Electrical Switching Memory Behaviors And Mechanisms Of Polyimides Containing Electron Donor-Acceptor Groups

With the rapid development of information technology, there is a growing demand for the memory devices characterized by high data storage density, fast write/read speed, low power consumption and multilevel designs. However, a number of physical and economic factors threaten the continued scaling of the conventional memory devices implemented on the silicon-based inorganic semiconductor materials. Polymeric materials possessing electrical bistabilities are likely to be the promising candidates for the future molecular-scale memory applications, due to their attractive features including easy processability, good mechanical properties, low-cost potential, the three-dimensional (3D) stacking capability and large capacity for data storage. Rather than encoding "0" and "1" as the amount of charge stored in a cell in silicon-based devices, polymer memory stores data in an entirely different way, for example, based on the high-and low-conductivity response to an applied voltage.Among various polymers, aromatic polyimides were considered to be one of the most promising candidates for memory applications due to their excellent thermal and chemical stabilities, and outstanding mechanical performances. And the design and synthesis of novel and solution-processable polyimides, which can provide desirable electronic properties within a single macromolecule and yet still possess good physical and chemical characteristics, were the most-intensively investigated research area. Although various PIs showing different memory properties have been prepared, the relationship between the PIs structure and their memory characteristics still remains unclear. Currently, predicting the memory behavior of a given structure or telling the possible memory differences between different structures prior to measurement is a considerably challenging task with great difficulties, and seems inaccessible so far. Most previously-reported memory PIs possess vastly different molecular structures, which makes it difficult and insufficient to establish a fine structure-property relationship. The design and synthesis of donor-acceptor-containing PIs with similar but elaborately-varied molecular structures and exploring their corresponding memory characteristics are supposed to be greatly helpful to clarify the effect of PIs structures, but were rarely reported.Herein, we report our work on the synthesis and electrical characterization of five novel aromatic polyimides (6F-CzTPA PI,6F-αNA PI,6F-βNA PI,6F-CzTZ PI and6F-OCzTZ PI), and divide them into three parts to discuss the realationships betweent structures and memory behaviors of polyimides. 6F-CzTPA PI, with structures in which the carbazole-tethered triphenylamine units (CzTPA) function as the electron-donating moieties and the hexafluoroisopropylidene phthalimide units (6FDA) serve as the electron-accepting species, exhibits a volatile SRAM characteristic. The roles of the donor and acceptor components in the memory mechanism of the electroactive polyimide were elucidated via molecular simulation. The strong electron-donating ability of the carbazole-thethered triphenylamine moieties and their stoichiometrically excess charge-trapping sites as compared to the6FDA units are suggested to play an important role in realizing the temporarily unerasable, but eventually volatile SRAM memory effect observed in the current system.In the structures of6F-aNA PI and6F-βNA PI, the diphenylnaphthylamine units (DPNA) function as the electron-donating moieties and6FDA serve as the electron-accepting species. By altering the naphthyl group in diphenylnaphthylamine from a-substitution to β-substitution, the memory behaviors of the synthesized polyimides were found to be tuned from the irreversible WORM type (6F-aNA PI) to the programmable flash type (6F-βNA PI). Theoretical analysis suggests that the electric-field-induced donor-acceptor electronic transition and the subsequent formation of charge-transfer complexes could be used to explain the electrical switching effects observed in the synthesized polyimides. And, the more non-coplanar conformation of the diphenylnaphthylamine species in the6F-aNA PI, as compared to the6F-βNA PI, is suggested to bring about a higher energy barrier that prevents the back charge transfer processes under applied electric field, leading to the irreversible WORM vs. programmable flash memory behaviors.6F-CzTZ PI and6F-OCzTZ PI, in which6FDA serves as the electron-accepting unit, the phenylcarbazole group in diamine DACzTZ or DAOCzTZ functions as the electron-donating species and the triazole group as the mediator, have been synthesized for memory device applications. The structures of6F-CzTZ PI and6F-OCzTZ PI are basically identical, with the only difference lying on the introducing of phenoxy linkage in6F-OCzTZ PI; nevertheless, they show vastly different memory behaviors. The6F-CzTZ PI displays nonvolatile WORM memory effect; however, the6F-OCzTZ PI exhibits volatile SRAM characteristic, due to the flexibility of molecular chain and dipole moments. The roles of the donor and acceptor components in the memory mechanism of the electroactive polyimides were elucidated via molecular simulation. Theoretical analysis suggests that the electric-field-induced donor-acceptor electronic transition and the subsequent formation of charge-transfer complexes could be used to explain the electrical switching effects observed in the synthesized polyimides.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. The memery behaviors of polyimides can be predicted by the method of molecular simulation, which is very important to the development of future polymeric information materials.