S. H. Jo

Effect of length and spacing of vertically aligned carbon nanotubes on field emission properties

The length and the spacing of carbon nanotube (CNT) films are varied independently to investigate their effect on the field-emission characteristics of the vertically aligned CNT films grown by plasma-enhanced hot filament chemical vapor deposition using pulsed-current electrochemically deposited catalyst particles. It is shown that, in general, the macroscopic electric field Emac,1, defined as the electric field when the emission current density reaches 1 mA/cm2, can be reduced by increasing the length and the spacing of CNTs. However, for the very-high-density CNT films, the increase of length increases Emac,1 slightly, whereas for the very short CNT films, the increase of spacing does not effectively reduce Emac,1.

Field emission of zinc oxide nanowires grown on carbon cloth

An extremely low operating electric field has been achieved on zinc oxide (ZnO) nanowire field emitters grown on carbon cloth. Thermal vaporization and condensation was used to grow the nanowires from a mixture source of ZnO and graphite powders in a tube furnace. An emission current density of 1mA∕cm2 was obtained at an operating electric field of 0.7V∕μm. Such low field results from an extremely high field enhancement factor of 4.11×104 due to a combined effect of the high intrinsic aspect ratio of ZnO nanowires and the woven geometry of carbon cloth.

Field emission of carbon nanotubes grown on carbon cloth

Field emission from carbon nanotubes grown on carbon cloth has been studied. An extremely low electric field of less than 0.4V∕μm is required to reach an emission current density of 1mA∕cm2. This ultralow operating electric field of carbon nanotubes grown on carbon cloth is mainly due to a very high field enhancement factor of 1.882×104, which is the result of geometrical configuration of the carbon nanotubes and the substrate. In addition to the field enhancement, the highly disordered microstructure of carbon nanotubes grown on carbon cloth plays an important role to field emission. This unexpected result indicates that the roughness of the substrates on which carbon nanotubes grow is very important. This result also brings us significantly closer to practical applications such as highly efficient lamps, field emission displays, micro vacuum electron sources, etc.

Correlation of field emission and surface microstructure of vertically aligned carbon nanotubes

Two kinds of distinctive field emission behaviors were observed on vertically aligned multiwall carbon nanotube (CNT) films grown by hot filament dc-plasma enhanced chemical vapor deposition. Some samples have stable emission current for more than 60 h (type I), while others degrade substantially in the first 16 h (type II). During the field emission measurement, a brief exposure to air led an abrupt decrease of emission current of all samples. But subsequent reevacuation made type I samples recover the emission current fully, whereas type II samples were damaged permanently reflecting on the irrecoverable emission current. Microstructure studies by transmission electron microscope clearly showed that the stable emission is due to a surface passivation of CNTs by a thin layer of amorphous carbon that prevents CNTs from reacting with ambient gases, e.g., oxygen, during air exposure.

Field emission of silicon nanowires

Field emission of single crystal silicon nanowires of 100nm in diameter grown at 480°C from silane using Au as catalyst has been investigated. An emission current density of 1mA∕cm2 over a 0.2cm2 area was obtained at an electric field of 3.4V∕μm with a turn-on field of 2V∕μm at 0.01mA∕cm2. The annealing of the as-grown samples at 550°C in vacuum has drastically improved the field emission performance. The low growth and annealing temperatures make the process applicable to glass substrates.

Nanomaterials fabrication and physics

We have studied growth of carbon nanotubes by chemical vapor deposition and zinc oxide nanowires by physical vapor deposition on carbon cloth with iron sulfate or stainless steel as a catalyst, and various combinations of carbon source and carrier gases. The field emission of these nanostructures shows a combined result of field enhancement from carbon nanotubes or Zinc oxide nanowires and carbon cloth. An emission current density of 1 mA/cm2 was obtained at 0.4 V/μm and 0.7 V/μm for nanotubes and nanowires, respectively. Field enhancement factor of ~4x104 has been observed. Moreover, electron transport characteristics and structural studies of carbon nanotubes have been investigated. Microscopic observations of electric wall-by-wall breakdown imply that transport in the nanotubes is not ballistic and that a significant scattering occurs as carriers traverse nanotubes length.