Urchin-structured zinc oxide(ZnO) nanorod(NR) gas sensors were successfully demonstrated on a polyimide(PI) substrate, using single wall carbon nanotubes(SWCNTs) as the electrode. The ZnO NRs were grown with ZnO shells arranged at regular intervals to form a network structure with maximized surface area. The high surface area and numerous junctions of the NR network structure was the key to excellent gas sensing performance. Moreover, the SWCNTs formed a junction barrier with the ZnO which further improved sensor characteristics. The fabricated urchin-structured ZnO NR gas sensors exhibited superior performance upon NO2 exposure with a stable response of 110, fast rise and decay times of 38 and 24 sec, respectively. Comparative analyses revealed that the high performance of the sensors was due to a combination of high surface area, numerous active junction points, and the use of the SWCNTs electrode. Furthermore, the urchin-structured ZnO NR gas sensors showed sustainable mechanical stability. Although degradation of the devices progressed during repeated flexibility tests, the sensors were still operational even after 10000 cycles of a bending test with a radius of curvature of 5 mm.

In order to produce size-controllable Ag nanoparticles and a nanomesh-patterned Si substrate, we introduce a rapid thermal annealing(RTA) method and a metal assisted chemical etching(MCE) process. Ag nanoparticles were self-organized from a thin Ag film on a Si substrate through the RTA process. The mean diameter of the nanoparticles was modulated by changing the thickness of the Ag film. Furthermore, we controlled the surface energy of the Si substrate by changing the Ar or H2 ambient gas during the RTA process, and the modified surface energy was evaluated through water contact angle test. A smaller mean diameter of Ag nanoparticles was obtained under H2 gas at RTA, compared to that under Ar, from the same thickness of Ag thin film. This result was observed by SEM and summarized by statistical analysis. The mechanism of this result was determined by the surface energy change caused by the chemical reaction between the Si substrate and H2. The change of the surface energy affected on uniformity in the MCE process using Ag nanoparticles as catalyst. The nanoparticles formed under ambient Ar, having high surface energy, randomly moved in the lateral direction on the substrate even though the etching solution consisting of 10 % HF and 0.12 % H2O2 was cooled down to -20˚C to minimize thermal energy, which could act as the driving force of movement. On the other hand, the nanoparticles thermally treated under ambient H2 had low surface energy as the surface of the Si substrate reacted with H2. That's why the Ag nanoparticles could keep their pattern and vertically etch the Si substrate during MCE.

An effect of thermal annealing on activating phosphorus (P) atoms in ZnO nanorods (NR) grown using a hydrothermal process was investigated. NH4H2PO4 used as a dopant source reacted with Zn2+ ions and Zn3(PO4)2 sediment was produced in the solution. The fact that most of the input P elements are concentrated in the Zn3(PO4)2 sediment was confirmed using an energy dispersive spectrometer (EDS). After the hydrothermal process, ZnO NRs were synthesized and their PL peaks were exhibited at 405 and 500 nm because P atoms diffused to the ZnO crystal from the Zn3(PO4)2 particles. The solubility of the Zn3(PO4)2 initially formed sediment varied with the concentration of NH4OH. Before annealing, both the structural and the optical properties of the P-doped ZnO NR were changed by the variation of P doping concentration, which affected the ZnO lattice parameters. At low doping concentration of phosphorus in ZnO crystal, it was determined that a phosphorus atom substituted for a Zn site and interacted with two VZn, resulting in a PZn-2VZn complex, which is responsible for p-type conduction. After annealing, a shift of the PL peak was found to have occurred due to the unstable P doping state at high concentration of P, whereas at low concentration there was little shift of PL peak due to the stable P doping state.

Gold have been used as an electrode materials having a good mechanical flexibility as well as electrical conductivity, however the stretchability of the gold on a flexible substrate is poor because of its small elastic modulus. To overcome this mechanical inferiority, the reinforcing gold is necessary for the stretchable electronics. Among the reinforcing materials having a large elastic modulus, carbon nanotube (CNT) is the best candidate due to its good electrical conductivity and nanoscale diameter. Therefore, similarly to ferroconcrete technology, here we demonstrated gold electrodes mechanically reinforced by inserting fabrics of CNTs into their bodies. Flexibility and stretchability of the electrodes were determined for various densities of CNT fabrics. The roles of CNTs in resisting electrical disconnection of gold electrodes from the mechanical stress were confirmed using field emission scanning electron microscope and optical microscope. The best mechanical stability was achieved at a density of CNT fabrics manufactured by 1.5 ml spraying. The concept of the mechanical reinforced metal electrode by CNT is the first trial for the high stretchable conductive materials, and can be applied as electrodes materials in various flexible and stretchable electronic devices such as transistor, diode, sensor and solar cell and so on.

Si Nanowire (NW) field effect transistors (FETs) were fabricated on hard Si and flexible polyimide (PI) substrates, and their electrical characteristics were compared. Si NWs used as channels were synthesized by electroless etching method at low temperature, and these NWs were refined using a centrifugation method to get the NWs to have an optimal diameter and length for FETs. The gate insulator was poly(4-vinylphenol) (PVP), prepared using a spin-coating method on the PI substrate. Gold was used as electrodes whose gap was 8 μm. These gold electrodes were deposited using a thermal evaporator. Current-voltage (I-V) characteristics of the device were measured using a semiconductor analyzer, HP-4145B. The electrical properties of the device were characterized through hole mobility, Ion/Ioff ratio and threshold voltage. The results showed that the electrical properties of the TFTs on PVP were similar to those of TFTs on SiO2. The bending durability of SiNWs TFTs on PI substrate was also studied with increasing bending times. The results showed that the electrical properties were maintained until the sample was folded about 500 times. But, after more than 1000 bending tests, drain current showed a rapid decrease due to the defects caused by the roughness of the surface of the Si NWs and mismatches of the Si NWs with electrodes.

As a growth-template of ZnO nanorods (NR), a hexagonal β-Ni(OH)2 nanosheet (NS) was synthesized with the low temperature hydrothermal process and its microstructure was investigated using a high resolution scanning electron microscope and transmission electron microscope. Zinc nitrate hexahydrate was hydrolyzed by hexamethylenetetramine with the same mole ratio and various temperatures, growth times and total concentrations. The optimum hydrothermal processing condition for the best crystallinity of hexagonal β-Ni(OH)2 NS was determined to be with 3.5 mM at 95˚C for 2 h. The prepared Ni(OH)2 NSs were two dimensionally arrayed on a substrate using an air-water interface tapping method, and the quality of the array was evaluated using an X-ray diffractometer. Because of the similarity of the lattice parameter of the (0001) plane between ZnO (wurzite a = 0.325 nm, c = 0.521 nm) and hexagonal β-Ni(OH)2 (brucite a = 0.313 nm, c = 0.461 nm) on the synthesized hexagonal β-Ni(OH)2 NS, ZnO NRs were successfully grown without seeds. At 35 mM of divalent Zn ion, the entire hexagonal β-Ni(OH)2 NSs were covered with ZnO NRs, and this result implies the possibility that ZnO NR can be grown epitaxially on hexagonal β-Ni(OH)2 NS by a soluble process. After the thermal annealing process, β-Ni(OH)2 changed into NiO, which has the property of a p-type semiconductor, and then ZnO and NiO formed a p-n junction for a large area light emitting diode.

Transparent conductive films of single wall carbon nanotube (SWCNT) were prepared by spray coating method. The effect of acid treatment on the SWCNT films was investigated. The field emission scanning electron microscope (FESEM) shows that acid treatment can remove dispersing agent. The electrical and optical properties of acid-treated films were enhanced compared with those of as deposited SWCNT films. Nitric acid (HNO3), sulfuric acid (H2SO4), nitric acid:sulfuric acid (3:1) were used for post treatment. Although all solutions reduced sheet resistance of CNT films, nitric acid can improve electrical characteristics efficiently. During acid treatment, transmittance was increased continuously with time. But the sheet resistance was decreased for the first 20 minutes and then increased again. Post-treated SWCNT films were transparent (85%) in the visible range with sheet resistance of about 162Ω/sq. In this paper we discuss simple fabrication, which is suitable for different types of large-scale substrates and simple processes to improve properties of SWCNT films.

Various thicknesses of Al-doped ZnO (AZO) films were deposited on glass substrate using pulsed dcmagnetron sputtering with a cylindrical target designed for large-area high-speed deposition. The structural,electrical, and optical properties of the films of various thicknesses were characterized. All deposited AZO filmshave (0002) preferred orientation with the c-axis perpendicular to the substrate. Crystal quality and surfacemorphology of the films changed according to the film thickness. The samples with higher surface roughnessexhibited lower Hall mobility. Analysis of the measured data of the optical band gap and the carrierconcentration revealed that there were no changes for all the film thicknesses. The optical transmittances weremore than 85% regardless of film thickness within the visible wavelength region. The lowest resistivity,4.13×10-4Ω·cm-1, was found in 750nm films with an electron mobility (µ) of 10.6cm2V-1s-1 and a carrierconcentration (n) of 1.42×1021cm-3.

Inorganic-organic composite thin-film-transistors (TFTs) of ZnO nanowire/Poly(3-hexylthiophene)(P3HT) were investigated by changing the nanowire densities inside the composites. Crystalline ZnO nanowireswere synthesized via an aqueous solution method at a low temperature, and the nanowire densities inside thecomposites were controlled by changing the ultrasonifiaction time. The channel layers were prepared withcomposites by spin-coating at 2000rpm, which was followed by annealing in a vacuum at 100oC for 10 hours.Au/inorganic-organic composite layer/SiO2 structures were fabricated and the mobility, Ion/Ioff ratio, andthreshold voltage were then measured to analyze the electrical characteristics of the channel layer. Comparedwith a P3HT TFT, the electrical properties of TFT were found to be improved after increasing the nanowiredensity inside the composites. The mobility of the P3HT TFT was approximately 10-4cm2/V·s. However, themobility of the ZnO nanowire/P3HT composite TFT was increased by two orders compared to that of theP3HT TFT. In terms of the Ion/Ioff ratio, the composite device showed a two-fold increase compared to thatof the P3HT TFT.

Indium Gallium Zinc Oxide (IGZO) thin films were deposited onto 300 nm-thick oxidized Si substrates and glass substrates by direct current (DC) magnetron sputtering of IGZO targets at room temperature. FESEM and XRD analyses indicate that non-annealed and annealed IGZO thin films exhibit an amorphous structure. To investigate the effect of an annealing treatment, the films were thermally treated at 300˚C for 1hr in air. The IGZO TFTs structure was a bottom-gate type in which electrodes were deposited by the DC magnetron sputtering of Ti and Au targets at room temperature. The non-annealed and annealed IGZO TFTs exhibit an Ion/Ioff ratio of more than 105. The saturation mobility and threshold voltage of nonannealed IGZO TFTs was 4.92×10-1cm2/V·s and 1.46V, respectively, whereas these values for the annealed TFTs were 1.49×10-1cm2/V· and 15.43V, respectively. It is believed that an increase in the surface roughness after an annealing treatment degrades the quality of the device. The transmittances of the IGZO thin films were approximately 80%. These results demonstrate that IGZO thin films are suitable for use as transparent thin film transistors (TTFTs).