Crystallographically altered (Mg,Sr)3(Al,Ga)2GeO8:Cr3+ phosphors with tuned emission width from the ultra-sharp red to profound NIR band

Abstract

Deep red to near-infrared (NIR) phosphors with extraordinary emission intensity, variable peak wavelength, and emission width are in great demand for smart next-generation lighting applications. Nonetheless, incorporating such features in any phosphor system is extremely difficult. Herein, a new Mg₃Al₂GeO₈:0.01Cr³⁺ phosphor with ultra-sharp emission band peaking at 693 nm with a full-width at half maximum (FWHM) of ∼4 nm has been initially developed. The emission intensity and the FWHM of the phosphor have been increased by substituting alkaline earth metal ions. The optimum Mg_2.9Al₂GeO₈:0.01Cr³⁺,0.1Sr²⁺ exhibits emission intensity that is three times greater than that for the Mg₃Al₂GeO₈:0.01Cr³⁺ sample. The additional [MgO₆] sites from the newly generated Sr impurity phase and the structural distortion in the Cr³⁺-occupied sites are responsible for the emission enhancement. In addition, Ga³⁺ co-doped Mg_2.9(Al_2−yGa_y)GeO₈:0.01Cr³⁺,0.1Sr²⁺ samples have been developed and the crystal field environment is weakened in [GaO₆] that is occupied by Cr³⁺ ions. The emission from the multiple sites with strong and weak crystal field environments resulted in a broadband emission (630 nm to 1050 nm and an FWHM of ∼150 nm), effectively covering the first biological window region. The pc-LED produced by combining the 410 nm LED with the Mg_2.9Al₂GeO₈:0.01Cr³⁺,0.1Sr²⁺ phosphor with 100% color purity can be used for display application. Meanwhile, the 410-nm LED combined with the Mg_2.9(AlGa)GeO₈:0.01Cr³⁺,0.1Sr²⁺ phosphor exhibits the potential for plant growth applications. Moreover, the NIR LEDs produced from the Mg_2.9(Al_0.5Ga_1.5)GeO₈:0.01Cr³⁺,0.1Sr²⁺ phosphor find application in night vision surveillance. Finally, the synthesized phosphor is utilized for in vitro cellular imaging, as demonstrated in cervical cancer HeLa cells.