Paulette Clancy

A critical study of the simulation of the liquid-vapour interface of a Lennard-Jones fluid

Despite the fact that the surface tension for a Lennard-Jones fluid has been simulated many times in the past, there is some considerable disagreement between the results. This paper calculates the surface tension and density profiles for the liquid-vapour interface of a Lennard-Jones fluid using molecular dynamics (MD) simulation techniques for a variety of system sizes, film thicknesses, interfacial areas, interatomic potential cut-offs, and temperatures. The results are compared with previous work in order to resolve some of the discrepancies of the past work. Combining this work with some reliable results from the past, the minimum system size, film thickness, and equilibration time necessary for the accurate description of the surface tension was determined. Using simulation results calculated for computationally-economic values of the potential cut-off, the surface tension was extrapolated to the full potential value using a tail correction and the results compared to simulations performed with long...

Computer Simulation of the Crystal Growth and Dissolution of Natural Gas Hydratesa

Natural gas hydrates are icelike solids in which the water molecules or “hosts” form a rigid cagelike structure enclosing voids in which light hydrocarbons or other small molecules, known as “guests,” are contained. These solids belong to a class of crystals known as clufhrutes.’ Gas hydrates are nonstoichiometric crystals. The number of water molecules per gas molecule, the hydration number, depends on the conditions in which the crystal was formed and the type of gas molecule in the hydrate. Usually, hydration numbers range from 5.75 to 19, as quoted by Sloan.2 There arc two types of well-characterized natural gas hydrate denoted as hydrate type I and hydrate type 11. The main factor deciding the type of gas hydrate formed is the diameter of the guest molecule. Propane forms the type I1 hydrate (with the larger cavity), while methane forms the type I (in which the cavity is smaller). Hydrates are important industrially since they form spontaneously in natural gas lines, even at temperatures above O’C, forming plugs that can reduce and stop the flow of gas. These plugs are difficult to remove. To avoid their formation, inhibitors like methanol are injected in the lines, increasing the cost of operation. Hence there is considerable economical incentive to try to understand how hydrates form and how to inhibit their growth. To do this, one needs to understand both the thermodynamics and the kinetics of hydrate growth and dissolution. There are extensive data on hydrate thermodynamic properties, and the phase diagram is well known. However, there are much fewer kinetic data available. Some hydrates exhibit an induction period, i.e., even when conditions are favorable for their formation, some time elapses before the hydrate forms. In contrast, if the hydrate is melted and the conditions reestablished for hydrate formation, there is essentially no induction period and the hydrate will form almost immediately.2 This hysteresis effect suggests that the water remembers its past hydrate history, presumably through the existence of subcritical sized hydrate crystal nuclei in the fluid. A hypothesis has been proposed for the existence of the induction period and the memory but this has yet to be proved.

Shape-anisotropy driven symmetry transformations in nanocrystal superlattice polymorphs.

Despite intense research efforts by research groups worldwide, the potential of self-assembled nanocrystal superlattices (NCSLs) has not been realized due to an incomplete understanding of the fundamental molecular interactions governing the self-assembly process. Because NCSLs reside naturally at length-scales between atomic crystals and colloidal assemblies, synthetic control over the properties of constituent nanocrystal (NC) building blocks and their coupling in ordered assemblies is expected to yield a new class of materials with remarkable optical, electronic, and vibrational characteristics. Progress toward the formation of suitable test structures and subsequent development of NCSL-based technologies has been held back by the limited control over superlattice spacing and symmetry. Here we show that NCSL symmetry can be controlled by manipulating molecular interactions between ligands bound to the NC surface and the surrounding solvent. Specifically, we demonstrate solvent vapor-mediated NCSL symmetry transformations that are driven by the orientational ordering of NCs within the lattice. The assembly of various superlattice polymorphs, including face-centered cubic (fcc), body-centered cubic (bcc), and body-centered tetragonal (bct) structures, is studied in real time using in situ grazing incidence small-angle X-ray scattering (GISAXS) under controlled solvent vapor exposure. This approach provides quantitative insights into the molecular level physics that controls solvent-ligand interactions and assembly of NCSLs. Computer simulations based on all-atom molecular dynamics techniques confirm several key insights gained from experiment.

Existence of a density maximum in extended simple point charge water

Simulations of water using the exnteded simple point charge (SPC/E) model at temperatures between 190 and 330 K were performed using molecular dynamics techniques. A maximum in the density at 1 bar pressure was found to occur at 235 K. The energies and diffusivities are also reported. The SPC/E‐modeled water exhibits a glass transition ∼177 K. No crystallization events were observed during the course of the long simulations.

Phase equilibria in extended simple point charge ice‐water systems

The characteristics of the solid/liquid transition for a modified Simple Point Charge model of water have been determined using free energy calculations supported by nonequilibrium Molecular Dynamics (NEMD) simulations. We have considered the behavior of liquid water and of a variety of ice phases. Unlike real water, the stable crystalline phase at 1 bar is not hexagonal ice I, but a denser new ice phase. The melting point of this ice was found to be near 295 K. The lower‐density ices, Ih and Ic, are less stable than water down to the glass transition temperature. The conclusions are supported by NEMD simulations of the behavior of the planar crystal–liquid interface for these different cases. The first report of the growth of ice from water using molecular simulation is shown here. The influence of the components of the intermolecular potential on the stability of the ice polymorphs is investigated. It is found that, for ice I to be the stable phase, the Lennard–Jones attractive part should be reduced, a...

Kinetic Monte Carlo simulation of the growth of polycrystalline Cu films

A kinetic Monte Carlo (KMC) technique has been developed for simulating the growth of polycrystalline thin Cu films on Cu. This method consists of an impact angle-based multiple-collision method for the deposition of incident atoms from an atomic beam and surface diffusion of the surface atoms, combined with a restriction of growth within identically oriented grains to simulate the surface morphology and porosity of a polycrystalline-like material. The effect of incident angle (measured from the substrate normal) on the growth morphology and characteristics was the main focus of the study here. This simulation scheme allowed us to observe both columnar growth, at incident angles below about 60°, and dendritic porous growth, at angles above this value. A comparison was made between results from the atomic-scale KMC simulation and existing macroscopic theories to relate the morphological features of the grown film to the angle of the incident beam. We observed that the relationship between the angle of growth and the angle of incidence depends sensitively on the rules governing the sticking probability. Comparison of the roughness of the films predicted by KMC to experimental atomic force microscopy and X-ray reflectance data shows similar behavior. The predicted density of the porous films was also in good agreement with experimental results. It was demonstrated that faceting phenomena is a function not only of temperature but also the angle of incidence of the incoming beam. At grazing angles of incidence, faceting can be observed at a temperature much lower than the minimum temperature necessary in experiments conducted at normal incidence.

Understanding Polymorphism in Organic Semiconductor Thin Films through Nanoconfinement

Understanding crystal polymorphism is a long-standing challenge relevant to many fields, such as pharmaceuticals, organic semiconductors, pigments, food, and explosives. Controlling polymorphism of organic semiconductors (OSCs) in thin films is particularly important given that such films form the active layer in most organic electronics devices and that dramatic changes in the electronic properties can be induced even by small changes in the molecular packing. However, there are very few polymorphic OSCs for which the structure-property relationships have been elucidated so far. The major challenges lie in the transient nature of metastable forms and the preparation of phase-pure, highly crystalline thin films for resolving the crystal structures and evaluating the charge transport properties. Here we demonstrate that the nanoconfinement effect combined with the flow-enhanced crystal engineering technique is a powerful and likely material-agnostic method to identify existing polymorphs in OSC materials and to prepare the individual pure forms in thin films at ambient conditions. With this method we prepared high quality crystal polymorphs and resolved crystal structures of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), including a new polymorph discovered via in situ grazing incidence X-ray diffraction and confirmed by molecular mechanic simulations. We further correlated molecular packing with charge transport properties using quantum chemical calculations and charge carrier mobility measurements. In addition, we applied our methodology to a [1]benzothieno[3,2-b][1]1benzothiophene (BTBT) derivative and successfully stabilized its metastable form.

One-dimensional self-confinement promotes polymorph selection in large-area organic semiconductor thin films

A crystals structure has significant impact on its resulting biological, physical, optical and electronic properties. In organic electronics, 6,13(bis-triisopropylsilylethynyl)pentacene (TIPS-pentacene), a small-molecule organic semiconductor, adopts metastable polymorphs possessing significantly faster charge transport than the equilibrium crystal when deposited using the solution-shearing method. Here, we use a combination of high-speed polarized optical microscopy, in situ microbeam grazing incidence wide-angle X-ray-scattering and molecular simulations to understand the mechanism behind formation of metastable TIPS-pentacene polymorphs. We observe that thin-film crystallization occurs first at the air-solution interface, and nanoscale vertical spatial confinement of the solution results in formation of metastable polymorphs, a one-dimensional and large-area analogy to crystallization of polymorphs in nanoporous matrices. We demonstrate that metastable polymorphism can be tuned with unprecedented control and produced over large areas by either varying physical confinement conditions or by tuning energetic conditions during crystallization through use of solvent molecules of various sizes.

A kinetic Monte Carlo study of the growth of Si on Si(100) at varying angles of incident deposition

Abstract Two- and three-dimensional kinetic Monte Carlo simulations were used to model the deposition of a hyperthermal molecular beam at varying angles of incidence. The simulations incorporate realistic deposition and diffusional moves, and feature many-layer growth for large systems containing up to 80 000 atoms. Kinetic Monte Carlo simulations for the three-dimensional model mimic the deposition and growth of Si on a Si(100) substrate at close to experimental length- and timescales. At high angles of incidence for the beam, the formation of porous columnar structures seen in the two-dimensional simulations evolve into flakes in the three-dimensional model. The growth angles of the columns and flakes follow the same general trends as previous ballistic deposition and molecular dynamics simulations, but existing theories do not adequately represent the simulation data. In the two-dimensional model, the effect of an additional step-edge reflection barrier increases the porosity of the deposited films at conditions for which columnar growth would be observed in the absence of the additional barrier. Raising the substrate temperature increases the widths of the columns and flakes perpendicular to the path of the beam. Increased substrate temperature also affects the depth to which grown films remain defect-free before columnar growth begins.

Tight-binding studies of the tendency for boron to cluster in c-Si. II. Interaction of dopants and defects in boron-doped Si

Clusters containing up to five boron atoms were considered as extended defects within a crystalline Si matrix. Tight-binding calculations suggest that a cluster containing two boron atoms occupying substitutional sites is stable, unlike any other small boron cluster that we studied. The formation energy increases when a third and fourth substitutional boron atom is added to the cluster. Estimates of the equilibrium concentration, using tight-binding-derived formation energies and formation entropies from the Stillinger–Weber model, indicate that B2 clusters become important when the boron doping level is ∼1018 cm−3, well below the solubility limit. In contrast, the formation energy of defect clusters involving an interstitial (BnI clusters, n=1–5, in their preferred charge states) decreases with increasing cluster size, down to 0.6 eV for B5I in a −5 charge state. None had formation energies that would lead to stable bound clusters. Several BnI clusters were found to be considerably more stable than isola...