• Korean Journal of Materials Research
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• v.31 no.7
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• pp.382-385
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• 2021

Well-crystallized vanadium pentoxide V₂O₅ thin films are fabricated on MgO single crystal substrates by using pulsed-laser deposition technique. The linear optical transmission spectra are measured and found to be in a wavelength range from 300 to 800 nm; the data are used to determine the linear refractive index of the V₂O₅ films. The value of linear refractive index decreases with increasing wavelength, and the relationship can be well explained by Wemple's theory. The third-order nonlinear optical properties of the films are determined by a single beam z-scan method at a wavelength of 532 nm. The results show that the prepared V₂O₅ films exhibit a fast third-order nonlinear optical response with nonlinear absorption coefficient and nonlinear refractive index of 2.13 × 10^-10 m/W and 2.07 × 10^-15 cm²/kW, respectively. The real and imaginary parts of the nonlinear susceptibility are determined to be 3.03 × 10^-11 esu and 1.12 × 10^-11 esu, respectively. The enhancement of the nonlinear optical properties is discussed.

• Journal of Electrochemical Science and Technology
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• v.14 no.3
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• pp.272-282
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• 2023

Ni-rich layered oxides cathode has recently gained attention as an advanced cathode material due to their applicable energy density. However, as the Ni component in the layered site is increased, the high reactivity of Ni⁴⁺ results in parasitic reaction associated with decomposing electrolyte, which leads to a rapid decreasing the lifespan of the cell. The electrolyte additive triallyl borate (TAB) improves interfacial stability, leading to a stable cathode-electrolyte interphase (CEI) layer on the LNCM83 cathode. A multi-functionalized TAB additive can produce a uniformly distributed CEI layer via electrochemical oxidation, which implies an increase in long-term cycling performance. After 100 cycles at elevated temperature, the cell tested by 0.75 TAB retained 88.3% of its retention ratio, whereas the cell performed by TAB-free electrolyte retained 64.1% of its retention. Once the TAB additive formed CEI layers on the LNCM83 cathode, it inhibited the decomposition of carbonate-based solvents species in addition to the dissolution of transition metal components from the cathode. The addition of TAB to LNCM83 cathode material is believed to be a promising way to increase the electrochemical performance.

• Journal of the Korean institute of surface engineering
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• v.57 no.2
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• pp.57-70
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• 2024

Nickel-cobalt-manganese (NCM) lithium-ion batteries(LIBs) are increasingly prominent in the energy storage system due to their high energy density and cost-effectiveness. However, they face significant challenges, such as rapid capacity fading and structural instability during high-voltage operation cycles. Addressing these issues, numerous researchers have studied the enhancement of electrochemical performance through the coating of NCM cathode materials with substances like metal oxides, lithium composites, and polymers. Coating these cathode materials serves several critical functions: it acts as a protection barrier against electrolyte decomposition, mitigates the dissolution of transition metals, enhances the structural integrity of the electrode, and can even improve the ionic conductivity of the cathode. Ultimately, these improvements lead to better cycle stability, increased efficiency, and enhanced overall battery life, which are crucial for the advancement of NCM-based lithium-ion batteries in high-demand applications. So, this paper will review various cathode coating materials and examine the roles each plays in improving battery performance.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.35 no.3
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• pp.297-302
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• 2022

We investigated the origin of magnetic behaviors induced by an asymmetric spin exchange interaction in Fe-site engineered lead iron niobate [Pb(Fe_1/2Nb_1/2)O₃, PFN], which exhibits a room-temperature multiferroicity. The magnitude of spin exchange interaction was regulated by the introduced transition metals with a distinct Bohr magneton, i.e., Cr, Co, and Ni. All compositions were found to have a single-phase perovskite structure keeping their ferroelectric order except for Cr introduction. We discovered that the incorporation of each transition metal imposes a distinct magnetic behavior on the lead iron niobate system; antiferro-, hard ferro-, and soft ferromagnetism for Cr, Co, and Ni, respectively. This indicates that orbital occupancy and interatomic distance play key roles in the determination of magnetic behavior rather than the magnitude of the individual Bohr magneton. Further investigations are planned, such as X-ray absorption spectroscopy, to clarify the origin of magnetic properties in this system.

• Membrane Journal
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• v.29 no.6
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• pp.299-307
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• 2019

Photocatalysis is an environment friendly technique for degrading organic dyes in water. Tungsten oxide is becoming an active area of research in photocatalysis nanomaterials for having a smaller bandgap than the previously favored titanium dioxide. Synthesis of hierarchical structures, doping platinum (Pt), coupling with nanocomposites or other semiconductors are investigated as valid methods of improving the photocatalytic degradation efficiency. These impact the reaction by creating a redshift in the wavelength of light used, effecting charge transfer, and the formation/recombination of electron-hole pairs. Each of the methods mentioned above are investigated in terms of synthesis and photocatalytic efficiency, with the simplest being modification on the morphology of tungsten oxide, since it does not need synthesis of other materials, and the most efficient in photocatalytic degradation being complex coupling of metal oxides and carbon composites. The photocatalysis technology can be incorporated with water purification membrane by modularization process and applied to advanced water treatment system.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.380-380
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• 2010

Currently, organic light-emitting diodes (OLEDs) have been proven of their readiness for commercialization in terms of lifetime and efficiency. In accordance with emerging new technologies, enhancement of light efficiency and extension of application fields are required. Particularly inverted structures, in which electron injection occurs at bottom and hole injection on top, show crucial advantages due to their easy integration with Si-based driving circuits for active matrix OLED as well as large open area for brighter illumination. In order to get better performance and process reliability, usually a proper buffer layer for carrier injection is needed. In inverted top emission OLED, the buffer layer should protect underlying organic materials against destructive particles during the electrode deposition, in addition to increasing their efficiency by reducing carrier injection barrier. For hole injection layers, there are several requirements for the buffer layer, such as high transparency, high work function, and reasonable electrical conductivity. As a buffer material, a few kinds of transition metal oxides for inverted OLED applications have been successfully utilized aiming at efficient hole injection properties. Among them, we chose 2 nm of $WO_3$ between NPB [N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine] and Au (or Al) films. The interfacial energy-level alignment and chemical reaction as a function of film coverage have been measured by using in-situ ultraviolet and X-ray photoelectron spectroscopy. It turned out that the $WO_3$ interlayer substantially reduces the hole injection barrier irrespective of the kind of electrode metals. It also avoids direct chemical interaction between NPB and metal atoms. This observation clearly validates the use of $WO_3$ interlayer as hole injection for inverted OLED applications.

• Journal of the Korean Magnetics Society
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• v.23 no.5
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• pp.149-153
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• 2013

The electronic structures of $TCr_2O_4$ (T = Fe, Co, Ni) spinel oxides have been investigated by employing synchrotron radiation-based soft X ray absorption spectroscopy (XAS). The measured 2p XAS spectra of transition-metal ions reveal that Cr ions are trivalent ($Cr^{3+}$), and all the T (T = Fe, Co, Ni) ions are divalent ($Fe^{2+}$, $Co^{2+}$, $Ni^{2+}$). It is also found that most of T (T = Fe, Co, Ni) ions occupy the A sites under the tetrahedral symmetry, while Cr ions occupy mainly the B sites under the octahedral symmetry. These findings show that the structures of $TCr_2O_4$ (T = Fe, Co, Ni) are very close to the normal spinel structures. Based on these findings, it is expected that Jahn-Teller (JT) effects are important in $FeCr_2O_4$ and $NiCr_2O_4$. In contrast, $CoCr_2O_4$ maintains the cubic structure without having the JT distortion since both $Cr^{3+}$ and $Co^{2+}$ ions are non-JT ions. This work suggests that the antiferromagnetic interaction between $Cr^{3+}$ and $T^{2+}$ ions plays an important role in determining the magnetic properties of $TCr_2O_4$ (T = Fe, Co, Ni).

• Korean Journal of Materials Research
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• v.22 no.6
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• pp.316-320
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• 2012

$Co_3O_4$, $Al_2O_3$ and $Co_3O_4$/$Al_2O_3$ mesoporous powders were prepared by a sol-gel method with starting matierals of aluminum isopropoxide and cobalt (II) nitrate. A P123 template is employed as an active organic additive for improving the specific surface area of the mixed oxide by forming surfactant micelles. A transition metal cobalt oxide supported on alumina with and without P123 was tested to find the most active and selective conditions as a heterogeneous catalyst in the reaction of styrene epoxidation. A bBlock copolymer-P123 template was added to the staring materials to control physical and chemical properties. The properties of $Co_3O_4$/$Al_2O_3$ powder with and without P123 were characterized using an X-ray diffractometer (XRD), a Field-Emission Scanning Electron Microscope (FE-SEM), a Bruner-Emmertt-Teller (BET) surface analyzer, and $^{27}Al$ MAS NMR spectroscopy. Powders with and without P123 were compared in catalytic tests. The catalytic activity and selectivity were monitored by GC/MS, $^1H$, and $^{13}C$-NMR spectroscopy. The performance for the reaction of epoxidation of styrene was observed to be in the following order: [$Co_3O_4$/$Al_2O_3$ with P123-1173 K > $Co_3O_4$/$Al_2O_3$ with P123-973 K > $Co_3O_4$-973 K>$Co_3O_4$/$Al_2O_3$-973 K > $Co_3O_4$/$Al_2O_3$ with P123-1473 K > $Al_2O_3$-973 K]. The existence of ${\gamma}$-alumina and the nature of the surface morphology are related to catalytic activity.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.318.2-318.2
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• 2016

FeM2X4 spinel structures, where M is a transition metal and X is oxygen or sulfur, are candidate materials for spin filters, one of the key devices in spintronics. Both the Fe and M ions can occupy tetrahedral and octahedral sites; therefore, these types of compounds can display various physical and chemical properties [1]. On the other hand, the electronic and magnetic properties of these spinel structures could be modified via the control of cation distribution [2, 3]. Among the spinel oxides, iron manganese oxide is one of promising materials for applications. FeMn2O4 shows inverse spinel structure above 390 K and ferrimagnetic properties below the temperature [4]. In this work, we report on the structural and magnetic properties of epitaxial FeMn2O4 thin film on MgO(100) substrate. The reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD) results indicated that films were epitaxially grown on MgO(100) without the impurity phases. The valance states of Fe and Mn in the FeMn2O4 film were carried out using x-ray photoelectron spectrometer (XPS). The magnetic properties were measured by vibrating sample magnetometer (VSM), indicating that the samples are ferromagnetic at room temperature. The structural detail and origin of magnetic ordering in FeMn2O4 will be discussed.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.392.1-392.1
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• 2016

Nowadays, research interest in developing the wearable devices are growing remarkably. Portable consumer electronic systems are becoming lightweight, flexible and even wearable. In fact, wearable electronics require energy storage device with thin, foldable, stretchable and conformable properties. Accordingly, developing the flexible energy storage devices with desirable abilities has become the main focus of research area. Among various energy storage devices, supercapacitors have been considered as an attractive next generation energy storage device owing to their advantageous properties of high power density, rapid charge-discharge rate, long-cycle life and high safety. The energy being stored in pseudocapacitors is relatively higher compared to the electrochemical double-layer capacitors, which is due to the continuous redox reactions generated in the electrode materials of pseudocapacitors. Generally, transition metal oxides/hydroxide (such as $Co_3O_4$, $Ni(OH)_2$, $NiFe_2O_4$, $MnO_2$, $CoWO_4$, $NiWO_4$, etc.) with controlled nanostructures (NSs) are used as electrode materials to improve energy storage properties in pseudocapacitors. Therefore, different growth methods have been used to synthesize these NSs. Of various growth methods, electrochemical deposition is considered to be a simple and low-cost method to facilely integrate the various NSs on conductive electrodes. Herein, we synthesized amorphous $NiWO_4$ NSs on cost-effective conductive textiles by a facile electrochemical deposition. The as-grown amorphous $NiWO_4$ NSs served as a flexible and efficient electrode for energy storage applications.