Fine-Tunable Self-Activated Luminescence in Apatite-Type (Ba,Sr)5(PO4)3Br and the Defect Process

Structures, photoluminescence, and principles of self-activated phosphors

Self-activated phosphors without any luminescent dopants, usually display excellent optical properties, such as high oscillator strength, large Stokes shift, and strong luminescence efficiency, and thus have been widely investigated by researchers for several decades. However, their recent advancements in scintillators, white-light illumination, displays and optical sensors compel us to urgently understand the basic principles and significant technological relevance of this worthy family of materials. Herein, we review the structures, photoluminescence principles, and applications of state-of-the-art self-activated phosphors, such as borate, gallate, niobate, phosphate, titanate, vanadate, tungstate, nitrides, oxyfluoride, perovskite, metal halides, and carbon dots. The photoluminescence principles of self-activated phosphors are mainly summarized as transitions between energy levels of rare-earth and transition metal ions, charge transfer transitions of some oxide compounds, and luminescence in all-inorganic semiconductors. The different self-activated phosphors exhibit various structures and site-dependent spectra. Additionally, we discuss the application prospect and main challenges of self-activated phosphors.

High stability ultra-narrow band self-activated KGaSiO4 long-persistent phosphors for optical anti-counterfeiting

Optical anti-counterfeiting has been developed as a promising optical-sensing technique. A self-activated KGaSiO₄ phosphor was successfully prepared using the traditional solid-state method. The photoluminescence spectra of the as-synthesized phosphors indicate that the ultra-narrow band emission with green light peak at 503 nm is obtained when phosphors are excited by 254 nm UV light. Additionally, the measured afterglow curve shows that the emission of this phosphor can last more than 1200 s after UV excitation stops, which indicates that KGaSiO₄ is a potential candidate for anti-counterfeiting materials. The luminescent and decay mechanism are discussed by theoretical calculation and thermo-luminescent spectra in detail. The theoretical model can provide support for explaining the mechanism of narrow band or persistent phosphor.

Thermally stable and highly efficient red-emitting Eu3+-doped Cs3GdGe3O9 phosphors for WLEDs: non-concentration quenching and negative thermal expansion

Red phosphor materials play a key role in improving the lighting and backlit display quality of phosphor-converted white light-emitting diodes (pc-WLEDs). However, the development of a red phosphor with simultaneous high efficiency, excellent thermal stability and high colour purity is still a challenge. In this work, unique non-concentration quenching in solid-solution Cs₃Gd_1 - xGe₃O₉:xEu³⁺ (CGGO:xEu³⁺) (x = 0.1-1.0) phosphors is successfully developed to achieve a highly efficient red-emitting Cs₃EuGe₃O₉ (CEGO) phosphor. Under the optimal 464 nm blue light excitation, CEGO shows a strong red emission at 611 nm with a high colour purity of 95.07% and a high internal quantum efficiency of 94%. Impressively, this red-emitting CEGO phosphor exhibits a better thermal stability at higher temperatures (175-250 °C, >90%) than typical red K₂SiF₆:Mn⁴⁺ and Y₂O₃:Eu³⁺ phosphors, and has a remarkable volumetric negative thermal expansion (coefficient of thermal expansion, α = -5.06 × 10^-5/°C, 25-250 °C). By employing this red CEGO phosphor, a fabricated pc-WLED emits warm white light with colour coordinates (0.364, 0.383), a high colour rendering index (CRI = 89.7), and a low colour coordinate temperature (CCT = 4508 K). These results indicate that this highly efficient red-emitting phosphor has great potential as a red component for pc-WLEDs, opening a new perspective for developing new phosphor materials.

Highly Efficient Cyan-Green Emission in Self-Activated Rb3RV2O8 (R = Y, Lu) Vanadate Phosphors for Full-Spectrum White Light-Emitting Diodes (LEDs)

Phosphor-converted white-light-emitting diodes (pc-WLEDs) rely on combining a near-ultraviolet (n-UV) or blue chip with trichromatic and yellow-emitting phosphors. It is challenging to discover cyan-green-emitting (480-520 nm) phosphors for compensating the spectral gap and producing full-spectrum white light. In this work, we successfully discovered two unprecedented bright cyan-green emitting Rb₃RV₂O₈ (R = Y, Lu) phosphors that gives emission bands centered at 500 nm upon 362 nm n-UV light excitation. Interestingly, the both self-activated compounds exhibit high internal quantum efficiencies (IQEs) of 71% for Rb₃YV₂O₈ and 85% for Rb₃LuV₂O₈, respectively. Moreover, controllable emission color can be successfully tuned from cyan-green to orange-red across the warm white light region by design strategy of VO₄^3- → Eu³⁺ energy transfer. The thermal quenching of as-prepared phosphors could be effectively mitigated by this design strategy. Finally, the as-fabricated n-UV (λ_ex = 370 nm) pumped phosphor-converted (pc) W-LED devices utilizing Rb₃RV₂O₈ (R = Y, Lu) along with commercial phosphors demonstrate well-distributed warm white light with high color-rendering index (CRI) of 91.9 and 93.5, and a low correlated color temperature (CCT) of 5095 and 4946 K. It suggests that the both vanadate phosphors have potential applications in full-spectrum pc-WLEDs.

Highly Efficient Metal-Free Two-Dimensional Luminescent Melem Nanosheets for Bioimaging

Two-dimensional (2D) luminescent materials have received tremendous attention for their intrinsic properties and promising practical applications. Achieving 2D luminescent materials with high photoluminescence (PL) efficiency is still a great challenge. Here, ultrathin metal-free 2D luminescent nanosheets of 2,5,8-triamino-tri-s-triazine (melem) are synthesized through a facile liquid exfoliation process assisted by ultrasound. The as-obtained melem nanosheets distribute in the size range from a few nanometers to around 150 nm with a thickness of about 5 to 6 atomic layers. Melem nanosheets exhibit efficient blue emission with a PL efficiency as high as 77.09%, much higher than the heavily explored 2D luminescent g-C₃N₄ nanosheets. The high efficiency of melem nanosheets comes from the absence of atom vacancies and the low carrier mobility. Benefiting from the easy synthesis, good stability, low cell toxicity, and high efficiency, melem nanosheets are successfully applied as bioimaging materials on human breast cancer cells, requiring no extra treatments such as surface coating or functionalization. These metal-free 2D luminescent melem nanosheets hold great potential for various applications including bioimaging and other biorelated applications.

Multispectral tunability in single Eu2+-doped (Ba,Sr)5(PO4)3Br phosphor

The dual emission centers of Eu²⁺ and oxygen vacancy defects endow the Ba₂Sr₃(PO₄)₃Br:xEu²⁺ phosphors with multicolor tunability and irradiation dependent dynamic chromaticity.