Jongsik Yoon

Strain control and spontaneous phase ordering in vertical nanocomposite heteroepitaxial thin films

Two-phase, vertical nanocomposite heteroepitaxial films hold great promise for (multi)functional device applications. In order to achieve practical devices, a number of hurdles need to be overcome, including the creation of ordered structures (and their formation on a large scale), achieving different combinations of materials and control of strain coupling between the phases. Here we demonstrate major advances on all these fronts: remarkable spontaneously ordered structures were produced in newly predicted compositions, vertical strain was proven to dominate the strain state in films above 20 nm thickness and strain manipulation was demonstrated by selection of phases with the appropriate elastic moduli. The work opens up a new avenue for strain control in relatively thick films and also promises new forms of ordered nanostructures for multifunctional applications.

High tunability of lead strontium titanate thin films using a conductive LaNiO3 as electrodes

Highly tunable films of dielectric PbxSr1−xTiO3 (with x=0.3 and 0.4) have been deposited on conductive LaNiO3 coated LaAlO3 substrates using a sol-gel technique. The processing condition was found to greatly influence the microstructure as well as the dielectric and electrical properties of the films. At room temperature, dielectric tunability values as high as 70% and 78.6% at an applied electric field of 223kV∕cm were achieved for the Pb0.3Sr0.7TiO3 and Pb0.4Sr0.6TiO3 films, respectively.

Flux pinning in YBa2Cu3O7−δ thin film samples linked to stacking fault density

In this paper, we report a strong correlation between the stacking fault (SF) density and the critical current density of YBa2Cu3O7−δ (YBCO) thin films in applied field (Jcin-field). We found that the Jcin-field (H∥c) increases as a clear linear dependence of the density of SF identified in the as-grown samples deposited on both SrTiO3 (STO) and LaAlO3 substrates. Detailed microstructural studies including cross-section transmission electron microscopy (TEM) and high resolution TEM were conducted for all the films deposited on STO substrates. This work suggests that the YBCO SF density plays an important role in the YBCO in-field transport performance.

Spontaneous Ordering, Strain Control, and Multifunctionality in Vertical Nanocomposite Heteroepitaxial Films

Two-phase nanocomposite heteroepitaxial films with vertical microstructures hold great promise for various (multi)functional (e.g. multiferroic) electronic device applications. With the aim of creating addressable arrays, it is necesary to form spontaneously ordered structures over large areas. However, such structures have not, so far, been demonstrated. We have recently shown remarkable spontaneously ordered phase assemblies and find that these structures form concomitantly with two-dimensional vertical strain control, i.e. strain in the two phases is controlled along the vertical interface between them rather than being influenced by the substrate [1]. In this extended abstract we report briefly on our findings.

Spontaneous ordering, strain control and mutlifunctionality in vertical nanocomposite heteroepitaxial films

Two-phase nanocomposite heteroepitaxial films with vertical microstructures hold great promise for various (multi)functional (e.g., multiferroic) electronic device applications. With the aim of creating addressable arrays, it is necessary to form spontaneously ordered structures over large areas. However, such structures have not, so far, been demonstrated. We have recently produced remarkable spontaneously ordered phase assemblies and find that these structures form concomitantly with 2-D vertical strain control, i.e., strain in the 2 phases is controlled along the vertical interface between them rather than being influenced by the substrate. In this paper, we report on our findings in the BiFeO3 and BaTiO3 ferroelectric systems.

Developing nanostructured cathode of thin film SOFC and its characteristics

Thin film Solid Oxide Fuel Cells (TFSOFC) have been intensively researched because of their potentials in compact and high-efficiency energy conversion applications. It was found that the microstructural variations in the electrolyte and electrode could affect the reaction kinetics of TFSOFCs dramatically. In this paper we present a study of the effects of nanostructured cathodes on their electrochemical performance. Nanostructured thin film cathodes were processed using a pulsed laser deposition technique (PLD). Using PLD we were able to process vertically aligned nanopores in cathode structures with enhanced oxygen-gas phase diffusivity, thus improving TFSOFCs performance. La0.5Sr0.5CoO3 (LSCO), La1¿xSrxCo0.8Fe0.2O3 (LSCFO) and other cathode materials were deposited on various substrates (YSZ, Si and pressed Ce0.9Gd0.1O1.95 disks). Microstructures and properties of the nanostructured cathodes were characterized by XRD, TEM, HRTEM, SEM and electrochemical meas! urements. We explored the oxygen-gas phase diffusion efficiency as a function of nanopore size using electrochemical impedance spectroscopy. We also investigated the thermal stress built in the cell as a function of nanopore sizes. These well-aligned nanopores could not only relieve or partially relieve the internal thermal stress and lattice strain caused by the differences of thermal expansion coefficients and lattice mismatch between the electrode and electrolyte but also facilitate the oxygen diffusion through the cathode electrode.

NA008 Deposition temperature dependence of YBCO transport properties

In this paper, we report a strong correlation between the stacking fault (SF) density and the critical current density of YBa2Cu3O7¿¿ (YBCO) thin films in applied field (Jcin-field). We found that the Jcin-field increases as the deposition temperature increases (775°C ¿ 825°C) for the samples grown on both SrTiO3 (STO) and LaAlO3 substrates. An interesting linear relation is observed between the SF density and the Jcin-field value, which suggests that the YBCO SF density plays an important role in the YBCO in-field transport performance.

Tunable dielectric properties of lead strontium titanate thin films by sol-gel technique

For the applications in tunable microwave devices, thin films of a potential candidate material, PbxSr1-xTiO3 (with ¿=0.3), were studied. To study dielectric proeprties in coplanar and metal-insulator-metal configuration, the PbxSr1-xTiO3 films were deposited on single crystalline LaAlO3 and conductive LaNiO3 coated LaAlO3 substrates using sol-gel technique. The film on LaAlO3 was highly c-axis oriented and epitaxial. At room temperature, dielectric tunability values as high as 18% (at 20kV/cm) and 70% (at 225kV/cm) were achieved for the Pb0.3Sr0.7TiO3 film on LaAlO3 and LaNiO3/LaAlO3, respectively.