Changhoon Jung

Silicon/Carbon Nanotube/BaTiO3 Nanocomposite Anode: Evidence for Enhanced Lithium-Ion Mobility Induced by the Local Piezoelectric Potential

We report on the synergetic effects of silicon (Si) and BaTiO3 (BTO) for applications as the anode of Li-ion batteries. The large expansion of Si during lithiation was exploited as an energy source via piezoelectric BTO nanoparticles. Si and BTO nanoparticles were dispersed in a matrix consisting of multiwalled carbon nanotubes (CNTs) using a high-energy ball-milling process. The mechanical stress resulting from the expansion of Si was transferred via the CNT matrix to the BTO, which can be poled, so that a piezoelectric potential is generated. We found that this local piezoelectric potential can improve the electrochemical performance of the Si/CNT/BTO nanocomposite anodes. Experimental measurements and simulation results support the increased mobility of Li-ions due to the local piezoelectric potential.

An electronic structure reinterpretation of the organic semiconductor/electrode interface based on argon gas cluster ion beam sputtering investigations

The effects of the Ar gas cluster ion beam (GCIB) sputtering process on the structural and chemical properties of organic material and the energy-level alignment at the organic semiconductor/electrode interface are studied. The Ar GCIB sputtering process causes no damage to the molecular orientation and structure of the pentacene layer. The thin-film phase (001 at 5.74°, 15.4 A) in the X-ray diffraction patterns and the terrace-like structure in the atomic force microscope images are maintained even after the Ar GCIB sputtering process. Furthermore, there is no change in the chemical bonding state in the organic materials, including pentacene and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS). Finally, to investigate the preservation of the interface properties after the Ar GCIB sputtering process, the valence band structures of the pentacene/PEDOT:PSS and pentacene/Au structures are characterized using bottom-up (in situ ultraviolet photoemission spectroscopy (UPS) analysis with phased pentacene deposition) and top-down (in situ UPS analysis with Ar GCIB sputtering) methods, and the energy levels and chemical states are compared using the same sample. The Ar GCIB sputtering process causes no variation in the primary valence band structure, including the chemical state and configuration. Therefore, the energy-level alignment determined using the top-down method is comparable to that obtained using bottom-up method, since the Ar GCIB sputtering process is damage-free.The effects of the Ar gas cluster ion beam (GCIB) sputtering process on the structural and chemical properties of organic material and the energy-level alignment at the organic semiconductor/electrode interface are studied. The Ar GCIB sputtering process causes no damage to the molecular orientation and structure of the pentacene layer. The thin-film phase (001 at 5.74°, 15.4 A) in the X-ray diffraction patterns and the terrace-like structure in the atomic force microscope images are maintained even after the Ar GCIB sputtering process. Furthermore, there is no change in the chemical bonding state in the organic materials, including pentacene and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS). Finally, to investigate the preservation of the interface properties after the Ar GCIB sputtering process, the valence band structures of the pentacene/PEDOT:PSS and pentacene/Au structures are characterized using bottom-up (in situ ultraviolet photoemission spectroscopy (UPS)...

TCNQ Interlayers for Colloidal Quantum Dot Light-Emitting Diodes.

CdSe/CdS/ZnS quantum dot light-emitting diodes (QD-LEDs) show increased brightness (from ca. 18 000 to 27 000 cd m(-2) ) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) between the QD and electron-transfer layers of ZnO nanoparticles. As QD/ZnO layers are known to have interface defects, our finding leads to the importance of interface engineering for QD-LEDs. Although the photoluminescent intensity and decay lifetime of ZnO/TCNQ/QD layers are similar to those of ZnO/QD layers, cyclic voltammetry suggests improved charge transfer of TCNQ/ZnO layers compared to that of pure ZnO layers. This helps us to understand the mechanism of electrically driven QD-LED behavior, which differs from that of conventional solid-state LEDs, and enables the rational design of QD-based optoelectronic devices.

A novel approach for the characterization of a bilayer of phenyl-c71-butyric-acid-methyl ester and pentacene using ultraviolet photoemission spectroscopy and argon gas cluster ion beam sputtering process

The material arrangement and energy level alignment of an organic bilayer comprising of phenyl-c71-butyric-acid-methyl ester (PCBM-71) and pentacene were studied using ultraviolet photoelectron spectroscopy (UPS) and the argon gas cluster ion beam (GCIB) sputtering process. Although there is a small difference in the full width at half maximum of the carbon C 1s core level peaks and differences in the oxygen O 1s core levels of an X-ray photoemission spectroscopy spectra, these differences are insufficient to clearly distinguish between PCBM-71 and pentacene layers and to classify the interface and bulk regions. On the other hand, the valence band structures in the UPS spectra contain completely distinct configurations for the PCBM-71 and pentacene layers, even when they have similar atomic compositions. According to the valence band structures of the PCBM-71/pentacene/electrodes, the highest unoccupied molecular orbital (HOMO) region of pentacene is at least 0.8 eV closer to the Fermi level than that of ...

High-Performance and Industrially Feasible Ni-Rich Layered Cathode Materials by Integrating Coherent Interphase

For developing the industrially feasible Ni-rich layered oxide cathode with extended cycle life, it is necessary to mitigate both the mechanical degradation due to intergranular cracking between primary particles and gas generation from the reaction between the electrolyte and residual Li in the cathode. To simultaneously resolve these two issues, we herein propose a simple but novel method to reinforce the primary particles in LiNi0.91Co0.06Mn0.03O2 by providing a Li-reactive material, whose spinel interphase is coherent with the surface of the cathode. The modified structure significantly outperforms analogous bare samples: they conserve more than 90% of the initial capacity after 50 cycles and also exhibit a greater rate capability. By tracking the same particle location during cycling, we confirmed that the current method significantly reduces crack generation because the provided coating material can penetrate inside the grain boundary of the secondary particle and help maintain the volume of the primary particle. Finally, first-principles calculations were implemented to determine the role of this spinel material, i.e., having intrinsically isotropic lattice parameters, superior mechanical properties, and only a small volume change during delithiation. We believe that the proposed method is straightforward and cost-effective; hence, it is directly applicable for the mass production of Ni-rich cathode material to enable its commercialization.

Understanding the structural, electrical, and optical properties of monolayer h-phase RuO2 nanosheets: A combined experimental and computational study

The structural, electrical, and optical properties of monolayer ruthenium oxide (RuO2) nanosheets (NSs) fabricated by chemical exfoliation of a layered three-dimensional form of K-intercalated RuO2 are studied systematically via experimental and computational methods. Monolayer RuO2 NS is identified as having a distorted h-MX2 structure. This is the first observation of a RuO2 NS structure that is unlike the t-MX2 structure of the RuO2 layers in the parent material and does not have hexagonal symmetry. The distorted h-MX2 RuO2 NSs are shown to have optical transparency superior to that of graphene, thereby predicting the feasibility of applying RuO2 NSs to flexible transparent electrodes. In addition, it is demonstrated that the semiconducting band structures of RuO2 NSs can be manipulated to be semi-metallic by adjusting the crystal structure, which is related to band-gap engineering. This finding indicates that RuO2 NSs can be used in a variety of applications, such as flexible transparent electrodes, atomic-layer devices, and optoelectronic devices.Two-dimensional materials: A more transparent way to get in contactNanosheets of ruthenium oxide could make excellent transparent electrical contacts, show researchers from Korea. Graphene is the wonder material of the last decade owing to its amazing electrical, mechanical and thermal properties. Scientists are thus keen to fabricate single layers of atoms other than carbon. Now, Dong-Su Ko, Jung-Hwa Kim and Jong Wook Roh from the Samsung Advanced Institute of Technology and co-workers have combined experiments and theory to fully characterize this unusual two-dimensional material. They created their nanosheets by exfoliating a three-dimensional block of ruthenium oxide. Transmission electron microscopy, X-ray diffraction experiments and first-principles calculations showed that the two-dimensional material has a distorted atomic arrangement, which makes it a semiconductor rather than a metal like its parent material. Furthermore, ruthenium oxide nanosheets are more transparent than graphene, making them useful for flexible transparent electrodes.As a new two-dimensional (2D) material, monolayer ruthenium oxide (RuO2) nanosheets (NSs) have distorted h-MX2 type crystal structures that lead to semiconducting properties and good optical transmittance. This study suggests that monolayer RuO2 can be useful in applications of flexible optoelectronics.

Nanotube and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) composite film for the electrode applications in organic thin-film transistor and dye-sensitized solar cells

In this study, composite films made of coiled carbon nanotubes (CCNTs) and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) were fabricated with different composition ratios. The variations in film properties (including surface morphology, work function, and electrical conductivity) in accordance with the amount of CCNT dosing were investigated. Subsequently, through HCl-methanol treatment, we achieved a significant enhancement in electrical conductivity with little damage to the CCNT features. The characteristics of CCNT/PEDOT:PSS composite film are generally comparable to those of PEDOT:PSS film, and some of them, such as catalytic activity and work function, are significantly higher. On the basis of these versatile features, the CCNT/PEDOT:PSS composite films exhibit excellent performance as source/drain electrode in organic thin-film transistors and as catalytic counter electrode in dye-sensitized solar cells.