Magnesium-Induced Strain and Immobilized Radical Generation on the Boron Oxide Surface Enhances the Oxidation Rate of Boron Particles: A DFTB-MD Study.

Despite their high gravimetric and volumetric energy densities, boron (B) particles suffer from poor oxidative energy release rates as the boron oxide (B2O3) shell impedes the diffusivity of O2 to the particle interior. Recent experiemental studies have shown that the addition of metals with a lower free energy of oxidation, such as Mg, can reduce the oxide shell of B and enhance the energetic performance of B by ∼30-60%. However, the exact underlying mechanism behind the reactivity enhancement is unknown. Here, we performed DFTB-MD simulations to study the reaction of Mg vapor with a B2O3 surface. We found that the Mg becomes oxidized on the B2O3 surface, forming a MgBxOy phase, which induces a tensile strain in the B-O bond at the MgBxOy-B2O3 interface, simultaneously reducing the interfacial B and thereby developing dangling bonds. The interfacial bond straining creates an overall surface expansion, indicating the presence of a net tensile strain. The B with dangling bonds can act as active centers for gas-phase O2 adsorption, thereby increasing the adsorption rate, and the overall tensile strain on the surface will increase the diffusion flux of adsorbed O through the surface to the particle core. As the overall B particle oxidation rate is dependent on both the O adsorption and diffusion rates, the enhancement in both of these rates increases the overall reactivity of B particles.

Enhancing the accuracy of density functional tight binding models through ChIMES many-body interaction potentials.

Semi-empirical quantum models such as Density Functional Tight Binding (DFTB) are attractive methods for obtaining quantum simulation data at longer time and length scales than possible with standard approaches. However, application of these models can require lengthy effort due to the lack of a systematic approach for their development. In this work, we discuss the use of the Chebyshev Interaction Model for Efficient Simulation (ChIMES) to create rapidly parameterized DFTB models, which exhibit strong transferability due to the inclusion of many-body interactions that might otherwise be inaccurate. We apply our modeling approach to silicon polymorphs and review previous work on titanium hydride. We also review the creation of a general purpose DFTB/ChIMES model for organic molecules and compounds that approaches hybrid functional and coupled cluster accuracy with two orders of magnitude fewer parameters than similar neural network approaches. In all cases, DFTB/ChIMES yields similar accuracy to the underlying quantum method with orders of magnitude improvement in computational cost. Our developments provide a way to create computationally efficient and highly accurate simulations over varying extreme thermodynamic conditions, where physical and chemical properties can be difficult to interrogate directly, and there is historically a significant reliance on theoretical approaches for interpretation and validation of experimental results.

Semi-Automated Creation of Density Functional Tight Binding Models through Leveraging Chebyshev Polynomial-Based Force Fields.

Density functional tight binding (DFTB) is an attractive method for accelerated quantum simulations of condensed matter due to its enhanced computational efficiency over standard density functional theory (DFT) approaches. However, DFTB models can be challenging to determine for individual systems of interest, especially for metallic and interfacial systems where different bonding arrangements can lead to significant changes in electronic states. In this regard, we have created a rapid-screening approach for determining systematically improvable DFTB interaction potentials that can yield transferable models for a variety of conditions. Our method leverages a recent reactive molecular dynamics force field where many-body interactions are represented by linear combinations of Chebyshev polynomials. This allows for the efficient creation of multi-center representations with relative ease, requiring only a small investment in initial DFT calculations. We have focused our workflow on TiH2 as a model system and show that a relatively small training set based on unit-cell-sized calculations yields a model accurate for both bulk and surface properties. Our approach is easy to implement and can yield reliable DFTB models over a broad range of thermodynamic conditions, where physical and chemical properties can be difficult to interrogate directly and there is historically a significant reliance on theoretical approaches for interpretation and validation of experimental results.

Guest-Dependent Stabilization of the Low-Spin State in Spin-Crossover Metal-Organic Frameworks.

Computer simulations are carried out to characterize the variation of spin-crossover (SCO) behavior of the prototypical {Fe(pz)[Pt(CN)4]} metal-organic framework (MOF) upon adsorption of chemically and structurally different guest molecules. A detailed analysis of both strength and anisotropy of guest molecule-framework interactions reveals direct correlations between the mobility of the guest molecules inside the MOF pores, the rotational mobility of the pyrazine rings of the framework, and the stabilization of the low-spin state of the material. On the basis of these correlations, precise molecular criteria are established for predicting the spin state of {Fe(pz)[Pt(CN)4]} upon guest adsorption. Finally, predictions of the SCO temperature upon adsorption of various toxic gases demonstrate that in silico modeling can provide fundamental insights and design principles for the development of spin-crossover MOFs for applications in gas detection and chemical sensing.

Ultrafast direct electron transfer at organic semiconductor and metal interfaces.

The ability to control direct electron transfer can facilitate the development of new molecular electronics, light-harvesting materials, and photocatalysis. However, control of direct electron transfer has been rarely reported, and the molecular conformation-electron dynamics relationships remain unclear. We describe direct electron transfer at buried interfaces between an organic polymer semiconductor film and a gold substrate by observing the first dynamical electric field-induced vibrational sum frequency generation (VSFG). In transient electric field-induced VSFG measurements on this system, we observe dynamical responses (<150 fs) that depend on photon energy and polarization, demonstrating that electrons are directly transferred from the Fermi level of gold to the lowest unoccupied molecular orbital of organic semiconductor. Transient spectra further reveal that, although the interfaces are prepared without deliberate alignment control, a subensemble of surface molecules can adopt conformations for direct electron transfer. Density functional theory calculations support the experimental results and ascribe the observed electron transfer to a flat-lying polymer configuration in which electronic orbitals are found to be delocalized across the interface. The present observation of direct electron transfer at complex interfaces and the insights gained into the relationship between molecular conformations and electron dynamics will have implications for implementing novel direct electron transfer in energy materials.

Pore Breathing of Metal-Organic Frameworks by Environmental Transmission Electron Microscopy.

Metal-organic frameworks (MOFs) have emerged as a versatile platform for the rational design of multifunctional materials, combining large specific surface areas with flexible, periodic frameworks that can undergo reversible structural transitions, or "breathing", upon temperature and pressure changes, and through gas adsorption/desorption processes. Although MOF breathing can be inferred from the analysis of adsorption isotherms, direct observation of the structural transitions has been lacking, and the underlying processes of framework reorganization in individual MOF nanocrystals is largely unknown. In this study, we describe the characterization and elucidation of these processes through the combination of in situ environmental transmission electron microscopy (ETEM) and computer simulations. This combined approach enables the direct monitoring of the breathing behavior of individual MIL-53(Cr) nanocrystals upon reversible water adsorption and temperature changes. The ability to characterize structural changes in single nanocrystals and extract lattice level information through in silico correlation provides fundamental insights into the relationship between pore size/shape and host-guest interactions.

Many-Body Interactions in Ice.

Many-body effects in ice are investigated through a systematic analysis of the lattice energies of several proton ordered and disordered phases, which are calculated with different flexible water models, ranging from pairwise additive (q-TIP4P/F) to polarizable (TTM3-F and AMOEBA) and explicit many-body (MB-pol) potential energy functions. Comparisons with available experimental and diffusion Monte Carlo data emphasize the importance of an accurate description of the individual terms of the many-body expansion of the interaction energy between water molecules for the correct prediction of the energy ordering of the ice phases. Further analysis of the MB-pol results, in terms of fundamental energy contributions, demonstrates that the differences in lattice energies between different ice phases are sensitively dependent on the subtle balance between short-range two-body and three-body interactions, many-body induction, and dispersion energy. By correctly reproducing many-body effects at both short range and long range, it is found that MB-pol accurately predicts the energetics of different ice phases, which provides further support for the accuracy of MB-pol in representing the properties of water from the gas to the condensed phase.

Spin Crossover in the {Fe(pz)[Pt(CN)4]} Metal-Organic Framework upon Pyrazine Adsorption.

The spin-crossover behavior of the {Fe(pz)[Pt(CN)4]} metal-organic framework (MOF) upon pyrazine adsorption is investigated through hybrid Monte Carlo/molecular dynamics (MC/MD) simulations. In contrast to previous theoretical studies, which reported a transition temperature of ∼140 K, the present MC/MD simulations predict that the high-spin state is the most stable state at all temperatures, in agreement with the experimental observations. The MC/MD simulations also indicate that the pyrazine molecules adsorbed in the MOF pores lie nearly parallel but staggered by 60° relative to the pyrazine ligands of the framework. The analysis of the magnetization curve as a function of the temperature demonstrates that the staggered configuration assumed by the guest pyrazine molecules within the framework is responsible for the stabilization of the high-spin state. Both the guest pyrazine molecules and the pyrazine ligands of the framework are effectively locked into the minimum-energy configuration and do not display any rotational mobility.

Molecular Mechanisms of Spin Crossover in the {Fe(pz)[Pt(CN)4]} Metal-Organic Framework upon Water Adsorption.

The rational design of multifunctional materials with properties that can be selectively controlled at the molecular level is key to the development and application of nanoscale devices. In this study, molecular dynamics simulations using ligand-field molecular mechanics are performed to elucidate, for the first time, the molecular mechanisms responsible for the variation of the spin-crossover properties of the {Fe(pz)[Pt(CN)4]} metal-organic framework upon water adsorption. The simulations demonstrate a direct relationship between the water loading adsorbed in the pores and the variation of the spin-crossover transition temperature, with the high-spin state of the material becoming gradually more stabilized as the number of adsorbed water molecules increases. The decrease of the spin-crossover temperature of {Fe(pz)[Pt(CN)4]} upon water adsorption predicted by the simulations is in agreement with the available experimental data and is traced back to the elongation of the bonds between the Fe(II) atoms and the organic linkers induced by interactions of the adsorbed water molecules with the framework.

Tunneling through finite graphene superlattices: resonance splitting effect.

An exact expression of the transmission probability through a finite graphene superlattice with an arbitrary number of potential barriers n is derived in two cases of the periodic potential: rectangular electric potential and δ-function magnetic potential. Obtained transmission probabilities show two types of resonance energy: barrier-induced resonance energies unchanged as n varies and well-induced resonance energies that have undergone the (n - 1)-fold splitting as n increases. Supported by numerical calculations for various types of graphene superlattices, these analytical findings are assumed to be equally applied to all of the finite graphene superlattices regardless of their potential nature (electric or magnetic) and potential barrier shapes.

Touching points in the energy band structure of bilayer graphene superlattices.

The energy band structure of the bilayer graphene superlattices with zero-averaged periodic δ-function potentials are studied within the four-band continuum model. Using the transfer matrix method, the study is mainly focused on examining the touching points between adjacent minibands. For the zero-energy touching points the dispersion relation derived shows a Dirac-like double-cone shape with the group velocity which is periodic in the potential strength P with the period of π and becomes anisotropic at relatively large P. From the finite-energy touching points we have identified those located at zero wave-number. It was shown that for these finite-energy touching points the dispersion is direction-dependent in the sense that it is linear or parabolic in the direction parallel or perpendicular to the superlattice direction, respectively. We have also calculated the density of states and the conductivity which demonstrates a manifestation of the touching points examined.

Massless Dirac fermions in a graphene superlattice: a T-matrix approach.

Using the T-matrix approach, we study the effect of a Kronig-Penney periodic potential on the electronic states and the transport properties of graphene. The energy band structure and the group velocity of charge carriers are calculated and discussed in detail for potentials with varying amplitudes and barrier-to-well width ratios. The periodic potential is shown to cause a resonant structure and to enhance the magnitude of the conductivity.