Multifunctional optical thermometry using dual-mode green emission of CaZrO₃:Er/Yb/Mo perovskite phosphors

Abstract

The weak emission intensity of rare-earth element-doped dual-mode materials leads to low-sensor sensitivity, which is a challenge in optical sensor applications. The present work achieved high-sensor sensitivity and high green color purity based on the intense green dual-mode emission of Er/Yb/Mo-doped CaZrO₃ perovskite phosphors. Their structure, morphology, luminescent properties, and optical temperature sensing properties have been investigated in detail. Phosphor shows a uniform cubic morphology with an average size of approximately 1 μm. Rietveld refinement confirms the formation of single-phase orthorhombic CaZrO₃. Under the excitation of 975 and 379 nm, the phosphor emits pure green up and down-conversion (UC and DC) emission at 525/546 nm corresponding to ²H_11/2/⁴S_3/2–⁴I_15/2 transitions of Er³⁺ ions, respectively. Intense green UC emissions were achieved because of energy transfer (ET) from the high-energy excited state of Yb³⁺–MoO₄²⁻ dimer to the ⁴F_7/2 level of Er³⁺ ion. Furthermore, the decay kinetics of all obtained phosphors confirmed ET efficiency from Yb³⁺–MoO₄²⁻ dimer to Er³⁺ ions, leading to strong green DC emission. Moreover, the DC of the obtained phosphor shows that a sensor sensitivity value of 0.697% K⁻¹ at 303 K is higher than the UC (0.667% K⁻¹ at 313 K) because the thermal effect generated by the DC excitation source light is ignored compared with UC luminescence. CaZrO₃:Er–Yb–Mo phosphor shows intense green dual-mode emission with high green color purity, 96.50% of DC and 98% of UC emissions, and high sensitivity, making it suitable for optoelectronic devices and thermal sensor applications.

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1. Introduction

Temperature is an essential physical parameter in science and daily life; thus, the accurate measurement of temperature is critical.^1,2 However, conventional contact temperature measurement using liquid-filled glass thermometers and thermocouples has many disadvantages, such as low sensitivity, limitation in response time, and inability to measure under harsh conditions (such as high temperature, strong acids, and strong magnetic fields).^3,4 Thus, the temperature sensor application of this method is hindered.³ At present, non-contact temperature measurement methods via optical temperature sensors based on temperature-dependent up and down-conversion (UC and DC) emission intensity are widely used because they could overcome the limitations of conventional contact temperature measurement.^5,6 Accordingly, several parameters of rare-earth (RE) element-doped phosphors, such as emission intensity, lifetime, and peak shift, depend on temperature,⁷ which is used for determining temperature sensing sensitivity.^7,8 Notably, the fluorescence intensity ratio (FIR) based on the emission spectra of the phosphor is widely used to estimate the sensor characteristics of phosphors. This technique is performed by observing the changing intensity emission of two thermally coupled levels of RE ions with an energy gap from 200 to 2000 cm⁻¹.⁸ The FIR is independent of an activator, spectrum losses, and fluctuation in excitation intensity.⁹ Therefore, this method has good measurement accuracy, high resolution, and broad range, and it requires low excitation power, leading to high reliability.^9,10 However, previous literature reports only explore the sensor sensitivity of a single UC or DC material instead of both. In addition, DC emission shows that a thermal effect generated by the excitation light could be ignored, resulting in high sensor sensitivity.⁶ Therefore, investigating the sensor sensitivity based on UC and DC emissions is necessary to extend the application of phosphors.

Er³⁺ ion is a widely used activator for UC/DC luminescent materials. It has an energy gap between ²H_11/2 and ⁴S_3/2 levels of approximately 800 cm⁻¹,¹¹ indicating that it is suitable as a luminescent center for phosphor-temperature sensor using the FIR.^11,12 However, due to small absorption cross-section in the near-infrared band led to its low emission intensity.¹³ Previous works showed that weak UC emission intensity of phosphors results in low sensor sensitivity, which could not be suitable for temperature sensor applications.^13,14 Recently, researchers have achieved good temperature sensing properties based on a selective enhancement of green UC emission using a sensitizer pair as Yb³⁺-transition metal (TM) to activate Er³⁺.^15,16 Energy transfer at a high-energy excitation state of the Yb³⁺-TM ion pairs to the ⁴F_7/2 level of Er³⁺ could reduce non-radiative transition and improve UC efficiency and sensor sensitivity. B. Dong et al. have reported that high-temperature sensor sensitivity was achieved by using Yb³⁺–MoO₄²⁻ dimer sensitizer to activate Er³⁺ on Yb₃Al₅O₁₂ (ref. 17) and Al₂O₃ (ref. 16) host lattices. Meanwhile, X. Yang et al.¹⁸ have observed an intense green UC emission of Er–Yb-doped NaY(MoO₄)₂ phosphor with good temperature sensing properties, thereby causing ET from Yb³⁺–MoO₄²⁻ dimer to Er³⁺. Besides, the MoO₄²⁻ groups (obtained by Mo⁶⁺ doped host) have a high absorption cross-section at the UV region; thus, they can strongly absorb radiation in this region and transfer energy to the RE doping ion, resulting in an improvement in the RE ion's emission efficiency.¹⁹ Furthermore, the presence of the Mo⁶⁺ ion in the host lattice leads to a decrease in the lattice symmetry and enhances stark energy splitting in the crystal field because they have a short radius and large electric charge, enabling a powerful polarization effect.^19,20 Most recently, our previous works explored the super enhancement of green UC emission by using the Yb³⁺–MoO₄²⁻ dimer sensitizer for Er³⁺ ion on HA/TCP (650 times)²¹ and HA (70 times),²² thereby achieving good temperature sensing properties in BCP host.²³ Based on previous works, the strong green UC emission of phosphors can be achieved by using Er–Yb–Mo tri-doped systems, leading to the enhancement of sensing sensitivity.^15,23 Furthermore, several works^1,3,23,24 have observed that temperature sensitivity depends on the host lattice.

Oxides are widely used for luminescent hosts because of their excellent properties, such as low phonon energy, high thermal stability, and wide-range transparency.⁷ The perovskite-type oxide CaZrO₃ has an orthorhombic structure with space group Pbnm, and it has received considerable attention as a host material because of its excellent physical–chemical properties.^25,26 It has relatively low phonon energy, resulting in reduced non-radiative transition, thereby enhancing UC and DC efficiency. A. Khan et al.²⁷ successfully prepared Dy³⁺/Eu³⁺/Mo⁶⁺-doped CaZrO₃ for white light emission through a solid-state reaction at 1200 °C for 6 h. V. Singh et al.²⁸ reported the intense green and red UC emission of Er–Yb-doped CaZrO₃ by solution combustion. A. Maurya et al.²⁹ investigated the effect of alkali ions on optical properties, and the temperature sensing efficiency of Er–Yb-doped CaZrO₃ was prepared by a solid-state reaction. They found that the enhancement of UC emission of the systems by adding alkali ions was caused by a change in the local crystal field and an increase the crystallinity of the obtained phosphors. The RE-doped CaZrO₃ host lattice was successfully prepared in previous works.^30,31 However, to our knowledge, the intense green UC and DC emission of CaZrO₃:Er/Yb/Mo phosphor with good optical temperature sensing properties has yet to be well documented.

Herein, CaZrO₃:Er/Yb/Mo microstructure phosphors with good sensor sensitivity are synthesized via a solid-state reaction. Their structure, morphology, and optical property were investigated in detail. The optical temperature sensing was characterized using the FIR. Their characteristics were analyzed using X-ray diffraction (XRD), high-resolution scanning electron microscopy (FE-SEM), photoluminescence (PL), and FIR.

2. Experimental

1% Er, 10% Yb, and x% Mo (x = 0, 1, 3, 5, and 8% mol) co-doped CaZrO₃ phosphors were prepared through a solid-state reaction. Er(NO₃)₃·5H₂O (99.99%), Yb(NO₃)₃·5H₂O (99.99%), (NH₄)₆Mo₇O₂₄ (99.9%), ZrO₂ (99.9%), and CaCO₃ (99.9%) were utilized as precursors. The precursors were ground using an agate mortar for 1 h and then treated at 500 °C for 5 h. Next, the powder mixture was ground again for 1 h and incubated at 900 °C for 5 h. Finally, the homogeneous powders were further calcined at 1300 °C for 5 h. All investigated samples were labeled (Table 1).

Table 1 Labels of all investigated samples

Materials	Mo^6+ content (mol%)	Label
1Er–10Yb doped CaZrO3	0	S0
1Mo–1Er–10Yb doped CaZrO3	1	S1
3Mo–1Er–10Yb doped CaZrO3	3	S3
5Mo–1Er–10Yb doped CaZrO3	5	S5
8Mo–1Er–10Yb doped CaZrO3	8	S8

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Their structural property was characterized using an X-ray diffractometer (Bruker D8 Advance). Their morphological property was analyzed using high-resolution scanning electron microscopy (JEM 1010, JEOL Technique, Japan). Diffuse reflectance spectra were recorded using a Varian Cary 5000 UV-vis-NIR spectrophotometer. The luminescence properties of UC and DC were recorded using a NANO LOG spectrometer (Horiba, USA) equipped with a 975 nm laser radiation source and heat controller. The Rietveld refinement structure of phosphors was obtained using FullProf software.

3. Results and discussion

3.1. XRD analysis

Fig. 1a shows the XRD pattern of all investigated samples CaZrO₃:Er–Yb without and with Mo-doped. Diffraction peaks of all patterns match well with the standard card (PDF # 00-061-0228) of orthorhombic single-phase CaZrO₃ with a Pbnm space group. Their intensity is sharp and strong, which indicates the high crystallinity of phosphors. Moreover, diffraction peaks intensity tends to increase with rise of Mo⁶⁺ doping content up to 3% mol, then slight decrease with higher Mo⁶⁺ content. These results confirm the presence of Mo⁶⁺ ion influency on structure and crystallinity of the host lattice. The structure of orthorhombic CaZrO₃ includes CaO₈ polyhedra and ZrO₆ octahedra, with the following lattice parameters (Fig. 1b): a = 5.60 Å, b = 5.75 Å, and c = 8.01 Å. On the other hand, according to A. Khan et al.,²⁷ the difference in ionic radii between doped and substituted ions with an acceptable percentage was lower than 30%. In this case, the radius percentage (D_r) between the doped ions (Er³⁺, Yb³⁺, and Mo⁶⁺) and the substituted ions (Ca²⁺ and Zr⁴⁺) in Ca_1−x−yZr_1−zO₃:xEr³⁺, yYb³⁺, zMo⁶⁺ was determined using eqn (1):^27,32where CN = 6 is the coordination number of the resulted ions. R_s(CN) and R_d(CN) are radius of the substituted and dopant ions, respectively. The ionic radius values such as Er³⁺ (0.89 Å), Yb³⁺ (0.87 Å), Mo⁶⁺ (0.60 Å),²¹ Zr⁴⁺ (0.72 Å), and Ca²⁺ (0.99 Å) can be taken from the literature.²⁷ According to eqn (1), the D_r values were calculated to be D_r(Er³⁺/Ca²⁺) 10%, D_r(Yb³⁺/Ca²⁺) 12%, and D_r(Mo⁶⁺/Zr⁴⁺) 17%. In contrast, the D_r(Mo⁶⁺/Ca²⁺) value is 39.4% that is much higher than 30%. Therefore, these results indicate that Er³⁺ and Yb³⁺ are substitute Ca²⁺ sites. Meanwhile, Mo⁶⁺ is substitute Zr⁴⁺ sites, which agree with the Rietveld refinement result in Table 2.

Fig. 1 (a) XRD pattern of all investigated samples; (b) crystal structure of CaZrO₃ from Rietveld refinement; (c) and (d) Rietveld refinement structure of the typical samples S0 and S3.

Table 2 Lattice parameters from Rietveld refinement of samples S0 and S3^a

Parameter	CaZrO3:Er–Yb	CaZrO3:Er–Yb–Mo
a GOF = (Rwp/Rexp)^2.
Space group	Pbnm	Pbnm
Symmetry	Orthorhombic	Orthorhombic
a, Å	5.5948	5.5938
b, Å	5.7554	5.7556
c, Å	8.0190	8.0170
V, Å^3	258.2139	258.1129
Rexp (%)	4.65	4.65
Rp (%)	6.20	6.08
Rwp (%)	7.58	7.06
GOF	2.65	2.30

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Furthermore, to confirm the influence of Mo⁶⁺ doped in the crystal structure of the material, the Rietveld refinement of the typical two samples (S0 and S3) was performed using the standard card PDF # 00-061-0228 as references (Fig. 1c and d). As shown in Fig. 1c and d, the experimental result is consistent with the calculated results, indicating the reliability of the refinement technique, and all refinement parameters (as atom coordinates, fraction factors, and thermal vibration) are consistent with the reflection condition.³³ As shown in Table 2, the change in lattice parameters (a, c, and V) of sample S3 compared with sample S0 confirms the successful replacement of Mo⁶⁺ for Zr⁴⁺ sites into the host lattice. In addition, impurity phases were not observed in all samples, indicating the successful synthesis of the resulting phosphors.

3.2. SEM and EDS analysis

Fig. 2a and b show FE-SEM images of samples S0 and S3. All samples show a cubic grain morphology with a uniform distribution and clear grain boundary. The sample S0 exhibits a grain size of approximately 1.2 μm (Fig. 2a), whereas the doped sample (S3) is approximately 1 μm (Fig. 2b), which is due to the substitution of small ionic radius (Mo⁶⁺, 0.6 Å) for larger ionic radius (Zr⁴⁺, 0.72 Å).²⁷ Meanwhile, Fig. 2c shows resulting peaks corresponding to Ca, O, Zr, Er, Yb, and Mo with element percentage (Fig. 2d) from elemental analysis of the EDS spectrum, indicating the presence of Er, Yb, and Mo doped in CaZrO₃ host lattice. In addition, as shown in Fig. 2c and d, other impure chemical elements were not observed, indicating the high purity of the investigated phosphors in this work. These results indicate the high crystallinity degree of resulting phosphor, which is consistent with the XRD results shown in Fig. 1a, leading to the improvement of UC efficiency of the phosphor.

Fig. 2 FE-SEM images of (a) S0 and (b) S3 (scale bar of 1 μm), (c) EDS spectra, and (d) element percentage of the investigated sample S3.

3.3. Optical characteristics

3.3.1. DRS spectral measurements. Fig. 3 shows the diffuse reflectance spectra of samples S0, S1, S3, and S8. As shown in Fig. 3, broad absorption bands from 230 to 350 nm are attributed to the O²⁻–Er³⁺/Yb³⁺/Mo⁶⁺ charge transfer band.³⁴ Significantly, these absorption band intensity increases with the rise of Mo⁶⁺ doping content due to the covalency of the Mo–O bond being higher than that of the Zr–O bond because of the difference in electronegativity between the Mo⁶⁺ (2.16) and the Zr⁴⁺ (1.4),³⁵ which confirms the presence of Mo⁶⁺ ion in the host. It can be seen in Fig. 3 typical sharp absorption bands such as 380, 492, 524, 546, 660, and 805 nm correspond to transitions from the ⁴I_15/2 level to ⁴G_11/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, and ⁴I_9/2 levels of Er³⁺ ion. Meanwhile, the broad absorption band at NIR (around 975 nm) corresponded to an overlap between the ⁵F_7/2–⁵F_5/2 transition of Yb³⁺ ions and the ⁴I_15/2–⁴I_11/2 transition of Er³⁺ ions. These results confirm the successful introduction of Er³⁺ and Yb³⁺ ions into the CaZrO₃ host lattice. Significantly, the absorption peaks of doped samples (S1, S3, and S8) are blue shifted compared with the undoped sample (S0), which is due to the difference between the ionic radius of the dopant and the host cation,³⁶ indicating the presence of Mo⁶⁺ dopant ions on the host lattice and leading to the formation of Yb³⁺–MoO₄²⁻ dimers in the system. These results supported the proposed energy level diagram of the Er–Yb–Mo-doped CaZrO₃ phosphor (Fig. 6).

Fig. 3 Diffuse reflectance spectra of investigated samples S0, S1, S3, and S8.

3.3.2. UC luminescence properties. Fig. 4a shows UC emission spectra of all samples doped with various Mo⁶⁺ contents. As shown in Fig. 4a, under excitation of 975 nm, all samples exhibit two typical emission bands: (i) strong green emission bands at 525/546 nm are attributed to the ²H_11/2/⁴I_3/2–⁴I_15/2 transition of Er³⁺ ion; (ii) the weak red emission band at 660 nm is due to the ⁴F_7/2–⁴I_15/2 transition of Er³⁺ ion. Green and red emission intensities depend on the Mo⁶⁺ doping content and reach the maximum intensity of 3% mol Mo⁶⁺ (Fig. 4c). By adding Mo⁶⁺ ion into the system, the green emission intensity of the phosphor has been enhanced up to fourfold. The energy transfer effectively process from the high-energy excited state (|²F_7/2, ³T₂〉) of the Yb³⁺–MoO₄²⁻ dimer to the ⁴F_7/2 level of the Er³⁺ ion is responsible for the strong green emission of phosphors.^15,23,37 When Mo⁶⁺ doping content is higher than 3% mol, the UC emission intensity decreases because of the energy migration between Yb³⁺–MoO₄²⁻ dimers^23,37 and between the MoO₄²⁻ groups as a donor in the energy transfer process.³⁸ On the contrary, the weak absorption cross-section of the ⁴I_13/2 state of the Er³⁺ ions led to weak red emission intensities. Moreover, the dominant UC ET process leads to the transfer of electrons from the ⁴I_11/2 level to the ⁴F_7/2 level, which is higher than that of non-radiative relaxation to the ⁴I_13/2 level formation of weak red emission (Fig. 6). Furthermore, the green/red (I₅₄₆/I₆₆₀) intensity ratio of all samples increased with enhanced Mo⁶⁺ doping content, indicating remarkable control-enhanced green emission using Mo⁶⁺ doping ions (Fig. 4d).

Fig. 4 (a) UC spectra of all samples with various Mo⁶⁺ content doping, (b) CIE chromaticity coordinates of all UC emission samples, (c) UC emission intensity of all samples as a function of Mo⁶⁺ doping content, and (d) I₅₄₆/I₆₆₀ intensity ratio of all samples.

Additionally, Commission International de l'Eclairage (CIE) chromaticity coordinates of investigated samples were calculated from their emission spectra presented in Fig. 4b. In this case, the obtained values of all samples are displayed in Table 3. As shown in Table 3, the chromaticity coordinates value of the S3 sample (0.2743, 0.7084) is close to the standard green chromaticity coordinated for the National Television Standard Committee (NTSC) system.³⁹ Significantly, the color purity of the green light emission phosphors in this work was calculated using eqn (2):⁴⁰

where (x, y) present the color coordinates of samples; x_i = 0.3333 and y_i = 0.3333 correspond to the color coordinate of the white light source; (x_d, y_d) means the color coordinates of the dominant wavelength.³⁹ Putting these values (x, y), (x_i, y_i), and (x_d, y_d) into eqn (2), the color purity of resulted samples is exhibited in Table 3. Notably, the color purity of samples increased with enhanced Mo⁶⁺ doping content and reached a maximum value of 96.50% (S3 sample), indicating pure green light emission. This result suggests the potential application of phosphor for optoelectronic devices. Consequently, the S3 sample with the highest emission intensity was selected to analyze the temperature sensor properties.

Table 3 The CIE chromaticity coordinates and color purity of different samples

Samples	Chromaticity coordinates (x, y)	Color purity (%)
S0	(0.2835, 0.6964)	93.11
S1	(0.2809, 0.7012)	94.41
S3	(0.2743, 0.7084)	96.50
S5	(0.2722, 0.7076)	96.35
S8	(0.2721, 0.7070)	96.21

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3.3.3. UC emission mechanism. The intense green UC emission mechanism (S0 and S3 samples) of the obtained phosphor can be explained through the relationship between the emission intensity (I) and excitation power (P) as⁴¹ I ∝ Pⁿ, where n is the number of photons absorbed for emitter UC. Fig. 5a and b show that I increase rapidly with the increase of P from 200 to 600 mW of the two samples (S0 and S3). In sample S0 (Fig. 5c), the slope values (n) are 1.48/1.68 and 1.52, corresponding to the ²H_11/2/⁴I_3/2–⁴I_15/2 (green) and ⁴F_7/2–⁴I_15/2 (red) transitions, respectively. In sample S3, the corresponding slopes are 1.94/1.84 and 1.42 (Fig. 5d). These results indicate the two-photon process for green/red emissions of the two obtained samples. Significantly, the n values corresponding to the green emission of sample S3 are higher than that of the S0 sample. By contrast, the n value of red emission is low, which confirms the effective ET from the Yb³⁺–MoO₄²⁻ dimer to the Er³⁺ ions, thereby enhancing green UC emission. This result was further used to build the energy level diagram and explain the UC emission mechanism (Fig. 6).

Fig. 5 Emission intensity-dependent excitation power of the samples S0 (a) and S3 (b) and the log–log plots of green/red emission as a function of the power of the samples S0 (c) and S3 (d).

Fig. 6 Energy level diagram of Er/Yb/Mo doped CaZrO₃ phosphor.

3.3.4. Energy level diagram of UC and DC green emission of Er–Yb–Mo tri-doped CaZrO₃ phosphor. In understanding the selective enhancement of green UC emission, the energy level diagram of the CZO:Er–Yb–Mo system is schematically explained in Fig. 6. The intense green UC emission (in the S3) of the system can be discussed using three main processes: ground-state absorption (GSA), excited-state absorption (ESA), and ET.⁴² For doped samples, GSA, ESA, and ET are responsible for the sensitizer of the Yb³⁺–MoO₄²⁻ dimer. The dominant green emission of CaZrO₃:Er–Yb–Mo is proposed as follows:

➢ Step 1: the energy level of the Yb³⁺–MoO₄²⁻ dimer, such as |²F_7/2, ¹A₁〉 (ground state), |²F_5/2, ¹A₁〉 (excited state), and |²F_7/2, ³T₁〉, |²F_7/2, ³T₂〉, |²F_7/2, ¹T₁〉, |²F_7/2, ¹T₂〉 (at highly excited state), was obtained by mixing the electron wavefunctions of Yb³⁺ and MoO₄²⁻.⁴³

➢ Step 2: the electron at the initial state |²F_7/2, ¹A₁〉 of the Yb³⁺–MoO₄²⁻ dimer absorbs one photon, and it is excited to the |²F_5/2, ¹A₁〉 level via GSA.

➢ Step 3: the electron at the |²F_5/2, ¹A₁〉 level absorbs a second photon, and then they are elevated to the |²F_7/2, ¹T₁〉 level (high-energy excited state) by ESA. Given the unstable |²F_7/2, ¹T₁〉 state, leading to the non-radiative relaxation of the electron to the |²F_7/2, ³T₂〉 state.

➢ Step 4: the electron at |²F_7/2, ³T₂〉 state ET to ⁴F_7/2 level of Er³⁺ and then relax NR down to two meta-stable levels, ²H_11/2/⁴S_3/2. Next, most of the electrons in ²H_11/2 and ⁴S_3/2 levels return to the ground-state ⁴I_15/2, producing intense green emission (530/546 nm).

Meanwhile, the red emission is originally from the ⁴F_7/2 NR to the ⁴S_3/2 level. However, the ⁴S_3/2 energy level is higher than the ⁴F_9/2 level, leading to a few electrons from the ⁴S_3/2 NR to the ⁴F_9/2 level,³⁷ causing red emission that is weaker (several orders of magnitude) than green emission. These results are used to explain the experimental evidence shown in Fig. 4a. Thus, the proposed mechanism for the intense green UC emission is suitable.

3.4. Optical temperature sensing characterization based on UC emission

The potential application in temperature sensors of the phosphor was evaluated by determining the temperature-dependent green UC emission of the S3 sample. The Er³⁺ ion has two thermal coupled levels, ²H_11/2/⁴S_3/2, with an energy gap of approximately ∼800 cm⁻¹.⁴⁴ Thus, the thermal equilibrium between these two levels can be achieved through thermal excitation. Fig. 7a shows the UC emission of the S3 sample under different temperatures ranging from 303 K to 523 K. The emission intensities of samples decrease with the increase of temperature. Notably, the emission intensity of the ⁴S_3/2–⁴I_15/2 transition rapidly decreases. Meanwhile, the ²H_11/2–⁴I_15/2 and ⁴F_7/2–⁴I_15/2 transitions slightly change, indicating the excellent thermal stability of the phosphor.

Fig. 7 (a) Green/red emission spectra of the S3 sample measured from 313 K to 523 K, (b) comparison of green emission of the S3 sample at several temperatures, (c) greed/red emission intensity as a function of temperature, (d) the relationship between the ln(FIR) value and inverse absolute temperature, (e) the sensitivity (absolute and relative) value of the sample using the FIR, and (f) the relationship between calculated and experimental temperature.

Based on the FIR technique, the emission intensity ratio corresponding to ²H_11/2–⁴I_15/2/⁴S_3/2–⁴I_15/2 transitions was used to estimate the optical temperature sensor characteristics of the phosphor using eqn (3):⁴⁵

where I₅₂₅ and I₅₄₆ are the emission intensities corresponding to the ²H_11/2–⁴I_15/2/⁴S_3/2–⁴I_15/2 transitions; A is a pre-exponential constant; k = 0.6950348 cm⁻¹ K⁻¹ is the Boltzmann constant; T is the absolute temperature, and ΔE represents the energy gap between the two-level thermal couple (²H_11/2 and ⁴S_3/2). Eqn (4) can be written by taking the natural logarithm on both sides of eqn (3):ΔE and A parameters can be determined by using a simple linear fit of eqn (4) based on the relationship between ln(FIR) and 1/T (Fig. 7d).

The correlation between the calculated temperature (using the FIR) and the actual experimental temperature (read directly from the thermal coupe) has been determined using eqn (5),¹³ which evaluates the reliability of the measurement, and results are shown in Fig. 7f. Fig. 7f reveals that the calculated temperature (green sphere) is consistent with the experimental temperature (red line) because of the high correlation coefficient R² = 0.999, indicating the reliability and accuracy of the FIR.

Furthermore, the key parameters such as relative sensitivity (S_r) and absolute sensitivity (S_a) values can be estimated using eqn (6) and (7), respectively, to evaluate the temperature sensing performance of the resulting phosphor:^46,47

Fig. 7e shows the maximum S_r of 0.445% K⁻¹ at 303 K, whereas S_a was 0.667% K⁻¹ at 343 K. Moreover, the phosphor shows high-intensity emission at high temperatures (523 K), indicating its temperature stability and applicability at high temperatures.

The sensor sensitivity of the material is compared with other RE-doped phosphors to check its applicability (Table 4). As shown in Table 4, the phosphor obtained in this work has presented competitive results compared with other host lattices such as phosphate,^11,50 fluoride,^48,51 and oxide.^4,5,16,49,52 Furthermore, CaZrO₃:Er–Yb–Mo phosphor shows a high sensitivity temperature (0.667% K⁻¹) based on the operating temperature range (303–523 K) that is prospective for temperature sensor applications.

Table 4 Comparison of the sensitivity sensor of our phosphor with other phosphor based on UC emission

Sample	Sr max (% K^−1)	Sa max (% K^−1)	Temperature (K)	References
SrWO4:Er, Yb	—	1.498	300–518	4
Ba5Y8Zn4O21:Er, Yb	0.39	1.36	293–563	5
NaBiF4:Er, Yb, Fe	—	0.53	300–550	48
Li2Ti2O7:Er, Yb	0.63	0.63	333–553	11
Oxyfluorotellurite glasses:Er, Yb	0.46	0.645	295–675	49
Sr3(PO4)2:Er, Yb	0.70	0.88	303–623	50
Al2O3:Er, Yb, Mo	—	0.51	294–973	16
NaGdF4:Er, Yb, Fe	0.27	0.78	303–523	51
Er1−xYbxO3	—	0.45	298–473	52
CaZrO3:Er, Yb, Mo	0.445	0.667	303–523	This work

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3.5. Downconversion luminescence properties

Fig. 8a displays the PL excitation spectra of typical samples S0 and S3. For two samples show the sharp absorption bands at 360 and 379 nm are assigned to the ⁴I_15/2–⁴G_9/2 and ⁴I_15/2–⁴G_11/2 transitions of Er³⁺ ions. Meanwhile, the doped sample (S3) shows a broad absorption band ranging from 250 to 300 nm (peak at 280 nm) corresponding to the ¹A₁–³T₂ transition of MoO₄²⁻ group involving the charge transfer state transition from O²⁻ to Mo⁶⁺,⁵³ which disappears in the undoped sample (S0). This result confirms the presence of Mo⁶⁺ ions in the host lattice. Notably, the absorption band intensity of sample S3 is higher than the S0 sample, which could enhance the DC emission intensity of studied phosphors. Under the excitation of 379 nm, the phosphors show strong green DC emission, as shown in Fig. 8b. All samples exhibited a strong green emission at 530/546 nm and weak red emission at 660 nm, corresponding to the ²H_11/2–⁴I_15/2/⁴S_3/2–⁴I_15/2 and ⁴F_7/2–⁴I_15/2 transitions of Er³⁺ ions, respectively. The emission intensity of all samples increases with the enhancement of Mo⁶⁺ doping, reaches a maximum value with 3% mol Mo⁶⁺, and then decreases with further Mo⁶⁺ doping content (inset of Fig. 8b). Significantly, the green emission intensity of sample S3 was six times higher than that of sample S0. The intense green UC emission of phosphor can be due to the ET from Yb³⁺–MoO₄²⁻ dimer to Er³⁺ ions, which is similar to the enhancement mechanism of the UC emission.^23,54 By contrast, the weak red emission may correspond to the relatively small population in the ⁴F_9/2 level of Er³⁺ (Fig. 6). The quenching luminescence by further doping Mo⁶⁺ is due to cross-relaxation and ET in the Yb³⁺–MoO₄²⁻ dimer.

Fig. 8 (a) Excitation spectra of resulted samples, (b) down-conversion spectra of all investigated samples annealed at 1300 °C for 4 h in air and inset of the figure is dependent DC emission intensity on Mo⁶⁺ doping content, (c) CIE chromaticity coordinates of obtained phosphors, and (d) decay curves of all samples by monitoring ⁴S_3/2–⁴I_15/2 transitions.

Furthermore, the intensely green DC emission and weak red DC emission can be explained as follows (Fig. 6):

➢ First, the ⁴I_15/2 ground state of Er³⁺ ions absorb a 379 nm light excitation from the xenon lamp source and moves to the ⁴G_11/2 level (Fig. 6).

➢ Next, most electrons of the ⁴G_11/2 level NR to the ²H_11/2/⁴S_3/2 level and then return to the ⁴I_15/2 ground state to produce two green emission bands. The intense green emission was achieved because the NR process has the fastest multi-phonon relaxation rate. Meanwhile, a few electrons are excited to the ⁴I_11/2 state (intermediate state of red emission), resulting in an impoverished electron population in the ⁴F_9/2 level, indicating fragile red emission intensity.

These results indicated that the behavior of UC and DC green emission of the CaZrO₃:Er–Yb–Mo phosphor was explained based on the suitably proposed mechanism because of the point of view of the electronic structure scale.

In addition, the decay curve of phosphor materials was determined to explain the DC emission mechanism of the system (including the luminescence process, type of occupancy of the dopant ion in the host lattice sites, and characterization of the luminescence center). Decay measurements of all investigated samples (S0, S1, S3, and S8) were performed using the 379 nm excitation and 546 nm emission, as shown in Fig. 8d. The curve fitting technique can determine the lifetime of an excited state from the decay curve, which can be written as follows:⁵⁵

y(t) = y₀ + Ae^−t/τ (8)

where y(t) and y₀ are the intensity at time t and infinite time, respectively; A is the weighting parameter, and τ is the decay component of Er³⁺ into the host lattice. Our experimental data are correctly fitted with a single exponential, indicating the substitution of Er³⁺ ions for Ca²⁺ sites in the host lattice rather than on the particle surface.⁵⁶ The fitting results are shown in Fig. 8d. The lifetime increases (from 118.17 to 167.04 μs) with the increase of Mo⁶⁺-doped content (from 0 to 8% mol), confirming the efficient ET from the Yb³⁺–MoO₄²⁻ dimer to Er³⁺ ions. A similar phenomenon has also been observed in previous works.^5,44

Additionally, the CIE chromaticity coordinates of DC emission samples with different Mo⁶⁺ doping contents were calculated, as shown in Fig. 8c. Notably, chromaticity coordinates (x, y) of the DC phosphors are most close to pure green emission. By accepting these values into eqn (2), the color purity of all samples was estimated and reached a maximum value of 98% (S3 sample), indicating the high pure green light emission of the obtained phosphor. Overall, the high color purity and long lifetime of the UC and DC phosphor suggest that it is suitable as a candidate material for optoelectronic devices.

3.6. Optical temperature sensing based on DC emission

Down-conversion emission intensities of sample S3 depending on the temperature were also investigated to estimate the temperature sensing characteristics of the phosphor, as shown in Fig. 9a. Notably, the ²H_11/2–⁴I_15/2 transition intensity slightly increases with the increase of temperature. By contrast, the ⁴S_3/2–⁴I_15/2 transition intensity significantly decreases from 303 K to 525 K (Fig. 9b). The emission intensity of the phosphor decreases with the increase of temperatures because of the thermal quenching effect.³ Moreover, the emission intensity of the phosphor at 523 K was 54% compared with 303 K, indicating the good thermal stability of the phosphor.

Fig. 9 (a) Photoluminescence excitation spectra of sample S3 with different temperatures ranging from 303 K to 523 K, (b) relationship between ln(FIR) and 1/T, (c) green and red emission intensities depend on temperature, and (d) absolute and relative sensitivities of the phosphor.

Furthermore, using eqn (4) and (5), the relationship of ln(FIR) versus 1/T was determined (Fig. 9c). The fitting results shown in Fig. 9c indicate that the slope value is −636.707, corresponding to the ΔE/k value. In addition, the absolute and relative sensitivity was calculated using eqn (6) and (7) (Fig. 9d). The absolute (S_a) and relative sensitivity (S_r) maximum were 0.417% K⁻¹ at 323 K and 0.697% K⁻¹ at 303 K. Table 5 shows the comparison of temperature sensitivity in this work and other phosphor reported in the literature.^57–60 As shown in Table 4, the CaZrO₃:Er–Yb–Mo phosphor in this work has presented a competitive result. In particular, the S_r maximum value of the DC emission is higher than the UC emission because the thermal effect generated by the DC excitation source light was ignored. Therefore, the high sensing sensitivity of DC phosphor was achieved, thereby extending material's application.

Table 5 Comparison of the sensitivity sensor of our phosphor with other phosphors based on down-conversion emission

Sample	Sr max (% K^−1)	Sa max (% K^−1)	Temperature (K)	References
Tellurite glass:Er	—	0.85	313–713	57
Germanate glass:Er	0.66	—	453–713	57
NaGd(WO4)2:Er	—	1.28	298–573	58
NaBiTiO3:Er	—	0.23	80–440	59
SrGdFGa3O7:Er	—	1.06	10–430	60
CaZrO3:Er, Yb, Mo	0.697	0.417	303–523	This work

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4. Conclusion

The structure, morphology, and luminescent behavior of the CaZrO₃:Er–Yb–Mo phosphor were investigated in detail to explore their temperature sensor properties. The XRD and Rietveld refinement confirmed the formation of the orthorhombic single-phase CaZrO₃ with a high crystallinity degree. SEM images show a uniform cubic morphology with an average size of approximately 1 μm. The strong green UC and DC emission of the phosphor was achieved under excitation of 975 and 379 nm. Consequently, the green UC and DC emission intensities of the doped sample were increased four and sixfold, respectively, compared with the undoped sample. The ET from the high-energy excited state of the Yb³⁺–MoO₄²⁻ dimer to the ⁴F_7/2 level of Er³⁺ is responsible for the enhancement of green UC and DC emissions, which is further confirmed by decay kinetics based on the DC emission of the phosphors. Additionally, the phosphor showed high green color purity, 96.50% of UC emission, and 98% of DC emission. Furthermore, the sensor sensitivity value based on DC emission was more than that of UC emission because the thermal effect generated by the DC excitation source light was ignored, thereby extending the application potential of the material. These results indicate that the CaZrO₃:Er–Yb–Mo phosphor is an efficient down and up-conversion luminescent material suitable for optoelectronic integrated devices and thermal sensor applications.

Author contributions

Hoang Tuan Nam: data curation, format analysis, experiment, writing – original draft. Phuong Dinh Tam: investigation, data curation. Nguyen Van Hai: methodology, format analysis. Hoang Nhu Van: conceptualization, format analysis, methodology, resources, supervision, writing – review & editing.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work is funded by the Ministry of Education and Training of Vietnam, under the project B2023-SPH-06.

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