A direct warm-white-light CaLa₂(MoO₄)₄: Tb³⁺, Sm³⁺ phosphor with tunable color tone via energy transfer for white LEDs

Abstract

A series of Tb³⁺, Sm³⁺ co-doped CaLa₂(MoO₄)₄ phosphors have been prepared via a solvothermal method without further sintering. The CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 0.0–5.0) phosphors were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) spectra. Upon 277 nm excitation, the phosphors exhibit strong green emission of the Tb³⁺ ions and red-orange emission of the Sm³⁺ ions, the quenching concentration of Sm³⁺ is determined to be about 4%. The critical distance between Tb³⁺ and Sm³⁺ has been calculated to be about 14.3 Å and the energy transfer from Tb³⁺ to Sm³⁺ occurs through a dipole–dipole interaction. The color tone of the obtained phosphors is easily modulated from blue to cool-white, green, and ultimately to warm-white-light. Furthermore, the relationship between the value of CCT for warm-white-light and the rare earth ion (Tb³⁺, Sm³⁺) concentration was investigated in detail. These results reveal that this kind of phosphor is a potential candidate for white LEDs.

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1 Introduction

In recent years, white light emitting diodes (w-LEDs) are hoped to be the fourth-generation lighting source because of their longer lifetime, higher energy efficiency, greater reliability, and more environmentally friendly characteristics than conventional incandescent and fluorescent lamps.^1,2 Currently, there are three different ways to achieve white light based on LEDs, as shown in Fig. 1a and b.^3,4 At present, the commercial white LEDs are fabricated 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 (R_a < 80) due to the lack of red component, which can not meet the warm-white-light demands.⁵ Warm-white-light LEDs can be achieved by utilizing a UV LED chip to excite a blend of blue, green, and red emitting phosphors. Unfortunately, the strong re-absorption of the blue light by the green and red phosphors may significantly low the conversion efficiency of the device.^6–9 Therefore, it is important to design a single-phase direct warm-white-light phosphor, which is expected to overcome the above-mentioned drawbacks. For this case, many excellent phosphors have been synthesized, such as Ca₃Sc₂Si₃O₁₂:Ce³⁺, Mn²⁺,¹⁰ KSr₄(BO₃)₃:Dy³⁺, Tm³⁺, Eu³⁺,¹¹ Na₃(Y, Sc)Si₃O₉:Eu²⁺.¹²

Fig. 1 Three ways of yielding white-light mechanism.

Owing to the characteristic emissions ⁴G_5/2 → ⁶H_J/2 (J = 5, 7, 9) of Sm³⁺ ions are in the orange-red region, they are expected to provide red color component in phosphor.^13,14 However, Sm³⁺ ions have very weak emissions under UV excitation. Tb³⁺ ions are expected to be used as green emitters in rare earth ions doped phosphors due to the high efficiency characteristic green emissions. It is well known that Tb³⁺ ions can exhibit strong blue and green emissions by adjusting the concentration of Tb³⁺ ions and excitation wavelengths. Obviously, the characteristic emissions of Tb³⁺ and Sm³⁺ ions almost cover the whole visible region. Warm-white-light can be easily realized by varying the relative composition of Tb³⁺/Sm³⁺ and excitation wavelengths in single-phase host. Furthermore, the ⁵D₃ and ⁵D₄ emission bands of Tb³⁺ have a good overlap with the excitation bands of Sm³⁺. It is expected that Tb³⁺ ions could transfer energy to Sm³⁺ ions, which contributes to realizing warm-white-light. In previous researches, it was reported that energy transfer played an important role in the luminescence properties of the phosphors.¹⁵ It can not only increase the PL intensities of the phosphor, but also tune the emission color by changing the concentrations of the sensitizer and activator. Some research work has been done to develop the single-phase phosphors based on the process of energy transfer from Tb³⁺ to Sm³⁺, such as in the host of NaGd(WO₄)₂,¹⁶ NaGdF₄,¹⁷ BaCeF₅,¹⁸ and Ca₂Gd₈Si₆O₂₆.¹⁹

As a large class of inorganic functional materials, molybdates have attracted great interest due to their wide use in catalysts, laser, and luminescence materials.^20–24 Recently, the research has focused on the double rare earth molybdates ARE(MoO₄)₂ (A = Li, Na and K, RE = rare earth cation), which share the sheelite-like (CaWO₄) isostructure, owing to their broad and intense absorption band in the near-UV region, excellent thermal and hydrolytic stability, they can be widely used in white light-emitting diodes (white LEDs).^25–28 However, only few researches have done on the synthesis of rare-earth molybdates with the structure of ARE₂(MoO₄)₄ (A = alkaline earth, RE = rare earth cation).^29–31 More importantly, there has been no reports on Tb³⁺, Sm³⁺ co-doped in CaLa₂(MoO₄)₄ host up to now.

In this work, we report on the synthesis, structure, luminescence properties and energy transfer of a series of molybdate-type phosphor, CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺, which can serve as a single-phase warm-white-light phosphor based on the UV-LEDs. The emission color of the obtained phosphors is easily modulated by changing the Sm³⁺ contents. The efficient energy transfer and critical distance between Tb³⁺ and Sm³⁺ were calculated. In addition, we have also investigated the relationship between the values of CCT and the rare-earth ions (Tb³⁺, Sm³⁺) concentration in detail.

2 Experimental section

2.1 Materials

Aqueous solutions of La(NO₃)₃, Tb(NO₃)₃, Sm(NO₃)₃ and Ca(NO₃)₂ were obtained by dissolving the rare earth oxides La₂O₃, Tb₄O₇, Sm₂O₃, and CaO 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 CaLa₂(MoO₄)₄ phosphors were synthesized by a facile solvothermal process without further sintering treatment. For the synthesis of CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺ phosphor, 1.0 mmol of RE(NO₃)₃ (including 0.98 mmol La(NO₃)₃, 0.01 mmol Tb(NO₃)₃, 0.01 mmol Sm(NO₃)₃) and 0.5 mmol Ca(NO₃)₂ were added into 100 mL flask. After vigorous stirring for 20 min, 2.0 mmol of Na₂MoO₄·2H₂O, 8.8 mL H₂O and 20 mL CH₃CH₂OH (according to the molar ratio of H₂O : CH₃CH₂OH = 1 : 1) was slowly added 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 20 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. Both of the excitation and emission slits were set at 2.5 nm. All of the measurements were performed at room temperature.

3 Results and discussion

3.1 Phase identification and morphology

Fig. 2a shows the XRD patterns of CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺. It is noted that all diffraction peaks of the samples are found to be well coincident with the standard CaMoO₄ (PDF#29-0351). The strong and sharp diffraction peaks indicate that the as-synthesized samples at low temperatures are still highly crystalline. It is also observed that the entire diffraction profiles shift to the lower angle, as depicted in Fig. 2b, owing to the larger ionic radius of La³⁺ (1.16 Å) compared with that of Ca²⁺ (1.12 Å).³² The diffraction profiles shift towards to the higher degree from CaLa₂(MoO₄)₄:1% Tb³⁺, CaLa₂(MoO₄)₄:1% Sm³⁺ to CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺, which can be assigned to the ionic radius for the Tb³⁺ ions (1.04 Å) and the Sm³⁺ ions (1.079 Å) are smaller than La³⁺ ions (1.16 Å).³² In view of the ionic radius and valence state, the Tb³⁺ and Sm³⁺ ions preferentially substitute the La³⁺ ions in the CaLa₂(MoO₄)₄ crystal. In other words, the rare-earth ions have been effectively built into the host.

Fig. 2 (a) XRD patterns of CaLa₂(MoO₄)₄:1% Tb³⁺, CaLa₂(MoO₄)₄:1% Sm³⁺, CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺. The standard pattern of CaMoO₄ (PDF#29-0351) is presented at the bottom for comparison. (b) The corresponding magnified XRD patterns in the 2θ region between 27 and 29°.

The morphology and size of the phosphors are important for their application in coatings on lighting devices.³³ Fig. 3 shows the FESEM images of CaLa₂(MoO₄)₄:1.0% Tb³⁺, 1.0% Sm³⁺ phosphor at different magnification, it is noted that the obtained phosphors take on a micro-sphere morphology with the diameter in the range of 0.2–0.8 μm and the average diameter is 0.437 μm, seen from the particle size histogram in the inset of Fig. 3b. The result can meet the requirement of commercial white LEDs phosphors.

Fig. 3 FESEM images (a and b) of CaLa₂(MoO₄)₄:1.0% Tb³⁺, 1.0% Sm³⁺ phosphor (inset of b is the particle size histogram).

3.2 Photoluminescence properties

Fig. 4 illustrates the PLE and PL spectra for CaLa₂(MoO₄)₄:1% Tb³⁺ (a) and CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺ (b) phosphors. As seen from Fig. 4a, we have observed that the excitation spectrum of CaLa₂(MoO₄)₄:1% Tb³⁺ (monitored at 545 nm) exhibits a strong excitation band from 200 to 350 nm with a maximum at 277 nm and two weak absorption peaks at 369 and 377 nm, which are assigned to the charge transfer (CTB) transition within Mo⁶⁺–O²⁻ and the transitions of Tb³⁺ ions from the ground level ⁷F₆ to the ⁵G₆ and ⁵D₃ excited levels, respectively. Upon 277 nm excitation, CaLa₂(MoO₄)₄:1% Tb³⁺ phosphor emits a series of luminescent peaks located at 490, 545, 586, 620 nm, which originate from the ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions of Tb³⁺. Meanwhile, it is clearly seen that two strong green emissions (490 and 545 nm) and weak yellow and red emissions (586 and 620 nm), which could be used for obtaining excellent and abundant tunable color emissions. As seen from Fig. 4b, the PLE spectrum of CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺ illustrates some absorption peaks corresponding to the characteristic transitions of Tb³⁺ (369 and 377 nm) and Sm³⁺ (405 nm) when monitored by the emission of Sm³⁺ ions (564 nm), demonstrating the existence of energy transfer from the Tb³⁺ to Sm³⁺ ions in the CaLa₂(MoO₄)₄ host. In addition, the CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺ presents a weaker CTB monitored at 564 nm than that monitored at 545 nm, suggesting that the energy transfer from CTB to Tb³⁺ is more efficient than that from CTB to Sm³⁺.³⁴ Upon 277 nm excitation, the CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺ phosphor exhibits the characteristic emissions of ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions of Tb³⁺ ions and ⁴G_2/5 → ⁶H_J (J = 5/2, 7/2, 9/2) transitions of Sm³⁺ ions, corresponding to 490, 545, 586, 625 nm for Tb³⁺ ions and 564, 594, 646 nm for Sm³⁺ ions, respectively. The emission spectrum nearly covers the entire visible region. Therefore, warm-white-light can be generated by combining the emissions of Tb³⁺ ions and orange-red emissions of Sm³⁺ ions in a single host by changing the Tb³⁺ and Sm³⁺ content via the process of energy transfer.

Fig. 4 Photoluminescence excitation (PLE) and photoluminescence (PL) spectra for CaLa₂(MoO₄)₄:1% Tb³⁺ (a) and CaLa₂(MoO₄)₄:1% Tb³⁺, 1% Sm³⁺ (b) phosphors.

In order to verify the energy transfer from Tb³⁺ to Sm³⁺, Fig. 5 presents the PL spectrum of CaLa₂(MoO₄)₄:1% Tb³⁺ (dash line) and the PLE spectrum of CaLa₂(MoO₄)₄:1% Sm³⁺ (solid line). The solid line is the f–f transitions of Sm³⁺ in the longer wavelength region at 405, 416, 444, 465 and 483 nm corresponding to the electronic transitions of Sm³⁺ ions from the ground level ⁶H_5/2 to the ⁴K_11/2, ⁶P_5/2, ⁴G_9/2, ⁴F_5/2 and ⁴I_11/2 excited levels, respectively.

Fig. 5 The PL emission spectrum of CaLa₂(MoO₄)₄:1% Tb³⁺ (dash line) and the PLE spectrum of CaLa₂(MoO₄)₄:1% Sm³⁺ (solid line).

Furthermore, we can find that the strongest peak is located at 405 nm, which is suitable to be used for near-UV LED excited phosphors.¹⁶ The dash line is the emission peaks of Tb³⁺, we also observe the strong peak at 490 nm. Due to the d–d forbidden transitions of Sm³⁺, their excitation transitions are difficult to excite and the emission intensity is very weak. Therefore, the energy transfer was expected to occur from Tb³⁺ to Sm³⁺. Based on the effective spectral overlap from Fig. 5, it is found that the ⁵D₃ and ⁵D₄ emission bands of Tb³⁺ overlap the excitation bands of Sm³⁺. Based on the above discussion, the energy transfer from Tb³⁺ to Sm³⁺ is expected to occur in CaLa₂(MoO₄)₄ system.

A series of CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0) samples have been prepared to further study the energy transfer phenomenon from Tb³⁺ to Sm³⁺. Fig. 6 illustrates the PL emission spectra of the Tb³⁺, Sm³⁺ co-doped CaLa₂(MoO₄)₄ phosphors with different Sm³⁺ doped concentrations. Upon 277 nm excitation, the PL spectra consist of the typical green emissions of Tb³⁺ and the typical orange-red emissions of Sm³⁺. Furthermore, we find that the PL intensities of ⁵D₄ → ⁷F₆ (490 nm) and ⁵D₄ → ⁷F₅ (545 nm) transitions for Tb³⁺ decrease monotonically with an increase in doping Sm³⁺ concentration. Meanwhile, the emissions of ⁴G_5/2 → ⁶H_5/2 (564 nm), ⁴G_5/2 → ⁶H_7/2 (594 nm) and ⁴G_5/2 → ⁶H_9/2 (648 nm) of Sm³⁺ increase gradually until the Sm³⁺ concentration is above 4% and then decrease for the concentration quenching, which can be easily observed from the inset of Fig. 6. These results give us another evidence to validate energy transfer from Tb³⁺ to Sm³⁺. In addition, as shown in Fig. 6, one can see that the emission peak of the Sm³⁺ shifts toward short wavelength with gradually increasing the doping concentration of Sm³⁺, which originates from the change of crystal field strength.^8,35

Fig. 6 Series of PL spectra of CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0) under UV excitation (λ_ex = 277 nm). The inset shows the dependence of Tb³⁺ and Sm³⁺ emission intensity on the Sm³⁺ concentrations.

Generally speaking, the critical distance (R_C) of energy transfer is calculated by using the concentration quenching method estimated by the following formula suggested by Blasse:^36,37

R_C = 2 × [3V/(4πx_CZ)]^1/3 (1)

where V is the volume of the unit cell, Z is the number of host cations in the unit cell. For CaLa₂(MoO₄)₄ host lattice, Z = 4, V = 312.2 ų. x_C is the total concentration of the sensitizer ions of Tb³⁺ and the activator ions of Sm³⁺ at which the luminescence intensity of Tb³⁺ is half of that the sample in the absence of Sm³⁺. According to the inset of Fig. 6, x_C is 0.051. From the above formula, the critical distance (R_C) of the energy transfer is estimated to be about 14.3 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.³⁸ The critical distance analysis indicates that the energy transfer from Tb³⁺ to Sm³⁺ has occurred via multipolar interaction.

In order to well understand the mechanism of the energy transfer process, the fluorescent decay curves of CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 1.0, 2.0, 3.0, 4.0) samples were measured by monitoring the emission of Tb³⁺ at 545 nm. As shown from Fig. 7a–d, it is easily found that all the decay curves of Tb³⁺ can be fitted to a single exponential function as:

I = I₀ + Ae^−t/τ (2)

where I represents the intensity at any time, I₀ is the intensity at t = 0, and τ is the decay lifetime. The lifetime values of Tb³⁺ are 0.9169, 0.8630, 0.8459, 0.8216, 0.7680 ms for CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 0.0, 1.0, 2.0, 3.0, 4.0), respectively. It can be found that the decay lifetimes of the Tb³⁺ decrease monotonically with an increase of Sm³⁺ concentration, which is strong evidence for the energy transfer between Tb³⁺ and Sm³⁺.

Fig. 7 Decay curves for the luminescence of Tb³⁺ ions in CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 0.0, 1.0, 2.0, 3.0, 4.0) phosphors displayed on a normalized intensity (a–d) (excited at 277 nm, monitored at 545 nm).

Fig. 8 shows the decay time of the Tb³⁺ and the energy transfer efficiency from Tb³⁺ to Sm³⁺ as a function of Sm³⁺ doping concentration in host. The energy transfer efficiency η_T from the sensitizer to the activator can be calculated by the following equation:³⁹

η_T = 1 − I_S/I_S0 (3)

where I_S0 and I_S are the luminescence intensity of a sensitizer in the absence and presence of an activator, respectively. For the CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺ phosphor, Tb³⁺ is the sensitizer and Sm³⁺ is the activator. As shown from Fig. 8, it can be seen that the fluorescence lifetime value of Tb³⁺ (τ) is straight reduced with increasing the Sm³⁺ content while the energy transfer efficiency (η_T) is not in the same trend which increases monotonically. In detail, the values of η_T were calculated to be 0.00, 15.52%, 33.07%, 45.33% and 49.20% for CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 0.0, 1.0, 2.0, 3.0, 4.0), respectively. These above results reveal that the energy migration from Tb³⁺ to Sm³⁺ is valid.

Fig. 8 Dependence of the decay time of the Tb³⁺ and energy transfer efficiency η_T on doped Sm³⁺ concentration in CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 0.0, 1.0, 2.0, 3.0, 4.0) phosphors.

Based on the discussion above, it is easy for us to draw the conclusion that the energy transfer mechanism from the Tb³⁺ to Sm³⁺ ions is a multipolar interaction. According to Dexter's energy transfer formula of exchange and multipolar interaction, the following relation can be given as:⁴⁰

I_S0/I_S ∝ C^n/3 (4)

where I_S0 and I_S are the luminescence intensity of Tb³⁺ ions in the absence and presence of Sm³⁺ ions, respectively. C is the sum of Tb³⁺ ions and Sm³⁺ ions concentration, and n = 6, 8, and 10, corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Fig. 9 presents the relationships between (I_S0/I_S) versus C^n/3, and the best linear relationship is obtained when n = 6. This result clearly demonstrates that the energy transfer mechanism from the Tb³⁺ to Sm³⁺ ions is a dipole–dipole interaction.

Fig. 9 The dependence of I_S0/I_S of Tb³⁺ on C_Tb+Sm^n/3 × 10⁶ in CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺phosphors.

To further research the dipole–dipole interaction, the energy transfer probability P_SA from a sensitizer to an accepter is given by the following equation:⁴¹

P_SA = (1/τ) − (1/τ₀) (5)

where τ and τ₀ are the decay lifetimes of Tb³⁺ presence and absence the Sm³⁺ ions. On the basis of above equation, the energy transfer probabilities from Tb³⁺ to Sm³⁺ are calculated to be 0.06812, 0.09154, 0.12651 and 0.21145 for the CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺ (x = 1.0, 2.0, 3.0, 4.0), respectively. This result demonstrates that with an increase in Sm³⁺ concentrations, the energy transfer probability from Tb³⁺ to Sm³⁺ increases.

The Commission International de I'Eclairage (CIE) chromaticity coordinates and the correlated color temperature (CCT) for the single-phase CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺ phosphors under different excitation wavelengths (277, 365, 370, 405 nm) are summarized in Table 1. As shown in Fig. 10, it can be observed that the color hue of the obtained phosphors can be easily modulated from blue (point 1, 5) to cool-white (point 2, 8), green (point 6, 7), and ultimately to warm-white (point 3, 4). Moreover, the points 3 and 4 show lower CCT values (4932, 4533 K), which meet the commercial warm-white-light requirement. All the above results reveal that the CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺ phosphors can be used as a potential multicolor phosphor for w-LED application.

Table 1 Comparison of the CIE chromaticity coordinates (x, y) and correlated color temperature for CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺ phosphors

Point	Sample	Excitation (nm)	CIE (x, y)	CCT (K)
1	0.05% Tb^3+, 5% Sm^3+	365	(0.232, 0.212)	108 458
2	0.05% Tb^3+, 5% Sm^3+	405	(0.282, 0.334)	8255
3	0.1% Tb^3+, 5% Sm^3+	277	(0.349, 0.377)	4932
4	0.1% Tb^3+, 6% Sm^3+	277	(0.360, 0.366)	4533
5	0.05% Tb^3+, 5% Sm^3+	277	(0.238, 0.298)	14 047
6	1% Tb^3+, 0.5% Sm^3+	277	(0.266, 0.485)	7208
7	1% Tb^3+, 4% Sm^3+	277	(0.315, 0.449)	5973
8	1% Tb^3+, 4% Sm^3+	370	(0.252, 0.226)	36 872

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Fig. 10 CIE chromaticity diagram of the selected CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺ phosphors under 277 nm (3, 4, 5, 6, 7), 365 nm (1), 370 nm (8) and 405 nm (2) excitation. The corresponding images under corresponding excitation wavelengths.

For the application of white LEDs, the correlated color temperature of the phosphor is one of the important factors. As we all known, the correlated color temperature for a warm-white-light should be less than 5000 K, which is popular in solid state lighting.¹¹ Fig. 11 presents the relationship of the correlated color temperature for CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺ phosphors with different Sm³⁺ and Tb³⁺ doping concentrations. Fig. 11a shows that the values of correlated color temperature are calculated to be 4932, 4630, 4164, 4534 K for the CaLa₂(MoO₄)₄:0.1% Tb³⁺, x% Sm³⁺ (x = 5.0, 6.0, 7.0, 8.0), respectively. With an increase in Sm³⁺ concentration, we can find that the CCT decreases gradually until the Sm³⁺ concentration arrives at 7%, this phenomenon should result from the Sm³⁺ concentration quenching. From Fig. 11b, the values of correlated color temperature are calculated to be 4510, 4823, 5225, 5242, 7207 K for the CaLa₂(MoO₄)₄:x% Tb³⁺, 5% Sm³⁺ (x = 0.2, 0.4, 0.6, 0.8, 1.0), respectively. Obviously, the values of CCT gradually increase as the concentration of Tb³⁺ increases. According to the above results, the values of CCT are closely related the Tb³⁺ and Sm³⁺ concentration, which have guiding significance for the application of commercial white LEDs.

Fig. 11 Correlated color temperature of the CaLa₂(MoO₄)₄:0.1% Tb³⁺, x% Sm³⁺ (x = 5.0, 6.0, 7.0, 8.0) (a) and CaLa₂(MoO₄)₄:x% Tb³⁺, 5% Sm³⁺ (x = 0.2, 0.4, 0.6, 0.8, 1.0) (b) phosphors under 277 nm excitation.

In order to further describe the excitation and emission mechanisms in detail, we depict the energy-level diagram of Tb³⁺ and Sm³⁺ co-doped CaLa₂(MoO₄)₄, shown in Fig. 12. It can be found that the electrons can absorb energies of photons from 277 nm UV light, when the electrons return to lower energy-level via multi-color emission and energy transfer from Tb³⁺ to Sm³⁺ ions, and some energy is lost by cross relaxation.

Fig. 12 Schematic energy-level diagram showing the excitation and emission mechanisms of CaLa₂(MoO₄)₄:Tb³⁺, Sm³⁺ phosphors (ET: energy transfer; NR: nonradiative).

4 Conclusions

In summary, a series of color tunable phosphors from blue to warm-white-light were successfully realized with Tb³⁺ and Sm³⁺ co-doped in a CaLa₂(MoO₄)₄ host. Upon 277 nm excitation, the phosphors show the intense green emissions of Tb³⁺ and the typical orange-red emissions of Sm³⁺. As for CaLa₂(MoO₄)₄:1% Tb³⁺, x% Sm³⁺, the quenching concentration of Sm³⁺ was about x = 4.0. The photoluminescence and fluorescence decay times demonstrate that the energy transfer from Tb³⁺ to Sm³⁺ is expected. The critical distance for Tb³⁺ to Sm³⁺ has been calculated to be 14.3 Å by the basis of Dexter theory. The analysis indicates that the dipole–dipole interaction should be responsible for this energy transfer. The CIE coordinates show that the color hue can be tunable from blue to cool-white, green, and ultimately to warm-white-light. The CCT of warm-white-light emission required were obtained, which closely related to the Tb³⁺ and Sm³⁺ concentration. The results indicate that the as-synthesized phosphors have huge potential to be a single host warm-white-light phosphor for white 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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