Introduction

In last decades rare earth materials have been widely used as high-performance luminescent devices, catalysts, and other functional materials based on the electronic, optical, and chemical characteristics arising from their 4f electrons1,2,3,4,5. The unique 4f electronic configuration of rare earth elements makes them perfect phosphors emitting light ranging from UV to NIR. Hence, rare earth doped materials are potential candidates for design of multicolor LEDs6. Among rare earth ions, the dysprosium has attracted much attention due to its white light emission7,8. Dy3+ ions emit several luminescence bands in blue, green, yellow and red ranges of the spectrum, which intensities depend on the host environment. As a host for Dy3+ doping, various matrices have been extensively studied such as phosphates9,10,11, tungstate12, vanadate13, molybdate14 niobate15, silicates16,17,18, aluminate19, and borates20,21. Orthovanadates have many exceptional characteristics including excellent thermal, mechanical, and optical properties; which play an important role in many optical devices, catalysts, and laser host materials22,23,24. In particular, yttrium vanadate (YVO4) is very significant oxide in materials science and technology, for example, its large single crystal has been extensively used as an excellent polarizer and laser host material, whereas its powder doped with several rare earth ions has been used as an attractive phosphor due to its high luminescence quantum yield25,26,27. The previous works mainly focused on the Eu3+-doped YVO4 with different synthetic methods28,29,30, because it is a commercial red-emitting phosphor used in color television, the high-pressure mercury lamp, and as a scintillator in medical image detectors31,32,33,34. To the best of our knowledge, the systematic study of calcination temperature and doping concentration effect on properties of YVO4:Dy3+ nanophosphors has not been reported so far.

Recently, great attention was attracted to developing of non-contact luminescence thermometers with submicrometric spatial resolution. One of the most promising thermal sensing techniques is based on luminescence intensity ratio between emission bands arising from two thermally coupled levels. These levels should be closely spaced (the energy gap generally ranges from 200 to 2000 cm−1) and it is assumed to be in the thermodynamic quasi-equilibrium35. The main advantage of the ratiometric approach for temperature sensing is independence on spectrum losses and fluctuations of the excitation intensity, which leads to a much higher accuracy36,37. Emission bands of Dy3+ ions were originated from two thermally coupled levels (4I15/2 and 4F9/2) with the energy difference around 1000 cm−1. So, Dy3+-doped nanoparticles is suitable for temperature sensing with high spatial resolution and good sensitivity, which can be used in organisms or in living cells36,38,39.

This paper is focused on the detailed study of calcination temperature and doping concentration effect on the structure, steady-state and kinetics luminescence properties of YVO4:Dy3+ nanoparticles. Influence of excitation mechanisms on optimum doping concentration and quenching was explored. Synthesized samples were successfully used as ratiometric thermal sensors in wide temperature range of 298–673 K. The thermometric performances including thermal sensitivities, temperature uncertainty and repeatability were obtained.

Results and Discussion

Figure 1a shows XRD patterns of YVO4:Dy3+ 1 at.% samples prepared at different calcination temperatures. All diffraction peaks of powders can be indexed by the reference standard YVO4 (tetragonal phase, space group I41/amd, JCPDS 17-0341). No other crystalline phase was detected. The cell parameters were calculated with UnitCell software. As it can be seen from Fig. 1b, unit cell volume decreases along with increase of calcination temperature. Such behavior can be elucidated by formation of better crystalline structure of nanoparticles with cell volume closer to the monocrystal40.

Figure 1
figure 1

(a) XRD patterns of YVO4:Dy3+ 1 at.% temperature series and the standard card YVO4; (b) unit cell volume as a function of calcination temperature; (c) XRD patterns of YVO4:Dy3+ 1000 °C concentration series and the standard card YVO4; (d) unit cell volume as a function of Dy3+-doping.

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Effect of Dy3+ doping concentration on YVO4 nanocrystalline powders is presented in Fig. 1c. XRD patterns confirm the presence of pure tetragonal phase without any structural impurities. Dysprosium ions substitute yttrium ions in D2d site symmetry. The single-crystal cell volume systematically increases with growth of dopant concentration due to the difference of ionic radii between yttrium (r = 0.893 Å) and dysprosium ions (r = 1.03 Å). Noteworthy, the unit cell volume has a linear relationship with the amount of Dy3+, which is consistent with Vegard’s law (Fig. 1d). This result demonstrates that the dysprosium ion has been efficiently and homogeneously incorporated into the host matrix of YVO4 due to the similar ionic radius and chemical reactivity of Dy3+ and Y3+ 41.

Figure 2a shows scanning electron microphotograph of the synthesized YVO4:Dy3+ 1 at.% 1000 °C nanopowder. As seen from the micrograph, the powder consists of weakly agglomerated nanoparticles with average size about 50 nm. Elemental analysis was studied using EDX technique. We observed signals from yttrium, vanadium, oxygen, and dysprosium (Fig. 2b).

Figure 2
figure 2

(a) SEM images of YVO4:Dy3+ 1 at.% 1000 °C nanopowder; (b) EDX analysis of YVO4:Dy3+ 1 at.% 1000 °C nanopowder.

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Structural properties of Dy3+-doped YVO4 nanopowders were studied also using vibrational spectroscopy. Figure 3 presents Raman spectrum of YVO4:Dy3+ 1 at.% 1000 °C measured in spectroscopic range from 100 to 1000 cm−1. As can be seen, this spectrum consists of several sharp Raman lines corresponding to the internal vibrations of VO43− group and external vibrations of VO43− tetrahedra and Y3+ ions in YVO4 unit cell42. The external vibration at 157 cm−1 (B1g(1)) can be attributed to the O–Y–O bending mode. The internal vibrations, which can be ascribed to the O–V–O bending and VO4 stretching modes, are located at higher frequencies: 259 (B2g), 376 (A1g(1)), 488 (B1g(3)), 812 (B1g(4)), 835 (Eg(5)) and 888 cm−1 (A1g(2))43. The observed spectrum is dominated by the totally symmetrical vibration of VO43− tetrahedron32. Narrow width of Raman lines indicates good crystallinity and homogeneity of synthesized powder.

Figure 3
figure 3

Raman spectrum of YVO4:Dy3+ 1 at.% 1000 °C nanopowder.

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Further investigations were devoted to the influence of synthesis conditions (calcination temperature and doping concentration) on luminescence properties. Figure 4a shows excitation spectra within spectral range of 260–560 nm (λem = 573 nm) for nanocrystalline powders YVO4:Dy3+ 1 at.% calcinated at different temperatures. The obtained spectra consist of broad and intense band in the UV region and several narrow lines in visible region. The broad band can be assigned to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO43− groups44. From the viewpoint of molecular orbital theory, it corresponds to transitions from the 1A2 (1T1) ground state to 1A1 (1E) and 1E (1T2) excited states of the VO43− ion45. Low peaks in visible region at longer wavelength are assigned to the intra-configurational 4f–4f transitions in dysprosium ions. The observed bands are attributed to the following transitions: 6H15/2–4P7/2 (352 nm), 6H15/2–4P5/2 (365 nm), 6H15/2–4I13/2 (387 and 390 nm), 6H15/2–4G11/2 (427 nm), 6H15/2–4I15/2 (449 and 453 nm) and 6H15/2–4F9/2 (473 nm).

Figure 4
figure 4

(a) Excitation spectra of YVO4:Dy3+ 1 at.% nanopowders calcinated at different temperatures (λem = 573 nm). Emission spectra of YVO4:Dy3+ 1 at.% nanopowders calcinated at different temperatures upon (b) λex = 310 nm and (c) λex = 365 nm excitation. The asterisk indicates the second order of excitation wavelength.

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Emission spectra of nanocrystalline powders YVO4:Dy3+ 1 at.% calcinated at different temperatures are shown in Fig. 4b,c. The measurements were conducted in the spectral range of 450–800 nm upon λex = 310 nm and 365 nm radiation, which presents various excitation mechanisms. The emission spectra are dominated by green-yellow band (573 nm) corresponding to the hypersensitive 4F9/2–6H13/2 transition. Other observed lines are attributed to the 4I15/2–6H15/2 (455 nm), 4F9/2–6H15/2 (484 nm), 4F9/2–6H11/2 (662 nm), 4F9/2–6H9/2 (752 nm) transitions. 4F9/2–6H13/2 is the forced electric dipole transition, which is hypersensitive and its intensity can vary by orders of magnitude depending on the local site symmetry, whereas 4F9/2–6H15/2 transition intensity is insignificantly affected by the environment46,47.

Integrated intensity of the most prominent 4F9/2–6H13/2 transition upon λex = 310 and 365 nm radiation versus calcination temperature is shown in Fig. 5. As one can see, increase of calcination temperature leads to the growth of emission intensity, which coincides with usual behavior of rare earth-doped nanomaterials48,49,50. The observed growth is explained by an improvement of the sample crystallinity and a decrease of the number of OH– groups on the surface of nanoparticles51,52. In addition, it is well known that a growth of the calcination temperature leads to an increase of nanoparticles size, which in turn also results in luminescence intensity enhancement due to reduction of doping ions fraction on the surface53.

Figure 5
figure 5

Dependence of 4F9/2–6H13/2 integrated intensity of YVO4:Dy3+ 1 at.% nanopowders on calcination temperature.

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To get more information about photoluminescence characteristics of prepared nanophosphors, fluorescence kinetics measurements were carried out. The photoluminescence decays of YVO4:Dy3+ temperature series were monitored at 4F9/2–6H13/2 transition upon 310 nm excitation (Fig. 6a). All experimental curves were fitted by single exponential function:

$$I={I}_{0}\cdot {e}^{-\frac{t}{{\tau }_{f}}}$$
(1)

where τf is the observed lifetime of 4F9/2 level. Figure 6b shows measured lifetimes dependence on calcination temperature. It should be noted that the increase of calcination temperature insignificantly affects the 4F9/2 lifetime.

Figure 6
figure 6

(a) Decay curves of YVO4:Dy3+ 1 at.% temperature series; (b) 4F9/2 lifetime as a function of calcination temperature.

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Next we discuss the effect of doping concentration on the luminescence properties of nanocrystalline powders YVO4:Dy3+. All samples of concentration series (1–15 at.%) were synthesized with calcination temperature of 1000 °C. Excitation spectra of nanocrystalline powders YVO4:Dy3+ 1000 °C with different doping concentrations are shown in Fig. 7a. These spectra consist of intense band attributed to the absorption of VO43− groups and weak Dy3+ 4f–4f transitions as it was described for temperature series. The spectral line positions of concentration series coincide with the temperature series. Emission spectra of nanocrystalline powders YVO4:Dy3+ 1000 °C with different doping concentrations upon 310 nm and 365 nm radiation are presented in Fig. 7b,c. The most intensive line in the measured spectra is attributed to the 4F9/2–6H13/2 transition.

Figure 7
figure 7

(a) Excitation spectra of YVO4:Dy3+ 1000 °C nanopowders with different doping concentrations (λem = 573 nm). Emission spectra of YVO4:Dy3+ 1000 °C nanopowders with different doping concentrations upon (b) λex = 310 nm and (c) λex = 365 nm excitation. The asterisk indicates the second order of excitation wavelength.

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Figure 8 presents the integrated luminescence intensity of the most intensive transition 4F9/2–6H13/2 as a function of doping concentration upon excitation wavelengths 310 and 365 nm. It is well-known that the optimum of Dy3+ concentration is determined by two competitive effects: on the one hand, an increase of the doping concentration means an increase of luminescence centers number and thus radiative recombination. On the other hand, there is also an increase of the probability of energy transfer between dysprosium ions, which enhances the efficiency of the nonradiative processes54. As can be seen, first emission intensity goes up with increasing doping concentration but further growth of Dy3+ ions number leads to intensity reduction. This indicates the concentration quenching effects for the luminescence of Dy3+ in YVO4 host. Noteworthy, the optimum doping concentration was found to be different for various excitation mechanisms. The optimum Dy3+ concentration for host excitation (310 nm) was determined to be 1 at.%, whereas for the direct Dy3+ ions excitation (365 nm) optimum concentration was 2 at.%. Similar difference in optimum concentration depending on excitation mechanism was previously observed for Eu3+ and Nd3+-doped YVO4 nanopowders32,55.

Figure 8
figure 8

Dependence of 4F9/2–6H13/2 integrated intensity of YVO4:Dy3+ nanopowders on doping concentration upon (a) 310 nm and (b) 365 nm excitation.

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To obtain information about Dy3+ concentration effect on the crystal structure and crystal field, ratio between 4F9/2–6H13/2 and 4F9/2–6H15/2 intensities (R) was calculated. This parameter is similar to the well-known asymmetry ratio for Eu3+ ions56,57. R value give information about the local surrounding and environmental changes near the Dy3+ ions. The higher the calculated parameter is, the more apart from a centrosymmetric geometry luminescent center is located. It is well-known that if Dy3+ is located at low symmetry without the inversion symmetry, the yellow emission is the most intense of all the transitions, as is the case with our synthesized nanocrystalline phosphors58. Figure 9 shows the dependence of R value, calculated from emission spectra obtained upon 365 nm excitation, on Dy3+ doping concentration for YVO4 host. As can be seen, the calculated ratio demonstrates nonmonotonical behavior, but main trend is decreasing of R value along with increasing of doping concentration. So, we can conclude that introduction of additional Dy3+ ions in YVO4 host results in growth of local symmetry.

Figure 9
figure 9

4F9/2–6H13/2 / 4F9/2–6H15/2 intensity ratio of YVO4:Dy3+ samples as a function of doping concentration.

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Luminescence decays of YVO4:Dy3+ concentration series monitored at 4F9/2–6H13/2 transition upon 310 nm excitation are presented in Fig. 10a. Experimental data of synthesized nanocrystalline powders doped with 0.5 and 1 at.% were approximated by mono-exponential function. Higher doped samples demonstrate non mono-exponential decay, therefore, to provide correct fitting we used biexponential model. The average lifetime was obtained according following formula59:

$${\tau }_{av}=\frac{{A}_{1}{\tau }_{1}^{2}+{A}_{2}{\tau }_{2}^{2}}{{A}_{1}{\tau }_{1}+{A}_{2}{\tau }_{2}}$$
(2)

where A1 and A2 are pre-exponential factors; τ1 and τ2 are lifetimes. The fitting model was changed due to growth of Dy3+ doping ions located on the nanopartilces surface60. The non mono-exponential behavior is originated from the different decay rates of the Dy3+ ions situated at the surface and in the volume of the nanoparticles61,62,63. Figure 10b shows dependence of the obtained lifetimes on Dy3+ doping concentration. One can see that the lifetime gradually declines from 163 μs to 69 μs along with increase of Dy3+ ions number.

Figure 10
figure 10

(a) Decay curves of YVO4:Dy3+ concentration series; (b) 4F9/2 level lifetime as a function of doping concentration.

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As it was previously observed, growth of the amount of Dy3+ ions in YVO4 host leads to the concentration quenching. According to Blasse et al., the energy transfer mechanism is defined by the critical energy transfer distance (Rc) if the doping ions occupy single crystallographic position. The critical energy transfer distance was calculated by the following equation64:

$${R}_{c}\approx 2{[\frac{3V}{4\pi {x}_{c}N}]}^{\frac{1}{3}}$$
(3)

where xc is the critical doping content (xc = 0.01 or 0.02), N is the number of cation sites in the unit cell (N = 4 for YVO4), and V is the volume of the unit cell (V ≈ 318.78 Å3). Using aforementioned parameters, Rc was found to be 24.8 and 19.6 Å for host and direct excitations, respectively. Upon both excitation wavelengths calculated value of Rc is bigger than 5 Å, so exchange interaction cannot control energy transfer between Dy3+ ions in YVO4 host65. Therefore, we can conclude that the concentration quenching is caused by the multipolar interaction mechanism66,67. Multipolar interaction includes dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interaction. An interaction type can be determined using formula proposed by Van Uitert68. Further, Ozawa and Jaffe modified it as follows69:

$$\frac{I}{x}=k{[1+\beta {(x)}^{\frac{\theta }{3}}]}^{-1}$$
(4)

where I is the integral intensity, x is the activator concentration, k and β are constant for the same excitation conditions for a given host crystal. According to the above equation θ = 3 for the energy transfer among the nearest neighbor ions, while θ = 6, 8 and 10 for d–d, d–q and q–q interactions, respectively70,71.

The critical concentration of Dy3+ ions was determined as 1 and 2% for 310 and 365 nm excitation, respectively. The dependence of the emission intensity of YVO4:Dy3+ nanopowders on the doping concentration was investigated. The multipolar character (θ) can be obtained by plotting log (I/x) vs log (x) as presented in Fig. 11. The slope θ/3 from approximation was determined to be −2.06 and −1.89 for 310 and 365 nm excitation, giving calculated value of θ is close to 6 in both cases. Therefore, concentration quenching in YVO4:Dy3+ nanocrystalline powders occurred due to dipole–dipole interaction regardless of the excitation mechanism.

Figure 11
figure 11

Logarithmic plot of YVO4:Dy3+ nanophosphors emission intensity vs doping concentration upon (a) 310 nm and (b) 365 nm excitation. Lines are a linear fit.

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In order to study the emission color dependence of the synthesized nanophosphors on calcination temperature and doping concentration, the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates were calculated and the results are presented in Fig. S1 and Table 1. The chromaticity coordinates were obtained using emission spectra measured upon 310 nm excitation. As can be seen, both calcination temperature and doping concentration do not strongly influence on the emission color of Dy3+-doped YVO4 phosphor. Moreover, it should be noted that the observed small change of CIE coordinates has nonmonotonic behavior along with growth of calcination temperature or Dy3+ concentration.

Table 1 CIE coordinates of YVO4:Dy3+ temperature and concentration series.
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During last decade a lot of attention is attracted to the search of convenient non-contact luminescence thermometer72,73,74. Due to the unique properties of rare earth ions, plenty organic and inorganic materials doped with lanthanides has been suggested for luminescence thermometry75,76,77. Here, we studied possibility to define local temperature using Dy3+-doped YVO4 nanophosphors.

Figure 12 shows normalized emission spectra of YVO4:Dy3+ 1 at.% and 2 at.% nanopowders measured at different temperatures (298, 423 and 673 K). The observed emission lines are originated from electron transitions from 4I15/2 and 4F9/2 excited states with energy separation of about 1000 cm−1 (Fig. 12c). According to definition, 4I15/2 and 4F9/2 are thermally coupled levels, therefore ratiometric approach for transitions from these excited states can be utilized to determine local temperature. Luminescence intensity ratio (LIR) between 4I15/2–6H15/2 and 4F9/2–6H15/2 transitions (R455/480), as well as ratio between 4I15/2–6H15/2 and 4F9/2–6H13/2 transitions (R455/575) were used for thermal sensing. It should be noted that calculation of luminescence ratios was based on the integrated intensities of transitions, because such approach gives better accuracy comparing with peak intensities78.

Figure 12
figure 12

Normalized emission spectra of (a) YVO4:Dy3+ 1 at.% and (b) YVO4:Dy3+ 2 at.% nanopowders at different temperatures. The colored areas are used for the integral intensity ratio calculations. (c) Energy levels scheme of YVO4:Dy3+ nanophosphors.

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The variations of the LIR values of YVO4:Dy3+ 1 at.% and YVO4:Dy3+ 2 at.% nanocrystalline powders as a function of the temperature are presented in Fig. 13. Temperature induced change of LIR is caused by electron re-distribution at the energy levels according to the Boltzmann formula:

$$R=A\cdot \exp (-\frac{{\rm{\Delta }}E}{kT})$$
(5)

where A is a temperature-independent constant, ΔE is the energy gap between thermally coupled energy levels, k is the Boltzmann’s constant and T is the absolute temperature. As can be seen, the experimental data were successfully fitted with eq. (5) and the obtained parameters were presented in the graphs.

Figure 13
figure 13

Luminescence intensity ratio (R455/480) of (a) YVO4:Dy3+ 1 at.% and (c) YVO4:Dy3+ 2 at.% nanopowders as a function of temperature. Luminescence intensity ratio (R455/575) of (b) YVO4:Dy3+ 1 at.% and (d) YVO4:Dy3+ 2 at.% nanopowders as a function of temperature. Red lines correspond to the best fitting with Eq. (5).

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To estimate the performance of the thermometer, absolute (Sa) and relative (Sr) thermal sensitivities are usually used. The absolute thermal sensitivity shows the absolute LIR change with temperature variation and is defined as follows:

$${S}_{a}=\frac{dR}{dT}=R\frac{{\rm{\Delta }}E}{k{T}^{2}}$$
(6)

It is obviously from Eq. (6) that Sa depends on absolute LIR value, which can be significantly changed by manipulating LIR calculation procedure (for instance, change of integration limit for calculation of integral intensity of emission transition). Therefore, absolute thermal sensitivity cannot be used for the fair comparison among different systems79. To compare thermometers irrespective to their nature and sensing parameter, the relative thermal sensitivity is introduced. Sr shows normalized change of LIR with temperature variation and is defined as follows:

$${S}_{r}=\frac{1}{R}\frac{dR}{dT}=\frac{{\rm{\Delta }}E}{k{T}^{2}}$$
(7)

The variation of the Sa and Sr value with temperature from 298 to 673 K for both studied LIRs is presented in the Fig. 14. The observed temperature dependences of Sa and Sr demonstrate opposite behavior: temperature increase leads to the gradual growth of Sa and monotonic decline of Sr. Temperature dependence in both studied LIRs is originated from the same excited levels: 4I15/2 and 4F9/2. So, it is unsurprising that the maximal relative thermal sensitivity (T = 298 K) for R455/480 and R455/575 ratios is similar: 1.8% K−1 (YVO4:Dy3+ 1 at.%) and 1.5% K−1 (YVO4:Dy3+ 2 at.%). However, the absolute thermal sensitivities differ significantly: 0.039 K−1@673 K (R455/480) and 0.0013 K−1@673 K (R455/575) in case of YVO4:Dy3+ 1 at.%. Noteworthy that increase of Dy3+ doping concentration results in worsening of thermal sensitivity.

Figure 14
figure 14

Variation of absolute (Sa) and relative (Sr) thermal sensitivity dependence on temperature for (a,b) YVO4:Dy3+ 1 at.% and (c,d) YVO4:Dy3+ 2 at.%.

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In comparison with Er3+, Tm3+ or Nd3+ ions, dysprosium ions are relatively rare utilizing for temperature sensing. Among other Dy3+ single doped materials, the obtained Sr values are comparable to the values reported in the literature: 1.7% K−1 for BaYF5:Dy3+ NPs80, 1.6% K−1 for Gd2Ti2O7:Dy3+ NPs81, 1.7% K−1 for YAG:Dy3+ microcrystals82, 1.7% K−1 and 1.3% K−1 for Dy:Y(acac)3 and Dy:Y(acac)3(phen) molecular crystals, respectively83.

The minimum temperature uncertainty (ΔT) provides information about accuracy of thermal sensing which can be derived using this material. There are several experimental techniques to obtain minimum temperature uncertainty, which were discussed and compared in our previous work78. Here, ΔT was estimated from consecutive emission spectra measured at fixed heating stage temperature (Figs S2 and S3). Due to the rather wide temperature sensing region, temperature uncertainty was obtained for 323 and 473 K. The obtained value of ΔT is lying in the range of 2–7 K depending on the measured temperature, used luminescence intensity ratio and doping concentration.

Third important factor for the assessment of the precision of a thermometric system is repeatability. We tested repeatability of YVO4:Dy3+ thermometers over cyclic heating-cooling measurements (Fig. 15). During the experiment, we increased and decreased the temperature within the thermal range of 323–473 K. Black squares indicate the actual temperature of heater, whereas red circles and blue triangles present temperature obtained with R455/480 and R455/575 luminescence intensity ratio, respectively. Taking into account temperature uncertainty, we can conclude good repeatability of considered YVO4:Dy3+ nanopowders: temperatures obtained using optical thermometry are repeated from cycle to cycle and they are in good agreement with the actual heater temperature.

Figure 15
figure 15

Calculated and measured temperature in heating-cooling cycles for (a) YVO4:Dy3+ 1 at.% and (b) YVO4:Dy3+ 2 at.%.

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Summary

Series of nanocrystalline Dy3+-doped YVO4 samples with different calcination temperature and doping concentration were prepared by modified Pechini method. XRD study defines that YVO4:Dy3+ nanoparticles had tetragonal phase without any impurities. Synthesized samples consisted of quite small nanoparticles with size about 50 nm. EDX technique revealed signals from yttrium, vanadium, oxygen, and dysprosium. Narrow width of Raman lines confirmed good homogeneity and crystallinity of synthesized nanoparticles. Excitation spectrum of Dy3+-doped YVO4 nanophosphors consisted of broad band and several narrow lines assigned to the pump through matrix host and direct pump of Dy3+ ion, respectively. Emission spectrum consisted of the characteristic narrow lines attributed to the intra-configurational 4f-4f transitions. The most intensive luminescence band corresponded to the hypersensitive 4F9/2–6H13/2 transition. Calcination temperature increase led to the growth of emission intensity due to the improvement of the crystallinity and decrease of the quenchers number. Study of YVO4:Dy3+ concentration series demonstrated usual concentration quenching effect. The optimal Dy3+ doping concentrations of 1 and 2 at.% were determined for different excitation mechanisms (λex = 310 and 365 nm, respectively). Concentration quenching in YVO4:Dy3+ nanocrystalline powders occurred due to dipole–dipole interaction regardless of the excitation mechanism. The calcination temperature variation did not affect 4F9/2 lifetime, whereas increase of doping concentration resulted in its gradual decline. CIE coordinates of YVO4:Dy3+ nanoparticles did not depend on either calcination temperature or doping concentration. YVO4:Dy3+ 1 at.% and YVO4:Dy3+ 2 at.% nanocrystalline powders were tested as ratiometric luminescence thermometers. Temperature sensing based on R455/480 and R455/575 luminescence intensity ratios was demonstrated within wide thermal range of 298–673 K. The maximum relative thermal sensitivity was 1.8% K−1@298 K, whereas the minimum temperature uncertainty was found to be 2 K. Thermal cycling experiments showed good repeatability of studied thermometer.

Methods

YVO4:Dy3+ samples were prepared by modified Pechini method32,55. The doping concentration of Dy3+ was 0.5, 1, 2, 5, 10, 15 at.% to Y3+ in YVO4 host. The starting materials were yttrium oxide (Y2O3), dysprosium oxide (Dy2O3), vanadium oxide (V2O5), concentrated nitric acid (HNO3), citric acid (C6H8O7) and ethylene glycol (C2H6O2). Y2O3 and Dy2O3 were dissolved in concentrated HNO3 with heated to form nitrates solution. Then aqueous solution of citric acid was added to yttrium-dysprosium nitrates mixture (with volume ratio 1:1). The chemical reaction is as follows:

$${\rm{Me}}{({{\rm{NO}}}_{3})}_{3}+{{\rm{3C}}}_{6}{{\rm{H}}}_{8}{{\rm{O}}}_{7}=[{\rm{Me}}{({{\rm{C}}}_{6}{{\rm{H}}}_{8}{{\rm{O}}}_{7})}_{3}]{({{\rm{NO}}}_{3})}_{3}$$

Then V2O5 was dissolved in citric acid to form VO(C6H7O7)2 according to the following reaction:

$${{\rm{V}}}_{2}{{\rm{O}}}_{5}+{{\rm{5C}}}_{6}{{\rm{H}}}_{8}{{\rm{O}}}_{7}={\rm{2VO}}{({{\rm{C}}}_{6}{{\rm{H}}}_{7}{{\rm{O}}}_{7})}_{2}+{{\rm{C}}}_{6}{{\rm{H}}}_{8}{{\rm{O}}}_{7}+1/2\,{{\rm{CO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}$$

After that previous metals citrate complex ([Me(C6H8O7)3](NO3)3) was added with stirring and heated. The green solution was formed. Then ethylene glycol was added to the above mixture (volume ratio with of ethylene glycol and citric acid solution was 1:4 respectively). The formation of citrate transparent gel was observed. The obtained polymer gel was transferred to a crucible and then was placed in a furnace maintained at a temperature of 500 °C/1 h to burn off the organic components. The brown powder was formed. It was grinded in mortar with adding the potassium chloride in weight ratio 1:1. Then powder mixture kept in a muffle furnace maintained at predefined temperature/1.5 h. We used 850, 900, 950, 1000, 1100 and 1200 °C as the second calcination temperature. After the second thermal treatment white powder was removed from the furnace, centrifuged, washed three times with distilled water to remove potassium chloride and, finally, dried. Thus, YVO4: Dy3+ nanocrystalline powders were synthesized.

X-ray diffraction patterns were registered with the powder diffractometer UltimaIV (Rigaku) in Bregg-Bretano geometry with CuKα1 radiation (λ = 1.54059 Å) in the 2θ range from 7° to 80°. Phase identification was carried out using a powder diffraction database PowderDiffractionFile (PDF-2, 2011). The unit cell parameters were estimated using UnitCell software. Electron micrograph images and elemental analysis were obtained using Zeiss Merlin electron microscope with Oxford Instruments INCAx-act accessory. Raman spectrum was measured on Bruker SENTERRA Raman Microscope with semiconductor laser 488 nm as an excitation source. Steady-state and kinetics photoluminescence properties were studied with modular fluorescence spectrometer Fluorolog-3 (HORIBA Jobin Yvon). All thermal measurements were performed by using T64000 Raman Spectrometer. The Dy3+-doped YVO4 NPs were optically excited with a 374 nm diode laser Coherent CUBE. The laser beam was focused into the sample by using a 4x long working distance microscope objective (NA 0.1). The fluorescence was collected by using the same microscope objective and was spectrally analyzed by single spectrometer and Peltier cooled Synapse CCD detector. The temperature was controlled with a heating stage controlled with heating stage Linkam TS1000 with 0.1 °C temperature stability and 0.1 °C set point resolution.