Self-Assembly of Chiral Nanoparticles into Semiconductor Helices with Tunable near-Infrared Optical Activity

Unique optical, electrical, and mechanical properties of continuous semiconductor helices with nanoscale and mesoscale dimensions represent a previously unexplored materials platform for various applications requiring near-infrared (NIR) optical activity. However, current methods of their synthesis limit the spectrum of chiral geometries, charge transport, and spectral response. Furthermore, the requirements of nearly perfect enantioselectivity, high uniformity, and high yield need to be attained as well. Here, we show that continuous semiconductor helices with tunable spectral response and high monodispersity can be made via self-assembly of semiconductor nanoparticles (NPs). Unraveling the interdependent effects of solvent, pH, ligand density, and coordination bridges between NPs allowed us to maximize the chiral bias for face-to-face particle particle interactions, control of the geometry of the helices, and increase assembly efficiency by 3 orders of magnitude. The self-limiting nature of NP association results in consistency of their geometries over the entire synthetic ensemble. The helices show chiroptical activity across a broad range of wavelengths from 300 to 1300 nm, and the maximum/sign of their polarization rotation in NIR part can be modulated by varying their pitch. The method described in this study can be extended to chiral semiconductor materials from a variety of other NPs and their combinations.