A vivid example of turning waste into treasure: persistent luminescence of Ca2Ga2(Si, Ge)O7:Pr3+, Yb3+ phosphor tailored by band gap engineering

We exhibit a vivid example of turning waste into treasure for the development of persistent luminescence (PersL) phosphor. Facing the original experimental failure in which the as-synthesized Ca2Ga2SiO7: Pr3+, Yb3+ phosphor showed very weak PersL, we did not give up but made a deep analysis. It reveals that the relatively deep depth of traps (0.72 eV) may be the key reason for the weak PersL of Ca2Ga2SiO7: Pr3+, Yb3+ phosphor. Accordingly, in order to make the depth of the traps more shallow, a band gap engineering method was applied by the partial substitution of Si4+ with Ge4+ ions. Investigation into the optical spectra and the electronic structures indicate that the band gap has been successfully reduced. Correspondingly, the depth of traps becomes shallower, and it is adjusted in an appropriate range (around 0.65 eV) when Si : Ge = 1 : 9. As a consequence, the original "waste'' phosphor with very weak PersL is successfully turned into a valuable PersL phosphor: Ca2Ga2(Si0.1Ge0.9)O-7: Pr3+, Yb3+, whose PersL duration time and initial PersL brightness have been increased by 74 times and 52 times, respectively, compared with the original Ca2Ga2SiO7: Pr3+, Yb3+ sample.