Comparative studies of upconversion luminescence and optical temperature sensing in Tm³⁺/Yb³⁺ codoped LaVO₄ and GdVO₄ phosphors

Abstract

Tm³⁺/Yb³⁺ codoped LaVO₄ and GdVO₄ phosphors are successfully synthesized using solid state reaction methods and then upconversion emission studies are performed. X-ray diffraction has confirmed a pure monoclinic phase of LaVO₄ and a tetragonal phase of GdVO₄. Upconversion emission through 980 nm laser diode excitation has shown a strong blue band at 475 nm and two weak red bands at 647 and 700 nm originating from ¹G₄ → ³H₆, ¹G₄ → ³F₄ and ³F₃ → ³H₆ transitions of Tm³⁺ ions, respectively. Non-thermally coupled levels viz. ³F₃ (700 nm) and ¹G₄ (475 nm) in both the phosphors are used for fluorescence intensity ratio based optical thermometric studies and a comparison is made. The FIR data against temperature were fitted with polynomial and exponential fittings. The results show that polynomial fitting has a higher absolute sensitivity of 21.2 × 10⁻³ K⁻¹ at 653 K for the LaVO₄: Tm³⁺/Yb³⁺ phosphor than the exponential fitting sensitivity of 19.0 × 10⁻³ K⁻¹ at 653 K, while in the case of the GdVO₄: Tm³⁺/Yb³⁺ phosphor both fitting functions provided the same value of absolute sensitivity, that is 13.0 × 10⁻³ K⁻¹ at 653 K. A comparison of the sensitivity values shows that the LaVO₄: Tm³⁺/Yb³⁺ phosphor provides higher sensitivity than the GdVO₄: Tm³⁺/Yb³⁺phosphor but the latter one is too high in upconversion emission.

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1. Introduction

Rare earth doped upconverting phosphors, which convert lower-energy photons to higher-energy photons, have triggered widespread interest in recent decades because of their excellent properties and vast potential applications in solar cells, display devices, light emitting diodes, bio-imaging, optical thermometry etc.^1–8 Nevertheless, upconversion (UC) luminescent materials are currently hampered by low emission efficiency which restricts their field applications in several cases. Therefore, it is crucial to find ways to improve their UC efficiency. Several methods for the improvement in upconversion emission efficiency have been proposed so far and the selection of appropriate host, doping of light ions, use of plasmonic particles etc.^9–11 are some popular ways for this purpose. For maximum upconversion efficiency a low phonon energy host is generally preferred that decreases nonradiative losses.^12–14 In this aspect, fluoride hosts are found to be good but unfortunately they suffer lower chemical and photo-physical stability than oxides.¹⁴ Hence, researchers are trying to improve the upconversion emission with oxide hosts.

Among various oxide matrices, lanthanide orthovanadates (LnVO₄; Ln: La, Gd, Y) are found crucial for doping of rare earth ions due to their distinct optical, chemical, and electronic properties. These lanthanide orthovanadates generally exist in tetragonal (t-) zircon type structure. The zircon type yttrium orthovanadate (YVO₄) and gadolinium orthovanadate (GdVO₄) have been studied for upconversion emission and strong upconversion luminescence is noted in these hosts.^15,16 The LaVO₄ host however, is found to exist in two polymorphs, either tetragonal (t-) zircon type structure or monoclinic (m-) monazite type structure depending upon the reaction methods.¹⁴ Lanthanide ions with a larger ionic radius prefer to choose monazite structure because of its higher oxygen coordination number (9).¹⁵ On the line of YVO₄ and GdVO₄ hosts it is expected that LaVO₄ can be a good candidate for strong upconversion emission which is also revealed by Shao et al.¹⁷ It is feasible to create multi-colored emission by doping with various rare earth (Ln³⁺) ions, such as red from Eu³⁺, green from Er³⁺, and blue from Tm³⁺ ions. The LaVO₄ is substantially less expensive and is based on a resource that is far more abundant than Y. The current objective is to synthesize LaVO₄-based phosphor and to compare it with popular GdVO₄ host. The thermodynamically stable monazite-type LaVO₄ can be prepared via conventional solid–state reaction method. However, problem lies in the preparation of zircon type LaVO₄ due to its metastable nature. Many researchers have synthesized zircon type LaVO₄ through various synthesis methods. For instance, Oka et al.¹⁸ have reported the synthesis of high crystalline zircon type tetragonal LaVO₄ using hydrothermal method. Similar observation is made by Jia et al., and according to them monazite and zircon phased LaVO₄ nanocrystals may be produced hydrothermally and in a controlled manner using additives like EDTA.¹⁹

Among lanthanide ions the thulium Tm³⁺ ion emits strong upconversion emission spanning from ultravoliate (UV) to near-infrared (NIR) region upon 980 nm excitation. As a result, it is widely used activator ion for upconversion emission. Taking advantage of the efficient energy-transfer from sensitizer and activator, the Yb³⁺ ion as sensitizer is used with Tm³⁺ ion. The energy transfer from Yb³⁺ to other ions is effectively facilitated by the fact that the ²F_7/2 → ²F_5/2 transition of Yb³⁺ is strongly resonant with the f–f transitions of common upconverting lanthanide ions including Er³⁺, Tm³⁺, and Ho³⁺.^20–24 Furthermore, the energy difference between the excited and the ground states of the Yb³⁺ ion is roughly 10 000 cm⁻¹, which corresponds to the low-cost 980 nm laser diode excitation. To the best of our knowledge, the literature does not have any reports on the upconversion and optical thermometric characteristics of either monazite-type or zircon-type LaVO₄ codoped with Tm³⁺/Yb³⁺ ions.

Temperature sensing is crucial in a variety of sectors including research, industrial application, medicine, and others. Traditional temperature detection techniques frequently involve contact measurement and these thermometers often fall short of the demands for their applications in a variety of challenging and harsh environments such as in tissue cells.²⁵ Therefore, temperature monitoring technique based on fluorescence intensity ratio (FIR) is regarded as promising due to its non-contact, high sensitivity, and broad detection range benefits.²⁶ Change in FIR with temperature is often caused by repopulation of electrons in thermally coupled levels (TCLs) upon thermal excitation. The energy gap (ΔE) between thermally coupled levels should be in the range 200–2000 cm⁻¹. In principle, a larger energy gap (ΔE) indicates higher sensitivity.²⁷ Consequently, it is a serious issue to increase sensitivity while taking the smaller (ΔE) between TCLs into account. For example, the energy difference between ³F₃ and ³H₄ excited energy levels of Tm³⁺ ion is about 1817 cm⁻¹, which is extremely near to the maximum limit range of TCLs. So these levels will give high temperature sensitivity.^28,29 Most of the energy level pairs in rare earth ions are non-thermally coupled levels (NTCLs). In actuality, the luminescence produced by NTCLs is also temperature-dependent since it results from the emission bands of two excited states that behave differently as a function of temperature. As a consequence, the FIR between these states is substantially temperature-dependent. NTCLs-based FIR technique, opposed to TCL-based FIR technique, is not restricted by difference in energy levels and may thus have better temperature sensitivity.^30,31

Herein, monoclinic LaVO₄: Tm³⁺/Yb³⁺ and tetragonal GdVO₄: Tm³⁺/Yb³⁺ phosphors were synthesized via conventional solid–state reaction method for comparison of upconversion emission and non-contact temperature sensitivity in the temperature range 300–653 K under 980 nm laser diode excitation. Non-thermally coupled level ³F₃ and ¹G₄ of Tm³⁺ ion are utilized for temperature sensing application in both the phosphors. Colour tuning is also studied with the help of energy level and CIE chromaticity diagram.

2. Experimental

2.1. Materials

To synthesize Tm³⁺/Yb³⁺ codoped LaVO₄ and GdVO₄ phosphors, La₂O₃ (99.99%, Alfa Aesar), Gd₂O₃ (99.99%, Alfa Aesar), V₂O₅ (99.99%, Alfa Aesar), Tm₂O₃ (99.99%, Alfa Aesar), Yb₂O₃ (99.99%, Alfa Aesar) were taken as initial materials.

2.2. Synthesis

Monazite type LaVO₄: Tm³⁺/Yb³⁺ and zircon type GdVO₄: Tm³⁺/Yb³⁺ phosphors were synthesized by high-temperature solid–state reaction technique. For both hosts the concentrations of Tm³⁺ and Yb³⁺ were taken as 0.3 mol% and 5 mol%, respectively based on literature.²⁸ The calculated amounts of Gd₂O₃, La₂O_3, V₂O₅, Tm₂O₃ and Yb₂O₃ were individually mixed and grinded homogeneously in an agate mortar for 1 h each using acetone as mixing medium. The obtained powder was kept in alumina crucible and then heated at a rate of 5° per min in an electrical furnace set to 1473 K for 8 hours. After cooling to ambient temperature, the materials were crushed to get fine powders for further characterizations.

2.3. Characterizations

Rigaku smartlab X-ray diffractometer with Cu Kα radiation source (λ = 0.15406 nm) was employed to determine the crystal phases of the produced phosphors. Agilent Cary 5000 UV-vis-NIR spectrophotometer in 200–1200 nm wavelength range was utilized to record the absorption spectra of the synthesized samples. A CCD-based spectrometer (Avantes, ULS2048 × 64) was used to record upconversion emission spectra of the prepared samples using 980 nm laser diode as the excitation source. A self-fabricated heating element was used to measure the temperature-dependent upconversion spectra in the temperature range of 300–653 K. To avoid the laser-induced optical heating of the material, the laser power was maintained at 66 mW. All the measurements were performed using the materials in powder form at room temperature.

3. Results and discussion

3.1. X-ray diffraction (XRD) analysis

The XRD analysis was carried out to ascertain the phase identity and purity of both the prepared samples and recorded patterns are shown in Fig. 1(a and b). Fig. 1(a) shows the XRD pattern of the LaVO₄: Tm³⁺/Yb³⁺ while Fig. 1(b) represents XRD pattern of GdVO₄: Tm³⁺/Yb³⁺ phosphor. The diffraction peaks were well matched with the typical monoclinic phase of LaVO₄ (JCPDS No: 01-070-2392) with the space group P2₁/n (14) and tetragonal phase of GdVO₄ (JCPDS No: 017-0260) with space group I₄₁/amd (141).^15,32 There were no traces of impurity phases present in the recorded patterns. Here it is interesting to note that both the phosphors were prepared under similar environmental conditions but both have resulted different crystal phases. The GdVO₄ sample is in its common crystal phase however, LaVO₄ sample is in less common monoclinic phase. The doping position of Tm³⁺/Yb³⁺ ions in LaVO₄ and GdVO₄ hosts can be calculated on the basis of percentage radius variance (Δ_r), which can be given by;³³
where R_h and R_d represents the ionic radii of host and doping ion, respectively. Using above formula, Δ_r (%) for V⁵⁺ (0.54 Å, CN = 6) with Tm³⁺ (0.88 Å, CN = 6) and Yb³⁺ (0.868 Å, CN = 6) ions are calculated to be 63% and 60.74% respectively. Whereas, Δ_r (%) for La³⁺ (1.032 Å, CN = 6) with Tm³⁺ (0.88 Å, CN = 6) and Yb³⁺ (0.868 Å, CN = 6) pairs are estimated to be 14.72% and 15.89% respectively. It is widely assumed that preferred replacement requires a radius variance (Δ_r) of about 15% between the dopant and host ions. So, this calculation favours the substitution of La³⁺ with Tm³⁺/Yb³⁺ ions. Similarly, for GdVO₄: Tm³⁺/Yb³⁺ phosphor the Δ_r (%) for Gd³⁺ (0.935 Å, CN = 6) with Tm³⁺ (0.88 Å, CN = 6) and Yb³⁺ (0.868 Å, CN = 6) ions comes out to be 5.88% and 7.16%, respectively which favours the substitution of Gd³⁺ ion with Tm³⁺/Yb³⁺ pairs.

Fig. 1 X-ray diffraction pattern of 0.3 mol% Tm³⁺/5 mol% Yb³⁺ codoped (a) monazite type LaVO₄ (b) zircon type GdVO₄ phosphors.

3.2. UV-vis-NIR absorption spectroscopy

Fig. 2(a) depicts the UV-vis-NIR absorption spectra of 0.3 mol%Tm³⁺/5 mol% Yb³⁺: LaVO₄ and 0.3 mol%Tm³⁺/5 mol% Yb³⁺: GdVO₄ phosphors recorded in diffuse reflectance mode in the 200–1200 nm wavelength range. The spectra of both the phosphors show broad absorption bands between 200 and 400 nm, with two peaks centred at 260 and 305 nm. These peaks are arising due to charge transfer state (CTS) transitions from O²⁻ to V⁺⁵ ions.^34,35. Apart from these bands, both the spectra contain three absorption peaks due to 4f–4f transition of Tm³⁺ and Yb³⁺ ions. The band centred at 695 and 797 nm are attributed to ³F₃ ← ³H₆ and ³H₄ ← ³H₆ transitions of Tm³⁺ ion while the broad absorption band at 976 nm is present due to ²F_5/2 ← ²F_7/2 transition of Yb³⁺ ion.³⁶

Fig. 2 (a) UV-vis-NIR absorption spectra in diffuse reflectance mode of Tm³⁺/Yb³⁺ codoped monazite type LaVO₄ and zircon type GdVO₄ phosphors; (b) Kubelka–Munk plots to estimate the optical band gap energies of the synthesized phosphors.

The above absorption spectra are further used to calculate the optical band gap of the samples. With the use of Wood–Tauc (W–T) formula and the Kubelka–Munk (K–M) function, the band gap of phosphor materials may be determined. The (W–T) formula for bandgap energy E_g is given by³⁷

where α is the linear absorption coefficient of the material, E_g, hν and A are the optical bandgap energy, incident photon energy and A is the proportionality constant, respectively. The K–M function is defined as³⁸where K, S and R are the absorption coefficient, scattering factor and R = R_sample/R_standard known as reflectance of material, respectively. The optical band gap energy is estimated by combining eqn (1) and (2) which is given by;

[F(R)hν] = B(hν − E_g)ⁿ (3)

where B is a constant called the band tailoring parameter and n is a constant that represents the nature of band transition and can have values 1/2, 2, 3/2 or 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. Previous reports indicate that monazite type LaVO₄ is an indirect band gap material while tetragonal GdVO₄ is a direct band gap material.^15,35,39,40 For estimation of band gap values, the plots of [F(R)hν]^1/n versus hν for indirect and direct band gap transitions are shown in Fig. 2(b). From the graph, the value of E_g is extracted by extrapolating the linear fitted regions to [F(R)hν]^1/n = 0. By this way, band gap for Tm³⁺/Yb³⁺ codoped monazite type LaVO₄ is determined to be 3.73 eV, and that of Tm³⁺/Yb³⁺ codoped zircon type GdVO₄ is estimated to be 3.36 eV. Both the calculated band gap values are in consistent with the reported results.

3.3. Upconversion emission and energy level diagram

Fig. 3(a) compares the UC emission spectra of 0.3 mol% Tm³⁺/5 mol% Yb³⁺: LaVO₄ and 0.3 mol% Tm³⁺/5 mol% Yb³⁺: GdVO₄ phosphors at 66 mW excitation power of 980 nm laser diode. In both the phosphors, three emission bands are observed at 475, 647 and 700 nm wavelengths. These bands are attributed to the ¹G₄ → ³H₆, ¹G₄ → ³F₄ and ³F₃ → ³H₆ transitions of Tm³⁺ ion, respectively. It is interesting to see that UC emission intensity of GdVO₄ phosphor is around 24 times higher than the LaVO₄ phosphor, although both samples were synthesized under similar conditions and contain same concentrations of the dopant ions. Moreover, it was expected that monoclinic phase should show higher emission compared to the tetragonal phase due to lower symmetry in monoclinic phase. The blue emission (475 nm) is found to dominant over red bands (647, 700 nm) in both the phosphors. The inset of Fig. 3(a) shows the enlarged view of the spectra in wavelength range 600–730 nm for better visibility of weak emission bands.

Fig. 3 (a) Comparison of upconversion emission spectra of 0.3 mol% Tm³⁺/5 mol% Yb³⁺ codoped LaVO₄ and GdVO₄ phosphors under 980 nm laser diode excitation; inset shows the enlarged spectra in 600–730 nm range; (b) energy level diagram of Tm³⁺ and Yb³⁺ ions with possible upconversion processes in both the hosts.

To better understand the observed UC emission bands in both the phosphors, energy level diagram is illustrated in Fig. 3(b). The Yb³⁺ ion works as sensitizer for this system as it has higher absorption cross-section for 980 nm excitation. After absorbing 980 nm photon energy Yb³⁺ ions excite to ²F_5/2 level and then transfer the photon energy to nearby Tm³⁺ ion via various UC processes. After getting energy from Yb³⁺ through ET(1) process, ground state (³H₆) Tm³⁺ ions are raised to excited state ³H₅ followed by non-radiative decay to ³F₄ level, while Yb³⁺ ion goes back to its ground state ²F_7/2. The Tm³⁺ ions in ³F₄ level again uplifted to ³F₂ excited state by absorbing next 980 nm photon energy transferred via ET(2) process of Yb³⁺ ion. Tm³⁺ ions while coming back to ³H₄ level non-radiatively, a part of them makes radiative transition from ³F₃ to ³H₆ by emitting red light of wavelength of 700 nm. Since Yb³⁺ ions continuously transfer their absorbed energy to Tm³⁺ ions resulting transition of Tm³⁺ ions from ³H₄ to ¹G₄ level via ET (3) process. Some part of Tm³⁺ ions in ¹G₄ level make radiative emission to ³H₆ ground state by emitting blue light at 475 nm while rest part of Tm³⁺ population in ¹G₄ state goes radiatively to ³F₄ state via emission of 647 nm wavelength. It can be seen that 475 nm and 647 nm UC emission belongs to three photon absorption processes while 700 nm emission is due to two photon process.

3.4. Pump power dependence study

Fig. 4(a and b) shows the UC emission spectra of LaVO₄ and GdVO₄ based phosphors at various pump powers of 980 nm laser diode, respectively. The UC emission in both the samples increases upon increasing pump power from 30 to 104 mW. Interestingly, the red emission (700 nm) is found to increase rapidly in LaVO₄ sample than the GdVO₄. For an unsaturated UC process, emission intensity is related to the pump power as;⁴¹

I ∝ Pⁿ (4)

where I and P are the UC emission intensity and excitation pump power. ‘n’ is the number of NIR photons engaged in populating the emitting levels. Inset of Fig. 4(a and b) shows the ln–ln plot of UC emission intensity versus pump power for ¹G₄ → ³H₆ (475 nm) and ³F₃ → ³H₆ (700 nm) transitions. It is found that for 475 and 700 nm emissions, the slopes are 1.19 and 1.57, respectively for LaVO₄ sample whereas slopes of 1.51 and 1.42 respectively are found in GdVO₄ sample. These values are presenting two photon processes for 475 and 700 nm emissions. However, the slope values for 700 nm in both the samples are in good agreement with two photon process as proposed by energy level diagram (Fig. 3(b)). But the observed slope values for 475 nm are less than expected value of ∼3. This may be due to the fact that the cooperative energy transfer (CET) process also takes part in UC emission. As represented in energy level diagram, ¹G₄ level of Tm³⁺ ion is populated from a virtual state (V) where two excited Yb³⁺ ion simultaneously transferred their energy. In this case, only 2 excitation photons are required to emit 475 nm photons. Hence, the slope values for 475 nm emission in both the systems are deviated from expected value of ∼3. Such kind of observations for Tm³⁺/Yb³⁺ doped systems are also reported by various researchers.^26,41,42

Fig. 4 Pump power dependent UC spectra of Tm³⁺/Yb³⁺ codoped (a) LaVO₄ (b) GdVO₄ phosphors. Inset of both figures represent the ln–ln plot of UC emission intensity versus excitation power.

3.5. Optical thermometry

To explore the possibility of synthesized 0.3 mol% Tm³⁺/5 mol% Yb³⁺: LaVO₄ and 0.3 mol% Tm³⁺/5 mol% Yb³⁺: GdVO₄ phosphors for optical temperature sensing, the temperature dependent UC spectra were recorded in the temperature range 300–653 K upon 980 nm laser diode excitation, as shown in Fig. 5(a and b). The laser pump power was kept at minimum (∼66 mW) to avoid laser induced heating of the sample. It can be seen that UC emission intensity of 475 nm (¹G₄ → ³H₆) and 647 nm (¹G₄ → ³F₄) bands decreases with increasing temperature, while 700 nm (³F₃ → ³H₆) emission intensity increases with increasing temperature in both the samples. Since, ³F₃ and ³H₄ levels are the thermally coupled levels with energy gap of ∼1817 cm⁻¹ (ref. 25) and hence thermal excitation increases the population of ³F₃ from ³H₄ level with enhancement of 700 nm band at elevated temperatures. Here authors have plotted the intensity ratio of red/blue bands (I₇₀₀/I₄₇₅) for both the phosphors against temperature and pump power. For the plot shown in Fig. 5(c and d) the intensity ratio (I₇₀₀/I₄₇₅) is found to increase faster for LaVO₄ sample than the GdVO₄ sample. Due to different intensity response of emission bands with temperature, the non-thermally coupled levels ³F₃ and ¹G₄ (700 and 475 nm) of both the samples were utilized for fluorescence intensity ratio (FIR) based optical thermometry.

Fig. 5 (a) Temperature dependent UC spectra upon 980 nm laser excitation in temperature range 300–653 K of (a) Tm³⁺/Yb³⁺: LaVO₄ (b) Tm³⁺/Yb³⁺: GdVO₄ phosphors. Variation in the ratio of red/blue emission band (I₇₀₀/I₄₇₅) in both the phosphors at varying (c) temperature (d) pump power.

For non-thermally coupled levels (NTCLs), the FIR data can be well fitted through following exponential equation;^27,30,43

where A, B and C are constants whose values can be found by fitting the experimental data. T denotes the absolute temperature. As illustrated in Fig. 6(a and b), the best fits of FIR to temperature are FIR = 75.03 × exp(−1860.44/T) + 0.26 and FIR = 1062.55 × exp(−4403.45/T) + 0.07 for LaVO₄: Tm³⁺/Yb³⁺ and GdVO₄: Tm³⁺/Yb³⁺ samples, respectively. The absolute sensitivity (S_a) is defined as the rate of change of FIR with temperature and expressed as;⁴³

Fig. 6 Exponential fitting of FIR data of non-thermally coupled levels (³F₃ and ¹G₄) as a function of temperature for (a) Tm³⁺/Yb³⁺: LaVO₄ (b) Tm³⁺/Yb³⁺: GdVO₄ phosphors; absolute sensitivity as a function of temperature for (c) Tm³⁺/Yb³⁺: LaVO₄ (d) Tm³⁺/Yb³⁺: GdVO₄ phosphors.

Fig. 6(c and d) shows the plot of absolute sensitivity as a function of temperature for both the samples. It is observed that the sensitivity increases from room temperature to studied (653 K) temperature. The maximum sensitivity for LaVO₄: Tm³⁺/Yb³⁺ phosphor is found to be 19.0 × 10⁻³ K⁻¹ at 653 K (Fig. 6(c)) whereas, maximum sensitivity for GdVO₄: Tm³⁺/Yb³⁺ phosphor is found to 13.0 × 10⁻³ K⁻¹ at 653 K (Fig. 6(d)). However, it seems that sensitivity of GdVO₄ will increase above 653 K. The observed value is compared with Tm³⁺/Yb³⁺ codoped samples in which TCLs are utilized for temperature sensing measurement as given in Table 1.

Table 1 Comparison of absolute sensitivity of Tm³⁺ doped luminescent materials

Samples	Transitions	Temperature range (K)	Sa-max (×10^−3 K^−1)	Ref.
LaVO4: Tm^3+/Yb^3+ (exponential fitting)	^3F3 → ^3H6	300–653	19.00 (653 K)	This work
	^1G4 → ^3H6
LaVO4: Tm^3+/Yb^3+ (polynomial fitting)	^3F3 → ^3H6	300–653	21.20 (653 K)	This work
	^1G4 → ^3H6
GdVO4: Tm^3+/Yb^3+ (exponential fitting)	^3F3 → ^3H6	300–653	13.00 (653 K)	This work
	^1G4 → ^3H6
GdVO4: Tm^3+/Yb^3+ (polynomial fitting)	^3F3 → ^3H6	300–653	13.00 (653 K)	This work
	^1G4 → ^3H6
Bi7F11O5: Tm^3+/Yb^3+	^3F3 → ^3H6	303–573	14.00 (303 K)	46
	^3H4 → ^3H6
SrWO4: Tm^3+/Yb^3+	^3F3 → ^3H6	308–573	6.17 (323 K)	5
	^3H4 → ^3H6
BaGd2ZnO5: Tm^3+/Yb^3+	^1G4(1) → ^3H6	313–573	5.50 (323 K)	47
	^1G4(2) → ^3H6
Na2Y2B2O7: Tm^3+/Yb^3+	^1G4(i) → ^3H6	300–623	4.54 (300 K)	48
	^1G4(j) → ^3H6
Y2O3: Tm^3+/Yb^3+	^1G4(a) → ^3H6	303–753	3.50 (303K)	26
	^1G4(b) → ^3H6
ZnWO4: Tm^3+/Yb^3+/Mg^2+	^1G4(1) → ^3H6	300–600	3.40 (300 K)	49
	^1G4(2) → ^3H6
Y2O3: Tm^3+/Yb^3+/Gd^3+	^1G4(a) → ^3H6	298–533	1.33 (298 K)	20
	^1G4(b) → ^3H6
CaZnOS: Tm^3+/Yb^3+	^1G4(a) → ^3H6	303–423	1.00 (303 K)	44
	^1G4(b) → ^3H6

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Some authors have also fitted FIR data of NTCLs with help of polynomial equation.^44,45 So, to examine the difference between both the fittings, we have fitted the same FIR data with the polynomial equation as given below;

where A, B, C and D are the constants. As shown in Fig. 7(a and b) the FIR versus temperature data can be well fitted by above polynomial equation. The best fit of FIR to temperature for LaVO₄: Tm³⁺/Yb³⁺ is FIR= (2.34 × 10⁻³) − (1.01 × 10⁻³)T + (3.22 × 10⁻⁶)T² + (1.41 × 10⁻⁸)T³ and for GdVO₄: Tm³⁺/Yb³⁺ is FIR = (−3.75) + (0.02)T − (7.21 × 10⁻⁵)T² + (6.14 × 10⁻⁸)T³. The absolute sensitivity using eqn (7)can be written as,

Fig. 7 Polynomial fitting of FIR data of NTCLs (³F₃ and ¹G₄) as a function of temperature for (a) Tm³⁺/Yb³⁺: LaVO₄ (b) Tm³⁺/Yb³⁺: GdVO₄ phosphors; absolute sensitivity as a function of temperature for (c) Tm³⁺/Yb³⁺: LaVO₄ (d) Tm³⁺/Yb³⁺: GdVO₄ phosphors.

The calculated sensitivity as a function of temperature is shown in Fig. 7(c and d) for both the samples. The maximum absolute sensitivity for LaVO₄: Tm³⁺/Yb³⁺ phosphor is found to be 21.2 × 10⁻³ K⁻¹ at 653 K (Fig. 7(c)) whereas, absolute sensitivity for GdVO₄: Tm³⁺/Yb³⁺ phosphor is calculated to be 13.0 × 10⁻³ K⁻¹ at 653 K (Fig. 7(d)). Only slight variation in sensitivity is seen for both the fittings and it can be concluded that both the techniques are equally well.

3.6. CIE chromaticity diagram

Colour coordinates study of prepared phosphors at various temperatures was done in the temperature range of 300–653 K under 980 nm laser excitation at fixed pump power of 66 mW. The coordinates are shown in CIE plot in Fig. 8(a and b). The colour tuning behaviour is prominent in GdVO₄ phosphor. Coordinates of LaVO₄: Tm³⁺/Yb³⁺ phosphor are only slightly shifted from light blue (0.31, 0.31) to pure white (0.33, 0.33) with increasing temperature as shown in Fig. 8(a). On other hand, GdVO₄: Tm³⁺/Yb³⁺ phosphor shows deep blue colour (0.18, 0.15) at 300 K and approaches nearly white light (0.29, 0.28) at 653 K, shown in Fig. 8(b).

Fig. 8 CIE colour chromaticity diagram of (a) LaVO₄: 0.3 mol% Tm³⁺/5 mol% Yb³⁺ (b) GdVO₄: 0.3 mol% Tm³⁺/5 mol% Yb³⁺ phosphors. Colour change is more prominent is GdVO₄ phosphor.

4. Conclusions

The LaVO₄: Tm³⁺/Yb³⁺ and GdVO₄: Tm³⁺/Yb³⁺ upconversion phosphors were successfully synthesized using solid state reaction method. The LaVO₄ sample is found in monoclinic crystal phase while GdVO₄ is found in tetragonal crystal phase. Upon 980 nm laser diode excitation the GdVO₄: Tm³⁺/Yb³⁺ has resulted several fold intense blue upconversion emission than the LaVO₄: Tm³⁺/Yb³⁺ phosphor. The non-thermally coupled levels viz. ³F₃ (700 nm) and ¹G₄ (475 nm) were utilized for optical thermometry in both the phosphors and two different functions were used for fitting the FIR versus temperature data. For LaVO₄: Tm³⁺/Yb³⁺ phosphor, exponential fitting gives a maximum absolute sensitivity of 19.0 × 10⁻³ K⁻¹ at 653 K while polynomial fitting provides a maximum value of 21.2 × 10⁻³ K⁻¹ at 653 K. Similarly, for GdVO₄: Tm³⁺/Yb³⁺ phosphor, maximum absolute sensitivity of 13.0 × 10⁻³ K⁻¹ at 653 K is observed using the both kind of fitting functions. It is concluded that LaVO₄: Tm³⁺/Yb³⁺ phosphor provides higher sensing sensitivity compared to GdVO₄: Tm³⁺/Yb³⁺ phosphor.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Madan M. Upadhyay is thankful to IIT(ISM) Dhanbad for providing financial support in terms of Senior Research Fellowship (SRF). Authors also acknowledge DST-SERB, New Delhi India (Project no: EMR/000228/2017) for the lab facilities.

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