Cheol Eui Lee

Structural study of nitrogen-doping effects in bamboo-shaped multiwalled carbon nanotubes

We have investigated nitrogen doping effects on the structure and crystallinity of bamboo-shaped multiwalled carbon nanotubes (BS-MWNTs) by means of x-ray photoemission spectroscopy (XPS) and transmission electron microscopy. By controlling the NH3/C2H2 flow ratio during the chemical vapor deposition, the nitrogen concentrations of 0.4% to 2.4% were obtained. According to the XPS measurements, the increasing nitrogen concentration gave rise to an increase of the N-sp3 C bonds as well as the deterioration of the crystallinity of the BS-MWNTs. Besides, the N-sp3 C bonds were found to prevail over the N-sp2 C bonds above 5% nitrogen concentration. At higher nitrogen concentrations, the BS-MWNTs showed shorter compartment distances, presumably due to the suppressed surface diffusion of carbon on the catalyst particles.

Modeling of growth kinetics for Pseudomonas spp. during benzene degradation

A modeling study was conducted on growth kinetics of three different strains of Pseudomonas spp. (Pseudomonas aeruginosa, Pseudomonas fluorescens, Pseudomonas putida) during benzene degradation to determine optimum substrate concentrations for most efficient biodegradation. Batch tests were performed for eight different initial substrate concentrations to observe cell growth and associated substrate degradation using benzene-adapted cells. Kinetic parameters of both inhibitory (Haldane–Andrews, Aiba–Edwards) and noninhibitory (Monod) models were fitted to the relationship between specific growth rate and substrate concentration obtained from the growth curves. Results showed that half-saturation constant of P. fluorescens was the highest among the three strains, indicating that this strain could grow well at high concentration, while P. putida could grow best at low concentration. The inhibition constant of P. aeruginosa was the highest, implying that it could tolerate high benzene concentration and therefore could grow at a wider concentration range. Estimated specific growth rate of P. putida was lower, but half-saturation constant was higher than those from literature study due to high substrate concentration range used in this study. These two kinetic parameters resulted in substantial difference between Monod- and Haldane-type models, indicating that distinction should be made in applying those models.

Effects of coercive voltage and charge injection on memory windows of metal-ferroelectric-semiconductor and metal-ferroelectric-insulator-semiconductor gate structures

We have prepared Pt/SrBi2Ta2O9 (SBT)/Si metal-ferroelectric-semiconductor (MFS) and Pt/SBT/Y2O3/Si metal-ferroelectric-insulator-semiconductor (MFIS) ferroelectric gate structures and investigated the changes in memory window with different thickness of SBT and Y2O3 in the MFS and MFIS. As a result, it is found that the memory window increases with increasing thickness of SBT and decreasing thickness of Y2O3. The experimental and theoretical analysis reveals that the memory window equals to the difference between the effective coercive voltage (2Vc) applied to the ferroelectric film and the flat band voltage shift due to charge injection (Vci). Increasing the thickness of SBT, the 2Vc seems to be saturated at higher voltage, whereas the Vci starts to increase exponentially at the higher gate voltage. In contrast, the Vci decreases with decreasing thickness of Y2O3, resulting in the enhancement of the memory window due to the reduction of charge injection.

Metallic conductivity in bamboo-shaped multiwalled carbon nanotubes

Temperature-dependent resistivity measurements were carried out on bamboo-shaped multiwalled carbon nanotubes (MWNTs) grown by thermal chemical vapor deposition. They were deposited on Al2O3/Ti and SiO2/Ti substrates using cobalt and iron catalysts, respectively. As a result, a metallic conductivity, i.e. resistivities with a positive temperature slope, was observed for the MWNTs grown on the SiO2/Ti substrates. The different temperature behaviors of the resistivity for the MWNTs grown on different substrates are discussed in view of the substrate morphology and crystallinity.

Evidence of hydrogen-mediated ferromagnetic coupling in Mn-doped ZnO

Hydrogen shallow donors in undoped and doped ZnO systems have been studied by means of the electron paramagnetic resonance and nuclear magnetic resonance measurements. Experimental evidence is given in this work for coupling of hydrogen shallow donors and Mn ions in Mn-doped ZnO mediating short-range ferromagnetic spin-spin interaction.

Interwall support in double-walled carbon nanotubes studied by scanning tunneling microscopy

Atomically resolved room temperature scanning tunneling microscopy images of carbon nanotubes were obtained, and actual values of the nanotube dimensions were estimated from a line profile fitting analysis. It was found that the cross sectional deformation induced by the van der Waals forces between the CNTs and the substrate is much smaller in the double-walled carbon nanotubes than in the single-walled nanotubes.

Half-metallic carbon nanotubes.

Half-metallicity in carbon nanotubes is achieved and controlled by hydrogen adsorption patterns. The edge states in carbon nanotubes are unstable under an electric field due to the spin-conserving electron transfer between the edges, but a large enough transfer barrier between the edge states, obtained by controlling the adsorption patterns, renders the CNTs half-metallic.

Enhanced electroluminescence in polymer-nanotube composites

Enhanced electroluminescence (EL) was observed with increasing carbon nanotube concentration in the MEH-PPV/SWCNT composites, where MEH-PPV and SWCNTs correspond to poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] and single-walled carbon nanotubes, respectively. Spatially separated conduction paths of the two distinct charge carriers, induced by blending SWCNTs with MEH-PPV, may explain the enhanced EL.

Oxygen plasma effects on sol-gel-derived lead-zirconate-titanate thin films

We studied effects of oxygen plasma treatment on the ferroelectric Pb(Zrx,Ti1−x)O3 (PZT) films prepared by a sol-gel method. Electrical characteristics of the films were found to be improved considerably by exposure to the O2 plasma, with enhanced remanent polarization and decreased leakage current densities. Chemical bonding analysis by means of x-ray photoelectron spectroscopy in the Pb 4f region indicated that the intensity from the perovskite PZT phase increased considerably after the O2 plasma treatment, giving rise to the improved performances of the PZT films.

Structure and magnetism in VO2 nanorods

The structural and magnetic properties in vanadium dioxide (VO2) nanorods prepared by the hydrothermal treatment are shown to be markedly different from the case of the bulk system. A peculiar mixed phase of an insulating monoclinic structure (M2) that is observed in the bulk systems under doping or stress and a rutile (R) structure was revealed and confirmed by transmission electron microscopy, x-ray diffraction, and electron spin resonance measurements. Besides, a broad transition from a high-temperature magnetic phase to a low-temperature one, with quite distinct magnetic correlation strengths, was observed.