Journal
ACS NANO
Volume 12, Issue 4, Pages 3681-3688Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.8b00862
Keywords
ferroelectric material; interface coupling; ultrathin PbTiO3 film; polarization rotation; aberration-corrected scanning transmission electron microscopy
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Funding
- National Natural Science Foundation of China [51571197, 51501194, 51671194, 51401212, 11627801]
- National Basic Research Program of China [2014CB921002]
- Key Research Program of Frontier Sciences CAS [QYZDJ-SSW-JSC010]
- Youth Innovation Promotion Association CAS [2016177]
- National Key Research Program of China [2016YFA0201001]
- IMR SYNL-T.S. Ke Research Fellowship [2016177]
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Multiple polar states and giant piezoelectric responses could be driven by polarization rotation in ferroelectric films, which have potential functionalities in modern material applications. Although theoretical calculations have predicted polarization rotation in pure PbTiO3 films without domain walls and strains, direct experiment has rarely confirmed such polar states under this condition. Here, we observed that interfacial oxygen octahedral coupling (OOC) can introduce an oxygen octahedral rotation, which induces polarization rotation in single domain PbTiO3 films with negligible strains. We have grown ultrathin PbTiO3 films (3.2 nm) on both SrTiO3 and Nb:SrTiO3 substrates and applied aberration-corrected scanning transmission electron microscopy (STEM) to study the interfacial OOC effect. Atomic mappings unit cell by unit cell demonstrate that polarization rotation occurs in PbTiO3 films on both substrates. The distortion of oxygen octahedra in PbTiO3 is proven by annular bright-field STEM. The critical thickness for this polarization rotation is about 4 nm (10 unit cells), above which polarization rotation disappears. First-principles calculations manifest that the interfacial OOC is responsible for the polarization rotation state. These results may shed light on further understanding the polarization behavior in ultrathin ferroelectrics and be helpful to develop relevant devices as polarization rotation is known to be closely related to superior electromechanical responses.
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