Kyoung Jin Choi

Novel fabrication of an SnO2 nanowire gas sensor with high sensitivity

We fabricated a nanowire-based gas sensor using a simple method of growing SnO(2) nanowires bridging the gap between two pre-patterned Au catalysts, in which the electrical contacts to the nanowires are self-assembled during the synthesis of the nanowires. The gas sensing capability of this network-structured gas sensor was demonstrated using a diluted NO(2). The sensitivity, as a function of temperature, was highest at 200u2009°C and was determined to be 18 and 180 when the NO(2) concentration was 0.5 and 5xa0ppm, respectively. Our sensor showed higher sensitivity compared to different types of sensors including SnO(2) powder-based thin films, SnO(2) coating on carbon nanotubes or single/multiple SnO(2) nanobelts. The enhanced sensitivity was attributed to the additional modulation of the sensor resistance due to the potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire.

Evolution of optical phonons in CdS nanowires, nanobelts, and nanosheets

We report Raman scattering from single and ensemble CdS nanowires, nanobelts, and nanosheets. The Raman spectra of nanobelts and nanosheets are notably different from those of nanowires, exhibiting a strong enhancement of the multiphonon response. Moreover, the first-order longitudinal optical (LO) phonon energy systematically increases with increasing lateral size from nanowires to nanobelts, and to nanosheets. These results suggest that the optical phonons in the CdS nanostructures are influenced by strain, crystallinity, and exciton-LO phonon coupling.

Metal/graphene sheets as p-type transparent conducting electrodes in GaN light emitting diodes

We demonstrate the use of graphene based transparent sheets as a p-type current spreading layer in GaN light emitting diodes (LEDs). Very thin Ni/Au was inserted between graphene and p-type GaN to reduce contact resistance, which reduced contact resistance from ∼5.5 to ∼0.6Ω/cm2, with no critical optical loss. As a result, LEDs with metal-graphene provided current spreading and injection into the p-type GaN layer, enabling three times enhanced electroluminescent intensity compared with those with graphene alone. We confirmed very strong blue light emission in a large area of the metal-graphene layer by analyzing image brightness.

Stable field emission performance of SiC-nanowire-based cathodes

In this paper we report the fabrication and testing of diode-type low-voltage field emission display (FED) devices with SiC-nanowire-based cathodes. The SiC-nanowire FEDs (flat vacuum lamps) were characterized by low emission threshold fields (∼2xa0Vxa0µm(-1)), high current density and stable long-term performance. The analysis of field emission data evidenced that the Schottky effect would have a considerable influence on the field emission from nanowire-based samples, leading to the true values of the field enhancement factor being significantly lower than those derived from Fowler-Nordheim plots.

Graphene as an Interfacial Layer for Improving Cycling Performance of Si Nanowires in Lithium-Ion Batteries

Managing interfacial instability is crucial for enhancing cyclability in lithium-ion batteries (LIBs), yet little attention has been devoted to this issue until recently. Here, we introduce graphene as an interfacial layer between the current collector and the anode composed of Si nanowires (SiNWs) to improve the cycling capability of LIBs. The atomically thin graphene lessened the stress accumulated by volumetric mismatch and inhibited interfacial reactions that would accelerate the fatigue of Si anodes. By simply incorporating graphene at the interface, we demonstrated significantly enhanced cycling stability for SiNW-based LIB anodes, with retentions of more than 2400 mAh/g specific charge capacity over 200 cycles, 2.7 times that of SiNWs on a bare current collector.

Photoluminescence behavior of Al 3+ , Pr 3+ doped perovskite La 2/3 TiO 3 and pyrochlore La 2 Ti 2 O 7

The purpose of this study is to develop an understanding of the photoluminescence properties of Al 3+ , Pr 3+ doped perovskite-type La 2/3 TiO 3 and pyrochlore-type La 2 Ti 2 O 7 phosphor, which is characterized by the red emission ( 1 D 2 → 3 H 4 transition) of the Pr 3+ ion. The explanation for the energy transfer and the corresponding critical distance is proposed on the basis of the role of Al 3+ ions in the perovskite-type La 2/3 TiO 3 :Pr phosphor. To clarify the distinction of photoluminescence properties between the perovskite-type La 2/3 TiO 3 and the pyrochlore-type La 2 Ti 2 O 7 , the trap-involved process and the charge transfer band have been investigated, respectively.

Preparation of TiO2 nanofibers immobilized on quartz substrate by electrospinning for photocatalytic degradation of ranitidine

The photocatalytic activity of TiO2 nanofibers immobilized on quartz substrates was investigated by evaluating the decomposition of organic pollutants. TiO2 nanofibers were synthesized by electrospinning the Ti-precursor/polymer mixture solution, followed by hot-pressing for enhancing the adhesion of TiO2-nanofiber films to the substrates. TiO2 started to crystalize in the anatase form at 500xa0°C and reached the optimal photocatalytic anatase/rutile phase ratio of 70:30 at a calcination temperature of 600xa0°C. The TiO2-nanofiber film was demonstrated to be an efficient photocatalyst by ranitidine decomposition under UV illumination and was proven to have a comparable photocatalytic activity with the well-known Degussa P25 nanoparticulate photocatalyst and excellent recyclability during 10 cycles of photocatalytic operation, indicating no loss of TiO2 nanofibers during photocatalytic operations.

Laterally grown SnO2 nanowires and their NO2 gas sensing characteristics

One dimensional semiconducting nanomaterials have attracted considerable interest for their potential as the building blocks for fabricating various nanodevices. In this paper, a simple and efficient way of preparing highly sensitive SnO2 nanowire-based gas sensor without an arduous lithography process was studied. The SnO2 nanowires could be grown laterally upon the Si substrate by separating the Au catalyst layer in the substrate. As the electric current is transported along the networks of the nanowires, not along the bottom layer on the substrate, the sensitivity to gases was maximized in this lateral-type structures.

Structural characteristics of ternary In(x)Ga(1-x)As nanowires on Si (111) grown via Au-catalyzed VLS.

We have characterized the structural properties of the ternary In(x)Ga(1-x)As nanowires (NWs) grown on silicon (Si) substrates using metalorganic chemical vapor deposition (MOCVD). Au catalyzed vapor-liquid-solid (VLS) mode was used for the NW growth. The density of the In(x)Ga(1-x)As NW array grown under optimized condition exceeds 1 x 10(8)/cm2. X-ray diffraction (XRD) spectra confirm the In composition (x = 0.9-0.3) of the In(x)Ga(1-x)As nanowires which bandgap energy can cover the entire near-infrared (NIR) range. Massive stacking faults and twin planes were observed but no misfit dislocation was found along the NWs as confirmed by transmission electron microscopy (TEM). The energy-dispersive X-ray spectroscopy (EDS) analysis shows the gradual variation of In composition along the NW.