Giwoong Nam

Structural and blue emission properties of Al-doped ZnO nanorod array thin films grown by hydrothermal method

ZnO seed layers were deposited on a quartz substrate using the sol-gel method, and Al-doped ZnO (AZO) nanorod array thin films were grown on the ZnO seed layers by the hydrothermal method with different Al concentrations ranging from 0 to 2.0 at. %. The structural and blue emission properties of the ZnO and AZO nanorod array thin films were investigated using scanning electron microscopy (SEM), x-ray diffraction, Ultraviolet-visible spectroscopy, and photoluminescence (PL). Al doping greatly affects the morphology of AZO nanorod array thin films. For an Al concentration of 2.0 at. %, it can be clearly seen from the SEM image that the hexagonal shape has changed into a prism-like shape. In the PL spectra, it is clear that the intensity ratio of the near-band-edge emission to the deep-level emission (DLE) increases as the Al concentration increases up to 2.0 at. %. The DLE peak (about 2.80 eV) in the blue emission region is found for the AZO nanorod array thin films. The transmittance spectra show that as compared to the ZnO nanorod array thin films, the AZO nanorod array thin films exhibited significantly improved transmittance in the visible region and a blue shift of the absorption edge.

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.

Growth and characterization of seed layer-free ZnO thin films deposited on porous silicon by hydrothermal method

Catalyst- and seed layer-free zinc oxide (ZnO) thin films were grown on porous silicon (PS) by a hydrothermal method. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and photoluminescence (PL) were carried out to investigate the structural and optical properties of the PS and the ZnO thin films. The ZnO thin films have an extraordinary tendency to grow along the a-axis with a hexagonal wurtzite structure. The growth rate of the ZnO thin films was increased with the increase in the precursor concentration. The crystal quality of the ZnO thin films was improved, and the residual stress was decreased as their thickness increased. Monochromatic indigo and red light emission peaks were observed from the ZnO thin films and the PS, respectively. At an excessively high precursor concentration, a green light emission peak was also observed in the ZnO thin films. The luminescent efficiency of the indigo light emission peak was enhanced with the increase in the precursor concentration.

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.

A novel regrowth mechanism and enhanced optical properties of Mg0.25Zn0.75O nanorods subjected to vapor-confined face-to-face annealing

Although the sol–gel spin-coating method is usually a technique for depositing thin films, here, we report on the sol–gel fabrication of one-dimensional Mg0.25Zn0.75O nanorods through the use of vapor-confined face-to-face annealing (VC-FTFA), in which mica is inserted between two films before annealing using the FTFA method. Mg0.25Zn0.75O nanorods are regrown when magnesium chloride hexahydrate is used as the solvent, because ZnCl2 and MgCl2 vapors are generated under these conditions. The near-band-edge emission intensity of the Mg0.25Zn0.75O nanorods is enhanced by VC-FTFA by a factor of 37 compared to that of the films annealed in open air at 700 °C. Our method may provide a route for the facile fabrication of Mg0.25Zn0.75O nanorods.

Facile Synthesis and Enhanced Ultraviolet Emission of ZnO Nanorods Prepared by Vapor-Confined Face-to-Face Annealing

In this study, we report a novel regrowth method of sol-gel-prepared ZnO films using a vapor-confined face-to-face annealing (VC-FTFA) technique in which mica was inserted between two films, followed by annealing with the FTFA method. The ZnO nanorods are regrown when zinc acetate dihydrate and zinc chloride (ZnCl2) are used as the solvent, because these generate ZnCl2 vapor. The near-band-edge emission intensity of the ZnO nanorods was enhanced through the VC-FTFA method, increasing significantly by a factor of 56 compared to that of ZnO films annealed in open air at 700 °C. Our method may provide a route toward the facile fabrication of ZnO nanorods.

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.

Synthesis and fast-response of a photodetector of hydrothermally grown ZnO nanorods through the use of a graphene oxide/ZnO seed layer

We used a hydrothermal method combined with a graphene oxide (GO)/ZnO seed layer to fabricate a ZnO nanorod based fast-response UV detector. The introduction of GO film decreases the excitonic photoluminescence intensity, which indicates the transfer of electrons from ZnO nanorods to the surface of the GO film (quenching).

Regrowth method for the enhancement in the photoluminescence, UV photoresponse, and electrical properties of n-ZnO:Sn films

We report the effects of vapor-confined face-to-face annealing (VC-FTFA), where mica is inserted between two films and annealed using FTFA method, on the optical, photoresponse, and electrical properties of Sn-doped ZnO (n-ZnO:Sn) films deposited on a p-Si substrate using the sol-gel spin-coating method. The near-band-edge photoluminescence emission intensity is increased by a factor of 445. The photocurrents of the sample 1, 2, and 3 were 1.95 × 10−4, 1.14 × 10−3, and 3.47 × 10−3 A, respectively. The turn-on voltage of the n-ZnO:Sn film/p-Si heterojunction annealed using the VC-FTFA method was measured to be ~3.6 V, which is smaller than that of the heterojunction annealed in open air (~4.2 V). Furthermore, in n-ZnO:Sn film/p-Si heterojunctions annealed in open air and using the VC-FTFA method, the currents at 4.5 V are 4.4 and 17.9 mA, respectively. Our method could provide pathway to the easy fabrication of optoelectronic devices based on an ZnO annealed using the VC-FTFA method.