Preparation of Eu³⁺-Y³⁺ co-doping red-emitting phosphors for white-light emitting diodes (W-LEDs) application and investigation of their optical characteristics

The phosphors were prepared by using the solid-state reaction at high temperature, of which the reactants include CaO(A.R.grade), SrCO₃(A.R.grade), Eu₂O₃(99.99% purity), MoO₃(A.R.grade), Y₂O₃(A.R.grade) and WO₃(A.R.grade). According to the nominal composition Ca_0.54Sr_0.34−1.5x Eu_0.08Y _x (MoO4) _y (WO₄)_1−y (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.12, 0.16, 0.20. y = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), the synthesized system may be as following:

The stoichiometric reactants were first ground and pre-fired at 500 °C for 2 h, and then heated at 600–1,100 °C in air for 3 h and the powders were obtained. Then the luminescence properties of the product were studied, NH₄Cl employed as flux.

The structure of the product was recorded by X-ray powder diffraction (XRD) employing CuK_α radiation at 40 kv and 250 mA. A step size of 0.02°(2θ) was used with a scan speed of 4°/min. Excitation and emission spectra, excitation and emission slits set at 2.5 nm, were measured by using a Hitachi F-4600 spectrometer equipped with a 150 W-xenon lamp under a working voltage of 500 V. All the measurements were performed at room temperature.

Figure 1a and b show XRD patterns of Ca_0.54Sr_0.34Eu_0.08(MoO₄)_0.2(WO₄)_0.8 and Ca_0.54Sr_0.46MoO₄, respectively. Both patterns are shown without unidentified diffraction peaks from impurity. The XRD patterns of Ca_0.54Sr_0.34Eu_0.08(MoO₄)_0.2(WO₄)_0.8 and Ca_0.54Sr_0.46MoO₄ are sheelite structures with space group I4₁/a and similar unit cell parameters, a = 0.5203 nm, c = 1.1330 nm for the former and a = 0.5352 nm, c = 1.1645 nm for the latter. It is demonstrated that the cell unit of the latter increases a little because of W⁶⁺(0.062 nm) substituting Mo⁶⁺(0.062 nm) and Eu³⁺(0.095 nm) substituting Sr²⁺(0.113 nm). Figure 1b depicts the situation of main reflection peaks, which is investigated that the situation of main reflection peaks are consistent and their intensities increase with Eu³⁺ and W⁶⁺ ions introduced into the host lattice.

Rainho et al. [14] studied that the cell unit size had great effect on the dispersion of UV and mentioned the correlated formula. The dispersion coefficient s and the average grain size g has a relation as follows:where k is a constant and σ is broaden standard deviation of granulometric normal distribution. It can be indicated that, the less the phosphors grain size is, the bigger granulometric distribution width and distribution coefficient s are. In Fig. 2 shown the SEM patterns of Ca_0.54Sr_0.34Eu_0.08(MoO₄)_0.2(WO₄)_0.8 with different flux dosages, the SEM patterns indicate that the grains diameters are approximately 6–8 μm as the amount of NH₄Cl flux reaches 4 mol%. The luminescent system appears melting and agglomeration when the amount of NH₄Cl flux is 8–12 mol%. The results show the appropriate dosage and category of flux can improve the grain size and crystal structures of luminescent system, of which can improve its luminescent properties.

Shown in Fig. 3 are the excitation spectra monitored at 616 nm and emission spectra under 394, 465 and 535 nm, respectively, of Ca_0.54Sr_0.22Eu_0.08Y_0.08(MoO₄)_0.2(WO₄)_0.8. The excitation spectrum for monitoring ⁵D₀ → ⁷F₂ emission of Eu³⁺ can be divided into two regions, the broad excitation band from ~210 nm extending up to 355 nm, its band edge located at 311 nm, attributed to the charge transfer transition of Eu–O, W–O and Mo–O group, and the narrow peaks located at wavelengths longer than 355 nm assigned to the f-f transitions of Eu³⁺. The f-f transitions of Eu³⁺ in excitation spectrum include sharp lines ⁷F₀ → ⁵L₆ at 394 nm, ⁷F₀ → ⁵D₂ at 465 nm, and ⁷F₀ → ⁵D₁ at 535 nm. For the Eu³⁺ doped phosphors, the intensity of charge transfer band is weaker than that of f-f transitions in the excitation spectrum. In the emission spectra of luminescent system, The typical emission spectra of Ca_0.54Sr_0.22Eu_0.08Y_0.08(MoO₄)₀₂(WO₄)_0.8 is composed of groups of several sharp lines, which belong to the intrinsic emission of trivalent Eu ion. The main emission line around 616 nm is assigned to the Eu³⁺ electric dipole transition of ⁵D₀ → ⁷F₂, which is sensitive to the site symmetry. The domain emission peak at 616 nm indicates that the Eu³⁺ is located at the site lack of inversion symmetry, breaking the parity-selection rules. The emission spectra excited upon 394 and 465 nm have the same profile as that excited upon 535 nm. Obviously, this phosphor can be excited upon f-f transition from the excitation spectra. It is a good sign that this novel phosphor can strongly absorb ultraviolet (394 nm) and visible blue light (465 nm), and transfer the excitation energy to the red radiation. The wavelengths at 394 and 465 nm are nicely in agreement with the widely applied UV or blue output wavelengths of GaN-based LED chips.

Shown in Fig. 4 is the emission intensity of Ca_0.54Sr_0.22Eu_0.08Y_0.08(MoO₄) _x (WO₄)_1−x (x = 0, 0.1, 0.2, 0. 4, 0. 6, 0. 8, 1.0) with different Mo⁶⁺ content under 394, 465 and 535 nm excitation, respectively. When the Mo⁶⁺ concentration is 0–20 mol%, with Mo⁶⁺ concentration increasing, the emission intensity of the samples strengthens obviously and reaches a maximum value, 20 mol%. Then, with Mo⁶⁺ concentration increasing, the emission intensity of samples can decrease in a certain extent when Mo⁶⁺ concentration is beyond 20 mol%. It is indicated that W⁶⁺ concentration and Mo⁶⁺ content may reach an optimum status in energy transfer, which can improve the crystal structures and luminescent properties of the phosphors.

Figure 5 shows the emission spectra of Ca_0.54Sr_0.34−1.5x Eu_0.08Y _x (MoO₄)_0.2(WO₄)_0.8 (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.20) under 394 and 465 nm excitation respectively, with different Y³⁺ content. Y³⁺-Eu³⁺ co-doped this system can improve its luminescent properties, when Y³⁺ concentration is 8 mol%, the emission intensity of luminescent system reaches the maximum value. Furthermore, a ratio between the integrated intensity of the two transitions, I_0–J/I_0–J, is used in lanthanide-based systems as a probe of the cation local surroundings [15]. As shown in Fig. 5, the transition ⁵D₀ → ⁷F₂ (616 nm) is much stronger than the transition ⁵D₀ → ⁷F₃ (655 nm) and ⁵D₀ → ⁷F₄(702.8 nm), and the ratio of I_0–2/I_0–3 and I_0–2/I_0–4 are about 45.44, 14.43 (394 nm), 43.73 nm and 14.12(465 nm) respectively, which suggests that Eu³⁺ is located in a distorted cation environment. That is also favorable to improve the color purity of the red phosphors.

To explore the highly efficient red-emitting phosphors for LED, two red-emitting LEDs were fabricated by combining Ca_0.54Sr_0.22Eu_0.08Y_0.08(MoO₄)_0.2(WO₄)_0.8:0.08Eu³⁺, 0.08Y³⁺ (b) or commercial available Y₂O₂S:Eu³⁺(a) with 390–405 nm LED chip. Shown in Fig. 6, the band at ~394 nm attributed to the LED chip and the sharp peaks at 616 nm due to the emissions of Ca_0.54Sr_0.22Eu_0.08Y_0.08(MoO₄)_0.2(WO₄)_0.8:0.08Eu³⁺, 0.08Y³⁺ (b) or Y₂O₂S:Eu³⁺(a). The CIE chromaticity coordinates of the phosphor Ca_0.54Sr_0.22Eu_0.08Y_0.08(MoO₄)_0.2(WO₄)_0.8 are calculated to be x = 0.64, y = 0.36 (394 nm) and Y₂O₂S:Eu³⁺ being x = 0.63, y = 0.37 (465 nm). The former is close to the standard of NTSC (x = 0.67, y = 0.33).

The intensive emission of LED chip at ~400 nm can be observed in Fig. 6, which is benefitable to obtain a white-light LED by combining this phosphor with appropriate blue and green phosphors. From the application angle, one good mono-color LED phosphor with luminescent property should own three characteristics. Above all, the phosphor must have efficient absorption at ~400 nm. Secondly, the phosphor exhibits high luminous intensity under ~400 nm excitation. Last but not least, the chromaticity coordinates of the phosphor is close to the NSTC standard values.

In a word, it is investigated that the luminescent property of Ca_0.54Sr_0.22(MoO₄)_0.2(WO₄)_0.8:0.08Eu³⁺, 0.08Y³⁺ phosphor is better than that of commercial available Y₂O₂S:Eu³⁺ phosphor in the above three facets.