Upconversion luminescence and favorable temperature sensing performance of eulytite-type Sr3Y(PO4)3:Yb3+/Ln3+ phosphors (Ln=Ho, Er, Tm)

ABSTRACT Phase-pure eulytite-type Sr3Y0.88(PO4)3:0.10Yb3+,0.02Ln3+ upconversion (UC) phosphors (Ln = Ho, Er, Tm) were synthesized via gel-combustion and subsequent calcination at 1250°C. Their UC luminescence, temperature-dependent fluorescence intensity ratio of thermally and/or non-thermally coupled energy levels, and performance of optical temperature sensing were systematically investigated. The phosphors typically exhibit green, orange-red and blue luminescence under 978 nm NIR laser excitation for Ln = Er, Ho and Tm, respectively, which were discussed from two- and three-photon processes. The 524 nm green (Er3+), 657 nm red (Ho3+) and 476 nm blue (Tm3+) main emissions were analyzed to have average decay times of ~52 ± 2, 260.6 ± 0.7 and 117 ± 1 μs, respectively. It was shown that (1) the Er3+ doped phosphor has a better overall performance of temperature sensing with thermally coupled 2H11/2 and 4S3/2 energy levels, whose maximum absolute (SA) and relative (SR) sensitivities are ~5.07 × 10−3 K−1 at 523 K and ~1.16% at 298 K, respectively; (2) the Ho3+ doped phosphor shows maximum SA and SR values of ~0.019 K−1 (298–573 K) and 0.42% at 573 K for the non-thermally coupled energy pairs of 5F5/(5F4,5S2) and 5I4/5F5, respectively; (3) the Tm3+ doped phosphor has a maximum SA of ~12.74 × 10−3 K−1 at 573 K for the non-thermally coupled 3F2,3/1G4 energy levels and a maximum SR of ~1.74% K−1 at 298 K for the thermally coupled 3F2,3/3H4 levels. Advantages of the current phosphors in optical temperature sensing were also revealed by comparing with other typical UC phosphors.


Introduction
Upconversion (UC) luminescence is a process of converting low-energy light, usually near-infrared (NIR) or infrared, into high-energy light (ultraviolet or visible) through multiple absorption and/or energy transfer [1][2][3]. UC materials are drawing extensive attention due to their wide applications in the fields of solid-state lasers, multi-color displays, optical communication, wavelength converters for solar cells, bio-imaging, optical temperature sensors, and so forth [1][2][3][4]. A UC phosphor is usually formed by doping a host lattice with a sensitizer/activator pair, and the Yb 3+ /Ln 3+ combination (Ln = Ho, Er, Tm) is the most popular since the 2 F 7/2 -2 F 5/2 transition of Yb 3+ possesses a large absorption cross-section for~980 nm NIR excitation light and can well resonate with the ladder-like energy levels of Ho 3+ , Er 3+ and Tm 3+ activators [1][2][3]. The host lattice for UC should assure not only a satisfactory luminescence efficiency, but also excellent physicochemical stability, safety, and low toxicity. A handful of inorganic compounds have been developed as UC host so far, typically including fluorides [5,6], oxides [7,8], oxysulfides [9], phosphates [10,11] and other oxygenates [12][13][14][15], and new hosts are also under active exploration and/or perfection.
Optical temperature sensing with UC phosphor, most frequently investigated in the range of~293-573 K, gained increasing research interest during recent years, which utilizes the fluorescence intensity ratio (FIR) of two emission bands that involve either thermally coupled or non-thermally coupled energy levels of the luminescent ion [4,16]. The FIR technique is usually independent of spectrum loss and excitation-power fluctuation, and may thus provide a high detection resolution and excellent sensitivity [17][18][19]. The FIR of thermally coupled emission levels is a function of temperature and obeys the Boltzmann distribution of electrons [11]. Since the energy separation ΔE of such levels is usually restricted to 200-2000 cm −1 to avoid strong overlapping of two emission bands [16][17][18][19], the sensitivity of temperature sensing, which is proportional to ΔE, can hardly be further improved to a higher level according to the Boltzmann distribution. For this reason, the use of non-thermally coupled energy levels is being considered as an effective complement to a better sensitivity of FIR. Wang et al. [4] have recently reviewed the rare-earth ions, host lattices, electronic transitions and emission wavelengths that have been used for the purpose of optical temperature sensing, together with the temperature ranges of sensing and the absolute/relative sensitivities of FIR.
We thus originally synthesized in this work a series of eulytite-type Sr 3 Y(PO 4 ) 3 :Yb 3+ /Ln 3+ UC phosphors (Ln = Ho, Er, Tm) via gel-combustion, followed by a thorough investigation of their UC luminescence and performance of optical temperature sensing with thermally coupled and/or nonthermally coupled energy levels. The high cation homogeneity of sol-gel processing allowed phase-pure products to form by calcination at the lower temperature of 1250°C for 4 h. The current phosphors were also compared with other typical UC systems to show their advantages in optical temperature sensing. In the following sections, we report the synthesis, characterization and optical properties of Sr 3 Y(PO 4 ) 3 3 and Sr(NO 3 ) 2 were dissolved in an aqueous solution (20 ml) of EDTA-NH 4 OH to chelate the RE 3+ and Sr 2+ cations (total cation to EDTA molar ratio = 1:1), followed by the addition of a stoichiometric amount of NH 4 H 2 PO 4 . The mixture was evaporated by heating at 85°C under continuous magnetic stirring to form a sol and then a viscous white gel. Auto-ignition of the gel took place upon increasing the temperature to~300°C on a resistance oven, which produced a loosely packed black precursor powder. The targeted phosphor was then produced by calcining the precursor in flowing oxygen (200 ml/min) at 1250°C for 4 h [20,21], using a heating rate of 8°C/min at the ramp stage.

Characterization
Phase identification was performed via X-ray diffractometry (XRD, SmartLab, Rigaku, Tokyo, Japan) under 40 kV/200 mA, using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm) and a scanning speed of 4.0º 2θ per minute. Crystal structure refinement of the product was carried out using the TOPAS 3.0 program [13], and the XRD data for this purpose were acquired in the step scan mode using a step size of 0.02°and an accumulation time of 1.8 s per step. Powder morphology was analyzed via field-emission scanning electron microscopy (FE-SEM, Model S-5000, Hitachi, Tokyo) under an acceleration voltage of 10 kV. UV-Vis spectroscopy was performed at room temperature on a UV-VIS-NIR spectrometer (Model UV-3600 Plus, Shimadzu Co., Kyoto, Japan) equipped with a 150mm diameter integrating sphere (Model ISR-1503, Shimadzu Co.). UC luminescence of the phosphor was analyzed under 978 nm CW-laser excitation (Model KS3-12322-105, BWT Beijing Ltd., Beijing, China) on an FP-8600 fluorospectrophotometer (JASCO, Tokyo) installed with a heating controller (Model HPC-836, JASCO). Fluorescence decay kinetics was analyzed under 980 nm pulsed laser excitation on a steady-state and transient photoluminescence spectrometer (Model FLS1000, Edinburgh Instruments Ltd., Livingston, UK).   indicate that the products were well crystallized. The synthesis temperature is about100°C lower than that (1360°C) needed for the synthesis of Ba 3 La(PO 4 ) 3 : Yb 3+ /Ln 3+ (Ln = Er, Tm) via solid reaction [14], which may originate from the better cation homogeneity of sol-gel processing. The crystal structure of Sr 3 Y(PO 4 ) 3 can be viewed as a three-dimensional connection of [PO 4 ] 3tetrahedrons and [(Sr/Y)-O] polyhedrons via corner sharing, where all the [PO 4 ] 3are totally independent while the Sr/Y polyhedrons share edges with each other to form a three-dimensional network [26]. In such a structure, the Sr 2+ /Y 3+ cations are randomly disordered over a single 16c crystallographic site (C 3 point symmetry) [30] but the [PO 4 ] 3tetrahedrons show three different orientations in response to three sets of partially occupied oxygen positions O 1 , O 2 and O 3 [31]. It is noteworthy that Sr 2+ and Y 3+ have different coordination environments although they occupy the same lattice site. Specifically, the Y 3+ ion resides in YO 6 octahedron distorted by three equally short and three equally long Y-O bonds [32] while the Sr 2+ ions have the two coordination environments of CN = 6 and CN = 9 (CN: coordination number) [20]. Accordingly, Sr 3 Y(PO 4 ) 3 presents not only cation disorder but also disorder in the oxygen sublattice [20,32]. In this work, the Yb 3+ and Ln 3+ dopants were expected to replace Y 3+ by valence and size preference (ionic radius r= 90.0, 90.1, 89.0, 88.0 and 86.8 pm for Y 3+ , Ho 3+ , Er 3+ , Tm 3+ and Yb 3+ under CN = 6; r= 118 and 131 pm for Sr 2+ under CN = 6 and 9, respectively) [33]. Based on this information, Rietveld refinement of the XRD pattern was performed using the standard crystallographic data of isostructural Sr 3 La(PO 4 ) 3 (ICSD No. 69432) as initial structure model. Figure 1(b) shows the experimental and calculated XRD profiles for the Sr 3 Y 0.88 (PO 4 ) 3 :0.10Yb 3+ ,0.02Er 3 + representative, while the derived coordinates and site occupancy factors (SOF) of atoms are summarized in Table S1. The refinement was ended up with the well-acceptable reliability factors of R wp = 8.28%, R p = 6.12%, R exp = 3.83% and χ 2 = 2.16, and yielded a lattice parameter (a= b= c) of~10.1043 ± 0.0002 Å and cell volume V of~1031.62 ± 0.05 Å 3 . Similar analysis found the a and V values of~10.1045 ± 0.0004 Å and 1031.68 ± 0.12Å 3 for the Yb 3+ /Ho 3+ doped and~10.1024 ± 0.0006 Å and 1031.04 ± 0.18 Å 3 for the Yb 3+ /Tm 3+ doped phosphor powders. The cell constants are all smaller than that (10.1091 Å) of Sr 3 Y(PO 4 ) 3 in the standard diffraction file, owing to the smaller average ionic radius of Yb 3+ /Ln 3+ pair, and tend to decrease toward a smaller Ln 3+ . The above results thus provided persuasive evidence of solid-solution formation.

Results and discussion
FE-SEM observations (Figure 2(a-c)) reveal that the Sr 3 Y 0.88 (PO 4 ) 3 :0.10Yb 3+ ,0.02Ln 3+ products contain aggregated primary particles/crystallites of~2.0-6.0 μm, which is typical of a gel-combustion product [20,21], and the type of Ln has no appreciable influence on overall morphology of the powder. Elemental mapping via energy dispersive X-ray spectroscopy (EDS), with the Sr 3 Y 0.88 (PO 4 ) 3 :0.10Yb 3+ ,0.02Er 3+ sample as a representative, found that all the elements of concern are quite evenly distributed among the particles ( The above EDS and XRD analyses confirmed that a solid-solution product with the intended chemical composition has been formed. Figure 3 shows the UV-Vis diffuse reflectance spectra of the Sr 3 Y(PO 4 ) 3 :Yb 3+ /Ln 3+ powders, where the broad band in the spectral range of ~200-360 nm and that centered at~978 nm, which are common to the three samples, can be assigned to absorption by the Sr 3 Y(PO 4 ) 3 host and 2 F 7/2 → 2 F 5/2 transition of Yb 3+ , respectively. In addition, the Ho 3 + activator clearly shows transitions its 5 F i → 5 I 8 (i= 2, 3, 4 and 5) transitions at~453, 486, 541 and 644/657 nm (Figure 3(a)), Er 3+ exhibits transitions from the 4 F 7/2 , 2 H 11/2 , 4 S 3/2 and 4 F 9/2 energy states to 4 I 15/2 ground state at~489, 523, 546, 654 nm ( Figure 3(b)), and Tm 3+ shows transitions from the 1 G 4 , 3 F 2,3 and 3 H 4 levels to 3 H 6 ground state at~474, 690 and 796 nm (Figure 3(c)), respectively. The results thus imply that the Sr 3 Y(PO 4 ) 3 :Yb 3+ /Ln 3+ powders can effectively absorb 978 nm laser excitation for UC luminescence. The energy bandgap of Sr 3 Y(PO 4 ) 3 :Yb 3+ /Ln 3+ can be estimated from the reflectance spectra according to Equation (1) [34,35] where hv is the incident photon energy, A is a proportional constant, E g is the value of bandgap, n= 2 for a direct transition or 1/2 for an indirect transition, and F(R ∞ ) is the Kubelka-Munk function which is defined as [36,37] where R, K and S are the reflection, absorption and scattering coefficients, respectively. The [F(R ∞ )hv] 1/2 vs hv plots are shown in Figure S2, where extrapolating the linear portions to [F(R ∞ )hv] 1/2 = 0 yielded the similar E g values of~3.37 eV. The E g value is also close to those reported for the isostructural Ba 3 La(PO 4 ) 3 (3.46 eV) [38], Ba 3 Y(PO 4 ) 3 (3.15 eV) [39] and Sr 3 Gd(PO 4 ) 3 (3.49 eV) [40] compounds synthesized by solid reaction.  [41,42], respectively. Increasing power of excitation did not bring about any change to peak position but monotonically raised the emission intensity of each band. The Commission International de L'Eclairage (CIE) chromaticity coordinates of UC luminescence are summarized in Figure 4(d) and  Figure S3(a)). Under 2.00 W laser pumping, vivid and strong green emission was observed for Sr 3 Y 0.88 (PO 4 ) 3 :0.10Yb 3+ ,0.02Er 3+ with naked eyes, as shown by the inset photograph taken for the appearance of luminescence in Figure 4(d).