Dongjo Kim

Influence of fluid physical properties on ink-jet printability.

Ink-jet printing is a method for directly patterning and fabricating patterns without the need for masks. To achieve this, the fluids used as inks must have the capability of being stably and accurately printed by ink-jetting. We have investigated the inter-relationship between ink-jet printability and physical fluid properties by monitoring droplet formation dynamics. The printability of the fluids was determined using the inverse (Z) of the Ohnesorge number (Oh) which relates to the viscosity, surface tension, and density of the fluid. We have experimentally defined the printable range as 4 < or = Z < or = 14 by considering characteristics such as single droplet formability, positional accuracy, and maximum allowable jetting frequency.

Direct writing of silver conductive patterns: Improvement of film morphology and conductance by controlling solvent compositions

The authors have investigated the influence of conductive ink compositions on the quality of ink-jet printed patterns. Controlling the solvent compositions plays an important role of reducing the so-called coffee ring effect. The use of a higher boiling point solvent with lower surface tension such as ethylene glycol enables the formation of a uniform deposit of silver nanoparticles due to surface tension gradient-induced inward Marangoni flow, which can compensate outward convective flow. It is demonstrated that the ink-jet printed film quality is directly related to the resistance difference in the conductive patterns.

Ink-jet printing of cu-ag-based highly conductive tracks on a transparent substrate.

We have developed a Cu-Ag-based mixed metal conductive ink from which highly conductive tracks form on a flexible substrate after annealing at low temperature. Addition of small Ag particles significantly improves the particle packing density by filling the interstices formed between the larger Cu particles, which in turn facilitates better conductivity compared to pure Cu metal film. The particle size and volume ratio of the Ag particles added should be carefully controlled to achieve maximum packing density in the bimodal particle system, which is consistent with the theoretical considerations of the Furnas model. In addition, we demonstrate direct writing of complex patterns that exhibit high conductivity upon annealing at sufficiently low temperature (175-210 degrees C) to not damage the transparent plastic substrate such as polyethersulphone (PES).

Inkjet-Printed Zinc Tin Oxide Thin-Film Transistor

Recently, there has been considerable interest in adapting printing approaches that are typically used in the graphic arts to the printing of electronic circuits and circuit components. We report the fabrication of solution-processed oxide transistors using inkjet printing. A zinc tin oxide sol-gel precursor is utilized as the ink for directly printing a thin uniform semiconducting layer. The printed device performance is significantly influenced by printing conditions such as the surface wettability and substrate temperature. The inkjet-printed transistors exhibit reproducible electrical performance, demonstrating their potential application in low-cost manufacturing of large-area flat panel displays.

High-performance low-temperature solution-processable ZnO thin film transistors by microwave-assisted annealing

Oxide semiconductors afford a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin film transistors (TFTs). We synthesized an aqueous inorganic precursor by a direct dissolution of zinc hydroxide in ammonium hydroxide solution from which a dense and uniform ZnO semiconducting layer is achieved. Solution-processed ZnO-TFTs prepared at 140 °C by microwave irradiation have shown enhanced device characteristics of ∼1.7 cm2 V−1s−1 mobility and a ∼107 on/off current ratio, with good air stability. Spectroscopic analyses confirmed that such a device improvement originates from accelerated dehydroxylation and better crystallization at low temperature by microwave irradiation. Our results suggest that solution-processable oxide semiconductors have potential for low-temperature and high-performance applications in transparent devices.

Bias-Stress-Stable Solution-Processed Oxide Thin Film Transistors

We generated a novel amorphous oxide semiconductor thin film transistor (AOS-TFT) that has exellent bias-stress stability using solution-processed gallium tin zinc oxide (GSZO) layers as the channel. The cause of the resulting stable operation against the gate bias-stress was studied by comparing the TFT characteristics of the GSZO layer with a tin-doped ZnO (ZTO) layer that lacks gallium. By photoluminescence, X-ray photoelectron, and electron paramagnetic resonance spectroscopy, we found that the GSZO layer had a significantly lower oxygen vacancy, which act as trap sites, than did the ZTO film. The successful fabrication of a solution-processable GSZO layer reported here is the first step in realizing all-solution-processed transparent flexible transistors with air-stable, reproducible device characteristics.

Solution processed invisible all-oxide thin film transistors

We report on a fully transparent solution processed thin-film transistor (TFT) device with oxide semiconductor and oxide electrode. Selective doping into the sol–gel derived ZnO materials tailors the electrical properties to range from metallic to semiconducting characteristics. Integration of a spin-coated zinc tin oxide (ZTO) semiconductor with an ink-jet-printed zinc indium oxide (ZIO) electrode creates a transparent TFT with high performance and good transparency (∼90%). Use of the same ZnO-based oxide materials in a TFT allows for the formation of good electrical contacts characterized by low contact resistance, comparable to those with a vacuum-deposited Al electrode. Our results suggest that the solution-processed ZnO-based TFT has great potential to work as a building block for future printed transparent electronics.

Compositional influence on sol-gel-derived amorphous oxide semiconductor thin film transistors

We investigated the influence of the chemical compositions of gallium and indium cations on the performance of sol-gel-derived amorphous gallium indium zinc oxide (a-GIZO)-based thin film transistors (TFTs). A systematic compositional study allowed us to understand the solution-processed a-GIZO TFTs. We generated a compositional ternary diagram from which we could predict electrical parameters such as saturation mobility, threshold voltage, and the on/off current ratio as the constituent compositions varied. This diagram can be utilized for tailoring solution-processed amorphous oxide TFTs for specific applications.

Celecoxib induces hepatic stellate cell apoptosis through inhibition of Akt activation and suppresses hepatic fibrosis in rats

Background and aims: Activated hepatic stellate cells (HSCs) but not quiescent HSCs express cyclo-oxygenase-2 (COX-2), suggesting that the COX-2/prostanoid pathway has an active role in hepatic fibrogenesis. However, the role of COX-2 inhibitors in hepatic fibrogenesis remains controversial. The aim of this study was to investigate the antifibrotic effects of celecoxib, a selective COX-2 inhibitor. Methods: The effects of various COX inhibitors—that is, ibuprofen, celecoxib, NS-398 and DFU, were investigated in activated human HSCs. Then, the antifibrotic effect of celecoxib was evaluated in hepatic fibrosis developed by bile duct ligation (BDL) or peritoneal thioacetamide (TAA) injection in rats. Results: Celecoxib, NS-398 and DFU inhibited platelet-derived growth facor (PDGF)-induced HSC proliferation; however, only celecoxib (⩾50 μM) induced HSC apoptosis. All COX inhibitors completely inhibited prostaglandin E2 (PGE2) and PGI2 production in HSCs. Separately, PGE2 and PGI2 induced cell proliferation and extracellular signal-regulated kinase (ERK) activation in HSCs. All COX inhibitors attenuated ERK activation, but only celecoxib significantly inhibited Akt activation in HSCs. Celecoxib-induced apoptosis was significantly attenuated in HSCs infected with adenovirus containing a constitutive active form of Akt (Ad5myrAkt). Celecoxib had no significant effect on PPARγ (peroxisome proliferator-activated receptor γ) expression in HSCs. Celecoxib inhibited type I collagen mRNA and protein production in HSCs. Oral administration of celecoxib (20 mg/kg/day) significantly decreased hepatic collagen deposition and α-SMA (α-smooth muscle actin) expression in BDL- and TAA-treated rats. Celecoxib treatment significantly decreased mRNA expression of COX-2, α-SMA, transforming growth factor β1 (TGFβ1) and collagen α1(I) in both models. Conclusions: Celecoxib shows a proapoptotic effect on HSCs through Akt inactivation and shows antifibrogenic effects in BDL- and TAA-treated rats, suggesting celecoxib as a novel antifibrotic agent of hepatic fibrosis.

Low Temperature Solution-Processed InZnO Thin-Film Transistors

We prepared indium zinc oxide (IZO) semiconductors for low temperature solution-processed thin-film transistors (TFTs). The sol-gel derived IZO films, annealed at 300°C, are uniform and have smooth surface morphology (root-mean-square roughness of 0.27 nm). Both the composition and the film thickness need to be optimized for high performance TFTs. With the composition of In/Zn equal to 50/50 in mol percent, the IZO TFTs with a thickness of 10 nm exhibited the best performance for a clear switching behavior (on/off current ratio of 1.2 × 10 7 and output characteristics (drain current of 3.7 × 10 -4 A), with a relatively high field-effect mobility (0.54 cm 2 V -1 s -1 ) and a low threshold voltage (1.9 V). The nonpassivated IZO-TFT stably operates over a two-month period without any significant change in the on/off current ratio and the mobility.