  The quest for realizing and manipulating ever smaller man-made movable
structures and dynamical machines has spurred tremendous endeavors, led to
important discoveries, and inspired researchers to venture to new grounds.
Scientific feats and technological milestones of miniaturization of mechanical
structures have been widely accomplished by advances in machining and
sculpturing ever shrinking features out of bulk materials such as silicon. With
the flourishing multidisciplinary field of low-dimensional nanomaterials,
including one-dimensional (1D) nanowires/nanotubes, and two-dimensional (2D)
atomic layers such as graphene/phosphorene, growing interests and sustained
efforts have been devoted to creating mechanical devices toward the ultimate
limit of miniaturization--genuinely down to the molecular or even atomic scale.
These ultrasmall movable structures, particularly nanomechanical resonators
that exploit the vibratory motion in these 1D and 2D nano-to-atomic-scale
structures, offer exceptional device-level attributes, such as ultralow mass,
ultrawide frequency tuning range, broad dynamic range, and ultralow power
consumption, thus holding strong promises for both fundamental studies and
engineering applications. In this Review, we offer a comprehensive overview and
summary of this vibrant field, present the state-of-the-art devices and
evaluate their specifications and performance, outline important achievements,
and postulate future directions for studying these miniscule yet intriguing
molecular-scale machines.
