Barium molybdate white emitting phosphor synthesized at room temperature by co-precipitation

Abstract

Crystalline BaMoO₄:Dy³⁺ and BaMoO₄:Sm³⁺ phosphors were synthesized by co-precipitation at room temperature. The main peak (112) phase and tetragonal structure were confirmed using X-ray diffraction analysis. The lattice constant and Raman signal on d₍₁₁₂₎ were changed by the rare earth doping. A strong absorption wavelength appeared in the UV region, and BaMoO₄:Dy³⁺ excited with UV wavelength showed a yellow spectrum. BaMoO₄:Sm³⁺ showed a reddish orange spectrum. BaMoO₄:[Sm³⁺]/[Dy³⁺] was synthesized for use as a white light phosphor, and the change in the emission characteristics of yellow, white, and reddish orange could be observed depending on the doping concentration of Sm³⁺ ions. The synthesized phosphor powder and PDMS polymer were mixed to form a flexible composite, and when applied on a UV-LED chip, the same color as the powder was realized, suggesting its use as an LED color filter.

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1. Introduction

White light emitting phosphors doped with rare earth ions are attracting research attention for various applications such as displays, lighting, and light emitting devices.^1,2 Typically, dysprosium ions are mainly used for the white light emitting phosphor. Dysprosium ions have a yellow peak (∼575 nm) and a blue peak (∼485 nm) when excited with ultraviolet light.^3,4 When synthesizing Dy³⁺ ions into a phosphor, the intensities of the blue and yellow light emissions are adjusted to provide a perfect white color. This adjustment is necessary because the intensities of the blue light emission from the ⁴F_9/2 → ⁶H_15/2 transition and the yellow light emission from the ⁴F_9/2 → ⁶H_13/2 transition are different, because of differences in site symmetry location, depending on the host crystal when synthesized as a phosphor.

There is another possible way to realize white light emission.^5,6 The mainstream approach to obtain white light in existing displays and light emitting devices has been to use white light emitting diodes and tricolor display devices, appropriately mixing blue, green, red, and ultraviolet wavelengths. The representative BaMgAl₁₀O₇:Eu²⁺ (ref. 7) phosphor, which emits blue light, has a disadvantage in that the light emitting characteristic is deteriorated by the heat treatment process performed in oxygen atmosphere during panel manufacturing. Zn₂SiO₄:Mn²⁺,⁸ which is used as a green phosphor, has a problem with overlapping images because the afterimage time is too long. In the (Gd, Y)BO₃:Eu³⁺ (ref. 9) red phosphor, the intensity of orange luminescence is stronger than the red, so there is some difficulty controlling color purity.

To compensate for these disadvantages, it would be useful to develop a phosphor doped with dysprosium ions that emits all colors rather than synthesizing and combining the phosphors of blue, green, and red respectively. To develop a phosphor doped with Dy³⁺ ions that has high luminous efficiency, various synthesis methods such as the solid-state reaction method,¹⁰ sol–gel method,¹¹ hydrothermal method,¹² Czochralski method,¹³ melt quenching method,¹⁴ and others have been used. Raju et al. synthesized a Gd₃Al₅O₁₂ phosphor doped with Dy³⁺ ions by solvothermal synthesis and reported that the ratio of yellow and blue emission intensity was dependent on the concentration ratio of Dy³⁺ ions, and thus could be controlled.¹⁵ Kuang et al. prepared a SrSiO₃ phosphor doped with Dy³⁺ ions using a solid-state reaction method. After irradiating the phosphor with a 254 nm ultraviolet lamp for 5 minutes and removing it, white light emission was observed for more than 1 hour. The white light emission was caused by a mixture of blue light emission at 480 nm and yellow light emission at 572 nm. Excited electrons in the electron trap site continuously transfer energy to the ⁴F_9/2 energy level of Dy³⁺ ions, so light emission occurs for the long time reported.¹⁶

Li et al. prepared a NaLa(MoO₄)₂:Dy³⁺ phosphor with a particle size of about 1 μm with a hydrothermal method, and observed a yellow signal at 574 nm and a blue emission spectrum at 486 nm when excited at 390 nm and 456 nm. The maximum luminescence intensity was observed when the concentration of Dy³⁺ ions was 0.02.¹⁷

A case of co-doping with Sm³⁺ ions to compensate for the insufficient red color of Dy³⁺ ions was also reported for preparing white light phosphors. Fan et al. synthesized a warm-white light-emitting phosphor using a solid-state reaction method to co-dope Sm³⁺ and Dy³⁺ ions into a BaY₂ZnO₅ host matrix. It was observed that the color coordinates shifted from cold white to warm white depending on the doping amount of Sm³⁺ ions.¹⁸ Yu et al. co-doped Sm³⁺ and Dy³⁺ ions with crystalline Sr₂SnO₄ as a host matrix and the synthesized phosphor exhibited enhanced red emission compared to a sample single doped with Dy³⁺ upon excitation at 254 nm, after being calcined at 900 °C.¹⁹ In the present study, a barium molybdate (BaMoO₄) phosphor with excellent thermal and chemical stability²⁰ was co-doped with Dy³⁺ and Dy³⁺ and Sm³⁺ ions at room temperature by co-precipitation without additional heat treatment, an easy synthesis method. The structure and emission characteristics of the synthesized phosphor were subsequently investigated. By mixing the synthesized phosphor with a polymer to make a flexible composite, the phosphor could be used as an LED color filter.

2. Experimental

2.1. Synthesis of BaMoO₄ and BaMoO₄ phosphors by co-precipitation

Starting materials: barium acetate ((CH₃COO)₂Ba, Sigma-Aldrich), sodium tungstate (Na₂MoO₄·2H₂O, Sigma-Aldrich), dysprosium(III) nitrate (Dy(NO₃)₃·5H₂O, Dy³⁺), and samarium(III) nitrate (Sm(NO₃)₃·5H₂O, Sm³⁺).

10 mmol of (CH₃COO)₂Ba was dissolved in beaker “A” containing 100 mL of distilled water. In beaker “B”, 10 mmol of Na₂MoO₄·2H₂O was dissolved in 10 mL distilled water (Fig. 1). The solution that was completely dissolved in beaker “B” was slowly poured into the stirring beaker “A” and remained there for about 20 minutes. After that, powder was recovered using a centrifuge (4000 rpm, 5 min), and the powder was prepared by rinsing with distilled water 3 times to remove any remaining sodium. The white powder was dried in an oven at 80 °C for 16 hours. The phosphor was synthesized with BaMoO₄ as a host, the precursor was made by simultaneously adding Dy(NO₃)₃·5H₂O and Er(NO₃)₃·5H₂O or Sm(NO₃)₃·5H₂O to beaker “A”. The BaMoO₄:[Sm³⁺]/[Dy³⁺] phosphor was synthesized. Rare earth [RE] ions of Dy³⁺ were fixed at 0.05 mmol, and the amount of added Sm³⁺ was changed ([Sm³⁺]/[Dy³⁺] ∼ 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1).²¹

Fig. 1 Schematic of co-precipitation experimental process.

2.2. Characterization

The crystal structure of the synthesized phosphor powder was measured using an X-ray diffraction apparatus (XRD, X'Pert PRO MPD, 40 kV, 30 mA) using Cu-Kα radiation (wavelength: 1.5406 Å) at a scan rate of 4° per minute at a diffraction angle of 10° to 70°. The size and microscopic surface shape of the crystal grains were photographed with a field emission scanning electron microscope (FE-SEM, CZ, MIRA I LMH, TESCAN), and Raman spectroscope (JP/NRS-3300, 532 nm 100 mW soli-state primary laser).

2.3. Fabrication of the LED color flexible composite

The synthesized BaMoO₄:Dy³⁺, BaMoO₄:Sm³⁺, and BaMoO₄:[Sm³⁺]/[Dy³⁺] ∼ 0.1 phosphors were mixed at 0.1 g each with 1 g of polydimethylsiloxane (PDMS) and then cast in a square mold. After curing in an oven at 80 °C for 2 hours, the unique luminescent color was checked with a UV lamp, and the color reproducibility was checked by placing it on the LED chip.

3. Results and discussion

3.1. Characteristics of the rare earth single doped BaMoO₄

Fig. 2(a) shows the XRD diffraction patterns of a sample doped with crystalline BaMoO₄ as a host material, rare earth ions single doped BaMoO₄:Dy³⁺, and BaMoO₄:Sm³⁺ synthesized at room temperature, respectively. Regardless of the rare earth ions doping, the samples matched BaMoO₄ of ICDD no. 00-008-0455 card and had a tetragonal structure. The main peak (112) phase was strongly detected, and peaks (004), (200), (202), (114), (204), (220), (116), (312), and (224) were observed. The MXO₄ (M = Ba, Sr and Ca, X = Mo and W) nanoparticles precipitated—M²⁺ cations as electron pair acceptors (Lewis acid) and reacted with XO₄²⁻ anions as electron pair donors (Lewis base). The reaction between these two species (M²⁺ ← :XO₄²⁻) resulted in bonding. The lowest molecular orbital energy of the Lewis acid interacted with the highest molecular orbital energy of the Lewis base, and MXO₄ nanoparticles were finally synthesized.²² To investigate the changes in crystal structure due to the rare earth doping, the lattice constant was calculated using the signal position on the (112) plane, which is the main peak, and substituting it into Bragg's equation,²³ as shown in Fig. 2(b). Rare earth ions with a relatively large ionic radius were doped,^24,25 and a shift in the peak of the phase (112) was observed. The lattice constant of phase d₍₁₁₂₎ was increased by the rare earth ions located in the crystal lattice of BaMoO₄. The full width at half maximum (FWHM) of the (112) phase was also increased by doping with rare earth ions (Fig. 2(c)).

Fig. 2 (a) XRD patterns, (b) change in d₍₁₁₂₎ spacing, (c) change in FWHM, and (d) Raman shift of synthesized BaMoO₄ and BaMoO₄:RE³⁺ powders.

Fig. 2(d) shows the Raman shift spectrum obtained by excitation of the sample with a 532 nm laser, to observe the change in molecular frequency caused by the rare earth ions located in the doped BaMoO₄ crystal lattice. Raman vibration modes can be divided into two groups: external and internal modes. The first one corresponds to the motion of phonons and Ba²⁺ cations in the rigid [MoO₄]²⁻ tetrahedron unit. The second belongs to [MoO₄]²⁻ vibration within a tetrahedral unit, considered to be the steady state of the center.²⁶ The synthesized BaMoO₄ had frequencies of 328, 362, 793, 840, and 893 cm⁻¹. A slight change in the position of the frequencies of the BaMoO₄:Dy³⁺ and BaMoO₄:Sm³⁺ samples doped with rare earth ions was observed, suggesting that the vibration of molecules was affected by energy transferred from the outside by doping with rare earth ions.²⁷ Fig. 3 shows the spectra of the absorption and emission characteristics of the BaMoO₄:Dy³⁺ and BaMoO₄:Sm³⁺ phosphor powders synthesized by doping with Dy³⁺ or Sm³⁺ ions, respectively. Under the 572 nm spectrum of the BaMoO₄:Dy³⁺ sample, a broad spectrum distributed over a 230–330 nm region with a peak at 291 nm were observed, as well as several narrow absorption spectra appearing in the 330–400 nm region. Absorbance with a rather wide bandwidth is a transition signal generated by a charge transfer band (CTB) between Dy³⁺ cations and O²⁻ anions, and absorbance with a narrow bandwidth is a 4f–4f transition signal generated within the ⁴F₉ electron arrangement of Dy³⁺ ions.²⁸ 350 nm, which was the strongest signal among the absorption peaks with a narrow bandwidth, is a signal generated while transitioning from the ⁶H_15/2 level, which is the ground state of the Dy³⁺ ions in the host lattice, to the ⁴P_7/2 energy level, which is the excited state. In contrast, wavelengths of 323 (⁶H_15/2 → ⁶P_3/2), 351 (⁶H_15/2 → ⁶P_7/2), 365 (⁶H_15/2 → ⁶P_5/2), and 387 (⁶H_15/2 → ⁴G_11/2) nm with relatively weak absorption intensity were observed. When the BaMoO₄:Dy³⁺ sample was excited at 291 nm, a yellow spectrum with a strong signal at 572 nm, a blue emission spectrum with a peak at 480 nm, and a red spectrum at 659 nm with a relatively weak signal were observed. These emission signals are emission spectra resulting from the ⁴F_9/2 → ⁶H_j transition of Dy³⁺ ions.²⁹ The yellow emission spectrum is at the ⁴F_9/2 → ⁶H_13/2 transition with j = 13/2, the blue emission at the ⁴F_9/2 → ⁶H_15/2 transition with j = 15/2, and the red emission at j = 11/2 known as the ⁴F_9/2 → ⁶H_11/2 transition signal.³⁰ It is known that the size of the emission wavelength and intensity varies depending on the degree the local environment around the rare earth ion doped in the host lattice in the phosphor is deformed in the inversion symmetry. The asymmetry ratio is defined as the ratio of the light emission intensity due to the electric dipole transition and the light emission intensity due to the magnetic dipole transition. It is known that the electric dipole transition, which generates a yellow light emission signal at 572 nm when Dy³⁺ ions are doped, belongs to a sensitive transition, and is significantly affected by the external environment of the Dy³⁺ ions. Conversely, the blue emission spectrum at 480 nm is generated by magnetic dipole transition and is hardly affected by the strength of the crystal field around the Dy³⁺ ions. That is, when blue light emission due to magnetic dipole transition is the main peak, Dy³⁺ ions are located at an inversion symmetric site, and when yellow light emission due to electric dipole transition is strong, Dy³⁺ ions are located at non-inversion symmetry sites.^31,32 In this study, since the intensity of 572 nm, which is yellow light emission, was stronger than that of 480 nm, which is the blue light emission, the Dy³⁺ ions located in the BaMoO₄ crystal were accordingly located in non-inversion symmetry (Fig. 3(a)). Fig. 3(b) shows the absorption and emission spectra of the BaMoO₄:Sm³⁺ phosphor single doped with Sm³⁺ ions. An absorption signal by CTB between Sm³⁺ cations and O²⁻ anions with a wide bandwidth at 230–340 nm and a peak at 286 nm, and several absorption signals at 340–400 nm with a rather narrow bandwidth, were observed. Absorption signals with relatively narrow bandwidths had peaks at 344 (⁶H_5/2 → ⁴K_17/2), 361 (⁶H_5/2 → ⁴H_7/2), and 375 (⁶H_5/2 → ⁶P_7/2) nm due to f–f transitions, which were generated when they transitioned from ⁶H_5/2 in the bottom state of Sm³⁺ ions, respectively.³³ When the BaMoO₄:Sm³⁺ sample was excited at 286 nm, spectra with peaks at 560, 597, 641, and 702 nm were observed. A strong red emission spectrum was exhibited, and these are the peak emission spectra due to the ⁴G_5/2 → ⁶H_j (j = 5/2, 7/2, 9/2) transition of Sm³⁺. The yellow and orange fluorescence spectra are magnetic dipole transition signals generated by the ⁴G_5/2 → ⁶H_5/2 and ⁴G_5/2 → ⁶H_7/2 transitions, and the red fluorescence spectrum is a signal due to the ⁴G_5/2 → ⁶H_9/2 electric dipole transition.^34,35 In the synthesized BaMoO₄:Sm³⁺ phosphor, the ratio of the emission intensity of the spectrum emitted by the magnetic dipole transition and the electric dipole transition was relatively strong due to the reddish orange electric dipole transition. In general, it is known that the main emission wavelength intensity produced by rare earth ions is determined by competition between the magnetic dipole transition and electric dipole transition.³⁶ As in the Dy³⁺-doped specimen, if the light emission intensity due to magnetic dipole transition is strong, it is located in inversion symmetry. The Sm³⁺ doped sample also has a stronger luminescence intensity because the red (641 nm) electric dipole transition is greater than that of the orange (597 nm) light emission, so Sm³⁺ ions located in BaMoO₄ crystals are located in non-inversion symmetric sites.

Fig. 3 Photoluminescence properties of (a) BaMoO₄:Dy³⁺ and (b) BaMoO₄:Sm³⁺.

3.2. Characteristics of rare earth co-doped BaMoO₄

To prepare a white light-emitting phosphor, Dy³⁺ and Sm³⁺ ions were co-doped and synthesized as BaMoO₄:[Sm³⁺]/[Dy³⁺]. To confirm the crystal structure of the synthesized phosphor, after the pattern according to the Sm³⁺ concentration change X-ray diffraction analysis is shown in Fig. 4(a). The synthesized phosphor was consistent with the ICDD no. 00-008-0455 card, and the tetragonal structure of BaMoO₄ was confirmed. A signal on the main peak (112) was strongly detected in the synthesized BaMoO₄:[Sm³⁺]/[Dy³⁺] phosphor, and (004), (200), (112), (213), (204), (220), (116), (312), (224) phases were observed. The amount of added Dy³⁺ ions was fixed, and as the doping concentration of Sm³⁺ ions increased, a small shoulder was observed on the left side of the main peak (112). From the BaMoO₄:[Sm³⁺]/[Dy³⁺] ∼ 6 sample, a shoulder was observed at the position indicated by diamonds. This was confirmed to be the Sm₂O₃ phase, which appeared due to the excessive doping of Sm³⁺ ions and oxidized rare earth ions. To investigate the particle size and formation of the BaMoO₄:[Sm³⁺]/[Dy³⁺] phosphor synthesized at room temperature by co-precipitation, FE-SEM imaging was performed, and the image is shown in Fig. 4(b). The synthesized particles of the phosphor powder had a long octahedral shape in the longitudinal direction. The size was about 1.08 μm in the horizontal direction and about 2.63 μm in the longitudinal direction. Muhammad et al. synthesized crystalline Co₃O₄ nanoparticles at a temperature of 60 °C in a process like the co-precipitation method used in this study and observed plate-shaped particles of several to several tens of microns using FE-SEM.³⁷ F. Sedighi et al. synthesized silver doped crystalline SrWO₄ at 70 °C by co-precipitation to observe the shape of a flower and particles such as a star, and reported a change in the shape of the particles depending on the amount of added Na(B(C₆H₅)).³⁸ Fig. 5(a) illustrates the PL spectrum of the co-doped BaMoO₄:[Sm³⁺]/[Dy³⁺] phosphor after fixing the concentration of Dy³⁺ ions and adjusting the amount of added Sm³⁺ ions. When the samples were excited at 291 nm, a signal at 572 nm, the main peak of Dy³⁺, was strongly detected, but the intensity of the signal decreased as the doping concentration of Sm³⁺ ions increased. From the BaMoO₄:[Sm³⁺]/[Dy³⁺] ∼ 0.3 sample, blue light emission at 480 nm (⁴F_9/2 → ⁶H_15/2) and yellow light emission at 572 nm (⁴F_9/2 → ⁶H_13/2) of Dy³⁺ ions, orange light emission at 597 nm (⁴G_5/2 → ⁶H_7/2) and red light emission at 641 nm (⁴G_5/2 → ⁶H_9/2) of Sm³⁺ ions were observed together. As shown in Fig. 5(b), the yellow signal at 572 nm, the main peak of Dy³⁺, decreased as the concentration of Sm³⁺ ions increased, but the red signal at 641 nm increased. This phenomenon means that the luminescence energy was converted from Dy³⁺ ions located in the BaMoO₄, the host crystal, to Sm³⁺ ions. The energy transfer from Dy³⁺ ions to Sm³⁺ ions can be expressed by eqn (1).³⁹

η = 1 − I/I₀ (1)

Here, I is the emission intensity of Dy³⁺ ions in the phosphor co-doped with Dy³⁺ and Sm³⁺ ions, and I₀ is the emission intensity of Dy³⁺ ions in the phosphor doped with Dy³⁺ single ions. Fig. 5(c) shows that the energy transfer (ET) efficiency increases sharply from the sample of BaMoO₄:[Sm³⁺]/[Dy³⁺] ∼ 0.6 with the doping concentration of Sm³⁺ ions, and the main peaks are 572 nm and 641 nm. It showed the same result as the change. As the amount of Sm³⁺ doped increased, the PL intensity decreased along with the distance between Sm³⁺ ions. This can be interpreted as a concentration quenching, in which emission intensity decreases due to the non-radiative energy transfer generated between the activator Sm³⁺ ions.⁴⁰ Photoexcited rare earth-doped semiconductors suffer from this so-called concentration quenching effect, in which the intensity of the rare earth emission decreases with increasing dopant concentration. This effect is dominant when the excitation energy moves between many ions within the time required for radioactive decay. This is often referred to as energy transfer. In such situations, the chances of reaching a path of non-radioactive decay are greatly increased. Concentration quenching is a typical effect at high concentrations, as the probability of energy transfer increases as the density increases with the decreasing distance between dopants.

Fig. 4 (a) XRD patterns of BaMoO₄:[Sm³⁺]/[Dy³⁺] according to Sm³⁺ ion concentration and (b) FE-SEM image of BaMoO₄:[Sm³⁺]/[Dy³⁺].

Fig. 5 BaMoO₄:[Sm³⁺]/[Dy³⁺] of (a) PL spectra under 291 nm, (b) change of 572 nm (Dy³⁺) and 641 nm (Sm³⁺), (c) energy transfer efficiency, and (d) CIE coordinate of the BaMoO₄ phosphors.

The critical distance R_C was calculated using the following equations:⁴⁰

R_C ≈ 2[3V/4πx_cN]^1/3 (2)

where V is the volume of the host unit cell (V = 399.20 ų), x_c is the total concentration of [Sm³⁺]/[Dy³⁺] maximum and minimum intensity (x_c = 0.1, 1) and N is the number of host cations in the unit cell (N = 4). The critical distance of [Sm³⁺]/[Dy³⁺] ∼ 0.1 was calculated to be 12.40 Å, and at [Dy³⁺]/[Ce³⁺] ∼ 1 was calculated to be 5.755 Å. These results indicate that concentration quenching occurs when the distance between the activator ions Sm³⁺ and Dy³⁺ approached. There are three types of well known interactions in which the electric multipolar interaction is involved in the energy transfer: dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions.⁴¹ Fig. 5(d) shows the CIE (Commission Internationale de L'Eclairage) color coordinates of the synthesized phosphors, BaMoO₄:Dy³⁺, BaMoO₄:Sm³⁺, and BaMoO₄:[Sm³⁺]/[Dy³⁺]. The BaMoO₄:Dy³⁺ phosphor was in yellow with coordinates (0.365, 0.424) and the BaMoO₄:Sm³⁺ phosphor was located in the orange position with coordinates (0.538, 0.373). The BaMoO₄:[Sm³⁺]/[Dy³⁺] phosphor moved from yellow to white through red as the doping concentration of Sm³⁺ ions increased, and the emission color of the phosphor could be controlled by the doping concentration of rare earth ions. The energy level structure of Dy³⁺ and Sm³⁺ as shown in Fig. 6. The electrons located at the ground state ⁶H_15/2 of Dy³⁺ ion absorb energy under 291 nm excitation energy and later jump to the excited state. Since the high energy level is unstable, these electrons will drop successively to the lower energy excited state ⁴F_9/2 by non-radiative transition (NR). With the populated ⁴F_9/2 level, the radiative transitions of Dy³⁺ occurred with yellow and blue emissions due to the ⁴F_9/2 → ⁶H_15/2, ⁶H_13/2, ⁶H_11/2 transitions, respectively. In the interim, partial electrons located at the ⁴F_9/2 level of Dy³⁺ are relaxed to the ⁶H_5/2 level of Sm³⁺ by the resonance between the two levels, which ultimately gives rise to the characteristic emissions of Sm³⁺.⁴²

Fig. 6 Schematic energy level structure of the Dy³⁺ and Sm³⁺ ions.

3.3. Flexible composite for LED color filter

When the synthesized BaMoO₄:Dy³⁺, BaMoO₄:Sm³⁺, BaMoO₄:[Sm³⁺]/[Dy³⁺] ∼ 0.1 phosphor powder was placed under a UV lamp, yellow, white, and reddish orange emission colors could be observed as shown in Fig. 7. The BaMoO₄:Dy³⁺ phosphor showed a yellow emission wavelength, but green emission was seen in photography. It looked yellowish close to white with the naked eye, but it was photographed as green due to a camera filter. Composites were made by mixing individual powders with PDMS, a silicon-based polymer, which were easily bent and exhibited a unique color emission like that of the powder. To determine whether the manufactured composite could be used as an LED filter, a voltage of about 10 V was applied to the LED chip by placing the composite on a UV-LED chip. The manufactured composite exhibited colors similar to those in UV lamps, and this suggests that the BaMoO₄ phosphor synthesized by doping with rare earth ions could be used as an LED color filter.

Fig. 7 Photographs of synthesized phosphors powder and flexible composite under UV lamp, and composite applied on the UV-LED chip.

4. Conclusion

Crystalline BaMoO₄ was synthesized at room temperature by co-precipitation. Using the BaMoO₄ as a phosphor host, Dy³⁺ and Sm³⁺, which are rare earth ions, were doped to synthesize the phosphor. Diffraction analysis confirmed a tetragonal structure, and a strong diffraction peak on the main peak (112). After doping with rare earth ions with relatively large ionic radii, the lattice constant of the (112) phase and the position of the Raman signal were changed. BaMoO₄:Dy³⁺ and BaMoO₄:Sm³⁺ phosphors excited with UV wavelengths with strong absorbance in the UV region showed yellow and reddish orange spectra. A sample was prepared, BaMoO₄:[Sm³⁺]/[Dy³⁺], by co-doping with Dy³⁺ and Sm³⁺ ions for white light fluorescence, and the main peak (112) phase was also confirmed by X-ray diffraction analysis. However, as the concentration of Sm³⁺ ions was increased, a secondary phase Sm₂O₃ peak was observed. When the sample was excited at 291 nm, the intensity of the reddish orange emission spectrum in the yellow emission spectrum increased, as the concentration of Sm³⁺ ions increased. The synthesized phosphor, BaMoO₄:[Sm³⁺]/[Dy³⁺] ∼ 0.1 showed white light emission, and a composite was prepared by mixing it with a PDMS polymer for use as an LED color filter. The prepared composite was easily bent, and when applied on a UV-LED chip, the light emission of yellow, white, and reddish orange, which are the intrinsic colors of the synthesized phosphor, were confirmed.

Data availability statement

The data presented in this study are available on request from the corresponding author.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1F1A1072676).

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