Response to “Comment on ‘Unusual Photoluminescence of CaHfO3 and SrHfO3 Nanoparticles’”

Abstract

The comment from Clavel et al. on our paper entitled “Unusual photoluminescence of CaHfO3 and SrHfO3 nanoparticles” raises questions on the synthesis, the photoluminescence, and the structure of these perovskite nanoparticles. The comment is based on several references used as support; most of which are self-citations. It surprisingly seems to miss the key point of our report, i.e., no luminescence from regular hafnia perovskite nanoparticles is expected under the experimental conditions described in our work, as there are no energy levels available for any radiative processes in contrast to many lanthanide and rare earth (RE) compounds. This is the reasoning behind entitling the observed strong visible luminescence as “unusual” and suggesting a surface-related effect as its plausible origin. We find the comment rather dubious, and at times unrealistic and inconsistent. More specifically, the authors open their comment by claiming that the photoluminescence (PL) studied in our paper is not a surface-related effect. However, in their conclusion, they self-contradictorily suggest that the PL observed in our study is related to benzoate species adsorbed on the surface of the nanoparticles, which in our opinion is a surface related effect. If the authors had contacted us directly, we would have been equally glad to answer their concerns. With respect to the perovskite phase: The structure of these hafnia perovskites was studied using Cs corrected TEM and synchrotron measurements. An X-ray diffraction (XRD) pattern of CaHfO3 sample fitted with Rietveld refinement (Figure 1a) highlights the XRD peaks of the perovskite structure and provides sizes <2 nm for the perovskite particles (smallest perovskites ever synthesized in the literature) and 6 nm for the HfO2. A size decrease induces peak broadening for the perovskite phase; the secondary cubic HfO2 phase (nanoparticles of average size ≈5–6 nm) then becomes more prominent on the XRD pattern. This explains why, the authors had incorrectly considered the XRD pattern to correspond to the HfO2 cubic phase. The authors have expressed disappointment in not seeing the XRD pattern of SrHfO3 in our paper; we have therefore included a synchrotron XRD pattern, in Figure 1b, which validates the