English

• 1. INTRODUCTION

  Optical thermometers based on rare earth ion-doped upconversion (UC) phosphors have been given more attention due to their excitation in the near-infrared (NIR) region and suitable two thermally coupled energy levels (TCLs)^[1-3]. Generally, the TCLs could be found in many rare earth ions, such as Er³⁺ (⁴S_3/2, ²H_11/2), Ho³⁺ (⁵F_{2, 3}/³K₈, ⁴F₁/⁵G₆), Tm³⁺ (³F_{2, 3}, ³H₄), etc^[4-6]. Among them, the temperature sensors based on the green upconversion (UC) emission from TCLs of ²H_11/2 and ⁴S_3/2 of Er³⁺ ions have gained wide attention. And the Yb³⁺ ion is usually combined and used as a sensitizer to enhance UC efficiency. Although several methods have been reported by researchers to enhance the sensitivity of Er³⁺-doped systems, most Er³⁺-doped materials are still suffering from low sensing sensitivity^{[7, 8]}.

  The Tb³⁺/Yb³⁺ codoped system deserves special attention due to its good performance in the UC processes^{[9, 10]}. The Tb³⁺ ion exhibits high quantum efficiencies for luminescence due to the large energy gap between the ⁷F_J ground state and the ⁵D₄ excited state, which results in the lack of multiphonon relaxation of the ⁵D₄ excited state and the high usage of this ion as a dopant in efficient phosphors^{[11, 12]}. But the Tb³⁺/Yb³⁺ codoped system shows poor thermal performance, which is the emission intensities that are decreased fast as the temperature increases. Therefore, the introduction of Er³⁺ in Tb³⁺/Yb³⁺ codoped system may be a valid strategy to improve sensitivity.

  According to the consideration stated above, herein Tb³⁺/Yb³⁺ co-doped and Er³⁺/Tb³⁺/Yb³⁺ tridoped NGW phosphors are directionally synthesized. The Na₅Gd(WO₄)₄ (NGW) crystal is a member of Na₅RE(WO₄)₄ (RE = Y, La, Eu, Lu) family, which were first reported by L. G. Sillén, et al, and have been considered to be efficient luminescent host lattices^[13-16]. The crystal structure of NGW crystal was determined by Wang^[17]. NGW crystal is related to the scheelite structure of CaWO₄ and the Gd site symmetry is the same as the cationic site symmetry in CaWO₄ (S₄)^[18]. In addition, the metal-to-metal intervalence charge transfer (IVCT) processes between lanthanide (Tb³⁺) and d⁰ electron configured transition metal ions (W⁶⁺) in oxide crystals have been demonstrated to be an effective pathway to excite the corresponding lanthanide ions.

  In the present work, we report the synthesis of Tb³⁺/Yb³⁺ co-doped and Er³⁺/Tb³⁺/Yb³⁺ tri-doped NGW phosphors using conventional high temperature solid state reaction process for the first time. The color-tunable emission in NGW: Er³⁺/Tb³⁺/Yb³⁺ phosphors by the modulation of temperature was realized for the first time. In summary, the Er³⁺/Tb³⁺/Yb³⁺ tri-doped NGW phosphors show wide-range-tunable blue-green and yellow-green emissions under 980 nm excited, which would provide a new strategy to design novel optical temperature sensing materials.

  2. EXPERIMENTAL

  2.1 Materials and synthesis

  Tb³⁺/Yb³⁺ co-doped and Tb³⁺/Er³⁺/Yb³⁺ tri-doped NGW polycrystalline powder samples were synthesized by means of the solid-state reaction method. The raw materials were Na₂CO₃ (A.R. (Analytical Reagent)), Gd₂O₃ (99.99%), WO₃ (A.R.), and the doping concentrations of Tb₄O₇ (99.99%), Er₂O₃ (99.99%) and Yb₂O₃ (99.99%) are 5%, 0.1% and 10% of Gd³⁺, respectively. The stoichiometric amount of raw materials was thoroughly mixed by ground in an agate mortar. The mixture of starting materials was then transferred to crucibles and had been subsequently heated at 973 K for 24 h in air atmosphere. As the muffle furnace was cooled to room temperature naturally, and the final products were collected and ground to powder for further characterization.

  2.2 Measurements and characterization

  Phase purity of the synthesized powders was checked by the X-ray powder diffraction (XRD), which was carried out in an angular range 15^o < 2θ < 65^o using Ni-filtered CuKα radiation on a Rigaku MiniFlex Ⅱ diffractometer working in Bragg-Brentano (θ/2θ) geometry. The measured X-ray powder diffraction patterns of the Tb³⁺/Yb³⁺ co-doped and Tb³⁺/Er³⁺/Yb³⁺ tri-doped NGW were in accordance with the simulated ones calculated from the single crystal data^[17].

  Photoluminescence excitation (PLE) and photoluminescence (PL) spectra were recorded by a spectrometer (FLS980, Edinburgh Instruments) equipped with both continuous (450 W) and pulsed xenon lamps as the light source and a Hamamatsu R955 photomultiplier tube (PMT) as the detector. UC luminescence spectra were measured upon 980 nm NIR excitation from a continuous-wave diode laser. The temperature related spectra measurements were performed on a temperature controlling stage (THMS 600). The temperature stabilization time was 90 s with a temperature tolerance set to ±5 K.

  3. RESULTS AND DISCUSSION

  3.1 Structure and phase identification

  The crystallographic analyses in ref. [17] manifested that NGW crystallizes in the tetragonal system with space group I4₁/a (No. 88). The lattice parameters are: a = 11.4937(1), b = 11.4937(1), c = 11.3998(3) Å, V = 1505.97(5) ų and Z = 4. In the refined structure as plotted in Fig. 1, the crystalline structure is constituted by completely isolated WO₄ tetrahedra and Na⁺/Gd³⁺ cations, which are residing between the WO₄ tetrahedra for charge balance.

  Figure 1

  

  Figure 1.  A view of the structure unit of the NGW crystal

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  The XRD measurements for all samples were carried out at room temperature. The XRD patterns of samples NGW: Tb³⁺/Yb³⁺, NGW: Tb³⁺/Er³⁺/Yb³⁺ and the calculated XRD pattern of NGW are shown in Fig. 2. All peaks of XRD for the synthesized samples matched well with the XRD pattern for pure NGW, which was calculated by Diamond program from the crystal parameters of NGW, while no other peaks can be observed^[17]. The results of XRD analysis confirmed that doping of Tb³⁺/Er³⁺/Yb³⁺ ions did not generate obvious distortion in the host structure. Furthermore, based on the effective ionic radii of cations reported by Shannon, as the ionic radius of Gd³⁺ (N = 8, 1.06 Å) is close to that of Tb³⁺ (N = 8, 1.04 Å), Er³⁺ (N = 8, 1.00 Å) and Yb³⁺ (N = 8, 0.98 Å), the Tb³⁺/Er³⁺/Yb³⁺ ions are expected to occupy the Gd³⁺ sites^[19].

  Figure 2

  

  Figure 2.  XRD patterns of the Tb³⁺/Yb³⁺ co-doped and Tb³⁺/Er³⁺/Yb³⁺ tri-doped NGW samples and simulated X-ray powder diffraction patterns of NGW crystal

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  3.2 Photoluminescent properties

  Fig. 3 shows the PLE spectra of NGW: Tb³⁺/Yb³⁺ and NGW: Tb³⁺/Er³⁺/Yb³⁺ in a range of 220~550 nm, monitored at 544 nm (Tb³⁺: ⁵D₄ → ⁷F₅). In Fig. 3, the broad excitation band located around 250 nm in both spectra is 4f⁸ → 4f⁷5d¹ transition of Tb³⁺ ions and Tb³⁺~W⁶⁺ intervalence charge transfer (IVCT) states^[20]. The remaining excitation peaks with narrow character appear in the range of 270~500 nm that are associated with typical intra-4f forbidden transitions of the Tb³⁺ ions, which are attributed to the ⁷F₆ → ⁵I₇ + ⁵F₃ (273 nm), ⁷F₆ → ⁵H₆ (305 nm), ⁷F₆ → ⁵H₇ (311 nm), ⁷F₆ → ⁵D₀ (318 nm), ⁷F₆ → ⁵L_6-8, ⁵G₂ (340 nm), ⁷F₆ → ⁵L₉ (352 nm), ⁷F₆ → ⁵G₅ (359 nm), ⁷F₆ → ⁵L₁₀ (369 nm), ⁷F₆ → ⁵D₃, ⁵G₆ (378 nm) and ⁷F₆ → ⁵D₄ (488 nm) transitions^[21]. Moreover, both spectra are almost the same, which indicate that the multiplet ⁵D₄ of Tb³⁺ in both NGW: Tb³⁺/Yb³⁺ and NGW: Tb³⁺/Er³⁺/Yb³⁺ could be excited by 4f⁸ → 4f⁷5d¹ transition, IVCT bands and intra-4f forbidden transitions of the Tb³⁺ ions, and unchanged by the introduction of Er³⁺ ions.

  Figure 3

  

  Figure 3.  PLE spectra of NGW: Tb³⁺/Yb³⁺, NGW: Tb³⁺/Er³⁺/Yb³⁺ monitored at 544 nm

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  The PL spectra of NGW: Tb³⁺/Er³⁺/Yb³⁺ measured under 365 nm excitation at 300 K are illustrated in Fig. 4. It can be observed intuitively that NGW: Tb³⁺/Er³⁺/Yb³⁺ demonstrates a typical green emission. The luminescence signals in the range of 400~640 nm are attributed to the characteristic ⁵D₃ → ⁷F_j (j = 4, 5, 6) and ⁵D₄ → ⁷F_j (j = 3, 4, 5) transitions of Tb³⁺. Among these peaks, that corresponding to the ⁵D₄→⁷F₅ transition at 545 nm (green emission) which shows maximum luminescence intensity is the most prominent transition in all emission spectra. As in the schematic configurational coordinate diagram of Tb³⁺ and Yb³⁺ ions shown in Fig. 5, when the sample is excited by 365 nm, corresponding to ⁷F₆→⁵D₃ transition, the populations at ⁵D₃ levels are relaxed to the ⁵D₄ level via multiphonon relaxations and cross relaxation. From the emission spectra in Fig. 4, the obvious emissions from ⁵D₃ and ⁵D₄ levels indicate these two relaxation pathways are efficient.

  Figure 4

  

  Figure 4.  PL spectra of NGW: Tb³⁺/Er³⁺/Yb³⁺ excited at 365 nm

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  Figure 5

  

  Figure 5.  PL spectra of NGW: Tb³⁺/Yb³⁺ and NGW: Tb³⁺/Er³⁺/Yb³⁺ excited at 980 nm

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  The UC emission spectra of NGW: Tb³⁺/ Yb³⁺ and NGW: Tb³⁺/Er³⁺/Yb³⁺ samples are shown in Fig. 5. Under 980 nm NIR laser excitation, typical blue (485 nm), green (545 nm), yellow (585 nm) and red (621 nm) emissions attributed to Tb³⁺: ⁵D₄→⁷F_j (j = 6, 5, 4, 3) transitions are easily detected in both samples. Besides, we observed that NGW: Tb³⁺/ Er³⁺/Yb³⁺ shows weak luminescent around 525 nm, which can be ascribed to the Er³⁺ (²H_11/2 → ⁴I_15/2). The emissions of Tb³⁺ ions are much stronger than the emissions of Er³⁺ due to the much higher concentration of Tb³⁺ ions than the Er³⁺ ions. The energy level diagram of Tb³⁺/Er³⁺/Yb³⁺ tri-doped system and possible UC pathways populating the emitting states were schematically illustrated in Fig.6. Under 980 nm laser excitation, the ground state absorption (GSA) of Yb³⁺ion: ²F_7/2 → ²F_5/2 and Er³⁺ ion: ⁴I_15/2 → ⁴I_11/2 take place by absorbing NIR photons. Usually, the population at the excited level ⁴I_11/2 mainly occurs through an efficient energy transfer (ET) from Yb³⁺ to Er³⁺: Yb³⁺ (²F_5/2) + Er³⁺ (⁴I_15/2) → Yb³⁺ (²F_7/2) + Er³⁺ (⁴I_11/2) because of the lager absorption cross section of Yb³⁺ than that of Er³⁺. For the green emission bands around 525 and 545 nm from ²H_11/2, ⁴S_3/2 → ⁴I_15/2 radiative transitions, the metastable ²H_11/2 and ⁴S_3/2 levels are populated by nonradiative (NR) transition from upper level ⁴F_7/2. The excited state of ⁴F_7/2 could be populated through the ET process: Yb³⁺ (²F_5/2) + Er³⁺ (⁴I_11/2) → Yb³⁺ (²F_7/2) + Er³⁺ (⁴F_7/2) or excited state absorption (ESA): Er³⁺ (⁴I_11/2) → Er³⁺ (⁴F_7/2). For the red emission band from the ⁴F_9/2 → ⁴I_15/2 transition, the population of levels ⁴F_9/2 could be realized through the NR from the ⁴S_3/2 level, and the ET process: Yb³⁺ (²F_5/2) + Er³⁺ (⁴I_13/2) → Yb³⁺ (²F_7/2) + Er³⁺ (⁴F_9/2) or ESA: Er³⁺ (⁴I_13/2) → Er³⁺ (⁴F_9/2) in which the ⁴I_13/2 level was populated through a ⁴I_11/2 → ⁴I_13/2 NR process.

  Figure 6

  

  Figure 6.  Schematic energy level diagram of Tb³⁺/Er³⁺/Yb³⁺ as well as the possible energy transfer mechanisms involved in the DC and UC processes under excitation of 365 and 980 nm, respectively

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  Besides, the Tb³⁺ ions have no excited states resonant with the Yb³⁺ transition ²F_7/2 → ²F_5/2 in the IR region, so it is generally assumed that the ET from Yb³⁺ to Tb³⁺ is a phonon-assisted cooperative sensitization process, where the energy of a pair of excited Yb³⁺ is transferred simultaneously to an adjacent Tb³⁺ (process CSU)^{[22, 23]}. Once Tb³⁺ is excited to ⁵D₄, it can be further excited to higher states through the absorption of a pump photon (excited-state absorption, ESA) or through ET from a third Yb³⁺ (ETU), but the absence of the emissions from ⁵D₃ indicates that the relaxation of higher states would not populate ⁵D₃ and might be through IVCT (Tb³⁺−W⁶⁺) to ⁵D₄.

  3.3 Thermal properties

  Fig. 7 presents the UC emission spectra of NGW: Tb³⁺/Er³⁺/Yb³⁺ and NGW: Tb³⁺/Yb³⁺ by exciting at 980 nm under different temperature. It can be noticed the transition intensity of Tb³⁺ shows a continuous decrease as the temperature rises, but the green transition intensities of Er³⁺ exhibit some difference. Fig. 8 depicts the normalized intensities of 525 nm emission by Er³⁺ (²H_11/2 → ⁴I_15/2), the Tb^{3+ 5}D₄ → ⁷F_j (j = 6, 5, 4, 3) transitions (490, 545, 586, 621 nm), and the overlapping 657 nm emission by Er³⁺ (⁴F_9/2→⁴I_15/2) and Tb³⁺ (⁵D₄ → ⁷F_/2) as a function of the absolute temperature. It can be noticed that the 525 nm (Er³⁺: ²H_11/2 → ⁴I_15/2) one has been largely enhanced with increasing temperature, which is due to the population of TCLs (²H_11/2 and ⁴S_3/2) abide by the Boltzmann distribution law. With the increase of internal temperature, enhanced lattice vibrations promote the nonradiative relaxation rate between two closely spaced energy levels, which keeps them quasi-thermal balanced. For Er³⁺ ions, the ΔE between ²H_11/2 and ⁴S_3/2 is about 800 cm⁻¹. The small energy gap leads to the fact that the ²H_11/2 level can be easily thermally populated from the ⁴S_3/2 level.

  Figure 7

  

  Figure 7.  Temperature dependence of photoluminescence intensities for NGW: Tb³⁺/Er³⁺/Yb³⁺ (a) and the corresponding normalized spectra (b), and NGW: Tb³⁺/Yb³⁺ (c) under 980 nm excitation

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  Figure 8

  

  Figure 8.  Intensity of 490, 525, 545, 586, 621 and 657 nm as the temperature increases

  DownLoad: Full-Size Img PowerPoint

  Besides, the thermal quenching mechanism of Tb³⁺ emissions has been researched, which is explained by thermally promoted crossover from the emitting ⁵D₄ level to the IVCT state and followed by feeding to ground states in a nonradiative way with raising the temperature^{[10, 24]}.

  In order to evaluate the colorimetric performance of the phosphors, the chromaticity coordinates for NGW: Tb³⁺/Er³⁺/Tb³⁺ were calculated on the basis of Commission International de L'Eclairage (CIE) 1931 color-matching functions. The chromaticity coordinates of NGW: Tb³⁺/Er³⁺/Tb³⁺ at various temperature under 980 nm excitation are listed in Table 1. The chromaticity coordinates are with slight variations around (0.334, 0.609) when the temperature is lower than 375 K. With the increase of temperature, the chromaticity coordinates change from (0.334, 0.609) at 375 K to (0.260, 0.585) at 550 K, and the changes of x and y values of CIE coordinate result in the color of photoluminescence changing from yellow-green to blue-green (Fig. 9). The results demonstrated that NGW: Tb³⁺/Er³⁺/Tb³⁺ phosphor may be used as optical temperature sensing materials.

  Table 1

  

  Table 1.  Chromaticity Coordinates of NGW: Tb³⁺/Er³⁺/Yb³⁺ at Various Temperature under 980 nm Excitation

  DownLoad: CSV

  Temperature (K)	CIE
  	x	y
  300	0.334	0.606
  325	0.334	0.609
  350	0.334	0.609
  375	0.332	0.609
  400	0.329	0.607
  425	0.323	0.607
  450	0.314	0.606
  475	0.300	0.604
  500	0.283	0.604
  525	0.263	0.606
  550	0.260	0.585

  Figure 9

  

  Figure 9.  CIE 1931 chromaticity diagram of NGW: Tb³⁺/Er³⁺/Yb³⁺ at various temperature under 980 nm excitation

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In summary, novel phosphors NGW with dopant Tb³⁺/Yb³⁺ and Tb³⁺/Er³⁺/Yb³⁺ have been successfully synthesized by the solid-state reaction method. Base on the TCL (²H_11/2 and ⁴S_3/2) of the Er³⁺ ion, the relative emission intensities are strongly temperature dependent. The introduction of Er³⁺ was used to modify the chromaticity coordinates for NGW: Tb³⁺/Yb³⁺, then providing a good color tunable property, which induces the shift of emission color from yellow-green to green for NGW: Tb³⁺/Er³⁺/Yb³⁺ as the temperature increases. All the results indicate that the NGW: Tb³⁺/Er³⁺/Yb³⁺ phosphor may be used as optical temperature sensing materials.

  
  
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• 

  Figure 1  A view of the structure unit of the NGW crystal

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  Figure 2  XRD patterns of the Tb³⁺/Yb³⁺ co-doped and Tb³⁺/Er³⁺/Yb³⁺ tri-doped NGW samples and simulated X-ray powder diffraction patterns of NGW crystal

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  Figure 3  PLE spectra of NGW: Tb³⁺/Yb³⁺, NGW: Tb³⁺/Er³⁺/Yb³⁺ monitored at 544 nm

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  Figure 4  PL spectra of NGW: Tb³⁺/Er³⁺/Yb³⁺ excited at 365 nm

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  Figure 5  PL spectra of NGW: Tb³⁺/Yb³⁺ and NGW: Tb³⁺/Er³⁺/Yb³⁺ excited at 980 nm

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  Figure 6  Schematic energy level diagram of Tb³⁺/Er³⁺/Yb³⁺ as well as the possible energy transfer mechanisms involved in the DC and UC processes under excitation of 365 and 980 nm, respectively

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  Figure 7  Temperature dependence of photoluminescence intensities for NGW: Tb³⁺/Er³⁺/Yb³⁺ (a) and the corresponding normalized spectra (b), and NGW: Tb³⁺/Yb³⁺ (c) under 980 nm excitation

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  Figure 8  Intensity of 490, 525, 545, 586, 621 and 657 nm as the temperature increases

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  Figure 9  CIE 1931 chromaticity diagram of NGW: Tb³⁺/Er³⁺/Yb³⁺ at various temperature under 980 nm excitation

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  Table 1.  Chromaticity Coordinates of NGW: Tb³⁺/Er³⁺/Yb³⁺ at Various Temperature under 980 nm Excitation

  Temperature (K)	CIE
  	x	y
  300	0.334	0.606
  325	0.334	0.609
  350	0.334	0.609
  375	0.332	0.609
  400	0.329	0.607
  425	0.323	0.607
  450	0.314	0.606
  475	0.300	0.604
  500	0.283	0.604
  525	0.263	0.606
  550	0.260	0.585

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