Constrution Of Naphthalimide Derivatives For Multilevel Electronic Memory Applications

With the coming of the era of information explosive,the widely used binary systems that based on "0" and "1" two digital signals cannot meet the requirement of ever-growing information explosive.Due to the limitation of physical factor and process technology,the storage capacity of these traditional memory devices has reached their limitation and it’s very difficult to further increase the density of the memory devices through downscaling.Thus,in order to overcome this problem,scientists are trying to design a multilevel memory device that can increase the storage density exponentially.In 2010,our group first reported a small molecule based ternary memory device,which is a breakthrough of the binary system.Additionally,we also put forward the "charge trap" mechanism,which could be used as the guidance for the synthesis of small molecules with ternary memory behaviors.Under the guidance of "charge trap" mechanism,a series of naphthalimide based conjugated small molecules were designed and synthesized during my Ph.D course.We focused on researching the influence between the performance of organic multilevel memory devices and the number of electron-acceptors,the depth of charge traps and the symmetry of the conjugated backbones,etc.The main research works are as following:(1)Investigation the effect of introducing electron-accepting cyano groups in molecular backbone on multilevel data storage performance:A conjugated organic molecules DPHCANA with electron-rich carbzole moiety and naphthalimide acceptor was first designed and synthesized,and exhibited binary flash memory property.The influence of cyano acceptor in the molecular backbone on the microstructure stacking in film state and the performance of memory devices was systematically investigated.Research results suggest that rationally introducing of cyano acceptor in the molecular backbone formed two distinct charge traps thus exhibited the ternary memory behaviors.Furthermore,the introducing of cyano group in the backbone lowered the LUMO energy level,which was beneficial for the stability of DPCNCANA molecules.Finally,the cyano substitution also improves the ordered stacking in film state through intromolecular or intermolecular H-bond interaction,facilitating the mobility of the charge carriers in the films thus lowering the power consumption and improving the stability of the memory device.(2)Investigation the tuning of electron-acceptors in the molecular backbone to realize quaternary data storage performance:Under the guidance of charge trap mechanism,two different electron-acceptors,benzo[1,2,5]thiadiazole and nitro,were introduced into the backbone to form an symmetrical molecule DNOBTDT with A2-D-A1-D-A2 structure,and it showed the ternary memory property.Subsequently,an electron-deficient moiety naphthalimide with relatively lower electron-accepting capability than nitro was introduced into the backbone of DNOBTDT to design an asymmetrical molecule NONIBTDT with the structure of A2-D-A1-D-A3.These electron-acceptors formed three charge traps with different depth,and the sandwich-structured memory device ITO/NONIBTDT/A1 firstly exhibited the quaternary memory behavior,which not only verified the "charge trap" mechanism we set forth previously,but further increased the data-storage density from 3n to 4n.This work will open a new way for the design of multilevel memory materials through tuning different electron-acceptors in the backbone.Quaternary memory device can match the binary systems much better than ternary memory device,thus it will make the computer operate much faster than we used nowadays.(3)Investigation the electron-withdrawing capabilities of the acceptors in the backbone to realize rewritable multilevel data storage performance:Based on the previous work,a symmetrical molecules with a same benzo[1,2,5]thiadiazole core and two naphthalimide end-capping electron-acceptors,DNIBTDT,was designed and synthesized.The influence of the electron-accepting capability of electron-accepting moieties in the molecular backbone on the performance of memory devices was systematically investigated.Electric measurement showed that DNIBTDT based memory device showed a WORM property from the OFF to ON1 transition,and exhibited flash memory behavior for the ON1/ON2 transition.The research of the relationship of I-V showed that this memory behavior was caused by different electron-acceptors and the various molecular stacking properties in film state.This is the first report about small molecule based memory device with rewritable multilevel memory properties.Meantime,DNIBTDT showed excellent molecular stacking performance in film state and ordered orientation to the substrate;which was beneficial for the mobility of the charge carriers through the films.This will open a way for the future fabrication of fully rewritable multilevel memory devices.(4)Investigation the tune of heteroatom in the molecular backbone on crystalline properties and multilevel data storage performance:Two A-D-A structure organic molecules consisting of the same carbzole donor and naphthalimide acceptor were designed and synthesized.Subsequently,benzothiazole containing one C=N bond and benzotriazole having two C=N bonds that can induce intramolecular H-bond interaction,were introduced as the end-capping acceptors to improve the microstructure of asymmetrical molecules.The research results showed that these two molecules based memory devices both exhibited nonvolatile ternary memory performance.It is worthy to mention that the molecule with the benzotriazole unit showed reduced switching threshold voltages compared with the molecule with benzothiazole group.AFM,XRD,and GISAXS results indicated that the molecule with the benzotriazole unit formed a better crystalline properties due to the two C=N bonds that can result in stronger intramolecular H bond interaction,which could promote the intramolecular packing in the film,thus could effectively reduce the charge carrier injection barrier from the electrode to the organic layer,leading to the reduced threshold voltage.Thus,through rational design,asymmetrical molecule can also perform highly ordered micro structural stacking in film state,and simplifies the synthesis procedure and molecular structure.This work will have a great potential for future portable low-cost nano electronic device applications.