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We have developed a machine learning (ML)-assisted Hybrid ReaxFF simulation method ("Hybrid/Reax"), which alternates reactive and non-reactive molecular dynamics simulations with the assistance of ML models to simulate phenomena that require longer time scales and/or larger systems than are typically accessible to ReaxFF. Hybrid/Reax uses a specialized tracking tool during the reactive simulations to further accelerate chemical reactions. Non-reactive simulations are used to equilibrate the system after the reactive simulation stage. ML models are used between reactive and non-reactive stages to predict non-reactive force field parameters of the system based on the updated bond topology. Hybrid/Reax simulation cycles can be continued until the desired chemical reactions are observed. As a case study, this method was used to study the cross-linking of a polyethylene (PE) matrix analogue (decane) with the cross-linking agent dicumyl peroxide (DCP). We were able to run relatively long simulations [>20 million molecular dynamics (MD) steps] on a small test system (4660 atoms) to simulate cross-linking reactions of PE in the presence of DCP. Starting with 80 PE molecules, more than half of them cross-linked by the end of the Hybrid/Reax cycles on a single Xeon processor in under 48 h. This simulation would take approximately 1 month if run with pure ReaxFF MD on the same machine.

Since their discovery in late 1940s, perovskite ferroelectric materials have become one of the central objects of condensed matter physics and materials science due to the broad spectrum of functional behaviors they exhibit, including electro-optical phenomena and strong electromechanical coupling. In such disordered materials, the static properties of defects such as oxygen vacancies are well explored but the dynamic effects are less understood. In this work, the first observation of enhanced electromechanical response in BaTiO3 thin films is reported driven via dynamic local oxygen vacancy control in piezoresponse force microscopy (PFM). A persistence in peizoelectricity past the bulk Curie temperature and an enhanced electromechanical response due to a created internal electric field that further enhances the intrinsic electrostriction are explicitly demonstrated. The findings are supported by a series of temperature dependent band excitation PFM in ultrahigh vacuum and a combination of modeling techniques including finite element modeling, reactive force field, and density functional theory. This study shows the pivotal role that dynamics of vacancies in complex oxides can play in determining functional properties and thus provides a new route toward- achieving enhanced ferroic response with higher functional temperature windows in ferroelectrics and other ferroic materials.

Ferroelectric materials such as barium titanate (BaTiO3) have a wide range of applications in nano scale electronic devices due to their outstanding properties. In this study, we developed an easily extendable atomistic ReaxFF reactive force field for BaTiO3 that can capture both its field- and temperature-induced ferroelectric hysteresis and corresponding changes due to surface chemistry and bulk defects. Using our force field, we were able to reproduce and explain a number of experimental observations: (1) the existence of a critical thickness of 4.8 nm below which ferroelectricity vanishes in BaTiO3; (2) migration and clustering of oxygen vacancies (OVs) in BaTiO3 and a reduction in the polarization and the Curie temperature due to the OVs; (3) domain wall interaction with the surface chemistry to influence the ferroelectric switching and polarization magnitude. This new computational tool opens up a wide range of possibilities for making predictions for realistic ferroelectric interfaces in energy-conversion, electronic and neuromorphic systems.

We propose a novel method to create vacancy defects on MoS2 structures. The method is similar to a potato stamp process, which involves creating vacancy defects on a graphene layer and stamping to the MoS2 surface. On the basis of nudged elastic band and density functional theory calculations, we predict that sulfur atoms on the surface will diffuse into the vacancy sites of the graphene layer. Separation of the graphene layer will carry away diffused sulfur atoms, leaving the MoS2 surface with sulfur vacancy defects. We carried out molecular dynamics simulations with the ReaxFF reactive force field to test the potato stamp concept and then functionalized the MoS2 surface defects with epoxy molecules. We observed dissociation of epoxy molecules at the vacancy site, in which the exposed metal atoms had catalytic activity.

We explore the thickness dependence of the converse piezoelectric coefficient (${d}_{33}$) in epitaxial thin films of ${\mathrm{BaTiO}}_{3}$ (BTO) grown on (001) ${\mathrm{SrTiO}}_{3}$ substrates. Piezoresponse force microscope was performed using an atomic force microscope equipped with an interferometric displacement sensor allowing direct quantification of electromechanical coupling coefficients in BTO free from unwanted background contributions. We find that 80-nm-thick films exhibit a ${d}_{33}$ of $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}20.5\phantom{\rule{0.16em}{0ex}}\mathrm{pm}/\mathrm{V}$, but as the thickness is reduced, the ${d}_{33}$ reduces to less than 2 pm/V for a 10 nm film. To explain the atomistic origin of the effect, we performed molecular dynamics simulations with a recently developed ab initio-derived reactive force field, constructed using the ReaxFF framework. Simulations predict that under applied electric fields thin films of ${\mathrm{BaTiO}}_{3}$ show an increasing thickness, with compressive strain, of the region screening the depolarization-field. This study confirms quantitatively the drop in piezoelectric performance in BTO ultrathin films and again highlights the importance of the screening mechanisms when films approach the ultrathin limits in dictating the functional behaviors.

Ferroelectric materials such as barium titanate (BaTiO3) have a wide range of applications in nano scale electronic devices due to their outstanding properties. In this study, we developed an easily extendable atomistic ReaxFF reactive force field for BaTiO3 that can capture both its field- and temperature-induced ferroelectric hysteresis and corresponding changes due to surface chemistry and bulk defects. Using our force field, we were able to reproduce and explain a number of experimental observations: (1) the existence of a critical thickness of 4.8 nm below which ferroelectricity vanishes in BaTiO3; (2) migration and clustering of oxygen vacancies (OVs) in BaTiO3 and a reduction in the polarization and the Curie temperature due to the OVs; (3) domain wall interaction with the surface chemistry to influence the ferroelectric switching and polarization magnitude. This new computational tool opens up a wide range of possibilities for making predictions for realistic ferroelectric interfaces in energy-conversion, electronic and neuromorphic systems.

Defects have a profound impact on the electronic and physical properties of crystals. For two-dimensional (2D) materials, many intrinsic point defects have been reported, but much remains to be understood about their origin. Using scanning transmission electron microscopy imaging, this study discovers various linear arrays of W-vacancy defects that are explained in the context of the crystal growth of coalesced, monolayer WS2. Atomistic-scale simulations show that vacancy arrays can result from steric hindrance of bulky gas-phase precursors at narrowly separated growth edges, and that increasing the edge separation leads to various intact and defective growth modes, which are driven by competition between the catalytic effects of the sapphire substrate and neighboring growth edge. Therefore, we hypothesize that the arrays result from combined growth modes, which directly result from film coalescence. The connections drawn here will guide future synthetic and processing strategies to harness the engineering potential of defects in 2D monolayers.

Failure mechanisms of poly(p-phenylene terephthalamide (PPTA) under extreme tensile deformation has been studied using reactive potentials with molecular dynamics simulations. Amorphous PPTA systems with different molecular weights generated using an in-house developed amorphous builder. Tensile modulus of amorphous PPTA has been calculated as up to 6.7 GPa. Nitrogen and carbon vacancy defects were introduced to both crystalline and amorphous systems. The tensile modulus of defects-free crystalline PPTA calculated as 350 GPa. Introduction of 5% nitrogen vacancy defects reduced the tensile modulus to 197 GPa. To estimate fiber modulus, PPTA fiber considered to be composed of amorphous and crystalline phases. Rule of mixtures formula modified to incorporate influence of defects on tensile modulus. Histograms of various quantities such as bond lengths, bond angles and phenyl ring diameters were calculated at different strain levels. Tensile load was mostly accommodated through stretching of bonds between amide group and phenyl groups. Under extreme tensile deformation PPTA chains failed at these C-N bonds. (C) 2015 Elsevier B.V. All rights reserved.

We modeled poly(p-phenylene terephthalamide) (PPTA) fibers in realistic size and structure and used Molecular Dynamics (MD) simulations with the ReaxFF reactive force field to investigate response to tensile deformation at extreme values. We constructed PPTA fibers in three forms: fully crystalline, fully unordered and core-shell structured with core and shell regions consisting of unordered and crystalline chains. The tensile deformation applied quasi-statically up to 15% strain. We calculated the average tensile modulus of fully unordered and crystalline fibers as 192 GPa and 289 GPa, indicating that crystallinity significantly increases this modulus. We introduced defects in the form of nitrogen vacancies with densities up to 5% and formulated an empirical equation to predict tensile modulus of the fiber based on defect density and crystallinity of the fiber. During the simulation, we observed domains in the crystalline region of fibers and failure of fibers started with breaking of chains located at these domain boundaries. As such, in this paper we present the first realistic-scale fully reactive molecular dynamics simulations of structure and failure in high performance fibers.

International audience; Structural transformations of nano-systems show properties different from their 3D bulk analogs. The mechanism of transformation is different due to the nano-object size or nanometric confinement. This field has not yet been fully explored and requires more fundamental studies which may lead to future application, e.g., in nanoporous systems characterization. Properties of methane adsorbed in confined geometries are interesting from both fundamental and practical points of view. At ambient temperatures supercritical adsorption is studied for possible methane storage applications. At the same time, the analysis of methane’s low temperature adsorption properties is essential for understanding the mechanism of adsorption as a function of temperature, pore size, pore topology and heterogeneity. The temperature dependence of structures of adsorbed phases allows one to understand evolution of confined phases in nano-space. In this work we compare the mechanism of adsorption and structural transformations of methane confined in (i) homogeneous carbon slit pores of widths between 1nm and 4nm and (ii) heterogeneous MOF pores. We discuss the mechanism of layering transition, melting and capillary condensation in subcritical conditions, for temperatures between 80 K and 180 K. The mechanism of layer formation is strongly temperature dependent. At the same time, a commensurability between the methane and the pore sizes is important factor. In slit pores it evolves from a sharp layers formation at 80 K to continuous adsorption at higher temperatures. The capillary condensation is observed clearly in slit pores bigger than 2 nm. The melting transition of methane in slit pores shows complicated mechanism where the interior of the pore melts at lower temperature than the layers adsorbed closer the pore wall. We compare this behavior with the mechanism of adsorption in MOF porous structures where the adsorbing walls are strongly heterogeneous both structurally and energetically. The comparison between the slit pores and the MOFs will allow us to discuss an influence of the heterogeneous wall topology and intermolecular interactions on the thermodynamic properties of nano-confined structures.

This article reviews recent advances in the development of reactive empirical force fields or potentials. In particular, we compare two widely used reactive potentials with variable-charge schemes that are desirable for multicomponent or multifunctional systems: the ReaxFF (reactive force field) and charge-optimized many-body (COMB) potentials. Several applications of these approaches in atomistic simulations that involve metal-based heterogeneous systems are also discussed.

International audience; It is well known that the nano-systems exhibit properties different from their bulk analogs. Typically, the phase diagrams are redefined because the position of coexistence lines depends on the size and shape of the nano-objects. This is related to the fact that nano-systems are characterized by high surface-to-volume ratio. The surface atoms are weakly bonded and their contribution to the latent heat is smaller. Consequently, the surface usually transforms at lower temperature and the whole transition may happen smoothly over a finite range of temperatures. This observation suggests that there is no temperature of melting (or any other struc-tural change) in the conventional sense because the structural (phase) changes are gradual and phases are no longer distinguishable.Here we discuss mechanism of methane melting [1-3] and its structural transformations when adsorbed in nanoporous systems. The general analysis of methane melting in slit pores was already discussed by Mi-yahara and Gubbins [1]. In this paper, we emphasize the influence of structural heterogeneity on the mecha-nism of structural transformations. As an example, we discuss the mechanism of melting of methane con-fined in two different structures: first, in 3 and 4 nm slit pores, then, in 2.8 nm square channels of SURMOF porous structure. Mechanism of melting transformation in both cases will be compared and the correlation between the nano-scale and heterogeneity will be emphasized and discussed. Fig. 1 presents an order pa-rameter calculated in different layers of the confined system as a function of temperature.Structural transformations of adsorbate are interesting phenomena from both fundamental and practical point of view. The mechanism of transformations is defined by the characteristic nanosize of the system and the influence of the interactions with the pore framework. This defines the unusual properties of the confined system which can be used for the system characterization.

International audience;

International audience; It is well known that the nano-systems exhibit properties different from their bulk analogs. Typically, the phase diagrams are redefined because the position of coexistence lines depends on the size and shape of the nano-objects. This is related to the fact that nano-systems are characterized by high surface-to-volume ratio. The surface atoms are weakly bonded and their contribution to the latent heat is smaller. Consequently, the surface usually transforms at lower temperature and the whole transition may happen smoothly over a finite range of temperatures. This observation suggests that there is no temperature of melting (or any other structural change) in the conventional sense because the structural (phase) changes are gradual and phases are no longer distinguishable.Here we discuss mechanism of methane melting [1-3] in confined nanoporous systems. The general analysis of methane in slit pores was already discussed by Miyahara and Gubbins [1]. In this paper, we emphasize the influence of structural heterogeneity on the mechanism of structural transformations. As an example, we discuss the differences in mechanism of melting of methane confined in two different structures: first, in 3 and 4 nm slit pores, then, in 2.8 nm square channels of SURMOF porous structure. Mechanism of melting transformation in both cases will be compared and the correlation between the nano-scale and heterogeneity will be emphasized and discussed.

International audience;

International audience; Properties of methane adsorbed in confined geometries are interesting from both fundamental and practical points of view. At ambient temperatures adsorption at the supercritical conditions is usually studied, as it is interesting from the point of view of methane storage. At the same time, the analysis of low temperature adsorption properties is essential for understanding the mechanism of adsorption, as a function of temperature and the pore size. In this work we study phase diagram of methane confined in carbon slit pores of the width between 1nm and 4nm. We analyze the mechanism of the layering transition(s) and capillary condensation at subcritical conditions, for temperatures between 80 K and 180 K, then we study the pore s storage capacity, up to the room temperature. The mechanism of the layers formation is strongly temperature dependent, and changes from a sharp adsorption at low temperature to more continuous one at higher temperatures. The size of pore defines the number of layers adsorbed: 1 nm pore allows adsorbing 2 layers of methane molecules. Capillary condensation is observed in 4 nm pores.