The mixed ionic–covalent nature of the Hf–O bonds in HfO 2 polymorphs contribute to a high Born effective charge tensor Z*, and the combination of large Z* with the presence of soft low-energy phonon modes yield large dielectric constants 6. Increasing the dielectric constant of gate oxides is imperative for continued reduction of the gate thickness, without deleterious leakage currents typical of ultrathin SiO 2 when it is reduced to the nanometre scale. The continued miniaturization of device components such as metal oxide semiconductor field-effect transistors 1, 2, 3, 4 has inspired a major push to replace silicon dioxide with high- κ dielectrics as the gate material 1, 2, 3, 4, 5. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation the nanorod is reduced to hafnium on cooling. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO 2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000☌ from bulk. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO 2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO 2. High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge.
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