A simple and generic post-treatment strategy for highly efficient Cr3+-activated broadband NIR emitting phosphors for high-power NIR light sources

Cr3+-activated broadband near-infrared (NIR) emitting phosphors have attracted more attention worldwide as they are strongly expected to be promising light convertors for phosphor-converted NIR light sources for NIR spectroscopy and imaging technologies. Unfortunately, few works have reported the co-existence of multi-valence Cr ions and its negative effects on the luminescence efficiency of the Cr3+ ion in NIR emitting phosphors. Herein, we systematically investigated and prove the existence of Cr6+ ions in novel LiInO2:Cr3+ NIR emitting phosphors based on optical absorption, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and inductive coupled plasma (ICP) measurements. The competitive absorption of Cr6+ ions is found to be the main reason for greatly limiting the high luminescence efficiency of Cr3+ ions in NIR emitting phosphors. More importantly, we proposed a generic post-treatment strategy to eliminate the Cr6+ ions in Cr3+-activated NIR emitting phosphors by simply soaking the phosphors in deionized water or dilute acid. For LiInO2:Cr, the ICP data show 82.7% of Cr ions present in a valence of +6 in LiInO2 and they were finally washed out, which consequently enhances its NIR emission intensity by 72.6% and increases its quantum efficiency by 21.3% from 14.8% to 36.1%. More interestingly, the as-proposed post-treatment strategy is highly versatile and successfully applied to other well-known Cr3+-activated NIR-emitting phosphors reported previously. Our findings suggest that this simple and generic post-treatment strategy can open up new avenues for developing highly efficient Cr3+-activated broadband NIR-emitting phosphors.

Automated summary

This study confirms the existence of Cr6+ ions in LiInO2:Cr3+ NIR emitting phosphors and identifies their competitive absorption as the main factor limiting the luminescence efficiency of Cr3+ ions. Moreover, a generic post-treatment strategy is proposed to eliminate Cr6+ ions and enhance NIR emission intensity and quantum efficiency.