Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films

Qi Zhang; Lin Xie; Guangqing Liu; Sergei Prokhorenko; Yousra Nahas; Xiaoqing Pan; Laurent Bellaiche; Alexei Gruverman; Nagarajan Valanoor

doi:10.1002/adma.201702375

Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films

Adv Mater. 2017 Dec;29(46). doi: 10.1002/adma.201702375. Epub 2017 Oct 24.

Authors

Qi Zhang¹, Lin Xie^{2

3}, Guangqing Liu¹, Sergei Prokhorenko^{4

5}, Yousra Nahas⁵, Xiaoqing Pan^{3

6}, Laurent Bellaiche⁵, Alexei Gruverman⁷, Nagarajan Valanoor¹

Affiliations

¹ School of Materials Science and Engineering, The University of New South Wales, Sydney, New South Wales, 2052, Australia.
² National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing, 210093, P. R. China.
³ Department of Chemical Engineering and Materials Science, University of California, Irvine, CA, 92697, USA.
⁴ Theoretical Materials Physics Q-MAT CESAM, University of Liège, Sart Tilman, B-4000, Belgium.
⁵ Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, AR, 72701, USA.
⁶ Department of Physics and Astronomy, University of California, Irvine, CA, 92697, USA.
⁷ Department of Physics and Astronomy, University of Nebraska, Lincoln, NE, 68588, USA.

PMID: 29064154
DOI: 10.1002/adma.201702375

Abstract

Observation of a new type of nanoscale ferroelectric domains, termed as "bubble domains"-laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix-is reported. The bubble domains appear in ultrathin epitaxial PbZr_0.2 Ti_0.8 O₃ /SrTiO₃ /PbZr_0.2 Ti_0.8 O₃ ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

Keywords: Néel-Bloch domain walls; aberration-corrected scanning transmission electron microscopy; nanoscale bubble domains; piezoresponse force microscopy; ultrathin ferroelectric films.