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

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.