Hyocheon Kweon

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.

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.

Electron spin resonance study of proton-irradiation-induced defects in graphite

Electron spin resonance measurements of proton-irradiated graphite have revealed detailed nature of proton-irradiation-induced defects. Our results indicate that proton-irradiation creates confined defect regions of a metallic island surrounded by an insulating magnetic region which “isolates” the metallic island inside from the metallic graphite background outside. We have thus come up with a picture of phase separation in proton-irradiated graphite comprising three regions of distinct electrical and magnetic properties.

Electron spin resonance of defect-rich vanadium oxide nanotubes

We have studied defect-rich vanadium oxide nanotubes, with reduced V4+ concentration, by means of x-ray diffraction, infrared spectroscopy, x-ray photoemission spectroscopy, and electron spin resonance (ESR) measurements. In particular, prominent anomalies at 320 and 370 K attributed to structural phase transitions were manifest in the ESR measurements. The ESR spin susceptibility exhibited a singlet-triplet transition of the spin dimers and an antiferromagnetic exchange coupling. In the high-temperature region above 370 K, where the two distinct ESR lines merged, an activation energy of the fast hopping polaron motion, 0.48 eV, was obtained.

Charge and spin dynamics in VO2 nanorods

Electrical conductivity and photoconductivity measurements were carried out on vanadium dioxide (VO2) nanorods prepared by the hydrothermal treatment of vanadium pentoxide gels. While the structural properties of the nanorods resembled those of the Cr-doped VO2, the charge and spin dynamics appears to resemble those of the Nb-doped VO2. The magnetic and (photo)electrical properties of the nanorods can be understood in terms of localization of itinerant electrons giving rise to a spin-polarized (S=3/2) V4+ pair, dominant at higher temperatures, or to a (S=1) V3+ ion out of a V4+ ion pair, dominant at lower temperatures.

Photoinduced reentrant insulator-metal-insulator transitions in vanadium oxide nanotubes

We report photoinduced reentrant insulator-metal-insulator transitions in a low-dimensional nanoscale system of vanadium oxide nanotubes, a Mott insulator in the dark with no metal-insulator transitions. The nature of the spatially inhomogeneous intermediate metallic phase was elucidated by the photocurrent decay analysis of the system.

Field-Induced Transition from Room-Temperature Ferromagnetism to Diamagnetism in Proton-Irradiated Fullerene

Room-temperature ferromagnetism in proton-irradiated C60 fullerene is demonstrated. The ferromagnetism turns into diamagnetism intrinsic to the fullerene as the magnetic field increases above a critical field.

Ferromagnetism in water-doped vanadium oxide nanotubes

We have investigated the magnetic properties of vanadium oxide nanotubes (VOx-NTs) of various V4+ concentrations. Ferromagnetic response was observed at ∼100 K only in the V4+-rich water-intercalated VOx-NTs, whose saturation moment is close to that of Li-doped VOx-NTs, a recently-reported room-temperature ferromagnet. The ferromagnetism may have to do with charge localization as well as the spin concentration giving rise to removal of spin frustration according to our previous study.