White light generation under violet-blue excitation from tunable green-to-red emitting Ca2MgSi2O7 : Eu,Mn through energy transfer

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White light generation under violet-blue excitation from tunable green-to-red emitting

Ca 2 Mg Si 2 O 7 : Eu , Mn through energy transfer

Chun-Kuei Chang and Teng-Ming Chen

Citation: Applied Physics Letters 90, 161901 (2007); doi: 10.1063/1.2722670

View online: http://dx.doi.org/10.1063/1.2722670

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/16?ver=pdfcov Published by the AIP Publishing

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White light generation under violet-blue excitation from tunable

green-to-red emitting Ca

₂

MgSi

₂

O

₇

: Eu, Mn through energy transfer

Chun-Kuei Chang and Teng-Ming Chena兲

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

共Received 19 December 2006; accepted 14 March 2007; published online 16 April 2007兲

Yellowish green-to-orange red emission can be generated by energy transfer from Eu2+to Mn2+in

the Ca2MgSi2O7 host matrix. Eu2+-doped Ca2MgSi2O7 shows a broad green emission band

centering at 528 nm and Mn2+-doped Ca2MgSi2O7exhibits a red emission at around 602 nm. The

authors have demonstrated that the mechanism of energy transfer from Eu2+ to Mn2+ in

Ca2MgSi2O7: Eu, Mn phosphor is a resonant type via a dipole-quadrupole mechanism. They have

also shown that the white light with varied hue, depending on contents of Mn2+, is generated by

combination of predesigned emission wavelength-tunable Ca2MgSi2O7: Eu, Mn phosphors and

White light can be produced by many approaches; one of the most common and easiest ways is the combination of the yellow-emitting phosphor made of cerium doped yttrium

alu-minum garnet共YAG:Ce3+_{兲共Ref.}₁_{兲 and blue-emitting GaN}

chips. Although this kind of white-light blending method has been well used for a few years, some problems have existed; one of them is that the variety of hue is insufficient compared with the white light mixed from three primary colors. To circumvent this disadvantage, the red, green, and blue mul-tiphased phosphors or single-phased dichromatic phosphor consisting of two emission bands in the green and red spec-tral regions are employed to generate white light under

ultra-violet共UV兲 or blue light-emitting diode 共LED兲 chip

excita-tion, respectively. However, in the three-converter system, the blue emission efficiency is poor because of the strong reabsorption of the blue light by the red or green-emitting phosphors. Therefore, many efforts have been made to de-velop the single-phased white-emitting phosphors which are

based on the mechanism of energy transfer from Eu2+ _to

Mn2+_{共abbreviated as ET}

Eu→Mnhereafter兲 that has been

stud-ied actively in the past few years.2–6 In this letter, we have demonstrated that the white light could be generated with combination of tunable yellow green-to-orange red emitting

phosphor Ca2MgSi2O7: Eu2+, Mn2+ 共CMSO:Eu,Mn兲 and

violet-blue 共380–420 nm兲 radiation source. We have also

proven that a warm white light can be achieved by increasing the contents of Mn2+.

The silicate compound of Ca2MgSi2O7 with akermanite

structure was reported to crystallize in the tetragonal crystal

system with the space group P-421m and the lattice

param-eters a = 7.8338共6兲Å and c=5.0082共5兲Å.7

Three crystallo-graphically independent cation sites, namely, one Ca2+_{, one}

Mg2+, and one Si4+exist in the crystal lattice. The Ca2+ion is coordinated by eight oxygen atoms with an average Ca–O

distance of 2.573 Å, and both Mg2+ _{and Si}4+ _{cations were}

reported to occupy in the tetrahedral sites with an average

M – O distance of 1.916 Å 共M =Mg兲 and 1.624 Å 共M =Si兲,

respectively.7 In view of the effective ionic radii of cations with different coordination numbers,8we have predicted that

Eu2+ _{and Mn}2+ _{prefer to occupy the Ca}2+ _{and Mg}2+ _sites,

respectively, because the ionic radii of Eu2+ 共1.25 Å兲 and

Mn2+ 共0.66 Å兲 are compatible with those of Ca2+ 共1.12 Å兲

and Mg2+ 共0.57 Å兲. Nevertheless, the Si4+ site 共0.26 Å兲 is too small for the substitution of Eu2+_{and Mn}2+_{to take place.}

High temperature solid-state reactions were employed to

synthesize CMSO:Eu,Mn. Powders of high-purity 共all in

99.99%, Aldrich Chemical Co., WI, USA兲 CaCO3, MgO,

SiO2, MnCO3, and Eu2O3 were weighed in stoichiometric

proportions, thoroughly ground and mixed in an agate mor-tar, pressed into pellets, and calcined at 1300 ° C for 8 h in an inner alumina crucible that was contained in a covered outer alumina crucible filled with graphite powder. After-wards, the product was reduced further at 900 ° C for 3 h

under an atmosphere of 5% H2/ 95% Ar. The phase purity of

all the samples was characterized and evaluated by powder

x-ray diffraction共XRD兲 analysis by using a Bruker AXS D8

advanced automatic diffractometer with Cu K_␣ radiation

operated at 40 kV and 20 mA. Figure 1 shows the

XRD profile for the optimized composition

共Ca0.996Eu0.004兲2共Mg0.65Mn0.35兲Si2O7, which agrees very well

with that reported in Ref.9, indicating that both of the host

a兲_{Author to whom correspondence should be addressed; FAX: 886}_⫹

35723764; electronic mail: [email protected]

FIG. 1. 共Color online兲 XRD profile of 共Ca0.996Eu0.004兲2共Mg0.65Mn0.35兲Si2O7

showing good agreement with that reported in Ref.9. APPLIED PHYSICS LETTERS 90, 161901共2007兲

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structure and single-phased composition are retained even at high codoping level of Eu2+ _{and Mn}2+_.

The measurements of photoluminescence共PL兲 and

pho-toluminescence excitation共PLE兲 spectra for CMSO:Eu,Mn

phosphors were performed by using a Spex Fluorolog-3

spectrofluorometer 共Instruments S.A., NJ, USA兲 equipped

with a 450 W Xe light source and double excitation mono-chromators, and the details have been described in our pre-vious work.5Figure2共a兲depicts the PLE and PL spectra of Eu2+- and Mn2+-doped Ca2MgSi2O7, respectively. In solely

Eu2+-activated system, the PL spectrum shows a broad green emission band centering at 528 nm attributed to the typical 4f6_5d1_共t

2g兲→4f7共8S7/2兲 transition of Eu2+, and the

absorp-tions observed in the PLE spectrum situate between the UV and blue region consisting of unresolved bands due to the

4f5d multiplets of the Eu2+ _{excited state. However, in the}

solely Mn2+_{-doped system, the PL spectrum exhibits a broad}

red emission around at 602 nm which is ascribed to the spin-forbidden4T₁共4G兲→6A₁共6S兲 transition of Mn2+and the PLE spectrum contains several bands centering at 325, 350, 408,

425, and 500 nm, corresponding to the transitions of Mn2+

from6A₁共6S兲 to4E共4D兲,4T₂共4D兲,关4A₁共4G兲,4E共4G兲兴,4T₂共4G兲,

and4T₁共4G兲 energy levels, respectively.

As also shown in Fig.2共a兲, based on the observed

sig-nificant overlap between the excitation spectrum of Mn2+_and

emission spectrum of Eu2+_{, the effective resonance-type}

en-ergy transfer is expected to take place from Eu2+ _{to Mn}2+_.

The PL spectra obtained by excitation at 381 nm for 共Ca0.996Eu0.004兲2共Mg1−nMnn兲Si2O7 with varied Mn2+ dopant

contents are presented in Fig.2共b兲 and it can be visualized clearly that the intensity of Eu2+ _{sensitizer was found to}

de-crease remarkably and simultaneously as the contents of Mn2+ _{increased gradually. The observed energy transfer}

effi-ciency共␩T兲 from Eu2+ to Mn2+ has been discussed by Pau-lose et al.10and can be expressed by

␩T= 1 − IS

IS0

, 共1兲

where I_S0and ISare the luminescence intensity of the

sensi-tizer共Eu2+兲 with and without activator 共Mn2+兲 present. The

␩T from Eu2+ to Mn2+ in共Ca0.996Eu0.004兲2共Mg1−nMnn兲Si2O7

was calculated as a function of n and is represented in the inset of Fig.2共b兲. With increasing Mn2+ _{dopant content, the} ␩Twas found out to increase and reach the saturation when n is above 0.2.

Based on Dexter’s energy transfer formula of multipolar interaction and Reisfeld’s approximation, the following rela-tion can be obtained:11

␩0

␩ ⬀ Cn/3, 共2兲

where␩0and␩ are the luminescence quantum efficiency of

Eu2+ _{in the absence and presence of Mn}2+_{, respectively; the}

values of␩0/␩can be approximately calculated by the ratio of related luminescence intensities共IS0/ IS兲; C is the content

of Mn2+; and n = 6, 8, and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The IS0/ IS-Cn/3 plots are further illustrated in

Figs.3共a兲and3共b兲, and the linear relation was observed only

when n = 8, implying that ET_Eu→Mnis the dipole-quadrupole

mechanism, which is similar to those previously investigated and observed in our group.5,6

The critical distance 共Rc兲 of ETEu→Mn for the

dipole-quadrupole mechanism can be calculated by using the

fol-lowing simplified equation for dipole-quadrupole

mechanism:5 Rc 8 = 0.63⫻ 1028fq␭S 2 QA fdES 4

冕

FS共E兲FA共E兲dE, 共3兲

where QA is the absorption coefficient of Mn2+that is equal

to 4.8⫻10−16 _f

d given by Blasse;12 fd= 10−7 and fq= 10−10

are the oscillator strengths of the activator共Mn2+_{兲 dipole and}

quadrupole electric transitions, respectively;␭S共in Å兲 and E

共in eV兲 are the emission wavelength and emission energy of Eu2+; and兰FS共E兲FA共E兲dE expresses the spectral overlap

be-tween the normalized shapes of Eu2+ emission FS共E兲 and

Mn2+ excitation FA共E兲, and it is estimated at about

1.79 eV−1. Therefore, the Rcfor ETEu→Mnin CMSO:Eu,Mn

was reckoned to be 11.97 Å, which is longer than 11.4 and

10.8 Å reported for SrZn2共PO4兲2: Eu, Mn 共Ref. 6兲 and

CaSi₂Al₂O₈: Eu, Mn,5respectively.

The diffuse reflectance spectra of selected samples

doped with different amounts of Mn2+ and the pristine

FIG. 2. 共Color online兲共a兲 PLE 共blue line兲 and PL 共red line兲 spectra for 共Ca0.996Eu0.004兲2MgSi2O7 共PLE monitored at 528 nm and PL excited at

381 nm兲 and Ca2共Mg0.65Mn0.35兲Si2O7 共PLE monitored at 602 nm and PL

excited at 408 nm兲. 共b兲 PL spectra for 共Ca_0.996Eu_0.004兲₂共Mg_1−nMnn兲Si2O7

phosphors共excited at 381 nm兲 and dependence of the energy transfer effi-ciency␩Ton Mn2+content n.

FIG. 3. Dependence of IS0/ ISof Eu2+on共a兲 C6/3and共b兲 C8/3.

161901-2 C. Chang and T. Chen Appl. Phys. Lett. 90, 161901共2007兲

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Ca2MgSi2O7 are depicted in Fig. 4. The Ca2MgSi2O7 host

shows an absorption edge only in the UV spectral region. However, the strong absorption in the near-UV to visible

spectral region共370–450 nm兲 was clearly observed not only

for Ca₂MgSi₂O₇: Eu2+ _{but also for Eu}2+_{- and Mn}2+_-codoped

systems, and the results were found to be consistent with the PLE spectra shown in Fig.2共a兲.

Figure 5 portrays the Commission International de

I’Eclairage 共CIE兲 chromaticity coordinates for

CM-SO:Eu,Mn phosphors with various dopant concentrations.

The chromaticity coordinates 共x,y兲 of

共Ca1−mEum兲2共Mg1−nMnn兲Si2O7vary from共0.36, 0.55兲共point G兲 through 共0.45, 0.44兲 and finally to 共0.57, 0.36兲共point R兲

corresponding to solely Eu2+_-doped, _Eu2+_- _and

Mn2+_{-coactivated, and solely Mn}2+_{-doped systems,}

respec-tively. As shown in Fig.5, points on the line GR represents the change of hue from yellow-green to orange-red and this

variation is also summarized in Table I. We have observed

that white light can be generated by exciting

共Ca1−mEum兲2共Mg1−nMnn兲Si2O7 phosphors under 381 nm

ra-diation source with chromaticity coordinates at共0.18, 0.00兲 共point B兲. Comparing the white light produced by our emission-tunable phosphors and violet-blue light source with that from commercial YAG:Ce and blue LED chip, our

sys-tem shows exceptionally flexible hues with increasing Mn2+

content than those of the commodity because the approach-ing RGB primary area can be effectively established in our system. The above observations hint enormously promising application of CMSO:Eu,Mn as a single-phased white-emitting phosphor under violet-blue irradiation.

In conclusion, the white light with varied hues and color

temperature has been obtained by combination of

wavelength-tunable yellowish green-to-orange red-emitting

Ca₂MgSi₂O₇: Eu, Mn phosphors and the violet-blue

共381–420 nm兲 radiation source. The energy transfer from Eu2+ to Mn2+ in Ca2MgSi2O7: Eu, Mn phosphors has been

demonstrated to be a resonant type via a dipole-quadrupole mechanism. Therefore, the Ca2MgSi2O7: Eu, Mn phosphors

can play an important role in a potential single-phased white-emitting phosphor for violet-blue white-emitting LEDs by utilizing the principle of energy transfer and properly designed acti-vator contents.

The authors acknowledge the generous financial support from the National Science Council of Taiwan under Contract No. NSC95-2113-M-009-024-MY3 and helpful suggestions from Woan-Jen Yang of PRL, NCTU are also acknowledged.

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FIG. 4.共Color online兲 Diffuse reflectance spectra for undoped Ca₂MgSi₂O₇, Ca2MgSi2O7: mEu2+, and Ca2MgSi2O7: mEu2+nMn2+

FIG. 5. 共Color online兲 CIE chromaticity diagram for 共Ca1−mEum兲2共Mg1−nMnn兲Si2O7phosphors with different Mn2+dopant

con-tents represented as line GR共m=0.004 excited at 381 nm; m=0 excited at 408 nm兲 and simulated white light generated with YAG:Ce 关excited at 467 nm with chromaticity coordinates at共0.15, 0.04兲兴.

TABLE I. Variation of CIE chromaticity coordinates from yellowish-green

共n=0兲 to orange-red 共m=0兲 as a function of n for

共Ca1−mEum兲2共Mg1−nMnn兲Si2O7.

␭ex

共nm兲共Ca1−mEum兲2共Mg1−nMnn兲Si2O7 共x, y兲

381 m = 0.004 n = 0 共0.36, 0.55兲 381 m = 0.004 n = 0.10 共0.37, 0.55兲 381 m = 0.004 n = 0.15 共0.38, 0.53兲 381 m = 0.004 n = 0.20 共0.40, 0.49兲 381 m = 0.004 n = 0.25 共0.42, 0.47兲 381 m = 0.004 n = 0.30 共0.45, 0.46兲 381 m = 0.004 n = 0.35 共0.45, 0.44兲 408 m = 0 n = 0.25 共0.57, 0.36兲 467

Simulated white light with

YAG:Ce 共0.31, 0.27兲

161901-3 C. Chang and T. Chen Appl. Phys. Lett. 90, 161901共2007兲