First-principles density-functional investigation of the effect of water on the field emission of carbon nanotubes

The geometrical structures and the field-emission properties of capped (5, 5) single-walled carbon nanotubes with water adsorbed on the tip with and without an applied electric field have been investigated using first-principles density-functional theory. It is found that the structures of carbon nanotubes with water molecules are stable under field-emission conditions. The dipole moments induced by the adsorption of water molecules point from the water molecules to the CNT tips. The Mulliken charges are redistributed and accumulated on the carbon nanotube tips. Under an applied electric field, the number of Mulliken charges that transfer from the carbon nanotube body to both its tip and water molecules increases with the increase of the number of water molecules. The local density of states at the Fermi level increases with the adsorption of water molecules. These results elucidate that the field-emission properties of carbon nanotubes can be enhanced by the adsorption of water molecules, and are consistent with the experimental results.