Anomalous Trapped Exciton and d–f Emission in Sr4Al14O25:Eu2+

Deep-red to near-infrared luminescence from Eu2+-trapped exciton states in YSiO2N

The valence state of Eu ions doped in inorganic compounds is easily influenced by the synthesizing conditions. In this study, X-ray absorption spectroscopy revealed that almost half of Eu ions incorporated in the YSiO₂N host were reduced into the divalent state through the sintering process at 1600 °C under a N₂ gas atmosphere without any annealing processes. The prepared Eu^2+/3+-doped YSiO₂N sample showed anomalous deep-red to near-infrared luminescence below 300 K under violet light illumination, whose luminescent properties are discussed through detailed spectroscopic analyses. In the photoluminescence spectra at 4 K, the broad luminescence band ranging from 550 to 1100 nm with a large Stokes shift of 5677 cm^-1 was observed, assigned to the recombination emission related to the Eu²⁺-trapped exciton state. The temperature dependence of luminescence lifetime suggests that the thermal quenching of Eu²⁺-trapped exciton luminescence takes place through complicated processes in addition to thermal ionization. The energy diagrams based on the spectroscopic results indicate that Eu²⁺-trapped exciton luminescence in the YSiO₂N:Eu^2+/3+ sample was observed because all the Eu²⁺: 5d excited levels are degenerated with the host conduction band, and the relatively stable Eu²⁺-trapped exciton state in the Y³⁺ sites is formed just below the conduction band bottom. A comprehensive discussion on the deep-red to near-infrared luminescence in the YSiO₂N host could give new insights into the mechanism of Eu²⁺-trapped exciton luminescence in Y³⁺ sites, which has potential in near-infrared emitting devices.

First-principles calculations to identify key native point defects in Sr4Al14O25

This article reports for the first time an in-depth ab initio computational study on intrinsic point defects in Sr₄Al₁₄O₂₅ that serves as host lattice for numerous phosphors. Defect Formation Enthalpies (DFEs) and defect concentrations were computed considering the supercell approach for different oxygen atmospheres. The charge transition levels have been determined for several point defects in their thermodynamically stable state and their impact on the electronic structure of the ideal unfaulted material is discussed. Our simulations demonstrated that the formation of most of native point defects is energy intensive under oxygen-rich, -intermediate or -poor synthesis conditions, except for the oxygen vacancies under O-poor atmosphere.

Tailoring of White Luminescence in a NaLi3 SiO4 :Eu2+ Phosphor Containing Broad-Band Defect-Induced Charge-Transfer Emission

Single-component materials with white-light emission are ideal for lighting applications. However, it is very challenging to achieve white luminescence in single-dopant activated solid phosphors. Herein, white NaLi₃ Si_{1- x} O₄ :Eu²⁺ materials are designed via defect engineering and synthesized by reducing the Si content (0.15 ≤ x ≤ 0.25). Stochiometric NaLi₃ SiO₄ :Eu²⁺ exhibits a narrow-band blue emission at 469 nm, ascribed to the 5d → 4f transition of Eu²⁺ at highly symmetric cuboid Na sites, while samples with Si content reduced by 15-25% display white emission with two peaks at 472 nm and 585 nm. The newly appeared broadband yellow peak arises from charge-transfer transitions involving Eu²⁺ and nearby defects, as verified by an unusual bandwidth, a large Stokes shift, and a long decay time. A single-component white light-emitting diode device is fabricated by employing a white phosphor to demonstrate a color-rendering index of 82.9. This result provides a new design strategy for single-component white-light materials with broad-band defect-induced charge-transfer emission.

Identification of Dy^{3+}/Dy^{2+} as Electron Trap in Persistent Phosphors

Laser excitation and x-ray spectroscopy are combined to settle a long-standing question in persistent luminescence. A reversible electron transfer is demonstrated, controlled by light and showing the same kinetics as the persistent luminescence. Exposure to violet light induces charging by oxidation of the excited Eu^{2+} while Dy^{3+} is simultaneously reduced. Oppositely, detrapping of Dy^{2+} occurs at ambient temperature or by infrared illumination, yielding afterglow or optically stimulated luminescence, respectively.

Insights into the complexity of the excited states of Eu-doped luminescent materials

Multiconfigurational ab initio embedded-cluster methods give a detailed view of the excited states of Eu-doped luminescent materials, improving the understanding of their structure and how it is affected by changing the host's chemical composition.

An Interfacial Europium Complex on SiO2 Nanoparticles: Reduction-Induced Blue Emission System

In this study, Eu-coated SiO₂ nanoparticles have been prepared, consisting of an interfacial complex of Eu and 1,10-phenanthroline (phen) at the solid surfaces of the SiO₂/Eu nanostructures. The as-prepared SiO₂/Eu/phen nanoparticles exhibits sharp red emission via energy transfer from the phen to the Eu^III. After sintering at 200 °C in air, the emission is tuned from red to blue. The blue emission is originated from Eu^II. This reduction-induced emissive phenomenon resulted from the electron-donating environment created by the surrounding phen and SiO₂, which is the first reported fabrication of a stable Eu^II-based emissive material using mild conditions (reaction in air and at low temperature) and an organic-inorganic hybrid nanostructure. The existence of two different stable oxidation states with characteristic emissions, blue emissive Eu^II and red emissive Eu^III, suggests significant potential applications as novel luminescent materials with inorganic-organic hybrid structures.

Eu2+ luminescence in strontium aluminates

The luminescence properties of Eu²⁺ doped strontium aluminates are reported and reviewed for a variety of aluminates, viz. SrAl₁₂O₁₉, SrAl₄O₇, Sr₄Al₁₄O₂₅, SrAl₂O₄ and Sr₃Al₂O₆.