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Materials Science and Engineering B 176 (2011) 1537– 1540

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering B

jou rna l h om epage: www.elsev ier .com/ locate /mseb

hort communication

ynthesis and blue to near-infrared quantum cutting of Pr3+/Yb3+ co-dopedi2TeO4 phosphors

an Lina, Xiaohong Yana,b,∗, Xiangfu Wanga

College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, People’s Republic of ChinaSchool of Electronic Science and Engineering, Nanjing University of Post and Telecommunications, Nanjing 210046, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 9 April 2011eceived in revised form 27 July 2011ccepted 4 September 2011

a b s t r a c t

Near-infrared (NIR) quantum cutting luminescent materials Li2TeO4 doped with Pr3+ and Yb3+ weresynthesized by solid-state reaction method. The dependence of Yb3+ doping concentration on thevisible- and NIR-emissions, decay lifetime, and quantum efficiencies of the phosphors are investi-gated. Quantum cutting down-conversion involving 647 nm red emission and 960–1050 nm broadband

vailable online 15 September 2011

eywords:uantum cuttingi2TeO4

ownconversion

near-infrared emission for each 487 nm blue photon absorbed is realized successfully in the result-ing phosphors, of which the process of near-infrared quantum cutting could be expressed as3P0(Pr3+) → 2F5/2(Yb3+) + 2F5/2(Yb3+). The maximum quantum cutting efficiency approaches up to 166.4%in Li2TeO4: 0.3 mol%Pr3+, 1.8 mol%Yb3+ sample corresponding to the 66.4% value of energy transfer effi-ciency.

nergy transfer

. Introduction

In recent decades, near-infrared (NIR) quantum cutting down-onversion, which is based on the principle that the quantumutting phosphors could generate two NIR photons from anbsorbed incident high-energy photon, has attracted extensivettention for application in solar cells [1–4]. Lanthanide rare earthons are the suitable candidates for the quantum cutting down-onversion processes owing to their abundant energy levels andarrow emission spectral lines. There were many investigationsn quantum cutting downconversion luminescence process in rarearth codoped systems, such as Tb3+–Yb3+ [4–9], Tm3+–Yb3+ [8,10],r3+–Yb3+ [11–14], and other ion-pairs [15–17], where Tb3+, Tm3+,nd Pr3+ act as the active centers ions to absorb high-energy pho-on. Especially, Pr3+ ion is thought as a very attractive ion, due tots special 4f2 energy configuration and the energy-level gap matchetween Pr3+ (3P0) and Yb3+–Yb3+ ion-pairs (22F5/2). Thus, the Pr3+,b3+ co-doped quantum cutting phosphors had been investigatedxtensively in many hosts.

Tellurite phosphate is considered potential candidate as opti-

al material [18]. It has certain advantage over many other oxidehosphates as host material because of its superior properties, suchs high chemical and thermal durability and mechanical strength

∗ Corresponding author at: College of Science, Nanjing University of Aeronauticsnd Astronautics, Nanjing 211106, People’s Republic of China.el.: +8602552145205; fax: +8602552145205.

E-mail address: xhyan@nuaa.edu.cn (X. Yan).

921-5107/$ – see front matter © 2011 Published by Elsevier B.V.oi:10.1016/j.mseb.2011.09.020

© 2011 Published by Elsevier B.V.

[19], wide band infrared transmittance, large refractive index andlarge third-order non-linear optical susceptibility [20,21]. Recently,framework materials based on lithiated phosphates of Li2MOx

(MOx−2 = TiO3

−2, SeO4−2, TeO3

−2, TeO4−2, etc.) systems [22,23]

have attracted significant interests due to their good lithium ionmobility and wide range of applications in LED. The stoichiometriccrystal Li2TeO4, was chosen as the host lattice based on its func-tional properties, which has an orthorhombic structure at roomtemperature and smaller activation energy in the higher tempera-ture range [23]. However, the photoluminescence investigation ofsuch an important material was seldom reported. Thus, the opti-cal character is necessary to be explored in rare earth ions dopedLi2TeO4 materials.

In the present work, near-infrared quantum cutting in Pr3+, Yb3+

co-doped Li2TeO4 phosphors by cooperative energy transfer (CET),were demonstrated. The reason for the near-infrared emission oftwo photons in the range of 960–1050 nm under the excitation ofone blue-visible photon is analyzed by the visible and NIR emis-sions, mean decay lifetimes, and quantum efficiency. The possiblemechanism involved in the near-infrared quantum cutting phe-nomenon has been discussed.

2. Experimental

All chemicals were directly used as received without further

purification. Samples of Li2TeO4: 0.3 mol%Pr3+, x mol%Yb3+ in aseries with x = 0.0, 0.3, 0.9, 1.8, and 3.0 were synthesized via hightemperature solid state reaction route as discussed below. Accord-ing to the stoichiometric formula, high-purity powders of Li2CO3

1538 H. Lin et al. / Materials Science and Engi

60555045403530252015

101

2θ (degree)

100

111

102 220 114214

322211

Inte

nsity

(a.u

.)

Fo

((taii

(wtntSdt

3

1wp

Fso

ig. 1. XRD pattern of the Li2TeO4: Pr3+/Yb3+. Bottom part is the standard line patternf the orthorhombic phase Li2TeO4 (JCPDS 70-0676).

analytical reagent) and TeO2 (99.5%), Pr6O11 (99.99%) and Yb2O399.99%) were pulverized thoroughly in agate mortars and thenransferred into corundum crucibles. The samples were calcinedt 650 ◦C for 6 h in air to complete the crystallization. After finish-ng the heating treatment, the samples were cooled down naturallyn the furnace to the room temperature.

Structures of the samples were investigated by X-ray diffractionXRD) using a Ultima-III (Rigaku) equipment provided with Cu tubeith K� radiation at 1.54060 A, scanning in the 2� range from 13◦

o 60◦ with 0.01◦ increments and 6 s swept time. The photolumi-escence excitation (PLE), the photoluminescence (PL) spectra, andhe luminescence decay curves were recorded by the Fluorescencepectrophotometer (FSP920) with a photomultiplier tube (InGaAsetector). All spectrum measurements were performed at roomemperature.

. Results and discussion

The XRD pattern for Li2TeO4 sample with 0.3 mol%Pr3+ and.8 mol%Yb3+ doping concentration is shown in Fig. 1(a). Comparedith standard data (JCPDS No. 70-0676), the sample exhibits theeaks of pure orthorhombic phase. No obvious shifting of peak or

500475450425 650600550500

3H6

3H5

3F2

Pr3+:3P0(b)

Wavelength

L

3P0

3P1,1I63P2

3H4(a)Excitation

Inte

nsity

(a.u

.)

Emission

ig. 2. (Color online) (a) Excitation spectra of Li2TeO4: Pr3+ (0.3 mol%) sample (�em = 647 nmample (�em = 978 nm, Yb3+: 2F5/2 → 2F7/2 transition, dotted line). (b) Visible-NIR PL spectrf 487 nm (Pr3+: 3H4 → 3P0).

neering B 176 (2011) 1537– 1540

second phase can be detected at the current doping level. Addi-tionally, the fairly narrow full width and intense diffraction peaksindicate the well crystallization of the sample. However, due tothe different experimental methods, the relative intensities of thepeaks are a little different from those in standard data [24]. The sim-ilar XRD patterns are obtained for the samples with different Pr3+

and Yb3+ doping concentrations, which suggests all the sampleshave been crystallized into the pure orthorhombic phase.

Fig. 2 depicts, as a typical example, the photoluminescence exci-tation (PLE) of Pr3+ single doped and Pr3+–Yb3+ codoped Li2TeO4samples, and photoluminescence (PL) spectra of the samples withvarious Yb3+ (0.0, 0.3, 0.9, 1.8, and 3.0) doping concentrations.From the PLE spectra of Pr3+ single doped Li2TeO4 (Fig. 2(a) solidline), intense bands observed at 487, 475, and 449 nm, respectively,ascribed to the 3H4 → 3Pj, 1I6 (j = 0, 1, and 2) transitions of Pr3+, havebeen measured when the 3P0 → 3F2 transition of Pr3+ at 647 nm ismonitored. In the Pr3+–Yb3+ codoped Li2TeO4 sample (Fig. 2(a) dashline), the excitation spectra of Yb3+: 2F5/2 → 2F7/2 infrared emissionat 978 nm is in good agreement with Pr3+ 3H4 → 3Pj, 1I6 (j = 0, 1,and 2) absorption, which indicates that energy transfer (ET) fromPr3+ to Yb3+ occurs. The emission spectra of Li2TeO4: Pr3+, Yb3+ inthe visible region from 500 to 800 nm and the NIR region from 800to 1100 nm are shown in Fig. 2(b). Upon excitation of 487 nm, themost intense emission bands are at around 647 nm, correspondingto 3P0 → 3F2 transition and the other peaks at 529, 615, 684, 709,and 730 nm in visible region are assigned to the electronic transi-tions of 3P0 → 3Hi (i = 5, 6), 3Fj (j = 2, 3, 4), respectively. Meanwhile,the characteristic emission band located at 960–1050 nm corre-sponding to the 2F5/2 → 2F7/2 transition of Yb3+ ions, are observed inthe right of Fig. 2(b). From the comparison of the emission spectraof the visible region and the NIR region under 487 nm excitation,we found that the visible emission intensities of Pr3+ significantlyreduced with increase of Yb3+ concentrations ranging from 0.0 to3.0 mol%. It is noticed that the PL intensity of the NIR emission at978 nm increases rapidly, while a red emission at 647 nm decreasesmonotonically. However, at higher Yb3+ concentrations reaching1.8 mol%, a decrease in the NIR emission has been clearly observed

due to the concentration quenching effect [8,11]. In this situation,cross-relaxation between Yb3+ ions will dominate at higher Yb3+

concentrations, which effectively reduces luminescence yield. Theobservation of both the 3P0 → 3Hi (i = 5, 6), 3Fj (j = 2, 3, 4) transitions

11001000900800750700

0.0 0.3 0.9 1.8 3.0

3F4

(nm)

i2TeO4: 0.3%Pr3+, x%Yb3+

Yb3+:2F5/22F7/2

, Pr3+: 3P0 → 3F2 transition, solid line) and Li2TeO4: Pr3+ (0.3 mol%), Yb3+ (0.9 mol%)a of Li2TeO4: Pr3+, Yb3+ with different Yb3+ concentrations (x mol%) upon excitation

H. Lin et al. / Materials Science and Engineering B 176 (2011) 1537– 1540 1539

F 3+ 3+

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snff

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wpc(cicftasb1

ces2

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im

euc

10.08.06.04.02.00.0

Li2TeO 4: Pr,Yb

0.00.30.91.83.0

Time(μs)

Inte

nsity

(a.u

.)

ig. 3. Energy levels diagram of Pr and Yb showing possible mechanisms for aear-infrared quantum cutting. One blue photon absorbed by Pr3+ is converted intowo near infrared photons Yb3+ through cooperative downconversion route.

f Pr3+, and the 2F5/2 → 2F7/2 transition of Yb3+ upon excitation with visible excitation at 487 nm, further confirm the occurrence ofnergy transfer (ET) from Pr3+ to Yb3+.

In Fig. 3, the energy levels of Pr3+ and Yb3+ ions are schematicallyhown to explain the quantum cutting downconversion mecha-ism in the Li2TeO4 codoped with Pr3+–Yb3+. The Pr3+ ion is excited

rom the 3H4 to the 3P0 level. The two possible energy transitionsrom the Pr3+ to Yb3+ are described as follow:

P0(Pr3+) + 2F7/2(Yb3+) → 1G4(Pr3+) + 2F5/2(Yb3+) (1)

P0(Pr3+) → 2F5/2(Yb3+) + 2F5/2(Yb3+) (2)

hich can both feed the Yb3+: 2F5/2 level, as previous works pro-osed [11–13]. On the one hand, there exists the possibility of theross-relaxation process (1). Though the 1G4 level slightly lower200 cm−1) than the 2F5/2 level of Yb3+, the energy gap of 200 cm−1

an be easily matched by phonon energy of Li2TeO4. Therefore,t is hard to exclude the possibility of the cross-relaxation pro-ess (1) in our samples. Meanwhile, as can be seen in Fig. 2(b)or the sample of Pr3+ single doped Li2TeO4, a NIR emission inhe region of 800–1100 nm can be observed, which should bessigned to the Pr3+: 3P0 → 1G4 transition. So, the downconver-ion luminescence of Li2TeO4: 0.3 mol%Pr3+, x mol%Yb3+ mighte due to a cross-relaxation process: 3P0(Pr3+) + 2F7/2(Yb3+) →G4(Pr3+) + 2F5/2(Yb3+). On the other hand, the cooperative down-onversion route (2) also exists in our samples. Notably, uponxcitation at the 3P0 level of Pr3+ ion, the Pr3+: 3P0 → 3H4 tran-ition is located at approximately twice the energy of the Yb3+:F5/2 → 2F7/2 transition and Yb3+ has no other levels up to the UVegion. According to the Miyakawa–Dexter theory [25], it is rea-onable to assume a cooperative downconversion route (2) for thisnergy transfer efficiency, in which the excitation energy at theP0 level of Pr3+ is transferred simultaneously to two nearby Yb3+

ons, is easy to occur and will be responsible for the relaxationechanism to achieve NIR emission.

3+ 3 3

In Fig. 4, the luminescence decay curves of Pr for the P0 → F2mission at 647 nm were recorded for various Yb3+ concentrationspon excitation at 487 nm, providing further insight into the effi-iency of the energy transfer process from the 3P0 level of Pr3+ to

Fig. 4. (Color online) Luminescence decay curves of Pr3+ 647 nm emission originatedfrom the 3P0 → 3F2 transition. Doping concentrations are 0.3 mol%Pr3+, x mol%Yb3+,with x = 0.0, 0.3, 0.9, 1.8 and 3.0, respectively.

the 2F5/2 level of Yb3+. One can evaluate the effective experimentallifetime by

� =∫

I(t)t dt∫

I(t)dt, (3)

where I(t) represents the luminescence intensity as a function oftime t. In the Pr3+ singlely doped Li2TeO4 sample, as expected, thedecay of the luminescence can be well-fitted to a nearly single expo-nential decay with a lifetime of 28.4 �s. However, when the Yb3+

ion concentration is increasing in the range of 0.0–3.0 mol%, energytransfer between Pr3+ and Yb3+ ions appears and the decay curvesbecome nonexponential with the lifetimes decline faster from 28.4to 9.5 �s.

As to the decay curves, the energy transfer efficiency (ETE) (�ETE)and the quantum efficiency (QE) (�QE) can be determined. Theenergy transfer efficiency �ETE is defined as the ratio of donorsthat are depopulated by ET to the acceptors over the total num-ber of the Pr3+ excited. For this system, Pr3+ acts as the donor andYb3+ as the acceptor. On the assumption that all excited Yb3+ decayradiatively, �ETE can be educed from Eq. (4). By dividing the inte-grated intensity of the decay curves of Pr3+, Yb3+ codoped samplesto Pr3+ single-doped sample, �ETE is obtained as a function of theYb3+ concentration as follows [1,4]:

�ETE = �x%Yb = 1 −∫

Ix%Ybdt∫

I0%Ybdt, (4)

where I denotes intensity and x% stands for the Yb3+concentration.In addition, the relation between the transfer efficiency �ETE andthe quantum efficiency �QE is linear and is defined as [1,4]:

�QE = �Pr(1 − �ETE) + 2�ETE, (5)

where the �QE stands for the Pr3+. Ignoring the nonradiate energyloss by defects and impurities, the quantum efficiency for Pr3+ ions�Pr is set to 1 [4,8].

Using Eqs. (4) and (5), the value of �QE and �ETE are gained.The decay lifetimes and ETE, QE of the phosphors are sum-marized in Table 1. It is presented that the ETE increasesmonotonously and achieves a maximum value of 66.4% in the0.3 mol%Pr3+/1.8 mol%Yb3+ co-doped Li2TeO4. However, taking intoaccount the concentration quenching of Yb3+, the actual QE would

be lower [8,11].

From Table 1, we observe that the quantum efficiencies rapidlyincreases with increasing Yb3+ concentration, until it reaches amaximum value of 166.4% for 1.8 mol%Yb3+ doping and then

1540 H. Lin et al. / Materials Science and Engi

Table 1Decay lifetimes, energy transfer efficiencies (ETE) and quantum efficiencies (QE) asa function of the Yb3+ doping concentration.

Yb3+ concentration (mol%) Lifetime (�s) ETE (%) QE (%)

0.0 28.4 0 100.00.3 21.5 24.4 124.40.9 14.1 50.2 150.21.8 9.5 66.4 166.4

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lightly decreases when the Yb3+ concentration increases to.0 mol%. This can be explained as follows: initially, increasing Yb3+

oping enhances the energy transfer efficiency from Pr3+ to Yb3+ asell as the corresponding quantum efficiency. However, when theb3+ concentration is high enough (more than 3.0 mol%), Yb3+–Yb3+

on-pairs mutual annihilation as follow

F5/2(Yb3+) + 2F7/2(Yb3+) → 2F7/2(Yb3+) + 2F5/2(Yb3+) (6)

ay occur and result in a lower quantum efficiency. According tour experimental data, the optimum Yb3+ concentration is 1.8 mol%n this system. Compared with Pr3+, Yb3+ co-doped quantum cut-ing host materials in matrices previously studied [11–13], the QEalue reaches a maximum value of 166.4%, and it is comparable tohe most efficient downconversion process.

. Conclusion

In summary, lithium tellurate samples doped with Pr3+ and Yb3+

ere synthesized and investigated as a quantum cutting downcon-

ersion phosphors. On excitation of Pr3+ ions with a visible photont 487 nm, Yb3+ ions emit two near-infrared photons at 978 nmhrough a cooperative energy transfer from Pr3+ to Yb3+, with max-mum quantum efficiency as high as 166.4%.

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neering B 176 (2011) 1537– 1540

Acknowledgements

This work was supported by the key project of National HighTechnology Research and Development Program of China (863Program) (No. 2011AA050526), National Natural Science Founda-tion of China (No. 51032002, and 10874089) and Natural ScienceFoundation of Jiangsu Province (No. BK2008398) as well as theFunding of Jiangsu Innovation Program for Graduate Education (No.CX10B 095Z).

References

[1] R.T. Wegh, H. Donker, K.D. Oskam, A. Meijerink, Science 283 (1999) 663–666.[2] T. Trupke, M.A. Green, P. Wurfel, J. Appl. Phys. 92 (2002) 1668–1674.[3] B.S. Richards, Solar Energy Mater. Solar Cells 90 (2006) 1189–1207.[4] P. Vergeer, T.J.H. Vlugt, M.H.F. Kox, M.I. den Hertog, J.P.J.M. van der Eerden, A.

Meijerink, Phys. Rev. B 71 (2005) 014119.[5] Q.Y. Zhang, C.H. Yang, Y.X. Pan, Appl. Phys. Lett. 90 (2007) 021107.[6] Q.Y. Zhang, C.H. Yang, Z.H. Jiang, X.H. Ji, Appl. Phys. Lett. 90 (2007) 061914.[7] S. Ye, B. Zhu, J. Chen, J. Luo, J. Qiu, Appl. Phys. Lett. 92 (2008) 141112.[8] Q.Y. Zhang, G.F. Yang, Z.H. Jiang, Appl. Phys. Lett. 91 (2007) 051903.[9] D.Q. Chen, Y.L. Yu, Y.S. Wang, P. Huang, F.Y. Weng, J. Phys. Chem. C 113 (2009)

6406–6410.10] L.C. Xie, Y.H. Wang, H.J. Zhang, Appl. Phys. Lett. 94 (2009) 061905.11] D.Q. Chen, Y.S. Wang, Y.L. Yu, P. Huang, F.Y. Weng, Opt. Lett. 33 (2008)

1884–1886.12] B.M.V. Ende, L. Aarts, A. Meijerink, Adv. Mater. 21 (2009) 3073–3077.13] X.P. Chen, X.Y. Huang, Q.Y. Zhang, J. Appl. Phys. 106 (2009) 063518.14] Y. Katayama, S. Tanabe, Opt. Mater. 33 (2010) 176–179.15] D.Q. Chen, Y.S. Wang, Y.L. Yu, P. Huang, F.Y. Weng, J. Appl. Phys. 104 (2008)

116105.16] D.Q. Chen, Y.L. Yu, H. Lin, A.P. Yang, Y.S. Wang, Opt. Lett. 35 (2010) 220–222.17] H. Lin, D.Q. Chen, Y.L. Yu, A.P. Yang, Y.S. Wang, Opt. Lett. 36 (2011) 876–878.18] L. D’Alessio, F. Pietrucci, M. Bernasconi, J. Phys. Chem. Solids 68 (2007) 438–444.19] A.K. Singh, S.B. Rai, A. Rai, Prog. Cryst. Growth Charact. Mater. 52 (2006) 99–106.20] H. Nasu, T. Uchigaki, K. Kamiya, H. Kanbara, K. Kubodera, Jpn. J. Appl. Phys. 31

(1992) 3899.21] S.H. Kim, T. Yoko, S. Sakka, J. Am. Ceram. Soc. 76 (1993) 2486.

22] J. Liang, W.Z. Lu, J.M. Wu, J.G. Guan, Mater. Sci. Eng. B 176 (2011) 99–102.23] N.K. Singha, S. Sharmab, R.N.P. Choudhary, Fermelectrics 242 (2000) 89–96.24] F. Daniel, J. Moret, E. Philippot, E. Maurice, J. Solid State Chem. 22 (1977)

113–119.25] T. Miyakawa, D.L. Dexter, Phys. Rev. B 1 (1970) 70–80.