Hafnium dioxide luminescent nanoparticles: structure and emission control through doping and thermal treatments

High density powder phosphors are of great interest in technological fields like imaging and ionizing radiation detection. The powder form is of choice when the material can hardly be synthesized as bulk single crystal by conventional techniques. This is the case of hafnium oxides having a melting point above 2500 °C. Moreover nanoscale dimensions are an important requirement for fabricating nanocomposites, in nanomedicine, and for the realization of optical ceramics. In this last field materials with cubic structure are foreseen since their isotropic optical response allow the minimization of light scattering at grain interfaces. This work focuses on the synthesis, structural and optical investigation of HfO2 nanoparticles obtained by non-aqueous sol-gel route. In a first investigation, particular attention was paid to doping with europium and with lutetium. Structure and morphology characterization by XRD, TEM/SEM, elemental analysis, and Raman/IR vibrational spectroscopies confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations exceeding a threshold value of nominal 5 mol%, for either Lu3+or Eu3+ [1]. The spectroscopic features of Ti3+ impurities have been recently analyzed by room temperature radio- and photo-luminescence, time resolved luminescence and scintillation experiments. In addition, we have detected an intrinsic blue emission peaking at 2.5 eV and exhibiting a fast photoluminescence decay time of a few nanoseconds. This emission is due to the presence of surface defects; its intensity, as well as that of an additional band peaking at 2.1 eV, can be varied by thermal treatments that lead to surface modifications and variations of particle dimensions. For temperatures between 500 and 650 °C, tuning of the bands intensities induces a white emission under 3.5 eV excitation. The results demonstrate that the control of intrinsic defects is a potential route to design the optical activity of a material at the nanoscale. [1] A. Lauria et al., ACS Nano 7, 7041 (2013).