Winadda Wongwiriyapan

Single-Walled Carbon Nanotube Thin-Film Sensor for Ultrasensitive Gas Detection

We demonstrated a gas sensor fabricated by growing a single-walled carbon nanotube (SWNT) thin film directly on a conventional sensor substrate. NO2 and Cl2 were detected down to the ppb level under room-temperature operation with a fast response. Using an electrical breakdown technique, gas response sensitivity was improved by an order of magnitude. The relationship between gas concentration and sensor response was derived based on the Langmuir adsorption isotherm, predicting a detection limit of 8 ppb for NO2. The SWNT thin-film gas sensor exhibits merits over other types of sensors by virtue of its simplicity in fabrication and feasible application.

Carbon-supported Pt–Ru nanoparticles prepared in glyoxylate-reduction system promoting precursor–support interaction

A high dispersion of carbon-supported Pt–Ru alloy nanoparticles have been prepared in an alkaline aqueous solution by using glyoxylate as a reducing agent. The glyoxylate monoanion is converted to oxalate dianion with the reduction of metal precursor ions. The surface-potential analysis of unsupported Pt–Ru black in the presence of all the anion species coexistent in the preparation system suggests a possibility of oxalate dianion as the most effective particle stabilizer. In the glyoxylate-reduction system, a precursor–support interaction is significantly promoted to affect the formation and stabilization of nanoparticles. The glyoxylate reduction at 20-wt% metal loading has provided a higher Pt(0)/Ru concentration ratio in the near-surface region than that of 60-wt% catalyst. The structural and morphological features and advantageous surface composition of the 20-wt% catalyst contribute to the high mass activity for methanol oxidation in the anode overpotential range of typical direct methanol fuel cells.

Single Crystalline Film of Hexagonal Boron Nitride Atomic Monolayer by Controlling Nucleation Seeds and Domains.

A monolayer hexagonal boron nitride (h-BN) film with controllable domain morphology and domain size (varying from less than 1 μm to more than 100 μm) with uniform crystalline orientation was successfully synthesized by chemical vapor deposition (CVD). The key for this extremely large single crystalline domain size of a h-BN monolayer is a decrease in the density of nucleation seeds by increasing the hydrogen gas flow during the h-BN growth. Moreover, the well-defined shape of h-BN flakes can be selectively grown by controlling Cu-annealing time under argon atmosphere prior to h-BN growth, which provides the h-BN shape varies in triangular, trapezoidal, hexagonal and complex shapes. The uniform crystalline orientation of h-BN from different nucleation seeds can be easily confirmed by polarized optical microscopy (POM) with a liquid crystal coating. Furthermore, seamlessly merged h-BN flakes without structural domain boundaries were evidence by a selective hydrogen etching after a full coverage of a h-BN film was achieved. This seamless large-area and atomic monolayer of single crystalline h-BN film can offer as an ideal and practical template of graphene-based devices or alternative two-dimensional materials for industrial applications with scalability.

Electronic Transport in Multiwalled Carbon Nanotubes Contacted with Patterned Electrodes

The electrical conductance of 0:8 � 5-mm-long multiwalled carbon nanotubes (MWCNT) was measured at room temperature in a multiprobe scanning tunneling microscope (STM)-scanning electron microscope (SEM) system and a conventional prober system, by bringing the MWCNTs into contact with patterned metal electrodes. The contact resistance between the CNTs and metal electrodes was sufficiently small. The conductance was proportional to A=L (and also to B=L, within our experimental error), where A, B, and L are the cross section, circumference, and length of CNTs. This indicates the occurrence of diffusive transport. A nonlinear current-voltage characteristic was obtained; the conductance increased steeply with current. A multiprobe STM-SEM system was very useful for measuring individual CNTs. [DOI: 10.1143/JJAP.43.L1081]

Highly Sensitive Detection of Carbon Monoxide at Room Temperature Using Platinum-Decorated Single-Walled Carbon Nanotubes

We demonstrated highly sensitive detection of carbon monoxide (CO) down to 1 ppm at room temperature using platinum-decorated single-walled carbon nanotubes (Pt-SWNTs). The obtained sensitivity to CO was 3–4 orders higher than the values reported for functionalized SWNTs, and was achieved by the controlled deposition of Pt nanoparticles on SWNTs. For 1–10 ppm of CO, the sensor response linearly increased with CO concentration, affording the quantitative detection of CO in a low-concentration range. Furthermore, Pt-SWNTs exhibited detection selectivity to CO against H2. The sensing mechanism was attributed to electron donation to the SWNTs as a result of CO oxidation on the Pt catalyst surface.

Ultrasensitive Ozone Detection Using Single-Walled Carbon Nanotube Networks

We demonstrated ultrasensitive detection of ozone using single-walled carbon nanotube (SWNT) networks directly grown on a conventional sensor substrate. Ozone was detected down to the ppb level (~6 ppb) at room-temperature operation with a fast response. Using an electrical breakdown technique, gas sensitivity was improved. This result clearly indicates that owing to its high sensitivity, simplicity in fabrication and compact size, an SWNT sensor provides a feasible route to ultrasensitive ozone detection surpassing existing methods.

Influence of the growth morphology of single-walled carbon nanotubes on gas sensing performance

We investigated the impact of the growth morphology of single-walled carbon nanotubes (SWNTs) on gas sensing performance. An SWNT film was directly synthesized on alumina substrate by thermal chemical vapour deposition. Different morphologies of the SWNTs in terms of density, diameter distribution and orientation were obtained by varying the growth temperature. Vertically aligned SWNTs with a high density were grown at 750 °C, while horizontally lying SWNT networks with a low density were grown in the temperature range 800–950 °C. The sensor response of the resultant SWNTs to NO2 was characterized at room temperature. It was found that the density of SWNTs strongly dominates sensor performance; the SWNT networks with the lowest density exhibited the highest sensor sensitivity. This was evidenced by characterization of density-controlled SWNTs synthesized using different thicknesses of an Fe/Al multilayer catalyst. The high sensor sensitivity for low-density SWNT networks is likely to be attributed to suppression of the formation of SWNT bundles and reduction of narrow-band-gap conduction paths, resulting in the enhancement of the adsorption probability and chemical gating efficiency of gas molecules on SWNTs.

High-yield synthesis of conductive carbon nanotube tips for multiprobe scanning tunneling microscope

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8 kOmega/microm. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7 kOmega. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements.

Direct Growth of Single-Walled Carbon Nanotube Networks on Alumina Substrate: A Novel Route to Ultrasensitive Gas Sensor Fabrication

We explored structural and electrical properties of single-walled carbon nanotube (SWNT) networks directly grown on alumina substrates. The netlike SWNTs were uniformly grown on the substrate at a high quality. From the Raman spectroscopy analysis it was found that the as-grown SWNT networks were a mixture of metallic and semiconducting SWNTs, while the SWNT networks after their electrical breakdown exhibited a predominance of the semiconducting property. The direct growth method and subsequent electrical breakdown facilitated high-throughput production of practical ultrasensitive sensors for pollutant gases with a high sensitivity, which was demonstrated by NO2 detection.

Selective Growth of Straight Carbon Nanotubes by Low-Pressure Thermal Chemical Vapor Deposition

Carbon nanotubes (CNTs) were grown by low-pressure thermal chemical vapor deposition using pure ethylene. It was found that straight CNTs, which were composed of bundled single- or double-wall CNTs and multiwall CNTs, were preferentially bridged between Fe nanoparticles under a low pressure of 100 Pa. Moreover, utilizing this method, we attempted to grow CNT bridges between patterned Ta electrodes. By nitriding the surface of the Ta electrodes, Fe nanoparticles with a moderate size were effectively formed, resulting in bridging CNTs between the electrodes.