Persistent energy transfer in ZGO:Cr³⁺,Yb³⁺: a new strategy to design nano glass-ceramics featuring deep red and near infrared persistent luminescence

ZnGa₂O₄:Cr³⁺, owing to its persistent luminescence properties in the deep red range, is an exceptional material in view of foreseen in vivo imaging applications. In the present work, we report the elaboration process and detailed investigations of the optical properties of nano glass-ceramics composed of spinel ZnGa₂O₄:Cr³⁺,Yb³⁺ nanocrystals embedded in a transparent, silica rich, glass matrix. The as-prepared materials show good incorporation of the dopants in the crystallites leading in both Cr³⁺ and Yb³⁺ emissions. These emissions occur while exciting in the Cr³⁺ bands, indicating an energy transfer process from Cr³⁺ to Yb³⁺. Furthermore, excitation in the Yb³⁺ band in the near-infrared (NIR) range suggests an interesting up-conversion process, which promotes the Cr³⁺ emission. Persistent luminescence of both Cr³⁺ and Yb³⁺ doping ions can be activated by charging the Cr³⁺ excitation bands, leading to persistent luminescence of zinc gallate nanocrystals in both first and second biological windows. The influence of Yb³⁺ co-doping on persistent luminescence properties has been investigated by persistent luminescence decay profiles and thermoluminescence studies. Indeed, thermoluminescence glow curves of Yb³⁺ exhibit similar shape to those of Cr³⁺ but appear broader and shifted towards higher temperatures. This temperature shift may be explained by the temperature dependence of the involved energy transfer process.