Preparation and photoluminescence properties of SrY2O4:Yb3+, Er3+ powders

Article abstract of DOI:10.1016/j.jallcom.2008.06.158

Yb³⁺/Er³⁺ co-doped SrY₂O₄ powders are prepared by developing a nitric-decomposition method. Under 980 nm laser excitation, the green and red up-conversion emissions are observed at around 549 and 661 nm, which a

Full text of DOI:10.1016/j.jallcom.2008.06.158

Journal of Alloys and Compounds 474 (2009) 424–427
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Preparation and photoluminescence properties of SrY2O4:Yb3+, Er3+ powders
Jikai Yang, Siguo Xiao, Jianwen Ding∗, Xiaoliang Yang, Xiangfu Wang
Key Laboratory of Low Dimensional Materials & Application Technology & Faculty of Materials, Photo-electronics and Physics,
Xiangtan University, Xiangtan, Hunan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Yb3+/Er3+ co-doped SrY2O4 powders are prepared by developing a nitric-decomposition method. Under
980 nm laser excitation, the green and red up-conversion emissions are observed at around 549 and
Article history:
Received 12 November 2007
Received in revised form 21 June 2008
Accepted 24 June 2008
661 nm, which are attributed to the transitions of 4S3/2
→
4I15/2 and 4F9/2
→
4I15/2 of Er3+ ions, respectively.
The intensity ratio of the red emission to the green one increases with Yb3+ doping concentration. A
quadratic dependence of the two emissions on excitation power is obtained, indicating that a two-photon
Available online 19 August 2008
4
4
process populates the S3/2 and F9/2 levels for the SrY2O4 powders. Especially, it is found that at lower
excitation power, Yb3+ doping concentration has an important influence on the slopes of the emission
intensity versus excitation power, similar with the excitation power. The results can be helpful for the
design and applications of low-cost near-infrared laser diodes.
Keywords:
Rare earth alloys and compounds
Optical properties
Luminescence
© 2008 Elsevier B.V. All rights reserved.
esting to explore the photoluminescence properties of Er3+ doped
SrY2O4.
1. Introduction
In this paper, Yb3+/Er3+ codoped SrY2O4 powders are prepared
by developing a nitric-decomposition method, in order to inves-
tigate the up-conversion properties of Er3+ in such an interesting
host. In order to enhance the up-conversion intensity, Yb3+ is also
introduced as the sensitizer due to its great absorption cross-
section and efficient energy transfer from Yb3+ to Er3+ [12–14].
Under 980 nm laser excitation, the green and red up-conversion
emissions are observed at around 549 and 661 nm. The relative
intensity of the red and green up-conversion emissions is inves-
tigated as a function of the Yb3+ doping concentration.
In recent years, practical application in optical devices, such as
color display, optical data storage, biomedical diagnostics and tem-
perature sensors have been produced on the basis of rare earth ion
doped materials [1,2]. Recently, the luminescence properties of rare
earth ions in SrY2O4 have attracted much attention.
SrY2O4 belongs to the ordered CaFe2O4 structure, which is com-
posed of a (R2O4)2− (R = rare earth metal) framework of double
octahedral with rare earth ions residing within the framework. Due
to the thermal and chemical stability, SrY2O4 have been used in
thermal barrier coating (TBC) materials [3,4]. It has been found that
SrY2O4: Eu3+ is one of promising red phosphors for FED application
[5–9]. Blass and Bril proved that SrY2O4:Ce3+ samples could emit
in the green region [10], while Manivannan et al. [11] showed that
SrY2O4:Ce3+ samples could emit in the blue region with a Stokes
shift of about 3400 cm−1. These works indicate that SrY2O4 may be
an important class of luminescent materials under ultraviolet (UV)
and vacuum ultraviolet (VUV) excitation.
Although the luminescence properties of some rare earth doped
SrY2O4 have been investigated, the properties of its up-conversion
are still lack of report. As one member of rare earth, Er3+ ion has a
favorable energy level structure of 4I15/2–4I11/2 in the near-infrared
region, which can be excited by 980 nm luminescence diode. Thus, it
is believed that Er3+ doped materials are one of the most important
luminescent materials for up-conversion. Therefore, it is very inter-
2. Experiment and measurement
All reagents were analytical grade. The starting materials were SrCO3, Y2O3,
Yb2O3, Er2O3 and nitric acid. R(NO3)3 (R = Sr, Y, Yb and Er) salts were freshly prepared
by reactions of R2O3 with dilute nitric acid. According to the nominal compositions of
compounds SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 (x = 1, 5, 10, 15 and 20 mol%, y = 1.5 mol%),
the corresponding nitrate solutions were mixed stoichiometrically. After being evap-
orated for several hours in a vessel, the solution turned to be a yellowish gel. Then,
the gel was continually heated until it was dried. The dried precursors were placed
in a corundum crucible to be preheated for 2 h at 400 ◦C in air and then sintered
at 1000 ◦C for 2 h and 1200 ◦C for 10 h in an electric furnace. After these processes
white powders were obtained.
The SEM (JSM-5600LV) was used to observe the morphology and sizes of
the particles. The X-ray diffraction (XRD) patterns were recorded in the range of
10◦ ≤ 2ꢀ ≤ 80◦ by using a D-500 equipment (50 kV, 100 mA, Cu K␣ = 0.154056 nm). UC
emission spectra were measured from 500 to 780 nm by exciting the samples with
980 nm diode laser at room temperature. The dependence of the up-converted emis-
sion intensities on pumping powers for different samples was obtained by changing
the excitation powers at room temperature. All the measuring conditions were kept
steady for different samples, so as to ensure that the relative luminescence intensity
was comparable.
∗
Corresponding author. Tel.: +86 7328293798; fax: +86 7328292468.
E-mail address: jwding@xtu.edu.cn (J. Ding).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.06.158

J. Yang et al. / Journal of Alloys and Compounds 474 (2009) 424–427
425
Fig. 1. X-ray diffraction pattern of SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 powder at
x = 10 mol% and y = 1.5 mol%.
Fig. 3. Up-conversion emission spectra in SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 powder
under 980 nm excitation at x = 10 mol% and y = 1.5 mol%.
3. Results and discussion
Fig. 1 shows the XRD patterns of 1.5 mol% Er3+ and 10 mol% Yb3+
codoped SrY2O4 powder at the sintering temperature of 1200 ◦C. All
the peaks can be indexed to the orthogonal phase of SrY2O4 without
other impurity phase, and are matched to the standard values of
SrY2O4 given in JCPDS (no. 32-1272). Fig. 2 shows the SEM image of
SrY2O4: Yb3+, Er3+ powder. The SEM shows that SrY2O4:Yb3+, Er3+
powder is mainly sphere-like particles with a diameter of about
1 ␮m.
Under 980 nm laser excitation, Fig. 3 shows the room temper-
ature up-conversion spectra of the SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4
sample at x = 10 mol% and y = 1.5 mol%. In the region of 500–600 nm,
the spectrum is enlarged, as shown in Fig. 3 inset. The visi-
ble emission bands in the region 510–540 nm, 540–588 nm and
630–710 nm are observed, which can be attributed to the tran-
emissions in the SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 samples at various
Yb3+ doping concentration (x) but a given Er3+ doping concentra-
tion (y = 1.5 mol%). For the red up-conversion emission (661 nm),
it is seen from Fig. 4 that the emission intensity firstly increases
and then decreases with x increasing, a maximum existing at
x = 15 mol%. A similar behavior has also been obtained in the green
up-conversion emission (549 nm), in which the strongest emission
occurs at x = 5 mol%.
At much high Yb3+ doping concentration (x > 15 mol%), the pair-
ing or aggregation might make some of Yb3+ ions into quenchers
and lead to the back energy transfer (BET) [19,20]. This leads to the
fact that the intensity of the red emissions at x = 20 mol% is less than
that at x = 15 mol%. The similar result is also obtained in the green
emission. To the naked eyes, especially, some bright and visible
emissions are observed at various Yb3+ doping concentration. The
green emission is obtained at x = 1 mol%, and the yellow emission
at 5 mol%. The red emission appears at x = 10 mol%, 15 mol% and
20 mol%, but no green and yellow emissions occur there. Clearly,
the relative intensity ratio of the red emission to the green one
depends strongly on the Yb3+ doping concentration. For a given
Er3+ doping concentration (y = 1.5 mol%), Fig. 5 shows the inten-
sity ratio of the SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 samples at various
Yb3+ doping concentration (x = 1, 5, 10, 15 and 20 mol%). With the
increase in Yb3+ doping concentration, it is seen from Fig. 5 that
2
4
4
4
4
sitions of H11/2 → I15/2
,
4S3/2 → I15/2 and F9/2 → I15/2 of Er3+
,
respectively. The red emission at 630–710 nm is much stronger
than other two emissions, which can be seen by naked eyes.
Also, the obvious splits are observed from the emission bands of
2H11/2 → I15/2
,
4S3/2 → I15/2 and F9/2 → I15/2 transitions, differ-
4
4
4
4
ent from previous reports [15,16]. It is well known that the crystal
field has a strong effect on the emissions of the rare earth doped
ions. The results show that the crystal field in SrY2O4 is stronger
than that in other hosts.
To explore the effects of Yb3+ doping concentration on the emis-
sions, Fig. 4 shows the integral intensity of both 549 and 661 nm
Fig. 4. Effects of Yb3+ doping concentration on peak intensity of the red
and green up-conversion emissions centered at about 661 nm and 549 nm for
SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 powders at x = 1, 5, 10, 15 and 20 mol%, and
y = 1.5 mol%.
Fig. 2. The SEM of SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 powder at x = 10 mol% and
y = 1.5 mol%.

426
J. Yang et al. / Journal of Alloys and Compounds 474 (2009) 424–427
ground state absorption (GSA) process and the energy transfer
2
process of 2F5/2(Yb3+) + 4I15/2(Er3+) → F7/2(Yb3+) + 4I11/2(Er3+
)
(ET1) are responsible for the population of Er3+ on I11/2
4
level. Since Yb3+ has
a larger absorption cross-section than
Er3+ in the 980 nm region, the ET1 process is dominant to
4
4
the I11/2 level population. Once the I11/2 level is excited,
three processes may occur: (1) the excited state absorption
4
4
of I11/2(Er3+) + a photon → F7/2(Er3+
)
(ESA1), (2) the cross
4
4
4
relaxation of I11/2(Er3+) + I11/2(Er3+) → I15/2(Er3+) + 4F7/2(Er3+
)
(CR1),
and
(3)
the
energy
transfer
process
(ET3).
of
The
2F5/2(Yb3+) + 4I11/2(Er3+) → F7/2(Yb3+) + 4F7/2(Er3+
)
2
three processes populate the Er3+ on F7/2 level, and then F7/2
level relaxes rapidly and nonradiatively to the next lower levels of
both 2H11/2 and 4S3/2. As a result, such a process produces the green
4
4
emissions of 2H11/2 → I15/2 and 4S3/2 → I15/2. In addition, it should
be noticed that the green up-conversion emissions depend mainly
on the ET3 process, due to the large absorption cross-section of
4
4
Fig. 5. The intensity ratio of the red emission to the green one for SrY2(1−x−y) = 2−2x−2y
Yb2xEr2yO4 powders at x = 1, 5, 10, 15 and 20 mol%, and y = 1.5 mol%.
Yb3+ and the efficient energy transfer from Yb3+ to Er3+
.
As for the red up-conversion emission, it originates from the
4
4F9/2 → I15/2 transition. The population of 4F9/2 level is also based
on the three processes as follows: (1) the excited state absorption of
4
4I13/2(Er3+) + a photon → F9/2(Er3+) (ESA2), (2) the energy transfer
2
process
(ET2),
of
and
2F5/2(Yb3+) + 4I13/2(Er3+) → F7/2(Yb3+) + 4F9/2(Er3+
)
of
(3)
the
cross
relaxation
4
4I11/2(Er3+) + 4I13/2(Er3+) → I15/2(Er3+) + 4F9/2(Er3+
)
(CR2). Here,
the ET2 process is dominant, just as the situation of ET3
process in the green up-conversion emission. The nonra-
4
4
diative relaxation from S3/2 to F9/2 can also contribute
to the red emission. In addition, the cross relaxation pro-
cesses of I9/2(Er3+) + 4I11/2(Er3+) → I13/2(Er3+) + 4F9/2(Er3+
)
and
4
4
4I11/2(Er3+) + 4F7/2(Er3+) → 24F9/2(Er3+) are believed to occur, as
indicated in refs. [15,18].
The up-conversion emission intensity IUP is proportional to
the nth power of infrared excitation intensity IIR, that is, IUP
∝
Fig. 6. Energy level diagram of the red and green up-conversion emissions from
InIR, where n represents the number of photons absorbed per up-
conversion emission [21,22]. One can obtain the photon number
n from the logarithmic dependence of the emission intensity on
the pump power. For the green and red emissions centered at
549 and 661 nm, Fig. 7 shows the logarithmic dependence of the
emission intensity as a function of the pump power at y = 1.5 mol%
but various x. At x = 1, 15, and 20 mol%, the slopes of n = 1.75, 1.60
and 1.53 were obtained at the 549 nm emission, while n = 1.64,
Yb3+/Er3+ co-doped SrY2O4 powders.
the intensity ratio increases from 0.48 to 31.61. At x > 5 mol%, the
up-conversion emission becomes to be almost entirely red. The
result is very similar with those reported in both the Al2O3 and
Y2O3 powders codoped with Yb3+ and Er3+ [15,17].
Fig. 6 shows the possible up-conversion emission mechanism
for SrY2O4: Yb3+/Er3+ powder under 980 nm laser excitation. The
Fig. 7. Dependence of up-conversion emission intensity on excitation power under 980 nm excitation for SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 powders at x = 1, 15 and 20 mol%, and
y = 1.5 mol%.

J. Yang et al. / Journal of Alloys and Compounds 474 (2009) 424–427
427
1.34 and 1.18 at 661 nm emission. The results indicate that a two-
the slopes of the up-conversion intensity versus excitation power,
similar with the excitation power. The results can be helpful for the
design and applications of low-cost near-infrared laser diodes.
4
4
photon process populates the S3/2,
2H11/2 and F9/2 levels for the
SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 samples at x = 1, 15, and 20 mol%,
respectively.
From Fig. 7, especially, it is found that the slope of the emis-
sion intensity versus pump power decreases with the increase in
Yb3+ doping concentration. This can be understood by the follow-
ing consideration. By increasing the pump power, on the one hand,
the up-conversion process dominates over the linear decay, leading
a decrease in the slope, just as previous reports [21,23,24]. On the
other hand, the energy transfer rate for dipole–dipole interaction
can be expressed as ref. [25]
Acknowledgments
This work is supported by National Natural Science Founda-
tion of China (no. 10674113), Program for New Century Excellent
Talents in University (NCET-06-0707), Foundation for the Author
of National Excellent Doctoral Dissertation of China (grant no.
200726), and partially by Scientific Research Fund of Hunan Provin-
cial Education Department (no: 06A071 and 07B073).
ꢀ
ꢁ
6
1
ꢁS
R0
R
PSA(R) =
,
(1)
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4. Conclusions
By developing a nitric-decomposition method, we have pre-
pared the SrY2(1−x−y) = 2−2x−2y Yb2xEr2yO4 samples with various
Yb3+ and Er3+ doping concentration. Under 980 nm laser excita-
tion, the green and red up-conversion emissions are observed at
around 549 and 661 nm, which are attributed to the transitions of
4S3/2 → I15/2 and F9/2 → I15/2 of Er3+ ions. Their relative inten-
sity ratio increases with the increase in Yb3+ doping concentration,
and a quadratic dependence of the emission intensity on excita-
tion power is obtained at 549 and 661 nm emissions. It is shown
that a two-photon process populates the S3/2 and F9/2 levels for
the SrY2O4 powders. Especially, it is found that at lower excitation
power, Yb3+ doping concentration has an important influence on
4
4
4
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