K. S. Kang

Effect of annealing temperature on the conduction mechanism for a sol–gel driven ZnO Schottky diode

The conduction mechanism of a sol?gel driven ZnO Schottky diode has been investigated with various annealing temperatures. A dense and pore-free ZnO film has been fabricated for a metal?semiconductor?metal structure via the sol?gel method with Zn acetate as a precursor, ethanol as a solvent and ethanol amine as a chelating agent. ZnO films were analysed by means of field emission scanning electron microscopy, ultraviolet visible absorption analysis and Fourier transform infrared (FTIR) analysis. The absorption shoulder appeared and shifted to a longer wavelength as the heating temperature and duration increased. The intensities of FTIR absorption peaks decreased as the heating temperature and duration increased. In particular, the strong and broad absorption peak of ?OH and ?NH at approximately 3400?cm?1 almost disappeared after the heating process. The intensity of the x-ray diffraction peak at 2? = 34? (2?0?0) was strongly dependent on the ZnO film thickness and annealing temperature. Schottky diodes comprising Al/ZnO/Au were fabricated with three different annealing temperatures, i.e. 100 (AZA100), 200 (AZA200) and 300??C (AZA300). The forward current of AZA300 has been increased more than twice with respect to that of AZA100. The plots of Ln(I) versus Ln(V), Ln(I) versus V1/2 and Ln(I/V) versus V1/2 based on the equations of the space charge limited conduction model, the Schottky conduction model and the Poole?Frenkel conduction model were employed to identify the conduction mechanism. All three plots showed a linear relationship. However, the slope and beta values do not match compared with the theoretical values. This result indicates that the conduction process is not a single conduction mechanism. Therefore, the forward current enhancement may be due to the reduction of ?OH and ?NH groups (acting as deep traps) instead of a fundamental conduction mechanism change.

The TiO2 nanoparticle effect on the performance of a conducting polymer Schottky diode

Among the conjugate polymers, poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) has been paid a great deal of attention for various application fields. The absorption intensity of the whole UV-visible range increases linearly, as the concentration of PEDOT:PSS increases. When a small amount of TiO(2) nanoparticles are dispersed in the PEDOT:PSS solution, the absorption in the visible range normally increases, but the UV range absorption (TiO(2) absorption area) is greatly depressed as the concentration of PEDOT:PSS increases. Various weight ratios of TiO(2) nanoparticles in PEDOT:PSS were prepared. The TiO(2)/PEDOT:PSS solution was spin-coated onto the Al electrode and thermally treated to remove water molecules and densify the film. These thermal processes generated nanocracks and nanoholes on the surface of the TiO(2)/PEDOT:PSS film. As the heating temperature increased, wider and longer nanocracks were generated. These nanocracks and nanoholes can be removed by subsequent coating and heating processes. Schottky diodes were fabricated using four different concentrations of TiO(2)-PEDOT:PSS solution. The forward current increased nearly two orders of magnitude by doping approximately 1% of TiO(2) nanoparticles in PEDOT:PSS. Increasing the TiO(2) nanoparticles in the PEDOT:PSS matrix, the forward current was continuously enhanced. The enhancement of forward current is nearly four orders of magnitude with respect to the pristine PEDOT:PSS Schottky diode. The possible conduction mechanisms were examined by using various plotting and curve-fitting methods including a space-charge-limited conduction mechanism [Ln(J) versus Ln(V)], Schottky emission mechanism [Ln(J) versus E(1/2)], and Poole-Frenkel emission mechanism [Ln(J/V) versus E(1/2)]. The plot of Ln(J) versus Ln(V) shows a linear relationship, implying that the major conduction mechanism is SCLC. As the concentration of TiO(2) increased, the conduction mechanism slightly detracted from the ideal SCLC mechanism.

Magnetic field effect for cellulose nanofiber alignment

Regenerated cellulose formed into cellulose nanofibers under strong magnetic field and aligned perpendicularly to the magnetic field. Well-aligned microfibrils were found as the exposure time of the magnetic field increased. Better alignment and more crystalline structure of the cellulose resulted in the increased decomposition temperature of the material. X-ray crystallograms showed that crystallinity index of the cellulose increased as the exposure time of the magnetic field increased.

The Effect of Residual Ionic Liquid for Cellulose Based Electro-Active Paper Actuator

The cellulose was directly dissolved in 1-butyl-3-methylimidazolium chloride at 80°C. The clear and transparent cellulose solution was cast on the glass substrate using tape-casting method. The cast films were soaked into various solvents such as methanol, methanol-deionized water, deionized water, and isopropyl alcohol-deionized water. UV-visible transmittances were approximately 74% to 50% depending on the curing solvents. High thermal stability was observed for the cellulose film regenerated with high ratio of water content. Using these cellulose films, EAPap actuators were fabricated by coating gold-electrodes on both sides. The resonance frequency shifted to lower frequency as the amount of residual ionic liquids increased. The resonance frequency shift to lower frequency might be due to the hygroscopic nature of the ionic liquids.

The effect of TiO2 nanoparticle concentration on conduction mechanism for TiO2-polymer diode

Polymer Schottky diodes using p-type poly(3,4-ethylenedioxythiophene:poly (styrenesulfonate) (PEDOT:PSS) doped with various concentrations of n-type TiO2 nanoparticles have been fabricated. Although Al/PEDOT:PSS/Au Schottky diode does not show a clear diode characteristics, the Al∕TiO2-PEDOT:PSS/Au Schottky diode exhibites excellent rectification characteristics. A Schottky diode with highly doped TiO2 nanoparticles (20wt%) shows high forward current having more than three orders of magnitude with respect to pristine PEDOT:PSS Schottky diode. The conduction mechanism of the TiO2 doped Schottky diodes shows best fit of space charge limited conduction process compared to the other mechanisms including Schottky emission and Poole–Frenkel emission.

Au micro-pattern fabrication on cellulose paper : Comparison of μ-contact printing and liftoff techniques

Micro-contact printing (μ-CP) and liftoff techniques were employed to fabricate Au micro-patterns onto electro-active paper (EAPap) for biodegradable and flexible microelectromechanical system (MEMS) application. Conventional lithography and etching techniques cannot be utilized to fabricate micropatterns on EAPap having interesting actuating properties due to its hydrophilic and flexible characteristics. The cause of nanodefects on the Au pattern after the μ-CP process was investigated to enhance the pattern quality. Three different solvents were utilized to investigate the consequence of the solvent polarity during the 3-mercaptopropyltrimethoxysilane (MPTMS) self-assembled monolayer (SAM) fabrication and after the μ-CP process. The defects, such as microcracks and nanograins, were closely related to the solvent polarity. The higher polar solvent turned out smaller defects than the lower polar solvent due to the reduction of the solvent swell onto the PDMS stamp during SAM layer formation. The liftoff technique exhibited a new way of fabricating various metal patterns on EAPap without any nanodefects.

Au-pattern fabrication on a cellulose film using a polyurethane acrylate mold

This paper deals with a gold micro-patterning process on a cellulose film using a polyurethane acrylate (PUA) mold. Recently, cellulose electro-active paper (EAPap) has been found to be a smart material that can be used for biodegradable sensors, actuators and MEMS devices. However, the hydrophilic and flexible characteristics of cellulose EAPap are major drawbacks for applying a conventional lithography process to fabricate MEMS devices. To overcome these drawbacks, an unconventional lithography process, the so-called micro-transfer printing technique based on a PUA mold, was employed. A master pattern for the PUA mold was fabricated using the conventional photolithography process with an SU-8 photoresist, and the replica of the master pattern was fabricated using PUA. Gold was deposited onto the PUA mold, and a mercaptopropyltrimethoxysilane (MPTMS) self-assembly monolayer was made on the gold surface to securely transfer the gold layer onto the cellulose film. The effect of MPTMS was investigated. Further investigation of the factors to optimize the repeated stamping process will lead to a practical, reusable mold.

Investigation of surface morphology of cellulose acetate micro-mould after deacetylation

Biodegradable honeycomb patterns are useful for a cell culture substrate and tissue engineering. Attaining control over pore size and depth is extremely important for many biological and optical applications. In this study, uniform sizes and various heights of cellulose acetate (CA) honeycomb patterns were fabricated using a photoresist micro-mould. The carbonyl peak was diminished by 60%, 90% and 98% after 1?h, 4?h and 7?h of deacetylation (DA) reaction, respectively, and disappeared after 31?h of DA reaction. The UV?visible transmittance decreased as the DA reaction time increased up to 17?h and increased after 60?h reaction. The transmittance reduction was due to the rough structure of the surface and the inside of the pattern created during the early DA reaction. The rough structure became dense and smooth as the reaction time further increased. This may be a result of uniform removal of the acetyl group at the surface and inside the film after enough time of DA reaction. The line width was increased by 39% and 56% after 1.5?h and 17?h DA reaction, respectively. The line width returned to the initial width after further DA reaction. These results imply that the structure of the CA mould became rough in the early stage of the DA reaction and became dense and smooth after further DA reaction.

Effect of Electrode Pattern on the Actuator Performance of Cellulose Electro-Active Paper

The effect of electrode patterns on the actuator performance of electro-active paper (EAPap) is studied. A rectangular electrode pattern has been typically used for EAPap actuator. Since the electrical field can be concentrated at the edge of electrodes, the electrode patterns of EAPap can influence the actuator performance. Thus, a fishbone pattern electrode was designed and fabricated on EAPap materials and its actuation characteristics were compared with the rectangular electrode ones. Two EAPap materials, DCell, which is made by dissolving cellulose pulp with N,N-Dimethylacetamide and LiCl followed by curing process, and cellophane were used. Bending displacements and the resonance frequencies of EAPap actuators were investigated. Although the bending displacements of the fishbone patterned EAPap actuators were slightly lower than the rectangular patterned actuators, the resonance frequencies of the fishbone patterned actuators were higher than the rectangular patterned actuators. Electrical field concentration around the edge of fishbone pattern electrode might result in increased bending stiffness of the actuator and electrical power consumption. Electrical power consumption and electrode damage of the fishbone pattern electrode were addressed.

Direct and indirect contact effect between Al and TiO2 on the conduction mechanism for polymer-TiO2 Schottky diodes

Two kinds of Schottky diodes comprising an of Al/a mixture of TiO2 nanoparticles and poly(3,4-ethylenedioxythiophene) : poly(4-styrenesulfonate) (PEDOT : PSS)/Au and Al/TiO2/PEDOT : PSS/Au were fabricated to investigate the effect of direct and indirect contacts of Al/TiO2 on the conduction mechanism. A mixture of TiO2 nanoparticles and PEDOT : PSS was coated to fabricate an Al/TiO2-PEDOT : PSS/Au Schottky diode (IATPA). TiO2 nanoparticles were directly coated on the Al electrode prior to PEDOT : PSS coating to fabricate an Al/TiO2/PEDOT : PSS/Au Schottky diode (DATPA). The forward current of DATPA increased by more than three-fold with respect to that of IATPA. Three conduction models including space charge limited conduction (SCLC), Schottky conduction and Poole–Frenkel conduction (PFC) models were employed to analyse the conduction mechanism. The major conduction mechanisms for both IATPA and DATPA closely follow the SCLC and PFC models. Higher slope and bigger beta value than those of the theoretical SCLC and PFC models for the DATPA indicate more involvement of the PFC mechanism in SCLC or vice versa. The reason for following the SCLC mechanism might be the low conductive property of PEDOT : PSS and the reason for following the PFC mechanism may be the partial positive and negative charges of the PEDOT : PSS.