Push-Pull Archetype Of Functional Polymer Materials For Bistable Electrical Switching And Nonvolatile Memory Devices

The molecular structure of polymers can be tailored by functionalizing them with electron donors and acceptors of different strengths, spacer moieties of different steric effects for the electroactive pendant groups, or nanostructured electroactive materials, to induce different memory behaviors in simple metal/polymer/metal devices. In comparison to inorganic materials-based memory devices, polymer memories are proposed to revolutionize electrical applications by providing extremely inexpensive, lightweight, and transparent modules that can be fabricated onto plastic, glass, or the top layer of CMOS hybrid integration circuits. On the other hand, with all the advantages including ultraminiature circuitry, mechanical flexibility, high data storage capacity through three-dimensional stacking, high operating speed and ON/OFF ratio, low power consumption, and the non-exotic nature of carbon raw material, over the current state-of-the-art memories and other upcoming technologies, graphene and its derivatives promise themselves great potential as an alternative, or at least a supplement, to silicon for the next generation information storage applications. In this thesis, by introducing graphene oxide (GO). reduced graphene oxide (RGO) and metal nanoparticles into the polymer systems. we designed and synthesized a series of donor-acceptor type polymer memory materials, and investigated their basic structures, nonvolatile memory performance and memory mechanisms as well. This Ph.D. thesis was divided into seven chapters, as follows:In chapter one, the basic concept and the memory mechanism of resistive polymer memory, as well as the progress of resistive polymer memory materials and the problems urgently to be solved were reviewed. On the basis of these reported literatures, the research topic and the main contents of the thesis were put forward.In chapter two. poly(N-vinylcarbazole)(PVK) covalently functionalized GO (GO-PVK) has been synthesized via "grafting to" and "grafting from" methods, respectively. In the first, GO-PVK was synthesized by reaction of carboxyl terminated PVK with GO-toluene-2.4-diisocynate (GO-TDI). The second approach is to grow PVK chains directly from the surface of GO by using reversible addition fragmentation chain transfer (RAFT) polymerization. The memory performance of the materials were discussed in the AI/GO-PVK/ITO structure.In chapter three, two conjugated polymers covalently functionalized GO materials:GO-PFCz and GO-PANI were synthesized, respectively. The basic structures, surface morphology, and memory effects of the materials were characterized. In the GO-PFCz based memory device, electrical field-induced charge-transfer (CT) from the polymer donor to the GO acceptor gives rise to a conductive CT state, resulting in the electrical transition from the initial OFF state to the ON state. However, a reverse bias can dissociate the CT state and reset the device back to the initial OFF state. The electrical field-induced CT process is supported by fluorescence quenching and recovery in the in-situ fluorescence spectra of the GO-PFCz film under electrical biases. The successful in-situ synthesis of the GO-PANI composites was confirmed by IR spectra, Raman spectra, XPS analysis and TEM. After growing PANI from the surface of GO, spindle-shaped PANI nanofibers with large length-to-diameter ratios appear to surround the GO nanosheets. The Al/GO-PANI/ITO device was fabricated and exhibited typical bistable electrical switching and nonvolatile rewritable memory effect, with an ON/OFF current ratio in excess of104.In chapter four, two solution-processable RGO-based functional polymer materials (RGO-PFTPA and RGO-PFCF) were synthesized by the1,3-dipolar cycloaddition between^-conjugated polymers with aldehyde groups, N-methylglycine and RGO. Bistable electrical switching and nonvolatile rewritable memory effects were demonstrated in a sandwich structure of Al/RGO-polymer/ITO. The switch-on threshold voltages of the two devices are comparable, while the switch-off threshold voltage and ON/OFF current ratio of the RGO-PFCF device are slightly larger than those of the RGO-PFTPA device. Incorporation of carbazole groups into the RGO-PFCF molecule were believed to account for the differences of electronic performance, the carbazole groups have strong electron-donating ability, further enhanced the charge transfer effects between RGO and electron-donating polymer, thus facilitated the electrical transition from the OFF state to the ON state.In chapter five, two poly(N-vinylcarbazole) derivatives with pendant donor-trap-acceptor (D-T-A) structures, PVK-AZO-2CN and PVK-AZO-NO2. were synthesized. The Al/polymer/ITO devices exhibit write-once read-many-times (WORM) memory effects with a switching threshold voltage of less than-1.9V and an ON/OFF current ratio of more than105. The electrical bistability and memory effects in the present devices are attributed to the electric-field-induced charge transfer between the carbazole electron donor and the terminal nitro or cyano electron acceptor entities, and subsequent charge trapping at the intermediate azobenzene chromophores. The proposed switching and conduction mechanism is supported by the molecular computation results. UV-vis spectra, XPS. and TEM images of the polymer thin films. The influence of the charge trapping effect of the azobenzene mediator is further explored by studying the electrical and electronic properties of the other two PVK derivatives in which the nitro or cyano acceptors are directly bonded to the carbazole donor moieties as control samplesIn chapter six. monodisperd hairy Au@air@PTEMA-g-P3HT hybrid nanorattles. with a movable Au nanocore in the cavity and electroactive P3HT polymer brushes on the exterior surface, have been synthesized by selective etching of the silica inner shell of the hairy Au@SiO2@PTEMA-g-P3HT core-double shell nanospheres, prepared from combined sol-gel reaction, distillation-precipitation polymerization and oxidative graft polymerization. The Au@air@PTEMA-g-P3HT hybrid nanorattles can be readily dispersed in toluene and uniformly integrated into polystyrene (PS) thin films. The memory performance of the Al/Au@air@PTEMA-g-P3HT+PS/ITO devices can be tuned by varying the Au@air@PTEMA-g-P3HT content (5,10,25and50wt%) in the active polymer layer. The switching mechanism in the present device can be ascribed to the field-induced charge transfer between the electron-donating PTEMA-g-P3HT shells and the electron-accepting Au nanocores.In chapter seven, conclusions and prospective were given.