Exploring
the structural transformation of nonhexagonal rings is
of fundamental importance for understanding the thermal stability
of nonhexagonal rings and revealing the structure–property
relationships. Here, we report on the thermally induced transformation
from the fused tetragon-hexagon (4–6) carbon rings to a pair
of pentagon (5–5) rings in the graphene-like nanoribbons periodically
embedded with tetragon and octagon (4–8–4) carbon rings.
A distinct contrast among tetragon, pentagon, hexagon, and octagon
carbon rings is provided by noncontact atomic force microscopy with
atomic resolution. The thermally activated bond rotation with the
dissociation of the shared carbon dimer between the 4–6 carbon
rings is the key step for the 4–6 to 5–5 transformation.
The energy barrier of the bond rotation, which results in the formation
of an irregular octagon ring in the transition state, is calculated
to be 1.13 eV. The 5–5 defects markedly change the electronic
local density of states of the graphene-like nanoribbon, as observed
by scanning tunneling microscopy. Our density functional theory calculations
indicate that the introduction of periodically embedded 5–5
rings will significantly narrow the electronic band gap of the graphene-like
nanoribbons
