Hye- Jin

Control of work function of MoS2 with ferroelectric polarization in honeycomb-like heterostructure

MoS2, one of the transition metal dichalcogenides (TMDs), has been utilized in heterostructures with functional oxide materials such as ferroelectrics. Here, we report MoS2-ferroelectric heterostructures, especially using the (111)-oriented PbTiO3 epitaxial thin films which have an unconventional polar axis. The mutual interplay between MoS2 and PbTiO3 is investigated via Kelvin probe force microscopy. On the ferroelectric thin films, an induced dipole effect is observed in MoS2, resulting in the variation of surface potential. From the surface potential, work function values of MoS2 are obtained on the ferroelectric polarizations. Also, a decrease and an increase in the work function of MoS2 are observed at in edge and interior regions, respectively. As a result, we performed an innovative approach to control the work function of MoS2 on the ferroelectric thin films. The honeycomb-like heterostructure consisting of MoS2 and PbTiO3 thin films gives the suggestion for the combination of TMDs and ferroelect...

Spatial Investigation on Structural Properties of GdBa 2 Cu 3 O 7 − x Coated Conductors Grown on IBAD-MgO Based Stainless-Steel Substrates

The performance of high-temperature superconducting (HTS) wires has been studied for various substrates and buffer layers. Progress is still being made to find new materials for use as superconducting layers, buffer layer, and substrates for HTS wires. Therefore, it is important to investigate the stacking structure of the commercial wires to improve their performance. Among the commercial wires, GdBCO coated conductors on stainless-steel (STS) were studied in this paper. STS can be purchased at a competitive price, and therefore it is a promising substrate for use in second generation (2G) HTS wires. To analyze structural properties of the GdBCO coated conductors on the STS substrate, several characterization tools are used. Scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (EDS) was used to investigate microstructure of films. In addition, we performed scanning laser microscopy on the conductors at room temperature to identify a map of the inhomogeneity positions. Raman scattering spectroscopy was also used to identify the secondary phases and to verify the phase formation in the films. The results of the analysis revealed the features of GdBCO coated conductors on STS for progress of HTS wires used particularly in rotating machines for high magnetic field applications.

Structural properties and phase formation of epitaxial PbVO3 thin films grown on LaAlO3(0 0 1) by pulsed laser deposition

We report epitaxial synthesis of PbVO3 (PVO) thin films on LaAlO3 (LAO) (0?0?1) substrates under reduction atmosphere from a Pb2V2O7 sintered target over a wide range of temperatures using laser ablation. Polar magnetic PVO has a non-centrosymmetric structure (tetragonality c/a?=?1.23). The effects of substrate temperature on structural properties and chemical composition of PVO thin films were studied by x-ray diffraction and energy-dispersive spectrometry. The c-axis of the PVO films was abnormally elongated as much as 5?7% on the well-lattice-matched LAO substrates. Using Raman scattering spectroscopy, single phase formation of PVO with the shift of phonon modes was confirmed and epitaxial growth of the PVO films was also confirmed with polarization-dependent measurements. The strained and epitaxial growth of the PVO films will lead to a new class of multiferroic materials with future electronic device applications.

Virtual out-of-plane piezoelectric response in MoS2 layers controlled by ferroelectric polarization

The MoS2 carrier distribution can be controlled with the use of a dielectric environment substrate. Ferroelectric thin films are used to investigate the electrical responses at the MoS2 layer. The MoS2/(111)-PbTiO3 vertical heterostructure is investigated, and the electrical responses, including piezoelectricity, are obtained using piezoresponse force microscopy. The piezoelectric response modifications obtained at the MoS2 layer on the ferroelectric thin films are a result of the depolarizing effect. In particular, the piezoelectricity enhancement is observed at the 19-layer MoS2 because of an induced dipole effect. By considering the polarization effects of ferroelectric thin films, the electrical responses at the MoS2 layers can be controlled, and the interfacial carrier distribution at the interface results in different electrical performances at the MoS2.

Hetero-epitaxial growth and large piezoelectric effects in (0 0 1) and (1 1 1) oriented PbTiO3–LaNiO3multilayers

Hetero-epitaxial thin films were obtained in PbTiO3/LaNiO3 thin films by using laser ablation for sequential deposition of each thin film. Structural variations were built to characterize the differences in local ferroelectric properties. For this study, 50 nm-thick-PTO thin films with (0 0 1) and (1 1 1) orientations were investigated. Piezoresponse force microscopy (PFM) was used to investigate the local ferroelectricity in the thin films. High piezoelectric responses were obtained in the thin films, with values of approximately 75 and 83 pm V−1 in the (0 0 1) and (1 1 1)-oriented thin films, respectively. It is notable that remarkable piezoelectric properties were obtained in the (1 1 1)-oriented thin films. The (1 1 1)-oriented thin films have the potential to be used as multi-bit memory because of the reduction of degradation in ferroelectricity and various directional piezoelectricity. We compared the functionalities of the (0 0 1)- and (1 1 1)-oriented thin films and obtained different performances in the thin films.

Surface potential mapping and n-type conductivity in organic-inorganic lead iodide crystals

Organic–inorganic lead halide perovskites have drawbacks in applications due to insufficient and inadequate understanding of the materials. In particular, it is a priority to investigate CH3NH3PbI3 due to its suitable properties and its band gap energy of about 1.59 eV. In this work, we investigated the perovskite crystal with respect to structural, electronic, and optical properties because a single crystal is an ideal platform for examining essential characteristics. The spatial distribution of the perovskite structure was identified by transmission electron microscopy and Kelvin probe force microscopy. The built electronic structure is an important index for the carrier types depending on the surface potential distribution. In particular, a potential variation showed a mixture of the perovskite phase and a decomposed phase on the surface, which also implied the role of surface imperfections as initiation sites for degradation. Therefore, it is critical to verify the carrier types on the surface to enhance the possibilities of improvements for applications.

Microstructural investigation of phases and pinning properties in MBa2Cu3O7−x (M = Y and/or Gd) coated conductors produced by scale-up facilitie

To expedite the commercialization of coated conductors, a robust stacking architecture of the wires must be developed and the performance of the critical currents improved. More importantly, the manufacturability, or large-scale delivery, and the capability of sustaining production at a high rate must be considered. The products of three companies, American Superconductor, Superpower Inc., and SuNAM Co., Ltd, were selected because these companies have announced commercial-grade production lines and delivered a significant amounts of wires to the open market that meet the standards demanded by power devices. X-ray diffraction patterns were used to verify the structural properties and the phase formation in the wires, and transmission electron microscopy with energy dispersive spectroscopy was used to investigate the microstructure and composition of the conductors. In addition, Raman scattering spectroscopy was used for the analysis of the phase formation and for the elucidation of secondary phases in the superconducting layers. The field dependence of the critical current was also studied to compare the transport characteristics under relatively low and medium magnetic field at 77 K and 60 K. Pinning forces were obtained from the field dependence of transport properties and pinning characteristics were investigated. The theoretical and experimental analyses were combined together using the Dew-Hughes formula to extract the scaling exponents and estimate the irreversibility lines of the fields. The results showed that the three conductors possess pinning mechanisms that originate from core pinning with a surface pinning geometry. It is remarkable that the wires discussed in this paper exhibit very similar pinning characteristics even though they have different characteristics in terms of chemical composition, microstructure, stacking architectures, and distribution of parasitic phases.