Yoon-Hwae Hwang

The growth mechanism and optical properties of ultralong ZnO nanorod arrays with a high aspect ratio by a preheating hydrothermal method

Well-aligned ultralong ZnO nanorod arrays with a length of 10 microm have been synthesized on glass substrates using a preheating hydrothermal method. The diameter of the nanorods is in the range from 50 to 80 nm, and the aspect ratio and alignment can be simply controlled by varying the preheating time. Based on the evolution of aspect ratio with preheating time, a possible growth mechanism was proposed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the nanostructures are well oriented with the c-axis perpendicular to the substrate. The photoluminescence (PL) spectrum of the as-grown ZnO nanostructure reveals a near-band-edge (NBE) emission peak and a yellow emission, and the origin of yellow emission was confirmed to be from the absorbed hydroxyl group. The ultralong nanorod arrays made in solution have a desirable diameter, length, density and orientation for ordered nanodevice applications.

Solution-derived 40 µm vertically aligned ZnO nanowire arrays as photoelectrodes in dye-sensitized solar cells

Well-aligned ZnO nanowire arrays with a long length of more than 40 microm were prepared successfully by using the polyethylenimine (PEI)-assisted preheating hydrothermal method (PAPHT). Several important synthetic parameters such as PEI content, growth time, preheating time and zinc salt concentration were found to determine the growth of ultralong ZnO nanowire arrays, including length, diameter, density and alignment degree. The photoluminescence (PL) spectrum of as-grown ultralong ZnO nanowire arrays revealed a UV emission and a yellow emission, which was attributed to the absorbed hydroxyl group based on the peak shift after annealing in various atmospheres. The performance of dye-sensitized solar cells (DSSCs) increased with increasing length of ZnO nanowire arrays, which was mainly ascribed to the aggrandized photocurrent and reduced recombination loss according to electrochemical impedance spectroscopy (EIS). A maximum efficiency of 1.3% for a cell with a short-circuit current density (J(sc)) = 4.26 mA cm(2), open-circuit voltage (V(oc)) = 0.69 V and (fill factor) FF = 0.42 was achieved with a length of 40 microm.

Photoluminescence of polycrystalline ZnO under different annealing conditions

We investigated polycrystalline zinc oxide (ZnO) with different annealing conditions in air by x-ray photoelectron spectroscopy and photoluminescence. We found that the concentration of antisite oxide (OZn) increases when ZnO ceramics were in an O-rich condition. As the concentration of antisite oxide (OZn) increased, the photoluminescence intensity of the green band emission increased. The crossover temperature of the free and bound excitons was roughly estimated as 100 K.

Color-tunable properties of Eu3+- and Dy3+-codoped Y2O3 phosphor particles.

Rare-earth phosphors are commonly used in display panels, security printing, and fluorescent lamps, and have potential applications in lasers and bioimaging. In the present study, Eu3+- and Dy3+-codoped uniform-shaped Y2O3 submicron particles were prepared using the urea homogeneous precipitation method. The structure and morphology of the resulting particles were characterized by X-ray diffraction, field emission scanning electron microscope, and field emission transmission electron microscope, whereas their optical properties were monitored by photoluminescence spectroscopy. The room-temperature luminescence color emission of the synthesized particles can be tuned from red to yellow by switching the excitation wavelength from 254 to 350 nm. The luminescence intensities of red and yellow emissions could be altered by varying the dopant concentration. Strong quenching was observed at high Eu3+ and Dy3+ concentrations in the Y2O3 host lattice.

A facile route to aligned TiO2 nanotube arrays on transparent conducting oxide substrates for dye-sensitized solar cells

TiO2 nanotube arrays (NTAs) on transparent conducting oxides (TCO) have attracted great attention due to the potential application in dye-sensitized solar cells (DSCs). Here, we introduce the template-assisted process for direct fabrication of aligned TiO2 NTAs on TCO substrates, involving layer-by-layer adsorption and reaction (LBL-AR) assembled TiO2 coating on ZnO nanorods (NR). Key factors of the fabrication process on the microstructures of TiO2 NTAs are analyzed, and the geometry effects of TiO2 NTAs on the performance and electron transport properties of DSCs are investigated by using electrochemical impedance spectra (EIS). An efficiency of 4.25% (under AM1.5 irradiation, 100 mW cm−2) is obtained from N719 sensitized 20 μm thick TiO2 NTAs with a wall thickness of 20 nm, with Jsc = 8.2 mA cm−2, Voc = 0.81 V and FF = 63%.

Bioinspired piezoelectric nanogenerators based on vertically aligned phage nanopillars

Bioinspired nanogenerators based on vertically aligned phage nanopillars are inceptively demonstrated. Vertically aligned phage nanopillars enable not only a high piezoelectric response but also a tuneable piezoelectricity. Piezoelectricity is also modulated by tuning of the proteins dipoles in each phage. The sufficient electrical power from phage nanopillars thus holds promise for the development of self-powered implantable and wearable electronics.

Impurity band characteristics near the band edge of Al-doped ZnO

The characteristics of impurity bands near the band edge of Al-doped and undoped ZnO ceramics were investigated by photoluminescence, photoluminescence excitation, and x-ray diffraction. We found that Al(0.6%)-doped ZnO had two impurity bands whose binding energies were roughly 13 and 99meV below the effective band edge. Also, we found that Al(1.1%)-doped ZnO had an impurity band of ∼80meV binding energy below the effective band edge. As the doping concentration of ZnO increases, Al-impurity bands degenerated from two localized levels to the single localized level. The green band emission of Al(0.6%)-doped ZnO is stronger than those of the pure and Al(1.1%)-doped ZnO because of the high charge transfer rate to the effective band.

Study of the structural evolution in ZnO thin film by in situ synchrotron x-ray scattering

The evolution of surface roughness and strain relaxation as a function of film thickness of ZnO films grown on sapphire(0001) were studied by in situ synchrotron x-ray scattering and atomic force microscopy measurements. The well-aligned two-dimensional (2D) planar layer dominated in layer-by-layer growth at the highly strained initial growth stage. As the film thickness increased, the discrete nucleations on the 2D planar layer continuously grew until the ZnO film reached the strain relaxed steady-state regime. When the 3D islands were quickly developed by the strain relaxation, the dynamic scaling exponent β was roughly 1.579. The strain relaxed steady-state regime was described as β∼0.234.

Advantages of using Ti-mesh type electrodes for flexible dye-sensitized solar cells

We used Ti meshes for both the photoanodes and counter electrodes of dye-sensitized solar cells (DSSCs) to improve the flexibility and conductivity of the electrodes. These mesh type electrodes showed good transparency and high bendability when subjected to an external force. We demonstrated the advantages of cells using such electrodes compared to traditional transparent conducting oxide based electrodes and back side illuminated DSSCs, such as low sheet resistance, elevated photo-induced current and enhanced sunlight utilization. Nanotube layers of different thicknesses were investigated to determine their effect on the photovoltaic parameters of the cell. The overall efficiency of the best cells was approximately 5.3% under standard air mass 1.5 global (AM 1.5 G) solar conditions. Furthermore, the DSSCs showed an efficiency of approximately 3.15% due to the all Ti-mesh type electrodes even after illumination from the back side.

Strain effects in ZnO thin films and nanoparticles

We grew Stranski-Krastanow-type ZnO thin film and Volmer-Weber-type self-assembled ZnO nanocrystals using magnetron sputtering methods. The evolution of surface roughness and strain effects in thin ZnO films on Al2O3(0001) substrate and ZnO nanocrystals on Pt(111) surface studied by synchrotron x-ray scattering. The well-aligned two-dimensional (2D) planar layer dominated in layer-by-layer growth at the highly strained initial growth stage in the thin films. As the film thickness increased, the discrete nucleations on the 2D planar layer continuously grew until the ZnO film reached the strain relaxed steady-state regime. The accumulated strain energy in the thin film grown at low temperature slowly relaxed while the strain energy in the high temperature system rapidly relaxed. When the three-dimensional islands on the 2D surface of thin ZnO film grown at the low and high temperatures were quickly developed by strain relaxation, the critical exponent β were roughly 0.693 and 1.579, respectively. The thickn...