GaSI: A wide gap non-centrosymmetric helical crystal

The complex non-centrosymmetric and chiral nature of helical structures endow materials that possess such motifs with unusual properties. However, despite their ubiquity in biological and organic systems, there is a severe lack of inorganic crystals that display helicity in extended lattices where these unusual properties are expected to be most pronounced. Here, we report a new inorganic helical structure, gallium sulfur iodide (GaSI), within the exfoliable class of III-VI-VII (1:1:1) one-dimensional (1D) van der Waals (vdW) crystals. Through detailed structural analyses, including single crystal X-ray diffraction and electron microscopy, we elucidate the apparent non-crystallographic screw axis and the first example of an atomic scale helical structure bearing a “squircular” cross-section in GaSI. Crystallizing in the non-centrosymmetric P-4 space group, we found that GaSI crystals exhibit pronounced second harmonic generation. From diffuse reflectance spectroscopy, GaSI displays a sizeable band gap of 3.69 eV, owing to weaker orbital overlap between conduction and the valence bands based on density functional theory calculations. These results position GaSI as a promising exfoliable non-linear optical material across a broad optical window.