Min-Su Yi

Epitaxial growth of ZnO nanowall networks on GaN/sapphire substrates

Heteroepitaxy of vertically well-aligned ZnO nanowall networks with a honeycomblike pattern on GaN∕c-Al2O3 substrates by the help of a Au catalyst was realized. The ZnO nanowall networks with wall thicknesses of 80–140nm and an average height of about 2μm were grown on a self-formed ZnO thin film during the growth on the GaN∕c-Al2O3 substrates. It was found that both single-crystalline ZnO nanowalls and catalytic Au have an epitaxial relation to the GaN thin film in synchrotron x-ray scattering experiments. Hydrogen-sensing properties of the ZnO nanowall networks have also been investigated.

Catalyst-free synthesis of ZnO nanowall networks on Si3N4∕Si substrates by metalorganic chemical vapor deposition

ZnO nanowall networks were synthesized on Si3N4∕Si (100) substrates at low growth temperature of 350°C by metalorganic chemical vapor deposition (MOCVD) without any help of metal catalysts. Depending on MOCVD-growth conditions, a large number of nanowalls with extremely small wall thicknesses below 10nm are formed into nanowalls with a thickness of about 20nm, resulting in the formation of two-dimensional nanowall networks. The ZnO nanowall networks were found to have a preferred c-axis orientation with a hexagonal structure in synchrotron x-ray scattering experiments. Room-temperature hydrogen incorporation into ZnO nanowall networks has been observed in photoluminescence measurements.

High-quality thin-film passivation by catalyzer-enhanced chemical vapor deposition for organic light-emitting diodes

The thin-film passivation of organic light-emitting diodes (OLEDs) by a SiNx film grown by catalyzer-enhanced chemical vapor deposition was investigated. Using a tungsten catalyzer connected in series, a high-density SiNx passivation layer was deposited on OLEDs and bare polycarbonate (PC) substrates at a substrate temperature of 50°C. Despite the low substrate temperature, the single SiNx passivation layer, grown on the PC substrate, exhibited a low water vapor transmission rate of (2–6)×10−2g∕m2∕day and a high transmittance of 87%. In addition, current-voltage-luminescence results of an OLED passivated with a 150-nm-thick SiNx film compared to nonpassivated sample were identical indicating that the performance of an OLED is not critically affected by radiation from tungsten catalyzer during the SiNx deposition. Moreover, the lifetime to half initial luminance of an OLED passivated with the single 150-nm-thick SiNx layer was 2.5 times longer than that of a nonpassivated sample.

Enhancement of hole injection using ozone treated Ag nanodots dispersed on indium tin oxide anode for organic light emitting diodes

The authors report the enhancement of hole injection using an indium tin oxide (ITO) anode covered with ultraviolet (UV) ozone-treated Ag nanodots for fac tris (2-phenylpyridine) iridium Ir(ppy)3-doped phosphorescent organic light-emitting diodes (OLEDs). X-ray photoelectron spectroscopy and UV-visible spectrometer analysis exhibit that UV-ozone treatment of the Ag nanodots dispersed on the ITO anode leads to formation of Ag2O nanodots with high work function and high transparency. Phosphorescent OLEDs fabricated on the Ag2O nanodot-dispersed ITO anode showed a lower turn-on voltage and higher luminescence than those of OLEDs prepared with a commercial ITO anode. It was thought that, as Ag nanodots changed to Ag2O nanodots by UV-ozone treatment, the decrease of the energy barrier height led to the enhancement of hole injection in the phosphorescent OLEDs.

Effect of Ag interlayer on the optical and passivation properties of flexible and transparent Al2O3∕Ag∕Al2O3 multilayer

We report on the characteristics of a flexible Al2O3∕Ag∕Al2O3 multilayer passivation grown on a polyethylene terephthalate (PET) substrate as a function of Ag thickness. Due to the surface plasmon resonance (SPR) effects and the ductility of the Ag layer that is sandwiched between the dielectric Al2O3 layer, the flexible Al2O3∕Ag∕Al2O3 multilayer passivation exhibits a high transparency of 86.44% and improved flexibility at a Ag thickness of 10nm. We found that SPR effects in the Ag layer occur at the transition region from distinct islands to a continuous film at a critical thickness (∼10nm). In addition, the water vapor transmission rate of the Al2O3∕Ag∕Al2O3∕PET sample decreased as the thickness of the Ag layer increased. Using synchrotron x-ray scattering and field emission scanning electron microscopy, we suggest a possible mechanism to explain the SPR in the Ag layer of the flexible and transparent Al2O3∕Ag∕Al2O3 multilayer passivation.

ZnO nanotips and nanorods on carbon nanotube/Si substrates: anomalous p-type like optical properties of undoped ZnO nanotips

ZnO nanotips and nanorods were grown on screen-printed multi-walled carbon nanotube (MWCNT) films via thermal chemical vapor deposition at relative low growth temperatures of 400 and 500u2009°C. Uniform formation of ZnO nanotips and nanorods occurred on MWCNT-printed Si substrates, but were rarely observed on bare Si substrates at the same growth temperatures. In photoluminescence (PL) measurements, it was found that ZnO nanorods exhibit typical intrinsic optical properties, while ZnO nanotips revealed p-type like luminescence behavior. Acceptor-related emission bands originating from neutral acceptor-bound exciton, free-to-acceptor and donor-acceptor pair transitions are clearly observed in temperature-dependent PL spectra of ZnO nanotips.

Optimal growth and thermal stability of crystalline Be0.25Zn0.75O alloy films on Al2O3 (0001)

The influence of growth temperature on the synthesis of BexZn1−xO alloy films, grown on highly-mismatched Al2O3(0001) substrates, was studied by synchrotron x-ray scattering, high-resolution transmission electron microscopy and photoluminescence measurements. A single-phase BexZn1−xO alloy with a Be concentration of xu2009=u20090.25, was obtained at the growth temperature, Tgu2009=u2009400u2009°C, and verified by high-resolution transmission electron microscopy. It was found that high-temperature growth, Tg≥600u2009°C, caused phase separation, resulting in a random distribution of intermixed alloy phases. The inhomogeneity and structural fluctuations observed in the BexZn1−xO films grown at high temperatures are attributed to a variation in Be composition and mosaic distribution via atomic displacement and strain relaxation.