UTS LIBRRRY
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`TUMTNHSCENT
`WERIALS
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`TCL 1016, Page 1
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`TCL 1016, Page 2TCL 1016, Page 2
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`G. Blasse, B. C. Grabmaier
`
`Luminescent
`Materials
`
`With l7l Figures and 3l Tables
`
`Springer-Verlag
`Berlin Heidelberg New York
`London Paris Tokyo
`Hong Kong Barcelona Budapest
`
`I
`
`/(
`
`

`

`Prof. Dr. G. Blasse
`
`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-81730 München
`Germany
`
`also with Debye Institute
`University Utrecht
`
`OF
`
`I 7 t{(lv 1994
`
`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 / G. Blasse, B'C. Crabmaier. p' cm'
`Includes bibliographical references and index.
`ISBN 3-540-58019-0. -- lsBN 0-387-58019-0 (u.s.)
`l. Phosphors. 2. Luminescence. I. Grabmaier, B. C., 1935- Il. Title'
`QC47ó.?.853 t994 620.t' t295--dc2o 94-20336 CIP
`
`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 staten'lent, that such names are exempt from the relevant protective laws
`and regulations and therefore free for general use.
`Typesetting with TgX: Data conversion by Lewis & Leins, Berlin
`SPIN: 10187460 0213020 - 5 4 3 2 I 0 - Printed on acid-free paper
`
`
`
`TCL 1016, Page 3TCL 1016, Page 3
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`ro patiently typed the
`)r corection appeared
`drawing some of the
`: disposal.
`r with and inspiration
`ìcts, some oral, some
`. In the preparation of
`, A. Bril, C.W.E. van
`ry useful.
`,minescence. We hope
`phenomena, to design
`rn in doing so.
`
`G. Blasse, Utrecht
`Grabmaier, München
`
`Table of Contents
`
`Chapter 1 A General Introduction to Luminescent Materials
`chapter 2 How Does a Luminescent Material Absorb Its Excitation Energy?
`
`2.1
`2.2
`2.3
`
`2.4
`
`General Considerations.. " .'
`The Influence of the Host Lattice
`The Energy Level Diagrams of Individual Ions
`2.3.1 The Transition Metal Ions (dn)
`2.3.2 The Transition Metal Ions with d0 Configuration" '
`2.3.3 The Rare Earth Ions (4f')..
`23.4 The Rare Earth Ions (4f-5d and charge-Transfer Transitions)
`2.3.5 Ions with s2 Configuration. . ' '
`2.3.6 Ions with dlo Configuration...
`23.7 OtherCharge-TransferTransitions
`2.3.8 Color Centers .. . ' . .
`Host Lattice AbsorPtion
`References
`
`Chapter 3 Radiative Return to the Ground State: Emission
`
`3.1
`3.2
`J.J
`
`3.4
`3.5
`3.6.
`
`Introduction
`General Discussion of Emission from a Luminescent Center
`Some Special Classes of Luminescent Centers
`3.3.1 Exciton Emission from Alkali Halides'
`3.3.2 Rare Earth Ions (Line Emission)
`3.3.3 Rare Eafh Ions (Band Emission)
`3.3.4 Transition Metal Ions
`3.3.5 d0 Complex lons....'..¡...
`3.3.6 dro lons.
`3.3.7 s2 lons.
`3.3.8 The U6+ ion ' . . '
`3.3.9 Semiconductors . , ,. .
`3.3,10 Cross-Luminescence
`Afterglow
`Thermoluminescence
`Stimulated emission
`References
`
`l0
`r6
`20
`20
`25
`25
`27
`28
`29
`30
`30
`30
`3l
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`33
`33
`38
`38
`40
`45
`50
`52
`53
`55
`59
`60
`64
`65
`66
`67
`70
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`vlil
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`Table of Contents
`
`Chapter 4 Nonradiative Tlansitions
`4.1 Introduction
`4.2 Nonradiative Transitions in an Isolated Luminescent Centre
`4.2.1 The Weak-Coupling Case
`4.2.2 The Intermediate- and Strong-Coupling Cases.....
`4.3 Efficiency
`4.4 Maximum Efficiency for High Energy Excitation tl3l ...
`4.5 Photoionization and Electron-Transfer Quenching.
`4.6 Nonradiative Transitions in Semiconductors . .
`References
`
`Chapter 5 Energy Ttansfer
`5.1 Introduction
`5.2 Energy Transfer Between Unlike Luminescent Centers. ,
`5.3 Energy Transfer Between Identical Luminescent Centers
`5.3.1 Weak-Coupling Scheme Ions . . .
`5.3.2 Intermediate- and strong-coupling scheme ions,
`5.4 Energy Transfer in Semiconductors,.
`Refèrences
`
`Chapter 6 Lamp Phosphors
`6.1 Introduction
`6.2 Luminescent Lighting tl-31..
`6.3 The Preparation of Lamp Phosphors
`6.4 Photoluminescent Materials......
`6.4.1 . Lamp Phosphors for Lighting.
`6.4.2 Phosphors for Other Larnp Applications.
`6.4.3 Phosphors for High-Pressure Mercury Vapour Lamps
`6.4.4 Phosphors with Two-Photon E¡nission
`6.5 Outlook
`References
`
`Chapter 7 Cathode-Ray Phosphors
`7.1 Cathode-Ray Tubes: Principles 4nd Display
`7.2 Preparation of Cathode-Ray Phosphors
`7.3 Cathode-Ray Phosphors
`'7.3.1 Some General Remarks
`'7.3.2 Phosphors forBlack-and-V/hite Television
`7.3.3 Phosphors for Color Television
`1.3.4 Phosphors for Projection Television......
`7.3.5 Other Cathode-Ray Phosphors . . . .
`1.4 Outlook
`References
`
`1l
`72
`14
`'77
`
`84
`85
`86
`88
`89
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`9l
`9'.|
`95
`9s
`103
`r06
`r06
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`r08
`r08
`ill
`112
`l2
`126
`121
`r30
`r30
`133
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`TCL 1016, Page 5TCL 1016, Page 5
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`entre
`
`7l
`72
`'14
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`77
`84
`85
`86
`88
`89
`
`91
`9l
`9s
`95
`r03
`r06
`r06
`
`.... 134
`,... 136
`. . .. t3'l
`.... 137
`..,. 138
`.... 138
`. ... l4t
`.,,, 143
`.... 145
`.... 14s
`
`Table of Contents
`
`tx
`
`Chapter 8 X-Ray Phosphors and Scintillators (Integrating Techniques)
`
`8.1
`
`8.2
`
`8.3
`
`8.4
`
`Introduction
`8.1.1 X-Ray Absorpii"" . . . . .. . ........ :...... ... :.. .. .. . .. . .. .
`8.1.2 The Conventional Intensifying Screen
`8.1.3 The Photostimulable Storage Phosphor Screen
`8.1 .4 Computed Tomography
`Preparation of X-ray Phosphors
`8.2.1 Powder Screens
`8.2.2 Cerarnic Plates.
`8.2.3 Single Crystals.....
`Materials
`8.3.1 X-Ray Phosphors for Conventional Intensifying Screens....
`8.3.2 X-Ray Phosphors for Photostimulable Storage Screens..,...
`8.3.3 X-Ray Phosphors for Computed Tomography
`Outlook
`References
`
`Chapter 9 X-Ray Phosphors and Scintillators (Counting Techniques)
`
`9.1
`9.2
`9.3
`9.4
`9.5
`
`9.6
`
`Introduction
`The Interaction of Ionizing Radiation with Condensed Matter
`Applications of Scintillator Crystals
`Material Preparation (Crysral Growth)
`Scintillator Materials
`9.5,1 Alkali Halides......
`9.5.2 Tungstates
`9.5.3 BiaGe3O¡2 (BGO)
`9.5.4 Gd2SiO-5 : Ce3+ and Lu2SiO5 : Ce3+.
`9.5.5 CeF3 ..
`9.5.6 Other Ce3+ Scintillators and Related Materials. . . . . .
`9.5.7 BaF2 (Cross Luminescence; Pa¡ticle Discrimination) .
`9.5.8 Other Materials with Cross Luminescence .
`Outlook
`References
`
`Chapter 10 Other Applications
`
`l0.l Upconversion: Processes and Materials...
`l0.l,l Upconversion Processes
`10.1.2 Upconversion Materials
`10.2 The Luminescent Ion as a Probe.
`10.3 Luminescence Immuno-Assay
`10.3.I Principle
`10.3.2 Materials.
`
`146
`146
`t48
`149
`153
`r56
`156
`157
`r59
`r59
`159
`t62
`t65
`r68
`t69
`
`t70
`170
`172
`r78
`t82
`182
`r83
`r83
`t84
`186
`t88
`r88
`r90
`r9r
`193
`
`r95
`195
`197
`203
`206
`206
`208
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`TCL 1016, Page 6TCL 1016, Page 6
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`x
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`Table of Contents
`
`10.4 Electroluminescence.
`10.4. I Introduction
`10.4.2 Light-Em¡tting Diodes and Semiconductor Lasers
`10.4.3 High-Field Electrolurninescence.
`10.5 Amplifiers and Lasers with Optical Fibers .
`10.6 Luminescence of Very SmalÌ Particles
`References
`
`Appendix l. The Luminescence Literature
`Appendix 2. From Wavelength to Wavenurnber and Some Other Conversions
`Appendix 3. Luminescence, Fluorescence, Phosphoresence
`Appendix 4.. Plotting Emission Spectra
`
`Subject Index . -
`ñt,-\\-z'-'--z-
`
`210
`2t0
`210
`212
`214
`216
`218
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`221
`223
`224
`225
`
`221
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`TCL 1016, Page 7TCL 1016, Page 7
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`ers
`
`ther Conversions
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`210
`210
`210
`212
`214
`216
`218
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`22t
`223
`224
`225
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`221
`
`Csnprnn I
`A General Introduction to Luminescent Materials
`
`This chapter addresses those readers for who luminescent rnaterials are a new chal-
`lenge. Of course you are farniliar with lurninescent ¡naterials: you rneet thern everyclay
`in youl laboratoly and in your home. If this should corne as a sulprise. switch on
`your fluorescent lighting, relax in front of your television set, or take a look at the
`screen of your cornputer. Perhaps you would like sonrething rnore specialized. Re-
`mernber then y<>u| visit to the hospital for X-ray photography. oI the laser in your
`institute; the hearl of this instrurnent consists of a lurninescent rnaterial. However,
`such a high degree of specialization is not necessary. The packet of washing powder
`in your supelrnalket also contains lulninescent rnaterial.
`Now that we have been rerninded of how till of lunrinescent rnaterials our society
`is, the question arises "How do we define a lulninescent rnaterial'J" The answer. is as
`f'ollows: A lurninescent rnaterial, also called a phosphor, is a solid which convens cer-
`tain types of energy into electlomagnetic radiation over and above thennal racliation.
`vy'hen you heat a soìid to a temperature in excess of about 600'c, it elnits (infra)red
`radiation. This is therrnal radiation (and not luminescence). The electrornagneric ra-
`diation ernitted by a lurninescent material is usually in the visible range, but can also
`be in other spectral regions, such as the ultraviolet or intiared.
`Lurninescence can be excited by rnany types of energy. photoruminescence is
`excited by electrolnagnetic (often ultraviolet) radiation, cathodoluminescence by a
`bealn of energetic electrons, electroluminescence by an electric voltage, tribolumines-
`cence by mechanical energy (e.g. grinding), X-ray lunrinescence by X rays, cherni-
`luminescence by the energy of a chemical reaction. and so on. Note that therrnolu-
`lninescence does not refèr to thermal excitation, but to sti¡nulation of lulninescence
`which was excited in a different way.
`
`EXC
`
`EM
`
`HEAT
`
`Fig. 1.1. A lulninescent ion A in its host lattice. EXC; excitation; ÇM: ernission (radiative return
`to the ground state): HEA'I: nonradiative leturn to the ground staþ
`
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`TCL 1016, Page 8TCL 1016, Page 8
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`2
`
`L A General Introduction to Lulninescent Materials
`
`A
`
`R
`
`NR
`
`A
`Fig. 1.2. Schcrnaric energy level schelne of the lurninescent ion A in Fig. l.l. The ¿sl¿rlsk
`incl"icates the excited sta(e, R the ladiative return and NR the nonradiative return to the ground
`state
`
`In Fig. l.l, we have jrawn schematically a crystal or a grain of a photoluminescent
`rnaterial in order to illustrate the clefinition. Our system consists of a host lattice and
`a lulninescent center, often called an activator. For example, consider the fãmous
`lulninescent materials Al2Oj:Cr3+ (ruby) and Y2O:: Eu3+. The host lattices ¿ue
`
`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,iemission spectrurn), and of the excitation (the exci-
`tation spectrum; which in this simple case is ofìen 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 conversíon efficiency of our lul¡inescent material'
`Ler us for a second return to ruby (Aì2O3: Cr3+). This is a beautiful red gem-
`stone wh
`tation with ultraviolet or visible
`radiation
`s early as 1867 by the famous
`scientist
`on source' He claimed that the
`color as
`perties of the host lattice' This
`
`than to hold the Crr+ ion tightlY.
`In many luminescent materials the situation is more complicated than depicted
`in Fig. l.l, because the exciting radiation is not absorbed by the activator, but else-
`where. For example, we can add another ion to the host lattice' This ion may absorb
`
`
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`TCL 1016, Page 9TCL 1016, Page 9
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`L A General lntroduction to Luminescent Materials
`
`-t
`
`EXC.
`
`EM
`
`Fig. f3. Energy transfer from a sensitiser S to an activator A. Energy transfer is indicated by
`E 7. For further notation, see Fig. l. l
`
`S
`
`S
`
`E.ï
`
`A,
`
`A,
`
`A
`
`Fig. f.4. Energy transfer fronr S to A. The S -+ S+ transition is the absorption (or excitation),
`the Ai + A'transition the enrission. The level Af, populated by energy transfer (8.r.), decays
`nonradiatively to the slightly lower A{ level. This prevents back transfer
`
`the exciting 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 Cas(pO¿)¡F : Sb3+, Mnl+.
`ultraviolet radiation is not absorbed by Mn2+, but only by Sb3+. under ultraviolet
`imadiation, the emission consists partly of blue Sbl+ emission, and partly of yellow
`Mn2+ emission. since the Mn2+ ion was not excited,directry, the eicitation energy
`was transferred from Sb3+ to Mn2+ (see Fig. l.Ð.,The luminescence processes can
`be written as follows, where hz indicates radiation'with frequency z and the asterisk
`an excited state:
`,
`Sb3+ + hz --+ (Sb3+)t
`(Sb3*)* * Mn2a --+ Sb3+ + (Mn2+)*
`(Mn'*)* -* Mn2+ + hz.
`These "equations" indicate absorption, ,n"rgy ounrf"r, and emission, respectively,
`If the sb3+ ion has no Mn2+ ions in its vicinity, it gives its own blue emission.
`For those of you who are not familiar with solids, please realize that in general the
`concentrations of the luminescent centers'are of the order of a I mol.vo, and that the
`centers are, in first approxirnation, distributed at random over the host lattice,
`Sometimes, however, the activator concentration canbe 1OO7o. This illustrates the
`rather complicated nature of luminescent materials. Actually these high activator con-
`centrations which occur in some cases were not understood for a long time, preventing
`
`)
`
`\ in Fig. l.l. The osterisk
`liative return to the ground
`
`in of a photoluminescent
`ists of a host lattice and
`le, consider the famous
`i. The host lattices a¡e
`
`¡s. The exciting radiation
`r. 1.2). The excited state
`;gests that every ion and
`reason for this is that the
`tive retum to the ground
`to excite the vibrations
`ile efficient luminescent
`t.
`r are the spectral energy
`the excitation (the exci-
`re absorption spectrum),
`turn to the ground state.
`:scent material,
`is a beautiful red gem-
`¡ith ultraviolet or visible
`as 1867 by the famous
`ce. He claimed that the
`of the host lattice, This
`:h is responsible for the
`rctral regions. The host
`es. In fact Al2O3 is col-
`re Cd+ ion and the host
`I has no other function
`
`nplicated than depicted
`the activator, but else-
`p. This ion may absorb
`
`
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`TCL 1016, Page 10TCL 1016, Page 10
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`4
`
`l. A General lntroduction to Luminescent Materials
`
`progress in our understanding of luminescent materials. A famous example of such a
`high-concentration material is CaWOq where the tungstate group is the luminescent
`center. Simultaneously it is a building unit of the host lattice which consists of Ca2+
`and WO]- ions. This material was used for 75 years in X-ray photography, and in
`tungsten mines its lurninescence is used to find CaWOq. 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 lutninescence outputs.
`In stead of exciting a low concentration of sensitizers or activators, we can also
`excite the host lattice. This is, fbr example, what happens if we excite with X rays
`or electron beams. ln 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 YVO¡ : Eu3+ ultraviolet radiation excites the vanadate groups, i.e. the host lattice.
`The emission, however, consists of Eu3+ er¡ission. This shows that the host lattice is
`able to transfèr its excitation energy to the Eu3+ ions. Another exarnple 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 fàct 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 frorn the activator (Chapter 3)
`- nonradiative return to the ground state, a process which reduces the lurninescence
`efficiency of the rnaterial (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-
`pressure mercury discharge generates ultraviolet radiation (85Vo of this radiation con-
`sists of 254 nm radiation). The lamp phosphor (or a mixture of larnp 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 actìvated 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 tirne have been considered as rare, peculiar, and hard to sep-
`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.
`
`
`
`TCL 1016, Page 11TCL 1016, Page 11
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`

`nous exarrple of such a
`|oup is the lulninescent
`which consists of Carl.
`ry photoglaphy, and in
`: miners use ultl'aviolct
`rce! Chapter 5 discusses
`lu rninescence, whereas
`cscence outputs.
`activators, wc calt also
`we excite with X lays
`ts excitation encrgy to
`\gain a f'ew exarnples.
`r¡rs, i.c. the host lattice.
`s that the host lattice is
`exarnple is ZnS : Agr',
`s. Ultraviolet radiation,
`transfêrs this excitation
`
`ental backgrouncl (this
`ole irnportant physical
`
`or itself. in anothel ion
`
`luces the lurninescence
`
`r lulninescent rnatelial,
`rninescent tìlaterials.
`ication was even used
`tube in which a low-
`/o of this radiation con-
`of larnp phosphors) is
`re ultraviolet radiation
`ight is in a fluorescent
`
`scent larnps during the
`ering drastically. As a
`cps and offices, but is
`t in this way chernical
`:uliar, and harcl to sep-
`contains the fblìowing
`r, terbiurn, yttriutn and
`of photolurninescence
`
`I A Ccncral Intloductiorr to [,r¡l¡incsccnt Matcrials
`
`-5
`
`One can hardly irnagine lifè today without cnthode-ray tubes. Think of youl tele-
`vision set ol'your conlputel'screen. ln a cathode-r'ay tube the luminescent tn¿tterial is
`appliecl ort thc innel side of thc glass tube and bornbalclecl with fast electrons frorn
`the electt'olt gtrn irt the t'ear end of the tube. When the electron hits the luminescent
`lttaterial, it ertlits visible light. In the case of a colour television tube there ale three
`cÌectron guns, one irradiating a blue-enritting lunrinescent tnater'ìal, so that it creates
`t bltte picttrlcs, wherctts two others cl'eate in a silnilal w¿ìy a green and a re(l picture,
`Onc fast eìcctron creates in the lurninescent rnatcri¿rl rnany electlon-hole pairs
`which recorlbine on the lurninescent center. This rnultiplication is one of the factols
`which have cletcl'¡ninecì the success of the cathode-ray tube as a clisplay. It will be
`clear that thc lulnirtescent rlraterials applied belong to the class of lllaterials whele
`excitation occut's ilt the host lattice . They will be cliscussed in Chapter 7 whele we will
`also cleal with rnater-ials fbr prtljection television. In this way the clisplay screen c¿ìn
`havc a diatnetcl of 2 rn. This application pr.rts lcquilernents on lunrinescent ¡naterials
`which ule hirrtl trr s¿rt¡sly.
`Let us now turn to lnaterials which are able to conveft X-ray irracliation into
`visible Iight. Röntgen cliscovcred X rays in lllg-5, and realiz.ecl alrnost irnrnecliately
`that this type of radiation is not very el'fìcient at exposing photographic fìlrn, because
`the fìlnl cloes not absorb the X rays ettectively. As a consec¡uence long irracliation
`tirncs are lequir'ccl. Nowadays we know that this is bacl fìl'the patient. Thele is also
`a t)racticaì objcction against long irradiation tirnes: the patient is a lnoving object (he
`bleathcs and rnay, in adrlition, rnake other rnovernents), so that sharp pictures can
`only bc obtaincd if the irracliation tilne is sho¡t.
`Therefìtre Röntgen initiated a sealch fbr lulninescent rnaterials which absorb
`X lays effìcientìy ancì convert their energy into racliation which is able to blacken
`the ¡rhotoglaphic fìlrn. Soon it was f'ound that cawo4 with a clensity of 6.06 g.cm-3
`was abÌe to do so. This conrpound was used ful a vely long time in the so-callecl X-
`ray intensifying screens. A schcrnatic picture of X-ray photography with this rnethocì
`is given in Fig. l.-5.
`Just as i¡r thc fìclcl of larnp phosphols and (paltly) cathocle-ray phosphors, cawoa
`lost its lcacling position to rare-earth activated X-r'ay ¡rhosphols (see chapter ti). As
`a salute to this old chanrpion, but also firr your infolnration, we give in Fig. 1.6 the
`clystal structtrre of CaWO¡ which illustl'ates the builcl-up of the lattice frorn Ca2+
`ions and lurninescent woj groups, ancl in Fig. 1.7. an electron micrograph of a
`corrrnercial CaWO,¡ powder.
`A recent developlnent in this fìelcl is the introduction of stol'¿ìge phosphors. These
`m¿ìtel'¡als have a "rnernory" for the arnount of X rays which has been absor.becl at a
`given spot of the screen. By scanning the irradiated screen with an (infi'a)red laser,
`visible lurninescence is stirnulated. Its intensity is proportional to the amount of X rays
`absorbed. Chapter tì cliscusses how to produce such rnaterials ancÌ the physics behincl
`these phenolnena.
`As atr exarnple of a lnore specialized character, we should l¡ention the case of X-
`ray conrputecl tornoglaphy, This rnethod of rnedical radiology generates closs-sectional
`irnages of the interior of the hurnan body (see Fig. 1.8). Besicles the X-ray source,
`the key conìponent is the detectol consisting of about 1000 pieces of a lur¡inescent
`solicl (crystals or cerarnics) which are connected to photocliocles or -rnultipliers. The
`
`
`
`TCL 1016, Page 12TCL 1016, Page 12
`
`I
`
`

`

`6
`
`l. A General lntroduction to Lunrinescent Materials
`
`-r f ilm
`
`X-roy source potient intensifying screen
`
`Fig. 1.5, Schernatic representation ol a nredical radioglaphy systenr based on the use of'an
`intensifying screen
`
`X
`
`oo
`
`Fig. 1.ó. Crystal structure of CaWOa (scheelite). The general formula is AXO¿ where A are the
`larger and X the smaller metal ions
`
`luminescent material has to show sharply defined properties in order to be acceptable
`for this application.
`After these examples of X-ray photography, you will not be surprised to learn that
`d and y radiation can also be detected by luminescent materials, which, in this case,
`are usually called scintillators and are often in the form of large single crystals. The
`applications range from medical diagnostics (for example positron emission tomog-
`
`
`
`TCL 1016, Page 13TCL 1016, Page 13
`
`

`

`l. A Ccncral lntroduction to Lu¡uincsccnt Matcrirls
`
`1
`
`'film
`
`s cr een
`
`nl bascd on thc usc ol ¿rn
`
`I,'ig. 1,7. CaWOa ¡rowdcr phos¡rhor (1000x)
`
`ian beam
`
`X-ray source
`
`detector
`
`I
`
`I I
`
`\
`
`r is AXOa where A al'c thc
`
`Itig. l.tt. The pr inciplc ol- X-riry corl¡rutcd tornogr aphy. Thc paticnt is in thc ccntcr of' the ¡rictur-e.
`Sourcc and dctcctor rotatc around thc pÍlticnt
`
`l order to be acceptable
`
`e surprised to learn that
`als, which, in this case,
`rge single crystals. The
`sitron emission tornog-
`
`raphy (PET)) to nuclear ancl high-energy physics. A spectaculiu application in the
`fatter fìeld is the use ol' 12000 crystals of BiqGe.¡Orz (3 x 3 x 24 cm3) as a detector
`fbr electrons and photons in the LEP rnachine at CERN (Ceneva). Scintillators will
`be discussed in Chapter'9.
`As a matter of fact there are rnany other types of application. Solne of these
`are discussed in Chapter 10. Befbre you get the irnpression that (the application of)
`
`
`
`TCL 1016, Page 14TCL 1016, Page 14
`
`

`

`8
`
`L A General Introduction to Lunrinescent Materials
`
`Ln3+
`
`Fig. 1.9. Thc [Ln c bpy.bpy.bpy]3+ cryptate
`
`lurninescence is restricted to solids, we should pick out one of these, viz. fluoro-
`irnnrunoassay, which is based on luminescent rnolecules. This is a method used in
`irnrnunology in order to detect biornolecules. The method is superior to other rnethods
`(such as those using radioactive molecules) as far as sensitivity and specificity are
`concerned. It is used parlicularly in the clinical investigation of corrpounds in low
`concentration and consists of the labelling of sarnples with luminescent species and
`the rneasurement of their luminescence.
`One of the molecules which plays a role in this field is depicted in Fig. 1.9. The
`luminescent species is the Eu3+ ion which we rnet already above. It is surrounded by
`a cage containing rnolecules of bipyridine. The whole complex is called a cryptate
`and its fonnula is written as [Eu c bpy.bpy.bpy]3+. The cage protects the Eu3+
`ion against the (aqueous) surroundings which tries to quench the luminescence. If
`this cryptate is excited with ultraviolet radiation, the bipyridine molecules absorb the
`exciting radiation and transfer their excitation energy subsequently to the Eu3+ ion
`which then shows its red luminescence.
`Coordination chernists call this transfer tiom bipyridine to Eu3+ an antenna eff'ect.
`It is of course exactly the same phenornenon which we described above (Fig. 1.3).
`In solid state research the effect is simply called energy transfer. In Chapter l0 we
`will see that the physics of molecules like cryptates is very similar to that of solids
`like those depicted in Fig. 1.3.
`Finally that intriguing application yielding the laser. In luminescence the radiative
`decay ofthe excited state to the ground state occurs by spontaneous emission, i.e. the
`emission processes on different activqtor ions a¡e not correlated. If, by some means,
`the majority of the luminescent ions are in the excited state (this situation is called
`population inversion), a single spontaneously emitted photon (quantum of radiation)
`may stirnulate otherexcited ions to emit. This process is called stimulated emission. It
`is monochromatic, coherent and non-divergent. Laser action depends on emission by
`a stirnulated process. Actually the word laser is an acronym for light amplification by
`stimulated emission of radiation. This book does not deal with lasers or laser physics.
`However, we will deal with the material where the stirnulated emission is generated
`if this is useful for our purpose. Every laser material is after all also a luminescent
`material.
`
`
`
`TCL 1016, Page 15TCL 1016, Page 15
`
`

`

`References
`
`9
`
`If you are surprised, remember the gemstone ruby (Al2O3 : CC+) mentioned in the
`beginning of this chapter. Becquerel started long ago to study its luminescence and
`spectroscopy. Ruby was and is the start of several interesting phenomena in solid state
`physics. Probably the rnost important of these is the first solid state laser which was
`based on ruby (Maiman, 1960). This illustrates the connection between luminescence
`and lasers. On the other hand, in more recent years it has only been possible to unravel
`and understand luminescence processes by using laser spectroscopy.
`This introductory chapter should have stimulated or excited you to read on and
`to become acquainted with the physics and the chemistry of luminescence and lumi-
`nescent materials.
`
`References
`
`Chaprer I does not have specific literature relerences in view of its general and introductory
`character. Those who are invited by this chapter to improve their knowledge ofthe basic physics
`and chernistry of energy levels, ions, spectloscopy and/or solid state chemistry, are referred to
`general textbooks in the field of physical and inorganic chemistry. Our personal prefèrences are
`the following: P.W. Atkins (1990) Physical Chemistry,4th edn, Oxford University Press; and
`D.F. Shriver, P.W. Atkins and C.H. Langfbrd (1990) lnorganic Chemistry, Oxford University
`Press (chapters l4and l8). The ruby history has been reviewed by C.F. Imbusch (1988) In:
`W.M. Yen and M.D. Levenson (eds) Lasers, Spectroscopy and New ldeas. Springer Berlin
`Heidelberg New York. Those who want to judge the great progress in luminescent nlaterials
`during the last few decades should cornpare this book with the review paper by J.L. Ouweldes,
`Lunrinescence and Phosphors, Modern Materials 5 (1965) p. l6l.
`
`ne of these, viz. fluoro-
`'his is a method used in
`;uperior to other methods
`tivity and specificity are
`rn of compounds in low
`luminescent species and
`
`Cepicted in Fig. 1.9. The
`rove. It is surrounded by
`plex is called a cryptate
`cage protects the Eu3+
`Lch the luminescence. If
`ne molecules absorb the
`quently to the Eu3+ ion
`
`¡ Eu3+ an antenna effect.
`cribed above (Fig. 1.3).
`nsfer. In Chapter l0 we
`similar to that of solids
`
`ninescence the radiative
`lneous emission, i.e. the
`ted. Il by some rneans,
`(this situation is called
`(quantum of radiation)
`I stimulated emission. It
`Jepends on emission by
`rr light amplification by
`r lasers or laser physics.
`d emission is generated
`: all also a luminescent
`
`
`
`TCL 1016, Page 16TCL 1016, Page 16
`
`

`

`Cnnprpn 3
`Radiative Return to the Ground state: Emission
`
`3.1 Introduction
`
`tation Energy?
`
`laro P (1982) C.R. Ac. Sci.
`
`ysics and chemistry of rare
`
`ry, Oxford University hess,
`
`edn. Wiley, Chichestener
`
`t. Phys. 37: 5l I
`re physics and chemistry of
`imsterdam
`rysics and chemistry of rare
`rdam; (l 987) Spectroscopy
`w York (1987) 179
`
`) Adv. Physics 32:823
`
`rdrecht
`several editions.
`
`3.2 General Discussion of Emission
`from a Luminescent Center
`
`Figure 3.1 shows again the configurationàl coordinate diagram. For the moment we
`assume that there is an offset between the parabolas of the ground and excited state.
`According to Chapter 2 absorption occurs in a broad optical bãnd and brings the center
`in a high vibrational level of the excited state. The center returns first to the lowest
`vibrational level of the excited state giving up the excess energy to the surroundings.
`Another way to describe this process is to say that the nuctef aa¡ust their positions
`to the new (excited) situation, so t¡at the interatomic distances equal the equilibrium
`distances belonging to the excited state. The configurational coordinate changes by
`AR. This process is called relaxation.
`
`
`
`TCL 1016, Page 17TCL 1016, Page 17
`
`

`

`34
`
`3. Radiative Return to the Ground State: Emission
`
`relaxation
`
`E
`
`R
`
`R"
`R:
`AR
`
`-F
`
`is' 2'3)' The absorption transition g -+ e
`; i: :::',ä'1, iå:lÏi,.Ï:;Ïî il"àl'::
`ssion e -> g occurs in a broad band' The
`
`-
`
`parabola offset is given bY AR
`
`"urity
`
`Duringrelaxationthereoccursusuallynoemission,andcertainlynotofhigh
`seen from the iates involved: whereas a very fast emission
`intensity. This can u"
`it"î"ìãt" "i 108 s-r, the vibrational rate is about l0r3 s-r'
`FromthelowestvibrationalleveloftheexcitedStatethesystemcanreturntothe
`ground state spontaneously under emission of radiation. The rules for this process
`afethesameasdescribedfortneabsorptionprocess.Thedifferenceisthatemission
`occurs spontaneously (ie' in ttt" uq'ence of a radiation field)' whereas absorption
`can only occur when a radiation fielà is present. The reverse process of absorption is
`stimulaied emission (see Sect' 3'6) and not spontaneous emission'
`By emission, tne cent"r .eacheá a high vibìational level of the ground state. Again
`relaxationoccurs'butnowtothelowestvibrationallevelofthegroundstate'The
`emission occurs at a lower energy than the absorption due to the relaxation pro-
`Fi ;ure 3.2 shows the emission and excitation
`cesses (see Fig' 3'l)' e, *
`"^utñile'
`(: absorption) spectra of the luminescençe of Bi3+ in LaOCI' The energy difference
`betweenthemaximumofthe(lowest)excitationbandandthatoftheemissionband
`is called the Stokes shift. It wiìl ue immediately clear that the larger the value of AR
`is, the larger the Stokes shift a¡rd the broader the optical bands involved'
`
`
`
`TCL 1016, Page 18TCL 1016, Page 18
`
`

`

`3.2. General Discussion of Emissionfrom a Lulninescent Center
`
`35
`
`STOKES SHIFT
`
`a 1
`
`qr
`
`1
`
`EXC.
`
`EM.
`
`2so
`
`300
`
`350 nm
`Fig,3.2. Emission and excitation.spectra of thc Bi3+ lurninescence of LaoCl :Bi3+. The Sfokes
`shift arnounts to about 9fi)0 cm-¡. In this and other ngures q. gives the rerative quantum output
`and <Þ the spectral radiant power per constant wavelength inieival, both in arbitrarv units
`
`4OO
`
`
`
`TCL 1016, Page 19TCL 1016, Page 19
`
`,lt
`
`R
`
`re absorption transition g + e
`th maxirnurn intensity). After'
`I state. Subsequcntly it rclaxcs
`I occurs in a broad band. Thc
`
`and certainly not of high
`hereas a very fast erniss