Roles of alkali-metal adsorption and defect position on work functions of capped single-wall carbon nanotubes

Summary form only given. The influence of alkali-metal adsorption positions and defects positions on work functions of (5, 5) single-walled carbon nanotubes (CNTs) with a capped edge had been investigated by first-principles calculations. An single-walled armchair (5, 5) CNT with a capped edge was assumed. A single vacancy defect was created by removing a carbon atom from different atomic layers (which were labeled as T1-T4 in FIG. 1(a)). The alkali-metal adatoms (Li/Na/Cs) were located above the center of the pentagons or hexagons (which were labeled as P1-P4 in Fig. 1(b)) on the caps for the perfect CNT (P-CNT), while they were associated with defective CNTs (D-CNTs) on the vacancy defects. After Li/Na/Cs adsorption, the work functions of the Pand D-CNTs along the Z-axis and the X-axis (X-WF) decrease significantly. Compared with adsorption of one Li/Na atom, the work functions of CNTs in axial or radial directions decreased more obviously after Cs adsorption. For comparison purpose, FIG. 1(c) summarizes the work functions of P-CNTs and D-CNTs (Ti) with alkali-metal adatoms on the top, plotted against the electronegativity of Li, Na and Cs. All the axial and radial work functions of (5, 5) Pand D-CNTs with Li/Na/Cs on P
₁
increase linearly with the electronegativity. The curves for the axial or radial work functions are almost parallel to each other. We plot the work functions of the (5, 5) P-CNT with alkali-metal adatoms on different positions in FIG. 1(d)-(e). For the adatom-P-CNT systems, there is no significant difference between the work functions in radial direction (except lower work functions in P
₃
), while the work functions in axial direction actually depend on the adsorption position of alkali-metal atoms. The adatom-P-CNT systems have the lowest work functions with alkali-metal adatoms on P
₁
in axial direction and on P
₃
in radial direction. The work functions of different D-CNTs and adatom-D-CNTs systems with one vacancy defect in different atomic layers are shown in FIG. 1(f)-(g). One vacancy defect could raise the work functions of the CNTs. For the adatom-D-CNTs systems, there is no visible trend between the work functions in axial direction, while the work functions in radial direction show a monotonously decrease from T
₁
to T
₄
. The adatom-D-CNTs systems have the lowest work functions with the Li adatom on T
₄
and the Na/Cs adatom on T
₂
in axial direction and with alkali-metal adatoms on T
₃
in radial direction. Since the electronegativity of Li/Na/Cs is less than carbon, the Li/Na/Cs adatoms on the CNTs are easily ionized. The charge density redistributions or charge transfer will lead to increase of the Fermi levels of the Pand D-CNT. The variation of work functions can be induced by either an enhanced (reduced) surface dipole moments, or a lowering (rising) of its intrinsic bulk Fermi levels [1]. Our results show that the changes of the work functions mainly come from the shifts of Fermi levels. The induced dipole moments lead to a minor decrease in the work functions.