Yoshikazu Homma

Atomic-Scale In-situ Observation of Carbon Nanotube Growth from Solid State Iron Carbide Nanoparticles

We have first observed the nucleation and growth process of carbon nanotubes (CNTs) from iron carbide (Fe 3C) nanoparticles in chemical vapor deposition with C 2H 2 by in situ environmental transmission electron microscopy. Graphitic networks are formed on the fluctuating iron carbide nanoparticles, and subsequently CNTs are expelled from them. Our atomic scale observations suggest that carbon atoms diffuse through the bulk of iron carbide nanoparticles during the growth of CNTs.

Role of Transition Metal Catalysts in Single-Walled Carbon Nanotube Growth in Chemical Vapor Deposition

We characterize the iron and cobalt catalysts for carbon nanotube growth in chemical vapor deposition (CVD) by using electron microscopy. Nanoparticles of iron and cobalt exhibit a melting point drop in the methane ambient. Nanoparticles after nanotube growth are identified as Fe3C and Co3C for iron and cobalt, respectively. Those results indicate that a eutectic compound of metal and carbon is formed in the methane ambient, resulting in the phase separation into graphite (nanotubes) and metal carbide as the carbon uptake in the catalyst melt increases. This supports the vapor−liquid−solid mechanism for nanotube growth by CVD. Iron or cobalt silicide formation causes the poisoning of the catalysts. However, the coexistence of oxygen due to native oxide on the silicon surface or the metal surface causes formation of a SiO2 base, which can prevent silicidation of iron particles.

Growth of suspended carbon nanotube networks on 100-nm-scale silicon pillars

We investigated carbon nanotube growth by means of methane chemical vapor deposition on ultrafine silicon patterns prepared by synchrotron-radiation lithography. Grown nanotubes formed suspended bridges between pillars when pillar spacing was comparable to pillar height. Network-like interconnections were obtained on pillar arrays. Nearest-neighbor bridging accounted for more than 80% of all the bridging nanotubes. The self-directed growth between neighboring pillars may be explained by the swing of the nanotube cantilever which contacts a catalyst particle in liquid phase as the nanotube grows. These results confirm the possibility of self-assembled wiring of nanostructures.

Carbon Nanotube Growth from Diamond

We demonstrate that nanosize diamond particles in a stable solid phase can act as nuclei for carbon nanotube (CNT) growth by chemical vapor deposition. This growth process is explained by a new growth mechanism, where the surface diffusion of carbon adatoms on the solid surface of nanoparticles plays an important role, contrary to the bulk diffusion in conventional metal catalysts. The use of diamond nanoparticles overcomes the issues of deactivation due to fusion with each other or reaction with substrate materials in the CNT growth using conventional metal catalysts and promotes high-density CNT growth from highly dense nanoparticles on any substrate.

Single-Walled Carbon Nanotube Growth on Silicon Substrates Using Nanoparticle Catalysts

We investigated the growth of carbon nanotubes (CNT) directly on silicon substrates by nanoparticle-catalyst-assisted chemical vapor deposition. Single-walled CNTs were almost selectively obtained when methane was used in combination with Fe2O3 nanoparticles at the growth temperature of around 950°C. In the growth of single-walled CNTs, this temperature is essential in order to avoid silicidation of the catalyst. The growth direction was parallel to the substrate surface, which is useful for device applications. For Fe-metal nanoparticles, grown nanotubes always contained thick multiwalled CNTs. The selective single-walled CNT growth for Fe2O3 nanoparticles is attributed to the fact that the particles remain small even after the chemical vapor deposition (CVD) process.

Mechanism of bright selective imaging of single-walled carbon nanotubes on insulators by scanning electron microscopy

Individual single-walled carbon nanotubes (SWNTs) produce highly bright images of the insulator surface around them when observed by scanning electron microscopy at low primary-electron voltage. We found that the insulator surface near SWNTs emits more secondary electrons due to electrons supplied through SWNTs connecting to the outside area of the primary-electron beam scanning. SWNTs are thus highlighted as bright lines corresponding to the electron-beam-induced current range around them. This technique provides a useful and effective way to investigate lateral growth morphology of SWNTs on the substrate.

Mechanism of gold-catalyzed carbon material growth.

We demonstrate that nanosized Au particles have carbon solubility. Au-catalyzed carbon material growth by chemical vapor deposition undergoes a structural change, either a carbon nanowire or a single-walled carbon nanotube, depending on the catalyst particle size. This carbon material growth from Au is derived by the formation of Au-C eutectic nanosized alloy.

Wetting of Si surfaces by Au–Si liquid alloys

The behavior of liquid Au–Si alloys on Si surfaces covered by a monolayer of gold has been investigated by ultrahigh-vacuum scanning electron microscopy. On the (111) surface, the alloy displays a constant contact angle with the surface from the eutectic temperature up to a temperature of 650 °C and thereafter the contact angle increases linearly with temperature. As observed in previous work, the shape of the liquid droplets changes from circular at lower temperature to hexagonal at higher temperature. In contrast, on the (100) surface, the contact angle increases linearly from the eutectic temperature to high temperature. The behavior of the shape of the droplets is, however, reversed: it is polygonal (octagonal) at lower temperature and becomes round at higher temperature. This behavior is explained in terms of the relative surface energy of the two surfaces and changing line tension of the liquid–solid–vapor phase line. In addition, the behavior of Au–Si droplets on vicinal and patterned surfaces of S...

Growth of Single-Walled Carbon Nanotubes from Ceramic Particles by Alcohol Chemical Vapor Deposition

Al2O3 ceramic nano-particles, which were regarded as an inactive catalyst in the growth of carbon nanotubes in the past, have been prepared as the catalyst for single-walled carbon nanotube (SWCNT) growth using an alcohol chemical vapor deposition method. Dense SWCNTs have been successfully synthesized, indicating that Al2O3 serves as an efficient catalyst. Moreover, it was found that many SWCNTs were also grown from irregular large Al2O3 particles ranging from several tens of nanometers to hundreds of nanometers, which has never been observed in the case of metallic catalyst particles. These results give more insights into the role of catalyst in SWCNT growth.

DC-Resistive-Heating-Induced Step Bunching on Vicinal Si (111)

Step bunching on a 1°-misoriented Si (111) surface induced by DC resistive heating is observed by ultrahigh-vacuum scanning electron microscopy. Step band regions of the DC-heating-induced bunching surface break up into finer step bands (subbands) and (111) facets below the (7×7)↔(1×1) phase transition temperature (Tc=830°C). The temperature dependence of bunching-inducing current direction on the vicinal surface is the same as that of nearly flat (111) surfaces previously reported, except that bunching is induced for the opposite current direction to nearly flat (111) surfaces below Tc.