Cylindrical line-feeding growth of free-standing silicon nanohelices as elastic springs and resonators.

Three-dimensional (3D) construction of free-standing silicon (Si) nanohelices has been a formidable challenge for planar lithography and etching technology. We here demonstrate a convenient 3D growth and integration of Si nanohelices (SiNHs) upon bamboo-like cylinders with corrugated sidewall grooves, where the indium catalyst droplets grow around the cylinders in a helical fashion, while consuming pre-coated amorphous Si (a-Si) thin film to produce crystalline Si nanowires on the sidewalls. At the end of each groove cycle, the droplets are enforced to linefeed/switch into the neighbor groove, to continue a spiral growth of SiNHs, with readily tunable diameter, pitch, aspect-ratio and chiral/achiral symmetries. In addition, the SiNHs can be reliably released as free-standing units to serve as elastic links, supports and vibrational resonators. These results highlight the unexplored potential of high precision 3D self-assembly growth in constructing a wide range of sophisticated electromechanical, sensor and optoelectronic functionalities.