Hyunsik Yoon

Hydrothermally grown boron-doped ZnO nanorods for various applications: Structural, optical, and electrical properties

The structural, optical, and electrical properties of ZnO and BZO nanorods were investigated using fieldemission scanning electron microscopy, x-ray diffraction (XRD), photoluminescence (PL), and van der Pauw Hall-effect measurements. All the nanorods had grown well on the ZnO seed layers and were hexagonal. The BZO nanorods were shorter than the undoped ZnO nanorods, and the BZO nanorods grew shorter with increasing concentration of B to 2.0 at. % while the average length of the nanorods doped with 2.5 at. % B increased from 1620 to 1830 nm. The XRD patterns suggest that the amount of residual stress in the nanorods decreased with increasing concentration of B in the nanorods. The PL spectra showed near-bandedge and deep-level emissions, and B doping also varied the PL properties of the ZnO nanorods. The Halleffect data suggest that B doping also varied the electrical properties such as the carrier concentration, mobility, and resistivity of the ZnO nanorods.

Structural, optical, and electrical properties of ZnO thin films deposited by sol-gel dip-coating process at low temperature

Sol-gel dip-coating was used to prepare ZnO thin films with relaxed residual stress by lowering the deposition temperature from room temperature (25°C) to −25°C. The effect of deposition temperature on the structural, optical, and electrical properties of the films was characterized using scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL), ultraviolet-visible (UV-Vis) spectroscopy and reflectance accessory, and the van der Pauw method. All the thin films were deposited successfully onto quartz substrates and exhibited fibrous root morphology. At low temperature, the deposition rate was higher than at room temperature (RT) because of enhanced viscosity of the films. Further, lowering the deposition temperature affected the structural, optical, and electrical properties of the ZnO thin films. The surface morphology, residual stress, PL properties, and optical transmittance and reflectance of the films were measured, and this information was used to determine the absorption coefficient, optical band gap, Urbach energy, refractive index, refractive index at infinite wavelength, extinction coefficient, single-oscillator energy, dispersion energy, average oscillator wavelength, moments M−1 and M−3, dielectric constant, optical conductivity, and electrical resistivity of the ZnO thin films.

Seed-layer-free hydrothermal growth of zinc oxide nanorods on porous silicon

Zinc oxide (ZnO) nanorods were grown on porous silicon (PS) using hydrothermal synthesis without a metal catalyst or a seed layer. Scanning electron microscopy, x-ray diffraction, and temperature-dependent photoluminescence (PL) were carried out to investigate the structural and optical properties of the ZnO-PS sample. Most of the nanorods had an average diameter about of 120 nm and an average length of 5 µm, and were assembled into flower-like clusters where several nanorods were joined at a central point. In some cases, ZnO nanorods were merged in parallel bundles. The ZnO nanorods exhibited an overall compressive residual stress. The Zn-O bond length was 1.953 Å. ZnO-PS exhibited one PL peak in the ultraviolet (UV) range, and two peaks in the visible range. The UV and green emission peak were generated from the ZnO nanorods, while the red emission peak was attributed to the PS. The fitting parameters for Varshni’s empirical equation were α = 8 × 10−4 eV/K, β = 186 K, and Eg(0) = 3.375 eV, and the thermal activation energy was about 32 meV.

Effects of annealing temperature on optical properties of ZnO nanorods with Mg0.2Zn0.8O capping layers

ZnO nanorods were grown on spin-coated ZnO seed layers by the hydrothermal method. The Mg0.2Zn0.8O capping layers were deposited on ZnO nanorods by the sol-gel method. The ZnO nanorods with Mg0.2Zn0.8O capping layers were annealed at 600°C. Temperature-dependent photoluminescence (PL) spectroscopy was carried out to investigate the mechanism governing the quenching behavior of the PL spectra. For the 12 K PL spectra, the peaks of the Mg0.2Zn0.8O capping layers, excitons bound to neutral donors (D0X), two-electron satellite transitions, donor-acceptor pairs, and free-to-neutral-acceptors and their longitudinal optical (LO) phonon replicas were observed at annealing temperatures from 600°C. At 12 K, the peak of the Mg0.2Zn0.8O capping layers was blue-shifted as the annealing temperature increased. The peaks of the D0X, free excitons and Mg0.2Zn0.8O capping layers merged and the PL intensity of the peaks decreased while the temperature increased from 12 to 300 K.

Photoluminescent properties of CdxZn1−xO thin films prepared by sol-gel spin-coating method

CdxZn1−xO thin films were prepared on Si (100) substrates by the sol-gel spin coating method. Temperaturedependent photoluminescence (PL) measurements were carried out to investigate the luminescent properties of the CdxZn1−xO thin films. The PL peaks of the CdxZn1−xO thin films decrease as the Cd concentration increases and the near-band edge emission (NBE) PL peaks of the CdxZn1−xO thin films are shifted toward the red region. In the temperature-dependent PL measurement, three components at 2.855, 3.038, and 3.148 eV in the PL emission peak of the Cd0.2Zn0.8O thin films were observed at 12 K. With increasing temperature, the emission peak at 3.148 eV at 12 K becomes red-shifted and the monotonic PL peak at 12 K divides into three clear peaks as the temperature increases. The activation energy for the 3.148 eV peak is 69.54 meV corresponding to the energy for the frozen-out donors.

Research on Thermal Conductivity of Electrospun Polyacrilonitrile-Multi-Walled Carbon Nanotubes Composite Carbon Nanofiber Papers

In recent decades, the rapid development of nanoscience and nanotechnology has accelerated materials with high thermal conductivity, such as composite carbon nanomaterials, since heat removal has become a crucial issue. In this work polyacrilonitrile (PAN) incorporated with alteration of mass fraction (0.3 wt%, 0.62 wt%, 0.92 wt%, 2.74 wt%) of the multi-walled carbon nanotubes (MWCNTs) were used to fabricate composite fibers via electrospinning. The stabilizing and carbonization reactions of composite nanofibers are conducted in tube furnace at 280 °C and 1000 °C. The structural features of flexible composite carbon nanofiber papers were characterized under scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and Raman spectroscopy. Special attention of composite carbon nanofiber papers is given to cross-plane thermal conductivity and in-plane thermal conductivity and were measured via Flash Line Analyzer. The results proved that flexible composite carbon nanofiber papers are promising heat sink radiator for a variety of applications including flexible optical and electronic devices.