  We study by high-resolution transmission electron microscopy the structural
response of bilayer graphene to electron irradiation with energies below the
knock-on damage threshold of graphene. We observe that one type of divacancy,
which we refer to as the butterfly defect, is formed for radiation energies and
doses for which no vacancies are formed in clean monolayer graphene. By using
first principles calculations based on density-functional theory, we analyze
two possible causes related with the presence of a second layer that could
explain the observed phenomenon: an increase of the defect stability or a
catalytic effect during its creation. For the former, the obtained formation
energies of the defect in monolayer and bilayer systems show that the change in
stability is negligible. For the latter, ab initio molecular dynamics
simulations indicate that the threshold energy for direct expulsion does not
decrease in bilayer graphene as compared with monolayer graphene, and we
demonstrate the possibility of creating divacancies through catalyzed
intermediate states below this threshold energy. The estimated cross section
agrees with what is observed experimentally. Therefore, we show the possibility
of a catalytic pathway for creating vacancies under electron radiation below
the expulsion threshold energy.
