Takahiko Muneyoshi

Structure of carbon nanotubes grown by microwave-plasma-enhanced chemical vapor deposition

Carbon nanotubes grown on a Ni substrate and an Fe–Ni–Cr alloy substrate by plasma-enhanced chemical vapor deposition were investigated by transmission electron microscope (TEM) and energy dispersive x-ray (EDX) analysis. TEM showed that the nanotubes on both substrates have a piled-cone structure with metal particles on top which determine the diameter of the nanotubes. Their diameter ranges from 60 to 80 nm. Moreover, EDX showed that the metal particles are composed of Ni when the nanotubes are grown on Ni substrate and of Fe and Ni in the case of the Fe–Ni–Cr alloy substrate.

In Situ Transmission Electron Microscope Observation of Carbon Nanotubes in Electric Fields

Transmission electron microscope is used to examine the movements of carbon nanotubes in electric fields. Carbon nanotubes lying along the surface of the cathode electrode start to move into alignment with the electric field vector when the field strength reaches 0.5 V/µm and become increasingly well-aligned with the vector as field strength increases. The carbon nanotubes return to their original positions when the electric field strength returns to zero. We also examine the abrupt breakdown of carbon nanotubes when the electric field is maintained at 5.5 V/µm. The corresponding breakdown emission current density is estimated as 3.4×107 A/cm2. The distance between the nearest nanotubes standing to align with the electric field vector is approximately 2 µm. This fact means that emission site density could be increased up to 3×107 points/cm2 (which corresponds to one tube for each 2 µm square).

Growth mechanism of carbon nanotubes grown by microwave plasma-assisted chemical vapor deposition

To investigate the most suitable deposition conditions and growth mechanism, we grew carbon nanotubes (CNTs) by microwave plasma-assisted chemical vapor deposition under various conditions. The experimental parameters we varied were (a) the mixture ratio of methane in hydrogen, (b) the total gas pressure, and (c) the bias electric current. We found that the bias electric current was the most influential parameter in determining the shape of CNTs. We believe that the growth process of CNTs can be explained by using the solid solubility curves of metal-carbon phase diagrams. Selective growth and low-temperature growth of CNTs can also be understood from these phase diagrams.

Growth mechanism of carbon nanotubes grown by microwave-plasma-enhanced chemical vapor deposition

To understand the growth mechanism of carbon nanotubes, we have investigated the initial stage of carbon nanotube growth by microwave-plasma-enhanced CVD on a metal substrate. Metal droplets with diameters of 20–180 nm appeared on the substrate surface after plasma cleaning. These metal droplets operate as a catalyst for the growth of carbon nanotubes. The grown nanotubes had a piled-cone structure with metal particles at the top. The diameters of the carbon nanotubes ranged from 60 to 80 nm and the metal particles at the top were the same sizes.