Probing into the conduction band and type of carriers/traps on red/orange persistent phosphors in vacancy & solid-solution induced (Sr/Ba)1−xCaxGe4−yO9:Mn2+

Abstract

Long-wavelength afterglow has been a key issue in the investigation of afterglow materials. Herein, the orange/red (Sr/Ba)_1−xCa_xGe_4−yO₉:0.01Mn²⁺ is prepared by the introduction of vacancy induction and solid solution. The (Sr/Ba)Ge₄O₉:Mn²⁺ hardly possesses an afterglow phenomenon and exhibits only red/orange photo-luminescence (PL) attributed to the d–d transition of Mn²⁺, while samples with reduction of the Ge⁴⁺ content and with replacement by Ca²⁺ show bright afterglow emission with the peak located at about 612 nm/620 nm. The emerging broadband peak comes from charge transfer involving Mn²⁺ and nearby defect clusters and the bottom of the conduction band (CB). The introduction of V′′′′_Ge creates a defective energy level above the valence band, but the ground state energy difference with Mn²⁺ is too large (>1 eV) to allow hole transfer, which was confirmed by ultraviolet photoelectron spectroscopy (UPS), spherical aberration-corrected transmission electron microscopy (AC-STEM), charge differential density (CDD) analysis, electron paramagnetic resonance (EPR) analysis, etc. With this achievement, we propose an original design strategy for long-wavelength afterglow and a more reasonable afterglow mechanism, which is of great importance for the investigation of long-wavelength afterglow materials.