John A. DeLuca General Electric Corporate Research and Development Center P.O. Box 8 Schenectad~,NY 12301
An Introduction to Luminescence in Inorganic Solids
When a solid absorbs vbotons or charzed particles, a number of energy conversion processes are possible, as illustrated in Figure 1. One of these processes, luminescence, is used to advantage in such familiar applications as fluorescent lamps and television screens. We can define luminescence as the absorption of photons or charged particles by a substance which is then followed by aphoton emission in excess of that due to thermal apitation (incandescence) and which is stronelv dependent upon-the nature of the emitting substance (unlike incandescence). This introdktion to luminescence will treat the class of materials which emit characteristic luminescence. These materials, or phospors, consist of a host material which constitutes the bulk of the phosphor. The characteristic luminescence properties are obtained by adding ("doping") to the host material relatively small amounts of foreign ions. An activator is a forrign ion which when incorporated intoa host lattice rives rise to acenter which can be excited to luminescence. A sensitizer is a foreign ion incorporated into a host lattice and is capable of transferring its energy of excitation to a neighboring activator, thus inducing luminescence. Figure 2 is a schematic representation of aphosphor which contains an activator. The activator creates a center which a t ~ o r h excitation s energy and convem it into vlsihle radiation. 'I'he role of a sensitizer is illustrated in Figure 3. It may occur that an activator with the desired emission does not have a significant absorption for the a\,ailable exritation energy. In such a m e it may he possible to use a sensitizer which absorbs the excitation energy and then transfers this energy to the activator, which can then emit its characteristic luminescence. Characterization of Phosphors Two primary characteristies which are usually determined for a phosphor are its excitation spectrum and emission spectrum. We will use the spectra of the phosphor CaFz:Mn2+ to illustrate these determinations. The designation CaFzMn2+ is a short hand notation to indicate that a phosphor has been obtained by incorporating the activator Mn2+ into a CaFz host. A typicalexperimental arrangement for determiningexcitation spectra is shown schematically in Figure 4. In this example theexcitation source is theoutput of a monochromator which, like a prism, resolves the exr~tationlight source into Presented at the ACS National Meeting, March 25,1980, as part of the State-of-the-ArtSymposium on Solid State Chemistry in the Undergraduate Curriculum sponsored by the Division of Chemical Education.
Excitation -Absorption-Conversion Luminescence (T< W C ) - Photons VUV-clnfrared l@&-12.4 x 10d A 124ev+lev
/ X-rays. Electrons, UV. Visible 0124 A 6 2 0 0 A lpev2ev Flgure I.
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Thermal Processes - monons (-.Ole") - motons (T 7 WOOC) ChemicallStructural Change
Ccnversion of Excitation Energy in Solids.
Figure 2. Diagrammatic representation of me role in the luminescenceprocess of an activator (A) doped in a host (H) lattice.
Figure 3. Diagrammatic representation 01 the role in the luminescence pracess sensitizer (S) and ih relationship to an activator (A) and lhe host lattice
of a (HI.
Volume 57. Number 8. August 1980 1 541
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T
F i l t e r Selected to P a s s Emission A's -Absorb Excitation A's Sample
~ctivstorConcentration Figure 7. The effenof activator concentration on phosphor efficiency.
F!gm 4. Schematic diagram of a typical experimental anangsment f a recarding me excitation spechum of a phosphor.
Phosphor Efficiency
Wawlengfh in Nanomefcrr
Figure 5. The excitation and emission spectrum of Mnz+ in a CaF2 host. Figure 8. The effect of poison centers on phosphor efficiency
t Excitation
Fpr SBiected to - Pass Emission A's Absorb Excitation A
I I
I 0
Figure 6. Schematic diagram of atypical expimental amangemem fw recading the emission spectrum of a phosphor. its comnonent wavelenzths. The excitation wavelength of interest illuminates thesample. The intensity of theluminescence emission is measured by a photomultiplier tuhe. The optical cutoff filter placed between the sample and the photomultiplier tube is selected so that it will pass the luminescence emission hut will absorb the reflected excitation radiation. The output of the photomultiplier tuhe is amplified and then fed into the v axw of an r-v recorder. The value of the excitation wavelength selected Ls plotted on the x-axis. Thus. one obtains an x-v . plot . which shows the intensitv of the luminescencr emission as a function of the wavelength o i the rxcitntlun radiation. The excitation swctrum oi the activator Mn2+ in the host CaFz is shown in Figure 5. An experimental arrangement for the determination of an emission spectrum is shown schematically in Figure 6. Usually a single excitation wavelength is selected. The optical cutoff filter serves the same purpose as previously descrihed. In this exneriment the emission of the sample is analyzed by means ofhamonochromator. The result i s m x-y plot showing the intensity of the emission as a function of the wavelength of the emission. The emission spectrum of Mn2+ in CaF2 is also shown in Figure 5. 542 1 Journal of Chemical Education
2W
400 T (Dk)
TB
, \ 8W , 600
Figure 9. The effect of temperature on phosphor efficiency. Inspertion of Figure 5 illustrates two other important chararteristics of luminescence emis%ion.It is apparent that t,he occurs at~loneer~waveleneths ~ - luminescence - ~ - ~ ~ emission ~ ~ ~than is referred to as the excitation radiation. This the Stokes shift and is more commonlv described bv. savine . .. that the energy of an emitted photon is usually less than that of the photon which excited the luminesr~nceOIU,.,~,.,~,. > h vemission). The other characteristic noticeable in Figure 5 is that the peaks in the t!xcitatim spectrum are nut of equal intensity. '['his occurs hecause the effirienry of con\wsion of the exritation energy into emission energyis not the same for the three absorption processes that give rise to the three peaks in the excitation spectrum. The conversion efficiency of a phosphor is an important consideration in commercial applications. This as either efficiencv can he determined and is usuallv. exnressed . an energy conversion efficiency p~ = Eemitted/Eineident. or a quantum conversion efficiency pg = Qemitted/Qincident. ~
~
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Phosphor Phenomena
Like most solid state properties, the luminescence characteristics of a phosphor are dependent upon such factors as the
~
Potential Energy
Re
Bond
Distance
Figure 10. Ground state potential energy of a luminescent center in a crystal lattice with its as~ociatedvibrational states.
I
Re
Bond Distance Figure 12. Thermal quenching in the configurational Mordlnate model. details of careful sample preparation can greatly minimize the chances of discovering a superior phosphor. Thermal quenching refers to the fact that phosphors are generally characterized by a transition temperature, Tg,above which luminescence efficiency decreases rapidly to vanishingly small values. The phenomenon of thermal quenching is illustrated graphically in Figure 9.
Potential Energy
I
Re
Bond Distance Figure 11. Oround and excited electronic states showing the excitation and emission processes of luminescence. composition of the material, the level of purity, and the temperature. These dependencies are reflected in the phenomena of concentration quenching, poisoning, and thermal quenching. The graph shown in Figure 7 illustrates concentration quenching. If one has a phosphor consisting of an activator incorporated into a host, then one might justifiably expect the luminescence emission intensity (at a given excitation intensitv) to increase with increases in the activator concentration. That is, by increasing the activator concentratioli one would he effectively increasina the ahorption of the incident excitation energy by the phosGhor. ~ e n e d l this y expected behavior is true at lower concentrations of activator is& . 10%).hut as the concentration of activator is increased still further one finds that there is an optimum concentration above which the phosphor efficiency actually decreases. Phosphors can be very sensitive to the presence of foreign ions other than the beneficial sensitizers and activators. The effect of these "poisons" or "killers" on phosphor efficiency is illustrated in Figure 8. If the phosphor is contaminated with the wrong kind of impurity, it may have only a fraction of its potential efficiencv. This phenomenon is especiallv important to keep in mind when ul;dertaking a program t o synthesize new phosphors. A cavalier attitude toward attention to the
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Configurational Coordinate Model of Characteristic Luminescence The configurational coordinate model of characteristic luminescence is especially valuable in helping one to gain a better insight into the various phenomena that we have discussed so far. The potential energy diagram shown in Figure 10 forms the basis of this model. The potential curve represents the binding (electronic) energy of an ion in a host lattice as a functinn of theinternuclear se~arationbetween it and its .~~ nearest neighbors. Inn h u n d system the vibrational energy is uuantized. 'l'he nuantized ril~ra~ional states are indicated by the levels V, inkigure 10. Of course. a hound ion mav exist in a number of electronic states ifsuitably excited from-its lowest or "ground" electronic state. Each of these states can he represented bv a potential energy curer and assoviated vibratibnal levels. I n i.'igurr 11 a gruund state and higher "rxcited" state are illuitrated. It dhould be noted that neithrr the shape ot'the pot~.ntinlenergy curves nor the position of their minimums (corresponding to the equilibrium internuclear distance) necessarily coincide. Using the configurational model as a means of focusing our thoughts, we'll now use i t to explain the phosphor characteristics described earlier. The processes of excitation and emission are illustrated in Figure 11. The hound activator is excited' hy absorbing a quantum of energy, causing it to un. dereo a transition from its lowest enerev state to an ... eround ,. rxcited state. Hecause the potential rurves of the two states are not swnmetrical. the activator "ends UD" in a hiah vihrational state within the excited electronic state. some energy is lost to the host lattice as vibrational enerw when the excited state system relaxes to its lowest vibrational level. At this ~ o i n t the , system returns to its ground electronic state by givingoff the energy difference as aquantum of luminescence ~~~
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'Transitions between electronic states of a bound system correspond to quantum energies of photons in theultraviolet through the visible spectrum. Volume 57, Number 8,August 1980 / 543
emission. Note that the energy required to excite the system (length of up arrow) is greater than the energy of the emitted quantum of energy (length of down arrow). Thus, the model accounts for the origin of the Stokes Shift in addition to the excitation and emission process. Thermal quenching may be explained in this model as illustrated in Figure 12. When there is sufficient thermal energy available (higher temperatures), the activator, after it has been excited to an upper vibrational level in the excited electronic state, can be excited to a still higher vibrational level. In the illustration in Figure 12 it can be seen that this vibrational level corresponds to a point of intersection (or equal energy) between the excited and ground electronic states. Given the possibility of two paths, the system will seek the path which allows it to return to its lowest possible energy. Thus, the system returns to its lowest state by losing energy t o the host lattice through vibrational transitions. The end result is that the excitation energy is lost to the lattice and does not contribute to luminescence emission. As indicated in Figure 11,the rather complicated potential energy curve and its associated vibrational levels can he represented simply by a line for each state. The line representation of the energy states is used in Figure 13 to illustrate the concept of energy transfer which was introduced earlier in our definition of a sensitizer (see Figure 3). We see in Figure 13 that a sensitizer is first excited to an upper energy state by absorbiue the radiation incident on the ~ r . enerw . h o -s ~ h oThis of excitation is transferred (sensitizer returns t o its ground state). bv. excitine a neiahhorine activator which then returns to its ground Zate with anHccompanying luminescence emission. This process of transfer is called non-radiative energy transfer. For it to ?cur, the sensitizer and activator must have excited state levels of approximately equal energy above their ground states. Also, the activator and sensitizer must he close enough in the host lattice for the transfer probability to he appreciable. Depending upon the mechanism of the transfer this "close enough" distance may range from 4 to 30
A.
The concept of energy transfer also permits us to gain a better insight into the phenomenon of poisoning in phosphors. In Figure 14 is shown the transfer of excitation energy from a desired site (sensitizer or activator) to a poison site at which the energy can be lost to the host lattice as vibrational energy. Concentration quenching can be understood in this model by combining the concepts of energy transfer and poisoning. As we have seen, the necessary conditions for energy transfer to occur are that the ions involved have excited states of aDproximately equal energy and be ut'ficiently close together. Bs rlrfinition all the activator ions in a ~ h o s ~ h have o r identical excited states, and as we increase the concentration of the activators in the phosphor we create more and more situations in which the; are close enough for energy transfer to occur. The result is that a t higher activator concentrations the energy of excitation can wander throughout the host lattice from activator to activator until it finds a poison site and is then lost to the luminescence emission. This process is illustrated schematically in Figure 15. Commercial Applications of Phosphors Phosphors are important constituents of energy efficient fluorescent lamps, cathode ray tubes used in televisions, oscillosco~es. . . and disolavs. . - . and in X-rav detectors used in diagnostic equipment. Of course not just any phosphur will do in these anulirnrionr: thus. the solid statc chemist must meet the challenge of developihg a phosphor with the necessary properties of a high efficiency for the excitation energy, desired emission spectrum, and long term chemical stability. I t will be instructive to consider the major application of phosphors today, their use in fluorescent lamps. The basic elements of a fluorescent lamp are shown in Figure 16. When current is passed through the lamp envelope, the gaseous mercurv atoms are excited to uoner .. enerw -. states which then return to thr ground itate with an accompanying emission of radiation. 1)eoendine on the lewl to whirh an atom is excited it may emit in the visible or in the ultraviolet (2537 A or 1850
Energy Transfer
Emission
Excitation
1 Activator
.w
Figure 13. Schematic diagram illustrating energy t'ansfer between a sensitizer and activator.
--,.
/
I
Excitation
Transfer
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Fiaure 15. Concentration ouenchina occurs when lhe activator concentration become5 s ~ l f l cenl) n gh lnat elf cent energy transfer perm ls me exc tslm energy lo mlgrats mro-gh the how ~ n t l lt strapped at a polson sate
Glass
Envelope
rPhosphor
Non-Radiative Dissipation
Figwe 14. An explanation of poisoning in terms of Uw configurational cwtdimte model.
544 / Journal of Chemical Education
Figure 16. Schematic representation of a fluorescent lamp.
Coating
Calcium Halophosphate Gas (PO,), Fx-. CI.. Mn*'. Sb" Sensitizer
Activators
Sb*l
Sb" Mn-
B ue-l/
Emission
1
Figure 18. The mle of Sb3+ and Mn2+ In the commercial fluorescent lamp phosphor, celcium halophosphate, C ~ S ( P O ~ ~ F ~ - . C I , : M ~ ~ + : S ~ ~ ~ . Flgue 17. l'k e m i s b n spechum of the commercial flmewem lamp phoaphw. calcium halophosphate, Cas(P04)sF1-xClx:Mn2+,Sbs+.
A). Most of the emission from the mercuw arc lies in the ultraviolet. Consequently, the glass envelope is coated on the inside with a ohosohor which efficiently - converts the ultraviolet radiation into visible radiation. Conditioned as we are to the visible spectrum of the sun, we would not react favorably to highly efficient lamps with emission spectra radically different from that spectrum. Consequently, a fluorescent lamp must contain a phosphor which provides an emission approximating white light. A nhosnhor used extensivelv in commercial fluorescent lamos in calcium halophosphate doped with manganese and ant]mony, Cas(P04)3F1,C1,:Mn2+:Sb3+. The emission spectrum of this phosphor is shown in Figure 17. In this phosphor the Sb3+ olavs the role both of a sensitizer and an activator and ~ n z ~ h e h a vonly e s as an activator. The contributions of Sh3+ and Mn2+ are illustrated in Figure 18. The blue emission of the Sb3+ and the yellow emission of the Mn2+ combine to produce an emission spectrum that approximates white light. In summary, this review has introduced the reader to luL
.
minescence in inorganic materials by first familiarizing him or her with the terminolow ohosohors. We then considered . . of . . two basic experimental measurements used to characterize luminescent solids and described pheoomenologically several important characteristics of phosphors. The configurational coordinate model of characteristic luminescence has been introduced and has been used to eain a better insieht into the various phosphor phenomena. ~Tnally,we have presented a brief introduction to a major application of phosphors. It is hoped that this paper has helped the reader to appreciate the multidisci~linarvnature of solid state investieations and the many chalienges"confronting a solid state chemist as he or she attemots to convert laboratow curiosities into useful products. Bibliography
Dram. " . R. S.. "Phvaieal Methods in lnoreanie Chemi8tw," Reinhold Publishing Co.. New Yark. 1965;chap. 6. Barrow. G.M. "The SLructurcof Malecules." W . A. Benjamin Ine., New York. 1964. Lcverenz, H. W., "An Introduction foLumineseenee in Solids,"John Wiley andSon. Inc., New York, 1950. Blsase, G., and Brill. A., "Characteristic Lumin-nee Revielu.311101 3M, 1970. Goldberg, P., Editor, "Luminescenee of Inorganic
1966.
I, 11, and 111," Philips Technical
Solids,"Academic Press, New York,
Garlick, C. F.J., in "Handhueh der Physik." (Editor:Fluge, 8.1 Springer-Verlag. Berlin, 1958. Vol. XXXVI, pp. 1-128.
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