Programmable Helicity in Optically Active One-Dimensional van der Waals Solids

Long-range helicity in condensed materials has sternly relied on the precise ordering of cooperative intra- and inter-molecular bonding interactions. The exclusivity of these interactions in natural, organic, or molecular systems has limited the conclusive demonstration of aperiodic helical motifs in densely packed solid state lattices. In this talk, we will present a class of 1D van der Waals solids that crystallize as weakly bound helical chains that possess atomic scale order consistent with an aperiodic tetrahelix. We will describe how precise atomic level control of the local coordination environment in these 1D vdW lattices induces helical aperiodicity in periodic helical motifs based on repeating screw axis symmetry motifs. We will also highlight herein that the modularity of vdW lattices allows for the systematic engineering of helical attributes (radius, rise, and twist angle) and band gap energies across the visible to the near-UV spectral window. Owing to the intrinsic non-centrosymmetry of aperiodic helices, we will show that these 1D vdW tetrahelices exhibit visible range second harmonic generation in freestanding crystals from the bulk down to the nanoscale. Lastly, we will demonstrate that the weak vdW interchain interactions in these 1D vdW tetrahelices enables the micromechanical exfoliation into single ~1 nm diameter helical chains. The realization of exfoliatable 1D vdW tetrahelices presents a unique materials platform towards understanding the synthetic design rules towards helicity and chirality in low-dimensional solids. Atomically-precise helicity along these length scales will facilitate new opportunities in designing solid state materials towards sub-nanoscale nonlinear chiroptics, long-range spin polarization via chiral induced spin selectivity, and 1D topological helicoid quantum states.