Helical structures on the micro- or nanoscale could have applications, e.g., in sensors or actuators. They can be prepared from a variety of organic or inorganic materials by manipulating the strain inside the material or between the layers of a multilayer structure. There are, for example, lithographic, solution-phase-, or vapor-deposition approaches for the synthesis of helical structures. However, a one-step colloidal synthesis of three-dimensional, rigid, nanosized helices in solution with high yields had not been reported so far.

Xingchen Ye, Indiana University, Bloomington, USA, and colleagues have performed a solution-phase synthesis of uniform Gd₂O₃-based nanohelices with a bilayer structure. The team heated a mixture of gadolinium(III) 2,4-pentanedionate hydrate, (Gd(acac)₃), lithium hydroxide (LiOH), oleic acid (OA), oleylamine (OAm), and 1-octadecene (ODE) stepwise up to 290 °C under a nitrogen atmosphere. The products were isolated and purified by ethanol precipitation and several rounds of centrifugation. Nanohelices were obtained in high yield.

Transmission electron microscopy (TEM) showed that the nanohelices are uniform in size and appear to have a rigid structure. Aberration-corrected scanning TEM (AC-STEM) allowed the team to investigate the structure in more detail. They found that the helices have a bilayer structure. The structure of the layers can be derived from the {111} plane of cubic-phase Gd₂O₃ for the outer layer and from the {100} plane for the inner layer. There is a misfit between the lattices if these two layers, which introduces strain, and thus, causes a bending of the bilayer material that results in the helical structure. According to the researchers, the work could help to better understand and control crystal growth on the nanoscale.

• Colloidal Synthesis of Nanohelices via Bilayer Lattice Misfit,
  Yang Liu, Yuda Li, Soojin Jeong, Yi Wang, Jun Chen, Xingchen Ye,
  J. Am. Chem. Soc. 2020.
  https://doi.org/10.1021/jacs.0c05175