Available online at www.sciencedirect.com

www.elsevier.com/locate/jnoncrysol

Journal of Non-Crystalline Solids 354 (2008) 659–664

Cr3+-doped macroporous Al2O3 monoliths preparedby the metal-salt-derived sol–gel method

Koji Fujita a,b,*, Yasuaki Tokudome a, Kazuki Nakanishi c, Kiyotaka Miura a,Kazuyuki Hirao a

a Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japanb PRESTO, Japan Science and Technology Agency (JST), 4-1-8, Honcho Kawaguchi, Saitama, Japan

c Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

Available online 7 November 2007

Abstract

Cr3+-doped alumina (Al2O3) monoliths with well-defined macropores have been synthesized from the aqueous and ethanolic solutionof aluminum and chromium salts in the presence of propylene oxide (PO) and poly(ethylene oxide) (PEO) using the sol–gel methodaccompanied by phase separation. The addition of PEO to the starting solution induces the phase separation, whereas the introductionof PO controls the gelation. The bicontinuous macroporous structures are obtained by inducing the phase separation parallel to the gela-tion, and the pore size can be controlled by adjusting the composition of starting solutions. The dried gel and that heat-treated at 700 �Care amorphous. As the heat-treatment temperature is increased over 700 �C, nanocrystalline c-Al2O3 is precipitated at 800 �C, a mixtureof h- and a-Al2O3 phases appears at 1100 �C, and a single of a-Al2O3 is obtained at 1200 �C, while keeping the bicontinuous macropo-rous structure. Cr3+-doped a-Al2O3 monoliths with well-defined macropores exhibit photoluminescence as observed for ruby, indicatingthat Cr3+ ions are homogenously dispersed into the skeletons of bicontinuous network and substitute uniformly for the Al3+ sites.� 2007 Elsevier B.V. All rights reserved.

Keywords: Crystallization; Luminescence; Sol–gels (xerogels)

1. Introduction

Materials couple strongly with light when the dielectricconstant of materials varies on a length scale of the orderof the wavelength of light. Such materials have a refractiveindex n that can be either periodic or random in space,showing interesting fundamental properties such as photonlocalization. Recently, the ordered and disordered dielec-tric systems have attracted considerable attention becauseof their potential photonic applications [1]. The photonlocalization appears due to multiple light scattering andinterference in strongly scattering and weakly absorbing

0022-3093/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2007.06.091

* Corresponding author. Address: Department of Material Chemistry,Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. Tel.: +81 75 383 2432; fax: +81 75 383 2420.

E-mail address: fujita@dipole7.kuic.kyoto-u.ac.jp (K. Fujita).

media. In the case of disordered media, the strength of scat-tering is largest for light having the wavelength comparableto the size and spatial separation of randomly distributedscatterers, and increases with an increase in refractive-indexcontrast.

Pore formation is a very promising technique not onlyfor obtaining strongly scattering disordered media but alsofor tailoring the scattering strength. Lagendijk et al. [2,3]found that macroporous semiconductor gallium phosphide(GaP, n � 3.2) prepared by electrochemical etching showsthe strong scattering without optical absorption in thered part of the visible spectrum. They also demonstratedthat the scattering strength of macroporous GaP can betuned in a wide range, depending on the density and sizeof pores [4]. On the other hand, we have fabricated macro-porous monoliths from silica (SiO2) and titania (TiO2)sol–gel systems, and investigated their light scattering

660 K. Fujita et al. / Journal of Non-Crystalline Solids 354 (2008) 659–664

properties [5–7]. The macroporous structure is formedwhen a transient structure of phase separation inducedby either the hydrolysis and polycondensation of metal alk-oxides or the aggregation of colloidal particles is frozen bythe sol–gel transition to produce the bicontinuous structure[8]. In these systems, the phenomena associated with inter-ference of multiply scattered light are observed such ascoherent backscattering and three-dimensional opticalmemory effects, and the scattering strength is controlledby the size and density of pores. The pore formation in avariety of metal oxides is highly desirable to study the lightbehavior in porous media in a systematic way.

a-alumina (Al2O3) has a refractive index of n � 1.7 andvery low optical absorption in the visible region [9]. The nof a-Al2O3 is lower than that of TiO2 (rutile, n � 2.7) butlarger than that of SiO2 (amorphous, n � 1.45). Hence,the fabrication of porous Al2O3 would enable us to com-pare the scattering strength among SiO2, Al2O3, andTiO2 systematically. Also, it has been recently reported thatphotoexcitation into the composite in which porous Al2O3

membrane prepared via the anodic oxidation of metallicaluminum [10–12] is filled with dye-TiO2 polymer showslasing without any cavity mirrors, so-called random laser[13]. This phenomenon is caused by spontaneous feedbackof photons emitted from the dye as a result of the multiplescattering and interference. It is thus important to developthe method of fabricating porous Al2O3 from the viewpointof photonic applications as well as for the understanding oflight propagation in porous materials.

Recently, we developed a route toward synthesizingmacroporous Al2O3 monoliths using a solution process,in contrast to the anodic oxidation of metallic aluminum.This approach is based on the new sol–gel technique asoriginally developed by Gash et al., which enables thesynthesis of metal oxide aerogels and xerogels from thecorresponding metal salts [14–17]. This metal-salt-derivedsol–gel technique involves the use of epoxides as gelationinitiators. We succeeded in obtaining the bicontinous mac-roporous Al2O3 gels by inducing the phase separation inthe course of the metal-salt-derived sol–gel reaction [18].In this paper, doping of Cr3+ ions into the skeletons ofbicontinuous network is demonstrated. Various crystallo-graphic phases such as c-, h- and a-Al2O3, as well as amor-phous Al2O3, are obtained by heating the resultant gels atdifferent temperatures. We also show that the characteristicR line transitions of ruby are observed for macroporousa-Al2O3:Cr3+ monoliths heat-treated at 1200 �C.

2. Experimental

Aluminum chloride hexahydrate, AlCl3 Æ 6H2O(Aldrich), and chromium chloride hexahydrate,CrCl3 Æ 6H2O (Aldrich), were used as the inorganic compo-nents, and a mixture of water and ethanol as the solvent.Propylene oxide (PO, Aldrich) was added to initiategelation, and poly(ethylene oxide) (PEO, Aldrich) having

viscosity average molecular weight of 1000000 was usedas a polymer to induce the phase separation.

We prepared gels with a nominal composition of99Al2O3 Æ 1Cr2O3 (mol%). The detail of gel preparation isas follows. First, 4.27 g of AlCl3 Æ 6H2O and 0.048 g ofCrCl3 Æ 6H2O, together with an appropriate amount ofPEO in the range of 0.03–0.10 g, were dissolved in a mix-ture of water (4.00 g) and ethanol (4.35 g). 3.11 g of POwas then added to the transparent solution under ambientconditions (25 �C). After stirring for 1 min, the resultanthomogeneous solution was transferred into a glass tube.The tube was sealed and kept at 40 �C for gelation. Aftergelation, the wet gel was aged for 24 h and evaporation-dried at 40 �C. Some of the dried gels were heat-treatedat various temperatures between 700 and 1200 �C for 5 hin air. Hereafter, the content of PEO added to the startingsolution will be often referred to as wPEO (unit: g) to distin-guish the samples prepared from different PEO contents.

A scanning electron microscope (SEM; S-2600N, Hit-achi Ltd., Japan, with Pt coating) was used to observethe morphology of gels. The size distribution of macrop-ores was measured by a Hg porosimetry (Poresizer9320,Micromeritics Co., USA). X-ray diffraction (XRD) analy-sis with Cu Ka radiation (RINT2500, Rigaku, Japan)was performed to identify the crystalline phases. The mea-surements were carried out for the powder specimens pre-pared by grinding macroporous monoliths. Fluorescenceand excitation spectra were measured at room temperaturewith a fluorescence spectrometer (Hitachi 850, Japan) usingan Xe lamp as a light source.

3. Results

3.1. Macroporous morphology of dried gels

Starting from a homogenous starting solution, the phaseseparation and gelation occurred spontaneously in a closedand static condition at a constant temperature (40 �C). Thesolution containing PO gelled quite quickly (�10 min),although gelation did not take place in the solution withoutPO within one weak at least. On the other hand, the addi-tion of PEO to the starting solution affected only weaklythe gelation time, but instead induced the phase separation.As a result, micrometer-range morphology of the resultantgels depended on wPEO in the starting solution. Fig. 1shows the morphology variation of dried gel with wPEO.When wPEO = 0.03, the resultant gel is nonporous in themicrometer range (Fig. 1(a)). Bicontinuous structure, inwhich each of gel skeletons and macropores is intercon-nected and highly continuous, can be obtained in the lim-ited compositional range of wPEO = 0.04–0.08, and thepore size increases with increasing wPEO (Fig. 1(b)–(f)).Further addition of PEO (wPEO = 0.10) brings about themorphology of particle aggregates (Fig. 1(g)). The depen-dence of micrometer-range morphology on wPEO is almostthe same as that observed for the Cr3+-undoped system aswe reported previously [18].

Fig. 1. SEM images of dried Al2O3:Cr3+ gels prepared with wPEO = 0.03 (a), 0.04 (b), 0.05 (c), 0.06 (d), 0.07 (e), 0.08 (f), and 0.10 (g).

K. Fujita et al. / Journal of Non-Crystalline Solids 354 (2008) 659–664 661

3.2. Effect of heat-treatment

Fig. 2 shows the pore size distributions for the dried gelprepared from the starting solution with wPEO = 0.05 andthat heat-treated at 1200 �C. The sharp pore size distribu-tion is observed for the dried gel, implying that the bicon-tinuous network forms via spinodal decomposition [8].Upon heating at 1200 �C, the size and volume of macrop-

0.05 0.1 0.5 1 5 100

0.1

0.2

0.3

0.4

0.5

Pore diameter, Dp / μm

Cum

ulat

ive

pore

vol

ume,

Vp

/ cm

3 g-1 as-dried1200 ºC

0.05 0.1 0.5 1 5 10

0.1

0.2

0.3

0.4

0.5

Pore diameter, Dp / μm

Cum

ulat

ive

pore

vol

ume,

Vp

/ cm

3 g-1 as-dried1200 ºC

Fig. 2. Pore size distributions for the dried Al2O3:Cr3+ gel prepared fromthe starting solution with wPEO = 0.05 and that heat-treated at 1200 �C.

ores are decreased by the shrinkage of gels, while the sharppore size distribution is maintained. Fig. 3(a) and (b) depictthe SEM image and the photo for the sample heat-treatedat 1200 �C, respectively. A large-dimension monolith withbicontinuous macroporous morphology can be obtainedeven after the heat-treatment. The color of the heat-treatedsample is pink, which indicates that Cr3+ ions are entirelyincorporated into the Al2O3 skeletons and uniformly sub-stitute for Al3+ sites, as described below.

In Fig. 4 are shown the XRD patterns for samples pre-pared from the starting solution with wPEO = 0.05 and thenheat-treated at various temperatures. It can be clearly seenthat the gel heat-treated at 700 �C remains amorphous.Heating at 800 �C results in the precipitation of a cubicc-Al2O3 phase (JCPDS No. 10-0425), although the diffrac-tion peaks are very broad. The broad diffraction peaks areindicative of the precipitation of nanometer-sized crystal-lites. Only the nanocrystalline c-Al2O3 phase is observedup to 1000 �C. The crystallite sizes evaluated by Scherrer’sequation are 4.7, 5.0, and 5.6 nm for the samples heat-trea-ted at 800, 900, and 1000 �C, respectively. At 1100 �C, themonoclinic h-Al2O3 (JCPDS No. 35-0121) and hexagonala-Al2O3 (JCPDS No. 46-1212) phases coexist. When theheat-treatment temperature is raised to 1200 �C, a singlephase of hexagonal a-Al2O3 is obtained with good crystal-lization. A similar sequence of phase transformationwas observed for other heat-treated samples doped withCr3+ ions. The phase evolution observed for the present

Fig. 3. (a) SEM image of the Al2O3:Cr3+ sample prepared from thestarting solution with wPEO = 0.05 and then heat-treated at 1200 �C. (b) Aphoto of the Al2O3:Cr3+ sample heat-treated at 1200 �C. The monolithiccircular cylinder of Al2O3:Cr3+ is obtained in large dimensions (diameterof top and bottom faces �5 mm, height �3 mm).

20 40 60 802 /deg.

as-dried

700 ºC

γ-Al2O3

800 ºC

900 ºC

1000 ºC

1100 ºC

1200 ºC

θ-Al2O3

α-Al2O3

20 40 60 802 /deg.

Inte

nsit

y (a

rb. u

nits

)

as-dried

700 ºC

γ-Al2O3

800 ºC

900 ºC

1000 ºC

1100 ºC

1200 ºC

θ-Al2O3

α-Al2O3

θ

Fig. 4. XRD patterns of the dried Al2O3:Cr3+ gel prepared from thestarting solution with wPEO = 0.05 and those heat-treated at varioustemperatures. The data of as-dried gel and that heat-treated at 700 �C aremagnified by a factor of 5 for clarity.

400 500 600 700

Wavelength (nm)

Em

issi

on in

tens

ity (

arb.

uni

ts)

Wavelength (nm)690 700 710

Fig. 5. Room temperature excitation (dashed curve) and fluorescencespectra (solid curve) for the Al2O3:Cr3+ sample prepared from the startingsolution with wPEO = 0.05 and then heat-treated at 1200 �C. An Xe lampwas used as a light source (bandwidth�1.0 nm); the fluorescence spectrumwas measured by exciting the sample at a wavelength of 400 nm, and theexcitation spectrum was obtained by monitoring the fluorescence near694 nm. The inset shows the fluorescence spectrum near 694 nm wave-length on an enlarged scale.

662 K. Fujita et al. / Journal of Non-Crystalline Solids 354 (2008) 659–664

Cr3+-doped system is different from that for the undopedsystem, in which a single phase of a-Al2O3 is obtained at

lower temperature (�1100 �C), without undergoing thephase transition of c- to h-Al2O3 [18]. The difference inphase evolution between doped and undoped systemsmay be related to the development of microstructures dur-ing heat-treatment.

3.3. Cr3+ fluorescence

Fig. 5 displays the room-temperature fluorescence andexcitation spectra for Al2O3:Cr3+ sample prepared fromthe starting solution with wPEO = 0.05 and then heat-trea-ted at 1200 �C. The inset is an enlargement of the fluores-cence spectrum, demonstrating well-known sharp R linesat 694 nm ascribed to the 2E! 4A2 transition of singleCr3+ ions in ruby. The fluorescence spectrum clearly indi-cates that Cr3+ ions are homogeneously incorporated intothe a-Al2O3 skeletons of bicontinuous network and occupythe octahedral sites to form the substitutional solid solu-tion. The two weak peaks observed at around 702 and705 nm are ascribed to N1 and N2 lines, which arise fromsecond and forth nearest-neighbor exchange-coupled pairsof Cr3+ ions, respectively, due to the high Cr3+ concentra-tion [19]. The excitation spectrum also confirms the occu-pancy of the octahedral sites in a-Al2O3 by Cr3+ ions.Two strong broad absorption bands with peak positionsat 410 and 563 nm are observed, corresponding to spin-allowed 4A2(4F)! 4T1(4P) and 4A2(4F)! 4T2(4F) transi-tions of Cr3+ in a-Al2O3, respectively [20].

4. Discussion

As Gash et al. [14–17] have previously shown, aerogelsand xerogels of main group and transition metal oxide

K. Fujita et al. / Journal of Non-Crystalline Solids 354 (2008) 659–664 663

can be prepared directly from the aqueous solution ofmetal salts using epoxides as gelation initiators. The epox-ide consumes the protons from the hydrated metal species,and the protonated epoxide then undergoes an irreversiblering-opening reaction with a nucleophilic anion of themetal salt (Cl� in this report). Namely, the epoxide actsas an irreversible proton scavenger, which induces hydroly-sis and condensation of the hydrated metal cations. Thisprocess is the basis for the preparation of macroporousAl2O3 gels presented here. In the starting solution, theAl3+ ion initially forms the hexahydrated complex[Al(OH2)6]3+. As time elapses, PO begins to consume pro-tons from [Al(OH2)6]3+, and the pH rises gradually andhomogenously throughout the solution. The hydratedcomplex then generates a series of hydroxylated species,[Al(OH)h(OH2)6�h](3�h)+, where h can vary from 0 to 4depending on the pH of the solution. After dimers and thenoligomers are uniformly formed in the solution via the con-densation, the further increase in solution pH eventuallybrings about polymerization to produce a monolithic gel.Here, it should be emphasized that the addition of strongbases such as OH�, CO2�

3 , NH3 is inadequate to obtainthe monolithic gel, because very rapid reaction rate dueto the drastic pH rise yields the regions of non-uniformparticle production or the precipitation of large clusters.The PO is not a strong enough base to induce the immedi-ate precipitation, but instead, it mixes well with the Al3+

species and gives a homogenous solution before the grad-ual increase in the solution pH occurs.

On the other hand, the addition of PEO to the startingsolution induces the phase separation in the course of thepolymerization reaction. According to our previousresults of thermogravimetry and differential thermal anal-ysis [18] PEO is preferentially distributed to the fluidphase composed mainly of solvent mixtures, rather thanto the gel phase, during the formation of phase-separateddomains, implying that the interaction between Al2O3

oligomers and PEO chains is very weak. In such a situa-tion, phase separation is induced by the reduction ofcompatibility between the polymerizing Al2O3 oligomersand the PEO dissolved in the fluid phase. When phaseseparation occurs later than gelation, transparent gelswith nanometer-sized pores, i.e, nonporous gels in themicrometer range, are formed (Fig. 1(a)). On the con-trary, when phase separation takes place earlier thangelation, the gel phase is fragmented to reduce the inter-face energy, leading to the morphology of particle aggre-gates (Fig. 1(g)). Under limited reaction conditions inbetween the above two cases, i.e., when phase separationand gelation are induced nearly concurrently, the bicon-tinuous structure can be obtained, where each of gelphase and fluid phase is three-dimensionally intercon-nected. After evaporation drying, the fluid phase com-posed mainly of solvent mixture turns into continuousmacropores, and the gel phase becomes skeletons(Fig. 1(b)–(f)). The size of macropores becomes largeras the amount of PEO in the starting solution is larger.

The pore size is determined by the timing of the onsetof phase separation relative to the gelation, since thephase separation involves a coarsening process [8]. Inthe present systems, the increase in wPEO reduces thecompatibility between the Al2O3 oligomers and the PEOdissolved in the fluid phase, and thus, the onset of phaseseparation relative to gelation is accelerated to producethe coarsened bicontinuous structure, with large macrop-ores being left behind after drying.

The sol–gel synthesis of mixed oxides usually suffersfrom the different reaction rates of the individual precur-sors. We preliminary compared the gelation time of theaqueous and ethanolic solution of AlCl3 in the presenceof PO with that of CrCl3 in the identical condition. Theformer, i.e., AlCl3 system, resulted in the formation ofmonolithic gel in approximately 10 min, while the gelationtime for the latter, i.e., CrCl3 system, was about 15 min.Namely, the chemical reactivity of AlCl3 and CrCl3 inthe present solution is not so significantly different fromeach other. Besides, the concentration of Cr3+ ions is aslow as 1 mol% in the present case. Thus, it is highly prob-able that in the course of the formation process of Al2O3

gels, Cr3+ species are homogeneously incorporated intothe network. During the heat treatment at 1200 �C,Cr3+ ions uniformly diffuse into a-Al2O3 to form thesubstitutional solid solution. Isolated Cr3+ ions locatedat the octahedral sites in a-Al2O3 exhibit the intenseand sharp fluorescence attributable to R lines as demon-strated in Fig. 5.

The introduction of optically active species into macro-porous materials has the possibility of controlling the lightemission due to multiple light scattering and interference[21,22]. Using the present technique, doping with variousoptically active ions would be feasible by the addition ofthe corresponding metal salts, so it is expected that theresultant macroporous materials find some applicationsin photonics, such as optical storage media and lasersource.

5. Conclusion

Monolithic Al2O3:Cr3+ gels with well-defined bicontinu-ous macropores were synthesized from the aqueous andethanolic solution of AlCl3 and CrCl3 using PO as a gela-tion initiator and PEO as a phase separation promoter,and macroporous Al2O3:Cr3+ with different crystallinephases were also obtained by subsequent heat treatmentat various temperatures. The well-defined macropores areformed by the concurrent phase separation and sol–geltransition induced by the polymerization reaction. Thephase transformation sequence on heating is amorphousAl2O3! nanocrystalline c-Al2O3 (800 �C)! a mixture ofh-Al2O3 and a-Al2O3 (1100 �C)! a-Al2O3 (1200 �C). Dur-ing heat treatment at 1200 �C, the skeletons of bicontinu-ous network are sintered into the dense body, whilekeeping the well-defined macroporous structures. The char-acteristic R line transitions of ruby can be observed in the

664 K. Fujita et al. / Journal of Non-Crystalline Solids 354 (2008) 659–664

macroporous a-Al2O3:Cr3+ monolith heat-treated at1200 �C, indicating that Cr3+ ions can homogeneouslyincorporated into the dense a-Al2O3 skeletons and substi-tute uniformly for the Al3+ sites.

Acknowledgements

This study was supported by the Grand-in-Aid for Sci-entific Research (B) (No. 18360316) from the Ministry ofEducation, Culture, Sports, Science, and Technology(MEXT) of Japan and the Industrial Technology ResearchGrant Program (04A25023c) from the New Energy andIndustrial Technology Development Organization(NEDO) of Japan. One of authors (K.F.) thanks a researchgrant from The Mazda Foundation. Y.T. also appreciatesthe research fellowship of global COE program, Interna-tional Center for Integrated Research and Advanced Edu-cation in Material Science, Kyoto University, Japan.

References

[1] C.M. Soukoulis (Ed.), Photonic Crystals and Light Localization inthe 21st Century, Kluwer, Dordrecht, 2001.

[2] F.J.P. Schuurmans, D. Vanmaekelbergh, J. van de Lagemaat, A.Lagendijk, Science 284 (1999) 141.

[3] F.J.P. Schuurmans, M. Megens, D. Vanmaekelbergh, A. Lagendijk,Phys. Rev. Lett. 83 (1999) 2183.

[4] J.G. Rivas, A. Lagendijk, R.W. Tjerkstra, D. Vanmaekelbergh, J.J.Kelly, Appl. Phys. Lett. 80 (2002) 4498.

[5] S. Murai, K. Fujita, K. Nakanishi, K. Hirao, Jpn. J. Appl. Phys. 43(2004) 5359.

[6] K. Fujita, S. Murai, K. Nakanishi, K. Hirao, J. Non-Cryst. Solids 352(2006) 2496.

[7] K. Fujita, J. Konishi, K. Nakanishi, K. Hirao, Appl. Phys. Lett. 85(2004) 5595.

[8] K. Nakanishi, J. Porous Mater. 4 (1997) 67.[9] E. Dorre, H. Hubner, Alumina-Processing, Properties, and Applica-

tions, Springer-Verlag, Berlin, 1984, p. 216.[10] H. Masuda, K. Fukuda, Science 268 (1995) 1466.[11] H. Masuda, K. Yada, A. Osaka, Jpn. J. Appl. Phys. 37 (1998) L1340.[12] W. Lee, R. Scholz, K. Nielsch, U. Gosele, Angew. Chem. Int. Ed. 44

(2005) 6050.[13] H.W. Shin, S.Y. Cho, K.H. Choi, S.L. Oh, Y.R. Kim, Appl. Phys.

Lett. 88 (2006) 263112.[14] A.E. Gash, T.M. Tillotson, J.H. Satcher, J.F. Poco, L.W. Hrubesh,

R.L. Simpson, Chem. Mater. 13 (2001) 999.[15] A.E. Gash, J.H. Satcher, R.L. Simpson, Chem. Mater. 15 (2003)

3268.[16] A.E. Gash, T.M. Tillotson, J.H. Satcher, L.W. Hrubesh, R.L.

Simpson, J. Non-Cryst. Solids 285 (2001) 22.[17] T.F. Baumann, A.E. Gash, S.C. Chinn, A.M. Sawvel, R.S. Maxwell,

J.H. Satcher Jr., Chem. Mater. 17 (2005) 395.[18] Y. Tokudome, K. Fujita, K. Nakanishi, K. Miura, K. Hirao, Chem.

Mater. 19 (2007) 3393.[19] S.P. Jamison, G.F. Imbusch, J. Lumin. 75 (1997) 143.[20] M.D. Lumb, Luminescence Spectroscopy, Academic, New York,

1978.[21] N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain,

Nature 368 (1994) 436.[22] H. Cao, Y.G. Zhao, H.C. Ong, S.T. Ho, J.Y. Dai, J.Y. Wu, R.P.H.

Chang, Appl. Phys. Lett. 73 (1998) 3656.