Advances in Materials Science and EngineeringVolume 2011 (2011), Article ID 790517, 7 pagesdoi:10.1155/2011/790517

Research Article

Near UV-Blue Excitable Green-Emitting Nanocrystalline Oxide

C. E. Rodríguez-García,¹ N. Perea-López,² and G. A. Hirata³

¹Programa de Posgrado en Física de Materiales, CICESE-UNAM, Carretera Ensenada-Tijuana No. 3918, Zona Playitas, C.P. 22860, Ensenada, BC, Mexico²Laboratorio de Investigaciones en Nanociencias y Nanotecnología (LINAN) y División de Materiales Avanzados, IPICYT, Camino a la Presa San José 2055, Lomas 4a. Sección, San Luis Potosí 78216, Mexico³Centro de Nanociencias y Nanotecnología Universidad Nacional Autonóma de México, Km 107 Carretera Tijuana-Ensenada, 22800 Ensenada, BC, Mexico

Received 1 October 2010; Accepted 4 April 2011

Academic Editor: Peter Majewski

Copyright © 2011 C. E. Rodríguez-García et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Green-emitting Eu-activated powders were produced by a two-stage method consisting of pressure-assisted combustion synthesis and postannealing in ammonia. The as-synthesized powders exhibited a red photoluminescence (PL) peak located at  nm when excited with  nm UV. This emission peak corresponds to the ⁵D₀→⁷F₂ transition in Eu³⁺. After annealing in ammonia, the PL emission changed to an intense broad-band peak centered at  nm, most likely produced by 4f⁶5d¹→4f⁷ electronic transitions in Eu²⁺. This green-emitting phosphor has excitation band in the near UV-blue region (–450 nm). X-ray diffraction analysis reveals mainly the orthorhombic EuAlO₃ and Al₂O₃ phases. Transmission electron microscopy observations showed that the grains are formed by faceted nanocrystals (~4 nm) of polygonal shape. The excellent excitation and emission properties make these powders very promising to be used as phosphors in UV solid-state diodes coupled to activate white-emitting lamps.

1. Introduction

Commercial white-light-emitting diodes (WLEDs) are composed of a blue-GaN chip which pumps YAG : Ce³⁺ yellow phosphor, and the blend of these two colors, the transmitted blue and luminescent yellow, results in white-light emission [1]. However, WLEDs fabricated in this way deliver di-chromatic white-light with poor color-rendering index quality () [2]. To solve this problem, WLEDs should be composed of blue, green, and red-emitting multiphase phosphors excited by a UV-blue (–450 nm) InGaN chip [3]. This type of WLED is advantageous because the white color is generated by phosphors and this produces an excellent color-rendering index () [3–5]. Based on the above, research in material science engineering has focused on the production of efficient green phosphors, considering that these can be combined with either the blue component from the InGaN chip or with one supplied by another phosphor, to produce artificial white-light with high CRI. Phosphors constituted by nanocrystalline and submicrocrystalline oxides doped with rare earths (REs) are advantageous because they have high chemical and thermal stability [6]. Another valuable property of the nanocrystalline oxide phosphors is their large surface area, which enhances luminescence intensity [7]. In addition, RE-doped oxides allow a wide range of technological applications.

Europium aluminates EuAlO₃ and EuAlO₃ : Eu²⁺ [8–10] which have been studied for their interesting luminescent properties applied to solid-state lighting. The precursors of EuAlO₃ and EuAlO₃ : Eu²⁺ are Al₂O₃ : Eu³⁺ and Al₂O₃ : Eu²⁺, respectively [8]. Ozuna et al. [11] reported two compounds, α-Al₂O₃ : Eu3+ and γ-Al₂O₃ : Eu3+ as candidates for the red component and specified that low concentration of europium (–5.0 at %) is required in order to obtain intense red emission. In addition, when the Eu-doping concentration at %, it produces EuAlO3 [8]. Few reports have referred to the Al₂O₃ : Eu²⁺ system [12] which produces green emission.

EuAlO₃ crystallizes in the distorted orthorhombic perovskite structure with general chemical formula ABO₃ [13]. The large A ion has 12-fold and the smaller B ion 6-fold coordination in the crystalline lattice. In this case the Eu³⁺ is at the A-site of EuAlO₃. When Eu³⁺ is reduced to Eu²⁺, it remains at the A-site producing defects in the lattice. The role of these defects and RE impurities is important to explain the red and green emissions of europium aluminates in powder form [8, 13]. However, the f-f forbidden transitions in Eu³⁺ are very sensitive towards the crystal-field environment in the host lattice [14, 15]. If the Eu³⁺ site is noninversion symmetry site, the ⁵D₀→⁷F₂ ( = 616 nm) electric-forced dipole transition is more probable [15]. This also means that if the crystal-field in the lattice is distorted, a forced electric dipole transition is promoted. The Goldsmith tolerance factor describes the stability of the perovskite structure, ABO₃, and is defined as , where rA, rB, and rO are the 2 average ionic radii of the A-, B-site cations and of the O²⁻ ion, respectively [16]. By means of the Goldsmith factor (t), it is possible to determine the distortion of the perovskite structure, that is an ideal perovskite structure has a , and if this value is reduced, the distortion of the perovskite increases. For the ionic radii of Å, Å, and Å, the Goldsmith factor (t) for the EuAlO₃ structure is . In this sense, EuAlO₃ shows a certain degree of distortion, hence the red emission due to the ⁵D₀→⁷F₂ transition is now possible. The Å,  , and EuAlO₃ : Eu²⁺ have a structure very close to the ideal perovskite.

In a previous report of our group, combustion synthesis plus postannealing was applied to produce EuAlO₃ and EuAlO₃ : Eu²⁺ [8]. However, the pressure-assisted combustion synthesis method has not been explored to produce these materials. In this context, the pressure effects on other europium aluminates, as well as on different materials, are an important factor that determines the nanostructured morphology [17].

In the present study, green phosphors were produced by means of pressure-assisted combustion synthesis (PACS) plus the rapid IN and rapid OUT (RIRO) process [8]. As a final result, we obtained luminescent nanocrystalline powders with intense green emission due to their content of Eu²⁺ ions.

2. Experimental Section

2.1. PACS Method

Nanocrystalline green-emitting powders were prepared in two stages. In stage 1, we applied the pressure-assisted combustion synthesis (PACS) method [18] as follows: the metal nitrates, europium nitrate [Eu(NO₃)₃·6H₂O reacton 99.9%] and aluminum nitrate [Al(NO₃)₃·H₂O Puratronic 99.9965%] were dissolved in 25 ml of deionized water in a quartz beaker, and hydrazine [N₂H₄ Alfa AEesar 98.5%] was added as a reductive free-carbon fuel to prevent contamination, forming a gelatinous homogeneous blend. The quartz beaker containing the blend was placed into a closed reactor. A flux of 50 sccm of argon (Ar) was established in order to create an inert atmosphere and temperature. Subsequently, the reactor temperature was increased from room temperature to 100°C and maintained for 15 min to expel water vapor. The exhaust valve was closed, allowing flowing Ar to pressurize the reactor to 2.75 MPa. The temperature of the reactor was gradually increased to 340°C. The pressure inside the reactor also increased progressively, and the hydrazine was ignited in the range of 190–280°C showing a peak in pressure. The final pressure value inside the reactor was 6 MPa. Subsequently, the exhaust valve was opened to release the pressure and give vent to residual vapors. A weaker argon flux (10 sccm) was maintained for approximately 3 h to allow a decrease in temperature. The chemical precursors were adjusted to different Eu concentration of , 5.0, 10.0, or 15.0 at %. As a result of this stage, as-synthesized red phosphor powder samples were obtained.

2.2. RIRO Process

Reducing atmospheres of H₂, N₂/H₂, NH₃, or their combinations have been used elsewhere to convert the Eu³⁺ into Eu²⁺ [8, 17]. In these studies, the phosphor powder was rapidly introduced and pulled out in a preheated tubular furnace with flowing NH₃ at 1100°C. This process is also called the rapid IN and rapid OUT process (RIRO) and is well-described elsewhere [8].

In stage 2 of the present study, the powders obtained by PACS (in stage 1) were placed in a quartz crucible inside a tubular furnace and heated at a temperature of 1100°C under constant flow (flow rate = 40 sccm) of ultra-high-purity ammonia (99.99% NH₃ analytical grade). The RIRO technique was applied during 45 minutes at 1100°C. As a result of stage 2, we obtained luminescent powders of EuAlO₃ : Eu²⁺ with brilliant green emission.

2.3. Characterization

The X-ray diffractograms of the powders were obtained with a Philips X’pert diffractometer with CuKα radiation ( nm). Measurements in a –80° range were taken with a step size of 0.02° and a 1 sec dwell per point. Scanning electron microscopy (SEM) images were captured with a JEOL electron microscope Model JSM-5300. Transmission electron microscopy (TEM) images were obtained with a JEOL-2010 operated at 200 kV accelerating voltage. Photoluminescence (PL) spectra were collected with a fluorescence spectrophotometer (Hitachi FL-4500). All measurements were performed at room temperature.

3. Results and Discussion

3.1. XRD Analysis

Figure 1(a) shows the comparison of the XRD patterns of EuAlO₃ powder samples produced by PACS (stage 1) for the different Eu concentrations. The XRD patterns of the samples with concentrations of 1.5 and 5.0 at % Eu were dominated by the mixture of the α-Al₂O₃ and γ-Al₂O₃ as reported elsewhere [11]. The residual phase of AlOOH also appeared and is labeled with the “*” symbol. This phase is attributed to the low temperature range (100–340°C) in which AlOOH formation occurs [19]. At the higher Eu concentrations of 10.0 and 15.0 at % the product contained mainly the orthorhombic phase (O), which corresponds to EuAlO₃. However, the 10.0 at % Eu-doped sample showed reflection peaks which correspond to the Eu₄Al₂O₉ monoclinic phase and are labeled with (m) in the XRD pattern. The principal planes of the orthorhombic phase (EuAlO₃) are indexed in the XRD pattern of the powder sample doped with 15.0 at %.

(a)

(b)

Figure 1: (a) XRD patterns of the as-synthesized red-emitting phosphor powders produced by the PACS method. Principal peaks are labeled and correspond to the crystalline phases: (α) α-Al₂O₃ (JCPDS #50-1496), (γ) γ-Al₂O₃ (JCPDS #10-0425), (*) AlOOH (JCPDS #21-1307). Peaks of the 10.0 at % Eu-doped sample are labeled with “m” (JCPDS #72-0683). Below, (b) XRD patterns of the green-emitting phosphor powders produced after RIRO. Principal peaks are labeled and correspond to the crystalline phases: (α) α-Al₂O₃ (JCPDS #50-1496) and (δ) δ-Al₂O₃ (JCPDS #46-1131). The principal planes of the orthorombic EuAlO₃ (JCPDS #09-0084) phase are indexed in both cases.

Regarding the phosphor powder, samples produced by RIRO (stage 2), XRD analysis of the sample containing 1.5 at % was dominated by the α-Al₂O₃ and δ-Al₂O₃ phases (see Figure 1(b)). At 5.0 at %, the Eu-doped sample presented the appearance of the orthorhombic phase. For the 10.0 at % Eu-doped sample, the formation of the orthorhombic EuAlO₃ with small traces of α-Al₂O₃ was mainly detected. At the highest Eu concentration (15.0 at %), it was clearly noticed that the α-Al₂O₃ and δ-Al₂O₃ phases were absent since only reflections corresponding to EuAlO₃ appeared in the XRD pattern. Table 1 summarizes the powder crystalline phases produced by PACS and RIRO and shows the temperature and pressure values of the PACS reaction peaks. These results are in agreement with a previous report [8], which states that the orthorhombic phase depends on Eu-doping concentration.

Table 1: Summary of crystalline phases obtained in both stages. Stage 1: PACS method. Stage 2: RIRO technique at 1100°C. The third column shows the pressure and reaction temperature parameters observed in PACS.

Eu-doping concentration (at)
XRD phases (PACS)
XRD phases (RIRO)
T(°C), P(MPa) in PACS

1.5%

215, 3.4

5%

190, 3.1

10%

280, 5.0

15%

210, 3.0

3.2. SEM Analysis

Figure 2 shows a representative SEM image corresponding to the 10.0 at % Eu-doped phosphor produced by PACS. The morphology of this sample reveals a large number of irregular pores of sizes in the range of 1.0–3.0 μm. This high porosity increases the amount of grain surface area which probably causes the enhancement of luminescence intensity of the 10.0 at % Eu-doped sample. Other studies have correlated porosity with the enhancement of optical properties [20]. However, the relation between porosity and luminescence intensity is still under study.

Figure 2: SEM image of the red-emitting porous powder ( at % Eu) produced by PACS performed at 200 kV.

3.3. TEM Characterization

Figure 3 shows a TEM micrograph of faceted nanocrystals of irregular polygonal shape of the 10.0 at % Eu-doped green-emitting powders obtained by RIRO. Mean size of the nanocrystals is ~4 nm. The nanocrystalline nature of the 10.0 at % Eu-doped sample increases the amount of surface area; both features of this sample are correlated with luminescence enhancement.

Figure 3: TEM image of the faceted nanocrystals observed in the green-emitting phosphor ( at % Eu ) powder produced by RIRO.

3.4. Photoluminescence Analysis

The (–15.0 at %) powders produced by PACS showed red emissions under long-UV excitation. Figure 4(a) illustrates the PL excitation spectra obtained with fixed emission at 616 nm. The weak broad excitation band centered at 275 nm is attributed to the well-known Eu-O charge-transfer transition [21]. The spectra show narrow excitation lines at longer wavelengths corresponding to the characteristic f→f transitions of Eu³⁺. These lines are spectroscopically assigned as follows: ⁷F₀→⁵H₆ (318 nm), ⁷F₀→⁵D₄ (362 nm), ⁷F₀→⁵G₄ (381 nm), the strongest excitation line ⁷F₀→⁵L₆ (395 nm), and, lastly, ⁷F₁→⁵D₃ (416 nm) [15, 21].

(a)

(b)

(c)

(d)

Figure 4: PL spectra of luminescent powders, –15.0 at % Eu, produced by PACS: (a) excitation spectra for all Eu-doping concentrations at fixed  nm (red) and (b) emission spectra for all powders excited with  nm. The inset shows the highest asymmetry ratio value at 10.0 at % Eu-doping concentration. All measurements performed at room temperature. After RIRO: (c) excitation spectra for all Eu-doping concentrations at fixed  nm (green) and (d) emission spectra for all powders excited with  nm.

Figure 4(b) shows the PL emission spectra of the powders produced by PACS with different Eu concentrations. The strongest contributions to the red emission come from the Eu³⁺ ions, mainly from the hypersensitive ⁵D₀→⁷F₂ (616 nm) transition forced by an electric dipole mechanism; of less intensity is a single peak located at 592 nm due to the ⁵D₀→⁷F₁ magnetic dipole. PL emission spectra of red-emitting powders were obtained under  nm excitation wavelength. If the forced electric dipole transition (⁵D₀→⁷F₂) is more intense than the magnetic dipole (⁵D₀→⁷F₁), the Eu³⁺ is localized in a noninversion symmetry site within the crystal lattice [15]. Therefore, the (⁵D₀→⁷F₂)/( 5D₀→⁷F₁) intensity ratio, also called the asymmetry ratio, provides a measure of the degree of distortion of the inversion symmetry of the local Eu³⁺ ion in the lattice [22]. A plot of the ratio values of (⁵D₀→⁷F₂)/( 5D₀→⁷F₁) calculated from the emission spectra is shown in the inset of Figure 4(b). It is important to note that the highest asymmetry ratio was obtained for the 10.0 at % Eu concentration value of the asymmetry ratio. This high value in the asymmetry ratio is indicative of low-symmetry crystal-fields around the Eu³⁺ sites [23]. Consequently, the 10.0 at % Eu-doped sample yields the highest luminescent intensity produced by the electric dipole ⁵D₀→⁷F₂ transition. Additionally, its high-intensity red emission was about 13 times higher in comparison with the 1.5 and 5.0 at % doped samples. Furthermore, it was 7 times more intense than the 15.0 at % Eu-doped sample. The 10.0 at % Eu-doped sample contained a mixture of a larger quantity of the orthorhombic EuAlO₃ and less amount of the monoclinic Eu₄Al₂O₉ crystalline phases. Red emission of the powder samples is mainly due to the ⁵D₀→⁷F₂ transition. The hypersensitive ⁵D₀→⁷F₂ transition occurs only if the crystal-field of the lattice is distorted and, as a consequence, the Eu³⁺ site of the ion is nonsymmetric [15]. In the case of EuAlO₃, the Eu³⁺ ion can emit because the nature of the structure is distorted; in fact, it is called distorted perovskite [8].

After RIRO, the phosphor powders showed intense green emission under long-UV excitation. Figure 4(c) compares the excitation spectra of all powder samples, monitoring the maximum emission wavelength ( nm). The maximum excitation bands for the 10.0 at % and the 5.0 at % Eu-doped samples were centered at  nm, while for other concentrations the excitation peaks were centered at  nm (1.5% at) and  nm (15.0 at %). The energy transfer from the host lattice to the Eu²⁺ sites was probably enhanced by the porous structure of the material. Figure 4(d) shows comparative emission spectra of the powder samples after RIRO. A strong broadband emission centered at  nm was observed for the 10.0 at % Eu-doped sample, while for the rest of the samples (1.5, 5.0, and 15.0 at %) showed reduced emission after the treatment in heated ammonia (RIRO process). The excitation wavelength light was fixed at  nm for all the powder samples. The luminescent intensity of the 10.0 at % Eu-doped sample was approximately 25-times higher compared with the rest of the samples. No characteristic Eu³⁺ transitions were detected in any of these samples and the broadband emission centered at 500 nm was associated to the 4f⁶5d¹–4f⁷ electronic transitions of Eu²⁺ ions. This is an indication that the Eu³⁺ to Eu²⁺ reduction process was successful. The broadband character of the Eu²⁺ emission is due to the nature of the electronic transitions: differing from 4f-4f forced electric dipole transition in Eu³⁺, 4f–5d transitions of Eu²⁺ are allowed electric dipole and the excited states of the 5d band are not strongly shielded from the crystal-fields by the 5s² and 5p⁶ bands. As a consequence, the 5d wave function is largely extended [15].

Red phosphor powders dominated by the EuAlO₃ distorted phase were found to possess a highly porous structure. The porosity produced by PACS in the 10.0 at % Eu-doped sample and preserved after RIRO treatment was induced by the pressure effect and temperature inside the reactor during synthesis. Future studies need to test if there is a direct relation between the high luminescence intensity and the porosity of this material [20]. The gelatinous blend of nitrates and hydrazine made ignition at a temperature of 280°C, and pressure increased from 4 MPa to 5 MPa in the case of the 10.0 at % Eu-doped sample; this drastic increase is probably what caused pore formation.

In the case of the 10.0 at % Eu-doped sample, the ignition temperature (280°C) of the gelatinous blend contained in the beaker inside the reactor was close to the natural ignition temperature (270°C) of hydrazine. In the PACS stage, the ignition temperature of the 10.0 at % Eu-doped sample was the highest in comparison with the other Eu-doped samples (see Table 1). Thus, the pressure delivered for this particular case (10.0 at % Eu) was also the highest. The combination of two factors, pressure and temperature, causes a drastic pressure increase, which presumably generated foam-like structures with abundant pores in the 10.0 at % Eu-doped oxide network sample.

Furthermore, within the 10.0 at % Eu-doped grains are formed cavities or voids. The cavities are surrounded by thin neighboring walls which increase the amount of active surface exposed to the UV light (395 nm). These thin walls readily absorb and transmit light, making available the excitation energy. This feature facilitates energy transfer to the luminescent centers, in this case, to the Eu³⁺ ion which receives the energy directly from the UV light.

EuAlO₃:Eu²⁺ green phosphor was obtained by the RIRO processing technique. When the 10.0 at % Eu-doped sample was reduced, the orthorhombic phase was preserved. The porosity of the material promoted NH₃ diffusion during the RIRO process; subsequently, the decomposition of the NH₃ facilitated the reduction of the Eu³⁺ to Eu²⁺ ions in the powder. The complete reduction at the surface of the grains increased the luminescent intensity of the green-emitting powder. As mentioned, after the RIRO process, the 10.0 at % Eu-doped sample showed a broadband emission located at 500 nm (green).

The 10.0 at % Eu-doped sample synthesized by PACS produced a red-emitting material, and after RIRO, a highly efficient green-emitting phosphor is obtained. The green phosphor showed a near UV-blue broadband PL excitation (300–450 nm). This near UV-blue excitable phosphor would be a desirable component for the construction of the WLEDs consisting of InGaN chip coated with triphosphor [5, 14].

The match of the excitation band of this green phosphor with the emission lines produced by the InGaN diode is very favorable for applications in solid-state white lamps. In this context, this phosphor is a promising candidate to complement the green spectral component of WLEDs composed of a UV-InGaN chip coated with phosphors. Quantum efficiency studies are under progress and will be reported in a future paper in order to further contribute to the technological viability of these phosphors in solid-state white-emitting devices.

4. Conclusions

Luminescent powders of europium aluminates (–15.0 at % of Eu concentrations) with green emission were successfully produced by pressure-assisted combustion synthesis (PACS). The 10.0 at % Eu concentration showed the highest luminescence intensity. For this sample, the red emission ( nm) was also detected under UV excitation of  nm. The diffusion of NH₃ was favored by the porosity in the 10.0 at % Eu-doped sample. Moreover, after treatment in a reducing atmosphere, a green luminescent powder with high intensity emission was obtained by exciting with UV radiation of  nm. Due to its high luminescence, it may be concluded that the 10.0 at % Eu-doped sample exhibits promising optical properties for applications as phosphors in devices coupled to solid-state UV diodes for the development of white-emitting lamps.

Acknowledgments

The authors are grateful for the technical assistance provided by E. Aparicio, M. Sainz, F. Ruíz, I. Gradilla, J. A. Díaz, V. García, P. Casillas, E. Flores, D. Domínguez, J. Peralta, J. Palomares and M. Sc. Isabel Pérez M. for revising the English version of the manuscript. We acknowledge the financial support from CONACYT (Grant no. 100555) and DGAPA UNAM (Grant IN-114010).

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