Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary-Condition Engineering

Lin Xie; Linze Li; Colin A Heikes; Yi Zhang; Zijian Hong; Peng Gao; Christopher T Nelson; Fei Xue; Emmanouil Kioupakis; Longqing Chen; Darrel G Schlom; Peng Wang; Xiaoqing Pan

doi:10.1002/adma.201701475

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary-Condition Engineering

Adv Mater. 2017 Aug;29(30). doi: 10.1002/adma.201701475. Epub 2017 Jun 6.

Authors

Lin Xie^{1

2}, Linze Li¹, Colin A Heikes³, Yi Zhang¹, Zijian Hong⁴, Peng Gao⁵, Christopher T Nelson⁵, Fei Xue⁴, Emmanouil Kioupakis⁵, Longqing Chen⁴, Darrel G Schlom³, Peng Wang², Xiaoqing Pan¹

Affiliations

¹ Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, USA.
² National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu, 210093, P. R. China.
³ Department of Materials Science and Engineering, Cornell University, Ithaca, NY, 14850, USA.
⁴ Department of Materials Science and Engineering, Pennsylvania State University, State College, PA, 16802, USA.
⁵ Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48105, USA.

PMID: 28585347
DOI: 10.1002/adma.201701475

Abstract

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO₃ ferroelectric films is reported. Using aberration-corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm-thick ultrathin BiFeO₃ film is abnormally increased up to ≈90-100 µC cm^-2 in the out-of-plane direction and a peculiar rumpled nanodomain structure with very large variation in c/a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase-field calculations, it is shown that it is the unique single atomic Bi₂ O₃_-_x layer at the surface that leads to the enhanced polarization and appearance of the MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.

Keywords: polarization; scanning transmission electron microscopy; surface effect; ultrathin ferroelectric films.