Structural and photoluminescence properties of Sm and Dy codoped HfO₂ nanophosphors: role of intersystem energy transfer

Herein, we report a detailed investigation on structural and photoluminescence properties of Sm and Dy codoped HfO₂ synthesized by a Pechini type sol-gel method followed by calcination at 900 °C in air. X-ray diffraction and Le-Bail profile refinement revealed that Sm and Dy codoped HfO₂ exhibits monoclinic phase with space group, P21/c. After incorporating Sm and Dy, the crystallite size of HfO₂ decreased substantially from ~25 to ~15 nm resulting in an exponential increase of lattice volume. The inter-planar lattice spacing (d) calculated from high resolution transmission electron microscopy (TEM) micrograph and selected area electron diffraction (SAED) pattern further endorse the formation of monoclinic phase of HfO₂. Photoluminescence spectra have shown that after exciting at 393 nm, Sm and Dy codoped HfO₂ nanophosphors show strong emissions in blue, yellow and near red spectral regions producing purplish red color light. The emissions from Sm and Dy co activated HfO₂ nanophosphors emerge primarily due to the energy transfer from Dy³⁺ to Sm³⁺ active ions through multipolar interaction confirmed using time resolved decay spectra. On the basis of excitation and emission spectra, we propose a schematic energy band diagram to illustrate different electronic transitions. (author)