  Our understanding of sp2 carbon nanostructures is still emerging and is
important for the development of high performance all carbon devices. For
example, in terms of the structural behavior of graphene or bi-layer graphene
at high bias, little to nothing is known. To this end we investigated bi-layer
graphene constrictions with closed edges (seamless) at high bias using in situ
atomic resolution transmission electron microscopy. We directly observe a
highly localized anomalously large lattice expansion inside the constriction.
Both the current density and lattice expansion increase as the bi-layer
graphene constriction narrows. As the constriction width decreases below 10 nm,
shortly before failure, the current density rises to 4 \cdot 109 A cm-2 and the
constriction exhibits a lattice expansion with a uniaxial component showing an
expansion approaching 5 % and an isotropic component showing an expansion
exceeding 1 %. The origin of the lattice expansion is hard to fully ascribe to
thermal expansion. Impact ionization is a process in which charge carriers
transfer from bonding states to antibonding states thus weakening bonds. The
altered character of C-C bonds by impact ionization could explain the
anomalously large lattice expansion we observe in seamless bi-layer graphene
constrictions. Moreover, impact ionization might also contribute to the
observed anisotropy in the lattice expansion, although strain is probably the
predominant factor.
