Kyohyeok Kim

Study on Ag mesh/conductive oxide hybrid transparent electrode for film heaters

Ag mesh-indium tin oxide (ITO) hybrid transparent conductive films were fabricated and evaluated for use in film heaters. PS monolayer templates were prepared using highly mono-dispersed PS spheres (11.2 μm) obtained by a filtering process with micro-sieves. At first, three Ag meshes with different sheet resistances (20, 100, and 300 Ω sq(-1)) and transmittances (70, 73, and 76%) were evaluated for film heaters in terms of voltage and long-term stability. Subsequently, in an effort to obtain better transmittance, Ag mesh-ITO hybrid heaters were fabricated utilizing finite ITO depositions. At the optimised ITO thickness (15 nm), the sheet resistance and the transmittance were 300 Ω sq(-1) and 88%, respectively, which indicates that this material is a good potential candidate for an efficient defroster in vehicles.

Fabrication of ordered anodic aluminum oxide with matrix arrays of pores using nanoimprint

Anodic aluminum oxide (AAO) with matrix arrays of pores was obtained using nanoimprint and anodizing. Si3N4 imprint stamps were fabricated using electron-beam lithography. The Si3N4 stamps were imprinted into Al films grown on Si wafers by applying a force of 250 kg for 10 s. Two different diameters (45 and 80 nm) and two different periodic distances (100 and 200 nm) of the square patterns in Si3N4 imprint stamp were transferred into an Al film as a form of shallow pores. The Al films were then anodized in 0.3M oxalic acid under conditions of 4 °C at 40 V. The authors found a linear relationship between the interpore distance and anodizing voltage (2.5 nm V−1) to obtain a square array in AAO. In addition, the matrix pores in AAO tend to form a natural hexagonal pattern as the anodizing time increases. The surface images are obtained using field-emission secondary-electron microscope and scanning probe microscopy.

Fabrication of vertically aligned Si nanowires on Si (100) substrates utilizing metal-assisted etching

Vertically aligned Si nanowires were fabricated utilizing a metal-assisted chemical etching scheme using a thin anodic aluminum oxide (AAO) template formed on (100) Si substrate. The diameter and length of the obtained Si nanowires were about 55 and 340 nm, respectively, when the thickness of the AAO template was about 600 nm. The diameters and shapes of the Si nanowires were determined by the hole size and shape of the Ag mesh on the AAO template. In addition, the lengths of the vertical Si nanowires depended on both the AAO thickness and the Ag film thickness.

Fabrication and characterization of ink jet processed organic thin film transistors with poly-4-vinylphenol gate dielectric

Organic thin film transistors (OTFTs) were fabricated on polyethersulphone substrate using 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene as an active layer and cross-linked poly-4-vinylphenol (PVP) as a gate dielectric. Prior to adoption of PVP as a gate dielectric, the PVP print condition was optimized using a metal-insulator-metal structure and making a comparison with spin coated devices in terms of the leakage current, breakdown voltage, and dielectric constant. Then, OTFTs were fabricated with an optimized PVP gate dielectric and a TIPS pentacene active layer using an ink jet printer. The electrical properties such as the field effect mobility, Ion/Ioff ratio, and threshold voltage had values of 0.055 cm2/V s, 103, and −2.6 V, respectively. The smaller Ion/Ioff ratio can be attributed to the smaller coverage of the TIPS pentacene layer due to the plateletlike crystalline structures.

Effect of Sol-Gel Prepared ZnO Electron Selective Layer on the Performance of Inverted Organic Solar Cells

ZnO buffer layer was prepared as electron selective layers (ESL) for photo-induced electron transport and hole blocking in inverted organic solar cells (IOSCs) by sol-gel process. The effects of thickness and surface roughness of ZnO ESL on the performance of IOSCs were investigated. Power Conversion Efficiency (PCE) of the IOSC strongly varied as a function of ZnO film thickness and surface roughness, in particular, when the film thickness was <70 nm, increase in the surface roughness enhanced photovoltaic performances. It is demonstrated that optimization of the electrode thickness and roughness is essential for obtaining better performance of organic photovoltaic cells.

Fabrication and characterization of metal-semiconductor-metal nanorod using template synthesis

The authors attempted to fabricate and characterize one dimensional metal-semiconductor-metal (MSM) nanorod using a template. Cadmium selenide (CdSe) and polypyrrole (Ppy) were chosen as n-type and p-type semiconductor materials, respectively, whereas Au was chosen as a metal electrode. The fabrication of the nanorod was achieved by “template synthesis” method using polycarbonate membrane. The structure of the fabricated nanorod was analyzed using scanning electron microscopy and energy dispersive spectroscopy. In addition, the electrical properties of MSM nanorods were characterized using scanning probe microscopy (Seiko Instruments, SPA 300 HV) by probing with a conductive cantilever. I-V characteristics as a function of the temperature give the activation energy, as well as the barrier height of a metal-semiconductor contact, which is useful to understand the conduction mechanism of MSM nanorods.

Polymerization of 3TUTS SAM Formed on ITO Substrates

Surface modification of ITO with polythiophene derivative, poly(3-TUTS), was done for molecular electronic device application purpose. In order to prepare a derivative of polythiophene having n-alkyltrichlorosilane at 3-position of the thiophene ring, the monomer of thienyl-functionalized n-alkyltrichlorosilane, 11-(3-thienyl)undecyltrichlorosilane (3TUTS), was synthesized. The monomer was identified by using NMR, UV/VIS spectroscopic methods. Surfaces of ITO substrates were modified with the synthesized 3TUTS to get SAM on them. Polymerization of the SAM was done by dipping this SAM into FeCl3 solution. The polymer have been identified by using Cyclic Voltammetry, UV/VIS spectroscopy, scanning electron microscopy (SEM), DFM (Dynamic Force Microscopy) image analysis.

Simulation and characterization of short-channel organic thin-film transistors fabricated using ink-jet printing and an imprint process

The authors fabricated short-channel organic thin-film transistors (OTFTs) on a polyethersulphone substrate using ink-jet printing combined with an imprint method. 6,13-bis(triisopropylsilylethynyl)pentacene and polyvinyl alcohol were used as an active material and a gate insulator, respectively, in the OTFT fabrication. The channel length was reduced remarkably by interfacing ink-jet printing with imprint technology. The authors obtained an OTFT mobility of approximately 0.25 cm2/V s in the saturation region, and the OTFT threshold voltage was approximately −2.5 V. The results were compared with the simulated results to understand the variations in the device structure and material properties.

Fabrication and characterization of nano-scaled Cr Schottky diodes using AAO templates

We fabricated Anodic Aluminum Oxide (AAO) templates with three different heights by controlling 1st and 2nd anodizing times. We obtained nano-scaled Cr Schottky diodes with different diameters (about 75 nm, 55 nm, and 35 nm) using AAO templates. Cr Schottky diodes with smaller diameters yielded higher current densities than those with larger diameters, due to enhanced tunnel current. It is caused by the reduced Schottky barrier thickness, as the diameters of Cr nanodots are much smaller than Debye length (156 nm).

Fabrication of quantum dots using multicoated self-assembled monolayer

The authors have fabricated quantum dots with various nanogaps by controlling the number of self-assembled molecular coatings. First, Au electrodes with a unique shape were obtained using conventional lithography. Then, a self-assembled multilayer, composed of alternating layers of 16-mercaptoalkanoic acids [HS(CH2)15COOH, 16-MHDA] and copper (II) ions, were deposited on Au electrode patterns to form the controllable gap between adjacent Au electrodes. After reaching a nanometer-scale gap, the second Au was deposited again using e-beam evaporation. Finally, both the second Au and molecular resist were removed by lift-off, thereby resulting in quantum dots with a nanogap between gold electrodes. Ellipsometry and cyclic voltammetry were used to find the number of self-assembled molecular layers. In addition, contact angle and x-ray photoelectron spectroscopy were used to analyze chemical properties between gold and the self-assembled multilayer. Field-emission scanning-electron microscopy was used for characterization of shapes of nanogaps and quantum dots.The authors have fabricated quantum dots with various nanogaps by controlling the number of self-assembled molecular coatings. First, Au electrodes with a unique shape were obtained using conventional lithography. Then, a self-assembled multilayer, composed of alternating layers of 16-mercaptoalkanoic acids [HS(CH2)15COOH, 16-MHDA] and copper (II) ions, were deposited on Au electrode patterns to form the controllable gap between adjacent Au electrodes. After reaching a nanometer-scale gap, the second Au was deposited again using e-beam evaporation. Finally, both the second Au and molecular resist were removed by lift-off, thereby resulting in quantum dots with a nanogap between gold electrodes. Ellipsometry and cyclic voltammetry were used to find the number of self-assembled molecular layers. In addition, contact angle and x-ray photoelectron spectroscopy were used to analyze chemical properties between gold and the self-assembled multilayer. Field-emission scanning-electron microscopy was used for chara...