Tuning the electrical memory characteristics from WORM to flash by α- and β-substitution of the electron-donating naphthylamine moieties in functional polyimides
作者:Lei Shi、Hebo Ye、Wenlu Liu、Guofeng Tian、Shengli Qi、Dezhen Wu
DOI:10.1039/c3tc31369g
日期:——
Two novel functional aromatic polyimides (PIs), 6F-αNA PI and 6F-βNA PI, in which the hexafluoroisopropylidene-diphthalic anhydride (6FDA) serves as the electron-accepting unit and the diphenylnaphthylamine (DPNA) functions as the electron-donating species, were synthesized for memory device applications. The 6F-αNA PI shows distinct electrical bistable states with an ON/OFF current ratio up to 106, and can be switched on bi-directionally with no polarity, which could be applied as the nonvolatile write-once read-many times (WORM) memory. Whereas, the 6F-βNA PI-based memory device exhibits flash type memory characteristics with a switching-on voltage at ca. 1.1 V and an ON/OFF ratio of 104. Both polyimides exhibit good long-term operation stability, survive up to 108 reading cycles with no current degradation, and show ultrafast switching with a response time less than 20 ns. Mechanisms associated with the electrical switching behaviors are discussed on the basis of the experimental and quantum simulation results. The electric-field-induced electronic transition from diphenylnaphthylamine units to hexafluoropropylidene phthalimide units and the subsequent formation of charge-transfer complexes are supposed to be responsible for the observed electrical memory effects. Molecular simulation suggests that α-tethering of the naphthyl group results in more non-coplanar conformation of the DPNA species in the 6F-αNA PI, as compared to that of the β-tethering in the 6F-βNA PI, therefore producing a higher energy barrier that prevents the back charge transfer processes, consequently leading to the WORM vs. flash memory behaviors. The 6F-αNA PI differs from 6F-βNA PI only in the substitution position of the naphthyl group, i.e., α-tethering vs. β-tethering. The distinct memory effects observed here suggest the significance of the electron-donating structures on the memory effects, and the tailorability of the memory characteristics through fine structure adjustment.
我们合成了两种新型功能芳香族聚酰亚胺(PI)--6F-δ±NA PI 和 6F-δ²NA PI,其中六氟异丙亚基二酞酸酐(6FDA)作为电子接受单元,二苯基萘胺(DPNA)作为电子供体,可用于存储器件。6F-αNA PI 显示出明显的电双稳态,导通/关断电流比高达 106,并且可以无极性双向导通,可用作非易失性一次写入多次读取(WORM)存储器。而基于 6F-βNA PI 的存储器件则具有闪存型存储器的特性,其开关电压约为 1.1 V,导通/关断电压约为 1.2 V。1.1 V,导通/关断比为 104。这两种聚酰亚胺都具有良好的长期运行稳定性,可在 108 个读取周期内保持稳定且无电流衰减,并显示出响应时间小于 20 ns 的超快开关特性。根据实验和量子模拟结果,讨论了与电开关行为相关的机理。电场诱导的从二苯基萘胺单元到六氟丙基邻苯二甲酰亚胺单元的电子转变以及随后形成的电荷转移复合物应该是观察到的电记忆效应的原因。分子模拟表明,与 6F-βNA PI 中的δ-拴系相比,萘基的δ-拴系导致 DPNA 物种在 6F-αNA PI 中形成更多的非共面构象,因此产生了更高的能障,阻止了反向电荷转移过程,从而导致了 WORM 与闪存行为。6F-αNA PI 与 6F-βNA PI 的区别仅在于萘基的取代位置,即δ-系链与δ-系链。这里观察到的截然不同的记忆效应表明了电子供体结构对记忆效应的重要影响,以及通过微调结构对记忆特性的可定制性。