Up/down conversion, tunable photoluminescence and energy transfer properties of NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors

Abstract

Er³⁺ or/and Eu³⁺ codoped NaLa(WO₄)₂ down conversion (DC) and up conversion (UC) phosphors were prepared by a facile hydrothermal process. For NaLa(WO₄)₂:Er³⁺ phosphors, the WO₄²⁻ group can efficiently absorb ultraviolet (UV) light, and emit bright blue and green emissions by the f–f transitions of Er³⁺ through the host sensitization effect. The critical distance of the Er³⁺ ions in NaLa(WO₄)₂ is calculated and the energy quenching mechanism is proven to be a resonant type dipole–dipole interaction. More significantly, in the Er³⁺ and Eu³⁺ codoped NaLa(WO₄)₂ phosphors, the bright green emissions of Er³⁺ ions and the red characteristic emissions of Eu³⁺ ions can be observed, and the Er³⁺–Eu³⁺ energy migration has been demonstrated to be a resonant type of a dipole–dipole mechanism. Color-tunable emissions of NaLa(WO₄)₂:Er³⁺,Eu³⁺ microcrystals are realized under different UV radiation, and this could make them good candidates for use as full-color DC phosphors for near UV-LEDs. More practically, under near infrared (NIR) laser excitation at 980 nm, these phosphors also exhibit intense green and red emissions from the Er³⁺–Eu³⁺ energy transfer process, which causes the observed UC of Eu³⁺. The mechanism of UC luminescence is proposed by the observed dependence of integral intensity on the power of the pumping laser.

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1 Introduction

White light-emitting diodes (WLEDs) have brought about a significant revolution in solid-state lighting and display areas owing to their attractive features such as excellent luminescence characteristics, good stability, high luminescence efficiency, and long lifetimes, as well as low cost.^1–3 Recently, conventional WLEDs have suffered from an unsatisfactorily high correlated color temperature (CCT ≈ 7750 K) and low color-rendering index (CRI ≈ 70–80) for room lighting due to the color scarcity of sufficient red emission.^4,5 Therefore a near ultraviolet/ultraviolet (nUV/UV)-LED chip combined with tri-band, i.e., RGB (red, green and blue), phosphors have been suggested to achieve white light with a high CRI and high power output,^6,7 but the poor luminescence efficiency is inescapably impacted by the reabsorption of blue light. In contrast, novel full-color phosphors especially warm-white-emitting phosphors for WLEDs utilizing a single component are expected to avoid such drawbacks, giving rise to a higher quality of white light. Therefore, the design of a single-component full-color or white-light emitting coactivated phosphor is becoming more and more attractive.^7–12

In consideration of the Eu^{3+ 5}D₀ → ⁷F_J (J = 1, 2, 3, 4) characteristic emissions in the orange-red region respectively, Eu³⁺ ions are expected to possess a superior red color, and are considered as one of the most frequently useful red emitters in rare earth ion doped materials.¹³ As an important dopant, Er³⁺ mainly presents high efficiency characteristic emissions between the emitting states ²H_11/2 and ⁴S_3/2 and the excited state ⁴I_15/2 for giving intense green compositions, which overlaps well with the absorption of Eu³⁺ (⁵D_3,0 → ⁷F₀ transitions),^7,14 suggesting that Er³⁺ ions can be utilized as donors to sensitize Eu³⁺ acceptors. It is feasible to produce Er³⁺ and Eu³⁺ co-activated phosphors in an inorganic host. At the same time, a sensitizer and an activator co-doped into the same host can exhibit efficient full-color and white emission by energy transfer between sensitizers and activators and the combination of RGB emissions. Remarkably, the Er³⁺ sensitization effect on Eu³⁺ ion DC emissions has only been reported in BaGd₂O₄:Tm³⁺/Er³⁺/Eu³⁺ phosphors, where tunable emissions were obtained for WLEDs as well as field emission displays (FEDs).⁷ Nevertheless, the mechanism of energy transfer between Er³⁺ and Eu³⁺ has not been analyzed yet. For the first time, we established the mechanism of Er³⁺–Eu³⁺ energy transfer and the possibility of white-light down conversion (DC) emission from the Er³⁺/Eu³⁺ codoped NaLa(WO₄)₂ phosphors in the present work. The NaLa(WO₄)₂ compound has a scheelite type CaWO₄ structure, adopting a tetragonal phase with excellent physical–chemical stability and an individual self-activation property from the WO₄²⁻ group, so it has been considered recently to be an efficient luminescent host candidate.

More practically, Er³⁺ ions not only present strong absorption in the nUV region (365–410 nm) but also could be efficiently pumped by 980 nm near infrared (NIR) radiation. Er³⁺ ions served as the first up conversion (UC) rare-earth ions, and were widely utilized for the conversion of NIR radiation to visible light.^14–17 In view of the fact that UC emission from Eu³⁺ activated phosphors is not possible due to the unavailability of energy levels corresponding to the suitable near infrared (NIR) excitation, another activator should be introduced into the system as a sensitizer for exciting the Eu³⁺ ions. Furthermore, as an excellent donor, an excited Er³⁺ ion could transfer energy to Eu³⁺ ion and sensitize it in an UC system.^14,18–20 Very few reports on Er³⁺–Eu³⁺ codoped phosphors and glasses have been studied until now. Nearly all those investigated are Yb³⁺–Er³⁺–Eu³⁺ UC systems, where Er³⁺ acts as the bridging ion for the energy transfer (ET) between Yb³⁺ and Eu³⁺ ions under NIR excitation.^14,19,20 In previous reports, the energy transfer (ET) mechanism from Er³⁺ to Eu³⁺ with NIR excitation has been studied in NaYF₄,¹⁴ tellurite glasses¹⁸ and Y₂O₃,²⁰ respectively. Multicolor light emissions have been observed from the reported work in Er³⁺, Eu³⁺ and Yb³⁺ codoped phosphors.

Following this, in this work, we aim to focus our attention on NaLa(WO₄)₂ as a host, and Eu³⁺ and Er³⁺ ions as activated ions. The DC and UC properties of the Eu³⁺ and Er³⁺ co-doped NaLa(WO₄)₂ phosphors with color-tunable emissions are investigated as well as the sensitization effect of Er³⁺–Eu³⁺ ions.

2 Experimental section

2.1 Materials

Aqueous solutions of La(NO₃)₃, Er(NO₃)₃ and Eu(NO₃)₃ were obtained by dissolving the rare earth oxides La₂O₃, Er₂O₃, Eu₂O₃ (99.99%) in dilute HNO₃ solution (15 mol L⁻¹) under heating with agitation in ambient atmosphere. All the other chemicals were of analytical grade and used as received without further purification.

2.2 Preparation

A series of rare earth-doped NaLa(WO₄)₂ phosphors were synthesized by a facile hydrothermal process without further sintering treatment. 1.0 mmol of RE(NO₃)₃ (including La(NO₃)₃, Er(NO₃)₃ and Eu(NO₃)₃) was added into a 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 resultant milky colloidal suspension was transferred to a 50 mL Teflon bottle 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 8000 rpm for 10 min, washed with deionized water and ethanol in sequence several times each, and then dried in air at 60 °C for 12 h.

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 a 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 and composition of the samples were observed by a FEI XL-30 field emission scanning electron microscope (FESEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The excitation and emission spectra, and the luminescence decay curves of samples were measured using a HITACHI F-7000 Fluorescence Spectrophotometer equipped with a Xe lamp as the DC excitation source and a power-adjustable laser diode (980 nm, 0–2 W) as the UC pump source, scanning at 1200 nm min⁻¹. All of the measurements were performed at room temperature.

3 Results and discussion

3.1 Crystallization behaviors and structures

The XRD patterns of the NaLa(WO₄)₂ phosphors together with the PDF card (no. 79-1119) are given in Fig. 1. All the diffraction peaks of these samples can be exactly indexed to pure tetragonal phase NaLa(WO₄)₂ which has a CaWO₄ type structure and matches well with the standard values of the PDF card (no. 79-1119) indicating that the as-prepared phosphors are single phase and that the doping ions do not impact significantly on the host structure.

Fig. 1 PDF card (79-1119) and XRD patterns of the NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors.

The morphology of NaLa(WO₄)₂:Er³⁺,Eu³⁺ was characterized by FESEM, as presented in Fig. 2a and b. It can be observed that the samples are microparticles composed of many irregular flake-like microcrystals with an average size ranging from 3 to 5 μm. The EDS pattern shows the chemical composition of the product containing Na, La, Er, Eu, W, and O (the silicon and chromium signals are from the silicon host and chromium spraying process) (shown in Fig. 2c). Combined with the above XRD patterns, the samples are further proved to be NaLa(WO₄)₂.

Fig. 2 FESEM images (a and b) and EDS pattern (c) for NaLa(WO₄)₂:0.03Er³⁺,0.03Eu³⁺ phosphor.

3.2 Down conversion luminescence properties of NaLa(WO₄)₂:Er³⁺ phosphors

Fig. 3 illustrates the photoluminescence excitation (PLE) spectrum for NaLa(WO₄)₂:0.03Er³⁺. It can be seen that the PLE spectrum consists of two components: the former is a strong and broad band from 200 to 350 nm centered at 285 nm, which can be assigned to the charge transfer (CT) transitions of O²⁻–W⁶⁺ within the WO₄²⁻ groups,^21,22 and the latter is the f–f transition of Er³⁺ in the longer wavelength region at 357, 365, 378, 407, 443, 451, 483 nm assigned to the electronic transitions of Er³⁺ ions from the ground level ⁴I_15/2 to the ²G_7/2, ⁴G_9/2, ⁴G_11/2, ²H_9/2, ⁴F_3/2, ⁴F_5/2 and ⁴F_7/2 excited levels, respectively. The strongest absorption peak is mainly at about 378 nm, indicating that the NaLa(WO₄)₂:Er³⁺ phosphors could be pumped by nUV and used as nUV LED excited phosphors. The intensity of the peak at 378 nm first increases with the increase of concentration (x) and reaches a maximum value at x = 0.03 (inset of Fig. 3). The existence of O²⁻–W⁶⁺ CT indicates energy transfer from WO₄²⁻ to Er³⁺ in the Er³⁺ ion activated NaLa(WO₄)₂ phosphors, and that Er³⁺ ions could be excited via energy transfer from the WO₄²⁻ groups to the Er³⁺ ions. Therefore, this is a feasible route to realize color-tunable emission in a single host under UV excitation by combining the emissions of the WO₄²⁻ groups and Er³⁺ ions with different Er³⁺ ion doping concentrations.

Fig. 3 PLE spectrum for the NaLa(WO₄)₂:0.03Er³⁺ phosphor (λ_em = 552 nm); inset is the dependence of the absorption intensity at 378 nm on different Er³⁺ concentrations.

To further validate the occurrence of the host sensitization effect, the luminescence intensities and the decay times of WO₄²⁻ in NaLa(WO₄)₂:xEr³⁺ phosphors with different Er³⁺ concentrations are investigated. Fig. 4a depicts the PL spectra for NaLa(WO₄)₂:xEr³⁺ excited by a CT transition of O²⁻–W⁶⁺ at 285 nm (¹A₁ → ¹B(¹T₂) transition). As revealed, the WO₄²⁻ emission intensity decreases with the increase of Er³⁺-dopant concentration, accompanied by the enhancement of emission from Er³⁺, which also supports the idea that there is an energy transfer between Er³⁺ and WO₄²⁻. Nevertheless, not all of the absorption energy of WO₄²⁻ can be transferred to the Er³⁺ ions, so it is noted that upon excitation with 285 nm, the NaLa(WO₄)₂:0.03Er³⁺ phosphor yields the broad blue emission of WO₄²⁻ in the short wavelength region and three high intensity emission peaks of Er³⁺ in the long wavelength region. The decay curves for the luminescence of WO₄²⁻ in the NaLa(WO₄)₂:xEr³⁺ phosphors excited at 285 nm and monitored at 469 nm are measured and displayed in Fig. 4b. The corresponding luminescent decay curves can be fitted by a single-exponential equation

I = I₀ + A exp(−t/τ) (1)

Fig. 4 PL spectra for NaLa(WO₄)₂:xEr³⁺ (λ_ex = 285 nm) (a) and decay curves for the luminescence of WO₄²⁻ in NaLa(WO₄)₂:xEr³⁺ phosphors (excited at 285 nm, monitored at 469 nm) (b).

On the basis of eqn (1), the decay times for WO₄²⁻ are calculated and determined to be 2.5987, 2.3235, 2.1509, 1.9438, 1.9150, and 1.7323 ms. The lifetimes for WO₄²⁻ are found to be drastically decreased with an increase of the Er³⁺ concentration.

The PL spectra of the NaLa(WO₄)₂:xEr³⁺ phosphors are obtained by excitation at 378 nm (Fig. 5). It is noted that NaLa(WO₄)₂:xEr³⁺ phosphors give blue and green emissions, corresponding to the ²H_9/2 → ⁴I_15/2 (408 nm), ²H_11/2 → ⁴I_15/2 (529 nm) and ⁴S_3/2 → ⁴I_15/2 (552 nm) characteristic emissions of Er³⁺ ions. The strongest of these is located at 552 nm (⁴S_3/2 → ⁴I_15/2) and does not change with varying concentration (x) except for the differences in intensity. The optimal Er³⁺-dopant concentration was found to be 0.03, which is shown in Fig. 5a. On increasing the Er³⁺ doping concentration to x < 0.03, the dominating emissions are gradually enhanced due to the increase of luminescence centers. These then decrease when x > 0.03 as a result of the concentration quenching, during which the excitation energy is lost to the killer sites non-radiatively,²³ shown in Fig. 5a. The concentration quenching of the Er³⁺ emission is mainly due to the cross relaxation between neighbouring Er³⁺ ions which are in resonance with their respective energy levels: Er³⁺ (⁴S_3/2) + Er³⁺ (⁴I_15/2) → Er³⁺ (⁴I_9/2) + Er³⁺ (⁴I_13/2).²⁴

Fig. 5 PL spectra of NaLa(WO₄)₂:Er³⁺ phosphors; dependence of relative emission intensity at 552 nm on Er³⁺ concentration (inset, a) and the relationship between log(I/x) and log(x) (inset, b).

As an important parameter to evaluate the luminescence properties, the critical distance R_Er–Er between Er³⁺ ions can be calculated using the concentration-quenching method²⁵

R_c = 2 × [3V/(4πx_cZ)]^1/3 (2)

where V is the volume of the unit cell, x is the concentration of Er³⁺, and Z is the number of available crystallographic sites occupied by the activator ions in the unit cell. For the NaLa(WO₄)₂ host lattice, V = 332.7 ų and Z = 4. The critical concentration x_c, at which the luminescence intensity of Er³⁺ is quenched, is 0.03. As a result, the critical distance (R_Er–Er) of energy transfer is calculated to be about 17.43 Å. With the increase of Er³⁺ concentration, the distance between the Er³⁺ ions becomes shorter than 17.43 Å, so resonant energy transfer occurs between neighboring Er³⁺ ions.

According to van Uitert’s report, the interaction type between sensitizers or between sensitizer and activator can be calculated by the following^26,27

I/x = k[1 + β(x)^m/3]⁻¹ (3)

in which x is the activator concentration; I/x is the emission intensity (I) per activator concentration (x); k and β are constants; and when the value of m is 6, 8, or 10, the interaction corresponds to dipole–dipole (d–d), dipole–quadrupole (d–q), or quadrupole–quadrupole (q–q), respectively. From eqn (3), it can be found that log(I/x) acts as a linear function of log(x) with a slope of −m/3. In order to understand the energy transfer mechanism well, we plotted the log(I/x) versus log(x) of Er³⁺ as shown in Fig. 5 (inset b). The result of linear fitting shows that the slope is approximately −1.77 for NaLa(WO₄)₂:xEr³⁺ samples with x varying from 0.01 to 0.03. Therefore, the calculated value of m is nearly coincident with the conventional value of m = 6, meaning that the dominant interaction mechanism for Er³⁺ quenching in the NaLa(WO₄)₂ host is based on a dipole–dipole interaction.

To further study the luminescence properties of Er³⁺, the fluorescence decay process of the Er³⁺ ions in the NaLa(WO₄)₂:xEr³⁺ (0.005 ≤ x ≤ 0.09) phosphors is investigated by monitoring the emission at 552 nm with irradiation of 378 nm. From Fig. 6, one can see that the decay behavior of Er³⁺ can be best fitted to the single-exponential model, and by employing eqn (1) the lifetimes of Er³⁺ ions are determined to be 2.5089, 2.4725, 2.3461, 2.2975, 2.2517 and 2.1183 ms. The lifetimes for Er³⁺ ions are found to decrease with increasing Er³⁺ concentration, which is ascribed to the increase of the nonradiative and self-absorption rate of the internal doped ions when the activators cross the critical separation between donor and acceptor.^28,29

Fig. 6 Decay curves for the luminescence of Er³⁺ ions in NaLa(WO₄)₂ phosphors displayed on a logarithmic intensity (excited at 378 nm, monitored at 552 nm).

3.3 Down conversion luminescence properties of NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors and energy transfer between Er³⁺ and Eu³⁺ ions

Fig. 7 shows the PLE spectra for NaLa(WO₄)₂:0.03Eu³⁺ (a) and NaLa(WO₄)₂:0.03Er³⁺,0.03Eu³⁺ (b) phosphors. In Fig. 7a, the excitation spectrum of NaLa(WO₄)₂:0.03Eu³⁺, found by monitoring the emission wavelength at 615 nm, exhibits some peaks at 323, 364, 385, 395, 416, and 466 nm. These are assigned to the transitions of Eu³⁺ ions from the ground level ⁷F₀ to the ⁵H₃, ⁵D₄, ⁵L₇, ⁵L₆, ⁵D₃ and ⁵D₂ excited levels respectively, simultaneously including a broad absorption band in the 200–350 nm region ascribed to the CTB of WO₄²⁻ groups and the O²⁻–Eu³⁺ charge transfer transition from an oxygen 2p state excited to an Eu³⁺ 4f state.^11,13,30 The f–f transitions of Eu³⁺ from 350 nm to 420 nm match well with UV-LED chips, indicating that NaLa(WO₄)₂:Eu³⁺ are suitable for nUV LED excited phosphors. In Fig. 7b, when monitoring by the green emission of Er³⁺ (552 nm) and red emission of Eu³⁺ (616 nm), the PLE spectra of NaLa(WO₄)₂:0.03Er³⁺,0.03Eu³⁺ illustrate some absorption peaks corresponding to the characteristic transitions of Er³⁺ and Eu³⁺, respectively. More significantly, as shown in Fig. 7b, it can be clearly seen that the excitation spectrum of Eu³⁺ ions at 385 nm in NaLa(WO₄)₂:0.03Er³⁺,0.03Eu³⁺ is stronger than that in NaLa(WO₄)₂:0.03Eu³⁺ (Fig. 7a) because it consists of typical Er³⁺ and Eu³⁺ f–f excitation bands giving direct evidence to demonstrate the Er³⁺–Eu³⁺ sensitization effect in the NaLa(WO₄)₂ host. Therefore, tunable color could be generated by combining the green emissions of Er³⁺ ions and red emissions of Eu³⁺ ions via codoping Er³⁺ and Eu³⁺ ions in a single component.

Fig. 7 PLE spectra for NaLa(WO₄)₂:0.03Eu³⁺ (a) and NaLa(WO₄)₂:0.03Er³⁺,0.03Eu³⁺ (b) phosphors.

To further elucidate the impact of dopant concentration on the color-tunable emissions in a single component and energy migration process between activators, a series of phosphors with a fixed Er³⁺ ion concentration were prepared. Fig. 8 illustrates a series of emission spectra for NaLa(WO₄)₂:0.03Er³⁺,yEu³⁺ (y = 0.00, 0.005, 0.01, 0.02, 0.025, 0.03, 0.04 and 0.05) under 364 excitation. It can be seen that the NaLa(WO₄)₂:0.03Er³⁺,yEu³⁺ phosphors yield the characteristic emissions of both Er³⁺ and Eu³⁺ ions. On fixing the doping concentration of Er³⁺ at 0.03, and increasing Eu³⁺ concentration, the emission intensities of the Eu³⁺ first increase to an optimum concentration at 0.04 and then decrease due to the concentration quenching caused by energy transfer between the Eu³⁺ luminescent centers. That of the Er³⁺, on the other hand, decreases monotonically, reflecting the result of energy transfer from Er³⁺ to Eu³⁺. Therefore, the luminescence intensities of various rare-earth ions can be enhanced or quenched by the energy transfer from other codoped rare-earth ions. The above illustrates the occurrence of energy transfer from Er³⁺ to Eu³⁺ when these ions are codoped in an NaLa(WO₄)₂ host, and they are provided with the necessary conditions for synthesizing single phase full-color phosphors.

Fig. 8 PL spectra for NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors (λ_ex = 364 nm); dependence of the absorption intensity at different wavelengths on Eu³⁺ concentration (inset).

The energy-transfer efficiencies (η_T) from Er³⁺ to Eu³⁺ were calculated using the following formula³¹

η_T = 1 − I/I₀ (4)

in which I and I₀ are the emission intensities for sensitizers (Er³⁺) with and without the acceptors ions (Eu³⁺). The energy transfer efficiency calculated as a function of Eu³⁺ concentrations is shown in Fig. 9. The efficiency η_T increases gradually and reaches approximately 70% at x = 0.05. Moreover, the critical distance R_Er–Eu of energy transfer from Er³⁺ to Eu³⁺ can be estimated using the concentration-quenching method according to eqn (2), but here, the critical concentration x_c is defined as the total concentration of Er³⁺ and Eu³⁺, at which the luminescence intensity of Er³⁺ is half of that in the absence of Eu³⁺; 0.068. Therefore, the critical distance (R_Er–Eu) of energy transfer is calculated to be about 13.24 Å. As Eu³⁺ concentration is increased, the distance between Er³⁺ and Eu³⁺ becomes small enough (shorter than 13.24 Å) that the resonant energy transfer occurs: ⁴S_3/2(Er³⁺) + ⁷F₀(Eu³⁺) → ⁴I_15/2(Er³⁺) + ⁵D₀(Eu³⁺); ²H_11/2(Er³⁺) + ⁷F₀(Eu³⁺) → ⁴I_15/2(Er³⁺) + ⁵D₁(Eu³⁺); ²H_9/2(Er³⁺) + ⁷F₀(Eu³⁺) → ⁴I_15/2(Er³⁺) + ⁵D₃(Eu³⁺).^14,20 The possible energy transfer mechanism is shown in Fig. 11.

Fig. 9 Dependence of energy-transfer efficiency (η_T) on Eu³⁺ concentration for NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors.

In order to analyze the mechanism of energy-transfer process, we employ Dexter and Reisfeld’s theory to deal with the luminescence intensities. The following equation can be used to analyze the potential mechanism^24,32,33

I_so/I_s ∝ C^n/3 (5)

where I_so is the intrinsic luminescence intensity of donors (Er³⁺), and I_s is the luminescence intensity of donors in the presence of acceptors (Eu³⁺); and C is the doped concentration of acceptors. When the value of n is 6, 8, or 10, this corresponds to a dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole interaction, respectively. The I_so/I_s plots using linear fitting are illustrated in Fig. 10. It can be clearly seen that the linear fitting result is the best when n = 6, clearly implying the predominate interaction mechanism for energy transfer process between Er³⁺ and Eu³⁺ ions in the NaLa(WO₄)₂ host is based on dipole–dipole interactions.

Fig. 10 The dependence Iso/Is of Er3+ on the CEu3+n/3 × 104 (n = 6, 8, 10) in NaLa(WO4)2:0.03Er3+,yEu3+ phosphors.

A schematic model proposed for the probable ways of energy transfer in the NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors is shown in Fig. 11. During the excitation process, the electrons situated in the oxygen 2p states absorb the energy of photons from the UV light. As a consequence of this phenomenon, the energetic electrons are promoted to tungsten 5d states located near the conductor band.³⁴ When the electrons fall back to lower energy states again via blue emission and energy transfer to Er³⁺ and Eu³⁺ ions, some energy is lost by cross relaxation. In addition, Er³⁺ ions in excited states such as ⁴S_3/2, ²H_11/2 and ²H_9/2 could transfer energy to Eu³⁺ ions in excited states such as ⁵D_J (J = 0, 1, 3) through phonon assisted dipolar–dipolar interactions,^7,14,18–20 then relax to the ⁵D₀ energy level, and finally transfer to the ⁷F₁ or ⁷F₂ level of Eu³⁺ ions by radiative transition.

Fig. 11 Schematic energy-level diagram showing the excitation and emission mechanism of NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors (ET: energy transfer; NR: nonradiative).

The energy transfer among activator ions (Er³⁺, Eu³⁺) offers an approach to tune emission colors. Therefore, the CIE chromaticity coordinates for the phosphors excited at different wavelengths were determined based on their corresponding PL spectra, which are represented in the CIE diagram in Fig. 12 and the data is given in Table 1. The NaLa(WO₄)₂:Er³⁺ phosphor yields a bluish green emission under 364 nm radiation, whereas it gives green light with excitation at 378 nm (Fig. 12, points a₁ and a₂). For the NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors, the Er³⁺ doping concentration is fixed at 0.03, as the concentration of Eu³⁺ increases from 0.005 to 0.05. It can be seen that when excited at 364 nm, the trend of the color tones changes from bluish green to pink through white with higher correlated color temperature. In addition, when excited at 378 nm the phosphors exhibit green emissions (point b₂, c₂, d₂ and e₂ in Fig. 12) and the color changes to white (point f₂, g₂ and h₂ in Fig. 12). The correlated color temperature lowers gradually with adjustment of the doping concentration of Eu³⁺ reflecting that increasing the Eu³⁺ concentration, the red component, could decrease the correlated color temperature at an exponential rate. In particular, there are two points at (0.356, 0.368) and (0.387, 0.364) with lower correlated color temperatures of 4673 and 3684 K, respectively, that have potential in applications in WLEDs.

Fig. 12 CIE chromaticity diagram (a) and correlated color temperature (b) of the NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors under different wavelength excitation.

Table 1 The CIE chromaticity coordinates for NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors under different wavelength excitation

Lab.	Samples NaLa(WO4)2:Er^3+,Eu^3+	λex = 364 nm		λex = 378 nm
		CIE (x, y)	CCT/K	CIE (x, y)	CCT/K
a	0.03Er	(0.228, 0.376)	10 551	(0.223, 0.481)	8677
b	0.03Er, 0.005Eu	(0.252, 0.362)	9495	(0.241, 0.426)	8753
c	0.03Er, 0.01Eu	(0.258, 0.342)	9689	(0.249, 0.409)	8676
d	0.03Er, 0.02Eu	(0.267, 0.308)	10 296	(0.272, 0.394)	7867
e	0.03Er, 0.025Eu	(0.274, 0.289)	10 607	(0.287, 0.389)	7271
f	0.03Er, 0.03Eu	(0.288, 0.268)	10 225	(0.314, 0.381)	6217
g	0.03Er, 0.04Eu	(0.323, 0.255)	6301	(0.356, 0.368)	4673
h	0.03Er, 0.05Eu	(0.351, 0.246)	3464	(0.387, 0.364)	3684

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3.4 Up conversion luminescence properties and mechanism of NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors

In view of Er³⁺ playing an indispensable role in UC luminescence, we study the UC luminescence properties and mechanism of the obtained codoped samples. Fig. 13 displays the UC luminescence spectra of NaLa(WO₄)₂:Er³⁺,Eu³⁺ under 980 nm excitation. From Fig. 13, it can be seen that the most of the emissions of the Er³⁺ ions correspond well to those observed with 378 nm excitation, remarkably including a red emission of Er³⁺ assigned to ⁴F_9/2 → ⁴I_15/2 transitions at 655 nm shown in the enlarged inset of Fig. 13. In addition, it is significant that the UC of Eu³⁺ is obtained near 612 nm by the effect of the energy transfer between the NIR absorbing Er³⁺ ions and Eu³⁺ emitters. As the doping concentration of Er³⁺ is fixed at 0.03, with increasing Eu³⁺ concentration, the UC emission intensities of the Eu³⁺ first increase until an optimum concentration at 0.02 is reached, then decrease due to the concentration quenching, whereas that of the Er³⁺ decreases monotonically, reflecting the result of energy transfer from Er³⁺ to Eu³⁺: ⁴S_3/2(Er³⁺) + ⁷F₀(Eu³⁺) → ⁴I_15/2(Er³⁺) + ⁵D₀(Eu³⁺); ²H_11/2(Er³⁺) + ⁷F₀(Eu³⁺) → ⁴I_15/2(Er³⁺) + ⁵D₁(Eu³⁺); ²H_9/2(Er³⁺) + ⁷F₀(Eu³⁺) → ⁴I_15/2(Er³⁺) + ⁵D₃(Eu³⁺). Simultaneously, the low energy levels ⁵D_3,2,1,0 of the Eu³⁺ ions could be populated through a series of nonradiative relaxations from the neighboring high excited levels.

Fig. 13 UC luminescence spectra for NaLa(WO₄)₂:0.03Er³⁺,yEu³⁺ phosphors (excited at 980 nm).

To understand the UC processes well, we investigated the excitation power at 980 nm and the dependence of the UC emission intensities. For an unsaturated UC process, the integrated UC luminescence intensity I is proportional to P^n35,36

I ∝ Pⁿ (6)

where P is the pumping laser power, and n is the number of laser photons required to populate the upper emitting state. Fig. 14 shows the typical pump-power dependence of UC luminescence of NaLa(WO₄)₂:0.03Er³⁺,0.02Eu³⁺. The values of photon number n are 2.05 for the ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺, 1.96 for the ²H_11/2 → ⁴I_15/2 transition of Er³⁺, and 2.08 for the ⁵D₀ → ⁷F₂ transition of Eu³⁺, indicating that all these transitions for Er³⁺ and Eu³⁺ ions are two-photon UC processes, respectively. A two-photon mechanism was also found previously in other Er³⁺ and Eu³⁺ ion codoped phosphors, such as that reported by Wang and Rai.^14,20 Power dependence analyses illustrate that these levels of Eu³⁺ have the same multi-photon UC characters with the corresponding levels of Er³⁺ ions and confirm that they are populated by the energy transfer from the corresponding levels of Er³⁺.

Fig. 14 Dependence of the logarithm of intensity on the logarithm of excitation power at 980 nm of the NaLa(WO₄)₂:0.03Er³⁺,0.02Eu³⁺ phosphor.

4 Conclusions

In summary, a series of novel, color-tunable, single-component NaLa(WO₄)₂:Er³⁺,Eu³⁺ phosphors were prepared by a one-step hydrothermal method at 180 °C for 20 h. For the NaLa(WO₄)₂:Er³⁺ phosphors, Er³⁺ ions generate intense emission owing to the host sensitization effect. Under ultraviolet excitation, individual Er³⁺ ions activated the NaLa(WO₄)₂ phosphors exhibit excellent bluish green or green emissions and the Er³⁺ ions are quenched at the concentration of 0.03 via a resonant type dipole–dipole interaction. In the case of Er³⁺ and Eu³⁺ co-doped systems under UV light excitation, the bright green emissions of the Er³⁺ ions and the red characteristic emissions of Eu³⁺ ions can be observed, the efficient energy transfer process between Er³⁺ → Eu³⁺ occurs via the dipole–dipole mechanism. These single-component phosphors exhibit abundant color-tunable emissions besides warm white light with low correlated color temperatures in the NaLa(WO₄)₂ host. Almost all of the prepared phosphors could find applications in WLEDs. Additionally, under 980 nm laser excitation, these phosphors also exhibit intense green and red emissions by the Er³⁺–Eu³⁺ energy transfer process, which causes the observed UC of Eu³⁺. The mechanisms of UC luminescence of both Er³⁺ and Eu³⁺ are determined to be two-photon UC processes.

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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