Multistep synthesis and upconversion luminescence of spherical Gd₂O₃:Er and Gd₂O₃:Er @ silica

Spherical Gd₂O₃:Er³⁺ and Gd₂O₃:Er³⁺ @ silica nanoparticles have been successfully synthesized by a multistep procedure including precipitation and sol–gel processes from rare-earth nitrates, TEOS and urea as starting precursors. The structure, morphology and chemical composition of the products were characterized by X-ray diffraction, field emission scanning electron microscopy, and fourier transform infrared spectroscopy. The results indicated that the obtained phosphors consist of separated, non-agglomerated spheres in nanoscale with an uniform, ideal spherical shape. The suitable amount of TEOS was addressed to obtain the spherical Gd₂O₃:Er³⁺ @ silica nanocomposites with the luminescence intensity almost unchanged with respect to the uncoated Gd₂O₃:Er³⁺ sample. Besides, the upconversion emission spectra showed intense green and red emission bands under 980-nm laser diode excitation with remarkable increase in the red emissions. The upconversion luminescence followed the two-photon mechanism for green and red emissions. With obtained results, the prepared nanospheres are expected to be a potential material to create functional groups for subsequent bio-conjugation with various biomolecules in medical diagnostics.