Sang-Kwon Lee

Focused ion beam-assisted manipulation of single and double β-SiC nanowires and their thermal conductivity measurements by the four-point-probe 3-ω method

Control of one-dimensional (1D) nanostructures is demonstrated in this paper by selectively placing and aligning silicon carbide (beta-SiC) nanowires (NWs). We developed a reliable and highly reproducible way of placing a single or double SiC NW on pre-patterned electrodes by using a focused ion beam and a nanomanipulator. 3-omega signals obtained by the four-point-probe method were used in measuring the thermal conductivity of the NWs. The thermal conductivities of the placed single and double beta-SiC NWs were obtained at 82 +/- 6 W mK( - 1) and 73 +/- 5 W mK( - 1), respectively. The proposed technique offers new possibilities for manipulating and evaluating 1D nanoscale materials.

3C-Silicon Carbide Nanowire FET: An Experimental and Theoretical Approach

Experimental and simulated I-V characteristics of silicon carbide (SiC) nanowire-based field-effect transistors (NWFETs) are presented. SiC NWs were fabricated by using the vapor-liquid-solid mechanism in a chemical vapor deposition system. The diameter of fabricated SiC NWs varied from 60 up to 100 nm while they were some micrometers long. Their I-V characteristics were simulated with SILVACO software, and special attention was paid to explore the role of NW doping level and NW/dielectric interface quality. The fabricated SiC-based NWFETs exhibit a mediocre gating effect and were not switched-off by varying the gate voltage. Based on the simulations, this is a result of the high unintentional doping (estimated at 1times1019 cm-3) and the poor NW/dielectric interface quality. Moreover, a homemade algorithm was used to investigate the ideal properties of SiC-based NWFETs in ballistic transport regime, with NW lengths of 5-15 nm and a constant diameter of 4 nm for which the carrier transport is fully controlled by quantum effects. This algorithm self-consistently solves the Poisson equation with the quantum nonequilibrium Green function formalism. In the ballistic regime, devices with undoped SiC NWs exhibit superior theoretical performances (transconductance: ~43.2times10-6 A/V and ION/IOFF=1.6times105 for a device with 9-nm NW length) based on their simulated characteristics.

Current transport mechanism in a metal–GaN nanowire Schottky diode

We investigated nano-Schottky diodes of gallium nitride nanowires with three Schottky metals (Cr, Ti, and Au) using current-voltage characteristics. All of the GaN nano-Schottky diodes showed a rectifying behavior. The abnormal electrical characteristics of a single GaN nanowire Schottky diode can be explained by a thermionic-field emission and an enhancement of the tunneling effects owing to both the relatively high concentration of the GaN nanowire itself and the nanoscale junction size of the GaN nanowire Schottky diodes.

Cell adhesion and migration on nanopatterned substrates and their effects on cell-capture yield

With scanning electron microscopy analysis, we investigated the role of nanoscale topography on cellular activities; e.g. cell adhesion and spreading by culturing A549 cells (human lung carcinoma cell line cells) for 1-48 h on three sets of nanostructures; quartz nanopillars (QNPs), silicon nanopillars and silicon nanowire (SiNW) arrays, along with planar glass substrates. We found that cells on QNP arrays developed a longer shape than those on SiNW arrays. In addition, we studied how cell morphologies influence the cell-capture yield on the three sets of nanostructures. This research showed that the filopodial formations were directing the cell-capture yield on nanostructured substrates. This finding implies the possibility of using nanoscale topography features to control the filopodial formation including extension and migration from the cells. Using streptavidin-functionalized SiNW substrate, we further demonstrated a substantially higher yield (~91.8 ± 5.9%) than the planar glass wafers (~24.1 ± 7.5%) in the range of 200-3000 cells.

ZnO Nanorod–TiO2-Nanoparticulate Electrode for Dye-Sensitized Solar Cells

Highly dense ZnO nanorods were synthesized on TiO2-nanoparticulate coated fluorine-doped tin oxide (FTO) substrates by the chemical vapor deposition method for dye-sensitized solar cells (DSSCs). The uniformly grown ZnO nanorod layer has a thickness of ~4 µm on the TiO2-nanoparticulate layer with a wurtzite structures as confirmed by the X-ray diffraction pattern. The DSSC fabricated with a ZnO nanorod/TiO2-nanoparticulate electrode had an overall light-to-electricity conversion efficiency η of 3.7% with a short-circuit current density JSC of 8.12 mA/cm2, open-circuit voltage VOC of 0.76 V, and fill factor FF of 0.59, whereas ZnO nanowire/TiO2-nanoparticulate-electrode-based DSSCs exhibited a low η of 1.1% with JSC of 2.14 mA/cm2 and slightly high VOC of 0.79 V. It is expected that the enhanced photovoltaic performance of the ZnO nanorod/TiO2-nanoparticulate electrode can be attributed to high dye loading and high light harvesting through large surface areas of ZnO nanorods incorporated with TiO2-nanoparticulate as compared with the ZnO nanowire/TiO2-nanoparticulate electrode.

Statistical analysis of immuno-functionalized tumor-cell behaviors on nanopatterned substrates

Laser scanning cytometry has been proven as a powerful technology for high-content, high-throughput quantitative analysis of cellular functions in a fully automated manner. It utilizes a large-area fluorescence imaging scheme and rigorous image quantitation algorithms to enable informative analysis of cell samples attached to solid substrates. While this technology represents a powerful approach for high-content screening using cell lines, it has not been applied to the study of tumor-cell behaviors on these solid nanopatterned substrates after several hours of incubation. Herein, we statistically demonstrated functional cellular morphology information, including size, shape, and distribution of the captured cells after 0.5 to 45 h of incubation on nanopatterned substrates, such as silicon nanowires and quartz nanopillars, along with planar glass substrates. With increasing incubation time up to 45 h, we observed that the nanopatterned substrates could have not only increased adhesion and traction forces between cells and nanopatterned substrates, but also limited cell spreading on the substrates compared to the planar glass substrates. On the basis of our results, we suggest that the most important factors to influence the cell behaviors on the three solid substrates are the degree of dimension on cell behaviors and cell traction force.

The formation and characterization of electrical contacts (Schottky and Ohmic) on gallium nitride nanowires

We report on the fabrication and characterization of Ti/Au Ohmic contacts to unintentionally doped gallium nitride (n-GaN) nanowires. The specific contact resistance and resistivity were determined to be ~1.1 × 10−5 ± 5 × 10−6u2009Ωu2009cm2 and ~6.9 × 10−3 ± 3 × 10−4u2009Ωu2009cm, respectively, with a diameter of ~140u2009nm using a transmission line model (TLM). We also present the electrical characterizations of metal/GaN nano-Schottky diodes with four Schottky metals (Al, Ti, Cr and Au) on unintentionally doped GaN nanowires using current–voltage (I–V) characteristics at room temperature. We observed the abnormal electrical characteristics of GaN nano-Schottky diodes for each Schottky metal.

Evaluation of Pt anode stability in repeated electrochemical oxide reduction reactions for pyroprocessing

The widespread use of Pt anodes in pyroprocessing for oxide reduction inspired us to investigate the stability of a representative Pt anode in repeated UO2 reduction experiments. The formation and subsequent exfoliation of an oxide layer on the Pt surface was shown to result in anode degradation, decreasing the electrode thickness from 2 to 1.22xa0mm and reducing its volume by 42.4% after 33 experiments. Although the above degradation exhibited almost no effect on the UO2 reduction performance of the Pt anode, Pt degradation during long-term oxide reduction was demonstrated to be an important factor for estimating the cost of a pyroprocessing facility.

Chemical behavior of grey phases in LiCl molten salt for oxide reduction in pyroprocessing

Chemical stabilities of SrZrO3, BaZrO3, and Sr0.5Ba0.5ZrO3 in LiCl molten salt were investigated to evaluate dissolution properties of Sr and Ba in grey phases of spent nuclear fuels during oxide reduction. Sr in SrZrO3 was partially insoluble in LiCl, even in the presence of metallic Li, while Ba in BaZrO3 was almost extracted. In contrast, both Sr and Ba in solid-solution phase (Sr0.5Ba0.5ZrO3) well dissolved in LiCl. Pre-treatment of the spent nuclear fuels before the oxide reduction should be carefully designed to prevent formation of the insoluble Sr-based compounds for efficient Sr separation.

Electrical characteristics of AC dielectrophoretically aligned ZnO nanowires

We report on a straightforward and successful AC dielectrophoresis (DEP) method that can be used to align and manipulate ZnO nanowires as well as for extracting the electrical properties of the semiconductor nanowires. The dielectrophoresis was accomplished at a frequency of 20 MHz with five different AC electric field (1, 5, 10, 15, 20 Vp.p). The DEP results indicated that the number of aligned ZnO nanowires increased with increasing AC voltages. From the transport measurements of our AC DEP prepared ZnO nanowires using conventional three-probe schemes in field-effect transistor structures, the estimated carrier mobility from the gate-modulation characteristics was on the order of~15.9 cm2/V dot dots. We found that AC DEP offers great opportunities for electrical characteristics of semiconductor nanowires and for further fundamental research in electronics and photonics device applications.