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Direct molecular dynamics simulation of atomic deposition under realistic conditions is notoriously challenging because of the wide range of time scales that must be captured. Numerous simulation approaches have been proposed to address the problem, often requiring a compromise between model fidelity, algorithmic complexity, and computational efficiency. Coarse projective integration, an example application of the "equation-free" framework, offers an attractive balance between these constraints. Here, periodically applied, short atomistic simulations are employed to compute time derivatives of slowly evolving coarse variables that are then used to numerically integrate differential equations over relatively large time intervals. A key obstacle to the application of this technique in realistic settings is the "lifting" operation in which a valid atomistic configuration is recreated from knowledge of the coarse variables. Using Ge deposition on amorphous SiO2 substrates as an example application, we present a scheme for lifting realistic atomistic configurations comprised of collections of Ge islands on amorphous SiO2 using only a few measures of the island size distribution. The approach is shown to provide accurate initial configurations to restart molecular dynamics simulations at arbitrary points in time, enabling the application of coarse projective integration for this morphologically complex system. [ABSTRACT FROM AUTHOR]

Nonequilibrium processes during solidification can lead to kinetic stabilization of metastable crystal phases. A general framework for predicting the solidification conditions that lead to metastable-phase growth is developed and applied to a model face-centered cubic (fcc) metal that undergoes phase transitions to the body-centered cubic (bcc) as well as the hexagonal close-packed phases at high temperatures and pressures. Large-scale molecular dynamics simulations of ultrarapid freezing show that bcc nucleates and grows well outside of the region of its thermodynamic stability. An extensive study of crystal-liquid equilibria confirms that at any given pressure, there is a multitude of metastable solid phases that can coexist with the liquid phase. We define for every crystal phase, a solid cluster in liquid (SCL) basin, which contains all solid clusters of that phase coexisting with the liquid. A rigorous methodology is developed that allows for practical calculations of nucleation rates into arbitrary SCL basins from the undercooled melt. It is demonstrated that at large undercoolings, phase selections made during the nucleation stage can be undone by kinetic instabilities amid the growth stage. On these bases, a solidification-kinetic phase diagram is drawn for the model fcc system that delimits the conditions for macroscopic grains of metastable bcc phase to grow from the melt. We conclude with a study of unconventional interfacial kinetics at special interfaces, which can bring about heterogeneous multiphase crystal growth. A first-order interfacial phase transformation accompanied by a growth-mode transition is examined. [ABSTRACT FROM AUTHOR]

We study sputtering by 100 eV deuterium irradiation on deuterated amorphous carbon layers at 300 K using molecular dynamics (MD) simulations. Two main results are reported here. First, a special mechanism for carbon release—additional to and distinct from the standard definitions for physical and chemical sputtering of carbon by hydrogen isotopes—has been identified and quantified. This process, here termed ion induced release of unsaturated hydrocarbons (IRUH’s), is primarily due to a recently identified atomic collision process where momentum from an impacting particle is transferred approximately perpendicular to the C–C bond, severing it. For the prescribed conditions, the IRUH yield has been found to be comparable to that of standard physical and chemical sputtering, the former being also consistently and simultaneously calculated here. IRUH release of single C atoms does not involve any hydrogenic chemistry and is therefore properly considered to be a distinct and additive type of physical sputtering to that of standard physical sputtering. For 100 eV D+ the single C yields of the two physical sputtering mechanisms have been found to be approximately equal. IRUH release of carbon is directly from the surface region of the solid and is separate from, and additional to, standard chemical sputtering (not included in these MD calculations), which typically produces saturated hydrocarbons such as CD4, from regions extending over the stopping depth of the deuteron in the solid. IRUH is evidently included in experimental measurements of total sputtering yield, e.g., by weight loss. The average energy of IRUH carbon products is about 1 eV and the angular distribution is consistent with a cosine distribution. Second, it is found that for the standard physically sputtered single C atoms the energy distribution is roughly consistent with the widely used Thompson distribution—this despite the fact that the assumptions on which the Thompson distribution is based are not satisfied for 100 eV D on C. The angular distribution of the standard physically sputtered single C atoms is also found to be consistent with the usually assumed cosine distribution. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations in the isobaric–isenthalpic ensemble are used to derive various important parameters related to nucleation from the melt. In particular, by using results from liquid–solid phase equilibria, we present a simple, yet reliable, approach to extract melting temperature, critical nucleus size, and liquid–solid interfacial free energy for six different embedded atom FCC systems. Our approach is based on the fundamental thermodynamic relation given by the Gibbs–Thomson equation that uses the heat released during solidification to drive growth of the solid phase until thermodynamic equilibrium is achieved. Equilibrium between the liquid and solid phases is identified by a steady-state evolution in the temperature of the two-phase system. • MD simulations of melting temperature, critical size, and interfacial free energy. • Gibbs–Thomson equation sets equilibrium of solid surrounded by its own liquid. • Robust & general methodology for calculation of thermodynamic liquid–solid properties. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were used to study Ge island nucleation and growth on amorphous SiO 2 substrates. This process is relevant in selective epitaxial growth of Ge on Si, for which SiO 2 is often used as a template mask. The islanding process was studied over a wide range of temperatures and fluxes, using a recently proposed empirical potential model for the Si–SiO 2 –Ge system. The simulations provide an excellent quantitative picture of the Ge islanding and compare well with detailed experimental measurements. These quantitative comparisons were enabled by an analytical rate model as a bridge between simulations and experiments despite the fact that deposition fluxes accessible in simulations and experiments are necessarily different by many orders of magnitude. In particular, the simulations led to accurate predictions of the critical island size and the scaling of island density as a function of temperature. The overall approach used here should be useful not just for future studies in this particular system, but also for molecular simulations of deposition in other materials. [ABSTRACT FROM AUTHOR]

We employ molecular dynamics simulations of defect-free nanopillars with realistic cylindrical geometries to obtain an atomic-level picture of their deformation behavior under compression. We find that dislocations are nucleated in the two outermost surface layers. Furthermore, plastic yield depends crucially on the particular arrangement of steps and facets at the surface of the nanopillars. We show that different facet orientations can differ dramatically in their response to external stresses. Freestanding nanopillars exhibit a highly nonuniform distribution of stresses along their height. This causes an elastic deformation that leads to a barrel-like shape attained by the nanopillars under compression. The stress concentration at the center of the pillars due to barreling causes dislocations to preferentially nucleate in this region. [ABSTRACT FROM AUTHOR]

We apply a hybrid Monte Carlo-molecular dynamics code to the study of grain boundary motion upon annealing of pure Cu and Cu with low concentrations of Fe. The hybrid simulations account for segregation and precipitation of the low solubility Fe, together with curvature-driven grain boundary motion. Grain boundaries in two different systems, a Σ7+U-shaped half-loop grain and a nanocrystalline sample, were found to be pinned in the presence of Fe concentrations exceeding 3%. [ABSTRACT FROM AUTHOR]