Composition-structure-luminescence and enhancement of Cr³⁺ activated broadband near infrared phosphors for night vision, bio-imaging, and noninvasive detection

In the construction of novel near-infrared (NIR) radiation sources, NIR phosphors equipped with a large emission range and thermally stable luminescence are highly desired. In this work, we designed and prepared a series of Cr³⁺ activated and Cr³⁺/Yb³⁺ co-activated KScP₂O₇ (KSPO) phosphors, with their composition-morphology-structure properties and interaction mechanisms systematically investigated. Excited by 475 nm, KSPO:0.03Cr³⁺ can emit 700-1200 nm NIR light peaking at 873 nm with a large full width at half maximum (FWHM, 163 nm) corresponding to the ⁴T₂ → ⁴A₂ transition among Cr³⁺ ions. Cr³⁺ ions occupy solo crystal sites with a weak local crystal-field environment, which is verified to be responsible for the NIR broadband emission. With the addition of Yb³⁺, extra NIR emission dominating at 968 nm was obtained, as a result of the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions. The internal quantum efficiency was improved from 27.3% of KSPO:0.03Cr³⁺ to 40.5% of KSPO:0.03Cr³⁺,0.004Yb³⁺ phosphor, mainly benefiting from the energy transfer of Cr³⁺ → Yb³⁺. The fabricated NIR pc-LED, utilizing KSPO:0.03Cr³⁺,0.004Yb³⁺ phosphor and a 470 nm LED chip, exhibits high output power (37.56 mW@320 mA). Taking this NIR pc-LED as a radiation source, the doll model, the cleanser content, and high-resolution human hand vein imaging are visualized with the assistance of a NIR camera. The present work suggests that Cr³⁺/Yb³⁺ co-activated KSPO phosphor has considerable application prospects in night-vision, bio-imaging, and noninvasive detection.