ARTICLE IN PRESS

Journal of Luminescence 129 (2009) 1654–1657

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Novel Ba–Sc–Si-oxide and oxynitride phosphors for white LED

Tomoyuki Nakano a,�, Yoshitaka Kawakami a, Kazuyoshi Uematsu b, Tadashi Ishigaki c, Kenji Toda a,Mineo Sato b

a Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japanb Department of Chemistry and Chemical Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japanc Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan

a r t i c l e i n f o

Available online 3 May 2009

Keyword:

Oxide

Oxynitride

Phosphor

LED

13/$ - see front matter & 2009 Elsevier B.V. A

016/j.jlumin.2009.04.028

esponding author.

ail address: f08b014d@mail.cc.niigata-u.ac.jp

a b s t r a c t

Alkaline earth silicates, which comprise a host material doped with rare-earth minerals, show excellent

luminescence properties with various crystal structures and high stability. From results of this study, we

report luminescence properties of Ba9Sc2Si6O24:Eu2+ and Ba9Sc2+dSi6O24�3dN3d:Eu2+ as a novel alkaline

earth silicate and silicon oxynitride phosphors for white LEDs. Using a conventional solid-state reaction,

Ba9Sc2Si6O24:Eu2+ samples were synthesized and Ba9Sc2+dSi6O24�3dN3d:Eu2+ samples were obtained by

nitrization of Ba9Sc2+dSi6O24:Eu2+. The samples can be excited by blue light, exhibiting green

(Ba9Sc2Si6O24:Eu2+) and yellow (Ba9Sc2+dSi6O24�3dN3d:Eu2+) efficiently, which are emissions for use in

white LEDs essentially.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

White LEDs are widely anticipated for use in new energy-saving lighting systems to solve environmental problems in thenear future. The blue-emitting InGaN-based white LED is themajor lighting component. The most dominant white LEDs use ablue-emitting LED that excites a yellow-emitting Y3Al5O12:Ce3+

(YAG:Ce3+) phosphor dispersed in epoxy resin on a blue LED chip[1]. The white light is produced by mixing the blue and yellowlight. This method is presently the most efficient among availabletechniques. However, the color is not true white because oflacking red and green color component. Therefore, for use in awhite LED, it is necessary to identify a novel phosphor that cancover for lacking color and excited by blue light.

Promising materials for use in a white LED phosphor aresilicates and alkaline earth silicon oxynitride doped with Eu2+ [2].These hosts of phosphor with various crystal structure and highstability have excellent luminescence properties for use in whiteLEDs. In particular, Eu2+-doped alkaline earth silicon oxynitridespresent the advantage of being excited by blue light because ofstrong covalency of its host materials [3].

In this study, we report the luminescence properties of novelalkaline earth silicon oxide Ba9Sc2Si6O24:Eu2+ and alkaline earthsilicon oxynitride Ba9Sc2+dSi6O24�3dN3d:Eu2+ phosphors for use inwhite LEDs.

ll rights reserved.

(T. Nakano).

The Ba9Sc2Si6O24:Eu2+ phosphors have broad absorption ofblue light (ca. 450 nm) by the allowed 4f–5d transition. Thesephosphors emit at about 460 and 580 nm, depending on thecoordination number and symmetry of emission ion sites in thehost lattice. The 4f65d energy level of the emission ion Eu2+ has awide distribution of the electron orbital. The excited state isaffected by strong crystal fields and distorted coordination.Consequently, the energy gap separating the ground state 8S andthe bottom of 4f65d level of Eu2+ component becomes smallerthan that of other oxide host materials because the 4f65d energylevel is split by the increased crystal field strength. Therefore, theluminescence wavelength shifts to the longer wavelength side[4–6].

The Ba9Sc2+dSi6O24�3dN3d:Eu2+ phosphors are excited by theblue light. The Ba9Sc2+dSi6O24�3dN3d:Eu2+ phosphors show yellowemissions of 530–650 nm after the nitrization samples shift to thelonger wavelength side because of the increased crystal fieldsplitting and covalency [7].

2. Experimental

Ba9Sc2Si6O24:Eu2+ and Ba9Sc2+dSi6O24�3dN3d:Eu2+ were synthe-sized using a solid-state reaction method. As raw materials, BaCO3

(Kanto Chemical Co., Inc. 3N), Sc2O3 (Shin-Etsu Chemical, Co., Inc.4N), SiO2 (Kanto Chemical Co., Inc. 3N), and Eu2O3 (Shin-EtsuChemical Co., Inc., 4N) were used.

In oxide Ba9Sc2Si6O24:Eu2+, a stoichiometric mixture was firedin an alumina crucible at 1573–1773 K for 12 h in air. The samplesafter firing were ground and fired in an alumina boat at

ARTICLE IN PRESS

Ba(1)

O(3.21)

O(2.87)

O(2.87)

O(2.87) O(2.87)

O(3.21)

O(3.21)

O(3.21)

T. Nakano et al. / Journal of Luminescence 129 (2009) 1654–1657 1655

1573–1773 K for 12 h under a weak reductive atmosphere of 5%H2–95% Ar gas.

The oxynitride Ba9Sc2+dSi6O24�3dN3d:Eu2+ was synthesized bynitrization of oxide Ba9Sc2+dSi6O24:Eu2+. The raw materialsweighed to be the nominal composition of (Ba1�xEux)9Sc2+dSi6O24

and mixed. The mixture was fired in an alumina crucible at1573–1773 K for 12 h in air. After the firing, the sample was groundand fired in an alumina boat at 1573–1773 K for 12 h in NH3 flow(50 ml/min).

Ba(1)12-coordination

O(3.21)O(3.21)

O(2.87)O(2.87)

O(2.76)O(2.76)

O(2.76)

O(2.81)O(2.81)

O(2.81)

Ba(2)O(2.96)

O(2.66)

O(2.58)

O(2.91)O(2.71)

O(2.96)

O(3.00)

O(3.09)

Ba(3)

3. Results and discussion

3.1. Ba9Sc2Si6O24:Eu2+

The crystal structure and coordination of Ba9Sc2Si6O24 arepresented in Fig. 1(a) and (b) and Fig. 2 [8]. The structure closelyresembles that of Ba-merwinite in the linkage of [SiO4] tetrahedraland [ScO6] octahedral. Three different Ba sites exist in thisstructure. The coordinations of the Ba2+ are 9-, 10- and 12-fold andthe bond length is irregular. The XRD patterns, excitation andemission spectra of (Ba1�xEux)9Sc2Si6O24 are shown, respectively,in Figs. 3 and 4. Samples were obtained in single phase with Eu

Ba (2)

Ba (3)

Ba (1)

b

a

ScO6

SiO4c

ab

c

Vacancy

ScO6

SiO4

Ba

Fig. 1. Schematic crystal structure model of Ba9Sc2Si6O24 viewed along the c-axis

(a) and b-axis (b).

Ba(3)10-coordination

Ba(2)9-coordination

O(3.08)O(3.08)

O(3.08) O(2.96)

O(3.14)

Fig. 2. Coordinate environment of Ba sites. The center of sites is Ba ions and

coordinate elements is O ions. The figure in parentheses ( � ) is the bond distance of

Ba–O bonds.

2� / deg.

Inte

nsity

/ ar

b.un

it

Eu-5%

Eu-3%

Eu-1%

Ba9Sc2Si6O24-simulation

10 20 30 40 50 60 70

Fig. 3. XRD patterns of (Ba,Eu)9Sc2Si6O24. The simulation sample is Ba9Sc2Si6O24 in

reference [8]. Eu ¼ 1%, 3%, 5% sample heated at 1300 1C.

composition x ¼ 0.05 at 1300 1C. Wide excitation spectra werenoticed from the ultraviolet region (ca. 300 nm) to the blue lightregion (ca. 450 nm), indicating the possibility of a wide range ofexcitation light such as ultraviolet, near-ultraviolet, violet, andblue LED light. The excitation band is wider than that of blue lightexcitable green phosphor Ca3Sc2Si3O12:Ce3+ [9]. However, theemission is high-intensity green emission with a wide range ofemission wavelengths: 450–580 nm. These emission wavelengthsare suitable to phosphor and can cover green color lacking for inwhite LEDs. The (Ba0.95Eu0.05)9Sc2Si6O24 sample exhibited thehighest emission intensity in single phase.

ARTICLE IN PRESS

wavelength

Eu-5%

Eu-3%

Eu-1%Nor

mal

ized

inte

nsity

/arb

.uni

t

300 400 500

Fig. 4. Excitation and emission spectra of (Ba,Eu)9Sc2Si6O24. The broken line shows

excitation spectra; the full line shows emission spectra.

Eucomposition/x

Inte

nsity

/arb

.uni

t

0.02 0.04 0.06

20

40

60

80

100

Fig. 5. Concentration quenching of (Ba1�xEux)9Sc2Si6O24.

y

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8x

(Ba0.95Eu0.05)9Sc2Si6O24

(0.2100.552)

Fig. 6. The CIE chromaticity diagram.

10 20 30 40 50 60 70

Inte

nsity

/ ar

b. u

nit

2� / deg.

Eu-5%

Fig. 7. XRD patterns of (Ba0.95Eu0.05)9Sc2Si6O24.

Wavelength / nm

Nor

mal

ized

Inte

nsity

/ ar

b.un

it

(Ba0.95Eu0.05)9Sc2Si6O24

(Ba0.95Eu0.05)9Sc2 + � Si6O24-3 � N3 �

200 300 400 500 600 700

Fig. 8. Excitation and emission spectra of (Ba0.95Eu0.05)9Sc2Si6O24 and

(Ba0.95Eu0.05)9Sc2+dSi6O24�3dN3d. The broken line shows excitation spectra; the

full line shows emission spectra.

T. Nakano et al. / Journal of Luminescence 129 (2009) 1654–16571656

Concentration quenching of Ba9Sc2Si6O24:Eu2+ is indicated inFig. 5. The emission intensity was increased (xo0.01) and nearlyconstant(x40.01). As shown in Fig. 5, the Ba sites, which areindependent in the structure, are replaced by the doped activatorEu2+, which suggests that Ba9Sc2Si6O24:Eu2+ has weakconcentration quenching. The chromatic coordinate of(Ba0.95Eu0.05)9Sc2Si6O24 is depicted in Fig. 6. High-color-rendering white light is expected using Ba9Sc2Si6O24:Eu2+ as agreen phosphor for white LEDs.

3.2. Ba9Sc2+dSi6O24�3dN3d:Eu2+

According to Fig. 1(b), the crystal structure of Ba9Sc2Si6O24 hasvacancies that can be replaced by Sc3+. Since theBa9Sc2+dSi6O24�3dN3d:Eu2+ is nitrided from Ba9Sc2+dSi6O24:Eu2+

by the Sc3+ substitution. The body color of the sample changesfrom light yellow (Ba9Sc2Si6O24:Eu2+) to deep yellow by nitriza-tion. Fig. 7 shows XRD patterns of (Ba0.95Eu0.05)9 Sc2Si6O24. The

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T. Nakano et al. / Journal of Luminescence 129 (2009) 1654–1657 1657

simulation patterns of Ba9Sc2+dSi6O24�3dN3d:Eu2+ have not beenreported yet. The XRD patterns of (Ba0.95Eu0.05)9Sc2Si6O24 werenearly identical to simulation patterns of(Ba0.95Eu0.05)9Sc3Si6O21N3 (Fig. 3), the obtained sample(Ba0.95Eu0.05)9Sc3Si6O21N3 was single phase. Excitation andemission spectra of (Ba0.95Eu0.05)9Sc2Si6O24 are presented in Fig.8. Wide excitation spectra were observed in the blue light region(ca. 450 nm); they were wider than that of YAG:Ce3+ [10]. Usingthis excitation band, blue LED light is useful as an excitationsource. In fact, (Ba0.95Eu0.05)9Sc3Si6O21N3 shows yellow emissionat wavelengths of 530–650 nm. These emission spectra showlonger wavelengths than the green emission of Ba9Sc2Si6O24:Eu2+

by increased crystal field splitting and covalency.Ba9Sc2+dSi6O24�3dN3d:Eu2+ emission wavelengths include lackingred color component in a white LED. Therefore,Ba9Sc2+dSi6O24�3dN3d:Eu2+ can be used as a novel phosphor forwhite LEDs. Consequently, we infer that the Ba9Sc2Si6O24:Eu2+ andBa9Sc2+dSi6O24�3dN3d:Eu2+ phosphors exhibit excellent luminanceproperties for use as phosphors with white LEDs.

4. Conclusion

For this study, Eu2+-activated Ba9Sc2Si6O24 andBaSc2+dSi6O24�3dN3d were synthesized. They were characterizedusing X-ray powder diffraction, excitation, and emission spectro-scopy. For excitation in the UV–blue range, Ba9Sc2Si6O24:Eu2+

exhibits efficient green emission at 508 nm, whereas yellowemission at 570 nm was found for Ba9Sc2+dSi6O24�3dN3d:Eu2+. Byvirtue of their intense absorption and excitation band in theUV–blue spectral region (370–460 nm), these materials areinferred to be useful as novel conversion phosphors for whiteLEDs.

Acknowledgement

This work was supported by the project of Center forTransdisciplinary Research, Niigata University.

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