- Email: [email protected]
Ho3+ phosphors
Optical Materials 98 (2019) 109452
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Tunable upconversion luminescence and optical temperature sensing based on non-thermal coupled levels of Lu3NbO7:Yb3þ/Ho3þ phosphors Jinsheng Liao *, Liyun Kong, Minghua Wang, Yijian Sun, Guoliang Gong School of Chemistry and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou, Jiangxi, 341000, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lu3NbO7 Phosphor Optical temperature sensing Upconversion luminescence
The color-tunable upconversion (UC) emission and optical temperature sensing based on non-thermal coupled levels (NTCL) were observed from the Yb3þ/Ho3þ codoped Lu3NbO7 phosphors synthesized by the solid-state method. The phosphors are capable of generating color tunable UC luminescence from green (yellow) to yel low (green) with the increase of the Yb3þ (Ho3þ) concentration. The tunable emission is due to the different energy back transfer processes from Ho3þ to Yb3þ. The temperature sensing performances are investigated in the temperature range of 293–573 K based on NTCL by using fluorescence intensity ratio technology. The maximum absolute sensitivities are 0.37%K 1, 0.94%K 1, 0.27%K 1 at 298 K, which are based on three pairs NTCL of (5F4/5S2→5I8)/(5F5→5I8), (5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/(5F4/5S2→5I7) of Ho3þ, respectively. The above results suggest that the as-prepared Lu3NbO7:Yb3þ/Ho3þ phosphors have great potential for the appli cation prospects of upconverter, color tunable device and optical temperature sensor.
1. Introduction Rare-earth (RE) doped upconversion (UC) inorganic luminescent materials have received more and more attention because of its broad potential applications, such as color display, optoelectronics, sensor technology, laser cooling, optical data storage and vivo imaging [1–6]. Recently, RE3þ doped UC inorganic luminescent materials as optical temperature sensors have attracted considerable attention, since it shows unique advantages in fast response, electromagnetic passivity and high temperature sensitivity [7,8]. At present, the popular strategy for the optical temperature sensing is based on the UC fluorescence intensity ratio (FIR) of thermal coupled levels (Er3þ: 2H11/2/4S3/2 [9]; Tm3þ: 3F2, 3 3þ 5 3 3/ H4 [10]; Ho : F3/ K8 [11]; etc). According to the formula of rela tive sensitivity (Sr, Sr ¼ △E/KBT2), a larger energy gap (△E) of thermal coupled levels (TCL)is favorable for achieving a higher relative sensi tivity. However, the tight restriction in the energy gap cannot exceed 2000 cm 1, which assures the strong coupling between the two levels. Therefore, further enhancement of thermometric sensitivity is restricted intrinsically. To solve the above disadvantage, an alternative strategy based on FIR between the non-thermally coupled levels (Er3þ: 2 4 H11/2F9/2 [12]; Tm3þ: 3H4/1G4 [13]; Ho3þ: 5F5/5F4,5S2 [14] has been adopted. Ding et al. have reported that the maximal relative sensitivity (Sr) of Yb3þ/Tm3þ:Y2Ti2O7 is ~0.81% K 1 by using FIR between the
non-thermally coupled levels (NTCL) of Tm3þ:3F2,3/1G4 [13]. Among the RE ions, Ho3þ doped luminescent materials are explored for tem perature sensing because of the ability of efficiently emit photons from ultraviolet to near infrared regions and its favorable intra-atomic 4f energy level structure and the relatively long-lived [15]. It was reported that the optical temperature sensitivity of Ho3þ doped phosphors de pends mainly on host types irrespective of the other conditions, such as selection of different non-thermal energy levels of Ho3þ as FIR tech nology. However, it is essential that the effects of different non-thermal energy levels of Ho3þ are considered for optical temperature sensitivity. The selection of the host matrix is an important factor to obtain desirable UC luminescence due to the different crystal fields caused by structural symmetry of the host materials [16,17]. So, it is important to select suitable host material for designing desirable UC phosphor. Over the last few years, extensive research has been done in the lanthanide doped UC phosphors, mostly on the fluoride based phosphors such as NaYF4:Yb3þ/Er3þ, LiYF4:Yb3þ/Tm3þ, LuF3:Yb3þ/Er3þ [18–20]. But the fluorides are usually poisonous, harmful to the environment and very sensitive to oxygen, which may influence the luminescence properties and limit their application [18]. Instead, oxides usually show high chemical stability and environmental-friendly characters [21]. So, more and more oxide-based UC phosphors have been paid attention in recent years. It is known the cubic fluorite-type rare-earth niobates with the
* Corresponding author. E-mail address: [email protected] (J. Liao). https://doi.org/10.1016/j.optmat.2019.109452 Received 10 July 2019; Received in revised form 25 September 2019; Accepted 14 October 2019 Available online 24 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
J. Liao et al.
Optical Materials 98 (2019) 109452
which produced red emission upon violet light excitation [22]. Ilhan et al. found that Nd3þ doped Lu3NbO7 phosphor exhibited excellent near-infrared emission under 800 nm excitation [23]. At present, a few reports mainly focused on the downshift luminescence properties of RE3NbO7 [21–23]. However, to the best of our knowledge, the UC properties of Lu3NbO7 based phosphors have not reported. In this paper, Lu3NbO7:Yb3þ/Ho3þ phosphors were synthesized by a simple solid state reaction method. UC luminescence properties of Lu3NbO7:Yb3þ/Ho3þ phosphors are discussed by change of pump power and Yb3þ (Ho3þ) ion concentration as well the measured temperature. Green, red and near-infrared (NIR) UC luminescence had been acquired upon a 980 nm laser excitation. The temperature sensing performances of the phosphors are investigated based on three pairs NTCL of (5F4/5S2→5I8)/(5F5→5I8), (5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/ (5F4/5S2→5I7) for Ho3þ by using FIR technology. The results indicate that Lu3NbO7:Yb3þ/Ho3þ phosphor as promising multifunctional ma terials can be potentially used for lightings and optical temperature sensors. 2. Experimental
Fig. 1. XRD pattern of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ samples at 1400 � C for different calcination time (6–12 h). The standard data for Lu3NbO7 (JCPDS NO.024-1089) is also presented in the figure.
2.1. Preparation of samples The Lu3NbO7:Yb3þ/Ho3þ phosphors were synthesized by a hightemperature solid state reaction. All the chemicals of Nb2O5 (99.9%), Lu2O3 (99.99%), Yb2O3 (99.99%), and Ho2O3 (99.99%) were used as the starting materials without any further purification. All raw materials
general formula RE3NbO7 (RE ¼ Lu, La, Gd) have attracted great attention due to their electro-optical and magnetic properties [22–24]. Eu3þ doped Y3NbO7 phosphor was initially investigated by Kim et al.
Fig. 2. The XRD patterns of Ho3þ/Yb3þ co-doped Lu3NbO7 samples with various Ho3þ (a) and Yb3þ (c) concentrations. The enlarge figures for (b) and (d) are corresponded to the figures of (a) and (c), respectively. 2
J. Liao et al.
Optical Materials 98 (2019) 109452
Fig. 3. (a) The Rietveld refinement of the Lu3NbO7:7% Yb3þ/1.5% Ho3þ with the content of calculated (cross), background (green), experiment (black solid line), difference (blue) and Bragg position (vertical line) results; (b) Schematic illustration of the Lu3NbO7:7% Yb3þ/1.5% Ho3þ, and the coordination environment of the [LuO8]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
were weighted according to the stoichiometry of the Lu3(1-x-y)Ho3x Yb3yNbO7 formula. Then, the raw powders were mixed and ground in an agate mortar with ethanol until ethanol volatilization, heated in a muffle furnace at 1400 � C for different time (6–12 h) in air. When naturally cooled to room temperature, the as-prepared phosphors were crushed and ground for the subsequent measurements.
order to investigate the temperature dependence of the UC emission, the sample was placed in a temperature-controlled copper cylinder, and its temperature was increased from 298 to 523 K (Temperature measure ment resolution is 0.1 K). The UC spectra of sample at various temper atures were obtained using a Horiba/Jobin Yvon Fluorolog-3 double monochromator equipped with a Hamamatsu R928 Photomultiplier under the excitation of a 980 nm diode laser with 150 mW and the excitation power density was about 3 W/cm2. The slits of emission and excitation were both set as 1 nm in the spectral measurement.
2.2. Characterization of samples The samples were characterized by powder X-ray diffraction (XRD) performed on a Panalytical X’Pert diffractometer using CuKα1 radiation (λ ¼ 0.154187 nm). All of the patterns within the 10–90� 2θ range were collected in a scanning mode with a step size of 0.02� . The morphology patterns of samples were obtained on a field emission scanning electron microscope (SEM JSM-6700F) equipped with an energy-dispersive X-ray spectra (EDS). Lu3NbO7:Yb3þ/Ho3þ phosphors was investigated by using Fourier transform infrared spectroscopy (FT-IR, Nicolet5700). In
3. Results and discussion 3.1. Structure and morphology To investigate the effect of calcination time on the phase purity of the as-prepared samples, XRD pattern of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ samples at 1400 � C for different calcination time (6–12 h) are shown in
Fig. 4. (a) SEM and (b) EDS of Lu3NbO7:7% Yb3þ/1.5%Ho3þ; (c) Elemental mapping of Lu, Nb, O, Yb and Ho in Lu3NbO7:7% Yb3þ/1.5%Ho3þ. 3
J. Liao et al.
Optical Materials 98 (2019) 109452
except for some weak diffraction peaks of Nb2O5. When calcinated time for 12 h, the diffraction peaks of the product matches well with the standard data of cubic phase Lu3NbO7 (JCPDS card No. 024-1089), and no traces of additional peaks from other phases were observed. The above-stated results indicate that the calcination time is very important for the formation of the pure cubic phase Lu3NbO7. Therefore, the calcination time (12 h) is selected as the preparation of the Ho3þ/Yb3þ co-doped Lu3NbO7 samples with various Ho3þ/Yb3þ concentrations. In addition, the XRD patterns of Ho3þ/Yb3þ co-doped Lu3NbO7 samples with various Ho3þ/Yb3þ concentrations are presented in Fig. 2. All the diffraction peaks of the samples are exactly assigned to the pure phase Lu3NbO7 and no impurities are identified, which clearly imply that Ho3þ/Yb3þ co-doping do not change the crystal structure of the host lattice. The effective ionic radii of Ho3þ ions (0.1015 nm CN ¼ 8) and Yb3þ ions (0.0985 nm CN ¼ 8) are very close to those of Lu3þ ions (0.0977 nm CN ¼ 8) [25]. Besides, due to the same valence state and the similar ionic radii of RE3þ, the Ho3þ and Yb3þ ions probably occupy the Lu3þ sites. As shown in Fig. 2(b) and (d), the diffraction angles of (1 1 1) crystal plane shift toward lower angle with increasing Ho3þ and Yb3þ concentrations. According to the Bragg equation (2dsinθ ¼ nλ), the phenomenon can be ascribed that smaller radii of Lu3þ ions are substituted by larger radii of Ho3þ and Yb3þ. The above results show that Ho3þ and Yb3þ ions successfully incorporate into the Lu3NbO7 host lattice. Rietveld refinement was further carried out based on the crystallo graphic data of Lu3NbO7 (JCPDS No. 024-1089) from the XRD pattern using the General Structure Analysis System (GSAS) program. The Rietveld refinement of the Lu3NbO7:7% Yb3þ/1.5% Ho3þ sample is shown in Fig. 3(a), The reliability factors of weighted profile R-factor (Rwp) and the R-factor (Rp) are converged to 9.72 and 8.40%, respec tively. The relatively smaller goodness of fit (�2 ¼ 1.39) indicates good reliability of the refinement results. The XRD peaks of Lu3NbO7 could be indexed in the cubic system with the space group Fm-3m with the cell parameters of a ¼ b ¼ c ¼ 5.1808(7) Å, β ¼ 90� , and is composed of 8fold coordinated Lu3þ and Nb5þ ions (Fig. 3(b)). In this structure, the Lu and Nb ions randomly occupy the cation site (4a site in the space group Fm-3m) in the ratio of 3:1, and 1/8 of the oxide ions are random defect at the anion site (8b site). Fig. 4(a) shows the scanning electron microscope (SEM) images of typical Lu3NbO7:7% Yb3þ/1.5%Ho3þ sample, which shows the morphology is rice-like and the particle size is about 1 μm. The element analysis of energy-dispersive spectroscopy (EDS) was used to further characterize the chemical composition of the as-prepared product, and the results shown in Fig. 4(b) confirm that element ratios consist with the chemical formula of Lu3NbO7:7%Yb3þ/1.5% Ho3þ, with the Lu:Yb: Ho molar ratio close to 183:14:3. The results confirm that Yb3þ and
Fig. 5. The FTIR spectra of Lu3NbO7 samples doped with Ho3þ (a) and Yb3þ (b) doping concentration.
Fig. 1. When calcinated time from 6 to 10 h, the main diffraction peaks are well consistent with the target product (Lu3NbO7, Joint Committee for Power Diffractions Standards, JCPDS standard card No. 024-1089),
Fig. 6. The upconversion luminescence spectra and (b) the CIE chromaticity coordinates (x, y) of Lu3NbO7:20% Yb3þ/a%Ho3þ (a ¼ 1–10). 4
J. Liao et al.
Optical Materials 98 (2019) 109452
centers at 668 nm (5F5→5I8 of Ho3þ) are observed in the emission spectra. Moreover, the weakest far-red emission centered at 759 nm (5F4/5S2→5I7 of Ho3þ) is also observed in the UC emission spectra. The intensity of green emission increases first with increasing Ho3þ con centrations and reaches a maximum at a ¼ 1.5, then the concentration quenching occurs. However, the intensity of red emission keeps decreasing continuously with the increasing Ho3þ contents. And the ratio of red to green (R/G) decreases with increasing Ho3þ (a%) content from 1 to 10, which indicates that there may be presence a pathway of back energy-transfer process (BET1): 5F5 (Ho3þ)þ2F7/2 (Yb3þ)→5I7 (Ho3þ)þ2F5/2(Yb3þ) [28]. The CIE chromaticity coordinates (x, y) for Lu3NbO7:20%Yb3þ/a% Ho3þ phosphors were calculated and presented in Fig. 6(b). The data of CIE (x,y) are displayed in Table 1. The representing features of chro maticity coordinates for Lu3NbO7:20%Yb3þ/a%Ho3þ phosphors could be tuned from primrose yellow (0.4087, 0.5839) to green (0.3684, 0.6227) by increasing the Ho3þ concentration. Similarly, the UC luminescence spectra of Lu3NbO7:b%Yb3þ/1.5% 3þ Ho (b ¼ 2–15) samples is depicted in Fig. 7(a). The spectra of all samples with different Yb3þ concentrations are similar in shape, but differ in luminescence intensity. Unlike the Yb3þ/Ho3þ co-doped Lu3NbO7 with different Ho3þ concentrations, the Yb3þ/Ho3þ co-doped Lu3NbO7 with different Yb3þ concentrations exhibits intense green (centered at 550 nm) and red emission peak (centered at 668 nm) under the excitation of 980 nm. The intensities of red and green emission grow with increasing Yb3þ contents (Ho3þ content remains constant at a ¼ 1.5), and reaches a maximum at b ¼ 7. When the Yb3þ concentration exceeds the threshold, the upconversion luminescence intensity de creases. It is due to the occurrence of the concentration quenching. Two factors result in quenching effect: (a) at higher Yb3þ concentrations, the energy may be mainly transferred between Yb3þ ions and then migrates to defects or other quenching centers; (b) the distances between Yb3þ and Ho3þ shortens at higher Yb3þ concentration which contribute to the increased rates of the back energy transfer from Ho3þ to Yb3þ ions BET2: 5 F4/5S2 (Ho3þ) þ 2F7/2 (Yb3þ)→5I6 (Ho3þ) þ 2F5/2 (Yb3þ) [29] and re sults in the decrease of UC luminescence intensity. Therefore, the 5 F4/5S2 intermediary level is depopulated, which may be the main reason for the increase of R/G ratio. And the CIE chromaticity coordinates (x, y) for Lu3NbO7: b%Yb3þ/ 1.5%Ho3þ phosphors were calculated and presented in Fig. 7(b). The representing features of chromaticity coordinates for Lu3NbO7: b%Yb3þ, 1.5%Ho3þ phosphors could be tuned from green (0.3241, 0.6680) to yellow (0.3750, 0.6169) by controlling the Yb3þ concentration.
Table 1 The chromaticity coordinates CIE (x, y) of Lu3NbO7:20% Yb3þ/a%Ho3þ and Lu3NbO7:b%Yb3þ/1.5% Ho3þ phosphors. a/b (Ho3þ/Yb3þ) content
Sample 3þ
3þ
Lu3NbO7:20% Yb /a%Ho 1 2 3 4 5 Lu3NbO7:b%Yb3þ/1.5% Ho3þ 6 7 8 9 10
a 1 1.5 3 5 10 b 2 5 7 10 15
CIE coordinates (x, y) (0.409, 0.584) (0.390, 0.602) (0.384, 0.608) (0.376, 0.616) (0.368, 0.623) (0.324, 0.668) (0.342, 0.650) (0.343, 0.649) (0.361, 0.631) (0.375, 0.617)
Ho3þ ions have been effectively incorporated into the Lu3NbO7.host lattice, which is agreement with the XRD analysis above. Moreover, the elemental mapping technique was used to confirm the composition uniformity of Lu3NbO7:Yb3þ/Ho3þ, as shown in Fig. 4(c–g). The elemental mapping images exhibit that Lu, Nb, O, Yb and Ho are ho mogeneously distributed within the phosphor, which improves homo geneously luminescence of Lu3NbO7: Yb3þ/Ho3þ phosphors in the field of lights and displays. 3.2. FTIR spectra Fig. 5(a) and (b) illustrate the FTIR spectra of Lu3NbO7 samples doped with Ho3þ and Yb3þ doping concentration, respectively. From Fig. 5, it can be clearly seen that the FTIR spectra of Lu3NbO7:Yb3þ/ Ho3þ with different doping contents have similar peak positions, which further confirms that the phosphors have the same composition. The weak broad band centered at about 3845 cm 1 and 1625 cm 1 are due to the symmetrical O–H stretching and bending vibration of water molecule absorbed on the surface of the phosphor from the air atmo sphere [26]. The main bands around 648 cm 1 and 492 cm 1 corre spond to Nb–O and Lu–O vibrations, respectively [27]. The low vibration energy of Lu3NbO7:Yb3þ/Ho3þ will help to improve the upconversion emission intensity. 3.3. UC luminescence properties Fig. 6(a) presents the upconversion luminescence spectra of Lu3NbO7:20% Yb3þ/a%Ho3þ (a ¼ 1–10). A weaker green emission band centers at 550 nm (5F4/5S2→5I8 of Ho3þ) and strong red emission band
Fig. 7. (a) The upconversion luminescence spectra and (b) the CIE chromaticity coordinates (x, y) of Lu3NbO7:b%Yb3þ/1.5% Ho3þ (b ¼ 2–15). 5
J. Liao et al.
Optical Materials 98 (2019) 109452
Fig. 10. Temperature-dependent emission spectra (under 980 nm excitation) of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor at different temperatures. (b) The normalized emission intensities of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor at different temperatures.
Fig. 8. (a) The UC emissions spectra and (b) dependences of the UC intensities (Iem) of 5F4/5S2→5I8 and 5F5→5I8 as well as 5F4/5S2→5I7 transitions on the 980 nm pumping laser power (Ip) for Lu3NbO7: Yb3þ/Ho3þ phosphors.
power ln(P). The values of the slope (n) of the fitting lines are listed in Fig. 8. As shown in Fig. 8(b), in Ho3þ doped Lu3NbO7 sample, the slopes for 550 nm green emission (5F4/5S2(Ho3þ)→5I8(Ho3þ)), 668 nm red emission (5F5(Ho3þ)→5I8(Ho3þ)) and 759 nm far-red emission (5F4/5S2(Ho3þ)→5I7(Ho3þ)) are fitting to be 2.23, 2.18 and 2.34, respectively, which indicates that two-photon processes are involved in producing the green and red UC emission in Ho3þ doped system. According to the pump power dependence of UC emission intensity, the energy level diagram with possible transition and population process in Ho3þ/Yb3þ co-doped Lu3NbO7 systems are illustrated to better comprehend the UC luminescence mechanism in Fig. 9. Under 980 nm excitation, the 5I6 intermediary level of Ho3þ has three ways to accomplish population: the ET1 process from the 2F5/2 state of Yb3þ to Ho3þ: 5I8(Ho3þ)þ2F5/2(Yb3þ)→5I6(Ho3þ)þ2F7/2(Yb3þ) and the ground state absorption (GSA) of Ho3þ(5I8→5I6). For the two processes mentioned above, ET1 process plays a dominant role due to the larger absorption cross section of Yb3þ around 980 nm and the energy transfer between Yb3þ (2F5/2 → 2F7/2) and Ho3þ (5I8→5I6). Subsequently, the population of the upper levels 5S2/5F4 for Ho3þ are obtained via ET3 process or excitation state absorption (ESA). Finally, green emission is generated by radiative transitions from the excited 5S2/5F4 states to the ground 5I8 state. For the red emission of Ho3þ (5F5→5I8), there are usually two pathways to populate in the excited state 5F5 level of Ho3þ. It could be realized by the non-radiative (NR) transition from upper 5 F4/5S2 levels to 5F5 level and the population of 5I7 intermediate level via
Fig. 9. Energy level diagrams of Yb3þ and Ho3þ ions and UC emission mech anism in Lu3NbO7: Yb3þ/Ho3þ phosphors.
3.4. Pump power dependence Generally, the UC emission intensity (I) depends on pump-power (P): I ∝ Pn, where n is approximately equal to the number of pump photons required to excite RE ions from the ground state to the excited state at relatively lower pump power. The value of n can be obtained from the slope of the straight line by linear fitting the plot of ln(I) and pump 6
J. Liao et al.
Optical Materials 98 (2019) 109452
NR1 transition of Ho3þ (5I6→5I7). In other words, part of the electrons in the 5I6 level will first relax to the 5I7 level by the NR2 process and then populate to the 5F5 level via the ET2 process [5I7(Ho3þ)þ2F5/ 3þ 5 3þ 2 3þ 2(Yb )→ F5(Ho )þ F7/2(Yb )]. Finally, the electrons in the excited 5 F5 state jump to the ground 5I8 state of Ho3þ to produce a 668 nm red emission. And the ratio of R/G decreases with increasing Ho3þ contents from 1 to 10, which indicates that there may be the process of BET1: [5F5(Ho3þ)þ2F7/2(Yb3þ)→5I7(Ho3þ)þ2F5/2(Yb3þ)]. It is noteworthy that BET2 process [5S2/5F4(Ho3þ)þ2F7/2(Yb3þ)→5I6(Ho3þ)þ2F5/2(Yb3þ)] can occur to decrease the green emission in high Yb3þ concentration doped samples, which is consistent with the variation of upconversion
Table 2 The relative intensity and R of different emission peaks with their temperature range. Emission peak (nm)
550
668
759
I298K I523K (I298k-I523k)/I298K R (K 1)
1 0.184 0.816 3.63 � 10
0.839 0.265 0.684 3.04 � 10
0.298 0.034 0.886 3.94 � 10
3
3
3
Fig. 11. Dependence of FIR for the 550, 668 and 759 nm emissions on the absolute temperature. 7
J. Liao et al.
Optical Materials 98 (2019) 109452
Table 3 SA and SR values, transitions and type of temperature sensing as well as temperature ranges of different RE ions doped temperature sensing materials. (TCL: thermal coupled levels, NTCL: non-thermally coupled levels.) RE3þ/Host 3þ
Er /SrWO4 Sm3þ/YNbO4 Ho3þ/CaMoO4 Ho3þ/Y2O3 Er3þ/Bi7Ti4NbO21 Er3þ/Y2WO6 Eu3þ/LiNbO3 Ho3þ/Ba3Y4O9 Ho3þ/Lu3NbO7 Ho3þ/Lu3NbO7 Ho3þ/Lu3NbO7
Transitions 2
4
4
H11/2/ S3/2 → I15/2 4 F3/2/4G5/2 → 6H5/2 5 F3/3K8→5I8 5 F3/3K8→5I8 4 F9/2/4S3/2 → 4I15/2 4 F9/2/2H11/2 → 4I15/2 5 D0/5D1→7F2/7F1 (5F4,5S2)/5F5→5I8 (5F4/5S2→5I8)/(5F5→5I8) (5F5→5I8)/(5F4/5S2→5I7) (5F4/5S2→5I8)/(5F4/5S2→5I7)
Type
Range (K)
SA (K 1)
SR (K
TCL TCL TCL TCL NTCL NTCL NTCL NTCL NTCL NTCL NTCL
299–518 303–773 303–543 299–673 153–553 303–563 303–723 294–573 298–523 298–523 298–523
0.015 (403 K) 0.0007 (700 K) 0.0066 (353 K) 0.0030 (673 K) 0.0044 (153–450 K) 0.0022 (303–563 K) 0.0007 (303–723 K) 0.0017 (573 K) 0.0024 (298–523 K) 0.0233 (298–523 K) 0.0090 (298–523 K)
0.0097 0.0043 0.0071 0.0024 / 0.0117 0.0427 0.0036 0.0037 0.0094 0.0027
Ref
(299 K) (500 K) (303 K) (673 K)
[36] [37] [38] [34] [39] [35] [40] [13] This work This work This work
(303 K) (303 K) (448 K) (523 K) (298 K) (298 K)
(5F4/5S2→5I8)/(5F4/5S2→5I7) NTCLs of Ho3þ are 0.0024 K-1, 0.0233 K-1 and 0.0090 K-1, respectively. As a comparison, the optical thermometry parameters of different RE ions doped temperature sensing materials based on the TCLs and the NTCLs are tabulated in Table 3. From Table 3, The maximum sensitivity of 5F5/5I4 NTCLs is 0.0233 K-1 among there pairs NTCLs in the Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor. The maximum sensitivity of 5F5/5I4 NTCLs for Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor is obviously higher than those of the 5F3/3K8 TCLs of Ho3þ for (0.0030 K-1) Y2O3:Ho3þ/Yb3þ/Zn2þ [34] and (0.0066 K-1) CaMoO4: Ho3þ/Yb3þ/Mg2þ phosphors [12], also higher than those of 4 F9/2/2H11/2 NTCLs of Er3þ for (0.0022 K-1) Y2WO6:Er3þ/Yb3þ phos phors [35]. For the same (5F4,5S2)/5F5 NTCL of Ho3þ, the sensitivity (0.0024 K-1) of Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor is slightly higher than that (0.0017 K-1) of Ba3Y4O9:Ho3þ/Yb3þ/Tm3þ [13]. The relative � � � 1 dðFIRÞ� � sensitivity (SR) was calculated by using equation [14]: SR ¼ ��FIR dðTÞ �,
3.5. Temperature sensing behavior The emission spectra of the typical Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor under 980 excitation at various temperatures are displayed in Fig. 10(a). The emission intensities of green 5F4/5S2→5I8 and red 5 F5→5I8 as well as far-red 5F4/5S2→5I7 of Ho3þ exhibit obvious change with temperature. Fig. 10(b) depicts the relative intensities of the 5 F4/5S2→5I8 transition of 550 nm (integrated from 520 to 580 nm) and 5 F5→5I8 transition of 668 nm (integrated from 620 to 700 nm) as well as 5 F4/5S2→5I7 transition of 759 nm (integrated from 730 to 780 nm) as a function of the absolute temperature. With increasing temperature, the intensities of the green and red as well as far-red emission peaks decrease gradually, but their decay rates of temperature-related luminescence intensity (R) are different. R is calculated by the following equation: R ¼ ðI1 I2 Þ=I ðT T Þ, where I1 2
)
� � �dðFIRÞ� 5 5 5 5 5 5 5 5 5 5 � � � dðTÞ �. The SA of ( F4/ S2→ I8)/( F5→ I8), ( F5→ I8)/( F4/ S2→ I7) and
luminescence intensity (See Fig. 5). Far-red emission(759 nm) is ob tained via the transition process [5S2/5F4(Ho3þ)→5I7(Ho3þ) ].
1
1
1
and the resultant curve as a function of the absolute temperature is shown in Fig. 11(b), (d) and (f). The SR for (5F4/5S2→5I8)/(5F5→5I8), (5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/(5F4/5S2→5I7) NTCLs of Ho3þ are 0.0037 K-1, 0.0094 K-1 and 0.0027 K-1, respectively. The maximum SA (0.0094 K-1) of the (5F5→5I8)/(5F4/5S2→5I7) NTCLs for Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor is higher than those of the 5 F3/3K8 TCLs of Ho3þ for (0.0024 K-1) Y2O3:Ho3þ/Yb3þ/Mg2þ and (0.0071 K-1) CaMoO4:Ho3þ/Yb3þ/Mg2þ phosphors. These results imply that Lu3NbO7:Yb3þ/Ho3þ material using FIR from the 5F5/5I4 NTCLs of Ho3þ is very promising materials for the application in the optical thermometry.
and I2 are the relative intensity of emission peak at low temperature (T1) and high temperature (T2), respectively. According to the data of Fig. 10 (b) and the formula of R, the relative intensity and R of different emis sion peaks with their temperature range were listed in Table 2. From Table 2, the order of R is R759nm > R550nm > R668 nm. This reason may be related to the non-radiative probability (Knr) [30–33]. Knr can be approximately described by the Mott-Seitz model by Knr ∝ € is the energy gap between two levels, ^eB is the expð ΔE =κB TÞ, where AE Boltzmann constant and T is the absolute temperature. Therefore, the increasing temperature could urge the occurrence of the nonradiative (NR) relaxations (NR1:5F4/5S2→5F5 and NR2:5I6→5I7) in Fig. 9, which reduces the population number of the electrons on the 5F4/5S2 levels of Ho3þ and is beneficial to the electron population on the 5F5 level. As a result, the intensity ratio of green/red FIR (I550/I668) has been decreased with the temperature increased. For emission peaks form the same en ergy level of the excited state, the smaller the energy gap of emission peak, the larger the non-radiative probability. With the temperature increased, far-red emission intensity (759 nm 5F4/5S2→5I7 of Ho3þ) de cays faster than that of green (550 nm 5F4/5S2→5I8 of Ho3þ). As a result, the intensity ratio of red/far-red FIR (I668/I759) and green/far-red FIR (I550/I759) have been increased with the temperature increased. Based on this finding, green/red FIR (I550/I668) and red/far-red FIR (I668/I759) as well as green/far-red FIR (I550/I759) are considered for use as a function of temperature at the given temperature range (298–523 K). The FIR as a function of temperatures in the range of 298–523 K were calculated and plotted in Fig. 11. Fig. 11(a), (c) and (e) depict the dependence of FIR for the two emissions based on NTCLs on the absolute temperature. The experimental data can be well fitted by linear rela tionship as FIR (I550/I668) ¼ -0.0024T þ 1.9261, FIR (I668/I759) ¼ 0.0233T-4.4461 and FIR(I550/I759) ¼ 0.0090T þ 0.7567. The absolute sensitivity (SA) is defined by the equation [14]: SA ¼
4. Conclusions A series of Yb3þ, Ho3þ codoped Lu3NbO7 phosphors were prepared by a high-temperature solid-state reaction method. Excited by 980 nm, multicolor visible emission including green, red, and NIR can be ob tained. The phosphors could be tuned from primrose yellow to green by increasing the Ho3þ concentration and could be tuned from green to primrose yellow by varying the concentration of Yb3þ. The possible UC mechanisms are proposed to be two-photon processes for various color formations, respectively. UC emission arising from Stark energy levels to ground state transitions (5F4/5S2→5I8 (550 nm), 5F5→5I8 (668 nm), 5 F4/5S2→5I7 (759 nm)) was observed. And using the FIR technique, multiple temperature sensing performances based on the three pairs of (5F4/5S2→5I8)/(5F5→5I8),(5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/ (5F4/5S2→5I7) NTCLs of Ho3þare investigated in a temperature range of 298–523 K. The maximum absolute sensitivity of the (5F5→5I8)/ (5F4/5S2→5I7) NTCLs of Ho3þ is calculated to be 0.0233 K 1 at 523 K. These results indicate that Lu3NbO7:Yb3þ/Ho3þ could be used in an optical thermometer in noncontact temperature measurements and display fields. 8
J. Liao et al.
Optical Materials 98 (2019) 109452
Declaration of competing interest
[17] H. Suo, C. Guo, J. Zheng, B. Zhou, C. Ma, X. Zhao, T. Li, P. Guo, E. Goldy, Sensitivity modulation of Upconverting thermometry through Engineering phonon energy of a matrix, ACS Appl. Mater. Interfaces 8 (2016) 30312–30319. [18] B. Dong, R. Hua, B. Cao, Z. Li, Y. He, Z. Zhang, O.S. Wolfbeis, Size dependence of the upconverted luminescence of NaYF4: Er, Yb microspheres for use in ratiometric thermometry, Phys. Chem. Chem. Phys. 16 (2014) 20009–20012. [19] M.S. Meijer, P.A. Rojas-Gutierrez, D. Busko, I.A. Howard, F. Frenze, C. Wurth, U. Resch-Genger, B.S. Richards, A. Turshatov, J.A. Capobianco, S. Bonnet, Absolute upconversion quantum yields of blue- emitting LiYF4: Yb3þ, Tm3þ upconverting nanoparticles, Phys. Chem. Chem. Phys. 20 (2018) 22556–22562. [20] Q. Wang, J.S. Liao, J.F. Guo, H.P. Huang, H.R. Wen, Upconversion emission and temperature sensing behavior of LuF3:Yb3þ,Er3þ microcrystals prepared by onestep hydrothermal synthesis, Chin, J. Inorg. Chem. 34 (2018) 579–588. [21] Y.Y. Du, Y. Zhang, K.K. Huang, S. Wang, L. Yuan, S.H. Feng, Hydrothermal synthesis and photoluminescence properties of rare-earth niobate and tantalate nanophosphors, Dalton Trans. 42 (2013) 8041–8048. [22] K.Y. Kim, A. Durand, J.M. Heintz, A. Veillere, V. Jubera, Spectral evolution of Eu3þ doped Y3NbO7 niobate induced by temperature, J. Solid State Chem. 235 (2016) 169–174. _ [23] M. Ilhan, M.K. Ekmekçi, R.G. Oraltay, A.S. Bas¸ak, Structural and near-infrared properties of Nd3þ activated Lu3NbO7 Phosphor, J. Fluoresc. 27 (2017) 199–203. [24] Y. Doi, Y. Harada, Y. Hinatsu, H. Unive, Crystal structures and magnetic properties of fluorite-related oxides Ln3NbO7 (Ln¼lanthanides), J. Solid State Chem. 182 (2009) 709–715. [25] Y.Q. Jia, Crystal radii and effective ionic radii of the rare earth ions, J. Solid State Chem. 95 (1991) 184–187. [26] V.C. Farmer (Ed.), The Infrared Spectra of Minerals, The Mineralogical Society, London, 1974. [27] N.T. Mcdevitt, W.L. Baun, Infrared absorption study of metal oxides in the low frequency region (700-240 cm-l), Spectrochim. Acta 20 (1964) 799–808. [28] Y. Yu, Y. Zheng, F. Qin, L. Liu, C. Zheng, G. Chen, Z. Zhang, W. Cao, Influence of Yb3þ concentration on upconversion luminescence of Ho3þ, Opt. Commun. 284 (2011) 1053–1056. [29] S.F. Liu, S.B. Liu, H. Ming, D.J. Hou, W.X. You, X.Y. Ye, Investigation on the upconversion luminescence in Ho3þ/Yb3þ co-doped Ba3Sc4O9 phosphor, Mater. Res. Bull. 98 (2018) 187–193. [30] J. Zhang, G.B. Chen, Z.H. Hua, Up-conversion luminescence of novel Yb3þ-Ho3þ/ Er3þ doped Sr5(PO4)3Cl phosphors for optical temperature sensing, Opt. Mater. Express 7 (2017) 2084–2089. [31] T. Pang, J.J. Wang, Controllable upconversion luminescence and temperature sensing behavior in NaGdF4:Yb3þ/Ho3þ/Ce3þ nano-phosphors, Mater. Res. Express 5 (2018) 015049–015057. [32] Z. J. Huang Pang, A novel upconverison phosphor Gd2Mo4O15:Yb3þ,Ho3þ: intense red emission and ratiometric temperature sensing under 980 nm excitation, Mater. Res. Express 5 (2018) 066204–066213. [33] Zhang, X.W. Li, G.B. Chen, Upconversion luminescence of Ba9Y2Si6O24:Yb3þ-Ln3þ (Ln¼ Er, Ho, and Tm) phosphors for temperature sensing, Mater. Chem. Phys. 206 (2018) 40–47. [34] A. Pandey, V.K. Rai, Improved luminescence and temperature sensing performance of Ho3þ-Yb3þ-Zn2þ:Y2O3 phosphor, Dalton Trans. 42 (2013) 11005–11011. [35] J. Zhang, B.W. Ji, G.B. Chen, Z.H. Hua, Upconversion luminescence and discussion of sensitivity improvement for optical temperature sensing application, Inorg. Chem. 57 (2018) 5038–5047. [36] A. Pandey, V.K. Raib, V. Kumara, V. Kumara, H.C. Swarta, Upconversion based temperature sensing ability of Er3þ–Yb3þcodoped SrWO4: an optical heating phosphor, Sens. Actuators B Chem. 209 (2015) 352–358. [37] L.R. Ða�eanin, S.R. Lukiæ-Petroviæ, D.M. Petroviæ, M.G. Nikoliæ, M. D. Dramiæanin, Temperature quenching of luminescence emission in Eu3þ- and Sm3þ-doped YNbO4 powders, J. Lumin. 6 (2014) 82–87. [38] R. Dey, A. Kumari, A.K. Soni, V.K. Rai, CaMoO4:Ho3þ–Yb3þ–Mg2þ upconverting phosphor for application in lighting devices and optical temperature sensing, Sens. Actuators B Chem. 210 (2015) 581–588. [39] H. Zou, J. Li, X.S. Wang, D.F. Peng, Y.X. Li, X. Yao, Color-tunable upconversion emission and optical temperature sensing behaviour in Er-Yb-Mo codoped Bi7Ti4NbO21 multifunctional ferroelectric oxide, Opt. Mater. Express 4 (2014) 1545–1554. [40] Z. Liang, F. Qin, Y.D. Zheng, Z.G. Zhang, W.W. Cao, Noncontact thermometry based on downconversion luminescence from Eu3þ doped LiNbO3 single crystal, Sens. Actuators A Phys. 238 (2016) 215–219.
The authors declare the following financial interests/personal re lationships which may be considered as potential competing interests: The National Natural Science Foundation of China (No.51862012) and the Major Project of Natural Science Foundation of Jiangxi Province (No. 20165ABC28010). Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.51862012) and the Major Project of Natural Science Foundation of Jiangxi Province (No. 20165ABC28010). References [1] R.R. Deng, F. Qin, R.F. Chen, W. Huang, M.H. Hong, X.G. Liu, Temporal full-colour tuning through non-steady-state upconversion, Nat. Nanotechnol. 10 (2015) 237–242. [2] J.S. Liao, Q. Wang, L.Y. Kong, Z.Q. Ming, Y.L. Wang, Y.Q. Li, L.X. Che, Effect of Yb3 þ concentration on tunable upconversion luminescence and optically temperature sensing behavior in Gd2TiO5:Yb3þ/Er3þphosphors, Opt. Mater. 75 (2018) 841–849. [3] Y.Y. Cheng, A. Nattestad, T.F. Schulze, R.W. MacQueen, B. Fückel, K. Lips, G. G. Wallace, T. Khoury, M.J. Crossley, T.W. Schmidt, Increased upconversion performance for thin film solar cells: a trimolecular composition, Chem. Sci. 7 (2016) 559–568. [4] J.S. Liao, L.L. Nie, Q. Wang, S.J. Liu, H.R. Wen, J.P. Wu, NaGd(WO4)2:Yb3þ/Er3þ phosphors: hydrothermal synthesis, optical spectroscopy and green upconverted temperature sensing behavior, RSC Adv. 6 (2016) 35152–35159. [5] D. Pugliese, N.G. Boetti, J. Lousteau, E. Ceci-Ginistrelli, E. Bertone, F. Geobaldo, D. Milanese, Concentration quenching in an Er-doped phosphate glass for compact optical lasers and amplifiers, J. Alloy. Comp. 657 (2016) 678–683. [6] J. Pichaandi, J.C. Boyer, K.R. Delaney, F.C.J.M. van Veggel, Two-photon upconversion laser (scanning and wide-field) microscopy using Ln3þ-doped NaYF4 upconverting nanocrystals: a critical evaluation of their performance and potential in bioimaging, J. Phys. Chem. C 115 (2011) 19054–19064. [7] B. Dong, B. Cao, Y. He, Z. Liu, Z. Li, Z. Feng, Temperature sensing and in vivo imaging by molybdenum sensitized visible upconversion luminescence of rareearth oxides, Adv. Mater. 24 (2012) 1987–1993. [8] X.D. Wang, O.S. Wolfbeis, R.J. Meier, Luminescent probes and sensors for temperature, Chem. Soc. Rev. 42 (2013) 7834–7869. [9] J.S. Liao, Q. Wang, L.F. Lan, J.F. Guo, L.L. Nie, S.J. Liu, H.R. Wen, Microwave hydrothermal synthesis and temperature sensing behavior of Lu2Ti2O7:Yb3þ/Er3þ nanophosphors, Curr. Appl. Phys. 17 (2017) 427–432. [10] L.L. Tong, X.P. Li, R.N. Hua, L.H. Cheng, J.S. Sun, J.S. Zhang, S. Xu, H. Zheng, Y. Q. Zhang, B.J. Chen, Optical temperature sensing properties of Yb3þ/Tm3þ codoped NaLuF4 crystals, Curr. Appl. Phys. 17 (2017) 999–1004. [11] J. Zhang, B.W. Ji, G.B. Chen, Z.H. Hua, Upconversion luminescence and discussion of sensitivity improvement for optical temperature sensing application, Inorg. Chem. 57 (2018) 5038–5047. [12] X.B. Tu, J.S. Xu, M.C. Li, T. Xie, H.P. Wang, S.Q. Xu, Color-tunable upconversion luminescence and temperature sensing behavior of Tm3þ/Yb3þ codoped Y2Ti2O7 phosphors, Mater. Res. Bull. 112 (2019) 77–83. [13] S.F. Liu, H. Ming, J. Cui, S.B. Liu, W.X. You, X.Y. Ye, Y.M. Yang, H.P. Nie, R. X. Wang, High sensitive Ln3þ/Tm3þ/Yb3þ (Ln3þ¼Ho3þ, Er3þ) tri-doped Ba3Y4O9 upconverting optical thermometric materials based on diverse thermal response from non-thermally coupled energy levels, Ceram. Int. 45 (2019) 1–10. [14] J. Zhang, Y.Q. Zhang, X.M. Jiang, Investigations on upconversion luminescence of K3Y(PO4)2:Yb3þ-Er3þ/Ho3þ/Tm3þ phosphors for optical temperature sensing, J. Alloy. Comp. 748 (2018) 438–445. [15] X.N. Chai, J. Li, X.S. Wang, Y.X. Li, X. Yao, Upconversion luminescence and temperature-sensing properties of Ho3þ/Yb3þ -codoped ZnWO4 phosphors based on fluorescence intensity ratios, RSC Adv. 7 (2017) 40046–40052. [16] S.F. Le� on-Luis, U.R. Rodríguez-Mendoza, P. Haro-Gonzalez, I.R. Martín, V. Lavín, Role of the host matrix on the thermal sensitivity of Er3þ luminescence in optical temperature sensors, Sens. Actuators B Chem. 174 (2012) 176–186.
9
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Tunable upconversion luminescence and optical temperature sensing based on non-thermal coupled levels of Lu3NbO7:Yb3þ/Ho3þ phosphors Jinsheng Liao *, Liyun Kong, Minghua Wang, Yijian Sun, Guoliang Gong School of Chemistry and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou, Jiangxi, 341000, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lu3NbO7 Phosphor Optical temperature sensing Upconversion luminescence
The color-tunable upconversion (UC) emission and optical temperature sensing based on non-thermal coupled levels (NTCL) were observed from the Yb3þ/Ho3þ codoped Lu3NbO7 phosphors synthesized by the solid-state method. The phosphors are capable of generating color tunable UC luminescence from green (yellow) to yel low (green) with the increase of the Yb3þ (Ho3þ) concentration. The tunable emission is due to the different energy back transfer processes from Ho3þ to Yb3þ. The temperature sensing performances are investigated in the temperature range of 293–573 K based on NTCL by using fluorescence intensity ratio technology. The maximum absolute sensitivities are 0.37%K 1, 0.94%K 1, 0.27%K 1 at 298 K, which are based on three pairs NTCL of (5F4/5S2→5I8)/(5F5→5I8), (5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/(5F4/5S2→5I7) of Ho3þ, respectively. The above results suggest that the as-prepared Lu3NbO7:Yb3þ/Ho3þ phosphors have great potential for the appli cation prospects of upconverter, color tunable device and optical temperature sensor.
1. Introduction Rare-earth (RE) doped upconversion (UC) inorganic luminescent materials have received more and more attention because of its broad potential applications, such as color display, optoelectronics, sensor technology, laser cooling, optical data storage and vivo imaging [1–6]. Recently, RE3þ doped UC inorganic luminescent materials as optical temperature sensors have attracted considerable attention, since it shows unique advantages in fast response, electromagnetic passivity and high temperature sensitivity [7,8]. At present, the popular strategy for the optical temperature sensing is based on the UC fluorescence intensity ratio (FIR) of thermal coupled levels (Er3þ: 2H11/2/4S3/2 [9]; Tm3þ: 3F2, 3 3þ 5 3 3/ H4 [10]; Ho : F3/ K8 [11]; etc). According to the formula of rela tive sensitivity (Sr, Sr ¼ △E/KBT2), a larger energy gap (△E) of thermal coupled levels (TCL)is favorable for achieving a higher relative sensi tivity. However, the tight restriction in the energy gap cannot exceed 2000 cm 1, which assures the strong coupling between the two levels. Therefore, further enhancement of thermometric sensitivity is restricted intrinsically. To solve the above disadvantage, an alternative strategy based on FIR between the non-thermally coupled levels (Er3þ: 2 4 H11/2F9/2 [12]; Tm3þ: 3H4/1G4 [13]; Ho3þ: 5F5/5F4,5S2 [14] has been adopted. Ding et al. have reported that the maximal relative sensitivity (Sr) of Yb3þ/Tm3þ:Y2Ti2O7 is ~0.81% K 1 by using FIR between the
non-thermally coupled levels (NTCL) of Tm3þ:3F2,3/1G4 [13]. Among the RE ions, Ho3þ doped luminescent materials are explored for tem perature sensing because of the ability of efficiently emit photons from ultraviolet to near infrared regions and its favorable intra-atomic 4f energy level structure and the relatively long-lived [15]. It was reported that the optical temperature sensitivity of Ho3þ doped phosphors de pends mainly on host types irrespective of the other conditions, such as selection of different non-thermal energy levels of Ho3þ as FIR tech nology. However, it is essential that the effects of different non-thermal energy levels of Ho3þ are considered for optical temperature sensitivity. The selection of the host matrix is an important factor to obtain desirable UC luminescence due to the different crystal fields caused by structural symmetry of the host materials [16,17]. So, it is important to select suitable host material for designing desirable UC phosphor. Over the last few years, extensive research has been done in the lanthanide doped UC phosphors, mostly on the fluoride based phosphors such as NaYF4:Yb3þ/Er3þ, LiYF4:Yb3þ/Tm3þ, LuF3:Yb3þ/Er3þ [18–20]. But the fluorides are usually poisonous, harmful to the environment and very sensitive to oxygen, which may influence the luminescence properties and limit their application [18]. Instead, oxides usually show high chemical stability and environmental-friendly characters [21]. So, more and more oxide-based UC phosphors have been paid attention in recent years. It is known the cubic fluorite-type rare-earth niobates with the
* Corresponding author. E-mail address: [email protected] (J. Liao). https://doi.org/10.1016/j.optmat.2019.109452 Received 10 July 2019; Received in revised form 25 September 2019; Accepted 14 October 2019 Available online 24 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
J. Liao et al.
Optical Materials 98 (2019) 109452
which produced red emission upon violet light excitation [22]. Ilhan et al. found that Nd3þ doped Lu3NbO7 phosphor exhibited excellent near-infrared emission under 800 nm excitation [23]. At present, a few reports mainly focused on the downshift luminescence properties of RE3NbO7 [21–23]. However, to the best of our knowledge, the UC properties of Lu3NbO7 based phosphors have not reported. In this paper, Lu3NbO7:Yb3þ/Ho3þ phosphors were synthesized by a simple solid state reaction method. UC luminescence properties of Lu3NbO7:Yb3þ/Ho3þ phosphors are discussed by change of pump power and Yb3þ (Ho3þ) ion concentration as well the measured temperature. Green, red and near-infrared (NIR) UC luminescence had been acquired upon a 980 nm laser excitation. The temperature sensing performances of the phosphors are investigated based on three pairs NTCL of (5F4/5S2→5I8)/(5F5→5I8), (5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/ (5F4/5S2→5I7) for Ho3þ by using FIR technology. The results indicate that Lu3NbO7:Yb3þ/Ho3þ phosphor as promising multifunctional ma terials can be potentially used for lightings and optical temperature sensors. 2. Experimental
Fig. 1. XRD pattern of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ samples at 1400 � C for different calcination time (6–12 h). The standard data for Lu3NbO7 (JCPDS NO.024-1089) is also presented in the figure.
2.1. Preparation of samples The Lu3NbO7:Yb3þ/Ho3þ phosphors were synthesized by a hightemperature solid state reaction. All the chemicals of Nb2O5 (99.9%), Lu2O3 (99.99%), Yb2O3 (99.99%), and Ho2O3 (99.99%) were used as the starting materials without any further purification. All raw materials
general formula RE3NbO7 (RE ¼ Lu, La, Gd) have attracted great attention due to their electro-optical and magnetic properties [22–24]. Eu3þ doped Y3NbO7 phosphor was initially investigated by Kim et al.
Fig. 2. The XRD patterns of Ho3þ/Yb3þ co-doped Lu3NbO7 samples with various Ho3þ (a) and Yb3þ (c) concentrations. The enlarge figures for (b) and (d) are corresponded to the figures of (a) and (c), respectively. 2
J. Liao et al.
Optical Materials 98 (2019) 109452
Fig. 3. (a) The Rietveld refinement of the Lu3NbO7:7% Yb3þ/1.5% Ho3þ with the content of calculated (cross), background (green), experiment (black solid line), difference (blue) and Bragg position (vertical line) results; (b) Schematic illustration of the Lu3NbO7:7% Yb3þ/1.5% Ho3þ, and the coordination environment of the [LuO8]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
were weighted according to the stoichiometry of the Lu3(1-x-y)Ho3x Yb3yNbO7 formula. Then, the raw powders were mixed and ground in an agate mortar with ethanol until ethanol volatilization, heated in a muffle furnace at 1400 � C for different time (6–12 h) in air. When naturally cooled to room temperature, the as-prepared phosphors were crushed and ground for the subsequent measurements.
order to investigate the temperature dependence of the UC emission, the sample was placed in a temperature-controlled copper cylinder, and its temperature was increased from 298 to 523 K (Temperature measure ment resolution is 0.1 K). The UC spectra of sample at various temper atures were obtained using a Horiba/Jobin Yvon Fluorolog-3 double monochromator equipped with a Hamamatsu R928 Photomultiplier under the excitation of a 980 nm diode laser with 150 mW and the excitation power density was about 3 W/cm2. The slits of emission and excitation were both set as 1 nm in the spectral measurement.
2.2. Characterization of samples The samples were characterized by powder X-ray diffraction (XRD) performed on a Panalytical X’Pert diffractometer using CuKα1 radiation (λ ¼ 0.154187 nm). All of the patterns within the 10–90� 2θ range were collected in a scanning mode with a step size of 0.02� . The morphology patterns of samples were obtained on a field emission scanning electron microscope (SEM JSM-6700F) equipped with an energy-dispersive X-ray spectra (EDS). Lu3NbO7:Yb3þ/Ho3þ phosphors was investigated by using Fourier transform infrared spectroscopy (FT-IR, Nicolet5700). In
3. Results and discussion 3.1. Structure and morphology To investigate the effect of calcination time on the phase purity of the as-prepared samples, XRD pattern of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ samples at 1400 � C for different calcination time (6–12 h) are shown in
Fig. 4. (a) SEM and (b) EDS of Lu3NbO7:7% Yb3þ/1.5%Ho3þ; (c) Elemental mapping of Lu, Nb, O, Yb and Ho in Lu3NbO7:7% Yb3þ/1.5%Ho3þ. 3
J. Liao et al.
Optical Materials 98 (2019) 109452
except for some weak diffraction peaks of Nb2O5. When calcinated time for 12 h, the diffraction peaks of the product matches well with the standard data of cubic phase Lu3NbO7 (JCPDS card No. 024-1089), and no traces of additional peaks from other phases were observed. The above-stated results indicate that the calcination time is very important for the formation of the pure cubic phase Lu3NbO7. Therefore, the calcination time (12 h) is selected as the preparation of the Ho3þ/Yb3þ co-doped Lu3NbO7 samples with various Ho3þ/Yb3þ concentrations. In addition, the XRD patterns of Ho3þ/Yb3þ co-doped Lu3NbO7 samples with various Ho3þ/Yb3þ concentrations are presented in Fig. 2. All the diffraction peaks of the samples are exactly assigned to the pure phase Lu3NbO7 and no impurities are identified, which clearly imply that Ho3þ/Yb3þ co-doping do not change the crystal structure of the host lattice. The effective ionic radii of Ho3þ ions (0.1015 nm CN ¼ 8) and Yb3þ ions (0.0985 nm CN ¼ 8) are very close to those of Lu3þ ions (0.0977 nm CN ¼ 8) [25]. Besides, due to the same valence state and the similar ionic radii of RE3þ, the Ho3þ and Yb3þ ions probably occupy the Lu3þ sites. As shown in Fig. 2(b) and (d), the diffraction angles of (1 1 1) crystal plane shift toward lower angle with increasing Ho3þ and Yb3þ concentrations. According to the Bragg equation (2dsinθ ¼ nλ), the phenomenon can be ascribed that smaller radii of Lu3þ ions are substituted by larger radii of Ho3þ and Yb3þ. The above results show that Ho3þ and Yb3þ ions successfully incorporate into the Lu3NbO7 host lattice. Rietveld refinement was further carried out based on the crystallo graphic data of Lu3NbO7 (JCPDS No. 024-1089) from the XRD pattern using the General Structure Analysis System (GSAS) program. The Rietveld refinement of the Lu3NbO7:7% Yb3þ/1.5% Ho3þ sample is shown in Fig. 3(a), The reliability factors of weighted profile R-factor (Rwp) and the R-factor (Rp) are converged to 9.72 and 8.40%, respec tively. The relatively smaller goodness of fit (�2 ¼ 1.39) indicates good reliability of the refinement results. The XRD peaks of Lu3NbO7 could be indexed in the cubic system with the space group Fm-3m with the cell parameters of a ¼ b ¼ c ¼ 5.1808(7) Å, β ¼ 90� , and is composed of 8fold coordinated Lu3þ and Nb5þ ions (Fig. 3(b)). In this structure, the Lu and Nb ions randomly occupy the cation site (4a site in the space group Fm-3m) in the ratio of 3:1, and 1/8 of the oxide ions are random defect at the anion site (8b site). Fig. 4(a) shows the scanning electron microscope (SEM) images of typical Lu3NbO7:7% Yb3þ/1.5%Ho3þ sample, which shows the morphology is rice-like and the particle size is about 1 μm. The element analysis of energy-dispersive spectroscopy (EDS) was used to further characterize the chemical composition of the as-prepared product, and the results shown in Fig. 4(b) confirm that element ratios consist with the chemical formula of Lu3NbO7:7%Yb3þ/1.5% Ho3þ, with the Lu:Yb: Ho molar ratio close to 183:14:3. The results confirm that Yb3þ and
Fig. 5. The FTIR spectra of Lu3NbO7 samples doped with Ho3þ (a) and Yb3þ (b) doping concentration.
Fig. 1. When calcinated time from 6 to 10 h, the main diffraction peaks are well consistent with the target product (Lu3NbO7, Joint Committee for Power Diffractions Standards, JCPDS standard card No. 024-1089),
Fig. 6. The upconversion luminescence spectra and (b) the CIE chromaticity coordinates (x, y) of Lu3NbO7:20% Yb3þ/a%Ho3þ (a ¼ 1–10). 4
J. Liao et al.
Optical Materials 98 (2019) 109452
centers at 668 nm (5F5→5I8 of Ho3þ) are observed in the emission spectra. Moreover, the weakest far-red emission centered at 759 nm (5F4/5S2→5I7 of Ho3þ) is also observed in the UC emission spectra. The intensity of green emission increases first with increasing Ho3þ con centrations and reaches a maximum at a ¼ 1.5, then the concentration quenching occurs. However, the intensity of red emission keeps decreasing continuously with the increasing Ho3þ contents. And the ratio of red to green (R/G) decreases with increasing Ho3þ (a%) content from 1 to 10, which indicates that there may be presence a pathway of back energy-transfer process (BET1): 5F5 (Ho3þ)þ2F7/2 (Yb3þ)→5I7 (Ho3þ)þ2F5/2(Yb3þ) [28]. The CIE chromaticity coordinates (x, y) for Lu3NbO7:20%Yb3þ/a% Ho3þ phosphors were calculated and presented in Fig. 6(b). The data of CIE (x,y) are displayed in Table 1. The representing features of chro maticity coordinates for Lu3NbO7:20%Yb3þ/a%Ho3þ phosphors could be tuned from primrose yellow (0.4087, 0.5839) to green (0.3684, 0.6227) by increasing the Ho3þ concentration. Similarly, the UC luminescence spectra of Lu3NbO7:b%Yb3þ/1.5% 3þ Ho (b ¼ 2–15) samples is depicted in Fig. 7(a). The spectra of all samples with different Yb3þ concentrations are similar in shape, but differ in luminescence intensity. Unlike the Yb3þ/Ho3þ co-doped Lu3NbO7 with different Ho3þ concentrations, the Yb3þ/Ho3þ co-doped Lu3NbO7 with different Yb3þ concentrations exhibits intense green (centered at 550 nm) and red emission peak (centered at 668 nm) under the excitation of 980 nm. The intensities of red and green emission grow with increasing Yb3þ contents (Ho3þ content remains constant at a ¼ 1.5), and reaches a maximum at b ¼ 7. When the Yb3þ concentration exceeds the threshold, the upconversion luminescence intensity de creases. It is due to the occurrence of the concentration quenching. Two factors result in quenching effect: (a) at higher Yb3þ concentrations, the energy may be mainly transferred between Yb3þ ions and then migrates to defects or other quenching centers; (b) the distances between Yb3þ and Ho3þ shortens at higher Yb3þ concentration which contribute to the increased rates of the back energy transfer from Ho3þ to Yb3þ ions BET2: 5 F4/5S2 (Ho3þ) þ 2F7/2 (Yb3þ)→5I6 (Ho3þ) þ 2F5/2 (Yb3þ) [29] and re sults in the decrease of UC luminescence intensity. Therefore, the 5 F4/5S2 intermediary level is depopulated, which may be the main reason for the increase of R/G ratio. And the CIE chromaticity coordinates (x, y) for Lu3NbO7: b%Yb3þ/ 1.5%Ho3þ phosphors were calculated and presented in Fig. 7(b). The representing features of chromaticity coordinates for Lu3NbO7: b%Yb3þ, 1.5%Ho3þ phosphors could be tuned from green (0.3241, 0.6680) to yellow (0.3750, 0.6169) by controlling the Yb3þ concentration.
Table 1 The chromaticity coordinates CIE (x, y) of Lu3NbO7:20% Yb3þ/a%Ho3þ and Lu3NbO7:b%Yb3þ/1.5% Ho3þ phosphors. a/b (Ho3þ/Yb3þ) content
Sample 3þ
3þ
Lu3NbO7:20% Yb /a%Ho 1 2 3 4 5 Lu3NbO7:b%Yb3þ/1.5% Ho3þ 6 7 8 9 10
a 1 1.5 3 5 10 b 2 5 7 10 15
CIE coordinates (x, y) (0.409, 0.584) (0.390, 0.602) (0.384, 0.608) (0.376, 0.616) (0.368, 0.623) (0.324, 0.668) (0.342, 0.650) (0.343, 0.649) (0.361, 0.631) (0.375, 0.617)
Ho3þ ions have been effectively incorporated into the Lu3NbO7.host lattice, which is agreement with the XRD analysis above. Moreover, the elemental mapping technique was used to confirm the composition uniformity of Lu3NbO7:Yb3þ/Ho3þ, as shown in Fig. 4(c–g). The elemental mapping images exhibit that Lu, Nb, O, Yb and Ho are ho mogeneously distributed within the phosphor, which improves homo geneously luminescence of Lu3NbO7: Yb3þ/Ho3þ phosphors in the field of lights and displays. 3.2. FTIR spectra Fig. 5(a) and (b) illustrate the FTIR spectra of Lu3NbO7 samples doped with Ho3þ and Yb3þ doping concentration, respectively. From Fig. 5, it can be clearly seen that the FTIR spectra of Lu3NbO7:Yb3þ/ Ho3þ with different doping contents have similar peak positions, which further confirms that the phosphors have the same composition. The weak broad band centered at about 3845 cm 1 and 1625 cm 1 are due to the symmetrical O–H stretching and bending vibration of water molecule absorbed on the surface of the phosphor from the air atmo sphere [26]. The main bands around 648 cm 1 and 492 cm 1 corre spond to Nb–O and Lu–O vibrations, respectively [27]. The low vibration energy of Lu3NbO7:Yb3þ/Ho3þ will help to improve the upconversion emission intensity. 3.3. UC luminescence properties Fig. 6(a) presents the upconversion luminescence spectra of Lu3NbO7:20% Yb3þ/a%Ho3þ (a ¼ 1–10). A weaker green emission band centers at 550 nm (5F4/5S2→5I8 of Ho3þ) and strong red emission band
Fig. 7. (a) The upconversion luminescence spectra and (b) the CIE chromaticity coordinates (x, y) of Lu3NbO7:b%Yb3þ/1.5% Ho3þ (b ¼ 2–15). 5
J. Liao et al.
Optical Materials 98 (2019) 109452
Fig. 10. Temperature-dependent emission spectra (under 980 nm excitation) of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor at different temperatures. (b) The normalized emission intensities of the Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor at different temperatures.
Fig. 8. (a) The UC emissions spectra and (b) dependences of the UC intensities (Iem) of 5F4/5S2→5I8 and 5F5→5I8 as well as 5F4/5S2→5I7 transitions on the 980 nm pumping laser power (Ip) for Lu3NbO7: Yb3þ/Ho3þ phosphors.
power ln(P). The values of the slope (n) of the fitting lines are listed in Fig. 8. As shown in Fig. 8(b), in Ho3þ doped Lu3NbO7 sample, the slopes for 550 nm green emission (5F4/5S2(Ho3þ)→5I8(Ho3þ)), 668 nm red emission (5F5(Ho3þ)→5I8(Ho3þ)) and 759 nm far-red emission (5F4/5S2(Ho3þ)→5I7(Ho3þ)) are fitting to be 2.23, 2.18 and 2.34, respectively, which indicates that two-photon processes are involved in producing the green and red UC emission in Ho3þ doped system. According to the pump power dependence of UC emission intensity, the energy level diagram with possible transition and population process in Ho3þ/Yb3þ co-doped Lu3NbO7 systems are illustrated to better comprehend the UC luminescence mechanism in Fig. 9. Under 980 nm excitation, the 5I6 intermediary level of Ho3þ has three ways to accomplish population: the ET1 process from the 2F5/2 state of Yb3þ to Ho3þ: 5I8(Ho3þ)þ2F5/2(Yb3þ)→5I6(Ho3þ)þ2F7/2(Yb3þ) and the ground state absorption (GSA) of Ho3þ(5I8→5I6). For the two processes mentioned above, ET1 process plays a dominant role due to the larger absorption cross section of Yb3þ around 980 nm and the energy transfer between Yb3þ (2F5/2 → 2F7/2) and Ho3þ (5I8→5I6). Subsequently, the population of the upper levels 5S2/5F4 for Ho3þ are obtained via ET3 process or excitation state absorption (ESA). Finally, green emission is generated by radiative transitions from the excited 5S2/5F4 states to the ground 5I8 state. For the red emission of Ho3þ (5F5→5I8), there are usually two pathways to populate in the excited state 5F5 level of Ho3þ. It could be realized by the non-radiative (NR) transition from upper 5 F4/5S2 levels to 5F5 level and the population of 5I7 intermediate level via
Fig. 9. Energy level diagrams of Yb3þ and Ho3þ ions and UC emission mech anism in Lu3NbO7: Yb3þ/Ho3þ phosphors.
3.4. Pump power dependence Generally, the UC emission intensity (I) depends on pump-power (P): I ∝ Pn, where n is approximately equal to the number of pump photons required to excite RE ions from the ground state to the excited state at relatively lower pump power. The value of n can be obtained from the slope of the straight line by linear fitting the plot of ln(I) and pump 6
J. Liao et al.
Optical Materials 98 (2019) 109452
NR1 transition of Ho3þ (5I6→5I7). In other words, part of the electrons in the 5I6 level will first relax to the 5I7 level by the NR2 process and then populate to the 5F5 level via the ET2 process [5I7(Ho3þ)þ2F5/ 3þ 5 3þ 2 3þ 2(Yb )→ F5(Ho )þ F7/2(Yb )]. Finally, the electrons in the excited 5 F5 state jump to the ground 5I8 state of Ho3þ to produce a 668 nm red emission. And the ratio of R/G decreases with increasing Ho3þ contents from 1 to 10, which indicates that there may be the process of BET1: [5F5(Ho3þ)þ2F7/2(Yb3þ)→5I7(Ho3þ)þ2F5/2(Yb3þ)]. It is noteworthy that BET2 process [5S2/5F4(Ho3þ)þ2F7/2(Yb3þ)→5I6(Ho3þ)þ2F5/2(Yb3þ)] can occur to decrease the green emission in high Yb3þ concentration doped samples, which is consistent with the variation of upconversion
Table 2 The relative intensity and R of different emission peaks with their temperature range. Emission peak (nm)
550
668
759
I298K I523K (I298k-I523k)/I298K R (K 1)
1 0.184 0.816 3.63 � 10
0.839 0.265 0.684 3.04 � 10
0.298 0.034 0.886 3.94 � 10
3
3
3
Fig. 11. Dependence of FIR for the 550, 668 and 759 nm emissions on the absolute temperature. 7
J. Liao et al.
Optical Materials 98 (2019) 109452
Table 3 SA and SR values, transitions and type of temperature sensing as well as temperature ranges of different RE ions doped temperature sensing materials. (TCL: thermal coupled levels, NTCL: non-thermally coupled levels.) RE3þ/Host 3þ
Er /SrWO4 Sm3þ/YNbO4 Ho3þ/CaMoO4 Ho3þ/Y2O3 Er3þ/Bi7Ti4NbO21 Er3þ/Y2WO6 Eu3þ/LiNbO3 Ho3þ/Ba3Y4O9 Ho3þ/Lu3NbO7 Ho3þ/Lu3NbO7 Ho3þ/Lu3NbO7
Transitions 2
4
4
H11/2/ S3/2 → I15/2 4 F3/2/4G5/2 → 6H5/2 5 F3/3K8→5I8 5 F3/3K8→5I8 4 F9/2/4S3/2 → 4I15/2 4 F9/2/2H11/2 → 4I15/2 5 D0/5D1→7F2/7F1 (5F4,5S2)/5F5→5I8 (5F4/5S2→5I8)/(5F5→5I8) (5F5→5I8)/(5F4/5S2→5I7) (5F4/5S2→5I8)/(5F4/5S2→5I7)
Type
Range (K)
SA (K 1)
SR (K
TCL TCL TCL TCL NTCL NTCL NTCL NTCL NTCL NTCL NTCL
299–518 303–773 303–543 299–673 153–553 303–563 303–723 294–573 298–523 298–523 298–523
0.015 (403 K) 0.0007 (700 K) 0.0066 (353 K) 0.0030 (673 K) 0.0044 (153–450 K) 0.0022 (303–563 K) 0.0007 (303–723 K) 0.0017 (573 K) 0.0024 (298–523 K) 0.0233 (298–523 K) 0.0090 (298–523 K)
0.0097 0.0043 0.0071 0.0024 / 0.0117 0.0427 0.0036 0.0037 0.0094 0.0027
Ref
(299 K) (500 K) (303 K) (673 K)
[36] [37] [38] [34] [39] [35] [40] [13] This work This work This work
(303 K) (303 K) (448 K) (523 K) (298 K) (298 K)
(5F4/5S2→5I8)/(5F4/5S2→5I7) NTCLs of Ho3þ are 0.0024 K-1, 0.0233 K-1 and 0.0090 K-1, respectively. As a comparison, the optical thermometry parameters of different RE ions doped temperature sensing materials based on the TCLs and the NTCLs are tabulated in Table 3. From Table 3, The maximum sensitivity of 5F5/5I4 NTCLs is 0.0233 K-1 among there pairs NTCLs in the Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor. The maximum sensitivity of 5F5/5I4 NTCLs for Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor is obviously higher than those of the 5F3/3K8 TCLs of Ho3þ for (0.0030 K-1) Y2O3:Ho3þ/Yb3þ/Zn2þ [34] and (0.0066 K-1) CaMoO4: Ho3þ/Yb3þ/Mg2þ phosphors [12], also higher than those of 4 F9/2/2H11/2 NTCLs of Er3þ for (0.0022 K-1) Y2WO6:Er3þ/Yb3þ phos phors [35]. For the same (5F4,5S2)/5F5 NTCL of Ho3þ, the sensitivity (0.0024 K-1) of Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor is slightly higher than that (0.0017 K-1) of Ba3Y4O9:Ho3þ/Yb3þ/Tm3þ [13]. The relative � � � 1 dðFIRÞ� � sensitivity (SR) was calculated by using equation [14]: SR ¼ ��FIR dðTÞ �,
3.5. Temperature sensing behavior The emission spectra of the typical Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor under 980 excitation at various temperatures are displayed in Fig. 10(a). The emission intensities of green 5F4/5S2→5I8 and red 5 F5→5I8 as well as far-red 5F4/5S2→5I7 of Ho3þ exhibit obvious change with temperature. Fig. 10(b) depicts the relative intensities of the 5 F4/5S2→5I8 transition of 550 nm (integrated from 520 to 580 nm) and 5 F5→5I8 transition of 668 nm (integrated from 620 to 700 nm) as well as 5 F4/5S2→5I7 transition of 759 nm (integrated from 730 to 780 nm) as a function of the absolute temperature. With increasing temperature, the intensities of the green and red as well as far-red emission peaks decrease gradually, but their decay rates of temperature-related luminescence intensity (R) are different. R is calculated by the following equation: R ¼ ðI1 I2 Þ=I ðT T Þ, where I1 2
)
� � �dðFIRÞ� 5 5 5 5 5 5 5 5 5 5 � � � dðTÞ �. The SA of ( F4/ S2→ I8)/( F5→ I8), ( F5→ I8)/( F4/ S2→ I7) and
luminescence intensity (See Fig. 5). Far-red emission(759 nm) is ob tained via the transition process [5S2/5F4(Ho3þ)→5I7(Ho3þ) ].
1
1
1
and the resultant curve as a function of the absolute temperature is shown in Fig. 11(b), (d) and (f). The SR for (5F4/5S2→5I8)/(5F5→5I8), (5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/(5F4/5S2→5I7) NTCLs of Ho3þ are 0.0037 K-1, 0.0094 K-1 and 0.0027 K-1, respectively. The maximum SA (0.0094 K-1) of the (5F5→5I8)/(5F4/5S2→5I7) NTCLs for Lu3NbO7:7%Yb3þ/1.5%Ho3þ phosphor is higher than those of the 5 F3/3K8 TCLs of Ho3þ for (0.0024 K-1) Y2O3:Ho3þ/Yb3þ/Mg2þ and (0.0071 K-1) CaMoO4:Ho3þ/Yb3þ/Mg2þ phosphors. These results imply that Lu3NbO7:Yb3þ/Ho3þ material using FIR from the 5F5/5I4 NTCLs of Ho3þ is very promising materials for the application in the optical thermometry.
and I2 are the relative intensity of emission peak at low temperature (T1) and high temperature (T2), respectively. According to the data of Fig. 10 (b) and the formula of R, the relative intensity and R of different emis sion peaks with their temperature range were listed in Table 2. From Table 2, the order of R is R759nm > R550nm > R668 nm. This reason may be related to the non-radiative probability (Knr) [30–33]. Knr can be approximately described by the Mott-Seitz model by Knr ∝ € is the energy gap between two levels, ^eB is the expð ΔE =κB TÞ, where AE Boltzmann constant and T is the absolute temperature. Therefore, the increasing temperature could urge the occurrence of the nonradiative (NR) relaxations (NR1:5F4/5S2→5F5 and NR2:5I6→5I7) in Fig. 9, which reduces the population number of the electrons on the 5F4/5S2 levels of Ho3þ and is beneficial to the electron population on the 5F5 level. As a result, the intensity ratio of green/red FIR (I550/I668) has been decreased with the temperature increased. For emission peaks form the same en ergy level of the excited state, the smaller the energy gap of emission peak, the larger the non-radiative probability. With the temperature increased, far-red emission intensity (759 nm 5F4/5S2→5I7 of Ho3þ) de cays faster than that of green (550 nm 5F4/5S2→5I8 of Ho3þ). As a result, the intensity ratio of red/far-red FIR (I668/I759) and green/far-red FIR (I550/I759) have been increased with the temperature increased. Based on this finding, green/red FIR (I550/I668) and red/far-red FIR (I668/I759) as well as green/far-red FIR (I550/I759) are considered for use as a function of temperature at the given temperature range (298–523 K). The FIR as a function of temperatures in the range of 298–523 K were calculated and plotted in Fig. 11. Fig. 11(a), (c) and (e) depict the dependence of FIR for the two emissions based on NTCLs on the absolute temperature. The experimental data can be well fitted by linear rela tionship as FIR (I550/I668) ¼ -0.0024T þ 1.9261, FIR (I668/I759) ¼ 0.0233T-4.4461 and FIR(I550/I759) ¼ 0.0090T þ 0.7567. The absolute sensitivity (SA) is defined by the equation [14]: SA ¼
4. Conclusions A series of Yb3þ, Ho3þ codoped Lu3NbO7 phosphors were prepared by a high-temperature solid-state reaction method. Excited by 980 nm, multicolor visible emission including green, red, and NIR can be ob tained. The phosphors could be tuned from primrose yellow to green by increasing the Ho3þ concentration and could be tuned from green to primrose yellow by varying the concentration of Yb3þ. The possible UC mechanisms are proposed to be two-photon processes for various color formations, respectively. UC emission arising from Stark energy levels to ground state transitions (5F4/5S2→5I8 (550 nm), 5F5→5I8 (668 nm), 5 F4/5S2→5I7 (759 nm)) was observed. And using the FIR technique, multiple temperature sensing performances based on the three pairs of (5F4/5S2→5I8)/(5F5→5I8),(5F5→5I8)/(5F4/5S2→5I7) and (5F4/5S2→5I8)/ (5F4/5S2→5I7) NTCLs of Ho3þare investigated in a temperature range of 298–523 K. The maximum absolute sensitivity of the (5F5→5I8)/ (5F4/5S2→5I7) NTCLs of Ho3þ is calculated to be 0.0233 K 1 at 523 K. These results indicate that Lu3NbO7:Yb3þ/Ho3þ could be used in an optical thermometer in noncontact temperature measurements and display fields. 8
J. Liao et al.
Optical Materials 98 (2019) 109452
Declaration of competing interest
[17] H. Suo, C. Guo, J. Zheng, B. Zhou, C. Ma, X. Zhao, T. Li, P. Guo, E. Goldy, Sensitivity modulation of Upconverting thermometry through Engineering phonon energy of a matrix, ACS Appl. Mater. Interfaces 8 (2016) 30312–30319. [18] B. Dong, R. Hua, B. Cao, Z. Li, Y. He, Z. Zhang, O.S. Wolfbeis, Size dependence of the upconverted luminescence of NaYF4: Er, Yb microspheres for use in ratiometric thermometry, Phys. Chem. Chem. Phys. 16 (2014) 20009–20012. [19] M.S. Meijer, P.A. Rojas-Gutierrez, D. Busko, I.A. Howard, F. Frenze, C. Wurth, U. Resch-Genger, B.S. Richards, A. Turshatov, J.A. Capobianco, S. Bonnet, Absolute upconversion quantum yields of blue- emitting LiYF4: Yb3þ, Tm3þ upconverting nanoparticles, Phys. Chem. Chem. Phys. 20 (2018) 22556–22562. [20] Q. Wang, J.S. Liao, J.F. Guo, H.P. Huang, H.R. Wen, Upconversion emission and temperature sensing behavior of LuF3:Yb3þ,Er3þ microcrystals prepared by onestep hydrothermal synthesis, Chin, J. Inorg. Chem. 34 (2018) 579–588. [21] Y.Y. Du, Y. Zhang, K.K. Huang, S. Wang, L. Yuan, S.H. Feng, Hydrothermal synthesis and photoluminescence properties of rare-earth niobate and tantalate nanophosphors, Dalton Trans. 42 (2013) 8041–8048. [22] K.Y. Kim, A. Durand, J.M. Heintz, A. Veillere, V. Jubera, Spectral evolution of Eu3þ doped Y3NbO7 niobate induced by temperature, J. Solid State Chem. 235 (2016) 169–174. _ [23] M. Ilhan, M.K. Ekmekçi, R.G. Oraltay, A.S. Bas¸ak, Structural and near-infrared properties of Nd3þ activated Lu3NbO7 Phosphor, J. Fluoresc. 27 (2017) 199–203. [24] Y. Doi, Y. Harada, Y. Hinatsu, H. Unive, Crystal structures and magnetic properties of fluorite-related oxides Ln3NbO7 (Ln¼lanthanides), J. Solid State Chem. 182 (2009) 709–715. [25] Y.Q. Jia, Crystal radii and effective ionic radii of the rare earth ions, J. Solid State Chem. 95 (1991) 184–187. [26] V.C. Farmer (Ed.), The Infrared Spectra of Minerals, The Mineralogical Society, London, 1974. [27] N.T. Mcdevitt, W.L. Baun, Infrared absorption study of metal oxides in the low frequency region (700-240 cm-l), Spectrochim. Acta 20 (1964) 799–808. [28] Y. Yu, Y. Zheng, F. Qin, L. Liu, C. Zheng, G. Chen, Z. Zhang, W. Cao, Influence of Yb3þ concentration on upconversion luminescence of Ho3þ, Opt. Commun. 284 (2011) 1053–1056. [29] S.F. Liu, S.B. Liu, H. Ming, D.J. Hou, W.X. You, X.Y. Ye, Investigation on the upconversion luminescence in Ho3þ/Yb3þ co-doped Ba3Sc4O9 phosphor, Mater. Res. Bull. 98 (2018) 187–193. [30] J. Zhang, G.B. Chen, Z.H. Hua, Up-conversion luminescence of novel Yb3þ-Ho3þ/ Er3þ doped Sr5(PO4)3Cl phosphors for optical temperature sensing, Opt. Mater. Express 7 (2017) 2084–2089. [31] T. Pang, J.J. Wang, Controllable upconversion luminescence and temperature sensing behavior in NaGdF4:Yb3þ/Ho3þ/Ce3þ nano-phosphors, Mater. Res. Express 5 (2018) 015049–015057. [32] Z. J. Huang Pang, A novel upconverison phosphor Gd2Mo4O15:Yb3þ,Ho3þ: intense red emission and ratiometric temperature sensing under 980 nm excitation, Mater. Res. Express 5 (2018) 066204–066213. [33] Zhang, X.W. Li, G.B. Chen, Upconversion luminescence of Ba9Y2Si6O24:Yb3þ-Ln3þ (Ln¼ Er, Ho, and Tm) phosphors for temperature sensing, Mater. Chem. Phys. 206 (2018) 40–47. [34] A. Pandey, V.K. Rai, Improved luminescence and temperature sensing performance of Ho3þ-Yb3þ-Zn2þ:Y2O3 phosphor, Dalton Trans. 42 (2013) 11005–11011. [35] J. Zhang, B.W. Ji, G.B. Chen, Z.H. Hua, Upconversion luminescence and discussion of sensitivity improvement for optical temperature sensing application, Inorg. Chem. 57 (2018) 5038–5047. [36] A. Pandey, V.K. Raib, V. Kumara, V. Kumara, H.C. Swarta, Upconversion based temperature sensing ability of Er3þ–Yb3þcodoped SrWO4: an optical heating phosphor, Sens. Actuators B Chem. 209 (2015) 352–358. [37] L.R. Ða�eanin, S.R. Lukiæ-Petroviæ, D.M. Petroviæ, M.G. Nikoliæ, M. D. Dramiæanin, Temperature quenching of luminescence emission in Eu3þ- and Sm3þ-doped YNbO4 powders, J. Lumin. 6 (2014) 82–87. [38] R. Dey, A. Kumari, A.K. Soni, V.K. Rai, CaMoO4:Ho3þ–Yb3þ–Mg2þ upconverting phosphor for application in lighting devices and optical temperature sensing, Sens. Actuators B Chem. 210 (2015) 581–588. [39] H. Zou, J. Li, X.S. Wang, D.F. Peng, Y.X. Li, X. Yao, Color-tunable upconversion emission and optical temperature sensing behaviour in Er-Yb-Mo codoped Bi7Ti4NbO21 multifunctional ferroelectric oxide, Opt. Mater. Express 4 (2014) 1545–1554. [40] Z. Liang, F. Qin, Y.D. Zheng, Z.G. Zhang, W.W. Cao, Noncontact thermometry based on downconversion luminescence from Eu3þ doped LiNbO3 single crystal, Sens. Actuators A Phys. 238 (2016) 215–219.
The authors declare the following financial interests/personal re lationships which may be considered as potential competing interests: The National Natural Science Foundation of China (No.51862012) and the Major Project of Natural Science Foundation of Jiangxi Province (No. 20165ABC28010). Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.51862012) and the Major Project of Natural Science Foundation of Jiangxi Province (No. 20165ABC28010). References [1] R.R. Deng, F. Qin, R.F. Chen, W. Huang, M.H. Hong, X.G. Liu, Temporal full-colour tuning through non-steady-state upconversion, Nat. Nanotechnol. 10 (2015) 237–242. [2] J.S. Liao, Q. Wang, L.Y. Kong, Z.Q. Ming, Y.L. Wang, Y.Q. Li, L.X. Che, Effect of Yb3 þ concentration on tunable upconversion luminescence and optically temperature sensing behavior in Gd2TiO5:Yb3þ/Er3þphosphors, Opt. Mater. 75 (2018) 841–849. [3] Y.Y. Cheng, A. Nattestad, T.F. Schulze, R.W. MacQueen, B. Fückel, K. Lips, G. G. Wallace, T. Khoury, M.J. Crossley, T.W. Schmidt, Increased upconversion performance for thin film solar cells: a trimolecular composition, Chem. Sci. 7 (2016) 559–568. [4] J.S. Liao, L.L. Nie, Q. Wang, S.J. Liu, H.R. Wen, J.P. Wu, NaGd(WO4)2:Yb3þ/Er3þ phosphors: hydrothermal synthesis, optical spectroscopy and green upconverted temperature sensing behavior, RSC Adv. 6 (2016) 35152–35159. [5] D. Pugliese, N.G. Boetti, J. Lousteau, E. Ceci-Ginistrelli, E. Bertone, F. Geobaldo, D. Milanese, Concentration quenching in an Er-doped phosphate glass for compact optical lasers and amplifiers, J. Alloy. Comp. 657 (2016) 678–683. [6] J. Pichaandi, J.C. Boyer, K.R. Delaney, F.C.J.M. van Veggel, Two-photon upconversion laser (scanning and wide-field) microscopy using Ln3þ-doped NaYF4 upconverting nanocrystals: a critical evaluation of their performance and potential in bioimaging, J. Phys. Chem. C 115 (2011) 19054–19064. [7] B. Dong, B. Cao, Y. He, Z. Liu, Z. Li, Z. Feng, Temperature sensing and in vivo imaging by molybdenum sensitized visible upconversion luminescence of rareearth oxides, Adv. Mater. 24 (2012) 1987–1993. [8] X.D. Wang, O.S. Wolfbeis, R.J. Meier, Luminescent probes and sensors for temperature, Chem. Soc. Rev. 42 (2013) 7834–7869. [9] J.S. Liao, Q. Wang, L.F. Lan, J.F. Guo, L.L. Nie, S.J. Liu, H.R. Wen, Microwave hydrothermal synthesis and temperature sensing behavior of Lu2Ti2O7:Yb3þ/Er3þ nanophosphors, Curr. Appl. Phys. 17 (2017) 427–432. [10] L.L. Tong, X.P. Li, R.N. Hua, L.H. Cheng, J.S. Sun, J.S. Zhang, S. Xu, H. Zheng, Y. Q. Zhang, B.J. Chen, Optical temperature sensing properties of Yb3þ/Tm3þ codoped NaLuF4 crystals, Curr. Appl. Phys. 17 (2017) 999–1004. [11] J. Zhang, B.W. Ji, G.B. Chen, Z.H. Hua, Upconversion luminescence and discussion of sensitivity improvement for optical temperature sensing application, Inorg. Chem. 57 (2018) 5038–5047. [12] X.B. Tu, J.S. Xu, M.C. Li, T. Xie, H.P. Wang, S.Q. Xu, Color-tunable upconversion luminescence and temperature sensing behavior of Tm3þ/Yb3þ codoped Y2Ti2O7 phosphors, Mater. Res. Bull. 112 (2019) 77–83. [13] S.F. Liu, H. Ming, J. Cui, S.B. Liu, W.X. You, X.Y. Ye, Y.M. Yang, H.P. Nie, R. X. Wang, High sensitive Ln3þ/Tm3þ/Yb3þ (Ln3þ¼Ho3þ, Er3þ) tri-doped Ba3Y4O9 upconverting optical thermometric materials based on diverse thermal response from non-thermally coupled energy levels, Ceram. Int. 45 (2019) 1–10. [14] J. Zhang, Y.Q. Zhang, X.M. Jiang, Investigations on upconversion luminescence of K3Y(PO4)2:Yb3þ-Er3þ/Ho3þ/Tm3þ phosphors for optical temperature sensing, J. Alloy. Comp. 748 (2018) 438–445. [15] X.N. Chai, J. Li, X.S. Wang, Y.X. Li, X. Yao, Upconversion luminescence and temperature-sensing properties of Ho3þ/Yb3þ -codoped ZnWO4 phosphors based on fluorescence intensity ratios, RSC Adv. 7 (2017) 40046–40052. [16] S.F. Le� on-Luis, U.R. Rodríguez-Mendoza, P. Haro-Gonzalez, I.R. Martín, V. Lavín, Role of the host matrix on the thermal sensitivity of Er3þ luminescence in optical temperature sensors, Sens. Actuators B Chem. 174 (2012) 176–186.
9