High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. 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. 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.
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Funding for this work was provided by NASA Kentucky under NASA Award No: NNX10AL96H (to B.M.H.), by the National Science Foundation under DMR 1504702 (to S.W.D., G.R.W. and S.B.), DMR 1455154 (to B.S.G.) and CMMI 1534534 (to R.A. and A.T.), and by the Air Force Office of Scientific Research under AFOSR-FA9550-16-1-0180 (to R.A. and A.T.).
The data that support the findings of this study are available from the corresponding authors (B.G. and S.B.) upon request.
Hudak, Bethany M.; Depner, Sean W.; Waetzig, Gregory R.; Talapatra, Anjana; Arroyave, Raymundo; Banerjee, Sarbajit; and Guiton, Beth S., "Real-time Atomistic Observation of Structural Phase Transformations in Individual Hafnia Nanorods" (2017). Chemistry Faculty Publications. 76.
Supplementary figures, supplementary tables, supplementary methods, and supplementary references
ncomms15316-s2.mov (4599 kB)
Video of the HfO2 nanorod shown in Figure 3 annealed at 600 °C for 85 mins in the STEM. Phase transformation from monoclinic to tetragonal HfO2 upon heating is visible
ncomms15316-s3.mov (224 kB)
Video of the HfO2 nanorod shown in Figure 3 cooled from 600 °C to 466 °C over 164 mins. Reduction of the nanorod from hafnia to hafnium metal upon cooling is visible