Room Temperature Synthesis of Various Color Emission Rare-Earth Doped Strontium Tungstate Phosphors Applicable to Fingerprint Identification

Abstract

Crystalline SrWO₄ was synthesized at room temperature using a co-precipitation method. To use the SrWO₄ as a phosphor, green and red phosphors were synthesized by doping with Tb³⁺ and Eu³⁺ rare earth ions. The synthesized samples had a tetragonal structure, and the main peak (112) phase was clearly observed. When the sample was excited using the absorption peak observed in the ultraviolet region, SrWO₄:Tb³⁺ showed an emission spectrum of 544 nm, and SrWO₄:Eu³⁺ showed an emission spectrum of 614 nm. When Tb³⁺ and Eu³⁺ ions were co-doped to realize various colors, a yellow-emitting phosphor was realized as the doping concentration of Eu³⁺ ions increased. When the synthesized phosphor was scattered on a glass substrate with fingerprints, as used in the field of fingerprint recognition, the fingerprint was revealed by green, red, and yellow emissions in response to a UV lamp.

1. Introduction

Crystalline tungsten has excellent thermal and chemical stability and has been applied in various fields. A material that is thermally and chemically stable has high energy transfer efficiency from tungsten ion to rare earth ion in the rare earth doped phosphor; thus, it is suitable for use as a host material [1,2,3]. Rare earth (RE) ions doped in the host lattice can generate high intensity emissions and various emission wavelengths, with a narrow bandgap due to energy transfer between the 4f-4f shells [4,5,6]. The type and site symmetry of the rare earth ions doped in a thermally and chemically stable host lattice are important factors in the performance of various types of lighting, laser, and display devices [7,8,9].

It has been reported that the emission wavelength of phosphors used in various types of light devices can vary depending on the type and concentration of the doped rare earth ions, the sintering temperature, crystal grain size, excitation wavelength, and synthesis conditions [10,11,12,13]. In particular, the main emission wavelength of the rare earth ions is determined by competition between electric dipole transitions and magnetic dipole transitions. If the electric dipole transition is strong, it reacts sensitively to the local environment around the rare earth ions located in the host lattice, but magnetic dipole transitions are hardly affected by external environmental factors [14,15,16]. For example, two types of emission wavelengths occur in a phosphor doped with europium (Eu³⁺) ions. In one emission spectrum, an orange emission (~597 nm) spectrum is generated by the ⁵D₀ → ⁷F₁ magnetic dipole transition, and the other is a rare red orange (~620 nm) emission signal from the ⁵D₀ → ⁷F₂ electric dipole transition. It is known that either the magnetic dipole transition or the electric dipole transition will become the main transition depending on whether Eu³⁺ rare earth ions located in the host lattice are in the inversion-doping region or not, and this determines the emission wavelength [17,18,19]. Yu et al. synthesized the BaWO₄:Eu³⁺, Bi³⁺ phosphor powder using a solid-state method and chemical immersion method and observed that the red emission at 613 nm increased as the doping concentration of Bi³⁺ ions increased [20]. Jung et al. synthesized crystalline BaWO₄ by preparing a precursor by co-precipitation and heat-treating it at 800 °C. By doping Dy³⁺, Tb³⁺, and Sm³⁺ rare earth ions, phosphors emitting yellow, green, and red were synthesized and applied to anti-counterfeiting [21]. Shinde et al. synthesized NaCaPO₄ phosphor doped with Ce³⁺, Eu³⁺, and Dy³⁺ rare earth ions using the combustion method. In the case of the Ce³⁺ ion-doped phosphor, an emission wavelength of 367 nm was obtained. Blue light emission at 482 nm (⁴F_9/2 → ⁶H_15/2, magnetic dipole transition) and light emission at 576 nm (⁴F_9/2 → ⁶H_13/2, electric dipole transition) were observed [22].

In this study, crystalline SrWO₄ was synthesized at room temperature by co-precipitation. Then, green and red phosphors were synthesized by doping with rare earth ions Tb³⁺ and Eu³⁺, respectively, and yellow phosphors were synthesized by co-doping the two rare earth ions. The structure of the synthesized phosphor, the size and shape of particles, and their luminescence characteristics were investigated. The synthesized phosphor was reacted with a UV lamp to visualize a fingerprint using the emission color, suggesting that it can be applied to the field of anti-counterfeiting.

2. Materials and Methods

2.1. Synthesis of SrWO₄:RE³⁺ by Co-Precipitation at Room Temperature

Starting materials: Strontium acetate ((CH₃CO₂)Sr, Sigma-Aldrich, reagent grade), Sodium tungstate (Na₂WO₄·2H₂O, Sigma-Aldrich, ≥99%), Terbium nitrate (Tb(NO₃)₃·_xH₂O, Tb³⁺, Sigma-Aldrich, 99.999%), Europium nitrate (Eu(NO₃)₃·6H₂O, Eu³⁺, Sigma-Aldrich, 99.9%)

The synthesis process was as follows. First 1 mmol (CH₃CO₂)Sr was placed in beaker ‘A’ and stirred with 50 mL distilled water. Then, 1 mmol Na₂WO₄·2H₂O was placed in beaker ‘B’ and stir with 50 mL distilled water (Figure 1). When the solutions in the ‘A’ and ‘B’ beakers are completely dissolved and become transparent, pour the ‘B’ solution into the ‘A’ beaker, and stir at room temperature for about 20 min. The reacted solutions change to a white opaque color and a powder is formed. The formed powder is recovered by centrifugation at 4000 rpm for 10 min. The recovered powder is washed 3 times with distilled water to remove unreacted substances and then centrifuged again to recover the powder and dried at 80 °C for 16 h. (Figure 1). To synthesize the phosphor, 0.25 mmol each of Tb³⁺ and Eu³⁺ was added to the ‘A’ beaker and processed in the same manner. White light phosphor was synthesized by co-doping by fixing the amount of Tb³⁺ and controlling the amount of Eu³⁺.

2.2. Fabricated Fingerprint Identification Application

To use the synthesized phosphor for fingerprint recognition, a thumb fingerprint was imprinted on a glass substrate. After spraying the synthesized phosphor on the glass substrate and removing the remaining powder with a brush, the fingerprint on the glass substrate could be visualized by illuminating it with a UV lamp, which revealed the unique luminous color of the phosphor.

2.3. Characterization

The crystal structure of the synthesized phosphor powder was measured using an X-ray diffraction apparatus (X’Pert PRO MPD, 40 kV, 30 mA) having 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 scanning electron microscope (TESCAN MIRA 3 LMH FE-SEM, TESCAN, Brno, Czech Republic), and a fluorescence photometer (FS-2, Scinco) with a xenon lamp was used as a light source to obtain emission and absorption characteristics.

3. Results & Discussion

3.1. Characteristics of SrWO₄ and SrWO₄:RE³⁺

Figure 2a shows the XRD measurement results of SrWO₄, SrWO₄:Tb³⁺, and SrWO₄:Eu³⁺. SrWO₄ synthesized by co-precipitation showed a tetragonal (a = 5.400 Å, b = 5.400 Å, c = 11.910 Å) structure consistent with ICDD # 01-089-2568. The (112) peak, which is the main diffraction peak, was clearly observed, and the sample to which the rare earth was added also clearly exhibited the main peak.

Based on the ‘Lewis’s acid–base’ reaction, the samples synthesized by the co-precipitation method showed an explosive reaction when the solution dissolved in beaker ‘A’ became ‘Homo’ and the solution dissolved in beaker ‘B’ became ‘Lumo’ [23,24], and crystalline SrWO₄ was easily synthesized at room temperature. Figure 2b shows the lattice constant change with and without rare earth doping with the (112) plane, which is the main peak of the sample, of the hosts SrWO₄, SrWO₄:Tb³⁺ and SrWO₄:Eu³⁺, respectively. The lattice constant of the (112) phase, which is the main peak of SrWO₄, was slightly changed by the rare earth doping (SrWO₄: 0.291 nm, SrWO₄:Tb³⁺: 0.2892 nm, SrWO₄:Eu³⁺: 0.2892 nm). It is considered that the change in the crystal lattice is due to the doping with rare earth ions, which have a relatively large ionic radius [25]. FE-SEM images of the synthesized samples are shown in Figure 3. The samples showed a long cylindrical shape with and without doping.

The particle size of SrWO₄ was about 5.78 μm in the longitudinal direction and about 2.36 μm in the transverse direction (Figure 3a). Rare earth doped SrWO₄:Tb³⁺ particles were about 3.57 μm in the longitudinal direction and about 2.29 μm in the transverse direction (Figure 3b), and SrWO₄:Eu³⁺ particles had a size of about 4.82 μm in the longitudinal direction and about 2.31 μm in the transverse direction (Figure 3c). Krishna at al. reported BaMoO₄ is synthesized by reacting with MoO₄⁻², which is a monomer of oxyanion and grows in the vertical direction immediately after mixing the Ba aqueous solution and Mo aqueous solution. It is reported that the shape of the shuttle was clearly visible due to the larger rift in Oswald. It was synthesized using basic materials and explained by the action of the bases [26].

The host SrWO₄ showed absorption in a wide range, from 220 to 340 nm, and peaked at 277 nm. When the sample was excited at the highest peak of 277 nm, it was broad from 350 to 650 nm, and the peak at 492 nm showed blue–white emission spectrum (Figure 4a). Figure 4b shows the emission spectrum of SrWO₄:Tb³⁺ phosphor synthesized by doping Tb³⁺ rare earth ions into SrWO₄. The absorption spectrum of the phosphor powder under 544 nm showed that the band of charge-transfer transition (CTB) generated between the O⁻² and W⁶⁺ of the WO₄²⁻ groups was widely distributed in the 210~290 nm region with a peak at 254 nm [27]. When the phosphor powder was excited with 254 nm, peaks at 487, 544, 586, 620, and 649 nm were observed in the emission spectrum. Among these peaks, the intensity of the green emission spectrum produced by the magnetic dipole transition was the strongest. This emission intensity was 2.99 times stronger than the blue emission intensity produced by the electric dipole transition. The Tb³⁺ ion in the SrWO₄ lattice is located at the inversion symmetric site because the emission intensity due to the magnetic dipole transition of green emission is strong [28].

Figure 4c shows the absorption and emission spectrum of the SrWO₄:Eu³⁺ phosphor synthesized by doping with the rare earth ion Eu³⁺. The absorption spectrum of the phosphor powder under 614 nm shows the absorption spectrum by CTB generated between O²⁻ and Eu³⁺ ions, which appear over the 230~310 nm region and have a peak at 277 nm, with Eu³⁺ observed over the 310~400 nm region. Absorption signals due to the 4f-4f transition of ions were observed [29]. The emission spectrum of the synthesized phosphor was measured by excitation at 277 nm. The phosphor powder showed a red–orange emission spectrum with a peak emission intensity at a wavelength of 614 nm and a spectrum with peaks at 590, 650, and 700 nm. Among these peaks, the 614 nm peak due to the electric dipole transition and the 590 nm peak due to the magnetic dipole transition signal had an intensity difference of about 7.94 times, indicating that the Eu³⁺ ions in the host are located in non-inversion symmetric sites [30].

3.2. Characteristics of the SrWO₄: [Eu³⁺]:[Tb³⁺] Phosphors

Figure 5a shows the X-ray diffraction peak of SrWO₄ co-doped with rare earth ions Tb³⁺ and Eu³⁺ as a white light-emitting phosphor. In the XRD pattern of the synthesized samples, a secondary phase caused by rare earth doping was not found, and the diffraction signal of the main peak (112) was clearly observed. Figure 5b shows the lattice constant change of the (112) phase, which is the main peak of the rare earth co-doped SrWO₄:[Eu³⁺]/[Tb³⁺] samples. Previously, the lattice constants of the SrWO₄:Tb³⁺ and SrWO₄:Eu³⁺ samples doped with a single rare earth decreased, but the lattice constants of the samples doped with both increased. It is believed that the crystal lattice is distorted, or the structure is changed by the amount of added rare earth ions, which have a relatively large ionic radius.

Figure 6 shows the FE-SEM images and energy dispersive X-ray spectroscopy (EDS) mapping component analysis results of the synthesized SrWO₄:[Eu³⁺][Tb³⁺] phosphor. The shape of the particles grew in the longitudinal direction with a cylindrical shape close to the shape of a dumbbell. The particles were about 3.18 μm in the longitudinal direction and about 1.45 μm in the transverse direction. In the EDS component analysis, Sr, W, O, Tb, and Eu components were detected, which confirmed that rare earth ions had been successfully doped.

Figure 7a shows the emission spectrum of the SrWO₄:[Eu³⁺]/[Tb³⁺] phosphor powder co-doped with changing Eu³⁺ ion concentrations, while the Tb³⁺ ion concentration remained fixed. The emission spectra of the two rare earth ions were shown when excited at a wavelength of 254 nm, as the doping concentration of Eu³⁺ increased. Green at 544 nm and orange–red at 614 nm were simultaneously observed. As the concentration of Eu³⁺ ions increased, the intensity of the green emission by Tb³⁺ ions decreased, which means that the emission energy was converted from Tb³⁺ ions in the host lattice to Eu³⁺ ions (Figure 7b). The energy transfer efficiency from Tb³⁺ to Eu³⁺ ions can be expressed by Equation (1) [31].

$$\eta =1-I/I_0$$

(1)

Figure 7. (a) Pl spectra under 254 nm, (b) change in PL intensity, (c) energy transfer efficiency, and (d) CIE coordination SrWO₄:[Eu³⁺]/[Tb³⁺].

Figure 7. (a) Pl spectra under 254 nm, (b) change in PL intensity, (c) energy transfer efficiency, and (d) CIE coordination SrWO₄:[Eu³⁺]/[Tb³⁺].

Here, I is the emission intensity of the Tb³⁺ ions in the SrWO₄:[Eu³⁺]/[Tb³⁺] phosphors, and I₀ is the emission intensity of Tb³⁺ ions in the SrWO₄:Tb³⁺ phosphors. As shown in Figure 7c, as the amount of added Eu³⁺ ions increases, the energy transfer efficiency tends to increase. However, the emission intensity decreased, which is a concentration-quenching phenomenon due to excessive rare earth doping [23]. In the CIE color coordinates, as the doping concentration of Eu³⁺ ions increased, the green coordinates moved to the yellow region (Figure 7d). According to Zhu et al. [32], among the Tb³⁺ and Eu³⁺ rare earth ions co-doped with the CaCO₃ cubic structure, the green emission of Tb³⁺ decreases and the intensity of the red emission of Eu³⁺ increases as the doping concentration of Eu³⁺ increases. Regarding the energy from Tb³⁺ to Eu³⁺ ions, it was reported that a transfer occurred. In this study, as the doping concentration of Eu³⁺ ions increased in the Tb³⁺ and Eu³⁺ ions co-doped with SrWO₄, the intensity of green emission decreased, and the intensity of red emission increased as the energy transfer occurred.

Figure 8 shows the schematic energy diagram of terbium and europium ions luminescence mechanisms in SrWO₄:[Eu³⁺]/[Tb³⁺] phosphors. The phosphors under 254 nm were excited, and ⁵D₃ states of Tb³⁺ can luminesce non-radiatively to the energetically lower ³D₄ excited states. Since the ⁵D₄ states of the Tb³⁺ and the ⁵D₃ states of Eu³⁺ are energetically closed to each other, excitation energy availability transferred to the ⁵D₃ states from Eu³⁺ by the path of resonance transmission. Thus, Tb³⁺ mainly emitted green peaks due to ⁵D₄ → ⁷F₅, and Eu³⁺ emitted red due to ⁵D₀ → ⁷F_J [32].

Table 1. Comparison of previous work in tungsten oxide phosphors.

No.	Host	Rare Earth	Type	Wavalength (nm)
1 [33]	SrWO4	Er3+/Yb3+	Up conversion	489, 525
2 [34]	SrWO4	Tm3+/Yb3+	Up conversion	684, 814
3 [35]	CaWO4	Sm3+/Eu3+	Down conversion	592, 615
4 [36]	CaWO4	Eu3+/Sm3+	Down conversion	622, 630
5 [37]	SrWO4	Eu3+/Sm3+	Down conversion	590, 613
This work	SrWO4	Eu3+/Tb3+	Down conversion	544, 614

shows a reported phosphor synthesized by adding various rare earth ions, with tungsten oxide as a host. Several types of phosphors have been reported, such as up-conversion pre-conversion phosphors co-doped with Yb³⁺ and Er³⁺ and red phosphors co-doped with Eu³⁺ and Sm³⁺. A phosphor was synthesized, and a yellow-emitting phosphor was synthesized by co-doping with Tb³⁺ and Eu³⁺ ions to produce various light-emitting materials as in the previously reported research [33,34,35,36,37].

3.3. Applied for Fingerprint Identification

To clearly observe the fingerprint of the author’s thumb on the glass substrate, the synthesized phosphor powder was scattered, and then the shape of the fingerprint was visualized by illuminating it with a UV lamp. The phosphor doped with rare earth ions revealed the fingerprints in emissions of green and red, which are their own colors, and the specimens co-doped with Eu³⁺ and Tb³⁺ revealed the fingerprints by emitting yellow light. The results suggest that the synthesized phosphor can be used for fingerprint identification (Figure 9).

4. Conclusions

Crystalline SrWO₄ was synthesized at room temperature by co-precipitation. Then, green and red phosphors were synthesized by doping with rare earth ions, Tb³⁺ and Eu³⁺, respectively. The synthesized samples clearly exhibited the (112) phase, which was the main peak in the X-ray diffraction pattern, and the lattice constant was changed by doping with rare earth ions. The synthesized specimens had a size of several microns and a cylindrical shape. In addition, when each specimen was excited using an absorption peak in the ultraviolet region, SrWO₄:Tb³⁺ exhibited green, and SrWO₄:Eu³⁺ emitted red due to the doped rare earth. In the specimen co-doped with Tb³⁺ and Eu³⁺ to obtain various color emissions, the color coordinates shifted to the yellow region as the doping concentration of Eu³⁺ ions increased. The synthesized phosphor was scattered on the glass substrate on which the fingerprint was printed, and when a UV lamp was lit, the green, red, and yellow emission colors were visualized so that the fingerprint could be clearly recognized.