Abstracts

Nowadays, the intense use of screens has stimulated the search for devices that require lower energy consumption. LEDs are considered the cutting-edge technology in solid-state lighting, meeting this requirement. Among the production methods for white LEDs (WLEDs), the one currently commercially used is the hybrid LED type, although it suffers from low color rendering index (CRI) and high correlated color temperature (CCT) due to the lack of red-emitting component. This study reports the photoluminescence properties of Eu³⁺-based MgAl₂O₄ co-doped or not with Li⁺ at 2%, red-emitting phosphor, synthesized by an adapted Pechini route at 1000 °C, at air atmosphere. MgAl₂O₄ doped Eu³⁺ at 4% was used to study the Eu³⁺ occupation sites, in comparation to another samples, i.e., MgAl₂O₄:Eu³⁺(2%) and MgAl₂O₄:Eu³⁺(2%)Li⁺(2%). The XRD data confirmed that all samples have the spinel-type MgAl₂O₄ structure and are therefore single-phase. XPS analysis for Eu³⁺-doped sample indicated the absence of surface contamination. Excitation spectra of samples monitoring the ⁵D₀→⁷F₂ transition at 612 nm displayed the O^2-→Eu³⁺ charge transfer band (CTB) as the most intense, while the narrow band assigned to ⁷F₀→⁵L₆ at 393 nm exhibited the highest relative intensity among the f-f transitions. Emission spectra under CTB excitation at low temperature (12 K) revealed the splitting of the ⁵D₀→⁷F₀ transition into two components, implying the insertion of Eu³⁺ in at least two non-equivalent low-symmetric sites lacking an inversion center for all samples. The two ⁵D₀→⁷F₀ components observed in the MgAl₂O₄:Eu³⁺(2 %) and MgAl₂O₄:Eu³⁺(2 %)Li⁺(2 %) emission spectra were located at 576 nm and 579 nm, being this last one more relatively intense. However, increasing Eu³⁺concentration, i.e., sample MgAl₂O₄:Eu³⁺(4%), an increase of the one at 576 nm was observed. When the excitation was fixed at 298 nm, the difference between these two components of the ⁵D₀→⁷F₀ transition became more evident and indicated the preference for occupation of this site with higher Eu³⁺ concentration. Finally, these samples were embedded in PMMA polymeric matrix, to produce luminescent films, aiming for their application in PC-LEDs with UV excitation. Although the absolute quantum yield of the films did not exhibit a significant increase with Li⁺ co-doping, staying around 16 % when the excitation was fixed in 260 nm, the 1931 Chromaticity Diagram (CIE) of all of them revealed color purity greater than 90% in the red spectral region. In this way, these results guarantee that the red-emitting phosphors were successfully synthesized, making it possible to use the Eu³⁺ probe properties to evaluate its occupancy sites. Furthermore, by incorporating the powders into a PMMA polymer matrix, it is evident that they constitute promising candidates for the development of a prototype of PC-WLED type architecture.