Influence of the grain sizes on Stokes and anti-Stokes fluorescence in the Yb3+ and Tb3+ ions co-doped nanocrystalline fluorapatite

In response to the need of new materials characterized by the spectroscopic properties in the near-infrared window in biological tissues (700–1100 nm), a new type of fluorapatites co-doped with Yb3+ and Tb3+ ions has been synthesized. The nanostructured fluorapatites were prepared using the so-called co-precipitation method and heat-treated in a broad temperature range (600–1100 °C). The structural and morphological properties of the obtained materials were determined by using XRPD (X-ray powder diffraction), TEM (transmission electron microscopy) and SEM (scanning electron microscopy) techniques. The crystallites size was verified and calculated by applying the Rietveld refinement. The primary size of the particles was estimated by TEM analysis and was found to be in the range from 30 nm × 15 nm for the sample annealed at 700 °C to 120 nm × 90 nm for the sample annealed at 1000 °C. Moreover, the formation of defects involving the doping process and their contribution to the charge compensation mechanism has also been discussed. Furthermore, the spectroscopic properties of the obtained materials have been investigated in detail with emission spectra, power dependence relations and luminescence kinetics. The co-doped fluorapatites demonstrated not only Stokes, but also anti-Stokes green luminescence (up-conversion (UC)) depending on the laser power. It was also found that there was weak emission from 5D3 energy level under UV irradiation, not detected under NIR excitation. The cooperative energy transfer (CET) was suggested as the main process responsible for the UC luminescence. •A new type of fluorapatite co-doped with Yb3+/Tb3+ ions has been successfully synthesized.•The fluorapatite was prepared using the so-called co-precipitation method.•The obtained materials demonstrated not only Stokes, but also anti-Stokes green luminescence.•The obtained fluorapatites could be used in theranostic application.