Combinatorial optimization of the atomic compositions for green-emitting YBO₃:Ce³⁺,Tb³⁺ and red-emitting YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ phosphors using a microplate reader

Abstract

Microplate readers are versatile devices that can rapidly measure the photoluminescence intensities of multiple samples, and are widely used in biological chemistry. In this work, using a commercial microplate reader, we attempted to optimize the atomic compositions of green-emitting phosphor Y_1−x−yCe_xTb_yBO₃ and red-emitting phosphor Y_{1−x−y−z}Ce_xTb_yEu_zBO₃. We filled 48 individual wells of an alumina microplate with aqueous solutions of nitrates of Y³⁺, Ce³⁺, Tb³⁺, and Eu³⁺ with different compositions, and then added an aqueous solution of boric acid to each well. After drying, the microplate was heated at 550 °C for 2 h in air, and then at 1100 °C for 3 h in a reducing atmosphere. Y_1−x−yCe_xTb_yBO₃ absorbed near ultraviolet light through 4f → 5d transitions of Ce³⁺ and emitted green fluorescence corresponding to 4f → 4f transitions of Tb³⁺ through Ce³⁺ → Tb³⁺ energy transfer. Moreover, Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ emitted red fluorescence corresponding to 4f → 4f transitions of Eu³⁺ through Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer under near-ultraviolet light. Measurement of the photoluminescence intensity of each well by a microplate reader revealed that the optimized green and red phosphors were Y_0.835Ce_0.025Tb_0.14BO₃ and Y_0.535Ce_0.005Tb_0.45Eu_0.01BO₃, respectively.

---

1 Introduction

Luminescent inorganic materials are widely used in various optoelectronic devices and bioassays, such as white light-emitting diodes (LEDs),^1–5 displays,⁶ and biological labels,^7–10 because of their high durability and thermal and chemical stabilities. Phosphors that emit visible colors under near-ultraviolet (near-UV) or blue excitation light are currently used as spectral conversion materials in white LEDs. White LEDs composed of a blue-emitting LED and Y₃Al₅O₁₂(YAG):Ce³⁺ phosphor that converts blue light to yellow are widely available.¹¹ However, this conventional combination exhibits a low color rendering index (CRI) because of the lack of a red component. Accordingly, red-emitting phosphors excited by blue LEDs have been used to improve the CRI of devices.³ White LEDs consisting of a near-UV LED and blue-, green-, and red-emitting phosphors have also been developed.³ Conventional red phosphors such as Y₂O₃:Eu³⁺ and Y₂O₂S:Eu³⁺ show poor photoluminescence (PL) efficiency under near-UV excitation.^12–15 The absorbance of these phosphors are small in the near-UV region because 4f → 4f transitions of Eu³⁺ are forbidden and their line widths are narrow.¹⁶ To realize efficient red emission of Eu³⁺, sensitizers with broad and strong absorption in the near-UV region are needed.

YBO₃ is used as a host crystal because of its high transparency and excellent optical damage threshold in the UV and visible regions.^17–20 YBO₃ has been synthesized via a solid-state reaction at high temperature.²¹ In addition, YBO₃ has also been synthesized at low temperature by combustion,^22,23,28 spray pyrolysis,^24–27 the sol–gel method,^28–33 and coprecipitation.^34,35 Nohara et al. and Sato et al.^36–38 reported that YBO₃:Ce³⁺,Tb³⁺ emits green light through Ce³⁺ → Tb³⁺ energy transfer under near-UV excitation. Setlur and colleagues found that YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ emits red light via Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer under near-UV excitation.³⁹ Sohal and co-workers reported that the PL intensities of Ce³⁺ and Tb³⁺ decreased and that of Eu³⁺ increased with increasing Tb³⁺ concentration in YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ at fixed concentrations of Ce³⁺ and Eu³⁺.⁴⁰ The intensity ratio of red emission corresponding to the electric dipole transition (⁵D₀ → ⁷F₂) relative to orange emission corresponding to the magnetic dipole transition (⁵D₀ → ⁷F₁) was improved because of the highly distorted symmetry around the Eu³⁺ local site. YBO₃:Ce³⁺,Tb³⁺ and YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ have various relaxation routes whose probabilities depend on the concentrations of individual dopants. For YBO₃:Ce³⁺,Eu³⁺, the Eu³⁺ emission is quenched by metal-to-metal charge transfer (MMCT) from Ce³⁺ to Eu³⁺ to form Ce⁴⁺ and Eu²⁺ under near-UV excitation.³⁹ When YBO₃:Ce³⁺,Eu³⁺ is codoped with a high concentration of Tb³⁺, Ce³⁺ absorbs near-UV light through the 4f → 5d transition, and then sequential energy transfer occurs from Ce³⁺ to Tb³⁺ to Eu³⁺, followed by red emission from Eu³⁺.

The combinatorial method is used to fabricate multiple products called a “library” by reacting various combinations of feedstocks. The library is analyzed to identify an optimal product. This method can drastically shorten the time for experimental planning, execution, and analysis. In recent years, combinatorial chemistry has been widely used for drug analysis and synthesis of functional inorganic materials including fluorescent compounds.^41–47 Chen et al.⁴⁶ developed a unique drop-on-demand inkjet delivery system, which they used to optimize the composition of red-emitting phosphor Y₂O₃:Bi³⁺,Eu³⁺. Su and colleagues determined the optimum composition of yellow-emitting phosphor (Lu_1−xGd_x)₃Al₅O₁₂:Ce_3y through a combinatorial procedure.⁴⁷ However, these methods used custom-made equipment to analyze each sample in the libraries.

Microplate readers are common commercial devices often used to simultaneously measure the fluorescence intensities of a number of samples, e.g., fluorescently-labeled biomolecules, in a library.^48–50 As a result, microplate readers can drastically shorten analysis time. In this work, we attempt to optimize the atomic compositions of green-emitting phosphor YBO₃:Ce³⁺,Tb³⁺ and red-emitting phosphor YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ excited by near-UV irradiation through combinatorial synthesis and simultaneous analysis using a microplate reader.

2 Experimental section

2.1 Preparation of YBO₃:Ce³⁺,Tb³⁺ and YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ libraries

Yttrium(III) nitrate hexahydrate (Kanto, 99.99%), cerium(III) nitrate hexahydrate (Kanto, 98.5%), terbium(III) nitrate hexahydrate (Kanto, 99.95%), and europium(III) nitrate hexahydrate (Kanto, 99.99%) were dissolved in ultrapure water to prepare 0.5 M aqueous solutions of RE(NO₃)₃ (RE = Y, Ce, Tb, Eu). Boric acid (Aldrich, 99.99%) was dissolved in ultrapure water to prepare a 0.6 M aqueous solution. Each of the 48 wells of an alumina microplate was filled with an aqueous nitrate solution (1 mL), which is a mixture of the prepared RE(NO₃)₃ solutions with a desired ratio and boric acid (4 mL), and then the microplate was dried at 80 °C. This process was repeated five times by human hands. After drying, the 48 mixtures in the microplate were ground by human hands and pre-heated in a muffle furnace at 550 °C for 2 h in air, following cooling to room temperature. Each sample was reground and then heated at 1100 °C for 3 h in a reducing atmosphere using carbon board to obtain YBO₃:Ce³⁺,Tb³⁺ and YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ phosphors.

2.2 Characterization

Powder X-ray diffraction (XRD) profiles were measured using an X-ray diffractometer (Rigaku, Rint 2200) with a Cu Kα radiation source. The elemental composition of YBO₃:Ce³⁺,Tb³⁺ and YBO₃:Ce³⁺,Tb³⁺,Eu³⁺ powder samples was determined by the fundamental parameter method using an X-ray fluorescence analyzer (Rigaku, ZSX mini II). Both PL and photoluminescence excitation (PLE) spectra were measured using a fluorescence spectrometer (JASCO, FP-6500) equipped with a 150-W Xe lamp. The PL intensity of each well of the microplate was measured using a multi-detection microplate reader (DS Pharma Biomedical, Powerscan HT) with an excitation band-pass filter (BT-7082220; 360 ± 40 nm). Emission band-pass filters [BT-7082210 (545 ± 40 nm) and BT-7082226 (645 ± 40 nm)] were used to detect green and red emission, respectively. Each well of a microplate was divided to 13 blocks, as shown in Fig. 1, and the PL intensity of each well was defined as an average of three strongest PL intensity among the 13 blocks. The PL measurement of 48 wells was completed in ∼8 min. To determine the precision of the procedure, Y_0.82Ce_0.03Tb_0.15BO₃ was synthesized in 48 wells of a microplate and the PL intensity of each well was evaluated from a representative value, which was the average of the three highest PL intensities among 13 blocks. The relative deviation of representative values of PL intensities was within 8.0%.

Fig. 1 Division of each well in a microplate into 13 blocks for PL intensity evaluation.

3 Results and discussion

3.1 Structural properties of the Y_1−x−yCe_xTb_yBO₃ library

XRD profiles of samples from the Y_1−x−yCe_xTb_yBO₃ library (0 ≤ x ≤ 0.1, 0 ≤ y ≤ 0.4) with a wide range of Ce³⁺ and Tb³⁺ concentrations synthesized in the wells of a microplate were measured (see Fig. S1†). All the XRD peaks except for that at 27.5° belonged to YBO₃ with hexagonal structure. The peak at 27.5° possibly originated from the byproduct Y₃BO₆, which accidentally formed through the evaporation of boron. Actual metallic compositions of typical samples (shown in Table S1†) were evaluated to be close to loading ones except for that of the sample taken from well A-7.

3.2 Photoluminescence spectra of the Y_1−x−yCe_xTb_yBO₃ library

Fig. 2 shows PL and PLE spectra of Y_0.98Ce_0.02BO₃ and Y_0.83Ce_0.02Tb_0.15BO₃ synthesized in the wells of a microplate. The PL intensity at 416 nm corresponding to the 5d → 4f transition of Ce³⁺ for Y_0.83Ce_0.02Tb_0.15BO₃, which exhibited Tb³⁺ emission in the wavelength region of 480–640 nm, was lower than that for Y_0.98Ce_0.02BO₃. In the PLE spectrum of Y_0.83Ce_0.02Tb_0.15BO₃ monitored at the Tb^{3+ 5}D₄ → ⁷F₅ emission wavelength λ_em of 544 nm, the excitation peak corresponding to the 4f → 5d transition of Ce³⁺ was observed at 360 nm. This result indicates that energy transfer occurred from Ce³⁺ to Tb³⁺.

Fig. 2 PL and PLE spectra of Y_0.98Ce_0.02BO₃ and Y_0.83Ce_0.02Tb_0.15BO₃ synthesized in the wells of a microplate.

Fig. 3 depicts the PL (excitation wavelength λ_ex = 360 nm) and PLE (λ_em = 544 nm) spectra of the samples from the Y_0.98−yCe_0.02Tb_yBO₃ library, in which the Ce³⁺ concentration was fixed at 2 at% and the Tb³⁺ concentration was varied from 0 to 30 at%. The maximum PL intensity at 544 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ was observed when the Tb³⁺ concentration was 15 at%. The PL intensity of the Ce³⁺ 5d → 4f emission simply decreased with increasing Tb³⁺ concentration. The efficiency of Ce³⁺ → Tb³⁺ energy transfer and PL intensity of the Tb^{3+ 5}D₄ → ⁷F₅ emission at 544 nm are plotted as a function of Tb³⁺ concentration in Fig. 4. Energy transfer efficiency, η, is given by:

where I₀ and I are the PL intensities of Ce³⁺ at 416 nm in the absence and presence of Tb³⁺, respectively.⁵¹ The PL intensity of Tb³⁺ increased with η and reached its maximum value when the Tb³⁺ concentration was 15 at%. Over 15 at% Tb³⁺, η approached unity, and the PL intensity of Tb³⁺ decreased. This decrease is attributed to concentration quenching of Tb³⁺ due to enhancement of probability of a non-radiative relaxation through the energy migration of Tb³⁺ → Tb³⁺.

Fig. 3 PL and PLE spectra of samples from the Y_0.98−yCe_0.02Tb_yBO₃ library (0 ≤ y ≤ 0.3).

Fig. 4 Dependence of the energy transfer efficiency and PL intensity of Y_0.98−yCe_0.02Tb_yBO₃ (0 ≤ y ≤ 0.3) samples on Tb³⁺ concentration.

3.3 Optimization of the atomic composition of the Y_1−x−yCe_xTb_yBO₃ phosphor

To roughly determine the optimum atomic composition range of the Y_1−x−yCe_xTb_yBO₃ phosphor, a Y_1−x−yCe_xTb_yBO₃ library with various Ce³⁺ and Tb³⁺ concentrations over wide ranges of 0 ≤ x ≤ 0.1 and 0 ≤ y ≤ 0.4 was prepared. Fig. 5(a) shows a fluorescence image of the library under near-UV excitation and measured PL intensities of each well measured by the microplate reader. Strong emission was observed when the Ce³⁺ concentration was 2–3 at% and Tb³⁺ concentration was 15 at%.

Fig. 5 Fluorescent images of Y_1−x−yCe_xTb_yBO₃ libraries under near-UV excitation and values of green emission intensity (λ_ex = 360 nm) in each well measured by a microplate reader. Y_1−x−yCe_xTb_yBO₃ libraries with (a) (0 ≤ x ≤ 0.10, 0 ≤ y ≤ 0.40) and (b) (0.020 ≤ x ≤ 0.045, 0.02 ≤ y ≤ 0.16).

Next, a Y_1−x−yCe_xTb_yBO₃ library with narrow ranges of 0.020 ≤ x ≤ 0.045 and 0.02 ≤ y ≤ 0.16 was prepared to find the optimum atomic composition for this phosphor. As illustrated in Fig. 5(b), Y_0.835Ce_0.025Tb_0.14BO₃ was determined as the optimum composition with the strongest green PL intensity.

3.4 Structural properties of the Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library

XRD profiles of samples from the Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library (0 ≤ x ≤ 0.05, 0 ≤ y ≤ 0.90, z = 0.05; synthesized with wide ranges of Ce³⁺ and Tb³⁺ concentrations and a fixed Eu³⁺ concentration of 5 at%) and Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library (x = 0.005, 0 ≤ y ≤ 0.795, 0 ≤ z ≤ 0.20; synthesized with wide ranges of Tb³⁺ and Eu³⁺ concentrations and a fixed Ce³⁺ concentration of 0.5 at%) were measured (see Fig. S2 and S3,† respectively). All the XRD peaks except for a weak peak attributed to Y₃BO₆ at 27.5° belonged to YBO₃ with hexagonal structure. Actual metallic compositions were close to loading ones (see Tables S2 and S3†).

3.5 Photoluminescence spectra of the Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library

Fig. 6 presents PL and PLE spectra of Y_0.97Ce_0.005Eu_0.025BO₃ and Y_0.82Ce_0.005Tb_0.15Eu_0.025BO₃ synthesized in the wells of a microplate. In the PLE spectrum monitored at the Eu^{3+ 5}D₀ → ⁷F₁ emission (λ_em = 593 nm), the excitation peak corresponding to the 4f → 5d transition of Ce³⁺ was not observed for Y_0.97Ce_0.005Eu_0.025BO₃. In contrast, for Y_0.82Ce_0.005Tb_0.15Eu_0.025BO₃, the excitation peaks ascribed to the Tb³⁺ 4f⁸ → 4f⁷5d¹ and Ce³⁺ 4f → 5d transitions were observed at 280 and 360 nm, respectively. This result indicates that Tb³⁺ relays the energy of excited Ce³⁺ to Eu³⁺. Judging from the PL spectra of Y_0.845Ce_0.005Tb_0.15BO₃ and Y_0.82Ce_0.005Tb_0.15Eu_0.025BO₃ synthesized in the wells of a microplate (Fig. S4†), PL intensities of Tb³⁺ and Ce³⁺ under excitation of Ce³⁺ decreased after co-doping with Eu³⁺, indicating that Ce³⁺ → Eu³⁺ and Ce³⁺ → Tb³⁺ → Eu³⁺ energy transfer processes occurred. However, Ce³⁺ → Eu³⁺ direct energy transfer did not contribute to the Eu³⁺ emission, because MMCT caused Ce³⁺ and Eu³⁺ to convert to Ce⁴⁺ and Eu²⁺, respectively, quenching the Eu³⁺ emission.

Fig. 6 PL and PLE spectra of Y_0.97Ce_0.005Eu_0.025BO₃ and Y_0.82Ce_0.005Tb_0.15Eu_0.025BO₃.

Fig. 7 shows the PL (λ_ex = 360 nm) and PLE (λ_em = 593 nm) spectra of samples from the Y_0.845−zCe_0.005Tb_0.15Eu_zBO₃ library, in which Ce³⁺ and Tb³⁺ concentrations were fixed at 0.5 and 15 at%, respectively, and Eu³⁺ concentration was varied from 0 to 20 at%. The maximum PL intensity of Eu³⁺ at 593 nm was reached at an Eu³⁺ concentration of 2.5 at%. The PL intensity of Tb³⁺ decreased with increasing Eu³⁺ concentration. Tb³⁺ → Eu³⁺ energy transfer efficiency and the PL intensity of Eu³⁺ at 593 nm are plotted as a function of Eu³⁺ concentration in Fig. 8. The η of Tb³⁺ → Eu³⁺ energy transfer was estimated using eqn (1); in this case, I₀ and I were the PL intensities of Tb³⁺ at 544 nm in the absence and presence of Eu³⁺, respectively. With increasing Eu³⁺ concentration, η increased until it approached unity over an Eu³⁺ concentration of 2.5 at%, while the PL intensity of Eu³⁺ decreased over this concentration. This result would indicate that concentration quenching of Eu³⁺,^52,53 caused by enhancement of probability of a non-radiative relaxation through the energy migration of Eu³⁺ → Eu³⁺, occurred when the Eu³⁺ concentration exceeded 2.5 at%. It should be noted that the MMCT effect between Ce³⁺ and Eu³⁺ was ignored for the calculation of η. The decrease in PL intensity might be affected by enhancement of the MMCT as well as the concentration quenching.

Fig. 7 PL and PLE spectra of samples from the Y_0.845−zCe_0.005Tb_0.15Eu_zBO₃ library (0 ≤ z ≤ 0.2).

Fig. 8 Dependence of the energy transfer efficiency and PL intensity of Y_0.845−zCe_0.005Tb_0.15Eu_zBO₃ (0 ≤ z ≤ 0.2) on Eu³⁺ concentration.

3.6 Optimization of the atomic composition of the Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ phosphor

To roughly determine the optimum concentration range of the Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ phosphor, a Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library with various Ce³⁺ and Tb³⁺ concentrations over wide ranges of 0 ≤ x ≤ 0.05 and 0 ≤ y ≤ 0.90 and a fixed Eu³⁺ concentration of 5 at% was prepared. Fig. 9(a) shows a fluorescence image of the library under near-UV excitation and the average value of the three strongest red emission intensities in each well measured by a microplate reader. The strong emission was observed for Ce³⁺ and Tb³⁺ concentrations of 0–2 and 40–90 at%, respectively. To determine the optimum concentration of Ce³⁺, a Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library with narrow ranges of 0 ≤ x ≤ 0.025 and 0.34 ≤ y ≤ 0.90 and at a fixed Eu³⁺ concentration of 5 at% was prepared. As illustrated in Fig. 9(b), the strong emission was found when the concentrations of Ce³⁺ and Tb³⁺ were 0.5 at% and 40–60 at%, respectively. Therefore, the optimum concentration of Ce³⁺ was 0.5 at%.

Fig. 9 Fluorescent images of Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ libraries under near-UV excitation and values of red emission intensity (λ_ex = 360 nm) in each well measured by a microplate reader. Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ libraries with (a) (0 ≤ x ≤ 0.05, 0 ≤ y ≤ 0.90, z = 0.05), (b) (0 ≤ x ≤ 0.025, 0.34 ≤ y ≤ 0.90, z = 0.05), (c) (x = 0.005, 0 ≤ y ≤ 0.795, 0 ≤ z ≤ 0.20), and (d) (x = 0.005, 0.40 ≤ y ≤ 0.80, 0.005 ≤ z ≤ 0.030).

A Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library with wide concentration ranges of 0 ≤ y ≤ 0.795 and 0 ≤ z ≤ 0.20 and at a fixed Ce³⁺ concentration of 0.5 at% was prepared. Fig. 9(c) reveals that strong emission was obtained for concentrations of Tb³⁺ of 40–80 at% and Eu³⁺ of 2.5 at%. Then, a Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ library with narrow ranges of 0.40 ≤ y ≤ 0.80 and 0.005 ≤ z ≤ 0.030 and a fixed Ce³⁺ concentration of 0.5 at% was prepared to determine the optimum composition of this phosphor. Fig. 9(d) indicates that Y_0.535Ce_0.005Tb_0.45Eu_0.01BO₃ is the optimum composition with the strongest red PL intensity. Red to orange ratios were calculated from the PL intensities attributed to ⁵D₀ → ⁷F₁ transition (O) at 593 nm and ⁵D₀ → ⁷F₂ transition of Eu³⁺ (see PL and PLE spectra shown in Fig. S5†). There were two large red peaks assigned to the ⁵D₀ → ⁷F₂ transition at 611 nm (R₁) and 627 nm (R₂) due to Stark splitting. Calculated ratios were R₁/O = 0.484 and R₂/O = 0.621. CIE coordinate obtained from the PL spectrum was (0.636, 0.359) (also shown in Fig. S5†).

3.7 Factors determining the optimum composition of the Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ phosphor

The optimum concentration of Ce³⁺ and Tb³⁺ in Y_1−x−yCe_xTb_yBO₃ were 2.5 and 14 at%, respectively. In contrast, the optimum concentrations of Ce³⁺, Tb³⁺, and Eu³⁺ in Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ were 0.5, 45, and 1.0 at%, respectively. We note that the optimum concentrations of Ce³⁺ and Tb³⁺ for Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ were lower and higher, respectively, compared with those in Y_1−x−yCe_xTb_yBO₃. This means the addition of Tb³⁺ suppresses the quenching of Eu³⁺ emission by inhibiting MMCT from Ce³⁺ to Eu³⁺ which results from enhancing probability of the Ce³⁺ → Tb³⁺ energy transfer. Fig. 10 summarizes energy transfer routes in the phosphors. Because of the lower concentration of Ce³⁺ and higher concentration of Tb³⁺ in Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ compared with those in Y_1−x−yCe_xTb_yBO₃, the possibility of Ce³⁺ → Tb³⁺ energy transfer could be higher than Ce³⁺ → Eu³⁺ energy transfer in Y_{1−x−y−z}Ce_xTb_yEu_zBO₃.

Fig. 10 Schematic illustration of the energy transfer routes in YBO₃:Ce³⁺,Tb³⁺ and YBO₃:Ce³⁺,Tb³⁺,Eu³⁺.

4 Conclusions

Metal nitrate aqueous solutions with different compositions and boric acid aqueous solution were reacted in alumina microplate wells to obtain libraries of the green-emitting phosphor Y_1−x−yCe_xTb_yBO₃ and red-emitting phosphor Y_{1−x−y−z}Ce_xTb_yEu_zBO₃. The compositions of both phosphors were optimized by evaluating the PL intensity of each microplate well measured by a versatile microplate reader. For the green-emitting phosphor, the concentrations of Ce³⁺ and Tb³⁺ were changed over wide ranges, and then over narrow ranges. The determined optimum composition of Y_1−x−yCe_xTb_yBO₃ was Y_0.835Ce_0.025Tb_0.14BO₃. For the red-emitting phosphor, the concentration of Eu³⁺ was fixed at 5 at% and the concentrations of Ce³⁺ and Tb³⁺ were optimized similarly. The PL intensity of this phosphor was more sensitive to Ce³⁺ concentration than Tb³⁺, and the determined optimum concentration of Ce³⁺ was 0.5 at%. Then, the concentrations of Tb³⁺ and Eu³⁺ were optimized at a constant Ce³⁺ concentration of 0.5 at%. The determined optimum composition of Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ was Y_0.535Ce_0.005Tb_0.45Eu_0.01BO₃. The optimum concentration of Ce³⁺ was lower and that of Tb³⁺ was higher for the red-emitting phosphor compared with those of the green-emitting phosphor. This is attributed to suppression of Ce³⁺ → Eu³⁺ MMCT, which causes quenching of Eu³⁺ emission, using a lower Ce³⁺concentration. The technique developed in this work will be useful to determine the optimum compositions of other phosphors.

References

1. D. A. Steigerwald, J. C. Bhat, D. Collins, R. M. Fletcher, M. O. Holcomb, M. J. Ludowise, P. S. Martin and S. L. Rudaz, IEEE J. Sel. Top. Quantum Electron., 2002, 8, 310–320.
2. H. A. Höppe, Angew. Chem., Int. Ed., 2009, 48, 3572–3582.
3. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma and Q. Y. Zhang, Mater. Sci. Eng., R, 2010, 71, 1–34.
4. A. Revaux, G. Dantelle, N. George, R. Seshadri, T. Gacoin and J. P. Boilot, Nanoscale, 2011, 3, 2015–2022.
5. W.-Y. Huang, F. Yoshimura, K. Ueda, Y. Shimomura, H.-S. Sheu, T.-S. Chan, H. F. Greer, W. Zhou, S.-F. Hu, R.-S. Liu and J. P. Attfield, Angew. Chem., Int. Ed., 2013, 52, 8102–8106.
6. S. Zhang, IEEE Trans. Plasma Sci., 2006, 34, 294–304.
7. B. Dong, J. Wang, J. Sun, S. Xu, X. Bai, Z. Jiang, L. Xia, L. Sun and H. Song, RSC Adv., 2012, 2, 3897–3905.
8. K. M. Kim and J. H. Ryu, J. Alloys Compd., 2013, 576, 195–200.
9. I. L. Medintz, H. T. Uyada, E. R. Goldman and H. Mattoussi, Nat. Mater., 2005, 4, 435–446.
10. H. Dong, L.-D. Sun and C.-H. Yan, Nanoscale, 2013, 5, 5703–5714.
11. V. Bachmann, C. Ronda and A. Meijerink, Chem. Mater., 2009, 21, 2077–2084.
12. J. Dhanaraj, R. Jagannathan, T. R. N. Kutty and C.-H. Lu, J. Phys. Chem. B, 2001, 105, 11098–11105.
13. J.-G. Li, X. Li, X. Sun and T. Ishigaki, J. Phys. Chem. C, 2008, 112, 11707–11716.
14. C. Guo, L. Luan, C. Chen, D. Huang and Q. Su, Mater. Lett., 2008, 62, 600–602.
15. J. Kuang, Y. Liu and D. Yuan, Electrochem. Solid-State Lett., 2005, 8, H72–H74.
16. R. Yu, H. Noh, B. Moon, B. Choi, J. Jeong, K. Jang, S. Yi and J. Jang, J. Alloys Compd., 2013, 576, 236–241.
17. D. Zou, Y. Q. Ma, S. B. Qian, B. T. Huang, G. H. Zheng and Z. X. Dai, J. Alloys Compd., 2014, 584, 471–476.
18. L. Jia, Z. Shao, Q. Lü, Y. Tian and J. Han, Ceram. Int., 2014, 40, 739–743.
19. K. S. Sohn, Y. Y. Choi and H. D. Park, J. Electrochem. Soc., 2000, 147, 1988–1992.
20. X. Zhang, X. Zhang, Z. Zhao and J. Chauduhuri, J. Mater. Sci., 2015, 50, 251–257.
21. G. B. Chadeyron, M. E. Ghozzi, D. Boyer, R. Mahiou and J. C. Cousseins, J. Alloys Compd., 2001, 317, 183–185.
22. J. T. Ingle, R. P. Sonekar, S. K. Omanwar, Y. Wang and L. Zhao, Combust. Sci. Technol., 2014, 186, 83–89.
23. A. B. Gawande, R. P. Sonekar and S. K. Omanwar, Combust. Sci. Technol., 2014, 186, 785–791.
24. K. Y. Jung, E. J. Kim and Y. C. Kang, J. Electrochem. Soc., 2004, 151, H69–H73.
25. N. Joffin, B. Caillier, A. Garcia, P. Guillot, J. Galy, A. Fernandes, R. Mauricot and J. D. Ghys, Opt. Mater., 2006, 28, 597–601.
26. G. Jia, P. A. Tanner, C. K. Duan and J. D. Ghys, J. Phys. Chem. C, 2010, 114, 2769–2775.
27. K. Y. Jung and H. K. Jung, J. Lumin., 2010, 130, 1970–1974.
28. D. Boyer, G. B. Chadeyron, R. Mahiou, C. Caperaa and J. C. Cousseins, J. Mater. Chem., 1999, 9, 211–214.
29. L. Lou, D. Boyer, G. B. Chadeyron, E. Bernstein, R. Mahiou and J. Mugnier, Opt. Mater., 2000, 15, 1–6.
30. Z. Wei, L. Sun, C. Liao, J. Yin, X. Jiang and C. Yan, J. Phys. Chem. B, 2002, 106, 10610–10617.
31. Z. G. Wei, L. D. Sun, C. S. Liao, X. C. Jiang, C. H. Yan, Y. Tao, X. Y. Hou and X. Ju, J. Appl. Phys., 2003, 93, 9783–9788.
32. D. Boyer, G. Bertrand and R. Mahiou, J. Lumin., 2003, 104, 229–237.
33. H. Zhu, L. Zhang, T. Zuo, X. Gu, Z. Wang, L. Zhu and K. Yao, Appl. Surf. Sci., 2008, 254, 6362–6365.
34. K. N. Kim, H. K. Jung, H. D. Park and D. Kim, J. Mater. Res., 2002, 17, 907–910.
35. X. C. Jiang, C. H. Yan, L. D. Sun, Z. G. Wei and C. S. Liao, J. Solid State Chem., 2003, 175, 245–251.
36. R. Sato, S. Takeshita, T. Isobe, T. Sawayama and S. Niikura, ECS J. Solid State Sci. Technol., 2012, 1, R163–R168.
37. A. Nohara, S. Takeshita and T. Isobe, RSC Adv., 2014, 4, 11219–11224.
38. A. Nohara, S. Takeshita, Y. Iso and T. Isobe, J. Mater. Sci., 2016, 51, 3311–3317.
39. A. A. Setlur, Electrochem. Solid-State Lett., 012, 15, J25–J27.
40. S. Sohal, M. Nazari, X. Zhang, E. Hassanzadeh, V. V. Kuryatkov, J. Chaudhuri, L. J. Hope-Weeks, J. Y. Huang and M. Holtz, J. Appl. Phys., 2014, 115, 183505.
41. B. Lee, S. Lee, H. G. Jeong and K.-S. Sohn, ACS Comb. Sci., 2011, 13, 154–158.
42. K. H. Son, S. P. Singh and K.-S. Sohn, J. Mater. Chem., 2012, 17, 8505–8511.
43. S. P. Singh, W. B. Park, C. Yoon, D. Kim and K.-S. Sohn, ECS J. Solid State Sci. Technol., 2016, 5, R3032–R3039.
44. W. B. Park, S. P. Singh, M. Kim and K.-S. Sohn, Inorg. Chem., 2015, 4, 1829–1840.
45. J. W. Park, B. Y. Han, Y. S. Kim and K.-S. Sohn, J. Nanosci. Nanotechnol., 2013, 6, 3935–3958.
46. T. Chan, C. Kang, R. Liu, L. Chen, X. Liu, J. Ding, J. Bao and C. Gao, J. Comb. Chem., 2007, 9, 343–346.
47. X. Su, K. Zhang, Q. Liu, H. Zhong, Y. Shi and Y. Pan, ACS Comb. Sci., 2011, 13, 79–83.
48. B. Bender, C. Condra, R. J. Gould and T. M. Connolly, Thromb. Res., 1995, 77, 453–463.
49. H. Wang and J. A. Joseph, Free Radical Biol. Med., 1999, 27, 612–616.
50. M. U. Kassack, B. Hofgan, J. Lehmann, N. Eckstein, J. M. Quillan and W. Sadee, J. Biomol. Screening, 2002, 7, 233–246.
51. X. Zhao, X. Wang, B. Chen, W. Di, Q. Meng and Y. Yang, Proc. SPIE, 2006, 6030, 60300N.
52. X. Zhang, L. Zhou, Q. Pang and M. Gong, J. Am. Ceram. Soc., 2014, 97, 2124–2129.
53. X. Zhang, L. Zhou, Q. Pang and M. Gong, Opt. Mater., 2014, 36, 1112–1118.

---

Footnotes

† Electronic supplementary information (ESI) available: XRD profiles, crystallite sizes, and actual metallic compositions of the samples taken from the libraries of Y_1−x−yCe_xTb_yBO₃ (0 ≤ x ≤ 0.1, 0 ≤ y ≤ 0.4) (Fig. S1 and Table S1), Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ (0 ≤ x ≤ 0.05, 0 ≤ y ≤ 0.90, z = 0.05) (Fig. S2 and Table S2), and Y_{1−x−y−z}Ce_xTb_yEu_zBO₃ (x = 0.005, 0 ≤ y ≤ 0.795, 0 ≤ z ≤ 0.20) (Fig. S3 and Table S3); PL spectra of Y_0.845Ce_0.005Tb_0.15BO₃ and Y_0.82Ce_0.005Tb_0.15Eu_0.025BO₃ synthesized in the wells of a microplate (Fig. S4); PL and PLE spectra, and CIE coordinate with a color diagram of Y_0.535Ce_0.005Tb_0.45Eu_0.01BO₃ (Fig. S5). See DOI: 10.1039/c7ra01356f

‡ Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan E-mail: E-mail: s.takeshita@aist.go.jp.

---