21

CHAPTER 2

SYNTHESIS, SURFACE COATING/FUNCTIONALIZATION AND

CHARACTERIZATION TECHNIQUES FOR IRON OXIDE

NANOPARTICLES

2.1 SYNTHESIS OF IRON OXIDE NANOPARTICLES

In the last several decades, numerous chemical methods have been

developed to synthesis iron oxide nanoparticles with various size and shape.

The most common methods including co-precipitation, thermal decomposition,

hydrothermal, gel-evaporation or polyol, micelles method and laser pyrolysis

technique can all be directed at the synthesis of high quality magnetic

nanoparticles. In this present research work, gel-evaporation method,

co-precipitation technique, reverse micelles method and hydrothermal method

were used. The relevant synthesis techniques and corresponding growth

mechanism are discussed in the following sections.

2.1.1 Polyol or Gel-Evaporation Method

The polyol processes also called as gel-evaporation method, it is a

versatile chemical approach for synthesize of nano and microparticles with

well-defined shapes and controlled sizes. Polyol-mediated preparation of

nanoscale oxides is carried out by dissolving a suitable metal precursor (for

example, acetate, nitrate and alcohol) in the solvent as polyol (for example,

polyethyleneglycol or ethylene glycol). The precipitation of metal oxide

nanoparticles was obtained while heating the solution (< 200 °C) [42]. Average

particles diameter can be tuned by adjusting the concentration of the metal

precursors. However, most polyol methods are carried out at high temperature

22

(> 200 °C) and under inert gas atmosphere to prevent an oxidation reaction by

O2 in the environment.

2.1.2 Reverse Micelles/Microemulsion Method

A microemulsion is a thermodynamically stable dispersion of two

immiscible liquids (water and oil) with the aid of surfactant. Small size droplets

of one liquid are stabilized in the other liquid by an interfacial film of

surfactant molecules. In the water-in-oil microemulsions, the aqueous phase

forms droplets (1-50 nm in diameter) in a continuous hydrocarbon phase.

Consequently, this system can impose kinetic and thermodynamic constraints

on particle formation, such as a nanoreactor. The surfactant-stabilized

nanoreactor provides a confinement that limits particle nucleation and growth

[43]. By mixing two identical water-in-oil emulsions containing the desired

reactants, the droplets will collide, coalesce and split and induce the formation

of precipitates (Fig. 2.1).

Adding a solvent like ethanol to the microemulsion, allows extraction of

the precipitate by filtering or centrifuging the mixture. The main advantage of

the reverse micelle or emulsion technology is better control on nanoparticles

size by varying the nature and amount of surfactant and cosurfactant, the oil

phase or the reacting conditions. The magnetite nanoparticles are formed by

reverse micelles method through oxidation of Fe2+ salts in γ-Fe2O3 and Fe3O4. The size of the magnetite particle can be controlled by the temperature and the

surfactant concentration. Variations in the temperature and concentration of

iron dodecyl sulfate Fe(DS)2 micelles allow to grow the particles of diameters

ranging from 3.7 to116 nm [44].

23

Figure 2.1 Mechanism for the formation of nanoparticles by

microemulsion method.

Although many types of magnetic nanoparticles have been synthesized

in a controlled manner using the microemulsion method, the obtained particle

size and shapes reported show very wide variations. The working window for

synthesis in microemulsions is usually quite narrow and the yield of

nanoparticles is low compared to other methods, such as hydrothermal and

co-precipitation methods. Furthermore, because large amounts of solvent are

necessary to synthesize appreciable amounts of material, microemulsion is not

a very efficient process and is rather difficult to scale-up.

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2.1.3 Co-Precipitation Technique

Co-precipitation is a facile and convenient way to synthesize iron oxides

(either Fe3O4 or γ-Fe2O3) from aqueous Fe2+/Fe3+ (1:2 molar ratio) salt

solutions by the addition of a base at room temperature or at elevated

temperature. Typical overall reaction may be written as follows:

3 23 4 28 4Fe Fe OH Fe O H O

According to the thermodynamics of this reaction, complete

precipitation of Fe3O4 should be expected at a pH between 8 and 14, with a

stoichiometric ratio of 2:1 (Fe3+/Fe2+) in a non-oxidizing oxygen environment

(Fig. 2.2).

Figure 2.2 Co-Precipitation set-up for the synthesis of nanoparticles.

25

The main advantage of the co-precipitation process is that a large

quantity of nanoparticles can be synthesized. In this process, two stages are

involved: a short burst of nucleation occurs when the concentration of the

species reaches critical supersaturation and then, there is a slow growth of the

nuclei by diffusion of the solutes into the surface of the crystal [45].

To produce monodispersed iron oxide nanoparticles, these two stages

should be separated; i.e., nucleation should be avoided during the period of

growth. Recently, significant advances have been adapted to preparing the

monodispersed magnetite nanoparticles through adding of organic additives as

stabilization and/or reducing agents. For example, Fe3O4 nanoparticles with

size range 4-10 nm were stabilized by aqueous solution of 1 wt %

polyvinlyalcohol (PVA). However, when using PVA containing 0.1 mol %

carboxyl groups as the stabilizing agent, magnetite nanoparticles in the form of

chainlike clusters precipitate [46]. This result indicates that the selection of a

proper surfactant is an important issue for the stabilization of such particles. In

general, biopolymers such as carbohydrates (dextran, chitosan, alginate,

arabinogalactan), proteins, etc., and synthetic polymers such as polyethylene

glycol (PEG), poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA),

poly(methylacrylic acid) (PMAA), poly(lactic acid), polyvinylpyrrolidone

(PVP), polyethyleneimine (PEI), AB and ABC-type block copolymers

containing the above polymers as segments are often used as precipitating

agents. Since carbohydrates are abundant in hydroxyl groups as well as

carboxylic groups (in alginate) and amino groups (in chitosan), therefore they

can firmly stick to the particle surface and effectively inhibit the growth of

crystal nuclei according to the adsorption mechanism mentioned above [47].

Although, co-precipitation is probably the most widely used method

used to obtain commercial magnetic nanoparticles for biomedical applications,

particles prepared by co-precipitation tend to be rather polydisperse and their

26

shapes are difficult to control. Other methods have been developed to provide a

better control over the shape and size distribution of magnetic nanoparticles

such as hydrothermal method.

2.1.4 Hydrothermal Method

Hydrothermal synthesis is a wet-chemical technology of crystallizing

substance which is carried out in water under supercritical conditions, that is, at

temperatures around or higher than 200 °C under a pressure higher than

14 MPa. Under these conditions, water plays the role of a hydrolytic reactant.

The photograph of reaction container, which was used in our labarotary for

materials synthesis, is shown in Fig. 2.3.

There are two main routes for the formation of ferrites via hydrothermal

conditions: hydrolysis and oxidation or neutralization of mixed metal

hydroxides. These two reactions are very similar, except that ferrous salts are

used in the first case [48]. In this process, the reaction conditions such as the

solvent, the temperature and time are having important effects on the quality of

products [49]. The size and morphology of the nanoparticles can be tuned by

controlling the reaction time and the temperature, surfactant concentration,

nature of the solvent, precursors, and addition of seeds.

2.2 SURFACE COATING/FUNCTIONALIZATION OF IRON OXIDE

NANOPARTICLES

Stabilization of magnetic iron oxide nanoparticles for a long time

without agglomeration is crucial for several applications. Especially Fe3O4

nanoparticles are not very stable under ambient conditions and can be easily

oxidized to γ-Fe2O3.

27

Figure 2.3 Photograph of Teflon-lined stainless steel autoclave.

Figure 2.4 Commonly used methods for coating of iron oxide

nanoparticles.

28

The main reason for the agglomeration of magnetic iron oxide

nanoparticles is the large surface area-to-volume ratio. In the absence of any

proper surface coating, the magnetic nanoparticles try to minimize their surface

energy by forming agglomerations. The clustering process takes place through

attractive interaction between hydrophobic surface of the the nanoparticles.

Additionally, in order to expand the scope of the iron oxide nanoparticles in

biological applications surface coating and functionalization is essential

(Fig. 2.4).

2.2.1 Polymer Coating

Polymers are often employed to coat the surface of small magnetic iron

oxide nanoparticles during or after the synthesis to avoid the agglomeration. In

addition, these coating provide a means of engineering the surface of the

magnetic iron oxide nanoparticles to tailor its characteristic such as surface

charge and chemical functionality. In general, polymers can be chemically

attached or physically adsorbed on magnetic iron oxide nanoparticles to form a

single or double layer structure resulting in a repulsive (mainly as steric

repulsion) force to prevent a magnetic attraction between the iron oxide

nanoparticles. A variety of natural and synthetic polymers has been used for

coating of nanoparticles. Synthetic polymers generally the functional groups,

such as carboxylic acids, phosphates and sulfates, which can be attached on the

surface of magnetic iron oxide nanoparticles, are used. Suitable polymers for

coating include poly(pyrrole), poly(aniline), poly(alkylcyanoacrylates),

poly(methylidene malonate) and polyesters, such as poly (lactic acid),

poly(glycolic acid) and polyethylene glycol, etc.

Polyethylenimine (PEI) is a polymer and synthetic organic

macromolecules, which contains the highest density of positive charge. Many

researchers have proved PEI has high gene transfection efficiency and good

29

biocompatibility in-vivo and in-vitro, while its limitation is that of poor

targeting. In this regard PEI, coated magnetic iron oxide nanoparticles could

be used for targeted gene delivery [50, 51]. Several groups have reported

preparation of PEI-coated magnetic particles using various methods for gene

transfections. For example, iron oxide nanoparticles are synthesized in situ

precipitation within the PEI solutions [52-54]. Kamau et al., directly mixed

maghemite dispersions with PEI solutions [55], whilst McBain and co-workers

exploited covalent bonds to attach PEI to magnetic particles [56]. The PEI

coated magnetic iron oxide nanoparticles prepared by these methods are all

capable of enhancing gene delivery.

In the recent years, researchers have been concentrating on synthesis of

protein coated magnetic iron oxide nanoparticles. Protein coated magnetic iron

oxide nanoparticles can be effectively used for biomedical applications due to

their biocompatibility and low toxicity [57, 58].

Recently, several approaches have been developed to coat magnetic iron

oxide nanoparticles by polymers and protein, including in situ coatings and

post-synthesis coatings. In situ coatings is one-pot synthesis method, have

several advantages over stepwise surface modification, including the reduced

agglomeration due to the immediate coating of the particles and less processing

procedure [59]. However, the presence of polymers or protein during the

nanocrystals nucleation and growth can have the significant impact on the

crystal structure and morphology of the nanoparticles obtained through this

process. For example, Mandeep et al., [60], found that the shape and the

structure of the nanocrystals is affected by thermal denaturation of the bovine

serum albumin (BSA) through one-pot aqueous route. Furthermore, utilizing

the polymer for coating of magnetic iron oxide surface may also reduce the

magnetic properties [61].

30

2.2.2 Metal Coating

Another facile route to protect the iron oxide nanoparticles is formation

of core-shell (magnetic iron oxide/metal) nanocomposite. Metallic shell of

nanocomposites such as gold, silver, platinum, palladium and gadolinium not

only provide the stability to the nanoparticles in solution but also help in

binding the various biological ligands on the nanoparticle surface for various

biomedical applications. The core-shell nanoparticles have been extensively

used as a matrix and cytochemical label for the immobilization and study of

macromolecules such as drugs, proteins, enzymes, antibodies, nucleotides

tumor in hyperthermia [62-64]. Recently, several reserachers have reported on

the magnetic iron or iron oxide nanoparticles coated with Au [65], Ag [66],

Gd [67] and carbon [68]. In contrast with Au colloids, colloidal Ag

nanoparticles have several advantages. These are: (i) Ag nanoparticles exhibit a

surface plasmon band between 390 and 420 nm; that is a spectral region

distinct from that of Au (510-560 nm) [69] and (ii) the extinction coefficient of

the surface plasmon band for an Ag particle is approximately four times as

large as that for an Au particle of the same size [70].

2.2.3 Silica Coating

Silica is the most common coating material for magnetic iron oxide

nanoparticles. A silica shell not only prevent the aggregation, but can also

prevents the direct contact of the magnetic core with additional agent linked to

the surface of the silica thus prevent the unwanted interaction. For example,

the direct attachment of the luminescence materials on the surface of magnetic

nanoparticles reduces the luminescence properties. Inorder to avoid this

problem, magnetic nanoparticles were first coated by silica and then

luminescence materials were grafted on silica shell [71]. Silica coated magnetic

nanoparticles have advantages such as improved chemical stability, good

31

biocompatability, ease in surface modification and a controlled inter particles

interaction through varying the thickness of the shell. In order to generate the

magnetic silica sphere different processes have been explored such as Stöber

process, sol-gel process and aerosol pyrolysis. Stöber process is a well known

process, in which silica is formed in situ through the hydrolysis and

condensation of a sol-gel precursor, such as tetraethyl orthosilicate (TEOS)

[72]. The thickness of the silica shell can be tuned by varying the concentration

of ammonium and the ratio of TEOS to H2O. Furthermore, silica contains free

silanol groups that can be subsequently reacted with additional appropriate

functional groups through relevant silanization reactions [73].

In the recent years, researchers are concentrating on development of

silica encapsulation of nanoparticles with dual functionalities such as

fluorescence and magnetism [74]. Such materials can be physically

manipulated by application of an external magnetic field and simultaneously

optically probed by monitoring their fluorescence in real time. Further, it has

recently been proposed that such nanocrystals could themselves be used as both

magnetic and optical subunits in these silica core-shell systems. Specifically,

superparamagnetic iron oxide nanoparticles have been used for magnetism

based protein harvesting, magnetic resonance imaging (MRI), hyperthermia

treatment, bioseparation and magnetic sensing of biomolecules [75], while

semiconducting quantum dots (QDs) or fluorescent dyes have been used in

fluorescence-based biolabeling and imaging applications at the subcellular

level [76].

The fluorescent nanoparticles for optical imaging can be divided into

two major categories: Dye doped nanoparticles and quantum dots [12].

Usually, dye molecules suffer from photobleaching and quenching due to

interactions with solvent molecules and reactive species such as oxygen or ions

32

dissolved in solution when they are exposed to a variety of harsh environments

[77]. Whereas, QDs are more photochemically stable and have narrower,

tunable emission spectra than fluorescent dye molecules [78-80]. However, to

overcome the toxicity (generally related to heavy metal ions such as Pb2+ or

Cd2+) associated with quantum dots remains challenge, which limits the

adoption of multimodal probes containing QDs, photo-oxidation and difficult

surface conjugation chemistry associated with QDs limiting their applications

[81]. A very promising direction is the use of heavy-metal-free like carbon-

based fluorescent materials, including carbon nanoparticles (CNPs) and carbon

quantum dots (CDs) have received particular attention. Compared with

conventional QDs, carbon based fluorescent materials are superior in chemical

stability and biocompatibility [82].

Several methods have been developed for synthesis of magnetic and

luminescent composite silica microspheres containing both magnetic iron oxide

nanoparticles and luminescnet QDs such as inverse suspension method,

simultaneous coating of the silica shell on magnetic iron oxide nanoparticles

and luminescent QDs. However, the drawback of this method is polydispersity

of the nanoparticles within the silica spheres. Recently, magnetic and

luminescent silica spheres were synthesised by using silica coated magnetic

iron oxide nanoparticles, followed by layer-by-layer assembly of QDs on

magnetic silica sphere through electrostatic interaction, which were finally

coated by silica shell [83]. However, polydispersity of nanoparticles was not

calculated. Monodispersed state of the nanoparticles is very essential to retain

both magnetic and luminescence properties. Insin et al., have developed sol-gel

method to synthesis the magnetic and luminescent silica sphere [84].

33

2.3 CHARACTERIZATION TECHNIQUES FOR IRON OXIDE

NANOPARTICLES

2.3.1 X-Ray Diffraction

X-ray diffraction (XRD) is a method used to characterize the crystal

structure and analyse the parrticular phase of the material. XRD is basically

used to identify unknown substances, by comparing diffraction data with a

database maintained by the International Centre for Diffraction Data (ICDD).

These techniques are based on collecting the scattered intensity of an X-ray

beam hitting a sample as a function of incident and scattered angle, polarization

and wavelength or energy. The peak intensity in XRD result can be used to

quantify the proportion of iron oxide forms in a mixture by comparing

experimental peak and a reference peak intensity. The crystal size also can be

calculated from line broadening from the XRD pattern using the Scherrer’s

equation. The crystalline structure of the products were identified using

Panalytical X'pert Pro diffractometer with CuKα radiation (λ = 0.15417 nm).

Approximately 10 mg of sample was placed onto a silicon zero-reflectance disc

adhered to an aluminum sample plate. In the present work, XRD data were

collected with 2θ ranging from 10 to 80 degree at a rate of 2 degree per minute.

2.3.2. Scanning Electron Microscopy

Scanning electron microscopy (SEM) is a technique which provides

high resolution three dimensional morphological and topographical information

of the solid surface. When the high intensity electron beam hits a point on the

sample, numerous collisions between the electron from the beam and atoms in

the sample will occur, which cause it to emit secondary electron. These

secondary electrons have relatively low energy and can easily attach by the

34

detector. The detector counts the number of electrons emitted from the sample

and resulting pattern produces a three dimensional image on the screen of a

cathode ray tube. In the present work, FESEM-Hitachi S4800, field emission

SEM was used for higher resolution. Samples were prepared by mounting them

to a piece of carbon tape on a designated sample holder and dried for at least 24

h in a desiccator. The samples could be mounted parallel or perpendicular to

the electron beam.

2.3.3 Transmission Electron Microscopy

Transmission electron microscopy (TEM) is a technique that is used to

characterize the morphology and size of materials such as nanoparticles. These

instruments are used because of the limited image resolution in light

microscopes imposed by the wavelength of visible light. Electrons have wave

like characteristics, with a wavelength substantially less than visible light.

Since electrons are smaller than atoms, TEM is capable of resolving atomic

level detail. Samples are prepared for TEM imaging by inserting a TEM grid

(copper coated) into dry or wet powder (usually dried over-night) using

tweezers to hold the grid. The sample grid is then lightly tapped to remove any

excess particles and the grid is placed in the TEM for imaging. This procedure

can be used to characterize the coated and uncoated magnetic particles.

A JEM 3010 (JEOL) operated at 200 kV was utilized to view particle

size and shape of nanoparticles. In the present work, the samples for TEM

measurements were prepared by weighing approximately 0.01 g of synthesized

samples, dispersed into 5 ml methanol under sonication. The suspension was

then dropped on a copper coated TEM grid and kept in desiccator over-night to

dry. Size analysis from the transmission electron micrographs was performed

using image processing software ImageJ.

35

2.3.4 Dynamic Light Scattering Study

Dynamic light scattering (DLS) is a common method to characterize

dilute and transparent dispersions of magnetic nanoparticles. DLS is also

known as photon correlation spectroscopy. This technique is based on the

analysis of fluctuations of the scattered intensity pattern caused by the

Brownian motion of particles and is used to determine the suspended particle

size from nanometer up to a few microns.

Hydrodynamic radius of objects can then be evaluated through the

Stokes-Einstein law,

.............................(2.9)6

B

h

k TDR

where D, the diffusion coefficient of particles, KB, the Boltzmann constant,

T, the temperature, η, the dynamic viscosity of the continuous phase and Rh, the

hydrodynamic radius.

The hydrodynamic size estimation and net ζ potential measurements

were carried out in the present research using Malvern Instrument-Zetasizers

2000, UK. These measurements were performed in water medium at 25 °C.

Sodium phosphate buffer was used to maintain the pH at 7.4. The ζ potential

measurements were measured for 70 s and Smuluchowsky approximation was

applied in the calculation of surface charge of the nanoparticles.

2.3.5 Energy Dispersive X-Ray Spectroscopy

Energy dispersive X-ray spectroscopy (EDS) is an analytical technique

which is used to identify the elemental compounds of sample by atomic and

weight percent. When the sample is bombarded by the electron beam of the

SEM, electrons are ejected from the atoms on the specimen surface. A resulting

electron vacancy is filled by an electron from a higher shell and an X-ray is

36

emitted to balance the energy difference between the two electrons. The X-ray

detector measures the number of emitted X-rays versus their energy. The

energy of the X-ray is characteristic of the element from which the X-ray was

emitted. A spectrum of the energy versus relative counts of the detected X-ray

is obtained and evaluated for qualitative and quantitative determinations of the

elements present.

2.3.6 Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy (FTIR) is a technique which is

used to determine the chemical functional groups in the sample. In infrared

spectroscopy, IR radiation is passed through a sample. Some of the infrared

radiation is absorbed by the sample and some of it is passed through

(transmitted). The resulting spectrum represents the molecular absorption and

transmission, creating a molecular fingerprint of the sample. Like a fingerprint

no two unique molecular structures produce the same infrared spectrum. This

makes infrared spectroscopy useful for several types of analyses. For infrared

spectroscopy, solid samples were analyzed on Perkin-Elmer 580B IR

spectrophotometer using KBr pellet technique between 4000 and 500 cm-1 for

16 scans at a resolution of 4 cm-1.

2.3.7 Raman Spectroscopy

Raman spectroscopy is important technique to identify organic

molecules and phase of the minerals. It is similar to FTIR spectroscopy but it

has several distinct advantages. For example, it can be used to study solids,

liquids, powders, gels, slurries and aqueous solutions. This technique is based

on inelastic scattering of monochromatic light, usually from a laser source.

37

Inelastic scattering is the frequency of photons of monochromatic light

changes upon interaction with a sample. Photons of the laser light are absorbed

by the sample and then reemitted. Frequency of the reemitted photons is shifted

up or down compared to original monochromatic frequency, which is called the

Raman effect. This shift provides information about vibrational, rotational and

other low frequency transitions in molecules.

The MicroRaman measurement was carried out with an HR 800

(Jobin-Yvon) Raman microscope equipped with 1800 grooves/mm holographic

grating. He-Ne laser of 633 nm was used as an excitation source. The laser spot

size focused on the surface was approximately 0.33 μm and the depth

resolution is 0.5 μm. The output of the filtered He-Ne laser was 20 mW.

2.3.8 UV-Visible Spectroscopy

Ultraviolet-visible (UV-Vis) spectroscopy is used to obtain the

absorbance spectra of a compound in solution or as a solid. Since the

absorption of ultraviolet or visible radiation by a molecule leads transition from

ground state to exited state of the molecule, it is also often called as electronic

spectroscopy. The absorbance data from the UV-Vis measurement can be

related to the concentration of the sample by Beer’s Law.

UV-Vis absorption spectra values were recorded using Perkin Elmer

Lambda 5 UV-visible spectrophotometer in the present research. Information

collected from the scans were stored and analyzed by the UV Probe Version

2.31 software. Scans were performed in the UV and visible range, 260-800 nm,

with a 3 nm slit size. Samples were prepared by dispersing rare earth

nanoparticles in tetrahydrofuran followed by sonication for 15 minutes. An

appropriate polymer was then added to the solution, sonicated and stirred until

38

completely dissolved. Measurements were carried out in a quartz cuvette with a

10 mm path length at room temperature.

2.3.9 Photoluminescence Spectroscopy

Photoluminescence (PL) is a non-destructive optical technique used for

the characterization, investigation and detection of point defects or for

measuring the band-gaps of materials. PL involves the creation of electron-hole

pairs by incident radiation (photo-excitation) and subsequent radiative

recombination photon emission. Photo-excitation causes electrons within a

material to move into permissible excited states. When these electrons return to

their equilibrium states, the excess energy is released and may give rise to

emission of light (a radiative process) or may not (a non-radiative process). The

energy of the emitted light (photoluminescence) relates to the difference in

energy levels between the two electron states involved in the transition between

the excited state and the equilibrium state. The quantity of the emitted light is

related to the relative contribution of the radiative process.

Photoluminescence proceeds via following three steps [85]:

1) Excitation: Excitation of electrons from lower energy state to higher

energy state by absorption of energy from external sources, such as

lasers, arc-discharge lamps and xenon lamp and in this process

electron-hole pairs are created.

2) Thermalization: Excited pairs relax towards quasi-thermal equilibrium

distributions.

3) Recombination: The energy can subsequently be released, in the form of

a lower energy photon, when the electron falls back to the original

ground state. This process can occur radiatively or non-radiatively.

39

Room temperature photoluminescence spectra were recorded using

Horiba Jobin Yvon photoluminescence system comprising of Xenon lamp as

excitation source, Gemini 180 as excitation monochromator, iHR 320 as

emission monochromator and liquid Nitrogen cooled (150 K) CCD detector.

Symphony software was used to run the system. The excitation scans were

carried out in a range from 200 to 450 nm, while the emissions scans were

mostly in the visible spectrum, from 400 to 800 nm. PL spectrum was recorded

for liquid samples. Colloidal nanoparticles were placed into the quartz cuvette

with a 10 mm path length. Standard photoluminescence scan were done at 50

nm/min., using 3 nm slit width for both excitation and emission band passes.

2.3.10 Vibrating Sample Magnetometer

A vibrating sample magnetometer (VSM) was used to measure the

magnetic properties of materials. A sample was placed inside a uniform

magnetic field to magnetize the sample. The sample was then physically

vibrated sinusoidally, typically through the use of a piezoelectric material.

Commercial systems use linear attenuators of some form and historically the

development of these systems was done using modified audio speaker, though

this approach was dropped due to the interference through the in-phase

magnetic noise produced, as the magnetic flux through a nearby pickup coil

varies sinusoidally. The induced voltage in the pickup coil is proportional to

the sample’s magnetic moment but does not depend on strength of applied

magnetic field. In a typical setup, the induced voltage is measured through the

use of a lock-in amplifier using the piezoelectric signal as its reference signal.

By measuring the field of an external electromagnet, it is possible to obtain the

hysteresis curve of materials.

The magnetization measurements were performed by VSM JDM-13

vibrating sample magnetometer with applied magnetic field in the range of

40

-15 to 15 kOe. The temperature dependence of the magnetization was

monitored by warming the sample with an applied field of 100 Oe. The shape

of the sample holder was cylindrical, with a diameter of 5.5 mm and a height of

2.7 mm.