Elevation from Binary to Ternary Resistive Switching Behaviors in Cubane-like Copper Iodide Clusters by Ligand Engineering

Multilevel memory is a feasible solution for the explosive demand for data storage in the Big Data era. In this work, ligand engineering has been implemented in cubane-type Cu₄I₄L₄ clusters to obtain three complexes: Cu₄I₄L₄ (L = P(Ph-p-Cl)₃ (1), PPh₃ (2), P(Ph-p-OCH₃)₃ (3)), in which the substitutes on the p-position of benzyl vary from electron-withdrawing (-Cl) to electron-donating (-OCH₃) groups. Interestingly, with enhancement of electron-donating ability, the trend of more distorted Cu₄I₄ cores, enhanced Cu···Cu interactions, and higher structural dimensions can be achieved. Consequently, in FTO/Cu₄I₄L₄/Ag memristors, the electron-withdrawing group (-Cl) and neutral group (-H) exhibit bistable resistive switching performances, and a strong electron-donating group (-OCH₃) can give ternary resistive switching behavior (OFF/ON1/ON2 current ratio: 1:102.74:103.73, VSet1/VSet2: 1.09/1.48 V). The mechanism about the elevation from binary to ternary memory behavior in these cubane-like Cu₄I₄L₄ clusters has been clarified: the external voltage-induced I···Cl halogen bonding and C-H···π interaction contribute to the binary resistive switching performances in FTO/1/Ag and FTO/2/Ag, and the presence of the second conductive state in FTO/3/Ag stems from the injected carrier delocalization on the Cu₄I₄ core due to strong Cu···Cu interactions. The rules drawn in this work can provide a feasible strategy for the design of novel multilevel memories.