Quantum Cutting in KGd(CO₃)₂:Tb³⁺ Green Phosphor

Abstract

Phosphors with a longer excitation wavelength exhibit higher energy conversion efficiency. Herein, quantum cutting KGd(CO₃)₂:Tb³⁺ phosphors excited by middle-wave ultraviolet were synthesized via a hydrothermal method. All the KGd(CO₃)₂:xTb³⁺ phosphors remain in monoclinic structures in a large Tb³⁺ doping range. In the KGd(CO₃)₂ host, ⁶D_3/2 and ⁶I_17/2 of Gd³⁺ were employed for quantum cutting in sensitizing levels. The excited state electrons could easily transfer from Gd³⁺ to Tb³⁺ with high efficiency. There are three efficient excited bands for quantum cutting. The excited wavelengths of 244, 273, and 283 nm correspond to the transition processes of ⁸S_7/2→⁶D_3/2 (Gd³⁺), ⁸S_7/2→⁶I_17/2 (Gd³⁺), and ⁷F₆→⁵F₄ (Tb³⁺), and the maximum quantum yields of KGd(CO₃)₂:Tb³⁺ can reach 163.5, 119, and 143%, respectively. The continuous and efficient excitation band of 273–283 nm can well match the commercial 275 nm LED chip to expand the usage of solid-state light sources. Meanwhile, the phosphor also shows good excitation efficiency at 365 nm in a high Tb³⁺ doping concentration. Therefore, KGd(CO₃)₂:Tb³⁺ is an efficient green-emitting phosphor for ultraviolet-excited solid-state light sources.

1. Introduction

Gd³⁺, one of the most popular ions, was used to sensitize Tb³⁺ [1,2,3,4], Eu³⁺ [5,6], Dy³⁺ [7,8], and Sm³⁺ [9,10] in luminescence emission. There are four excited energy bands, ⁶G_J, ⁶D_J, ⁶I_J, and ⁶P_J, below 5200 cm⁻¹ in a Gd³⁺ ion [11]. Energy level matching enables excited electrons to transfer between different excited states by resonance, cross-relaxation, and phonon assistance [9,12]. The higher the energy of the exciting light, the greater the possibility of the energy level matching. In much of the literature, vacuum ultraviolet and short-wave ultraviolet are widely used for efficient quantum cutting [4,13]. In the energy range of 4500–5200 cm⁻¹, the higher energy band is ascribed to Gd³⁺ absorption of the ⁶G_J multiplet, which corresponds to the excitation light of vacuum or short-wave ultraviolet. Efficient emission of quantum cutting can be easily achieved with the assistance of ⁶G_J [13,14,15,16,17,18]. When the excited state energy is lower than 3900 cm⁻¹, energy level matching is difficult to achieve for quantum cutting. Some of the literature describes quantum cutting assisted with the ⁶D_J and ⁶I_J energy bands in the Tb³⁺ emission. However, excited state ions can easily transfer from Gd³⁺ to Eu³⁺ and Dy³⁺ by nonradiative relation and energy transfer in the ⁶D_J and ⁶I_J energy bands [19,20]. Additionally, the ⁶I_J energy band consists of six energy levels: ⁶I_15/2 (36,725 cm⁻¹), ⁶I_13/2 (36,711 cm⁻¹), ⁶I_11/2 (36,525 cm⁻¹), ⁶I_17/2 (36,461 cm⁻¹), ⁶I_9/2 (36,231 cm⁻¹), and ⁶I_7/2 (35,878 cm⁻¹), which correspond to the theoretical excitation wavelengths of 272.4, 272.5, 273.9, 274.4, 276.1, and 278.8 nm in Gd³⁺ [11], respectively. The Gd³⁺ absorption of the ⁶I_J level matches well with the 275 nm LED solid-state light source, which can be used to break through the limitation of the mercury lamp for miniaturization and portability of the product.

Rare earth carbonate is an excellent optical material with high photoluminescence [21,22] and birefringence [23,24]. In this work, we reported on the quantum cutting of Gd³⁺ sensitized KGd(CO₃)₂:Tb³⁺ phosphor in a relatively lower lever of ⁶D_J and ⁶I_J. In the phosphor, three important quantum cutting levels, ⁶I_J (Gd³⁺), ⁶D_J (Gd³⁺), and ⁵F_J (Tb³⁺), were systematically studied by the luminesce spectra, quantum yields, and decay curves. The detailed transfer processes of excited electrons between different energy levels are discussed.

2. Materials and Methods

KGd_1-x(CO₃)₂:xTb³⁺ (x = 0, 0.005, 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, note as KGdC:xTb³⁺) phosphors were synthesized via a hydrothermal method with the raw materials Gd(NO₃)₃·6H₂O (99.99%) and Tb(NO₃)₃·6H₂O (99.99%). First, the nitrates of Gd(NO₃)₃ and Tb(NO₃)₃ were dissolved into deionized water. Second, the mixture was added to the K₂CO₃ solution (0.55 mol/L) under vigorous stirring. Third, diluted nitric acid was used to adjust the pH value (9.5) of the mixture solution. Finally, the reaction solution was transferred to an autoclave and heated at 200 °C for 8 h. Then, the precipitate was washed with deionized water and ethanol three times. The phosphor was dried at 60 °C (40 min) for the final product.

The crystal structures of KGd(CO₃)₂:xTb³⁺ were analyzed by the X-ray diffractometer in the range of 10–80° (PANalytical, Almelo, the Netherlands). The morphologies and energy dispersive spectrum (EDS) were imaged via cold field emission scanning electron microscopy (Regulus 8220, Hitachi High-Tech Co., Tokyo, Japan). The luminescent properties were measured by an FLS920 fluorescence spectrophotometer equipped with 450 W Xe-lamp (Edinburgh Instruments, Livingston, UK). Using BaSO₄ as a reference, powder samples were placed in the PTFE powder vessels and slotted into the sample holder, and then the absolute quantum yields were measured by the integrating sphere within the FLS920 sample chamber in an indirect method. The lifetimes of KGd(CO₃)₂:xTb³⁺ were tested by the 60 W microsecond flashlamp (Edinburgh Instruments, Livingston, UK).

3. Results and Discussion

3.1. Crystal Structures

Figure 1 shows the diffraction patterns of Tb³⁺-doped KGdC at x = 0.1 and 0.9. In the variation in Tb³⁺ doping concentration, the diffraction peaks of samples are almost the same. The crystal structures remain stable in a larger Tb³⁺ doping range. The most intense diffraction peak is located at the (110) crystal plane for all samples. All the diffraction patterns are consistent with the standard card of monoclinic KGd(CO₃)₂ (JCPDS:1-88-1422) [25]. As the Tb³⁺ concentration increased, the diffraction peak of the (110) crystal plane shifted to a large angle. With a similar ion radius, Gd³⁺ (1.05 Å) could be easily substituted by Tb³⁺ (1.04 Å) at an arbitrary proportion [26]. Pure phase KGdC:xTb³⁺ phosphors were successfully synthesized without any second phase.

3.2. Morphology

Figure 2 shows the morphologies of KGdC:xTb³⁺ with different Tb³⁺ doping concentrations. As shown in Figure 2a–f, most of the particles exhibit well-developed monoclinic crystal grains with a size of about 80–350 μm. When the Tb³⁺ doping concentration is increased, the morphology of grains changes slightly. All grains show good monoclinic structure in samples with different Tb³⁺ doping ratios. The size of monoclinic grains is relatively large, especially at x = 0.3. Although large grains are easy to fracture, the type of grains could be clearly identified. A small area of monoclinic grain was selected to demonstrate the existence of elements in KGdC:0.5Tb³⁺. In the element distribution maps, K, Gd and Tb were well-dispersed (Figure 2g–i). As shown in Figure 2j, the element content was similar to the original stoichiometric ratio.

3.3. Luminescence Spectra

In the low Tb³⁺ doping concentration, the interactions between the emission ions are weak, and the transfer process of the excited state electrons is easily traced. Figure 3 shows the luminescence properties of KGdC:xTb³⁺ at x = 0.005. The efficiently excited wavelengths are clearly shown in Figure 3a for the 542 nm emission. On the excitation spectrum, the intense excitation peaks are located at 244, 253, 273, 306, and 312 nm, corresponding to the Gd³⁺ transitions from ⁸S_7/2 ground state to ⁶D_3/2, ⁶D_9/2, ⁶I_17/2, ⁶P_5/2, and ⁶P_7/2 excited states [11], respectively. Additionally, the weak excitation peaks are located at 351, 368, and 376 nm, corresponding to the Tb³⁺ transitions from ⁷F₆ to ⁵L₉, ⁵L_10, and ⁵G₆ [27,28], respectively. However, the intense peaks of the entire exciting spectrum mainly consist in the absorption of Gd³⁺, which indicates a high transfer efficiency from Gd³⁺ to Tb³⁺. The contribution of Gd³⁺ is indicated by the emission spectra in Figure 3b. Under the excitations of 244, 253, 273, and 306 nm, most of the Gd³⁺ excited state electrons could transfer to Tb³⁺ with the typical luminescence emission of 542 nm. Another typical emission of Gd³⁺ at 312 nm could be observed in all spectra, which indicates that a fraction of high excited state electrons could depopulate to ground state through the ⁶P_7/2 intermediate excited state.

As shown in Figure 4a, the peak intensities at 273 nm decreased rapidly with the increasing Tb³⁺ concentration in KGdC:xTb³⁺ phosphor monitored at 312 nm. The excitation intensities at 244, 253, and 273 nm are intense in low Tb³⁺ doping. When x is larger than 0.005, the excited intensities at 244 and 253 nm almost disappear. Furthermore, the exciting intensity at 273 nm also decreases rapidly. The excited intensity quenching of Gd³⁺ indicates that most excited state electrons cannot release energy through Gd³⁺ itself in a 312 nm emission at a high Tb³⁺ concentration.

Figure 4b shows the variations in exciting spectra under different Tb³⁺ concentrations in KGdC:xTb³⁺ phosphor monitored at 542 nm. At the lowest Tb³⁺ doping, the contribution to the Tb³⁺ emission is effective, with the intense absorption peaks of Gd³⁺ at 244, 253, 273, 306, and 312 nm. When the Tb³⁺ doping concentration is increased, the exciting peaks of short wavelength are slightly shifted to the longer wavelength from 244 to 249 nm, corresponding to the Tb³⁺ spin-allowed transition [29,30]. The other excitation peaks remain in the same position except for the variation in excitation intensity. The most obvious change is that the excitation intensities at 283 nm increased with the Tb³⁺ concentration, which corresponded to the abolishment of the Tb³⁺ spin-forbidden transition [22]. In the range of 300–380 nm, the exciting peaks were related to the f–f transition of Tb³⁺ [30]. Two excitation bands show strong absorption at high Tb³⁺ concentrations, which is suitable for the excitation of commercial 275 and 365 nm UV LED.

The typical emission intensities of KGdC:0.3Tb³⁺ at different excitation wavelengths are shown in Figure 4c. In the emission spectra of KGdC:0.3Tb³⁺, the 312 nm emission peak almost disappears for the excitation at 245, 273, and 283 nm. Meanwhile, the efficient emission intensities at 542 nm are about four times higher than those excited at 312 and 351 nm at x = 0.3. The relative emission intensities of KGdC:xTb³⁺ at 542 nm are shown in Figure 4d. As seen in the curves, the emission intensities at 245, 273, and 283 nm are relatively high. Especially for 245 nm excitation, the medium intensity of exciting light emitted the strongest emission light, which indicated that a higher efficient transfer existed between Gd³⁺ and Tb³⁺. For the excitation wavelengths of 273 and 283 nm, the phosphors exhibited a higher emission intensity in a wide range of Tb³⁺ doping. Moreover, the excitation spectrum of the phosphor is continuous in the range of 273 to 283 nm, which effectively matches the excitation of the 275 nm LED. For the excitation wavelength at 351 nm, the green emission intensity of phosphors slightly increased with increasing concentrations of Tb³⁺ ions without any concentration quenching. In addition, the decrease in emission intensity excited at 312 nm indicates that there is concentration quenching at this energy level in a high Tb³⁺ doping concentration.

3.4. Energy Level Diagram

Figure 5 shows the energy level diagram for the transitions of excited state electrons in the emission process. First, 244 nm photons are absorbed by Gd³⁺ in the ⁸S_7/2→⁶D_3/2 transition [31]. One of the excited state electrons in ⁶D_3/2 depopulates to ⁶P_7/2 in a non-radiation way; then, the electrons continually transfer to the ⁸S_7/2 ground state, and emit 312 nm photons. Owing to the similar energy levels of 40,851 cm⁻¹ (Gd³⁺, ⁶D_3/2) [11] and 40,749 cm⁻¹ (Tb³⁺, ⁵K₈) [27], the other parts of the excited state electron transfer to the neighboring Tb³⁺ by resonance vibration. Then, the cross-relaxation occurs between two Tb³⁺: ⁵K₈ + ⁷F₆→⁵D₄ + ⁵D₄ (process 1) [21]. Finally, two excited state electrons in ⁵D₄ are stimulated by one excited photon for the quantum cutting of ⁵D₄→⁷F_J (J = 6, 5, 4, 3) with typical Tb³⁺ emission at 542 nm.

In the excitation of 253, 273, and 306 nm, the ground state electrons of Gd³⁺ are excited to ⁶D_9/2, ⁶I_17/2, and ⁶P_5/2 levels, respectively. One of the excited electrons returns to the ⁶P_7/2 in a non-radiation way for the 312 nm emission. Most excited state electrons are transferred to Tb³⁺ with high transfer efficiency [32]. The similar energy gaps of ⁶D_9/2-⁵F₃ (3200 cm⁻¹) and ⁷F₆-⁷F₃ (3270 cm⁻¹) are beneficial to the ⁶D_9/2 electronic cross relation for the accumulation of ⁵F₃ excited state electron populations in process 2 [27]. Meanwhile, ⁶I_17/2 (Gd³⁺) excited state electrons also transfer to the adjacent ⁵F₃ (Tb³⁺) level by resonance migration. Then, ⁵F₃ excited state electrons relax to the ⁵F₄ level. In process 3, the energy gaps between ⁵F₄-⁵D₄ (14,835 cm⁻¹) and ⁷F₀-⁵D₄ (14,842 cm⁻¹) are close enough for the cross-relation: ⁵F₄ + ⁷F₆→⁵D₄ + ⁵D₄ [27]. This is also another effective way to realize quantum cutting in KGdC:xTb³⁺ phosphor. As for the Gd³⁺, the excited state electrons in ⁶P_7/2 reach the ⁵D₄ level of Tb³⁺ by cross-relaxation and non-radiation transition in processes 4 and 5. Another exciting peak at 283 nm is attributed to the Tb³⁺ absorption from ⁷F₆ to ⁵F₄. The excited state electrons are directly transferred to the ⁵D₄ level by cross-relaxation for the two-photon emission.

3.5. Quantum Yield

Figure 6 shows the quantum yields (QY) of KGdC:xTb³⁺ (x = 0.05, 0.1, 0.3, 0.5, 0.7, 0.9) under different excitation wavelengths. The results show that there are three efficient excitation bands in the quantum cutting process of KGdC:xTb³⁺. On the top curve, the initial value of QY reaches 132% at x = 0.05 under the 245 nm excitation. Moreover, under the optimal excitation wavelength, all QY values of KGdC:xTb³⁺ are greater than 100% and the maximum value is 163.5% at x = 0.5. This QY value is close to those of Tb³⁺-doped Ca₉Y(PO₄)₇ (157%) [4], K₂GdF₅ (177%) [33], and KY(CO₃)₂ (177%) [22]. On the middle and lower curves, the exciting efficiency at 273 nm is greater than that at 283 nm only at x = 0.05. Quantum yields are greater than 100% in the Tb³⁺ doping range of 0.3–0.7. As for KGdC:0.5Tb³⁺, the maximum QYs are 119 and 143% under the excitations at 273 and 283 nm, respectively. In addition, the series of weak excitation peaks caused by the f–f transition of Tb³⁺ ions is also worthy of discussion. In the range of 320–390 nm, 351 nm is the typical excitation wavelength for Tb³⁺ emission. QY values increase with the increasing Tb³⁺ doping concentration. Compared with the other luminescent hosts, phosphate [34], fluoroborates [35], and silicate [36], concentration quenching does not occur in the KGC host with a high Tb³⁺ doping concentration. The unique structure of [REO₈] polyhedral enables the sufficient distance between adjacent Tb³⁺ to avoid energy loss [21,23]. The maximum QY was 91.44% for KGdC:0.9Tb³⁺. It is interesting to confirm that the highly efficient excitation bands of quantum cutting are broad in KGdC host material, which is beneficial for choosing an appropriate exciting source for the application of solid-state lighting [37].

3.6. Decay Curve

Figure 7a shows the decay curves of KGdC:xTb³⁺ under excitation at 273 nm. The electronic depopulation of the ⁵D₄ level is characterized by the decay curve. As shown in the figure, all decay curves could be fitted by the single exponential. The fitted lifetimes mainly originate from the decay process of excited state electrons at the ⁵D₄ level. The values of a lifetime were shown in Figure 7b. In the excitation at 273 nm, the lifetime values of the ⁵D₄ level first increase and then decrease with an increasing Tb³⁺ concentration. The dramatic change originates from the competition between the ⁶I_17/2 (Gd³⁺) and ⁵F₃ (Tb³⁺) levels. The lifetime value is small in a small number of Tb³⁺. When the Tb³⁺ doping concentration is increased, the energy transfers between Gd³⁺ and Tb³⁺ are efficiently enhanced [38]. The lifetimes increase with the increasing number of excited state electrons. When x is larger than 0.3, the high concentration of Tb³⁺ improves the cross-relaxation efficiency between two Tb³⁺. The rapid accumulation of excited state electrons decreases the energy lifetime of the ⁵D₄ level. As for the excitation at 245 and 283 nm, there are two lower excitation intensities for the 312 nm emissions than that of 273 nm in low Tb³⁺ doping concentration (as shown in Figure 4a). The competition of excited state electrons between Gd³⁺ and Tb³⁺ is weak. The Gd³⁺ emission consumes a small number of excited state electrons, which results in a slight reduction in lifetime. When the Tb³⁺ doping concentration is further increased, the lifetime decreases rapidly in a shortened distance between Tb³⁺ ions [39].

4. Conclusions

Tb³⁺-doped KGd(CO₃)₂ phosphors with high quantum yields were prepared via a hydrothermal method. Tb³⁺ could substitute for Gd³⁺ in the KGd(CO₃)₂ crystal lattice at any proportion. In the Tb³⁺-doped KGd(CO₃)₂ material, Gd³⁺ is both an activated and sensitized ion. The energy transfer between Gd³⁺ and Tb³⁺ is highly efficient. Quantum cutting can be effectively triggered at three excitation bands: 245, 273, and 283 nm. Under excitation oat 245 nm, the quantum yield of KGdC:xTb³⁺ phosphor could reach 132% at x = 0.05, and the maximal value is 163.5% in the quantum cutting process of KGdC:0.5Tb³⁺. The excitation bands at 273 and 283 nm are continuous, which is consistent with the emission wavelength of commercial 275 nm LED. The highly efficient KGdC:Tb³⁺ phosphor could be potentially applied to the ultraviolet-excited solid-state light sources.