Nanometer‐Scale Uniform Conductance Switching in Molecular Memristors

Abstract

energy-efficient, high-density data storage, and computing are the subject of intense research. [1-3] Memristors are promising building blocks for the next generation of electronics because they enable emerging highly efficient computing platforms such as in-memory and neuromorphic computing. [4,5] Of all genres of memristors, nonvolatile oxide materials are believed to be the most successful candidates in terms of their commercial potential. All the oxide memristors in use today are primarily based on filamentary mechanisms, which are inherently nonuniform [6-8] and thus exhibit performance variability and compromised device-yield. [9] In fact, the issue of nonuniformity and stochasticity is not only specific to oxides but is common in other genres of memristors like phase change memories, [10] nitrides, [11] or those working on metal-ion migration. [12-14] One of the problematic features inherent to any filamentary mechanism is electroforming, which is the first-time switching process involving dissolution, injection, and orientation of the active conducting atoms/ions into the dielectric layer. [7,15-17] The forming process usually requires much higher voltage and current than the reading/writing processes, resulting in One common challenge highlighted in almost every review article on organic resistive memory is the lack of areal switching uniformity. This, in fact, is a puzzle because a molecular switching mechanism should ideally be isotropic and produce homogeneous current switching free from electroforming. Such a demonstration, however, remains elusive to date. The reports attempting to characterize a nanoscopic picture of switching in molecular films show random current spikes, just opposite to the expectation. Here, this longstanding conundrum is resolved by demonstrating 100% spatially homogeneous current switching (driven by molecular redox) in memristors based on Ru-complexes of azo-aromatic ligands. Through a concurrent nanoscopic spatial mapping using conductive atomic force microscopy and in operando tip-enhanced Raman spectroscopy (both with resolution <7 nm), it is shown that molecular switching in the films is uniform from hundreds of micrometers down to the nanoscale and that conductance value exactly correlates with spectroscopically determined molecular redox states. This provides a deterministic molecular route to obtain spatially homogeneous, forming-free switching that can conceivably overcome the chronic problems of robustness, consistency, reproducibility, and scalability in organic memristors.

Memristor Circuits for Colloidal Robotics: Temporal Access to Memory, Sensing, and Actuation

“…[ 126 ] In the context of memristor array operation, skipping this pre‐treatment may give rise to faster sequential switching and thus a shorter timing window. Another example is the recently reported “discrete state” memristor whose resistance switches stepwise [ 127,128 ] ; Our simulations, in contrast, describe a continuously changing “analog” resistance. Interestingly, these discrete state memristors may work even better in a memristor array with a more pronounced and abrupt ON/OFF boundary.…”

“…The deceleration thus limits the array's worst-case temporal resolution and is mathematically a direct result of the uniform fixed resistors and constant voltage. As an alternative to the probe station, conductive atomic force microscope has become a common tool for memristor characterization, [128,131,132] offering reduced measurement errors and more importantly, a nanometer-range resolution. [133,134] This, then, translates to a temporal resolution of merely AE0.03 s at t ¼ 400 s in the same benchmarking array simulated.…”

A Marr's Three‐Level Analytical Framework for Neuromorphic Electronic Systems

“…In the last two sections, we have already seen examples of how advanced technologies enable electronic mimics of individual parts of the neural system with demonstrated simple computational functionalities. We do not intend to survey the neuromorphic hardware implementation again as this has been done in numerous reviews, just to name a few, at materials level, [ 48,661–722 ] at device level, [ 10,244,263,723–790 ] at more circuit level, or above. [ …”

The Importance of Electrical Impedance Spectroscopy and Equivalent Circuit Analysis on Nanoscale Molecular Electronic Devices

“…[ 15–19 ] Besides thiols, several other anchoring groups such as amine, isocyanide, cyanide, carboxylic acid, and pyridine moieties, have been investigated to study the effect of interfacial electronic properties. [ 20–24 ] Molecular electronic devices grown via electrochemical reduction of aromatic diazonium salts find an attractive alternative to SAMs‐based tunnel junctions. [ 25–28 ] However, controlling molecules to carbon contact at the nanoscale level remains a challenge.…”