Our ability to exploit the benefits of metallic glasses depends on identifying alloys of high glass-forming ability (GFA). So far, the established empirical correlations of GFA (ref. ) are statistical guides at best and lack a microscopic rationale. Although simulations have the potential to provide this physical insight into the maximum crystallization rate, crystal nucleation is often too slow to be observed. In contrast, measuring the growth rate of a planar crystal surface represents an accessible route to understanding ordering kinetics. Here we use molecular dynamics simulations to show that the crystal growth rate for an important binary glass former, CuZr, is significantly slower than that of a poor glass former, NiAl. In accounting for this difference, we find that the crystal/liquid interface in NiAl exhibits a significantly greater width than that of CuZr. Our results suggest that the crystal/liquid interfacial structure exerts an important influence on the GFA of alloys.

Modern quantum chemical electronic structure methods typically applied to localized chemical bonding are developed to predict atomic structures and free energies for meso-tetraalkylporphyrin self-assembled monolayer (SAM) polymorph formation from organic solution on highly ordered pyrolytic graphite surfaces. Large polymorphdependent dispersion-induced substrate−molecule interactions (e.g., −100 kcal mol −1 to −150 kcal mol −1 for tetratrisdecylporphyrin) are found to drive SAM formation, opposed nearly completely by large polymorph-dependent dispersion-induced solvent interactions (70-110 kcal mol −1 ) and entropy effects (25-40 kcal mol −1 at 298 K) favoring dissolution. Dielectric continuum models of the solvent are used, facilitating consideration of many possible SAM polymorphs, along with quantum mechanical/molecular mechanical and dispersion-corrected density functional theory calculations. These predict and interpret newly measured and existing high-resolution scanning tunnelling microscopy images of SAM structure, rationalizing polymorph formation conditions. A wide range of molecular condensed matter properties at room temperature now appear suitable for prediction and analysis using electronic structure calculations.self-assembled monolayers | density functional theory | dispersion | free energy | polymorphism A priori calculations of the free energies of chemical reactions using density functional theory (DFT) and/or ab initio methods are now well established for gas-phase processes (1, 2), gas surface reactions (3), and, using continuum self-consistent reaction field (SCRF) methods, for condensed phase processes also (4). Over the last few years, a major advance in computational methods has occurred, however, allowing for rapid and accurate evaluation of intermolecular dispersion interactions. This makes feasible similar SCRF calculations for physisorption, macromolecule structuring, and other self-assembly processes in condensed phases. We present the first application of this type, to our knowledge, considering the free energy of formation of various polymorphs of tetraalkylporphyrin self-assembled monolayers (SAMs) at solid/liquid interfaces on highly ordered pyrolytic graphite (HOPG) surfaces. Calculated structures and free energies are used to interpret new and existing high-resolution scanning tunneling microscopy (STM) images, focusing on the critical roles played by the dispersion interaction in driving SAM formation and by entropy and dispersion-based desolvation effects that oppose it.Calculating a priori free energies for SAM formation from solution is a significant challenge. Accurate representations of the substrate−molecule energies, the intermolecular energies, the solvent interaction energies, and the effects of solvent structure are required, a set of tasks that, with a few exceptions (see e.g., refs. 5 and 6), has remained prohibitive a priori. Alternatively, model calculations have been useful for identifying key qualitative features (7-11), whereas some full molecular dynamics si...

The calculation of the accurate surface energies for (0001) surfaces of wurtzite ZnO is difficult because it is impossible to decouple the two inequivalent (0001)-Zn and (0001¯)-O surfaces. By using a heterojunction model we have transformed the uncertainty of the surface energies into that of interface energies which is much smaller than the former and hence estimated the surface energies to a high degree of accuracy. It is found that the oxygen terminated (0001¯)-O face of the wurtzite phase and (1¯1¯1¯O of the zinc blende phase are more stable than their Zn-terminated counterparts within the major temperature and oxygen partial pressure range accessible to experiment. The instability of Zn-terminated polar surfaces explains the experimentally observed high activity of these surfaces. The effects of native surface vacancies on the surface energies have also been discussed. These results provide insights into the modification of the surface stability and activity of ZnO nanoparticles.

The rich stereochemistry of the self-assembled monolayers (SAMs) of four butanethiols on Au(111) is described, the SAMs containing up to 12 individual C, S, or Au chiral centers per surface unit cell. This is facilitated by synthesis of enantiomerically pure 2-butanethiol (the smallest unsubstituted chiral alkanethiol), followed by in situ scanning tunneling microscopy (STM) imaging combined with density functional theory molecular dynamics STM image simulations. Even though butanethiol SAMs manifest strong headgroup interactions, steric interactions are shown to dominate SAM structure and chirality. Indeed, steric interactions are shown to dictate the nature of the headgroup itself, whether it takes on the adatom-bound motif RS(•)Au(0)S(•)R or involves direct binding of RS(•) to face-centered-cubic or hexagonal-close-packed sites. Binding as RS(•) produces large, organizationally chiral domains even when R is achiral, while adatom binding leads to rectangular plane groups that suppress long-range expression of chirality. Binding as RS(•) also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs.

No abstract