1 research outputs found
Size-Dependent Luminescence in HfO<sub>2</sub> 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 HfO<sub>2</sub> 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 HfO<sub>2</sub> under near-UV excitation