Enhancing thermometric precision: modulating the temperature of maximum sensitivity via erbium dopant addition in Ba₂GdV₃O₁₁:Tm³⁺/Yb³⁺ nano phosphors

Abstract

Developing luminescence sensors often prioritizes maximizing relative sensitivity to achieve optimal performance. However, a critical parameter often overlooked is the temperature at which maximum sensitivity occurs. In this study, we delve into this crucial aspect by exploring the impact of erbium doping in Tm³⁺/Yb³⁺ co-doped Ba₂GdV₃O₁₁ nano phosphors. The crystal structure, microscopic morphology, and luminescence mechanism of BGVO:Yb³⁺/Tm³⁺ and Er³⁺/Tm³⁺/Yb³⁺ up conversion nanoparticles, as well as the temperature sensing characteristics are investigated. Under 975 nm laser excitation, the BGVO:Yb³⁺/Tm³⁺ and BGVO:Er³⁺/Tm³⁺/Yb³⁺ nano phosphors exhibited strong blue and green upconversion luminescence, respectively. The luminescence intensity ratio (LIR) approach was used to analyze the temperature-dependent luminescence spectra in the 300–600 K temperature range. The thermometry strategies were based on thermally coupled energy levels (TCLs) and non-thermally coupled energy levels (NTCLs) of Er³⁺ and Tm³⁺ for temperature sensing performance. In the Tm³⁺/Yb³⁺ codoped samples, the relative sensitivity typically peaks around 350 K, attributed to TCLs (1.7% K⁻¹, 700 nm/800 nm) with generally lower relative sensitivity compared to non-TCLs (5.39% K⁻¹, 700 nm/475 nm). However, non-TCL sensitivities in the 300–600 K range lack a clear maximum. In contrast, Er³⁺/Tm³⁺/Yb³⁺ samples exhibit distinct maxima in non-TCL sensitivities within this temperature range (1.91% K⁻¹, 700 nm/550 nm), offering precise temperature determination for specific applications. Our findings underscore the potential of erbium doping to modulate temperature sensitivity peaks, crucial for optimizing performance in tailored luminescence nanosensors and offering fresh concepts for investigating alternative superior optical temperature sensing nano materials.

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Introduction

Temperature is a fundamental parameter in thermodynamics, essential for scientific inquiry, climatology, industrial production, and routine tasks. However, traditional temperature sensors often face limitations due to susceptibility to electromagnetic interference and potential errors from component heating.^1–5 Consequently, the development of non-contact temperature detection methods, such as fluorescence thermometry, has gained momentum.⁶ A key spectral characteristic utilized in fluorescence thermometry is the luminescence intensity ratio (LIR), which undergoes changes with temperature variations.^7–10 LIR thermometry, also known as self-referencing radiometric thermometry, reduces reliance on external factors like excitation intensity fluctuations or electromagnetic interference.¹¹ Luminescence nano thermometry predominantly employs optical sensors based on Ln³⁺ ions due to their advantageous properties.¹² These sensors commonly utilize radiometric techniques, particularly assessing band intensity ratios.^7,13,14 While ions like Er³⁺, Tm³⁺, and Nd³⁺ are frequently utilized due to their thermally coupled levels (TCLs), their relative sensitivity (S_r) to temperature is limited to specific energy differentials between TCLs, resulting in relatively low S_r values. To address this limitation, recent research has focused on utilizing band intensities from nonthermally coupled levels (non-TCLs) of different Ln³⁺ ions, offering theoretically limitless temperature sensitivity.^7,13,14

Luminescent nanothermometers use a wide range of materials, including inorganic fluorides, phosphates, vanadate doped with optically active d-block metal ions, and lanthanide ions, along with carbon dots and quantum dots.^8,15,16 These matrices offer strong photoluminescence signals and can be employed to create highly efficient phosphors due to their low phonon energies and thermal stability. Lanthanide-based inorganic compounds, including nanoparticles (NPs), possess a multitude of advantageous characteristics, such as broad-range emission induced by UV, visible, or NIR irradiation, long luminescence lifetimes, narrow emission bands, large spectral shifts, stability in the (+3) oxidation state, resistance to photobleaching, and high-temperature treatment.^17–19 These advantageous characteristics are primarily attributed to several factors: the forbidden nature of 4f–4f transitions in Ln³⁺ ions, crystal-field effects, and the shielding of 4f electrons by 5s and 5p electrons.²⁰ Moreover, lanthanide ions (Ln³⁺) exhibit up conversion (UC) luminescence, which involves the emission of higher-energy photons due to multiphoton absorption of lower-energy radiation, typically in the NIR range.^21,22 Lanthanide ions serve as both luminescence activators (e.g., Eu³⁺, Tb³⁺, Nd³⁺, Er³⁺, and Tm³⁺) and sensitizers (mostly Ce³⁺ and Yb³⁺) due to their advantageous spectroscopic characteristics, including long luminescence lifetimes, wide range of emission states, large Stokes shift, and the capacity for upconversion.²³ Upconverting nanomaterials find applications in biological sensing, particularly for subcutaneous sensing, owing to their small size and the use of NIR laser excitation to avoid damaging UV radiation.²⁴

This study investigates the influence of erbium (Er³⁺) doping in Tm³⁺/Yb³⁺ codoped Ba₂GdV₃O₁₁ (BGVO) nano phosphors, focusing on its impact on luminescence and thermometric properties. While maximizing relative sensitivity is a primary goal in luminescence-based temperature sensing, the temperature at which maximum sensitivity occurs and the temperature uncertainty are equally crucial but often overlooked. By incorporating erbium ions as a third dopant alongside Tm³⁺ and Yb³⁺ ions, the study reveals insights into how this addition can significantly affect luminescence and thermometric properties.

We present here the inorganic nano phosphors BGVO:Yb³⁺/Tm³⁺ and tridoped Er³⁺/Tm³⁺/Yb³⁺, which were created for temperature sensing and exhibit strong, perceptible UC luminescence (λ_ex = 975 nm). The produced nanomaterial exhibits both phonon-assisted emission from ytterbium ions (above ≈900 nm) and UC luminescence from thulium ions. The material's luminescence characteristics were examined in the 400–960 nm range, which includes the first biological window. The relative sensitivities and temperature resolutions for the produced nanomaterial were determined using the band intensity ratios of the emission bands of Yb³⁺, Er³⁺, and Tm³⁺ ions situated in that wide spectrum range.

Synthesis procedure

Synthesis of BaGdV₃O₁₁ doped Yb³⁺, Tm³⁺ and Er³⁺

The Ba₂GdV₃O₁₁ structures doped with 2%Er³⁺, 1%Tm³⁺ and 15%Yb³⁺ were synthesized using the Pechini sol–gel technique. Nano phosphors are prepared by acquiring multiple chemicals, including barium nitrate [Ba(NO₃)₂ (99.0%)], gadolinium nitrate hexahydrate [Gd(NO₃)₃·6H₂O (99.9%)], erbium nitrate pentahydrate [(Er(NO₃)₃·5H₂O) (99.9%)], thulium nitrate pentahydrate [Tm(NO₃)₃·5H₂O (99.9%)], ytterbium nitrate pentahydrate [Yb(NO₃)₃·5H₂O (99.9%)], ammonium metavanadate [NH₄VO₃ (99.96%)] and citric acid [C₆H₈O₇ (99.0%)]. To initiate the process, all the precursors are placed into an Erlenmeyer flask and stirred using a magnetic stirrer at 70 °C. Following that, citric acid is introduced, causing the solution to transition from green to blue in color. The mixture is gently swirled and then heated to 80 degrees Celsius until it undergoes hydrolysis, transforming into a sol, and ultimately forming a gel. Subsequently, the gel is subjected to calcination through annealing at 700 °C. The final product is further ground for additional analysis.

Characterization

The XRD patterns were measured in the 2θ range of (20–50°) on an X-ray diffractometer (Bruker D8, Germany) with Cu Kα radiation (1.5405 Å), working at 40 kV and 30 mA for phase detection. A UV vis-NIR spectrometer (PerkinElmer Lambda 950) was used to measure UV-vis-NIR absorption. The samples’ morphologies were examined using a scanning electron microscope (JEOL, JSM 6510LV). The sample is positioned within a homemade nano heater chamber and here heating is regulated with an accuracy of ±0.5 K. Utilizing 975 nm diode laser stimulation at a consistent pump power of 30 mW, emission spectra were captured using a monochromator spectrometer (Horiba Jobin Yvon, iHR320).

Results and discussion

Structural properties and particle morphology

Phase composition and crystallinity of luminous materials play a crucial role in their performance. In general, luminophores with high crystallinity exhibit both stronger and weaker luminescent characteristics. The crystallinity of the powder composed of all-nano phosphors was assessed using X-ray diffraction (XRD). Fig. S1 (ESI†) displays the powder X-ray diffraction (XRD) pattern of the BYVO:2%Er³⁺/1%Tm³⁺/15%Yb³⁺ and BGVO:1%Tm³⁺/15%Yb³⁺ nano phosphor. The XRD patterns are closely matched the JCPDS card no. 81-1778 for all the synthesized nano phosphors.^25,26 Rietveld refinement displayed in Fig. 1a and b was done based on the XRD data of all samples. The Rietveld findings indicate that these compounds crystallized in a monoclinic system with the space group P2₁/c. The unit cell parameters are calculated to be a = 12.376 Å, b = 7.732 Å, c = 11.179 Å and β = 103.486°. The absence of impurity peaks in the prepared nano phosphors is confirmed by the XRD pattern. The XRD patterns remained unchanged even when the Er³⁺, Yb³⁺, and Tm³⁺ ions were substituted for the Y³⁺ site, indicating the successful incorporation of these ions into the lattice structure. This outcome can be attributed to the similarity in ionic radii among Y³⁺ (CN = 6; r = 0.90 Å), Er³⁺ (CN = 6; r = 0.95 Å), Yb³⁺ (CN = 6; r = 0.95 Å), and Tm³⁺ (CN = 6; r = 0.88 Å) ions.²⁷

Fig. 1 (a) Rietveld refinement of XRD data of BGVO:1%Tm³⁺/15%Yb³⁺ and (b) Rietveld refinement of XRD data of BGVO:2%Er³⁺/1%Tm³⁺/15%Yb³⁺.

A transmission electron microscopy (TEM) technique was employed to examine the surface morphology and distribution of grain sizes. In Fig. 2, we have provided TEM images of both doped and undoped samples. It is evident that the irregular shapes observed in the images are due to the aggregation of grains with sizes ranging from 300 to 500 nm.

Fig. 2 SEM images for BGVO doped (a) 1%Tm³⁺/15%Yb³⁺ and (b) 2%Er³⁺/1%Tm³⁺/15%Yb³⁺.

The diffuse reflectance spectra are displayed in Fig. 3 of BGVO co-doped Tm³⁺/Yb³⁺ and tri-doped Er³⁺/Tm³⁺/Yb³⁺ in the range (200–1100 nm). The spectrum presents an intense broad band peaking at 280 nm, which is attributed to the O²⁻–V⁵⁺ entity. However, this spectrum shows four distinct absorption peaks at 525, 648, 682 and 796 nm. These peaks correspond to the transitions of the Er³⁺ and Tm³⁺ ions, respectively. The ²F_7/2 → ²F_5/2 (Yb³⁺) absorption transition produces a broad absorption band with a peak at 975 nm.⁷

Fig. 3 The reflectance spectra of BGVO doped Tm³⁺, Er³⁺, Yb³⁺.

Luminescence measurements

Characteristics of emissions at ambient temperature

After being stimulated by a 975 nm laser, the synthesized nanomaterial exhibits a pronounced, easily discernible blue up-conversion (anti-Stokes) luminescence to the naked eye. Fig. 4a represents the emission spectra for the sample co-doped with Yb³⁺/Tm³⁺ when it is excited with 975 nm wavelength. The spectrum is the result of several sharp bands typical of Tm³⁺ ions in the spectral region of 450–850 nm and one band more than 900 nm typical of Yb³⁺ ions that is partially filtered by the short-pass 950 nm filter used.² All detected bands are associated with the 4f–4f intra-configurational radiative transition of Tm³⁺:¹G₄ → ³H₆ (≈470 nm), ¹G₄ → ³F₄ (≈640 nm), ³F_2,3 → ³H₆ (≈700 nm), ³H₄ → ³H₆ (≈800 nm), and Yb³⁺:²F_5/2 → ²F_7/2 (>900 nm).^28–30 This system employs Yb³⁺ as a sensitizer, pumping the excited states of Tm³⁺ (UC emitter) primarily via energy transfer UC processes when absorbed NIR light is transferred to neighboring Tm³⁺. The BGVO:Tm³⁺/Yb³⁺ nanomaterial demonstrates heightened near-infrared (NIR) emission at low excitation densities, offering potential utility in applications such as deep tissue imaging, optical signal amplification, and various other fields.^31,32 In another way, the UC emission spectra shown in Fig. 4b of the BGVO tri-doped Tm³⁺, Er³⁺, and Yb³⁺ nano phosphors with a 975 nm laser diode were detected in the 450–960 nm wavelength range. Five UC emission peaks can be detected at 470, 487, 523, 538, 655, and 800 nm, associated to the ¹G₄ → ³H₆ (Tm³⁺), ²H_11/2 → ⁴I_15/2 (Er³⁺), ⁴S_3/2 → ⁴I_15/2 (Er³⁺), the combined ¹G₄ → ³F₄ (Tm³⁺)/⁴F_9/2 → I_15/2 (Er³⁺) and ³H₄ → ³H₆ (Tm³⁺) transitions, respectively, and of Yb³⁺:²F_5/2 → ²F_7/2 (>900 nm).^28–30

Fig. 4 Up-conversion emission spectra (a) of the BGVO:Tm³⁺/Yb³⁺ and (b) of the BGVO:Er³⁺/Tm³⁺/Yb³⁺ upon 975 nm excitation.

Fig. S2 (ESI†) shows these results and a rise time and a decay time for the Tm³⁺/Yb³⁺ and Er³⁺/Tm³⁺/Yb³⁺ doped BGVO nano phosphors have been measured using the 975 nm excitation. These curves are characteristics of upconversion processes, and the temporal evolution depends on the lifetimes of the involved intermediate levels.

To evaluate the transition mechanism of different emission peaks, we also examine the correlation between the luminous intensity of the UP (upconversion) and the power of the pump. The intensity of each emission peak in the upconversion process is determined by a power function of the power of infrared excitation. The formula is as follows:

I = K·Pⁿ (1)

P denotes the pump energy, and I denotes the UC fluorescence intensity, while n signifies the number of excitation photons required to generate a single UC photon. Fig. 5a illustrates the correlation between the natural logarithm of UC fluorescence intensity and the natural logarithm of BGVO:Tm³⁺/Yb³⁺ pump power. The slope of the curve is 2.3 (475 nm), and 1.2 (800 nm), indicating that the blue UC is a three-photon process, while NIR emission of BGVO:Tm³⁺/Yb³⁺ is a two-photon process. Fig. 4b demonstrates the presence of blue and red emissions upon 975 nm excitation for the BGVO:Er³⁺/Tm³⁺/Yb³⁺ tri doped nano phosphor. The relationship between the natural logarithm of UC intensity and the natural logarithm of pump power is represented in Fig. 5b. The slope of the curve is 2.10 (475 nm), 1.26 (545 nm) and 1.48 (650 nm), and 1.12 (800 nm), indicating that all emission peaks are a two-photon process, except for the 475 nm one that can be produced by a three-photon process. These values (see Fig. S2, ESI†) could indicate that in the codoped sample there is transfer from the Er to Tm ions.

Fig. 5 Dependence of the intensity with the pump power at 975 nm of (a) BGVO:Tm³⁺/Yb³⁺ and (b) BGVO:Er³⁺/Tm³⁺/Yb³⁺.

Fig. 6 shows the schematic diagram of the energy levels of Yb³⁺, Er³⁺ and Tm³⁺, as well as energy transfer and non-radiative transition processes. It can be seen from the figure that more effective energy conversion successively occurs from Yb³⁺ to Er³⁺ and Tm³⁺ under 975 nm excitation. For the UP mechanism of Yb³⁺ and Er³⁺ ions, the ²F_7/2 ground state of Yb³⁺ ion absorbs a 975 nm photon energy and transitions to the ²F_5/2 excited state. The ²F_5/2 excited state of the Yb³⁺ ion transfers energy to the ⁴I_11/2 energy level of Er³⁺, and the electrons at the ⁴I_11/2 level of Er³⁺ rapidly relax to the ⁴I_13/2 level by non-radiative transition. Er³⁺ at the ⁴I_13/2 level can transfer energy with one Yb³⁺ in the excited state and be excited to the ⁴I_9/2 level. Er³⁺ at the ⁴I_9/2 level rapidly transitions to the ground state to produce upconverted red light. Simultaneously, Yb³⁺ reaches the excited state under 975 nm excitation. The energy transfer between the excited Yb³⁺ and Er³⁺ makes the Er³⁺ of the ⁴I_11/2 level transition to the ²H_11/2 and ⁴S_3/2 levels. Er³⁺ at the ²H_11/2 and ⁴S_3/2 levels rapidly transitions to the ground state to produce upconverted green light. For the UC mechanism of Yb³⁺ and Tm³⁺ ions, the ²F_7/2 ground state of Yb³⁺ ion absorbs a 975 nm photon energy and transitions to the ²F_5/2 excited state, the ²F_5/2 excited state of the Yb³⁺ ion transfers energy to the ³H₅ ground state of the Tm³⁺ ion. The electrons at the ³H₅ level of Tm³⁺ rapidly relax to the ³F₄ level by non-radiative transition. Tm³⁺ at the ³F₄ level can transfer energy with one Yb³⁺ in the excited state and be excited to ³F_2,3 levels. Tm³⁺ at the ³F_2,3 levels rapidly relaxes to the ³H₄ level or transitions to the ground state to produce upconverted red light, respectively. Tm³⁺ at the ³H₄ level can transfer energy with one Yb³⁺ in the excited state and be excited to the ¹G₄ level. Tm³⁺ at the ¹G₄ level transitions to the ground state to produce UC blue fluorescence. The resonant cross relaxation process (CR) ³F₄ (Tm³⁺) + ⁴F_9/2 (Er³⁺) → ¹G₄ (Tm³⁺) + ⁴I_15/2 (Er³⁺) between Tm³⁺ and Er³⁺ ions leads to the population of the ¹G₄ state of Tm³⁺ ions and depopulation of the ⁴F_9/2 state of Er³⁺ ions, and then to the ²H_11/2/⁴S_3/2 state through the multiphonon-assisted relaxations.

Fig. 6 Energy levels diagram of Tm³⁺, Yb³⁺, and Er³⁺ ions in the BGVO host.

Temperature sensing

Temperature is an important parameter in scientific research, healthcare, and manufacturing, and reliable temperature measurement is necessary for practical applications. First, it is important to note that BGVO was chosen as a host for the optically active lanthanide ions (Yb³⁺, Tm³⁺ and Er³⁺) primarily due to its resistance to high-temperature treatment, and the ability to dope with various lanthanide ions. This resulted in a product that was well-crystallized, nano-sized, and displayed bright and intense luminescence. Changes in temperature affect the intensity of the emission bands of Er³⁺, Tm³⁺ and Yb³⁺ ions that are observed.

Codoped BGVO:Tm³⁺/Yb³⁺ luminescent sensors. Fig. 7 displays the UC spectra as the temperature increases. The intensity of the Tm³⁺ bands gradually decrease due to the thermal heating process, except for the thermalized band around 700 nm, whose intensity increases. The Yb³⁺ band also shows a similar effect of decreasing intensity. In contrast to Tm³⁺ UC emissions, the rate of decrease is much lower. The alteration of color with varying temperature aligns with the chromaticity coordinates demonstrated in the CIE diagram (Fig. 7). These findings suggest that the BGVO:Tm³⁺/Yb³⁺ materials can also find applications as visual optical temperature sensors, leveraging their thermochromic properties. Furthermore, the temperature-sensitive and adjustable luminescence of BGVO:Tm³⁺/Yb³⁺ materials can be harnessed for safety signage purposes in high-temperature settings.

Fig. 7 (a) Upconversion spectra, and (b) chromaticity diagram of BGVO:Tm³⁺/Yb³⁺ obtained upon 975 nm excitation at different temperatures.

From the point of view of temperature sensing, we have focused on two diverse band intensity ratios, corresponding to the thermally coupled levels (TCLs) of Tm³⁺ and the non-TCLs of Yb³⁺ and Tm³⁺. Herein, the thermalization processes between the ³F_2,3 and ³H₄ excited levels of Tm³⁺ are well-established in the literature.³³ These TCLs are separated by relatively large energy (usually ΔE ≈ 1500–2000 cm⁻¹), resulting in effective thermalization in a high-temperature range and good relative sensitivity. As the temperature increases the lower energy I₁ band (800 nm) decreases, and the intensity of the higher energy I₂ band (700 nm) increases, according to the Boltzmann-type distribution.³⁴

where A is a parameter that depends on the host lattice, T is the absolute temperature, K_B is the Boltzmann constant, and ΔE is the energy gap between these two thermalized states. The temperature variation of the LIR (700/800 nm) value within the range of 300–603 K is illustrated in Fig. 8a. By employing eqn (2), we established a correlation between the calculated 700/800 nm band intensity ratio and temperature, yielding ΔE = 1527 cm⁻¹ and R₂ = 0.996, as shown in Fig. 8a.

Fig. 8 LIR variations of Tm³⁺/Yb³⁺; (a) 700/800 nm, (b) 700/650 nm, (c) 700/475 nm and (d) 700/950 nm as a function of temperature.

Meanwhile, in the case of uncoupled levels (NTCL), it is challenging to populate them through thermal excitation because of their significant energy gaps. Consequently, traditional LIR is not suitable for NTCL. Experimental LIR data can be effectively modeled using a flowing function.^35,36

the LIR_NTCL is linked to the non-thermally coupled levels (non-TCLs) of Tm³⁺, specifically referring to the intensity ratio of the 700/650 nm and 700/475 nm bands as displayed in Fig. 8b and c with (R² = 0.997 and R² = 0.987), respectively.

In contrast, the LIR parameter, linked to Yb³⁺ and Tm³⁺ NTCL, which corresponds to the 700/950 nm band intensity ratio, exhibited a correlation with temperature through the utilization of the following multi-exponential function.

where A₁, A₂, A₃, C₁, C₂, C₃ and B are fitting constants. The empirical function was used in this context due to the lack of a suitable physical model that could explain the observed temperature-dependent variations in the LIR parameter. It is worth mentioning that a similar empirical exponential function was employed by Marcin Runowski² to establish a correlation between the LIR of NTCL containing Tm³⁺ and Yb³⁺ ions at various temperatures. With the LIR derived from dual center Yb³⁺ and Tm³⁺ associated with the 700/950 nm band intensity ratio shown in Fig. 8d, we have achieved excellent fitting results with R² = 0.9906.

Sensitivity was a crucial and notable attribute when it came to quantitatively evaluating the appropriateness of materials for use as optical sensors in real-world applications. Moreover, the absolute sensitivity (S_a) and relative sensitivity (S_r) can be computed using the following formula. These sensitivities are defined by these two equations:

In all cases, regardless of whether it is TCL, NTCL, single-center, or dual-center, the absolute sensitivity increases with the rise in temperature, with a maximum of 0.38 × 10⁻³, 0.056 × 10⁻³, 0.014 × 10⁻³, and 0.048 × 10⁻³% K⁻¹ for 700/800 nm, 700/650 nm, 700/475 nm, and 700/950 nm, respectively. It's observed that in the case of TCL, the relative sensitivity decreases with an increase in temperature, reaching a maximum equal to 1.71% K⁻¹ at 348 K as it can be seen on Fig. 9a. In the cases of NTCL, Fig. 9b–d show that the relative sensitivity ratio decreases with increasing temperature. It's obvious that the S_r of the NTCL is higher than that of the TCL, and when comparing the NTCL in the single center (Tm³⁺/Tm³⁺) and dual center (Yb³⁺/Tm³⁺), we can conclude that S_r in the single center (Tm³⁺/Tm³⁺) has excellent relative sensitivity, which is equal to 5.39% K⁻¹ at 300 K (Fig. 9c).

Fig. 9 The calculated S_a and S_r values based on Tm³⁺/Yb³⁺ (a) LIR 700/800 nm, (b) LIR 700/650 nm, (c) LIR 700/475 nm and (d) LIR 700/950 nm as a function of temperature.

Temperature resolution (δT) is a crucial parameter for evaluating the effectiveness of a temperature sensor. It denotes the smallest temperature variation that the sensor is capable of detecting. It is calculated by dividing the relative uncertainty in determining the ratiometric FIR value, δFIR/FIR, by the relative thermal sensitivity S_r.

The value of δT is influenced by the material's effectiveness, which is determined by S_r, as well as the conditions of optical thermal detection and calibration, such as the signal-to-noise ratio. To optimize the resolution limit, one possibility is to employ enhanced acquisition methods and improve measurement performance. One way to achieve this is by increasing the integration time and taking the average of multiple measurements to minimize experimental noise.

Based on eqn (6), we have displayed all the computed temperature resolutions as a function of temperature in the range 300–600 K (see Fig. 10), the values obtained are below 0.5 K. This shows that both LIR methods demonstrate great accuracy over the whole temperature range. This statement offers an approximate sense of the thermal resolution that may be attained in temperature measurements with this luminous substance. It is worth noticing that the LIR (700/475 nm) (Fig. 10c), offers the best thermal resolution values (0.1 K). Based on the gathered data, it can be stated that the developed thermometer is appropriate for temperature sensing in both biological and industrial systems. The results reveal that the BGVO-based nano phosphor displays improved temperature resolution compared to the other ceramics, within the required operating temperature range.³⁷

Fig. 10 Temperature uncertainty of BGVO:Tm³⁺/Yb³⁺, (a) δT (700/800) nm, (b) δT (700/650) nm, (c) δT (700/475) nm and (d) δT (700/950) under 975 nm.

Tridoped BGVO Er³⁺/Tm³⁺/Yb³⁺ luminescent sensors. By including the Er³⁺ as an additional luminescent center to augment the overall emission bands, the FIR of multiple emission bands can be calculated using various FIR fitting techniques, the approach towards multi-rate optical thermometry can be actualized, thus facilitating the objective of self-referenced temperature measurement.

To investigate how up-conversion luminescence changes with temperature, we collected emission spectra from the BGVO:Er³⁺/Tm³⁺/Yb³⁺ nano phosphors. The up-conversion emission spectra at various temperatures are presented in Fig. 11a. It is evident that the temperature has a noticeable impact on the relative intensities of the measured up-conversion (UC) emission bands of Er³⁺ and Tm³⁺ ions. The gradual decrease in luminescence intensity with increasing temperature can be attributed to nonradiative relaxation, energy transfer, and crossover processes, all playing a role in this effect. It's important to highlight that the emission band around ∼700 nm (corresponding to the ³F_2,3 → ³H₆ transition of Tm³⁺) is initially absent (close to the noise level) at room temperature but becomes visible and intensifies as the temperature increases. The 950 nm band of Yb³⁺ exhibits a similar trend of decreasing intensity as well.

Fig. 11 (a) Temperature-dependence of the emission spectra; (b) chromaticity diagram of the BGVO:Er³⁺/Tm³⁺/Yb³⁺ sample.

The determined FIR values of the emission bands originating from TCLs of Er³⁺ ions (²H_11/2/⁴S_3/2i.e. 525/550), and Tm³⁺ ions (³F_2,3/³H₄i.e. 700/800) are well-fitted with a value of R² = 0.999 and 0.995 based on eqn (1), as depicted in Fig. 12a and b, respectively. Furthermore, the 700/475, 700/650, 700/550 and 700/950 ratios are analyzed using nonlinear fits (see Fig. 12c–f).

Fig. 12 LIR variations of Er³⁺/Tm³⁺/Yb³⁺; (a) 525/550 nm, (b) 700/800 nm, (c) 700/475 nm, (d) 700/650 nm, (e) 700/550 nm and (f) 700/950 nm as a function of temperature.

In all analyzed cases, regardless of the energy level type (TCL, NTCL, single-center, or dual-center), the absolute sensitivity increased with rising temperature. The maximum S_a observed was: 11 × 10⁻³ K⁻¹ (525/550 nm), 0.22 × 10⁻³ K⁻¹ (700/800 nm), 10.0 × 10⁻³ K⁻¹ (700/475 nm), 0.8 × 10⁻³ K⁻¹ (700/650 nm), 0.6 × 10⁻³ K⁻¹ (700/550 nm), and 17.0 × 10⁻³ K⁻¹ (700/950 nm) shown in Fig. 13a–f. Notably, the S_a values were significantly higher for Er³⁺/Tm³⁺/Yb³⁺ tri-doped BGVO nano phosphors compared to Tm³⁺/Yb³⁺ co-doped samples. This observation highlights the role of Er³⁺ doping in enhancing this crucial parameter.

Fig. 13 The calculated S_a and S_r values based on Er³⁺/Tm³⁺/Yb³⁺: (a) 700/800 nm, (b) 700/650, (c) 700/475, (d) 700/550 nm, (e) 525/550 nm and (f) 700/950 nm as a function of temperature.

The variation of the S_r parameter with temperature exhibits similar behavior for both Tm³⁺/Yb³⁺ and Er³⁺/Tm³⁺/Yb³⁺ co-doped samples in the case of TCLs (see Fig. 13a and b). However, for non-thermally coupled energy levels (NTCL), a significant difference is observed. Unlike Tm³⁺/Yb³⁺ co-doped samples, Er³⁺/Tm³⁺/Yb³⁺ co-doped samples exhibit a well-defined maximum S_r value (S_rmax) at specific temperatures. This behavior holds promise for the design of high-sensitivity sensors within a well-defined temperature range.

The LIR values of non-TCLs for single centers (700/650), (700/475), and dual centers (Tm³⁺/Er³⁺) (700/550) and (Tm³⁺/Yb³⁺) (700/950) at temperatures ranging from 300 K to 600 K are displayed in Fig. 13c–f. Therefore, eqn (3) can be successfully utilized to fit the data for (700/650), (700/475), and (700/550), while eqn (4) is suitable for fitting (700/950). The resulting variations of S_a and S_r with temperature are depicted in Fig. 13c–e.

Remarkably, the utilization of NTCL resulted in a significantly higher S_rmax value of 2.35% K⁻¹ at 300 K. This achievement was made possible by employing the combination of upconversion (UC) emission bands at 700/950 nm, originating from the transitions of Tm³⁺/Yb³⁺ in the tridoped BGVO:Er³⁺/Tm³⁺/Yb³⁺ nano phosphor.

The maximum S_r values (S_rmax) for the Er³⁺/Tm³⁺/Yb³⁺ co-doped samples were determined to be: 1.3, 1.12, 1.67, 1.45, 1.91 and 2.35 K⁻¹ for emission wavelengths of (525/550 nm), (700/800 nm), (700/475), (700/650), (700/550), and (700/950), respectively (shown in Fig. 13a–f). These values are slightly lower than those observed in the Tm³⁺/Yb³⁺ co-doped samples, but with the advantage of a well-defined S_rmax at a specific temperature.

Fig. 14a–f illustrates the temperature uncertainties (δT) for all the samples. Notably, the addition of Er³⁺ doping to BGVO:Tm³⁺/Yb³⁺ significantly improves the temperature resolution. In particular, for the 700/550 and 700/445 ratios, δT reaches an impressive value of 0.02 K. This observation further emphasizes the crucial role of Er³⁺ doping in enhancing the performance of these optical temperature sensors.

Fig. 14 Temperature uncertainty of BGVO:Er³⁺/Tm³⁺/Yb³⁺: (a) δT (525/550) nm, (b) δT (700/800) nm, (c) δT (700/475) nm, (d) δT (700/650), (e) δT (700/550) nm, and (f) δT (700/950) under 975 nm excitation.

Table 1 shows that compared to other lanthanide-based optical thermometers, the Ba₂GdV₃O₁₁:Er³⁺/Tm³⁺/Yb³⁺ and Ba₂GdV₃O₁₁:Tm³⁺/Yb³⁺ sensors have higher S_r values, in a wide temperature sensing range (300–600 K). This makes them perfect for a wide range of luminescence thermometry uses.

Table 1 Sensitivities and temperature range for the temperature-sensing luminescence material

Materials	T-range (K)	S r (% K^−1) at 300 K	S r (% K^−1) at Tmax	Ref.
NaBiF4:Tm^3+/Yb^3+	303–443	0.7	1.1	38
Y2Ti2O7:Tm^3+/Yb^3+	300–400	0.4	0.87	39
GdVO4:Tm^3+/Yb^3+	297–673	0.4	1.54	40
NaY2F7:Tm^3+/Yb^3+	300–567	0.5	1.2	41
Na3GdV2O8Tm^3+/Yb^3+	298–565	1.34	2.01	42
Ba2GdV3O11:Er^3+/Yb^3+	298–573	1.1	1.1	43
Gd2O3:Yb^3+/Er^3+	300–900	0.83	0.20	44
NaSrY(MoO4)3:Er^3+/Yb^3+	298–335	1.19	1.19	45
YPVO4:2%Er^3+/15%Yb^3+	300–440	0.937	0.937	46
Ba2YV3O11:Er^3+/Yb^3+/Bi^3+	300–440	1.43	1.43	37
Er^3+/Yb^3+Na3Gd(VO4)2	300–600	0.83	0.83	47
Ba2GdV3O11:Tm^3+/Yb^3+	303–603	5.397	5.397 (700/475)	This work
Ba2GdV3O11:Er^3+/Tm^3+/Yb^3+	303–603	0.2	2.35 (700/950)	This work

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Our findings underscore the potential of erbium doping to modulate temperature sensitivity peaks, and improve the temperature resolution, crucial for optimizing performance in tailored luminescence sensors. This research highlights the profound influence of erbium (Er³⁺) doping on the temperature sensitivity characteristics of thermometric systems, paving the way for improved precision in temperature measurements. This adjustment not only sheds light on the underlying mechanisms of tunability, but also unlocks novel opportunities for customizing thermometric devices to meet specific application needs.

The repeatability (R) is another crucial metric for assessing the accuracy of the temperature sensing techniques employed. The thermometric parameters, specifically the LIR values, were measured multiple times as the sample was cycled from a low to a high temperature, as illustrated in Fig. S3(a–d) and S4(a–f) (ESI†). The repeatability (R) was calculated using the following formula:

where Mi(T)_c is the measured parameter (LIR or band centroid) in the ith cycle and M(T)_c is the mean value of the M(T)_c over 10 cycles. The LIR values determined change reversibly with temperature, and the LIR values for all LIRs were above 97% over the 300–600 K temperature range, confirming the good repeatability and reliability of the thermometric methods applied.

Conclusion

In this study, the influence of erbium (Er³⁺) doping on the temperature sensitivity profile of upconversion luminescent nanoparticles based on Ba²GdV³O¹¹ co-doped with Tm³⁺ and Yb³⁺ was investigated. Both BGVO:Yb³⁺/Tm³⁺ and BGVO:Er³⁺/Tm³⁺/Yb³⁺ nanoparticles exhibited strong blue and green upconversion luminescence, respectively, under 975 nm laser excitation. The fluorescence intensity ratio (FIR) method was employed to analyze the temperature-dependent luminescence spectra across a broad temperature range (300–600 K). Our analysis was focused on both thermally coupled energy levels (TCLs) and non-thermally coupled energy levels (NTCLs) of Er³⁺ and Tm³⁺ for their impact on temperature sensing performance. The key finding of this work is the ability to modulate the temperature of maximum sensitivity through Er doping. In Tm³⁺/Yb³⁺ co-doped samples, the peak sensitivity (1.7% K⁻¹) occurred around 350 K primarily due to TCLs. Interestingly, non-TCL sensitivities lacked a clear maximum within the 300–600 K range. In contrast, Er³⁺/Tm³⁺/Yb³⁺ tri-doped samples displayed a distinct maximum in non-TCL sensitivity within this temperature range, offering superior temperature determination for specific applications. Notably, Er³⁺ doping also improved the temperature resolution from 0.1 K to 0.02 K. Remarkably, the utilization of non-TCLs resulted in a significantly higher S_rmax value of 2.35% K⁻¹ at 300 K. This achievement was made possible by employing the combination of upconversion (UC) emission bands at 700/950 nm, originating from the transitions of Tm³⁺/Yb³⁺ in the tridoped BGVO Er³⁺/Tm³⁺/Yb³⁺ nano phosphor. Thus, erbium doping has emerged as a powerful tool for fine-tuning temperature sensitivity in luminescent nanosensors. This precise control over sensitivity translates to enhanced temperature resolution, a critical factor for optimizing sensor performance. By revealing the underlying mechanism, our research unlocks the potential for designing customized thermometric devices with superior accuracy.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Ministry of Higher Education and Scientific Research Tunisia, the Ministerio de Ciencia e Innovación of Spain (MICIIN) under the National Program of Sciences and Technological Materials (PID2019-106383GB-C44 and PID2019-107335RA-I00) and Gobierno de Canarias (ProID2020010067) and EU-FEDER funds.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00699b

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