Mu-Huai Fang

Synthesis of Na2SiF6:Mn4+ red phosphors for white LED applications by co-precipitation

A one-step approach to synthesize Na2SiF6:Mn4+and K2SiF6:Mn4+ red phosphors by co-precipitation is reported in this paper. The phosphors were precipitated from a silicon fluoride solution with NaF and Na2MnO4 (Na2SiF6:Mn4+ preparation) or KF and K2MnO4 (K2SiF6:Mn4+ preparation) using H2O2 to reduce Mn7+ to Mn4+ at room temperature. Na2SiF6:Mn4+ was also prepared through a convenient two-step route with K2MnF6 as a raw material. The obtained Na2SiF6:Mn4+ phosphors have hexagonal structures with space group D32-P321 and no impurity phase when they were examined via X-ray diffraction. Photoluminescence, photoluminescence excitation, thermal luminescence, and luminescence decay time were considered to determine the optical properties of the fluoride complexes. The prepared phosphors exhibited bright red emission under 460 nm light excitation and low-thermal quenching (∼92% of the luminescent intensity at 423 K). Increasing the concentration of Mn4+ enhanced the luminescence intensity. A warm white light LED with high color rendering index (Ra = 86 and R9 = 61) was fabricated by employing Na2SiF6:Mn4+ as red phosphors and commercial Y3Al5O12:Ce3+ as yellow phosphors on a blue-InGaN chip.

Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White Light-Emitting Diodes

Red phosphors AMF6:Mn(4+) (A = Na, K, Cs, Ba, Rb; M = Si, Ti, Ge) have been widely studied due to the narrow red emission bands around 630 nm. The different emission of the zero-phonon line (ZPL) may affect the color rendering index of white light-emitting diodes (WLED). The primary reason behind the emergence and intensity of ZPL, taking KNaSiF6:Mn(4+) as an example, was investigated here. The effects of pressure on crystal structure and luminescence were determined experimentally and theoretically. The increase of band gap, red shift of emission spectrum and blue shift of excitation spectrum were observed with higher applied pressure. The angles of ∠FMnF and ∠FMF(M = Si, Ti, Ge) were found clearly distorted from 180° in MF6(2-) octahedron with strong ZPL intensity. The larger distorted SiF6(2-) octahedron, the stronger ZPL intensity. This research provides a new perspective to address the ZPL intensity problem of the hexafluorosilicate phosphors caused by crystal distortion and pressure-dependence of the luminescence. The efficacy of the device featuring from Y3Al5O12:Ce(3+) (YAG) and KNaSiF6:Mn(4+) phosphor was 118 lm/W with the color temperature of 3455 K. These results reveal that KNaSiF6:Mn(4+) presents good luminescent properties and could be a potential candidate material for application in back-lighting systems.

Photoluminescent Evolution Induced by Structural Transformation Through Thermal Treating in the Red Narrow-Band Phosphor K2GeF6:Mn4+

This study explored optimal preparation conditions for K2GeF6:Mn(4+) red phosphors by using chemical coprecipitation method. The prepared hexagonal P3̅m1 K2GeF6:Mn(4+) exhibited efficient red emission, high color purity, good Mn(4+) concentration stability, and low thermal quenching. Structural evolution from hexagonal P3̅m1 to P63mc and then P63mc to cubic Fm3m occurred after thermal treatment at approximately 400 and 500 °C, respectively. Hexagonal P63mc phase showed an obvious zero phonon line peak at 621 nm, whereas cubic Fm3m phase showed no red emission. Yellowish K2GeF6:Mn(4+) with both hexagonal P3̅m1 and P63mc symmetries are promising commercial red phosphors for white light-emitting diodes.

Preparation of a novel red Rb2SiF6:Mn4+ phosphor with high thermal stability through a simple one-step approach

A new Rb2SiF6:Mn4+ phosphor has been successfully synthesized through a one-step co-precipitation method. The addition of H2O2 into the solution with appropriate reaction temperature results in the reduction of Mn7+ to Mn4+. Mn4+ is excited by UV and blue light, providing sharp red emission at approximately 630 nm. Rb2SiF6:Mn4+ has a Fmm (225) cubic phase crystal structure as determined using X-ray diffraction and Total Pattern Analysis Solutions. No apparent impurity phase is found in the X-ray diffraction spectrum. All luminescence properties, namely, emission, excitation, and thermal luminescence spectrum, have been investigated in depth.

Enhanced Photoluminescence Emission and Thermal Stability from Introduced Cation Disorder in Phosphors

Optimizing properties of phosphors for use in white-light-emitting diodes (WLEDs) is an important materials challenge. Most phosphors have a low level of lattice disorder due to mismatch between the host and activator cations. Here we show that deliberate introduction of high levels of cation disorder leads to significant improvements in quantum efficiency, stability to thermal quenching, and emission lifetime in Sr1.98-x(Ca0.55Ba0.45)xSi5N8:Eu0.02 (x = 0-1.5) phosphors. Replacing Sr by a (Ca0.55Ba0.45) mixture with the same average radius increases cation size variance, resulting in photoluminescence emission increases of 20-26% for the x = 1.5 sample relative to the x = 0 parent across the 25-200 °C range that spans WLED working temperatures. Cation disorder suppresses nonradiative processes through disruption of lattice vibrations and creates deep traps that release electrons to compensate for thermal quenching. Introduction of high levels of cation disorder may thus be a very useful general approach for improving the efficiency of luminescent materials.

Aluminate Red Phosphor in Light-Emitting Diodes: Theoretical Calculations, Charge Varieties, and High-Pressure Luminescence Analysis

Searching for a non-rare-earth-based oxide red-emitting phosphor is crucial for phosphor-converted light-emitting diodes (LEDs). In this study, we optimized a blue and UV-light excited Sr4Al14O25:Mn phosphor exhibiting red emission peaked at ∼653 nm, which was successfully synthesized by solid-state reaction. The crystal structure, micromorphology, and luminescent properties of Sr4Al14O25:Mn phosphors were characterized by X-ray Rietveld refinement, high-resolution transmission electron microscopy, and photoluminescence spectra. The band gap and electronic structure of Sr4Al14O25 were analyzed by density functional theory calculations using the hybrid exchange-correlation functional. The crystal field environment effect of Al sites from introducing activator Mn ions was investigated with the aid of Raman 27Al nuclear magnetic resonance spectra and electron spin resonance. The pressure dependent luminescent properties and decay time of this compound were presented. The tricolor display spectrum by combining blue InGaN chips, commercial β-SiAlON:Eu2+ green phosphor, and Sr4Al14O25:Mn red phosphor were evaluated for commercial applications: using the present Sr4Al14O25:Mn red phosphor converted LED as a backlighting source.

Ultrafast Self-Crystallization of High-External-Quantum-Efficient Fluoride Phosphors for Warm White Light-Emitting Diodes

In this study, we used HF (as good solvent) to dissolve K2GeF6 and K2MnF6 and added ethanol (as poor solvent) to cause ultrafast self-crystallization of K2GeF6:Mn4+ crystals, which had an unprecedentedly high external quantum efficiency that reached 73%. By using the red phosphor, we achieved a high-quality warm white light-emitting diode with color-rendering index of Ra = 94, R9 = 95, luminous efficacy of 150 lm W-1, and correlated color temperature at 3652 K. Furthermore, the good-poor solvent strategy can be used to fast synthesize other fluorides.

Pressure-controlled synthesis of high-performance SrLiAl3N4:Eu2+ narrow-band red phosphors

SrLiAl3N4:Eu2+ (SLA) phosphors have gained increasing attention worldwide due to their narrow emission bandwidth and excellent thermal stability. In this study, a series of SLA phosphors are synthesized at gas pressures ranging from 0.1 MPa to 100 MPa using gas pressure sintering and hot isostatic press systems. Upon elevating the gas pressure in synthesis, the volume of a unit cell slightly decreases while both the quantum efficiency and the particle size of SLA increase. The Eu2+/Eu3+ ratio also increases with applied pressure, thereby enhancing the luminescence intensity of the phosphor materials. The presence of Li defects in the host material is evidenced by inductively coupled plasma with atomic emission spectroscopy. The mechanism of controlling the oxidation state of Eu and defect formation is proposed to elucidate the behavior of phosphors under different applied pressure levels during synthesis. Finally, the LED package shows that SLA could be a potential candidate for backlighting.

Integrated Surface Modification to Enhance the Luminescence Properties of K2TiF6:Mn4+ Phosphor and Its Application in White-Light-Emitting Diodes

Narrow-band Mn4+-doped fluoride phosphors have become a research hotspot worldwide. In this study, we propose integrated surface modification processes to enhance the performance and stability of the luminescence properties of K2TiF6:Mn4+ (KTF) phosphor. These integrated process are applied in the initial synthesis step, coating of the as-synthesized powder post-treatment process, and during the application of the phosphor in the white-light-emitting diode (WLED) device. Surface etching is conducted to remove impurities and small particles in KTF. Double-shell coating forms a stable protective layer outside the KTF. Atomic layer deposition is employed for the surface of the WLED device.