Probing The Working Mechanism of Organic Nanowire Memristors Based On a Series of Oxocarbon Derivatives

Neuromorphic computation is a promising way to overcome the limits of the von Neumann chip architecture in modern computing. The current problems computation faces are due to the bottlenecks in the current Von Neumann architecture. These include storage, resolution and speed in which data can be processed. Memristors (memory + resistance) have been explored as an option to replace current computing chips due to their memory storage capabilities and their ability to dynamically change their physical properties when applying a bias. [1] The memristor was mathematically derived by L. Chua in 1971 and has been demonstrated in the lab since. [2] Organic memristors have been an area of interest because of the ability to tune the structure and functional groups of a molecule to enhance the electrical performance of a device, including memory and volatility performance. [3][4]<br/> <br/>Recently our group demonstrated a working analog memristor based on 2,4-bis[4-(diethylamino)-2-hydroxyphenyl]squaraine (SQ) nanowires on gold interdigitated electrodes. [5] The device showed exceptional analog switching behaviour with large hysteresis loops and increased conductivity with successive sweeps. The device showed non-volatile memory behaviour which can be compared and is on par with benchmark transition metal oxide devices. The working mechanism of the device, however, was seen to differ from transition metal oxide (TMO) memristors. The concentration of oxygen vacancies in TMOs is responsible for the memristive behaviour in TMO memristors. Initially we proposed that protons in SQ could be responsible for the memristive behaviour, especially as squaric acid and another oxocarbon, croconic acid, undergo inter-intra-molecular proton tautomerism. [6] To test this hypothesis, we performed a series of experiments using impedance spectroscopy and current-voltage measurements under a variety of environmental conditions. Hydration studies were employed to determine whether proton conduction occurred in SQ nanowires, which would be analogous to oxygen vacancies in TMOs. [7] Hydration studies, while ruling out intermolecular proton transport in these devices, reveal interesting observations on what controls the magnitude of hysteresis in these devices. We have since began to test alternative oxocarbon derivatives and salts in order to both develop deeper insights into the working mechanism of these devices as well as realise enhanced performance. Indeed, there are certainly a number of exciting routes to take in order to fully explore, and exploit, the potential of squaraine and related small molecule mixed conductors in the fields of bioelectronics, organic electrochemical transistors, organic batteries and neuromorphic computing.<br/> <br/> <br/> <br/>(1) Zidan, M. A.; Strachan, J. P.; Lu, W. D., The future of electronics based on memristive systems, <i>Nature Electron.,</i> 2018, <b>1</b> (1), 22−29<br/>(2) Chua, L., Memristor-The missing circuit element. <i>IEEE Transactions on Circuit Theory </i>1971<b>,</b> <b>18</b> (5), 507-519.<br/>(3) van de Burgt, Y.; Melianas, A.; Keene, S. T.; Malliaras, G.; Salleo, A, Organic electronics for neuromorphic computing, <i>Nature Electron</i>., 2018, <b>1</b> (7), 386−397<br/>(4) Sangwan, V. K.; Hersam, M. C., Neuromorphic nanoelectronic materials. <i>Nat. Nanotechnol</i>., 2020, <b>15</b> (7), 517−528<br/>(5) O’Kelly, C. J., Nakayama, T. & Ryan, J. W., Organic Memristive Devices Based on Squaraine Nanowires. <i>ACS Appl. Electron. Mater.</i>, 2020, <b>2</b>, 3088-3092<br/>(6) Horiuchi S, Kobayashi K, Kumai R, Ishibashi S. Proton tautomerism for strong polarization switching. Nat Commun. 2017<br/>(7) Strukov, D., Snider, G., Stewart, D. <i>et al.</i> The missing memristor found. <i>Nature</i> <b>453</b>, 80–83 (2008).