Preparation Of Electrical Bistable Memory Materials Based On Porphyrinated Polyimides And The Mechanism Investigation

Since the beginning of of the 21st century,the explosive growth of information and continuous miniaturization of electronic devices make higher requirements for the development of memory material technology.The device linewidth and memory site of memory device based on traditional silicon-based memory materials have approached their upper limits of application.Therefore,it is a crucial problem to be solved urgently for researchers to develop novel memory materials and technologies possessing characteristic of high density and rapid memory storage.Organic memory materials,owning advantages of low cost,flexibility,easy process,3D stacking ability and adjustable performance through chemical structure designing,become the research hotpots.What’s important,the functional unit size of memory meterials with intrinsic memory behavior is below several nanometers,leading to the significantly enhanced memory capacity of devices.Among organic materials,polyimide memory materials based on field-induced charge transfer mechanism possesses excellent high-temperature resistance,chemical stability and well dimensional stability,which makk them one of the most potential materials for applications.In these years,through designing of chemical structures,the polyimides bearing electron donor-acceptor(D-A)structure have shown dynamic random access memory(DRAM),static random access memory(SRAM),flash and write-once-read-many(WORM)memory behaviors.However,there are still several crucial problems.Firstly,the type of electron donor or acceptor is limited and it is difficult to design and synthesize novel ones,leading to the poor adjustable memory performance.The strategy of regulating memory behaviour of polyimide by simple chemical decoration is still lack.And during the investigation,the established relationship between chemical structure and memory behavior of polyimides is not clear enough.There are still some ignored and unconsidered influencing factors.It is necessary to introduce more characterizations and rational theoretical simulation to propose corresponding memory mechanism and perfect the relationship between chemical structure and memory performance.What’s more,compared with other organic materials,polyimides based on field-induced charge transfer mechanism are hard to exhibit ternary even multilevel memory behaviors,which limits its application potential in ultra-high density information storage technology.Porphyrin,a type of compounds with conjugated system of 26 electrons,owns good electron donating ability.There are multiple structural modification sites at porphyrin ring,which is feasible to attach other decorated groups or be aminated as monomer for the polymerization of polyimide.What’s more prominent,a variety of metal ions could be complexed into central cavity in porphyrin.Its electronic structure could be regulated flexibly in a wide range by changing the type of metal ion,leading to distinct characteristics and various applying situations.In this paper,based on the above properties of porphyrin,it is employed as electron donor.We expect to flexibly and broadly regulate the electronic structure of porphyrinated polyimides and realize precise control of their memory behavior by limited chemical decoration,and then propose rational memory mechanism and establish corresponding relationship between chemical structure and memory performance.We design and prepare three series of porphyrinated polyimides and test their optical,electrochemical and memory behavior characterizations.And combining the molecular simulation on the ground and excited states of the model compounds,the influence and regulation mechanism of type,content of metal ion and spatial position of metallporphyrin group on memory performance are investigated.The research content and achievements of this paper are divided into three parts as followed:(1)A series of porphyrinated polyimides(DATPP-DSDA and M-DATPP-DSDA,M=Zn,Cu,Ni,Co,Mn)were prepared by polymerization of 3,3’,4,4’-diphenyl sulfonetetracarboxylic dianhydride(DSDA)and porphyrinated diamine(DATPP and M-DATPP).Optical and electrochemical propertied indicate that,for the electronic structure and excitation process,Cu2+/Ni2+ and Zn2+/Co2+make different moderate effects,respectively,but a distinct role for Mn2+.Current-voltage curves demonstrate that DATPP-DSDA,Cu-DATPP-DSDA and Ni-DATPP-DSDA exhibit non-volatile WORM memory behaviors,Zn-DATPP-DSDA and Co-DATPP-DSDA possess volatile SRAM and DRAM performance,but Mn-DATPP-DSDA didn’t own memory property.Molecular simulation indicates that,compared with DATPP-DSDA,the introduction of Cu2+and Ni2+could make few effect on charge transfer process of porphyrinated polyimides:Zn2+and Co2+play a metal "bridge" role in enhancing the hybridization of charge transfer and local excitations and promoting the electrons to migrate between donor and acceptor,endowing polyimides with volatility;However,the Mn2+could play a metal "trap" role.Its electron-deficiency feature hinders the electrons from being excited to electron acceptor,resulting in the failure formation of charge transfer complex and the lack of memory performance.(2)Five porphyrinated copolyimides coPI-Znx(x=5,10,20,50,80)containing different contents of zinc ion were prepared by copolymerization of DSDA and two porphyrinated diamines(DATPP and Zn-DATPP)with different mole ratios to investigate the influence of proportion of "zinc bridge"effect on the memory behaviors.The characterizations of optical performance and electrochemical behavior demonstrate that the electronic structures and excitation processes of coPI-Znx could be slightly regulated by altering the content of Zn2+.Measurement of memory performance indicates that all of several coPI-Znx exhibit volatile memory behaviors,and the only difference is the retention ability of its high conductivity state.When the proportion of"zinc bridge" effect is relatively low(coPI-Zn5 and coPI-Zn10),the memory devices possess DRAM behavior with relatively short retention time(about 20 and 30 seconds);When the proportion of "zinc bridge" effect is relatively high(coPI-Zn20,coPI-Zn50 and coPI-Zn80),the memory devices possess SRAM behavior with relatively long retention time(about 150,300 and 380 seconds).Molecular simulation indicates that the "zinc bridge" effect could produce extended influence on adjacent repeat unit containing empty porphyrin and promote the occurance of its charge transfer process.When the content of Zn2+of coPI-Znx increases,the more charge transfer process controlled by "zinc bridge" effect is formed and the established "zinc bridge" is more stable.And then the dissociation of charge transfer complex needs more time,leading to the longer retention time of coPI-Znx.(3)The diamine containing covalently linked porphyrin-triphenylamine(TPA)moiety as well as its zinc complex(TPA(P)and TPA(ZnP))was synthesized.Then four polyimides containing pendant porphyrin group,TPA(P)-6FDA,TPA(ZnP)-6FDA,TPA(P)-DSDA and TPA(ZnP)-DSDA were prepared through polymerization of the two diamine and two electron acceptors,DSDA and 6FDA(4,4’-(hexafluoroisopropylidene)diphthalic anhydride).Measurements of memory behavior indicate that TPA(P)-6FDA and TPA(P)-DSDA exhibit typical flash and WORM electrical bi-stable memory performance,respectively.When introducing "zinc bridge" effect into the pendant porphyrin group,TPA(ZnP)-6FDA and TPA(ZnP)-DSDA possess novel DRAM/flash and DRAM/WORM ternary memory behaviors,respectively.Characterizations of electrochemical performance and molecular simulation of the ground state of model compound demonstrate that Zn2+could effectively weaken the electronic interaction between TPA and zinc porphyrin,which causes that under external fields the two moieties are inclined to independently participate in electron transfer process.The molecular simulation of the excited states indicates that the "zinc bridge"effect could substantially reduce the degree of participation of TPA to characteristic local excitation of porphyrin,which could also prove this inference.Therefore it is considered that under electrical filed,the "zinc bridge" effect could activate the charge transfer process in electron donor and lead to the appearance of extra intermediate conductivity for memory device,endowing polyimides with novel ternary memory behaviors.