Design And Synthesis Of Heteroatom-containing Conjugated Organic Small Molecules For Electronic Memory Device Performance Research

With therapiddevelopmentofinformationtechnologyandeconomyglobalization, the theoretical maximum storage capacity of current binary data storage systems based on “0” and “1” two digital signals are far behind the requirement of big data era. Therefore, pushing through the conventional binary memory limitations, design and synthesis of multilevel data storage materials and nanodevices of multi-conductive states(ternary, quaternary and more) in a single storage cell are becoming the focus of scientists. In 2010, our research group first reported organic molecule-based ternary memory device via tailoring the molecular structural design and synthesis, successfully break through organic multilevel memories’ “zero record”. From the point of molecular design principles, most reported multilevel organic memory materials have been designed to contain “N, O, P, Si, or S” heteroatoms in the conjugated backbone, which are essential to realize multiple conductive states. However, these heteroatoms cause the molecule to be vulnerable to oxidation, moisture, and phase transitions in different environments and long-term stability of systemic aren’t enough, more experimental and theoretical aspects of issues need to be explored and studied. In this report, we have designed and synthesized a series of heteroatom-containing organic materials via tailoring the molecular heteroatom valence and element types or numbers to study the effects of heteroatom in molecules on the device electrical performance and long-term stability. These molecular design and synthesis would lay the foundation for future Multilevel Electronic Memory Applications. The main research works are described as following:1.By regulating the oxidation state of the central group to achieve the molecular charge "trap" depth and memory performance matching:Based our previous studies. A theory we found that the conjugated molecules contain two groups with different electron withdrawing strength is a necessary condition for formation of ternary information storage, but not sufficient condition.Sometimes, different electron-withdrawing groups were chosen blindly, leading to devices based on the designed molecule with two different electron-withdrawing groups that could not exhibit ternary memory behavior. Therefore, we speculate that a suitable trap depth difference between the two electron-withdrawing groupsto realize a ternary data storage material plays a key role.We designed and synthesized three symmetrical conjugated molecules derived from phenothiazine. By gradual oxidation of the sulphur atom in phenothiazine into sulfoxide or sulfone, a differentiated trap depth was achieved between the central sulphur-containing group and the terminal cyano groups. The results show that when the Sulphur atom isn’t oxidation, the cyano group is the only one electron-withdrawinggroup, the PTZ-CN based devices exhibit binary storage. Because the levels of the sulfone and cyano moieties trap depth are almost the same, the PTZDO-CN based devices also exhibit binary memory performance. However, the PTZO-CN based devices exhibit ternary storage, because of the sulfoxide and cyano groups with different depths.Therefore, by adjusting charge trap number and depth in molecular backbone can be achieved tunable multilevel data storage performance. The work has improved and enriched the previously proposed "charge trap" theory.2.Adjustment of central group atomic changesto achieve closely intermolecular stacking and low-power consumption in data-storage.we designed and synthesized three benzochalcogenadiazole derivatives where the chalcogen of the benzodiazole moiety is systematically varied from oxygen to sulfur to selenium and studied the atomic effect on the performance of sandwich structure storage device. The results show that with increasing atomic radius, the molecular planarity, electron affinities, intra- and intermolecular interactions and film morphology etc. are changed.Three compounds based devices all exhibit non-volatile ternary memory characteristics while PBOP showed the lowest switching threshold voltages(1.2 V) and highest Ion/ Ioff ratios(105 and 103). According to thecharacterized by the atomic force microscopy(AFM), X-ray diffraction, single crystal structure analysis and theoretical calculated, the smaller chalcogen atomic radius, resulting in the greater conjugate plane and the stronger intramolecular and intermolecular interactions, which is favourable to formation of closer ordered π-π stacking in the produced film and significantly enhance the smooth of the film surface, thus effectively reducing the barrier of the carrier injection from the electrode to the organic layer, so that compared to PBTP / PBSeP, the switching threshold voltage of PBOP based devices are significantly reduced. It has great guiding significance for the future develop low-power portable electronic nano-devices by changing the atomic radius.3.Adjustment of conjugated molecular fluorine content to achieve long-term environmental stability of the memory devices.1) In this work we synthesized two phenothiazine-cored, cyano-substituted diphenylethene derivatives with and nitro groups(PTZ-CF3 and PTZ-NO2) and the photoelectric property, environmental stability, micro structure of film and performance of electroresistive memory devices based on these two molecules is systematically investigated. Both molecules can realize ternary memory data storage with distinct long-term stability and device yields. the PTZ-CF3 molecules introduction of trifloromethyl group into molecular backbone can achieve better solubility, lower moisture absorption, lower surface energy, higher chemical resistance, higher thermal stability, higher chemical resistance compared to the PTZ-CF3 compounds.Such superior performance can be attributed to the intra- and intermolecular F…H-C,F…F interactions in solid film,which leads to a better solubility, fim-forming ability, higher thermostability and lower surface energy facilitating the electron injection and formation of closer ordered π-π stacking, resulting in PTZ-CF3 molecular devices switching voltage decreased obviously. Furthermore, the introduction of fluorine atoms increased the water contact Angle, improved the film hydrophobic, facilitating improvement of device long-term stability in the environment. These results may serve as a guide to improve the performance and promote the development of organic memories.2) To study the fluorine content factor on memory device performance, four azo-imide derived organic small molecules with the same electron-donating and accepting groups but different fluorine number were designed and synthesized, named as IDAZO、FIDAZO、F2IDAZO and F4 IDAZO. The photophysical, electrochemical properties and switching behaviors of the four molecules based devices were comparatively investigated. Atomic Force Microscope(AFM) and X-ray diffraction(XRD) characterization results indicated that with increasing the number of fluorine atoms in the molecule, nanofilm morphology gradually becomes smoother, nano-particle size and layer by layer stacking distance between molecules become smaller. The four compounds based devices all exhibitnonvolatile ternary memory performance, but the F4 IDAZO based devices exhibit the lowest switching voltage and the best long-term stability. Note that the UV absorption peaks of four molecules in the solid states showed a larger bathochromic shift than that of solvent states, and the more fluorine atom, the greater red shift, indicating that fluorine containing molecules stack in a more orderly and closer fashion, which would be beneficial for improving the charge-carrier mobility through the films. This comparative study of tuning the properties of azo-imide conjugated molecules via change the substituted fluorine number may be an alternative approach for the design of future advanced organic electronic devices.