Browsing by Author "Zhang, Yi"

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Atomic-Scale Mechanisms of Defect-Induced Retention Failure in Ferroelectrics
(2017-06-01)
Li, Linze
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Zhang, Yi
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Xie, Lin
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Jokisaar, Jacob R.
;
Beekman, Christianne
The ability to switch the ferroelectric polarization using an electric field makes ferroelectrics attractive for application in nanodevices such as high-density memories. One of the major challenges impeding this application, however, has been known as retention failure, which is a spontaneous process of polarization back-switching that can lead to data loss. This process is generally thought to be caused by the domain instability arising from interface boundary conditions and countered by defects, which can pin the domain wall and impede the back-switching. Here, using in situ transmission electron microscopy and atomic-scale scanning transmission electron microscopy, we show that the polarization retention failure can be induced by commonly observed nanoscale impurity defects in BiFeO3 thin films. The interaction between polarization and the defects can also lead to the stabilization of novel functional nanodomains with mixed-phase structures and head-to-head polarization configurations. Thus, defect engineering provides a new route for tuning properties of ferroelectric nanosystems.
Characterization of domain distributions by second harmonic generation in ferroelectrics
(2018-07-24)
Zhang, Yuan
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Zhang, Yi
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Guo, Quan
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Zhong, Xiangli
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Chu, Yinghao
Domain orientations and their volume ratios in ferroelectrics are recognized as a compelling topic recently for domain switching dynamics and domain stability in devices application. Here, an optimized second harmonic generation method has been explored for ferroelectric domain characterization. Combing a unique theoretical model with azimuth-polarization-dependent second harmonic generation response, the complex domain components and their distributions can be rigidly determined in ferroelectric thin films. Using the proposed model, the domain structures of rhombohedral BiFeO3 films with 71 degrees and 109 degrees domain wall, and, tetragonal BiFeO3, Pb( Zr0.2Ti0.8)O-3, and BaTiO3 ferroelectric thin films are analyzed and the corresponding polarization variants are determined. This work could provide a powerful and all-optical method to track and evaluate the evolution of ferroelectric domains in the ferroelectric-based devices.
Deterministic, Reversible, and Nonvolatile Low-Voltage Writing of Magnetic Domains in Epitaxial BaTiO3/Fe3O4 Heterostructure
(2018-09-01)
Zhong, Gaokuo
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An, Feng
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Bitla, Yugandhar
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Wang, Jinbin
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Zhong, Xiangli
The ability to electrically write magnetic bits is highly desirable for future magnetic memories and spintronic devices, though fully deterministic, reversible, and nonvolatile switching of magnetic moments by electric field remains elusive despite extensive research. In this work, we develop a concept to electrically switch magnetization via polarization modulated oxygen vacancies, and we demonstrate the idea in a multiferroic epitaxial heterostructure of BaTiO3/Fe3O4 fabricated by pulsed laser deposition. The piezoelectricity and ferroelectricity of BaTiO3 have been confirmed by macro- and microscale measurements, for which Fe(3)O(4 )serves as the top electrode for switching the polarization. X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra indicate a mixture of Fe2+ and Fe3+ at O-h sites and Fe3+ at T-d sites in Fe3O4, while the room-temperature magnetic domains of Fe3O4 are revealed by microscopic magnetic force microscopy measurements. It is demonstrated that the magnetic domains of Fe(3)O(4 )can be switched by not only magnetic fields but also electric fields in a deterministic, reversible, and nonvolatile manner, wherein polarization reversal by electric field modulates the oxygen vacancy distribution in Fe3O4, and thus its magnetic state, making it attractive for electrically written magnetic memories.
Discovery of a magnetic conductive interface in PbZr0.2Ti0.8O3 /SrTiO3 heterostructures
(2018-02-15)
Zhang, Yi
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Xie, Lin
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Kim, Jeongwoo
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Stern, Alex
;
Wang, Hui
Emergent physical properties often arise at interfaces of complex oxide heterostructures due to the interplay between various degrees of freedom, especially those with polar dis-continuities. It is desirable to explore if these structures may generate pure and controllable spin currents, which are needed to attain unmatched performance and energy efficiency in the next-generation spintronic devices. Here we report the emergence of a spin-polarized two-dimensional electron gas (SP-2DEG) at the interface of two insulators, SrTiO3 and PbZr0.2Ti0.8O3. This SP-2DEG is strongly localized at the interfacial Ti atoms, due to the interplay between Coulomb interaction and band bending, and can be tuned by the ferroelectric polarization. Our findings open a door for engineering ferroelectric/insulator interfaces to create tunable ferroic orders for magnetoelectric device applications and provide opportunities for designing multiferroic materials in heterostructures.
High quality 10.6 mu m AIN grown on pyramidal patterned sapphire substrate by MOCVD
(2019-01-28)
Long, Hanling
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Dai, Jiangnan
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Zhang, Yi
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Wang, Shuai
;
Tan, Bo
In this letter, we demonstrate a crack and strain free AlN epilayer with a thickness of 10.6 mu m grown on a pyramidal patterned sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of the X-ray rocking curve was 165/185 arcsec for (002)/(102) planes, respectively. The total threading dislocation density was less than 3 x 10(8) cm(-2). The dislocation evolution and the coalescence process were probed by transmission electron microscopy and scanning electron microscopy. A dual coalescence of the AlN epilayer was observed, which can effectively relax strain during the heteroepitaxy process. Owing to the approximately entire strain relaxation demonstrated by reciprocal space mapping and Raman shift, the surface morphology was crack-free and atomically smooth with a root-mean-square roughness of 0.14 nm. Temperature dependent Raman spectra showed the Raman linewidth of 4.3 cm(-1) at 300 K which was comparable to that of bulk AlN; it also demonstrated good crystalline quality of the AlN epilayer. Published under license by AIP Publishing.
Interface Anchored Effect on Improving Working Stability of Deep Ultraviolet Light-Emitting Diode Using Graphene Oxide-Based Fluoropolymer Encapsulant
(2018-03-07)
Liang, Renli
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Dai, Jiangnan
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Xu, Linlin
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Zhang, Yi
;
He, Ju
The graphene oxide (GO)-based fluoropolymer is first proposed as an interface encapsulant to improve the light extraction efficiency and achieve the ultralong working stability of deep ultraviolet light-emitting diodes (DUV-LEDs), benefitting from its superior interface performance based on an anchored effect. For the GO-based fluoropolymer composite, the anchored structure is designed to effectively and tightly rivet the quartz lens on the DUV-LED chip by using the interface reaction between GO embedded in fluoropolymer and 3-aminopropyltriethoxy-silane grafted on the surfaces. Experimental results show that on the basis of the interface anchored effect, the air voids in the interface layer of DUV-LED are reduced by 84%, leading to an improvement of the light output power by 15% and a decrease of the junction temperature by 5%, by virtue of the sealing characteristics of the 0.10 wt % GO-based fluoropolymer. In addition, the steady working time is dramatically improved by 660% and it was attributed to the good interface anchored bonding of the 0.10 wt % GO-based fluoropolymer. This novel graphene oxide-based fluoropolymer is believed to provide a feasible and effective interface encapsulant to improve the performance of DUV-LEDs.
Self-Assembled Ferroelectric Nanoarray
(2019-01-16)
Jiang, Jie
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Yang, Qiong
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Zhang, Yi
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Li, Xiao-Yu
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Shao, Pao-Wen
Self-assembled heteroepitaxial nanostructures have played an important role for miniaturization of electronic devices, e.g., the ultrahigh density ferroelectric memories, and cause for great concern. Our first principle calculations predict that the materials with low formation energy of the interface (E-f) tend to form matrix structure in self-assembled heteroepitaxial nanostructures, whereas those with high E-f form nanopillars. Under the guidance of the theoretical modeling, perovskite BiFeO3 (BFO) nanopillars are swimmingly grown into CeO2 matrix on single-crystal (001)-SrTiO3 (STO) substrates by pulsed laser deposition, where CeO2 has a lower formation energy of the interface (E-f) than BFO. This work provides a good paradigm for controlling self-ssembled nanostructures as well as the application of self-assembled ferroelectric nanoscale memory.
Simple silver nanowire patterning using a DUV lamp direct write with sol-gel IZO capping
(2017-01-01)
Liu, Hung-Chuan
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Zan, Hsiao-Wen
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Soppera, Olivier
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Zhang, Yi
;
Yang, Bo-Ru
Herein, we developed a simple method to pattern a silver nanowire (AgNW) electrode using a DUV lamp to directly write on an indium-zinc-oxide capped AgNW. The patterning was achieved via directly transferring the DUV-write IZO pattern to the AgNW pattern because the IZO film protected the AgNWs from melting during the subsequent thermal annealing step. The patterned AgNW electrodes retained good conducting properties on a flexible polyimide substrate after 1000 bending cycles. This DUV-lamp-write pattern method is promising to form a AgNW pattern on a large area and flexible substrate for sensor or photovoltaic applications.