Extension of Spectral Shift Controls from Equivalent Substitution to an Energy Migration Model Based on Eu2+/Tb3+-Activated Ba4-xSrxGd3-xLuxNa3(PO4)6F2 Phosphors

Boosting Red Luminescence of Mn4+ in Tantalum Heptafluoride Based on an Ab Initio-Facilitated Sensitizer and Hydrophobic Surface Modification

A narrow-band red-light component is critical to establish high color rendition and a wide color gamut of phosphor-converted white-light-emitting diodes (pc-WLEDs). In this sense, Mn⁴⁺-doped K₂SiF₆ fluoride is the most successful material that has been commercialized. As with K₂SiF₆:Mn⁴⁺ phosphors, Mn⁴⁺-doped tantalum heptafluoride (K₂TaF₇:Mn⁴⁺) fulfills a similar luminescence behavior and has been brought in a promising narrow-band red phosphor. But the limited brightness and low moisture-resistant performances have inevitably blocked its practical application. Herein, we employed the density functional theory (DFT)-based ab initio estimation approach to quickly identify the proper sensitizer by systematically investigating the electronic-band coupling between the several possible sensitizers (Rb, Hf, Zr, Sn, Nb, and Mo) and the luminescent center (Mn). Combined with experimental results, Mo was demonstrated to be the optimal sensitizer, which resulted in a 60% enhancement of the emission. On the side, the moisture sensitivity has been effectively improved via grafting the hydrophobic octadecyltrimethoxysilane (ODTMS) layer on the phosphor surface. Through employing the K₂TaF₇:Mn⁴⁺,Mo⁶⁺@ODTMS composite as a red component, warm WLEDs with good performance were achieved with a correlated color temperature (CCT) of 4352 K, a luminous efficacy (LE) of 90.1 lm/W, and a color rendering index (Ra) of 83.4. In addition, a wide color gamut reaching up to 102.8% of the NTSC 1953 value could be realized. Aging tests at 85 °C and 85% humidity for 120 h on this device manifested that the ODTMS-modified phosphor had much better moisture stability than that of the unmodified one. These studies provided viable tools for optimizing Mn⁴⁺ luminescence in fluoride hosts.

Facile Microwave Synthesis of a Narrow-Band Green-Emitting Phosphor Cs3MnBr5 and the Effect of Anion Substitution on Its Luminescence Properties

A bright blue light excitable and narrow-band green-emitting phosphor Cs₃MnBr₅ has been synthesized by a facile microwave radiation method within 2 min. The influence of the matrix on its steady-state and transient-state luminescence properties is investigated by partial substitution of Br^- ions by Cl^- ions. The incorporation of Cl^- ions in Cs₃Mn(Br_1-xCl_x)₅ resulted in almost no change in the emission maxima of Mn²⁺, which is attributed to the synergistic effect of reduced covalency and increased crystal field strength caused by the replacement of Br^- ions by Cl^- ions. Meanwhile, the emission of Mn²⁺ decreases with the increasing Cl^- content, which is caused by different thermal quenching of Mn²⁺ emission in the mixed Cl^-/Br^- coordination. Moreover, the incorporation of Cl^- in Cs₃Mn(Br_1-xCl_x)₅ was found to have different effects on the lifetime of Mn²⁺ at different temperatures, that is, at room temperature, the lifetime of Mn²⁺ decreases with the increasing Cl^- content, while at liquid nitrogen temperature, the lifetime of Mn²⁺ increases upon increasing the Cl^- content. The former is due to the different thermal quenching for different coordinations of Mn²⁺ with Cl^- and Br^-, while the latter is due to the weaker spin-orbit coupling of the Mn²⁺ ion caused by the interaction with the lighter Cl^- ions, which makes the spin selection rule stricter and leads to a longer lifetime of Mn²⁺ consequently.

Efficient Ce3+ → Tb3+ energy transfer pairs with thermal stability and internal quantum efficiency close to unity

The incorporation of Ce3+–Tb3+ pairs has been reported in the Ca3Lu2Si6O18 (CLSO) host for identifying a novel green-emitting material with extremely high internal quantum efficiency (close to unity) and excellent thermal stability.

Realization of an Optical Thermometer via Structural Confinement and Energy Transfer

The influence of temperature on a variety of physiological or chemical processes has generated considerable interest, and recently noninvasive lanthanide-incorporated optical thermometers have been considered as promising candidates for monitoring its changes at different scales. Herein, a novel Bi³⁺-activated Sr_3-xGd_xGaO_4+xF_1-x phosphor with tunable color has been constructed by a cooperative cation-anion substitution strategy with to the replacement of [Sr²⁺-F^-] by [Gd³⁺-O^2-]. When x = 0, the sample Sr₃GaO₄F/Bi³⁺ possesses a peak wavelength at 438 nm, and this value will shift to 470 nm if x is equal to 1 (Sr₂GdGaO₅/Bi³⁺). In addition, photoluminescence tuning from blue to red has been realized successfully by an efficient Bi³⁺ → Eu³⁺ energy migration model in Sr_2.6Gd_0.4GaO_4.4F_0.6 samples. The specific Bi³⁺ → Eu³⁺ energy transfer has been explained by dipole-dipole interactions derived from a model of the Dexter pathway. Intriguingly, the two dopants (a blue signal from Bi³⁺ and a red signal from Eu³⁺) possess different thermal responses to increasing temperature. Accordingly, the intensity ratio values are sensitive to the temperature changes. The energy level cross relaxation causes the quenching effect of Bi³⁺, and the multi-phonon de-excitation mode leads to the thermal quenching of Eu³⁺. At room temperature (298 K), the determined maximum relative sensitivity (S_r) is 1.27% K^-1. Moreover, the absolute sensitivity (S_a) is 0.067 K^-1 since the temperature is elevated to 523 K. The collected results are superior to most of the reported optical thermometry materials.