QC 475
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`.7
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`.B53
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`1994
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`LAN DOVER
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`Gencofl
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`

`
`G. Blasse, B. C. Grabmaier
`
`Luminescent
`Materials
`
`With 171 Figures and 31 Tables
`
`Springer-Verlag
`Berlin Heidelberg New York
`London Paris Tokyo
`Hong Kong Barcelona Budapest
`
`Vizio EX1018 Page 0004
`
`

`
`Prof. Dr. G. Biasse
`
`Debye Institute
`University Utrecht
`Postbox 80.000
`3508 TA Utrecht
`The Netherlands
`
`Prof .. Dr. B. C. Grabmaier
`
`Siemens Research Laboratories
`ZFE BT MR 22
`D-81 730 Miinchen
`Germany
`
`also with Debye Institute
`University Utrecht
`
`ISBN 3-540-58019-0 Springer-Verlag Berlin Heidelberg New York
`ISBN 0-387-58019-0 Springer-Verlag New York Berlin Heidelberg
`
`Library of Congress Cataloging-in-Publication Data
`Blasse, G. Luminescent materials I G. Blasse, B.C. Grabmaier. p. em.
`Includes bibliographical references and index.
`ISBN 3-540-58019-0. --ISBN 0-387-58019-0 (U.S.)
`I. Phosphors. 2. Luminescence. I. Grabmaier, B. C., 1935- II. Title.
`QC476.7.B53 1994 620.1' 1295--dc20 94-20336 CIP
`
`This work is subject to copyright. All rights are reserved, whether the whole or part of the
`material is concerned, specifically the rights of translation, reprinting, re-use of illustrations,
`recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks.
`Duplication of this publication or parts thereof is only permitted under the provisions of the
`German Copyright Law of September 9, 1965, in its current version, and a copyright fee must
`always be paid.
`
`© Springer.Verlag Berlin Heidelberg 1994
`Printed in Germany
`
`The use of registered names, trademarks, etc. in this publication does not imply, even in the
`absence of a specific statement, that such names are exempt from the relevant protective laws
`and regulations and therefore free for general use.
`
`Typesetting with TEX: Data conversion by Lewis & Leins, Berlin
`SPIN: 10187460
`02/3020- 5 4 3 2 1 0 - Printed on acid-free paper
`
`Vizio EX1018 Page 0005
`
`

`
`Preface
`
`Luminescence is just as fascinating and luminescent materials (are) just as important
`as the number of books on these topics are rare. We have met many beginners in
`these fields who have asked for a book introducing them to luminescence and its
`applications, without knowing the appropriate answer. Some very useful books are
`completely out of date, like the first ones from the late 1940s by Kroger, Leverenz and
`Pringsheim. Also those edited by Goldberg ( 1966) and Riehl (1971) can no longer
`be recommended as up-to-date introductions.
`In the last decade a few books of excellent quality have appeared, but none of
`these can be considered as being a general introduction. Actually, we realize that it
`is very difficult to produce such a text in view of the multidisciplinary character of
`the field. Solid state physics, molecular spectroscopy, ligand field theory, inorganic
`chemistry, solid state and materi·als chemistry all have to be blended in the correct
`proportion.
`Some authors have tried to obtain this mixture by producing multi-authored books
`consisting of chapters written by the specialists. We have undertaken the difficult task
`of producing a book based on our knowledge and experience, but written by one
`hand. All the disadvantages of such an approach have become clear to us. The way in
`which these were solved will probably not satisfy everybody. However, if this book
`inspires some of the investigators just entering this field, and if it teaches him or her
`how to find his way in research, our main aim will have been achieved.
`The book consists of three parts, although this may not be clear from the table of
`contents. The first part (chapter 1) is a very general introduction to luminescence and
`luminescent materials for those who have no knowledge of this field at all. The second
`part (chapters 2-5) gives an overview of the theory. After bringing the luminescent
`center in the excited state (chapter 2: absorption), we follow the several possibilities
`of returning to the ground state (chapter 3: radiative return; chapter 4: nonradiative
`return; chapter 5: energy transfer and migration). The approach is kept as simple as
`possible. For extensive and mathematical treatments the reader should consult other
`books.
`Part three consists of five chapters in which many of the applications are discussed,
`viz. lighting (chapter 6), cathode-ray tubes (chapter 7), X-ray phosphors and scintil(cid:173)
`lators (chapters 8 and 9), and several other applications (chapter 10). These chapters
`discuss the luminescent materials which have been, are or may be used in the appli(cid:173)
`cations concerned. Their performance is discussed in terms of the theoretical models
`presented in earlier chapters. In addition, the principles of the application and the
`preparation of the materials are dealt with briefly. Appendices on some, often not-well(cid:173)
`understood, issues follow (nomenclature, spectral units, literature, emission spectra).
`
`Vizio EX1018 Page 0006
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`

`
`vi
`
`Preface
`
`We are very grateful to Mrs. Jessica Heilbrunn (Utrecht) who patiently typed the
`manuscript and did not complain too much when correction after correction appeared
`over many months. Miss Rita Bergt (Miinchen) was of help in drawing some of the
`figures. Some of our colleagues put original photographs at our disposal.
`This book would not have been written without discussions with and inspiration
`from many colleagues over a long period of time. These contacts, some oral, some
`via written texts, cover a much wider range than the book itself. In the preparation of
`this book our communication with Drs. P.W. Atkins, F. Auzel, A. Bril, C.W.E. van
`Eijk, G.F. lmbusch, C.K. J0rgensen, and B. Smets has been very useful.
`For many years we have enjoyed our work in the field of luminescence. We hope
`that this book will help the reader to understand luminescence phenomena, to design
`new and improved luminescent materials, and to find satisfaction in doing so.
`
`Spring 1994
`
`G. Blasse, Utrecht
`B.C. Grabmaier, Miinchen
`
`Vizio EX1018 Page 0007
`
`

`
`Table of Contents
`
`Chapter 1 A General Introduction to Luminescent Materials
`Chapter 2 How Does a Luminescent Material Absorb Its Excitation Energy?
`
`10
`2. 1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`16
`2.2 The Influence of the Host Lattice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.3 The Energy Level Diagrams of Individual Ions......................... 20
`2.3.1
`The Transition Metal Ions (d 11
`) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
`2.3.2
`The Transition Metal Ions with d° Configuration. . . . . . . . . . . . . . . 25
`2.3.3
`The Rare Earth Ions (4j 11
`) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
`2.3.4
`The Rare Earth Ions (4j-5d and Charge-Transfer Transitions) . . .
`27
`Ions with s 2 Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
`2.3.5
`Ions with d 1° Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
`2.3.6
`2.3.7
`Other Charge-Transfer Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`30
`2.3.8
`Color Centers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`30
`2.4 Host Lattice Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3 1
`
`Chapter 3 Radiative Return to the Ground State: Emission
`
`3.1
`3.2
`3.3
`
`Introduction ... ... . . . ....... .... ................ ...... . .. ........... .
`33
`General Discussion of Emission from a Luminescent Center ... ......... .
`33
`Some Special Classes of Luminescent Centers ................ ..... ... .
`38
`3.3.1
`Exciton Emission from Alkali Halides ..... .. ....... . ......... .
`38
`40
`3.3.2
`Rare Earth Ions (Line Emission) ..................... .. .. .... .
`3.3.3
`Rare Earth Ions (Band Emission) .............. . ..... .. ...... .
`45
`Transition Metal Ions . ... .................. .... ..... . ....... .
`50
`3.3.4
`52
`3.3.5
`d° Complex Ions ........................... ·················
`d 10 Ions ..... . ...... . ............ . ...... . .... . ... . . . . . .... · .. .
`53
`3.3.6
`s 2 Ions ... .. . ...... .... ...... . . . ................... . ......... 55
`3.3.7
`The U 6+ ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`59
`3.3.8
`Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`60
`3.3.9
`3.3.1 0 Cross-Luminescence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
`3.4 Afterglow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
`3.5 Thermoluminescence.. . .... .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
`3.6. Stimulated emission... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
`References.............................. . ........ . ......... .. ....... 70
`
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`
`viii
`
`Table of Contents
`
`Chapter 4 Nonradiative Transitions
`
`71
`Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4. 1
`4.2 Nonradiative Transitions in an Isolated Luminescent Centre. . . . . . . . . . . . . 72
`4.2.1
`The Weak-Coupling Case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`74
`4.2.2
`The Intermediate- and Strong-Coupling Cases. . . . . . . . . . . . . . . . . . 77
`4 .3 Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
`4.4 Maximum Efficiency for High Energy Excitation [13] . . . . . . . . . . . . . . . . . .
`85
`4.5
`Photoionization and Electron-Transfer Quenching. . . . . . . . . . . . . . . . . . . . . . . 86
`4.6 Nonradiative Transitions in Semiconductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`89
`
`Chapter 5 Energy Transfer
`
`91
`Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5. 1
`5.2 Energy Transfer Between Unlike Luminescent Centers.................. 91
`5.3 Energy Transfer Between Identical Luminescent Centers................ 95
`5.3.1 Weak-Coupling Scheme Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
`5.3.2
`Intermediate- and strong-coupling scheme ions ................. I 03
`5.4 Energy Transfer in Semiconductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 06
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 06
`
`Chapter 6 Lamp Phosphors
`
`Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
`6.1
`6.2 Luminescent Lighting [1-3] ....... ..... ....... .... .................... 108
`6.3 The Preparation of Lamp Phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 11
`6.4
`Photoluminescent Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 12
`6.4. 1 .
`Lamp Phosphors for Lighting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 12
`6.4.2
`Phosphors for Other Lamp Applications. . . . . . . . . . . . . . . . . . . . . . . 126
`6.4.3
`Phosphors for High-Pressure Mercury Vapour Lamps ........... 127
`6.4.4
`Phosphors with Two-Photon Emission . .......... .. ............ 130
`6.5 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
`
`Chapter 7 Cathode-Ray Phosphors
`
`7.1 Cathode-Ray Tubes: Principles and Display. . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
`7.2
`Preparation of Cathode-Ray Phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
`7.3 Cathode-Ray Phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
`7.3.1
`Some General Remarks .... . ........................... . . .. .. 137
`7.3 .2
`Phosphors for Black-and-White Television ..................... 138
`7 .3.3
`Phosphors for Color Television . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
`7 .3.4
`Phosphors for Projection Television. . . . . . . . . . . . . . . . . . . . . . . . . . . 141
`7.3.5
`Other Cathode-Ray Phosphors ................................ 143
`7.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
`
`Vizio EX1018 Page 0009
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`
`Table of Contents
`
`ix
`
`Chapter 8 X-Ray Phosphors and Scintillators (Integrating Techniques)
`
`8.1
`
`Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
`8.1.1
`X-Ray Absorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
`8.1.2
`The Conventional Intensifying Screen. . . . . . . . . . . . . . . . . . . . . . . . . 148
`8.1.3
`The Photostimulable Storage Phosphor Screen . . . . . . . . . . . . . . . . . 149
`8.1.4
`Computed Tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
`8.2 Preparation of X-ray Phosphors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
`8.2. 1
`Powder Screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
`8.2.2
`Ceramic Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 57
`8.2.3
`Single Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
`8.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 59
`8.3.1
`X-Ray Phosphors for Conventional Intensifying Screens. . . . . . . . 159
`8.3.2
`X-Ray Phosphors for Photostimulable Storage Screens .......... 162
`8.3.3
`X-Ray Phosphors for Computed Tomography.. . . . . . . . . . . . . . . . . 165
`8.4 Outlook.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 69
`
`Chapter 9 X-Ray Phosphors and Scintillators (Counting Techniques)
`
`Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
`9.1
`9.2 The Interaction of Ionizing Radiation with Condensed Matter. . . . . . . . . . . 170
`9.3 Applications of Scintillator Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
`9.4 Material Preparation (Crystal Growth)........ . . . . . . . . . . . . . . . . . . . . . . . . . 178
`9.5
`Scintillator Materials... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
`9.5.1
`Alkali Halides.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
`9.5.2
`Tungstates.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
`9.5.3
`Bi4Ge3012 (BGO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
`: Ce3+ and Lu2Si05
`9.5.4
`: Ce3+ . . . . . . . . . . . . . . . . . . . . . . . . . . 184
`Gd2Si05
`9.5.5
`. . . . . . . . . . • . . . . . . . . . . . . . . . • . . . . . . . . . . . • . . . . . • . . • . . . . . . . 186
`CeF3
`Other Ce3+ Scintillators and Related Materials... . . . . . . . . . . . . . . 188
`9.5.6
`9.5.7
`BaF2 (Cross Luminescence; Particle Discrimination) . . . . . . . . . . . 188
`9.5.8
`Other Materials with Cross Luminescence..................... 190
`9.6 Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 93
`
`Chapter 10 Other Applications
`
`10.1 Upconversion: Processes and Materials......... .. ... . . .. .. . . .. .... . ... 195
`1 0.1.1 Upconversion Processes... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
`1 0.1.2 Upconversion Materials... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
`10.2 The Luminescent Ion as a Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
`I 0.3 Luminescence Immuno-Assay ....................... . ................. 206
`1 0.3.1
`Principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
`1 0.3.2 Materials .................................................... 208
`
`Vizio EX1018 Page 0010
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`

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`X
`
`Table of Contents
`
`10.4 E1ectro1uminescence ................................................ . . 210
`1 0.4. 1
`Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
`10.4.2 Light-Emitting Diodes and Semiconductor Lasers .............. 210
`1 0.4.3 High-Field Electroluminescence ............................... 212
`I 0.5 Amplifiers and Lasers with Optical Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
`I 0.6 Luminescence of Very Small Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 8
`
`I. The Luminescence Literature .................................. 221
`Appendix
`Appendix 2. From Wavelength to Wavenumber and Some Other Conversions . 223
`Appendix 3. Luminescence, Fluorescence, Phosphoresence ................... 224
`Appendix 4 .. Plotting Emission Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
`
`Subject Index ............................................................. 227
`
`Vizio EX1018 Page 0011
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`

`
`CHAPTER 1
`
`A General Introduction to Luminescent Materials
`
`This chapter addresses those readers for who luminescent materials are a new chal(cid:173)
`lenge. Of course you are familiar with luminescent materials: you meet them everyday
`in your laboratory and in your home. If this should come as a surpri se. switch on
`your fluorescent lighting, relax in front of your television set, or take a look at the
`screen of your computer. Perhaps you would like something more specialized. Re(cid:173)
`membe r then your visit to the hospital for X-ray photography . Or the laser in your
`institute; the heart of this instrument consists of a luminescent material. However,
`such a high degree of specialization is not necessary. The packet of washing powde r
`in your supermarket also contains luminescent material.
`Now that we have been reminded of how full of luminescent materials our society
`is, the question arises "How do we define a luminescent material'?" The answer is as
`follows: A luminescent material, also called a phosphor, is a solid which converts cer(cid:173)
`tain types of energy into electromagnetic radiation over and above thermal radiation.
`When you heat a solid to a temperature in excess of about 600"C , it emits (infra)red
`radiation. This is thermal radiation (and not luminescence) . The electromagnetic ra(cid:173)
`diation emitted by a luminescent material is usually in the visible range, but can also
`be in other spectral regions, such as the ultraviolet or infrared.
`Luminescence can be excited by many types of energy. Photoluminescence is
`excited by electromagnetic (often ultraviolet) radiation, cathodoluminescence by a
`beam of energetic electrons, electroluminescence by an electric voltage, tribolumines(cid:173)
`cence by mechanical energy (e.g. grinding), X-ray luminescence by X rays, chemi(cid:173)
`luminescence by the energy of a chemical reaction, and so on. Note that thermolu(cid:173)
`minescence does not refer to thermal excitation, but to stimulation of luminescence
`which was excited in a different way.
`
`EXC.
`
`EM .
`
`Fig. 1.1. A luminescent ion A in its host lattice. EXC: excitation; EM: emission (radiative return
`to the ground state); HEAT: nonradiative return to the ground state
`
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`2
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`I. A General Introduction to Luminescent Materials
`
`Fig. 1.2. Schematic energy level scheme of the luminescent ion A in Fig. 1.1. The asterisk
`indicates the excited state, R the radiative return and NR the nonradiative return to the ground
`state
`
`In Fig. 1.1, we have drawn schematically a crystal or a grain of a photoluminescent
`material in order to illustrate the definition. Our system consists of a host lattice and
`a luminescent center, often called an activator. For example, consider the famous
`luminescent materials Al 2 0 3 : c~+ (ruby) and Y 2 0 3 : Eu 3+. The host lattices are
`Al 2 0 3 and Y 2 0 3 , the activators the Cr'+ and the EuH ions.
`The luminescence processes in such a system are as follows. The exciting radiation
`is absorbed by the activator, raising it to an excited state (Fig. 1.2). The excited state
`returns to the ground state by emission of radiation. This suggests that every ion and
`every material shows luminescence. This is not the case. The reason for this is that the
`radiative emission process has a competitor, viz. the nonradiative return to the ground
`state. In that process the energy of the excited state is used to excite the vibrations
`of the host lattice, i.e. to heat the host lattice. In order to create efficient luminescent
`materials it is necessary to suppress this nonradiative process.
`The obvious characteristics to be measured on this system are the spectral energy
`distribution of the emission (the emission spectrum), and of the excitation (the exci(cid:173)
`tation spectrum; which in this simple case is often equal to the absorption spectrum),
`and the ratio of the radiative and the nonradiative rates of return to the ground state.
`The latter determines the conversion efficiency of our luminescent material.
`Let us for a second return to ruby (A1 2 0 3 : CrH). This is a beautiful red gem(cid:173)
`stone which shows a deepred luminescence under excitation with ultraviolet or visible
`radiation. Its spectroscopic properties were studied as early as 1867 by the famous
`scientist Becquerel, who used sunlight as the excitation source. He claimed that the
`color as well as the luminescence were intrinsic properties of the host lattice. This
`time, however, Becquerel was wrong. It is the CrH ion which is responsible for the
`optical absorption of ruby in the visible and ultraviolet spectral regions. The host
`lattice Ah0 3 does not participate at all in the optical processes. In fact A1 2 0 3 is col(cid:173)
`orless. In the case of the ruby, the activator A in Fig. 1.1 is the Cr'+ ion and the host
`lattice is Al 2 0 3 . The host lattice of this luminescent material has no other function
`than to hold the Cr3+ ion tightly.
`In many luminescent materials the situation is more complicated than depicted
`in Fig. 1.1, because the exciting radiation is not absorbed by the activator, but else(cid:173)
`where. For example, we can add another ion to the host lattice. This ion may absorb
`
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`

`
`I . A General Intro duction to Luminescent M ate rial s
`
`3
`
`Fig. 1.3. Energy tran sfer from a sensiti ser S to an activator A. Energy tran s fer is indicated by
`£. T. Fo r further notatio n, see Fig . 1. 1
`
`-.J\JV\f\1'-
`E.T.
`
`-..,,.------ A~
`
`- --+---A;
`
`_ _ __,__ _ _ A
`
`Fig. 1.4. Energy tran sfer from S to A. The S ------7 S* transition is the absorpti o n (or excitation) ,
`the A; ------7 A transition the emission. The level A i, populated by energy trans fer (£. T. ), decay s
`nonradiatively to the slightly lower A~ level. Thi s prevents back transfe r
`
`the exc1tmg radiation and subsequently transfer it to the activator. In this case the
`absorbing ion is called a sensitizer (see Fig. 1.3) .
`Another well-known example, viz. the lamp phosphor C a 5 (P04 )JF: Sb3 +, Mn 2 +.
`Ultraviolet radiation is not absorbed by Mn 2+, but only by Sb3+ . Under ultraviolet
`irradiation, the emission consists partly of blue Sb3+ emission, and partly of yellow
`Mn 2 + emission. Since the Mn 2+ ion was not excited directly, the e xcitation energy
`was transferred from Sb3+ to Mn 2+ (see Fig . 1.4). The luminescence processes can
`be written as follows , where hv indicates radiation with frequency v and the asterisk
`an excited state:
`Sb3+ + hv ~ (Sb3+)*
`(Sb3+) * + Mn2 + ~ Sb3+ + (Mn2+)*
`
`These "equations" indicate absorption, energy transfer, and emission, respectively.
`If the Sb3+ ion has no Mn 2+ ions in its vicinity, it gives its own blue emission .
`For those of you who are not familiar with solids, please reali ze that in general the
`concentrations of the luminescent centers· are of the order of a 1 mol.%, and that the
`centers are, in first approximation, distributed at random over the host lattice.
`Sometimes, however, the activator concentration can be 100%. This illustrates the
`rather complicated nature of luminescent materials. Actually these high activator con(cid:173)
`centrations which occur in some cases were not understood for a long time, preventing
`
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`4
`
`I . A General Introduction to Luminescent Materials
`
`progress in our understanding of luminescent materials. A famous example of such a
`high-concentration material is CaW04 where the tungstate group is the luminescent
`center. Simultaneously it is a building unit of the host lattice which consists of CaH
`and wo~- ions. This material was used for 75 years in X-ray photography, and in
`tungsten mines its luminescence is used to find CaW04 . The miners use ultraviolet
`lamps to find the tungstate-rich ores by their visible luminescence! Chapter 5 discusses
`why a high concentration of activators is sometimes fatal for luminescence, whereas
`in other cases such high concentrations yield very high luminescence outputs.
`In stead of exciting a low concentration of sensitizers or activators, we can also
`excite the host lattice. This is, for example, what happens if we excite with X rays
`or electron beams. In many cases the host lattice transfers its excitation energy to
`the activator, so that the host lattice acts as the sensitizer. Again a few examples.
`In YV04 : Eu 3+ ultraviolet radiation excites the vanadate groups, i.e. the host lattice.
`The emission, however, consists of Eu3+ emission. This shows that the host lattice is
`able to transfer its excitation energy to the Eu 3+ ions. Another example is ZnS : Ag+ ,
`the blue-emitting cathode-ray phosphor used in television tubes. Ultraviolet radiation,
`electron beams and X rays excite the sulfide host lattice which transfers this excitation
`energy rapidly to the activators (the Ag+ ions).
`In spite of the fact that we did not discuss any fundamental background (this
`will be done in Chapters 2-5), you have met by now the more important physical
`processes which play a role in a luminescent material:
`absorption (excitation) which may take place in the activator itself, in another ion
`(the sensitizer), or in the host lattice (Chapter 2)
`emission from the activator (Chapter 3)
`nonradiative return to the ground state, a process which reduces the luminescence
`efficiency of the material (Chapter 4)
`energy transfer between luminescent centers (Chapter 5).
`After this short, general introduction into the operation of a luminescent material,
`we now turn to a similar introduction to the applications of luminescent materials.
`Photoluminescence is used in fluorescent lamps. This application was even used
`before the Second World War. The lamp consists of a glass tube in which a low(cid:173)
`pressure mercury discharge generates ultraviolet radiation (85% of this radiation con(cid:173)
`sists of 254 nm radiation). The lamp phosphor (or a mixture of lamp phosphors) is
`applied to the inner side of the tube. This phosphor converts the ultraviolet radiation
`into white light. The efficiency of conversion of electricity to light is in a fluorescent
`lamp considerably higher than in an incandescent lamp.
`The introduction of rare-earth activated phosphors in fluorescent lamps during the
`last decade has improved the light output and the colour rendering drastically. As a
`consequence this type of lighting is no longer restricted to shops and offices, but is
`now also suitable for living rooms. It is interesting to note that in this way chemical
`elements which for a long time have been considered as rare, peculiar, and hard to sep(cid:173)
`arate, have penetrated our houses. A modern fluorescent lamp contains the following
`rare earth ions: divalent europium, trivalent cerium, gadolinium, terbium, yttrium and
`europium. You will find more about this important application of photoluminescence
`in Chapter 6.
`
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`I. A General Introduction to Luminescent Materials
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`5
`
`One can hardly imagine life today without cathode-ray tubes. Think of your tele(cid:173)
`vi s ion set or your computer screen. In a cathode-ray tube the luminescent material is
`applied on the inner side of the glass tube and bombarded with fast electrons from
`the electron gun in the rear end of the tube. When the electron hits the luminescent
`material, it emits visible light. In the case of a colour television tube there are three
`electron guns, one irradiating a blue-emitting luminescent material, so that it creates
`a blue pictures, whereas two others create in a similar way a green and a red picture.
`One fast electron creates in the luminescent material many electron-hole pairs
`which recombine on the luminescent center. This multiplication is one of the factors
`which have determined the success of the cathode-ray tube as a display. It will be
`clear that the luminescent materials applied belong to the class of materials where
`excitation occurs in the host lattice. They will b e discussed in Chapter 7 where we will
`also deal with materials for projection television. In thi s way the di splay screen can
`have a diam e ter of 2 m. This application puts requirements on luminescent materials
`which are hard to satisfy.
`Let us now turn to materials which are able to convert X-ray irradiation into
`visible light. Rontgen di sco vered X rays in 1895, and realized almost immediately
`that this type of radiation is not very efficient at exposing photographic film, because
`the film does not absorb the X rays effectively. As a consequence long irradiation
`times are required. Nowadays we· know that this is bad for the patient. There is also
`a practical objection against long irradiation times: the patient is a moving object (he
`breathes and may, in addition, make other movements), so that sharp pictures can
`only be obta ined if the irradiation time is short.
`Therefore Rontgen initiated a search for luminescent materials which absorb
`X rays efficiently and convert their energy into radiati o n which is able to blacken
`the photographic film. Soon it was found that CaW0 4 with a density of 6.06 g.cm - 3
`was able to do so. This compound was used for a very long time in the so-called X(cid:173)
`ray intensifying screens. A schematic picture of X-ray photography with this method
`is given in Fig. 1.5.
`Just as in the field of lamp phosphors and (partly) cathode-ray phosphors, CaW04
`lost its leading position to rare-earth activated X-ray phos phors (see chapter 8). As
`a salute to thi s old champion, but also for your information, we give in Fig. 1.6 the
`crystal structure of CaW04 which illustrates the build-up of the lattice from Ca2+
`ions and luminescent wo~- groups, and