Upconversion fluorescence studies of sol-gel-derived Er-doped KNN ceramics

Article abstract of DOI:10.1016/j.jallcom.2013.05.096

Er-doped K0.5Na0.5NbO3 luminescent ceramics have been prepared using a sol-gel method. The results of X-ray diffraction show that Er3+ diffuses into the K_0.5Na_0.5NbO₃ lattices, forming an orthorhombic-structured perovskite. Under an excitation of 980 nm, the ceramics exhibit luminescent emission bands at 527 nm (green), 548 nm (green), 660 nm (red) as well as 487 nm (blue), which are attributed to the transitions ²H_11/2 ⁴I_15/2, ⁴S _3/2 ⁴I_15/2, ⁴F_9/2 ⁴I_15/2 and ⁴F_7/2 ⁴I _15/2, respectively. Our Raman results also reveal that the ceramics have low phonon energy (～ 860 cm^-1) and thus slow multi-phonon relaxation rate. The sol-gel process and Er³⁺-doping can produce ceramics with better compositional homogeneity as well as finer and uniform grains. All these are beneficial for improving both the upconversion and luminescent efficiencies of the ceramics, and thus leading to very strong green emissions even at a low quenching concentration of 2 mol%. These suggest that even without using sensitizers, the sol-gel-derived Er-doped KNN ceramics are promising candidates for multifunctional optoelectronic applications.

Full text of DOI:10.1016/j.jallcom.2013.05.096

Journal of Alloys and Compounds 580 (2013) 88–92
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Upconversion fluorescence studies of sol–gel-derived Er-doped KNN
ceramics
⇑
Xiao Wu, K.W. Kwok , Faliang Li
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Er-doped K0.5Na0.5NbO3 luminescent ceramics have been prepared using a sol–gel method. The results of
X-ray diffraction show that Er diffuses into the K0.5Na0.5NbO lattices, forming an orthorhombic-struc-
3
tured perovskite. Under an excitation of 980 nm, the ceramics exhibit luminescent emission bands at
Received 11 March 2013
Received in revised form 29 April 2013
Accepted 15 May 2013
3+
5
27 nm (green), 548 nm (green), 660 nm (red) as well as 487 nm (blue), which are attributed to the tran-
Available online 23 May 2013
2
4
4
4
4
9/2 ? I15/2 and 4
4
4
sitions
H
11/2 ? I15/2
,
S
3/2 ? I15/2
,
F
F
7/2 ? I15/2, respectively. Our Raman results also
ꢁ1
reveal that the ceramics have low phonon energy (ꢀ860 cm ) and thus slow multi-phonon relaxation
Keywords:
Er-doped KNN ceramics
Sol–gel process
3+
rate. The sol–gel process and Er -doping can produce ceramics with better compositional homogeneity
as well as finer and uniform grains. All these are beneficial for improving both the upconversion
and luminescent efficiencies of the ceramics, and thus leading to very strong green emissions even at
a low quenching concentration of 2 mol%. These suggest that even without using sensitizers, the
sol–gel-derived Er-doped KNN ceramics are promising candidates for multifunctional optoelectronic
applications.
Upconversion photoluminescence
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
Upconversion of infrared into visible light by lanthanide-doped
line-splitting, lift-time and luminescent efficiency. It has been
shown that compositional homogeneity, fine grains and good crys-
tallinity of the host materials are essential for enhancing the PL
intensity [5–7].
Recently, ferroelectric materials such as lead magnesium nio-
bate-lead titanate, lanthanum-modified lead zirconate titanate,
bismuth titanate, barium titanate and potassium sodium niobate
have been used as host materials to prepare lanthanide-doped PL
materials for multifunctional optoelectronic applications [8–12].
materials has attracted considerable attention because of their po-
tential applications in solid-state lasers, three-dimensional color
display, optical data storage and medical diagnostics [1–3]. Activa-
tors having metastable states for trapping the excited electrons are
required for efficient upconversion process. Either by the subse-
quent excited state absorption or cooperative energy transfer, the
electrons in the metastable states are further excited to the emit-
ting levels and then relax back to the ground states following with
fluorescence emission. Owing to the long-lifetime metastable
states in the 4f manifold, lanthanide ions have been considered
as the ideal activators for the upconversion process. Among them,
3
Potassium sodium niobate K0.5Na0.5NbO (KNN) has inherent
advantages over the other ferroelectric host materials. It has a high
Curie temperature, good piezoelectric properties and low anisot-
ropy in electromechanical coupling coefficients [13–15]. It is also
environmental friendly owing to the lead-free compositions and
low sintering temperature (ꢀ1100 °C). In the present work, Er-
doped KNN multifunctional ceramics have been prepared with
the aim of enhancing the PL intensity without using sensitizers.
A sol–gel process has been used to fabricate the ceramics. As the
precursors are mixed at the molecular level, the sol–gel process
can provide a high degree of homogeneity and effective doping
of activators. It can also produce ceramics with a dense and fine-
grained structure.
3
+
Er is one of the most popular and efficient ions for obtaining
upconversion luminescence [4]. However, owing to the relatively
low absorption cross-section of Er , Yb is normally added as a
sensitizer to enhance the pumping efficiency and then the photo-
3+
3+
3+
2
2
6
2
6
10
2 6 10
luminescence (PL) intensity. Er has a 1s 2s p 3s p d 4s p d
1
1
2 6
f
5s p electron configuration, in which the 4f electrons are
effectively shielded by the filled 5s and 5p electron shells. As a re-
sult, the wavelengths of the PL emissions that are arisen from the
electronic transitions between the partially filled 4f levels are nor-
mally not affected by the host materials. Nevertheless, the host
materials have important effects on the PL properties, such as
2. Experimental
Er-doped KNN ceramics, (K0.5Na0.5
abbreviated as KNN:Er-x), were fabricated from sol–gel-derived fine powders. A
niobium precursor solution was first prepared from Nb (99.9%) and KOH
(95%). The powders were mixed thoroughly and heated at 350 °C for 2 h to obtain
x 3
)1ꢁ3xEr NbO with x varying from 0 to 0.03
(
⇑
Corresponding author. Tel.: +852 27665667; fax: +852 23337629.
E-mail address: apkwkwok@polyu.edu.hk (K.W. Kwok).
2 5
O
0
925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.05.096

X. Wu et al. / Journal of Alloys and Compounds 580 (2013) 88–92
89
water-soluble K
with nitric acid to form a Nb(OH)
the precipitate was chelated with oxalic acid to form the niobium precursor solu-
tion. After that, stoichiometric amounts of K CO (99.9%), Na CO (99.5%) and
Er(NO O (99.9%) were weighted according to the formula of (K0.5Na0.5
ꢂ6H
NbO and dissolved in a mixture of the niobium precursor solution and dilute citric
acid. The mixture was then stirred vigorously for 4 h and heated at 120 °C for 24 h
to form a gelatin. After calcination at 800 °C for 5 h, the resulting ultrafine powders
were mixed thoroughly with a PVA binder solution and pressed into disk samples.
The disk samples were sintered at 1080 °C for 4 h.
The crystalline structure of the sintered samples was examined using an X-ray
diffraction (XRD) analysis with Cu Kb radiation (SmartLab, Rigaku Co., Japan). The PL
spectra were measured using a spectrophotometer (FLSP920, Edinburgh Instru-
ments, UK) under a 980-nm diode laser excitation. The Raman spectra were mea-
sured in backscattering geometry using the 488-nm line of an Ar ion laser
3 4
NbO , which was then dissolved in distilled water and titrated
increasing x (Fig. 1b). As also shown in Fig. 1b, the two diffraction
peaks (220) and (002) near 46° become merged together, forming
a broadened peak at x P 0.3. This suggests that the lattice constants
of the ceramics become close to each other. Similar results have
been observed for other KNN-based ceramics, e.g., La-doped KNN
and Ta-modified KNN ceramics [18,19]. A small amount of impurity
+
5
precipitate. After removal of the residual K ,
2
3
2
3
3
)
3
2
)1ꢁ3xEr
x
3
phase K5.75Nb10.85O30 (JCPDS No. 38-0297) is observed in the
3+
Er-doped ceramics. This may be due to the ‘‘deficiency’’ of Er ions.
3
+
For maintaining the charge neutrality, one Er is substituted for
three A-site ions. Moreover, because of the similar ionic radius,
3+
+
+
Er may preferentially occupy the Na sites rather than the K sites.
+
5+
K and Nb may then become slightly ‘‘excessive’’, and thus leading
to the formation of the impurity phase.
(
Spectra-Physics) as the excitation source. Spectra were recorded by a grating spec-
trometer (HORIBA JOBIN YVON, HR800). The microstructures were observed using a
scanning electron microscope (SEM) (JSM-6490, JEOL Ltd., Japan).
The upconversion PL emission spectra of the KNN:Er-x ceramics
are shown in Fig. 2. Under the excitation of 980 nm, all the Er-
doped ceramics exhibit two strong green emission bands at 548
4
3
. Results and discussion
and 527 nm (Fig. 2a), which are attributed to the transitions
S
3/2
4
2
4
?
I15/2 and H11/2 ? I15/2, respectively (Fig. 3). Clear stark-split-
3+
The XRD patterns of the KNN:Er-x ceramics are shown in Fig. 1.
ting is observed, revealing the interaction between Er and the
host material. No shifting of the split peaks with increasing x is ob-
served. The observed PL intensity remains weak and almost the
same at x 6 0.01, and then increases significantly and reaches a
maximum at x = 0.02 (Fig. 2). It has been observed that the emis-
sion intensity of the KNN:Er-0.02 ceramic is very strong, giving a
All the ceramics possess an orthorhombic perovskite structure
16], suggesting that Er3 has diffused into the KNN lattices. Proba-
+
[
bly due to the lattice shrinkage arisen from the replacement of large
+
+
3+
K (CN = 8, 1.51 Å) and Na (CN = 8, 1.18 Å) with small Er (CN = 8,
.004 Å) [17], the diffraction peaks shift toward higher angles with
1
Fig. 1. XRD patterns of the KNN:Er-x ceramics.
Fig. 2. Upconversion photoluminescence spectra of the KNN:Er-x ceramics.

9
0
X. Wu et al. / Journal of Alloys and Compounds 580 (2013) 88–92
Fig. 3. Schematic diagram for the upconversion mechanism of Er3+ ions under an
excitation of 980 nm.
Fig. 4. CIE chromaticity coordinates of the KNN:Er-x ceramics.
very bright green spot on the sample during the measurement
(Fig. 2a). Probably due to the increase in the defect density and
concentration-quenching effect [20], the PL intensity decreases at
x > 0.02. Above the critical concentration for quenching (i.e.,
3
+
2
mol% for the KNN:Er-x ceramics), the inter-distance of Er be-
comes shorter and the transfer of non-radiative energy from one
to another becomes easier, having a probability larger than that
of radiative emission. As a result, the radiative emission is
quenched and the PL intensity decreases. According to Blasse, the
c
critical distance R between the activators for energy transfer can
be estimated by [21]:
ꢀ
ꢁ
1
3
3
V
R
c
ꢃ 2
ð1Þ
C
4pX Z
where V is the volume of the unit cell, X
c
is the critical concentration
of the activator, and Z is the number of host cations in the unit cell.
3
By using V ꢃ 126.9 Å and Z = 2 for the KNN host [22] and X
observed from our samples), R is calculated to be about 18.2 Å.
As shown in the enlarged PL spectra (Fig. 2b and c), the KNN:Er-
c
= 0.02
(
c
x ceramics also exhibit two emission bands at 487 nm (blue) and
Fig. 5. Raman spectra of the KNN:Er-x ceramics excited by 488 nm.
4
4
6
and
60 nm (red), which are attributed to the transitions
F
7/2 ? I15/2
4
4
F9/2 ? I15/2, respectively (Fig. 3). Unlike the green emission
4
bands, the intensities of these two bands are low, in particular
for the blue emission band that has rarely been reported for the
Er-doped systems. As illustrated in Fig. 3, the upconversion process
On the other hand, due to the large band gap between S3/2 and
4F9/2, there is only a small amount of Er in the F9/2 level, and thus
3+
4
leading to the weak emission band at 660 nm for the KNN:Er-x
ceramics with x 6 0.01. As x increases, the inter-distance of Er de-
creases and the ET process becomes more effective. For example,
involves the excitation of Er3 in the I15/2 ground level to the F7/2
level, via the ground state absorption (GSA) following by excited
state absorption (ESA) or cooperative energy transfer (ET). Due to
+
4
4
3+
3
+
4
7/2 level may interact with those in the 4
Er in the
F
I
11/2 level,
3
+
4
inducing an ET process between the two transitions 4F7/2 ? F9/2
4
the small energy gaps, the excited Er in the F7/2 level will decay
2
11/2 and then 4
4
4
3+
4
non-radiatively to the
H
S
3/2 level very quickly by
and I11/2 ? F9/2. As a result, the population of Er in the F9/2 le-
vel increases and the red emission at 660 nm is enhanced. The re-
sults also suggest that the large increase in PL intensity of the green
bands at x = 0.02 should be partly attributed to the effective energy
multi-phonon relaxation. As a result, the probability for the excited
Er relaxing radiatively from the 4
3
+
F
7/2 level directly to the I
15/2
4
ground level becomes very low. The corresponding emission is
then very weak and hard to be observed unless the population of
3
+
transfer between different Er . Owing to the enhancement of the
red emission, the Commission Internationale de L’Eclairage (CIE)
chromaticity coordinates (X, Y) of the KNN:Er-x ceramics show a
red shift (Fig. 4), changing from (0.2402, 0.7355) at x = 0.005 to
(0.3443, 0.6245) at x = 0.03. Nevertheless, the color remains as yel-
lowish green because of the strong green emission.
3
+
4
Er in the F7/2 level is very large or the overall PL intensity is very
strong. Accordingly, the observation of the blue emission band in
this work attests the strong PL intensity of the KNN:Er-x ceramics.
It should be noted that the quenching concentration of Er3 for pro-
ducing such a strong PL intensity is only 2 mol%, which is lower
than those for the other Er-doped ferroelectrics, e.g. 3 mol% for
+
The Raman spectra of the KNN:Er-x ceramics are shown in Fig. 5.
ꢁ1
Er-doped KNbO
films [24], and 5 mol% for Er-doped 0.75Pb(Mg1/3Nb2/3)O
.25PbTiO
KNN:Er-x ceramics have a higher pumping efficiency, which should
3
polycrystalline [23], 3 mol% for Er-doped BaTiO
3
All the ceramics exhibits three main scattering peaks at 258 cm
ꢁ1
ꢁ1
3
–
(
m
5
), 614 cm
due to the similar ionic radius of Er and Na , the scattering peak
does not shift towards higher frequencies [25]. Moreover, no
splitting of the coupled peak ( ) is observed, suggesting that
(
m
1
) and 860 cm
1 5
(m + m ), respectively. Probably
3
+
+
0
3
transparent ceramics [8]. This suggests that the
m
1
be partly due to the uniform distribution of Er3
+
.
1 5
m + m

X. Wu et al. / Journal of Alloys and Compounds 580 (2013) 88–92
91
4
27 cmꢁ1, respectively (Fig. 5). The former is associated with the
translation vibration modes of the cations [26], while the latter is
2
4
arisen from the H11/2 ? I15/2 hypersensitive transition (DJ = 2) of
the Er ions rather than the Raman shifts [27].
As also shown in Fig. 5, the maximum phonon energy for the
KNN:Er-x ceramics is 860 cm , which is lower than those of the
commonly used PL materials such as borate (1400 cm ), phos-
3
+
ꢁ1
ꢁ1
ꢁ1
ꢁ1
phate (1200 cm ), silicate (1100 cm ), and germinate
ꢁ1
(
900 cm ) [28]. Host materials with low phonon energy are gen-
erally desirable for achieving efficient upconversion luminescence
29,30]. Accordingly to the Miyakawa–Dexter theory, the multi-
[
phonon relaxation rate W
¼ W exp ðꢁ E=ꢀh
where W and
p
is given as
W
p
o
aD
x
max
Þ
ð2Þ
o
a
are constants dependent on the host,
D
E is the en-
ergy gap to the next lower energy level and ꢀh xmax is the maximum
Fig. 6. Upconversion photoluminescence spectra of the KNN:Er-0.02 ceramics
prepared by the sol–gel and conventional mixed-oxide method.
phonon energy of the host. Although phonons with other energies
can be emitted in the relaxation process, the one emitting the few-
est number of phonons (i.e., the smallest
nate, and hence ꢀh max is used in Eq. (2). As illustrated by Eq. (2),
a lower hꢀ max leads to a slower W , and thus increasing the popu-
lations of some excited levels and then the probability of radiative
emission from those excited levels, e.g. the emission at 487 nm in
the KNN:Er-x ceramics. The strong PL intensity observed at other
wavelengths (Fig. 2) should also be partly due to the low maximum
phonon energy.
Fig. 6 compares the PL intensities of the KNN:Er-0.02 ceramics
prepared by the sol–gel and conventional mixed-oxide method. It
is clearly seen that the observed PL intensity for the mixed-
oxide-derived ceramic is much lower. It should be noted that the
experimental conditions, e.g., the power of the diode laser, were
kept the same for all the PL measurements. The difference in PL
intensities, which is also demonstrated by the green spots
DE/ ꢀh xmax) will predomi-
x
the secondary phase detected in XRD tests is of insignificant amount
x
p
14
[
26]. Belonging to the Amm2-C2 space group, KNN has Raman-ac-
t
tive optical modes of 4A
tional modes of cations and internal modes of NbO
Among the vibration modes of octahedra, A1g ), E
) are the stretching mode, whereas F1u ), F2g
1
+ 4B
1
+ 3B
2
+ A
2
that comprising transla-
octahedra.
) and F1u
) and F2u
6
(m
1
g 2
(m
(m
3
(m
4
(m
5
6
(m )
are the bending mode. It has been shown that a substitution of A-
site ions with a smaller (lighter) ion will shorten the distance be-
5
+
tween Nb and its coordinated oxygen atoms, causing an increase
in the binding strength and then a shift of the stretching mode
to higher frequencies [25]. On the other hand, impurity phases such
as K Nb will induce a splitting of the coupled peak ( ) [26].
Two weak scattering peaks are also observed at 147 cm
m
1
4
6
O
7
1 5
m + m
ꢁ
1
and
Fig. 7. SEM micrographs of the KNN:Er-x ceramics, (a) sol–gel, x = 0, (b) sol–gel, x = 0.01, (c) sol–gel, x = 0.02, and (d) mixed-oxide method, x = 0.02.

9
2
X. Wu et al. / Journal of Alloys and Compounds 580 (2013) 88–92
observed on the two ceramics during the measurements (inset of
Fig. 6), should be partly attributed to the better compositional
homogeneity of the host material and dopants in the sol–gel-de-
rived ceramics. The SEM micrographs of the sol–gel-derived and
mixed-oxide-derived KNN:Er-x ceramics are shown in Fig. 7. All
the ceramics possess a dense structure. As shown in Fig. 7a–c,
the grain growth of the ceramics is suppressed effectively by the
08E), the Research Committee (A/C code A-PK78) and the Centre
for Smart Materials of The Hong Kong Polytechnic University.
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2
4
4
4
which are attributed to the transitions H11/2 ? I15/2, S3/2 ? I15/2
4
4
and F9/2 ? I15/2, respectively. Owing to the slower multi-phonon
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and finer grains resulted from the sol–gel process, both the upcon-
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4
4
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This work was supported by the Research Grants Council of the
Hong Kong Special Administrative Region (Project No. PolyU 5194/