Synthesis Of Nitrogen-containing Electronic Memory Materials Based On D-A Conjugated Frameworks And Property Manipulation Through Structural Design

With the explosive development of information, a large gap emerges between the theoretically maximum capacity of current binary storage systems based on “0” and “1”and the ever-increasing requirement for ultrahigh density data storage. Therefore, design and synthesis of multilevel data storage materials and nanodevices of multi-stable(ternary, quaternary and more) states in a single storage cell are becoming more and more significant. In 2010, our group first reported an organic molecule-based ternary memory device via tailoring the molecular structures, demonstrating the groundbreaking results of organic-based ternary memory materials. On the other hand, although many materials based on D-A skeletons have been demonstrated to show memory properties, the relationship between the molecular structure and memory behaviors was not clearly revealed. There are still many puzzles yet to be explored, the answer of these puzzles will help the design of future organic memory materials.In this dissertation, random copolymers with different composition and a series of conjugated small molecules were designed and synthesized. We focused on investigating the influence of copolymer composition and molecular structure, such as the terminal acceptor strength adjustment, the D-A arrangement design and the aromatic spacer variation, on the performance of fabricated polymeric/organic data storage device. Our work was performed in the following research points:(1) Incorporation of dual mechanisms to obtain ternary data storage devices: In the designed copolymers of poly(9-(4-vinyl benzyl)-9H-carbazole)(PVCz)-random-poly(1-(4-nitro-azo-phenyl)-pyrrole-2, 5-dione)(PMIDO3), carbazole donor and nitro-azobenzene acceptor are introduced into the lateral chains to induce charge transfer under an electric field and the resulting copolymers have a progressive increase in the nitrogen content to induce filamentary conduction. The fabricated devices with simple sandwich-like configuration distinctively exhibit three conductivity states when a negative bias was applied which can be encoded as “0”, “1” and “2” for future ternary data storage. It is worth noting that the switch-OFF voltages and OFF-currents are significantly affected by the MIDO3 content within the polymers. The experimental results indicate that charge transfer between the donor and acceptor is responsible for the first switching, and the second switching can be attributed to the filamentary rupture. This approach of achieving multi-stable states through combination of different switching mechanisms in one device may provide a strategy for the design and selection of multilevel electric memory materials in future research.(2) Investigation of the terminal acceptor strength on multilevel device data storage performance: Triphenylamine(TPA) donor based molecules with progressively weaker terminal acceptors strength(i.e., nitro, acetyl and bromine) were synthesized, and their photophysical and electrochemical properties and memory characteristics were studied. The influence of terminal electron acceptor strength on the film morphology and the devices storage performance was investigated. Nonvolatile ternary(“0”, “1” and “2” states) memory devices for high-density data storage could be achieved with a simple ITO/D-A molecule/Al sandwich-like configuration for TPA-NAP and TPA-AAP. It is noteworthy that memory device based on TPA-AAP exhibits a better reproducibility and stability with lower operation voltages than TPA-NAP, promising low-power consumption in data-storage. These obtained results demonstrate that it is able to adjust the film morphology and the device performances via altering the terminal electron accepting strength in D-A molecules, which is useful for the design of future advanced organic electronic devices.(3) Investigation of effect of D-A arrangement on data storage performance: Two D-A molecules NACANA and CANACA, based on carbazole(CA) donor and naphthalimide(NA) acceptor, with different D-A arrangement(A-D-A and D-A-D) were synthesized. The photophysical and electrochemical properties, microstructure and memory behaviors of both A-D-A and D-A-D molecules were systematically investigated. The fabricated devices ITO/NACANA or CANACA layer/Al with a simple sandwich configuration both exhibited volatile nature after removing the external electric field. Interestingly, NACANA showed an ON-state retention time of ca. 12 min, longer than that of CANACA(ca. 6 min). The difference in retention ability of the programmed states could be assigned to the difference of the D-A arrangement. This type of retention ability adjustment by varying the arrangement of donor and acceptor segments may provide a guide of structure design for future organic-based specific memory devices with tunable volatile property.(4) Investigation of the aromatic spacer between donor and acceptor units on memory device performance: Two imidazole-π-triphenylamine derivatives TPAPPI and TPATPI, linked by different aromatic spacers(i.e., phenyl or thienyl) were synthesized. The photophysical, electrochemical properties and memory behaviors of the two donor-π-acceptor molecules were comparatively investigated.The substitution of phenyl with thienyl leads to much better nanoscale morphology after thermal treatment, as characterized by the atomic force microscopy(AFM). Sandwich devices based on TPAPPI and TPATPI both exhibited the nonvolatile WORM characteristic but the TPATPI-based device showed a higher ON/OFF ratio and a lower switching voltage. Simulation results showed that the insertion of the thienyl spacer between the donor and acceptor moieties leads to smaller torsion between the imidazole ring and TPA moiety, which indicates a smaller charge transfer barrier and a higher extent of charge transfer(CT).This comparative study of tuning the properties of conjugated D-π-A molecules via aromatic π-spacers may be an alternative approach for the design and study of future high-performance memory devices based on new D-π-A type materials.