Size-dependent polymorphism in HfO₂ nanotubes and nanoscale thin films

Many metal oxides exhibit size-dependent phase transitions among multiple polymorphs. In this work, the microstructure and crystallinity of ultrathin HfO₂ films and utrathin-wall nanotubes were investigated by high-resolution electron microscopy and electron diffraction after high-temperature annealing. Nanotubes were formed by atomic layer deposition of HfO₂ on epitaxial Ge 〈 111 〉 nanowire arrays on Si (111) substrates followed by selective etching of the Ge wires. A size-dependent phase transition sequence from amorphous (a ?HfO₂) to tetragonal (t ?HfO₂) and from tetragonal to monoclinic (m ?HfO₂) phase was observed with increasing film and nanotube wall thickness. These results are analyzed in light of recent predictions of surface energy-driven phase transitions in nanoscale fluorite-structure oxides.