Tadashi Fujieda

Direct Observation of Field Emission Sites in a Single Multiwalled Carbon Nanotube by Lorenz Microscopy

Emission sites were observed as bright spots near the tip end of a multiwalled carbon nanotube (MWNT) by means of Lorenz microscopy. The bright spots appeared above electric fields as electrons were emitted. A marked fluctuation was observed in the emission current above 20–30 µA, which was closely related to structural changes at the tip of the MWNT. The layers of the MWNT were peeled off during field emission and they functioned as the second emission sites for the concentration of electric field.

In situ observation of field emissions from an individual carbon nanotube by Lorenz microscopy

In situ observation of field emissions from an individual carbon nanotube (CNT) was performed by Lorenz microscopy. A bright spot appeared by Lorenz microscopy at the end of the CNT tip during field emission. The bright spot is assumed to be related to the emission site on the CNT. A drastic fluctuation was observed in the emission current above a few tens of microamperes, which was closely related to structural changes at the tip of the CNT. The layers of the CNT were peeled off and they worked as a second emission site by concentration of the electric field.

Energy Spread of Field Emission Electrons from Single Pentagons in Individual Multi-Walled Carbon Nanotubes

We investigated the dependence of tip radius on the field emission energy spread of electrons emitted from clean single pentagons in individual multi-walled carbon nanotubes (MWNTs) in a wide range of total emission currents (10–2000 nA). We found that the full width at half maximum of the field emission energy distribution decreases in inverse proportion to the involution of the radius of curvature at a constant total emission current. This is because as the radius of curvature increases, the space between adjoining pentagons becomes wider, and therefore the stochastic Coulomb interactions between electrons emitted from adjoining pentagons become weaker. The full widths at half maximum of the field emission energy distributions of MWNTs with tip radii of 1.8–45.0 nm were 0.38–0.60 eV at a total emission current of 2000 nA.

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).

Field Emission Stability of Individual Multi-Walled Carbon Nanotubes

We investigated the emission stability of individual multi-walled carbon nanotubes (MWNTs) and clarified the mechanism of emission current instability. An initial decrease in the emission current, which is generally seen in the case of metal emitters, was hardly observed. Furthermore, the current fluctuation was much lower than that for a metal emitter, and the peak-to-peak fluctuation was less than 2% when the emission pattern was pentagonal. However, spikelike and steplike noises occurred, with a frequency approximately proportional to the product of the emission current and the background pressure. These noises may be caused by physical adsorption and ion impact desorption of residual gas molecules. The number of these noise events depended on the emission pattern: it was much greater in the case of a nonpentagonal emission pattern than in the case of a pentagonal emission pattern. This type of current noise is considered to be due to ionic-collision-induced damage at the surface of the tip when the emission pattern is nonpentagonal.

Behavior of Catalyst Particle at Tip of Carbon Nanotube during Field Emission

A catalyst particle at the tip of a multi-walled carbon nanotube (MWNT) during field emission inside a transmission electron microscope was observed in-situ. The particle streamed from the tip like a liquid as the emission current abruptly increased from 20 to 40 µA. This was due to the temperature rise at the tip of the MWNT, resulting from the increased emission current and dipole moment in the particle caused by the electric field. Maintenance of this high emission current led to an electrical discharge, which severely damaged the MWNT electron emitter. Under high emission currents, in particular, the catalyst particle caused an unstable emission.