  Highly efficient and widely applicable working mechanisms that allow
nanomaterials and devices to respond to external stimuli with controlled
mechanical motions could make far-reaching impact to reconfigurable, adaptive,
and robotic nanodevices. Here, we report an innovative mechanism that allows
multifold reconfiguration of mechanical rotation of semiconductor nanoentities
in electric (E) fields by visible light stimulation. When illuminated by light
in the visible to infrared range, the rotation speed of semiconductor Si
nanowires in electric fields can instantly increase, decrease, and even reverse
the orientation depending on the intensity of the applied light and the AC
E-field frequency. This multifold rotation configuration is highly efficient,
instant, and facile. Switching between different modes can be simply controlled
by the light intensity at an AC frequency. An array of experimentations,
theoretical analysis, and simulations are carried out to understand the
underlying principle, which can be attributed to the optically tunable
polarization of Si nanowires in aqueous suspension and an external electric
field. Finally, leveraging this newly discovered effect, we successfully
differentiate semiconductor and metallic nanoentities in a non-contact and
non-destructive manner. This research could inspire a new class of
reconfigurable nanoelectromechanical and nanorobotic devices for optical
sensing, communication, molecule release, detection, nanoparticle separation,
and microfluidic automation.
