Bismuth Oxychalcogenides: A New Class of Ferroelectric/Ferroelastic Materials with Ultra High Mobility

Atomically thin Bi₂O₂Se has been recently synthesized, and it possesses ultrahigh mobility (Nat. Nanotechnol. 2017, 12, 530; Nano Lett. 2017, 17, 3021). Herein, we show first-principles evidence that Bi₂O₂Se and a related class of bismuth oxychalcogenides, such as Bi₂O₂S and Bi₂O₂Te, not only are novel semiconductors with ultrahigh mobility but also possess previously unreported ferroelectricity/ferroelasticity. Such a unique combination of semiconducting with ferroelectric/ferroelastic properties enables bismuth oxychalcogenides to potentially meet a great challenge, that is, integration of room-temperature functional nonvolatile memories into future nanocircuits. Specifically, we predict that bulk Bi₂O₂S is both ferroelastic and antiferroelectric and that a thin film with odd number of layers can even be multiferroic with nonzero in-plane polarization, and this polarization can be switchable via ferroelasticity. Moreover, Bi₂O₂Te possesses intrinsic out-of-plane ferroelectricity, while Bi₂O₂Se possesses piezoelectricity and ferroelectricity upon an in-plane strain. The in-plane strain on Bi₂O₂Se can induce giant polarizations (56.1 μC/cm² upon 4.1% strain) with the piezoelectric coefficient being about 35 times higher than that of MoS₂ monolayer. The in-plane strain can also enhance the bandgap or even convert indirect to direct bandgap beyond a critical value. The good match among the lattice constants of bismuth oxychalcogenides is also desirable, rendering the epitaxial growth of heterostructure devices free of fabrication issues related to lattice mismatch, thereby allowing high-quality bismuth oxychalcogenide heterostructures tailored by design for a variety of applications.