Temperature-dependent light upconversion and thermometric properties of Er³⁺/Yb³⁺-codoped SrMoO₄ sintered ceramics

Abstract

A series of Sr_1−x−yEr_xYb_yMoO₄ phosphor compositions codoped with Er³⁺ (x = 1 mol%) and varying Yb³⁺ (y = 0–9 mol%) dopants have been evaluated for luminescence-based thermometry. Sintered ceramics exhibit enhanced grain growth at an optimum Yb³⁺ content (y = 0.03) and exhibit improved upconversion luminescence and a relatively better thermometric performance over the phosphors in the powder form. Strong upconversion (UC) luminescence at ~ 529, ~ 552, and ~ 662 nm assisted by a 2-photon process, cooperative luminescence from Yb³⁺ ion pair at 495 nm, and weak UC emissions at ~ 380 and ~ 410 nm due to 3-photon process are identified. Luminescence quenching is observed in all the UC emission bands for Yb³⁺ content y > 3 mol%. Sensitization from Yb³⁺ to Er³⁺ and Yb³⁺–(MoO₄)²⁻ dimer to Er³⁺ ions results in selective enhancement of the green emission. Variation of UC emission intensity with increasing dopant concentration is analyzed using Dexter’s energy transfer formula, which supports the dipole–dipole interaction between Yb³⁺ and Er³⁺ ions. Changes in fluorescence intensity ratio (I₅₂₉/I₅₅₂) with temperature reveal the potential usefulness of the optimized Sr_0.96Er_0.01Yb_0.03MoO₄ phosphor composition in the ceramic form for non-contact optical thermometry and exhibits good repeatability for temperature sensing applications.

Introduction

Trivalent lanthanide ions (Ln³⁺) have shown remarkable adaptability in different host materials to up-convert near-infrared radiation into visible radiation with larger luminescence lifetimes and a high signal-to-noise ratio [1,2,3,4]. The applicability of the fluorescence intensity ratio method as a non-contact probe for temperature sensing has been recognized for spatial mapping of temperature fluctuations at the sub-micron scale in areas such as microscopic cellular interactions, biochemical processes, and medical diagnostics [5, 6]. Current research concentrates on a large number of host materials with different rare-earth ion combinations for achieving improved sensitivity over a wide temperature range, good linearity, and a fast response speed [7, 8].

For efficient UC emission, the non-radiative (NR) energy transfer (ET) process is either due to multipolar interaction or exchange interaction mechanism between the selected activator and the sensitizer ions at an appropriate distance between them [9]. The Er³⁺ ion acts as an activator due to its intra f-f transitions and Yb³⁺ as a sensitizer provides a large absorption cross section for the near-infrared radiation [10]. The stability of the host material is of crucial importance for achieving a controlled structural arrangement that favors efficient energy transfer and gives a controlled emission profile. The reported temperature sensing performance based on light upconversion in many host materials can be summarized as follows: (1) the sensitivity varies with temperature and reaches a maximum at a certain temperature, (2) different host materials exhibit maximum sensitivity at different temperatures, and (3) only a few host materials have been characterized for applicability over a wide temperature range [6, 8]. So far, tungstates and molybdates such as Er³⁺/Yb³⁺-codoped SrWO₄ and SrMoO₄ have revealed improved functional performance for high-temperature applications over a wide range (80–775 K), and Gd₃Ga₅O₁₂ has exhibited an exceptionally high sensitivity in the cryogenic (4.2–300 K) range [11,12,13].

SrMoO₄ is an attractive material due to its intense absorption near the UV region, low phonon energy (850 cm⁻¹), and the strong polarization of Mo⁶⁺ ion which distorts the symmetry and enhances Stark splitting [2, 14]. Studies on RE ion-doped SrMoO₄ in different forms including single crystals [15], nanocrystals/nanoparticles [16, 17], core–shell structures and nanopowders [18, 19] were reported. Synthesis of other Er³⁺/Yb³⁺-codoped CaMoO_4, and CaWO₄ phosphors prepared by different methods including sol–gel [20] and solid-state reaction [21] method has been attempted, and recently, the significant influence of Ce³⁺ and Sm³⁺ dopants in SrMoO₄ has been reported [22, 23]. For temperature sensing applications, Du et al. [12] reported a stable performance over a wide temperature range (93–773 K) with a maximum sensitivity (0.0128 K⁻¹) at 480 K using Er³⁺/Yb³⁺-codoped SrMoO₄ powders. Soni et al. [24], showed optical heating effects and color tunability in co-precipitated Er³⁺/Yb³⁺-codoped SrMoO₄ powders and reported a high sensitivity for optical thermometry over a limited range (300–543 K). Zhang et al. [25] emphasized the influence of large grain growth that was achieved in Er/Yb-codoped SrMoO₄ phosphors which were further modified with aliovalent doping of Li⁺ and Ga³⁺. Strong upconversion luminescence, enhanced temperature sensitivity, and a high measurement accuracy with good repeatability were reported in the temperature range (300–600 K) prompting the usefulness of such molybdate phosphors for thermometric application.

Photonic materials in the form of bulk ceramics are also gaining lot of attention. Ceramics are robust and simple to make and allow superior control over their thermal and mechanical properties. They can be formed into different shapes and allow the chemical composition to be tailored for combining several functional properties with optical control. Comparison studies on glasses and glass-ceramics have revealed the beneficial influence of crystal field effects due to inherent microscopic crystallinity [26]. Recent studies have shown improvements in upconversion luminescence relating to crystallinity with the introduction of Bi³⁺ in rare-earth-codoped phosphors [27, 28]. Ceramics regardless of being opaque have shown interesting light upconversion properties (visible to ultraviolet) on their surface, and microstructural features including material thickness, increasing crystallite/grain size have shown a profound effect on the antimicrobial activity [29]. Motivated by the potential applicability of opaque ceramic surfaces, the present work is focused on the processing, microstructure, and light upconversion properties of Er/Yb-codoped SrMoO₄ ceramics which have not been reported earlier. Effects of exchange interaction between the Er³⁺ and Yb³⁺ ions in the SrMoO₄ lattice, cooperative luminescence from Yb³⁺ ion pair, and increased sensitization from Yb³⁺–(MoO4)²⁻ dimer complex formation are analyzed. The present study compares the performance of the phosphors in powder and ceramic form and examines the influence of Er/Yb dopant concentration on the structural and thermometric properties. The performance of sintered ceramics is found to be superior with an optimum content of Yb³⁺ (3 mol%). The usefulness of the optimized composition for temperature sensing is evaluated over a wide range of temperature (90–775 K) in terms of linearity, sensitivity, repeatability, and measurement errors.

Experimental procedure

Modified Sr_1−x−yEr_xYb_yMoO₄ ceramic compositions were prepared by reacting a mixture of oxide powders having a pre-set Er³⁺ content (x = 0.01) and varying Yb³⁺ content (y = 0.01, 0.03, 0.05, 0.07, 0.09). Fine powders of Er₂O₃, Yb₂O₃ (from Alfa Aesar), SrCO_3, and MoO₃ (from Sigma-Aldrich) of high purity (99.9%) in stoichiometric proportions were mixed with a motorized agate pestle and mortar for 6 h. The initial calcination step was carried out at 750 °C for 3 h, and the powder mixture was then pressed into pellets which were sintered at 950 °C for 5 h. BRUKER D8-X-ray diffraction equipment was used in the 2θ range ($25^{^\circ } - 80^{^\circ }$) and to collect the XRD data at a step-size of $0.02^{^\circ }$. The light upconversion (UC) data were measured on a HORIBA PTI Quanta-master (8450–11) with 980 nm excitation produced by a continuous-wave (CW) solid-state laser. The lifetime measurements at room temperature (RT) were taken using a pulsed Xe light source. The UC spectra at different temperatures were recorded on a polished ceramic surface mounted on a programmable temperature-controlled stage (LinkamT95). The temperature of the sample was stabilized for 8 min before recording the UC emission, and a cooled photo-multiplicator (R928P Hamamatsu) was used.

Results and discussion

X-ray diffraction

The X-ray diffraction measurements revealed single phase formation in the sintered Sr_1−x−yEr_xYb_yMoO₄ phosphors as shown in Fig. 1a. A good match with the reported reference data on SrMoO₄ (JCPDS No. 00-008-0482) was achieved and found to correspond with the tetragonal structure of SrMoO₄ with space group I41/a (No.88) [30, 31]. The well-reacted phosphors upon the initial calcination treatment at 750 °C/3 h itself showed single phase formation as shown in Fig. 1b. Further post-annealing treatment at 950 °C/5 h and the use of calcined powders for making ceramics which were sintered at 950 °C/5 h did not show any changes in the phase purity. No additional phases were observed confirming the solubility of Er³⁺ and Yb³⁺ in the host lattice. Table 1 presents the variations in the lattice constants a, c, and the unit-cell volume (V) with varying dopant concentration [x (Er³⁺) + y (Yb³⁺)]. A slight expansion in the lattice is noted with just the introduction of Er³⁺ (ESMO) dopant alone, and further addition of Yb³⁺ content showed lattice contraction. The changes in the lattice due to the incorporation of Er and Yb at the Sr²⁺ site are attributed to the variation of ionic radii, electronegativity, and atomic crystal structures of Sr, Er, and Yb, respectively. Sr and Yb atoms are known to exist with bcc structure, whereas Er exists in the hcp structure [32]. Considering the ionic radii differences of [Sr²⁺ (0.138 nm), Er³⁺ (0.103 nm Å), Yb³⁺ (0.1008 nm)], it is inferred that Er³⁺ is substituted at the Sr²⁺ site in hcp structure having a coordination number 12 CN. Therefore, substitution of hcp structured Er³⁺ in place of bcc structured Sr²⁺ results in the slight expansion of the lattice (Table 1) leading to increased unit-cell volume [33]. In all the other compositions (EYSMO) having a fixed 0.01 mol of Er³⁺ content, the substitution of Yb³⁺ is favored at the Sr²⁺ site as both the ions have a bcc structure, and both are in the same state with the same coordination number 8 CN.

Figure 1

a XRD patterns recorded on Sr_1-x–y Er_xYb_yMoO₄ ceramics having fixed Er³⁺ (x = 0.01) and different Yb³⁺ content (y = 0.00–0.09). b Comparision of XRD patterns recorded on (i) calcined powder, (ii) calcined + post-annealed powder, and (iii) sintered ceramic

Table 1 Lattice parameters (a, b, c), unit-cell volume, and density of Sr_1-x-yEr_xYb_yMoO₄ ceramics with varying Yb content. (The numbers in the parentheses show the error in the last two digits

Varying Yb³⁺ content is found to influence the grain growth in sintered ceramics as seen in the micrographs shown in Fig. 2a–f. An optimum Yb³⁺ content y = 0.03 produces a densely packed microstructure structure with a large grain size (~ 10 μm), and further increase in Yb³⁺ content increases the porosity and induces the development of non-uniform grain size in the ceramic microstructure.

Figure 2

a–f Microstructure of Sr_1-x–y Er_xYb_yMoO₄ ceramics having fixed Er³⁺ (x = 0.01) content, and different Yb³⁺ content (y = 0.00–0.09)

Light upconversion and quenching

Comparison of phosphor powder and sintered ceramic

The upconversion luminescence in the developed phosphor Sr_1−x−yEr_xYb_yMoO₄ with Er³⁺ (x = 0.01), and Yb³⁺ (y = 0.03) was compared in three different forms;

(1) Calcined powders, (2) calcined and annealed powder, and (3) dense ceramic pellets prepared from calcined powders. Although they were subject to varying heat treatment, the structural analysis (Fig. 1b) showed single phase formation without any significant changes in the crystallite size; however, they showed a significant change in the light upconversion spectra as shown in Fig. 3. The observed improvement in the upconverted light intensity is believed to arise from the remarkable increase in the grain size of the ceramics for the optimum concentration of Yb³⁺ (y = 0.03).

Figure 3

Comparision of UC emission intensity on calcined, calcined + post-annealed powder and sintered ceramics prepared from calcined powder of the phosphor with composition Sr_1-x–y Er_xYb_yMoO₄: x = 0.01/y = 0.03

The observed dependence of UC emission intensity on grain size is in close agreement with the recent results of Zhang et al. [25], where enhanced upconversion luminescence, an increased FIR ratio, and improved temperature sensitivity were observed in Er/Yb-codoped SrMoO₄ phosphors when further modified with Li⁺/Ga³⁺ dopants. Both Li⁺/Ga³⁺ were suggested as sintering aids which promoted enhanced grain growth in the developed phosphors. Motivated from this earlier study which was primarily concentrated on using additional dopants (Li⁺/Ga³⁺), we have rather concentrated on sintered bulk ceramics and observed a similar enhancement in the grain growth for an optimum Yb³⁺ concentration of y = 0.03 as seen from the scanning electron micrographs presented in Fig. 2c.

The light upconversion spectra measured on different Sr_1−x−yEr_xYb_yMoO₄ ceramic compositions are shown in (Fig. 4a, b) when excited with 980 nm at a constant pump power (0.50 W). With single Er³⁺ dopant (x = 0.01, y = 0), only a weak UC emission in the green and red bands is observed as indicated by the black colored trace in (Fig. 4b). Addition of Yb³⁺ content (y = 0.01 to 0.09), along with the fixed Er³⁺ content (x = 0.01) reveals UC emissions at ~ 380, ~ 410 nm and ~ 495 nm due to the ⁴G_11/2 → ⁴I_15/2, ²H_9/2 → ⁴I_15/2, and cooperative luminescence of Yb³⁺ ion pair, respectively (Fig. 4a), in agreement with earlier reported studies [34,35,36,37]. The strong upconversion emissions in the green band at ~ 529 nm (²H_11/2→ ⁴I_15/2), and ~ 552 nm (⁴S_3/2 → ⁴I_15/2) are followed by the weak emission in the red band at 662 nm (⁴F_9/2 → ⁴I_15/2) as shown in Fig. 4b [25, 38,39,40].

Figure 4

Upconversion emission spectra of Sr_1-x–y Er_xYb_yMoO₄ phosphors excited at 980 nm, in the range a 350–500 nm, b 500–680 nm, inset: plot of log (I/X) versus log (X)

The emission intensity at 552 nm being stronger than the emission at 529 nm (Fig. 4b) stems from the redistribution of energy transfer within the respective levels as they are coupled thermally. The upconverted emission intensity at ~ 380, ~ 410, ~ 495, ~ 529, ~ 552, and ~ 662 nm increases up to an optimum Yb³⁺ concentration y = 0.03. Further increase in Yb³⁺ dopant (y > 0.03) results in a sharp fall in the intensity (Fig. 4a, b) due to concentration quenching effect [12, 14, 16]. Beyond the quenching concentration (y = 0.03), the critical distance R_c between the Yb³⁺ and Er³⁺ ions is found to reduce considerably due to the shortened interatomic distance between these two ions and gives rise to non-radiative transitions. As the exchange interaction and multipole–multipole interaction have greater probability over radiative emission, the UC emission intensity thereby decreases [41]. It is noted that the presence of both the activator and the sensitizer ions play an important role in the calculation of the critical distance (R_c) for evaluating the quenching process [42],

$$R_{C} \approx 2\left[ {\frac{3V}{{4\pi X_{{\text{C}}} Z}}} \right]^{1/3}$$

(1)

where V is the unit-cell volume, $X_{{\text{C}}}$ = (x + y) is the dopant concentration at which quenching is observed. Er³⁺ (x) and Yb³⁺ (y) is the dopant concentration and (Z = 4) represents the number of host cations per unit cell. For ${X}_{C}=0.04$, with x = 0.01 and y = 0.03, the critical distance in the presence of Yb³⁺ ion is ~ 16.08 Å which is smaller than the value 25.56 Å in the absence of the Yb³⁺ ions. The efficient transfer of energy from the Yb³⁺ sensitizer ion to the activator Er³⁺ ion occurs, when R_c in the presence of sensitizer is less than the value obtained in the absence of the sensitizer. Normally, the exchange interaction plays a key role for R_c < 5 Å for concentration quenching, and, alternately, the electric multipole interaction mechanism is dominant for R_c > 5 Å. For the optimum concentration of Yb³⁺ (y = 0.03), as the calculated value of R_c is 16.08 Å, it is implied that a multipolar interaction is accountable for the observed concentration quenching. This was further understood on the basis of Dexter’s energy transfer formula [43, 44] which governs the UC emission (I) and the concentration X = (x + y) of the sensitizer and the activator as:

$$\log \left( \frac{I}{X} \right) = b - \left( \frac{S}{3} \right) \log \left( X \right)$$

(2)

The fitting parameter S = 6, 8, and 10 corresponds to (1) dipole–dipole, (2) dipole–quadrupole, or (3) quadrupole–quadrupole interactions, respectively, and “b” is a coefficient. The slope (S/3) obtained from a linear fit of the $\mathrm{log}\left(I/X\right)$ versus $\mathrm{log}\left(X\right)$ shown in the inset of Fig. 4b yields $S\sim 6$ (5.83) and confirms the observed concentration quenching due to dipole–dipole interaction.

UC emission mechanism

The pathways for the energy transfer in the present Er³⁺/Yb³⁺: SrMoO₄ system are depicted in Fig. 5 and show the sensitization from both (1) Yb³⁺ to Er³⁺ ions, and (2) Yb³⁺–(MoO₄)²⁻ dimer complex to Er³⁺ ions, and their combined influence is understood as follows:

Figure 5

Energy transfer pathways between Yb³⁺- Er³⁺ ions and Yb³⁺–MoO₄²⁻ dimer complex to Er³⁺ ions under 980 nm excitation

Yb³⁺ to Er³⁺ ions

The luminescence mechanism triggered by Yb³⁺ to Er³⁺ is well known, where Er³⁺ levels (⁴G_11/2 ²H_9/2, ²H_11/2, ⁴S_3/2, and ⁴F_9/2) get populated, due to energy transfer from Yb³⁺ to Er³⁺ ion. The resultant sensitization due to successive energy transfers and relaxations gives rise to several UC emission bands at (380, 410, 495, and 662 nm) which remain weak despite the increase in Yb³⁺ concentration in contrast to the intense green emission bands at 529 and 552 nm as shown in Fig. 4a, b.

Primarily with the absorption of 980 nm photon, the population at the metastable level ⁴I_11/2 of Er³⁺ increases via ground state absorption (GSA) and is supplemented by the matched resonant absorption from the (²F_7/2–²F_5/2) of Yb³⁺ and (⁴I_15/2–⁴I_11/2) levels of Er³⁺ [41, 45]. A non-radiative relaxation (NRR) process populates the ⁴I_13/2 level. Yb³⁺ ions, which absorb energy from another photon actively transfer energy to nearby Er³⁺ ions at ⁴I_13/2 and ⁴I_11/2 levels and thereby populates the ⁴F_9/2 and ⁴F_7/2 levels via excited-state absorption (ESA).

As the energy gap between ⁴F_7/2 and the underlying levels is small, the Er³⁺ ions rapidly de-excite non-radiatively via multiphonon relaxation (MPR) to the lower levels ²H_11/2 and ⁴S_3/2 which are thermally coupled. This leads to an instantaneous radiative relaxation from the thermally coupled levels to the ⁴I_15/2 ground state and gives rise to the green emission bands at 529 and 552 nm, respectively. Likewise, relaxation from ⁴S_3/2 to ⁴F_9/2 and consequent de-excitation gives the red emission band at ~ 662 which is relatively weak due to the reduced possibility of transitions between ⁴S_3/2 and ⁴F_9/2 levels as the energy gap is large [18, 19, 41].

The Er³⁺ ions at the excited ²H_11/2 or the ⁴S_3/2 levels can get further excited to ⁴G_7/2 by absorbing energy transferred from Yb³⁺ ions and via multiphonon non-radiative relaxation populate the ⁴G_11/2 and ²H_9/2 levels. Moreover, as the ⁴G_11/2 level can also get populated by cross-relaxation: (Er³⁺) ⁴F_9/2 + (Er³⁺) ²H_11/2 → (Er³⁺) ⁴I_13/2 + (Er³⁺) ⁴G_11/2, the UC emissions at ~ 380 nm and ~ 410 are observed [46,47,48,49]. Cooperative luminescence from simultaneous radiative de-excitation of the Yb³⁺ ions from the virtual level V leads to the weak UC emission at ~ 495 nm in the blue band as explained in earlier reported studies [34,35,36] (Fig. 4a).

Yb³⁺–(MoO₄)²⁻ dimer formation

It is noted from Fig. 6 that an increasing Yb³⁺ concentration induces a selective increase in the intensity of the green band (I₅₂₉) by nearly 36 times at the optimum Yb³⁺ content, whereas the enhancement in the other UC emission levels is very small. This could be understood by the additional sensitization provided by the Yb³⁺–(MoO₄)²⁻ dimer complex which is complementary and has a greater probability, because of the harmonizing match between the ⁴F_7/2 level of Er³⁺ion and (|²F_7/2, ³T₂ >) of the dimer complex. Effectual energy transfer is expected because the high-excited-state-energy-transfer process (HESET) at the (|²F_7/2, ³T₂ >) state (T–IV) is relatively at a much higher level in comparison with the energy transfer levels (T–I, T–II, and T–III) available from Yb³⁺ to Er³⁺ (Fig. 5). As a result, the intensity ratio (I₅₂₉ + I₅₅₂)/I₅₂₉ + I₅₅₂ + I₆₆₂)% tends to increase and reach a saturation at higher Yb³⁺ content (y = 0.09) indicating that Er/Yb-codoped SrMoO4 ceramics are essentially green phosphors (Fig. 6). The HESET from the |²F_7/2, ³T₂> state of the Yb³⁺–(MoO₄)²⁻ dimer effectively reduces the losses due to the lattice phonon quenching process and reinforces the UC green emission. These observations are in agreement with earlier studies on Er/Yb-codoped molybdates [16, 18], and Mo⁶⁺-doped oxides (TiO₂, ZnO, Yb₃Al₅O₁₂) which have highlighted the selective increase in the green emission due to additional sensitization effect arising from Yb³⁺–MoO₄²⁻ dimer to Er³⁺ ions [50, 51]. In one of our recent papers, the selective enhancement in the green upconversion luminescence seen for Yb³⁺ concentration of y = 0.03 has been explained to originate directly from [Yb³⁺–MoO₄]²⁻ dimer complex and correlates with the increased degree of lattice distortion occurring at an optimum concentration of Yb³⁺ dopant concentration as evidenced through the changes in the lattice parameters and the unit-cell volume changes presented in Table 1 [52].

Figure 6

Enhancement in the green and red emission band intensities, and the ratio (I₅₂₉ + I₅₅₂)/I₅₂₉ + I₅₅₂ + I₆₆₂)% as the function of the Yb³⁺ content

Pump power dependence

The Sr_0.96Er_0.01Yb_0.03MoO₄ composition with optimum concentration of Yb³⁺ ion (y = 0.03) was examined further at different power levels as shown (Fig. 7a, b). The upconverted intensity ($I_{{{\text{uc}}}} )$ in all the observed emission bands increased with increasing pump power (P) (350–600 mW), and the number of incident photons absorbed in the light upconversion process is estimated from the following relation [53]:

$$I_{{{\text{uc}}}} \left( P \right) \propto P^{n}$$

(3)

Figure 7

UC emission profiles at different values of pump power under 980 nm excitation at 300 K in the range a 360–505 nm, b 500–680 nm, inset: logarithmic plots of pump power dependence of the UC emission

The slope determined from the linear fit of the ln(I) versus ln(P) plots (inset of Fig. 7b) for each one of the UC emissions at ~ 380, ~ 410, ~ 495, ~ 529, ~ 552, and ~ 662 nm is found to be ~ 3.12, ~ 2.76, ~ 1.99, ~ 2.31, ~ 1.94, and ~ 1.96, respectively. A two-photon process is inferred for the green and red emission bands, and a three-photon process for the observed emission at ~ 380 and ~ 410 nm, as foreseen from the energy transfer process discussed in Fig. 5.

Decay curve analysis

Figure 8 compares the normalized decay profiles for the ⁴S_3/2 → ⁴I_15/2 transition (552 nm) transition in Er³⁺ doped (y = 0.00) and the Er³⁺ + Yb³⁺-codoped (y = 0.03) compositions under 380 nm excitation. Decay profile for y = 0 shows a good fit for the single exponential function; however, for y = 0.03 a close fit is not obtained, and the slight departure from the exponential behavior is observed for all other compositions with increasing Yb³⁺ content [54], and the respective decay profiles for all the other compositions are shown in Fig.S1 in the supplementary file. The time dependence of the emission intensity $I(t)$ is expressed as [16]:

$$I\left( t \right){ } = { }A_{1} {\text{exp}}\left( { - t/\tau_{1} } \right){ } + { }I_{{\text{o}}}$$

(4)

where A₁ is a fitting parameter and I_o is the offset parameter, and τ₁ is an exponential component of lifetime.

Figure 8

UC emission intensity (λ_em = 552 nm) decay profiles for two different Sr_1-x–y Er_xYb_yMoO₄ compositions having fixed Er³⁺ (x = 0.01) and a varying Yb³⁺(y = 0.00 and 0.03) content. Measurements at room temperature under λ_ex = 380 nm. Inset: Variation in the calculated lifetime for Sr_1-x–y Er_xYb_yMoO₄ compositions with varying Yb³⁺ (y) content

The estimated lifetime of Er³⁺ corresponding to ⁴S_3/2 → ⁴I_15/2 (552 nm) transition increases from 0.117 to 0.265 ms (inset of Fig. 8) for a change in Yb³⁺ content from y = 0 to 0.09 and suggests an increase of radiative emission. As the interatomic distance between the dopant species reduces with increasing Yb³⁺ concentration, the energy transfer is expedited more efficiently from Yb³⁺ to Er³⁺ [16, 55].

Noninvasive temperature sensing

The fluorescence intensity ratio method is used for studying the optimized Sr_0.96Er_0.01Yb_0.03MoO₄ composition by comparing the emission intensity of the thermally interacting (I₅₂₉ & I₅₅₂) levels. The intensity ratio between the thermally coupled electronic levels (FIR_TCEL) is expressed as [11, 24, 56]:

$${\text{FIR}}_{{{\text{TCL}}}} = \frac{{{\text{I}}_{{\text{H}}} }}{{{\text{I}}_{{\text{S}}} }} = \frac{{{\text{I}}_{529} }}{{{\text{I}}_{552} }} = {\text{C}}\exp \left( {\frac{{ - \Delta {\text{E}}}}{kT}} \right)\, + \,D$$

(5)

where the intensities ${\mathrm{I}}_{529}$ and ${\mathrm{I}}_{552}$ relate with the respective ²H_11/2→ ⁴I_15/2 (~529 nm) and ⁴S_3/2 → ⁴I_15/2 (~ 552 nm) transitions, $\Delta \mathrm{E}$ is the energy difference connecting the two thermally coupled levels, T, k, and D correspond to absolute temperature, Boltzmann constant, and an offset parameter, respectively. The pre-exponential coefficient C depends upon the degeneracy factor of the two levels and the relative probability of the transitions.

Using Eq. (5), a linear fit can be obtained between ln(FIR) versus (1/T) and the obtained thermometric parameter is used to determine the absolute and relative sensitivity, respectively. The temperature dependence of absolute sensitivity (S_a) was determined in accordance with relation defined commonly in the published literature [6, 57, 58]:

$$S_{a} = \frac{{{\text{d}}\left( {{\text{FIR}}_{{{\text{TCL}}}} } \right)}}{{{\text{d}}T}} = {\text{FIR}}_{{{\text{TCL}}}} \times \frac{\Delta E}{{kT^{2} }}$$

(6)

Alternately, the relative or comparative sensitivity (S_r) which is considered to be more informative for non-contact thermometry is defined as [6, 59]:

$$S_{r} = \frac{1}{{{\text{FIR}}_{{{\text{TCL}}}} }} \times \frac{{{\text{d}}\left( {{\text{FIR}}_{{{\text{TCL}}}} } \right)}}{{{\text{d}}T}} = \frac{\Delta E}{{kT^{2} }}$$

(7)

A comparison of the thermometric properties for the optimized phosphor composition in different forms (calcined + post-annealed powder (Fig. S2) and sintered ceramic pellets (Fig. S3)) shows a much higher sensitivity value for the sintered ceramic (Fig. 9a–d) in comparison with the powders as shown in Table 2,

Figure 9

a UC emission spectra recorded at different temperatures for optimum composition Sr_1-x-yEr_xYb_yMoO₄: x = 0.01/y = 0.03 under 980 nm excitation; b temperature dependence of fluorescence intensity ratio FIR(I₅₂₉/I₅₅₂) for Sr_0.96Er_0.01Yb_0.03MoO₄ ceramic; c Ln (FIR) vs. temperature (T); d variation in the absolute (S_a) and relative (S_r) sensitivity with temperature for Sr_0.96Er_0.01Yb_0.03MoO₄ ceramic

Table 2 Comparison of structural and thermometric performance of the optimized phosphor composition Sr_1-x–y Er_xYb_yMoO₄ with x = 0.01 and y = 0.03 in powder and sintered ceramic form

In the present study, the impact of Yb³⁺ concentration has been studied on the thermometric properties of ceramic with varying Yb³⁺ content as shown in Fig. S3 (a-g) supplementary information, and the results are summarized in Table 3. It is noted that all the three compositions with different Yb³⁺ dopant concentrations show the maximum sensitivity around the same temperature 473 K, and an optimum content of Yb³⁺ ( y = 0.03) yields the maximum sensitivity.

Table 3 Influence of Yb³⁺ content in the Sr_1-x–y Er_xYb_yMoO₄ composition on the energy gap of thermally coupled levels, pre-exponential factor C and maximum sensitivity and temperature of the maximum sensitivity

The calculated sensitivity values are considered correct when the error $\delta$ corresponding to the difference between $\Delta E_{f}$ and $\Delta E_{m}$ is low enough [25]. The energy difference $\Delta E_{f}$ is calculated from fitting parameter ${\raise0.7ex\hbox{${\Delta E}$} \!\mathord{\left/ {\vphantom {{\Delta E} k}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$k$}}$ obtained from the linear fit of ln(FIR) versus (1/T) (Fig. 9c), and $\Delta E_{m}$ is the energy difference obtained from the deconvolution of the UC emission spectra between the ²H_11/2 and ⁴S_3/2 levels. The values of $\Delta E_{f}$, $\Delta E_{m}$ and $\delta = \frac{{\left| {\Delta E_{f} - \Delta E_{m} } \right|}}{{\Delta E_{m} }} \times 100\%$ for the three ceramics with varying Yb³⁺ content are given in Table 3. It is noted that an optimum Yb³⁺ content indicates the high values of C that corresponds to the high temperature sensitivity, and the low enough $\delta$ value indicates the correctness of the determined sensitivity values.

Temperature sensing stability and repeatability

Since the FIR versus temperature data serve as the master data for calibration, it needs to be carefully measured by collecting much finer data at shorter temperature intervals and also demands special care towards the stabilization of the ceramic surface temperature before recording the upconversion luminescence for calculating the fluorescence intensity ratio (FIR). FIR values at different temperatures were determined at temperature intervals of 10 K, and the sample was stabilized for 7 min before recording the UC luminescence spectra (Fig. 9a) and it is presented in Fig. 9b.

The FIR value increased from 0.00224 to 5.78533 with increasing temperature in the range 93–773 K. The calculated FIR when plotted over the entire temperature range (93–773 K) revealed an “S-type” plot in the FIR vs. temperature T(K) graph (Fig. 9b) and is similar to (thermocouple voltage vs. temperature) plots obtained for standard thermocouple junctions (e.g., Type-J thermocouple) as shown in Fig. 9b. Such (S-type) variation has not been discussed so far in the reported literature due to the limited temperature range in the reported studies [11, 12, 24, 25]. The present observation of (S-type plot) in fact suggests that much more rigorous experimental data need to be examined in future studies with repeated measurements to establish a rich database for carrying out a reliable mathematical treatment and establish a good calibration curve. Alternately, the phosphor material composition also needs further improvisation to achieve a good linearization over a much wider temperature range for practical applications. In the present study, we have limited our analysis and utilized only that portion of (FIR vs. Temp.) plot which exhibits good linearization as shown in Fig. 9c and extends from 203 to 403 K.

In order to evaluate the temperature sensing performance and the stability, two observation points (273 and 373 K) were selected for the calibration study which correspond to the freezing and boiling temperatures of water in accordance with the standard practice followed for carrying out temperature sensor calibration.

The ceramic sample (0.5 mm thick) was fixed using a heat conducting grease (No.73174, Anton Paar, Germany) onto a temperature-controlled silver block in our temperature controller module which provides a temperature stability of < 0.1 °C. Upconversion emission spectra were logged at intervals of 1 min for an extended duration of ~ 100 mins, and the distribution in calculated FIR values is shown in Fig. 10a, c separately for the two temperatures (273 and 373 K). The temperature calibration curve in Fig. 9c was used to convert the FIR data into temperature as shown in Fig. 10b, d. The deviation in the measured temperature (ΔT) from FIR data and the holding temperature of the sample as maintained by the temperature controller is shown in Fig. 11a, b, respectively. The measurement accuracy (ΔT) at the two selected temperatures 273 and 373 K is found to be within − 0.2 to + 0.2 K with a standard deviation SD = 0.107 and 0.130 K respectively as shown in the histograms (Fig. 11b, d) and corresponds to the measurement uncertainty.

Figure 10

a–b FIR values from repeated measurements at every 1 min recorded for a total duration of 100 min under continuous laser irradiation at 273 and 373 K; c–d temperature data determined from each FIR measurements at 273 and 373 K

Figure 11

a–b Temperature deviation between the measured temperature obtained from FIR data and the controlled set temperature of the sample by the temperature controller at the two temperatures 273 and 373 K. c–d Statistical distribution showing the measurement uncertainty at 273 and 373 K

The reversibility test with heating and cooling cycles was driven at a rate of 100 °C/min, by cycling ceramic sensor temperature between 150 and 523 K as shown in Fig. 12. The FIR values remain unchanged, and the calculated relative standard deviation (RSD) values are found to be 1.362% and 0.73% at 150 K and 583 K, respectively.

Figure 12

Repeatability test for temperature measurement under reversible heating and cooling cycles between the temperatures 153 to 523 K

The present investigations on the ceramic pellets over a wide temperature are in good agreement with the observations reported by Chai et al. [60] and Du et al. [12] on the high sensitivity of Er/Yb-codoped SrMoO₄ and MgWO₄ in the range 0.0093–0.0128 K⁻¹. The close agreement in the high sensitivity values reported for Er/Yb: SrMoO₄ prepared by different techniques establishes the reproducible performance of SrMoO₄ host lattice for high-temperature applications. Therefore, the prepared Er/Yb-doped SrMoO₄ ceramic composition may be used in wide range noninvasive temperature sensing applications.

Conclusions

Substitution of Er³⁺ at the Sr²⁺ sites in the singly doped SrMoO₄ composition shows slight lattice expansion, and further addition of increasing Yb³⁺ content decreases the unit-cell volume and density. Sintered ceramics due to enhanced grain growth show superior performance over the corresponding phosphors in the powder form. An optimum Yb³⁺ content (y = 0.03) shows improved properties. An alternate energy transfer mechanism relating to Yb³⁺–(MoO₄)²⁻ dimer formation to Er³⁺ is seen to assist the selective increase in the UC green emission intensity. The luminescence quenching is found to occur beyond a critical Yb³⁺ content of y = 0.03. The occurrence of weak UC emission at ~ 380 and ~ 410 nm is confirmed in the codoped Er/Yb: SrMoO₄ system due to a 3-photon process. The energy transfer between Yb³⁺ and Er³⁺ ions occurring due to dipole–dipole interaction is substantiated through Dexter’s energy transfer formula. The temperature sensing by fluorescence intensity ratio method shows a maximum sensitivity (0.0139 K⁻¹) at 473 K for the optimized Sr_0.96Er_0.01Yb_0.03MoO₄ ceramic composition. Good linearity and high sensitivity are seen in the temperature range (203–403 K) with a measurement accuracy of − 0.2 to + 0.2 K, and thermometric performance shows good repeatability.