ACS PRF | ACS
All e-Annual Reports
43966-AC10
Effective Infrared and Field-Emission Properties of Composites and Forests of Carbon Nanotubes
1. We had develped a theory of response of a single-walled carbon nanotube (NT) to the external electric field directed along the NT. Field-induced charge density distribution is approximately linear along the axis of a metallic NT and depends rather weakly on the NT length.
In a semiconducting nanotube with a gap, Eg, charge separation occurs as electric field, F, exceeds the threshold value Fth=Eg/eh. For F>Fth, positively and negatively charged regions at the ends of nanotube are separated by a neutral strip in the middle. Properties of this neutral strip, length and induced charge distribution near the ends, are studied in detail. We also consider a bent nanotube and demonstrate that the number of neutral strips can be one or two depending on the direction of F.
2. We had developed an analytical theory of polarization of a vertically aligned array of NTs in external electric field. Such arrays are commonly utilized in field-emission devices, due to the known electrostatic effect of strong field enhancement near the tip of an individual NT. A small ratio of the NT radius to the separation between neighboring NTs allowed us to obtain asymptotically exact solution for the distribution of induced charge density along the NT axes. For a regular array, this solution allows us to trace the suppression of the field penetration with increasing the density of NTs in the array. We demonstrate that for a random array, fluctuations in the NT density terminate the applicability of our result at distances from the NT tips much larger than the field penetration depth, where the induced charge density is already exponentially small. Our prime conclusion is that, due to collective screening of the external field by the array, the field-emission current decreases drastically for dense arrays compared to an individual NT. We argue that the reason why the strong field emission, described by the Fowler-Nordheim law and observed in realistic arrays, is the strong dispersion in heights of the constituting NTs.
