Tunable white-light emission via energy transfer in single-phase LiGd(WO₄)₂:Re³⁺ (Re = Tm, Tb, Dy, Eu) phosphors for UV-excited WLEDs

Abstract

A series of novel single-phase color tunable LiGd(WO₄)₂(LGW):Re³⁺ (Re = Tm, Tb, Dy, Eu) phosphors have been synthesized by a solid-state reaction. X-ray diffraction (XRD), FT-IR, photoluminescence (PL) and fluorescence decay curves were utilized to characterize the as-prepared samples. Under the excitation of UV light, LGW:Tm³⁺, LGW:Tb³⁺, LGW:Dy³⁺ and LGW:Eu³⁺ exhibit the characteristic emissions of Tm³⁺ (blue), Tb³⁺ (green), Dy³⁺ (yellow) and Eu³⁺ (red), respectively. On the one hand, by simply adjusting the doping concentration of Eu³⁺ ions in the LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ system, a white emission in a single-phase was achieved by blending simultaneously the blue, green, and red emissions of Tm³⁺, Tb³⁺, and Eu³⁺ ions in the LGW host, in which the energy transfer from Tb³⁺ to Eu³⁺ ions was found to play an important role. On the other hand, a white emission can also be realized based on the energy transfer from Tm³⁺ to Dy³⁺ ions in the LGW:2% Tm³⁺, x% Dy³⁺ system. The energy transfer mechanism from Tm³⁺ to Dy³⁺ ions has been demonstrated to be through dipole–quadrupole interaction and the critical distance (R_Tm–Dy) calculated by the concentration quenching method is 16.03 Å. The PL properties of as-prepared materials indicate that LGW:Re³⁺ (Re = Tm, Tb, Dy, Eu) may be potentially applied as single-phase white-light-emitting phosphors for UV-excited WLEDs.

---

1. Introduction

Nowadays, with the continuous increase of global electricity consumption for artificial lighting, the development of new high-efficiency lighting sources has become particularly important for reducing the world’s energy consumption.^1–4 As a hot spot in the field of solid-state lighting applications, white light-emitting diodes (WLEDs) are expected to be the most promising long-term solution due to their attractive merits of energy savings, longer lifetimes (>100 000 h), faster response, higher luminous efficiency and environmental friendliness compared with conventional incandescent and fluorescence lamps.^5–7 The 2014 Nobel Prize in Physics, awarded to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”, adequately affirmed this technology.⁸ Currently, the most common method for producing commercial WLEDs is based on a blue-emitting InGaN LED chip with a yellow-emitting phosphor (YAG:Ce³⁺). Despite their wide applications and high luminous efficacy (>100 lm W⁻¹), the major disadvantage of such a combination is that they are limited to a high correlated color temperature (CCT > 6000 K) and low color rendering index (R_a < 80) due to the lack of a sufficient red spectral component, which restricts their widespread commercialization in the general lighting market.^9–11 This limitation can be solved by utilizing an ultraviolet (UV) light-emitting diode (LED) chip coated with multiphased phosphors emitting blue, green, and red light. However, the fatal shortcoming of this method is the relatively low luminescence efficiency owing to the strong reabsorption of the blue light by the red and green phosphors.^12–14 Moreover, multi-phosphor systems are difficult to make, because the phosphors have to be prepared separately, the particle sizes of individual phosphor materials have to be adapted to one another in order to avoid agglomeration or sedimentation, and the final product has to be mixed homogeneously in exact ratios.⁸ To solve the above challenges, recently, single-phase white-light-emitting phosphors pumped by UV or near-UV LED chips have attracted great interest and been considered as an effective approach for phosphor-converted WLEDs, since they show significant advantages compared with the multiphased phosphors, including higher luminescence efficiency, higher color rendering index, better reproducibility, lower manufacturing costs and an easier fabrication process.^15–18

The most widely used method to realize white light in a single-phased phosphor is by codoping a sensitizer and an activator into the same host matrix. It should be pointed out that energy transfer plays a critical role in achieving white light emission in a single-phase phosphor. As a result, single-phase white-light-emitting phosphors based on the mechanism of energy transfer from a sensitizer to an activator have been realized in many inorganic hosts, such as NaCaBO₃:Ce³⁺, Tb³⁺, Mn²⁺,¹⁹ Na₂Ca₄Mg₂Si₄O₁₅:Eu²⁺, Mn²⁺,²⁰ Ca₄(PO₄)₂O:Ce³⁺, Eu²⁺,²¹ BaY₂Si₃O₁₀:Ce³⁺, Tb³⁺, Eu³⁺,²² Sr₇La₃[(PO₄)_2.5(SiO₄)₃(BO₄)_0.5](BO₂):Ce³⁺, Mn²⁺,²³ and so on. However, some harsh reaction conditions such as high temperature (above 1000 °C) and reducing atmosphere especially for Eu²⁺ or Ce³⁺ ion doped phosphors, lead to a high investment and energy costs. So, it will be interesting to find a new single-phase white-light phosphor which can be synthesized at a moderate temperature and in ambient atmosphere. Recently, scheelite-like double tungstate phosphors with the general formula of MRe(WO₄)₂ (M = alkali metal, Re = rare earth) for WLED applications have gained increasing interest for their promising luminescence properties, excellent physical and chemical stability, low synthetic temperature, and environmentally friendly characteristics.^24–27 In addition, the optical properties of these materials can be well improved due to their strong ultraviolet absorption capacity and effective transfer of the absorbed energy to the doping ions.

In this research, a series of novel single-phase LiGd(WO₄)₂(LGW) phosphors with tunable emission color through singly doping or codoping rare earth ions (Tm³⁺, Tb³⁺, Dy³⁺, and Eu³⁺) have been successfully synthesized by solid-state reaction. We find that LGW:Tm³⁺ phosphors show blue emission under the excitation of 360 nm, which match well with the emission light of UV-LED chips. Furthermore, two routes are discovered to realize the white emission in a single LGW host. When codoping Tm³⁺, Tb³⁺, and Eu³⁺ ions into a LGW host, a white emission is realized by simply adjusting the doping concentration of Eu³⁺ ions, in which the energy transfer from Tb³⁺ to Eu³⁺ ions was found to play an important role. On the other hand, a white emission can also be achieved by utilizing the principle of energy transfer and the appropriate tuning of Tm³⁺ and Dy³⁺ contents in LGW:Tm³⁺, Dy³⁺. Moreover, we investigate the energy transfer mechanism between Tm³⁺ and Dy³⁺ ions in LGW:Tm³⁺, Dy³⁺, which results in tunable emission colors from such phosphor materials. These results indicate that the as-prepared products may be potentially applied as candidates of single-phase white-light-emitting phosphors for application in WLEDs.

2. Experimental

2.1 Materials and synthesis

A series of rare-earth-doped LiGd(WO₄)₂ phosphors were synthesized through the solid-state reaction. Li₂CO₃ (A.R.), WO₃ (A.R.), Gd₂O₃ (99.99%), Tm₂O₃ (99.99%), Tb₄O₇ (99.99%), Dy₂O₃ (99.99%) and Eu₂O₃ (99.99%) were used as the starting materials. Stoichiometric amounts of the raw materials were first thoroughly mixed by grinding them in an agate mortar with an appropriate amount of ethanol and then dried at 80 °C for 0.5 h. After regrinding for 10 min, the powder mixtures were loaded into a crucible and transferred to a box furnace to calcine at 800 °C for 6 h in air with a heating rate of 3 °C min⁻¹. Finally, the prepared samples were cooled to room temperature and then ground once more for further measurements.

2.2 Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku D/max 2400 X-ray diffractometer using Cu Kα radiation (λ = 0.15405 nm). Fourier transform infrared (FTIR) spectra were performed on a JASCO FT/IR-460 plus spectrometer using a KBr disk technique. Photoluminescence (PL) emission and photoluminescence excitation (PLE) spectra as well as quantum efficiency were carried out on a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curves were measured using an FLS920 spectrofluorometer (Edinburgh Instruments). All the measurements were performed at room temperature.

3. Results and discussion

3.1 Phase identification and structure

The representative XRD patterns of LGW and Tm³⁺, Tb³⁺, Dy³⁺, and Eu³⁺ ion doped LGW samples as well as the standard data of LiGd(WO₄)₂ (JCPDS no. 04-008-0355) are presented in Fig. 1. It is obvious that the diffraction peaks of all these samples are well assigned to the pure tetragonal phase of LiGd(WO₄)₂ [space group: I4₁/a (88)] according to the JCPDS file 04-008-0355, and no traces of impurity phases are observed, indicating that the Tm³⁺, Tb³⁺, Dy³⁺ and Eu³⁺ ions can be completely dissolved into this host and their introductions do not cause any significant changes to the crystal structure. LiGd(WO₄)₂ crystals belong to a scheelite-type tetragonal structure, which have the cell parameters of a = b = 5.20 Å, c = 11.17 Å, V = 302.03 ų and Z = 2.²⁸ Fig. 2 shows the unit cell structure of LiGd(WO₄)₂ and the coordination environments of the Gd³⁺ (Li⁺) and W⁶⁺ sites. It can be seen that the central W⁶⁺ is coordinated by four oxygen atoms at a tetrahedral site and the Li⁺ and Gd³⁺ cations are randomly distributed over the same site positions and are coordinated by eight oxygen atoms to form a distorted polyhedron with symmetry S₄ (without an inversion center).^29–31 Based on the similar effective ionic radii and charge balance, the Tm³⁺, Tb³⁺, Dy³⁺ and Eu³⁺ ions are expected to occupy Gd³⁺ sites in LiGd(WO₄)₂.

Fig. 1 XRD patterns of representative LGW host (a), LGW:5% Tm³⁺ (b), LGW:5% Tb³⁺ (c), LGW:5% Dy³⁺ (d), LGW:5% Eu³⁺ (e), LGW:2% Tm³⁺, 8% Dy³⁺ (f), and LGW:2% Tm³⁺, 4% Tb³⁺, 7% Eu³⁺ (g) samples as well as the standard data of LiGd(WO₄)₂ (JCPDS no. 04-008-0355).

Fig. 2 The unit cell structure of LGW and the coordination environment of cations in the host lattice.

To determine the chemical constituents of the sample, FT-IR spectroscopy was performed on the pure LGW phosphor, as shown in Fig. 3. A broad absorption band at 3395 cm⁻¹ is ascribed to the stretching vibration of O–H, which belongs to physically absorbed water on the surface of the product from air. And a weak absorption band at 1652 cm⁻¹ is attributed to the bending vibration of O–H. The set of bands below 1000 cm⁻¹ are characteristic vibrations of W–O bands (from the WO₄²⁻ group). A strong absorption band at 808 cm⁻¹ and a weak band at 443 cm⁻¹ are caused by the stretching vibration of W–O and bending vibration of W–O, respectively.³²

Fig. 3 FT-IR spectra of pure LGW.

3.2 Photoluminescence properties

Fig. 4 illustrates the PLE and PL spectra of LGW:5% Tm³⁺, LGW:5% Tb³⁺, LGW:5% Dy³⁺ and LGW:5% Eu³⁺ samples, respectively. The excitation spectrum of LGW:5% Tm³⁺ shown in Fig. 4a contains a sharp absorption peak at 360 nm attributed to the ³H₆ → ¹D₂ transition of Tm³⁺,³³ which matches well with the UV-LED chips. Upon excitation at 360 nm, the LGW:5% Tm³⁺ phosphor shows an intense blue emission attributed to the ¹D₂ → ³F₄ transition (455 nm),³⁴ indicating it could be an outstanding candidate as a blue phosphor for UV-LED chips. Fig. 4b shows the PL excitation and emission spectra of LGW:5% Tb³⁺ phosphors. The excitation spectrum was obtained by monitoring the emission of Tb³⁺ (547 nm, ⁵D₄ → ⁷F₅). It is observed that the excitation spectrum can be divided into two parts: one is a broad band with a maximum at 272 nm, which is attributed to the charge-transfer absorption from the 2p orbitals of oxygen to the 5d orbitals of tungsten within the WO₄²⁻ groups and the 4f⁸ → 4f⁷5d transition of Tb³⁺;³⁵ the other one is composed of a series of narrow bands from 300 to 400 nm, which correspond to the characteristic f–f transitions of Tb³⁺ within its 4f⁸ configuration.²⁵ Under 272 nm excitation, the obtained emission spectrum of LGW:5% Tb³⁺ consists of f–f transition lines within a 4f⁸ electron configuration of Tb³⁺, i.e., ⁵D₄ → ⁷F₆ (490 nm) in the blue region and ⁵D₄ → ⁷F₅ (547 nm) in the green region, as well as ⁵D₄ → ⁷F₄ (590 nm) and ⁵D₄ → ⁷F₃ (623 nm) in the red region.³⁶ The strongest one is located at 547 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺. As for LGW:5% Dy³⁺ phosphors, the excitation spectra monitoring the emission at 576 nm corresponding to the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ is shown in Fig. 4c. It can be seen that the excitation spectrum of LGW:5% Dy³⁺ consists of a strong broad band from 200 to 320 nm with a maximum at 267 nm due to the WO₄²⁻ groups as discussed above. In the longer wavelength region from 310 to 500 nm, some sharp lines due to the f–f transitions of Dy³⁺ are observed, which are ascribed to the transitions from the ⁶H_15/2 ground state to the different excited states of Dy³⁺, i.e., 327 nm (⁶P_3/2), 353 nm (⁶P_7/2), 367 nm (⁶P_5/2), 388 nm (⁴I_13/2), 428 nm (⁴G_11/2), 455 nm (⁴I_15/2), and 477 nm (⁴F_9/2), respectively.³⁷ Upon the excitation at 267 nm, LGW:5% Dy³⁺ phosphors obtain blue and yellow luminescence, corresponding to the ⁴F_9/2 → ⁶H_15/2 (478 and 488 nm) and the ⁴F_9/2 → ⁶H_13/2 (576 nm) transitions of Dy³⁺ ions, respectively. The ⁴F_9/2 → ⁶H_15/2 transition is the magnetic dipole transition, which hardly varies with the crystal field strength or coordination environment around Dy³⁺ ions. However, the forced electric dipole transition ⁴F_9/2 → ⁶H_13/2 transition (ΔJ = 2) is hypersensitive to the chemical environment around Dy³⁺ ions in the host lattice.³⁸ When Dy³⁺ ions locate at low-symmetry sites without an inversion center, the yellow emission ⁴F_9/2 → ⁶H_13/2 transition is often prominent in the emission spectra. From the results in Fig. 4c (right), the intensity of the ⁴F_9/2 → ⁶H_13/2 transition is much stronger than that of the ⁴F_9/2 → ⁶H_15/2 transition, indicating that Dy³⁺ ions are located in an asymmetric cation environment, which is consistent with a symmetric S₄ without an inversion center. At last, Fig. 4d shows the PL excitation and emission spectra of LGW:5% Eu³⁺ phosphors. When monitoring the red emission of Eu³⁺ (617 nm, ⁵D₀ → ⁷F₂), the PLE spectrum of LGW:5% Eu³⁺ reveals a broad band ranging from 200 to 350 nm ascribed to the CTB of the WO₄²⁻ groups and the O²⁻–Eu³⁺ charge transfer transition from an oxygen 2p state excited to an Eu³⁺ 4f state.²⁶ In addition, we can easily distinguish the characteristic lines located at 362, 383, 395, 417, and 466 nm corresponding to the transitions of Eu³⁺ ions from the ground level ⁷F₀ to the ⁵D₄, ⁵L₇, ⁵L₆, ⁵D₃, and ⁵D₂ excited levels, respectively.³⁹ The PL spectrum of the LGW:5% Eu³⁺ phosphor is obtained by exciting at 275 nm. It can be found that the strongest emission peak of LGW:5% Eu³⁺ is located at 617 nm, which originates from the ⁵D₀ → ⁷F₂ transition of Eu³⁺. Two other weak peaks at 594 and 657 nm are attributed to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₃ transitions of Eu³⁺, respectively. It is well-known that Eu³⁺ is an excellent structural probe for investigating the local environment in a host lattice. The electric dipole transition ⁵D₀ → ⁷F₂ is a hypersensitive one, and the emission intensity is strongly influenced by the local environment surrounding Eu³⁺ ions. When Eu³⁺ ions are located at a low symmetry site, the ⁵D₀ → ⁷F₂ transition often dominates in the emission spectrum. Nevertheless, the magnetic dipole transition (⁵D₀ → ⁷F₁) is independent to the local crystal field environment.⁴⁰ In the present case, the emission intensity of electric transition is much stronger than that of magnetic dipole transition, indicating that the Eu³⁺ ions occupy the low symmetry sites, which is in accordance with the emission spectrum of LGW:5% Dy³⁺. As illustrated in Fig. 4a–d, there are some excitation peaks from 350 to 420 nm in the PLE spectrum of Tm³⁺, Tb³⁺, Dy³⁺ and Eu³⁺ doped LGW samples, which indicate that those phosphors can match well with the dominant emission band of a UV-LED chip. In addition, the presence of the excitation peak for WO₄²⁻ groups in the excitation spectrum of Tb³⁺/Dy³⁺/Eu³⁺ implies that there is an energy transfer from WO₄²⁻ groups of the host to Tb³⁺/Dy³⁺/Eu³⁺ ions in phosphors.⁴¹ However, we do not observe a charge transfer band for WO₄²⁻ groups in the PLE spectrum of LGW:Tm³⁺, certifying the inexistence of the energy transfer from WO₄²⁻ to Tm³⁺.

Fig. 4 PL and PLE spectra of (a) LGW:5% Tm³⁺, (b) LGW:5% Tb³⁺, (c) LGW:5% Dy³⁺ and (d) LGW:5% Eu³⁺ samples.

A better understanding of the actual emission color of the phosphor is very important for application in lighting and display devices. Fig. 5 shows the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates and the corresponding digital luminescence photographs of LGW:5% Tm³⁺, LGW:5% Tb³⁺, LGW:5% Dy³⁺ and LGW:5% Eu³⁺ phosphors. The CIE chromaticity coordinates of LGW:Tm³⁺/Tb³⁺/Dy³⁺/Eu³⁺ phosphors can vary from (0.160, 0.061) for LGW:5% Tm³⁺ to (0.277, 0.583) for LGW:5% Tb³⁺, (0.406, 0.431) for LGW:5% Dy³⁺, and (0.619, 0.345) for LGW:5% Eu³⁺, which correspond to the color changes from blue to green, yellow, and red, respectively.

Fig. 5 CIE color coordinate diagram of LGW doped with 5% (1) Tm³⁺, (2) Eu³⁺, (3) Dy³⁺, and (4) Tb³⁺. The insets are their corresponding digital photographs taken under 365 nm excitation.

The aim of this study is to realize the white light emission in a single-phase host for UV-excited WLEDs. For this purpose, an appropriate combined amount of the luminescent ions Tm³⁺, Tb³⁺, and Eu³⁺ as emitters of blue, green, and red light codoped into a LGW host was selected for white light generation. Therefore, a series of LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ (x = 0, 1, 3, 5, 7) samples have been prepared. The PL spectra of the as-prepared samples under excitation at 360 nm and the corresponding CIE chromaticity diagram are presented in Fig. 6. Under excitation at 360 nm, the emission color varies from light green (which is represented by point 1) to rose red (point 5), and in particular the white light is realized (point 3) by simply controlling the doping concentration of Eu³⁺ ions (Fig. 6b). Therefore, a novel single-phase white-emitting phosphor is successfully obtained by simultaneously blending the blue, green, and red emissions of Tm³⁺, Tb³⁺, and Eu³⁺ ions in a LGW host under UV light excitation.

Fig. 6 (a) PL emission spectra of LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ samples with different Eu³⁺ concentrations (0 ≤ x ≤ 7). (b) CIE coordinate diagram of LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ with different Eu³⁺ concentrations: (1) x = 0; (2) x = 1; (3) x = 3; (4) x = 5; and (5) x = 7. The insets are their corresponding digital photographs taken under 365 nm excitation.

Detailed information of LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ (x = 0, 1, 3, 5, 7) phosphors for the CIE chromaticity coordinates and CCTs are listed in Table 1. Moreover, it is worth noting that although the concentration of Tb³⁺ was fixed at 4%, as the Eu³⁺-doping concentration increased, the emission intensity of Eu³⁺ was enhanced clearly whereas the emission intensity of Tb³⁺ decreased steadily (Fig. 6a), indicating that there is an energy transfer process from Tb³⁺ to Eu³⁺ ions. As reported, the luminescence intensities of various rare earth ions can be enhanced or quenched by an energy transfer from other codoping rare earth ions.⁴² The energy transfer between Tb³⁺ and Eu³⁺ is a well-known phenomenon since the ⁵D₄ → ⁷F_6,5,4,3 emissions of Tb³⁺ are effectively overlapped with the ⁷F_0,1 → ⁵D_0,1,2 absorptions of Eu³⁺.⁴³ So far, the energy transfer phenomenon from Tb³⁺ to Eu³⁺ has been extensively investigated in many inorganic hosts, such as molybdates,³⁶ fluorides,⁴⁴ germanates,⁴⁵ tungstates,⁴⁶ oxyhalides,⁴⁷ niobates,⁴⁸ and so on. In order to provide further evidence to validate the energy transfer from Tb³⁺ to Eu³⁺ ions in the LGW host lattice, the fluorescence decay curves of Tb³⁺ in LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ (0 ≤ x ≤ 7) samples excited at 272 nm, monitoring the emission of Tb³⁺ ions at 547 nm, were measured and shown in Fig. 7. The decay curves fit well with a single exponential formula as follows:⁴⁹

I = I₀ exp(−t/τ) (1)

where I and I₀ are the luminescence intensities at time t and 0, and τ is the luminescence lifetime. For the LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ (0 ≤ x ≤ 7) samples, the lifetime of Tb³⁺ decreases with increasing Eu³⁺ concentration, which are 5.26, 4.55, 4.31, 4.14 and 3.87 ms for x = 0, 1, 3, 5 and 7, respectively. This is powerful proof for the energy transfer from Tb³⁺ to Eu³⁺ ions.

Table 1 Comparison of the CIE chromaticity coordinates and CCT (K) for LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ phosphors excited under 360 nm UV radiation

Sample no.	Sample composition	CIE coordinates (x, y)	CCT (K)
1	x = 0	(0.279, 0.418)	7344
2	x = 1	(0.321, 0.356)	5993
3	x = 3	(0.343, 0.327)	4980
4	x = 5	(0.377, 0.296)	3159
5	x = 7	(0.435, 0.280)	1961

---

Fig. 7 The decay curves for the luminescence of Tb³⁺ ions in LGW:2% Tm³⁺, 4% Tb³⁺, x% Eu³⁺ samples with changing Eu³⁺ concentrations (excited at 272 nm, monitored at 547 nm).

Interestingly, white light emission can also be realized by combining the blue and yellow emissions of Tm³⁺ and Dy³⁺ ions in a single host under UV light excitation. As shown in Fig. 8, the comparison of the PL spectrum of LGW:Tm³⁺ and the PLE spectrum of LGW:Dy³⁺ reveals a significant spectral overlap between the emission of Tm³⁺ and the excitation of Dy³⁺. Therefore, an effective resonance-type energy transfer from Tm³⁺ to Dy³⁺ in a LGW host is expected. In order to further investigate the energy transfer process from Tm³⁺ to Dy³⁺ and to study the impact of doping concentration on the luminescence properties of phosphors, a series of LGW:2% Tm³⁺, x% Dy³⁺ (x = 0, 2, 4, 6, 8, 10) samples were prepared. The PL spectra of LGW:2% Tm³⁺, x% Dy³⁺ under 360 nm excitation are illustrated in Fig. 9. The doping concentration of Tm³⁺ was fixed at 2%, while the Dy³⁺ content changes from 0% to 10%. We can observe that as the amount of Dy³⁺ content increases, the emission intensity of Tm³⁺ gradually decreases, whereas the emission intensity of Dy³⁺ first increases to an optimum concentration of 6%, and then begins to decrease because of the concentration quenching effect, which confirms the existence of energy transfer from Tm³⁺ to Dy³⁺ ions. Meanwhile, those single-phase LGW:2% Tm³⁺, x% Dy³⁺ phosphors exhibit abundant color-tunable emissions through an effective energy transfer process and controlling the doping concentration of Dy³⁺. The most interesting part of this approach is its ability to realize white light emission from a particular sample of LGW:2% Tm³⁺, 6% Dy³⁺. This has been confirmed by direct exposure to ultraviolet (UV) light as well as from the CIE coordinate calculation (discussed later). In addition, the fluorescence decay lifetime measurements for Tm³⁺ emission (λ_ex = 360 nm, λ_em = 455 nm) with increasing Dy³⁺ concentration in LGW:2% Tm³⁺, x% Dy³⁺ were conducted to further confirm the occurrence of energy transfer from Tm³⁺ to Dy³⁺. Fig. 10 shows the decay curves of Tm³⁺ emission in LGW:2% Tm³⁺, x% Dy³⁺ samples. The decay curves can be fitted successfully by a second-order exponential function as given by the following equation:⁵⁰

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I represents the luminescence intensity; A₁ and A₂ are constants; t is the time; τ₁ and τ₂ are rapid and slow decay times for exponential components, respectively. As a result, the average decay times (τ*) can be determined using the following equation:

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (3)

Fig. 8 Spectral overlap between the PL spectrum of LGW:Tm³⁺ (red line) and PLE spectrum of LGW:Dy³⁺ (black line).

Fig. 9 PL emission spectra of LGW:2% Tm³⁺, x% Dy³⁺ samples with different Dy³⁺ concentrations (x = 0, 2, 4, 6, 8, and 10).

Fig. 10 Photoluminescence decay curves of Tm³⁺ in LGW:2% Tm³⁺, x% Dy³⁺ with changing Dy³⁺ concentrations.

On the basis of eqn (3), the average lifetimes of Tm³⁺ are determined to be 9.56, 7.82, 6.63, 5.31 and 4.54 μs for Dy³⁺ concentrations of 0%, 4%, 6%, 8%, and 10%, respectively. The decrease in the lifetimes of Tm³⁺ with increasing Dy³⁺ concentration strongly demonstrates the existence of energy transfer from Tm³⁺ to Dy³⁺ ions. Fig. 11 illustrates a schematic diagram of the energy transfer process from Tm³⁺ to Dy³⁺ in a LGW:Tm³⁺, Dy³⁺ phosphor. As shown in Fig. 11, the energy gap between ¹D₂ and ³F₄ of Tm³⁺ matches well with that between ⁶H_15/2 and ⁴I_15/2 of Dy³⁺, which makes the energy transfer from Tm³⁺ to Dy³⁺ ions efficient. It illustrates the existence of energy transfer from Tm³⁺ to Dy³⁺ when they are codoped into the LGW host and provides a necessary condition for synthesizing the single-phase color-tunable phosphors.

Fig. 11 A simple model expressing the energy transfer from Tm³⁺ to Dy³⁺ in LGW:Tm³⁺, Dy³.

3.3 Energy transfer mechanism

The energy transfer efficiency (η_T) from Tm³⁺ to Dy³⁺ in LGW:2% Tm³⁺, x% Dy³⁺ systems can be calculated using the following equation:⁵¹

η_T = 1 − I_S/I_S0 (4)

where η_T is the energy transfer efficiency, and I_S and I_S0 are the luminescence intensities of Tm³⁺ ions in the presence and absence of Dy³⁺, respectively. As shown in Fig. 12, the intensity of Tm³⁺ emission and the energy transfer efficiency from Tm³⁺ to Dy³⁺ in a LGW:2% Tm³⁺, x% Dy³⁺ system are plotted as a function of Dy³⁺ concentration. One can find that with the increase of Dy³⁺ concentration, the intensity of Tm³⁺ emission decreases gradually, while the energy transfer efficiency steadily increases to reach a maximum at 84% when using UV irradiation of 360 nm as the excitation wavelength, indicating that the energy transfer from Tm³⁺ to Dy³⁺ becomes more and more efficient.

Fig. 12 Dependence of the emission intensity of Tm³⁺ and the energy transfer efficiency (η_T) on Dy³⁺ concentration in LGW:2% Tm³⁺, x% Dy³⁺ (x = 0, 2, 4, 6, 8, 10) phosphors.

In order to determine the energy transfer mechanism from Tm³⁺ to Dy³⁺ ions, it is necessary to know the critical distance (R_c) between the sensitizer (Tm³⁺) and activator (Dy³⁺). With the increase of Dy³⁺ content, the distance between Tm³⁺ and Dy³⁺ ions becomes shorter and shorter, thus the probability of energy migration increases. When the distance becomes small enough, concentration quenching occurs and energy migration is hindered. Therefore, the critical distance (R_c) for energy transfer from Tm³⁺ to Dy³⁺ ions can be calculated using the concentration quenching method. According to Blasse,^52,53 the critical distance between Tm³⁺ and Dy³⁺ ions can be expressed by

where V refers to the volume of the unit cell, N is the number of available sites for the dopant in the unit cell, and X_c is the total content of Tm³⁺ and Dy³⁺, at which the luminescence intensity of Tm³⁺ is half that of the sample in the absence of Dy³⁺ ions. For the LGW host, V = 302.03 ų, N = 2, and X_c = 0.07. Therefore, the R_c value is calculated to be about 16.03 Å. Generally, non-radiative energy transfer from a sensitizer to an activator usually takes place through an exchange interaction or electric multipolar interactions.⁵⁴ The value of R_c calculated above implies a small possibility of an exchange interaction because the exchange interaction is predominant only for about 5 Å.⁵⁵ As a consequence, we can conclude the energy transfer mechanisms from Tm³⁺ to Dy³⁺ ions are dominated by electric multipolar interactions. According to Dexter’s energy transfer formula of multipolar interaction and Reisfeld’s approximation, the following relationship can be used:⁵⁶where η_S0 and η_S represent the luminescence quantum efficiencies of Tm³⁺ ions in the absence and presence of Dy³⁺ ions, respectively. C is the total doping concentration of Tm³⁺ and Dy³⁺ ions. The value of α = 6, 8, and 10 corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The values of η_S0/η_S also can be approximately calculated by the ratio of related luminescence intensities as⁵⁷where I_S0 and I_S refer to the luminescence intensity of Tm³⁺ ions without and with Dy³⁺ ions, respectively. The relationship between I_S0/I_S and C^α/3 based on the above equation is shown in Fig. 13. It can be clearly seen that when α = 8, the linear fitting result is the best. Therefore, the energy transfer from Tm³⁺ to Dy³⁺ ions takes place through the dipole–quadrupole interaction mechanism.

Fig. 13 Dependence of I_S0/I_S of Tm³⁺ emission on Tm³⁺ and Dy³⁺ ion concentration: (a) C^6/3, (b) C^8/3, and (c) C^10/3.

Fig. 14 shows the CIE chromaticity coordinates and the representative digital luminescence photographs of single-phase emission-tunable phosphors LGW:2% Tm³⁺, x% Dy³⁺. The CIE chromaticity coordinates for the samples were determined on the basis of their corresponding PL spectrum, and were calculated as (0.258, 0.207), (0.294, 0.257), (0.335, 0.321), (0.349, 0.348) and (0.368, 0.368) for LGW:2% Tm³⁺, x% Dy³⁺ with x = 2, 4, 6, 8 and 10. For WLED applications, the quantum efficiency (QE) is an important parameter for phosphors. Therefore, the white-light-emitting phosphors, LGW:2% Tm³⁺, 4% Tb³⁺, 3% Eu³⁺ and LGW:2% Tm³⁺, 6% Dy³⁺, were measured. The QE of LGW:2% Tm³⁺, 4% Tb³⁺, 3% Eu³⁺ is 9%, while that of LGW:2% Tm³⁺, 6% Dy³⁺ is 28%. It is believed that the quantum efficiency of these white-light-emitting phosphors could be further improved by optimizing the synthetic conditions.

Fig. 14 CIE color coordinate diagram of LGW:2% Tm³⁺, x% Dy³⁺ with different Dy³⁺ concentrations: (1) x = 2; (2) x = 4; (3) x = 6; (4) x = 8; and (5) x = 10. The insets are their corresponding digital photographs taken under 365 nm excitation.

4. Conclusion

In summary, a series of LiGd(WO₄)₂:Re³⁺ (Re = Tm, Tb, Dy, Eu) phosphors have been prepared by the conventional solid-state reaction method. Under UV light excitation, the LGW:Tm³⁺, LGW:Tb³⁺, LGW:Dy³⁺ and LGW:Eu³⁺ phosphors show the characteristic emissions of Tm³⁺ (blue), Tb³⁺ (green), Dy³⁺ (yellow) and Eu³⁺ (red). More interestingly, two routes are disclosed to realize the white emission via the energy transfer process between different rare earth ions in a single LGW host. By codoping Tm³⁺, Tb³⁺, Eu³⁺ into the LGW host, white light emission was obtained under 360 nm excitation which is accessible with UV-LED chips. In addition, the LGW:2% Tm³⁺, x% Dy³⁺ phosphors show tunable color from light blue to light yellow, and in particular the white light emission is also realized based on the energy transfer from Tm³⁺ to Dy³⁺ ions under UV excitation. In detail, the energy transfer mechanism from Tm³⁺ to Dy³⁺ ions has been demonstrated to be through dipole–quadrupole interaction. These results indicate that the present materials could potentially serve as single-phase white-light-emitting phosphors for application in UV-excited WLEDs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21476045) and the Foundation of key laboratory for micro/nano technology and system of Liaoning province (20140401).

Notes and references

1. T. R. Kuykendall, A. M. Schwartzberg and S. Aloni, Adv. Mater., 2015, 27, 5805.
2. Z. Zhang, D. Liu, D. Li, K. Huang, Y. Zhang, Z. Shi, R. Xie, M.-Y. Han, Y. Wang and W. Yang, Chem. Mater., 2015, 27, 1405.
3. A. Layek, P. C. Stanish, V. Chirmanov and P. V. Radovanovic, Chem. Mater., 2015, 27, 1021.
4. P. Pust, V. Weiler, C. Hecht, A. Tucks, A. S. Wochnik, A. K. Henss, D. Wiechert, C. Scheu, P. J. Schmidt and W. Schnick, Nat. Mater., 2014, 13, 891.
5. J. Chen, Y. Liu, L. Mei, H. Liu, M. Fang and Z. Huang, Sci. Rep., 2015, 5, 9673.
6. M. M. Shang, C. X. Li and J. Lin, Chem. Soc. Rev., 2014, 43, 1372.
7. S. Nakamura, Angew. Chem., Int. Ed., 2015, 54, 7770.
8. A. Marchuk and W. Schnick, Angew. Chem., Int. Ed., 2015, 54, 2383.
9. X. Li, J. D. Budai, F. Liu, J. Y. Howe, J. Zhang, X.-J. Wang, Z. Gu, C. Sun, R. S. Meltzer and Z. Pan, Light: Sci. Appl., 2013, 2, e50.
10. K. Li, M. Shang, Y. Zhang, J. Fan, H. Lian and J. Lin, J. Mater. Chem. C, 2015, 3, 7096.
11. M. H. Fang, H. D. Nguyen, C. C. Lin and R. S. Liu, J. Mater. Chem. C, 2015, 3, 7277.
12. W. Lu, Y. Jia, Q. Zhao, W. Lv and H. You, Chem. Commun., 2014, 50, 2635.
13. J. Ding, Q. Wu, Y. Li, Q. Long, C. Wang and Y. Wang, Dalton Trans., 2015, 44, 9630.
14. G. Li, Y. Zhang, D. Geng, M. Shang, C. Peng, Z. Cheng and J. Lin, ACS Appl. Mater. Interfaces, 2012, 4, 296.
15. H. Liu, Y. Luo, Z. Mao, L. Liao and Z. Xia, J. Mater. Chem. C, 2014, 2, 1619.
16. W. R. Liu, C. H. Huang, C. W. Yeh, J. C. Tsai, Y. C. Chiu, Y. T. Yeh and R. S. Liu, Inorg. Chem., 2012, 51, 9636.
17. W. Tang and F. Zhang, Eur. J. Inorg. Chem., 2014, 3387.
18. P. Li, Z. Wang, Z. Yang and Q. Guo, J. Mater. Chem. C, 2014, 2, 7823.
19. X. Zhang and M. Gong, Dalton Trans., 2014, 43, 2465.
20. W. Lv, Y. Jia, Q. Zhao, M. Jiao, B. Shao, W. Lü and H. You, RSC Adv., 2014, 4, 7588.
21. Y. Jia, R. Pang, H. Li, W. Sun, J. Fu, L. Jiang, S. Zhang, Q. Su, C. Li and R. S. Liu, Dalton Trans., 2015, 44, 11399.
22. J. Zhou and Z. Xia, J. Mater. Chem. C, 2015, 3, 7552.
23. Z. Ci, Q. Sun, M. Sun, X. Jiang, S. Qin and Y. Wang, J. Mater. Chem. C, 2014, 2, 5850.
24. A. M. Kaczmarek and R. van Deun, Chem. Soc. Rev., 2013, 42, 8835.
25. X. Liu, W. Hou, X. Yang and J. Liang, CrystEngComm, 2014, 16, 1268.
26. Y. Liu, G. Liu, J. Wang, X. Dong and W. Yu, Inorg. Chem., 2014, 53, 11457.
27. Z. Wang, J. Zhong, H. Jiang, J. Wang and H. Liang, Cryst. Growth Des., 2014, 14, 3767.
28. B. Rekik, M. Derbal, O. Benamara and K. Lebbou, J. Cryst. Growth, 2014, 405, 11.
29. X. Huang, Z. Lin, L. Zhang, J. Chen and G. Wang, Cryst. Growth Des., 2006, 6, 2271.
30. J. M. Postema, W. T. Fu and D. J. W. Ijdo, J. Solid State Chem., 2011, 184, 2004.
31. L. Li, Y. Liu, R. Li, Z. Leng and S. Gan, RSC Adv., 2015, 5, 7049.
32. A. J. Peter and I. B. Shameem Banu, J. Mater. Sci.: Mater. Electron., 2014, 25, 2771.
33. G. Li, C. Li, C. Zhang, Z. Cheng, Z. Quan, C. Peng and J. Lin, J. Mater. Chem., 2009, 19, 8936.
34. L. Wu, Y. Zhang, M. Y. Gui, P. Z. Lu, L. X. Zhao, S. Tian, Y. F. Kong and J. J. Xu, J. Mater. Chem., 2012, 22, 6463.
35. H. Qian, J. Zhang and L. Yin, RSC Adv., 2013, 3, 9029.
36. L. Hou, S. Cui, Z. Fu, Z. Wu, X. Fu and J. H. Jeong, Dalton Trans., 2014, 43, 5382.
37. L. Jing, X. Liu, Y. Li and Y. Wang, J. Lumin., 2015, 162, 185.
38. Y. Zhang, W. Gong, J. Yu, H. Pang, Q. Song and G. Ning, RSC Adv., 2015, 5, 62527.
39. T. Ishigaki, A. Torisaka, K. Nomizu, P. Madhusudan, K. Uematsu, K. Toda and M. Sato, Dalton Trans., 2013, 42, 4781.
40. Y. Luo, Z. Xia, B. Lei and Y. Liu, RSC Adv., 2013, 3, 22206.
41. Y. Su, L. Li and G. Li, J. Mater. Chem., 2009, 19, 2316.
42. X. Li, Y. Zhang, D. Geng, J. Lian, G. Zhang, Z. Hou and J. Lin, J. Mater. Chem. C, 2014, 2, 9924.
43. E. Nakazawa and S. Shionoya, J. Chem. Phys., 1967, 47, 3211.
44. C. Lorbeer and A. V. Mudring, J. Phys. Chem. C, 2013, 117, 12229.
45. J. Zhou and Z. Xia, J. Mater. Chem. C, 2014, 2, 6978.
46. Z. Hou, Z. Cheng, G. Li, W. Wang, C. Peng, C. Li, P. Ma, D. Yang, X. Kang and J. Lin, Nanoscale, 2011, 3, 1568.
47. G. Li, Z. Hou, C. Peng, W. Wang, Z. Cheng, C. Li, H. Lian and J. Lin, Adv. Funct. Mater., 2010, 20, 3446.
48. K. Li, Y. Zhang, X. Li, M. Shang, H. Lian and J. Lin, Phys. Chem. Chem. Phys., 2015, 17, 4283.
49. S. P. Lee, C. H. Huang and T. M. Chen, J. Mater. Chem. C, 2014, 2, 8925.
50. C. H. Huang and T. M. Chen, J. Phys. Chem. C, 2011, 115, 2349.
51. W. Wu and Z. Xia, RSC Adv., 2013, 3, 6051.
52. S. Das, C. Y. Yang, H. C. Lin and C. H. Lu, RSC Adv., 2014, 4, 64956.
53. G. Blasse, Philips Res. Rep., 1969, 24, 131.
54. R. Reisfeld and L. Boehm, J. Solid State Chem., 1972, 4, 417.
55. W. J. Yang, L. Y. Luo, T. M. Chen and N. S. Wang, Chem. Mater., 2005, 17, 3883.
56. D. L. Dexter and J. A. Schulman, J. Chem. Phys., 1954, 22, 1063.
57. J. Sun, Z. Lian, G. Shen and D. Shen, RSC Adv., 2013, 3, 18395.

---