(Invited) Electronic and Ionic Mechanisms in Conjugated Polymer Memristor for Mimicking Synaptic Plasticity

Synapse is a connection between two neurons (or a sensory cell and a neuron) used to transmit excitations. Memristors designed for neuromorphic applications must exhibit reliable analog characteristics, in particular, a switching transition with a continuously variable resistance state and a predictable response. These properties are required in order for the memristor to be used in devices for the construction of physical autonomous artificial neural networks that are independent of programming by other external software means. Synaptic plasticity (SP) is central to Hebbian learning. SP can be divided into a short-term (STP), and long-term plasticity (LTP).1 Both are important for the functioning of the human brain. Fundamental characteristics of the functioning of neuronal synapses in nervous systems include spike-timing dependent plasticity (STDP), whose emulation in an artificial device is critical for simulating biological systems. According to Hebb's rule, a synapse increases its efficiency if it is consistently involved in firing a postsynaptic target neuron. Similar functionality can be achieved on thin films of ditopic π-conjugated bis-terpyridine (tpy-) ligands that form supramolecular structures, in which the ligands are interconnected by complexation with metal ions. These molecules offer a low energy operation with improved reproducibility of memristive characteristics. They possess low-lying molecular energy levels and relatively narrow bandgap enabling the low-power consuming resistive switching and longer stability. We used a bis-terpyridine ligands based on 9-phenyl carbazole, Cbz, and its complex with cobalt Co2+ ions as an active layer of a memristor device. A two-terminal memristor architecture was employed, consisting of the ligand or its cobalt complex deposited on an ITO substrate, and aluminum, gold, or gallium as a top electrode. The Cbz molecule, when deposited in a form of thin film by vacuum sublimation, exhibits an excellent nonvolatile bistable memory behavior, while Co-Cbz exhibits both electronic memory and synaptic plasticity, depending on the amplitude of applied voltage.. The cobalt complex displayed a synaptic effect characterized by subsequent potentiation and depression cycles.2 To measure spike timing-dependent plasticity (STDP), an electrical circuit was employed with the measured element connected in a test setup to pulse generator. Measurements was performed by sending a presynaptic excitation (electrical pulse) at time t = 0 and then sending a postsynaptic excitation at time Dt. This was followed by a weak electrical pulse to measure conductivity. Voltage-induced modulation of synaptic weight was observed with a low applied voltage below 500 mV and short pulse duration below 20 ms. Pair pulse facilitation and pair pulse depression demonstrated significant synaptic weight changes, showing the cobalt complex ability to emulate biological neurons. The polymer structure is shown in the Figure, together with the examples of its STP and LTP. The figure shows the ability to mimic the paired-pulse facilitation and depression. The time courses of the read current decay after the subtraction of the current in equilibrium, without excitation, shown also in the Figure, could be fitted by a stretched exponential function, There are two processes clearly distinguished, running in different timescales, the slower one extended to thousands of seconds. It shows that the diffusion of counter ions participates on the mechanism. The operational mechanism of this memristor was attributed to a combination of the redox effect in the ligand and complex, and to the voltage induced migration of perchlorate counter ions, and subsequent redox processes, altering the metal-to ligand charge transfer band and changing the conducting state of the active layer. Acknowledgments: The work was financially supported by the MEYS of the Czech Republic, project LUAUS24032 References Chang, T., Jo, S. H. & Lu, W.: Short-term memory to long-term memory transition in a nanoscale memristor. ACS Nano 5, 7669–7676 (2011). Pandey, A., Chernyshev, A., Panthi, Y.R., Zedník, J., Šturcová, A., Konefal, M., Kočková, O., Foulger, S.H., Vohlídal, J., and Pfleger, J.: Synapse-Mimicking Memristors Based on 3,6-Di(tpy)-9-Phenylcarbazole Unimer and Its Copolymer with Cobalt(II) Ions. Polymers 2024; 16. Figure 1