Down-conversion luminescence and its temperature-sensing
properties from Er
3+-doped sodium bismuth titanate ferroelectric thin films
Shanshan Wang1• Shanshan Zheng1•Hong Zhou1•Anlian Pan1• Guangheng Wu2•Jun-ming Liu2
Received: 22 April 2015 / Accepted: 4 September 2015 / Published online: 10 September 2015 Springer-Verlag Berlin Heidelberg 2015
Abstract Here, we demonstrate outstanding temperature- sensing properties from Na0.5Bi0.49Er0.01TiO3 (NBT:Er) thin films. The perovskite phase for them is stable in the temperature range from 80 to 440 K. Interestingly, the Er doping enhances the ferroelectric polarization and intro- duces local dipolar, which are positive for temperature sensing. Pumped by a 488-nm laser, the NBT:Er thin films show strong green luminescence with two bands around 525 and 548 nm. The intensity ratioI525/I548can be used for temperature sensing, and the maximum sensitivity is about 2.3910-3K-1, higher than that from Er-doped silicon oxide. These suggest NBT:Er thin film is a promising candidate for temperature sensor.
1 Introduction
In recent years, luminescence from rare-earth ions (Ln3?) in ferroelectric hosts draws increasing interest in scientific community [1–5]. Multifunctional devices are developed based on these compounds having both luminescence and ferroelectric properties. Several ferroelectric materials were investigated as the host for the luminescence from Ln3?, including PbTiO3[6], BaTiO3[7], (Ba,Sr)TiO3[8], and Bi4Ti3O12 [4, 9, 10]. Among them, bismuth-based
ferroelectric materials are suggested to be promising can- didates. Firstly, charge compensation is not concerned when Bi3?is substituted by Ln3?[11], and secondly, Bi3?
can act as sensitizer and enhance the emission from Ln3?
[12,13].
Sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) is one of the well-investigated bismuth-based ferroelectric mate- rials [14,15]. At room temperature, NBT crystallizes in a rhombohedral perovskite structure withR3cspace group. It has large remnant polarization Pr of 38lC/cm2, high piezoelectric strain constant d33 of 73 pC/N, and high Curie temperature Tc of 320C [11]. More recently, luminescence from rare-earth ions in NBT was reported, such as strong red luminescence from Pr3?-doped NBT thin films [11,16]. These studies suggest that the NBT is a good host for trivalent Ln3?luminescence.
On the other hand, investigations show the luminescence from Ln3?has strong coupling effect to their ferroelectric host. The up-conversion luminescence from Er, Yb co- doped BaTiO3thin film is enhanced by 2.7 times after the thin film is electrical poled [3]. The reason for this strong coupling effect is that the luminescence from Ln3? is highly sensitive to the crystal field, which is modified as the polarizations in the ferroelectric host are changed.
Given the fact that the polarizations in the ferroelectrics are sensitively respond to the external triggers, including temperature, electric field, and/or stress, researchers pro- pose several sensor applications using the luminescence from Ln3?in ferroelectric hosts [17–19].
Among them, temperature sensor is the most attractive.
Luo reported good temperature-sensing properties from Er- doped NBT ceramics [20]. On the other hand, nano-sized thin films are more interesting to their ceramics counter- parts due to their potentials for integrated micro-electro- mechanical system. This motivates us to investigate the
& Hong Zhou
& Guangheng Wu
1 School of Physics and Electronics, Hunan University, Changsha 410082, China
2 State Key Laboratory for Solid State Microstructures, Nanjing University, Nanjing 210093, China
DOI 10.1007/s00339-015-9480-x
DCL and its temperature-sensing properties from Er3?- doped NBT ferroelectric thin films.
In this letter, we demonstrate strong green DCL and outstanding temperature-sensing performance from Na0.5- Bi0.49Er0.01TiO3 thin films. The Na0.5Bi0.49Er0.01TiO3 is chosen for optimized polarization properties for the Er3?- doped Na0.5Bi0.5TiO3ferroelectric thin films [21].
2 Experimental
Both the NBT and Na0.5Bi0.49Er0.01TiO3 thin films were prepared using a modified CSD method [11]. The solutions were adjusted to a concentration of 0.3 mol/L and spin- coated onto the Pt/Ti/SiO2/Si substrates. Each layer was annealed at 700C for 5 min in air using a rapid thermal annealing method. The spin-coat and annealing process was repeated several times to obtain the desired thickness.
The crystallization and crystal structure of the thin films were identified using X-ray diffraction (XRD) unit (Rigaku D/MAX 2200 VPC; Rigaku Corp., Sendagaya, Shibuya-Ku, Tokyo, Japan) with a Cu-Karadiation (k =1.54050 A˚ ). The morphology of the Er3?-doped NBT thin film was charac- terized using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). To measure the electrical properties, Pt top electrodes were deposited on the surfaces of the thin films through shadow mask. Dielectric properties were measured using Agilent 4284a (Santa Clara, CA) with the applied ac signal amplitude of 100 mV. Ferroelectric properties were characterized using a Radiant Precision Workstation ferroelectric tester (Albuquerque, NM). Tem- perature-dependent Raman spectra, the DCL, and its tem- perature-sensing properties were performed on a system which consists of a confocal micro-photoluminescence system (WITec, alpha-300), an air-cooled continuous wave argon laser (k =488 nm), a long-working-distance objec- tive, and a micro-optical cryostat.
3 Results and discussion
The synthesized films are phase-pure, dense, and smooth with a thickness of 400 nm. Figure1a shows the XRD patterns for both the NBT and Na0.5Bi0.49Er0.01TiO3 thin films. The positions for the diffraction peaks are corrected corresponding to the (111) peak of platinum bottom elec- trodes. All diffraction peaks raise from the perovskite structure of NBT except for one from silver, which is used for the contact between Pt bottom electrode and probe in electrical measurements. Furthermore, the peaks from the Na0.5Bi0.49Er0.01TiO3thin films shift slightly toward higher angles, resulting from smaller ionic radius of Er3?(0.88 A˚ ) than that of Bi3? (0.96 A˚ ) [22]. These suggest the
synthesized thin films of either NBT or Na0.5Bi0.49Er0.01- TiO3present a pure perovskite structure with no impurity phase like Bi2Ti2O7and Bi4Ti3O12.
Fig. 1 aXRD patterns of the NBT and NBT:Er thin films annealed at 700C. Theinset of (a) is the cross section of NBT:Er thin film.
bRaman spectrum of NBT:Er thin film at room temperature. Spectral deconvolution was performed according to several Lorentzian modes.
cRaman spectra of NBT:Er thin film as a function of temperature
In order to confirm the structure phase for the thin films, we further conduct the Raman measurement, which is more sensitive than XRD. Figure1b shows the room temperature Raman spectrum with its Lorentzian-shaped deconvolution for Er-doped NBT thin film in the range from 200 to 1100 cm-1. Due to the disorder of A-site ions and the overlapping of Raman modes, all of the vibration bands are broad. The first band at around 288 cm-1is dominated by the vibration of TiO6 octahedron. The second band (450–700 cm-1) is related to the vibrations and/or rotations of the oxygen octahedral. The band above 680 cm-1 has been linked to A1(LO) and E(LO) overlapping bands. The observed Raman spectrum for Na0.5Bi0.49Er0.01TiO3 thin film is consistent with those from NBT ceramic [23]. No other Raman mode is observed, implying that the Er-doped NBT thin film is phase-pure, consistent with the result from XRD measurement mentioned above.
Thermal stability is central for temperature-sensing applications, because a phase transition during warming and/or cooling may cause mechanical fatigue. Interest- ingly, within our measurement condition with temperature range from 80 to 440 K, the Na0.5Bi0.49Er0.01TiO3thin film remains in the perovskite phase. The corresponding Raman spectra are presented in Fig.1c. As the temperature increases, the thermally induced broadening for the Raman spectrum is observed. This is in qualitative agreement with the theory that higher structural disorder exists in the Ti–O bond and TiO6octahedra with increasing temperature [24].
No additional Raman mode is observed in spite of the measurement temperatures, suggesting good thermal sta- bility for the Er-doped NBT thin films.
Temperature-sensing application based on Er-doped NBT thin film also requires large polarizations. Figure2a shows the polarization–electric field (P–E) loops for both NBT and Na0.5Bi0.49Er0.01TiO3 thin films. The remnant polarizationPris 22lC/cm2for Na0.5Bi0.49Er0.01TiO3thin films. These values are larger than those for undoped NBT thin films (14lC/cm2).
Er doping not only enhances ferroelectric polarization, but also introduces local dipolar, resulting in enhanced dielectric constant. Figure2b shows the frequency depen- dence of the dielectric constant and loss at room temperature for both NBT and Na0.5Bi0.49Er0.01TiO3 thin films. The Na0.5Bi0.49Er0.01TiO3thin film has relatively high dielectric constant of 1166 and low loss of 0.05 at 1 k Hz, while the values for NBT thin film are 528 and 0.07, respectively. The enhancement of dielectric constant may result from the local dipolar caused by Er3? doping. Similar results with enhanced ferroelectric polarization and dielectric contact in rare-earth-doped NBT and other Bi-based ferroelectric thin films are reported elsewhere [10,25,26]. These are helpful to obtain good temperature-sensing properties, because these polarization and dipolar are very sensitive to temperature.
The third requirement to obtain good temperature- sensing properties is efficient emission under light pump- ing. At room temperature, pumped with a 488-nm laser, the Er-doped NBT thin film shows bright green color. The corresponding DCL spectra are shown in Fig.3a. It con- sists of two strong green emission bands (centered at 525 and 548 nm) and a weak red emission band (centered at 658 nm). The inset of Fig.3a shows the linear relationship between the DCL intensity and the pump power, indicating a single-photon process is involved.
The expected mechanism is shown in Fig.3b. Pumped by 488-nm laser, the Er3?ion is directly excited into the
4F7/2 level. Then, the exited Er3? ion non-radiatively relaxes to other excited states, 2H11/2, 4S3/2, and 4F9/2. When it decays to the ground state from one of these exited states, the emission from the Er3?ion occurs. The strong green emission bands centered at 525 and 548 nm are ascribed to the 2H11/2?4I15/2 and 4S3/2 ?4I15/2 transi- tions, respectively [27]. At the same time, the weak red emission band centered at 658 nm is ascribed to the
4F9/2?4I15/2transition.
We further investigate the DCL properties at different temperatures from 80 to 480 K. Figure4a shows two of them at 80 and 440 K, respectively. As the temperature Fig. 2 a Polarization–electric field (P–E) loops and b frequency dependence of dielectric constant and loss for NBT and NBT:Er thin films
(T) increases, the intensity for the 525-nm band increases, while the intensity for the 548-nm band decreases, which is due to the increasing in electrons excited from 4S3/2 to
2H11/2level by thermal excitation when temperature getting higher. This leads to an increase in the luminescence intensity ratio R=I525/I548 (where I525 and I548 are the integrated intensities of the 2H11/2?4I15/2 and 4S3/
2?4I15/2transitions, shown in the inset of Fig.4a). Thus, this characteristic can be used for the temperature-sensitive thin film (thin-film thermometer).
The luminescence intensity ratio of green DCL at 525 and 548 nm can be written as follows, which follow a thermal equilibrium Boltzmann law [28]:
R¼I525
I548¼CexpðDE=kTÞ ð1Þ whereCis a constant, DEis the energy gap between the two excited states,k is the Boltzmann constant, and T is the absolute temperature. Thus, we can use this charac- teristic for the temperature-sensitive thin film (thin-film thermometers).
It is necessary to investigate the temperature sensitivity S forR, where Sis defined as:
Fig. 3 aThe corresponding PL spectrum andbschematic energy- level diagram. The inset of (a) shows a photoluminescence pho- tograph of the thin film and the pump power dependence of PL intensity
Fig. 4 Temperature sensing based on NBT:Er thin film. a Green DCL spectra at two different temperatures.bA plot of Ln(I525/I548) as a function of inverse absolute temperature.cSensor sensitivity as a function of the temperature
S¼dR
dT ¼RDE
kT2 ð2Þ
The corresponding sensitivity curve is shown in Fig.4c.
The sensitivity S reaches maximum value of about 2.3910-3K-1at 310 K. The sensitivitySreaches maxi- mum value of about 2.3910-3K-1at 310 K, higher than those from Er-doped oxide materials, such as Er-doped SiO2[29], whose S value is 1 910-3K-1. The highSfrom NBT:Er thin films is benefited from the ferroelectric nature of NBT:Er thin films and the local dipolar caused by Er doping. It is known that the photoluminescence (PL) from Er3?and other rare-earth elements is affected by the crystal symmetry of the host materials [30]. Since ferroelectric materials are non-centrosymmetric, the forbidden f–f tran- sitions in Er3?are allowed when Er3?ion is in ferroelectric hosts. Furthermore, Hao et al. [3] demonstrated the PL intensity from rare-earth elements is affected with the magnitude of the ferroelectric polarization. Meanwhile, the magnitude of the ferroelectric polarization is not only affected with the applied electric field, but also affected with the environment temperatures. Therefore, the PL from Er3?in ferroelectric is very sensitive to the temperature.
4 Conclusions
In conclusion, NBT:Er thin films were prepared using a modified CSD method. The thin films are phase-pure, dense, and smooth. The perovskite phase for the thin films is stable for the temperature range from 80 to 440 K.
Enhanced ferroelectric polarization and doping-induced local dipolar are observed in these NBT:Er thin films.
Pumped by a 488-nm laser, the NBT:Er thin films show strong green luminescence with two bands around 525 and 548 nm. The intensity ratioI525/I548can be used for tem- perature sensing, and the maximum sensitivity is about 2.3910-3K-1, higher than that from Er-doped silicon oxide. These suggest NBT:Er thin films are promising candidate for temperature sensors and multifunctional fer- roelectric thin-film devices.
Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51302077), the Hunan Provincial Natural Science Foun- dation of China (Grant No. 2015JJ3049), and the Fundamental Research Funds for the Central Universities.
References
1. Y.B. Wei, Z. Wu, Y.M. Jia, J. Wu, Y.C. Shen, H.S. Luo, Appl.
Phys. Lett.105, 042902 (2014)
2. P. Molina, H. Loro, S. Alvarez-Garcia, L.E. Bausa, E.M.
Rodriguez, O. Guillot-Noel, P. Goldner, M. Bettinelli, P. Ghigna, J.G. Sole, Phys. Rev. B80, 05411 (2009)
3. J. Hao, Y. Zhang, X. Wei, Angew Chem. Int. Ed.50, 6876–6880 (2011)
4. F. Gao, G.J. Ding, H. Zhou, G.H. Wu, N. Qin, D.H. Bao, J.
Electrochem. Soc.158, G128–G131 (2011)
5. H. Zhou, X.M. Chen, G.H. Wu, F. Gao, N. Qin, D.H. Bao, J. Am.
Chem. Soc.132, 1790–1791 (2010)
6. R.C. Lima, J.W.M. Espinosa, M.F.C. Gurgel, E.C. Paris, E.R.
Leite, M.R. Joya, P.S. Pizani, J.A. Varela, E. Longo, J. Appl.
Phys.100, 034917 (2006)
7. Z. Wu, Y. Zhang, G. Bai, W. Tang, J. Gao, J. Hao, Opt. Express 22, 29014–29019 (2014)
8. R. Reshmi, M.K. Jayaraj, K. Jithesh, M.T. Sebastian, J. Elec- trochem. Soc.157, H783–H786 (2010)
9. X. Liu, H. Zhou, G.H. Wu, D.H. Bao, Appl. Phys. Express4, 032103 (2011)
10. H. Zhou, G.H. Wu, N. Qin, D.H. Bao, J. Am. Ceram. Soc.93, 2109–2112 (2010)
11. H. Zhou, X. Liu, N. Qin, D.H. Bao, J. Appl. Phys.110, 034102 (2011)
12. X.Y. Huang, X.H. Ji, Q.Y. Zhang, J. Am. Ceram. Soc. 94, 833–837 (2011)
13. B. Xu, G. Bai, J. Wang, Q. Guo, D. Tan, W. Chen, X. Liu, S.
Zhou, J. Hao, J. Qiu, Opt. Lett.39, 3022–3025 (2014) 14. B.N. Rao, R. Ranjan, Phys. Rev. B86, 134103 (2012) 15. D. Schu¨tz, M. Deluca, W. Krauss, A. Feteira, T. Jackson, K.
Reichmann, Adv. Funct. Mater.22, 2285–2294 (2012) 16. H. Zhou, G.H. Wu, N. Qin, D.H. Bao, J. Am. Ceram. Soc.95,
483–486 (2012)
17. J.-C. Zhang, X. Wang, X. Yao, C.-N. Xu, H. Yamada, J. Elec- trochem. Soc.157, G269–G273 (2010)
18. X.S. Wang, C.N. Xu, H. Yamada, K. Nishikubo, X.G. Zheng, Adv. Mater.17, 1254–1258 (2005)
19. P. Zhang, M. Shen, L. Fang, F. Zheng, X. Wu, J. Shen, H. Chen, Appl. Phys. Lett.92, 222903–222908 (2008)
20. P. Du, L.H. Luo, W.P. Li, Q.Y. Yue, J. Appl. Phys.116, 014102 (2014)
21. H. Zhou, S.S. Wang, G.H. Wu, X.L. Zhu, X.X. Wang, A.L. Pan, Appl. Phys. A119, 937–940 (2015)
22. N. Niu, F. He, S. Gai, C. Li, X. Zhang, S. Huang, P. Yang, J.
Mater. Chem.22, 21613–21623 (2012)
23. T. Huang, Z.G. Hu, G.S. Xu, X.L. Zhang, J.Z. Zhang, J.H. Chu, Appl. Phys. Lett.104, 111908 (2014)
24. M. Otonicˇar, S.D. Sˇkapin, B. Jancˇar, D. Suvorov, J. Appl. Phys.
113, 024106 (2013)
25. H.B. Zhang, S.L. Jiang, K. Kajiyoshi, J.Z. Xiao, J. Am. Ceram.
Soc.93, 750–757 (2010)
26. K.B. Ruan, X.M. Chen, T. Liang, G.H. Wu, D.H. Bao, J. Appl.
Phys.103, 086104 (2008)
27. F. Gao, G.J. Ding, H. Zhou, G.H. Wu, N. Qin, D.H. Bao, J. Appl.
Phys.109, 043106 (2011)
28. F. Vetrone, R. Naccache, A. Zamarron, A.J.D.L. Fuente, F. Sanz- Rodriguez, L. Martinez Maestro, E. Martin Rodriguez, D. Jaque, J. Garcia Sole, J.A. Capobianco, ACS Nano4, 3254–3258 (2010) 29. E. Maurice, G. Monnom, B. Dussardier, A. Saı¨ssy, D.B.
Ostrowsky, G.W. Baxter, Appl. Opt.34, 8019–8025 (1995) 30. S.Y. Gong, M. Li, Z.H. Ren, X. Yang, X. Li, G. Shen, G.R. Han,
J. Phys. Chem. C119, 17326–17333 (2015)
