Angstrom Scale Ionic\nMemristors’ Engineering\nwith van der Waals Materials: A Route to Highly Tunable Memory States

Memristors that mimic brain functions are crucial for\nenergy-efficient\nneuromorphic devices. Ion channels that emulate biological synapses\nare still in the early stages of development, especially the tunability\nof memory states. Here, we demonstrate that cations such as K<sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>, and Al<sup>3+</sup> intercalated\nin the interlayer spaces of vermiculite result in highly confined\nchannels of size 3–5 Å. They host exotic memristor properties\nthrough ion exchange dynamics, even at high salt concentrations of\n1 M. The bipolar memristor characteristics observed are tunable with\nfrequency, geometric asymmetry, ion concentration, and intercalants.\nNotably, we observe polarization-flipping memristor behavior in two\ncases: one with Al<sup>3+</sup> ions and another with devices having\na geometric asymmetry ratio greater than 15. This inversion is attributed\nto the overscreening of counterions due to their accumulation at the\nchannel entrance. Our results suggest that ion exchange dynamics,\nion–ion interactions, and ion accumulation/depletion mechanisms,\nparticularly with multivalent ions, can be harnessed to develop advanced\nmemristor devices.