Doping Eu³⁺/Sm³⁺ into CaWO₄:Tm³⁺, Dy³⁺ phosphors and their luminescence properties, tunable color and energy transfer

Abstract

In order to obtain warm white light emission, Eu³⁺ or Sm³⁺ ions were incorporated into CaWO₄:Tm³⁺, Dy³⁺ phosphors via a one-step hydrothermal method without further sintering. The as-synthesized samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) spectra. Upon UV light (355 nm and 365 nm) excitation, the phosphors exhibit blue emission of Tm³⁺ ions, yellow-blue emission of Dy³⁺ ions and red emission of Eu³⁺ ions (or red-orange emission of Sm³⁺ ions). The quenching concentration of Dy³⁺ is determined to be about 1.0%. The critical distance between Tm³⁺ and Dy³⁺ has been calculated to be about 16.4 Å and the energy transfer from Tm³⁺ to Dy³⁺ occurs through the quadrupole–quadrupole interaction. The color tone of the obtained phosphors is easily modulated to warm-white-light by co-doped Eu³⁺ or Sm³⁺. Furthermore, the influence of the rare earth ion emission intensities under different excitation wavelengths (355 nm and 365 nm), and the doped concentration of Eu³⁺ or Sm³⁺ ions on the luminescence properties for the CaWO₄:Tm³⁺, Dy³⁺ phosphor were measured and discussed. These results reveal that this kind of phosphor is a potential candidate for white LEDs.

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1 Introduction

Currently, white light emitting diodes (w-LEDs) are widely used due to their longer lifetime, higher energy efficiency, greater reliability, and more environmentally friendly characteristics compared with conventional incandescent and fluorescent lamps.^1,2 Up to now, the most popular method for manufacturing w-LEDs is by combing a blue InGaN chip with the yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. However, this combination has a high correlated color temperature (CCT = 7756 K) and low color rendering index (Ra < 80) due to the lack of red component, which can not meet the warm-white-light demands.³ Consequently, w-LEDs fabricated using UV LED with a single host emission color tone phosphor have been investigated to improve the Ra and to tune the CCT.^4–7

The tungstate families have been long known as self-activated materials, and have extensive practical applications in our daily life. There are intensive researches on properties such as luminescence and scintillation, because of the higher light output and low intrinsic radioactivity of the compound.^8–12 Calcium tungstate (CaWO₄) crystal is a naturally occurring luminescent mineral with scheelite structure. It has been well known for its strong visible luminescence, and the optical properties of the scheelite crystal have been studied extensively for decades.^13,14 WO₄²⁻ group have broad and intense absorption bands, because of charge transfer from oxygen to metal in the near-UV region, most importantly, the WO₄²⁻ group can transfer the energy to the activator ions.^15–19 In the rare earth activated tungstate luminescence materials, WO₄²⁻ group can serve as not only hosts of lighting materials but also sensitizers to activators. Rare earth ions can act as sensitizers and activators because of high efficiency and abundance of emission colors based on 4f–4f or 5d–4f transitions.^20,21 The Dy³⁺ ions can emit yellow and blue light by UV light. It is of interest to obtain white phosphors by doping Dy³⁺, owing to the combination of blue and yellow emissions.²² The Tm³⁺ ions were usually used as blue light emission activators. Meanwhile, the ¹D₂ → ³F₄ transition of Tm³⁺ ions overlaps well with the ⁶H_15/2 → ⁴I_15/2 transition of Dy³⁺ ions in tungstate materials.²³ Hence, the energy transfer from Tm³⁺ to Dy³⁺ is expected so that it can enhance the yellow light emission intensities of Dy³⁺ ions. Recently, Tm³⁺ and Dy³⁺ co-doped phosphors had been prepared, the energy transfer and cool white light can be obtained for w-LEDs.^24–26 However, these phosphors can not meet the warm-white-light emissions of the indoor lighting demands for a lack of red component.²⁷ In order to generate the warm white light, the common red emission ions of Eu³⁺ and Sm³⁺ can be introduced, owing to the characteristic emissions of Sm³⁺ ions in the orange-red region and the efficiency characteristic emissions related to the large energy gap between the emitting state ⁵D_{0, 1, 2} and the excited states ⁷F_J (J = 1, 2, 3, 4, 5) for offering intense red composition of Eu³⁺,^28,29 they are expected to provide red emission in phosphor.^30–32 In the previous reports, using Eu³⁺ to tune the emission color tones of the phosphors has been reported.^23,33–36 However, there have been no reports on Sm³⁺ or Eu³⁺ ions co-doped in CaWO₄:Tm³⁺, Dy³⁺ phosphors to tune the colors up to now.

In this work, we report on the synthesis, structure, luminescence properties and energy transfer of a series of Tm³⁺, Dy³⁺, Eu³⁺/Sm³⁺ co-doped CaWO₄ phosphors, which can serve as a single-phase warm-white-light phosphor based on the UV-LEDs. The color emission of the obtained phosphors is easily modulated by changing the Eu³⁺ or Sm³⁺ contents. The efficient energy transfer and critical distance between Tm³⁺ and Dy³⁺ were calculated. In addition, the influence of the rare earth ions emission intensities under different excitation wavelengths (355 nm and 365 nm) and the effect of addition of Eu³⁺ or Sm³⁺ on the luminescence properties for CaWO₄:Tm³⁺, Dy³⁺ phosphor were investigated in detail.

2 Experimental section

2.1 Materials

Aqueous solutions of Tm(NO₃)₃, Sm(NO₃)₃, Dy(NO₃)₃ and Eu(NO₃)₃ were obtained by dissolving the rare earth oxides Tm₂O₃, Sm₂O₃, Dy₂O₃ and Eu₂O₃ in dilute HNO₃ solution (15 mol L⁻¹) under heating with agitation in ambient atmosphere. All the other chemicals were of analytic grade and used as received without further purification.

2.2 Preparation

A series of rare earth ions-doped CaWO₄ phosphors were synthesized by a facile hydrothermal process without further sintering treatment. For the synthesis of CaWO₄:0.5% Tm³⁺, 1.0% Dy³⁺, 1.0% Eu³⁺ phosphor, 0.05 mmol of RE(NO₃)₃ (including 0.01 mmol Tm(NO₃)₃, 0.02 mmol Dy(NO₃)₃, 0.02 mmol Eu(NO₃)₃) and 1.95 mmol Ca(NO₃)₂ were added into 100 mL flask. After vigorous stirring for 20 min, 2.0 mmol of Na₂WO₄·2H₂O was slowly added dropwise into the above solution. After additional agitation for 30 min, the as-obtained white mixing solution was transferred to a 50 mL Teflon bottle (filled up to 80% of its total volume) held in a stainless steel autoclave, and then heated at 180 °C for 16 h. Finally, as the autoclave was naturally cooled to room temperature, the precipitates were separated by centrifugation at 9000 rpm for 3 min, washed with deionized water and ethanol in sequence each three times, and then dried in oven at 60 °C for 12 h. The other samples were prepared in a similar procedure except for the doping appreciate content of rare earth ions.

2.3 Characterization

X-ray diffraction (XRD) was performed with a Rigaku D/max-RA X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) and Ni filter, operating at a scanning speed of 10° min⁻¹ in the 2θ range from 10 to 90°, 20 mA, 30 kV. The morphology of the samples was observed by field emission scanning electron microscope (FESEM) using a FEI XL-30 instrument. The excitation and emission spectra, and the luminescence decay curves of samples were measured using a HITACHI F-7000 Fluorescence Spectrophotometer equipped with a 150 W Xe lamp as the excitation source, operating at 700 V, scanning at 1200 nm min⁻¹. The excitation and emission slits were set at 5.0 nm and 2.5 nm, respectively. All of the measurements were performed at room temperature.

3 Results and discussion

3.1 Phase identification and morphology

Fig. 1a shows the XRD patterns of CaWO₄:Tm³⁺, Dy³⁺, Eu³⁺ or Sm³⁺. All diffraction peaks of the as-synthesized samples can be well indexed to the standard data of CaWO₄ (PDF#85-0443), indicating that all the prepared samples are purity phase and the rare earth ions have been successfully co-doped into the CaWO₄ host lattice without making significant changes to the host lattice. As shown in Fig. 1b, it is worth pointing out that the positions of the (112) peaks slightly shift to the higher angle, owing to the smaller ionic radius for Tm³⁺ ions (0.994 Å, CN = 8), Dy³⁺ ions (1.027 Å, CN = 8), Eu³⁺ ions (1.066 Å, CN = 8) and Sm³⁺ ions (1.079 Å, CN = 8) compared with that of Ca²⁺ (1.12 Å).³⁷ Therefore, based on the comparison of the ionic radius, it could offer a great opportunity for Tm³⁺, Dy³⁺, Eu³⁺ and Sm³⁺ doping ions to enter into the site of Ca²⁺ ions in the CaWO₄ crystal.

Fig. 1 (a) XRD patterns of CaWO₄:2.0% Tm³⁺, 1.0% Dy³⁺ (A), CaWO₄:0.5% Tm³⁺, 1.0% Dy³⁺, 5.0% Eu³⁺ (B), CaWO₄:0.5% Tm³⁺, 1.0% Dy³⁺, 5.0% Sm³⁺ (C). The standard pattern of CaWO₄ (PDF#85-0443) is presented at the bottom for comparison. (b) Magnified XRD patterns in the region between 27 and 30 degree for the phosphors.

The morphology and size of the phosphors are important for their application in coatings on lighting devices.³⁸ The FESEM images shown in Fig. 2a–c present the morphology of CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, 5% Eu³⁺ phosphor at different magnification, it takes on a uniform dumbbell-like shape with the average length of 2 µm and the average diameters of 1 µm. Kniep et al. have pioneered the formation of fluorapatite crystallites by a double diffusion technique in gelatin. They proposed that the growth of dumbbell-like fluorapatite begins with elongated hexagonal-prismatic seeds and then proceeds with a fractal branching stage of outgrowths of needle-shaped prisms. Finally, dumbbell-shaped aggregates were formed. The physical mechanism of this process has also been explored, revealing that an intrinsic electric dipolar field along the elongated seeds causes the fractal growth of the dumbbell-like particles.^39–41 EDS shown in Fig. 2d reveals the chemical composition of the sample, containing O, W, Ca, Tm, Dy and Eu. Combined with the above XRD patterns, the samples were further confirmed to be CaWO₄:Tm³⁺, Dy³⁺, Eu³⁺.

Fig. 2 FESEM images (a–c) and EDS spectrum (d) of CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, 5% Eu³⁺ phosphor.

3.2 Photoluminescence properties and energy transfer of CaWO₄:Tm³⁺, Dy³⁺ phosphors

Fig. 3 exhibits the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Tm³⁺ ions doped CaWO₄ phosphors. In the PLE spectrum, monitored the emission at 454 nm, a broad band near 250 nm is due to the charge transfer band (CTB) of WO₄²⁻ (ref. 42 and 43) and a strong peak at 359 nm is due to ³H₆ → ¹D₂ transition of Tm³⁺ ions, which is matched well with the near-UV LEDs chips. Upon the excitation of 359 nm, the PL spectrum show a strong emission band centered at 454 nm, which is attributed to the electronic dipole transition of ¹D₂ → ³F₄ of Tm³⁺. The inset exhibits the dependence of the emission intensity of the 454 nm on the doping concentration of the Tm³⁺, the intensity reaches the maximum when the concentration of the Tm³⁺ ions is 2.0%. The concentration quenching may be caused by the cross-relaxation mechanism.⁴⁴

Fig. 3 PL and PLE spectra of CaWO₄:Tm³⁺ phosphor. Inset: the dependence of emission intensity on Tm³⁺ concentration.

Fig. 4a shows the PLE and PL spectra of CaWO₄:2% Tm³⁺, 1% Dy³⁺ phosphor. When monitored by the characteristic emission of Tm³⁺ (454 nm), the PLE spectrum of CaWO₄:2% Tm³⁺, 1% Dy³⁺ is similar to that of CaWO₄:2% Tm³⁺ (monitored at 454 nm) except for the difference of the relative intensity. Meanwhile, the PLE spectrum of CaWO₄:2% Tm³⁺, 1% Dy³⁺ (monitored at 575 nm) show a broad excitation band attributed to CTB transition within the WO₄²⁻ groups and some absorption peaks at 325, 353, 366 and 389 nm corresponded to the ⁶H_15/2 → ⁶P_3/2, ⁶H_15/2 → ⁶P_7/2, ⁶H_15/2 → ⁶P_5/2, ⁶H_15/2 → ⁴I_13/2 transition of Dy³⁺ ions, respectively. Upon 359 nm excitation, the PL spectrum of CaWO₄:2% Tm³⁺, 1% Dy³⁺ shows a strong blue emission assigned to the ¹D₂ → ³F₄ transition (454 nm) and two very weak yellow-blue emissions assigned to the ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2 transitions (575 and 487 nm), which provides an evidence for energy transfer from Tm³⁺ to Dy³⁺. Upon 353 nm excitation, the PL spectrum of CaWO₄:2% Tm³⁺, 1% Dy³⁺ consists of a very weak blue emission of Tm³⁺ (454 nm) and two strong yellow-blue emissions of Dy³⁺ (575 and 487 nm). As shown in Fig. 4a, upon 353 and 359 nm excitation, the emission intensities of Tm³⁺ (454 nm) and Dy³⁺ (575 nm) were very weak, respectively. In order to enhance the emission intensities of Tm³⁺ and Dy³⁺ in the CaWO₄ phosphors, selecting a suitable excitation wavelength is very necessary. Fig. 4b shows the cross-sectional area of the excitation bands of Tm³⁺ and Dy³⁺ ions with equal energy levels. Thus, the most effective excitation is estimated to this cross-sectional area of 355 and 365 nm as the excitation wavelength to generate relative stronger emissions of Tm³⁺ as well as Dy³⁺ ions. The PL emission spectra shown in Fig. 4a excited by 355 and 365 nm indicate the relative stronger emissions of Tm³⁺ as well as Dy³⁺ ions.

Fig. 4 PL and PLE spectra for CaWO₄:2% Tm³⁺, 1% Dy³⁺ under different excitation and emission wavelengths (a), PLE spectrum of the CaWO₄:2% Tm³⁺ and CaWO₄:1% Dy³⁺ (b).

In order to evaluate the energy transfer between the Tm³⁺ and Dy³⁺, the PLE spectrum of CaWO₄:1% Dy³⁺ (dash line) and the PL spectrum of CaWO₄:2% Tm³⁺ (solid line) were measured. As shown in Fig. 5, the PLE spectrum of Dy³⁺ ions consists of some peaks located at 428, 455 and 477 nm, corresponding to the transitions of Dy³⁺ ions from the ⁶H_15/2 to the ⁴G_11/2, ⁴I_15/2 and ⁴I_9/2 levels, respectively, which are in good agreement with the Dy³⁺ ion excitation wavelength. The solid line is the ³H₆ → ¹D₂ emission of Tm³⁺, we can observe the strong peak at 454 nm. From Fig. 5, it is reveals that there is a significant spectral overlap between the emission band of Tm³⁺ and the excitation bands of Dy³⁺. Therefore, the energy transfer from Tm³⁺ to Dy³⁺ is expected to occur in CaWO₄ system.

Fig. 5 The PL spectrum of CaWO₄:2% Tm³⁺ (solid line) and PLE spectrum of CaWO₄:1% Dy³⁺ (dash line).

In order to further study the energy transfer phenomenon from Tm³⁺ to Dy³⁺, series of CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0, 2.0) samples have been prepared. The PL spectra of the Tm³⁺, Dy³⁺ co-doped CaWO₄ phosphors under 355 nm are displayed in Fig. 6. The PL spectra of the CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0, 2.0) phosphors appear not only the blue emission due to the Tm³⁺ ions but also the yellow-blue emission due to the Dy³⁺ ions. With increasing Dy³⁺ concentration, the emission intensity of yellow-blue band reaches a maximum as x equals about 0.01 and begins to decrease due to concentration quenching, which can be easily observed from the inset of Fig. 6a. These results give us another evidence to validate energy transfer from Tm³⁺ to Dy³⁺. Upon 365 nm excitation, it is observed that the PL spectrum is similar to that upon 355 nm excitation. With increasing Dy³⁺ content, the emission intensity of Tm³⁺ (454 nm) decreases, reflecting the result of energy transfer from Tm³⁺ to Dy³⁺. By comparing Fig. 6a with Fig. 6b, we can see that the change trend for Dy³⁺ (575 nm) emission intensity of Fig. 6a is higher than that of Fig. 6b, which is a good sign for obtaining yellow color emission.

Fig. 6 Series of PL spectra of CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0, 2.0) under UV excitation (λ_ex = 355 nm). The inset shows the dependence of Tm³⁺ and Dy³⁺ emission intensity on the Dy³⁺ concentrations.

The decay curves of the Tm³⁺ ions in CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0, 1.5 and 2.0) samples were measured with excitation at 359 nm and monitored at 454 nm, and which were presented in Fig. 7. From Fig. 7, the decay curves of Tm³⁺ present single exponential phenomenon. The lifetime of Tm³⁺ can be estimated based on the following single exponential equation:

I = I₀ + Ae^−t/τ (1)

where I and I₀ are the luminescence intensity at times t and 0, respectively, and τ is the decay lifetime. The lifetime values of Tm³⁺ are 1.63475, 0.92673, 0.82099, 0.77444 and 0.60375 ms for CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0, 1.5 and 2.0), respectively. The decay lifetimes of the Tm³⁺ decrease gradually with an increase of Dy³⁺ concentration, which is strong evidence for the energy transfer between Tm³⁺ and Dy³⁺.

Fig. 7 Decay curves for the luminescence of Tm³⁺ ions in CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0, 1.5 and 2.0) phosphors and the fitted lifetimes are also given (excited at 359 nm, monitored at 454 nm).

In the view of Blasse, the critical distance between sensitizer and activator can be estimated from the activator concentration at which the sensitizer emission intensity has dropped to half the initial intensity. Meanwhile, the R_C is calculated by using the concentration quenching method estimated by the following formula suggested by Blasse:^45,46

R_C = 2 × [3V/(4πx_CZ)]^1/3 (2)

where V is the volume of the unit cell and Z is the number of host cations in the unit cell. For CaWO₄ host lattice, Z = 4, V = 312.2 ų. x_C is the total concentration of the sensitizer ions of Tm³⁺ and the activator ions of Dy³⁺ at which the luminescence intensity of Tb³⁺ is half of that the sample in the absence of Dy³⁺. According to the inset of Fig. 6, x_C is 0.034. Base on the formula (2), the critical distance (R_C) of in CaWO₄:Tm³⁺, Dy³⁺ is calculated to be about 16.4 Å. The energy transfer process can be defined as two interaction models: multipolar and exchange interaction. While the critical distance in exchange interaction model should be less than 4 Å.⁴⁷ It can be inferred that the mechanism of exchange interaction make no difference in energy transfer between Tm³⁺ to Dy³⁺, so the energy transfer is a resonant type via a nonradiative multipolar interaction.

On the base of the Dexter's energy transfer formula of multipolar interaction, the following relation can be given as:⁴⁸

τ_S0/τ_S ∝ C^n/3 (3)

where τ_S0 and τ_S are the lifetime of Tm³⁺ ions in the absence and presence of Dy³⁺ ions, respectively. C is the sum of Tm³⁺ ions and Dy³⁺ ions concentration, and n = 6, 8 and 10, corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The τ_S0/τ_S ∝ C^n/3 plots are further illustrated in Fig. 8, and the best linear relationship is obtained when n = 10, which means that the energy transfer from Tm³⁺ to Dy³⁺ takes place via the quadrupole–quadrupole mechanism.

Fig. 8 The dependence of τ_S0/τ_S of Tm³⁺ on C_Tm+Dy^6/3 × 10⁴, C_Tm+Dy^8/3 × 10⁵ and C_Tm+Dy^10/3 × 10⁶ in CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.5, 1.0, 1.5, 2.0) phosphors.

The energy transfer efficiency η_T from the sensitizer to the activator can be calculated by the following equation:⁴⁹

η_T = 1 − I_S/I_S0 (4)

where I_S0 and I_S are the luminescence intensity of a sensitizer in the absence and presence of an activator, respectively. For the CaWO₄:Tm³⁺, Dy³⁺ phosphor, Tm³⁺ is the sensitizer and Dy³⁺ is the activator. Fig. 9 shows the energy transfer efficiency from the Tm³⁺ to Dy³⁺ ions in CaWO₄, which is calculated as a functional of Dy³⁺ ion concentration. From Fig. 9, the efficiency is found to increase gradually with an increase in the Dy³⁺ doping contents, which is another evidence for the energy transfer from Tm³⁺ to Dy³⁺. In detail, upon 355 nm excitation, the values of η_T are calculated to be 0.0, 17.3%, 38.1% and 66.3% for CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0 and 2.0), respectively. Upon 365 nm excitation, the values of η_T are calculated to be 0.0, 12.5%, 41.5% and 59.5% for CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0 and 2.0), respectively. Combined with the above discussions, this conclusion is explained to be the change trend of Dy³⁺ emission intensities. These above results reveal that the energy migration from Tm³⁺ to Dy³⁺ is valid.

Fig. 9 Dependence of the energy transfer efficiency η_T on doped Dy³⁺ concentration in CaWO₄:2% Tm³⁺, x% Dy³⁺ (x = 0.0, 0.5, 1.0, 2.0) phosphors.

In order to further describing the excitation and emission mechanisms in detail, we depict the energy-level diagram of Tb³⁺, Dy³⁺ co-doped CaWO₄, shown in Fig. 10. It can be found that when the electrons return to lower energy-level again via multi-color emission and the energy transfers from Tm³⁺ to Dy³⁺ ions, and some energy is lost by cross relaxation.

Fig. 10 Schematic energy-level diagram showing the excitation and emission mechanisms of CaWO₄:Tm³⁺, Dy³⁺ phosphors (ET: energy transfer; NR: nonradiative).

3.3 Photoluminescence properties and tunable color study of CaWO₄:Tm³⁺, Dy³⁺, Eu³⁺/Sm³⁺ phosphors

In order to control the color tone, Eu³⁺ or Sm³⁺ was introduced to CaWO₄:Tm³⁺, Dy³⁺ samples. Fig. 11a and c presents the PL spectra of the CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Eu³⁺ (or Sm³⁺) (x = 0.0, 1.0, 3.0, 5.0, 7.0) samples under 355 nm excitation. From Fig. 11a, it can be observed that the PL spectra consist of the typical blue emission of Tm³⁺ (¹D₂ → ³F₄), the typical yellow-blue emissions of Dy³⁺ (⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2) and the red emission of Eu³⁺ (⁵D₀ → ⁷F₂). Furthermore, we find that the PL intensities of ¹D₂ → ³F₄ (454 nm) and ⁴F_9/2 → ⁶H_15/2 (575 nm) transitions decrease monotonically with an increase in doping Eu³⁺ concentration. Meanwhile, the emission intensity of ⁵D₀ → ⁷F₂ (616 nm) of Eu³⁺ increases gradually until the Eu³⁺ concentration is above 5.0% and then decreases for the concentration quenching, which can be easily observed from the inset of Fig. 11a. As shown in Fig. 11c, the PL intensities of Tm³⁺ and Dy³⁺ decrease monotonically with an increase in doping Sm³⁺ concentration, but the emissions intensities of ⁴G_5/2 → ⁶H_7/2 (597 nm) and ⁴G_5/2 → ⁶H_9/2 (648 nm) for Sm³⁺ are very weak, which can be clearly seen from the inset of Fig. 11c. Fig. 11b and d show the PL spectra of the CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Eu³⁺ (or Sm³⁺) (x = 0.0, 1.0, 3.0, 5.0, 7.0) phosphors excited at 365 nm. The PL spectra of Fig. 11b and d are similar to those of Fig. 11a and c, respectively. However, the emission intensities of Eu³⁺ and Sm³⁺ for Fig. 11b and d are stronger than those in Fig. 11a and c.

Fig. 11 Series of photoluminescence emission spectra of CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Eu³⁺ (x = 0.0, 1.0, 3.0, 5.0, 7.0) under 355 nm and 365 nm excitation (a and b), CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Sm³⁺ (x = 0.0, 1.0, 3.0, 5.0, 7.0) under 355 nm and 365 nm excitation (c and d). The inset shows the dependence of Tm³⁺, Dy³⁺ and Eu³⁺ (or Sm³⁺) emission intensity on the Dy³⁺ concentrations.

The correlated color temperature (CCT) of the phosphor is one of the important technological factors for phosphors used in solid-state lighting, especially in w-LEDs. As we all know, the correlated color temperature for a warm-white-light should be less than 5000 K, which is popular in solid state lighting.³⁰ The Commission International de I'Eclairage (CIE) chromaticity coordinates and the CCT of the single-phase CaWO₄:Tm³⁺, Dy³⁺, Eu³⁺ or Sm³⁺ phosphors were determined by their corresponding PL spectra under different excitation wavelength (355 nm and 365 nm) are summarized in Table 1. Table 1 shows the calculated CIE coordinates and their positions are represented in Fig. 12. Fig. 12A and B depicts the CIE for the single-component CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Eu³⁺ or x% Sm³⁺ (x = 0.0, 1.0, 3.0, 5.0, 7.0) phosphors under 355 nm excitation. As shown in Fig. 12A and B, the color tone of the as-synthesized for Fig. 12A (point 2, 3, 4, 5) could be easily tuned to white light than that for Fig. 12B (point 7, 8). Meanwhile, upon the excitation of 365 nm, the points of Fig. 12D (point g, h, i) are close to red region but the points of Fig. 12C (point b, c, e) are concentrated on white region. In conclusion, Eu³⁺ can supply a better red component than that of Sm³⁺ in CaWO₄:Tm³⁺, Dy³⁺ phosphors. Moreover, the points 4 and e show lower CCT values (4761 and 4185 K), which meet the commercial warm-white-light requirement. Therefore, the warm-white-light emitting phosphors with single-phase are obtained by adjusting the concentration of the rare earth ions and under different excitation wavelength.

Table 1 Comparison of the CIE chromaticity coordinates (x, y) and correlated color temperature for CaWO₄:Tm³⁺, Dy³⁺, Eu³⁺, Sm³⁺ phosphors

Point	Sample CaWO4:Tm^3+, Dy^3+, Eu^3+/Sm^3+	Excitation (nm)	CIE (x, y)	CCT (K)
1	0.5% Tm^3+, 1% Dy^3+	355	(0.276, 0.273)	11 521
2	0.5% Tm^3+, 1% Dy^3+, 1% Eu^3+	355	(0.304, 0.290)	7626
3	0.5% Tm^3+, 1% Dy^3+, 3% Eu^3+	355	(0.310, 0.279)	7340
4	0.5% Tm^3+, 1% Dy^3+, 5% Eu^3+	355	(0.343, 0.280)	4761
5	0.5% Tm^3+, 1% Dy^3+, 7% Eu^3+	355	(0.339, 0.278)	5014
6	0.5% Tm^3+, 1% Dy^3+, 1% Sm^3+	355	(0.322, 0.355)	5932
7	0.5% Tm^3+, 1% Dy^3+, 3% Sm^3+	355	(0.318, 0.318)	6282
8	0.5% Tm^3+, 1% Dy^3+, 5% Sm^3+	355	(0.309, 0.209)	16 281
9	0.5% Tm^3+, 1% Dy^3+, 7% Sm^3+	355	(0.289, 0.260)	10 785
a	0.5% Tm^3+, 1% Dy^3+	365	(0.244, 0.218)	60 080
b	0.5% Tm^3+, 1% Dy^3+, 1% Sm^3+	365	(0.285, 0.252)	12 357
c	0.5% Tm^3+, 1% Dy^3+, 3% Sm^3+	365	(0.294, 0.244)	11 650
d	0.5% Tm^3+, 1% Dy^3+, 5% Sm^3+	365	(0.307, 0.238)	9674
e	0.5% Tm^3+, 1% Dy^3+, 7% Sm^3+	365	(0.351, 0.273)	4185
f	0.5% Tm^3+, 1% Dy^3+, 1% Eu^3+	365	(0.269, 0.203)	100 430
g	0.5% Tm^3+, 1% Dy^3+, 3% Eu^3+	365	(0.317, 0.216)	9864
h	0.5% Tm^3+, 1% Dy^3+, 5% Eu^3+	365	(0.379, 0.233)	1821
i	0.5% Tm^3+, 1% Dy^3+, 7% Eu^3+	365	(0.523, 0.334)	1717

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Fig. 12 CIE chromaticity diagram of CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Eu³⁺ (x = 0.0, 1.0, 3.0, 5.0, 7.0) under 355 nm and 365 nm excitation (A and D), CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Sm³⁺ (x = 0.0, 1.0, 3.0, 5.0, 7.0) under 355 nm and 365 nm excitation (B and C). The corresponding images under corresponding excitation wavelengths.

4 Conclusions

In summary, a series of color tunable phosphors from blue to warm-white-light were successfully realized with Tb³⁺, Dy³⁺, Eu³⁺ or Sm³⁺ co-doped into CaWO₄ host. Upon UV light (355 nm and 365 nm) excitation, the CaWO₄:2% Tm³⁺, x% Dy³⁺ phosphors show the intense blue emission of Tm³⁺ and the yellow-blue emissions of Dy³⁺, the quenching concentration of Dy³⁺ is about x = 1.0. The photoluminescence and fluorescence decay times demonstrate that the energy transfer from Tm³⁺ to Dy³⁺ is expected. The critical distance for Tm³⁺ to Dy³⁺ has been calculated to be 16.4 Å by the basis of Dexter theory. The analysis indicates that the quadrupole–quadrupole interaction should be responsible for this energy transfer. As for CaWO₄:0.5% Tm³⁺, 1% Dy³⁺, x% Eu³⁺ or x% Sm³⁺ phosphors, the PL spectra present that the emission intensities of Eu³⁺ or Sm³⁺ under 365 nm excitation are stronger than that under 355 nm excitation. The CIE coordinates show that the Eu³⁺ ion can supply a better red component than that of Sm³⁺ ion. The above results indicate that the as-synthesized phosphors have a certain potential to be used as a single host warm-white-light phosphor for w-LEDs.

Acknowledgements

This work was supported by the National Natural Science Foundation of P. R. China (NSFC) (Grant No. 51072026, 51573023) and the Development of Science and Technology Plan Projects of Jilin Province (Grant No. 20130206002GX).

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