Enhanced up-conversion persistent luminescence of Zn₂GeO₄:Mn phosphors by non-equivalent doping and sensitization of up-conversion nanocrystals

Up-conversion persistent luminescence (UCPL) nanoparticles have captured tremendous attention due to their unique optical operating performance such as the absence of auto-fluorescence and the charging capacity of near-infrared (NIR) light. However, the weak afterglow intensity as well as low duration limits their practical applications including multimode anti-counterfeiting and biological imaging, etc. Herein, typical green persistent luminescence (PersL) and photo-stimulated luminescence (PSL) of Zn₂GeO₄:Mn²⁺ phosphors were enhanced via non-equivalent Li⁺ co-doping, in which the preferred site selection of Mn²⁺ and Li ⁺ have been verified by using the first-principles theory. The possible luminescent enhancement mechanisms were proposed based on improving the oxygen vacancy density and promoting the formation of Mn⁴⁺ defect energy level in bandgap structure by non-equivalent doping of Li⁺. Interestingly, UCPL of Zn₂GeO₄:Mn²⁺,Li ⁺ can be achieved after charging of Vis to NIR light. It is worth noting that this UCPL can be further enhanced by the sensitization of NaYF₄:Yb,Tm nanoparticles via simply mixing Zn₂GeO₄:Mn²⁺,Li⁺ and NaYF₄:Yb,Tm under 980 laser stimulation. As a result, Zn₂GeO₄:Mn²⁺,Li⁺ phosphor exhibits the enhanced quintuple-mode luminescence including photoluminescence, PersL, PSL, photo-stimulated persistent luminescence and UCPL. In particular, the achieved NIR rechargeable UCPL hybrid phosphors exhibit enormous potential for higher order anti-counterfeiting and rewriteable data encryption and decryption applications.