Structural diversity and topological property of I-based two-dimensional inorganic molecular crystals

Two-dimensional inorganic molecular crystals (2D IMCs) consisting of compound molecules are emerging as a new branch of the 2D materials family. Based on first-principles calculations, here we propose a 2D IMC class assembled from diatomic molecules whose building blocks are derived from a single element, iodine. We reveal that 2D monolayers of halogen-bonded ${\mathrm{I}}_{2}$ molecules can be stabilized with distinct structures of $\ensuremath{\alpha}$-I, $\ensuremath{\beta}$-I, and $\ensuremath{\gamma}$-I, and these allotropes are collectively named iodene. Intriguingly, the intermolecular angle between two neighboring ${\mathrm{I}}_{2}$ units demonstrate magic angles of ${90}^{\ensuremath{\circ}},{120}^{\ensuremath{\circ}}$, and ${180}^{\ensuremath{\circ}}$ for $\ensuremath{\alpha}$-I, $\ensuremath{\beta}$-I, and $\ensuremath{\gamma}$-I, respectively, which serves as a degree of freedom to modulate the structural and electronic properties of iodene monolayers. The $\ensuremath{\gamma}$-I behaves as a quantum spin Hall insulator with a band gap of 0.18 eV, which offers a candidate for potential topological 2D IMCs. The $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-I phases demonstrate trivial semiconductors with gaps of 2.30 and 1.94 eV. These findings broaden the scope of 2D IMCs stabilized by halogen bonding, with appealing application potentials in molecular physics and optoelectronics.