Preparation Of The Electro Active Polyimides,Characterization Of The Electrobistable Memory Behavior And Research Of The Switching Mechanism

The explosive growth of information data makes demands on the new-generation memory materials and devices characterized by ultra-high storage density, ultra-fast reading and response speed, low starting voltage,low cost potential and easy processability. However, the conventional memory technology based on the silicon and germanium cannot meet the need because of its limitation in individual cell size and line width. The organic and polymeric materials thus attract considerable attention due to the outstanding features, such as flexibility, low cost, easy processability, large capacity, low power consumption and performance adjustability through molecular tailoring.Considering the property of heat resistance and mechanical strength,polyimide is regarded as the most promising candidate on account of its excellent thermal stability, chemical resistance and dimensional stability.The research on polyimide based memory materials mainly focus on the synthesis of new type polyimide containing electron donor and electron acceptor in single macromolecular chain. This structure can facilitate the microcosmic charge transfer process between the electron donor and acceptor and the macroscopic electrobistability. Up to now, various kinds of electroactive polyimides with distinct chemical structure have been synthesized and different memory behaviors have been achieved, such as the volatile dynamic random access memory (DRAM), volatile static random access memory (SRAM), nonvolatile write-once read-many time memory(WORM) and Flash type memory behavior. However, the switching mechanism remains unclear because of the complicated factors such as the molecular frontier orbital levels, band-gap, molecular conformation, the electron-donating ability of the electron donor, the electron-withdrawing ability of the electron acceptor, the charge-trapping ability and population of the charge-trapping sites, thickness and surface morphology of the active layer and the electrodes type. Thus, although there have been many studies on this topic, further in-depth research and detailed analysis are still necessary. And the most important is to predict and adjust the memory performance by demystifying the structure-property relationship and microcosmic switching mechanism.In this paper, we report our work and achievements as follows:(1) A series of functional copolyimides were designed and synthesized in this work by copolymerizing the 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA) with the (9,9'-bis(4-aminophenyl)fluorene (BAPF) and new synthesized diamine, N,N-bis(4-aminophenyl)aminopyrene (DAPAP).The obtained copolyimides DSDA/(DAPAP/BAPF) were denoted as coPI-DAPAPx (x=100, 50, 20, 10, 5, 1, 0), where x% represents the molar fraction of the DAPAP unit in the diamines. All of the polyimides possess good solubility and excellent thermal stability. The polyimides were fabricated to be memory devices with the configuration of ITO/Polyimide/Au through spin-coating and vacuum evaporation. Characterization results indicate that the DAPAP acts as the main electron donor and the coPI-DAPAPx exhibits tunable electrical switching behaviors from the nonvolatile WORM(coPI-DAPAP100, coPI-DAPAP50, coPI-DAPAP20, coPI-DAPAP10) to the volatile SRAM (coPI-DAPAP5, coPI-DAPAP1) with the decreasing of the DAPAP content. Optical and electrochemical characterization and molecular simulation result show gradually decreasing of the highest occupied molecular orbital levels (HOMO) and enlarged energy gap with the decrease of the DAPAP moiety, suggesting decreasing charge-transfer (CT) effect in the copolyimides, which can account for the observed WORM-SRAM memory conversion.(2) Three novel functional diamines containing anthracene moiety, i.e.,N,N-bis(4-aminophenyl)-1 -anthracenamine (1-DAPAA),N-bis(4-aminophenyl)-2-anthracenamine (2-DAPAA), and N,N-bis(4-aminophenyl)-9-anthracenamine (9-DAPAA), have been synthesized and used to polymerize with 4, 4'-hexafluoroisopropylidene dianhydride (6FDA). The electroactive products, which were denoted as 1-DAPAA-6FDA, 2-DAPAA-6FDA and 9-DAPAA-6FDA, contain the same electron donor. The only difference between the three polyimides was that the anthracene group in DAPAA was attached to the nitrogen atom through different tethering positions (1-, 2-, and 9-). ?-? curves show that the 1-DAPAA-6FDA and 9-DAPAA-6FDA based memory device exhibit nonvolatile WORM behavior, while the 2-DAPAA-6FDA based memory device exhibit volatile SRAM behavior. The influence of electron effect on memory behavior was eliminated by similar electron-donating ability of the three diamines and almost the same quantity of charges transfer from the ground state to the excited state, as shown in the quantum chemical calculation results. And analysis on spatial conformation demonstrates that lower dihedral angle between anthracene group and the molecular backbone of 2-DAPAA-6FDA caused better coplanar structure for CT band back CT process, accounting for the observed volatile SRAM memory performance,while the 1-DAPAA-6FDA and 9-DAPAA-6FDA possess nonvolatile feature because of the large dihedral angle, which will hinder the CT process.(3) Four novel functional polyimides (PIs) for electrical memory applications,DATP6Cz-DSDA, DATP2Cz-DSDA, DATP6Cz-NTDA, and DATP2Cz-NTDA, were synthesized through condensation polymerization of two novel diamines containing unconjugated aliphatic chain of different length,N,N-bis(4-amino)phenyl-6-(9-carbazol)-ethylamine (DATP2Cz) and N,N-bis(4-amino)phenyl-2-(9-carbazol)-hexamine (DATP6Cz), with two dianhydrides, diphenylsulfone-3,3',4,4'-tetracarboxylic dianhydride (DSDA)and 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA). The ethyl and hexyl spacer were intentionally inserted into the diamines to alter the spatial position of the electron donor in the PIs and then to elucidate the effect on the memory behavior. Experimental results show that DATP6Cz-DSDA and DATP2Cz-DSDA exhibit nonvolatile WORM memory behavior, while DATP6Cz-NTDA and DATP2Cz-NTDA exhibit volatile SRAM and DRAM behavior, respectively. Theoretical simulation results indicate much stronger charge-trapping effect of the sulphone moiety in DSDA than that of the carbonyl moiety in NTDA, accounting for the nonvolatile feature of the DSDA-based PI memories and the volatile feature of the NTDA-based PI memories. Meanwhile, spatial position effect was observed in the NTDA-based PIs. The spacer of varied length between carbazole group and diphenyl amino group in DATP6Cz and DATP2Cz, i.e., hexyl vs. ethyl, has significantly altered the spatial position of the electron donor and the charge transportation path in the PIs, resulting in different retention time and corresponding SRAM and DRAM behavior.Above all, the project and results of this paper have proved that the electron-donating ability of the electron donor, depth of the charge-trap ping sites, population and spatial conformation of the electron donor and acceptor can be well adjusted through molecular tailoring directed by quantum chemical simulation. Thus the polyimide memory materials with expected memory performance can be achieved. The combination of theory and practice and critical conclusions in this paper provide instructive principals for the research on novel polymeric memory materials, which are of great importance to the development of information technology.