Dong-Hee Park

Carrier transport in flexible organic bistable devices of ZnO nanoparticles embedded in an insulating poly(methyl methacrylate) polymer layer

The bistable effects of ZnO nanoparticles embedded in an insulating poly(methyl methacrylate) (PMMA) polymer single layer by using flexible polyethylene terephthalate (PET) substrates were investigated. Transmission electron microscopy (TEM) images revealed that ZnO nanoparticles were formed inside the PMMA polymer layer. Current-voltage (I-V) measurement on the Al/ZnO nanoparticles embedded in an insulating PMMA polymer layer/ITO/PET structures at 300 K showed a nonvolatile electrical bistability behavior with a flat-band voltage shift due to the existence of the ZnO nanoparticles, indicative of trapping, storing, and emission of charges in the electronic states of the ZnO nanoparticles. The carrier transport mechanism of the bistable behavior for the fabricated organic bistable device (OBD) structures is described on the basis of the I-V results by analyzing the effect of space charge.

Cu-doped ZnO-based p–n hetero-junction light emitting diode

Copper-doped p-ZnO thin films (Cu:ZnO) were grown on α-Al2O3(0 0 0 1) and 6H:SiC(0 0 0 1) single crystal substrates by plasma-assisted molecular beam epitaxy. A p–n hetero-junction with p-Cu:ZnO/n-6H:SiC was successfully fabricated and demonstrated as a greenish-blue light emitting diode (LED). The rectifying I–V curve along with the matching photoluminescence and electroluminescence emissions characterizes the fabricated p–n hetero-junction LED. The Cu cell temperature (TCu) and the post-deposition annealing environment greatly influence the Cu oxidation state, and hence the electrical conversion from n-type to p-type and carrier concentration in the films. The higher TCu and post-annealing in O-plasma were observed to be the favorable conditions for Cu2+ and hence the p-type nature of the films.

Single active-layer structured dual-function devices using hybrid polymer-quantum dots.

We demonstrate hybrid polymer-quantum dot dual-function devices with a single active-layer structure consisting of CdSe/ZnS semiconductor quantum dots dispersed with poly N-vinylcarbazole (PVK) and 1,3,5-tirs-(N-phenylbenzimidazol-2-yl) benzene (TPBi) fabricated on an indium-tin-oxide (ITO)/glass substrate by using a simple spin-coating technique. The dual-function devices are composed of light-emitting diodes (LED) on the top side and nonvolatile organic bistable memory devices (OBD) on the bottom side and can show electroluminescence (EL) along with electrical bistability concurrently. Both the functionality of LEDs and OBDs can be successfully achieved by adding an electron transport layer (ETL) TPBi to the OBD to attain an LED in which the lowest unoccupied molecular orbital (LUMO) level of TPBi is positioned at the energy level between the conduction band of CdSe/ZnS and the LiF/Al electrode. Through transmission electron microscopy (TEM) study, it is revealed that CdSe/ZnS QDs distributed on the interface of the hole transport layer (HTL) and ETL significantly take part in the electroluminescence process rather than those existing at the outer surface of the ETL.

Electroluminescence of a single active layer polymer―nanocrystal hybrid light-emitting diode with inversion symmetry

A hybrid polymer-nanocrystal (NC) light-emitting diode (LED) device with a single active layer structure is simply fabricated by a spin coating. From a high-resolution transmission electron microscopy (HRTEM) study, each PVK polymer particle is observed to be capped with TPBi molecules and CdSe/ZnS NCs are mainly distributed along the circumference of PVK and TPBi surfaces, resulting in a core-shell polymer-NC hybrid of [CdSe/ZnS]/TPBi/[CdSe/ZnS]/PVK. An Al/[CdSe/ZnS]/TPBi/[CdSe/ZnS]/PVK/indium-tin oxide(ITO)/glass LED shows electroluminescence (EL) centered at around 585 nm at the forward bias of +10 V, which clearly reveals that CdSe/ZnS NCs existing at the interface between PVK and TPBi act as recombination centers for excitons. In particular, EL can be observed at both forward bias and reverse bias, and this means that this device with an isotropic distribution of NCs has an inversion symmetry.

Linear ion source with closed drift and extended acceleration region

Ion source with closed drift, which is caused by E x B field, and extended acceleration region is discussed. Though conventional circular-type closed drift ion source has advantages of high efficiency of gas ionization and low ion beam energy, there is a limitation in enlarging the beam size. Linear ion source with horse-track shape with 270 mm ceramic channel width is newly designed and tested. Inert gas (Ar) and reactive gas (O(2)) are discharged. Discharge is ignited with voltage of 90 V. Discharge current is proportional to discharge voltage and increases up to 16.3 A in argon and 15.6 A in oxygen at discharge voltage of 320 V. Extracted ion beam current is also proportional to discharge voltage and is saturated after 280 V for both gases. It is measured up to 0.78 mA/cm(2) in argon beam and 0.73 mA/cm(2) in oxygen beam at a distance of 100 mm from the ion source. Argon ion beam shows better space uniformity than oxygen across the beam extraction region.

Characterization of Al-Doped ZnO Thin Film Grown on Buffer Layer with RF Magnetron Sputtering Method

The optimal condition of low temperature deposition of transparent conductive Al-doped zinc oxide (AZO) films is studied by RF magnetron sputtering method. To achieve enhanced-electrical property and good crystallites quality, we tried to deposit on glass using a two-step growth process. This process was to deposit AZO buffer layer with optimal growth condition on glass in-situ state. The AZO film grown at rf 120 W on buffer layer prepared at RF shows the electrical resistivity , Carrier concentration , and mobility in these results, The crystallinity of AZO film on buffer layer was similar to that of AZO film on glass with no buffer later but the electrical properties of the AZO film were 30% improved than that of the AZO film with no buffer layer. Therefore, the cause of enhanced electrical properties was explained to be dependent on degree of crystallization and on buffer layer`s compressive stress by variation of ion impinging energy.

Formation and Interface Analysis of Ti ∕ Ni ∕ Ti ∕ Au Ohmic Contacts on n -Type 6H–SiC

We present the results of TiNiTiAu multilayer ohmic contacts on n -type 6H-SiC and their interface analysis. The as-deposited contacts show rectifying behavior and, with the increase in annealing temperature, they gradually transform to high-quality ohmic contacts exhibiting linear current-voltage characteristics. The interface evolution was analyzed through glancing angle X-ray diffraction, Auger electron spectroscopy, and atomic force microscopy. The Ti Si2 and Ni2 Si formed at the interfaces during the low-temperature annealing initiate the conversion from Schottky to ohmic behavior, while the increased Ni2 Si formation at high-temperature annealing makes the perfect ohmic contacts. The results were interpreted through the thermodynamic reaction mechanisms. © 2007 The Electrochemical Society.

Colossal Resistivity Change of Polycrystalline NiO Thin Film Deposited by RF Magnetron Sputtering

Polycrystalline NiO thin films were deposited on glass substrate by RF magnetron sputtering using only Ar as a plasma sputter gas. based on the analysis of x-ray diffraction (XRD), NiO films had a polycrystalline cubic (NaCl type) structure. NiO thin films grown below and above showed preferred orientation of (111) and (220) respectively. It showed colossal change in electrical resistivity as much a order form an insulating state of below to a conducting state of above such a Mott metal-insulator transition (MIT) in polycrystalline.

Effect of Indium–Tin-Oxide Schottky Contact on the Resistance Switching of NiO Film

The effect of electrode materials on resistance switching was evaluated on the Pt/NiO/electrode (EL) structures where the EL contacts were Pt, Al, and indium–tin-oxide (ITO). It was confirmed that ohmic Pt contact needs to induce the effective electric field for resistance switching across the NiO film. For the Pt/NiO/Al structure, the barrier height of the Al Schottky contact was measured as 0.66 eV and no resistance switching was observed owing to a large voltage drop at the rectifying interface induced by the reduction of NiO resulting from the formation of Al oxide. In the ITO (EL)/NiO/Pt structure, the barrier height of the Schottky contact between ITO and NiO was about 0.52 eV and it did not show any resistance switching, either. Through the depth-profile study by X-ray photoelectron spectroscopy, chemical reactions at the interface ITO/NiO was identified to be not too much evolved compared with that of NiO/Al, which might due to be abundant oxygen on the ITO surface. Such Schottky barrier heights 0.52–0.66 eV were considered too high to induce a sufficient electric field in the NiO film causing the resistance switching.

ZnO Nanoparticle Based Dye-Sensitized Solar Cells Devices Fabricated Utilizing Hydropolymer at Low Temperature

To fabricate nanoparticle-based dye sensitized solar cells (DSSCs) at a low-temperature, DSSCs were fabricated using hydropolymer and ZnO nanoparticles composites for the electron transport layer around a low-temperature (). ZnO nanoparticle with 20 nm and 60 nm diameter were used and Pt was deposited as a counter electrode on ITO/glass using an RF magnetron sputtering. We investigate the effect of ZnO nanoparticle concentration in hydropolymer and ZnO nanoparticle solution on the photoconversion performance of the low temperature fabricated () DSSCs. Using cis-bis(isothiocyanato)bis(2,20 bipyridy1-4,40 dicarboxylato) ruthenium (II) bis-tetrabutylammonium (N719) dye as a sensitizer, the corresponding device performance and photo-physical characteristics are investigated through conventional physical characterization techniques. The effect of thickness of the ZnO photoelectrode and the morphology of the ZnO nanoparticles with the variations of hydropolymer to ZnO ratio on the photoconversion performance are also investigated. The morphology of the ZnO layer after sintering was examined using a field emission scanning electron microscope (FE-SEM). 60 nm ZnO nanoparticle DSSCs showed an incident photon-to-current conversion efficiency (IPCE) value of about 7% higher than that of 20 nm ZnO nanoparticle DSSCs. The maximum parameters of the short circuit current density (), the open circuit potential (), fill factor (ff), and efficiency () in the 60 nm ZnO nanoparticle-based DSSC devices were 4.93 mA/, 0.56V, 0.40, and 1.12%, respectively.