  Despite decades of extensive research on mechanical properties of diamond,
much remains to be understood in term of plastic deformation mechanisms due to
the poor deformability at room temperature. In a recent work in Advanced
Materials, it was claimed that room-temperature plasticity occurred in
<001>-oriented single-crystal diamond nanopillars based on observation of
unrecovered deformation inside scanning electron microscope. The plastic
deformation was suggested to be mediated by a phase transition from sp3 carbon
to an O8-carbon phase by molecular dynamics simulations. By comparison, our
in-situ transmission electron microscopy study reveals that the
room-temperature plasticity can be carried out by dislocation slip in both
<100> and <111>-oriented diamond nanopillars. The brittle-to-ductile transition
is highly dependent on the stress state. We note that the surface structure may
play a significant role in the deformation mechanisms as the incipient
plasticity always occurs from the surface region in nanoscale diamonds.
