Color-tunable long persistent luminescence (LPL) phosphors are more strongly desired for intelligent anti-counterfeiting and information storage compared with single color types.
Near-infrared (NIR) long persistent luminescent (LPL) materials have attracted the interest of many researchers as they have potential applications in many aspects. However, majority of studies on Cr3+ ion-doped LPL materials have focused on Cr3+ in an octahedral site, and the luminescence is limited to the short-wavelength NIR-I region (700–900 nm), which is detrimental to fully explore Cr3+ ion-doped LPL materials with potential applications. In this work, a novel ultra-broadband NIR LPL material, Na2CaGe6O14 (NCGO):x%Cr3+, was successfully designed and synthesized, covering the luminescence range of 600–1200 nm and having the best afterglow duration of more than 10 h. Combining the luminescence lifetime with the low-temperature spectrum, it was concluded that the luminescence of NCGO:Cr3+ consists of the co-emission of Cr3+ in octahedra and tetrahedra. And it was confirmed by electron paramagnetic resonance (EPR) spectrum and X-ray absorption near-edge spectrum (XANES). The application prospects of NCGO:x%Cr3+ in many aspects were investigated in detail. This work could not only give a reference for researchers to study Cr3+ luminescence in multiple coordination but also provide a new strategy for obtaining new ultra-broadband NIR LPL materials.
chip + yellow phosphors, (b) NUV LED chip + red-green-blue (RGB) phosphors. However, the former type features a low color rendering index (Ra) and residual rich blue light, which is not suitable to meet the increasingly prominent demand for human-centric health lighting. [2] NUVactivated w-LED shows a preponderance, for the acquisition of high-quality white light through the combination of different phosphors, meanwhile, the blue light can be minimized.In addition, during the working process, the Ohm effect caused by the current will unavoidably increase the temperature, then the emission intensity of phosphors will drop, and Stokes shift will drift, which is so-called thermal quenching. [3] This effect seriously drags down the overall luminescent efficiency and the total life of the devices, as well as the imbalance of white light. Therefore, the development of phosphors with high brightness and thermal stability is still a key challenge.Very recently, researchers have made positive efforts to explore low thermal quenching phosphors and delved novel strategies which can be summarized as follows: i) defect level regulation: as the defect levels make the supplement with emission center during high-temperature emission. [4] Kim et al. obtained a blue emitting zero-thermal quenching phosphor Na 3 Sc 2 (PO4) 3 :Eu 2+ via its polymorphism transition (α−β−γ) and their defects correspondingly. [5] Wu et al. explored an antithermal quenching red emitting NaZn(PO 3 ) 3 :Mn 2+ and discovered the trap compensation effect during the self-reduction process. [6] However, one must concern about the lack of certain common principles and the uncertainty of traps in different hosts. ii) Structural modulation, which begins from the structure itself, searching or enhancing the rigidness of crystal structure to reduce thermally activated phonons-assisted non-radiative process under high temperature. [7] For instance, this is discovered in some UCr 4 C 4 type structures (e.g., RbLi(Li 3 SiO 4 ) 2 :Eu 2+ , NaK 2 Li(Li 3 SiO 4 ) 4 :Eu 2+ , NaK 7 (Li 3 SiO 4 ) 8 :Eu 2+ et al.), however, this is limited in its poor chemical stability. [8] iii) Surface and interface engineering, which is constructed by making phosphors into a core-shell structure or making phosphors-in-glass for higher chemical and thermal stability. Although effective, such as Ca 3 SiO 4 Cl 2 :Eu 2+ @SiO 2 , RbLi(Li 3 SiO 4 ) 2 :Eu 2+ @ Al 2 O 3 @ODTMS, the complexity, and extensibility of the process still limit practical production. [9] Therefore, newly efficient
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