Free-Standing 2D Ferro-Ionic Memristor via Nanoscale Flexoelectric Engineering

2D van der Waals ferroelectric materials, which have non-volatile switching with switchable polarization, have emerged as significant platforms for next-generation functional devices. Among these 2D ferroelectric materials, ferro-ionic CuInP₂S₆ has received considerable attention due to its phase tunability. Depending on the interlayer Cu⁺ ion dynamics, CuInP₂S₆ offers the versatile advances for neuromorphic computing <i>via</i> ferro-ionic characteristics. While the CuInP₂S₆ exhibits ferroelectric phase with insulating properties at the room temperature, external temperature or electric field are required to facilitate the ferro-ionic conduction. However, the external conditions inevitably cause the stochastic conductive filament formation, which completely limits the practical application of the memristor devices. Herein, we present the programmable flexoelectric engineering for nano-confined conductive filaments in free-standing 2D ferro-ionic memristor. To spatially modulate the Cu⁺ ion, an ultra-high vertical shear strain was site-selectively facilitated in the free-standing area to activate the flexoelectric energy conversion, which allowed local positioning of the CuInP₂S₆ phase transition. Moreover, we experimentally demonstrated the concept of a free-standing 2D ferro-ionic memristor with nanoscale shear strain engineering. As the local flexoelectric engineering results in a fully reversible paraelectric <i>V</i>_th shift, a 6.25×10²-fold increased <i>I</i>_{max, with strain}/ <i>I</i>_{max, without strain} ratio was experimentally observed owing to the vertical shear strain 720 nN, which is theoretically supported by the 3D flexoelectric simulation. Additionally, a topographical Cu⁺ ion extraction was locally confined within 232.42 nm width and 58.98 nm height, while the upward polarized bottom In₂Se₃ suppresses the undesirable ionic conduction in the suspended junction area. In conclusion, we envision that our free-standing 2D ferro-ionic memristor provides the extendable geometric solution for ultra-high strain gradient and reliable neuromorphic computing systems.