Counterions Drive Memory Functionality in Two-Terminal Viologen-based Molecular Junctions

Molecular memory devices have emerged as promising hardware components for future neuromorphic technology. However, their optimization for industrial applications remains a challenge. Viologen offers bistable redox states, making them ideal for non-volatile resistive switching memory elements in electronic devices. However, it is crucial to understand the key roles of counterions in the viologen-based memory devices. This study explores the potential for tuning memory parameters-including threshold voltage, ON/OFF current ratio (I_ON/OFF), retention time, and stability in redox-active benzyl viologen dichloride system by four counter anions such as tetrafluoroborate (BF₄¯), perchlorate (ClO₄¯), triflate (OTf¯), and bis(trifluoromethanesulfonyl)imide (TFSI¯). Electrical studies reveal that larger counter anions require a higher threshold voltage but produce larger I_ON/OFF. Among the four specific examples, OTf¯ has shown a good balance in terms of switching voltage (± 1.5 V), with I_ON/OFF ̴ 10⁵, 1050 cycle endurance, and stability up to 2000 s. Thickness-dependent studies identify ≈70 nm of viologen film as optimal for memory performance. Counterions in the molecular films undergo movement due to the high electric field, which originates hysteresis. The study highlights the impact of counter-anion selection on the performance and parameters of viologen-based non-volatile memory devices, providing crucial insights for designing resistive switching characteristics.