Electrochemical rewiring through quantum conductance effects in single metallic memristive nanowires.

Memristive devices have been demonstrated to exhibit quantum conductance effects at room temperature. In these devices, a detailed understanding of the relationship between electrochemical processes and ionic dynamic underlying the formation of atomic-sized conductive filaments and corresponding electronic transport properties in the quantum regime still represents a challenge. In this work, we report on quantum conductance effects in single memristive Ag nanowires (NWs) through a combined experimental and simulation approach that combines advanced classical molecular dynamics (MD) algorithms and quantum transport simulations (DFT). This approach provides new insights on quantum conductance effects in memristive devices by unravelling the intrinsic relationship between electronic transport and atomic dynamic reconfiguration of the nanofilment, by shedding light on deviations from integer multiples of the fundamental quantum of conductance depending on peculiar dynamic trajectories of nanofilament reconfiguration and on conductance fluctuations relying on atomic rearrangement due to thermal fluctuations.

Stability and Bandgap Engineering of In1-xGaxSe Monolayer.

Bandgap engineering of semiconductor materials represents a crucial step for their employment in optoelectronics and photonics. It offers the opportunity to tailor their electronic and optical properties, increasing the degree of freedom in designing new devices and widening the range of their possible applications. Here, we report the bandgap engineering of a layered InSe monolayer, a superior electronic and optical material, by substituting In atoms with Ga atoms. We developed a theoretical understanding of In1-xGaxSe stability and electronic properties in its whole compositional range (x=0-1) through first-principles density functional theory calculations, the cluster expansion method, and kinetic Monte Carlo simulations. Our findings highlight the possibility of modulating the InGaSe bandgap by ≈0.41 eV and reveal that this compound is an excellent candidate to be employed in many optoelectronic and photonic devices.

Molecular dynamics study of the pore formation in single layer graphene oxide by a thermal reduction process.

Nanoporous graphene is considered the next-generation material for reverse osmosis water desalination providing both high water permeability and almost complete salt rejection. The main problem with graphene is the difficulty of synthesizing membranes with a consistent subnanometer pore size distribution. A recently proposed solution involves processing as-grown graphene oxide (GO) monolayers via a mild temperature annealing pre-treatment causing GO functional groups to cluster into small oxidized islands. A following harsh thermal reduction process creates pores only in the small oxidized regions. However, a suitable relationship between the area of the GO islands and the pore dimension is still missing. Here, we study in detail the effects of such a thermal reduction process on the graphene oxide sheet by means of molecular dynamics simulations, particularly highlighting and analysing the process parameters affecting the final pore area. Besides proving that epoxides represent the most suitable functional group to induce carbon removal and, thus, pore generation in reduced GO, we find a twofold way to achieve control over the pore size: tuning the dimension and shape of the initial clustered GO areas or changing the harsh reduction process temperature. An accurate balance of these parameters consistently gives rise to targeted pore dimensions in graphene membranes.

Water-Mediated Ionic Migration in Memristive Nanowires with a Tunable Resistive Switching Mechanism.

Memristive devices based on electrochemical resistive switching effects have been proposed as promising candidates for in-memory computing and for the realization of artificial neural networks. Despite great efforts toward understanding the nanoionic processes underlying resistive switching phenomena, comprehension of the effect of competing redox processes on device functionalities from the materials perspective still represents a challenge. In this work, we experimentally and theoretically investigate the concurring reactions of silver and moisture and their impact on the electronic properties of a single-crystalline ZnO nanowire (NW). A decrease in electronic conductivity due to surface adsorption of moisture is observed, whereas, at the same time, water molecules reduce the energy barrier for Ag+ ion migration on the NW surface, facilitating the conductive filament formation. By controlling the relative humidity, the ratio of intrinsic electronic conductivity and surface ionic conductivity can be tuned to modulate the device performance. The results achieved on a single-crystalline memristive model system shed new light on the dual nature of the mechanism of how moisture affects resistive switching behavior in memristive devices.

Controlled Pore Generation in Single-Layer Graphene Oxide for Membrane Desalination.

Nanoporous graphene was proposed as an efficient material for reverse osmosis water desalination membranes because it allows water molecules to pass at high flux while rejecting hydrated salt ions. Nevertheless, from an experimental point of view it is still difficult to control the pore size. A scalable method to generate pores is urgently required for the diffusion of this technology. We propose, by theoretical calculations, an innovative and scalable strategy to better control the dimension of the pores in graphene-based membranes by reduction of single-layer graphene oxide (GO). The latter is first annealed at a controlled mild temperature to induce the aggregation of its randomly distributed oxygen-containing functional groups into small nanometric clusters. The layer then undergoes a high-temperature reducing treatment that causes the desorption of the functional groups along with carbon removal only in the oxidized areas, producing subnanometric pores while leaving unchanged the remaining pristine graphene areas.

Unveiling the Fundamental Role of Temperature in RRAM Switching Mechanism by Multiscale Simulations.

Even though resistance switching memories (RRAMs) can be potentially employed in a broad variety of fields, such as electronics and brain science, they are still affected by issues that prevent their application in circuitry. These problems are a consequence of the lack of detailed knowledge about the physical processes occurring in the device. In this work, we propose multiscale simulations, combining kinetic Monte Carlo and finite difference methods, to shed light on the yet-unclear switching process occurring in the valence change RRAMs, which are believed to work as a consequence of the drift and diffusion of crystalline defects that act as dopants. Results show that the height of the defect diffusion barrier influences the switching process, the retention, and the switching time. In particular, nonvolatile switching can be achieved only by means of the fundamental role of temperature variations induced by Joule heating if the diffusion barriers of the defects are larger than ∼1 eV. High barriers prevent defects from hopping when no voltage is applied. During the transition from the high-resistance to the low-resistance state of the device, a heating stage of the material precedes the defect drift because the applied electric field by itself is not enough to lead to a drift velocity such that switching is achieved within microseconds. The temperature increase has, therefore, the double effect of activating the motion of the defects and enhancing their drift velocity. The switching process can occur only if a sufficiently high temperature is reached thanks to the Joule effect. On the basis of these findings, the RRAM design could aim at a better temperature management to achieve at the same time reproducibility and reliability.

MoS2 Enhanced T-Phase Stabilization and Tunability Through Alloying.

Two-dimensional MoS2 is a promising material for nanoelectronics and catalysis, but its potential is not fully exploited since proper control of its multiple phases (H, T, ZT) and electronic properties is lacking. In this theoretical study, alloying is proposed as a method to stabilize the MoS2 T-phase. In particular, MoS2 is alloyed with another material that is known to exist in a monolayer MX2 T-structure, and we show that the formation energy difference among phases decreases even for low impurity concentrations in MoS2, and a relationship between impurity concentration and alloy band gap is established. This method can be potentially applied to many two-dimensional materials to tune/enhance their electronic properties and stabilities in order to suit the desired application.

A New Theoretical Insight Into ZnO NWs Memristive Behavior.

Resistive switching memory operation is generally described in terms of formation and rupture of a conductive filament connecting two metal electrodes. Although this model was reported for several device types, its applicability is not guaranteed to all of them. On the basis of density functional theory calculations, we propose a novel switching mechanism suitable to nanowire-based resistive switching memories. For thick devices in particular, the current is highly unlikely to flow through a metallic filament connecting the electrodes. We demonstrate that in the case of ZnO nanowires metal adatoms, spread on the nanowire surface, locally dope the insulating oxide allowing surface conductance even for small metal concentrations.