Developing near-infrared long-lasting phosphorescence of Yb3+ through a medium: insights into energy transfer in the novel material Zn1.98Li0.02P2O7:Yb3+

Near-infrared (NIR) long-lasting phosphorescence (LLP) phosphors have potential applications in biological and military fields. However, the properties of these phosphors are insufficient to satisfy the demands. Herein, we report a series of novel LLP phosphors Zn1.98-xLi0.02P2O7:xYb3+ with strong NIR LLP. By investigating the O-Yb3+ charge transfer (CT) band and host thoroughly, it can be confirmed that distinct photoluminescence (PL) of the two can be realized by selecting an appropriate excitation. For NIR emission, considering that the emission of Yb3+ originates from the characteristic 4f-4f transition between 4F5/2 and 4F7/2 levels and the difference between these two levels is too small to be excited by UV light, it is practicable to use the energy transfer of the host rather than that of the O-Yb3+ CT band as its energy level does not match with that of Yb3+. LLP is different from PL of the O-Yb3+ CT band, host or Yb3+, and it is strongly dependent on the appropriate depths of the traps; therefore, the selection and regulation of the traps are crucial. Considering the practicability and the demand of NIR LLP materials, we focus on how to excite Yb3+ and obtain LLP.