photoluminescence properties of sral o:eu...
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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: [email protected]
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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