Room-temperature visible upconversion luminescence of Ni²⁺ sensitized by Yb³⁺ in transparent glass ceramics

Upconversion luminescence of transition metal Ni²⁺ ions is seldom at room temperature (RT) due to large non-radiative transition probability. Here, a green Ni²⁺ upconversion luminescence at RT is obtained by the near-infrared excitation of Yb^{3+ 2}F_7/2 → ²F_5/2 at 980 nm in Ni²⁺/Yb³⁺ codoped transparent wide bandgap semiconductor γ-Ga₂O₃ glass ceramics, which can be assigned to the Ni^{2+ 1}T₂(¹D) → ³A₂(³F) transition. Lifetime measurement and upconversion power dependence data reveal energy transfer upconversion as the underlying upconversion mechanism for the Yb³⁺-Ni²⁺ systems incorporated into the γ- Ga₂O₃ nanocrystals. It is suggested that the low thermal quenching effect of the wide bandgap semiconductor γ-Ga₂O₃ and resonant sensitizing of the Yb^{3+ 2}F_5/2 state to the Ni²⁺ upconversion ³T₂(³F) intermedia state are responsible for the achievement of roomtemperature upconversion luminescence of Ni²⁺. The results demonstrate that wide bandgap semiconductor nanocrystal (γ-Ga₂O₃, TiO₂, SnO₂, et al.) glass ceramics may be a good candidate for hosting Ni²⁺ room-temperature upconversion luminescence.