Papers by Author: Nam Hee Cho

Abstract: Nanoscale BaTiO3 powders were prepared by hydro-thermal synthesis as a function of solvent conditions. The size of the BaTiO3 powders was in the range of 20 ~ 100 nm. The variation in the relative volume fraction of the tetragonal phase was analyzed quantitatively by means of XRD and Raman spectroscopy. It was found that the maximum volume fraction of the tetragonal phase was ~ 29 %; this was obtained when the synthesis was performed at a solvent condition R (H2O/(H2O + EtOH)) = 0.25.

Abstract: This study reported the fabrication of tin oxide (SnO2) nanostructures on Co-coated Si substrates by the thermal heating of Sn powders. The microstructures and morphologies of the resultant nanostructures were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning electron microscopy (SEM). The product mainly comprised the tangled nanowires with average diameters in the range of 50-180 nm. The nanostructures were single-crystalline rutile structure of SnO2. The PL measurement with the Gaussian fitting exhibited visible light emission bands centered at 576 nm and 638 nm, respectively. We have discussed the possible growth mechanism of the nanostructures.

Abstract: Amorphous and nanocrystalline Si films were prepared by plasma enhanced chemical vapor deposition (PECVD). The films were deposited with a RF power of 100 W, while substrates were under DC biases varying from 0 to -600 V. The size as well as the concentration of Si nanocrystallites increased with raising the DC bias; the PL emission wavelength was shifted from 400 to 750 nm. A model for the nanostructural variation in the nc-Si:H films was suggested to describe the change in the size and concentration of the nanocrystallites as well as the amorphous matrix depending on the DC bias conditions.

Abstract: The nanostructural and optical features of Al-doped Si thin films, which were prepared by co-sputtering Al-chips and a Si main target, were investigated in terms of Al-doping and post-deposition heat-treatment conditions; the heat treatment was carried out at temperatures of 400 ~ 1100 °C. The structural and chemical features are related with the photoluminescence (PL) phenomena of the films. The PL intensity as well as the concentration of Si nanocrystallites were increased by doping particular amount of Al in the films.

Abstract: Porous alumina bodies were successfully prepared by spark plasma sintering of alumina powders with different amounts of graphite, and by subsequently burning out the graphite. Highly porous bodies were fabricated by spark plasma sintering at 1000°C for 3 min under a pressure of 30 MPa. The heating rate was 80°C/min, and the pulse pattern (on-off) was 12:2. For example, alumina bodies prepared by the addition of 10 ~ 30 vol% graphite showed high porosity of 50 ~ 57%. Porous alumina bodies prepared by the addition of 10 ~ 30 vol% graphite had a high compressive strength of 200 ± 55 MPa, about 35 times higher than those obtained on samples prepared by pressureless sintering, and about 2.5 times higher than those in samples prepared by hot-pressing. The significant improvement in strength relative to values obtained with conventional sintering was attributed to better sintering resulting from the rapid heating between particles.

Abstract: ~100 nm-sized BaTiO3 powders were prepared by hydro-thermal techniques, and the structural features of the powders were investigated as a function of synthesis temperature and time. The spectral features of (200) XRD peaks and FT-Raman peaks at frequencies of 305 and 710 cm-1, which are sensitive to the presence of tetragonal BaTiO3, indicate that the nano-sized BaTiO3 powders are dominantly of a cubic phase, but the tetragonal phase is also exhibited at room temperature. The tetragonal and cubic phases exist in the core and shell regions in the powder, respectively; the difference in lattice spacing between the core and the shell regions is 0.061 Å, and the thickness of the cubic-phased shell region of the powders prepared at 180°C for 24 hrs is 3 nm.

Abstract: The nanostructural and optical features of hydrogenated nanocrystalline silicon (nc-Si:H) thin films, which were prepared by plasma enhanced chemical vapor deposition (PECVD), were investigated as a function of deposition conditions. It was found that the crystallite size varied with the relative fraction of Si-H3 bonds in the films, [ ] eger n n n H Si H Si int 3 1 3 / ] [ = = ∑ − − , which was sensitively related with the flow rate of SiH4 reaction gas. The silicon nanocrystallites in the films enlarged from ~2.0 to ~8.0 nm in their size with increasing gas flow rate, while the PL emission energy varied from 2.5 to 1.8 eV; the relative fractions of the Si-H3, Si-H2, and Si-H bonds in the amorphous matrix were also varied sensitively with the SiH4 flow rate. A model for the nanostructure of the nc-Si:H films was suggested to discribe the variations in the size and chemical bonds of the nanocrystallites as well as the amorphous matrix depending on the deposition conditions.

Abstract: We studied photoluminescence (PL) and electroluminescence (EL) properties of hydrogenated nanocystalline silicon (nc-Si:H) thin films prepared by applying the plasma enhanced chemical vapor deposition (PECVD) techniques. . A prototype of ITO/nc-Si:H/P-type Si wafer/Al EL devices was illustrated with its fundamental electrical and optical features. The nc-Si:H films exhibited PL spectra in a wavelength range of 350 ~ 700 nm with the maximum intensity at ~ 530 nm, which is attributed to quantum confinement effects (QCE) owing to the presence of nanocrystalline Si. The EL device produced EL spectra with their maximum intensity at ~ 525 nm which are similar to the PL spectra. The light emission is attributed to radiative recombination related to nanocrystalline Si contained in the hydrogenated amorphous Si (a-Si:H).

Abstract: Hydrogenated amorphous Si (a-Si:H) films were prepared by plasma enhanced chemical vapor deposition (PECVD) techniques, and the effect of nano-structure on the photoluminescence (PL) phenomena of the films was investigated. The films, which were prepared at R.T., contain both amorphous and crystalline phases of 1 ~ 3 nm size nano-crystallites with {100} orientation preference while the films prepared at 500°C are composed of about 6 nm and 150 nm size crystallites. The former exhibit a strong PL intensity near blue light region, while the latter exhibiting little PL phenomena; also, the optical band gap of the former was calculated at 4.2 eV.