Preparation Of Functionalized Polyimides And Research On Their Electrical Bistable Memory Characteristics

The booming development of information technology demands the electrical memory device toachieve non-volatile, high-density data storage, fast respondence and low cost. The fabrication andapplication of the traditional silicon-based memory device have met many difficulties. Polymernon-volatile memory devices, which not only overcome the disadvantage of silicon-basedsemiconductor device but also possess the advantages such as flexible, light-weight, easy fabricationand most likely breaking potential limiting scaling difficulties, have become the active research topicin new non-volatile data storage technology. Polymer memory materials exhibit different conductivitystates at the same applied voltage to fulfill ultra-high density and ultra-capacity of data storage andprocess, thus they are promising candidates for next generation non-volatile memories.Functionalized polyimides, due to their excellent thermal stability, optical transparency, chemicalstability and film-developing ability, have been widely used in optoelectronic devices. In thisdissertation, five series of novel functionalized polyimides have been synthesized and characterized.The electrical switching characteristics of the prepared polymers have also been investigated. Theeffects of the different aryl pendant groups on the electrical switching characteristics were furtherstudied by quantum chemistry calculation. The obtained results were as follows:1. New synthetic routes of2,2’-biaryl-4,4’,5,5’-biphenyltetracarboxylic dianhydride derivatives and2,2’-biaryl-4,4’-biphenyl diamine derivatives were developed. Elementary reaction in these routescan be controlled and easily manipulated. Reaction products were easily separated and of highyields. These new synthetic routes are suitable for industrial scale production.2.2,2’-Diphenyl-4,4’,5,5’-biphenyltetracarboxylic dianhydride (DPBPDA),2,2’-bis[4’’-(3’’’,4’’’,5’’’-trifluorophenyl)phenyl]-4,4’,5,5’-biphenyltetracarboxylic dianhydride(BTFBPDA) and2,2’-bis[4’’-(3’’’,4’’’,5’’’-trifluorophenyl)phenyl]-4,4’-biphenyl diamine (BTFBPD)were synthesized and characterized. Two polyimides, PI(BTFBPD-DPBPDA) andPI(BTFBPD-BTFBPDA) were prepared via a two-step procedure, and they are soluble incommon organic solvents and possesses high thermal stability. Devices with the configuration ofITO/PI/Al were fabricated and investigated. It was found that these undoped PI films can exhibittwo accessible conductivity states. PI(BTFBPD-DPBPDA) exhibited bipolar Flash memorycapability, while PI(BTFBPD-BTFBPDA) presented bipolar WORM memory behaviour;ON/OFF current ratio of both devices were up to103. Mechanism analysis indicated thatdifferent memory characteristics may be attributed to the difference in the fluorine atompopulation per unit volume and the difference in the twisted molecular conformation.3. Two new compounds,2,2’-bis(biphenyl)-4,4’,5,5’-biphenyl tetracarboxylic dianhydride(DPPBPDA) and2,2’-bis[4’’-(naphthalen-1-yl)phenyl]-4,4’,5,5’-biphenyl tetracarboxylicdianhydride (DNPBPDA), and a series of new functionalized polyimides BTFBPD-BPDA(PI-3a), BTFBPD-DPPBPDA (PI-3b) and BTFBPD-DNPBPDA (PI-3c) were synthesized andfully characterized. The memory characteristics of three polyimides were also investigated. PI-3awas determined to present bipolar Flash memory behavior, while PI-3b and PI-3c exhibitedbipolar WORM memory capability. These results indicated that biphenyl and naphthyl-phenylfrom the biphenyltetracarboxylic dianhydride moieties can act as hole trapping sites; thereforethey affect the polyimide’s electrical bistable memory characteristics. 4. PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA), consisting of alternating electron-donating2,2’-bis[4’’-(9H-carbazol-9-yl)phenyl]-or2,2’-bis[4’’-(diphenylamino)phenyl]-substitutedbiphenyl moieties and electron-accepting phthalimide moieties were synthesized andcharacterized. PI(CzBD-BTFBPDA) exhibited unipolar WORM memory character, whereasPI(TPABD-BTFBPDA) showed bipolar Flash memory capability. These devices showed verygood memory performance, with ON/OFF current ratios of more than105and a low misreadingrate through the precise control of the ON and OFF state. The molecular simulation of electronicand geometry structure of the study polyimides were further elucidated that the switchingmechanism depends on the geometric conformation and electronic properties. With theintroduction of carbazole and triphenylamine groups, energy levels of the elctron-donor and theelectron-acceptor from structural unit matched better with each other, improving the ON/OFFcurrent ratio of the devices.5. Fluorine-free aromatic polyimides PI(CzBD-DPBPDA) and PI(TPABD-DPBPDA) weresynthesized from CzBD and TPABD reacting with DPBPDA, respectively. As far as we know, itwas the first report of fluorine-free aromatic polyimides with electrical switching characteristics.With the change of current compliance, PI(CzBD-DPBPDA) demonstrated bipolar Flashmemory characteristics. However, under the same current compliance, PI(TPABD-DPBPDA)demonstrated excellent unipolar ON and OFF switching behavior, independent of the initialvoltage scanning direction. Both polyimides exhibit good performance with an ON/OFF currentratio up to104and stable ON and OFF states under a constant voltage stress and read pulses. Theresults implied that the current controlled electrical switching behavior of this reliable PI-baseddevice can be explained with trap-limited SCLC model.6.2,2’-diphenyl-4,4’-biphenyl diamine (DPBD),2,2’-bis(biphenyl)-4,4’-biphenyl diamine (BBPBD),2,2’-bis[4’’-(naphthalen-1-yl)phenyl]-4,4’-biphenyl diamine (BNPBD) and2,2′-bis(3’’,5’’-dimethoxyphenyl)-4,4’-biphenyl diamine (BMPBD) were synthesized by Suzukireaction. These diamines were reacted with BTFBPDA to obtain four novel polyimidesBPBD-BTFBPDA (PI-6a), BBPBD-BTFBPDA (PI-6b), BNPBD-BTFBPDA (PI-6c) andBMPBD-BTFBPDA (PI-6d). PI-6a, PI-6d and PI-6c exhibited bipolar Flash memorycharacteristics. Whereas, PI-6d demonstrated a unipolar WORM memory capability. ON/OFFcurrent ratio of all polyimides were up to106. Electronic properties of the basic units of PI-6cand PI-6d in the ground state were calculated from DFT at the B3LYP level with the6-31G(d)basis set. Based on the ground state optimized geometry, the optimized geometry, ESP surfaceand dipole moments of the first singlet excited state were calculated with configurationinteraction with single excitations (CIS) method. The results suggested that intramolecularcharge-transfer mechanism could explain the memory characteristics of the studied polyimides.The relatively dipole moments of PI-6d compared to PI-6c provided a stable charge-transfercomplex for the WORM memory performance.