Toshiya Murakami

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.

Catalytic mechanism of a Fe–Co bimetallic system for efficient growth of single-walled carbon nanotubes on Si∕SiO2 substrates

Single-walled carbon nanotubes (SWCNTs)were grown on Si∕SiO2 substrates by catalytic chemical vapor deposition using ethanol as a carbon source. We found that a bimetallic catalyst, Fe–Co, worked efficiently in growing SWCNTs in contrast to Fe or Co catalyst only. The underlying mechanism was carefully studied by observing the catalyst-substrate interactions by transmission electron microscopy. It is shown that preferential diffusion of Fe into a SiO2 layer occurs in the bimetallic case, which suppresses aggregation of nanometer- sized Co particles that play key roles in growing SWCNTs.

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.

Growth of Single-Walled Carbon Nanotubes Rooted from Fe/Al Nanoparticle Array

We explored the growth of single-walled carbon nanotubes (SWNTs) from nanoparticle array made of an Fe/Al multilayer catalyst by thermal chemical vapor deposition. Fe nanoparticles with a high number density and a narrow size distribution (1–5 nm) were efficiently formed by annealing the Fe/Al thin layer, resulting in the high-yield growth of SWNTs. Moreover, it was found that isolated SWNTs are rooted from patterned Fe/Al islands. The SWNTs bridged between electrodes exhibited semiconducting and metallic properties.

Isochronal annealing study of X-ray induced defects in single- and double-walled carbon nanotubes

X-ray induced defects in single-walled (SWCNTs) and double-walled carbon nanotubes (DWCNTs) were characterized by Raman scattering spectroscopy. Frenkel defects, interstitial-vacancy pairs, were revealed to form in both SWCNTs and DWCNTs after X-ray irradiation because these defects were entirely healed by thermal annealing. In order to clarify the structure of the X-ray induced defect in SWCNT and DWCNT, isochronal-annealing experiments were performed on the irradiated samples and the activation energy for defect healing was estimated. The intensity of D band (defect induced band) on Raman spectra was used as a measure of the density of X-ray induced defects. The experimental results were in good agreement with the simulated values using second order reaction model, which indicated that the defect healing was determined by the migration energy of interstitials on the carbon layer. We also found that the activation energy for defect healing of SWCNT and DWCNT were around 0.5 eV and 0.32 eV, respectively. The X-ray induced defects in SWCNTs were more stable than those in DWCNTs. Compared these estimated activation energies to previous theoretical reports, we concluded that bridge and/or dumbbell interstitials are formed in both SWCNT and DWCNT by X-ray irradiation.

Direct Growth of Single-Walled Carbon Nanotubes on W Tip Apex

We demonstrated the direct growth of single-walled carbon nanotubes (SWNTs) on a W tip by Fe/Al catalyst-assisted chemical vapor deposition (CVD). A high-purity network of SWNTs with diameters of 0.9–3.7 nm was grown on the W tip. Under optimum CVD conditions, the growth of isolated SWNTs protruding from the W tip apex was achieved. We found that the circumference of the tip region affected the morphology of the resultant SWNTs. The direct growth method provides a potential application of SWNTs to a novel tip for scanning tunnelling microscopy (STM).

Studies on the growth of pure double-walled carbon nanotube and its phonon spectra

Double-walled carbon nanotubes (DWCNTs) with a purity higher than 99% were synthesized by chemical vapor deposition, and their Raman spectra were observed at different excitation wavelengths λex. The spectra had a unique feature compared with single-walled carbon nanotubes (SWCNTs): the G-band shape was distinctly different from that of SWCNTs and showed a clear λex dependence. The pure DWCNT samples showed complex radial breathing modes (RBM) spectra. The mode peaks were unambiguously classified into those for the inner and outer tubes by applying a simple analytic model considering the interwall interaction. After isolation treatment of the pure bundled samples, we observed RBM signals of DWCNTs having an identical inner tube with different outer tubes. The peculiar behavior of the G-band shape was interpreted by resonance enhancement of the outer tube.

Simultaneous Observation of Single-Walled Carbon Nanotubes and Catalyst Particles on SiO2 Substrate by Transmission Electron Microscopy

Single-walled carbon nanotubes (SWNTs) were grown by catalytic chemical vapor deposition on SiO2 substrates. By thinning the substrate before growth, the morphologies of the SWNTs and their seed catalyst particles were simultaneously observed in the as-grown state by plan-view transmission electron microscopy (TEM). Analysis over a wide area of the substrate showed that the diameters of the SWNTs d and those of the catalyst particles D had different distributions. The catalytic activity was size-dependent: fine particles (D\lesssim4 nm) yielded the same diameter SWNTs (d~D), and intermediate sizes (~4 < D\lesssim6 nm) yielded finer tubes (d < D), while larger particles (D\gtrsim6 nm) had no catalytic activity. We also found that the small catalyst particles with D of less than 5 nm that appeared during the growth process by chemical reactions with the carbon source mainly contributed to the growth of SWNTs.

Diameter-dependent annealing kinetics of X-ray-induced defects in single-walled carbon nanotubes

Diameter-dependent annealing effects of X-ray-induced defects in single-walled carbon nanotubes (SWNTs) were studied by Raman scattering spectroscopy. We found that X- ray-induced defects were formed in thin SWNTs at higher density than that in thick SWNTs. The X-ray-induced defects, distant pairs of interstitials and vacancies (I-V), were eliminated by thermal annealing. The recovery temperature of the X-ray-induced defects in the thin SWNTs were higher than that of the thick ones, indicating the thermal stability of the defects in thinner SWNTs are higher. In order to simulate the annealing behaviours of the X-ray-induced defects in SWNTs with diameters of ~0.9 and ~1.4 nm, we suggested the competitive reactions of diffusion of interstitials, formation of close I-V pairs and their recombination. The simulated results showed that the reaction-rate constant of elimination of close I-V pairs is dependent on tube diameter, which is presumably derived from nano-structure effects of SWNTs.

Structural modification of single-walled carbon nanotube induced by X-ray irradiation and subsequent annealing studied by Raman scattering spectroscopy in radial breathing mode

The formation of X-ray-induced defects changes the spectral shape of the radial breathing mode (RBM) and defect-induced mode (D band) in the Raman spectra of single-walled carbon nanotubes (SWNTs). X-ray-induced defects have been found to be annealed by thermal treatment, indicating that they are Frenkel pairs (vacancy and interstitial pairs). We found that the spectral shape of RBM is not entirely recovered after post-irradiation annealing. The temperatures for the complete annealing of X-ray-induced defects were within the range of 200–600 °C depending on the tube geometry. From these results, we suggest that the stability of X-ray-induced defects depends on the tube geometry and that the combination of X-ray irradiation and post-irradiation annealing causes a chirality change in SWNTs.