Scheme 1
From: Internal OH− induced cascade quenching of upconversion luminescence in NaYF4:Yb/Er nanocrystals

Schematic diagram of the internal OH− influences on energy migration-induced UCL, which can be visualized as “survivor effect.” Typically, the shorter the migration path of an excited state of Yb3+, the larger the possibility of its surviving from OH−-induced quenching