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Ferroelectric order in individual nanometre-scale crystals

Abstract

Ferroelectricity in finite-dimensional systems continues to arouse interest, motivated by predictions of vortex polarization states and the utility of ferroelectric nanomaterials in memory devices, actuators and other applications. Critical to these areas of research are the nanoscale polarization structure and scaling limit of ferroelectric order, which are determined here in individual nanocrystals comprising a single ferroelectric domain. Maps of ferroelectric structural distortions obtained from aberration-corrected transmission electron microscopy, combined with holographic polarization imaging, indicate the persistence of a linearly ordered and monodomain polarization state at nanometre dimensions. Room-temperature polarization switching is demonstrated down to ~5 nm dimensions. Ferroelectric coherence is facilitated in part by control of particle morphology, which along with electrostatic boundary conditions is found to determine the spatial extent of cooperative ferroelectric distortions. This work points the way to multi-Tbit/in2 memories and provides a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.

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Figure 1: Ferroelectric distortions of the GeTe and BaTiO3 conventional unit cells.
Figure 2: Atomic-resolution reconstructed phase images and polar displacement maps of individual GeTe monocrystalline nanoparticles.
Figure 3: Atomic-resolution reconstructed phase images and titanium displacement maps of individual BaTiO3 monocrystalline nanocubes.
Figure 4: Direct polarization imaging of individual BaTiO3 nanocubes with off-axis electron holography.
Figure 5: Temperature-dependent PFM measurements of individual BaTiO3 nanocubes.
Figure 6: Atomic PDF analysis of GeTe and BaTiO3 nanocrystal ensembles.

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Acknowledgements

The authors would like to thank Shiva Adireddy for the synthesis of the BaTiO3 nanomaterials used in this manuscript; P. Ercius, T. Duden, Y. Ren and A. Gautam for technical assistance and helpful discussions; and H. Park for critical feedback on the manuscript. In addition, the authors gratefully acknowledge M. R. McCartney for providing scripts for the analysis of the holographic images. Access to the electron microscopy facility at the Center for Functional Nanomaterials, Brookhaven National Laboratory, is acknowledged. Work at the National Center for Electron Microscopy was supported by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under contract no. DE-AC02-05CH11231. Electron holography experiments at Brookhaven National Laboratory were supported by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under contract no. DE-AC02-98CH10886 and were carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory. Synchrotron X-ray diffraction measurements at the Advanced Photon Source were supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract DE-AC02-06CH11357. Work on piezoresponse force measurements and synthesis of BaTiO3 nanostructures was supported by the National Science Foundation through grants no. NSF-MSN CAREER-1157300, no. EPS-1003897 and no. NSF-DMR-1004869. All other work was supported by the Physical Chemistry of Nanocrystals Project of the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under contract no. DE-AC02-05CH11231. M.J.P. was supported by a National Science Foundation Graduate Research Fellowship and by a National Science Foundation Integrative Graduate Education and Research Traineeship fellowship.

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M.J.P. conceived the experiment, performed atomic-resolution TEM studies of GeTe and BaTiO3 nanocrystals and analysed the results, participated in holographic imaging experiments with M-G.H. and Y.Z., interpreted data, and wrote the manuscript. A.Y. acquired piezoresponse force data on BaTiO3 nanocubes under the supervision of G.C. Analysis of the PFM data was performed by G.C. and A.Y. V.P. acquired atomic PDF data and analysed the results, and C.F.K. assisted in the analysis of the TEM data. V.V.V performed phase image simulations for the holographic images. R.R. and A.P.A. supervised the work and provided critical feedback on the manuscript.

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Correspondence to A. Paul Alivisatos or Ramamoorthy Ramesh.

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Polking, M., Han, MG., Yourdkhani, A. et al. Ferroelectric order in individual nanometre-scale crystals. Nature Mater 11, 700–709 (2012). https://doi.org/10.1038/nmat3371

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