  A WSe2 monolayer of 1T' phase is a large band gap quantum spin Hall
insulator, supporting dissipationless charge and spin transports through the
topologically protected edge states. In this work, we explore the nanoribbon
forms of 1T' phase WSe2 by first-principles density functional calculations and
the many-body perturbation GW and Bethe-Salpeter equation method. We found that
the 1T' WSe2 nanoribbon can show topological edge states with a ribbon width of
~4-6 nm. Those edge bands show crossing through the Fermi level an odd number
of times, with one kind of spin-polarization connecting the valence band
continuum and conduction band continuum. The topological features of the edge
bands hold even under small and medium bending in the nanoribbon, while large
bending induces large band splitting, resulting in a topological switch-off in
the edge bands. The semiconducting 1T' WSe2 nanoribbon shows a large tunability
with bending in optical absorption spectra and exciton states. The
lowest-energy exciton is changed from optically dark in the flat nanoribbon to
bright in the bent nanoribbons. These properties in the 1T' WSe2 nanoribbons
suggest potential applications in controllable quantum electronics and
exciton-based quantum information processes.
