Shin-ichi Honda

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.

Ultra-Low-Threshold Field Electron Emission from Pillar Array of Aligned Carbon Nanotube Bundles

We observed the field electron emission of the technologically useful current density of 10 mA/cm2 at an extremely low threshold electric field (Eth) of 1.0 V/µm, from an array of pillars of aligned carbon nanotube bundles, which were grown on a Si substrate by thermal chemical vapor deposition. Adjusting the distance between the neighboring pillars (R) and the pillar height (H) to the optimal condition (R/H = 2) can effectually enhance the field concentration, resulting in a highly efficient electron emission. The obtained Eth is 1/2–1/3 times lower than the best values that have been reported to date.

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.

Improvement in Field Emission Uniformity from Screen-Printed Double-Walled Carbon Nanotube Paste by Grinding

We have improved field emission uniformity in a screen-printed paste composed of double-walled carbon nanotubes (DWNTs), which were ground by planetary ball milling (PBM). Results showed that after grinding pristine DWNTs, carbonaceous particles that affected the roughness of the DWNT paste were atomized and long DWNTs were cut. Ground DWNTs were dispersed uniformly on a stainless steel substrate, thereby improving field-emission uniformity. This simple grinding technique is expected to be applicable to low-cost and high-yield processing that produces a DWNT paste for field emitters.

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.

Low-Temperature Synthesis of Aligned Carbon Nanofibers on Glass Substrates by Inductively Coupled Plasma Chemical Vapor Deposition

Vertically aligned carbon nanofiber (CNF) films were successfully grown on glass substrates at 450 °C with metal buffer layers by inductively coupled plasma chemical vapor deposition (ICP-CVD). The diameter and number density of the aligned CNFs can be controlled by changing the type and thickness of the metal buffer layers deposited on the glass substrates. The metal buffer layers play an important role in reducing the thermal expansion coefficient difference between the catalyst metal film and the glass substrate, resulting in the enhancement of the formation of catalyst nanoparticles so as to grow the aligned CNFs with high number density.

Structure analysis of ZrB2(0001) surface prepared by ex situ HF treatment

The surface structure of ZrB2(0001) was investigated using coaxial impact-collision ion scattering spectroscopy (CAICISS). The ZrB2(0001) 1×1 surface was prepared by HF solution treatment and annealing in vacuum. It was directly evidenced that the ZrB2(0001) surface is terminated with a Zr layer. Moreover, we found that the interlayer spacing between the first Zr layer and the second B layer is expanded by 20% with regard to c-axis lattice constant. Since small amount of O atoms were detected after the cleaning, the incorporation of the residual O atoms into the subsurface region of the ZrB2(0001) surface is likely to be the origin of the large expansion.

Scanning Tunneling Spectroscopy Study of the ZnO(0001)?Zn Surface

The electronic structure of the ZnO(0001)–Zn surface was studied using scanning tunneling spectroscopy (STS) and first principles molecular dynamics. The STS spectrum indicated that the clean surface is n-type semiconducting with a band gap of about 3.3 eV. The local density of states (LDOS) calculated using ZnO slab model was in qualitative agreement with the STS spectrum, and revealed that occupied and unoccupied peaks originate from O and Zn atoms at the top bilayer of the surface, respectively. From the contour plots of LDOS, it was found that Zn atoms dominantly contribute to both occupied and unoccupied LDOS distributions and their broadening on the surface, which prevents atom-resolved scanning tunneling microscopy imaging of ZnO(0001) surface.