Spectral Modulation Study Of Cr³⁺-Doped Garnet Near-infrared Phosphors

With the rapid development of artificial lighting technology,near-infrared phosphor-converting light-emitting diodes（NIR pc-LEDs）are widely used in security monitoring,biomedical and plant lighting due to their energy-saving,low thermal radiation,high efficiency and spectral control advantages.However,the existing near-infrared phosphors for plant lighting face problems such as insufficient matching with the absorption range of plant photosensitive pigments P_FR and insufficient luminous intensity.Therefore,the development of phosphors with high luminescence intensity and emission spectra matching the absorption spectrum of P_FR is of great significance to boost the development of NIR plant lamps.It has been shown that Cr³⁺is an activated ion that can achieve near-infrared emission,while its emission wavelength and spectral profile are easily tunable.In comparison with other matrix materials,garnet has the advantages of adjustable composition,high structural rigidity and abundant polyhedral sites.Therefore,in this paper,two highly efficient garnet structured near-infrared phosphors for plant lighting were prepared using Cr³⁺as the activating ion and garnet structured materials as the matrix,and using chemical unit co-substitution and energy transfer bridge construction strategies,respectively.The main results of the study are as follows:（1）Lu_3-xCa_xAl_5-xSi_xO₁₂:0.05Cr³⁺（0.5≤x≤2.5）garnet structured NIR phosphors based on Lu₃Al₅O₁₂:Cr³⁺phosphors were designed by a chemical unit co-substitution strategy.After a series of optimization experiments,the sample LCAS:Cr³⁺whose spectral distribution and luminescence intensity best matched the absorption curve of the plant photosensitive pigment P_FR was selected.Under the blue light excitation at 430 nm,LCAS:Cr³⁺exhibits a broadband NIR emission with the main peak at 740 nm and a half-height width of 160 nm.The optimal Cr³⁺-doped sample LCAS:0.07Cr³⁺has relatively high internal/external quantum yields of80%/30.5%,respectively.Meanwhile,the phosphor can maintain 66.5%of its luminescence intensity at 423K compared to room temperature,demonstrating good thermal stability.The emission spectrum of LCAS:Cr³⁺phosphor can cover a large area of the absorption spectrum of the plant photochrome PFR,indicating that the phosphor-encapsulated light source can be used as a supplemental light for plants and has potential applications in the agricultural field.（2）The second part of the study aims at enhancing the luminescence intensity of Lu₂Ca Mg₂Si₃O₁₂:Cr³⁺,improving the external quantum yield and modulating its spectrum.By introducing an efficient sensitizer Ce³⁺into the system,the luminescence intensity and external quantum efficiency in the NIR band are improved by constructing a special Ce³⁺→Cr³⁺energy transfer bridge,and the overall FWHM of the phosphor is also broadened to achieve efficient VIS-NIR luminescence.Under blue light excitation at 470 nm,LCMS:Ce³⁺,Cr³⁺showed double-peak emission with 590 nm and 734 nm as the emission centers,and their luminescence intensity in the near-infrared range was increased by 1.05 times compared with that of single-doped Cr³⁺,and the overall external quantum yield（EQY）was greatly increased to 43.1%.Also,LCMS:Ce³⁺,Cr³⁺have a favorable thermal stability,and their combined luminescence intensity at 425 K can be maintained at 65.1%of that at room temperature.Similarly,VIS-NIR pc-LEDs were also encapsulated with co-doped phosphors LCMS:0.05Ce³⁺,0.01Cr³⁺,and the optimized phosphors were shown to be better for plant lighting applications by optoelectronic performance tests.