This chapter surveys the field of solid-state lu-minescent materials, beginning with a discussionof the different ways in which luminescence canbe excited. The internal energy-level structures ofluminescent ions and centres, particularly rare-earth ions, are then discussed before the effectsof the vibrating host lattice are included. Hav-ing set the theoretical framework in place, thechapter then proceeds to discuss the specific exci-tation process for photo-stimulated luminescenceand thermally stimulated luminescence beforeconcluding by surveying current applications, in-cluding phosphors for compact fluorescent and LEDlighting, long-term persistent phosphors, x-raystorage phosphors, and scintillators.
Luminescent materials are substances which convert anincident energy input into the emission of electromag-netic waves in the ultraviolet (UV), visible or infraredregions of the spectrum, over and above that due toblack-body emission. A wide range of energy sourcescan stimulate luminescence, and their diversity providesa convenient classification scheme for luminescencephenomena, which is summarised in Table 38.1. Pho-toluminescence, where the luminescence is stimulatedby UV or visible light, is a widely used materialsscience technique for characterising dopants and im-purities, and finds applications in lighting technologiessuch as fluorescent and solid state lamps. Radiolumi-nescence involves excitation by ionising radiation, andis used in scintillators for nuclear particle detection;the special case of stimulation by energetic electronsis called cathodoluminescence, the name arising fromearly atomic physics experiments involving gas dis-charges. Electroluminescence involves collisional exci-tation by internal electrons accelerated by an appliedelectric field, and with a much lower energy than
in the case of cathodoluminescence. Electrolumines-cence finds applications in panel lighting used in someliquid-crystal display (LCD) back-planes, in inorganiclight-emitting diodes (LEDs) and organic light-emittingdiodes (OLEDs).
There are other forms of luminescence which wemention for completeness but will not discuss further:bioluminescence and chemiluminescence where the en-ergy input is from chemical or biochemical reactions,sonoluminescence (sound wave excitation), and tribolu-minescence (strain or fracture excitation). Both organicand inorganic systems can display luminescence, buthere we focus primarily on inorganic systems.
There are several bookswhich describe the lumines-cence and spectroscopy of inorganic materials [38.1–4]and applications to phosphors and scintillators [38.5, 6]in more depth than is possible in a short article.
The terms phosphorescence and fluorescence are of-ten used in connection with luminescent materials. Thisclassification is based on the time-domain response ofthe luminescent system. Figure 38.1 shows a generic
photoluminescent system where incident UV radiationexcites a system from a ground state 1 with probabilityper unit time P12 into an excited state 2. The system de-cays with probability P23 to a luminescing level 3, fromwhich there is a probability P31R and P31NR of radia-tive and nonradiative decay, respectively, to the groundstate. Nonradiative decay generally involves phononemission.
First suppose that the transition probabilities aresuch that P12, P21 � P31.D P31RCP31NR/. If the opticalpumping (excitation) is abruptly stopped, the popula-tion of the luminescing state 3 decays as,
N3 D N3.0/ exp .�P31t/ (38.1)
and the rate of luminescent energy emission is�.h�P31R=P31/dN3=dt for an energy differenceE31 D h� between states 1 and 3. Hence the lumines-cence intensity at distance r from the sample is,
I D .h�/N3.0/P31Rexp.�P31t/
4 r2(38.2)
and the characteristic luminescence lifetime is� D .P31/
�1. Thus the lifetime is governed by both ra-diative and nonradiative processes, whilst the intensityof the luminescence depends on the relative magnitudeof P31R.
This discussion provides the basis for understandingthe terms fluorescence and phosphorescence applied toluminescent materials. A material is often classified asone or the other according to the relative magnitude of� D .P31/
�1, with 10 ns being set in a relatively arbi-
Luminescing state
Ground state
Excited state
P12
2
1
P31NR P31R
3
P23
Fig. 38.1 Optical pumping cycle for a generic photolumi-nescent system
trary way as the boundary between a fast fluorescentsystem and a slow phosphorescent one. For compari-son, the theoretical lifetime for spontaneous emissionfor a strongly allowed hydrogen atom 2p ! 1s transi-tion is about 0:2 ns.
However, by this definition phosphorescence canalso arise from luminescent states with short lifetimeswhich are populated through ones with long lifetimes.In Fig. 38.1, if state 2 is long-lived in the sense thatP23 � P31 then the measured lifetime for emissionfrom the luminescing state will be � D .P23/
�1, and thesystem will be labelled phosphorescent, even thoughthe luminescing level itself has a very short lifetime.In consequence, a second classification [38.2] is basedon whether or not the luminescing level is fed bya metastable state which sets the lifetime. Sometimesthe metastable state is a long-lived intermediate formof energy storage which can be triggered by an exter-nal stimulus to undergo a transition to a fluorescentlevel. Thus in Table 38.1, thermally stimulated and op-tically stimulated luminescence involve a metastablestate consisting of trapped electrons and holes, whichcan be triggered to recombine by heating or by opticalstimulation; the recombination energy is transferred toa fluorescing centre.
Overall, the fluorescence/phosphorescence classi-fication is somewhat nebulous, and it is debateablewhether the classification is necessary or desirable.
A wide variety of centres give rise to luminescencein semiconductors and insulating materials, includingrare-earth ions, transition-metal ions, excitons, donor–acceptor pairs, and ions with a d10 or s2 electronic con-figuration ground state. Organic molecules also displayluminescence on two different time scales, fast fluo-rescence associated with spin-allowed singlet-to-singlettransitions, and slow phosphorescence from triplet-to-singlet transitions. Some luminescence spectra consistof broad emission bands arising from the interaction be-tween the electronic system of the luminescent centreand the vibrations of the atoms or ions, which surroundit; the broad bands arise from simultaneous transitionsof both electronic and vibrational systems. For others,such as the rare earths, the spectra comprise sharp linesarising from purely electronic transitions, and the effectof the environment is felt mainly through their effectson the lifetimes of the states. Thus in discussing thephysical background to luminescence, it is simplest tostart with a discussion of rare-earth luminescence, as anexample where the effect of vibrations can be initiallyignored.
38.1.1 Rare-Earth Ions
The trivalent rare-earth ions have n electrons (n D1� 14) in the 4f shell. In a free ion, the eigenstatesresulting from the various atomic interactions are la-belled by the total spin S and orbital angular momentaL. Spin–orbit coupling breaks up each L; S multipletof degeneracy .2SC 1/.2LC 1/ into sub-multiplets la-belled by the total angular momentum J D LCS, whereJ can range from L� S to LC S. The 4fn orbitalslie within the outer 5s2 and 5p6 filled shells, whichpartly shield them from the effects of a crystallineenvironment. The effects of the latter are quantita-tively described by the crystal field [38.3, 7], and thisterm in the Hamiltonian splits the J multiplets into2J C 1 sublevels, the so-called crystal-field splitting.Some of these crystal-field levels may still be two-or threefold degenerate, depending upon the symme-try of the environment. Odd-electron systems alwayshave at least twofold (Kramers) degeneracy. The re-sulting energy-level structures are complicated, andare summarised in the classic Dieke diagram [38.7].The original has been updated by Carnall et al. [38.8]and is reproduced in many books and papers on rare-earth ion spectroscopy [38.9–11] which should beconsulted for a more extensive presentation of thesubject than is possible here. In Fig. 38.2, we showa schematic version (not accurately to scale) of the di-
agram appropriate for Pr3C with n D 2, which servesfor our discussion. The crystal-field splitting is usu-ally smaller than the spin–orbit splitting, and is il-lustrated schematically by the vertical extent of thebands in Fig. 38.2. The multiplet labels follow the usual2SC1LJ scheme.
In the figure we also show a generic highest excitedband which does not belong to the 4f2 configuration.In the specific case of the rare earths this can be theexcited-state configuration resulting from promotion ofone electron to the 5d state, giving an overall 4fn�15d1
configuration; more generally it could be a so-calledcharge transfer band, which corresponds to the trans-fer of one electron from the ligands to the luminescention. The relative location and importance of these bandsvaries with the luminescent ion and the crystallineenvironment, but they play an important role in the ex-citation of luminescence.
Excitation and luminescence transitions within thevarious levels in Fig. 38.2 are governed by the goldenrule of quantum mechanics [38.12] for interactionswith the electromagnetic field; in summary the prob-ability of a transition between two states i and j isproportional to square of the matrix element < ijHjj>,
4f 5d or charge transfer
1S0
Energy (104cm–1)
40
20
0
3P23P13P0
1I6
1G4
1D2
3F4 3F33F2 3H63H5
3H4
Fig. 38.2 Schematic energy-level diagram for Pr3C
PartD|38
.1
1000 Part D Materials for Optoelectronics and Photonics
where H is the time-dependent perturbation Hamilto-nian representing the interaction of the electrons withthe electromagnetic field. The proportionality constantcontains the light intensity in the case of excitation.The perturbation can be expanded in a power seriesinvolving electric and magnetic multipoles of the elec-tronic system, but of these the electric dipole term isdominant, with the magnetic dipole term being muchsmaller by a factor of more than five orders of mag-nitude. Since the electric dipole operator er has oddparity, the matrix element for transitions rij D< ijerjj>is necessarily zero unless i and j have opposite par-ity. This is the most important selection rule governingluminescence: transitions between states of the sameparity have zero transition probability and so are for-bidden (Laporte’s rule). In the case of the rare earths,all states of a single 4fn configuration have the sameparity, and so all optical transitions within the config-uration are strictly forbidden. But this rule is relaxedby several considerations. First, if the crystalline en-vironment lacks inversion symmetry, the crystal fieldadmixes a small fraction of the excited configura-tions (e.g., 4f15d1 for Pr3C) of opposite parity intothe ground configuration, which makes such transi-tions weakly allowed. Secondly, the selection rule formagnetic dipole transitions is that they are allowedbetween states of the same parity, although they are typ-ically about five orders of magnitude weaker than forelectric dipole ones. Finally, odd-parity vibrations andan electron–phonon interaction produce a similar con-figuration admixture effect to static lattice odd-paritymixing although this effect is more important for 3dions than for 4f ones.
With only weak transitions possible within the 4fconfiguration, one might wonder how it would be possi-ble to optically excite any significant luminescence. Theanswer lies in the 5d or charge-transfer bands whicheither lie at higher energies, or overlap with the upperlevels of the 4f configuration. These give rise to strongabsorption and efficient pumping. Relaxation can oc-cur via the parity-allowed transitions to the upper levelsof the 4f configuration, and from there via single ormultiple radiative emissions back to the ground state.Because these intra-configurational transitions are onlyweakly allowed, the lifetimes are generally quite long,of the order of �s–ms. Figure 38.2 shows some of theobserved transitions in the case of the Pr3C ion. Thereare further constraints on possible transitions whicharise from an analysis of the angular momenta of theinitial and final states. For example, a transition be-tween two states both of which have J D 0 is forbiddensince there is no angular momentum change as requiredfor a photon; similarly for dipole transitions we require J D 0;˙1.
The transition probability per second for sponta-neous emission [38.13] is given by
Pij D 64 4�3
3hc3ˇ̌rijˇ̌2
; (38.3)
where � is the frequency of the transition, h is Planck’sconstant, c is the velocity of light, and jrijj is the ma-trix element of the electric dipole operator er betweenthe two states i and j, and e is the electronic charge.For absorption, this must be multiplied by N, the meannumber of photons with energy h�, which thus in-corporates the effect of the incident beam intensity.Experimentally, one measures an absorption coefficientk as a function of energy k.E/ [38.13] which is linkedto Pij through,
Zk.E/dE D Ni
� e2h
nmc
��n2 C 2
3
�2
fij ; (38.4)
where n is the refractive index of the crystal environ-ment,m is the electronic mass,Ni is the concentration ofthe luminescent centres, and f is the oscillator strengthfor the transition. For both absorption and emission, thedimensionless oscillator strength fij defined as [38.13]
fij D 8 2m�
3he2ˇˇrij
ˇˇ2 (38.5)
is often quoted to compare the relative transition prob-abilities. For an electron harmonic oscillator,
Pfij D 1,
and so oscillator strengths of the order of 0:1�1 arestrongly allowed transitions.
38.1.2 Transition-Metal Ions
Transition-metal ions from the 3d series are charac-terised by a much stronger interaction with the crys-talline environment than the 4f ions since there is noequivalent of screening by the 5s, 5p outer shells.In addition, the spin–orbit coupling is weaker, andso the order of perturbation is reversed: the atomicL, S multiplets are split by the crystal field, withspin–orbit coupling being a smaller interaction. Intra-configurational transitions are again strictly forbidden,but become weakly allowed by inter-configurationalmixing through odd-parity crystal fields, and by odd-parity vibrations. The result of this is that the strongestselection rule after parity is that transitions should have S D 0, since the electric dipole operator does not in-volve spin. The other major difference, again due to thestrength of the crystal-field interaction, is that transi-tions which are purely electronic, the so-called zero-phonon lines, are rarely observed. Rather what are seen
Luminescent Materials 38.2 Interaction with the Lattice 1001Part
D|38.2
are broad bands which correspond to the simultane-ous excitation of an electronic transition and vibrationaltransitions, which overlap to give the broad observedbands. In particular, transitions involving odd-parity vi-brations have a high transition probability through theeffect of configuration admixing. This will be con-sidered in a following section. The most commonlyobserved luminescent ions are those from the d3 con-figuration (Cr3C, Mn4C) and from the d5 configuration(Mn2C). Avram and Brick [38.14] give a recent andcomprehensive account of transition metal ion spec-troscopy and fluorescence.
38.1.3 s2 Ions
The 5s2 (e.g., Sn2C and Sb3C) ions and 6s2 (e.g., TlC,Pb2C, Bi3C) ions are of considerable importance be-cause transitions to and from the excited s1p1 statesare Laporte-allowed. The interaction of the p state withthe crystalline environment can be very strong, and sobroad spectra are often observed.
38.1.4 Semiconductors
Luminescence in inorganic semiconductors is dom-inated by near-band-gap luminescence arising fromrecombination of electrons and holes. This process ismost efficient in direct-band-gap materials such as ZnSand GaP rather than indirect-gap materials such asSi and Ge because the transition probability requiresconservation of wavevector, but the photon wavevec-tor is � 0 on the scale of the Brillouin zone. Hencecreation or destruction of a phonon is required forband-to-band luminescence in indirect-gap materials,which is less probable. The near-edge emission maycorrespond to luminescence from a variety of shallow
energy-level structures such as free or trapped exci-tons, or from donor–acceptor recombination. These areboth example of electronic systems with spatially ex-tensive wavefunctions, in contrast to the atomicallylocalised 3d and 4f wavefunctions considered earlier.However, it is also possible to observe deep-level lu-minescence from transition-metal ions and rare earthsin semiconductors provided that the electron affini-ties and band-gap energies are such that the perti-nent energy levels fall in the band gap. In recentyears, recombination emission in structured PN junc-tions based on GaN and InGaN which emit in theUV/blue spectral region has become of substantial prac-tical importance in solid state lighting. Since inorganicsemiconductors are discussed elsewhere in this vol-ume, we shall not consider them further here, exceptas pump sources for down-conversion phosphors inSect. 38.6.2.
Organic semiconductors are the basis for organiclight emitting diodes (OLEDs) which are now findingapplications in television and mobile phone displays,and in lighting. The advantages of OLEDS are thesimple processing required, and ease of large area pro-duction and elelctrode patterning, together with a widerange of available emission colours. The operation isin principle similar to that of inorganic diodes but withvery different materials. A diode structure is is engi-neered with an emissive polymer or small moleculelayer sandwiched between between a polymer whichis a good hole transporting material and one which isa good electron transporting material. The two trans-port layers inject charge carriers into the emissivelayer, where they form excitons, which subsequentlydecay through luminescent molecular units in the emis-sive layer. OLEDs are described in more detail inChap. 51.
38.2 Interaction with the Lattice
For rare-earth ions, the interaction with the vibrationsof the crystal lattice can be ignored for most purposes;the observed luminescence spectrum consists of sets ofsharp electronic transitions. But for other luminescentions which interact strongly with the vibrating ions ofthe surrounding crystal, the incorporation of the latter iscritical to explaining the observed spectra. The simplestmodel of ion–lattice interactions is to consider only theN nearest neighbour ions and their atomic displace-ments Xn, Yn, Zn, (n D 1;N) in Cartesian coordinates.The vibrational Hamiltonian involves cross terms inthese coordinates, but may be transformed to harmonicform if symmetry-adapted forms of these coordinates
(normal modes) are used instead of the actual displace-ments. For example, the so-called breathing mode Qb,for an octahedrally coordinated ion takes the form
Qb D .Z1 � Z2 CX3 �X4 C Y5 �Y6/
6: (38.6)
If all the other modes have zero amplitude, the ionsmove radially towards or away from the central lumi-nescent ion. The key point in considering the influenceof the crystal lattice is that the vibrational potentialenergy is just the variation of the electronic energywith ionic displacement, or equivalently with the nor-mal modes (within the spirit of the Born–Oppenheimer
1002 Part D Materials for Optoelectronics and Photonics
Energy
Coordinate Q
ExcitationEmission
Fig. 38.3 Configuration coordinate diagram for excita-tion/emission cycle
approximation). Put another way, the crystal field de-pends on the ion positions so that the electron and latticequantum-mechanical systems are linked through thiselectron–lattice coupling.We can therefore expect a dif-ference in the harmonic vibrational potential from oneelectronic state to another, so that it will in general havethe form .1=2/m!2
i .Q�Q0i/2; i. e., both the magnitude
of the potential and the position of the minima Q0i willdepend on the electronic state i. The vibrational statesin the harmonic approximation are just the usual simpleharmonic oscillator states with energies (nC 1=2/h�š,with n an integer. Thus we arrive at a configuration co-ordinate diagram such as that shown in Fig. 38.3, withthe potential energies for a ground state g and excitedstate e being offset parabolas of different curvature.The square of the vibrational wavefunction is shownfor the ground state and that with n D 11. We note thatfor a classical oscillator there would be a peak in theprobability function at the extreme lengths of travel,corresponding to the maxima for n D 11 at the outerlimits of the wavefunction in Fig. 38.3.
We consider first luminescence from the excitedelectronic state/lowest vibrational state. The transi-tion probability may be calculated using the Franck–Condon principle, that is we assume that the durationof the electronic transition is much shorter than a vi-brational period, so that the Q remains constant dur-ing a transition. The transition is therefore taken tobe vertical on the configuration coordinate diagram.The transition probability is just h�ej�gi2, the square
Energy
AbsorptionEmission
Stokes shift
Fig. 38.4 Absorption and emission line shapes for strongelectron–lattice coupling
of the vibrational wavefunction overlap, multiplied bythe electronic transition probability considered earlier.From Fig. 38.3, this overlap will be a maximum forsome vibrational state other than the ground state, un-less the positions of the minima of the two potentialenergy curves coincide accidentally. (In Fig. 38.3, thismaximum would be for the n D 11 ground vibrationalstate). Put another way, the maximum transition prob-ability is not for the zero-phonon transition (no changein vibrational state), but corresponds to the creation ofa finite number of phonons. A range of transitions isallowed, and the result in a semiclassical analysis, al-lowing for finite line widths, is a Gaussian-shaped band.The analysis has to be extended to include finite tem-peratures and other modes, as described in detail byStruck and Fonger [38.15], but the overall result is thatthe emission line shape is approximately a Gaussiancentred at an energy lower than that of the differencebetween the minima of the two potential curves. Thesame argument can be applied to the excitation process,and again an approximately Gaussian line shape resultsbut this time centred on an energy above that of the dif-ference in potential-energy minima. Figure 38.4 showsthe overall result; the difference between the maxima ofthe two curves, known as the Stokes shift, is an indica-tor of the degree of electron–lattice coupling.
It is clear from the diagram that, in cases wherethere is strong electron–lattice coupling of this type,that: (a) there will be very little intensity in the zero-phonon line, and (b) there will be a large Stokes shiftbetween the energies for maximum absorption andmaximum emission. Thus the luminescence spectra oftransition-metal ions, colour centres, and closed-shellground-state ions (s2, d0), which have strong interac-tions with the lattice in the excited states, are typicallybroad bands with only occasionally weak, sharp zero-
phonon lines being observed. For luminescence fromwithin the 4f states of the rare earths, the reverse is true;
we are in the weak-coupling regime, and zero-phononlines are the predominant features of the spectrum.
38.3 Thermally Stimulated Luminescence
Thermally stimulated luminescence (TSL), or sim-ply thermoluminescence (TL), refers to luminescenceinduced by thermally stimulated recombination oftrapped electrons and holes in materials which havebeen subject to prior irradiation. The irradiation, whichmay be in the form of UV light, x-rays, gamma rays,or energetic electrons, creates free electrons and holes,most of which promptly recombine, but some of whichare locally trapped at defect centres such as impuritiesand vacancies. If the trap binding energies are suffi-ciently large, thermal promotion of the electron or holeto the conduction band or valence band, respectively, isimprobable at the irradiation temperature, and so thesecharge carriers remain trapped after irradiation. How-ever, if the sample is then heated, thermally assistedrecombination becomes increasingly probable, and theresult is an initially increasing light output with increas-ing temperature until the traps are depleted, whereuponthe light intensity drops. The curve of light intensityversus temperature I.T/ is known as a glow curve, andmay be analysed to extract the trap depths and concen-trations. A comprehensive review of the field has beengiven byMcKeever [38.16], and a shorter discussion byVij [38.1].
The process is shown schematically for a simplesystem comprising a single electron trapping level Tand a single recombination centre R in Fig. 38.5.
Irradiation results in a trapped electron at trap T anda trapped hole at R. The trapped hole binding energyis larger than that of the trapped electron, so the latteris depopulated first, with a probability P which has the
Conduction band
Valence band
T
R
Fig. 38.5 Thermally stimulated luminescence process
form,
P D s exp
� �E
kBT
�; (38.7)
where s and E are the attempt frequency and the activa-tion energy respectively. We have shown the hole trapand recombination centre as being one and the same inFig. 38.5, but it is also possible that the recombinationenergy is transferred to a separate luminescent centre.
In the measurement process, the sample is heatedat a fast and linear rate, typically 1�10K=s, whilstthe light emission is monitored by a sensitive fil-ter/photomultiplier combination, or by a monochroma-tor system. The resulting glow curve may be fitted toa theoretical curve to extract the trap parameters. Gen-erally the light output is quite weak, and TSL systemshave sensitivity as one of their prime design factors,so cooled detectors and photon counting are common-place. Above about 400 ıC, thermal radiation from thesample/heater is a problem and must be eliminated byfiltering or by subtraction of a glow curve recorded us-ing a thermally bleached sample.
The main uses of TSL are in determining trapdepths and irradiation doses. In archaeological and geo-logical applications, a comparison is made between ac-cumulated natural dose and a dose from a radioisotope;by combining this with a measurement of the activityof the surroundings, a date since last thermal or opti-cal erasure of the object can be determined. The natureof the traps is often poorly understood for these chem-ically complex samples. For medical dosimetry, room-temperature stable traps with high thermoluminescentoutput are required. At present, the material of choiceis LiF doped with a few hundred ppm of Mg and Ti.
The mathematical form of the TSL glow curve de-pends on the physical model used for the TL process. Inthe simplest case, assuming first-order kinetics [38.1,16], the light intensity I at temperature T is given by,
I.T/ D n0s exp
� �E
kBT
�
� exp
2
4� s
ˇ
TZ
T0
exp
� �E
kBT
�dT
3
5 ; (38.8)
where n0 is the number of occupied traps at time t D t0when the temperature is T0, ˇ is the heating rate in K=s,
PartD|38.4
1004 Part D Materials for Optoelectronics and Photonics
1.2
1.0
0.8
0.6
0.4
0.2
0500450400350300
Intensity (arb. units)
Temperature (K)
0.9 eV1.0 eV1.1 eV
Fig. 38.6 Computed glow curves for first-order kineticsfor fixed escape frequency and various trap depths
and kB is Boltzmann’s constant. The result is a glowcurve whose peak position varies approximately lin-early with trap depth E, as shown in Fig. 38.6 (for fixedescaped frequency s D 3:3�1011 s�1 and heating rate1K=s).
Of course, the peak position also depends on the es-cape frequency s, but is less sensitive to s than to E. Forsecond-order kinetics [38.1, 16], the glow curve equa-tion becomes
I.T/ Dn20sN
�exp
�EkBT
�
h1C
n0sNˇ
� R TT0exp
�EkBT
�dT
i2 ; (38.9)
1.2
1.0
0.8
0.6
0.4
0.2
0
–0.2500300 450400350
Intensity (arb. units)
Temperature (K)
E = 1 eVs = 3.3 x 10–11 s–1
ß = 1 K/s
Second order
First order
Fig. 38.7 Comparison of first- and second-order glowcurves
where N is the number of available traps. The two formsboth give glow curves of similar shape, but the first-order form shows an asymmetric form with a sharpfall off on the high-temperature side of the peak, whilstthe second-order form is more symmetric as shown inFig. 38.7. The parameters are extracted by least-squaresfitting of these expressions to the experimental glowcurves, which is cumbersome due to the integral, butKitis et al. [38.17] have given analytical approxima-tions to (38.8) and (38.9). In practice, many glow curvesdo not follow first- or second-order kinetics precisely.Furetta [38.18] describes the simulation of glow curvesfor a variety of TL analytical models.
38.4 Optically (Photo-)Stimulated Luminescence
Thermally stimulated luminescence is sometimes alsoaccompanied by optically stimulated luminescence(OSL), in which one of the trapped carriers is ex-cited by optical stimulation to a level from whichit can recombine with the conjugate carrier by tun-nelling, or completely to one of the bands so thatrecombination is achieved through what is essentiallya photoconductivity effect. For OSL to be significant,
there must be an appreciable optical transition prob-ability, so not all TSL centres are OSL active. Thestimulation energy measured in OSL generally dif-fers from that determined from TSL because of theFranck–Condon principle. The OSL effect finds prac-tical application in dosimetry, e.g., [38.19, 20] and inx-ray storage phosphors used for medical imaging, asdescribed later.
A typical traditional photoluminescencemeasuring sys-tem involves a broad-spectrum source, either a com-bined tungsten filament for the visible spectrum anddeuterium lamp for the UV, or a xenon flash lamp. Thelamp emission is passed through a grating monochro-mator and so selectively excites the luminescence.Band-pass or band-edge filters are generally required toeliminate unwanted second- and higher-order diffrac-tion maxima from the grating. The luminescence isefficiently gathered by low-f/number optics and fed toa second grating monochromator, also equipped withfilters, to monitor and analyse the luminescence. Thefinal detector may be a photomultiplier, or prefer-ably a charge-coupled device or photodiode array forimproved data collection efficiency at multiple wave-lengths.
This arrangement relies on good monochroma-tion/filtering to remove what is sometimes a relativelystrong component of scattered light from the beam anal-ysed by the emission monochromator. An alternativemethod of removing scattered light is to use time dis-crimination, by replacing the source by a xenon flashlamp. The flash has a duration of about 10�s, so anyscattered light has decayed away to an insignificantlevel when the emitted beam is sampled some time(0:1�10ms) after the flash. A timed electronic gate isused to sample the emission immediately after a flashand just before the next flash; the difference betweenthese two sampled signals gives the short-term lumines-cence whilst the second sample alone gives the long-term luminescence with long and short being relativeto the flash repetition period. Of course, luminescencewith lifetimes shorter than the pulse width (� 10�s)cannot be readily measured with this system. The lu-minescence intensity is normalised with respect to theexcitation intensity by steering a sample of the excita-tion beam to a rhodamine dye cell which has a quantumefficiency of essentially unity for wavelengths belowabout 630nm. The fluorescence from the dye is mea-sured with a second photomultiplier.
To minimise the effect of scattered light, a con-ventional laser with its intrinsically narrow linewidthand high intensity is a very convenient replacement fora broad-spectrum lamp, but suffers from the disadvan-tage of a fixed wavelength. Typical lasers of interestare nitrogen (pulsed), argon (UV lines, or frequency-doubled visible lines), krypton, and the new generationsof GaN/GaInN blue/violet/UV laser diodes. For rare-
earth spectroscopy, or other systems which are charac-terised by narrow absorption lines, it is very useful tohave a scanning dye laser as the excitation source. Thisselective excitation facility enables tagging of particularluminescent levels with excited states belonging to thesame centre, so that a picture of the energy-level struc-ture of each luminescent centre can be built up in caseswhere several such centres contribute to the overall lu-minescence.
For decay kinetics on faster time scales, fluorime-ters such as those developed initially by the Spexcompany (now Horiba) use a fast modulator and phase-sensitive detection to measure the phase shift betweenfluorescence and excitation; it is claimed that fluores-cence decays can be measured with a resolution of 25 psthis way. An alternative is the time-correlated single-photon-counting technique which can measure decayconstants in the ps–ns range. In this method, the exci-tation comes from a fast laser pulse, and the light levelreaching the photomultiplier or micro-channel plate de-tector is reduced to such a low level that less than onephoton per excitation pulse is detected. The time de-lay between the photon detection and the time of thepulse is measured, and a histogram produced of num-bers of detected photons versus arrival time taken overa large number of excitation pulses. For efficient datacollection, a high repetition rate and fast-pulse laser arerequired, often a Ti–sapphire laser. The wavelength forTi–sapphire is too large for stimulating many materialsdirectly with single-photon excitation, but stimulationis nonetheless possible by a two-photon excitation pro-cess, or by the use of a nonlinear crystal acting asa frequency doubler to produce laser output at one halfthe wavelength of the basic laser. Alternatively, a rangeof fixed wavelength LEDs and laser diodes with pulsewidths in the range of ns to ps have become available.
There are a number of specialist techniques, suchas hole-burning, fluorescence line-narrowing, and pho-ton echo methods associated with the use of laserswith either very narrow line widths or short pulseduration which have developed in a parallel way totechniques first introduced in nuclear magnetic reso-nance, and which are mainly used to investigate thedynamics and quantum mechanics of the lumines-cent species rather than the material in which theyare contained. Meijerink includes a short discussionof these specialist techniques in the book edited byVij [38.1].
PartD|38
.6
1006 Part D Materials for Optoelectronics and Photonics
38.6 Applications
The largest market for luminescent materials has tradi-tionally been in the areas of lighting, through fluores-cent tubes, and in cathode-ray-tube screen phosphorsfor image display. Both of these areas can now be re-garded as mature in terms of materials development,and in the case of cathode ray tubes, in decline as al-ternative display technologies have appeared. However,new discoveries in the past decade, and the advent ofnew technologies, have rekindled interest in phosphormaterials. Some of the current areas of activity in appli-cations are outlined below.
38.6.1 Compact Fluorescent Lamps
Compact Fluorescent Lamps (CFLs) are simply con-densed versions of the traditional fluorescent lamp witha built-in ballast/starter in the lamp base. The tube con-tains an argon/mercury gas mixture, and an electric dis-charge between two electrodes results in the emission of(predominantly) 254 nmUV light from the ionised mer-cury. This light falls on the powder phosphormixture onthe inside surface of the glass tube, and excites lumines-cence from the rare earth ions containedwithin the phos-phors. Typically Eu2C is used for blue emission, Eu3C
for red, and Ce-activated Tb3C for green. In the lattercase, theCe3C ion efficiently absorbs theUVand the en-ergy migrates to the Tb3C ions. A variety [38.6] of hostcrystals are used, typically alkaline earth aluminates andyttria, selected on the basis of their performance andstability at high temperatures under intense UV irradi-ation and ion bombardment. A major challenge is thatof good colour rendition, that is to achieve a light out-put thatmatches the spectrum of a conventional tungstenbulb in the visible region (which is close to the contin-uous spectrum expected for a black body radiator) withthe multiline output of a rare-earth phosphor. AlthoughCFLs offer a substantially higher energy efficiency andlifetime compared to conventional tungsten bulbs, albeitat a much higher cost, they face strong competion fromthe newer solid state LED technology as described be-low. The efficiencies of LED and CFL lamps are similar,but CFL’s suffer from the disadvantages of having a slowstart characteristic (as mercury liquid is progressivelyconverted into gas as the lamp warms up), they are notreadily dimmable, and they are an environmental hazardwhen they come to the end of their useful life due to the(small) mercury content.
38.6.2 Phosphors for Solid State Lighting
The development of blue, violet and UV LEDs basedon GaN, InGaN and other semiconductors and alloys
has laid the foundation for the new industry of solidstate lighting, where high powered blue or UV LEDexcites intense visible light emission from an adja-cent phosphor or phosphors [38.21, 22]. These so calledwhite-light LEDs are making large inroads into the do-mestic and industrial lighting market as high-efficiencyreplacements for traditional incandescent bulbs, andare at present in direct competition with CFLs. Themost common commercial design for a white LED usesvisible blue LED emission to stimulate yellow lumines-cence from a single phosphor, so that when that yellowlight is mixed with residual blue light from the LED, theresult is simulated white light. For UV emitting LEDs,the emission is used to stimulate separate blue, red andgreen phosphors, or a single phosphor containing red,green and blue emitting centres, whose collective out-put approximates white light. The phosphors used forwhite light LEDs are quite different to those used forCFL’s because of the need for efficient optical pumpingin the range of 370�450 nm rather than 254 nm.
The most common single phosphor/single centretechnology is based around a YAG:Ce garnet phos-phor powder, embedded in a transparent polymer resin,which converts some of the emission from a 465 nmblue GaInN LED into an orange/yellow emission cen-tred at 550 nm; the combination of blue and yellowsimulates white light. A very strong crystal field split-ting in the 5d state of Ce3C shifts the lowest emittinglevel from the UV or blue where it normally occurs,to the yellow/orange region of the spectrum. YAG:Cesatisfies the considerations of optical and thermal stabil-ity in high power applications, but the colour renditionis not optimal, particularly in the red spectral region.Other single phosphors [38.21, 22] which have beeninvestigated for this application include compositionalvariations on YAG:Ce, silicates, borates, sulphides, allwith various rare earth dopings. The nitrides, oxyni-trides and other complex nitrides [38.23, 24] are ofparticular interest from a colour rendition and thermalstability perspective. However, the original YAG:Ce hasundergone performance improvements which shouldmaintain its dominance in single phosphor white LEDs.The pumping and light extraction efficiency has beenimproved, for example through the development ofnanoparticle forms such as transparent glass ceram-ics [38.25] which minimise back-scattering.
Whilst the single phosphor/single centre phosphorimplementation has many advantages, including sim-plicity, improved colour rendition is obtained by usingmore phosphors or centres, with optimised balance be-tween the various emissions, for example with bluediode pumped SrGa2S4:Eu (green), SrY2S4:Eu (red),
Luminescent Materials 38.6 Applications 1007Part
D|38
.6
complemented by residual blue LED light. Recent de-velopments based on 375 nm UV LEDs have used mul-tiple emissions to achieve even better colour balance;for example Kim et al. [38.26] use Sr3MgSi2O8:Eu2C,Mn2C, which has blue (Eu2C), yellow (Eu2C) and redemissions (Mn2C). The europium ions presumably oc-cupy two different sites with very different crystal fieldsplittings. A major problem with multi-phosphor whiteLEDs is finding a satisfactory red emitter – usuallyEu3C fills this role, but it is not efficiently pumped bythe available LED wavelengths, and so Mn2C or Mn4C
are also under consideration [38.27].Other directions in research include the possible
use of photoluminescent quantum dots (QDs) [38.28]so as to improve colour rendition and achieve a warmwhite light output. QDs are nanometer-sized particles,in this context they are based on a semiconductor suchas CdSe or CdS whose band gap lies in the visible re-gion. The quantum confinement effect means that theenergy of recombination emission in these direct bandgap materials has a wavelength which is dependent onparticle size, resulting in a size-tuneable phosphor. Theemission colour can also be modified by chemical mod-ification of the nanoparticle surface with luminescentions such as rare earths; the recombination energy ofelectron-hole pairs excited by LED emission can mi-grate to the surface and excite these ions. The surfacelayer is a significant volume fraction for a nanoparticle,and so the surface emission can dominate the volumeemission. The combination of tailored absorption byadjusting the nanoparticle size and composition, andtailored emission by chemical modification of the sur-face, gives a very flexible design palette. However, thetoxicity of cadmium is an issue, and so other semicon-ductors which can be fabricated as nanoparticles areunder active consideration.
The efficiencies of white LEDs are comparable withCFLs and (currently) about five times larger than forincandescent bulbs, and with a lifetime which is abouttwenty five times longer. The major problems relate tothe cost, colour rendition, and stability at the high tem-peratures produced by incident optical powers of theorder of tens of watts=cm2, but continuing improve-ments can be expected in all of these aspects in contrastto the mature state of CFL development.
38.6.3 Long-Persistence Phosphors
It has been discovered in the past twenty years thatsome materials show a very long-lived but intense after-glow, arising from thermal emission of charge carriersfrom deep traps, followed by electron–hole recombina-tion at or near a luminescent ion, i. e., room-temperaturethermoluminescence. The persistence is for periods
of several hours, much longer than the well-knownZnS:Cu persistent phosphor. This new class of ma-terials [38.29], have widespread applications in areassuch as signage and passive emergency lighting forpublic buildings, aircraft cabins etc. The material isactivated or charged by the blue/UV content of solarradiation or fluorescent indoor lighting during normalconditions; in subsequent dark conditions the energy isreleased as an afterglow. No power supply is required,the operation is entirely passive, and so a high degreeof reliability is assured. Generally the phosphor pow-der (� 10�m grain size) is mixed with a resin binderand applied as a thick (� 1mm) coating. The firstmaterials which were used were rare-earth-doped stron-tium or calcium aluminate [CaAl2O4:Eu=Nd (blue),SrAl2O4:Eu=Dy (green), Sr4Al14O25:Eu=Dy (blue–green), and Y2O2S:Eu=Mg=Ti (orange)]. Figures 38.8and 38.9 show the decay and emission/excitation char-acteristics of a commercial phosphor strip of this type,based on Luminova SrAl2O4:Eu=Dy powder sourcedfrom the Nemoto company. The decay is clearly non–exponential, but after the initial rapid decay the inten-sity may be fitted with an exponential with a decayconstant of 3:1 h.
The persistent phosphor effect has now been foundin a wide variety of materials [38.30, 31] includingthe nitrides and oxynitrides (also of interest for LEDlighting), although the aluminates remain as the mostsignificant materials for all-round performance froma commercial perspective. The mechanismmust involve
1000
100
104.00 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Intensity (arb. units)
Time (h)
Fig. 38.8 Dark decay of persistent luminescence in a com-mercial lighting strip based on Nemoto SrAl2O4:Eu=Dymaterial
PartD|38
.6
1008 Part D Materials for Optoelectronics and Photonics
shallow charge carrier traps from which electrons orholes can recombine with the conjugate charge carrierthrough thermal excitation to the appropriate band, bytunnelling, or by thermally assisted tunnelling. A usefulpersistent phosphor requires both a substantial bright-ness, and a time constant for recombination which is ofthe order of hours. The traps which are responsible forthe long-lived decays in these systems have not beenclearly identified, and it is likely that there are severalinvolved. It would seem that progress in identifying themechanisms and traps will require corellated optical,ESR and TSL studies.
38.6.4 X-Ray Imaging
X-ray images have traditionally been recorded withphototographic film since the time of Roentgen, butit was noted in the very early days that photographicfilm itself was not very sensitive to x-rays. However,if a phosphor is used as an intermediate x-ray to visiblephoton converter, the sensitivity is increased by approx-imately two orders of magnitude. Consequently, x-rayfilm cassettes are usually outfitted with two intensifyingscreens which sandwich the photographic film. The op-erating principle of the screens is that an incident x-rayphoton generates a large number (tens of thousands) ofelectron-hole pairs through the photoelectric effect orCompton scattering. When the electrons and holes re-combine, the recombination energy can be efficientlytransferred to a luminescent centre, resulting in the
1.2
1.0
0.8
0.6
0.4
0.2
0700200 600500400300
Intensity (arb. units)
Wavelength (nm)
Excitation Emission
Fig. 38.9 Excitation and emission spectra of a commerciallighting strip based on Nemoto SrAl2O4:Eu=Dy material
emission of a large number of visible light photons. Themost common phosphor material [38.2] is powderedgadolinium oxysulphide doped with terbium, embed-ded in an organic binder, and which emits in the greenregion of the spectrum. However, this long standingtechnique of phosphor-enhanced film for x-ray radio-graphy is being superceded by a variety of new meth-ods, some of which are phosphor or scintillator based.Among these, optically stimulated or photo-stimulatedluminescence is the basis for an x-ray imaging tech-nology known commercially as computed radiography(CR). Several other techniques such as those basedon amorphous selenium photoconductor/flat panelsand a-Si arrays, and scintillator CCD/complementarymetal–oxide–semiconductor (CMOS) arrays have alsoemerged in recent years, but CR remains as a substan-tive technology on the basis of numbers of installedunits in medical and dental radiography. The manyadvantages of CR over photographic film, and the prin-ciples of the method are detailed in [38.32, 33], but thebasic mechanism is that incident x-rays create electron–hole pairs in the material. Most of these promptlyrecombine, but some are trapped at defects and impu-rities, and remain trapped for periods of hours after thex-ray source is turned off. In an x-ray storage phosphor(XRSP), one of the trapped carriers is optically stim-ulable, and can be excited to the conduction band orvalence band, or to a level from which it can recombineby tunnelling with the conjugate trapped carrier. Theresulting recombination energy is transferred to a lumi-nescent ion, and the intensity of the photo-stimulatedluminescence (PSL) is in direct proportion to the in-cident x-ray intensity. The dominant material used incurrent XRSP systems is BaFBr1�xIx:Eu2C, where theelectron traps are F-centres, the hole traps are unidenti-fied, and the luminescence is the 5d–4f transition of theEu2C ion. One disadvantage of systems based on this(powder) phosphor is that when the image is extractedwith a raster-scanned He–Ne laser beam, light scatter-ing from the powder grains means that material outsidethe focussed laser spot is also stimulated, limiting thespatial resolution to around 200�m, which is inade-quate for applications such as mammography. Severalways to overcome this are currently under develop-ment. CsBr:Eu is also an x-ray storage phosphor [38.34]and can be grown by vapour deposition in a colum-nar form. The columnar structure has a light-guidingproperty, restricting the scattering effect, and improvingthe resolution. A second development is that of glass-ceramic storage phosphors, where PSL active crystalsare embedded in a glass [38.35–37]; the combination ofparticle size, separation and refractive-index mismatchmeans that these composite materials are semitrans-parent and the problem of scattering of read-out light
Luminescent Materials 38.6 Applications 1009Part
D|38
.6
Fig. 38.10PSL image ofa BC549 transis-tor recorded ona glass-ceramicimaging plate
is reduced. Figure 38.10 shows an image of a BC549transistor recorded in a glass-ceramic x-ray storagephosphor.
The quest for high resolution x-ray imaging has alsoled to the development of transparent glasses in whichthe image is recorded by a quite different lumines-cent ion technique. Some rare earth ions, particularlyEu and Sm can show a valence change under x-ray orUV irradiation. For example, Sm present in the divalentstate in fluorophosphate and fluoroaluminate glasseshas been shown to transform in part from the trivalentto the divalent state under x-irradiation [38.38]. Theextra electron is sourced from the simultaneous cre-ation of colour centres. The concentration of divalentsamarium can be monitored by the Sm2C photolu-minescence spectrum, which is readily distinguishedfrom Sm3C as there is minimal overlap, and can befurther accentuated by an appropriate choice of excita-tion wavelength. One-dimensional x-ray imaging basedon the monitoring of the Sm2C photoluminescencehas been reported by Okada and Morell et al. [38.38,39] using the methodology of confocal microscopy toachieve spatial resolutions of the order of ten microns.This particular application of one dimensional imagingwas to beam profile monitoring in microbeam ther-apy, a proposed oncology treatment. Edgar et al. haveshown [38.40] that two dimensional imaging can beachieved with the same materials using a simple cameratechnique, and demonstrated resolution of the 25�mbonding wires on a semiconductor chip. However, thex-ray dose required for the fluorophosphate and fluoroa-luminate glasses is too high for conventional medicalradiography, and further work on alternative materialsor dopants is required to extend the range of applica-tions to the low dose regime.
38.6.5 Phosphors for Displays
Several new technologies have been developed to re-place cathode ray tubes for domestic televisions, includ-ing plasma display panels (PDPs). In these units, eachpixel is a sealed cell containing a mixture of Xe and Ne
in a dielectric-shielded electrode structure (for a review,see Boeuf [38.41]). An alternating current (AC) voltageapplied between the electrodes results in a glow dis-charge being set up in the gas, and a Xe dimer vacuumUV (VUV) emission predominantly between 147 and190 nm occurs. (In comparison, the mercury dischargein a conventional fluorescent tube emits primarily at254 nm.) The UV discharge excites red, blue, or greenphosphors coated on one of the cells; each colour isactivated by an adjacent electrode. The requirementsfor efficient output from these phosphors differ fromconventional tubes since the latter were chosen onthe basis of their luminescence efficiency at a 254 nmpump wavelength, and their resistance to degradationby the UV light and chemical attack by HgC ions.These requirements are evidently different for the PDPtechnology; in addition the phosphors must have a sig-nificant reflection coefficient in the visible to optimisethe light output [38.42], and the surface quality is ofgreater significance due to the short penetration depthof the VUV. The phosphors which have been used sofar include BaMgAl10O17:Eu2C (blue), Zn2SiO4:Mn2C
(green), and .Y;Gd/BO3:Eu3C and Y2O3:Eu3C (red).The blue phosphor is prone to degradation.
The widespread introduction of Xe excimer excita-tion in PDPs can be expected to stimulate applicationsin other lighting technologies. In this regard, the possi-bility of so-called quantum cutting is of much interest.This recognises that the energy of a VUV photon isequivalent to two or more visible photons, so that quan-tum efficiencies in excess of 100% can in principlebe achieved. The difficulty lies in finding a lumines-cent ion system whose energy-level system provides forboth efficient pumping and two-photon luminescence inthe visible. One example which has been reported tohave a quantum efficiency of up to � 145% is Pr3C inYF3 and other hosts [38.43, 44], where the excitation isthrough the allowed 4f2 ! 4f15d1 or host transitions.The system then decays to the 1S0 excited state of the4f2 configuration from which two-photon decay is pos-sible through successive 1S0 ! 1I6 and 3P0 ! 3HJ,3FJ transitions, as shown in Fig. 38.2. (The intermedi-ate step from 1I6 to 3P0 is provided by a nonradiativetransition.) A difficulty is that the photon for the firsttransition is in the UV region of the spectrum, and soit is necessary to incorporate a second luminescent ionpumped by this transition to convert the UV to visibleoutput, and the visible quantum efficiency is necessarilyreduced.
As at 2016, it would appear that PDP displays arelosing ground in comparison with LCD and OLEDbased display technologies, which are discussed else-where in this handbook. In contrast, a rising tech-nology is so-called QLEDs, or quantum dot OLEDs,
PartD|38
.6
1010 Part D Materials for Optoelectronics and Photonics
in which an OLED structure is modified to incorpo-rate quantum dots within the electron-hole recombi-nation zone [38.28]. The recombination energy mi-grates to the quantum dots, which can be customisedfor colour output by adjustments of size, semicon-ductor, and coating. A significant improvement in ef-ficiency has been achieved by replacing the organichole-transport layer with an inorganic one. The currentapplication of QLEDs is primarily in backlighting forLCD displays, where the existing cold cathode fluo-rescent or white LED panels suffer from poor colourrendition.
38.6.6 Scintillators
Although semiconductor detectors of ionising radiationhave made large inroads into the particle detection mar-ket, traditional scintillators are still widely used andare indispensable for some applications. The operatingprinciple is that an incident gamma ray creates a largenumber of electrons and holes in the scintillating ma-terial directly or indirectly through the photoelectriceffect, Compton scattering, or pair production. Theseenergetic electrons and holes lose energy to the latticeon a picosecond time scale through thermalisation pro-cesses, and their energy approaches that of the bandedges. They can then associate as free or bound ex-citons, and the energy of these electron-hole pairs isthen localised on luminescent ions which have beendeliberately doped into the material. The luminescention is promoted by this energy into an excited state,from which it can decay by photon emission. Onegamma or x-ray photon typically creates tens of thou-sands of electron-hole pairs, and a similar number ofvisible region photons which are emitted in one ag-gregate pulse or scintillation. Charged particles suchas protons produce electron–hole pairs through theCoulomb interaction with the band electrons. The scin-tillation or multiphoton bursts which signal the eventis detected by a photomultiplier, and the height of theoutput pulse from the photomultiplier is proportionalto the energy of the particle. A pulse-height analysersorts the pulses according to energy, and so an en-ergy spectrum can be obtained. Key figures of merit fora scintillator material include the light yield, i. e., thenumbers of photons per MeV of particle energy, the ra-diative lifetime of the luminescent ion (since possiblepulse overlap limits the maximum count rate which can
be measured), and the weighted density �Z4eff, which
reflects the gamma sensitivity. (Here Zeff is the effec-tive atomic number.) The most widely used scintillatorfor many years has been NaI:Tl, but many differentscintillators are being investigated, driven by the needfor improved performance and lower cost for medi-cal applications such as positron emission tomography(PET) and single-photon emission computed tomogra-phy (SPECT), and for large-scale elementary-particlefacilities such as the Large Hadron Collider at the Cen-tre Européen pour la Recherche Nucléaire (CERN).In the latter regard, the Crystal Clear collaborativeproject and other programs have resulted in several newmaterials such as LaBr3, LaCl3, Lu2SiO5, Gd2SiO5,Lu3Al5O12 and LuAlO3, all Ce-doped, CeBr3, andundoped Bi4Ge3O12 and PbWO4, with typical per-formance figures of 10 000�70 000 photons=MeV andlifetimes of 10�50 ns. For gamma spectroscopy thepulse-height resolution is critical and LaBr3:Ce hastwice the resolution of NaI:Tl. The fastest scintillatorsare based on core-valence luminescence where a holecreated in a core level recombines with an electron inthe valence band. For example, BaF2 shows this cross-luminescence effect with a lifetime as short as 600 ps.The effect is only shown by materials with a core-valence (CV) band energy gap less than the usual bandgap, otherwise CV luminescence is absorbed. Neutron-sensitive scintillators can be made by incorporating anelement which has a large cross section for thermal neu-tron capture such as Li, B, Gd, or Cd; the nuclear decayfollowing neutron capture results in gamma, alpha, orbeta emission which again results in electron–hole pairproduction and scintillation. Plastics are also useful forneutron detection because of the large neutron crosssection of hydrogen.
Most scintillator materials are single crystal inor-ganics, typically halides or oxides, doped with rareearth ions, usually Ce or Eu. However, it has beenshown in the past decade that high-performance scintil-lators can also be made in the form of transparent glassceramics [38.25] which has advantages in terms of pro-cessing and manufacturing flexibility with regard to thegeometrical form.
Rodnyi [38.45], Lecoq et al. [38.46], and vanEijk [38.47] discuss scintillator physics and perfor-mance in detail, whilst Nikl and Yoshikawa [38.48]present a recent update on inorganic single crystal scin-tillators.
Luminescent Materials References 1011Part
D|38
38.7 Representative PhosphorsTo conclude, we present in Table 38.2 a list of sev-eral luminescent materials of practical significance. Thetable is intended to be representative rather than com-
prehensive. It is noticeable from the table that just a fewions are responsible for a large number of applications,and primarily as oxides.
Table 38.2 Some luminescent materials of practical significance
Host Dopants Colour Excitation ApplicationBi4Ge3O12 – Blue Ionising radiation ScintillatorY3Al5O12 Ce3C Yellow Blue, violet White LEDGd2SiO5 Ce3C UV Ionising radiation ScintillatorLu3Al5O12 Ce3C Green Ionising radiation ScintillatorLaBr3 Ce3C UV Ionising radiation ScintillatorBaFBr Eu2C UV/blue X-rays X-ray imagingCsBr Eu2C Blue X-rays X-ray imagingBaMgAl10O17 Eu2C Blue UV Fluorescent lamps, plasma displaysGd2O2S Tb3C Green X-rays Intensifying screensSr3SiO5 Eu2C Blue UV White LEDSrGa2S4 Eu2C Green UV White LEDSrAl2O4 Eu2C, Dy3C Green UV, violet Persistent phosphorCaAl2O4 Eu2C, Nd3C Blue UV, violet Persistent phosphorY2O3 Eu3C Red Electrons, UV Plasma displays, fluorescent lampsSr2SiO4 Eu3C Yellow UV White LED.Y;Gd/BO3 Eu3C Red UV Plasma displaysSrY2S4 Eu3C Red UV White LEDLiF Mg2C and Ti4C UV/blue Ionizing radiation TL dosimetryZnS Mn2C Yellow Electric field Panel displaysZn2SiO4 Mn2C Green UV Plasma displaysCeMgAl11O19 Tb3C Green UV Fluorescent lamps
Acknowledgments. The author would like to ac-knowledge financial support from the New Zealand
Ministry of Business, Innovation, and Employment, andthe Foundation for Research Science and Technology.
References
38.1 D.J. Vij (Ed.): Luminescence of Solids (Plenum, NewYork 1998)
38.2 G. Blasse, B.C. Grabmeier: Luminescent Materials(Springer, Berlin, Heidelberg 1994)
38.21 J. McKittrick, L.E. Shea-Rohwer: J. Am. Ceram. Soc.97, 1327 (2014)
38.22 S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang: Mater.Sci. Eng. R 71, 1 (2010)
38.23 Y. Tian: J. Solid State Light. 1, 11 (2014)38.24 R.J. Xie, N. Hirosaki: Sci. Technol. Adv. Mater. 8, 588
(2007)38.25 A. Edgar: Optical properties of glasses. In: Optical
Properties of Condensed Matter, ed. by J. Singh (Wi-ley, Chichester 2006)
38.26 P.L. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, L.P. Park:Appl. Phys. Lett. 85, 3696 (2004)
38.27 J. Meyer, F. Tappe: Adv. Opt. Mater. 3, 424 (2015)38.28 V. Wood, V. Bulovic: Nano Rev. 1, 5202 (2010)38.29 K. van den Eeckhout, D. Poelman, P.F. Smet: Mate-
rials 3, 2536 (2010)38.30 K. van den Eeckhout, D. Poelman, P.F. Smet: Mate-
rials 6, 2789 (2013)38.31 P.F. Smet, J. Botterman, K. van den Eeckhout,
K. Korthuit, D. Poelman: Opt. Mater. 36, 1913 (2014)38.32 S. Schweizer: Phys. Status Solidi 187, 335 (2001)38.33 J.A. Rowlands: Phys. Med. Biol. 47, R123 (2002)38.34 P. Leblans, D. Vandenbroucke, P. Willems: Materials
4, 1034 (2011)
38.35 S. Schweizer, L. Hobbs, M. Secu, J.-M. Spaeth,A. Edgar, G.V.M. Williams: Appl. Phys. Lett. 83, 449(2003)
38.36 M. Secu, S. Schweizer, A. Edgar, G.V.M. Williams,U. Rieser: J. Phys. C 15, 1 (2003)
38.37 A. Edgar, G.V.M. Williams, S. Schweizer, M. Secu, J.-M. Spaeth: Curr. Appl. Phys. 4, 193 (2004)
38.38 G. Okada, B. Morell, C. Koughia, A. Edgar, C. Varoy,G. Belev, T. Wysokinski, D. Chapman, S. Kasap: Appl.Phys. Lett. 99, 121105 (2011)
38.39 B. Morrell, G. Okada, S. Vahedi, C. Koughia, A. Edgar,C. Varoy, G. Belev: J. Appl. Phys. 115, 063107 (2014)
38.40 A. Edgar, C.R. Varoy, C. Koughia, G. Okada, G. Belev,S. Kasap: J. Non-Cryst. Solids 377, 124 (2012)
38.41 J.P. Boeuf: J. Phys. D 36, R53 (2003)38.42 H. Bechtel, T. Juestel, H. Glaeser, D.U. Wiechert:
J. Soc. Inf. Disp. 10, 63 (2002)38.43 S. Kuck, I. Sokolska, M. Henke, M. Doring, T. Schef-
fler: J. Lumin. 102/103, 176 (2003)38.44 A.B. Vink, P. Dorenbos, C.W.E. Van Eijk: J. Solid State
