Site preferences of dopant Eu on the locations of K, Ba, and Ca in the mixed cation phosphate KBaCa(PO) (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs.

Near-infrared luminescent materials exhibit unique photophysical properties that make them crucial components in photonic, optoelectronic and biological applications. As broadband near infrared phosphors activated by transition metal elements are already widely reported, there is a challenge for next-generation materials discovery by introducing rare earth activators with 4f-5d transition. Here, we report an unprecedented phosphor K3LuSi2O7:Eu2+ that gives an emission band centered at 740 nm with a full-width at half maximum of 160 nm upon 460 nm blue light excitation. Combined structural and spectral characterizations reveal a selective site occupation of divalent europium in LuO6 and K2O6 polyhedrons with small coordination numbers, leading to the unexpected near infrared emission. The fabricated phosphor-converted light-emitting diodes have great potential as a non-visible light source. Our work provides the design principle of near infrared emission in divalent europium-doped inorganic solid-state materials and could inspire future studies to further explore near-infrared light-emitting diodes.

Near‐infrared (NIR) luminescence materials with broadband emissions are necessary for the development of light‐emitting diodes (LEDs) based light sources. However, most known NIR‐emitting materials are limited by their low external quantum efficiency. This work demonstrates how the photoluminescence quantum efficiency of europium‐activated calcium oxide (CaO:Eu) NIR phosphor can be significantly improved and stabilized at operating temperatures of LEDs. A carbon paper wrapping technology is innovatively developed and used during the solid‐state sintering to promote the reduction of Eu3+ into Eu2+. In parallel, the oxygen vacancies in the CaO lattice are repaired utilizing GeO2 decomposition. Through this process, a record‐high external quantum efficiency of 54.7% at 740 nm is obtained with a thermal stability greatly improved from 57% to 90% at 125 °C. The as‐fabricated NIR‐LEDs reach record photoelectric efficiency (100 mA@23.4%) and output power (100 mA @ 319.5 mW). This discovery of high‐performance phosphors will open new research avenues for broadband NIR LED light sources in a variety of photonics applications.

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Near-infrared (NIR) light-emitting diodes (LEDs) light sources are desirable in photonic, optoelectronic, and biological applications. However, developing broadband red and NIR-emitting phosphors with good thermal stability is always a challenge. Herein, the synthesis of Eu 2+ -activated SrY 2 O 4 red phosphor with high photoluminescence quantum efficiency and broad emission band ranging from 540 to 770 nm and peaking at 620 nm under 450 nm excitation is designed. Sr/Ba substitution in SrY 2 O 4 :Eu 2+ has been further utilized to achieve tunable emission by modifying the local environment, which facilitates the giant redshifted emission from 620 to 773 nm while maintaining the outstanding thermal stability of SrY 2 O 4 :Eu 2+ . The NIR emission is attributed to the enhanced Stokes shift and crystal field strength originated from the local structural distortions of [Y1/Eu1O 6 ] and [Y2/Eu2O 6 ]. The investigation in charge distribution around Y/Eu provides additional insight into increasing covalency to tune the emission toward the NIR region. As-fabricated NIR phosphor-converted LEDs demonstration shows its potential in night-vision technologies. This study reveals the NIR luminescence mechanism of Eu 2+ in oxide-based hosts and provides a design principle for exploiting Eu 2+ -doped NIR phosphors with good thermal stability.