Enhancing upconversion luminescence and thermal sensing properties of Er/Yb co-doped oxysulfide core-shell nanocrystals

The development of noncontact thermal probe based on stable inorganic materials of trivalent lanthanide (Ln³⁺) doped phosphors with nontoxicity is of vital importance for their promising applications in bio-medical fields. Here we explore the upconversion luminescence and thermal sensing properties of Er³⁺, Yb³⁺ co-doped oxysulfide in a broad temperature range of 300-583 K. It was found that constructing an active shell with an optimum concentration of sensitizers is an efficient way to improve both the luminescent intensities and thermal sensitivity. Compared with the core-only sample, the luminescent intensity of the Y₂O₂S: Er³⁺, Yb³⁺@ Y₂O₂S: 5%Yb³⁺ sample is significantly enhanced by 12-fold at excitation of 980 nm. While further increasing the Yb³⁺ concentration in the shell activates new quenching pathways of Er³⁺ → Yb³⁺ → quencher from the core to the shell. Similar quenching mechanisms are also observed at excitation of 1550 nm. These energy transfer processes and luminescence mechanisms are verified in the fluorescence decay measurements. Furthermore, coating the core sample with an active shell doped by 10% Yb³⁺ enhances the thermal sensitivity by 30%, holding a high and stable sensitivity more than 50 × 10⁻⁴ K⁻¹ in a broad temperature range of 423-573 K at 980 nm excitation. In addition, at the much safer excitation wavelength of 1550 nm, this sample achieved the maximum sensitivity of 45 × 10⁻⁴ K⁻¹ at 503 K. Our work contributes a feasible and versatile way to promote the luminescence and thermal sensing properties of Ln³⁺-based materials, combining with the nontoxic oxysulfide host, indicating their potential applications as safe fluorescent and temperature nano-probes in bio-field.