Ce3+/Eu2+ codoped Ba2ZnS3: A blue radiation-converting phosphor for white light-emitting diodes

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Ce 3 + Eu 2 + codoped Ba 2 Zn S 3 : A blue radiation-converting phosphor for white

light-emitting diodes

Woan-Jen Yang and Teng-Ming Chen

Citation: Applied Physics Letters 90, 171908 (2007); doi: 10.1063/1.2731685

View online: http://dx.doi.org/10.1063/1.2731685

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/17?ver=pdfcov Published by the AIP Publishing

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Ce

3+

/ Eu

2+

codoped Ba

₂

ZnS

₃

: A blue radiation-converting phosphor

for white light-emitting diodes

Woan-Jen Yang and Teng-Ming Chena兲

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, Republic of China

共Received 1 February 2007; accepted 27 March 2007; published online 24 April 2007兲 The Ce3+_{/ Eu}2+_{codoped Ba}

2ZnS3phosphor shows intense blue absorption and tunable green-to-red

emission. The energy transfer from Ce3+ _{to Eu}2+ _{in this phosphor has been demonstrated to be a}

resonant type via an electric dipole-dipole mechanism. The Ba2ZnS3: Ce3+, Eu2+phosphor would be

White light-emitting diodes 共LEDs兲 could be produced by blue chip-pumped yellow Y₃Al₅O₁₂: Ce3+ _{共YAG:Ce兲,}1

whereas color rendering index共CRI兲 of white light made by the complementary blue and yellow emission is deficient due to the lack of red light contribution. Hence, several red phos-phors were developed to add into the above-mentioned sys-tem in order to improve CRI.2–6 Unfortunately, the extreme difference in degradation between different host phosphors will produce color aberration. Accordingly, it is important to investigate a single-host phosphor with green-to-red emis-sion bands for blue LEDs. A phosphor could emit a couple of radiation by codoping activators with f-d or d-d electron configurations,7such as Eu2+_{/ Mn}2+_,8–11

Ce3+_{/ Mn}2+_,12,13

and Ce3+/ Eu2+;13–16 the energy transfer 共ET兲 would occur be-tween activator/coactivator couples by effective resonant type via a multipolar interaction and the ET to Mn2+ _{can be}

of exchange interaction.8–16 Nevertheless, in the past few years, coactivated single-host phosphors with blue absorp-tion or for blue LEDs were rarely investigated. In this work, we have explored and discovered a single-host phosphor, Ce3+_{/ Eu}2+ _{codoped Ba}

2ZnS3, which shows

ultraviolet-to-blue absorption and green-to-red emission, exhibiting great potential application in white LEDs while a blue chip is coupled.

Ba2ZnS3was prepared and reported in 1959 by Hoppe;17

until 1961, its crystal structure was determined by Schnerung et al.;18the luminescence of Ba₂ZnS₃: Ce3+_{was reported by}

Lin et al.19 Nonetheless, the luminescence properties of Ce3+_{/ Eu}2+_{codoped Ba}

2ZnS3were not reported in literatures.

Therefore, we report herein the investigation of the lumines-cence properties and ET phenomenon between activators and demonstrate that white light emission can be achieved in Ba2ZnS3: Ce3+, Eu2+ 共BZS:Ce,Eu兲.

Ba2ZnS3has two crystallographically independent cation

sites in a unit cell: a seven-coordinated Ba2+_{site and a}

four-coordinated Zn2+site.18We could propose that Ce3+and Eu2+ are both expected to occupy the Ba2+ _{sites preferably}

be-cause the ionic radii of Ce3+_{共1.07 Å兲 and Eu}2+_{共1.20 Å兲 are}

close to that of Ba2+ _{共1.38 Å兲.}20

However, the Zn2+ _sites

共0.60 Å兲 are too small for Ce3+ _{and Eu}2+_{to occupy.}20

The absorption spectrum of Ba₂ZnS₃ host reveals a di-rect band gap of about 3.3 eV, as shown in Fig.1; the

pho-toluminescence 共PL兲 spectrum of that shows an emission band centered at 623 nm. As presented in Fig.2, the photo-luminescence excitation 共PLE兲 spectrum of Ba₂ZnS₃: Ce3+

shows two excitation bands centered at 364 and 420 nm, assigned to host-lattice absorption and 4f1→5d1transition of Ce3+_{, respectively. In addition, as expected the PLE intensity}

ratio共I364 nm/ I420 nm兲 of that is found to decrease as the Ce3+

concentration increases, indicating that Ce3+ _ions

undoubt-edly substitute for Ba2+ sites. The PL spectrum of Ba2ZnS3: Ce3+ shows an asymmetric band emission

de-convoluted into two peaks centered at 489 and 540 nm, at-tributed to the transitions from 5d1_to2_F

5/2and 2_F

7/2,

respec-tively. The spin-orbit splitting of ground state共2FJ兲 for Ce3+ was estimated at about 1900 cm−1_{, approximating to that}

re-ported in Ref.7. As depicted in Fig.3, the PLE spectrum of Ba₂ZnS₃: Eu2+ _{shows a host-lattice absorption centered at}

357 nm and an unresolved band from 375 to 570 nm as-signed to the 4f65d1multiplets of Eu2+excited states; the PL spectrum of that displays a broad band deconvoluted into two peaks centered at 623 and 671 nm, attributed to host-lattice emission and 4f65d1→4f7共8S_7/2兲 transition of Eu2+, respectively. Moreover, the host-lattice emission still retains under 420 nm excitation because the conduction band edge of host lattice is close to the lowest energy level of Eu2+

excited state; at higher Eu2+ _{concentrations, the emission}

band does not show obvious change because the emission intensity of host lattice and Eu2+ decreases simultaneously under the influence of substitution of Eu2+_{for Ba}2+_and

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

[email protected]

FIG. 1. Absorption spectrum of Ba2ZnS3 host; the inset for PLE and PL

spectra of Ba2ZnS3 host 共PLE monitored at 623 nm and PL excited at

352 nm兲.

APPLIED PHYSICS LETTERS 90, 171908共2007兲

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centration quenching, respectively. In fact, the red emission of Eu2+ is the effect of the larger crystal-field splitting and the lower energy of the center of gravity of 5d level. The Stokes shift of Eu2+ _{was roughly calculated to be about}

7800 cm−1_{, and the large shift might be the result of the}

lower lattice stiffness.7Besides, a significant spectral overlap was observed between the emission band of Ce3+ and the excitation band of Eu2+exhibited in Fig.4共a兲, with the result that the effective resonance-type ET from Ce3+ _{to Eu}2+ _is

expected. Consequently, Ce3+ _{and Eu}2+ _{are regarded as}

en-ergy donor and enen-ergy acceptor, respectively.

The ET evidence from Ce3+to Eu2+excited at 420 nm is shown in Fig.4共b兲; the red-emission intensity of Ce3+_{/ Eu}2+

codoped Ba2ZnS3 is higher than that of Eu2+ doped one.

Furthermore, with increasing Eu2+ _{concentration, the PL}

in-tensity of Ce3+as well as the total intensity of BZS:Ce,Eu are found to decrease gradually as displayed in Fig.5. The ET efficiency共␩T兲 from Ce3+ to Eu2+ can be expressed by12

␩T= 1 − IS

I_S0, 共1兲

where IS0and ISare the luminescence intensity of Ce3+in the absence and presence of Eu2+_{, respectively. As Eu}2+ _content

increases, the ␩T is found to increase and finally saturate. Based on Dexter’s ET formula of multipolar interaction and Reisfeld’s approximation, the following relation can be obtained:21

IS0 IS

⬀ C␣/3_, _共2兲

where C is the content of Eu2+_and_␣_{= 6 and 8 corresponding}

to electric dipole-dipole and dipole-quadrupole interactions, respectively. The plots represented in Figs. 6共a兲 and 6共b兲 both exhibit linear relationships; moreover, electric dipole interaction usually accompanies electric dipole-quadrupole interaction because the Coulombic effect of the former is larger than that of the latter. Therefore, the electric dipole-dipole interaction is dominant in the ET mechanism from Ce3+ _{to Eu}2+ _{in BZS:Ce,Eu, which is similar to that}

observed in several references.13–16

For electric dipole-dipole mechanism, the critical dis-tance共Rc兲 of ET from Ce3+ to Eu2+ can be expressed by22

Rc6= 0.63⫻ 1028 QA

E4

冕

FS共E兲FA共E兲dE, 共3兲 where QA= 4.8⫻10−16fd is the absorption cross section of Eu2+_{, f}

d⬇0.02 is the electric dipole oscillator strength for Eu2+_, _兰F

S共E兲FA共E兲dE represents the spectral overlap be-tween the normalized shapes of Ce3+ _{emission F}

S共E兲 and Eu2+ _{excitation F}

A共E兲, estimated at about 0.96 eV−1, and E 共in eV兲 is the maximum energy of spectral overlap. There-fore, the Rcof ET was calculated to be about 32.7 Å, which is longer than that共25 Å兲 reported in BaLiF3: Ce, Eu because

the spectral overlap共0.96 eV−1_{兲 in BZS:Ce,Eu is larger than}

FIG. 2. PLE and PL spectra of Ba2ZnS3: 0.1% Ce3+ 共PLE monitored at

498 nm and PL excited at 420 nm兲; the inset for dependence of intensity ratio I364 nm/ I420 nmin Ba2ZnS3: m % Ce3+on m.

FIG. 3. PLE and PL spectra of Ba₂ZnS₃: 0.8% Eu2+ _{共PLE monitored at}

655 nm and PL excited at 357 nm兲; the inset for PL spectra of Ba2ZnS3: n % Eu2+excited at 420 nm.

FIG. 4. 共a兲 Spectral overlap between PLE spectrum of Ba2ZnS3: Eu2+

共dashed line兲 and the PL spectrum of Ba2ZnS3: Ce3+共solid line兲; 共b兲 PL

spectra for Ba2ZnS3: 0.8% Eu2+and Ba2ZnS3: 0.1% Ce3+, 0.8% Eu2+excited

at 420 nm.

FIG. 5. PL spectra for Ba₂ZnS₃: 0.1% Ce3+_{, n % Eu}2+_{excited at 420 nm; the}

inset showing dependence of ␩T and total intensity in

Ba2ZnS3: 0.1% Ce3+, n % Eu2+on n.

171908-2 W.-J. Yang and T.-M. Chen Appl. Phys. Lett. 90, 171908共2007兲

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that共0.77 eV−1_{兲 in BaLiF}

3: Ce, Eu.14 When Ce3+– Eu2+ was

supposed to form a close pair at a distance of 4.210 Å, the shortest distribution length between Ce3+_{and Eu}2+_{, the R}

cof ET was approximated about eight lattice sites.

Because a commercial blue chip providing 420 nm was unavailable, the samples for blue LED application were simulated by measuring with a Xe light source with the same excitation wavelength. The Commission International de I’Eclairage 共CIE兲 chromaticity coordinates for BZS:Ce,Eu excited at 420 nm were also measured, and the results are shown in Fig.7. The CIE chromaticity coordinates of Ce3+

-and Eu2+_{-activated Ba}

2ZnS3are共0.34, 0.49兲 and 共0.64, 0.33兲,

respectively, corresponding to hues of green-yellow and red. Furthermore, with increasing Eu2+ content, we have ob-served that the hues of BZS:Ce,Eu locate in the yellow to

orange range. As a consequence, the tunable emission of BZS:Ce,Eu coupled with blue LEDs could generate various white lights, which are more numerous than single-emitting YAG:Ce coupled with ones. In addition, the CRI of white light generated by BZS:Ce,Eu would be higher than that pro-duced by YAG:Ce because the former contains the red light component. Indeed, BZS:Ce,Eu has the promising applica-tion for white LEDs.

In conclusion, the unprecedented Ba₂ZnS₃: Ce3+_{, Eu}2+

shows intense blue absorption and two emission bands: the one at 498 nm is attributed to Ce3+, and the other at 655 nm is assigned to host lattice and Eu2+_{. We have demonstrated}

that the energy transfer from Ce3+ _{to Eu}2+ _{is of a resonant}

type via an electric dipole-dipole mechanism. All in all, Ba2ZnS3: Ce3+, Eu2+has been proven to be potentially useful

as a blue radiation-converting phosphor for white light-emitting diodes.

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

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FIG. 6. Dependence of IS0/ ISof Ce3+on共a兲 C6/3and共b兲 C8/3.

FIG. 7. CIE chromaticity diagram for Ba2ZnS3: m % Ce3+, n % Eu2+exited at

420 nm.共1兲 m=0.1, n=0; 共2兲 m=0.1, n=0.2; 共3兲 m=0.1, n=0.4; 共4兲 m = 0.1, n = 0.6; and共5兲 m=0, n=0.8.

171908-3 W.-J. Yang and T.-M. Chen Appl. Phys. Lett. 90, 171908共2007兲