  Hafnia-based nanostructures and other high-k dielectrics are promising
wide-gap materials for developing new opto- and nanoelectronics devices. They
possess a unique combination of physical and chemical properties such as
insensitivity to electrical and optical degradation, radiation damage
stability, a high specific surface area, and an increased concentration of the
appropriate active electron-hole centers. The present paper aims to investigate
the structural, optical, and luminescent properties of anodized
non-stoichiometric $HfO_2$ nanotubes. As-grown amorphous hafnia nanotubes and
nanotubes annealed at 700{\deg}C with a monoclinic crystal lattice served as
samples. It has been shown that the bandgap $E_g$ for direct allowed
transitions amounts to $5.65\pm0.05$ eV for amorphous and $5.51\pm0.05$ eV for
monoclinic nanotubes. For the first time, we have studied the features of the
intrinsic cathodoluminescence and photoluminescence of the obtained nanotubular
$HfO_2$ structures with an atomic deficiency in the anion sublattice at
temperatures of 10 and 300 K. A broad emission band with a maximum of 2.3-2.4
eV has been revealed. We have also conducted an analysis of the kinetic
dependencies of the observed photoluminescence for synthesized $HfO_2$ samples
in the millisecond range at room temperature. It showed that there are several
types of optically active capture and emission centers based on vacancy states
in the $O_{3f}$ and $O_{4f}$ positions with different coordination numbers and
a varied number of localized charge carriers ($V^0$, $V^-$, and $V^{2-}$). The
uncovered regularities can be used to optimize the functional characteristics
of developed-surface luminescent media based on nanotubular and nanoporous
modifications of hafnia.
