1. Introduction

Scheelite-structured compounds belonging to the tungstates family have been interested and attracted great application due to their unique spectroscopic properties and dramatically excellent upconverted(UC) optical properties. ¹⁾ These UC characteristics of the scheelitestructured compounds have current applications in many fields, including optoelectronic devices and luminescence imaging, due to their excellent UC luminescencent behaviors. They could overcome the present limitations and enhance the applications of the traditional photoluminescence materials.^2,3) In particular, rare-earth doped binary NaLn(WO₄)₂ (Ln = La³⁺, Gd³⁺ and Y³⁺) compounds belong to the space group I4₁/a with the tetragonal phase, and have the family of sheelite-type structure.^4,5) The trivalent lanthanide ions are partially substituted into the crystalline lattices of the tetragonal double tungstate crystal phase. The possible doping could be attributed to the very similar radii of the trivalent lanthanide ions and bring to the excellent properties for UC photoluminescence. ⁶⁾ Multicolor and white light emissions can be generated via tri-doping system based on blue, green and red emission bands. Many lanthanide doping materials such as laser active Ho³⁺ and Tm³⁺ are employed as an activator in luminescent centers for Yb³⁺ as a sensitizer, because their unique electronic energy levels. The tridoped Yb³⁺, Ho³⁺ and Tm³⁺ ions can remarkably enhance the UC efficiency for the shift from infrared to visible light due to the efficiency of the energy transfer from Yb³⁺ to Ho³⁺ and Yb³⁺ to Tm³⁺. Ho³⁺ exhibits ⁵S₂/⁵F₄→ ⁵I₈ transitions in the green region, ⁵F₅→ ⁵I₈ transitions in the red region in upconversion process, while Tm³⁺ shows the ¹G₄→ ³H₆ transitions in the blue region, and ¹G₄→ ³F₄ and ³H₄→ ³H₆ transitions in the red region.^7-9) These ions are effectively doped into the crystal lattices of the tetragonal phase due to the similar radii of the trivalent rare-earth ions, this results in high red emitting efficiency, and superior thermal and chemical stability in white emitting diode.

NaY(WO₄)₂ phosphors have been developed via specific preparation processes, including solid-state reactions, ^10,11) the hydrothermal method,^12,13) the Czochralski method.^14,15) Compared with the usual methods, microwave synthesis has the advantages of a very short reaction time, small-size particles, narrow particle size distribution, and high purity of final polycrystalline samples. Microwave heating is delivered to the material surface by radiant and/or convection heating, which is transferred to the bulk of the material via conduction.¹⁶⁾ A microwave sol-gel process is a cost-effective method that provides high homogeneity and is easy to scale-up, and it is emerging as a viable alternative approach for the quick synthesis of high-quality luminescent materials. However, the synthesis of Ho³⁺/Yb³⁺/Tm³⁺ tri-doped NaY (WO₄)₂ phosphors via the microwave sol-gel route has not been reported. In this study, the double tungstate NaY(WO₄)₂ phosphors with the proper doping concentrations of Ho³⁺, Yb³⁺ and Tm³⁺ (x = Ho³⁺ + Yb³⁺ + Tm³⁺, Ho³⁺ = 0.04, 0.03, 0.02, 0.01, Yb³⁺ = 0.35, 0.40, 0.45, 0.50 and Tm³⁺ = 0.01, 0.02, 0.03, 0.04) were successfully prepared by the microwave sol-gel method, followed by heat treatment. The synthesized particles were characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM). The pump power dependence of the UC emission intensity and Commission Internationale de L’Eclairage(CIE) chromatic coordinates were evaluated in detail. The optical properties were examined comparatively using photoluminescence(PL) emission and Raman spectroscopy.

2. Experimental Procedure

Appropriate stoichiometric amounts of Na₂WO₄·2H₂O (99 %, Sigma-Aldrich, USA), Y(NO₃)₃·6H₂O (99 %, Sigma-Aldrich, USA), (NH₄)₆W₁₂O₃₉·xH₂O (99 %, Alfa Aesar, USA), Ho(NO₃)₃·5H₂O (99.9 %, Sigma-Aldrich, USA), Yb(NO₃)₃·5H₂O (99.9 %, Sigma-Aldrich, USA), Tm(NO₃)₃·5H₂O (99.9%, Sigma-Aldrich, USA), citric acid (99.5 %, Daejung Chemicals, Korea), NH₄OH (A.R.), ethylene glycol (A.R.) and distilled water were used to prepare the compounds. Table 1 shows the chemical compositions and sample notations of NaY_1-x(WO₄)₂: Ho³⁺/Yb³⁺/Tm³⁺(x =Ho³⁺+Yb³⁺+Tm³⁺). To prepare NaY_0.60 (WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, 0.2 mol% Na₂WO₄·2H₂O and 0.067 mol% (NH₄)₆W₁₂O₃₉·xH₂O were dissolved in 20 mL of ethylene glycol and 80 mL of 5M NH₄OH under vigorous stirring and heating. Subsequently, 0.24 mol% Y(NO₃)₃·6H₂O with 0.016 mol% Ho(NO₃)₃·5H₂O, 0.14 mol% Yb(NO₃)₃·5H₂O and 0.004 mol% Tm(NO₃)₃·5H₂O, and citric acid (with a molar ratio of citric acid to total metal ions of 2:1) were dissolved in 100 mL of distilled water under vigorous stirring and heating. Then, the solutions were mixed together under vigorous stirring and heating at 80-100 °C. Finally, highly transparent solutions were obtained and adjusted to pH = 7-8 by the addition of 8M NH₄OH. In order to prepare NaY_0.55 (WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, the mixture of 0.22 mol% Y(NO₃)₃·6H₂O with 0.012 mol% Ho(NO₃)₃·5H₂O, 0.16 mol% Yb(NO₃)₃·5H₂O and 0.008 mol% Tm(NO₃)₃·5H₂O was used for the creation of the rare-earth solution. In order to prepare NaY_0.50 (WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, the mixture of 0.20 mol% Y(NO₃)₃·6H₂O with 0.008 mol% Ho(NO₃)₃·5H₂O, 0.18 mol% Yb(NO₃)₃·5H₂O and 0.012 mol% Tm(NO₃)₃·5H₂O was used for the creation of the rare-earth solution. In order to prepare NaY_0.45(WO₄)₂: Ho_0.01/Yb_0.50/Tm_0.04, the rare-earth containing solution was generated using 0.18 mol% Y(NO₃)₃·6H₂O with 0.004 mol% Ho(NO₃)₃·5H₂O, 0.20 mol% Yb(NO₃)₃·5H₂O and 0.016 mol% Tm(NO₃)₃·5H₂O.

Table 1

Chemical compositions and sample notations of NaY_1-x(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ (x = Ho³⁺ + Yb³⁺ + Tm³⁺).

The transparent solutions were placed into a microwave oven operating at a frequency of 2.45 GHz with a maximum output-power of 1250 W for 30 min. The working cycle of the microwave reaction was controlled very precisely using a regime of 40 s on and 20 s off for 15 min, followed by further treatment of 30 s on and 30 s off for 15 min. The samples were treated with ultrasonic radiation for 10 min to produce a light yellow transparent sol. After this, the light yellow transparent sols were dried at 120 °C in a dry oven to obtain black dried gels. The black dried gels were ground and heattreated at 900 °C for 16 h with 100 °C intervals between 600-900 °C. Finally, pink particles were obtained for the doped compositions. The phase composition of the synthesized particles was identified using XRD (D/MAX 2200, Rigaku, Japan). The microstructure and surface morphology of the synthesized particles were observed using SEM (JSM-5600, JEOL, Japan). The PL spectra were recorded using a spectrophotometer (Perkin Elmer LS55, UK) at room temperature. Raman spectroscopy measurements were performed using a LabRam Aramis (Horiba Jobin-Yvon, France). The 514.5-nm line of an Ar ion laser was used as the excitation source, and the power on the samples was kept at 0.5 mW.

3. Results and Discussion

Fig. 1 shows the X-ray diffraction patterns of the (a) JCPDS 48-0886 data of NaY(WO₄)₂, the synthesized (b) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (c) NaY_0.55(WO₄)₂: Ho_0.03/Yb_0.40/Tm_0.02, (d) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (e) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04 particles. All of the XRD peaks could be assigned to the tetragonalphase NaY(WO₄)₂ with the space group of I41/a, which was in good agreement with the crystallographic data of NaY (WO₄)₂ (JCPDS 48-0886). It is observed that the diffraction peaks of the doped samples in Fig. 1(b-e) shift slightly to the higher angles, as compared to that of the standard data NaY(WO₄)₂ shown in Fig. 1(a). It is well known that the relationship of the interplanar space(d_hkl), diffraction angle(θ) and wavelength of Xray(λ) is expressed by the Bragg’s equation:

Fig. 1

X-ray diffraction patterns of the (a) JCPDS 48-0886 data of NaY(WO₄)₂, the synthesized (b) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (c) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, (d) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/ Tm_0.03, and (e) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04, particles.

In pure NaY(WO₄)₂ crystals, the unit cell decrease occurs due to the substitution of Ho³⁺(R = 1.015 Å), Yb³⁺ (R = 0.985 Å) and Tm³⁺(R = 0.994 Å) ions in the Y³⁺(R = 1.019 Å) sites.¹⁷⁾ According to Bragg equation (1), the diffraction peaks shift to higher angles with the decrease of d_hkl values. Post heat-treatment plays an important role in a well-defined crystallized morphology. To achieve a well-defined crystalline morphology of NaY_0.60(WO₄)₂: Ho_0.04/Yb_0.35/Tm_0.01, NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and NaY_0.45(WO₄)₂: Ho_0.01/Yb_0.50/Tm_0.04 particles, phases need to be heat treated at 900 °C for 16 h. It is assumed that the doping amount of Ho³⁺/Yb³⁺/Tm³⁺ has a great effect on the crystalline cell volume of the NaY(WO₄)₂, because of the different ionic sizes. This means that the obtained samples possess a tetragonal-phase after partial substitution of Y³⁺ by Ho³⁺, Yb³⁺ and Tm³⁺ ions, and the ions are effectively doped into crystal lattices of the NaY(WO₄)₂ phase due to the similar radii of Y³⁺, Ho³⁺, Yb³⁺ and Tm³⁺.

Fig. 2 shows SEM images of the synthesized (a) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (b) NaY_0.55(WO₄)₂:Ho_0.03 /Yb_0.40/Tm_0.02, (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (d) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04 particles. The assynthesized samples are very similar morphologies and have no discrepancy in aspect of morphological feature, showing well crystallized and homogeneous microcrystalline morphology with particle size of 1-2 μm. The agglomerated particles are induced by the atom interdiffusion between the grains. It should be noted that the morphological feature is insensitive to the Ho³⁺/Yb³⁺/ Tm³⁺ doping concentrations. The microwave sol-gel method in application to the double tungstates provides the energy to synthesize the bulk of the material uniformly, so that fine particles with controlled morphology can be fabricated in a short time period. The method is a costeffective way to fabricate highly homogeneous products with easy scale-up. It is a viable alternative for the rapid synthesis of UC particles. This suggests that the microwave sol-gel route is suitable for the creation of homogeneous NaY_1-x(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ crystallites. This suggests that the microwave sol-gel route is suitable for the creation of homogeneous NaY_1-x(WO₄)₂:Ho³⁺/Yb³⁺/ Tm³⁺ crystallites and it can be successfully applied for other tungstaes from this crystal family.

Fig. 2

Scanning electron microscopy images of the synthesized (a) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (b) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (d) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04, particles.

Fig. 3 shows the UC photoluminescence emission spectra of the as-prepared (a) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/ Tm_0.01, (b) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, (c) NaY_0.50 (WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (d) NaY_0.45(WO₄)₂:Ho_0.01 /Yb_0.50/Tm_0.04 particles. particles excited under 980 nm at room temperature. Under excitation at 980 nm, the doped particles exhibited white emissions based on blue, green and red emission bands, which correspond to the ¹G₄→ ³H₆ transitions of Tm³⁺ in the blue region, the ⁵S₂/ ⁵F₄→ ⁵I₈ transitions of Ho³⁺ in the green region, the ⁵F₅→ ⁵I₈ transitions of Ho³⁺ as well as the ¹G₄→ ³F₄ and ³H₄→ ³H₆ transitions of Tm³⁺ in the red region. The UC intensity of (d) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04 exhibits the strongest 475-nm emission band in the blue region due to higher content of Tm³⁺, while the UC intensity of (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03 provides the strongest 545-nm emission band in the green region and the strongest 655-nm emission band in the red region due to appropriate ratio of Yb³⁺:Ho³⁺ + Tm³⁺ = 9:1. Thus, the optimal Yb³⁺:Ho³⁺ + Tm³⁺ ratio is as high as 9:1 for the white emitting diode based on the blue, green and red emissions.

Fig. 3

Upconversion photoluminescence emission spectra of (a) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (b) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/ Tm_0.02, (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (d) NaY_0.45(WO₄)₂: Ho_0.01/Yb_0.50/Tm_0.04 particles excited under 980 nm at room temperature.

The logarithmic scale dependence of the UC emission intensities at 475, 545, 655 and 695 nm on the working pump power over the range of 20 to 110 mW in the NaY_0.50(WO₄)₂:Yb0.02/Ho0.45/Tm_0.03 sample is shown in Fig. 4. In the UC process, the UC emission intensity is proportional to the slope value n of the irradiation pumping power, where n is the number of pumped photons required to produce UC emission:¹⁸⁾

Fig. 4

Logarithmic scale dependence of the upconversion emission intensity on the pump power in the range from 20 to 110 mW at 475, 545, 655 and 695 nm in the NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03 sample.

where value n is the number of the pumped photons required to excite the upper emitting state, I is the UC luminescent intensity and P is the laser pumping power. As evident from Fig. 4, the slope value calculations indicate n = 2.23 for green emission at 475 nm, n = 1.95 for green emission at 545 nm, and n = 1.73 and 1.69 for red emissions at 655 and 695 nm, respectively. These results show that the UC mechanism of the blue, green and red emissions can be explained by the multi-step energy transfer process in Ho³⁺/Yb³⁺/Tm³⁺ tri-doped phosphors. ^7-9,22-25)

Fig. 5 shows schematic energy level diagrams of Ho³⁺/ Tm³⁺(activators) and Yb³⁺ ions(sensitizer) ions in the NaY_1-x(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ system and the UC mechanisms accounting for the blue, green and red emissions during 980 nm laser excitation. The UC emissions are generated by the multi-step process through excited state absorption(ESA) and energy transfer(ET). For the emissions for Ho³⁺ activator, initially the Yb³⁺ ion sensitizer is excited from the ²F_7/2 level to the ⁴F_5/2 level under excitation of 980 nm pumping, and transfers its energy to Ho³⁺ ions. Then, Ho³⁺ ions are populated from the ⁵I₈ ground state to the ⁵I₆ excited state. This is a phonon-assisted energy transfer process because of energy mismatch between the ²F_5/2 level of Yb³⁺ and the ⁵I₆ level of Ho³⁺. Secondly, the Ho³⁺ in the ⁵I₆ level is excited to the ⁵S₂ or ⁵F₄ level by the same energy transfer from Yb³⁺. In addition, the ⁵S₂ /⁵F₄ level of Ho³⁺ can be also populated through excited state absorption. Finally, the green emission around 545 nm corresponding to ⁵S₂/⁵F₄→ ⁵I₈ transition takes place. For red emission of Ho³⁺, the population of the ⁵F₅ level is generated by two different channels. One channel is that Ho³⁺ in the ⁵S₂/⁵F₄ level relaxes nonradiatively to the ⁵F₅ level. Another channel is closely related to the ⁵I₇ level populated by non-radiative relaxation from the ⁵I₆ excited state. The Ho³⁺ in the ⁵I₇ level is excited to the ⁵F₅ level by the energy transfer from Yb³⁺. Therefore, the red emission around 655 nm is corresponding to the ⁵F₅→ ⁵I₈ transition.^19,20)

Fig. 5

The schematic energy level diagrams of Ho³⁺/Tm³⁺ (activators) and Yb³⁺ ions (sensitizer) ions in the NaY_1-x(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ system and the upconversion mechanisms of the blue, green and red emissions under 980 nm laser excitation.

(3)

LnI∝nLnP

For the emissions for Tm³⁺ activator, the Yb³⁺ ion sensitizer is excited from the ²F_7/2 level to the ⁴F_5/2 level under excitation of 980 nm pumping, and transfers its energy to Tm³⁺ ions. Then, Tm³⁺ ions is populated from the ³H₆ ground state to the ³H₅ excited state. During the phonon-assisted energy transfer process, the energy mismatch occurs between the ²F_5/2 level of Yb³⁺ and the ³H₅ level of Tm³⁺. Continuously, the Tm³⁺ in the ³H₅ level relaxes non-radiatively to the ³F₄ level. The Tm³⁺ in the ³F₄ level is excited to the 3F3 level by the energy transfer from Yb³⁺. Furthermore, the Tm³⁺ in the 3F3 relaxes also non-radiatively to the ³H₄ level, and is generated to the red emission of 695 nm. Subsequently, the Tm³⁺ in the ³H₄ level is excited to the ¹G₄ level by the energy transfer from Yb³⁺. Finally, the red emission at 655 nm corresponding to the ¹G₄→ ³F₄ transitions is developed, and the blue emission at 475 nm corresponding to the ¹G₄→ ³H₆ transitions is created.^21,22)

Fig. 6 shows (A) calculated chromaticity coordinates(x, y) values and (B) CIE chromaticity diagram for (a) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (b) NaY_0.55(WO₄)₂:Ho_0.03 /Yb_0.40/Tm_0.02, (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (d) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04 particles. In Fig. 6, (A) the calculated chromaticity coordinates(x, y) and (B) CIE chromaticity diagram are shown for the compositions (a) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (b) NaY_0.55 (WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45 /Tm_0.03, and (d) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04. The triangle depicted in Fig. 6(B) indicates standard coordinates for blue, green and red colors. The inset in Fig. 6(B) shows the chromaticity points for the samples (a), (b), (c) and (d). The chromaticity coordinates(x, y) are strongly dependent on the Ho³⁺/Yb³⁺/Tm³⁺ concentration ratio. As shown in Fig. 6(A), the calculated chromaticity coordinates x = 0.332 and y = 0.345 for (a) NaY_0.60(WO₄)₂: Ho_0.04/Yb_0.35/Tm_0.01, x = 0.324 and y = 0.332 for (b) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, x = 0.325 and y = 0.325 for (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and x = 0.313 and y = 0.312 for (d) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/Tm_0.04 are corresponding to the standard equal energy point in CIE diagram in Fig. 6(B).

Fig. 6

(A) Calculated chromaticity coordinates(x, y) values and (B) CIE chromaticity diagram for NaY_1-x(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ phosphors. The inset shows the emission points for the sample synthesized (a) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (b) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40 /Tm_0.02, (c) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (d) NaY_0.45(WO₄)₂: Ho_0.01/Yb_0.50/Tm_0.04, particles.

Fig. 7 shows the Raman spectra of the synthesized (a) pure NaY(WO₄)₂, (b) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (c) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/Tm_0.02, (d) NaY_0.50(WO₄)₂: Ho_0.02/Yb_0.45/Tm_0.03, and (e) NaY_0.45(WO₄)₂:Ho_0.01/Yb_0.50/ Tm_0.04 particles excited by the 514.5-nm line of an Ar ion laser at 0.5 mW. The internal modes for the (a) pure NaY(WO₄)₂ particles were detected at 332, 396, 768, 836, 878, 918 and 936 cm⁻¹, respectively. The well-resolved sharp peaks for the NaY(WO₄)₂ indicate a high crystallinity state of the synthesized particles. The internal vibration mode frequencies are dependent on the lattice parameters and the strength of the partially covalent bond between the cation and molecular ionic group WO₄. The Raman spectrum of the NaY(WO₄)₂ crystal in Fig. 7(a) shows the typical tungstate compounds, which is divided into two parts with a wide empty gap of 400-750 cm⁻¹.^23-25) The stretching vibrations of W-O bonds are observed at 768~936 cm⁻¹. For these stretching vibrations, strong mixing occurs between the W-O bonds and the WO₄. The bands at 332 and 396 cm⁻¹ could be assumed to originate from vibrations of the longer W-O bonds, which are employed in the formation of the W-W bridge. The translational vibration motion of the Na³⁺ ions is observed around 200 cm⁻¹, whereas the Y³⁺ translations were located below 180 cm⁻¹.^26,27) The Raman spectra of the doped particles indicate the very strong and dominant peaks at higher frequencies of 838, 918, 1110 and 1284 cm⁻¹ and at lower frequencies of 212, 334 and 415 cm⁻¹. These strong disordered peaks at higher and lower frequencies are attributed the strong mixing between the W-O bonds and the WO₄ stretching vibrations as well as the concentration quenching effect of Ho³⁺ and Tm³⁺ ions. It is assumed that the highly modulated structure has strong absorption in the near ultraviolet region, so that energy transfer processes from the WO_4-x group to rare earth ions can easily occur, which can greatly enhance the external quantum efficiency of rare earth ions doped materials. These results lead to high emitting efficiency and superior thermal and chemical stability, and these materials, which can be considered potentially active components in new optoelectronic devices and in biomedical applications.

Fig. 7

Raman spectra of the synthesized (a) pure NaY(WO₄)₂, (b) NaY_0.60(WO₄)₂:Ho_0.04/Yb_0.35/Tm_0.01, (c) NaY_0.55(WO₄)₂:Ho_0.03/Yb_0.40/ Tm_0.02, (d) NaY_0.50(WO₄)₂:Ho_0.02/Yb_0.45/Tm_0.03, and (e) NaY_0.45(WO₄)₂: Ho_0.01/Yb_0.50/Tm_0.04, particles excited by the 514.5-nm line of an Ar ion laser at 0.5 mW.

4. Conclusions

The double tungstate NaY_1-x(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ phosphors were successfully synthesized by microwave solgel method. The well-crystallized particles formed after heat-treatment at 900 °C for 16 h showed the fine and homogeneous microcrystalline morphology with particle sizes of 1-2 μm. Under excitation at 980 nm, the UC doped particles exhibited white emissions based on blue, green and red emission bands, which correspond to the ¹G₄→ ³H₆ transitions of Tm³⁺ in the blue region, the ⁵S₂/ ⁵F₄→ ⁵I₈ transitions of Ho³⁺ in the green region, the ⁵F₅ → ⁵I₈ transitions of Ho³⁺ as well as the ¹G₄→ ³F₄ and ³H₄→ ³H₆ transitions of Tm³⁺ in the red region. The calculated slope value n indicated n = 2.23 for green emission at 475 nm, n = 1.95 for green emission at 545 nm, and n = 1.73 and 1.69 for red emissions at 655 and 695 nm, respectively. The calculated chromaticity coordinates were corresponding to the standard equal energy point in CIE diagram. The strong disordered peaks at higher and lower frequencies in Raman spectroscopy were attributed the strong mixing between the W-O bonds and the WO₄ stretching vibrations as well as the concentration quenching effect of Ho³⁺ and Tm³⁺ ions. The results led to high emitting efficiency and the involved materials can be considered as potentially active components in new optoelectronic devices and in the field of biomedical applications.