Dimensionally Resolved Nanostructures of an Atomically Precise and Optically Active 1D van der Waals Helix.

Inorganic freestanding helices are rare and sought-after for their unusual physical states endowed by chirality. To this end, III-VI-VII solids have emerged as a distinct class of ternary 1D van der Waals (vdW) crystals which bear atomically precise helical motifs. However, the physical understanding of the intrinsic and size-dependent properties of these materials is limited by the lack of synthetic strategies to directly access freestanding nanocrystals in high volumes. Using GaSI as a representative phase, a bottom-up strategy is presented to grow high yields of ultrathin nanostructures based on this helical materials class. With this strategy, it is possible to grow single crystals of 1D nanowires with thicknesses in the 10-100 nm range at high temperature conditions, as well as quasi-2D nanoribbons at lower temperatures. The bandgap of the nanowires is established in the UV region and demonstrates the persistence of nonlinear optical behavior as evidence of the persistence of the noncentrosymmetric crystal structure of GaSI at the nanoscale. Inspired by these results, the effect of the helical nature of GaSI on the electronic structure of hypothetical single chains is probed from first principles and shows the pronounced handedness-dependent and helicity-imposed spin polarization at the single helix regime.