Sr3B2O6 : Ce3+,Eu2+: A potential single-phased white-emitting borate phosphor for ultraviolet light-emitting diodes

(1)

Sr 3 B 2 O 6 : Ce 3 + , Eu 2 + : A potential single-phased white-emitting borate phosphor

for ultraviolet light-emitting diodes

Chun-Kuei Chang and Teng-Ming Chen

Citation: Applied Physics Letters 91, 081902 (2007); doi: 10.1063/1.2772195

View online: http://dx.doi.org/10.1063/1.2772195

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/8?ver=pdfcov Published by the AIP Publishing

Articles you may be interested in

Sr 3 Al 2 O 5 Cl 2 : Ce 3 + , Eu 2 + : A potential tunable yellow-to-white-emitting phosphor for ultraviolet light emitting diodes

Appl. Phys. Lett. 94, 091902 (2009); 10.1063/1.3094753

Vacuum ultraviolet spectroscopic properties of rare earth ( RE = Ce , Tb , Eu , Tm , Sm ) -doped hexagonal K Ca Gd ( P O 4 ) 2 phosphate

J. Appl. Phys. 102, 093514 (2007); 10.1063/1.2800172

Enhanced luminescence of Sr Si 2 O 2 N 2 : Eu 2 + phosphors by codoping with Ce 3 + , Mn 2 + , and Dy 3 + ions

Appl. Phys. Lett. 91, 061119 (2007); 10.1063/1.2768916

Ce 3 + Eu 2 + codoped Ba 2 Zn S 3 : A blue radiation-converting phosphor for white light-emitting diodes Appl. Phys. Lett. 90, 171908 (2007); 10.1063/1.2731685

On Ba Mg Al 10 O 17 : Eu 2 + phosphor degradation mechanism by vacuum-ultraviolet excitation J. Appl. Phys. 98, 113528 (2005); 10.1063/1.2132094

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Wed, 30 Apr 2014 07:08:51

(2)

Sr

₃

B

₂

O

₆

: Ce

3+

, Eu

2+

: A potential single-phased white-emitting borate

phosphor for ultraviolet light-emitting diodes

Chun-Kuei Chang and Teng-Ming Chena兲

Phosphors Research Laboratory, National Chiao Tung University, Hsinchu 30010, Taiwan and Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan

共Received 1 May 2007; accepted 25 July 2007; published online 20 August 2007兲 The Ce3+_{/ Eu}2+ _{coactivated Sr}

3B2O6 phosphors exhibit varied hues from blue through white and

eventually to yellow-orange by resonance-type energy transfer from Ce3+ _{to Eu}2+ _{and tuning the}

relative proportion of Ce3+_{/ Eu}2+_{properly. The authors have demonstrated that electric dipole-dipole}

interaction dominates the energy transfer mechanism in Sr3B2O6: Ce3+, Eu2+ phosphor, and the

critical distance of energy transfer has been estimated to be about 30 Å by both spectral overlap and concentration quenching methods. They have also shown that under the excitation of UV radiation, white light is generated by coupling 434 and 574 nm emission bands attributed to Ce3+ _{and Eu}2+

A primitive white LED lamp consisting of three LED dices of red, green as well as blue is easy to fabricate, but some disadvantages exist:共1兲 high cost, 共2兲 drive voltages are different to each other, and 共3兲 thermal properties and degradation trends are also different, and, thus, restrict its wide application. A good choice to assemble white LEDs in low cost is to couple a blue or near-ultraviolet 共UV兲 LED with a downconverting phosphor. For example, the commer-cial YAG 共yttrium aluminum garnet兲:Ce 共Ref. 1兲 yellow

phosphor excited by a blue GaN LED results in two-band white light emission. In spite of white light that is easily achieved in YAG:Ce based system, the individual degrada-tion rate between the blue LED and yellow phosphor will cause chromatic aberration and poor white light performance after long period of working. Developing a single-phased white-emitting phosphor using the principle of energy trans-fer共ET兲 from a sensitizer 共energy donor兲 to an activator 共en-ergy accepter兲 in a single host lattice for UV LEDs is an excellent option to replace YAG:Ce based white-emitting system. The ions possessing f-d or d-d electron configura-tions are good candidates to be selected as activators in phos-phors because they could emit visible and broadband light under the influence of crystal-field and nephelauxectic effects.2Moreover, white light can be produced by codoping these ions together under effective resonance-type ET in a single host, such as Eu2+_{/ Mn}2+ _{coactivated systems.}3–10

Al-though there were sweeping studies on Eu2+_{/ Mn}2+ _codoped

white-emitting phosphors in the past years, yet, investiga-tions on Ce3+_{/ Eu}2+_{codoping white-emitting phosphors were}

rarely reported. In certain Eu2+_{-doped oxide matrices, the}

yellow-orange emission and UV to blue-light excitation are commonly observed through the strong crystal-field splitting and the lower energy of centroid of 5d level. Hence, Eu2+ can be sensitized by a well-known blue-emitting Ce3+ _in

such oxide lattices because of the spectral overlap between emission band of Ce3+ and excitation band of Eu2+. Conse-quently, white light with blue and yellow-orange irradiations can be produced in this kind of Ce3+_{/ Eu}2+_-codoped

single-composition oxide host.

The crystal structure of Sr3B2O6 was first determined

and reported by Richter and Muller11in 1980; the lumines-cence of Sr3B2O6: Ce3+ and Sr3B2O6: Eu2+ was reported to

exhibit blue and yellow-orange emissions, respectively, at 4.2 K by Schipper et al. in 1993.12 However, the lumines-cence properties of Ce3+_{/ Eu}2+_{-codoped Sr}

3B2O6 have not

been reported in the literature. Therefore, according to the design principles discussed above, we have explored and dis-covered a single-composition Sr3B2O6: Ce3+, Eu2+

共SB-O:Ce,Eu兲 white phosphor and investigated its luminescence properties as well as ET phenomenon between the sensitizer and activator for potential application in white LEDs.

SBO:Ce,Eu phosphors studied in this work were synthe-sized by a solid-state reaction route. The reactants SrO, H₃BO₃, CeO₂, and Eu₂O₃ with high purity of 99.99% were mixed in the requisite proportions and calcined at 900– 1000 ° C under 15% H2/ Ar atmosphere. The detailed

measurements of photoluminescence 共PL兲, photolumines-cence excitation 共PLE兲, Commission International de I’Eclairage共CIE兲 chromaticity, and diffuse reflectance 共DR兲 spectra have been carried out at room temperature and de-tailed in our previous work.6

The PL, PLE, and DR spectra for solely Ce3+ _共or

Eu2+兲-doped and Ce3+/ Eu2+-codoped Sr3B2O6 are

repre-sented in Fig.1. As shown in Fig.1共a兲, under UV of 351 nm excitation, a broad asymmetric blue emission band centering at 434 nm, which is attributed to the 5d1→4f1 transition of Ce3+_{, is observed in the PL spectrum. The asymmetry of the}

emission band of Sr₃B₂O₆: Ce3+ _{results from the spin-orbit}

coupling into two levels of ground state, and it can be de-convoluted into two Gaussians centering at 427 and 470 nm that originated from the lowest 5d level to 2F_5/2and 2F_7/2, respectively. The energy difference between spin-orbit split-ting of 4f ground state in Sr₃B₂O₆: Ce3+ _{is about 2142 cm}−1

which is in good agreement with the theoretical value13 of 2000 cm−1_{. As depicted in Fig.} _1共b兲_{, the PL spectrum of}

Sr₃B₂O₆: Eu2+ _{shows a broad yellow-orange emission band}

centering at 574 nm attributed to the typical 4f6_5d1_→4f7_共8_S

7/2兲 transition of Eu2+, and the PLE spectrum

shows a broad absorption from UV to blue region with a maximum at 377 nm. The low energy of Eu2+_{emission band}

in Sr₃B₂O₆: Eu2+_{is due to the strong crystal-field and}

nephe-a兲_{Author to whom correspondence should be addressed; electronic mail:}

[email protected]

APPLIED PHYSICS LETTERS 91, 081902共2007兲

(3)

lauxetic effects originated from the dodecahedral activator center and the crystal structure of anticorundum,14 respec-tively. A good evidence of absorption from activator ions is obtained through comparison of the DR spectra of host and activators doped Sr3B2O6; on the other hand, high

lumines-cent efficiency is expected in all samples because of the high reflectance in emissive regions, all observations are depicted in Fig.1. Moreover, a conspicuous spectral overlap between the emission band of Ce3+ and the excitation band of Eu2+ was observed clearly in Figs.1共a兲and1共b兲 and the effective resonance-type ET from a sensitizer Ce3+ _{to an activator}

Eu2+ _共ET

Ce→Eu兲 is expectable based on the significant

spec-tral overlapping.

In this work the concentration of Ce3+ is fixed at the optimal doping level of 0.01 with varied Eu2+_codoping

con-tents to investigate the energy transfer phenomenon. Figure 2共a兲 shows the PL spectra for Sr3B2O6: 1 % Ce3+, n % Eu2+ phosphors with different n’s of

0, 0.10, 0.15, 0.20, 0.25, and 0.70, which were measured at excitation wavelength of 351 nm, corresponding to the opti-mal excitation wavelength of the energy donor Ce3+_{. With}

increasing Eu2+ _content _{共n兲, the PL intensity of Eu}2+ _was

observed to increase systematically from n = 0.10 to 0.15, and reach saturation as n equal to or larger than 0.2, whereas that

of Ce3+was simultaneously found to decrease gradually from

n = 0.10 to 0.70. The observed saturation in Eu2+ intensity might be attributed to the results of other nonradiative relax-ation present in the host matrix or Eu2+_{– Eu}2+ _internal

con-centration quenching effect. The above-described observa-tions confirmed that the ETCe→Eu in SBO:Ce,Eu and the

calculated Ce3+_→Eu2+_{energy transfer efficiency}_共␩

T兲 can be

expressed by15

␩T= 1 − IS I_S0,

where IS0and ISare the luminescence intensities of the

sen-sitizer共Ce3+_{兲 with and without activator 共Eu}2+_{兲 present. The}

␩Tof Sr3B2O6: 1 % Ce3+, n % Eu2+ was calculated as a

func-tion of n and represented in the inset of Fig.2共a兲, in which

␩T was found to increase gradually with increasing Eu2+

dopant content. Figure 2共b兲 portrays the detailed PLE/PL spectra for Sr3B2O6: 1 % Ce3+, n % Eu2+, where n is equal to

0.15. As indicated in Figs.1共a兲and1共b兲, the PLE spectra for Sr3B2O6: 1 % Ce3+, nEu2+ obtained by monitoring␭emat 434

and 574 nm, respectively, are different. When the emission wavelength was monitored at 434 nm, we have found the PLE spectrum of Sr3B2O6: 1 % Ce3+, n % Eu2+ to be identical

to that of solely Ce3+_{-doped Sr}

3B2O6; on the other hand,

when the emission wavelength was monitored at 574 nm, the observed PLE spectrum resembles that of Sr3B2O6: Eu2+;

however, two broad emission bands centering at 434 and 574 nm ascribed to the emission of Ce3+ _{and Eu}2+_,

respec-tively, are observed in PL spectrum. Furthermore, the con-version efficiency portrayed in PL spectra of the phosphor under UV excitations of 340, 350, 360, and 370 nm is also shown in the inset of Fig.2共b兲. Actually, the conversion ef-ficienies under the excitations of 360 and 370 nm are about 87% and 50%, respectively, of the optimal excitation wave-length 共i.e., 351 nm兲, indicating that our phosphor is also effective under longer UV excitation.

Based on Dexter’s energy transfer formula of multipolar interaction and Reisfeld’s approximation, the following rela-tion can be obtained:15,16

␩0

␩ ⬀ Ca/3,

where␩0and␩are the luminescence quantum efficienies of

Ce3+ in the absence and presence of Eu2+, respectively; the values of␩0/␩can be approximately calculated by the ratio

of related luminescence intensities共I_S0/ IS兲, C is the content

of Eu2+_{, and a = 6 and 8 correspond to dipole-dipole and}

dipole-quadrupole interactions, respectively. The IS0/ IS-Ca/3

plots are further illustrated in Figs. 3共a兲 and 3共b兲, and the linear relationship were observed when n = 6 and 8. Electric dipole-dipole interaction with larger Columbic effect usually accompanies electric dipole-quadrupole interaction; there-fore, the electric dipole-dipole interaction predominates in the ET mechanism from Ce3+to Eu2+ in SBO:Ce,Eu, which is similar to the results of our previous study.17

For electric dipole-dipole interaction, the critical dis-tance共Rc兲 of ETCe→Eucan be expressed by16

Rc

6

= 0.63⫻ 1028QA

E4

冕

FS共E兲FA共E兲dE,

where QA= 4.8⫻10−16 fd is the absorption cross section of

Eu2+ ions, fd⬇0.02 is the electric dipole oscillator strength

FIG. 1. 共Color online兲 PLE, PL, and DR spectra for 共a兲 Sr3B2O6: Ce3+

phosphor共PLE monitored at 434 nm, PL excited at 351 nm, and DR spectra shown in blue curve兲, 共b兲 Sr3B2O6: Eu2+ phosphor 共PLE monitored at

574 nm, PL excited at 377 nm, and DR spectra shown in blue curve兲, and 共c兲 DR spectra for Sr3B2O6host共blue solid line兲 and Sr3B2O6: Ce, Eu共blue

dashed line兲. Spectral overlaps between PLE of Sr3B2O6: Eu2+and PL of

Sr₃B₂O₆: Ce3+_{are also shown.}

FIG. 2.共Color online兲共a兲 PL spectra for Sr3B2O6: 1 % Ce, n % Eu phosphors

excited at 351 nm. Inset: dependence of the energy transfer efficiency␩Ton Eu2+ _{content n.} _{共b兲 PL and PLE spectra for Sr}

3B2O6: 1 % Ce, n % Eu 共n

= 0.15兲. Inset: the conversion efficiency under UV excitation.

081902-2 C.-K. Chang and T.-M. Chen Appl. Phys. Lett. 91, 081902共2007兲

(4)

for Eu2+ _{ions, and} _兰F

S共E兲FA共E兲dE represents the spectral

overlap between the normalized spectral shapes of Ce3+

emission FS共E兲 and Eu2+excitation FA共E兲, and it is estimated

to be about 1.13 eV−1; E 共in eV兲 is the maximum energy of spectral overlap. Therefore, the Rcof ETCe→Euin SBO:Ce,Eu

was calculated to be about 30.67 Å, which is slightly shorter than that 共i.e., 32.7 Å兲 in Ba₂ZnS₃: Ce, Eu reported by our group.17When Ce3+_{– Eu}2+_{was supposed to form a close pair}

at a distance of 3.59 Å, which is the nearest distance between Sr and Sr sites, and, hence, Rcwas approximated as 8.5 times

of lattice dimension in length.

In many cases, concentration quenching is due to energy transfer from one activator to another until energy sink in the lattice is reached.18Blasse suggested that the critical distance 共Rc兲 of energy transfer can be expressed by16

Rc⬇ 2

冉

3V 4␲xcN

冊

1/3

,

where V is the volume of the unit cell, xc is the critical

concentration, at which the luminescence intensity of sensi-tizer共Ce3+_{兲 is half that in the sample in the absence of}

acti-vator共Eu2+_{兲, namely, x}

coccurs when␩Tequals 0.5, and N is

the number of host cations in the unit cell. For Sr₃B₂O₆host,

V is 889.93 Å,3N is 18,11and xcis 0.002 07 determined from

the inset of Fig.2共a兲. Therefore, the critical distance for en-ergy transfer Rcis estimated to be about 35 Å, which agrees

approximately with that obtained by using the spectral over-lap method.

The CIE chromaticity coordinates with varied hues for SBO:Ce,Eu excited at 351 nm were measured and shown in Fig.4. With increasing Eu2+ _{content, the color tone changes}

from blue, which is represented by point 1 共solely Ce3+

doped兲 through white 共Ce3+_{/ Eu}2+ _{coactivated兲 and finally to}

yellow, which is represented by point 7共solely Eu2+ doped兲, corresponding to chromaticity coordinates 共x,y兲 varying from 共0.20, 0.16兲 to 共0.31, 0.24兲 and ultimately to 共0.53, 0.46兲. Hence, by properly tuning the ratio of Ce3+_{/ Eu}2+_{, the}

diversified white light with different hues can be achieved under UVLEDs radiation. Higher CRI values can be antici-pated in our single-phased white phosphors because of more red-light component contribution from Eu2+ _emission

in-cluded than that of the commercial YAG:Ce based white-emitting system.

In conclusion, we have demonstrated that the energy transfer from Ce3+ _{to Eu}2+ _{in Sr}

3B2O6: Ce3+, Eu2+ is

domi-nated by resonance-type electric dipole-dipole interaction. In addition, the tunable color hues from blue through white and finally to yellow-orange is achieved by properly tuning the relative Ce3+_{/ Eu}2+_{ratio in Sr}

3B2O6: Ce, Eu.

The authors acknowledge the generous financial support from the National Science Council of Taiwan under Contract No. NSC95-2113-M-009-024-MY3.

1_{Y. Shimizu, K. Sakano, Y. Noguchi, and T. Moriguchi, US Patent No.}

5998925共December 7, 1998兲.

2_{G. Blasse and B. C. Grabmaier, Luminescent Materials}_{共Springer, Berlin,}

Germany, 1994兲, p. 46.

3_{J. S. Kim, P. E. Jeon, J. C. Choi, H. L. Park, S. I. Mho, and G. C. Kim,}

Appl. Phys. Lett. 84, 2931共2004兲.

4_{J. S. Kim, P. E. Jeon, Y. H. Park, J. C. Choi, H. L. Park, G. C. Kim, and}

T. W. Kim, Appl. Phys. Lett. 85, 3696共2004兲.

5_{J. S. Kim, K. T. Lim, Y. S. Jeong, P. E. Jeon, J. C. Choi, and H. L. Park,}

Solid State Commun. 135, 21共2005兲.

6_{W. J. Yang, L. Luo, T. M. Chen, and N. S. Wang, Chem. Mater. 17, 3883}

共2005兲.

7_{W. J. Yang and T. M. Chen, Appl. Phys. Lett. 88, 101903}_共2006兲. 8_{S. H. Lee, J. H. Park, S. M. Son, and J. S. Kim, Appl. Phys. Lett. 89,}

221916共2006兲.

9_{J. S. Kim, A. K. Kwon, Y. H. Park, J. C. Choi, H. L. Park, and G. C. Kim,}

J. Lumin. 122-123, 583共2007兲.

10_{C. K. Chang and T. M. Chen, Appl. Phys. Lett. 90, 161901}_共2007兲. 11_{L. Richter and F. Muller, Z. Anorg. Allg. Chem. 467, 123}_共1980兲. 12_{W. J. Schipper, D. van der Voort, P. van den Berg, Z. A. E. P. Vroon, and}

G. Glasse, Mater. Chem. Phys. 33, 311共1993兲.

13_{G. Blasse and B. C. Grabmaier, Luminescent Materials}_{共Springer, Berlin,}

Germany, 1994兲, p. 45.

14_{A. Vegas, Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 41, 1689}

共1985兲.

15_{Y. Tan and C. Shi, J. Phys. Chem. Solids 60, 1805}_共1999兲. 16_{G. Blasse, Philips Res. Rep. 24, 131}_共1969兲.

17_{W. J. Yang and T. M. Chen, Appl. Phys. Lett. 90, 171908}_共2007兲. 18_{D. L. Dexter and J. A. Schulman, J. Chem. Phys. 22, 1063}_共1954兲.

FIG. 3.共Color online兲 Dependence of I_S0/ ISof Ce3+on共a兲 C6/3and共b兲 C8/3.

FIG. 4. 共Color online兲 CIE chromaticity diagram for Sr3B2O6: m % Ce, n % Eu excited at 351 nm. 共1兲 m=1, n=0; 共2兲 m=1,

n = 0.1;共3兲 m=1, n=0.15; 共4兲 m=1, n=0.20; 共5兲 m=1, n=0.25; 共6兲 m=1, n = 0.7; and共7兲 m=0, n=1. The corresponding CIE chromaticity coordinates

共x,y兲 are 共1兲共0.20, 0.16兲, 共2兲共0.25, 0.19兲, 共3兲共0.31, 0.24兲, 共4兲共0.43, 0.36兲, 共5兲共0.44, 0.37兲, 共6兲共0.49, 0.44兲, and 共7兲共0.53, 0.46兲. The Ce:YAG and GaN based white-emitting systems are compared with this work and portrayed in red line.

081902-3 C.-K. Chang and T.-M. Chen Appl. Phys. Lett. 91, 081902共2007兲