Griffin Karl, Redmond Gareth
School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland.
ACS Appl Mater Interfaces. 2024 Jan 17;16(2):2539-2553. doi: 10.1021/acsami.3c13735. Epub 2024 Jan 4.
In this work, the discovery of volatile memristive devices that exhibit analog resistive switching (RS) and synaptic emulation based on squaraine materials is presented. Specifically, organic microtubes (MTs) based on 2,4-bis[(4-(,-diisobutyl)-2-6-hydroxyphenyl]squaraine (SQ) are prepared by evaporation-induced self-assembly (EISA). The MTs are ca. 2 μm in diameter (aspect ratio: 10-130). While powder X-ray diffraction data for MTs identify monoclinic and orthorhombic polymorphs, optical data report the monoclinic phase with energetic disorder. By favorable energetic alignment of the Au work function with the SQ HOMO energy, unipolar (hole-only) symmetric metal-insulator-metal devices are formed by EISA of MT meshes on interdigitated electrodes. The DC characteristics acquired exhibit pinched hysteretic loops, indicative of memristive behavior. Analysis indicates Ohmic transport at low bias with carrier extraction by thermionic emission. At high bias, space-charge-limited conduction in the presence of traps distributed in energy, enhanced by a Poole-Frenkel effect and with carrier extraction by Fowler-Nordheim tunneling, is observed. These data indicate purely electronic conduction. hysteresis attenuates at smaller voltage windows, suggesting that carrier trapping/detrapping underpins the hysteresis. By applying triangular voltage waveforms, device conductance gradually increases sweep-on-sweep, with wait-time-erase or voltage-erase options. Using square waveforms, repeated erase-write-read of multiple distinct conductance states is achieved. Such analog RS behavior is consistent with trap filling/emptying effects. By waveform design, volatile conductance states may also be written so that successive conductance states exhibit identical current levels, indicating forgetting of previously written states and mimicking the forgetting curve. Finally, advanced synaptic functions, i.e., excitatory postsynaptic current, paired-pulse facilitation, pulse-dependent plasticity, and a transition from short- to long-term memory driven by post-tetanic potentiation, are demonstrated.
在这项工作中，展示了基于方酸材料的具有模拟电阻开关（RS）和突触仿真功能的挥发性忆阻器件的发现。具体而言，通过蒸发诱导自组装（EISA）制备了基于2,4-双[(4-(,-二异丁基)-2-6-羟基苯基]方酸（SQ）的有机微管（MTs）。这些微管的直径约为2μm（长径比：10 - 130）。虽然微管的粉末X射线衍射数据确定了单斜和正交多晶型，但光学数据表明单斜相存在能量无序。通过使金的功函数与SQ的最高占据分子轨道（HOMO）能量实现有利的能量对齐，通过在叉指电极上对微管网格进行EISA形成了单极（仅空穴）对称金属-绝缘体-金属器件。所获得的直流特性呈现出收缩的滞后回线，表明具有忆阻行为。分析表明在低偏压下为欧姆传输，通过热电子发射进行载流子提取。在高偏压下，观察到在存在能量分布陷阱的情况下的空间电荷限制传导，通过普尔-弗伦克尔效应增强，并通过福勒-诺德海姆隧穿进行载流子提取。这些数据表明是纯电子传导。滞后在较小的电压窗口中减弱，这表明载流子俘获/去俘获是滞后的基础。通过施加三角电压波形，器件电导在逐次扫描时逐渐增加，具有等待时间擦除或电压擦除选项。使用方波，可实现对多个不同电导状态的重复擦除-写入-读取。这种模拟RS行为与陷阱填充/清空效应一致。通过波形设计，也可以写入挥发性电导状态，使得连续的电导状态呈现相同的电流水平，表明忘记了先前写入的状态并模拟了遗忘曲线。最后，展示了先进的突触功能，即兴奋性突触后电流、双脉冲易化、脉冲依赖性可塑性以及由强直后增强驱动的从短期记忆到长期记忆的转变。
