Fig. 2

Schematic illustration of the effect of ion concentration on photoluminescence intensity. E is energy. E_c, E_v, and ε_F are conduction band edge, valance band edge, and Fermi level respectively. a Surface depletion region caused by the trapped electrons in the surface states. b Depletion region neutralized by the attachment of positives ions H⁺ or K⁺. c Electron transfer to the redox level of H⁺