Engineering dual-mode pure green emission in cerium oxide-based phosphors for optical thermometry applications

In this work, selective enhancements in green upconversion (UC) and downconversion (DC) emissions are achieved in Ho³⁺/Yb³⁺/Mo⁶⁺ tri-doped CeO₂ cubic phosphors exhibiting good optical thermometry performance. Rietveld refinement analysis confirmed the formation of a highly crystalline single-phase cubic CeO₂ structure. X-ray photoelectron spectroscopy further verified the valence states of both dopant ions and host-lattice elements. Under excitations at 342 nm and 975 nm, the phosphors exhibit intense green DC and UC emissions centered at 540/550 nm, respectively, with emission intensity significantly dependent on Mo content. The Mo-doped DC samples showed enhanced green emissions without obvious concentration quenching, which can be attributed to the energy transfer from the charge-transfer band (O^2--Mo⁶⁺) to Ho³⁺. In contrast, the Mo-doped UC samples exhibited an optimal emission intensity at 3 mol%, which is approximately three times higher than that of the undoped sample. This enhancement is associated with the formation of oxygen vacancies that act as energy-transfer centers, together with improved crystallinity induced by Mo incorporation. These effects are further supported by luminescence decay kinetics analysis. Furthermore, the UC phosphor demonstrated excellent optical thermometry performance, with a maximum relative sensitivity of 1.24% K^-1 and an absolute sensitivity of 7.2% K^-1. Overall, the introduction of Mo ions into the CeO₂:Ho³⁺/Yb³⁺ phosphors enabled intense green dual-mode emissions, high color purity, and superior thermal sensitivity, making these phosphors promising candidates for optical thermometry and optoelectronic applications.