Host matrix impact on Er³⁺ upconversion emission and its temperature dependence

Abstract

By synthesizing Y_(1.9−2x)Yb_0.1Er_2xO₃, Y_(0.95−x)Yb_0.05Er_xVO₄ and Y_(0.95−x)Yb_0.05Er_xPO₄ phosphors, with phonon frequency maxima at 560, 826 and 1050 cm⁻¹, respectively, we present the impact of phonon energy and crystal structure of the host matrix on upconversion and temperature sensing behavior. The spectral upconversion characteristics of all three phosphors reveal noticeable differences. The temperature sensing studies reveal that the phosphors have maximum sensitivity at ∼490 K, which is found to be highest (0.0105 K⁻¹) in Y_0.947Yb_0.05Er_0.003VO₄ followed by Y_1.894Yb_0.1Er_0.006O₃ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors. We found that the temperature sensitivity basically depends on the intensity ratio of two thermally coupled emission bands, ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2, of Er³⁺. Further, the intensity ratio depends on phonon energy of the host lattice, crystal structure, surface quenching centers and the temperature dependence of non-radiative decay rate.

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1. Introduction

Conventional methods for temperature measurements, which employ, for example liquid-filled thermometers, thermocouples, and thermistors are not applicable for temperature measurement of objects below 10 μm size.¹ Consequently, temperature sensing in the microcosm requires altered, non-traditional probing techniques. This can be done with luminescence-based temperature sensors, which can be used for biological and technological applications, such as measuring the temperature of microfluids and microcircuits, to name a few.^2,3 The here pursued principle basically uses lanthanide ions as a luminescence center, whilst the emission intensity, lifetime or intensity ratio of two-thermally coupled bands is monitored as a function of temperature to record the temperature of an object.^4–10

Luminescence intensity and lifetime-based temperature sensing is generally performed in lanthanide complexes or lanthanide-based inorganic–organic hetero-paired nanostructures. However, the use of ultraviolet light as an excitation source limits their biological applicability,^11,12 and additionally, power fluctuations and other environmental effects might influence the performance. On the other hand, temperature probing by means of luminescence intensity ratio variations overcomes these problems. The lanthanide which is commonly used for this purpose are Er³⁺, Ho³⁺, Tm³⁺, etc. The advantage with these ions is that they show efficient upconversion (UC) under near infrared (NIR) irradiation exposure, and therefore, are suitable for biological applications as well. It is important to note that temperature measurements relying on luminescence intensity ratios contain the precise information about the temperature in a single emission spectrum and are not influenced by intensity variations of the excitation source and photo detector drifts. In literature mainly Er³⁺, Ho³⁺, Nd³⁺ and Tm³⁺ doped phosphors have been used for temperature sensing.^8–10 Among them Er³⁺ activated phosphors are preferred because of strong UC emission intensities for the thermally-coupled levels (²H_11/2 and ⁴S_3/2). The nature of temperature versus sensitivity plots reported in literature varies in different phosphors.^1–10 Bearing in mind that the host lattice phonon energy affects the UC emission, it may also alter the sensitivity of temperature sensor.^13–16 Therefore, it is important to investigate the role of host matrix (phonon energy, crystal structure, etc.) on UC emission and temperature sensing characteristics. In order to perform this study, which – to the best of our knowledge – cannot be found in the literature, we have synthesized Y_(1.9−2x)Yb_0.1Er_2xO₃, Y_(0.95−x)Yb_0.05Er_xVO₄ and Y_(0.95−x)Yb_0.05Er_xPO₄ phosphors by a solid state reaction method and studied the UC emission and its potential for temperature sensing.

2. Experimental

2.1 Materials

Analytical reagent (AR) grade yttrium oxide (Y₂O₃, 99.99%, Himedia), vanadium pentaoxide (V₂O₅, 99.9%, Loba Chemicals), phosphorus pentaoxide (P₄O₁₀ (empirical formula P₂O₅), 99.9%, Loba Chemicals), ytterbium oxide (Yb₂O₃, 99.9%, Aldrich) and erbium oxide (Er₂O₃, 99.9%, Aldrich) were used for the synthesis of Y_(1.9−2x)Yb_0.1Er_2xO₃, Y_(0.95−x)Yb_0.05Er_xVO₄ and Y_(0.95−x)Yb_0.05Er_xPO₄ phosphors.

2.2 Synthesis

For the synthesis of Y_(1.9−2x)Yb_0.1Er_2xO₃, Y_(0.95−x)Yb_0.05Er_xVO₄ and Y_(0.95−x)Yb_0.05Er_xPO₄ (x = 0.003, 0.01, and 0.02) phosphors the following, stoichiometric amounts of raw materials were mixed homogeneously in an agate mortar using AR grade acetone as a mixing medium:

Afterwards, the samples were kept at ambient temperature for overnight drying. The mixture was then placed in an alumina crucible and calcined in high temperature furnace at the optimized temperature of 1523 K for 5 h. For the temperature sensing experiments, phosphor samples were produced in pellet form with diameter and thickness of 12 and 1.5 mm, respectively, and were sintered at 1573 K for 5 h.

2.3 Characterization

X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) transmission spectra (from 400 to 4000 cm⁻¹ with a resolution of 2 cm⁻¹) of the samples were measured with a Rigaku (MiniFlex II DEXTOP) powder diffractometer (CuKα radiation (1.5404 Å)) and a Thermo Scientific FTIR spectrometer (NICOLET 6700), respectively. The down-shifting (DS) PL measurements were performed employing the fluorolog-3 spectrofluorometer (FL3-11, Horiba Jobin Yvon) equipped with a 450 W xenon flash lamp. The DS lifetime measurements were carried out with a pulsed xenon lamp (25 W) by the same setup. The UC luminescence was excited by the 976 nm emission of a tunable continuous wave (cw) 2 W diode laser, while the data collection was addressed via the conjunction of aiHR 320 (Horiba JobinYvon) spectrometer with an attached R928P photomultiplier tube (PMT). For the lifetime measurement of the ²H_11/2 and ⁴S_3/2 levels the laser beam was mechanically chopped at 10 Hz and the data were acquired using analog digital scope-HM1507 running software SP107. To measure the UC emission at different temperatures, the pellets were mounted on a homemade heating device, whereas the laser was focused near to the pellet edge keeping the thermocouple in close proximity (within ∼4 mm) to the focal spot.

3. Results and discussion

3.1 Structural analysis

In order to check the phase purity and to determine the crystal structure parameters, Rietveld analyses of the powder diffraction data have been carried out using the crystal structure refinement program “FULLPROF”.¹⁷ The Pseudo-Voigt function has been used to define the peak profile. In case of the Y_1.894Yb_0.1Er_0.006O₃ phosphor all the diffraction peaks match well using Ia3̄ (206) space group (JCPDS no. 41-1105) of cubic structure. The structure provides two cation symmetry sites, C₂ and C_3i, with six-fold oxygen coordination. The Wyckoff positions for these two cationic sites are 8b and 24d with local symmetry C_3i and C₂, respectively, whilst oxygen ions are located at 48e Wyckoff positions. The Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors have zircon type tetragonal crystal structure with space group I41/amd (141). The Wyckoff positions for Y, V(P) and O are 4a, 4b and 16h, respectively. In this structure, Y and V(P) are surrounded by eight oxygen atoms having two different bond lengths and by four oxygen atoms with equal bond lengths, respectively, and between YO₈ and VO₄ (PO₄) units two oxygen atoms are corner-sharing. The VO₄ (PO₄) (tetrahedron) has S₄ symmetry, whilst YO₈ (dodecahedron) has D_2d symmetry. The Yb/Er atoms substitute Y sites in the lattice. Fig. 1 depicts the Rietveld fits for Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors, indicating a good agreement between observed and calculated XRD patterns. The refinement parameters are given in Table 1. The XRD analysis clearly depicts that Y_1.894Yb_0.1Er_0.006O₃ and Y_0.947Yb_0.05Er_0.003VO₄ are phase pure, whereas Y_0.947Yb_0.05Er_0.003PO₄ has small surplus of unreacted Y₂O₃.

Fig. 1 X-ray diffraction patterns of the Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors: the experimental results are represented by dots, and the solid lines show the calculation and difference (bottom) profiles after Rietveld refinement. Vertical tick marks above the difference plot show the positions of the Bragg peaks.

Table 1 Refine structural parameters for Y_1.894Yb_0.1Er_0.006O₃ (space group = Ia3̄), Y_0.947Yb_0.05Er_0.003VO₄ (space group = I41/amd) and Y_0.947Yb_0.05Er_0.003PO₄ (space group = I41/amd) phosphors

Sample	Atoms	Positional Coordinates			Occupancy	Thermal parameters
		X	Y	Z		B (Å^2)
Y1.894Yb0.1Er0.006O3	Y/Yb/Er	0.25(0)	0.25(0)	0.25(0)	1.0	0.63(7)
	Y/Yb/Er	0.968(2)	0.00(0)	0.25(0)	1.0	0.45(5)
	O	0.392(1)	0.156(2)	0.376(2)	3.0	0.41(8)
a = b = c = 10.5935(2) Å, Rp = 12.0, Rwp = 16.9, Rexp = 11.74, χ^2 = 2.06
Y–O = 2.4441 Å (48), Y–Y = 3.5138 Å (48), Y–Y = 3.9923 Å (24), Y–Y = 3.5302 Å (48), Y–O = 2.2717 Å (48), Y–O = 2.3904 Å (48), O–Y = 2.2437 Å (24), Y–Y = 4.0064 Å (24)
Y0.947Yb0.05Er0.003VO4	Y/Yb/Er	0.00(0)	0.75(0)	0.125(0)	1.0	0.70(4)
	V	0.00(0)	0.25(0)	0.375(0)	1.0	0.08(1)
	O	0.00(0)	0.435(1)	0.203(1)	4.0	0.13(2)
a = b = 7.1112(2) Å, c= 6.2860(2) Å, RB = 8.9, Rwp = 11.3, Rexp = 4.98, χ^2 = 5.14
Y–Y = 3.8842 Å (5), Y–V = 3.1405 Å (6), Y–O = 2.2882 Å (14), Y–O = 2.4352 Å (12), V–O = 1.7088 Å (16)
Y0.947Yb0.05Er0.003PO4	Y/Yb/Er	0.00(0)	0.75(0)	0.125(0)	1.0	0.60(4)
	P	0.00(0)	0.25(0)	0.375(0)	1.0	0.09(1)
	O	0.00(0)	0.426(1)	0.219(1)	4.0	0.23(2)
a = b = 6818(2) Å, c = 6.0187(2) Å, RB = 9.16, Rwp = 12.7, Rexp = 4.69, χ^2 = 7.3
Y–Y = 3.7555 Å (5), Y–P = 3.0092 Å (6), Y–O = 2.30122 Å (14), Y–O = 2.3964 Å (12), P–O = 1.1.5325 Å (16)

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To determine the phonon frequency of the host lattices, we monitored the FTIR spectra of Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors (see Fig. 2). The Y_1.894Yb_0.1Er_0.006O₃ shows a strong sharp peak due to the stretch of the Y–O bond at 560 cm⁻¹, and Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ show intense peaks due to stretches of the V–O and P–O bonds at 826 and 1050 cm⁻¹, respectively.^14,18 In Y_0.947Yb_0.05Er_0.003PO₄ phosphor, the FTIR peaks at 523 and 638 cm⁻¹ correspond to bending vibrations of P–O bonds.¹⁹ Thus, the FTIR studies reveal that all these host matrices possess considerably different phonon frequencies.

Fig. 2 FTIR spectrum of Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors.

3.2 Upconversion

The UC emission spectra of Y_0.947Yb_0.05Er_0.003PO₄, Y_0.947Yb_0.05Er_0.003VO₄ and Y_1.894Yb_0.1Er_0.006O₃ are shown in Fig. 3. The Y_1.894Yb_0.1Er_0.006O₃ and Y_0.947Yb_0.05Er_0.003VO₄ phosphors show blue emission bands around 410 nm because of three-photon processes.⁸ The blue emission is more intense in Y_1.894Yb_0.1Er_0.006O₃ than in Y_0.947Yb_0.05Er_0.003VO₄ phosphor, most likely owing to the higher phonon energy of YVO₄ host. The equivalent argument explains the absence of blue emission of Y_0.947Yb_0.05Er_0.003PO₄. The emission bands observed at 380, 410, 488, 523, 548 and 660 nm correspond to the ⁴G_11/2 → ⁴I_15/2, ²H_9/2 → ⁴I_15/2, ²H_9/2 → ⁴I_15/2, ⁴F_7/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively.^5–8 It can also be noticed that all the emission bands show Stark components. The energy level splitting is the largest in Y_1.894Yb_0.1Er_0.006O₃ because of the difference in crystal field potential/distance between Er³⁺ and O²⁻ (ligands) due to different crystal structure of the phosphors.^20,21

Fig. 3 Room temperature UC emission spectrum of Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors.

In order to explain the origin of the UC emission transitions of the phosphors, Fig. 4 displays the energy level diagram of the Er³⁺ and Yb³⁺ ions. Basically, there are two distinct processes, i.e., the excited state absorption (ESA) and energy transfer (ET), which are responsible for the UC emission. The Er³⁺ ion in its ground state absorbs the incident NIR photon (ground state absorption, GSA) resulting in the resonant excitation of the ⁴I_11/2 level, from where, via the absorption of a second photon the ⁴F_7/2 level gets excited. As a result, the excitation scheme refers to a non-coherent two photon process. Instead re-excitation (from ⁴I_11/2 level), however, the ion can relax to ⁴I_13/2 level from where it can absorb a second photon and populate ⁴F_9/2 level. The ion in ⁴F_9/2 level can absorb a third photon populating the ²H_9/2 level, or relaxes into the ⁴I_15/2 level by emitting light at 660 nm. The ion in the ⁴F_7/2 level can emit at 488 nm by relaxing to level ⁴I_15/2, or through multiphonon non-radiative relaxation can populate the ²H_11/2 and ⁴S_3/2 levels. There are three possibilities for depopulation from these two levels: (i) emission at 523 and 548 nm via the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively, (ii) further relaxation to the ⁴F_9/2 level, or (iii) the ion gets re-excited in the ⁴G_7/2 level by the absorption of a third photon. The latter transition may result in blue and ultraviolet (UV) emissions.

Fig. 4 Schematic energy level diagram of Er³⁺–Yb³⁺ showing mechanism involved in UC emissions.

It is important to stress at this point that the Er³⁺ ion itself has a very weak emission due to the low absorption cross section (1.7 × 10⁻²¹ cm²). However, by considering the factor 7 times greater absorption cross section (11.7 × 10⁻²¹ cm²) of the Yb³⁺ ion, the emission of the Yb³⁺/Er³⁺ co-doped samples can be increased by the energy transfer from Yb³⁺ to Er³⁺ ion.⁵ Additionally, keeping a concentration ratio of Yb³⁺/Er³⁺ = 16.7, the possibility of photon absorption by Yb³⁺ is considerably gained in comparison to Er³⁺. Fig. 4 shows that the Yb³⁺ can transfer single photon energy through different ways or it can transfer energy of two photons simultaneously through cooperative energy transfer (CET) process to ⁴F_7/2 level of Er³⁺ ion. The UC emission in Yb³⁺/Er³⁺ co-doped phosphors for the given concentrations is basically due to ET process, while the probability of ESA and CET processes are very low.²²

It is clearly of importance to discuss the reasons for the different green/red (G/R) emission intensity ratios in Fig. 3, which are ∼46.88, ∼2.66, and ∼5.18 (calculated by taking the ratio of integrated UC emission intensity of 500–586 nm and 625–720 nm regions) for Y_0.947Yb_0.05Er_0.003VO₄, Y_0.947Yb_0.05Er_0.003PO₄, and Y_1.894Yb_0.1Er_0.006O₃, respectively. To study the scatter of G/R intensity ratios, we prepared phosphors with different concentrations and recorded their DS and UC emission spectra, which are shown in Fig. 5 for Y_1.86Yb_0.1Er_0.04O₃, Y_0.93Yb_0.05Er_0.02VO₄, and Y_0.93Yb_0.05Er_0.02PO₄ phosphors (spectra for x = 0.02 composition are shown, because of higher possibility of red emission). The DS emission spectra of phosphors do not show red emission band, whereas it is observed in the UC emission spectra. Fig. S1† reveals that the intensity of the red emission band changes very little with Er³⁺ ion concentration (x = 0.003 to 0.02), but it significantly affected the G/R intensity ratio in Y_(0.95−x)Yb_0.05Er_xVO₄ phosphors. The UC emission spectrum of Y_1.994Er_0.006O₃ phosphor, shown in Fig. S2,† reveals only green emission bands, but an addition of Yb³⁺ ion in this phosphor (Y_1.894Yb_0.1Er_0.006O₃) causes the prominent red emission band. This observation suggests that – along with the overall UC emission change – the Yb³⁺ ion is also responsible for the G/R intensity alterations. The absence of the red emission band in the DS spectra clearly shows that mainly the ET process is the origin of the red emission band in UC spectrum, i.e. ion in the ⁴I_13/2 level receives the energy via non-resonant energy transfer (most likely from Yb³⁺) and populates ⁴F_9/2 level from where relaxation to the ⁴I_15/2 level occurs – during the emission of red light emission at 660 nm.^22–25 Thus, the difference in the G/R intensity ratio is basically due to the difference in lifetime of ⁴I_11/2 and ⁴I_13/2 levels in different host matrices. The lifetime of Er³⁺ levels in YVO₄ host is significantly different than in other host matrices. Fig. S3† reveals that the lifetime of the ⁴S_3/2 level of Er³⁺ ion in YVO₄ is an order of magnitude less than in Y₂O₃ and YPO₄ host matrices. The shorter lifetime of the ⁴S_3/2 level of Er³⁺ in YVO₄ could be attributed to wavefunction coupling between 3d orbital of V⁵⁺ and 4f orbital of Er³⁺. In YVO₄ the energy levels of ²H_11/2 and ⁴S_3/2 of Er³⁺ are close to the 3T₁ level of the V 3d orbital. These adjacent positions provoke strong coupling between the ²H_11/2, ⁴S_3/2 orbit of Er³⁺ and 3T₁ level of the V 3d orbital, leading to oscillator strength increase in of the Er³⁺ ions.²³ We should stress that Tolstik et al.²⁴ measured lifetimes of 28 μs and 2.3 ms of the ⁴I_11/2 and ⁴I_13/2 levels of the Er³⁺ ion in YVO₄ hosts, respectively. Both lifetimes are significantly lower than in Y₂O₃ and other host matrices^22,25 and could be responsible for the relative intensity reduction of the red emission band in samples hosted by YVO₄, causing the stronger green emission.

Fig. 5 Room temperature DS and UC emission spectra of the Y_1.86Yb_0.1Er_0.04O₃, Y_0.93Yb_0.05Er_0.02VO₄, and Y_0.93Yb_0.05Er_0.02PO₄ phosphors.

The CIE (international commission on illumination) diagram provide the parameters x and y to demonstrate the color perception. This includes the hue and saturation on a two dimensional chromaticity diagram. We have given the color coordinates of Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ in the CIE diagram shown in Fig. 6(a). The color perception reveals green (0.23, 0.74), yellowish (0.31, 0.67) blue-yellowish (0.30, 0.47) colors for the Y_0.947Yb_0.05Er_0.003VO₄, Y_1.894Yb_0.1Er_0.006O₃ and Y_0.947Yb_0.05Er_0.004PO₄ phosphors, respectively. It suggests that the color of the UC emission can also be tuned by modifications of the host lattices.

Fig. 6 (a) The CIE chromaticity diagram for the UC emission in Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄. (b) The UC emission of Y_0.947Yb_0.05Er_0.003PO₄ measured at different temperatures.

3.3 Temperature sensing

In the Er³⁺ ion, the ²H_11/2 and ⁴S_3/2 levels are thermally coupled to each other and the intensity ratio (R) of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 emission bands is expressed as a function of temperature for temperature measurements. Fig. 6(b) shows the UC emission temperature dependence of Y_0.945Yb_0.05Er_0.005PO₄ in the wavelength range 500–575 nm. While the peak positions do not reveal sensitivity to temperature alterations, R considerably changes. At temperatures below ∼370 K the emission of ²H_11/2 → ⁴I_15/2 band is less intense than the one of ⁴S_3/2 → ⁴I_15/2 transition. For elevated temperatures beyond ∼470 K, however, the situation is opposite. The relative populations of the ²H_11/2 and ⁴S_3/2 levels follow the Boltzmann type distribution function, which leads to,where, I₅₂₃ and I₅₄₈ are the integrated intensity of the ²H_11/2 → ⁴I_15/2 (515–540 nm) and ⁴S_3/2 → ⁴I_15/2 (540–565 nm) bands, N is the number of ions, and B = ω_HA_Hg_H/ω_SA_Sg_S, with ω, A, g being the transition frequency, spontaneous emission rate from ²H_11/2 and ⁴S_3/2 to the ground level ⁴I_15/2, and the level degeneracy, respectively – and k is the Boltzmann factor, T is the temperature, and ΔE is the energy gap between the ²H_11/2 and ⁴S_3/2 levels.

Fig. 7(a) shows R(T) for (i) Y_0.947Yb_0.05Er_0.003VO₄, (ii) Y_1.894Yb_0.1Er_0.006O₃, and (iii) Y_0.947Yb_0.05Er_0.003PO₄. As expected, caused by the above discussed differences of the recombination behaviors in Fig. 3, R varies for all the three samples. Additionally, the R(T) trend can also be affected by change of non-radiative relaxation rates during the temperature increase, causing variations of the luminescence intensities and lifetimes of the single transitions.

Fig. 7 Temperature sensing performance of the samples: (a) R numbers of the 523 and 548 nm emission bands in (i) Y_0.947Yb_0.05Er_0.003VO₄, (ii) Y_1.894Yb_0.1Er_0.006O₃ and (iii) Y_0.947Yb_0.05Er_0.003PO₄, as a function of temperature. Temperature sensitivity in (b) Y_0.947Yb_0.05Er_0.003VO₄, (c) Y_1.894Yb_0.1Er_0.006O₃, and (d) Y_0.947Yb_0.05Er_0.003PO₄ vs. temperature.

The R fits displayed by the solid lines in Fig. 7(a) results in ΔE/k fit parameters of (i) 598, (ii) 599, and (ii) 581 for Y_0.947Yb_0.05Er_0.003VO₄, Y_1.894Yb_0.1Er_0.006O₃, and Y_0.947Yb_0.05Er_0.003PO₄, respectively. The result demonstrates the proximity of the average energy gap between the levels ²H_11/2 and ⁴S_3/2 in all three investigated phosphors. The sensitivity (S) of the sensor is defined as,

where the symbols have the same meanings as in eqn (1). Fig. 7(b)–(d) presents the calculated S(T) trend. The maximum sensitivity in Y_0.947Yb_0.05Er_0.003VO₄, Y_1.894Yb_0.1Er_0.006O₃, and Y_0.947Yb_0.05Er_0.003PO₄ phosphors is found to be 0.0105, 0.0077, 0.0035 K⁻¹ at 499, 488, and 486 K, respectively.

It is important to mention that the temperature sensitivity in Y_0.947Yb_0.05Er_0.003PO₄ phosphor is significantly lower than in other two phosphors. This might be because of the higher phonon energy of the host lattice, which increases non-radiative relaxation from the ²H_11/2 to the ⁴S_3/2 level, resulting in the increase in the intensity of 548 nm emission with respect to the 523 nm emission (leading to smaller R). The value of 0.0105 maximum sensitivity observed in Y_0.947Yb_0.05Er_0.003VO₄ is probably the highest sensitivity reported for the luminescence intensity ratio based temperature sensors.^1–12 The maximum sensitivity in all the three phosphors is observed ∼490 K, while, above this temperature, sensitivity starts to decrease. This might be because of the resonance of thermal phonon energy with the energy separation of the levels and/or the population saturation of the ²H_11/2 level at increasing temperatures.

4. Conclusion

Summarizing, X-ray data analysis reveals the cubic structure for Y_1.894Yb_0.1Er_0.006O₃ and the tetragonal structure for Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄. The optical studies show that the phonon energy and crystal structure of the host lattices affect the UC emission intensity of the blue, green and red emission bands, as well as the Stark splitting of the levels. The R value in Y_0.947Yb_0.05Er_0.003PO₄ is significantly lower than in Y_1.894Yb_0.1Er_0.006O₃ and Y_0.947Yb_0.05Er_0.003VO₄ because of the higher phonon energy in Y_0.947Yb_0.05Er_0.003PO₄, enforcing faster relaxation from the ²H_11/2 to the ⁴S_3/2 level. The maximum temperature sensitivity in all the three phosphors takes place in the vicinity of 490 K, which is the highest (0.0105 K⁻¹) in Y_0.947Yb_0.05Er_0.003VO₄.

Authors' contributions

The manuscript was written through the contributions of all the authors. A.K.S. and P.K.S. have contributed equally to this work and therefore both are the first author of this manuscript. All the authors have given approval to the final version of the manuscript.

Acknowledgements

PKS acknowledges UGC, India for the award of JRF fellowship. AKS thankfully acknowledges the DGAPA-UNAM program of post-doctoral fellowship. The work was partly supported by the DGAPA-UNAM PAPIIT project TB100213-RR170213 (PI Dr Bruno Ullrich). The authors also thankfully acknowledge DST, New Delhi for experimental facilities. The authors also acknowledge S. K. Singh for his support during the experiments and valuable discussions.

References

1. C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Millán, V. S. Amaral, F. Palacio and L. D. Carlos, New J. Chem., 2011, 35, 1177–1183.
2. F. Vetrone, R. Naccache, A. Zamarrón, A. J. de la Fuente, F. Sanz-Rodríguez, L. Martinez Maestro, E. M. Rodriguez, D. Jaque, J. G. Solé and J. A. Capobianco, ACS Nano, 2010, 4, 3254–3258.
3. L. H. Fischer, G. S. Harms and O. S. Wolfbeis, Angew. Chem., Int. Ed., 2011, 50, 4546–4551.
4. H. Peng, M. I. J. Stich, J. Yu, L. Sun, L. H. Fischer and O. S. Wolfbeis, Adv. Mater., 2010, 22, 716–719.
5. A. K. Singh, S. K. Singh, B. K. Gupta, R. Prakash and S. B. Rai, Dalton Trans., 2013, 42, 1065–1072.
6. B. Dong, B. Cao, Y. He, Z. Liu, Z. Li and Z. Feng, Adv. Mater., 2012, 24, 1987–1993.
7. R. Dey and V. K. Rai, Dalton Trans., 2014, 43, 111–118.
8. S. K. Singh, K. Kumar and S. B. Rai, Sens. Actuators, A, 2009, 149, 16.
9. X. Wang, J. Zheng, Y. Xuan and X. Yan, Opt. Express, 2013, 21, 21596–21606.
10. A. Pandey and V. K. Rai, Dalton Trans., 2013, 42, 11005–11011.
11. J. B. Yu, L. N. Sun, H. S. Peng and M. I. J. Stich, J. Mater. Chem., 2010, 20, 6975–6981.
12. B. Zelelow, G. E. Khalil, G. Phelan, B. Carlson, M. Gouterman, J. B. Callis and L. R. Dalton, Sens. Actuators, B, 2003, 96, 304–314.
13. F. Wang and X. Liu, Chem. Soc. Rev., 2009, 38, 976–989.
14. A. K. Singh, S. K. Singh, P. Kumar, B. K. Gupta, R. Prakash and S. B. Rai, Sci. Adv. Mater., 2014, 6, 405–412.
15. F. Auzel, Chem. Rev., 2004, 104, 139–173.
16. A. K. Singh, S. K. Singh and S. B. Rai, RSC Adv., 2014, 4, 27039–27061.
17. J. Rodriguez-Carvajal, FULLPROF a Rietveld and pattern matching analysis program, Laboratoire Leon Brillouin (CEA-CRNS), France, 2007.
18. K. Mishra, S. K. Singh, A. K. Singh and S. B. Rai, Mater. Res. Bull., 2013, 48, 4307–4313.
19. A. K. Parchur, A. I. Prasad, S. B. Rai, R. Tewari, R. K. Sahu, G. S. Okram, R. A. Singh and R. S. Ningthoujam, AIP Adv., 2012, 2, 032119.
20. B. F. Aull and H. P. Jenssen, Phys. Rev. B: Condens. Matter Mater. Phys., 1986, 34, 6640–6646.
21. L. Sorace, C. Benelli and D. Gatteschi, Chem. Soc. Rev., 2011, 40, 3092–3104.
22. H. Lu, W. P. Gillin and I. Hernandez, Phys. Chem. Chem. Phys., 2014, 16, 20957–20963.
23. W. Xu, B. Chen, W. Yu, Y. Zhu, T. Liu, S. Xu, X. Min, X. Bai and H. Song, Dalton Trans., 2012, 41, 13525–13532.
24. N. A. Tolstik, A. E. Troshin, S. V. Kurilchik, V. E. kisel, N. V. Kuleshov, V. N. Matrosov, T. A. Matrosova and M. I. Kupchenko, Appl. Phys. B: Lasers Opt., 2007, 86, 275–278.
25. A. Jaffrès, P. Loiseau, G. Aka, B. Viana, C. Larat and E. Lallier, Laser Phys., 2014, 24, 125801.

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Footnote

† Electronic supplementary information (ESI) available: UC emission spectra of Y_1.994Er_0.006O₃, Y_(1.9−2x)Yb_0.1Er_2xO₃, Y_(0.95−x)Yb_0.05Er_xVO₄ and Y_(0.95−x)Yb_0.05Er_xPO₄ (x = 0.003, 0.01 and 0.02) phosphors and luminescence decay curves of ⁴S_3/2 level of Er³⁺ ion in Y_1.894Yb_0.1Er_0.006O₃, Y_0.947Yb_0.05Er_0.003VO₄ and Y_0.947Yb_0.05Er_0.003PO₄ phosphors are given. See DOI: 10.1039/c4ra12637h

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