Atomistic simulations of materials chemistry: from nanoelectronics to energetic materials

Predictive, physics-based modeling with quantified uncertainties has the potential to revolutionize the design and certification of materials and devices. Accomplishing this requires not only advances in modeling and simulation across scales, but also their synergistic combination with experiments via rigorous methods to quantify uncertainties and arrive at the desired decision in an optimal manner. I will illustrate our recent progress in the field with two applications: Atomistic simulations of electrochemical reactions. I will describe a method that combines reactive interatomic potentials with a modified charge equilibration method to simulate electrochemical cells of interest in nanoelectronics and energy. We apply the method to electrometalization cells on interest for nonvolatile memory applications. These resistance-switching devices operate via the electrochemical formation and disruption of metallic filaments and our simulations predict switching timescales ranging from hundreds of picoseconds to a few nanoseconds for device dimensions corresponding to the scaling limit. The simulations provide the first atomic-scale picture of the operation of these devices and show that stable switching proceeds via the formation of small metallic clusters and their progressive chemical reduction as they become connected to the cathode. Molecular dynamics chemical reactions under extreme conditions. We use reactive force fields to characterize chemical reactions in high-energy density materials under dynamical mechanical and electromagnetic insults. Under such conditions chemical reactions can occur under nonequilibrium conditions and our goal is to understand and, eventually, exploit such chemistry. The impact of predictive simulations in materials design would be enormously accelerated if advanced simulation tools were universally available and useful. I will describe nanoHUB.org, a web-portal that provides users from around the world access to simulation tools free of charge and simply using a web-browser, with no need to download and install any software or provide hardware resources. I will illustrate the use of online materials simulations for research and education

Cloud computing in nanoHUB powering education and research

The synergistic integration of data from physics-based simulations and experiments within a decision-making framework has the potential to revolutionize the discovery, optimization, and certification of materials and devices. Transforming this vision into a reality requires the rapid transition of cutting-edge research codes from developers to researchers who can use them in design and optimization and to instructors who are training next generations of engineers and scientists. NSF’s nanoHUB empowers simulation tool developers to make their codes universally accessible and useful via cloud computing, and users who can run these tools directly from their web browsers or iPads neither without the need to download or install any software nor to provide compute cycles. In this discussion, I will illustrate the use of nanoHUB tools in materials education and research focusing on ab initio electronic structure and molecular dynamics simulations. In the area of education, I will discuss learning modules for undergraduates designed to help students develop a more intuitive and deeper understanding of how materials look and work at atomic scales. Our research shows improved learning via hands-on simulations and powerful visualization tools. In the area of research I will demonstrate nanoHUB tools for electronic structure and thermal transport calculations with examples from the recent scientific literature

Role of nanoscale coherent precipitates on the thermo-mechanical response of martensitic materials

White lines represent the free energy as a function of lattice parameter in the martensitic phase and in the coherent precipitates. Their combined landscapes (bottom) can exhibit properties not achievable otherwise. Martensitic phase transitions are first order, diffusionless, solid-to-solid, transformations that underlie shape memory, superelasticity, and strengthening in modern steels, impacting a wide range of technologies. While shape memory and superelasticity in traditional alloys are well understood from a mechanistic point of view, recent unexpected results by our group and others indicate a much richer set of phenomena yet to be fully characterized and with significant potential to result in improved or tunable properties. Specifically, martensitic materials with coherent second phases or nanoscale variations in composition have been shown to exhibit uncharacteristic, and some unprecedented, properties. I will discuss recent work in our group that combines theory with high-fidelity atomistic simulations to demonstrate that a tailored, coherent second phase can modify the free energy landscape that governs the martensitic transformation and achieve notable changes in response, see Figure on the right. We demonstrated that a coherent second phase can reduce the energy barrier that separates the martensite and austenite phases and reduce the hysteresis of the transformation.1 More importantly, we demonstrated ultra-low stiffness metallic alloys with high strength.2 We predicted Young’s moduli as low as 2GPa, a value typical of soft materials, in full density metallic nanomaterials. This remarkable result is possible by the stabilization of a thermodynamically unstable state with negative stiffness via interfacial stresses caused by the coherent second phase. MD simulations further revealed how the size and shape of the second phase affects the hysteresis and temperature of the phase transition as well as the martensitic microstructure.3 In addition, we showed that coherency stresses from an appropriately chose second phase can also change the nature of the martensitic transformation in metallic alloys from first order (discontinuous transitions) to second order or continuous transformations. Large scale MD simulations showed a remarkable change in the character of the martensitic transformation in Ni-Al alloys near the critical point. We observed continuous transformation, uncharacteristic martensitic microstructures, and scaling behavior described by power-law exponents compatible to those of similar second-order transitions. Please click Additional Files below to see the full abstract

Role of Strain on Electronic and Mechanical Response of Semiconducting Transition-Metal Dichalcogenide Monolayers: an ab-initio study

We characterize the electronic structure and elasticity of monolayer transition-metal dichalcogenides MX2 (M=Mo, W, Sn, Hf and X=S, Se, Te) with 2H and 1T structures using fully relativistic first principles calculations based on density functional theory. We focus on the role of strain on the band structure and band alignment across the series 2D materials. We find that strain has a significant effect on the band gap; a biaxial strain of 1% decreases the band gap in the 2H structures, by as a much 0.2 eV in MoS2 and WS2, while increasing it for the 1T materials. These results indicate that strain is a powerful avenue to modulate their properties; for example, strain enables the formation of, otherwise impossible, broken gap heterostructures within the 2H class. These calculations provide insight and quantitative information for the rational development of heterostructures based on these class of materials accounting for the effect of strain.Comment: 16 pages, 4 figures, 1 table, supplementary materia

Reply to “Comment on ‘Phase diagram of MgO from density-functional theory and molecular-dynamics simulations’”

In answer to a Comment by Belonoshko [Phys. Rev. B 63, 096101 (2001)], we show that the B1-liquid melting curve of MgO obtained using two-phase simulations is in good agreement with the published one obtained using the Clausius-Clapeyron equation in conjunction with separate single phase calculations of liquid and solid