Design And Synthesis Of Nitrogen-containing Conjugated Organic Small Molecules For Multilevel Electronic Memory Applications

With the explosive development of information, current binary data storage systemswith “0” and “1” two digital signals based on optical or magnism materials suffered fromthe challenge that maximum storage capacity is far behind the intense demand for ultrahighdensity data storage nowadays. Compared with binary memory system, the storagecapacity of the same cell size for multilevel memory system could be exponentiallyincreased, therefore, design and synthesis of multilevel data storage materials andfabrication of nanodevices able to record “0”,“1”,“2”,… multi-digital in a single storagecell (ternary, quaternary and…) are strongly recommended. In2010, our research groupfirstly reported the organic molecule-based ternary memory device via tailoring themolecular structures, which are highlighted by ACS Chemical&Engineering website titledas “Data Storage Goes Organic”.On the basis of the achievement in organic ternary memory, a series of conjugatedsmall molecules were designed and synthesized during my Ph.D course. We focused oninvestigating the influence of molecular chemical structure factors, such as the conjugationlength, molecular planarity, depth and number of charge traps, etc., on organic multileveldata storage device performances. The main research works are as following:(1) Investigation of the effect of the number and strength ofelectron-withdrawing groups in molecular backbone on multilevel data storageperformance: A series of conjugated push-pull organic molecules with electron-richtriphenylamine moiety as donor and azobenzene chromophore/cyano group as acceptorswere designed and synthesized. The influence of the number and strength ofelectron-withdrawing group in molecular backbone on devices’ switching behaviors fordata storage was systematically investigated. Research results suggest that the number ofdata storage levels and memory type are highly-dependent on the strength and number ofthe electron-withdrawing units. The memory devices based on the molecule containing two acceptors exhibited excellent ternary memory characteristics. However, the devices basedon the molecules containing one acceptor showed binary storage. Furthermore, the bistablememory effects varied from WORM to Flash when the acceptor changed from a cyanogroup to an azobenzene chromophore. Therefore, tunable multilevel memory performancehas been achieved by adjusting the number and electron-withdrawing strength of acceptormoieties in the molecular backbone, not only providing a solid evidence for our proposed“charge trap” mechanism, but also offering guidance for the rational design of superiormolecules for ultrahigh density data storage applications.(2) Investigation of the molecular length factor on multilevel data storageperformance: Two imide-derived organic small molecules with the sameelectron-donating and accepting groups but different molecular length were designed andsynthesized. Atomic Force Microscope (AFM) and X-ray diffraction (XRD)characterization results indicated that longer molecule formed close packing invacuum-deposition and thermal-annealing process, resulting in the high-performancenonvolatile ternary memory devices. Shorter molecule, however, showed amorphousmorphology and its sandwiched device had no obvious electrical switching behavior. Theseresults suggested that molecular conjugation length and thermal-annealing temperatureplay significant roles in molecular π-π stacking in the solid thin-film, and thus in datastorage devices’ properties. Moreover, writing and read-out of ternary storage signals onthe newly-prepared nano-film through C-AFM conductive probe, from the perspective ofdevices’ microminiaturization, offers a unique opportunity for future fabrication ofsubnano-and even molecular-scale memory devices, and could further improve theinformation storage capacity via further reducing the cell size on the basis of achievingorganic ternary memories.(3) Investigation of the molecular planarity effect on multilevel data storageperformance: Two conjugated azo molecules based on diaryl-ketone scaffold withdifferent co-planarity were designed and synthesized, and the multilevel memory devices’properties were systematically studied. The research results showed that under an appliedbias, sandwiched devices which employed these two molecules as electro-active layersexhibited nonvolatile ternary memory performance. It is worthy to mention that the devicesof the planar molecule showed reduced switching threshold voltages (Vths) compared withthe poorly-planar molecule. AFM, XRD, Transmission Electron Microscope (TEM) and simulation results indicated that the improved planarity of central electron-withdrawinggroup could promote the intermolecular packing in the film, improve thin-film morphology,and effectively reduce the charge carrier injection barrier from the electrode to the organiclayer, thereby leading to the reduced Vths compared with the poorly-planar molecule. Thus,this device has a great potential for future portable low-cost nanoelectronic deviceapplications.(4) Investigation of the impact of incorporating core unit into the molecularscaffold on the storage-levels of memory device: The reported study resultsdemonstrated that the device based on the molecule formed by combination ofnaphthalimide (NI) and carbazole (Cz) functional groups only afforded binary memorybehavior. Herein, novel symmetric bat-shaped A-D-A conjugated molecule were designedand synthesized by introducing a dicyanomethlene-4H-pyran (DCM) unit into themolecular backbone based on NI and Cz groups. The solution-processedsandwich-structure device with this molecule as the active material exhibited outstandingnonvolatile ternary memory performance (such as high ON/OFF ratio, low thresholdvoltages), successfully breaking through original molecule’s bistability behavior andmanifesting that the DCM core segment serving as the “bat’s brain” could control thewhole molecular electrical properties and the corresponding memory device performanceas expected.(5) Investigation of the influence of “charge trap” depth modulation onmultilevel data storage types: In order to fabricate organic multilevel information storagedevices with various memory types and expand their function, linear D-π-A-π-A’conjugated small molecules based on benzothiadizole (BT) skeleton incorporating Cz asdonor and a weak electron-withdrawing pyridine group (Py) as another acceptor weredesigned and synthesized. The three functional groups, Cz, Py and BT, were connectedwith two alkynyl spacers so as to ensure molecular complete-coplanarity. TheITO/molecule/Al sandwiched device exhibited fine-tunable nonvolatile multilevel memoryperformance which varied from binary Flash to ternary WORM storage characteristics asapplied voltages increasing. In addition, from the investigation of end-capping alkyl chainimpact on molecular film-forming property, morphology, and thus memory deviceperformance, a preliminary conclusion could be drawn that two basic requirements of“acceptor diversity” and “good film-forming property” must be met for organic molecules to achieve the organic ternary data storage, which, not only improve the “charge trapping”mechanism proposed in our previous work, but also lay a solid theoretical foundation forthe future fabrication of stable multilevel memory devices.