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In the last decade, many efforts have been devoted to the development of smart multifunctional materials. Among them, inorganic nanostructures have gained importance because of their outstanding luminescence properties and their potential applications as new building block materials for the next generation electronics and in a variety of lighting applications. In particular, many recent researches have been addressed on the achievement of progress in the synthesis of nanosized metal oxides. In this field, a significant attention is paid to hafnium dioxide (hafnia or HfO2), which can be employed in optical protective and thermal barrier coatings. Thanks to its mechanical resistance, hafnia finds applications as ceramic, super hard materials and catalysts, or as component in gas sensors and fuel cell electrolytes. Lastly, HfO2 is now evaluated as potential alternative gate dielectric to replace SiO2 in the future generation of electronic nanodevices. Regarding the scintillating properties, the high atomic number Z=72 and the quite high density (9.6 g cm-3) make HfO2 nanocrystals good hosts for phosphor and scintillating applications where a large stopping power for ionizing radiation (X-rays, γ-rays) is required (1). Bulk hafnia is very difficult to grow due to its very high melting point (2774 °C). Actually, HfO2 can be synthetized also in the nanocrystals form to fabricate thin films, optical ceramics and nanocomposite materials (2). Indeed, besides the well-known luminescence of HfO2 nanocrystals activated by the incorporation of rare earth ions, some recent studies evidenced the occurrence of a blue fluorescence from undoped nanocrystals upon UV excitation; remarkably, a broad bluish luminescence appears upon X-ray illumination (RL). These findings suggest the potential of HfO2 nanocrystals as radiation detectors, but the lack of a detailed model that relates their structural and RL properties still hinders the development of efficient nanoscintillators with optimized structure and chemical composition. We studied the RL features of undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale in order to elucidate their origin. Upon X-ray irradiation, the nanocrystals show six emission bands in the near UV/visible spectral range, detectable between 10 K and 300 K. The visible luminescence bands at 2.2 eV, 2.5 eV and 2.8 eV are similar to those detected in our previous PL studies (3), while the UV emission at 4.2 eV and 4.6 eV have been observed for the first time. The excitonic behavior of the UV luminescence is evidenced. The strong increase of the 2.5 eV blue luminescence in annealed samples is likely related to the presence of titanium and it might be used for the design of highly efficient blue scintillating materials. 1. LeLuyer C. et al. (2008), HfO2:X (X = Eu3+, Ce3+, Y3+) Sol Gel Powders for Ultradense Scintillating Materials. J. Phys. Chem. A, 112, pp. 10152-10155. 2. Lange S. et al. (2006), Luminescence of Re-Ions in HfO2 Thin Films and Some Possible Applications. Opt. Mater., 28, pp. 1238-1242. 3. Villa I. et al. (2016), Size-Dependent Luminescence in HfO2 Nanocrystals: Toward White Emission from Intrinsic Surface Defects. Chem. Mater., 28(10), pp. 3245–3253.

Luminescent nanoparticles are researched for their potential impact in medical science, but no materials approved for parenteral use have been available so far. To overcome this issue, we demonstrate that Eu3+-doped hafnium dioxide nanocrystals can be used as non-toxic, highly stable probes for cellular optical imaging and as radiosensitive materials for clinical treatment. Furthermore, viability and biocompatibility tests on artificially stressed cell cultures reveal their ability to buffer reactive oxygen species, proposing an anti-cytotoxic feature interesting for biomedical applications.

International audience; The recent trends in scintillator technologies stimulate research efforts toward the development of novel materials morphologies, such as nanoparticles, able to efficiently convert ionizing radiations into light. For example, scintillating nanoparticles attract great interest in medical oncological therapies. In this work, the structural and morphological properties of HfO2:Ti nanoparticles with Ti concentrations from 0.03 to 10 mol% and subjected to calcination up to 1000 °C are thoroughly characterized; moreover, X‐ray photoelectron spectroscopy reveals the incorporation of Ti in both Ti (III) and Ti (IV) chemical states in as prepared samples, while the exclusive presence of Ti(IV) is unambiguously identified in calcined nanoparticles. The optical emission under X‐ray excitation evidences an intense Ti (IV)‐related luminescence at 2.5 eV in high temperature calcined samples with a few microseconds scintillation lifetime, and efficiency comparable to that of Bi4Ge3O12 reference scintillator. Finally, the competitive role of defects in charge carriers capture is demonstrated by the monotonic increase of the 2.5 eV band during prolonged X‐ray irradiation, more evident for nanoparticles with titanium concentration below 1 mol%. HfO2:Ti may also find application in X‐ray triggered oncological therapies by using the Ti (IV)‐related bright radioluminescence to excite photosensitizer molecules for singlet oxygen generation.

Defect engineering operated on metal oxides by chemical and structural modifications may strongly affect properties suitable for various applications such as photoelectrochemical behavior, charge transport, and luminescence. In this work, we report the tunable optical features observed in undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale. Transmission electron microscopy together with X-ray diffraction and surface area measurements were used to determine the fine structural modifications, in terms of crystal growth and coalescence of crystalline domains, occurring during a calcination process in the temperature range from 400 to 1000 °C. The fit of the broad optical emission into spectral components, together with time-resolved photoluminescence, allowed us to identify the dual nature of the emission at 2.5 eV, where an ultrafast defect-related intrinsic luminescence (with a decay time of a few nanoseconds) overlaps with a slower emission (decay of several microseconds) due to extrinsic Ti-impurity centers. Moreover, the evolution of intrinsic visible bands during the material transformation was monitored. The relationship between structural parameters uniquely occurring in nanosized materials and the optical properties was investigated and tentatively modeled. The blue emissions at 2.5 and 2.9 eV are clearly related to defects lying at crystal boundaries, while an unprecedented emission at 2.1 eV enables, at relatively low calcination temperatures, the white luminescence of HfO2 under near-UV excitation.

International audience; In this work, we investigate the optical properties of hafnium dioxide nanocrystals, upon X-ray irradiation, looking for spectral evolution following thermal treatments in air up to 1000 °C that modify the crystal size as well as their point defect concentrations. Radio-luminescence measurements from 10 K up to room temperature reveal a rich and evolving picture of the optical features. A complete spectral analysis of the broad luminescence spectra reveals the presence of several emission components in the visible and UV regions. The lower energy components peaking at 2.1, 2.5, and 2.9 eV are characterized by a thermal quenching energy of 0.08 eV, while the corresponding value for the UV bands at 4.1 and 4.7 eV is close to 0.23 eV. We tentatively assign the components ranging from 2 to 3 eV to the presence of optically active defects of an intrinsic nature, together with the occurrence of titanium impurities; conversely, the bands at higher energies are likely to be of an excitonic nature. The comparison with previous photo-luminescence studies allows evidencing characteristic differences between the features of luminescence emissions caused by intra-centre excitation and those occurring under ionizing irradiation. Finally, scintillation measurements in the visible range reveal the existence of a fast decay in the nanosecond time scale for the smallest hafnia nanocrystals. This study offers a clear description of HfO2 luminescence characteristics upon excitation by X-rays and can lead to a better comprehension of the structure–property relationship at the nanoscale in metal oxides.

In this work a strategy for the control of structure and optical properties of inorganic luminescent oxide-based nanoparticles is presented. The nonaqueous sol-gel route is found to be suitable for the synthesis of hafnia nanoparticles and their doping with rare earths (RE) ions, which gives rise to their luminescence either under UV and X-ray irradiation. Moreover, we have revealed the capability of the technique to achieve the low-temperature stabilization of the cubic phase through the effective incorporation of trivalent RE ions into the crystal lattice. Particular attention has been paid to doping with europium, causing a red luminescence, and with lutetium. Structure and morphology characterization by XRD, TEM/SEM, elemental analysis, and Raman/IR vibrational spectroscopies have confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations exceeding a threshold value of nominal 5 mol %, for either Lu3+ or Eu3+. The optical properties of the nanopowders were investigated by room temperature radio- and photoluminescence experiments. Specific features of Eu3+ luminescence sensitive to the local crystal field were employed for probing the lattice modifications at the atomic scale. Moreover, we detected an intrinsic blue emission, allowing for a luminescence color switch depending on excitation wavelength in the UV region. We also demonstrate the possibility of changing the emission spectrum by multiple RE doping in minor concentration, while deputing the cubic phase stabilization to a larger concentration of optically inactive Lu3+ ions. The peculiar properties arising from the solvothermal nonaqueous synthesis here used are described through the comparison with thermally treated powders. © 2013 American Chemical Society.