Residual-strain-induced nanocoils of metallic nanowires

Metallic nanocoils are attractive nanowire (NW) structures, which are expected to have an application as small inductors, but have not been reported before. This study proposes a coating technique for permanently bending a straight, metallic NW into a helix. A physical vapor deposition is applied to oblique NWs standing on a substrate. The deposition produces a biased thickness of the coating on NWs, resulting in a mismatch in internal strains, namely thermal strain and intrinsic strain, between a NW and the coating. These residual strains are driving forces for the bending process of NWs. In particular, the intrinsic strain of the overlayer contributes the bending deformation. In addition, elastic anisotropy of NWs couples bending with a twist, contributing to the formation of helixes. We have demonstrated nanocoils, comprised of Cr-coated Cu NWs, with a coil diameter of about 300 to 500 nm.