Competing charge transfer and screening effects in two-dimensional ferroelectric capacitors

Two-dimensional (2D) ferroelectrics offer the potential for ultrathin flexible nanoelectronics, typically utilizing a metal-ferroelectric-metal sandwich structure as the functional unit. Electrodes can either contribute free carriers to screen the depolarization field, enhancing nanoscale ferroelectricity, or they can induce charge doping, disrupting the long-range crystalline order. Here, we explore the dual roles of electrodes in 2D ferroelectric capacitors, supported by extensive first-principles calculations covering a range of electrode work functions. Our results reveal volcano-type relationships between ferroelectric-electrode binding affinity and work function, which are further unified by a quadratic scaling between the binding energy and the transferred interfacial charge. At the monolayer limit, the charge transfer dictates the ferroelectric stability and switching properties. This is a general characteristic confirmed in various 2D ferroelectrics including $α$-In$_2$Se$_3$, CuInP$_2$S$_6$, and SnTe. As the ferroelectric layer's thickness increases, the stability of the capacitor evolves from a charge transfer-dominated to a screening-dominated state. The delicate interplay between these two effects will have important implications for the applications of 2D ferroelectric capacitors.