Enhancement of field emission in carbon nanotubes through adsorption of polar molecules

First-principle’s quantum-mechanical density functional theory (DFT) calculations are carried out to analyze the effect of single water-vapor molecules, three- and five-molecule water-vapor clusters and several other single-molecules with high dipole moments absorbed at the tip of capped (5,5) metallic armchair nanotubes on the ionization potential which is a measure of the ease at which electrons are extracted from carbon nanotubes during field emission. The results obtained show that in the absence of an externally applied electric field, the adsorption energies of both single- and multi-molecule clusters are quite low (typically less than 0.7 kcal/mol or 0.03 eV/molecule) suggesting that these adsorbates are not stable and would most likely desorb before a typical field-emission temperature of ∼900 K is reached. In sharp contrast, under a typical field-emission electric field of 1 eV/Å, the adsorption energy is substantially higher (typically around 20 kcal/mol or 0.867 eV/molecule) making the adsorbates stable. The increased stability of the adsorbates is found to be the result of electrostatic interactions between dipole moments of the adsorbates and the applied electric field. These interactions increase the energy of the highest occupied molecular orbital in the nanotube and, in turn facilitate field emission. These findings are generally consistent with the available experimental results.