Under 980 nm excitation, the temperature dependence of five-photon UV (256 and 276 nm) upconversion luminescence in Yb3+-Er3+ codoped β-NaLuF4 nanocrystals was studied from 303 K to 523 K. The 4D7/2 and 4G9/2 levels of Er3+ are confirmed to be thermally coupled levels. They are the highest energy states for optical thermometry known so far. By using fluorescence intensity ratio technique, optical temperature sensing characteristics based on the 4D7/2/4G9/2 → 4I15/2 transitions of Er3+ were reported here for the first time. The obtained sensitivity of this UV-based sensor is higher than that of green-based optical thermometer in low temperature range.
© 2015 Optical Society of America
The measurement of temperature is crucial in countless scientific investigations and industrial productions. Among all the methods for determining temperature, considerable interests have focused on luminescence-based measurements because they can offer excellent accuracy, resolution, repeatability, and stability [1,2]. Optical temperature sensors based on the fluorescence intensity ratio (FIR) technique takes advantage of the temperature-dependent fluorescence intensities of two thermally coupled emitting levels of rare earth ions (RE3+) and have garnered much attention in recent years due to their potential applications in electrical power stations, oil refineries, coal mines, and biocompatible temperature probe, etc [3–6]. Compared with other temperature measuring methods, luminescent thermometry technique can improve the sensitivity and reduce the dependence of measurement condition, such as fluorescence loss and electromagnetic compatibility problems. More importantly, by using the FIR method, researchers have acquired the temperature of a single living cell .
RE3+ doped upconversion luminescence (UCL) is of importance in lasers, solar cells, three-dimensional displays, and luminescent probes in microscopy owing to the unique optical properties arising from the intra 4f transitions of RE3+. Among all the RE3+, Er3+ is an ideal candidate for UCL since its rich and ladder-like electronic energy level structure. By comparing the emission intensities from thermally coupled electronic levels of 2H11/2 and 4S3/2, Er3+ doped UCL materials have been widely investigated as optical thermometry over the past few years [8–10]. Moreover, in 2012, Zhang and associates reported that the upconversion emissions from the 4G11/2 → 4I15/2 (384 nm) and 2H9/2 → 4I15/2 (408 nm) transitions of Er3+ could be used for measuring temperature as well . Er3+ ions have rich energy level structures in UV region. In our previous studies, UCLs of Er3+ in the range from 240 nm to 350 nm were obtained with NIR (980 nm or 1560 nm) laser excitation [12,13]. However, until now, no results about optical thermometry have been reported mainly concerning the UCLs of Er3+ in the UV region.
In this letter, we presented an observation of temperature sensor based on the UV UC emissions of Er3+. Upon excitation with NIR radiation of 980 nm, UV UC emissions of Er3+ in the region of 240 − 350 nm were observed in hexagonal phase sodium lutetium fluoride (NaLuF4) nanocrystals. The emission intensity ratios of 256 nm to 276 nm of Er3+ were investigated systematically by changing the temperature of the sample. The 4D7/2 and 4G9/2 states of Er3+ were verified to be thermally coupled levels. Especially, this UV-based sensor of Er3+ showed a higher sensitivity than the corresponding green-based nanothermometer from room temperature to 330 K, which demonstrated their great potential for application as high-performance optical thermometry.
Yb3+-Er3+ codoped β-NaLuF4 sample was synthesized via the high-temperature thermal decomposition method according to the procedure described in Ref . X-ray powder diffraction (XRD) analysis revealed that the sample was hexagonal phase, all diffraction peaks can be indexed as the β-NaLuF4 (JCPDS No. 27−0726). The corresponding morphological analysis with transmission electron microscope (TEM, Hitachi H-600) showed that the nanocrystals were uniform nanospheres with the particle size of about 50 nm. The UCL spectra in the UV region were recorded by a spectrophotometer (Hitachi F-4500) which equipped with a power-controllable 980 nm CW diode laser as the excitation source. The temperature of the sample was controlled by using a set of home-made equipment.
3. Results and discussion
Figure 1(a) shows the UCL spectra of NaLuF4:Yb3+, Er3+ nanocrystals at room temperature with the laser excitation of 980 nm. Apart from the well-known green and red upconversion emissions of Er3+, the sample emitted UV UC fluorescence in the region of 230 − 350 nm, as shown in Fig. 1(a). The emissions that peaked at 244 nm, 256 nm, 276 nm, 288 nm, 305 nm, 317 nm, and 355 nm were originated from the 2I11/2 → 4I15/2, 4D7/2 → 4I15/2, 4G9/2 → 4I15/2, 2D5/2 → 4I15/2, 2K13/2 → 4I15/2, 2P3/2 → 4I15/2, and 4D7/2 → 4I11/2 transitions of Er3+ ions, respectively [13,15]. In addition, Fig. 1(b) depicts the double logarithmic plots of the emission intensity If as a function of pump power IIR. For an unsaturated UC process, If ∝ IIRn, where n is the number of IR photons absorbed per upconverted photon emitted . Under 980 nm excitation, both the n values were around 5 (4.87 ± 0.09 and 4.69 ± 0.07) for the UV UC emissions of 256 nm and 276 nm, which indicated that the 4D7/2 → 4I15/2 and 4G9/2 → 4I15/2 transitions of Er3+ in the Yb3+-Er3+ codoped NaLuF4 nanocrystals came from five-photon UC processes.
To further recognize the UC mechanisms in the sample, possible UC processes are schematically given in the energy level diagrams of Yb3+ and Er3+, as shown in Fig. 2. In Yb3+-Er3+ codoped systems, both the energy transfer (ET) from Yb3+ ions to Er3+ and the nonradiative relaxation (NR) from excited Er3+ are involved and played important roles in populating the high-energy states of Er3+. The possible UC population processes for the UV levels of Er3+ can be clearly described as follow: 4I15/2 4I11/2 4F7/2 2H11/2, 4S3/2 2G7/2 4G11/2, 2H9/2 2D5/2 2K13/2 2I11/2 4D7/2, 4G9/2.
To verify that the 4D7/2 and 4G9/2 levels were thermally coupled, the 256 nm and 276 nm UCLs of Er3+ were studied by changing the sample temperature from 303 K to 523 K. Figure 3 shows the normalized UV UCL spectra of the NaLuF4:Yb3+, Er3+ nanocrystals in the wavelength range from 250 nm to 284 nm. They were recorded at 303, 398, and 503 K, respectively, and normalized at 256 nm. There was nearly no overlap of two UV UC emission bands originating from the 4D7/2 → 4I15/2 and 4G9/2 → 4I15/2 transitions of Er3+, which was in favor of the measurement accuracy of their emission intensities. In addition, it is obvious that the relative intensity ratio of I256/I276 increased dramatically with the increase of sample temperature. This is because the electronic states of 4D7/2 and 4G9/2 of Er3+ are thermally coupled. Similar to the thermally coupled levels of 2H11/2 and 4S3/2 levels of Er3+, the FIR of the emissions from the 4D7/2 → 4I15/2 and the 4G9/2 → 4I15/2 transitions are in accord with Boltzmann distribution, which could be written as:6,8,10].
The detailed changes of UV UCLs (256 nm and 276 nm) of Er3+ with temperature change from 303 K to 523 K were investigated, as illustrated in Fig. 4(a), where the fluorescence intensities have been normalized at 256 nm as well. Upon increasing the sample temperature, we found that the relative emission intensity of I276 decreased gradually. Moreover, the appropriate energy separation between the 4D7/2 and 4G9/2 allows the upper (4D7/2) level to be populated by thermal excitation from the excited 4G9/2 level. Figure 4(b) shows a monolog plot of the fluorescence intensity ratio of UV emissions of Er3+ as a function of inverse absolute temperature in the range of 303 − 523 K. The experimental data could be fitted by a straight line with the slope of about 384.
Furthermore, the relationship between the FIR of UV emissions from the 4D7/2/4G9/2 → 4I15/2 transitions of Er3+ and the temperature in the region of 303 − 523 K is shown in Fig. 4(c). The value of FIR increased from 1.17 to 2.06 with increase the temperature from 303 K to 523 K. By fitting the experimental data according to equation as described above, the obtained coefficient C and the energy gap ΔE12 were about 4.448 and 384 cm−1, respectively. To better understand the temperature sensing performance of this UV-based nanothermometer, it is necessary for us to investigate its sensor sensitivity S. The value of S is determined by the rate of FIR varies with temperature and it can be defined as:Fig. 4(d) for the temperature range of 303 – 523 K. It is clearly that the sensor sensitivity decreased gradually with the increase of temperature. At the temperature of 303 K, the value of S reached the maximum that is slightly larger than 0.0052 K−1. Several testing cycles were performed and a good repeatability was obtained in NaLuF4:Yb3+, Er3+ nanocrystals. To the best of our knowledge, it is the first time that optical thermometer was obtained by using the UV UC emissions of Er3+ ions in the wavelength short than 350 nm.
It is then very important to compare the sensitivities of this UV-based optical thermometer with that of green-based temperature sensor of Er3+. Figure 5(a) displays the temperature-dependent green UCLs of Er3+ under the excitation of 980 nm. The corresponding sensitivity curve was depicted in Fig. 5(b) (black solid line). Obviously, the S value for the green-based thermometer keeps increasing in the experimental temperature range (303 – 503 K) and reaches the maximum of 0.0073 K−1 at the temperature of 503 K. For comparison, the sensor sensitivities based on the green UC emissions and UV UC emissions of Er3+ are also exhibited, as shown in Fig. 5(b). With increasing the experimental temperature, the changes of sensitivities of these two (UV-based and green-based) sensors presented completely opposite trends. In particular, the sensitivities of UV-based sensor are prior to those of the green-based optical thermometer in low temperature range (from room temperature to 330 K). Therefore, the coupled UV emissions of Er3+ is more suitable to be applied in low temperature measurement than its green thermal coupled levels, and this property made β-NaLuF4:Yb3+, Er3+ nanoparticles better candidate for high-performance optical thermometry.
In conclusion, an optical temperature sensor based on the shortest wavelength to date has been developed in Yb3+-Er3+ codoped β-NaLuF4 nanocrystals. The neighboring 4D7/2 and 4G9/2 levels were verified to be thermally coupled levels and their population ratios were fitted well by the Boltzmann distributions. By using FIR technology, optical temperature sensing based on the thermal coupled levels of 4D7/2 and 4G9/2 was reported in this work for the first time. The obtained sensor sensitivity of this UV-based sensor is prior to that of the green-based optical thermometer in low temperature range (from room temperature to 330 K). This novel property made Er3+-based UV UCL materials promising platforms for high-performance optical thermometer.
This work was supported by the National Natural Science Foundation of China (NNSFC) (grants 11404136, 11474132, and 11274139), China Postdoctoral Science Foundation (2012M520668), and Scientific and Technological Developing Project of Jilin Province (20150520028JH).
References and links
3. S. F. León-Luis, U. R. Rodríguez-Mendoza, E. Lalla, and V. Lavín, “Temperature sensor based on the Er3+ green upconverted emission in a fluorotellurite glass,” Sens. Actuators B Chem. 158(1), 208–213 (2011). [CrossRef]
4. R. K. Verma and S. B. Rai, “Laser induced optical heating from Yb3+/Ho3+:Ca12Al14O33 and its applicability as a thermal probe,” J. Quant. Spectrosc. Radiat. Transf. 113(12), 1594–1600 (2012). [CrossRef]
5. D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek, and M. Samoc, “Neodymium(III) doped fluoride nanoparticles as non-contact optical temperature sensors,” Nanoscale 4(22), 6959–6961 (2012). [CrossRef] [PubMed]
6. K. Z. Zheng, Z. Y. Liu, C. J. Lv, and W. P. Qin, “Temperature sensor based on the UV upconversion luminescence of Gd3+ in Yb3+-Tm3+-Gd3+ codoped NaLuF4 microcrystals,” J. Mater. Chem. C 1(35), 5502–5507 (2013). [CrossRef]
7. F. Vetrone, R. Naccache, A. Zamarrón, A. Juarranz de la Fuente, F. Sanz-Rodríguez, L. Martinez Maestro, E. Martín Rodriguez, D. Jaque, J. García Solé, and J. A. Capobianco, “Temperature Sensing Using Fluorescent Nanothermometers,” ACS Nano 4(6), 3254–3258 (2010). [CrossRef] [PubMed]
8. M. Quintanilla, E. Cantelar, F. Cussó, M. Villegas, and A. C. Caballero, “Temperature Sensing with Up-Converting Submicron-Sized LiNbO3:Er3+/Yb3+ Particles,” Appl. Phys. Express 4(2), 022601 (2011). [CrossRef]
9. B. Dong, B. S. Cao, Y. Y. He, Z. Liu, Z. P. Li, and Z. Q. Feng, “Temperature Sensing and In Vivo Imaging by Molybdenum Sensitized Visible Upconversion Luminescence of Rare-Earth Oxides,” Adv. Mater. 24(15), 1987–1993 (2012). [CrossRef] [PubMed]
10. N. Rakov and G. S. Maciel, “Three-photon upconversion and optical thermometry characterization of Er3+:Yb3+ co-doped yttrium silicate powders”, Sens. Actuat. B 164, 96–100 (2012).
11. W. Xu, Z. G. Zhang, and W. W. Cao, “Excellent optical thermometry based on short-wavelength upconversion emissions in Er3+/Yb3+ codoped CaWO4.,” Opt. Lett. 37(23), 4865–4867 (2012). [CrossRef] [PubMed]
12. K. Z. Zheng, D. Zhao, D. S. Zhang, N. Liu, and W. P. Qin, “Temperature-dependent six-photon upconversion fluorescence of Er3+,” J. Fluor. Chem. 132(1), 5–8 (2011). [CrossRef]
13. K. Z. Zheng, D. Zhao, D. S. Zhang, N. Liu, and W. P. Qin, “Ultraviolet upconversion fluorescence of Er3+ induced by 1560 nm laser excitation,” Opt. Lett. 35(14), 2442–2444 (2010). [CrossRef] [PubMed]
14. F. Shi, J. S. Wang, X. S. Zhai, D. Zhao, and W. P. Qin, “Facile synthesis of β-NaLuF4: Yb/Tm hexagonal nanoplates with intense ultraviolet upconversion luminescence,” CrystEngComm 13(11), 3782–3787 (2011). [CrossRef]
15. G. Chen, H. Liang, H. Liu, G. Somesfalean, and Z. Zhang, “Near vacuum ultraviolet luminescence of Gd3+ and Er3+ ions generated by super saturation upconversion processes,” Opt. Express 17(19), 16366–16371 (2009). [CrossRef] [PubMed]
16. M. Pollnau, D. R. Gamelin, S. R. Lüthi, H. U. Güdel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61(5), 3337–3346 (2000). [CrossRef]