Visual color modulation and luminescence mechanism studies on Mn/Eu codoped Zn-Mg-Ge-O long afterglow system

In this work, Mn/Eu co-doped Zn-Mg-Ge-O composite system was prepared by high temperature solid phase method. After 375 nm external excitation, Zn-Mg-Ge-O system emitted four visual colors of red, orange, yellow and green afterglow. Phase analysis proved that these four visual colors were derived from both red long afterglow emitted from Mn/Eu co-doped MgGeO3 (expressed as MgGeO3:Mn,Eu) and green afterglow from Mn/ Eu co-doped Zn2GeO4 (Zn2GeO4:Mn,Eu). The change of Zn content could modulate the intensity ratio of red and green afterglow, ultimately resulted in different visual colors. The luminescence mechanism of MgGeO3:Mn,Eu and Zn2GeO4:Mn,Eu was quantitatively studied by DFT simulation. Although the emission centers of MgGeO3:Mn,Eu and Zn2GeO4:Mn,Eu were same Mn ion, electron-hole separation and electronic transition path of emission were different. The electron-hole separation of MgGeO3:Mn, Eu occurred at the 6A1(S) level of Mn ion, and that of Zn2GeO4:Mn,Eu occurred in the valence band (VB). Downward transition path of electrons in MgGeO3:Mn,Eu was T-4(2)(D)->(6)A(1)(S), while that in Zn2GeO4:Mn,Eu was T-4(2)(D)-> VB. In addition, Eu ion in no matter MgGeO3 or Zn2GeO4 worked as trap center, however the trap depth was different.