Synthesis of tetraphenylphosphonium/polysulfide hybrids and their temperature-induced switchable WORM/flash electrical memory behavior

Abstract

The development of innovative non-volatile storage materials is essential for the progression of next-generation high-performance storage devices. In this study, a hybrid material (Ph₄P)₂S₇ was synthesized, and single crystal structure analysis demonstrated that the organic tetraphenylphosphonium cation (Ph)₄P⁺ adopts a regular tetrahedral conformation and forms one-dimensional chains through CH···π interactions. The (S₇)^2- anion, characterized by quasi-bicentric symmetry and a right-handed helical cluster, is confined within the one-dimensional quantum well established by the (Ph)₄P⁺ cation chain in the organic matrix. A device composed of FTO/(Ph₄P)₂S₇/Ag was fabricated through spin-coating, demonstrating remarkable temperature-induced switchable binary write-once-read-many-times (WORM)/Flash electrical memory behavior. At room temperature, the material demonstrated nonvolatile binary WORM-type resistance switching behavior with a switching ratio of 1.95 × 10³ and an onset voltage of 1.01 V. Conversely, at 150 °C, it transits into binary Flash-type resistance switching behavior characterized by current ratio of 3.34 × 10² and onset/reset voltages of 1.05/−1.99 V. The mechanism underlying binary resistive switching is identified as spatial charge-limited charge capture. Furthermore, the observed reversible transition between WORM- and Flash- type electrical memory behavior at elevated temperatures can be attributed to lattice expansion of the (Ph)₄P⁺ cations at high temperatures, resulting in denser packing of organic molecules and deeper trapping potential wells for electrons within the active layer. This study provides theoretical insights into developing novel high-performance information storage materials.