Luminescent properties of Eu2+, Eu3+ and Dy3+ co-activated SrAl2O4
nanocrystals
Yuan Ming Huanga, Qing-lan Ma and Bao-gai Zhai
School of Mathematics and Physics, Changzhou University, Jiangsu 213164, China
a
dongshanisland@126.com
Keywords: Strontium aluminate oxide; Europium ions; Dysprosium ions; Photoluminescence;
Phosphor
Abstract. By the control of the reducing atmospheres in the one-pot combustion reactions, Eu2+,
Eu3+ and Dy3+ co-activated strontium aluminate oxide were synthesized. X-ray diffraction
confirmed the formation of monoclinic SrAl2O4 nanocrystals. The photoluminescence spectra of the
phosphors were measured. It is found that three sharp emissions at 483, 570 and 615 nm were
superimposed onto the broad green emission band at about 520 nm. The origins of the broad green
luminescent band, the sharp blue at 483 nm, and sharp yellow emissions at 570 nm and the sharp
red emissions at 615 nm can be attributed to the 4f65d1→4f7 transitions of Eu2+ ions, the 4F9/2→
6
H15/2 transition of Dy3+ ions, the 4F9/2→6H13/2 transition of Dy3+ ions, and the 5D0–7F2 transition of
Eu3+ ions in the SrAl2O4 nanocrystals, respectively. The results indicate that the white light emitting
phosphors are possible by tuning the relative molar percentages of Eu2+, Eu3+ and Dy3+ in the
phosphors.
Introduction
Europium and dysprosium co-activated strontium aluminate oxides (SrAl2O4:Eu2+,Dy3) are ideal for
luminescent infrastructure materials with high brightness and persistent phosphorescence [1-4].
Compared to its predecessor copper-activated zinc sulfide, strontium aluminate is a superior
phosphor due to its brighter and longer glows in darkness. SrAl2O4:Eu2+,Dy3+ phosphors absorb the
ultraviolet light, and then store the absorbed energy in the medium before its release in the form of
green emission. The wavelengths for its green emission at 520 nm, its blue-green emission at 505
nm and its blue emission at 490 nm were reported. In general, the produced wavelengths depend on
the internal crystal structure of the material, and slight modifications in the crystal structure and
minute addition of rare-earth halides can significantly influence the emission wavelengths [5, 6].
That is why researchers incorporated extra rare-earth elements (i.e., Er3+, Pr3+, Sm3+) into SrAl2O4
to tune the luminescent properties of the phosphors SrAl2O4:Eu2+,Dy3+.
Europium is one of the most important rare earth elements. Both of its divalent and trivalent ions
can give off strong emissions in the visible spectral region, and it is widely utilized as red
phosphors in cathode ray tubes and key components in laser optics. It is reported that the valence
state of Eu incorporated in the host of SrAl12O19 can generate an important effect on the
luminescence of the synthesized compound [1]. In this work, we investigate the photoluminescence
(PL) of Eu2+, Eu3+ and Dy3+ activated SrAl2O4 nanocrystals. It is found that PL spectrum and color
of SrAl2O4:Eu2+,Eu3+,Dy3 are quite different from those of its counterpart SrAl2O4:Eu2+,Dy3+.
Experiment
All chemicals were in analytical grade and were used as received without further purification.
Strontium aluminate oxide was prepared from the mixture of strontium nitride (10 mmol, 2.171 g),
aluminum nitrate nonahydrate (20 mmol, 7.537 g), urea (200 mmol, 12.056 g) and boric acid (8
mmol, 0.495 g), ethanol (5 ml) and water (40 ml). The boric acid was added to the mixture as a
flux. Co-activators of 0.2 mmol (73 mg) of europium oxide (Eu2O3) and 0.4 mmol (158 mg) of
dysprosium oxide (Dy2O3) were dissolved into the mixture at 50oC. The designed composition of
the phosphors was SrAl2O4:Eu0.08Dy0.16. About 20 ml of the solutions were transferred into each of
three alumina crucibles. The phosphors were prepared by conventional combustion method in an
alumina crucible [4, 6-8]. The reference phosphor was fired at 600oC in an alumina crucible
covered with an alumina cap to reduce Eu3+ ions into Eu2+ ions. The resulted phosphor
SrAl2O4:Eu2+,Dy3+ was denoted as phosphor I. The phosphor under investigation was fired at 800oC
in an alumina crucible which had been partially covered with an alumina cap. The release of a
portion of the reducing atmosphere from the crucible rendered the reduction of Eu3+ into Eu2+ ions
incomplete. The resulted phosphor SrAl2O4:Eu2+, Eu3+,Dy3+ was denoted as phosphor II.
The crystal structure of the sample was identified by X-ray diffraction (XRD) analysis with a
Bruker AXS (Japan) D8 ADVANCE diffractometer using Cu K radiation (λ = 1.5405 Å). The PL
spectra of the phosphors were recorded with a spectrophotometer (Aipha Technologies, China). The
excitation wavelength for the PL measurement was the 325 nm emission line from a
helium-cadmium laser (Kimmon Koha Co. Ltd., China).
Results and discussions
Figure 1. (a) X-ray diffraction patterns of the as-prepared
SrAl2O4:Eu2+,Eu3+,Dy3+ powders that that was fired at 600oC; (b)
XRD reference pattern of SrAl2O4 (JCPDS file no.34-0379).
Both kinds of phosphors were characterized with X-ray diffraction to identify their phases and
crystal structures. The two phosphors synthesized in complete and in incomplete reducing
atmospheres exhibit almost identical X-ray diffraction patterns corresponding to the pure phase of
SrAl2O4 since the level of impurity doping is low in the host lattice. Figure 1(a) shows the X-ray
diffraction pattern of the phosphor SrAl2O4:Eu2+, Eu3+,Dy3+ synthesized by the combustion method.
All the peaks could be indexed to the SrAl2O4 phase. Figure 1(b) depicts the X-ray diffraction
reference pattern of SrAl2O4 (JCPDS file no.34-0379). Comparison of the results in Fig. 1 reveals
that the diffraction peaks of the phosphor SrAl2O4:Eu2+, Eu3+,Dy3+ match perfectly with those of the
standard SrAl2O4. The crystallographic parameters of synthesized nanocrystalline SrAl2O4 have
been compared with standard JCPDS values, which are listed in Table 1. The average crystallite
size estimated using Scherrer formula is about 50 nm for the nanocrystals form prepared by
combustion method. As dopant concentration is low, the lattice strain of the host lattice is not
appreciable and Scherrer formula provides a reasonable estimate of average crystallite size.
Table 1. Crystallographic parameters of SrAl2O4:Eu2+,Eu3+,Dy3+.
Value of hkl
d values
(JCPDS data in Å)
2θ
(JCPDS data)
d values
(our case in Å)
2θ (our case)
(our case)
011
4.447
19.951
4.449
19.942
120
3.907
22.740
3.908
22.735
-211
3.141
28.386
3.141
28.392
220
3.048
29.275
3.047
29.287
211
2.983
29.922
2.984
29.924
-311
2.432
36.924
2.432
36.924
-231
2.156
41.860
2.213
40.735
400
2.106
42.892
2.107
42.890
240
1.954
46.431
1.954
46.432
441
1.479
62.770
1.478
62.805
Figure 2. PL spectra of SrAl2O4:Eu2+,Dy3+.
SEM micrographs were taken for the two kinds of phosphors. Our results showed that the two
types of synthesized SrAl2O4 phosphors shared quite similar branched and porous microstructures
as reported in our previous publications [4, 6-8]. However, the PL spectra of the two kinds of
phosphors are quite different. In phosphor I, the Eu3+ ions were reduced to Eu2+ in the complete
reducing atmosphere. Therefore, strong green emission is expected for phosphor I. Figure 2
represents the typical PL spectra of the SrAl2O4:Eu2+,Dy3+ nanocrystals. It is clear that the PL
spectra of Eu2+ and Dy3+ co-doped SrAl2O4 are broad. Its maximal intensity is located at about 520
nm, indicating that the lowest excited states are 4f65d1 (f–d). The green PL spectrum of
SrAl2O4:Eu2+,Dy3+ phosphor has been characterized by the transitions of Eu2+ ions from its 4f65d
excited state to its ground state 4f7. Both the absorption and emission spectra of Eu2+ in
SrAl2O4:Eu2+,Dy3+ are due to electronic transitions between the 4f7 and 4f65d1 electronic
configuration.
Figure 3. Schematic energy level
diagram of Eu3+ and Eu2+ in crystal
lattices. Horizontal lines represent
narrow energy states of 4f levels.
Shaded areas represent broad charge
transfer states in the case of Eu3+ and
4f65d1 states for Eu2+.
The energy level diagrams of Eu3+ and Eu2+ in the crystal lattices are schematically shown in
Fig. 3. Horizontal lines represent narrow energy states of 4f levels. Shaded areas represent the
4f65d1 states for Eu2+ and the broad charge transfer states for Eu3+. For Eu2+ the 5d band covers
almost all 4f levels, leading to a broad emission spectrum found in many lattices. The emission
wavelength is determined by the position of the 4f65d1 levels, which is a function of the crystal
field. Therefore, the crystal field splitting of the 5d level is responsible for the broad PL band of
SrAl2O4:Eu2+Dy3+. When phosphor I was fired in complete reducing atmosphere, Eu3+ had been
reduced to Eu2+ state. For the divalent dopant Eu2+ ions, overall charge compensation in the lattice
could be fulfilled by one to one substitution. The signature of the divalent state of Eu in phosphor I
is confirmed by the characteristic PL spectra shown in Fig. 2. Hence the results in Fig. 2 showed
that SrAl2O4 nanocrystals were prepared by the combustion synthesis with successful incorporation
of europium ions in its reduced state Eu2+.
Figure 4. PL spectra of SrAl2O4:Eu2+,Eu3+,Dy3+.
When the phosphor was fired in incomplete reducing atmosphere, only a portion of Eu3+ has
been reduced to Eu2+, leaving the residual Eu3+ ions untouched. As a result of the incomplete
reduction, both Eu2+ and Eu3+ ions could be incorporated into the host matrix. Dopant Eu3+ ions are
accommodated in substitutional sites of Sr2+. The radius of host Sr2+ (0.118 nm) differ from Eu3+
(0.0947 nm). For Eu3+ dopant, charge compensation would require that two Eu3+ ions are
substituted for three Sr2+ ions. For Eu3+ dopant state, there could be two ways to maintain overall
charge neutrality in the lattice, by creating one Sr2+ vacancy for each two Eu3+ incorporation or
introducing one oxygen interstitial (Oi2−) defect as reported by Chawla et al. [1]. As the radius of
substitutional Eu3+ is smaller than Sr2+ and the substitution demands the presence of vacancy or
interstitial in neighboring position for charge compensation, hence the strain in the SrAl2O4 lattice
will be more for Eu3+ substitution than for Eu2+ substitution.
The substitution of Eu3+ in the crystal lattice will generate significant effects on the emissions.
Figure 4 exhibits the PL spectra of SrAl2O4:Eu2+,Eu3+,Dy3+. As shown in Fig. 4, the signature of the
Eu2+ is confirmed by the broad green PL band whose peak is located at about 520 nm. More
importantly, three sharp peaks are superimposed upon the broad green PL spectrum. The maxima of
the three peaks are located at 483, 573 and 613 nm, respectively. It is established that the 4f
electrons in the Eu3+ and Dy3+ ions are well shielded by the 5s25p6 outer shells, so the 4f to 4f
transitions in Eu3+ and Dy3+ are likely responsible for the recorded sharp emissions because the
crystal field experienced by 4f electrons is weak. First of all, the sharp peak at 613 nm can be
assigned to the emissions of the residual Eu3+ ions in the host SrAl2O4. In principle, radiative
recombination can happen by transitions from the excited 5D0 level to the ground 7FJ (J = 0-6) levels
of the 4f6 configuration in Eu3+ ions. The data of the typical transitions of Eu3+ in SrAl2O4 host
materials are listed in Table 2. As the transitions are between the states of same parity, parity
selection rules forbid electric dipole transitions. Only magnetic dipole transitions can occur between
ΔJ = ±1 states. However, for small deviation from inversion symmetry, ΔJ = ±2, ±4 forced electric
dipole transitions become significant. This is clearly seen from the strong 5D0–7F2 (615 nm)
transition in Fig. 4. The emission from the 5D0–7F4 (698 nm) transition was reported in the literature
[9-11]. The relative intensity of 5D0–7F2 transition in Eu3+ is linked to the local site symmetry of the
Eu3+ luminescent centre in the lattice. If Eu3+ occupies the inversion centre site, the magnetic dipole
transition (5D0–7F1) should be relatively strong whereas electric dipole transition (5D0–7F2) is parity
forbidden and should be very weak. The emissions from the magnetic dipole transition (5D0–7F1)
are hard to be indexed unambiguously in Fig. 4 because it is likely merged into the asymmetric
sharp emission at 570 nm.
Table 2. Typical transitions of trivalent Eu in host materials.
Emission Peak (nm)
573
Transition
s
5D –7F
0
0
591
5D
0–
615
5D
7
0– F2
647
5D
0–
683
5D
7
0– F4
698
5D
707
5D
Transition Type
Polarization
Forbidden
Magnetic dipole
π
Forced electric dipole
σ
Magnetic dipole
σ
Forced electric dipole
π
0–
7F
4
Forced electric dipole
σ
7
0– F4
Forced electric dipole
π
7F
1
7F
3
The sharp and intense emission at about 570 nm (2.16 eV) cannot be originated from the 5D0–7F0
electronic transitions of Eu3+ ions because such transitions are forbidden. Although Barros et al.
reported the 5D0–7F0 electronic transitions of Eu3+ ion in SrWO4 [12], the emission intensity was
many times weaker than the emission from the 5D0–7F2 transitions. Dy3+ is also one of the important
rare-earth ions for the preparation of phosphors and also plays a major role in the production of
light luminescent materials [3]. In Dy3+ doped phosphors, the strongest emissions are observed in
the yellow and blue spectral ranges. Specifically the most intense yellow line is centered at 582 nm
and can be assigned to the 4F9/2→6H13/2 transition, while the blue emission peaked at 483 nm is due
to its 4F9/2→6H15/2 transition. The two following lines at 676 and 762 nm associated with
4
F9/2→6H11/2 and 4F9/2→6F11/2 transitions are hardly perceptible. Comparison of the characteristic
emissions of Dy3+ ions in the literature shows that the blue emissions observed at 483 nm (2.57 eV)
and the yellow emissions at about 570 nm in Fig. 4 can be attributed to the characteristic emissions
of Dy3+ ions from their 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions, respectively.
Figure 5. Chromaticity coordinates of the phosphors SrAl2O4:Eu2+,Dy3+ (phosphor
I) and SrAl2O4:Eu2+,Eu3+,Dy3+ (phosphor II) in the CIE 1931 XYZ color space.
The results in Fig. 4 have demonstrated that sharp emissions of Eu3+ and Dy3+ ions in red,
orange, yellow and blue colors were achieved in addition to the broad green luminescent Eu2+ ions
in the phosphors. The results indicate that the luminescent properties of the phosphors can be tuned
by tuning the relative molar percentages of Eu2+, Eu3+ and Dy3+ in the phosphors. To quantitatively
study the change of the colors of the two types of phosphors, we performed colorimetric
investigations on the phosphors by employing the method reported in our previous publications [13,
14]. The chromaticity coordinates x and y were obtained for the phosphors I and II in the CIE 1931
XYZ color space, and the results are shown in Fig. 5. The results in Fig. 5 shows that the
chromaticity coordinates of the phosphors can be effectively shifted from the initial point (0.183,
0.480) to its final point (0.247, 0.349) as some Eu3+ ions are incorporated into the crystal lattices of
the host. Consequently the Eu2+, Eu3+ and Dy3+ co-activated SrAl2O4 represents one kind of solid
state luminescent materials with tunable emission properties. In principle, white colored emissions
at the point (0.333, 0.333) can be expected in the Eu2+, Eu3+ and Dy3+ co-activated SrAl2O4 if the
molar ratios of the Eu2+, Eu3+ and Dy3+ ions in the host are finely adjusted.
Summary
Phosphors SrAl2O4:Eu2+,Eu3+,Dy3+ have been synthesized by the control of the reducing
atmospheres in the reactions to reduce a portion of Eu3+ into Eu2+ ions. The PL spectra of the
synthesized phosphors have characteristic of three sharp emissions at 483, 570 and 615 nm which
are superimposed onto the broad green luminescent band at 520 nm. The origins of the broad green
luminescent band, the sharp blue at 483 nm, and sharp yellow emissions at 570 nm and the sharp
red emissions at 615 nm can be attributed to the 4f65d1→4f7 transitions of Eu2+ ions, the 4F9/2→
6
H15/2 transition of Dy3+ ions, the 4F9/2→6H13/2 transition of Dy3+ ions, and the 5D0–7F2 transition of
Eu3+ ions in the SrAl2O4 nanocrystals, respectively. The results indicate that tunable luminescent
properties of the phosphors can be achieved by tuning the relative molar percentages of Eu2+, Eu3+
and Dy3+ in the SrAl2O4 nanocrystals. White colored emissions are expected from the Eu2+, Eu3+
and Dy3+ co-doped SrAl2O4, which can be used as a potential matrix material under the excitation
of ultraviolet light-emitting diode.
Acknowledgements
The authors are grateful to Changzhou University for the financial support under the grant no.
ZMF1002132.
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