Transition-pathway models of atomic diffusion on fcc metal surfaces. II. Stepped surfaces

Action-derived molecular dynamics was demonstrated in the companion paper (Paper I) to be effective for the analysis of atomic surface diffusion. The method is here applied to the search of minimum-energy paths and the calculation of activation energy barriers in more complex single-adatom diffusion processes on fcc metal surfaces containing steps. Diverse diffusion routes are investigated along and across one- or two-layer steps on different surface orientations. Fundamental diffusion mechanisms near the step corners are also studied. Results are analyzed in relation to the island growth mechanism, which is of importance to surface nanoengineering.open221

Adatom-assisted structural transformations of fullerenes

Microscopic mechanism of autocatalytic structural transformations of fullerenes is investigated by the action-derived molecular dynamics. Dynamic pathways and the corresponding activation energies are obtained for the Stone-Wales transformation in fullerene and the fullerene coalescence, under the presence of extra carbon atoms. The adatom-assisted Stone-Wales transformation is proved to be a highly probable process unit for the structural transformations and annealing treatments of carbon-based graphitic networks. The complex processes of adatom-assisted fullerene coalescence, yielding very low activation energies, are presented.open271

Size Dependence of the Nonlinear Elastic Softening of Nanoscale Graphene Monolayers under Plane-Strain Bulge Tests: A Molecular Dynamics Study

The pressure bulge test is an experimental technique to characterize the mechanical properties of microscale thin films. Here, we perform constant-temperature molecular dynamics simulations of the plane-strain cylindrical bulge test of nanosized monolayer graphene subjected to high gas pressure induced by hydrogen molecules. We observe a nonlinear elastic softening of the graphene with an increase in hydrogen pressure due to the stretching and weakening of the carbon-carbon bonds; we further observe that this softening behavior depends upon the size of the graphene monolayers. Our simulation results suggest that the traditional microscale bulge formulas, which assume constant elastic moduli, should be modified to incorporate the size dependence and elastic softening that occur in nanosized graphene bulge tests

Coalescence and T-junction formation of carbon nanotubes: Action-derived molecular dynamics simulations

The mechanisms of coalescence and T-junction formation of carbon nanotubes are analyzed using action-derived molecular dynamics. The control of kinetic energy in addition to the total energy leads to the determination of the minimum-energy atomistic pathway for each of these processes. Particularly, we find that the unit merging process of two carbon nanotubes consists of four sequential generalized Stone-Wales transformations occurring in four hexagon-heptagon pairs around the jointed part. In addition, we show that a single carbon atom may play the role of an autocatalyst, which significantly reduces the global activation energy barrier of the merging process. For T junction formation, two different models are chosen for simulation. One contains defects near the point of junction formation, while the other consists of two perfect nanotubes plus two additional carbon atoms. Our results indicate that the coalescence and junction formation of nanotubes may occur more easily than theoretically predicted in the presence of additional carbon atoms at moderate temperatures.open9

Transition-pathway models of atomic diffusion on fcc metal surfaces. I. Flat surfaces

Numerical calculation of minimum-energy paths and activation energy barriers for various atomic diffusion processes on fcc metal surfaces are presented. The computational method employed is the action-derived molecular dynamics that searches the approximate Newtonian trajectory on potential-energy surfaces. The minimization of a modified action, which facilitates the conservation of total energy and the control of kinetic energy, enables us to find efficiently the minimum-energy paths of complex microscopic processes. Diverse diffusion mechanisms on flat fcc substrates are investigated in this first part of the series. More complicated systems including surface steps are simulated in paper II.open2

Atomistic calculations of interface elastic properties in noncoherent metallic bilayers

The paper describes theoretical and computational studies associated with the interface elastic properties of noncoherent metallic bicrystals. Analytical forms of interface energy, interface stresses, and interface elastic constants are derived in terms of interatomic potential functions. Embedded-atom method potentials are then incorporated into the model to compute these excess thermodynamics variables, using energy minimization in a parallel computing environment. The proposed model is validated by calculating surface thermodynamic variables and comparing them with preexisting data. Next, the interface elastic properties of several fcc-fcc bicrystals are computed. The excess energies and stresses of interfaces are smaller than those on free surfaces of the same crystal orientations. In addition, no negative values of interface stresses are observed. Current results can be applied to various heterogeneous materials where interfaces assume a prominent role in the systems' mechanical behavior.open322

Action-based pathway Modeling for atomic surface diffusion

Action-derived molecular dynamics is applied to the simulation of self-diffusion processes on copper substrates. By minimizing a modified action with an energy conservation constraint, the method enables effective computations of minimum energy paths and activation energy barriers for the broad range of multiple timescale problems, including infrequent events and slow-mode systems. Single-adatom diffusions of hopping and exchange moves are first presented to demonstrate its performance. More complex diffusion mechanisms are simulated for hopping and exchange motions across a double-layer step on the Cu(111) surface, which are very difficult to explore by conventional molecular dynamics. Strain effects on diffusion energy barriers are also investigated for a Cu(001)flat surface. Finally, we propose an algorithm to incorporate a multiple length scale scheme into the current method, i.e., the combination of the action-derived molecular dynamics with the nonlocal quasicontinuum method. This hybrid scheme is expected to provide an efficient route to the simultaneous coupling of multiple length and timescales within a single algorithmic framework.open1

Large-scale molecular dynamics simulations of Al(111) nanoscratching

Molecular dynamics simulations of nanoscratching are performed with emphasis on the correlation between the scratching conditions and the defect mechanism in the substrate. More than six million atoms are described by the embedded atom method (EAM) potential. The scratching process is simulated by high-speed ploughing on the Al(111) surface with an atomic force microscope (AFM) tip that is geometrically modelled to be of a smoothed conical shape. A repulsive model potential is employed to represent the interaction between the AFM tip and the Al atoms. Through the visualization technique of atomic coordination number, dislocations and vacancies are identified as the two major defect types prevailing under nanoscratching. Their structures and movements are investigated for understanding the mechanisms of defect generation and evolution under various scratching conditions. The glide patterns of Shockley partial dislocation loops are obviously dependent upon the scratching directions in conjunction with the slip system of face-centred cubic (fee) single crystals. It is shown that the shape of the AFM tip directly influences the facet formation on the scratched groove. The penetration depth into the substrate during scratching is further verified to affect both surface pile-up and residual defect generations that are important in assessing the change of material properties after scratching.close403