48 research outputs found
Molecular dynamics study of resistance-switching in nanoscale electrometallization cells
In the search of new memory devices, conductive bridge random access memory have been of particular interest due to their low power consumption, fast write/read capability, high endurance, and scaling limits reaching nanometers. These devices consist of a metal–insulator–metal structure and switch between high and low conductivity states with the application of voltage due to the formation and dissolution of a metallic conductive bridge. We present the first molecular dynamics simulations of switching in nanoscale devices based on Cu as the active electrode and amorphous SiO2 electrolyte. The interactions between atoms are described by the reactive force field (ReaxFF) and the charges are calculated self-consistently at each step of the simulation using a modified charge equilibration method. This last method describes the voltage applied between two electrodes and the electrochemistry that occurs at the interfaces by adjusting of the local electronegativities of the atoms, identified on-the-fly during the simulation. The simulations predict the ultrafast switching observed in these devices. We find that single-atom-chain bridges often form during device operation but they are metastable with lifetimes below a nanosecond. We propose an atomic-level mechanistic understanding of the switching and provide insight into their ultimate scaling and performance. Finally, the method proposed to describe electrochemical processes is generally applicable and can also be used to study nanoscale batteries and capacitors
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 interactions with metals are needed in
order to accelerate their incorporation into commercial products. Interatomic
potentials enable large-scale atomistic simulations at the device level, beyond
the range of applications of first principle methods. 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 properties
of layered MoTe2 in various phases, the intercalation of Cu atoms and clusters
within its van der Waals gap, including a proper description of energetics,
charges and mechanical properties. The training set consists of an extensive
set of first principle calculations computed from density functional theory. We
use the force field to study the adhesion of a single layer MoTe2 on a Cu(111)
surface and the results are in good agreement with density functional theory,
even though such structures were not part of the training set. We characterized
the mobility of the Cu ions intercalated into MoTe2 under the presence of an
external electric fields via molecular dynamics simulations. The results show a
significant increase in drift velocity for electric fields of approximately 0.4
V/A and that mobility increases with Cu ion concentration.Comment: 21 pages, 9 Figure
Molecular Exploration Tool
Density Functional Theory (DFT) which is based on quantum mechanics theory has been broadly used to compute the energy and the structure of molecules and solids. However, the DFT method is limited when running calculations for a large system and only thousands of atoms can be solved. Alternatively, Molecular Dynamics (MD) simulation can be used to investigate the properties of the atomic system for large systems in the classical mechanics approximation. When running the MD simulation, the electronic structure is approximated by Force Fields (FF) which can be parametrized against DFT calculations. Nevertheless, the accuracy of the MD results and the FF is suspicious for the scientists because of the variety and complexity of the FF. Hence, a free web-browser based tool has been developed to allow the user upload a force field, run MD simulations and compare the results with the DFT calculations. Users can select desired molecules and solids in the database, run MD simulation, plot the corresponding energies and visualize the atomic structures. So that users can find out if they can trust the FF results according to the comparison with DFT calculations
Building Predictive Chemistry Models
Density Functional Theory (DFT) simulations allow for sophisticated modeling of chemical interactions, but the extreme computational cost makes it inviable for large scale applications. Molecular dynamics models, specifically ReaxFF, can model much larger simulations with greater speed, but with lesser accuracy. The accuracy of ReaxFF can be improved by comparing predictions of both methods and tuning ReaxFF’s parameters. Molecular capabilities of ReaxFF were gauged by simulating copper complexes in water over a 200 ps range, and comparing energy predictions against ReaxFF. To gauge solid state capabilities, volumetric strain was applied to simulated copper bulk and the strain response functions used to predict elastic constants, which were then compared against experimental data and ReaxFF predictions. Results suggest ReaxFF’s predictions are fairly robust, making it useful for molecular simulations. Training ReaxFF with this data can improve the accuracy of molecular dynamics simulations, providing wider application of molecular modeling software
Molexpl: a tool for ab initio data exploration and visualization
Density functional theory (DFT) based on ab initio theory, is a powerful method to resolve the electronic structure of atoms, molecules and solids. However, in practical, DFT is limited to few hundreds of atoms. To overcome this limitation, researchers have developed empirical interatomic potentials implemented in molecular dynamics (MD) simulations. MD ignores the movements of electrons and describes bonding and non-bonding interaction as a function of the distance between atoms called force fields (FF) or interatomic potentials. These empirical potentials are optimized against large datasets of DFT calculations relevant to describe the interactions between the atoms included in the training set. The Molexpl tool has been created to allow the user to explore a database of DFT calculations and to compare the results against classical MD simulations. The tool displays molecular structures and energy curves such as dissociation curve or equation of state. The initial database includes a set of elements that can be updated dynamically. The user can select multiple elements, and can choose between running MD or explore the database of DFT calculations. Moreover, Molexpl allows the user to upload a force-field(FF)or a training set for more specific research purpose. Finally, this tool can serve as a portal to share fundamental properties of molecules and crystals with the scientific community or be used as a data visualizer in the classroom. Beyond the academic purpose, this tool can help to judge the quality of a FF applied to a specific problem
Breakdown of superfluidity of an atom laser past an obstacle
The 1D flow of a continuous beam of Bose-Einstein condensed atoms in the
presence of an obstacle is studied as a function of the beam velocity and of
the type of perturbing potential (representing the interaction of the obstacle
with the atoms of the beam). We identify the relevant regimes:
stationary/time-dependent and superfluid/dissipative; the absence of drag is
used as a criterion for superfluidity. There exists a critical velocity below
which the flow is superfluid. For attractive obstacles, we show that this
critical velocity can reach the value predicted by Landau's approach. For
penetrable obstacles, it is shown that superfluidity is recovered at large beam
velocity. Finally, enormous differences in drag occur when switching from
repulsive to attractive potential.Comment: 15 pages, 6 figure
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 approximate to 80% and full-cell energy efficiency of 20.3% with a specific current density of 261.4 mA cm(-2) for C2+ products.
Strategies to systematically tune CO2 electroreduction to multicarbon products are of high interests. Here the authors report electron withdrawing functional group alters the reaction pathway towards C2+ products by adjusting the oxidation state of surface copper.D.V., K.Q., and H.L.W. acknowledge funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 804320). L.L., D.V., and H.L.W acknowledge the use of TEM instrumentation provided by the Nation Facility ELECMI ICTS (`Division de Microscopia Electronica', Universidad de Cadiz, DME-UCA). L.L. acknowledges funding from the Andalusian regional government (FEDER-UCA-18-106613), the European Union's Horizon 2020 research and innovation program (grant agreement 823717-ESTEEM3), and the Spanish Ministerio de Economia y Competitividad (PID2019-107578GA-I00). K.Q. and Y.Z. acknowledge financial support from the China Postdoctoral Science Foundation (2018M633127) and the Natural Science Foundation of Guangdong Province (2018A030310602). J.L. acknowledge financial support from the National Natural Science Foundation of China (21808134). We thank Soleil Synchrotron and Andrea Zitolo for allocating beamtime at beamline Samba within the proposal 20200732