Band structure, ferroelectric instability, and spin–orbital coupling effect of bilayer α-In2Se3

Abstract

Recently, two-dimensional van der Waals ferroelectrics have been receiving much interest with continuous exploration of the underlying physics and device applications. While α-In2Se3 in an atomically thin crystal form is believed to have nonzero out-of-plane polarization, its ferroelectric (FE) instability in competition with the antiferroelectric (AFE) ground state is highly concerned. Along this line, a bilayer α-In2Se3 structure should be a good object for clarifying this issue since it is the simplest 2D lattice accommodating an AFE state, possibly allowing the AFE–FE competition. In this work, we employ the first-principles calculation to investigate the lattice and electronic structures of bilayer α-In2Se3, and special attention is paid to the FE instability in competition with the AFE ground state. It is found that the energy difference between the AFE ground state and FE state is small, thereby allowing an electric field modulation of the AFE–FE inter-conversion. More importantly, the Hyed–Scuseria–Ernzerhof algorithm predicts that the FE state is indeed semiconducting rather than metallic, removing the inconsistency between experimental observation and theoretical prediction. The spin–orbital coupling effect can further enlarge the bandgap and drive the indirect-to-direct bandgap transition, and thus appears to be an important ingredient of the underlying physics.

Two-dimensional ferroelectric metal for electrocatalysis

“…Before addressing the switchability issue of these metallic multilayers, we note that the critical n L at which the gap closure occurs obtained with PBE should only be taken semiquantitatively because of the wellknown issue of band gap underestimation in (semi-)local DFT. For example, Heyd-Scuseria-Ernzerhof (HSE) hybrid density functional predicts a band gap of ≈1.50 eV compared with PBE value of ≈0.80 eV for monolayer α-In 2 Se 3 [34,47]; the bilayer is a semiconductor with a small gap of ≈0.20 eV within HSE [47,48]. Nevertheless, both PBE and HSE predict a substantial reduction in E g with increasing n L , confirming the robustness of our design principle.…”

Designing Ultra-flat Bands in Twisted Bilayer Materials at Large Twist Angles: Theory and Application to Two-Dimensional Indium Selenide

“…We can see that these valence band states fulfill our condition: they are located at the outermost atomic layer and extended outward with anisotropic character. Now, we construct a bilayer of α-In2Se3 (23) . According to our proposal, in the bilayer, we should have the octahedra in each layer face to each other (end-to-end polarization alignment), as shown in Figure 3a.…”

Two-Dimensional Ferroelectric Ga2O3 Bilayers with Unusual Strain-Engineered Interlayer Interactions

“…The group analysis reveals that the polarization reversal transition of a monolayer is equivalent to a flipping operation against ab -plane followed by a 60° rotation around c -axis through the site C O , as illustrated in Supporting Information Figure S4. We note that this symmetry group, in principle, can be generalized to other FE-ZB′ III 2 –VI 3 bilayer systems such as In 2 Se 3 . ,, In fact, a vertically flipping operation on a 2D monolayer is difficult to conduct experimentally, since the operation usually needs multistep exfoliation and transfer of the 2D monolayer. However, by nature, the polarization reversal transition does not involve any sophisticated exfoliation and transfer operation to vertically flip the monolayer.…”