Sang-Hun Nam

METAL-DOPED ZnO THIN FILMS: SYNTHESIS, ETCHING CHARACTERISTIC, AND APPLICATION TEST FOR ORGANIC LIGHT EMITTING DIODE (OLED) DEVICES

Metal-doped ZnO films with various metal contents (Al, Ag and Li of 0–10 wt.%) were prepared by RF magnetron sputtering system with specially designed ZnO targets. The structural, optical and electrical properties of MZO films depended on the type and content of doping in target. Electrical resistivity of LZO thin films increased with increasing Li doping amounts between 0 and 4 wt.%, suggesting that an epitaxial LZO film has high resistivity. We observed morphology in pure ZnO films by using different etchant. In addition, etching rate were contrasted with the etchant concentration and pH. The etching rate is proportional exponentially to pH value. These data will be the technical basis for TCO application. Also, the dry etching rate decreased with increasing the Cl2 concentration in CH4/H2/Ar + additive Cl2 gas mixture but metal dopants were etched effectively.

Growth behavior of titanium dioxide thin films at different precursor temperatures

The hydrophilic TiO2 films were successfully deposited on slide glass substrates using titanium tetraisopropoxide as a single precursor without carriers or bubbling gases by a metal-organic chemical vapor deposition method. The TiO2 films were employed by scanning electron microscopy, Fourier transform infrared spectrometry, UV-Visible [UV-Vis] spectroscopy, X-ray diffraction, contact angle measurement, and atomic force microscopy. The temperature of the substrate was 500°C, and the temperatures of the precursor were kept at 75°C (sample A) and 60°C (sample B) during the TiO2 film growth. The TiO2 films were characterized by contact angle measurement and UV-Vis spectroscopy. Sample B has a very low contact angle of almost zero due to a superhydrophilic TiO2 surface, and transmittance is 76.85% at the range of 400 to 700 nm, so this condition is very optimal for hydrophilic TiO2 film deposition. However, when the temperature of the precursor is lower than 50°C or higher than 75°C, TiO2 could not be deposited on the substrate and a cloudy TiO2 film was formed due to the increase of surface roughness, respectively.

Improvement on the Electron Transfer of Dye-Sensitized Solar Cell Using Vanadium Doped TiO2

In dye-sensitized solar cells, nanoporous structure of TiO2 is very important for efficient cell because lots of dye molecules are adsorbable and they are the source of the photocurrent. However, the internal impedance of TiO2 is relatively large and it limits the performance. For better performance, vanadium was doped into TiO2 in this work. Doping different material generally improves the characteristics and functions of original materials. Vanadium doping has some advantages such as the reduction of internal resistance, the improvement of chemical stability and high absorption. Especially, reduced internal resistance is so helpful for better electron transfer in TiO2 network. Various amounts of vanadium were applied and photovoltaic performance, internal impedance and absorbance were measured in order to verify the effect of vanadium doping. As a result, vanadium doping improved the overall performance from 6.01 to 6.81% with decreased internal resistance although adsorbed dye amount was reduced by decreased surface area and open circuit voltage was also decreased by the change of band-gap energy.

Physical properties of metal-doped zinc oxide films for surface acoustic wave application

Metal-doped ZnO [MZO] thin films show changes of the following properties by a dopant. First, group III element (Al, In, Ga)-doped ZnO thin films have a high conductivity having an n-type semiconductor characteristic. Second, group I element (Li, Na, K)-doped ZnO thin films have high resistivity due to a dopant that accepts a carrier. The metal-doped ZnO (M = Li, Ag) films were prepared by radio frequency magnetron sputtering on glass substrates with the MZO targets. We investigated on the optical and electrical properties of the as-sputtered MZO films as dependences on the doping contents in the targets. All the MZO films had shown a preferred orientation in the [002] direction. As the quantity and the variety of metal dopants were changed, the crystallinity and the transmittance, as well as optical band gap were changed. The electrical resistivity was also changed with changing metal doping amounts and kinds of dopants. An epitaxial Li-doped ZnO film has a high resistivity and very smooth surface; it will have the most optimum conditions which can be used for the piezoelectric devices.

Enhanced Visible Transmittance of Thermochromic VO2 Thin Films by SiO2 Passivation Layer and Their Optical Characterization

This paper presents the preparation of high-quality vanadium dioxide (VO2) thermochromic thin films with enhanced visible transmittance (Tvis) via radio frequency (RF) sputtering and plasma enhanced chemical vapor deposition (PECVD). VO2 thin films with high Tvis and excellent optical switching efficiency (Eos) were successfully prepared by employing SiO2 as a passivation layer. After SiO2 deposition, the roughness of the films was decreased 2-fold and a denser structure was formed. These morphological changes corresponded to the results of optical characterization including the haze, reflectance and absorption spectra. In spite of SiO2 coating, the phase transition temperature (Tc) of the prepared films was not affected. Compared with pristine VO2, the total layer thickness after SiO2 coating was 160 nm, which is an increase of 80 nm. Despite the thickness change, the VO2 thin films showed a higher Tvis value (λ 650 nm, 58%) compared with the pristine samples (λ 650 nm, 43%). This enhancement of Tvis while maintaining high Eos is meaningful for VO2-based smart window applications.

Synthesis of ZnO nanoparticles by spray-pyrolysis method and their photocatalytic effect

ZnO nanoparticles were synthesized by spray pyrolysis method using the zinc acetate dihydrate as starting material at various synthesis temperatures. The structures of the synthesized ZnO were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transformation infrared (FT-IR), and UV-vis spectroscopy. With increasing synthesis temperature, the mean diameter of ZnO nanoparticles increased, and their crystallinity was improved. The photocatalytic activity of ZnO was studied by the photocatalytic degradation of methyleneblue (MB) under UV irradiation (365nm) at room temperature. The results show that the photocatalytic efficiency of ZnO nanoparticles was enhanced by increasing synthesis temperature.

SPRAY PYROLYSIS OF MANGANESE DOPED ZINC SILICATE PHOSPHOR PARTICLES

Spherical-shape Zn2SiO4:Mn phosphor particles with the mean particle size from submicron to micron sizes were prepared by ultrasonic spray pyrolysis method. A droplet separator was introduced to control the size distribution of the phosphor particles with spherical shape. The Zn2SiO4:Mn phosphor particles with 2 mol% doping concentration of manganese have decay time and have photoluminescence intensities comparable with those of the latest commercial product prepared by the solid state reaction method. The size of the phosphor particles was decreased from 1000 to 200 nm as the inorganic salt solution concentration was changed from 0 to 5 M. The phosphor particles prepared from the solutions above 0.5 M have photoluminescence intensities comparable with that of the latest commercial product.

Optical Sensing Properties of ZnO Nanoparticles Prepared by Spray Pyrolysis

We studied the optical sensing properties of ZnO nanoparticles prepared by spray pyrolysis. To investigate their optical sensing performance, we incubated peptides on ZnO nanoparticles. The photoluminescence (PL) peak intensity of peptides on the ZnO nanoparticles was higher than that of peptides on the ZnO film or on the glass plate. This observed PL enhancement is attributed to the optical confinement of ZnO nanoparticles. The low-temperature spectra displayed a strong exciton emission peak with multiple sidebands, attributed to the bound exciton and its longitudinal optical phonon sidebands. The strong exciton emission is thought to be the combined effect of optical confinement due to the nanoparticle geometry, reduction of defect emission by thermal annealing, and reduction of non-radiative relaxation at low temperatures.

Improved Electrochromic Characteristics of a Honeycomb-Structured Film Composed of NiO

Color changes controlled by electronic energies have been studied for many years in order to fabricate energy-efficient smart windows. Reduction and oxidization of nickel oxide under the appropriate voltage can change the color of a window. For a superior nickel oxide (NiO) electrochromic device (ECD), it is important to control the chemical and physical characteristics of the surface. In this study, we applied polystyrene bead templates to nickel oxide films to fabricate a honeycomb-structured electrochromic (EC) layer. We synthesized uniform polystyrene beads using the chemical wet method and placed them on substrates to create honeycomb-structured NiO films. Then, the EC characteristics of the nickel oxide films with a honeycomb structure were evaluated with UV-Visible and cyclic voltammetry. FE-SEM and AFM were used to measure the morphologies of the nanostructures and the efficiencies of the redox reactions related to the specific surface area.

Synthesis and characterization of vanadium doped TiO 2 for the visible light-driven photocatalytic activity

V-doped TiO 2 nanoparticles (NPs) was synthesized by a sol-gel hydrolysis method using titanium butoxide (Ti(OBu) 4 ) and vanadyl acetylacetonate (VO(acac) 2 ) as a precursor and dopant, respectively. The effects of different V : Ti ratios on the crystal structure, band gap energy and photocatalytic activity have been investigated. The photocatalytic activity of V-doped TiO 2 NPs was evaluated using UV-vis absorption spectroscopy through degradation of a methyleneblue (MB) solution under visible light irradiation. Characterization by Raman and X-ray diffraction pattern shows that V was doped effectively and oxygen vacancies and various defects or Ti3+ centers were formed. Energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) results indicated the presence of V in the TiO 2 . The band gap energy of V-doped TiO 2 NPs was determined to be 2.3 eV, with a distinct red-shift confirmed by diffuse reflectance spectroscopy (DRS). The photocatalytic activity of V-doped TiO 2 NPs exhibited highly enhanced visible-light-induced photocatalytic performance compared with pure TiO 2 NPs.