Zhiguo Liu

Nanjing University, Nan-ching, Jiangsu Sheng, China

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Publications (143)271.19 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The multiferroic properties of BiFeO3-based ceramics were improved through optimizing their sintering method and doping with certain rare earth elements in pure BiFeO3. Some methods, especially liquid-phase sintering method has largely decreased the densities of oxygen vacancies and Fe2+ in BiFeO3-based ceramics, and thus their resistivity became high enough to measure the saturated polarization and the large piezoelectric d 33 coefficient under the high electric field of >150 kV/cm. Besides, multiferroic properties were improved through the rare earth elements’ doping in pure BiFeO3. Magnetization commonly increases with the proportional increase of Nd, La, Sm and Dy contents up to ~30 %, while ferroelectric phase can transform to paraelectric phase at a certain proportion. An improved magnetoelectric coupling was often observed at ferroelectric phase with a relatively large proportion. Besides, an enhanced piezoelectric coefficient is expected in BiFeO3-based ceramics with morphotropic phase boundaries as they are already observed in thin epitaxial BiFeO3 films.
    Chinese Science Bulletin 12/2014; 59(36). · 1.37 Impact Factor
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    ABSTRACT: The charge-trapping memory devices with the structures p-Si/Al2O3/AlTi4Ox/Al2O3/Pt were fabricated by using atomic layer deposition and RF magnetron sputtering techniques, and a memory window of 6.61 V and a high charge-trapping density of 1.29 × 1013 cm−2 at gate voltage of ±11 V have been obtained. The remarkable charge-trapping effect in the high-k composite oxide layer was ascribed to the electron-occupied defect states formed by the inter-diffusion at the interface of TiO2/Al2O3. An Al2O3 layer intercalated in the charge-trapping layer AlTi4Ox enlarged the memory window to 14.59 V and also improved the data retention property by suppressing the vertical charge migration.
    Japanese Journal of Applied Physics 07/2014; 53(8S3):08NG02. · 1.07 Impact Factor
  • Applied Physics A 05/2014; 115(2). · 1.69 Impact Factor
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    ABSTRACT: The stability, the electronic and the magnetic properties of the hydrogen-terminated zigzag graphitic C3N4 nanoribbons (ZCNNRs) were investigated by using first-principles computations. The semiconductor to half-metal transition could be realized by controlling the hydrogen content of the environment, which is independent of nanoribbon width. The semiconducting ZCNNRs are stable at low hydrogen pressure, while the ZCNNRs are half-metallic at high hydrogen concentration. The hydrogenation acts as the electron doping at high concentration, which significantly alters the electronic and magnetic properties of ZCNNRs. The transition metal-free magnetism in ZCNNRs endows them many applications in novel integrated functional nanoelectronic devices, especially spintronics.
    physica status solidi (b) 05/2014; · 1.49 Impact Factor
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    ABSTRACT: Molecular ferroelectric thin films are highly desirable for their easy and environmentally friendly processing, light weight, and mechanical flexibility. A thin film of imidazolium perchlorate processed from aqueous solution is an excellent molecular ferroelectric with high spontaneous polarization, high Curie temperature, low coercivity, and superior electromechanical coupling. These attributes make it a molecular alternative to perovskite ferroelectric films in sensing, actuation, data storage, electro-optics, and molecular/flexible electronics.
    Angewandte Chemie International Edition 04/2014; · 11.34 Impact Factor
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    ABSTRACT: Multiferroic Bi1−xLaxFeO3 [BLFO (x)] ceramics with x = 0.10–0.50 and Mn-doped BLFO (x = 0.30) ceramics with different doping contents (0.1–1.0 mol%) were prepared by solid-state reaction method. They were crystallized in a perovskite phase with rhombohedral symmetry. In the BLFO (x) system, a composition (x)-driven structural transformation (R3c→C222) was observed at x = 0.30. The formation of Bi2Fe4O9 impure phase was effectively suppressed with increasing the x value, and the rhombohedral distortion in the BLFO ceramics was decreased, leading to some Raman active modes disappeared. A significant red frequency shift (~13 cm−1) of the Raman mode of 232 cm−1 in the BLFO ceramics was observed, which strongly perceived a significant destabilization in the octahedral oxygen chains, and in turn affected the local FeO6 octahedral environment. In the Mn-doped BLFO (x = 0.30) ceramics, the intensity of the Raman mode near 628 cm−1 was increased with increasing the Mn-doping content, which was resulted from an enhanced local Jahn–Teller distortions of the (Mn,Fe)O6 octahedra. Electron microscopy images revealed some changes in the ceramic grain sizes and their morphologies in the Mn-doped samples at different contents. Wedge-shaped 71° ferroelectric domains with domain walls lying on the {110} planes were observed in the BLFO (x = 0.30) ceramics, whereas in the 1.0 mol% Mn-doped BLFO (x = 0.30) samples, 71° ferroelectric domains exhibited a parallel band-shaped morphology with average domain width of 95 nm. Dielectric studies revealed that high dielectric loss of the BLFO (x = 0.30) ceramics was drastically reduced from 0.8 to 0.01 (measured @ 104 Hz) via 1.0 mol% Mn-doping. The underlying mechanisms can be understood by a charge disproportion between the Mn4+ and Fe2+ in the Mn-doped samples, where a reaction of Mn4+ + Fe2+→Mn3+ + Fe3+ is taken place, resulting in the reduction in the oxygen vacancies and a suppression of the electron hopping from Fe3+ to Fe2+ ions effectively.
    Journal of the American Ceramic Society 04/2014; · 2.43 Impact Factor
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    ABSTRACT: Bismuth ferrite nano- and microcrystals were prepared by a facile molten salt technique in two kinds of molten-salt-based systems (NaCl–KCl and NaCl–Na2SO4). In the NaCl–KCl salt system, a systematic study indicating the effects of process parameters (e.g., calcination temperature, holding time as well as the molten salt ratios) on the bismuth ferrite formation mechanism and structural characteristics is reported. The results show that almost pure phase BiFeO3 powders with minimum impurity phase of Bi2Fe4O9 were synthesized at temperatures of 700°C–800°C, whereas high calcination temperature (e.g., 900°C) resulted in the formation of almost pure phase Bi2Fe4O9 powders. The prolonged holding time increased the particle size via the Ostwald ripening mechanism; however, there was little effect on the particle morphology. Similar phenomenon occurred as increasing the molten salt ratios. In the NaCl–Na2SO4 salt systems, it is found that low NP-9 (nonylphenyl ether, NP-9) surfactant content (0–5 mL) led to the formation of almost pure phase BiFeO3 powders, whereas high NP-9 surfactant content (e.g., 20 mL) resulted in pure phase Bi2Fe4O9 powders. The average particle size of the BiFeO3 powders was decreased as increasing the NP-9 surfactant content, whereas their morphologies did not change significantly. Because of the simplicity and versatility of the approach used, it is expected that this methodology can be generalized to the large-scale preparation of other important transitional metal oxides with controllable sizes and shapes.
    Journal of the American Ceramic Society 03/2014; · 2.43 Impact Factor
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    ABSTRACT: Perovskite ZnZrO3 nanoparticles were synthesized by hydrothermal method, and their microstructures and optical properties were characterized. The crystallinity, phase formation, morphology and composition of the as-synthesized nanoparticles were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), high-resolutiontransmission electron microscopy (HRTEM), and energy-dispersive X-ray (EDX) spectroscopy analysis, respectively. TEM images demonstrated that the average particle size of the ZnZrO3 powders was increased with increasing the Zn/Zr molar ratios in the precursors, and more large ZnZrO3 particles with cubic morphology were observed at high Zn/Zr molar ratios. In addition, the phase structures of the ZnZrO3 particles were also evolved from a cubic to tetragonal perovskite phase, as revealed by XRD and SAED patterns. HRTEM images demonstrate that surface structures of the ZnZrO3 powders synthesized at high Zn/Zr molar ratios, are composed of corners bound by the {100} mini-facets, and the surface steps lying on the {100} planes are frequently observed, whereas the (101) facet isoccasionally observed. The formation of such a rough surface structure is understood from the periodic bond chain theory. Quantitative EDX analyses demonstrated that the atomic concentrations (at.%) of Zn:Zr:O in the particles were 20.70:21.07:58.23, as close to the composition of ZnZrO3. In the optical spectra, a significant red shift of the absorption edges (for the ZnZrO3 nanopowders) from UV to visible region (from 394 to 417 nm) was observed as increasing the Zn/Zr molar ratios in the precursors, which corresponds to that the band gap energies of the ZnZrO3 nanopowders can be continuously tuned from 3.15 to 2.97 eV. This opens an easy way to tune the band gap energies of the ZnZrO3 nanopowders.
    Journal of the American Ceramic Society 03/2014; · 2.43 Impact Factor
  • Hao Zhu, Jiang Yin, Yidong Xia, Zhiguo Liu
    Applied Physics Letters 02/2014; 104(12). · 3.52 Impact Factor
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    ABSTRACT: Fully transparent indium-tin-oxide/high-k-rare-earth-oxide Eu2O3/F-doped SnO2 devices that show stable bipolar resistance switching have been successfully fabricated. In addition to the transmittance of above 86% for visible light, high resistance ratio, good data retention and initial forming-free resistance switching behaviour were obtained in the transparent memory. The results of high-resolution spectroscopy and x-ray photoelectron spectroscopy analyses combining with the temperature dependence of resistance suggest that metallic Eu filaments are formed in the low-resistance state. Mechanism analysis indicates that the coexistence of oxygen vacancies and metallic Eu in the Eu2O3 films plays an important role in the forming-free resistive switching performance. The switching mechanism was attributed to formation/oxidation of filamentary and oxygen ion migration.
    Journal of Physics D Applied Physics 01/2014; 47(6). · 2.53 Impact Factor
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    ABSTRACT: The impact of thermal stress on the polarization, as well as dielectric and piezoelectric properties of (001) oriented PbTiO3 (PTO) thick films deposited on various substrates was investigated based on Landau-Devonshire thermodynamic model. The results showed that dielectric and piezoelectric properties of PTO films depend strongly on the thermal stress in PTO films decided by the deposition temperature TG and the thermal expansion coefficients' difference between PTO films and substrates. For IC-compatible substrates such as Si, c-sapphire, and a-sapphire that induce tensile in-plane thermal stresses, the dielectric and piezoelectric responses and tunabilities of PTO films were enhanced. Whereas for PTO films on MgO, compressive thermal in-plane stresses can degraded the dielectric and piezoelectric responses and tunabilities of the films.
    Journal of Applied Physics 01/2014; 116(5):054103-054103-5. · 2.21 Impact Factor
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    ABSTRACT: The prototypical charge-trapping memory devices with the structure p-Si/Al2O3/(Ta2O5)x(Al2O3)1−x/Al2O3/Pt(x = 0.5, 0.3, and 0.1) were fabricated by using atomic layer deposition and RF magnetron sputtering techniques. A memory window of 7.39 V with a charge storage density of 1.97 × 1013 cm−2 at a gate voltage of ±11 V was obtained for the memory device with the composite charge trapping layer (Ta2O5)0.5(Al2O3)0.5. All memory devices show fast program/erase speed and excellent endurance and retention properties, although some differences in their memory performance exist, which was ascribed to the relative individual band alignments of the composite (Ta2O5)x(Al2O3)1−x with Si.
    Applied Physics Letters 01/2014; 105(12):123504-123504-5. · 3.52 Impact Factor
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    ABSTRACT: Here, we report the atomic-scale microstructural characterization and dielectric properties of crystalline cubic pyrochlore Bi1.5MgNb1.5O7 (BMN) nanoparticles with mean size of 70 nm, which were synthesized by sol–gel method. The crystallinity, phase formation, morphology, and surface microstructure of the BMN nanoparticles were characterized by X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM), respectively. The phase evolution of the BMN nanoparticles investigated by XRD patterns showed that uniform cubic pyrochlore BMN nanoparticles were obtained after calcination at temperature of 800 °C, and their structural information was revealed by Raman spectrum. TEM images demonstrated that the BMN nanoparticles had a spherical morphology with an average particle size of 70 nm, and their crystalline nature was revealed by HRTEM images. In addition, HRTEM images also demonstrate a terrace–ledge–kink (TLK) surface structure at the edges of rough BMN nanoparticles, where the terrace was on the (100) plane, and the ledge on the (001) plane. The formation of such a TLK surface structure can be well explained by a theory of periodic bond chains. Due to the surface structural reconstruction in the BMN nanoparticles, the formation of a tetragonal structure in a rough BMN nanoparticle was also revealed by HRTEM image. The BMN nanoparticles exhibited dielectric constants of 50 at 100 kHz and 30 at 1 MHz, and the dielectric loss of 0.19 at 1 MHz.
    Journal of Nanoparticle Research 01/2014; 16(1). · 2.18 Impact Factor
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    ABSTRACT: The as-prepared BiFeO3 ceramic shows a piezoelectric d33 coefficient of −14 pC/N, that is, an obvious ferroelectric self-poling phenomenon. The temperature gradient between the two surfaces of BiFeO3 ceramic was intentionally enlarged when BiFeO3 was prepared with a rapid liquid sintering method. This temperature gradient and the corresponding thermal strain can introduce defect dipoles through separating bismuth vacancies from oxygen vacancies. A mass of these dipoles introduce a macroscopic internal electric field (Ein) which downward poles BiFeO3 ceramic during its cooling down process. As expected, an Ein of >10 kV/cm is confirmed by the asymmetrical polarization/strain versus electric field curves.
    Journal of the American Ceramic Society 12/2013; 96(12). · 2.43 Impact Factor
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    ABSTRACT: The evolution of ferroelectric domains in BaTiO3 film on (001) SrTiO3 substrate is studied at 20–120 °C. The dominant domains orient in (001) plane due to strong internal strain when BaTiO3 island grains are embedded in the film, most of which is of layer-by-layer growth. These domains are commonly larger than grains at 20 °C; however, they are close to grain size at 60–80 °C, which largely enhances the coercive field. The in-plane domains are very stable at 20 °C; however, they can transform to out-of-plane domains at 40–80 °C under electric field, which is likely the cause of enhanced piezoelectric response.
    Applied Physics Letters 08/2013; 103(6). · 3.52 Impact Factor

Publication Stats

479 Citations
271.19 Total Impact Points

Institutions

  • 1995–2014
    • Nanjing University
      • • National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering
      • • Department of Materials Science & Engineering
      • • Department of Physics
      Nan-ching, Jiangsu Sheng, China
  • 2011
    • University of Science and Technology of China
      Luchow, Anhui Sheng, China