Browsing by Author "Li, Mingqiang"

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Atomic-scale mechanism of internal structural relaxation screening at polar interfaces
(2018-05-25)
Li, Mingqiang
;
Cheng, Xiaoxing
;
Li, Ning
;
Liu, Heng-Jui
;
Huang, Yen-Lin
The effective screening of the polarization bound charge is a prerequisite to stabilize the ferroelectricity in ferroelectric thin films. Here, by combining annular bright field imaging and electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope with phase-field simulations, we investigate the screening mechanism by quantitatively measuring the structural relaxation at Pb(Zr0.2Ti0.8)O-3/SrTiO3 interfaces. We find that the thickness of the interfacial layer is similar to 3.5 unit cells (similar to 1.4 nm) in a domain with upward polarization and similar to 5.5 unit cells (similar to 2.2 nm) in a domain with downward polarization. Phase-field simulations, an EELS analysis, and a lattice parameter analysis verify the existence of interfacial oxygen vacancies accounting for the narrower interfacial layer in the domain with upward polarization. Our study indicates the internal structural relaxation at the interface is the dominant mechanism for the polarization charge screening for ferroelectric films grown on insulating substrates and has implications for our understanding of domain switching in ferroelectric devices.
Direct observation of weakened interface clamping effect enabled ferroelastic domain switching
(2019-06-01)
Li, Mingqiang
;
Wang, Bo
;
Liu, Heng-Jui
;
Huang, Yen-Lin
;
Zhang, Jingmin
Reversible switching of non-180 degrees ferroelastic domains that largely alters the local strain distribution enables many electromechanical, electromagnetic and electroacoustic applications. However, in thin films, the ferroelastic domain walls are usually believed to be immobile because of the interface clamping and/or dislocation pinning. Here, using in situ and aberration-corrected transmission electron microscopy, we directly observe reversible switching of individual 90 degrees domains in dislocation-free PbTiO3 thin films and uncover the weakened interface clamping effect. We find the tetragonality is suppressed to similar to 1.017 while the polarization vectors rotate 45 degrees in the a-domain near the interface. These huge structural distortions at the interface is mainly responsible for the weakened clamping effect and thus the ability to switch ferroelastic domains. The switching is fully reversible (i.e., either electric field or mechanical stress can re-establish the erased domain) regardless of polarization orientation of the c-domain matrix. Phase-field modeling also shows excellent agreement with experimental observations. Our study reveals the mechanism of controllable and reversible ferroelastic domain switching, enabling the design of new actuators, sensors, and electromagnetic devices. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Possible absence of critical thickness and size effect in ultrathin perovskite ferroelectric films
(2017-06-06)
Gao, Peng
;
Zhang, Zhangyuan
;
Li, Mingqiang
;
Ishikawa, Ryo
;
Feng, Bin
Although the size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate. Here, we directly measure the thickness-dependent polarization in ultrathin PbZr0.2Ti0.8O3 films via quantitative annular bright field imaging. We find that the polarization is significantly suppressed for films <10-unit cells thick (similar to 4 nm). However, approximately the polarization never vanishes. The residual polarization is similar to 16 mu Ccm(-2) (similar to 17%) at 1.5-unit cells (similar to 0.6 nm) thick film on bare SrTiO3 and similar to 22 mu Ccm(-2) at 2-unit cells thick film on SrTiO3 with SrRuO3 electrode. The residual polarization in these ultrathin films is mainly attributed to the robust covalent Pb-O bond. Our atomic study provides new insights into mechanistic understanding of nanoscale ferroelectricity and the size effects.