  Molecular dynamics simulations show that a graphene nanoribbon with
alternating regions which are one and three hexagons wide can transform into a
hybrid 1D nanoobject with alternating double chains and polycyclic regions
under electron irradiation in HRTEM. A scheme of synthesis of such a nanoribbon
using Ullmann coupling and dehydrogenation reactions is proposed. The reactive
REBO-1990EVC potential is adapted for simulations of carbon-hydrogen systems
and is used in combination with the CompuTEM algorithm for modeling of electron
irradiation effects. The atomistic mechanism of formation of the new hybrid 1D
nanoobject is found to be the following. Firstly hydrogen is removed by
electron impacts. Then spontaneous breaking of bonds between carbon atoms leads
to decomposition of narrow regions of the graphene nanoribbon into double
chains. Simultaneously thermally activated growth of polycyclic regions occurs.
Density functional theory calculations give barriers along the growth path of
polycyclic regions consistent with this mechanism. Electronic properties of the
new 1D nanoobject are shown to be strongly affected by the edge magnetism and
make this nanostructure promising nanoelectronic and spintronic applications.
The way of synthesis the 1D nanoobject proposed here can be considered as an
example of the general three-stage strategy of production of nanoobjects and
macromolecules. 1) precursors are synthesized by traditional chemical method,
2) precursors are placed in HRTEM with the electron energy that is sufficient
only to remove hydrogen atoms, 3) as a result of hydrogen removal, the
precursors become unstable or metastable and transform into new nanoobjects or
macromolecules.
