First-principles study of two-dimensional ferroelectrics using self-consistent Hubbard parameters

The discovery of two-dimensional (2D) materials possessing switchable spontaneous polarization with atomic thickness opens up exciting opportunities to realize ultrathin, high-density electronic devices with potential applications ranging from memories and sensors to photocatalysis and solar cells. First-principles methods based on density functional theory (DFT) have facilitated the discovery and design of 2D ferroelectrics (FEs). However, DFT calculations employing local and semilocal exchange-correlation functionals failed to predict accurately the band gaps for this family of low dimensional materials. Here, we present a $\mathrm{DFT}+U+V$ study on 2D FEs represented by $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ and its homologous ${\mathrm{III}}_{2}\text{\ensuremath{-}}{\mathrm{VI}}_{3}$ compounds with both out-of-plane and in-plane polarization, using Hubbard parameters computed from first principles. We find that ACBN0, a pseudohybrid density functional that allows self-consistent determination of $U$ parameters, improves the prediction of band gaps for all investigated 2D FEs with a computational cost much lower than the Heyd-Scuseria-Ernzerhof hybrid density functional. The intersite Coulomb interaction $V$ becomes critical for accurate descriptions of the electronic structures of van der Waals heterostructures such as bilayer ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$/InTe. Pertinent to the study of FE-based catalysis, we find that the application of self-consistent $U$ corrections can strongly affect the adsorption energies of open-shell molecules on the polar surfaces of 2D FEs.