Krista G. Steenbergen

Electronic effects on the melting of small gallium clusters

Motivated by experimental reports of higher-than-bulk melting temperatures in small gallium clusters, we perform first-principles molecular dynamics simulations of Ga(20) and Ga(20)(+) using parallel tempering in the microcanonical ensemble. The respective specific heat (C(V)) curves, obtained using the multiple histogram method, exhibit a broad peak centered at approximately 740 and 610 K--well above the melting temperature of bulk gallium (303 K) and in reasonable agreement with experimental data for Ga(20)(+). Assessment of atomic mobility confirms the transition from solid-like to liquid-like states near the C(V) peak temperature. Parallel tempering molecular dynamics simulations yield low-energy isomers that are ~0.1 eV lower in energy than previously reported ground state structures, indicative of an energy landscape with multiple, competing low-energy morphologies. Electronic structure analysis shows no evidence of covalent bonding, yet both the neutral and charged clusters exhibit greater-than-bulk melting temperatures.

Method of increments for the halogen molecular crystals: Cl, Br, and I

Method of increments (MI) calculations reveal the n-body correlation contributions to binding in solid chlorine, bromine, and iodine. Secondary binding contributions as well as d-correlation energies are estimated and compared between each solid halogen. We illustrate that binding is entirely determined by two-body correlation effects, which account for >80% of the total correlation energy. One-body, three-body, and exchange contributions are repulsive. Using density-fitting (DF) local coupled-cluster singles, doubles, and perturbative triples for incremental calculations, we obtain excellent agreement with the experimental cohesive energies. MI results from DF local second-order Møller-Plesset perturbation (LMP2) yield considerably over-bound cohesive energies. Comparative calculations with density functional theory and periodic LMP2 method are also shown to be less accurate for the solid halogens.

Quantum Size Effects in the Size–Temperature Phase Diagram of Gallium: Structural Characterization of Shape‐Shifting Clusters

Finite temperature analysis of cluster structures is used to identify signatures of the low-temperature polymorphs of gallium, based on the results of first-principle Born-Oppenheimer molecular dynamics simulations. Pre-melting structural transitions proceed from either the β- and/or the δ-phase to the γ- or δ-phase, with a size- dependent phase progression. We relate the stability of each isomer to the electronic structures of the different phases, giving new insight into the origin of polymorphism in this complicated element.

Dense or Porous Packing? Two‐Dimensional Self‐Assembly of Star‐Shaped Mono‐, Bi‐, and Terpyridine Derivatives

The self-assembly behavior of five star-shaped pyridyl-functionalized 1,3,5-triethynylbenzenes was studied at the interface between an organic solvent and the basal plane of graphite by scanning tunneling microscopy. The mono- and bipyridine derivatives self-assemble in closely packed 2D crystals, whereas the derivative with the more bulky terpyridines crystallizes with porous packing. DFT calculations of a monopyridine derivative on graphene, support the proposed molecular model. The calculations also reveal the formation of hydrogen bonds between the nitrogen atoms and a hydrogen atom of the neighboring central unit, as a small nonzero tunneling current was calculated within this region. The title compounds provide a versatile model system to investigate the role of multivalent steric interactions and hydrogen bonding in molecular monolayers.

A Two-Dimensional Liquid Structure Explains the Elevated Melting Temperatures of Gallium Nanoclusters.

Melting in finite-sized materials differs in two ways from the solid-liquid phase transition in bulk systems. First, there is an inherent scaling of the melting temperature below that of the bulk, known as melting point depression. Second, at small sizes changes in melting temperature become nonmonotonic and show a size-dependence that is sensitive to the structure of the particle. Melting temperatures that exceed those of the bulk material have been shown to occur for a very limited range of nanoclusters, including gallium, but have still never been ascribed a convincing physical explanation. Here, we analyze the structure of the liquid phase in gallium clusters based on molecular dynamics simulations that reproduce the greater-than-bulk melting behavior observed in experiments. We observe persistent nonspherical shape distortion indicating a stabilization of the surface, which invalidates the paradigm of melting point depression. This shape distortion suggests that the surface acts as a constraint on the liquid state that lowers its entropy relative to that of the bulk liquid and thus raises the melting temperature.

How a single aluminum atom makes a difference to gallium: First-principles simulations of bimetallic cluster melting

First principles molecular dynamics simulations of Ga19Al(+) have been performed in the microcanonical ensemble using parallel tempering. We perform a thorough investigation of the changes induced by the presence of an Al atom in the Ga dominated cluster. Dynamic analysis indicates that the Al atom prefers to occupy the internal sites of the cluster structure, at all temperatures, and above 450 K, the Al atom is less mobile than the central Ga atom throughout the simulation. Using the multiple histogram method, canonical specific heat curves are obtained that compare well with previous experimental measurements of the specific heat and equivalent simulations for the Ga20 (+) cluster. The first-principles melting temperature agrees well with the experimental value for Ga19Al(+). Analysis of the root mean squared fluctuation in bond length, velocity auto-correlation function, and the corresponding power spectrum, confirms the solid-liquid-like phase transition in Ga19Al(+), as for Ga20 (+).

Two worlds collide: Image analysis methods for quantifying structural variation in cluster molecular dynamics

Inspired by methods of remote sensing image analysis, we analyze structural variation in cluster molecular dynamics (MD) simulations through a unique application of the principal component analysis (PCA) and Pearson Correlation Coefficient (PCC). The PCA analysis characterizes the geometric shape of the cluster structure at each time step, yielding a detailed and quantitative measure of structural stability and variation at finite temperature. Our PCC analysis captures bond structure variation in MD, which can be used to both supplement the PCA analysis as well as compare bond patterns between different cluster sizes. Relying only on atomic position data, without requirement for a priori structural input, PCA and PCC can be used to analyze both classical and ab initio MD simulations for any cluster composition or electronic configuration. Taken together, these statistical tools represent powerful new techniques for quantitative structural characterization and isomer identification in cluster MD.