“…In oxide and fluoride host lattices with lanthanide activators, successful examples of boosting upconversion emission have been reported with co-doping TM n+ for the following ions: Mn 2+ , Cu 2+ , Co 2+ , Ni 2+ , Cr 3+ , Fe 3+ , Zn 2+ , Cd 2+ , and Mo 3+ . [42][43][44][45][46][47][48][49] UC luminescence can be tuned by controlling the doping concentration of the transition metal ions. For instance, Wei et al 50 reported that the strongest UC emission intensity was obtained in Fe 3+ co-doped NaGdF 4 :20 mol%Yb 3+ ,2 mol%Er 3+ , when the stoichiometric ratio of Fe 3+ was 20 mol%.…”