  Molecular dynamics simulations are performed to investigate edge effects on
the quality factor of graphene nanoresonators with different edge
configurations and of various sizes. If the periodic boundary condition is
applied, very high quality factors ($3\times10^{5}$) are obtained for all kinds
of graphene nanoresonators. However, if the free boundary condition is applied,
quality factors will be greatly reduced by two effects resulting from free
edges: the imaginary edge vibration effect and the artificial effect. Imaginary
edge vibrations will flip between a pair of doubly degenerate warping states
during the mechanical oscillation of nanoresonators. The flipping process
breaks the coherence of the mechanical oscillation of the nanoresonator, which
is the dominant mechanism for extremely low quality factors. There is an
artificial effect if the mechanical oscillation of the graphene nanoresonator
is actuated according to an artificial vibration (non-natural vibration of the
system), which slightly reduce the quality factor. The artificial effect can be
eliminated by actuating the mechanical oscillation according to a natural
vibration of the nanoresonator. Our simulations provide an explanation for the
recent experiment, where the measured quality factor is low and varies between
identical samples with free edges.
