Tuning the electrical memory characteristics from WORM to flash by α- and β-substitution of the electron-donating naphthylamine moieties in functional polyimides

Abstract

Two novel functional aromatic polyimides (PIs), 6F-αNA PI and 6F-βNA PI, in which the hexafluoroisopropylidene-diphthalic anhydride (6FDA) serves as the electron-accepting unit and the diphenylnaphthylamine (DPNA) functions as the electron-donating species, were synthesized for memory device applications. The 6F-αNA PI shows distinct electrical bistable states with an ON/OFF current ratio up to 10⁶, and can be switched on bi-directionally with no polarity, which could be applied as the nonvolatile write-once read-many times (WORM) memory. Whereas, the 6F-βNA PI-based memory device exhibits flash type memory characteristics with a switching-on voltage at ca. 1.1 V and an ON/OFF ratio of 10⁴. Both polyimides exhibit good long-term operation stability, survive up to 10⁸ reading cycles with no current degradation, and show ultrafast switching with a response time less than 20 ns. Mechanisms associated with the electrical switching behaviors are discussed on the basis of the experimental and quantum simulation results. The electric-field-induced electronic transition from diphenylnaphthylamine units to hexafluoropropylidene phthalimide units and the subsequent formation of charge-transfer complexes are supposed to be responsible for the observed electrical memory effects. Molecular simulation suggests that α-tethering of the naphthyl group results in more non-coplanar conformation of the DPNA species in the 6F-αNA PI, as compared to that of the β-tethering in the 6F-βNA PI, therefore producing a higher energy barrier that prevents the back charge transfer processes, consequently leading to the WORM vs. flash memory behaviors. The 6F-αNA PI differs from 6F-βNA PI only in the substitution position of the naphthyl group, i.e., α-tethering vs. β-tethering. The distinct memory effects observed here suggest the significance of the electron-donating structures on the memory effects, and the tailorability of the memory characteristics through fine structure adjustment.