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
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 [
Europium aluminates EuAlO3 and EuAlO3 : Eu2+ [
EuAlO3 crystallizes in the distorted orthorhombic perovskite structure with general chemical formula ABO3 [
In a previous report of our group, combustion synthesis plus postannealing was applied to produce EuAlO3 and EuAlO3 : Eu2+ [
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 [
Nanocrystalline green-emitting powders were prepared in two stages. In stage 1, we applied the pressure-assisted combustion synthesis (PACS) method [
Reducing atmospheres of H2, N2/H2, NH3, or their combinations have been used elsewhere to convert the Eu3+ into Eu2+ [
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.
The X-ray diffractograms of the powders were obtained with a Philips X’pert diffractometer with CuK
Figure
(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: (
Regarding the phosphor powder, samples produced by RIRO (stage 2), XRD analysis of the sample containing 1.5 at % was dominated by the
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 |
Figure
SEM image of the red-emitting porous powder (
Figure
TEM image of the faceted nanocrystals observed in the green-emitting phosphor (
The (
PL spectra of luminescent powders,
Figure
After RIRO, the phosphor powders showed intense green emission under long-UV excitation. Figure
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 [
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
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 [
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.
Luminescent powders of europium aluminates (
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).