the broadband excitation in the blue region (≈450 nm) and efficient narrowband red emission at ≈630 nm, these phosphors are the ideal candidates as red components in LED backlight displays and warm WLEDs lighting. [5] However, these phosphors are always suffering from poor moisture resistance and lower external quantum efficiency (EQE). [6] Though the moisture resistance can be effectively improved by coating inorganic or organic layer, it inevitably increases the surface defects, which would eventually reduce the EQE, as well as affect the thermal stability and heat dissipation. [7] Generally, higher Mn 4+ concentrations lead to higher absorption and retain high EQE. [8] Moreover, Mn 4+ ions are exposed to the outside of the particle surface, which in turn leads to Mn 4+ hydrolysis. [9] Therefore, it is a fundamental challenge to realize the balance between the high EQE and excellent moisture resistance for Mn 4+-doped fluoride red phosphors. [6,10] Single crystal phosphor represented by YAG was regarded as a promising alternative to powder counterpart due to its high crystallization, fewer defects, and higher thermal conductivity. [11] Especially, the effective heat dissipation compared to powder phosphor in encapsulants, facilitates the dissipation of heat in the devices. [12] In this regard, high quantum efficiency, moisture resistance, and thermal stability can be expected in Mn 4+-doped fluoride single crystals, which are essential for high power applications. Herein, micrometer-size cubic K 2 SiF 6 :Mn 4+ (KSFM) single crystals were grown by a saturated crystallization method. The formation of Mn 4+-rare surface layer on KSFM single crystal particles was studied considering the crystallization process. The excellent performance of KSFM single crystal on moisture resistance, high quantum efficiency, thermal quenching stability, and effective heat dissipation were evaluated in detail. The as-fabricated KSFM single crystal phosphors converted white LEDs or LDs devices have the potential to be used for high-power and high-brightness backlight displays or lighting devices, and the result could initiate the exploration of new single crystal materials and Mn 4+-doped fluoride single crystals for other emerging applications. 2. Results and Discussion The schematic flow chart of this saturated crystallization method is shown in Figure 1a, and the typical KSFM single crystals Narrow-band red-emitting fluoride phosphors play a crucial role in lighting and backlight displays, whereas the stability issue is a major challenge. Here, highly stable K 2 SiF 6 :Mn 4+ (KSFM) single crystal phosphors with tunable sizes (20-1000 µm) are grown by a facile saturated crystallization method. The small specific surface area together with the unique Mn 4+-rare interface layer in KSFM single crystals enables the simultaneous fulfillment of high external quantum efficiency, excellent moisture, improved thermal quenching, and dissipation for high-power and laser-driven lighting applications. As a prototype, a warm white laser diode dev...

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