A new class of crystal structure for two-dimensional materials

Two-dimensional (2D) layered materials have recently been studied extensively owing to their unusual physical properties. Not only they offer superior mechanical, transport, energy harvesting properties, they also host intriguing electronic structures of Dirac Fermions. Together with various exfoliation techniques available, diverse families of 2D materials and their heterojunctions will enrich the structure-property relations. Here, we demonstrate a new class of 2D materials, which are composed of six group IV (C, Si, Ge, Sn) and two group VI (O, S, Se, Te) elements. The microscopic crystal structure of the new 2D materials with a space group of Cmme (No. 67) are remarkably distinctive from the previously known 2D materials such as transition metal dichalcogenides, metal halides, and layered III-VI and V-VI compounds. Furthermore, we find that their electronic structures vary significantly depending on the group IV elements consisting of the crystal. Some of the crystals are direct bandgap semiconductors with sizes up to 1.2 eV, while others are quantum spin Hall insulators with their spin-orbit bandgap sizes ranging from a few tens to hundreds of meV. We believe that the new 2D materials presented in this work can extend the library of 2D materials.