Design,synthesis And Luminescence Properties Of Mn⁴⁺-Doped Red-Emitting Fluoride Phosphors With Weather Resistance

The Mn⁴⁺-doped fluoride phosphors can be effectively excited by the blue light?⁴60nm?and exhibit narrow-band red emission?⁶30 nm?,which have been widely used in white light-emitting diodes?WLEDs?as a red component.However,the Mn⁴⁺ions hydrolyze easily in the humid environment,which limits practical applications.This work is dedicated to synthesize the stable and efficient fluoride red phosphors from the physical and chemical properties of fluoride compounds,which provides a new feasible perspective for the design of efficient and stable material.With the background mentioned above,the first chapter of the thesis introduces the phosphors for white LEDs and their research status.Besides,the synthesis methods,phase structure properties,luminescence properties,WLEDs applications and unsolved scientific problems of Mn⁴⁺-doped fluoride red phosphors are summarized.The second chapter introduces the raw materials and the synthesis and characterization methods of samples.Besides,the results and analysis of experiments are discussed from chapter three to seven.The specific research contents are as follows:?1?The reason for the instability of Mn⁴⁺-doped fluoride phosphors is the hydrolysis of surface Mn⁴⁺.Therefore,reducing the exposure of Mn⁴⁺can improve the moisture resistance of phosphors.In chapter 3,a waterproof narrowband fluoride phosphor K₂TiF₆:Mn⁴⁺?KTF?has been demonstrated via a facile super-hydrophobic surface modification strategy.Using the super-hydrophobic surface modification with Octadecyltrimethoxysilane?ODTMS?on KTF surfaces,the moisture resistance performances and thermal stability of the phosphor KTF can be significantly improved.Meanwhile,the absorption,quantum efficiency showed without obvious changes.The surface modification processes and mechanism,as well as moisture resistance performances and luminescence properties of the phosphors have been carefully investigated.It was found that the luminous efficiency?LE?of the modified KTF was maintained at 83.9%or 84.3%after being dispersed in water for 2 h or aging at high temperature?85°C?and high humidity?85%?atmosphere?HTHH?for 240 h,respectively.The WLEDs fabricated with modified KTF phosphor showed excellent color rendition with lower color temperature?2736 K?,higher color rendering index?CRI,Ra=87.3,R9=80.6?and high luminous efficiency?LE=100.6 lm/W?at 300 mA.These results indicate that hydrophobic SCAs surface modification was a promising strategy for enhancing moisture resistance of humidity sensitive phosphors,exhibiting great potential for practical applications.?2?In chapter 4,the dissolution and hydrolysis characteristics of fluoride compounds were studied.However,complete hydrolysis of[MnF₆]^2-groups was found at aqueous solutions,while the K₂XF₆?X=Ti,Si,Ge?was reversible.Based on the differences in the physical and chemical properties of host materials and active Mn⁴⁺,we proposed a facile and general postsynthetic hydrogen peroxide?H₂O₂?surface passivation strategy to treat the K₂XF₆:Mn⁴⁺?KXF,X=Ti,Si,Ge?phosphors,in which a Mn⁴⁺-rare surface protective layer appears and further covers the corresponding particles.The environment moisture can be effectively isolated by the Mn⁴⁺-rare surface passivation layer with low solubility,which will be sacrificed to protect the phosphor particles even if under extreme hydrolysis conditions.The relative external quantum efficiency of the optimized phosphors still maintained over96%after the passivation treatment,and the relative luminous intensity still remained 97%even soaked in water after 12 h.The correlated color temperature?CCT?of high-power WLEDs fabricated by the passivated phosphors has no remarkable change during the aging process?100 days?in the high temperature?85°C?and high humidity atmosphere?85%?.It is expected that such a surface-redox strategy can be expanded to other doped materials systems,and also opening a new perspective for the development of luminescence materials with enhanced surface stability and device duration.?3?In chapter 5,we reported a cubic single-crystal fluosilicate(K₂SiF₆:Mn⁴⁺,KSF)prepared with a facile one-pot solvent evaporation method,which can ideally resolve the issues discussed above.It showed an excellent moisture resistance,highly quantum efficiency,better thermal quenching stability and broader photoluminescence excitation?PLE?spectrum compared with the KSF powder phosphor.To evaluate its feasibility for high-power white light-emitting diodes?WLEDs?,the KSF single crystals were applied to a 10 W blue InGaN chips with a chips-on-board?COB?structure.It reached an extremely higher luminous efficiency?LE?of 130.2 lm/W at 3930 K than that of KSF powder phosphor?117.8 lm/W at3924 K?driven by 300 mA.Furthermore,it has lower equilibrium temperature at 106.5°C after 30 min operating in comparison with the KSF powder phosphor?119.6°C?.The shortcomings of existing fluoride phosphors can be well improved by using KSF single crystal,such as poor moisture resistance,low quantum yield and poor thermal stability.Meanwhile,the operating temperature of the device can be reduced and the service lifetime will be significantly prolonged.We can foresee that the Mn⁴⁺-doped fluoride single crystal red-emitting materials have great prospect of research,and it is expected to promote the application on high-power white LEDs and white laser LEDs.?4?The weather resistance for fluoride red phosphors is related to the host materials solubility.In chapter 6,host materials with lower solubility were selected as the research objects.Herein,a novel red-emitting double-perovskite A₂NaScF₆:Mn⁴⁺?A=K,Rb,Cs??ANSF?phosphor with short fluorescence lifetimes??<4 ms?and obvious zero phonon line?ZPL?emission have been developed through Mn⁴⁺ions non-equivalent doping strategy.The Effect of A⁴⁺ion on crystal field and local symmetry which results in the variation of luminescence characteristics has been investigated.Density functional theory?DFT?calculations show that the observations of short fluorescence lifetime??<4 ms?and obvious ZPL are mainly ascribed to the slightly distorted environment of Mn⁴⁺,and the effects of Na⁺vacancy induced by Mn⁴⁺non-equivalent doping.Using the ANSF as red-light component and?-SiALON as green one,all of the fabricated WLEDs can reach a wide color gamut above105%of National Television Standards Committee?NTSC?standard.Furthermore,when applying as-prepared WLEDs in LCD backlights,the image-retention phenomenon can be effectively avoided.Such Mn⁴⁺non-equivalent doping strategy and cation substitution can be expanded to design expected phosphors with desire fluorescence lifetimes and ZPL intensity,and this research also open a new perspective for the optimization of local symmetry of Mn⁴⁺-doped red-emitting fluoride phosphors.