• Korean Journal of Materials Research
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• v.18 no.12
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• pp.655-659
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• 2008

The NO gas sensing properties of ZnO-carbon nanotube (ZnO-CNT) composites fabricated by the coaxial coating of single-walled CNTs with ZnO were investigated using pulsed laser deposition. Upon examination, the morphology and crystallinity of the ZnO-CNT composites showed that CNTs were uniformly coated with polycrystalline ZnO with a grain size as small as 5-10 nm. Gas sensing measurements clearly indicated a remarkable enhancement of the sensitivity of ZnO-CNT composites for NO gas compared to that of ZnO films while maintaining the strong sensing stability of the composites, properties that CNT-based sensing materials do not have. The enhanced gas sensing properties of the ZnO-CNT composites are attributed to an increase in the surface adsorption area of the ZnO layer via the coating by CNTs of a high surface-to-volume ratio structure. These results suggest that the ZnO-CNT composite is a promising template for novel solid-state semiconducting gas sensors.

• Korean Journal of Materials Research
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• v.19 no.11
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• pp.631-636
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• 2009

We investigated the NO gas sensing characteristics of ZnO-carbon nanotube (ZnO-CNT) layered composites fabricated by coaxial coating of single-walled CNTs with a thin layer of 1 wt% Al-doped ZnO using rf magnetron sputtering deposition. Morphological studies clearly revealed that the ZnO appeared to form beadshaped crystalline nanoparticles with an average diameter as small as 30 nm, attaching to the surface of the nanotubes. It was found that the NO gas sensing properties of the ZnO-CNT layered composites were dramatically improved over Al-doped ZnO thin films. It is reasoned from these observations that an increase in the surface-to-volume ratio associated with the numerous ZnO “nanobeads” on the surface of the CNTs results in the enhancement of the NO gas sensing properties. The ZnO-CNT layered composite sensors exhibited a maximum sensitivity of 13.7 to 2 ppm NO gas at a temperature of 200${^{\circ}C}$ and a low NO gas detection limit of 0.2 ppm in dry air.

• Korean Journal of Materials Research
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• v.19 no.11
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• pp.607-612
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• 2009

We investigated the effects of Co doping on the NO gas sensing characteristics of ZnO-carbon nanotube (ZnO-CNT) layered composites fabricated by coaxial coating of single-walled CNTs with ZnO using pulsed laser deposition. Structural examinations clearly confirmed a distinct nanostructure of the CNTs coated with ZnO nanoparticles of an average diameter as small as 10 nm and showed little influence of doping 1 at.% Co into ZnO on the morphology of the ZnO-CNT composites. It was found from the gas sensing measurements that 1 at.% Co doping into ZnO gave rise to a significant improvement in the response of the ZnO-CNT composite sensor to NO gas exposure. In particular, the Co-doped ZnO-CNT composite sensor shows a highly sensitive and fast response to NO gas at relatively low temperatures and even at low NO concentrations. The observed significant improvement of the NO gas sensing properties is attributed to an increase in the specific surface area and the role as a catalyst of the doped Co elements. These results suggest that Co-doped ZnOCNT composites are suitable for use as practical high-performance NO gas sensors.

• Journal of the Semiconductor & Display Technology
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• v.8 no.4
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• pp.65-69
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• 2009

Carbon nanotubes (CNTs) were coated with undoped zinc oxide (ZnO) or 5 wt% gallium-incorporated ZnO (GZO) using various deposition conditions. The CNTs were directly grown on conical-type tungsten substrates at $700^{\circ}C$ using inductively coupled plasma-chemical vapor deposition. The pulsed laser deposition technique was used to deposit the ZnO and GZO thin films with very low stress. Field-emission scanning electron microscopy and high-resolution transmission electron microscopy were used to monitor the variations in the morphology and microstructure of CNTs prior to and after ZnO or GZO coating. The formation of ZnO and GZO films on CNTs was confirmed using energy-dispersive x-ray spectroscopy. In comparison to the as-grown (uncoated) CNT emitter, the CNT emitter that was coated with a thin (10 nm) GZO film showed remarkably improved field emission characteristics, such as the emission current of $325\;{\mu}A$ at 1 kV and the threshold field of $1.96\;V/{\mu}m$ at $0.1\;{\mu}A$, and it also exhibited the highly stable operation of emission current up to 40 h.

• Korean Journal of Materials Research
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• v.22 no.5
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• pp.237-242
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• 2012

We report on the NO gas sensing properties of Al-doped zinc oxide-carbon nanotube (ZnO-CNT) wire-like layered composites fabricated by coaxially coating Al-doped ZnO thin films on randomly oriented single-walled carbon nanotubes. We were able to wrap thin ZnO layers around the CNTs using the pulsed laser deposition method, forming wire-like nanostructures of ZnO-CNT. Microstructural observations revealed an ultrathin wire-like structure with a diameter of several tens of nm. Gas sensors based on ZnO-CNT wire-like layered composites were found to exhibit a novel sensing capability that originated from the genuine characteristics of the composites. Specifically, it was observed by measured gas sensing characteristics that the gas sensors based on ZnO-CNT layered composites showed a very high sensitivity of above 1,500% for NO gas in dry air at an optimal operating temperature of $200^{\circ}C$; the sensors also showed a low NO gas detection limit at a sub-ppm level in dry air. The enhanced gas sensing properties of the ZnO-CNT wire-like layered composites are ascribed to a catalytic effect of Al elements on the surface reaction and an increase in the effective surface reaction area of the active ZnO layer due to the coating of CNT templates with a higher surface-to-volume ratio structure. These results suggest that ZnO-CNT composites made of ultrathin Al-doped ZnO layers uniformly coated around carbon nanotubes can be promising materials for use in practical high-performance NO gas sensors.

• Korean Journal of Food Science and Technology
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• v.44 no.4
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• pp.472-477
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• 2012

As people are being exposed to many types of fast food, rancid oil is a factor affecting public health. Monitoring of rancidity in frying oils needs to be done adequately. The chemical methods that are currently used require long periods of time and expertise. The development of a device that quickly and easily measures rancidity would be helpful to manage rancidity in frying oils adequately. Due to the fact that carbon nanotube (CNT) is sensitive to acid value, we used CNT as a sensing material for detecting oil rancidity. Polyethylenimine (PEI) was coated on CNT for stable measurements. Experiments were conducted at $100^{\circ}C$ after samples were cooled from $180^{\circ}C$. The results showed a strong correlation between acid values and resistances using CNT sensors. As the acid value of oils increased, the resistance of CNT sensors increased. Development of sensors using CNT may make it possible to determine the rancidity of frying oils in real-time and on site.

• Korean Journal of Materials Research
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• v.19 no.1
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• pp.50-53
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• 2009

Multi-walled carbon nanotubes (MWNTs) were synthesized on different substrates (bare Si and $SiO_2$/Si substrate) to investigate dye-sensitized solar cell (DSSC) applications as counter electrode materials. The synthesis of MWNTs samples used identical conditions of a Fe catalyst created by thermal chemical vapor deposition at $900^{\circ}C$. It was found that the diameter of the MWNTs on the Si substrate sample is approximately $5{\sim}10nm$ larger than that of a $SiO_2$/Si substrate sample. Moreover, MWNTs on a Si substrate sample were well-crystallized in terms of their Raman spectrum. In addition, the MWNTs on Si substrate sample show an enhanced redox reaction, as observed through a smaller interface resistance and faster reaction rates in the EIS spectrum. The results show that DSSCs with a MWNT counter electrode on a bare Si substrate sample demonstrate energy conversion efficiency in excess of 1.4 %.

• Korean Journal of Materials Research
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• v.20 no.11
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• pp.623-627
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• 2010

Semiconducting metal oxides have been frequently used as gas sensing materials. While zinc oxide is a popular material for such applications, structures such as nanowires, nanorods and nanotubes, due to their large surface area, are natural candidates for use as gas sensors of higher sensitivity. The compound ZnO has been studied, due to its chemical and thermal stability, for use as an n-type semiconducting gas sensor. ZnO has a large exciton binding energy and a large bandgap energy at room temperature. Also, ZnO is sensitive to toxic and combustible gases. The NO gas properties of zinc oxide-single wall carbon nanotube (ZnO-SWCNT) composites were investigated. Fabrication includes the deposition of porous SWCNTs on thermally oxidized $SiO_2$ substrates followed by sputter deposition of Zn and thermal oxidation at $400^{\circ}C$ in oxygen. The Zn films were controlled to 50 nm thicknesses. The effects of microstructure and gas sensing properties were studied for process optimization through comparison of ZnO-SWCNT composites with ZnO film. The basic sensor response behavior to 10 ppm NO gas were checked at different operation temperatures in the range of $150-300^{\circ}C$. The highest sensor responses were observed at $300^{\circ}C$ in ZnO film and $250^{\circ}C$ in ZnO-SWCNT composites. The ZnO-SWCNT composite sensor showed a sensor response (~1300%) five times higher than that of pure ZnO thin film sensors at an operation temperature of $250^{\circ}C$.

• The Journal of Korea Institute of Information, Electronics, and Communication Technology
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• v.8 no.1
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• pp.37-44
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• 2015

In this paper, we have investigated a selective assembly method of single-walled carbon nanotubes (SWCNTs) on a silicon substrate using only photolithographic process and then proposed a fabrication method of field effect transistors (FETs) using SWCNT-based patterns. The aminopropylethoxysilane (APTES) patterns, which are formed for positively charged surface molecular patterns, are utilized to assemble and align millions of SWCNTs and we can more effectively assemble on a silicon (Si) surface using this method than assembly processes using only the 1-octadecyltrichlorosilane (OTS). We investigated a selective assembly method of SWCNTs on a Si surface using surface-programmed APTES and OTS patterns and then a fabrication method of FETs. photoresist(PR) patterns were made using photolithographic process on the silicon dioxide (SiO2) grown Si substrate and the substrate was placed in the OTS solution (1:500 v/v in anhydrous hexane) to cover the bare SiO2 regions. After removing the PR, the substrate was placed in APTES solution to backfill the remaining SiO2 area. This surface-programmed substrate was placed into a SWCNT solution dispersed in dichlorobenzene. SWCNTs were attracted toward the positively charged molecular regions, and aligned along the APTES patterns. On the contrary, SWCNT were not assembled on the OTS patterns. In this process, positively charged surface molecular patterns are utilized to direct the assembly of negatively charged SWCNT on SiO2. As a result, the selectively assembled SWCNT channels can be obtained between two electrodes(source and drain electrodes). Finally, we can successfully fabricate SWCNT-based multi-channel FETs by using our self-assembled monolayer method.

• Korean Chemical Engineering Research
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• v.51 no.4
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• pp.443-454
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• 2013

Various metal ions (transition and base metals) incorporated MCM-41 catalysts can be synthesized using colloidal and soluble silica with non-sodium involved process. Transition metal ion-typically $V^{5+}$, $Co^{2+}$, and $Ni^{2+}$-incorporated MCM-41 catalysts were synthesized by isomorphous substitution of Si ions in the framework. Each incorporated metal ion created a single species in the silica framework, single-site solid catalyst, showing a substantial stability in reduction and catalytic activity. Radius of pore curvature effect was investigated with Co-MCM-41 by temperature programmed reduction (TPR). The size of metallic Co clusters, sub-nanometer, could be controlled by a proper reduction treatment of Co-MCM-41 having different pore size and the initial pH adjustment of the Co-MCM-41 synthesis solution. These small metallic clusters showed a high stability under a harsh reaction condition without serious migration, resulting from a direct anchoring of small metallic clusters to the partially or unreduced metal ions on the surface. After a complete reduction, partial occlusion of the metallic cluster surface by amorphous silica stabilized the particles against aggregations. As a probe reaction of particle size sensitivity, carbon single wall nanotubes (SWNT) were synthesized using Co-MCM-41. A metallic cluster stability test was performed by CO methanation using Co- and Ni-MCM-41. Methanol and methane partial oxidations were carried out with V-MCM-41, and the radius of pore curvature effect on the catalytic activity was investigated.