Antiparallel interlayer polarization alignments in bilayer van der Waals ferroelectrics

Two-dimensional (2D) van der Waals (vdW) ferroelectrics with robust polarization hold great promise for next-generation nanoscale electronics. While interlayer sliding-induced symmetry breaking has been extensively studied, the polarization alignment between layers has received less attention. Here, by means of first-principles calculations, we study the interlayer polarization alignment in three representative bilayer vdW ferroelectrics, ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}, {\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$, and SnSe. Surprisingly, antiparallel alignments, tail-to-tail for ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, head-to-head for ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$, and interlayer antiparallel for SnSe, emerge as the lowest-energy configurations. Contrary to conventional bulk ferroelectrics, where dipole-dipole electrostatic energy dominates, we find that vdW energy governs the interlayer polarization alignment in out-of-plane (OP) vdW ferroelectrics (${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$), while dipole-dipole interactions dictate for in-plane (IP) ferroelectric SnSe. This distinction arises from the unique atomic arrangement near the vdW gap in OP systems, which lowers the vdW energy in antiparallel configurations. The existence of these antiparallel interlayer alignments opens up possibilities for multiple accessible states, offering exciting opportunities for high-density miniaturized memory devices.