Yangsoo Kim

Effects of post-heated ZnO seed layers on structural and optical properties of ZnO nanostructures grown by hydrothermal method

ZnO nanostructures were grown by the hydrothermal method on ZnO seed layers post-heated in the range 350°C–500°C. The effects of the post-heated ZnO seed layers on the structural and optical properties of the ZnO nanostructures were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD) spectroscopy, and photoluminescence (PL) spectroscopy. The average grain sizes in the ZnO seed layers increased with increasing post-heating temperature, and nano-fibrous structures were observed on the surface of the ZnO seed layers post-heated at 450°C. The ZnO seed layers post-heated in the range 350°C–500°C affected the residual stress, lattice distortion in the ZnO nanostructures and the intensity, positions, and full widths at half maximum of 2θ and PL peaks in the XRD and PL spectra for the ZnO nanostructures.

Inhomogeneous carrier transport at contact/GaN interfaces and thermal instability of vertical light-emitting diodes.

The carrier-transport behavior at the interface of a contact and n-type GaN was investigated for group III nitride vertical light-emitting diodes (LEDs). Three types of samples were investigated including dry-etched flat Ga-polar n-GaN (GA), dry-etched flat N-polar n-GaN (NF), and wet-etched roughened N-polar n-GaN (NR). Schottky diodes fabricated using a palladium contact revealed that carrier transport at the contact/GaN interface could be understood in terms of the barrier inhomogeneity model, in which the local shallow barriers dominated the overall carrier transport. Kelvin probe force microscopy showed that the peculiar crystallographic structures and native defects (acting as the local shallow barriers) led to the largest tunneling probability of the NR samples. The study was followed up by forming a TiN/Al contact on the NR surfaces to study the thermal instability of the vertical LEDs, revealing that the vertical LEDs were degraded after a thermal annealing process. This could be related to the n-contact size, indicating that the degradation of vertical LEDs was likely due to the failure of Ohmic patches (contacts formed on the defects and/or the tip/edge of hexagonal cones) at elevated temperature.

Two phase mass transfer model for the semibatch melt polymerization process of polycarbonate

A twophase mass transfer model has been developed to predict the polymer molecular weight for the semibatch melt polymerization process of polycarbonate, in which the reaction melt viscosity is high and the mass transfer of volatile by-product is the rate-controlling step. The model simulation results have been compared with the experimental data reported in the published literature regarding the number average molecular weight and the reaction product composition. The mass transfer coefficient for phenol [(kla)p] in the melt polymerization process of polycarbonate has been estimated.

Removal of hexavalent chromium ion from aqueous solution using nanoscale zero-valent iron particles immobilized on porous silica support prepared by polymer template method

Porous silica supported nanoscale zero-valent iron was prepared by a polymer template method in order to effectively remove a hexavalent chromium ion (Cr(VI)) in an aqueous solution. It did not show a deterioration of Cr(VI) removal efficiency, which could be caused by the surface oxidation and agglomeration of nanoscale zero-valent iron (NZVI) particles. Porous silica by the polymer template method showed quite unique structure, which we named as quasi-inverse opal silica (QIOS), and it showed high surface area (375.4m2/g) and fine pore size (76.5 nm). NZVI immobilized on the surface of QIOS (NZVI@QIOS) was added to an aqueous Cr(VI) solution at 0.025 g/L, and it showed over 96% Cr(VI) removal efficiency. Such a high removal efficiency of Cr(VI) was maintained over two weeks after preparation (92% after 16 days). Morphology of porous silica supported nanoscale zero-valent iron was analyzed by TEM and FE-SEM. Identification of the reaction compounds produced by the reaction of Cr(VI) and zero-valent iron (Fe(0)) was made by the application of XPS.