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Title: Electrical Conductivity Switching Behavior and Memory Effects in Electroactive Polymers and Nanocomposites
Authors: LIU GANG
Keywords: switching, non-volatile, memory, electroactive, polymer, nanocomposite
Issue Date: 21-Jan-2010
Source: LIU GANG (2010-01-21). Electrical Conductivity Switching Behavior and Memory Effects in Electroactive Polymers and Nanocomposites. ScholarBank@NUS Repository.
Abstract: Organic and polymeric materials can exhibit electrical conductivity switching behavior and memory effects. The field-induced electrical bistability, together with the low-cost potential, light weight, mechanical flexibility, and the most important of all, tunable electronic properties via molecular design, make organic and polymer materials promising alternatives or supplements to inorganic semiconductors in data storage technologies. In this work, a series of polymers and polymer composite materials were explored for memory applications. The focus of this work is concentrated on studying the electrical properties and the underlying switching and conduction mechanisms of the electroactive polymers and nanocomposites. The electrical behavior of three conjugated fluorene copolymers of PFPCz, PFPTPA and PFPPy was first studied and found to be dependent on the molecular structure of the macromolecules. Both WORM memory effects (PFPCz and PFPTPA) and insulator (PFPPy) behavior are demonstrated in the J-V characteristics of the devices with Al/polymer/ ITO structure. The electrical conductivity switching behavior of these fluorene polymers is ascribed to electric field-induced conformational ordering and/or charge transfer interaction of the polymer film in the devices. To achieve a higher ON/OFF state current ratio and thus a lower misreading rate of the polymer memories, two non-conjugated imide polymers, PCz6FDA and PNa6FDA were prepared. The incorporation of a stronger electron withdrawing 6FDA group can significantly enhance the field-induced CT characteristics of the imide polymers. The Al/PCz6FDA/ITO device exhibits electrical bistability and WORM memory effects with an ON/OFF ratio of 100 k, while the PNa6FDA device behaves as an electrical insulator. The electrical bistability of PCz6FDA device is well maintained at elevated temperatures and thermally stable electronic memory device is thus demonstrated. Capitalizing on the charge trapping ability of azobenzene chromophores, electrical bistability with enhanced ON/OFF ratios in excess of ~ 100 k were demonstrated in two donor-trap-acceptor (D-T-A) structure carbazole-azobenzene polymers. PVK-AZO-NO2 and PVK-AZO-2CN possess pendant electron D-A pair (carbazole-nitro/cyano) and charge trapping center (azobenzene) spontaneously, allowing effective stabilizing of the intra-molecular CT state, and leading to WORM memory. In addition to molecular design and organic chemistry, the electronic properties of polymers can also be controlled by forming composites with other electroactive materials. The bistable switching behavior and memory effect of the fluorene polymer PFPTPA can be enhanced upon mixing with carbon nanotubes (CNTs). Furthermore, by varying the carbon nanotube content in poly(N-vinylcarbazole) (PVK) composite films, the electrical conductivity of the PVK-CNT doping system can be tuned deliberately. The Al/PVK-CNT/ITO sandwich structure exhibits insulator, bistable electrical conductivity switching (WORM memory and flash memory effects), and conductor behaviors, when the CNT content in the composite film is increased from 0 to 3%. The conductivity switching effects of the PVK-CNT composite films are ascribed to electron trapping in the CNTs of the hole-transporting PVK matrix. Similar to its consanguinity of C60 and carbon nanotube, the large numbers of hexagonal aryl make graphene material a good electron acceptor. The atomic nanosheets of graphene enhance its potential application in ultrathin electronic devices. A solution-processable and electroactive complex of GO-PVK exhibits bistable electrical conductivity switching and non-volatile rewritable memory effects. Both the OFF and ON states of the memory device are stable under a constant voltage stress of -1 V for up to 3 h, or under a pulse voltage stress of -1 V for up to 100 million read cycles, with an ON/OFF state current ratio in excess of 1000.
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