage of the particles surface in a good solvent, the

produced potential between two particles is purely repulsive. The polymer layers

interact at separations somewhat greater than twice the end-to-end distance of the free

chains. Since the coverage of the surface is approximately full there is no significant

bridging.

With strong adsorption but only partial coverage, the situation differs considerably.

Then the layers interpenetrate more easily and excess surface is available for adsorption

of tails from the opposing layer leading to attraction via bridging of the particles. But

still the potential stays repulsive at small separations, demonstrating that strongly

adsorbed polymers do not desorbs and migrates out of the gap between the particles.

With still weaker adsorption and a solvent only slightly better than a theta solvent, even

interaction at full coverage produces attraction. But there is still a repulsion at very

small separations indicating that complete desorption of the polymer from the surface

does not occur [Russel et al., 1992].

1.8 NASCENT/DOPED CORE-SHELL ZnS QUANTUM DOTS FOR

BIOMEDICAL APPLICATIONS

The doped Core-shell quantum dots are highly functional materials with tailored

properties. They are in the size range of 20nm-200nm [Sounderya and Zhang, 2008].

The necessity to develop and synthesize doped core/shell nanoparticles is to customize

the properties such as increased stability, surface area, magnetic, optical, catalytic and

luminescent properties. By simply tuning the core to shell ratio, the properties can be

altered. With emerging new techniques it is now possible to synthesize these

nanostructures in desired shape, size and morphology [Xu et al., 1998; Yang et al.,

INTRODUCTION

26

2001; Yang et al., 2004]. The doped core-shell nanostructures have applications in

luminescent devices, light emitters, phosphors optical sensors and biological detections

etc.

The role that an activator plays in a doped core-shell quantum dot depends on its

position in the crystal structure. If it occupies a substitution position, i.e. when it is in a

correct metal site, it behaves differently than when it is incorporated interstitially.

Each of the II-VI semiconductors demonstrates unique properties, making them useful

for different applications but ZnS is selected in the present studies. ZnS has a band gap

of 3.68 eV for the zinc blend phase and 3.91 eV for wurtzite phase. It displays a high

refractive index of 2.2 and a high transmittance of light in the visible region of the

spectrum [Elidrissi et al., 2001; Jiang et al., 2001; Yang et al., 2003]. Also ZnS is a

nontoxic, chemically more stable and technologically better than other semiconductor

materials. As we know that nontoxicity of quantum dots is essential for their successful

use in medical applications. All these properties make ZnS as a strong candidate for

biomedical applications. ZnS is an II-VI compound semiconductor, which contain the

elements of group II and group VI, bonded together. In general, elements of group II

have two electrons in the s2 outermost shell of the atom, while group VI elements have

six electrons due to s2p

4 configuration. Group II elements are further divided into IIA

(Be, Mg, Ca, etc.) and IIB (Zn, Cd, Hg) groups. Due to the difference in electron

affinity and electro negativity between groups IIA and VI, the outermost electrons of

group IIA atoms are fully donated to those of group VI atoms, during the bond

formation. Then they form octahedral configuration like in case of NaCl. However due

to high ionization potential, group IIB cannot fully donate their electrons to group VI

atoms during bond formation. Therefore, in this case covalent bonds are formed with

INTRODUCTION

27

tetrahedral based zincblende and wurtzite crystal structures. [Jain, 1993; Yang et al.,

2002]

In its bulk form, ZnS is typically found in the zinc blend crystal structure at room

temperature [Joannopoulos and Cohen, 1973]. The zinc blend structure is cubic, with

four sulfur anions per unit cell located at the corners and centers of each face and with

four zinc cations situated in half of the tetrahedral sites (the ¼, ¼, ¼ positions). At

elevated temperatures, bulk ZnS can undergo a phase transformation from the cubic zinc

blend structure to a hexagonal Crystal structure known as the wurtzite structure [Lee et

al., 2004]. This transformation has been shown to occur at 1020º C. The zinc blend and

wurtzite structures are very similar. The stacking sequence of the close-packed planes of

zinc blend (111) planes is represented by the ABCABCABCABC repeating pattern.

However, if the close-packed planes stack themselves in the ABABABABAB repeating

pattern, they would form the (0001) planes of the wurtzite structure. Both the zinc blend

and the wurtzite structure are shown in Figure 1.9.

ZnS is categorized as a semiconductor material which is suitable as host matrix for a

large variety of dopants. Doped ZnS had attracted a lot of attentions from research

community due to its versatile luminescent properties. By controlling the size of doped

ZnS in nanometer range we can improve its luminescent efficiency in comparison to

bulk but as the particles become smaller, the surface to volume ratio and surface states

increased rapidly which leads to surface defects and particle aggregation. These act as

non-radiative pathways for excited electrons. To eliminate the non-radiative

contribution from the surface states and particle aggregation, doped core-shell ZnS

quantum dots are used. Doped core-shell ZnS quantum dots are extremely promising

candidates as luminescent probes and labels in biological applications as they are

nontoxic and also shows stable luminescence.

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28

Figure 1.9: The zinc blend and wurtzite crystal structures. The blue represents the

zinc atoms and the black represents the sulfur atoms

ZnS quantum dots doped with activator ions is known to have efficient light emitting

properties, especially Mn2+

doped core-shell ZnS. The identical ionic states of the

migrating Mn2+

and the host Zn2+

, their ionic radii are also similar, i.e. 0.83 Ao

(Zn2+

)

and 0.80 Ao (Mn

2+). It is therefore anticipated that Mn

2+ migrations takes place by the

simultaneous replacement of Zn2+

[Gan et al., 1997]. Mn2+

produces localized levels

(4T1 and

6A1) in the energy gap of the semiconductor ZnS particles due to crystal field

effects [Bhargava et al., 1994] similar to those in the bulk material [Gumlich, 1981]. An

electron can undergo photo excitation in the host ZnS lattice and subsequently decay via

a nonradiative transition to the 4T1 level from which it then makes a radiative transition

to the 6A1 level which results in an emission at about 580-600 nm (Figure 1.10). This is

the same transition as in the bulk material and it has been reported by several groups

[Bhargava et al., 1994; Bol and Meijerink, 2000] that the transition does not seem to be

much influenced by the particle size. But the quantum size effect does influence the

lifetime and fluorescence efficiency of the ZnS:Mn2+

particles.

INTRODUCTION

29

Figure 1.10: Recombination mechanism for the ZnS:Mn2+

system

To account for the phenomena it was suggested that with decreasing particle size a

strong hybridization of the s-p states of the ZnS host and the d states of the Mn2+

impurity could occur [Bhargava et al., 1994; Bhargav, 1996; Chung et al., 2002]. This

hybridization results in a faster energy transfer between the ZnS host and the Mn2+

impurities. Due to this fast energy transfer, the radiative recombination at the Mn2+

impurity, which competes with nonradiative decay at the surface, becomes more

efficient. This results in increased quantum efficiency. This advantage make ZnS:Mn2+

quantum dots as ideal candidates for fluorescent labeling agents especially in biology.

1.9 ZnS CORE-SHELL NANOPARTICLES: BACKDROP

The research work in the field of nanotechnology in last two decades has explored

outstanding results and properties of nanomaterials leading to the promising application

in diverse fields. Among various nanomaterials core-shell ZnS nanocrystals have drawn

significant attention of various researchers due to its versatile applications in the fields

of electronics, optics and biomedical. As the present work focuses on the synthesis of

core-shell ZnS nanocrystals, therefore the literature of last 15 years on synthesis and

INTRODUCTION

30

characterization of ZnS nanocrystals pertaining to various capping agents, dopants and

other experimental conditions has been reviewed.

Numerous techniques [Liz-Marzan et al., 2001; Davies et al., 1998; Templeton et al.,

2000; Caruso, 2001; Xia et al., 2000] have been developed to synthesize the core-shell

nanoparticles including chemical precipitation [Imhof, 2001; Ocana et al., 1991],

grafted polymerization [Okaniwa, 1998], microemulsion, reverse micelle [Li et al.,

1999; Chen et al., 2005; Lin et al., 2001, Huang et al., 1999], sol-gel condensation [See

et al., 2005], layer-by-layer absorption technique [Caruso et al., 1999; Caruso et al.,

2001] etc. Various synthesis techniques have their own limitations and draw backs.

However chemical precipitation method has been graded as a better technique for

producing efficiently luminescent nanophosphors in terms of process simplicity,

effectiveness of doping, higher yield and has a very simple procedure with a low cost

apparatus [Khosravi et al., 1995; Kim et al., 2006; Jindal and Verma, 2008].

Jin et al., [1995] observed the influence of the surfactant (Polymethylmethacrylate) on

the Mn2+

luminescence of ZnS:Mn nanocrystals. It was found that Mn2+

luminescent

intensity is enhanced significantly as that of uncoated ZnS.

Torres-Mart´ýnez et al., [1999] had studied the influence of temperature and

precipitation on the formation of ZnS nanocrystals capped with Glutathione (GSH).

UV–visible spectra showed prominent peaks at 268 and 290 nm. The intensity of these

peaks increased with time.

Lu et al., [2000] has studied the ZnS:Mn coated with 3-methacryloxypropyl

trimethoxysilane and found 30 fold increase in the photoluminescence intensity.

Jaehun et al., [2002] have found that the luminescence, shape, and stability of ZnS

nanoparticles in ethanol were enhanced by capping the surface with 3-mercaptopropyl

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31

trimethoxy silane (MPS) or by treating the surface with SiO2. While MPS forms an

exterior network to encapsulate the ZnS surface, SiO2 surrounds ZnS nanoparticles to

smoothen the surface.

Kubo et al., [2002] have found modification of ZnS:Mn nanocrystal suspension by a

surfactant with a phosphate or carboxyl group. They reported increase in

photoluminescence intensity and quantum efficiency due to d–d transition of Mn2+

ions.

Wageh et al., [2003] had synthesized two sizes of mercaptoacetic acid capped ZnS

nanoparticles of an average diameter 3.3 nm and 4.4 nm using Na2S.9H2O and Thiourea

as source of chalcogenide(S2-

). UV-visible absorption spectra showed maximum

absorption at 291 for and 306 nm for both the nanocrystallites.

Karar et al., [2004] have prepared ZnS:Mn nanocrystals by using PVP as a capping

agent. They have varied Mn2+

concentration from 0 to 40% and studied the

photoluminescence properties.

Tang et al., [2004] had studied the UV-visible absorption spectra of ZnS nanoparticles

capped with AOT− and pyridine. A red shift of 70 meV in absorption peak of pyridine

capped ZnS as compared to AOT− capped ZnS is attributed to the influence of dielectric

confinement on the band gap of ZnS nanoparticles.

Zhu et al., [2004] have synthesized L-Cysteine capped nanoparticles for the

determination of total protein in the human serum samples. Koneswaran and

Narayanaswamy [2009] had studied the absorption and fluorescence spectra of L-

Cysteine capped ZnS. The absorption peak occurred at 288 nm and emission maxima

obtained at 390 nm with the excitation of 290 nm. The optimum fluorescence intensity

was obtained in the pH range between 4.9 and 5.5.

INTRODUCTION

32

Manzoor et al., [2004] had studied the photoluminescence properties of doped ZnS

nanoparticles capped with pyridine and polyvinyl pyrrolidone. In case of pyridine

capped nanocrystals photoluminescence excitation band shows a sharp peak maximum

at 340 nm.

Warad et al., [2005] have synthesized nanoparticles of manganese doped ZnS by a

simple precipitation reaction using aqueous route. The nanoparticles were stabilized

using sodium tripolyphosphate (STTP) and sodium hexametaphosphate (SHMP). It was

observed that the particle sizes were dependent on the amounts of stabilizing agents.

Ghosh et al., [2006] have synthesized polyvinyl pyrrolidone (PVP) encapsulated ZnS

nanoparticles and studied the effect of capping through TEM to investigate the particle

size. Blue shift of absorption edge compared to bulk ZnS explained quantum

confinement effect of ZnS nanoparticles. Kumbhojkar et al., [2000] have synthesized

ZnS (QDs) using wet chemical routes with 1- thioglycerol, thiophenol and

mercaptoethanol to passivate the nanoclustors. The excitonic absorption peaks for all

the samples are blue shifted compared to the bulk. The sharp optical absorption peaks

observed for 1-thioglycerol and thiophenol capped samples manifest the high

monodispersity of these samples.

Rathore et al., [2008] have synthesized ZnS nanoparticles with different molar

concentrations of precursors. It is observed that the particle size depends on molar

concentration of reactant solution. A decrease in the size of particle is observed with the

increase of molar concentration of reactants. The UV-visible absorption studies showed

the blue shift with decreasing particle size which arises from quantum confinement

effect in nanoparticles.

INTRODUCTION

33

Borah et al., [2008] reported that ZnS nanoparticles of different crystalline sizes can be

synthesized by chemical route. The particle size is controlled by stirring rate,

temperature of the solution, pH and time of stirring. The structural and optical

characterization of the samples showed formation of ZnS nanoparticles, having size

between 5-7nm.

Sharma et al., [2008] have synthesized Core-Shell ZnS nanocrystals by chemical

precipitation method. Optical and morphological measurements on ZnS nanoclusters

were carried out to investigate the effect of capping on ZnS nanoparticles. XRD results

showed the crystallite size to be from 1.5-2.5 nm depending upon the peaks. Band gap

of PVP capped ZnS nanoparticles were increased in comparison with uncapped ZnS

nanoparticles indicating quantum size effects. Increase of emission intensity in case of

capped ZnS as compared to uncapped ZnS depicted better surface passivation.

He et al., [2008] had studied the phosphorescence characteristics of the L-Cysteine

capped Mn doped ZnS quantum dots for the detection of enoxacin in biological fluids.

The fluorescence spectra of the Mn-doped ZnS quantum dots showed two well-defined

emission bands, at 435 and 590 nm, while the phosphorescence spectra exhibited only a

single emission peak at 590 nm.

Mehta et al., [2009] had studied the stabilization mechanism of ZnS nanoparticles in

aqueous micellar solution of cationic surfactants. UV-visible studies presented

absorption edge located at 326 nm in cetyltrimethylammonium chloride (CTAC) and

318 nm in cetylpyridinium chloride (CTAC).

Zhang et al., [2010] had synthesized mercaptoacetic coated ZnS:Mn2+

quantum dots. An

absorption peak at 294 nm in UV– visible spectra was observed. Photoluminescence

spectra showed emission peaks at 400 nm and 590 nm at 300 nm excitation wavelength.

INTRODUCTION

34

The weak emission at 400 nm originated from the defect-related emission of the ZnS

and the strong emission peak at 590 nm was attributed to the 4T1−

6A1 transition of Mn

2+

impurities.

Geszke-Moritz et al., [2012] prepared 3-Mercaptopropionic acid-capped core/shell

ZnS:Cu/ZnS and ZnS:Mn/ZnS doped quantum dots (QDs) through hydrothermal

methods. Navaneethan et al., [2012] synthesized highly size confined ZnS QDs through

aqueous route using N-Methylaniline. Strong blue shift was observed from the UV

visible absorption spectrum with increase in band gap and enhanced photoluminescence

near band edge emission was observed at 346 nm.

1.9.1 Gaps in the Reported Literature

1. It is evident from the review of literature that the properties of organically capped

doped and undoped ZnS nanostructures are very sensitive to preparation

conditions such as pH, temperature, molar concentration of the precursors,

synthesis technique, nucleation atmosphere, lattice mismatching etc. Moreover for

the functionalization of any type of nanostructures for industrial applications, the

characteristic data of synthesized ZnS nanoparticles is of paramount importance.

Therefore there is a huge potential of research to study the effect of preparation

conditions on the nanoparticles characteristics.

2. Steric hindrance and electrostatic stabilization are two different approaches to

passivate nanostructures. The properties of any type of core-shell nanostructures

depend on the type of capping agent whether it is organic and inorganic. The

properties vary with the change in nature and amount of capping agent . It is also

very important to study variable capping agents, their concentration and other

synthesizing parameters on the behavior of ZnS quantum dots.

INTRODUCTION

35

3. Very less work is reported on the study of core-shell nanostructures involving

doped ZnS. Doping in semiconductors leads to the generation of optoelectronic

material. But in the case of nanomaterials, defects play a vital role. Therefore

suitable selection of surfactant and dopant pair along with their appropriate

concentration plays a crucial role in the suitability for the desired application.

4. The ZnS nanoparticles are the potential candidates for the biomedical applications

and thus need to be passivated with the biocompatible surfactants. The organic

capping agents like Glutathione, Pyridine and L-Cysteine are biocompatible but in

the literature very few studies have been carried out using these surfactants.

1.9.2 Objectives of Present Work

In light of the above mentioned literature gaps, following are the objectives for the

present research work.

1. To synthesize the nascent ZnS core-shell QDs using organic surfactants like

pyridine, glutathione and L-cysteine.

2. To synthesize manganese doped ZnS core-shell QDs using organic

surfactants like pyridine, glutathione and L-cysteine.

3. To study the effect of organic capping on the optical (Band Gap,

Photoluminescence) properties of nascent and manganese doped ZnS

quantum dots.

4. To study the effect of organic capping on the morphological (shape, size and

structure) properties of nascent and manganese doped ZnS quantum dots.

INTRODUCTION

36

1.9.3 Methodology

1. For synthesizing the ZnS core-shell nanoparticles, chemical precipitation method

is used in the present research work.

2. To characterize the synthesized ZnS core-shell nanoparticles. XRD, TEM, SAED,

Photoluminescence spectroscopy, UV-visible absorption spectroscopy and FTIR

spectroscopy has been used.

Detailed methodology has been discussed further in the next chapter regarding the

synthesis and the characterization of the nanostructures. It contains the techniques for

synthesis of nanoparticles along with the basic principle and working of the

characterization equipments used to determine the morphological and optical properties

of nanomaterials. The equipments used in the present work have also been explained

briefly.