Phosphor-converted white-light-emitting diodes (pc-WLEDs) have been of great interest for the nextgeneration lighting technology considering their potential to reduce the electricity consumption of conventional lighting sources [1–3]. Furthermore, pc-WLEDs exhibit large lifetimes, robustness, high luminous efficacies, and environment-friendly characteristics [4–6]. The most common method for the fabrication of pc-WLEDs is based on the blue emitting LED chips and combined with yellow light (red and green emitting phosphors). However, because the red-light component is not sufficient, the commercial pc-WLEDs exhibit low color rendering index (CRI) values [7,8], which hinders their applications. The performances of pc-WLEDs are dependent on the luminescence properties of the used phosphorus. Thus, advanced pc-WLEDs require improved quantum yield and thermal stability.

Among the oxide materials, yttrium oxide (Y₂O₃) has been extensively studied owing to its high thermal conductivity, good physical and chemical stabilities, high transparency in the ultraviolet to mid-infrared ranges, and low phonon energy [9–11]. Among the various rare-earth ions, praseodymium (Pr³⁺)-doped materials are of interest, because Pr³⁺ ions have an energy level diagram containing several multiplets. This imply that Pr³⁺-doped materials can emit light simultaneous blue, green, and red emissions depending on the hosts [12,13]. Therefore, Pr³⁺-doped materials can be used as luminescent materials for WLEDs, scintillators, and three-dimensional (3D) visual displays [14–16]. Over the past few years, the syntheses and structural and luminescent properties of Pr³⁺-doped phosphors such as CaTiO₃:Pr³⁺, LaBMO₆:Pr³⁺ (M = W, Mo), and Sr₂Ge₂O₄:Pr³⁺ have been investigated [17–19]. Alarcon-Flores et al. elucidated the structural and luminescent characteristics of Pr³⁺-doped Y₂O₃ powders using the solvent evaporation method [20].

In this study, we report the structural and optical properties of Pr³⁺-doped Y₂O₃ nanopowders obtained high-energy ball-milling (HEBM). The structural and optical properties of Pr³⁺-doped Y₂O₃ nanopowders were investigated as a function of the Pr³⁺ concentration and annealing temperature. The HEBM method is environmentally friendly, cost-effective, simple, and reliable. This technique also does not require high-temperature treatment for the production of nanopowders [21,22].

1. Sample preparation

Y₂O₃:Pr³⁺ powders were prepared by mixing yttrium oxide (Y₂O₃), followed by addition of 0.1, 0.5, and 1.0 mol% praseodymium (III, IV) oxide (Pr₆O₁₁). The mixtures were then milled using a planetary ball mill at room temperature. The mixture weight ratio of the Y₂O₃:Pr³⁺ powder and zirconia (ZnO) balls was 50:1. They were rotated in an 80-mL zirconia jar at 500 rpm for a milling time (tm) of 300 min. The milling was conducted for 15 min, and then rested for 15 min to avoid an excessive increase in temperature inside the mill pot. The as-prepared powders were annealed at 600, 800, 1000, and 1200 °C for 1 h.

2. Characterization

The structure of synthesized samples was characterized by X-ray diffraction (XRD) (PANalytical, X’pert PRO MPD) with Cu Kα (λ = 1.54060 Å) radiation in the 2θ measurement rage of 5 – 65° in scanning rate of 0.03°/s. The acceleration voltage and current were 40 kV and 30 mA, respectively. The morphology of the sample was observed by field-emission scanning electron microscopy (FE-SEM, S-4800 Hitachi) at an acceleration voltage of 15.0 kV. Raman spectra were acquired using an XperRam F1.4 system (Nanobase Inc., Seoul, Korea) equipped with a 785-nm diode laser source. The laser power incident on the sample surface was 6.2 mW. The spectra were recorded by scanning the 100-2800 cm^-1 region, which was accumulated for 10 scans with a 5s acquisition time. The photoluminescence excitation (PLE) and emission (PL) spectra were measured using an FL 6500 fluorescence spectrometer (Perkin-Elmer Inc., Waltham, USA) at room temperature.

1. Structural properties

Figure 1(a) presents XRD patterns of the Pr³⁺-doped Y₂O₃ powders for different doping levels (0.1, 0.5, and 1 mol%) for a milling time of 300 min, annealing temperature of 1200 °C, and annealing time of 1 h. A good match with the standard XRD peaks of the cubic Y₂O₃ phase (Inorganic Crystal Structure Database (ICSD) card #086817) plotted at the bottom of the diagram was obtained, without impurity peaks. The absence of secondary phases confirmed the efficacy of the synthesis in transferring the Pr³⁺ ions into the Y₂O₃ matrix in the entire range of Y³⁺ substitution levels. The 2θ values were 29.2, 33.9, 48.7, and 57.8°, corresponding to the (222), (400), (440), and (622) planes, respectively. The increase in Pr content shifted the main diffraction peak of the (222) plane at approximately 2θ ~ 29.2° toward smaller angles (Fig. 1(b)), and the lattice constant increased (Fig. 1(c)), which suggests lattice expansion. These phenomena can be attributed to the replacement of Y³⁺ ions with a smaller ionic radius (r = 0.90 Å) by Pr³⁺ ions with a larger ionic radius (r = 1.13 Å) in the Y₂O₃ matrix. These results are consistent with previous researches [23–26]. Figure 1(d) presents the XRD patterns of the 0.1-mol%-Pr³⁺-doped Y₂O₃ powders annealed at different temperatures for 1 h. The XRD spectra of these samples also agreed with the standard data of the cubic phase of the Y₂O₃ powders. The XRD peaks increased and broadening of peaks decreased with increasing annealing temperature. This indicate that the crystallinity of the powder is improved. As the annealing temperature increases, the characteristic (220) diffraction peak around 2θ ~ 29.2° slightly shifted toward larger angles (Fig. 1(e)) and the lattice constant decreased (Fig. 1(f)). This behavior suggests lattice contraction. Y₂O₃:Pr³⁺ structure may typically have a number of defects such as oxygen vacancies, lattice disorders, etc. As a result of annealing these defects are removed and the lattice contracts.

Figure 1. (Color online) (a) XRD patterns of the Y₂O₃:Pr³⁺ 1200-°C concentration series and standard data of the cubic phase of Y₂O₃ (ISCD card no. 086817); (b) expanded 2θ view of the XRD patterns; (c) lattice parameter as a function of the Pr³⁺ doping concentration; (d) XRD patterns of the Y₂O₃:Pr3 0.1-mol% temperature series; (e) expanded 2θ view of the XRD patterns; (f) lattice parameter as a function of the annealing temperature.

Figure 2(a)–(d) present SEM images of the 0.1-mol%-Pr³⁺-doped Y₂O₃ powders fabricated at the different annealing temperatures (600, 800, 1000, and 1200 ℃). At 600 °C, the SEM image of the sample shows irregular microstructures with fairly small highly agglomerated grains. It can be seen that the shapes of the particles became clearer with obvious facets and round edges with increasing annealing temperature. The particle size of the Pr³⁺-doped Y₂O₃ powder increased from 50 to 110 nm with the increase in the annealing temperature from 600 to 1200 ℃. During the annealing, individual nanostructures mixed and coalesced with one another, yielding larger particles. An elemental energy-dispersive spectroscopy (EDS) analysis of the Pr³⁺-doped Y₂O₃ powders is shown in Fig. 2(e). The EDS spectrum confirms the existence of Y, O, and Pr in good agreement with the initial elements.

Figure 2. (Color online) SEM images of the Y₂O₃:Pr³⁺ powders at different annealing temperatures of (a) 600, (b) 800, (c) 1000, and (d) 1200 ℃; (e) ED spectrum of the Y₂O₃:Pr³⁺ powders at the annealing temperature of 1200 ℃ and milling time of 300 min.

To further investigate the crystal structures of the Pr³⁺-doped Y₂O₃ powders, we measured the Raman spectra of the samples. Raman spectroscopy can be used to characterize the phonon energies of materials. Figure 3(a) and (b) show the Raman spectra of the Pr³⁺-doped Y₂O₃ powders with different doping contents of Pr³⁺ (0.1, 0.5, and 1 mol%) fabricated at different annealing temperatures (600, 800, 1000, and 1200 °C) in the range of 100–2800 cm^-1. In addition, the Raman spectrum of a pure Y₂O₃ powder is shown in Fig. 3(c). As shown in Fig. 3, no significant differences are observed between the Raman spectra of the pure Y₂O₃ powder and Pr³⁺-doped Y₂O₃ powders, which implies that the introduction of Pr³⁺ may not lead to additional modes. In the range of 100–500 cm^-1, the major peak assigned to the F_g + A_g mode is observed at 375 cm^-1 originated from the Y³⁺–O^2- vibration [27–29]. The intensity of the Raman peak decreased with increasing the Pr³⁺ concentration. This result is consistent with the XRD results for the Y₂O₃:Pr³⁺ powders with different doping contents of Pr³⁺. As the annealing temperature increased, the Raman bands became more intense and sharper. It is likely that better and larger crystallites are produced in the host material with the increase in the annealing temperature. Therefore, a better arrangement of the Pr³⁺ ions at the crystallographic sites of the Y₂O₃ cubic lattice is achieved. This is also supported by the XRD results.

Figure 3. (Color online) Raman spectra of the Y₂O₃:Pr³⁺ powders at different (a) Pr³⁺ doping concentrations and (b) annealing temperatures and (c) pure Y₂O₃ powder.

2. Optical properties

The photoluminescence efficiencies of rare-earth phosphor materials can be enhanced by the doping concentration and annealing temperature. Thus, it is important to determine the optimal dopant concentration and annealing temperature. Figure 4(a) and (b) present the PLE and PL spectra of the Y₂O₃:Pr³⁺ powders as a function of the Pr³⁺ doping concentration, respectively. The PLE spectra contain an intense excitation band at 285 nm at an emission wavelength of 630 nm. This implies that the Pr³⁺-doped Y₂O₃ powders can be used as red-emission phosphors in WLEDs based on ultraviolet or blue-lightemitting diode chips. The inset of Fig. 4(b) presents photoluminescence photographs of the Pr³⁺-doped Y₂O₃ powders for different doping levels (0.1, 0.5, and 1.0 mol%) annealed at 1200 °C under a 365-nm ultraviolet lamp radiation. The 0.1-mol%-Pr³⁺-doped Y₂O₃ powder exhibited a stronger fluorescence than those at 0.5 and 1 mol%. The PL spectra showed the characteristic peaks at 618 (³P₀ → ³H₆), 630 (²D1 → ³H₄), 644 (³P₀ → ³F₂), and 509 nm (³P₀ → ³H₅) at an excitation wavelength of 285 nm, which is in good agreement with the literature [30, 31]. Compared to other emission transitions from the ³P₀ excited state, the emission transition starts 1D2 state shows high intensity. Because the ³P₀ state is relaxed non-radiatively to the 2D1 state via the deep lying 4f5d level, and quenches the emission from the ³P₀ manifolds [32]. An energy level diagram of the Pr³⁺ ion is shown in Fig. 5 [33]. The intensities of the emission bands decreased with increasing the Pr³⁺ concentration. This result is attributed to concentration quenching, caused by the increase in the ion–ion interaction triggered by the smaller distance between the interacting activators with the increase in concentration [34, 35]. Therefore, the PL intensities decreased with increasing the Pr concentration. The maximum PL intensity of the Y₂O₃:Pr³⁺ powders was obtained at an annealing temperature of 1200 °C, annealing time of 1 h, and Pr³⁺ doping concentration of 0.1 mol%.

Figure 4. (Color online) (a, b) Excitation and emission spectra of the Y₂O₃:Pr³⁺ powders at different Pr³⁺ doping concentrations; (c, d) excitation and emission spectra of the Y₂O₃:Pr³⁺ powders at different annealing temperatures. The inset shows luminescence photographs.

Figure 5. (Color online) Energy level structure of the trivalent praseodymium ion (with wavelengths for Y₂O₃:Pr³⁺).

Figure 4(c) and (d) present the PLE and PL spectra of the 0.1-mol%-Pr³⁺-doped Y₂O₃ powders as a function of the annealing temperature, respectively. The inset of Fig. 4(d) presents luminescence photographs of the 0.1-mol%-Pr³⁺-doped Y₂O₃ powders at different annealing temperatures (600, 800, 1000, and 1200 °C) under the 365-nm ultraviolet lamp radiation. The fluorescence of the Pr³⁺-doped Y₂O₃ powder was enhanced with the increase in the annealing temperature. The PLE spectrum monitored at 630 nm exhibited a broad band centered at 285 nm. The PL spectrum consisted of a strong red peak at 630 nm and very weak green peak at 509 nm at an excitation wavelength of 285 nm. The PL emission intensity increased with the annealing temperature owing to the increased crystallinity, which is proved by the XRD results. Therefore, the Pr³⁺ ions were sufficiently dispersed at the crystallographic sites in the Y₂O₃ cubic lattice. The PL intensity of the Y₂O₃:Pr³⁺ sample annealed at 1200 °C at 630 nm was approximately 3.8 times that of the sample annealed at 600 °C (Fig. 4(d)).

In this study, we synthesized a series of Pr³⁺-doped Y₂O₃ powders using the HEBM method, and investigated their structural and optical properties as functions of the Pr³⁺ ion doping concentration and annealing temperature. These powders showed the standard Y₂O₃ cubic phase without structural impurities. As the annealing temperature increased, the XRD peak intensity increased, due to the improvement in crystallinity. The emission spectra showed a strong red emission at 630 nm (²D1 → ³H₄) and weak green emission at 509 nm (³P₀ → ³H₅). The PL emission intensity was proportional to the annealing temperature, which agreed well with the XRD results. With the increase in the Pr³⁺ doping concentration, the PL emission intensity decreased owing to the quenching effect. In our experiments, photoluminescence presented maximum emission intensity at 0.1 mol% Pr³⁺ concentration and at the annealing temperature of 1200 °C. The Pr³⁺-doped Y₂O₃ powders can potentially be used in WLEDs and displays.

This study was supported by the 2018 Research Fund from Chosun University.