Size-Dependent Luminescence in HfO₂ Nanocrystals: Toward White Emission from Intrinsic Surface Defects

Abstract

Defect engineering operated on metal oxides by chemical and structural modifications may strongly affect properties suitable for various applications such as photoelectrochemical behaviour, charge transport and luminescence. In this work we report the tuneable optical features observed in undoped monoclinic HfO 2 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 decay time of few ns) overlaps with a slower emission (decay of several µs) due to extrinsic Ti -impurity centres. 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 modelled. 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 HfO 2 under near-UV excitation.

The Bright X‐Ray Stimulated Luminescence of HfO2 Nanocrystals Activated by Ti Ions

“…These observations well agree with previous results, where the broad luminescence of undoped HfO 2 NPs, treated at low temperature, has revealed an intrinsic origin both under near UV excitation and ionizing radiation . Spectroscopic investigations and computational calculations by DFT have evidenced that oxygen vacancies play a major role as defect centers responsible for the optical emissions in hafnia.…”

“…After the reaction between Hf t‐butoxide and Ti (III) chloride in benzyl alcohol, crystalline monoclinic NPs were obtained for different doping levels (Figure a–c,f) and calcination treatment (Figure d,e,h). The elemental analysis in Figure g of pressed powder samples, confirms a good agreement of the experimentally measured concentrations of Ti with the nominal ones; the slightly higher values of the experimental data with respect to the nominal ones might be ascribed to an additional content of Ti ions deriving from unintentional impurities (up to hundreds of ppm) in the Hf precursor itself, as already reported for similar materials . No additional segregated phases, such as titanium oxide are visible, regardless of the Ti nominal content, even after calcination, confirming that Ti ions are incorporated in the HfO 2 lattice in all samples (Figure S1, Supporting Information).…”

“…The HfO 2 crystals average size, calculated by the Scherrer formula on the diffractogram, lies in the 3–8 nm range and is weakly dependent on the dopant concentration both in the as‐synthesized as well as in the Ti doped NPs calcined at 500 °C (Figure i, blue circles and green triangles, respectively). At this temperature the decomposition of the organic ligand shell bound to the NPs surface is usually observed . Figure d,e shows representative TEM images of 0.3 mol% Ti doped HfO 2 powders after calcination at 500 and 1000 °C, respectively.…”

“…The observed clustering of the NPs (Figure d), starting at a temperature of 500 °C, is due to severe reconfiguration of the material at the nanoscale. The degradation of the organic ligands on the surface certainly contributes to the agglomeration of the NPs …”

“…For instance, defect‐engineering strategies can be applied in wide bandgap nanomaterials. The tuning of their intrinsic luminescence properties, which are mostly due to optically active intrinsic defects and self‐trapped excitons, results from chemical and structural modifications, surface restructuring, and lattice reconfiguration (i.e., crystal growth, sintering, and rearrangement of lattice defects) . In turn, other studies have revealed the possibility to modify the materials emission color through doping routes.…”