Shuangyi Liu

Intrinsic dielectric frequency dependent spectrum of a single domain tetragonal BaTiO3

The intrinsic dielectric frequency dependent spectrum of single domain barium titanate (BaTiO3) at room temperature is investigated by considering the vibration of phonons and the conductivity of the tetragonal system in a wide frequency range up to THz. The proposed model combines Debye type of dissipation, soft mode theory, and the influence of conductivity on the dielectric loss to obtain a more precise dielectric frequency spectrum. The calculated results were compared with experimental data on single domain nanocrystals of BaTiO3, both free standing and suspended in a low dielectric medium. The comparisons provide insight into the mechanism for the dielectric behavior, which can be extended to apply to a range of composites that comprise single domain dielectrics embedded in continuous media. At the lower frequency range, conductivity plays a dominant role in the contribution to the dielectric loss along both a- and c-axes, while the phonon vibration controls the dielectric behavior of the system at ...

Three-dimensional hierarchical nickel–cobalt–sulfide nanostructures for high performance electrochemical energy storage electrodes

To meet the ever-growing global demand for highly efficient and reliable energy storage systems, novel three-dimensional (3D) hierarchical porous cobalt–nickel–sulfide, H-(Co, Ni)3S2, nanostructures were designed and fabricated. The electrodes, based on a 3D hierarchical, porous nanoarchitecture, exhibit outstanding comprehensive performance with ultra-high specific capacitance of 4840 F g−1 (7.3 F cm−2) at current density of 1 A g−1, excellent rate capability of 3984 F g−1 (6.0 F cm−2) even at 20 A g−1, and superior cycling stability with as high as 93% capacitance retention after 5000 cycles at 10 A g−1. The performance greatly exceeds most previously reported faradaic electrodes for supercapacitors, due to the hierarchical, porous nanostructures, large active ion accessible surface area, varied and efficient faradic redox reactions, as well as strong mechanical stability and robust adhesion to the conductive matrix. Supercapacitors based on the 3D hierarchical H-(Co, Ni)3S2 nanostructured electrodes possess not only outstanding power and life performance, but also competitive energy densities, compared to the current, popular batteries.

Hollandites as a new class of multiferroics

Discovery of new complex oxides that exhibit both magnetic and ferroelectric properties is of great interest for the design of functional magnetoelectrics, in which research is driven by the technologically exciting prospect of controlling charges by magnetic fields and spins by applied voltages, for sensors, 4-state logic, and spintronics. Motivated by the notion of a tool-kit for complex oxide design, we developed a chemical synthesis strategy for single-phase multifunctional lattices. Here, we introduce a new class of multiferroic hollandite Ba-Mn-Ti oxides not apparent in nature. BaMn3Ti4O14.25, designated BMT-134, possesses the signature channel-like hollandite structure, contains Mn4+ and Mn3+ in a 1:1 ratio, exhibits an antiferromagnetic phase transition (TN ~ 120u2005K) with a weak ferromagnetic ordering at lower temperatures, ferroelectricity, a giant dielectric constant at low frequency and a stable intrinsic dielectric constant of ~200 (1-100u2005MHz). With evidence of correlated antiferromagnetic and ferroelectric order, the findings point to an unexplored family of structures belonging to the hollandite supergroup with multifunctional properties, and high potential for developing new magnetoelectric materials.

Structure and performance of dielectric films based on self-assembled nanocrystals with a high dielectric constant.

Self-assembled films built from nanoparticles with a high dielectric constant are attractive as a foundation for new dielectric media with increased efficiency and range of operation, due to the ability to exploit nanofabrication techniques and emergent electrical properties originating from the nanoscale. However, because the building block is a discrete one-dimensional unit, it becomes a challenge to capture potential enhancements in dielectric performance in two or three dimensions, frequently due to surface effects or the presence of discontinuities. This is a recurring theme in nanoparticle film technology when applied to the realm of thin film semiconductor and device electronics. We present the use of chemically synthesized (Ba,Sr)TiO3 nanocrystals, and a novel deposition-polymerization technique, as a means to fabricate the dielectric layer. The effective dielectric constant of the film is tunable according to nanoparticle size, and effective film dielectric constants of up to 34 are enabled. Wide area and multilayer dielectrics of up to 8xa0cm(2) and 190xa0nF are reported, for which the building block is an 8xa0nm nanocrystal. We describe models for assessing dielectric performance, and distinct methods for improving the dielectric constant of a nanocrystal thin film. The approach relies on evaporatively driven assembly of perovskite nanocrystals with uniform size distributions in a tunable 7-30xa0nm size range, coupled with the use of low molecular weight monomer/polymer precursor chemistry that can infiltrate the porous nanocrystal thin film network post assembly. The intercrystal void space (low k dielectric volume fraction) is minimized, while simultaneously promoting intercrystal connectivity and maximizing volume fraction of the high k dielectric component. Furfuryl alcohol, which has good affinity to the surface of (Ba,Sr)TiO3 nanocrystals and miscibility with a range of solvents, is demonstrated to be ideal for the production of nanocomposites. The nanocrystal/furfuryl alcohol dispersions are suitable for the fabrication of thin films by chemical deposition techniques, including spin-coating, printing or a spraying process. To demonstrate the application of this technique to device fabrication, a multilayer capacitor with capacitance of 0.83xa0nFxa0mm(-2) at 1xa0MHz is presented.

Influence of size-dependent electrostatic energy on the ferroelectric nanodot domain structure by phase field method

A three-dimensional phase field model is proposed to study the effects of size-dependent electrostatic energy on ferroelectric nanodots at room temperature. The simulation results demonstrate that three domain structures exist in different sizes of nanodots, i.e. vortex structure, 90° domain structure and single domain. The periodic distributions of the electrostatic potential and its energy with positive and negative signs across the domain body are the main causes of the formation of the vortex structure. With decreasing nanodot size, the change in the competitiveness between the electrostatic energy and other intrinsic energies induces the formation of a 90° domain structure or a single domain.

Magnetoelectricity in CoFe2O4 nanocrystal-P(VDF-HFP) thin films

Transition metal ferrites such as CoFe2O4, possessing a large magnetostriction coefficient and high Curie temperature (Tc > 600 K), are excellent candidates for creating magnetic order at the nanoscale and provide a pathway to the fabrication of uniform particle-matrix films with optimized potential for magnetoelectric coupling. Here, a series of 0–3 type nanocomposite thin films composed of ferrimagnetic cobalt ferrite nanocrystals (8 to 18 nm) and a ferroelectric/piezoelectric polymer poly(vinylidene fluoride-co-hexafluoropropene), P(VDF-HFP), were prepared by multiple spin coating and cast coating over a thickness range of 200 nm to 1.6 μm. We describe the synthesis and structural characterization of the nanocrystals and composite films by XRD, TEM, HRTEM, STEM, and SEM, as well as dielectric and magnetic properties, in order to identify evidence of cooperative interactions between the two phases. The CoFe2O4 polymer nanocomposite thin films exhibit composition-dependent effective permittivity, loss tangent, and specific saturation magnetization (Ms). An enhancement of the effective permittivity and saturation magnetization of the CoFe2O4-P(VDF-HFP) films was observed and directly compared with CoFe2O4-polyvinylpyrrolidone, a non-ferroelectric polymer-based nanocomposite prepared by the same method. The comparison provided evidence for the observation of a magnetoelectric effect in the case of CoFe2O4-P(VDF-HFP), attributed to a magnetostrictive/piezoelectric interaction. An enhancement of Ms up to +20.7% was observed at room temperature in the case of the 10 wt.% CoFe2O4-P(VDF-HFP) sample.

Vortex structure transformation of BaTiO3 nanoparticles through the gradient function

Phase field method has been used to simulate the vortex structures in BaTiO3 (BTO) nanoparticles. Through the modulation of the gradient coefficients, vortices are found to transform in a path of monoclinic MA→orthorhombic→monoclinicu2002MC→tetragonal. Although the gradient coefficients vary significantly, the change in gradient energy is remarkably small. The simulation results show that the rotation and magnitude reduction in polarization dipoles increase the bulk energy, which induces the vortex transformation process in BTO nanoparticles. Moreover, the existence of monoclinic phase is a necessity to start the polarization rotation as well as the vortex transformation process.

Fabrication and characterization of CoxGd1-x nanowire arrays and core-shell-like CoxGd1-x/CoO nanowire arrays

Amorphous CoxGd1-x nanowire arrays and core-shell-like CoxGd1-x/CoO nanowire arrays with composite amorphous and crystalline structures, where 0.75 < x < 0.99, are fabricated successfully by employing electrochemical deposition with the assistance of AAO templates. Based on the experiments, dissolving of the organic salt of Gd in dimethylsulfoxide and the deposition potentials are the critical factors for formation of amorphous structures and different morphologies. Investigation of the magnetic properties shows that CoxGd1-x nanowire arrays have superparamagnetic characteristics, and core-shell-like CoxGd1-x/CoO nanowire arrays possess better soft ferromagnetic properties. Moreover, these magnetic behaviors are attributed to spin disorder of amorphous structures and shape anisotropy, which arise from the large aspect ratio of one-dimensional nanoscale materials.

The Solvent Induced Inter-Dimensional Phase Transformations of Cobalt Zeolitic-Imidazolate Frameworks

Studying inter-dimensional phase transitions of zeolitic-imidazolate frameworks (ZIFs) is essential for developing strategies in controlling morphology and properties. Herein, the inter-dimensional topotactic phase transformations of 3D ZIF-67 to 2D ZIF-L are investigated in detail by employing a simple and efficient solvent-induced growth method. In addition to ZIF-67 and ZIF-L, a series of novel core-shell composites of ZIF-67@ZIF-L, with unprecedented morphologies, are also obtained and well-defined. The different behaviors of the amine hydrogen of 2-MIM in the solvents play a pivotal role for inter-dimensional phase transformations, and in combination with the concentration of 2-MIM, the 2D to 3D phase transformations are also revealed. The findings are very beneficial for morphological design of the ZIFs, along with exploration of the corresponding properties. Impressively, Co-ZIFs exhibit interesting tunable CO2 adsorption behaviors with the phase evolution, which might bring broader understanding for designing CO2 detection and adsorption devices.

Frequency-dependent ferroelectric behavior of BaMn3Ti4O14.25 at room temperature

We report the activation field and selective frequency-dependent ferroelectric behavior of BaMn3Ti4O14.25 (BMT-134) at room temperature. BMT-134, a recently discovered multiferroic complex oxide, exhibits antiferromagnetic and ferroelectric behavior and belongs to the hollandite crystal class. The microstructure can be manipulated through processing conditions to prepare a nanocrystalline textured tablet. We measured polarization-electric field (P-E) hysteresis loops and strain-electric field butterfly loops as a function of frequency in order to investigate the AC dynamics of domain switching and strain behavior. Under an electric field loading condition, a clear hysteresis loop of the electric field-displacement curve is obtained at 50u2009Hz, indicating that room temperature ferroelectricity is attainable under the right processing conditions. When the frequency is increased to 500u2009Hz, the coercive field also increases, until the frequency reaches 5 kHz, at which point the electric field versus electric displacement becomes linear indicating the limit of domain switching at high frequency.