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Optical temperature sensor based on upconversion emission in Er-doped ferroelectric
0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ceramic
Peng Du, Laihui Luo, Weiping Li, Qingying Yue, and Hongbing Chen
Citation: Applied Physics Letters 104, 152902 (2014); doi: 10.1063/1.4871378
View online: http://dx.doi.org/10.1063/1.4871378
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APPLIED PHYSICS LETTERS 104, 152902 (2014)
Optical temperature sensor based on upconversion emission in Er-doped
ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ceramic
Peng Du,1 Laihui Luo,1,a) Weiping Li,1 Qingying Yue,1 and Hongbing Chen2
1
Department of Microelectronic Science and Engineering, Ningbo University, Ningbo 315211, China
Institute of Materials Science and Engineering, Ningbo University, Ningbo 315211, China
2
(Received 21 February 2014; accepted 2 April 2014; published online 14 April 2014)
Optical temperature sensing properties based on upconversion emission of Er-doped
0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ferroelectric ceramics are reported. The fluorescence
intensity ratio of green upconversion emissions at 525 and 550 nm in the temperature range of
200–443 K was investigated. The maximum sensing sensitivity and temperature resolution were
found to be 0.0044 K1 and 0.4 K, respectively, suggesting that the Er-doped 0.5Ba(Zr0.2Ti0.8)O30.5(Ba0.7Ca0.3)TiO3 ferroelectric ceramic possesses potential application in optical temperature sensing.
Ferroelectric and piezoelectric properties were also investigated. These results reveal that the Er-doped
0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ferroelectric ceramic is a promising multifunctional sensing
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4871378]
material. V
Enormous interest over recent years in rare-earth
(RE)-doped materials has been gaining as a result of their wide
applications in bio-imaging in vivo, color displays, and temperature sensors.1–3 Among these applications, optical temperature sensors based on the fluorescence intensity ratio (FIR)
technique have drawn much attention, because this technique
can reduce the dependence on measurement conditions and
increase measurement accuracy and resolution.4,5 To avoid the
different emission overlaps and allow the upper levels of optically active ions to have a minimum population in the temperature range of interest, the energy gap between the emission
levels of RE ions should be in the range of 200–2000 cm1. It
is well known that the optical temperature sensors using the
FIR technique, which is based on upconversion (UC) emissions of RE ions, mainly focus on glass and fluoride host materials.6,7 However, there are some disadvantages with these
materials. Fluorides, for example, clearly have toxicity levels
harmful to human beings and environment. Moreover, both
glasses and fluorides have relatively poor physical, chemical,
and thermal stability. These two drawbacks limit their further
applications in temperature sensors. Therefore, it is necessary
to search for new materials to replace both.
In recent years, ferroelectrics doped with RE have
attracted great attention owing to their excellent multifunctional properties. Multi-property coupling, such as electromechano, electro-optic, and mechano-optic couplings, has
been realized in ferroelectrics.8–10 For sensing applications,
ferroelectrics are widely used in piezoelectric and infra-red
sensors.11,12 Furthermore, RE-doped lead-free ferroelectrics
were found to exhibit strong UC emissions,13–15 with intensities that are significantly dependent on temperature.16,17
Additionally, compared with glasses and fluorides, lead-free
perovskite ferroelectrics have excellent thermal and chemical stability and are environmentally friendly. Therefore,
the RE-doped lead-free ferroelectrics are expected to be
prominent in the next generation optical temperature sensors
a)
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0003-6951/2014/104(15)/152902/4/$30.00
using the FIR technique. However, investigations on
temperature-dependent sensing performances of the REdoped ferroelectrics are scant.
The 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT)
ferroelectric, which was discovered by Ren and Liu,18 is one
of the more promising lead-free ferroelectrics. This BZT-BCT
ferroelectric exhibits excellent piezoelectric properties with
d33 620 pC/N. The RE Er has also drawn much attention
because of its unique characteristic in UC emissions.15,19,20
Moreover, considering its energy level distribution, the Er ion
has a pair of thermally coupled emission levels. As a result of
these characteristics, its UC emission can be used in optical
temperature sensors that use the FIR technique.
We have reported that the RE ions occupying the
A-sites in ferroelectrics exhibit stronger UC emission intensity compared with their counterparts occupying the
B-sites.13 In the present work, the BZT-BCT:0.015Er (0.015
is mol content) ferroelectrics were prepared with Er ions
occupying the A-sites by substituting Ca and Ba ions. The
ferroelectric, piezoelectric, and optical properties were
investigated. To investigate the optical temperature sensing
properties of the BZT-BCT:0.015Er ferroelectric, the FIR
technique based on UC emission was used.
The BZT-BCT:0.015Er ferroelectrics were prepared by
a conventional solid-state reaction method. High-purity powders of CaCO3, BaCO3, TiO2, ZrO2, and Er2O3 were used as
raw materials. The powders in the stoichiometric ratio were
mixed in alcohol using agate balls for 12 h, and then dried
and calcined at 1200 C for 3 h. They were then remilled and
thoroughly mixed with polyvinyl alcohol (PVA) binder solution and pressed into pellets. Finally, the pellets were sintered at 1450 C for 4 h in air. All the sintered pellets were
ground into samples 1 mm in thickness. For performing
measurements of the electrical property, silver electrodes
were pasted on both sides of the samples.
The crystal structure of the ceramics was checked using
a X-ray diffraction (XRD) analyzer (Bruker D8 Advance)
with Cu Ka radiation. The morphology of calcined ceramics
was investigated using a scanning electron microscope
104, 152902-1
C 2014 AIP Publishing LLC
V
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Du et al.
FIG. 1. XRD pattern of BZT-BCT:0.015Er ferroelectric ceramic. The inset
is a SEM micrograph of the ceramic.
(SEM) (Hitachi SU-700). The polarization vs. electric field
(P-E) hysteresis loop was obtained at 1 Hz using a RT
Premier II ferroelectric workstation. The strain vs. electric
field (S-E) hysteresis loop was recorded using a MTI-2100
Photonic Sensor. UC emissions were recorded using a spectrofluorometer (Ocean Optics USB4000) under excitation by
a 980-nm diode laser with a fixed pump power of 1000 mW.
The temperature of the samples, ranging from 93 to 443 K,
was controlled using a temperature-controlled stage (Linkam
LNP95).
From the XRD pattern of the BZT-BCT:0.015Er ferroelectric (Fig. 1), the sample exhibits a pure perovskite structure
and no secondary impure phase is observed. The result indicated that the Er ions had diffused into the BZT-BCT host.
The SEM micrograph of the BZT-BCT:0.015Er ferroelectric
(inset of Fig. 1) indicates that the samples were densely sintered. Grain size is inhomogeneous for this BZT-BCT ceramic
but is consistent with the previous report.21 The largest grains
were 25 lm, whereas the smallest were only 5 lm.
Under 980-nm excitation, the BZT-BCT:0.015Er ferroelectric exhibits strong UC emission at room temperature
that can be easily seen by the naked eye. The UC emission
spectrum of BZT-BCT:0.015Er ferroelectric (Fig. 2) is composed of two bands, a green emission band from the mixed
(2H11/2, 4S3/2) levels to the ground state 4I15/2 located at
approximately 525 and 550 nm, and a relatively weak red
emission band from 4F9/2 level to the ground state 4I15/2
located at about 660 nm. The observed emission peaks coincides well with earlier reports.15–17
FIG. 2. UC emission spectrum of BZT-BCT:0.015Er ferroelectric ceramic
under 980-nm optical excitation at room temperature.
Appl. Phys. Lett. 104, 152902 (2014)
Under 980-nm laser excitation, electrons populating the
ground state are excited to 4I11/2 level through ground-state
absorption (GSA, see simplified energy level diagram of Er
in Fig. 3). After multiphonon relaxation (MPR), the electrons
partly decay to the 4I13/2 level. Next, the laser pumps the
electrons from 4I13/2 and 4I11/2 to 4F9/2 and 4F7/2, respectively. Subsequently, electrons located in the 4F7/2 level nonradiatively relax to the 4S3/2 and 4F9/2 levels via MPR. At the
same time, the 2H11/2 level is populated by electrons from
the 4S3/2 level through thermal agitation (TAG), as indicated
in Fig. 3. Finally, these electrons relax to the ground state
4
I15/2. In consequence, green and red emissions are observed.
The variation of green UC emissions of BZTBCT:0.015Er ferroelectric with the temperature over the
range 93 to 443 K is depicted in Fig. 4(a), where the emission
intensity has been normalized at 550 nm. Clearly, the positions of the green emission peaks do not change with temperature, whereas the relative intensity of I525 and I550 at 525
and 550 nm, respectively, increases gradually. Note that,
when the temperature is below 223 K, the green UC emission
located at 525 nm is not obvious (Fig. 4(a)). At low temperatures, populating the 2H11/2 level by TAG is difficult owing
to the low TAG energy.22 According to the UC emission
spectra, one finds the energy gap between the 2H11/2 and
4
S3/2 levels to be about 850 cm1. Because of this low energy
separation, the 2H11/2 level can be populated from 2S3/2 level
by TAG. Consequently, the relative intensity ratio of
I525/I550 increases as temperature increases. This thermal
coupling characteristic enables the BZT-BCT:0.015Er ferroelectrics to have potential applications in temperature sensing. With the thermalization of the population at the two
levels, and ignoring effects from self-absorption of emissions, the FIR of the UC emissions from the 2H11/2 ! 4I15/2
and the 4S3/2 ! 4I15/2 transitions are4,22
2
I525 Nð H11=2 Þ c1 ðvÞA1 g1 hv1 b1
DE
¼
;
¼
exp
I550
c2 ðvÞA2 g2 hv2 b2
kT
Nð4 S3=2 Þ
DE12
;
(1)
¼ C exp
kT
R¼
where I525 and I550 are the integrated intensities for 2H11/2
! 4I15/2 (500–530 nm) and 4S3/2 ! 4I15/2 (530–570 nm),
respectively. N(2H11/2) and N(4S3/2) represent the population
numbers of the 2H11/2 and 4S3/2 levels. The values of ci(v) are
FIG. 3. Energy level diagram of Er under 980-nm optical excitation and possible UC processes.
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Du et al.
Appl. Phys. Lett. 104, 152902 (2014)
FIG. 4. Temperature sensing performances of the BZT-BCT:0.015Er ferroelectric ceramic. (a) green UC emission spectra at different temperatures; (b) lognormal plot of the FIR as a function of inverse absolute temperature; (c) FIR relative to temperature; and (d) sensor sensitivity as a function of temperature.
related to the response of the detection system. Ai, gi, hvi, and
bi are the spontaneous radiative rate, the degeneracy, the photon
energy, and the branching ratio, respectively, for the transitions
from the excited 2H11/2 and 4S3/2 levels to the ground states.
DE12 is the energy gap between the 2H11/2 and 4S3/2 levels. k is
the Boltzmann constant and T is the absolute temperature.
The log plot of the FIR for the green UC emissions at 525
and 550 nm as a function of inverse absolute temperature in
the range of 223–443 K is given in Fig. 4(b). The experimental
data give a linear fit with slope 1135.5. Furthermore, the
temperature dependence of these emissions at 530 and 550 nm
in the range of 93–443 K (Fig. 4(c)) show a clear rise in FIR
value with temperature, reaching a maximum value when the
temperature approaches the maximal experiment temperature
443 K. From a curve fitting of the experimental data, one
obtains values for coefficient C and energy gap DE12 in Eq.
(1) of 9.97 and 789 cm1, respectively. The fitted 789 cm1
value is of similar order as the experiment value of 850 cm1.
To further understand the temperature response of the
BZT-BCT:0.015Er ferroelectric, it is important to investigate
the sensing sensitivity; this can be defined from4,22
dR
DE12
:
(2)
¼R kT 2
dT
The sensitivity as a function of temperature (Fig. 4(d)) is
seen to reach its maximum value of 0.0044 K1 at 443 K in
the temperature range of interest, indicating that the temperature sensing resolution for BZT-BCT:0.015Er is about 0.4 K,
obtained by employing single division circuitry with a precision of four digits or better.23
The optical temperature sensing performances of other Erdoped materials, which include oxides, fluorides, and glasses,
are compared with the present results, (Table I). In the comparison, a favorable result is obtained for BZT-BCT:0.015Er ferroelectric with an operation temperature range of 200–443 K
and a maximum sensitivity of 0.0044 K1. Note that most of
the Er-doped oxides, fluorides, and glass sensors can only be
operated at or above room temperature, which means that these
sensors can only be used in high-temperature measurements.
In contrast, the BZT-BCT:0.015Er ferroelectric can operate at
very low temperatures, hence is suitable in monitoring temperatures of cold stores and glaciers, for example. Additionally,
synthesizing the Er-doped BZT-BCT ferroelectrics is easy and
cheap compared with other RE doped materials. These characteristics make the Er-doped BZT-BCT ferroelectrics more
promising for applications in optical temperature sensors.
For piezoelectric and infra-red sensing applications, ferroelectric and piezoelectric performances of the prepared
multifunctional sensing materials require investigating. The
P-E hysteresis loop and S-E field butterfly curve of BZTBCT:0.015Er ferroelectric ceramic (Figs. 5(a) and 5(b),
respectively) give values for the remnant polarization (Pr)
and the maximum strain of approximately 13.02 lC/cm2 and
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152902-4
Du et al.
Appl. Phys. Lett. 104, 152902 (2014)
TABLE I. The comparison of optical temperature sensing properties of different Er doped materials.
RE ions: host
Er: BZT-BCT
Er: fluorotellurite glass
Er-Mo: Yb3Al5O3
Er-Yb: silicate glass
Er: BaTiO3 nanocrystal
Er-Yb: fluoride glass
Er: fluorozirconate glass
Temperature range (K)
Maximum sensitivity (K1)
Excitation wavelength (nm)
References
200–443
293–540
295–973
296–732
322–466
303–823
100–300
0.0044
0.0054
0.0048
0.0033
0.0052
0.0047
0.0006
980
800
976
978
980
980
805
This work
7
22
24
25
26
27
can be operated over a wide temperature range of
223–443 K. Compared with other Er-doped materials,
Er-doped BZT-BCT ferroelectrics have superior temperature
sensing properties. Moreover, these ferroelectrics exhibit
excellent piezoelectric and ferroelectric properties.
This work was supported by the National Natural
Science Foundation of China (No. 61378068), and the K.C.
Wong Magna Foundation in Ningbo University.
1
FIG. 5. (a) Polarization-electric field hysteresis loop; (b) bipolar strain loop
of the BZT-BCT:0.015Er ceramic at room temperature.
0.104%, respectively, indicating that this ferroelectric possesses excellent ferroelectric and piezoelectric properties
raising expectations that this ferroelectric would perform
favorable in applications in piezoelectric and infra-red
sensing.
In summary, BZT-BCT:0.015Er ferroelectric samples
were prepared by a conventional solid-sate reaction technique. The green UC emissions at 525 nm (2H11/2 ! 4I15/2)
and 550 nm (4S3/2 ! 4I15/2) were investigated under 980-nm
optical excitation in a wide temperature range from 90 to
443 K. This investigation revealed that the value of FIR for
I525/I550 increases gradually with increasing temperature, and
a maximum sensitivity for the BZT-BCT:0.015Er ceramic of
0.0044 K1 was reached. Furthermore, BZT-BCT:0.015Er
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