Color-tunable properties of Eu³⁺- and Dy³⁺-codoped Y₂O₃ phosphor particles

Abstract

Rare-earth phosphors are commonly used in display panels, security printing, and fluorescent lamps, and have potential applications in lasers and bioimaging. In the present study, Eu³⁺- and Dy³⁺-codoped uniform-shaped Y₂O₃ submicron particles were prepared using the urea homogeneous precipitation method. The structure and morphology of the resulting particles were characterized by X-ray diffraction, field emission scanning electron microscope, and field emission transmission electron microscope, whereas their optical properties were monitored by photoluminescence spectroscopy. The room-temperature luminescence color emission of the synthesized particles can be tuned from red to yellow by switching the excitation wavelength from 254 to 350 nm. The luminescence intensities of red and yellow emissions could be altered by varying the dopant concentration. Strong quenching was observed at high Eu³⁺ and Dy³⁺ concentrations in the Y₂O₃ host lattice.

Background

The development of novel luminescent phosphor materials with a controllable size and morphology has been a major focus in the field of photonics and optoelectronics [1]. Phosphor nanocrystals are exceptionally promising materials in many fields of technology including photonics, luminescent displays, fluorescent lamps, lasers, cathodoluminescence, and biotechnology [2]. Moreover, the emission wavelength of rare-earth-doped nanoparticles is independent of the particle size and depends only on the dopant type, leading to lower synthesis cost. They also offer excellent chemical stability as well as high quantum yield. Different methods have been used to fabricate nanocrystalline phosphor particles, such as flame spray pyrolysis [3], co-precipitation method [4], sol–gel method [5], and solvothermal method [5]. The urea homogeneous precipitation method was recognized to be a green route for the high-yield mass production of spherical ceramic submicron particles with controllable sizes. Spherical-shaped particles can improve the optical performance due to the high packing density and reduction of light scattering [6].

Yttrium oxide (Y₂O₃) has been investigated widely as a host material for rare-earth (RE) ion doping in optical applications [3, 4, 6, 7] on account of its excellent chemical stability, broad transparency range (0.2 to 8 μm) with a band gap of 5.6 eV, high refractive index, and low phonon energy [1]. Furthermore, the similarities in the chemical properties and ionic radius of RE ions and Y₂O₃ make it an attractive choice as a host material [6, 8].

The color tunability of yttria-based phosphors can be achieved by codoping the host material with some specific rare-earth elements. For example, in our previous report we investigated the color-tunability effect of Eu³⁺- and Tb³⁺-codoped Y₂O₃ submicron particles [8]. We showed that the color emission of synthesized particles could be tuned precisely from red to green by a simple variation of the Tb/Eu ratio and excitation wavelength. Strong energy transfer (ET) from Tb to Eu ions was observed in Tb/Eu-codoped Y₂O₃ submicron particles, but back ET from Eu³⁺ to Tb³⁺ was not significant. Ishiwada et al. investigated the Tb/Tm-codoped Y₂O₃ phosphor for high-temperature thermometry application [9]. The synthesized Tb/Tm-codoped Y₂O₃ phosphor showed a distinct change of visible emission colors from green to blue with increasing temperature. Therefore, research into Y₂O₃ codoped with other different RE activators is important because the color-tunable properties can be used in a wide range of applications. Although many studies have examined the optical properties of RE ion-doped Y₂O₃ phosphors, only a few have investigated the codoping of two or more different ions in the same yttria host material [6, 8, 9].

In recent years, the Eu/Dy codoping in a single host material has attracted a great deal of attention and has been extensively investigated. For example, the SrAl₂O₄ host material codoped with Eu³⁺ and Dy³⁺ is known as a new generation long-lasting luminescent phosphor material [10]. It is well known that Eu³⁺ doping into Y₂O₃ host material results in a red emission, whereas doping with Dy³⁺ results in blue-yellow emission [11]. To the best of the authors' knowledge, there are no reports of an Y₂O₃ phosphor codoped with Eu³⁺ and Dy³⁺. In addition, there is an optimum concentration of dopant ions for all RE phosphors, but the optimum concentration also depends on several parameters, such as the size of the phosphors and synthetic route.

In the present study, urea homogeneous precipitation synthesis method was used for the preparation of Eu³⁺- and Dy³⁺-codoped Y₂O₃ submicron particles. The morphology and particles characteristics were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), and energy dispersive X-ray (EDX) spectroscopy. The optical properties of the synthesized particles were explored by photoluminescence spectroscopy (PL). Y₂O₃ submicron particles with different concentrations of codoped Eu³⁺ and Dy³⁺ were investigated, and the luminescence intensity of these particles were found to be strongly dependent on the activators concentration. The emission color of the Eu³⁺- and Dy³⁺-codoped Y₂O₃ particles could be switched from red to yellow by variation of the excitation wavelength.

Methods

Chemical synthesis

Analytical grade Y₂O₃ (99.9%), europium oxide (Eu₂O₃; 99.9%), dysprosium oxide (Dy₂O₃; 99.9%), nitric acid (HNO₃; 70%), and urea (99% to 100.5%) were purchased from Sigma-Aldrich Corporation (MO, USA) and were used without further purification.

Uniform-shaped sub-micron Eu³⁺- and Dy³⁺-codoped Y₂O₃ particles were synthesized according to the reported protocols [6, 8]. Phosphor precipitates were prepared by heating the corresponding RE nitrates (0.001 mol each sample) in aqueous solution of urea (40 ml H₂O and 0.5 g urea). The concentration of Eu³⁺ varied between 1 to 3 mol%, whereas the Dy³⁺ concentration varied from 1 to 2 mol%.

Physical characterization

The structure of the prepared powders was examined by XRD using a Bruker D8 Discover diffractometer (Bruker Optics Inc., MA, USA) with Cu Kα radiation (λ = 0.15405 nm) and a 2θ scan range of 20 to 60°. The structural properties were also analyzed using Fourier transform infrared spectroscopy (Jasco FT/IR6300, JASCO Corp., Easton, MD, USA). The morphologies of the particles were characterized by FESEM (Hitachi S-4700, Hitachi, Ltd., Tokyo, Japan) and FETEM (JEOL JEM-2100F, JEOL Ltd., Tokyo, Japan). Elemental analysis was carried out by EDX (Horiba 6853-H, HORIBA Jobin Yvon Inc., Edison, NJ, USA). The PL measurements were performed with a Hitachi F-7000 spectrophotometer equipped with a 150-W xenon lamp as an excitation source. All the measurements were performed at room temperature.

Results and discussion

Morphology and structure

The luminescence intensity depends strongly on the phosphor crystallinity [6, 11]. Therefore, all synthesized particles were calcinated at the temperature of 1,000°C. The morphology of the synthesized phosphor particles after calcination at 1,000°C was examined by FESEM. Figure 1a shows FESEM image of the Y₂O₃ phosphor particles codoped with a 1%Eu³⁺-1%Dy³⁺ composition. From the FESEM image, it is apparent that the Y₂O₃:1%Eu³⁺-1%Dy³⁺ phosphor particles consists of relatively uniform, spherical-shaped submicron particles, 100 ± 20 nm in size.

Figure 1

Y ₂ O ₃ : 1%Eu³⁺-1%Dy³⁺phosphor particles ( T _cal. = 1,000°C). FESEM image (a) and EDX spectra (b).

Codoping with different Eu³⁺ and Dy³⁺ concentrations did not alter the morphology of the final phosphor product, and all particles had a spherical morphology with a size distribution of 100 ± 20 nm (not shown for other samples). On the other hand, we showed that the sizes of the final Y₂O₃ phosphor particles can be tuned by altering the reaction time, reaction temperature, and concentration of starting materials [8]. The EDX analysis of 1%Eu³⁺-1%Dy³⁺-codoped Y₂O₃ particles confirmed the presence of dysprosium and europium elements in the yttria host material (Figure 1b). Figure 2 shows the XRD patterns of Y₂O₃ submicron particles codoped with different Eu³⁺ and Dy³⁺ compositions, and the standard peak positions of a pure cubic Y₂O₃ structure (JCPDS No. 86–1107).

Figure 2

X-ray diffraction patterns of Y ₂ O ₃ phosphor particles. The Y₂O₃ particles are codoped with different RE ion (Eu³⁺ and Dy³⁺) concentrations (T_cal. = 1,000°C).

It is obvious that all the diffraction peaks could be indexed directly to the cubic Y₂O₃ phase with the space group Ia3 (206) according to the standard card (JCPDS No. 86–1107). No additional peaks from the doped components could be detected due to a relatively low concentration of dopant ions indicating the formation of a pure cubic Y₂O₃ phase. Table 1 lists the mean crystallite sizes of synthesized particles estimated from the well-known Debye-Scherrer's equation. The diffraction data of the three strongest peaks ({222}, {440}, and {622} planes) were used to calculate the mean crystallite sizes. The crystallite sizes showed a slightly increasing tendency, which can be attributed to the effect of the increased dopant concentration [8].

Table 1 Calculated mean crystallite sizes of Eu³⁺- and Dy³⁺-codoped Y ₂ O ₃ particles

Figure 3a shows a FETEM image of a single Y₂O₃:1%Eu³⁺-1%Dy³⁺ phosphor particle. It is obvious that a single Y₂O₃:1%Eu³⁺-1%Dy³⁺ particle has a relatively spherical shape consisting of smaller crystallites (approximately 35 ± 12 nm) associated with each other, which is in good agreement with that calculated from the XRD patterns using the Debye-Scherrer's equation. The lattice fringes in the FETEM image also confirm the high crystallinity of the phosphor product. Figure 3b shows the corresponding selected area electron diffraction (SAED) image of the Y₂O₃:1%Eu³⁺-1%Dy³⁺ particle. The clear concentric rings from the inside to outside were indexed directly to the {211}, {222}, {400}, {440}, and {622} planes of cubic Y₂O₃, demonstrating the highly crystalline nature of the phosphor particles.

Figure 3

Single Y ₂ O ₃ :1%Eu³⁺-1%Dy³⁺phosphor particle ( T _cal. = 1,000°C). FETEM image (a) and corresponding SAED image (b).

Luminescent properties

As we mention, the luminescence emission of phosphor materials depends strongly on the synthetic route, size of the phosphor materials, and concentration of dopant ions. The particle sizes were similar for all Eu³⁺- and Dy³⁺-codoped Y₂O₃ samples which were fabricated under identical conditions. This allows a comparison of the luminescence emission properties of Y₂O₃ phosphor particles codoped with different concentrations of Eu³⁺ and Dy³⁺ activators. Figure 4a shows the emission spectra of Y₂O₃ phosphor particles codoped with different Eu³⁺ and Dy³⁺ concentrations under ultraviolet (254 nm) irradiation. The emission spectrum showed several main groups of emission lines, which were assigned to the ⁵D₁ → ⁷F₁ and ⁵D₀ → ⁷F _j (where j = 0, 1, 2, and 3) transitions within Eu³⁺[8, 12]. Obviously, the emission spectrum is dominated by a red ⁵D₀ → ⁷F₂ (612 nm) transition within the Eu³⁺. Figure 4b shows the ⁵D₀ → ⁷F₂ (612 nm) transition peak height as a function of the codoped Eu³⁺ and Dy³⁺ concentration under ultraviolet (254 nm) irradiation. According to the recent literature, the best doping value of Eu³⁺ was reported to be approximately 5 mol% in Y₂O₃ host material [5, 13]. On the other hand, the luminescence intensity of the ⁵D₀ → ⁷F₂ (612 nm, hypersensitive to the environment) transition decreased significantly (Figure 4a,b) with increasing codoped Eu³⁺ and Dy³⁺ concentrations in Y₂O₃ host.

Figure 4

PL emission spectra and integrated luminescence emission intensity (612-nm peak). PL emission spectra of Y₂O₃ phosphor particles codoped with different Eu³⁺ and Dy³⁺ contents (a) and integrated luminescence emission intensity of 612-nm peak as a function of RE ion concentration (b) upon constant 254 nm excitation.

Figure 5a shows the same samples under 350-nm excitation. In this case, the emission spectrum exhibited luminescence spectra assigned to the characteristic ⁴F_9/2 → ⁶H_15/2 (blue region) and ⁴F_9/2 → ⁶H_13/2 (greenish-yellow region) transitions within Dy³⁺[11]. The intensity of the Dy³⁺ characteristic emission transitions also decreased strongly with increasing codoped Eu³⁺ and Dy³⁺ concentration, as it shown in Figure 5a,b. The strong quenching behavior at high codoped Eu³⁺ and Dy³⁺ concentrations was related to a cross-relaxation mechanism (nonradiative decay of the two ions to the ground state) within the dopant ions.

Figure 5

PL emission spectra and integrated luminescence emission intensity (573-nm peak). PL emission spectra of Y₂O₃ phosphor particles codoped with different Eu³⁺ and Dy³⁺ content (a) and integrated luminescence emission intensity of 573-nm peak as a function of RE ions concentration (b) upon constant 350-nm excitation.

The mean distance between the dopant ions at high concentrations (R = 0.62/(N)^1/3, where N is the concentration of ions) was much shorter. Therefore, ions can interact through an electric multipolar process leading to energy migration. The dipole-dipole quenching process is inversely proportional to the sixth power of ion-ion separation and, thus, to the square of the dopant concentration [14, 15]. In Figures 4a and 5a, except for a decrease of the luminescence intensity, the emission spectrum of Y₂O₃ particles codoped with different Eu³⁺ and Dy³⁺ concentrations was similar due to the same f-f transitions within the specific RE ion. Therefore, the concentration of codoped RE ions plays an important role and should be strongly considered during the phosphor fabrication process.

The excitation spectra of Y₂O₃:1% Eu³⁺-1% Dy³⁺ particles are shown in Figure 6. The excitation spectra of Y₂O₃:1% Eu³⁺-1% Dy³⁺ (λ_em. = 612 nm) consists of a band extending from approximately 230 to 260 nm, which is related to the charge-transfer band (CTB) of O²⁻-Eu³⁺ bond. Upon excitation into the CTB at 254 nm, the emission spectrum exhibited several emission lines that were assigned to the ⁵D₁ → ⁷F₁ and ⁵D₀ → ⁷F _j (where j = 0, 1, 2, and 3) transitions within Eu³⁺, as shown in the Figure 7. On other hand, the weak signal at 573 nm, which was assigned to ⁴F_9/2 → ⁶H_13/2 (hypersensitive transition of Dy³⁺), was also observed, as shown in Figures 7 and 8. Figure 6 also shows the excitation spectra of Y₂O₃:1% Eu³⁺-1% Dy³⁺ particles measured in the 200- to 400-nm range by monitoring the yellow emission transition of Dy³⁺ (λ_em. = 573 nm). In this case, when the excitation wavelength was switched to 350 nm, the hypersensitive ⁶P_7/2 level of Dy³⁺ was excited resonantly, which then quickly relaxes nonradiatively to populate the ⁴F_9/2 level [11]. Radiative emission occurred from ⁴F_9/2 to a lower ⁶H_15/2 and ⁶H_13/2, emitting at 488 and 573 nm, respectively, with feeble ET to Eu³⁺. The energy transferred to Eu³⁺ cascades rapidly via nonradiative transitions to the ⁵D₀ state, from which luminescence associated with Eu³⁺ occurs. Therefore, the feeble signal of the ⁵D₀ → ⁷F₂ (612 nm) transition within Eu³⁺ was also detected in Eu³⁺ and Dy³⁺ codoped phosphor particles upon 350-nm excitation.

Figure 6

PL excitation spectra of Y ₂ O ₃ :1%Eu ³⁺ -1%Dy ³⁺ phosphor particles ( T _cal. = 1,000°C) .

Figure 7

Schematic diagram illustrating the Dy ³⁺ /Eu ³⁺ energy level diagram and transitions between the levels.

Figure 8

PL emission spectra of Y ₂ O ₃ :1%Eu³⁺-1%Dy³⁺phosphor particles ( λ _exc. = 254 and 350 nm, respectively) with assigned transitions. Inset digital photographs showed the eye-visible red and yellow luminescence emissions.

Such behavior suggests that there is some energy transfer (ET) occurring between the two codoped ions. On the other hand, the ET observed in Eu³⁺- and Dy³⁺-codoped Y₂O₃ was not as strong as that observed in Tb³⁺-Eu³⁺-codoped Y₂O₃ phosphor [8]. Therefore, the emission wavelength and color output of the same Y₂O₃:1% Eu³⁺-1% Dy³⁺ particles can be adjusted by switching the radiation from 254 to 350 nm. The digital photographs of eye-visible luminescence emissions from Y₂O₃:1% Eu³⁺-1% Dy³⁺ particles upon 254 and 350 nm excitations are shown in the Figure 8. It is obvious that the same host material emit red or yellow color, depending on the excitation wavelength.

Conclusion

In conclusion, Eu³⁺- and Dy³⁺-codoped Y₂O₃ submicron spherical particles were synthesized using the urea homogeneous precipitation method. The crystal structure and morphology of synthesized particles were characterized by XRD, FESEM, EDX, and FETEM. The PL spectroscopy was used to examine luminescent properties of Eu³⁺- and Dy³⁺-codoped Y₂O₃ particles. PL measurements revealed strong concentration quenching at high-codoped Eu³⁺ and Dy³⁺ concentrations. The luminescence color emission could be controlled by the excitation wavelength and the incorporation of Dy³⁺ and Eu³⁺ at the appropriate concentrations into Y₂O₃ structure. Weak energy transfer between the codoped ions was observed. These color-tunable Y₂O₃:1%Eu³⁺-1%Dy³⁺ phosphor particles can be used for security printing, solid state illumination, or for optical displays.