Charge equilibration model with shielded long-range Coulomb for reactive molecular dynamics simulations.

Atomic description of electrochemical systems requires reactive interaction potential to explicitly describe the chemistry between atoms and molecules and the evolving charge distribution and polarization effects. Calculating Coulomb electrostatic interactions and polarization effects requires a better estimate of the partial charge distribution in molecular systems. However, models such as reactive force fields and charge equilibration (QEq) include Coulomb interactions up to a short-distance cutoff for better computational speeds. Ignoring long-distance electrostatic interaction affects the ability to describe electrochemistry in large systems. We studied the long-range Coulomb effects among charged particles and extended the QEq method to include long-range effects. By this extension, we anticipate a proper account of Coulomb interactions in reactive molecular dynamics simulations. We validate the approach by computing charges on a series of metal-organic frameworks and some simple systems. Results are compared to regular QEq and quantum mechanics calculations. The study shows slightly overestimated charge values in the regular QEq approach. Moreover, our method was combined with Ewald summation to compute forces and evaluate the long-range effects of simple capacitor configurations. There were noticeable differences between the calculated charges with/without long-range Coulomb interactions. The difference, which may have originated from the long-range influence on the capacitor ions, makes the Ewald method a better descriptor of Coulomb electrostatics for charged electrodes. The approach explored in this study enabled the atomic description of electrochemical systems with realistic electrolyte thickness while accounting for the electrostatic effects of charged electrodes throughout the dielectric layer in devices like batteries and emerging solid-state memory.

Stable Liquid-Sulfur Generation on Transition-Metal Dichalcogenides toward Low-Temperature Lithium-Sulfur Batteries.

The electrochemical formation of liquid sulfur at room temperature on the basal plane of MoS2 has attracted much attention due to the high areal capacity and rapid kinetics of lithium-liquid sulfur chemistry. However, the liquid sulfur is converted to the solid phase once it contacts the solid sulfur crystals generated from the edge of MoS2. Thus, stable liquid sulfur cannot be formed on the entire MoS2 surface. Herein, we report entire liquid sulfur generation on hydrogen-annealed MoS2 (H2-MoS2), even under harsh conditions of large overpotentials and low working temperatures. The origins of the solely liquid sulfur formation are revealed to be the weakened interactions between H2-MoS2 and sulfur molecules and the decreased electrical polarization on the edges of the H2-MoS2. Progressive nucleation and droplet-merging growth behaviors are observed during the sulfur formation on H2-MoS2, signifying high areal capacities by releasing active H2-MoS2 surfaces. To demonstrate the universality of this strategy, other transition-metal dichalcogenides (TMDs) annealed in hydrogen also exhibit similar sulfur growth behaviors. Furthermore, the H2 annealing treatment can induce sulfur vacancies on the basal plane and partial oxidation on the edge of TMDs, which facilitates liquid sulfur formation. Finally, liquid sulfur can be generated on H2-MoS2 flakes at an ultralow temperature of -50 °C, which provides a possible development of low-temperature lithium-sulfur batteries. This work demonstrates the potential of a pure liquid sulfur-lithium electrochemical system using functionalized two-dimensional materials.

Improved electrochemical conversion of CO2 to multicarbon products by using molecular doping.

The conversion of CO2 into desirable multicarbon products via the electrochemical reduction reaction holds promise to achieve a circular carbon economy. Here, we report a strategy in which we modify the surface of bimetallic silver-copper catalyst with aromatic heterocycles such as thiadiazole and triazole derivatives to increase the conversion of CO2 into hydrocarbon molecules. By combining operando Raman and X-ray absorption spectroscopy with electrocatalytic measurements and analysis of the reaction products, we identified that the electron withdrawing nature of functional groups orients the reaction pathway towards the production of C2+ species (ethanol and ethylene) and enhances the reaction rate on the surface of the catalyst by adjusting the electronic state of surface copper atoms. As a result, we achieve a high Faradaic efficiency for the C2+ formation of ≈80% and full-cell energy efficiency of 20.3% with a specific current density of 261.4 mA cm-2 for C2+ products.

A dual mode electronic synapse based on layered SnSe films fabricated by pulsed laser deposition.

An artificial synapse, such as a memristive electronic synapse, has caught world-wide attention due to its potential in neuromorphic computing, which may tremendously reduce computer volume and energy consumption. The introduction of layered two-dimensional materials has been reported to enhance the performance of the memristive electronic synapse. However, it is still a challenge to fabricate large-area layered two-dimensional films by scalable methods, which has greatly limited the industrial application potential of two-dimensional materials. In this work, a scalable pulsed laser deposition (PLD) method has been utilized to fabricate large-area layered SnSe films, which are used as the functional layers of the memristive electronic synapse with dual modes. Both long-term memristive behaviour with gradually changed resistance (Mode 1) and short-term memristive behavior with abruptly reduced resistance (Mode 2) have been achieved in this SnSe-based memristive electronic synapse. The switching between Mode 1 and Mode 2 can be realized by a series of voltage sweeping and programmed pulses. The formation and recovery of Sn vacancies were believed to induce the short-term memristive behaviour, and the joint action of Ag filament formation/rupture and Schottky barrier modulation can be the origin of long-term memristive behaviour. DFT calculations were performed to further illustrate how Ag atoms and Sn vacancies diffuse through the SnSe layer and form filaments. The successful emulation of synaptic functions by the layered chalcogenide memristor fabricated by the PLD method suggests the application potential in future neuromorphic computers.

Enhanced sieving from exfoliated MoS2 membranes via covalent functionalization.

Nanolaminate membranes made of two-dimensional materials such as graphene oxide are promising candidates for molecular sieving via size-limited diffusion in the two-dimensional capillaries, but high hydrophilicity makes these membranes unstable in water. Here, we report a nanolaminate membrane based on covalently functionalized molybdenum disulfide (MoS2) nanosheets. The functionalized MoS2 membranes demonstrate >90% and ~87% rejection for micropollutants and NaCl, respectively, when operating under reverse osmotic conditions. The sieving performance and water flux of the functionalized MoS2 membranes are attributed both to control of the capillary widths of the nanolaminates and to control of the surface chemistry of the nanosheets. We identify small hydrophobic functional groups, such as the methyl group, as the most promising for water purification. Methyl- functionalized nanosheets show high water permeation rates as confirmed by our molecular dynamic simulations, while maintaining high NaCl rejection. Control of the surface chemistry and the interlayer spacing therefore offers opportunities to tune the selectivity of the membranes while enhancing their stability.

Exceptional catalytic effects of black phosphorus quantum dots in shuttling-free lithium sulfur batteries.

Lithium sulfur batteries with high energy densities are promising next-generation energy storage systems. However, shuttling and sluggish conversion of polysulfides to solid lithium sulfides limit the full utilization of active materials. Physical/chemical confinement is useful for anchoring polysulfides, but not effective for utilizing the blocked intermediates. Here, we employ black phosphorus quantum dots as electrocatalysts to overcome these issues. Both the experimental and theoretical results reveal that black phosphorus quantum dots effectively adsorb and catalyze polysulfide conversion. The activity is attributed to the numerous catalytically active sites on the edges of the quantum dots. In the presence of a small amount of black phosphorus quantum dots, the porous carbon/sulfur cathodes exhibit rapid reaction kinetics and no shuttling of polysulfides, enabling a low capacity fading rate (0.027% per cycle over 1000 cycles) and high areal capacities. Our findings demonstrate application of a metal-free quantum dot catalyst for high energy rechargeable batteries.

The dynamics of copper intercalated molybdenum ditelluride.

Layered transition metal dichalcogenides are emerging as key materials in nanoelectronics and energy applications. Predictive models to understand their growth, thermomechanical properties, and interaction with metals are needed in order to accelerate their incorporation into commercial products. Interatomic potentials enable large-scale atomistic simulations connecting first principle methods and devices. We present a ReaxFF reactive force field to describe molybdenum ditelluride and its interactions with copper. We optimized the force field parameters to describe the energetics, atomic charges, and mechanical properties of (i) layered MoTe2, Mo, and Cu in various phases, (ii) the intercalation of Cu atoms and small clusters within the van der Waals gap of MoTe2, and (iii) bond dissociation curves. The training set consists of an extensive set of first principles calculations computed using density functional theory (DFT). We validate the force field via the prediction of the adhesion of a single layer MoTe2 on a Cu(111) surface and find good agreement with DFT results not used in the training set. We characterized the mobility of the Cu ions intercalated into MoTe2 under the presence of an external electric field via finite temperature molecular dynamics simulations. The results show a significant increase in drift velocity for electric fields of approximately 0.4 V/Å and that mobility increases with Cu ion concentration.

Atomistic simulations of electrochemical metallization cells: mechanisms of ultra-fast resistance switching in nanoscale devices.

We describe a new method that enables reactive molecular dynamics (MD) simulations of electrochemical processes and apply it to study electrochemical metallization cells (ECMs). The model, called EChemDID, extends the charge equilibration method to capture the effect of external electrochemical potential on partial atomic charges and describes its equilibration over connected metallic structures, on-the-fly, during the MD simulation. We use EChemDID to simulate resistance switching in nanoscale ECMs; these devices consist of an electroactive metal separated from an inactive electrode by an insulator and can be reversibly switched to a low-resistance state by the electrochemical formation of a conducting filament between electrodes. Our structures use Cu as the active electrode and SiO2 as the dielectric and have dimensions at the foreseen limit of scalability of the technology, with a dielectric thickness of approximately 1 nm. We explore the effect of device geometry on switching timescales and find that nanowires with an electroactive shell, where ions migrate towards a smaller inactive electrode core, result in faster switching than planar devices. We observe significant device-to-device variability in switching timescales and intermittent switching for these nanoscale devices. To characterize the evolution in the electronic structure of the dielectric as dissolved metallic ions switch the device, we perform density functional theory calculations on structures obtained from an EChemDID MD simulation. These results confirm the appearance of states around the Fermi energy as the metallic filament bridges the electrodes and show that the metallic ions and not defects in the dielectric contribute to the majority of those states.

Voltage equilibration for reactive atomistic simulations of electrochemical processes.

We introduce electrochemical dynamics with implicit degrees of freedom (EChemDID), a model to describe electrochemical driving force in reactive molecular dynamics simulations. The method describes the equilibration of external electrochemical potentials (voltage) within metallic structures and their effect on the self-consistent partial atomic charges used in reactive molecular dynamics. An additional variable assigned to each atom denotes the local potential in its vicinity and we use fictitious, but computationally convenient, dynamics to describe its equilibration within connected metallic structures on-the-fly during the molecular dynamics simulation. This local electrostatic potential is used to dynamically modify the atomic electronegativities used to compute partial atomic changes via charge equilibration. Validation tests show that the method provides an accurate description of the electric fields generated by the applied voltage and the driving force for electrochemical reactions. We demonstrate EChemDID via simulations of the operation of electrochemical metallization cells. The simulations predict the switching of the device between a high-resistance to a low-resistance state as a conductive metallic bridge is formed and resistive currents that can be compared with experimental measurements. In addition to applications in nanoelectronics, EChemDID could be useful to model electrochemical energy conversion devices.

Atomic origin of ultrafast resistance switching in nanoscale electrometallization cells.

Nanoscale resistance-switching cells that operate via the electrochemical formation and disruption of metallic filaments that bridge two electrodes are among the most promising devices for post-CMOS electronics. Despite their importance, the mechanisms that govern their remarkable properties are not fully understood, especially for nanoscale devices operating at ultrafast rates, limiting our ability to assess the ultimate performance and scalability of this technology. We present the first atomistic simulations of the operation of conductive bridging cells using reactive molecular dynamics with a charge equilibration method extended to describe electrochemical reactions. The simulations predict the ultrafast switching observed in these devices, with timescales ranging from hundreds of picoseconds to a few nanoseconds for devices consisting of Cu active electrodes and amorphous silica dielectrics and with dimensions corresponding to their scaling limit (cross-sections below 10 nm). We find that single-atom-chain bridges often form during device operation but that they are metastable, with lifetimes below a nanosecond. The formation of stable filaments involves the aggregation of ions into small metallic clusters, followed by a progressive chemical reduction as they become connected to the cathode. Contrary to observations in larger cells, the nanoscale conductive bridges often lack crystalline order. An atomic-level mechanistic understanding of the switching process provides guidelines for materials optimization for such applications and the quantitative predictions over an ensemble of devices provide insight into their ultimate scaling and performance.

Ferromagnetic Spin Coupling through the 3,4'-Biphenyl Moiety in Arylamine Oligomers-Experimental and Computational Study.

This report describes the study of a dimer d2+ and a linear trimer t3+ of amminium radical cations coupled by 3,4'-biphenyl spin coupling units. The synthesis of the parent diamine and triamine and their optical and electrochemical properties obtained by UV-visible and cyclic voltammetry are presented. The chemical doping of the parent diamine d and triamine t was performed quantitatively to obtain samples containing the corresponding dimer d2+ and trimer t3+ in almost pure high-spin states as evidenced by pulsed EPR nutation spectroscopy. The J coupling constants of the corresponding S = 1 and S = 3/2 spin states were measured (J/k = 135 K) and compared quantitatively to DFT calculations.

Magnetic properties of a doped linear polyarylamine bearing a high concentration of coupled spins (S = 1).

Magnetic properties of a doped linear polyarylamine (PA2), whose chain includes alternating para-phenylene and meta-phenylene groups, and of two cyclic and linear model compounds (C2 and D2) were explored by pulsed-EPR nutation spectroscopy, SQUID magnetometry and DFT calculations. Stoichiometrically doped PA2 samples exhibit a pure S = 1 state (exchange coupling constant J = 18 K) with a high spin concentration (0.65) corresponding to 65% of mers bearing holes. Such properties were already observed for doped reticulated polyarylamines but are quite unusual for doped linear polyarylamines. In order to better understand the properties of PA2, model compounds C2 and D2 were also investigated: pure S = 1 spin states could also be obtained, but with higher J (respectively 57 K and 35 K) and, surprisingly, with high but still limited spin concentrations (respectively 0.77 and 0.65).

Analysis of the singlet-triplet splitting computed by the density functional theory-broken-symmetry method: is it an exchange coupling constant?

We derive an analytical expression of the density functional theory (DFT)-broken-symmetry (BS) estimation J(BS) of the singlet-triplet gap at the "3 sites-4 electrons" level, that is, two S = (1)/(2) metallic sites + one diamagnetic bridge orbital. As originally designed by Noodleman and Davidson (Chem. Phys.1986, 109, 131), J(BS) contains the residual ferromagnetic contribution, single ligand-to-metal and metal-to-metal charge-transfer terms, but no double ligand-to-metal charge-transfer terms or intra/interligand spin-polarization terms. As revealed by the present analysis, the triplet and BS states computed by DFT differ, not only perturbatively (as expected) because of the various physical mechanisms involved (i.e., differential charge-transfer terms) but mainly because of a spurious and unphysical symmetry breaking of the bridge orbitals in the BS state. We examine the consequences of such a difference by deriving two analytical expressions of the exchange coupling constant, one from the BS orbitals designed to match J(BS) and another one from triplet orbitals only. Following and extending on the first paper in the series (J. Phys. Chem. A 2010, 114, 6149), we propose a simple procedure to extract appropriate parameters filling in our analytical expressions. Moreover, we derive the equivalent "3 sites-4 electrons" exchange coupling constant in the configuration-interaction approach, J(CI), for the purpose of comparison. These analytical expressions have been applied to various copper dimers and compared to experimental values.

Valence bond/broken symmetry analysis of the exchange coupling constant in copper(II) dimers. Ferromagnetic contribution exalted through combined ligand topology and (singlet) covalent-ionic mixing.

In this paper we aim at presenting a full-VB (valence-bond) analysis of the DFT broken symmetry (BS) exchange coupling constant J(BS). We extend Kahn and Briat's "two sites-two electrons" VB original formalism (Kahn, O.; Briat, B. J. Chem. Soc. Farady Trans. II, 1976, 72, 268) by taking into account the covalent-ionic singlet state mixing, here translated into intersite magnetic orbital delocalization. In this way, two explicit contributions to the magnetic orbital overlap appear, one from the purely covalent state, and the other one from the covalent-ionic mixing. This scheme allows us to relax the strict orthogonality constraint of Kahn and Briat's chemically heuristic model resulting into ferromagnetism. Moreover, we show how DFT-BS calculations applied to various copper(II) dimers yield effective parameters that can be injected into the full-VB model, allowing for a breaking down of J(BS) into various contributions, one of which being either ferromagnetic or antiferromagnetic depending on the bridging ligand topology. Two classes of systems emerge from this analysis and the exceptional ferromagnetic coupling property of the "end-on" azido-bridged copper dimer is especially emphasized.