Intense one-band near-infrared upconversion luminescence induced by using spontaneous polarization BiOCl sheet crystals as hosts for Yb3+ and Tm3+ ions

Abstract

Rare earth (RE) doped nanophosphors, with nearly pure near-infrared to near-infrared (NIR-to-NIR) upconversion (UC), have great potential to revolutionize biological luminescent labels due to their deep tissue penetrability and high signal-to-noise ratios. Here, we report a new scheme that utilizes the anisotropic polarization of layered hosts to easily produce an efficient single-band NIR UC emission of Tm³⁺ ions. To verify this concept, BiOCl:Yb³⁺,Tm³⁺ microsheets and nanosheets are prepared, and their photoluminescence properties are characterized. Under excitation at 980 nm, Tm³⁺–Yb³⁺ co-doped BiOCl can produce a high NIR-to-visible UC emission ratio (I_{808 nm}/I_{481 nm}, over 1200) of Tm³⁺ ions, which can be carefully modified with the Yb³⁺ doping concentration. The experimental results indicate that the high environmental sensitivity of the ³H₄ → ³H₆ electric dipole transition (808 nm) of Tm³⁺ ions and its response to the oriented spontaneous internal electric field (IEF) of the BiOCl-layered host are mainly responsible for the intense, tunable one-band NIR UC emission property. This work provides new insight into the method for controlling the emission variance of energy levels of RE-doped UC systems and provides new directions for achieving nearly pure NIR-to-NIR UC in luminescent labelling probes for the field of biology.