Jie Jian

Strongly enhanced oxygen ion transport through samarium-doped CeO2 nanopillars in nanocomposite films.

Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO2 embedded in supporting matrices of SrTiO3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness.

Dielectric relaxation, resonance and scaling behaviors in Sr3Co2Fe24O41 hexaferrite.

The dielectric properties of Z-type hexaferrite Sr3Co2Fe24O41 (SCFO) have been investigated as a function of temperature from 153 to 503 K between 1 and 2 GHz. The dielectric responses of SCFO are found to be frequency dependent and thermally activated. The relaxation-type dielectric behavior is observed to be dominating in the low frequency region and resonance-type dielectric behavior is found to be dominating above 108 Hz. This frequency dependence of dielectric behavior is explained by the damped harmonic oscillator model with temperature dependent coefficients. The imaginary part of impedance (Z″) and modulus (M″) spectra show that there is a distribution of relaxation times. The scaling behaviors of Z″ and M″ spectra further suggest that the distribution of relaxation times is temperature independent at low frequencies. The dielectric loss spectra at different temperatures have not shown a scaling behavior above 108 Hz. A comparison between the Z″ and the M″ spectra indicates that the short-range charges motion dominates at low temperatures and the long-range charges motion dominates at high temperatures. The above results indicate that the dielectric dispersion mechanism in SCFO is temperature independent at low frequencies and temperature dependent at high frequencies due to the domination of resonance behavior.

Textured metastable VO2 (B) thin films on SrTiO3 substrates with significantly enhanced conductivity

Textured metastable VO2 (B) thin films with a layered structure were grown on SrTiO3 (001) by pulsed laser deposition. The X-ray diffraction and transmission electron microscopy results indicate that VO2 (B) films exhibit c-axis out-of-plane, while the films have 4 possible in-plane matching relations. In addition, a small amount of VO2 (M) phase can co-grow in the VO2 (B) phase when the film thickness exceeds a threshold. The thick VO2 films on STO exhibit a sharp metal-insulator transition with an increase of electrical conductivity in two orders of magnitude. This study may provide an alternative approach to enhance the performance of insulating VO2 (B) based batteries with increased electrical conductivity by incorporating VO2 (M) phase in the VO2 (B) phase layered network.

Strain relaxation and enhanced perpendicular magnetic anisotropy in BiFeO3:CoFe2O4 vertically aligned nanocomposite thin films

Self-assembled BiFeO3:CoFe2O4 (BFO:CFO) vertically aligned nanocomposite thin films have been fabricated on SrTiO3 (001) substrates using pulsed laser deposition. The strain relaxation mechanism between BFO and CFO with a large lattice mismatch has been studied by X-ray diffraction and transmission electron microscopy. The as-prepared nanocomposite films exhibit enhanced perpendicular magnetic anisotropy as the BFO composition increases. Different anisotropy sources have been investigated, suggesting that spin-flop coupling between antiferromagnetic BFO and ferrimagnetic CFO plays a dominant role in enhancing the uniaxial magnetic anisotropy.

Self-assembled oxide films with tailored nanoscale ionic and electronic channels for controlled resistive switching

Resistive switches are non-volatile memory cells based on nano-ionic redox processes that offer energy efficient device architectures and open pathways to neuromorphics and cognitive computing. However, channel formation typically requires an irreversible, not well controlled electroforming process, giving difficulty to independently control ionic and electronic properties. The device performance is also limited by the incomplete understanding of the underlying mechanisms. Here, we report a novel memristive model material system based on self-assembled Sm-doped CeO2 and SrTiO3 films that allow the separate tailoring of nanoscale ionic and electronic channels at high density (∼1012 inch−2). We systematically show that these devices allow precise engineering of the resistance states, thus enabling large on–off ratios and high reproducibility. The tunable structure presents an ideal platform to explore ionic and electronic mechanisms and we expect a wide potential impact also on other nascent technologies, ranging from ionic gating to micro-solid oxide fuel cells and neuromorphics.

Sharp semiconductor-to-metal transition of VO2 thin films on glass substrates

Outstanding phase transition properties of vanadium dioxide (VO2) thin films on amorphous glass were achieved and compared with the ones grown on c-cut sapphire and Si (111) substrates, all by pulsed laser deposition. The films on glass substrate exhibit a sharp semiconductor-to-metal transition (∼4.3 °C) at a near bulk transition temperature of ∼68.4 °C with an electrical resistance change as high as 3.2 × 103 times. The excellent phase transition properties of the films on glass substrate are correlated with the large grain size and low defects density achieved. The phase transition properties of VO2 films on c-cut sapphire and Si (111) substrates were found to be limited by the high defect density.

Roles of grain boundaries on the semiconductor to metal phase transition of VO2 thin films

Vanadium dioxide (VO2) thin films with controlled grain sizes are deposited on amorphous glass substrates by pulsed laser deposition. The grain boundaries (GBs) are found as the dominating defects in the thin films. The semiconductor to metal transition (SMT) properties of VO2 thin films are characterized and correlated to the GB density. The VO2 films with lower GB density exhibit a sharper SMT with a larger transition amplitude. A high resolution TEM study at GB area reveals the disordered atomic structures along the boundaries and the distorted crystal lattices near the boundaries. The VO2 SMT amplitude and sharpness could be directly related to these defects at and near the boundaries.

Self-Assembled Epitaxial Au–Oxide Vertically Aligned Nanocomposites for Nanoscale Metamaterials

Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques with limited paths toward reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned gold (Au) nanopillars (∼20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales.

Impedance spectroscopy and scaling behaviors of Sr3Co2Fe24O41 hexaferrite

The impedance spectroscopy of Z-type hexaferrite Sr3Co2Fe24O41 (SCFO) has been investigated as a function of temperature from 303 to 503 K. The frequency dependent impedance ( Z″) and modulus ( M″) spectra show that for the air annealed SCFO, the electrical responses of SCFO are thermal activated and there is a distribution of relaxation times. The scaling behaviors of Z″ and M″ spectra further suggest that the distribution of relaxation times is temperature independent. The Cole-Cole plots in impedance formalism show that the electrical response of SCFO originates from both the grain and the grain-boundaries. The activation energies for grain and grain boundary are 0.66 eV and 0.67 eV, respectively. The frequency dependent conductivity ( σ′) spectra follow the universal power law. The fitting results of σ′ spectra show that the small polaron hopping is the most probable conduction mechanism for SCFO. Moreover, the scaling behavior of σ′ spectra further confirms that the distribution of local electrical r...

Continuous Tuning of Phase Transition Temperature in VO2 Thin Films on c-Cut Sapphire Substrates via Strain Variation

Vanadium dioxide (VO2) thin films with controlled thicknesses are deposited on c-cut sapphire substrates with Al-doped ZnO (AZO) buffer layers by pulsed laser deposition. The surface roughness of AZO buffer layers is varied by controlling oxygen pressure during growth. The strain in the VO2 lattice is found to be dependent on the VO2 thickness and the VO2/AZO interface roughness. The semiconductor-to-metal transition (SMT) properties of VO2 thin films are characterized and the transition temperature (Tc) is successfully tuned by the VO2 thickness as well as the VO2/AZO interface roughness. It shows that the Tc of VO2 decreases with the decrease of film thickness or VO2/AZO interface roughness. Other SMT properties of the VO2 films are maintained during the Tc tuning. The results suggest that the strain tuning induced by AZO buffer provides an effective approach for tuning Tc of VO2 continuously.