  The method for production of atomic chains by heating of graphene nanoribbons
(GNRs) is proposed and studied by molecular dynamics simulations. The Brenner
potential is revised to adequately describe formation of atomic chains, edges
and vacancy migration in graphene. A fundamentally different behavior is
observed for zigzag-edge GNRs with 3 and 4 atomic rows (3 and 4-ZGNRs) at 2500
K: formation of triple, double and single carbon chains with the length of
hundreds of atoms in 3-ZGNRs and edge reconstruction with only short chains and
GNR width reduction in 4-ZGNRs. The chain formation mechanism in 3-ZGNRs is
revealed by analysis of bond reorganization reactions and is based on the
interplay of two processes. The first one is breaking of bonds between 3 zigzag
atomic rows leading to triple chain formation. The second one is bond breaking
within the same zigzag atomic row, which occurs predominantly through
generation of pentagons with subsequent bond breaking in pentagons and results
in single or double chain formation. The DFT calculations of the barriers for
relevant reactions are consistent with the mechanism proposed. The possibility
of chain-based nanoelectronic devices with a controllable number of chains is
discussed.
