Raman and Er3+ spectroscopy of hafnia single crystals and nanocrystals

In this work, we report on the spectroscopic characterization of erbium doped HfO2 nanocrystals in SiO2-HfO2 glass-ceramic waveguides, fabricated by sol-gel process. The results are compared with those of single crystals grown using the flux growth technique. XRD and Raman measurements showed that the single crystals are monoclinic, while in the glass-ceramic waveguides, depending on the heat treatment, the nanocrystals are present in both tetragonal and monoclinic phases. The size distribution of nucleated nanocrystals was studied by transmission electron microscopy. The Raman peaks of the nanocrystals are broadened and shifted with respect to those observed in the bulk samples. The luminescence measurements in monoclinic nanocrystals and bulk single crystals of the same structure are quite dissimilar, evidencing that the rare earth ions are subjected to a different crystal field