本實驗利用固相合成法製備Al2(WO4)3、La2(WO4)3以及La10W22O81，分別摻雜過渡金屬和稀土元素作為發光中心，研究其光譜性質，並評估其作爲紫外光激發之螢光粉的潛力。實驗結果顯示，以378nm波長作為激發光，摻Er3+之La2(WO4)3最強的兩個發光躍遷為2H11/2→4I15/2 (523nm)和4S3/2→4I15/2 (552nm)，混光呈黃綠色。與之相比，La10W22O81:Er3+的4S3/2→4I15/2躍遷其相對強度更強、譜帶更寬，因此混光也更偏黃。在394nm波長激發下，La2(WO4)3:Eu3+的發光幾乎都集中在5D0→7F2 躍遷(615nm 紅光)，而在La10W22O81:Eu3+中，通常很弱的兩個躍遷，即5D0→7F0 (591nm)、5D0→7F1(593nm)，其強度有大幅度增強，使得整體發光色度偏橘色。
摻Dy3+的La2(WO4)3和La10W22O81最強的紫外吸收位於386nm，在此波長激發可觀察到4F9/2→6H15/2 (478nm)和4F9/2→6H15/2 (573nm)兩個躍遷，兩者的混光呈黃白色，而La10W22O81:Dy3+較La2(WO4)3:Dy3+更接近白色。若再添加Mo與Dy共摻雜，隨著Mo濃度的增加，La2(WO4)3的發光色度則愈趨偏黃，而La10W22O81的發光色度則愈趨近白色，在摻雜40 mol% Mo時，CIE色度座標為(0.364,0.392)，已屬暖色系白光。摻雜Sn或Mn之Al2(WO4)3，沒有在可見光區域觀察到相關的吸收和發光，原因可能是Al位置的晶體場太弱，以至於3d軌域的分裂太小，其能量差小於可見光之能量。

SUMMARY
In this study, the transition-metal ion doped Al2(WO4)3 and the rare-earth ion doped La2(WO4)3 and La10W22O81 were synthesized by the solid-state reaction method. Their spectroscopic property and the potential as the phosphor for UV light conversion were investigated. Under the 378nm excitation, La2(WO4)3:Er3+ showed two dominant emission transitions, 2H11/2→4I15/2 (523nm) and 4S3/2→4I15/2 (552nm), the blend of which gave out a yellowish-green light. The 4S3/2→4I15/2 transition was relatively stronger and broader in La10W22O81:Er3+, so the overall emissions appeared more yellowish. Under the 394nm excitation, the contribution to the La2(WO4)3:Eu3+ emissions came almost solely from the 5D0→7F2 transition (615nm, red), while in La10W22O81:Eu3+ the intensities of two usually very weak transitions, i.e. 5D0→7F0 (591nm) and 5D0→7F1 (593nm), increased greatly, resulting in a color shift of the blended light to orangish. For the Dy3+ doped La2(WO4)3 and La10W22O81, the most intense absorption in UV region took place at 386nm. Under the excitation of this wavelength, two strong emissions were observed due to the transitions 4F9/2→6H15/2 (478nm) and 4F9/2→6H15/2 (573nm). Although the color tones were both yellowish-white, the blended light from La10W22O81:Dy3+ was more on the white side. For the Mo and Dy co-doped samples, the color tone of La2(WO4)3 shifted continuously to the yellow side as the Mo doping concentration increased, while the color tone of La10W22O81 moved more and more close to pure white. At the doping level of 40 mol% Mo, the CIE chromaticity coordinate of the total emissions from co-doped La10W22O81 was located at (0.364, 0.392), which corresponds to the warm-white color. Sn or Mn doped Al2(WO4)3 did not show any relevant absorption/emission in the visible light region, presumably due to a very week crystal field at the Al site that causes a very small splitting of the 3d orbitals.
Key words: Tungstate, optical spectroscopy, phosphor

INTRODUCTION

During last ten years or so, white light-emitting-diodes (LEDs) have gained an increasing attention in the solid state lighting and display systems by the virtue of their long life time, high efficiency, good safety, etc. The first technique to create such a lighting device was to combine a blue LED (i.e. ~460 nm) with a yellow phosphor (Y3Al5O12:Ce3+). However, although this technique has been used widely and is efficient and low-cost, the blended light does not have a satisfactory color rendering index, and a small amount of a red phosphor has to be added. Currently, there are large numbers of compounds being studied as the replacement phosphors for such a purpose. The types of compounds under study include oxides, sulfides, oxysulfides, nitrides, oxynitrides, etc., among which the oxides apparently have some superiorities, such as low toxicity and high chemical and thermal stability. Indeed, in the early stage of the development of inorganic solid-state lasers, most of the compounds studied were oxides, which included various tungstates, e.g. CaWO4:Nd3+, Al2(WO4)3:Cr3+, La2(WO4)3:Pr3+/Nd3+, KY(WO4)2:Er3+/Nd3+, etc. However, so far there has been very little study on the tungstates for the application as the white-LED conversion phosphors. In this study, we synthesized a number of rare-earth or transition-metal ions doped tungstates, including Al2(WO4)3, La2(WO4)3 and La10W22O81. Their spectroscopic properties were characterized and discussed in terms of the potential phosphor application for the UV LED conversion.

MATERIALS AND METHODS

Polycrystalline samples of Al2(WO4)3:0.25%M (M=Mn and Sn), La2(WO4)3:2%R and La10W22O81:2%R (R=Er, Eu and Dy) were synthesized by the solid state reaction method. The starting materials were the oxide powders of WO3, Al2O3, La2O3, R2O3, and MO2, which were weighed according to the stoichiometric ratios and doping concentrations and then, well mixed in an agate mortar by grinding with the addition of some ethanol. After drying, the powder mixtures were pressed into the pellets of 10 mm in diameter by 2~3 mm thick, which were then placed in the alumna crucible and sintered in air at 8001100 oC for 1216 h. The sintered pellets were crushed, re-ground, re-pressed and re-sintered to ensure a good homogeneity of the composition. La2(WO4)3:2%Dy and La10W22O81:2%Dy were also co-doped with Mo up to the concentration, i.e. Mo/(Mo+W), of 40%. The co-doped samples were prepared with the addition of a proper amount of MoO3 powders and the process was similar to what is described above. The phase purity of the synthesized samples was examined by the powder X-ray diffraction (XRD). The spectroscopic property was characterized via the measurements of the photoluminescence and excitation spectra.

RESULTS AND DISCUSSION

Figure 1 shows the XRD patterns of the synthesized Al2(WO4)3. All the recorded lines can be identified with the known structure in the powder diffraction database (PDF #70-1041), indicating that the sample had a pure phase. Doping small amount (0.25%) of Mn or Sn did not cause the formation of any secondary phase, However, the Sn or Mn doped Al2(WO4)3 did not show any relevant absorption/emission in the visible light region. Owing to the high valence of W6+, the electron cloud of O2- is highly polarized and as the consequence, the Al site (at which the M ion should replace) must feel a very weak crystal field. So, the splitting of the 3d orbitals may be so small that the transition between the t2g and eg states is located at a very long wavelength beyond the visible region.
Figure 2 shows the XRD patterns of the La2(WO4)3:2%R (R=Er, Eu and Dy) samples. There is no unknown reflection line compared to the standard pattern in the database (PDF #82-2068), confirming the pure phase of the obtained samples. Similarly, after comparing with the database (PDF#34-0651), the XRD patterns in Figure 3 confirm the phase purity of the synthesized La10W22O81:2%R (R=Er, Eu and Dy) samples. Figure 4 shows the emission spectra of the Er doped samples under the excitation of 378 nm UV light. The spectra have the typical character of trivalent Er ions, where the two emissions peaked at 523 nm and 552 nm can be attributed to the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions, respectively. The blend of these two emissions gave out the yellowish-green light in La2(WO4)3:Er3+, but in La10W22O81:Er3+ where the 4S3/2→4I15/2 transition had a higher relative intensity and a broader linewidth, the overall emissions appeared more yellowish. Figure 5 shows the emission spectra of the Eu doped samples under 394 nm excitation. The spectrum for La2(WO4)3 is dominated by the 5D0→7F2 transition of Eu3+, which has the peak wavelength of 615nm and the chromaticity close to pure red. The 5D0→7F0 and 5D0→7F1 transitions of Eu3+ are forbidden or partially forbidden by the Judd-Ofelt theory, which are therefore usually very weak. In La10W22O81, however, the intensities of these two transitions (peaked at 591nm and 593nm, respectively) increased greatly, resulting in a color shift of the blended light to orangish.
For the Dy doped samples, the most intense absorption in the UV region was located at 386nm due to multiple transitions of Dy3+, i.e. 6H15/2→4M21/2+4I13/2+4K17/2+4F7/2. Figure 6 shows the emission spectra under the excitation of this wavelength, in which two strong emissions are observed at 478nm and 573nm, arising from the 4F9/2→6H15/2 and 4F9/2→6H15/2 transitions of Dy3+, respectively. The color tones were yellowish-white for both La2(WO4)3:Dy3+ and La10W22O81:Dy3+, although the latter looked more on the white side. For the Mo and Dy co-doped La2(WO4)3, the relative intensity of the 573nm emission increased slightly with the increasing Mo concentration and as the consequence, its color tone also shifted continuously to the yellow side. However, for the Mo and Dy co-doped La10W22O81, the relative intensity of the 573nm emission decreased as the Mo concentration increased, as shown in Figure 7, and therefore the color tone moved more and more close to pure white. At the doping level of 40% Mo, the CIE chromaticity coordinate of the total emissions from co-doped La10W22O81 was located at (0.364, 0.392), which corresponds to the warm-white color.

CONCLUSION

The transition-metal ion doped Al2(WO4)3 and the rare-earth doped La2(WO4)3 and La10W22O81 were synthesized by the solid state reaction method. XRD confirmed that all the prepared samples had a pure phase. Owing to a weak crystal field that resulted in a small 3d obit splitting, the Mn or Sn doped Al2(WO4)3 did not show a relevant absorption or emission in the visible wavelength region. In contrast, under a UV light excitation, the Er, Eu and Dy doped La2(WO4)3 and La10W22O81 showed strong emissions in the visible wavelength region. Their spectroscopic properties were discussed in terms of the potential application as a phosphor. The effects on the photoluminescence property and chromaticity of the overall emissions due to the co-doping of Mo and Dy in La2(WO4)3 and La10W22O81 were also investigated.

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