art persistent luminescence beats the afterglow 400 years of persistent luminescence.pdf

8/10/2019 ART Persistent Luminescence Beats the Afterglow 400 Years of Persistent Luminescence.pdf

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42 The Electrochemical Society Interface Winter 2009

Persistent luminescence is aphenomenon where the materialis emittingusually in the visible

rangefor hours after the irradiation(or excitation) source has been switchedoff.1The irradiation used may be visiblelight or UV, X-ray, or gamma radiation.Persistent luminescence has been, andstill is, unfortunately in a misleadingmanner, called phosphorescencebecause of the long emission time.Phosphorescence may be an appropriateterm to be used in the context ofluminescence from organic compoundsinvolving triplet-to-singlet transitions.These are forbidden with a long decaytime but are otherwise perfectly natural.

The long decay time of persistentluminescence, however, is due tothe storage of the excitation energyby traps and is released from themwith thermal energy. Thus the termthermally stimulated luminescence(TSL) is appropriate but for the sake ofbriefness, the phenomenon is called, inwhat follows, persistent luminescence.

The Glorious Past

The well-documented historyof persistent luminescence as aphenomenon dates from the beginning

of the 17thcentury.2In 1602, an Italianshoemaker, V. Casciarolo, observedstrong luminescence from a mineralbarite, BaSO4, later to be known asthe famous Bologna stone. It appears,however, that the material actually wasnot the barite mineral itself but ratherthe reduced product, barium sulfide,BaS. According to modern knowledge,the reduction process of the sulfidecannot be held responsible for theluminescence since this luminescenceoccurred long after the process had beenterminated. On the other hand, takinginto account the low purity of both the

original barite and the charcoal used forthe reduction, it is evident that a lot ofdifferent (trace) impurities were present.That is probably the reason for thevarying color of the persistent emissionreported, though mainly orange or redemission in the visible range prevailsin the literature. Whatever the emissioncolor, the cause for the persistent emissionremained unknown then and, to certainextent, up to the 21st century. At thattime, i.e., during the decades followingthe discovery of the bright emissionfrom the (reduced) Bologna stone, thephenomenon did not cease to arouse theinterest of both scientists and laymen,

and even several books were written onthis miraculous phenomenon (see Fig. 1).

Persistent Luminescence Beats the Afterglow:400 Years of Persistent Luminescence

by Jorma Hls

FIG. 1.The book Litheosphorus Sive de Lapide Bononiensiby Fortunius Licetus (Bologna, Italy,1640) on the persistent luminescence of the Bologna stone.

The matter clearly went beyond theknowledge of the scientists of the time,and, maybe for comprehensible reasons,even the famous interdisciplinarygenius Galileo refused to get involved.Probably Galileo had had enoughtrouble with the Catholic Church andhe preferred to leave the shine of theBologna stone alone since it may havebeen of celestial origin. Since the 17thcentury, only scattered reports areavailable on persistent luminescenceand the research receded considerablybecause no explanation was found forthe persistent luminescence producedwithout any evident excitation source.

From the above, it may well be

concluded that persistent luminescencewas one of the first genres ofluminescence ever discovered andstudied to some scientific extent.Despite no explanation being foundfor the persistent luminescence, theapplications were taken into use in the20th century. Luminous paints werebased on persistent luminescence fromthe different sulfide materials as ZnSdoped with e.g., Cu.1The emission wasmodified with the partial substitutionof Zn with Cd. However, at its best,persistent luminescence from thesematerials was both weak and short,

lasting for a few hours only. Someenvironmentally dubious tricks, suchas the doping of ZnS with radioactiveelements (one of the few uses of the

artificial radioactive lanthanide,promethium), were used in order toprolong the duration of persistencewith external excitation. These trickswere only retarding the final rejectionof the ZnS based phosphors, also dueto their pronounced instability againsthumidity.

The leading role of persistentluminescence as the number oneluminescence subject was finally lostwith the advent of the introduction ofthe rare earth based phosphors in the1960s. This state of affairs seemed toremain even while the other fields ofluminescence flourished, for example inthe areas of lighting, cathode ray tubes,

and scintillator material applications.The hectic research and developmentwhich followed the lanthanides nowbeing commercially available in apure form (incidentally as a byproductof the Manhattan project!) producedboth new information about thedifferent luminescence processes andever better phosphor materials. Noneof this took place in the persistentluminescence field. As a result of thespeeded-up introduction of many newphosphor materials, also some withserious problems with the resistanceto the some times hostile operating

conditions (e.g. mercury vapor andelectron bombardment) appearedin the commercial market. Thus thenegative side of persistent luminescence


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The Electrochemical Society Interface Winter 2009 43

was introduced: the afterglow. As aresult of the formation of traps3 tostore the excitation energy (e.g., UVradiation, electrons) the emission wasretarded3 and, in many cases, alsosignificantly weakened.4 This was notalways completely condemning thephosphor material since different waysto solve these problems were finallydeveloped, e.g., post-treatment in inert/reactive atmosphere or introduction of

some additives.4,5However, in general,the afterglow problem seemed to givepersistent luminescence the final coupde grceand practically nothing, neitherin understanding the phenomenon norin developing new materials, took placebefore the mid 1990s.

The New PersistentLuminescence Era

Then, out of the blue, the firstgeneration of the modern persistentluminescence materials, the Eu2+dopedand rare earth (R3+) co-doped alkalineearth aluminates (MAl2O4:Eu

2+,R3+;M: Ca and Sr) appeared in both thecommercial market and in researchlaboratories as well.6 With this, thefirst new persistent luminescence erasince 1995, there rapidly appeared notonly new materials and mechanismsbut also new methods of analysisand applications. As a result of theincreased research activity, thereemerged also hundreds of papers,reports, and meeting communications.

Unfortunately, since the persistentluminescence phenomenon seemed tobe rather complicated, the quality ofthe reports has not always been of thebest caliber.

As the new persistent luminescencematerials have been developed, thereare presently persistent luminescencephosphors for each of the main colors:blue, green, and red that fulfillperhaps with the exception of redthe

requirements for not only sufficientlystrong and long persistent luminescencebut also for the stability of a commercialphosphor. Figure 2 shows the behavior ofthree different persistent luminescencephosphors in daylight, under UVradiation excitation, and in dark afterUV irradiation. Despite the high initialpersistent luminescence of the redemitting material, Y2O2S:Eu

3+,Mg2+,TiIV,its long term luminescence is muchweaker than the duration of the blue(Sr2MgSi2O7:Eu

2+,Dy3+; 25+ hours) andgreen (SrAl2O4:Eu

2+,Dy3+; 15+ hours)emitting counterparts.

The idea of mutating a well-established and commercially availablephosphor for other luminescenceapplications such as Y2O2S:Eu

3+though reinforced with co-dopants suchas Mg2+and TiIVdoes not always resultin a superior persistent luminescencematerial, as was the case withSrAl2O4:Eu

2+. The design of new efficientpersistent luminescence phosphorsrequires more than this; or a lot of luck.As a result of perhaps the latter, themost recent persistent luminescencephosphors, the Eu2+ doped and rareearth (R3+) co-doped alkaline earthdisilicates7 (M2MgSi2O7:Eu

2+,R3+; M: Caand Sr) were discovered to be much moreefficient and stable than corresponding

aluminates. Their manufacture is alsosomewhat less complicated, as no fluxmaterial is needed. The host materials(both MAl2O4 and M2MgSi2O7, butnot Y2O2S) are basically rather cheap,though the high purity requirementsincrease the price tag. So far there hasnot been any serious alternative for theemitting Eu2+dopant (the price of whichis sky-rocketing), mainly because of thevery favorable position of its ground

electronic energy level vis--visthe hostband structure.8The low concentrationof this element somewhat lowers theprice, which is inherently high becauseeuropium is used in most luminescenceapplications, including as securitymarkers in Euro bank notes.

The idea of making a white emittingpersistent light source by combiningthe three individual blue, green, and redemitting phosphors, in a way similar tothe tricolor fluorescent tubes, seemspossible when the emission spectra ofthese phosphors are considered alone(see Fig. 3). However, the unbalanced

duration of the three colors, especiallydue to the weak and short red persistentemission, will delay or even precludethis application. Instead, there aremany more or less well established ones,such as sensor applications includingtemperature sensing but also changesin pressure (on airplane wings, forexample). The use of nanoparticulatepersistent luminescence materials forbiomedical applications with direct insitu imaging sounds both interestingand feasible. However, it is quite certainthat the commonplace, less excitingapplications using persistent luminouspaints will dominate the market for atleast the near future.

FIG. 2.The persistent luminescence behaviorof the blue emitting Sr2MgSi2O7:Eu

2+,Dy3+(E),green emitting SrAl2O4:Eu

2+,Dy3+(C) and redemitting Y2O2S:Eu3+,Mg2+,TiIV(S) phosphors inday light, under UV excitation, and in dark.

FIG. 3.The luminescence spectra of the blue emitting Sr2MgSi2O7:Eu2+,Dy3+, green emittingSrAl2O4:Eu

2+,Dy3+, and red emitting Y2O2S:Eu3+,Mg2+,TiIVphosphors after UV excitation.


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44 The Electrochemical Society Interface Winter 2009

FIG. 5.The effect of crystal field strength on the energy levels and emission color of the Eu2+ion insolid state.

FIG. 4.The persistent luminescence mechanism of the blue emitting Sr2MgSi2O7:Eu2+

,R3+

phosphors.

The Mechanism

The design of new persistentluminescence materials would probablybe much facilitated if the mechanism(s)of the phenomenon were known. Theresearch of persistent luminescence has

thus been focused on two intertwinedobjectives: the development of newmaterials, especially the red emittingones, and the mechanism(s) of persistentluminescence. The latter studies haveproduced more papers than good ideasin the past. In most cases, one of thetwo most important factors have beenignored: the energetics deciding thenature of charge carriers or the natureand energetic positions of the defectswhere the excitation energy is stored forfurther use as persistent luminescence.Since 2005, persistent luminescencemechanisms have somewhat converged

into what is known now, though thereis not really any widespread agreementon the details. In Fig. 4 these factorsare presented in a schematic andsimplified way for one of the bestpersistent luminescence phosphors,Sr2MgSi2O7:Eu

2+,R3+, modified from thatof CaAl2O4:Eu

2+,Dy3+.9

The irradiation of the material byblue light (or UV radiation) results inthe photoexcitation of Eu2+ via the4f7 4f65d1transitions which, as statedabove, overlap with the conductionband of Sr2MgSi2O7. The capture ofthe excited electron by the conductionband may take place directly or may

be assisted by thermal energy sincethe life time of the conventional Eu2+luminescence in aluminates is ratherlong, about 1 s.10 The electron canmove in the conduction band untilit returns to the europium centeror is captured by the traps close tothe bottom of the conduction band.The actual persistent luminescenceinvolves the temperature controlledgradual release of the trapped electronsfollowed by the migration of electronsto the europium center through theconduction band. The recombinationproduces the persistent emission. No

evident pitfalls exist in this mechanismthough proving that is hard due tothe ubiquitous uncertainty about thethermally controlled mechanism.Despite the apparent incompatibility inenergies between the thermal energyat room temperature (ca. 25 meV)stimulating the persistent luminescenceand those used in synchrotronradiation methods (UV - VUV: 5 eV;XANES and EXAFS: 5-20 keV), thesemethods have been found very usefulin the study of persistent luminescencemechanisms.11,12

The challenge to find an efficientred emitting persistent luminescencematerial has been found quite hardsince the best candidate as the emitting

center in persistent luminescencematerials is the Eu2+ ion. The strengthof the crystal field effect (i.e., thepredominantly electrostatic effect ofneighboring ions onto Eu2+) requiredto lower the lowest emitting level of the4f65d1electron configuration to energieslow enough to produce red emission isvery high (Fig. 5).

Among the few potential candidates,the Ca2Si5N8:Eu

2+,R3+ materials13 seem

to be the most promising ones. If theuse of Eu2+ as the emitting center isdiscarded, the Mn2+ red emission,despite the very difficult excitation,can be used. The challenge of weakexcitation can be circumvented by theuse of the Eu2+ ion as the absorbingand energy storage species. Persistentemission can be obtained with the aidof Eu2+to Mn2+energy transfer.14

Hls(continued from previous page)


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The Electrochemical Society Interface Winter 2009 45

Conclusion and Future

Persistent luminescence phosphorsare here to stay and their applicationsare rapidly expanding from bulkmaterials visible to everyone (e.g.,exit signalization on airplane cabinfloors) to high tech products (e.g.,biomedical imaging). The elaborationof persistent luminescence mechanismsis advancing at a rapid pace and these

have been basically solved, thoughthe refinement of the details is stillneeded. On the application side, thechallenge of finding an efficient redemitting persistent phosphor is stillwaiting. Finally, the rapid developmentof theoretical methods, mainly basedon density functional theory (DFT) andincreased computational capabilities,have given promising results in thecalculation of practically all thoseissues of persistent luminescencenow determined experimentally. Thehangover caused by the afterglowshown by many a commercial phosphor

is now overcome. As a result, persistentluminescence has resumed its placein the front line in the research ofluminescence phenomena. At thesame time, persistent luminescenceresearch has offered the means (e.g.,thermoluminescence) and knowledge(e.g., defect chemistry and physics) tosolve the afterglow problems.

Acknowledgments

The author would like to expresshis gratitude to Dr. Mika Lastusaarifor help in preparing this manuscript.

The financial support of the Academyof Finland and different EuropeanUnion financing bodies for the work onpersistent luminescence materials andmechanisms is also acknowledged.

About the Author

JORMA HLS is Professor of InorganicChemistry at the University of Turku(Turku, Finland). His research interestsare in the areas of powder (nano)materials, luminescence, and phosphormanufacturing. He focuses his researchon the chemistry, physics, and

spectroscopy of lanthanides/rare earths.He is a member of several internationalscientific and professional organizationsand has extensive international co-operation with different universities aswell as research institutes. He may bereached at [email protected].

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Luminescence: From the EarliestTimes until 1900, p. 305, Amer.Phil. Soc., Philadelphia (1957).

2. Y. Murayama, in PhosphorHandbook, S. Shionoya and W. M.Yen, Editors, p. 651, CRC Press,Boca Raton (1999).

3. T. Jstel, H. Lade, W. Mayr, A.

Meijerink, and D. U. Wiechert, J.Lumin.,101,195 (2003).

4. P. Fabeni, A. Krasnikov, T. Krner,V. V. Laguta, M. Nikl, G. P. Pazzi,and S. Zazubovich,J. Lumin.,129,767 (2009).

5. G. Bizarri and B. Moine,J. Lumin.,113,199 (2005).

6. T. Matsuzawa, Y. Aoki, N. Takeuchi,and Y. Murayama, J. Electrochem.Soc.,143,2670 (1996).

7. Y. Lin, Z. Tang, Z. Zhang, X. Wang,and J. Zhang,J. Mater. Sci. Lett.,20,1505 (2001).

8. P. Dorenbos, J. Electrochem. Soc.,

152,H107 (2005). 9. T. Aitasalo, J. Hls, H. Jungner,M. Lastusaari, and J. Niittykoski,J.

Phys. Chem. B,110,4589 (2006).10. T. Aitasalo, J. Hls, H. Jungner,

J.-C. Krupa, M. Lastusaari, J.Legendziewicz, and J. Niittykoski,

Radiat. Meas.,38,727 (2004).11. T. Aitasalo, J. Hassinen, J. Hls,

T. Laamanen, M. Lastusaari, M.Malkamki, J. Niittykoski, andP. Novk, J. Rare Earths, 27, 529(2009).

12. J. Hls, T. Laamanen, M.Lastusaari, M. Malkamki, J.Niittykoski, and P. Novk, Rad.

Phys. Chem.,78,511 (2009).13. K. Van den Eeckhout, P. Smet, and

D. Poelman, J. Lumin., 129, 1140(2009).

14. X. J. Wang, D. Jia, and W. M. Yen,J.Lumin.,102-103,34 (2003).

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