Ratiometric Optical Thermometry Based on Emission and Excitation

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C: Physical Processes in Nanomaterials and Nanostructures

Ratiometric Optical Thermometry Based on Emission and Excitation Spectra of YVO:Eu Nanophosphors 4

3+

Ilya E. Kolesnikov, Alexey A. Kalinichev, Mikhail A. Kurochkin, Daria V. Mamonova, Evgeniy Yu. Kolesnikov, and Erkki Lahderanta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00284 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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The Journal of Physical Chemistry

Ratiometric

Optical

Thermometry

Based

on

Emission and Excitation Spectra of YVO4:Eu3+ Nanophosphors I.E. Kolesnikov*,1,2, A.A. Kalinichev1, M.A. Kurochkin1, D.V. Mamonova1, E.Yu. Kolesnikov3, E. Lähderanta2 1 St.

Petersburg State University, St. Petersburg, Russia

2 Lappeenranta 3

University of Technology LUT, Lappeenranta, Finland

Volga State University of Technology, Yoshkar-Ola, Russia

Corresponding Author *E-mail: [email protected] ABSTRACT Development of new approaches to the non-contact optical thermometry is of great importance for modern science and technology. In the current work, single phase YVO4:Eu3+ nanoparticles prepared via modified Pechini technique were studied as luminescence thermometers. Thermal sensing was performed using two different ratiometric approaches: utilizing the luminescence intensity ratio between transitions emitted from two thermally coupled excited levels (emission spectrum) and between transitions originating from different thermally coupled low lying levels (excitation spectrum). First technique allows determining temperature within 298–873 K range, whereas the second one within 298–473 K. The spectral position of 5D0(1)–7F1(2) band was also suggested as temperature dependent parameter. Thermometric performance of YVO4:Eu3+ nanophosphors including absolute and relative sensitivity, minimum temperature uncertainty and repeatability was obtained and discussed. INTRODUCTION Nanomaterials have become a unique tool which found applications in different areas of science, from cancer treatment 1 to surface enhanced Raman scattering (SERS) 2. The size, morphology, and physicochemical properties of nanoparticles strongly depend on the synthesis conditions, for example, even 10 oC difference during the synthesis of metal nanoparticles can lead to completely different results 3. To reach desirable properties, it is necessary to control the

1

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experimental conditions and their variations on micro- and nanolevel. Special attention should be paid to the temperature control, because it is one of the most important parameter 4. Nowadays appear a lot of new methods and instruments of maintaining and detecting the synthesis parameters on nanoscale

5,6.

One of such methods is temperature control by non-

contact optical thermometry. Among plenty materials

7–11,

rare earth doped metal oxides are

often used as optical thermometers in recent years. Metal oxides are characterized by high melting points and by the ability to absorb UV radiation in the long and medium wavelength ranges. They do not dissolve in water, organic acids, weak inorganic acids, which allows them to be used in aggressive environments. Doping the oxide crystal lattice with a fluorescent ion allows one to obtain a contactless optical response to heating or cooling

12–16.

Unlike other non-

contact thermometric methods, luminescence thermal sensing demonstrates some advantages, such as continuous real-time probing of temperature, high spatial and temperature resolution, easy implementation and tolerance to electromagnetically or thermally harsh environments

17,18.

Nanoparticles of such crystal structures can have a reversible temperature response within the wide temperature range. Many optical thermometers are based on ratiometric approach, when temperature is defined from luminescence intensity ratio (LIR) between two thermally coupled levels

19–22.

There are two

mechanisms of LIR thermometry: the first mechanism is based on the thermoequilibrium between excited energy levels, whereas the second one on the thermoequilibrium between two ground energy levels. Regarding the first mechanism, 5D1 and 5D0 excited energy levels of Eu3+ ions have the largest energy gap among all trivalent rare earth ions, which can be used for LIR thermometry. Consequently, in this case intensity ratio thermometry reaches the highest relative sensitivity and the largest high-temperature measurement bound 23. The second mechanism was realized utilizing 7F0, 7F1 and 7F2 ground levels of Eu3+ ions. To the best of our knowledge, we proposed thermal sensing based on LIR calculated from excitation spectrum for the first time. In this work, we considered both ratiometric mechanisms of optical thermometry involving the excited and low-lying energy levels. Moreover, temperature sensing was provided using spectral position of 5D0(1)–7F1(2) band. Performance of YVO4:Eu3+ nanothermometers utilizing different temperature dependent parameters were obtained and compared. Doping concentration effect on thermal sensing properties was also investigated. The obtained results make YVO4:Eu3+ nanoparticles prospective tool for contactless temperature sensing using all regarded mechanisms. EXPERIMENTAL

2


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Eu3+-doped YVO4 nanocrystalline powder was synthesized using modified Pechini technique. This method has already been used for synthesis of various oxides systems activated by rare earths ions and has been studied in detail in our previous works

24–26.

The modified Pechini

technique has some advantages over standard Pechini method, particularly it enables to reduce particle agglomeration during sintering processes because of using molten potassium chloride. All used reagents (Y2O3, Eu2O3, V2O5, HNO3, C6H8O7, C2H4(OH)2 and KCl) were chemically pure. To form yttrium and europium nitrates solution, corresponding oxides (Y2O3, Eu2O3) were dissolved in concentrated nitric acid (HNO3) while heating. Then prepared saturated citric acid solution was added to mentioned nitrates mixture (with volume ratio 1:1). Further, the solution containing vanadium ions (VO(C6H7O7)2) was produced by dissolution vanadium (V) oxide (V2O5) in citric acid. [Y1-xEux(C6H8O7)3](NO3)3 solution was mixed with VO(C6H7O7)2 while heating. After that, ethylene glycol was added to it (with the ethylene glycol to citric acid solution volume ratio of 1:4). As a result of the etherification reaction between ethylene glycol and metals citrate complex, polymer citrate gel was formed. Then this gel was calcined at 500 °C for 1 hour to burn off the organic components. The obtained powder was grinded in mortar and the potassium chloride was added to it in weight ratio 1:1 and milled together in mortar. Then resulting powder mixture was calcined for the second time at 950 °C for 1.5 hours. The used calcination temperatures, durations and Eu3+ doping concentration were optimal and have been found in our earlier works 27–29. To remove potassium chloride, the powder was washed off with distilled water. The synthesized particles were collected by centrifugation at 2500 rpm for 5 min. This procedure was repeated three times. Then the resulting samples were dried naturally. Finally, YVO4:Eu3+ 4 at.%, 10 at.% and 16 at.% nanophosphors were prepared according to described procedure. X-ray diffraction patterns were registered with Rigaku «Miniflex II» diffractometer with CuKα-radiation (λ = 1.5406 Å) in the 2θ range from 15o to 60o. Phase identification was carried out using a powder diffraction database PowderDiffractionFile (PDF-2, 2011). Scanning electron micrograph (SEM) images were made using SUPRA 40VP WDS scanning electron microscope. Emission and excitation spectra were measured with modular fluorescence spectrometer Fluorolog-3. Temperature experiments dealing with emission spectra were carried out using setup that includes solid-state laser Coherent Matrix 355-1-60 (355 nm, 20 ns), double monochromator MDR6, PMT Hamamatsu H7844 and heating stage Linkam TS1000 with 0.1 °C temperature stability and 0.1 °C set point resolution. Temperature experiments dealing with excitation spectra were performed on modular fluorescence spectrometer Fluorolog-3 equipped with optical fibers and heating stage controlled with ThorLabs TC200 with a resolution of 0.1 °C. 3



Page 4 of 26

RESULTS AND DISCUSSION Figure 1a presents XRD patterns of YVO4:Eu3+ 4, 10 and 16 at.% nanoparticles synthesized via modified Pechini method. All the peaks in diffraction patterns are well matched to the YVO4 tetragonal phase (JCPDS 17-0341) and no impurity lines were observed. One can see that diffraction lines position shifts towards a lower angle along with doping concentration increase. Such behavior is typical for substitution of smaller ion (Y3+, r=0.89 Å) with larger one (Eu3+, r=0.95 Å)

30.

Scanning electron microscope image of the synthesized YVO4:Eu3+ 16 at.%

phosphor is shown in Figure 1b. As can be seen, the powder consists of weakly agglomerated nanoparticles with average size about 80–90 nm. Photoluminescence spectra of YVO4:Eu3+ nanophosphors doped with different amount of Eu3+ ions, which were obtained upon 355 nm, are shown in Figure 1c. The observed spectra consist of narrow lines corresponding to the electron transitions inside 4f shell of Eu3+ ions. Almost all bands are originated from metastable 5D0 level. Only low intensity lines centered at 538 and 609 nm are assigned to the transitions from higher excited levels: 5D1–7F1 and 5D2–7F6, respectively. As Eu3+ ions situate at lattice site without inversion symmetry in YVO4 host (D2d symmetry), the forced electric dipole transitions 5D0–7F2,4 are stronger than the magnetic dipole one 5D0–7F1 31. It should be noted that increase of doping concentration (4, 10, 16 at.%) leads to the monotonic growth of emission intensity.

Figure 1. a) XRD pattern of YVO4:Eu3+ nanoparticles with different concentrations; b) SEM image of YVO4:Eu3+ 16 at.% nanophosphors; c) emission spectra of YVO4:Eu3+ nanoparticles with different concentrations (λex = 355 nm). Figure 2a presents the temperature effect on the emission spectra of YVO4:Eu3+ 16 at.% nanoparticles. The temperature was varied within wide range of 298–873 K. As can be seen, temperature affects emission line intensity differently. Special attention should be paid to the transitions from thermally coupled 5D0 and 5D1 excited levels (Figure 2b). Luminescence intensity ratio between 5D1–7F1 and 5D0–7F1 emission lines can serve as a temperature dependent parameter. The excited states are populated according to the Boltzmann distribution, and since 4


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the emission intensities are proportional to the population of each energy level, LIR of two thermally coupled levels can be descried by following equation 32,33: 𝐼5𝐷1

(

)

∆𝐸

𝐿𝐼𝑅 = 𝐼5𝐷0 = 𝐵 exp ― 𝑘𝑇

(1)

where B=A5D1ν5D1g5D1/A5D0ν5D0g5D0, νi and gi are the frequency and the degeneracies of the levels, Ai are spontaneous emission rates, ΔE is the energy gap between the two thermally coupled levels, and k=0.695 cm-1 K-1 is the Boltzmann constant. B is temperature independent constant, defined purely by the material properties. Figure 2c shows evolution of LIR355 as a function of temperature for YVO4:Eu3+ 16 at.%. The spectral limits of the integrated emission are 534–542 nm and 591–597 nm for 5D1–7F1 and 5D0– 7F

1

transitions, respectively. The experimental data is accurately fitted by the exponential

Boltzmann formula with adj. R2 better than 0.99. An effective gap ΔEeff obtained from Eq. 1 was determined to be 1665 cm-1, whereas real energy gap ΔEreal obtained from emission spectrum is 1749 cm-1. Due to the close values of effective and real energy gap between 5D1 and 5D0 levels, we can make a conclusion that thermal behavior of LIR355 is governed by Boltzmann’s redistribution only. Emission spectra measured at different temperatures together with evolution of LIR355 as a function of temperature for YVO4:Eu3+ 4 at.% and 10 at.% nanophosphors are presented in Figure S1 and S2. The theoretical fit of LIR355 was performed using Boltzmann formula in both cases. It was found that effective gap ΔEeff was slightly less than the real one. Quantitatively the performance of thermometer can be described with absolute and relative thermal sensitivity. Absolute thermal sensitivity shows the absolute LIR change with temperature variation and is defined as follows: 𝑆𝑎 =

𝑑𝐿𝐼𝑅 𝑑𝑇

∆𝐸

= 𝐿𝐼𝑅𝑘𝑇2

(2)

One can see that Sa depends on absolute LIR value, which can be simply changed by manipulating LIR calculation procedure (for instance, broadening of spectral boundaries for integral intensity calculation). To provide fair comparison of thermometers performance irrespective to their nature and sensing parameter, the relative thermal sensitivity is introduced. Relative sensitivity demonstrates normalized change of LIR with temperature variation: 1 𝑑𝐿𝐼𝑅

𝑆𝑟 = 𝐿𝐼𝑅

𝑑𝑇

∆𝐸

∙ 100% = 𝑘𝑇2 ∙ 100%

(3)

The variation of the Sa and Sr value with temperature from 298 to 873 K for YVO4:Eu3+ 16 at.% nanoparticles is presented in the Figure 2d. As can be seen, the temperature dependences of absolute and relative sensitivities demonstrate opposite behavior: Sa increases along with temperature growth, whereas Sr gradually decline. The maximal absolute and relative thermal 5



Page 6 of 26

sensitivities were found to be 0.00328 K-1@823 K and 2.70 % K-1@298 K, respectively. Eu3+ doping concentration effect on calculated Sa and Sr values is summarized in Table 1. Along with sensitivity an important parameter, which is used for characterization of any thermometer, is the minimum temperature uncertainty (ΔT)

34.

This parameter defines how

accurate can be thermal sensing using particular thermometer. It was reported earlier that value of minimum temperature uncertainty can be obtained using different methods 1𝛿𝐿𝐼𝑅 𝛿𝐿𝐼𝑅

paper ΔT was determined from calibration curve (𝛥𝑇𝑐𝑎𝑙𝑖𝑏𝑟 = 𝑆 𝐿𝐼𝑅 ,

𝐿𝐼𝑅

35.

In the present

is the relative uncertainty

in the determination of luminescence intensity ratio obtained from three measurements) and also from acquisition of several consecutive emission spectra at a fixed temperature. Both temperature uncertainties were obtained at T = 323 K. The calculated values are listed in Table 1. Noteworthy, all calculated parameters show non-monotonic behavior with doping concentration. The best thermometric performance was demonstrated by YVO4:Eu3+ 16 at.% nanophosphor.

Figure 2. a) Emission spectra of YVO4:Eu3+ 16 at.% NPs obtained at different temperatures (λex = 355 nm); b) thermal sensing scheme based on emission spectra of Eu3+, c) temperature evolution of luminescence intensity ratio LIR355; d) absolute and relative thermal sensitivity of YVO4:Eu3+ 16 at.% NPs (LIR355).

6


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Table 1. Maximal absolute and relative thermal sensitivity and minimum temperature uncertainty obtained with different methods for YVO4:Eu3+ nanothermometers based on LIR355. Material

Sa (max), K-1

Sr, % K-1

ΔTcalbr, K

ΔTdistr, K

YVO4:Eu3+ 4 at.%

0.00262@873 K

2.85

3

3

YVO4:Eu3+ 10 at.%

0.00244@773 K

2.34

2

2

YVO4:Eu3+ 16 at.%

0.00328@823 K

2.70

1

2

Excitation spectra of YVO4:Eu3+ nanophosphors doped with different amount of Eu3+ ions, which were obtained by monitoring forced electric dipole transition 5D0–7F4 (698 nm), are shown in Figure 3. The observed spectra consist of narrow lines corresponding to the electron transitions inside 4f shell of Eu3+ ions. As can be seen, the spectra consist of broad band and several narrow lines in the longer-wavelength region. The observed broad band centered at about 300 nm can be attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO43- ion and to the charge transfer transition between Eu3+ and O2-

36,37.

The

narrow lines in excitation spectra corresponded to the typical transitions inside Eu3+ ion: 7F0–5D4 (362 nm), 7F0–5L7 (382 nm), 7F0–5L6 (394 nm), 7F0–5D3 (416 nm), 7F0–5D2 (465 nm), 7F1–5D2 (474 nm), 7F2–5D2 (490 nm), 7F0–5D1 (526 nm), 7F1–5D1 (537 nm), and 7F1–5D0 (593 nm).

Figure 3. Excitation spectra of YVO4:Eu3+ nanoparticles with different concentrations (λem = 698 nm). Taking into account that transitions from several low lying levels (7F0, 7F1, and 7F2) were observed in excitation spectra and that the energy gap between 7F0 and 7F2 levels is less than 2000 cm-1, we suggested to use these levels for thermal sensing. Noteworthy, to the best of our knowledge, it is the first demonstration of thermal sensing based on excitation spectra of luminescence nanoparticles. 7



Page 8 of 26

Figure 4a presents part of excitation spectra of YVO4:Eu3+ 16 at.% nanoparticles of the most prominent transition 5D0–7F2 (618 nm). These spectra were measured at different temperatures from the range of 299–466 K. The luminescence intensity of 7F0–5D2 (465 nm) and 7F2–5D2 (490 nm) transitions demonstrates opposite dependence on temperature. Growth of temperature results in monotonic decline of 7F0–5D2 transition, whereas 7F2–5D2 intensity increases. LIR between 7F2–5D2 and 7F0–5D2 transitions obtained from excitation spectrum was tested as temperature dependent parameter (Figure 4b). To obtain transition intensity, the excitation spectrum was integrated within spectral range of 2 nm around central wavelength. The integration limits were chosen in accordance with spectral slit width during measurement. Calibration curve of LIR618 for YVO4:Eu3+ 16 at.% nanophosphor is presented in Figure 4c. As in case of LIR355, the experimental data was accurately approximated by the exponential Boltzmann formula, Eq. (1), with adj. R2 better than 0.99. An effective gap ΔEeff obtained from Eq. (1) was found to be 740 cm-1, which is much smaller than the 1097 cm-1 real energy gap ΔEreal between 7F0 and 7F2 obtained from emission spectrum. The error δ is a key parameter which shows how much the effective energy gap ΔEeff differs from the real energy gap ΔEreal and it is defined as 𝛿 = |∆𝐸𝑟𝑒𝑎𝑙 ― ∆𝐸𝑒𝑓𝑓|/∆𝐸𝑟𝑒𝑎𝑙. The δ value of 33% was obtained in case of YVO4:Eu3+ 16 at.% nanoparticles. It was found that the calculated value is originated from the variation of the population of 7F2 level, because the positions of the Eu3+ energy levels changed with temperature. Some other factors such as absorption efficiency and luminescence quantum efficiency may have also changed with temperature 23. Excitation spectra measured at different temperatures together with evolution of LIR618 as a function of temperature for YVO4:Eu3+ 4 at.% and 10 at.% nanophosphors are presented in Figure S3 and S4. The theoretical fit of LIR618 was performed using Boltzmann formula, Eq. (1), in both cases. The variation of the Sa and Sr value with temperature from 298 to 473 K for YVO4:Eu3+ 16 at.% nanoparticles is presented in the Figure 4d. The thermometric performance including maximal absolute and relative thermal sensitivity and minimum temperature uncertainty of YVO4:Eu3+ nanoparticles using LIR618 as temperature dependent parameter is summarized in Table 2. As can be seen, Sa and Sr values are less compared with values for LIR355. The minimum temperature uncertainties which can be obtained using LIR618 are comparable with thermal resolution for LIR355.

8


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Figure 4. a) Excitation spectra of YVO4:Eu3+ 16 at.% NPs obtained at different temperatures (λem = 618 nm); b) thermal sensing scheme based on excitation spectra of 5D0–7F2 transition, c) temperature evolution of luminescence intensity ratio LIR618; d) absolute and relative thermal sensitivity of YVO4:Eu3+ 16 at.% NPs (LIR618). Table 2. Maximal absolute and relative thermal sensitivity and minimum temperature uncertainty obtained with different methods for YVO4:Eu3+ nanothermometers based on LIR618. Material

Sa (max), K-1

Sr, % K-1

ΔTcalbr, K

ΔTdistr, K

YVO4:Eu3+ 4 at.%

0.00033@466 K

1.23

3

3

YVO4:Eu3+ 10 at.%

0.00039@466 K

1.08

3

3

YVO4:Eu3+ 16 at.%

0.00038@441 K

1.19

2

2

Excitation spectra of YVO4:Eu3+ 16 at.% nanoparticles monitored at forced electric dipole transition 5D0–7F4 (698 nm) measured at different temperatures are shown in Figure 5a. The studied temperature range was the same as in case of 5D0–7F2 (618 nm) excitation spectra. Unlike previously discussed excitation spectra of 5D0–7F2 transition, these spectra consist of many bands, which can be used for thermal sensing based on electron population redistribution of low lying levels. As 7F0, 7F1, and 7F2 levels are situated within 2000 cm-1, the following luminescence lines were utilized to calculate LIR: 7F0–5D1 (526 nm), 7F1–5D1 (537 nm), 7F2–5D1 (555 nm), 7F1– 9



5D

0

Page 10 of 26

(593 nm), and 7F2–5D0 (613 nm) (Figure 5b). LIRs based on thermal electron population

redistribution between 7F0 and 7F1, 7F0 and 7F2, 7F1 and 7F2 levels were compared. Moreover, effect of excited level on LIR sensing properties was also studied. We calculated following 698 7 7 5 7 5 5 7 luminescence intensity ratios: 7F1–5D1/7F0–5D1 (𝐿𝐼𝑅698 1 ), F1– D0/ F0– D1 (𝐿𝐼𝑅2 ), F2– D1/ F0– 5D

1

698 7 698 698 7 5 7 5 5 7 5 7 5 7 5 (𝐿𝐼𝑅698 3 ), F2– D0/ F0– D1 (𝐿𝐼𝑅4 ), F2– D1/ F1– D1 (𝐿𝐼𝑅5 ), and F2– D0/ F1– D1 (𝐿𝐼𝑅6 ).

Integral intensity of each band was collected within spectral range equal to spectral slit width during measurement (3 nm). Evolution of the obtained LIRs as a function of temperature is presented in Supplementary Information (Figure S5). In all cases experimental data were fitted by the exponential function, Eq. (1), confirming that LIR thermal dependence is governed by Boltzmann process. All suggested LIRs are suitable to provide thermal sensing. However, comparing the effective gap ΔEeff obtained from fitting procedure, we can conclude that 𝐿𝐼𝑅698 4 would have the best thermometric performances among all calculated 𝐿𝐼𝑅𝑠698. It was found that ΔEeff value almost does not depend on excited level, to which electron transfers. Slight difference of ΔEeff (for example, for 𝐿𝐼𝑅698 and 𝐿𝐼𝑅698 3 4 ) is most probably explained with different starting Stark sublevels of transitions. Calibration curve of the most promising 𝑅698 for YVO4:Eu3+ 16 at.% nanophosphor is shown in 4 Figure 5c. As in case of LIR618, the effective gap ΔEeff was found to be less than the real one ΔEreal. The calculated δ value is 21%. The variation of the Sa and Sr value with temperature from 298 to 473 K for YVO4:Eu3+ 16 at.% nanoparticles is presented in the Figure 5d. Excitation spectra measured at different temperatures and calibration curves of 𝐿𝐼𝑅698 for 4 YVO4:Eu3+ 4 at.% and 10 at.% nanoparticles are presented in Fig . S6 and Figure S7. The thermometric performance including maximal absolute and relative thermal sensitivity and minimum temperature uncertainty of YVO4:Eu3+ nanoparticles using 𝐿𝐼𝑅698 as temperature 4 dependent parameter is summarized in Table 3. YVO4:Eu3+ 10 at.% nanophosphor has the highest Sa and Sr values, whereas its temperature uncertainty is the worst.

10


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Figure 5. a) Excitation spectra of YVO4:Eu3+ 16 at.% NPs obtained at different temperatures (λem = 698 nm); b) thermal sensing scheme based on excitation spectra of 5D0–7F4 transition, c) temperature evolution of luminescence intensity ratio 𝐿𝐼𝑅698 4 ; d) absolute and relative thermal sensitivity of YVO4:Eu3+ 16 at.% NPs (𝐿𝐼𝑅698 4 ). Table 3. Maximal absolute and relative thermal sensitivity and minimum temperature uncertainty obtained with different methods for YVO4:Eu3+ nanothermometers based on 𝐿𝐼𝑅698 4 . Material

Sa (max), K-1

Sr, % K-1

ΔTcalbr, K

ΔTdistr, K

YVO4:Eu3+ 4 at.%

0.00473@466 K

1.25

1

2

YVO4:Eu3+ 10 at.%

0.00581@466 K

1.28

2

2

YVO4:Eu3+ 16 at.%

0.00475@466 K

1.25

1

2

Additional important parameter assessing precision of a thermometric system is repeatability. Repeatability refers to the variation in repeat measurements made under identical conditions 38. Figure 6 presents a thermal cycling experiment with YVO4:Eu3+ 16 at.% nanophosphor, where the temperature was determined in consecutive complex heating-cooling cycles. The temperature was defined by two independent methods: luminescence nanothermometry and thermocouple measurements. Determination of temperature using luminescence nanothermometry was 11



Page 12 of 26

performed using intensity ratios obtained from excitation spectra: LIR618 and 𝐿𝐼𝑅698 4 . As can be seen from Figure 6, measured and calculated temperatures are in good agreement taking into account thermal uncertainties.

Figure 6. Complex heating-cooling cycles of YVO4:Eu3+ 16 at.% NPs. Temperature was determined using a) LIR618 and b) 𝐿𝐼𝑅698 4 . During study of emission and excitation spectra of YVO4:Eu3+ nanoparticles at different temperatures, it was found that variation of temperature affects also spectral positions of the observed bands, not only the luminescence intensity. So, spectral position of line could be used as a temperature dependent parameter. Some luminescence bands demonstrate more pronounced thermally induced spectral shift. As a proof of concept, we monitored 5D0–7F1 transition, which consists of two Stark splittings. We carried out deconvolution procedure to obtain spectral position of the most intense line. Normalized emission spectra around 5D0–7F1 transition of YVO4:Eu3+ 16 at.% nanophosphor measured at different temperatures are shown in Figure 7a. The temperature was varied within range of 298–723 K. As can be seen, two Stark lines can be distinguished up to 573 K, however further temperature increase leads to their merge due to the thermally induced broadening. The temperature evolution of the spectral position of 5D0(1)– 7F

1(2)

line is presented in Figure 7b. The experimental data were well-fitted with linear function.

The relative thermal sensitivity for line shift was calculated according to earlier reported definition 39 (Figure 7c). Noteworthy, obtained Sr value (0.34 % K-1 at T = 298 K) is comparable with sensitivity of Nd3+-doped nanothermometers based on line shift

39,40.

In this case the

absolute thermal sensitivity did not depend on temperature and it was found to be 0.03 K-1.

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Figure 7. a) Normalized emission spectra of YVO4:Eu3+ 16 at.% NPs obtained at different temperatures (λex = 355 nm); b) temperature evolution of 5D0(1)–7F1(2) spectral line position; c) relative thermal sensitivity of spectral line shift for YVO4:Eu3+ 16 at.% NPs. CONCLUSIONS Single phase Eu3+-doped YVO4 nanocrystalline powders with average particle size of 80-90 nm were synthesized using modified Pechini technique. Emission and excitation spectra revealed narrow characteristic lines corresponding to the intra-configurational transitions inside 4f shell of Eu3+ ions. Thermal sensing using YVO4:Eu3+ nanoparticles was performed based on thermally coupled excited (5D1 and 5D0) and low lying (7F0, 7F1, and 7F2) energy levels. We applied ratiometric approach to obtain temperature using emission and excitation spectra. Thermometric performance of YVO4:Eu3+ nanophosphors based on different LIRs in terms of absolute and relative sensitivity and minimum temperature uncertainty was studied and compared. Doping concentration effect on thermal sensing properties was also investigated. The highest relative thermal sensitivity of 2.85 % K-1 was determined for LIR355, whereas the best thermal resolution of 1 K was found for LIR355 and LIR698. Thermal cycling experiments showed good repeatability of YVO4:Eu3+ nanothermometer. The spectral position of 5D0(1)–7F1(2) line was successfully used for thermal sensing. This parameter could provide accurate temperature determination within range of 298–573 K with thermal sensitivities Sr = 0.34 % K-1@298 K and Sa = 0.03 K-1. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: Emission spectra of YVO4:Eu3+ 4 at.% and 10 at.% NPs at different temperatures (λex = 355 nm), temperature evolution of LIR355, excitation spectra of YVO4:Eu3+ 4 at.% and 10 at.% NPs at different temperatures (λem = 618 nm), temperature evolution of luminescence intensity ratio LIR618, temperature evolution of calculated luminescence intensity ratios 𝐿𝐼𝑅698 1 ― 6, excitation

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spectra of YVO4:Eu3+ 4 at.% and 10 at.% NPs at different temperatures (λem = 698 nm), temperature evolution of luminescence intensity ratio 𝐿𝐼𝑅698 4 . ACKNOWLEDGMENTS This research has been supported by the Russian Science Foundation (№ 17-72-10055). Experimental investigations were carried out in «Center for Optical and Laser materials research», «Research Centre for X-ray Diffraction Studies», «Innovative Technologies of Composite Nanomaterials» (St. Petersburg State University). REFERENCES (1)

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Ratiometric Optical Thermometry Based on Emission and Excitation

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