Abstract
Theoretical problems arising in connection with development and operation of electron field emitters on the basis of carbon nanotubes are reviewed. The physical aspects of electron field emission that underlie the unique emission properties of carbon nanotubes (CNTs) are considered. Physical effects and phenomena affecting the emission characteristics of CNT cathodes are analyzed. Effects given particular attention include: the electric field amplification near a CNT tip with taking into account the shape of the tip, the deviation from the vertical orientation of nanotubes and electrical field-induced alignment of those; electric field screening by neighboring nanotubes; statistical spread of the parameters of the individual CNTs comprising the cathode; the thermal effects resulting in degradation of nanotubes during emission. Simultaneous consideration of the above-listed effects permitted the development of the optimization procedure for CNT array in terms of the maximum reachable emission current density. In accordance with this procedure, the optimum inter-tube distance in the array depends on the region of the external voltage applied. The phenomenon of self-misalignment of nanotubes in an array has been predicted and analyzed in terms of the recent experiments performed. A mechanism of degradation of CNT-based electron field emitters has been analyzed consisting of the bombardment of the emitters by ions formed as a result of electron impact ionization of the residual gas molecules.
Highlights
Carbon nanotubes (CNTs) have unique emission characteristics [1,2,3,4,5] which are caused by their high aspect ratio and good electrical conductivity
The main physical factors affecting the emission current density include the screening of an electric field by neighboring nanotubes, which decreases the electric field gain with increasing intertube distance [28,29,30]; thermal instability, that restricts the emission current from a nanotube [31]; and a statistical scatter of the individual parameters of a nanotube, which changes the character of the I–V characteristic of a cathode [30,31]
Where N is the concentration of the particles. This expression is valid if Nl3 >> 1. As it follows from Equation (37), the characteristic time for attachment of a small particle to an elongated structure of length l is inversely proportional to the value of this parameter, which is due to the effect of the electric field enhancement in the vicinity of the carbon nanotubes (CNTs) tip
Summary
Carbon nanotubes (CNTs) have unique emission characteristics [1,2,3,4,5] which are caused by their high aspect ratio and good electrical conductivity. In contrast with conventional electron field emitters, such cathodes operate at a relatively low applied voltage (at level 1 kV) which permits the development of miniature devices for wide application This possibility is provided by electric field enhancement: due to this effect the electric field near a nanotube tip can be several hundred times higher than the average electric field strength in the interelectrode gap. The forbidden band gap of semiconductor SWNTs is rather narrow (less 1 eV) and inversely proportional to their diameter, so that their conductivity at a temperature of 300–1000 K is quite high to provide the electron emission under the action of the electrical field. Taking into account these peculiarities, one can believe that all the.
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