Multiple Energy Transfer in Luminescence-Tunable Single-Phased Phosphor NaGdTiO₄: Tm³⁺, Dy³⁺, Sm³⁺

Abstract

Advances in solid-state white-light-emitting diodes (WLEDs) necessitate the urgent development of highly efficient single-phase phosphors with tunable photoluminescence properties. Herein, the Tm³⁺, Dy³⁺, and Sm³⁺ ions are incorporated into the orthorhombic NaGdTiO₄ (NGT) phosphors, resulting in phosphors that fulfill the aforementioned requirement. The emission spectrum of Tm³⁺ ions overlaps well with the adsorption spectra of both Dy³⁺ and Sm³⁺ ions. Under the excitation at 358 nm, the single-phase NaGdTiO₄: Tm³⁺, Dy³⁺, Sm³⁺ phosphor exhibits tunable emission peaks in the blue, yellow, and red regions simultaneously, resulting in an intense white-light emission. The coexisting energy transfer behaviors from Tm³⁺ to Dy³⁺ and Sm³⁺ ions and the energy transfer from Dy³⁺ to Sm³⁺ ions are demonstrated to be responsible for this phenomenon. The phosphors with multiple energy transfers enable the development of single-phase white-light-emitting phosphors for phosphor-converted WLEDs.

1. Introduction

White-light-emitting diodes (WLEDs) are considered as the next generation of green lighting sources because of their low energy consumption, high output efficiency, and environmental friendliness, compared with traditional incandescent and fluorescent lamps [1,2,3,4]. At present, there are two methods are available for generating white light: the multichip combination method, in which red, green, and blue LED chips are used in combination, and the light conversion method, in which various phosphors are excited by blue, ultraviolet (UV), or near-UV LED chips [5,6]. The multichip combination method is limited by high costs and low performance. In the light conversion method, a yellow yttrium aluminum garnet phosphor powder such as (Y_1-aGd_a)₃(Al_1-bGa_b)₅O₁₂:Ce³⁺ is excited by a blue LED chip [7,8,9]; the white-light is finally obtained when the blue and yellow light are combined, which is a more popular approach in high-efficiency commercial LEDs. However, the lack of a red-light component in the combined white-light results in a low color-rendering index, which is the main weakness of this approach for lighting [10,11]. This disadvantage appears to be solvable by combining red, blue, and green tricolor phosphors excited by UV or near-UV LED chips. However, energy reabsorption between various phosphors results in unsatisfactory luminous efficiency [12]. Therefore, the development of a single-phase white phosphor that is excited by UV and near-UV chips and that exhibits high luminous efficiency, high color stability, and good reproducibility remains a challenge.

The Dy³⁺ ion is commonly used as an activator for white-light-emitting phosphors because of its two emission peaks in the blue and yellow regions. The blue-light emission peak is located at 486 nm and corresponds to the ⁴F_9/2→⁶H_15/2 transition of Dy³⁺ ions, whereas the yellow-light emission at 579 nm is related to the ⁴F_9/2→⁶H_13/2 transition [13,14,15]. Both of these transitions are highly sensitive to the crystal-field environment. Therefore, the yellow- and blue-light emissions can be well regulated by manipulating the crystal-field environment of the Dy³⁺ ion, thus enabling white-light emission with a perfect color-rendering. However, because the blue-emission peak at 486 nm is not a pure blue emission, the co-doping of Tm³⁺ ions, which emit blue light under UV excitation, can lead to better white-light emission upon adjustment of the doping concentrations of the Tm³⁺ and Dy³⁺ ions [16,17]. However, the white light produced by the Tm³⁺/Dy³⁺ co-doped phosphor still lacks a red component; thus, the color-rendering index and color temperature are low, resulting in cool white-light emission. Among all the other rare-earth ions, Sm³⁺ can exhibit an orange-red-light emission under UV excitation [18], suggesting that it might be a suitable supplement for providing a red-light component to improve the color-rendering index and transferring the emission from the cool to the warm white-light region. Therefore, in the present work, Tm³⁺, Dy³⁺, and Sm³⁺ ions were co-doped into a NaGdTiO₄ (NGT) matrix to obtain a single-phase white phosphor with improved luminescence properties.

Among the matrix compounds used for phosphors, which include phosphates, tungstates, molybdates, and vanadates, titanates offer several advantages, including low cost, easy preparation, and good thermal stability [19,20,21,22,23,24]. NGT is a typical perovskite-structured titanate with a two-dimensional (2D) layered structure (Figure 1b). In this structure, the ionic radius of Gd³⁺ is similar to those of Tm³⁺, Dy³⁺, and Sm³⁺ ions, which theoretically enables the formation of a continuous solid solution without changing the original crystal structure [25,26]. In addition, NGT exhibits excellent chemical and thermal stability and strong absorption in the near-UV region [27,28]. Compared with the traditional inorganic phosphor materials, NGT has a relatively high critical concentration because of the limited energy transfer in the semi-2D sublattice [29]. Thus, we prepared the NGT-based phosphors doped with various activator ions (Tm³⁺, Dy³⁺, and Sm³⁺) via a conventional solid-state reaction and studied their luminescence properties, energy transfer behavior, and tunable emission properties in detail.

2. Material Synthesis and Characterization

The NGT: Tm³⁺, NGT: Dy³⁺, and NGT: Sm³⁺ single-doped phosphors; NGT: Tm³⁺/Dy³⁺, NGT: Tm³⁺/Sm³⁺ co-doped phosphors; and NGT: Tm³⁺/Dy³⁺/Sm³⁺ tri-doped phosphors were prepared via a high-temperature solid-state reaction method. High-purity TiO₂, Gd₂O₃, Dy₂O₃, and Sm₂O₃ produced by Tianjin Guangfu Fine Research Institute (Tianjin, China); high-purity Tm₂O₃ produced by Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China); and analytically pure Na₂CO₃ produced by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) were used as raw materials.

First, appropriate amounts of the reactants were weighed according to the desired stoichiometric ratio, and an excess of Na₂CO₃ (30%) was added as a flux. The weighed compounds were transferred to a mortar and thoroughly mixed and ground for a few minutes. The resultant mixture was transferred to a crucible, which was subsequently placed in the center of a muffle furnace. The mixture was heated to 1000 °C at a rate of 10 °C/min and maintained at this temperature for 2 h under an air atmosphere. After naturally cooling to room temperature, the product was collected and thoroughly ground; the excess Na₂CO₃ in the product was removed by centrifugation with deionized water. After centrifugation, the product was placed in an oven and dried at 120 °C. After drying sufficiently, the sample was again ground for characterization.

Samples were first scanned using an X-ray powder diffractometer (DRU Advance, BRUKER, Billerica, MA, USA) over the 2θ range of 5–80° at a scan step width of 0.02° and a dwell time of 0.05 s per step. The phase behavior and purity of the samples were determined by comparing the XRD pattern of each sample to that reported in the standard card (JCPDS #86-0830). The morphology of the sample was observed using a Zeiss SUPRA-55 field-emission scanning electron microscope (Oberkochen, Germany). The elemental distribution of the sample area was qualitatively and quantitatively studied using energy spectra recorded with a field-emission scanning electron microscope (JSM-7001F, JEOL, Ltd., Tokyo, Japan). The photoluminescence excitation (PLE) spectra and photoluminescence (PL) emission spectra were recorded at room temperature using a Hitachi F-4600 fluorescence spectrometer (Hitachi, Japan). The excitation light source was a 150 W ozone-free Xe lamp; the scanning speed was 240 nm/min, and the photomultiplier tube voltage was 600 V. The PL quantum yields (QY) were recorded by a 10-inch integrating sphere (LMS-100, Labsphere Inc., North Sutton, NH, USA) with a multichannel CCD detector (USB QE Pro-65, Ocean Optics Inc., Edinburgh, UK). The decay curves and lifetimes of the samples were recorded using an Edinburgh FLS1000 fluorescence spectrometer (Edinburgh, UK).

3. Results and Discussion

3.1. Phase Composition and Morphologies

Figure 1a shows the XRD patterns of the as-prepared samples NGT: 3%Tm³⁺, NGT: 5%Dy³⁺, NGT: 3%Sm³⁺, NGT: 3%Tm³⁺/5%Dy³⁺, NGT: 3%Tm³⁺/3%Sm³⁺, and NGT: 3%Tm³⁺/5%Dy³⁺/3%Sm³⁺. All of the XRD patterns substantially match the standard card (JCPDS #86-0830) of NaGdTiO₄. The results indicate that the phase compositions for both the single-doped and co-doped phosphors were stable, which means that the rare-earth dopants Tm³⁺, Dy³⁺, and Sm³⁺ did not change the crystal structure of the products. According to the crystal structure of NGT shown in Figure 1b, the ionic radius of Gd³⁺ (0.1053 nm) in coordination with oxygen atoms is similar to the radii of Tm³⁺ ion (0.0994 nm), Dy³⁺ ion (0.1027 nm), and Sm³⁺ ion (0.1079 nm) in the same coordination environment [30]. However, the charge of the Na⁺ ion and the radius of the Ti⁴⁺ ion vary dramatically from those of rare-earth ions. Thus, the Tm³⁺, Dy³⁺, and Sm³⁺ ions likely substitute Gd³⁺ ions, maintaining the stability of the NGT structure.

The SEM images of the single-doped phosphors, co-doped phosphors, and tri-doped phosphor (Figure 2a–f) show that their morphologies are similar. All of the samples are composed of flakes with an average length of 1.5–2.5 μm and thickness of ~200–600 nm. To further confirm the doping of Tm³⁺, Dy³⁺, and Sm³⁺ ions into the NGT flakes, EDS mapping images were collected (Figure 2g–n). The results indicate that the Tm³⁺, Dy³⁺ and Sm³⁺ are uniformly distributed on the surface of the NGT: Tm³⁺/Dy³⁺/Sm³⁺ flakes, further confirming that these elements are effectively doped into NGT host.

3.2. PL Properties of NGT: Dy³⁺, NGT: Tm³⁺, and NGT: Sm³⁺

Figure S1a shows the PLE spectra of NGT: y%Dy³⁺ (y = 1, 3, 5, 7, 9) phosphors when monitoring at 579 nm. The phosphor with a doping amount of 3% exhibits the highest excitation intensity. The PLE spectra comprise a series of sharp excitation peaks in the region from 310 nm to 500 nm. These excitation peaks, centered at 326, 352, 366, 388, 427, 453, and 473 nm, are attributed to the transitions from ⁶H_15/2 to ⁴M_17/2, ⁹P_7/2, ⁴P_5/2, ⁴I_13/2, ⁴G_11/2, ⁴I_15/2, and ⁴F_9/2, respectively [14,15]. These excitation peaks are caused by the f-f transitions in the 4f⁶ shell of Dy³⁺ ions.

The PL spectra of the NGT: y%Dy³⁺ phosphors excited at an excitation wavelength of 352 nm are shown in Figure S1b. Two distinct characteristic emission peaks are observed at 486 nm and 579 nm, along with one weak emission peak at 670 nm. These emission peaks correspond to the ⁴F_9/2→⁶H_15/2, ⁴F_9/2→⁶H_13/2, and ⁴F_9/2→⁶H_11/2 transitions of Dy³⁺ ions, respectively [14,15]. Moreover, with increasing Dy³⁺ concentration from 1% to 7%, the luminescence intensity gradually increases, reaches a maximum at 3%, and then rapidly decreases with a further increase in concentration. The concentration quenching mainly arises from an increase in probability of nonradiative interaction with increasing distance between Dy³⁺ ions at higher concentrations.

To explore the main mechanism of concentration quenching of NGT: Dy³⁺ phosphors, the van Uitert equation (Equation (S1)) was used to fit the emission intensity at different concentrations [31]. In this equation, Q values of 3, 6, 8, and 10 represent exchange interactions, electric dipole-electric dipole interactions, electric dipole–electric quadrupole interactions, and electric quadrupole–electric quadrupole interactions, respectively. The emission intensity as a function of the concentration of Dy³⁺ ions is shown in Figure S1c. According to the fitted curves, the Q values of the yellow-light and blue-light emission are 5.89 and 6.13, respectively, both of which are very close to 6. This result indicates that the main mechanism of the concentration quenching of the NGT: Dy³⁺ phosphors is the electric dipole–electric dipole interaction. Furthermore, the color coordinates of the NGT: 3%Dy³⁺ phosphor were calculated to be (0.3504, 0.3783).

Figure S1d,e shows the PLE and PL spectra, respectively, of the NGT: x%Tm³⁺ (x = 1, 2, 3, 5, 7) phosphors. One strong blue-emission peak at 459 nm due to the ¹D₂→³F₄ transition is obvious [32]. The emission intensity increases with increasing Tm³⁺ doping amount from 1%, reaches a maximum at 3% (Figure S1f), and then begins to decrease because of concentration quenching. The color coordinates of the NGT: 3%Tm³⁺ phosphor are calculated to be (0.1673, 0.0953), which fall within the blue region.

The PLE and PL spectra of the NGT: z%Sm³⁺ (z = 1, 2, 3, 4, 5) phosphors were also recorded (Figure S1g,h). Under excitation at 409 nm, the phosphors show distinct characteristic peaks at 568, 606, and 653 nm in their PL spectra, corresponding to the ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2, and ⁴G_5/2→⁶H_9/2 transitions of Sm³⁺, respectively [18]. Among these emission peaks, the one at 606 nm is much more intense than the others, resulting in a strong orange-red emission. As the Sm³⁺ concentration increases from 1% to 5%, the luminescence intensity first increases and then begins to decrease when the concentration is greater than 3% (Figure S1i), which is again attributed to concentration quenching. The calculated color coordinates of NGT: 3%Sm³⁺ are (0.5737, 0.4189), indicating that this phosphor can potentially be used as the red emitter for WLEDs.

As shown in Figure S1 and Figure 3b, the emission peak at 459 nm of NGT: Tm³⁺ overlaps well with the excitation peaks of both NGT: Dy³⁺ and NGT: Sm³⁺ phosphors in the absorption region from 440 to 480 nm, which may result in efficient energy transfer form Tm³⁺ to Dy³⁺ or Sm³⁺ ions [33]. Thus, we co-doped Dy³⁺ and Sm³⁺ ions with Tm³⁺ ions in an NGT host. First, the PL spectra of NGT: 3%Tm³⁺/m%Dy³⁺ (m = 1, 3, 5, 7) phosphors at an excitation wavelength of 360 nm were recorded (Figure 4a). These spectra have distinct characteristic emission peaks of Dy³⁺ and Tm³⁺ ions at 459, 483, and 578 nm, corresponding to the ¹D₂→³F₄ transition of Tm³⁺ and the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺, respectively [17]. As the concentration of Dy³⁺ ions increases, the intensity of the emission peaks of Dy³⁺ gradually increases until the Dy³⁺ concentration reaches 5%. By contrast, the intensity of the emission peak corresponding to Tm³⁺ ions decreases with increasing concentration of Dy³⁺ ions, confirming the energy transfer behavior from Tm³⁺ ions to Dy³⁺ ions. As a result, the PL spectra and color coordinates can be tuned by controlling the doping concentration of Dy³⁺ ions (Figure 3a and Table S1).

All of the NGT: 3%Tm³⁺/m%Dy³⁺ phosphors exhibit cool white-light emission because of their lack of red emission. Thus, doping with rare-earth ions that can compensate for the emission in the red region, such as Sm³⁺ ions, is necessary. On the basis of this approach, we prepared NGT: 3%Tm³⁺/n%Sm³⁺ phosphors; the PL spectra at an excitation wavelength of 360 nm are presented in Figure 4c. Obvious characteristic emission peaks are observed at 459, 567, 606, and 652 nm, which are attributed to the ¹D₂→³F₄ transition of Tm³⁺ and the ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2, and ⁴G_5/2→⁶H_9/2 transitions of Sm³⁺ ions, respectively [18]. When the Sm³⁺ concentration increases, the characteristic emission peak intensity of Tm³⁺ decreases, whereas the characteristic emission intensity of Sm³⁺ increases and reaches a maximum when the Sm³⁺ doping concentration is 4% because of the energy transfer between Tm³⁺ and Sm³⁺. Thus, the color coordinates are also tunable; those of the NGT: 3%Tm³⁺/4%Sm³⁺ phosphor are calculated to be (0.2891, 0.2171) because of the red-emission contribution of the Sm³⁺ ions.

To further confirm the energy transfer from the Tm³⁺ to Dy³⁺ and Sm³⁺ ions, we recorded the decay curves and average lifetimes of Tm³⁺ ions in the NGT: 3%Tm³⁺/m%Dy³⁺ and NGT: 3%Tm³⁺/n%Sm³⁺ phosphors (Figure 4b,d). All of the decay curves indicate that the emission intensities decrease with increasing decay time. With increasing concentrations of Dy³⁺ and Sm³⁺ ions, the emission intensity decays much more quickly. Furthermore, the decay curves were fitted through a double-exponential function based on Equations (S2) and (S3) [34]. The average lifetimes τ were calculated and are shown in the inset tables in Figure 4. The lifetimes decrease with increasing concentration of both Dy³⁺ and Sm³⁺ ions, implying that the diminished lifetimes mainly result from the increasing energy transfer from Tm³⁺ ions to Dy³⁺ and Sm³⁺ ions at high Dy³⁺ and Sm³⁺ doping concentrations.

Equation (S4) was used to calculated the energy transfer efficiencies (η) from the sensitizer Tm³⁺ to the activators Dy³⁺ and Sm³⁺ according to the intrinsic decay lifetime of the sensitizer with (I_s) and without (I₀) an activator [35]. The calculated η values are shown in the inset of Figure 4b,d, where the η values are observed to increase gradually with increasing activator dopant concentration. The η values also reached a relatively large value when the doping concentration was suitable for intense emission.

3.3. PL Properties and Energy Transfer Behavior of NGT: Tm³⁺/Dy³⁺/Sm³⁺

To further improve the emission properties of the as-prepared phosphors, we co-doped the Sm³⁺ ion Tm³⁺ and Dy³⁺ ions in the NGT host. The emission spectra of NGT: 3%Tm³⁺/5%Dy³⁺/h%Sm³⁺ phosphors excited at an excitation wavelength at 360 nm are shown in Figure 5a. The characteristic emission peaks of Tm³⁺, Dy³⁺, and Sm³⁺ ions are observed at 459 nm, 483 and 578 nm, and 603 nm, respectively. The emission intensity of the Sm³⁺ ions increases with increasing doping concentration of Sm³⁺ and reaches a maximum when the Sm³⁺ doping amount is 3%. The intensity of all of the other emission peaks decreases with increasing doping concentration of Sm³⁺. The decreasing emission intensity of Tm³⁺ ions is easily explained by the ET from Tm³⁺ to Dy³⁺ and Sm³⁺ ions. The overlap of the emission peak of Dy³⁺ and the excitation peak of Sm³⁺, as shown in Figure S2, enable the transfer the energy from Dy³⁺ ions to Sm³⁺ ions, which might result in decreased emission of Sm³⁺ with increasing concentration of Sm³⁺ ions. To investigate this ET mechanism, we plotted the decay curves as a function of dopant concentration (Figure 5b,c). Under excitation at 360 nm, the emission intensities of the Tm³⁺ and Dy³⁺ decay quickly with increasing Sm³⁺ concentration.

The lifetimes of Tm³⁺ at 459 nm were calculated according to the Equation (S5) [29]; those of Dy³⁺ ions at 487 nm were fitted by a biexponential temporal dependence (Table S2), and the average lifetimes τ are calculated through Equations (S2) and (S3). The average lifetimes of Tm³⁺ and Dy³⁺ are reported in the inset tables in Figure 5b,c, where both are observed to decrease slightly with increasing Sm³⁺ concentration. On the basis of ET of the NGT: 3%Tm³⁺/5%Dy³⁺ sample, the ET efficiencies are further enhanced by more than 20%, when the Sm³⁺ ions are co-doped with Tm³⁺ and Dy³⁺. These results confirm the ET from Tm³⁺ to Dy³⁺ and Sm³⁺ and the ET from Dy³⁺ to Sm³⁺ in the NGT host under excitation of Tm³⁺ at a wavelength of 360 nm.

The process for the ET of NGT: Tm³⁺/Dy³⁺/Sm³⁺ phosphors can be described in detail using the energy-level diagram in Figure 6. When the phosphors are excited by UV light at 358 nm, energy is absorbed by the Tm³⁺ ions, which emit blue-light with a peak at 459 nm. At the same time, the relaxed energy could also be absorbed by Dy³⁺ and Sm³⁺ ions, thus achieving ET from Tm³⁺ to Dy³⁺ and Sm³⁺ and resulting in blue, yellow, and red-emission peaks. When the electrons fall to the ground state from the excited states of Dy³⁺, energy can also be transferred to the Sm³⁺ ion, resulting in red-emission. The blue, yellow, and red-emissions combine, resulting in white-light.

We obtained the CIE chromaticity diagram for the NGT: 3%Tm³⁺/5%Dy³⁺/h%Sm³⁺ phosphors (Figure 3a). The chromaticity coordinates were tunable from blue-, red-, and yellow-light regions to the white-light region. The chromaticity coordinates of NGT: 3%Tm³⁺/5%Dy³⁺/2%Sm³⁺ sample were calculated to be (0.2767, 0.2536), which are near the equal-energy point (0.3333, 0.3333) and comparable with similar results in in previous literatures (Figure S3 and Table S3). Meanwhile, the QY for the phosphors NGT: 3%Tm³⁺/5%Dy³⁺ and NGT: 3%Tm³⁺/5%Dy³⁺/2%Sm³⁺ were measured to be 0.25 and 0.21, respectively. These results indicate that the NGT: Tm³⁺/Dy³⁺/Sm³⁺ phosphor is suitable for commercial applications.

4. Conclusions

A series of Tm³⁺-, Dy³⁺-, Sm³⁺-, Tm³⁺/Dy³⁺-, Tm³⁺/Sm³⁺-, and Tm³⁺/Dy³⁺/Sm³⁺-doped NGT phosphors were synthesized via a solid-state reaction method, and their luminescence properties were investigated in detail. The Tm³⁺ single-doped NGT phosphor emits blue-light at 459 nm under excitation by UV light. The Dy³⁺ single-doped NGT phosphor emits blue-light at 486 nm and yellow-light at 579 nm under excitation by UV light, showing white-light emission. The single-doped Sm³⁺ phosphor shows red-light emission under excitation by 409 nm near-UV light. When Dy³⁺ and Sm³⁺ ions are co-doped into the phosphor NGT: 3%Tm³⁺, color-tunable emission is achieved, with chromaticity coordinates changing from (0.1673, 0.0953) to (0.2468, 0.2359) and (0.2891, 0.2171), respectively. White-light with chromaticity coordinates of (0.2767, 0.2536) was finally obtained by doping Sm³⁺ ions into the NGT: 3%Tm³⁺/5%Dy³⁺ phosphor. The results are may be the simultaneous ET from Tm³⁺ to Dy³⁺ and Sm³⁺ ions and ET from Dy³⁺ to Sm³⁺ ions.