Electrode Effects on Ultrathin van der Waals Ferroelectric CuInP₂S₆

Out-of-plane polarized ferroelectric materials placed in a capacitive structure provide a foundation for several technological applications. CuInP₂S₆ (CIPS) is a van der Waals layered ferrielectric with out-of-plane polarization that can be in a low- or high-polarization (LP or HP) state and is also an ionic conductor with both phenomena controlled by the Cu ions. At the ultrathin limit, many ferroelectrics become paraelectric or antiferroelectric due to depolarizing fields. Here, using density-functional-theory calculations, we explore the effects of several prototypical electrodes (Gr, Ni, Cu, Au, and Ag) on the stabilization of the ferroelectric state compared with possible antiferroelectric states. We find that the ferroelectric state with electrodes can be stabilized with fewer layers compared to the contact-free case and that Au and Ag stabilizing ferroelectricity at the bilayer limit. Furthermore, interactions with the Au and Ag electrodes stabilize the HP state in the ultrathin limit. Quantum-molecular-dynamics simulations show that even at the bilayer limit, the system is switchable despite contact doping of the CIPS layers.