GaSI: A Wide-Gap Non-centrosymmetric Helical Crystal

Abstract

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, electron microscopy, and density functional theory (DFT), we elucidate the apparent noncrystallographic screw axis and the first example of an atomic scale helical structure bearing a "squircular" cross-section in GaSI. Crystallizing in the non-centrosymmetric P4̅ space group, we found that GaSI crystals exhibit pronounced second-harmonic generation. From diffuse reflectance spectroscopy, GaSI displays a sizeable bandgap of 3.69 eV, owing tostrong covalent interactions arising from the smaller sulfur atoms within the helix core. These results position GaSI as a promising exfoliable nonlinear optical material across a broad optical window.

Figure 1

Crystal structure of the helical GaSI phase with the squircular motif projected along the basal plane (A) and long-axis (B) directions. (C) GaSI bulk crystals grown from the melt. Scale bar, 5 mm. (D) SEM image of micromechanically unbundled GaSI nanowires. Scale bar, 50 μm (E) AFM image of ∼10 nm-thick exfoliated GaSI nanowire. Scale bar, 500 nm. (F) HRTEM image of micromechanically exfoliated GaSI. Inset is the FFT with indexed (003) and (−110) periodic spots in red. Scale bar, 5 nm.

Figure 2

Cross-sections of the tetrahelices corresponding to the concentric Ga (A), chalcogen (S or Se; B), and I (C) elements in the GaSI (blue) and GaSeI (red) crystal structures. Filled (GaSI) and empty (GaSeI) circles correspond to experimental crystallographic positions, while lines correspond to fitted cross-sections. (D) Average experimental bond angles around the [GaS₃I]_n and [Ga₃S]_n polyhedral building units. (E) Calculated adhesive formation energies of the experimental GaSI structure bearing the squircular motif compared to several hypothetical structural models and packing motifs.

Figure 3

(A) DRS spectra of GaSI, GaSeI, and InSeI plotted in terms of the Kubelka–Munk function, F(R)_∞. (B) DFT band structure and corresponding partial densities of states of GaSI, calculated with the PBE (top) functional and the HSE06 (bottom) functional. (C) Composite SHG micrograph of the forward (yellow) and epi (blue) channels taken at 400 nm from a micromechanically exfoliated GaSI microcrystallite. The forward (yellow) and epi (blue) channels are shown on the right. Scale bars: 40 μm.