The enhanced field emission properties of K and Rb doped (5,5) capped single-walled carbon nanotubes

The field emission properties of alkali metal K and Rb (AM) doped (5,5) capped single-walled carbon nanotubes (CNTs) have been investigated using first-principles theory. Our results showed the work function of (5,5) capped CNTs doped with AM at the cap (3.60-4.00 eV) decreases significantly in comparison with pristine CNTs (4.44 eV), due to a rise in the Fermi level (mainly) and a drop in the vacuum level. The reduction of the work function in AM-doped CNTs was more evident than when doped with N. All doped (5,5) capped CNTs were semiconductors and their energy gaps (0.12-0.55 eV) were considerably smaller than those of pristine CNTs (1.14 eV). The HOMO and LUMO of pristine (5,5) capped CNTs were evenly distributed at the side wall of the tube. However, the HOMO and LUMO of CNTs doped with AM at the cap were concentrated in the cap of the tube. The emission current of CNTs doped with AM at the cap was increased, because the LDOS (of the cap) at the Fermi level was increased and there emerged new localized states near the Fermi level. The abovementioned results suggest that the field emission properties of (5,5) capped CNTs can be enhanced significantly by doping K and Rb at the cap. Our research findings provide a new dopant that can reduce the work function of CNTs more effectively.