Electromechanical effects in carbon nanotubes: Ab initio and analytical tight-binding calculations

We perform ab initio calculations of charged graphene and single-wall carbon nanotubes (CNTs). A wealth of electromechanical behaviors is obtained. (1) Both nanotubes and graphene expand upon electron injection. (2) Upon hole injection, metallic nanotubes and graphene display a nonmonotonic behavior. Upon increasing hole densities, the lattice constant initially contracts, reaches a minimum, and then starts to expand. The hole densities at minimum lattice constants are 0.3 \e\/atom for graphene and between 0.1 and 0.3\e\/atom for the metallic nanotubes studied. (3) Semiconducting CNT's with small diameters (dless than or similar to20 Angstrom) always expand upon hole injection. (4) Semiconducting CNT's with large diameters (dgreater than or similar to20 Angstrom) display a behavior intermediate between those of metallic and large-gap CNT's. (5) The strain versus extra charge displays a linear plus power-law behavior, with characteristic exponents for graphene, metallic, and semiconducting CNT's. All these features are physically understood within a simple tight-binding total-energy model.