Room temperature quantum spin Hall states in two-dimensional crystals composed of pentagonal rings and their quantum wells

Quantum spin Hall (QSH) insulators are a peculiar phase of matter exhibiting excellent quantum transport properties with potential applications in lower-power-consuming electronic devices. Currently, among all predicted or synthesized QSH insulators, square and hexagonal atomic rings are the dominant structural motifs, and QSH insulators composed of pentagonal rings have not yet been reported. Here, based on first-principles calculations, we predict a family of large-gap QSH insulators in SnX2 (X=S, Se, or Te) two-dimensional (2D) crystals by the direct calculation of Z2 topological invariants and edge states. Remarkably, in contrast to all known QSH insulators, the QSH insulators predicted here are composed entirely of pentagonal rings. Moreover, these systems can produce sizeable nontrivial gaps ranging from 121 to 224 meV, which is sufficiently large for practical applications at room temperature. Additionally, we propose a quantum well by sandwiching an SnTe2 2D crystal between two BiOBiS2 sheets and reveal that the considered 2D crystal remains topologically nontrivial with a sizeable gap. This finding demonstrates the robustness of its band topology against the effect of the substrate and provides a viable method for further experimental studies. A family of materials with exotic quantum properties ideal for low-power electronics has been predicted by an international team. Yandong Ma from Jacobs University Bremen in Germany and his colleagues have theoretically shown that two-dimensional materials made of tin and sulphur, selenium or tellurium can exist in an unusual state of matter known as a quantum spin Hall insulator. Such systems conduct electrical current around their edges but not in their centres, making them useful in electronics because they are robust against impurities and require only low applied voltages to control electron flow. Ma and the team used density functional theory calculations to show that such a state exists at room temperature in SnS2, SnSe2 and SnTe2; materials that, unlike previous quantum spin Hall insulators, have a pentagonal atomic structure. Currently, among all known quantum spin Hall (QSH) insulators, square and hexagonal atomic rings are the dominant structural motifs, and QSH insulators composed of pentagonal rings have not yet been reported. Here, we propose a family of large-gap QSH insulators in the SnX2 (X=S, Se, Te) two-dimensional (2D) crystals (121–224 meV). Remarkably, different from all the known QSH insulators, the QSH insulators predicted here are composed entirely of pentagonal rings. Additionally, the considered 2D crystals retain their QSH properties in a quantum well obtained by sandwiching monolayers between two BiOBiS2 sheets, thus providing a viable way for further experimental studies.