Lanthanide Doped Coreshell Nanoparticles for Solid State Lighting Applications

8/17/2019 Lanthanide Doped Coreshell Nanoparticles for Solid State Lighting Applications

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Lanthanide-doped core-shell nanoparticles for

solid state lighting applications

by

Saurabh Singh

Chemical Engineering Department

Indian Institute of Technology Kanpur

A Thesis Submitted in Partial Fulfillment of theRequirements for the Degree of

Master of Technology


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CERTIFICATE

It is certified that the work contained in the thesis entitled “Lanthanide-

doped core-shell nanoparticles for solid state lighting applications” has been

carried out by Saurabh Singh under my supervision and that it has not been

submitted elsewhere for a degree.

Dr. Sri Sivakumar

Assistant Professor

Department of Chemical Engineering

Indian Institute of Technology Kanpur

India, 208016


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ABSTRACT

There is a large interest in the production of cheap and efficient white light for the

applications such as liquid crystal display, display devices, general lighting and active

materials in laser. The optical materials that have been developed include mercury vapour

lamps; fluorescent light tubes and the latest to the field are Organic LED’s and Inorganic

LED’s which exploit electricity for the light generation. There are issues of long term

stability of emitters and low efficiency of incandescent light sources which produce light

by heating. To overcome these issues, lanthanide ions are gaining popularity these days

which can produce light by two processes 1) Up-conversion which is based on sequential

absorption of photons followed by energy transfer leading to the conversion of near

infrared photons to visible photons. 2) Down-conversion which is the conversion of UV

into visible light.

This thesis describes the incorporation of luminescent lanthanide ions-doped

nanoparticles into an inorganic matrix to improve the optical properties of lanthanide ions

and these materials can potentially be used in white light devices, optical amplifiers,

lasers, and biolabeling. Bright white light has been generated from luminescent

lanthanide ions doped LaVO4 core-shell nanoparticles through down-conversion of a

single 280 nm light source. The down-conversion mechanisms involved in the generation

of light have been discussed in detail. Preparation of luminescent lanthanide ions-doped

LaVO4 core-shell nanoparticles has been discussed and they show energy transfer from

the semiconductor matrix to the lanthanide ions.

Supervisor: Dr. Sri Sivakumar (Chemical Engineering Department)


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edicated to my

family


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I have been fortunate to have made very good friends at IIT

Kanpur and the times I have spent with them are the best in my life.

Starting with a long list, I am grateful to Tiwari , Ramu , Bhuwan , Raze

da , Piyush , Lala , Gunjan , Rehul , Bajpai , Sugga , Laddu , GMD ,

Rupesh and Babbar .

I would like to thank everybody who was important to the

successful completion of my stay at IIT Kanpur.

Last, but not least, I thank my family: my parents, father and

mother for giving me life in the first place, for educating me with

aspects from both arts and sciences, for unconditional support and

encouragement to pursue my interests, my sister Swati for giving her

sort of useful advices, for listening to my complaints and frustrations,

and for believing in me and Priyanka , my soul mate for her dedication,

support and for reminding me that my research should always be useful

and serve good purposes for all humankind.


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Table of Contents

Certificate…………………………………………………………………………….. .1

Abstract……………………………………………………………………………… ...2

Acknowledgement……………………………………………………………………. .4

List of tables……………………………………………………………………………8

List of figures…………………………………………………………………………..8

1. CHAPTER 1: General Introduction…………………………………………….10

1.1. Introduction…………………………………………………………………..11

1.2. Optical properties of lanthanide ions…………………………………………14

1.2.1. Lanthanides in the Periodic Table……………………………………..14

1.2.2. Luminescence of trivalent lanthanide ions…………………………….15

1.2.3. Phenomena of fluorescence……………………………………………19

1.2.4. Phenomena of phosphorescence…………………………………….…19

1.3. Jablonski Diagram, Lifetime and Quantum Yield……………………………20

1.3.1. Jablonski Diagram……………………………………………………..20

1.3.2. Quantum yield…………………………………………………………22

1.3.3. Lifetime………………………………………………………………..22

1.4.

Quenching process…………………………………………………………….23

1.4.1. Multi-phonon emission…………………………………………………23

1.4.2. Energy transfer between lanthanide ions……………………………….24

1.4.3. Cross relaxation.......................................................................................25

1.5 Objective……………………………………………………………………….31

2. CHAPTER 2: Synthesis of lanthanide (III)-doped nanoparticles………………33

2.1.

Introduction…………………………………………………………………….342.2. Preparation of oleic acid stabilised lanthanide doped nanoparticles …………..35

2.3. Characterization………………………………………………………………...36


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3. CHAPTER 3: Results and Discussions……………………………………………39

3.1. Characterization ………………………………………………………………...40

3.2. Eu3+

emission……………………………………………………………………45

3.3.

Tb

3+

emission……………………………………………………………………503.4. Dy

3+ Emission…………………………………………………………………..53

3.5. Tm3+

emission…………………………………………………………………..55

3.6. White light through LaVO4: 0.5% Eu3+

, 12.5% Tb3+

& 20% Tm3+

core-shell

nanoparticles……………....................................................................................57

3.7. White light through LaVO4: 0.125% Dy3+ & 20% Tm3+ core-shell

nanoparticles……………………………………………………………………. 59

3.8. Control Experiment……………………………………………………………..62

CONCLUSION…………………………………………………………………………64

REFERENCES…………………………………………………………………………66


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List of tables and figures

CHAPTER 1

Table 1.1 Important emission lines of some lanthanide ions…………………………...18

Figure 1.1 Periodic table showing the position of the elements………………………..14

Figure 1.2 Energy level diagram of Eu3+

, Dy3+

, Tb3+

& Tm3+

…………………………17

Figure 1.3 Jablonski Diagram showing …………………………………………….…..20

Figure 1.4 Energy level diagram showing energy transfer between Tb3+

and Eu3+

…....24

Figure 1.5 Cross-relaxation processes in Eu3+

& Pr 3+

………………………………….26

Figure 1.6 Scheme showing the generation of white light ……………………………..32

CHAPTER 2

Figure 2.1 Schematic for the preparation of Ln3+

: LaVO4 core-shell NP……………..36

CHAPTER 3

Figure 3.1 Structure of oleic acid………………………………………………………40

Figure 3.2 XRD scan of the sample…………………………………………………….41

Figure 3.3 FTIR spectrum of the sample ………………………………………………42

Figure 3.4 NMR spectrum of the sample ………………………………………………43

Figure 3.5 TEM image of the sample ………………………………………………….44

Figure 3.6 a) Emission Spectrum of LaVO4: Eu3+ core nanoparticles………………….46

Figure 3.6 b) Emission Spectrum of LaVO4: Eu3+

core-shell nanoparticles……………46

Figure 3.7 a) Excitation Spectrum of LaVO4: Eu3+ core nanoparticles…………………47

Figure 3.7 b) Excitation Spectrum of LaVO4: Eu3+

core-shell nanoparticles…………...47


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Figure 3.8 a) Lifetime decay curve of LaVO4: Eu3+

core nanoparticles ………………..48

Figure 3.8 b) Lifetime decay curve of LaVO4: Eu3+

core-shell nanoparticles …………49

Figure 3.9 Emission Spectrum of LaVO4: Tb3+

core-shell nanoparticles …………...….51

Figure 3.10 Excitation Spectrum of LaVO4: Tb3+

core-shell nanoparticles ……...….....52

Figure 3.11 Luminescence decay curve of Tb3+ emission……………….…...…………52

Figure 3.12 Emission Spectrum of LaVO4: Dy3+

core-shell nanoparticles …...………..53

Figure 3.13 Excitation Spectrum of LaVO4: Dy3+ core-shell nanoparticles …..……….54

Figure 3.14 Luminescence decay curve of Dy3+

emission……………….……..……….54

Figure 3.15 Emission Spectrum of LaVO4: Tm3+ core-shell nanoparticles ……..……..55

Figure 3.16 Excitation Spectrum of LaVO4: Tm3+

core-shell nanoparticles ……..….....56

Figure 3.17 Luminescence decay curve of Tm3+

emission……………….………....…..56

Figure 3.18 Scheme ……………………………………………………………..…...….57

Figure 3.19 Emission Spectrum of LaVO4: Eu3+

/Tb3+

/Tm3+

core-shell nanoparticles..…58

Figure 3.20 Excitation Spectra for all the major emissions…………………………......59

Figure 3.21 Emission spectrum of LaVO4: Tm3+

, Dy3+

core-shell nanoparticles…….....60

Figure 3.22 Excitation Spectra for 474 nm (Tm3+) & 572 nm (Dy3+) emission…….…..61

Figure 3.23 Emission Spectrum of control sample ………………………………….. ...62


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Chapter 1

General Introduction


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1.1 Introduction

Most of the lanthanide ions were discovered in the early 19th

and some in the 20th

century,[1]

but since this fairly recent discovery the technological importance of the ions

has been growing rapidly. Although they are called rare earth ions, they are not as rare as

the name would suggest the ions are abundant in the earth’s crust, but they do not have

the tendency to form concentrated ore deposits.[2]

A wide variety of minerals, which can

be found on a few places in the world, do contain rare earth elements at relatively high

concentration, in different compositions.[3]

The lighter ions have a higher abundance in

these ores and consequently have lower prices. The ions have a wide variety of

technological importance such as in permanent magnets, catalysis, batteries, and optics.[4]

The optical properties of lanthanide ions became important when techniques were

developed to separate the different lanthanide ions to high purity. Cathode ray tubes of

computers and colour televisions use europium as the red phosphor [5]

and in fiber optic

telecommunication, erbium ions are used in laser amplifiers to enhance optical signals.[6]

There is a large interest in the production of cheap and efficient white light for the

applications such as liquid crystal display (LCD), display devices, general lighting and

active materials in laser. The optical materials that have been developed include mercury

vapour lamps, fluorescent light tubes and the latest to the field are Organic Light

Emitting Diodes (OLED’s)[7]

, Inorganic Light Emitting Diodes (ILED’s)[8]

and Ploymer

Light Emitting Diodes (PLED’s) which exploit electricity for the light generation. After

being introduced in 1879 by Thomas Alva Edison, incandescent light sources which


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produce light using electricity have still very low efficiencies (10-12%)[9],[10]

. Later on,

mercury vapour lamps and fluorescent light tubes were developed which still in use

having low efficiencies (~28). Source of emission used in fluorescent tubes are

lanthanide ions because of their stable photocycle, increased brightness, and high

quantum yield. To overcome the issues of efficiency, organic light-emitting diodes

(OLEDs) and polymer light-emitting diodes (PLEDs)[11]

are being developed. However,

there are issues of long term stability of emitters, high fabrication costs, low

photostability and bias dependent colour variation which limits the use of

OLEDs/PLEDs. To overcome these issues, lanthanide ions are gaining popularity these

days which can produce light by two processes 1) Up-conversion[12]

which is based on

sequential absorption of photons followed by energy transfer leading to the conversion of

near infrared photons to visible photons. 2) Down-conversion[13]

which is the conversion

of UV into visible light. White light for these uses can also be generated using various

combinations of different lanthanide ions in different concentrations. Generation of white

light by different combinations of various lanthanide ions is the subject of this thesis.

The Ln3+

ions used in the experiments are extremely stable, because no chemical

bonds are involved so degradation does not occur. The solubility of the ions in organic

materials is low due to which they can’t be processed further via spin coating technique

and coating it over LED of the required range. One way to solve this problem is by the

synthesis of lanthanide complexes, which are soluble in organic materials. However, a

disadvantage of the use of these complexes is that the good optical properties these

lanthanide ions have in inorganic materials are largely reduced in the organic


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complexes.[14]

The high vibrational energies of the chemical bonds in organic complexes

are efficient quenchers of lanthanide luminescence. In case of an organic matrix,

vibrational energy of the bonds is sufficient enough to quench the luminescence of the

lanthanide ion so the preferred matrix for doping is an inorganic one with the addition of

a ligand to make the product dispersible in organic solutions. In order to use lanthanide

ions in an inorganic matrix, they have to be shielded from the organic environment in

order to have good luminescence properties.

Light can be generated from the lanthanide ions by either by up-conversion or by

down-conversion by incorporating the lanthanide ions into an organic/inorganic matrix

and then exciting the sample with IR/UV light depending on the type of process we are

using for light generation. Luminescent NPs that are soluble in organic solvents attract a

great deal of interest because these materials can be easily processed via spin-coating

techniques to form a uniform film from an organic solution. Among the known

luminescent materials, Ln3+ - doped inorganic materials offer a range of compounds with

unique versatility.

Bright white light has been generated from luminescent lanthanide-doped core-

shell LaVO4 nanoparticles through down-conversion of a single 280 nm light source. The

down-conversion mechanisms involved in the generation of light has been discussed.

Preparation of luminescent lanthanide ions-doped LaVO4 core-shell nanoparticles has

been discussed and they show energy transfer from the semiconductor matrix to the

lanthanide ions.


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1.2 Optical properties of lanthanide ions

1.2.1 Lanthanides in the Periodic Table

Rare Earth Elements: Series of chemical elements of the periodic table. The rare earth

elements (or rare earth metals) include the elements with atomic numbers 57 through 71,

and, named in order, are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium

(Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),

dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and

lutetium (Lu). Yttrium (Y, atomic no. 39) and scandium (Sc, atomic no. 21) are also

included in the group of rare earth elements. The elements cerium (Ce, atomic no. 58)

through lutetium (Lu, atomic no. 71) is commonly known as the lanthanide series.

F ig. 1.1 Periodic table showing the position of the elements

Oxides of the rare earth elements are called rare earths, and are found in minerals

that are actually more abundant than those of some other metals, such as those in the

platinum group. The principal source of rare earths is the mineral monazite. Some other


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rare minerals that also contain small amounts of rare earths include cerite, gadolinite, and

samarskite.

The lanthanides usually exist as trivalent cations, in which case their electronic

configuration is (Xe) 4fn, with n varying from 1 (Ce

3+) to 14 (Lu

3+). The transitions of

the f-electrons are responsible for the interesting photo physical properties of the

lanthanide ions, such as long-lived luminescence and sharp absorption and emission

lines. The f-electrons are shielded from external perturbations by filled 5s and 5p orbitals,

thus giving rise to line-like spectra.

The f-f electronic transitions are forbidden, leading to long excited state lifetimes,

in the micro- to millisecond range. The forbidden nature of the f-f transitions is also

reflected in low extinction coefficients, making direct photo-excitation of lanthanide ions

difficult. This can be overcome by using energy transfer from organic chromophores to

lanthanide ions.

1.2.2 Luminescence of tr ivalent lanthanide ions

The lanthanides are the elements following lanthanum in the periodic table. In this

range of elements the 4 f shell is successively filled. Since the valence electrons are the

same for all the ions, they all show very similar reactivity and coordination behavior.[4]

Few ions in the series also show luminescence in the divalent state (Eu2+

, Sm2+

).[15]

Since

luminescence of the trivalent lanthanide ions arises from transitions within the 4 f shell

and because this shell is shielded by filled 5 s and 5 p shells, the absorption and emissions

of the ions are only slightly affected by the environment. The transitions within the 4 f


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state are parity forbidden, but due to mixing with allowed transitions, like the 4 f -5d

transitions, they do occur. As a result of the forbidden character, absorption coefficients

are low and luminescence lifetimes are long, ranging from microseconds up to several

milliseconds. Energy level diagram of the four lanthanide ions used in the synthesis for

white light generation is shown here. Three lanthanide ions which do not show any

emission in visible spectrum are La3+

, Lu3+

& Ce3+

. La3+

and Lu3+

have a completely

empty and a completely filled 4 f shell, respectively, and therefore have no optical

transitions and Ce3+

has one electron and one 4 f level just above the ground state. Ce3+

has

the lowest oxidation potential of the lanthanide ions making the allowed 4 f -5d transitions

possible in the UV.

The energy levels are denoted as(2S+1)

ГJ (Russel-Saunders notation), where S is

the spin multiplicity, Г the orbital angular momentum, and J the total angular momentum.

Due to the effective shielding of the 4 f electrons, the crystal field has almost no effect on

the energy of the levels. For this reason this energy level diagram can be used for

lanthanide ions in all sorts of host materials. In principle this could lead to very similar

emission and absorption spectra for the same lanthanide ion in a range of different hosts,

but symmetry and quenching will have an effect on the emission properties as discussed

further in this chapter. The selection rules for the different transitions are influenced by

the symmetry of the environment. The nature of the transitions varies from pure magnetic

dipole transitions to pure electric dipole transitions and mixtures of the two.


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Figure 1.2 Energy level diagram of Eu3+

, Dy3+

, Tb3+

& Tm3+

Y3+ is usually also treated as a lanthanide ion, because of similar reactivity and

coordination behaviour. This ion also has no optical transitions, but luminescent

lanthanide ions are often incorporated in host materials with Y3+ ions.

The emission spectrum of the Eu3+ ion is strongly influenced by the symmetry of

the surroundings. The main emissions of this ion occur from the5D0 to the

7FJ (J = 0-6)

levels. The

5

D0→

7

F1

transition is a pure magnetic dipole transition, which is practically

independent of the symmetry of the surroundings and the strength can be calculated

theoretically. The transitions to the7F0, 3, 5 levels are forbidden both in magnetic and

electric dipole schemes and are usually very weak in the emission spectrum. The


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remaining transitions to the7F2, 4, 6 levels are pure electric dipole transitions and they are

strongly dependent on the symmetry of the environment. In a crystal site with inversion

symmetry the electric dipole transitions are strictly forbidden and the5D0→

7F1 transition

is usually the dominant emission line. In a site without inversion symmetry the strength

of the electric dipole transitions is higher. The 5D0→7F2 transition is usually the strongest

emission line in this case, because transitions with ΔJ = ±2 are hypersensitive to small

deviations from inversion symmetry.[16] The symmetry around the lanthanide ion can thus

be obtained from the shape of the emission spectrum of the Eu3+

ion. The other lanthanide

ions have transitions that are usually mixtures of electric and magnetic dipole transitions

and the effects of the symmetry are less pronounced.

Table 1.1: Important emission lines of some lanthanide ions.

Ion Transition Wavelength (nm) Application

Pr + G4→ H5 1300 Optical amplifier

Nd F3/2→ I11/2 1064 Solid state lasers

Eu+ D0→ F2 615 Displays, lighting

Tb + D4→ F5 545 Lighting

Dy + F11/2 + H9/2→ H15/2 1300 Optical amplifier

Er + I13/2→ I15/2 1530 Optical amplifier

Tm H4→ F4 1480 Optical amplifier

Yb+ F5/2→ F7/2 980 Sensitizer


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The symmetry also has an influence on the radiative lifetime of the5D0 level. The

radiative lifetime is the time for the luminescence to drop to 1/e in intensity in absence of

quenching. In the case of a Eu3+

ion without inversion symmetry the rate of the forced

electric dipole transition is higher than in the case of a Eu3+

ion with inversion symmetry.

This automatically means that the radiative lifetime of a Eu3+ ion in a site with inversion

symmetry is longer.

1.2.3 Phenomena of f luorescence

Luminescence is the emission of light from excited states of any substance.

Luminescence is divided in to two types – fluorescence and phosphorescence, which

depends on the nature of excited states.

Fluorescence – emission of light when an electron in excited singlet state returns

to the ground state where it is paired to another electron of opposite spin. Return to the

ground state is spin allowed and occurs rapidly by emission of a photon, thus

fluorescence has a very short lifetime (~ 10 ns).

1.2.4 Phenomena of Phosphorescence

Phosphorescence – emission of light from triplet excited state where electrons in

excited and ground state have the same spin and thus transition is spin forbidden leading

to low emission rates and consequently longer lifetimes of the order of seconds.

Phosphorescence is usually not seen in fluid solutions at room temperature because of

many deactivation processes that compete with emission, such as non-radiative decay and

quenching processes. Fluorescence typically occurs from organic molecules, e.g. quinine.


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1.3 Jablonski Diagram, L if etime and Quantum Yield

13.1 Jablonski Diagram

The processes that usually occur between the absorption and emission of light are

illustrated by Jablonski diagram.

F ig. 1.3 Jablonski Diagram showing molecular processes involved in excited states

A typical Jablonski diagram is shown in Figure 1.3. The singlet ground, first, and

second electronic states are depicted by S0, S1, and S2, respectively. At each of these

electronic energy levels the fluorophores can exist in a number of vibrational energy

levels, depicted by 0, 1, 2, etc. In this Jablonski diagram a number of interactions such as

quenching, energy transfer, and solvent interactions have been excluded. The transitions


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between states are depicted as vertical lines to illustrate the instantaneous nature of light

absorption. Transitions occur in about 10-5

s, a time too short for significant displacement

of nuclei. This is the Franck-Condon principle. At room temperature thermal energy is

not adequate to significantly populate the excited vibrational states.

Absorption and emission occur mostly from molecules with the lowest vibrational

energy. The larger energy difference between the S0 and S1 excited states is too large for

thermal population of S1. For this reason light is used and not heat to induce fluorescence.

Following light absorption, several processes usually occur. A fluorophore is usually

excited to some higher vibrational level of either S 1 or S2. With a few rare exceptions,

molecules in condensed phases rapidly relax to the lowest vibrational level of S1. This

process is called internal conversion and generally occurs within 10-12

s or less. Since

fluorescence lifetimes are typically near 10-8

s, internal conversion is generally completed

prior to emission. Hence, fluorescence emission generally results from a thermally

equilibrated excited state, that is, the lowest energy vibrational state of S1. Return to the

ground state typically occurs to a higher excited vibrational ground state level, which

then quickly (10-12

s) reaches thermal equilibrium (Figure 1.3). Return to an excited

vibrational state at the level of the S0 state is the reason for the vibrational structure in the

emission spectrum of perylene.

An interesting consequence of emission to higher vibrational ground states is that

the emission spectrum is typically a mirror image of the absorption spectrum of the S 0

S1 transition. This similarity occurs because electronic excitation does not greatly alter

the nuclear geometry. Hence the spacing of the vibrational energy levels of the excited


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states is similar to that of the ground state. As a result, the vibrational structures seen in

the absorption and the emission spectra are similar. Molecules in the S1 state can also

undergo a spin conversion to the first triplet state T1. Emission from T1 is termed

phosphorescence, and is generally shifted to longer wavelengths (lower energy) relative

to the fluorescence. Conversion of S1 to T1 is called intersystem crossing. Transition from

T1 to the singlet ground state is forbidden, and as a result the rate constants for triplet

emission are several orders of magnitude smaller than those for fluorescence. Molecules

containing heavy atoms such as bromine and iodine are frequently phosphorescent. The

heavy atoms facilitate intersystem crossing and thus enhance phosphorescence quantum

yields.

1.3.2 Quantum Yield

Quantum yield of a fluorophore can be defined as the ratio of the number of photons

emitted to the number of photons absorbed.

1.3.3 Lifetime

Lifetime of an excited state is defined by average time the molecule spends in excited

state prior to return to the ground state. Generally fluorescence lifetimes are near 10 ns.

Lifetime of a fluorophore is given by the equation:

where, Γ is the emissive rate of the fluorophore and K nr is the rate of non-radiative decay


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The lifetime of a fluorophore in the absence of non-radiative processes is called the

intrinsic lifetime.

1.4 Quenching processes

1.4.1 Multi -phonon quenching

Non-radiative processes can also play an important role. The energy of the excited

state can be taken up by the surroundings in the form of vibrational energy, often referred

to as phonon emission. The effectiveness of this process depends on the availability of

high-energy vibrations in the surroundings and the energy difference between the energy

levels of the lanthanide ion. The fundamental vibrations of the chemical bonds in the

surroundings and the energy of the vibration are determined by the reduced mass of a

bond. Especially bonds with hydrogen have a small reduced-mass and therefore high

vibrational energies. These bonds are therefore able to take up large amounts of energy

and effectively quench lanthanide ions with large separations between the energy levels.

The visible emitting ions Eu3+

and Tb3+

have large gaps between the emissive5D0 and

5D4

level of 12,000 and 15,000 cm-1

, respectively, but still these ions and especially Eu3+

are

quenched substantially when the ions are dissolved in water (vibrational energy: νmax

3500 cm-1

). The quenching efficiency is strongly dependent on the number of vibrational

quanta that are needed to bridge the gap between the lowest emitting level and the highest

non-emitting level of the lanthanide ion. The observation of luminescence of a lanthanide

ion in solvents with high vibrational energies (water) is dependent on the energy

difference between the lowest radiative level and the highest non-radiative level. For


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example, Tb3+

in water shows reasonable luminescence but Eu3+

luminescence is almost

completely quenched. The Eu3+

ion is quenched by energy transfer to the 4th

overtone of

the OH bonds, while the Tb3+

ion is quenched by energy transfer to the 5th overtone of the

OH bond. Another important factor governing the efficiency of quenching is the distance

between the lanthanide ion and the quencher group. Quenching occurs through a dipole-

dipole interaction in a Förster-type mechanism.[17]

In this Förster mechanism the

interaction between the lanthanide ion (donor) and the quenching site (acceptor) only

occurs when the energy levels of the donor and acceptor are resonant. The donor and

acceptor do not have to have overlap of their wave functions, but the process is strongly

distance dependent. For dipole-dipole interactions the rate of quenching has a distance

dependence of r -6

.[18]

1.4.2 Energy transfer between lanthanide ions

Fig. 1.4 Energy level diagram showing energy transfer between Tb3+ and Eu3+.


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Another factor in the quenching of lanthanide ions is the interaction between the

lanthanide ions, of the same or different type. Two different lanthanide ions can transfer

energy when they have similar separations between the energy levels. The small

mismatch in energy can be compensated for by the emission or uptake of a phonon.

Energy transfer of one lanthanide ion can be used to enhance luminescence of the other

lanthanide ion. For example, the lanthanide couple Tb3+

- Eu3+

, when excited at


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F ig. 1.5 Cross-relaxation processes in Eu3+

& Pr 3+

Energy migration is another form of cross-relaxation between two ions of the

same sort. The excited state energy levels of two identical ions are resonant, so the

energy can be transferred to the neighbouring ion by cross-relaxation and travel through

the material hopping from one ion to the other. An increase in the doping concentration

leads to a faster energy migration through the material, making the chance of meeting a

quenching site higher. For reasons of cross-relaxation and energy migration, high doping

concentrations often lead to a decrease in luminescence intensity and luminescence

lifetime.

It is known that the luminescent spectra of Ln3+

-doped nanocrystals (and Ln3+

-

doped materials in general) vary little from host to host.[20]

However, the crystal field


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exerted on the Ln3+

dopant ions by the host (nano) matrix plays a pivotal role in

determining transition probabilities, lifetimes of the excited states, luminescence

efficiency, as well as energy transfer efficiency.[21]

In this thesis, we focus mainly on lanthanide-doped LaVO4 nanoparticles and

study their optical properties. Many research groups have investigated the optical

properties of lanthanide ions in detail in various nanoparticles matrices such as

oxides[22],[23]

, fluorides[24],[25]

, phosphates[26]

, vanadates[27],[28]

and semiconductor

nanoparticles[29],[30].

Capobianco and his co-workers have investigated the effect co-doping of Yb3+

ion

with Tm3+

and Er 3+

ions in up-conversion of Lu3Ga5O12 nanocrystals[31]

. They prepared

the nanocrystals by a simple sol-gel method exhibiting bright white light following

excitation with lower energy near-infrared light ( λexc) 980 nm) via an upconversion

process. The combination of upconverted blue (from Tm3+

), green, and red (from Er 3+

)

emissions resulted in the white luminescence, which is intense and visible to the naked

eye at a laser power less than 30 mW (3.4 W/cm2). The calculated CIE colour coordinates

fell within the white region and changed only very little with the incident pump power.

These two significant characteristics in combination with the thermal stability of the

lutetium garnets made this material an ideal candidate for the development of white light

based lasers and LEDs.


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They have also studied the up-conversion properties of effect of doping of Er 3+

ions in Gd3Ga5O12[32]

at different concentration (1% and 5%). Green emission from the

(2H11/2,

4S3/2)

4I15/2 and red emission from the

4F9/2

4I15/2 transitions were observed

following excitation with 800 nm into the4I9/2 state. They determined that up-conversion

occurred via excited-state absorption in the 1% sample while energy transfer up-

conversion took over as the dominant mechanism as the concentration was increased to

5%. An enhancement of the red (4F9/24I15/2) emission was observed and hypothesized

to occur via the concentration dependent (4I9/2,

4I11/2) (

4I13/2,

4F9/2) ion pair process,

which directly populated the 4F9/2 state.

Sivakumar et al. have demonstrated the generation of white light through Up-

conversion of a Single NIR Source from Sol-Gel-Derived Thin Film Made with Ln3+

-

Doped LaF3 Nanoparticles.[33]

White light was generated from SiO2, ZrO2 sol-gel thin

film made with Ln3+

-doped nanoparticles co-doped with Yb3+

ions. They also prepared

the core-shell samples to increase the lifetime values by surrounding a doped LaF 3 core

with an undoped LaF3 shell.[34]

Patra et al. have reported the bright white light emission from Eu3+ doped In2S3

nanoparticles by single wavelength light excitation (350 nm).[35]

They found that the

Energy Transfer efficiency from In2S3 nanoparticles to Eu3+ increases from 0.27% to

0.42% with increasing dopant concentration with quantum efficiency 24.2% for 1.0

mol% Eu doped In2S3 nanoparticle and the CIE coordinates 0.27 and 0.29, which fall

within the white region of the 1931 CIE diagram.

A highly efficient blue-green-emitting cationic iridium complex [Ir(dfppz)2(tp-

pyim)]PF6, which contains a bulky side group in the ancillary ligand, has been


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synthesized and fully characterized. LECs based on [Ir(dfppz)2(tppyim)] PF6 showed

highly efficient blue-green electroluminescence. White LECs based on [Ir(dfppz)2(tp-

pyim)]PF6 showed warm white light, with CIE coordinates of (0.37, 0.41).[36]

Herland et al. demonstrated the use of self-assembled bionanostructures in

polymer light-emitting diodes. Amyloid fibrils formed by protein misfolding were

decorated with a soluble luminescent conjugated polymer.[37]

This conjugated polymer

complex with amyloid fibrils was used as the active layer in a light emitting diode,

resulting in a 10-fold increase in external quantum efficiency compared with pristine

polymer, because of improved carrier injection.

They have also shown that a negatively charged conjugated polyelectrolyte can be

used as a novel optical probe for the detection of the formation of amyloid fibrils. The

formation of amyloid fibrils reflected as an alteration of the geometry and the electronic

structure of the bound polyelectrolyte chains has so far been detected by absorption and

emission, but electrical detection of these transitions will most likely be possible. Their

method is fast and simple and is based on noncovalent assembly between the anionic

polyelectrolyte and the protein. They suggested that their method may be used for a wide

range of proteins, biosensors, and bioelectronic devices.[38]

Chung Hsu et al. developed a novel series of blue and yellowish-green light-

emitting single polymers by end-capping of low contents of 4-bromo-7H-benzo

[de]naphtha[2’,3’:4,5] imidazo[2,1-a]isoquinolin-7-one into polyfluorene.

Electroluminescence (EL) spectra of these polymers exhibit blue emission (λ max =

430/460 nm) from the fluorene segments and yellowish-green emission (λ max = 510/530

nm) from the 4-bromo-7H-benzo [de]naphtha [2’,3’:4,5] imidazo[2,1-a]isoquinolin-7-one


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units. For the polymer (PFNAP-0.06) with the 4-bromo-7H-benzo [de]naphtha [2’,3’:4,5]

imidazo[2,1-a]isoquinolin-7-one unit content of 0.06 mol%, its EL spectrum shows

balanced intensities of blue emission and yellowish-green emission with Commission

Internationale de l’Eclairage (CIE) coordinates of (0.25, 0.34).

A new white polymer-light-emitting-diode (WPLED) can be developed from the

single polymer (PFNAP-0.06) system blended with a red phosphorescent iridium

complex [Bis(2-[20-benzothienyl)-pyridinato-N,C30] iridium (acetylacetonate) (BtpIr)].

They were able to obtain a white-light-emission device by adjusting the molar ratio of

BtpIr to PFNAP-0.06 with a structure of indium tin oxide (ITO)/poly(3,4-

ethylenedioxythiophene): poly(styrene sulfonic acid) [PEDOT:PSS]/PVK/emission

layer/Ca/Ag. The brightness in such a device configuration is 4030 cd/m2

at 9 V with CIE

coordinates of (0.32, 0.34).[39]

Hung Lu et al. demonstrated a high-efficiency white polymer light-emitting diode

(WPLED) based on carbazolegrafted poly(para-phenylene) (CzPPP) doped with green-

and red-emitting Ir complexes as the emitting layer. The device exhibits pure white light

yet with stable sharp blue (430 nm), green (512 nm), and red (613 nm) emissions and has

the aximum current and external quantum efficiencies of 16.8 cd/A and 8.65%,

respectively. The overall performance is resulted from the excellent shielding effect of

CzPPP host to prevent back transfer of triplet energy especially from the green

phosphor.[40]

Dong Ba and his co-workers demonstrated upconversion (UC) white light hybrid

thin films containing Ln3+

-tridoped (Yb3+

, Er 3+

and Tm3+

) NaYF4 nanoparticles and

poly(vinyl pyrrolidone) (PVP, M(w) approximately 1300000) prepared by a spin-coating


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method and characterized by X-ray diffraction (XRD), field emission scanning electron

micrograph (FE-SEM) and Fourier transform infrared spectra (FT-IR).[41]

They generated

white light by two different lanthanide ions, Er 3+

(red and green) and Tm3+

(blue) under

excitation by a 980-nm laser diode. They modified PVP to the UC populating processes,

the color stability of the white light in the hybrid films was remarkably improved.

They also demonstrated white light-emitting thin films containing Ln3+

-doped

NaYF4 nanoparticles prepared by a simple spin-coating method. They generated white

light by using two different lanthanide ions, Er 3+

(red and green) and Tm3+

(blue), by

upconversion process under the excitation of a 980-nm laser diode. [42] The ratio of the

intensity of the three main emissions was tuned by controlling the concentration of the

nanoparticles in the thin film and the concentration of the lanthanide ions in the

nanoparticles. The color coordinates corresponding to emissions of different nanoparticle

concentrations and with the different pump powers were investigated. When the pump

power was fixed at 900 mW, the thin film with a concentration ratio of 2.5:1 emitted pure

white light with coordinates of (0.333, 0.339).

1.5 Objective

The goal of this thesis was to prepare bright white light emitting nanoparticles via

down-conversion of UV-light prepared by doping LaVO4 with various luminescent

lanthanide ions and are dispersible in organic solvents like chloroform, etc. Core-shell

approach was used as shown in the figure 1.6, to separate the various lanthanide ions

from each other as well as outer environment so as to prevent different types of


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quenching processes described earlier in this chapter. We successfully prepared such

nanoparticles followed by their characterization using various techniques.

Fig. 1.6 Scheme showing the generation of white light


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Chapter 2Synthesis of lanthanide(III)-doped nanoparticles


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2.1 I ntroduction

The luminescence of lanthanide ions has found important commercial

applications in displays,[43]

optical amplifiers,[44]

and lasers,[45]

as described in chapter 1.

There is a growing interest to use this luminescence in polymer-based materials, because

of the easy processing of polymers and ease of integrating different optical components.

The luminescence of these lanthanide ions arises from transitions within the 4 f electron

shell. These transitions are parity forbidden, leading to low absorption cross-sections and

long luminescence lifetimes. However, this long-lived excited state can be quenched very

efficiently in the presence of the high-energy vibrations of organic solvents, polymers, or

ligands, thus hampering the application in polymer-based devices. Therefore, to use

lanthanide luminescence in an organic environment it is important to shield the

lanthanide ion from the organic surroundings. Shielding of the lanthanide ion can be

achieved by doping it in the inorganic part of nanoparticles that should still be soluble in

organic solvents.

Good solubility is generally achieved by having organic groups on the outside of

the nanoparticles. Most nanoparticles doped with lanthanide ions are made in high

temperature procedures leading to nanoparticles without organic groups on the surface

and, therefore, they have no solubility in solvents.[46] Only very few examples of these

lanthanide-doped nanoparticles are there that have a good solubility in organic solvents

have been reported.[47] Nanoparticles with dimensions of a few nanometers are small

enough to minimize scattering in polymer films.[48]

The largest contribution to scattering

in polymer films originates from particles with a size comparable to the wavelength of


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light, or bigger (Mie scattering). Nanoparticles have a size much smaller than the

wavelength of light and when clustering of the nanoparticles in a polymer film is

prevented, scattering should be minimal.

All the lanthanide salts were obtained from Sigma-Aldrich. Ethanol was obtained

from Merck’s chemicals and chloroform, oleic acid and tri-ethyl amine were obtained

from Qualigens. We used all the chemicals and salts without any further purification.

Distilled water was applied for all synthesis and treatment processes.

2.2 Preparation of oleic acid stabilised lanthanide doped core-shell

nanoparticles

A solution of distilled water – ethanol (1:1) mixture (70 ml) with Oleic acid (2 ml)

was prepared and was allowed to heat to 750C with the pH of the solution was adjusted to

6 by the addition of (CH3)3 N (30% w/v). Stoichiometric amount of Na3VO4 was added to

the solution, followed by the addition of stoichiometric amounts of the nitrate salts of

lanthanide ions (99.99%) dissolved in 2 ml of water and added drop wise. For Tm3+

, Tb3+

& Eu3+

doped LaVO4 core-shell nanoparticles, Tm3+

doped La(NO3)3 makes the core,

followed by the addition of undoped La(NO3)3 shell which will protect Tm3+

from the

interaction with other lanthanide ions. Then Tb3+

doped La(NO3)3 shell is added followed

by undoped La(NO3)3 shell with the subsequent addition of Eu(NO3)3 doped La(NO3)3

and undoped La(NO3)3 shell.


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F ig. 2.1 Scheme for the preparation of white light emitting Ln3+

: LaVO4 core-shell NP

Formation of precipitate indicates the completion of reaction after two hours of

stirring and heating at 750C. The solution was then cooled to room temperature for

another two hours then precipitate formed was collected by centrifuge at 5000 rpm,

washed with water and ethanol, and dried under ambient conditions. After drying the

particles were dispersible in chloroform.

2.3 Character ization

The down conversion fluorescence analysis was done using FLSP920 Edinburgh

Instrument. The excitation source used was 450W Xe arc lamp and the detector was a


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Red-sensitive Peltier element cooled Hamamatsu R928-P PMT. For recording decay

curves μF-flash lamp was used. Sample was prepared by dissolving the precipitate

obtained earlier in to chloroform (CHCl3) followed by the excitation with 280 nm UV

light.

The formation of core-shell structure was visualized from Transmission Electron

Microscope (TEM) images obtained using FEI Technai Twin microscope with high

contrast and resolution at 20kV to 120kV. Sample for TEM was prepared by dissolving

the precipitate in CHCl3, followed by the sonication of the solution. The sonicated

solution was then loaded on the copper coated TEM grid which was further used.

NMR: The NMR graph was recorded using Bruker AC 300 instrument. The basic

frequency for1H nucleus is 500 MHz. Sample was prepared by dissolving the precipitate

in CDCl3 not in CHCl3 to prevent C-H peak in the NMR spectrum.

XRD analysis: Approximately 20-25 mg of the sample was powdered in an

alumina mortar to break up lumps. The powder was smeared on to a zero-diffraction

quartz plate using ethanol. Step-scan X-ray powder-diffraction data were collected over

the 2θ range 3 - 100° with CuKα (40 kV, 40 mA) radiation on a Siemens D5000 Bragg-

Brentano θ-2θ diffractometer equipped with a diffracted-beam graphite monochromator

crystal, 2 mm (1°) divergence and anti-scatter slits, 0.6 mm receiving slit, and incident

beam Soller slit. The scanning step size was 0.04°2θ with a counting time of 1.5 s/step. X-

ray powder diffraction data for different phases were refined with the Rietveld program

Topas 2.1 from Bruker using the fundamental parameters approach.


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FTIR: The Fourier Transform Infra Red (FTIR) spectra were obtained from Vertex

70V spectrometer using KBr pellet technique. Sample preparation was similar to that

done for fluorescence characterization.


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Chapter 3

Results and Discussions


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3.1 Character ization

LaVO4 nanoparticles were synthesized using a procedure described in chapter 2.

Oleic Acid is depicted in Figure 3.1 was used as a ligand for the synthesis of the

lanthanide-doped luminescent nanoparticles. To a solution of Na3VO4 in a water/ethanol

mixture was added drop wise a solution of the La(NO3)3 salts in water. The oleate head-

group of the ligand coordinates weakly to the lanthanide ions allowing the growth of the

nanoparticles, but coordinates strongly enough to prevent the nanoparticles from

aggregating. The 1H NMR spectrum of the nanoparticles obtained after the synthesis

shows broadened signals of the ligand due to the co-ordination of the ligand to the

nanoparticle surface. This broadening can be ascribed to the inhomogeneous distribution

of the magnetic environment around the nanoparticle and a reduction in rotational degree

of the ligand. No free ligand is present because no sharp signals are observed for unbound

ligand in the solution. Peak of oleate at 5.2 ppm of the protons on the double bond is

observed.

Fig. 3.1 Structure of oleic acid used as a ligand for the synthesis of LaVO4 nanoparticles


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The X-ray powder diffraction pattern of LaVO4 was refined in space group D19

4h with the

Rietveld refinement method using the fundamental parameters approach.

Figure 3.2 Rietveld refinement plot of LaVO4: Eu, Tm & Tb nanoparticles

The pattern fits well with the xenotime structure of LaVO4, in contrast to the

monazite structure that is generally observed for bulk LaVO4. The sharp peaks in XRD

pattern indicate the good crystallinity of the sample. The difference in crystal structure of


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this nano crystalline LaVO4 and bulk LaVO4 is most likely a result of the increased

amount of surface ions in these materials.

FTI R Spectrum

Figure 3.3 FTIR spectrum of the sample dissolved in CHCl 3

FTIR spectrum of the sample was carried to prove the presence of oleic acid

molecules. Absorbance peak at 789.40 cm-1

is the characteristic peak of the tetrahedral

VO43- (range is 780 – 920 cm-1). Due to La – O vibration, there is a small peak at 442.11

cm-1

. For m-LaVO4, absorbance peak are split in to 3 peaks while for that of t-LaVO4


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phase there is only one peak near 800 cm-1

which indicates that the sample prepared is in

t-LaVO4 phase. Peaks at 2924 cm-1

and 2853 cm-1

are assigned to anti-symmetric and

symmetric methylene stretches (Vas(CH2)), (Vs(CH2)) of the oleic acid molecule.

Sharpness of the peaks indicates that the hydrocarbon chains are well arranged. Peak at

3006 cm-1 is due to V(C-H) made of the C-H bond adjacent to the C=C bond. 2500 cm -1 –

3500 cm-1

is assigned to (O-H) stretch which is visible at 3414 cm-1

. It also imply that

CH=CH group of oleic acid does not interact with oleic acid surface thus, the oleic acid

molecules are adsorbed on the LaVO4 nanoparticles.

NMR Spectrum.

Figure 3.4 NMR spectrum of the sample dissolved in CDCl 3


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The1H NMR spectrum of the nanoparticles obtained after the synthesis shows

broadened signals of the ligand due to the co-ordination of the ligand to the nanoparticle

surface. This broadening can be ascribed to the inhomogeneous distribution of the

magnetic environment around the nanoparticle and a reduction in rotational degree of the

ligand. No free ligand is present because no sharp signals are observed for unbound

ligand in the solution. Peak of oleate at 5.2 ppm of the protons on the double bond is

observed.

TEM Image

F ig. 3.5 TEM image of the sample


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TEM image of the sample shows the agglomeration of the nanoparticles with typical size

around 20 nm.

3.2 Eu 3+ emission

5% Eu3+

: LaVO4 core and core-shell nanoparticles were prepared following the

method discussed in chapter 2. The Eu3+

emission originating from the5D0, and

5D1

levels was observed after excitation at 280 nm for LaVO4: Eu. The LaVO4: Eu

nanoparticles can be excited in a charge transfer band. In the LaVO 4 nanoparticles the

charge transfer is in the V-O bond. Deactivation of the charge transfer state leads to an

excited Eu3+ ion. The LaVO4 nanoparticles have the advantage that the charge transfer

band of the VO4 group is independent of the doping ion, making it possible to excite

almost all lanthanide ions using the charge transfer band of the VO 4 group. The ratio of

the different peaks of the5D0→

7F j (J = 1, 2) transitions in the Eu

3+ emission spectrum

gives information about the symmetry of the crystal site in which the ion is located. In the

LaVO4 nanoparticles the Eu3+

ion has no inversion symmetry, so the5D0→

7F2 transition

is clearly the dominating emission band.

In LaVO4: Eu nanoparticles the5D1 emission could be observed, but due to multi-

phonon relaxation the emissions were very weak. The emission of the 5D1 level is

however strongly concentration dependent. A cross-relaxation process as described in

chapter 1 is responsible for quenching of the 5D1 emission at higher concentrations of

Eu3+

. Difference in the emission and excitation spectra of core and core-shell samples can

be seen easily.


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Fig. 3.6 a) Emission Spectrum of LaVO4: Eu3+ (5%) core nanoparticles with 280 nm UV excitation and 395 nm filter

3.6 b) Emission Spectrum of LaVO4: Eu3+ (5%) core-shell nanoparticles with 280 nm UV excitation and 395 nm filter


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Fig. 3.7 a) Excitation Spectrum of LaVO4: Eu3+ (5%) core nanoparticles for 612nm emission using 395 nm filter

3.7 b) Excitation Spectrum of LaVO4: Eu3+ (5%) core-shell nanoparticles for 612nm emission using 395 nm filter


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As can be seen from the excitation spectra for the two nanoparticles, there is an energy

transfer band from LaVO4 from 250 nm to 350 nm along with a small peak at 395 nm

which is due to self excitation of Eu3+

. As can be seen energy transfer is more effective in

case of core-shell sample as compared to core sample as well as charge transfer from CT

VO4 is more efficient as compared to the self excitation of Eu3+

ions.

Luminescence lifetime: The luminescence lifetime is an important parameter indicative

of the efficiency of the luminescence of the lanthanide ion. The observed luminescence

lifetime is the same as the radiative lifetime when quenching does not play a role.

Fig. 3.8 a) Lifetime decay curve of LaVO4: Eu3+

core nanoparticles


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3.8 b) Lifetime decay curve of LaVO4: Eu3+ core-shell nanoparticles

Figure 3.8 shows the luminescence decays of the 5 D0 level of the Eu3+

ion doped in thetwo nanoparticles.

Nanoparticle Τ av (ms)

LaVO4 : Eu+ (5%) core NP 0.98

LaVO4 : Eu

+

(5% )core-shell NP 1.20

We can see the increase in the lifetime of the particular level because of the

reduced quenching from the surrounding environment which is because of the formation


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of an inert layer. The luminescence lifetimes were fitted using a bi-exponential decay. In

the LaVO4 nanoparticles the Eu3+

the ion is in a crystal site that has no inversion

symmetry as was concluded from the emission spectrum. As a result the radiative lifetime

of the Eu3+

ion in LaVO4 is much shorter than in the other matrices. The radiative

lifetime of the Eu3+ ion is strongly dependent on the symmetry of the crystal site.

3.3 Tb 3+ emission:

The emission and excitation spectra of nanoparticles doped with Tb3+

ions are

shown in Figure 3.9 and 3.10. The excitation occurs through the charge transfer band of

the VO4 groups, the excitation spectra all show the broad vanadate absorption band. Four

emission peaks are observed of the5D4→

7FJ (J = 3 - 6) transitions after excitation at 280

nm. The positions of the major peaks are the same, because of the good shielding of the

4 f electrons from the environment. Small differences can occur in the peak splitting and

the intensity ratio between the peaks. These small differences are a result of the

difference in symmetry of the doping site.


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Fi g. 3.9 Emission Spectrum of LaVO4: Tb

3+

(5%) core-shell nanoparticles with 280 nm UV excitation and 395 nm filter

In the excitation spectrum monitoring the emission at 545 nm, the 4 f absorption

bands are clearly visible, together with the allowed 4 f -5d absorption. Charge Transfer

from VO4 group can be seen clearly, which shows the efficient energy transfer from

LaVO4 to Tb3+

ions.


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Fi g. 3.10 Excitation Spectrum of LaVO4: Tb3+ (5%) core-shell nanoparticles for 545nm emission using 395 nm filter

F igure 3.11 Luminescence decay of the 5 D4 level of the LaVO4: Tb3+ core-shell nanoparticles


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3.4 Dy 3+ Emission:

Figure 3.12 demonstrated the emission spectrum of 5% Dy3+ doped LaVO4 core-

shell nanoparticles. Dy3+

shows three emission bands in the visible from the4F9/2 level to

the6H15/2 (475 nm),

6H13/2 (570 nm), and the

6H11/2 (655 nm) levels, after excitation at 280

nm. The charge transfer band of the VO4 group is observed in the excitation spectrum for

the LaVO4 nanoparticles alongwith very small self excitation peaks of Dy3+

near 370 nm

which shows efficient energy transfer from VO4 to Dy3+.

Fig. 3.12 Emission Spectrum of LaVO4: Dy3+ (5%) core-shell nanoparticles with 280 nm UV excitation & 395 nm filter


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Fig. 3.13 Excitation Spectrum of LaVO4: Dy3+ (5%) core-shell nanoparticles for 572nm emission using 395 nm filter

F igure 3.14 Luminescence decay of the 4 F 9/2 level of the LaVO4: Dy3+ (5%) core-shell nanoparticles


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3.5 Tm 3+ emission:

Tm3+

emission was effectively observed in the doped LaVO4 nanoparticles. The

Tm3+

ion shows a broad absorption band around 360 nm. Luminescence occurs from the

1G4 level at 475 nm, and 650 nm from transitions to the

3H6,

3H4 levels, respectively, after

excitation in the broad absorption band of the LaVO4:Tm3+

nanoparticles. The excitation

spectrum of the LaVO4 nanoparticles shows the charge transfer band of the VO4 groups.

Fig. 3.15 Emission Spectrum of LaVO4: Tm3+

for 280 nm excitation


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Fig. 3.16 Excitation spectrum of LaVO4: Tm3+ for 475 nm emission

Fig. 3.17 Lifetime decay curve of LaVO4: Tm3+

for 475 nm emission


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3.6 Scheme 1: Whi te light thr ough LaVO 4 : 0.5% Eu 3+ , 12.5% Tb

3+ & 20%

Tm 3+ core-shell nanopar ticles

For getting the white light by doping red, blue and green light emitting lanthanide

ions in to LaVO4 matrix, core-shell nanoparticles stabilized by oleic acid were prepared.

After several experiments using trial and error method, above composition of Ln3+

ions

was obtained. For the LaVO4 nanoparticles the quenching seems to have a longer range,

going deeper into the nanoparticle. Therefore Tm3+

which is a very weak emitter and is

easily quenched is placed in the core followed by an inert shell then Tb3+

doped LaVO4

shell which is again covered with an inert shell. Eu3+

being the strongest emitter is placed

in the penultimate shell over which another inert shell is placed.

F ig. 3.18 Scheme for the preparation of white light emitting Ln3+

: LaVO4 core-shell NP

All shells are under the influence of quenching for the LaVO4 nanoparticles,

because the first shell in the core of the nanoparticle has a lifetime that is substantially

lower than the radiative lifetime. Quenching from the surface is stronger in the LaVO4


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nanoparticles because in the LaVO4 nanoparticles, more shells are influenced by surface

quenching. As the absorption coefficient of the vanadate charge-transfer transition is

higher than that of the lanthanide 4f transitions, this property can be exploited to excite

all the lanthanide Ln3+

ions present in the matrix simultaneously using a single excitation

source so 280 nm UV source was used to excite all the Ln3+ ions simultaneously for

getting the white light.

F ig. 3.19 Emission Spectrum for LaVO4: Eu3+ , Tb3+ & Tm3+core-shell nanoparticles using 280

nm UV source and 395 nm filter

As can be seen from the above emission spectrum, all the characteristic peaks of

the three Ln3+ ions are there. CIE co-ordinates of the white light obtained were (0.35,

0.35) as compared to that of the pure white light (0.33, 0.33).


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F ig. 3.20 Excitation Spectra for all the major emissions

Excitation spectra for all the three major emissions show the broad absorption

range from 260 – 330 nm which is the typical absorption range for LaVO 4. As all the

spectra are of similar shape it means that the energy transfer from LaVO4 is the main

source of all the Ln3+

emission also the energy transfer is efficient.

3.7 Scheme 2: White li ght through LaVO 4 : 0.125% Dy

3+

& 20% Tm

3+

core-shell nanoparticles

For this type of nanoparticles, number of shells was reduced to three with Tm3+

again at core, followed by an inert/undoped LaVO4 shell surrounded by Dy3+

doped

LaVO4 which is finally covered by an inert/undoped LaVO4 shell. On excitation with 280


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nm UV source, emission spectrum showed all the characteristic emission peaks of Ln3+

ions doped into the LaVO4 matrix.

Fig. 3.21 Emission spectrum of LaVO4: Tm3+

, Dy3+

core-shell nanoparticles

For the above spectrum, CIE co-ordinates obtained were (0.34, 0.35) as compared

to that of pure white light (0.33, 0.33) with the major emission peaks of 474 nm (Tm3+

)

and 572 nm (Dy3+). Background emission in the spectrum is possibly due to LaVO 4

emission which is reported to emit in visible region when excited with 280 nm UV light.


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F ig. 3.22 Excitation Spectra for 474 nm (Tm3+

) & 572 nm (Dy3+

) emission

Excitation spectra for both the major emissions showed the broad absorption band

of LaVO4 which is the main cause for these emissions. Also the energy transfer is highly

efficient as can be seen from the excitation spectrum for 474 nm (Tm3+

) emission where

there is a narrow absorption peak at 350 nm due to its self excitation. In case of 572 nm

(Dy3+

) emission, emission occurs solely from the charge transfer from VO4 group.


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3.8 Control Exper iment: Preparation of 12.5% Tb 3+ , 20% Tm

3+ & 0.5%

Eu 3+ doped core LaVO 4 nanoparticles

Control sample was prepared by doping all the lanthanide ions in the above

composition into LaVO4 to prepare LaVO4: Ln3+

core nanoparticles.

Fig. 3.23 Emission spectrum of the LaVO4: Eu3+ , Tm3+ & Tb3+ core nanoparticles using 280 nm

UV source and 395 nm filter

For the above spectrum, CIE co-ordinates obtained were (0.30, 0.37) as compared

to (0.34, 0.35), obtained for core-shell nanoparticles of the same composition. Here, Tm3+


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emission is not present because Tm3+

emission is quenched due to the presence of other

Ln3+

ions in the same vicinity without any separation. It shows the need of core-shell

structure where all the Ln3+

ions are separated from each other using an inert shell.


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Conclusion

Generally, the generation of white light can be done by mixing red, green and

blue emission together and this can also be achieved by simply adding blue and yellow

light. We successfully prepared. Bright white light emitting Tm3+

, Tb3+

& Eu3+

: LaVO4

and Tm3+ & Dy3+: LaVO4 core-shell nanoparticles using oleic acid as a stabilizing agent

to control the size of the nanoparticles and making them dispersible in organic solvents

like chloroform so that they can be further processed via spin coating technique to coat

over UV LED having emission range in the absorption range of LaVO 4, i.e. (260 – 320

nm) to get white light. CIE co-ordinates of the light obtained from the two nanoparticles,

i.e. LaVO4: Eu3+

, Tb3+

& Tm3+

and LaVO4: Tm3+

& Dy3+

were (0.35, 0.35) & (0.34,

0.35) respectively which is very close to that of the pure white light (0.33, 0.33). Control

experiments showed the need of core-shell structure. The use of strongly coordinating

group to the surface of the nanoparticles decreases the non-radiative effects and high

quantum yields can be obtained under certain circumstances. The quantum yield of the

nanoparticles could be improved significantly when the surface is coated with an

inorganic shell. These core-shell particles consist of a core of small band-gap

semiconductor surrounded by a shell of larger band-gap semiconductor.[49]

In these core-

shell nanoparticles, the hole is confined to the core of the particles while the electron can

travel over the whole particle.


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Stability of nanoparticles was an issue which tends to agglomerate after some

time of preparation, even after dissolving them in chloroform. This problem has to work

out in future before coating the nanoparticles dispersed solution on a UV LED.


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Lanthanide Doped Coreshell Nanoparticles for Solid State Lighting Applications

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