Strategies of Manipulating D-A Molecular Conjugated Plane and Their Effects on Multilevel Memory Device Performance
Supervisor:lu jian mei
With the arrival of big-data era and rapid development of information techmology,a new information-storage technology with larger capacity,fast speed,and higher data-storage density has been the goal of future data-driven computation.Currently,the theoretically maximum capacity of binary storage systems based on“0”and“1”signals lags far behaind the ultrahigh-density requirement of upcoming information storage.Therefore,design and synthesis of multilevel memory materials with multi-stage(e.g.“0”,“1”and“2”)read-out signals has been considered to be the forefront.In 2010,our group firstly demonstrated an organic small-molecule-based ternary memory and proposed the rational“chage-trapping”mechanism,which provides an important guideline for design and synthesis of ultrahigh-density data-storage device.Most recent studies of organic multilevel memory are based on D-A type molecules.However,whereas most research efforts are devoted to report the novel organic multilevel memory materials,few strategies pay attention to the effects of rational D-A molecular design on the film packing and device performace.Yet,for D-A type molecules,rationally manipulating their conjugated frameworks is decisive for affording the most efficient molecular stacking in films,which is a key parameter to obtain optimal device performance.Such investigation of strucuture-packing-performance relationship is critical for preparing high-performance organic-based multilevel memory materials in the future.In this dissertation,a series of D-A type organic conjugated small molecules were designed and synthesized.We focused on examining the strategies of manipulating D-A molecular conjugated plane,such as molecular plarnarity,conjugated length and symmetry,and investigating their effects on the molecular crystalline orientation in films as well as the corresponding multilevel memory device performance.Our work was carried out from the standpoint of the following six aspects:(1)Investigating the effect of tailoring molecular planarity on multilevel memory device performance:For the organic memory device with vertically arranged electrodes,controlling the film-packing to achieve highly oriented crystallite arrangement is critical for obtaining the satisified performance,but there is still no reliable guideline for molecular design.Here,the effect of backbone planarity on the crystallite orientation is studied.Two A1-D-A2-D-A1 molecules are synthesized with increasing planarity by furan substitution for phenyl rings.The orientations of these crystallites in films are demonstrated to be well controlled through tailoring molecular planarity.The nonplanar molecule displays less ordered packing with a broad orientation distribution relative to the substrate while the highly planar molecule in film prefers out-of-plane crystallite orientation with respect to the substrate normal,which favors uniformity in thin film,thus,the device displays higher reproducibility of memory effects.Therefore,the backbone planarity could be increased by tailoring steric effect between donor and acceptor,which both enhances the molecular packing and the multilevel memory device peformance.(2)Investigating the effect of tuning molecular conjugated length on multilevel memory device performance:Practical application of organic memory devices requires low power consumption and reliable device quality rather than prototype demonstration.In this study,we discovered that inserting thienyl units in the D-A molecules could improve these device parameters via tuning film texture.Theoretical calculations reveal that introducing thienylπbridges increases the D-A effective conjugated length.Thus molecules with more thienyl spacers in film state become well-stacked and highly-oriented relative to the substrate,which show enhanced ternary memory behaviors with lower threshold voltages and better repeatability.The conductive switching and variation trend of device performances were interpreted by an extended charge-trapping mechanism.Our study illustrates an efficient molecular engineering design that can control the crystallite orientation in solid state to achieve superior multilevel memory performanc.(3)Investigating the effect of modulating molecular symmetry on multilevel memory device performance:For the organic resistive memory device,how to establish the strucuture-packing-performance relationship remains challenging,which hinders the development of multilevel memory materials.Herein,an exceptional organic layer transition from randomly-oriented state to highly-aligned crystallization is facilely realized by asymmetric-to-symmetric modification.More importantly,the molecular specific packing is clearly detected,which provides promising opportunities to creat the strucuture-packing-performance relationship.Whereas the asymmetric molecule shows an isotropic texture,the symmetric molecule conforms to a highly-oriented out-of-plane laminar crystalline motif,which demonstrates excellent multilevel memory character and device reproducibility.This strucuture-packing-performance relationship provides an important design guideline to actuate high-performance multilevel memory circuits.(4)Investigating the effect of photoelectrical cooperatation on multilevel memory device performance:Currently,the integration of multi-functionalities into a microelectronic platform has been regarded as an alternative solution for increasing information storage capacity.However,recent strategies are usually confined to a single electrical stimulus,which largely hinder the exploration of combining multiple physical channels(e.g.,optical,electrical,and magnetic multi-input)into one electronic device.Herein,a photoelectrical cooperative strategy is proposed to effectively modulate the performance of multilevel data storage device.Through taking advantages of organic photoelectronic molecule as storage media,the fabricated device exhibits enhanced working parameters under the action of both optical and electrical input,with decreased operating voltages and increased ON/OFF current ratios.This study reveals an encouraging prospect for achieving multi-functionalities integrated high-density data storage devices.(5)Investigating the potential of large 10-ring fused N-heteroacene in multilevel memory device application:As revealed by the above studies,D-A type molecules with large molecular palanarity and high conjugation is favorable for proming charge transport efficiency.In this work,we synthesize a 10-ring fused N-heteroacene that possess large molecular palanarity and high conjugation,and investigate its potential application in multilevel memory materials.The diverse functionalities of large N-heteroacenes continue pushing their strategic synthesis and application in organic electronic field.Herein,we report a novel large stable pyrene-containing N-heteroacene with ten linearly-annulated rings in one row.Remarkably,it exhibits excellent tri-state resistive memory property with ternary device yield as high as 78%and retention stability longer than 10~4 s.To the best of our knowledge,it is the first demonstration of organic multistate memory device based on large N-heteroacene(n≥10),which provides guidelines for designing more proof-of-concept larger N-heteroacene based multilevel memory electronics.(6)Investigating the number and strength of acceptors in N-heteroacene and their effects on multilevel memory device performance:On the basis of the previous work,here we further investigate the number and strength of acceptors in N-heteroacene and their effects on multilevel memory device performance.We reported a thiadiazoloquinoxaline(TQ)-based D-A type N-heteroacene,which possesses two different types of acceptors namely as pyrazine and TQ.Meanwhile,we also synthesized two counterparts with similar structures,which only have one type of acceptor as pyrazine and TQ,respectively.Compared with its counterparts that show FLASH-type binary memory behaviors,N-heteroacene with two different types of acceptors exhibits nonvolatile WORM-type ternary memory effects.These results suggest that rationally tailoring the number and strength of acceptors within N-heteroacenes can effectively modulate the multilevel memory types,which provides a novel design guideline for preparing ultrahigh density data-storage devices.