Luminescence : From theory to apllications

Luminescence

Edited by

Cees Ronda

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Luminescence

From Theory to Applications

Edited by

Cees Ronda

The Editor

Prof. Dr. Cornelis (Cees) R. Ronda

Philips Research

Weißhausstrasse 2

52066 Aachen

Germany

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ISBN: 978-3-527-31402-7

Foreword

Since Joseph Swan in Britain and Thomas Edison in the USA invented the light bulb

independently in 1879, illumination technology has become indispensable in our

daily lives. In the 20th century, more efficient illumination technologies of gas

discharge lamps, halogen tungsten lamps and LEDs were commercialized for

lighting. Liquid crystal displays and plasma display panels have become the most

promising technologies for display screens. Excellent luminescent materials have

been developed for lighting and display with greater performance in brightness,

color gamut, efficiency, and lifetime. Therefore, it is critical to understand the

mechanism behind the new technologies of luminescence.

The book you are about to read addresses this need. It contains 9 chapters. From

Chapter 1 to Chapter 8, each chapter is on one kind of phosphors, while Chapter 9 is

on experimental techniques. The authors describe clearly the physical principles,

related backgrounds and research directions for almost every popular luminescent

material. Various subjects are covered, such as physics, nonlinear optics, chemistry,

quantum mechanism and spectroscopy. Many clear diagrams and illustrations are

given to help readers understand and remember the principles well. Definitions are

made precisely and much attention has been paid to the differences (even small)

the among various concepts. All the equations used in this book are very basic as the

authors want to give readers a clear insight into the related physics. No puzzling

mathematics or complicated calculations are involved. The book is easy to read and

thus very suitable for students who want to get an overall picture of luminescence.

Cees Ronda is both an academic professor and a research fellow of Philips. He has

given a series of excellent lectures on luminescence in our center during the past

several years and we enjoyed very much his style of making everything crystal clear.

His personal experience and contacts in industry and academia are essential in

putting together such an impressive book. Each theory explained here has practical

applications. Many classical papers and books have been cited, as well as the latest

developments. R & D histories, current markets and future trends and challenges in

luminescence technology are given. Therefore, the book is also very suitable for

researchers.

Luminescence: From Theory to Applications. Edited by Cees Ronda

Copyright
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-31402-7

V

My students and I enjoyed very much in reading this book. I am sure you will also

enjoy it.

Sailing He

Professor and Chief Scientist

Joint Research Center of Photonics of Royal Institute of Technology

(Sweden) and Zhejiang University (China)

July 2007

VI Foreword

Contents

Foreword V

Preface XIII

List of Contributors XV

1 Emission and Excitation Mechanisms of Phosphors 1

Cees R. Ronda

1.1 Introduction 1

1.2 General Considerations – Fluorescent Lamps 1

1.3 General Considerations – Cathode Ray Tubes 2

1.4 Luminescence Mechanisms 3

1.4.1 Center Luminescence 4

1.4.2 Charge Transfer Luminescence 8

1.4.3 Donor Acceptor Pair Luminescence 8

1.4.4 Long Afterglow Phosphors 11

1.5 Excitation Mechanisms 12

1.5.1 Optical Excitation of Luminescence and Energy Transfer 12

1.6 Energy Transfer Mechanisms between Optical Centers 14

1.6.1 Mechanisms Underlying Energy Transfer 14

1.6.2 Energy Transfer Governed by Electrostatic Interaction 15

1.6.3 Energy Transfer by Higher-order Coulomb Interaction 18

1.6.4 Energy Transfer Governed by Exchange Interactions 19

1.6.5 Cross-relaxation and Energy Transfer 19

1.6.6 Practical Implications 20

1.7 Excitation with High-energy Particles 21

1.8 Electroluminescence (EL) 24

1.8.1 High-voltage Electroluminescence 24

1.8.2 Low-voltage Electroluminescence 26

1.9 Factors Determining the Emission Color 27

1.10 Energy Efficiency Considerations of Important Luminescent

Devices 29

1.11 Luminescence Quantum Yield and Quenching Processes 29

Luminescence: From Theory to Applications. Edited by Cees Ronda

Copyright
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-31402-7

VII

1.11.1 The Energy does not Reach the Luminescent Ion 31

1.11.2 The Absorbed Energy Reaches the Luminescent Ion but there are

Nonradiative Channels to the Ground State 31

1.11.3 The Luminescence Generated is Absorbed by the Luminescent

Material 33

1.12 Acknowledgement 34

2 Quantum Dots and Nanophosphors 35

Cees R. Ronda and Thomas Ju¨stel

2.1 Introduction 35

2.1.1 Optical Properties of Quantum Dots 35

2.1.2 Particle in a One-dimensional Potential Well 36

2.1.3 Particle in Three-dimensional Potentials 40

2.1.3.1 Particle in a General Three-dimensional Potential 40

2.1.3.2 Electron in a Coulomb Potential 41

2.1.3.3 The Hydrogen Atom 42

2.2 Density of States in Low-dimensional Structures 43

2.3 Electrons, Holes, and Excitons 45

2.4 Low-dimensional Structures 46

2.4.1 The Weak Confinement Regime 46

2.4.2 The Strong Confinement Regime 47

2.5 Quantum Confinement in Action 49

2.6 Photoluminescence of Quantum Dots Prepared by Wet-chemical

Precipitation 52

2.7 Photoluminescence from Doped Quantum Dots 53

2.8 Luminescence of Nano Particles of Rare-Earth Phosphors 55

2.9 Nanoscale Particles for Molecular Imaging 56

2.10 Conclusions 58

2.11 Acknowledgements 58

3 Phosphors for Plasma Display Panels 61

Thomas Ju¨stel

3.1 Introduction 61

3.2 Principle of Operation of Plasma Display Panels 61

3.3 Performance of Applied Phosphors in PDPs 65

3.3.1 Phosphor Efficiency 66

3.3.2 Electronic Transitions Involved in Europium Luminescence 68

3.3.3 Color point and efficiency of the red phosphors 68

3.3.4 Stability and Color Point of BaMgAl10O17:Eu 70

3.4 Summary and Prospects 72

4 Quantum-Splitting Systems 75

Alok M. Srivastava and Cees R. Ronda

4.1 Introduction 75

VIII Contents

4.2 Quantum-splitting Phosphors Based on Pr3þ-activated Fluoride

Materials 76

4.3 Quantum-splitting Phosphors Based on Pr3þ-activated

Oxide Materials 82

4.3.1 SrAl12O19: Pr3þ 83

4.3.1.1 LaMgB5O10 and LaB3O6 Doped with Pr3þ 85

4.4 The Quantum Efficiency of the Quantum-splitting Process 88

4.5 Limitations of Pr3þ-based Quantum-splitting Phosphors 91

4.6 Quantum-splitting Phosphors Based on Gd3þ and Rare Earth

Ion-Activated Fluoride Materials 92

4.6.1 The Electronic Energy Level Structure of the Gd3þ Ion 92

4.6.2 Quantum Splitting in the Gd3þ-Eu3þ System 94

4.6.3 Quantum Splitting in the Er3þ-Gd3þ-Tb3þ System 97

4.7 Multiphoton Emission through High-energy Excitation 98

4.8 Applications of Quantum-splitting Phosphors 99

4.9 Conclusions 100

4.10 Acknowledgements 101

5 Scintillators 105

Cees R. Ronda and Alok M. Srivastava

5.1 Introduction 105

5.2 Positron Emission Tomography and Computed Tomography 106

5.2.1 Physical Principles of Positron Emission Tomography (PET) 106

5.2.2 Computed Tomography (CT) 107

5.3 General Requirements for Scintillating Materials used in

Medical Imaging 107

5.4 Scintillators for Pet Application 112

5.4.1 General Description of Phosphors for PET Scintillators 112

5.4.2 Scintillating Composition Used in PET 114

5.4.2.1 Bi4Ge3O12 (BGO) 115

5.4.2.2 NaI:Tlþ 116

5.4.2.3 Lu2SiO5:Ce3þ (LSO) 116

5.4.2.4 Lu2Si2O7:Ce (Lutetium Pyrosilicate, LPS) 117

5.4.2.5 LaBr3:Ce 118

5.4.2.6 LuI3:Ce 119

5.4.3 Other PET Scintillators 119

5.5 Scintillators for CT Application 120

5.5.1 General Description of Scintillators for CT 120

5.5.2 Scintillating Compositions Used in CT 120

5.5.2.1 CdWO4 120

5.5.2.2 (Y,Gd)2O3:Eu3þ 121

5.5.2.3 Gd2O2S:Pr3þ (GOS) 122

5.6 X-ray Intensifying Screens 123

5.6.1 General Description of Scintillators for Intensifying Screens 123

5.6.2 Phosphor Compositions for Use in X-ray Intensifying Screens 123

Contents IX

5.7 FDXD Detectors 124

5.8 Storage Phosphors 124

5.8.1 General Description of Storage Phosphors 124

5.9 Semiconductor Scintillators 127

6 Upconversion Phosphors 133

J. Freek Suijver

6.1 Introduction 133

6.2 Theory of Upconversion 137

6.2.1 Absorption and Excitation Spectroscopy 139

6.2.2 Time Evolution of UC Emission 143

6.2.3 Power Dependence of Upconversion 146

6.2.4 Photon Avalanche Effects in Upconversion 150

6.2.5 Determination of the Upconversion Efficiency 153

6.3 Examples 154

6.3.1 Rare Earth Upconverters 155

6.3.2 Transition Metal Upconverters 162

6.3.3 Mixed Rare Earth/Transition Metal Upconverters 165

6.3.4 Organic Upconverters 169

6.3.5 Nanocrystalline Upconverters 171

6.4 Conclusions and Outlook 175

6.5 Acknowledgements 176

7 Luminescent Materials for Phosphor–Converted LEDs 179

Thomas Ju¨stel

7.1 Inorganic Light-Emitting Diodes (LEDs) 179

7.2 White and Colored LEDs 180

7.3 Phosphor-Converted LEDs 183

7.4 Future Trends 188

8 Organic Electroluminescence 191

Joseph J. Shiang and Anil R. Duggal

8.1 Introduction 191

8.2 OLED Fundamentals 192

8.3 Key OLED Trends and Innovations 197

8.3.1 Electroluminescence from Vapor-deposited Organic Films 197

8.3.2 Electroluminescence from Solution-Deposited Organic Films 202

8.4 Prospects for General Illumination 207

8.4.1 A First OLED Lighting Demonstration 208

8.4.1.1 Downconversion for White Light Generation 209

8.4.1.2 Scattering for Outcoupling Efficiency Enhancement 210

8.4.1.3 A Scalable Monolithic Series Architecture 211

8.4.2 Efficiency Challenge for General Illumination 212

8.5 Conclusions 213

8.6 Acknowledgements 214

X Contents

9 Experimental Techniques 219

Peter Vergeer

9.1 Introduction 219

9.2 Energy of Optical Transitions: Absorption, Excitation, and Emission

Spectroscopy 220

9.2.1 Broadband Light Sources 223

9.2.2 Dispersing Elements 224

9.2.2.1 Gratings 224

9.2.2.2 Interferometers 227

9.2.3 Detectors 229

9.3 The Transition Dipole Moment: Absorption Strengths and

Luminescence Lifetimes 233

9.3.1 Lasers 235

9.3.2 Luminescence Lifetimes 237

9.4 Quantum Efficiency and Nonradiative Relaxation 238

9.5 Homogeneous Broadening and Dephasing 240

9.6 Detection of Luminescence from Individual Optical Centers 244

9.7 Acknowledgement 248

Index 251

Contents XI

Preface

Modern society relies heavily on mankind’s ability to produce light. In the early days,

light was produced by chemical means. Though this is a rather inefficient way, the

heat produced also enabled our predecessors to develop metal tools and to cook their

foods. This is a very early demonstration how increased technological capabilities

improved people’s life. Later on, dedicated light sources were developed, such as

candles and oil lamps.

Electrically generated light is only a few centuries old and the developments in

these light sources is a beautiful example of how our increased understanding of

physical and chemical processes led to new light generation principles.

In incandescent lamps, in which light generation is still rather inefficient, a

conducting body is heated and the spectrum of the radiation generated corresponds

to the temperature of the heated body (black body radiation). It is also interesting to

note that understanding the operation principles of incandescent lamp requires

quantum mechanics and in fact black body radiation played a very important role in

the early development of quantum mechanics. The large-scale introduction of

incandescent lamps in addition required glass- and vacuum technology and metal￾lurgy. Finally, the availability of electricity was a decisive prerequisite. The vacuum

technology developed was very important in the development of valves, X-ray tubes,

gas discharge lamps and picture tubes later on.

In gas discharge lamps, light is generated by exciting atoms or molecules in the

gas phase. Gas discharge lamps require knowledge of electronic states of individual

excited atoms or molecules, which reflects our increased understanding of electro￾nic states in these moieties. In addition, they use emitters (materials releasing

electrons), developed to certain maturity in the early 20th century. Many gas

discharge lamps use luminescent materials, which absorb light generated by the

discharge and convert it to light with a different frequency. The development of

luminescent materials, also called phosphors, requires high purity materials and

sophisticated materials science. Very important is the description of electronic states

of ions interacting with their environment. This reflects a further increased under￾standing of the electronic states in matter: ions, which interact with their environ￾ment. It is this level of understanding, which has also enabled the development of

solid-state lasing materials.

Luminescence: From Theory to Applications. Edited by Cees Ronda

Copyright
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-31402-7

XIII

Also in the first half of the 20th century luminescent materials, excitable with

electron beams with large kinetic energy or with high-energy photons were devel￾oped. Application areas are picture tubes and detection of X-rays or g-rays. These

achievements were instrumental in reaching our current level of medical care and

our current ability to distribute and receive information. Understanding the excita￾tion mechanism of this kind of emission requires some knowledge of the electronic

band structure, the electronic states of the emitting ions and of lattice vibrations

(phonons) in luminescent materials.

In the same period, Destriau discovered high voltage electroluminescence1

. High

voltage electroluminescence has a number of applications, mainly in displays. The

materials properties that govern high voltage electroluminescence are not yet well

understood, but they also involve electronic band states and electronic states of the

emitting ions.

A further increase of our understanding of electronic states has led to emission in

quantum dots and in materials showing electroluminescence under low voltage

excitation. In quantum dots, the electronic states depend on the size of the particles.

Quantum dots typically have a diameter between 1–10 nm. Application opportu￾nities are in e.g. molecular imaging. Low voltage electroluminescence involves

charge transport in extended molecular orbitals and recombination in such states

or on e.g. ions.

In this book, the luminescence mechanisms underlying important applications

will be dealt with. This makes this book very interesting for people working in both

an academic and an industrial environment. Experts in their respective fields have

written the chapters. All chapters start at a fundamental level and finally deal with the

state of the art. This also makes this book very useful for teaching purposes.

Cees Ronda

Research Fellow, Royal Philips Electronics

Professor of Chemistry, Utrecht University, the Netherlands

Professor of Materials Science, Zhejiang University, China

1

G. Destriau, J. Chem. Phys. 33, 620 (1936).

XIV Preface