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

Abstract 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 (BF ₄ ¯), perchlorate (ClO ₄ ¯), 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 ̴ 10 ⁵ , 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.