Preparation And Properties Of Six-coordinated Cr³⁺ And Bi³⁺ Activated Luminescent Materials

Phosphor converted light emitting diodes（pc-LEDs）have become the most competitive solid-state light source and widely used in the field of lighting and display due to their advantages of low cost,high efficiency,energy conservation,compact structure,long service lifetime and reliability.As a key component of pc-LED,phosphor has a significant impact on the spectral output and overall performance of the final pc-LED.With the evolution of demand in the development of the times,different types of inorganic phosphor materials have been developed and applied.At present,the development of new inorganic phosphor materials is mainly focused on the application of new near-infrared light sources,and backlight light sources for wide color gamut display devices.In order to meet the needs of these different applications,inorganic phosphor materials need to adjust their luminescent properties,including emission wavelength,spectral shape,thermal stability,chemical stability,absorption,and color rendering.This article mainly selects different matrix lattices,use transition metal Cr³⁺ions and post transition metal Bi³⁺ions as activators to occupy representative hexagonal octahedral lattice sites（with the same coordination number）,exploring the effects of the number of occupied sites,symmetry,and co-occupancy on luminescence,and studying the relationship between local crystal structure and photoluminescence performance of inorganic materials.This article provides theoretical guidance for predicting and designing the required optical properties in inorganic materials.It mainly includes the following four work contents:（1）Cr³⁺ion is an ideal near-infrared emission activator ion because of its broadband absorption that matches well with blue LED chips,and it can exhibit a wide emission band in weak octahedral crystal fields.However,the application of most NIR phosphors is now restricted due to their poor photoluminescence thermal stability.Here,a blue light excitable Ba Ca₂Mg Si₂O₈:Cr³⁺phosphor was synthesized,with a broadband NIR emission spectrum spanning from 650 to 1100 nm,peaking at 803 nm and a full width at half maximum（FWHM）of 128 nm.It was demonstrated that Cr³⁺ions occupy a single site in BCMS using crystal structure analysis,varied excitation and emission tests,time-resolved emission spectroscopy,and luminescence lifetime fitting.Notably,Cr³⁺ions occupy the Mg O₆ octahedron in the structural unit,which is firmly encircled and compressed by the Si O₄ tetrahedron to create a rigid framework that is conducive to achieving good PL thermal stability.90.84%of the PL intensity is retained at 423 K compared to that at room temperature,which turns it into one of the best thermal stability of Cr³⁺activated phosphor with longest wavelength broadband NIR emissions yet discovered.The pc-LED devices fabricated with BCMS:Cr³⁺exhibit a linear increase in NIR output power with increasing current,demonstrating their enormous potential for application in high-power NIR pc-LED devices.（2）Near-infrared phosphor conversion light-emitting diode（NIR pc-LED）is a widely used ideal NIR light source due to its unique advantages of low cost,energy saving,compactness and long operational lifetime.However,there is currently a scarcity of appropriate phosphor for NIR light sources utilized in spectral detection,with emission peak larger than 850 nm and steady spectrum output of ultra broadband spectra.Here,a broadband Cr³⁺activated Li Sc Sn O₄（LSS）phosphor(λ_ex-max=460 nm)matching the pump band of commercial blue LED chips has been developed,which shows a long wavelength NIR emission peaking at 900 nm.Cr³⁺simultaneously replace the Wyckoff site co-occupied by Sc and Sn in LSS.Due to the difference of local environment of the specific crystal field,two different wide emission bands are caused.These two broad emission bands both contribute to the broadband luminescence with a full width at half maximum（FWHM）of 227 nm.Benefiting from the crystallography consistent Wyckoff position occupation of Cr³⁺ions,the spectral pattern of LSS:Cr³⁺exhibit excellent stability over a wide temperature span（298-423 K）.The broadband NIR pc-LED prototype based on LSS:Cr³⁺shows great a potential in NIR spectrum detection,night vision imaging applications.（3）Ultra-efficient broadband near-infrared（NIR）phosphor-converted light-emitting diodes（pc-LEDs）are urgently needed to improve the detection sensitivity and spatial resolution of current smart NIR spectroscopy-based techniques.Nonetheless,the performance of NIR pc-LED has severely limited owing to the external quantum efficiency（EQE）bottleneck of NIR light-emitting materials.Herein,we advantageously modified a blue LED excitable Cr³⁺-doped tetramagnesium ditalate（Mg₄Ta₂O₉,MT）phosphor through lithium ion as a key efficient broadband NIR emitter to achieve high optical output power of NIR light source.The emission spectrum encompasses the 700-1300 nm electromagnetic spectrum of first biological window(λ_max=842 nm)with a full-width at half-maximum（FWHM）of～2280 cm^-1（～167 nm）,and achieves a record EQE of 61.25%detected at 450 nm excitation through Li-ion compensation.A prototype NIR pc-LED was fabricated with MT:Cr³⁺,Li⁺to evaluate its potential practical application,which revealed a NIR output power of 53.22m W at a driving current of 100 m A,and a photoelectric conversion efficiency of 25.09%at 10 m A.This work provides an ultra-efficient broadband NIR luminescent material,which shows great promise in practical applications and presents a novel option for the next generation of high-power compact NIR light sources.（4）Due to the negative impact on human health caused by the strong and sharp blue light emitted by traditional blue LED chips,the photoluminescence spectrum of w LED structures obtained by excitation of tricolor（blue,green,and red）phosphor materials using near-ultraviolet（n-UV）chips contains significant cavity in the deep blue region（400-430 nm）,which reduces the color rendering index.Herein,we synthesized a n-UV excitable narrow-band blue-emitting Ba₃Lu₂B₆O₁₅:Bi³⁺(BLB:Bi³⁺)phosphor.It exhibits a narrow band emission（FWHM=35.5 nm）,high color purity of97.76%,surprisingly high internal quantum efficiency（IQE）which reached up to90.6%,excellent thermal stability（85%@423 K of the integrated emission intensity at room temperature）and reversibility and high consistency.The outstanding photoluminescence properties of Bi³⁺are attributed to a highly symmetrical crystal field environment and tightly connected polyhedral network of Ba₃Lu₂B₆O₁₅.In addition,the energy level scheme is constructed from the calculated electronic structure to explore the difference in luminescence stability of Ce and Bi doped BLB phosphors.The deep blue phosphor Ba₃Lu₂B₆O₁₅:Bi³⁺can compensate the deep blue gap of n-UV chip pumped blue Ba Mg Al₁₀O₁₇:Eu²⁺,green（Ba,Ca）₂Si O₄:Eu²⁺,and red Ca Al Si N₃:Eu²⁺phosphors.The device exhibits a high color rendering index（CRI）of95.4 and a low correlation color temperature（CCT）of 4807 K.These characteristics indicate that BLB:Bi³⁺phosphors have potential application prospects in full spectrum high-quality w LED lighting based on n-UV chips.