Design And Preparation Of Novel Polymeric Data Storage Materials

Advantages of polymer/organic memories include simplicity in device structure, good scalability, low cost potential, low power operation, multiple state accessibility, three dimensional (3D) stacking capability and large capacity for data storage. There is increasing interest in polymer memories during the last few years, however, the response time, ON/OFF ratio, read cycles and retention time are far beyond practical requirements comparing with inorganic, and especailly silicon based memory devices. Up on this, design and preparation of novel organic/polymeric data starage materials and probe into the mechanisms become the extremely interesting topics in the field of ultra-high density information storage. There are a few emerging area required to study, such as (1) new polymeric functional materials and device films process better electric properties and technic compatibility and (2) further preparation devices based on the materials and/or films. Thus, we need to comprehend, study and develop many technologies, such as molecular design, materials preparation technology, self-assembly technology, device preparation and the mechanism of memory device etc. In order to carry out all of these, it depends on the close cooperation among the scientists who are studying in the field of physics, chemistry, materials and device processes, etc. Base on these, several novel electrical active functional polymeric materials were successfully prepared. Besides, the electrical memory properties of the materials were also studied and some positive and original results were obtained.Chapter 1:The kinds of organic/polymeric memory devices, the mechanisms of the memory effect, the progress of polymeric memory applications using organic/polymeric/hybrid materials as active materials were reviewed and the problems which are still under studying were also discussed in this chapter.Chapter 2:The photoinduced intramolecular processes of a novel polymer, namely bis Dispersed Red 1(DR1) and C6o end-capped poly[9,9-bis(4-diphenylaminophenyl)-2,7-fluorene] (abbreviated as DR1-PDPAF-DR1 and C60-PDPAF-C60) are reported. The polymers are soluble in organic solvents and possesses high thermal stability. The geometric and electronic structures using ab intio B3LYP/6-31G methods show that the fluorene backbone acts as conjugated channel for charge carriers, the lateral triphenylamine groups serve as the electron-donors, and the DR1/C60 terminals serve as electron acceptors. The molecular orbital calculations show the sequential hole-migration along the PDPAF units. The charge-separated configuration of DR1-PDPAF-DR1 was found to be long lived up to 2.3 ms in the polar benzonitrile, as inferred from the studies of the time-resolved emission and absorption techniques in the picosecond, nanosecond and millisecond region, reflecting the significant effect of the charge carrying of the polymer chain on prolonging the charge-separated states. Polymer memory devices based on these two polymers were fabricated. While, only the devices of DR1-PDPAF-DR1 exhibit electrical bistability in the I-V characteristics and can be used to perform read-write-erase memory functions. The memory devices exhibit good performance with an on/off current ratio up to 103 and stable on and off states under a constant voltage stress and read pulses. Furthermore, memory retention tests show that it is possible to preserve both states at 150℃under ambient atmosphere for about 1 h when using Cu as the top electrode.Chapter 3:A nanoaggregated DR1-grafted poly(N-vinylcarbazole) (abbreviated PVDR) is self-assembled viaπ-πstacking interactions of the carbazole groups in the polymer system after adding a solution of PVDR in N,N’-dimethylformamide to dichloromethane. Upon self-assembly, the nanoaggregated PVDR film displays helical columnar stacks with large grain sizes, whereas a non-aggregated PVDR film exhibits an amorphous morphology with smaller grain size. A write-once read-many-times (WORM) memory device is shown whereby a pre-assembled solution of PVDR is spin-coated as the active layer and is sandwiched between an aluminum electrode and an indium-tin-oxide (ITO) electrode. This device shows very good memory performance, with an ON/OFF current ratio of more than 105 and a low misreading rate through the precise control of the ON and OFF states. The stability of the nanoaggregated PVDR device is much higher than that of the non-nanoaggregated PVDR device. This difference in device stability under constant voltage stress can be mainly attributed to the difference in the film crystallinity and surface morphology. No degradation in current density was observed for the ON- and OFF-states after more than one hundred million (108) continuous read cycles indicating that both states were insensitive to the read cycles. These results render the nanoaggregated PVDR polymer as promising components for high-performance polymer memory devices.Chapter 4:A highly soluble polyfluorene-based copolymer containing electron-rich triphenylamine (TPA) and electron-poor 9,9’-bis(3,4-bis(3,4-dicyanophenoxy)phenyl side chains in the C-9 position of the fluorene unit was synthesized under Yamamoto conditions. By applying 306 nm as excitation wavelength, the resultant polymer exhibits strong photoluminescence with maximum emission peaks centered at 413 and 433(sh) nm in chloroform. The calculated highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), energy bandgap, ionization potential, and electron affinity are-5.66,-3.44,2.22,5.92, and 3.70 eV, respectively. The as-fabricated polymer film exhibited typical stable write-once-read-many times (WORM) memory characteristics, which are desirable for ultra low-cost permanent storage of digital images. The currents in both ON and OFF states did not show any degradation, suggesting good device stability. The ON/OFF current ratio observed in the sweep I-V characteristics at+1.0 V is 6.1×103. The conduction mechanism through ITO/polymer/Al device is discussed.Chapter 5:A novel conjugated-polymer-modified graphene oxide (TPAPAM-GO) was, for the first time, synthesized and was successfully used to fabricate a TPAPAM-GO based nonvolatile memory device. This device exhibited a typical bistable electrical switching and nonvolatile rewritable memory effect, with a turn-on voltage of about-1V and an ON/OFF current ratio of more than 103. Both the ON and OFF states are stable under a constant voltage stress and survived up to 108 read cycles at a read voltage of-1.0 V.Chapter 6:The results from chapter 2 to chapter 5 were summerized.