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Tetragonal BiFeO3 on yttria-stabilized zirconia
(2015-11-01)
Liu, Heng-Jui
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Du, Yu-Hao
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Gao, Peng
;
Huang, Yen-Chin
;
Chen, Hsiao-Wen
High structural susceptibility of multiferroic BiFeO3 (BFO) makes it a potential replacement of current Pb-based piezoelectrics. In this study, a tetragonal phase is identified based on a combination of x-ray diffraction, scanning transmission electronic microscopy, x-ray absorption spectroscopy, and Raman spectroscopy when BFO is grown on yttria-stabilized zirconia (YSZ) substrates. To distinguish the discrepancy between this tetragonal phase and common cases of monoclinic BFO, piezoelectric force microscopy images and optical property are also performed. It shows a lower electrostatic energy of ferroelectric domains and a large reduction of band gap for BFO grown on YSZ substrate comparing to the well-known one grown on LaAlO3 substrate. Our findings in this work can provide more insights to understand the structural diversity of multiferroic BFO system for further applications. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
Photostriction of strontium ruthenate
(2017-04-24)
Wei, Tzu-Chiao
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Wang, Hsin-Ping
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Liu, Heng-Jui
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Tsai, Dung-Sheng
;
Ke, Jr-Jian
Transition metal oxides with a perovskite crystal structure exhibit a variety of physical properties associated with the lattice. Among these materials, strontium ruthenate (SrRuO3) displays unusually strong coupling of charge, spin and lattice degrees of freedom that can give rise to the photostriction, that is, changes in the dimensions of material due to the absorption of light. In this study, we observe a photon-induced strain as high as 1.12% in single domain SrRuO3, which we attribute to a nonequilibrium of phonons that are a result of the strong interaction between the crystalline lattice and electrons excited by light. In addition, these light-induced changes in the SrRuO3 lattice affect its electrical resistance. The observation of both photostriction and photoresistance in SrRuO3 suggests the possibility of utilizing the mechanical and optical functionalities of the material for next-generation optoelectronics, such as remote switches, light-controlled elastic micromotors, microactuators and other optomechanical systems.
Magnetic-coupled phase anomaly in mixed-phase BiFeO3 thin films
(2017-08-01)
Huang, Yen-Chin
;
Liou, Yi-De
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Liu, Heng-Jui
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Lee, Hsin-Hua
;
Chen, Yi-Chun
The rich phase diagram of rhombohedral (R)-like and tetragonal (T)-like monoclinic polymorph in strained BiFeO3 (BFO) films brings on various functionalities. Finding correlations of physical ordering parameters in this system is generally difficult because T-like and R-like phases are undistinguishable in many aspects. In this study, the magnetic-coupled structural transitions of the mixed-phase BFO at low temperatures were investigated by atomic force microscopy (AFM), X-ray diffraction (XRD), and Raman spectroscopy. To resolve the complexity resulted from the similarity between T-like and R-like phases, we analyzed the Raman spectra on a micro-scale region with various T/R ratios, which is in situ manipulated by an AFM tip carrying a dc bias. Phonons of T-like and R-like phases were thus successfully separated. Based on temperature-dependent XRD and resolved Raman spectra, we observed two isostructural transitions at around 225 K and 150 K, and they are strongly correlated with the magnetic ordering in the mixed-phase BFO film. Moreover, through the effective spin-lattice coupling, the evolution of the T/R polymorph is changed by the magnetic cooling process at low temperatures. This study provides a pathway to modulate phonon behaviors by magnetic fields in a highly strained system. (C) 2017 Author(s).
Periodic elastic nanodomains in ultrathin tetragonal-like BiFeO3 films
(2013-08-26)
Luo, Zhenlin
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Chen, Zuhuang
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Yang, Yuanjun
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Liu, Heng-Jui
;
Huang, Chuanwei
We present a synchrotron grazing incidence x-ray diffraction analysis of the domain structure and polar symmetry of highly strained BiFeO3 (BFO) thin films grown on LaAlO3 substrate. We reveal the existence of periodic elastic nanodomains in the pure tetragonal-like BFO ultrathin films down to a thickness of 6 nm. A unique shear strain-accommodation mechanism is disclosed. We further demonstrate that the periodicity of the nanodomains increases with film thickness but deviates from the classical square root law in the ultrathin thickness regime (6-30 nm). Temperature-dependent experiments further reveal the disappearance of periodic modulation above similar to 90 degrees C due to a M-C-M-A structural phase transition.
Flexible ferroelectric element based on van der Waals heteroepitaxy
(2017-06-01)
Jiang, Jie
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Bitla, Yugandhar
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Huang, Chun-Wei
;
Thi Hien Do
;
Liu, Heng-Jui
We present a promising technology for nonvolatile flexible electronic devices: A direct fabrication of epitaxial lead zirconium titanate (PZT) on flexible mica substrate via van der Waals epitaxy. These single-crystalline flexible ferroelectric PZT films not only retain their performance, reliability, and thermal stability comparable to those on rigid counterparts in tests of nonvolatile memory elements but also exhibit remarkable mechanical properties with robust operation in bent states (bending radii down to 2.5 mm) and cycling tests (1000 times). This study marks the technological advancement toward realizing much-awaited flexible yet single-crystalline nonvolatile electronic devices for the design and development of flexible, lightweight, and next-generation smart devices with potential applications in electronics, robotics, automotive, health care, industrial, and military systems.
Atomic mechanism of polarization-controlled surface reconstruction in ferroelectric thin films
(2016-04-01)
Gao, Peng
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Liu, Heng-Jui
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Huang, Yen-Lin
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Chu, Ying-Hao
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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.
Revealing the flexoelectricity in the mixed-phase regions of epitaxial BiFeO3 thin films
(2015-01-28)
Cheng, Cheng-En
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Liu, Heng-Jui
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Dinelli, Franco
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Chen, Yi-Chun
;
Chang, Chen-Shiung
Understanding the elastic response on the nanoscale phase boundaries of multiferroics is an essential issue in order to explain their exotic behaviour. Mixed-phase BiFeO3 films, epitaxially grown on LaAlO3 (001) substrates, have been investigated by means of scanning probe microscopy to characterize the elastic and piezoelectric responses in the mixed-phase region of rhombohedral-like monoclinic (M-I) and tilted tetragonal-like monoclinic (M-II,M-tilt) phases. Ultrasonic force microscopy reveal that the regions with low/ high stiffness values topologically coincide with the M-I/M-II,M-tilt phases. X-ray diffraction strain analysis confirms that the M-I phase is more compliant than the M-II,(tilt) one. Significantly, the correlation between elastic modulation and piezoresponse across the mixed-phase regions manifests that the flexoelectric effect results in the enhancement of the piezoresponse at the phase boundaries and in the M-I regions. This accounts for the giant electromechanical effect in strained mixed-phase BiFeO3 films.
Possible absence of critical thickness and size effect in ultrathin perovskite ferroelectric films
(2017-06-06)
Gao, Peng
;
Zhang, Zhangyuan
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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.
Tuning the functionalities of a mesocrystal via structural coupling
(2015-07-14)
Liu, Heng-Jui
;
Liu, Yun-Ya
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Tsai, Chih-Ya
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Liao, Sheng-Chieh
;
Chen, Ying-Jiun
In the past decades, mesocrystal, a kind of nanocrystals with specific crystallographic orientation, has drawn a lot of attention due to its intriguing functionalities. While the research community keeps searching for new mesocrystal systems, it is equally crucial to develop new approaches to tune the properties of mesocrystals. In this work, a self-organized two-dimensional mesocrystal composed of highly oriented CoFe2O4 (CFO) nano-crystals with assistance of different perovskite matrices is studied as a model system. We have demonstrated that the strain state and corresponding magnetic properties of the CFO mesocrystal can be modulated by changing the surrounding perovskite matrix through their intimate structural coupling. Interestingly, this controllability is more strongly correlated to the competition of bonding strength between the matrices and the CFO mesocrystals rather than the lattice mismatch. When embedded in a matrix with a higher melting point or stiffness, the CFO mesocrystal experiences higher out-of-plane compressive strain and shows a stronger magnetic anisotropy as well as cation site-exchange. Our study suggests a new pathway to tailor the functionalities of mesocrystals.
Permanent ferroelectric retention of BiFeO3 mesocrystal
(2016-10-26)
Hsieh, Ying-Hui
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Xue, Fei
;
Yang, Tiannan
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Liu, Heng-Jui
;
Zhu, Yuanmin
Non-volatile electronic devices based on magnetoelectric multiferroics have triggered new possibilities of outperforming conventional devices for applications. However, ferroelectric reliability issues, such as imprint, retention and fatigue, must be solved before the realization of practical devices. In this study, everlasting ferroelectric retention in the heteroepitaxially constrained multiferroic mesocrystal is reported, suggesting a new approach to overcome the failure of ferroelectric retention. Studied by scanning probe microscopy and transmission electron microscopy, and supported via the phase-field simulations, the key to the success of ferroelectric retention is to prevent the crystal from ferroelastic deformation during the relaxation of the spontaneous polarization in a ferroelectric nanocrystal.

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