Up-Conversion Emissions from HfO 2 : Er 3+ , Yb 3+ Nanoparticles Synthesized by the Hydrothermal Method.

Up-conversion emission from HfO 2 nanoparticles, as a host lattice, doped with Er 3+ and Yb 3+ ions and codoped with alkaline cations Li + and Na + obtained. The HfO 2 nanoparticles, about 80 nm in diameter, were synthesized by the hydrothermal method at 200 °C for 1.3 h, and an additional heat treatment at 1000 °C was necessary to ensure the dopants incorporation into the host lattice. These nanoparticles were studied by means of XRD, Raman Spectroscopy, SEM, EDS, PL, CL, and up-conversion luminescence. First, the doping was performed with Er 3+ ions in different percentages. The photoluminescence and cathodoluminescence studies showed an inefficient emission, and only at 7 at % Er 3+ ions, the sample presented emissions at 522, 545, and 656 nm corresponding to the transitions of the Er 3+ ions. So, codoping was carried out, and HfO 2 : Er 3+ /Yb 3+ generated an efficient conversion process. The atom percentage of Yb 3+ ions was fixed (7 at % Yb 3+ ), and the Er 3+ content was varied, showing the highest emission intensity at 3 at % Er 3+ ions. Subsequently, the up-conversion emission intensity was optimized by varying the percentage of Yb 3+ ions and keeping the Er 3+ ion content fixed (3 at %). Adding cations such as Na + and Li + in different percentages, a notable improvement of the up-conversion emission intensities in the HfO 2 : Er 3+ /Yb 3+ nanoparticles was obtained. The up-conversion emission bands observed were located at ∼523 and 544 nm, corresponding to the electronic transitions 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 , respectively. While the bands at ∼652 and 673 nm correspond to the transition 4 F 9/2 → 4 I 15/2 , respectively. The excitation of these materials with infrared radiation (980 nm) produced noticeable emission bands in the red spectral range, whereas excitation with accelerated electrons (CL) generated prominent bands in the green region.