Insight into the Upconversion Luminescence of Highly Efficient Lanthanide-Doped Bi₂O₃ Nanoparticles

A series of Bi₂O₃ nanoparticles doped with Yb³⁺ and Ln³⁺ (Ln³⁺ = Er³⁺, Ho³⁺, Tm³⁺) ions were prepared by means of a Pechini-type sol–gel synthesis in order to develop novel approaches for the realization of high-performing upconverting nanophosphors, with controlled chromaticity output and enhanced emission efficiency. The overall upconversion mechanism originating the observed luminescence spectra is strongly influenced by the narrow bandgap of the Bi₂O₃ matrix (about 2.6 eV when doped at 10–12 at %) since the occurrence of optical band-to-band transitions sets such an upper energy threshold to the activation of those upconversion features characterizing the spectrum of the different Yb³⁺–Ln³⁺ systems. Moreover, as emerging from diffuse reflectance analysis performed on a series of Yb³⁺, Er³⁺ codoped samples with Yb content in the 0–20 at % range, the Bi₂O₃ energy gap can be properly tuned by varying the overall dopant concentration. This evidence suggests a strategy to achieve (i) chromaticity output control and (ii) the realization of single-band emitters. Concerning the last point, important results were achieved for Yb³⁺–Er³⁺ and Yb³⁺–Tm³⁺ codoped samples that behave nearly monochromatic in NIR-to-red and NIR-to-NIR upconverters under 980 nm light exposure, respectively, with significant damping of those radiative components in the blue-green part of the visible spectrum. Furthermore, the emission mechanism for the investigated systems is characterized by a remarkable quantum efficiency value, a fundamental parameter in view of possible application in bioimaging or anticounterfeiting fields

Revealing trap depth distributions in persistent phosphors with a thermal barrier for charging

The performance of persistent phosphors under given charging and working conditions is determined by the properties of the traps that are responsible for these unique properties. Traps are characterized by the height of their associated barrier for thermal detrapping, and a continuous distribution of trap depths is often found in real materials. Accurately determining trap depth distributions is hence of importance for the understanding and development of persistent phosphors. However, extracting the trap depth distribution is often hindered by the presence of a thermal barrier for charging as well, which causes a temperature-dependent filling of traps. For this case, we propose a method for extracting the trap depth distribution from a set of thermoluminescence (TL) curves obtained at different charging temperatures. The TL curves are first transformed into electron population functions via the Tikhonov regularization, assuming first-order kinetics. Subsequently, the occupation of the traps as a function of their depth, quantified by the so-called filling function, is obtained. Finally, the underlying trap depth distribution is reconstructed from the filling functions. The proposed method provides a substantial improvement in precision and resolution for the trap depth distribution compared with existing methods. This is hence a step forward in understanding the (de)trapping behavior of persistent and storage phosphors

Strontium Aluminate Persistent Luminescent Single Crystals: Linear Scaling of Emission Intensity with Size Is Affected by Reabsorption

The green-emitting SrAl2O4:Eu,Dy phosphor is the most widely used and well-studied persistent luminescent phosphor available today. Recent efforts to boost its performance in terms of luminescence intensity and duration are challenged by complex loss mechanisms, including the optically stimulated release of previously trapped charges by excitation light. Here, we present minimally scattering SrAl2O4:Eu,Dy single crystals, which, as opposed to powder phosphors, allow to profit from a so-called volume effect, resulting in a significantly increased emission intensity. Additionally, they allow for the identification of the reabsorption of the afterglow emission by trapped charges as an important loss mechanism, leading to a nonlinear scaling of the emission intensity with the crystal size. If circumvented, the emission intensity could be further increased, in persistent luminescent powders, ceramics, and single crystals

First-Principles Study of Antisite Defect Configurations in ZnGa₂O₄:Cr Persistent Phosphors

Zinc gallate doped with chromium is a recently developed near-infrared emitting persistent phosphor, which is now extensively studied for in vivo bioimaging and security applications. The precise mechanism of this persistent luminescence relies on defects, in particular, on antisite defects and antisite pairs. A theoretical model combining the solid host, the dopant, and/or antisite defects is constructed to elucidate the mutual interactions in these complex materials. Energies of formation as well as dopant, and defect energies are calculated through density-functional theory simulations of large periodic supercells. The calculations support the chromium substitution on the slightly distorted octahedrally coordinated gallium site, and additional energy levels are introduced in the band gap of the host. Antisite pairs are found to be energetically favored over isolated antisites due to significant charge compensation as shown by calculated Hirshfeld-I charges. Significant structural distortions are found around all antisite defects. The local Cr surrounding is mainly distorted due to a Zn_Ga antisite. The stability analysis reveals that the distance between both antisites dominates the overall stability picture of the material containing the Cr dopant and an antisite pair. The findings are further rationalized using calculated densities of states and Hirshfeld-I charges

Red Mn⁴⁺-Doped Fluoride Phosphors: Why Purity Matters

Traditional light sources, e.g., incandescent and fluorescent lamps, are currently being replaced by white light-emitting diodes (wLEDs) because of their improved efficiency, prolonged lifetime, and environmental friendliness. Much effort has recently been spent to the development of Mn⁴⁺-doped fluoride phosphors that can enhance the color gamut in displays and improve the color rendering index, luminous efficacy of the radiation, and correlated color temperature of wLEDs used for lighting. Purity, stability, and degradation of fluoride phosphors are, however, rarely discussed. Nevertheless, the typical wet chemical synthesis routes (involving hydrogen fluoride (HF)) and the large variety of possible Mn valence states often lead to impurities that drastically influence the performance and stability of these phosphors. In this article, the origins and consequences of impurities formed during synthesis and aging of K₂SiF₆:Mn⁴⁺ are revealed. Both crystalline impurities such as KHF₂ and ionic impurities such as Mn³⁺ are found to affect the phosphor performance. While Mn³⁺ mainly influences the optical absorption behavior, KHF₂ can affect both the optical performance and chemical stability of the phosphor. Moisture leads to decomposition of KHF₂, forming HF and amorphous hydrated potassium fluoride. As a consequence of hydrate formation, significant amounts of water can be absorbed in impure phosphor powders containing KHF₂, facilitating the hydrolysis of [MnF₆]^2– complexes and affecting the optical absorption of the phosphors. Strategies are discussed to identify impurities and to achieve pure and stable phosphors with internal quantum efficiencies of more than 90%