Wenrui Zhang

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.

Strong oxygen pressure dependence of ferroelectricity in BaTiO3/SrRuO3/SrTiO3 epitaxial heterostructures

The oxygen pressure effect on the structural and ferroelectric properties have been studied in epitaxial BaTiO3 (BTO)/SrRuO3/SrTiO3 (001) heterostructures grown by pulsed laser deposition. It is found that oxygen pressure is a sensitive parameter, which can influence the characteristics of oxide films in many aspects. The out-of-plane lattice parameter, tetragonality, (c/a) and Ti/Ba ratio monotonously decrease as the oxygen pressure increases from 5 mTorr to 200 mTorr. Microstructural study shows that the growth of BaTiO3 varies from a dense large grained structure with a smooth surface to a small columnar grain structure with rough surface as the deposition pressure increases. Electrical measurements show that the 40 mTorr deposited BTO films present maximum remanent polarization (Pr) (14 μC/cm2) and saturation polarization (Ps) (27 μC/cm2) due to the stoichiometric cation ratio, very smooth surface, and low leakage current. These results demonstrate that the controlling of cation stoichiometry, surface...

Novel Electroforming‐Free Nanoscaffold Memristor with Very High Uniformity, Tunability, and Density

A novel device structure is developed, which uses easy-to-grow nano scaffold films to localize oxygen vacancies at vertical heterointerfaces. The strategy is to design vertical interfaces using two structurally incompatible oxides, which are likely to generate a high-concentration oxygen vacancy. Non-linear electroresistance at room temperature is demonstrated using these nano scaffold devices. The resistance variations exceed two orders of magnitude with very high uniformity and tunability.

Integration of Self-Assembled Vertically Aligned Nanocomposite (La0.7Sr0.3MnO3)1–x:(ZnO)x Thin Films on Silicon Substrates

Epitaxial (La0.7Sr0.3MnO3)(1-x):(ZnO)x (LSMO:ZnO) in vertically aligned nanocomposite (VAN) form was integrated on STO/TiN-buffered silicon substrates by pulsed-laser deposition. Their magnetotransport properties have been investigated and are systematically tuned through controlling the ZnO concentration. The composite film with 70% ZnO molar ratio exhibits a maximum magnetoresistance (MR) value of 55% at 70 K and 1 T. The enhanced tunable low-field MR properties are attributed to structural and magnetic disorders and spin-polarized tunneling through the secondary ZnO phase. The integration of LSMO:ZnO VAN films on silicon substrates is a critical step enabling the application of VAN films in future spintronic devices.

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.

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.

Role of scaffold network in controlling strain and functionalities of nanocomposite films

The tuning of functional properties in thick oxide films via nanoscaffolds induced large vertical lattice strain. Strain is a novel approach to manipulating functionalities in correlated complex oxides. However, significant epitaxial strain can only be achieved in ultrathin layers. We show that, under direct lattice matching framework, large and uniform vertical strain up to 2% can be achieved to significantly modify the magnetic anisotropy, magnetism, and magnetotransport properties in heteroepitaxial nanoscaffold films, over a few hundred nanometers in thickness. Comprehensive designing principles of large vertical strain have been proposed. Phase-field simulations not only reveal the strain distribution but also suggest that the ultimate strain is related to the vertical interfacial area and interfacial dislocation density. By changing the nanoscaffold density and dimension, the strain and the magnetic properties can be tuned. The established correlation among the vertical interface—strain—properties in nanoscaffold films can consequently be used to tune other functionalities in a broad range of complex oxide films far beyond critical thickness.

Evolution of microstructure, strain and physical properties in oxide nanocomposite films

We, using LSMO:ZnO nanocomposite films as a model system, have studied the effect of film thickness on the physical properties of nanocomposites. It shows that strain, microstructure, as well as magnetoresistance strongly rely on film thickness. The magnetotransport properties have been fitted by a modified parallel connection channel model, which is in agreement with the microstructure evolution as a function of film thickness in nanocomposite films on sapphire substrates. The strain analysis indicates that the variation of physical properties in nanocomposite films on LAO is dominated by strain effect. These results confirm the critical role of film thickness on microstructures, strain states, and functionalities. It further shows that one can use film thickness as a key parameter to design nanocomposites with optimum functionalities.

Ferroelectric Properties of Vertically Aligned Nanostructured BaTiO3–CeO2 Thin Films and Their Integration on Silicon

Epitaxial (BaTiO3)0.5(CeO2)0.5 films have been deposited in vertically aligned nanocomposite form on SrTiO3/TiN buffered Si substrates to achieve high-quality ferroelectrics on Si. The thin TiN seed layer promotes the epitaxial growth of the SrTiO3 buffer on Si, which in turn is essential for the high-quality growth of the vertically aligned nanocomposite structure. X-ray diffraction and transmission electron microscopy characterization show that the films consist of distinct c-axis oriented BaTiO3 and CeO2 phases. Polarization measurements show that the BaTiO3-CeO2 films on Si are actually ferroelectric at room temperature, and the ferroelectric response is comparable to pure BaTiO3 as well as the BaTiO3-CeO2 films on SrTiO3 single-crystalline substrates. Capacitance-voltage measurements show that, instead of decreasing, the Curie temperature increases to 175 and 150 °C for the samples on SrTiO3 and Si substrates, respectively. This work is an essential step towards integrating novel nanostructured materials with advanced functionalities into Si-based devices.