Ionic control over ferroelectricity in 2D layered van der Waals capacitors

Originating from their intriguing ferroelectric and ionic properties, van der Waals layered CuInP₂S₆ provides many interesting opportunities for utilization in memory devices and energy applications. Ferroelectric properties of CuInP₂S₆ comprise electromechanical activity even in ultrathin samples, large electrostrictive coefficients and the existence of four uniaxial polarization states. These polar properties are underpinned by the position of Cu ions. However, those ions are also highly mobile. Here, we demonstrate how ionic conductivity can be leveraged to control the ferroelectric behavior in CuInP₂S₆ capacitors. Activating ionic currents through tailored direct voltage pulses impacts coercive voltages as well as the switchable polarization in subsequent probing of ferroelectric switching during fast triangular voltage sweeps. Theoretical calculations indicate that the existence of an internal electric field emanating from inhomogeneous Cu distribution leads to the observed modifications of hysteresis loops. Activating ionic currents can even result in a complete deactivation of hysteretic behavior. Notably, hysteretic polarization switching can be fully restored by activating ionic migration in the opposite direction. This phenomenon provides a path to memory devices beyond binary limitation where information can be encoded in three events: (i) switching from positive to negative polarization, (ii) switching from negative to positive polarization and (iii) no switching within the same voltage range.