methods are utilized as computational tools in many areas of chemical physics. In this paper, we present the theoretical basis for a dynamical Monte Carlo method in terms of the theory of Poisson processes. We show that if: (1) a "dynamical hierarchy" of transition probabilities is created which also satisfy the detailed-balance criterion; (2) time increments upon successful events are calculated appropriately; and (3) the effective independence of various events comprising the system can be achieved, then Monte Carlo methods may be utilized to simulate the Poisson process and both static and dynamic properties of model Hamiltonian systems may be obtained and interpreted consistently.

We use density functional theory to resolve the role of polyvinylpyrrolidone (PVP) in the shape-selective synthesis of Ag nanostructures. At the segment level, PVP binds more strongly to Ag(100) than Ag(111) because of a surface-sensitive balance between direct binding and van der Waals attraction. At the chain level, correlated segment binding leads to a strong preference for PVP bind to Ag(100). Our study underscores differences between small-molecule and polymeric structure-directing agents.

We describe a theoretical study of the role of adsorbate interactions in island nucleation and growth, using Ag/Pt(111) heteroepitaxy as an example. From density-functional theory, we obtain the substrate-mediated Ag adatom pair interaction and we find that, past the short range, a repulsive ring is formed about the adatoms. The magnitude of the repulsion is comparable to the diffusion barrier. In kinetic Monte Carlo simulations, we find that the repulsive interactions lead to island densities over an order of magnitude larger than those predicted by nucleation theory and thus identify a severe limitation of its applicability.

Shape-control is used to tune the properties of metal nanostructures in applications ranging from catalysts to touch screens, but the origins of anisotropic growth of metal nanocrystals in solution are unknown. We show single-crystal electrochemistry can test hypotheses for why nanostructures form and predict conditions for anisotropic growth by quantifying the degree to which different species cause facet-selective metal deposition. Electrochemical measurements show disruption of alkylamine monolayers by chloride ions causes facet-selective Cu deposition. An intermediate range of chloride concentrations maximizes facet-selective Cu deposition on single crystals and produces the highest aspect ratio nanowires in a solution-phase synthesis. DFT calculations similarly show an intermediate monolayer coverage of chloride displaces the alkylamine capping agent from the ends but not the sides of a nanowire, facilitating anisotropic growth.

Density, temperature, and bond-length dependence of dynamic friction on a molecular bondWe present a new method for accelerating molecular-dynamics simulations of infrequent events. The method targets simulation of systems that spend most of the time in local energy minima, with slow transitions in between, as is the case with low-temperature surface diffusion. The potential-energy surface is modified by adding a boost potential in regions close to the local minima, such that all transition rates are increased while relative rates are preserved. The boost potential is an empirical function determined by the deviation of the bond lengths of a specified set of atoms from equilibrium. The method requires no previous knowledge of the processes involved and it can be applied to a wide variety of interaction potentials. Application to the diffusion of Cu atoms on the Cu͑100͒ surface using an embedded-atom potential yields correct rates for adatom hopping, exchange, as well as vacancy and dimer diffusion with speed-ups up to several orders of magnitude.

We present a theoretical model for predicting equilibrium wetting configurations of two-dimensional droplets on periodically grooved hydrophobic surfaces. The main advantage of our model is that it accounts for pinning/depinning of the contact line at step edges, a feature that is not captured by the Cassie and Wenzel models. We also account for the effects of gravity (via the Bond number) on various wetting configurations that can occur. Using free-energy minimization, we construct phase diagrams depicting the dependence of the wetting modes (including the number of surface grooves involved in the wetting configuration) and their corresponding contact angles on the geometrical parameters characterizing the patterned surface. In the limit of vanishing Bond number, the predicted wetting modes and contact angles become independent of drop size if the geometrical parameters are scaled with drop radius. Contact angles predicted by our continuum-level theoretical model are in good agreement with corresponding results from nanometer-scale molecular dynamics simulations. Our theoretical predictions are also in good agreement with experimentally measured contact angles of small drops, for which gravitational effects on interface deformation are negligible. We show that contact-line pinning is important for superhydrophobicity and that the contact angle is maximized when the droplet size is comparable to the length scale of the surface pattern.

Oriented attachment (OA) of nanocrystals is now widely recognized as a key process in the solution-phase growth of hierarchical nanostructures. However, the microscopic origins of OA remain unclear. We perform molecular dynamics simulations using a recently developed ReaxFF reactive force field to study the aggregation of various titanium dioxide (anatase) nanocrystals in vacuum and humid environments. In vacuum, the nanocrystals merge along their direction of approach, resulting in a polycrystalline material. By contrast, in the presence of water vapor the nanocrystals reorient themselves and aggregate via the OA mechanism to form a single or twinned crystal. They accomplish this by creating a dynamic network of hydrogen bonds between surface hydroxyls and surface oxygens of aggregating nanocrystals. We determine that OA is dominant on surfaces that have the greatest propensity to dissociate water. Our results are consistent with experiment, are likely to be general for aqueous oxide systems, and demonstrate the critical role of solvent in nanocrystal aggregation. This work opens up new possibilities for directing nanocrystal growth to fabricate nanomaterials with desired shapes and sizes.

Chloride (Cl–) is often used together with polyvinylpyrrolidone (PVP) in the polyol synthesis of Ag nanocubes. In the literature, shape control is attributed predominantly to the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets, whereas the role of Cl– has not been well studied. Several hypotheses have been proposed regarding the role of Cl–; however, there is still no consensus regarding the exact influence of Cl– in the shape-controlled synthesis of Ag nanocubes. To examine the influence of Cl–, we undertook a joint theoretical–experimental study. Experimentally, we examined the influence of Cl– concentration on the shape of Ag nanoparticles (NPs) at constant H+ concentration. In the presence of H+, in situ formed HNO3 etches the initially formed Ag seeds and slows down the overall reduction of Ag+, which promotes the formation of monodisperse Ag NPs. Ex situ experiments probed the evolution of Cl– during the growth of Ag nanocubes, which involves the initial formation of AgCl nanocubes, and their subsequent dissolution to release Cl–, which adsorbs onto the surfaces of single crystal seeds to impact shape evolution through apparent thermodynamic control. The formation of cubes is independent of the source of AgCl, indicating temporal control of the Cl– chemical potential in solution leads to high-yield synthesis of Ag nanocubes. Increasing the concentration of Cl– alone leads to a progression in shape from truncated octahedra, to cuboctahedra, truncated cubes, and ultimately cubes, directly demonstrating the importance of Cl– in Ag NP shape control. We used ab initio thermodynamics calculations based on density functional theory to probe the role of Cl– in directing shape control. With increasing Cl chemical potential (surface coverage), calculated surface energies γ of Ag facets transition from γ111 < γ100 to γ100 < γ111 and predict Wulff shapes terminated with an increasing (100) contribution, consistent with experimental observations. The combination of theory and experiment is beneficial for advancing the understanding of nanocrystal formation.