- Email: [email protected]
Yb3+: Improvement by Yb3+ codoping and temperature sensitivity for optical temperature sensors
Accepted Manuscript Title: Upconversion of SrWO4 :Er3+ /Yb3+ : improvement by Yb3+ codoping and temperature sensitivity for optical temperature sensors Author: Zhen Wei Wei Zheng Zhiyong Zhu Xiongfei Guo PII: DOI: Reference:
S0009-2614(16)30115-4 http://dx.doi.org/doi:10.1016/j.cplett.2016.03.006 CPLETT 33692
To appear in: Received date: Revised date: Accepted date:
5-2-2016 1-3-2016 3-3-2016
Please cite this article as: Z. Wei, W. Zheng, Z. Zhu, X. Guo, Upconversion of SrWO4 :Er3+ /Yb3+ : improvement by Yb3+ codoping and temperature sensitivity for optical temperature sensors, Chem. Phys. Lett. (2016), http://dx.doi.org/10.1016/j.cplett.2016.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The successful codoping of Er2+ and Yb3+ ions into SrWO4 Enhancement of green emission intensities by the Yb3+ ions codoping Obvious temperature-dependence of green emission on temperature
Ac ce p
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an
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The sensitivity value of 0.01282 K-1 at 489 K for SrWO4:1%Er3+/6%Yb3+
1
Page 1 of 20
Upconversion of SrWO4:Er3+/Yb3+: improvement by Yb3+ codoping and temperature sensitivity for optical temperature sensors Zhen Wei a, , Wei Zheng b, Zhiyong Zhu c, Xiongfei Guo d
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a. Department of Chemistry, College of Science, Hebei North University,
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Zhangjiakou 075000, China
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b. Department of Physics, College of Science, Hebei North University, Zhangjiakou 075000, China
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c. College of Foreign Languages, Hebei North University, Zhangjiakou 075000, China
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d. The Nineteenth Middle School of Zhangjiakou, Zhangjiakou 075000, China Abstract: SrWO4:Er3+/Yb3+ phosphors are synthesized by a solid state reaction. The
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XRD patterns indicate that the doping ions will not change the phase of SrWO4.
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Under the excitation at 980 nm, SrWO4:Er3+/Yb3+ phosphors show emission bands in
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green and red regions. The temperature-dependence of upconversion efficiency and temperature-sensing properties of SrWO4:Er3+/Yb3+ phosphors have been discussed
according to the fluorescence intensity ratio of green emissions from 2H11/2/4S3/2 → 4
I15/2 transitions of Er3+ in the range of 95–775 K under the excitation at 980 nm. The
maximum sensitivity of SrWO4:1%Er3+/6%Yb3+ phosphor is found to be 0.01282 K-1 at 489 K. Keywords: SrWO4:Er3+/Yb3+; Upconversiton; Sensor sensitivity.
1. Introduction
[email protected] (Corresponding author: Zhen Wei) 2
Page 2 of 20
Contact temperature measurement is one of effective methods to study some phenomena that change with temperature. However, it is difficult to measure the temperature directly in some fields, such as high-voltage power stations, coal mines,
ip t
and volcanic and corrosive circumstances [1-4]. On the basis of monitoring emission
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changes of phosphors induced by temperature variation when they interact with
us
physical systems, optical temperature sensing is a promising method to reach the contactless measurement and large-scale imaging [5]. As a result, many efforts have
an
been done to design optical temperature sensors, in which optical thermometry based on the fluorescence intensity ratio (FIR) technique using temperature-dependent
M
upconversion luminescence intensities from two thermally coupled energy levels of coupled energy levels of rare earth ions has attracted more attention.
te
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Generally, in these systems, phosphors are composed of a host and some rare earth ions and they emit lights with different wavelengths dependent on the ion
Ac ce p
selection and excitation parameters. Among various ions, Er3+ ion is one of widely
used ions due to its two intense green emission bands originating from the 2H11/2 → 4
I15/2 and 2S3/2 → 4I15/2 transitions, and Yb3+ ion is often codoped with Er3+ since it has
an intense broad absorption in the range of 850-1050 nm [6-12]. In the luminescence process, the adjacent 2H11/2 and 2S3/2 levels of Er3+ ions can be populated and depopulated thermally through changing the environmental temperature around the phosphors. These adjacent energy levels can be called thermally coupled energy levels (TCL). The intensity ratio of the emission originating from these two levels will change regularly with the temperature variation. Moreover, the effective energy 3
Page 3 of 20
transfer from Yb3+ to Er3+ will occur under excitation from a commercial 980 nm near-infrared laser diode, which highly enhances the intensity of Er3+ emission and thus improves the sensitivity. Therefore, Er3+/Yb3+ codoped phosphors are regarded as
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an ideal choice for detection of temperature in optical temperature sensors.
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SrWO4 belongs to a body-centered tetragonal system with scheelite crystal
structure where WO42- molecular ions are loosely bound to Sr2+ cations, and its space
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group is denoted by C4h1 [13]. It has been reported that SrWO4 is an ideal host for rare
an
earth ions [8, 14-19]. In this work, the influence of codoping Yb3+ concentration and temperature on the upconversion emission of Er3+ is evaluated. Comparing with
2. Materials and methods
d
wider temperature range is given.
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earlier report [19], the sensitivity of Er3+ upconversion emission to temperature in a
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SrWO4:Er3+/Yb3+ phosphors were synthesized through a solid state reaction.
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SrCO3 (strontium carbonate, 99.95%), WO3 (tungstate trioxide, 99.9%), Er2O3 (erbium oxide, 99.99%) and Yb2O3 (ytterbium oxide, 99.99%) were used as raw
materials. All of these materials were purchased from Aladdin and used directly without further purification. In a typical synthesis, the stoichiometric amounts of raw materials were weighted and mixed in an agate mortar by grinding. Then, the mixture was transferred into a corundum crucible and precalcined at 400 ºC for 3 h in air. Finally, the precalcined powders were regrinded and calcined again at 950 ºC for 4 h in air. After the system being cooled to room temperature naturally, the product was collected and regrinded. For comparison, the series of SrWO4:1mol%Er3+/xmol%Yb3+ 4
Page 4 of 20
(x=0, 2, 4, 6, 8) phosphors were synthesized. The phase purity of the prepared phosphors was carried out by a Rigaku D/max-IIIA X-ray Diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and
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30 mA. The upconversion spectra at room temperature were recorded by a
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BLK-CXRSR spectrofluorometer under the excitation from a 980 nm laser diode. The
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data were collected by a lens-coupled monochromator of 1 nm spectral resolution with an attached photomultiplier tube. The uperconversion spectra at different
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temperatures were measured by an Ocean Optics spectrofluorometer (USB 4000, USA) and the temperature ranging from 95 to 775 K was controlled by a
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temperature-controlled stage (Linkam HFS600E-PB2, UK). 3. Result and discussion
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The phase of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6, 8) phosphors are confirmed
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by the XRD. Fig.1 shows the XRD patterns of SrWO4:Er3+/Yb3+ phosphors. Clearly,
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all phosphors can be indexed to the scheelitr-type tetragonal structure with space group of I41/a, which is well in agreement with the JCPDS card No. 08-0490. This suggests that SrWO4:Er3+/Yb3+ phosphors are synthesized successfully under the
synthesis condition. No extra peaks rather than the tetragonal SrWO4 phase were
observed in the XRD patterns, suggesting that Er3+ and Yb3+ ions were incorporated into the SrWO4 matrix and the doping concentration of Er3+ and Yb3+ has no influence
on the phase of the phosphors. Because of the small ionic radius of W6+ (0.421 Å) and the similar ionic radii of Sr2+ (1.130 Å), Er3+ (1.004 Å) and Yb2+ (0.985 Å) ions, Er3+ and Yb2+ ions will substitute the sites of Sr2+ ions in SrWO4 matrix. However, the 5
Page 5 of 20
substitution will induce the formation of Sr2+ site vacancy because of the different oxidation states. The formation of Sr2+ site vacancy can be described as 3Sr2+ → 2Er3+ + VSr and 3Sr2+ → 2Yb3+ + VSr, where VSr is a Sr2+ site vacancy. The similar
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phenomenon has been reported in Eu3+ doped CaWO4 materials [20]. Fig.2 shows the
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SEM images of SrWO4:1%Er3+ (Fig.2A) and SrWO4:1%Er3+/8%Yb3+ (Fig.2B)
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phosphors. It can be seen that phosphors containing irregular grains are obtained by the solid state synthesis. The results also show that the obtained phosphors have good
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crystallinity.
Fig.3 shows upconversion spectra of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6 and
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8) phosphors at room temperature. Under the excitation at 980 nm and pump power of 320 mW, SrWO4:1%Er3+/x%Yb3+ (x= 0, 2, 4, 6 and 8) phosphors show three emission
H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F7/2 → 4I15/2 transitions of Er3+ ions. Herein, Yb3+
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2
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bands in green (529 and 551 nm) and red (656 nm) regions, which originate from
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ion plays a role of sensitizer to increase the green emission intensity through the energy transfer 2F5/2 (Yb3+) + 4I15/2 (Er3+) → 2F7/2 (Yb3+) + 4I11/2 (Er3+) and 2F5/2 (Yb3+)
+ 4I11/2 (Er3+) → 2F7/2 (Yb3+) + 4I7/2 (Er3+). The intensities of two green emission bands increase gradually with increasing Yb3+ concentrations, and reach a maximum at x=6
due to efficient energy transfer from Yb3+ to Er3+. But they decrease if the Yb3+ concentration is further increased up to 8mol%, which is induced by the concentration quenching effect [21]. The distance between the doping ions decreases with the increase of Yb3+ concentration, which results in the nonradiative energy transfer between nearest Er3+ and Yb3+ ions. As a result, concentration quenching occurs. 6
Page 6 of 20
Moreover, the energy back transfer, namely 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2
F5/2 (Yb3+) , also plays a role for the decrease of emission intensity [22]. It is well known that the logarithmic dependence of the pump power versus
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upconversion emission intensity plot gives the number of required pump photons ( n ),
I Pn
[23].
The
intensities
of
three
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as the UC emission intensity ( I ) and pump power ( P ) is related by formula of upconversion
emissions
for
shows
the
log-log
plot
of
intensity
versus
pump
power
for
an
Fig.4
us
SrWO4:1%Er3+/6%Yb3+ have been measured as a function of the excitation power.
SrWO4:1%Er3+/6%Yb3+. The slopes of the fitted lines (n) are 1.96, 1.58 and 1.62
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respectively for emissions originating from the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F7/2 → 4I15/2 transitions of Er3+ ions. These values of n suggest that two photons are
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responsible for populating 2H11/2, 4S3/2 and 4F7/2 states. The energy level diagrams of
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Er3+ and Yb3+ ions as well as the upconversion mechanism are shown in Fig.5. Firstly,
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Yb3+ is excited to 2F2/5 state by absorbing a near infrared photon, and then energy transfer occurs from Yb3+ to Er3+ populating the 4I11/2 level. Besides, Er3+ ion can also
absorb a 980 nm photon directly from the laser. Another energy transfer between Yb3+ and Er3+ can populate the 4F7/2 level of Er3+ ion. Then Er3+ ions relax nonradiatively to 2
H11/2 and 4S3/2 levels. When 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions
occur, bright green light can be observed. As for red light emission, it comes from 4
F9/2 → 4I15/2 transition. There are two pathways for the population of 4F9/2 level. One
is the relaxation of 2F7/2 and the other is the excitation from 4I13/2 by the energy transfer of Yb3+ or absorption of a 980 nm photon. 7
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In order to investigate the optical temperature sensing properties, the temperature-dependence of emission intensities for green emission has been measured. Fig.6 shows the normalized emission spectra for SrWO4:1%Er3+/6%Yb3+ at different
H11/2 and 4S3/2 levels, the 2H11/2 level can be populated easily from the 4S3/2 level by
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2
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temperatures in the range of 95-775 K. Owing to the small energy separation between
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thermal agitation, resulting in the variation of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions at elevated temperature. At lower temperature, the integrated intensity of
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emission band originating from 4S3/2 → 4I15/2 transitions is higher than that of emission band originating from 2H11/2 → 4I15/2 transitions. However, the character is
2
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reversed at higher temperatures. The reverse is because a fact that the population of H11/2 level from 4S3/2 level through thermal excitation is difficult at low temperature.
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As a result, the relative intensity ratios of two normalized emission bands increase dramatically with the increasing temperatures from 95 to 775 K. Moreover, it should
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be noted that there is a blue shift for the emission band originating from the 2H11/2 → 4
I15/2 transitions. The slight blue shift can be ascribed to the thermally active phonon
assisted excitation from a lower energy sublevel to a high energy sublevel in the excited state of Er3+ ions [24, 25]. The energy gap ΔE between 2H11/2 and 4S3/2 is about 780 cm-1, which locates in
the range of 200 cm-1 < ΔE < 2000 cm-1 [26]. That is to say, this energy gap matches well with the requirement of the thermally coupled levels for FIR-based optical temperature sensors. The relative population of the two thermally coupled multiplets fits a Boltzmann distribution law and the relative population of the thermally coupled 8
Page 8 of 20
energy levels can be expressed as
, where IH and IS
are integrated intensities of emissions originating from 2H11/2 → 4I15/2 and 4S3/2 → I15/2 transitions of Er3+ ions, KB is the Boltzmann constant, ΔE is the energy gap
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4
between 2H11/2 and 4S3/2 levels, A is a proportionality constant, and T is the absolute
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temperature [27]. Sensitivity is a crucial parameter for optical temperature sensors,
sensitivity
can
be
calculated
by
the
formula
of
an
The
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and thus it is necessary to investigate the variation of sensitivity with temperature.
, where S is the temperature-dependent
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sensitivity of phosphor and other terms have their usual meaning as mentioned above. The calculated values of S are plotted as a function of absolute temperature, as shown
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in Fig.7. It can be seen that the sensitivity of SrWO4:1%Er3+/6%Yb3+ phosphor increases gradually with the increase of temperature and it reaches the maximum of
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0.01282 K-1 at 489 K. That is to say, the considered material is promising for temperature sensing application with very good sensor sensitivity. 4. Conclusion
In summary, SrWO4:Er3+/Yb3+ phosphors have been synthesized successfully by
a solid state reaction and these phosphors show intense emission in green region and high sensitivity to temperature. All of SrWO4:Er3+/Yb3+ phosphors crystallize in pure
tetragonal phase, suggesting the doping ions are not affected the host phase state. Several upconversion emission bands have been detected and assigned via a two-photon process. The green emissions originating from 2H11/2 → 4I15/2 and 4S3/2 → 9
Page 9 of 20
4
I15/2 transitions of Er3+ have been used for optical thermometry via the fluorescence
intensity ratio technique. The results indicate a potential of the phosphors for using in temperature sensor with a highest sensor sensitivity of 0.01282 K-1 at 489 K.
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Acknowledgement
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This work is supported financially by the Science and Technology Research and
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Development Programs of Hebei Province, Zhangjiakou City (No. 1411070B, No. 1511075B and No. 1411058I-23), Science and Technology Department of Hebei
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Province Project (No. 16961301D), the Youth Fund of Natural Science of Hebei North University (No. Q2014006), the Major Project of Hebei North University (No.
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ZD201304), Hebei Provincial Education Department Youth Fund Projects (No. QN2015148 and No. QN2015007) and the Project of Hebei Planning Office of
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Ac ce p
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7. P. Du, L. Luo, J.S. Su, Curr. Appl. Phys. 15 (2015) 1576. 8. K. Song, H. Chen, Supperlattice. Microst. 85 (2015) 842. 9. Y. Tang, X. Yang, Y. Xu, J. Mater. Sci.: Mater. Electron. 26 (2015) 2311.
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10. C.S. Lim, Mater. Res. Bull. 75 (2016) 211.
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16. H. Feng, Y. Yang, X. Wang, Ceram. Int. 40 (2014) 10115. 17. J. Zhang, Y. Wang, Z. Zhai, G. Chen, Opt. Mater. 38 (2014) 126. 18. P.F.S. Pereira, I.C. Nogueira, E. Longo, E.J. Nassar, I.L.V. Rosa, L.S. Cavalcante, J. Rare Earth. 33 (2015) 113.
19. A. Pandey, V.K. Rai, Vinod Kumar, Vijay Kumar, H.C. Swart, Sensor. Actuat. B-Chem. 209 (2015) 352. 20. G. Chen, F. Wang, W. Ji, Y. Liu, X. Zhang, Supperlattice. Microst. 90 (2016) 30. 21. J.H. Chung, J.H. Ryu, J.W. Eun, J.H. Lee, S.Y. Lee, Y.H. Heo, B.G. Choi, K.B. Shim, J. Alloys Compd. 552 (2012) 30. 11
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22. D. He, C. Guo, S. Jiang, N. Zhang, C. Duan, M. Yin, T. Li, RSC Adv. 5 (2015) 1385. 23. J. Zhang, Z. Zhai, G. Chen, J. Alloys Compd. 610 (2014) 409.
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24. W.W. Wu, Z.G. Xia, RSC Adv. 3 (2013) 6051-6057.
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25. Y. Liu, A. Lan, Y. Jin, G. Chen, X. Zhang, Opt. Mater. 40 (2015) 122.
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26. D. He, C. Guo, S. Jiang, D.K. Agrawal, T. Li, RSC Adv, 4 (2014) 6391.
27. M.D. Shinn, W.A. Sibley, M.G. Drexhage, R.N. Brown, Phys. Rev. B 27 (1983)
M
an
6635.
Figure captions
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Fig.1 XRD patterns of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6, 8) phosphors
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Fig.2 SEM images of SrWO4:1%Er3+ (A) and SrWO4:1%Er3+/8%Yb3+
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Fig.3 Upconversion spectra of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6, 8) phosphors
Fig.4 Log-log plot of the upconversion emission intensity of SrWO4:1%Er3+/6%Yb3+ as a function of pumping power Fig.5 Energy level diagram of the Er3+ and Yb3+ ions and the proposed upconversion
mechanisms in SrWO4:1%Er3+/6%Yb3+ Fig.6
Temperature-dependent
green
upconversion
emission
spectra
of
SrWO4:1%Er3+/6%Yb3+ under 980 nm excitation Fig.7 Sensitivity of SrWO4:1%Er3+/6%Yb3+ as a function of temperature
12
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S0009-2614(16)30115-4 http://dx.doi.org/doi:10.1016/j.cplett.2016.03.006 CPLETT 33692
To appear in: Received date: Revised date: Accepted date:
5-2-2016 1-3-2016 3-3-2016
Please cite this article as: Z. Wei, W. Zheng, Z. Zhu, X. Guo, Upconversion of SrWO4 :Er3+ /Yb3+ : improvement by Yb3+ codoping and temperature sensitivity for optical temperature sensors, Chem. Phys. Lett. (2016), http://dx.doi.org/10.1016/j.cplett.2016.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The successful codoping of Er2+ and Yb3+ ions into SrWO4 Enhancement of green emission intensities by the Yb3+ ions codoping Obvious temperature-dependence of green emission on temperature
Ac ce p
te
d
M
an
us
cr
ip t
The sensitivity value of 0.01282 K-1 at 489 K for SrWO4:1%Er3+/6%Yb3+
1
Page 1 of 20
Upconversion of SrWO4:Er3+/Yb3+: improvement by Yb3+ codoping and temperature sensitivity for optical temperature sensors Zhen Wei a, , Wei Zheng b, Zhiyong Zhu c, Xiongfei Guo d
ip t
a. Department of Chemistry, College of Science, Hebei North University,
cr
Zhangjiakou 075000, China
us
b. Department of Physics, College of Science, Hebei North University, Zhangjiakou 075000, China
an
c. College of Foreign Languages, Hebei North University, Zhangjiakou 075000, China
M
d. The Nineteenth Middle School of Zhangjiakou, Zhangjiakou 075000, China Abstract: SrWO4:Er3+/Yb3+ phosphors are synthesized by a solid state reaction. The
d
XRD patterns indicate that the doping ions will not change the phase of SrWO4.
te
Under the excitation at 980 nm, SrWO4:Er3+/Yb3+ phosphors show emission bands in
Ac ce p
green and red regions. The temperature-dependence of upconversion efficiency and temperature-sensing properties of SrWO4:Er3+/Yb3+ phosphors have been discussed
according to the fluorescence intensity ratio of green emissions from 2H11/2/4S3/2 → 4
I15/2 transitions of Er3+ in the range of 95–775 K under the excitation at 980 nm. The
maximum sensitivity of SrWO4:1%Er3+/6%Yb3+ phosphor is found to be 0.01282 K-1 at 489 K. Keywords: SrWO4:Er3+/Yb3+; Upconversiton; Sensor sensitivity.
1. Introduction
[email protected] (Corresponding author: Zhen Wei) 2
Page 2 of 20
Contact temperature measurement is one of effective methods to study some phenomena that change with temperature. However, it is difficult to measure the temperature directly in some fields, such as high-voltage power stations, coal mines,
ip t
and volcanic and corrosive circumstances [1-4]. On the basis of monitoring emission
cr
changes of phosphors induced by temperature variation when they interact with
us
physical systems, optical temperature sensing is a promising method to reach the contactless measurement and large-scale imaging [5]. As a result, many efforts have
an
been done to design optical temperature sensors, in which optical thermometry based on the fluorescence intensity ratio (FIR) technique using temperature-dependent
M
upconversion luminescence intensities from two thermally coupled energy levels of coupled energy levels of rare earth ions has attracted more attention.
te
d
Generally, in these systems, phosphors are composed of a host and some rare earth ions and they emit lights with different wavelengths dependent on the ion
Ac ce p
selection and excitation parameters. Among various ions, Er3+ ion is one of widely
used ions due to its two intense green emission bands originating from the 2H11/2 → 4
I15/2 and 2S3/2 → 4I15/2 transitions, and Yb3+ ion is often codoped with Er3+ since it has
an intense broad absorption in the range of 850-1050 nm [6-12]. In the luminescence process, the adjacent 2H11/2 and 2S3/2 levels of Er3+ ions can be populated and depopulated thermally through changing the environmental temperature around the phosphors. These adjacent energy levels can be called thermally coupled energy levels (TCL). The intensity ratio of the emission originating from these two levels will change regularly with the temperature variation. Moreover, the effective energy 3
Page 3 of 20
transfer from Yb3+ to Er3+ will occur under excitation from a commercial 980 nm near-infrared laser diode, which highly enhances the intensity of Er3+ emission and thus improves the sensitivity. Therefore, Er3+/Yb3+ codoped phosphors are regarded as
ip t
an ideal choice for detection of temperature in optical temperature sensors.
cr
SrWO4 belongs to a body-centered tetragonal system with scheelite crystal
structure where WO42- molecular ions are loosely bound to Sr2+ cations, and its space
us
group is denoted by C4h1 [13]. It has been reported that SrWO4 is an ideal host for rare
an
earth ions [8, 14-19]. In this work, the influence of codoping Yb3+ concentration and temperature on the upconversion emission of Er3+ is evaluated. Comparing with
2. Materials and methods
d
wider temperature range is given.
M
earlier report [19], the sensitivity of Er3+ upconversion emission to temperature in a
te
SrWO4:Er3+/Yb3+ phosphors were synthesized through a solid state reaction.
Ac ce p
SrCO3 (strontium carbonate, 99.95%), WO3 (tungstate trioxide, 99.9%), Er2O3 (erbium oxide, 99.99%) and Yb2O3 (ytterbium oxide, 99.99%) were used as raw
materials. All of these materials were purchased from Aladdin and used directly without further purification. In a typical synthesis, the stoichiometric amounts of raw materials were weighted and mixed in an agate mortar by grinding. Then, the mixture was transferred into a corundum crucible and precalcined at 400 ºC for 3 h in air. Finally, the precalcined powders were regrinded and calcined again at 950 ºC for 4 h in air. After the system being cooled to room temperature naturally, the product was collected and regrinded. For comparison, the series of SrWO4:1mol%Er3+/xmol%Yb3+ 4
Page 4 of 20
(x=0, 2, 4, 6, 8) phosphors were synthesized. The phase purity of the prepared phosphors was carried out by a Rigaku D/max-IIIA X-ray Diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and
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30 mA. The upconversion spectra at room temperature were recorded by a
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BLK-CXRSR spectrofluorometer under the excitation from a 980 nm laser diode. The
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data were collected by a lens-coupled monochromator of 1 nm spectral resolution with an attached photomultiplier tube. The uperconversion spectra at different
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temperatures were measured by an Ocean Optics spectrofluorometer (USB 4000, USA) and the temperature ranging from 95 to 775 K was controlled by a
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temperature-controlled stage (Linkam HFS600E-PB2, UK). 3. Result and discussion
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The phase of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6, 8) phosphors are confirmed
te
by the XRD. Fig.1 shows the XRD patterns of SrWO4:Er3+/Yb3+ phosphors. Clearly,
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all phosphors can be indexed to the scheelitr-type tetragonal structure with space group of I41/a, which is well in agreement with the JCPDS card No. 08-0490. This suggests that SrWO4:Er3+/Yb3+ phosphors are synthesized successfully under the
synthesis condition. No extra peaks rather than the tetragonal SrWO4 phase were
observed in the XRD patterns, suggesting that Er3+ and Yb3+ ions were incorporated into the SrWO4 matrix and the doping concentration of Er3+ and Yb3+ has no influence
on the phase of the phosphors. Because of the small ionic radius of W6+ (0.421 Å) and the similar ionic radii of Sr2+ (1.130 Å), Er3+ (1.004 Å) and Yb2+ (0.985 Å) ions, Er3+ and Yb2+ ions will substitute the sites of Sr2+ ions in SrWO4 matrix. However, the 5
Page 5 of 20
substitution will induce the formation of Sr2+ site vacancy because of the different oxidation states. The formation of Sr2+ site vacancy can be described as 3Sr2+ → 2Er3+ + VSr and 3Sr2+ → 2Yb3+ + VSr, where VSr is a Sr2+ site vacancy. The similar
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phenomenon has been reported in Eu3+ doped CaWO4 materials [20]. Fig.2 shows the
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SEM images of SrWO4:1%Er3+ (Fig.2A) and SrWO4:1%Er3+/8%Yb3+ (Fig.2B)
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phosphors. It can be seen that phosphors containing irregular grains are obtained by the solid state synthesis. The results also show that the obtained phosphors have good
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crystallinity.
Fig.3 shows upconversion spectra of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6 and
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8) phosphors at room temperature. Under the excitation at 980 nm and pump power of 320 mW, SrWO4:1%Er3+/x%Yb3+ (x= 0, 2, 4, 6 and 8) phosphors show three emission
H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F7/2 → 4I15/2 transitions of Er3+ ions. Herein, Yb3+
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2
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bands in green (529 and 551 nm) and red (656 nm) regions, which originate from
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ion plays a role of sensitizer to increase the green emission intensity through the energy transfer 2F5/2 (Yb3+) + 4I15/2 (Er3+) → 2F7/2 (Yb3+) + 4I11/2 (Er3+) and 2F5/2 (Yb3+)
+ 4I11/2 (Er3+) → 2F7/2 (Yb3+) + 4I7/2 (Er3+). The intensities of two green emission bands increase gradually with increasing Yb3+ concentrations, and reach a maximum at x=6
due to efficient energy transfer from Yb3+ to Er3+. But they decrease if the Yb3+ concentration is further increased up to 8mol%, which is induced by the concentration quenching effect [21]. The distance between the doping ions decreases with the increase of Yb3+ concentration, which results in the nonradiative energy transfer between nearest Er3+ and Yb3+ ions. As a result, concentration quenching occurs. 6
Page 6 of 20
Moreover, the energy back transfer, namely 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2
F5/2 (Yb3+) , also plays a role for the decrease of emission intensity [22]. It is well known that the logarithmic dependence of the pump power versus
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upconversion emission intensity plot gives the number of required pump photons ( n ),
I Pn
[23].
The
intensities
of
three
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as the UC emission intensity ( I ) and pump power ( P ) is related by formula of upconversion
emissions
for
shows
the
log-log
plot
of
intensity
versus
pump
power
for
an
Fig.4
us
SrWO4:1%Er3+/6%Yb3+ have been measured as a function of the excitation power.
SrWO4:1%Er3+/6%Yb3+. The slopes of the fitted lines (n) are 1.96, 1.58 and 1.62
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respectively for emissions originating from the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F7/2 → 4I15/2 transitions of Er3+ ions. These values of n suggest that two photons are
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responsible for populating 2H11/2, 4S3/2 and 4F7/2 states. The energy level diagrams of
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Er3+ and Yb3+ ions as well as the upconversion mechanism are shown in Fig.5. Firstly,
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Yb3+ is excited to 2F2/5 state by absorbing a near infrared photon, and then energy transfer occurs from Yb3+ to Er3+ populating the 4I11/2 level. Besides, Er3+ ion can also
absorb a 980 nm photon directly from the laser. Another energy transfer between Yb3+ and Er3+ can populate the 4F7/2 level of Er3+ ion. Then Er3+ ions relax nonradiatively to 2
H11/2 and 4S3/2 levels. When 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions
occur, bright green light can be observed. As for red light emission, it comes from 4
F9/2 → 4I15/2 transition. There are two pathways for the population of 4F9/2 level. One
is the relaxation of 2F7/2 and the other is the excitation from 4I13/2 by the energy transfer of Yb3+ or absorption of a 980 nm photon. 7
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In order to investigate the optical temperature sensing properties, the temperature-dependence of emission intensities for green emission has been measured. Fig.6 shows the normalized emission spectra for SrWO4:1%Er3+/6%Yb3+ at different
H11/2 and 4S3/2 levels, the 2H11/2 level can be populated easily from the 4S3/2 level by
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2
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temperatures in the range of 95-775 K. Owing to the small energy separation between
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thermal agitation, resulting in the variation of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions at elevated temperature. At lower temperature, the integrated intensity of
an
emission band originating from 4S3/2 → 4I15/2 transitions is higher than that of emission band originating from 2H11/2 → 4I15/2 transitions. However, the character is
2
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reversed at higher temperatures. The reverse is because a fact that the population of H11/2 level from 4S3/2 level through thermal excitation is difficult at low temperature.
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d
As a result, the relative intensity ratios of two normalized emission bands increase dramatically with the increasing temperatures from 95 to 775 K. Moreover, it should
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be noted that there is a blue shift for the emission band originating from the 2H11/2 → 4
I15/2 transitions. The slight blue shift can be ascribed to the thermally active phonon
assisted excitation from a lower energy sublevel to a high energy sublevel in the excited state of Er3+ ions [24, 25]. The energy gap ΔE between 2H11/2 and 4S3/2 is about 780 cm-1, which locates in
the range of 200 cm-1 < ΔE < 2000 cm-1 [26]. That is to say, this energy gap matches well with the requirement of the thermally coupled levels for FIR-based optical temperature sensors. The relative population of the two thermally coupled multiplets fits a Boltzmann distribution law and the relative population of the thermally coupled 8
Page 8 of 20
energy levels can be expressed as
, where IH and IS
are integrated intensities of emissions originating from 2H11/2 → 4I15/2 and 4S3/2 → I15/2 transitions of Er3+ ions, KB is the Boltzmann constant, ΔE is the energy gap
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4
between 2H11/2 and 4S3/2 levels, A is a proportionality constant, and T is the absolute
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temperature [27]. Sensitivity is a crucial parameter for optical temperature sensors,
sensitivity
can
be
calculated
by
the
formula
of
an
The
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and thus it is necessary to investigate the variation of sensitivity with temperature.
, where S is the temperature-dependent
M
sensitivity of phosphor and other terms have their usual meaning as mentioned above. The calculated values of S are plotted as a function of absolute temperature, as shown
te
d
in Fig.7. It can be seen that the sensitivity of SrWO4:1%Er3+/6%Yb3+ phosphor increases gradually with the increase of temperature and it reaches the maximum of
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0.01282 K-1 at 489 K. That is to say, the considered material is promising for temperature sensing application with very good sensor sensitivity. 4. Conclusion
In summary, SrWO4:Er3+/Yb3+ phosphors have been synthesized successfully by
a solid state reaction and these phosphors show intense emission in green region and high sensitivity to temperature. All of SrWO4:Er3+/Yb3+ phosphors crystallize in pure
tetragonal phase, suggesting the doping ions are not affected the host phase state. Several upconversion emission bands have been detected and assigned via a two-photon process. The green emissions originating from 2H11/2 → 4I15/2 and 4S3/2 → 9
Page 9 of 20
4
I15/2 transitions of Er3+ have been used for optical thermometry via the fluorescence
intensity ratio technique. The results indicate a potential of the phosphors for using in temperature sensor with a highest sensor sensitivity of 0.01282 K-1 at 489 K.
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Acknowledgement
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This work is supported financially by the Science and Technology Research and
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Development Programs of Hebei Province, Zhangjiakou City (No. 1411070B, No. 1511075B and No. 1411058I-23), Science and Technology Department of Hebei
an
Province Project (No. 16961301D), the Youth Fund of Natural Science of Hebei North University (No. Q2014006), the Major Project of Hebei North University (No.
M
ZD201304), Hebei Provincial Education Department Youth Fund Projects (No. QN2015148 and No. QN2015007) and the Project of Hebei Planning Office of
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Figure captions
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Fig.1 XRD patterns of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6, 8) phosphors
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Fig.2 SEM images of SrWO4:1%Er3+ (A) and SrWO4:1%Er3+/8%Yb3+
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Fig.3 Upconversion spectra of SrWO4:1%Er3+/x%Yb3+ (x = 0, 2, 4, 6, 8) phosphors
Fig.4 Log-log plot of the upconversion emission intensity of SrWO4:1%Er3+/6%Yb3+ as a function of pumping power Fig.5 Energy level diagram of the Er3+ and Yb3+ ions and the proposed upconversion
mechanisms in SrWO4:1%Er3+/6%Yb3+ Fig.6
Temperature-dependent
green
upconversion
emission
spectra
of
SrWO4:1%Er3+/6%Yb3+ under 980 nm excitation Fig.7 Sensitivity of SrWO4:1%Er3+/6%Yb3+ as a function of temperature
12
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