Ji Ho Kang's research while affiliated with Myongji University and other places

The structure, morphology, and electrochromic properties of sputtered-WO3 thin films from ceramic and metallic targets were investigated. The films were produced using different types of power sources, namely, direct current (DC), pulsed DC, and radio frequency (RF) generators. Highly transparent as-deposited WO3 thin films grown at room temperature using each target changed to dark blue upon introduction of an electric field in electrolyte regardless of types of sputtering power. WO3 films obtained by sputtering the ceramic target using a pulsed DC power exhibited a large transmittance difference of 97.5% and high coloration efficiency of 55.2 cm²/C compared to the films using DC and RF powers, these films had higher response speeds and stable transmittance modulation after long-term cycle tests. WO3 thin films produced by sputtering a metallic tungsten target showed enhanced electrochromic performance when grown under DC power and a coloration efficiency of 69.1 cm²/C due to their porous microstructure. This article presents methods for the fabrication of low-cost and large-scale oxide electronic devices for various applications, such as electrochromic displays, that can be applied for mass production.

The development of highly efficient and stable red phosphors fabricated using cost-effective and simple mass-production methods is essential for applications in future displays and lighting devices. Herein, we investigate the phase formation, microstructures, and luminescence properties of Sr3Al2O6:Eu powders synthesized using a solid-state reaction process. Single-phase Sr3Al2O6:Eu phosphors exhibiting excellent photoluminescence with color-tunable orange-red and red colors under different excitation wavelengths were successfully synthesized. The optimal Eu doping concentration of Sr3Al2O6, yielding strong photoluminescence with phase stability, was 5 at. %, with the concentration quenching point determined by systematic exploration. The addition of H3BO3 as a flux significantly affected the grain size and improved the emission intensity of the synthesized (Sr0.95Eu0.05)3Al2O6 phosphors up to 12.5 wt. %, while secondary phases were formed at higher flux concentrations. The photoluminescence intensities of the ⁵D0 → ⁷F1 and ⁵D0 → ⁷F2 transitions of 12.5 wt. % H3BO3 added (Sr0.95Eu0.05)3Al2O6 increased by 3.4 and 4.2 times compared with the flux-free powders under the 394-nm-wavelength excitation, while the decay time decreased from 7.35 to 5.40 ms. Our report of the enhanced photoluminescence intensity and shortened decay time of (Sr0.95Eu0.05)3Al2O6 phosphors following the flux addition is promising for production of single-phase phosphors emitting multi-colors, which is needed for optical devices activated by various excitation frequencies.

In this study, a promising method for the modulation of the microstructure, composition, and magnetic properties of thin films fabricated by co-sputtering metallic Fe and oxide SrTiO3 targets is proposed. The proposed method can potentially be applied in multifunctional electronics. The flat surface morphology was observed in the thin film sputtered under a low power for the Fe target, whereas micro-height rods were fabricated on an uneven layer at a higher power. Strong magnetic anisotropy was not observed in the co-sputtered thin films because of the randomly oriented growth of the magnetic rods. Detailed microstructural and compositional studies using high-resolution transmission electron microscopy and scanning transmission electron microscopy revealed that self-assembled nanocomposites with three components were grown with a core–shell structure, in which Fe and FeTiO3 phases grew in the Sr(Ti,Fe)O3 matrix. This report provides an insight into designing multifunctional metal–metal oxide nanocomposites with single-step co-sputtering, which is capable of adjusting the composition and geometry of the nanocomposites.

This article reports on the control of interface morphology degradation between acid-based electrolytes and electrochromic WO3 thin films by stacking Nb2O5 layers to develop highly sustainable and durable electrochromic devices. Thin films of Nb2O5 sputtered at room temperature did not exhibit electrochromic properties, whereas WO3 thin films showed a typical electrochemical response by manifesting reversible color changes from dark blue (under negative potential) to transparent (under reverse potential). As the number of electrochromic cycles in the H2SO4 electrolyte increased, the columnar-structured WO3 thin film considerably changed to a nanoflake structure; however, the microstructure did not significantly change when Nb2O5 layers were stacked on the WO3 layers. The electrochromic performance of bilayer thin films was critically dependent on the stacking order and thickness of each layer. The thin film stack of 100-nm thick Nb2O5/200-nm thick WO3 exhibited enhanced transmittance modulation, response speed, and coloration efficiency compared with that of the 300-nm thick WO3 thin film. Furthermore, up to 1300 cycles, the bilayer thin film exhibited transmittance modulation exceeding 40%; it lost electrochromic switching at the 3500th cycle. In contrast, the single-layer WO3 lost its electrochromic properties at the 700th cycle. This study provides an excellent opportunity for fabricating devices with enhanced electrochromic properties and long-term cyclic durability that can be achieved by stacking Nb2O5 layers.

Eu³⁺-activated WO3-Eu2(WO4)3 luminescent composite thin films are grown on low-cost slide glass substrates by radio frequency sputtering and then annealing in an air atmosphere furnace. The films obtained by sputtering 12 at.% of Eu-added WO3 target afforded a two-phase composite structure comprising Eu-doped WO3 and Eu2(WO4)3 after annealing. The composite thin films annealed at 600 °C exhibited strong red emission after growing at room temperature compared to those annealed at 500 °C under the excitation of 254, 394, and 465 nm corresponding to ultraviolet, near-ultraviolet, and blue light, respectively. The thin films grown at low temperatures without annealing did not emit under ultraviolet light. The crystalline structure, surface morphology, and optical properties, such as transmittance and photoluminescence, of the films grown at 500 °C were almost similar to those of the thin films grown at room temperature when they were annealed at the same temperature. Pure WO3 thin films without Eu doping showed similar crystallization behavior under the same annealing conditions as Eu-added composite films; however, they did not exhibit photoluminescence properties. Our report on the integration of thin-film red phosphors on glass substrates is promising for the development of low-cost and large-area optical devices activated at various excitation wavelengths.