Browsing by Author "Wang, En-Ge"

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Atomic mechanism of polarization-controlled surface reconstruction in ferroelectric thin films
(2016-04-01)
Gao, Peng
;
Liu, Heng-Jui
;
Huang, Yen-Lin
;
Chu, Ying-Hao
;
Ishikawa, Ryo
At the ferroelectric surface, the broken translational symmetry induced bound charge should significantly alter the local atomic configurations. Experimentally revealing the atomic structure of ferroelectric surface, however, is very challenging due to the strong spatial variety between nano-sized domains, and strong interactions between the polarization and other structural parameters. Here, we study surface structures of Pb(Zr0.2Ti0.8)O-3 thin film by using the annular bright-field imaging. We find that six atomic layers with suppressed polarization and a charged 180 degrees domain wall are at negatively poled surfaces, no reconstruction exists at positively poled surfaces, and seven atomic layers with suppressed polarization and a charged 90 degrees domain wall exist at nominally neutral surfaces in ferroelastic domains. Our results provide critical insights into engineering ferroelectric thin films, fine grain ceramics and surface chemistry devices. The state-of-the-art methodology demonstrated here can greatly advance our understanding of surface science for oxides.
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.