Directional scrolling of SiGe/Si/Cr nanoribbon on Si(111) surfaces controlled by two-fold rotational symmetry underetching.

The controllable fabrication of self-scrolling SiGe/Si/Cr helical nanoribbons on Si(111) substrates is investigated. The initial lateral etching profile of the Si(111) substrates shows a 2-fold rotational symmetry using 4% ammonia solution, which provides guidance for initial scrolling of one-end-fixed nanoribbons to form helical structures. The chirality of the SiGe/Si/Cr helices with isotropic Young's moduli is governed by the anisotropic underetching in the initial stage, which can be precisely judged, as the orientation of the ribbon is predesigned. Furthermore, the helicity angle and radius of the formed helices are investigated by the lateral etching behavior and Cosserat curve theory of the Si(111) substrates, respectively, which are consistent with the experimental data. The present work provides the scrolling rule of nanoribbons with an isotropic Young's modulus and anisotropic underetching in the formation of micro-/nanohelices.