In-Hui Hwang

Anomalous structural disorder and distortion in metal-to-insulator-transition Ti2O3

Mott proposed that impurity bands in corundum-symmetry Ti2O3 at high temperatures caused a collapse in the bandgap. However, the origin of the impurity bands has not yet been clarified. We examine the local structural properties of metal-to-insulator-transition Ti2O3 using in-situ x-ray absorption fine structure (XAFS) measurements at the Ti K edge in the temperature range from 288 to 739 K. The Ti2O3 powder is synthesized by using a chemical reaction method. X-ray diffraction (XRD) measurements from Ti2O3 with a Rietveld refinement demonstrate a single-phased R-3c symmetry without additional distortion. Extended-XAFS combined with XRD reveals a zigzag patterned Ti position and an anomalous structural disorder in Ti-Ti pairs, accompanied by a bond length expansion of the Ti-Ti pairs along the c-axis for T > 450 K. The local structural distortion and disorder of the Ti atoms would induce impurity levels in the band gap between the Ti 3d a1g and egπ bands, resulting in a collapse of the band gap for T > 450 K.

Linear defects and electrical properties of ZnO nanorods

Proton irradiation (17–34 MeV at flux values ranging from 1011 to 1012 cm−2) was used to assess the influences of orientation-dependent linear defects in a current passing through ZnO nanorods. Compared with the pristine ZnO nanorods, there was a significant increase in the current passing through ZnO nanorods that were irradiated with a proton beam kept in parallel with the nanorod length. The current was gradually decreased with a corresponding increase in the angle of the proton beams relative to the nanorod length. Calculations using the density functional theory demonstrated a substantial reduction and a lack of influence on the bandgap due to linear defects along the respective c- and the a-axes of the ZnO nanorods. Linear defects likely play roles as channels or traps of conduction electrons or holes in wide-bandgap materials.Proton irradiation (17–34 MeV at flux values ranging from 1011 to 1012 cm−2) was used to assess the influences of orientation-dependent linear defects in a current passing through ZnO nanorods. Compared with the pristine ZnO nanorods, there was a significant increase in the current passing through ZnO nanorods that were irradiated with a proton beam kept in parallel with the nanorod length. The current was gradually decreased with a corresponding increase in the angle of the proton beams relative to the nanorod length. Calculations using the density functional theory demonstrated a substantial reduction and a lack of influence on the bandgap due to linear defects along the respective c- and the a-axes of the ZnO nanorods. Linear defects likely play roles as channels or traps of conduction electrons or holes in wide-bandgap materials.

Polarization-dependent DANES study on vertically-aligned ZnO nanorods

The local structural and local density of states of vertically-aligned ZnO nanorods are examined by using polarization-dependent diffraction anomalous near edge structure (DANES) measurements from c-oriented ZnO nanorods at the Zn K edge at the geometry of the incident x-ray electric field parallel and perpendicular to the x-ray momentum transfer direction. Orientation-dependent local structures determined by DANES are comparable with polarization- dependent EXAFS results. Unlike other techniques, polarization-dependent DANES can uniquely describe the orientation-dependent local structural properties and the local density of states of a selected element in selected-phased crystals of compounds or mixed-phased structures.