Journal of Ceramic Processing Research. Vol. 18, No. 10, pp. 697~700 (2017)

697

J O U R N A L O F

CeramicProcessing Research

Photoluminescence properties of SrAl2O4:Eu2+ phosphors

Byoung Su Choia, You Shin Ahnb, Jin Kon Kimb, Jeong Ho Ryuc and Hyun Chob,*

aDepartment of Nano Fusion Technology, Pusan National University, Gyeongnam 50463, KoreabDepartment of Nanomechatronics Engineering, Pusan National University, Busan 46241, KoreacDepartment of Materials Science and Engineering, Korea National University of Transportation, Chungbuk 27469, Korea

An alkaline earth aluminate-based SrAl2O4:Eu2+ phosphor was prepared by solid-state reaction at 1300-1500 oC in a reducedatmosphere. The prepared SrAl2O4:Eu2+ phosphors were found to exhibit a monoclinic phase in crystal structure and anysecondary phase formation by the Eu2+ addition was not detected. Symmetric single broad band photoluminescence (PL)emission spectra centered at ~514 nm due to the 4f65d1 → 4f7 (8S7/2) transition wsere obtained from the SrAl2O4:Eu2+

phosphors, which indicates the doped Eu2+ ions occupied only one type of site in the SrAl2O4 lattice and the formation of onecorresponding Eu2+ emission luminescent center. PL emission intensity showed a strong dependence on the Eu2+ dopingconcentration and the strongest emission was observed for the 7 mol% Eu2+-doped SrAl2O4 phosphor. Dynamic light scattering(DLS) and field-effect scanning electron microscopy (FE-SEM) characterization revealed that the 7 mol% Eu2+-doped SrAl2O4

phosphor particles have an irregularly round shape and an average particle size of ~4 μm.

Key words: SrAl2O4:Eu2+ phosphors, Solid-state reaction, Green phosphors, Photoluminescence properties.

Introduction

The alkaline earth aluminates MAl2O4 (M = Ca, Sr,

and Ba) are the most widely used host material systems

in phosphors for display devices, signage and medical

applications due to their structural flexibility and

distinguishing features that separate them from other

inorganic compounds. Strontium aluminate (SrAl2O4),

belongs to the stuffed tridymite structure and the

framework consists of AlO4 tetrahedra with Sr2+ ions in

the cavities to balance the charge, has recently gathered

much attention due to its ability to provide excellent

luminescence properties such as high quantum efficiency,

high brightness and long persistence when it is doped

with appropriate activators. It is also chemically more

stable than the conventional sulfide phosphors [1-5].

It is well known that rare earth ions play a very

important role as efficient emitter in a variety of solid-

state phosphor matrices. The emission of light from the

rare earth ions is mostly due to electric and magnetic

dipole optical transitions within the 4fn manifold, but it

may also be interconfigurational in nature, involving

configurations such as 4fn-15d [6]. Among various rare

earth ions, divalent or trivalent europium (Eu) have

been widely used as an activator in phosphors and

exhibited very good luminescence properties in the

blue to red regions of the visible spectrum. It has been

reported that the luminescence of Eu2+ ions generally

shows a broad band due to the transitions between the

4f7 ground state and the 4f65d excited state and this is

likely to lead to different peak positions ranging from blue

to yellow region [7-11]. The Eu2+ activator ions can be

incorporated into the SrAl2O4 lattice by substituting the

Sr2+ ions in the cavities owing to the similarity of ionic

radius (Sr2+ : 0.127 nm, Eu2+ : 0.130 nm) and valence

electrons [12].

In this paper, we report Eu2+-doped SrAl2O4 green

phosphors for light emitting diode applications. The

SrAl2O4:Eu2+ phosphors were synthesized via a solid-

state reaction route and the effects of solid-state

reaction temperature and doping concentration on the

photoluminescence properties were studied.

Experimental

SrAl2O4 phosphors doped with Eu2+ ions were

synthesized by a solid-state reaction method using high

purity strontium carbonate (SrCO3, Alfa Aesar, 99.99%),

aluminum oxide (Al2O3, Alfa Aesar, 99.99%) and

europium oxide (Eu2O3, Alfa Aesar, 99.99%) as starting

materials. Doping concentration of Eu2+ was controlled

from 1.0 to 9.0 mol%. Stoichiometric mixtures of

SrCO3, Al2O3 and Eu2O3 powders were homogeneously

mixed by ball-milling and then calcined at temperatures of

1200 to 1500 oC for 12hrs in reduced atmosphere of 5%

H2/95% N2 gas mixtures. After heat treatment, the phosphor

samples were mildly ground before photoluminescence

measurements. The crystalline phase of the synthesized

phosphor powders was identified by X-ray diffraction

*Corresponding author: Tel : +82-51-510-6113Fax: +82-51-514-2358E-mail: hyuncho@pusan.ac.kr

698 Byoung Su Choi, You Shin Ahn, Jin Kon Kim, Jeong Ho Ryu and Hyun Cho

(XRD) analysis with Cu-Kα radiation operated at

40 kV and 30 mA. The photoluminescence emission

and photoluminescence excitation spectra were collected

at room temperature using a Fluorescence spectrometer

(FluoroMate FS-2, SCINCO) in the range of 380-700 nm

and 360-500 nm, respectively. Particle morphology and

size distribution were recorded using scanning electron

microscopy (SEM) and particle size analyzer (PSA).

Results and Discussion

Fig. 1 shows the XRD patterns of the prepared

7 mol% Eu2+-doped SrAl2O4 phosphors calcined for 12hrs

at temperatures of 1200 oC, 1300 oC, 1400 oC and 1500 oC,

respectively. The SrAl2O4 phosphors synthesized at

temperatures higher than 1300 oC show the characteristic

diffraction peaks which correspond well to the

monoclinic phase SrAl2O4 (JCPDS Card No. 74-0794)

and the crystallinity of the synthesized SrAl2O4

phosphors was improved by increasing the calcination

temperature [13, 14]. No peaks other than those from

the monoclinic phase SrAl2O4 are resolved in the XRD

pattern, which suggests that the formation of single

phase material and no effect of the incorporation of

europium ions on the SrAl2O4 phase composition. The

XRD patterns were similar for all the prepared samples

doped with Eu2+ concentration other than 7 mol% and

hence are not presented to avoid repetition. However,

the sample synthesized at 1200 oC shows peaks of the

unreacted starting materials at diffraction angle of ~25,

~32, ~44, and ~56 degrees, indicating that the solid-

state reaction of SrAl2O4 can be completed at

temperatures above 1200 oC.

The photoluminescence (PL) spectra of the prepared

SrAl2O4:Eu2+ phosphors with variation of Eu2+ doping

concentration are presented in Fig. 2. The PL spectra of

the SrAl2O4: Eu2+ excited by a 360 nm pumping source

consists of a single broad green emission band peaked at

~514 nm and the PL properties of the SrAl2O4:Eu2+

phosphors show a strong dependence on the Eu2+ doping

concentration. No apparent emission peaks of Eu3+ ions

(sharp lines between 580 and 650 nm) are observed in

the spectra, suggesting that Eu3+ was reduced to Eu2+ in

a reduced atmosphere. The broad band emission

centered at ~514 nm is commonly ascribed to the parity-

allowed 4f65d1 → 4f7 (8S7/2) electric dipole transition

between two electronic configurations of the divalent

europium ion.[15-17] Furthermore, the SrAl2O4:Eu2+

phosphors display only one symmetric emission band

regardless of the Eu2+ doping concentration. This implies

that the doped Eu2+ ions occupied only one type of site

in the SrAl2O4 host lattice and one corresponding Eu2+

emission luminescent center was formed.

Fig. 3 shows the dependence of PL emission peak

intensity and peak position of the respective emission

bands on the Eu2+ doping concentration. The PL emission

intensity of the prepared SrAl2O4:Eu2+ phosphors increases

as the Eu2+ doping concentration increases and reaches

a maximum at 7 mol%, and then decreased at the

concentration beyond 7 mol% because of concentration

quenching. This is most likely due to the probability of

the energy transfer from the Eu2+ ions at higher levels

of 5d to those at the lower levels of 5d increases with

Fig. 1. XRD patterns of 7 mol% Eu2+-doped SrAl2O4 phosphorssynthesized at different temperatures.

Fig. 2. Photoluminescence emission (λex = 360 nm) spectra ofSrAl2O4 green phosphors with variation of Eu2+ dopingconcentration.

Fig. 3. Relative photoluminescence emission intensity andemission peak position as a function of Eu2+ doping concentration.

Photoluminescence properties of SrAl2O4:Eu2+ phosphors 699

the increase of the Eu2+ doping concentration. This

makes it possible that higher Eu2+ concentration lowers

the emission energy for transfer from the low 5d

excited state to the 4f ground state and also causes a

shift of emission band to longer wavelength.

Fig. 4 presents the PL excitation and emission

spectra of the 7 mol% Eu2+-doped SrAl2O4 phosphor. It

is demonstrated that the excitation spectrum shows a

broad band over the wavelength of 280-450 nm and

three shoulders appear at around 320 nm, 360 nm, and

420 nm, respectively. The broadness of this excitation

spectrum indicates that the prepared SrAl2O4:Eu2+

phosphors can be well excited in the range from 320 to

420 nm. The excitation spectrum ranging from 320 to

420 nm is ascribed to the partly-allowed 4f7 → 4f65d

transition of the Eu2+ ions. More strong emission

intensity in the green region is obtained with excitation

at 360 nm since the most intensive excitation peak

appears at around 360 nm.

Fig. 5 shows a particle size distribution profile

examined by dynamic light scattering (DLS) and a

typical SEM micrograph for the 7 mol% Eu2+-doped

SrAl2O4 phosphor powder calcined at 1400 oC. The

prepared SrAl2O4:Eu2+ phosphor was found to have an

average particle size of ~4 μm with a relatively broad

distribution profile in the range of ~1.2-8.7 μm. The

morphology of the SrAl2O4:Eu2+ phosphor displayed an

irregular spherical shape. The performance of the

phosphors is also dependent on the particle shape.

Considering the morphology of the SrAl2O4 phosphors

synthesized in this work, less scattering effect on their

photoluminescence efficiency is expected.

Conclusions

Strontium aluminate green phosphors (SrAl2O4:Eu2+)

have been synthesized via a solid-state reaction route

and the effects of heat treatment temperature and

Eu2+ doping concentration on their photoluminescence

properties have been studied. Monoclinic SrAl2O4

phosphors with high crystallinity were prepared at

temperatures higher than 1300 oC and no effect of the

incorporation of europium ions on the SrAl2O4 phase

composition was observed. Under excitation at 360 nm

source, the SrAl2O4:Eu2+ phosphors exhibited a strong

single band of green emission peaking at ~514 nm due to

the parity-allowed 4f65d1 → 4f7 (8S7/2) electric dipole

transition and the optimum doping concentration of

Eu2+ was determined to be 7 mol%. The SrAl2O4:Eu2+

phosphors showed a broad excitation band ranged from

280 nm to 450 nm and more strong emission intensity

was obtained with an excitation at near UV region. It

has been demonstrated that the SrAl2O4:Eu2+ phosphors

has a good potential as a green phosphor for white

LED using near UV or blue LEDs as the excitation

source.

Acknowledgments

This work was supported by a 2-Year Research

Grant of Pusan National Unversity.

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Fig. 4. PL excitation (λem = 514 nm) and PL emission(λex = 360 nm and 420 nm) spectra of SrAl2O4:Eu2+ phosphors.

Fig. 5. Particle size distributions and a typical SEM image (inset)of 7 mol% Eu2+-doped SrAl2O4 phosphor.

700 Byoung Su Choi, You Shin Ahn, Jin Kon Kim, Jeong Ho Ryu and Hyun Cho

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