Anne M. Chaka

Elongation and breaking mechanisms of gold nanowires under a wide range of tensile conditions

Semistatic density functional theory is used to explore the evolution of [1 1 0] and [1 1 1] gold nanowires during tensile deformation under a wide range of conditions, including different tensile axes (along high- and low-symmetry directions), nanowire shapes, and effective strain rates. Large structural changes are observed during the elongation. The analysis of such low-energy intermediate configurations provides quantitative information about the underlying energy landscape that cannot be obtained through experiments or more approximate modeling methods, and four stable intermediate atomic structures are identified. A rich diversity of deformation pathways is uncovered that converge to only two final local configurations with reproducible breaking strengths, in agreement with experimental results. Such a high reproducibility in the breaking force makes gold nanowires excellent candidates as intrinsic force standards at the nanolevel.

Geochemistry of Mineral Surfaces and Factors Affecting Their Chemical Reactivity

Publisher Summary This chapter focuses on mineral surfaces and some of the factors affecting their chemical reactivity with water and aqueous species. It presents a brief overview of the geochemistry of mineral surfaces, focusing on metal-oxide and metal-(oxy)hydroxide surfaces, including their dissolution mechanisms, development of electrical charge when in contact with aqueous solutions, and uptake of aqueous cations and anions. It also discusses some of the factors that control their chemical reactivity, including defect density, cooperative effects among adsorbates, intrinsic differences in surface properties such as isoelectric points, and differences in surface structure under hydrated conditions. It begins with a discussion of the most common minerals present in Earths crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is it provides a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH; and the nature of the electrical double layer. It also describes how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid. Furthermore, it deals with dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. Finally, it explores some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces.

Chemical, magnetic and charge ordering in the system hematite–ilmenite, Fe2O3–FeTiO3

Spin polarized electronic structure calculations of total energies for ordered supercells in the system Fe2O3–FeTiO3 suggest that some layered ordered phases are more stable than an isocompositional mechanical mixture of hematite, Fe2O3, and ilmenite, FeTiO3. This result contradicts established ideas about hematite–ilmenite phase relations because it suggests that there is at least one stable ordered phase with a bulk composition intermediate between hematite and ilmenite. It is not clear if this result is an artifact of the approximations made in generalized gradient spin density functional calculations, or if an intermediate phase, or phases, is in fact stable. The electronic structure of a 30-atom layered supercell was studied by a variety of techniques. The supercell structure is FTFFFT, where F is an Fe layer and T is a Ti layer perpendicular to the hexagonal c axis. The idea was to investigate possible charge ordering on Fe sites, that is a postulate of the ‘lamellar magnetism hypothesis’, but significant Fe2+–Fe3+ordering is not predicted.

An ultrastable platform for the study of single-atom chains

We describe a surface probe instrument capable of sustaining single atomic bond junctions in the electronic quantum conduction regime for tens of minutes, and present results for Au junctions that can be “locked” stably in n=1 and n=2 quantum conduction states with electrical conductivity nG0 (G0=2e2/h) and switched in a controlled way. The instrument measures and controls the gap formed between a probe and a flat surface with better than 5 pm long-term stability in a high-vacuum chamber at 4 K using a high-sensitivity fiber-optic interferometer that forms a Fabry–Perot cavity immediately adjacent and parallel to the probe. We also report the experimental observation of stable noninteger conduction states, along with preliminary density functional theory-based calculations of one-dimensional (1D) and two-dimensional Au “bridges” that produce comparable noninteger conduction states. Finally, we report the observation of novel stochastic processes related to nonballistic electron transport through strained ...

Simulation approaches for studying the conductance behavior of gold nanowires during tensile deformation

Under tensile deformation at 0 K, gold nanowires progressively thin through a series of metastable ordered structures down to a single atom chain. The conductances of these one-, two- or three-dimensional evolving self-ordered atomic structures are examined and used as test cases to explore several critical factors that must be considered when performing such calculations. These factors include the level of theory (tight binding, density functional theory, choice of basis set), the electrode geometry, and finally, the correspondence between conductance properties and the electronic band structure. Several example cases are explored.

A Hybrid, Quantum-Classical Approach For The Computation Of Dislocation Properties In Real Materials: Method, Limitations And Applications

In this work we introduce a hybrid ab initio-classical simulation methodology designed to incorporate the chemistry into the description of phenomena that, intrinsically, require very large systems to be properly described. This hybrid approach allows us to conduct large-scale atomistic simulations with a simple classical potential (embedded atom method (EAM), for instance) while simultaneously using a more accurate ab initio approach for critical embedded regions. The coupling is made through shared atomic shells where the two atomistic modeling approaches are relaxed in an iterative, self-consistent manner. The magnitude of the incompatibility forces arising in the shared shell is analyzed, and possible terminations for the embedded region are discussed, as a way to reduce such forces. As a test case, the formation energy of a single vacancy in aluminum at different distances from an edge dislocation is studied. Results obtained using the hybrid approach are compared to those obtained using classical methods alone, and the range of validity for the classical approach is evaluated.

Chapter 17 The Challenges in Developing Molecular Simulations of Fluid Properties for Industrial Applications

Publisher Summary This chapter examines the challenges faced in developing molecular simulations of fluid properties for industrial applications. Molecular simulation methods, both molecular dynamics and Monte Carlo, and computer speeds have developed to the point where it is possible to envision these methods as being able to provide reliable estimates of the thermal properties of industrially interesting fluids. These methods are considered to be one of the enabling technologies of computational chemistry that are expected to facilitate the application of chemical science knowledge of condensed phase properties in the chemical industry for conditions where experimental data are sparse. In molecular simulations, the potential functions describing the intermolecular interactions determine the way the phase space of a system is sampled, as well as being used to evaluate the physical properties of the system. For the sampling barrier, the short-term task is to critically evaluate the novel nonequilibrium methods. For the longer term, the challenge is to develop user-friendly simulation packages so that the learning curve cost of using simulations is reduced to a level that encourages industrial use.