Ab initio calculation of ideal strength and phonon instability of graphene under tension

Graphene-based sp(2)-carbon nanostructures such as carbon nanotubes and nanofibers can fail near their ideal strengths due to their exceedingly small dimensions. We have calculated the phonon spectra of graphene as a function of uniaxial tension by density functional perturbation theory to assess the first occurrence of phonon instability on the strain path, which controls the strength of a defect-free crystal at 0 K. Uniaxial tensile strain is applied in the x (nearest-neighbor) and y (second nearest-neighbor) directions, related to tensile deformation of zigzag and armchair nanotubes, respectively. The Young's modulus E=1050 GPa and Poisson's ratio nu=0.186 from our small-strain results are in good agreement with previous calculations. We find that in both x and y uniaxial tensions, phonon instabilities occur near the center of the Brillouin zone, at (epsilon(xx)=0.194, sigma(xx)=110 GPa, epsilon(yy)=-0.016) and (epsilon(yy)=0.266, sigma(yy)=121 GPa, epsilon(xx)=-0.027), respectively. Both soft phonons are longitudinal elastic waves in the pulling direction, suggesting that brittle cleavage fracture may be an inherent behavior of graphene and carbon nanotubes at low temperatures. We also predict that a phonon band gap will appear in highly stretched graphene, which could be a useful spectroscopic signature for highly stressed carbon nanotubes.