Enhanced one-band near infrared upconversion luminescence of Yb3+-Tm3+ co-doped BiOCl1-xBrx nanosheet by tuning band gap

Tm3+-Yb3+ codoped nanophosphor with highly efficient near-infrared (NIR) upconversion (UC), which is nearly pure NIR-to-NIR luminescence, exhibited great potential in revolutionary biological luminescence labels and super resolution fluorescence imaging on account of its depth of penetrability into tissue and high signal-to-noise ratio. Herein, we reported a different strategy to enhance the one-band NIR UC of Tm3+ ion by tuning the band gaps of polarized semiconducting nanosheets. Under excitation at 980 nm, increasing Brxe213; concentration from 0 to 0.5, the one-band NIR UC emission generated by the H-3(4)-> H-3(6) electric dipole transition (801 nm) in Tm3+Yb3+ co-doped BiOCl1-xBrx nanosheets, was gradually enhanced accompanied by prolonged decay time, which was as a result of the high environmental sensitivity of the spontaneous host; however further increase of Br resulted to a contrasting effect. The results show that band gap tuning with Br- improved the separation efficiency of the photocarriers of the semiconducting nanosheets and led to asymmetrical local crystal field, which enabled the prolonged decay time of the excited energy levels of Tm3+ ions, and consequently, the UC enhancement. This work presents a new direction for the realization of efficient and pure NIR-to-NIR UC in fluorescent labelling probe and super resolution fluorescence imaging.