First-principles study of work functions of double-wall carbon nanotubes

Using first-principles density functional calculations, we investigated work functions (WFs) of thin double-walled nanotubes (DWNTs) with outertube diameters ranging from 1 to 1.5 nm. The results indicate that a work function change within this diameter range can be up to 0.5 eV, even for DWNTs with the same outer diameter. This is in contrast with single-walled nanotubes, which show negligible WF changes for diameters larger than 1 nm. We explain the WF change and the related charge redistribution in DWNTs using the charge equilibration model. The predicted work function variation of DWNTs indicates a potential difficulty in their nanoelectronic device applications.

Figure 1

(a) Heat of formation per unit cell for the DWNTs; solid square for (3,3)@(n,n) DWNTs and empty circle for (4,0)@(n,0) DWNTS. (b) Work functions for DWNTs (dots) and SWNTs (triangles) of different diameters.

Figure 2

Contour plots of charge accumulation (left column) and depletion region (right column) for (4,0)@(13,0), (8,0)@(17,0), and (3,3)@(8,8) DWNTs, respectivly. The contour plots are shown in a plane normal to the tube axis, with black dots indicating positions of carbon atoms. The contour value spacings are set to be equal.

Figure 3

Site-projected DOS for (4,0)@(13,0), (8,0)@(17,0), and (3,3)@(8,8) DWNTs, respectively. Empty circles for the outer tube and filled triangles for the inner tube.