Title Visible to near infrared conversion in Ce3+-Yb3+ Co-dopedYAG ceramics

Author(s) Ueda, Jumpei; Tanabe, Setsuhisa

Citation Journal of Applied Physics (2009), 106(4): 043101

Issue Date 2009-08-17

URL http://hdl.handle.net/2433/150457

Right Copyright c 2009, American Institute of Physics

Type Journal Article

Textversion publisher

Kyoto University

Visible to near infrared conversion in Ce3+–Yb3+ Co-doped YAGceramicsJumpei Ueda and Setsuhisa Tanabe Citation: J. Appl. Phys. 106, 043101 (2009); doi: 10.1063/1.3194310 View online: http://dx.doi.org/10.1063/1.3194310 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v106/i4 Published by the American Institute of Physics. Related ArticlesMicrocavity effects in SiGe/Si heterogeneous nanostructures prepared by electrochemical anodization of SiGe/Simultiple quantum wells J. Appl. Phys. 110, 103101 (2011) Microstructure, optical property, and electronic band structure of cuprous oxide thin films J. Appl. Phys. 110, 103503 (2011) Suppression of luminescence quenching at the nanometer scale in Gd2O3 doped with Eu3+ or Tb3+: Systematiccomparison between nanometric and macroscopic samples of life-time, quantum yield, radiative and non-radiative decay rates J. Appl. Phys. 110, 094317 (2011) Influence of local atomic configuration in AlGdN phosphor thin films on deep ultra-violet luminescence intensity J. Appl. Phys. 110, 093108 (2011) Comparative investigation on the 2.7m emission in Er3+/Ho3+ codoped fluorophosphate glass J. Appl. Phys. 110, 093106 (2011) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Visible to near infrared conversion in Ce3+–Yb3+ Co-doped YAG ceramicsJumpei Uedaa� and Setsuhisa TanabeGraduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-choSakyo-ku, Kyoto 606-8501, Japan

�Received 25 June 2009; accepted 7 July 2009; published online 17 August 2009�

In Ce3+–Yb3+ co-doped Y3Al5O12 �YAG� ceramics, possibility of quantum cutting mechanismconverting one visible photon into two NIR photons with optimum quantum efficiency approaching200% have been investigated. In this material, Yb3+ emissions due to the 2F5/2-

2F7/2 in the range of1 �m were observed upon the excitation of 5d level of Ce3+. In addition, excitation spectra of Yb3+

emission corresponded to that of Ce3+ emission completely. Lifetime of the 5d level of Ce3+

decreased with increasing Yb3+ content. These results indicate the energy transfer �ET� from the 5dlevels of Ce3+ to the 5F5/2 level of Yb

3+. In �Y0.945Ce0.005Yb0.05� 3Al5O12 sample, the directlymeasured quantum yield �QY� of Yb3+ emission upon the excitation of 5d level of Ce3+ was about12% and lower than QY �96%� that estimated from the ET efficiency which was calculated with themeasured lifetime of Ce3+. © 2009 American Institute of Physics. �DOI: 10.1063/1.3194310�

I. INTRODUCTION

Luminescent materials doped with rare earth ions areused for many devices such as optical amplifiers in telecom-munication, phosphors for white light emitting diodes�LEDs�, displays, and so on. Recently, they also have at-tracted a great interest for photovoltaic applications to im-prove solar cell efficiency by modifying solar spectrum.1

Figure 1 shows the solar spectrum and the spectral responseof crystalline silicon solar cell. Crystal silicon �c-Si� solarcells most effectively convert photons of energy close to thesemiconductor band gap. The mismatch between the incidentsolar spectrum and the spectral response of solar cells is oneof the main reasons to limit the cell efficiency. The efficiencylimit of the c-Si have been estimated to be 29% by Shockleyand Queisser.2 However, this limit is estimated to be im-proved up to 38.4% by modifying the solar spectrum by aquantum cutting �downconverting� phosphor which convertsone photon of high energy into two photons of lowerenergy.3

The phenomenon such as the quantum cutting or thedownconversion of rare earth ions have been investigatedsince Dexter4 reported the possibility of a luminescent quan-tum yield greater than unity in 1957. In the past, the quantumcutting from a vacuum ultraviolet photon to visible photonsfor Pr3+,5,6 Gd3+,7 Gd3+–Eu3+,8 and Er3+–Tb3+9 had beenstudied. Recently, a new quantum cutting phenomenon fromvisible photon shorter than 500 nm to two infrared photonsfor Tb3+–Yb3+,10–13 Pr3+–Yb3+,14 and Tm3+–Yb3+15 hasbeen reported. The Yb3+ ion is suitable as an acceptor andemitter because luminescent quantum efficiency of Yb3+ isclose to 100% and the energy of the only excited level ofYb3+ ��1.2 eV� is roughly in accordance with the band gapof Si ��1.1 eV�. However, absorption transitions of Pr3+,Tb3+, and Tm3+ as a donor are due to forbidden f-f transi-tions. Therefore, the absorption linewidth and cross sectionsare not so wide and large, respectively. On the other hand,

optical transitions of Ce3+ in the UV to visible regions aredue to allowed f-d transitions. As a result the absorptionlinewidth and cross sections are wide and large, respectively.In addition, the Ce3+-doped Y3Al5O12 �YAG�, used as aphosphor for white LED, has broad absorption bands in therange of 300–500 nm due to strong ligand field and highluminescent quantum efficiency.16,17 Therefore, the Ce3+ ionsin the YAG can be suitable as an excellent sensitizing donorfor down conversion materials of Si solar cells.

In this study, Ce3+–Yb3+-codoped YAG ceramics wereprepared and the energy transfer �ET� including down con-version mechanism in Ce3+–Yb3+ codoped YAG ceramicshave been evaluated by the photoluminescence �PL�, thephotoluminescence excitation �PLE�, the lifetime and thequantum yield �QY�, which was measured directly using anintegrating sphere.

II. EXPERIMENT

Polycrystalline YAG ceramics with composition of�Y0.995−xCe0.005Ybx� 3Al5O12 �x=0, 0.005, 0.02, 0.05, and

a�Electronic mail: j.ueda@at3.ecs.kyoto-u.ac.jp.

300 400 600 1000 1600

Photon Energy (eV)

Solar spectrum Spectral responseof c-Si

Intensity(arb.unit)

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

2Eg~Eg�2Eg

feasible area ofquantum cutting

hν

2hν

Eg=1.2eV

quantum cutting

Wavelength (nm)

SpectralResponse(arb.unit)

Eg

FIG. 1. Solar spectrum and spectral response of c-Si.

JOURNAL OF APPLIED PHYSICS 106, 043101 �2009�

0021-8979/2009/106�4�/043101/5/$25.00 © 2009 American Institute of Physics106, 043101-1

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http://dx.doi.org/10.1063/1.3194310http://dx.doi.org/10.1063/1.3194310

0.1� were prepared by using reagent grade Y2O3 �99.99%�,Al2O3 �99.99%�, CeO2 �99.99%�, and Yb2O3 �99.99%� asstarting materials. The powders were mixed well with tetra-ethyl orthosilicate �0.5 wt %� in an alumina mortar to facili-tate solid state reaction, pressed into a pellet of 20 mm di-ameter and sintered at 1600 °C for 6 h. The first obtainedpolycrystalline pellets were crushed, pressed and sinteredagain to obtain complete single phase of YAG. The crystalphases of obtained samples were identified by an x-ray dif-fraction �XRD� measurement �Shimadzu, XRD6000�. Forthe PL spectra, samples were excited by using 450 nm lightthat was obtained by combining a band pass filter and a Xelamp �Asahi Spectra Co., Ltd., MAX-302�. The lumines-cence spectra were measured with a monochromator �Nikon,G250� and a Si photodiode �Electro-Optical System Inc.,S-025-H�. The obtained PL spectra were calibrated by a stan-dard halogen lamp �Labsphere, OGL-600�. In the PLE spec-tra measurement monitoring near infrared luminescence, theluminescence was detected by combining an 850 nm shortcut filter and an InGaAs �Electro-Optical System Inc., IGA-010-H� photodiode. Samples were excited by monochro-matic light obtained by combining the Xe lamp �350–800nm� and the monochromator. For the PLE spectra monitoringvisible luminescence, a 550 nm bandpass filter and the Siphotodiode were used. For the luminescent decay measure-ment, samples were excited by using a 466 nm dye �Exciton,LD466� laser pumped with a nitrogen laser pulse excitation.The decay curves of fluorescence at 550 and 1030 nm weredetected by a high-speed silicon detector �Thorlabs, Inc.,DET110� and averaged on a digital oscilloscope �Yokogawa,DL1620�. For the QY of emission upon the excitation ofCe3+ :5d level, the PL spectra were measured under the 440nm LD excitation �Nichica, NDHB510APA� by using an in-tegrating sphere �Labsphere� which was connected to acharge coupled device �CCD� detector of visible range�Ocean Optics, USB2000� and a CCD detector of infraredrange �Ocean Optics, USB2000+� with an optical bifurcatedfiber of 400 �m core. The obtained PL spectra were cali-brated by using the standard halogen lamp and an auxiliarylamp �Labsphere, AUX-30�, and then the total radiant fluxand photon distribution were obtained. The QY was evalu-ated as the ratio of the emission photon number to the ab-sorption photon number. For the QY of emission upon theexcitation of Yb3+, the 935 nm laser diode �LD� �Qphotonics,QLD-945-100S� was used as excitation.

III. RESULTS

A. YAG samples

The XRD patterns of obtained samples are shown in Fig.2. The XRD patterns of all samples almost corresponded tothat of the YAG.18 The XRD peaks were slightly shifted tohigher angle �2�� with increasing Yb3+ content �inset in Fig.2�.

B. Optical property of Ce3+ and Yb3+ in YAG

1. Photoluminescence and photoluminescenceexcitation

Figure 3 shows the PL spectrum by 450 nm excitationand the PLE spectra monitoring 550 and 1030 nm of the

sample with x=0.05 composition. The emission bandspeaked at 550 nm �Ce3+:5d-4f� and 1030 nm�Yb3+: 2F5/2-

2F7/2� were observed by exciting the 5d level ofCe3+ with 450 nm light. For the PLE spectrum monitoring550 nm, the broad excitation bands were located at 330 and450 nm. For the PLE spectrum monitoring 1030 nm, a broadexcitation band located at 450 nm was also observed. Thesetwo PLE spectra were consistent between 400 and 500 nm.

2. Decay curves

Figure 4 shows the decay curves of Ce3+ emission at 540nm under the 466 nm dye laser. Each decay curve was fittedby the following single exponential function:

I = I0 exp�− t/�decay� . �1�

It can be seen that the decay times were decreased with in-creasing Yb3+ content. Figure 5 shows the decay curves ofemission at 1030 nm due to Yb3+ transition �2F5/2→ 2F7/2�.In this measurement, both rise and decay components wereobserved. Each decay curve was fitted by the followingdouble exponential function:

20 30 40 50 60

YAG JCPDS# 33-0040

x=0

x=0.1

x=0.05

x=0.005

Intensity(arb.unit)

x=0.02

32 33 34

x=0.1

x=0.05

x=0.02

FIG. 2. XRD patterns of Ce3+–Yb3+ codoped YAG with different Yb3+

contents. �Inset is extend figure of main peak in some samples�.

PL(λex=450nm)PLE(λem=550nm)PLE(λem=1030nm)

x=0.05

Ce3+:4f→5d

Ce3+:5d→4f

Yb3+:2F5/2→2F7/2

300 400 500 600 700 800 900 1000 1100 1200

Intensity(arb.unit)

Wavelength (nm)

FIG. 3. PL spectrum by the 450 nm LD excitation and PLE spectra moni-tored at 550 and 1030 nm in Ce3+–Yb3+ codoped YAG.

043101-2 J. Ueda and S. Tanabe J. Appl. Phys. 106, 043101 �2009�

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I = I0�exp�− t/�decay� − exp�− t/�rise�� . �2�

IV. TOTAL RADIANT FLUX

Figure 6 shows the total radiant flux spectra of somesamples by 440 nm LD excitation. In the sample with x=0composition, the intense emission at around 540 nm wasobserved, while no emission at around 1030 nm was ob-served. The intensity of emission at around 540 nm was de-creased with increasing Yb3+ content, while that at around1030 nm was increased with increasing Yb3+ content untilx=5 composition. Figure 7 shows the radiant flux spectra ofsamples in the range of near infrared by 935 nm LD excita-tion.

V. DISCUSSIONS

A. YAG samples

From the obtained XRD peaks, the lattice constant, a, ofthe YAG ceramics was estimated by the following equationthat combined the Bragg’s equation with a relation of cubiclattice constant with the Miller’s indices:

a =��h2 + k2 + l2

2 sin �, �3�

where � is the wavelength of Cu K� radiation, �h ,k , l� is theMiller’s indices and � is the angle of diffraction peak. Figure8 shows the calculated lattice constant and reference crystalof Y3Al5O12:YAG

18 and Yb3Al5O12:YBAG.19 The lattice

constant of obtained YAG were decreased with increasingYb3+ content following Vegard’s law. Therefore, it was con-firmed that Yb3+ ions were incorporated into YAG crystal.

B. Optical property of Ce3+ and Yb3+ in YAG

1. Energy transfer from Ce3+ to Yb3+

Results of the PL and PLE spectra of the sample withx=0.05 composition in Fig. 3 indicated evidence of energytransfer from the 5d levels of Ce3+ to the 2F5/2 level of Yb

3+.The excitation bands of Ce3+ correspond to the peak energyof solar spectrum, which is in a range of low spectral re-sponse of c-Si. In addition, the emission band peaked at 1030nm corresponds to a range of high spectral response. There-fore, the ET from Ce3+ to Yb3+ in YAG ceramics is suitablefor solar spectrum convertor from viewpoint spectral charac-teristics of the PL and the PLE. From decreasing of lifetimeof Ce3+ :5d level with increasing Yb3+ content, it is consid-ered that energy transfer from Ce3+ to Yb3+ occurred. Thetotal decay rate �Wtot� of 5d levels in Ce3+ single doped YAGsample is given by

FIG. 5. Yb3+ content variation of decay curve, which monitored emissionfrom the 2F5/2 of Yb

3+ by 466 nm excitation.

400 500 600 700 800 900 1000 1100

0

10

20

30 x=0

x=0.02

x=0.05

Totalradiantflux(mW/nm)

Wavelength (nm)

Ex. 440 nm

x=0

x=0.02

x=0.05

FIG. 6. Yb3+ content variation total radiant flux spectra by 440 nmexcitation.

900 950 1000 1050 1100 1150

0

50

100

150

200

Ex. 935 nmx=0.005x=0.02x=0.05x=0.1

Totalradiantflux(μW/nm)

Wavelength (nm)

FIG. 7. Yb3+ content variation total radiant flux spectra by 935 nmexcitation.

0 100 200 3000.01

0.1

1

x=0x=0.005x=0.02x=0.05x=0.1

Intensity(arb.unit)

Time (ns)

Ex. 466nm

FIG. 4. �Color online� Single logarithmic chart of decay curves, whichmonitored emission from 5d level of Ce3+ by 466 nm excitation, in sampleswith different Yb3+ contents.

043101-3 J. Ueda and S. Tanabe J. Appl. Phys. 106, 043101 �2009�

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Wtot = A + WMP = �Ce−1, �4�

where A is the radiative rate, WMP is the multiphonon relax-ation rate, and �Ce is the lifetime of 5d level in Ce

3+ singledoped YAG. In Ce3+, Yb3+-codoped YAG, the extra decaypathway from 5d level of Ce3+ to 2F5/2 level of Yb

3+ wasgenerated. Therefore, the total decay rate is given by

Wtot = A + WMP + WET = �Ce,Yb−1 , �5�

where WET is the energy transfer rate and �Ce,Yb is the life-time of 5d level in Ce3+–Yb3+ codoped YAG. Therefore theenergy transfer efficiency is given by

�ET =WET

A + WMP + WET= 1 −

�Ce,Yb�Ce

. �6�

Figure 9 shows the Yb3+ content dependences of lifetimefor the 5d level of Ce3+ and the ET efficiency from the 5dlevel of Ce3+ to the 2F5/2 of Yb

3+. The lifetime decreasedwith increasing Yb3+ content, the ET efficiency increasedwith increasing Yb3+ content. The ET efficiency exceededabout 50% above x=0.05 composition. Therefore, the quan-tum yield of Yb3+ will be over 100% if ideal quantum cuttingoccurred as the equation given by

QY = 2�Yb�ET, �7�

where �Yb is the emission quantum efficiency of Yb3+, usu-

ally about 100% because of low multiphonon relaxation ratedue to large energy gap to the next lower level, the ground

state. In the sample with x=0.05 composition, the QY wasestimated high value �96%�.

VI. TOTAL RADIANT FLUX

From the total radiant flux spectrum by 440 nm LD ex-citation, the photon distribution spectrum can be obtained.Therefore the emission and absorption photon number wasestimated, and then the quantum efficiency was calculated.Figure 10 shows the Yb3+ content dependences of the QY ofCe3+ emission at around 550 nm and that of Yb3+ emission ataround 1030 nm. The QY of Ce3+ emission decreased andthat of Yb3+ emission increased with increasing Yb3+ con-tent. The maximum value of QY of Yb3+ was 12.8% in thex=0.05 sample and lower than that estimated from the ETefficiency, which was calculated with the measured lifetimeof Ce3+ :5d level. It is considered that other nonradiative pro-cesses existed. From the radiant flux spectrum by 935 nm LDexcitation, the QY of Yb3+ was calculated. Figure 11 showsthe Yb3+ content dependence of QY of the Yb3+ emission by935 nm LD excitation. At least the QY indicated higher valuethan 80% until x=0.05. Therefore it is considered that thenonradiative processes are not due to concentration quench-ing of Yb3+.

VII. ENERGY TRANSFER MECHANISM

Figure 12 shows the Yb3+ content dependence of riseand decay time of the Yb3+ emission by 466 nm excitation.

0 10 95 10011.90

11.95

12.00

12.05

Ce,Yb:YAG samples

YAG JCPDS# 33-0040YBAG JCPDS# 23-1476

Latticeconstatnt(�)

Yb3+ content (%)

FIG. 8. Yb3+ content dependence of lattice constant of Ce3+–Yb3+ codopedYAG. Solid line is lattice constant estimated from Vegard’s law.

0 2 4 6 8 10

0

20

40

60

80

Celifetime(ns)

Energytransferefficienc y(%)

0

20

40

60

80

Energy transfer efficiencyCe3+ lifetime

Yb3+ content (%)

FIG. 9. Yb3+ content dependence of lifetime at 5d level of Ce3+ and energytransfer efficiency from Ce3+ to Yb3+.

0 2 4 6 8 10

0

20

40

60

80

100

Ce3+QYYb3+QY

Quantum

Yield(%)

Yb3+ content (%)

Ex. 440 nm

FIG. 10. Yb3+ content dependence of quantum yield of Ce3+ emission andYb3+ emission by 440 nm excitation.

0 2 4 6 8 1060

70

80

90

100

Yb3+ QY

Quantum

Yield(%)

Yb3+ content (%)

Ex. 935 nm

FIG. 11. Yb3+ content dependence of quantum yield of Yb3+ emission andby 935 nm excitation.

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The decay time decreased at x=0.1 composition. It is con-sidered that concentration quenching of Yb3+ occurred. Therise time should correspond to relaxation time from the 5dlevels of Ce3+ to the 2F7/2 of Yb

3+. However, the rise timewas about 0.2 ms in all the samples and longer than lifetime��100 ns� of Ce3+ :5d level estimated from measurement ofdecay curve by 440 nm LD excitation. These results indi-cated that the energy transfer from the 5d level of Ce3+ to the2F5/2 of Yb

3+ is not through direct process. It is consideredthat the longer rise time is due to slow nonradiative relax-ation from a charge transfer state �CTS� to the 2F5/2 level. InYb3+-doped YAG, the charge transfer absorption and thecharge transfer luminescence have been reported.20–23 In ad-dition, it had been known that the emission of Ce3+ wasquenched by the process of charge transfer state�Ce4+–Yb2+� in Ce3+-doped crystal that consists of Yb, suchas LiYbF4,

24 YbSiO5.25,26 That is because the Ce3+ and Yb3+

changes to Ce4+ and Yb2+ easily. In a similar system, it wasreported that Yb3+ IR emission in Tb3+–Yb3+ codoped Y2O3by UV excitation was caused through Tb4+–Yb2+ chargetransfer state.12 Therefore, it is considered that the energytransfer from Ce3+ to Yb3+ may be caused through a CTSCe4+–Yb2+ �Fig. 13�.

VIII. CONCLUSION

The energy transfer from Ce3+ and sensitized lumines-cence of Yb3+ in polycrystalline YAG ceramics was ob-served. The quantum yields of Yb3+ emission upon Ce3+ :5dlevel excitation were lower than that value expected for theideal quantum cutting mechanism. In addition the relaxationtime, which was estimated from measurement of Yb3+ decaycurve by 440 nm LD excitation, from 5d levels of Ce3+ to2F7/2 of Yb

3+ was not consistent to the lifetime of Ce3+ esti-mated from measurement of Ce3+ decay curve by 440 nm LDexcitation. From the QY of Yb3+ emission by 935 nm exci-tation, the concentration quenching did not occur until x=0.5 Yb3+ concentration. Therefore, we indicated that theenergy transfer from Ce3+ to Yb3+ in YAG was not directprocess and has some nonradiative processes that were notthe concentration quenching Yb3+.

ACKNOWLEDGMENTS

These investigations were supported by the Toray Sci-ence Foundations and the Nippon Sheet Glass Foundationfor Materials Science and Engineering.

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0 2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Decay timeRise time

Lifetime(ms)

Ex. 466nm

Yb3+ content (%)

FIG. 12. Yb3+ content dependence of lifetime at 2F5/2 of Yb3+ by 466 nm

excitation.

Ce3+

5d

450nm

550nm

2F5/2

2F7/2

Yb3+

1030nm

Charge transfer stateCe4++Yb2+

2F7/2

2F5/2

R R

E E

FIG. 13. Configuration coordinate model of emission and energy transferfrom Ce3+ to Yb3+. Dashed lines are Ce4+–Yb2+ charge transfer state.

043101-5 J. Ueda and S. Tanabe J. Appl. Phys. 106, 043101 �2009�

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