7,7,8,8-tetracyanoquinodimethane (TCNQ: ${\mathrm{C}}_{12}{\mathrm{H}}_4{\mathrm{N}}_4$), a well known electron acceptor organic molecule, has recently been quite considered for practical applications with carbon nanotubes, such as chiral selectivity and nanoelectronics. In this paper, we present the results of an extensive ab initio theoretical investigation of TCNQ-doped single-walled carbon nanotubes (SWCNTs). We have obtained the energetically most stable geometries (namely, “bridge-I,” for zigzag nanotubes, and “bridge-II,” for armchair ones), and the respective equilibrium distances (between 0.30 and $0.32\mathrm{nm}$) for external functionalization of SWCNTs by TCNQ, as well as the minimum diameter (around $0.9\mathrm{nm}$) a SWCNT should have so that it is able to exothermically encapsulate a TCNQ molecule. In addition, the electronic properties of TCNQ-doped small semiconducting zigzag carbon nanotubes were examined in details, clarifying the doping effects near the bandgap and revealing a sizable charge transfer from the nanotube toward the molecule.

• Received 19 December 2005

DOI:https://doi.org/10.1103/PhysRevB.73.245422