The effective workfunctions of single-walled carbon nanotubes (5,5) (SWNTs) with various geometries and adsorbates under external electric field have been calculated by the ab initio plane-wave, pseudopotential method. The capped, open-ended, and close-ended nanotubes show the workfunctions of 4.8 eV, 4.43 eV and 3.75 eV, respectively, and these results exhibit a good agreement with experiments. Under external electric field, the effective workfunction is further reduced due to the charge redistribution at the nanotube tip. In addition, the effects of participation of foreign adsorbates on the nanotube surface both physically and chemically on the variations of workfunctions have also been studied. In the physisorption process, the electrostatic interaction between adsorbates and nanotubes plays an important role under external electric field. The polar molecules like water have a large binding energy with the nanotube under electric field. These molecules act as tunneling states for electrons emitting from the nanotube tip into the vacuum. In the chemisorption process, the variations of effective workfunctions can be understood in terms of the surface dipole of the terminated bond due to the different electronegativity between nanotubes and adsorbates.
Diamond and Related Materials 13(4-8), pp.1306-1313