Fluoride-upconversion transparent ceramic materials doped with Er ions have attracted extensive interest due to their higher stability, lower phonon energy, and higher upconversion efficiency. xEr:Sr 5 (PO 4 ) 3 F (S-FAP) (x=0.5–5 at%) transparent ceramics have been synthesized by hot pressing sintering in this work, and the maximum optical transmittance of the sample reached 74.52 and 82.45% at 500 and 1000 nm, respectively. The XRD results of the powder and ceramic samples were consistent with the diffraction characteristics of the hexagonal S-FAP crystal structure. The SEM and TEM images of powder revealed that the morphology of the sample was composed of mostly rice grains with a diameter of around 20 nm. The XPS and EDS results confirmed the presence of Er 3+ and the uniform distribution of Er elements. The thermally etched surface and cross section of ceramics revealed residual pores as the most important scattering source, and the SEM images of the ceramics show that Er doping can obviously promote the growth of the average grain size of S-FAP transparent ceramics. Furthermore, the upconversion luminescence characteristics and fluorescence lifespan of S-FAP transparent ceramics with varied Er doping concentrations were examined, as well as the upconversion luminescence process. The findings in this work indicate that Er:S-FAP transparent ceramics can be employed as a potential upconversion fluorescent material in display technology, biomedicine, temperature sensing, and other applications.
Optical temperature sensing based on the fluorescence intensity ratio (FIR) of red emission for lanthanide ions holds significant relevance in non-contact temperature measurement for biological application. In this study, the perovskite-structured KZnF 3 is utilized as a host material for Er 3+ to achieve a high-purity upconversion (UC) red emission. The observed Stark splitting of the red emission peak provides evidence of the energy level splitting of Er 3+ . Group theory is employed to decompose the spectral branching of Er 3+ under the point group symmetry of KZnF 3 , allowing for the derivation of Stark splitting energy levels induced by the crystal field effect. The optical temperature-sensing behavior of the red UC luminescence was investigated, specifically examining the FIR of the splitting sub-peaks, which exhibited an exponential relationship with temperature. The KZnF 3 : Yb 3+ , Er 3+ demonstrated a relative sensitivity ( S r ) of 0.00182%·K −1 at 298 K, highlighting its excellent response to temperature. Ex vivo bio-thermometry experiments conducted on chicken breast validated the material's ability to penetrate biological tissues and showed its significant sensitivity of the FIR to temperature. These results establish KZnF 3 :Yb 3+ , Er 3+ as a promising material for optical thermometry in various biological applications. Graphic abstract
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