“…Er 3+ , Tm 3+ , Nd 3+ , Eu 3+ , and Ho 3+ ), Er 3+ has been widely used as an activator for FIR studies [6][7][8][9] because of its suitable TCELs ( 2 H 11/2 and 4 S 3/2 ) of energy difference (∼350-700 cm −1 ). 10 However, since the quantum yield of lanthanide based upconversion emission is low, therefore, a lot of effort is still being devoted to enhance the upconversion (UC) emission at the rst step and eventually enhance the sensing ability of the lanthanides thereof.…”