Jin Jung Kweon

Infrared-wave number-dependent metal–insulator transition in vanadium dioxide nanoparticles

We have studied the metal–insulator transition (MIT) taking place at 340 K in vanadium dioxide nanoparticles. A peculiar nanosize effect on the 10-nm-sized VO2 nanoparticles is reported. While the infrared transmittance at high wave numbers displayed a broad transition behavior, the magnetic susceptibility and infrared transmittance at low wave numbers showed a sharp first-order MIT. Our results suggest that the size effect on the MIT is due to the surface region while the core region undergoes the same MIT as that in the bulk.

Impedance spectroscopy of the superprotonic conduction in LiH2PO4

Superprotonic conduction in the LiH2PO4 (LDP) system has been studied by means of impedance spectroscopy. A dielectric dispersion associated with room-temperature superionic conduction is manifested, as the temperature-dependent dielectric constant displays a broad peak shifting toward the high temperatures with increasing measuring frequency. In particular, our impedance spectroscopic analysis in distinct temperature ranges has revealed the evolution of the microscopic proton dynamics and inhomogeneous electric environments associated with the dynamics of the two distinct hydrogen-bond types in LDP.

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.

Significant increase of the ferroelectric phase transition temperature in partially deuterated KH2PO4 by proton irradiation

The ferroelectric phase transition temperature was significantly raised by 5K in partially deuterated KH2PO4 irradiated by a proton beam. Increase in the hydrogen bond length was indicated by the dielectric constant analysis. Deuteron nuclear magnetic resonance (NMR) measurements of the electric field gradient tensor showed atomic displacement after the proton irradiation, and P31 NMR measurements of the chemical shift tensor revealed phosphorous displacement in the hydrogen-bonded direction and the PO4 tetrahedral distortion. Increase of the phase transition temperature can be closely related to the structural modification involving the hydrogen-bond geometry.

Nuclear magnetic resonance study of the superprotonic conduction in LiH2PO4

Superprotonic conduction in the LiH2PO4 system has been studied by means of high-resolution nuclear magnetic resonance measurements, which enabled us to distinguish dynamics of the two different hydrogen bonds in the structure. The protonic motion, primarily associated with the longer hydrogen bond, rather than the Li ionic motion, was revealed to dictate the extraordinarily high electrical conductivity of the system.

Charge dynamics in KH2PO4 systematically modified by proton irradiation

Our systematic study employing high-resolution nuclear magnetic resonance measurements shows that the hydrogen bonds and proton transport in the KH2PO4 (KDP) system may be tuned sensitively by proton irradiation. In particular, the hydrogen-bond length in KDP increased by a properly chosen dose of proton irradiation is shown to give rise to a minimum in the activation energy of proton hopping in the hydrogen-bond direction.

Low-energy D+ and H+ ion irradiation effects on highly oriented pyrolytic graphite

We have investigated the low-energy (100 eV) D+ and H+ ion irradiation effects on the structural and chemical properties of highly oriented pyrolytic graphite (HOPG). Structural disorder due to the ion irradiation was identified by the Raman spectroscopy, the D+ irradiation giving rise to greater structural disorder than the H+ irradiation. Only sp2 bonding was identified in the X-ray photoemission spectroscopy of the D+-irradiated HOPG, indicating no change in the surface chemical structure. The H+ irradiation, on the other hand, gave rise to sp3 bonding and π−π* transition, the sp3 bonding increasing with increasing irradiation dose. It is thus shown that the chemical properties of the HOPG surface may be sensitively modified by the low-energy H+ ion irradiation, but not by the low-energy D+ ion irradiation.

Electric dipole moment in KH2PO4 systematically modified by proton irradiation

We have carried out an impedance spectroscopy study on a series of proton-irradiated KH2PO4 (KDP) systems. A systematic modification was observed in the transverse dipole moment of the proton-irradiated KDP systems, associated with hydrogen-ion displacements, as obtained from dielectric constant measurements by using a mean-field approximation. Besides, intercorrelation of the charge transport with the dielectric properties was revealed, both having closely to do with the hydrogen-bond modification.We have carried out an impedance spectroscopy study on a series of proton-irradiated KH2PO4 (KDP) systems. A systematic modification was observed in the transverse dipole moment of the proton-irradiated KDP systems, associated with hydrogen-ion displacements, as obtained from dielectric constant measurements by using a mean-field approximation. Besides, intercorrelation of the charge transport with the dielectric properties was revealed, both having closely to do with the hydrogen-bond modification.

Structural defects in LiCoO2 studied by L7i nuclear magnetic relaxation

Microscopic environments and dynamics in LiCoO2 systems were probed by L7i nuclear magnetic relaxation measurements with regard to the structural defects as revealed by x-ray diffraction and electron spin resonance measurements. Thus, the structural defects of differing degrees, associated with Li vacancies, were well accounted for in the temperature-dependent spin-lattice relaxation rate analysis, and in the structural and dynamical inhomogeneity as derived from the relaxation patterns.