Judith A. Harrison

A second-generation reactive empirical bond order (REBO) potential energy expression for hydrocarbons

A second-generation potential energy function for solid carbon and hydrocarbon molecules that is based on an empirical bond order formalism is presented. This potential allows for covalent bond breaking and forming with associated changes in atomic hybridization within a classical potential, producing a powerful method for modelling complex chemistry in large many-atom systems. This revised potential contains improved analytic functions and an extended database relative to an earlier version (Brenner D W 1990 Phys. Rev. B 42 9458). These lead to a significantly better description of bond energies, lengths, and force constants for hydrocarbon molecules, as well as elastic properties, interstitial defect energies, and surface energies for diamond.

A reactive potential for hydrocarbons with intermolecular interactions

A potential function is presented that can be used to model both chemical reactions and intermolecular interactions in condensed-phase hydrocarbon systems such as liquids, graphite, and polymers. This potential is derived from a well-known dissociable hydrocarbon force field, the reactive empirical bond-order potential. The extensions include an adaptive treatment of the nonbonded and dihedral-angle interactions, which still allows for covalent bonding interactions. Torsional potentials are introduced via a novel interaction potential that does not require a fixed hybridization state. The resulting model is intended as a first step towards a transferable, empirical potential capable of simulating chemical reactions in a variety of environments. The current implementation has been validated against structural and energetic properties of both gaseous and liquid hydrocarbons, and is expected to prove useful in simulations of hydrocarbon liquids, thin films, and other saturated hydrocarbon systems.

Investigation of the atomic-scale friction and energy dissipation in diamond using molecular dynamics

We have used molecular dynamics simulations to examine friction when two diamond (111) surfaces are placed in sliding contact. The essence of atomic-scale friction was shown to be the mechanical excitation (in the form of vibrational and rotational energy) of the interface lattice layers upon sliding. This excitation was propagated to the rest of the lattice, and eventually dissipated as heat. In general, this excitation increases with increasing applied load; therefore, the atomic-scale friction also increases with load. Flexible hydrocarbon species, chemically bound to the diamond surface, can lead to a significant reduction of mechanical excitation upon sliding at high loads, leading to lower friction. In addition to clarifying the effects of chemically-bound hydrocarbon groups on atomic-scale friction at diamond interfaces, these simulations might also yield insight into more complicated systems, e.g. Langmuir-Blodgett films, and aid in the design of low-friction coatings.

Understanding collision cascades in molecular solids

Abstract This paper describes simulations of the sputtering of a molecular solid that uses a reactive potential with both covalent bonding and van der Waals interactions. Recently, the adaptive intermolecular REBO (AIREBO) potential has been developed, which incorporates intermolecular interactions in a manner that maintains the reactivity of the original reactive empirical bond-order (REBO) potential. Preliminary simulations of the keV bombardment of a molecular solid have been performed using the AIREBO potential. Molecules that are initially struck by the bombarding particle break into fragments. The fragments initiate molecular collision cascades leading to the ejection of intact molecules and molecular fragments from the surface.

Elucidating atomic-scale friction using molecular dynamics and specialized analysis techniques

Because all quantities associated with a given atom are known as a function of time, molecular dynamics simulations can provide unparalleled insight into dynamic processes. Many quantities calculated from simulations can be directly compared to experimental values, while others provide information not available from experiment. For example, the tilt and methyl angles of chains within a self-assembled monolayer and the amount of hydrogen in a diamond-like carbon (DLC) film are measurable in an experiment. In contrast, the atomic contact force on a single substrate atom, i.e., the force on that atom due to the tip atoms only, and the changes in hybridization of a carbon atom within a DLC film during sliding are not quantities that are currently obtainable from experiments. Herein, the computation of many quantities, including the ones discussed above, and the unique insights that they provided into compression, friction, and wear are discussed.

Periodicities in the properties associated with the friction of model self-assembled monolayers

The classical molecular dynamics simulations presented here examine the periodicities associated with the sliding of a diamond counterface across a monolayer of hydrocarbon chains that are covalently bound to a diamond substrate. Periodicities observed in a number of system quantities are a result of the tight packing of the monolayer and the commensurate structure of the diamond counterface. The packing and commensurability of the system force synchronized motion of the chains during sliding contact. This implies that the size of the simulations for this special case can be reduced so that the simulations can be conducted with sliding speeds and time durations that may bridge the gap between theory and experiment.

Friction between solids

The theoretical examination of the friction between solids is discussed with a focus on self-assembled monolayers, carbon-containing materials and antiwear additives. Important findings are illustrated by describing examples where simulations have complemented experimental work by providing a deeper understanding of the molecular origins of friction. Most of the work discussed herein makes use of classical molecular dynamics (MD) simulations. Of course, classical MD is not the only theoretical tool available to study friction. In view of that, a brief review of the early models of friction is also given. It should be noted that some topics related to the friction between solids, i.e. theory of electronic friction, are not discussed here but will be discussed in a subsequent review.

Atomic-Scale Wear of Amorphous Hydrogenated Carbon during Intermittent Contact: A Combined Study Using Experiment, Simulation, and Theory

In this study, we explore the wear behavior of amplitude modulation atomic force microscopy (AM-AFM, an intermittent-contact AFM mode) tips coated with a common type of diamond-like carbon, amorphous hydrogenated carbon (a-C:H), when scanned against an ultra-nanocrystalline diamond (UNCD) sample both experimentally and through molecular dynamics (MD) simulations. Finite element analysis is utilized in a unique way to create a representative geometry of the tip to be simulated in MD. To conduct consistent and quantitative experiments, we apply a protocol that involves determining the tip-sample interaction geometry, calculating the tip-sample force and normal contact stress over the course of the wear test, and precisely quantifying the wear volume using high-resolution transmission electron microscopy imaging. The results reveal gradual wear of a-C:H with no sign of fracture or plastic deformation. The wear rate of a-C:H is consistent with a reaction-rate-based wear theory, which predicts an exponential dependence of the rate of atom removal on the average normal contact stress. From this, kinetic parameters governing the wear process are estimated. MD simulations of an a-C:H tip, whose radius is comparable to the tip radii used in experiments, making contact with a UNCD sample multiple times exhibit an atomic-level removal process. The atomistic wear events observed in the simulations are correlated with under-coordinated atomic species at the contacting surfaces.

Simulated engineering of nanostructures

Results are reported from two molecular-dynamics simulations designed to yield insight into the engineering of nanometre-scale structures. The first is the initial stages of the indentation of a silicon substrate by an atomically-sharp diamond tip. Up to an indentation depth of approximately 0.6 nm the substrate responds elastically and the profile of the disturbed region of the substrate normal to the surface reflects the shape of the tip apex. The disturbed region in the plane of the surface, however, reflects the symmetry of the substrate rather than that of the tip. As indentation progresses the damage to the substrate becomes irreversible, and the profile of the damage normal to the substrate surface approximately matches that of the tip, while the in-plane profile appears roughly circular rather than displaying the symmetry of either the tip or substrate. The tip maintains its integrity throughout the simulation, which had a maximum indentation depth of 1.2 nm. The second study demonstrates patterning of a diamond substrate using a group of ethynyl radicals attached to a diamond tip. The tip is designed so that the terrace containing the radicals has an atomically-sharp protrusion that can protect the radicals during a tip crash. At contact between the tip and substrate the protrusion is elastically deformed, and five of six chemisorbed radicals abstract hydrogen atoms during the 1.25 ps the tip is in contact with the surface. Displacement of the tip an additional 2.5 A, however, results in permanent damage to the protrusion with little deformation of the substrate.

Simulated adhesion between realistic hydrocarbon materials: effects of composition, roughness, and contact point.

The work of adhesion is an interfacial materials property that is often extracted from atomic force microscope (AFM) measurements of the pull-off force for tips in contact with flat substrates. Such measurements rely on the use of continuum contact mechanics models, which ignore the atomic structure and contain other assumptions that can be challenging to justify from experiments alone. In this work, molecular dynamics is used to examine work of adhesion values obtained from simulations that mimic such AFM experiments and to examine variables that influence the calculated work of adhesion. Ultrastrong carbon-based materials, which are relevant to high-performance AFM and nano- and micromanufacturing applications, are considered. The three tips used in the simulations were composed of amorphous carbon terminated with hydrogen (a-C-H), and ultrananocrystalline diamond with and without hydrogen (UNCD-H and UNCD, respectively). The model substrate materials used were amorphous carbon with hydrogen termination (a-C-H) and without hydrogen (a-C); ultrananocrystalline diamond with (UNCD-H) and without hydrogen (UNCD); and the (111) face of single crystal diamond with (C(111)-H) and without a monolayer of hydrogen (C(111)). The a-C-H tip was found to have the lowest work of adhesion on all substrates examined, followed by the UNCD-H and then the UNCD tips. This trend is attributable to a combination of roughness on both the tip and sample, the degree of alignment of tip and substrate atoms, and the surface termination. Continuum estimates of the pull-off forces were approximately 2-5 times larger than the MD value for all but one tip-sample pair.