Mn-Activated Fluoride Phosphors Modified by Surfactant with Outstanding Water Resistance and Luminescent Thermal Properties

Although Mn4+-activated fluoride phosphors have high luminescence quality, their poor water resistance and thermal fluorescence properties significantly limit their practical applications. Here, we propose a surfactant modification strategy by adding the surfactant cetyltrimethylammonium bromide (CTAB) to the synthesis and modifying the surface of the phosphor with ethylene diamine tetraacetic acid (EDTA) to obtain a phosphor with excellent luminescence thermal properties and water resistance, K2TiF6:Mn4+-xCTAB-EDTA (KTFM-xC-E) phosphors. The experimental and X-ray diffraction Rietveld refinement results confirm that the phosphor has higher structural rigidity and thus improved thermal stability. The surface modification with EDTA resulted in the formation of a dilute Mn4+ shell layer on the phosphor surface, which prevented the inward hydrolysis of the phosphor and resulted in excellent water resistance. Therefore, we have successfully modified K2TiF6:Mn4+ (KTFM) phosphors using low-cost surfactants, which also provides new ideas for other commercial high-quality phosphors

High Water Resistance and Luminescent Thermal Stability of Li_<i>y</i>Na_{(2–<i>y</i>)}SiF₆: Mn⁴⁺ Red-Emitting Phosphor Induced by Codoping of Li⁺

Mn4+-doped fluoride phosphors are efficient narrowband red-emitting phosphors for white light-emitting diodes (WLEDs) and backlight displays. However, erosion by moisture is the main obstacle that limits their application. In this work, LNSF:Mn4+ (Li0.06Na1.94Si0.94Mn0.06F6) with high quantum yield (QY), luminescent thermal stability, and waterproofness was synthesized using the H2O2-free reaction method at room temperature. Compared to NSF:Mn4+(Na2Mn0.06Si0.94F6), the QY value, luminescence thermal stability, and water resistance of LNSF:Mn4+ are obviously improved by codoping of Li+ because of the formation of charge-carrier transfer (CT) and rare-Mn4+ layer induced by codoping of Li+. The former produces the negative thermal quenching (NTQ) effect, which results in the improvement of the luminescent thermal stability. The latter can inhibit the hydrolysis of Mn4+ on the surface of the sample, which leads to the enhancement of waterproofness. The formation mechanism of the rare-Mn4+ layer is discussed. A prototype WLED emitting the ideal warm white light (CCT = 3173 K, Ra = 90.4) was assembled by coating a mixture of LNSF:Mn4+, yellow emitting phosphor (YAG:Ce3+), and epoxy resin on the blue light InGaN chip, indicating that the performance of the WLED can be improved by using LNSF:Mn4+

Design of Eu³⁺-Doped Fluoride Phosphor with Zero Thermal Quenching Property Based on Density Functional Theory

Although being applied in various fields, white light emitting diodes (WLEDs) still have drawbacks that urgently need to be conquered: the luminescent intensity of commercial phosphors sharply decreases at working temperature. In this study, we calculated the forming energy of defects and confirmed that the VNa defect state can stably exist in β-NaGdF4, by density functional theory (DFT) calculation. Furthermore, we predicted that the VNa vacancies would provide a zero thermal quenching (ZTQ) property for the β-NaGdF4-based red-light phosphor. Then, a series of β-NaGdF4:xEu3+ and β-NaGdF4:0.25Eu3+,yYb3+ red-light phosphors were synthesized by the hydrothermal method. We found that β-NaGdF4:0.25Eu3+ and β-NaGdF4:0.25Eu3+,0.005Yb3+ phosphors possess ZTQ properties at a temperature range between 303–483 K and 303–523 K, respectively. The thermoluminescence (TL) spectra were employed to calculate the depth and density of the VNa vacancies in β-NaGdF4:0.25Eu3+ and β-NaGdF4:0.25Eu3+,0.005Yb3+. Combining the DFT calculation with characterization results of TL spectra, it is concluded that electrons stored in VNa vacancies are excited to the exited state of Eu3+ to compensate for the loss of Eu3+ luminescent intensity. This will lead to an increase of luminescent intensity at high temperatures and facilitate the samples to improve ZTQ properties. WLEDs were obtained with CRI = 83.0, 81.6 and CCT = 5393, 5149 K, respectively, when phosphors of β-NaGdF4:0.25Eu3+ and β-NaGdF4:0.25Eu3+,0.005Yb3+ were utilized as the red-light source. These results indicate that these two phosphors may become reliable red-light sources with high antithermal quenching properties for WLEDs