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다양한 전기감응 고분자의 디지털 메모리 특성과 화학구조와의 상호연계성에 대한 연구

Title
다양한 전기감응 고분자의 디지털 메모리 특성과 화학구조와의 상호연계성에 대한 연구
Authors
고용기
Date Issued
2015
Publisher
포항공과대학교
Abstract
Electrically bistable switching polymeric materials have recently attracted significant attention because they have important advantages over inorganic silicon-based materials for memory device applications in that their dimensions can easily be miniaturized and their properties can easily be tailored through chemical synthesis. Moreover, Polymer materials can be processed at low cost and have high flexibility, high mechanical strength, and good scalability. A series of aromatic polyimides (PIs) were synthesized via the polymerization of 3,3′,4,4′-diphenylsulfonyltetracarboxylic dianhydride with 4,4′-diaminotriphenylamine derivatives containing hydrogen, cyano, methoxy, or dimethylamine substituents. These PIs were thermally and dimensionally stable and produced high-quality thin films when applied in a conventional spin-coating process. Their structure and properties were characterized. Nanoscale thin films of the PIs demonstrated excellent electrical memory performance, with high stabilities and ON/OFF current ratios. The memory characteristics were found to be tunable by varying the substituents; nonvolatile write-once−read-manytimes memory behavior, nonvolatile ON/OFF switching type memory behavior, and volatile dynamic random access memory behavior were observed. The memory characteristics were substantially influenced by the electron-accepting cyano- and electrondonating dimethylamine substituents but were apparently not affected by the electron-donating methoxy substituent. In addition, the film density was a significant factor influencing the observed memory behaviors, with larger film densities causing lower OFFcurrent levels. However, the critical switching-on voltage varied very little as the substituents were changed and was measured to be approximately ±2 V. All of the memory behaviors were found to be governed by a mechanism involving trap-limited space charge-limited conduction and local filament formation. Overall, all of the PIs assessed in the present work were found to be suitable active materials for the low-cost mass production of high-performance, programmable unipolar memory devices that can be operated with very low power consumption, high ON/OFF current ratios, and high thermal and dimensional stability. An ionic conjugated polymer with propargyl side groups (poly(2-ethynylpyridinum bromide); PEP-P) was synthesized. High-quality thin films with smooth surfaces were prepared from this polymer on silicon substrates and metal electrodes, via a conventional, simple solution-coating and drying process. Synchrotron X-ray scattering analysis showed that the polymer in the nanoscale thin films was amorphous, but was somewhat preferentially oriented in the film plane, rather than randomly oriented. Using synchrotron X-ray reflectivity analysis, the electron densities and interfaces between the polymer film layers and the silicon substrate and metal electrodes were examined in detail. PEP-P is the first ionic conjugated polymer to be shown to exhibit electrically nonvolatile memory behavior. The polymer in the nanoscale thin films showed excellent write-once-read-many times (WORM) memory characteristics, without any polarity. The switching-ON voltage was lower than 1.5 V. WORM memory devices based on PEP-P were highly stable, even under ambient air conditions. PEP-P therefore has great potential as a candidate material for the low-cost mass production of high-performance, programmable unipolar WORM memory devices with very low power consumption. The fully π-conjugated donor−acceptor hybrid polymers Fl-TPA, Fl-TPA-TCNE, and Fl-TPA-TCNQ, which are composed of fluorene (Fl), triphenylamine (TPA), dimethylphenylamine, alkyne, alkyne-tetracyanoethylene (TCNE) adduct, and alkyne-7,7,8,8-tetracyanoquinodimethane (TCNQ) adduct, were synthesized. These polymers are completely amorphous in the solid film state and thermally stable up to 291−409 °C. Their molecular orbital levels and band gaps vary with their compositions. The TCNE and TCNQ units, despite their electron-acceptor characteristics, were found to enhance the π-conjugation lengths of Fl-TPATCNE and Fl-TPA-TCNQ (i.e., to produce red shifts in their absorption spectra and significant reductions in their band gaps). These changes are reflected in the electrical digital memory behavior of the polymers. Moreover, the TCNE and TCNQ units were found to diversify the digital memory modes and to widen the active polymer layer thickness window. In devices with aluminum top and bottom electrodes, the Fl-TPA polymer exhibits stable unipolar permanent memory behavior with high reliability. The Fl-TPA-TCNE and Fl-TPA-TCNQ devices exhibit stable unipolar permanent memory behavior as well as dynamic random access memory behavior with excellent reliability. These polymer devices were found to operate by either hole injection or hole injection along with electron injection, depending on the polymer composition. Overall, this study demonstrated that the incorporation of π-conjugated cyano moieties, which control both the π-conjugation length and electron accepting power, is a sound approach for the design and synthesis of high-performance digital memory polymers. The TCNE and TCNQ polymers synthesized in this study are highly suitable active materials for the low-cost mass production of high performance, polarity-free, programmable, volatile, and permanent memory devices that can be operated with very low power consumption, high ON/OFF current ratios, and high reliability. Covalent incorporations into polymers of fullerene were achieved via the Cu(I)-catalyzed azide−alkyne click polymerizations of a fullerene derivative monomer functionalized with 5-(trimethylsilyl)pent-4-yn-1-yl groups and a comonomer functionalized with azidomethyl groups, producing the novel fullerene polymers P1-C60 and P2-C60. Despite their extremely high fullerene loading levels, the polymers were soluble in common organic solvents and exhibited no aggregation of fullerene units in films. Moreover, devices containing these fullerene polymers were easily fabricated with common coating processes that exhibit excellent unipolar and bipolar flash memory characteristics as well as unipolar permanent memory characteristics, with high ON/OFF current ratios, long retention times, and low power consumption. These electrical switching behaviors were favorably operated by electron injection. Overall, these devices are the first n-type bipolar and unipolar digital polymer memory devices which can be operated in flash and write-once-read-many-times modes. Amphiphilic brush-linear poly(4-di(9,9-dihexylfluoren-2-yl)styrene)-b-poly(2-vinylpyridine)s (PStFl2m-b-P2VPn) in various compositions were synthesized, yielding copolymers with a high thermal stability and excellent processability. The immiscibility of the blocks induced the formation of a variety of nanostructures, depending on the composition and fabrication conditions, which differed significantly from the nanostructures observed among common diblock copolymers. Interestingly, the orientations of the nanostructures could be controlled. The nanostructured polymers yielded distinct electrical memory properties when incorporated as the active layer into a digital memory device. Overall, the PStFl2m-b-P2VPn polymers are suitable active materials for use in tunable digital memory devices that can be operated at very low power consumption levels. The devices displayed excellent unipolar switching modes with a high ON/OFF current ratio.
URI
http://postech.dcollection.net/jsp/common/DcLoOrgPer.jsp?sItemId=000001910912
http://oasis.postech.ac.kr/handle/2014.oak/93708
Article Type
Thesis
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