Kijung Yong

Highly efficient visible light photocatalysis of novel CuS/ZnO heterostructure nanowire arrays.

Here, a facile approach for the fabrication of CuS nanoparticle (NP)/ZnO nanowire (NW) heterostructures on a mesh substrate through a simple two-step solution method is demonstrated. Successive ionic layer adsorption and reaction (SILAR) was employed to uniformly deposit CuS NPs on the hydrothermally grown ZnO NW array. The synthesized CuS/ZnO heterostructure NWs exhibited superior photocatalytic activity under visible light compared to bare ZnO NWs. This strong photocatalytic activity under visible light is due to the interfacial charge transfer (IFCT) from the valence band of the ZnO NW to the CuS NP, which reduces CuS to Cu(2)S. After repeated cycles of photodecolorization of Acid Orange 7 (AO7), the photocatalytic behavior of CuS/ZnO heterostructure NWs exhibited no significant loss of activity. Furthermore, our CuS/ZnO NWs/mesh photocatalyst floats in solution via partial superhydrophobic modification of the NWs.

A Light Incident Angle Switchable ZnO Nanorod Memristor: Reversible Switching Behavior Between Two Non‐Volatile Memory Devices

A light incident angle selectivity of a memory device is demonstrated. As a model system, the ZnO resistive switching device has been selected. Electrical signal is reversibly switched between memristor and resistor behaviors by modulating the light incident angle on the device. Moreover, a liquid passivation layer is introduced to achieve stable and reversible exchange between the memristor and WORM behaviors.

Self-assembled foam-like graphene networks formed through nucleate boiling

Self-assembled foam-like graphene (SFG) structures were formed using a simple nucleate boiling method, which is governed by the dynamics of bubble generation and departure in the graphene colloid solution. The conductivity and sheet resistance of the calcined (400°C) SFG film were 11.8u2005S·cm–1 and 91.2 Ω□−1, respectively, and were comparable to those of graphene obtained by chemical vapor deposition (CVD) (~10u2005S·cm–1). The SFG structures can be directly formed on any substrate, including transparent conductive oxide (TCO) glasses, metals, bare glasses, and flexible polymers. As a potential application, SFG formed on fluorine-doped tin oxide (FTO) exhibited a slightly better overall efficiency (3.6%) than a conventional gold electrode (3.4%) as a cathode of quantum dot sensitized solar cells (QDSSCs).

Non-vacuum deposition of CIGS absorber films for low-cost thin film solar cells

Thin film solar cells composed of chalcopyrite Cu(In1−xGax)(Se1−ySy)2 (CIGSSe) absorbers have gained considerable attention in recent years in an effort to develop sustainable technologies for harnessing clean energy. Nonvacuum solution methods can reduce production costs by replacing vacuum-based deposition methods with large-scale, high-throughput processes. The efficient use of materials can reduce production costs. Non-vacuum processes generally rely on two sequential steps: solution-coating, followed by a post-annealing process. Depending on the point at which the CIGS phase evolves, non-vacuum processes can be categorized as nanoparticle (NP) approaches or molecular precursor approaches. These two types of liquid processes are believed to be compatible with a variety of applications, such as roll-to-roll coating for the production of flexible, portable devices. Additional thermal treatments using a gaseous chalcogen or oxygen can improve the absorber quality. This review describes the current status of chalcopyrite thin film solar cells fabrication methods via low-cost solution routes. An analysis of recently published reports describing liquid-based deposition methods is introduced, and the features of the development steps are compared. Finally, a discussion and future outlook are offered.

Highly efficient photoelectrochemical hydrogen generation using a quantum dot coupled hierarchical ZnO nanowires array.

A highly efficient photoelectrochemical cell was developed using a quantum dot semiconductor coupled hierarchical ZnO nanostructure for hydrogen generation. The hierarchical ZnO nanostructure consists of ZnO nanowire as a core and 2-dimensional ZnO nanosheets shelled on the ZnO nanowire surface. This multi-dimensional nanostructured photoanode was optimized with solution process conditions for efficient charge separation and transportation, and the visible light harvesting was effectively enhanced using the low bandgap semiconductor cadmium chacogenide quantum dots. Our hierarchical ZnO nanowire photoelectrode has greatly improved the saturated photocurrent density (17.5 mA/cm(2)) at 0.4 V vs. RHE under 1 sun illumination condition. In addition, we have modified the photoelectrode with IrO(x)·nH2O deposition to enhance the photostability, showing a noticeable improvement in steady generation of photocurrent under the light illumination condition.

Highly durable and efficient quantum dot-sensitized solar cells based on oligomer gel electrolytes.

For stable quantum dot-sensitized solar cells, an oligomer-contained gel electrolyte was employed with a carbon-based counter electrode and a hierarchically shelled ZnO photoelectrode. Poly(ethylene glycol) dimethyl-ether (PEGDME) was added to the polysulfide electrolyte to enhance the stability of the methanol-based electrolyte. In addition, the nanocomposite gel electrolyte with fumed silica was used, which provided a solid three-dimensional network. A quantum-dot-modified ZnO nanowire photoanode enhanced the visible light harvesting, and a Pt/CNT-RGO counter electrode increased the catalytic activity. The oligomer gel electrolyte prevented the liquid electrolyte from leaking, and the carbon-based counter electrode retarded chemical poisoning at the counter electrode. The optimized cell exhibited 5.45% photoelectric conversion efficiency with long-term stability demonstrated over 5000 s operation time.

An exceptionally facile method to produce layered double hydroxides on a conducting substrate and their application for solar water splitting without an external bias

An exceptionally facile process is presented for in situ formation of zinc chromium layered double hydroxide (ZnCr:LDH) nanosheets on a conducting substrate. Thus, ZnCr:LDH nanosheets were synthesized from a metallic Zn film/fluorine-doped tin oxide (FTO) glass by simply dipping into a Cr nitrate solution for only one minute at room temperature. Then, ZnCr:LDHs were converted into zinc chromium mixed metal oxide (ZnCr:MMO) nanoparticles by calcination. Under visible light irradiation (λ > 420 nm), the in situ synthesized ZnCr:MMO photoanode exhibited a stable and an order-of-magnitude higher activity for photoelectrochemical water splitting than that of a ZnCr:MMO film fabricated ex situ by electrophoretic deposition of already-synthesized ZnCr:MMO powders. More significant was that it generated anodic photocurrents even without an externally applied bias potential, which is an unprecedented result for an oxide photoanode-driven PEC system working under visible light.

Freestanding CdS nanotube films as efficient photoanodes for photoelectrochemical cells

Freestanding CdS nanotube films were synthesized for the first time using ZnO nanorod arrays as a sacrificial template. The CdS nanotube films exhibited excellent photo-anodic activities in a photoelectrochemical (PEC) cell.

Low-voltage bendable pentacene thin-film transistor with stainless steel substrate and polystyrene-coated hafnium silicate dielectric.

The hafnium silicate and aluminum oxide high-k dielectrics were deposited on stainless steel substrate using atomic layer deposition process and octadecyltrichlorosilane (OTS) and polystyrene (PS) were treated improve crystallinity of pentacene grown on them. Besides, the effects of the pentacene deposition condition on the morphologies, crystallinities and electrical properties of pentacene were characterized. Therefore, the surface treatment condition on dielectric and pentacene deposition conditions were optimized. The pentacene grown on polystyrene coated high-k dielectric at low deposition rate and temperature (0.2-0.3 Å/s and R.T.) showed the largest grain size (0.8-1.0 μm) and highest crystallinity among pentacenes deposited various deposition conditions, and the pentacene TFT with polystyrene coated high-k dielectric showed excellent device-performance. To decrease threshold voltage of pentacene TFT, the polystyrene-thickness on high-k dielectric was controlled using different concentration of polystyrene solution. As the polystyrene-thickness on hafnium silicate decreases, the dielectric constant of polystyrene/hafnium silicate increases, while the crystallinity of pentacene grown on polystyrene/hafnium silicate did not change. Using low-thickness polystyrene coated hafnium silicate dielectric, the high-performance and low voltage operating (<5 V) pentacene thin film transistor (μ: ~2 cm(2)/(V s), on/off ratio, >1 × 10(4)) and complementary inverter (DC gains, ~20) could be fabricated.

Molecular conformation dependent emission behaviour (blue, red and white light emissions) of all-trans-β-carotene-ZnS quantum dot hybrid nanostructures

A novel hybrid material consisting of all-trans-β-carotene and ZnS quantum dots (QDs) was prepared by two different controlled experiments and their photophysical properties were investigated in detail. Depending upon the preparation method, the β-carotene molecule had formed either an upright or wrapped conformation around the ZnS QDs and consequently the interaction between these two hybridizing systems was either charge transfer induced or electrostatic in type, respectively. Optical absorption, Raman and FTIR investigations have confirmed the two different conformations of the molecule around the ZnS QDs. Because of the two different conformations and the consequent interactions, different emission colours, such as blue and red wavelengths were obtained from these hybrids. We were also able to obtain white light emission by using cadmium doped ZnS QDs for the hybrid preparation.