  Observations of topological defects associated with Stone-Wales-type
transformations (i.e., bond rotations) in high resolution transmission electron
microscopy (HRTEM) images of carbon nanostructures are at odds with the
equilibrium thermodynamics of these systems. Here, by combining
aberration-corrected HRTEM experiments and atomistic simulations, we show that
such defects can be formed by single electron impacts, and remarkably, at
electron energies below the threshold for atomic displacements. We further
study the mechanisms of irradiation-driven bond rotations, and explain why
electron irradiation at moderate electron energies (\sim100 keV) tends to
amorphize rather than perforate graphene. We also show via simulations that
Stone-Wales defects can appear in curved graphitic structures due to incomplete
recombination of irradiation-induced Frenkel defects, similar to formation of
Wigner-type defects in silicon.
