Coordination Compound Materials for Neuromorphic Computing and Brain-Computer Interfaces

As we near the limits of conventional digital Si based transistor technology, neuromorphic computing using analog devices based on functional materials that emulate neuronal and synaptic characteristics has emerged as a powerful approach to realize the energy efficiency and dynamic adaptability of animal brains. While most efforts have focused on inorganic materials due to their compatibility with Si CMOS processing and scalability, a wide range of neuromorphic functionality has also been demonstrated in devices based on organic polymers, coordination polymers and molecular coordination complexes. In my presentation I will give a snapshot of how these different materials are being explored for analog neuromorphic computing, and then focus on our recent work with the mixed valence coordination polymers like the Prussian blue analogues (PBAs), attractive because they have an open framework structure and ability to conduct both ionic and electronic charge. Using inkjet-printing, we demonstrate flexible artificial synapses that reversibly switch electronic conductance by more than four orders of magnitude based on electrochemically tunable oxidation state. Retention of programmed states is improved by nearly two orders of magnitude compared to the extensively studied organic polymers, thus enabling in-memory compute and avoiding energy costly off-chip access during training. We demonstrate dopamine detection using PBA synapses and biocompatibility with living neurons, evoking prospective application for brain-computer interfacing. By application of electron transfer theory to in-situ spectroscopic probing of intervalence charge transfer, we elucidate a switching mechanism whereby the degree of mixed valency between N-coordinated Ru sites controls the carrier concentration and mobility, as supported by DFT.