Electron field emission from carbons and their emission mechanism.

This thesis is concerned with the research of the electron field emission properties of carbon based materials. Low emission threshold fields have been observed from both amorphous carbon thin films and carbon nanotubes. The emission mechanism can be subdivided into two groups depending on the type of electric field enhancement. These are the amorphous carbon flat films with non-geometric field enhancement and carbon nanotubes with high surface geometric field enhancement. Amorphous carbon thin films are deposited using an rf-plasma enhanced chemical vapour deposition technique. Changing the deposition conditions such as the addition of Argon or Nitrogen modifies the electronic properties. This induces variations in the sp2 concentration and its distribution within the films. The electron field emission properties from amorphous carbon thin films show a close relationship to its sp2 configuration. A model based on non-geometric field enhancement is proposed to explain the variation in the field emission characteristics. Nano-structured amorphous carbon films custom "designed" using ion beam assisted deposition with sp2 cluster sizes of around 60 nm have also been investigated. The field emission threshold field was shown to be controlled by the film's intrinsic stress and the local carbon density. With increasing stress, there is a concomitant increase in the local density, which is postulated to decrease the distance between the carbon graphitic "planes". This results in enhancement of the electron emission at lower fields. Stress within the films also induces changes to the band structure of the nano-structured carbon which are beneficial to the field emission process. Field emission from carbon nanotubes that are embedded in a polymer matrix has been investigated. The emission threshold fields are observed to be dependent on the nanotube density. The effect of electric field screening is used to explain the reduction of field enhancement observed in these films with increasing nanotube density. The field emission properties are compared with those films which have vertically aligned and in e-beam fabricated nanotube arrays. Results indicate that field emission properties from non-aligned nanotube films are comparable in performance to the best designed arrays in the literature. Although this study shows carbon based materials to have superior field emission properties, integrating the cathodes to fabricate commercial devices could prove to be very challenging